Selected Commercially Developed Projects for Recovery of Water from Acid Mine Drainage
This section discusses a few selected commercially developed projects that are either in operation, being piloted or under evaluation. These processes were also discussed by Simate and Ndlovu (2014) in detail.
The Council for Scientific and Industrial Research (CSIR) of South developed the CSIR ABC (alkali-barium-calcium) process. The CSIR ABC desalination process, developed for AMD neutralisation and the removal of total dissolved solids from 2 600 to 360 mg/L, was demonstrated at a pilot plant in 2010. This precipitation process developed by CSIR uses barium carbonate to
Process flow diagram for the CSIR-ABC process. (From Maree et al., 2012; Simate and Ndlovu, 2014.)
precipitate dissolved sulphate from AMD. It consists of the following three stages: pre-treatment, treatment with barium carbonate, and the waste processing stage as shown in Figure 9.5 (de Beer at al., 2010; Maree et al., 2012). In the pre-treatment stage the feed water is treated with CaS, Ca(HS)2, or Ca(OH)2 to remove free acid and metals. Ideally, metals are precipitated to low values as either hydroxides or sulphides, depending on the precipitation agent used. During this stage, the sulphate content is lowered from about 4500 mg/L to 1250 mg/L (de Beer et al., 2010; Maree et al., 2012). In the water treatment stage, BaCO, is added into the water thus producing barium sulphate as the solid waste and clean water. The alkalinity of the calcium bicarbonate-rich water was reduced from 1000 to 110 mg/L (as CaC03). The water treatment stage is integrated with a sludge processing stage to recover the alkali, barium, and calcium (ABC) from the sludge through reduction in a coal-fired kiln. In this process, good quality water containing less than 100 mg/L of sulphate was obtained in a cost-effective way from polluted mine water (de Beer et al., 2010; Maree et al., 2012). This process is a strong candidate for cost-effective treatment of mine water. However, the major limitation of this technology is the amount of sludge produced which is expensive to dispose. High capital and operating costs associated with the thermal reduction of waste to produce CaS, gypsum, and other solids for disposal also make the technology less cost-effective.
THIOPAQ process developed by PAQUES company is a biotechnological process which uses two distinct microbiological populations and stages (Boonstra et al., 1999): (1) conversion of sulphate to sulphide by using hydrogen gas (from the conversion of ethanol/butanol to acetate and hydrogen) as the electron donor and precipitation of metal sulphides, and (2) conversion of any excess hydrogen sulphide produced to elemental sulphur, using sulphide-oxidizing bacteria. In this way sulphate is removed from AMD to produce water of reusable quality. This process has lost attractiveness over the years due to an increase in the price of ethanol and butanol which are energy sources for the process (Boonstra et al., 1999).
SAVMIN process flow diagram. (From Smith, 1999; Sibiliski, 2001; INAP, 2003; Simate and Ndlovu, 2014.)
The SAVMIM process was developed by the Council for Mineral Technology (Mintek) of South to treat polluted mine water (Smith, 1999; Sibiliski, 2001; Naidoo, 2018). The process was patented by Mintek in 1998 (Naidoo,
2018). Figure 9.6 shows the process flow diagram, and the main process stages are as also discussed below (Smith, 1999; Sibiliski, 2001; INAP, 2003; Simate and Ndlovu, 2014).
Stage 1 - Metal precipitation: Using lime, the pH of the feed water is raised to between 12.0 and 12.3 to precipitate metals (trace) and magnesium.
Stage 2 - Gypsum “de-supersaturation": Using gypsum seed crystals, gypsum is precipitated from the supersaturated solution and removed.
Stage 3 - Ettringite precipitation: Using aluminium hydroxide, dissolved calcium and sulphate are removed from the solution by the precipitation of ettringite (a calcium-aluminium sulphate mineral).
Stages 4 and 5 - Recycling of aluminium hydroxide: Using sulphuric acid, the ettringite slurry from stage 3 is decomposed at pH 6.5 in a solution supersaturated with gypsum (no precipitation). The resulting aluminium hydroxide is recycled to the third stage and the solution that is supersaturated with gypsum is contacted with seed crystals (stage 2) to precipitate and remove gypsum. The remaining solution saturated with gypsum is recycled.
Stage 6 - Carbonation and calcite precipitation: Using carbon dioxide, the pH of the solution from the third process stage (pH 11.2-12.4) is lowered to precipitate and remove calcite.
The end products of the SAVMIN process are potable water and a number of potentially saleable by-products (metal hydroxides, gypsum, and calcite). One of the major advantages of SAVMIN process is that high quality products can be obtained (Smith, 1999). A major disadvantage of this process is the vast amount of sludge produced which is expensive to dispose (Smith, 1999; INAP, 2003).
A process developed by Aveng Water called HiPRO (high-pressure reverse osmosis) process is capable of consistently achieving greater than 97% water recovery (Aveng Water, 2009). The final products from this process are portable water, a liquid brine solution (less than 3% of the total feed), and solid waste. The solid waste products are saleable grades of calcium sulphate and less pure calcium sulphate and metal sulphates (Aveng Water, 2009). The main disadvantages of this process are that it produces waste brine and sludge which are expensive to dispose.