Beneficiation of wastewater streams from gold mine process water systems with recovery of value-adding liquid waste products

Abstract

A strategy for beneficiation of wastewater streams from fissure and process water developed for a gold mine operation in the west of Johannesburg was tested for viability in a pilot study. The investigation was aimed at evaluating the compliance of the finally discharged effluent streams with the current Water Use Licence (WUL). The core of the water recovery process consisted of softening to remove divalent cationic species, followed by ion exchange processes employing Strong Acid Cationic (SAC) resins and Weak Acid Cationic (WAC) resins. An operational design limitation was that the crystalactor used in the softening stage had a minimum capacity of 20 000 L/h, whereas the rest of the system could be operated at flow rates of as low as 2 000 L/h. For this reason, the softening step was done in semi-batch mode. Calcium hardness was decreased from 70 mg/L to values lower than 40 mg/L (as Ca2+). During the ion exchange (water recovery) process, columns using SAC resin produced better quality water than the WAC resins. The SAC columns produced water compatible with South African Water Quality Standards. Additionally, the use of SAC proved to be a more financially favourable option, since the regenerant stream contained high concentrations of calcium nitrate, magnesium nitrate and sodium nitrate fertilizer. The latter could be sold as a liquid fertilizer to farmers. In addition to the above findings, the pilot system reduced the concentration of toxic and radiotoxic metals such as uranium. The final concentration of the uranium in the effluent (0.01 mg/L) was below the regulation limit 0.07 mg/L. The selective removal of uranium is crucial in order to produce high-quality fertilizer from the ion exchange regeneration streams. Copyright