The Leachox™ Process

The Leachox™ Refractory Gold Process – The Testing, Design, Installation and Commissioning of a large Scale Plant at the VASGOLD Gold Mine, Kazakhstan. Presented at the MEI Precious Metals ‘10 Conference, Falmouth, Cornwall, UK, June 2010.

The Leachox™ Process

The Leachox process involves recovery of the sulphides containing gold to a high grade, low mass concentrate using Imhoflot G-Cell centrifugal pneumatic flotation or gravity concentration followed by ultra-fine grinding and oxidative treatment using Aachen Reactors™ and MMS Leach Reactors.

The use of ultra fine grinding of a gold concentrate is not a new development. In fact the fundamentals – the finer you grind, the more you liberate, the more valuable metal you recover has been the basis of mineral processing since time immemorial. A number of mining operations world-wide over the last 30 years have used ultra fine grinding followed by oxidative cyanide leaching to treat refractory gold ores with varying economic success. The Salsigne gold mine in France successfully operated a process from the early 1990’s until 2003 that involved grinding refractory gold sulphide concentrate in a ball mill to 95% passing 20 microns followed by atmospheric preoxidation and cyanide leaching in tanks.

However it is well known that ultra fine grinding in a tumbling mill is highly inefficient and incurs considerable power costs that make the process nearly always uneconomic. The development of grinding mills specifically for ultra fine grinding over the past ten years has now made the liberation of fine particles far more efficient. These mills such as the Isamill, the Deswik Mill and the Stirred Mill Detritor (SMD Mill) have their origins in equipment developed for the industrial minerals and chemical industries but have been adapted for the generally higher throughputs and more abrasive conditions

found in mineral processing applications. This is why grinding to below 20 microns for liberation is not seen as prohibitive anymore.

Grinding refractory gold ores to below 20 microns and finer, whilst liberating more gold, also liberates many other mineral species which affect the oxidative and cyanide downstream process.  In many cases when only ultra fine grinding is used a maximum recovery is reached and grinding finer actually reduces the precious metal recovery. It also dramatically increases the reagent consumptions for the leach process and consequently the costs. The Leachox process, using Aachen Reactors and MMS Leach Columns has found to negate to a high extent both these effects of limited recovery and high reagent consumptions. Laboratory and pilot plant test comparisons of Leachox against open tank oxidative leach and cyanide leach after ultra fine grinding for a number of concentrates in many cases resulted in higher gold recoveries with Leachox. In all cases it resulted in a considerable reduction in cyanide, oxygen and lime consumptions. In some cases it has led to an order of magnitude reduction in consumption. In one pilot plant study Leachox on a gold sulphide concentrate achieved 90.1% gold recovery. However it reduced cyanide consumption from 10 kg/t to 0.8 kg/t with oxygen and lime consumptions being reduced by a similar magnitude when compared with a similar ultra fine ground non-Leachox test. Another great advantage is that the Leachox process dramatically accelerates the leach kinetics, reducing the leach time down to a few hours from 24 hours for open tank leaching.