It is important to understand the different related to operating costs of gold projects and to learn how these costs are widely variable due to process recovery, gold bearing minerals, and accounting methods used to calculate the cost. It has been detected in several gold projects that confusion may result in making cost comparisons even with current information from different operations without complete knowledge of the factors involved in their calculation. The totals costs of an operation are usually classified into three classes: direct, indirect and fixed. These are further subcategorized into mining and milling costs, and these are subcategorized. Basically, the distribution of operating cost expenses to the mining and processing operations must necessarily vary due to they are variable. For these reasons, real comparisons can only be performed from detailed accounting information. This is especially true when the designer tray to compare unit processing costs such as crushing, grinding, flotation or cyanidation.
For the gold ore processing plant some costs are usually considered as direct operating costs, although in accounting records these are classified in different ways such as by unit operations (e.g. crushing, cyanidation) or by commodity (e.g. labor, supplies). In general direct operating cost comprises the following items: operating labor, maintenance, operating supplies, maintenance supplies and utilities such as power, fuel and water. Indirect costs comprise the following items: overhead labor (e.g. supervision, accounting, warehousing), supplies (e.g. health, safety, water treatment), utilities, payroll overhead (e.g. social security, paid vacations, etc) and taxes. It is important to mention that the cost of amortization the investment is considered as a fixed cost. Some items of taxes may be included in this class. The usual criterion for classification as a fixed cost is that the expense is constant regardless of the continuity of the operation.
The total costs of producing the final gold concentrate or Dore bar included expenses of mining, ore haulage, and processing cost. Depending on the accountability procedure, the indirect costs are usually combined with the direct costs and the total given as the direct costs of production. Considering the unit cost at any given throughput rate, the cost in any event declining with increasing the processing capacity, approaching some minimum value that depends upon certain basic, irreducible costs per ton of ore. Mill operating cost may be expressed either in terms of cost per ton of ore or cost per ounce of gold produced. Essentially, much of the direct expense is incurred in proportion to the tons of ore processed. Higher grade ore cost somewhat more per ton to process than low grade due to reagents and handling costs in the latter stages of the processing process are in proportion to quantity of gold involved. Gold recovery tend to increase with ore grade and the processing costs expressed on a per-ounce-of-gold basis, varies in inverse proportion to the gold ore grade.
Other factors that may affect the operating costs include the gold ore properties, the recovery process used and the efficiency with which is applied, cost of utilities, reagents, condition of the processing plant and the cost of labor supply. Also, extra special costs might include such items as the need for cyanide destruction and for any other measures to avoid pollution of the environment by effluents. The characteristics of the ore which affect the process cost comprise ore hardness, the fineness of primary grind needed for satisfactory gold dissolution, the presence of cyanicides and the presence of any mineral or compound that interfere with the recovery process. Depending on the process, they could be oxide copper minerals, carbonaceous matter, or clays. Hard ores and those requiring fine grinding add to the process cost due to increase the power and steel consumption in crushing and grinding. For example, in some operations in America the steel consumption during grinding ranges from 0.05 kg/t for loosely sandstone ores to about 0.5 kg/t for harder ores. The presence of quartz in the deposit tends to increase the steel consumption.