Extraction Efficiency Factors
Aside from chemical requirements several key factors influence extraction efficiency:
- Retention time - refers to the time spent in the leaching system by the solids. This is calculated as the total volumetric capacity of the leach tank/s divided by the volumetric throughput of the solid/liquid slurry. Retention time is commonly measured in hours for precious metals recovery. A sequence of leach tanks is referred to as a leach "train", and retention time is measured considering the total volume of the leach train. The desired retention time is determined during the testing phase, and the system is then designed to achieve this.
- Size - The ore must be ground to a size that exposes the desired mineral to the leaching agent (referred to as “liberation”), and in tank leaching this must be a size that can be suspended by the agitator. In vat leaching this is the size that is the most economically viable, where the recovery achieved as ore is ground finer is balanced against the increased cost of processing the material.
- Slurry density - The slurry density (percent solids) determines retention time. The settling rate and viscosity of the slurry are functions of the slurry density. The viscosity, in turn, controls the gas mass transfer and the leaching rate.
- Numbers of tanks - Agitated tank leach circuits are typically designed with no less than four tanks and preferably more to prevent short-circuiting of the slurry through the tanks.
- Dissolved gas - Gas is often injected below the agitator or into the vat to obtain the desired dissolved gas levels – typically oxygen, in some base metal plants sulphur dioxide may be required.
- Reagents - Adding and maintaining the appropriate amount of reagents throughout the leach circuit is critical to a successful operation. Adding insufficient quantities of reagents reduces the metal recovery but adding excess reagents increases the operating costs without recovering enough additional metal to cover the cost •of the reagents.
The tank leaching method is commonly used to extract gold and silver from ore.
Read more about this topic: Tank Leaching
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