Recovery of NaCl from saline mine water in the ED-MSF system

A considerable part of water obtained by drainage of Polish coal-mines is saline which creates substantial ecological problems. The load of salt (mainly sodium chloride) amounts to 5 min t/year. Despite the utilisation of saline coalmine waters is considered to be the most adequate method of solving ecological problems caused by this kind of water in Poland there are only two installations utilising coal-mine waters and producing 100,000 t salt per year. In the case of the most concentrated waters, the so-called coal-mine brines, the method of concentrating by evaporation in twelve-stage expansion installation or vapour compression is applied, after which sodium chloride is manufactured. In the case of low salinity waters they are preconcentrated first by RO method. High energy consumption in above-mentioned methods of evaporation is a considerable restriction in the utilisation of coal-mine brines. An obstacle in the application of low energy evaporation processes, e.g. multi-stage flash, is the high concentration of calcium and sulphate ions in the coal-mine waters. A method of pre-treatment of coal-mine saline waters is proposed consisting in reducing the concentration of calcium or sulphate ions. The product of the concentrations of Ca2+ and SO42- ions, expressed in kmol/m(3), in saline water being evaporated should not exceed (C-1/C-2)(2) . 1.45 . 10(-3) (where: c(1) - concentration of salt (NaCl) in saline water before evaporation, c(2) - concentration of salt in concentrated brine). Electrodialysis with membranes of low transport number for divalent ions is proposed for low salinity waters and nanofiltration for so-called coal-mine brines. Saline water treated in this way may be evaporated without any risk of sulphate crystallisation. Low energy evaporation process may be then applied, resulting in a reduction of the unit costs of the concentration process. In the case of ED the efficiency of separation of Ca2+ and SO42- ions from NaCl has been tested. The cascade of electrodialysers and the concentration of the ED concentrate have been optimised, the latter being the input solution to the MSF process, in order to minimise the costs of a comprehensive utilisation.