A one-step ambient temperature ferrite process for treatment of acid mine drainage waters

Acid mine drainage (AMD) waters are low in pH, very high in dissolved iron, high in non-ferrous (mainly heavy) metals and very high in sulphate salinity. As such, AMD waters present a serious environmental problem in the mining regions of South Africa and elsewhere in the world. Treatment of AMD therefore requires pH neutralisation, removal of ferrous and non-ferrous metals and sulphate removal. This thesis addresses the problem of metals removal from AMD. The approach involves pH elevation and partial oxidation of ferrous iron present in AMD so as to precipitate the ferrite magnetite (Fe3O4). Magnetite in turn has the property of incorporating non-ferrous metals into its crystal lattice by cation substitution. Magnetite has several other properties which make its formation an ideal means of metals removal from AMD. Magnetite settles and dewaters extremely well and it is also stable at low pH, making remobilisation of metals into the environment unlikely. Ferrite formation at elevated temperatures (>65°C) is well established and has been successfully used in Japan to remove dissolved metals from laboratory wastes. The challenge insofar as massive volumes of AMD are concerned is to form ferrites at ambient temperatures. A second challenge is to form ferrites at ambient temperature in the presence of calcium, as lime (Ca(OH)2) is the most economical agent available for raising pH, but calcium is known to interfere with magnetite formation. To this end a series of batch and steady-state experiments have been performed in order to establish ambient temperature conditions for ferrite formation from AMD-like waters in both the presence and absence of calcium; as well as in the presence of both calcium and some non-ferrous metals commonly found in AMD (Co, Ni, Zn, Mn). The results represent the first well-described proven demonstration of a continuous flow, steady-state ambient temperature ferrite process which works successfully in the presence of calcium. The process relies upon ( 1) the property of magnetite/ferrite seed to channel the end- products of oxidation of ferrous solutions in the presence of calcium towards magnetite/ferrite formation; and (2) upon a contact stabilisation reactor -- settler sequence which serves to enhance the ferrous intermediate : dissolved calcium ratio in the oxidation reactor so as to overcome the problem of calcium interference in ferrite formation. The feasibility of the process for metals removal from AMD is borne out by a number of robust results pertaining to the quality of the effluent and the density and stability of the resulting sludge. There are several features of the process described here which render it economically attractive in comparison with existing technologies. Salient among these is the role of the contact stabilisation reactor - settler sequence in separating all the metals in the AMD from ~70% of the bulk AMD volume during the first stage of the process. Thus only ~30% of the bulk AMD volume requires further processing resulting in significantly reduced energy, infrastructure and chemical costs. Another feature of economic significance is the production of a commercially valuable end-product.

Grantor dc:publisher.institution

Department of Civil Engineering

Year dc:date.issued

2005

Author dc:creator

• Morgan, Barak Eli

Advisor dc:contributor.advisor

• Loewenthal, Richard Eric

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/11427/8725

OAI identifier oai:identifier

oai:open.uct.ac.za:11427/8725