Removal of sulfate ions from process water by ion exchange resins

Abstract

High concentrations of sulfate and thiosulfate ions in the process water may cause significant problems in the flotation of sulfide ores through changing the pulp chemistry, surface chemistry of sulfide minerals and froth stability. In addition to that formation of colloidal gypsum and scaling problems in process plants are observed when lime is used for pH regulation. Therefore, reducing sulfate and thiosulfate ion concentration in the process water is required for minimizing gypsum formation and stabilizing pulp chemistry. In this study, strong base ion exchange resins; Amberlite IRA-400 and Dowex 1X2 were used to reduce the amount of sulfate and thiosulfate in the process water.Adsorption and regeneration behavior of resins were determined by bench scale laboratory experiments using synthetic water simulating the process water of a zinc flotation plant from an Iberian Mine (Portugal). In the second phase of the study, the resins were tested using the process water in the mine site. Batch scale flotation tests were conducted by using the process water treated with Amberlite IRA-400 to determine the effects of water treatment on flotation efficiency of the Zinc ore.Amberlite IRA-400 has higher degree of cross-linkage in its structure and higher adsorption capacity. More than 90% of the sulfate could be removed from the water in the first 5 min of the adsorption test. The resin has high adsorption kinetics and can remove majority of the anions by adjusting the required quantity of resin according to the concentration of dissolved ions in the process water. The resins were regenerated effectively by using 4% w/v NaOH solution as a regenerant and used multiple times for adsorption. The same regeneration solution could be used with stage addition of NaOH to achieve the balance between hydroxide ions and adsorbed ions.Effects of resin treated water on flotation efficiency and water chemistry profile in the grinding and flotation stages were investigated using an ore sample taken from the Zn flotation plant. The results show that concentration of all ions increased significantly after the grinding stage due to dissolution of the minerals. Water treatment by resins removed majority of sulfate and thiosulfate ions from the flotation water. Water treatment by ion exchange resins increased the pulp pH up to 12, due to the ion exchange reaction between the hydroxide ions and anions. However, high pH did not significantly affect Zn flotation efficiency.