The term gangue is referred to non-valuable minerals present in the ore. These minerals are variable and the potential is related to the specific mineral type. Carbonates are characterized by reacting with acid solutions and its relative softness. If we need to include an acid stage in the flowsheet, the acid consumption (e.g. sulfuric acid, nitric acid) will be extremely high and the chemical compounds produced by these reaction will be deposited on several parts creating some disturbs. If the ore needs to be treated by flotation, the excessive quantity of fine material is a problem because flotation reagents are taken by these tiny particles. Then generation of slimes must be noted during the test work program and the mineralogy study must indicate the type of carbonates present in the ore.
Carbon or any type of carbonaceous matter creates problems during flotation and cyanidation, especially for the second. Carbon absorbs flotation reagents (xanthates, frothers) and the dosage must be increased in order to have the appropriate addition. If we consider cyanidation, carbon and carbonaceous matters are responsible for a phenomenon called pre-robbing. This phenomenon is produced during ore processing when this gangue takes gold from the pregnant cyanide solution during leaching. When this situation occurs, gold recovery is extremely low. The solution to this problem is not simple and a pre-treatment before cyanidation is necessary.
Quartz and iron oxides are responsible of gold losses into flotation tails. When gold is present as fine inclusions or encapsulated, the fineness of primary grind is not enough to liberate gold particles. This is a special case of many polymetallic operations such as copper-zinc or lead-zinc that are not able to recover gold into the copper or lead concentrate.
Gold and pyrite locked by quartz Gold locked by gangue