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.
Key Basic 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.
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.
MMSA will ensure satisfactory commissioning of the units.
Defining Criteria (Example)
|Typical hyper-oxygenation level tank example using Aachen REA 450 unit (s)|
|Tank volume (process dependent)||1000||m3|
|Minimum residence time during hyper-oxygenation||To be defined||hours|
|REA 450 Feed Pump requirement, nominal flow (per unit)||600-700||m3/h|
|REA 450 Feed Pump requirement, head pressure||50||m water head|
|REA 450 Feed Pump installed power||200 kW||(per unit)|
|Oxygen delivery, capacity range (per unit)||30 – 150||kg/h|
|Aachen unit pressure drop||4||bar|
|Oxygen delivery excess head pressure||2||bar|
|Interlocks (oxygen pressure and flow) to stop slurry pump||DP to be defined||bar|
|Interlocks (DP slurry to O2 P)||>0.5||bar|
|Piping diameter (inlet and exit)||DN250|