Ahead of the water management article in the March issue of International Mining, one of the many interesting papers at this week’s SME Annual Meeting was presented by J. A. Lombardi of Jalema Tech. He notes that the use of membranes for the treatment of acid mine drainage (AMD) water has been a 35 year mining industry quest that has only recently produced commercial scale Zero-Liquid-Discharge (ZLD) plants. The process of development of the membrane treatment of AMD has been defined by efforts to control gypsum scaling of the membranes, an artifact of the concentration of calcium and sulphate ions on the concentrate or “brine” side of the membranes. Improved anti-scalants, changes to membrane surface geometry and a lowered permeate flux rate were all part of solving the calcium sulphate (gypsum) scale formation problem.

Whether used singly or as part of a multi-stage membrane treatment process, the production of “left over” membrane concentrate or brine, in any volume large or small, was a second barrier to adoption of the method by the mining industry. The two identified “final brine” treatments were a) solar evaporation ponds and b) mechanical evaporators. Brine disposal by solar means is typically site limited to flat terrains or high net evaporation locations. Mechanical evaporation-crystallization is applicable at all sites but, typically, at onerously high ownership and operating costs.

Miwatek, a South African water treatment start-up, has entered the mining AMD ZLD marketplace with two new, pilot-scale tested, processes, both of which are predicated on the innovative treatment of a reasonably concentrated AMD and neither of which produces a final brine that requires disposal.

In the Miwatek Brine Treatment, MBT, method the membrane brine is caustic pH adjusted and de-calcified precedent to passage through a second membrane plant. The caustic is generated on-site by a lime treatment of the second-membrane concentrate. The first- and second-stage permeates are blended for discharge. The process usually yields no “final brine” for disposal.

In the Miwatek Ettringite Treatment (MET) method a low pH first-stage membrane brine is lime treated in two or three steps in the presence of aluminium in a way that metal hydroxide and calcium sulphate solids are formed and purged from the system. Sufficient aluminium is often present in the feed water to, with judicious recycling, enable operation of the lime-aluminium treatment without the purchase of outside aluminium reagent. The first-stage membrane permeate is blended with the lime-aluminium treated membrane concentrate for discharge. The indications are that the method can comfortably meet the low 250 -500 ppm SO4 discharge water quality standards that have most recently been proposed in South America. The process yields no “final brine” that requires disposal.

Projected own-operate costs for the MBT and MET systems are less than the estimated costs for all identified alternative treatments.