Sustainable Treatment of Acid Mine Drainage

Omya Group is a leading global producer of industrial minerals. Sustainability is the key to future success on our journey to achieving our objectives. In the course of the “Total Product Life Cycle” thinking we strive to optimize resource efficiency.

Omya strives to extract high purity reserves with a minimum of impurities to optimize resource use. The right choice of raw materials is essential for efficient energy optimized production. The prevention and reduction of waste throughout the entire production process is the primary focus. Due to the nature of the extracted stone the lower brightness parts of our quarries may not work for applications in paints or paper. We are constantly seeking for applications for these materials. We believe that the success of our products will increasingly depend upon minimizing our footprint in the future.

We identified the Acid Mine Drainage (AMD) in the metal and coal mining industry as an area of potential synergies. AMD is a well-known phenomenon which refers to the outflow of acidic water from (usually abandoned) metal or coal mines. If the rock contains sulphides a natural oxidation process can acidify the water.

Gold mine in South Africa – case study
Gold occurs in association with pyrite which produces sulphuric acid when exposed to water and oxygen. The acid dissolves the surrounding or in flooded parts of the mine, slime dumps and tailings releasing the heavy metals within. The ferric hydroxide formed in this reaction is also called “yellow boy”, a yellowish orange precipitate that covers streams or river beds with a slimy sludge. By adding calcium carbonate to AMD heavy elements can be precipitated and acid can be neutralized.

The process flow is basically very simple and results in clear cleaner water and a sludge which could be sent to or put in a disposal site. The calcium carbonate could be either regular products (for instance based on limestone) or the lower brightness material which could contribute to the “circular economy” concept.

Acid effluents from the surface treatment industry
An even more advanced technology could so far be demonstrated using the case study of Printed Circuit Board (PCB) which releases a volume of waste effluent of about 2.3 million m3 per year in Europe. The current treatment of the PCB acid waste is neutralization of the acids and precipitation of the heavy metals by using a lot of chemicals (precipitation agents, complexing agents and flocculants). The solids removal has to happen within one single sludge which has to be dewatered and disposed according to local regulatory rules. The chemicals contribute to 25% and the sludge disposal to up to 75% of the OPEX costs!

The alternative treatment process is a selective precipitation of valuable heavy metals or substances present in the acid waste (e.g. copper sludge, white gypsum). The precipitation is a continuous process with stepwise neutralization of the acids by adding especially developed calcium carbonate product and initiates a selective precipitation of added value substances.

Mr. Ekhard Beuleke
Omya International AG