One of the main methods of iron ore briquetting. Iron concentrate obtained by beneficiation of lean iron ore, fine ore obtained by crushing and screening of rich iron ore, and iron-containing materials recovered in production (blast furnace and converter dust, continuous casting and rolling iron sheet, etc.). ), flux (limestone, quicklime, hydrated lime, dolomite, magnesite, etc. ) and fuel (coke powder and anthracite) are mixed according to the required proportion, and mixed with water to make granular sintering mixture, which is laid flat on the sintering trolley.
Iron ore sintering
1887 brief history T. Huntington and F. Heberlein of Britain applied for the patent of blast sintering method of sulfide ore and sintering disc equipment used in this method for the first time. 1906, Americans A. Dwight and R. Lloyd obtained the patent of exhaust belt sintering machine in the United States. 19 1 1 year, the first continuous belt exhaust sintering machine (also known as DL sintering machine) with an effective area of 8m2 was built and put into operation in Broken Iron and Steel Company, Pennsylvania, USA. As soon as this equipment appeared, it quickly replaced the briquetting machine (see square briquetting), sintering plate (see sintering plate) and other briquetting equipment. With the development of iron and steel industry, the output of sinter has also increased rapidly. By the 1980s, the world's sintered mineral output had reached more than 500 million tons. The earliest belt exhaust sintering machine in China was built and put into operation in Anshan in 1926, with an effective area of 2 1.8 1m2. From 1935 to 1937, four 50m2 sintering machines were put into operation one after another, and the maximum annual output of sinter in 1943 reached 247,000 t. After the founding of the People's Republic of China, the iron and steel industry developed rapidly, and the sintering capacity and output were greatly improved. By the end of 199 1, the total effective area of sintering machines in China reached 9064m2, the annual output of sinter reached 96.54 million tons, and the clinker rate of blast furnaces in key enterprises reached 90%.
After the appearance of belt ventilation sintering method, not only the production scale and output of sinter have been greatly improved, but also the production technology has been greatly improved: (1) The treatment of sintering raw materials has been strengthened, such as the mixing of mineral powder, the crushing of fuel and flux, the accurate batching of mixture, granulation and preheating. (2) Various new technologies have been developed to increase production, save energy and improve quality, such as thick bed sintering, low temperature sintering, small ball sintering, double ball sintering, fine concentrate sintering, double-layer sintering, hot air sintering, new ignition technology and sinter classification. (3) The sintering equipment is large-scale, mechanized and automated, and the computer is used for production management and operation control; (4) Application of environmental protection technologies such as dust removal, desulfurization and removal of nitrogen oxides.
Sintering of mineral powder includes many physical and chemical reaction processes. No matter which sintering method is adopted, the sintering process can basically be divided into three stages: drying and dehydration, preheating of sintered materials, fuel combustion, high-temperature consolidation and cooling. These processes are carried out in the sinter in the order of layers. Figure 1 shows the reaction of each layer during sintering under ventilation. The pumped air is preheated by sintering hot sintering bed, and the solid fuel is burned in the combustion bed, giving off heat to obtain high temperature (1250 ~ 1500℃). The high-temperature waste gas discharged from the combustion layer will preheat and dehydrate the sinter. According to the temperature and atmosphere conditions, each layer has different physical and chemical reactions: evaporation and decomposition of free water and crystal water, decomposition of carbonate, decomposition, reduction and oxidation of iron oxide, removal of impurities such as sulfur and arsenic, and solid-phase and liquid-phase reactions of some oxides (CaO, SiO _ 2, FeO, Fe2O3, MgO); Cooling crystallization and consolidation of liquid phase.
Combustion and heat transfer The combustion of solid carbon can provide more than 80% heat income in the sintering process and the high temperature of 1250 ~ 1500℃ (in the combustion layer), which ensures the physical and chemical reactions such as dehydration, limestone decomposition, iron oxide decomposition and reduction, desulfurization, liquid phase generation and consolidation in the sintering process. Combustion reaction also affects the output of sintering machine.
The combustion reaction of carbon in the sinter bed is complicated, which can be generally expressed as: c+O2 = CO2; 2C+O2 = 2CO； CO2+C = 2CO； Carbon dioxide+oxygen = carbon dioxide. In the area where carbon is concentrated, the concentration of CO in the gas phase is high, the concentration of CO2 is low, and the atmosphere is reducing. In areas with little carbon and no carbon, the concentration of CO is low, the concentration of CO2 is high, and the atmosphere is oxidized. The two most important conditions for carbon combustion in the bed are that the surface of fuel particles is heated to the ignition temperature and the hot fuel surface needs to be in contact with the airflow with sufficient oxygen concentration. Increasing oxygen concentration, gas temperature, gas flow rate and the reaction surface area of fuel are all helpful to improve the combustion reaction speed. The commonly used fuels for sintering are coke powder and anthracite; High volatile coal is not suitable for sintering, because a large amount of volatile volatilizes before fire, which is easy to block the pipeline.
The heat transfer rate during sintering is very fast. Sintered materials are all small particles with high heat transfer efficiency, and there are endothermic processes such as evaporation and decomposition of water, so the heat transfer in sintered materials is very fast. Good heat utilization in sintering process is mainly manifested in low exhaust temperature and "automatic heat storage" in sintering process. The latter means that the pumped air is preheated to above 1000℃ when it passes through the thermal sintering bed (equivalent to "regenerator"), which increases the heat income in the combustion bed (about 40% ~ 60% of the total heat income of the combustion bed) and increases the temperature of the combustion bed. With the thickening of sinter bed, this part of heat income increases; With the increase of combustion layer temperature, the sintering liquid phase increases and the sintering strength increases, but the sintering speed decreases. The temperature of combustion layer is affected by fuel consumption, automatic heat storage and thermal effects of various chemical reactions in combustion layer. Increasing carbon content, increasing exothermic reaction and reducing endothermic reaction are beneficial to increase the temperature of combustion layer, as well as the temperature of material layer.
All the reactions and changes in the sintering process are carried out under the condition that the airflow continuously passes through the material layer. The airflow movement has a great influence on the output and quality of sinter. The vertical sintering speed is proportional to the gas flow through the bed. The gas flow rate is related to suction negative pressure, combustion layer temperature and material layer permeability. Because the layers are constantly changing during the sintering process, the permeability and gas flow rate of the material layers are also changing. The sintering seam has many pores and good air permeability; The combustion layer has high temperature, poor liquid phase and poor 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 destroys the material ball and produces great resistance to air flow. If the particles are broken after drying, the drying layer and preheating layer will also produce greater resistance. The permeability index p of the sintered material can be expressed by the following formula:
P=F / A(h/S) n
Where f is air volume, m3/min；; ; A is the exhaust area, m2; H is the thickness of material layer, m; S is suction negative pressure, kPa；; N is the coefficient related to gas flow property, raw material property and material state in the sintering process, generally n = 0.5 ~ 1.0. The permeability of sinter bed is related to the particle size of ore powder, the quantity and quality of returned ore, the amount of water added to the mixture, the balling performance of ore powder, the use of binder, the preheating and sintering temperature of sinter. Whether the air flow is evenly distributed along the material surface will affect the uniformity of sintering process, especially for large sintering machines. Uneven air distribution leads to uneven sintering, which leads to a decline in yield, more ore returns, poor quality and reduced sinter quality. Uniform distribution, reasonable and complete structure of sintering trolley, which is beneficial to uniform distribution of air flow.
The evaporation and condensation of water and the addition of a certain amount of water to the sinter are the needs of powder particles. When the sintering temperature reaches 100℃ or higher, the water evaporates violently and the humidity of sintering waste gas increases. When the exhaust gas leaves the dry material layer and enters the wet material layer, the temperature drops below the dew point due to cooling, and the water vapor in the exhaust gas condenses in the wet material layer, so that the humidity of the wet material layer exceeds the original humidity, which is the "over-humidity phenomenon". Excessive humidity will destroy particles and reduce the permeability of the material layer. Preheating sinter can reduce or eliminate excessive humidity. The over-wetting phenomenon of fine concentrate during sintering is more serious than that of rich ore powder. Water in the form of crystal water is a kind of chemically bound water, which can only be separated and removed at a higher temperature.
The main decomposition reaction in the process of decomposition, oxidation and reduction sintering is the decomposition of carbonate (CaCO3, MgCO3 and FeCO3, etc.). ) and some oxides. When the partial pressure of carbonate is 10 1.325 kPa, its temperature is CaCO3 9 10℃, MgCO3 630℃ and FeCO3 400℃. Therefore, they can be completely decomposed during sintering. If the particle size of limestone is coarse, it will not only prolong the decomposition time, but also not completely decompose and fully mineralize with other oxides. The free CaO remaining in the sinter will lead to the pulverization of the sinter. Therefore, the particle size of limestone is required to be less than 3 mm, carbonate decomposition is an endothermic reaction, and increasing the amount of limestone generally requires increasing the amount of carbon.
In the sintering process, iron oxide can be decomposed, reduced or oxidized according to its shape, temperature and gas phase composition. The partial pressure of Fe2O3 at 1383℃ is 20.6 kPa (0.2 1 atmospheric pressure), and the partial pressure of oxygen during sintering is low (6.8 ~ 18.6 kPa), so it can occur at1300 ~130. The partial decompression of Fe3O4 and FeO is very small, so it is impossible to produce thermal decomposition during sintering. The partial decompression of Fe2O3 is high, and the sintering waste gas often contains a small amount of CO, which can be reduced at 300 ~ 400℃, so Fe2O3 is reduced in the preheating layer and combustion layer. The decomposition pressure of Fe3O4 is low, and it can only be reduced in the atmosphere with high CO concentration, so the reduction is only carried out in the area with high temperature and CO concentration near the fuel particles in the combustion layer. FeO only works when the fuel ratio is high (>: 10%) and can be reduced to some metallic iron. Under the condition of low fuel ratio, there are relatively few thermal decomposition and reduction reactions of Fe2O3. Because there is no carbon in the sintered ore bed, Fe3O4 and FeO can be partially oxidized to Fe2O3.
The behavior of non-ferrous elements in sintering process MnO2 _ 2 _ 2 and mn2o _ 3 have high partial pressures (when the partial pressure is 20.6kPa, the temperature is 460℃ and 927℃), so they can be decomposed and reduced in the preheating layer, and the generated Mn3O4 and SiO2 _ 2 form Mn2SiO4 with low melting point. FeS2 starts thermal decomposition at 565℃ (2FeS2 = 2FeS+S2), but it can be oxidized before decomposition (4fes2+1kloc-0/O2 = 2fe2o3+8so2). At 565 ~ 1383℃, oxidation and thermal decomposition are carried out at the same time, and the oxidation products are at a higher temperature. FeS2(FeS) can also be oxidized by Fe2O3, and the generated SO3 can be absorbed by CaO to generate CaSO4. Reducing the particle size of mineral powder, matching with appropriate fuel amount, maintaining sufficient oxidation atmosphere and high temperature are beneficial to removal; Increasing alkalinity will reduce the desulfurization rate, and more than 90% of sulfur can be removed in general sintering process. Decomposition temperature of sulfate (BaSO4, etc.). ), the desulfurization rate is 80% ~ 85%. As2O3 is easy to remove, but it is very stable. PbS and ZnS can be oxidized to PbO and ZnO, and dissolved in silicate slag phase. Therefore, As, Pb and Zn are difficult to be removed during sintering, and some of them can be removed under the condition of high fuel ratio. Add a small amount of chloride (CaCl2, etc. ) can generate volatile AsCl3, PbCl 2 and ZnCl2, and can remove 60% of As, 90% of Pb and 60% of Zn. K2O, Na2O and P2O5 are difficult to remove during sintering.
There is a solid reaction between the melting of mineral powder and the solidified mineral powder before melting. It is a reaction of migration, diffusion and mutual combination into new compounds caused by the increase of ionic kinetic energy on the particle surface when mineral powder is heated to a certain temperature below its melting point. The appearance temperature of solid reaction product 2 Cao·SiO 2 is 500 ~ 690℃. The temperature of CaO Fe2O3 is 400 ~ 600℃; 2CaO Fe2O3 is 400℃; 2 FeO SiO 2 is 970℃. These reactions can be carried out in preheating layer and combustion layer, but they will not develop greatly because of the short time. 2CaO SiO2 _ 2 can be completely preserved in high-temperature melt, 2FeO SiO2 _ 2 is partially decomposed, while CaO Fe2O3 and 2CaO Fe2O3 are completely decomposed. Solid-state reaction is exothermic, and its reaction degree is not only affected by temperature, but also by contact conditions and chemical affinity. In the process of reduction, oxidation and solid-state reaction, some substances with low melting point will appear in sintered ore, such as 2feo SiO 2 (melting point is 1205℃) and its * * * crystal mixture (1177 ~1178). FeO-2CaO SiO2 is a * * crystal mixture (1280℃), CaO Fe2O3-CaO 2Fe2O3 is a * * crystal mixture (1200℃), and CaO Fe2O3-2CaO Fe2O3-Fe3O4 is a * * crystal mixture (1200℃) The composition of the melt is influenced by the composition of sinter and the degree of reduction-oxidation reaction, but the melt can basically be divided into silicate system and ferrite system. Sinter with high grade (i.e. low SiO2 _ 2 content), high alkalinity and high oxidation degree is beneficial to the formation of ferrite melt. On the contrary, it contributes to the formation of silicate melt. Sinters with different structures are formed after the melt cools and solidifies. In the process of cooling and solidification, minerals such as hematite (Fe2O3), magnetite (Fe3O4), calcium ferrite (CaFe2O3 and 2CaO Fe2O3), calcium silicate (2CaO SiO2 and 3CaO SiO2), etc. ) and fayalite can be crystallized according to different melt compositions. In the sinter containing TiO _ 2 and caf _ 2, perovskite (CaO TiO2 _ 2 _ 2) and cuspidine(3 CaO 2 SiO 2 caf _ 2) can be formed. Finally, the glass body with low melting point is solidified, and its composition is mainly silicate, which is complex. Different mineral compositions have great influence on the properties of sinter. For example, the reducibility of calcium ferrite is better than that of calcium fayalite and fayalite (2feo SiO 2); During the cooling process, the crystal form of 2cao SiO 2 changed (β 2cao SiO 2 → γ 2cao SiO 2), and the volume expanded by about 65,438+00%, which led to the pulverization of sintered powder. The strength of amorphous glass is worse than that of crystalline minerals. During solidification, due to volume shrinkage, pores with different sizes and numbers are produced. Small and many pores are beneficial to improve strength and reducibility, while large pore structure is not conducive to improve strength and reducibility.
Sintering methods and equipment sintering methods can be divided into two types according to the flow direction of gas in the material layer: exhaust sintering method and blowing sintering method. Although blast sintering method has the function of loosening the material layer and improving the permeability of the material layer, its main disadvantages are serious environmental pollution and large blast loss of mineral powder. Therefore, the blowing sintering method has been completely replaced by the exhaust sintering method. Sintering equipment includes belt sintering machine and intermittent disc sintering machine. Belt sintering machine has replaced intermittent disc sintering machine for its high output, mechanization and automation. In the total output of sinter in the world, more than 99% is produced by belt exhaust sintering machine (see belt sintering machine sintering). Some township enterprises in China also adopt indigenous sintering (see open-air sintering).
Sintering process is the process of sintering iron ore (concentrate, rich ore powder) into sinter. Modern sintering process includes three parts: raw material preparation, sintering and sinter treatment. Each part consists of several processes (see Figure 2). The preparation of raw materials includes the storage and mixing of raw materials (see ore mixing), the processing, batching, mixing and granulating of flux and fuel, and the distribution of materials. The sintering part includes ignition and exhaust sintering. Sinter treatment includes cooling, crushing, screening and classification.
The main fluxes for the processing and sintering of fluxes and fuels are lime and dolomite, which are carbonates. In the sintering process, not only must it be completely decomposed, but also the decomposed CaO and MgO should be fully combined with other oxides to generate new minerals. Otherwise, the sinter will contain free CaO, causing pulverization, which is not conducive to storage. Therefore, the particle size of flux should be less than 3 mm; However, the particle size of limestone and dolomite is generally 40 ~ 0 mm or thicker, so it must be crushed. The flux crushing process is basically closed-circuit crushing, and the crushing operation mostly adopts hammer crusher or impact crusher; Self-centering vibrating screen is used for screening operation. Generally, the particle size of quicklime and hydrated lime is relatively fine when they enter the factory, so they don't need to be broken again. However, quicklime will burn human skin, so it is advisable to transport it by gas and strengthen the sealing of the work area.