Sulphate control by ettringite precipitation in textile industry wastewaters.

In the present study, ettringite precipitation was theoretically and experimentally evaluated as a means of sulphate removal. The results showed that ettringite precipitation is an effective and reliable method for sulphate removal. Synthetically prepared samples which were simulated to total wastewaters originating from the textile industry (sulphate concentration of 0.06 M) and to dye bath effluent (sulphate concentration of 0.22 M) were subjected to ettringite precipitation using the systems with Na2SO4-AlCl3-Ca(OH)2-NaOH, Na2SO4-AlCl3-Ca(OH)2, and Na2CO3-Na2SO4-AlCl3-Na(OH)2. An equilibrium model involving precipitation more than one solid phase and with ionic strength correction was used to predict the sulphate removal efficiency as well as solution composition. The optimum pH for ettringite precipitation in all systems was found to be around 12.0. By the application of the method, 0.06 M initial sulphate concentration was reduced down to 60 mg/L for synthetically prepared samples and 325 mg/L for real wastewater. For the concentrated samples of 0.22 M initial sulphate, remaining sulphate levels varying between 230 and 280 mg/L were obtained for both synthetic and real wastewater samples.