Potash from Polyhalite by Reduction Process I. Preliminary Experiments with Hydrogen F. FRAAS AKD EVERETT P. PARTRIDGE Nonmetallic Minerals Experiment Station, Bureau of Mines, New Brunswick, IC'. J. KE recovery of potash
T
interpretation of r e s u l t s a n d The reduction by hydrogen of the double salt, from polyhalite (KQSOIbecause it would a c t u a l l y be potassium magnesium sulfate, io a mixture of MgS04.2CaSOd. 2H20) possible to produce it for use on potassium monosulfide and magnesia was rehas b e e n u n d e r investigation an industrial scale if it proved ported in 1877. Since it was possible that a since 1928 a t the Nonmetallic markedly superior to n a t u r a1 similar reduction might be obtained with polyMinerals Experiment Station of gas. Work with natural gas as the United States B u r e a u of the r e d u c i n g a g e n t is being halite, a study of the subject was undertaken Mines as a result of the extensive carried on a t present. and preliminary experiments are reported in and commercially significant deThe first r e d u c t i o n s w i t h this paper. I t was found that polyhalite could posits of this material revealed hydrogen were made in small be reduced at a rapid rate by hydrogen at temby core-drilling explorations in porcelain boats i n s e r t e d in a peratures above approximately 800" C. Subthe Permian basin of Texas and silica combustion tube, within an e le c t r i c a l l y he a t e d furnace New Mexico. T h e s e explorasequent hot extraction of the residue with water tions were conducted both by whose temperature was autoyielded solutions high in potash and total sulfur, m a t i c a l l y controlled. With a private i n t e r e s t s and b y the and extremely low in Ca++, Mg++, and SO4--. Bureau of Mines in cooperation moderate stream of h y d r o g e n Concentrat ion of these extract solutions caused passing through and with a rate with t h e G e o l o g i c a l Survey. the separation of small amounts of potassium of temperature increase of 15' C. Storch and his co-workers have per minute, it was found that appublished a n u m b e r of papers sulfate and potassium sulfite. Evaporation to preciable amounts of hydrogen (2-7) dealing with processes for dryness yielded an extremely hygroscopic prodsulfide began to be evolved a t the extraction of potash from uct consisting essentially of potassium mono650' C. After continued heatcalcined p o l y h a l i t e a n d the suljide, potassium hydrosu&de, and poiassium ing for 16 hours a t 675" C., hyrecovery of potassium sulfate, drogen sulfide and water \yere schonite (KzSO4.MgS04.6Hz0), sulfite, and containing 51 per cent potassium still being evolved in measurable or syngenite (K2S04CaS04.Hz0) and approximately 20 per cent water. This q u a n t i t i e s . At the end of 40 from the extract liquors. This water was removed by fusion, yielding a product h o u r s t h e s a m p l e was rework is being continued and excontaining 64 per cent potassium equivalent to moved and analyzed. Analysis tended. 77 per cent potassium oxide. showed sulfate e a u i v a l e n t to The recovery of potash from 22.8 per cent su1f;r and 7.6 per polyhalite b y a n y process invilves a t some point aseparation of calcium and magnesium cent sulfur in some lower state of oxidation, indicating that compounds from potassium compounds. I n nearly all of the the reduction was far from complete. A much higher degree processes hitherto described, this separation depends upon of reduction was obtained in a second experiment made in the the crystallization of simple or double salts from complex same manner but carried on a t 910" C. for 21 5 hours. This solutions obtained by the extraction of calcined polyhalite. yielded a product which contained 25.05 per cent total sulfur The experimental work described in this paper points to a and no SO4--. During this reduction, hydrogen sulfide and process in which reduction a t approximately 900' C. is water vapor were evolved and elemental sulfur wag deposited substituted for calcination at approximately 500" C., and from the exit gases on cooling. A series of reductions was subin which an apparently simple and efficient extraction is sequently made a t 900" C. for 2-hour periods, primarily for the purpose of studying the extraction of potash from the substituted for a more complicated series of process steps. Schumann (1) in 1877 had found that calcium sulfate reduction product. Additional preliminary experiments heated in a stream of hydrogen yielded a mixture of calcium showed a sharp rise in the rate of reduction in the temperature sulfide and calcium oxide, while the double salt, potassium range 800-900' C. Operations with the combustion tube produced only small magnesium sulfate, was reduced to a mixture of magnesia and potassium monosulfide. It seemed possible that polyhalite amounts of reduced polyhalite. Since it was found possible to might behave in a similar manner, and the availability of obtain water extracts highly concentrated with respect to cheap natural gas in large quantities close to the Texas-New potash, thorough investigation of the reduced material Mexico potash region made the reduction process seem worth demanded its production in larger quantities. For this reason investigation. The experiments described in this paper were in further experiments a large fire-clay crucible heated in a accordingly carried out, yielding results so favorable that gas-fired furnace was used for the reducing chamber as indifurther work is being actively continued. While the data cated in Figure 1. This crucible was 9 inches (23 cm.) high presented should be regarded only as preliminary and indica- and had an internal diameter of 5.5 inches (14 cm.) a t the top. tive, they show very definite possibilities of producing a A standard 1/8 inch (0.3-cm.) iron pipe introduced through a hole in the bottom of the crucible and fastened to the latter concentrated potash material from polyhalite. with alundum cement served as the hydrogen inlet. A tank of REDUCTION OF POLYHALITE WITH HYDROGEN compressed hydrogen was attached to this line through a flow Hydrogen was chosen as the reducing agent for the first indicator. Gaseous products of the reaction escaped through experiments on polyhalite, both in order to simplify the a hole in the clay cover of the crucible. Sulfur vapor and 1028
September, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
hydrogen sulfide were therefore oxidized to sulfur dioxide after leaving the crucible and were removed with the furnace gases. With the size of crucible used, a charge of 2000 grams of polyhalite could be reduced at one time. Temperature control in the gas-fired furnace proved more difficult than in the early experiments, but, by careful regulation of the gas supply and the maintenance of a long flame by a slight deficiency of air to the burners, it was possible to obtain fairly uniform temperature distribution in the furnace. Measurements of temperature in these experiments were made with an optical pyrometer on the outside of the crucible near the top. Owing to the heat evolved by the reaction, the temperatures within the polyhalite may have been appreciably higher. Analyses of the products from the reductions made in this type of apparatus are summarized in Table I. Two separate reductions, run A and run B, are shown. In each case the reduced polyhalite was removed from the crucible in three separate layers of approximately equal amount, 1 being the top and 3 the bottom layer next to the hydrogen inlet pipe. Run A was carried on for 70 minutes a t approximately 1050O C., and B for 100 minutes a t 950" C. crucible temperature, the rates of flow of hydrogen being approximately 22 and 13 liters of hydrogen per minute, respectively. The last column of Table I indicates the amount of material taken from each layer for a subsequent extraction experiment. TABLEI. REDUCTION OF
POLYH.4LITE
BY
around 1000O C., an estimate of the loss of potash by volatilization was desired. The potash lost by volatilization from different regions in the crucible could not be obtained by a direct material balance. However, a t the temperature of the reduction it may be assumed that very little magnesia is volatilized because of the low vapor pressure of any of its compounds which might be formed during the reduction. Relative values of magnesium and potassium before and after reduction of the polyhalite should therefore allow an approximate estimate of the potash volatilized. The data, as summarized in Table 11, indicate a high loss of morelthanta
I 4
L
AII'I'A
TAXEX RESCLTSF R O M F O R STANDARD COMEXTRACTIOX POSITE AXALYSIS O F REDUCTION K Ex- ExCRCPRODCCT con- trac- TRACCIBLE S O I - - Total S tent of tion T I O N R E S TEMP.LASER (as S) ( 8 8 S) M g K residue of K SAMPLE c. % % % % % % Grams .. 0.282 0.214 0.365 0.296 0.170 0.277
950
27.00 28.72 30.80 28.25 27.34 30.73
9:94 9.73 9.3s 8.62 8.75 8.35
19.95 19.62 17.90 25.72 24.70 23.70
0.243 0.267 0.396 0.087 0.070 0.146
98.8 98.6 97.7 99.7 99.7 99.4
72 158 58 28 135 179
XI1 the polyhalite used in these investigations was 10-30 mesh material which had been previously dehydrated by calcination a t a temperature of about 500" C. The original mineral contained halite (NaCl) and anhydrite (CaS04)as its chief impurities, together with very small amounts of magnesite (MgC03), iron oxide, and silicates. The halite content was reduced to a low value before calcination by washing with cold water. Analysis of the material after calcination and before reduction showed 20.15 per cent potassium sulfate, 21.25 per cent magnesium sulfate, and 0.18 per cent sodium chloride, the remainder being calcium sulfate (originally both combined in the polyhalite and present as anhydrite) plus m a l l amounts of the impurities noted. It should be noted that preliminary dehydration of the polyhalite in a separate step is unnecessary. TABLE11. ESTIMATED VOLATILIZATION REDUCTION
OF
POTASH DURING
CALCULATED VLLUES ANALYSIS OF TheoREDCCTION PRODUCT retical K RUN TEXP. L.AYER M g K K content A I( volatilmed O c. 7 0 % % % 1 (top) 9.94 19.95 30.25 -10.30 A 1050 34.05 CRIJCIBLE
A A B B B
950
2 3 1 (top) 2 3
9.73 9.38 8.62 8.75 8.35
19.62 17.90 25.72 24.70 23.79
29.63 28.55 26.25 26.65 25.43
A
HYDROGES RIAL
1050
-
P
MATE-
A d d B B B
-10 01 -10.65 - 0.53 - 1.95 - 1.73
33.75 37.30 2.10 7.32 6.81
Since the potassium compounds present during reduction may show an appreciable vapor pressure a t temperatures
1029
-
FIGURE1. GAS-FIRED FURNACE AND FIRE-CLAYCRUCIBLE USEDIN REDUCTION OF POLYHALITE BY HYDROGES
Furnace walls Fire-clay crucible C. Hydrogen inlet pipe D. Lock nuts E. Alundum cement F Broken.fireLbrick .-I. B.
G.
Polyhalite
Ti. Fire-clay orucible cover
I. Vent for gaseoua reaction products
.I.
Furnace vent
K. Fire-brick support for crucible L. Gas burner
third of the potash during run A a t 1050" C., but a relatively low loss during run B at 950" C. crucible temperature. The volatilization loss is being checked by actual material balances in further reductions in which the actual temperature of the the charge will be measured. It is apparent that the temperature for the reduction of polyhalite should be as low as possible in order t o minimize loss of potash while still gking a sufficiently rapid rate of reaction.
EXTRACTION OF REDUCED POLYHALITE Extractions of potash were first carried out on polyhalite which had been reduced in the combustion tube a t a temperature of 900" C. for a period of 2 hours. A series of these reductions was made in order t o collect 76 grams of the reduced material. Because of the high solid-liquid ratio and the resulting mechanical difficulties in separation when an attempt v a s made to reach the maximum top concentration in a single-stage extraction, the operation was carried out in two separate stages. These consisted of extracting, under reflux a t the boiling point, 40 grams of reduced polyhalite with 50 grams of water for 30 minutes, filtering off the leached residue and then using the filtrate to extract the remaining 36 grams. The results of these extractions are given in Table 111. The solid designated as residue is the filter cake from stage 1, which was washed with 50 cc. of water and dried a t 110' C. The two analyses of the extract liquors in stage 1 indicate that the potash content of the reduction product goes into
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 ICI I S T R Y
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Vol. 24, No. 9
solution very rapidly, while the content of 28 per cent potassium in the extract from stage 2 shows that high top concentrations should be possible. The fact that the residue from stage 1 after a simple wash contained but 0.22 per cent potassium is good evidence that complete removal of potash from the reduction product may be obtained. These data, combined with the absence of Ca++,Mg", and Sod-- in the extract liquor, indicate that the potash contained in t.he reduced polyhalite may be extracted simply and efficiently.
water a t the boiling point for 10 minutes and then filtered. The filter cake was washed with a second 630 grams of water a t room temperature. The extract and wash were then separately subjected to evaporation a t a temperature close to the boiling point a t atmospheric pressure, yielding the two series of crystal crops summarized in Table IV. From the data presented, a general idea may be gathered as to the quantity and the solubility of the various compounds present. No attempt was made to obtain a quantitative recovery of the filtrate a t each filtration, so that there was a cumulative TABLE 111. TWO-STAGE EXTRACTION OF REDUCTION PRODUCTloss due to the successive separations. However, the data BY WATERAT 100' C. suffice to show that the potash compounds of limited solu-ANALYSIS OF EXTRACTSbility are present only in small amounts in both the extract STAQD TIMW K Total S Mg a n d C a so4-and the wash solutions. Minutes % ' % % % 1 1 2
Q 30 10
14.01 13.93 28.00
SOLID Reduced polyhalite Residue, stage 1
6.88 8.76 16.88
None None None
None None None
-ANALYSIS OF SOLIDSTotal9 sod--
K
%
%
%
21.00 0.22
30.55 26.40
None None
When the reduction experiments were transferred from the combustion-tube furnace to the larger gas-fred furnace, it seemed desirable to develop some means of comparing the results obtained with different batches. Absence of SO4-in the solid reduction product would not be considered satisfactory evidence of complete reduction, since it was possible that oxysulfur compounds other than the sulfate might be present which would retard subsequent extraction of the potash content. Since the completeness of extraction is the final practical measure of the efficacy of the reduction process, a standard extraction procedure was adopted for testing samples from different batches. Standard Extraction. Ten grams of reduced olyhalite are extracted with 25 grams of water for 5 minutes at t i e boiling point, The mixture is filtered by suction, and the residue washed with another 25 grams of water at room temperature. The entire filter cake is anal zed for potash, and the percentage extraction is calculated on txe basis of the weight of potash in the sample of reduced polyhalite taken for analysis.
TABLEIV. EVAPORATION OF EXTRACT SOLUTIOXS FROM COMPOSITE SAMPLEOF REDUCED POLYHALITE WEIGHTOQ CRYSTAL APPROX. CRYSTAL CROP" EVAPN. CROP Grams
PDTROQRAPHIC IDENTIFICATION OF CRYBTAL CROP
E X T R A C T F I L T R A T E . 480 QRAMS
lb
0
2c
0
2.05 0.81
3d 4d
60 80
0.48 2.80
56
100
124
KzSOr in very thin crystals KzSo4 with traces of adsorbed FeS, rendering crystals pleochroic &SO4 KzSOz KzSOa (less t h a n in 3) KzSOa (more t h a n in 3) Probably KzS
+
+
WASH FILTRATE. 6SO Q R A M S
0 0.54 C a and Mg compounds present 72 0.92 K&O4 in ver thin crystals 80 1.57 Kz804 K2S103 95 2.07 K1S KzSOs KzSOa (less t h a n in 3) 5We 100 90.8 Probably KzS " All crystal crops were washed with 95% ethyl alcohol. b Precipitate from extract filtrate after cooling to room temperature. C Precipitate from extract after standing 2 days a t room temperature. d Crystal crop filtered from solution a t temperature near boiling point. 0 Residuea left after complete evaporation a n d drying at 1 10' C. for 15 hours. f Precipitate from wash filtrate after standing 3 days.
1WI
2 Wd 3Wd 4Wd
++
+
Analysis of the final crystal crops 5 and 5W from the filtrate and wash, respectively, yielded the results shown in Table V. The sulfur present as S--,So,--, and s&-- was determined by direct titration with iodine according to the method of Wollak (9). As indicated in the table, the total sulfur calculated from this titration checked well with the value With the relatively low solid-water ratio used in the stand- obtained by complete oxidation to sulfate and gravimetric ard extraction, there is a sacrifice of concentration for in- determination as barium sulfate. The total sulfur found by creased ease in manipulation. However, the results ob- the two methods was more than equivalent to the potassium tained with the short extraction period of 5 minutes show, as for the monosulfide (K2S). The excess sulfur could not be may be seen from Table I, that all of the potash com- attributed to polysulfides, since the excess sulfur in the polypounds present must have a high solubility and rate of solu- sulfides would not have appeared in the iodine titration. tion. High top concentrations, such as that of 28 per cent It seemed probable, therefore, that the final crystal crops conpotassium in the two-stage extraction reported in Table 111, tained potassium hydrosulfide (KHS) as well as potassium cannot be obtained in a single-step standard extraction be- monosulfide. This was checked by evolving the combined cause of the mechanical difficulties resulting from a high hydrogen sulfide, using nitrogen as the purging gas (8). The solid-water ratio. The question of whether all the potash experimentally determined values for combined hydrogen may be extracted in a countercurrent system to give a high sulfide are shown in the last column of Table V, together with the theoretical values Calculated on the assumptions that top concentration is under investigation. The standard extractions on the three different layers of the mixture contained both potassium monosulfide and material from each of the reductions in the gas-fired furnace potassium hydrosulfide, that in the acidified solution the indicate complete removal of the potash content in every case, thiosulfate was decomposed to sulfite and that this sulfite as but it is evident that the material from run B at 950' C. was well as that originally present was decomposed to free sulfur. somewhat more readily or more completely extracted than The agreement between the calculated and actual values for that from run A a t 1050" C. Since the results were uniformly combined hydrogen sulfide is very good in the case of crop 5, favorable, it was decided to extract a composite batch of the but the calculated is below the actual in crop 5W. Even in material to obtain sufficient solution for a preliminary the case of this poorer check, the value for combined hydrogen crystallization test to determine 17-hat potassium compounds sulfide definitely proves the presence of hydrosulfide. Although both crystal crops had been dried a t 110' C. for might be recovered by direct evaporation of the extract 15 hours, it was suspected that they still contained appreciliquors. A composite sample of 630 grams of reduced polyhalite was able amounts of water, probably as water of hydration. A made up, as indicated in the last column of Table I: from the direct determination was made a t 440" C., using a current of various portions of the reduction products of runs A and B. nitrogen to sweep the evolved water vapor into a calcium This material was extracted under reflux with 630 grams of chloride absorption tube. The values shown in Table V were
September, 1932
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 ST R Y
obtained. A simultaneous determination of hydrogen sulfide evolved during these analyses showed values of less than 0.1 per cent of the weight of the original samples.
1031
BY-PRODUCTS FROX REDUCTION PROCESSES
The more obvious possible by-products from the reduction of polyhalite are magnesium carbonate and sulfuric acid, the TABLEV. CHEMICAL ANALYSISOF POTASH SCLFIDE PRODUCTS former to be obtained by treatment of the solid residue after extraction, and the latter by oxidation of the sulfur and S COYPOUNDS CRYSTAL POTISB Y IODISE TOTlL C O M B I X E D hydrogen sulfide contained in the exit gases from the reducCROP SIC11 TITR$TIOX Sa WATER HnS tion. 70 %S % S % HzO %S 5 51 00 ( S-- f H S - 2 4 . 8 0 ) 26 29 20.43 Detd. 21.85 The specific substances present in the solid residue from the s03-1.41 I Calcd.21.90 extraction step have not yet been identified, but this residue s203--. 0.00 i __ must contain calcium and magnesium compounds equivalent Total 26.21 1 to the calcium sulfate and magnesium mlfate present in the HS- 2 4 . 1 5 I 2 6 . 3 519.79 Detd. 2 2 . 2 3 1.33 I C d c d . 20.65 original polyhalite. Table VI1 shows the potassium, calcium, 1 szo3-0.96 } magnesium, and sulfur determined by actual analysis after I Total 26.44 J drying a t 105" C., as well as the hypothetical composition Gravimetric determination. calculated on the assumptions that the 24.04 per cent not accounted for in the analysis is hydroxide, and that only simple The composition of the two crystal crops, as calculated from sulfides and hydroxides are present, While the composition the iodine titrations and the determinations of potassium and indicated is admittedly hypothetical, it agrees rather well water, are shown in Table VI. I t is apparent that the two with the analytical data. crops are very nearly identical in composition, the only difference being the indicated presence of a small amount of FROM EXTRACTION O F REDUCED potassium thiosulfate in crop 5W and the absence of this TABLEVII. SOLID RESIDUE POLYH.4LITE substance in crop 5. e
~
5
ANALYsIs
Element
TABLEVI. CALCULATED COMPOSITION OF POTASFI SULFIDE
K
PRODUCTS COMPONEXT
PRODCCT F R O M F I LT n A T E
% KrS KHS KzS203 Hi0
48.96 23.75 6.98 0.00 20.43 --
Total
100.11
Cs M . 9
PRODTJCT Fl
