Without any doubt the relation of fineness of primary grind to the degree of liberation of the gold bearing minerals from the gangue is factor to be considered in optimizing the grinding circuit. This aspect largely determines the grind-recovery curve and the point of maximum gold recovery that is usually at a very fine grind. Nevertheless, it is obviously not the only factor to be studied. Essentially, how closely the optimum grind approaches the maximum recovery grind is influenced by sacral factors such as grindability, operating costs, capital cost, product value or mining factors. The lower the resistance to grinding, the finer will be the optimum grind. The resistance is measured by the Bond ball mill work index for several different meshes of grind.
If the operating costs per kilowatt of grinding capacity are lower. Then the closer the optimum grind will be to the maximum recovery grind. In this way the major factors that have influence on the operating cost per kilowatt of capacity are the unit cost of electrical energy and the unit cost of grinding media. It has been noted that low capital cost per kilowatt of grinding capacity tends to favor a finer grind, more closely approaching the maximum recovery grind. Also, the higher the unit market price of gold and silver in the concentrate, the closer the optimum design grind will approach the maximum recovery grind.
Regarding the mining aspects, the optimum grind for a specific gold project is related to the cut off grade and the life of the project. Many times, for simplicity it is considered that these factors are vary variable and the final projection must be done during the operation. In any gold ore processing plant, considerable fluctuation usually occurs in gold grade, grindability and fineness of dissemination. Nevertheless, since grinding is the most expensive unit operation in a gold concentrator, the grinding capacity is usually designed for average rather than extreme conditions, with provisions for varying the processing rate and fineness of grind to utilize the capacity of the primary grinding circuit as fully as possible as the gold ore varies.
Basically, the degree of variation of the gold ore characteristics is a factor many times overlooked during the design of the circuit. The auxiliary equipment in the grinding section and the rest of equipment in the plant should be designed to handle the range of process flows that will result from operating the grinding circuit at maximum efficiency on variable gold ore. Other considerations have a minor influence on the optimum grind. In some cases, some of these may have major significance and must be considered. Grinding is a flexible process treating variable ore under changing conditions and the fuller recognition of the variable nature in project planning may lead to improved optimization of project design. The effective optimization requires close cooperation among specialists on project planning, economics, geology, metallurgy and marketing.