Removal of pollutants from acid mine wastewater using metallurgical by-product slags
Introduction
Various methods exist for the removal of toxic metal ions from aqueous solution, viz. ion exchange, reverse osmosis, precipitation and adsorption, among others. Adsorption is by far the most versatile and widely used process. Activated carbon has been the standard adsorbent for the reclamation of municipal and industrial wastewaters. Owing to the high-cost of activated carbon, production of its low-cost alternatives has been the focus of research in this area for years. These sorbents for the heavy metals sorption ranged from natural materials to industrial and agricultural by-products, such as fly ash, carbonaceous material, metal oxides, zeolites, moss, hydroxides, lignin, clays, biomass, peanut hulls, pyrite fines, goethite and coral sand.
Furnace slags as metallurgical by-products are being used as fillers or in the production of slag cement. It has been reported that granulated furnace slag can be converted into an effective adsorbent and used for the removal of dyes [1], [2] and metal ions [3], [4]. Alkaline-based slags as non-conventional sorbents for various heavy metal ions combine ion-exchange and sorption properties with an acid-neutralising ability. Acid mine water is an unavoidable by-product of the mining and mineral industry, especially as far as the oxidation of sulphide minerals is concerned. Acid mine waters typically contain high concentrations of dissolved heavy metals and sulphate and can have a high turbidity and pH values as low as 2. These conditions may prohibit discharge of untreated acid mine waters into public streams, as they have a detrimental effect on aquatic plant and fish life. Similarly, ground water pollution caused by the drainage of acid mine water is an equally serious problem. Traditionally, acid mine water is neutralised by treatment with lime, resulting in concomitant precipitation of iron, aluminium and other metal hydroxides. However, since the minimum solubilities for the different metals usually found in the polluted water occur at different pH values and the hydroxide precipitates are amphoteric in nature, maximum removal efficiency of mixed metals cannot be achieved at a single precipitation pH level. Conventional sorbents are not acceptable in such a mal-condition as acidic high-turbidity mine drainage. Slags can be used as low-cost adsorbents and neutralising agents and viable alternatives to the combination of much more expensive activated carbon or ion exchange resins and lime.
Slags exist often in a powdered form and are mainly applied as dispersions. Downstream of the reaction tank, a suitable solid/liquid separation is generally necessary. Flotation offers various advantages for the scope of separation, compared with other processes such as filtration, sedimentation or centrifugation [5] and constitutes a known method in effluent and water treatment [6], [7]. Combination of adsorption and subsequent flotation has proven to be an effective method for the removal of heavy metals from wastewater streams [8], [9], [10].
The present study involves an examination of the sorption capacities of two different slags for Cu and Pb removal from wastewater streams. Batch sorption/pH/kinetic studies were conducted in laboratory scale using simulated Cu and Pb bearing wastewater. Flotation of slags following ion sorption offers an effective way for solid/liquid separation. Also reported is the successful use of the novel technique in the treatment of an acid drainage from a gold mine.
Section snippets
Experimental work
Two furnace slags, viz. iron making slag and steel making slag, were obtained from Saldanha Steel South Africa in the form of powder with a mean particle size of 24.5 and 24.1 μm, respectively. The size distribution was: 100% and100 μm; 90% and 45 μm; 22% and 10 μm; and 1.2% and 1 μm for the iron slag, compared to 100% and 100 μm; 90% and 45 μm; 23% and 10 μm; and 1.5% and 1 μm for the steel slag. The chemical composition of the slags expressed as oxides in mass percentage is shown in Table 1. The XRD
Sorption equilibria
Two important physico–chemical aspects for the evaluation of the sorption process as a unit operation are the equilibria of sorption and the kinetics. Sorption equilibrium is established when the concentration of metal in a bulk solution is in dynamic balance with that of the interface. Fig. 1 shows typical sorption isotherms of Cu2+ and Pb2+ on the two slags, respectively. The slurry pH was maintained at 5.5 and the slag doses were 2 g/l.
As can be seen from Fig. 1, the iron slag had a much
Conclusions
The iron and steel making slags were appropriate sorbents for heavy metal removal from aqueous solutions. The slags combined ion-exchange and sorption properties with an acid-neutralising ability. The iron slag had a much higher sorption capacity for metals than the steel slag owing to its higher surface area, higher porosity and higher ion-exchange ability. Flotation following the slag sorption could effectively separate the slags, yielding very low solution turbidities. Advantages when
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