New methods for recovery of inorganic salts from waste water in the petroleum industry
Oosthuizen, Lydianna Maria
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In this study three novel methods for the removal/recovery of inorganic salts from aqueous solution were explored to make a contribution to ongoing efforts by the petroleum industry to upgrade waste water for reuse by immobilising and removing inorganic substances from such contaminated water. These methods were targeted precipitation, supercritical treatment and eutectic freeze crystallisation. The feasibility of these methods for waste water treatment was investigated by using both simple laboratory prepared solutions and complex real-world waste water samples (TRO and EDR brine) from the petroleum industry. Different analytical techniques, including simple and complexometric titrations (AgNO3, EDTA), ICP-MS and IC were utilised to analyse original solutions and filtrates collected after precipitation by the different methods for several key anions/cations typically found in the waste water of a petroleum industry. The method of targeted precipitation entailed an adjustment of the molar ratio of species in laboratory and industrial solutions to effect precipitation of a target compound having a favourable stoichiometry to remove large amounts of anions/cations from solution. It was successful for the removal of sulphate ion from both simple synthetic solutions (> 85%) and complex real-world solutions (50% for TRO and 75% for EDR). The optimum chloride ion removal from synthetic solutions (60%) was obtained by using a slightly different molar ratio than planned, but from the industrial brines disappointingly low chloride ion removal (25% for TRO, 12% for EDR) was achieved. The second method required laboratory and industrial solutions to be subjected to conditions (typically 218 atm and 400°C) at which water is a supercritical fluid. The polar character, hydrogen bonding and ion solvating capability of water are destroyed under such conditions, so that ionic species are forced out of solution. The removal of sulphate ion from synthetic solutions was just as successful (80%) as targeted precipitation, though redissolution occurred over time as a result of competition with other ions for hydrating water molecules. A vast amount of sulphate (1000 mg/L or 28% for TRO and 3500 mg/L or 35% for EDR) was removed under supercritical water conditions, and large amounts of sodium (450 mg/L or 31% for TRO, 1300 mg/L or 41% for EDR) and calcium (450 mg/L or 85% for TRO, 600 mg/L or 90% for EDR) precipitated in addition to sulphate. Chloride ion removal was disappointingly low and never exceeded 15% for synthetic and 8% for real-world solutions. Eutectic freeze crystallisation was used in successive fractionations to remove substantial amounts of simple inorganic salts, such as Na2SO4 (67%) and Na2CO3 (31%), from pre-prepared eutectic solutions. Application of the technique to industrial waste water samples in seven successive fractionations led to collective removal of 20% of sulphate and 18% of chloride content of TRO and 60% of sulphate and 15% of chloride content of EDR waste water samples. The reliability of the operation to precipitate and collect ice-salt mixtures in several successive fractionations was proven by a proper mass balance enabled by analysis of the original solution, the filtrates after each collection and the residual mother liquid. The study rendered a modest contribution to the treatment of industrial waste water for the sake of mineral recovery and water restoration. It identified, applied and evaluated novel methods of waste water treatment in order to expand knowledge in this field and to broaden the capability of industry to help conserve water as a scarce resource. The inclusion of a method related to supercritical technology demonstrated its applicability to industrial processes and its relevance to green chemistry by utilising environmentally friendly supercritical fluids.