The effects of inorganic impurities on the crystallization of calcium sulfates in strong HCl (6.3 mol L–1)-CaCl2 (1.8 mol L–1) solutions were investigated. The impurities considered relate to hydrochloric acid leaching of apatite-type ores for the extraction of rare earth elements. The impurities investigated were K+, Mg2+, Sr2+, Ba2+, Al3+, Fe2+, Fe3+, La3+, Y3+, F– (fluoride), and PO4 3– (phosphate). The investigation was done in the context of a continuous steady-state crystallization process. Therefore, temperature-controlled, semibatch crystal growth experiments with regulated reagent addition, to ensure nearly constant supersaturation, were performed. The experiments were conducted at 40 and 80 °C corresponding, respectively, to crystallization of calcium sulfate dihydrate (DH) and calcium sulfate hemihydrate (HH). Among all impurities investigated, phosphate and strontium were found to have the most significant effects, with La3+ and Y3+ having some modest effects. Phosphate (added as phosphoric acid) was found to accelerate the growth kinetics of dihydrate up to a certain concentration level (0.3 mol L–1), subsequently causing a retardation effect over the concentration range from 0.3 to 1.0 mol L–1. In contrast, phosphate had no effect on the growth kinetics of hemihydrate. In the meantime, phosphate uptake increased with increasing impurity concentration in the range up to 0.2 mol L–1 and then plateaued at 0.02 molphosphate molsolid –1. X-ray photoelectron spectroscopy (XPS) analysis provided evidence of the presence of a surface calcium phosphate species. On the other hand, the uptake of Sr2+ by dihydrate was much more extensive than that of phosphate (≈5–10×). In this case, substitution rather than adsorption was the mechanism of uptake, reflecting the similar ionic radii between calcium and strontium. At phosphate and strontium concentrations >≈ 0.2 mol L–1, partial transformation of dihydrate to hemihydrate was induced. Finally, La3+ and Y3+ were found to be incorporated at trace level amounts into dihydrate crystals causing crystal morphology changes but not promoting phase transformation.

The crystal growth kinetics of calcium sulfate α-hemihydrate (α-HH) in nearly constant supersaturated HCl-CaCl 2 solutions were investigated. Two types of solutions were used, the first had a low HCl (1.4 mol/L) and high CaCl 2 (2.8 mol/L) concentration and the second had a high HCl (5.6 mol/L) and low CaCl 2 (0.7 mol/L) concentration. These conditions were chosen to represent the first and last stage of a newly developed stage-wise HCl regeneration process. The seeded growth experiments were carried out in a stirred, temperature controlled semi-batch reactor in which supersaturation was kept constant by simultaneous addition of CaCl 2 and Na 2 SO 4 solutions. The influence of the following parameters on α-HH crystal growth was studied: temperature (70-95°C), specific power input of stirring (0.02-1.29 W/kg) and equimolar inflow rate of CaCl 2 and Na 2 SO 4 (0-0.6 mol/h). The crystal growth rate was derived from particle size distribution measurements made with the laser light diffraction technique. It was found that the surface area normalized crystal growth rate increased linearly with the molar inflow rate up to 0.3 mol/h, at higher inflow rates no further increase of the growth rate was observed. Temperature and specific power input, within the investigated ranges, did not show a marked effect on the growth rate, attributable to a diffusion/adsorption controlled growth process. An interesting finding of the present research is the establishment of a positive relationship between the narrowing of the width of the particle size distribution with increasing crystal growth rate. The results show that the resulting particle size distribution is positively related to the reagent inflow rate, a finding that can be applied to the industrial design and scale-up of the α-HH crystallization/HCl regeneration process.

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BACKGROUND This work deals with the effect of several additives on the morphology of calcium sulfate dihydrate (DH) crystals, produced by reactive crystallization involving strong CaCl2–HCl solutions. Experiments were conducted at 40 °C under various conditions, namely homogeneous and seeded (heterogeneous) crystallization in CaCl2/HCl solutions ranging from very dilute (0.42 mol L−1/0.04 mol L−1) to very concentrated (1.8 mol L−1/6.3 mol L−1). RESULTS In the absence of additives a distinct difference in crystal morphology was observed depending on solution composition. Homogeneously formed DH crystals from dilute solution were in the form of needles. DH crystals formed in concentrated acid solution were thick elongated plates. The most influential additives in concentrated solutions were CTAB (cetyltrimethylammoniumbromide), PAA (polyacrylic acid) and PVS (polyvinyl sulfonic acid). CTAB favoured formation of industrially interesting thick slab crystals. In contrast PAA favoured the formation of elongated and thinner crystals as a result of preferential crystal plane adsorption. PVS had a detrimental effect on DH crystal nucleation and growth explained by its polymeric structure. CONCLUSION Apart from PVS, CTAB and PAA no influence was seen for the majority of additives tested. This was attributed to lack of ionization caused by the highly acidic solutions. © 2013 Society of Chemical Industry

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