Important economic attributes of the metal-bearing material that affect income and cost for extraction include size of the deposit, metal grade, complexity, eventual co-products, metal value as well as existing infrastructure on site. Metals may be extracted by different technologies and methods within extractive metallurgy. (Also, factors, not related to the mineral resource)
Bioleaching competes with and complements other methods for metal extraction. Generally speaking, low grade ores are used for bioleaching and high grade ores are still more economical to treat using pyrometallurgical processes. Bioleaching becomes less economically competitive with an increase in grade as the ore becomes more suited to concentrating and smelting.
Heap leaching is ideally suited to small deposits, low grade ores and disseminated ores rather than massive sulfide ores. It is vital to maintain the structure of the heap in order for an even distribution of the air and solution flow throughout the heap. Massive sulfides have very little gangue mineralogy and so tend to collapse. When contaminants are present in the sulfide ore, heap leaching may become the only economical processing route due to the stringent environmental restrictions placed on smelters and roasters.
Heap leaching is a much slower process than pyrometallurgical processes and the metal values associated with the ore are recovered over a period of months compared to a period of hours.
United States Patent 6063158
"The bulk solids fed to a particular reaction process must meet the economic constraints of the selected system. Low grade (low metal content) sulfide ores are typically bioleached in dumps or lined heap piles, whereas low grade sulfide concentrates must be processed in low cost reaction vessels, such as passive vat leach tanks or, for higher grades, stir tanks or fluidized reaction vessels. Processing variables which further affect materials handling and processing costs are material feed size, whole ore or milled ore, metal content, solids retention time in reaction vessel (reaction rates), solid to liquid ratios, ie., slurry viscosities or pulp densities; and feed density, amenability of solids suspension (passive, stirring)."
Source: United States Patent 6063158