Method for preparing high-strength silicate clinker by using high-silicon low-grade limestone

Abstract

This invention discloses a method for preparing high-strength silicate clinker using high-silica, low-grade limestone. The method includes optimizing raw material batching ratios, using efficient composite grinding aids, composite mineralization calcination, and adapting to industrial processes. It is applicable to high-silica, low-grade limestone with SiO2 content >10 wt.% and CaO content 45 wt.%~48 wt.%. By improving grinding efficiency through composite grinding aids and lowering calcination temperature and promoting C3S mineral phase development through fluorite-steel slag composite mineralizers, a high proportion of low-grade raw material utilization and high-strength clinker preparation can be achieved. This invention transforms high-silica, low-grade limestone from "inefficient, low-value waste" to "efficient, high-value raw material," significantly improving limestone mine resource utilization, reducing solid waste emission risks, and offering a stable process suitable for large-scale dry rotary kilns. It combines significant resource benefits, energy-saving and carbon-reducing benefits, and economic benefits, and can be widely applied to green and low-carbon production in the cement industry and the efficient utilization of low-grade mineral resources.

Description

Technical Field

[0001] This invention specifically relates to a complete set of technologies for preparing high-strength silicate cement clinker using high-silica, low-grade limestone. It is particularly suitable for the efficient utilization of low-grade limestone resources with SiO2 content >10 wt.% and CaO content only meeting Grade II requirements, as well as for the large-scale production of low-carbon, energy-saving, high-strength silicate clinker in new dry-process cement production lines. It belongs to the field of green preparation technology for inorganic cementitious materials. Background Technology

[0002] Silicate cement, as the most widely used and applied inorganic cementitious material globally, is an indispensable core material for infrastructure construction. Its production primarily uses limestone as a calcareous raw material. According to DZ / T 0213 "Specification for Geological Exploration of Limestone, Dolomite, and Cement Raw Material Minerals in Metallurgy and Chemical Industry," limestone used in cement must meet the following requirements: CaO ≥ 45wt.%, quartz SiO2 ≤ 6wt.%, and flint SiO2 ≤ 4wt.%. For a long time, the industry has generally relied on high-grade limestone resources.

[0003] With the continuous advancement of infrastructure construction, the cement industry's demand for high-quality limestone raw materials is constantly increasing. High-grade limestone resources are becoming increasingly depleted, and mining is gradually shifting towards low-grade, high-impurity ore sections. A large amount of limestone produced from these mines exhibits high silica and low calcium characteristics, with SiO2 content generally exceeding 10 wt.% and existing in the form of crystalline quartz. CaO content is only 45 wt.% to 48 wt.%, barely meeting the Grade II standard, and cannot directly meet the requirements for traditional cement clinker production. This type of high-silica, low-grade limestone presents significant problems such as difficulty in grinding, poor calcinability, low clinker strength, and high energy consumption during firing. For a long time, it has been stored as mine stripping waste, causing not only a serious waste of mineral resources but also environmental and safety risks such as land occupation, dust pollution, and geological disasters.

[0004] The industry currently faces prominent challenges such as the depletion of high-grade limestone resources, low utilization rate of low-grade ore, high energy consumption and carbon emissions in production, and difficulty in meeting clinker strength standards. Specific technical problems are as follows:

[0005] (1) High-silicon, low-grade limestone is difficult to grind, and the grinding efficiency and particle size distribution are difficult to meet the requirements for calcination.

[0006] The SiO2 content in high-silicon, low-grade limestone is generally higher than 10 wt.%, and it exists in the form of high-hardness quartz crystals, resulting in poor grindability and high grinding resistance of the raw material. The effect of a single grinding aid is limited and cannot effectively improve particle dispersibility and surface wettability. The residue on the 80μm sieve of raw material is too high and the proportion of qualified particles of 5~45μm is insufficient, which directly causes insufficient subsequent calcination and poor burnability.

[0007] (2) The raw materials have poor combustibility, the limestone has a high decomposition temperature, and the energy consumption for firing remains high.

[0008] High silicon content significantly increases the initial decomposition temperature and liquid phase formation temperature of CaCO3, while also increasing the viscosity of the high-temperature liquid phase. This hinders ion diffusion and solid-phase reaction, resulting in high clinker firing temperature, long holding time, and a significant increase in coal and electricity consumption, making it difficult to meet the energy conservation, carbon reduction, and first-level energy consumption standards for the cement industry.

[0009] (3) The clinker mineral phases are poorly developed, the free calcium oxide content is too high, and the strength cannot be stably achieved to a high strength grade.

[0010] Under traditional calcination conditions, high-silicon, low-calcium raw materials have insufficient C3S formation, irregular grain development, and uneven distribution, which easily leads to the formation of B mineral nests and a large amount of f-CaO. This results in low 3-day and 28-day compressive strength of the clinker, with the 28-day strength usually below 55 MPa, which cannot stably meet the requirements of high-strength silicate clinker.

[0011] (4) The single adjuvant has limited function and lacks a composite regulation technology system for synergistic effect enhancement.

[0012] Traditional single grinding aids only improve grinding, and single mineralizers only partially optimize firing, failing to simultaneously achieve the synergistic goals of "easy grinding, easy firing, high strength, and low consumption." Furthermore, the mineralizers have poor compatibility with the three ratios of raw materials (lime saturation coefficient KH, silica ratio SM, and alumina ratio IM), making it difficult to apply stably on large-scale new dry rotary kilns.

[0013] (5) Low-grade resources have low utilization rates, large solid waste emissions, and high environmental burden and safety risks.

[0014] Existing processes cannot achieve a high proportion of high-silicon, low-grade limestone substitution, resulting in large-scale stockpiling of mine overburden, which leads to land occupation, dust pollution, and risks of secondary geological disasters. At the same time, non-traditional industrial solid wastes (such as copper slag and steel slag) have not been effectively utilized, and the industry lacks key technological support for green and low-carbon transformation.

[0015] (6) Poor industrial adaptability, making it difficult to achieve stable large-scale production on large production lines.

[0016] The lack of dedicated batching schemes, precise metering and feeding devices, and process control parameters for new dry process production lines with a capacity of 6000t / d and above results in large fluctuations in kiln conditions and unstable quality when using low-grade raw materials, making it impossible to achieve engineering demonstration and large-scale promotion.

[0017] Based on this, developing a key technology that can efficiently utilize high-silica, low-grade limestone, significantly reduce calcination energy consumption, and stably produce high-strength silicate clinker has become an urgent need to solve the problems of resource shortage, high environmental pressure, and high energy consumption in the cement industry. Summary of the Invention

[0018] Technical Problem: To address the problems of low grinding efficiency of high-silicon, low-grade limestone, high energy consumption in clinker firing, and difficulty in achieving the required strength, this invention provides a method for preparing high-strength silicate clinker using high-silicon, low-grade limestone, achieving a high proportion of high-silicon, low-grade limestone substitution, improved clinker strength, and significant reduction in energy consumption and environmental impact.

[0019] Technical Solution: This invention provides a method for preparing high-strength silicate clinker using high-silica, low-grade limestone, including raw material batching optimization, efficient composite grinding aid, and composite mineralization calcination. It is applicable to high-silica, low-grade limestone with SiO2 content >10 wt.% and CaO content 45 wt.%~48 wt.%. The method improves grinding efficiency through composite grinding aids and lowers calcination temperature and promotes C3S mineral phase development through fluorite-steel slag composite mineralizers, achieving high-proportion utilization of low-grade raw materials and preparation of high-strength clinker. Specific steps are as follows:

[0020] Step 1. Raw material batching ratio optimization: High-silica, low-grade limestone, sandstone, ferroalumina, and ferrous materials are used for raw material batching. The three ratios of raw material batching are optimized and matched to control the lime saturation coefficient KH=0.880±0.005, the silicon ratio SM=2.45±0.05, and the aluminum ratio IM=1.45±0.05.

[0021] Step 2. High-efficiency composite grinding aid: In the raw material grinding process, a multi-component high-efficiency composite grinding aid is added for grinding. The multi-component high-efficiency composite grinding aid consists of triethanolamine, ethylene glycol, calcium lignosulfonate, and sodium hexametaphosphate at a ratio of 0.05% to 0.15% of the total mass of the raw material.

[0022] Step 3. Composite mineralization and calcination: A composite mineralizer made of fluorite and steel slag is introduced into the raw material, followed by preheating decomposition, high-temperature calcination and rapid cooling; high-strength silicate clinker is produced; the mass ratio of fluorite to steel slag is 6:4~8:2.

[0023] in,

[0024] The raw materials are made from high-silicon, low-grade limestone, sandstone, ferroalumina, and ferrous materials. The mass ratio of each raw material is as follows: high-silicon, low-grade limestone 88wt.%~93wt.%, sandstone 4wt.%~7wt.%, ferroalumina 2wt.%~3wt.%, and ferrous materials 1wt.%~2wt.%.

[0025] The raw material batching ratios are controlled as follows: lime saturation coefficient KH = 0.9, silicon ratio SM = 2.5, and aluminum ratio IM = 1.4; lime saturation coefficient KH = 0.880 ± 0.005, silicon ratio SM = 2.45 ± 0.05, and aluminum ratio IM = 1.45 ± 0.05.

[0026] The mass percentages of each component in the high-efficiency composite grinding aid are as follows: triethanolamine 25-35 wt.%, ethylene glycol 4-6 wt.%, calcium lignosulfonate 8-12 wt.%, sodium hexametaphosphate 12-18 wt.%, and the balance being water.

[0027] The raw material grinding process includes:

[0028] Raw material pretreatment: High-silica, low-grade limestone, sandstone, ferroalumina, and ferrous materials are dried and then crushed to ≤3mm;

[0029] Ingredient ratio: high-silica, low-grade limestone 88wt.%~93wt.%, sandstone 4wt.%~7wt.%, bauxite 2wt.%~3wt.%, ferrous materials 1wt.%~2wt.%;

[0030] Grinding conditions: Φ300mm×300mm ball mill, media filling rate 40%, steel ball ratio Φ30mm:Φ20mm:Φ15mm=2:4:4, grinding time 30min; liquid grinding aid is sprayed in, and solid grinding aid is fed into the mill simultaneously with the raw materials.

[0031] The composite mineralizer, which is a mixture of fluorite and steel slag, has a mass ratio of fluorite to steel slag of 7:3, and the amount of the composite mineralizer is 1.0 wt.% to 2.0 wt.% of the raw material mass.

[0032] The calcination process is as follows: raw materials are pressed into sheet-like samples of 60mm×60mm×6mm~80mm×80mm×10mm under 15~25MPa and dried at 100~110℃; the temperature is increased to 850~950℃ at 8~12℃ / min and held for 20~40min, and then the temperature is increased to 1420~1480℃ and held for 45~75min. After calcination, the samples are quickly air-cooled to room temperature.

[0033] The fluorine element introduced by the composite mineralizer is solidified in the clinker in the form of calcium fluoride, without secondary pollution, and is suitable for large-scale production of green and low-carbon cement clinker.

[0034] The parameters of the high-strength silicate clinker are as follows:

[0035] 28-day compressive strength ≥ 60.4 MPa, 3-day compressive strength ≥ 32.3 MPa, free calcium oxide f-CaO ≤ 0.58 wt.%; clinker comprehensive coal consumption 93.14 kgce / t, comprehensive energy consumption 98.87 kgce / t, reaching level 1 energy consumption, comprehensive environmental load reduction of 16.6%, CO2 emission per ton of clinker < 835 kg.

[0036] The preferred ratio of high-silica, low-grade limestone is: high-silica, low-grade limestone: sandstone: iron-aluminate: iron material = 90.75: 5.78: 2.31: 1.16.

[0037] Beneficial Effects: This invention achieves breakthrough improvements in multiple aspects, including raw material utilization, clinker performance, production energy consumption, environmental benefits, and industrial application, through a complete set of technologies combining high-efficiency composite grinding aid, composite mineralization and calcination, rate optimization, and industrial adaptation. It exhibits significant and stable beneficial effects.

[0038] 1. Significantly improve the utilization rate of high-silica, low-grade limestone, and realize the high-value utilization of mine solid waste.

[0039] This invention can directly utilize high-silica, low-grade limestone with SiO2 content >10% and CaO only reaching grade II, achieving a utilization rate of up to 82.27%. It significantly reduces the use of mine overburden and low-grade ore, transforming traditionally unusable waste minerals into high-quality raw materials. This significantly improves the overall resource utilization rate of limestone mines, reduces the land occupation and geological disaster risks associated with solid waste storage, and realizes the transformation of low-grade resources from "inefficient and low-value" to "efficient and high-value".

[0040] 2. Significantly improves grinding and sintering performance, reducing production energy consumption.

[0041] The composite grinding aid significantly reduces the residue on the 80μm sieve of raw meal and increases the proportion of qualified particles in the 5~45μm range, resulting in a substantial improvement in grinding efficiency. The composite mineralizer can reduce the initial decomposition temperature of CaCO3 by up to 55.97℃ and the liquid phase formation temperature by 32.3℃, making calcination easier. After industrial application, the comprehensive coal consumption of clinker is reduced to 93.14kgce / t, and the comprehensive energy consumption is 98.87kgce / t, meeting the first-level standard of GB 16780-2021 for energy consumption limits per unit product of cement, demonstrating outstanding energy-saving effects.

[0042] 3. The clinker strength is significantly improved, and the indicators of high-strength silicate clinker are stably achieved.

[0043] Under high-silicon, low-calcium raw material conditions, the 3-day compressive strength of the clinker increased from 29.3 MPa to 32.3 MPa, and the 28-day compressive strength increased from 55.4 MPa to 60.4 MPa, reaching a maximum of 62.6 MPa. The strength was stable and met the standards with a high margin of safety. The C3S mineral phase was more fully developed and more uniformly distributed, with regular crystal morphology and a dense clinker structure, fully meeting the requirements for high-strength cement clinker.

[0044] 4. Significantly reduces free calcium oxide, improving clinker stability and calcination stability.

[0045] The synergistic effect of composite mineralization reduces f-CaO in clinker from 1.25 wt.% in traditional processes to below 0.58 wt.%, with the best laboratory results as low as 0.27 wt.%. This significantly improves the calcinability of raw materials, makes in-kiln calcination more stable, reduces the risk of ring and skin formation, and enhances the stability of clinker, meeting the requirements of large-scale continuous production.

[0046] 5. Significant energy conservation and carbon reduction effects, resulting in a substantial decrease in overall environmental impact.

[0047] By lowering the firing temperature, improving resource utilization, and replacing some traditional raw materials, the overall environmental load was reduced by 16.6%, and CO2 emissions per ton of clinker were controlled below 835 kg, achieving significant carbon reduction. At the same time, the replacement rate of copper slag in industrial solid waste reached 3wt.%~5wt.%, and steel slag was efficiently utilized as a mineralizing agent component, further enhancing the co-processing capacity of solid waste.

[0048] 6. Fluorine is safely solidified, causing no secondary pollution and making it environmentally friendly.

[0049] The fluorine introduced by the composite mineralizer is stably solidified in the clinker mineral structure in the form of calcium fluoride, and will not be released into the atmosphere or water bodies, posing no risk of fluorine pollution. The production process is clean and environmentally friendly, in line with the green and low-carbon cement production guidelines.

[0050] 7. Adaptable to large-scale new dry process production lines, with stable process and strong scalability.

[0051] This technology can be directly applied to a 6000t / d new dry process clinker production line. Only a simple mineralizer metering and feeding device needs to be added. The transformation investment is low, the operation is simple, and the kiln condition is stable. It can be quickly replicated and promoted, providing a mature and feasible green transformation solution for a large number of low-grade limestone mines and cement enterprises across the country.

[0052] 8. Synergistic economic, social, and environmental benefits, with outstanding industry demonstration value.

[0053] This technology reduces reliance on high-grade limestone, lowers raw material and energy costs, and increases product added value. At the same time, it promotes the low-carbonization and high-value utilization of resources in the cement industry. It has been selected as an advanced green and low-carbon technology achievement and has significant industry demonstration effect and promotion value. Attached Figure Description

[0054] Figure 1 The effect of composite grinding aids on the compressive strength of clinker; Figure 1 (a) in the figure represents the 3-day compressive strength. Figure 1 (b) in the figure represents the 7-day compressive strength. Figure 1 (c) in the figure represents the 28-day compressive strength;

[0055] Figure 2 The effect of composite grinding aids on burnability;

[0056] Figure 3 The effect of composite grinding aids on clinker facies: Figure 3 In the examples (a) and (b), no grinding aid was added; Figure 3 In the text, (c) and (d) are grinding aid H7;

[0057] Figure 4 XRD patterns of clinker calcined with added mineralizers: Figure 4 (a) represents a composite mineralizer prepared from fluorite and steel slag with an admixture concentration of 0.5 wt.%. Figure 4 (b) represents a composite mineralizer prepared from fluorite and steel slag with an admixture concentration of 1.0 wt.%. Figure 4 (c) represents the dosage of the composite mineralizer prepared from fluorite and steel slag, which is 1.5 wt.%.

[0058] Figure 5 The effect of composite mineralizer on the decomposition temperature of CaCO3 in raw materials (DSC curves).

[0059] Figure 6 The effect of the dosage ratio of composite mineralizer on the f-CaO content in clinker: Figure 6 (a) represents a composite mineralizer prepared from fluorite and steel slag with an admixture concentration of 0.5 wt.%. Figure 6 (b) represents a composite mineralizer prepared from fluorite and steel slag with an admixture concentration of 1.0 wt.%. Figure 6 (c) represents the dosage of the composite mineralizer prepared from fluorite and steel slag, which is 1.5 wt.%.

[0060] Figure 7 A quantitative analysis diagram of the mineral composition of clinker prepared for the demonstration production line (U-Net identification). Detailed Implementation

[0061] This invention provides a complete technical solution for preparing high-strength silicate clinker using high-silica, low-grade limestone. Using high-silica, low-grade limestone with SiO2 > 10 wt.% and CaO 45 wt.%~48 wt.% as the main calcareous raw material, it systematically solves the technical problems of low-grade raw materials being difficult to grind, difficult to burn, having low strength, and high energy consumption through a four-in-one synergistic control of efficient composite grinding technology, fluorite-steel slag composite mineralization and calcination technology, precise matching of raw material ratio, and adaptation to industrial production line processes.

[0062] A method for preparing high-strength silicate clinker using high-silica, low-grade limestone includes optimizing raw material batching ratios, using efficient composite grinding aids, and composite mineralization calcination. This method is suitable for high-silica, low-grade limestone with SiO2 content >10 wt.% and CaO content 45 wt.%~48 wt.%. The method improves grinding efficiency through composite grinding aids and lowers calcination temperature and promotes C3S mineral phase development through fluorite-steel slag composite mineralizers, achieving high-proportion utilization of low-grade raw materials and preparation of high-strength clinker. The specific steps are as follows:

[0063] Step 1. Raw material batching ratio optimization: High-silica, low-grade limestone, sandstone, ferroalumina, and ferrous materials are used for raw material batching. The three ratios of raw material batching are optimized and matched to control the lime saturation coefficient KH=0.880±0.005, the silicon ratio SM=2.45±0.05, and the aluminum ratio IM=1.45±0.05.

[0064] Step 2. High-efficiency composite grinding aid: In the raw material grinding process, a multi-component high-efficiency composite grinding aid is added for grinding. The multi-component high-efficiency composite grinding aid consists of triethanolamine, ethylene glycol, calcium lignosulfonate, and sodium hexametaphosphate at a ratio of 0.05% to 0.15% of the total mass of the raw material.

[0065] Step 3. Composite mineralization and calcination: A composite mineralizer made of fluorite and steel slag is introduced into the raw material, followed by preheating decomposition, high-temperature calcination and rapid cooling; high-strength silicate clinker is produced; the mass ratio of fluorite to steel slag is 6:4~8:2.

[0066] First, this invention employs a multi-component, high-efficiency composite grinding aid, which is a compound of triethanolamine, ethylene glycol, calcium lignosulfonate, and sodium hexametaphosphate in a fixed ratio. It is added at 0.05% to 0.15% of the raw material mass. By improving the wettability of the particle surface, increasing the surface electrostatic repulsion, and enhancing the adsorption and dispersion effect, it significantly improves the grinding efficiency of high-silica limestone, reduces the 80μm sieve residue, and increases the proportion of qualified particles with a diameter of 5 to 45μm. This results in more uniform raw material particle size and higher reactivity, creating favorable conditions for subsequent calcination.

[0067] Secondly, this invention innovatively employs a composite mineralizer with a fluorite to steel slag mass ratio of 7:3 and an external admixture amount of 1.0%. Fluorite is used to lower the clinker liquid phase formation temperature and reduce the liquid phase viscosity, promoting ion diffusion and solid-phase reaction. C3S and C2S minerals contained in steel slag are used as seed crystals to lower the nucleation activation energy and accelerate C3S formation. The synergistic effect of the two can significantly reduce the CaCO3 decomposition temperature, shorten the firing cycle, greatly reduce the free calcium oxide content in the clinker, promote the full development and uniform distribution of high-strength mineral phase C3S, and increase the total amount of silicate mineral phase by about 10%.

[0068] Meanwhile, this invention optimizes and matches the raw material batching system with the three ratio values. In the laboratory, the raw materials are mixed with high-silica, low-grade limestone, sandstone, ferroalumina clay, and ferrous materials in a fixed proportion, controlling the lime saturation coefficient KH=0.9, the silica ratio SM=2.5, and the alumina ratio IM=1.4. In industrial production, the lime saturation coefficient KH=0.880±0.005, the silica ratio SM=2.45±0.05, and the alumina ratio IM=1.45±0.05. In addition, 3wt.%~5wt.% copper slag is added as a non-traditional raw material substitute, so as to achieve a high proportion of utilization of low-grade raw materials and stable kiln operation.

[0069] In terms of the calcination process, this invention adopts a segmented heating, high-temperature holding and rapid cooling system. After the raw material is pressed into tablets and dried, it is heated to 900℃ at 10℃ / min and held for 30min, then heated to 1450℃ and held for 60min. After calcination, it is rapidly air-cooled to room temperature to ensure that the C3S mineral phase is fully developed, the crystal morphology is regular and the clinker structure is dense.

[0070] In industrial applications, this invention is adapted to a 6000t / d new dry clinker production line, and is equipped with a dedicated composite mineralizer storage, metering and feeding device to achieve precise and controllable addition of the mineralizer; the utilization rate of high-silica low-grade limestone can reach 82.27%, the comprehensive coal consumption of clinker is reduced to 93.14kgce / t, the comprehensive energy consumption reaches the national standard level 1 energy consumption, the 28d compressive strength is stable at over 60.4MPa, the comprehensive environmental load is reduced by 16.6%, the fluorine element is effectively solidified in the clinker, and there is no secondary pollution, ultimately realizing the efficient, low-carbon and high-value utilization of low-grade limestone resources.

[0071] In the raw meal batching, 3wt.% to 5wt.% copper slag is added according to the total mass of raw meal as a silicon-aluminum-iron correcting material, which partially replaces sandstone, iron-aluminum clay and iron correcting materials, balances the chemical composition and three ratio values ​​of raw meal, and stabilizes the calcination conditions in the kiln; so that the utilization rate of high-silicon low-grade limestone can reach 80wt.% to 85wt.%, and the heat consumption, power consumption and mechanical properties of clinker all meet the national standard level 1 energy consumption and high-strength clinker indicators.

[0072] Example 1: Laboratory-scale test - Preparation of high-silica, low-grade limestone raw meal and clinker calcination

[0073] Using high-silicon, low-grade limestone as raw material, its chemical composition is as follows: Loss on ignition (LOI) 39.42 wt.%, SiO2 11.04 wt.%, Al2O3 2.02 wt.%, Fe2O3 0.37 wt.%, CaO 45.89 wt.%, MgO 1.26 wt.%. The SiO2 content far exceeds the standard limit, making it a typical difficult-to-grind and difficult-to-burn raw material.

[0074] The raw materials are prepared according to the following mass ratio: low-grade limestone: sandstone: iron-bauxite: iron material = 90.75: 5.78: 2.31: 1.16. The three ratios of the raw materials are controlled as follows: lime saturation coefficient KH = 0.9, silicon content SM = 2.5, and aluminum content IM = 1.4.

[0075] Add 0.1 wt.% of composite grinding aid (triethanolamine 30 wt.%, ethylene glycol 5 wt.%, calcium lignosulfonate 10 wt.%, sodium hexametaphosphate 15 wt.%), crush the raw material to ≤3 mm, and grind it for 30 min in a Φ300 mm×300 mm experimental ball mill with a media filling rate of 40 wt.% and a steel ball ratio of Φ30 mm:Φ20 mm:Φ15 mm=2:4:4.

[0076] After grinding, the residue on the 80μm sieve of the raw material decreased from 8.64% to 6.84%, and the content of 5~45μm particles increased by 2.6%, laying a good particle size foundation for calcination.

[0077] After the raw material is compressed into tablets, it is calcined in a high-temperature furnace: the temperature is increased to 900℃ at 10℃ / min and held for 30min, then the temperature is increased to 1450℃ and held for 60min, and then air-cooled to room temperature.

[0078] At the same time, 1.0 wt.% of composite mineralizer (fluorite: steel slag = 7:3) is added externally.

[0079] The properties of the obtained clinker are as follows: 28-day compressive strength is 66.8 MPa, f-CaO is 0.77wt.%, which is 45.77wt.% lower than that of the blank sample; C3S grains are developed into regular long plates with a size of 20~38μm, the mineral phase is evenly distributed, and there are no obvious B mineral nests and pores, realizing the preparation of high-strength clinker under laboratory conditions.

[0080] Example 2:

[0081] System commissioning was carried out on a 6000t / d cement clinker production line. The raw material used was high-silica, low-grade limestone with SiO2 content of 11.37wt.%~13.34wt.% and CaO content of 45.49wt.%~46.42wt.%. The raw meal was ground by a vertical mill and equipped with a special silo for composite mineralizer, a metering device and an automatic feeding system to achieve precise control of the dosage.

[0082] (1) Raw material preparation:

[0083] The proportion of high-silica, low-grade limestone is 82.27 wt.%; 3 wt.%~5 wt.% copper slag is added as a non-traditional iron correction raw material; the amount of composite mineralizer is stably controlled at around 1.0 wt.%; raw material ratio values: KH=0.911±0.010, SM=2.46±0.05, IM=1.39±0.05.

[0084] (2) Calcination process control:

[0085] The outlet temperature of the decomposition furnace is stable at 870~890℃, ensuring a decomposition rate of ≥95% in the kiln; the temperature of the kiln head firing zone is 1420~1450℃; the grate cooler achieves rapid cooling of the clinker, with an outlet temperature of <100℃.

[0086] (3) Key indicators for continuous operation for 19 days:

[0087] The clinker has a 28-day compressive strength of 59.3~62.6 MPa, with an average of 60.4 MPa; f-CaO is stable at 0.35%~0.58%; the standard coal consumption of clinker is 93.14 kgce / t, and the comprehensive energy consumption is 98.87 kgce / t, meeting the first-level energy consumption standard of GB 16780-2021; most of the fluorine is solidified in the clinker, and there is no fluorine pollution in waste gas or wastewater.

[0088] This embodiment verifies the feasibility and stability of this technology on large-scale dry-process lines.

[0089] Example 3:

[0090] Based on successful debugging, we will enter long-term large-scale production, further optimize process parameters and rate values, and form a standardized operation mode.

[0091] (1) Raw material and ingredient control

[0092] The limestone used is all high-silicon, low-grade ore from our own mines, with SiO2 of 11wt.%~13.5wt.% and CaO of 45.5wt.%~46.5wt.%. A combined mining system, online component detection, multi-storage homogenization, and pre-homogenization stockpile system for material stabilization were established. The rate values ​​were optimized and set as follows: KH=0.880±0.005, SM=2.45±0.05, IM=1.45±0.05. Composite grinding aids were continuously incorporated, and the fineness of the vertical mill outlet was stably controlled, significantly improving the burnability of the raw material.

[0093] (2) Optimization of firing system

[0094] Appropriately reduce the kiln speed, extend the material residence time, and increase the secondary air temperature to 1150~1200℃; strengthen the ventilation and flame shape in the kiln, stabilize the firing temperature, and avoid short flame rapid firing and low temperature underfiring; continuously feed the composite mineralizer to stably reduce the liquid phase generation temperature, improve the liquid phase viscosity, and promote C3S development.

[0095] (3) Stable indicators for large-scale production

[0096] The 3-day compressive strength increased from 29.3 MPa to 32.3 MPa; the 28-day compressive strength increased from 55.4 MPa to 60.4 MPa; the f-CaO content decreased from 1.25 wt.% to below 0.58 wt.%; the C3S content was 54.68%, C2S 24.51%, and mesophase 20.82%, with a uniform and dense petrographic structure; the standard consistency water requirement for clinker was 24.5%~25.5%, the initial setting time was 140~160 min, and the final setting time was 180~220 min, which fully met the national standard requirements.

[0097] Example 4: Integrated Implementation of Energy Conservation, Carbon Reduction, and Green Development

[0098] This embodiment, based on embodiment 3, adds energy conservation and carbon reduction with the synergistic utilization of solid waste to form a complete green manufacturing system.

[0099] (1) Energy consumption and emission control

[0100] The comprehensive coal consumption of clinker is 93.14 kgce / t, and the comprehensive power consumption is 46.69 kW·h / t; CO2 emissions per ton of clinker are controlled below 835 kg; and the comprehensive environmental load is reduced by 16.6%.

[0101] (2) Co-utilization of solid waste

[0102] Copper slag replaces 3% to 5% of traditional iron raw materials; steel slag is used efficiently as a mineralizing agent component; high-silicon, low-grade limestone has a utilization rate of 82.27%, significantly reducing the amount of mine waste rock stockpiling and stripping, and lowering the risk of geological disasters.

[0103] (3) Synergy between quality and environmental protection

[0104] Fluorine is stably solidified in the clinker minerals in the form of CaF2, without secondary pollution; the kiln conditions are stable, with less crusting and less flying sand, and nitrogen oxide emissions are stable and meet the standards, making it valuable for industry promotion and demonstration.

[0105] The above four sets of examples fully demonstrate that the present invention, on a large-scale new dry process production line of 6000t / d, uses high-silica, low-grade limestone as the main raw material to stably produce high-strength silicate clinker with a compressive strength of ≥60MPa after 28 days. At the same time, it achieves the integrated goals of low energy consumption, low carbon emissions, high resource utilization, and high environmental safety. The process is mature, reliable, replicable, and can be promoted on a large scale.

[0106] The strength and free calcium oxide content of the clinker were analyzed, and the results are as follows: Figure 1 , Figure 2 As shown, Figure 1 In the figure, (a), (b), and (c) represent the 3-day, 7-day, and 28-day compressive strengths of the clinker, respectively. Figure 1 It can be seen that, compared with H0 without grinding aid, H7 achieved a 28-day compressive strength of 66.8 MPa, representing an increase of 6.11 MPa (10.2%). The f-CaO content decreased from 1.42 wt.% to 0.77 wt.%, a decrease of 45.77% (see...). Figure 2 ).

[0107] Figure 3 The images show the petrographic diagrams of clinker before and after the addition of composite grinding aid H7. Figure 3 Images (a) and (b) are petrographic structures magnified at 500x and 1000x respectively, without grinding aids. Figure 3(c) and (d) in the diagram represent petrographic structures with H7-doped composite grinding aid, magnified at 500x and 1000x magnification, respectively. Figure 3 As shown in (a) and (b), without grinding aids, the C2S and C3S in clinker made from low-grade raw materials are unevenly distributed and always appear in clusters. C2S aggregates to form B ore nests, with less intermediate phase formation and more pores in the lithofacies. C3S has irregular morphology and its size is mostly between 15 and 25 μm. Figure 3 As shown in (c) and (d), the mineral distribution of clinker is more uniform after the addition of grinding aid. C3S and C2S are interspersed. The occurrence of B mineral nests caused by C2S aggregation is reduced, the number of pores is reduced, the intermediate phase is increased, and the morphology of C3S is more regular, mostly in the form of long plate-shaped grains with a size distribution mostly between 20 and 38 μm.

[0108] Figure 4 The mineral composition of the blank group and other clinker incorporating different types of composite mineralizers is presented. By comparing and analyzing the XRD patterns of the three different incorporation groups, the characteristic diffraction peaks of C3S in the clinker can be clearly observed. In these XRD patterns, the intensity of the characteristic diffraction peaks of C3S in the clinker samples with added mineralizers is significantly higher than that in the blank group without added mineralizers. This phenomenon indicates that the incorporation of mineralizers has a significant promoting effect on C3S formation, while also reducing the f-CaO content in the clinker, effectively improving the stability of the clinker. The enhanced intensity of the characteristic diffraction peaks of C3S reflects its increased content in the clinker, while the diffraction peak intensities of C3A are reduced, indicating that the addition of mineralizers leads to a decrease in C3A content, which to some extent affects the early hydration of the clinker.

[0109] Figure 5The DSC curves of raw meals with different mineralizers added under a N2 atmosphere show that the maximum exothermic peak temperature was highest in the control group, reaching an extreme value of 793.7℃. This indicates that without any mineralizer, the decomposition reaction of CaCO3 in the raw meal reaches its fastest rate at 793.7℃. The maximum exothermic peak temperatures of the other groups, from lowest to highest, are 1SS, 1.5(7F+3SS), 1(7F+3SS), 1F, and 0.5(7F+3SS), which are 22℃, 20℃, 19℃, 10℃, and 6℃ lower than the control group, respectively. This demonstrates that the addition of fluorite and steel slag can effectively promote the decomposition reaction of CaCO3 in the raw meal, reducing the temperature and energy requirements of the decomposition stage during clinker firing. The raw meal with 1% steel slag added had the lowest maximum exothermic peak temperature at 771.7℃, while the raw meal with 1% fluorite added had a maximum exothermic peak temperature of 783.7℃, a difference of 12℃. This indicates that while both steel slag and fluorite can promote the decomposition reaction of CaCO3 in raw materials, steel slag has a more significant effect on lowering the decomposition temperature of CaCO3. Subsequently, the exothermic peaks of the 1.5(7F+3SS) group and the 1(7F+3SS) group appeared sequentially, with maximum exothermic peak temperatures of 773.7℃ and 774.7℃, respectively, differing by only 1℃, and delayed by 2-3℃ compared to the steel slag group. This indicates that the composite mineralizer can also effectively promote the decomposition reaction of CaCO3 in raw materials. However, when the composite mineralizer dosage increased from 1% to 1.5%, the fastest decomposition temperature of CaCO3 did not advance significantly, while the fastest decomposition temperature of the 0.5(7F+3SS) composite group was 787.7℃, only 6℃ lower than the blank group and 13℃ higher than the 1(7F+3SS) group. This indicates that increasing the dosage of the composite mineralizer from 0 to 0.5 wt.% has no significant effect on reducing the decomposition temperature of CaCO3, suggesting that the composite mineralizer has little impact on the decomposition temperature of CaCO3 at low dosages. However, its effect is significantly enhanced when the dosage is increased from 0.5 wt.% to 1 wt.%. Therefore, if the goal is to reduce the decomposition temperature of CaCO3 with a relatively low dosage of composite mineralizer, the optimal dosage is approximately 1 wt.%.

[0110] The clinker samples were ground to ensure all samples passed through an 80μm sieve. The f-CaO content in the samples was then determined using the benzoic acid-ethylene glycol method. Figure 6As shown, the blank group had a higher f-CaO content, reaching 2.99 wt.%. With the addition of mineralizers, the f-CaO content in the clinker decreased, indicating that under the same calcination conditions, both fluorite and steel slag can improve the burnability of the clinker to some extent. In the 0.5 wt.% admixture group, compared with the blank group, the f-CaO content in the fluorite group and the steel slag group decreased by 1.74 wt.% and 1.18 wt.%, respectively. The addition of fluorite caused the liquid phase to appear earlier, while reducing the viscosity of the clinker liquid phase and increasing the amount of liquid phase, thus providing a better environment for the reaction of C2S with f-CaO and promoting the participation of more f-CaO in the formation of C3S. The reason for the decrease in f-CaO content in the steel slag group is that the C3S crystals contained in the steel slag played a crystal induction role in the C3S formation reaction, lowering the activation energy of the reaction between C2S and f-CaO to form C3S, making the reaction easier to proceed, thereby reducing the f-CaO content.

[0111] In the composite mineralizer group with a dosage of 0.5 wt.%, when the total amount of externally added composite mineralizer remained constant, the effect of reducing f-CaO content showed a trend of first decreasing and then increasing with the increase of fluorite / steel slag ratio. Figure 6 (a) When the ratio of fluorite to steel slag is 7:3, the effect of reducing f-CaO is most significant, with the f-CaO content decreasing by 0.62 wt.% compared to the fluorite group. This indicates that fluorite and steel slag have a synergistic effect in reducing f-CaO content and promoting C3S formation.

[0112] like Figure 6 As shown in (b), in the 1% admixture group, the effect and trend of each group in reducing f-CaO content were similar to those in the 0.5% admixture group. Compared with the blank group, the f-CaO content in the fluorite group and the steel slag group decreased by 1.88 wt.% and 2.03 wt.%, respectively. In the composite mineralizer group, the effect of reducing f-CaO was most significant when the ratio of fluorite to steel slag was 7:3, with the f-CaO content decreasing by 0.53 wt.% compared to the fluorite group. This indicates that the 1 wt.% admixture is still within a reasonable admixture range.

[0113] like Figure 6As shown in (c), in the 1.5% admixture group, the mineralization effect was more significant due to the increased mineralizer dosage, and the decrease in f-CaO content was more pronounced. Compared with the control group, the f-CaO content in the fluorite group and the steel slag group decreased by 2.66 wt.% and 2.20 wt.%, respectively, both reaching low levels, indicating that fluorite and steel slag had reached near-optimal effects within their respective dosage ranges. In the composite mineralizer group, at a 1.5% admixture dosage, the highest f-CaO content was 0.39 wt.%, and the lowest was 0.27 wt.%. This indicates that at high dosages, small adjustments to the ratio of fluorite to steel slag had little effect on reducing f-CaO content, and also shows that at a high dosage of 1.5%, the three composite admixture groups in the figure had a better effect on reducing clinker f-CaO content.

[0114] To accurately quantify the various mineral phases of clinker with composite mineralizers, a clinker mineral facies identification method based on the deep learning model U-Net was used for calculation. This method involved selecting over 100 particles (2.5–5.0 mm), curing them with epoxy resin, and polishing them until the surface was free of obvious scratches. Ten randomly selected areas were then photographed using a laser confocal microscope at a resolution of 1161 × 1161 μm. 2 The lithofacies images were used to identify clinker minerals using a model (model learning such as...). Figure 7 (As shown) to quantify clinker minerals.

[0115] After grinding with the compounded grinding aid at a dosage of 0.1 wt.% of the total raw material mass, the residue on the 80 μm sieve and the particle size distribution of the raw material were ground for 30 min. Table 1 shows the largest decrease in 80 μm sieve residue with H7, a relative decrease of 20.9% compared to the blank group. As can be seen from the particle size distribution in Table 1, the use of the compound grinding aid increased the content of particles with a diameter of 5–45 μm in the raw material. Compared to the blank group H0, the addition of H7 increased the content of fine particles with a diameter of 5–45 μm by 2.6 wt.%, which is beneficial for subsequent calcination.

[0116] Table 1. Grinding effect of composite grinding aid

[0117]

[0118] The other raw material ratios for the experiment are shown in Table 2. After being mixed uniformly in a mixer for 24 h, the raw materials were then mixed uniformly with 5% deionized water. The raw material samples were then pressed into 70 mm × 70 mm × 8 mm sheets using a tablet press at a pressure of 20 MPa. The samples were then dried in an oven at 105 °C. The dried samples were then placed in a high-temperature furnace and heated from room temperature to 900 °C at a rate of 10 °C / min, and held at that temperature for 30 min. Subsequently, the temperature was increased to 1450 °C at a rate of 10 °C / min and held for 60 min. After the holding time, the cooked samples were removed and rapidly cooled to room temperature using a fan.

[0119] Table 2 Chemical composition of raw meal and proportion of mineralizers

[0120]

[0121] Note: KB represents the blank group, F represents fluorite, SS represents steel slag, and the numbers outside the brackets in the sample name indicate the content of the mineralizer added, in wt.%. The numbers inside indicate the ratio of fluorite to steel slag.

[0122] Table 3 shows the mineral content of clinker based on the U-Net deep learning model for mineral facies identification. The contents of C3S, C2S, and intermediate phase in clinker with added composite mineralizer are 54.68 wt.%, 24.51 wt.%, and 20.82 wt.%, respectively. This method identifies the main mineral phase contents of cement clinker that are close to those obtained by the Bogue method, and provides a more intuitive observation of the morphology and distribution of clinker mineral phases.

[0123] Table 3. Mineral content of each mineral in clinker based on the U-Net deep learning model for petrographic identification.

[0124]

[0125] Table 4 shows the clinker strength data during the commissioning phase. The results indicate that the strength ranged from 59.3 to 62.6 MPa at 28 days, demonstrating that the performance of the prepared clinker met the expected targets. Furthermore, the application of the composite mineralizer successfully controlled the carbon dioxide emissions per ton of clinker to below 835 kg, demonstrating the effectiveness of this process in reducing its carbon footprint.

[0126] Table 4 Cement clinker strength during the commissioning phase of the cement plant demonstration line / MPa

[0127]

[0128] Table 5 shows the test results of fluorine content in raw materials and clinker. As can be seen from the table, the vast majority of fluorine is effectively fixed in the clinker, indicating that this process not only facilitates fluorine recovery and utilization but also reduces potential environmental impact. Therefore, the analysis of intensity data and fluorine content during the commissioning phase demonstrates the superiority and sustainability of using composite mineralizers in clinker preparation.

[0129] Table 5 Fluorine content in raw and clinker (calculated as calcium fluoride, wt.%)

[0130]

[0131] Based on the chemical composition of the clinker, the mineral composition of the clinker before and after the addition of the composite mineralizer was calculated, and the results are shown in Table 6. After adding the composite mineralizer, the C3S content was adjusted from 53.09 wt.% to 48.28 wt.%, the C2S content increased from 21.65 wt.% to 26.37 wt.%, and the C3A and C4AF contents did not change significantly. Combined with the strength of the clinker with the composite mineralizer in Table 6, the 3-day compressive strength of the clinker increased from 29.3 MPa to 32.3 MPa, and the 28-day compressive strength increased from 55.4 MPa to 60.4 MPa after the addition of the composite mineralizer. It can be seen that although the C3S content decreased, the compressive strength increased significantly. This may be because the addition of the composite mineralizer promoted the growth and development of the clinker mineral phases, allowing the clinker to maintain a high compressive strength at a lower KH.

[0132] Table 6. Clinker mineral composition and compressive strength before and after addition of composite mineralizing agent.

[0133]

Claims

1. A method for preparing high-strength silicate clinker using high-silica, low-grade limestone, characterized in that, This includes optimizing raw material batching ratios, high-efficiency composite grinding aids, and composite mineralization calcination. It is suitable for high-silica, low-grade limestone with SiO2 content >10 wt.% and CaO content 45 wt.%~48 wt.%. The grinding effect is improved through composite grinding aids, and the calcination temperature is lowered and C3S mineral phase development is promoted through fluorite-steel slag composite mineralizers, achieving high-proportion utilization of low-grade raw materials and preparation of high-strength clinker. The specific steps are as follows: Step 1. Raw material batching ratio optimization: High-silica, low-grade limestone, sandstone, ferroalumina, and ferrous materials are used for raw material batching. The three ratios of raw material batching are optimized and matched to control the lime saturation coefficient KH=0.880±0.005, the silicon ratio SM=2.45±0.05, and the aluminum ratio IM=1.45±0.

05. Step 2. High-efficiency composite grinding aid: In the raw material grinding process, a multi-component high-efficiency composite grinding aid is added for grinding. The multi-component high-efficiency composite grinding aid consists of triethanolamine, ethylene glycol, calcium lignosulfonate, and sodium hexametaphosphate at a ratio of 0.05% to 0.15% of the total mass of the raw material. Step 3. Composite mineralization and calcination: A composite mineralizer made of fluorite and steel slag is introduced into the raw material, followed by preheating decomposition, high-temperature calcination and rapid cooling; high-strength silicate clinker is produced; the mass ratio of fluorite to steel slag is 6:4~8:

2.

2. The method for preparing high-strength silicate clinker using high-silica, low-grade limestone according to claim 1, characterized in that, The raw materials are made from high-silicon, low-grade limestone, sandstone, ferroalumina, and ferrous materials. The mass ratio of each raw material is as follows: high-silicon, low-grade limestone 88wt.%~93wt.%, sandstone 4wt.%~7wt.%, ferroalumina 2wt.%~3wt.%, and ferrous materials 1wt.%~2wt.%.

3. The method for preparing high-strength silicate clinker using high-silica, low-grade limestone according to claim 2, characterized in that, The raw material batching ratios are controlled as follows: lime saturation coefficient KH = 0.9, silicon ratio SM = 2.5, and aluminum ratio IM = 1.4; lime saturation coefficient KH = 0.880 ± 0.005, silicon ratio SM = 2.45 ± 0.05, and aluminum ratio IM = 1.45 ± 0.

05.

4. The method for preparing high-strength silicate clinker using high-silica, low-grade limestone according to claim 3, characterized in that, The mass percentages of each component in the high-efficiency composite grinding aid are as follows: triethanolamine 25-35 wt.%, ethylene glycol 4-6 wt.%, calcium lignosulfonate 8-12 wt.%, sodium hexametaphosphate 12-18 wt.%, and the balance being water.

5. The method for preparing high-strength silicate clinker using high-silica, low-grade limestone according to claim 4, characterized in that, The raw material grinding process includes: Raw material pretreatment: High-silica, low-grade limestone, sandstone, ferroalumina, and ferrous materials are dried and then crushed to ≤3mm; Ingredient ratio: high-silica, low-grade limestone 88wt.%~93wt.%, sandstone 4wt.%~7wt.%, bauxite 2wt.%~3wt.%, ferrous materials 1wt.%~2wt.%; Grinding conditions: Φ300mm×300mm ball mill, media filling rate 40%, steel ball ratio Φ30mm:Φ20mm:Φ15mm=2:4:4, grinding time 30min; liquid grinding aid is sprayed in, and solid grinding aid is fed into the mill simultaneously with the raw materials.

6. The method for preparing high-strength silicate clinker using high-silica, low-grade limestone according to claim 5, characterized in that, The composite mineralizer, which is a mixture of fluorite and steel slag, has a mass ratio of fluorite to steel slag of 7:3, and the amount of the composite mineralizer is 1.0 wt.% to 2.0 wt.% of the raw material mass.

7. The method for preparing high-strength silicate clinker using high-silica, low-grade limestone according to claim 6, characterized in that, The calcination process is as follows: raw materials are pressed into sheet-like samples of 60mm×60mm×6mm~80mm×80mm×10mm under 15~25MPa and dried at 100~110℃; the temperature is increased to 850~950℃ at 8~12℃ / min and held for 20~40min, and then the temperature is increased to 1420~1480℃ and held for 45~75min. After calcination, the samples are quickly air-cooled to room temperature.

8. The method for preparing high-strength silicate clinker using high-silica, low-grade limestone according to claim 7, characterized in that, The fluorine element introduced by the composite mineralizer is solidified in the clinker in the form of calcium fluoride, without secondary pollution, and is suitable for large-scale production of green and low-carbon cement clinker.

9. A method for preparing high-strength silicate clinker using high-silica, low-grade limestone according to claim 8, characterized in that, The parameters of the high-strength silicate clinker are as follows: 28-day compressive strength ≥ 60.4 MPa, 3-day compressive strength ≥ 32.3 MPa, free calcium oxide f-CaO ≤ 0.58 wt.%; clinker comprehensive coal consumption 93.14 kgce / t, comprehensive energy consumption 98.87 kgce / t, reaching level 1 energy consumption, comprehensive environmental load reduction of 16.6%, CO2 emission per ton of clinker < 835 kg.

10. A method for preparing high-strength silicate clinker using high-silica, low-grade limestone according to claim 9, characterized in that, The preferred ratio of high-silica, low-grade limestone is: high-silica, low-grade limestone: sandstone: iron-aluminate: iron material = 90.75: 5.78: 2.31: 1.16.

Images