| Flue gas desulfurization (FGD) gypsum which is mainly composed of DH was used as raw material to obtainα-calcium sulfate hemihydrate (α-HH) through dehydration in CaCl2-MgCl2-KCl solution medium at 95℃under atmospheric pressure. Effect of pH and carboxylic acids on the dehydration process and the habit of the final products were investigated.The initial pH values of the suspensions were adjusted from 1.2 to 8.0 to explore the influence of pH on the dehydration process and the product characteristics. The variations of crystal water content, the phase constituents and the habit of the crystals were examined and compared. The concentrations of calcium and sulfate ion were also measured to indicate the relative supersaturation. The results showed thatα-HH crystal was the only dehydration product with pH range from 1.2 to 8.0. With the increase of initial pH, the dehydration rate decreased and the formedα-HH crystal had larger particle size. The length/width (l/w) ratio decreased markedly from 4.8 to 2.9 as the initial pH increased from 1.2 to 7.3. pH had a profound influence on the dehydration of DH and the morphology ofα-HH via its effect on the supersaturation and the precipitation of Ca(OH)2 in alkaline environment.The influence of different concentration of tartaric acid, citric acid and succinic acid on the dehydration process and product characteristic were investigated. The results showed that the dehydration rate was unchanged with the concentration of tartaric acid below 1.0×10-2 M, while the dehydration was retarded in the case of 2.5×10-2 M tartaric acid. The products were allα-HH with tartaric acid. Low concentration of citric acid and succinic acid had little effect on the dehydration rate and the final products were mainlyα-HH, co-existed with a little amount of AH. The dehydration was retarded in the case of 1.0×10-4 M citric acid or 5.0×10-4 M succinic acid and the dehydration phase was only AH or mainly AH co-existed with a little amount ofα-HH. High concentration of tartaric acid increased the l/w ratio ofα-HH crystal, while low concentration of citric acid or succinic acid decreased the l/w ratio ofα-HH crystal. The effect of carboxylic acids on the dehydration process and the habit of the final products is possibly due to the selective adsorption on the facets of the crystal. The different effect of carboxylic acids on the dehydration process might be attributed to the structure difference.The research shows thatα-HH can be prepared by dehydration of FGD gypsum in CaCl2-MgCl2-KCl salt solution and under mild conditions (95℃and atmospheric pressure). It is feasible to control the dehydration rate and the morphology ofα-HH crystals with the adjustment of the solution's pH and addition of citric acid or succinic acid.
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