Abstracts of the International Mine Water Conference 19 th – 23 rd October 2009 Proceedings ISBN Number: 978-0-9802623-5-3 Pretoria, South Africa Produced by: Document Transformation Technologies cc Conference organised by: Cilla Taylor Conferences TREATMENT OF ACID MINE DRAINAGE USING MAGNESIUM HYDROXIDE V. BOLOGO, J.P. MAREE and C.M. ZVINOWANDA Department of Environmental, Water and Earth Sciences, Tshwane University of Technology, Private Bag X680, Pretoria, 0001, South Africa; e-mail: mareej@tut.ac.za ABSTRACT Acid drainage from mining activities is of major environmental concern in South Africa. These effluents require neutralisation and metal removal prior to release to public watercourses. A novel process is proposed whereby magnesium hydroxide is being used for neutralization of free acid and subsequent raising of the mine wastewater pH to above 7 to facilitate rapid iron (II) oxidation and precipitation as ferric hydroxide. This is followed by lime treatment for removal of magnesium as Mg(OH) 2 . By using magnesium hydroxide instead of Ca(OH) 2 or CaCO 3 , gypsum precipitation is avoided and metal hydroxides can be precipitated separately from gypsum. Magnesium hydroxide can be recovered from the magnesium hydroxide /gypsum mixture through dissolution of magnesium hydroxide as magnesium bicarbonate, with carbon dioxide. This study showed that magnesium hydroxide can be used for treatment of acid mine drainage rich in iron (II)-and magnesium hydroxide recovered from the sludge. Pilot-plant studies are in the planning stage to demonstrate the suitability of the magnesium process for full-scale application in the treatment of acid mine drainage. 1. INTRODUCTION Acid mine drainage, AMD, from mining activities has become a major environmental concern. It has become a threat not only to the environment but also to human health. AMD is characterized by low pH (2-3), high salinity levels, containing a broad range of heavy metals ions and high concentrations of sulphate, iron, aluminium and manganese. Iron disulphide (FeS 2 ), commonly known as pyrite, is a major constituent of the strata being mined and large rock surfaces become exposed to air and water during mining activities (Sasowsky et al., 2000). Pyrite is oxidized to soluble iron complexes and sulphuric acid, catalysed by sulphur oxidizing bacteria (Sawyer et a.l, 1994). 2FeS 2 (s) + 7O 2 (aq) + 2H 2 O ﾆ 2Fe 2+ + 4SO 4 2- (aq) + 4H + (aq) (1) Traditionally, lime has been used for neutralization of acid mine water (Herrera et al., 2007; Sibrell et al.,2005, Watten et al., 2005; Feng et al., 2000; Semerjian et al., 2003). Several new processes have been developed, based on the use of precipitated calcium carbonate or lime pre-treatment for neutralization of acid mine drainage and partial desalination (Maree et al., 1992; Maree et al., 1994; Maree et al., 1996; Maree et al., 1998; Maree et al., 2004; Maree et al., 1994). The Council for Scientific and Industrial Researchers (CSIR) in South Africa has developed the fluidised-bed limestone neutralization process for the treatment of acid mine water (Maree & Clayton; 1991). Their studies showed that complete neutralization of discard leachate, containing 10 000 mg/L acid as CaCO 3 and 4 000mg/L Fe(II), can be achieved in a limestone neutralization, fluidized-bed reactor, provided that Fe(II) is oxidized beforehand. The process involves the neutralization of Fe(II)-rich water with lime to obtain iron(II) oxidation. This process can also speed up the settling process of ferric sludge (Semerjian et al., 2003). The integrated limestone and Fe(II)-oxidation process was developed, which allows the oxidation of Fe(II) when limestone alone was used for neutralization (Maree, 1997). In this process powdered limestone was used for Fe(II)- oxidation at pH 5.5, neutralization of free acid, metal precipitation (e.g. Fe 3+ and Al 3+ ) and gypsum crystallization, all in the same reactor. The novelty of this development lies in the fact that conditions were identified where Fe(II) can be oxidized at pH 5.5, by the addition of limestone. Previously, lime was used to raise the pH to 7.2 where the rate of Fe(II)-oxidation is rapid. A handling and dosing system was developed for using waste limestone containing 25% moisture, from the paper industry which (Maree, 2000) The integrated limestone and lime process was developed for the treatment of acid and sulphate-rich effluents (Maree, 2003). The process consists of various stages. The bulk of the acid content is neutralized in first stage with limestone. CO 2 is produced and stripped off through aeration and transported to the third stage. In the second stage the water is treated with lime to allow precipitation of magnesium and other metals and the sulphate associated with these metals. The level to which sulphate is removed via gypsum crystallization is controlled by the solubility product of gypsum. In the third stage, the CO 2 that is produced in the first stage, is contacted with the high pH of the water from the second stage to adjust the pH to 8.3. This affords CaCO 3 precipitation. Due to its high purity, this CaCO 3 can be sold as a by- product or be recycled to the first stage, to supplement the limestone addition. This process offers the following benefits: (i) The treated water is under-saturated with respect to gypsum and, (ii) if the feed water contains aluminium, 371