There are some changes that metallurgical are used to consider in the grinding circuit. A change that sometimes is justified that that indicated by standardization of equipment. Basically, a continuous gradation of capital and operating costs as grinding Kw-h/t is usually considered at the moment of taking a decision. In practice, the actual capital cost curve may be somewhat irregular and equipment delivery time may also be a factor. It is not practical or necessary to take these irregularities into account to optimize the design, but the final decision may vary slightly in either direction because of such factors.
Since calculation of the grinding circuit capacity is not an exact science, it is common to apply a safety factor to the calculated requirements on the assumption that overdesign is a lesser evil that underdesign. The concept of an economic optimum grinding capacity in a gold flotation or cyanidation plant implies that either overdesign or underdesign will results in an economic loss relative to the optimum design. In this way, if the economic optimum grind is specified as the design criterion for grinding, it must be made clear that this is the target grind from which error in either direction is to be minimized, rather than a minimum specification that must be obtained or safely exceeded.
It is important to mention that grinding mills are designed for operation at a moderate speed, with provision for later increasing the speed to provide additional capacity at some increase in operating and maintenance costs. In such cases, it must be recognized that practically the total capital cost for the higher capacity is incurred at the start. Whether it is ever used or not. In some cases, particularly where the new plant will mill ore similar to that being milled in an existing plant, the existing operation may be useful as a check. If there are major differences, between the existing plant and the new plant design or plant expansion, the reasons for them must be analyzed. Nevertheless, in designing any new plant, it must not be assumed that any existing plant represents the economic optimum design, especially with regard to grinding capacity. Optimized operation is not equivalent to optimized design.
In the case of an anticipated future decrease in mill feed rate and increase in rate, overall changes and optimization may well indicate a constant grinding capacity in terms of installed Kilowatts and equipment, but not in terms of feed tonnage.