Operation Of Tube Mills: Complete Technical Guide

The operation of tube mills is the classical heart of the cement grinding: the slowly rotating steel drum, the body of grinding balls, the clinker fed at one end and the fine powder leaving the other: the tube mill has been grinding cement for a hundred and fifty years and remains the workhorse of the industry because its operation is transparent, robust and endlessly tunable: the operator of the tube mill commands the load, the ventilation, the speed and the media charge: the product fineness, the capacity and the mill’s energy bill answer his commands: this guide walks the complete machine: the shell, the media, the drives, the control and the daily routines.

The Complete Cement Technical Package (931 files including the courses, the books, the Excel tools and the operating manuals: $249.99 one-time: instant download via the PayPal payment) includes the full Operation of Tube Mills file with its charging tables, the control loops, the maintenance plans and the troubleshooting chart: the mill operators learn the shifts, the designers the dimensions: the honest framework of the package: the tube mill knowledge, measured.

This article follows the order of the file: the machine anatomy first, the grinding media second, the operating parameters third, the daily routine and the maintenance last: each section with the lists, the tables and the numbers of the mill floor: the reader can work beside the mill panel: the intention of the document.

1. The Tube Mill and Its Place in the Grinding Circuit

The tube mill is a horizontal rotating cylinder, 3 to 6 meters in diameter and 8 to 20 meters long, filled to about 28 to 36 percent of its volume with the grinding media: the feed enters at one trunnion, the drum rotates at the critical speed of roughly 70 to 80 percent, the balls are lifted and cascade, the material is crushed and ground between the balls and the liner: the fine material leaves the other trunnion with the air draft: the mill operates in the open circuit (once through) or in the closed circuit with the separator (the ground discharge classified, the oversize returned): the closed circuit with the third generation separator is the modern standard of the finish grinding:

• The open circuit: the mill discharge is the product: the fineness controlled by the feed rate and the mill internals: the simple circuit of the small plants;
• The closed circuit: the discharge goes to the separator, the coarse returns: the flexible and the efficient: the general solution of the large mills;
• The multi-compartment design: the two and three compartment mills with the different sizes of the media in sequence: the coarse grinding, the fine grinding and the finishing sections;
• The airswept and the drying grinding: the high-volume air through the mill dries the wet raw material in the same drum: the raw mills of the plants, the coal mills;
• The central drive and the girth gear: the motor powers the mill through the girth gear on the shell or through the central drive: the differences of the maintenance and the speed control;
• The final grinding: 55 to 70 percent of the plant’s finish grinding energy: the class vertical, the horsepower of the plant: the reduction of that energy is the biggest single saving of the plant.

The tube mill works on the simple laws: the comminution by the crushing, the attrition and the impact of the falling balls: the operating energy 30 to 45 kWh/t for the clinker, versus 25 to 35 for the vertical mill: the tube mill reigns through the simplicity, the flexibility and the robustness of its mechanics: the plants that keep the tube mills well-run and the class separates keep the cost of the finished ton at the level the vertical machines need: the file teaches the operation and the boundary values of the machine.

2. The Mill Shell and Its Anatomy: Compartments, Diaphragms and Liners

The shell of the tube mill is not an empty drum: it is an engineered sequence of the zones: the inspection of the internals decides the operation range and the longevity of the machine: the anatomy of the typical 4-meter mill:

• The shell: the rolled steel plate 40 to 80 mm thick, with the inlet and outlet head bearings: the mill rotates on the support bearings under the heads or at the shell: the diameter governs the energy and the peripheral speed of the media;
• The liner: the wear surfaces that protect the shell and impart the motion to the media: the lifter liners (the corrugated, the wave, the step) lift the balls: the smooth liners at the fine end cascade the balls: the liners of the first compartment are the heavy lifters;
• The first compartment: the 25 to 35 percent of the length: the largest media 60 to 90 mm: the coarse crushing of the feed: the liners with the lifting action;
• The diaphragm: the wall between the compartments: the baffle with the slots 6 to 8 mm: the first diaphragm retains the coarse material in the first compartment: the mill discharge between the compartments: the materials flow control of the mill;
• The second compartment: the fine grinding with the media 15 to 60 mm: the smooth liners and the finer balls: the “finishing zone” of the cement;
• The outlet: the discharge diaphragm with the grate keeps the balls inside: the product leaves with the airborne dust and the flow: the outlet openings pattern decides the hold-up of the fine material.

The internals set the possible: the liner profile decides the ball action, the diaphragm the level of the material in the compartments: the plant that measures the shell position of the mill with the level measurements and the peeps of the running mill seeds understands the interior of the beast: the maintenance section of this guide returns to the internals, because the operation of the trough mill is the operation of a box the operator never sees directly: the discipline of the indirect views: the power, the sound, the temperature.

3. The Grinding Media: The Charge, the Size Distribution and the Top-Up

The grinding media are the tools of the mill: their size, the charge weight and the wear pattern decide the breakage of the two compartments: the complete mill charge is the product of two decisions: the initial filling and the continuous top-up: the plants control four grades of the balls in the first compartment and two or three grades in the second:

• The charge weight: the sum of the balls: measured by the mill bearing load or the motor mill: 28 to 36 percent of the shell volume: the high filling raises the power and the production but floods the impact:
• The ball size program: from the consumption of the feed: the largest ball = the cube root of the largest feed particle: the simplified rule: D = 28 to 33 times the root of the largest feed size in millimeters:
• The top-size decision: the first compartment 80 to 90 mm for the clinker mill: the second 15 to 25: the new charge of the mill lifetime: the wear media inspect the size of the biggest ball:
• The top-up schedule: the daily ball top-up by the weigh: 0.6 to 1.0 kg per ton of cement: the ball consumption of the clinker grinding: the top-up maintains the charge curve:
• The sorting: the ball sorting during the liner change: the removal of the deformed and the broken balls: the sorted bins: the charge rebuild:
• The charge curve: the distribution of the remaining ball sizes: the balanced curve: the single large and the pebble clusters: the production falls: the pattern audits the two-year cycle of the charge.

Typical media profile of a 4.2 m x 13 m two-compartment cement mill:

Compartment	Length m	Ball sizes mm	Charge t	Filling %
First (crushing)	3.0 to 3.5	60 and 70 + 80 to 90 mix	45 to 60	30 to 33
Second (fine)	7.5 to 8.0	15 to 30 + 40 mix	130 to 160	30 to 34
Full charge	—	—	180 to 220	30 to 33

The filling degree is measured at the overhauls by the length chords of the ball surface: in the running, the operator follows the amperage of the main motor: the mill power is proportional to the charge weight: a falling motor current at the constant load means the balls are wearing away or the charge is washing out: the file provides the charge curve forms and the calibration of the motor against the ball level for the monthly audit: the media: the muscle mass of the mill: the operation must keep it whole.

4. The Drive and the Speed: The Speed of the Drum and the Drive Systems

The correct rotational speed of the mill is the percent of the critical speed: the critical speed is the speed at which the balls stick to the shell by the centrifugal force: 70 to 75 percent of the critical is the classic operating speed, the range where the balls cascade and fall through the feed: the drive systems transmit the mechanical power:

• The critical speed: Nc = 42.3 / sqrt(D) rpm (D in meters): the mill radius determines: the typical 4.2 m mill critical ≈ 20.6 rpm: the operating 70-75%: 14.4 to 15.5 rpm:
• The girth gear drive: the pinion on the reducer drives the cast gear wheel on the shell: the majority of the mills: the wheel and pinion lubrication: the alignment: the drive, the simplest:
• The central drive: the motor, the gear reducer in the axial line: the shell driven through the girth flange: no rim gear: the stability of the mill, the heavy reducer under the mill: the maintenance;
• The ring-gear and the pinion: the standard arrangement with the lubrication of oil: the girth gear wheel 4.0 to 8.0 m pitch diameter: the pinion: the backlash 1 to 1.5 mm:
• The mill motor: the synchronous three-phase, the moderate speed: the direct-on-line start with the fluid clutch or the slip-ring rotor: the control varis: the typical installed power: 2500 to 7000 kW per mill of 3000 to 7000 t/d plant.

The speed is not a daily trim: the 3-4% change of the speed is a modification of the mill: the fixed drive: the mill designer sets the operating speed at the critical percentage with the internal liner profile: the operator area: the power draw, the load: the speed of the future variable-speed drives allows the lining the speed of the load: the file includes the design checklists and the pinion alignment rules: the drive group is the first mechanical chapter of any mill’s overhaul story.

5. The Operating Parameters: The Power, the Sound and the Level

The operator of the tube mill reads the machine through the indirect instruments: the main motor current, the sound of the mill, the outlet temperature, the separator load and the feed weigh: the correct interpretation of the five signals is the craft of the mill operator, and the file organizes the craft:

• The main motor current (the load): the mirror of the charge weight and the material level: the rising current at the rising feed: the falling current at the empty mill: the overload protects the drive;
• The mill sound: the ear on the inlet head: the full mill thuds, the empty mill rings: the older operators tune the feed by the ear: the modern plants automate the sound spectrum analysis: the acoustic sensor of the mill load;
• The outlet temperature: the 80 to 120 °C typical for the finish mill: above 125 the cement quality suffers and the gypsum dehydrates: the cooling water on the mill body and the ventilation control the heat;
• The differential of the separator: the inlet separator pressure: the load of the classifier: the coarse return flow: the loop tuning variables;
• The feed weighing: the belt scales and the weigh feeders of the clinker, the gypsum and the additives: the total feed rate and the ratio of the mix: the hourly record;
• The mill ventilation volume: the air flow through the mill at the inlet and the outlet fans: 0.8 to 1.2 m/s velocity through the mill cross section: the carry of the fine particles and the cooling.

The operating parameter table of a typical 4.2 x 13 m finish mill:

Parameter	Typical value	Limit / alarm	Correction
Feed rate	100 to 140 t/h	max by the drive	trim the load set point
Main motor current	70 to 85% of the rating	90% sustained	reduce the feed, check the charge
Outlet temperature	95 to 115 °C	125 °C	water spray, more ventilation
Mill ventilation	0.8 to 1.0 m/s shell velocity	—	dampers of the inlet and outlet fans
Blaine product	3600 to 4200 cm2/g	per the spec	separator speed, feed rate
Ball charge	30 to 34% filling	25% minimum	top-up the daily weigh

The parameters are the vocabulary of the mill: the file includes the trend charts of a shift, showing how the operator balances the feed against the current and the temperature: the “three knobs” of the classic operation: the feed rate, the ventilation, the separator speed: the discipline is to change one knob at a time and to wait for the settling: the tube mill is slow, the results arrive in 20 to 40 minutes, and the fast fingers destabilize the loop: the file preaches the patience of the operator.

6. The Ventilation of the Mill: The Air, the Cooling and the Dew Point

The air through the tube mill serves three masters: it carries the fine particles out of the drum, it cools the grinding zone and the cement, and it removes the water vapor from the feed and the mill atmosphere: the ventilation volume is a real operating variable: too little air, the mill chokes with the fines and the temperature rises; too much air, the coarse particles are lifted and the separator overloads and the dust collection load grows:

• The velocity: the air velocity through the mill shell 0.5 to 1.2 m/s: the fine grinding the higher velocities, the raw drying the highest: the cross-section area times the velocity equals the volume:
• The dew point control: the mill exhaust above the dew point at the baghouse: 60 to 70 °C minimum: below the acid dew point the bags condense and the cement dust cakes: the winter operation of the mills;
• The water injection: the internal or external water spray in the second compartment at 0.5 to 1.5% of the feed: the evaporative cooling holds the 95 to 115 °C target: the spray nozzles and the air purge to keep them clear:
• The material temperature: the hot clinker from the silo at 60 to 100 °C: the gypsum dehydration above the 120: the ventilation and the water injection protect the set of the cement:
• The baghouse balance: the ventilation air exits through the separator and the filter: the pressure balance of the mill inlet (slightly negative) and the filter delta: the false air of the seals and the flap:

The mill ventilation is the breathing of the tube: the file includes the ventilation calculation for a given mill: the required volume from the material rate, the temperature and the moisture: and the measurement of the actual flow with the anemometer at the duct: the gap between the calculated and the measured is the leakage hunt: the ventilation savings of the sealed systems are a documented case in the file’s optimization chapter: the breath of the mill, disciplined.

7. The Control Loops of the Mill: The Ear, the Level and the Feed

The modern tube mill control is a pyramid: at the base the mechanical protections, in the middle the load loops, at the top the quality loop:

• The protections: the motor temperature, the bearing temperature, the lubrication pressure, the mill current limit: the trips and the interlocks: the first line of the safety;
• The mill load loop: the feed rate adjusted so the mill power (the fill level) holds the set point: the classical cascade: the sound-based fill control or the power-based: the response slow, the stability the goal;
• The quality loop: the fineness of the product at the separator: the separator speed and the feed trim: the closed loop with the on-line fineness instrument: the slow outer loop;
• The mill sound analysis: the frequency analysis of the shell noise: the fill level estimate: the modern “electronic ear” as the standard of the load control on the old mills: the retrofit cost vs the manual trim;
• The expert systems: the rule-based controllers that mimic the best operator: the fuzzy loops and the model-based optimization: the mature plants run the expert control with the 1 to 3 percent capacity gain.

The file presents the control block diagrams with the tuning parameters of the PID loops: the integral time of the load loop (20 to 60 minutes) and the derivative constraints: the tuning of the mill loops is a chapter of its own because the loop that is tuned too aggressively oscillates the whole line: the plants document the loop tuning and re-verify it at every campaign: the automation of the tube mill is the best student of the operator: it learns the manual discipline and then replaces it: the control loop, the invisible operator.

8. The Start and Stop Sequences: The Complete Operating Procedure

The tube mill is a heavy inertial machine: the start and the stop are the moments of the highest risk: the procedures of the file follow the mill manual and the plant practice:

• The preparation: the lockout of the electrical and the mechanical services, the inspection of the trunnion bearings, the lubrication levels, the cooling water, the auxiliary drives: the checklist of the start:
• The auxiliaries first: the mill fan and the separator, the baghouse, the lubrication pumps: the mill only after the auxiliaries reach the running state: the interlock sequence of the DCS:
• The start: the mill motor with the empty-to-partially-filled charge: the feed starts after the mill reaches the speed and the current settles: the slow ramp of the feed to the set point over 30 to 60 minutes:
• The normal stop: the feed stops first, the mill runs 10 to 20 minutes to empty the chamber, the separator purges, then the mill motor stops, then the auxiliaries:
• The emergency stop: the motor trip, the bearing temperature, the fire: the immediate actions per the procedure: the restart rules after the emergency: the mill must not restart with the material in the boot:
• The blocked mill: the material overload after a trip: the unblocking procedure: the slow rotation by the auxiliary drive (the turning gear), the removal of the material at the inlet: the file’s emergency chapter.

The start and the stop are the moments when the mill temperature and the lubrication decide the survival: the file includes the start-stop checklists printed for the control room, and the log of the starts and stops with the dates: the operators sign the checklists: the discipline of the procedures is the reliability of the plant: the heavy drum, handled by the sequence, not the heroics.

9. The Daily Operation: The Shift Routines and the Hourly Records

The shift is the basic unit of the mill life, and the file codifies the routine of the shift: the operator comes to the panel, reads the overnight trends, and steps through the checks:

• The handover reading: the previous shift log: the feed rate, the fineness, the events: the pending problems: the state of the mill before the takeover;
• The hourly checks: the current, the temperatures, the sound, the product fineness and the rejects: the entries in the shift log: the deviations from the target trended;
• The laboratory interface: the product samples every few hours: the mortar dust and the fineness: the cement strength samples weekly or per the plant plan: the lab report against the production record;
• The media top-up: the daily weigh of the balls charged to the mill: the standard consumption rate: the accumulated weigh vs the predicted: the audit of the charge;
• The housekeeping: the leakages, the spillage, the dust deposits at the mill floor: the fire risks and the drives observed: the lubricants at the right level;
• The events: the trips, the alarms, the interventions: every event logged with the time and the action: the event log of the file is the memory of the plant.

The daily routine builds the mill database: the trends of the power, the fineness, the temperature and the top-up become the diagnosis tool of the month: the measured plant compares the performance of the shifts and the campaigns: the routine is the discipline of the many small actions, the sum of which is the reliable plant: the file closes the chapter with the shift log form, printable, and the instructions for the comparative analysis of the shifts: the data of the daily routine is the intelligence of the mill.

10. The Optimization Campaign: The Higher Capacity and the Lower Energy

Once per year, or when the market demands, the plant runs the mill optimization campaign: the systematic test series that move the mill to the best operating point: the campaign follows the methods of the file:

• The mill test matrix: the series of the feed rates and the fineness targets: the capacity, the fineness and the specific energy measured at each point: the operating envelope of the mill drawn;
• The media rebalancing: the sorting and the top-up with the new charge curve: the tests before and after the media change: the media change typically gains 3 to 8 percent of the capacity;
• The separator tuning: the speed and the vane sweeps: the Tromp curves at each setting: the sharpest curve selected: the separator retrofit considered at the same campaign;
• The water and the ventilation: the water spray and the ventilation volumes optimized for the specific product: the temperature control at the minimum energy;
• The liner review: the wear measurements and the profile check of the liners: the profile change recommendations for the next lining campaign: the lift and the cascade behavior;
• The control loop retune: the response times and the loop gains re-verified: the expert system thresholds updated with the new baseline: the automation taught the new mill.

The results of the optimization campaign in the industry: 3 to 10 percent capacity gains and 3 to 8 kWh/t energy savings from the classification and the media: the payback of the campaign in weeks: the file includes the campaign template with the test matrix, the measurement plan and the report format: the optimization is not a miracle: it is the disciplined application of the parameters: the campaign: the review of the mill’s operating full potential: the file’s method: measure, change, measure again.

11. The Maintenance of the Mill: The Lining Exchange and the Structure Care

The mill maintenance is the train of the cyclic overhauls: the planned maintenance of the tube mill follows the hours or the months, and its heart is the liner exchange with the media sorting:

• The lining exchange: the complete liner replacement at the end of the wear life (5000 to 12000 operating hours at the mill, depending on the lining material): the crane and the mill closure: the new liners with the lifter profile: the bolt tightening to the torque:
• The media sorting: the sorting of the ball sizes during the outage with the clinker screens: the damaged balls to scrap: the cleaned charge rebuilt per the charge curve:
• The trunnion and the bearings: the white metal (babitt) bearing inspection: the level of the bearing, the wedge and the oil condition: the bearing replacement per the manufacturer cycles:
• The drive group: the pinion and the girth gear teeth inspection: the backlash and the alignment were recorded: the grease and the oil analysis: the spares of the large gears:
• The shell cracks: the inspection of the shell welds and the head to shell joints: the ultrasonic thickness sampling: the crack repair plan at the next outage:
• The instrumentation: the sensors, the interlocks and the controls verified after the reassembly: the functional tests of the protection chain before the restart.

The outage planning is the heart of the maintenance: the critical path of the liner exchange, the manpower and the spares: the file includes the outage plan template of the 4.2 m mill with the timeline and the tool lists: the tube mill maintenance is 30% work and 70% preparation: the plant that prepares the spares, the tools and the procedures before the first bolt release restarts the mill on schedule: the maintenance of the tube mill: the discipline of the shutdown.

12. The Troubleshooting of the Tube Mill: The Matrix of the Common Faults

The operating problems recur with the familiar names and the file asserts the classic matrix: the symptom: the cause: the correction:

Symptom	Probable cause	Correction
Mill current high, sound muffled	overfeed, slurry, media overload	reduce the feed, purge, check the charge
Mill current falling, sound ringing	charge wash out or feed shortage	top-up, raise the feed, check the weigh
Outlet temperature rising	low ventilation, hot feed, water failure	raise the draught, repair the water spray
Coarse product despite the fineness	diaphragm flow, media worn, separator bypass	inspect the diaphragm, sort the balls, check the separator
Plate overheating at the gear	misalignment, lubrication loss	check the backlash, oil pressure, alignment
Mill vibration at the foundation	foundation loose, liner breaking, charge shift	foundation check, bolt torque, charge audit

The matrix is the beginning, and the file’s troubleshooting tree, with the photographs, the flow charts and the documented cases, is the end: the operator who recognizes the first symptom column and follows the procedures avoids the escalation: the mill’s fault is solved by the procedure of the file: the deviation, the diagnosis, the action, the follow-up: the complete control cycle of the failures: the same matrix returns the mill to the optimum.

13. The Frequently Asked Questions

How many balls do I add daily?

The ball wear of the clinker grinding is 0.5 to 1.2 kg per ton of cement: the plant computes its own rate from the average daily consumption and the weigh records: the daily top-up = the tonnage x the rate per ton: the rate is verified at the sorting: the charge curve held means the stable operation: the top-up is the cheapest insurance of the mill performance.

Why does my mill sound differently on the night shift?

The night shifts often run the different feeds: the cooler clinker, the moist additive, the different blend: the sound follows the material: the operators with the electronic ear and the history of the machine decide: the deviations logged against the normal ranges: the file’s appendix contains the sound spectra of the normal loads: the “different sound” logged and the cause found, not just the sound ignored.

The cement is hot at 130 degrees: what now?

The first action: the water spray and the ventilation raised: the second: the feed structure checked (the clinker temperature, the gypsum addition at the mill): the gypsum dehydrated past 120 loses the set control: the third: the fineness possibly too high: the separator trimmed: the hot cement is the warning of the quality breakdown, never accept it.

How long does the lining last?

With the chrome-molybdenum alloy lining 60,000 to 120,000 hours on the clinker mills: the stepped liners at the first compartment less, at the second compartment more: the rubber liners shorter but lighter and quieter: the wear measurement by the spot gauges during the outage: the replacement at 60% remaining: the lining the biggest consumable of the mill, planned ahead.

The mill trips on the high current during the start: why?

The charge is heavy or the mill started with the material in the drum: the correct start empties the chamber first (the running down), then the fresh feed: the check also the charge level: the heavy charge assists the start: the auxiliary drive: the soft start of the motor: the file’s start sequence: the correct answer to the current trip.

14. Conclusion

The operation of the tube mills is the foundational craft of the cement grinding: the charge, the ventilation, the temperature and the load: the four ends of the operator’s control: the mill there, a century of experience wrapped in the steel and the balls: the plant that runs its tube mills instrumented, measured and maintained gets the reliable, flexible grinding the industry’s flexibility: the tube mill remained the workhorse because the discipline of its operation is the discipline of the plant.

The Complete Cement Technical Package includes the Operation of Tube Mills guide with the charge tables, the control logics, the maintenance plans and the start lists: the one-time 249.99: the instant download: the library of the cement plant: the ball mill of the knowledge: the operation of the drums: the cement of the careers: the complete package of the mill.

This file is part of the Complete Cement Technical Package (931 files) available from cementequipment.org. Respective rights holders; library copy for the licensed single user.