Gangue Resource Utilization: 3-stage Suspension + Rotary Kiln Calcined Kaolin Process

Gangue, a major industrial solid waste generated during coal mining, typically contains 30%–60% kaolin, with impurities primarily consisting of carbon, quartz, and iron and titanium oxides. The combined process of "three-stage suspension pre-calcination followed by deep calcination in a rotary kiln" allows for high-value utilization of gangue, transforming it into high-quality calcined kaolin.

The core of this process is to first complete the preheating, decontamination (decarbonization, dehydration) and preliminary calcination of the raw materials through a three-stage suspension kiln, and then achieve deep calcination and crystal optimization through a rotary kiln to produce functional mineral materials suitable for papermaking, coatings, ceramics and other fields.

1. Raw Material Pretreatment: Remove Impurities and Optimize Material Properties

Since gangue ore contains a high proportion of carbon, quartz and iron minerals, direct calcination will result in a black product with low whiteness and poor activity. Therefore, it must undergo systematic pretreatment to improve the purity and physical properties of the raw materials. This is the key step that distinguishes it from the calcination of natural kaolin.

First, large pieces of gangue (particle size 500-1000mm) are fed sequentially into a jaw crusher for coarse crushing. A cone crusher or impact crusher then performs secondary crushing, reducing the particle size to 20-50mm for subsequent grinding. After crushing, the particle size must be uniform to avoid over-crushing and reduce dust generation. The particle size after secondary crushing is controlled within 30mm.

Washing and decarbonization are then performed. Gangue typically contains 5%–15% residual coal. If not removed, the product will darken in color after calcination, significantly affecting its whiteness. Using a jig or heavy media separator for roughing, combined with a flotation cell for fine separation, effectively removes most of the carbonaceous components, achieving a carbon removal rate of ≥90% and ensuring a carbon content of ≤1% after washing. A wastewater recycling system is also implemented to effectively utilize water resources and reduce environmental pollution.

After washing, the material enters the grinding and classification stage. It is ground to a fine powder in a ball mill (dry or wet), and then sized using a hydrocyclone or cyclone classifier to ensure uniform particle size distribution. If dry grinding is used, the finished product must have a fineness of at least -200 mesh (i.e., a particle size ≤74μm) of at least 90%. If wet grinding is used, the finished product must be dehydrated using a filter press to reduce the moisture content to 20%–25%.

For the dry process, pre-drying is also required to remove free water from the material. Use a drum dryer or flash dryer to reduce the material moisture content to below 2%. Keep the drying temperature below 80°C to prevent premature heating of the material, which could cause agglomeration or caking and affect the stability of subsequent suspension calcination.　

To further enhance the product's whiteness potential, magnetic separation is used for pre-iron removal before entering the calcination system. This initial iron removal process, using a permanent magnetic drum separator, effectively removes strongly magnetic minerals such as Fe₂O₃ and Fe₃O₄, achieving an iron removal rate of at least 60%, reducing the iron content to below 0.8%, laying the foundation for subsequent deep purification.

2 & 3 Stage Suspension Calcination: Preheating, Impurity Removal and Shallow Calcination Integrated

The three-stage suspension calcination system consists of three vertical calcination chambers connected in series. It utilizes high-velocity hot airflow to suspend fine powder, creating a countercurrent flow of hot flue gas. This results in a heat transfer efficiency far superior to that of traditional rotary kilns, reaching 3–5 times that of conventional rotary kilns. This stage primarily accomplishes three key tasks: removing free water and some water of crystallization, burning off residual carbon, and achieving the initial crystal transformation of kaolin to metakaolin.

Materials enter the first-stage suspension kiln from the top, while high-temperature flue gas (800–900°C) from the rotary kiln enters the third-stage suspension kiln from the bottom, creating a countercurrent heat exchange. As the materials descend through the stages, their temperature gradually increases, reaching 700–800°C by the time they exit the third-stage kiln bottom. The flue gas exits the first-stage kiln top, having cooled to 300–400°C. This can be used to preheat combustion air or as a drying heat source, achieving cascaded energy utilization.

In the first-stage suspension kiln (300–400°C), the primary goal is to remove residual free water from the material (reducing moisture from 2% to below 0.5%) and incinerate a small amount of volatile impurities. Air velocity is controlled at 10–12 m/s, with a residence time of approximately 3–5 seconds, ensuring that the material is fully suspended and prevents sedimentation.

After entering the secondary suspension kiln (500–600°C), the residual carbon in the material is deeply burned, reducing the carbon content from 1% to below 0.2%, preventing the subsequent formation of coke deposits or rings in the rotary kiln. During this stage, the air velocity is increased to 12–14 m/s, the residence time is extended to 5–8 seconds, and the CO concentration in the exhaust gas is monitored in real time to ensure it is below 500 ppm, indicating that the carbon has been essentially burned.

The three-stage suspension kiln (700–800°C) is a critical region for crystal transformation. Within this temperature range, kaolin undergoes a dehydroxylation reaction: Al₂O₃·2SiO₂·2H₂O → Al₂O₃·2SiO₂ + 2H₂O↑, removing most of the water of crystallization, with a removal rate of ≥80%, to produce metakaolin. Simultaneously, some siliceous impurities begin to soften. Airflow velocity is maintained at 14–15 m/s, with a residence time of 8–10 seconds to ensure the reaction proceeds.

Key equipment includes a three-stage suspension kiln (made of heat-resistant steel and lined with refractory castables) and a matching cyclone separator, which recovers fines entrained in the kiln flue gas and returns them to the system for re-calcination. Airflow velocity must be strictly controlled during operation: too low a velocity can easily cause material to settle and clog the kiln; too high a velocity can result in insufficient residence time. A flow meter should be used to adjust the flow in real time to ensure stable system operation.

3. Deep Calcination in Rotary Kiln: Crystal Form Optimization and Impurity Harmlessness

The material (metakaolin, still containing a small amount of unremoved crystal water and iron-titanium impurities) pre-calcined in the three-stage suspension kiln enters the rotary kiln and completes deep calcination under high temperature (900-1050℃) and long residence time (1-2 hours). This is the core link that determines the whiteness, activity and purity of the product.

After exiting the three-stage suspension kiln (temperature 700–800°C), the material is evenly fed into the upper end (kiln tail) of the rotary kiln via a kiln tail distributor. The rotary kiln is a tilted cylindrical device (3°–5° inclination, 2.5–4m diameter, 20–30m length) lined with high-alumina refractory bricks, offering excellent high-temperature and wear resistance. The kiln rotates slowly at a speed of 0.5–2 rpm. The material slowly moves toward the kiln head (lower end) under the influence of gravity and friction from the kiln wall, achieving "rolling and calcining" simultaneously.

The heating method is to install a coal or gas burner at the kiln head. The hot flue gas flows from the kiln head to the kiln tail, contacting the material in countercurrent. The temperature inside the kiln gradually rises from 700℃ at the kiln tail to 1050℃ at the kiln head, forming a reasonable temperature gradient.

At the inlet of the kiln (700–850°C), the remaining crystal water is completely removed. By extending the residence time (15–20 minutes), the crystal water removal rate is ensured to be complete and the material is converted into anhydrous kaolin.

The middle section of the kiln (850–950°C) is a critical area for crystal form optimization. By controlling the heating rate (5–10°C/minute), the metakaolin is gradually converted into structurally stable calcined kaolin, significantly increasing its chemical activity and making it suitable for functional applications such as paper fillers and coatings.

The kiln outlet section (950–1050°C) achieves harmless impurity treatment. By introducing a small amount of pulverized coal to create a weak reducing atmosphere, Fe₂O₃ is reduced to FeO, which is easier to remove during subsequent magnetic separation. At the same time, TiO₂ remains in a stable form, without affecting product performance.

The high-temperature flue gas (800–900°C) from the rotary kiln is first fed into a three-stage suspension kiln as a heat source, recovering waste heat. It then passes through a bag filter (dust removal efficiency ≥99.9%) to remove dust. If the feedstock contains sulfur, it enters a desulfurization tower to remove SO₂, achieving standard emissions and controlling dust concentration to ≤10mg/m³.

4. Cooling: Temperature Control to Prevent Explosion and Heat Recovery

After calcination, the material is discharged from the rotary kiln head at a temperature of 800-1000°C. If it is directly cooled rapidly, the thermal stress will cause the particles to explode, affecting the fineness and integrity of the product. Therefore, a gradient cooling method is required to control the cooling rate.

Equipped with a multi-drum cooler (6–8 cooling drums rotating around a central axis), hot materials enter the kiln and come into countercurrent contact with the incoming cold air, gradually cooling it to below 100°C. During the cooling process, the air is heated to 300–400°C and returned to the rotary kiln burner as combustion air, significantly reducing fuel consumption and achieving energy savings of 15%–20%.

The cooling rate should be controlled at ≤50℃/min. 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 accelerated.

5. Grinding, Grading and Purification: Optimizing Particle Size and Removing Impurities in Depth

The cooled material may contain a small amount of agglomerates and requires further purification to meet the demands of high-end applications.

First, a hammer crusher (lined with rubber to prevent iron contamination) is used to coarsely crush and disagglomerate any agglomerates (particle size ≤ 50 mm) after cooling, reducing the particle size to less than 5 mm.

Subsequently, ultrafine grinding is performed using a jet mill (media-free grinding to minimize contamination) or a ceramic ball mill (using alumina balls as the grinding medium) to achieve the target fineness. For example, papermaking fillers require a minimum of 95% -325 mesh (particle size ≤ 44 μm).

After grinding, the material enters the fine classification stage, where an airflow classifier is used to remove coarse particles with a classification efficiency of ≥90%. The coarse particles are returned to the grinding process for reprocessing to ensure a uniform particle size distribution.

Second, secondary iron removal is performed using a high-gradient magnetic separator (magnetic field strength 1.2–1.5 T) to thoroughly remove trace iron impurities introduced during the grinding process due to equipment wear. The iron content is required to be ≤0.3% (whiteness ≥85%); for high-white products, this must be further reduced to ≤0.1%, achieving whiteness above 90%.

Finally, flotation purification is performed using a flotation cell with a fatty acid collector and NaOH to adjust the pH to 8–9, removing silicate impurities such as quartz and feldspar and increasing the Al₂O₃ content. After flotation, the Al₂O₃ content can be increased from 30%–35% to 35%–40%, meeting industrial-grade kaolin standards.

6. Finished Product Processing: Dehydration, Drying and Packaging

If the moisture content of the material after flotation is high (20%–30%), dehydration and drying are required to prevent moisture absorption and clumping during storage.

First, dehydrate the filter cake using a chamber filter press to reduce its moisture content to 15%–20%. Dry the cake using a spray dryer (suitable for ultrafine powders) or a drum dryer, controlling the drying temperature at 150–200°C and the moisture content to 0.5%–1%.

After drying, quality testing is performed for whiteness (spectrophotometric), Al₂O₃ content (X-ray fluorescence analysis), particle size distribution (laser particle size analyzer), and moisture content (drying method) to ensure that all indicators meet standards.

Qualified products are packaged in PE film-lined valve bags (25kg per bag or ton bags) and stored in a dry, well-ventilated warehouse to protect against moisture.

7. Core Advantages and Key Control Points

This process offers significant technical and economic advantages. First, it transforms coal gangue into high-value-added calcined kaolin, which is widely used in papermaking, coatings, rubber, ceramics, and other fields, effectively alleviating the environmental pressure caused by solid waste storage. Second, it is energy-efficient: the three-stage suspension kiln utilizes waste heat from the rotary kiln exhaust, and the cooler recycles hot air to aid combustion, reducing overall energy consumption by 25%–30% compared to traditional rotary kiln processes. Furthermore, due to the uniform heat transfer during suspension calcination and the deep calcination in the rotary kiln, which ensures crystal transformation, the product achieves high whiteness, excellent activity, and consistent quality, surpassing single-equipment calcination processes.

Key control points include: the raw material carbon removal rate must be ≥90%, otherwise the product will easily turn black; the airflow velocity in the suspension kiln should be controlled at 10–15 m/s to ensure stable material suspension; the rotary kiln head temperature must not exceed 1050°C to prevent overheating and loss of activity; and the iron content must be controlled to ≤0.3%. Every 0.1% reduction in iron content increases the product whiteness by 1%–2 percentage points. Through the above-mentioned full-process treatment, coal gangue can be effectively converted into qualified calcined kaolin products, which have both environmental and economic benefits. It is one of the current mainstream technical paths for the resource utilization of coal-based kaolin.

Through the above-mentioned full-process treatment, coal gangue can be effectively converted into qualified calcined kaolin products, which has both environmental and economic benefits. It is one of the current mainstream technical paths for the resource utilization of coal-based kaolin.