Pellet

In connection with a decrease in the share of rich ore mined and a constant increase in the share of ore subjected to enrichment , more and more iron ore raw materials containing 80-90% or more fractions <0.07 mm, and in some cases <0.05 mm, are involved in the metallurgical redistribution . As a rule, mining and processing enterprises are located at a considerable distance from metallurgical plants . During agglomeration of finely ground (with deep enrichment) iron ore concentrates , the process speed noticeably decreases (due to a sharp decrease in the gas permeability of the charge). To transport wet concentrate to metallurgical plants for its subsequent agglomeration is unprofitable because of the costs associated with transporting water, and difficult because of the freezing of the concentrate in the winter. The manufacture of sinter directly at mining and processing plants is impractical due to its lack of mechanical strength.

A successful solution to the problem of sintering fine iron ore concentrates was the production of iron ore pellets, first proposed in 1912 by Anderson (Sweden) and in 1913 by Brackelsberg (Germany). The production of iron ore pellets developed at the beginning of the 20th century in many countries of the world at a fast pace and exceeded 300 million tons / year by the beginning of the 2000s.

Pellet production schemes are a combination of two stages of pellet formation by pelletizing a wet mixture in special machines - pelletizing machines (production of raw pellets) and hardening granules (by firing or non-firing methods) to give the pellets the strength necessary for storage, transportation to blast furnaces and their melting in stoves.

Crude pellets form when pellets of finely dispersed iron ore are wetted to a certain degree. Fine iron ore powder refers to hydrophilic disperse systems characterized by intense interaction with water. In such a system, the desire to reduce energy is realized by reducing the surface tension at the phase boundary (when interacting with water) and the enlargement of particles (as a result of their adhesion). We can assume that, on the whole, the dispersed system of iron ore material — water has a definite thermodynamic tendency to pelletization ^[3] .

The process of granule formation from moistened iron ore concentrate is a combination of various phenomena of wetting , capillary saturation, osmosis , swelling, surface dispersion, etc. The most harmonious system of granule formation was developed by V. I. Korotich ^[4] .

As a rule, iron-rich ores and various iron-containing wastes are used for the production of pellets. To remove mineral impurities, the initial (raw) ore is finely ground and enriched in various ways .

The manufacturing process of raw (unburnt) pellets is often called pelletizing (or pelletizing). The mixture, i.e. a mixture of finely ground concentrates of iron-containing minerals, flux (additives that regulate the composition of the product), and hardening additives (usually bentonite clay ), is moistened and subjected to rolling in rotating bowls or pelletizing drums ( granulators ).

Raw pellets should be strong enough to prevent deformation and destruction upon their delivery to the kiln (usually a kiln ), as well as good heat resistance, that is, the ability to not break when fired. To enhance these properties, binders (mainly bentonite, as well as its mixture with water, lime , calcium chloride , and iron sulfate ) are introduced into the mixture of pellets.

The most widespread in production was bentonite , which in the amount of 0.5-1.5% is introduced into the mixture before pelletizing. Bentonite is a clay characterized by fine dispersion, ion-exchange ability, a high degree of swelling when wet, connected, the ability to gradually release water when heated. Bentonite mainly consists of montmorillonite (AI, Mg) _2-3 (OH) ₂ • (Si ₄ O ₁₀ ) • nH ₂ O and minerals close to it in composition. When moistened, bentonite intensively absorbs water, increasing in volume by 15 - 20 times. The choice of bentonite is due to its ability to form gels when wet, with an extremely developed specific surface area (600–900 m ² / t), which is about 7 times larger than the surface of particles of other types of clay . Bentonite increases the porosity of raw pellets, which favorably affects the rate of moisture removal during drying of the pellets without reducing their strength.

As a result of rolling in special aggregates - granulators , particles close to spherical with a diameter of 1 ÷ 30 mm are obtained. They are dried at temperatures of 150 ÷ ​​200 ° C and fired at temperatures of 1200 ÷ 1300 ° C on special installations - roasting machines . Roasting machines (usually conveyor type) are a conveyor of roasting carts (pallets) that move along rails. In the upper part of the roasting machine above the roasting trolleys there is a furnace for burning gaseous, solid or liquid fuels and forming a coolant for drying, heating and roasting pellets. Distinguish roasting machines with cooling pellets directly on the machine and with an external cooler.

Factors affecting the quality of raw pellets:

• humidity coming to pelletizing charge;
• its particle size distribution associated with the total surface area of ​​the particles;
• the quality of mixing the components of the charge;
• constancy of the charge in terms of moisture, grain size and load on the pelletizer .

The purpose of roasting raw pellets is to give them strength that provides:

• minimal destruction and fines formation during transportation of pellets from the factory to the consumer, while taking into account the possibility of storing pellets in an open warehouse, which is associated with additional overloads and exposure to precipitation ;
• minimal softening and destruction of pellets in the conditions of reduction smelting in a blast furnace .

In the process of firing raw pellets, a number of physicochemical transformations occur:

• moisture removal (hydrated and hygroscopic);
• decomposition of carbonates (MgCO ₃ ; CaCO ₃ );
• oxidation of magnetite (Fe ₃ O ₄ );
• sintering (firing);
• hematitis dissociation .

As a result of these transformations, the pellets acquire the necessary strength. Also, during firing, a significant portion of sulfur contaminants is removed.

In world science, work does not stop searching for new additives and technological methods for the manufacture of pellets.

When using pellets, separate loading of ore and fluxes into the blast furnace is excluded, the amount of slag is significantly reduced when processing ores with a low iron content. In addition, the productivity of smelting cast iron in a blast furnace is increased. Pellets are also used in the smelting of steels in induction and electric arc furnaces.

With the relatively equal properties of the pellets and sinter ^[6] , the pellet can be used in the case of remoteness of mining from consumers.

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  Screening raw pellets before loading onto a roasting machine

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  Loading raw pellets into a roasting machine

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  Calcined pellets in the unloading zone of the roasting machine

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  Pellets in the hold of a ship

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  Pellets in the ore yard of the blast furnace

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  Iron ore pellets factory in Kiruna ( Sweden )

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  Iron Ore Pellets, Sherman Factory, Ontario , Canada

• Agglomerate (metallurgy)
• Iron ore
• Iron ore industry
• Mining and processing plant

• Yusfin Yu.S. , Bazilevich T.N. Firing of iron ore pellets. - Moscow: Metallurgy, 1973.- 272 p.
• Zhuravlev F.M., Malysheva T. Ya. Pellets from concentrates of ferruginous quartzites. - Moscow: Metallurgy, 1991 .-- 127 p.
• Maerchak S. Production of pellets. - Moscow: Metallurgy, 1982. - 232 p.
• Korotich V. I. Fundamentals of the theory and technology of preparation of raw materials for blast furnace smelting: Textbook. for universities. - Moscow: Metallurgy, 1978. - 208 p.
• Pellets // Encyclopedia of Kryvyi Rih . In 2 t. T. 2. L — I: [ Ukrainian ] / comp. V.F. Bukhtiyarov. - Kryvyi Rih: Java, 2005 .-- S. 205.