High quality assaying of precious metals is an important proof of the comptence of the laboratory
Gold and PGE’s
Ore grade assays of gold, palladium and platinum are performed by a high-precision classical Pb-Fire Assay methods using either 25 g or 50 g subsamples (704/705), combined with alternative finishes, FAAS, ICP-OES, gravimetry. If Au, Pd and Pt are to be analysed we recommend the Method 704P (or 705P).
Gravimetric determination after the fire assay (704G) gives the best precision and accuracy for high-grade (50 –1000 ppm) gold samples and low-level concentrates. For metallurgical samples Labtium recommends method 740G.
Fire Assay fusion method is very sensitive for matrix problems particularly with ore samples where the matrix can be challenging. This requires experience and practical skills from the laboratory staff. Special precautions need to be taken if samples contain appreciable amounts of graphite, S, As, Te, Se, Ni, Cu. For a sample with high concentrations of these metals a smaller subsample weight may have to be used deviating from the original request.
As a routine method for cyanide soluble gold Labtium recommends method 236A which involves the use of PAL1000 –machine. The method enables the simultaneous pulverising and cyanide leach (using Assay Tabs™-reagent) of crushed rock samples, percussion samples or soil samples. A 0,500 kg subsample can be used. The leaching is very effective due to aggressive leaching conditions which promote the liberation and breaking of gold nuggets. Graphite, organic matter (humus) and sulphides interfere in the cyanide leach, lowering the recovery of gold. The concentration of cyanide soluble free gold may also be evaluated using the traditional three hour bottle-roll tumbling with the Assay Tabs™- reagent (235A). This method requires pulverising of the sample before leaching. The results attained by this partial extraction are comparable to technical CIP- and CIL- extraction techniques and are of benefit in the metallurgical testing of the mineralisation. The method is not suitable when the total content of gold is needed.
Other methods for gold analysis include screen fire assay for coarse grained gold, diagnostic leaches to evaluate mode of occurrence of gold in different mineralogical phases and analysis of the free and refractory gold, which includes cyanide leach and analysis of the tailing and head, if requested, sample by fire assay (see Chemical phase analyses).
The most suitable method for silver is by acid digestion with aqua regia and finish with FAAS (510A/514A) or ICP-OES (510P/514P). That said,, there are potential limitations in the solubility of Ag in high concentrations i.e. Ag > 200ppm. Fire Assay and gravimetric finish (704G) with a more representative sample and better precision is recommended for high grade ore samples i.e. Ag > 50ppm. In addition to metallic gold, metallic silver or silver in electrum can be analysed with cyanide leach methods 235A and 236A .