Browsing by Author "Mophethe, Moletsane"

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    The recovery of aluminium utilising the Donnan Membrane process
    (Stellenbosch : Stellenbosch University, 2018-12) Mophethe, Moletsane; Pillay, V. L.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: The recovery of aluminium from Water Treatment Residuals (WTRs) is both an economic and environmental advantage. Its economic attractiveness lies in the fact that the aluminium can be recycled and reused in municipal potable water treatment plants, which consequently reduces their operational costs. The source of this metal is from a coagulation salt called Alum that is subsequently precipitated after flocculation occurs. The precipitated sludge is discarded into water bodies such as rivers and lakes, as well as land fill sites. Large concentrations of aluminium have been found to be toxic to both wildlife and aquatic life. In the case of humans, research has indicated a strong correlation between high aluminium concentrations and contracting Alzheimer’s disease. Three current methods that exist to recover this aluminium are namely: acid digestion, alkalization and Donnan Dialysis. While both digestion and alkalization have been shown to recover up to 80% of the aluminium, research has shown that the recovery is highly non selective as organics have been found to be recovered in the process as well. The aims and objectives of this project are to identify and investigate the optimal flow rates and concentrations that maximize the recovery of aluminium ions from Water Treatment Residuals using Donnan Dialysis. Secondly, to establish if Donnan Dialysis can selectively recover aluminium ions whilst simultaneously rejecting organics. In order to achieve these objectives, a laboratory scale Donnan Dialysis rig was set up. Experimental runs were performed on synthetic feed in order to quantify the effects of the manipulated variables (feed flow rate, sweep flow rate, feed concentration and sweep concentration) on the recovery of aluminium ions with the final objective of maximizing aluminium ion recoveries. Once the effects were quantified, further experimentation was conducted using real Water Treatment Residuals (WTRs) from Blackheath treatment plant. The objective of this was to establish if Donnan Dialysis could selectively recover aluminium ions whilst rejecting organics. It was found that water transport in the system from the feed to the sweep side diluted final aluminium ion concentrations. Further investigation revealed that the water flux was linearly proportional to the concentration of acid used. The higher the acid concentration, the higher the water flux. In addition, acids at the same concentrations with higher Van’t Hoff factors attributed to a higher water flux than using those with a smaller Van’t Hoff factor. Based on experimental results from synthetic feed utilizing a Box Behnken design, a statistically significant second order model was fitted, allowing optimisation of the operating parameters. It was found that a combination of low feed concentrations and moderate acid concentrations (with flow rate having no effect) were found to yield the highest recoveries of aluminium. The highest recovery was found to be 98%.Statistical analysis at a confidence interval of 95% revealed that the feed concentration was found to be the most significant parameter. Both sweep concentration and feed flow rate were found to be statistically insignificant. For the real WTR feed analysis, two acids concentrations were used for digestion, namely 0.5M and 0.05M HCl. The stronger acid was found to breakdown the sludge more efficiently. The starting concentration of aluminium in the 0.5M sludge was found to be 600 mg/L while that of the 0.05M acid was 300 mg/L. The concentration of organics in the stronger acid digestion was also higher than that of the weaker. Donnan Dialysis was found to selectively recover aluminium whilst rejecting organics by up to 97% in the first 24 hours of operation. The maximum recovery of aluminium using real WTRs feed was found to be 97%. When synthetic feed was used, a 98% recovery of aluminium was obtained in the first 24 hours of operation. Donnan Dialysis is a promising alternative for the recovery and reuse of aluminium from municipal potable water treatment residuals. The two limiting factors of the technique are the slow kinetics of the process and that the subsequently recovered aluminium requires further concentration before it can be directly reused in the water treatment process.