Characterization and Chemical Speciation Modelling of Saline Effluents at Sasol Synthetic Fuels Complex-Secunda and Tutuka Power Station
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Date
2009
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University of the Western Cape
Abstract
Chemical speciation and the evaluation of species distribution is the key to understanding the potential of brines to form scale or corrode the water circuit as well as the potential of mobility and release trends of the pollutants into the environment. It is important to identify highly soluble free ions in water chemistry because toxicity of ions is related to mobility and consequently bioavailability. The chemical composition, character and chemical speciation modelling of saline effluents (brines) at Tutuka Power Station and Sasol Synthetic Fuels Complex in
Secunda were studied. The form in which chemical species exist (chemical speciation) and the physical and chemical interactions of species in saline effluents at these two study sites is not fully understood. This study investigated how pH, temperature, alkalinity and chemical composition influenced chemical speciation, species distribution, scale forming and corrosion potentials of the different saline effluent streams at the two sites using computer programs PHREEQC and Aq.QA. Characterizations of the results were presented in Stiff and Piper diagrams generated by the Aq.QA computer software. Chemical speciation modelling of the brines showed that scale-forming minerals
aragonite, calcite, hematite, anhydrite and gypsum have positive saturation indices between 0 and 20 in mine water, RO brine at Tutuka and Sasol Secunda, EDR brine at Sasol Secunda and VC brine at Tutuka Power Station. The water types at Tutuka Power Station were found to be mainly Na-S04 water types and those at Sasol Secunda were a mixture of Na-Cl and Na-S04 water types. Water treatment chemicals playa major role in increase were absent in the intake water. It was found that Sasol Secunda water streams are much more heavily contaminated than Tutuka water streams. The study also found that the mine water utilised at Sasol Secunda is two-fold more polluted than the mine water utilised at Tutuka although these sites are a mere 40 km apart. The sodium adsorption ratios showed that all the saline effluent streams at Tutuka and Sasol Secunda were unsuitable for irrigation, except for desalination product waters. Chemical speciation showed that the predominant species in the most concentrated saline effluent (VC brine) at Tutuka were the free cr ion at approximately 100 % with very minute quantities of FeCI+ and ZnCI+ and the predominant sodium species were the free Na+ ion which existed at 85 %. Magnesium species had the predominant form as the ionic compound MgS04 at 73 % and the carbonates were mainly in the form of NaC03- (53 %), HC03- (28 %) and CO{(7 %). The most concentrated brine analysed at Sasol Secunda was the TRO brine. PHREEQC did not predict the precipitation of CaC03 from the TRO brine at Sasol Secunda. The most abundant calcium species were Ca2+(59 %) ions and CaS04 (40 %). The brine was at
a pH of 5.76 with dissolved CO2 at 73 % of the carbonate species. Trace elements were evaluated and the toxic trace elements varied from 0.07 mg/L (As) to 26.75 mg/L (Sr) at Sasol Secunda. At Tutuka Power Station the toxic trace elements in brines varied from 0.02 mg/L (As/Se) to 16.85 mg/L (Sr). Sr and B were found to be the most highly concentrated toxic elements. The major and trace ion chemistry, alkalinity, pH, sodium adsorption ratios, change in concentration of the water streams and the brine chemical composition after contact with ash was also evaluated. When saline effluents at Tutuka Power Station and Sasol Secunda are combined with ash, pH, Ca content and alkalinity of the
resulting solution increased. The chemical composition of saline effluents can be influenced by the ingress of CO2 from the atmosphere.
The study shows conclusively that brine composition and concentration is highly variable at these South African power utilities and processes such as RO, contact with ash and C02 ingress can have an impact upon the overall brine quality. Aq.QA was found to be a more accurate tool for classifying waters according to dominant ions than Stiff diagrams but Stiff diagrams still have the superior advantage of being a
mapping tool to easily identify samples of similar composition as well as quickly identify what has been added or what has been removed from a water stream. Chemical speciation could identify effluent streams where C02 dissolution had taken place.
Description
>Magister Scientiae - MSc
Keywords
RO brine, EDR brine, Sasol Secunda, Magnesium, Iron chemistry, Alkalinity, pH, Sodium adsorption ratios, Tutuka Power Station, Sasol Synthetic Fuels Complex