APRIL, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
previous data indicates either a pure sample or a constantboiling mixture. The latter possibility may be checked by the melting point. The good agreement obtained between physical constants speaks for the effectiveness of this fractionating column and technic. A similar column used by Booth and Willson (4) of this laboratory to purify boron -trifluoride, contaminated with silicon tetrafluoride, was shown by H. M. Strong of the Physics Department of Ohio State University to yield a boron fluoride spectroscopically pure. The significance of this accomplishment may be realized when it is recalled t h a t Ruff (9) stated that he could not purify boron trifluoride b y ampoule-to-ampoule distillations and that C. F. Rumold, working in this laboratory, found that it required about forty ampoule-to-ampoule distillations to obtain boron trifluoride of constant density (i. e., pure boron trifluoride). Experience here has shown that a sample taken from a fractionating column, operating as described above a t an adequate reflux ratio and a t constant still head temperature,
475
yielding a gas of constant molecular weight, and showing a sharp freezing point, is in a high state of purity. Such samples will yield reliable and comparable vapor pressures, analyses, and critical constants.
Literature Cited (1) Booth and Carter, J . Phys. Chem., 34, 2801 (1930). Anal. Ed., 2, 12 (1930). (2) Booth and McIntyre, IKD.ENQ.CBEM., (3) Booth and Swinehart, J . A m . Chem. SOC.,57, 1333 (1935). (4) Booth and Willson, Ihid., 57, 2273 (1935). (5) Dufton, S. F., J . SOC.Chem. Ind., 38, 45T (1919). (6) Edwards, J. D., Natl. Bur. Standards, Tech. Paper 89 (1917). (7) Germann, A. F.O . , J . A m . Chem. SOC.,36, 2456 (1914). (8) Podbielniak, W. J., IND.ENO.CHEM.,Anal. Ed., 3, 177 (1931); 5, 119, 172 (1933). (9) Ruff, Otto, Z.anorg. allgem. Chem., 206, 60 (1932). R E C ~ I V ESeptember D 23, 1935. Presented as part of the Symposium on the Chemistry of Fluorine before the Division of Physical and Inorgania Chemistry at the 88th Meeting of the American Chemioal Society. Cleveland, Ohio, September 10 to 14. 1934.
POTASH FROM POLYHALITE
M
ANY of the processes suggested for the utilization of polyhalite MgS04.2CaSG2HzO) depend upon calcination of this complex salt to cause its rapid decomposition when treated with water, What happens to polyhalite during calcination and what combination of conditions yields the most favorable results have not, however, been determined, a 1t h o u g h from the early experiments of Rose (10) down through the series of more recent investigations (2, 6, IS, 16) there has been a definite trend toward lower temperatures and shorter times of calcination. Conley, Fraas, and Davidson (6) suggested that a period of only 2 or 3 minutes in passing t h r o u g h t h e temperature range from 300" to a maximum somewhat below 500" C. would y i e l d optimum results. Their conclusions, based on experimentsin a 5-em. (2inch) laboratory rotary kiln, have now been checked by calc i n a t i o n tests on a I Present address, U. S. Department of A g r i c u l t u r e , S o i l Conservation Bureau, Washington, D. C. 2 Present address, Hall Laboratories, Inc., Pittsburgh, Pa.
Relation between Calcination Conditions and Extraction Behavior J. M. DAVIDSON,I A. A. BERK, J. E. CONLEY, AND EVERETT P. PARTRIDGE* Nonmetallic Minerals Experiment Station, U. S. Bureau of Mines, Rutgers University, New Brunswick, N. J.
larger scale e m p l o y i n g a 15-em. (6-inch) k i l n w i t h rates of feed up to 80 kg. (170 pounds) per hour coupled with time-concentration extraction e x p e r i m e n t s on the calcined product. A description of these tests and a correlation of time and temperature of calcination with density of calcine on one hand, and with behavior of the calcine during extraction on the other hand are presented. The effects of sodium chloride content and of particle size are also considered.
Experimental Rotary Kiln The essential details of the externally fired rotary kiln are s h o w n in Figures 1 and 2. The seamless tube, 3.35 meters (11 feet) long, with an inside d i a m e t e r of 152 mm. (6 inches) and a wall thickness of 6.35 mm. (0.25 inch), was carried at each end on supporting rolls. B y changing the slope and the rate of rotation, the t i m e of retention of polyhalite in the kiln was varied between 1 and 36 minutes in the tests described in this paper.
The furnace was fired b y means of fifteen 19-mm. (0.75-ino h )
FIQURE 1. VIEW OF R O T A R YK I L N FROM FEEDEND
VOL. 29, NO. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
476
The duplicate analyses in Table I were obtained after each batch had been thoroughly mixed, divided in half, and each half carefully sampled. The good checks indicate that each batch was practically uniform in composition. The apparent density of the polyhalite in each batch, as determined with the standard LeChatelier flask, is given in Table I, together with the hypothetical density of the completely dehydrated material if no change in the volume of the solid occurred d u r i n g calcination. This represents a theoretical minimum density with which the observed densities of calcined samples in Table 111may be compared. FIGURE 2. LONQITUDINAL CROSS SECTION OF ROTARY KILN As may be seen in Table 11,the two batches A . Rotary furnace b. Refractory lining B . Shelby tubing C. Flues of -10 mesh polyhalite had roughly the C. Thermocouple manifold d . Insulating fill D . Helical-screw feeder e. Refractory baffle same size distribution when charged to the E. Variable-drive pulleys f. Removable packing kiln. The same is true of the two batches F. Recording potentiometer Q. Roller bearings 1-8. Thermocouples la. Burner holes of -20 mesh material. The size distribua. Furnace walls IC. Tube sprocket drive tion of the calcine leaving the kiln was not determined, but in all cases some shift Venturi burners operating on city gas. A partial horizontal toward the fines undoubtedly occurred as a result of attribame prevented the flames from impinging on the steel tube but tion both in the helical-screw feeder and in the kiln. allowed ready transfer of heat by radiation and convection. The temperature history of the polyhalite during EL number Temperatures were controlled by manual regulation of the of typical calcination tests, as measured by the thermocouple burners. Temperatures in the material moving through the kiln were system within the rotary kiln, is shown in Figure 3. Because measured by means of eight chromel-alumel thermocouples and of unavoidable variatio,ns in kiln operation, the temperature recorded continuously on two Leeds & Northrup potentiometer distribution during each run fluctuated somewhat, but the recorders. A 38-mm. (1.5-inch) stainless steel tube extending through the kiln was equi ped with eight nipples, the ends of which cleared the wall of tge rotating tube by approximately 13 mm. (0.5 inch). Into each nipple a thermocouple was packed OF FOUR BATCHES OF POLYKALITE TABLE11. S l Z E DISTRIBUTION with asbestos, so that the bead would project slightly beyond --Screen Sizethe end of the nipple, while the separately insulated wires were Material on Screen, %-Meshes per Mm. in. opening A B C D carried out of the kiln through the stainless steel tube. The axis of the nipples was slanted so that the hot junctions of the 20 0.833 42.8 1.7 39.0 0.8 28 0.589 13.2 20.8 12.4 16.8 thermocouples would be immersed as deeply as possible in the 35 0.417 16.9 8.8 17.1 polyhalite. 65 0.208 2i:2 20.7 13.0 25.5 Polyhalite was supplied to the kiln at predetermined rates 100 0.147 4.1 9.1 6.7 9.9 150 0.104 2.7 2.7 1.3 3.0 rangin from 25 to 80 kg. (50 to 170 lb.) per hour by a helicalThrough 150 ... 15.0 28.1 18.8 26.9 screw feeder discharging directly into the end of the tube. The calcine was discharged at the other end into enameled pans for weighing. Sam les of the hot calcine taken from these pans were sealed in dason jars for subsequent extraction tests. curves as given probably represent the average conditions To obviate a tendency toward the formation of rings of fine for the specific runs within + 5 O C. Although the temmaterial on the walls in the hot zone, a tapping device, improvised perature distribution varied even more widely for different from a Ro-Tap machine, was set up at the discharge end of the kiln. runs, the curves of Figure 3 may be regarded as approximations of the conditions in other runs with corresponding times Material and Measurements of retention and maximum temperatures. Calcination tests were conducted on four standard batches Effect of Time and Temperature upon Density of polyhalite, the chemical composition and size distribution of Calcine of which are given respectively in Tables I and 11. These The process of calcination may be regarded in the case of batches were prepared from crude polyhalite by grinding, polyhalite as comprising the transfer of heat into a particle, washing to remove sodium chloride (B), and mixing to produce the transfer of water out of the particle, and a simultaneous the desired sodium chloride contents. or subseauent rearrangement of the internal structure bf the material (9, 15). The extent of the changes produced by any specified treatment TABLEI. CHEMICAL CONPOSITION OF FOURBATCHES OF POLYHALITE must be a rather complex function of both time -Apparent Density--Screen Size-Grams/Cc. and temperature. For a first approximation, the Ne8he8 DehyMm. Compn., % Original drated per average time of retention in the kiln and the Batch in. opening KzSOa MgSOr CaSOn NaCl (measured) (calcd.)" maximum temperature attained by the material A -10 1.651 26.14 19.18 ... 0.80 2.751 2.603 during calcination may be used as measures of 26.08 19.15 ... 0.80 B -20 0.833 25.37 18.96 47.99 1.32 2.746 2.601 these factors. Similarly, the apparent density of 25.41 18.91 47.93 1.33 the calcine may be regarded as a measure of the C -10 1.651 24.61 18.32 46.32 5.02 2.729 2.590 24.65 18.28 46.11 5.02 changes produced in the polyhalite. Figure 4 D -20 0.833 24.58 18.27 46.09 5.09 2.731 2.592 from Conley, Fraas, and Davidson (6) indicates 24.63 18.30 46.11 5.12 how a calcination period of 1 hour yielded matea On assumptions that initial polyhalite contained no free moisture, and that water of rial of different density for different maximum hydration could be removed without change in volume of solid. temperatures. Dehydration produced a marked 1
--
--
-
APRIL, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
decrease in density in the range from 300" C., a t which loss of water is first apparent, to 450" C., but above this temperature a progressive increase in density indicated a rearrangement in structure. Polyhalite calcined a t a temperature slightly higher than that corresponding to the minimum density yields better results during extraction than that treated a t either a higher or a lower temperature ( 5 ) . The present investigation was therefore limited to a range from 400" to approximately 525" C., as indicated on Figure 4. I n Figure 5 the apparent density of the calcine is plotted as a function of the maximum temperature for successive samples from each of several runs, the time of retention being indicated on each curve. The theoretical minimum density of the corresponding batch of polyhalite, taken from Table I, is also shown. It is evident that increase in the time of retention tends to shift the density-temperature curves upward and to the left in each group. This means that for tempera-
477
-
TIME IN KILN
MINUTES
CURVESFOR CALCIFIQURE3. TYPICALTIME-TEMPERATURE NATION OF POLYHALITE
tures above approximately 430" C., the degree of calcination, as measured by the density, increases with time of retention as- well as with temperature. Two questions immediately propose themselves : Will a longer time of retention a t a lower temperature produce 'TABLE 111. COMPARISON O F CALCINATION CONDITIONS WITH the same behavior during extraction as a shorter COMPOSITION OF EXTRACT LIQUOR time a t a higher temperature; and what is the -Calcination Conditionsoptimum combination of time and temperature? Ext. Liquor Compn. at Time of retention
Lot
12-1 4 10 11 10-2 4
Min.,
Min.
1.2
0.9 0.8 1.0 0.7 1.4 1.5 1.4 2.2 2.2 5.8 5.8 9.1 14.5 13.6 27.2 26.2 24.9
1.9
5
13-1 4 8-2 7-2
11-1 5-2
a
6-2 5 4
3.5
8.3
8.2 9.8 18.5 37
22-2 20-1 2 3
1.1
16-3 4 26-2 3 28-3 4 5
3.2
1.7
4
19-1 2 3 4 14-2 3 4 27-2 3 4
7.8 11.6
1.9
3.6 7.7
23-2 21-1 2 4
1.0 1.6
15-2 3 5 25-2 3 4
3.7
29-2 3 4 31-3
14.5
5
8.3
5
4
Time above 300° C.
21.5
0.9 1.3 1.3 1.3 1.1 3.1 2.9 5.8 5.6 8.3 8.0 7.8 1.7 1.6 1.8 1.8 2.7 2.5 2.5 5.4 5.3 5.3 0.9 1.3 1.3 1.2 1.1 2.5 2.5 2.4 6.3 6.0 6.0 5.6 10.4 10.1 10.0 14.3 13.9
__
Max. temp.
DenSity ot
Max. &SO4 Concn.
,
P
Calcine KzSOc MgsO4 C . Grams/cc. Grams/lOO grams Batch A, -10 Mesh, 0.8% NaCl 6.30 2.632 8.26 500 5.76 2.633 7.65 480 6.80 5.18 2.627 420 4.33 2.649 5.68 400 8.92 6.92 500 2.649 8.76 6.82 475 2.642 6.68 2.639 8.88 465 7.99 6.55 515 2.661 8.51 6.63 2.643 455 8.40 6.97 2.663 510 8.80 6.77 2.647 475 7 . 0 1 5.70 2.730 595 6.56 2.654 8.65 475 6.54 2.646 8.43 440 6.56 2.653 8.50 475 8.25 6.53 2.636 425 7.76 5.99 2.642 400 Batch B, -20 Mesh, 1.3% NaCl 495 2.637 9.44 7.28 6.84 2.674 8.75 515 7.15 2.654 9.51 495 9.19 6.87 2.637 470 6.43 2.626 8.65 455 7.28 2.637 9.93 480 7.00 2.636 9.15 475 9.22 7.20 490 2.653 9.48 7.06 2.646 475 7.35 2.650 9.16 465 7.22 2.642 9.44 440 9.54 7.22 425 2.636 Batch C, -10 Mesh, 5% NaCl 5.85 2.653 9.76 515 7.27 2.626 10.21 490 7.18 2.622 10.19 480 6.76 9.61 460 2.613 7.22 2.640 10.03 485 7.01 2.640 9.76 470 6.95 460 2.627 9.64 7.24 2.637 10.17 495 10.30 7.23 2.627 475 7.20 10.11 465 2.622 Batch D, -20 Mesh, 5% NaCl 495 2.619 10.56 7.42 9.90 7.34 2.630 525 10.53 7.45 2.615 495 10.18 7.19 465 2.611 9.82 6.98 440 2.619 7.31 10.00 500 2.635 7.26 10.20 475 2.617 7.09 2.612 9.77 440 7.61 2.649 10.12 500 7.49 2.636 10.44 480 10.43 7.54 460 2.622 7.28 2.618 10.01 440 7.50 10.24 475 2.639 7.51 2.622 10.40 460 7.46 10.33 440 2.619 10.37 7.49 2.625 460 7.25 2.620 10.06 440
NaCl Hz0 0.33 0.33 0.31 0.31 0.32 0.32 0.33 0.32 0.32 0.33 0.31 0.30 0.32 0.33
Max. Extn. of
KzSOr
%
0:32 0.32
75 70 62 52 81 80 81 73 77 76 80 64 79 77 77 71 75
0.57 0.57 0.57 0.57 0.56 0.58 0.58 0.58 0.59 0.58 0.58 0.58
86 80 86 83 79 90 83 84 86 83 86 87
2.32 2.25 2.27 2.25 2.27 2.21 2.16 2.19 2.35 2.35
89 93 93 88 92 89 88 93 94 92
2.32 2.32 2.33 2.30 2.33 2.31 2.31 2.31 2.35 2.31 2.33 2.30 2.32 2.37 2.28 2.29 2.28
96 90 96 93 89 91 93 89 92 95 95 91 93 95 94 94 92
Relation between Time and Temperature of Calcination and Extraction Behavior In place of the standard batch extraction employed in previous studies (2, 5 ) to meamre the extractability of calcined polyhalite, a more detailed study of the extraction behavior of material prepared under different conditions seemed essential. The change in concentration of potassium sulfate and magnesium sulfate during extraction was accordingly measured in the following manner: The potassium content of a sample of calcined polyhalite was determined by the chloroplatinate method (8). Two hundredJgramsof the calcine were then placed in a 1-liter, 3-neck, roundbottom flask equipped with a stirrer, a reflux condenser, and a connection to the bottom of a second flask which also carried a reflux condenser. After adding some water to this second flask and blowing it out to waste through the discharge line with compressed air, a
2.7 6
1
1
_-_-__ .___------- -------
THEORETICAL MINIMUM DENSIT!>----
2,50
0
100
ZOO
300
400
TEMPERATURE
- 'C.
500
600
_. 700
FIQURE4. RELATIONBETWEEN TEMPERATURE OF CALCINATION AND DENSITY OF CALCINE FOR POLYHALITE CALCINED ONEHOUR(5)
478
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I
curves for lots 12-4, 10-4, 7-2, 5-2, and 6-2, which represent the range from undercalcination to overL O T 25 calcination, with lot 7-2 close to 8.3 MIN. 2.67the optimum condition. A similar comparison for lots 12-1 and 10-2 2.66with a maximum temperature of 500" C. indicates that the former was inadequately calcined during 1.2 minutes in the kiln. Extreme overcalcination is evident in the case of lot 11-1 in Figure 6, which attained a maximum temperature of 595" C. during its 2 2.62 period of 9.8 minutes in the kiln. The low maximum concentration of magnesium sulfate reached during extraction and the rapid decrease in concentration with time indicate an accelerated formation THEORETICAL MINIMUM DENSITY ______,________ ________460_____________ of polyhalite from solution, as ) 420 440 480 500 520 540 '"%OO 420 440 460 480 500 ,520 540 compared with the behavior of MAXIMUM TEMPERATURE MAXIMUM TEMPERATURE C. lot 7-2, which was in the kiln BETWEEN MAXIMUM TEMPERATURE OF CALCINATION AND DENSITY OF FIQURE 5. RELATION CALCINE FOR VARYING PERIODS OF CALCINATION for a period only slightly shorter but was not heated above 475" C. weight of water was introduced sufficient t o produce a concenThe relative effects of time and temperature of calcination tration of 11 grams of potassium sulfate per 100 grams of water upon the extraction behavior of calcined - 20 mesh polyhalite if the polyhalite were completely extracted. This was heated low in sodium chloride are shown in Figure 7. Postponing to the boiling point and then, at zero time on a stop watch, was discussion of the effect of particle size to a later section, it blown over by compressed air into the flask containing the polyhalite, approximately 30 seconds being required t o complete the is evident from the eight curves in this group that a time transfer. This method of adding the hot water to the stirred of retention of 3.2 minutes with a maximum temperature polyhalite produced complete dispersion of the latter. of 480" C. (lot 16-3) gave optimum results during extraction. The mixture, stirred at 450 r. p. m., was maintained at the A longer time with a slightly lower maximum temperature boiling point throughout the extraction, loss of water being prevented by the reflux condenser. Approximately 10-cc. samples of the mixture were blown out at intervals by com ressed air onto a small filter, care being exercised to flush out tEe delivery tube each time before sampling. The filtrate of each sample 7 1.2 MIN a2 MIN. 1.9 UIN. was analyzed for potassium by the chloroplatinate method (8) and for magnesium by the usual gravimetric procedure (7). 9 LOT 12 When calcium was also determined in a number of tests, the procedure developed by Blasdale (1) was followed. The residue on the filter after sampling was washed with 50 per cent and then with 95 per cent ethyl alcohol, air-dried, and examined under the petrographic microscope. 2,691
/
_.l_--_--__
~
OC.
7 d
Duplicate extractions run at the same time to check the precision of the method did not deviate by more than 2 per cent a t any point during a 90-minute extraction. Typical time-concentration curves, based on data obtained in the foregoing manner, are shown in Figures 6 to 9. Study of these reveals definite effects of calcination conditions, particle size, and sodium chloride content upon the extraction process.
. 1 20
40
60
Effect of Calcination Conditions Because particle size and sodium chloride content influence the extraction process, the effect of time of retention and maximum temperature must be considered individually with respect to each of the four batches of material put through the calcination tests. A wide range of calcination conditions is represented by the curves in Figure 6 for - 10 mesh polyhalite containing approximately 0.8 per cent of sodium chloride. If under-, over-, and optimum calcination are defined in tterms of the maximum concentrations of potassium sulfate and magnesium sulfate obtained during extraction, it is apparent that lots 10-2 and 7-2 were the best calcined of the group. The short time of 1.9 minutes in the kiln, combined with a maximum temperature of 500" C . , produced nearly the same results as 8.2 minutes with a maximum of 475" C. At this latter temperature, the effect of increasing the time of retention may be seen by comparing the extraction
o
20
40
I
60
I
o
TIME O F
eo 40 EXTRACTION-MINUTES
FIGURE6. CHANQES IN SOLUTION COMPOSITION DURING ExCALCINED -10 MESHPOLYHALITE CONTAINING 0.8 PERCENTSODIUM CHLORIDE
TRACTION OF
APRIL, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
1.65 MIN.
a
io
io
TIME
3.2 MIN.
20' 0 OF EXTRACTION
- MINUTES
7.5 M I N .
L9 MIN.
0
0 -
b
479
7.7 MIN.
3.6MIN.
I
I
I
I
0 20 40 60 0 T I M E OF E X T R A C T I O N - M I N U T E S
20
40
I
60
7. CHANGES IN SOLUTION COMPOSITION DURINQ ExCALCINED -20 MESH POLYHALITE CONTAINING FIGURE 8. CHANQES IN SOLUTION COMPOSITION DURINQ Ex1.3 PERCENTSODIUM CHLORIDE TRACTION OF CALCINED - 10 MESH POLYHALITE CONTAININQ 5 PERCENTSODIUM CHLORIDE (lot 26-3) and a shorter time with a higher maximum of 495' C. (lot 20-2) yielded the next highest concentrations. The fact that even a brief exposure of -20 mesh material % II to a temperature above 500" C. will result in overcalcination I is brought out by comparison, in Figure 7, of the extraction d 0 behavior of the various lots from run 20 in the kiln. Lots e 10 P 20-4 and 20-3 are obviously undercalcined with respect to > 5 lot 20-2, which reached a temperature of 495" C.; lot 20-1 I which was heated to 515' C. can owe its comparatively poor 2 9 2 extraction only to overcalcination. As in the case of overI4 a calcined lot 11-1in Figure 6, the magnesium sulfate concentra5 8 tion for lot 20-1 drops sharply after its early maximum, in0 dicating excessive formation of polyhalite from solution. 0 Trends similar to those already discussed may also be oba 7 served in Figures 8 and 9 for polyhalite containing approximately 5 per cent of sodium chloride. It seems safe to conclude that polyhalite as coarse as - 10 mesh may be adequately calT I M E OF E X T R A C T I O N MINUTES cined by a heating cycle lasting only 2 or 3 minutes with a maxiFIGURE 9. CHANGES IN SOLUTION COMPOSITION DURING) EXmum temperature of 470" to 490" C., as suggested by Conley, TRACTION OF CALCINED -20 MESH POLYHALITE CONTAININQ Fraas, and Davidson (6). A temperature of 500' C., instead 5 PERCENTSODIUM CHLORIDE of being the lower limit as recommended by Schoch (11, l a ) , appears to be the upper limit for the best results. amount of sodium chloride, but these concentrations are maintained over a much longer period. Effect of Storage The same improvement in extraction is produced in the Samples of calcine taken hot from the kiln in Mason jars case of -20 mesh material, as may be seen by comparison were stored for periods ranging from 20 to 150 days before of the curves in Figures 7 and 9 for polyhalite containtheir extraction behavior was tested. Correlation of all the ing, respectively, 1.33 and 5.11 per cent of sodium chloride. data indicates no tendency for the degree of extraction to A slightly greater tendency for the concentrations to drop decrease on protracted storage, as long as the container reafter attaining maximum values is shown by the -20 mesh mained tightly closed. I n two cases, however, a second sample material represented in Figure 9 than by the -10 mesh tested a t an interval of several months after the first sample material in Figure 7, owing perhaps both to the higher conshowed increased density and decreased extraction. This centrations reached and to the greater area of contact between change was probably due to absorption of water vapor from the solid phase and the solution. the air during the period when the first sample was removed The extraction experiments consistently showed that all from the container. of the sodium chloride present in the calcined polyhalite was dissolved almost immediately after addition of the water. Effect of Sodium Chloride Since the beneficial effect of sodium chloride upon the extracThe presence of sodium chloride during extraction of caltion process is apparently due to the fact that it decreases cined polyhalite has been shown by Conley and Fraas (3, 4) the tendency for syngenite and polyhalite to form from soluto increase the concentrations of potassium sulfate and tion, equally good results might be anticipated whether the magnesium sulfate by decreasing the tendency toward forsodium chloride entered as an impurity in the polyhalite mation of syngenite and polyhalite from solution. This or was added to the water used for extraction. The effect effect is strikingly evident on comparison of Figures 6 and 8 of the latter treatment upon different types of calcine is from - 10 mesh material containing, respectively, 0.8 and shown in Figure 10. The highly overcalcined lot 11-1 showed 5 per cent of sodium chloride, corresponding to average a much greater improvement in concentration than the underconcentrations of 0.32 and 2.32 grams of this constituent per calcined lots 12-11, 12-10, and 6-4. Material A was a com100 grams of water in the extract solution. Not only are posite derived from several lots, one of which was underhigher ooncentrations attained as a result of the increased calcined. Lots 18-3 and 20-3 were both slightly undercalcined;
FIQURE
TRACTION OF
(D
VOL. 29, NO. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
480
19-3 and 21-4 represent material nearer the optimum condition. The doubleend arrows indicate the range of concentrations obtained in the three or four best e x t r a c t i o n s for each of the four original batches of polyhalite. It seems reasonable t o c o n elude from the combined data in Figure 10 t h a t s o d i u m chloride present during extraction in a concentration of 2 grams per 100 grams NoCI 'CONCENTZRATlON GlOOG.H, of water will yield a FIGURE 10. EFFECT OF SODIUM CHLO- maximum c o n c e n RIDE CONCENTRATION IN IMPROVING tration of potassium EXTRACTION sulfate higher by 0.5 to 1 gram per 100 grams of water than would be obtained in the substantial absence of sodium chloride, and that little further gain is to be expected by using a sodium chloride concentration greater than 2 grams per 100 grams of water. ~~
fraction yielding the highest extraction of potassium sulfate, with the coarse fraction barely below the composite. The approach to a common solution composition after 60 minutes is probably fortuitous. The best estimate based on the available data is that reduction of the upper size limit from -10 to -20 mesh will increase the maximum concentration attained during extraction by less than 5 per cent. The optimum conditions of calcination seem to be about the same for the two sizes. That even as short a time of retention as 1 minute may be adequate for -20 mesh polyhalite is indicated in Figure 9 by the results for lot 23-2, which was heated to a maximum temperature of 495" C.
Comparison of Effects I n Figure 12 are shown four pairs of extraction curves for samples of well-calcined material from each of the four batches of polyhalite. Although these are intended particularly to compare the results obtained by calcination for 2 minutes with maximum temperatures slightly below 500" C . and for 8 minutes with temperatures some 20" lower, they also illustrate the small effect of particle size and the much more important effect produced by sodium chloride.
Effect of Particle Size Comparison of Figures 6 and 7 shows a t once that higher concentrations and hence, from the method of the test, correspondingly higher recoveries were obtained from the - 20 mesh material containing 1.3 per cent sodium chloride than from -10 mesh material with the lower salt content of 0.8 per cent. Although part of the improvement shown by the finer polyhalite was probably caused by the higher concentration of sodium chloride during extraction, as discussed in the preceding section, this should have amounted to an increase in the maximum potassium sulfate concentration of only about 0.3 gram per 100 grams of water. Direct comparison of individual curves in Figure 6 with those in Figure 7 is scarcely possible, but the three highest concentrations actually measured in the series of tests with the -20 mesh material actually exceed the three highest found with the -10 mesh material by two to three times the amount ascribable to the difference in sodium chloride content. Figures 8 and 9, representing data from batches of polyhalite with the same sodium chloride content of approximately 5 per cent, show only a small effect of particle size. The three highest concentrations actually measured for the - 20 mesh material exceed the three highest for the - 10 mesh material by approximately 0.3 gram of potassium sulfate per 100 grams of water. l n the preceding studies of calcined polyhalite with different upper limits of particle size, much of the material in all cases was in the range of finer sizes. A comparison of -10 +20 mesh and of -20 mesh fractions with the composite lot of -10 mesh calcined polyhalite from which they were screened is presented in Figure 11, in a manner which indicates the course of the changes in solution composition. When the sodium chloride concentration was approximately 0.3 g r a m per 100 grams of water, the coarse fraction yielded a slightly higher, and the fine fraction a slightly lower maximum concentration of potassium sulfate than the original composite. In the presence of 2 grams of sodium chloride per 100 grams of water the case was just reversed, the finer
2
a
4
MgSOa CONCENTRATION
-
6
7
0. PER 100 G.HsO
FIaURE 11. COMPARATIVE CHANQES IN SOLUTION COMPOSITION DURINQ EXTRACTION OF SIZEDSAMPLES OF CALCINED
POLYHALITE
Samples taken successively at 2.6, 5, 16, 30 45, and 60 minutes except two samples at lower left taken at 38 and 46 seconds, respectively.
The major difference between the two calcination conditions is a greater tendency toward early formation of syngenite in the case of materials calcined for the longer period with the lower maximum temperature. This is indicated by the displacement downward and to the right of the curve for this material in each pair in Figure 12. The effect of sodium chloride in decreasing the rate of polyhalite formation is illustrated clearly by the limited reversal of the two upper pairs of curves after attainment of the maximum concentration of potassium sulfate, as compared with the way in which the two lower pairs of curves turn downward parallel to the
APRIL, 1937
INDUSTRIAL AND ENGINEERmG CHEMISTRY
line representing a mole ratio of unity. I n this figure all curves are displaced somewhat to the right as a result of including calcium sulfate with magnesium sulfate.
Mechanism of Calcination
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of the petrographic microscope, has led to the following picture of the processes occurring during extraction of wellcalcined polyhalite. When the calcine, presumably a finely dispersed mixture of the solid solution KzSO4.MgSO4.CaSO4and anhydrite, is agitated with hot water, it decomposes rapidly, sending potassium sulfate, magnesium sulfate, and calcium sulfate into solution. Supersaturation with respect to calcium sulfate occurs in less than a minute, and gypsum tends to be pre-
If the experimental evidence in the preceding sections is considered, together with that previously available (2-5, 1316) and that from the contemporary investigation by Ramsdell (9),a reasonably complete and consistent theory may be developed to account for the behavior of polyhalite during calcination and extraction. Ramsdell (9) found by x-ray studies that a continuous series of solid solutions exists between potassium dicalcium sulfate (K2S04.2CaS04)and langbeinite (K2S04.2MgS04),and that when fused polyhalite is cooled the solid apparently contains only two phases, one of which is anhydrite (CaS04) and the other a solid solution belonging to this series, which accordingly may be represented as K2SO4.MgSO4CaSO~. Since the x-ray pattern of calcined material is the same as that of the material crystallized from fusion, it,seems reasonable to believe that the dehydration of polyhalite produces an unstable structure which readily rearranges into the two new structures of anhydrite and K2SO4.MgSO4.CaS04. This is in substantial agreement with the conclusions of Storch and Clarke (16) based upon petrographic studies of polyI I 0.2 0.3 halite calcined a t different temperatures. MggOs CONCENTRATIO: G/IOO 0. HIO :aso. CONCN. G ~ O G. O HP Granted this combined process of dehydration and reFIGURE13. CHANQES IN CONCENTRATION OF MAGNESIUM arrangement of structure, it is not difficult to interpret the vaAND CALCIUM SULFATES RELATIVE TO POTASSIUM SULFATE DURINQ EXTRACTION OF CALCINED POLYHALITE riation in density of calcined polyhalite illustrated in Figures 4 and 5. At temperatures lower than that corresponding to Samples taken successively at 5, 15, 30, 45, 60, and 90 minutes, except sample of lot 6-2 taken at 7 minutes. the minimum density for each curve, dehydration is predominant; a t higher temperatures rearrangement of struccipitated as an unstable solid phase, which then disappears ture must be responsible for the increase in density. In this within the first 5 minutes. Syngenite in quantity also is latter range it is evident that increase in either temperature formed during the first few minutes, usually being most or time of calcination results in increased density. abundant in the 5-minute sample. It is likewise unstable Mechanism of Extraction under the conditions, and slowly disappears during a period of approximately 30 minutes. Detailed correlation of the changes in solution composition The solid phase which is stable in contact with the extract with the formation of new solid phases, as identified by means liquor after the first minute or two is polyhalite, as indicated by the polyhalite-anhydrite boundary3 superimposed on CALCINATION LOT I' MESH NICI TEMP TIME NO, Figure 11. Polyhalite does not appear as a solid phase in /' .5 -IO 0.8 500 1.9 10-2 noticeable amounts, however, until after about 15 minutes. A -10 0.8 475 8.2 7 - 2 v -10 5.0 490 1.S 19-2 Thereafter it forms continuously, causing the typical bending 6.0 475 '1.7 27-3 T -10 , 0 -20 1.3 4 9 5 1.85 20-2 ,/ back of the extraction curves in Figure 11 close to the line representing an equimolecular ratio of potassium sulfate to magnesium sulfate. The calcium sulfate used up in the formation of polyhalite must pass through the solution phase containing not more than 0.2 gram of this substance per 100 grams of water. The rate of formation of polyhalite should therefore depend to a considerable extent upon the rate a t which calcium sulfate is supplied to the solution. This calcium sulfate must come from the dissolution of the original calcine and from the decomposition of syngenite. At the same time that these various processes are taking place, part of the calcium sulfate in solution is apparently being removed to form compact anhydrite from the residual particles of extracted calcine. At the start of extraction these are opaque, but as extraction proceeds they become progressively more transparent and their optical properties approach closely those of natural anhydrite. It appears as if the finely dispersed anhydrite in the particles of calcine pro5 6 7 vided nuclei for crystallization. Mg SO4 CONCENTRATION - G/IOO 0. H20 The changes in concentration of calcium sulfate as well FIGURE 12. COMPARISON OF CHANGES IN SOLUas of magnesium sulfate relative to potassium sulfate shown TION COMPOSITION DURING EXTRACTION OF WELLin Figure 13 for three samples illustrate the high early conCALCINEDSAMPLESOF POLYHALITE, SHOWINQ centration of calcium sulfate corresponding to the preOF CALCINATION CONDITIONS A N D SODIUM EFFECT CHLORIDE CONCENTRATION a From unpublished work from this station. I
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INDUSTRIAL AND ENGINEERING CHEMISTRY
dominance of syngenite as a new solid phase, the subsequent rapid decrease in calcium sulfate and increase in potassium sulfate as syngenite decomposes and anhydrite forms, and the final slow decrease in calcium sulfate and rapid decrease in potassium sulfate and magnesium sulfate resulting from the extensive formation of polyhalite. It is evident that the degree of extraction obtained depends upon the net effect of a number of processes proceeding a t varying rates, and that satisfactory separations of the potassium and magnesium sulfates from the calcium sulfate can be obtained only because of the slow rate a t which the mixture during extraction approaches ultimate equilibrium.
Optimum Conditions for Calcination and Extraction On the basis of the calcination and extraction tests described in this paper, the following recommendations may be made concerning particle size, time and temperature of calcination, and sodium chloride concentration, for the most effective extraction of potassium and magnesium sulfates from calcined polyhalite. PARTICLE SIZE. It is unnecessary to grind finer than -10 mesh to obtain satisfactory extraction if the material is properly calcined and a concentration of 2 grams of sodium chloride per 100 grams of water is present during extraction. Not more than a 5 per cent improvement has been obtained under these conditions by decreasing the size to -20 mesh. Material sized to remove the finer fractions will yield satisfactory results, as indicated in Figure 11. CALCINATION CONDITIONS.The maximum temperature of the polyhalite during calcination should not exceed 500' C. At this temperature level, a complete calcination cycle of only 2 minutes will produce optimum results; a longer time will result in overcalcination. If a lower maximum temperature of 475" C. is used, a slightly longer time is desirable (about 8 minutes is adequate). The results obtained by these alternative procedures are compared in the extraction curves of Figure 12. Calcined polyhalite rapidly absorbs water vapor. If completely protected from atmospheric moisture it may, however, be stored for several months without deleterious effect upon its extraction behavior. EXTRACTION CONDITIONS.The previous work of Conley and Fraas (3, 4 ) showed that a temperature of 100" C. yields better extractions than lower temperatures. Their conclusion that ti concentration of 2 grams of sodium chloride per 100 grams of water in solution during extraction was desirable has been further verified by the results of the timeconcentration tests described in the present paper. One mechanical factor of prime importance is the complete dispersion of the calcined polyhalite on addition to the water or solution used for extraction. Only moderate subsequent agitation is necessary or desirable.
VOL. 29, NO. 4
Summary A laboratory study of the effects of calcination conditions, particle size, and sodium chloride on the extraction of potassium and magnesium sulfates by hot water from polyhalite showed that optimum conditions for calcination and extraction are those which yield the highest concentrations of potassium and magnesium sulfate in the hot extract solution. The highest feasible concentrations are 10.5 grams of potassium sulfate and 7.5 of magnesium sulfate per 100 grams of water with a 96 per cent extraction. The practical temperature range for calcination lies between 450' and 500" C.; a short time a t 500" C. produces essentially the same effects as a longer time at the lower temperature. Two minutes' heating a t 500" C. is satisfactory, and 8 minutes are required a t 475" C. Temperatures higher than 500" or lower than 450" C. are undesirable because over- or undercalcination results. Grinding to a particle size of -10 mesh is sufficient but slightly inferior to treatment of -20 mesh material. The Iarger size extracts slightly more slowly but also favors a flatter time-concentration curve owing to a lesser tendency toward formation of secondary solid phases. The mechanism of calcination is the dehydration of the complex molecule with liberation of the individual compounds, followed by the gradual formation of a series of solid solutions involving dicalcium sulfate (K2S04.2CaS04) and langbeinite (K2S04.2MgS04). On extraction by hot water, dissolution occurs, with potassium, magnesium, and calcium sulfates going into solution with the simultaneous precipitation of syngenite ( K 2 S O 4 C a S 0 ~ H ~and 0 ) polyhalite (KzSOaMgS04.2CaS04.2H20). The latter compound forms concurrently with and later a t the expense of the syngenite. Sodium chloride tends to decrease the rate and amount of syngenite and polyhalite appearing as secondary phases.
Literature Cited (1) Blasdale, W. C., J . Am. Chem. Soc., 31, 917-22 (1909). (2) Clarke, L.,Davidson, J. M., and Storch, H. H., U. S.Bur. Mines, Repts. Investigations 3061 (1931). (3) Conley, J. E . , and Fraas, F.,IND.ENQ. CHEM.,25, 1002-9 (1933). (4) Conley, J. E . , and Fraas, F., U. S. Bur. Mines, Repts. Investigations 3210 (1933). (5) Conley, J. E . , Fraas, F., and Davidson, J. M., Ibid., 3167 (1932). (6) Davidson, J. M.. and Fraas, F., Ibid., 3237 (1934). (7) Gibbs, W., Am. J . Sci., [3]5, 114 (1873). (8) Hicks, W. B., J. IND.ENQ.CHIM., 5, 650-3 (1913). (9) Ramsdell, L. S.,Am. Mineral., 20, 569-74 (1935). (10) Rose, H., Ann., [4]3, 1-14, 594-614 (1854). (11) Schoch, E. P., IND.ENG.CHEM.,27,467-73 (1935). (12) Schoch, E. P.,U. S. Patent 1,794,551(March 3, 1931). (13) Storch, H.H . , IND.ENG.CHEM.,22, 934-41 (1930). (14) Storch, H. H.,U. S. Bur. Mines, Repts. Investigations 3032 (1930). (15) Storch, H.H., and Clarke, L., Ibid., 3002 (1930). REOEIVBID
September 28, 1936, Published b y permission of the Director, (Not subject to copyright.)
U. S. Bureau of Mines.
