Fundamentals of the elution of gold cyanide from activated carbon

Van der Merwe, Pieter F.(Pieter Francois) (1991)

Dissertation (Ph. D. (Ing.)) -- University of Stellenbosch, 1991.

Thesis

ENGLISH ABSTRACT: Most research work on activated carbon systems has been concerned only with the adsorption process. Nevertheless, controversy still exists on the mechanism by which metal cyanide complexes such as Au(CN)2 are adsorbed onto activated carbon, and whether these complexes are decomposed to irreversibly adsorbed species, such as AuCN and Au( 0l. As these species determine directly the need for cyanide during the elution process, the first part of this thesis was devoted to a study of the reversibility 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 type of carbon used and that oxygen did not promote the decomposition of Au(CN)2 to AuCN. Although the dissolved oxygen level affected the equilibrium loading of anionic metal cyanides, it did not participate directly in the adsorption reaction. Au(Dl was formed only under drastic conditions of low pH's and high temperatures. It was now possible to select a carbon on which no irreversibly adsorbed species were formed in order to study the mechanism of the desorption of- gold cyanide and to determine the need for cyanide in the elution process. Although the loaded carbon contained only Au(CN)2, the presence of cyanide was found to drastically enhance the elution of the gold. It was shown that the cyanide present in the elution step of an AARL elution was less important than the cyanide in the pretreatment solution. It was concluded that the cyanide passivates the carbon for the adsorption of anionic metal cyanides by reacting with the functional groups on the carbon surface. However, the presence of high concentrations of cations promotes the formation of neutral ion-pairs with the anionic metal cyanide on the carbon and thereby hinders the desorption of the metal cyanide in the pretreatment solution. Once the bulk of the cations are removed in the elution stage, the desorption of the gold increases rapidly. As the concentrations of cyanide and the spectator cations, as we 11 as their 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 and potassium concentrations during the elution of the gold cyanide. This enabled the calculation of a shifting gold cyanide equilibrium isotherm as a function of time and position in the column. The elution of the gold cyanide was found to be diffusion controlled only under conditions of weak desorption. Under these conditions, the desorption of the gold cyanide was simulated with a model accounting for mass transfer between the micropores and the macropores of the carbon, surface diffusion in the macropores, and film diffusion through the liquid layer surrounding the carbon particles. Under strong elution conditions (i.e. high temperatures and efficient pretreatments) the resistanc• to mass transfer becam~ negligible and the desorption of the gold could be modelled with an equilibrium mode 1. The equilibrium mode 1 was shown to be applicable to both the Zadra .and the AARL elution processes. Simul at i ans of exp·erimental data were presented as evidence of the validity of the models. Furthermore~ the models were used to perform a parametric sensitivity analysis and to predict the elution profiles during a continuous elution process.

AFRIKAANSE OPSOMMING: Die meeste navorsingswerk op geaktiveerde koolstof het tot dusver gekonsentreer op die adsorpsie proses. Daar heers egter nog heelwat polemiek oor die meganisme waarmee metaalsianied komplekse soos Au(CN)2 op koolstof geadsorbeer word, en of hierdie komplekse afgebreek word na onomkeerbaar geadsorbeerde spesies soos AuCN en Au(Ol. Aangesien die noodsaaklikheid van sianied in die elueringsproses bepaal word deur die teenwoordigheid van hierdie spesies, is die eerste gedeelte van hierdie proefskrif afgestaan aan 'n studie van die omkeerbaarheid van die adsorpsie van metaalsianiede op geaktiveerde koolstof, en die rol wat suurstof in die adsorpsi~reaksies speel. Daar is gevind dat die vorming van AuCN hoofsaaklik 'n funksie is van die tipe koolstof wat gebruik word en dat die afbreking van Au(CN)2 na AuCN 1nie deur suurstof bevorder word nie. Alhoewel die opgeloste suurstofvlak die ewewigslading van anioniese metaalsianiede be1nvloed het, het dit nie direk deelgeneem aan die adsorpsiereaksies nie. Au(Ol is slegs onder die uiterste kondisies van lae pH en hoe temperatuur gevorm. Met hierdie kennis was dit toe moontlik om 'n koolstof te kies waarop geen onomkeerbaar geadsorbeerde spesies gevorm word nie ten einde die meganisme van desorpsie en die noodsaaklikheid van sianied in die elueringsproses te bestudeer. Dit is gevind dat selfs met net Au(CN)2 teenwoordig op die gelaaide koolstof, die desorpsie van goud drasties verhoog word deur die teenwoordigheid van sianied. Daar is verder aangetoon dat die sianied in die elueringstap van 'n AARL elueringsprosedure nie so belangrik is as die sianied wat in die voorafbehandeling gebruik word nie. Hieruit is afgelei dat die sianied die koolstof passiveer vir die adsorpsie van anioniese metaalsianiede deur die reaksie daarvan met die funksionele groepe op die oppervlak van die koolstof. Min desorpsie van goud vind egter in die voorafbehandelingstap plaas, omdat die hoe katioon konsentrasie die vorming van neutrale ioonpare met die anioniese metaalsianied bevorder. Dit is eers nadat die grootste gedeelte van die katione verwyder is in die elueringstap, dat die desorpsie van die goud vinnig toeneem. Aangesien dit gevind is dat die toeskouer kat i one, asook hul 1 e beinvloed, moes hierdie faktore in konsentrasies van die sianied en reaksies, die goudsianied ewewig ag geneem word in die modellering van die desorpsie van die goudsianied. Om die model te vereenvoudig, is slegs een katioon, naamlik kalium, gebruik. Madelle is toe ontwikkel waarmee die veranderinge in die stanied- en kaliumkonsentrasie gedurende die desorpsie van die goudsianied beskryf kon word. Hierdie modelle het dit moontl ik gemaak om 'n verskuiwende goudsianied ewewigsisoterm te bereken as 'n funksie van tyd en hoogte in die kolom. Dit is gevind dat die desorpsie van die goudsianied slegs ender kondisies van swak desorpsie diffusiebeherend was. Onder sulke toestande kon die desorpsie van die goud beskryf word met 'n model wat voorsiening maak vir massaoordrag tussen die mikro~ en makroporiei van die koolstof, oppervlakdiffusie in die makroporiei, en dfffusie deur die vloeistoflagie om die koolstofkorrels. Hoe temperature en effektiewe voorafbehandelings (d.w.s. sterk elueringskondisies) het die weerstand teen massa-oordrag sodanig verlaag dat die desorpsie van die goudsianied met 'n ewewigsmodel beskryf kon word. Die ewewigsmodel was van toepassing op beide die AARL en Zadra elueringsprosesse. Simulasies van eksperimenteel gemete elueringsprofiele is voorgele as bewys van die geldigheid van die verskillende modelle. Die modelle is verder gebruik in 'n sensitiwiteitsanalise en ook om die elueringsprofiele van 'n kontinue elueringsproses te voorspel.

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