The influence of sodium carboxymethylcellulose and sodium chloride on the precipitation of calcium sulfate from its supersaturation solutions

The crystallization of gypsum from supersaturated solutions in dependence on the molar [Ca2+] : [SO42−] ratio 0,26 : 0,15 and 0,35 : 0,15 in the presence of the sodium carboxymethylcellulose (CMC, with the degree of polym- erization of 1000 units) and NaCl was investigated at 25C. The efficiency of inhibition was estimated from measure- ments of change in Ca2+ concentration during the precipitation process. It was found that increasing the calcium ions content in solution compared to sulfate ions results in decrease in the induction period of crystallization. At a polymer concentration of 1,0 g/L and sodium chloride concentration of 0,7 mol/l the induction period of gypsum crystallisation in a supersaturation of the calcium sulfate solution with the molar rate of [Ca2+] : [SO42−] of 0,35 : 0,15 increases from several minutes to one hour. Further increasing the sodium chloride concentration up to 2,0mol/l did not result in change in the induction period of crystallization. This enables the preparation of supersaturated calcium chloride solu- tions using brines for the sealing of porous rocks, cracks, and joints of soil formations.The results of the phase analysis confirmed that salt precipitates in the form of gypsum.