PART I PROJECT DEVELOPMENT
3. METHODOLOGY FOR STUDYING GOLD MINERALS
Gold minerals are few, with somewhat unique properties (high specific gravity, brightness and average atomic number) making it relatively easy to pre-concentrate them for characterization and the identification under the optical and electron microscopes. For quantitative gold deportment investi- gations, it is important to remember the different forms of gold, measure each gold form independently and make sure limitations of the measuring method/
device do not compromise results, or at least be aware of that possibility.
Diagnostic leaching (Tumilyet al., 1987;Lorenzen, 1995), the first method for complete gold disposition evaluations, was warmly embraced by industry and metallurgical laboratories because of its simplicity, speed, need for spe- cialized instrumentation and relatively low cost. It consists of a series of cyanidation steps in between a series of progressively more aggressive acid-digestion steps. Thus, it apportions a gold assay into water-soluble, cyanidable-exposed gold, and gold enclosed in carbonates, in sulfides and in silicates (see also Chapter 4)
Gravity-recoverable gold (GRG) tests at three successively finer grinds coupled with automated image analysis (AIA) search for gold minerals in each of the three gravity concentrates and the gravity tails (Guerneyet al., 2003) and provides information on the gravity-recoverable gold at different grind finenesses, the gold grain size distribution, as well as their association and composition.
Gravity concentration using heavy media, screening of the heavy mineral fraction followed by microscopist-assisted or automated search for gold minerals and assaying of thelightfraction (Zhouet al., 2004) is a variant of the GRG approach. In this method gold is differentiated into: free, visible gold associated with sulfide minerals and the heavier rock/sulfide binaries;
and gold associated with rock and light binaries.
Gold deportments address all forms of gold as part of the same procedure.
Unlike the other methods, each form is independently assessed (not by dif- ference unlike the other methods) and where possible are confirmed using a complementary method. Special care is exercised to not miss important frac- tions of gold, such as free gold in the slimes fraction. Different deportment procedures have been developed to address the specifics of different gold beneficiation processes. Thus, the term goldcarriers has been introduced to better characterize gold losses in flotation tails. A typical example of clas- sification of unfloated gold would be: free gold of floatable size classes (o100, 45mm), free gold in the slimes (o5mm), gold associated with val- uable sulfides, with pyrite, with middlings and with rock mineral particles;
submicroscopic gold in pyrite and arsenopyrite (see Fig. 27). The latter determination includes, when required, differentiation between solid-solution
Mineralogical investigation of gold ores 55
and colloidal-size (o0.5mm) particulate gold (which is pertinent to a UFG/
CIL follow-up treatment of the concentrate) and the measurement of any significant variation in submicron gold concentration between the pyrite and arsenopyrite morphological types, which can exhibit very different degrees of liberation in grinding and respond differently to flotation. Good examples are the high/low arsenic and gold pyrites of Olympias (Greece) and the nu- merous Carlin-type ores.
Additional characterization includes the size distribution of the associated gold minerals and determining if there is a specific target grind for liberation, as well as the identification of surface contaminants, which hampered re- covery of free gold of floatable size classes. A more recent tool to help find ways to augment the recovery of free gold is the matching of a secondary collector to the rejected gold grain composition (Chryssoulis et al., 2003a).
In the case of leach tails the following types of gold are used to characterize the unleached gold: water-soluble gold salts; free and exposed cyanidable gold; enclosed cyanidable gold (including determination of a target grind to expose such gold); submicroscopic gold in sulfides and surface gold on car- bonaceous matter. If the sample has a high pyrite or clay mineral content then surface gold is measured on both. Surface contaminants interfering with gold extraction are determined directly by analyzing the surfaces of residual
Free gold/electrum
>5 µm
0 0.05 0.1 0.15 0.2 Au (g/t)
Final Tails 0.195g Au/t
8%
50%
<5 µm
9%
7%
24%
Free pyrite Magnetite
Rock/sulphide binaries
Rock minerals
Forms and Carriers of Gold Improving Au recovery
understanding/enhancing free gold flotation kinetics
scavenging for regrind (assuming regrind capacity availability)
Fig. 27. Typical graphic illustration of gold disposition in flotation tails, tailored as required to address specific issues.
S.L. Chryssoulis and J. McMullen 56
free gold grains. Speciation of surface gold on carbonaceous matter provides valuable information on the mechanism of gold dissolution and the source of the unwanted gold ligand.
In the case of pre-oxidized samples the following additional carriers of unrecovered gold are evaluated: gold encapsulated by calcium sulfate (and for certain processes elemental sulfur), gold in precipitated iron hydroxyl-oxides and on oxidized portions of grinding media (seeFig. 28), as well as gold in roaster calcine particles of distinct morphological and compositional type.
The gold deportment procedure was developed to provide a clear and accurate picture of the disposition of the unrecovered gold in tailing samples, provide a ranking of the causes for the observed losses, identify the means for augmenting recovery and set a realistic target for the tails grade. An abbre- viated version is used for studying gold deportments in feed samples to pre- dict metallurgical behaviour.
Given the low grade of the tailings, large samples (1–100 kg, depending on gold assay) are used, with particular attention paid to characterizing gold in the slimes fraction. This provides accurate assessments of free gold losses within and below optimum floatable size classes. The main drawbacks of gold Fig. 28. Gold plating on completely oxidized high-chrome steel from the grinding media.
Inset: high resolution SEM image of plated gold showing concentric growth bands.
Sample collected from Neutralization Tank No. 1. Gold plates selectively on areas where a chromium sulfate salt is more abundant.
Mineralogical investigation of gold ores 57
deportment studies is that they are time consuming, involving several steps and a number of microprobe techniques. These are discussed in the next section.