High free lime in clinker — Causes, Diagnosis & Operating Targets

Free lime above the operating window is one of the few symptoms that shows up in the lab before it shows up on the kiln board. By the time the result is back from QC, the clinker that produced it has already left the cooler, and the next decision is whether to chase the cause or absorb it. Under-burning, raw-mix LSF drift, a coating disturbance, or a homogeneity problem in the silo can each push free lime up. Each has a different time-to-recovery, so the diagnosis is partly about what is wrong now and partly about how fast the next sample can confirm the fix worked.

Common Causes

1. Under-burning from low burning zone temperature

If the burning zone is colder than the raw-mix burnability requires, calcination completes but combination of CaO into C₃S does not. Free lime rises with no obvious feed change, and pyrometer trends usually confirm the cause.

2. High LSF in raw mix

Above the burnability threshold for the current operating point, the clinker simply needs more energy than the kiln is delivering. The lab sees free lime climbing while the kiln board looks normal.

3. Insufficient residence time in burning zone

Kiln speed too high relative to feed rate shortens residence in the burning zone. Combination reactions are time-dependent; cutting time below what burnability requires is enough to push free lime up.

4. Coating disruption causing temperature drop

A piece of coating breaking off carries stored heat into the cooler and leaves a colder local zone behind it. Free lime rises within one or two retention cycles of the disturbance.

5. Poor raw mix homogeneity

High LSF standard deviation at the kiln inlet means some moments are easy to burn and some are not. Free lime tracks the worst pockets, not the average.

6. Excessive kiln feed rate

Pushing feed beyond what the burning zone can handle overloads it. Free lime rises and stays high until feed is cut and the kiln catches up.

How to Diagnose

1. 01
   Confirm the lab result against a duplicate sample and the calibration of the free-lime apparatus — instrumentation drift is a real cause.
2. 02
   Pull recent LSF data: average and standard deviation. A drift in either points at the raw-mix side rather than the kiln side.
3. 03
   Check burning zone temperature trend on the radiation pyrometer and compare against the historical operating point.
4. 04
   Review kiln speed and feed rate; calculate effective residence time and compare against the design value for current burnability.
5. 05
   Inspect the burning zone for visible coating disturbance — a recent fall is often visible from the camera and explains a single bad shift.
6. 06
   Increase burning zone temperature in 20–40°C steps while reducing feed by 5%; allow at least one full retention cycle (typically around 30 minutes) before judging the response.

Process Impact

High free lime is a quality risk that propagates straight to cement performance. Above target, soundness fails the Le Chatelier limit, 28-day strength softens, and the cement mill has to be pushed harder on Blaine to recover early-age strength. The plant ends up running both the kiln and the cement mill less efficiently to manage a problem that started in the burning zone. Each shift of high free lime also costs in repeat sampling, hold-and-test silos, and the schedule pressure of clinker that may need to be blended away. Sustained free lime drift also tells you that the operating window has narrowed — usually because raw-mix homogeneity, coating, or burnability has changed in ways the control room has not yet caught up with.

Operating Targets