Integration of electrodeposition with other separations

The effectiveness of metal deposition can be impeded in the presence of other ionic species, organic or neutral species and also when relevant ionic species are in low concentrations. To improve the efficiency a combined, or integrated , approach to metal recycling can be used which incorporates other chemical and physical separation processes in conjunction with electrodeposition. This approach is used in the recycling of batteries and metal catalysts. 

1 The recycling of batteries. A commercial process [30] for the recovery of lead is based on the dissolution of the spent batteries in a fluoroboric acid electrolyte. The lead is deposited on the cathode in tank cells with insoluble lead dioxide coated graphite anodes. Of the metal ion impurities present in the electrolyte formed by battery solution only triva-lent Sb is found to co-deposit significantly with the Pb. By oxidising Sb(III) to Sb(IV), the amount of co-deposition becomes acceptably small for the process. 

Another hydrometallurgical process [31] consists of crushing batteries and separating metal pieces from electrodic pastes. The sludge is then solubilised in a double stage reduction process (alkaline and acid, respectively) and the purified solution is submitted to electrowinning from the fluoroborate solution onto stainless steel blank cathodes. 

In the process the Ni-Cd batteries are first shredded and leached of the metal components in concentrated hydrochloric acid. This leachate consists of a solution of the dissolved metals, mainly Fe, Ni, Cd with small amounts of Co, Cu and Hg at a pH of approximately 0. The Cd is extracted with a commercial metal ion extractant, then stripped into an aqueous electrolyte solution from which cadmium is electrowon. After extraction of the cadmium, the leachate is contacted with sodium hypochlorite, at pH 4, to oxidise Fe(II) to Fe(III) and induce precipitation of ferric hydroxide. The filtrate now contains mainly Ni which is recovered by electrodeposition. The processing of 1000 kg of batteries will produce 200 kg of Ni metal and 159 kg of Cd metal and 500 kg of ferro-nickel scrap. An alternative process [33], is operated in China for the recovery of Ni and Cd from Ni/Cd batteries. 

The processing of alkaline batteries can also proceed in a similar way to that of the Ni-Cd battery. 

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