Iron ore enrichment involves purifying raw iron ore to increase its iron content and separate gangue minerals. High-grade ore must have an iron concentration of 65% or higher. Hematite and magnetite are the most common types used for refining. Enrichment methods include crushing, milling, and magnetic separation. Hematite is preferred for its response to flotation and gravity separation. The global iron purification industry has perfected refining hematite, offering the highest yield in net iron content. Hematite deposits are considered the best form of iron ore available.
Iron ore enrichment is a multi-step process that raw iron ore undergoes to purify it prior to the smelting process, which involves smelting the ore to remove the metal content. The process of enriching iron ore has two complementary objectives and these define the methods used to refine it. The iron content of the ore must be increased and the gangue, which is native rock and less valuable minerals within the ore itself, must be separated. Methods such as winnowing, crushing, and milling of iron ore are often used in a variety of ways to purify it, along with several magnetic separation steps.
The iron ore industry classifies the material based on the concentration of the metal present after the enrichment of the iron ore has been completed. High-grade iron ore must have an iron concentration of 65% or higher and an average quality of 62% to 65%. Low-grade iron ore includes all mixtures with an iron concentration of less than 62%, which are not considered viable ore types for use in metallurgy. There are several types of natural iron ore, but the two most common types used for refining metals are hematite, Fe2O3, which is usually 70% iron, and magnetite, Fe3O4, which is 72% iron. There are also low-grade iron ores, such as limonite, which is hematite bonded to water molecules of 50% to 66% iron, and siderite, FeCO3, which is 48% iron.
One of the approaches to iron ore enrichment involves first a basic sieving or filtering of the ore and then crushing it using equipment such as a jaw crusher to break the rock from its natural state down to the size of individual blocks or rocks with dimensions in length or height not exceeding 3.3 feet (1 meter). This rock is then further pulverized in medium- and fine-level cone crushers or fine jaw crushers and screened to particle sizes of 0.5 inches (12 millimeters) or less, and is then passed to a flotation process for separation. Separation involves the use of low-power magnetic fields to extract the ore with a high metal content from the lower grade metal particles. Lower grade ore at this point is returned to the crude flotation stage for further refining.
The final product emerging from the crushing and magnetic separation equipment is then ground to a powder-like consistency in a ball mill. This material is then further refined through enrichment of the iron ore using a dewatering tank to remove the water content and applying high intensity magnetic fields generated by a magnetic disc separator. In this stage, the low-grade ore that still contains some metal’s worth is put back to the beginning of the cycle, and the tailings, which are also lower-grade tailings, are removed as waste.
Iron ore mining often focuses on finding 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 enriching iron ore than magnetite, so it is the preferred ore type. It also responds best to what is known as gravity separation, and several types of gravity-fed equipment can be used to refine it, including jiggers, centrifugal separators, and vibrating tables.
The global iron purification industry has perfected the methodology for refining hematite as of 2011 more than other types of iron ore, and thus offers the highest yield in net iron content of any ore mined up to Today. Hematite deposits around the world are considered to be the best form of iron ore available, although how such deposits were formed is unclear. Deposits are a dwindling natural resource believed to have formed on Earth approximately 1,800,000,000 to 1,600,000,000 years ago.
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