Batch Precipitation of Potash Alum Crystals by Mixing Aqueous Solutions of the Component Salts

The effects of temperature, supersaturation, excess of reacting component salts and method of precipitation on the batchwise precipitation of potash alum crystals, by mixing aqueous solutions of potassium sulphate and aluminium sulphate, have been investigated. From the primary characteristics of the process (crystal size, size distribution and yield) relative crystal growth rates and relative nucleation rates are calculated and interrelation of these rates with the corresponding supersaturation leads to a value of the nucleation order. It is concluded that secondary nucleation plays an important role in the batch precipitation of potash alum crystals.     

Supersaturation and crystal size distribution changes during the precipitation of Nickel ammonium sulphate hexahydrate

The desupersaturation of nickel ammonium sulphate aqueous solutions, during the precipitation of the hydrated salt, has been followed by refractive index measurements. The addition of seed crystals has a considerable effect on the desupersaturation process: the induction and latent periods and the crystal size distribution are all greatly reduced. The precipitated crystal size follows V . WEIMARN'S rules, viz, the median size (a) passes through a maximum with increasing supersaturation for a given crystallization time, and (b) decreases with increasing supersaturation, for precipitations which have virtually ceased. The crystal yield increases with both supersaturation and time, but the size distribution remains fairly constant with time for supersaturations, S > 2.     

Precipitation studies in an air-slug crystallizer: Potash alum from aqueous solution using 2-propanol

An air-slug crystallizer is described and its utility demonstrated with the precipitation of potash alum from aqueous solution using 2-propanol as the water-miscible precipitant. Basic equipment dimensions are specified and several modes of operation explored. Best results are obtained with a controlled step-wise addition of alcohol.     

A mechanism for the habit modification of ammonium dihydrogen phosphate crystals in the presence of ionic species in aqueous solution

Ammonium dihydrogen phosphate (ADP) crystals develop new high index faces when grown from aqueous solution containing up to 1000 ppm of added CrCl₃ · 6 H₂O. The crystallographic nature of these faces has been studied as a function of the CrCl₃ · 6 H₂O concentration in the growth solution (24.0°C, σ = 0.06) and their high morphological importance is explained, using a modification of the Hartman-Perdok theory of crystal morphology, but postulating that the adsorption of impurity species at specific sites transform K-faces into pseudo-F-faces.