The procedures and methodology employed for determining the gold content in refined bullion and in fixing the purity of proof plate are important part of the Refinery performance. The assay pieces of correct mass are wrapped in lead cases of 4-5 g each, together with 40-45 mg copper and sufficient silver to bring its concentration to 2.5 times that of the gold. Cupellation is performed at 1040 oC for 25-30 minutes in magnesia cupels. The prills are then prepared for parting by hammering, rolling, annealing, followed by rolling into cornets. Parting is carried out by successive treatment with nitric acid solutions of specific gravities 1.2-1.3, respectively, followed by washing, annealing and mass determination. Electronic balances have replaced micro and free-swinging balances and electric furnaces give the appropriate control to the process. The outlet flue from each furnace passes to a condenser box and the required constant air supply of 0.20-0.30 m3/min could be controlled by a 10-15 mm diameter movable orifice. Fume from all furnaces must be ducted to a common manifold and extracted by a fan of 15-20 m3/min capacity.
In practice, assays of samples of mine bullion and fine gold are made in the same batch. Due to the composition of mine bullion varies, the ideal procedure of working assay pieces with proof samples of like composition is not practicable. The problem is solved by controlling all fires with assay pieces of high purity proof gold and applying appropriate corrections to the mean surcharge of the proof pieces. These corrections can be obtained from information derived from the results of numerous assays on gold/silver alloys of varying composition, prepared under the same rigidly controlled conditions prevailing in routine practice. The system of corrections is acceptable because the range of gold and silver contents of the majority of mine bullion is not always high, and control of operating procedures and parameters is very precise.
For instance, the average supercharge given by the proof pieces is 1.00 part per thousand. For mine bullion containing 87-90% gold, this average supercharge would be corrected by minus 0.15-0.18 part per thousand. Samples of mine bullion which do not fall within the normal range of composition, such as those containing excessive amount of base metal or those having appreciable osmiridum content are treated by special methods of assay, mainly by cupellation and parting. Dore bullion produced by the By-Products recovery, which has a gold content of 11-23% and silver content of 78-88%, also needs special assay. The normal 1000 millieme assay piece is given a double inquartation. Before cupellation, 750 millieme of proof gold with sufficient silver to give normal silver/gold ratio of 2.5 to 1 is added. Allowance is made for this gold in calculating the final result.
The silver content of the incoming bullion is determined by a fire assay in which 1000 millieme assay pieces are wrapped in pre-prepared 8.0-9.0 lead cases and cupelled for 40-45 minutes in magnesia cupels. The difference between this gold-cum-silver assay and the parted gold assay gives the silver content. Cupellation is controlled by proof pieces of high purity gold and secondary corrections are applied for the fineness of each of the several assay pieces worked in the batch. These corrections, like those applied in the assay of gold, must be derived from the results obtained from numerous experimental assays made on alloys of varying fineness. Due to the fact that base metals are not totally removed during cupellation and because the amount of lead retained in the cupelled button is irregular, the method is of limited value and cannot be applied successfully to all grades of mine bullion. If the base metal content of samples under assay is fairly high, a re-cupellation with 2-3 g of lead is needed. Samples of bullion containing large amounts of base metals and those having a higher silver content are best assayed by cupellation with like proof pieces.
The wet chemical method applied in the estimation of the silver content of crude silver chloride is of sufficient accuracy and precision for the determination of silver in mine bullion. However, being a multi operation procedure, it is not well suited for making large numbers of assays on a routine basis. Spectrometric techniques provide an accurate and much more rapid method of analysis. The Gay Lussac method could be employed to determine fine silver content, Dore bullion produced by the By-Products recovery area and other predominantly silver bullion samples. 0.1 N sodium chloride is adjusted to have a silver equivalent of approximately 998.5 mg per 100 mL. the exact silver equivalent of one discharge from each of the two pipettes used is determined by working proof pieces of fine silver with each batch of samples assayed.