The recovery of aluminium utilising the Donnan Membrane process

Mophethe, Moletsane (2018-12)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Die herwinning van aluminium uit waterbehandelingoorblyfsels (WBOs) het beide ’n ekonomiese en omgewingsvoordeel. Die ekonomiese aantreklikheid lê in die feit dat die aluminium herwin en hergebruik kan word in munisipale drinkbare waterbehandelingsaanlegte, wat gevolglik die operasionele kostes verminder. Die bron van hierdie metaal is van ’n koagulering sout, genaamd aluin, wat vervolgens presipiteer nadat flokkulering plaasvind. Die gepresipiteerde slyk word weggegooi in watermassas soos riviere en damme, asook vullisterreine. Groot konsentrasies aluminium is toksies vir beide natuurlewe en akwatiese lewe. In die geval van mense, het navorsing getoon dat daar ’n sterk korrelasie tussen hoë aluminiumkonsentrasies en die opdoening van Alzheimersiekte is. Drie metodes bestaan om hierdie aluminium te herwin: suurvertering, alkalisasie en Donnan Dialise. Terwyl beide vertering en alkalisasie al gewys het dat dit tot 80% van die aluminium kan herwin, het navorsing bewys dat die herwinning hoogs nie-selektief is as gevolg van die organiese komponente wat ook in die proses herwin word. Die mikpunt en doelstellings van hierdie projek is om die optimale vloeitempo’s en konsentrasies te identifiseer en ondersoek, wat die herwinning van aluminiumione van waterbehandelingoorblyfsels sal maksimeer deur Donnan Dialise te gebruik. Tweedens, om vas te stel of Donnan Dialise selektief die aluminiumione kan herwin terwyl dit terselfdertyd organiese komponente verwerp. Om hierdie doelstellings te bereik, is ’n laboratoriumskaal Donnan Dialise opstelling opgerig. Eksperimentele lopies is gedoen op sintetiese voer om die effek van die gemanipuleerde veranderlikes (voervloeitempo, veevloeitempo, voerkonsentrasie en veekonsentrasie) op die herwinning van aluminiumione met die finale doel om die aluminiumioon-herwinning te maksimeer. Sodra die effekte gekwantifiseer was, is verdere ekperimentasie gedoen deur gebruik te maak van werklike waterbehandelingoorblyfsels (WBOs) van ’n Blackheath behandelingsaanleg. Die doel hiervan was om vas te stel of Donnan Dialise aluminiumione selektief kan herwin, terwyl dit organiese komponente verwerp. Dit is vasgestel dat wateroordrag in die stelsel van die voer na die veekant finale aluminium-ioonkonsentrasies verdun. Verdere ondersoek het bekendgemaak dat die watervloed liniêr proporsioneel aan die konsentrasie van suur gebruik, is. Hoe hoër die suurkonsentrasie, hoe hoër die watervloed. Sure met dieselfde konsentrasies wat hoër Van’t Hoff faktore het, het ook bygedra tot hoër watervloed, eerder as die met ’n kleiner Van’t Hoff faktor. Gebaseer op eksperimentele resultate van sintetiese voer wat ’n Box Behnken ontwerp gebruik, is ’n statisties beduidende tweede orde model gepas, wat optimering van die bedryfparameters toelaat. Dis gevind dat ’n kombinasie van lae voerkonsentrasies en gematigde suurkonsentrasies (met vloeitempo wat geen effek het nie) die hoogste opbrengs van herwinde aluminium gehad het. Die hoogste herwinning was 98%. Statistiese analise by ’n vertrouensinterval van 95% het gewys dat die voerkonsentrasie die mees beduidende parameter was. Beide veekonsentrasie en voervloeitempo is as statisties onbeduidend gevind. Vir die werklike WBO voer analise is twee suurkonsentrasies vir vertering gebruik, naamlik 0.5 M en 0.05 M HCl. Dis gevind dat die sterker suur die slyk meer effektief afbreek. Dis gevind dat die begin konsentrasie van aluminium in die 0.5 M slyk 600 mg/L was, terwyl dít van die 0.05 M suur 300 mg/L was. Die konsentrasie van organiese komponente in die sterker suur vertering was ook hoër as dié van die swakker. Dis gevind dat Donnan dialise selektief aluminium herwin terwyl dit organiese komponente afgekeur het, tot en met 97% in die eerste 24 uur van bedryf. Die maksimum herwinning van aluminium wat werklike WBO voer gebruik, is gevind om 97% te wees. As sintetiese voer gebruik is, is ’n 98% herwinning van aluminium verkry in die eerste 24 uur van bedryf. Donnan Dialise is ’n belowende alternatief vir die herwinning en hergebruik van aluminium uit munisipale drinkbare waterbehandelingoorblyfsels. Die twee beperkende faktore van die tegniek is die stadige kinetika van die proses en dat die herwinde aluminium steeds verdere konsentrasie vereis voor dit direk in die waterbehandelingsproses hergebruik kan word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105045
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