In practice, the mill feed varies considerably in gold grade and grindability from hour to hour and from year to year and any grinding circuit must be optimized with certain flexibility in throughput. This variability is recognized in operation and some provision is made in design for some variations in feed rate. Nevertheless, there are considerable potential for improving optimization of grinding design, and of the overall project, by entire recognition and utilization of ore variability and concentrator or cyanidation circuit flexibility in the planning stage. Since the optimum grind, mill feed rate, flotation time, leaching time, and reagents dosage depend on the ore grade, grindability and operation control, the optimization must be focused on these factors. If each type of gold ore can be treated efficiently at its optimum feed rate, grind and other conditions, greater profit may results it the ore was blended and prepared properly. The greater the variability of the ore, the greater the potential advantage of this approach.
In cases where the low grade gold ore is harder to grind than the higher grade ore, the advantage of separate processing would be enhanced. In this way, the grinding design must be optimized on a dynamic basis, with continuously changing conditions and evaluating the possible blending before milling. Where nature has provided a partial pre-concentration of the gold ore, it is important to take advantage of it. The concept of designing for variable operation runs somewhat counter to the idea that steady and easily controlled operation means efficient operation. The better control system, the more advantage could be taken of variability of the ore and flexibility of the equipment.
Then, considering a flexible design idea, the grinding section could be optimized and rated for design purposes in terms of kilowatts capacity and nominal tons per day of feed. The crushing and material handling must be designed considering the probable range of throughput that would result continuously optimizing the plant feed rate. It has been noted that a special case for flexible design is when the mill feed grade is expected to decrease significantly after a number of years of operation. In such cases, it is assumed that any provision must be made to increase the grinding capacity at that time in order to maintain the gold production rate with constant targets such as grind size and gold recovery. The flexible design implies that overall project optimization such as on a Net Present Value basis may require no additional capacity at that time, except possible mining, crushing and tailings disposal.