Title: Fluoride removal from wet-process phosphoric acid reactor gases
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Title: Fluoride removal from wet-process phosphoric acid reactor gases
Physical Description: 1 online resource (xv, 201 leaves.) : ill. ;
Language: English
Creator: Craig, John Munro, 1938-
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 1970
Copyright Date: 1970
 Subjects
Subject: Phosphate industry -- Florida   ( lcsh )
Fluorides   ( lcsh )
Air -- Pollution   ( lcsh )
Air -- Purification   ( lcsh )
Genre: bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )
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Thesis: Thesis--University of Florida, 1970.
Bibliography: Bibliography: leaves 191-199.
General Note: Manuscript copy.
General Note: Vita.
General Note: Description based on print version record.
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Bibliographic ID: UF00097712
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 - 004816948
oclc - 503002112

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FLUORIDE REMOVAL FROM WET-PROCESS

PHOSPHORIC ACID REACTOR GASES













By
JOHN MUNRO CRAIG













A DISSERTATI1ON 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
1970












































UNIVERSITY OF FLORIDA\

3 1262 08552 4733







ACKN~OWLEDGMENT:N S


The author wishes to express his appreciation to

the many people and firms who made this project possible.

Gratitude is extended to the members of the

writer's supervisory committee, Dr. R. S. Sholeos,

chairman, Dr, R. 0. McCaldin, Dr. J. E. Singley, and



Special r-ecognlition is due to the many companies

who provided the items necessary for this work. The

following are singled out for m~ention~ing: Tampa El~ectric

Company, Tri-Coat, Agrico Chemical Corporation, Airetron

Engineering Corporation, Amecrican Air Filter Company,

Inc., Peabodyv Engineer-ing Corporation, and the Occidlental

Chemical Company. Special recognition is due Mlr. P.

Flanagan, Occidental Chemical Company, for his efforts

in coordinating thy author' s requirements with the mai~n-

tenance andi operating departments at the plant site.

The author is indebted to Mr. H. McGraw for his

efforts in erecting the research equipment and to Mrs, J.

Hunt for typing this manuscript.

The author wishes to express deep appreciation

to the Department of Healthl, Education, and Welfare,

National Air Polluti~on Control Administration for financing

his graduate studies and to the Agrico Chemical Company

for partial funding of this research project.







The author wishe~s to thanked the many- University

of Florida faculty an~d staff members, whose names are

too numerous to list, for their kind and patient assis-

tance.

In conclusion, the author would like to thank

his wife, Jo, and his children, John, Li~nda, and K~imi

for their patience and understanding during this period

in their lives.














TABLE OF CONTENTS
Pa~e

ACKNPUOWLEDGN~ENTS .................... ............... 11

LIST OF TABLES ...,. ....,,.. ..... .. ....,,.,..... vIii

LIST OF FIGURES ,................... .... ......... ...viin

KiEY TO EUIN ABBREVIATIONS .,,,............... ........ xit

ABSTRACT ..,,.......,.., .. .. ............. ...... xiii

CHAPTER
I, INTRODUCTION ................... ........ 1

II. FROSPHATE INDUSTRY IN FLORIDA .......... 6

Ph~osphpte Fertilizer Mlanufacturing
Processes ................... ....... 6

Geology ................... .... 6
Mining .............. .,. 9
Washing and Flotation ..... 9
Beneficiation ......... 12
Acid Manufacturing ..:............ 12
Fertilizer Manufactulrin ..... 18
Elemental Phosphorous Manufactur-
ing ............,.. .......... 18
Air Pollution Associated With the
Phosphate Industry ................. 21
Fluoride Toxicity ............... 24
Vegetation Damage ............... 25
Cattle Damage ................... 29
Human Health Damage ............ 31
Governmental Controls ........... 34

III, WET-PROCESS BROSPHORIC ACID ....,,...... 37

History ................ .. ........... 37
Prior to 19000 ...............,,,. 37
1900 to 1930 .................... 39
1930 to 1968 ................. ... 41

iv







Page
Fundamenta-l Prinrciples of the D~ihydirate
Process ........ .................. 43
Raw Materials .......,.......... 45
Process Chemistry .......,,..,... 49
Process Technlology ........,.... 52
Fluoride Evolutionl ,............ 57

IV, ABSORPTION THEORIES AND EMPIRICAL
RELATIONSHIPS ........................ 60

Absorption Theories ............. ... 60
General Principles ........,..... 61
Two-Film Theory ,,............... 66
Penetration Theory ............. 68
Surface Renewal Theory ,,......., 68
Film Penetration Theory ......... 69
Absorption with- Chemical Reaction 69
Fluoride Absorption ............... 72
General Design Concepts ......... 72
Absorption in the Cyclonic Spray
Chamber .....,............., ... 79
Absorption in the Venturti .,..... C1
Absorption in the Baffle Plate
Impingement Column ..,,,,. 82
Correlation of Scrubber E-fficien1-
cies ....,,,..,,................ 83

V. TH~E EXPERIMIENT ..,,...................,. 86

Experiment Design ................... 87
Operating Variabl~es ,........... 87
Preliminary Inv~estigation~s ,, 89
Response ,.............- . 90
Replication of Experiments .,,.., 90
Equipment Description ..,.............. 90
Wet-Process Phosphoric Acid Plant 93
Liquid Handling System ........ 95
Gas Handling System ,,.,.. 100
Cyclonic Spray Chamber Scrubber .103
Venturi -Cyclonic Scrubber ..... 105
Impingement Baffle Plate Column 105
Variable Throat Venturi ,,...... 108
Instrum~entation ....,... 109

Experimental and Analytical Procedures 110
Preparation for the Study ....... 110






Paee
Typical Expedi:;cntal Run ...:.... 111
Fluoride Samnp.ing and Anlalysis 114

VJ. RESULTS OF THE~ EXP1ERIMENT ............ 134;

Cy-clonic Spray Chamtber ..... 152
Venturi Cyclonice Scrubber ,. 156i
Baiflfl Plate Imnpingemrent Colulmn 58
Variable Throat Venturt .......16

VIT. CONCLUSIONS AND RECOMMENDATIONS ,.... .. 163

APPENDTX I ............. ..... ....... .,,,... ..... 170
APPENDIX II . . . . ., , .. . .. 18)7

LIST OF REFERENVCES .................. ..,........,,.,, 191:

BIOGRAPHICAL, SKIETCH ............... . ............. 200











LIST OF TABLES


Table Page
1 FLUORIDE EVOLUTION ACCORDING TO SPECHT
AND CALACETO ..... ...,,........,.,.......... 23

2 FLUORIDE EMISSIONS ACCORDING TO HUFFSTUTLER .... 24

3 TYPICAL COMPOSITION OF COMMERCIAL GRADES OF
FLORIDA PHOSPHATE ROCK ................... .... 46

4 TYPICAL COMPOSITION OF OCCIDENTAL BROSPHAITE
ROCK ................ ,,,.,......... ,,.,,...,, 46

5 FLUORIDE BALANCE IN SWIFT & CO. WET-PROCESS
PHOSPHIORIC ACID PLANT .. ................ ..,.. 56

6 LIQUID HANDLING SYSTEM ORIFICES ,...,........ .. 99

7 GAS HANDLING SYSTEM ORIFICES ,............ ... .102

8 OCCIDENTAL LABORATORY ANALYSIS .,..............125

9 STATISTICAL ANALYSIS OF CALIBRATION CURVE DATA .126

10 SPRAY CHAMBER EXPERIMEZNTAL DATA SCRUBBING
LIQUID WELL WATER ...,,........... .,......71

11 SPRA~Y CHAMBER EX'PERIMENTAL DATA SCRUBBING
LIQUID GYPSUM POND WATER ,,. ............ .,173

12 VENITURI EXPERIMENTAL DATA SCRUBBING LIQUID
WELL WATER ,................ ........ ......,175

13 VENTURI EXPERfIMENTAL DATA SCRUBBING LIQUID-
GYIPSUM POND WATER ....,,.,,...,..,... .........177

14 BAFFLE PLATE IMPINGEMENT COLUMN SCRUBBING
LIQUID WELL WATER ..............,....,.......179

15 BAFFLE PLATE IMLPINGEPMET COLUMN SCRUBBINiG
LIQUID GYPSUM! POND WATER, ............... ..181

16 VARIABLE THROAT VENTURI EXPERIMENTAL DATA
SCRUBB1N!G L1QUID WELL WATER ................183

17 VARIABLIE THROAT VENTURI EXPERIMENTAL DATA
SCRUBBING LIQUID -GYPSUM POND WATER .... ...185

V11













LIST OF FIGURES


.gure Page
1 TYPICAL PHIOSPHATE ROCK PROCESSING
FACILITY .. .... ... .... ...............,.,. ]0

2 WASHER AND FLOTATION PLANT FLOW DIAGRAM ...... 11

3 TYPICAL PHOSPHA~TE ROCK BENEFICATION FLOW
DIAGRAM ................... ................. 13

4 TYPICAL ACID-FERTILIZER MANUFACTURING COMPLEX
FLOW DIAGRAM ................... ........... 15

5 TYPICAL CONTACT SULFURIC ACID PROCESS FLOWV
DIAGRAM1 ................... ................. 16

6 TYPICAL WET-PROCESS PHOSPHIORIC ACID FLOW
DIAGRAM .................... ..............._ 17

7 TYP~ICAL TSRIPLE-SUPERP;OSPHIATE FACILITY FLOW
DIAGRAM ................... ................. 19

8 TYPICAL. DIAMMONIUM PHOSPHIATE FACILITY FLOW
DIAGRAMI ................... ................. 20

9 TYPICAL ELEMENTAL PHF~OSPH-ORUS FACILITY FLOWj
DIAGRAM .................................... 22

10 RELATION OF CONCENTRATION AND DURATION OF
EXPOSURE TO EFFECTS OF ATMOSPHERIC FLUORIDE
ONr TOMATO PLANTS .,,...... .,.,............. 28

11 PRECIPITATION AND STABILITY OF CALCIUM SULFATES
IN PH~OSPHORIC ACID ..............,,... ...... 51

12 EQUILIBRIUM DIAGRAM ................... ....... 62

13 DIFFUSION OF A\ THROUGH STAGNANT B ......... 64

14 INTERFACIAL CHARACTERISTICS IN PHYSICAL
ABSORPTION .............. ................. 70

15 INTERFACIAL CHAIRACTERISTICS INU ABSORPTION
WITH CHEMIICAL REACTION .,,,,,.,,..... ...,,.. 71






igure Page
16 VAPOR PRESSURE OF HYDROFLU0RIC ACID OVER
DILUTE AQUEOUS SOLUTIONS ...,,,.. ......,,,. 77

17 VAPOR PRESSURE OF SILICON TETRAFLU0RIDE OVER
AQIUEOUS SOLUTIONS OF FLUOSI.ICIC ACID ... .. 77

18 EQUIPMENT TRAILER FROM ROOF OF ACID PLANT
REACTOR TANK ...,,.......... ............ ... 91

19 SIDE VIEW OF EQUIPMENT TRAILER ............... 92

20 FLOW DIAGRAM OCCIDENTAL WET-PROCESS PHOSPHORIC
ACID PLANT ,....,........,.,,.,,,,,..... 94

21 OCCIDENTAL WET-PROCESS BROSPH1ORIC ACID PLANT
EFFLUENT CONTROL SYSTEM ..,,,.........,,,,... 96

22 SCHEMATIC DIAGRAM OF LIQUID SYSTEM ..,,,,,,... 97

23 LIQUID STORAGE TANK TRAILER ............,,...., 98

24 SCHE:MATIC DIAGRAM OF GA4S HANDLING SYSTEM ..... 101

25 DIAGRAMMATICVIEW OF CYCLONIC SPRAIY CHAMBER ... 104

26 DWIAGRMMATICVIEW OF VENTURI .................. 106

27 DLAGRAMMTICVIEW OF IMPINGEMENT BAFFLE PLATE
COLUMN ..,,,,............. ............ ...... 107

28 FRACTION O7F FREE FLUORIDE AS A FUNCTION OF
SOLUTION pH, WHERE HYDROGEN ION IS THE ONLY
COMPLETING SPECIES ..., ,.118

29 EFFECT OF pH ON ELECTRODE RESPONSE (SODIUM
FLUORIDEC` STANDARD) ....,.... ...118
30 CALIBRATION CURVE USING 2 PERCENT SODIUM
ACETATE SOLUT'ION ,,.........,,... .... 121
31 FLUjORIDE SAMPLING TRAIN ...., ..... 127

32 EFFECT OF LIQUID/GAS RATIO ON FLUORIDE REMOVAL
EFFICIENCY IN SPRAY CHAMBERS, .,,............. 135

33 EFFECT OF TOTAL CONTACTING POWER ON NUMBER OF
TRANSFER UNITS IN SPRAY CHAMIBER ........... 136

34 EFFECT OF LIQUID/GAS RATIO ONJ NUMBER OF
TRANSFER UNITS FOR SPRAY CHAMBER USING WELL
WA1TER ... . . . .- * * * * 37







igur-e Page
35 E;FFE:CT OF` L1QiUiI/GAS RATIO ON NiUNlUFR OF
TJRANSFERP UNIT'S FOR: SPRAY CHAZMBERI USINiG
CYIPSLUM POND WArtTERP .....,,.,...... ... ....... 138

361 EFFECT OF: LIQUID/GArS RATIO1 OlN F'LUORIDE REMOVAL
EFF~ICIEN~CY ;N VENTURI ,.,....... ....... ... 139

37 EFFECT OF TOTAL CONITACTING POWER ON N~UMBER OF'
TR'ANSFERI UNITS IN VE:NTURI .......,,......... 140

38 EFFECT OF: GAS CONTACTING POWER ON NUMBER OF
TRANDSFER UNITS IN VEN~TUiR1 .................. 141

39 EFFECT OF` L1QUID/GAS RATIO ON- NUMBERS OF TRAN\S-
FER UNITS FOR, VEN\TURtI U~SING WEI~l WATER ..,.. 142

40 EFFECT OF LIQUID/GAS RATIO ON NUMB~IER O3F TRANiS-
FER UNITS FOR VENTURI USING GYPSUM POND
WA4TER ............... ..... .................. 143

41 EFFECT OF LIQUID/G-AS RATIO ON FLUORIDE
REMOVAL:I EFFICIENCY IN BAFFLE PLATE: IMPINGE-
MENT COLU~MN ..,,,.,,....... ...... ........., 144

42 EFFECT OF TOTAL CONTRACTING FLOWER OiN NUMBERR OF
TRAINSFERZ UNI1rS IN\ BAFFLE PLATlE INFINUG;lEMENT
COLUMN? ....,,,............................... 145

43 EFFECT OF LIQUID/GAS RATIO ON NUMBlER~ OF TRANS-
FER UNITS FOR: BAFFLE PLATE IMPFINGEMENT COLUiMN
USING WELL WATER .,,,................ ...,.... 146

44 EFFECT OF LIQU1D/GAS PATIO ON NUMBER OF TRA~NS-
FER UNITS FOR, BAFFLE PLATE IMPINGEMENT
COLUMNU USING GYPSUM POND WATER ..........., 147

45 EFFECT OF LIQUID/GAS RATIO ON FLUORIDE REMG~VAL
EFFICIENCY IN VARIABLE ITHROAT VENTURI ...... 148

46 EFFECT OF TOTAL CONTACTING POWER. ON NUMBER OF
TRANSFER UNITS IN VARIABLE T;HROAT VENTURI .. 149

47 EFFECT OF LIQUID/GAS RATIO ON NrUMIBER OF
TRANSFER, UNITS FOR VA~IABL.E THRPOA\T VENTURI
USING WELL WA~TER .............,,. .. ..,,... 150

4;8 EF;E.CT OF LIQUID/GAS RATIO ON NUMBER OF TRANS-
FER UNITS FOCR VARFIABLE TRROAT VENTURI USING
GYPSUMH POND WXTIER ................... ....... 151







Figure P'age
49 COMPARISON OF: TtEl EFFECT OF TOTaL. CONTACT-
ING POW~iER ON NUMBER, OF TRANSFER UNITS IN
EQUIPMENT STUDIED .....,..................... 164







KEY 10 KllN ABDREVIATIONS


BPL bone phosphate of lime~, commonly uised to express

the content of tricalcium phosphate, Ca3(PO )2'

in phosphatre rockr, and used to express the

grade of ore.

ofm gas flow;, cubic feet per minute.

gpm liquid flow, gallons per minute.

HP horsepower.

mg/std cu m milligrams per standards cubic meter
(standard conditions of 250OC and 760 mlm mercury;).

my millivol~ts.

ppb parts per billion.






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



FLUORIDE REMOVAL. FROM W:ET-PROCESS
PHOSPHORIC ACID REACTOR GASES

By

John Munro Craig

March, 1970



Chairman: Dr. R. S. Sholtes
Major Department: Environmental Engineering

Due, in part, to the growth of the phosphate

fertilizer industry, fluoride pollution of the air has

become an area of major concern. Since the state of

Florida is the center of this industry in the United

States, air pollution from the phosphate fertilizer

industry is a major problem in certain areas of the state.

Fluoride air pollution is a matter of great concern

since even at very low levels it is known to produce

detrimental effects on vegetation and animals.

Phosphate fertilizer manufacturing complexes

normally have acid plants associated with them which

produce the acid necessary to convert the highly in-

soluble fluorapatite in phosphate rock to a more readily

available form of a nutrient phosphate. One of the most

common methods of producing fertilizer grade phosphoric

acid is via thie wet-process phosphoric acid process.


X111






In~ this process, the reaction between the phosphate rock,

sulfuric acid, and recycled weak phosphoric acid releases

fluoride gases which may be harmful to plants and

animals if not closely controlled.

With this in mind, a research project was int-

tiated to study the effects of operating variables upon

certain types of air cleaning equipment and the efft-

ciency of fluoride removal from wet-process phosphoric

acid reactor gases.

For this study, a portable pilot plant was constyuct-

ed on two semi-trailers, taken to a 650 tons/day wet-

process phosphoric acid plant, and connected into the

center compartment of the reactor tank, The air clean.-

ing equipment used in this study included a 2600 cfm

capacity cyclonic spray chamber, a 2600 ofm capacity

venturi-cyclonic demister scrubber, a 350 ofm capacity

baffle plate impingement column, and a 1000 ofm capacity

variable throat venturi, The effects of liquid and gas

flow rates and type of scrubbing liquid uponl fluoride

removal efficiency were studied. The types of scrubbing

liquid used-were well water and gypsum pond water.

In the case of scrubbing with well water, fluoride

removal efficiencies of 63.2 to 96.1 percent, for less

than 1.5 inches of H20j, and for an energy input of less

than 0.7 horsepower/1000 std efm, are possible using the

cyclonic spray chamber. Efficiencies of 88.3 to 99.5

percent, for less than 15 inches of H20, and for an
xiv






energy input of less than 3.7 horsepower/1000 std efm,

are possible using the venturi-cyclonic demister scrub-

ber. Efficiencies of 90,1 to 99.0 percent, for less

than 8.5 inches of H20 pressure drop, and for an energy

input of less than 1.0 horsepower/1000 std ofm, are

possible using the baffle plate impingement column,

Efficiencies of 92.9 to 97.8 percent, for less than

40.0 inches of H20 pressure drop, and for an energy

input of less than 4.3 horsepower/1000 std efm, are

possible using the variable throat venturi.

In all instances, scrubbing with gypsum pond

water produced lower removal efficiencies than scrubbing

with well water at approximately the same pressure drops

and energy requirements.

The fluoride removal efficiencies developed by

the equipment studied were not sufficiently high

to meet the requirements necessary for emission control

from this process. Two-stage scrubber installations are

required if the fluorides from the reactor are to be

brought under control. Since two-stage scrubber installa-

tions are necessary, gypsum pond water would be the

preferable scrubbing liquid because the use of this

liquid will not disturb the existing water balance

established in the process.












CHAPTER I


INTRODUCTION


The emission of fluorides into the ambient air has

long been a concent of many people and with the growth

of certain industries fluoride pollution of the air has

become more of a problem. The accepted threshold limit

value for human exposure under industrial conditions is

3 ppm; however, atmospheric pollution would involve

concentrations far below this level, Criteria for ambient

fluoride levels are being prepared by the National Air

Pollution Control Administration under provisions of the

Clean Air Act of 1967, The American Industrial Hygiene

Association has recommended the following air quality

levels for gaseous fluorides, expressed as HF by volume:

4.5 ppb average for 12 hours

3.5 ppb average for 24 hours

2.0 ppb average for 1 week

1.0 ppb average for 1 month

There has been no demonstrated case of chronic fluorosis,

or acute poisoning, resulting from humans breathing air

containing these low concentrations of fluorine com-

pounds.1 Although some have suggested that fluorine

compounds may have been the prime agents in the M~euse








Valley and Donora air pollution disasters, no evidence

has been presented to substantiate these clainCs.23,4

Fluoride contaminants may be emitted to the ambie-nt

air in a wide vJariety of industrial processes in which

fluorine compounds are manufactured, utilized, or are

present as impurities in the process materials. The

processes include but are not limited to:

Major sources

1, hosphate fertilizer manufacturing

2. Aluminum manufacturing

3. Steel manufacturingn

Minor sources

1 Brick and tile manufacturlng

2. En~amel fric manufacturing

3. Manufacture of fluorin~e compoundss

4. Power generations;

5, Use of fluorine co~mpounds as catalysts

6, Manufacture of motor fuels

Although the previously mentioned industries have been

recognized as producing damage in some instances, numer-

ous other industries may emit fluoride compounids to some

extent in proce=ssing of mlinerals and ores. ~This is due

to the wide distribution, frequently in small conconl-

tratijons, of fluocrin~e throughout thie earnh's crust.

Recent estimates have- placed flu:orine as the thirteenth

most abundant eleme7nc in thep earth's crust.5








Somnrau and Specht6 consider the mianufacture of

phosphate fertilizers as the most important source of

fluoride contamination of the atmospheree. Since Florida

produces approximately 70 percent of the nation' s phos-

phate rock, air pollution from this industry is a major

problem in certain areas of the state. The objective of

this research was to study fluoride emissions and their

control from one of the processes involved in the phos-

phate fertilizer industry located in Florida and all

discussion from this point on will be limited to this

industry and its manufacturing processes.

The recovery of fluorine effluent, gaseous and

particulate, from the phosphate industrial operations

presents three equally demanding problems. First, level

of recovery; second, operational dependability; and third,

capital investment and operating costs. The recovery of

gaseous fluori~ne compounds is, at present, most econlomical-

ly achieved by absorption in water or 'In other aqueous

solutions. The design of a fluoride recovery system is

not without many problems, some of which are:

1. Multiple contaminants within an effluent

stream.

2. L~ow concentration of inlet. gas and the

extremely low concentration requirement

for effluentc.

3. Limitation in choice of scrubbing medium.








4. Low pressure drop requirements for the

system.

5. Disposal of scrubbing solution,.

In order to optimize the costs involved in con-

structing and mraintaininlg recovery systems, the following

steps are necessary:

1. Determination of optimum scrubbing solu-

tions.

2. DeterminationI of optimum scrubbing equiip-

ment configurations.

3, Determination of optimum liquid-gas ratios.

This research effort dealt with specific gas-

liquid absorption processes utilizing four types of

scrub'oers and two scrutbbing solutions. The types o~f

scrubbers studied were:

1 A~iretron Engineering Corpora-ion modifiedc

low pressure drop venturi.

2. Airetron Engineering Corporation modified

cyclonic spray chamber.

3. Peabody Engineering Company impin:gement

baffle plate column.

4. American Air Filter Company variable

throat venturi,

The scrubbing solutions studied were:

1. G~ypsumn pond water, pH 2, available at

all plant sites.

2. Well water, pH 5 to 7, available at all








planc sites and lowest cost buffer for

gy~psuml pondc %(aters.

This study sought to answerL tlhe following que~stions:

1. ihat is the relationship between the

operat~ing varianbles and removal effi-

ciency?

2. Wdhat are thie mrass transfer characteristics?

3, W~hat ar-e the optimumn Energy r~equijrements??

4. What is the m~axjimuml luoride removal?

The major independent variables studcied for each

systemr were:

1. Scrubbing solution.

2. Li~ui~d and gas flow rates,












CHAPTER II


PRIOSPHA~TE INDUSTRY IN FLORID.


Phos1ht Fer-tilizer Manufactu-ring Procese

Florida's phosphate industry had its start in the

1880's when phosphate pebbles and fossil bones were dis-

covered in the Peae River south of Fort Mede6.7 Early

mining efforts soon were shifted from thle river bed to

near Bartow an~d Mul~berry, wiher land-lock~ed phosphate of

much highor grade could be objtainred at far less cost.

From this smrall beginnings, the ph~osphate indulstrl in

Florida has grown to an industry which at tne: present

time exceeds 40 million long tonls a year or approxrirately

70 percent of the nations output.

The principal use of phosphate rock is in the

production of fertilizer. Other products are phosphorus,

phosphoric acid and ferrophosphate, By-products of the

p~hosphate indiustry- include g~psuml, fluorine compounds,
sulfur dioxide and carbon dioxide.


Geolcry

Tiwo general types of phosph!ate are deposits serve

as sour-ces of high~-grade material. Onep is of ignreous

origins ; ith other sedi;mentar-y, Both havie essentially the








same phosphate mlineral, calciumn phosphate of the apatite

class of minerals.

The igneous deposits are not so extensive in

number or as easily and economically processed as the

sedimentary deposits,8 This type of deposit accounts

for approximately 10 percent of the world's production

from mines located in Russia, South Africa and South

America.

The major phosphate deposits of the world are

sedimentary in origin and nearly all are calcium phlos-

phate. There are two types of sedimentary phosphate

deposits -- guano and pellet, H~utchinson 0has dis-

cussed the guano type in somne detail in his work. The

pellet deposits are the major deposits in the world.

They furnish over 80 per-cent o~f the world's phosphate

demands at the present time. According to Emigh9 they

exist in many parts of the world andi new deposits are

periodically being discovered. A recent discussion

of the geology of these deposits is given by Emigh.9

Recognizing that there is little phosphate in

sea water, past theories have tried to establish how

sufficient quantities of phosphate in solution could

be concentrated in any one place to furnish the large

amounts now present in the phosphate deposits, Some of

the theories that have been postulated are:

1. Accumulation of remains of organisms

due to local destruction of sea life.








2'. Chemijcal precipi~tation..

3. Phosphatizatlion of animal excrecment.

Emigh~ indicates th~e most accepted theory at this date

is the one postulating chemical precipitation.

The phosph~ate deposits in Florida are divided

into three types: land pebble, hard rock and soft rock,

The land pebble phosphlate deposits, centered in Polk

and H-illsborough counties, account for more than 95

percent of the total Florida production. These de-

posits are apparently ancient placer deposits concen-

trated by wave action along a former shoreline or shore-

lines.

The hard rock deposits occur in a narrow belt

extending southward from northern Florida through the

western half of the peninsula and parallel to its axis

for about 100 miles to a point about halfway down the

state. in this type of deposit the phosphate has ap-

parently replaced other types of rock and occurs as

fragmentary rock, boulders, plate rock, pebbles, and

soft phosphatic clay.

Soft rock phosphate production is from the waste

pounds of former hard rock oper~ations and from colloidal

clay deposited at the margins of the land-pebble phos-

phate area. The deposits currently beinge mined yield

phosphate ro-k containing from 65 to SO percent BP:..

BPL denotes bone phosphate of 11me; Ca3(PO4)2, and is used

as an expression of thec grade of deposit.








Minin

In the land pebble area of Florida, phiosphate

is covered over by deposits of quartz sand known as the

overburden ranging in thickness from 5 to 50 feet,

averaging about 15 feet. The phosphate ore-bearing

material known as "matrix" consists of approximately

30 to 60 percent phosphate rock, 15 to 40 percent clay

and colloidal phosphate, and 10 to 25 percent quartz

sand, is found beneath the overburden.

In mining, the overburden is removed by drag-

lines. After thie overburden is stripped off, the matrix

is removed and deposited into a pit where it is sluiced

into a sump. The slurry formed during the sluieinlg

operation is pumped from the sump to the flotation

washer plant, Figure 1 depicits the flow of the phos-

phater rock from the mine through shipment.


Washing and Flotation

At the washing and flotation plant, the slurry

of water, phosphate, quartz sand, and clay passes

through two processing stages, washing and flotation.

Figure 2 describes the flow of rock through the washer

and flotation plant.

Phosphatic material is scrubbed, washed, and

screened. The screening separates the material into

two sizes. less than and larger than 1 mm, Material

not passing a 14-mesh screen comprises pebble rock




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which is more or less free from quartz sand and clay

when delivered to loadingp bins fo;- shipment. The smaller

material is sent to feed storage bins for further

processing in the flotation plant.

In the flotation plant reagents are used to

separate the quaertz sand from the phosphatric material.

Reagents used in the process are caustic soda, fuel

oil, and a mixture of fatty and resin acids known as

tall oil. The caustic soda cleans the phosphate parti-

cles and regulates the pH so that the tall cil can bJe

properly absorbed. The phosphate rock particles then

take on a film of oil and when agitst~ed float to the

top of the water while the sand sinks to the batcom,


Beneficiation

Various grades of rock are blended on a conveyor

belt to meet specific manufacturing and sales require-

ments. The rock is fed into either rotary or fluidized

bed dryers. Dried rock is then conveyed to grinders

for final sizing before shipment to fertilizer- plants

for the manufacturing of triple supe~rpousphat-e and

phosphatic fecrilizer mixtures, phosphoric acid, or to

produce elemrental phosphoru~s. Figure 3 illustrates

the rock flow in a typical beneficiation facility.


Acid Mlanuactugin

M~ost phospChate imanufacturing facilties have

sulfuric antd phosphocic a-id masnufacturing~ plants










































































ro~
uzo
~
w> r
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~S








associated with them. Figure 4 illustrates the acid

flow in a typical acid-fertilizer manufacturing complex.

Sulfuric acid is manufactured. by the conventional

Contact Sulfuric Acid process.12 In this operation

elemental sulfur is burned in a combustion chamber to

form sulfur dioxide gas, The sulfur dioxide is then

passed through a series of converters, charged with

catalysts; there it interacts with air to form sulfur

trioxide gas, This gas passes on to the absorption

tower, where it interacts with water and weak sulfuric

acid to form the strong product sulfuric acid. Figure

5 is a typical flow diagram of a Contact Sulfuric Acid

process.
In the manufacture of phosphoric acid by the

Wet-Process Phosphoric Acid process,12ground phosphate

rock is mixed with the strong sulfuric acid in a reactor

to form weak phosphoric acid and gypsum. The weak

phosphoric acid is separated from the gypsum in the
filtering step, and the gypsum is pumped to ponds for

storage. The clean phosphoric acid is then concen-

trated to 54 percent P2C05 strength in vacuum evapora-

tors. The 54 percent strength acid is then stored for

use in fertilizer manufacture or for further concen-

tratirng to makte superphosphoric acid. Figure 6 is a

flow diagram depicitin~g a typical phosphoric acid

process.


















E
Q
4
8 M




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ct
a

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93% ACID COOLERS L _Y
98% D 93% ACID
99% ACID COOLERS L STORAGE TAINKS
COOLING WATER OUT c


Figure 5: Typical Contact Sulfuric Acid Process
Flow Diagram




















SWATER FAN
FUFME HOOD~ /t WATER
uasHWITEWISHCA(E DRY CAKE DISCHARG I CEL


Figure 6: Typical Wet-Process Phosphoric Acid
Flow Diagram








Fertilizer Manufacturing

Triple superphosphate (TSP) is manufactured

by reacting phosphate rock with phosphoric acid in a

cone-type continuous mixer. The cone discharges to a

series of enclosed moving belts called the "Den,"

where the mixture completes its chemical reaction and

solidifies. From these belts the triple superphos-

phate is sent to the storage pile, where after further

"curing" it is ready for shipment, At this stage it is

called run-of pile (ROP) triple superphosphate. Figure

7 illustrates a typical triple superphosphate plant,

Granular TSP is made from ROP triple sup3erphos-

phate.,2 In this process ROP with a small amount of

water is rolled in a granulating drum to forml small

round pellets. These are dried in a rotary or fluid-

ized bed dryer, cooled, screened to size and sent to

finished produce' storage.

Diammonium phosphate (DAP) is made by reacting

phosphoric acid and anhydrous ammonia in a reactor,

to form a hot liquid DAP,91 This liquid is pumped to

a granulator where it mixes with recycled material

and solidifies, It is then dried, cooled, screened

to size and sent to storage. Figure 8 illustrates

a typical diamrmonium phosphate plant.


Elemnental Phosohorous M~a~nufacturino

At present there are only two elemental phos-

phorous manufacturing facilities in Florida,6 Elemental




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phosphorous is manufactured at those facilities by the

electrothecrmal process.12 In this process the phosphate

rock is mixed with cokec which acts as a reducing agent

and silica which acts as a flux and convoyed to electric

fur-naces. As thle furnace charge is melted the ele-

mental phosphor-us is released from the rock and passes

off as a vapor. The vapor is condensed and collected

as a liquid in pans under water below th-e condenser

pipes. The elemental phosphorus is then stored under

water because of its instability and is ready for uise

or sale. Figure 9 illustrates a typical elemental

phosphorous plant.


Air Pol~lution Associated Wi~jth the Phosrphate indutr

Florida phosphate rock is a non-crystalline

phosphorite consisting principally of fluorapatite

(a complex of tricalcium phosphate and calcium fluoride)

having the approximate formulation 3Ca (PO )2 aF2'13

The tricalcium phosphate is only slightly soluble and

when combined with calcium fluoride is nearly insoluble

in water. The fluorine represents 3.5 to 4.0 percent by

weight of the total compound. In order to make the

phosphate available to plants or animals and at the same

timle non-toxic, the fluoride must be removed.

The release of fluorides from the phosphate rock

is usually accomplished by treatment with heat or acid,

Unfor-tunately, this treatment produces fluorides in the































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0


O
.0


o








for-m of H:F, SiF4, and H2;SiF6, In addition to the gases

andc vap-ors released fromT someC of the chem'incal processes,

insoluble dust containing upiwards of 6 percent~ fluoridet

may be dischiargod: from other processing facilities such

as drying, r-i~nding. and materiall handling,

Genierally in the presence of reactive silica

from the rock, fluoride is released as gaseous silicon

tetr~afluor-ide (SjF ), wjh~clh readily is hydrolyzed in

the presence of water to H2SiF6. Silicon tetrafluoride
evolution is not restricted to any one process in m~anu-

facture of fei-tilizer. It is normally given off in t~he

processes of (1) acidulation, (2) concentration by

evaporation, and (3) calcination when~ silica and wat-er

are present.

Specht and Calacetob have reported the fluoride

evolutions from fertilizer processes given in Table 1.


TABLE 1

FLUORIDE EVOLUTION'
ACCORDING; TO SPECHIT AND CALACETO

Pocess _F /_std cu m*;
Acidulation
Batch 10.6 20.6
Continuous 2.1 5.3
Triple superphlosphate
Granular 1.8 3.6
Phosphoric acid
Digestion (Acidul-tion) 0,3 !.2
Superphosphloric 7.0 17.5
a Standard conditions not- stated








Huffstutlerl41 reported fluoride emissions "that

are probably in the form of silicon tetrafluoride" as

noted in Table 2,


TABLE 2

FLUORIDE EMISSIONS
ACCORDING TO HUFFSTUTLER

Process fim F1/ht

Phosphoric acid plant 1 .8 79.5
Phosphoric acid reactor 1 .2 6.3
Run-of-pile triple superphosphate 23.4 331.5
Diammonium phosphate 8.5 39.0
Granular triolQe spephspat 4.2 102.6

Sherwinl6 has reported an evolution of 540 mg

SiF4/std cu ft from a 28 ton per hour superphosphate
manufacturing plant using phosphate rocki fr'om Morocco.


Fluoride Toxicity

Over the years there have been an increasing

number of reports of injury to livestock and vegetation

due to atmospheric pollution by fluorides, The impor-

tance of fluoride as an atmospheric pollutant was

emphasized by a number of investigators at the U. S.

Technical Conference on Air Pollution in 1950.

When present in sufficient concenltrations,

fluorides in gaseous forma are highly toxic to grow-ing

vegetation, humans anld aniimals. The President's Science

Advisory Comitticeel8 in its report to the President

ranked iLnvesti~gations into thle systemic effects oni








humans, animals, plants, and materials in the highest

priority category along with sulfur dioxide, carbon

monoxide, and carbon dioxide. The minutes of the

Florida Air Pollution Conltroll Commission have innumer-

able pages of testimony regarding the damages to vege-

tation, animals and humans caused by fluoride emissions

from the phosphate industry. The International Clean

Air Conference2 meeting in London, Englanid discussed

the damage done to fruit, vegetation, gladiolus, and

cattle in the Florida phosphate belt due to the toxicity

of the fluoride emissions. The President's Science

Advisory Committee21 pointed out the severity of the

fluoride damage inl Southern Florida in its statement

regarding the effects of phosphate fertilizer effluents

on citrus, vegetables, flower crops and cattle. Largent22

has compiled a detailed reviiew of the reported effects

of fluorides, from all sources, on vegetation, animals

and humans.

Vegggtation Damaze

Florida's phosphate belt unfortunately is located

in the same general area that for years has been used for

raising citrus, truck crops and gladiolus among other

vegetation.

The leading agricultural industry in Florida

involves the growing and processing of citrus. Heavy

concentrations of citrus groves are found in the Polk-








Hlillsborough arca within a relatively short distance of

the ph~osphate area, Citrus damnage wh~ich has been at-

tributed to gaseous fluorides includes defoliation of

trees, reduction of yield, reduction of growth, leaf

burn and severe chlorosis.

Citrus trees have formantions of flushes of leaves

periodically through the year which rendersa the plants

continuously susceptible to atmospheric fluoride pollu-

tion. Studies at low concentrations and long periods

of exposure have beenl used to determine leaf injury,

growth, productivity, anld physiological effects in

honcsituations232,52 These studies indicate

that for a mean concentration of atmrospheric fluoride

of 10 mg/cu mn for 2 months to 2 reg/cu m for 12 months

significantr? injury occurs.

Relativelyr high concentrations and shorer exposure-

periods have been used to simulate a fumigation situation.

Acute effects of leaf injury and defoliation have occur-

red at 8,000 mg/cu m; for 15 minutes to 400 mg/cu m

for 2 hours.2 Leonard2 reported on controlled experi-

ments using HF for fumigation of citrus but Taylor28

in testimony before the Florida Air Pollution Control

Commission indicated that "data concerning yield and

leaf-fluoride content is sufficiently scarce and con-

tradictory that LL offers little scientific basis for

establishing a threshold limit of fluoride in foliage."

Weinstin ad Mcue" at the samre hearing indicated








that fluoride would affect plants differently at dif-

ferenit levels due to:

1. A critical stago in the development of the

plant occurs in which yield is most

likely to be affected. TheG stage is

usually during the flowering period.

2. The age of the foliage affects the

sensitivity. Younger foliage is con-

sistenltly more sensitive to fluoride

than older foliage.

3. Dilution by/ growth and losses to the

soil decrease the fluoride level in a

plant with time.

4. Some fluoride in the plant will be con-

vertedl to an inactive starre inI time.

5. Some investigations have shown that

intermittent fumigat-ions m~ay be less

effective than continuous exposures

since plants show a recovery process

during low-fluoride p~eriods.

Winter truck crops such as tomatoes, lettuce,

corn, endive, cabbage and other vegetables are also

produced in the Polk-Hillsboroughi County area. Although

vegetation of this type is fairly resistant to fluoride

damage, damage from flu~ori-de air pollution; has been

alleged more than once.l








A relati~vely large body of data i~s available on

the effects of atmospheric fluoride on tomato plants.

McCune3 has summarized the available~ data in trhe fornl

of Figure 10, Line a represents his criterion with

respect to the presence or absence of foliar markings.

Line b represents the times and concentrations that are

known to have produced economic damage, namely a re-

duction in the quantity or quality of the crop.











LI10 I 0 0



Fiur 10 eain fCnetrto n
Duaino xouet Efcso topei
Floieo omt lns

Beeic3 ha eotd hta on slae







floidues fumig nations of foronths ratio 10 m/cmo
HFshowed no sinficant ue t effect s on th gothof hesec

plants.nToat Pans








M~cCune0 in h;is summanryi of available data oni thle

effects of atmosp~heric fluoride onI corn inldicatfs thlat

the data are scarce on chronic exposuro-s but in the

available data; no significant reductions in growth or

yield were foundc in the limited r-ange of concentrations

and exposure times.

Gladiolus is one of the more susceptible plants

to atmospheric fluoridie damage. McCune30 reports on

the considerable data available on experiments concerning

both chronic and acute fluoride damage. In general,

atmospheric fluoride levels above 6 mg/cu m for 1 day to

1 ag/cu m for 10 days would cause permanent injury to

the gladiolus leaf.

Cattle Dmag

Cattle have been raised in the Polk-Hillsborough

County area for wany years. In recent years veter-

inarians have diagnosed fluorosis in some of the cattle

raised in this area. In most cases, the diagnosis

is chronic fluoride poisoning and not acute poisoning

indicating a long exposure to fluoride.

-Cattle develop fluorosis by feeding on pastures

contamin~ated cy fluorides hence the mode of entry into

the animals is through the digestive sy~stem. U~p to a

certain level fluorides are excreted, and when This level

is exceeded fluoride is then deposited in their bones

due o th afinit offluor-ine for calcium.








In order to combat fluorosis in domestic animals,

the Florida Air Pollution Control Commission3 see the

standard of a maximum of 40 ppm of soluble fluoride

(dry weight basis) for acceptable forage or grass that

is to be used for cattle, They stated that grasses

containing more than 40 ppm F will, "if consistently

used as feed or forage over a substantial period of time,

produce harmful effects." However, Hobbs and Merriman9

have reported that cattle grazing on pastures with up

to 44 ppm F and cattle consuming hay averaging up to

66 ppm F showed nlo significant damage during 10 years

of testing. Hendrickson3 has reported that reduced

milk production, a decrease in the reproduction process,

and a reduction in appetite will result from fluorosis,

Evidence published by the Natioanal Research Council3

indicated that the tolerance level for lactating dairy

cows lies between 30 and 50 ppm F. But Suttie and

Phillips3 have indicated that mature cattle could tole-

rate 50 ppm F for 3 years and show no adverse effects.

Largent22 is his text Fluorosis devotes an entire

chapter on the chronic effects of fluoride intoxication
on animals. In general, the typical effects of chronic

fluoride poisoning on cattle are:

1 Incipient dental changes

2, Staining, with detectable wearing of teeth

3, Decreased consumption of food

4, Loss of body weight








5. Docreanse in calf production

6. Decrease in milk and butter fat production

7. Overgr-owth at joints

Largent also discusses the effects of both

chronic and acute poisoning on rats, sheep, poultry,

swine,, rabbits, and h-ousehold pets.

The toxicity of fluoride appears to be greater

in forage contaminated with gaseous effluents such as

silicon tetrafluorlde rather than wjith phosphate dust.

Thirty to 50 ppm (by weight) of soluble fluoride compounlds

in the total diet of dairy cattle may cause deleterious

effects, while it takes 60 to 100 ppm of insoluble fluo-

rides to produce thel same effect,
38
Lewis, while taking the p~ospha~te industry to

task, brings out an inter-esting statistic regarding th-e

damage that fluorosis can cause. He states that in

1954 Polk County w~as the largest cattle producing

county in Florida with 120,000 head. In 1965, there

were 90,000 head of cattle and the number continues to

decrease and he states "it is not unusual to come

upon cattle crawling across pasturelanid on their knees,

starved from their inability to chew,"



Fluorides that are highly reactive chemically

will be irriLtating to exposed areas of the human body

whe~ln they are present in the ai~r in~ sufficient quantities.








Normally, exposures to such highly reactive fluorides
is limited to ocuainlepsrLess is known

about the effects on humans of irnhaled flucri~des that

are not surface irritants and whose action depends upon

their absorption from the lungs by the blood.

The fact the cattle grPzing in an area have had

health effects does not mean that people living in

the same area would have thle same effects, Cattle

obtain most of their fluoride fromn the forage and

water rather than from the air. The California Stale

Department of Public Health39 has estimated that 1 .0

to 1 .2 milligrams per day of fluoride is the optimurr

ingestion for the control of cavities in children.

The mean concentration of fluoride found in the

air of communities ranges fromr 0.0031 ppm in Charlestown

to 0.018 ppm in Baltimore.0 The maximum concentration

in most cities is about 0.025 ppm with a high of 0.08

ppm being reported in Baltimore, Thomas1 reported

a value of 0.29 pp~ i~n the vicinlity of a superphos-

phate plant.

Largent424 has observed the followiing effects

when hydrogen fluoride was inhaled by man under labora-

tory conditions:

1 At concentrations less than 5 ppm, no

local and immiediate systemic effects.

A sizable portion of t~he inhaled dose

was promptly ex-cr-eted.








2. At 10 ppm, many~ exposed per-sons com-

plained of discomfort.

3. At 30 ppmn, all exposed persons com-

plained about environment.

4. At 60 ppm, definite irritation of con-

junction anld nasal passages and diis-

comlfort of pharynx and trachea.

Derryborry44 reported on a study of a large group

of men exposed to a phosphate fertilizer plant for

periods of up to 25 years and the few suggestions of

toxicity "were not found to be formidable in light of

the fact that other factors may have been responsiblee"

A Florida State Board of H~ealth study of school

children in Polk County found no Evidence of fluorosis

in their teeth. The same conclusion wa~s reached by

the Bureau of Dental Health and the National Institute

of Dental Research after they toured schools in Polk

County,45

While individual cases of temporary illness have

been well documented by physicians and civic organiza-
!9
tions^ in the Polk-Hillsborough area, it appears that

to date there have been no permanent human defects

directly attributable to the phosphlate industry in this
ara34,46








Gover~nentaol Cnrl

Perhaps the occurance of illnesses alleged to be

caused by the emissions from the phosphate processing

industry gave the impetus to the formation of the

Polk-Hillsborough Air Pollution Control District.

Perhaps it was the economic loss to the area such as

$1 ,000,000 damage to new citrus groves, loss of 1 ,500,000

boxes of fruit per year, or a reduction of citrus

property value of $20,000,000 in the Polk-Hillsborough

Area.4 Perhaps, it was the decline in cattle produc-

tion in the two county area.

For whatever thle reason or combination of reasons,

the Polk Air Pollution Control District was formed in

March 1958 to handle complaints regarding air pollution,

mainly from the phosphate industry, in the county.

The district was formed under the 1957 Florida Air

Pollution Control Act which established an air pollu-

tion control commission in the State Board of Health.

The district was created with the understanding that

it would permit an "orderly study of air pollution in

the county, and that members of agricultural, live-

stock, industrial, and political interests would be

considered and the cooperation of those interests

encouraged."

Hillsborough~ County Air Pollution Control Dis-

trict was formed in July, 1959.8 The two control dis-

tricts were merged into the Polk-Hillsborough Air








Pollutioni Control District in June, 1960.9 It was

hoped that the merger would provide greater efficiencies
and better control for the areas involved.

The Polk-Hillsborougeh Control District did not

live up to its expectations due to a number of reasons

including: lack of proper legal counsel; delays in

taking lawful action due to the procedure of conference,

conciliation, and persuasion; inadequate funding; a

defeatist acttiude where the commission felt "hamstrung"50

due to not enough direction, responsibility, and authority

from the legislature regarding its desires; pressures

from! the industry; and lack of adequate anld properly

trained per~sonnlel

In 1967, the enabling legislation for the act

forming this district was repealed and the F'lorida Air

and Water Pollution Control Act was passed. One of the

purposes of this act was to strengthen the pollution

control agencies in the state. The act had been enacted

with the hopes of eliminating some of the major problems

associated with Florida Air Pollution Act. It gave the

control commission powers which if used properly would

provide the incentive for positive action in controlling

air polluting industries.

On June 30, 1969 the Florida Air Pollution Control

Commission was out due to governmental reorganization.

On July 1, 1969 the Florida Department of Air and Water

Pollution. Control. was created and it is under this




36



department that all present surveillance of the phas-

phate industry takes place. T'he requirements of the

department regarding fluoride emissions are "the unit

emissions of fluoride expressed as pounds of fluoride

per ton of P 05 or equivalent produced, shall not exceed

0.4 poundss" taking into consideration the latest

technology, existing pollution levels, the lowest value

attained by other plants, and location. This require-

ment necessitates that some method of fluoride recovery

be utilized.












CHAPTER III

WET-PROCESS PHOSPHORIC ACID


Mlost wet-processi phosphoric acid produced at the

present time is: used in fertilizer production since the

acid meets the critical requirements of cost and grade.

Until quite recently, the general availability of sul-

furic acid gave the wer-process acid a favorable economic

position and since the fertilizer industry can. use a

relatively impure phosphori- alcid, the wet-process

phosphoric acid process is used in Florida instead of

the furnace process. This chapter will be confined to

the process of producing fertilizer grade, orthophosphorcl

acid (H3O


Hi smy


Prior to 1900

The development of wet-process phosphoric acid

manufacture is closely related with the early production

of chemical fertilizers.

Chinese farmers are said to have used calcined

bones somo 2,000 years ago.5 and Fritish5 has stated

that bones have been applied for centuries to the vine-

yards In southern F'rance. Guan~o, a fertilizing material

consisting almost wholly of the extcreta of sea birds,








was used by the Incas as early as 200 B.C. Bird guano,

bones, and other organic substances formed the basis

for the early fertilizer industry and during thle 19th

century the use of these materials was limited only by

their availability.53

Brandt prepared elemental phosphorus from urine

in 1669, and Boyle made phosphoric acid from phosphorus

in 1698, Gahn in 1769 first associated these materials

with bones. Scheele in 1775 prepared elemental phos-

phorus from bones by treatment with mineral acids and

reduction with heated charcoal, In 1840, phosphorus

was recognized as the major component of bone manure.

Justus von Liebig,in 1840, proposed that bones be dis-

solved in sulfuric acid to make the phosphorus content

more available to crops." J. B. Lawes was issued a

patent in 1842 for making superphosphate from "bones

and other phosphoritic substances" and sulfuric acid by

"setting free such phosphoric acid as will hold in solu-

tion the undecomrposed phosphate of lime," Lawes modified

his patent, and confined his claim to apatite, phos-

phorite;, and other substances containing phlosphoric

acid. Laws essentially founded the mineral superphos-

phate industry.

In 1851 Albright and Wilsonl Ltd., at present

England's largest producer of phosphoric acid, w~as

formed at Oldbury, England.55 In 1870, a plant was

established in Germany to manufacture an improved grade








of supertphosphatet by reactinge low-grade rock with phos-

pho-ic acid in place of the sulfuric acid that had been

used to date. By 1900, at leaste 12 Europ~ean companies

were manufacturing phosphoric acidl for fertilizer

process use by batch oper-ations from low-grade ores.56
Commercial fertilizer operations in the United

States predate those of Europe. William Davidson

erected the first sulfuric acid plant in 1832 to acid-

ulate bones and oyster shells.57 The first triple

superphosphate plant was built by the American Phosphate
and Chemical Company in 1890 at Ba(ltimore, Maryland.

This plant was built using knowledge gained in 1867

when phosphate rock from South Carolina was used in the

acidulatio.: process in place of bones and oyster shells.58

1900 to 1930

In the United States wet-process phosphoric acid

production was insignificant prior to 1900, and the total

production, w-hich1 was mainly concentrated in the Blaltimore

area, did not exceed 2000 tons/year. The leaders in

the Industry in the period from 1900 to 1915 were Stauffer

Chemic-al Company, Virginia-Carolina Company (now a

division of M~obil Oil Company) and American Agricultural

Chemical Company (nowj a division of Continental Oil

Company). These companies and many other smaller con-
cerns were acidulating bones to mak~e the acid In! the

1900 to 1905 period, and then sw~itchedd over to the use








of phosphate rock inl the 1905 to 1915 period. The process

was a batch operation in which digestion of low-grade

rock took place in wooden tanks of 1 to 2 tons/batch

and the resulting slurry was filtered on lead-lined,

wooden filtering pans,59 The filtrate, containing 10

to 15 percent P205, was concentrated in rock or lead-

lined pans.

In 1915, the continuous process was introduced

into thie phosphoric acid industry by the Dorr Company.

By 1924, this process made possible an acid cont~aining

22 to 23 percent P 05, as compared with the 10 to 15

percent with the batch process, Briefly, the process

consisted of the reaction of a phosphate rock.-phosphoric

acid mixture in a series of primary digestion tanks

with sulfuric acid (66.6 or 77.7 percent H2SO4). Agita-

tion of the slurry produced was maintained and after

the slurry left the last tank it was washed counter-

currently in thickeners. Thle overflow from the primary

thickener normally analyzed 22 to 23 percent P 05'

The calcium sulfate dihydrate which settled in the

primary thickener was washed in the remaining thickeners
to remove th~e entrained acid.6

In the period 1916 to 1929, Dorr built strong

acid plants for some 31 companies in the United States

and Europe. The smallest of those plants processed 25

tons of rock per day. During this same period, the

Chemical Colnstruction Company (Chemico) built wet phos-








phoric acid plants for U. S. Export Chemical Corporation

(now U. S. Phosphloric Chemical Corporation) and American

Cyanamid.

1930 to 1968

During th~e 1930 to 1940 period, Nordengren and

associates in Sweden developed strong acid processes

that produced a calcium sulfate hemibydrate and a very

strong acid, 40 to 50 percent P205. However, after exper-
ience was gained with this process, the conclusion was

reached that the economies in producing the strong acid

did not justify the complications involved in the fil-

tration of the slurry.61 However, Nordengren and his

associates performed basic studies to determine the

conditions under which the anhydrite (CaSO ), hemnihy-

'drate (CaSO4 \HI20), and dehydrate (CaSO4 2H20) were

formed in phosphoric acid slurries and established the

relationship between crystal filterability and acid

concentrations for each of these crystal forms, Dorr

and other companies in the United States were also

working on producing an acid in the 40 to 50 percent

P205 rangei however, due to the same type of problems
encountered by Nordengren, they were not able to produce

a high strength acid economically.

During the same period, Dorr built the largest

wet phosphoric acid plant up to that time for Consolidated
Mining and Smelting Company at Trail. British Columbia.








The plant consisted of three trains, each having a

processing capacity of 150 tons/day of phosphate rock.

This plant was probably the major contribution of Dorr

Company during this period since it involved (1) the

recycle of gypsum slurry for better control of crystal

growth, thus giving larger crystals and improved filter-

ability, and (2) the separation and countercurrent

washing of the gypsum on continuous filters. The product

acid from the filters contained 30 to 32 percent P205'

This process gained the name of the Dorr Strong Acid

Process since it produced the highest strength commer-

cial acid to that date. The acid produced was already

strong enough for the production of ammonium or sodium

phosphates but it was not used in the manufacture of

triple superphosphate or food grade calcium phosphate

since a concentrated acid was required for these products.

This process produced a dibydrate gypsum and was the

one used for many other plants throughout the world,

In the 1940 to 1950 period, processes yielding

calcium sulfate anhydrite were studied by Davison

Chemical Company; however, none of these processes were

ever commercialized.6 Tennessee Valley Authority

(TVAC) also began work on a process producing an anhy-

drite at their Muscle Shoals, Alabama facility.

-In the 1950 to 1968 period, interest was renewed

in the hemihydrate process of Nordengen and commercial

installations have been built in Japan. Better recovery








of P205 has beenl obtained and the by-productgypssum is
of better quality in the hemihydr-ate process, A~n acid

containing 40 percent or more of P205 has been obtained

using this process in large-scale tests.63

During this same period, due to a shortage of sulfur,

interest was developed in substitution of hydrochloric

acid for sulfuric acid. The Israel Mining Company devel-

oped a process using solvent extraction to separate the

phosphoric acid from the calcium chloride formed. Small

commercial plants have been built in Israel and Japan

using this process, Other companies have used nitric

acid as a replacement for sulfuric acid.6

The major process used at the present time is: the

dibydrate process and most large commercial plants con-

structed today utilize this process with the latest im-

provements which include (1) slurry recycle, (2) single-

tank reactor, and (3) tilting-pan filter. The capacity

of the largest single-train plant in Florida is a nominal

1500 tons P205/day. According to Slack,6 as of 1968
this plant is also the largest single-tratn plant in the

world. The Occidental Chemical Corporation plant at

which all experimentation for this study took place had

a nominal 650 tons P205/day capacity.

Fundamental Princioles of the Dihydrate Process

The primary objective in the phosphate fertilizer

industry is to convert the fluorapatite in phosphate rock








to a form available to plants. The fluorapatite is

quite insoluble and is not normally used as a fertilizer

in its natural state because it has little value as a

supplier of nutrient phosphate. 'The most widely used

method for making the nutrient phosphate available is

treatment with a mineral acid such as sulfuric, phos-

phoric, nitric, or hydrochloric.

Since phosphoric acid can be readily manufactured

at fertilizer plants, it is normally used to treat

phosphate rock for production of triple superphosphate

and in the reaction with ammonia to make ammonium phos-

phates. The growing importance of both of these products

makes phosphoric acid a very important intermediate in

the fertilizer industry.

Production of phosphoric acid by the wet-process

involves the steps of (1) dissolving phosphate rock in

sulfuric acid, (2) holding the acidulate slurry until the

calcium sulfate crystals grow to adequate size, (3)

separating the acid and calcium sulfate by filtration,

and (4) concentrating the acid to the desired level.

The principal difference in processes for manufacturing

phosphoric acid is the degree of hydration of the cal-

cium sulfate, which can be changed by altering the tem-

perature and the P205 concentration of the acidulate

slurry. The dihydrate, CaSO4 2H20, is precipitated

under conditions of low temperature and concentration

(see Figure 11). Because of these two reasons, the








amount of wat:-prce)Css ci;d produ61cedL by methods other

thlan trec dibydrare process is not significa;nt.6

Raw M~aterints

The two major raw materials in wct-process phos-

phoric acid manufacture are phosph~ate rock and sulfuric

acid, The phosphate rock fed to the process is as high

in grade as economically feasible, usually ranging from

30 to 35 per-cent P205, The sulfur-ic acid normnall.y is
660 Be (93 to 98 percent).

The phosphate rock used exists principally in
the form of a fluorapatite, which is a broad term

given to a relatively complex mnl'reral whose properties

can vary widely, depenlding uponl origin. Slack656

normally uses the cheicf~al! formula Cal0(PO )6'2, in his
descriptions of the pr-ocess, while Wraggamnan61 and

Sauchelli use 3Ca (PO )2 CaF2 in their. descriptions
of, the process. Phosphate rock used for production

of acid by the wet-process varies fromt about 65 to 85

BPL, depending upon the origin and extent of beneficia-

tion. All rocks contain varying amounts of other com-

pounds, either physically mixed or substituted in the

mineral itself. Table 3 gives a typical analysis of

Florida phosphate rock. Table 4 gives a typical analysis

of the rock used by Occidental at their W~h~ie Springs

plant,







TABLE 3
TYPICAL COMPOSITION; OF COMMErRCIAL GRADES
OF FLORIDA PHOSPHATE ROCK62,a


S _pecified BP Ran
68L66A 2/70 77/76
BPL 68.15 72.14 77,12
P205 equiv. 31.18 33.00 35.28
H20 1 .3 1 .0 1 .0
Fe203 1.33 1,07 0,84
Al203 1.76 0.83 0,56
Organic 2,18 1,76 1.70
SiO2 9.48 6.46 2.02
CO2 3.48 2.87 2.98
F2 3.60 3.62 3,89
Ca0 45.05 48.10 51.53
SO3 1.05 1.11 0,66
a Dry basis

TABLE 4

TYPICAL COMPOSITION OF OCCIDENTAL
PHOSPHA.TE ROCK"

BPL 74.00
P205 33.80
Fe203 0.49
Al203 1.49
Si02 4.30
CO2 4.72
F2 3.93
Ca0 50.0
SO3 1.32
C 0.28
Mgo) 0.31








TABLE 4 CONTINUED)

Na20 0.69
K20 0.10
a Process Report, P. Flanagan,
9/27/68, Occidental Chemical
Company, White Springs Plant.

While the performance of a given rock cannot

be completely predicated from the chemical analysis,68

it is possible to draw certain general conclusions.

Normally, a preferred rock is the one that yields

phosphoric acid at the lowest cost; therefore, the

primary consideration is the price of the rock per unit

of P205, Other items which may affect production
include:

1. Ca0 to P205 ratio. This ratio eventually

determines the amount of H2SO4 used per

unit of P205 produced. A low Ca0 to P205

ratio is desired since the cost of 112S04
is a major factor in the production cost of

phosphoric acid.

2. Fluorine. Most companies do not consider

this an item of significance, however, high

values may increase the air pollution gene-

rated in the process and may increase plant

corrosion.

3. Iron and a~um~inum. They are major problems

in acid manufacture and are nlormally rem~oved








by beneficiation to a major extent.

Residual amounts dilute the phosphate content ,

may interfere with crystal growth, cause

sludge to form in the product acid, and

in general may cause a poor acid for use in

making fertilizer products.

4, Carbonates. A high carbonate content is

accompanied by a high Ca0 to P205 ratio

reducing the grade of rock (P205 content)

and causing foaming during acidulation.

5, Magnesium and potassium. Normally both

give rise to complexes that lead to sludge

problems when the acid is concentrated.

6, Silica. Silica is partially attacked by the

fluorine released during digestion. Normally,

it is not considered a problem.

7, Sulfur compounds. Sulfur, if present as

sulfates, can usually be credited against

H2SOq consumption.
8. Organic matter. High levels of organic

compounds will cause foaming problems and

tend to reduce filtration rates by blinding

the filter cake.

The rapidity of digestion is influenced by the

particle size of the rock since the attack takes place

primarily on the surface of the particles therefore,

the time required to dissolve a rock particle is pro-








portional to thie diameter so long as other factors are
held constant. Normally, the rock is ground to 60

to 70 percent minus 200 mesh; however, ungroundl phos-

phate rock can also be used in the process.
The sulfuric acid used in the wet-process is 660

Be (93 to 98 percent). The strong acid is employed so
that as much water as possible can be used in washing

the gypsumn filter cake. The amount of sulfuric acid
used in the wet-process is approximately stoichiometri-

cally equal to the CaO in the rock. The amount of
free sulfuric acid in the digestion system is an im-

portant operating variable, since it affects the size
and shape of the gypsum crystals formed.

Process Chemistry

Waggaman and Slack0 have reported the fol-
lowing reactions taking place throughout the process:

Ca3(PO )2 + 3H2SO4--2H3PO4 + 3CaSO (1

CaF2 + H2SO --- CaSO4 + 2HF (2)
CaCO3 + H2SO4--->CO2 + CaSO4 + H20 (3)

6.F + Si02--' H2SiF6 + 2H120 (4)

Na2(or K20) + H2SiF6 --Na2SiF6(or K2SiF6) i20 (5)

H2SiF6 + Heat and/or Acid----SiFq + 2HF (6)
3SiFq + 2H 0--->2H2SiF6 + SiO2 (7)

Fe203(or A12 3) + 2H3PC)4---*2FePO4(or A1PO ) + 3K20 (8)
Phosphiate rock is a very complex mineral; the

principal mineral constituent, fluorapatite, contains







calcium, phosphate, fluoride, carbonate, and other

elemetnts or groups bound together. When the rock is

treated with sulfuric acid, the apat-ite lattice is

destroyed and the phosphate is solubilized as phos-

phoric acid. Th!e principal reaction, Equation 1 is

the reactionI between tricalcium phosphate and sulfuric

acid to give soluble phosphoric acid and insoluble

calcium sulfate. The calcium fluoride reacts with

sulfuric acid to produce hydrogen fluoride and calcium

sulfate, Equation 2. The calcium carbonate is con-

verted to carbon dioxide and calcium sulfate, Equation

3. The entire reaction between the major constituents

and sulfuric acid has been described by Slack0 as:

Cal0(PO ) F2 CaCO3 + 11H2SO~-- -6H3PO4 + 11CaSO4 +

2HF + H20 +CO2 9

The hydrogen fluoride produced may react with

Bte silica present to form silicon tetrafluoride, which

then hydrolyizes to form fluqgsilicic acid as shown in

Equations 4 and 5.
The calcium sulfate formed inl the reactions can

be in three stages of hydration: anhydrite (CaSO4 '

semihydrate (CaSO4 HI20), or dibydrate (CaSO4 2HI20),

depending on the reaction temperature and the phosph~oric
acid concentration (see Figure 11), However, since

the dihydrate occurs with low reactor temperatures and

low P205 concentrations in the acidulate slurry, it has



















Hemihydrate
00 Precipitated; Anhydrate
Stable
Dihydrate
80 Precipitated.
Anhydrite Stable

60



40~ Dihydrate
Precipitated and
Stable
20



S10 20 30 40 50
Acid Concentration, percent P205

Figure 11: Precipitation and Stability
of Calcium Sulfates in Phosphoric Acid


been antd still is the favored method of producing wet-

process phosphoric acid.


By increase in temperature or concentration,

conditions can be reached under which dihydrate is pre-

cipiltated but is converted in time to anhydrite (CaSO4)








In1 practice, however, the crystallizati~on is affected

by so many factors that p-rodiction of the result i~s dif-

ficult. Free sulfuric acid changes t-he stability regions

and impurities affect the rate of conversion of one

crystal form to another, Since there are so many factors

affectinlg crystallization, the dihydrate process is

favored due to the lower temperatures and the lower

concentrations. There are many variations in dthydrat~e

processes, mainly in regard to type of filter and in

design anld arrangement of digestion acidulationn)

equipment.


Process Techno~logy

There are many variations in dihydrate processes,

mainly in regard to the type of filter and in the de-

sign and arrangement of the digestion acidulationn)

equipment. The major areas in any phosphoric acid

process are raw material feeding, reactor system, filtra-

tion, and effluent control.

Raw material in the form of phosphate rock and

sulfuric acid is closely controlled, Gravimetric

feeders or automatic batch scales are normally used for

rock feeding. Sulfuric acid, recycled phosphoric acid,

and washwat~er are normally controlled by flowme~ters.

The reactor system varies from process to process.

The most recent type is the single tank reactor with

the necessary cooling, fum~e remo~val, and recycle facili-








ties. Older prucesses use a system of tanks. The

single tank reactor is normally equi~ppedl iith multiple

agitators that promote the reaction. One process has

a single tank with two com~partmentcs, an outer annulus

in which the reaction takes place, and an inner core

where the sIlurry formed in the reaction is held for

cryst-al growth and stabilization. Average retention time

in the reactor system is approximately eight hours.

Foaming in the reactor often is a problem when the rock

has a relatively high content of organic m~atter or car-

bonate. Various types of antifoaming agents are used

to control foaming.

The heat produced by acidulaltion of rock and

dilution of sulfuric acid is normally removed by one

of the three following methods (1) blowing air into

thle slurry, (2) blowing air. across the slurry, and (3)

flash: cooling under vacuum. Vacuum cooling is probably

the most commonly used method at present since the ef-

fluent control system is smaller and less expensive.

Filtration of the acid-gypsum slurry requires

that gypsum crystals of adequate size and shape be

produced in the reactor, Satisfactory crystals depend

upon:

1.Sulfate concentration. Insufficient sulfate

produces thin, platelike crystals that

are difficult to filter. Excessive sulfate

produces crystals that are easy to filter








but are difficult to wash free of phosphate.

The optimum sulfate concentration depends

upon many factors, but normally is approx;-

mately 1.5 to 3.0 percent H2So This

level produces rhownboid crystals agglome-

rated together which filter and wash well.

2. Slurry recirculation. Recirculation of

slurry is required in all systems in order

to (1) reduce the effects on the process of

surges, (2) reduce the effects of localized

high concentrations of rock and acid, and

(3) control the degree of supersaturation

necessary for good crystal growth.

3. Phosphoric acid concentration. Crystals

decrease rapidly in size as the acid con-

centration is increased above 32 percent

P205. This is one of the major limiting
factors on product acid concentration in

the dihydrate process.

4, Rock impurities, Slack7 reports that this

area is not understood at present but is

currently being studied.

Normally filtration occurs in three stages.

The slurry, containing 30 to 45 percent solids, is

filtered in the first stage to produce the product

acid. Wa~sh water Is added to the last stage to wash

traces of acid from the gypsum cakie. Filtrate from








this staeo is use~d as cash- water for the second stage?,

wlhere recycle acid of approximately 21 percent P205

is produced. The filter caklle formied ma~y vary from i Lo

4 inches. The type of filter varies widely depending

upon the process requirements. Early plants used

plate-and-frame presses and rotary drum vacuum filters.

At present, four types of horizontal vacuumr filters

are used:

1 Belt filter.

2. Horizontal rotary table filter.

3. In-line pan filter.

4, Horizontal rotary tilting-pan filter.

The horizontal rotary tilting-pan filter has become

thle standard! in ~the new large-capacity pl~ants since

very large units are feasible.

Approximately two-thirds of the weight of phos-

phrate rock and 98 percent of the sulfuric acid used in

the process consist of unsalable by-products.7 The

result of these unusable by-products is the production

of approximately 5 tons of by-product material per ton

of P205 produced. Effluent gases from the process
contain fluoride compounds which must be removed, Nor-

mally, these compounds are removed in somae type of

liquid scrubbers such as spray chambers, imrpingement

columns, packed towers, and venturi scrubbers, Fluorine

recovery is practiced? in somec planr;s where the resulting

fluosilicic acid liquor is recovered for sale. In other








plants, heliquor ispumped tothe gypsum settling

pond where it is recycled back to the process, Slack

reports that the fate of the fluoride in the pond is

not well u~nderstood at present, and that it "presumn-

ably remains with the gypsum in the pond or is evolved

from the pond surface as hydrogen fluoride." His

statement is based upon operating data which show

that the fluoride concentration in the recycled liquor

builds up to a certain level, depending upon operating

conditions, and stops there. Table 5 gives the fluoride

balance for one plant using Florida phosphate rock.

TABLE 5

FLUORIDE BALANCE IN SWIFT & CO.
WET-PROCESS BROSPHORIC ACID PLANT73ea


Material F content % F/ton P205, lb

Vapors from reactor 5.5 13
Gypsum cake 27.8 66
Vapors from vacuum evaporator 41.9 100
Concentrated Acid (54% P205) 24,8 60
Total 100.0 239
a Phosphate rock used contained 32.57% P205 nd3,8%

The quantity of gypsum produced in a wet-process

acid plant is normally 1.5 to 1.6 tons/ton of rock

digested or about 4.6 to 5.2 tons of gypsum per ton of

P205 produced. Approximately 1 acre-foot of gypsum
will be accumulated yearly as a result of the produc-

tion of 1 ton of P205 per day. In Florida, the gypsum








discharged from the filter is re~slurried with water and

pumrped to the settling pond where the solids settle

and the gypsumr~ pond water is recovered for reuse. Thle

settled solids are then used to build up the walls of

the gypFsum pond such that older plants have gypsum

piles exceeding 90 feet above grade.

The effluent water from coolers, scrubbers, and

the gypsum slurry passes through the pond, or ponds

depending upon the plant design, and is recycled back

into the process, N~ormally, little effluent water is

dumped into water courses unless it is absolutely

necessary due to high water conditions caused by heavy

rain and/or plant imlbalances. Before the water is

dumped into fresh water sources, it is treated with

lime to raise the pH.


Fluoride Evolut~ion.

The quantity of fluoride evolved as gaseous sili-

can tetrafluoride during the acidulation of phosphate

rock is usually only a fraction of the total fluorine

in the rock, as shown in Table 5. However, this arnlmoun

of fluoride can do considerable damage to animals and

vegetation if allowed to escape from the confines of

the plant.

The traditional view of fluoride evolution during

the mixinge of p~ohosphat rock and sulfuric acid gave

rise to the following reactions:








CaF2 2iSO4---*CaSOI + 2HF (2
6HF + SLO2---*H2SiF6 + H20 (II

4HiF +Si02-- =*SiF4 +- 2H20 (9

3SiF4 + 4H20---*SiO2 2H20 +; 2H2SiF6 (10
SiF + 2HF---rH2SiF6 (11

The formation of hydrofluoric acid by Equation 2

was generally considered to be an intermediate step since
further reaction within silica would give gaseous silicon

tetrafluoride (SiF ) and aqueous fluosilicic acid

(H2SiF6), Some of the silicon tetrafluoride produced in
Equlation 9 was released in the early stages of the proj-
cess while Equations 10 and 11 were liquid-gas reactions

that caused retention of the- remaining volatile silicon

tetrafluoridie. Th~e gases released in Equation 9 were

normally carried to the scrubbing system where water

or gypsum pond w~ater was used as the scrubbing media to
recover the fluoride as fluosilicate by Equation 10.

Recent studies ,776suggest that the tai

tional view wherein the formation of silicon te~rc-

fluoride is a secondary reaction between hydrofluoric

acid and silica in the rock, Equation 9, may not be

entirely correct, These studies indicated that fluo-

silicic acid is formed directly in acidulation of phos-

phate rock and that the reaction between moderately
strong hydrofluoric acid and either silica or- sili-

cates is too slow to account for the somewhat rapid

evolution of silicon Letrafluoride that is experienced




59



unlder normal operating conditions. The studies suggest

that fluoride behaves more as fluosilicic acid than

hydrofluoric acid in acidulated rock.

In a study of factors affecting the evolution of

silica tctmrafuoride during the acidulation of phas-

phate rock~, Fox and Hill71 found that the amount of gas

evolved on contact between the rock and acid increased

with increasing concentration and temperature of the

acid and with the degree of acidulation of the rock.

Other minor factors affecting fluoride evolution in-

cluded th~e order of mixing t~he rock, acid, and water

and the amount of water taken up in the formation of

the hydrates of the reaction products,












CHAPTER IV

ABSORPTION THEORIES AND
EMPIR1CAL RELATIONSHIPS


Treybal7 has defined gas absorption as "an

operation in which a gas mixture is contacted with a

liquid for the purposes of preferentially dissolving

one or more components of the gas and to provide a

solution of these in the liquid." A detailed discussion

of the absorption process is beyond the scope of this

dissertation which is concerned with the application

of this method to gas purification and so it deals

only briefly with the fundamental processes of absorp-

tion. A full treatment of gas absorption may be found

in specialized texts by Treybal,7 Kohl and Riesenfeld,7

Sherwood and Pigford, Morris and Jackson,80 and

Perry.8 Those aspects which are of particular interests

in the control of fluoride air pollution will be dealt

with in detail.




The theoretical principles involved in the ab-

sorption process are quite complex and controversial.

The newer theories attempt to resolve the apparent

discrepancies involved in using the older theories.










Absorption deals with the transfer of material

between two phases. In the case of gas absorption,

when a soluble gas comes in contact with a liquid,

molecules of the gas will enter the liquid to form a

solution, Gas molecules also will tend to leave the

solution to re-enter the gas phase at a rate which in-

creases as the liquid phase concentration increases,

until the gas pressure exerted by the solution is equal

to the pressure on the system. When this point is

reached, a dynamic equilibrium between the phases is

established and the gas concentration in solution will

no longer change. The cojncenltration of the gas in

solution at this point is termed the solubility of

the gas in the liquid and varies with the temperature,

pressure, and system components, At this equilibrium

point, the partial pressure of the soluble gas in the

main gas is related to the dissolved gas in th-e liquid.

With this idea In mind, it can be stated that a very

soluble gas will require a lower partial pressure than

a slightly soluble gas in order to obtain a given

concentration in the liquid. Figure 12 shows a typical

equilibrium curve illustrating this idea, Equilibrium

curves are normally determined by experimentation;

however, if the solution and gas are considered ideal,

certain gas laws may be applied.













lightly
Solbl
oa,


'nZ; Curquilibrium




Very Soluble





Concentration of gas in liquid

Figure 12: Equilibrium Diagram


Raoult's Law is normally applied to sys~tes that

approach equilibrium and states that

p = Ex

where:

p = partial pressure of solute gas

P = vapor pressure of solute gas at the same

temperature

x = mole fraction of solute gas in solution

When liquid solutions can not be considered

ideal, Hienr:'s Law is normally applied. Henry's Law

related the solubility of gases to the partial pressure

of the gas inl the gaseous phase

S =Hp(

where:


(12a)


:12b)








S = concentration of gas in liquid

H L- proportionality constant, sometimes called

Henry's solubility coefficient

p = partial pressure of gas in gaseous phase

The conlstant may have an~y units required; for example,

if S is in ppm, and p is in atmospheres, H will be

in ppm/atm. The constant is normally determined exper-

imentally for the particular system under study and is

applicable to that system. The law holds for the solu-

bility of nonreactive gases in wa~ter.

Since absorption of gases depends upon the trans-

fer of molecules from the bulk of the gas to the bulk

of the liquid, the case of a stagnant gas in contact

with a liquid surface has been widely studied. When

the liquid surface is brought into contact with the gas,

the diffusion of the gas molecules is by molecular

diffusion, and the rate of this depends on the pressure

and temperature of the gas and the molecular species

in the gas. The rate of molecular transfer is given

by Fick's Law

N =- (12c)

where:

N = rate of molecular transport, molar unit/

unit area/unit time

= concentration gradient in the direction of

diffusion,

D = molecular diffusivity, (length)2/time






Values for the diffusivity may be calculated using the

Stokces-Einstein equation but normally they~ are determined

experimentally and can be found in the Interatina

Critical Tables8 when available, Normally gas diffusi-

vity values are considerably higher, approximately 105

times as large, as liquid diffusivity values.

Equation 12chas been integrated for various cases

and the two most used cases are:

Case 1.

Steady-state equimolal counter diffusion

which frequently pertains to distillation opera-

tions and will not be described further.

Case 2.

Steady-state diffusion of A through non-

diffusing B. This occurs, for example, wheni

silicon tetrafluoride (A) is absorbed from air

(B) into water. In the gas phase if the evapora-

tion of water is neglected, only the silicon

tetrafluoride diffuses, since air does not dis-

solve appreciably in water, Figure 13 illustrates

this condition.




PB\PB

Pressure P1
PA2



Z1 Distance, Z Z2

Figure 13: Diffusion of A Through
Stagnant BI








Troybal8i3 reports the integrated form of Equation

12c for this case to be


In

where :

Na = rate of transport, [moles/(time)(area)]
D = diffusivity, (area/time)

P = absolute pressure

R = gas constant

T = absolute temperature

Pin = log mean pressure of component B
y = mole fraction

yi = mole fraction at interface
k = mass transfer film coefficient,
C(moles)/(time) (area) (mole fractiorn)]

The use of the mass transfer coefficient in Case

2 is generally accepted since most practical situations

involve turbulent flow in which it is generally not

possible to describe the flow conditions mathematically.
The mass transfer coefficient includes, in one quantity~,

effects which are the result of both molecular and

turbulent diffusion and which differ from situation to

situation, Mass transfer coefficients can, in a few

limited situations, be deduced from theoretical princi-

ples, but in the great majority of the cases, direct

measurement under known conditions is used for design

purposes. Many theories have been postulated In order








to attempt to interpre' or ex-plain the behavior of the

mass transfer coefficients. The most widely known are

the two-film, penetration, and surface-renewal.


TwofmThory

The oldest and most widely used model for inter-

pretation of mass transfer coefficients is the two-

film theory proposed by Lewis and Whbitman. This

model relies on a series of assumptions for its utili-

zation:

1 Steady-state conditions exist in both phases,

2. Rate of transfer is proportional to the

concentration gradient.

3. Equilibrium exists between liquid and vapor

at the interface with no interfacial resis-

tance to flow.

4. Hold-up at the interface is zero.

With these assumptions, when steady-state conditions

of transfer have been reached, the rate of transfer Na

from gas stream to the interface, and from the interface

to the liquid must be equal, and


Na = k (p p ) = k (Ci C) (14)

where:

p = partial pressure of the transferring com-

ponent in the gas stream

pi = partial pressure at the interface
C. = concentration at the interface








C = concentration in the liquid

kg = gas film mass transfer coefficient
kl = liquid film mass transfer coefficient

However, since the interface values pi and Ci are very
difficult to determine, the equation was redefined in

two equations:

Na = K (p p") = K (Cn C) (15)

where:

p* = equilibrium partial pressure of gas over a
solution having the same concentration

C as the liquid

C" = concentration of a solution which would be

in equilibrium with the gas partial

pressure existing in the main gas stream

K_ = over-all mass transfer coefficient for gas

phase

K1 = over-all mass transfer coefficient for

liquid phase

'and K_1
g m 1 m
kg 1l
where:

m = slope of solubility equilibrium line.

This model has been misapplied in highly soluble

gas systems where the slope m approaches zero. Equation

16 would then become Kg = ke (17)








indicating that the gas phase controls, and the liquid

phase offers no resistance to mass transfer. Teller

has pointed out that in absorption with chemical reaction

occurring a liquid is selected so that no equilibrium

partial pressure exists for the gas when dissolved and m

approaches zero, However, experimentation has shown

that Equation 17 does not hold and that the liquid

phase resistance can not be assumed to be negligible
when chemical reaction occurs.


Peneration TheorY

In 1935, Higbie8 emphasized that in many situa-

tions the time of exposure of a fluid to mass transfer

is short, so that the concentration gradient of the

film theory, characteristic of steady-state, would not

have time to develop. As an approximation to actual

conditions, he applied Fick's second law equation for

a one-dimensional case and with appropriate boundary

conditions found that even with a constant concentration

gradient, the rate of absorption decreases with time of

exposure. However, he did not apply his results to

absorption with chemical reaction which is the case in

m~any applications at present.

Surface Renewal Theory

In 1951 Danckwerts pointed out that the

Highte theory was a special case of a situation where

the liquid edd?- motion continuously brings fresh liquid









to the surface, increasing the rate of absorption. This

motions would tend to increase maEss transfer to a greater

degree than that predicted by Higbi~e's penetration

theory. While it expanded H~igbie's theory, it still

did not apply to absorption with chemical reaction and

it could not be applied directly to highly turbulent

flow conditions.


Film Penetration T~heory

Toor and M~arcello proposed their film penle-

tration model in 1958. They showed that the film and

penetration models were not separate concepts, but

only the limiting cases of their model. In doing this,

their theory seemed to better explainl some physical.

conditions than did previous theories.


Absorption with Chemical Reaction

When mass transfer occurs between phases across

an interface, the resistance to mass transfer in each

phase creates a concentration gradient in each phase.

In the case of purely physical absorption, the rate of

mass transfer is dependent upon concentration gradients

and the diffusivities of the gas in both phases as was

stated in the "film theory" and as is shown in Figure 14.


















Ci

Liquid


Characteristics in


With the addition of a rapid chemical reaction to the

process, the distance through which the gas m~ust diffuse

in the liquid phase is generally decreased because

upon meeting the liquid,reaction occurs. Therefore,

the need for further diffusion no longer exists, in-

creasinge the over-all mass transfer rate. Figure 15

Lllustr~ates this concept.


Figue 14 Intrfaial

Physcal bsortio













8
m
lcl
L1
a,
LJ
C


CABL


Figure 15: Interfacial Characteristics in

Absorption with Chemical Reaction


When a slow chemical reaction occurs, the distance

through which the gas must traverse is only slightly

affected and the process approaches that of physical

absorption,

Teller indicates that as a result of the many

complexities involved in the mechanism of absorption

accompanied by chemical reaction, no rigorous analyti-

cal method of design has been developed; but for a

first order reaction, the rate of absorption is pro-

portional to the unreacted gas concentration at the

interface. For a second order reaction, an increase

in the concentration of unreacted gas at the interface


Reaction
Zone








tends to remove unavailable unreacted liquid from the

interfacial region and moves the reaction zone deeper

into the liquid. With this in mind, an economical pro-

cess dictates the selection of the proper liquid and

the occurrence of a first order reaction or of a pseudo-

first order reaction, an excess of liquid or low gas

concentrations in the gas stream.


Fluoride Absorction

Generally, the two-fi~lm theory is used for the

design of flu~oride absorption units and the interpre-

tation of their performance.


General D~esign Concepts

Simplified equations have been developed for

design calculations where low gas and liquid concentra-

tions are involved and where the equilibrium curve is

linear or can be assumed linear over the range of con-

centrations used in the design. Equation 18 is the

most widely used equation for design calculations and it

is based upon the two-film theory

h =H N (18)
og og

where:

h = height of tower

Ho height of transfer unit
N = numbSer of transfer units
og
This equation is arrived at using Equation 15 as the








starting point and a differential section of an absorp-

:ion column dv, in which the rate of absorption is


Nadv = K (P p") dv = K (C- C) dv (19)

If a is the interfacial area per unit volume of absorber,

and S is the tower cross-section, then dv = aSdh (20)

However, the value of a is usually not known and diffi-

cult to determine, therefore, it is usual to combine

the mass transfer coefficient with the interfacial area

to form a composite coefficient Kga and K a, the Equa-
tion 19 becomes:


NaaSdh = K a(p, p*) Sdh = K a(CA C) Sdh (21)

For gas absorption, where the molar rates of flow

of gas and liquid phases are not constant, due to the

changes in composition which occur over the length of the

absorber, it has been shown that

d(Vy) = V--dy (22)

where:

V = molar rate of flow of gas phase

y = mole fraction solute in gas stream.

Combining Equations 21 and 22 and utilizing simplifing

assumptions yields the equation for the height of the

absorption tower with gas film coefficients


b = d (23)


where:








G = molar mass velocity of gas, lb moles/(hr)

(ft ) ata

P = total pressure of system, atm

Y = mole fraction of solute in gas stream

Y, = mole fraction of solute in gas in equili-

brium with liquid

Y1 = solute concentration of gas entering the

column, lb mole solute/1b moles inert gas

Y2 = salute concentration of gas leaving the
column, Ib moles solute/1b mole inert gas

This equation assumes that:

1. The couilibrium curve is linear over the

range of concentrations encountered, there-

fore, over-all ccefficients may be used.

2. The partial pressure of the inert gas is

essentially constant over the length of the

column ,

3. The solute contents of gaseous and liquid

phases are sufficiently low that the partial

pressure and liquid concentration values

may be assumed proportional to the correspond-

ing values when expressed in terms of moles

of solute per mole of inert gas,

Chilton and Colburn8 introduced the concept that

the calculation of column height invariably requires the

integration of a relationship such as (fromn Equation 23)










yr2 'YeY

The dimensionless value obtained f-rom the integration
was called the number of transfer units based on an over-

all gas dr-iving force, Nog, and is a measure of the
difficulty of the gas absorption operation. By substitu-
tion~ Equation 23 becomes

h $ E --- (25)

and the height of the transfer unit, Hog, can then
be defined as


Hog h K af (26)
og g
Values of N~og are" particularly uIseful for expressing
the performance of absorption equipment in which the
volume is not of fundamental importance.
In fluoride absorption uni-cs, where a large

excess of water is used, the concentration of the acid

formed in the solution is low and Ye may be neglected
and Equation 24 is simplified to

N, = S1 = In (27)
y2
Since the absorption efficiency, E, is directly related

To y1 and y2 by

P1 7
E = ( ) x 100 (28)
'1


then Ecuation 27 can be rewritten asi









Nog =n 1- E1/100 \ (29)

The necessity for fluoride removal from effluent

gases has been stated in previous chapters. Fortunately,

both hydrogen fluoride and silicon tetrafluoride are very
soluble in water and most commercial installations for

control of fluoride emissions make use of this fact.

The vapor pressure of hydrogen fluoride over aqueous

solutions is shown in Figure 16, which is based on the

data of Brosheer,90 Whynes91 has presented the data given

in Figure i7 on the concentration of fluosilicic acid in

equilibrium with silicon tetrafluoride vapors.
When silicon tetr~afluoride is absorbed by water,

it reacts to form fluosilicic acid, The mechanism of

the absorption process has been studied by Whynes,9

who suggested that the reaction probably occurs in steps,

as represented by Equations 30 and 31.

SiF4 + 2H20:a-d Sto2 + 4HIF (30)

2HF +SiF4mr=EH2SiF 6 (31)

The simple fluosilicic acid probably reacts with addition-

al SiF4 or S102 to form a more complex form of this

compound, The reaction given by: Equation 31 is reversi-

ble to the point that solutions of fluosilicic acid

exert a definite vapor pressure of HF and SiF ,
As Figures 16 and 17 indicate, both hydrogen

fluoride and silicon tetrafluoride are very soluble in










1.5





1 .0 0




L~0.5




0 L -
0 5 ~10 15
Solution Concentration, WrT Percent HF

Figure 16: Vapor Pressure of H drofluoric Acid
Over Dilute Aqueous Solutionsv















O C



20 30 40
Solution Concentration, WT Percent H2SiF6


Figure 17: Vapor Pressure of Silicon Tetrafluyoride
Over Aqueous Sol-tions of Fluosilicic Acid.9








water and the gas film resistance would be expected

to be the controlling factor in their absorption. This

has generally been verified by experimental evidence

although Whynes9 found silicon tetrafluoride absorption

to be complicated by a tendency to form miist droplets

in the gas and the tendency of the silica, produced in

Equation 30, to form a solid film on the outside of the

water droplets, thus hindering absorption.

Whynes91 reported that even though the absorption

of silicon tetraflooride in water is complex and involves

a chemical reaction, the precipitation of silica, the

results could be explained in terms of the two-film

theory.

Types of absorption equipment which have been

used or proposed for the removal of fluoride vapors

from gas streams with water include the following general

types:

1 Spray chambers (vertical and horizontal)

2, Packed towers (10w-pressure grid packing)

3, Venturi scrubbers

4. Wet cells (beds of wetted fiber)

5. Plate columns

6, Ejectors

Since the scope of this research was limited to three

types of absorption equipmnat; namely, spray ch-ambers,

venturi scrubbers. and plate columns, the discussion

from this point on will be limited to the types studied.








Absoration in thec Cyclonic Snray Chamber

A spray chamber is thte simpliest type of absorp-

tion equipment. In general, it consists of a cylindri-

cal tank in which spray nozzles are mounted in the upper

section and the droplets generated fall domwnwrds with

some liquid flowing downward on the interior walls.

Although the liquid and gas streams may be concurrent,

the most common type is a count~ercurrent flow where

the gas stream is introduced at the lower section of

the tower tangentially so that a spiral motion is im-

parted to the gas and the liquid sprays are directed

radially. True, countercurrent flow is difficult to

maintain due to flow characteristics and spray entrain-

ment, which tend to limit the mass transfer available.

Some spray towers have been successful in the absorp-

tion of slightly soluble gases, but since the mixing

of spray and gas is not as vigorous as in other equip-

ment, the application of spray towers is best suited

to operations involving easily absorbed gases.

In spray chambers, liquid dispersion is created

by liquid phase mechanical power consumption. Due to

scrubber design, gas phase power consumption is usually

quite low, and Lunde9 found that removal efficiency

depends largely upon the power expended in the liquid

phase. Hie stated that the number of transfer units

would be related to power introduced in the liquid anld

gas phase by









N : - -(32)
og p 0.1

where:

Pi = power introduced in liquid

Pg = power introduced in the gas
Per, Sherwood and Pigford,7anTryl

have summarized various studies mainly pertaining to sul-

fur dioxide removal and have presented curves for the

number of transfer units versus liquid and gas mass

flow rates.

In any attempt to compare the performance of

different types of absorption equipment, fixed operating

conditions for each installation present a problem, and

an uncertainty exists as to the controlling factors in

the performance of the equipment, unless these factors

have been explored over a wide range on a pilot plant

scale. Unfortunately, laboratory or pilot plant data

on the absorption of fluorides are practically non-existant.

Sherwrin9 reported on 14 full-size widely differing

installations ranging from continuous spray towers to

continuous ejection systems. Efficiencies up to 99.3

percent were obtained and values for Kga were reported.
However,? the volume coefficient K~a has little value

as a means of correlating spray equipment because the

area for mass transfer a varies with liquid rate, noz-

zle design, liquid pressure, distance from nozzle, and

other factors. Performance of different fluoride








vapor absorbers may be compared on the basis of the

number of transfer units in a given piece of equipment,

or, more simply, on the basis of percentage removal

efficiency.


Absorption in the Venturi

In the venturi scrubber, the velocity of the gases

causes the atomization of the scrubbing liquid. A typi-

cal venturi scrubber has the gas stream pass at a high

velocity thr-ough the venturi with the scrubbing liquid

being introduced at relatively low pressure at the venturi

throat where shearing of the liquid and droplet formation

take place, Large areas of liquid are made available

to contact the gas as a result of a high degree of dis-

persion which takes place at the throat, due to an in-

crease in the throat velocity. The energy required for

forming droplets and intimate mixing with the gas is

furnished by the gas stream. The resulting spray of

liquid mixes with the gas and absorption takes place.

It is necessary that an entrainment separator be pro-

vided at the discharge of the venturi to remove the en-

trained liquid from the gas. The entrainment separator

is usually of cyclonic design and normally the venturi -

cyclonic spray chamber is installed as a single unit.

Venturi scrubbers were designed originally for

dust and mist removal, and considerable research949

has been done in this area. Lunde has reported on








venturi installations for sulfur dioxide removal and has

suggested that their performance on fluoride vapors

would be controlled by the same factors as SO2. The

literature contains isolated cases reporting on the per-

formance of various venturi scrubber installations in

removing gaseous fluoride from process gases.91,29

Lunde9 reported on venturi units using water as the

scrubbing liquid with removal efficiencies generally

in the 95 to 98 percent range when employed for hydrogen

fluoride absorption,

In general, the average pressure drop across a

venturi varies from 10 to 50 inches of water depending

upon scrubber design. The liquid requirement has been:

found to be between 3 to 15 gpm per 1000 ofm with power

requirements in the range from 1 to 10 horsepower per

1000 cfm, Lundle2 also reports on attempts to correlate

the number of transfer units with power requirements and

he proposed that

N : : (P 0.3) 7 0.8) (
og 1 g
This indicates the dependence of the efficiency of the

venturi on the energy introduced to the gas phase.


Absorption in the Baffle Plate Imnineement Column

In plate towers, the gas-liquid contacting takes

place on a series of plates installed in a cylindrical

column. The plates are perforated by small holes through

which the gas passes while the liquid flows across the









plate to a dow~ncomner to the next plate in the column.

Thle gas stream palsses up thirough the perforated holes

in the formr of irregular bubbles and through the liquid

layor on top of the plate and mixes with the liquid.

Rapid absorp~tion occurs by virtue of the large interphase

surface produced by the small bubbles generated.

The literature abounds with studies on this type

of scrubber and the American Institute of Chemical

Engineers has published a design manual relating many

of the variables of these studies. Studies have related

mass transfer efficiency to plate characteristics such

as weir height, to gas flow rates and velocities, and

to liquid flow rates. However, as Perry indicates,

studies must be performed under similar conditions and

this has generally not been the case. Since this type

of scrubber is not generally employed in fluoride re-

moval processes, no information was available in the

litera'-ure regarding fluoride scrubbing. However, if

a parallel may be drawnm with sulfur dioxide scrubbing,

efficiencies above 90 percent have been reported with

high liquid to gas ratios (50 to 100 gpm per 1000 cfm).


Correlation of Scrubber Efficiencies

Absorption data are usually presented in terms

of the effect of liquid or gas mass flow rates on the

mass transfer coefficient, or on, the height of transfer

unit. The economic evaluation of equipment requires








translating these data into over-all performance as

measured by eificiency or number of transfer units.

Perry anid Lunde2 have indicated that it is difficult

to compare two basically different types of equipment

inl terms of mass transfer coefficients, mass flow rates,

or eight of transfer unit.

Semrau, Lunde,9 and others have correlated

the efficiency of a scrubber expressed in number of

transfer units, with total power consumption, the pres-

sure loss of the gas passing through the scrubber and

the pressure loss required for atomization of the scrub-

bing liquid. To date this seems to be the moost accepted

method of making comparisons between different ty-pes of

equipment, Semrau nas plotted the number of transfer

units against total power consumption on a log-10g plot

and expressed the results in the form

Nt =GPB (34)

where a and F are characteristic parameters of the

material being removed, He applied this equation to

dusts and mists and determined that the efficiency of

collection could be expressed in terms of total power

used and the characteristics for the dust being collected,

and independently of the actual type of scrubber being used.

For this study, since the prediction of exact mass

transfer characteristics would be very difficult due to

the unavailability of data, the procedure described by




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