Barium and Strontium Sulfate Decomposition in Aqueous
Solution R. NORRIS SHREVE AND H. F. WIEGANDTI Purdue University, Lafayette, Ind.
The decomposition of barium and strontium sulfates by the action of strong solutions of calcium chloride results in the formation of the soluble modification of anhydrite (CaSO,). Dilution of the reaction mass at this point with water results in immediate reversion to the initially reacting, materials, due to the reactivity of the soluble modification of the anhydrite. The soluble anhydrite is converted to the much less reactive insoluble modification
HE production of inorganic salts is often carried out by ordinary double decompositions. Usually such reactions, involving the formation of insoluble salts, are considered essentially quantitative in one direction. A few double decomposition reactions involving insoluble salts reverse their equilibrium position as the solution is changed from dilute to concentrated. Among them are barium sulfate and calcium nitrate in equilibrium with barium nitrate and calcium sulfate (6,l l ) , barium sulfate and calcium chloride in equilibrium with barium chloride and calcium sulfate (6),strontium sulfate and calcium nitrate in equilibrium with strontium nitrate and calcium sulfate (IO), and barium sulfate and an alkali carbonate in equilibrium with barium carbonate and an alkali sulfate ( l a ) . The change in equilibrium position as the solution medium is changed from dilute to concentrated has been studied in the Purdue Chemical Engineering Laboratories especially in the cases of barium and strontium sulfates decomposed by calcium chloride and calcium nitlate. As the solution is made more concentrated, the reaction proceeds farther in the direction of obtaining the soluble strontium and barium salts. Efforts to produce soluble strontium and barium compounds were reviewed in previous publications (4, 6, 7 ) . I n this work only enough water was used to make the ionic reaction possible. The consistency of the reaction mass was that of a thick mush. The experiments of Shreve and Toner (8) and Shreve and Watkins (9) indicated a possible mechanism for these equilibrium reactions. They and others observed that the solubility of calcium sulfate decreases with a rise in tempera-
T
Present address, Standard Oil Company (InJiana:, Whiting, Ind. After this work had been oompleted, a paper appeared on “Thermodynamic Properties of G y p ~ u mand Its Dehydration Products” [Kelley, Southard, and Anderson, U. S Bur. Mines, Tech. P a p e r 625 (1941) I. 1
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by heating the reaction products at elevated temperatures. This procedure makes practical the use of aqueous methanol as an extraction solvent for the recovery of the barium chloride made. Under these circumstances even water alone will extract most of the barium chloride initially formed before the insoluble anhydrite has had time to react. The effect of aqueous extractions on decompositions carried out by fusing the reactants also gives similar results*.
ture, the solubilities of strontium and barium sulfates increase with a rise in temperature, calcium chloride lowers the solubility of calcium sulfate, and barium and strontium chlorides are virtually insoluble in concentrated calcium chloride solutions. Under the conditions in which these experiments were carried out, highly concentrated solutions of calcium chloride were employed; since this double decomposition was carried out in the presence of only a small amount of water, the problem of separating the reaction products without causing reversion (the opposite of conversion) had to be solved. A number of methods might be possible for the separation of two inorganic salts which have been produced by evaporation of an aqueous solution media or by fusion. Extractions with water would be the first obvious choice. However, if the properties of the salts are such that the reversion reaction takes place (for example, the reaction products of barium and strontium sulfates with calcium chloride), water alone may not be satisfactory. I n this case an organic solvent has been used successfully. Previous work on the decomposition of barium and strontium sulfates with calcium chloride and nitrate (6-10) describes the use of organic solvents for extracting the reaction products. As the salts are essentially un-ionized in the organic solvents, no reversion takes place. The chlorides are soluble and are separated from the insoluble sulfates by filtration. If certain modifications could be made so that aqueous methanol or even water could be substituted for the organic solvent under special conditions without obtaining too much reversion, the process would be more economical. A large part of the experimental work described here is a study of these possibilities on the double decomposition products of barium and strontium sulfates with calcium chloride.
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anhydrite form. Under conditions of sustained elevated temperatures the pattern changed; although the ultimate stable phase above 42" C. is insoluble anhydrite, the metastable soluble anhydrite forms a t temperatures from just below the boiling point to approximately 150" C. (4). Since the reaction was carried out in the aqueous phase and a t atmospheric pressure, the molecular form of the calcium sulfate was that of soluble anhydrite. This is a reactive form of calcium sulfate readily reacting in aqueous solutions of strontium and barium salts. The form in which calcium sulfate exists after fusion is the insoluble anhydrite (8),and the fusion products may be extracted with water but with some reversion because of the low reactivity of this insolubilized calcium sulfate. The transformation of gypsum to insoluble anhydrite by heating suggested subjecting the products of these aqueous reactions to elevated temperatures. In the experimental work this method of changing calcium sulfate from a reactive form to the less reactive insoluble anhydrite form was used.
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AQUEOUS DECOMPOSITION OF BARIUM AND STRONTIUM SULFATES
A heated ball mill to carry out inorganic reactions when one or more reactants are insoluble has been used frequently in this laboratory (8). For mixing purposes, steel balls of 100
80
n
c y >
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PERCENT M E T H A N O L IN C H3OH-HzO SO LV E NT Figure 1. Effect of Extraction Solvent Composition on Aqueous Reaction Product of Barium Sulfate and 10 Per Cent Molar Excess of Calcium Chloride
When the reaction was carried out by fusing the reactants, aqueous methanol gave a satisfactory extraction. When a similar procedure was tried on the ball mill reaction mass (in stirred saturated solution as described in a following paragraph) , Shreve and Toner (8) found th-at almost complete reversion took place a t once. These results indicated that there was a difference between the two types of reaction products which was responsible for the change in behavior. On the basis of synthetic reaction mixtures representing different reaction products, it was pointed out (8) that the difference in reversion tendencies between the fusion and ball mill products was a result of a difference in the properties of calcium sulfate formed. CALCIUM SULFATE AND ITS HYDRATES
There are four modifications of calcium sulfate: gypsum (CaSO4.2H20), plaster of Paris (CaS01.%H20), soluble anhydrite, and insoluble anhydrite (g), Newman and Wells (3) observed the conditions of heating necessary to change the x-ray pattern from the soluble anhydrite to the insoluble
52 i?
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20 40 60 PERCENT METHANOL IN C H,OH - H20 SOLVENT
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Figure 2. Effect of Extraction Solvent Composition on Aqueous Reaction Product of Barium Sulfate and 25 Per Cent Molar Excess of Calcium Chloride
20
IO
80,
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100 120
,
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TEMPERATURE
700'
C.
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40
U n w
20
I 20
10 TIME
I 30
40 IN
OF EXTRACTION
60 80 MINUTES
100 120
Figure 3. Effect of Time on Extraction with Water of .iqueous Reaction Product of Barium Sulfate and Calcium Chloride Above, 10 per cent molar excess CaCh; below, 25 per cent excess CaCla. The volume of water refers to that used for extractions.
assorted sizes are placed inside with the reacting substances. As the ball mill rotates, these balls keep the materials ground and well intermixed. The reaction conditions were as follows: The finely ground reactants were placed in the ball mill, and sufficient water was added to give a thin creamy mass. Heat was gradually applied to the rotating mill, and 24-36 hours were allowed for the water to be driven off. The heat input was such that the equilibrium temperature finally reached was a little over 200" C. The product was removed, ground in a mortar, and stored in a bottle from which the samples were taken. B A m u b i SULFATE. The reaction, Bas04
+ CaCIL= BaC& + Cas04
was carried out in a ball mill for two diffexent molar ratios of reactants] one with 10 per cent and the other with 25 per cent molar excess of calcium chloride. Three moles of barium sulfate were used in each experiment. The reaction product consisted of a mixture of four salts: calcium chloride, barium chloride, barium sulfate, and calcium sulfate. The reaction products were separated for analysis by extraction with a mixed solvent of ethylene glycol and methanol in 1:3 ratio. The methanol was added to give the solvent satisfactory fluidity properties, but it was not used alone be-
cause of its limited solubility for barium chloride. The extraction resulted in a separation of the soluble chlorides from the insoluble sulfates. The analysis was based on the amount of barium present in the extract compared to the total amount of barium present in a duplicate unextracted sample. The barium in the latter was determined by adding water t o the sample; revetsion t o barium sulfate resulted. The method of analysis of the filtrate mas similar to that described by Shreve and Toner ( 8 ) which is a modification of the standard analysis for alkaline earths. Barium was determined as barium chromate. A 95 per cent conversion was obtained using 10 per cent excess calcium chloride, and a 97 per cent conversion using 25 per cent excess calcium chloride. Aqueous methanol extractions were carried out on the heated reaction product (thereby insolubilizing the calcium sulfate by changing it to the insoluble anhydrite form) which had been furnaced for periods of 1to 24 hours a t temperatures from 300" to 700" C. Aqueous extractions were carried out on sampIes after heating at 700" C. for 12 hours; various amounts of water were used for different time periods. The apparatus for heating the samples consisted of a small vertical cylindrical furnace calibrated for temperature against current. Two-gram samples were weighed out and transferred to flat-bottomed crucibles. At the end of the heating periods the crucibles were removed from the furnace; their contents were ground in a mortar and transferred to 100-ml. tincture bottles for extraction. The bottles were mounted on a rotary shaker and rotated for the timed extraction period. At the end of this interval the bottles were removed and their contents allowed to settle for 10 minutes previous t o their filtration through Gooch crucibles. The results for the 500" and the 700" C. furnace temperatures are pIotted for the aqueous methanol extractions in Figures 1 and 2; 100 ml. of extraction solvent were used in each case. An example of the data follows for Figure 1 (at 500" C.). The extraction time was one hour with the exception of the runs marked" in which case it was 10 minutes. A settling time of 10 minutes was allowed before filtration. The volume of extraction solvent was 100 ml. The sample weight was 2 grams: yo Llethanol Heating in Extn. Solvent 0 40 60 80 0 40 60 80 0 60 80 a
Time, Hr. 1
v0 BaCli
1
1
1 3 3 3
3
G 6
G
Yield 0 12 24 51 0 23 48
72
0 56 80
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ethanol 'i;'Extn. Solvent 0 20 60 80 0 40 60 80 OQ
Heating Time,
Hr. 12 12 12 12 24 24 24
60;
24 1 1
80
1
% BaCh Yield 0 22 68 73 2 56 75 89 0 49
58
Ten-minute extraction.
The curves show increased yield as the temperature was increased and the heating time was extended. The 10minute extraction for a sample heated 1 hour a t 500" C. gave a better yield than a duplicate sample extracted for an hour. The corresponding comparison a t the 700" C. furnace temperature showed a smaller yield for the 10-minute extraction. This behavior seems to indicate that, although the hour extraction was more complete than the 10-minute one, with the product heated a t the lower temperature the reversion was the more important factor; with the product heated a t the higher temperature, the greater insolubilizing of the calcium sulfate lessened the reversion and made the completeness of extraction relatively more important. Increasing the excess of calcium chloride in the initial reaction mass (25 as compared to 10 per cent excess) made considerable
INDUSTRIAL AND ENGINEERING CHEMISTRY
May, 1943
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PERCENT M E T H A N O L I N CH30H-H20 SOLVENT
Figure 4. Effect of Extraction Solvent Composition on Aqueous Reaction Product of Strontium Sulfate and 10 Per Cent Molar Excess of Calcium Chloride
difference a t the higher furnace temperatures when the extraction solvent contained larger quantities of water. At lower furnace temperatures or a t the higher furnace temperatures when the solvent was high in methanol, the effect of increasing the calcium chloride content of the reaction mass resulted in only a slight increase in yield. The results for the aqueous extractions of samples heated a t 700" C. for 12 hours using 10 to 200 ml. of water for extraction are plotted in Figure 3. For the reaction mass containing 10 per cent excess calcium chloride, the maximum volume of water used for extraction was 200 ml.; for the mass containing 25 per cent excess, it was 100 ml. The extraction times ranged from 10 minutes to 2 hours. The low yields in the IO-ml. extractions were probably caused by mechanical difficulties. When enough solvent was used for good mechanical contact, a further increase in volume did not have much effect. I n all cases the yield dropped as the extraction time was increased. There was an optimum value for the volume of extraction solvent, as indicated by the fact that the 50-ml. and the 200-ml. extractions of the upper graph gave yields less than those obtained with 100 ml. of water. The balance between completeness of extraction and extent of reversion would vary empirically with the equipment; but extraction was favored by the use of sufficient
solvent and a short extraction period. Increasing the calcium chloride content (from 10 to 25 per cent in excess of that theoretically required) of the initial reaction mam made a marked difference in yield for the shorter extraction peiiods. As the extraction time was lengthened, the effect of the increased calcium chloride content became progressively less significant. STRONTIUM SULFATE.The reaction, SrSOc
+ CaCL = SrCL + CaSOc
was carried out in the same manner as that for barium chloride, with 10 and 25 per cent molar excesses of calcium chloride for the two molar ratios of reactants. Three moles of strontium sulfate were used in each experiment. The reaction product consisted of a mixture of four salts: calcium chloride, strontium chloride, strontium sulfate, and calcium sulfate. Separation for analysis was by extraction with methanol, in which strontium and calcium chloride are both soluble; the result was separation of the soluble chlorides from the insoluble sulfates. Analysis was based on the amount of strontium present in the extract compared with the total amount of strontium present in a duplicate unextracted sample. The strontium in the latter was determined by adding water and causing a reversion to strontium sulfate. The method of analyzing the filtrate was that of Shreve and Watkins (0). The stIontium and calcium chlorides were precipitated as carbonates, the carbonates were converted to nitrates, and the nitrates were extracted with acetone in which the strontium nitrate is virtually insoluble and the calcium nitrate soluble. A 92 per cent conversion was
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obtained with 10 per cent excess calcium chloride, and a 97 per cent conversion with 25 per cent excess calcium chloride. Aqueous methanol extractions were carried out on the reaction product, which had been heated for 1 to 24 hours a t 300" to 650" C. Aqueous extractions were carried out on samples, which had been heated a t 650" C. for 12 hours, with various volumes of water for different extraction times. The furnace was the same as that used for the barium salts. The results for the 500' and the 650' C. furnace temperatures are plotted in Figures 4 and 5 ; 100 ml. of solvent were used in these extractions which were all for one hour. The curves show an increased yield as the temperature was increased and the heating time extended Increasing the excess calcium chloride in the initial reaction mass (25 as compared t o 10 per cent excess) did not increase the yield notably as in the reaction with the barium salts. The explanation is that the furnace temperature was higher for the barium salts, and it has already been mentioned that at the lower heating temperatures the effect of increasing the calcium chloride content was only slight. The results for the aqueous extractions of samples heated a t 650" C. for 12 hours are plotted in Figure 6, using 10 to 200 ml of water for extraction for periods of 10 minutes to 2 hours. The lowest yields again were with the 10-ml. extractions, probably because of lack of thorough contact of the water with the salts. An optimum value for extraction is indicated by the fact that both the 50- and 200-ml volumes of water for extraction gave values less than those obtained for the 100-mi. volume. -4few exceptionally low values resulted from some of the solid salts adhering to the sides of the tincture bottle and thus not being thoroughly extracted.
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FUSION REACTIOUS WITH BARIUM AND STRONTIUM SULFATES
The fusions of barium sulfate Tyith calcium chloride and of strontium sulfate with calcium chloride mere made in tvideform No. 60 porcelain crucibles. The furnace was brought t o 900" C., which was 176-225" C. above the fusion temperature of either of the mixtures. Five minutes after the contents had completely melted, the crucibles were removed and cooled, and the solidified contents ground in a mortar. The fusion reaction, Bas04
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FURNACE
65OoC.
TEMPERATURE
+ CaCll = Cas04 + BaClz
was investigated in detail by Shreve and Toner (8). The reaction has been shown to be one of equilibrium, and the effects of time, temperature, and ratios of reactants were determined. The fusion here was carried out with a 25 per cent molar excess of calcium chloride in a charge containing 0.2 mole of barium sulfate. The conversion of the fusion was 98 per cent. Extractions of 2-gram samples of the fusion product were made with various volumes of water for different extraction times. The results are plotted in the upper graph of Figure 7. The fusion reaction, SrSO4
20
+ CaCla = CaSOl + SrClz
was investigated by Shreve and Watkins (9). This results in more decomposition than does the fusion of barium sulfate with calcium chloride. It was carried out for a 25 per cent molar excess of calcium chloride in a charge containing 0.2 mole of strontium sulfate. The conversion was 91 per cent. Extractions were cariied out in the same manner as for the fusion with barium sulfate. The results are also plotted in Figure 7 . In the fusions of both the strontium and the barium salts, the mechanical limitations favored the-larger volumes of
water for complete extraction. A short extraction time gave less chance for reversion t o take place and thus favored a higher yield. The over-all yield of barium chloride was higher than for strontium chloride, but this was due probably to the higher yield in the initial fusion, in which barium chloride i s less volatile than strontium chloride, and not to the amount of reversion. CONCLUSIONS
Extractions of samples from the furnace were made with both water and aqueous methanol. Higher yields were obtained with aqueous methanol than with water alone. I n the aqueous decomposition of barium sulfate, the effect of increased molal ratio of calcium chloride in the reaction mass did not give a pronounced increase in yield when a solvent of high methanol concentration was used. In the extractions with water or with solvents of low methanol concentration, the product containing the higher molar ratio of calcium chloride gave a yield as much as 25 per cent greater. The aqueous extractions of the fused product resulted in yields up to 88 per cent if a short extraction time of 10 minutes and sufficient water were used. A short time was desired such ~
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
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cent methanol. Aqueous extractions of the heated ball mill product gave results somewhat higher than those observed in the barium chloride production. With a rapid 10-minute extraction using 100 ml. of water on a 2-gram sample of the fused product, a reversion of only 7 per cent t o give an over-all yield of 84 per cent was observed. A comparison of the fusion method with the aqueous decomposition method in the barium chloride production shows that, under the most favorable conditions, the 88 per cent yield on the aqueous extraction of the fusion product was 15 per cent greater than any obtained from the other method. Organic extractions were not carried out on the fusion products, but the 95 per cent yield for the aqueous decomposition method using 80 per cent aqueous methanol extraction solvent was higher than any direct aqueous extraction of the fused product. In the strontium chloride production by the fusion method, yields in the aqueous extractions of 84 per cent were 6 per cent higher than those possible with the aqueous decomposition method; but again the organic extractions with the solvent as 80 per cent methanol made possible the higher yield of 88 per cent and, with pure methanol, the theoretical (same as initial conversion) yield of 91 per cent. The latter did not require that the products be heated after conversion, as the insolubilized form of calcium sulfate is not necessary for the pure methanol extraction.
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LITERATURE CITED
(1) Averko-Antonovich, I. N., Trans. Kirov. Inst. Chem. Tech. Kazan, No. 7, 3-6 (1938); Chem. Abs., 35, 2431 (1941). (2) Broughton, G., IND. ENG.CHEM.,31, 1002-6 (1939). (3) Newman, E. S., and Wells, L. S., J . Research Nutl. Bur. Standurds, 20, 825 (1938) (Research Paper 1107). (4) Sohooh, E. P., and Cunningham, W. A,, Trans. Am. Inst. Chem. Engrs., 37, 1-18 (1941). (5) Shreve, R. N., et al., U. S . Patent 2,030,659 (Feb. 11, 1936). (6) Shreve. R. N., and Pritchard, W. N., IND.ENG.CHEM.,27, 1488
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20 TIME O F
100 120
Figure 7. Effect of Time on Extraction with Water of Fusion Product of Barium Sulfate (above) and of Strontium Sulfate (below) with 25 Per Cent Molar Excess of Calcium Chloride
that the soluble chlorides could be dissolved and yet not have opportunity to begin a reversion reaction. A large volume of water aids in dissolving the chlorides readily, and the reversion tendencies are apparently not much affected. The 88 per cent yield is somewhat higher than the 81.52-83.7 per cent yield reported by Averko-Antonovich (1). With the extraction solvent of 80 per cent methanol, little reversion was observed for the ball mill product heated at 700' C. for 3 or more hours. Aqueous extractions of 73 per cent yield were possible. I n the aqueous decomposition of strontium sulfate the effect of increased molar ratio of calcium chloride was not so pronounced. Theoretical recovery is possible without heating the reaction product when extracting with pure methanol, in which the strontium chloride is soluble, and the calcium sulfate insoluble. When extracting with aqueous methanol, heating the reaction product was necessary to get good yields. Heating is necessary to convert the calcium sulfate from the soluble to the insoluble anhydrite form. Heating for 12 hours or more at 650" C. gave a yield of 88 per cent or better for the aqueous solvent containing 80 per
(1935). ' (7) Shreve, R. N., Pritchard, W. N., and Watkins, C. H., Purdue Univ., Eng. Bull. 22, No. l a (Feb., 1938). (8) Shreve, R. N., and Toner, R. K., IND.ENG.CHEM.,32, 568
(1940). (9) Shreve, R. N., and Watkins, C. H., Ibid., 31, 1173 (1939). (10) Shreve, It. N., and Watkins, C. H., Purdue Univ., Eng. Bull., 23, No. 3a (June, 1939). (11) Thorpe, J. F., and Whitely, M. A., "Dictionary of Applied Chemistry", 4th ed., Vol. I, p. 639, London, Longmans, Green and Co., (1937). (12) Ibid., 4th ed., Vol. I, p. 641 (1937). PRESBNTED before the Division of Industrial and Engineering Chemistry at the 102nd Meeting of the AMERICAN C"IM1c.a SOCIRITY.Based upon 8 thesis submitted by Herbert F. Wiegandt to the faculty of Purdue University in partial fulfillment of the requirements for the degree of doctor of philnsophy.
Tar Elimination in Pyroligneous Acid (Correction) An error in Figure 9 on page 294 of this article in the March, 1943,issue of INDUSTRIAL AND ENGINEERING CHEMISTRY has been called to our attention. The arrows pointing to the respective scales are drawn incorrectly. Curve I should have the arrows pointing to the left and down, indicating the pH of acetic solution plotted against added amounts of sulfuric acid. The arrows of curve I1 should oint upward and to the right, indicating the pH of acetic acid soyutions (free of sulfuric acid) plotted against per cent acetic acid in such solutions.
DONALD F. OTHMER RAPHAEL KATZEN POLYTECHNIC INBTiTUTl BROOKLYN, N. Y.
