Gaseous carbon dioxide (CO2), provided by gas products and services company Afrox, has been successfully integrated into a water treatment system at coal miner Optimum Coal’s water reclamation plant near Hendrina, in Mpumalanga. The system treats acid mine drainage (AMD) during the remineralisation phase, using CO₂, with lime, to produce a valuable clean water supply for the local community.

Afrox gas applications engineer Gareth Jones tells Engineering News that the company was selected in 2011 by South African water treatment company Aveng Water to deliver its CO2 _{dosing system, called Solvocarb, to Optimum Coal for the treatment of desalinated mine-impacted water. The system removes all impurities to ensure that the water is potable.}

Using CO2 _{is part of a common method of remineralisation of desalinated water whereby lime and CO2 are added to reach certain water quality values necessary for hardness, alkalinity and final pH. Carbon dioxide is first dissolved in water to form carbonic acid. This slightly acidic solution reacts with a concentrated lime solution.}

The combination of lime and CO2 _{gas provides two essential ingredients of bicarbonate alkalinity and calcium hardness for the water to increase its ability to resist change in pH or buffering capacity. This makes the changes in pH levels more controlled when the water is exposed to pipework materials encountered in the distribution network, such as iron and concrete.}

A combination of milky lime and gaseous CO2 is being used at Optimum Coal’s water reclamation plant.

“Owing to the nature of reverse osmosis plants, the recovered water is stripped of the minerals required for domestic consumption,” explains Jones, adding that the soft water is unpalatable, chemically aggressive and will not form a lather when using soap or detergents.

This high-purity water also tends to be highly reactive and can cause severe corrosion within conventional pipelines, such as in the cement mortar lining of water pipes. This is mitigated with the introduction of bicarbonite alkalinity and calcium hardness, which stabilises the water.

Afrox’s parent company, Linde, has several CO2 _{remineralisation systems worldwide.}

Jones explains that Optimum Coal decided to treat its mine water with lime and CO2 _{as part of the overall treatment strategy to sell potable water to the Steve Tshwete local municipality.}

This has been a successful project, with the cost of CO2 _{forming a minimal percentage of the overall operating cost.}

The water at Optimum’s Hendrina colliery currently meets the Department of Water and Sanitation’s Blue Drop drinking standards. The bulk of this water is sold to the municipality, often acting as a supplementary resource during water shortages.

The rest of the water is released into the Klein Olifants river and other surrounding streams, which helps recharge and sustain the aquatic environment.

In addition, pure gypsum sludge, which is used as a building material, is also made available at this plant as part of the purification process stages.

Aveng was selected to develop the 15-million-litre-a-day reverse osmosis plant, which was completed in August 2010, to treat and purify the mine-affected water. It currently recovers more than 95% of the water pumped from the mine.