Proposed Integrated Processes and Technologies for Recovery of Water from Acid Mine Drainage
In a study by Simate and Ndlovu (2014) it was suggested that the best way to working towards a sustainable solution of the AMD challenge is to take a business approach and consider the integration of existing technologies as well as technologies under development so as to come up with a solution that has the potential to address the problem in a holistic and sustainable manner. In other words, coupling different processes together as two- or three-stage processes would be more appropriate. Such measures would possibly include the integration of both the active and passive water treatment systems. In fact, previous studies have shown that combinations of physico-chemical treatments may be able to partially or completely remove some organic and inorganic contaminants (Dobias, 1993; Harrelkas et al., 2009). The first proposed option is to couple the AMD fuel cell with the cyclic electrowinning/ precipitation (CEP) process. The first part of the process consists of the fuel cell where ferrous iron is completely removed through oxidation to insoluble Fe(III), forming a precipitate in the bottom of the anode chamber and on the anode electrode (Cheng et al., 2007). The iron contained in the precipitate or sludge could be marketed as a pigment for paint (Hedin, 2003), cosmetics, and possibly other uses. The electricity produced could be used to supplement the power in the electrowinning stage. The second part of the integrated process would consist of the CEP process. A pH swing (using NaOH or H2S04) would be applied to the water coming from the fuel cell so as to precipitate the heavy metals such as cadmium, nickel, copper, etc. The precipitation and redissolution of metals would be repeated until the concentration of the heavy metals (<100 ppm) has reached a point where electrowinning can be efficiently done (Brown University, 2011). In electrowinning stage, heavy metal ions are converted using electric current to stable metal ions which can be recovered and separated from water. However, the metal barren solution would still contain high amount of sulphate ions. The solution could be concentrated and then reacted with sodium monochromate (from roasting and leaching of chromite ore) to form a mixture of sodium dichromate and sodium sulphate. This mixture can then be recovered and separated from the water. Depending on the purity, the product can be marketed as a fertiliser or as a metal finish and other uses. The proposed integrated process flow diagram is shown in Figure 9.7.
The second proposed integrated process by Simate and Ndlovu (2014) is shown in Figure 9.8. In this process the AMD is fed to the first part of the
First proposed integrated process for the recovery of water from acid mine drainage. (From Simate and Ndlovu, 2014.)
Second proposed integrated process for the recovery of water from acid mine drainage. (From Simate and Ndlovu, 2014.)
process in which the CH-collector (e.g., amino bisphosphonate adsorbent) adsorbs some of the heavy metals directly from the wastewater. The resulting solution is then passed through a vacuum evaporator where the water component is vaporised thus producing re-usable water while sulphuric acid remains in solution. A vacuum evaporator is used because it has the advantage of producing a large separation factor in the sulphuric acid/water system (Nleya et al., 2016). The remaining solution from the vacuum evaporator that contains dilute sulphuric acid and some remaining heavy metals such as ferrous iron is fed to a fuel cell that produces electricity, iron oxide and metal-barren dilute sulphuric acid.