Performance and optimisation of a jet loop reactor's pilot plant for the treatment of acid mine water using South African coal fly ash

Abstract

South Africa's freshwater resources are constantly under threat due to the pollution of surface and ground water as a result of different mining activities. Acid Mine Drainage (AMO) which is formed through pyrite oxidation and Coal Fly Ash (CFA) a by-product from coal burning process in thermal power stations, are both environmental pollutants that occur as a result of mining activities; and are causing long-term damage to waterways and biodiversity. Many technologies for treating mine water have been developed in order to generate water appropriate for domestic, industrial, and irrigation purposes. This study aim's at evaluating the performance of the hydrodynamic cavitation jet loop reactor pilot plant at 1000 L industrial capacity, for the treatment of mine water from Eyethu coalmine with CFA from Lethabo and Kendal power stations, and some chemical reagents (lime and aluminium hydroxide). Mine water is conventionally treated with chemical such Cao, CaC03, NH3 and Ca(OH)2 etc., however, the use of these chemicals makes the process expensive. Therefore, ways are sought to solve this problem, and CFA has been found to be of the alternative ways to address this problem. Treatment of EAMD with Lethabo Fly Ash (LFA) or Kendal Fly Ash (KFA) using different AMD:FA ratios of 7:1, 6:1 and 5:1 resulted in 46.11%, 48.16% and 45.25% of SO/· removal respectively when using LFA, and 42.74%, 47.87% and 48.66% of SOi· removal were observed when KFA was used. Using both LFA and KFA, the optimum amount of FA was obtained at an AMO: FA ratio of 6: 1. To further improve the performance in cleaning up of the mine water with respect to SO/· and heavy metals, lime (0.5, 1.0 or 1.5 kg) was added to the mixture of EAMD and FA (LFA or KFA). SO/·% removal for the mixtures of EAMD and LFA containing 0.5 kg, 1.0 kg or 1.5 kg of lime was 74.63%, 73.54% and 74.64% respectively. In the case of EAMD and KFA mixtures, this was 73.82%, 73.97% and 74.58%. The concentration of elements such as Mg, Fe, Al and Mn was reduced to within the Target Water Quality Range (TWQR) required for domestic water in all the cases investigated. It was discovered that the removal of these elements was pH dependent. It was also observed that adding Al(OH)J to the solution (AMD+FA+lime) did not increase the performance in cleaning up of the mine water with respect to SOi· removal. Al(OH)3 addition was found to be an unnecessary expense, since SO/· % removal was in the same range (73 - 75%) as when EAMD was neutralised with FA and lime. The success of FA treatment of mine water has been found to be site specific, i.e., the composition of the FA and the chemistry of the mine water to be treated will determine the efficacy of this treatment method. After neutralisation of 1000 L of EAMD with 167 kg of FA (LFA or KFA) and 1.0 kg of lime, the treated water was separated from the solid residues (SR) by gravity settling for approximately 30 min. After separation by gravity settling, about 69.5% of treated water was recovered. The energy consumed during the entire process (from feeding the AMO to the slurry discharge) was found to be 9.674 kWh. The power needed to neutralise 1000 L of EAMD with 167.0 kg of FA and 1.0 kg of lime was found to be 7.284 kW after 138 min. The SR recovered after the treatment of EAMD with FA (LFA or KFA) was successfully synthesised into a geopolymer backfill material. The strength (0.68 MPa) developed by the material was within the acceptable range for mine backfill material strength requirement. The findings of this study demonstrated that this treatment procedure will lead to successful solutions to environmental challenges, including a process to fill (seal) mine voids and prevent the production of acid mine drainage utilising the SR recovered after AMO treatment.