Effect of high pit temperature and of preheating of the wood on the grinding of loblolly pine


Material Information

Effect of high pit temperature and of preheating of the wood on the grinding of loblolly pine
Physical Description:
Schafer, E. R ( Earl R )
Pew, J. C
Knechtges, R. G
Forest Products Laboratory (U.S.)
University of Wisconsin
United States Dept. of Agriculture, Forest Service, Forest Products Laboratory ( Madison, Wis )
Publication Date:

Record Information

Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 29607086
oclc - 757823910
System ID:

Full Text
13 Z-yl .

U. S. Department of Agriculture, Forest Service


In cooperation with the University of Wisconsin




Associate Engineer
Assistant Engineer
Assistant Scientific Aid

- -- -


O0 T ul 1 1972

I.F.A.S. Univ. of Florida

Published in
July 9, 1936

Digitized by the Internet Archive
in 2013





E. R. SCHAFER, Associate Fm:-ineer
J. C. FE7, Assistant Engineer
ER. G. .':uCHTLES, Assistant Scientific Aid

The results retorted here of grinding loblolly pine at high
temperatures are believed to be applicable to any coniferous species.
2!evation of grinder pit temperatures, with other variables constant, re-
sults in longer fibered and stronger pulps in relation to the pow-er con-
sumed. This trend previously reported to l00 F., continues on up to 21(c
F. Preheating of the wood in water has relatively s-,rall effect as com-
pared with the pit temperature. The small beneficial effect noted seems
to be due not merely to the elevation in temperature of the wood, but to
a mild chemical or cooking action talking place during the preheating
period. The increasedplasticity of the wood when charged at higher tempera-
tures a-. ears to be more of a detriment to quality than an aid. With a
stone carrying sufficient water in its surface, the temperature at the wood-
stone interface is not likely to exceed the boiling point of water at
atmos iheric pressure. Control of temperature by controlling the pulp carry-
ing capacity of the stone surface is possible, but is not so practicable or
so easily done as is controlling the pit temperature. Since the results
noted cannot be attributed alone to the effect of heat on the wood it is
suggested that the viscosity of the water may be largely responsible.


A previous publication on the effect of temperature in pulpwood
grinding!- pointed out that high temperatures in the grinder pit were

-Presented at the Annual Meeting of the Technical Association of the pu1p :
Paper Industry, Waldorf-Astoria Hotel, New York City, Feb. 17-20, I135.
-Effect of Temrerature and Consistency in Mechanical Pulping, by E. H.
Schafer and J. C. Pew. F->r Trade J. 101, No. 13; 71, Sept. 26, 13.


advantageouas in producing longer fibered, stronger pulps, generally with
lower power consumption and. higher g-rindin2 rate. The results reported
here a-'e an extension of the prior work. In general three types of excper-
iments were -:ade using (1) higher pit temperatures than previously
employed, (2) wood preheated external of the grinder, and (3) steam
projected on the stone just before it passed in rotation under the wood.

The loblolly pine used was from the same shipment as that used
in prt of the previous work. The selection of this species for the pres-
ent experiments was prompted by the current studies now bein- made on the
effect of growth variables in the pulping of the southern pines. The re-
sults of the temperature studies on this species are believed to be
applicable to any coniferous species.


The equipment and the general methods of operation were the same
as previously used with loblolly pine.2- In the present experimrnt.s, how-
ever, the wood was in all cases brought to a definite temperature before
cnarging into the grinder. This was accomrnlished by placing the 9-inch-
lon: wood bolts in a tank in which water was circulated at the required
tempeI.ture for a-proximately 5 hours. A control bolt with a thermometer
inserted to the center was used to determine when the wood was heated to
the desired temperature. The wood was left in the tank until required for
char_-in: the grinder. No material change in moisture content of the wood
or the color of the pulp was caused by this heat treatment. The unit
pressure of wood against the stone surface was constant at l14 pounds per
square inch and the temperature of the shower water was adjusted so as
to give a consistency of pulp in the pit of about 5 percent in all experi-
ments. The. surface of the artificial stone, from past experience, was
known to have remained constant throughout the relatively short time
covered by the experiments.

Discussion of Results

The data obtained are summarized in Table 1.

Series 1 shows the results of charging; wood of a temperature of
60 and grinding it with pit temperatures of 160, 1900, and 210 ?. This
series is similar in part to series 7 of the prior tests 2 but because of
the slightly different grinding conditions the actual values of pulp strength
and power are not comparable with the previous work. Comparison can, hovrw-
ever, be made in the general trends. It is noted in series 1 that increas-
ing the pit temperatures caused the pulp to become longer fibered and to
have greater strength, this tendency continuing above the 190Cc maximum
t-moerature previously employed.



As previously observed tht increase in pit temperature (with
other variables constant) caused a consistent decrease in the power inrut,
but contrary to the former work the production did not consistently. in-
crease. This resulted in irregularity in the power consumed per unit of
production which, within relatively narrow limits ap-oeared to increase at
19C and decrease at 2100. From the standpoint of economy of production
it is of interest to consider the quality per unit of power consumption,
obtained by dividing the strength values by the horsepow r days per ton.-
The str.,.th values thus factored are shown in Table 1. The most economi-
cal use of *power seems to be definitely indic-ted at high pit temperatures.
As previously pointed out, however, hi.--h pit temperatures must be
acconimanid with a sufficiently low consistency that the pulp flows freely
over the dam.

It is evident that the relation between power consumption and
quality is empirical. The energy absorbed in the grinding process
represents the resistance of the wood to the various forces made to act
upon it and therefore has an influence on the quality only as these forces
affect quality. The conditions that bring about improvement in quality
often require the consumption of more energy, but one does not neces-
sarily follow the other. Therefore, since it is conceivable that pulps
of equal quality may be produced with the consumption of different amounts
of energy, the evaluation 1of quality by means of the factoring of strength.
values by the power consumed has no theoretical significance and is
justified only when used for economic considerations.

The effect of varying the temperature when the wood was preheated
to a:-d charged at the pit temperature is shown in series 2. The tre]d of
increased strength with increased temperature is much as shown in series 1.
In series 2, however, the bursting-power factor values are slightly hig-her
and the tearing and tensile strengths, except in two instances, slightly
lower than in series 1. At 190 F. and 210 F. the fiber lengths are con-
siderably greater in series 2 than in series 1. The power consumption was
markedly higher at the tw-o higher temperatures when the wood was re-' heated
to the grinding temperature. The only advantages of preheating the wood
are a slight improvement in bursting strength and a fairly marked increase
in fiber length, but to accomplish this the preheating and grinding must
be done at high temperatures.

This is borne out further in series 3 in which the pit temperature
was maintained at 160c F. and the temperature of wood varied from 60 to 21C
F. in 50 steps. Considering this wide ranje of temperature variation
the results showed surprisingly small differences in pulp quality. T:1e
fiber lengths remained unc.-.anged except for a slight increase at 210 F.
The strength values factored by the power showed little or no change in
bursting strength and a slight decrease in tearing and tensile stre--ths

-This factor bears a reciprocal relationship to the "quality-price" number
used bor Brecht in determining the cost of maintaining a given quality.
Papier 7alrikant 13, 13; 113, Mar. 31, 1935; 33, 14; 121 A-r. 7, )1i35;
33, 15; 129, Apr. 14, 1935.



with increase in the temperature of the wood chare-ed. Prior heating
of the wood alone, even to a higher degree than the grnairn temperature,
a-ouarep.tly had no helpful influence on the pulp produced, but tended for
the most part in a direction opposite to that generally observed on the
effect of increased temperatures. This would indicate that certain
advanta--cs claimed in practice as due to the mild warming and ste-aminr-
that the wood receives in the grinder pocket, either do not exist or must
be attributed to other causes in addition to heat and moisture. If
benefits accrue it would seem that something more than softenin:- of the
wood must be the cause.

The foregoing indicates definitely that (1) improvement in
quality results as the temperature at the wood-stone contact increases,
(2) a slight improvement in certain pulp properties with an accomrany:ing
detriment to other properties results if in addition to raising pit
temp-eratures the entire mass of wood is brought slowly to the grinding
temperature in water external to the grinder, and (3) no appreciable
benefit is derived by raising the temperature of the wood only.

In connection with the last point it became of interest to
determine whether wood so treated is permanently affected or whether the
softening action is reversible upon cooling. For run No. 19- the wood
was heated to 2100 F. according to the procedure adopted and then rapidly
cooled t, 60 F. and ground at a pit temperature of 160. The pulp from
this run (see series 4) was not only better than that of a similar run
(No. 1S5) in which the wood was not heated, but also better than the run
(No. 1S8) in which the wood was heated to 210 and not cooled before grind-
ing. The strength-power factors indicate further that the wood in this
run was ground with the most economical use of power as compared with the
other tw'o. The experiment indicates that the x'ood is permanently affected
by slowly heating in water to approximately the boiling point. This
permanent alteration is undoubtedly due to a mild chemical reaction or
extraction of the wood constituents. An additional fact is that the
higher plasticity of the wood charged at high tem-roerature is not so
beneficial as hitherto supposed.

In series 5 the effect of heating the stone surface just before
it traveled under the wood was noted when the wood was charred at room
temperature and the grinder pit maintained at the normal operating tempera-
ture of 160. The heating was accomplished by projecting steam on the
stone at that point. Measurement of the amount of steam was not made,
therefore,the results are only qualitative. That the temperature at the
interface was higher than normal is shown by the lowered consistency. A
distinct increase in fiber length, bursting power and tensile-power factors
is evident in run 'o. i9 in which steam was a prled as compared with run
No. 185 in which no steam was used.

In attempting to account for the effects noted it seems that the
controlling causes are not attributable alone t4 the effect of temperature
on the physical properties of the wood. Brecht- has also reported that

Loc. cit.

the heatin,- of pulp, produced by the "cold" gririi:- process (Etropean
practice) for 30 minutes in water at various temperatures has but little
effect on any of the ph:,sical properties. Similar experiments at the
Forest Products Laboratory with commercial pulp, produced by the "hot"
grinding process (American practice), heated at various intervals in
boili-: water confirm this statement except that it was noted that boiling
for only 1 minute caused an appreciable increase in the freeness. Cer-
tainly contact with hot water cannot have any effect on the fiber length
of the pulp. Hence, as an alternative, for which, at present, there are
no data to submit as supporting evidence, it is sW-:ested that the princi-
pal factor producing the effects may be the viscosity of the water used
as the grinding medium. Certain experiments are in mind that may give
information on this point.

Theoretical Discussion of the Heat Developed in Grinding

In the work carried out so far consideration has been given only
to the temperature of the pulp in the pit and prior thermal treatment of
the wood. The actual temperatures attained between the wood and stone are
probably of most importance, but unfortunately these temperatures are very
difficult if not impossible to measure. However, the amount of heat
developed may be estimated by analysis from several angles of approach.
The following example serves to give an idea of the magnitude of these

Assume a grinder stone operating with a pressure of 30 pounds per
square inch of wood-stone contact. The grinding or friction coefficient as
calculated from operating data may be shown to vary from 0.15 to 0.35 or
evwn more. Assume an average value of 0.25. Allow one square inch of stone
surface to advance one foot under the wood at the above pressure. Then
the energy expended will be 30 x 0.25 x 1 = 7.5 foot pounds per square inch
of stone surface. This energy is almost entirely transformed into heat,
yielding 7." x 0.324 = 2.43 calories. The heat is absorbed by the stone,
by the pulp suspension carried under the wood in the grooves in the stone,
and by the new wood ground off. Lwing to its mobility and high specific
heat the major portion of the heat is probably absorbed by the pulp sus-
pension in the grooves. It is likely that the grooves in the stone suir-
face are full of pulp as they advance under the wood, since pulp from the
grinder pits is seen to be doctored off at the first block of wood in the
first pocket. It may be estimated by rough calculation that the volume of
the grooves on a moderately rough stone will be from 0.2 to 0.3 cc. per
square inch of stone surface. Assuming 0.25 cc. as an average and that the
heat is absorbed entirely by this pulp in the stone surface, the temperature
rise attained in one foot of travel would be 2.43 + (0.25 := 9.7 C. or
9.7 x 1.S = 17.50 F. Actually some of the heat is absorbed by the stone
and by newv fibers ground off the wood so that the increase in teynoerature
attained would be less than this amount. Thus from the star.dToint of
energy input it is seen that with a fairly deeply grooved stone runniing
at ordinary pit temperatures the temperature between the wood and stone



could nev-r reach the extreme values sometimes postulated. In the fore-
goin-- exarrole, if the pit temperature is 160, about 3 feet of continuous
wood-stone contact would be necessary for the pulp suspension to reach
the boiling point. Th ex-olosion theory sug -': ted by Schoen-ut5 would
require steam pressures in excess of atmospheric. A temperature s'Uffi-
cient to cause such pressures is probably seldom if ever attained in the
grindin" process. High.r temperatures are, of course, attained when the
stone surface does not carry sufficient water to prevent burning of the
wood, but this is not normal operation.

It is further evident from these considerations that the depth
and character of the stone grooves has a decided effect on the tempera-
ture produced between the wood and stone. It is possible, by varying the
depth and type of burring, that is, by varying the free space under the
periphery of the stone, to control the average temperature between the
wood and stone without changing the pit temperature. If the amount of
pulp-carrying space is decreased the average temperature between wood and
stone is increased. However, when this is done the range of temperature
from the point where the wood and stone first come into contact to the
point where the stone emerges is also extended.

The relative advantages of varying the stone surface or the pit
temperature may be illustrated by the following example. Assume the pre-
ceeing grinder stone (which carried 0.25 cc. of pulp per square inch) to
be running in a pit at 1500 F. and that all of the heat developed is
absorbed by the pulp in the stone surface. When a given area of the sur-
face first makes contact with the wood the temperature of the pulp in the
grooves is 1500 F. When the area has traversed one foot the temperature
e'uld be 1500 + 17.5 = 167.50. The average temperature for the one foot
t,'avel would be 159.C ). Now to raise the temperature suppose the pulp.-
cairrying soace in the stone is decreased by one-half. The pulp in the
grooves on entering the wood area would still be 150, but (since to absorb
the same quantity of heat the temperature would be doubled) after travers-
ing a distance of one foot the temperature would be 1500 + 35 = 1I5 ?.
and the average 167.50 F. On the other hand, if instead of chan.-ing the
soace in the stone surface the pit temperature had been raised to 1530
then the temperature after one foot of travel would be 155 + 17.5 = 176.5
with an average of 167.-. Thus by raising the oit temperature 9 the
average temperature under the wood has been increased an amount equal to
that caused by reducing the pulp-carrying space in the stone surface one-half.
Furthermore, by raising the pit temperature the range of temperature under
the wood is only half of that caused by changing the stone surface. It
appears that uniform temperatures within narrow limits are more easily ob-
tained by controlling pit temperature than by varying the pulp-carryir>-
capacity of the stone surface.

5-Schoengut, J. Papier-fabr. 3j, No. 3; 25, Jan. 20, 1935.


Raising the temperature of the grinder pit, other variables
rpmnaini:'. constant, has been found to have marked effects in mechanical
pulpin Preheating of the wood in water external to the grinder caused
slight effects that may be attributed to a mild coo:in, or extraction. The
increased plasticity of the wood caused by increased temperature had but
little influence on the resulting pulp. The e:q.-periments as well as the
theoretical considerations presented indicate that preheating the wood
has only an insignificant effect on the temperature of the wood-stone
interface, since the amount of new fiber added during< the passa :; of
an area of stone surface through the grinding zone is small in comparison
with amount of pulp alre.ady:r in the surface. Consideration of the enerj'
absorbed in the production of groundwood fiber leads to the conclusion
that the temperature of the interface is not likely to exceed the boiling
point of water at atmospheric pressure.

Since the marked effects observed in controlling the temperature
of the grinder pit or of the wood cannot be attributed alone to changes
in either the physical properties of the wood or of the pulp it is sug-
gested th-at the viscosity of the water may be the controlling factor.
This may be difficult to prove em-perimentally, but it is believed
further work along this line will yield valuable information.


Properties of pulp suspension
Grinding conditions 2 Screen analysis
Service Temperature of Retained between- Properties of pulp test sheets
Sof --- Yield per 100 Power Re- A-------- __
stone Wood lbs. dry wood Dry --- --- Free- stained 24- 42- 80- Pass- Aver- Bursting Tearing
S since Grinder - "----- Con- r--- ------. wood Per ton ness on and and and ir., age per per Tensile Color analysis
" last ----x- lre. Charg- sist- Screen- r iiri.l of dry Schop- 24- 42- 80- ISO- 1 screen pound pound per Ives readings S'rer.gth power factors'
tU burr- Shower heated ed ency edl Screen- i .-i wood' r' rresh mesh mesh mesh mesh open- per per square ,&---I--I-- ---_ ------
ug" water Pit to, at Per pulp ings hours' Input HI.P.- I ,-.I Per Per Per Per Per ing* ream7 reamr inch Red Green Blue Burst- Tear- Ten-
No. Hours F. F. F. OF. cent Lbs. Lbs. Tons H.P. days Cc. cent cent cent cent cent Mm. Points Grams Pounds Parts Parts Parts ing ing sile
185 50.0 80 160 60 60 5.8 91.4 0.2 0.56 41.3 53.7 363 2.3 7.0 14.6 20.5 55.6 0.078 0.09 0.43 697 76 67 63 0.17 0.80 1300
194 61.7 127 190 60 60 5.5 90.2 .3 .46 27.3 58.7 350 6.5 9.6 15.2 20.1 48.6 .086 .16 77 888 81 69 61 .27 1.31 1510
192 58.8 144 210 60 60 5.2 91.2 .4 .46 25.1 54.9 365 9.1 10.7 14.6 19.4 45.6 .090 .17 .92 927 76 69 62 .31 1.68 1690
187 52.2 80 160 160 160 5.1 91.0 .2 .51 27.6 54.0 348 2.4 6.9 14.4 20.7 55.6 .077 .10 .37 611 76 69 63 .19 .69 1130
191 57.2 80 190 190 190 6.3 89.8 .4 .36 25.3 70.5 358 12.3 11.2 14.9 17.7 43.9 .093 .21 .73 1008 77 70 62 .30 1.03 1530
193 60.2 144 210 210 210 5.4 92.8 .6 .40 25.4 62.7 360 16.3 10.6 13.6 16.4 43.1 .096 .21 1.07 906 77 69 60 .33 1.71 1440
185 50.0 80 160 60 60 5.8 91.4 .2 .56 41.3 53.7 363 2.3 7.0 14.6 20.5 55.6 .078 .09 .43 697 76 67 63 .17 .80 1300
186 51.0 80 160 110 110 5.4 94.6 .3 .51 28.4 55.3 348 2.0 6.2 13.3 20.7 57.8 .075 .11 .43 784 79 69 63 .20 .78 1420
187 52.2 80 160 160 160 5.1 91.0 .2 .51 27.6 54.0 348 2.4 6.9 14.4 20.7 55.6 .077 .10 .37 611 76 69 63 .19 .69 1130
188 53.4 80 160 210 210 4.8 95.5 .3 .48 31.2 64,3 368 4.5 9.3 15.3 20.3 50.6 .083 .12 .44 686 75 67 60 .19 .68 1060
185 50.0 80 160 60 60 5.8 91.4 .2 .56 41.3 53.7 363 2.3 7.0 14.6 20.5 55.6 .078 .09 .43 697 76 67 63 .17 .80 1300
1 55.8 80 160 210 60 5.3 94.0 .3 .46 25.9 560 377 6.8 10.1 15.9 20.2 47.0 086 .15 .49 795 74 67 59 27 .88 1420
1,S 53.4 80 160 210 210 4.8 95.5 .3 .48 31,2 64.3 368 4.5 9.3 15.3 20.3 50.6 .083 .12 .44 686 75 67 60 .19 .68 1060
185 50.0 80 160 60 60 5.8 91.4 .2 .56 41.3 53.7 363 2.3 7.0 14.6 20.5 55.6 .078 .09 .43 697 76 67 63 .17 .80 1300
189 54.7 80 160 60 60 3.8 94.4 .2 .49 28.4 57.7 348 2.6 6.9 15.4 22.5 52.6 .079 .13 .42 879 76 67 60 .23 .73 1520

The wood from shipment 1425 had the following average properties: age, 19 years; diameter, 6.6 inches; volume of springwood, 60 per cent; growth rate. 5.9 rings per inch; specific gravity, based on weight
when oven dry and volume when green, 0.445; dryness. 46 per cent.
The pressure of the wood against the stone was 14 pounds per square inch based on the actual thrust delivered by the pressure foot and the area represented by the product of the average diameter of the wood
bolts and their length.
SThe grinder stone was composed of Aloxite abrasive, 406 grit, grade K, bond 30, burred with a 9-cut. 4-inch lead spiral burr followed by a 4-cut straight burr.
The wood was preheated in water at atmniospheric pressure to the predetermined temperature.
Power and production are expressed on the basis of wood ground instead of pulp produced since the latter figures are somewhat inaccurate.
SThis value, calculated frim the screen analysis and the size of the screen openings, is a function of the average fiber length of the pulp.
I Based on a 25 x 40 x 5001) ream.
IThese factors are obtained 1 dividing the strength values by the horsepower days per ton of wood ground and multiplying the quotient by 100 so as to give numbers of the magnitude of the usual strength values.
SSteam used in grinder run IP. and not used in grinder run 185.
Z M 30198 F

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