How to Manufacture Premium Grade Fired Bricks from Inferior Coal Gangue？
Recycling inferior coal gangue for fired brick manufacturing is a profitable circular‑economy solution for mining areas. Nevertheless, many new‑built brick plant project suffer massive quality failures because operators skip raw‑material assessment and adopt improper equipment or firing workflows. To convert low‑grade coal gangue into qualified solid bricks, hollow bricks and hollow blocks, systematic technical controls must be implemented across raw‑material testing, internal‑combustion firing, extrusion forming, drying‑firing configuration and defect elimination.
Raw‑material characterization is the very first step before you invest in a fully‑automatic fired brick production line. Complete chemical and physical tests plus sintering thermal experiments must be carried out for coal gangue samples. Test scope covers chemical composition, heat value, plasticity index, drying sensitivity coefficient, particle gradation, hardness, drying linear shrinkage, firing linear shrinkage, sintering temperature window, post‑firing water absorption, lime bursting test and efflorescence test. Mineral composition analysis shall be performed when needed.
Only test data can validate whether coal gangue is suitable for brick‑making, determine target brick categories, and guide the selection of raw material preparation system, forming machinery and tunnel kiln. Never launch brick‑plant construction based on empirical judgment only.
Special notes for plasticity‑index testing: crush all test samples down to below 1 mm and apply liquid‑plastic limit combined tester. Particle dimension greatly changes plasticity performance. For instance, shale sample crushed <1 mm shows plasticity index 7.2; the same shale crushed below 0.5 mm reaches plasticity index 11.2. Lab‑test particle size 1 mm simulates real‑world production target 2 mm. Larger particles will obstruct the test cone and produce misleading test outcomes.
There exists no dedicated national standard for coal‑gangue calorific‑value measurement. The industry uniformly applies GB/T 213‑2003 Test Method for Calorific Value of Coal. Do not use empirical industrial calculation formulas, which are not fit for coal gangue feedstock.
Coal gangue fired bricks run on internal‑combustion firing technology. Calorific value of coal gangue largely defines the whole production scheme. When calorific‑value hits approximately 450 × 4.17 kJ/kg, pure coal‑gangue raw material can support full internal‑combustion brick‑making without extra fuel additions. The maximum allowable calorific‑value for 100 % coal‑gangue feedstock is 600 × 4.17 kJ/kg.
Under over‑internal‑combustion circumstances, you have to modify process parameters and extend firing duration. A lengthened customized tunnel kiln is required, fitted with double‑layer kiln gates at inlet and outlet. Supporting subsystems including waste‑heat recovery, flue‑gas re‑ignition, flue‑gas damper adjustment, circulating‑air circuit, rapid‑cooling combustion‑aid, external forced air supply, kiln‑bottom balanced cooling and surplus‑heat releasing shall be installed. These systems enable slow burnout of redundant combustibles inside green brick green body, delivering end‑products with compressive strength above MU20.
If coal gangue calorific‑value is insufficient, mix appropriate raw coal or middling coal. Adding fuel outside kiln is discouraged due to higher particulate emissions. If heat value is excessively high, blend non‑combustible brick‑making materials such as shale, clay or burnt coal gangue. Without available blending materials, adopt screening, air separation, flotation or low‑temperature decarbonization treatment, and tune comprehensive calorific‑value close to 450 × 4.17 kJ/kg.
Most inferior coal gangue appears as massive rock with low plasticity index (except montmorillonite‑rich ore), creating obstacles for wet plastic extrusion forming. Five proven technical solutions can resolve low‑plasticity challenges:
- Optimize crushing fineness: crush feedstock below 2 mm for standard solid & hollow bricks; crush below 1 mm for thin‑wall high‑void‑ratio products. Re‑crush over‑size particles to 0.5 mm and remix them to optimize particle distribution and improve overall plasticity.
- Adopt heavy‑duty hard‑extrusion vacuum extruder. Modern brick‑making vacuum extruder offers extrusion pressure over 4.0 MPa, forming moisture as low as 13 %. It can produce sound green bodies even for raw materials whose plasticity index drops to 5.
- Incorporate high‑plasticity additives like silt, shale or clay. Ensure precise metering and homogeneous blending. For materials with big density gaps, deploy wheel roller mill and disc kneader for intensive mixing besides conventional mixers.
- Equip steam‑heating system for mud mixer before vacuum extrusion. Steam can be supplied by waste‑heat boiler connected to tunnel kiln. Steam‑treated mud gains better plasticity, reduces extruder load and shortens green‑body drying cycle.
- Implement dual vacuum mud refining. Mud receives primary vacuum kneading in vacuum pug mill before entering main brick extruder. Multi‑stage degassing enhances mud plasticity, increases green‑body density and improves wet‑brick mechanical strength and surface quality.
One‑time stacking‑firing requires multi‑layer stacking of green brick green body on kiln cars, which tends to produce black spot pressing marks on contact surfaces. Apart from low‑temperature slow‑firing process, spread a thin sand layer atop each wet brick to boost inter‑layer air circulation. Alternatively change stacking pattern from upright‑setting to flat‑laying, so pressing marks appear on cutting surfaces and will not affect visible brick faces.
Inferior coal gangue contains harmful mineral components that trigger lime bursting and efflorescence.
- Lime bursting originates from calcite and limestone impurities. During firing they convert into quicklime CaO. After being discharged from kiln, quicklime absorbs ambient moisture and turns into slaked lime with sharp volume expansion, destroying brick structure. Crushing raw‑material particle size below 2 mm and saturating hot fresh‑out‑of‑kiln bricks with water can efficiently avoid lime bursting damage.
- Efflorescence comes from soluble salts such as magnesium sulfate and sodium sulfate generated during firing. These salts migrate with water inside bricks and precipitate white crystalline powder on brick exterior after water evaporation. Improve raw‑material fineness and prolong firing & heat‑preservation time, transforming soluble salts into water‑insoluble silicates to suppress efflorescence.
Black‑core cross‑section is typical for internal‑combustion coal‑gangue bricks. Two root causes: insufficient firing temperature & holding time leading to incomplete internal combustion; rapid temperature rise causing early surface sintering and oxygen‑deficient reduction inside bricks. Regulate heating‑up gradient and extend low‑temperature firing period, securing 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 characteristics, customized process solutions can be deployed to manufacture high‑quality fired brick products.