Fouling of carbon, regeneration and Elution

In all plant circuits, the carbon activity quickly deteriorates from stage to stage but the reasons why it should do so are complex. It is known that the carbon activity is enhanced by lower pH, higher temperature; the presence of Ca2*, Mg2*, Na*; and by small particle size. On the other hand, its activity is inhibited by excess CN~, build-up of CaCOa and silica, adsorption of xanthates, oils, frothers, huraic acids; and degradatin of active sites [33]. When inorganic foulants are considered, it was found that CaC03 and Mg(OH>2 in particular are adsorbed at high pH, and that copper is adsorbed when the CN~ concentration is low [28]. Silica and iron are invariably present as fine quartz, clay or calcine, which can block the pores of the carbon.

Acid washing with 5-10% HC1 removes many of these foulants but does not restore the carbon activity entirely. As recently shown by La Brooy et al [33] the organic foulants are particularly important, especially the flotation agents, frothers and humic acids. Fortunately most of these get burnt off by heating the carbon, but some require temperatures of 750° to remove them completely.

Carbon regeneration

There is no doubt that the temperature of carbon during reactivation is critical in removing organics and restoring carbon activity and that most plant carbons are not properly regenerated. Reactivation not only removes organics, but also reams out pores, and regenerates surface oxide sites. But as recent studies show, it is important to control temperature, added water/steam, and salts [34]. The key reaction is the water-gas reaction which is rapid above 800° and catalysed by Ca2-, Fe2-. However the type of functional groups produced are also determined by temperature.

Through a proper understanding of key parameters, improved Rotary Kiln designs are evolving, as well as the Electrical Resistive furnace (Rintoul), the Vertical Tubular furnace. Most gold plants currently use rotary kilns with poor control over the carbon temperature. Carbon temperatures vary according to how much water is fed into the furnace with the carbon. It is therefore of interest to compare the various kiln performances.carbon

Elution of gold from carbon

Over the last 10 years there has been much research and development into elution procedures, all trying to improve on the traditional Zadra Process [37] which takes days to complete. Recent studies by Adams and Nicol on the kinetics of elution [38] show that NaCN is more effective than NaOH as an eluant and that high ionic strength solutions retard gold desorption. The optimum NaCN concentration proves to be about 1% w/v

To date, the pressurised Zadra, Anglo, Duval c;nd Micron processes have been commercialised whilst an improved solvent elution system devised at Murdoch University is yet to be developed. All elute the carbon in less than 12 hours using a variety of eluants.

The Anglo process is slowly gaining acceptance over the Zadra process because of its lower overall cost, but the quality of water used is critical to the process [43] which is an important aspect for arid regions. On the other hand the W. Australian Micron process does offer cost advantages in the gold recovery step (see below) and is suited to organically fouled carbons. In South Africa, for example, it is being installed at gold plants which operate a solvent extraction circuit for uranium. The unique advantage of the Micron system (Figure 3) is that less than 1 bed volume of refluxing solvent is used to extract the gold from the carbon charge, and the gold is concentrated in the distillation pot.

The reason why organic solvents are so good at eluting gold is that they increase the reactivity of CN- and stabilise the auro-cyanide in solution.It was the more detailed fundamental knowledge about solvents at Murdoch led to the discovery that acetonitrile elutes gold more efficiently than methanol.

Because the gold is concentrated in the eluant, the Micron process lends itself to elegant gold recovery techniques which give 99.9% pure gold directly. The process selectively discards copper and silver as insoluble cyanides by acidification to pH 3 in the presence of thiourea. The aurocyanide/ thiourea complex is oxidised by chlorine to auric chloride, which is then readily reduced to gold sand by SO2 or sulfites leaving other impurities in solution. Figure 4 shows how a decrease in pH, precipitates out Zn, Ni and Cu cyanides around pH 2-6 leaving gold in solution.