Potassium Sulfate Production from Potassium Chloride and Sulfuric Acid E. J. Fox . ~ N DJ. W. TURRENTINE, Fertilizer Investigations, Bureau of Chemistry and Soils, Washington, D. C. results are obtained by mixing the salt and acid cold and allowing the first stage of the reaction to take place before heating the mixture. The acid should be added slowly to the salt, with eficient stirring. The mixture can, and should, be kepi in a granular, solid state throughout fhe contiersion. Conversion in the second stage will occur as low as 260" C., but the rate of reaction is greatly accelerated with increasing temperatures, reaching a maximum between 500" and 550" C . The application of hydrochloric acid, produced in this manner, to phosphate rock to yield the dibasic calcium phosphate represents a n eficiency of the sulfuric acid employed which is twice that realized in superphosphate manufacture.
American agriculture during the past ten years has consumed a n average of 68,000 tons of potassium sulfate, valued at $2,925,000 per annum, which represents the demonstrated domestic market for this commodity. High-grade potassium suuate m a y be easily produced f r o m potassium chloride of American origin. The increase in value of the potash resulting f r o m the conversion should bear the major portion of the cost of conversion. By-product hydrochloric acid should be produced at a cost s u 6 ciently low for its use in many industries where its present high cost makes such use prohibitive. The conversion of potassium chloride to potassium sulfate proceeds in two stages, the first of which is exothermic and requires no heat treatment. Best
T
HE demand for potassium sulfate within the United
States is measured by the importations of this commodity. I n the following table are presented these imports for the 10-year period 1923-32, inclusive, in the principal form in which potassium sulfate is importednamely, 90 to 95 per cent potassium sulfate: Year 1923 1924 1925 1926 1927
Long tons 63,741 75,696 68,952 69,783 68,904
Value $2,599,695 2,858,063 2,686,408 2,823,438 2,947,969
Year 1928 1929 1930 1931 1932 Average
LOILQ
tons 86,458 79,510 86,257 56,842 28,071 68,430
Value $3,908,221 3,647,839 3,947,479 2,628,316 1,201,571 2,924,993
or on a potassium oxide basis, of $87 per ton as sulfate, compared with $67 per ton for the chloride. At the same time there is the agronomic fact that the residual sulfate radical, in soils containing lime, can be largely eliminated from the soil solution as gypsum, while a chlorine residue can be removed only by leaching. The sulfate of potash-magnesia offers, as a selling point, a contribution of magnesium which is without a rated commercial value but is valued because of the occasional soil found to be deficient in this element.
PRESEKT SOURCES
I n addition, there are imports of the so-called sulfate of potash-magnesia, containing 48 to 53 per cent potassium sulfate, which in 1930 and 1931 amounted to 12,000 and 7000 tons, equivalent to 6000 and 3500 tons of potassium sulfate, respectively. These imports represent that portion of the total American market not satisfied from the domestic products. Since the latter a t present are rather small, these figures may be taken as representative of this market a t prevailing price levelsan average of 68,480 tons, valued at $2,925,000 c. i. f. Atlantic and Gulf ports, for the 10-year period. The marked decrease in this business during 1932 must be taken, for the time being at least, as anomalous and as attributable to depressed agriculture and to the 50 per cent decrease in fertilizer sales volume resulting from that distress. The essential function of fertilizers in American agriculture and the increasingly important role they will play in a rehabilitated and remodeled agriculture leaye no basis for doubting the complete restoration of this essential industry. With the exception of that year, this international trade has shown substantial stability. The basis is the demand that fertilizers designed for certain crops, notably tobacco, shall be largely chlorine-free. It is this fact rather than the plant food value of sulfur, or the less objectionable properties of the sulfate as compared with the chloride radical, that assigns a 1-alueof $42 per ton for 90 per cent potassium sulfate, as compared with $34 per ton for the 80 per cent potassium chloride,
The principal world source of potassium sulfate is Germany where abundant natural supplies of carnallite (KC1.MgC12.6H20) and kieserite (MgS04.H20) provide convenient raw materials for its manufacture. Concentrated solutions of potassium chloride and magnesium sulfate, when mixed, precipitate sulfate of potash-magnesia, which in turn, when boiled with potassium chloride, yields potassium sulfate. The size of this industry is indicated by German exports in these two commodities as follows: COUNTRY
SULFATE OF
&SO4 1930
POTASH-MAQNlO0IA
19315
Metric tons Great Britain Japan Netherlands United States All others Total salts Total KzSOa 0
11,729 63,723
.... 62,986
19,124 33,237
.... 43,145
1930 193Ia Metric tons
...
...
34545 6,895 579 41,729 20,860
47,672
36,866
43;ii5 12,026 693
186,110 167,500
132,372 119,135
55,844 27,900
-
-
First 11 months.
This international trade of some 200,000 metric tons of potassium sulfate (1930) of German origin is supplemented with some contributions from France (3000 metric tons in 1932) \There it is produced by the interaction of potassium chloride and sulfuric acid. Present sources of domestic sulfate are represented by small current contributions as by-products of the fermentation and cement industries.
493
494
INDUSTRIAL AND ENGINEERING CHEMISTRY
POTENTIAL SOURCES ~1~~ United States possesses large potential sources of tassium sulfate, Some of which at presentare of speculative interest while others are of much greater importance. representative of the latter may be mentioned the potash saline mineral, polyhalite, occurringin commercial quantities in Texas and xewMexico (8), and by-product potash from the cement industry. polyhalite is now accessible in the strata penetrated by the two shafts of the u. 8. potash company near Carlsbad, N. Mex., and newly developed processes indicate the feasibility of its commercial extraction; isolation with respect to present markets and resulting high distribution costs represent the chief deterrent factor. Cement-potash recovery has now been advanced to the stage of an economical, commercial process (6), representing, with a logical expansion, the potentiality of 50,000 tons POtassium oxide, as the sulfate, from this source. However, since the use of chloride promoters (6) doubles the yield of the potash volatilized from the cement mix, the potash, in that case, appearing as the chloride instead of the sulfate, it is not certain that this slight modification in processing will not be adopted, in part at least, the increased yield as chloride outweighing the higher unit value as the sulfate (9). Within these potentialities there does not Seem to lie the assurance that the domestic requirements in this essential commodity will be supplied in quantities that are ample or a t prices that will preempt the domestic market. m'ith a normal market for 68,000 tons of potassium sulfate existing within our own boundaries there appears to be an opportunity for the domestic industry to expand into this field, provided, of course, that such an expansion is economically feasible.
-
-~
i7me in Minutes
FIGURE 1. CONVERSION OF POTASSIUM CHLORIDE PLUS POTASSIUM ACID SULFATETO POTASSIUM SULFATEPLUS HYDROCHLORIC ACID AT VARIOUS TEMPERATURES
Sinceannualpotashsalesbythedomesticindustryinrecent years have approximated $3,000,000 in value, and importations of potassium sulfate have approached the same figure, the relative magnitude of the expansion potentially possible is evident. I n advocating this expansion, specifically by the application of sulfuric acid to potassium chloride to form the sulfate, and by the application of the resultant by-product hydrochloric acid to the extraction of low-grade phosphate rock for the production of converted phosphate, or to other useful purposes as dictated by local circumstances, consideration is given the following facts: We now have a production capacity of 150,000 tons of high-grade muriate (99 per cent potassium chloride) per annum, constituting an excellent raw material from which to produce potassium sulfate and other potash chemicals for which domestic sources are inadequate. Our supplies of sulfuric acid are most abundant and are widely distributed. The current price differential between sulfate and chloride, favoring the former, offers a margin substantially to cover an estimated conversion charge. Where the
Vol. 26, No. 5
production of hydrochloric acid is a desired objective, potassium chloride represents as convenient a raw material as does sodium chloride; otherwise, its utilization in extracting the cheaper grades of phosphate rock to yield fertilizer phosphates is both Simple and economical. Herein is represented a source of hydrochloric acid easily producible a t competitive costs with the COlhteI'al production of potassium sulfate for which a large market is already a t hand. At this time when the nation's attention is centered on its economic rehabilitation through increased employment, the establishment of new industries, Or the expansion of old, the present suggestion should win more than passing consideration as a potential contribution t o Ohis end*
EXPERIMENTAL PROCEDURE The proposal reported here was of only theoretical interest prior to the recent establishment of a n extensive French industry on a similar basis, and the literature has little to contribute to the subject (3). It therefore became necessary to resort to experimentation to test its feasibility and to determine the optimum conditions for inducing the simple reaction involved. Attempts to produce hydrochloric acid from potassium chloride and sulfuric acid by mixing the two reactants in stoichiometric quantities, after the manner made familiar from long use in the production of that acid from sodium chloride and sulfuric acid, results in a mixture of low melting point from which, at low temperatures, the hydrochloric acid is evolved relatively slowly, and which, a t higher temperatures, froths excessively. The reaction proceeds in two stages with widely differing thermal relationships (4): KCl, H2S04 = KHS04, HC1, 6.03 kilojoules (1) KCl, KHS04, = K2S04c HCI, - 69.2 kilojoules (2)
++
+ +
+
From these values it is obvious that the first stage proceeds without a heat requirement and that i t is only the second stage that is promoted by heating. Further, the preliminary experimentation teaches that the evolution of hydrochloric acid, which is a function of surface exposure, ,proceeds more readily from a granular than from a molten mixture, indicating the desirability of maintaining the mix a t temperatures below the melting points of the several systems represented by the varying relative proportions of the components present throughout the course of the reaction. TABLEI. COMPOSITION OF MIXTURES DURIPV'G CONVERSION AT VARIOUSTEMPERATURES TIME KC1 KHS04 K2SOd Min. % % % 29,6 26.6
54,0 48.6
c . 7 16,4 24.8
11.3 6.5 2.1
20.6 11.9
68.1 81.6 94.1
-AT
30
g:
120 150
270
::::;:::i::: -AT
ig 30
260'
3.8 416'
::::$:i;:;:! 2.3
4.2
KC1 KHSOI KnSO4
% -AT
2.8 2.0
..
:A' ii:: 93.5
826'
33.8 61,7 23.4 42.7
--AT
C-.
% 0.-
%
4,5
31.0 6.2
33.9
't:; zi:;
5.1 9 2 . 1 3.6
..
660° C
KCl KHSO4
%
94.4
.. .
7
-AT
$::PO
% 400'
56.6 11.0
% C
.
7
12.4 82.8
::$ 9":::
0.15
0.27 9 9 . 6
..
..
..
-AT
..
626' C
.. ..
.
7
Ai: 'i::!3:? ;8:: 2":; :;:! "8 6::" 8Q:3
:; 0.15
0.27 99.6
..
..
..
It has been found that a granular state is readily maintained throughout the first, or disulfate, stage by the simple expedient of adding, with stirring, the sulfuric acid to the POtassium chloride, instead of in the reverse order, 2 kg. (20 moles) sulfuric acid being added to 3 kg. (40 moles) potassium chloride. The product obtained in this manner, on analysis, is found t o have an acidity equivalent to 17.51 per cent hydrochloric acid and a chlorine content equivalent to 17.53 per cent hydrochloric acid (theoretically, 17.3 per cent), showing
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
May, 1934
a practically complete formation of the disulfate and evolution of hydrochloric acid. To determine the course Of the second Stage of the reaction, with heating, portions of the above resulting product have been heated a t graduated temperatures for varying lengths of time and the rate of conversion has been determined by titration with sodium hydroxide solution. Check analyses of the chlorine content have been made with standard silver nitrate. The compositions of the resulting mixtures as calculated from these determinations are given in Table I. The results extrapolated back to zero concentration of potassium sulfate are shown graphically in Figure 1. The figures indicated are not intended to represent the actual composition of the material. as no cognizance is taken of the impurities present in the salts. They are based on the determination of acidity in the product, assuming that the product is the result of the reaction indicated by the equation, KHSO, KCI = KtSOi HCl
+
tion of a low-melting mixture which tended to flow through the furnace without being completely converted. Thereafter a twostage procedure was adopted, consisting as the first step in mixing the salt and acid in the proper proportions without heating and then charging the resulting product into the apparatus described to effect the second step. This mixture when passed through the furnace remained granular throughout, and the conversion was, on the whole, satisfactory. Resulting data are presented in the following table, where for comparison are given likewise the results of other methods of mixing the charge: (1) mixing the salt and acid in the furnace itself; (2) premixing outside the furnace,
+
in which the hydrochloric acid is removed as fast as formed, and assuming that the acidity 1 of the product is due to potassium disulfate which has not reacted. From the amount of potassium disulfate found to be present is calculated the equivalent of potassium chloride, and the remainder is taken to be the normal potassium sulfate. As a matter of fact, a slight loss in sulfuric acid, due to the decomposition of potassium disulfate, is indicated by comparing the potassium chloride content estimated to be equivalent to the potassium disulfate with that calculated from the chlorine content, as determined by analysis. The following table gives the results of this comparison :
FIGURE2. DIAGRAM OF CONVERTER
adding salt to acid, and heating to molten state; (3) premixing, adding salt to acid, and no heating; and (4) premixing, adding acid to salt, and no heating. The temperature of the furnace was in each case between 550" and 600" C. The average length of time required for passage through the furnace was 20 minutes.
KC1 (EQUIVA- KC1 (DE.rN. LENT OF
TIME
Min. 0
30 180 270 160 30 30
495
TEUP.
c.
...
260 260 260 325 475 550
KHSOI) % 33; 2 26.6 4.8 2.1 2.0 2.3 0.15
OF
CHLORINE) D I F F E R E N C E % %
331 3 27.3 6.9 4.1 4.0 4.4 3.7
EXPT. 1 2
2.1 2.0 2.0 2.1 2.5
3 4
The loss of sulfuric acid is 1.3 per cent (the equivalent of about 2 per cent potassium chloride) during the conversion and does not vary greatly with the temperature so long as i t is above the decomposition point of the disulfate. A simple converter, shown in Figure 2, was designed to test the possibilities of a one-stage, continuous transformation of potassium chloride to potassium sulfate. It consisted of an iron pipe 4.5 inches (11.4 cm.) in diameter and 8 feet (2.4 meters) long, heated by an electrical heating unit, E, encircling the pipe and properly insulated. The salt and acid mixture was moved through the apparatus by a spiral cut-flight conveyor driven by an electric motor through a series of reducing pulleys and worm gears. Potassium chloride was charged at a constant rate from hopper A by means of a constantfeed mechanism driven by a belt from the conveyor shaft. Acid was run in from a conetant-level bottle, B. Hydrochloric acid gas was withdrawn at D and the solid product a t C . A thermocouple, F , was inserted through the hollow shaft of the conveyor from the exit end. A suction pump (not shown) was connected through a series of scrubbing bottles (also not shown) to D,and a current of air was drawn through the furnace t o remove the hydrochloric acid gas. Preliminary attempts to complete the two stages in a single o eration yielded unsatisfactory results, owing to the fact that tfe water and acid vapors liberated in the hot zone (about two-thirds of the way through near the position indicated by the arrow, E ) were drawn over the cooler incoming mixture by which the gases were partly condensed and absorbed, causing sticking, uneven distribution, imperfect mixing, and the forma-
MIXINQ
COMPOSITION ALFTERHEATINQ
KCI o/,
0,l
0.7
MloTEOD OF
5
In furnace Premixing Premixing' Premixing' Premixing:
salt to acid, heated salt t o acid cold
acid to salt: cold acid to d t , cold
26:O 11.0 16.4 4.1 2.2
KHSOi
V! ,_ 47.2 20.0 30.0
7.5
4.0
KsSOr % ._ 26.8 69.0 53.6 88.4 93.8
DISCUSSION OF RESULTS The results indicate that seemingly insignificant variations in methods of procedure affect the rate of conversion and the composition of the final product. In explanation it is suggested that in experiments 1 to 3, inclusive, because the method of mixing leads to the formation of a liquid phase, the free escape of hydrochloric acid is inhibited. In experiments 4 and 5 the liquid phase is avoided by adding the acid gradually to the solid salt, with stirring, thus facilitating the free escape of the hydrochloric acid as formed. Under these conditions the reaction proceeds promptly to the disulfate stage at ordinary temperatures, and an intimate mixture of this intermediate salt with the residual potassium chloride is automatically effected. This mixture, upon heating to the required temperature, remains granular and the reaction proceeds to practical completion without fusion of the salts. The above data indicate that satisfactory conversion in the second step can be obtained in 3 hours a t 260" C., or in 30 minutes a t 475" C.; thus a wide latitude in operating conditions is offered. On the basis of the foregoing, the recommendation seems justified that the operation be carried out in two steps, the first in a batch mixer of conventional design and the second in a heater also of conventional design or one analogous to that described in the foregoing paragraphs. However, the two could easily be combined, no doubt, to admit of a single
I N D U S T R I A L ,4N D E N G I N E E R I N G C H E M I S T R Y
496
operation and be further developed to admit of continuous operation. Chemical equipment already available could doubtless be applied without further developmental experimentation. For this reason, further work in this laboratory did not seem warranted. The observations made are offered in their present form in the belief that they cover the fundamentals required for a n appraisal of the recommendations embodied here.
ECONOMIC CONSIDERATIONS No factors appear here which would indicate that the proposed conversion represents difficulties or costs exceeding those encountered in the present manufacture of hydrochloric acid from sodium chloride and sulfuric acid, or from sodium chloride and sodium acid sulfate. Accordingly, there are offered comparisons of the three methods based on current quotations ( 7 ) of the raw materials and finished products involved (Table 11). It is obvious that these quotations do not accurately express the prices at which some of these commodities are purchasable. However, they do serve to illustrate the relative values involved. Further, it is assumed that operating costs are the same in each case, although it appears that these may prove to be least in the process dealt with here. While these estimates are based on 95 per cent potassium chloride, it is assumed that the 99 per cent domestic (or imported) product would be employed, since it is purchasable a t the same price per unit of potassium oxide as the lower grades, and there would be no objective in processing a diluent having no commercial value.
TABLE11. RELATIVEVALUESIN HYDROCHLORIC ACID PRODUCTION (Basis. . . one ton hydrochloric acid) KC1 HISO&: 2 02 tons 9 5 7 KC1 a t $40 50 1:73 tons 60 % X S Oa~t Sli.00
+
Total 2.36 tons 95% KzSOr a t $44.85 Differential Der ton HC1 NaCl HzSOa: 1.60 tons NaCl a t $11.40 1.73 tons 60” HzSOa a t $11.00
+
9
$ 81.80
19.00 100.80 105.50
-4.70 18.25 19.00
Tots1 1.95 tons NazSOa a t $12.00
37.2.5 23.40
Differential per ton HCl NaCl NaHSO4: 1.60 tons NaCl a t $11.40 4.32 tons NaHSOd a t $12.00
13.85
+
Total 4.33 tons NazSO4 a t $12.00 Differential per ton HC1
18.25
51.84 70.09 51.96
18.13
I n 1931 (1) thirty-one establishments within the United States produced a total of 55,000 tons anhydrous hydrochloric acid, of which only 14,000 tons were consumed where produced, the balance of 41,000 tons being sold a t a valuation of $2,422,000. As acid of commercial (20’ Be.) strength, each ton of hydrochloric acid involved 2 tons of water on which freight and handling charges were paid. There is no impressive reason why intraplant hydrochloric acid production could not be extended with the installation of units of appropriate capacities, employing such a method asis here described; this enterprise would be undertaken, however, in terms of advantageous supplies of raw materials and markets for potassium sulfate. To promote fertilizer use through the simplest expedient of reducing costs, freight charges must be maintained at the lower levels. The potassium chloride which can be intercepted for conversion enroute to the fertilizer market represents the best economic advantage. UTILIZATION OF HYDROCHLORIC ACID I n chemical plants employing hydrochloric acid as a reagent, the product thus yielded (largely in the gaseous form)
Vol. 26, No. 5
is deliverable to the process when required in this form or in any desired concentration, continuously or intermittently as wished. If it requires marketing, it should be able to enter the market a t competitive prices. However, the proposition does not depend for its merits upon already established usages for hydrochloric acid and warrants consideration quite independently of such usages since there is a potential outlet for large quantities in the manufacture of agricultural phosphates. I n this application the proposal, in essence, is that a part of the sulfuric acid now being applied to phosphate rock to yield superphosphate, should instead be applied to potassium chloride to yield the sulfate, and that the resulting hydrochloric acid should be applied to phosphate rock to yield an agronomically available phosphatic fertilizer ingredient. The calcium chloride, a byproduct in the latter, is largely removable and therefore, unlike the calcium sulfate in the former, need not remain as an unavoidable diluent. High-analysis phosphates result. The competitive disposal of hydrochloric acid is thereby avoided, and, in fact, the entire enterprise can be established and conducted without interfering with existing domestic industry. It is obvious that hydrochloric acid released from potassium chloride by sulfuric acid is equivalent to the latter and, when applied to phosphate rock, renders available an equivalent proportion of PzOs ( 2 ) . From the viewpoint of the present superphosphate manufacturer, if this were the whole story, profits would be derivable only from the price differential between potassium chloride and potassium sulfate. However, it now appears entirely feasible to recover the PzOs, made available by hydrochloric acid, as the dibasic calcium salt, if not even in the more basic form, from which it becomes apparent that the hydrochloric acid thus employed has a t least twice the efficiency of the equivalent sulfuric acid as employed in superphosphate manufacture to yield the monobasic salt. Per unit weight of sulfuric acid purchased, there is yielded accordingly the equivalent of potassium sulfate, and two equivalents of available PzOs. The details are to be presented in a subsequent contribution. LITERATURE CITED (1) Bureau of t h e Census, personal communication. (2) Fox, E. J., a n d Whittaker, C. W., IXD.ENQ.CHEM.,19, 349 (1927). (3) Frydlander, Rev. prod. chim., 27, 5 (1924); Saccharinfabrik Akt.-Ges. vorm. Fahlberg, List & Co., Geyman P a t e n t 261,411 (Oct. 20, 1911); Meyer and Klages, U. S. P a t e n t 1,099,382 (June 9, 1914); Fabrique produits chimiques T h a n n Mulhouse, British P a t e n t s 137,296 (Dee. 29, 1919), 137,519 (Dee. 30, 1919). (4) International Critical Tables, Vol. V, pp. 169 ff., McGraw-Hill, 1926. (5) Landolt, P. E., Chem. & M e t . Eng., 40, 345 (1933). (6) Madorsky, S. L., IND. ENG.CHEM.,24, 233 (1932). (7) Oil, Paint Drug Eeptr., 124, 5 6.(1933). (8) Partridge, E. P., IND.Ex+.CHEM.,24, 895 (1932); Mansfield, G. R., J . Chem. Education, 7, 737 (1930); Hill, J. R., a n d Adams, J. R., IND.EKG.CHEM.,23, 658 (1931). (9) Turrentine, J. W., “Potash,” Wiley, 1926. RECEIVED November 23, 1933. Presented before the Division of Industrial and Engineering Chemistry a t the 87th .Meeting of the American Chemical Society, St. Petersburg, Fla., March 25 to 30, 934.
I n the article, “Predicting Stability of GasoCORRECTION. lines to Aging,” by Winning and Thomas, IND.ENQ.CHEM., 2 5 , 511-16 (1933), an error in statement appears at the bottom of the fist column of page 514. The sentence reading ‘ I . . .the logarithm of the stability may be plotted as a linear function of the absolute temperature. . .” should read I‘. . as a linear function of the reciprocal of absolute temperature. . .” CARLWINNINQ
.
