Fundamentals of the elution of gold cyanide from activated carbon

[摘要] ENGLISH ABSTRACT:Most research work on activated carbon systems has been concerned onlywith the adsorption process. Nevertheless, controversy still exists onthe mechanism by which metal cyanide complexes such as Au(CN)2 areadsorbed onto activated carbon, and whether these complexes aredecomposed to irreversibly adsorbed species, such as AuCN and Au( 0l. Asthese species determine directly the need for cyanide during the elutionprocess, the first part of this thesis was devoted to a study of thereversibility of adsorption of metal cyanides on activated carbon and the·role that oxygen plays in the adsorption reactions.It was found that the formation of AuCN was mainly a function of the typeof carbon used and that oxygen did not promote the decomposition ofAu(CN)2 to AuCN. Although the dissolved oxygen level affected theequilibrium loading of anionic metal cyanides, it did not participatedirectly in the adsorption reaction. Au(Dl was formed only under drasticconditions of low pH's and high temperatures. It was now possible toselect a carbon on which no irreversibly adsorbed species were formed inorder to study the mechanism of the desorption of- gold cyanide and todetermine the need for cyanide in the elution process.Although the loaded carbon contained only Au(CN)2, the presence ofcyanide was found to drastically enhance the elution of the gold. It wasshown that the cyanide present in the elution step of an AARL elution wasless important than the cyanide in the pretreatment solution. It wasconcluded that the cyanide passivates the carbon for the adsorption ofanionic metal cyanides by reacting with the functional groups on thecarbon surface. However, the presence of high concentrations of cationspromotes the formation of neutral ion-pairs with the anionic metalcyanide on the carbon and thereby hinders the desorption of the metalcyanide in the pretreatment solution. Once the bulk of the cations areremoved in the elution stage, the desorption of the gold increasesrapidly.As the concentrations of cyanide and the spectator cations, as we 11 astheir reactions, were found to affect the gold cyanide equilibrium, these factors had to be accounted for in modelling the gold elution process.The system was simplified by using only one cation, namely potassium.Models were then developed to simulate the changes in· cyanide andpotassium concentrations during the elution of the gold cyanide. Thisenabled the calculation of a shifting gold cyanide equilibrium isothermas a function of time and position in the column. The elution of thegold cyanide was found to be diffusion controlled only under conditionsof weak desorption. Under these conditions, the desorption of the goldcyanide was simulated with a model accounting for mass transfer betweenthe micropores and the macropores of the carbon, surface diffusion in themacropores, and film diffusion through the liquid layer surrounding thecarbon particles. Under strong elution conditions (i.e. hightemperatures and efficient pretreatments) the resistanc• to mass transferbecam~ negligible and the desorption of the gold could be modelled withan equilibrium mode 1. The equilibrium mode 1 was shown to be applicableto both the Zadra .and the AARL elution processes.Simul at i ans of exp·erimental data were presented as evidence of thevalidity of the models. Furthermore~ the models were used to perform aparametric sensitivity analysis and to predict the elution profilesduring a continuous elution process.

[发布机构] Stellenbosch University