Coal gangue, as the main industrial solid waste generated during coal mining, typically contains 30% to 60% kaolin. The impurities mainly consist of carbon, quartz, and iron-titanium oxides. By adopting the combined process of "three-stage suspension pre-sintering + rotary kiln deep sintering", the high-value utilization of coal gangue can be achieved, converting it into high-quality sintered kaolin.
The core of this process lies in: Firstly, the raw materials are preheated, decontaminated (decarbonization and dehydration), and preliminarily calcined through three-stage suspension kilns. Then, the deep calcination and crystal structure optimization are achieved through a rotary kiln, resulting in functional mineral materials suitable for applications in papermaking, coatings, ceramics, and other fields.
I. Raw material pre-treatment: Remove impurities and optimize material properties
Since the raw coal gangue contains a high proportion of carbon, quartz and iron minerals, direct calcination would result in blackening of the product, low whiteness and poor activity. Therefore, a systematic pre-treatment is necessary to improve the purity and physical properties of the raw materials, which is a key step different from the calcination of natural kaolin.
Firstly, the large pieces of coal gangue (with a particle size of 500 - 1000mm) are successively fed into the jaw crusher for coarse crushing. Then, the material is further crushed by a cone crusher or a impact crusher to achieve medium crushing, reducing the particle size to 20 - 50mm, which is suitable for the subsequent grinding operation. After crushing, the particle size should be uniform to avoid over-crushing and reduce dust generation. The particle size of the material after medium crushing should be controlled within 30mm.
Subsequently, the coal gangue undergoes washing and carbon removal treatment. Generally, the coal gangue contains 5% to 15% residual coal. If not removed, after calcination, it will cause the product color to darken, seriously affecting the whiteness. The coarse selection is carried out using a jig or a heavy medium separator, and the fine selection is combined with a flotation machine. This can effectively remove most of the carbonaceous components, achieving a carbon removal rate of ≥ 90%, ensuring that the carbon content of the washed and selected materials is ≤ 1%. At the same time, a wastewater recycling system is also constructed to achieve the effective utilization of water resources and reduce environmental pollution.
The washed and selected materials then enter the grinding and classification stage. The materials are ground to a fine powder state through ball mills (either dry or wet method), and particle size classification is carried out using hydrocyclones or cyclone classifiers to ensure uniform particle size distribution. If dry grinding is used, the fineness of the finished product should be controlled to be above 90% for -200 mesh (i.e., particle size ≤ 74 μm); if wet grinding is used, subsequent dehydration through a filter press is required to reduce the moisture content to 20% - 25%.
For the dry process, pre-drying treatment is also required to remove the free water from the materials. Using a drum dryer or a flash dryer, the moisture content of the materials should be reduced to below 2%, with the drying temperature controlled within 80℃ to prevent the materials from agglomerating or caking due to premature heating, which would affect the stability of the subsequent suspension calcination.
To further enhance the potential of product whiteness, magnetic separation for pre-removal of iron is carried out before entering the calcination system. A permanent cylindrical magnetic separator is used to initially remove iron from the materials, effectively removing strong magnetic minerals such as Fe₂O₃ and Fe₃O₄. The iron removal rate is no less than 60%, reducing the iron content to below 0.8%, laying the foundation for subsequent deep purification.
II. Three-level Suspension Calcination: Integration of Preheating, Dross Removal and Light-level Calcination
The III. level suspension calcination system consists of three vertically arranged calcination chambers connected in series. Utilizing high-speed hot air flow, fine powder materials operate in a "suspended state", coming into direct contact with the high-temperature flue gas in counter-flow, resulting in a heat transfer efficiency much higher than that of traditional rotary kilns, up to 3 to 5 times. This stage mainly accomplishes three core tasks: removing free water and some crystalline water, burning off residual carbonaceous substances, and achieving the initial crystal transformation of kaolin to bentonite.
The materials enter the primary suspension kiln from the top, while the high-temperature flue gas (with a temperature range of 800 - 900℃) from the rotary kiln is introduced from the bottom of the tertiary suspension kiln, forming counter-flow heat exchange. As the materials descend step by step, the temperature gradually increases. When they are discharged from the bottom of the tertiary kiln, the temperature reaches 700 - 800℃; the flue gas is discharged from the top of the primary kiln, with the temperature dropping to 300 - 400℃. This temperature can be used to preheat the combustion air or as a drying heat source, achieving energy cascade utilization.
In the primary suspension kiln (300 - 400℃), the main task is to remove the residual free water from the materials (reducing the moisture content from 2% to below 0.5%), and to burn off a small amount of volatile impurities. The airflow speed is controlled at 10 - 12 m/s, with a residence time of approximately 3 - 5 seconds, ensuring that the materials remain suspended and do not settle.
After entering the secondary suspension kiln (at 500 - 600℃), the residual carbon in the materials is deeply burned, reducing the carbon content from 1% to below 0.2%. This prevents the formation of coke deposits or agglomerations in the rotary kiln in the subsequent process. During this stage, the airflow speed is increased to 12 - 14 m/s, the residence time is extended to 5 - 8 seconds, and the CO concentration in the exhaust gas needs to be monitored in real time to ensure it is below 500 ppm, indicating that the carbon has been largely burned out.
The third-level suspension kiln (700 - 800℃) is the key area for crystal transformation. Within this temperature range, kaolin undergoes a dehydration reaction: Al₂O₃·2SiO₂·2H₂O → Al₂O₃·2SiO₂ + 2H₂O↑, removing most of the crystalline water, with a removal rate of ≥ 80%, resulting in a modified kaolin. At the same time, some siliceous impurities begin to soften. The gas flow speed is maintained at 14 - 15 m/s, with a residence time of 8 - 10 seconds, ensuring the reaction proceeds.
The key equipment includes the three-level suspension kiln body (made of heat-resistant steel with a refractory mortar lining) and the accompanying cyclone separator, which is used to recover the fine powder entrained in the exhaust gas from the kiln and return it to the system for re-calcination. During operation, the airflow speed must be strictly controlled: too low a speed may cause the materials to settle and block the kiln body; too high a speed will result in insufficient residence time. The speed should be adjusted in real time using a flow meter to ensure the stable operation of the system.
III. Deep calcination in rotary kiln: Crystal type optimization and harmless treatment of impurities
The materials that have undergone pre-calcination in the three-stage suspension kiln (high kaolin, still containing a small amount of undissolved crystalline water and iron-titanium impurities) enter the rotary kiln. Under high temperature (900 - 1050℃) and long residence time (1 - 2 hours), they complete the deep calcination, which is the core link determining the whiteness, activity and purity of the product.
After being discharged from the three-level suspension kiln (with a temperature range of 700 - 800℃), the materials are evenly sent into the upper end (the kiln tail) of the rotary kiln through the kiln tail distributor. The rotary kiln is a cylindrical device installed at an angle (with an inclination angle of 3° - 5°, a diameter of 2.5 - 4m, and a length of 20 - 30m). It is lined with high-alumina refractory bricks and has excellent high-temperature resistance and wear resistance. The kiln body rotates slowly at a speed of 0.5 - 2 revolutions per minute. Under the action of gravity and the friction force of the kiln wall, the materials slowly move towards the kiln head (the lower end) to achieve "rolling and calcining simultaneously".
The heating method is that a coal or gas burner is installed at the kiln head. The hot flue gas flows from the kiln head to the kiln tail and comes into contact with the materials in an opposite direction. The temperature inside the kiln gradually rises from 700°C at the kiln tail to 1050°C at the kiln head, forming a reasonable temperature gradient.
In the tail section of the kiln (700 - 850℃), the remaining crystalline water is completely removed. By extending the residence time (15 - 20 minutes), the crystalline water removal rate is ensured to be completed, and the material is transformed into anhydrous kaolin.
The middle section of the kiln (850 - 950℃) is the key area for crystal type optimization. By controlling the heating rate (5 - 10℃/minute), the high-calcium kaolin is gradually transformed into a structurally stable calcined kaolin, significantly enhancing its chemical activity, and suitable for functional applications such as paper fillers and coatings.
The kiln head section (950 - 1050℃) achieves harmless treatment of impurities. By introducing a small amount of coal powder to create a weakly reducing atmosphere, Fe₂O₃ is reduced to FeO, which is easier to be removed in the subsequent magnetic separation process; meanwhile, TiO₂ remains in a stable form and does not affect the product performance.
The high-temperature flue gas (800 - 900℃) discharged from the rotary kiln is first sent to the three-stage fluidized bed kiln as a heat source to achieve the recycling and utilization of waste heat. Subsequently, it passes through the bag filter (with a dust removal efficiency of ≥ 99.9%) to remove dust. If the raw material contains sulfur, it will enter the desulfurization tower to remove SO₂, achieving compliance with emission standards. The dust concentration is controlled at ≤ 10mg/m³.
IV. Cooling: Temperature control to prevent cracking, heat recovery
After calcination, the material is discharged from the top of the rotary kiln at a temperature of 800 - 1000℃. If it is suddenly cooled, it will cause the particles to crack due to thermal stress, which will affect the fineness and integrity of the product. Therefore, a gradient cooling method should be adopted to control the cooling rate.
The equipment is equipped with a multi-tank cooling machine (6 to 8 cooling tanks rotating around the central axis). After the high-temperature materials enter, they come into head-on contact with the blown-in cold air, gradually cooling down to below 100℃. During the cooling process, the air is heated to 300-400℃ and then returned to the rotary kiln burner as combustion air, significantly reducing fuel consumption and achieving an energy-saving effect of 15% - 20%.
The cooling rate should be controlled at ≤ 50℃/minute. For products used in coatings or papermaking, slow cooling is recommended to maintain activity; if used in the ceramic field, the cooling rate can be appropriately increased.
V. Grinding, Classification and Purification: Optimizing particle size, thorough impurity removal
The cooled materials may contain a small amount of lumps, and further purification is required to meet the demands of high-end applications.
First, the cooled and agglomerated materials (with a particle size of ≤ 50mm) are subjected to coarse crushing and disintegration through a hammer crusher (with an inner rubber lining to prevent iron contamination). The particle size after crushing is controlled to be within 5mm.
Subsequently, ultrafine grinding is carried out. Either an air jet mill (without media grinding, avoiding contamination) or a ceramic ball mill (with grinding media being alumina balls) is used to grind the material to the desired fineness. For example, for paper filling materials, the -325 mesh size should account for more than 95% (particle size ≤ 44 μm).
After grinding, it proceeds to the fine classification stage. An airflow classifier is used to remove the coarse particles, with a classification efficiency of ≥ 90%. The coarse particles are then returned to the grinding process for reprocessing to ensure uniform particle size distribution of the product.
Then, a second iron removal process is carried out. A high-gradient magnetic separator (with a magnetic field strength of 1.2 - 1.5 T) is used to deeply remove the trace iron impurities brought in during the grinding process due to equipment wear. The iron content is required to be ≤ 0.3% (whiteness ≥ 85%); for high-whiteness products, it needs to be further reduced to ≤ 0.1%, and the whiteness can reach over 90%.
Finally, the flotation purification process is carried out. The flotation machine is used in conjunction with fatty acid type collectors and NaOH to adjust the pH to 8-9, thereby removing silica-based impurities such as quartz and feldspar and increasing the content of Al₂O₃. After the flotation process, the content of Al₂O₃ can be increased from 30%-35% to 35%-40%, meeting the industrial-grade kaolin standard.
VI. Finished Product Processing: Dehydration, Drying and Packaging
If the moisture content of the material after flotation is high (20% - 30%), dehydration and drying treatment should be carried out to prevent moisture absorption and caking during storage.
First, the sludge is dehydrated using a box-type filter press, reducing the moisture content of the filter cake to 15% - 20%. Then, it is dried using a spray dryer (suitable for ultrafine powder) or a drum dryer. The drying temperature is controlled at 150 - 200℃, and the moisture content is maintained at 0.5% - 1%.
After drying, conduct quality tests, including whiteness (measured by spectrophotometer), Al₂O₃ content (analyzed by X-ray fluorescence spectroscopy), particle size distribution (measured by laser particle size analyzer), and moisture (dried method), to ensure that all indicators meet the standards.
Qualified products are packed in valve bags lined with PE film (25kg per bag or bulk bags), and stored in a dry and well-ventilated warehouse to prevent moisture absorption.
VII. Core Strengths and Key Control Points
This process has significant technical and economic advantages. Firstly, it has achieved the transformation of coal gangue from waste to treasure, converting it into high-value calcined kaolin, which is widely used in papermaking, coatings, rubber, ceramics and other fields, effectively alleviating the environmental pressure caused by solid waste storage. Secondly, energy utilization is efficient: the three-stage suspension kiln utilizes the residual heat from the rotary kiln, and the cooling machine recovers hot air for combustion assistance, reducing the comprehensive energy consumption by 25% - 30% compared to the traditional pure rotary kiln process. Moreover, due to the uniform heat transfer in the suspension calcination process, the deep calcination in the rotary kiln ensures the crystal transformation, resulting in products with high whiteness, good activity and stable quality, which are superior to the single equipment calcination process.
The key control points are as follows: The carbon removal rate of raw materials must be ≥ 90%; otherwise, the product is prone to discoloration; the airflow velocity in the suspension kiln should be controlled within 10 - 15 m/s to ensure stable suspension of the materials; the kiln head temperature of the rotary kiln must not exceed 1050℃ to prevent over-burning and loss of activity; the iron content should be controlled at ≤ 0.3%; for every 0.1% reduction in iron content, the product's whiteness can increase by 1% - 2 percentage points.
Through the above-mentioned complete process, coal gangue has been effectively transformed into qualified calcined kaolin products, achieving both environmental benefits and economic benefits. It is one of the mainstream technical paths for the resource utilization of coal-derived kaolin at present.
