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The crystal size distribution (CSD) of sodium sulfate from the batch crystallization of sodium sulfate from concentrated sulfuric acid (spent acid) by salting-out with methanol was analyzed. The effects of amount of methanol added, the agitation rate, temperature and residence time (sampling time) on CSD were investigated. The mean crystal size and the coefficient of variation (c.v.) were the criteria used and the Rosin–Rammler distribution model was assumed. The mean crystal size varied from 0.03 to 0.07 mm while the range of c.v. was 5–45%, though most of the runs gave a c.v. of 20% or less, thus a size independent growth rate could be assumed. Larger mean crystal sizes were obtained for the following conditions: small amount of added methanol (low level of supersaturation), low temperature, low agitation rates and longer residence time. Reduction in coefficient of variation was enhanced by the same factors. A 2 4 factorial design analysis confirmed these findings.

The solubility of sodium sulfate in the concentrated sulfuric acid + methanol system was determined at temperatures of (273, 293, 303, 313, and 323) K. The saturation densities were also determined. The solubility increased with increasing temperature until about 323 K. When the result of this study was compared with an earlier study7 of solubility for the sodium sulfate + water + aqueous methanol system, the solubility values of the present system were greater at all comparable temperatures. This may be due to the fact that “like dissolves like”. In this case, the SO42- anion is common to both sulfuric acid and sodium sulfate, whereas in the former study, it is not. Moreover, compounds containing the SO42- anion are known to be less soluble in water.

The solubility of sodium sulfate in water and aqueous methanol has been determined over the temperature range 10 °C to 50 °C. Also the saturated solution densities were measured. The presence of methanol was observed to reduce the solubility of the salt greatly. In fact, in pure methanol, sodium sulfate was essentially insoluble. The solubility data for the sodium sulfate−water solution were within ±1% of values in the literature. No attempt was made in the study to differentiate between the two stable salts of sodium sulfate (decahydrate and anhydrous) in the temperature range studied.