30 april 2015
Lowers sulfate levels in mine water effluent below 100 mg/l
A new technology solution to the challenge of removing sulfate from mine effluent by combining chemical precipitation and nanofiltration is drawing attention on four
continents. Recently completed pilot projects in North and South America have generated interest from mining companies in search of cost-effective breakthroughs. In Europe, nanofiltration at iron mines delivered cost, performance and maintenance benefits that outperformed the original design. In Africa, the technology’s water recovery capability has created a stir in drought-stricken countries.
The proprietary process, developed by Veolia Water Technologies, responds to increasingly stringent regulations limiting sulfates in surface water discharges as well as increased
mining industry interest in the reuse of mine water for beneficial purposes. The improved method lowers sulfate levels in mine water effluent below 100 mg/l, generating a clean water effluent for reuse or discharge.
Recovery: affordability key
The first step in the process uses Veolia’s Multiflo™
system equipped with its Turbomix™ reactor to reduce sulfate to less than 1,800 mg/l. The first-stage effluent is then treated with calcium and an aluminumbased salt in a second system. This secondstage process precipitates sulfate as a highly insoluble calcium sulfoaluminate mineral known as Ettringite. This step reduces the dissolved sulfate in the effluent to less than 100 mg/l. The excess sludge from the system is pumped to a third smaller system, which serves as a chemical regeneration tank. This step recovers more than 95% of the aluminum-based salt from the precipitated sludge for reuse in the treatment process.
This is key to cost-effectiveness. Reusing the recovered aluminum in the treatment process instead of fresh aluminum avoids a six-fold increase in chemical costs.
The technology’s performance is turning heads, both for its efficiency and its speed. At a major copper mine in South America, water from a mine tailings pond with average sulfate concentrations up to 2,900 mg/l were reduced to less than 50 - 60 mg/l, after blending the treated membrane reject with the permeate. In North America, water from
a coal mine with levels as high as 1,800 mg/l were reduced below 100 mg/l within 25 minutes. The recent developments have been built on experience with nanofiltration at subsurface iron mines in eastern France where high-quality drinking water was produced from treating high sulfate concentrations formed in groundwater following the mines’ closure.
In Africa, particular interest has been expressed in the technology’s ability to increase water recovery from 60% to as high as 98%, depending on the original water quality. In addition to the positive response at the North and South American pilot sites, other companies have expressed strong interest in testing the technology in coming months.