Chemistry of CIP and related processes.
Most of the development and optimization work in CIP plants has been dome empirically. Simultaneously, various parties (including the activated-carbon industry) have put much effort into increasing the basic understanding of the process. The adsorption mechanism in particular has been given detailed attention. From this work it now appears that a logical, consistent picture of what is happening on a molecular scale has emerged. This article aims to provide a short overview of the chemistry of gold recovery by activated carbon.
Although the details of leaching are beyond the scope of this article, the following states the basic characteristics since they have an impact on the subsequent carbon performance.
Gold is a precious metal, and not normally oxidized by air. However, cyanide strongly stabilizes the oxidation product |Au.sup.+~ by complexation, and thus allows the reaction to proceed.
To prevent the formation of (toxic) hydrocyanic gas, the pH of the ore pulp is kept alkaline, usually by lime addition.
Activated carbons are mostly used as physical adsorbents, adsorbing a wide range of species onto a large pore surface area by physical attraction forces. Hence the usual characterization and specification in terms of pore structure, total surface area, and adsorption capacity for model compounds like iodine and carbon tetrachloride.
However, the adsorption of gold cyanide (and several other similar metal complexes) appears to be more like a chemical reaction than a physical adsorption. This poses different requirements on the activated carbon.
In solution, gold cyanide exists as a linear complex in which the central gold atom is bonded to two cyanide groups. During the adsorption by activated carbon, a third chemical bond is formed. This bond involves an electron from the activated carbon, that now spends part of its time on the gold atom. The gold atom accommodates this electron using an empty energy level, called the Lowest Unoccupied Molecular Orbital, or LUMO. Strong bonding of gold cyanide thus occurs when a 'part-time' electron transfer from the carbon to the gold atom is energetically favorable. This requires the activated carbon to have an electron available at a relatively high energy level.
The highest energy level available is called the Highest Occupied Molecular Orbital, or HOMO. The height of this level depends of the carbon structure. In activated carbons, most carbon atoms are located in small graphite-like structures--the basal planes. In a basal plane, each of the atoms contributes one electron to the common pi-electron system. The result is a large number of electrons, more or less spread out over the basal plane, having closely adjacent energy levels. The energy at the top level (HOMO) depends on the size of basal planes.
Thus an activated carbon can only be used in CIP processes when its basal planes are sufficiently large. The size of these planes depends on several factors (among others the raw material), which are determined in the production process of the activated carbon.
This explains why several types of commercially available activated carbons do not adsorb gold cyanide at all, notwithstanding their high porosity and total surface area.
Place of adsorption: Theoretical calculations have shown that high-energy electrons are predominantly found near the edges of the basal planes. In the activated carbon, these edges are mostly found at the walls of mesopores (2-50 nm in size).
Furthermore, under normal operating conditions, the adsorption process is kinetically controlled. This means that the adsorbed gold is found in pores in the outer shell of the carbon particle.
Counter ions: The gold-cyanide complex ||Au|(CN).sub.2~~.sup.-~ is negatively charged. So positively charged counter ions (cations) are co-adsorbed by the carbon to insure overall electro-neutrality. The larger the charge of the counter ion, the stronger the adsorption of the combination cation/gold complex. In practice, the predominant cation is calcium (|Ca.sup.2+~), which is abundantly present from lime addition.
The exact structure of the combination cation/gold complex is not known. It is usually discussed as either an ion-pair or as a double-layer structure (although the latter is difficult to envisage in a 10-nm sized pore). This discussion is, however, not of much practical importance.
Inorganic fouling and hydrochloric acid washing: The presence of calcium also has a less advantageous effect. In the activated carbon granules, cyanide is oxidized into carbonate. This results in the precipitation of calcium carbonate in the carbon granules. In extreme cases, up to 10% calcium has been found in CIP carbons.
The calcium carbonate precipitate causes a partial blocking of the pores, which adversely affects the gold adsorption kinetics. Therefore an acid wash is regularly applied, which effectively solubilizes and washes out all of the precipitate.
Hydrochloric acid washing does not result in elution of the gold cyanide. There is, however, a chemical change--the gold cyanide is converted into a different complex. The gold monocyanide formed is a chain-like compound, in which all gold atoms are still chemically bonded to the carbon surface.
Elution: Although gold cyanides are very strongly adsorbed by properly selected carbons, any adsorption is an equilibrium and can thus be reversed. In the elution process this is achieved primarily by increasing the temperature.
Cyanide is added to allow the reformation of gold cyanide from the chain-like polymer. The cyanide addition also makes up for some thermal decomposition of cyanide ions at the high elution temperature.
Alkalis are also added during elution, which results in an ionization of acidic (e.g. phenolic) groups of the carbon and thus a negative surface charge. This facilitates the stripping of the also negatively charged gold complex. The high pH also prevents the evolution of hydrocyanic acid.
Organic fouling and reactivation: All activated carbons readily adsorb organic material from the pulp, which may eventually hinder the gold adsorption. Therefore the barren carbon is thermally reactivated.
In contrast to the reactivation of spent potable-water carbons, for example, the reactivation of gold-recovery carbons is achieved under relatively mild conditions. This is a consequence of the different types of adsorption. For a potable-water carbon the reconstitution of a vast micropore system is essential, whereas in the case of gold-recovery carbons this is much less important. Also the degree of fouling is usually much lower in the case of CIP carbons.
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|Publication:||E&MJ - Engineering & Mining Journal|
|Date:||Jun 1, 1993|
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