Magnetic Concentration

a method for separating minerals from waste rock and undesirable impurities. The method is based on the action of a magnetic field on mineral particles with various levels of magnetic susceptibility.

The first magnetic separators date to the 18th century; the perfection and industrial use of the method date to 1892-1906 (in Sweden and in other countries). The first magnetic separator in Russia was built in 1911; series production of separators and the construction of magnetic concentration plants did not begin until the establishment of Soviet power. Concentration plants in the USSR treat about 500 million tons of mineral products annually by means of magnetic concentration (as of 1973).

The raw materials for direct magnetic concentration are poor iron ores (mainly magnetite) and minerals of manganese, titanium (containing ilmenite and titano-magnetite), and tungsten (wolframite); in the process, valuable minerals are separated into the magnetic fraction (magnetic concentrate). As a result, magnetic separation increases the content of useful components severalfold. In magnetic concentrates this content may be 95 percent or higher; at the same time, the content of undesirable impurities is reduced. The proportion of minerals that passes into the concentrate (the magnetic fraction) is usually not less than 75 percent of the original content, and for strongly magnetic ores it may be 95 percent or more. A magnetic concentration process also exists in which the action of a magnetic field separates magnetic or strongly magnetic materials into the magnetic fraction and weakly magnetic or nonmagnetic materials into the nonmagnetic fraction.

“Reverse” magnetic concentration is also used; in this case the minerals of the magnetic fraction are undesirable impurities (for example, in repurification of tin, zirconium, lithium, beryllium, feldspar, and quartz concentrates).

A diagram of the magnetic concentration process is shown in Figure 1. In dry magnetic concentration, the ore is loaded onto the upper drums of a magnetic separator, which contain open permanent magnets that generate a field with an intensity of about 90 kiloamperes per meter (kA/m) on the surface of the drum. The magnetite ore is attracted to the poles (the surface of the drum), and the weakly magnetic fraction is removed and passes to the lower drums, which have a stronger field (110 kA/m), for repurification. Less magnetic pieces of ore from the tailings are extracted here.

In magnetic washing (wet concentration), finely ground magnetite ore mixed with water passes under the drums, which rotate opposite the flow of the slurry and extract from it any ferromagnetic materials. In washing manganese and other weakly magnetic ores the separators operate with a much stronger field (1,500 kA/m), which is generated in the gaps between the rolls and the poles by a closed electromagnetic system. Ore particles are extracted from the slurry by the rolls, which carry them to the concentrate compartment of the bath. The less magnetic fractions undergo repurification on the lower rolls.

The design and operational parameters of magnetic separators

Figure 1. Diagram of magnetic concentration of magnetite ore at the Sokolovskaia-Sarbai Combine (Kazakh SSR)

are determined by a large number of interconnected factors, such as the type of magnetic system; the number, shape, and location of the poles; the composition of magnetic materials; the diameter and speed of rotation of the rotors; the grain size of the ore; the content and dissemination of magnetic minerals; and, for wet processes, the quantity of water and type of bath.

As of 1971, a wide variety of magnetic separators, cones, iron separators, and magnetizing and demagnetizing equipment was manufactured in the USSR for dry concentration and washing of strongly magnetic ores (magnetic susceptibility, more than 3 ×10 ^-5), for recovery of suspended matter, and for concentration of weakly magnetic materials whose susceptibility barely exceeds 1.2 ×10^-7 . Original designs have been developed for magnetic drum separators with electromagnetic systems and permanent magnets (for magnetite ores and suspensions) and for separators of the roll, rotor, and poly gradient drum-and-groove types (for weakly magnetic ores). Such equipment is used for production not only of ore concentrates but also of metallized concentrates. The output of concentrates of the latter type is now increasing markedly in connection with direct ore reduction methods, such as cokeless and powder metallurgy.