Reduced iron electrically heated rotary kiln

Abstract

A reduced iron electric heating rotary kiln is used for reducing iron ore into direct reduced iron. A plurality of heating sections are arranged in the reduction section of the furnace of the rotary kiln, and each heating section is heated by a plurality of direct current power sources with independent adjustable power output. The heating power sources of each heating section are in contact with the solid materials in the furnace through positive and negative electrodes in the furnace, and the current is fed into the solid materials to heat the materials by the resistance heating of the solid materials. One or more of the heating power sources are rectified only once after passing through a local transformer from the power grid. The advantage is that for the large electric heating rotary kiln required for industrial production of direct reduced iron, the adaptability to low-grade iron ore is strong, the structure of the heating power source is simple, the interference to the power grid is small, and the cost is greatly reduced.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy and relates to an electric rotary kiln equipment for producing direct reduced iron. Background Technology

[0002] my country's steel industry accounts for about 5% of the country's GDP and about half of the world's steel production. Currently, my country's steelmaking process is mainly a long process using blast furnaces and converters, resulting in huge carbon emissions. The steel industry accounts for about 15% of the country's total carbon emissions. Of this, blast furnace ironmaking alone accounts for 60-70% of the steel industry's carbon emissions. In other words, the ironmaking process accounts for about 9% of the country's total carbon emissions.

[0003] Coal-based direct reduced iron (FRE) generally employs a rotary kiln process. During reduction, secondary air is blown into the center of the rotary kiln and combusts with CO in the atmosphere, generating heat to fuel the reduction reaction. Since the reduction reaction between CO and iron oxides is slow below 800℃, the kiln temperature is typically controlled above 800℃. At this temperature, CO2 reacts with solid carbon to produce CO. Additionally, the CO2 generated from the reduction of iron oxides by CO also undergoes the same reaction. Therefore, in essence, solid carbon reduces iron oxides to produce direct reduced iron, and solid carbon reacts with oxygen to produce CO. The reaction of solid carbon reducing iron oxides to produce metallic iron and CO is a strongly endothermic reaction. Thus, a large amount of carbon is burned to heat the main reduction reaction and other heat dissipation processes. Only a small portion of solid carbon is used as a reducing agent. This results in a relatively low content of reducing gases in the rotary kiln exhaust gas, and the presence of a large amount of nitrogen introduced by the secondary air, making the recycling of the exhaust gas costly and inefficient.

[0004] In the process of producing direct reduced iron in a coal-based rotary kiln, the material filling rate inside the rotary kiln is relatively low in order to ensure effective heat and mass transfer, generally not exceeding 20%.

[0005] In summary, coal-based rotary kilns for producing direct reduced iron suffer from low mass and heat transfer efficiency, low gas conversion rate, low furnace utilization rate, and low equipment capacity. They consume approximately one ton of coal per ton of iron produced, and their carbon emissions are higher than those of blast furnace ironmaking.

[0006] The gas-based rotary kiln direct reduction iron process uses gases such as natural gas, coal gas, and hydrogen as reducing agents. The reaction of reducing iron oxides to produce metallic iron is still an endothermic reaction. Generally, the reduction reaction of iron oxides is heated by introducing oxygen for combustion or by preheating the reducing gas to about 1000℃ to introduce sensible heat.

[0007] Due to the massive scale of direct reduced iron (DRI) production, existing DRI rotary kilns suffer from low filling rates and low heat transfer efficiency. Even with a kiln length of over 60m and a furnace inner diameter of over 2.5m, the annual production capacity of a single rotary kiln is only around 150,000 tons, resulting in extremely low production efficiency.

[0008] Existing self-heating rotary kilns, which use electricity to heat the material, can overcome problems such as low material filling rate and low heat transfer efficiency. However, due to factors such as limited heating sections, unreasonable electrode settings, and the use of single-phase or three-phase AC heating, large-scale production is difficult. Current self-heating rotary kilns use high-frequency switching inverter power supplies, which are DC power supplies obtained from the power grid through a local transformer, primary rectification, full-bridge IGBT high-frequency switching or other power switching devices, secondary transformation, and secondary rectification. For industrial-scale self-heating rotary kilns producing direct reduced iron (DRI), the equipment is large-scale, with a single kiln annual capacity of 200,000 to 600,000 tons or even higher. Producing each ton of DRI requires 500 to 1000 kWh of electricity. Therefore, the investment cost of the heating power supply equipment for existing direct-heating rotary kilns (calculated from the primary rectification) accounts for 80% or more of the total cost of the rotary kiln, resulting in huge investment and high costs.

[0009] In summary, given the massive production capacity of the steel industry, gas-based reduction ironmaking processes are generally planned to reach an annual capacity of over 500,000 tons, a scale that existing rotary kiln direct reduced iron production cannot easily achieve. Coal-based rotary kilns, due to their reliance on secondary air combustion to provide heat for the reduction reaction, suffer from drawbacks such as difficulty in recycling exhaust gas and low filler ratios, resulting in low production efficiency and high carbon emissions. Existing self-heating rotary kilns, influenced by factors such as power supply type, electrode arrangement, and heating section setup, are not yet suitable for large-scale direct reduced iron production. Regardless of whether a vertical shaft furnace or rotary kiln is used, and regardless of whether coal-based, gas-based, or all-hydrogen reducing agents are employed, heat supply and heat transfer present significant challenges for large-scale endothermic reactions. Hydrogen is expensive, and coke oven gas and blast furnace gas are not readily available; using water gas as the reducing gas essentially still relies on coal or coke as the reducing agent. The requirement to reduce carbon emissions has made replacing coal with electricity and using coal-based raw materials merely as reducing agents an inevitable way to reduce carbon emissions. A direct reduction iron electric rotary kiln that can provide an annual production capacity of hundreds of thousands to millions of tons has become the preferred equipment method. However, the high cost of its heating power supply cabinet is a major obstacle to its realization. Summary of the Invention

[0010] The purpose of this invention is to provide an electric rotary kiln with a large production scale, which can meet the production needs of direct reduced iron with a single unit annual output of hundreds of thousands of tons. It is highly adaptable to low-grade iron ore, and its heating power supply has a simple structure, low interference with the power grid, and low cost.

[0011] The objective of this invention is achieved as follows: A rotary kiln for reducing iron ore to direct reduced iron (DRI), characterized in that: the rotary kiln has a nearly cylindrical or prismatic internal cavity as a furnace; one end of the kiln has a solid material inlet and a tail gas outlet, and the other end has a solid material outlet. The solid material entering the furnace is iron ore. Solid reducing agents such as coal, charcoal, coke, and organic carbon can also enter through the solid material inlet or be injected through the solid material outlet. The furnace is equipped with a reduction section. A preheating section is provided at the solid material inlet end of the reduction section. After entering the rotary kiln furnace from the inlet end, the iron ore passes through the preheating section and the reduction section sequentially. A heating power electrode is provided in the reduction section to energize the conductive material in this section to heat the material and provide heat for the reduction reaction of iron oxide in the iron ore.

[0012] The heating power supply is a DC power supply with adjustable output power. The heating power supply electrodes in the reduction section have two or more positive electrodes and two or more negative electrodes. These two or more positive and negative electrodes divide the reduction section furnace into two or more heating sections along the length of the rotary kiln. Each heating section is heated by two or more heating power supplies, and the electric heating power of each section is controlled separately. The heating sections feed current to the material in the furnace in parallel; that is, the positive electrodes of adjacent heating sections are adjacent to each other or share a positive electrode; or the negative electrodes of adjacent heating sections are adjacent to each other or share a negative electrode.

[0013] One or more of the two or more heating power sources are rectified once after passing through a local transformer from the power grid.

[0014] The electrodes of each heating power source inside the rotary kiln are made of conductive material embedded in the inner wall of the refractory lining of the furnace. For this electrode arrangement, two or more heating power sources are installed. The positive terminal of each power source is connected to the positive terminal of the corresponding heating section, and the negative terminal of each power source is connected to the negative terminals on both sides of its positive terminal, i.e., the negative terminals at both ends of that heating section. Multiple negative terminals are connected outside the rotary kiln furnace by conductive busbars. Each heating power source is a DC power source, and its output power can be adjusted individually or automatically according to the temperature of that section.

[0015] The iron ore contains iron oxides, with a total iron content of over 40%. After entering the rotary kiln furnace, the iron ore is preheated in the preheating section, where the iron oxide is partially reduced and metallized. This results in the ore containing some metallic iron before entering the reduction section, or the iron elements within it being partially reduced to contain metallic iron during the preheating section. This makes the iron ore conductive, allowing it to be electrically heated in the reduction section.

[0016] The raw materials for blast furnace ironmaking, including oxide pellets and sinter, can be used as raw materials for direct reduced iron in the rotary kiln of this invention. The iron ore material enters the rotary kiln furnace from the feed end and passes sequentially through a preheating section and a reduction section. In the preheating section of the rotary kiln, the material is preheated and pre-reduced by gases from the reduction section, where some iron oxide is reduced to metallic iron. This makes the solid raw material conductive, allowing current to flow through the material pile and generate its own heat when it enters the reduction section.

[0017] The preheating section is set in the rotary kiln for two reasons: first, to make full use of the high-temperature gas withdrawn from the reduction section to preheat the iron ore raw material; and second, to partially reduce the iron oxides in the iron ore in the preheating section so that the material has a certain electrical conductivity, so that electricity can be passed through the material to make the material itself heat up and provide the required heat of reaction for the endothermic reduction reaction.

[0018] Existing vertical shaft furnace gas-based direct reduction iron (DRE) processes use high-grade iron ore with a total iron content as high as 66-69% and very low impurity content (pure Fe2O3 has an iron content of 70%). In contrast, vanadium-titanium magnetite has an iron content of only about 55%. The lower the impurity content in the iron ore, the higher the metallization rate during reduction, and the lower the resistivity of the iron ore material. With the same aspect ratio and material filling rate in the rotary kiln, lower resistivity requires the heating power supply to operate at a lower voltage and a higher current. Existing DC-heated self-heating rotary kilns operate at low voltage and high current. Their heating power supply consists of: AC power from the grid passing through a transformer and rectification, then inverted into high-frequency AC power by power switching devices (IGBTs or MOSFETs), then transformed again by a high-frequency transformer, and finally rectified again by diodes to obtain the DC heating power supply delivered to the rotary kiln. The output power of the heating power supply is adjusted by controlling the duty cycle of the switching devices (pulse width modulation, PWM).

[0019] This invention utilizes the chemical and physical properties of low-grade iron ore with high impurity content, or high-grade iron ore with only a small portion of iron metallized, resulting in high resistivity during the reduction reaction. Combined with a suitable length-to-diameter ratio of the furnace chamber in the heating section, a higher voltage and a lower current are used to heat the material in the reduction section of the rotary kiln. A technical solution is proposed to use AC power from the grid after one transformation and one rectification by a local transformer as the heating power source for the rotary kiln of reduced iron. This significantly simplifies the circuit structure of the heating power source and reduces the investment and operating costs of the heating power source.

[0020] Furthermore, the rotary kiln is equipped with a cooling section at the solid material discharge end of the reduction section, and the iron ore material enters the cooling section after passing through the reduction section.

[0021] Furthermore, an air inlet is provided at the solid material discharge end of the furnace.

[0022] Furthermore, the furnace outlet is equipped with a baffle plate to improve the filling rate of solid materials in the furnace.

[0023] Furthermore, the local transformer is a rectifier transformer. Alternatively, the local transformer is a conventional transformer.

[0024] Furthermore, one or more of the aforementioned heating power sources are DC power supplies obtained by primary transformation of the power grid through a local transformer, rectification by diodes, and then pulse-width modulation (PWM) by power switching devices. This DC power supply is supplied to the material fed into the heating section of the rotary kiln to heat the material. The power switching devices can be IGBTs or MOSFETs, and can be silicon-based devices, SiC devices, GaN or GaO devices. Compared with existing heating power supplies that undergo high-frequency switching pulse-width modulation, secondary transformation, and secondary rectification, the technical solution of this invention eliminates the need for a secondary transformer and secondary rectification diodes, and reduces the number of power switching devices by half or three-quarters, resulting in a significant cost reduction.

[0025] Alternatively, one or more of the two or more heating power sources can be obtained from the power grid, rectified and regulated by a thyristor or silicon controlled rectifier (SCR) or diode, or by a diode and SCR or thyristor, to supply DC power to the material fed into the heating section of the rotary kiln for heating. Compared with existing technologies, this further reduces costs, but increases harmonic interference to the power grid.

[0026] Alternatively, one or more of the two or more heating power sources are obtained from the power grid, rectified and regulated by a local transformer, and then supplied as DC power to the material fed into the heating section of the rotary kiln for heating. The power switching devices can be IGBTs or MOSFETs, and can be silicon-based, SiC, GaN, or GaO devices. Compared to diode rectification, this method causes less interference to the power grid and has higher power efficiency.

[0027] Furthermore, one or more of the aforementioned heating power sources, after being connected to the power grid via a local transformer, undergo one and only one rectification, and power adjustment by a thyristor, silicon controlled rectifier, or power switching device, the resulting DC power is supplied to the material fed into the rotary kiln heating section to heat the material. Under resistive load conditions, the power adjustment range of the material heated by this power source in the rotary kiln heating section is 70-100% of the full-load power. By designing and controlling the power control range of the heating power source, the current during thyristor rectification is relatively small, which can reduce harmonic pollution to the power grid caused by rectification and power adjustment. It also meets the requirements for rotary kiln heating power control and temperature control.

[0028] Furthermore, the power adjustment range of the heating power supply is 90-100% of the full load power. This can further reduce the pollution of the power grid caused by rectification and power adjustment. It also meets the requirements for adjusting the heating power and controlling the temperature of the rotary kiln.

[0029] It should be noted that the present invention utilizes and controls factors such as the degree of reduction of iron ore in each heating section, the rotary kiln speed, the material filling rate in the rotary kiln reduction section, and the length-to-diameter ratio of each heating section to control the resistance of the material pile in each heating section. This allows the DC power supply to be rectified only once, eliminating the need for thyristor, IGBT, or MOSFET power adjustment, thus enabling control of the heating power.

[0030] Compared with the prior art, the present invention has the following advantages: It uses a DC power supply, which has high electrothermal efficiency and high safety.

[0031] Dividing the reduction section into two or more segments for controlled heating allows for more accurate temperature control throughout the entire reduction section. This approach addresses the challenges of varying resistivity of solid materials within the furnace during direct reduced iron production, as well as the differing voltage and current requirements of the heating power supply in each segment. This allows the heating power supply to operate at higher voltages and lower currents, enabling the use of a DC heating power supply with only one rectification cycle. The heating power supply circuit is simple in structure and low in cost.

[0032] The rotary kiln of this invention is equipped with a preheating section, which allows cold iron ore oxide pellets to be directly fed into the kiln as raw materials. The iron ore is pre-reduced in the preheating section, and has a certain degree of conductivity and high resistivity, making it possible to use a heating power supply with only one rectification.

[0033] The power adjustment range of the heating power supply is controlled at 70~100% of the full load power or the highest output voltage, preferably at 90~100%, so that the controllable rectifier device conducts when the current is small, thereby reducing the interference and harmonic pollution of the rectifier circuit to the power grid.

[0034] In summary, the electric rotary kiln for producing direct reduced iron of the present invention has high equipment efficiency, large production capacity, safe and reliable operation, low carbon emissions due to the replacement of coal (or natural gas or coal gas) with electricity, simple heating power supply circuit structure, and significantly reduced cost of heating power supply part for large-scale industrial equipment, and can realize the industrialization of direct reduced iron production by direct electric heating. Detailed Implementation

[0035] This invention uses vanadium-titanium magnetite oxide pellets as raw material, and tests the resistivity of material beds with different degrees of reduction, as well as the influence of dynamic and static changes in motion on the resistivity of the material bed. Based on these tests and other influencing factors, the technical solution of this invention is proposed.

[0036] The vanadium-titanium magnetite oxide pellets, characterized by XRD, showed that iron existed in the form of hematite and brookite, with no diffraction peaks observed in other iron-containing crystal structures. Elemental analysis revealed a total iron content of approximately 54%. Assuming all iron was reduced to metallic iron, the weight loss rate (weight reduction of the sample after reduction / weight of the raw sample) was approximately 23%. The degree of reduction was defined as: weight loss rate of the sample after reduction / 23%.

[0037] The resistance of the material bed is determined by measuring the voltage across the material bed and the current flowing through the material bed at a reaction temperature of 850~1100℃.

[0038] When the reduction degree is ~30%, the resistivity of the material is ~0.0082Ωm. When the reduction degree is ~50%, the resistivity of the material is ~0.0036Ωm.

[0039] Because the material in the rotary kiln is in motion, the resistivity change of vanadium-titanium magnetite oxide pellets reduced to a certain degree in the rotary kiln was measured under the conditions of a heating section length of 6m, a rotary kiln furnace inner diameter of 0.72m, and a filling rate of 40-50%. Within the required reaction speed range, the resistivity of the dynamic material bed is 1.7-2.5 times that of the static material bed.

[0040] The specific technical solutions of the present invention are described below with reference to embodiments. Attached Figure Description

[0041] Appendix Figure 1 This is a schematic diagram of the rotary tube of the reducing iron electric rotary kiln in Example 1, and a cross-sectional view of the rotation axis through the rotary tube.

[0042] Appendix Figure 2 This is a schematic diagram of the main circuit of the heating power supply for the reducing iron electric rotary kiln in Example 1.

[0043] Appendix Figure 3 This is a schematic diagram of the main circuit of the heating power supply for the reducing iron electric rotary kiln in Example 2.

[0044] Appendix Figure 4 This is a schematic diagram of the main circuit of the heating power supply for the reducing iron electric rotary kiln in Example 3.

[0045] Appendix Figure 5 This is a schematic diagram of the main circuit of the heating power supply for a comparative self-heating rotary kiln. Example 1

[0046] An electrically heated rotary kiln is used for large-scale industrial production of direct reduced iron. The rotary kiln has a rotary tube length of 32m, a preheating section length of 6m, a reduction section length of 20m, a cooling section length of 6m, and a furnace inner diameter of 1.0m. (See attached diagram.) Figure 1 As shown in the diagram. 1 represents the insulation and / or refractory material, 2 is the rotary tube, 3 is the furnace chamber enclosed by the refractory material, 9 is the preheating section of the furnace chamber, and 10 is the cooling section of the furnace chamber. 4, 6, and 8 are molybdenum electrodes, serving as the negative terminals of the power supply; 5 and 7 are the positive terminals of the power supply, also made of molybdenum. The electrodes 4-1, 5-1, 6-1, 7-1, and 8-1, corresponding to the five electrodes, are connected to their respective molybdenum electrodes and, outside the rotary tube, are connected to conductive busbars, then to copper slip rings and carbon brushes, and further to the corresponding heating power supply electrodes. The molybdenum electrodes inside the furnace chamber are all annular, embedded in the refractory material, and insulated from the rotary tube 2. The two positive terminals 5 and 7 are connected to the positive terminals of the two heating power supplies, and the negative terminals on either side of each positive terminal are connected to the negative terminals of the corresponding power supply. Since electrodes 4 and 6 are connected to the negative terminal of the same heating power supply, and negative electrodes 6 and 8 are simultaneously connected to the negative terminal of another heating power supply, the three negative terminals 4, 6, and 8 are short-circuited outside the rotary tube. In this embodiment, all three negative terminals of the power supply are reliably grounded. It should be noted that the accompanying drawings in this invention are only for illustrating the technical solution and technical features; the remaining mechanical and electrical control parts of the rotary kiln are not shown. The unshown parts include the kiln head and kiln tail seals, the feeding and discharging mechanism, the tires, the support rollers, the large and small transmission gears, the electrical switches, the electrical connection between the heating power supply and the brushes and copper slip rings on the outer wall of the rotary tube, etc. These parts are not shown in the drawings simply because they are unnecessary for expressing the technical features of this invention.

[0047] Both heating power supplies are three-phase full-wave rectified pulse-width modulated adjustable DC power supplies, with adjustable power supply voltage and output power, and their output power is controlled by thermocouple temperature. The two heating power supplies heat the reduction section. The reduction section consists of two heating sections: the furnace chamber between negative electrodes 4 and 6 is the first heating section, and the furnace chamber between negative electrodes 6 and 8 is the second heating section.

[0048] After entering the furnace, vanadium-titanium magnetite oxide pellets or sintered ore from ordinary ore pass through the preheating section, reduction section, and cooling section in sequence, and then exit the rotary kiln furnace.

[0049] In this invention, the lengths of the heating sections of the electrothermal rotary kiln are not equal. The length of each heating section is determined based on the resistance of the material in that section under stable operating conditions. This ensures that when the AC power from the grid, after passing through a local transformer to obtain 380V three-phase power, and then undergoing primary rectification (6 pulses) to obtain ~530V DC power, is directly supplied to the material in each heating section without secondary transformation and secondary rectification, the current will not be too large and could damage the rectifier diodes and switching transistors. (See attached diagram) Figure 2 As shown. The positive terminal of the DC power obtained from the first rectification is connected to the collector of a group of IGBT switches connected in parallel. The emitters of this group of IGBT switches are connected in parallel to the positive terminals 5-1 and 5 of the rotary kiln, while the negative terminals 4-1 and 6-1 of the rotary kiln are connected to the negative terminal of the aforementioned 530V DC power supply. The IGBT switches within the group operate synchronously at a switching frequency of ~100kHz. Under operating conditions, the power factor of the 380V three-phase power supply can reach 98%. The total harmonic distortion (THD) of the power supply is <10%.

[0050] Based on the resistivity values ​​above, the furnace length between electrodes 4 and 5 is set at 4m, and the length between electrodes 5 and 6 is set at 6m. When the rotary kiln is operating stably, with a material filling rate of ~50% in the furnace, and a DC voltage of 530V after primary rectification, the full-load power of both furnace sections is approximately 1000kW. The primary transformer for the first heating section can have a capacity of 2500kVA.

[0051] The IGBT switch can be replaced with other power switching devices such as silicon carbide MOSFETs.

[0052] By adjusting the total material filling rate of the rotary kiln furnace, which varies from 40% to 60%, and by adjusting the rotary kiln speed within a 20% range, the heating power and current can be adjusted in conjunction with this adjustment. This allows the rotary kiln heating power to be adjusted within a wide range when the DC voltage obtained from the primary rectification does not pass through a transformer and the current does not exceed the limit. Example 2

[0053] The rotary tube section in this embodiment is the same as that in Embodiment 1. (See attached image) Figure 3 This is a schematic diagram of the heating power supply in this embodiment. (See attached diagram.) Figure 3 As shown, the three-phase AC power from the power grid is transformed once by the local rectifier transformer T, then rectified and regulated by a 12-pulse thyristor (or other controllable rectifier devices), and connected to the positive terminal 5 via 5-1 on the rotary tube, and to the negative terminals 4 and 6 via 4-1 and 6-1. (See attached diagram) Figure 3 In the diagram, LP is the equalization inductor, and L is the chopper inductor, yoke inductor, or filter inductor.

[0054] By adjusting the total material filling rate of the rotary kiln furnace (ranging from 40% to 60%) and regulating the kiln speed within a 20% range, the heating power and current can be adjusted accordingly. This allows for a wide range of adjustment of the rotary kiln heating power, provided the DC voltage obtained from the primary rectification has been transformed by a transformer and the current does not exceed the allowable current. Specifically, a power modulation range of 70% to 100% of the full load power, preferably 90% to 100%, allows for control of the temperature in that heating section of the rotary kiln furnace, providing heat for the reduction reaction. Furthermore, because the power adjustment range is close to the full load power, the current when the thyristor is turned on is relatively small, resulting in less high-order harmonic interference and significantly reducing interference to the power grid. Additionally, due to the use of 12-pulse rectification, although the circuit structure is simple, the total harmonic distortion rate of the power supply is <10%.

[0055] Under otherwise unchanged conditions, the higher the rotational speed of the rotary kiln, the greater the resistance of the material bed. Therefore, increasing the rotational speed results in a decrease in heating power. Conversely, increasing the rotational speed also increases the material's velocity, leading to a greater amount of material passing through the kiln chamber per unit time (increased space velocity), thus requiring more heating power. Therefore, adjusting the electric heating power by rotating the kiln is a negative feedback process. As mentioned earlier, the technical solution of this invention utilizes this characteristic to synergistically regulate the heating power of the rotary kiln. Example 3

[0056] The rotary tube section in this embodiment is the same as that in Embodiment 1. (See attached image) Figure 4 This is a schematic diagram of the heating power supply in this embodiment. (See attached diagram.) Figure 4 As shown, the three-phase AC power from the grid is transformed once by the local rectifier transformer T, then rectified and regulated by MOSFETs, and connected to the positive terminal 5 via 5-1 on the rotary tube, and to the negative terminals 4 and 6 via 4-1 and 6-1. The MOSFET rectification and power regulation can also use Vienna type 1 and type 2 PFC circuit diagrams as the main circuit. Alternatively, the MOSFETs can be replaced with IGBT devices. The rectification and power regulation circuit in this embodiment has a wide power adjustment range and causes less interference to the power grid.

[0057] Under otherwise unchanged conditions, the higher the rotational speed of the rotary kiln, the greater the resistance of the material bed. Therefore, increasing the rotational speed results in a decrease in heating power. Conversely, increasing the rotational speed also increases the material's velocity, leading to a greater amount of material passing through the kiln chamber per unit time (increased space velocity), thus requiring more heating power. Therefore, adjusting the electric heating power by rotating the kiln is a negative feedback process. As mentioned earlier, the technical solution of this invention utilizes this characteristic to synergistically regulate the heating power of the rotary kiln.

[0058] Comparative Example Existing self-heating rotary kilns generally use low-voltage, high-current heating power supplies with a wide power adjustment range, as shown in the attached figure. Figure 5 As shown, the three-phase power from the grid is transformed once by a local ordinary transformer to obtain three-phase 380V AC power (A, B, C). This is then rectified by a three-phase full-bridge rectifier to obtain ~530V DC power. Next, it is inverted by four or more IGBT switches (VT1-4) to obtain pulse-width modulated high-frequency AC power. After a second transformation by a high-frequency transformer (HFT), it is further rectified by two diodes (VD9 and VD10) to obtain the DC power supply. The positive terminal of this DC power supply is connected to the positive terminal 5-1 on the rotary tube of the electric heating rotary kiln, and the negative terminal is connected to the negative terminals 4-1 and 6-1 on the rotary tube of the electric heating rotary kiln.

[0059] Compared with existing self-heating rotary kiln technologies, the technical solution of this invention eliminates the need for a high-frequency transformer secondary transformer and secondary rectifier diodes in the heating power supply. This significantly simplifies the circuit structure of the heating power supply, reduces the number of power devices, substantially lowers costs, and improves reliability.

Claims

1. A rotary kiln for reducing iron ore to direct reduced iron, characterized in that: The rotary kiln has an internal cavity that is nearly cylindrical or prismatic, serving as the furnace chamber. One end of the rotary kiln has a solid material inlet and a tail gas outlet, while the other end has a solid material outlet. The furnace chamber is equipped with a reduction section. A preheating section is provided at the solid material inlet end of the reduction section. After the iron ore enters the rotary kiln furnace chamber from the inlet end, it passes through the preheating section and the reduction section in sequence. A heating power electrode is provided in the reduction section to supply electricity to the conductive material in this section to heat the material. The heating power supply is a DC power supply with adjustable output power. The heating power supply electrodes set in the reduction section have two or more positive electrodes and two or more negative electrodes. The multiple positive and negative electrodes divide the furnace of the reduction section into two or more heating sections along the length of the rotary kiln, and each heating section is electrically heated by two or more heating power supplies. Each heating section feeds current to the material in the furnace in parallel, that is, the positive electrodes of the heating power supplies of adjacent heating sections are adjacent to each other or share a positive electrode, or the negative electrodes of adjacent heating sections are adjacent to each other or share a negative electrode. One or more of the two or more heating power sources are DC power sources obtained by transforming AC power from the grid through a local transformer once and then rectifying it only once. The iron ore is a material containing oxidized iron with an iron content of more than 40% by mass. After entering the rotary kiln furnace, the iron ore is preheated in the preheating section, where the iron oxide is partially reduced and metallized, so that it already contains some metallic iron when entering the reduction section, or the iron element in it has been partially reduced and contains some metallic iron when entering the preheating section of the rotary kiln furnace, thus making the iron ore conductive.

2. The reducing iron electrothermal rotary kiln according to claim 1, characterized in that: The rotary kiln is equipped with a cooling section at the solid material discharge end of the reduction section. The iron ore material enters the cooling section after passing through the reduction section.

3. A rotary kiln for reducing iron electrothermal heating according to any one of claims 1 to 2, characterized in that: An air inlet is provided at the solid material discharge end of the furnace.

4. A rotary kiln for reducing iron electrothermal heating according to any one of claims 1 to 3, characterized in that: The furnace outlet is equipped with a baffle plate to improve the filling rate of solid materials in the furnace.

5. A rotary kiln for reducing iron electrothermal heating according to any one of claims 1 to 4, characterized in that: The local transformer mentioned is a rectifier transformer.

6. A rotary kiln for reducing iron electrothermal heating according to any one of claims 1 to 4, characterized in that: The local transformer mentioned is a standard transformer.

7. A rotary kiln for reducing iron electrothermal heating according to any one of claims 5 to 6, characterized in that: One or more of the two or more heating power sources are AC power from the grid, which is transformed once by a local transformer, rectified by a diode, and then pulse-width modulated by a power switching device to obtain DC power. This DC power is supplied to the material in the heating section of the rotary kiln to heat the material.

8. A rotary kiln for reducing iron electrothermal heating according to any one of claims 5 to 6, characterized in that: One or more of the two or more heating power sources are AC power from the grid, which is transformed once by a local transformer, and then rectified and regulated once by a thyristor or a diode together with a thyristor or a diode. The resulting DC power is then supplied to the material in the heating section of the rotary kiln to heat the material.

9. A rotary kiln for reducing iron electrothermal heating according to any one of claims 5 to 6, characterized in that: One or more of the two or more heating power sources are AC power from the grid, which is transformed once by a local transformer, rectified and regulated once by an IGBT or MOSFET, and the resulting DC power is supplied to the material in the heating section of the rotary kiln to heat the material.

10. A rotary kiln for reducing iron electrothermal heating according to any one of claims 5 to 9, characterized in that: One or more of the two or more heating power sources are connected to the power grid through a local transformer, and after one and only one rectification, power adjustment by a thyristor or power switching device, the resulting DC power is supplied to the material in the heating section of the rotary kiln to heat the material; the material heated by the power source in the heating section of the rotary kiln is a resistive load, and its power adjustment range is 70~100% of the full load power.

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