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Quality and Use Value of DRI



Source: internal company

Direct reduced iron (abbreviated as DRI) is a metallic iron product produced by the reduction of iron oxides in the solid state at a temperature where there is no melting or slagging. Since DRI has a sponge-like structure and its volume density is much smaller than that of ordinary pig iron, direct reduced iron is also called "sponge iron". When produced with oxidized pellets as raw materials, the product is still in the form of pellets, but the main component is metallic iron. In order to distinguish it from oxidized pellets, this product is called "metalized pellets". In order to improve the oxidation resistance of the product and to make it easier to transport, direct reduced iron is extruded in the hot state, where the product is called hot briquette (HBI), or in the cold state, where the product is called cold briquette (CBI).

1. Applications of DRI
• It is an important substitute for scrap steel in steel production and an important way to solve the shortage of scrap steel in China;
• It is a diluent for residual elements in scrap steel, and an indispensable raw material for controlling residual elements in Electric Arc Furnace smelting of high-quality pure steel and high-quality steel;
• It is an indispensable pure raw material for the equipment manufacturing industry to produce petroleum, synthetic chemicals, nuclear facilities and other equipment (such as the production of oil casing, hydrogenation reactors and nuclear industry equipment, etc.);
• It is the best coolant for converter steelmaking. Using DRI as cold material in converter production can increase the usage of cold materials in the converter, which is beneficial to improving the converter operation rate and output, and effectively improves the heat utilization rate of converter production;
• DRI uses non-caking coal as energy source. The development of DRI can reduce the dependence of steel production on coking coal and help steel production get rid of the fetters of coking coal resource shortage.

2. Factors affecting the quality of DRI
The quality of DRI is mainly composed of the chemical composition and metallurgical properties. The main indicators of chemical composition include: total iron (TFe), metallization rate (MFe/TFe), SiO2 content, carbon content (C), total gangue content, harmful element content, etc. The main indicators of metallurgical properties include: shape, size, particle size and particle size composition, density, bulk density, melting characteristics, antioxidant performance, strength (compressive strength, anti-wear strength), etc.

2.1 Main indicators of chemical composition
2.1.1 Total iron content (TFe)
Existing in the DRI in various forms of iron elements, including metallic iron (MFe), a variety of iron oxides in the iron, as well as a variety of iron compounds (such as FeC) in the total amount of iron elements in the total mass as a percentage of the total mass.

TFe is the most important indicator of the quality of DRI. TFe content of DRI products for steelmaking should be as high as possible. The higher the TFe, the lower the gangue and impurities other than iron in the direct reduced iron product, which is beneficial to reducing the amount of electric furnace smelting slag and reducing power consumption. For DRI produced from the same raw material, the higher the TFe, the higher the metallization rate.

2.1.2 Metalization rate (MFe/TFe)
Metallization rate for the direct reduction of iron products in the degree of iron metallization index, that is, the product of iron into metallic iron degree of indicators.

Metalization rate = [(MFe)/(TFe)]xl00% In the formula, TFe is the total iron content in DRI product, %; MFe is the metallic iron content in DRI product, % .

The metallization rate of DRI has a significant impact on steelmaking production, according to the steel production of raw materials and smelting of different steel types, the metalization rate of DRI requirements are very different. For example: with molten iron, pig iron lumps as the main raw material of the electric furnace, DRI metallization rate requirements to be appropriately low; and to the lightweight materials as the main material of the electric furnace, the requirements of the DRI must be maintained at a higher rate of metallization. This is due to the molten iron, pig iron block as the main raw material of the electric furnace, the charge melting, the liquid steel containing high carbon needs decarburization, this time to join the low metallization rate of DRI is conducive to decarburization of the conduct of the DRI, low metallization rate of DRI can be brought into a certain amount of oxygen, which is conducive to improving the boiling state of the molten pool. While the electric furnace with the lightweight material as the main material, especially when the charge oxidation degree is high, the oxygen level in the molten steel after the charge melting is high, then the DRI is required to maintain a high metallization rate to maintain the iron yield.

2.1.3 SiO2 content and gangue amount
The SiO2 content in DRI is an important factor affecting the quality of DRI products and the use value of DRI. Under the condition that the whole iron content of DRI is the same (TFe), the lower the content of SiO2 in DRI, the higher the quality and value of use.The total amount of vein stones in DRI and the composition of vein stones have a direct influence on the value of use of DRI, the lower the total amount of vein stones, the less acidic components and refractory minerals in vein stones, the higher the value of use of DRI.

2.1.4 Content of harmful elements (Pb, Zn, Sn, As, Cu, B, Bi, etc.)
Since DRI is a raw material for smelting high-quality steel and a diluent for the residual elements in the scrap, the harmful elements of DRI must be low.The content of harmful elements of DRI is mainly determined by the raw materials for production. Therefore, the raw materials used to produce DRI must be pure.

2.1.5 Carbon content of DRI
The carbon content of DRI has a significant effect on the use value of DRI. Different smelting processes and process conditions have different requirements for the carbon content of DRI. The carbon content of the smelted steel grade determines the appropriate carbon content of DRI. A high carbon content in the batch will result in a low carbon content in the DRI and vice versa.

The carbon content of DRI depends on the production process and process conditions of DRI, usually the carbon content of coal-based method (rotary kiln, tunnel kiln) products are lower, generally 0.30%. The carbon content of the products of gas-based shaft furnace process can be adjusted within the range of 1.00~4.00%.

2.2 Main indicators of metallurgical properties
2.2.1 Density, particle size and particle size composition of DRI
In the steelmaking process, with different raw material conditions and charging systems, the requirements for the density, particle size and particle size composition of DRI are different. Usually, the density of DRI should be >5.Og/mm3, to ensure that DRI can pass through the slag layer directly in contact with the steel for rapid melting. It also avoids DRI mixing into the slag, which is discharged with the slag causing iron loss.

2.2.2 Strength of DRI (compressive strength, anti-wear strength)
The strength of DRI should ensure that DRI is not crushed and pulverized during transportation and charging to reduce the loss of DRI during transportation and charging.

2.2.3 Antioxidant property of DRI
The antioxidant property of DRI has an obvious influence on the use value of DRI, which is poor in antioxidant property, and protective measures must be taken to prevent reoxidation of DRI and "spontaneous combustion" due to rapid reoxidation in the process of transportation and preservation. Adopting briquetting treatment to increase the density of DRI and reduce the porosity of DRI is an effective way to improve the antioxidant capacity of DRI.

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