Leachox™ Process for flotation concentrates

Aachen reactors form an integral part of the Leachox process for the treatment of gold bearing sulphide flotation concentrates.

These flotation concentrates are often refractory to cyanide or require very high reagent additions to achieve even low recoveries as the gold is often locked within pyrite or arsenopyrite.

It is recommended that you first read our website section on the Aachen reactors to provide some background before reading further about the Leachox™ process. Links to these pages are provided below

Leachox™ Process for flotation concentrates

The Leachox™ process consists of several Maelgwyn proprietary processes linked together including: –

Imhoflot G-Cell flotation

Ultra-fine grinding using Ro-Star mill

Aachen Reactors

Aachen assisted cyanide destruction

Ultra-fine grinding can assist in the liberation of the sulphide minerals and is used as one of the unit process steps in the Leachox process.


The majority of Aachen reactors are employed on Run of Mine (ROM) feed applications primarily to increase gold recovery through increasing the kinetics. In the Leachox™ process the role of the Aachen reactor is somewhat different in that the reactor is used to facilitate partial oxidation of the sulphide minerals encapsulating  gold particles and is therefore normally used to improve the gold recovery on sulphide flotation concentrates.

There are several process alternatives to Leachox™ for the treatment of refractory flotation

Processes such as roasting, pressure oxidation, and bacterial oxidation are all aimed at breaking down the sulphide matrix to liberate gold. Ultra-fine grinding performs the same function particularly where gold is locked in silicates or other minerals. Many of these processes are well developed and can yield very high gold recoveries but unfortunately all tend to have inherent issues. 

Roasting for instance is an environmentally unfriendly process and presents permitting issues in many countries; pressure oxidation requires a fairly high degree of operator skill and control not to mention often exotic materials of construction. Bacteria used in bacterial oxidation whether heap of tank leaching are susceptible to changes in environmental conditions and require careful control. Pressure oxidation works well but requires high levels of technical expertise and exotic materials of construction

Whilst it is not possible here to go into the relative merits and demerits of each process the biggest issue that they all have in common to some degree is the very high associated capital and operating costs limiting their application to large high grade deposits which can accommodate the high capex and opex

For many deposits where the resource base is too small to justify Pressure oxidation etc then Leachox™ can be used. Leachox™ is a partial oxidation process for refractory ores centred around the Aachen reactors. Whilst it does not yield as high a gold recovery as POX and Roasting it is order of magnitudes cheaper resulting in an overall improved project economics

Ultra fine grinding:-

Often when gold is locked in sulphides and silicates it is necessary to reduce the particle size to a size where gold is liberated or partially liberated. For refractory ores this generally translates into grinding below 10 microns and often to as low as 3-4 microns. Historically this was cost prohibitive as the only mills that were available to do this were tumbling mills which become highly inefficient at these low sizes particularly below 20 microns. The last 10-15 years however, have seen the development of a number of grinding mills specifically designed for ultra-fine grinding in the minerals industry. This has lead to a commensurate reduction in cost to grind fine and has been the catalyst for the development of many refractory ore treatment processes.

One of the drawbacks of grinding finer is that in addition to liberating the desired mineral it also increases the surface area of other host minerals .This can result in order of magnitude increases in reagent consumption particularly cyanide for the subsequent leaching process unless cognisance of this is taken in the final process design.

How Aachen Reactors work in the Leachox™ process

Whilst the installation is similar to that used for pre-oxidation or Aachen assisted leaching in the Leachox process the flotation concentrate is pumped through the reactor multiple times perhaps as many as 30 passes in contrast to 1-2 passes for the pre-oxidation role .Depending upon the mineralogy ultrafine grinding of the flotation concentrate may be required prior to cyanidation

In Leachox™ rather than just raising the DO level and providing shear to remove passivating species to enhance cyanidation kinetics the sulphide matrix is partially oxidised (60-70%) resulting in the liberation of gold particles from the host sulphide matrix.

Unfortunately the breakdown of the sulphide matrix along is associated with a significant increase in the surface area of the various cyanide consuming minerals and can result in order of magnitude increases in cyanide consumption often as high as 20-30kg/t which in itself is uneconomic. In addition, significant passivation also can occur rendering the mineral surface refractory to cyanide

The Aachen reactor solves both of these problems by accelerating the leach kinetics such that gold is able to dissolve into solution prior to the cyanide being consumed and also continuously removing the passivating layers forming on the gold particles (See information on Aachen reactors for pre-oxygenation and Aachen assisted Leaching)

Whilst as mentioned previously oxygen lances might be suitable for readily cyanidable oxide ores their limitations become apparent with the more demanding refractory leaching applications and their use is associated with very high cyanide and oxygen consumptions and poor gold recoveries


Whilst the Aachen reactors are central and pivotal to the success of the Leachox process the other components utilising Maelgwyn’ s innovative technologies further enhance the process

Imhoflot G-Cells are used to produce the flotation concentrate. The reason for this is that the patented G-Cell is able to produce a higher grade flotation concentrate than conventional mechanical flotation cells resulting in a small concentrate mass and so reduced treatment costs (see under Imhoflot flotation for more information)

As previously mentioned ultra fine grinding is often required on the flotation concentrates. Grinding is by its very nature an expensive process and more so for fine grinding where particle size reduction is through abrasion rather than impact. Whilst there are a number of commercial ultrafine grinding mills available Maelgwyn specifically designed its own mill the Ro-Star mill to reduce the capex and opex costs for ultra- fine grinding where this is required

The cyanide consumption in refractory gold leaching is generally much higher than in ROM ore cyanidation circuits and so efficient cyanide destruction is important. The Aachen reactors can be used to enhance the well-known sulphur dioxide based cyanide detox process

Finally, for smaller orebodies in remote locations Contubes can be used to dewater the tailings