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 0.06M) and to dye bath effluent (sulphate concentration of 0.22M) 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, 0.06M initial sulphate concentration was reduced down to 60mg/L for synthetically prepared samples and 325mg/L for real wastewater. For the concentrated samples of 0.22M initial sulphate, remaining sulphate levels varying between 230 and 280mg/L were obtained for both synthetic and real wastewater samples.