When the sulfate ion content is sufficiently high to warrant recovery for reuse or sale, a crystallization approach should be evaluated. Water can be removed by evaporation and the sodium sulfate crystallized out (Cosgrove) at about 5°C as Na2SO4 • 10 H2O (Glauber's salt). The crystals can then be removed by filtration. Metal impurities can be removed by neutralizing with sodium hydroxide solution and refiltering, evaporating, crystallizing, and drying. Depending on the relative concentrations of metal ions and sulfates, an alternative approach first removes heavy metals by ion exchange.
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