Beneficiation is a process which removes the particles like Alumina, Silica from the iron ore. Basically, it separates Fe2O3 or Fe3O4 from other impurities in the iron ore. In this process the Fe content is improved to maximum possible extent. The highest can be 70%.
Most iron ores can be classified into three types. (1) High grade iron ores (Direct shipping), which contain enough iron to be feed to the blast furnace directly and the process may only require crushing, screening, and blending. (2) Low grade iron ores, which require minor upgrading by washing, or gravity separation techniques to increase the iron content. (3) The underlying iron formations, or taconite, a hard, dense and low grade material that requires extensive crushing, grinding and concentration to produce an acceptable concentrate.
Taconite (Primary Ore) contains only 25–35% recoverable iron that is present as either magnetite or hematite. Typically the ore is very hard and the magnetite or hematite requires very fine grinding for liberation, so the processing is complex and mining, crushing, grinding, and concentration costs are very high.
1) Drilling and blasting. In this period, the taconite rock is exposed, and then is hauled to crushers.
2) Primary, Secondary and Tertiary Crushing. Because of the hardness of the ore, three or even four stages of crushing are used in magnetite taconite plants to reduce the iron ore to grinding mill feed size. The crushed material is transferred by belt for milling, separating and concentrating.
3) Milling. Grinding mills are used for fine grinding. There are normally five types of mills: AG mills, SAG mills, ball mills, rod mills and pebble mills. Water is added at this point for transporting. When the ores are reduced to 3/4" or less, it moves out of the mill in a slurry solution.
4) Screening. The grinded ore is screened on trommel screens attached to the mill. Ore smaller than 1/4" is pumped in slurry solution to the magnetic separator, which begins the process of separating the iron from the non-iron material.
Magnetic separation is universally used for concentration of magnetite bearing iron ores and magnetite-taconite by separating the iron from the non-iron material. The magnetic iron ore is then washed in slurry tanks while the non-magnetics (silica/sand) go to the tailings disposal area.
Hydraulic separation is an important step in the magnetic separation plant. The feed is passed through a magnetic coil that causes the individual magnetite particles to coagulate by magnetic flocculation. The magnetic floccules settle very rapidly in a hydraulic separation tank and the silica slimes and some fine middlings will be rejected as final tailings in the overflow. Dewatering in this stage is as important as the silica rejection and the densified classifier discharge then is fed to the final magnetic separators.
The hydraulic separator underflow is pumped to the finisher magnetic separators. Once again, the magnetic separators grab the iron and discard the silica and sand. Thus, the ore is "concentrated" by removing the waste materials. The concentrate from the separators is pumped to fine screening.
The efficiency of fine grinding circuits is greatly affected by the efficiency of the classification system used to close the circuit. Because of the high gravitational forces in the cyclone, both particle size and particle specific gravity affects the size separation. Either stationary slotted screens or vibrating sandwich deck screens can be used to make separations.
Froth flotation is effective for the concentration of fine iron ores. It can be used for the supplementary upgrading of magnetite concentrates or as the primary recovery method for hematite ores.
Flotation processes depend on the fact that certain reagents added to water suspensions of finely ground iron ore selectively cause either iron oxide minerals or gangue particles to exhibit an affinity for air. The minerals having this affinity attach to air bubbles passing through the suspension and are removed from the suspension as a froth product.
By now, beneficiation of iron ore is finished and then the agglomeration processes begins, which is to sinter and pelletize slurries for transportation.