It is a good practice to classify samples in two groups according to their particle size. The first group consists of fines 100% passing 125 µm, and the second group comprises metallics coarser than 125 µm and fines 100% passing 125 µm. The minimum number of assays made on the -125 microns fines in each lot sampled can be four, two on the first group and two on the second group. The mass of the assay portions of the fines and the fluxes employed for each of the several classes of material are 30-35 g and the reducing power of the charge is normally adjusted to give 50-60 g lead button. In the case of slags, which are very low in gold and silver, a larger lead button is required for efficient collection of gold and silver.
Basically, the treatment of metallics is based on their nature, bulk and particle size. Some are subjected to fusion assay without pre-treatment. Others which are bulk and have a high iron content, are given a preliminary treatment with dilute sulphuric acid to remove most iron, the filtered residue being fluxed and fused in the usual way. Metallics that consist of copper or the precious metals are best dealt by melting to produce and ingot. Highly refractory metallics such as those containing tungsten carbide particles are scorified and rescorified with large portions of lead. Fusion is carried out in an electric furnace at a temperature of 1,200 oC. The cold crucibles containing the fluxed charges are ranged on the furnace hearth which can be lowered by a special system for the purpose of inspecting the progress of the fusion process.
When the fusion is complete, each assay is given a wash with a mixture of litharge and maize metals. Once this reaction is complete and the melt is again in a state of quiet fusion, the process is complete. The assayer responsible for the work is required to check each metal as it is poured and, after the melt has solidified, examine the slag and the lead button from each melt in order to be sure that the fusion has been complete and effective. Cupellation is performed as low temperature as possible in order to avoid underassessment of the silver content of the samples. Proof control of the cupellation process is resorted to when working samples of high silver content. Materials in high copper such copper sweep from cementation tanks and copper matte needs special treatment. It is important to mention that copper sweeps contains up to 65% copper, which is partially oxidized and ha slow gold and high silver assay. An aliquot of 10-15 g is treated with dilute sulphuric acid to remove the bulk of copper, filtered, dried and fused with a flux prepared with 20-25 g sodium carbonate, 10-15 g borax, 55-65 g litharge and 2-5 g maize meal.
Concentrate copper matte that contains 28-38% copper, 18-22% sulphur, 1.0-1.5% silver and a low gold content is assayed with 25-35 g sodium carbonate, 15-20 g borax, 100-120 g litharge, 5-10 g silica and 1-2 g maize meal. Due to the nature of the contaminants and because of the high copper content, slag retention and cupel absorption of gold and silver are both high and irregular. Since the ratio of silver to gold in the cupelled prills is extremely high, it is almost impossible to decant the nitric acid and wash the parted gold residue without losing some gold in these decantations. Then, slags and cupels are crushed, fluxed, fused, cupelled and parted. The original parting liquor is diluted with distilled water and few drops of a very dilute solution of ammonium sulphide. This precipitates a portion of the silver as silver sulphide that acts as collector of any gold particles. The liquor is filtered, the residue dried, and burnt to ash. The burnt residue is wrapped in lead foil and cupelled at 1,100-1,150 oC. The cupelled bead is parted and the mass of gold residue measured. The mass of gold and silver recovered from the slags and cupels and the mass of the gold recovered from the parting solutions are added to the masses of gold and the silver obtained in the prills to give the final result.