By-products materials are constituted by materials with a bulk density of 2-3 g/cm3, which contain gold bullion particles with a density of up 17.5. To get representative samples from this material is not an easy task. A situation has to be attained whereby the gold particles are randomly distributed in the bulk and where the sample is representative. Theoretically, it is necessary to take 50 kg sample from a material assaying 1000 g/t and containing spherical particles of gold that just pass through 1.7 mm to ensure that 99% of the assays are within 2.5% of the mean. Sampling the materials when they are completely dry, leads to segregation of the gold particles, and this situation leads to wrong results. A 500 kg sample is rather impracticable, but in some refineries has been noted that moistening the materials to the point where they are still flow, causes the gold particles to adhere to the bulk and remain randomly distributed. Because the larger metallic particles have been flattened by crushing prior to sampling, a smaller sample is sufficient. In practice 40-45 kg sample has been found appropriate to provide good accuracy.
In this way, a good practice is to add 2-8% of water to the material according to its absorbency and type, after which it is given a preliminary mix. The consignment is then transferred onto a conveyor, which deliver it to an enclosed automatic riffle sampler. Basically, this consists of a series of compartments containing splitters that divides the material into two sample fractions (usually 1/64 each) and a bulk fraction. Moisture determinations are made from the two samples, while the bulk of the consignment goes forward to have its mass determined. The samples are now split, one third being taken from each and combined to form a referee sample. They are prepared for assay.
It is important to separate the sample into three fractions due to the large range in size and density. Usually, the + 0.6mm fraction metallics portion contains the larger gold beads, leaving the main sample more uniform in value. Once the assay and dry mass of the material have been determined, gold and silver contents can be calculated and the material can be treated. It has been noted that base bars and jewelers sweeps vary in composition. Generally such deposits are melted with or without fluxes such galena and borax, and dip samples taken. Any matte or slag formed is crushed and sampled separately. In this way, to assay materials for gold and silver, certain chemical components are determined, especially those which are important to subsequent furnace operations. Among these are silica, iron, lead, alumina, calcium oxide, copper, zinc, sulphur and arsenic.
By-Products Sample Preparation Flowsheet