settling rate of calcium carbonate in the causticizing of soda ash

SETTLING RATE OF CALCIUM CARBONATE IN THE CAUSTICIZING OF SODA ASH

JOHN C. OLSEN AND OTTO G. DIRENGA' Polytechnic Institute, Brooklyn, N. Y.

I

The factors which affect the settling rate of calcium carbonate in the causticizing process of converting soda ash to caustic soda have been studied; they are: temperature, agitation, particle size of lime, amount of water used in slaking, presence of soda ash in the water used for slaking, previous exposure of the lime to air, reaction rate of the lime, relation between particle size of slaked lime, and the calcium carbonate produced.

A n experimental study of the equilibrium in the reaction Na~C03 + Ca(OH)2 e 2NaOH + CaC03 showed that the values given in Lunge's table are inaccurate and that the values obtained by Good- win are correct.

The outstanding result of the investigation has been to show the effect of preliminary exposure of the lime to air, which results in agglomeration of small particles to larger size particles and rapid

N THE production of caustic soda by the reaction between hydrated lime and soda ash, i t is important that the resulting calcium carbonate shall settle rapidly and that a

clear solution of the caustic liquor be produced. Since there are conflicting data in the literature concerning the factors which iduence the settling rate of calcium carbonate in this process, it was decided to study the conditions under which a rapidly settling calcium carbonate may be obtained.

A chemical quicklime with a high percentage of calcium oxide was selected. Before entering the causticizing vessel, the lime is slaked or hydrated with water or a weak solution containing sodium carbonate alone. or sodium carbonate and u

caustic soda together in solution. The causticization is carried out in a vessel equipped with an agitator, at a temperature approaching 100" C. After the conversion is completed, the calcium carbonate produced in the reaction is permitted to settle out, and the clear caustic soda liquor is drawn off. Modern practice, however, calls for a continuous causticizer and continuous separation of liquid from solids in thickeners of the type of the Dorr thickener. In either case the remaining sludge, composed mainly of calcium carbonate, is filtered and washed in a continuous rotary vacuum filter. After filtration, the calcium carbonate cake is reburned in lime kilns to quicklime which, mixed with fresh quicklime, serves to causticiee additional soda ash solutions. The caustic soda liquor, which is used in the manufacture of paper pulp from wood, is recovered as green liquor containing sodium carbonate. This liquor is recausticized according to the above method.

1 Present address, Natural Products Refining Company. Jersey City, N. J.

settling of the hydrate and carbonate. The size of the hydrate particles and the carbonate particles increases as the size of the quicklime particles decreases.

Experimental data show that the use of the theoretical amount of water does not produce the fastest settling hydrate or carbonate. Rather, the fastest settling hydrate or carbonate is obtained by using an excess over the theoretical amount, which was found to be twice the theoretical amount. The results of other investigators, that the relative particle size of the hydrate persists throughout the causticizing stage, were confirmed.

The rate of reaction decreases with time of exposure of the quicklime before slaking, with an increase in the particle size of the quicklime and an increase in the concentration of sodium carbonate in the slaking water.

For the economic operation of the causticizing process it is important to observe several points, with the object of obtaining: (a) the highest possible conversion of sodium carbonate to sodium hydroxide, (b) the clearest possible solution of caustic soda, (c) the highest possible settling rate of the calcium carbonate mud consistent with (b) in order to cut down equipment size, and (d ) the greatest possible recovery of sodium hydroxide from the calcium carbonate mud.

Much previous work has been done on the variables affecting the operation of the causticizing process, particularly on the factors pertaining to the behavior of the lime in this process.

THREE^ CAUSTICIZING AGITATORS ARRANGED IN SERIES FOR COKTISU- ous PROCESSING, IP; A DORR CONTINUOUS RECAVSTICIZIKG SYSTEM

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February, 1941 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

TOP VIEW OF THREE BALANCED-TRAY THICKENERS IN A DORR CONTINUOUS RECAUSTICIZING SYSTEM

Purpose of the Investigation

A thorough study of the general topic related to the manufacture of caustic soda by the causticization of sodium carbonate solutions with lime revealed the necessity of limiting the scope of this work to a few particular phases of the prob- lem.

A . The equilibrium in the causticizing process. According to Goodwin ( 1 1 ) the accepted figures of Lunge (18) for the equilibrium are inaccurate. As the literature revealed no other data confirming the claim of Goodwin, it seemed worth while to in-

It was decided to investigate the following:

vestigate zhis important point. B. The rate of settling of calcium hydroxide and of calcium

carbonate after slaking or after slaking and causticizing, and the effect of different variables in the slakin process on the rate of settling. According to Dorr and Bull (13 the factors influencing the settling rate are:

1. Source of lime:

2.

3. Method of slaking:

a. Chemical constitution b. Physical nature

a. Temperature of burning b. Length of burning period

a. Amount of water used b. Temperature during slaking c. Degree of agitation durin slaking d. Use of caustic soda or sofa ash solutions for slaking

4. Method of causticizing: a. Period of agitation b. Violence of agitation c. Temperature d.

Method of burning the lime:

Excess of lime or soda ash

used

5. Causticity and concentration of caustic soda desired 6. Presence of impurities particularly when reburned lime is

This paper discusses the settling rates of various high- calcium limes, and changes in the settling rate caused by different methods of slaking. It is admitted that the other factors enumerated above exert a definite influence on the settling rate. However, the methods of slaking and causticizing seem to be the more important factors with respect to the settling rate. This particular investigation was pur- posely limited to the effect of variables in the slaking process by keeping all other factors constant and unchanged.

In addition to the four variables mentioned under method of slaking, another variable-namely, the effect of weathering or exposure of the quicklime to the atmosphere before slaking- was found and investigated during the course of this work.

205

Furthermore, during the e x p e r i m e n t a l w o r k we noticed that changes in the settling rate caused by varying the procedure of slaking were always accompanied by a change in the rate of reaction or rate of hydration, as evidenced by the length of time required to bring the mixture of lime and water to its maximum temperature. The time re-

quired varied from a fraction of a second to several minutes. Since these changes in the rate of reaction were not fully re-

corded while the settling tests were carried out, another set of experiments was conducted with the sole purpose of determining qualitatively the change in the rate of reaction caused by differences in the method of slaking.

Types of Lime Used

Lime samples A , B, and C were supplied by the courtesy of the Standard Lime and Stone Company. They represent typical commercial types of chemical quicklime which have been used successfully in the causticizing process :

A , Martinsburg B. Knoxville C, Bakerton Pebble Lump Pebble

SiOa 1.06% 0.23% 1,107"

MgO

Fedh + Ah03 0.96 0.50 0.40 CaO 96.18 97.63 96.85

0.80 0.60 1.00 90.80 95.73 94.82 Available CaO

Sample D represents a c. P. grade of calcium oxide produced from marble: & 0 3 , 0.10 per cent; MgO, 0.36; volatile material, 6.3.

Equilibrium in the Causticizing Process

PREVIOUS WORK. Lunge (18) showed that the reaction

NazCOd + Ca(OH)z 2NaOH + CaCOs

is reversible and that, according to the law of mass action, the yield of caustic soda decreases with the concentration of sodium hydroxide. This subject was exhaustively treated from the physical-chemical point of view by Bodlander ( 6 ) .

According to Goodwin (11), reporting on the equiIibrium in the causticizing process:

The equilibrium is given by the expression,

K kJkz 3 (OH-)2/COs--

since a t temperatures above 80" C. the Ca(OH)2 and CaCOa are the only solids present. The value of the constant K can be calculated theoretically from the solubility product of Ca(0H)Z and CaC03 where Kt = Ca X (OH-)2 and kz = Ca X COa. There is incomplete agreement on values thus calculated and the yields on causticizin due to the error introduced by incomplete dis- sociation of in solution. A value calculated for this gives only an approximation. The same a plies to the CaC03 in solution, which is, in addition, considerabg hydrolyzed. There

206 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 33, No. 2

/ I i / / ' , 1% N ~ ~ C U ~ , UR/(;/NAL SOLUTIONI i ~ ' i j , i j I ~ i S.1

w

is, however, qualitative agreement between K as calculated from experimental data and from the ratio k l / k z .

The yield can be readily shown to be inversely proportional to the concentration of caustic soda.

The results of Goodwin's work are as follow: Excess

Original NarCOa, Conversion, Mean, Used,

Lime ---Summarya ----- % % % % Nan808 Causticity 7 %.

FIQURB 1. APPARATUS FOR EQUILIBRIUM DE-

TERMINATIONS

Goodwin's experiments were carried out in a 250-cc. nickel crucible provided with a fitting cover and stirrer. The solution was stirred for 2 hours at 96-98" C. and then permitted to settle for 5-20 minutes. A sample of clear solution was withdrawn with a pipet and made up to 250 cc., and an aliquot titrated for total alka- linity. Winkler's method was used for the determination of the sodium carbonate (addition of barium chloride to an aliquot part, making up to 250 cc., and titrating the sodium hydroxide in the clear pipetted solution; filtration is not permis- sible).

... 50 2 99.4 4 99.2 98.83

25 6

98.7 6 98.41

25 6

25 8 97.8 9 97.31 10 96.5 97.15

10 9

94.6 94.30

12 12 10

14.7 92.4 12 94.39 16 89.9 94.35

50 12

86.7 91.31

25 18 15 91.19 . . . 20 83.5 91.01

25 15

25 15 15 91.30 91.2

98.6

97.2 . . . ...

94 : 35 25 . . . ...

* These values are read from a graph plotted b y Goodwin from his experimental results.

The apparatus described by Goodwin appeared to be simple and efficient, and was duplicated in this work (Figure 1). The nickel crucible was immersed in a water bath which was thermostatically controlled. A tight-fitting cover was provided with a center-mounted bearing for the agitator shaft. The shaft and blades were made of nickel. The hollow cover contained a half-inch (1.27-cm.) layer of insulating asbestos, in order to prevent condensation on the inside of the cover. The solution in the crucible was maintained at 94-95" C., whereas Goodwin used a temperature of 96' C.

The experimental work was carried out as follows: The solutions were made up by weight to approximately 2, 5, 10, 15, and 20 per cent sodium carbonate. The solutions were cooled to 20' C., and a sample of each was withdrawn by a pipet and diluted. An aliquot was titrated with 0.5 N hydrochloric aFid, using methvl orange as indicator. The calculated concentration of the

EXPERIMENTAL WORK.

quot is titrated with standard acid, using methyl orange as indicator, for the total alkali. T o another aliquot are added 20 cc. of 20 per cent barium chloride solution, and the solution is titrated t o the phenolphthalein end point with constant stirring. This latter titration gives the value for the alkali in the form of hydroxide. The difference between the two figures represents the amount of unchanged sodium carbonate in the solution.

Another modification of the Winkler metho ( was investigated. It calls for the precipitation of the carbonate ion as before, by addition of barium chloride. The actual titration, however, is made on the clear solution from which the barium carbonate has been permitted to settle out overnight. This procedure is to prevent the action of the acid on the barium carbonate, which in the other modification of the Winkler method is permitted to remain in suspension in the solution to be titrated. It was found, however, that this modification gave the same values as the first one; consequently it was not used during this investigation on account of the length of time required for the test.

The standard acid used for titration was 0.5 N hydrochloric for the more concentrated solutions and 0.1 N hydrochloric for the weaker concentrations.

TABLE I. EQUILIBRIUM RESULTS Expt. 1 Expt. 2 Expt. 3,

50% Excess'CaO 10% Excess'CaO 5y0 Excess CaO 70 NaaCOa % NaaCOa % N+Oa in original Conversion, in original Conversion, in original Conversion,

2.30 99.38 2.4 99.30 3.0 98.30 5.0 98.85 5.2 98.65 5.1 98.80 9.8 96.51 10.1 96.40 10.2 96.15

15.1 91.50 14.8 91.40 15.0 91.20 19.6 83.47 18.6 85.75 19.2 84.40

soln. % soln. % soln. %

Percentage causticity, or conversion, is the percentage of the total alkali present which is converted to sodium hydroxide in the causticizing process. If A represents cubic centimeters of standard acid used in the determination of the total alkali, and B the cubic centimeters of standard acid used in the determination of alkali hydroxide, then the percentage conversion or causticity i s B / A X 100.

The final results are shown on Figure 2 and in Table I.

CONCLUSION. A study of Figure 2 reveals that the values for a causticity obtained by this investigator check the values given by Goodwin (11) rather closely. Therefore we may conclude that Goodwin's contention as to the inaccuracy of Lunge's table of causticity is correct, and that these newer values should be used instead.

Apparently i t is not important whether 50 or 5 per cent excess lime is used in the conversion. The period of 2 hours allowed for the establishment of equilibrium seems sufficient for either case.

The values obtained by Lunge (18) follow: % NaZCOs % Conversion % NaaCOs % Conversion

2 99.4 5 99.0 10 97.2 12 96.8

14 94.5 16 93.7 20 90.7

so1ut:on in gams per liter was then converted to per cent sodium carbonate by weight. The lime required was weighed, roughly owdered, and added to the solution in the crucitle. The C. P. grade of lime, sample D, was used. Determina- tions were made with 50, 10, and 5 per cent excess lime over the theoretical amount required. In calculating the excess lime, due allowance was made for the impurities present.

Themixture wasagitatedfor2 hoursat94-95' C. After completion of the run, the mixture was quickly transferred t o a stoppered bottle and ermitted to settle overnight. A sample of clear soKtion was pipetted out and diluted, and aliquot portions were analyzed for total alkali and for sodium hydroxide.

ANALYTICAL METHODS. The simplest method- namely, titrating of the solution with standard acid, using methyl orange and phenolphthalein as indi- cators and calculating the percentage sodium carbonate and sodium hydroxide by the difference in the respective end points-gave high values for the sodium carbonate. Therefore we decided to use the Winkler method. A given ali-

96

92

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February, 1941 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

Temperature Control during Settling Numerous investigators have made

determinations of the rate of settling of calcium hydroxide and carbonate. Two methods have been used. In both methods the sample, which is either calcium hydroxide or calcium carbonate in suspension in water or caustic soda. resoectivelv. is

EXPERIMENTAL METHOD.

PP 43 I I

FIGURE 3. APPARATUS FOR SETTLINQ RATE DETERMINA-

TIONS

transferred to a 160-cc. gAdu- ated cylinder, and the settling rate is timed with a stop watch. Some investigators permit the suspension to cool duringtheex eriment; others (Haslam, Acfams, Kean, I S ) take the precaution of maintaining a constant temperature by immersion of the cylinder in a water bath or thermostatically controlled air chamber.

We decided to determine whether the maintenance of a constant temperature was necessary for the accurate determination of settling rates. A 4-liter beaker served as a water bath and was equipped with a stirrer (Figure 3). A Nichrome heating coil sus- pended on the bottom of the beaker was connected in series with a thermoregulator. A condenser was connected across the terminals of the regulator to prevent excessive sparking at the contact points. The source of cur- rent was the 110-volt a. c. electric service line. This apparatus permitted maintenance of a constant tem- Derature within 4' C. and was

used in this form during the earlier- experiments only. Later a better type thermoregulator was employed which permitted a temperature regulation of 0.5' C.

We ran the first settling tests on the calcium hydroxide in water suspension, rather than on the calcium carbonate obtained after causticization, because of the greater ease with which the hydroxide could be prepared. This decision was justified by the conclusions of Piper (26) who states: "Experience shows that the particle size of the hydrate persists throughout the causticizing stage." It was assumed, therefore, for the time being that the settling rates of the calcium hydroxide would be similar to the settling rate of the calcium carbonate which would be obtained from the same hydroxide after the causticization was com leted. This assumption was found to be correct in later worf

Whereas' it is usual practice in these settling tests to make a suspension having a total volume of 100 cc., it was decided to work with a volume of only 80 cc. because of the limited size of the 4-liter water bath. Using this smaller volume, the 80 oc. mark on the 100-cc. cylinder was below the water level of the bath. Since the concentration of soda ash solution for causticization is about 12 per cent sodium carbonate in general plant practice, the amount of lime required to causticize 80 cc. of 12 per cent soda ash solution was calculated and found to be 6.5 grams of lime, which supplied an excess of 3 per cent over the theoretical amount required.

EXPERIMENT 1. Two 6-gram samples of lime C were weighed into two 250-cc. beakers, and 30 cc. of water at 20' C. were added. This amount of water is fifteen times the theoretical amount of water required for slaking. The samples were stirred gently for 15 minutes and at the same time heated up to about 90" C. in a water bath. At the end of this time they were transferred to 100-cc. cylinders, diluted with hot water to 80 cc., mixed well, and permitted to settle. Whereas one of the cylinders was immersed in the constant- temperature bath at 91" C., the second cylinder was placed on the laboratory table and permitted to cool. The extent of settling of each suspension was noted at given intervals, using a stop watch for correct timing. The results of this first run are represented by curves A1 and Az on Figure 4. Obviously

207

the rate of settling is influenced by the temperature during the settling period; the suspension which was kept at 91" C. settled much faster than the one which was permitted to cool. Two more runs were made during the day to confirm this conclusion. One of them is represented on Figure 4 b y curves B1 and Bz. I n each case there is a marked difference in the rate of settling between the sample maintained at 91 O C. and the sample which was permitted to cool during settling.

CONCLUSION. On the basis of experiment 1, i t may be con- cluded that a settling test must be carried out at constant temperature (preferably about 91' C.) which most closely corresponds to the temperature that would be encountered in plant practice. Settling tests which neglect this important requirement are of no value for comparison.

The Effect of Weathering or Exposure of Quicklime before Slaking

Whereas test runs AI, Az, B1, BZ (Figure 4) confirmed without exception the conclusion reached with respect to the necessity of maintaining constant temperature during the settling test, the plotted data of these runs show that the tests carried out at a constant temperature did not indicate the same rate of settling. Since all the samples were weighed from the same bottle containing the same ground quicklime and were consequently slaked in exactly the same manner, i t became obvious that an unknown factor was responsible for the divergent results; this made further investigation necessary.

TIME OF SETTLING, MINUTES

SETPLINQ (LIME SAMPLE C) FIQURE 4. EFFECT OF COOLING ON THE RATE OF

A careful check of the steps involved in the preceding experiment showed the following: The sample of lime C was in the form of big pebbles about 1 inch (2.5 cm.) in size. To obtain representative samples for the experiments, the pebbles were ground in a mortar and the powdered material was transferred to a wide-necked bottle superficially closed with a watch glass. This was done early in the morning, and the


three different runs were made during the day; the last was completed a t 9 P. AI. Hence, whereas the first run was conducted on a sample weighed out just after grinding, the last run was made on a sample which had been standing in the loosely closed bottle for nearly 12 hours. The next step was to find out if the standing and exposure of the quicklime was responsible for the wide difference in the results of the settling tests.

LT w 0 z -1 2.

0 0

I 0

- 0.

2 z 0

v)

zs 0

c U 3 a 4 a:

cl z

-1 t- +. id VJ

-

‘? -

T I M E O F SETTLING, MINUTES FIQURE 5 . EFFECT OF TIME OF EXPOSURE OF QUICKLIME TO THE AIR, BEFORE SLAKING, ON THE SETTLING RATE OF THE HYDRATE (LIME C)

EXPERIMENT 2. For this purpose a new quantity of the same quicklime was ground and immediately transferred to a tightly stoppered bottle. Six 6.5-gram samples were weighed out a t approximately the same time and transferred to six 250-cc. beakers, numbered 1 to 6. Number 1 was slaked immediately, in the manner previously described, for 15 minutes and simultaneously heated, diluted to 80 cc., and trsns- ferred to the settling cylinder; the settling rate of the suspension was recorded. Samples 2 to 6 were permitted t o stand, evenly spread over the bottoms of the beakers, which were covered with a watch glass. I n turn, these samples were slaked after standing for 45 minutes, 1.75 hours, 3.75 hours, 5.75 hours, and 24 hours, respectively. The slaking tests were carried out identically in each case.

Obviously, the time of exposure of the quicklime has a decided influence on the settling rate of the slaked lime; this is most marked in the first

The results are shown on Figure 5 .

2 or3 hours.

ture and carbon dioxide during standing. As may be expected, the samples of quicklime absorb mois-

To ascertain the .-~.

amounts absorbed, a seventh sample 0: the quicklime had been weighed out along with samples 1 to 6, transferred to a 250-cc. beaker, spread evenly on the bottom of the beaker (covered with a watch glass), and allowed to stand for the duration of the test. The beaker was weighed from time to time, and the increase in weight noted. Figure 6 shows the percentage increase in weight plotted against time of exposure.

The increase in weight after 45 minutes was only 0.75 per oent. A comparison with the rate of settling after these respective periods of exposure shows the effect on the rate of settling produced by

After 1.75 hours i t was 1.25 per cent.

the absorption of such relatively small amounts of water and carbon dioxide.

Quicklime must absorb 32.0 per cent of its weight in water for complete hydration, and 78.6 per cent of its weight in carbon dioxide for complete conversion to the carbonate. Therefore i t is reasonable to state that the amount of absorption by the samples of quicklime exposed for the time periods of this experiment is very small, and that any hydration or carbonation caused by this absorption must of necessity be superficial. K e conclude, therefore, that during the exposure of the samples to the atmosphere a partial hydration and carbonation of the lime takes place, but only on the surface of the particles, and that this surface reaction is responsible for the change in the settling rate of the slaked lime. Microscopic inspection revealed that the slaked particles of sample 1 were very small, whereas samples 2 to 6 showed much larger particles which appeared to be more or less in cluster form.

Another difference in the behavior of the six samples was noted during the slaking period. This particular brand (Wash- ington pebble lime) is a highly reactive quicklime. Khen water is added to a pebble, steam is evolved immediately and the lime swells to a considerable extent. When sample 1 was slaked with the usual amount of water, the temperature of the mixture immediately rose to the boiling point. In the case of sample 2, about 15 seconds elapsed before the rise in temperature took place. With sample 3 nearly a minute passed, and so forth. Sample 6, which had been exposed for 24 hours, did not show any temperature rise upon slaking until after 15 minutes of contact with the water.

Apparently the surface hydration and carbonation form a protective layer of calcium hydroxide and carbonate around the particle, which is penetrated by the slaking water with

d i f f i c u l t y a n d o n l v a f t e r a

TIME, H O U R S

period of time, depending on the thickness of this layer. Just why the particles of s l a k e d l i m e should increase in size with time exposure of the quicklime is a s u b j e c t f o r further investigation. At any rate, this phe- nomenon must be taken into account when con- d u c t i n g com- -

FIGURE 6. INCRE.4SE IN WEIGHT V S . parable settling TIME OF QUICKLIME EXPOSED TO THE rnte tpst,s. . .. . - . . . .

A t h o r o u g h s e a r c h o f t h e

ATMOSPHERn Lime sample C. room temperature 23’ C.;

relative’humidity. 20 per oedt literature back to 1925 indicates

that this particular behavior of lime was not known to other investigatbrs.

To ascertain whether this behavior of the lime was a characteristic Deculiar to the samole used through-

EXPERIMENT 3.

out these experiments (sample C), similar kxperiments were conducted on the other two brands available-pebble lime B from Knoxville, Tenn., and pebble lime A from Martinsburg, Va., respectively; the results are plotted on Figure 7. The similar results obtained seem to indicate that the effect on the settling rate of slaked lime due to the exposure of the

February, 1941 I N D U S T R I A L A N D

A . N O EXPOSURE I- 2 70 cl B. 3 I / 2 -HR.EXPOSURE - p: m

W r

-I c I- W rn

-

FIGURE 7. EFFECT OF EXPOSURE OF QUICKLIME TO AIR, BEFORE SLAKING, ON SETTLING RATE OF

HYDRATE

quicklime before slaking is a general characteristic of lime. The curves 5 and 6 show that the effect is less pronounced in the case of sample B. This lime was less reactive and slower in slaking than the other two investigated.

The photomicrographs of Figure 8 illustrate the effect of exposure to the air before slaking on the particle size of the resulting slaked lime.

Effect of Particle Size

In view of the conflicting statements in the literature, it was decided to investigate the relation between the particle size of the quicklime, and of the slaked lime after hydration and the calcium carbonate after oausticization, and their respective settling rates.

Knowles (16) states: “No relation exists between the particle size of the CaO and the particle size of the resulting Ca(OH)% or CaCOs produced by :slaking or by slaking and caustioizing.”

Adams (2) supplies the following in- formation: “The size of hydrate particles decreases with the diameter of the quicklime particles from which produced. A

E N G I N E E R I N G C H E M I S T R Y

more reactive hydrate with lower settling rate is obtained from a finely ground quicklime.”

EXPERIMENT 4. A quantity of lime (sample A ) was ground in a mortar and passed through a 60- and a 150-mesh screen. The three p o r t i o ns-name1 y , retained o n 60-mesh s c r e e n .

209

-- through 60- but retained on 150-mesh, and through 150-mesh- were kept in well-stoppered bottles. Samples of each portion (6.5 grams) were slaked as described previously with 30 cc. of water a t 20” C., and the settling rates of the resulting hydrates were observed. These samples were not exposed to the atmosphere before slaking. The results are shown on the upper graph of Figure 9; the same experiment was repeated on the following day and the results are plotted on the lower graph.

No definite conclusion can be drawn from the results of these two experiments. One definite fact may be noted. Four different settling tests were performed with that portion which passed the 150-mesh screen, and the settling rates do not check so closely as might be expected from the fact that all four tests were conducted in an identical manner. To show the extent of these deviations, the four curves were replotted; there are two pairs of two curves each which re- semble each other. These pairs were formed by results obtained on different days. No explanation of these irregulari- ties could be found.

To obtain more conclusive results, the settling tests on hydrates produced from different particle size quicklime samples were repeated in experiment 5.

A fresh sample of lime A was powdered and passed through a set of screens-namely, 20-, 40-, BO-, 80-, loo-, 150-, and 200-mesh. The resulting eight portions

EXPERIMENT 5.

THREE CAUSTICIZING AGITATORS AT UPPER RIGHT AND Two BALANCED- TRAY THICKENERS FOR LIME MUD WASHING IN THE BACKGROUND, IN A DORR

CONTINUOUS RECAUSTICIZING SYSTEM

210 Vol. 33, No. 2 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

I I1 111

I V V V I

VI1 VI11

FIGURE 8. EFFECT O F EXPOSURE TO AIR BEFORE SLAKING ON PARTICLE SIZE OF SLAKED LIME

1-111.

11. 111.

IV-VIII. IV. Quicklime before slaking. V.

VI. VII.

VIII.

Lime C, through 60, on SO mesh: slaked with 10 times the theoretical amount of water:

rlfter exposure of 21/2 hours before slaking (note increase in particle size). After exposure of 24 hours before slaking (note formation of large clusters).

I. Without exposure before slaking.

Lime A ; through 60, on SO mesh; except IV, all were slaked with 10 times the theoretical amount of water:

Slaked lime with no exposure before slaking. ll/p-Hour exposure before slaking; shows the beginning of cluster formation. &Hour exposure before slaking: very large clusters have formed but a large number of small particles are still present. 24-Hour exposure before slaking: though the particles are not so large as those shown in VI, the ratio of large t o small particles has

increased greatly.

werertransferred to stoppered bottles, again avoiding exposure to the atmosphere. The slaking and settling tests were carried out as before.

The settling rates are represented by the upper eight curves of Figure 10. The results are again such as not to permit of any definite conclusions. If anything, the contention of Adams

(2) that “the size of the hydrate particles decreases with the diameter of the quicklime particles from which produced” appears to be incorrect; for in this experiment the sample passing the 200-mesh screen has the highest settling rate, whereas the sample retained on the 20-mesh screen shows the lowest rate.

February, 1941 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 211

FIQURE 9. EFFECT OF THE PARTICLE SIZE OF QUICKLIME ON SETTLING) RATE OF HYDRATE ON

Two SUCCESSIVE DAYS

These results seem to be in direct opposition to Adams' results (2). An explanation why the coarsest particles of quicklime form the finest particles of slaked lime may be found on further research.

We realize that no conclusions should be drawn until after the experimental technique has been refined to a point where i t is possible to duplicate the settling rate of a given sample of lime a t any time. The first step in this direction seemed to be a closer temperature regulation during the settling test. In all the experiments carried out so far, the temperature regulation was = t 4 O C.

For this purpose a more sensitive thermoregulator was ac- quired. A high-speed stirrer was employed for circulating the water in the bath, and the water bath itself was insulated completely with the exception of a vertical strip which served as an observation window.

Since experiment 5 did not yield very satisfactory data, it was repeated with one change-namely, exposing the quicklime samples before slaking for 2.5 hours. The results are given in Figure 10 (lower curves). They seem to indicate that the smallest quicklime particles yield the fastest settling hydrate. This general rule holds for all particle sizes.

At this point in the investigation i t was decided to extend the settling tests to the carbonate formed after the causticization.

The same kind of lime (sample A ) was used, but it was not exposed to the atmosphere before slaking. The procedure adopted was as follows: 6.5 grams of quicklime were slaked with 30 cc. of water at 20" C. and heated for 10 minutes; 30 cc. of soda ash solution, containing 10.8 grams of sodium carbonate, were added. The mixture was heated to 90' C. and stirred slowly for 15 minutes to ensure

EXPERIMENT 6.

EXPERIMENT 7.

complete causticization. The sample was transferred to a settling test cylinder, diluted to 80 cc. with hot water, and mixed well prior to the settling test.

Unless otherwise noted, this method of slaking and causticizing was adhered to in all experiments dealing with the settling rate of calcium carbonate. The weights were calculated to yield a 12 per cent sodium carbonate solution, with 3 per cent excess of lime over the theoretical amount required. This closely parallels plant practice.

The results of experiment 7 are shown by the upper curves of Figure 11. There is not much difference in the settling rates of the carbonate produced from different size quicklime particles. Moreover, whatever differences exist do not indicate any definite trend. This experiment is in line with experiment 5 , in which the settling rates of the hydrates were determined under the same conditions-namely, no exposure of the samples to the atmosphere before slaking.

EXPERIMENT 8 was run under the same conditions as experiment 7, except that the samples were exposed for 2.5 hours before causticization. The results on the lower graph of Figure 11 demonstrate that the smaller quicklime particles yield not only a faster settling hydrate but also a faster set-

FIQURE 10. EFFECT OF PARTICLE SIZE OF QUICK- LIMB ON RATE OF SETTLING OF HYDRATE

Vol. 33, No, 2 212 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

EXPERIMENT 9. A test was carried out to determine the percentage increase in weight due to absorption of carbon dioxide and water vapor during exposure of the quicklime, in relation to the particle size of the latter. The results given in Table I1 show that the rate of absorption is the same, for all practical purposes, for the different particle sizes of quicklime. An appreciable difference is noticeable only after an exposure of 24 hours. Since all experiments carried out so far in connection with particle size of quicklime had an exposure of not more than 2.5 hours, i t may safely be said that the amount of such absorption was constant for all particle sizes

A . R E T A I N E 0 ON 20 MESH B.THROUGH 20, ON 40 C.THROUGH 40, ON 60 0.THROUGH 6 0 , ON 80

.x

0 z A encountered.

0 0 TABLE 11. RATE OF ABSORPTION OF WATER AXD CARBON

2

w

>. U

I DIOXIDE OF LIME EXPOSED TO ATMOSPHERE 0 (Lime sample A , room temperature, 22’ C.; relative humidity, 21%)

Particle Size, Mesh % Absorbed after Exposure of: Retained

z 0

Through: by: 1 hr. 2 hr. 3.75 hr. 6 hr. 24 hr. w z . . . 20 0.20 0.32 0.40 0.60 1.81

20 40 0.16 0.25 0.39 0.58 1.98 0

40 60 0.12 0.22 0.34 0.52 1.97 c 4 60 80 0.10 0.20 0.35 0.52 2.12

SO 100 0.14 0.21 0.35 0.52 2.28 3

100 150 0.15 0.26 0.46 0.65 2.54 U U 0: 150 200 0.08 0.16 0.32 0.53 2.32 a 200 . . . 0.14 0.27 0.41 0.63 2.36 m t z -I

c w

-

- c w

‘,.At this point i t was decided to find whether the time allowed for causticization was of sufficient length and whether the degree of conversion of the soda ash depended on the particle size of the lime. It was found experimentally that the time allowed was sufficient and that the conversion was fairly constant for all particle sizes:

Conversion by inkler Method

92.5 40 80.2 60 90.7

T I M E OF S E T T L I N G , M I N U T E S 80 90.6 100 93.2 150 92.3 200 92.8

Average 91.8

Particle Size, Mesh ?& Retained on 20

FIQURE 11. EFFECT OF QUICKLIME PARTICLE SIZE ON SETTLING RATE O F CALCIUkf CARBONATE Through 200 91.9

OBTAINED AFTER SLAKING AND CAUSTICIZING

tling carbonate. This result was to be expected from the data of experiment 6.

CONCLUSIONS. The statements of neither Knowles (16) nor Adams (2) appear to be quite correct. We have shown that under certain conditions a definite relation exists between particle size of the quicklime and particle size of the hydrate and carbonate resulting from the slaking and causticizing of the quicklime, as evidenced by the settling rates of the hydrate and carbonate.

The experiments carried out without exposure before slaking refute Adams’ claim that the size of the hydrate particles decreases with the size of the quicklime particles from which they were produced. These experiments would rather sup- port Knowles’ contention that there is no relation between the particle size of quicklime and the particle size of the hydrate produced therefrom. However, the experiments conducted on quicklime which had been exposed before slaking show conclusively that under certain conditions (namely, after exposure of the quicklime to the atmosphere) the particle size of the quicklime bears a direct relation to the settling rate of the hydrate and Carbonate and their respective particle sizes.

This relation may be formulated as follows: The size of hydrate or of carbonate particles increases as the size of the quicklime particles decreases. A higher settling rate is obtained from a finely ground quicklime. This contention is in direct contradiction to Adams’ findings, nor does i t agree with those of Knowles.

The average of 91.8 per cent agrees fairly well with the maximum possible conversion of 94.4 per cent for a 12 per cent sodium carbonate solution, which is the strength used in these experiments, The higher conversion of 94.4 per cent was obtained under ideal conditions for the establishment of equilibrium, as described previously.

Effect of the Amount of Water Used for Slaking Many investigators have studied the effect of the amount

of water used for slaking on the settling rate of the hydrate and of the carbonate obtained therefrom after causticization. Dorr and Bull (10) determined the settling rate of the carbonate after causticization and obtained a settling rate of 0.16 foot per hour, using thirteen times the theoretical amount of water in the slaking process, and a settling rate of 1.41 foot per hour, using only twice the theoretical amount of water.

Knowles (16) states that “the use of the theoretical amount of water required for slaking yields a readily settleable product, and after causticization, a readily settleable carbonate.”

Piper (26) reports: “Milk of lime prepared by slaking the lime with all the water available yields, after causticization, an extremely finely divided, dispersed precipitate which settles a t the low rate of 0.16, 0.19, and 0.28 foot per hour. When the above procedure was modified slightly, by slaking in just enough water to produce the dry hydrate and then repulping the dry hydrate with the balance of the water before causticizing, the settling rate of the calcium carbonate was acceler- ated to 1.41, 4.06, and 1.48 feet per hour in three cases.”


Bonnell (7) says that “excess water is required in the slaking process in order to obtain fine particles. Coarse agglomerations are obtained when using the theoretical amount of water.”

Whitman and Davis (37) come to the following conclusion: “An excess of water increases the fineness of the hydrate; a n increase of from five to twenty times the theoretical amount of water increases the fineness only a little. Boil- ing increases the fineness markedly. An increase in temperature increases the fineness slightly, when an excess of water is employed. Vapor hydration yields about the same results as water hydration. The reaction rate is a function of the surface area, while the settling rate is determined by the size of agglomerates of the particles. Thus, a hydrate con- sisting of very fine particles might react rapidly with acid, but these fine particles might also be clustered in lumps which settle rapidly. Excess water separates the individual crys- tals and decreases the tendency for agglomeration. Excess water also prevents local overheating and increases the rate of hydration.”

The unanimous finding of the different investigators seems to be that an excess of water produces fine hydrate particles.

A . T H E O R E T I C A L AMOUNT

B.50R EXCESS OVER

FIQURE 12. EFFECT OF AMOUNT OF WATER USBID IN SLAKINQ ON SETTLINQ RATE OF HYDRATE

All samples through 40, on 60 mesh

A check of this theory was conducted, and the results are set forth below. In view of the fact that “the particle size of the hydrate persists throughout the causticizing stage” (66) had been found correct in our own experimental work to date, we decided to determine experimentally the rate of settling of the hydrate us. the amount of water used in the slaking process. The rate of settling of the carbonate was studied later. It was anticipated that the results obtained with the settling rates of the hydrate would be directly proportional to the results which would be obtained in the settling rates of the carbonate. This method has two advantages: First, any differences in the settling rate show up much more clearly during the settling of the hydrate, because the settling takes place in a solution of lower viscosity. Secondly, the introduction of unknown factors due to method of causticizing is avoided.

EXPERIMENT 10 was conducted on a sample of lime A , ground to pass the 40-mesh screen but retained by the 60- mesh screen. The sample was exposed to the air 2.5 hours prior to slaking. The theoretical amount of water, as used in this paper, refers to that amount which is theoretically required for complete hydration according to the equation: CaO + HzO + Ca(OH)2. A certain amount of water is always lost during the slaking period because of evaporation as the lime becomes hot.

One sample was hydrated with the theoretical amount of water, the other with 50 per cent excess water over the theoretical amount. The results (Figure 12, upper curves) indicate that a faster settling lime hydrate was obtained from the sample slaked with 50 per cent excess water. This result is not in accordance with the findings of other investigators.

EXPERIMENT 11 was run on samples similar to those in experiment 10, but in this case ten times and four times the theoretical amount of water were used, respectively. At the same time, we decided to find out whether exposure before slaking would make any difference and if such a difference could be determined experimentally. Hence two of the samples were exposed to the air for 2.5 hours; the other two samples were not exposed.

The results are plotted on the lower graph of Figure 12. The sample slaked with four times the theoretical amount of water settled much more rapidly than the sample slaked with ten times the theoretical amount. The curves also indicate that in each case the sample exposed before slaking for 2.5 hours had a higher settling rate than the unexposed sample. The difference, however, is not nearly so great as that caused by different amounts of water. Nevertheless, even under these conditions the exposure factor is important.

The results of experiments 10 and 11 reveal that the use of only four times the theoretical amount of water yields a faster settling hydrate than that obtained by ten times the theoretical amount. On the other hand, the use of 50 per cent excess over the theoretical amount of water yields a faster settling hydrate than that from the theoretical amount only. In other words, the settling rate of the hydrate is not necessarily a direct function of the amount of water used in the slaking process.

EXPERIMENT 12 was performed with eight samples of lime A . All samples were ground to pass the 20-mesh and be retained by the 40-mesh screen; they were not exposed before slaking. The amount of water used.in the slaking process varied from the theoretical amount to twenty times the theoretical. The results are given on Figure 13 (upper curves).

These results confirm the statement made in the preceding

Vol. 33, No. 2 214 I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

T I M E O F S E T T L I N G , MINUTES

FIGURE 13. EFFECT OF AMOEXT OF WATER USED IN SLAKING AND OF EXPOSURE TO AIR BEFORE SLAKING, ON SETTLING RATE OF

HYDRATE All samples through 20, on 40 mesh

section., The highest settling rate was obtained by using twice the theoretical amount of water; the next highest by four times the theoretical amount; the third by one and a half times the theoretical amount; whereas the sample slaked with the theoretical amount of water only is in fourth place. An increase of water over four times the theoretical amount, however, yields progressively lower settling rates, with the exception of the samples slaked with six times and with ten times the theoretical amount, which have reversed positions on the graph. The difference in the settling rates of these two samples is not great, however, and does not refute the fact tha t slower settling hydrates are obtained by slaking the lime with progressively larger amounts of water, provided a certain minimum amount (in this case twice the theoretical) is present already.

In general, our findings agree with those of other investigators, except that the use of the theoretical amount of water only does not produce the fastest settling hydrate. The fastest settling hydrate is obtained by using an excess over

TIME O F S E T T L I N G , MINUTES

FIGURE 14. EFFECT OF AMOUST OF WATER USED IN SLAKING AND OF EXPOSURE TO AIR BEFORE SLAKISG ON SETTLING RATE

OF CALCIUM CARBONATE AFTER CAUSTICIZATIOX

All samples through 20, on 40 mesh

the theoretical amount, which in this case is twice the theoretical. This required excess may vary with different samples of lime tested. When twice the theoretical amount of water is used, the slaked lime is dry. Using four times the theoretical amount, a damp-dry hydrate is obtained; using six times the theoretical amount, a stiff paste results; and when using ten times the theoretical amount, a wet paste is obtained. The use of more water results in excess liquid after hydration is completed.

EXPERIMENT 13. Although experiment 11 showed that the effect of exposure of the quicklime to the air before slaking is noticeable under varying methods of hydration, another test was made to confirm this effect. Two samples of the same lime used in experiment 12 were slaked with that amount of water which produces the fastest settling hydrate-namely, twice the theoretical. One sample was exposed for 2 hours to the air before slaking, the other was not exposed.

The settling rates are plotted on Figure 13 (lower curves). It is evident that even under the most favorable conditions

February, 1941 I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

with respect to the amount of water used in slaking, the settling rate is influeneed by the presence or absence of a p r e vious period of exposure of the quicklime.

To confirm the assunlption that the “particle size of the hydrate persists throughout the causticizing stage” (96), and to show that the experiments carried out so far on the effect of amount of water used in slaking on the settling rate of the hydrate are of value in determining the corresponding settling rates of the carbonate after causticization, a few tests were made on the carbonate.

Six samples of lime A (through 20-mesh retained by 40-mesh screen) were slaked with different amounts of water, ranging from theoretical to twenty times the theoretical. After slaking, the samples were repulped with additional water and then mixed with the amount of sodium carbonate solution which was calculated to leave an excess of 3 per cent lime over that required for complete conversion. The samples were not exposed before slaking.

The results are plotted in the upper graph of Figure 14 and confirm the deductions made from the settling tests of the hydrates with respect to the amount of water used in the slaking process. The fastest settling carbonate is obtained by slaking the lime with twice the theoretical amount of water; the next fastest by using four times the theoretical amount; the third by using one and a half times the theoretical amount; the fourth by the theoretical amount only. An excess of water over twice the theoretical amount yields progressively slower settling carbonates. The results are in agreement with those of experiments 10, 11, and 12.

EXPERIMENT 15 was conducted to confirm the fact that exposure of the quicklime before slaking will influence the settling rate of the calcium carbonate produced by slaking and causticizing. Two samples of lime A (through 40-mesh

EXPERIMENT 14.

screen, retained by 60-mesh screen) were slaked with four times the theoretical amount of water and subsequently causticized as described above. One sample had been exposed to the air before slaking, the other had not.

The result of the settling test is shown on the lower graph of

215

Figure 14. The obvious conclusion is that, other factors being equal, the sample of lime which had been exposed before slaking yielded the faster settling carbonate, even under conditions most favorable to the settling with respect to the amount of water used in the slaking process, CONCLUSIONS. The results confirm the fact that exposure

before slaking has a dehi te influence on the settling rate of the hydrate and on the settling rate of the carbonate produced therefrom. Increasing amounts of water used in the slaking process, provided they are higher than a certain necessary minimum amount which is higher than the theoretical, will yield a slower settling hydrate and carbonate.

Effect of Slaking Lime with Sodium Carbonate

The effect of using soda ash solutions instead of pure water for the slaking process has been investigated by various work- ers. Knowles (16) states that “lime slaked in soda ash liquor settles much more rapidly than lime slaked in excess fresh water and, accordingly, the calcium carbonate produced has a better settling rate. A sample of lime slaked in excess water showed a settling rate of 1.48 feet per hour, whereas the same sample slaked in 15 per cent sodium carbonate solution had a

Two ROTARY LIME SLAKERS AND A BOWL CLASSIFIER, PREPARING AND REMOVIKQ GRIT FROM MILK OF LIME, IN A DORR CONTINU-

OUS RECAUSTICIZING SYSTEM

- settling rate of 2.92 feet per hour.” Harrup and Forrest (12) state that “slaking the lime in water before causticization reduced the settling rate of the calcium carbonate from 0.53 to 0.11 inch per minute, but increased the rate of conversion.”

Anable and Knowles (4) find that “lime should be slaked in green liquor with soluble salts in solution, and that lime should be slaked in a low dilution, but not with all of the green liquor. Note: Green liquor is a solution of the product of the smelter, in which the concentrated waste liquors are thickened to dryness. Green liquor consists mainly of sodium carbonate in solution, with iron sulfides and other impurities.” Piper ($66) recommends the slaking of the lime with the theoretical amount of weak liquor to form the hydrate, and repulping with the balance of weak liquor. Dorr and Bull (IO) find that “slaking with 14.8” BB. caustic soda solution yields an extremely fast-settling hydrate.”

The purpose of these experiments was to determine the effect on the settling rate of calcium carbonate of employing sodium carbonate solutions in the slaking process.

EXPERIMENT 16. 4 1 the variables affecting the settling rate of the carbonate were kept constant: temperature during slaking, time and method of slaking, exposure before slaking, particle size of quicklime, and amount of water present during slaking. The last variable is probably the most important.

This experiment deals with the changes in the settling rate produced by slaking the lime with soda ash solutions of varying concentrations. The least amount of water was used which is needed to assure good mix- ing of the quicklime and the slaking solution during slaking; according to previous experience, this is about four times the theoretical. Since this amount of


THROUGH 150, ON 200

FIGURE 15. EFFECT OF SODA ASH SLAKING ON SETTLING RATE O F CALCIUM CARBONATE OB-

No exposure before slaking TAINED BEFORE CAUSTICIZATION

water produces a dry hydrate, the latter was repulped with a solution of sodium carbonate of the same concentration as that employed in slaking, prior to the final causticization. The volume of slaking solution during the slaking process was maintained constant for all samples. Lime sample A was used throughout these experiments. The procedure was as follows:

8.4 cc. of the sodium carbonate solution of desired strength, at 20' C., are added to 6.5 grams of the quicklime sample of a given particle size. The mixture is stirred gently and slaking is permitted to proceed for 10 minutes.

The dry slaked lime is repulped with 25 cc. of sodium carbonate solution of the same concentration as that employed in slakinz. The mixture is warmed to 90" C.

The results are plotted on the upper graph of Figure 15. The conclusion is that a faster settling carbonate is obtained when the lime is slaked with pure water and that the settling rate of the calcium carbonate appears t o be inversely proportional to the concentration of sodium carbonate in the slaking liquor.

Since these results contradict the findings of other investigators, i t was thought that they might have been due to the particular conditions under which our experiment was conducted. The variable might have been the amount of water present during slaking. The amount used in slaking in plant practice should be sufficient to produce easy flow of the slaked lime from the hydrator. The amount of water used in this experiment was approximately four times the theoretical amount, which produced a dry hydrate. It is possible that

T I M E OF S E T T L I N G , MINUTES

FIGURE 16. COMPARISON OF SODA ASH AND WATER SLAKINQ IN THE PRESENCE OF DIF-

FERENT AMOUNTS OF WATER

slakef'with: No exposure before slakin through 10. on 20 mesh;

A Water 4 X theor. Ha0 B 8% NaaCOs soln. 4 X theor. H20 C Water 8 X theor. Ha0 D S% NarCOa soln. 8 X theor. Hz0

the experiments of the other investigators were carried out in the presence of a larger amount of water, which more closely approaches plant practice. Therefore experiment 16 was repeated but a different amount of water was used in the slaking procedure.

Eight times the theoretical amount of water was used in slaking; a fairly free-flowing sludge re- sulted. All other conditions were the same as in experiment 16. The results are given on the lower graph of Figure 15; they seem to be the reverse of the results obtained in experiment 16 and are in accord with the results of other investigators. These data show that the rate of settling of the carbonate is directly moportional to the concentration of sodium

EXPERIMENT 17.

Thcremainder of the soda ash solution required for complete causticization is added; 15 minutes are allowed for the completion of the reaction while the mixture is gently stirred and maintained at 90" C. in a water bath.

The mixture is transferred to the settling cylinder, diluted to 80 CC. with hot water, and mixed well, and the settling rate is

carbonate in thLsfakfng liquor. to depend On the

amount of water present during slaking. Since the results of experiments 16 and 17 do not permit a definite conclusion without further verification, another experiment was carried

The effect Of soda ash

observed.

per cent sodium carbonate. The concentrations used were pure water, and 1, 4, 8, and 12 Out.

EXPERIMENT 18. A sample of much larger particle size


lime was employed. Lime A , through 10-mesh but retained by 20-mesh screen, was used.

The samples were slaked with four times and with eight times the theoretical amount of water. In one case water was used for slaking, in the other, a solution of sodium carbonate of 8 per cent strength.

I n the presence of four times the theoretical amount of water, water slaking gave a faster settling carbonate than that produced by slaking with an 8 per cent soda ash solution. However, when eight times the theoretical amount of water was present, the carbonate produced by slaking with the 8 per cent soda ash solution had a higher settling rate than that obtained by slaking with water.

The use of soda ash in the slaking solution may or may not produce a faster settling carbonate, depending on the amount of water present during slaking. This conclusion is an elaboration of the findings of previous investigators.

The results are given on Figure 16.

CONCLUSION.

Rate of Reaction of Quicklime with Water

Differences in the treatment of the quicklime before slaking and in the method of slaking result in differences in the rate of reaction of the lime with water. The purpose of this experiment was to obtain a t least a rough idea of the actual causes and determine the difference in rate of reaction in a qualitative manner.

Harrup and Forrest (12) state that slaking the lime in water before causticizing reduced the settling rate of the calcium carbonate but increased the rate of conversion. Whitman and Davis (37) found that hydration in a large excess of boiling water assures the most reactive hydrate. Dorr and Bull (IO) conducted the following experiment: 125 grams of lime were slaked with 47 cc. of 14.8" BB. caustic soda. The lime took longer to reach maximum temperature than when slaked with water.

I n order to obtain some infor- mation as to the rate of reaction, 3 grams of lime were placed in a test tube and slaked as outlined below. The temperature was measured, by a thermometer, and the time required to reach the maximum temperature was noted. All samples were lime A, through 60- and retained on 80-mesh screen, unless otherwise noted.

EXPERIMENT 19. Data on the effect of time of exposure before slaking on the reaction rate, with four times the theoretical amount of water, are as follows:

EXPERIMENTAL METHOD.

Time of Time to Reach Exposure Max. Temp. Max. Temp.

Hours c. Min.:aec. 0 1 2

101 101 101

0:30 1:05 1:46

The effect of partial size of the quicklime on the reaction rate, with four times the theoretical amount of water, is shown by the following: .

Time t o Reach Particle Sine Max. Temp. Max. Temp.

Mesh. 0 c. Seconds Through 30, on 40 101 30 Through 200 101 15

The effect of slaking with soda ash solutions on the rate of reaction is as follows:

8 x Theoretical Amount of 4 X Theoretical Amount of

S o h Time t o reach Time t o reaah yo NanCOs Max. temp. mas. temp. Max. temp. max. temp.

S 1 a k i D g Water Water

0 c. Min.:aec. c. Min.:sec. 0 101 0:30 101 0:30 4 90 6:O 101 6:O 8 90 12:o 101 12:o

12 75 21:o 101 1 7 : O

The results are self-explana- tory. In summary, the time required to reach maximum temperature is increased by exposure before slaking and, in a larger degree, by slaking with soda ash solutions. I n the latter case it is probable that as the soda ash solution comes in contact with the quicklime particles during the slaking process, i t forms a layer of calcium carbonate on the surface of the particles; this layer is penetrated with difficulty by the slaking solution and only after a certain time has elapsed. This time increases with the concentration of sodium carbonate in the slaking solution.

Summary and Conclusions The results obtained in a study of the behavior of lime, as

used in the causticizing process for the manufacture of sodium hydroxide, have been recorded. They include an experimental determination of the equilibrium in the reaction, NaPCOt + Ca(OH)* $ 2NaOH + CaCOa. The results prove that the values given in Lunge's table of causticity (18) are inaccurate and that the values obtained by Goodwin (11) are correct.

It has been shown that settling tests must be conducted a t a constant temperature and that failure to do so will give meaningless results.

A new factor has been found-the effect of exposure to the air of the quicklime before slaking on the particle size and on the settling rate of the hydrate and carbonate. This factor which was not known to other investigators, has a marked influence and persists under all methods of slaking.

The relation between the particle size of the quicklime and the particle size of the slaked lime and of the carbonate produced therefrom, and the settling rates of the latter two, has been investigated. The size of hydrate or carbonate particles increases as the size of the quicklime particles decreases. This effect is more or less pronounced, depending on the method of slaking employed. The findings of the authors are a t variance with those of other investigators.

The effect of using excess water in the slaking process has been studied. The claim of other investigators that the use of the theoretical amount of water produces the fastest settling hydrate or carbonate, and that successive increases in the amount of water used caused successively finer hydrate or carbonate particles has been found to be in need of modification. Our experimental data show that the use of the theoretical amount of water does not produce the fastest settling hydrate or carbonate. Rather, i t was found that the fastest settling hydrate or carbonate is obtained by using an excess over the theoretical amount which was twice the theoretical amount. An excess over this amount was found to produce finer particles, in proportion to the amount of water used.

The claim of others that the relative particle size of the hydrate persists throughout the causticizing stage, has been found to be correct.

The effect of using sodium carbonate solutions instead of water in the slaking process has been investigated. The accepted theory that soda ash slaking produces faster settling hydrate or carbonate is correct, with the modification that, depending upon the amount of water present during slaking, the effect may be reversed. Soda ash slaking produced faster settling hydrate and carbonate if eight times the theoretical amount of water was present. The reverse effect was found in the presence of only four times the theoretical amount of water.

218 I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY Vol. 33, No. 2

A study of the reaction rate of lime with water was made. The rate of reaction decreases with time of exposure of the quicklime before slaking, with an increase in the particle size of the quicklime and with an increase in the concentration of sodium carbonate in the slaking water.

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(26A) Piper, W. E., personal comm;nication, 1932. '

Pozin, M. E., J . Chem. Ind. (U. S . S . R.), 12, 43-5 (1934). Ray, K. W., and Mathers, C. F., IND. ENG. CHEM., 20, 475-7

(1928). Rogers, J. S., Ibid., 20, 1355-6 (1928). Shaw, W. M., MacIntire, W. H., and Underwood, J. E., Ibid.,

Ibid.. 20. 315-19 11928). 20, 312-14 (1928).

Squire, M. E., Rock Pioducts, 39, 57-60 (1936). Stockett, Paper Ind., 17, 652-4 (1935). Ullmann, Enzyklopaedie der technischen Chemie, 2nd ed.,

Voronchikhin, V. E., J . Chem. Ind. Moscow, 13, 154-8 (1936). Voronchikhin, V. E., and Plathotnyuk, G. S., Ibid., 10, 33-9

Whitman and Davis, IND. ENC. CHEM., 18, 118 (1926).

Vol. 8, p. 60 (1931).

(1934).

AMINATION IN LIQUID AMMONIA R. NORRIS SHREVE AND D. R. BURTSFIELD'

Purdue University, Lafayette, Ind.

This paper is a continuation of the study of the action of sodium amide on alkyl halides in liquid ammonia, particularly on various amyl chlorides and bromides, as well as on n-hexyl, n-octyl, and n-dodecyl bromides, at -50' C. Amines were obtained as high as 30 to 80 per cent of the halide converted. The by-products were largely olefins, though the carbon balance was not very good owing to the difficulty in the recovery of these volatile materials, on the small scale of these experiments. With the reasonable price now prevailing for liquid ammonia and for sodium, this process promises to be an economical method of making certain amines.

HE reduction in cost of ammonia, together with efficient ammonia recovery systems, has increased the possibilities of the commercial use of liquid ammonia as a reaction

medium. The use of lower alkyl amines in the preparation of rubber chemicals has been important for some time. Re- cently the use of alkyl amines of five or six carbon atoms per molecule in the preparation of pharmaceuticals has stimulated interest for the preparation of primary alkyl amines in good yields from inexpensive intermediates.

Alkyl amines have been prepared by the action of liquid ammonia upon alkyl bromides by Braun (1) who reports a yield

T

of 10 per cent of primary amyl amine and 80 per cent of secondary amyl amine. Octyl bromide gave approximately equal amounts of the primary and secondary amines while n- dodecyl bromide gave a yield of 90 per cent of the primary amine.

Recent work on the reaction between alkyl halides and sodium amide in liquid ammonia was reported by Shreve and Rothenberger (4) and shown to give a larger portion of primary amine. This work has been continued with the object of obtaining a more complete recovery of reaction products. The following over-all reactions are known to take place:

(1) (2)

CsHllBr + NaNH2+ C ~ H X + NaBr + NH3 C6HllBr + A7aNHz + CbHnNH2 + NaBr

2CsHllBr + 2Na5Hz + (CsHll)t;\?TH + 2NaBr + KH3 (3) 3CaHllBr + 3XahH2 + (CgH11)A + 3NaBr + 2"s (4)

The equations do not necessarily account for the mecha- nisms for the reactions. Reactions 3 and 4 have been found to be much more prominent when water is added to the amination mixture following the usual amination process.

Experimental Procedure PREP.4R.4TION OF SODIUM AMIDE. About 240 ml. Of liquid

ammonia were drawn from a 25-pound cylinder into an open De- war beaker. Only commercial grade ammonia was used. The liquid ammonia was then poured through a Pyrex glass funnel into a cold 500-ml. three-neck round-bottom flask which was immersed in a chloroform-carbon tetrachloride mixture containing 50 per cent chloroform by weight and cooled by solid carbon di-

1 Present address, Merck and Company, Ino., Rahway, N. J.

settling rate of calcium carbonate in the causticizing of soda ash

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