(January 2019)

It’s now over 10 years since the European Patent Office advised its decision to grant Maelgwyn Mineral Services Ltd (MMS) a European Patent for its Leachox process for the treatment of refractory gold ores. Since that time the process route has become the industry leader in the low-cost recovery of gold from refractory and sulphide orebodies. There is now world-wide installed capacity of Leachox to recover over 3 million ounces per annum of gold contained in sulphides.

Leachox installed at the 730,000 oz Au/Annum Kibali Gold Mine in the DRC. A JV between Barrick Gold, AngloGold Ashanti and the DRC government.

Historically there have been two principle process routes to recover gold locked in sulphides. These two routes require pre-treatment to breakdown the sulphide matrix, either by pressure oxidation or bacterial leaching. Both these process routes come with a high installed capital cost and ongoing high operating costs, but both can achieve very high gold recoveries. The problem is that to justify the high capital and operating costs, mining projects require multi-million-ounce resources to be proven with a long mine life to get a return on capital. There are very few of these world-class deposits.

Another problem with pressure and bacterial leaching, which use either high temperature and pressure in titanium autoclaves, or bacteria that are required to be maintained under strict conditions, is the requirement for a high level of operation and support. Therefore, there is a hesitancy by mining companies to use them in remote areas.

The Leachox process uses simple ultra-fine grinding equipment combined with the Aachen Reactor; a proprietary low pressure, high shear mass transfer device utilising oxygen to partially oxidise the sulphides. When combined with the liberation of gold by the ultra-fine grinding of concentrates it results in acceptable but, more importantly economic gold recoveries.

To compare process routes and recoveries, pressure oxidation can completely breakdown the sulphides and then achieve high gold recoveries, maybe greater than 95%. Where as with the Leachox process and the partial oxidation, depending on the nature of the mineralogy, recoveries in the range 80% to 90% can be expected.

However, the Leachox process has an order of magnitude lower capital cost than pressure oxidation and much lower operating cost. The Aachen Reactor, which is central to the Leachox process has been shown to be extremely versatile and can also be used to advantage to treat oxide and then transition ores that normally are processed before the deeper sulphide ore bodies are reached.

The use of the Reactor accelerates the leach kinetics allowing for higher throughputs through the CIL/CIP plant and reduces cyanide consumption by initiating efficient oxidation of cyanicides in the pulp that react with cyanide in preference to gold and reduces the leaching effect. Whilst in parallel the Aachen Reactor removes passivating layers on the mineral surface that can otherwise impair the leach reaction.

The relatively fast acceptance of the Leachox process is in no small part down to the collaboration the MMS associate company, Maelgwyn Mineral Services Africa Pty Ltd (MMSA), has had with Randgold Resources since 2010. MMSA is an independently operated company (48% owned by MMS) who has expertise in gold processing and has top of the range metallurgical laboratory and pilot plant facilities.

Randgold utilised MMSA’s expertise when developing a number of mines across Africa where the majority of the gold in the ore deposits was in the sulphides and refractory by nature. These reserves were not large enough to justify pressure oxidation or bacterial leaching and the deposits were all situated in remote areas of Africa. Randgold undertook a due diligence on the Leachox process and installed some trial Aachen Reactors at its Loulo mine in Mali. The trial proved successful (the Aachen Reactors are still currently in operation at Loulo) and so Randgold used the Leachox process as its basis of design for the Tongon operation in the Cote d’Ivoire.

Today all the Randgold Resources operations utilise Leachox or Aachen Assisted Leach in their process plants and there are currently 49 Aachen Reactors installed across all their operations. The nature of the agreement between MMSA and Randgold means than MMSA maintain all the Reactors on site with replacement wear parts and keep them in good working order.

The flagship operation of Randgold is the Kibali gold mine in the Democratic Republic of Congo (DRC). A joint venture with AngloGold Ashanti. Leachox was again the chosen technology and installed in the initial design that saw the operation move from oxide, through transition material to the sulphide ore.

The 2018/19 outlook is for the Kibali mine to produce 730,000 ounces of gold for the year through the Leachox process.

Installation at the delivery side of a bank of Aachen Reactors

Over the time of collaboration between MMS/MMSA and Randgold Resources, Randgold had been the held up globally as being the most innovative gold mining leader. It has also been the best performing gold mining stock over this period. Some part of this might be attributed to their faith in the new technology offered by MMS. On January 1st, 2019 Randgold Resources was merged with Barrick Gold to become the largest gold mining company and the Kibali operation is considered one of the “Tier One” gold mines in the world.

The use of Aachen Assisted Leach and Leachox has expanded further into other operations world-wide and recently into the high tonnage tailings retreatment plants in South Africa operated by Evander Gold Mines and AngloGold Ashanti, where the application of the Aachen Reactors is in pre-oxidation before cyanide leaching to accelerate the leach and reduce reagent consumption.

A Leachox installation with a bank of Aachen Reactors on the suction side of the leach tanks

The uses of the Aachen Reactor as a high shear mass transfer device are not limited to gold leaching. The technology has undertaken various roles in a number of processes including cyanide destruction, silver and copper leaching, ferric ion generation and pyrite suppression. It is also envisaged that the reactor has applications in uranium leaching and lithium production. The majority of applications of the Aachen Reactor have been in gold operations. However, MMS’s first silver application is at the Gumustas silver mine in Turkey where retrofitting an Aachen Reactor is already yielding excellent results with an immediate recovery increase of 8% and a 30% reduction in cyanide consumption.