Resource utilization method for roasting red mud by adopting novel tunnel type microwave kiln

Abstract

The invention discloses a resource utilization method for roasting red mud by adopting a novel tunnel type microwave kiln, which comprises the following steps: refining the red mud, a carbonaceous reducing agent and an additive, mixing according to a certain ratio, uniformly mixing by virtue of a uniform mixing machine, and pressing into balls by virtue of a ball pressing machine; the pressed balls are screened, the balls meeting the requirements after screening are dried through a drying machine, then arranged on a trolley through a material distributing machine, fed into a tunnel type microwave kiln, preheated in a natural gas flame isolation heating mode, heated in a microwave heating mode, subjected to heat preservation reduction and cooled through a cooling section, and roasting reduction material balls are obtained; and the cooled roasting reduction material balls are unloaded through an unloader, then enter a ball mill to be subjected to ball milling and then are subjected to magnetic separation through a magnetic separator, and a high-grade reduced iron powder product is obtained. The method for treating the red mud is low in energy consumption, high in reduction rate and metallization rate, low in production cost, high in intelligent degree and environmentally friendly, and the product is reduced iron powder or reduced iron powder blocks with the higher additional value.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of solid waste disposal and comprehensive utilization, and particularly relates to a method for resource utilization of red mud by using a new type of tunnel microwave kiln. BACKGROUND

[0002] With the rapid development of the global alumina industry, the overexploitation of bauxite resources has led to a continuous decline in its grade (the average grade of bauxite in China has decreased from 65% in 2010 to 58% in 2023), and 1.0-1.8 tons of red mud are discharged for producing 1 ton of alumina (the specific discharge amount depends on the grade of bauxite and the production process). According to industry statistics, as of 2024, the cumulative discharge amount of red mud worldwide has exceeded 5 billion tons, and is increasing at a rate of 120 million tons per day; China, as the largest producer of alumina in the world (the output reached 81 million tons in 2023, accounting for 58% of the global total), produces 1.1-1.3 million tons of red mud per year, and the cumulative storage amount has exceeded 700 million tons, with a storage area of more than 12,000 hectares (equivalent to 16,800 standard football fields).

[0003] The main components of red mud include Fe2O3 (25%-40%), Al2O3 (10%-20%), SiO2 (15%-25%), CaO (5%-15%), and Na2O (2%-8%), among which Fe2O3 is a metal oxide with high recycling value. If it can be efficiently recycled, not only can it alleviate the dependence on foreign resources for iron resources in China (the dependence on foreign iron ore was 78% in 2023), but also can solve the environmental problem of red mud storage. However, the comprehensive utilization rate of red mud in China has been less than 8% for a long time, which is much lower than the global average of 12%. The core bottleneck is that the existing red mud resource utilization technology has defects that are difficult to overcome:

[0004] 1. The existing reduction technology has high energy consumption and poor economic efficiency

[0005] The current red mud iron resource recycling mainly adopts the "reduction roasting-magnetic separation" process, which is divided into gas-based reduction and coal-based reduction:

[0006] • Gas-based reduction technology: CO or H2 is used as the reducing agent (such as the technology disclosed in Chinese patent CN109868043A), which requires separate construction of a coal gasification or hydrogen production device, and the cost of the reducing agent accounts for more than 40% of the production cost; and CO and H2 are both flammable and explosive gases, which require the setting of explosion-proof and leak detection systems, and the safe investment increases by 20%-30%; in addition, the gas-based reduction has strict requirements on the reaction temperature control (±5℃), and the operation difficulty is large, with a single fault downtime repair time of more than 12 hours and an annual effective operation time of less than 7000 hours.

[0007] • Coal-based reduction technology: This is a widely used technology (e.g., Chinese patent CN108587423A). It uses pulverized coal as a reducing agent, heating it to 1300-1400℃ in a rotary kiln for 4-6 hours. This technology has two major problems: First, the excessively high temperature leads to a surge in energy consumption, with energy consumption per unit of red mud treatment reaching 850-950 kWh / t (accounting for more than 35% of production costs), far exceeding the average energy consumption of sintered ore in the steel industry (450 kWh / t). Second, at high temperatures, CaO and SiO2 in the red mud easily react with the refractory materials of the kiln wall to form low-melting-point compounds (such as Ca2SiO4, melting point 1544℃, but molten at 1300℃), causing ring formation on the inner wall of the rotary kiln. This requires shutdown for cleaning 2-3 times per month, with each cleaning lasting 8-12 hours, resulting in an annual capacity loss of 15%-20%.

[0008] 2. Low grade and low recovery rate of iron products

[0009] In existing technologies, red mud mixed with reducing agents is mostly introduced into the roasting equipment in bulk form. This results in limited material contact area and uneven heating (temperature differences within the rotary kiln can reach ±80℃), leading to insufficient Fe2O3 reduction with a reduction rate of only 70%-75% and a metallization rate of less than 80%. After magnetic separation, the iron concentrate typically has a total iron content of 65%-75%, making it only suitable as an auxiliary raw material for blast furnace ironmaking (requiring a total iron content of ≥60%). Its added value is low (market price approximately 800 RMB / ton), making it difficult to cover processing costs (approximately 900 RMB / ton). Furthermore, dust emissions during bulk roasting reach 50-80 mg / m³, far exceeding the 30 mg / m³ limit in GB16297-1996 "Integrated Emission Standard for Air Pollutants," necessitating additional investment in dust removal equipment and further increasing costs.

[0010] 3. Frequent equipment malfunctions and significant environmental risks.

[0011] Traditional rotary kiln calcination equipment has structural defects: First, the tilted rotation of the kiln (3°-5° inclination angle) leads to severe material wear, resulting in annual equipment maintenance costs of 500,000-800,000 yuan per unit. Second, the problem of ring formation cannot be completely solved. During the cleaning process, waste refractory materials mix with unreduced red mud, forming new solid waste and posing a high risk of secondary pollution. Simultaneously, during the storage of red mud, the Na2O (alkaline substances) it contains will seep in with rainwater, causing the pH value of the surrounding soil to rise to 9-11 (exceeding the suitable range for crop growth, pH 6.5-7.5), and groundwater alkalinity to exceed standards (2-5 times the standard). This has already caused salinization of farmland in several villages in major alumina producing areas such as Shanxi and Henan provinces in my country, with ecological restoration costs reaching 200,000-300,000 yuan per mu.

[0012] 4. Long process flow and low level of automation

[0013] Existing technologies require seven core steps: "red mud drying, crushing, batching, roasting, cooling, ball milling, and magnetic separation." The process length is 50-80 meters, and the equipment occupies a large area (a 100,000-ton / year production line requires 15,000 square meters). Furthermore, each step requires manual monitoring of parameters (such as roasting temperature and reducing agent ratio), with labor costs accounting for 15%-20%, making it difficult to achieve automated control.

[0014] In summary, existing red mud resource utilization technologies suffer from drawbacks such as "high energy consumption, low efficiency, high pollution, and low added value." There is an urgent need to develop a new technology that is low in energy consumption, high in reduction efficiency, stable in equipment, environmentally friendly, and has high added value, in order to promote the transformation of red mud from "solid waste" to "resource." Summary of the Invention

[0015] This invention aims to overcome the shortcomings of existing red mud resource utilization technologies and provides a method for the resource utilization of red mud by roasting in a novel tunnel microwave kiln, specifically addressing the following technical problems:

[0016] 1. Reduce reaction temperature and energy consumption: Solve the problems of excessively high coal-based reduction temperature (1300-1400℃) and high energy consumption (850-950kWh / t) by reducing the reaction temperature by 150-200℃ and reducing energy consumption by more than 30%;

[0017] 2. Improve the grade and recovery rate of iron products: Solve the problems of low reduction rate (70%-75%) and metallization rate (less than 80%) in existing technologies, and achieve a reduction rate ≥90%, a metallization rate ≥90%, and a total iron content of ≥85% in the reduced iron powder after magnetic separation;

[0018] 3. Eliminate equipment ring formation and dust pollution: Solve the problems of ring formation and frequent shutdowns in traditional rotary kilns, achieving continuous equipment operation time ≥7800 hours / year; at the same time, solve the problem of dust pollution from bulk material roasting, with dust emissions ≤10mg / m³;

[0019] 4. Streamline processes and enhance intelligence: Optimize process steps and reduce equipment footprint; achieve automated control of parameters such as temperature, pressure, and microwave power to reduce labor costs;

[0020] 5. Solve the problem of red mud stockpiling pollution: Increase the utilization rate of red mud from 8% to over 90%, reduce the stockpiling area, and reduce the risk of soil and groundwater pollution.

[0021] To achieve the above objectives, the present invention provides the following technical solution:

[0022] A method for the resource utilization of red mud by roasting in a novel tunnel-type microwave kiln includes the following steps:

[0023] S1: The red mud, carbonaceous reducing agent, and additives are refined separately to obtain red mud powder, carbonaceous reducing agent powder, and additive powder with a fineness of ≤100 mesh; the red mud powder, carbonaceous reducing agent powder, and additive powder are mixed in a mass ratio of 100:16-20:3-5, and 1-3% water by mass is added to the mixture. The mixture is then mixed for 20-50 minutes by a twin-shaft paddle mixer, and then cold-pressed into shape under a pressure of 80-100t by a roller briquetting machine to obtain composite lumps with a particle size of 30mm×50mm~50mm×80mm.

[0024] S2: The composite agglomerates are screened to select qualified composite agglomerates with a particle size of 30mm×50mm~50mm×80mm; the qualified composite agglomerates are arranged on the push trolley by the material distribution machine, and the push trolley carries the qualified composite agglomerates into the preheating section, heating and reduction section and cooling section of the tunnel microwave kiln in sequence;

[0025] In the preheating section, qualified composite agglomerates are preheated to 600-800℃ using natural gas flame-retardant heating, with a natural gas consumption of 10-25 m³ / h.

[0026] In the heating and reduction section, the preheated composite agglomerate is heated to 1000-1100℃ using microwave heating at a frequency of 2450MHz and kept at that temperature for 1-3 hours. The microwave heating power is 200-1500kW.

[0027] S3: In the cooling section, a combined cooling method of "upper air cooling + lower water cooling" is used to cool the calcined reducing material balls after heat preservation and reduction to ≤100℃, and the cooling rate is controlled at 5-10℃ / min. Then, the material is unloaded by a crawler unloader.

[0028] S4: The roasted reducing material balls after unloading are fed into a grid-type ball mill for ball milling for 20-40 minutes to obtain a powdered reducing material with a -200 mesh content of ≥85%; the powdered reducing material is fed into a wet permanent magnet drum separator for magnetic separation under a magnetic field strength of 3000-5000GS to obtain a reduced iron powder product with a total iron content of ≥85%.

[0029] As a further aspect of the present invention: in step S1, the total iron content of the red mud is ≥34%, and the moisture content is ≤30%; if the moisture content of the red mud is >30%, it is first dried in a rotary dryer at 120-150℃ to a moisture content of 25%-30%, and then refined.

[0030] As a further aspect of the present invention: in step S1, the carbonaceous reducing agent is one or more of coke, pulverized coal, and petroleum coke, and the fixed carbon content of the carbonaceous reducing agent is ≥80%; the refining treatment of the carbonaceous reducing agent includes: first crushing it to a particle size ≤5mm by a hammer crusher, and then refining it to ≤100 mesh by a ball mill.

[0031] As a further aspect of the present invention: in step S1, the additive is a composite system of "flux + catalyst", wherein the flux is CaO with a purity ≥90%, the catalyst is Fe3O4 with a purity ≥95%, and the mass ratio of flux to catalyst is 1:1-2:1; the additive is refined to ≤100 mesh by an air jet mill.

[0032] As a further embodiment of the present invention: in step S1, the mixing uniformity of the dual-shaft paddle mixer is ≥95%, and the mass deviation of each component during sampling and testing is ≤2%; the compressive strength of the composite agglomerate is ≥15MPa, and the yield of qualified composite agglomerates after sieving is ≥90%.

[0033] As a further aspect of the present invention: In step S2, the tunnel-type microwave kiln has a total length of 30-50m, a preheating section of 8-12m, a heating and reduction section of 15-25m, and a cooling section of 7-13m. Each section is isolated by an Al2O3 ceramic sealing door. The preheating section shell is lined with a 100mm thick aluminum silicate fiber insulation layer, and 4-6 gas burners with a heat load of 300,000 kcal / h are arranged on the top. The gas supply pipeline of the gas burners is connected in series with a solenoid valve, a flow regulating valve, a pressure gauge, and a flame arrester. The top of the preheating section is also equipped with a gas leak meter with a detection limit of ≤0.5%LEL and a K-type thermocouple temperature sensor.

[0034] As a further aspect of the present invention: In step S2, the inner lining of the heating and reduction section shell is a 150mm thick high-alumina brick insulation layer, and 8-12 microwave generators are evenly arranged on both side walls; each microwave generator is connected to a brass straight waveguide, and the waveguide end is equipped with a power distributor and a three-screw tuner; a water-cooled power feed port with a cooling water flow rate of 10-15L / min is provided at the connection between the waveguide and the kiln body; the inner wall of the heating and reduction section is covered with an 8mm thick Al2O3 ceramic microwave penetrating layer with a microwave transmittance ≥95%, and a 5mm thick SiC wave-inducing material; the top of the heating and reduction section is equipped with a dual-directional coupler and a water-loaded circulator for detecting and absorbing microwave reflected power, with a reflected power ≤5%.

[0035] As a further aspect of the present invention: In step S2, the propulsion trolley is made of HT350 high-temperature resistant cast iron, with dimensions of 2000mm×1200mm×800mm, a load capacity of 500kg / unit, and Si3N4 ceramic rollers at the bottom; the propulsion speed of the propulsion trolley is 500-800mm / min, driven by a chain propulsion machine, and the propulsion speed is adjusted by programming through a PLC control system.

[0036] As a further aspect of the present invention: in step S3, the specific parameters of the composite cooling method are as follows: air cooling speed of 3-5 m / s, cooling the calcined reducing material balls from 1000-1100℃ to 300-400℃; water cooling flow rate of 15-20 L / min, cooling the calcined reducing material balls from 300-400℃ to ≤100℃; the unloading capacity of the crawler unloader is 500 kg / time, and a PVC sealing cover is used to collect dust during the unloading process.

[0037] As a further aspect of the present invention: in step S4, the grinding media of the grid-type ball mill is high-chromium steel balls with a diameter of 20-50mm, and the steel ball ratio is 30% for 50mm, 50% for 30mm, and 20% for 20mm; the drum speed of the wet permanent magnet drum separator is 30-40r / min, and the flushing water flow rate is 5-8m³ / h; the non-magnetic tailings after magnetic separation are used to prepare building bricks or cement admixtures, and the overall utilization rate of red mud is ≥90%.

[0038] Compared with the prior art, the beneficial effects of the present invention are:

[0039] 1. The method of the present invention uses natural gas plus microwave heating for roasting and reduction. The material is first heated to 600-800℃, and then the selective, instantaneous, penetrating, non-contact, and low thermal inertia characteristics of microwaves are utilized to achieve high thermal efficiency. Compared with traditional methods, the extraction temperature is reduced by 150-200℃ and energy consumption is reduced by about 30%. Furthermore, the reduction rate and metallization rate are higher.

[0040] 2. The method of the present invention uses a tunnel kiln, which does not have the technical problems of ring formation on the inner wall of traditional rotary kilns, and is energy-saving, safe and environmentally friendly.

[0041] 3. The final product produced by the method of the present invention is reduced iron powder with higher added value.

[0042] 4. The reduced spheres produced by the reduction roasting method of the present invention are refined by ball milling and then subjected to magnetic separation to obtain high-grade reduced iron powder.

[0043] 5. The method of the present invention heats the material in a static environment and uses pellet form, resulting in almost no dust pollution and exhibiting low-carbon and environmentally friendly characteristics. Attached Figure Description

[0044] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0045] Figure 1 This is a process flow diagram of a new type of tunnel microwave kiln for the roasting and utilization of red mud resources. Detailed Implementation

[0046] In the description of this invention, it should be understood that the terms "center", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0047] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "configuration" should be interpreted broadly. For example, they can refer to a fixed connection or configuration, a detachable connection or configuration, or an integral connection or configuration. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0048] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] Please see Figure 1 In this embodiment of the invention, a method for the resource utilization of red mud by roasting in a novel tunnel microwave kiln is presented. The process route is "material pretreatment - tunnel microwave kiln composite heating and reduction - cooling - ball milling and magnetic separation." The core of this method lies in combining "natural gas flame-insulating preheating" with "microwave selective heating," along with tunnel kiln static roasting and pelletizing technology. The specific steps are as follows:

[0050] Step S1: Material pretreatment and preparation of composite agglomerates

[0051] 1. Material selection and refinement:

[0052] Red mud: Select red mud with a total iron content ≥34% and moisture content ≤30% (if the moisture content of the red mud is >30%, it needs to be dried to 25%-30% in a rotary dryer at a drying temperature of 120-150℃ to avoid clumping during refining due to excessive moisture); refine the red mud to a fineness ≤100 mesh (particle size ≤150μm) using a Raymond mill. After refining, the specific surface area of ​​the red mud increases from 0.5m² / g to 2.8m² / g, increasing the contact area with the reducing agent.

[0053] Carbonaceous reducing agent: Select one or more of coke, pulverized coal, and petroleum coke, with a fixed carbon content of ≥80% (e.g., 85%-90% fixed carbon in coke, 80%-85% fixed carbon in pulverized coal, and 95%-98% fixed carbon in petroleum coke); after being crushed by a hammer crusher, it is further refined to ≤100 mesh by a ball mill to ensure uniform dispersion of the reducing agent.

[0054] Additives: A composite system of "flux + catalyst" is selected. The flux is CaO (purity ≥90%), which can reduce the melting point of gangue (SiO2, Al2O3) in red mud (from 1600℃ to below 1200℃) and reduce the formation of high-temperature melts. The catalyst is Fe3O4 (purity ≥95%), which can accelerate the reduction reaction rate of Fe2O3→Fe3O4→Fe (reaction activation energy from 180kJ / mol to 120kJ / mol). The additives are refined to ≤100 mesh by air jet mill to avoid coarse particles affecting the strength of the agglomerates.

[0055] 1. Ingredient preparation and mixing:

[0056] Mix red mud powder : carbonaceous reducing agent powder : additives in a mass ratio of 100 : 16-20 : 3-5 (e.g., 1000 kg red mud, 180 kg coke, 20 kg CaO, 10 kg Fe3O4); feed the mixture into a twin-shaft paddle mixer, and simultaneously add 1%-3% deionized water by mass of the mixture (moisture control standard: it should clump together when squeezed in hand, but crumble when dropped), and mix for 20-50 minutes (the specific time should be adjusted according to the amount of mixture: 20 minutes for 500 kg of material, 50 minutes for 2000 kg of material), ensuring that the uniformity of the material is ≥95% (sampling and testing, deviation of each component ≤2%).

[0057] 1. Cold pressing:

[0058] The mixed material is fed into a roller briquetting machine, and the briquetting pressure is set to 80-100t (pressure selection standard: 30mm×50mm briquette with compressive strength ≥15MPa to avoid breakage during transportation and roasting). It is pressed into rectangular composite briquettes with a particle size of 30mm×50mm~50mm×80mm (rectangular structure facilitates uniform material distribution and increases the heating area by 15% compared to spherical briquettes). After forming, the briquettes are conveyed to a vibrating screen (screen holes 30mm×50mm and 50mm×80mm) by a belt conveyor to remove briquettes that are too fine (<30mm×50mm) or too coarse (>50mm×80mm) (too fine briquettes are easily carried away by airflow, and too coarse briquettes are unevenly heated). The yield of qualified briquettes is ≥90%.

[0059] Step S2: Combined heating and reduction in a tunnel microwave kiln

[0060] The core equipment in this step is a new type of tunnel microwave kiln. The total length of the kiln is 30-50m (adjusted according to the processing capacity: 30m for a 50,000-ton / year production line and 50m for a 200,000-ton / year production line). It is divided into three sections: a preheating section (8-12m long), a heating and reduction section (15-25m long), and a cooling section (7-13m long). Each section is isolated from the other by a high-temperature resistant sealing door (material: Al2O3 ceramic + asbestos gasket) to prevent temperature crosstalk and microwave leakage.

[0061] 1. Equipment structural details:

[0062] Preheating section: The inner wall of the shell is made of 16MnR heat-resistant steel (temperature resistance ≤900℃), and lined with a 100mm thick aluminum silicate fiber insulation layer (thermal conductivity ≤0.03W / (m・K)); 4-6 gas burners (model: RX-30, heat load 300,000 kcal / h) are evenly arranged on the top. The burners are connected to the natural gas supply system through pipelines. The pipelines are connected in series with a solenoid valve (model: ZCS-10, response time ≤0.5s), a flow regulating valve (accuracy ±2%), a pressure gauge (range 0-1MPa, accuracy 0.4 grade), and a flame arrester (model: GZW-1, explosion-proof rating ExdIIBT4); the top of the preheating section is also equipped with a gas leak meter (detection limit ≤0.5%LEL) and a temperature sensor (K-type thermocouple, accuracy ±1℃) to monitor the natural gas concentration and preheating temperature in real time.

[0063] Heating and reduction section: The shell is made of 304 stainless steel (temperature resistance ≤1200℃), with a 150mm thick high-alumina brick insulation layer (thermal conductivity ≤0.04W / (m・K)); 8-12 microwave generators (model: MW-200, single unit power 200kW, frequency 2450MHz, conforming to GB / T14507-1993 "Safety Requirements for Microwave Energy Application Equipment") are evenly arranged on both side walls. Each microwave generator is connected to a straight waveguide (material: brass, cross-sectional area 240mm×120mm), and a power distributor is installed at the end of the waveguide (to evenly distribute the microwave power into the kiln). The waveguide is equipped with a power uniformity of ≥90% and a three-screw tuner (to match microwave impedance and reduce reflected power by ≤5%). A water-cooled feed port (cooling water flow rate 10-15L / min, inlet water temperature ≤30℃) is provided at the connection between the waveguide and the kiln body to prevent the waveguide from overheating. The inner wall of the kiln is covered with a microwave penetration layer (material: Al2O3 ceramic plate, thickness 8mm, microwave transmittance ≥95%) and a wave-inducing material (material: SiC, thickness 5mm, to enhance microwave absorption). The top is equipped with a dual-directional coupler (to detect incident and reflected power) and a water-loaded circulator (to absorb reflected microwaves and avoid damaging the generator).

[0064] Cooling section: The shell is made of Q235 steel and lined with a 50mm thick rock wool insulation layer; the upper part is arranged with air-cooled pipes (wind speed 3-5m / s, wind temperature 25-30℃), and the lower part is arranged with water-cooled coils (cooling water inlet temperature ≤25℃, outlet temperature ≤50℃), forming a "top air-cooling + bottom water-cooling" composite cooling system; a temperature sensor (accuracy ±1℃) is installed at the end of the cooling section to monitor the outlet temperature of the material ball.

[0065] The propulsion system includes a propulsion trolley (material: HT350 high-temperature resistant cast iron, dimensions 2000mm×1200mm×800mm, load capacity 500kg / unit) and a chain propulsion machine (propulsion speed 500-800mm / min, adjustable via PLC programming). The bottom of the trolley is equipped with high-temperature resistant rollers (material: Si3N4 ceramic, temperature resistance ≤1200℃) to ensure the trolley moves smoothly inside the kiln. The surface of the trolley is covered with refractory bricks (material: high-alumina brick, thickness 50mm) to prevent high-temperature damage to the trolley.

[0066] Control system: The system adopts a PLC control system (model: S7-1200), which connects to sensors such as temperature, pressure, flow rate, and microwave power to collect data in real time and display it on the touch screen. Automatic control logic can be set (such as automatically increasing the natural gas flow rate when the preheating temperature is below 600℃; automatically adjusting the three-screw mixer when the microwave reflection power exceeds 10%) to achieve unattended operation.

[0067] 1. Heating reduction process parameters:

[0068] Preheating stage: The propulsion trolley carries qualified composite agglomerates into the preheating section. The natural gas burners are started, and the agglomerates are preheated to 600-800℃ using indirect heating (the high-temperature flue gas generated by natural gas combustion does not directly contact the agglomerates, but is transferred through radiation from the kiln wall). The preheating time is 30-60 minutes (36 minutes when the trolley propulsion speed is 500 mm / min). The natural gas consumption is adjusted according to the preheating temperature: 10-15 m³ / h at 600℃, 15-20 m³ / h at 700℃, and 20-25 m³ / h at 800℃. The purpose of preheating is to evaporate the moisture in the agglomerates (reducing the moisture content from 1%-3% to ≤0.5%), and at the same time to initially activate the carbonaceous reducing agent (increasing the reactivity of carbon by 30%), laying the foundation for subsequent microwave reduction.

[0069] Heating and Reduction Stage: After preheating, the trolley enters the heating and reduction section. The microwave generator is started, with an output power of 200-1500kW (500kW for a processing capacity of 5t / h, and 1000kW for 10t / h), raising the agglomerate temperature from 600-800℃ to 1000-1100℃ and maintaining this temperature for 1-3 hours (the holding time is adjusted according to the total iron content of the red mud: 3 hours for 34% total iron, and 1 hour for 36%). The advantage of microwave heating lies in "selective heating"—microwaves preferentially act on Fe2O3 (dielectric constant) in the red mud. The reduction is achieved by absorbing microwaves from the carbonaceous reducing agent (ε'=15-20) and the gangue (SiO2ε'=3-5, Al2O3ε'=2-4), while the gangue absorbs less microwaves, thus achieving "targeted reduction" and reducing ineffective energy consumption. During the heat preservation reduction process, Fe2O3 undergoes the following reactions in sequence: 3Fe2O3+C→2Fe3O4+CO↑ (600-800℃), Fe3O4+C→3FeO+CO↑ (800-1000℃), FeO+C→Fe+CO↑ (1000-1100℃), ultimately producing metallic iron.

[0070] Exhaust gas treatment: CO (volume fraction 5%-8%) and water vapor generated in the heating and reduction section are introduced into the burner through the exhaust pipe, mixed with air and then burned (combustion temperature 800-900℃). The flue gas after combustion (mainly containing CO2 and N2) recovers heat through a waste heat boiler (generating 0.3MPa saturated steam for red mud drying), and then is treated by a bag filter (dust removal efficiency ≥99.5%). The final exhaust gas dust emission is ≤10mg / m³, which meets the national standard.

[0071] Step S3: Cooling and unloading

[0072] 1. Composite Cooling: The calcined reducing pellets after heating and reduction enter the cooling section with the trolley. They are first cooled by air (wind speed 3-5m / s) to reduce the temperature from 1000-1100℃ to 300-400℃ (cooling time 20-30min), and then cooled by water (cooling water flow rate 15-20L / min) to ≤100℃ (cooling time 15-25min). The cooling rate is controlled at 5-10℃ / min to avoid the pellets from cracking due to thermal stress caused by excessive temperature difference (cracking rate ≤5%).

[0073] 2. Climbing unloading: After cooling, the trolley exits the tunnel microwave kiln and enters the climbing unloading machine (model: PX-500, unloading capacity 500kg / time); the unloading machine drives the trolley to tilt (tilt angle 30°-45°) via a chain, unloading the calcined reducing material balls onto the belt conveyor. After cleaning, the trolley returns to the material distribution area for reuse (trolley turnaround time ≤120min); during the unloading process, a sealed cover (material: PVC) is used to collect dust, which is then sent to a bag filter through a pipeline to avoid secondary pollution.

[0074] Step S4: Ball milling and magnetic separation

[0075] 1. Ball milling refinement: The calcined reducing material balls are fed into a grid-type ball mill (model: MQG1530, effective volume 40m³), and the grinding media are high-chromium steel balls with a diameter of 20-50mm (steel ball ratio: 30% Φ50mm, 50% Φ30mm, and 20% Φ20mm). The ball milling time is 20-40min (the ball milling endpoint is judged by the proportion of -200 mesh (particle size ≤74μm) in the powdered reducing material being ≥85%). The purpose of ball milling is to dissociate the reduced metallic iron particles (particle size 10-50μm) from the gangue, with a dissociation degree ≥90% (observed under a microscope, the separation rate of metallic iron particles from gangue is ≥90%).

[0076] 2. Magnetic separation: The powdered raw material is fed into a wet permanent magnet drum separator (model: CTB1240, magnetic field strength 3000-5000GS). The drum speed is 30-40 r / min, and the washing water flow rate is 5-8 m³ / h. The magnetic field strength is adjusted according to the target iron grade: if ≥85% iron is required, the magnetic field strength is set to 3500-4000GS; if ≥88% iron is required, the magnetic field strength is set to 4000-5000GS. Two products are obtained after magnetic separation:

[0077] Magnetic products: Reduced iron powder, with a total iron content of ≥85% and a moisture content of ≤10%, can be directly used as a raw material for powder metallurgy (for manufacturing mechanical parts) or as a high-quality raw material for blast furnace ironmaking (replacing part of the iron ore).

[0078] Non-magnetic products: tailings, mainly containing Al2O3 (15%-20%) and SiO2 (20%-25%), can be used to prepare building bricks (compressive strength ≥10MPa) or cement admixtures (dosage ≤15%), realizing the full utilization of red mud components.

[0079] It should be noted that this invention is a method for the resource utilization of red mud by roasting in a novel tunnel microwave kiln. All components are general standard parts or parts known to those skilled in the art. Their structure and principle can be learned by those skilled in the art through technical manuals or conventional experimental methods.

[0080] Example 1: Red mud treatment using high-grade petroleum coke as a reducing agent

[0081] 1. Material parameters

[0082] • Red mud: Total iron content 35%, moisture 27%, particle size 200 mesh (refined to 100 mesh by Raymond mill);

[0083] • Carbonaceous reducing agent: Petroleum coke, fixed carbon content 97%, refined to 100 mesh;

[0084] • Additives: CaO (flux, 92% purity) + Fe3O4 (catalyst, 96% purity), mass ratio 1:1, refined to 100 mesh.

[0085] 2. Process steps and parameters

[0086] •S1 Pretreatment: The ingredients are prepared according to the ratio of red mud: petroleum coke: additives = 100:16:3 (mass ratio) (1000kg red mud, 160kg petroleum coke, 1.5kg CaO, 1.5kg Fe3O4); 3% water (35.4kg) is added, and the mixture is mixed for 20min using a twin-shaft mixer; the mixture is then pressed into 30mm×50mm composite briquettes using a roller briquetting machine at a pressure of 85t, and the yield of qualified briquettes after sieving is 92%.

[0087] •S2 calcination and reduction: trolley propulsion speed 500mm / min, natural gas consumption in the preheating section 25m³ / h, preheating temperature 800℃ (preheating time 36min); microwave power in the heating and reduction section 200kW, heating to 1050℃, holding for 1h; waste heat recovery after CO combustion of the exhaust gas generates 0.3MPa steam, and dust emission after dust removal is 8mg / m³.

[0088] •S3 Cooling and Unloading: Air cooling speed 3m / s, cooling to 350℃; water cooling flow rate 15L / min, cooling to 80℃; unloading by crawler unloader, trolley turnaround time 100min.

[0089] •S4 ball mill magnetic separation: Grid-type ball mill for 20 minutes, -200 mesh accounts for 88%; wet magnetic separator with magnetic field strength of 3000GS and washing water flow rate of 5m³ / h.

[0090] 3. Implementation Results

[0091] • The reduction rate of the roasted reducing pellets was 92.3%, and the metallization rate was 91.7%;

[0092] • Reduced iron powder has a total iron content of 87.3%, an S content of 0.04%, and a P content of 0.02%;

[0093] • The tailings contain 18% Al2O3 and 22% SiO2, and are used to prepare building bricks with a compressive strength of 12MPa.

[0094] • The energy consumption for treating red mud is 560 kWh / t, and the amount of reducing agent is 16%, which meets the design target.

[0095] Example 2: Red mud treatment using a pulverized coal-coke mixed reducing agent

[0096] 1. Material parameters

[0097] • Red mud: Total iron content 36%, moisture 25%, refined to 100 mesh;

[0098] • Carbonaceous reducing agent: Coal powder (85% fixed carbon) + coke (88% fixed carbon), mass ratio 1:1, after mixing, the fixed carbon is 86.5%, and it is refined to 100 mesh;

[0099] • Additive: CaO (90% purity), refined to 100 mesh.

[0100] 2. Process steps and parameters

[0101] •S1 Pretreatment: Prepare materials according to the ratio of red mud: mixed reducing agent: CaO = 100:18:4 (mass ratio) (1000 kg red mud, 180 kg mixed reducing agent, 40 kg CaO); add 2% water (24.4 kg), mix in a mixer for 30 min; press into 40 mm × 60 mm composite briquettes at a pressure of 90 t, with a qualified briquette yield of 95%.

[0102] •S2 calcination and reduction: trolley propulsion speed 600mm / min, natural gas consumption in the preheating section 18m³ / h, preheating temperature 700℃ (preheating time 30min); microwave power in the heating and reduction section 800kW, heating to 1080℃, holding for 2h; exhaust dust emission 7mg / m³.

[0103] •S3 Cooling and Unloading: Air cooling speed 4m / s, cooling to 300℃; water cooling flow rate 18L / min, cooling to 70℃; trolley turnaround time 90min.

[0104] •S4 ball mill magnetic separation: ball milling for 25 minutes, -200 mesh accounting for 90%; magnetic separator magnetic field strength 3500GS, rinsing water flow rate 6m³ / h.

[0105] 3. Implementation Results

[0106] • Reduction rate 93.5%, metallization rate 92.8%;

[0107] • The reduced iron powder has a total iron content of 88.1%, an S content of 0.03%, and a P content of 0.01%, meeting the standard for iron powder used in powder metallurgy (GB / T4136-2022).

[0108] • Tailings are used as cement admixtures (12% dosage), with a cement compressive strength of 42.5 MPa (compliant with GB175-2020).

[0109] • With a unit energy consumption of 580 kWh / t and a reducing agent dosage of 18%, the economic benefits are significant.

[0110] Example 3: High-efficiency treatment of low-grade red mud

[0111] 1. Material parameters

[0112] • Red mud: Total iron content 34%, moisture 28%, refined to 100 mesh;

[0113] • Carbonaceous reducing agent: Coke (82% fixed carbon), refined to 100 mesh;

[0114] • Additives: CaO (91% purity) + Al2O3 (dispersant, 95% purity), mass ratio 2:1, refined to 100 mesh.

[0115] 2. Process steps and parameters

[0116] •S1 Pretreatment: The ingredients are prepared according to the ratio of red mud:coke:additive = 100:20:5 (mass ratio) (1000kg red mud, 200kg coke, 3.3kg CaO, 1.7kg Al2O3); 1% water (12.05kg) is added, and the mixture is mixed for 50min in a mixer; the briquetting pressure is 100t, and the briquetting is made into 50mm×80mm composite briquettes, with a qualified briquette yield of 91%.

[0117] •S2 roasting and reduction: trolley propulsion speed 800mm / min, natural gas consumption in the preheating section 10m³ / h, preheating temperature 600℃ (preheating time 24min); microwave power in the heating and reduction section 1500kW, heating to 1100℃, holding for 3h; exhaust gas waste heat recovery efficiency 80%.

[0118] •S3 Cooling and Unloading: Air cooling speed 5m / s, cooling to 400℃; water cooling flow rate 20L / min, cooling to 90℃; trolley turnaround time 120min.

[0119] •S4 ball mill magnetic separation: ball milling for 40 minutes, -200 mesh accounts for 85%; magnetic separator magnetic field strength 4000GS, rinsing water flow rate 8m³ / h.

[0120] 3. Implementation Results

[0121] • Reduction rate 91.2%, metallization rate 90.5% (meeting design target ≥90%).

[0122] • Reduced iron powder has a total iron content of 86.7% and can be used as a raw material for blast furnace ironmaking, replacing 20% ​​of iron ore;

[0123] • Tailings residue contains 15% Al2O3 and 25% SiO2, and the brick compressive strength is 10MPa;

[0124] • The unit energy consumption is 630 kWh / t, which is higher than that of Examples 1 and 2, but still 34% lower than that of traditional technology, making it suitable for the treatment of low-grade red mud.

[0125] Comparative Analysis of Examples

[0126] Indicators Example 1 Example 2 Example 3 Conventional technology Red mud total iron content (%) 35 36 34 35 Reduction agent type Petroleum coke Coal + coke Coke Coal powder Microwave power (kW) 200 800 1500 - Roasting temperature (°C) 1050 1080 1100 1350 Soaking time (h) 1 2 3 4 Reduction rate (%) 92.3 93.5 91.2 72 Metalization rate (%) 91.7 92.8 90.5 78 Reduced iron powder total iron (%) 87.3 88.1 86.7 70 Unit energy consumption (kWh / t) 560 580 630 880 Dust emission (mg / m³) 8 7 9 65

[0127] As shown in the table above, the present invention can achieve a reduction rate of ≥90%, a metallization rate of ≥90%, and a total iron content of ≥85% in reduced iron powder under different red mud grades and different types of reducing agents. Moreover, the energy consumption and dust emissions are far lower than those of traditional technologies, proving the stability and universality of the present invention.

[0128] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0129] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for roasting red mud resource utilization by using a novel tunnel microwave kiln, characterized in that, The method comprises the following steps: S1: refining the red mud, carbonaceous reducing agent and additive respectively to obtain red mud powder, carbonaceous reducing agent powder and additive powder with fineness ≤100 mesh; mixing the red mud powder, carbonaceous reducing agent powder and additive powder according to a mass ratio of 100:16-20:3-5, adding 1-3% water to the mixture, mixing the mixture in a double-shaft paddle mixer for 20-50 min, and cold-pressing the mixture into a composite briquette with a particle size of 30mm×50mm-50mm×80mm under a pressure of 80-100t by means of a roller-type ball press machine; S2: screening the composite briquette to select qualified composite briquettes with a particle size of 30mm×50mm-50mm×80mm; and arranging the qualified composite briquettes on a pushing trolley by means of a distributing machine, and then sequentially feeding the pushing trolley carrying the qualified composite briquettes into a preheating section, a heating and reducing section and a cooling section of a tunnel-type microwave kiln; In the preheating section, the qualified composite briquettes are preheated to 600-800℃ by means of natural gas flame shielding heating, and the natural gas consumption is 10-25m³ / h; In the heating and reducing section, the preheated composite briquettes are heated to 1000-1100℃ by means of microwave heating with a frequency of 2450MHz, and are kept at the temperature for 1-3h, and the microwave heating power is 200-1500kW; S3: in the cooling section, the roasted and reduced material balls are cooled to ≤100℃ by means of combined cooling of "upward air cooling+downward water cooling" at a cooling speed of 5-10℃ / min, and then the material balls are unloaded by means of a climbing unloading machine; S4: feeding the roasted and reduced material balls after unloading into a lattice-type ball mill for ball milling for 20-40min to obtain powdery reducing material with a proportion of-200 mesh ≥85%; and feeding the powdery reducing material into a wet-type permanent magnetic cylinder magnetic separator for magnetic separation treatment under a magnetic field intensity of 3000-5000GS to obtain reduced iron powder products with a total iron content ≥85%.

2. The method for roasting red mud for resource utilization by using the novel tunnel microwave kiln according to claim 1, characterized in that, In step S1, the total iron content of the red mud is ≥34%, and the moisture content is ≤30%; if the moisture content of the red mud is >30%, the red mud is first dried by a rotary drum dryer at 120-150℃ until the moisture content is 25%-30%, and then refined.

3. The method for roasting red mud for resource utilization by using the novel tunnel microwave kiln according to claim 1, characterized in that, In step S1, the carbonaceous reducing agent is a mixture of one or more of coke, coal powder and petroleum coke, and the fixed carbon content of the carbonaceous reducing agent is ≥80%; the refining of the carbonaceous reducing agent comprises: first crushing by a hammer crusher to a particle size ≤5mm, and then refining by a ball mill to ≤100 mesh.

4. The method for roasting red mud for resource utilization by using the novel tunnel microwave kiln according to claim 1, characterized in that, In step S1, the additive is a composite system of "fluxing agent+ catalyst", the fluxing agent is CaO with a purity ≥90%, the catalyst is Fe3O4 with a purity ≥95%, and the mass ratio of the fluxing agent to the catalyst is 1:1-2:1; the additive is refined to ≤100 mesh by means of an air flow pulverizer.

5. The method for roasting red mud for resource utilization by using the novel tunnel microwave kiln according to claim 1, characterized in that, In step S1, the mixing uniformity of the double-shaft paddle mixer is ≥95%, and the mass deviation of each component is ≤2% when sampling and testing; the compressive strength of the composite briquette is ≥15MPa, and the yield of the qualified composite briquette after screening is ≥90%.

6. The method for roasting red mud for resource utilization by using the novel tunnel microwave kiln according to claim 1, characterized in that, In step S2, the total length of the tunnel microwave kiln is 30-50 m, the length of the preheating section is 8-12 m, the length of the heating reduction section is 15-25 m, and the length of the cooling section is 7-13 m, and the sections are separated by Al2O3 ceramic sealing doors; the preheating section is lined with a 100 mm thick silica aluminum fiber insulation layer, and 4-6 gas burners with a heat load of 300,000 kcal / h are arranged at the top, and an electromagnetic valve, a flow regulating valve, a pressure gauge and a flame arrestor are connected in series on the gas supply pipeline of the gas burner; a gas leakage detector with a lower limit of ≤0.5% LEL and a K-type thermocouple temperature sensor are also provided at the top of the preheating section.

7. The method for roasting red mud for resource utilization by using the novel tunnel microwave kiln according to claim 1, characterized in that, In step S2, the heating reduction section is lined with a 150 mm thick high alumina brick insulation layer, and 8-12 microwave generators are evenly arranged on the two side walls; each microwave generator is connected to a straight waveguide made of brass, and the end of the waveguide is provided with a power distributor and a three-screw adjuster; a water-cooled energy feeding port with a cooling water flow rate of 10-15 L / min is provided at the connection between the waveguide and the kiln body; an Al2O3 ceramic microwave penetration layer with a thickness of 8 mm and a microwave transmittance of ≥95% is laid on the inner wall of the heating reduction section, and a SiC waveguide material with a thickness of 5 mm is laid on the inner wall of the heating reduction section; a double directional coupler and a water-loaded ring-shaped device are provided at the top of the heating reduction section for detecting and absorbing microwave reflected power, and the reflected power is ≤5%.

8. The method for roasting red mud for resource utilization by using the novel tunnel microwave kiln according to claim 1, characterized in that, In step S2, the advancing trolley is made of HT350 high-temperature-resistant cast iron, with dimensions of 2000 mm x 1200 mm x 800 mm, a load capacity of 500 kg per trolley, and Si3N4 ceramic rollers at the bottom; the advancing speed of the advancing trolley is 500-800 mm / min, driven by a chain advancing machine, and the advancing speed is adjusted by programming the PLC control system.

9. The method for roasting red mud for resource utilization by using the novel tunnel microwave kiln according to claim 1, characterized in that, In step S3, the specific parameters of the composite cooling method are as follows: air cooling at a speed of 3-5 m / s to cool the calcined and reduced material balls from 1000-1100℃ to 300-400℃; water cooling at a flow rate of 15-20 L / min to cool the calcined and reduced material balls from 300-400℃ to ≤100℃; the unloading capacity of the climbing unloader is 500 kg per time, and a PVC sealing cover is used to collect dust during the unloading process.

10. The method for roasting red mud for resource utilization by using the novel tunnel microwave kiln according to claim 1, characterized in that, In step S4, the grinding medium of the lattice-type ball mill is Φ20-50 mm high-chromium steel balls, with a ratio of Φ50 mm balls accounting for 30%, Φ30 mm balls accounting for 50%, and Φ20 mm balls accounting for 20%; the drum rotation speed of the wet-type permanent magnet drum magnetic separator is 30-40 r / min, and the flushing water flow rate is 5-8 m³ / h; the non-magnetic tailings after magnetic separation are used to prepare building bricks or cement admixtures, and the overall utilization rate of red mud is ≥90%.

Images