INDUSTRIAL AND ENGINEERING CHEMISTRY
934
Vol. 22, No. 9
Extraction of Potash from Polyhalite' H. H. Storch NONMETALLIC MINERALS EXPERIMENT STATION, U. S. BUREAU OF MIXES,NEW BRUNSWICK, N. J.
Some data on the rate of decomposition of polyhalite by water a t 25 ', 50 ",and 100' C. are presented. The rate a t 25" C. is sufficiently rapid to indicate the value of polyhalite, per se, as a fertilizer. The rates a t the three temperatures are too slow to justify percolating or heap leaching, for the manufacture of K2S04. Upon heating polyhalite to temperatures between 298 O and 500' C. the water of crystallization is driven off, leaving an amorphous mixture of K1S04, MgS04, and CaS04. Two transition points were located-namely, one a t 298" C. which coincides with the vigorous evolution of steam, and one a t 551 'C. which results i n the formation of a hard mass of crystals. The heat of decomposition of polyhalite between 298" and 400" C. was found to be approximately 66,000 calories by direct measurement. The optimum temperature range for calcination with regard to subsequent extraction with 2 parts of water a t 100" C. is 430-465" C. if hour's retention a t the calcination temperature is allowed. The maximum concentration of K2S04obtainable by leaching calcined polyhalite with boiling water appears to be 11.4 grams per LOO grams of water. Countercurrent leaching is not desirable, for the reasons set forth below. A study of the mechanism of the extraction of calcined
polyhalite with water a t 100" C. led to the following conclusions: (1) The rate of extraction of K2S04and MgS04 is practically independent of the rate of agitation, and is probably controlled by some hydration phenomenon. (2) The activation of the anhydrite (formed during calcination) by the potassium sulfate solution obtained immediately after mixing calcined polyhalite with water, results in the formation of potassium calcium pentasulfate and later of polyhalite, thereby decreasing the and MgS04. This behavior concentrations of makes countercurrent leaching undesirable. Three possible industrial processes, two producing &So4 and the third both &SO( and KzSOa.MgSO4, are outlined, and a brief statement of the production costs is given. Preliminary data are presented on the rate of reaction of both calcined and uncalcined polyhalite with boiling saturated NaCl solutions. These data indicate that it may be possible to produce KC1 by starting with polyhalite and NaCI. Some data on the synthesis of syngenite starting with calcined polyhalite are given. Syngenite would be a new potash fertilizer for which a market may be developed.
ORE drilling by the United States Government in the
the tubes by hand. I n the 100" C. experiments the same mixture was gently boiled under a reflux condenser. Further data a t 25" C. showing the rate of decomposition using different ratios of water to polyhalite are shown in Figure 2. It will be observed from Figure I that there is a continuous supersaturation so far as the potassium sulfate concentration is concerned a t all temperatures (3) up to as long as 100 hours contact time. The highest concentration obtained a t 25" C. was 5.07 grams of potassium sulfate per 100 grams of water, using 1 part of polyhalite to 1 part of water and 504 hours' contact time. At 100" C. the same concentration was obtained in 120 hours (using 2 parts of water). This concentration is less than one-half of that which may be obtained by treating 1 part of calcined 65- to 100-mesh polyhalite with 2 parts of water at 100" C. for 45 minutes. It is therefore unlikely that either heap or percolating leaching will be useful in any industrial process for the extraction of potash from polyhalite. The temperature coefficient of the rate of decomposition in the 25" to 50" C. range during the first hour of contact is 1.1 to 1.3 but appears to become negative after about 4 hours. The coefficient between 50" and 100" C. appears to be negative during the first hour, but subsequently reaches a positive value of about 1.1. This behavior may be due partly to differences in rate of agitation. I n going from agitation by occasional shaking by hand to 150 r. p. m. of a screw stirrer a t 25" C. the rate of decomposition during the first 15 minutes was increased about 10 per cent, and at 900 r. p. m. about 30 per cent. Petrographic examination of the mixtures used in the foregoing experiments yielded the following information :
C
Texas-New Mexico potash fields has revealed the presence of large deposits of polyhalite (KzSO4.MgS04. 2CaS04.2H20). A description of the findings is given by Mansfield and Lang (7) and the mineralogy of the cores is discussed by Schaller and Henderson (8). During the past two years an extensive study of the chemical and physical properties of polyhalite has been conducted by the Nonmetallic Minerals Experiment Station of the U. S. Bureau of Mines a t Rutgers University, New Brunswick, N. J. Some of the results have been published by Storch and Clarke ( 2 1 ) . A preliminary estimate of the costs of mining and treatment of polyhalite so as to produce K2S04 and K2S04. MgS04, utilizing two processes developed by Storch and Clarke ( I I ) , has been published by Wroth ( I S ) . I n this paper all the work pertaining to the extraction of potash which has been done to date on the properties of polyhalite, and the various processes which are indicated as a result of this work, will be discussed briefly. Rate of Decomposition of Polyhalite by Water a t Various Temperatures The rate a t which polyhalite is attacked by water is of importance in the use of polyhalite, per se, as a fertilizer, and in answering questions concerning the possibility of developing processes for the extraction of potash based on percolating or heap leaching. The rates a t 25", 50", and 100" C. are graphically presented in Figure I. I n the 25" and 50" C. experiments 40 grams of water and 20 grams of minus 325-mesh polyhalite (23.7 per cent K2S04, 16.3 per cent MgS04, 10.8 per cent NaCl) were placed in stoppered test tubes and occasionally agitated by vigorous shaking of 1
Received May 21, 1930. Published by permission of the Director, (Not subject to copyright.)
U. S. Bureau of Mines.
(1) Gypsum crystals appear a few minutes after mixing at 25" and 50" C., but not at 100' C. (2) Potassium calcium pentasulfate ( K ~ S O ~ . ~ C ~ S O I . H ~ O ) crystals were observed in the 100" C. samples after several hours.
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
September, 1930
(3) It was found inlpossible to determine concliisively the Presence Or absence of SYngefite (KzS04.CaSO4.HzO) in the 25" and 50" C. samples because of the imperfect form of the crystals. However. the marked chance in slooe of the 25" C. ootash curve of Figure I after 340 hours indicates formation of syngenite in appreciable quantity.
for the decomposition plus radiation losses (assuming that the specific heats of calcined and uncalcined polyhalite are and constant in the temperature range used). The radiation losses were determined by measuring the amount of heat necessary to keep the inner tube at a constant temperature for various temperatures between 250" and 400" C. Two experiments yielded the values 68,600 and 64,000 calories per mol. Hence the heat of decomposition may be approximated as 66,000 * 5ooo calories per mol.
By using 6 parts of water to Of 'I), 62.8 per cent decomposition was obtained in 24 hours. For agricultural purposes this rate is sufficiently rapid to warrant the use of polyhalite, per se, as a source of potash.
Leaching of Calcined Polyhalite with Water at
~ 4 P ~ ~ , ~ ~ M R
Ruff o f &omp@,f/on
f /GURC Z o f F'o/&4&@f%f/nus325 Mesh) f'okhhte)
6~ wufe-/Twoporn ro om of
--
-T-'
7inw
N)
XourJ
Calcination of Polyhalite
935,
100"
c.
During the preliminary experiments on the extraction of potash from calcined polyhalite by water a t 100" C., it was found that countercurrent leaching was futile because of the existence of a maximum concentration of 11.4 grams (about one-half the saturation value of potassium sulfate in the absence of calcium sulfate) of potassium sulfate per 100 grams of water and because prolonged contact of water with calcined polyhalite resulted in a decrease in the maximum potassium sulfate concentration. Hence experiments were confined to the study of the variables involved in obtaining a high percentage extraction of potash with the maximum concentration of potassium sulfate in the leach liquors. Using a 4-inch (10-cm.) horizontal tube furnace and a uniform rate of heating (approximately 10" C. per minute) to the maximum temperature to be investigated, the results given in Table I were obtained. I n all the experiments listed in this table 450-455" C. was the maximum calcination temperature employed. This was found, by preliminary experiments, to be the optimum temperature for 30 minutes' time of retention in the furnace, lower yields being obtained at temperatures appreciably above or below this temperature. A longer time of retention lowers the optimum temperature, and a shorter time increases it to points dangerously near the sintering range (525-551" (2,).
Using a differential method (temperature difference between a sample of quartz and one of polyhalite heitted slowly in the same furnace) two transition points of polyhalite were detected. One occurs at 298" C. and corresponds with a vigorous evolution of' steam. The other transition is at 551" C. and absorbs only about one-tenth the heat 6o absorbed a t 298" C. The 551" C. point corresponds to a rapid sintering and recrystallization of the originally finely 54 powdered polyhalite. Petrographic examination2 shows that upon heating above 298" C. the originally transparent crystal fragments e of polyhalite change to an amorphous material which is probably a solid solution of potassium, magnesium, and calcium sulfates. The CY refractive index changes progressively from 1.548 below 298" C. to 1.520 a t 460" C., where it starts to increase, again reaching 1.535 at 551" C. At this temperature a totally new crystalline material appears, probably a mixture of langbeinite (K2S04.2MgSOJ, potassium dicalcium sulfate (K2S042Ca2S04),and anhydrite (CaS04). An attempt was made to determine the vapor pressures of water vapor over polyhalite a t various temperatures in 6 order to calculate the heat of decomposition. However, it was soon found that polyhalite wasnotinequilibriumwith o zs 50 75 /a0 /z5 Ns a. pzs A---/oo 3w m E r n /n Hours water vapor-i. e.,that calcined polyhalite reacts with water vapor to form several ComPounds~none of which is Poly- Shorter times of retention have been successfully employed halite. Hence the heat of decomposition was measured with a laboratory-size (2 x 30 inch or 5 x 76 cm,) rotary directly, using an electrically heated P F e x tube containing kiln, and the results of these tests will be published as soon finely powdered polyhalite and insulated by a silvered Dewar as sufficient data have been obtained. tube which was itself carefully insulated. The amount of Preliminary experiments also showed that it was not d e heat absorbed from 250" to 400" c. Was measured first with sirable to calcine very coarse polyhalite and subsequently the original PolYhallte and second with the dehydrated grind before, or during, extraction. The efficiency of the PolYhalite, the difference being the quantity of heat necessary extraction of potash from calcined polyhalite depends, among other things, upon practically complete desiccation of the. 1 A. C. Hawluns, consulting petrographer, New Brunswick, N J.
INDUSTRIAL A N D ENGINEERING CHEiMISTRY
936 Table I-E5ciency
of Extraction of Potash from Calcined Polyhalite a8 Function of Particle Size a n d T i m e of Extraction w i t h 2 Parts of Water a t looo C. (All calcinations made at 450-455' C. for 30 minutes using 100 grams polyhalite.)
EXTN. TIME
EXPT.
100 EXTN.A GRAMS WASHINGSH r o POLYB
RATIO K ~ / M ~ Found MgSO4
Cas04
NaCl
Grams
Grams
Grams
Grams
10.3 4.8 9.1 9.9 4.06 8.85 10.9 5.16 9.68 11.0 4.9 9.68 11.2 5.06 9.9 10.9 5.32 9.66 11.3 4.6 9.93 11.4 5,72 10.7 11.2 7.04 10.3
7.3 2.9 6.3 7.06 2.58 6.27 7.75 4.04 6.96 7.8 3.5 6.9 7.92 3.92 7.06 7.77 3.54 6.85 7.96 3.66 7.05 8.08 4.04 7.18 8.02 4.90 7.3
0.16
2.3 1.0 2.0 2.25 0.88 2.01 2.02 1.08 1.81 2.05 0.96 1.82 2.06 1.06 1.84 2.12 1.02 1.88 2.4 1.0 2.13 2.62 1.26 2.31 2.63 1.64 2.4
HALITE
Mesh
%
Hours
Grams
1C-20
26.7
1.0
203
156
10-20
26.7
2.0
207
159
48-65
26.0
0.75
216
161
48-65
26.0
160
48-65
26.0
163
48-66
26.0
2.0
213
164
65-100
26.2
0.75
217
165
65-100
26.2
1.00
203
166
65-100
26.2
2.00
210
EXTRACTION OF K ~ .SO .I
MOL
PER
'
K2S04
I
CONCENTRATIONS PER 100 GRAMSHzO
U5ED
155
-
I
H20
POLYHALITE
Partial size
Vol. 22, No. 9
Grams A B A&B A B A&B A B A&B
A B A&B A B A &B A B A&B A B A&B
180 50 230 188 50 238 182 50 232
179 50 229 187 50 237 172 50 222 171 50 221
original polyhalite without sintering, and in order to accomplish this with a comparatively coarse material a very long period of heating a t a low (relatively) temperature is necessary. Hence the main purpose of the experiments of Table I was to ascertain the maximum particle size of calcined polyhalite which would yield an efficient extraction. For the experiments of Table I a 2-inch (5-cm.) Pyrex tube containing the sample of polyhalite (usually 100 grams), a thermocouple, and a gas inlet and outlet tube was surrounded by a tube of sheet iron and the whole placed inside the 4-inch (lo-cm.) alundum tube furnace. Carbon dioxide was passed through the tube a t the rate of approximately 6 liters per hour. Somewhat better results appear to have been obtained using carbon dioxide rather than air to displace the steam given off by the polyhalite, but the differences are well within the limit of experimental error. After calcination, the polyhalite was quenched by dropping it in a steady stream into 2 parts of boiling water. Quenching appears to be desirable, for 5 to 10 per cent lower concentrations are obtained if the polyhalite is cooled to below 100" C. before mixing with 2 parts of boiling water. Boiling was continued under a reflux condenser for various time intervals. The mixture was subsequently weighed, filtered on a suction filter, and washed with 0.5 part (based on weight of uncalcined polyhalite) of cold water. It was essential to weigh the mixture before filtering in order to avoid errors due to water lost during the quenching operation. By comparison with tests made using a steam-jacketed funnel for filtering the mixture, it was found that washing with 0.5 part of cold water was just sufficient to displace retained mother liquor without any appreciable further extraction of potash. The filtrate, washings, and filter cake were analyzed. The filtercake data are not listed in Table I, for they were used merely as a means of obtaining a potash mass balance which served as a check upon the accuracy of the analytical data. The last column of Table I gives the extraction efficiencies when a glass screw stirrer rotating a t about 1000 r. p. m. was employed, I n all the other experiments the only agitation was that due to gentle boiling. Somewhat lower yields were obtained when mechanical agitation was employed.
0.27 0.18 0.14 0.26 0.17 0.16 0.37 0.21 0.14 0.32 0.18 0.15 0.35 0.19 0.13 0.23 ,0.16
0.16 0.37 0.21 0.14 0.29 0.17 0.12 0.29 0.15
poly-
I
I
halite
% 0.97 1.18 0.995 0.966 1.08 0.975 0.97 0.88 0.96 0.975 0.966 0.968 1.00 0.89 0.987 0.97 1.04 0.975 0.981 0.876 0,972 0.980 0.976 0.978 0.964 0.990 0.973
Calcd. for 2 : l
69.4 9.0 78.4 69.6 7.4 77.0 76.2 9.9 86.1 78.0 9.5 87.5 79.6 9.6 89.2 75 10.1 85.1 81.0 9.0 90.0 75.2 10.8 86.0 73.4 13.3 86.7
%
Using agita-
tion
%
77.0
64.0
74.8
..
79.5 80.8
75.0
81.6
..
79.6
..
82.5 84.8
80.4
82.5
This behavior may be explained by the relative rates of extraction of potassium sulfate and the approach toward equilibrium. The solutions obtained are, in fact, highly supersaturated with respect to the stable solid phases potassium calcium pentasulfate, polyhalite, and anhydrite, for prolonged boiling decreases the concentration of potash and magnesia. The optimum time of extraction for 65- to 100mesh material is 60 minutes, and extraction with 2.3 parts of water would be necessary for a yield of over 90 per cent. I n order to obtain a better insight into the processes occurring during the extraction of calcined polyhalite, experiments on the rate of reaction of calcined and uncalcined plaster of Paris (CaS04.1/2Hz0)with KZS04-MgS04 solutions were performed. The results of these tests are given in Table 11, which also includes experiments with calcined polyhalite on the variation of extraction efficiency with time of extraction. I n all cases, unless otherwise mentioned, the solid materials consisted of 100- to 200-mesh particles. All calcined materials were quenched in boiling water, and the reactions conducted in a flask connected to a reflux condenser; 100- to 200-mesh material was employed because it could be kept in fairly uniform suspension by agitation due to boiling, and thus avoid changing the ratio of solids to water when a sample of the mixture was withdrawn. The samples obtained were filtered on a steam-jacketed funnel, and the filtrates were analyzed. A comparison of experiment E-3 with E-5 indicates that mixtures of calcined polyhalite and water behave very much like mixtures of calcined plaster of Paris and KzS04-MgSOd solutions. Thus in experiment E-3 the concentration of K2S04a t the end of 1 hour is almost identical with that of E-5. The MgS04 concentrations are not comparable b e cause the initial concentration of MgS04 in E-3 was too low. Experiments E-10 and E-13 show that for calcined polyhalite of 100 to 200 mesh the optimum time of extraction is 30 minutes as compared with 1 hour for coarser materials (of Table I). Upon comparing experiments E-5 with E-IO, it is observed that the decrease of KzS04 concentration is much more rapid in the case of a 1.5:l ratio of water to polyhalite than in that of a 2:l ratio. Experi-
I S D U S T R I A L A S D ENGILVEERIAYGCHEMISTRY
September, 1930 Table 11-Rate
937
of Approach to Equilibrium of Various Mixtures of CaSO4, KzSOs, MgSO4, a n d Water a t 100' C.
EXPT.
WATER
SOLID
E-2
80 grams CaS01.'/zHz0, 45 grams KPSOP,45 grams MgS04.7HzO
E-3
80 grams CaS04.'/zHzO calcined at 445' C. 45 grams MgSOc.7HzO
I/?
Grams 300
CONCENTRITIOX TIME PER 100 GRAMS AFTER MIXING &SO4WATER MgSOi
Hours 0 1 20
Grams
Grams
15.0 5.75 5.35
7.32 4.18 3.24
1.41 0.95 1.14
0
15.0 10.7 10.1 6.7 10.55 9.90 5.36 10.77 11.11 11.02 10.90 11.04
7.32 6.95 6.35 2.76 8.9 8.1 3.8 8.32 8.16 7.97 8.83 7.85
1 0 . 75 05 10.35 10.33 9.7 11.3 11.3 11.1 10.8
7 . 56 67 7.42 7.37 7.9 8.4 8.3 8.0 7.7
1.41 1.06 1.10 1.67 0.815 0.842 0.9YZ 0.893 0.940 0.955 0.961 0.971 0.964 0.963 0.963 0,968 0.848 0.928 0.940 0.957 0.969
hour, 45 grams K z S O I , 300
1
E-5
200 grams polyhalite calcined at 440' C. */z hour
300
E-10
200 grams polyhalite calcined at 445' C. 1/z hour
400
33-13
180 grams polyhalite, 150-200 mesh, calcined at 450' C. 1 / z hour
ment E-2 as compared with E-3 affords a clue as to why cooling of calcined polyhalite in humid air is detrimental. An examination of the KzSO4 and MgS04 concentrations of experiments E-10 and E-13 shows that the MgS04 concentration starts to decrease before the KzS04 concentration does. This is especially apparent in experiment E-10 where a decrease in MgSO4 concentration occurs between 0.25 and 0.5 hour, whereas the KSSOd concentration increased in the same interval. These observations are verified by an analysis of the residue a t various intervals during the extraction (Table 111). Table 111-Variation
of P o t a s h a n d Magnesia C o n t e n t s of Extraction Residue w i t h T i m e POTASH AS
CONTENT
Minutes 15 30 45 60 75 90 105 120
%
%
%
%
6.7 4.8 6.7 8.3 9.4 10.5 11 1 12.2
1.0 1.0 5.0 40.0 48.0 40.0 26.0 20.0
0.2 0.2 1.0 8.0 9.6 8.0 5.2 4.0
0.5 2.6 4.0 5.2 5.8 6,6 7.0 7.5
A petrographic study was made of the changes occurring during the extraction of calcined (455" C. 30 minutes) polyhalite (250-325 mesh) with 2 parts of water at 100" C. (using a reflux condenser). The procedure adopted was to remove a sample of the mixture at 15-minute intervals, filtering rapidly on a suction filter, washing first with 50 per cent and then with 90 per cent alcohol, and finally evaporating the alcohol in the residue by exposure to air a t room temperature. The samples thus obtained were analyzed for their KzS04 content, and examined with the aid of a petrographic microscope and a set of oils of known refractive index. The changes observed were as follows:
360
MOL
RATIO Kz/Mg
2 20 1 2 22 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.0 2.25 0.25 0.50 0.75 1.00 2.00
It will be observed, on comparing the second and fourth columns of Table 111,that some potash remains in the original grains of calcined polyhalite during the first 45 minutes of the extraction, but that all of it appears as potassium calcium pentasulfate in the 60- to 75-minute interval. After 75 minutes the percentage of potash as the pentasulfate decreases steadily. This decrease is undoubtedly caused by the formation of polyhalite. It was very difficult to obtain a petrographic estimate of the amount of polyhalite present in the samples, because the polyhalite forms in crystal plates which cannot be readily distinguished from such anhydrite particles as have been partly transformed to potassium calcium pentasulfate. A comparison of the second and fifth columns verifies the statement, made in a previous paragraph, that the MgS04 content of the residue starts to increase before the KzSO4 content does. The literature concerning potassium calcium pentasulfate is very meager, consisting only of van't Hoff's work ( d ) , in which he describes its preparation by boiling precipitated gypsum with 5 per cent KzS04 solution for 2 days. No optical data are given. It was found possible to prepare this compound more readily (in a few hours) by using plaster of Paris instead of gypsum. The potassium calcium pentasulfate thus prepared consisted of short, chisel-edged narrow prisms of refractive index 1.560. Its identification in the extraction residue was facilitated by its high refractive index as compared with that of syngenite or of gypsum. The processes occurring during the quenching and extraction of calcined polyhalite seem, from the evidence thus far submitted, to be as follows:
(1) Quenching is desirable in order to retard the hydration of calcined polyhalite. Hydration of the calcined polyhalite results in a more rapid approach to equilibrium of the solution in contact with it, and hence a lower concentration of KzSO,. (2) The extraction of soluble salts from 1 part of calcined polyhalite with 2 parts of water at 100' C. proceeds rapidly, resulting in a supersaturated solution which upon prolonged boiling reacts with the calcium sulfate residue to form potassium calcium pentasulfate; this subsequently reacts further to form polyhalite, thereby decreasing the concentrations of KzSOI and (1) The originally partly amorphous grains of the calcined MgSOd. polyhalite became increasingly transparent as the extraction (3) The rate pf extraction of calcined polyhalite with modproceeded. This change is due to t h e formation of anhydrite erate amounts of water at 100" C. appears to be largely independas the K&Od and MgSOd are extracted. ent of diffusion factors, for vigorous mechanical agitation has (2) Potassium calcium pentasulfate (K~S0a.5CaSOd.Hz0) only a small effect on the rate of extraction. Furthermore, the crystals were observed in all the samples in amounts as given in effect of vigorous agitation is to decrease the percentage of Table 111. K&O, extracted.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
938
(4) The maximum concentration of KzSO4 obtainable by the extraction of calcined polyhalite appears to be 11.4 grams per 100 grams of water. The existence of such a maximum may be explained by the competitive rates of reaction mentioned in the previous conclusions. Possible Indystrial Processes
Polyhalite as mined may possibly contain as high as 10 per cent of sodium chloride. It has been found possible to separate over 90 per cent of the NaCl with a loss of about 4 per cent of the K2S04 by rapid washing with water a t 20-25" C. The success of this method depends upon the relatively slow rate of decomposition of polyhalite as compared with the rate of solution of sodium chloride. Hence, it seems feasible to remove the bulk of the NaCl by a rapid wash before calcination of the polyhalite, and the solution resulting from the extraction of washed and calcined polyhalite may therefore be assumed to contain 11.4 grams of KBO4, 8.1 grams MgS04, 0.15 grams of CaSOI, and 0.2 grams of NaCl per 100 grams of water. Table IV-Lime
EXPT.
c-5 C-8 c-9 c-10 (2-15 (2-16 (2-18 c-19 (2-20 c-21" c-224 C-23 (2-26 '2-27
CONCN. OF Ca(0H)z S L U R R Y PER 100 GRAMSHzO
Ca (OH)OF z
Grams
%
Added as solid Ca(0H)z Added as solid Ca(0H)z Added as solid Ca(0H)z 20 18 18 36 36 9.8 9.0 17.1 18.0 18.0 18.0
1.1 1.1 1.1 13.0 1.1 1.2 1.2 1.2 0.0 3.7 1.2 1.2 1.2 1.2
VOI. 22, KO.9
and extraction. Hence for purposes of further discussion the extracts will be assumed to contain 10.5 grams K2S04, 7.5 grams MgS04, 0.15 gram CaS04, and 0.2 gram NaCl per 100 grams of water, the K2/Mg ratio being 0.978. Since the amounts of calcium sulfate and of sodium chloride are relatively small, the behavior of this system should be essentially that of the system KzS04-MgS04-HzO. This system has been studied by several investigators (1,4,5,6,12) and the results a t lower temperatures are fairly concordant, with the exception of the work of Weston. This investigator reported considerably higher concentrations of K2S04 in the solutions which are in equilibrium a t the two invariant points with the solid phases: K2S04 schonite (KzS04.MgS04.MgS04.7H20. Weston also re8H20); and schonite ported that mixed crystals were formed in the neighborhood of the second invariant point. Because of the discordant nature of Weston's results, it was found desirable to determine the equilibrium diagram for this system a t 30" C. Weston's results were found to be in error, and that the
+
+
Treatment of KzSOeMBSO, Solutions
INITIALCONCENTRATION PER 100 GRAMSHz0
TIMBOF TIMEOF ADDITION ADDITIONAL RECOVERY REMOVAL SLURRY OF MgSO4 STIRRING OF KiSO4 MIXTURE
OF
KzSO4
MgSO4
NaCl
Grams 10.0 6.7 5.7 8.0 8.0 13.3
Grams
Grams
6.9 4.6 3.9 5.5 5.5 9.4
0.0
0.0 0.0 0.0 0.0 0.0
Minutes
All All All All
at once at once at once at once 20 15 6 8
8.0 8.0 8.0
5.7 5.6 5.6
0.0 2.5 0.3
32 32 34 25 25 25
Minutes
%
%
None None None None None None 11 10 10
47 76 76 76 96 69 54.2 71.4 74.7 86.1 81.3 93.6 96.7 93.0
96.0 94.0 99.7 99.9 94.0 91.2 96.4 93.9 94.8 90.8 98.4 94.7 96.0 93.0
10
10 None None h'one
a Hot salt solution sdded t o the boiling Ca(0H)z slurry instead of the reverse as was done in the other experiments.
I n order to obtain 90 per cent (or greater) extraction of K2S04 from calcined 65- to 100-mesh polyhalite, Table I indicates that 2.3 parts of water followed by washing with 0.5 part of water are necessary. If this wash water is returned as part of the water for a subsequent extraction, a somewhat lower concentration of KzS04 is obtained, because the presence of a small amount of K2S04 in the initial quenching water speeds up the hydration of Cas04 and hence the formation of solid phases containing Cas04 and K2S04. Thus in experiment 170 an extraction of 100 grams of 65- to 100-mesh calcined (445' C., 30 minutes) polyhalite was made using 2.54 parts of water containing 2.3 grams of and 1.83 grams of MgSO4. This would correspond to using the wash waters of a previous extraction and fdtration in the make-up water of the next extraction, allowing an additional 0.24 part of water for the potash and magnesium salts present in the wash water. The concentration of KZSO4 obtained was 10.5 grams per 100 grams of water, the Kz/Mg mol ratio being 1.01 and the per cent extraction, 93.1. I n experiment 171 the conditions of experiment 170 were duplicated except that the potassium and magnesium sulfates were omitted and 2.30 parts of water used instead of 2.54. The concentration of K z S O ~obtained was 10.65 grams per 100 grams of water, the K2/Mg mol ratio being 0.965 and the per cent extraction, 94.6. These data indicate that if the wash waters are returned as make-up water for the next extraction, and if a 90 to 95 per cent extraction of the KzS04in calcined polyhalite is desired, the maximum concentration of K2S04 obtainable is 10.5 grams per 100 grams of water with a K2/Mg ratio varying from 0.96 to 1.0 depending upon the exact conditions of the calcination
results obtained (10) agree very well with the data of the other investigators. Upon evaporation of calcined polyhalite extracts until a concentration of MgS04 slightly less than that corresponding to the first invariant point (at 25" C.) is reached, and then cooling to 25" C., K2S04 will be deposited; the theoretical yield is 39.8 per cent of the total potash in solution. This yield is calculated for a Kz/Mg ratio of 0.978. During this process practically all of the Cas04 will be deposited as syngenite (CaS04.K2S04.HzO), but the amount of Cas04 is such that the K2S04 product would contain only 1 to 2 per cent of this impurity. The syngenite crystallizes in wellformed needles and seems to have no scaling tendencies. I n order to avoid the formation of leonite (KZS04.MgS04.4H20) with the K2SO4,it was found necessary to cool very slowly in the temperature range between 50" and 35" C. and that adding a very small amount of K2S04 seeds (about 0.2 per cent of the total finally obtained) was desirable. This formation of leonite is probably due to a metastable extension of the leonite solubility curve in the temperaturerange from 50" to 35' C. Evaporation of the mother liquor from the &SO4 crystallization until a concentration of MgSO4 slightly less than that corresponding to the second invariant point is reached and subsequent cooling a t 25' C. results in the crystallization of schonite, the theoretical yield being 48.7 per cent of the total potash in the original solution (before removal of the K2S04). The mother liquor would contain 11.5 per cent of the total potash (in the original extract) and would probably be discarded. During this second evaporation con-siderable quantities of leonite (KzS04.MgS04.4HzO) would
September, 1930
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
be deposited in the evaporators, and hence a salt removal system would be necessary. The first evaporation (to yield KzS04) may obviously be performed with safety in a triple-effect evaporator (or one containing a larger number of effects); but the second evaporation step entails the possibility of langbeinite (KzS04.21fgSo4) deposition if the concentration toward the latter part of the evaporation is done at too high a temperature. The equilibrium diagram for the system KzSO4-MgSO4-HzO has been determined for 85" C. and 100" C. (IO). An inspection of these curves indicates that, while the langbeinite regions are fairly extensive, it is possible to use triple-effect evaporation. The use of a larger number of effects is, however, not permissible. The above process (which will hereafter be called process 1) would produce K2S04 and K2S04.MgS04. It is. however, possible to produce only K2S04 by adding such an amount of previously prepared schonite as would bring the extract of calcined polyhalite up to a concentration of MgS04 corresponding to a point close to the first invariant point (of the 25" C. diagram). This addition results, upon cooling to 25" C., in the deposition of most of the KzS04 present in the original extract. The amount of schonite added must be so adjusted that it may be completely recovered upon evaporation of the mother liquor from the &So4 crystallization. This process will hereafter be called process 2. Process 3 for the extraction of potash from polyhalite consists in the addition of slaked-lime slurry to calcined polyhalite extracts. This results in the precipitation of the magnesium as Mg(OH)z along with gypsum. The chief difficulty involved in this process concerns the simultaneous formation of syngenite (KzS04. CaSO4. HzO) with the magnesia-gypsum precipitate. Table IV gives the results of a number of experiments which indicate the conditions that yield the smallest amount of syngenite crystals, while a t the same time removing the bulk of the magnesium sulfate from solution. I n all cases the mixture was heated a t 100" C. and mechanically agitated during the mixing of the slaked lime with the potash solution. After completed mixing, and heating for a short while to coagulate the precipitate, the mixture was filtered on a suction filter and residue was washed with small amounts of water. From Table IV the following conclusions may be drawn: (1) If the KzSOl and MgS04 concentrations are adjusted so as to be about 8.0 and 5.6 grams per 100 grams of water, respectively, the lime slurry concentration being 18.0 grams per 100 grams of water, and if the time of addition of the lime slurry is about 25 minutes, 93 to 98 per cent of the magnesia can be removed with 93 t o 97 pe.r cent recovery of the K2SO4. ( 2 ) The presence of as high as 2.5 grams of NaCl per 100 grams of water does not affect the efficiency of the separation of the magnesia.
Flowsheet and cost estimates of processes 1 and 2 have been prepared by the author in collaboration with J. Wroth (IS). The'estimates are $16 per ton of 90 per cent KzS04, and $8 per ton of K2S04.MgS04, as the costs of production at the plant. These costs are complete and include mining, crushing and grinding, depreciation, supervision, etc. The freight charges to the eastern markets are estimated a t about $8 per ton of K2S04or of K2S04.MgS04. The flow sheet for process 3 is identical with those of processes 1 and 2 up to the gypsum filtration. The remainder of the flow sheet for process 3 is given in Figure 111. It should be noted that processes 1 and 2 return the wash waters as part of the make-up water for a subsequent extraction, whereas process 3 mixes the wash waters with the extraction liquors. The details of the cost estimates for process 3 will not be given here, but will probably be subsequently published in a Bureau of Mines bulletin. These estimates lead
939
to a production cost of $20.30 per ton of 90 per cent K2S04 at the plant. Process 3, although considerably more costly than either of the other two processes, is nevertheless more efficient than No. 2 and is the simplest one from a chemical engineering viewpoint. Thus the over-all recovery of potash in the three processes is 86.0 per cent for KO. 1, 79.8 per cent for No. 2, and 83.2 per cent for No. 3. Both processes 1 and 2 mill necessitate very careful chemical control during the evaporation steps, and as yet it is uncertain whether this control will be sufficiently accurate to obtain the assumed yields. An additional engineering difficulty with process 2 consists in the circulation of a large tonnage of schonite.
/o
NaC/
202 +#osWufer @a.- wushmg)
F/GUR€
lU
PROCESS N U . 3 Beyund qyp5 urn f//frut/onl
Processes 1 and 3 afford a possible saving of from 20 to 30 per cent of the steam costs involved, not only by the withdrawal of steam for heating purposes from the evaporation systems, but also by the use of a larger number of effects. The latter economy would be larger for process 3 than for process 1, and could not be effected at all in process 2 owing to the possibility of langbeinite formation. There is also the possibility of finding a market for the magnesia-gypsum residue of process 3. This residue is perfectly white and, after drying a t 200" C. and subsequent mixing with water, it has good setting qualities, and hence might be useful as a plaster. Reaction between Calcined Polyhalite a n d Water a t 25' C.-Synthesis of Syngenite
h consideration of the equilibrium diagram for the system K2S04-MgS04-CaS04-Hz0 at 25" C. indicates that it should be possible (using 1 part of water a t 25" C. to 1 of polyhalite) to extract all of the MgS04 and convert the bulk of the K2S04 to syngenite. Table V gives the results of such an experiment, using minus 325-mesh polyhalite calcined at 450" C. for 30 minutes. The rate of reaction of 65- to 100-mesh calcined polyhalite with a solution containing 13.5 grams of K?SOd and 15 grams of MgS04 per 100 grams of water is important in the possible manufacture of K2S04 and K2SO4.Cas04 from polyhalite. This solution would be obtained as the solution to be evapo-
IXDL'STRIAL A N D EL17GI-VEERISGCHEMISTRY
940
rated for the production of schonite in process 1. Preliminary experiments have shown that, if this liquor is mixed with a slight excess of calcined 65- to 100-mesh polyhalite and the mixture agitated for about 16 hours a t 25" C., practically all of the magnesium sulfate is extracted from the calcined polyhalite, leaving a residue which is practically pure syngenite. The latter may then be dried to give anhydrous potassium calcium sulfate. This product may be found more desirable than kali-magnesia for some fertilizer purposes. Table V-Rate of Approach t o Equilibrium of a Mixture of Calcined Polyhalide (1 Part) with Water (1 Part) a t 25" C.
1
TIME
1
Hours
72 168
CONCENTRATION PER 100 GRAMSWATER MgSO,
NaCl
Grams
Grams
Grams
3.11 3.12 3.38 3.70 3.69
14.76 '15.61 17.00 19.05 19.20
3.55
;4:"
4:Ol
EXTRAC- TOTAL TION O F MgS04
I
1
Vol. 22, No. 9
and the corresponding SOi-- and Mg++ concentrations obtainable with the use of boiling saturated salt solutions and uncalcined polyhalite. Rate of Decomposition of Calcined Polyhalite by Saturated Salt Solutions at 106" C.
The rate of decomposition of 1 part of calcined (minus 325-meshJ 450" C. for 30 minutes) polyhalite by 2.4 parts of saturated salt solution is shown graphically in Figure IV. It will be observed that practically complete decomposition occurs very rapidly-i. e., within 1 hour after mixing. The compositions of the leach liquors obtained a t various times
POTASH I N
LEACHLIQUOR
%
%
76.5 81.3 88.5 99.2 100.0
11.20 11.23 12.15 13.31 13 27
The data available on this process are not sufficient to justify detailed cost estimates, but it is probable that the cost per ton of potassium-calcium sulfate will be somewhat less than that of potassium-magnesium sulfate. Further research on this process is in progress, and the results will be published giving flow-sheet and cost estimates as soon as sufficient data have been obtained. 0
Rate of Decomposition of Polyhalite by Saturated Salt Solutions at 25" and 106' C.
The rate of decomposition of finely ground (minus 325mesh) polyhalite by 2.4 parts of saturated NaCl solution a t 25" C. is shown graphically in Figure 11. This rate is apparently approximately equal to that of a corresponding amount (1.75 parts) of water. The rate of decomposition a t 106" C., which is shown in Figure IV, is, however, very much more rapid than that of water a t 100" C. Thus 2.4 parts of saturated NaCl solution (corresponding to 1.75 parts of water) will decompose 73.8 per cent of 1 part of polyhalite in 24 hours. The compositions of the leach liquors obtained by using 1 part and 2 parts of polyhalite to 2.4 of saturated KaC1 solution are given in Table VI. It will be noted that, while the mol ratio K2/Mg remains constant and close to unity, the mol Mg)/S04-- increases markedly. Hence it is ratio (Kp apparent that anhydrous Na2SO4is deposited during the boiling; whereas no new solid phase containing MgS04 is formed in appreciable quantity. The significance of these results from an industrial point of view cannot be satisfactorily discussed until further data are obtained concerning the maximum concentration of KzO
+
Table VI-Composition
Uncalcined polyhalite 199-A 199-B 199-c 199-D 201-A 201-B 201-c
201-D Calcined polyhalite 202-A 202-B 202-c 202-D 203-A 203-B
50
7S
/00
7m
NS
/50
,"H w a
/75
ZOO
2.95
216
are given in Table VI. It will be observed that the K2/Mg mol ratio is persistently somewhat higher than unity, indicating a slight deposition of a new solid phase containing MgS04 (probably a double salt of sodium and magnesium Mg)/SO4-- ratio is greater than sulfates). The (Kg unity, indicating the formation of considerable anhydrous NazS04. The rate of increase of this ratio is, however, not so rapid as in the case of uncalcined polyhalite. The potash content of the solution drops upon continued boiling, probably owing to potassium calcium pentasulfate formation. The rate of decomposition using 1.2 parts of saturated KaC1 solution is also very rapid, 89.0 per cent extraction being obtained in 30 minutes. I n this case, however, the formation of new solid phases containing potassium (probably potassium calcium pentasulfate) and magnesium (probably a double salt with sodium sulfate) is very much more rapid than in the case of 2.4 parts of saturated NaCl solution. The curve of Figure IV for calcined polyhalite plus 1.2 parts of saturated NaCl solution is arbitrarily constructed, for the maximum might have been reached before the first sample was taken. The data presented are of a preliminary nature, but they do indicate that it may be possible to
+
of Leach Liquors f r o m Decomposition of Polyhalite b y Boiling (Atmospheric Pressure) Saturated NaCl Solutions
1
RATIO EXFT.
25
COMPOSITION O F
LIOTJORS P E R 100 GRAMSW A T E R
I
MOLRATIO
I
+ Mg) /sod--
OF P O T A S H
1.67 1.86 4.11 5.12 1.77 2.70 4.86 4.20
39.3 42.2 55.5 73.8 20.1 23.5 28.5 44.1
2.06 2.37 2 42 2.70 1.93 2.63
96.6 97.1 100.0 98. 1 89.0 72.3
K ~ / M ~(K2
NaCl TO POLYHALITE
Grams
Grams
Gyams
Grams
2.4 2.4 2.4 2.4 1.2 1.2 1.2 1.2
Hours 0.25 0.50 1.0 24.0 0.25 0.50 1 0 24.0
2.87 3.09 4.06 5.38 2.93 3.43 4.16 6.44
1.26 1.35 1.71 2.33 1.28 1.47 1,75 2.81
25.8 23.7 25.5 26.8 25.0 24.6 25.6 27.9
3.82 3.43 1.92 2.05 3.43 2.69 1.94 3.18
2.4 2.4 2.4 2.4 1.2 1.2
0.25 0.50 1.0 3.0 0.50 1.0
7.81 7.87
3.30 3.24 3.18 3.24 4.95 3.9s
26.8 26.7 25.9 25.0 27.5 26.4
7.77 6.68 6.56 5.89 13.8
8.08
1
7.93 14.38 11.70
70 0.974 0.973 1.00 0,982 0.970 1.00 1.00
0.987 1.03 1.04 1.06 1.04
I
September, 1930
IiL'DUSTRIdL A S D EXGINEERIIL'G CHEMISTRY
develop an industrial process for producing KCl from polyhalite by extraction with saturated salt solutions. Acknowledgment
The help of L. Clarke and B. A. Starrs, assistarit chemists of the New Brunswick station, in connection with the work on process 3 is gratefully acknowledged. Literature Cited (1) Hoff, van't, "Zur Bildung der ozeanischen Salzablagerugen," p. 16 (1905).
94 1
(2) HOE, van't, "Untersuchungen iiber die Bildungsverhaltnisse der ozeanischen Salsablagerungen," Akademische Verlagsgesellschaft. Leipzig, 1912. (3) International Critical Tables, Vol. IV, p. 349; gives equilibrium data. (4) Ibid., p. 362. (5) Klooster, van, J . Phys. Chem., 21, 613 (1917). (6) Levi, Z . phys. Ckem.,106, 93 (1923). (7) Mansfield and Lang, Am. Inst. Mining Met. Eng.. Tech. Pub. 212 (February, 1929). (8) Schaller and Henderson, M i n i n g Met., 10, 197 (1929). (9) Starrs and Clarke, J. Phys. Ckem.,34, 1058 (1930). (10) Starrs and Storch, submitted to J . Phys. Chem. (11) Storch and Clarke, Bur. Mines, Repfs. of Investigations 3002 (1930). (12) Weston, J . Chem. Soc., 121, 1223 (1922). (13) Wroth, Bur. Mines, Bull. 316 (1930).
The Cooking Process 11-Cooking Wood with Sodium Carbonate' S. I. Aronovskyz and Ross Aiken Gortner DIVISION OF AGRICULTURAL BIOCHEMISTRY, MINNESOTA AGRICULTURAL EXPERIMENT STATIOX, ST.PAUL,511".
H E first paper of this Aspen sawdust was cooked with 20 and 40 per cent w0rk-i. e., 3 to 100. The sodium carbonate (based on the weight of the ovencooking procedure, methods series ( 1 ) showed that cooking wood w i t h dry wood) at temperatures of 170" and 186' C. and of preparing the productsfor analysis, and m e t h o d s of water has a very appreciable for 2 and 12 hours. The residues were analyzed for lignin, pentosans, cellulose, and abha-cellulose. analysis were the same as effect on the various constituents of the wood. 'It was Total organic matter, volatile acids (as acetic acid), outlined in the r e p o r t on found that the pentosans and lignin (72 Per cent HzSO4) determinations were the water cooks. were almost completely remade on the residual black liquors. The results of The data are tabulated moved from the wood, being these analyses were compared with those obtained in T a b l e s 1 to VI and partly reduced to furfural by cooking wood with water only, and it was found shown graphically in Figures and p a r t l y d e s t r o y e d to that, under the conditions employed in this work, 1 to 3, form gaseous products. sodium carbonate has a considerable effect on the Results and Discussion Some of the total celluresultant yields of residual wood and its main conlose, as well as the alphastituents. Sodium carbonate cannot be considered RESIDUALLIQUORS-The as an inert ingredient in a cooking liquor. residual liquors from all cellulose, was hydrolyzed t h e sodium c a r b o n a t e to sugars and broken up to form gaseous products. The lignin was prtrtially re- cooks were reddish black and alkaline to litmus paper. moved by the water, but there was practically no loss After standing for 2 weeks no solid matter had sepaof this constituent, as determined by the 72 per cent rated. On filtering these liquors the filter paper assulfuric acid method. It was found, however, that part of sumed a red color similar to that obtained in the water the lignin remaining in the residual woods had become soluble cooks. Total Organic Matter. The total organic matter was obin alcohol, pointing tJo a probable depolymerization of this tained by determining first the total solids and then the total substance taking place in the cooking process. Sodium carbonate, which is present in small amounts in soda as sodium sulfate, and finally subtracting the latter as t'he soda, sulfate, and neutral sodium sulfite cooking liquors, sodium carbonate from the total solids. The quantities of is considered a t present to be inert in so far as the cooking total organic matter thus obtained were substantially higher process is concerned. However, since it is found in all the than in the corresponding water cooks, as shown in Table I alkaline cooking liquors used in commercial practice, it was and Figure 1. Increasing the quantity of sodium carbonate used as the first chemical in a continuation of the investiga- and the time and temperature of cooking resulted in increased yields of organic matter in the residual liquors. tions on the cooking process. Lignin. The quantities of lignin (as determined by the Experimental 72 per cent sulfuric acid method) in the residual black liquors Eight cooks were run, using temperatures of 170" or 186" C. of the sodium carbonate cooks were much larger than in the (100 and 150 pounds :pressure, respectively), cooking duration corresponding water cooks, as shown in Tables I1 and I11 of 2 or 12 hours, and concentrations of 20 or 40 per cent and in Figure 1. The increase in the time and temperature sodium carbonate, on the basis of the oven-dry weights of of cooking and in the concentration of the salt all tended to the wood. The wood used was aspen, in sawdust form, from increase the quantity of lignin in the residual liquor. Reducing Sugars. Only a trace of reducing sugars could the same batch prepared for the series of water cooks ( 1 ) . The ratio of wood to liquor was the same as in the previous be found in the residual liquors of the sodium carbonate cooks. This was to be expected, however, since sugars are very 1 Received June 14, 1930. To be presented before the Division of Celluquickly destroyed in an alkaline medium. lose Chemistry at the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September 8 to 12, 1930. Published with the approval of T/'o/,ati/,eacids (as acetic acid). The total volatile acids the Director as Paper 945. Journal Series, Minnesota Agricultural Experi(calculated as acetic acid) in the black liquors of this series ment Station. * Cloquet Wood Products Fellow, University of Minnesota. Fellow- of cooks were remarkably high, increasing to Some extent with the increase in the quantity of sodium carbonate (Table ship established by the Ihrthwest Paper Company of Cloquet, Minn.
T
