Process optimization of brine purification and evaporation for combined crystallization of NaCl and Na_(2)SO_(4) by means of mechanical vapour recompression

摘要

Raw brines from solution mining process contain not only dissolved Na~(+) and Cl~(-) but also Ca~(2+), SO_(4)~(2-) and other ions like K~(+) and Br~(-). There are three different types of raw brine: 1) low concentration of SO_(4)~(2-) and other ions (Colombia, Thailand), 2) medium concentration of SO_(4)~(2-) and high concentration of other ions (Europe), 3) high concentration of SO_(4)~(2-) (China). Brines with low SO_(4)~(2-) concentration are normally treated with NaOH / CaO and Na_(2)CO_(3)/CO_(2) to precipitate SO_(4)~(2-) and Ca~(2+). Raw brines from Chinese brine wells contain higher concentrations of SO_(4)~(2-) and make the process of precipitation of SO_(4)~(2-) expensive. Therefore the combined process is used to crystallize not only NaCI but also Na2SO4 in the crystallization plant. In the past all Chinese plants which are based on the combined process used multiple effect evaporation process with additional Na_(2)SO_(4) loop. SEP Salt and Evaporation Plants developed a new process which uses mechanical vapour recompression as main process step for NaCI crystallization. The optimal configuration of the process was found after cost comparison for all process steps. 1) brine purification, 2) crystallization of NaCI by means of mechanical vapour recompression, 3) crystallization of NaCI by means of multiple effect cooling of mother liquor and additional loop for crystallization of Na_(2)SO_(4). Based on the composition of the raw brine different options of mother liquor recirculation from crystallization to brine purification were investigated. Recirculation of mother liquor on one hand leads to savings in consumption of chemicals in the brine purification process. On the other hand the concentration of SO_(4)~(2-) in the purified brine is increased. Subsequently the brine can be concentrated less in the mechanical vapour recompression part of the crystallization plant. Less electrical power but more steam has to be used to crystallize NaCI which has an impact on the utility costs in the crystallization plant. Depending on the composition of the raw brine and the cost of the utilities and chemicals the optimal configurations of the different process steps vary for each specific salt manufacturer. Detailed process studies are required to determine the most economic solution.