Lime DRI rotary kiln:
The rotary kiln direct reduction method is a direct reduction ironmaking method that uses a continuously rotating rotary kiln as a reactor and solid carbon as a reducing agent to refine iron ore into iron through solid-phase reduction reaction. The main methods for producing steel sponge iron include SL/RN method, CODIR method, DRC, TDR, ACCAR, etc.
Introduction to the Direct Reduction Process of Rotary Kiln:
Direct reduction in rotary kiln is a solid-phase carbon reduction reaction carried out at 950-1100 ℃. The material layer inside the kiln is thin, with considerable free space, and the airflow can escape freely without obstruction. The high temperature at the kiln tail is conducive to selective reduction and gasification of iron containing multi-element coexisting minerals. Elements and oxides with low gasification temperatures are discharged in gas form and then recovered to achieve comprehensive resource utilization.
Due to the low reduction temperature, the gangue in the ore is retained in the product and not fully carburized. Due to the formation of a large number of micropores caused by oxygen reduction, the microstructure of the product resembles that of a sponge, hence it is also known as sponge iron.
Process flow of rotary kiln direct reduction method:
The rotary kiln process is a cylindrical high-temperature reactor that is slightly inclined from the horizontal and placed on several sets of supporting rollers, lined with refractory materials that can continuously rotate. During operation, a certain particle size of iron ore (block ore, ball closed ore), partially reduced coal (including return coal), and desulfurizer are continuously added from the kiln feeding end (tail end) in proportion. As the kiln body rotates (0.5-1.2r/min), the material is lifted to a certain height by frictional force and rolls down due to heavy force, while moving forward a small distance towards the kiln discharge end (lower end). At the discharge end of the kiln, there is also a reducing coal injection device, which uses high-pressure air to inject suitable particle size reducing coal into the kiln. Adjusting the injection air volume can effectively control the injection distance and distribution. The heating and reaction heat of the materials in the kiln are supplied by the discharge end and the supply air duct installed along the length of the kiln, which extends into the kiln and sends in air (primary and secondary air). The volatile matter released by the reduction coal in the kiln, as well as the CO and carbon generated by the reduction reaction, are provided by the combustion. If there is insufficient heat, a coal powder burner can be added at the kiln head to supplement. The material is gradually heated by the reverse hot air flow during the forward movement process, completing drying, preheating, carbonate decomposition, desulfurization, reduction of iron oxides (or other elements), and carburizing reactions. By adjusting the air supply volume of each duct, the quantity, particle size, and distribution of coal powder and reducing coal, the temperature and distribution inside the kiln can be flexibly controlled. Allow the iron ore to remain in the kiln for 8-10 hours and transform into sponge iron at 950-1100 ℃.
Some rotary kilns install buried burners in the preheating section to expand the length of the high-temperature reduction zone. Air is sent into the material layer to burn the volatile matter released from the reduced coal at the kiln tail, thereby increasing the temperature in the preheating section. The high-temperature material discharged from the discharge end falls into the cooling cylinder through the chute. Cool the material to below 120 ℃ by spraying water outside the cylinder (or simultaneously spraying water inside and outside). To improve material movement and enhance cooling, a lifting plate is installed inside the cylinder. Sealing devices are installed at the discharge end of the rotary kiln and at both ends of the cooling cylinder to maintain a slight positive pressure during production, preventing air from being sucked in and causing re oxidation. After cooling, the material is screened, classified, and separated by magnetic separation to obtain magnetic particles (direct reduced iron), magnetic powder, non-magnetic particles, and non-magnetic powder. Magnetic powder is mixed with binder and pressed into blocks, which are then used together with direct reduced iron to power the furnace for steelmaking. Non magnetic particulate materials contain high levels of fixed carbon and can be reused as reducing agents.
Technical indicators of rotary kiln direct reduction method:
Due to the high reduction temperature of the rotary kiln (950-1100 ℃), the product is relatively stable and usually does not require passivation treatment. The rotary kiln directly reduces iron with low carbon content (0.05%~0.3%), S and P both<0.03%, and the metallization rate is controlled at 88%~93% as required.
Working principle of rotary kiln direct reduction method:
The material that enters the rotary kiln casing through the preheating device or directly exchanges heat with the combustion gas at the kiln head in a countercurrent manner. The material undergoes drying, preheating, decomposition, calcination, and cooling stages from the kiln tail to the discharge end. The combustion device is installed at the kiln head, and fuel is injected. The combustion flue gas is affected by the negative pressure caused by the smoke exhaust device (chimney or smoke exhaust fan) and flows upward along the cylinder. It meets the material in the opposite direction for heat exchange and is discharged from the kiln tail.
The rotary kiln is installed at a slope of 3% to 3.5% and rotates slowly at a rate of 1 to 2 r/min. The material added from the tail of the kiln moves towards the discharge end of the kiln head in a rolling and sliding manner along the kiln body, and is finally discharged from the discharge end. The distribution and length of each section in the kiln vary depending on the properties of the added materials and their physical and chemical changes during heating. The calcination zone is mainly characterized by radiative heat transfer, while the preheating zone is mainly characterized by convective and conductive heat transfer. The temperature of the flue gas discharged from the kiln tail is relatively high, generally reaching 900-1100 ℃.
In order to effectively utilize waste heat, various raw material preheating devices or waste heat boilers can be installed at the feeding end of the kiln. The temperature of the clinker exiting the kiln can be as high as 1000-1300 ℃. Install a clinker cooling device at the discharge end of the kiln to recover waste heat. When the clinker cools down to below 250 ℃, it enters the next process.
Composition of rotary kiln equipment:
The rotary kiln equipment mainly consists of a cylinder, rolling rings, supporting devices, transmission devices, kiln head covers, sealing devices, dust collection chambers, combustion devices, and hot smoke chambers.
Requirements for the selection of main raw materials for rotary kiln direct reduction method:
Iron ore (including oxidized ball ore), coal for reduction and combustion, and desulfurizer are the main raw materials for direct reduction production in coal based rotary kilns, and are the material basis for direct reduction production. The quality of raw materials not only has a direct impact on technical and economic indicators such as production efficiency, product quality, and energy consumption of direct reduction, but also determines the success or failure of the direct reduction process. Therefore, proper selection of raw materials and preparation for processing are crucial foundational work for direct reduction production, and are the key to producing direct reduced iron.
The iron containing raw materials used for direct reduction production in rotary kilns can be natural iron ore (i.e. lump ore) or oxidized pellets. Iron ore suitable for direct reduction production in rotary kilns must have high iron content, low gangue content, few harmful impurities, stable chemical composition, suitable particle size, and good reducibility and certain strength. The main factors determining the quality of iron containing raw materials are chemical composition, physical properties, and metallurgical properties.
Iron content: Iron containing raw materials are mainly iron oxides, usually requiring an iron content of over 66%. Hematite should be>66.5%, and magnetite should be>67.5%. The main chemical change that occurs during the rotary kiln reduction process is the removal of oxygen from the iron containing raw materials in the solid state, but not the removal of gangue components and other impurities. Therefore, the selected iron containing raw materials must have high iron content and low gangue content.
Vein stone content: It is usually required that the total amount of vein stones be less than 8%. Vein stones are generally composed of SiO2, Al2O3, CaO, MgO and other components. CaO and MgO are usually present in small amounts in ores and are not harmful components in steelmaking processes, nor do they have a significant impact on rotary kiln processes. Generally, it is required that CaO in the raw materials be less than 2.5%, Mgo<1.5％。 The acidic gangue SiO2 and Al2O3 retained in direct reduced iron lead to increased steelmaking power consumption, decreased productivity, increased slag and material consumption, and shortened furnace lining life. The content of SiO2+Al203 in iron ore should be less than 5.5%.
Sulfur content: Taking into account the smelting effect, the sulfur content of iron containing raw materials should be as low as possible, generally less than 0.03%. Sulfur is the most harmful component to steel because it makes it thermally brittle. In the direct reduction production of rotary kiln, the sulfur brought in by the raw materials evaporates partially into the exhaust gas and is absorbed by the desulfurizer. The process itself has a certain desulfurization ability.
Phosphorus content: The phosphorus content in iron ore raw materials should be as low as possible, preferably below 0.03%. Phosphorus is also a harmful component of steel, which gives it cold brittleness. Phosphorus in iron ore cannot be removed during the rotary kiln reduction process. The phosphorus content of directly reduced iron depends on the amount of ore carried in. High phosphorus direct reduction of iron for steelmaking forces an increase in slag content, enhances slag alkalinity, increases steelmaking energy consumption, and reduces yield.
Other components: lead, zinc, copper, arsenic, and tin are controlled components in high-quality steel. Generally, it is required that the total content of these elements and other non-ferrous metal elements in iron containing raw materials should not exceed 0.02%; Alkali metal oxides such as potassium and sodium have strong corrosiveness and are prone to form low melting point silicates with the kiln lining, causing the lining to bond and form rings, which affects the normal operation of the rotary kiln. In addition, iron containing raw materials contain alkali metal oxides, which will cause expansion and pulverization during the reduction process, resulting in dust loss. The content of alkali metal oxides in iron containing raw materials is generally less than 0.02%.
