Lime kiln

A lime kiln in a kraft pulp mill calcines washed lime mud, mainly calcium carbonate, back to burnt lime, mainly calcium oxide. The burnt lime is then slaked and reused in the recausticising cycle to convert green liquor into white liquor for pulping.

The kiln is a long, inclined, rotating cylinder. It resembles a cement rotary kiln, but its process duty, feed moisture, dust chemistry and integration with the pulp-mill chemical cycle are different. Lime kilns usually operate with hot combustion gases flowing counter-current to the lime mud, drying, heating and calcining it before discharge.

Process zones

• Feed end and drying zone - wet lime mud enters and water evaporates.
• Chain section or preheater - chains or heat-transfer internals lift and shower mud through hot gas to improve drying.
• Calcining zone - calcium carbonate decomposes into calcium oxide and carbon dioxide.
• Burning end - fuel combustion supplies the heat and the product exits as reburned lime.
• Gas-cleaning train - entrained lime dust is captured in an ESP, scrubber or other collector.

Fouling and operating issues

Lime kilns foul through mud rings, chain-section build-up, dust carryover, dam rings, refractory coating instability, ESP hopper bridging and duct deposition. Wet feed, poor mud filtration, high sodium, high sulphur, fuel changes and unstable draft can all intensify build-up.

A fouled chain section reduces heat transfer and narrows the gas path. That lowers kiln capacity, raises fuel use, increases differential pressure and may push more dust into the downstream ESP. Fine lime dust in the ESP hopper is cohesive and can bridge even when the upstream kiln is operating well.

Measurement and maintenance

Operators monitor mud solids, residual carbonate, lime availability, kiln draft, fuel rate, chain temperature, shell temperature, ESP differential pressure, stack opacity and dust return. Maintenance includes chain inspection, refractory repair, ring removal, burner tuning, seal repair and hopper cleanout.

Sonic-horn applications

Sonic horns are used where dry or weakly bonded lime dust can be kept moving:

• chain-section or preheater areas where early mud-ring formation begins;
• lime-kiln ESP inlet zones, plates and hoppers;
• ducts and transitions where fine lime settles;
• storage and return hoppers handling dry lime dust.

Acoustic cleaning is preventive. It can reduce the rate at which deposits consolidate, but it cannot fix poor mud washing, severe ring chemistry, wet sludge, refractory damage or an undersized gas-cleaning system.

Process and maintenance detail

Lime-kiln operation is tied to the whole recausticising loop. Mud dryness, residual sodium, sulphur input, fuel ash, burner stability, draft and chain-section condition all influence ring formation and dust carryover. A kiln that receives wet or poorly washed mud consumes more heat and can generate unstable flame conditions, which then affect refractory life and particulate loading into the gas-cleaning system.

Operators track mud feed rate, mud solids, kiln torque, shell temperature, draft, oxygen, fuel rate, lime quality, residual carbonate and ESP or scrubber performance. Maintenance checks include refractory wear, chain condition, tyre and roller alignment, burner position, seals, hood pressure and dust discharge from hoppers. Ring growth, dust build-up and poor gas distribution can force reduced throughput even when the rotating shell is mechanically sound.

Acoustic cleaning is usually applied around the gas-cleaning and dust-handling side rather than inside the wet mud system. It can help keep dry lime dust moving in ESP hoppers, ducts and return systems, reducing manual cleanout. It cannot correct poor mud washing, inadequate kiln heat input or refractory failure. The useful role is preserving clean gas paths and reliable dust discharge so process control changes have a chance to work.

Related terms

• Recausticising
• Recovery boiler
• Rotary kiln
• Sonic horn