March, 1943
INDUSTRIAL AND ENGINEERING CHEMISTRY
vious runs, the sulfuric acid was poured into the kerosene and aniline over a period of 2 minutes. The results are shown in Table IV and Figure 4. It is evident that a combination of the optimum values of all the variables causes the production of high yields of sulfanilic acids. These yields are somewhat better than those obtained in industry. The most favorable yields of sulfanilic acid were obtained when operating a t 183" C., using no excess
323
sulfuric acid, and removing the theoretical amount of water (in this case 11 cc.). These conditions are those of the last run in Table IV. Literature Cited (1) Groggins, P.H.."Aniline and I t a Derivatives", New Y o r k , D. Van Nostrand Co., 1926. (2) Kokatnur, V. R.,u. 8. Patent 2,111,973(1938). (3) Othmer, D. F., IND. ENQ.CHBM.,33, 1106, (1941).
STRONTIUM CARBONATE Conversion from Strontium Sulfate by Metathesis with Alkali Carbonate Solution' Kenneth
A. Kobe* and Norman 1. Deiglmeier
UNIVERSITY OF WASHINGTON, SEATTLE, WASH,
TRONTIUM carbonateis b b The optimum conditions for the converthe hydrogen sulfide thus the material from which sion of strontium sulfate to the carbonate b y the formed. One of t h e most soluble strontium action of solutions of alkali carbonates have been objections i s t h e h y dt roo gthis e n s process ulfide salts are made. The chief gletermined. The operation of a pilot plant for the military use of the latter is in evolved. chemical beneficiation of a mixed strontium sulfataThe metathetical conversion tracer bullets and other pyroof strontium sulfate to the carbonate ore is described. technics. Practically all of the strontium minerals and carbonate with solutions of alcompounds have been imkali carbonates has been studied ported in the past, but during a national emergency domesby a number of workers. Piva (12) reacted a slight excess tic sources must be scrutinized and methods developed' to (5 to 10 per cent) of ammonium carbonate solution with celesutilize the materials directly available. tite and precipitated strontium sulfate, transposing 82.85 Several of the western states have important deposits of per cent of the former and 98.75 per cent of the latter in 3 hours, celestite (SrSOr) which sometimes has been weathered to Gallo (6) investigated similar materials and found 90.5 and strontianite (SrC03). Arizona has reserves estimated a t 98.5 per cent transposed, using the optimum conditions. 200,000 tons. California has the largest known reserve of Thomas (13) used about 5 per cent excess of sodium carbon600,000 tons, A large area of Texas is underlaid with celestite. ate in solutions and ground in a ball mill a t 60" C. for one Washington has a small deposit highly weathered, estimated hour, filtered, washed, and then repulped and reground for as 10,000tons (7). another hour to remove soluble sulfates. Elledge and Hirsch Thus, strontium sulfate is available domestically in large (4) similarly agitated ground celestite with sodium carbonate quantities. The same is true of available imports, the bulk solution, but they added sodium hydroxide in an amount from of which has been celestite imported from England. The 27 to 100 per cent by weight of the strontium sulfate; inconversion of this material to the acid-soluble strontium creased conversion is claimed. An interesting medium for the carbonate is an important problem. reactions is fused sodium chloride used by Booth (1) who claims 99.9 per cent conversion with 17 per cent excess Conversion to Carbonate sodium carbonate. At equilibrium, the constant for the metathesis, The standard method of producing a soluble strontium salt has been to reduce the celestite with coal breeze in a reverSrSOc Na&Os e SrCOs Na&Oc beratory type furnace to produce the soluble sulfide, which is then leached from the black ash. The reduction is approxican be calculated from the solubility products. The solubilities of strontium carmately 60 to 80 per cent complete, and some carbonate and oxide are produced. The sulfide solution is a t e r e d to remove bonate a t 25" and 40" C. ash particles and then treated with acid to produce the deas determined by Townley, WhitneY, and Felsing (14) are sired salt. Dilute nitric acid can be used, but considerable and 0.7026 X free sulfur is formed by the oxidation of the hydrogen sulfide 0.5525 x gram mole Per 1000 grams evolved. The more usual process is to precipitate the carbonate, either by adding soda ash or directly carbonating the of water. The corresponding solution with kiln gases rich in carbon dioxide and removing Values for strontium sulfate as determined by Gallo (6) are 1 This is the third paper in a series on Metathesis between Solids and 0.735 X and 0.772 x 10-3 Solutions: for previous papers, see literature citations ( 8 ) and (9). gram mole. * Present address, IJniversity of Texas, Austin, Texas.
S
+
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
324
Figure
7,
Vol. 35, No. 3
Effect of Time, Concentration, Excess Sodium Carbonate at Constant Volume of Solution, and Temperature on Conversion
From the solubility products, K1 and Kz, calculated from the solubility data, the ratio of sulfate to carbonate ions in a solution saturated with respect to both solid phases can be obtained:
At 40" C. a similar calculation shows that the sulfate ion concentration is 121 times greater than the carbonate concentration. These formulas show qualitatively that the reaction can be expected to proceed almost completely to the conversion to strontium carbonate. However, the changing ionic strength of the solution causes the value of the solubility product t o increase so that no quantitative value is expected. Effect of Variables on Conversion
The major part of this investigation was made on natural celestite, ground to pass 200 mesh. It analyzed 97.9 per cent strontium sulfate. Baker's c. P. grade,, precipitated strontium sulfate was used in other runs. The sodium and potassium carbonates were Baker's c. P. grade, and the ammonium carbonate was a commercial product. In the majority of runs agitation was provided by mounting the heater and flask on a mechanical shaker, as shown by Doumani and Kobe (8). Some runs were made in a Charlotte laboratory-type colloid mill, in which the temperature was controlled by the water jacket surrounding the grinding head. After each run the slurry was filtered and a sulfate determination made using the volumetric barium chromate paste method described by Kolthoff (11). Attempts t o use internal indicators, such as THQ, were unsuccessful because traces of strontium ion interfered with the end point. EFFECTOF TIME.In the shaker 1.0 mole of strontium sulfate as celestite was shaken with 1.15 moles of sodium carbonate in 1 liter of solution at the boiling point for various times. The results are shown in Figure 1A; the reaction is
quite rapid under these conditions, and equilibrium is attained in one hour, which time was used in subsequent runs. EFFECT OF CONCENTRATION. Equivalent amounts of strontium sulfate as celestite were shaken with one liter of boiling sodium carbonate solution; the concentration of the latter was increased each run (Figure 1B). This decrease in conversion is relatively small and undoubtedly due to the decreased concentration of strontium sulfate in solution. Gallo (6) recommended at least five parts of water per part of celestite, which is a reacting solution about 1 molar in carbonate. Thomas (IS) used a concentrated solution almost 2.5 molar in carbonate, which increases the capacity of a given piece of apparatus. EFFECTOF EXCESSREACTANT.The effect of an excess of sodium carbonate in a constant volume of boiling solution was studied by treating 1.0 mole of strontium sulfate as celestite with 1 liter of sodium carbonate solution containing from 1.0 to 2.0 moles. Figure IC shows that the conversion is a maximum a t 50 per cent excess sodium carbonate. The helpful effect of excess reactant is retarded by the deleterious effect of increased concentration shown in Figure 1B. EFFECTOF TEMPERATURE. The effect of temperature of reaction was studied by shaking 0.25 mole each of strontium sulfate as celestite and of sodium carbonate in 1 liter of solution for 2 hours at various temperatures. It is apparent in Figure 1D that the reaction must be carried out at as high a temperature as practicable. Although strontium sulfate has an inverted solubility curve (6) above 40" C., the decrease in solubility due to increased temperature apparently is a minor factor in comparison with other factors. No data are available for the solubility of strontium carbonate above 40" C. EFFECTOF TYPEOF STRONTIUM SULFATE.The conversion was studied with four types of strontium sulfate: natural celestite; precipitated and calcined strontium sulfate; precipitated, dried, and powdered strontium sulfate; and freshly precipitated strontium sulfate that was not allowed
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
March, 1943
Table 1.
Conversions with Ammonium Carbonate Tp&,
(NHSzCO:
8rS04 0.25 0.25 0.25 0.25 0.25
325
0.25 0.50 0.25 0.25 0.25
Conversion. %
Time, Hr.
26.9 34.2 62.7 57.0 73.4
2 2 2 1 1
22 22 44 27 46
Method Shaker Shaker Shaker Colloid mill Colloid mill
~~~~
~
Other Carbonates
Although the conversion should be a function of the carbonate ion activity only, previous work showed that other factors frequently enter to prevent this equality. As shown on Figure lA, somewhat higher conversions are obtained with potassium carbonate under similar conditions. Conversions of 90 to 98 per cent were reported by Gallo (6) and Piva (If?), using ammonium carbonate. In this work the best conversion obtained with ammonium carbonate under the optimum conditions of Gallo was only 73.4 per cent (Table I). It was thought that the low conversion might be due to the decomposition and volatilization of ammonium carbonate; however, an ammonia balance after each run accounted for a t least 0.24 mole from 0.25 mole. Pilot-Plant Operation
I n the state of Washington a deposit of celestite is available which is highly weathered with formation of considerable strontianite. Analysis of a sample from a 5-ton lot shows: Fez08 CaO
0.64 6.03 60.08 0.49
SrO BaO
Figure
2.
Si01
0.52 12.30 20.08 0.96
SOs
coz
Not analyzed
Pilot Plant for Chemical Beneficiation of Celestite-Strontisnite Ore
to dry or cake. Equimolar amounts (0.25 mole) of the strontium sulfate and sodium carbonate in 1 liter of solution were allowed to react for one hour a t the boiling temperature. The conversionsa differed markedly, as had previously been found with barium sulfate (10): % Conversion
Type Freshly precipitated Celestite Preoipitated, dried, powdered Preoipitated and calcined
95.2
96.8 91.7 90.3
The natural celestite was soft and easily crushed, whereas the dried precipitate was hard and gritty. A similar result had been observed with calcium phosphate by Kobe and Doumani (8). 01
Equilibrium in the System
0
The equilibrium point in this metathetical reaction should be obtained by starting from either side of the equation, SrSOI NazCOs e SrCOs NaaSOc
Figure
+
+
The forward reaction was studied with 0.25 mole each of strontium sulfate and sodium carbonate in 1 liter of solution a t boiling temperature. The reverse reaction used 0.25 mole of precipitated strontium carhonate and sodium sulfate under the same conditions. The percentage of total carbonate that appeared as sodium carbonate in the final reaction solution was 4.2 in the forward reaction and 6.1 in the reverse reaction. This small difference in concentration of sodium carbonate a t equilibrium may be due to a difference in physical structure between the strontium salts used and those formed in the reaction. 8 The percentage conversion m a y well have been the function of the rate of solubility which would be influenced by the physical condition of the
strontium sulfate.
I
10
I
eo TIME
I
I
30 40 IN MINUTES
I
50
I
60
3. Rate of Conversion in Rotating Mixer
This corresponds to approximately 54 per cent strontium carbonate and 28 per cent strontium sulfate, after allowances are made for barium sulfate and calcium carbonate, This ground ore has a number of uses, such as the removal of iron from caustic solution (9). However, if the sulfate is converted to Carbonate, the ore will have the value of strontianite, whereas without chemical beneficiation it has only the value of celestite, despite the large amount of strontium carbonate present.
.
