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Iron ore beneficiation is a multi-stage process that raw iron ore undergoes to purify it prior to the process of smelting, which involves melting the ore to remove the metal content. The process of iron ore beneficiation has two complementary goals and these define the methods used to refine it. The iron content of the ore needs to be increased and gangue, which is native rock and minerals of lesser value within the ore itself, must be separated out. Methods such as screening, crushing, and grinding of iron ore are often used in various ways to purify it, along with several stages of magnetic separation.
The iron ore industry classifies the material by the concentration of the metal that is present after iron ore beneficiation has been completed. High-grade iron ore must have a concentration of 65% iron or higher, and medium grade of 62% to 65%. Low-grade iron ore includes all mixtures below 62% iron concentration, which are not considered to be viable types of ore for use in metallurgy. Several different types of natural iron ore exist, but the two most common types used for metal refining are hematite, Fe2O3, which is usually 70% iron, and magnetite, Fe3O4, which is 72% iron. Low-grade iron ores also exist, such as limonite, which is hematite bonded to water molecules at 50% to 66% iron, and siderite, FeCO3, that is 48% iron.
One of the approaches to iron ore beneficiation first involves a basic screening or filtering of the ore and then crushing it using equipment like a jaw crusher to break up the rock from its natural state down to individual block or rock sizes with dimensions of length or height no greater than 3.3 feet (1 meter). This rock is then further pulverized in medium and fine level cone crushers or fine jaw crushers, and screened down to particle sizes of 0.5 inches (12 millimeters) or less, and is then passed on to a flotation process for separation. Separation involves using low-power magnetic fields to pull the ore with high-metal content away from lower-grade metal particles. The lower-grade ore at this point is cycled back into the rough flotation stage for further refining.
The end product that emerges from crushing and magnetic separation equipment is then ground into a powder-like consistency in a ball mill. This material is then further refined through iron ore beneficiation by using a dehydration tank to remove water content and by applying high-intensity magnetic fields generated by a disc magnetic separator. At this stage, low-grade ore that still contains metal value is placed back at the start of the cycle, and tailings, which are even lower-grade residues, are removed as waste.
Iron ore mining often focuses on looking for hematite deposits known as red iron ore, and magnetite, as they have naturally weak magnetic fields that aid in their purification. Hematite, however, responds better to the flotation process in iron ore beneficiation than magnetite, so it is the preferred type of ore. It responds best to what is known as gravity separation as well and several types of gravity equipment can be used to refine it, including jiggers, centrifugal separators, and shaking tables.
The global industry for iron purification has perfected the methodology for refining hematite as of 2011 more than other types of iron ore, and it therefore offers the highest yield in net iron content of any ore mined to date. Deposits of hematite around the world are considered to be the best form of iron ore available, though it is not clearly understood how such deposits were formed. The deposits are a diminishing natural resource that are believed to have formed on Earth approximately 1,800,000,000 to 1,600,000,000 years ago.
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