The reprocessing of old gold tailings dams has a number of advantages over ROM ores as the expensive mining and comminution costs have already been incurred resulting in a significantly reduced operating cost. The associated disadvantage is of course that these tailings are often an order of magnitude lower in head grade than the original ore body. Generally high throughputs are required to compensate for the lower grades. Also, in contrast to ROM ores, reagent costs particularly cyanide and oxygen can now represent up to 50% of the operational cost so it is important to utilise these efficiently.

Maelgwyn Mineral Services (MMS) Aachen Shear reactors offer a cost-effective solution to enhance cyanidation in gold processing. The typical benefits include enhanced kinetics, reduced cyanide consumption, reduced lime consumption, increase in gold recovery and downstream benefits for carbon plants due to steeper leach profile.

The Aachen shear reactor is essentially a highly efficient mass transfer device developed and refined over many years out of experience with Maelgwyn’s Imhoflot flotation technology. The versatility of the reactor is evident by the ever- growing list of different process applications. For the direct cyanidation of gold ores, the Aachen reactors are used in two different ways:

• Mild pre-oxygenation to reduce cyanide consumption through oxidation of cyanide consuming species combined with efficient oxygen utilisation
• Aachen assisted leaching (AAL). As a highly efficient mass transfer device and also a shear reactor Aachen reactor are finding increasing applications within gold and silver leach circuits to accelerate leach kinetics, reduce cyanide consumption and reduce surface passivation

Currently there are over 60 Aachen reactors installed across various operations for a combination of pre-oxidation and Aachen assisted leaching including the majority of Barrick Gold Corporation’s African operations. Aachen reactors have also recently been installed at Pan African Resources’ Elikhulu Tailings Retreatment Plant, which operates a conventional carbon-in-leach circuit including the proprietary Aachen pre-oxidation process.

Having achieved the nameplate processing target of 1.2 Mt a month of tailings, the low-cost, long-life Elikhulu project is expected to reduce the Group’s average all-in cost of production. The three tailings facilities will, after processing, be consolidated into a single enlarged Kinross facility, thus reducing Evander Mines’ environmental footprint and associated environmental impact.

The Aachen units can be manufactured in a range of sizes to suit the site-specific flow rate requirements up to a maximum flow of 800 m³/hr for a single reactor and, enable efficient oxygen dispersion under high shear (velocities of up to 10 m/s within the unit) and pressure (3 to 6 bar) generated by the feed pump. Furthermore, the shear exposure reduces boundary layer resistances that influence most solid/liquid reaction rates. This can enhance the reaction kinetics tremendously and enable process options not feasible otherwise. Essentially all of the requirements of Elsner’s equation governing gold leaching are met ensuring that rapid dissolution of gold can take place.