In most cases gold mining operations have used power purchased from public utilities wherever it was feasible to do so. For example, at one gold operation this entailed building of a 50-mile 60,000-volt transmission line. Usually, various primary voltages such as 10,000; 30,000; 40,000 and 50,000 volts have been used. At plant substations these may be transformed to secondary voltages such as 4,000 or 2,000 volts for very large motors, such as for grinding mills, and to 440 volts for all other motors, except those of fractional horsepower. Basically, all voltage references are for 3-phase power. It has been noted that the cost of purchased power is influenced upon such factors as average demand, peak load, power factor, and amortization of special lines and equipment installed for the customer at the expense of the power company.
At gold mining companies using purchased power, it has been common practice to install stand-by diesel electric generators, large enough to provide power for agitators, thickeners, lighting, and for any other service needing and emergency power supply in the event of an interruption to the main source of power. These units can be started and put on the line within a few minutes. Means of starting these stand-by diesel engines have included electric starts that run with permanently charged storage batteries, gasoline engines, hydraulic motors and compressed air. Steam-turbine generators also have been an interesting alternative.
Essentially, self-generated power has been employed in some gold plants, usually because of the remoteness of their locations, lack of sufficient public power supply in the area or an abundant source of low-cost fuel such as natural gas. Some published indicators on capacities of these plants have ranged from 125 kilowatts for a small upgrading plant to about 10,000 kilowatts for a large plant installation. For example a large self-generation facility in Canada includes seven diesel engine driven 1,000-kilowatt generators housed in a central building along with steam generating and air compressing equipment. The 1,500 HP engines have been operated with a mixture of gas and diesel fuel.
Most of the power needed by gold mills is for crushing and grinding, and most of the variation encountered will be due to these functions rather than to the type of process employed. Obviously, variations in gold ore hardness and in the fineness of grind required for satisfactory exposure of the gold-bearing minerals will account for much of this difference. In some gold plants where limestone has been crushed and ground for neutralization, a substantial amount of extra power has been required. In this way, where public utilities are not immediately to the mill site, studies must be made to determine the most practicable means of providing power. The extent to which outside services and sources of supplies are available will determine how extensive the maintenance shop and crew has to be, as well as the size of the repair parts inventory and other operating supplies which must be kept on hand.