Pelletising kiln

A pelletising kiln is the high-temperature induration section of an iron-ore pellet plant. It converts moist green pellets made from concentrate, binder, flux, and water into hardened pellets that can survive handling, transport, and blast furnace or direct reduction duty.

Two common arrangements are travelling grate plants and grate-kiln plants. In a grate-kiln system, pellets dry and preheat on a grate, are fired in a rotary kiln, and are then cooled. In a straight grate system, drying, preheating, firing, and cooling occur along the same moving grate. Both designs depend on controlled gas flow through a deep bed of pellets.

Operating mechanisms

Induration drives moisture removal, oxidation of magnetite where present, recrystallisation, and bonding between iron oxide grains. Temperature must be high enough to create strength but not so high that pellets soften, fuse, or form excessive rings. Fuel, burner design, bed depth, pellet chemistry, and airflow all influence temperature uniformity.

Dust sources include green pellet degradation, fired pellet abrasion, burner ash, recirculated fines, and carryover from windboxes or hood areas. Alkali, chloride, and sulphur compounds can form sticky deposits in cooler off-gas sections. If dust returns to the process unevenly, it can change pellet chemistry and increase deposit tendency.

Failure modes

Common operating problems include kiln rings, grate blinding, windbox build-up, fan erosion, duct plugging, ESP or baghouse hopper hang-up, and unstable pressure balance. Poor pellet size distribution increases bed resistance and fines carryover. Over-firing damages pellet quality, while under-firing produces weak pellets that generate more downstream dust.

Cleaning and control context

Pelletising off-gas systems often use cyclones, ESPs, baghouses, scrubbers, or combinations depending on plant age and permit basis. Cleaning equipment must handle abrasive iron oxide dust and sometimes sticky salt-rich deposits. Sonic horns are relevant on ESP inlet zones, collecting surfaces, baghouse hoppers, ducts, and bins where frequent deposit control prevents heavy manual cleaning. They are not a substitute for correcting pellet degradation or gas maldistribution, but they can stabilise collection equipment between outages.

Operating and inspection context

Pelletising kilns sit in a tightly coupled process with the grate, annular cooler, burners, windboxes, fans, gas cleaning equipment, and product screening. Operators track bed depth, green-ball quality, burner heat release, draught, off-gas temperature, recycle dust, and pellet tumble strength because each variable changes the amount and character of entrained dust. A small upset in feed moisture or binder addition can show up later as coating growth, higher fan load, or unstable emissions.

Maintenance inspections look for tyre and roller condition, refractory loss, shell hot spots, seal leakage, burner tile damage, discharge-end dust carryover, and build-up in the ductwork to cyclones or ESPs. Acoustic cleaning is best treated as a gas-path housekeeping tool around dry deposits, not as kiln-shell or refractory repair. If deposits are plastic, fused, or part of a process coating, sound energy will have limited effect until the chemistry or temperature window is corrected.

Related terms

• Electrostatic precipitator
• Baghouse
• Rotary kiln
• Sonic horn