Lime Treatment of Acid Mine Water and Associated Solids/Liquid Separation

Abstract

Mine waters become contaminated to a varying degree with the oxidation products of pyritic materials associated with the minerals. Waters containing from 6 000 to 13 000 mg/L of sulphates and up to almost 5 000 mg/L of iron are reported. For practical or economical reasons it is not yet possible to prevent this contamination of mine water. Prevention of pollution of surface water in South Africa is mostly confined to the neutralisation of acid mine water with lime. The neutralisation reaction results in the precipitation of a voluminous sludge containing mostly iron hydroxide and calcium sulphate, when the latter is formed in excess of its solubility in water. The sludge usually settles to volumes equal to 10 percent of the volume of the water treated, but volumes as high as 50 per cent have been found. Through application of the “High Density Sludge” process, which comprises sludge recirculation and conditioning with lime in addition to conventional neutralisation and sedimentation, the volume of these sludges has been reduced to 0,4-2,0 per cent of the volume of water treated. The seeding of calcium sulphate resulting in a reduction of scale build-up on plant equipment seems to be an advantage of the process. Dissolved air flotation was found an attractive alternative means of solids/liquid separation to gravity settling for water producing relatively low amounts of sludge (100 to 200 mg solids per litre). At hydraulic loadings of 7 m3/m2.h (seven times higher than gravity settling) the flotation process produced an effluent containing less than 15 mg/L suspended matter and a sludge with a density of 50 g/L.