This report reviews published literature concerning the particle-size distribution of gold occurring in alluvial and bed-rock deposits and the various methods used for its recovery. The aim of this review is to recommend methods for the recovery of fine-grained gold (generally less than 100 µm in size) as an alternative to the environmentally damaging use of mercury amalgamation. This work has been carried out as part of an ODA I BGS Technology Development Research (TDR) project R6226 \"Mitigation of mining-related mercury pollution hazards\".
Information on the particle-size distribution of gold is sparsely scattered throughout the literature and presented in various different forms. Many gold assays are carried out on sieved heavy mineral concentrates (HMC's). This is of limited use, to the current review, as coarse-grained gold present in the ore will not be included in the analysis. This is simply due to the practicalities involved, often samples were pre-sieved on site to remove coarse material (for example >2 mm) and only the fine material was evaluated. Often laboratory evaluations will be geared toward the use of specific gold recovery techniques. Many of these techniques have their effectiveness limited to specific size ranges. Therefore the evaluation will often only be applicable to gold within that size range, regardless of the presence of finer (or coarser) gold.
Even where a complete gold particle-size distribution is quoted the information should be treated with caution. Gold generally occurs in minute amounts which requires (for statistical accuracy) large bulk samples to be collected for size analysis, especially to determine weight percentages of coarse gold grains (>500 µm). Various physical methods are used in order to separate gold for analysis, most commonly laboratory-scale gravity separators.
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