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Studies of anodising sludge and its uses

Anodising is an important finishing technique used to produce controlled oxide coating on aluminium extrusions which impart aesthetic finish, longer life and resistance to corrosion and abrasion. The anodising process however leads to generation of significant amounts of acid and alkaline waste containing mainly aluminium. Normally these wastes are neutralised producing aluminium hydroxide sludge the disposal of which is problematic. Finex is New Zealand's most recently developed anodising plant established in Hamilton in 2004. During the first period of operation the company was faced with a costly sludge disposal problem. The present project was devised to study their production plant and wastewater treatment plant. A brief study of the sludge showed that even under optimum conditions and with use of polyelectrolyte, gravity settling produced excessive volumes of sludge. The decision was made to install a plate and frame filter press. With the reduction of its bulk by dewatering, the problem of sludge disposal was essentially solved. However the 12 tonnes per week of dewatered sludge production still posed the challenge of devising a way of using the material beneficially. The characteristics of the sludge were determined and routes to commodity products were investigated. Commodity alum was produced in good yield at 4 C by treatment of the sludge with sulphuric acid followed by crystallisation with anhydrous potassium sulphate. The product was very similar to the commercial product. Under basic conditions, sodium aluminate was produced by adding stoichiometric amount of OH-. Under acid conditions at the H+/Al(OH)3 stoichiometric ratio of 3, 95% of the aluminium was recovered as aluminium salt solution. When acid addition was less than the stoichiometric ratio of 3, the polyelectrolyte containing sludge showed little tendency to re-suspend. A variety of mechanical and thermal methods was used in an attempt to disaggregate and re-suspend the sludge. These included ultrasonic treatment at 80 C at frequency 14 kHz for 6 hours, autoclaving the sludge in electrical furnace at 150 C for 6 hours, homogenisation at 3000 rpm for 30 minutes at 20 C and mechanical grinding using mortar and pestle at 20 C. The ultrasonic treatment was effective in re-suspending 30% of the sludge. The other treatments were largely ineffective. On comparing the heavy metal concentration present in the original anodising sludge with the heavy metal concentration in the recovered commodity products, it was found that heavy metal content reduction was achieved with the alum crystallisation process. On producing aluminate, Cd, Mg and Ni were reduced through the removal of insoluble hydroxides. Minimal reduction of heavy metal content was achieved in the acidic aluminium chloride solution. The acidic and alkaline products prepared from anodising sludge were tested for their usefulness as coagulation agents. Critical coagulation concentrations for kaolin suspensions for all products were determined to be 5 mg/L. For all systems except the ultrasonic re-suspended sludge, clear solutions of low turbidity resulted after 1 hour standing. An analysis of the cost recovery of various commodity products from waste sludge indicated that recovery of acid aluminium chloride solution was most cost effective while the recovery of alum produces the purest product. Assuming that the acid aluminium chloride solution could be substituted for and sold at the same per kg of aluminium price as polyaluminium chloride, a 52% margin over consumables cost allows the possibility of a modest profit. The main benefit to Finex from proceeding with a sludge recovery program however is likely to be enhancement of the companies green credentials.
Type of thesis
Patel, P. V. (2008). Studies of anodising sludge and its uses (Thesis, Master of Science (Technology) (MSc(Tech))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/4800
The University of Waikato
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