I
DONALD E. GARRETT and GERHARD P. ROSENBAUM American Potash & Chemical Corp., Trona, Calif.
CrystaIIization of Borax This systematic experimental study presents results sufficiently defined to serve as a reference for pilot plant tests and future plant design
IN
MANY industrial crystallization processes closely sized and specifically shaped crystals of high purity are required. With well designed equipment and proper process control, the desired product may usually be obtained. However, establishing the optimum conditions and design with a difficult system may require a detailed study of the numerous crystallization variables, especially for the most effective and economical enlargement of an existing plant's capacity. The laboratory study of borax crystallization reported here was undertaken for this reason. The industrial crystallization of borax has been thoroughly described (4, as have the generalities of industrial crystallization theory and practice (2). Buckley has presented numerous examples of how the borax crystal habit may be modified ( I ) , and others (3, .5-7) have discussed the use of additives in
Figure 1.
industrial control of borax supersaturation. Experimental Procedure
Reagent grade sodium tetraborate decahydrate was dissolved in a measured quantity of hot deionized water, stirred, heated to 100' C. (to dissolve all the borax), brought to constant temperature in a 60' C. water bath, and then transferred to an electronically controlled bath a t the crystallization temperature. When the solution had reached lo above crystallization temperature, a stirrer was positioned exactly on the center line of the beaker, and close enough to the bottom to prevent crystals from settling. When the crystallization temperature was reached, seeds were added. Later the resulting crystals were filtered in a Buchner without washing, weighed, reened. Their bulk density was determined, and in some
instances photomicrographs were taken. Two sizes of beakers were used. When 100 ml. of water was added, a 300-ml. beaker about 3 inches in inside diameter was used; when 300 ml. or more water was added, a 1000-ml. beaker about 4 inches in inside diameter was used. The seed addition rate was normally 0.5 gram of reagent grade borax, carefully screened to -100- +140- U. S. Standard mesh, per 250-ml. excess water. Investigators are prone to define crystal structure by some qualitative measure such as shatterability between the thumb nails. In this work, four measurements were used: yield, screen analysis, poured bulk density, and photomicrographs. The yield was measured by weighing the dried crystal crop from each experiment. The screen analysis was run with 10 grams of crystals in U. S. Standard sieves 3 inches in diameter, shaken for 3 minutes on a Tyler vibrator. The poured bulk den-
Trend of changing crystal characteristics with increasing supersaturation was very consistent
0.02 g./100 ml.
1.27 g./lOO ml.
7.93 g./100 mi. VOL. 50, NO. 11
NOVEMBER 1958
1681
0.2%
seed
Figure 2. Improved crystal structure (less agglomeration) occurred with increasing seed concentration 25%
sity was determined by pouring the -20-mesh fraction through a funnel 2 inches in diameter (without stem) into a container exactly 1 cubic inch in size. The weight of the crystals contained in this cubic inch was reported in grams. Photomicrographs were made of the crystals formed in many of the experiments. In some instances a shatterability test was performed. Five grams of -20- f40-mesh crystals were milled for 5 minutes on a 50-mesh U. S. screen 3 inches in diameter with 23 grams of steel balls 1/3z inch in diameter. The 23-gram steel balls covered about nine tenths of the area of the 3-inch diameter screen. The - 50-mesh portion of the product of the milling test was screened on a 80-mesh U. S. sieve. The percentages of the original 5 grams which were milled to -80-mesh and -50- f80-mesh were calculated and reported.
seed
saturations between about 1.80 and 8.0 grams per 100 ml. of water, agglomeration occurred, and the presence of new small crystals was evidenced. With increasing supersaturation the less stepped (230) faces, which are characteristic of tablets, began to grow. At higher supersaturations the crystals appeared to become “chunkier” (more nearly spherical). but the size steadily decreased until finally spontaneous nucleation was excessive and the crystallization was out of control.
Table I. Effect of Stirrer Speed and Size (In all teste, the more vigorous the agitation, the greater yield per unit time and the smaller the crystal produced)
”
Excess Water, M1.
Supersaturation, G./lOO 311.
Stirrer, R.P.M.
Yield,
%”
+20
250
3.78 3.78 3.78 4 . 67h 4 . 67h 4.67b 3.78
36 100 230 300 300 300 220d
38 51 58 50” 4lC 3 7c
39 3 0 1 8 3 1
Discussion of Results Supersaturation. At 30” C., Nith low supersaturation, the crystals formed were single and comparatively large (Figure 1). The highly stepped pyramid faces grew preferentially, so that elongated crystals predominated. At super-
1 682
A number of other series of runs on supersaturation were made at different degrees of agitation, at different temperatures, and with additives present. The general trend of changing crystal characteristics with increasing supersaturation was very consistent. even though the levels at which crystal changes took place were different Agitation. A series of experiments was performed with different agitators, agitator speeds, and ratios of agitator size to solution volume. Table I summarizes the results of several of these runs; in all tests, the more vigorous the agitation, the greater the yield per unit time. Agitation played a large factor in breaking borax supersaturation and stimulating nucleation. This is the normal expectation with any solution, but it is probably more pronounced in borax solutions. which tend to supersaturate very easily. In the runs with variable agitator speed, larger crystals Mere always obtained at lower speeds and there was generally less agglomeration. The same was true as the agitators were changed from larger to smaller and gentler ones. These changes resulted in a changing bulk densitv. Throughout this study, high bulk densities (12 8 to 14 0) indicated large, chunky. single crystals, medium densities (10.0 to 12.7), agglomerated crystals: and low densities (6.0 to 9.9), a high degree of agglomeration and/or very small crystals. Mechanical shock causes an effect similar to that caused by agitation. If the stirrer inadvertently touched the wall of the beaker during a run, spontaneous nucleation invariably occurred, causing an increased yield and producing a crystal much smaller than normal. It also often appeared to cause increased agglomeration. Seeding. I n most of the runs a small amount of borax seed was added to the crystallization solution. However, to test its influence upon the crvstal size and shape. a few series of runs were made under varying seeding conditions (Figure 2). In general, the runs shoxed a gain
500 750 500
32 68 31 49 58 71 19
15 26 57 47 32 22 35
14 3 12 3 2 4 45
Poured Bulk Density, G /Cu. In. 12.6 11.7 10.2 10.5 10.7 10.7 9.3
Borax concn., 15.66 wt. %; 15.07 in other runs. Per cent of total borax crystallized. Stirrer touched wall. 15-min. retention time; other runs, 40 min. a
c
...
Screen Analysis, % -20 -40 -80 +40 180
INDUSTRIAL AND ENGINEERING CHEMISTRY
B O R A X CRYSTALLIZATION in yield and a loss in crystal size with increasing seed concentration. Raising the seed concentration very significantly improved crystal structure; the higher the seed concentration, the lower the degree ofagglomeration. In fact, with 25 weight yo seed, only single crystals were produced. As seed concentration increased, the strength of the forces cementing the agglomerates decreased. The agglomerates fell apart easily under the milling action of the shatterability test. The effect of seeding is undoubtedly due to the increased crystal surface area available for crystallization. Increased seed densities promote a rapid release of supersaturation on the seed crystals, tending to prevent the conditions of high supersaturation that promote the formation of nuclei. Temperature. A survey of crystallization theories indicates that temperature can be a variable influencing crystallization. Therefore, a series of experiments was run comparing crystallization at 35' and 55" C. The results (Table 11) showed that the temperature of crystallization within the range 35' to 55' C. had no significant influence on the yield at the same degree of supersaturation. The effect of crystallization temperature on the crystal habit or degree of agglomeration appeared to be small. Viscosity. From a theoretical point of view, viscosity is one of the important physical factors in controlling crystallization rate. Therefore, to determine its influence on this system, tests were made in the presence of several viscosity-increasing agents. Sodium silicate reacted with borax, precipitating colloidal silica, which in turn caused' excessive nucleation. The result was a crystal crop of low bulk density containing a high percentage of fines. Cornstarch made the borax crystals sticky and almost impossible to wash and dry. The crystals were elongated. Methylcellulose, with its inverse solubility
Table
II.
NasB407 Concn.,
we. % 5.6 13.0 8.2 15.2
Crystallization Temperature Had No Significant Influence on Yield Cryst. Temp., O
C.
35 55 35 55
Supersaturation,
G./100Ml. 1.0 1.0 4.0 4.0
Yield,
G.a
5.2 5.1 23.9 23.4
Screen Analysis, -20 -40 +20 +40 +SO 0 0
6
0 0
30 58
4
59 36 66 37
Poured
70
Bulk -80
Density, G./Cu. In.
35 60 4 5
b
a Shaken in stoppered bottle to avoid evaporation losses. 30-min. retention time. crystals.
curve, precipitated out in the hot borax solution. In one run, improved single crystals were made, but the product foamed slightly when redissolved. Carboxymethylcellulose produced solutions with viscosities UP to 32 cp. and was the only additive that presented no technical difficulties. However, the results were inconsistent. Two runs showed an increase in crystal size; one showed a decrease, None of the runs showed any decrease in degree of agglomeration; with 0.09% carboxymethylcellulose tbere was an increase in bulk density, but microscopic examination indicated that the product was as agglomerated as that from the control run. Retention Time. This is a variable of considerable interest in designing continuous crystallizers, but difficult to study on a beaker scale. However, several runs were made, varying retention time only. It appeared that for retention times up to 5 minutes an increase in retention time gave an increased yield and an increased crystal size. Within 1 minute the crystals were agglomerated and the degree of agglomeration increased with increasing retention time. Runs made a t retention times of 15 and 30 minutes showed that after 15 minutes the supersaturation in the solution had dropped to a low value and in the following 15 minutes increase in yield was very small. However, the bulk density figures indicated that the degree of agglomeration in-
b
13.2 13.2
Single
creased significantly over the latter 15minute period. Chemipal Additives. A number of chemical additives were tested with borax solutions to examine their effectiveness in improving crystallizing conditions. The predictions of Buckley ( 7 ) were generally correct, for comparatively large quantities of the reagents were required to alter the crystal habit or growth rate. The results found with the additives are listed in Table 111; a nurriber of the materials tested were very effective in improving the borax crystal structure. Simulated Vacuum Crystallizer. A series of tests was run to study borax crystallization under conditions of vacuum cooling. A three-necked distillation flask, 51/, inches in diameter was used, and the solution was agitated and controlled to simulate the action of plant crystallizers. The feed solution was either added under the liquid surface or allowed to drop (and flash) in from above. The latter method produced very small agglomerates of still smaller needles (Figure 3). The data of these runs showed very clearly the detrimental effect on borax crystal structure that can be caused by flashing. The needles and dendrites formed under conditions of flash evaporation differed from the results obtained previously. When high supersaturation a t constant temperature in an agitated vessel gave chunky crystals, they prob-
Figure 3. Flashing has a detrimental effect on borax crystal structure
Normal
Flash evapcration
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ably represented a form caused by much higher growth rates than encountered previously. Buckley ( 7 ) gave two possible explanations for “dendritic” crystals that may apply in the present case. The first suggested that during rapid crystallization the heat of crystallization will tend to be transferred more rapidly from the corners of a crystal face than from the edges or face; the more rapidly heat escapes from a given area, the more rapidly that area will grow. I n this way the corners of a face tend to grow rapidly while the edges and face grow slowly, if a t all. This mechanism of growth would result in needles radiating from a central core. The second explanation involves the concept of metastable and labile types of supersaturation. During rapid cooling a portion of the solution becomes labile and a nucleus forms. Where the nucleus is in contact
Table 111.
with labile solution, deposition occurs. The growing portion of the nucleus is thus extended into more labile solution, so more deposition occurs and the process is repeated. Meanwhile the solution along the sides of the extending needle has become metastable and so no deposition occurs on the sides of the needle. This process is repeated at a number of points on the original nucleus, producing a crystal that consists of a number of needles extending from a central body. Buckley points out that dendritic growth is a phenmeonon not adequately explained. Conclusions
The above experiments provide a fairly comprehensive picture of the influence of the major variables affecting the crystallization of borax from its solutions. They do not directly lead to
Effect of Chemical Additives on Borax Crystallization
Concn. Range, P.P.M., 0 to
Additive NaOH Na~C03 NHiOH Na9S04
20, OOOQ 49,000* 40, OOOc 28,000
Na3POd
10,000
NaCl
30,000
KC1 Ca++
30,000 285
Mg++
1,400
Al+++
1,100
Zn++, Ba++ Hg++, P b f f KMnOl Fe+++ Li + cut+
500 1,200 1,810 24
Na?S:Oa Cr+++, Mn++ Bi++, NaBiOa, S n + + Oleic acid
2,500 40 1,400 570
Alcanol D W Pontamine Sky Blue 5 BX Lebcol defoamer, Silicon SX-1, Silicon Fluid-200, Avitex C Pontamine Fast Blue BLL, Merpol C Aviton T Pontamine Fast Blue 4GL, Turquoise 8GL, Deep Blue B H Fast Pink BL
1,200 1,400
820
1,330
Pontamine Fast Blue BRLN, Blue BBCC Ocenol, Product BDO, Nalco 71Na Pontamine White BR, BP, BT, and BTS Nigrosine W S B pH 9.46 t o 10.24.
1 684
pH 9.46 t o 9.75.
200
Results
Increased pH changed shape from elongated to chunky. Less dependent on supersaturation Greater elongation a s concentration increased, except a t high pH Made clean, brilliant surfaces; agglomerates more brittle. Low phosphate (0.3 to 0.7% P2Oj) with high pH gave larger, less agglomerated crystals Increased yield. Mild habit improvement Decreased yield Increased size under mild crystallization conditions Similar, but stronger effect than Ca. Less agglomeration at mild conditions Decreased yield. Larger crystals at high concentrations with mild crystallizing conditions Mild improvement Mild improvement (high concentrations) Mild improvement. Colored crystals Increased yield, agglomeration Little effect. Decreased yield, size Decreased yield. Elongated crystals at low pH. Colored crystals Smaller crystals Little effect Little effect Decreased yield, improved structure, primarily by reduced agglomeration. Most effective with mild crystallization conditions Improved crystallization Improved crystallization Mild improvement
1,400 200 1,400
Mild improvement Reduced agglomeration Reduced agglomeration
2,800
Reduced agglomeration, crystals, elongated, brittle Increased agglomeration
1,400
the design of continuous large-scale crystallizinq equipment, but they do define the general influence of each variable and the limits that can be worked within. Using these data as a framework, bench-scale and pilot plant tests can be accomplished more rapidly and with more confidence, and a firmer final design achieved. Control of borax supersaturation was the most important single variable influencing borax crystal habit, size, and degree of agglomeration. This in turn could be controlled by the only slightly independent variables : rate of cooling, concentration levels (feed and discharge) of the solution, retention time, and type and amount of seed used. The latter factor had a rather surprising importance, for utilization of the optimum seed density and optimum seed crystal size gave additional control to agglomeration and particle size. In many tests there was an apparent interdependence of supersaturation and crystal shape. At low degrees of supersaturation, the stepped pyramid of lowest energy levelsfaces-those appeared to grow preferentially, and elongated crystals predominated. With more supersaturation the less stepped faces [such as ( 2 3 0 ) ]began to grow, and tablet formation was favored. Finally, with increasing supersaturation, chunky crystals began to appear, indicating that the comparative growth rates of the various faces were becoming less of a factor. The degree of turbulence and chemical environment were also major variables affecting the crystallization. Turbulence accelerated the supersaturation forces to an extent that could easily dominate the entire process of crystallization. Shock and flashing were good examples of this. As for chemical environment, a number of additives were found that could simplify the formation of good crystals. The most important was p H control. Other additives could prove useful under the correct operational circumstances. literature Cited (1) Buckley, H. E., “Crystal Growth,”
Wilev. New York. 1951.
(2) Ga&tt, D. E.’, Rosenbaum, G. P., Chem. Enng. 65,125 (Aug. 11, 1958). (3) Peet, R. B., U. S. Patent 1,792,863 (Feb. 17, 1931). 141 Sawver. D. L.. Bixler. G. H.. IND.
‘ ENG.&EM. 49, 322 (1957). (5) Taylor, D. S., Connell, G. .4., U. S. Patent 2,662,810 (Dec. 15, 1953). (6) Zbid.,2,774,070(Dec. 11, 1956). (7) Zbid.,2,785,952 (March 19, 1957).
Little effect Little effect
RECEIVED for review January 20, 1958 ACCEPTED July 28, 1958
2,800 Little effect pH 9.46 to 9.84.
Division of Industrial and Engineering Chemistry, Chemical Processes Symposium, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
600
1,400
INDUSTRIAL AND ENGINEERING CHEMISTRY
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