Key Points to Produce High Quality Fired Bricks from Low Grade Coal Gangue
Low‑grade coal gangue is a common industrial solid waste from coal mining. With proper process design, it can be turned into high‑performance fired brick products including solid bricks, hollow bricks and hollow blocks. Building a coal gangue brick plant without sufficient raw‑material testing will easily trigger frequent quality defects and heavy economic losses. Below are core technical guidelines for stable production.
First of all, complete chemical‑physical inspection and sintering thermal test for coal gangue raw materials are mandatory before factory construction. Major test items cover chemical composition, calorific value, plasticity index, drying sensitivity coefficient, particle size distribution, hardness, drying linear shrinkage, firing linear shrinkage, firing temperature range, water absorption after sintering, lime bursting test and efflorescence test. Mineral composition analysis shall be added if necessary.
Test data decides whether coal gangue is suitable for brick‑making, which brick types can be manufactured, and what raw material preparation system, forming equipment and tunnel kiln should be adopted. Never launch construction blindly.
Special attention shall be paid to plasticity index measurement: crush all samples below 1 mm and test with liquid‑plastic limit combined tester. Particle size greatly affects plasticity. For instance, shale measured at particle <1 mm gets plasticity index 7.2, while the same material crushed below 0.5 mm reaches 11.2. Lab‑controlled 1 mm maximum particle size is close to practical production target 2 mm. Larger particles will block the cone and cause wrong test results.
There is no national standard for coal gangue calorific value testing. Operators shall follow GB/T 213‑2003 Test Method for Calorific Value of Coal. Empirical industrial calculation methods are not applicable for coal gangue.
Production of coal gangue fired bricks adopts the internal combustion firing process. Calorific value dominates the whole technical scheme. When calorific value reaches around 450 × 4.17 kJ/kg, full coal‑gangue internal‑combustion bricks can be produced without extra fuel or additives. The upper calorific‑value limit for pure coal‑gangue feedstock is 600 × 4.17 kJ/kg.
For over‑internal‑combustion condition, optimize manufacturing workflows and extend firing time. Custom‑built lengthened tunnel kiln is required, with double kiln gates at both inlet and outlet. Auxiliary systems including waste‑heat recovery, flue gas reburning, flue‑gas damper regulation, circulating air, rapid‑cooling combustion‑supporting, outside‑kiln forced air, kiln‑bottom balanced cooling and surplus‑heat discharge shall be configured. These systems burn excess combustibles inside green brick green body slowly within proper temperature zones, so as to obtain final bricks above MU20 strength grade.
If raw‑material calorific value is too low, mix proper raw coal or middling coal. Supplementing fuel outside kiln is not recommended because it raises particulate emission. If calorific value is excessively high, blend non‑calorific brick‑making materials such as shale, clay or burnt coal gangue. When blending materials are unavailable, apply screening, air separation, flotation or low‑temperature decarbonization, and adjust comprehensive calorific value near 450 × 4.17 kJ/kg.
Most coal gangue exists as massive rock with low plasticity index (except materials containing montmorillonite), bringing difficulties for wet plastic extrusion forming. Five practical solutions are available:
- 1. Improve crushing fineness: crush feedstock below 2 mm for standard bricks; below 1 mm for high‑void thin‑wall products. Re‑grind oversize fractions to 0.5 mm and remix to optimize particle gradation and lift plasticity.
- 2. Deploy heavy‑duty vacuum extruder: modern brick‑making vacuum extruder delivers extrusion pressure above 4.0 MPa, with forming moisture as low as 13 %. It can produce qualified green bodies even for raw materials with plasticity index as low as 5.
- 3. Mix high‑plasticity additives such as silt, shale and clay. Guarantee accurate proportioning and homogeneous mixing. For materials with large density difference, adopt wheel roller mill and disc‑screw kneader for intensive kneading besides ordinary mixers.
- 4. Steam‑heat mud before extrusion. Obtain steam from waste‑heat boiler attached to tunnel kiln. Heated mud gains better plasticity, reduces extruder load and shortens drying cycle of green bodies.
- 5. Apply double‑stage vacuum mud refining. Feed mud into vacuum kneader for primary degassing kneading before entering main brick extruder. Repeated vacuum treatment improves mud plasticity, increases green‑body density and wet‑brick strength.
For full coal‑gangue high‑quality fired brick, select one‑time stacking‑firing technology. Adopt separated tunnel drying chamber and tunnel kiln with smoke‑heat separation design. Independent drying and firing chambers support separate parameter adjustment, avoid brick cracking and facilitate flue‑gas treatment to meet emission standards.
One‑time stacking‑firing brings multi‑layer stacked green bodies, which easily generate black spot pressing marks. Apart from low‑temperature slow‑firing, spread thin sand layer on top of each wet brick to increase inter‑layer ventilation. Or change stacking posture from standing to lying down, so pressing marks appear on cutting surface without damaging visible brick faces.
Low‑grade coal gangue contains harmful impurities that trigger lime bursting and efflorescence.
- 1.Lime bursting comes from calcite and limestone. During firing they turn into CaO, which absorbs moisture and expands violently after kiln discharge. Crush raw material below 2 mm and soak hot‑out‑of‑kiln bricks with saturated water to prevent lime bursting.
- 2.Efflorescence originates from soluble salts including magnesium sulfate and sodium sulfate. Salts migrate with water and precipitate white crystal powder on brick surfaces. Enhance raw‑material fineness and extend firing & holding time, converting soluble salts into insoluble silicates to eliminate efflorescence risk.
Black core cross‑section is common for internal‑combustion coal‑gangue bricks. Two root causes: insufficient firing temperature & holding time leading to incomplete combustion; rapid temperature rise causing early surface sintering and internal oxygen shortage. Control heating‑up rate and extend low‑temperature firing period to ensure sufficient oxygen penetration inside green bodies, reducing over‑fired black‑core bricks and eliminating under‑fired black‑core defects.
For special low‑grade coal gangue with unique chemical, physical and mineral features, customized technical measures can be adopted to manufacture premium fired brick products.
FAQ
Q1: What pre‑tests are required before building a coal gangue brick plant?
A: Chemical‑physical property analysis, calorific‑value test, plasticity index, drying & firing shrinkage, lime bursting and efflorescence test are essential. Sintering thermal simulation is highly recommended.
Q2: What is the ideal calorific‑value range for full coal‑gangue internal‑combustion bricks?
A: Optimal value is around 450 × 4.17 kJ/kg; maximum allowable limit is 600 × 4.17 kJ/kg.
Q3: How to handle low‑plasticity coal gangue for extrusion forming?
A: Fine crushing, high‑pressure vacuum extruder, blending high‑plastic materials, steam heating for mud and double vacuum kneading are five mainstream solutions.
Q4: Why does coal gangue brick have lime bursting and efflorescence defects?
A: Lime bursting relates to calcium‑bearing minerals; efflorescence is caused by soluble sulfate salts. Finer crushing and optimized firing workflow can mitigate these issues.
Q5: What kiln technology suits full coal‑gangue fired brick production?
A: One‑time stacking‑firing process with smoke‑heat separation: independent tunnel drying chamber and tunnel kiln.
Q6: What causes black‑core defects in coal gangue fired bricks?
A: Under‑firing incomplete combustion or oxygen shortage inside green bodies due to fast heating rate. Slow heating and prolonged low‑temperature firing are effective countermeasures.
