Cement Process Stages Best: Complete Technical Guide

The cement process stages, benchmarked against the international best practice, are the backbone of every plant improvement program: the quarry and the crushing, the preblending, the raw grinding, the clinker making, the finish grinding and the dispatch each consume their share of the electricity and the heat, and the international best practice figures define what each stage should cost in energy per tonne when the plant is operated and maintained to the world standard. The Cement Process Stages workbook of this package carries the stage-by-stage best practice benchmark sheet: the process, the international best practice intensity and the notes, from the preblending at 0.5 kWh per tonne of raw material to the quarrying at 1% of the total facility electricity, together with the flow chart of the stages and the diagram of the process. This article explains the complete picture: every stage of the cement process, its function, its equipment, its energy consumption, the international best practice figures and the actions that close the gap between the average plant and the best.

The Complete Cement Technical Package (931 files including the Cement Process Stages BEST workbook, engineering tools, books, courses and presentations, $249.99 one-time, instant download) includes the stage benchmark spreadsheet with its flow chart, its stage sheets for the raw materials preparation, the clinker making, the finish grinding and the miscellaneous energy consumption, and the international best practice figures for every process step. This article follows the workbook’s structure stage by stage: the reader ends with the full map of the cement process, the best practice numbers for each stage and the improvement actions that move the plant toward the world class, with the energy accounting explained in the detail that the plant engineer needs.

1. The Structure of the Cement Manufacturing Process

The cement manufacturing process is a continuous chain of stages that converts the raw materials of the quarry into the finished cement of the dispatch silos. The chain is conventionally divided into six groups: the quarrying and the raw material extraction, the raw material preparation including the crushing, the preblending and the raw grinding, the clinker making in the pyro system, the finish grinding of the clinker with the additives, the storage and the packing, and the auxiliary services of the plant. Each stage receives the output of the previous stage, transforms it and passes it forward, and the energy and the mass of every stage are measured in the per-tonne units that make the stages comparable.

• The quarry stage: the drilling, the blasting, the ripping, the loading and the hauling of the limestone and the clay to the crusher;
• The raw material preparation: the crushing, the preblending and the storage of the raw materials, the proportioning and the raw grinding to the raw meal;
• The pyro process: the preheating, the calcination and the sintering in the preheater, the calciner and the rotary kiln, and the clinker cooling;
• The fuel preparation: the drying and the grinding of the coal or the pet coke and the handling of the alternative fuels;
• The finish grinding: the grinding of the clinker with the gypsum and the additives to the finished cement fineness;
• The storage and the dispatch: the clinker and the cement silos, the packing plants and the bulk loading;
• The services: the compressors, the dedusting, the lighting, the water supply and the workshop loads that support the whole plant;

The stage map of the workbook’s flow chart sheet presents the chain in the order of the process, and the stage sheets carry the benchmark figures: the process in one column, the international best practice intensity in the next and the notes in the third. The structure is the same structure that the plant’s energy accounting uses, so the benchmark comparison of the workbook can be applied directly to the measured consumptions of the plant.

2. The Quarrying Stage: The Extraction of the Raw Materials

The quarrying stage delivers the limestone and the corrective materials from the deposit to the crusher. The process includes the drilling of the blast holes, the blasting of the rock, the loading with the excavators and the wheel loaders, the hauling with the trucks to the crusher, and the secondary breaking where the boulders exceed the crusher opening. The energy of the stage is the diesel of the mobile equipment and the electricity of the fixed installations: the crusher feeder, the conveyors and the water pumps.

The international best practice of the workbook records the quarrying at roughly 1% of the total electricity of the facility, a figure that places the stage’s power consumption in its true context: the quarry is a small electricity consumer because its energy is mostly the diesel of the mobile fleet, and the truck haulage dominates the cost of the stage through the fuel, the tyres and the maintenance. The best practice operation of the quarry minimizes the haul distance, selects the blast design that maximizes the fragmentation and minimizes the secondary breaking, and matches the crusher feed to its capacity so the crusher never idles.

• The blast design: the burden, the spacing, the stemming and the specific charge that produce the fragment size distribution the crusher prefers, reducing the crushing energy and the wear;
• The haul optimization: the truck fleet sized to the crusher demand, the haul roads maintained to the rolling resistance targets and the load factors tracked;
• The in-pit crushing option: the mobile or the semi-mobile crusher moved into the pit, cutting the haul distance and the fuel by 30–60% on the deep deposits;
• The waste management: the overburden and the interburden stripped to a plan that minimizes the double handling and the re-handling;

The quarry’s energy figures are tracked per tonne of extracted material and per tonne of transported material, and the best practice benchmarking compares the plant’s figures against the local norms adjusted for the deposit geometry. The quarry is the beginning of the chain, and the discipline of the stage is the beginning of the plant’s energy culture.

3. The Crushing Stage: The First Size Reduction

The crushing stage reduces the blasted rock to the crusher product of 0–75 mm (or 0–120 mm for the roller press circuits), ready for the preblending and the raw mill. The crusher types follow the material: the hammer crushers for the soft and the medium limestone, the impact crushers for the medium materials, the jaw and the gyratory crushers for the hard rock, and the roller crushers for the clay and the sticky materials. The stage’s electricity consumption runs at 0.5–1.5 kWh per tonne of crushed material depending on the crusher type, the feed size and the work index of the rock.

The best practice of the crushing stage is set by the fragment size of the blasting, the crusher operation at full load, the correct gap and the hammer or the liner management, and the reduction of the circulating oversize through the screens. The workbook’s benchmark sheet carries the 0.5 kWh per tonne figure for the preblending as the international best practice, and the same precision applies to the crushing and the conveying steps, with the notes explaining the basis of each figure.

4. The Preblending Stage: The 0.5 kWh/t Benchmark

The preblending stage homogenizes the crushed raw materials in the longitudinal or the circular prehomogenization stockpiles: the material is stacked in the layers and reclaimed across the layers, so the chemical variation of the quarry is averaged out before the proportioning. The stage’s electricity consumption of 0.5 kWh per tonne of raw material, the international best practice of the workbook, covers the stacker, the reclaimer, the conveyor drives and the auxiliary equipment, and it is a benchmark that the well-designed and well-operated preblending systems meet with margin.

• The stacker: the boom stacker or the bridge stacker that lays the material in the layers, the number of the layers determining the homogenization effect;
• The reclaimer: the bridge scraper or the side scraper that cuts the layers across the full face, averaging the composition of the stack;
• The blending efficiency: the ratio of the standard deviation of the feed to the standard deviation of the reclaim, typically 3–10 for the well-designed stockpiles;
• The bed management: the full-face reclaiming, the correct layer count and the avoidance of the segregation at the stack ends;
• The energy optimization: the reclaimer at full load, the stacker cycles coordinated with the quarry supply and the conveyors stopped when the flow stops;

The preblending stage is the first quality instrument of the plant: a stable raw mix entering the raw mill reduces the kiln feed variation, stabilizes the burning zone and the clinker quality, and reduces the fuel and the refractory costs of the pyro stage. The 0.5 kWh/t benchmark is the price of this stability, and the stage’s design and operation should never be starved to save the electricity that the downstream stages repay many times over.

5. The Raw Grinding Stage: The Mill Systems and Their Energy

The raw grinding stage dries and grinds the proportioned raw materials to the raw meal fineness of 12–20% residue on the 90 micron sieve, with the moisture of the feed evaporated by the kiln exit gas or the auxiliary hot gas generator. The stage’s electricity consumption is 15–30 kWh per tonne of raw meal: the ball mill circuits at the high end of the range, the vertical roller mills at 15–20 kWh/t, and the roller press circuits between the two, with the drying demand and the feed grindability shifting the figures.

• The vertical roller mill: the best practice system of the modern plants, drying, grinding and classifying in one machine at 15–20 kWh/t of raw meal;
• The ball mill circuit: the robust classical system at 22–30 kWh/t, with the mill internals, the ball charge and the separator efficiency determining the actual figure;
• The roller press circuit: the pre-grinding and the finish grinding combination that reduces the specific power by 15–30% against the ball mill;
• The grinding aid: the small dosage that reduces the coating and the agglomeration, saving 5–10% of the grinding energy in the humid climates;
• The separator efficiency: the third generation separators that classify at 75–85% efficiency, reducing the over-grinding and the circulating load;

The best practice of the raw grinding is the balance of the drying, the grinding and the classifying: the mill runs at the optimum bed, the grinding pressure or the ball charge matches the feed size, the separator speed holds the fineness target and the false air is minimized. The stage’s energy is the largest variable power consumer after the finish grinding, and the benchmark comparison of the workbook against the plant’s measured kWh/t is the first screening of the raw mill improvement program.

6. The Homogenization and the Kiln Feed Stage

The raw meal from the raw mill is stored and homogenized in the raw meal silos before it enters the preheater: the homogenizing silo (the continuous blending silo with the aeration cones) averages the remaining chemical variation, and the kiln feed system meters the meal to the preheater at the rate that the kiln demands. The stage’s electricity consumption of 1.0–1.5 kWh per tonne of meal covers the aeration compressors, the extraction and the transport air, and the best practice minimizes the compressed air consumption through the correct aeration cycle.

• The homogenizing silo: the continuous blending with the aeration rings, the blending efficiency of 5–10 depending on the silo type and the aeration regime;
• The aeration control: the pulsed aeration cycles that maintain the fluidization with the minimum air, the biggest lever of the stage’s energy;
• The kiln feed stability: the weigh feeders and the flow gates that deliver the meal at the constant rate, the basis of the pyro stability;
• The silo evacuation: the mass flow extraction that prevents the ratholes and the dead zones, keeping the silo’s homogenizing effect alive;

The stage is the interface between the raw grinding and the pyro process, and its quality performance matters more than its energy: a kiln fed with the meal that swings in its chemistry and its fineness burns with a higher specific consumption and a higher refractory wear than a kiln fed with the stable meal. The best practice of the stage is therefore defined in the two dimensions of the chemistry stability and the energy per tonne, and the workbook records both.

7. The Clinker Making Stage: The Pyro Process

The clinker making stage is the heart of the cement process: the preheater dries and heats the meal, the calciner releases 55–65% of the carbonate CO2, the rotary kiln sinters the material into the clinker at 1,450°C, and the cooler recovers the clinker heat. The stage consumes the heat of the fuel, 700–800 kcal/kg clinker for the modern dry process, and the electricity of the fans, the drives and the compressed air, 25–35 kWh per tonne of clinker, and it is the stage where the energy cost of the plant is decided.

The international best practice of the pyro stage is achieved through the six-stage preheater for the exit gas recovery, the high-efficiency cooler for the heat recovery into the secondary and the tertiary air, the low false air of the whole system, the stable kiln operation at the optimum excess air, and the alternative fuel substitution where the fuel economics favor it. The stage’s benchmark sheet of the workbook carries these figures with the notes on the system configuration, so the comparison of the plant’s own figures against the best practice is made on the same configuration basis.

8. The Fuel Preparation Stage: The Coal and the Alternative Fuels

The fuel preparation stage dries and grinds the coal or the pet coke to the fineness that the kiln and the calciner burners require, and it handles the alternative fuels that the modern plants increasingly burn. The coal mill is typically a vertical roller mill or a ball mill with the hot gas drying, consuming 20–35 kWh per tonne of coal, and the alternative fuel preparation includes the crushing, the shredding, the blending and the dosing of the waste fuels, with their own energy and safety demands.

• The coal grinding: the fineness of 1–3% residue on the 90 micron sieve for the kiln coal, with the moisture dried to 1–2%;
• The inerting and the safety: the CO and the temperature monitoring, the inert gas systems and the explosion protection that the stage demands;
• The alternative fuel chain: the receipt, the analysis, the storage, the preprocessing and the dosing of the RDF, the tyres, the bone meal and the other wastes;
• The substitution curve: the ramp of the alternative fuel share toward the 50–90% total substitution rate that the best plants achieve, with the CO2 and the cost savings following;
• The fuel blending: the mix of the coal, the pet coke and the alternative fuels that optimizes the cost, the burnability and the process stability;

The energy of the fuel preparation stage is small against the fuel cost it controls, and the best practice of the stage is measured in the fuel substitution rate and the fuel cost per tonne of clinker rather than the kWh alone. The workbook records the stage’s energy benchmark and the substitution benchmarks together, because the purpose of the stage is the cheapest reliable fuel delivered to the burners.

9. The Finish Grinding Stage: The Largest Power Consumer

The finish grinding stage grinds the clinker with the gypsum and the additives to the finished cement, and it is the largest single electricity consumer of the plant: 30–45 kWh per tonne of cement for the ball mill circuits, 25–35 kWh/t for the vertical roller mills and 20–30 kWh/t for the high-pressure grinding roll circuits. The stage’s energy follows the product fineness, the clinker grindability, the mill system and the separator performance, and the best practice of the stage is the domain of the package’s ball charge and separator optimization tools.

• The ball mill circuit: the closed circuit with the third generation separator at 30–40 kWh/t of OPC at 3,200–3,500 Blaine, with the ball charge, the liner profile and the ventilation determining the actual figure;
• The vertical roller mill: the finish grinding alternative at 25–35 kWh/t, with the lower power and the higher product temperature and the moisture limits;
• The HPGR combination: the pre-grinding roller press feeding the ball mill, cutting the circuit power by 15–30% and the steel consumption;
• The grinding aids: the dosage that reduces the agglomeration and the coating, saving 5–12% of the finish grinding energy;
• The cement temperature control: the mill ventilation and the water injection that hold the cement below 110–120°C for the gypsum performance;

The finish grinding benchmark is compared per tonne of cement at a defined fineness and a defined cement type, because the grinding energy rises with the Blaine fineness and the clinker share: the blended cements with the limestone and the slag additions grind with a lower specific energy per tonne of cement, and the comparison must hold the product constant. The workbook’s stage sheet carries the benchmark with the product basis stated in the notes.

10. The Storage, the Packing and the Dispatch Stage

The storage and the dispatch stage holds the clinker and the finished cement and delivers them to the customers: the clinker hall and the cement silos, the packers with the bagging lines, the bulk loading spouts for the tankers, the truck and the rail loading, and the compressed air and the dedusting of the whole area. The stage’s electricity consumption runs at 1–3 kWh per tonne of cement, and the best practice of the stage is the efficient packing operation with the high packer availability and the low bag damage and the low dust re-circulation.

• The cement silos: the aeration and the fluidization of the silo cones and the extraction gates, with the compressed air consumption controlled by the level and the flow;
• The packing plants: the rotary and the in-line packers at 1,500–3,000 bags per hour, with the bag dust collectors and the palletizers;
• The bulk loading: the loading spouts with the dust collection hoods, loading the tankers and the rail wagons at the rates that the fleet demands;
• The compressed air system: the plant-wide air network, with the leaks, the pressure and the capacity management that can waste 10–30% of the compressor energy;
• The dispatch logistics: the weighbridges, the gate control and the loading bays scheduled against the truck arrivals to minimize the idling and the queuing;

The dispatch stage is the customer face of the plant, and its energy best practice is the least glamorous and the most easily forgotten part of the benchmark sheet: the air leaks, the idling compressors and the conveyor over-runs that quietly add up to 1–2 kWh per tonne across the year. The workbook’s miscellaneous energy sheet carries the stage’s share in the context of the total, so the small consumer is not ignored in the improvement program.

11. The Miscellaneous Energy Consumption: The Small Consumers That Add Up

The miscellaneous energy sheet of the workbook gathers the consumers that do not belong to a single process stage: the quarrying at 1% of the total facility electricity, the lighting, the workshops, the offices, the water supply and the sewage, the laboratories and the canteens. The consumers are small individually, but together they typically add 2–5% to the plant’s electricity bill, and their best practice is the discipline of the plant’s energy management rather than the technology of a single machine.

• The lighting: the LED conversion and the daylight and the presence controls, cutting the lighting load by 50–70%;
• The compressed air leaks: the ultrasonic leak surveys and the repair programs, recovering 10–30% of the compressor energy;
• The idling equipment: the conveyors, the fans and the pumps left running during the stops, the silent waste of the shifts;
• The peak demand: the load shifting and the demand control that reduce the maximum demand charges of the tariff;
• The power factor: the capacitor banks that hold the power factor above 0.95 and avoid the penalty charges;

The miscellaneous sheet of the workbook records these consumers with their best practice shares of the total, and the plant’s energy accounting system reports them monthly: the energy manager who watches the miscellaneous sheet catches the drift of the plant’s discipline before it becomes a budget line. The 1% quarrying benchmark of the workbook is the example of the precision: even the smallest stage has its international best practice figure, and the comparison applies to it as to the kiln.

12. The Plant-Wide Energy Accounting: From the Stages to the Total

The stage benchmarks become the plant’s energy budget when they are combined with the production plan: the electricity of each stage is the specific consumption times the material flow of the stage, and the sum of the stages is the plant-wide consumption of 85–120 kWh per tonne of cement, with the world class at 85–95 kWh/t. The same accounting structure is the structure of the energy metering: the plant’s electricity meters are allocated to the stages, the monthly consumptions are compared with the stage budgets, and the variances trigger the investigation.

The stage accounting reveals the structure of the opportunity: the finish grinding and the raw grinding together hold half of the plant’s electricity, the pyro fans a quarter, and the remaining stages the balance. An improvement program that saves 10% of the finish grinding energy is worth more than a 50% saving in the lighting, and the plant-wide budget of the workbook directs the effort by the size of the lines. The heat side of the budget is simpler: the clinker making stage carries nearly all the fuel, 700–800 kcal/kg clinker, and the fuel efficiency of the plant is the pyro efficiency.

13. The Benchmarking Methodology: Comparing the Plant with the Best Practice

The comparison of the plant’s stage consumptions with the international best practice figures follows a strict methodology, because the naive comparison misleads. The specific consumptions must be normalized to the same basis: the same material flow (the raw material, the meal, the clinker or the cement), the same product quality (the fineness, the strength class, the cement type), the same utilization and the same ambient conditions.

• Normalize the product: the finish grinding kWh/t must be quoted at the same Blaine fineness and the same cement type, because the energy follows the product spec;
• Normalize the utilization: the stage consumptions at the partial load carry the fixed share of the drives, and the comparison is made at the similar load factors;
• Normalize the material: the raw grinding kWh/t depends on the feed moisture, the grindability and the feed size, and the benchmark notes record these parameters;
• Normalize the vintage: the comparison against the best practice is the measure of the improvement potential, not the judgment of the past design;
• Track the trend: the stage consumptions are compared month to month against the plant’s own history, because the drift from the plant’s own baseline is the first signal of the degradation;

The workbook carries the comparison columns for the plant’s figures against the best practice and the notes that state the normalization basis of each figure, so the gap analysis that the improvement program uses is defensible. The gap is then valued: the kWh per tonne of the gap multiplied by the annual flow and the tariff is the annual saving of the stage, and the stages are ranked by the value of their gaps.

14. The Improvement Actions per Stage: Closing the Gap

The stage benchmarks point to the improvement actions, and the workbook’s notes column records the classic measures per stage. In the raw material preparation, the actions are the crusher operation at full load, the correct blast fragmentation, the reclaimer scheduling and the conveyor discipline. In the raw grinding, the actions are the mill system upgrades, the separator tuning, the grinding aid dosage and the false air reduction. In the pyro process, the actions are the false air sealing, the excess air control, the cooler optimization and the alternative fuel ramp.

• The raw grinding: the mill internals and the ball charge optimization, the separator tuning, the feed moisture control and the grinding aid, saving 10–25% of the stage energy;
• The pyro stage: the false air reduction, the combustion tuning, the cooler performance and the preheater tuning, saving 5–15% of the heat and 10–20% of the fan power;
• The finish grinding: the HPGR or the VRM conversion, the separator upgrade, the grinding aid and the cement formulation, saving 10–30% of the stage energy;
• The dispatch stage: the compressed air repair, the lighting conversion and the scheduling discipline, saving 20–40% of the stage energy;
• The cross-cutting measures: the variable speed drives on the fans and the conveyors, the power factor correction and the energy monitoring, saving 3–8% of the plant total;

The improvement program of a plant is the staged execution of these measures, funded by the savings that the earlier stages of the program generate, and the workbook’s tracking sheet records the before and after figures of every measure. The benchmark is the target, the gap is the opportunity and the program is the execution, and the international best practice sheet of the workbook is the reference that keeps the program honest.

15. The Link Between the Stage Benchmarks and the Other Tools of the Package

The stage benchmarks of the workbook connect directly to the other engineering tools of the Complete Cement Technical Package: the kiln pyro balance computes the actual heat consumption of the clinker making stage, the cooler calculation computes the pyro fans and the recovery, the ball charge and the separator tools optimize the finish grinding, the fan calculations size and check the stage fans, and the financial modelling workbook converts the stage savings into the cash flow of the company. The package is designed so that the stage benchmark sheet is the map and the detailed tools are the instruments.

The workflow of the plant improvement: the stage consumptions are measured and compared with the benchmark sheet, the largest gaps are selected, the detailed tool of the package is applied to the selected stage, the measure is designed and costed, the financial model tests the return, and the tracking sheet records the result. The benchmark without the tools is a report; the tools without the benchmark are a toolbox; and the package provides both, so the improvement program runs from the map to the machine without leaving the desk.

16. Frequently Asked Questions

What is the best practice electricity consumption of the preblending stage?

The international best practice for the preblending and the stacking is 0.5 kWh per tonne of raw material, covering the stacker, the reclaimer and the conveyor drives of the prehomogenization stockpile. Typical plants run at 0.6–1.2 kWh/t.

What is the share of the quarry in the plant’s electricity?

The quarrying stage consumes about 1% of the total electricity of the facility, because its energy is dominated by the diesel of the mobile equipment rather than the electricity. The figure is recorded in the miscellaneous energy sheet of the workbook as the international best practice share.

What is the plant-wide electricity consumption at best practice?

A world-class dry process plant consumes 85–95 kWh per tonne of cement including every stage from the quarry to the dispatch. The average modern plant runs at 95–120 kWh/t, and the gap is the improvement opportunity.

Which stage consumes the most electricity?

The finish grinding consumes 30–45% of the plant’s electricity, followed by the raw grinding at 20–28% and the pyro process fans at 22–30%. The two grinding stages together hold half of the plant’s power budget.

What is the best practice heat consumption of the clinker making stage?

The international best practice is 700–740 kcal/kg clinker for a six-stage preheater system with a calciner and a high-efficiency cooler. The average dry process plant runs at 750–800 kcal/kg, and the old wet process at 1,300–1,500 kcal/kg.

How should the stage benchmarks be used in the plant?

Measure the stage consumptions, normalize them to the same product and utilization basis, compare them with the benchmark sheet, value the gaps at the tariff and the flows, rank the stages by the value of the gaps, and execute the improvement measures with the detailed tools of the package, tracking the before and after in the workbook.

17. Conclusion and Summary

The cement process stages, measured against the international best practice, are the complete map of the plant’s energy: the quarrying at 1% of the total, the preblending at 0.5 kWh per tonne of raw material, the raw grinding at 15–30 kWh per tonne of meal, the clinker making at 700–740 kcal/kg and 20–28 kWh/t of clinker, the finish grinding at 28–40 kWh per tonne of cement and the dispatch and the services that complete the plant-wide budget of 85–95 kWh/t at the world class.

The Cement Process Stages BEST workbook of the package carries the flow chart, the stage sheets and the benchmark figures in the structure of this article, and the engineer uses it as the map of the improvement program: the stages are measured, normalized, compared, valued and improved, with the detailed tools of the package applied to the largest gaps and the financial model testing the returns. The best practice is a moving frontier, and the discipline of the stage benchmarking is what keeps the plant moving with it.

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.