Processing of current gold mining projects are generally more difficult than their historical counterparts, mainly because of mineralogy. Processing difficulties could include:

  • Excessive cyanide consumption due to severe side reactions from sulphide mineral dissolution, often inadequately suppressed through inefficient pre-oxidation regimes;
  • Slow leach kinetics as a function of inadequate oxygen supply during the leach (thus leaving an imbalance between available cyanide and oxygen for the gold dissolution to proceed); and
  • Partial or complete lock-up of the gold within specific sulphide host minerals such as pyrite, arsenopyrite or pyrrhotite.

The scope of objectives could range from:

  • Oxidising solution species (Fe2+ to Fe3+) to save reagents;
  • Passivation of sulphide solids to prevent further dissolution (saving reagent);
  • Purposefully oxidising sulphide matrices to liberate locked-up gold; and
  • Boosting of dissolved oxygen (DO) levels during the leach to values promoting fast oxidation reactions (either gold dissolution or sulphide phases).

Whilst oxygen needs for a solution species oxidation often only requires modest oxygen input masses, oxidising solid sulphide phases requires fairly high levels of oxygen addition.  The oxygen required is often much higher than the stoichimetric oxygen requirement for gold dissolution only.

Furthermore, the reaction kinetics are often a function of film diffusion layers and their relative thickness. Decreasing this film thickness through the application of elevated shear levels can double or triple the reaction rate.


Aachen Pre-oxidation to reduce cyanide consumption through oxidation of cyanide consuming species combined with efficient oxygen utilisation. Simple pre-oxidation, designed to oxidise cyanide consumers to prevent excess reagent consumption, may be more efficiently achieved using Aachen reactors where the oxygen mass requirements have been identified as relatively high. Elevated cyanide addition is often an indication for attempted compensation to overcome insufficient leach kinetics due to lack of oxygen at micro level through higher cyanide concentrations.

 Aachen Assisted Leach (AAL): As a highly efficient mass transfer device and also a shear reactor Aachen reactors are finding increasing applications within gold and silver leach circuits to accelerate leach kinetics, reduce cyanide consumption and reduce surface passivation. AAL involves the concept of combining oxygen, shear and cyanide to boost the leach kinetics (to increase throughput or reduce tank volume) as a function of drastically reduced film boundary layers, thus enhancing diffusion controlled reactions

Leachox™ involves the oxidative treatment of refractory, sulphidic ores or concentrates with or without UFG (Ultra Fine Grinding) to liberate locked up or at least leach hindered gold. This may involve only particle surface cleaning immediately prior to the leach or actual oxidation of sulphide minerals. At the most severe level, high percentage levels of sulphides will be converted to oxidation products and associated gold made leach amenable.

Key Design Criteria

  • The centrifugal slurry pump must be able to deliver against a water head of 50 m; and
  • The oxygen delivery system to the Aachen™ reactors must operate with a delivery pressure of 2 bar higher than the Aachen back-pressure, which is approximately 4 bar i.e. 6 bar requirement.
Tank volume (process dependent)


Minimum residence time during hyper-oxygenation
Minimum residence time during hyper-oxygenation


REA 450 Feed Pump requirement, nominal flow (per unit)


REA 450 Feed Pump requirement, head pressure

m water head

REA 450 Feed Pump installed power
200 kW

(per unit)

Oxygen delivery, capacity range (per unit)
30 – 150


Aachen unit pressure drop


Oxygen delivery excess head pressure


Interlocks (oxygen pressure and flow) to stop slurry pump
DP to be defined


Interlocks (DP slurry to O2 P)


Piping diameter (inlet and exit)
Piping diameter (inlet and exit)

Oxygen Feed

The feed system of each reactor should be equipped with:

  • Needle valves for flow regulation;
  • A pressure regulator; and
  • An oxygen flowmeter.

Safety Systems Requirements

Each reactor should be equipped with:

  • Pressure trip switch;
  • Flow trip switch; and 
  • Both interlocked with the relevant pumps.

Pumping of slurry without supporting gas flow must be avoided.

Physical Address

1331 Staal Street (corner Spokeshave)
Stormill Ext 2
South Africa

Postal Address

PO Box 2437
South Africa

Contact Details

Tel: +27 11 474 0705
Fax: +27 11 474 5580
Maelgwyn Mineral Services Africa (Pty) Limited | South Africa | Company Reg No: 2003/007416/07
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