In practice the electrowinning step becomes easier with high silver because a stainless steel wool cathode can be used. The deposited silver/gold slime is simply washed off the wool, filtered, dried and directly smelted with minimum flux usage. New cell designs also allow metal sludge removal without taking the cathode out of the electrowinning cell, thus reducing labour input. The final flowsheet selection is made through the economic comparison of capital and operating costs for the various options. The factors included in the assessment have previously depended very much on the degree of understanding of the metallurgical process. The following sections describe the testwork and modelling required to make a comprehensive assessment of the f!owsheet options. A comprehensive computer model has been developed to study and optimise the CIP flowsheet. (Ref 2). The model includes the leach, adsorption and elution processes using data generated in specifically designed testwork. Leach and carbon design parameters are derived from the testwork which are fed directly into the process model. Capital and operating information is contained within the model which then allows a full optimisation study to be carried out for the cyanidation and metal recovery areas. Variables examined are Leach time (number and size of leach tanks); Adsorption stage (number and size of tanks); Degree of integration between CIP and CIL; Carbon concentrations and loadings and Elution plant size. The specific testwork techniques to derive the appropriate model parameters are now well established. The laboratory scale test equipment has proven scaleup factors which have been successfully demonstrated on numerous plants, with both high and low silver values. A bulk leach test is first undertaken to produce sufficient sample for the adsorption testwork. The leach curve derived from this test is described by an equation which demonstrates the fast and slow leaching and unleachable components of the ore. Testwork & Modelling
The adsorption testwork examines the carbon kinetics, equilibrium loadings and fouling factors for both gold and silver. The derived data is then used along with the leach parameters to optimise the circuit design. Gold and silver are examined independently. The silver flowsheet is first optimised as this tends to determine the size of the plant. The gold parameters are then checked to ensure acceptable performance. The Merrill-Crowe flowsheet is not modelled in the same ways since its' performance is fairly well fixed by a zinc consumption rate and metal recovery percentage. Testwork is carried out to confirm the precipitation parameters and to check for problem elements, such as copper.
Process Selection
The carbon option is often eliminated because of high capital and operating costs. Two major misconceptions have significant effects on the final costings and therefore the final flowsheet selection. These refer to carbon loadings and plant sizing.
Carbon Loading
The size of the elution plant is determined by the silver loading assumed for the loaded carbon. For conservative reasons, design silver loadings on carbon have traditionally been kept below 7500g/t, whereas in practice levels of two or three times that are achieved. If higher loadings are used for design, then the plant size can be proportionally reduced, thus reducing the plant and operating costs. The capital and operating costs for the recovery plant are made up of a contribution from several areas: Acid wash, Elution, Electro-winning, Smelting and Regeneration. Traditionally it has been the practice to size each individual area to match the carbon elution rate. This, however, is not logical since different factors affect different areas, especially when related to high metal values. Plant Sizing
Acid Washing removes inorganic fouling of the carbon. The level of fouling decreases as the turnaround time increases at
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