In a mineral processing plant when mechanical methods of separation cannot secure optimal metal value of an ore, leaching is an alternative, either as a complement or as an overall process. Most of the leaching process is feed preparation by crushing, grinding and in some cases pre-concentration and roasting.

The separation is normally done by creating retention time for the chemicals to penetrate the feed.

The waste and heap leaching operations are typically low in investment but high in cost of chemicals. Recovery is normally low (below 60%).

The agitation methods (con current or counter current) are high in investment but are paid back in higher recovery.

Leaching by size

In leaching there is always an optimum in the relation feed size versus recovery.

This balance between cost for size reduction and value recovery is important to establish for high value ores, see below.

Leachants

Leaching of Metals

Below find the flow sheets for classical leaching circuits, heap leaching for coarse fractions (crushing only) of low grade ores and agitation leaching for finer fraction of high value ores.

In the Heap leaching process the ground is protected by a sealed surface, collecting the leaching chemicals, re- circulated by pumping. When the solution is “pregnant” its clarified by sedimentation or sand filtration and taken to metalrecovery by electrowinning.

In the Agitation leaching circuit the feed is finer (typical -200 microns) and the slurry moves in the same direction as the chemicals (con current flow). In this case, the pregnant solution has to be recovered from the solids by mechanical dewatering due to particle size

Gold Leaching

Enrichment by leaching is mainly used for recovery of gold often in combination with pre separation by gravity. If free coarse gold is released during size reduction, this fraction (typical – 1mm) is recovered in gravity spirals chutes. If finer fractions of free gold are present centrifuge technology can be applied (not covered here). Alternatively, when processing an ore containing gold metal only,leaching with carbon adsorption are frequently used.

Gold leaching - Carbon Adsorption

Leaching reaction

Process stages

1. Leaching by agitation to dissolve Au by reaction above. Leaching Agitators are arranged in con current flow.

CIL (Carbon in leach) is a method where the gold adsorption by carbon is done in the leaching circuit. The method, seldom used due to high operating costs, is not covered here

2. CIP (Carbon in pulp) adsorption by slow agitation using active carbon granules to adsorb the Au solution from the pulp. Agitators are arranged in counter current flow (carbon travelling towards the pulp flow). Carbon granules must be:

a. Hard to resist abrasion (made from coconut shells)

b. Coarse to be separated from slurry by sizing (1-3 mm, 16-6 Mesh)

c. High in specific surface

3. Carbon recovery is done by sizing over a screen (cut at 0,7 mm, 24 Mesh) bringing the loaded carbon out of the pulp system

4. Au stripping is the process of removing the gold solution by “washing” the carbon granules in a solution (cyanide + Na OH) at 135°C. (retention time 6-8 hours).

5. Carbon reactivation is needed after the washing to restore the active surface of the granules. This is done in a kiln at 125° C

6. Cyanide destruction of the pulp leaving the CIP circuit is done in an agitator

adding an oxidant (typical hypocloride) bringing remaining cyanide to a harmless state.

7. Au electrowinning by plating out the gold metal is done with steel wool cathodes. These cathodes are sent to fluxing and smelting(8).