ALTA Nickel-Cobalt-Copper, Uranium and Gold Conference, Perth Australia, 2011

Mike Adams
ABSTRACT
Modern gold plants are under increasing pressure to adopt world-class cyanide management practices due to escalating environmental pressures, particularly for signatories to the International
Cyanide Management Code. Operations are lowering the levels of weak-acid dissociable (WAD) cyanide reporting to spigot discharge and to any eventual discharges from the tailings storage facility (TSF). Gold operations are increasingly adopting the inclusion of cyanide oxidation processes into their circuits, particularly for greenfields projects. These processes predominantly use sodium meta-bisulfite (SMBS, or SO2) with air, Caro’s acid (H2SO5), or hydrogen peroxide (H2O2). Maelgwyn Mineral Services (MMS) has developed the MMS CN-D TM process, which utilizes the Aachen Reactor TM, a high energy mass transfer superoxygenation system, in conjunction with an actvated carbon based catalyst, to increase the rate of cyanide oxidation to cyanate, the thermodynamically more stable form. These four processes are compared with respect to stoichiometric reagent costs, chemical reaction efficiencies and risks, as well as logistical, operability, safety and environmental issues, for both remote sites and those close to urban areas. Considerable potential upside benefit is demonstrated for MMS CN-D TM compared with the alternative cyanide destruction processes. Additional potential features of the MMS CN-D TM process are also considered, such as the potential to recover additional gold otherwise lost to tailings, to derisk implementation and lower capital costs via staged integration of Aachen Reactors TM into existing oxygenation and SO2/Air equipment and to modify existing plant tankage in CIL trains to become part of the MMS CN-D TM cyanide destruction plant component. Finally, the current development and commercial status of the MMS CN-D TM process is described. A brief overview is also given of recent pilot -scale and bench- scale testwork conducted at the Maelgwyn Australia testwork facility in Perth Australia.
The MMS CN-D TM Process is shown to have consider able potential qualitative upside in comparison to the three most common oxidative cyanide destruction process routes. Oxidative reagent transportation, storage, mixing, dosing and safe handling are clear potential issues for the SMBS/Air, Caro’s acid and peroxide processes. Environmental issues such as elevated salt and sulphate loads due to sulphur addition, as well as therisk of corrosive contamination of wildlife habitats on overdosing excursions, are additional factors to be considered when implementing conventional chemical dosing cyanide destruction methods.  These factors become particularly important in both remote sites and sites located close to urban areas. From the reagent cost model outputs for a nominally typical gold plant requiring a comparison of the four options considered in this paper, it is concluded that costs for the Caro’s acid route are higher than the other routes; this effect is substantially magnified in the case of the sulphide ore (300 mg/LSCN) as compared with the oxide ore (0 mg/L SCN). SMBS/Air and hydrogen peroxide routes report somewhat reduced reagent costs. MMS CN-D TM reagent costs are significantly lower than those of the other three cyanide destruction process routes considered. Additional upside potential exists in the case of the MMS CN -D TM Process, given that additional gold leaching is expected in some cases, due to mineral surface cleaning in the high- shear cavitation zone of the Aachen Reactor TM, along with enhanced gold adsorption equilibrium onto the CN-D TM carbon, subject to carbon movement. Significant bench and pilot -scale testwork has been completed . The first full -scale MMS CN -DTM plant has been constructed at a gold plant in East Africa and is currently awaiting CIP plant stabilization and commissioning.

Comparison of the MMS CN-D Cyanide destruction Process with Caro’s Acid/SMBS/Air and Hydrogen Peroxide Options