What Is Involved in the Processing of Iron Ore?

Iron sintering is one of the main methods of iron ore agglomeration. Iron concentrates obtained through beneficiation of iron ore, fine ore produced during the crushing and screening of iron-rich ore, iron-containing powders recovered during production (blast furnace and converter dust, continuous casting and rolling of steel skin, etc.), flux (Limestone, quicklime, slaked lime, dolomite, magnesite, etc.) and fuel (coke powder and anthracite), etc., according to the required proportions, mixed with water to make a granular sintered mixture, tiled on a sintering trolley, ignited and ventilated Sintered into pieces.

Iron sintering is one of the main methods of iron ore agglomeration. Iron concentrates obtained through beneficiation of iron ore, fine ore produced during the crushing and screening of iron-rich ore, iron-containing powders recovered during production (blast furnace and converter dust, continuous casting and rolling of steel skin, etc.), flux (limestone,

In 1887, the British T. Huntin gton and F. Heberlein first applied for a patent for the sulphide blast sintering method and the sintering disk equipment used for this method. In 1906, Americans D. Dwight and L. Lloyd obtained a patent for a suction belt sintering machine in the United States. In 1911, the first continuous belt air extraction sintering machine with an effective area of 8m ² (also known as DL type sintering machine) was completed and put into operation at Broken Iron and Steel Company in Pennsylvania, USA. This equipment soon replaced block making equipment such as briquetting machines and sintering discs. With the development of the steel industry, the output of sintered ore has also increased rapidly. By the 1980s, the output of sintered ore in the world reached more than 500 million tons. The earliest belt exhaust sintering machine in China was completed and put into production in Anshan in 1926. The effective area of the sintering machine is 21.81m ² . From 1935 to 1937, four 50m ² sintering machines were put into production one after another. In 1943, the maximum annual output of sintered ore reached 247,000 tons. After the founding of the People's Republic of China, the iron and steel industry developed rapidly, and the sintering capacity and output have greatly improved. By the end of 1991, the total effective area of sintering machines nationwide had reached 9064 m ² , the annual output of sintering ore had reached 96.54 million tons, and the clinker rate of blast furnaces in key enterprises had reached 90%.

After the emergence of the belt exhaust sintering method, not only the production scale and output of sintered ore has been greatly improved, but also the production technology has been greatly improved:

(1) Strengthening the processing of sintering raw materials, such as mixing of slag powder, crushing of fuel and flux, accurate batching of the mixture, granulation and preheating, etc .;

(2) Developed various new processes to increase production, save energy and improve quality, such as thick layer sintering, low temperature sintering, small ball sintering, double ball sintering, fine concentrate sintering, double sintering, hot air sintering, new ignition processes, Sintered ore whole grains, etc .;

(3) Large-scale, mechanization and automation of sintering equipment, and computers for production management and operation control;

(4) Application of environmental protection technologies such as dust removal, desulfurization and nitrogen oxide removal ^[2]

Mineral sintering involves many physical and chemical reaction processes. Regardless of the sintering method used, the sintering process can be basically divided into the following stages: dehydration, preheating of the sinter, fuel combustion, high-temperature consolidation, and cooling. These processes are carried out successively in layers in the sinter. Figure 1 shows the reactions of the various layers during the sintering process under the condition of exhaust air. The sucked-in air is preheated through the sintered hot sintered ore layer, and the solid fuel is burned in the combustion layer to release heat to obtain a high temperature (1250-1500 ° C). The high-temperature exhaust gas extracted from the combustion layer preheats and dehydrates the sintered material. Depending on the temperature and atmosphere conditions, different physical and chemical reactions occur in each layer: evaporation and decomposition of free water and crystal water, decomposition of carbonates, decomposition, reduction and oxidation of iron oxide and iron tie, sulfur, arsenic, etc. Removal of impurities, solid and liquid reactions of some oxides (CaO, SiO ₂ , FeO, Fe ₂ O ₃ , MgO); cooling crystallization and consolidation of the liquid phase, etc. ^[3]

The combustion of solid carbon can provide more than 80% of the heat in the sintering process and a high temperature of 1250 to 1500 ° C (in the combustion layer), which ensures dehydration, limestone decomposition, decomposition and reduction of iron oxides, desulfurization, Physical and chemical reactions such as liquid phase formation and consolidation. The combustion reaction also has an effect on the output of the sintering machine.

The combustion reaction of carbon in the sinter layer is more complicated and can be generally expressed as: C + O ₂ = CO ₂ ; 2C + O ₂ = 2CO; CO ₂ + C = 2CO; 2CO + O ₂ = 2CO ₂ . In the region where the carbon is concentrated, the CO concentration in the gas phase is high, the CO ₂ concentration is low, and the atmosphere is reducing; in the regions with little and no carbon, the CO concentration is low, the CO ₂ concentration is high, and the atmosphere is oxidizing. The two most important conditions for carbon combustion in the material layer are that the surface of the fuel particles is heated to the ignition temperature and the hot fuel surface is exposed to a gas flow with sufficient oxygen concentration. Increasing the concentration of oxygen in the air stream, the temperature of the air stream, the speed of the air stream, and increasing the reaction surface area of the fuel all contribute to the improvement of the combustion reaction speed. The fuels commonly used for sintering are coke powder and anthracite; high-volatile coal types, because a large amount of volatiles are volatilized before catching fire, and it is easy to block the pipeline, so it is not suitable for sintering.

The heat transfer rate is fast during the sintering process. The sintering material is a small particle material, which has a high heat transfer efficiency, and there are also endothermic processes such as evaporation and decomposition of water, so heat conduction proceeds quickly in the sintering material. The heat is used well during the sintering process, which is mainly reflected in the low temperature of the exhaust gas and the "automatic heat storage effect" of the sintering process. The latter means that the air is preheated to above 1000 ° C when it is evacuated through the hot sintered ore layer (equivalent to the function of "reservoir"), which increases the heat income in the combustion layer (about 40% of the total heat income of the combustion layer) To 60%), increasing the temperature of the combustion layer. As the sinter layer thickens, this part of the heat income increases; the temperature of the combustion layer increases, the sintering liquid phase increases, the sintering strength increases, but the sintering speed decreases. The combustion layer temperature is affected by factors such as fuel dosing and automatic heat storage, as well as the thermal effects of various chemical reactions in the combustion layer. Increasing the carbon content, increasing the exothermic reaction, and decreasing the endothermic reaction are conducive to increasing the temperature of the combustion layer, and increasing the material layer has the same effect.

Airflow movement in the sintering layer All reactions and changes during the sintering process are carried out under the condition that the airflow continuously passes through the layer. Airflow motion has a great impact on the yield and quality of the sinter. The vertical sintering speed is directly proportional to the air flow through the layer. The air flow is related to the negative pressure of the exhaust air, the temperature of the combustion layer and the permeability of the material layer. Because the layers are constantly changing during the sintering process, the air permeability and air flow of the material layer are also changing. The sintered ore layer has more pores and good air permeability; the combustion layer has high temperature, has liquid phase, and has poor air permeability. The wet material layer with good sphericity has good air permeability, but sometimes the material layer is too wet due to water vapor condensation, which damages the material ball and generates a large resistance to the air flow. If the pellets are broken after drying, the dried layer and the preheated layer will also generate greater resistance.

The permeability of the sintering material layer is related to the particle size of the slag, the amount and quality of returned ore, the amount of water added to the mixture, the sphericity of the slag, the use of the binder, the preheating of the sintering material, and the sintering temperature. Whether the air flow is evenly distributed along the material surface will affect the uniformity of the sintering process, especially for large sintering machines. The uneven air flow distribution leads to uneven sintering, which reduces the yield, and returns to the mine with poor quality, which reduces the quality of the sintered mineral. The cloth is uniform, and the structure of the sintering trolley is reasonable and intact, which is conducive to the uniform distribution of the air flow.

Evaporation of water and coagulation of sintering materials by adding a certain amount of water are required for powder granulation. When the temperature of the sintering material reaches 100 ° C or higher, the moisture evaporates violently, and the humidity of the sintering exhaust gas increases. After the exhaust gas leaves the dry layer and enters the wet material layer, the temperature of the exhaust gas is condensed in the wet material layer due to the cooling to lower the temperature below the dew point, which causes the humidity of the wet material layer to exceed the original humidity. This is the "over-humidity phenomenon". Excessive humidity will damage the ball and reduce the air permeability of the layer. The use of preheated sinters can reduce or eliminate over-wetting. The phenomenon of over-wetting during sintering of fine concentrates is more severe than that during sintering of rich ore powders. The water in the form of crystal water is a kind of chemically bound water, which needs to be decomposed and removed at a higher temperature ^[3]

The main decomposition reaction in the sintering process is the decomposition of carbonates (CaCO3, MgCO3, FeCO3, etc.) and some oxides. When the decomposition pressure of carbonate is 101.325 kPa, the temperatures are: CaCO ₃ 910 ° C, MgCO ₃ 630 ° C, and FeCO ₃ 400 ° C. Therefore, they are completely decomposable during the sintering process. If the particle size of limestone is coarse, not only will the decomposition time be extended, but it will not be completely decomposed and fully mineralized with other oxides. Free CaO remaining in the sintered ore will cause sintered ore to be pulverized. Therefore, the particle size of limestone is required to be less than 3mm. Carbonate is decomposed into an endothermic reaction. Increasing the amount of limestone generally increases the carbon content accordingly.

During the sintering process, iron oxide can undergo decomposition, reduction or oxidation reaction according to its morphology, temperature and gas phase composition. The decomposition pressure of Fe ₂ O ₃ is 20.6 kPa (0.21 atm) at 1383 ° C. The partial pressure of oxygen during the sintering process is low (6.8 to 18.6 kPa), so thermal decomposition can occur at 1300 to 1350 ° C (combustion layer). (6Fe ₂ O ₃ = 4Fe ₃ O ₄ + O ₂ ). The decomposition pressure of Fe ₃ O ₄ and FeO is very small, and thermal decomposition is unlikely to occur during the sintering process. Fe ₂ O _{3 has a} high decomposition pressure, and the sintering exhaust gas often contains a small amount of CO, which can be reduced at 300-400 ° C. Therefore, Fe ₂ O ₃ is reduced in the preheating layer and the combustion layer; the decomposition pressure of Fe ₃ O ₄ is low, only The reduction can be performed in an atmosphere with a high CO concentration, so the reduction is performed only in a region where the temperature near the fuel particles and the CO concentration are high in the combustion layer. FeO can be reduced to some metallic iron only under the condition of high fuel ratio (> 10%). Under the condition of low fuel ratio, Fe ₂ O _{3 has} relatively few thermal decomposition and reduction reactions. In the sintered ore layer, Fe ₃ O ₄ and FeO can be partially oxidized to Fe ₂ O ₃ due to the absence of carbon.

Behavior of non-ferrous elements during the sintering process The decomposition pressures of MnO ₂ and Mn ₂ O ₃ are very high (temperatures of 460 ° C and 927 ° C, respectively, when the decomposition pressure is 20.6 kPa), so they can decompose in the preheating layer And is reduced, and the generated Mn ₃ O ₄ and SiO ₂ form a low melting point Mn ₂ SiO ₄ . FeS ₂ starts thermal decomposition at 565 ° C (2FeS ₂ = 2FeS + S ₂ ), but can be oxidized before decomposition (4FeS ₂ + 11O ₂ = 2Fe ₂ O ₃ + 8SO ₂ ), at 565 to 1383 ° C, oxidation Simultaneous with thermal decomposition, the oxidation product is Fe ₃ O _{4 at} higher temperatures; FeS ₂ (FeS) can also be oxidized by Fe ₂ O ₃ , and the generated SO ₃ can be absorbed by CaO to generate CaSO ₄ . Reducing the particle size of ore powder and matching the appropriate amount of fuel to maintain a sufficient oxidizing atmosphere and a higher temperature are favorable for de-removal; increase the alkalinity to reduce the desulfurization rate, and the general sintering process can remove more than 90% of sulfur. The decomposition temperature of sulfate (BaSO ₄ etc.) is high, and the desulfurization rate is 80% to 85%. As ₂ O ₃ is volatile and removed, but As ₂ O ₅ is very stable. PbS and ZnS can be oxidized to form PbO and ZnO, which melt in the silicate slag phase. Therefore, As, Pb, and Zn are difficult to remove during the sintering process. Under the condition of high fuel ratio, some can be removed. Adding a small amount of chloride (CaCl ₂ etc.) can generate volatile AsCl ₃ , PbCl ₂ and ZnCl ₂ , which can remove 60% As, 90% Pb and 60% Zn. K ₂ O, Na ₂ O and P ₂ O ₅ are difficult to remove during sintering.

There is a solid phase reaction between the melting of the ore powder and the solidified ore powder. It is the reaction of migration, diffusion and mutual combination of new compounds caused by the increase of ion kinetic energy on the particle surface when the ore powder is heated to a certain temperature below its melting point. The temperature at which the solid reaction product 2CaO · SiO ₂ appears is 500-690 ° C; the temperature at which CaO · Fe ₂ O ₃ appears is 400-600 ° C; 2CaO · Fe ₂ O ₃ is 400 ° C; 2FeO · SiO ₂ is 970 ° C. These reactions can be carried out in the preheating layer and the combustion layer, but due to the short time, they will not develop much. 2CaO · SiO ₂ can be completely stored in the high temperature melt, 2FeO · SiO ₂ is partially decomposed, and CaO · Fe ₂ O ₃ and 2CaO · Fe ₂ O ₃ are all decomposed. Solid-phase reactions are exothermic reactions, and the degree of reaction is affected by the contact conditions and chemical affinity in addition to temperature. During the reduction, oxidation, and solid-phase reactions, low-melting substances such as 2FeO · SiO ₂ (melting point 1205 ° C) and its eutectic mixture (1177 1178 ° C) and CaO · Fe ₂ will appear in the sinter. O ₃ (1216 ° C), FeO-2CaO · SiO ₂ based eutectic mixture (1280 ° C), CaO · Fe ₂ O ₃ -CaO · 2Fe ₂ O ₃ based eutectic mixture (1200 ° C) and CaO · Fe ₂ O ₃ -2CaO · Fe ₂ O ₃ -Fe ₃ O ₄ based eutectic mixture (1180 ° C). These materials first melt, and continue to melt the remaining materials, changing their composition and forming new melts. The composition of the melt is affected by factors such as the composition of the sintering material and the degree of the reduction and oxidation reaction, but the melt can be basically divided into two major categories: silicate systems and ferrite systems. Sintered ore with high grade (ie low SiO _{2 content} ), high alkalinity and high degree of oxidation contributes to the formation of ferrite melts; on the contrary, it contributes to the formation of silicate melts. After the melt has cooled and solidified, sintered ore of different structures is formed. During the cooling and solidification process, hematite (Fe ₂ O ₃ ), magnetite (Fe ₃ O ₄ ), and calcium ferrite (CaO · Fe ₂ O ₃ and 2CaO · Fe) can be crystallized according to the melt composition. ₂ O ₃ ), calcium silicate (2CaO · SiO ₂ and 3CaO · SiO ₂ etc.), and minerals such as peridotite (CaO · FeO · SiO ₂ ). In the sintered ore containing TiO ₂ and CaF ₂ , perovskite (CaO · TiO ₂ ) and mullite (3CaO · 2SiO ₂ · CaF ₂ ) can be formed. The final solidification is a low-melting glass body, whose composition is mainly a silicate with a complex composition. Different mineral compositions have a great influence on the properties of sintered ore. For example, calcium ferrite is more reducible than peridotite, and better than peridotite (2FeO · SiO ₂ ); 2CaO · SiO ₂ undergoes crystal transformation during cooling (2CaO · SiO ₂ 2CaO · SiO ₂ ) A volume expansion of about 10% occurs, causing sintering ore powdering; the strength of the amorphous glass body is worse than that of the crystalline minerals. During the solidification process, pores of different sizes and numbers are generated due to volume shrinkage. Small and many pores are conducive to improving strength and reducing properties, and the structure of atmospheric pores is not conducive to improving strength and reducing properties. ^[1]

The sintering method is divided into two types, the air sintering method and the air sintering method, according to the flow direction of the gas in the material layer. Although the air sintering method has the effect of loosening the material layer and improving the air permeability of the material layer, this method has major shortcomings such as serious environmental pollution and high blow loss of mineral powder. Therefore, the blown sintering method has been completely replaced by the exhaust sintering method. The sintering equipment includes a belt sintering machine and an intermittent disc sintering machine. The belt sintering machine has replaced the intermittent disc sintering machine due to its high output and high degree of mechanization and automation. Of the total output of sintered ore in the world, more than 99% is produced by a belt exhaust sintering machine. Some township and village enterprises in China also use sintering ^[1]

The process of sintering iron ore (concentrate, rich ore powder) into sintered ore. The modern sintering process includes three parts: raw material preparation, sintering, and sintering ore processing. Each part consists of several processes. Raw material preparation includes storage and mixing of raw materials (see ore mixing), processing of fluxes and fuels, ingredients, mixing and granulation, and cloth. The sintering part includes processes such as ignition and air sintering. The sintering ore processing part includes the steps of cooling, crushing, sieving, and granulation ^[3]

The main sintering fluxes are lime and dolomite, which are carbonates. In the sintering process, not only should it be completely decomposed, but also the decomposed CaO and MgO should be able to fully combine with other oxides to form new minerals; otherwise, the sintered ore will contain free CaO, causing pulverization and not conducive to storage. Therefore, the particle size of the flux should be less than 3mm; however, the incoming particle size of limestone and dolomite is generally 40 ~ 0mm or larger, so it must be crushed. The crushing process of flux is basically a closed-circuit crushing. Most crushing operations use hammer crushers or impact crushers; screening operations use self-centering vibrating screens. Quick lime and slaked lime are generally finer in the factory and do not need to be broken, but quick lime burns the human skin. It is advisable to use gas to transport and strengthen the sealing of the working area ^[2] .