Material that is relatively high grade may be left unmined in some awkward spot extending back too deeply beneath waste. The typical large open pit mining operation that has been in production for 10 years and more is operating under conditions that could not possibly have been foreseen by the original planners of the mine. Metal prices, machinery, and milling methods are constantly changing so that the larger operations must be periodically reevaluated, and several have been completely redeveloped from time to time as entirely different kinds of mining and milling operations. Sometimes the preliminary stripping of the waste overburden is contracted to firms specializing in earthmoving. Scrapers are sometimes used in special situations.
Haulage is usually by truck, although railroads, inclined rails, and conveyor belts have been used.
The conveyance unloads directly into a primary crusher and crushed material is stored in coarse ore bins prior to shipment to the mill. Bench level intervals are to a large measure determined by the type of shovel or loader used, and these are selected on the basis of the character of the ore and the manner in which it breaks upon blasting and supports itself on the working face. Bulk explosives are loaded in the holes and large volumes of ore are broken in a single blast. Sometimes the drill holes are routinely sampled and assayed to help plan the position of the shovels in advance of mining. Leaching Methods Solution mining techniques are used for extracting soluble ores such as potash and salt in situations where conventional mining methods would not be economic.