The prevention of scaling by decreasing the concentration of scale-forming ions in the vacuum evaporation salt-making process

Abstract

Carbonate ions (CO32–) play a vital role in CaCO3 scaling. The main goal of the present research was to study the inhibition of CaCO3 scaling during the vacuum evaporation salt-making process by regulating the doses of NaOH and Na2CO3 to promote the utilization efficiency and reaction activity of CO32–, and then to decrease the concentration of scale-forming ions, especially CO32–, in the refined brine obtained from purification via the NaOH-Na2CO3 method. The results demonstrate that the employed method not only achieved the improved efficiency of calcium (Ca2+) and magnesium (Mg2+) ion removal, but also effectively decreased the concentration of CO32– in the refined brine. The total concentration of Ca2+ and Mg2+ was decreased from 486.1 to 12.3 mg/L, and the concentration of residual CO32– during the brine purification process was 128 mg/L at the optimal doses of NaOH = 0.25 g/L and Na2CO3 = 1.3 g/L. The optimal parameters of the NaOH–Na2CO3 method were used in industrial production experiments and operated for a production period. The degree of accumulation of CO32– in the circulating feed brine was decreased, and the deposition ratio of CaCO3 was inhibited effectively. Additionally, the heat-transfer efficiency of the vacuum evaporation system was improved, and the energy consumption level was decreased. Moreover, large NaCl crystals with a narrow particle size distribution were prepared, thereby improving the anticaking property of the crystals. The scanning electron microscopy (SEM) results demonstrate that the surfaces of calcite CaCO3 were adsorbed by many spherical Mg(OH)2 particles, and the growth rate of calcite was retarded. Furthermore, the loose structure of the scale layer is beneficial to later cleaning treatment.