Quick answer
When raw‑mill rejects rise but the finished product’s Blaine fineness stays within target, the most common cause is a shift in the raw‑material size distribution or moisture that overloads the separator, allowing more coarse particles to be rejected while the downstream grinding circuit still produces the same fineness. Verify raw‑mill feed size, moisture, and separator settings; adjust feed classifiers, check wear on the separator deck, and confirm that the raw‑mill feed to the kiln is within the design specification. If the raw‑mill feed is correct, examine the kiln feed system for blockage or uneven distribution that could be forcing the separator to reject more material.
Problem / Job
The plant reports a steady increase in raw‑mill reject (RM‑R) percentages over several weeks, while the final cement’s Blaine fineness remains on target. The objective is to identify the root cause, restore the reject rate to its normal range, and ensure the raw‑mill feed to the kiln meets design specifications without compromising product quality.
Evidence
- RM‑R has risen from the typical 5‑7 % to 12‑15 %.
- Product fineness (Blaine) unchanged at 3 500 m²/kg (or plant‑specific target).
- Raw‑mill feed moisture content stable within the last month.
- Separator deck wear observed on visual inspection.
- Feed size distribution from the raw‑mill shows a shift toward larger particles.
- No recent changes to raw‑material sourcing or kiln feed rates.
Decision
Proceed with a systematic diagnostic workflow that starts at the raw‑mill feed, checks separator performance, validates kiln feed distribution, and ends with verification of the grinding circuit. Adjustments are made only after confirming the OEM‑specified limits for each component.
7‑step diagnostic workflow
- Secure the area and isolate equipment. Before any inspection, lock‑out/tag‑out the raw‑mill motor, separator drive, and associated conveyors. Verify zero voltage and confirm that all moving parts are immobilized. Use standard LOTO procedures and post isolation signs.
- Collect baseline data. Retrieve the last 30 days of RM‑R percentages, kiln feed rates, raw‑mill feed size distribution, and moisture records. Compare these trends with the plant’s historical baseline to quantify the deviation.
- Inspect raw‑mill feed size and moisture. Sample the raw‑mill inlet and outlet streams. Perform sieve analysis (e.g., 0.5 mm, 1 mm, 2 mm cuts) and moisture determination. Verify that the feed size distribution matches the OEM‑specified range for the mill and that moisture is within the design limit. If values differ, adjust raw‑material pre‑crushing or drying circuits.
- Examine the separator deck and airflow. With the equipment still isolated, visually inspect the separator for wear, fouling, or blockage of the deck plates. Measure the separator airflow using a calibrated anemometer and compare with the OEM‑recommended setting. Adjust the airflow regulator or replace worn deck plates as required.
- Validate the raw‑mill to kiln feed system. Check the feed chute, rotary valve, and distribution spreader for buildup, uneven wear, or mis‑alignment that could cause localized over‑feeding. Perform a flow‑rate check on the feed conveyor and confirm that the feed rate is within the design envelope.
- Review grinding circuit performance. Although the Blaine fineness is stable, verify that the ball‑mill or vertical roller mill is operating at the correct speed, load, and classifier settings. Confirm that the classifier’s cut‑size is unchanged and that the mill’s power draw is within normal limits.
- Implement corrective actions and monitor. After adjustments, restart the plant following standard start‑up procedures. Record RM‑R, raw‑mill feed size, and separator airflow for at least three consecutive shifts. If the reject rate returns to the baseline range, document the change; otherwise, repeat the workflow focusing on the next most likely cause.
Diagnostic matrix
| Evidence | Likely direction of the problem | Next check / action |
|---|---|---|
| Shift toward coarser particles in raw‑mill outlet sieve analysis | Raw‑mill feed size too large → separator rejects more material | Verify raw‑material pre‑crushing settings; adjust feed crusher set‑point |
| Separator deck wear or fouling observed | Reduced separation efficiency → higher reject rate | Inspect deck plates, replace worn sections, clean fouling, re‑calibrate airflow |
| Airflow measurement below OEM‑specified value | Insufficient lift on fine particles → coarse particles carried to reject stream | Adjust fan speed or clean inlet filters; re‑measure |
| Uneven raw‑mill feed distribution (e.g., localized over‑feeding) | Localized overload on separator → increased rejects | Check rotary valve, distribution spreader; clear blockages; realign chute |
| Grinding circuit operating at unchanged parameters | Unlikely primary cause (finest unchanged) | Document status; focus on upstream raw‑mill and separator |
| Moisture content stable but near upper limit | Potential for agglomeration affecting separation | Confirm moisture is within OEM limit; consider slight drying if near limit |
Acceptance / verification criteria
- Raw‑mill reject percentage returns to the plant’s historical range (typically 5‑7 %).
- Sieve analysis of raw‑mill outlet shows particle size distribution within the OEM‑specified limits for each cut size.
- Separator airflow measured at the inlet matches the OEM‑recommended value (verify with calibrated instrument).
- No abnormal vibration or temperature rise on the separator motor during a full‑load run (monitor for at least 2 hours).
- Product Blaine fineness remains at the target value with no increase in variability (standard deviation within normal range).
- All LOTO tags removed only after confirming that equipment is safe to restart and that corrective actions are documented.
Common mistakes and repeat‑failure traps
- Adjusting separator airflow without confirming deck condition. Changing fan speed can mask wear but will not solve the underlying inefficiency; the problem re‑appears once the fan is returned to nominal.
- Focusing solely on the grinding circuit. Since Blaine fineness is stable, the mill is unlikely the root cause; misdirected effort wastes time and may lead to unnecessary wear on grinding media.
- Neglecting LOTO compliance. Performing visual inspections or minor adjustments while the separator is energized creates a high risk of injury and can damage equipment.
- Changing raw‑material feed rate without checking size distribution. A higher feed rate can overload the separator, but if the particle size remains within spec, the reject rate may not improve.
- Replacing separator deck plates without measuring airflow. New plates can alter pressure drop; without re‑setting airflow, the separator may become too aggressive, increasing rejects again.
- Over‑drying raw material to “solve” moisture concerns. Excessive drying can create dust explosions or increase energy consumption; always stay within OEM moisture limits.
Related Infinity guides
Build a stronger cement maintenance library
The Infinity for Cement Organization technical package contains 900+ technical files. It is a one-time $249.99 purchase, with instant download immediately after payment and lifetime updates forever at no additional cost.