Tank leaching

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Refractory gold leaching

Tank leaching, also known as reactor leaching, is dissolving soluble minerals from a solid ore or concentrate within specially designed vessels. Tank leaching is one of several leaching methods.

Contents

Procedure

The ore or concentrate is fed as slurry or pulp into the leaching tank where it is mixed with a lixiviant, usually an aqueous solution, which acts as a solvent. The duration of the leaching operation can range from hours up to 5 days, the latter retention time being normal when refractory gold concentrates are bioleached.

Tank leaching can be performed as a batch process but is normally operated as a continuous process. Leaching is usually carried out in a series of tanks. In a continuous stirred tank reactor (CSTR) series leaching is co-current, i.e. a pulp of ore/concentrate and leaching reagents are pumped into the first reactor and then allowed to overflow to the following tanks in the reactor series. In the case of sulfide leaching as in bioleaching operations aeration is needed and is accomplished by blowing air under the stirrer.

To obtain good mixing in the reactors, propeller stirrers with downward pumping are normally used and to avoid vortex formation baffles with a width of 1/12 tank diameter are placed symmetrically in the reactor.

CSTR
CSTR

In oxidative leaching processes where air or oxygen is blown into the reactors the stirrer might be a turbine that has a better oxygen mass transfer capacity than a propeller but at the cost of a higher energy consumption. In some cases also air-lift reactors (Pachuca tanks) are used.

Tank leaching processes require fine ground ores/concentrates and are more costly to operate than in-situ, heap and vat leaching operations, therefore tank leaching is performed on materials with higher metal values.

Process conditions can be carefully controlled

Process conditions are carefully controlled in order to achieve required leaching efficiency. Examples of process parameters that can be controlled in tank leaching are:

Example of process conditions in a tank

The air supply was enriched with 0.3% CO2 and supplied to the reactors by means of a sparger situated below the impeller. The operating temperature was 45°C and the pH was controlled at ± 1.6. Feed slurry, fine-milled to a grind size of d90 = 12 μm, was added to the reactor at a residence time of 3 days.

Advantages

Tank leaching allows for good process control and contact between leaching reagent and material to be leached which therefore results in faster leaching kinetics and better recoveries.

  • rapid recovery of the desired metal
  • relatively high extraction

Disadvantages

  • higher capital and operating costs than heap leaching

Comments on tank bioleaching

Stirred tank bioleaching is mainly applied on refractory gold concentrates with gold concentrations in the range 10-150 g/tonne. In a refractory gold concentrate very fine gold particles are locked into the sulfide matrix and to liberate the gold for subsequent recovery by cyanide leaching the sulfides have to be leached. Alternative pre-treatment methods to bioleaching are roasting or pressure leaching.

Due to the relatively slow bioleaching kinetics, typically 5 days of treatment are necessary, the bioleaching tanks have to be very large to obtain a good production capacity. Bioleaching tanks have capacities of up to 1300 m3). Therefore the main operation costs are for mixing and air-supply, but also cooling of the reactor and neutralisation of the bioleach liquid are major costs in stirred tank bioleaching. In many of the recently started refractory gold treatment plants, bioleaching is the preferred technology.

Examples of tank bioleaching operations/Table ...

See also

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