Mintek technology successfully used in water-purification industry

Mintek has commissioned a Minfurn carbon-regeneration furnace at the Rietvlei water treatment plant near Tshwane in South Africa’s Gauteng Province.

The furnace, which has a capacity of 25 kilograms of dry regenerated product per hour, restores the efficiency of the granular activated carbon (GAC) that the plant uses to adsorb dissolved organic matter before the final chlorination step in the purification process.

This is the first adaptation of the Minfurn’s unique direct resistive heating technology to an area outside the gold industry.

The Minfurn was originally designed and developed by Mintek specifically for the regeneration of GAC used to adsorb gold from solution in the carbon in pulp (CIP) and carbon in solution (CIS) recovery processes. The furnace employs a technique of direct resistive heating by means of an electrical current passed directly through the carbon bed. This results in a high degree of regeneration efficiency, with significant advantages over other regeneration technologies including low consumption of electricity, ease of operation, minimal moving parts, and low maintenance.

About 30 Minfurns have been installed in industry, mainly at small and medium-sized gold mines in Latin America. Recently, further opportunities were identified for the application of the technology in two new industry sectors – potable water treatment and food processing.

Since GAC has an affinity for organic substances and repels water, it represents and effective way of separating traces of organic matter from water. “This is particularly important in the light of recent changes in raw water quality and supply in the southern African region,” said Hylton Gidish, Mintek’s product manager for the Minfurn. “Locally, acceptable final organics levels are 20 nanograms per litre. Previously, this level was relatively easily maintained by most water works in South Africa, but with recent deterioration in both quality and supply of raw water, increasing organic matter has begun to be a problem, particularly at the smaller plants.”

The Rietvlei plant, which produces 40 million litres of drinking water a day (about 6 per cent of Tshwane’s daily requirements), was the first plant in South Africa to apply a GAC filtration system for drinking water treatment. The project, which was completed in 1999, was awarded the SA Association of Consulting Engineers national award for technical excellence in 2000. The plant was recently singled out as the best medium-sized water purification facility in South Africa.

The Minfurn installation at Rietvlei presents an ideal opportunity for commercial demonstration of the technology in the potable water treatment industry. Similarly, a food-processing company in the USA has successfully tested a Minfurn for regenerating spent GCA used in the removal of unwanted colour from their lactic acid product.

“The GAC used in the water treatment and food industries is typically finer-grained and softer than the material used for gold recovery,” explained Gidish. “The Minfurn is ideally suited for these applications, since it results in minimal abrasion of the carbon, and hence fewer losses of fine material.”

Development work is ongoing at Mintek to adapt the Minfurn and the operating procedure to GAC samples with different physical and electrical characteristics, as well as in unexplored areas of GAC application.