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GOLD HEAP LEACHING STUDIES: MECHANISMS AND NEW EXTRACTION TECHNIQUES

Objective

The objective is to improve heap leaching kinetics and gold recovery yields.
The processing of low grade ores by the heap leaching method suffers from an insufficient knowledge of its mechanisms and many operations simply fail due to a lack of understanding of the phenomena involved.
Research was carried out in order to improve heap leaching kinetics and precious metals recovery.
The Rouez en Champagne Gold mine provided a source of data for statistical analysis and was the site for field tests on the process modifications developed from the initial data analysis.

The gold deposit is a gossan located over a massive orebody of sulphides. Today total gold production exceeds 2 tons of refined gold out of 250000 metric tons of oxidized ore.

Pure statistical work lead to the following hypotheses:
a heap reacts to cyanide solutions with a delay time of 4 days (gold extracted from a heap depends on the cyanide concentration of the leaching solution 4 days earlier);
submitted to cyanide leaching a heap progressively reaches an equilibrium state where gold extraction becomes very low;
zinc introduced by the Merryll-Crowe process is a very good tracer of internal chemical activity of the heap (it accumulates inside the heap).

Experiments were then carried out to see whether optimization of the surface flowrate of sprayed solution could improve the heap leaching process. A hypothesis was put forward that spraying surfacic flowrate could be too strong and that part of the solution could percolate through the heap without any leaching action. Measured parameters at the outlet could be masked by the influence of the nonefficient part of the sprayed solution.
Experimentation has shown that overspraying of the heap leads to poor leaching efficiency and masks outlet data. It was also shown that zinc grade in pregnant solution and used as tracer leads to the optimization of the surfacic flowrate of sprayed solutions.

An important part of the gossan is composed of silicified breaches. Column tests run on this type of ore after standard crushi ng has given gold recovery of approximately 50%. Studies on breachic ore have shown that gold is trapped in microfractures filled with calcite. Instead of freeing gold by increasing crushing, washing out calcite using polyacrylates has given gold recovery of 62% on average.
Heap leaching processing of low grade gold ores suffers from an insufficient knowledge of its mechanisms and there are many examples of failed operations in small scale mining. The only heap leaching operation in western Europe (the French Rouez mine) will be studied, comparing geological and mineralogical observations to leaching parameters and to the conventional gold extraction techniques results (on activated carbon followed by an elution and zinc reduction).

The different work milestones and targets are as follows.

Firstly, the mineralogical study involving: open pit ore sampling; ore sampling during construction of the heap; and ore sampling during the leaching cycle. Visual observations mineralogical and chemical analyses to be able to monitor adequately all geology, mineralogy, physicochemical and mineralogical changes. More precisely the following studies will be undertaken: mineralogical studies; gold microspatial relationships; and gold accompanying elements. The following measurements will be carried out: IM, scanning electron microscopy-energy dispersive X-ray analysis(SEM-EDAX), IMMA, Mossbauer. Data interpretation will mainly aim at a better understanding of the changes in components during the leaching process.

Secondly, preliminary laboratory and full industrial scale experimentation involving the following.
Systematic sampling of palletized ore and laboratory analysis.
Follow up and inspection of palletized operation and construction of the heap.
Follow up and inspection of heap leaching operation by plotting graphs indication on precious metals extraction, contents and characteristics of solutions, etc.
Sampling of ore during leaching process in order to verify extraction performances of precious metals.
Comparative tests of static leaching in laboratory column in order to: determine new hydraulic binders and new reagents so as to reduce the presence of carbonates, improve heap permeability, and use wind and frost resistant spraying systems; determine a carbonate elimination system (electrophysical procedure); and conduct other experiments based on the latest knowledge of the elements.

Thirdly, heap leaching interpretation involving:
study of the leaching impact in terms of mineralogy and metalogenia and establishment of corresponding relationships;
data interpretation and comparison with known causes capable of influencing leaching;
study of the real dependent relationships between variables in the heap leaching operation regarding recuperation capacities of precious metals;
and study of corresponding relationships.
The studies require data processing methods such as principal component analysis(PCA), cluster analysis, and multiple and partial regression analysis.

Fourthly, the feasibility study. At the end of the research work, a synthesis of the work conducted will incorporate the most salient industrial points into a technical and economical feasibility study.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Sofimines-Parmines France
Address
Chante Pie - Rouez En Champagne
72400 Sillé-le-guillaume
France

Participants (3)

Euromin Srl
Italy
Address
Via Donota 1
34121 Trieste
Géosum SARL
France
Address
24 Bis Rue Durival
54000 Nancy
Institut National Polytechnique de Lorraine
France
Address
Rue Du Doyen Marcel Roubault
54501 Vandoeuvre-les-nancy