Determination of the equilibrium and film diffusion constants of the platinum cyanide anions during the elution from activated carbon
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Various leach approaches have been developed for the alkaline cyanide leaching of platinum group metals (PGMs) from ores, concentrates and from secondary wastes such as spent automobile catalytic converters. Recently, the adsorption and elution of PGM cyanide complexes onto activated carbon has also been demonstrated and is similar to the activated carbon process for gold. The stripping of adsorbed tetracyanoplatinate(II) ([Pt(CN)4]2−) from activated carbon consists of a two-step batch process which involves the pre-treatment of the metal-loaded activated carbon with a relatively strong sodium cyanide and sodium hydroxide solution prior to the elution step with de-ionized water at 80 °C. This paper models the elution and describes the mechanism of platinum elution from activated carbon. The equilibrium and film transfer constants were determined by fitting the model to experimental results and then validated by comparing to additional experiments. It was found that the rate of release of the platinum ions is governed by the amount of platinum and sodium on the activated carbon and the concentrations of these ions in the bulk of the liquid. It was found that the adsorption-elution of the single component system can be adequately described by using a modified Freundlich isotherm and a mass transfer diffusion equation.The rate of platinum elution interchangeably depends on the equilibrium of the Pt ions at the carbon-liquid interface and the mass transfer of these Pt ions from the interface to the bulk liquid. As both of these rate limiting factors were found to depend on the sodium concentration, the dominant platinum elution rate limiting factor shifts as the sodium concentrations change as the elution progresses. The elution process could be modelled accurately and it is shown that the second step (elution) of the two-step batch process (pre-treatment and elution) can be broken down into a sequence of four consecutive stages. The benefits of fundamentally understanding this process can ultimately lead to improved elution, better process control, shorter elution times, smaller elution columns or assist in the development of a continues elution process.
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