Conserve and recycle

April 16, 2010
by David Kratochvil

Tanks in the Breckenridge plant treat metal-contaminated water emanating from a closed mine site.

Photo: BioteQ

Sustainability has become the watchword as plants seek efficient and environmentally friendly technologies to address day-to-day operations. One area of focus is wastewater treatment.

Water conservation and recycling is a major concern for any industry requiring large volumes of water for its process streams. Contaminants, such as metals, minerals and the presence of sulphates in wastewater streams are under legislative scrutiny in many countries.

Much of the wastewater from industrial processes is too contaminated for reuse or discharge into the environment, and therefore must be stored and treated at considerable expense. For example, power plant wastewater may be directed to zero liquid discharge systems or evaporators (solar ponds and/or evaporator crystallizer systems). Mining operations have relied extensively on lime treatment for acid mine drainage, a process that creates a residual sludge that must be stored in perpetuity.

The presence of metal contaminants and/or sulphates in wastewater creates a significant long-term environmental and economic liability. Add to that increased restrictions on water quality and discharge combined with diminishing access rights to fresh water resources, and it comes as no surprise that many operations need to consider alternative ways to reduce consumption and maximize reuse of existing resources.

But the sustainability issue extends beyond the environmental realm to include economic and energy considerations. Conventional treatment systems such as reverse osmosis effectively remove contaminants, yet they consume high amounts of energy, are costly to maintain and barely recapture half of the water used. While they continue to play a key role in water treatment, ultra-filtration systems have their limitations.

There are new technologies for safely removing metal and mineral contaminants without producing residual waste sludge. For example, sulphide precipitation effectively removes metal contaminants, while recovering up to 95% of water for reuse or safe discharge to the environment. And by-products (such as high-grade metal concentrates) can be extracted and sold to offset the cost of water treatment, thus converting a waste stream from a cost centre to a revenue generator while eliminating long term environmental liabilities.

Sulphide precipitation is already used successfully by mining industry water treatment plants to remove contaminants from wastewater. In this process, sulphide is mixed with the water under controlled conditions to selectively precipitate metals as a high-grade metal sulphide. The precipitated metals and treated water are then pumped to a clarifier tank where the clean water is separated from the metal solids and either discharged to the local environment or recycled. At that point, the metal solids are filtered to remove excess water, producing a high-grade metal sulphide product that’s suitable for refining.