Salt production from coal-mine brine in NF - evaporation - crystallization system

At "Debiensko" Desalination Plant salt crystallizer is supplied with untreated brine concentrated by RCC evaporation method that results in high energy consumption and huge amount of post-crystallization lye. Nanofiltration pre-treatment of coal-mine brine was then considered to improve plant outcome. Desal-5L (Osmonics) nanofiltration membrane was found to be suitable for the above purpose. The rejection coefficients were found as follows (%): Ca²⁺ - 68.8, Mg²⁺ - 71.6, SO4²- - 98.0, Cl^- - 11.9, that lets us to estimate the composition of NF permeate at 85% recovery to be as follows (g/L): Ca²⁺ - 0.242, Mg²⁺ - 0.188, SO4^2- - 0.064, Cl^- - 32.2. The performance of crystallization step was then estimated based on chemical composition of NF permeate and compared with operational data from "Debiensko" Desalination Plant. The energetic efficiency of the presently operated coal mine brine treatment system was compared with the one involving NF softening as the pretreatment for low energy VC thermal desalination in place of seeded VC (RCC presently applied). This comparison shows that unit energy consumption decreases from ca 970 kWh per 1 t of evaporated salt for untreated brine concentrated by RCC method to ca 450 kWh/t when low energy VC evaporation is applied. At the same time the amount of salt in post-crystallization lye decreases from approx. 110 kg per 1 t of evaporated salt produced to ca 50 kg.