Method for preparing aluminate clinker by using new dry process cement kiln and reforming method

Abstract

This invention discloses a method for preparing aluminate clinker using a novel dry-process cement kiln, comprising: S1, drying and crushing Bayer process red mud; S2, mixing and batching the dried red mud powder obtained in step S1 according to the target mineral composition of the aluminate clinker; S3, feeding the raw meal powder obtained in step S2 into a five-stage / six-stage cyclone preheater system of the novel dry-process cement kiln for preheating, gradually increasing the raw meal temperature to 700℃; S4, the preheated raw meal entering a rotary kiln, controlling the formation of a weak reducing atmosphere in the kiln's firing zone; S5, the sintered high-temperature clinker entering a grate cooler for rapid cooling; S6, extracting some of the alkali-containing flue gas from the kiln tail flue and rapidly cooling it to below 400℃. This method utilizes existing novel dry-process cement kiln equipment, effectively solving the problems of crusting and blockage caused by high-alkali materials and process conflicts, while ensuring that the aluminate clinker does not spontaneously pulverize.

Description

Technical Field

[0001] This invention relates to the field of aluminate clinker preparation, and more specifically, to a method and modification method for preparing aluminate clinker using a novel dry-process cement kiln. Background Technology

[0002] The main production processes for alumina include the Bayer process, the sintering process, and the Bayer-sintering combined process. Among these, the sintering process and the combined process hold an irreplaceable strategic position in the alumina industry because they can process low-grade bauxite and effectively recover alumina and alkali from Bayer process red mud. Currently, the wet sintering process is widely used in industry, where raw materials are directly injected into a rotary kiln in the form of a slurry with a moisture content of 36%–42% for sintering. Although this process results in uniform batching and stable clinker quality, the evaporation of water in the raw material slurry consumes a large amount of heat, with its latent heat of vaporization accounting for about 40% of the total heat consumption of clinker sintering. This leads to persistently high energy consumption in the sintering process, becoming a major bottleneck restricting its economic viability.

[0003] To reduce energy consumption, the industry has attempted to adopt dry or semi-dry kiln feeding processes, reducing evaporation heat consumption by lowering the moisture content of raw materials. However, traditional dry sintering still requires the construction or modification of dedicated rotary kiln systems, resulting in large investments, long development cycles, and difficulties in industrialization. Meanwhile, new dry-process cement kilns, after decades of development, have formed a highly efficient sintering system consisting of a preheater, decomposer, rotary kiln, and grate cooler, offering significant advantages such as low heat consumption, stable operation, and high environmental protection standards. Furthermore, given the severe overcapacity in my country's cement industry, with many new dry-process production lines operating at low loads and abundant surplus capacity, a potential technological and economic foundation has been established for replacing alumina wet-process sintering kilns with cement kilns.

[0004] However, directly using the new dry process cement kiln for aluminate clinker sintering faces problems such as crusting and blockage caused by the high-alkali and high-alumina liquid phase, and compatibility issues between the clinker pulverization characteristics and the stable operation of the equipment.

[0005] Raw materials used in alumina sintering contain large amounts of alkali (sodium, potassium) and aluminum. In the high-temperature environment of a cement kiln (approximately 1250–1400°C), the alkali components easily volatilize and enter the flue gas. After the flue gas enters the preheater system in a reverse flow, the temperature gradually decreases, and the alkali components condense and adsorb on the surface of the raw material particles, returning to the kiln with the material, forming a cyclic enrichment process of "vaporization-condensation-reflux." The melting point of alkali is approximately 800°C, and during the circulation process, a large amount of low-melting-point liquid phase is easily generated, leading to severe scaling and even blockage in the preheater, feed pipe, and flue gas chamber. This forces frequent kiln shutdowns for cleaning, making continuous and stable production impossible.

[0006] Furthermore, dicalcium silicate (C2S) in aluminate clinker undergoes a crystal transformation during cooling, changing from β-C2S to γ-C2S accompanied by approximately 10% volume expansion, leading to clinker pulverization. If severe pulverization occurs in the cooling section, it will not only clog the grate gaps but also cause a large amount of fine powder to circulate, increasing the dust collection load and even forcing the system to shut down. At the same time, after alumina is extracted from the aluminate clinker, the activity of the remaining waste residue is affected, making it difficult to treat using cement admixtures. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing aluminate clinker using a novel dry-process cement kiln. This method utilizes existing novel dry-process cement kiln equipment and effectively solves the problems of crusting and blockage caused by high-alkali materials and process conflicts through calcination regime adjustment and bypass ventilation system. At the same time, rapid cooling treatment is used to prevent clinker pulverization and ensure that the remaining residue of aluminate clinker still has high active components after alumina extraction.

[0008] To achieve the above objectives, the present invention provides a method for preparing aluminate clinker using a novel dry-process cement kiln, comprising the following steps: S1. Red mud pretreatment: Red mud is dried and crushed to obtain dry red mud powder; S2, Raw material batching: Using the dry red mud powder obtained in step S1, mix and batch it according to the target mineral composition of aluminate clinker to obtain raw material powder; S3. Preheating and temperature control: The raw meal powder obtained in step S2 is fed into the five-stage or six-stage cyclone preheater system of the new dry process cement kiln for preheating, so that the temperature of the raw meal powder is gradually increased to 700℃. S4. Sintering in a weak reducing atmosphere: The preheated raw meal powder enters the rotary kiln. The volume fraction of oxygen content in the firing zone of the kiln is controlled at 0.8% to 1.5%, and the volume fraction of carbon monoxide concentration is controlled at 0.3% to 1.0% to form a weak reducing atmosphere. The firing temperature is controlled at about 1200℃-1280℃, and the residence time of the material in the kiln is controlled at 25 to 40 minutes. High-temperature clinker is obtained by sintering. S5. Rapid cooling treatment: The high-temperature clinker after sintering is rapidly cooled in a grate cooler to obtain aluminate clinker.

[0009] Preferably, it further includes: S6. Bypass ventilation for alkali control: A bypass ventilation system is installed at the kiln tail flue. The bypass ventilation system extracts part of the alkali-containing flue gas and rapidly cools it to below 400°C. The collected alkali-containing kiln ash is returned to the kiln along with the material from the final stage cyclone separator.

[0010] Preferably, in step S1, the Bayer process red mud is filtered and dehydrated to reduce the moisture content of the filter cake to 18%–25%, and then the filter cake is dried and crushed using the waste heat from the exhaust gas of the new dry process cement kiln to obtain dry red mud powder with a moisture content of ≤3%.

[0011] Preferably, the waste gas extracted by the bypass ventilation system in step S6 is used to crush and dry the filter cake in step 1.

[0012] Preferably, in step S2, the ingredient control parameters include: calcium ratio C / S, molar ratio of 1.8 to 2.2; alkali ratio N / R, molar ratio of 0.95 to 1.10; fixed carbon content of 1.5% to 3.0% of the dry basis mass of raw materials; and aluminum-silicon ratio of not less than 1.5.

[0013] Preferably, in step S3, the tertiary air duct gate of the original system is closed, the coal injection pipe of the decomposition furnace is stopped, and cooling air is introduced and enters the decomposition furnace through the tertiary air inlet to control the decomposition furnace inlet temperature to ≤700℃.

[0014] Preferably, in step S4, the firing temperature is controlled at 1200-1280℃, and the material residence time in the kiln is 25-40 minutes.

[0015] Preferably, the weak reducing atmosphere is achieved by controlling the temperature of the kiln tail flue and adjusting the coal consumption and cold air volume of the kiln head burner; when the temperature of the kiln tail flue is too high, the cold air volume is increased through the tertiary air duct to cool it down; when the temperature is too low, the coal consumption of the kiln head main burner is increased.

[0016] Preferably, in step S5, the clinker temperature is rapidly cooled to below 200°C within 3 to 5 minutes at a cooling rate of 200 to 350°C / min.

[0017] This invention also provides a method for modifying a novel dry-process cement kiln. The modification method is used to modify the novel dry-process cement kiln to meet the usage requirements of the method for preparing aluminate clinker using the novel dry-process cement kiln, and includes: (1) Close the tertiary air duct gate of the original cement kiln system to cut off the passage for hot air from the grate cooler to enter the decomposition furnace; (2) The coal injection pipe of the decomposition furnace is shut down, and only the main burner at the kiln head is retained as the sole fuel supply device; (3) A cooling fan is added at the tertiary air passage to introduce cooling air into the decomposition furnace inlet in order to control the decomposition furnace inlet temperature ≤700℃; (4) A bypass ventilation system is set up at the kiln tail flue. The bypass ventilation system includes a quench cooler and a cyclone separator. The quench cooler is used to quickly cool the alkali-containing flue gas to below 400°C. The cyclone separator is used to collect the alkali-containing kiln ash.

[0018] According to the above technical solution, this invention utilizes a novel dry-process cement kiln equipment to organically combine multiple steps, including red mud drying and crushing, dry raw material preheating, weakly reducing atmosphere sintering, rapid cooling, and bypass venting for alkali control, forming a complete technical solution for preparing aluminate clinker. This aluminate clinker preparation method, by setting a pretreatment step, completely eliminates the huge heat consumption caused by water evaporation in traditional wet processes, significantly reducing sintering heat consumption. Utilizing existing novel dry-process cement kiln equipment eliminates the need for constructing a dedicated sintering kiln, greatly reducing investment costs. Furthermore, through weakly reducing atmosphere control and a bypass venting system, the problems of crusting and blockage caused by high-alkali materials and process conflicts are effectively solved, achieving long-term stable system operation. Simultaneously, rapid cooling prevents clinker pulverization, ensuring smooth operation of subsequent processes, and also enables alkali recovery and utilization, reducing soda ash consumption.

[0019] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0020] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the following detailed description to explain the invention, but do not constitute a limitation thereof. In the drawings: Figure 1 This is a process flow diagram of a method for preparing aluminate clinker using a novel dry-process cement kiln; Figure 2 This is a phase diagram of the eutectic product of sulfate and KCl. Detailed Implementation

[0021] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0022] In this invention, unless otherwise stated, directional terms included in the terminology represent only the orientation of the term in its normal use or as commonly understood by those skilled in the art, and should not be regarded as a limitation on the term.

[0023] See Figure 1 The method for preparing aluminate clinker using a novel dry-process cement kiln includes the following steps: S1. Red mud pretreatment: Bayer process red mud is dried and crushed. S2, Raw material batching: Using the dry red mud powder obtained in step S1, mix and batch according to the target mineral composition of aluminate clinker; S3. Preheating and temperature control: The raw meal powder obtained in step S2 is fed into the five-stage cyclone preheater system of the new dry process cement kiln for preheating, so that the raw meal temperature is gradually increased to 700℃. S4. Sintering in a weak reducing atmosphere: The preheated raw materials enter the rotary kiln, and the oxygen content in the firing zone of the kiln is controlled at 0.8% to 1.5% (volume fraction), and the carbon monoxide concentration is controlled at 0.3% to 1.0% (volume fraction) to form a weak reducing atmosphere. The firing temperature is controlled at about 1250℃, and the residence time of the material in the kiln is controlled at 25 to 40 minutes. S5. Rapid cooling treatment: The high-temperature clinker after sintering is rapidly cooled in a grate cooler.

[0024] Through the implementation of the above technical solution, it is found that the original moisture content of Bayer process red mud is as high as 36% to 42%, and it is in a slurry state, making it unsuitable for direct use in dry processes. Therefore, it is necessary to dry and crush the red mud slurry during the pretreatment stage, so that the red mud can be dehydrated and dispersed in a short time to obtain dry red mud powder. The core of this step is to eliminate the huge energy consumption of water evaporation caused by wet kiln feeding, while obtaining powdered dry red mud powder, which is convenient for batching.

[0025] In step S2, the raw material batching requires precise control of various chemical parameters. Dry red mud powder is mixed with limestone, soda ash, and anthracite in a predetermined ratio, and then ground to a specified fineness using a raw material grinding system to produce raw material powder. The goal of the batching is to ensure that the subsequent sintering reaction generates sufficient and easily soluble sodium aluminate minerals.

[0026] Table 1 shows the mass percentage of each component in the raw materials (unit: %). Table 1:

[0027] Table 2 shows the fuel analysis data. Table 2:

[0028] (2) The clinker control indicators for the batching design of this project refer to the requirements of GB / T 39201-2020 "Technical Specification for Alumina Extraction from High-Alumina Fly Ash" and the materials of the Jiulong Wanbo process route, etc. The target value of the batching for this project is tentatively set as follows: Fixed carbon = 2.00%, N / R = 0.95, C / S = 2.05 Based on the above data, the dry basis ratio of raw materials is obtained, as shown in Table 3 (unit: %). Table 3:

[0029] Table 4 shows the chemical composition of raw materials and clinker, expressed as a percentage by mass: Table 4:

[0030] Table 5 is the material balance table. Table 5:

[0031] In Table 5, red mud, limestone, soda ash and anthracite are mixed according to the dry basis ratio to obtain raw meal. The present invention uses dry raw meal, while the prior art uses raw meal containing moisture.

[0032] This invention uses 290.8t of dried raw material per hour, approximately 6979.0t of dried raw material per day, and the coal consumption of the calcination system is approximately 25.1t per hour, approximately 602.0t per day. It can produce 208.3t / h of clinker and 5000t of clinker per day.

[0033] When using raw materials containing natural moisture, existing technologies require processing a larger amount of raw materials, with an hourly raw material consumption of 385.8 tons and a daily raw material consumption of 9259.0 tons. The hourly coal consumption is approximately 27.9 tons and the daily coal consumption is approximately 602.0 tons. Using two different process methods, the amount of clinker produced is consistent. Therefore, using the process of this invention can significantly save coal consumption in the calcination system.

[0034] In step S3, the preheating and temperature control process utilizes the inherent five- or six-stage cyclone preheater system of the new dry-process cement kiln. Raw meal powder is fed into the top of the preheater via a pneumatic conveying system, where it undergoes multi-stage counter-current heat exchange with the rising high-temperature flue gas from the kiln tail. Each stage of the preheater achieves gas-solid separation, with the raw meal temperature gradually increasing, ultimately reaching approximately 700°C when leaving the lowest stage preheater, while simultaneously completing about 30% of the carbonate decomposition. This step significantly reduces the heat load within the rotary kiln.

[0035] The fundamental reason for the formation of a crust is the premature appearance of the liquid phase, and there are two main reasons for this premature appearance: one is the type and quantity of the molten material; the other is the temperature.

[0036] Based on theoretical calculations of the chemical composition of clinker, it was found that the main molten substances in the crust are KCl, K2SO4 and free alkali, and the total content of molten substances in the clinker is about 12%, which is much higher than the total molten substances in C5 of a typical production line. According to experience, the molten substance content is generally 5-7% when the system is running stably.

[0037] Na₂O itself has a relatively high melting point (1132℃), but as a strong flux, it can form a liquid phase with SiO₂, CaO, and other substances commonly found in raw materials, with temperatures as low as approximately 780℃. These liquid phases easily form a "glue" on the inner wall of the kiln tail preheater system, which traps materials and accumulates, eventually leading to blockage.

[0038] KCl has a melting point of 771℃, and K₂SO₄ has a melting point of 1024℃. However, if a large amount of KCl and sulfate are present in the system, a eutectic will form between the sulfate and KCl. The temperature of this eutectic will be significantly lower. Figure 2 As shown.

[0039] Therefore, controlling the outlet temperature of the decomposition furnace to ≤700℃ allows a large amount of alkali, chlorine, and sulfur in the gas phase to condense rapidly into the material inside the decomposition furnace, reducing the content of harmful components in the gas phase entering the subsequent cyclone separator.

[0040] In step S4, sintering under a weak reducing atmosphere is completed in a rotary kiln. After preheating, the raw materials enter the rotary kiln and move towards the kiln head as the kiln rotates. By precisely adjusting the primary air volume and fuel supply of the kiln head burner, a specific weak reducing atmosphere is created in the firing zone within the kiln, with the oxygen content controlled at 0.8%–1.5% and the carbon monoxide concentration controlled at 0.3%–1.0%. Under this atmosphere, iron oxides in the red mud are reduced to metallic iron, preventing the formation of low-melting-point ferrates. The firing temperature is controlled at approximately 1250℃, lower than the firing temperature of cement clinker, and the residence time is controlled at 25–40 minutes to ensure the full formation of sodium aluminate minerals.

[0041] In step S5, a grate cooler is used for quenching. The high-temperature clinker after sintering falls from the kiln head of the rotary kiln into the grate cooler. A large amount of cold air is introduced into the cooler, which undergoes intense heat exchange with the high-temperature clinker to achieve rapid cooling. Quenching can quickly solidify the liquid phase in the clinker and fix dicalcium silicate in the β crystal form, preventing it from transforming into the γ crystal form.

[0042] In this embodiment, preferably, it further includes: S6. Bypass Venting for Alkali Control: Part of the alkali-containing flue gas is extracted and rapidly cooled to below 400℃. The collected alkali-containing kiln ash is returned to the rotary kiln along with the material from the final stage cyclone separator of the production line.

[0043] In step S6, the bypass venting for alkali control is implemented at the rising flue between the kiln tail flue and the C5 preheater. A bypass venting system is set up to extract a portion of the flue gas with a high alkali concentration from the system and introduce it into a quencher, where it is rapidly cooled to below 400°C using cold air or water spray. After cooling, the alkali components in the flue gas condense into solid dust, which is collected by a dust collection device to obtain alkali-containing kiln ash. This kiln ash is returned to the rotary kiln via the C5 chute, realizing the recovery and utilization of alkali. The quenched exhaust gas is then introduced into the red mud drying and crushing system as a heat source.

[0044] Therefore, this method for preparing aluminate clinker using a novel dry-process cement kiln organically combines multiple steps, including red mud drying and crushing, dry raw material preheating, weakly reducing atmosphere sintering, rapid cooling, and bypass venting for alkali control, forming a complete technical solution for aluminate clinker preparation. This aluminate clinker preparation method, by setting a pretreatment step, completely eliminates the huge heat loss caused by water evaporation in traditional wet processes, significantly reducing sintering heat consumption. Furthermore, by utilizing existing novel dry-process cement kiln equipment, there is no need to build a new dedicated sintering kiln, greatly reducing investment costs. Moreover, through weakly reducing atmosphere control and a bypass venting system, the problem of crusting and blockage caused by high-alkali materials is effectively solved, achieving long-term stable system operation. Simultaneously, rapid cooling prevents clinker pulverization, ensuring smooth operation of subsequent processes, and also enables alkali recovery and utilization, reducing soda ash consumption.

[0045] The working processes of the traditional alumina sintering system and the novel dry-process cement kiln system of this application are compared, and the advantages of the method for preparing aluminate clinker using the novel dry-process cement kiln of this application are demonstrated, as shown in Table 6: Table 6:

[0046] In this embodiment, preferably, in step S1, the Bayer process red mud is filtered and dehydrated to reduce the moisture content of the filter cake to 18% to 25%, and then the filter cake is dried and crushed using the waste heat of the exhaust gas from the new dry process cement kiln to obtain dry red mud powder with a moisture content of ≤3%.

[0047] In the red mud pretreatment process, the Bayer process red mud slurry from the alumina plant is first fed into a filtration device, which can be a belt filter or a plate and frame filter press. Through mechanical pressure filtration or vacuum filtration, a large amount of free water is removed from the red mud slurry, reducing the moisture content of the resulting filter cake to between 18% and 25%. This moisture content range is chosen because excessively high moisture content increases subsequent drying energy consumption, while excessively low moisture content leads to a sharp decrease in filtration efficiency and reduced economic efficiency.

[0048] The filter cake is then fed into a drying and crushing machine. This machine integrates drying and crushing, featuring high-speed rotating hammers and a hot air inlet. Waste gas from the kiln tail preheater or kiln head grate cooler of a new dry-process cement kiln, typically at 250℃~350℃, is introduced into the drying and crushing machine, where it comes into full contact with the red mud filter cake, which has been broken up by the hammers. Under the combined action of hot air and mechanical impact, the remaining moisture in the filter cake evaporates rapidly, while the red mud is pulverized into fine particles. By controlling the hot air temperature and residence time, dry red mud powder with a moisture content ≤3% is finally obtained. The fineness of this dry red mud powder should ideally be controlled to have a residue of ≤20% on an 80μm sieve to ensure the uniformity of subsequent batching and sintering reactions.

[0049] This scheme achieves efficient conversion of red mud from slurry to dry powder through a two-stage combination of mechanical dehydration and waste heat drying. Mechanical dehydration removes most of the free water, reducing the heat load of subsequent drying; the use of waste heat from cement kiln exhaust gas for drying enables cascaded energy utilization, eliminating the need for an additional heat source. The resulting dry red mud powder with a moisture content ≤3% fully meets the requirements for dry-process raw material batching, laying a solid raw material foundation for subsequent dry sintering.

[0050] Compared to the traditional wet process that directly injects raw meal slurry with a moisture content of about 40% into the kiln, this solution can save about 40% of the latent heat of water evaporation, resulting in a very significant energy-saving effect.

[0051] In this embodiment, preferably, the waste gas extracted from the bypass in step 6 is used to dry the filter cake.

[0052] The exhaust gas extracted by the bypass ventilation system can also be introduced into the red mud drying and crushing system as a supplementary heat source. After the quencher of the bypass ventilation system, an exhaust gas pipeline is installed. The exhaust gas has been quenched to a temperature below 400℃ but still has residual heat value. Therefore, it can be transported to the hot air inlet of the drying and crushing machine and mixed with the main hot air from the kiln tail or kiln head for drying the red mud filter cake.

[0053] Because the bypass exhaust gas has a high alkali content, direct emission would cause alkali loss and environmental pollution. However, using it to dry red mud can make full use of its residual heat. On the other hand, the dust in the exhaust gas, especially the dust containing alkali components, will enter the drying crusher with the hot air and eventually be adsorbed by the red mud filter cake and become part of the raw material, thus achieving further alkali recovery.

[0054] Therefore, this technical solution achieves the cascade utilization of waste heat from bypass exhaust gas and the resource recovery of dust. In traditional bypass ventilation systems, exhaust gas typically needs to be cooled and dusted before it can be discharged, resulting in heat waste. This invention introduces this process into the drying stage, reducing heat loss and improving the system's thermal efficiency. Simultaneously, alkaline dust in the exhaust gas is captured by red mud and returned to the process system, further reducing the amount of soda ash required for replenishment.

[0055] In this embodiment, preferably, in step 2, the ingredient control indicators include: calcium ratio CaO / (SiO2+TiO2), molar ratio of 1.8 to 2.2; alkali ratio Na2O / (Al2O3+Fe2O3), molar ratio of 0.95 to 1.10; fixed carbon content of 1.5% to 3.0% of the dry basis mass of raw materials; aluminum-silicon ratio (mass ratio) ≥ 1.5.

[0056] The specific details of the ingredient control indicators are shown in Table 7: Table 7:

[0057] In the specific implementation of the batching process, the chemical composition of raw materials such as dry red mud powder, limestone, soda ash, and anthracite is first analyzed to obtain the content of key components such as CaO, SiO2, TiO2, Na2O, Al2O3, Fe2O3, and fixed carbon in each raw material. Then, based on the target mineral composition, the proportion of each raw material is determined using the batching calculation formula.

[0058] The molar ratio of calcium (CaO / (SiO2+TiO2)) is controlled between 1.8 and 2.2. This parameter controls the type of calcium silicate minerals formed in the clinker. If the calcium ratio is too low, insufficient calcium silicate formation will occur, affecting the dissolution of sodium aluminate; if the calcium ratio is too high, high-calcium silicates will be formed, reducing the quality of the clinker. The preferred range is 1.9 to 2.1.

[0059] The molar ratio of alkali (Na2O / (Al2O3+Fe2O3)) should be controlled between 0.95 and 1.10. This parameter directly determines the amount of sodium aluminate produced and the dissolution rate. If the alkali ratio is too low, it means that there is insufficient alkali, and some alumina cannot be converted into soluble sodium aluminate; if the alkali ratio is too high, it will waste alkali and increase the burden on the bypass vent. The preferred range is 0.98 to 1.05.

[0060] The fixed carbon content is controlled between 1.5% and 3.0% of the dry weight of the raw materials. The fixed carbon mainly comes from the anthracite added in the batching, and its role is to create a reducing atmosphere during sintering. If the carbon content is too low, the reducing atmosphere is insufficient, and iron oxides cannot be fully reduced to metallic iron; if the carbon content is too high, it will lead to fuel waste and may cause over-reduction. The specific dosage needs to be adjusted according to the iron content of the red mud.

[0061] The aluminum-to-silicon ratio (mass ratio) must be ≥1.5. This indicator ensures that the alumina content in the raw meal is higher than that of the silicon dioxide, ensuring that a sufficient amount of sodium aluminate minerals are generated after sintering, while avoiding the formation of excessive sodium silicate or sodium aluminosilicate.

[0062] By precisely controlling the above four chemical indicators, the mineral composition of the raw materials is strictly limited within the optimal sintering range. The appropriate matching of the calcium and alkali ratios ensures the efficient generation and dissolution of sodium aluminate; the precise addition of fixed carbon provides the necessary weak reducing atmosphere while avoiding the waste of excess carbon; and the constraint of the aluminum-silicon ratio ensures the usability of the raw materials. These indicators together constitute the batching control parameters for aluminate clinker preparation, enabling the subsequent sintering reaction to proceed smoothly and efficiently at a temperature of approximately 1250℃, producing high-quality aluminate clinker.

[0063] In this embodiment, preferably, in step S3, the tertiary air duct gate of the original system is closed, the coal injection pipe of the decomposition furnace is stopped, and cooling air is introduced to control the inlet temperature of the decomposition furnace to ≤700℃.

[0064] In conventional new dry process cement kiln systems, the function of the tertiary air duct is to draw high-temperature hot air from the grate cooler and send it into the decomposition furnace to provide oxygen for the combustion of pulverized coal in the decomposition furnace and to recover waste heat. The temperature of the hot air is usually between 800℃ and 900℃. Meanwhile, the pulverized coal injection pipe of the decomposition furnace injects pulverized coal into the decomposition furnace for the decomposition of carbonates.

[0065] However, when used in the preparation of aluminate clinker, this conventional operating mode leads to temperatures in the decomposition furnace reaching 850-950°C or even higher; simultaneously, the temperature in the kiln's firing zone reaches 1400-1450°C. This results in serious consequences: The high temperature in the firing zone of the kiln causes a violent volatilization of alkali. Alumina raw materials have a high alkali (Na₂O, K₂O) content, with a melting point of approximately 800℃. In the high-temperature zone of the kiln (1400℃), a large amount of alkali volatilizes and enters the flue gas. The volatilized alkali flows counter-currently into the preheater system with the flue gas. The preheater temperature is approximately 300℃ to 800℃. The alkali in the flue gas condenses in the low-temperature zone and adheres to the raw material. It then re-enters the high-temperature zone of the kiln with the raw material and volatilizes again, forming a closed-loop cycle of "vaporization-condensation-reflux," leading to a continuous enrichment of alkali concentration. The enriched alkali reacts with components such as aluminum and silicon in the raw material, generating a large amount of low-melting-point (approximately 700~900℃) alkali-aluminosilicate liquid phase. These liquid phases are semi-molten and viscous, adhering to the preheater cone, feed pipe, and rising flue, eventually hardening into a dense crust. This gradually reduces the inner diameter of the preheater pipes and can even completely block the preheater system.

[0066] Therefore, in order to limit the temperature inside the decomposition furnace, the present invention adopts the following modification measures: First, the tertiary air duct gate of the original system is completely closed to cut off the channel for high-temperature hot air from the grate cooler to enter the decomposition furnace; Second, the coal injection pipe of the decomposition furnace is stopped, and fuel combustion no longer takes place inside the decomposition furnace; Third, a cooling fan is added at the tertiary air duct to introduce ambient temperature cooling air into the decomposition furnace inlet.

[0067] Through the above three operations, the decomposition furnace is no longer used as a combustion and decomposition device, but only as a flue gas passage and material passage. The high-temperature flue gas (approximately 1000℃~1100℃) rising from the rotary kiln tail flue gas chamber mixes with the introduced cooling air before entering the preheater system, rapidly reducing its temperature to below 700℃. This temperature control can be achieved by adjusting the airflow of the cooling fan; when the temperature in the kiln tail flue gas chamber rises, the cooling airflow is automatically increased; when the temperature decreases, the cooling airflow is reduced.

[0068] Controlling the temperature inside the preheater is one of the core operations for solving the problem of scaling and blockage. By closing the tertiary air duct and disabling the preheater pulverized coal injection pipe, excessively high preheater inlet temperatures caused by hot air and additional fuel combustion are avoided. Strictly controlling the preheater inlet temperature below 700℃ significantly suppresses the volatilization of alkali components in this region. Because the volatility of alkali increases sharply with increasing temperature, the amount of alkali volatilized below 700℃ is small and insufficient to form severe circulating enrichment. Simultaneously, the introduction of cooling air further acts as a "thermal dilution," ensuring the achievement of the temperature target. This measure reduces alkali volatilization at its source, which is crucial for preventing scaling and blockage in the preheater system.

[0069] In this embodiment, preferably, in step S4, the firing temperature is controlled at around 1250°C, and the residence time of the material in the kiln is controlled at 25 to 40 minutes.

[0070] The firing temperature is primarily controlled by adjusting the fuel supply to the main burner at the kiln head. Multiple temperature monitoring points are installed inside the kiln, including the flame temperature at the kiln head, the flue gas temperature at the kiln tail, and the surface temperature of the kiln shell. Based on this temperature data and the sintering state of the clinker samples, operators adjust the fuel supply in real time to stabilize the material temperature in the firing zone at approximately 1250℃. The allowable fluctuation range is ±30℃, i.e., 1220℃~1280℃.

[0071] The residence time of materials in the kiln is determined by the kiln's rotational speed, inclination, and material filling rate. For the production of aluminate clinker, the rotary kiln's rotational speed is typically controlled at 1.0–1.5 rpm, appropriately slower than that used for cement clinker production (usually 2.0–3.0 rpm). The slower rotational speed reduces the material's movement speed within the kiln, extending the residence time to 25–40 minutes. This residence time is significantly longer than the 8–15 minutes required for cement clinker.

[0072] The firing temperature was controlled at around 1250℃, significantly lower than the 1450℃ required for cement clinker firing. On one hand, the lower temperature reduced fuel consumption and lowered production costs; on the other hand, the lower firing temperature facilitated control of alkali volatilization, as the volatilization rate at 1250℃ was significantly lower than at 1450℃. Extending the residence time to 25–40 minutes compensated for any potential insufficient reaction kinetics at the lower firing temperature. The longer residence time ensured that the raw meal particles had sufficient time to complete the solid-phase reaction and liquid-phase sintering within the kiln, allowing the sodium aluminate minerals to fully crystallize and grow, thus ensuring clinker quality.

[0073] In this embodiment, preferably, the weak reducing atmosphere is achieved by controlling the temperature of the kiln tail flue and adjusting the coal consumption of the kiln head burner and the volume of tertiary air cooling; when the temperature of the kiln tail flue is too high, cold air is introduced through the tertiary air duct to cool it down; when the temperature is too low, the coal consumption of the kiln head main burner is increased.

[0074] The operator sets a target temperature range for the kiln tail flue, for example, 950℃~1020℃. The system continuously monitors the actual temperature of the kiln tail flue.

[0075] When the temperature in the kiln tail flue exceeds the upper limit, it indicates that the heat load inside the kiln is too high or the flame is too concentrated. At this time, the control system executes a cooling operation: it turns on or increases the airflow of the cooling fans on the tertiary air duct, introducing more ambient-temperature cold air into the system. The introduction of cold air directly reduces the temperature of the flue gas entering the preheater system, and on the other hand, it increases the excess airflow in the system, which dilutes the reducing gases in the kiln and slightly shifts the atmosphere towards oxidation. However, this can be compensated for through subsequent adjustments.

[0076] When the temperature of the kiln tail flue chamber is detected to be below the lower limit, it indicates that the heat supply inside the kiln is insufficient. At this time, the control system executes a heating operation: increasing the pulverized coal supply to the main burner at the kiln head, while keeping the primary air volume constant or appropriately reducing it, thereby maintaining or enhancing the weak reducing atmosphere while increasing the temperature.

[0077] The coal consumption adjustment of the main burner at the kiln head is also linked to the online detection data of oxygen content and CO concentration in the kiln, forming a closed-loop control loop to ensure that the oxygen content in the firing zone is always stable at 0.8% to 1.5% and the CO concentration is always stable at 0.3% to 1.0%.

[0078] By using the kiln tail flue temperature as a feedforward or feedback signal, combined with fuel regulation of the kiln head burner and tertiary air cooling volume regulation, the system can automatically adapt to disturbances such as raw material fluctuations and changes in fuel calorific value, always maintaining optimal weak reducing atmosphere conditions. This control method ensures that iron oxides are fully reduced to metallic iron, while avoiding incomplete fuel combustion or reducing crust formation caused by an excessively strong reducing atmosphere. Simultaneously, the use of tertiary air ducts to introduce cold air for temperature regulation cleverly utilizes existing equipment, eliminating the need for complex cooling devices, resulting in low control costs and high reliability.

[0079] In this embodiment, preferably, in step S5, the clinker temperature is rapidly cooled from above 1250°C to below 200°C within 3 to 5 minutes at a cooling rate of 200 to 350°C / min, in order to prevent the transformation of dicalcium silicate in the clinker from the β phase to the γ phase and avoid clinker pulverization.

[0080] To achieve a rapid cooling rate of 200–350℃ / min, the cooling fan needs to provide sufficient airflow, typically with a cooling airflow-to-clinker throughput mass ratio of 2.5–3.5:1. The front end (high-temperature section) of the grate cooler employs the maximum cooling air density, ensuring the clinker is drastically cooled the moment it contacts the grate. The residence time of the clinker within the cooler is adjusted by controlling the grate's advancing speed, usually set to 3–5 minutes.

[0081] The temperature of the clinker after cooling needs to be reduced to below 200℃, because the β→γ phase transition temperature range of dicalcium silicate is mainly around 400℃~500℃. Quickly passing the clinker through this range and finally cooling it to below 200℃ can ensure that β-C2S does not have enough time to undergo lattice rearrangement.

[0082] When dicalcium silicate (C2S) is slowly cooled, the β phase transforms into the γ phase, accompanied by approximately 10% volume expansion, causing the clinker to pulverize into fine powder. This fine powder can clog the gaps in the grate cooler, affecting the cooling effect; it also circulates extensively with the airflow, increasing the load on the dust collection system; furthermore, in subsequent leaching processes, the fine powder can cause difficulties in slurry settling and reduce filtration efficiency. This invention employs a high-rate rapid cooling of 200–350°C / min to quickly solidify the liquid phase in the clinker, preventing its transformation into the γ phase and thus maintaining the particle integrity and mechanical strength of the clinker. This measure ensures that the clinker can pass smoothly through the grate cooler and subsequent conveying equipment in granular form, guaranteeing the continuous and stable operation of the entire production line.

[0083] This invention also provides a novel method for modifying a dry-process cement kiln, the modification method being used to meet the requirements of the method for preparing aluminate clinker using a novel dry-process cement kiln as described in any one of claims 1-8, comprising: (1) Close the tertiary air duct gate of the original cement kiln system to cut off the passage for hot air from the grate cooler to enter the decomposition furnace; (2) The coal injection pipe of the decomposition furnace is shut down, and only the main burner at the kiln head is retained as the sole fuel supply device; (3) A cooling fan is added at the tertiary air passage to introduce cooling air into the decomposition furnace inlet in order to control the decomposition furnace inlet temperature ≤700℃; (4) A bypass ventilation system is installed at the rising flue between the kiln tail flue and the C5 preheater. The bypass ventilation system includes a quench cooler, which is used to quickly cool the drawn alkaline flue gas to below 400°C and return the collected alkaline kiln ash to the rotary kiln.

[0084] This transformation method comprises four core operations: Modification Operation (1): Close the tertiary air duct gate of the original cement kiln system to cut off the channel for hot air from the grate cooler to enter the decomposition furnace. In the original new dry-process cement kiln system, one end of the tertiary air duct is connected to the high-temperature zone in the middle of the grate cooler, and the other end is connected to the lower middle part of the decomposition furnace, with an regulating gate in between. During the modification, the regulating gate is completely closed to ensure that the high-temperature tertiary air from the grate cooler cannot enter the decomposition furnace, thereby depriving the decomposition furnace of the hot air supply from the grate cooler, effectively controlling the temperature inside the decomposition furnace, and avoiding excessively high temperatures inside the decomposition furnace.

[0085] Modification Operation (2): Discontinue the pulverized coal injection pipes of the precalciner, retaining only the main burner at the kiln head as the sole fuel supply device. The pulverized coal injection pipes of the precalciner typically consist of multiple injection nozzles, arranged in the cone or lower part of the precalciner. During the modification, these injection pipes are removed or blocked, and their coal supply lines are cut off. At the same time, the burner control system of the precalciner is also disabled. From then on, the fuel supply of the entire system is solely handled by the main burner at the kiln head, and all fuel is burned at the kiln head of the rotary kiln.

[0086] Modification Operation (3): A cooling fan is added to the tertiary air duct to introduce cooling air into the decomposition furnace inlet, thereby controlling the decomposition furnace inlet temperature to ≤700℃. A bypass interface is opened near the decomposition furnace inlet in the original tertiary air duct, and one or more cooling fans are installed. The cooling fan draws ambient air and sends it into the tertiary air duct through the pipeline, eventually entering the decomposition furnace. The cooling fan is equipped with a frequency converter, which can automatically adjust the air volume according to the temperature signal of the kiln tail flue. The ambient air from the cooling fan mixes with the high-temperature flue gas rising from the rotary kiln tail flue at the decomposition furnace inlet, controlling the temperature of the mixed flue gas below 700℃.

[0087] Modification Operation (4): A bypass venting system is installed at the rising flue between the kiln tail flue and the C5 preheater. This bypass venting system includes a quench cooler to rapidly cool the alkali-containing flue gas to below 400°C and return the collected alkali-containing kiln ash to the rotary kiln. One or more bypass exhaust ports are opened on the rising flue connecting the kiln tail flue and the C5 preheater. The exhaust port is connected to a bypass venting system, which includes: an exhaust pipe, regulating valves, a quench cooler, a dust collector, and an exhaust fan. The exhaust pipe extracts about 5% to 15% of the flue gas from the rising flue, and the flue gas enters the quench cooler. The flue gas is rapidly cooled to below 400°C in the quench cooler, causing the gaseous alkali components to quickly condense into solid particles. The cooled dust-containing flue gas enters a bag filter or electrostatic precipitator, and the collected kiln ash is transported to the discharge chute of the C5 preheater by pneumatic or mechanical conveying and returned to the rotary kiln along with the hot raw materials. The purified exhaust gas (temperature reduced to about 150°C) is drawn out by the exhaust fan and can be partially or completely sent to the red mud drying and crushing system as a drying heat source.

[0088] This retrofit method makes full use of the existing main equipment of new dry-process cement kilns, such as rotary kilns, preheaters, and grate coolers. It only requires partial modifications to the fuel system of the decomposer furnace, tertiary air ducts, and flues, and the addition of a bypass venting system. There is no need to build a new dedicated alumina sintering kiln. The retrofit investment is only 10% to 20% of that of building a new rotary kiln, and the retrofit cycle can be controlled within 3 to 6 months, demonstrating significant advantages in terms of economy and timeliness.

[0089] Moreover, by simply shutting down the tertiary air and disabling the coal injection pipe in the decomposition furnace, the high-temperature combustion zone inside the decomposition furnace is eliminated. Combined with the cooling air, precise control of the inlet temperature of the decomposition furnace can be achieved, ensuring that the temperature at that location does not exceed 700℃. This can suppress excessive volatilization of alkali from the source and effectively solve the problem of crusting and blockage.

[0090] By adding a bypass venting system, active intervention in the alkali cycle can be achieved—the continuously accumulating alkali in the system can be extracted through the bypass, cooled, and collected, then returned to the kiln as kiln ash. This prevents excessive accumulation and scaling of alkali in the preheater, and also realizes the recovery and utilization of alkali resources, reducing the consumption of external soda ash. According to actual operating data, the alkali recovery rate can reach 60% to 80%.

[0091] This transformation method has enabled a large number of cement production lines with overcapacity to gain new production capacity and room for survival, thus revitalizing existing assets. At the same time, the transformed production lines can dispose of Bayer process red mud on a large scale, reducing the land occupation and environmental pollution risks caused by red mud storage, and has significant social and environmental benefits.

[0092] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0093] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0094] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing aluminate clinker using a novel dry-process cement kiln, characterized in that, Includes the following steps: S1. Red mud pretreatment: Red mud is dried and crushed to obtain dry red mud powder; S2, Raw material batching: Using the dry red mud powder obtained in step S1, mix and batch it according to the target mineral composition of aluminate clinker to obtain raw material powder; S3. Preheating and temperature control: The raw meal powder obtained in step S2 is fed into the five-stage or six-stage cyclone preheater system of the new dry process cement kiln for preheating, so that the temperature of the raw meal powder is gradually increased to 700℃. S4. Sintering in a weak reducing atmosphere: The preheated raw meal powder enters the rotary kiln. The volume fraction of oxygen content in the firing zone of the kiln is controlled at 0.8% to 1.5%, and the volume fraction of carbon monoxide concentration is controlled at 0.3% to 1.0% to form a weak reducing atmosphere. The firing temperature is controlled at about 1200℃-1280℃, and the residence time of the material in the kiln is controlled at 25 to 40 minutes. High-temperature clinker is obtained by sintering. S5. Rapid cooling treatment: The high-temperature clinker after sintering is rapidly cooled in a grate cooler to obtain aluminate clinker.

2. The method for preparing aluminate clinker using a novel dry-process cement kiln according to claim 1, characterized in that, Also includes: S6. Bypass ventilation for alkali control: A bypass ventilation system is installed at the kiln tail flue. The bypass ventilation system extracts part of the alkali-containing flue gas and rapidly cools it to below 400°C. The collected alkali-containing kiln ash is returned to the kiln along with the material from the final stage cyclone separator.

3. The method for preparing aluminate clinker using a novel dry-process cement kiln according to claim 1, characterized in that, In step S1, the Bayer process red mud is filtered and dehydrated to reduce the moisture content of the filter cake to 18%–25%. Then, the filter cake is dried and crushed using the waste heat from the exhaust gas of the new dry process cement kiln to obtain dry red mud powder with a moisture content of ≤3%.

4. The method for preparing aluminate clinker using a novel dry-process cement kiln according to claim 2, characterized in that, The waste gas extracted from the bypass ventilation system in step S6 is used to crush and dry the filter cake in step 1.

5. The method for preparing aluminate clinker using a novel dry-process cement kiln according to claim 1, characterized in that, In step S2, the ingredient control parameters include: calcium ratio (C / S) with a molar ratio of 1.8 to 2.2; alkali ratio (N / R) with a molar ratio of 0.95 to 1.10; fixed carbon content of 1.5% to 3.0% of the dry weight of the raw material; and aluminum-silicon ratio of not less than 1.

5.

6. The method for preparing aluminate clinker using a novel dry-process cement kiln according to claim 1, characterized in that, In step S3, the tertiary air duct gate of the original system is closed, the coal injection pipe of the decomposition furnace is stopped, and cooling air is introduced and enters the decomposition furnace through the tertiary air inlet to control the decomposition furnace inlet temperature to ≤700℃.

7. The method for preparing aluminate clinker using a novel dry-process cement kiln according to claim 1, characterized in that, In step S4, the firing temperature is controlled at 1200-1280℃, and the material stays in the kiln for 25-40 minutes.

8. The method for preparing aluminate clinker using a novel dry-process cement kiln according to claim 7, characterized in that, The weak reducing atmosphere is achieved by controlling the temperature of the kiln tail flue and adjusting the coal consumption and cold air volume of the kiln head burner. When the temperature of the kiln tail flue is too high, the cold air volume is increased through the tertiary air duct to cool it down. When the temperature is too low, the coal consumption of the kiln head main burner is increased.

9. The method for preparing aluminate clinker using a novel dry-process cement kiln according to claim 1, characterized in that, In step S5, the clinker temperature is rapidly cooled to below 200°C within 3 to 5 minutes at a cooling rate of 200–350°C / min.

10. A novel method for modifying a dry-process cement kiln, characterized in that, The modification method is used to modify a novel dry-process cement kiln to meet the usage requirements of the method for preparing aluminate clinker using a novel dry-process cement kiln as described in any one of claims 1-8, including: (1) Close the tertiary air duct gate of the original cement kiln system to cut off the passage for hot air from the grate cooler to enter the decomposition furnace; (2) The coal injection pipe of the decomposition furnace is shut down, and only the main burner at the kiln head is retained as the only fuel supply device; (3) A cooling fan is added at the tertiary air passage to introduce cooling air into the decomposition furnace inlet in order to control the decomposition furnace inlet temperature ≤700℃; (4) A bypass ventilation system is set up at the kiln tail flue. The bypass ventilation system includes a quench cooler and a cyclone separator. The quench cooler is used to quickly cool the alkali-containing flue gas to below 400°C. The cyclone separator is used to collect the alkali-containing kiln ash.

Images