Gold Pregnant solution containment

The pregnant solution contains the metal values and thus it is economically imperative that no leakage takes place from the pregnant pond. A liner system similar to that used for the barren ponds usually employed. A low permeability line r is required to contain the solution for economic values as well as potential environmental impact caused by solution releases. It is common practice to place the barren solution and pregnant solution ponds adjacent to each other. This confines the large volumes of solution to one area of the facility and reduces construction and operational costs.

 Interconnection of the pond also allows the combined pond volume to be available for drain dam operation and design storm event conditions.

The pregnant solution pond is directly connected to the heap. As such, drainage from the heap of both the applied leachate and direct precipitation will take place. The design of the pregnant pond, therefore, requires the inclusion of project hydrology, i.e. storm events.

In the case of heap designed to retain solution such as valley leaches, the pregnant solution is stored within the space between the ore particles. The liner and heap earthwork are required to function as water retaining structures in this instance. Abstraction of solution for processing or recycling to the heap can be performed by pumping of sumps within the heap or drainage from low points within the heap.

Metal recovery.

The metal recovery plant consists of:

• A system to remove the precious metals from the solution, and
• Facilities for smelting of the metal.

A schematic of potential metal recovery circuit is shown on Figure 1.3. Recovery can be through a process of zinc precipitation (Merrill-Crowe Process) or through carbon adsorption.

Barren solution containment.

The barren solution pond, like the pregnant solution pond, must have a low permeability liner for containment purposes. The solution must be contained for economic reasons (chemical make-up, water make-up, etc.), as well as for the minimization potential environmental hazards caused by solution releases. Alternative containments such as tanks have been used; however, such alternatives are generally more appropriate for small operations where solution volumes are not large.

The barren solution pond contains the leachate after metal removal in the recovery plant and so liquid in this pond contains the residual chemicals used in the process. In addition, the pH has been adjusted to about 10, and cyanide is present. Cyanide is nor necessarily present at the level required for leaching and is usually added prior to discharge to the barren pond or in the pipeline to the heap. Although the latter can be successful, addition in the barren pond results in more consistent cyanide levels in the leach solution.