The Kamyr countercurrent tower leach process consists of a true countercurrent leach with a flocculated ore and fibre mixture entering the top of the tower and a cyanide solution or alternative lixiviant as well as a barren wash solution entering the bottom of the tower.
The Kamyr countercurrent tower leaching system has been developed for processing gold, silver and other leachable ores. A wide range of ore sizes may be treated from as coarse as minus 300 micron or finer than minus 75 micron.
The Kamyr system can be used for any hydrometallurgical process in which ore is contacted with chemical solutions. It is especially suited for those processes where heap or vat leaching or agitation leaching methods are currently practiced. The Kamyr system can even be designed for processes which require elevated temperatures and pressures. Initial development of the Kamyr leaching system has concentrated on gold and silver cyanidation.
In this system, the ore is fed to the top of a column and moves downward in the column while the leach solution moves upward, resulting in true countercurrent leaching. This technology offers certain significant advantages over percolation leaching and agitation leaching systems. The fresh leaching solution is in contact with the ore just before discharge, enabling more effective leaching of gold residual values than can be obtained in co-current processes.
Indications are that the capital required for a Kamyr tower leach process is substantially less than that required for conventional Merril Crowe and CIP type processes. Figure 1 illustrates how this new generation process compares with the other two conventional processes.
The objective of this demonstration plant is to prove the process of gold extraction developed in the laboratory on a semi full scale plant. Scale-up factors and operating costs required for a full scale plant are also to be obtained. Once oxygen is introduced, this plant will be utilised to increase the treatment rate five-fold. All information and results obtained from this plant will be compared and evaluated with that of more conventional plants.
Process Description
The heart of the Kamyr leaching system is the leaching tower as schematically shown in Figure 2.
In this process, the ore slurry is first mixed with a small amount of wood fibre. This ore-fibre mixture is fed to a drum mixer at the top of the leaching tower where flocculant is added. These additives serve to make the ore behave as a cohesive but permeable mass, so that countercurrent flow of solids and solution is possible. This mass is then discharged from the bottom of the tower after the metal values have been leached. At the same time, a barren cyanide solution is fed to the bottom of the tower and the pregnant solution is removed at the top by an overflow weir.
A definite liquid-solid interface is maintained near the top of the tower. The flocculant prevents colloidal particles from leaving the ore mass so that a clear pregnant solution is obtained at the top of the tower.
Inside the leaching tower, the flocculated ore-fibre mixture moves downward as a plug. The leaching solution moves upward countercurrently, leaching the gold and silver. The controlled upward flow of leaching solution also prevents the ore-fibre floes from settling to a state of compression. This allows the solution to diffuse uniformly through the solids bed and to thoroughly contact the ore particles.
At the bottom of the tower is a rotating arm mechanism with a hollow shaft. The barren cyanide solution with a high dissolved oxygen content is pumped through the hollow shaft and enters the tower through solution inlet nozzles which are distributed across the upper rotating arm. As the arm rotates, the leaching solution is distributed across the entire cross-section of the tower and begins to flow up through the ore bed.
The lower rotating arm also has inlet nozzles to supply wash water which removes most of the cyanide solution from the ore slurry. The bottom section of the lower rotating arm includes a rake that serves to discharge the ore-fibre mixture.
Figure 3 illustrates the flow of material around the tower.
The clear solution, at the top of the tower consists partly of feed solution that disengages from the strongly flocculated pulp and partly of the column upflow solution, then overflows into a weir at the top of the column and advances to the metal recovery section. A conventional carbon adsorption system, zinc precipitation, or any other metal recovery system may be utilized for this purpose.
After recovery of metal, the barren solution is then oxygenated, cyanide concentration and pH are adjusted, and the solution is recycled to the bottom of the leach tower.