Group Title: study in the oxidation of Kraft black liquor
Title: A Study in the oxidation of Kraft black liquor
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Title: A Study in the oxidation of Kraft black liquor
Physical Description: viii, 140 l. : illus. ; 28 cm.
Language: English
Creator: Ricca, Peter Mauro, 1936-
Publication Date: 1962
Copyright Date: 1962
 Subjects
Subject: Oxidation   ( lcsh )
Air -- Pollution   ( lcsh )
Paper industry   ( lcsh )
Civil Engineering thesis Ph. D
Dissertations, Academic -- Civil Engineering -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis--University of Florida.
Bibliography: Bibliography: l. 136-139.
Additional Physical Form: Also available on World Wide Web
General Note: Manuscript copy.
General Note: Vita.
 Record Information
Bibliographic ID: UF00097974
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000421870
oclc - 11020771
notis - ACG9868

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A STUDY IN THE OXIDATION

OF KRAFT BLACK LIQUOR












By
PETER MAURO RICCA












A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY










UNIVERSITY OF FLORIDA


February, 1962











Abstract of Dissertation Presented to the Graduate Council
in Partial Fulfillment of the Requirements for the
Degree of Doctor of Philosophy

A STUDY IN THE OXIDATION OF KRAFT BLACK LIQUOR

By

Peter Mauro Ricca

February, 1962


This dissertation was undertaken as a fundamental research project

at the University of Florida Air Pollution Laboratory. Black liquor

oxidation,a process which reduces odors and other air-borne chemical

losses, has seen limited use in the Kraft paper industry. Technical

difficulties caused by unique characteristics of southern black liquor

have impeded the employment of this process in the southeastern United

States. In this research an attempt was made to provide some general

process design criteria which are applicable regardless of location or

liquor characteristics. The relative importance of temperature, oxidant

pressure, method of contact, and type of gaseous oxidizing agent was

determined. The chemical reactions that occurred during oxidation were

also investigated.

A southern Kraft black liquor containing 6.1 grams per liter

of sodium sulfide was oxidized using pure oxygen. This alkaline

sodium base black liquor also contained polysulfide, thiosulfate,

sulfate, carbonate, hydroxide, and soluble organic matter. A non-

foaming bench scale contactor was used and certain physical and chemical

parameters varied. The predominant inorganic reactions that occurred

were the oxidation of sodium hydrosulfide to sodium polysulfide and

sodium thiosulfate. The thiosulfate reaction was slightly reversible











although the polysulfide reaction was not. The mole ratio of poly-

sulfide to thiosulfate formed by complete oxidation of the hydrosulfide

was relatively independent of temperature. Approximately 1.0 moles

of sodium polysulfide was formed for every 1.6 moles of sodium thio-

sulfate produced. Oxidation below 600C precipitated small quantities

of amorphous sulfur droplets from the liquor. These droplets upon

aging crystallized to rhombic sulfur which in turn decomposed and

re-dissolved in the alkaline black liquor.

The method of gas-liquid contact affected the rate of reaction

but not the quality nor quantity of the major inorganic reaction

products. Oxidation at 60C required the least volume of oxygen, but

oxidation proceeded most rapidly at 750C. Therefore the optimum

oxidation temperature lay between 600C and 750C. When adequate gas-

liquid contact was made available, the reaction rate was limited by

the oxidation rate of hydrosulfide. This oxidation very closely

approximated a first-order reaction.

Evidence collected suggested that the organic material present

in the black liquor catalyzed the oxidation of the hydrosulfide.

Oxidation took place in two distinct steps: first, absorption of

oxygen and, second, the utilization of this oxygen to oxidize the

hydrosulfide. Undesirable side reactions that also utilize some of

the absorbed oxygen were increasingly prevalent above 600C.

Pure oxygen at atmospheric pressures had only limited effective-

ness in oxidizing the odorous constituents of black liquor. Oxi-

dation fixed the inorganic hydrosulfide but at least a portion of the








organic still remained volatile. These materials were subject to loss

when the black liquor was recycled to reclaim the unused chemicals.

Thus it was necessary to further treat the oxidation exhaust gases to

completely eliminate odors. These residual odors were identified and

effective means to destroy them with ozone were investigated. Ap-

proximate costs and suggestions for mill-scale application of pure

oxygen and ozone were discussed.















ACKNOWLEDGMENTS


I wish to express my appreciation to the Graduate Committee

chaired by Dr. E. R. Hendrickson, under whose direction and guidance

this study was conducted. The Committee, which included Professors

W. O. Ash, A. P. Black, F. W. Gilcreas, T. deS. Furman, and J. E.

Kiker, aided me greatly with continued encouragement and assistance.

To Professor Gilcreas, I owe special thanks for his help in obtaining

necessary equipment and materials for this study.

Appreciation is expressed to Mrs. Marjorie DuMez and Miss

Janice Brockett who aided in the preparation of the manuscript.

This investigation was supported in part by the National

Council for Stream Improvement, the United States Public Health

Service, and the University of Florida.















TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS . . . . .. . . . . . . ii

LIST OF TABLES . . . . . . .... . . . . .. iv

LIST OF FIGURES. .. . . . . . . . . . . vi

LIST OF PLATES . . . . . . . . . .. . viii

CHAPTER

I. INTRODUCTION . . . . . . . . .. .... 1

II. PURPOSE AND SCOPE . . . . . . . . . 12

III. SURVEY OF PREVIOUS WORK . . . . . . ... .15

IV. THEORY . . . . . . . . .. . . . 31

V. EXPERIMENTAL . . . . . . . . ... . 39

VI. DISCUSSION OF RESULTS. .. . . . . . . 53

VII. SUMMARY AND CONCLUSIONS. . . . . . ... 98

VIII. SUGGESTIONS FOR FURTHER STUDY . . . . .... 104

APPENDICES

I. METHODS OF CHEMICAL ANALYSIS . . . . . .. 107

II. DETERMINATION OF REACTION CONSTANTS . . . .. 129

III. COMPLETE BLACK LIQUOR ANALYSIS . . . . ... .133

BIBLIOGRAPHY . . . . . . . . .. . . . 136

BIOGRAPHICAL SKETCH . . . . . . . . .. .. . 140









iii



r















LIST OF TABLES


Table Page

1. SOURCES OF AIR-BORNE GASEOUS AND PARTICULATE EMISSIONS
IN THE KRAFT PROCESS . . . . . . . . . 7

2. EFFECT OF OXIDATION ON THE SULFUR COMPOUNDS IN A NORTHERN
BLACK LIQUOR . . . . . . . .... ...... 25

3. MOLAR SULFUR BALANCE OF DATA IN FIGURE 3 . . . .. 29

4. EFFECT OF OXIDATION ON HYDROGEN ION, HYDROXIDE, AND CARBONATE
CONCENTRATIONS IN BLACK LIQUOR . . . . . ... 55

5. MATERIAL BALANCE OF KNOWN AND UNKNOWN SULFUR COMPOUNDS . 58

6. SULFUR BALANCE BEFORE AND AFTER COMPLETE OXIDATION . .. 62

7. SULFUR BALANCE DURING COMPLETE OXIDATION . . . ... 63

8. CONCENTRATION OF AMORPHOUS SULFUR FORMED BY BLACK LIQUOR
OXIDATION . . . . . . . . ... ...... 66

9. RATIO OF THIOSULFATE TO POLYSULFIDE PRODUCED BY COMPLETE
OXIDATION . . . . . . .. . .. . . ... 72

10. EFFECT OF PURE-OXYGEN OXIDATION ON THE SULFUR COMPOUNDS IN
BLACK LIQUOR . . . . . . . . ... . . 73

11. EFFECT OF AIR OXIDATION ON THE SULFUR COMPOUNDS IN BLACK
LIQUOR . .. . . . . . . . . . . . 74

12. REVERSION OF POLYSULFIDE AND THIOSULFATE UPON ANAEROBIC
STORAGE . . . . . . . . ... . . . . 76

13. REGENERATION OF SULFIDE UPON ANAEROBIC STORAGE . . .. 78

14. PERCENTAGE REGENERATION OF SULFIDE UPON ANAEROBIC STORAGE 80

15. KINETICS OF BLACK LIQUOR OXIDATION . . . . ... 84

16. VOLUME OF OXYGEN ABSORBED BY BLACK LIQUOR DURING OXIDATION 87

17. UTILIZATION OF DISSOLVED OXYGEN . . . . . ... 89












Table Page

18. ODOR THESHOLDS OF ORGANIC SULFUR COMPOUNDS FOUND IN BLACK
LIQUOR . . . . . . . . ... . . . . . 93

19. GAS CHROMATOGRAPHY INSTRUMENT CALIBRATION . . . . 128



















































V














LIST OF FIGURES

Figure Page

1. THE KRAFT (SULFATE) PROCESS FOR PULP PRODUCTION . .. 5

2. UTILIZATION OF OXYGEN BY BLACK LIQUOR . . . .. 18

3. FATE OF SULFUR COMPOUNDS DURING OXIDATION AT 100 120 PSI
PRESSURE USING PURE OXYGEN . . . . . ..... 27

4. HIGH VOLTAGE SILENT DISCHARGE OZONE GENERATOR . . . 41

5. FLOW DIAGRAM OF STATIC SYSTEM OXIDATION EQUIPMENT . . 43

6. FLOW DIAGRAM OF DYNAMIC SYSTEM OXIDATION EQUIPMENT . . 44

7. SAMPLING EQUIPMENT FOR CONCENTRATING AND COLLECTING
ODOROUS GASES . . . . . . . . ... . .. 52

8. EFFECT OF OXIDATION ON THE SULFUR COMPOUNDS IN BLACK
LIQUOR. STATIC OXIDATION WITH PURE OXYGEN AT 75C . . 56

9. RATE OF FORMATION OF KNOWN AND UNKNOWN SULFUR COMPOUNDS.
STATIC BLACK LIQUOR OXIDATION WITH PURE OXYGEN AT 750C . 59

10. THE OXIDATION OF SULFIDE TO POLYSULFIDE AND THIOSULFATE.
STATIC BLACK LIQUOR OXIDATION WITH PURE OXYGEN AT 750C . 61

11. SULFIDE OXIDATION AND REGENERATION IN BLACK LIQUOR. STATIC
OXIDATION WITH PURE OXYGEN AT 750C AND 12-HOUR ANAEROBIC
REGENERATION AT 75C . . . . . . . . .. 79

12. THE KINETICS OF SULFIDE OXIDATION IN KRAFT BLACK LIQUOR.
STATIC OXIDATION WITH PURE OXYGEN AT 750C . . . . 82

13. THE KINETICS OF SULFIDE OXIDATION OF KRAFT BLACK LIQUOR.
DYNAMIC OXIDATION AT 75C WITH PURE OXYGEN AND PURE OXYGEN
PLUS OZONE . . . . . . . . ... . . . 85

14. OXIDATION OF SULFIDE IN BLACK LIQUOR AND SYNTHETIC LIQUOR.
STATIC OXIDATION WITH PURE OXYGEN AT 750C . . ... 90

15. OXIDATION OF BLACK LIQUOR IN A STORAGE TANK. SUGGESTED
EQUIPMENT REQUIRING LITTLE CAPITAL INVESTMENT . . .. 95











Figure


16. OXIDATION OF BLACK LIQUOR IN A PRESSURE-DIFFUSER
CONTACTOR. SUGGESTED EQUIPMENT REQUIRING LITTLE CAPITAL
INVESTMENT . . . . . . . . . . . 96

17. OXIDATION OF BLACK LIQUOR IN A RECIRCULATING PACKED
TOWER . . . . . . . . .. . . . . . 97

18. SAMPLE STANDARD CURVE FOR SODIUM SULFIDE ANALYSIS . .. 109

19. SAMPLE STANDARD CURVE FOR COLORIMETRIC OZONE ANALYSIS 117

20. CURVE FITTING PROCEDURES FOR DETERMINING THE OXIDATION
KINETICS OF SODIUM SULFIDE IN BLACK LIQUOR. SYSTEM USING
STATIC OXYGEN FEED AND TEMPERATURES OF 600C TO 890C . 131

21. CURVE FITTING PROCEDURES FOR DETERMINING THE OXIDATION
KINETICS OF SODIUM SULFIDE IN BLACK LIQUOR. SYSTEM USING
DYNAMIC OXYGEN FEED AT A TEMPERATURE OF 750C ...... 132


Page




W- -


LIST OF PLATES


Plate Page

1. PHOTOGRAPH OF EXPERIMENTAL EQUIPMENT . . . . ... 42

2. PHOTOMICROGRAPH OF FRESHLY PRECIPITATED AMORPHOUS SULFUR
DROPLETS . . . . . . . ... . . . 68

3. PHOTOMICROGRAPH OF PRECIPITATED SULFUR AFTER AGING FOUR
HOURS AT 250C. . . . . .. . . . .... 68

4. PHOTOMICROGRAPH OF PRECIPITATED SULFUR AFTER AGING THREE
DAYS AT 250C . . . . . . . . ... . . 69

5. PHOTOMICROGRAPH OF COMMERCIAL GRADE ELEMENTAL SULFUR . 69


viii














CHAPTER I


INTRODUCTION


Atmospheric Pollution and the Pulp and Paper Industry

Only in recent years have the expansion of industrial activity

and concentration of population created severe and widespread problems

known as atmospheric pollution. The Manufacturing Chemists Association

defines atmospheric pollution as "the presence in the air of substances

put there by the acts of man, in concentrations sufficient to interfere

with the comfort, safety, or health of man, or with the full use and

enjoyment of his property." Therefore, in order to have atmospheric

pollution problems, three conditions must exist simultaneously: there

must be a source of pollutants; a susceptible population or other
2
receptor; and a mechanism for transport between the two.

"The source may be any activity of man or nature which releases

to the atmosphere any dust, fume, gas, mist, odor, smoke, or vapor. The

population or receptor can be people, vegetation, domestic animals,

homes or manufacturing plants. Susceptibility of the population is

measured in terms of health, economics, aesthetics, and toxicity. For

a problem to exist, source and susceptible population must be brought

together by the common bond of transport which is controlled by topography

and meteorology."2

Numerous sources of pollution may be active in any given locality.

Industrial operations, steam-electric generating plants, food processing





-2-




operations, incinerators, and automobiles are some of the major contrib-

utors of air-borne material to the environment. Since atmospheric

pollution cannot be legislated out of existence, control activities must

be governed by sound economic, nuisance, and health factors.
3
Pulp and paper manufacturing is the fifth largest manufacturing

industry in the United States. Except in areas where low cost water and

rail transportation are available, pulp mills are frequently situated

near their sources of raw materials. The vast forests of the southern

Appalachian mountains and the gulf coast states have prompted industrial

growth in the wood and wood-products field. Today the southeastern

United States is one of the greatest pulp and paper producing areas of

the world.

Associated with pulp and paper making are various problems of

waste treatment and environmental change. Over the past few years the

Air Pollution Research Laboratory at the University of Florida has

studied the pollutional aspects of the air-borne emissions from the

sulfate paper industry. During this time methods have been developed
4, 44
to identify and monitor these effluents.

The most prevalent complaint concerning the sulfate pulp and

paper industry is the emission of odorous gases into the atmosphere.

Under stable meteorological conditions the typical "rotten cabbage"

Kraft mill odor can be detected tens of miles from a plant site. The

odorous gases commonly emitted from the Kraft process result mainly

from the recovery of black liquor. These gases contain sulfur in

the form of hydrogen sulfide, sulfur dioxide, alkyl mercaptans, and

their oxidation products.


IMF- -










Commercial devices are available for controlling gaseous

emissions. However, the emission of odor-producing materials,because

of the many points of release and the low concentrations involved ,is

not amenable to reduction by conventional control devices. The heart

of the problem may be found at the point where these materials are

initially formed rather than where they are emitted into the atmosphere.

It is often easier to improve a process by preventing the evolution of

an elusive material than to try to collect the material after it is

formed. This study concerns the fundamental chemistry of black liquor

oxidation -- a process which may be used to reduce the quantities of

odorous gases lost from the chemical recovery system of the sulfate

process.


Kraft Process

The Kraft pulping process was introduced in Germany in 1879 as

an improvement over the soda method. It was discovered that if wood

chips are cooked in a liquor containing a mixture of sodium hydroxide

and sodium sulfide instead of plain sodium hydroxide (caustic soda),

the resulting pulp could be made into a high strength paper sheet.

Hence, the German word Kraft, meaning strength, was the name given to

the new process. This process is interchangeably known as the sulfate

process because the chemical make-up is added to the process in the

form of sodium sulfate or salt cake.

In Kraft pulping the wood is barked, chipped, and placed into

large pressure digesters along with the solution of cooking chemicals.

The chips are cooked at about 115 pounds pressure(3440F) for one to






I





-4-


six hours. Following the digestion period, the material is dumped into

a blowpit or blowtank where the liquor containing the noncellulosic

portions of the wood is drained from the pulp. During digestion the

sulfide in the cooking liquor combines with organic and inorganic

materials to form noxious gases which are released into the air when the

digester is opened.

The pulp is washed with hot water to remove the remaining chemicals.

The drainage from the blowpit and pulp washers is known as black liquor

and is stored hot in insulated tanks. The pulp is screened, refined,

thickened, sometimes bleached, and then converted into paper.

In the chemical recovery cycle the black liquor is concentrated

in multiple-effect and direct-contact evaporators, then burned, and the

heat used to produce steam. During evaporation and handling volatile

sulfur compounds continue to be lost. Evaporator exhaust contains gases

similar to those released in the digester blow tank. Effluent gases

from the recovery furnace contain both sulfurous gases and particulate

matter.

The ash or smelt from the recovery boiler contains the inorganic

chemicals, but in a chemically reduced form, that were present in the

black liquor. This smelt is dissolved in water to produce a green liquor

containing sodium sulfide and sodium carbonate. During solution some

gases are produced and these are vented into the atmosphere. The green

liquor is causticized by the addition of slaked lime and clarified to

produce white liquor which is returned to the digesters to process more

wood. A flow diagram of the Kraft process is shown in Figure 1.















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Kraft paper making has expanded greatly in the past 20 years.

This growth has been prompted by the economy of the process and the ever

increasing uses for its high-strength pulp. Presently more than 75 per

cent of the paper-grade pulp produced in the United States is made by

the Kraft process.


Pollution Control and Black Liquor Oxidation

Building and operating a completely odorless Kraft mill is

possible but expensive because a small amount of material must be

eliminated from a large amount of hot gas. For a Kraft mill to be

completely odorless all sources of emission must be controlled, mainte-

nance and operation must be excellent, and equipment must not be

overloaded. However, when sulfate mills are not located in densely

populated areas, 100 per cent control is neither necessary nor economical.

The sources of air-borne gaseous and particulate emissions from

a Kraft mill are tabulated in Table 1.

Chemical losses from the recovery boiler can be reduced by

installation of cyclones and electrostatic precipitators or wet-type

scrubbers. In some instances all three devices are used. Evaporator

chemical losses can be reduced by black liquor oxidation or gas scrubbers,

while digester losses may be reduced by by-product recovery, solution of

gases in liquor in a black liquor oxidation tower, or oxidation. Lime

kiln control devices include wet scrubbers and bag filters while on

power boilers electrostatic precipitators, cyclones, and wet scrubbers

may be used. Procedures intended to reduce chemical losses also reduce

environmental pollution.














TABLE 1


SOURCES OF AIR-BORNE GASEOUS AND PARTICULATE

EMISSIONS IN THE KRAFT PROCESS6


Gaseous Particulate


Importance Source and Compounds Importance Source and Compounds


Recovery furnace -
H2S, SO2, and some
CH3SH and (CH3)2S2.

Evaporators H2S
and some CH3SH and
(CH3)2S2'

Digesters H2S, CH3SH,
(CH3)2S, and (CH3)2S2.

Lime kiln some H2S.


Auxilliary
(including
SO2, NO2.


power boilers
bark burning)


Major


Recovery furnace -
Na2SO4 and Na2CO3
fumes.

Evaporators none.



Digesters none.


Lime kiln CaO dust.


Auxilliary
(including
Fly ash.


power boilers
bark burning)


Minor Smelt dissolving tank. Minor Salt cake and lime
unloading systems.
Lime slaking.

Lime mud filter.

Pulp washers.


I


Major











Odor reduction may be achieved through black liquor oxidation,

absorption of gases in water or caustic spray, chlorination, or burning.

Black liquor oxidation and oxidation with chlorine are, to date, the

most practical systems for odor reduction. One progressive west coast

Kraft mill stores its dilute digester gases in a vaporsphere until a

sufficient amount is collected to be passed through a chlorination

system. Minor losses from dissolving tank, lime slaker, lime filter,

or pulp washers are seldom treated. The ideal piece of control equipment

is one which will recoup initial investment quickly by chemical savings

or by-product recovery.

Black liquor oxidation prior to concentration is effective in

reducing volatile sulfur loss in multiple-effect and direct-contact

evaporation. The idea was first patented by the Swedish engineers

Bergstroem and Trobeck. It is believed that black liquor oxidation

transforms the inherent malodors into products which are less volatile

and do not give rise to foul-smelling decomposition products. Because

of the conservation of sulfur in the recovery system, the amount of

sodium sulfate or sulfur make-up is decreased. This in turn decreases

the percentage of sodium carbonate present in the green liquor which,

therefore, reduces the quantity of lime needed for causticizing. By

conserving sulfur in the system, black liquor oxidation produces a

white liquor with high sulfidity (i.e., percentage of sodium sulfide in

the active chemicals) which means a better, more buffered cooking cycle.

Black liquor oxidation also improves the suitability of precipitated

lignin for by-product use and enables the evaporators to be operated

more uniformly and efficiently.











To date, the main drawbacks of black liquor oxidation have been

the expensive equipment cost and excessive foaming attributed to the

soap-like materials present in the liquor. Because of these drawbacks

at least part of the cost of such a system is frequently justified on

the basis of reduced odor and corrosion. At the present time some

full-scale black liquor oxidation units are operating in the north and

northwest United States, Canada, Mexico, and Sweden, although the

mechanisms of the process are not thoroughly understood.

Oxidation using pure oxygen rather than air has been suggested

by previous researchers.7' 8, 9 A closed cycle is necessary economically

to ensure that all of the oxygen introduced eventually is assimilated by

the black liquor. In such a system the foaming problem is minimized if

contact time is sufficient to enable any entrained bubbles to be

completely absorbed. Defoaming agents become unnecessary and equipment

size can be minimized since high oxygen tensions will shorten contact

time. Attempts have been made to add oxygen directly to a digester

towards the end of a cook.9 This study was abandoned at the bench scale

primarily because the oxygen costs exceeded the salt cake and lime savings.

Under present economic conditions black liquor oxidation in

general is a marginal operation. Future adjustments in chemical prices,

increased expansion, and social pressures caused by environmental

pollution could change this situation radically. The principal advantage

of the oxygen system over the air system is that the gas can be

introduced directly into existing equipment, such as pipe lines or

storage tanks, and achieve a degree of oxidation without expensive

equipment.





-10-


At present black liquor oxidation has not met with success

in the southeastern United States partly because of the pronounced

foaming tendencies of southern Kraft black liquor. Although many

equipment patents have been registered, voids still exist in the basic

understanding of the mechanisms of the process. The technology has

advanced to a point where further refinements are difficult without a

better understanding of the fundamental aspects of the problem.

Although it generally is accepted that black liquor oxidation reduces

volatile sulfur losses and, hence, atmospheric pollution, the true

reasons underlying this stabilization are unknown.

Technical information applicable to the problem can be divided

into two general categories: the application and technology of black

liquor oxidation, and the chemistry of pure sulfur compounds. Neither

the chemical engineering nor the pure chemistry approach adequately

answers certain basic questions concerning black liquor oxidation.

These questions are: what changes occur during oxidation; in what

quantities do these changes occur; and why do the changes occur?

The previous research reported in the literature has been

conducted for very specific process applications, hence much of this work

cannot be applied comprehensively to the general case of black liquor

oxidation without verification and modification.

Dealing with black liquor presents some unique technical problems.

The black liquor contains approximately 5 per cent inorganic

and 10 per cent organic matter, and the interaction between the mineral

and the complex organic constituents is largely unknown. There are





-11-


some controversial factors and more knowledge about them is necessary

before the mechanisms of black liquor oxidation can be truly understood.

Tle main purpose of black liquor oxidation is to stabilize the

volatile sulfur compounds in the liquor. The effectiveness of this

oxidation in reducing gaseous losses depends on the method and degree

of oxidation. Oxidation stabilizes the liquor for evaporation but some

gases may be lost during oxidation.

Advantages and Disadvantages of Black Liquor Oxidation

Summarizing the advantages and disadvantages of black liquor

oxidation, the advantages include:

1. Reduction of salt cake make-up and lime required for

causticizing.

2. Reduction of corrosion in multiple-effect and direct-contact

evaporators and scrubbers as well as at the plant site.

3. Better and more buffered cooking liquor.

4. Improved by-product recovery.

5. Reduction of odors released from evaporators.

The disadvantages are:

1. Capital investment in equipment which will not directly

increase production.

2. Additional operational costs to maintain oxidation equipment.

3. Increased chemical costs if pure oxygen is used.















CHAPTER II


PURPOSE AND SCOPE


Purpose

This work was conducted as a fundamental research project in

atmospheric pollution control at the Air Pollution Research Laboratory,

University of Florida. The purpose of the study was to investigate

completely the oxidation of Kraft black liquor using pure oxygen and

a mixture of oxygen plus ozone. The fate of the principal sulfur

compounds during oxidation was determined and their technological

significance evaluated. The technical and economic feasibility of

mill scale oxidation using pure oxygen was discussed. The possibilities

of eliminating pulp mill odors with ozone was also investigated.

Through this paper the relative importance of the parameters

-controlling black liquor oxidation has been established. Where

conflicting reports existed in the literature, experimentation was

conducted to define the process accurately. It is hoped that in

putting black liquor oxidation on a more quantitative basis, this study

aids the development of a commercial system applicable to Kraft mills

in the southeastern United States.


Scope

Black liquor oxidation was investigated under a variety of

conditions to determine the effect of different parameters on the process.


-12-







-13-


The principal physical and chemical parameters examined were:

1. Temperature.

2. Oxygen partial pressure.

3. Method of contact.

4. Oxidizing agent.

The practical temperature range considered was from 60 C to
o
90 C. Reaction pressures were not raised above one atmosphere since

previous studies9' 10 indicated that elevated pressures were conducive

to excessive oxygen consumption. Several different methods of contact

were used and the oxidation efficiency of oxygen, and oxygen enriched

with ozone were contrasted.

By varying the pertinent parameters, the following areas were

investigated:

1. The nature of the predominant inorganic reactions that occur

during oxidation and the effect of temperature, oxygen pressure, and

method of contact on the products of reaction.

2. The precipitation of elemental sulfur from 1e black liquor.

3. The completeness of these predominant reactions and a sulfur

balance for the entire system.

4. The degree of reversibility of these reactions.

5. The kinetics of oxidation and the effect of temperature,

oxygen pressure, method of contact, and oxidizing agent on the rate of

reaction.

6. The oxygen requirements of the black liquor and the

utilization thereof.






-14-





7. The catalysis of sodium sulfide in black liquor.

8. The emission of oderiferous gases during oxidation.

9. The elimination of these gases by oxidation with ozone and

the application and approximate cost of similar oxidation to other

odor sources.

10. The application and the approximate costs of a mill scale,

pure oxygen oxidation system.












CHAPTER III


SURVEY OF PREVIOUS WORK


Literature Survey

The idea of stabilization of some of the sulfur compounds in
11
Kraft black liquor was first suggested by M. G. Schmitt in 1938.

Schmitt noted that in the recovery operation sulfur losses exceeded

sodium losses by a ratio of four to one. The high sulfur loss was

thought to be due to thermal instability of residual sodium sulfide

in the black liquor. To remedy this situation Schmitt proposed

aeration of the black liquor as well as other methods of treatment.

Since its inception, most of the work on black liquor oxidation

has been conducted by a few groups of investigators in Sweden, Canada,

and the United States. To date, studies of black liquor oxidation have

been aimed primarily at reducing chemical make-up rather than odor or

atmospheric pollution control. From this aspect the economics of the

system have been marginal.

In the period between 1939 and 1942, Bergstroem and Trobeck11

studied the changes that occur in black liquor after storage. They

found that at elevated temperatures in the presence of air the sulfide

content decreased. Based on this work, Bergstroem pioneered the first

installation of a black liquor oxidation system at the Norrsundet,

Sweden, mill in 1952. Bergstroem and Trobeck11 reported reduction in

sulfur losses from 26 to 10 per cent at this installation. They also


-15-



/1





-16-


noted decreased evaporator corrosion, increased white liquor sulfidity,

increased viscosity, and no decrease in heat value of black liquor.

They found, however, that oxidation efficiency dropped off when the

solids content went above 30 per cent.

The Bergstroem-Trobeck system was developed in conjunction with

high-vacuum evaporation of black liquor to dryness (i.e., 10 per cent

moisture). The weak black liquor was run into a tower containing

perforated shelves. Air blown into the bottom of the tower converted

much of the liquor into foam which was carried up through the unit and

out into a cyclone to separate foam and droplets. The liquid from the

cyclone was evaporated to dryness and burned. A number of Swedish,

Canadian, and American patents have been obtained covering the Bergstroem-

Trobeck equipment.

A Bergstroem-Trobeck oxidation system has been installed in the

Kraft mill of Loreto and Pena Pobre near Mexico City and its operation
12
investigated by Trobeck, Lenz, and Tirado2 in 1959. In 1961, Tirado

et al.,10 conducted bench scale oxidation studies using air under

pressure. The reactor consisted of a vertical pipe with an air diffuser

at the bottom. Liquor and air in variable quantities were fed through

the bottom and the products of reaction escaped through a pressure relief

valve at the top. At 50 pounds per square inch and 1490F the liquor was

well oxidized if air was supplied at a rate of 2.3 times the theoretical

requirement. The oxidation of black liquor by air at 25 to 75 pounds per

square inch showed promise, especially regarding the stabilization of

mercaptans which are difficult to oxidize in conventional processes.





-17-


11, 13
Trobeck, 13 stated that complete oxidation would eliminate

sulfur lost as hydrogen sulfide and mercaptans in the evaporators. He

suggested that the sulfide present as sodium sulfide may be oxidized

according to the following formula:

(1) Na2S + 202 + H20 --- Na2S203 + 2NaOH.

The author mentioned, however, that it is likely that some of the

hydrogen sulfide was tied up in the liquor in the form of organic

compounds. Trobeck said that corrosion was caused by gaseous hydrogen

sulfide which oxidized to form droplets of sulfuric acid in the evaporator

tubes:

(2) H2S + 202 -- H2S4.

Reducing the amount of hydrogen sulfide in the evaporators increased the

tube life almost ten-fold.

Trobeck also presented data on the oxygen absorbed during

oxidation of black liquor (see Figure 2).

The sharp increase in oxygen uptake after 30 minutes was

accompanied by a decrease in liquor heating value which indicated

oxidation of organic matter. Since this loss in heat value was not

found in less than 30 minutes of oxidation, it was concluded that oxida-

tion of sulfide is possible without wasteful destruction of organic

matter.
14
Tomlinson4 and co-workers in Canada developed a nonfoaming

oxidation tower as part of their recovery process. Tomlinson placed

great emphasis on the fact that hydrogen sulfide was lost from the

concentrated black liquor in direct contact (disc, cyclone, or cascade)

evaporators due to the reaction of sodium sulfide with acidic gases
1







-18-


0



0
0



o







0



0




0c
0








o


ZI O1 8 9 V7 z 0



aonbl-[ pwoq 5o im 3ad Im 'uojdumnsuoo u9sXxo





-19-


such as sulfur dioxide, sulfur trioxide, and carbon dioxide. They

found that when a completely oxidized black liquor was evaporated by

hot flue gases in a venturi scrubber, the effluent gas contained only

180 parts per million of hydrogen sulfide.

Tomlinson et al., suggested that stabilization was achieved

because dissolved hydrogen sulfide was oxidized to sulfur, sulfites, or

sulfates which are no longer subject to destructive hydrolysis.

Laboratory pilot plant work employed a counter-current tower packed with

coke or Raschig rings and the optimum reaction temperature was found to
o
be about 70 C. Designs based on this study supplied five times the

theoretical quantity of air necessary to react with the sodium sulfide.

Full-scale units have been erected in Windsor and Marathon, Canada. At

Marathon, the gaseous effluent from the counter-current oxidation tower

is fed into the recovery furnace and the residual odorous material

burned. Several United States and Canadian patents have been issued

covering these systems.

A chemical recovery system of the Tomlinson type, including

black liquor oxidation, was installed at the South African Pulp and

Paper, Ltd., mill in Springs, S. A.15 Eucalyptus is one of the species

being pulped and it has been noted that isopropyl mercaptan and

isopropyl disulfide are detectable in digester relief gases. Hisey

reported that oxidation tests at Springs showed 85 per cent sulfide

reduction and that 48 hours additional contact did not completely fix

the remaining sulfur.

A combination oxidation and absorption tower used to strip

sulfurous gases from the digester exhaust has been investigated.





-20-


Collins passed noncondensible evaporator and digester gases along with

fresh air into an experimental Tomlinson oxidation tower. He noted that

the odor of both the gas and the black liquor was reduced while the

biochemical oxygen demand of the liquor was not markedly decreased.

In addition to the Bergstroem-Trobeck and Tomlinson systems

for oxidizing black liquor, it has been recognized that oxidation

occurred in the Industrikemiska Aktiebolaget 1(INKA) system used

for evaporating sulfate liquor. This system was independently patented
91 (
by Oman and Goth, and Naucler and Ledin in 1927 and 1928, respectively.

In the INKA evaporator the black liquor was heated indirectly by the

recovery furnace stack gases; the hot liquor then flowed over a

perforated plate through which dry air was blown. The dry air evaporated

the black liquor and oxidized it to a certain extent. Actually, many of

the INKA patents concerned the design of an aerating and evaporating

device which brought the liquor in contact with air without causing

excessive foam. The sodium sulfide in the liquor was reduced

approximately 80 per cent in its 20 passes through the INKA evaporator.

A detailed study of black liquor oxidation was carried out by

Bialkowsky and DeHaas7 18 of the Weyerhauser Timber Company. These

workers noted a very definite decrease in sulfide content when black

liquor was passed through the pulp washers. From their observation that

additions of sodium sulfide to oxidized black liquor proceeded to oxidize

rapidly and from unpublished polarographic data, the authors concluded

that organic compounds in the black liquor catalyzed the oxidation

reaction. They also found that more oxygen reacted with the black

liquor than was necessary to oxidize the sulfide to thiosulfate according





-21-


to reaction 1. No increase in sodium hydroxide was detected,

however, and it was thought that this increase may have been masked

by the buffering action of the organic compounds. Bialkowsky and

DeHaas also investigated the addition of air to a pilot plant digester.

They found that the sulfide concentration of the black liquor was

lowered and the bleachability of the pulp was increased.

The basic oxidation studies of Bialkowsky and DeHaas were

carried out in a concurrent-flow glass tower. Using "wetted wall"

surfaces with careful control of the temperature, humidity, air, and

liquor flow rates, they determined the optimum transfer coefficients for

oxidizing the sulfide. They found that foaming decreased and oxygen

consumption increased as the temperature rose. Studies in a cascade-

type evaporator showed that unoxidized liquor lost all of its hydrogen

sulfide while oxidized liquor lost little hydrogen sulfide when

evaporated to dryness. Next, the possibility for absorbing and oxidizing

noncondensable gases from digester relief and blow gases with black

liquor was studied in a laboratory oxidation tower. Hydrogen sulfide,

methyl mercaptan, and dimethyl sulfide were mixed with air and passed

through a packed tower. Fresh black liquor was passed concurrently

through the packed column. Hydrogen sulfide was completely absorbed

and oxidized; 5 per cent of the methyl mercaptan passed unchanged,

40 per cent left the tower as methyl di-sulfide,and the rest was absorbed

in the liquor. Over 90 per cent of the methyl di-sulfide passed through

the tower unchanged.

Other tests showed that when temperature was increased to around

250 C,some hydrogen sulfide was evolved from the black liquor in spite



fl





-22-




of the fact it did not contain any residual sodium sulfide. These

results indicated that a reduction of hydrogen sulfide losses could be

expected as far as the direct contact evaporators were concerned, but

any localized overheating in the evaporators and high temperature

combustion in the furnace would release hydrogen sulfide gas.

Wright and associates8 at the British Columbia Research Council

began work on odor control in 1948. Methods of analysis for hydrogen

sulfide and methyl mercaptan were developed and the kinetics of the

oxidation process were studied using a respirometer. The effect of

interfacial mass transfer was minimized by making the gas-liquid

interface very large. This was achieved by absorbing black liquor on

filter paper, thus presenting maximum surface area. They found that

sodium sulfide and mercaptans in black liquor were oxidized readily and

that the oxygen absorption continued indefinitely thereafter but at a

steadily diminishing rate. Tests also showed that oxygen consumption

increased with temperature in the range of 400C to 900C. Wright also

indicated that 1 to 3 volumes of oxygen were needed to oxidize completely

1 volume of black liquor. Since the black liquor contained approximately

1.7 grams per liter of sodium sulfide, this meant each mole of this

sulfide consumed 2 to 5 moles of oxygen during oxidation.

It was found that with infinite area the oxidation of sulfur

compounds still required from 2 to 3 minutes and hence nothing was

gained by increasing the superficial area beyond a certain point.

To test laboratory data a pilot plant oxidation unit was built
19, 20
at the Port Alberni,1 20 British Columbia, mill of Bloedel, Stewart,

and Welch. Concurrent air and liquor flow was used in the tower and





-23-


noncondensable digester gases were added to the air supply. These gases

were practically completely absorbed and the exit air carried little, if

any, Kraft odor. This statement contradicted the findings of Bialkowsky

and DeHaas.

In 1959, Murray at the British Columbia Research Council

published a comprehensive study of the kinetics of oxidation of Kraft

black liquor using both air and pure oxygen, Murray reported that the
o
maximum rate of oxidation occurred at 72 C using pure oxygen and that
o
sulfur formation was favored by temperatures below 80 C, low sulfide

concentration, and high oxygen pressures. Murray found that oxidation

rates were higher when pure oxygen rather than air was used, but a

five-fold increase in oxygen pressure did not bring about a proportional

increase in oxidation rate. It was noted that storing oxidized black

liquor at elevated temperatures in the absence of air caused some of

the sulfide to be regenerated. This was especially true when sulfur was

an oxidation product. It was also reported that oxidation rates were

not greatly affected by changes in solids content. Experiments showed

that an increase in thiosulfate concentration reduced the black liquor

oxidation rate while an increase in sodium hydroxide increased the

oxidation rate. The addition of elemental sulfur to a digester produced

sulfide according to equation 3; the over-all effect was to lower the

sodium hydroxide concentration and increase the thiosulfate and sulfide

concentrations.

(3) 4S + 6NaOH----- Na2S + Na2S203 + 3H20.

On the basis of the kinetic data Murray concluded that the oxidation

takes place in two definite steps: the first a rapid oxidation and then





-24-


a slower reaction. Therefore, he suggested that a two-stage oxidation

system should be more efficient than a one-stage system.

Collins 11, 16, 22 published a series of papers on black liquor

oxidation. On the basis of preliminary studies he presented data on

the effect of oxidation on the sulfur compounds in black liquor (see

Table 2). Collins found that the sulfide, thiosulfate, and sulfite

oxidized to thiosulfate and sulfate. Collins also showed that chemical

oxidation of black liquor with sodium peroxide was impractical. Based

on the results of pilot plant studies at Thilmany, Wisconsin, Collins

developed a commercial system for black liquor oxidation. In this

system air was blown into the black liquor to form a stable foam. The

oxidized foam was then collected in a cyclone and converted back into

liquor. A material balance on the Thilmany evaporators indicated that

the use of oxidized black liquor greatly reduced sulfur losses.

A comprehensive literature review published by Collins1 in 1953,

included most of the significant basic and related papers on sulfate

black liquor oxidation. He mentioned an early patent in which alkaline

waste liquors were purified by concentrating and heating with hot air

in the presence of caustic until the organic substances carbonized.

Addition of an oxidizing agent such as saltpeter was also suggested.

Collins noted another source which pointed out that sodium sulfide in

weak wash water discharged to the sewers was partially oxidized by the

dissolved oxygen in the water. He also mentioned Basberg who discovered

that at room temperature in the absence of air, black liquor sulfide

content did not change after 5 weeks of storage.





-25-


TABLE 2


EFFECT OF OXIDATION ON THE SULFUR COMPOUNDS

IN A NORTHERN BLACK LIQUOR22


Material Concentration
in Fresh Liquor in Oxidized Liquor
Grams per Liter Grams per Liter


Na2S 9.01 1.02

Na2S203 4.07 11.75

Na2SO3 0.18 0.00

Na2SO4 3.97 5.48

Total Sulfur (by analysis) 6.56 6.45

Total Sulfur (by addition) 6.29 6.42





-26-


11
According to Collins, Venemark stated that some of the sulfur

compounds in black liquor were subject to air oxidation at room temper-

ature. Venemark also reported that when evaporation was carried out in

direct contact with flue gases, sulfur losses occurred due to the action

of acidic gases on the alkaline black liquor.

In his literature survey Collins11 discussed the laboratory

findings of Bozza and Colombo. Their kinetic studies showed that the

oxidation of sulfide was a first order reaction, but the oxidation of

mercaptan was intermediate between a first and a second order reaction.

To determine the retention of sulfur in black liquor after oxidation

of the liquor, they distilled various samples in a current of nitrogen

and analyzed the gases evolved. Oxidation resulted in the reduction of

the amount of hydrogen sulfide lost during distillation but mercaptans

were not affected.
22
Heath, Bray, and Curran22 referred to the early work of Klason

and Segerfelt in which the oxidation of black liquor when stored was

first noted. This observation was never followed up and no attempt was

made to apply black liquor oxidation.
9
Fones and Sapp oxidized Kraft black liquor at digester temper-

atures using pure oxygen. Their experiment was divided into three phases:

the oxidation of pulp alone; the oxidation of pulp and black liquor at

the end of a normal Kraft cook; and the oxidation of black liquor alone.

The first two phases produced undesirable reduction in pulp quality.

On oxidizing the black liquor alone at 100 to 120 pounds per square inch,

the sodium sulfide was oxidized to sodium thiosulfate, which in turn was

oxidized to sodium sulfate (see Figure 3). Unfortunately, a sulfur







-27-


60

x
0







0
-4













C
4
00























0
u4
& O


7 o



























o 00






-4
dy .y / o o

















4.4 o
01 0\






















-4
o











b0





-28-


balance on the data in Figure 3 does not account for all the sulfur

compounds (see Table 3). After 150 minutes only 76 per cent of the

sulfur initially present as sulfide was accounted for by the thiosulfate

and sulfate formed. After 217 minutes, however, all of the sulfur was

accounted for. It would appear that some other compound or compounds,

either organic or inorganic, were formed during oxidation.


Conclusions Based on Previous Literature

On the basis of the literature survey of the information

available on black liquor oxidation, certain conclusions can be drawn:

1. Although the possibility of oxidizing black liquor has been

known for at least 20 years, little theoretical information is available

on the basic mechanisms of the process. The majority of the literature

covered the application of a few basic ideas to specific mill problems.

2. Full scale units have been successfully installed and

operated in areas other than the southeastern United States.

3. The principal reason for installation of a black liquor

oxidation system is to reduce chemical losses and corrosion rather

than to improve cooking liquor or reduce atmospheric pollution.

4. Knowledge of the chemistry of the stabilization of the

sulfur compounds in black liquor is indefinite. It is suspected that

the dissolved organic material catalyzes the oxidation of the sulfide

but little has been reported about this mechanism.

5. The relative importance of the physical parameters of the

system, such as temperature, contact method, and choice and quantity of

oxidizing agent have not been reported.





-29-


TABLE 3


MOLAR SULFUR BALANCE OF DATA IN FIGURE 3


Concentration
Percentage
Time Sulfide Thiosulfate Sulfate Sum of the
Expressed as Expressed as Expressed as Sulfur
Moles per Moles per Moles per Accounted For
Liter Sulfur Liter Sulfur Liter Sulfur


0 0.155 0.077 0.058 0.290 100.0

27 0.146 0.080 0.063 0.289 99.7

38 0.098 0.076 0.068 0.242 83.4

45 0.107 0.095 0.070 0.272 93.8

55 0.069 0.128 0.077 0.274 94.5

62 0.055 0.134 0.070 0.259 89.3

78 0.026 0.150 0.074 0.250 86.2

95 0.023 0.136 0.084 0.243 83.8

114 0.017 0.099 0.118 0.234 80.7

150 0.014 0.039 0.168 0.221 76.2

191 0.010 0.019 0.250 0.279 96.2

217 0.006 0.006 0.278 0.290 100.0




ww


-30-




6. There is some doubt as to whether or not black liquor can be

oxidized without the emission of odors.

7. There is also some doubt as to whether or not black liquor

oxidation using pure oxygen is technologically or economically feasible.

The work on black liquor oxidation over the past 20 to 30 years

has added knowledge as well as conflicting information to the technology

of the pulp and paper industry. Black liquor oxidation as a means of

reducing chemical losses and atmospheric pollution from the recovery

cycle of the Kraft process has definite potential. In some instances

oxidation has been made to work, while in others, technical and economic

limitations have hindered its application.

Oxygen, if properly utilized, may prove feasible on a mill scale

although it is a more expensive oxidizing agent than air. The potential

advantages of an oxygen system over an air system are:

1. The elimination of the foam problem.

2. Higher oxygen pressures over the liquor, which will decrease

the contact time and hence the size of the equipment.

3. Improved by-product recovery from black liquor.













CHAPTER IV


THEORY


Oxidation States of Sulfur
Sulfur in various degrees of oxidation is found in black liquor.

The more common oxidation states with their corresponding oxidation

potentials24 in basic solution are shown below:


--0.61 --i 0.76 1
(4) S= 0.48 S 0.74 S203 0.58 S03= 0.93 S04

Polysulfides are also stable in alkaline solution but have not been

included in the above diagram because their oxidation potentials are

indefinite. Polysulfides are intermediates between sulfide and elemental

sulfur and include the disulfide, S2", and the tri-, S3=, tetra-, S4",

and penta-, S5", sulfides. The couples shown in equation 4 are not

subject to rapid reversibility and hence they cannot be used to calculate

equilibrium constants.

Other oxidation states of sulfur include hyposulfite, S204 ,

dithionate, S206", and the persulfates or thionates such as S306",

S406", and S506=. The problem of assigning polar oxidation states

to these compounds is similar to that encountered with the polysulfides,

since polar numbers are rather ambiguous because the sulfur atoms

are linked by covalent bonds.


-31-



\





-32-


Evaporation of Sulfate Black Liquor

The active chemicals in Kraft white liquor are sodium sulfide

and sodium hydroxide in concentrations of 15-30 and 40-65 grams per

liter, respectively. During cooking the lignin and other noncellulosic

portions of the wood are hydrolyzed into soluble compounds, freeing the

fiber bundles. The products of hydrolysis include carbohydrates,

alcohols, acids, thiolignin, mercaptides, and other soluble organic;

the black liquor contains these constituents as well as unutilized

sodium sulfide, sodium hydroxide, and other soluble mineral matter.

The sulfide in the liquor serves a dual purpose5 by buffering

the system during cooking and by supplying sodium hydrosulfide (see re-

action 5) which combines with lignin to form alkaline soluble thiolignin.

(5) Na2S + H20 --- NaHS + NaOH.

At the end of the cooking period the sodium sulfide is at

least partially hydrolyzed and the concentrations of active chemicals

(i.e., Na2S and NaOH) are substantially reduced.

When the black liquor is concentrated in vacuum multiple-effect

evaporators, some sodium sulfide is lost as hydrogen sulfide gas.1

(6) Na2S + 2H20 --- 2NaOH + H2S.

Another odorous evaporator loss, but a relative minor one in

terms of volume, is methyl mercaptan. Methyl mercaptan and its sodium

aalt, sodium mercaptide, are formed by the reaction of hydrosulfide

on free methyl groups during the cooking process. During evaporation

some of the sodium mercaptide hydrolyzes and liberates methyl mercaptan.

(7) CH3SNa + H20---- NaOH + CH3SH.





-33-


Methyl sulfide is another compound which is formed in the digesters

possibly by the liberation of hydrogen sulfide from two molecules of

methyl mercaptan.

(8) 2CH3SH--- CH3SCH3 + H2S.

The methyl sulfide and any other low boiling gases such as

hydrogen sulfide or methyl mercaptan are stripped readily from the

black liquor during vacuum evaporation.

When unoxidized black liquor is passed through direct-contact

evaporators and heated with flue gases containing carbon dioxide,

hydrogen sulfide is lost.

(9) Na2S + CO2 + H20 --- Na2CO3 + H2S.

Thus hydrolysis, desorption, and reaction with acidic gases as

shown in reactions 6 through 9 are the generally accepted causes of

volatile sulfur losses during evaporation.


Black Liquor Oxidation

Black liquor oxidation reduces sulfur losses since the products

formed are apparently nonvolatile and stable at evaporator conditions.
Reactions i and 109, II, 13, 22
Reactions 1 and 10 11, 13, 22 have been suggested to explain the

formation of stable inorganic sulfur compounds.

(1) 2Na2S + 202 + H20-----Na2S203 + 2NaOH.

(10) Na2S203 + 2NaOH + 202 2Na2SO4 + H20.

Neither sodium thiosulfate nor sodium sulfate is subject to destructive

hydrolysis.

Prior to the experimental work, the following hypotheses were

drawn from studies of the previous literature.





-34-


It seems likely that when black liquor is oxidized, several

different oxidation states of sulfur will be formed. Certain compounds

are more likely than others to be present in the strongly alkaline

black liquor. Elemental sulfur, for example, is unstable in alkaline

solution and slowly decomposes into sulfide and thiosulfate.24' 25

(11) 4S + 6NaOH---- Na2S203 + 2Na2S + 3H20.

It is unlikely, therefore, that elemental sulfur is a stable oxidation

product of sodium sulfide. Further circumstantial evidence that elemental

sulfur is not to be expected in black liquor is the fact that in an

alkaline solution containing sodium sulfide and sulfur, the two compounds

combine to form polysulfides according to reactions 12 through 15.25

(12) Na2S + S -- Na2S2'

(13) Na2S + 2S --- Na2S3.

(14) Na2S + 3S --- Na2S4.

(15) Na2S + 4S --- Na2S5.

Sulfur also reacts with sodium sulfite to form sodium thiosulfate.25

(16) S + Na2SO3 -Na2S203.
8
Under certain conditions, however, sulfur is formed and is

apparently stable in the black liquor. The reasons for this sulfur

formation will be investigated and discussed in this paper.
25
Yeoman found that pure soluble metal sulfides in dilute

solution slowly hydrolyze and in the presence of air produce both

polysulfides and thiosulfates.

(17) Na2S + H20 NaHS + HaOH.

(18) 4NaHS + 02 -- 2Na2S2 + 2H20.

(19) 2NaHS + 202 ---- Na2S203 + H20.





-35-


It may be possible that the sulfide in the black liquor reacts similarly

when subjected to oxidation.

Sodium sulfite with an oxidation number of +4 is another compound

which may be formed when black liquor is oxidized. Near the boiling

point, however, sodium sulfite and disodium disulfide react quickly to
25
form sodium sulfide and sodium thiosulfate.2

(20) Na2SO3 + Na2S2 -- Na2S + Na2S203.

Hence, it is unlikely that both disodium disulfide and sodium sulfite

exist simultaneously in the hot black liquor.

It is possible that only thiosulfate and sulfate, as suggested

in reactions 1 and 10, are formed during oxidation. It is more

likely, however, that a mixture of the many oxides of sulfur are

formed during black liquor oxidation.

The free alkyl mercaptans, their derivatives,and salts are all

subject to partial or complete oxidation. Under mild oxidation mercaptans

form a stable disulfide.2

(21) 2RSH + 0----- RSSH + H20.
26
A more vigorous oxidation produces the corresponding sulfonic acid.2

(22) RSH + 30---- RS3H.

Pure oxygen normally oxidizes methyl mercaptan to methyl disulfide while

a peroxide oxidizes methyl mercaptan completely to methyl sulfonic acid.

In a similar manner aliphatic sulfides are oxidized to sulfoxides

and sulfones by mild and strong oxidation respectively.26










i




w


-36-




(23) RSR + 0 ----RSOR.

(24) RSR + 20 ---- RSO2R.

There is some doubt as to just how efficient black liquor

oxidation is in attacking the mercaptan fraction. Tirado10 et al.,

reported that oxidation using air at a pressure of 70 pounds per square

inch increased the oxidation of mercaptans from 45 to 85 per cent.

There is some question as to whether or not some of the oxidation

products of methyl mercaptan are stripped from the black liquor during

oxidation. It appears that methyl disulfide, CH3SSCH3, and methyl

sulfonic acid, CH3SO3H, with boiling points of 117C and 1600C will

remain condensed. Methyl sulfide, CH3SCH3, methyl sulfoxide, CH3SOCH3,

and residual methyl mercaptan, CH3SH, with boiling points of 380C, 1000C,

and 7.6C, respectively, possibly will be volatilized from the liquor.

during oxidation. This indicates that black liquor oxidation may emit

some odoriferous sulfur gases.


Black Liquor Oxidation with Oxygen Plus Ozone

Stabilization of the soluble sulfur compounds may be accelerated

by use of a stronger oxidizing agent than pure oxygen. A mixture of

ozone and oxygen provides additional oxidizing power. Ozone is a strong
24
electron acceptor and its oxidation potential is second only to fluorine.2

(25) 02 + 20H" 03 + H20 + 2e" EB = -1.24V.

Ozone has a very characteristic pungent odor and is more soluble

than oxygen in a pure alkaline solution. Upon absorption in basic

solution the ozone hydrolyzes and releases the powerful perhydroxyl ion.24

(26) 03 + 220H 2 + H20 AF = 5.1 Kcal.





-37-


Ozone is produced by a high voltage corona discharge according
25
to reaction 27.2

(27) 302 -~ 203 AF -69 Kcal.

It is thermally unstable and cannot exist at temperatures above

2000C. High concentrations decompose violently into oxygen with the

liberation of heat.

Ladenberg27 produced 86 per cent ozone in liquid oxygen, but
28
Saunders and Silverman2 found the equilibrium concentration of ozone

at room temperature to be 16.5 per cent. The latter produced this

concentration using a silent electrical discharge of 5.5 Kv, across a

2 mm gap, in an atmosphere of pure oxygen. Equilibrium was attained after

a corona discharge of 70 to 90 minutes. Saunders and Silverman found

that, using a 2 mm gap, 1.8 to 2.0 mg of ozone could be produced per

coulomb of electrical energy--a value which was relatively independent

of the potential used.

Ozone may also be useful in oxidizing odoriferous gases from a

pulp mill. Ozone will oxidize methyl mercaptan directly to methyl

sulfonic acid.25, 26

(28) CH3SH + 03 CH3SO3H.

Ozone will probably deodorize by oxidation other similar sulfurous

organic gases.


Oxidation Kinetics

The oxidation of weak black liquor cannot be described by any

simple chemical or absorption rate theory. The complexities of the

oxidation occur because the temperature region of practical interest





-38-


is a transition region in which the products of reaction, sulfide

concentration, and oxygen partial pressure act together to influence

the oxidation rate.

Using the semi-empirical equation 29, the effects of chemical

reaction and oxygen absorption can be separated.8

(29) dC K (C + bP).
dt
where: C = concentration of Na2S in grams per liter-at any time t, in

minutes.

K = velocity constant for oxidation reaction.

P partial pressure of oxygen in atmospheres.

b constant representing the rate of oxygen absorption.

By varying experimental conditions the effect of temperature,

oxygen pressure, method of contact,and oxidizing agent can be determined.

If the gas-liquid interface offers no resistance to oxygen absorption,

constant b will be zero and the oxidation is a first-order reaction.

Similarly, if oxidation is independent of sulfide concentration,the

reaction is of zero order, depending solely on the factor K (bP). These

are the two limiting cases of equation 29 and in reality the actual

situation probably lies somewhere between these two extremes.














CHAPTER V


EXPERIMENTAL


Equipment and Procedures

Experimentation on black liquor oxidation requires a ready

supply of liquor, an oxidizing agent, and a method of bringing the two

reactants into intimate contact.

The black liquor used in this study was obtained from the Hudson

Pulp and Paper Company Kraft mill at Palatka, Florida. Black liquor was

drawn off from a point between the pulp washers and the multiple-effect

evaporators. It was collected in 5-gallon glass carboys which were

sealed and allowed to cool. The space above the black liquoY was

filled with inert nitrogen or helium and the liquor was displaced as

needed into a 4-liter flask. Analyses showed that liquor could be

stored in these carboys up to five weeks with no apparent change in the

initial sulfide concentration.

Compressed oxygen with a rated purity of 95 per cent was obtained

in 200 cubic foot cylinders from the Matheson Company. When an oxidizing

agent stronger than pure oxygen was required, a mixture of ozone and

oxygen was produced and supplied to the system.

The ozone was produced by passing a stream of pure dry oxygen

through a group of high voltage aluminum plates. The plates were

insulated from each other with mica and a high voltage transformer was

used to create a silent corona discharge between alternate plates.


-39-





-40-




A portion of the oxygen was ionized by the corona discharge and recombined

as ozone. The ozone concentration was controlled by varying the voltage

across the plates. The oxygen flow-rate was held constant at 450 cubic

centimeters per minute. Concentrations from zero to 1450 parts per

million of ozone could be produced in the generator shown in Figure 4.

The actual concentration of ozone being fed into the chamber was

determined by passing a known volume of the ozone-oxygen mixture through

an alkaline solution of potassium iodide. The quantity of ozone collected

in the alkaline scrubbing solution was analyzed photometrically (see

Appendix I).

The oxidation chamber consisted of a 12-liter cylindrical stain-

less steel chamber (see Plate 1 and Figures 5 and 6). Eight bolts around

the perifery of the unit fasten the plexiglass lid to the chamber. A

gasket made from 1/16 inch Cordet sheet packing provided a gas-tight

seal between the plexiglass and the stainless steel. A fan which blows

the oxygen down onto the exposed liquor surface and a stirring disc

which agitates the liquor were both mounted on a stainless steel shaft.

This pulley-driven shaft was mounted in a gas-tight bearing in the center

of the plexiglass top. The shaft was turned at 98 revolutions per

minute by a 1/30 horsepower, 110 volt, AC motor. The oxidizing agent

was fed into the center of the chamber through an 11-millimeter glass

tube and circulated by the fan. The liquor was introduced into the

chamber through a funnel mounted on a length of 15-millimeter glass

tubing. A three-way valve mounted on an auxilliary piece of 11-millimeter

glass tubing served as a relief and exhaust valve.


























































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-45-


Black liquor samples of 0.15 to 20.0 milliliters were withdrawn

from the chamber through a sampling port with a hypodermic syringe.

The sampling port consisted of a length of 15-millimeter glass tubing

capped with a serum-bottle stopper. Liquor samples taken at different

time intervals during oxidation were analyzed for sulfide, polysulfide,

thiosulfate, sulfite, and sulfate.

The oxidation chamber was kept at a constant temperature by

immersion in a thermostatically controlled water bath. A 2000-watt

immersion heater kept the 15 gallons of bath water at the desired

temperature, and the water was circulated by an electrically-driven

stirrer.

Three methods, one static and two dynamic, were used to supply

gas to the oxidizing chamber. In the static system the vessel was

purged with oxygen, liquor poured in, and the chamber sealed. Oxygen

was made available at a constant pressure of 50 millimeters of water

and, as the liquor utilized the oxygen above it, more gas entered the

chamber through a wet-test meter. The fluid in the meter previously

had been saturated with oxygen. With this static system the volume of

the consumed oxygen was determined. The static oxygen system is shown

diagramatically in Figure 5.

A dynamic or circulating system was also employed to oxidize

the black liquor. An oxygen flow of 450 cubic centimeters per minute

was used so that the gas above the liquor was changed approximately

every ten to fifteen minutes. The oxygen flow rate was kept low in

order to simulate static conditions as much as possible and not to

upset the thermal equilibrium in the chamber. An oxygen-ozone mixture





-46-


was also supplied using this dynamic or circulating system (see Figure 6).

When plain oxygen was utilized dynamically, the ozone generator was not

energized.

Detailed descriptions of the operating procedures and testing

sequences for the static and dynamic flow systems are given below.


Static Flow Procedures

1. Fix the oxidation chamber in place and bring the bath up to

the desired working temperature.

2. Close the chamber, flush thoroughly with oxygen, and test

for leaks.

3. Withdraw samples from the 4-liter black liquor flask and

analyze for sulfide, polysulfide, thiosulfate, sulfite, and sulfate

(see Appendix I).

4. Open the relief valve and quickly pour 4 liters of black

liquor at room temperature into the oxidation chamber.

5. Reseal the chamber, start the stirrer, open the gas valve,

and make oxygen available to the system at a constant head of 50

millimeters of water. The liquor is heated in the reaction chamber

and temperature equilibrium is achieved in less than 10 minutes.

6. Read the wet-test gas meter at 5-minute intervals to determine

the oxygen utilized by the black liquor. Withdraw liquor samples at 10-

to 30-minute intervals during oxidation and analyze for sulfide,

polysulfide, thiosulfate, sulfite, and sulfate.


Dynamic Flow Procedures

1. Fix the oxidation chamber in place and bring the bath up to

the desired working temperature.




-47-


2. Close the chamber, flush thoroughly with oxygen, and test
for leaks.

3. Open the relief valve and adjust the flow rate of oxygen or
ozone-oxygen mixture to 450 cubic centimeters per minute.

4. Divert the ozone-oxygen stream from the chamber after

equilibrium has been established in the ozone generator (after 2 to 5

minutes). Bubble the gas through two midget impingers containing alkaline

iodide scrubbing solution. Analyze the contents of the impingers (Appen-

dix I) and calculate the ozone concentration in the oxygen stream.

5. After sampling, re-direct the gas through the chamber and
vent the exhaust to the hood. (If ozone is not used omit steps 4 and 5.)

6. Connect "freeze-out" traps to exhaust line to condense and
concentrate the odorous sulfurous compounds in the off-gases. (If

exhaust gases are not monitored for odors omit step 6.)

7. Withdraw samples from the 4-liter black liquor flask and

analyze for sulfide, polysulfide, thiosulfate, sulfite, and sulfate

(see Appendix I).

8. Quickly pour the 4 liters of black liquor at room temperature
into the oxidation chamber.

9. Reseal the chamber except for the relief valve and start the
stirrer. The liquor is heated in the reaction chamber and temperature

equilibrium is achieved in less than 10 minutes.

10. Adjust the oxygen flow to 450 cubic centimeters per minute.
Check this flow periodically throughout oxidation. Withdraw liquor

samples at 10-to 30-minute intervals during oxidation and analyze for

sulfide, polysulfide, thiosulfate, sulfite, and sulfate (see Appendix I).





I





-48-


Other oxidation studies were conducted using an impingement-

type glass bubbler. This unit was mounted in the constant temperature

bath and to minimize foaming the oxidizing gas impinged upon, rather

than under, the surface of the black liquor. Placing the impinger tube

under the surface of the liquor was found to be undesirable since the

bubbles formed forced large quantities of foam into the exhaust line.

This rapid "excess-air" technique was used to verify some tests conducted

by a previous research worker in the field. The impinger was also used

to supply small quantities of well-oxidized black liquor for miscellaneous

other studies.


Analytical Methods

The methods of chemical analysis used were obtained from a

variety of sources and in some cases had to be modified for use with

black liquor. Methods for sulfate, thiosulfate, and sulfite in black
29
liquor were obtained from TAPPI standards29 T625m-48. I

The determination of soluble sulfates in black liquor is a

gravimetric analysis. The organic matter in a diluted sample of black

liquor is precipitated by the addition of hydrochloric acid. These

interference are removed by double filtration. A dilute solution of

barium chloride is added to the sample to precipitate barium sulfate.

The precipitation is made in acidified solution near the boiling point.

The precipitate is filtered off, washed with water, ignited to redness,

and weighed as barium sulfate.

The thiosulfate and sulfite analyses are volumetric iodine-starch

titrations. The sulfide and organic matter in the liquor is precipitated




-49-


with zinc carbonate solution and the precipitated interference removed

by filtration. The filtered sample is acidified with acetic acid and

titrated with standard iodine solution to the blue-starch end point.

This titration determines the total quantity of thiosulfate and sulfite

present. A similar filtered sample is acidified and titrated with

standard iodine solution after the sulfite has been destroyed by the

addition of formaldehyde. This titration determines the total amount

of thiosulfate present in the black liquor. The sulfite concentration

is the difference between the two titrations. In general, the values

for these titrations are somewhat high due to organic reducing agents

invariably present. This is, however, the method recommended by TAPPI

and apparently the best available at this time.

Sulfide is determined colorimetrically by the method of Strickland

and Risk.30 The dissolved sulfide reacts quantitatively with p-Phenylene-

diamine dihydrochloride in the presence of ferric chloride forming a deep

purple color. The color is formed in a fixed volume of aqueous solution

containing dye, oxidizing agent (ferric chloride), and wetting agent

alkyll dimethyl benzyl ammonium chloride). The sample is appropriately

diluted and the absorption measured at 600 millimicrons against a

similarly prepared blank. A reference absorption curve is prepared using

a standard solution of sodium sulfide. This method was selected over the

TAPPI standard method because it required less time and produced more

reliable results.

Polysulfides are determined gravimetrically by a method modified

from Scott's Standard Methods of Chemical Analysis.31 Soluble polysulfides

quantitatively precipitate sulfur in a strongly acidified solution.





-50-


Interfering sulfides are precipitated with zinc chloride and interfering

organic materials with hydrochloric acid; then both are removed by

filtration. The pH is then lowered to 1.0 with hydrochloric acid to

precipitate flowers of sulfur. The precipitate is filtered off, washed

with water, dried,and weighed.

Ozone is analyzed colorimetrically using a variation of the
32
method of Smith and Diamond. The oxidants are absorbed in an alkaline

iodide solution and form hypoiodite ion. The hypoiodite quantitatively

liberates iodine when the solution is acidified with acetic acid. The

color is formed in a fixed volume of alkaline iodide solution and the

absorption is measured at 352 millimicrons against a distilled water

blank. A reference absorption curve is prepared using a standard

solution of sodium iodate. Possible interference in the oxygen stream

include nitrogen dioxide, sulfur dioxide, and peroxides. The nitrogen

dioxide interference is eliminated by adjusting the sample to the proper

pH immediately before measuring the absorbence. The possibility of

sulfur dioxide or any peroxides being present is unlikely since pure dry

oxygen is used to produce the ozone.

Methods for total solids, specific gravity, percentage inorganic

matter (sulfated ash), percentage organic matter, pH, hydroxide, and
29
carbonate were also obtained from TAPPI standards29 T625m-48.

The specific gravity and total solids are determined by weighing

a known volume of black liquor before and after drying. The specific

gravity is the ratio of the weight of one volume of black liquor to an

equal volume of distilled water. The total solids are the oven-dry

residue after all the water is completely driven off. This residue is





-51-


ashed, treated with sulfuric acid, ignited to redness, cooled,and weighed.

The material remaining is the inorganic matter (sulfated ash), while the

loss in weight is the volatile or organic matter.

The hydrogen ion concentration is determined on a pH meter

equipped with a glass and a calomel electrode. The hydroxide concentration

is determined from a potentiometric titration of a diluted liquor sample

with standard hydrochloric acid to a phenolphthalein end point. The

carbonate concentration is determined from a potentiometric titration

of a diluted liquor sample with standard hydrochloric acid to a methyl

orange end point.

Odorous organic gases were analyzed by gas chromatography

according to procedures developed by Adams33' 34, 35, 36 for the National

Council for Stream Improvement. The gases are concentrated by freezing

them out on silica gel at -78.50C. Moisture is removed prior to freeze-

out by passing the gas through an ice water condenser and a Drierite

adsorption trap (see Figure 7). The sample is desorbed from the silica

gel at 1000C and analyzed by gas chromatography (see Appendix I).

Detailed particulars covering the analytical procedures previously

described are presented in Appendix I beginning on page 107.







-52-









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CHAPTER VI


DISCUSSION OF RESULTS


Black liquor oxidation was investigated at different conditions

to determine the relative importance of certain variables on the system.

In a typical mill black liquor is kept near boiling; hence the temper-

ature range of practical interest is 600C to 900C. Oxidation was

conducted at atmospheric pressure using pure oxygen. As the temperature

increased, thus increasing the water vapor pressure, the oxygen partial

pressure was in turn decreased. Therefore, temperature and oxygen

pressure are not independent but are dependent variables which vary

inversely.

The contact surface, quantity of agitation, and other contact

variables have an influence on the oxidation rate. When the temperature

and oxygen pressure were studied, the contact variables were held fixed.

When the method of contact was investigated, the temperature and oxygen

pressure were held constant. In this manner the effect of temperature,

oxygen pressure, and method of contact on the underlying chemistry of

the system was determined.

The use of ozone to eliminate odor has been suggested by persons

experienced in the field of sanitary engineering.37 Ozonating pulp mill

odors and accelerating black liquor oxidation with ozone have been

evaluated.


-53-





-54-


Chemistry of Black Liquor Oxidation

The initial phase of this investigation dealt with the inorganic

reactions that transpire during oxidation and the effect of temperature

and oxygen pressure on the products formed. Analysis conducted according

to standard procedures (TAPPI T625m-48) showed considerable concentrations

of sulfide, thiosulfate, and sulfate but not sulfite present. Other

analyses conducted on the unoxidized black liquor included total solids,

specific gravity, percentage inorganic and organic matter, hydrogen ion

concentration (pH), hydroxide, and carbonate. Once a clear picture of

the major mineral constituents of black liquor was obtained, oxidation

tests were conducted and the changes in the inorganic compounds observed.

Initial tests were conducted at 750C which was the median

condition in the temperature range (600 to 900C) under consideration.

Samples were withdrawn at 10- to 15-minute intervals during static

oxidation and analyzed for sulfide, thiosulfate, sulfite, and sulfate.

The hydrogen ion, hydroxide, and carbonate concentrations were determined

before and after oxidation. Chemical changes produced by oxidation are

shown in Table 4 and Figure 8.

No change in pH, hydroxide, or carbonate was found to occur with

oxidation. The sulfur compounds, however, were affected by oxidation

(Figure 8). Thiosulfate was formed by the oxidation of sulfide but the

sulfate content remained unchanged. Apparently the conditions studied

were not severe enough to drive sulfide all the way to its sulfate form.

No sulfite was detected before, during, or after oxidation and it was

suspected that thiosulfate was the highest common oxidation state of

sulfur formed under normal conditions.





-55-


TABLE 4


EFFECT OF OXIDATION ON HYDROGEN ION, HYDROXIDE, AND

CARBONATE CONCENTRATIONS IN BLACK LIQUOR


Static Oxidation for 180 Minutes With Pure Oxygen at 750C


Concentration
Material ___ Difference
Before Oxidation After Oxidation


Hydrogen Ion (pH) 11.8 11.8 0

Hydroxide as Grams per
Liter NaOH 11.0 10.9 -0.1

Carbonate as Grams per
Liter Na2CO3 11.7 11.7 0






-56-


0"
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-57-


A material balance was calculated to determine whether or not

all the sulfur initially present was accounted for in oxidized form

after stabilization. Table 5 shows that 25.6 per cent of the sulfide

sulfur was not accounted for by the thiosulfate produced. The rate of

formation of the unknown compound or compounds was calculated by

difference and plotted in Figure 9.

The question of identifying these unknown compounds and

establishing routine methods of analysis was next approached. The

situation was complicated by the fact that no fewer than five stable

oxidation states are known to exist between sulfide sulfur with an

oxidation number of -2 and thiosulfate sulfur with an oxidation number

of +2. If the more oxidized forms than thiosulfate are considered and,

excluding the sulfate and sulfite which have been accounted for, the

matter is further complicated by the persulfates, hyposulfate, and

dithionate.

Since no sulfite or sulfate was produced, it seemed likely that

at least some of the unaccounted for sulfur was present in oxidation

states between the sulfide and thiosulfate form. The stable oxidation

states in this region fall into two categories, elemental sulfur and the

polysulfides. Elemental sulfur is insoluble in aqueous solution although it

decomposes slowly under alkaline conditions. Some visual evidence of

sulfur, if present, should be detectable in the oxidized black liquor.

At 750C no sulfur formation was observed. Therefore it seemed unlikely

that sulfur was a major stable product of oxidation. The polysulfides are

stable in alkaline solution and are a possible decomposition product of

sulfide.







-58-


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-59-


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-60-


Of the polysulfides four stable oxidation forms -- the di, tri,

tetra, and penta-sulfide -- are known. Separation of the four forms is

difficult and since the disulfide is believed to be the most predominant

form, quantitative analysis usually determines total polysulfides and

expresses the results as disodium disulfide, Na2S2. Scott's Standard
31
Methods of Chemical Analysis suggests a quantitative method for sodium

polysulfide. Since the analysis is based on the fact that the soluble

polysulfides will precipitate sulfur quantitatively in a strongly acid

solution, it is not directly applicable to black liquor without elimina-

tion of certain interference. The method was adapted by precipitating

the sulfides and lignin in the black liquor with zinc chloride and

hydrochloric acid, respectively. Filtering out these precipitates left

a fairly pure solution from which polysulfide sulfur could be precipitated

by acidification and digestion. The sulfur was collected, dried, and

weighed in sintered glass crucibles. To check the purity of the

precipitate, several of the crucibles were extracted with carbon disulfide

and reweighed. The precipitate was completely soluble in carbon disulfide

indicating that the sulfur was relatively free from impurities.

This analysis showed polysulfides were present in both the fresh

and the oxidized liquor. Once a second oxidation product was established,

the oxidation at 750C was repeated to determine the changes that occurred

in sulfide, thiosulfate, and polysulfide concentrations. At 600C and

890C the above mentioned sulfur compounds as well as pH, hydroxide, and

carbonate concentrations were determined before and after oxidation.

The oxidation of sulfide to polysulfide and thiosulfate is shown in

Figure 10, and the sulfur balances over the entire temperature range is

presented in Tables 6 and 7.



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-64-




Under the conditions studied, the sulfur balance (see Table 6)

accounted for all of the sulfide oxidation products in the system with

an average error of 3.9 per cent. The sulfur balance calculated at

30-minute intervals throughout oxidation (Table 7) confirmed the fact

that no appreciable amounts of unknown inorganic sulfur compounds were

formed during any stage of the oxidation. Therefore, it was thought

that most of the experimental error was caused by inaccuracies in the

thiosulfate analysis. Since the thiosulfate determination tended to

produce high results (see Appendix I) and the error of closure was in

the same direction, a considerable portion of the error may be attributed

to the determination. The remaining differences were attributed to

various other forms of experimental error.

It is entirely possible, of course, that compounds of sulfur

other than polysulfide and thiosulfate were formed by oxidation. These

compounds, however, were not formed in significant concentrations from

the inorganic hydrous sulfide. Thus, these trace compounds were of

little practical importance. Because all the sulfur was accounted for

within reasonable limits of experimental error, the presence of trace

compounds other than elemental sulfur was not determined.

The matter of trace quantities of sulfur present in the black

liquor was investigated for two distinct reasons. The first was the

theoretical consideration that elemental sulfur should not be stable in

a strongly alkaline solution. The second reason was that the precipi-

tation of sulfur during oxidation was reported by a previous worker in

the field.8





-65-


To determine the conditions conducive to sulfur formation,

samples of black liquor were oxidized in the reaction chamber (Figure 5)

at 890C, 750C, 60C, 450C, and 300C. No sulfur was formed at 89C and

750C. Traces of sulfur were noticed at 600C while at the two lower

temperatures a definite scum formed. The sulfur was very finely divided

and continued stirring led to agglomeration. To determine the order of

magnitude of the precipitated sulfur, 100-milliliter samples were placed

in a gas impinger and oxidized with oxygen. When analysis indicated that

the sodium sulfide content had dropped to 0.05 grams per liter or less,

the samples were filtered through sintered glass crucibles and thoroughly

washed. The precipitate was dried at 75 C and the weight of sulfur

expressed as grams per liter (see Table 8). The concentration of sulfur

precipitated during oxidation was in the range of 0.004 to 0.005 grams

per liter. This meant that the amount of sulfur formed was about one

thousand times smaller than the polysulfide and the thiosulfate formed,

Since this sulfur formation was negligible compared to other reactions,

it is easy to see why a major discrepancy did not appear in the sulfur

balance in Table 6.

To investigate the stability of the precipitated sulfur, a

portion was collected and washed free of black liquor. Examination under

a microscope showed the precipitate to be comprised of irregular bunches

of tiny spherical sulfur droplets. These droplets, which were insoluble

in carbon disulfide, exhibited no crystalline structure under 1455X

magnification. When the precipitated sulfur was aged about four hours

at room temperature, the drops began to grow into irregular crystals.

At room temperature (250C) about 3-days storage in aqueous solution was





-66-


TABLE 8


CONCENTRATION OF AMORPHOUS SULFUR FORMED

BY BLACK LIQUOR OXIDATION


Oxidation in Gas Impinger With Oxygen
Sulfur Concentrations Expressed as Grams per Liter Sulfur


Temperature
oC


30 45 60


0.004 0.001 0.007

0.006 0.006 0.004

0.003 0.005 0.005

Average 0.004 Average 0.004 Average 0.005





-67-


required to complete this crystallization. The resulting crystals were

completely soluble in carbon disulfide. Photomicrographs of this

metamorphosis from amorphous droplets to rhombic crystals are shown in

Plates 2 through 4. The comparison between the crystal structure of

the aged droplets and the commercial-grade elemental sulfur can be seen

in Plates 4 and 5. From the 0.5- to 1.0-micron particle size it appears

that the sulfur was precipitated very slowly, possibly as a decomposition

product of an organic complex.

The trace of amorphous sulfur will eventually disappear if aged

long enough in the oxidized black liquor. Experimentation showed it was

apparently immaterial whether the liquor was stored aerobically or

anaerobically. The controlling factor was storage temperature. Above

60 C the re-solution of sulfur was definitely accelerated. The probable

reason for this disappearance was that the amorphous or p sulfur crystal-

lized into rhombic or o< sulfur which is unstable in alkaline solution

(see reaction 30).
02 Heat
(30) REDUCED SULFUR COMPOUNDS ---- S (amorphous) Te- So (rhombic).
P Time oc
The crystalline rhombic sulfur will decompose probably according to

reaction 11.

(11) 4S (rhombic) + 6NaOH----- 2Na2S + Na2S203 + 3H20.

Under aerobic conditions the sodium sulfide formed will in turn be

oxidized to thiosulfate and polysulfide.

On the basis of the previous discussion and the data presented

in Tables 4 through 8, and Figures 8 through 10, certain generalizations

can be made regarding the reactions that take place during oxidation.





-68-


PL. 2.-Photomicrograph of freshly precipitated amorphous sulfur
droplets.


PI. 3.-Photomicrograph of precipitated sulfur after aging four
hours at 250C.






-69-


n4





PI. 4.-Photomicrograph of precipitated sulfur after aging three
days at 250C.


PI. 5.-Photomicrograph of commercial grade elemental sulfur.





-70-


The sulfide in black liquor was completely hydrolyzed and present

in the form of hydrosulfide ion, HS If this was not true and the

sulfide ion, S was present, an increase in hydroxide concentration

according to reaction 1 would occur during oxidation. Since no such

increase was found (Table 4), the conclusion was drawn that all the

sodium sulfide was hydrolized to sodium hydrosulfide and sodium

hydroxide during the digestion period (see reaction 4).

On the basis of the curves in Figure 8 and the sulfur balance in

Table 6, the premise that sulfate ion was a product of black liquor

oxidation cannot be verified. These findings are in direct contradiction

to the work of Collins (Table 2). Collins' work was conducted in an air

bubbler at 82 C, and it is difficult to explain how he could have oxidized

some of the soluble sulfide to sulfate. These results may be accounted

for by differences in the black liquor or possibly by different methods

of analysis. It is believed, therefore, that equations 31 and 32 describe

the primary inorganic reactions that take place during black liquor

oxidation.

(31) 4NaHS + 02 2Na2S2 + 2H20.

(32) 2NaHS + 202 P Na2S203 + H20.

It is also believed that reactions 31 and 32 more precisely describe the

fate of the dissolved sulfide than the unconfirmed reactions 1 and 10.

A relatively minor reaction that also occurs during oxidation is the

formation of amorphous elemental sulfur from the liquor.

(33) REDUCED SULFUR COMPOUNDS + 02 S (amorphous).

The oxidation states of sulfur as disodium disulfide and sodium

thiosulfate are -1 and +2, respectively. It was initially suspected that





-71-


in the absence of any data to the contrary, thiosulfate might be an

oxidation product of the more reduced polysulfide form. The shapes of

the thiosulfate and polysulfide curves in Figure 10 indicate, however,

that these two products were formed simultaneously rather than

consecutively. The flat slope of the polysulfide curve and the lack of

a maximum point showed that the thiosulfate was not an oxidation product

of polysulfide. Therefore, oxidation reactions 31 and 32 were parallel

rather than consecutive as might be suspected. The latter reaction

proceeded at a faster velocity than the former and produced a greater

concentration of oxidation products from the black liquor hydrosulfide.

Optimum conditions of temperature and oxygen pressure are

important considerations in black liquor oxidation. In a static oxygen

contact system (Figure 5) at a pressure of 1.0 atmosphere, a rise in

temperature from 60 C to 890C lowered the oxygen tension above the liquor

from 0.803 atmospheres to 0.334 atmospheres. These two dependent parame-

ters appeared to have little or no over-all effect on the products of

reaction. Table 9 shows the ratio in mole fraction of the thiosulfate

to polysulfide produced. The variation in this ratio which averaged

1.60 was small and it was concluded that the oxidation products formed

were relatively independent of temperature.

To determine whether or not the method of contact influenced the

oxidation products, experiments were performed at 750C using a dynamic

oxygen flow (Figure 6). Tests were also conducted to verify the work of

Collins.16 This was done by oxidizing black liquor at 82 C in an air

bubbler. At these experimental conditions large excesses of oxygen and

air were provided. Tables 10 and 11 show the change of certain sulfur
















































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-72-


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-73-


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-74-


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-75-


compounds upon oxidation. Even with great excesses of oxidizing agent

sulfate was not a product of oxidation. It was concluded that in the

temperature range considered the rate of oxidation may depend on the

method of contact but the ultimate oxidation products formed do not.

Miscellaneous other studies showed that application of 1450 parts per

million (0.1450 per cent) of ozone was insufficient to oxidize any of

the reduced sulfur compounds to sulfate.


Reversibility of the Oxidation Reactions

When in contact with oxygen at elevated temperatures, the sodium

hydrosulfide in black liquor spontaneously oxidizes to disodium

disulfide and sodium thiosulfate. The degree to which these reactions

are reversible when the oxygen is removed was investigated. This

reversibility may create odor problems since in practice the black liquor

may be stored anaerobically for a length of time after oxidation and

before evaporation. If volatile sulfur compounds are regenerated, some

of the effect of oxidation will be lost. Murray reported that sulfide

was regenerated when black liquor was stored anaerobically, but he did

not explore this observation or attempt to explain it. This regeneration

can be explained if reactions 31 and 32 are reversible.

To test this hypothesis, samples of partially oxidized liquor

were withdrawn periodicallyfrom the oxidizing chamber, analyzed for

sulfide, polysulfide, and thiosulfate, and then stored anaerobically at

75 C for 12 hours. At the end of that period the samples were again

analyzed for sulfide, polysulfide, and thiosulfate, and the results

reported in Table 12. From these data it was concluded that the






-76-


un

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-77-


regeneration of the sulfide is due mainly to the reduction of some of

the thiosulfate while the polysulfide formed is relatively stable. The

more completely oxidized the black liquor was, the more irreversible was

the sodium thiosulfate reaction. Therefore, the oxidation of hydro-

sulfide (reaction 32) to thiosulfate is partially reversible whereas

the oxidation to polysulfide is not.

A more detailed picture of the regeneration is shown in Table 13

and Figure 11. The solid line in Figure 11 shows the rate of oxidation

of black liquor at 750C using a static oxygen system. The dotted curve

shows the hydrosulfide concentration if oxidation were stopped at that
o
point and liquor stored anaerobically at 75 C for 12 hours. The

reactions involved were much more reversible in the early stages of

oxidation when the system was still actively absorbing oxygen. After the

oxygen transfer ceased, the hydrosulfide regenerability of the liquor

dropped sharply. This would seem to indicate that one mechanism of

reaction predominated early in the oxidation while a different mechanism

controlled during the final stages. The average degree of regeneration

to be expected with this particular liquor at various stages of oxidation

is shown in Table 14. To prevent less than 10 per cent regeneration after

12 hours of storage, 80 per cent or more of the black liquor hydrosulfide

had to be oxidized. This pointed up the importance of evaporating the

liquor immediately after aeration to derive the fullest benefits of

oxidation. Regeneration was not pronounced in samples stored anaerobically


at room temperature.





-78-


TABLE 13


REGENERATION OF SULFIDE UPON ANAEROBIC STORAGE


Static Oxidation With Pure Oxygen at 75 C
Anaerobic Storage for 12 Hours at 750C

Initial Sulfide Final Sulfide Percentage Sulfide
Concentration Concentration Regeneration
Expressed as Expressed as Based on Initial
Grams per Liter Grams per Liter Concentration of 6.1
Na2S Na2S Grams per Liter Na2S


6.10

5.95

5.42

5.31

4.52

3.83

3.71

3.45

2.50

2.26

2.03

1.79

0.98

0.60

0.32

0.13

0.12

0.00


7.05

6.65

6.99

6.59

6.71

5.11

5.57

3.85

3.43

4.10

3.20

2.18

1.99

0.89

0.94

0.40

0.38

0.03


+15.6

+11.5

+25.8

+21.0

+33.8

+37.4

+30.5

+ 6.5

+14.9

+30.2

+19.2

+ 6.1

+15.8

+ 4.8

+10.2

+ 4.4

+ 4.1

+ 0.5







-79-


01 6 8 L 9 7 c 1 0

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00

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IN
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-80-


TABLE 14


PERCENTAGE REGENERATION OF SULFIDE UPON ANAEROBIC STORAGE


0
Static Oxidation With Pure Oxygen at 75 C
Anaerobic Storage for 12 Hours at 750C
Based on Curve in Figure 12


Percentage Sulfide Percentage Sulfide Total Percentage
Oxidized Regenerated o Sulfide Present After
Based on Initial After 12 Hour Storage at 75 C Regeneration
Concentration of Based on Initial Based on Initial
6.1 Grams per Liter Concentration of 6.1 Concentration of 6.1
Na2S Grams per Liter Na2S Grams per Liter Na2S



0 16 116

20 24 104

40 24 84

60 21 61

80 9 29

90 7 17





-81-


Oxidation Kinetics

For effective oxidation sufficient quantities of oxygen or air

must be brought into intimate contact with the black liquor. Wright7

found that infinite contact area did not produce an instantaneous

reaction, and he proposed that nothing is gained by increasing the area

past a certain point. Assuming that adequate gas-liquid contact can be

provided on a mill scale, the reaction kinetics and stoichemetric oxygen

requirements become the paramount considerations. In the studies

conducted by the author on the kinetics of oxidation, the effect of gas-

liquid contact was not eliminated but merely held at a fixed value. This

was achieved by holding the stirring rate and gas feed constant. By

varying the temperature and hence the oxygen pressure within the system,

their effect on the reaction rate could be evaluated. Although it would

be difficult to extrapolate the reaction kinetics to actual plant design

criteria, comparison of these data at different physical conditions

showed the relative importance of the controlling parameters. Once these

parameters are evaluated, the gathering of design data for an actual

installation is greatly simplified.

The first step was to determine the effect of temperature on

oxidation of the dissolved sulfide. Experimental temperatures of 600C,

75 C, and 89 C were investigated to determine the optimum reaction

temperature. A comparison of oxidation at these three temperatures is

shown in Figure 12.

When the semi-empirical equation 29, dC/dt K (C + bP) was

fitted to the curves in Figure 12, some quantitative comparisons can be

made about the relationship between temperature, reaction rate, and






-82-


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-83-


oxygen absorption. The details of determining the reaction constants K

and b in the Appendix II.

A tabulation of the velocity and absorption constants K and b

is presented in Table 15. If the oxidation is a true first-order

reaction, oxygen absorption will not affect the rate and constant b will

be zero. Similarly, if oxidation is independent of the sulfide concen-

tration, the reaction will be of zero order depending solely on the

factor K (bP). As can be seen from Table 15, the static system with

very low values of bP is very nearly a first-order reaction. This was

especially true at 890C.

A normal "rule of thumb" in chemical kinetics is that the rate

constant is doubled with every 100C rise in temperature. In the case of

black liquor oxidation the velocity constant, K, varied only slightly

with temperature. The value of K at 750C was only 23 per cent higher

than the value at 600C. When the temperature was raised to 89 C, the

velocity constant began to drop off. Thus, of the three temperatures
o
tested, the reaction velocity was fastest at 75 C.

In connection with the ozone experiments black liquor was

oxidized dynamically at 750C. The oxidation of soluble hydrosulfide

in this dynamic system, both with and without ozone, is shown in Figure

13. Apparently 1450 parts per million (0.1450 per cent) of ozone was

insufficient to affect the oxidation of the dissolved sulfide, but the

ozone did react with the odoriferous gases above the black liquor. A

more thorough treatment of the odor problems involved with black liquor

oxidation is discussed later. Equation 21 was fitted to this curve and

the reaction constants (Table 15) were compared to those of the static





-84-


TABLE 15


KINETICS OF BLACK LIQUOR OXIDATION


Based on Curves in Figures 12 and 13
See Appendix II for Details of Calculation


Type of Kinetic Constants
Temperature
Oxygen o Velocity Oxygen Absorption Oxygen Absorption
FlConstant, K Constant, b Product, bP



60 -0.0241 0.175 0,140

Static 75 -0.0312 0.177 0.110
System
89 -0.0284 0.029 0.010



Dynamic
System 75 -0.0100 5.480 3.400
450 ml
per Minute







-85-


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0

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S00-
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y -^


/ o *r<
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S BN la3^^ lad suro-i 'UOT3BJ3U30UOO





-86-


system. In dynamic oxidation the reaction velocity was only one-third

as great as that of the static system. The oxygen absorption factor,

however, was increased by a factor of 30. Thus, the over-all reaction

was less efficient dynamically than statically and more dependent on the

rate of oxygen absorption. Apparently escaping water vapor hindered the

oxygen transfer at the gas-liquid interface.

The indirect inferences which may be drawn from the comparison

between the two systems should not be overlooked. In both cases the

oxidation was conducted under the same conditions except for the method

of oxygen feed. The static system was very nearly a first-order reaction,

but the dynamic system was significantly influenced by oxygen absorption.

This confirms that it was relatively easy for the contact parameters to

be the limiting factor in oxidation. In properly designed large-scale

equipment, however, contact should be sufficient so that the chemical

reaction rate which is a relatively fixed quantity is the limiting factor.


Black Liquor Oxygen Requirements

Additional tests were conducted using a closed static system

which enabled the utilized oxygen to be metered. Table 16 shows the

variation of oxygen absorbed with changes in temperature. Oxygen

absorption was almost doubled as the temperature was raised from 600C to
o
89 C. Since the black liquor contained 6.1 grams per liter of sodium

sulfide, the theoretical amount required to convert the sulfide to

polysulfide and thiosulfate according to reactions 31 and 32 was

approximately 1.1 milliliters of oxygen per milliliter of black liquor.

At temperatures above 6000C considerably more oxygen was absorbed than





-87-


TABLE 16


VOLUME OF OXYGEN ABSORBED BY BLACK LIQUOR DURING OXIDATION


Static Oxidation for 180 Minutes With Pure Oxygen


Temperature Volume of Oxygen Absorbed by Black Liquor
o Milliliters of Oxygen per Milliliter of
C Black Liquor


60 1.2

75 1.5

89 2.1



Volume of oxygen corrected to 25 C and 760 millimeters of mercury.





-88-


was required to oxidize the sodium sulfide. Thus, as the temperature

increased, more and more oxygen was used in side reactions. The

percentage of absorbed oxygen used in these side reactions is shown in

Table 17. Sixty degrees centigrade was the most desirable temperature

of the temperatures tried because most of the oxygen absorbed was

utilized in oxidizing the inorganic sulfides, This was not the case at

the higher temperatures and at 890C almost half of the absorbed oxygen

was used in extraneous reactions.


Catalysis in the Oxidation of Sodium Sulfide

It has been suggested that the organic matter dissolved in black

liquor catalyzes the oxidation of sodium sulfide. This author found
o o
that in the range of 60 C to 89 C oxygen absorption practically ceased

after 40 to 50 minutes when 1.5 to 1.7 grams per liter of sulfide had

yet to be oxidized. This remaining sodium hydrosulfide continued to

oxidize with little apparent oxygen absorption. The time lag between

oxygen absorption and utilization could be due to chemisorption of oxygen

by the liquor.


To test the hypothesis that soluble materials in black liquor

catalyze oxidation, a synthetic liquor containing 6.1 grams per liter

of sodium sulfide, 5.4 grams per liter of sodium thiosulfate, and 11.0

grams per liter of sodium hydroxide in distilled water was prepared.

This liquor contained the same concentration of the principal inorganic

compounds found in the black liquor. In a static contact system at

750C the synthetic liquor oxidized much more slowly (see Figure 14)

than the black liquor. Thus it is evident that the black liquor contained




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