Alkaline by-products as amendments for remediation of historic mine sites

Sammanfattning: Mining has been and still is an important industry in Sweden, it has strongly contributed to the standard of living we have today. Extraction of ore began in the 12th century, but did not come about frequently until the 16th century, which is often seen as the starting age for ore processing and metal extraction. Leaching from sulphidic mine waste deposits is a serious environmental issue. When some of the sulphide minerals come into contact with oxygen they produce acid. Low pH in the waste then promotes leaching of primarily iron and cationic trace elements like copper, zinc and lead and hence affects the surrounding environment by bringing these metals in solution. Mine waste produced today is therefore immediately treated to prevent further oxidation and metal leaching. Cost-effective solutions for the historic mine waste are though still needed, especially as legal responsibilities for the historic waste are in many cases unclear. The historic mine waste is often highly weathered due to long-time exposure to oxygen and water, and also contains higher metal concentrations than modern waste. Also, consideration has to be taken to cultural and historical interests. Covering of the waste is commonly applied for acid rock drainage (ARD) control. By minimizing the oxygen supply, the waste is controlled from further oxidation. Covers can consist of layers of highly impermeable materials or water. Reactive materials (pH-increasing or reducing) can be used as covers or amendments. ARD can also be controlled and treated with reactive barriers and filters. Generally, when neutralizing materials are used for improvement of mine waste or ARD, pure materials such as lime is used. A lot of alkaline by-products in Sweden are deposited as waste and the increasing amount of wastes is also a main environmental problem. The use of alkaline by-products to amend acidic mine waste or ARD therefore both saves natural resources and reduces costs. In the present study, various alkaline by-products have been used in laboratory- and field-scale experiments, either as amendments to historic mine waste or for treatment of ARD. They include: lime mud, green liquor dregs, fly ash, steel slag, lime kiln dust and water works granules. Differences between the alkaline source (carbonate or hydroxide) were shown to be a crucial factor, especially in ARD treatment. Slow dissolution rates for carbonate materials in combination with high iron concentrations resulted in iron precipitation and coating of neutralizing carbonate surfaces. Hydroxide materials was hence found to be superior to carbonate materials in ARD applications and also as solid amendments to oxidized mine waste, in the case where the alkaline material was added as discrete layers. The latter enabled formation of so called hard pans, which work as flow rate reducers and allowing longer contact time with the neutralizing source. Though, when alkaline materials were homogeneously mixed with oxidized mine waste, carbonate materials were able to generate a higher pH and alkalinity, followed by lower trace metal concentrations. Key Words: Oxidized mine waste, lime mud, green liquor dregs, fly ash, lime kiln dust, steel slag, water works granules