The gold contained in an ore is usually occurs in different mineralogical form. Understanding the gold ore mineralogy and its deportment is an important stage for metallurgists and process design engineers to determine the best processing route and design the suitable flowsheets for a specific ore. As normally gold is present in an ore in very low concentration (ppm) and different forms, the challenge for gold deportment studies via instrumental mineralogical techniques is the fact that the gold particles are so few compared to the gangue minerals; so the sample might not be representative. Also the gold particles are sometimes so fine or part of the sulphide mineral structures that the mineralogy analytical tools and techniques cannot detect the gold accurately. Anglo American Research Laboratories (AARL) in an attempt to understand which minerals the gold is associated and to get at least some information on how to process the gold; developed diagnostic leach technique.
Diagnostic leaching is a practical laboratory procedure that assists with the understanding of the nature and occurrence of gold within ore samples or any type of plant product. This technique is used to quantify the extraction efficiency that can be achieved by various unit operations in an existing plant. Also the information gained from a diagnostic leach indicates what changes in the process parameters are necessary or whether the process should be modified for a specific reagent when anomalies in reagents consumptions happen in operation.
The technique involves sequentially liberating and subsequently leaching gold associated with specific minerals. The concept of diagnostic leaching is very simple. Typically the least stable mineral is first digested and eliminated using a selective oxidative leach followed by filtering, washing and drying prior to cyanidation of the gold liberated by the destruction of this mineral. The process is sequential thus the residue from this first stage will be subjected to a more oxidative acid leach and the process repeated. The resulting information is used to interpret the deportment of gold in the ore. The test results is used to determine the best process flow sheet for a new project or solve issues occurring at an existing operating plant. The pre-treatment stages are varied depending upon the mineralogy of the sample, the source and the process options available. The diagnostic leach yields an empirical mineralogical analysis with process information on a relatively large and hence representative sample whereas an image analysis yields a mineralogical analysis with inferred process information on a relatively small sample. This test can be performed on ROM ore samples (feed), concentrate and residue samples based on steps described in the following paragraphs (except bacterial oxidation residues that have slightly different steps).
It is important to note that diagnostic leaching does not replace mineralogical evaluations as it cannot selectively leach only one mineral. Diagnostic leaching can only suggest with which group of minerals the gold is associated. For example during the acid digestion steps it can be identified that the majority of the gold is associated either with hydrochloric acid digestible minerals (Pyrrhotite, calcite, dolomite, galena, goethite, calcium carbonate, could include calcine, hematite and ferrites) or with nitric acid digestible minerals (Pyrite, arsenopyrite and marcasite). Also it should be note on the following:
- Some Information on distribution of gold like speciation, grain size and mode of occurrence (liberation, exposure, and mineral association) can only be determined by means of instrumental mineralogical techniques like QEMSCAN/MLA.
- The results acquired from the two methods should be interpreted separately but should agree with each other in certain aspects.
- In the present of high preg-robbing material the results of two methods might be different.
- If gold is coarse grained or if it occurs as slow leaching minerals like gold tellurides, it may not be effectively recovered in the cyanidation after diagnostic leaching.
There are many reasons why gold is not easily recovered from its ore. The most common reasons would include one or a combination of the following:
- Refractory ore: An ore from which the gold cannot be optimally recovered through direct cyanidation.
- Preg-robbing: Gold occurs with organic carbon or carbonaceous material which absorb the gold during cyanidation.
- Chemical interferences: Gold occurs with minerals that cause uneconomically high reagent consumptions (oxygen, cyanide, etc.)
- Leach kinetics: Gold occurs with minerals that cause very slow cyanidation leach kinetics.
- Physical encapsulation: Gold is locked inside gangue minerals (usually sulphide minerals) which are not liberated even with fine grinding.
Diagnostic leaching approach provides information into the cause of the refractoriness of the ore by following these sequential processing steps (for feed ROM ore, concentrate and residue samples):
- Step 1: Free milling gold is determined by gravity concentration and direct cyanidation.
- Step 2: Free gold and Preg-robbing gold is determined by carbon-in-leach (CIL)
- Step 3: The resulting residue form step 2 is subjected to mild oxidative acid pre-leach (HCL) followed by CIL to determine gold associated with pyrrhotite, calcite, dolomite, haematite, etc.
- Step 4: The resulting residue form step 3 is subjected to another oxidative pre-leach (H2SO4) followed by CIL to determine gold associated with sphalerite and reactive pyrite and other labile copper sulfide minerals (not all the lab will perform this step).
- Step 5: The resulting residue form step 3 is subjected to severe oxidative pre-leach (pressure oxidation, bacterial oxidation, roasting) – HNO3 – followed by CIL to determine gold associated with pyrite, arsenopyrite, etc.
- Step 6: The resulting residue from previous step is subjected to complete oxidation (roasting) followed by CIL to find out the gold associated with carbonaceous material such as kerogen.
- The undissolved gold appearing in the residue is assumed to be associated with silicates or fine sulphides.
Some laboratories propose fire assay as a final step for gold locked in silicates whereas others use HF leach followed by assay of final residue.
The leach procedure for bacterial oxidation residues is slightly different and involves following sequential processing steps:
- Step 1: Same as other type of samples.
- Step 2: Same as other type of samples.
- Step 3: The resulting residue form step 2 is pre-leached in hot sodium carbonate followed by CIL to determine gold associated with sulphur, sulphates and jarosite etc.
- Step 4: The resulting residue form step 3 is subjected to severe oxidative pre-leach in hot aqua regia followed by CIL to determine gold associated with pyrite, arsenopyrite, etc.
- Step 5: Same as other type of samples.
- The undissolved gold appearing in the residue is assumed to be associated with silicates or fine sulfides.
Normally two diagnostic leach programmes are available in commercial laboratories:
- a) Procedural method – this is based on a standard technique developed in industry. In this case, the findings infer that certain digestion steps carried out on the ore liberate the gold associated with certain leachable minerals, however, no specific minerals are identified.
- b) Quantification method – this is based on the above mentioned technique but the minerals digested are defined by both mineralogical (including microprobe and MLA) and chemical analysis.
A minimum of 2 kg of sample required to complete diagnostic leach test.
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- SGS Technical Bulletin 2004-03
- SGS Technical Paper 2011-04