Treatment of industrial effluents for neutralization and sulphate removal
Maree, Johannes Philippus
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1.1 Background: Acid mine water containing sulphate and high concentrations of dissolved heavy metals, including iron(II), can have pH values as low as 2.5. Environmental pollution caused by such effluents are major contributors to the salinisation of receiving water, and may prove toxic to both fauna and flora. Acid, sulphate-rich solutions are produced bacteriologically from pyrite present in waste dumps from mining and metallurgical operations and from spent sulphuric acid used in chemical or metallurgical plants. The following large mine water treatment projects are currently receiving attention in South Africa on a national level: 1. Amanzi Water Project. The Amanzi project deals with the treatment of mine water (potentially 240 Ml/d) for the recovery of potable water and by-products (e.g. gypsum). Participating mines in the project are Randfontein Estates, First Wesgold, Durban Roodepoort Deep, Rand Leases, ERPM and Grootvlei. The pH of these waters varies fiom 2.8 to 6.0 and the sulphate concentrations h m 600 to 3 000 mg/l (SWaMP Steering Committee, 1998). 2.Olifants Forum. Polluted mine water, estimated at a volume of 130 Ml/d, is currently discharged to water courses on the Highveld. The mine water has a pH level between 2 and 4 and contains high sulphate concentrations (> 700 mg/l) (Van Zyl, et al., 2000). Unless neutralized, such water may not be discharged into water courses. Lime is generally used for neutralization. Neutralization costs could be reduced significantly should lime be replaced with limestone. The cost of limestone is currently R130/t compared to R700/t for lime. Furthermore, increasing pressure is being exerted by the Department of Water Affairs and Forestry to enforce sulphate removal from effluent. Extensive studies have already been carried out by the mining industry to evaluate possible sulphate removal technologies. The high cost of these technologies are considered a major obstacle. Therefore, efforts to develop a cost-effective treatment process for the recovery of re-usable water from sulphate-rich effluents, is of national importance. 1.2 Objectives: The objectives of this investigation were to develop processes whereby acid and/or sulphate-rich water can be treated. The specific aims of the investigation were to: 1.Develop the integrated iron(II)-oxidation and limestone neutralization process where powdered limestone is used for the neutralization of iron(II)-rich acid water in a completely-mixed reactor (Chapters 3 and 4 and Patents 1 - 3). 2.Develop the biological sulphate removal process for treatment of sulphate-rich effluents (Chapters 5 and 6). 3.Develop the barium sulphide process for treatment of sulphate-rich effluents (Chapter 7). 4.Develop a water flow and chemical mass balance model to identify the most cost-effective treatment option for a water network (Chapter 8). 1.3 Findings: The following innovative processes/models were developed for neutralization and sulphate removal from industrial effluents: 1. A limestone handling and dosing system. 2. A limestone neutralization and iron(II)-oxidation process for the removal of free acid, iron and aluminium. 3. A biological sulphate removal stage which includes biological sulphate reduction, H2S-stripping and aerobic treatment for the removal of residual organic material, and calcium carbonate precipitation. The barium process, which is similar to the biological sulphate removal process, can also be used for sulphate removal. 4. Modelling of a typical water network of a mining operation. 1.3.1 Limestone neutralization: In order to develop the limestone neutralization technology to the stage of full-scale implementation it was necessary to understand its limitations, study its kinetics, develop design criteria for full-scale plants and to protect the intellectual property through patents. 126.96.36.199 The limestone neutralization process. Limestone was not used previously on a large scale for neutralization of iron(II)-rich acid water. The reasons were: 1. The pH of iron(II)-rich water could not be raised sufficiently with limestone to rapidly allow iron(II) to be oxidized to iron(II). Rapid oxidation of iron(II) occurs only at pH 7 and higher. This can however be achieved with lime, while limestone only raises the pH of iron(II)-rich water to pH 6. 2. The reactivity of limestone is too low to neutralize acid water completely within an acceptably short residence time when stoichiometric dosages are applied. 3. Iron(II) passivates limestone particles due to Fe(OH)3 preferentially precipitating on the surface of the limestone particles, where the pH is the highest. 188.8.131.52 Kinetics of limestone neutralization. Stumm and Lee (1961) investigated the rate equation for biological iron(II)-oxidation and determined that it is a function of the pH, iron(II) and oxygen concentrations. This rate equation was investigated for the case where limestone was used as the neutralization agent. Special attention was given to the effect of suspended solids concentration on the rate of iron(II)- oxidation. 184.108.40.206 Full-scale implementation of limestone neutralization For the present investigation a demonstration plant was constructed and evaluated for iron(II)- Oxidation/limestone neutralization (Maree, et al., 2004). A plant with a capacity of 1 Ml/d was constructed at BCL, a nickel and copper mine in Botswana. Ore tailings leachate, with an acid concentration of 10 g/l (as CaC03), was treated. Limestone, available at a cost of R150/t, was used for neutralization of the acid water. Previously, leachate with a high acid concentration was combined with less acidic streams before it was neutralized with lime. The result of this approach was that a large volume of product water was slightly over-saturated with respect to gypsum, resulting in scaling of pipelines and other equipment. The leachate was neutralized separately from the less acidic streams. The over-saturated fraction was first allowed to crystallize from solution in the fluidized-bed reactor before being combined with the other streams. The following patents were registered, following the investigation: 1. A patent on the integrated limestone and iron(II)-oxidation process. 2. A patent for a limestone handling and dosing system was registered where powdered precipitated CaC03 was dumped onto a concrete slab, slurried to constant density with an automatic control, and used for neutralization of the acid water. 3. A patent on an integrated limestone and lime process for the treatment of acid and sulphate-rich effluents. This allows the following: - Stage 1 : The bulk of the acid is neutralized with limestone while C02 is produced and stripped off by aeration. - Stage 2: Lime is added to allow precipitation of magnesium and other metals as well as sulphate associated with these metals. - Stage 3: The C02 that is produced in Stage 1 is used to adjust the high pH of the water from Stage 2 to 8.3. This allows CaC03 precipitation. 1.3.2 Biological sulphate removal A biological process was developed whereby sulphate reduction to sulphide and sulphide oxidation to elemental sulphur occur in the same reactor. The following aspects were investigated: the reaction rate of biological sulphate reduction, the effect of various parameters on the reaction rate such as temperature, sulphide and sulphate concentrations and the identification of intermediate products formed. Pilot scale evaluation of the following stages of the biological sulphate removal process were evaluated: 1. Heating stage. Feed water to the anaerobic stage was first contacted directly with hot coal gas to raise the temperature of the water to 30 °C. 2. Anaerobic stage. A pilot plant with a capacity of 8 m³/h was operated, using ethanol or sugar as energy source. H2S-stripping and processing stage. A laboratory unit was operated to evaluate the suitability of the following reactor types for H2S-stripping and processing: Venturi device and a packed-bed reactor. 1.3.3 Integrated Bas process for sulphate removal Laboratory studies were carried out to demonstrate that the integrated Bas-process is technically and economically viable for sulphate removal. The Bas process consists of the following stages: 1.Thermal stage where barium sulphate is reduced to barium sulphide at 1 050°C, using coal as the reductant. 2.Sulphate removal stage 3.Sulphide stripping and processing stage 4.Softening stage where limestone is precipitated. 1.3.4 Modeling The water network of a coal mine was audited and simulated by an interactive, steady state model to determine the optimum effluent treatment process configuration. The findings from this investigation were used to optimize the mine's water management strategy. Simulation of the interactions in the water network was used to show the following: (i) Powdered CaC03 can be used as an alternative to lime for the neutralization of acid water at a cost saving. (ii) The amount of gypsum crystallization that occurred in the primary neutralization and coal processing plants. This information was needed to plan for sludge disposal. (iii) The benefits associated with separate treatment of the most polluted stream versus combined treatment of all streams during mine water treatment. By treating the higher polluted streams separate from the lesser polluted streams, higher salt removal efficiencies are achieved. (iv) The OSI (gypsum over saturation index) value can be controlled effectively at 1 by treating the feed water to the coal processing, for sulphate removal. The capacity of the sulphate removal plant required was determined as well as the associated capital and running costs. 1.4 Benefits The treatment approach outlined offers the following benefits: (i) The cheapest alkali, a by-product from the paper industry, can be used for neutralization of the acid and for the removal of the bulk of the sulphate concentration through gypsum crystallization. The more advanced biological process is then used only for removal of the remaining sulphate, to low concentrations. (ii) A robust biological process is used for sulphate removal to produce process water which is non-scaling and suitable for discharge into public streams. (iii) This is an integrated process as CO2 produced during limestone-neutralization is used for H2S-stripping in the biological stage. The stripped H2S-gas is utilized in the limestone-neutralization stage for precipitation of iron as iron sulphide. Iron is also removed as inert Fe(OH)3 together with gypsum in the limestone neutralization stage, after oxidation.
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