Magnetic separation of typically, hard, dense, magnetic minerals of iron from aggregates is seldom necessary to assure the mechanical competence of the aggregate. Hydrated iron oxide minerals e.g. hematite, geothite are only very weakly magnetic and may be separated, if of economic value or to prevent iron staining of concrete, by density separation. High-intensity magnetic separators employing alloy permanent magnets or superconducting magnets are now available to remove such weakly magnetic mineral particles from more valuable ores such as iron ores, china clays, glass sands and bauxite but have not been applied to aggregates. Pyrite, which occurs in some deposits of sand and gravel, is so weakly magnetic that very little success has been achieved in removing it, even from coal where the incentive is so great, by magnetic separation.
Low intensity magnetic separators containing permanent ceramic (ferrite) magnets or electromagnets are commonly used to remove tramp iron and steel from quarried rocks in order to protect process equipment. It should be noted that many modern steel alloys, including stainless steels and manganese steels, are 'non-magnetic'. Therefore, many plants employ detector loops surrounding conveyor belts to initiate alarms or automatically stop the belt in the event that such material is detected in the feed to the process plant.