Cement Manufacturing Process: Step by Step Guide

The cement manufacturing process is the complete chain of operations that transforms limestone, clay and corrective raw materials from their natural state in the quarry into the finished cement that is packed in bags and dispatched to the construction market, and it is one of the largest-volume industrial processes in the world, with a single integrated plant producing several thousand tonnes of clinker per day. The process is a combination of mechanical, thermal and chemical operations that can be grouped into a continuous chain: the quarrying of the raw materials, their crushing and pre-homogenisation, the precise proportioning and grinding of the raw mix, the homogenisation of the raw meal in silos, the preheating and calcination of the raw meal in the preheater tower, the clinkering of the material in the rotary kiln at temperatures around 1450 degrees Celsius, the rapid cooling of the clinker, the grinding of the clinker together with gypsum and additives into cement, and the storage, packing and dispatch of the finished product. This article expands the original introduction to the cement manufacturing process into a complete technical reference, covering every stage of the chain in engineering detail, with the process data, the equipment, the reactions, the temperatures, the mass and energy flows, and the process control logic that define a modern dry-process cement plant. It is written for process engineers, plant operators, graduate engineers and technical students who need a single, structured, technically rigorous account of how cement is made.

1. Overview of the Process and the Dry Process Technology

The cement manufacturing process exists in two fundamentally different technology generations: the wet process, in which the raw materials are ground with a large quantity of water into a slurry and fed to a long wet kiln, and the dry process, in which the raw materials are ground dry and fed to a preheater and a kiln. The wet process has become obsolete in modern plants because of its very high heat consumption, around 5.0 to 6.0 gigajoules per tonne of clinker, of which a large fraction is used simply to evaporate the water of the slurry, while the modern dry process with a preheater and a precalciner consumes about 3.0 to 3.6 gigajoules per tonne of clinker, roughly half the energy of the wet process. The dry process is therefore the technology of every modern integrated cement plant, and it is the process described in this article, with the alternative processes, the semi-dry Lepol process and the semi-wet process, mentioned only as historical and transitional technologies.

The process chain of the dry process plant is a sequence of unit operations, each with its own equipment and its own process control. The quarry supplies the limestone and the clay or marl; the crushing and pre-homogenisation reduce the particle size and average out the natural variability of the quarry material; the raw grinding produces a fine, well-mixed raw meal with the correct chemical composition; the homogenisation silos further average the composition and feed a constant-quality stream to the process; the preheater tower dries, heats and begins to decompose the raw meal; the precalciner completes the calcination of the limestone before the kiln; the rotary kiln completes the clinkering reactions and forms the clinker; the cooler rapidly quenches the clinker and recovers its heat as secondary and tertiary air; the cement grinding mill reduces the clinker to the fine cement powder together with gypsum and additives; and the packing and dispatch systems deliver the cement to the customer. Each of these stages is described in detail in the following sections, together with the process data that define their operation.

The economics and the environmental performance of the plant are set by the design choices at each stage. The heat consumption is set by the efficiency of the preheater, the precalciner and the kiln; the electrical energy consumption is set by the efficiency of the mills, the fans and the material transport; the emissions are set by the combustion process and by the dust collection equipment; and the clinker quality is set by the chemistry of the raw mix and by the burning and cooling conditions. The process engineer views the plant as a single, coupled system, in which a change at any stage propagates through the whole chain: a change in the raw mix chemistry changes the burning behaviour, which changes the cooler operation, which changes the secondary air temperature, which changes the flame, and so on. This systems view, and the process data that support it, is what this article develops stage by stage.

2. Quarrying and Raw Material Extraction

The process begins at the quarry, where the raw materials, primarily limestone and clay or marl, are extracted from the earth. The limestone, which provides the calcium oxide of the cement, is the dominant raw material, typically 75 to 85 percent of the raw mix, and the clay or marl provides the silica, alumina and iron oxide. The quarry is developed in benches, with the limestone drilled and blasted or, where the geology allows, extracted directly by ripping and loading; the blasted rock is loaded by hydraulic excavators or wheel loaders into trucks or directly into a primary crusher. The quality control of the quarry is the first chemical control point of the process: the limestone is sampled and analysed as the quarry face advances, and the mining plan is adjusted so that the material fed to the crusher has a stable composition, because the natural variability of a limestone deposit is far larger than the variability that the process can tolerate downstream.

The extraction is planned on the basis of the geological model of the deposit, which divides the deposit into zones of different chemistry and defines the sequence in which they are mined. The operator blends the material from the different zones so that the average composition fed to the plant matches the target raw mix, and the quarry stockpiles provide a buffer between the intermittent extraction and the continuous demand of the plant. The environmental management of the quarry includes the control of dust and noise, the management of the groundwater, and the progressive rehabilitation of the worked-out areas, and the quarry is operated under a mining permit that defines the extraction limits, the restoration obligations and the environmental controls. The key process data of the quarry are the annual tonnage of raw material, typically 1.5 to 1.7 tonnes of raw material per tonne of clinker, the particle size of the blasted rock, which is typically up to one metre and must be reduced by the crushers, and the chemistry of the materials, which is tracked continuously through the drill-hole analyses and the face sampling.

3. Crushing and Pre-Homogenisation

The crushed rock from the quarry must be reduced to a size that the raw grinding mills can accept, typically 80 percent passing 50 to 75 millimetres, and this size reduction is performed by the crushers. The primary crusher, a single-stage hammer crusher or a jaw or gyratory crusher, reduces the blasted rock, with a feed size of up to one metre, to a product of typically 100 to 150 millimetres; the secondary crusher, when installed, reduces the material to the mill feed size. The hammer crusher is the most common choice in the cement industry because it performs the coarse and the medium crushing in a single stage, has a high capacity and a high reduction ratio, and is well suited to the soft-to-medium-hard limestone of most deposits; the jaw and gyratory crushers are used for harder rock and for the primary stage where the feed size is extreme. The crusher is selected on the basis of the feed size, the throughput, the abrasiveness of the rock and the required product size, and it is protected by a metal detector and a tramp iron separator, because a piece of steel in the feed would damage the crusher and the downstream mills.

The crushed limestone is then stacked on a pre-homogenisation stockpile, a large, carefully built pile that averages out the short-term variability of the crusher product. The pre-homogenisation stockpile is built by a stacker that travels along the pile and layers the material in horizontal or chevron layers, so that the pile contains a sequence of thin layers representing the material arriving at different times; the material is then reclaimed by a scraper or a bridge reclaimer that cuts across the layers, mixing them together as it takes the material from the face of the pile. The result is that a fluctuation of several percentage points in the chemistry of the arriving material is averaged down to a fraction of a percentage point in the reclaimed material, which is the accuracy that the raw mill needs. The stockpile also provides the storage buffer between the discontinuous quarry and crusher operation and the continuous demand of the raw mill, and the capacity of the stockpile, typically three to seven days of mill feed, protects the plant against quarry and crusher downtime.

The pre-homogenisation of the clay and the corrective materials follows the same principle on a smaller scale, and the corrective materials, iron ore or iron slag for the iron oxide, sand or silica for the silica, and the aluminium sources, are stored and dosed separately because they are added in small, precise quantities. The plant layout arranges the raw materials in a storage and proportioning area, where each material is stored in its own bunker or stockpile and is withdrawn by a weigh feeder that doses the material onto the raw mill feed belt at the rate set by the raw mix proportioning system. The proportioning system, which controls the feed of the limestone, the clay, the iron corrective and the silica corrective, is the chemical heart of the raw material preparation, because the ratio of these feeds determines the chemistry of the raw meal and therefore the quality of the clinker, and it is described in the next section.

4. Raw Mix Proportioning and Chemistry

The chemistry of the raw meal is the fundamental determinant of the cement quality, and it is defined by the proportions of the four main oxides: calcium oxide, silica, alumina and iron oxide, together with the minor oxides. The raw meal is designed by a set of modulus values that capture the chemical balance of the clinker: the lime saturation factor (LSF), which measures the ratio of the calcium oxide to the sum of the silica, alumina and iron oxide, and which typically lies in the range of 0.90 to 0.98 for portland cement clinker; the silica ratio (SR), the ratio of silica to the sum of alumina and iron oxide, typically 2.0 to 3.0; and the alumina ratio (AR), the ratio of alumina to iron oxide, typically 1.3 to 2.5. These three moduli, together with the minor components such as the magnesia, the sulphur and the alkalis, define the burnability of the raw mix, the quality of the clinker, the heat consumption of the kiln and the formation of the clinker phases.

The four clinker phases, whose proportions determine the cement properties, are the tricalcium silicate (alite, C3S), the dicalcium silicate (belite, C2S), the tricalcium aluminate (C3A) and the tetracalcium aluminoferrite (C4AF). The alite, typically 55 to 70 percent of the clinker, is the main strength-giving phase and the one that reacts with water in the first days of hydration; the belite, typically 15 to 30 percent, contributes the later strength development; the aluminate and the ferrite, together the interstitial phases, contribute to the early heat release and the setting behaviour. The target proportions are set by the cement type being produced: a high-early-strength cement has a high alite content, achieved by a high lime saturation factor, while a low-heat cement has a lower alite content and a higher belite content. The raw mix proportioning system therefore translates the target clinker composition into the raw material feed rates, using the continuously measured chemistry of the raw materials, and it adjusts the feed rates as the raw material chemistry drifts.

Process Stage	Key Equipment	Representative Process Data	Control Variable
Quarry	Drills, blasting, excavators, trucks	1.5–1.7 t raw material per tonne clinker	Blending plan, face sampling
Crushing	Hammer or jaw crusher	Product 80% < 50–75 mm	Crusher gap, feed rate
Raw grinding	Vertical roller mill or ball mill	Fineness 10–16% R90, moisture < 1%	Mill feed rate, temperature, fineness
Homogenisation	Blending silo with air	LSF standard deviation < 0.02	Aeration cycle, blending ratio
Preheater	5–6 stage cyclone tower	Gas exit 280–350 °C, meal to kiln 830–880 °C	Gas flows, draft, fan speed
Precalciner	Inline or separate calciner	Calcination 90–95% at kiln inlet	Calciner fuel rate, tertiary air
Rotary kiln	Kiln shell, refractory, drive	1450 °C material, 1600–2000 °C flame	Kiln speed, fuel rate, burning zone T
Cooler	Grate cooler	Clinker to storage 100–150 °C, 2nd air 800–1000 °C	Cooling air, grate speed
Cement grinding	Ball mill or VRM with classifier	Fineness 90–99% < 90 μm, Blaine 300–400 m²/kg	Mill feed, separator speed, gypsum rate
Packing and dispatch	Rotary packer, palletiser, silo	50 kg bags, 25/35 t bulk trucks	Weight accuracy, despatch rate

5. Raw Material Grinding and the Raw Mill

The raw grinding stage grinds and dries the proportioned raw materials into the fine, dry raw meal that is fed to the preheater, and it is performed by either a vertical roller mill or a ball mill, with the vertical roller mill now dominant in modern plants because of its lower energy consumption and its ability to use the preheater waste gas for drying. The raw mill receives the proportioned limestone, clay and corrective materials, dries them with the hot kiln waste gas, grinds them to a fineness of typically 10 to 16 percent residue on a 90 micrometre sieve, and conveys the finished raw meal to the homogenisation system. The vertical roller mill grinds the material between a rotating table and a set of hydraulically loaded grinding rollers, separates the fine material in an internal classifier, and returns the coarse material to the grinding table, and it combines the grinding, drying and classification in a single unit with a specific energy consumption of about 15 to 20 kilowatt-hours per tonne of raw meal.

The grinding process is controlled to achieve the required fineness and the required chemical composition at the minimum energy consumption. The fineness is controlled by the classifier speed and the mill ventilation, and the chemistry is controlled by the proportioning of the feed materials, with the process laboratory analysing the raw meal every hour or continuously by x-ray fluorescence and adjusting the feed proportions to hold the moduli on target. The moisture content of the raw meal must be below about one percent for the dry process, because the water must be evaporated in the preheater, and the drying in the mill uses the waste heat of the kiln gas, which limits the raw material moisture that a plant can process: with a five or six-stage preheater, the gas at the mill inlet is at 280 to 350 degrees Celsius, sufficient to dry raw materials with up to about 8 to 10 percent moisture, while wetter materials require either a partial bypass of the gas or a supplementary drying system.

The operation of the raw mill is closely integrated with the kiln, because the raw mill uses the kiln gas for drying and the kiln demands a continuous supply of raw meal. The raw mill is operated so that its production balances the kiln demand, with the surplus raw meal stored in a silo; the mill is often stopped during the peak electricity tariff periods and the kiln runs on the stored raw meal, an operating strategy known as raw mill scheduling, which saves energy cost but requires a raw meal storage capacity of several hours of kiln feed. The mill stop and start transitions are critical process events, because the mill, the classifier, the fans and the hot gas system must be brought into balance without overloads or excursions, and the mill control system manages the feed rate, the grinding pressure, the temperature and the differential pressure through these transitions. The raw mill is the largest consumer of electrical energy in the raw material preparation, and its efficient operation is a major element of the plant energy cost.

6. Homogenisation of the Raw Meal

The homogenisation stage is the final chemical averaging step before the raw meal enters the process, and it ensures that the kiln receives a stream of raw meal whose composition is constant enough for the clinkering process to run stably. The raw meal from the mill is stored in a blending silo, typically a conical or cylindrical silo equipped with an aeration system at the bottom, in which the meal is fluidised by air and mixed. The continuous blending silo operates on a simple but effective principle: the raw meal is fed continuously into the silo while the aerated bottom fluidises the meal and draws it off continuously, so that the incoming meal is mixed with the large mass of meal already in the silo, averaging out the short-term fluctuations of the mill product. The result is a reduction of the standard deviation of the lime saturation factor from about 0.5 to 1.0 percent at the mill to about 0.02 to 0.05 percent at the silo outlet, which is the stability that the kiln needs.

The homogenisation is supported by the process control system through the raw meal analysis. The process laboratory analyses the raw meal from the mill and from the silo on a continuous or high-frequency basis, and the proportioning system adjusts the feed materials to correct the drift of the composition. The control strategy is a two-layer system: the proportioning control holds the mill feed chemistry on target, and the blending silo averages out the residual short-term variability, with the two layers together delivering the constant-quality raw meal that is the precondition for a stable kiln. The blending silo also provides the storage capacity between the mill and the kiln, decoupling the intermittent operation of the mill from the continuous demand of the kiln, and the silo level is managed as part of the raw meal scheduling.

The capacity of the homogenisation system is matched to the kiln demand and to the operating strategy of the plant. The blending silo typically holds several hours of kiln feed, and a separate storage silo provides the additional capacity for the raw mill scheduling. The aeration system of the silo, with its air compressors, its valves and its control system, is a significant auxiliary consumer, and its operation is optimised to achieve the required homogenisation at minimum air consumption. The failure modes of the homogenisation system, the bridging of the meal in the silo, the blockage of the aeration nozzles and the segregation of the meal in the draw-off, are managed by the design of the silo bottom and by the operational monitoring, and the raw meal is transported from the silo to the preheater by a closed conveying system, typically a bucket elevator or a pneumatic transport system, that delivers the meal to the preheater feed at a rate set by the kiln feed control.

7. The Preheater: Heating and Starting the Decomposition

The preheater tower is the first thermal stage of the process, and it transfers the heat of the kiln exit gas to the raw meal, raising the meal temperature from ambient to about 830 to 880 degrees Celsius before it enters the kiln, and starting the decomposition of the limestone. The modern preheater is a suspension preheater, consisting of a tower of cyclone stages, typically five or six, arranged so that the raw meal flows downwards through the stages against the upward flow of the hot kiln gas. In each stage the raw meal is dispersed into the gas stream, heated almost instantly to the gas temperature by the excellent heat transfer of the suspended particles, and separated from the gas in the cyclone, and the cycle is repeated in the next, hotter stage. The heat transfer in the suspension preheater is so efficient that the meal reaches within about 50 to 80 degrees Celsius of the gas temperature at each stage, which is why the gas exits the top of the preheater at only 280 to 350 degrees Celsius, while the meal enters the kiln at 830 to 880 degrees Celsius.

The chemical change in the preheater is the start of the calcination reaction, in which the calcium carbonate of the limestone decomposes into calcium oxide and carbon dioxide. The calcination reaction, CaCO3 becomes CaO plus CO2, is strongly endothermic, requiring about 1780 kilojoules per kilogram of calcium carbonate, and it proceeds at a significant rate only above about 700 degrees Celsius, becoming rapid above 800 to 850 degrees Celsius. In the preheater, the meal reaching the lower stages begins to calcine, so that a portion of the calcination, typically 30 to 50 percent with a five-stage preheater, is already completed in the preheater stages and the downcomer to the kiln inlet, and the remaining calcination is completed in the precalciner and the kiln. The progress of the calcination through the preheater is controlled by the temperature and by the residence time of the meal in the hot stages, and it is an important element of the heat and mass balance of the plant.

The operation of the preheater is governed by the draft, the temperatures and the flow balance of the stages. The kiln gas, the calciner gas and the auxiliary air all flow upwards through the preheater, and the fans, principally the induced draft fan at the top of the tower, control the total gas flow and the pressure profile. The temperatures at each stage are monitored, and the process control holds the stage temperatures at their design values by the fuel rates, the raw meal feed and the gas flows, because a stage temperature that is too high indicates a gas flow imbalance, a meal flow problem or a build-up in the tower, while a temperature that is too low indicates an excess of meal or a shortage of heat. The preheater is also the location of the most important operational hazard of the plant: the build-up and blockage of the towers, cones and downcomers by the sticky, semi-molten deposits of the meal, which are caused by the alkalis, the sulphates and the chlorides that condense and accumulate in the lower stages, and the operation and maintenance of the preheater include the monitoring, the cleaning and the prevention of these blockages.

8. The Precalciner: Calcination Outside the Kiln

The precalciner is a combustion chamber between the preheater and the kiln in which a large fraction of the fuel is burned and a correspondingly large fraction of the calcination reaction is carried out, before the material enters the kiln. The precalciner receives the raw meal from the last preheater stage, receives a portion of the kiln fuel, typically 55 to 65 percent of the total kiln system fuel, receives the hot tertiary air from the cooler, and burns the fuel in a stream of the preheated meal, so that the meal is calcined to about 90 to 95 percent before it enters the kiln. The precalciner may be inline, integrated into the riser duct between the kiln and the last cyclone stage, or separate, a vessel of its own beside the tower, and its design and position determine the flow of the meal, the fuel and the gas through the calcination zone.

The advantage of the precalciner is that it removes the bulk of the endothermic calcination from the kiln, so that the kiln is left with the clinkering task alone, which allows a much larger production rate for a given kiln size, because the flame heat is no longer consumed by the calcination inside the kiln. The precalcined meal enters the kiln at 850 to 900 degrees Celsius with only 5 to 10 percent of its carbonate remaining, and the kiln then brings the material up to the clinkering temperature, completing the residual calcination in the first part of the kiln. The burning in the precalciner also reduces the fuel consumption per tonne of clinker, because the calcination at moderate temperature in the calciner is thermally more efficient than the calcination in the kiln, where the heat must first pass through the flame, and because the precalciner allows a higher heat recovery from the cooler through the tertiary air.

The control of the precalciner is the control of the calcination degree, which is measured indirectly by the temperature at the calciner exit and at the kiln inlet, and by the analysis of the meal. The calciner fuel rate is controlled to hold the calcination at the design level: too little fuel leaves too much carbonate for the kiln, which cools the kiln and can cause snowmen and kiln ring formation, while too much fuel overheats the calciner, risks the formation of sticky deposits and wastes fuel. The tertiary air flow to the calciner is controlled to provide the combustion air at the correct temperature and rate, and the calciner is operated with a slight excess air so that the combustion is complete and the CO concentration at the calciner exit is low. The precalciner is also the principal location where alternative fuels are burned in modern plants, because the moderate temperature and the long residence time of the meal in the calciner make it an ideal combustion zone for the low-grade alternative fuels, and the calciner fuel rate therefore reflects the full fuel mix of the plant.

9. The Rotary Kiln: The Clinkering Process

The rotary kiln is the heart of the process, the large refractory-lined rotating cylinder, typically 4 to 6 metres in diameter and 60 to 90 metres in length for a preheater kiln line, in which the precalcined meal is brought to the clinkering temperature and converted into clinker. The kiln is inclined at 2.5 to 4.5 percent and rotates at 2.5 to 4.5 revolutions per minute, so that the material fed at the upper end slowly travels down the kiln under the rolling and sliding of the charge, while the fuel is burned at the lower, discharge end by the main burner, and the hot combustion gas flows upwards through the kiln against the material flow. The kiln shell is supported on tyres and rollers at the support stations, is driven through a girth gear and a pinion, and is lined internally with refractory bricks and castables that protect the shell and resist the process temperatures.

The chemical and physical transformation of the material along the kiln is the clinkering process. In the first part of the kiln the residual calcination is completed; in the transition zone the material is heated to the clinkering temperature, the melt phase begins to form as the aluminate and the ferrite melt, and the kiln gas, at temperatures above 1500 degrees Celsius, transfers heat to the charge; and in the burning zone, the hottest zone near the flame, the material is held at 1400 to 1450 degrees Celsius, where the calcium oxide reacts with the silica to form the alite and the belite, and the liquid phase binds the material into the nodules of clinker. The formation of the clinker phases is controlled by the temperature, by the residence time of the material in the burning zone and by the composition of the raw meal: a high lime saturation factor and a high burning temperature favour the formation of the alite, while a lower temperature and a lower lime saturation favour the belite, and the operator controls the burning zone temperature through the kiln fuel rate, the kiln speed and the burner settings.

The control of the kiln is the most demanding process control task in the plant, because the kiln has a long response time and is subject to the disturbances of the raw meal chemistry, the fuel quality and the process conditions. The primary control variables are the kiln fuel rate, which sets the heat input, the kiln speed, which sets the material residence time, and the burning zone temperature, which is measured by a pyrometer or a camera system and is controlled by the fuel rate. The secondary variables are the kiln inlet gas temperature, the shell temperatures along the kiln, the kiln torque and the free lime content of the clinker, and the modern kiln control system, increasingly an expert system or a model-based controller, uses all of these to hold the kiln at the operating point that gives the required clinker quality at the minimum heat consumption. The refractory lining of the kiln, particularly in the burning zone, is protected by the coating of clinker that forms on the hot face, and the management of the refractory life, through the stable operation, the monitoring of the shell temperatures and the planned relining at the maintenance stops, is a major element of the kiln operation.

10. The Clinker Cooler: Rapid Quenching and Heat Recovery

The clinker leaving the kiln at 1300 to 1400 degrees Celsius is cooled rapidly to about 100 to 150 degrees Celsius above ambient in the clinker cooler, and the cooling serves three purposes: it recovers the heat of the clinker as the secondary air for the kiln flame and the tertiary air for the precalciner, it fixes the microstructure of the clinker, because the rapid cooling of the alite preserves its reactivity and improves the grindability of the clinker, and it makes the clinker suitable for transport, storage and grinding. The grate cooler is the dominant technology: the clinker is deposited on a moving grate, typically in a series of grate sections, and a large volume of ambient air is blown up through the clinker bed, so that the air is heated to 800 to 1000 degrees Celsius and the clinker is cooled by the convective heat transfer. The heated air is recovered as the secondary air, drawn into the kiln through the hood, and as the tertiary air, drawn to the precalciner, and the surplus air, after the heat recovery, is exhausted to the atmosphere after dedusting or is used for the drying in the raw mill and the coal mill.

The design of the cooler and the control of the cooling air are critical to the process performance. The air is distributed across the width and the length of the grate so that the clinker bed is cooled evenly and the air is recovered at the highest possible temperature, and the cooling air is staged, with a high-pressure air supply for the first, hottest grate section and lower-pressure supplies for the cooler sections. The clinker bed height is controlled by the grate speed and the air pressure, and the cooler is controlled to achieve the target clinker discharge temperature and the target heat recovery, which together determine the kiln fuel consumption. The temperature profile of the clinker in the cooler is monitored, and the cooler operator controls the grate speed, the air flows and the clinker crusher, which breaks the largest clinker lumps in the mid-cooler to improve the air distribution in the later sections.

The cooler is also the source of the dust that returns to the process and the location of several classic operational problems. The fine clinker dust carried by the cooling air is collected and returned, often to the kiln feed or to the cooler inlet, so that the material balance of the cooler is closed; the snowmen, the large, fused masses of clinker that form at the cooler inlet and block the feed, are the most serious cooler problem, caused by over-burning, by the presence of liquid in the clinker or by the reformation of the melt in the hot air stream; and the “red rivers”, the local zones of still-hot, liquid clinker that flow through the cooler and damage the grate, are controlled by the air distribution and the grate speed. The cooler operation is therefore an integral part of the kiln operation, and the cooler is operated, monitored and maintained as part of the kiln system rather than as an independent unit.

11. Clinker Storage, Additives and Cement Chemistry

The cooled clinker is transported to the clinker silo or the clinker stockpile, where it is stored and aged before the cement grinding. The storage provides the buffer between the continuous clinker production and the intermittent operation of the cement mills, and the ageing of the clinker, its exposure to the air and moisture, improves the grindability and the cement quality by the reaction of the free lime with the moisture and by the partial carbonation. The cement grinding then combines the clinker with gypsum and, in blended cements, with the supplementary cementitious materials, the granulated blast furnace slag, the fly ash, the natural pozzolana or the limestone filler. The gypsum is essential to the cement: it is added at a rate of 3 to 5 percent to control the setting of the cement, because the gypsum reacts with the tricalcium aluminate in the presence of water to form the ettringite that retards the setting, and the correct gypsum content is determined by the chemistry of the clinker and by the required setting and strength development of the cement.

The cement types are defined by their composition, and the plant produces the types demanded by its market. The ordinary portland cement (CEM I) contains the clinker and the gypsum with at most 5 percent of the minor constituents; the portland composite cements (CEM II) contain up to 35 percent of the supplementary materials such as the fly ash, the slag, the limestone or the pozzolana; the blast furnace cements (CEM III) contain 36 to 95 percent of the slag; and the pozzolanic and composite cements (CEM IV and V) contain the corresponding proportions of the pozzolanic materials. The choice of the cement type is driven by the availability and the cost of the supplementary materials and by the requirements of the market, and the dosing of the additives is controlled so that the cement composition is held within the tight tolerances of the product specification, with the process laboratory verifying the composition continuously.

The cement chemistry is completed by the minor components that enter with the raw materials and the fuels. The magnesia, the sulphur as sulphate, the alkalis as sodium and potassium, and the chloride are the most important, because they influence the clinker formation, the cement quality and the process operation. The magnesia is limited in the clinker because the periclase, the crystalline magnesia that forms in slowly cooled clinker, expands in the hardened cement and can cause soundness problems; the alkalis affect the setting time, the strength and the alkali-silica reaction of the concrete, and they are limited in the cement specification; and the chloride must be controlled because it attacks the steel reinforcement of the concrete and because it concentrates in the kiln system, where it forms the volatile recirculating load that causes the preheater blockages. The management of the minor components, through the raw mix design, the fuel selection and the process operation, is a permanent part of the cement plant chemistry.

12. Cement Grinding: The Final Size Reduction

The cement grinding is the final size reduction stage of the process, in which the clinker, the gypsum and the additives are ground together into the fine powder that is the finished cement, and it is performed by the closed-circuit ball mill or the vertical roller mill, with the ball mill in closed circuit with a high-efficiency separator being the classic configuration and the vertical roller mill and the horizontal roller press-based systems the modern alternatives with lower energy consumption. The cement fineness is the fundamental quality parameter, expressed either as the specific surface area, the Blaine value, typically 300 to 400 square metres per kilogram for the ordinary cement, or as the residue on the 45 and 90 micrometre sieves, and the fineness is controlled by the classifier and the mill operation. The particle size distribution of the cement is equally important, because it determines the water demand, the workability and the strength development of the concrete, and the modern grinding systems are designed to produce a steep, well-defined particle size distribution.

The grinding process is controlled for the fineness, the temperature, the quality and the energy consumption. The mill temperature is controlled by the mill ventilation and the water injection, because a mill that runs too hot dehydrates the gypsum, changing the setting behaviour of the cement, and a mill that runs too cool grinds inefficiently and can coat the balls and the liners; the finish mill is therefore operated within a defined temperature window, typically with the cement discharge at 100 to 120 degrees Celsius. The quality is controlled by the process laboratory, which analyses the cement for its fineness, its composition, its setting time and its strength, and the mill control adjusts the feed rate, the separator speed and the ventilation to hold the quality on target. The energy consumption of the finish grinding is the largest single item of the plant electrical energy, typically 25 to 35 kilowatt-hours per tonne of cement for the modern systems, and the choice of the grinding system and its efficient operation are major elements of the plant operating cost.

The finish mills are operated in a schedule that matches the cement demand, the electricity tariff and the availability of the clinker, and the cement is stored in the cement silos, which are differentiated by cement type and by quality. The cement silos hold the finished cement for the packing and dispatch, and the cement is transferred from the mills to the silos by pneumatic or mechanical conveying, with the quality of each silo filling batch recorded so that the cement dispatched to the market is traceable to its production history. The grinding of the cement is the last opportunity to correct the product quality before dispatch, and the mill and the quality control laboratory work together to ensure that every tonne of cement leaving the plant meets its specification.

13. Packing and Dispatch

The packing and dispatch stage delivers the finished cement to the customer, in bulk to the ready-mix concrete plants and the construction sites, and in bags, normally 50 kilograms, to the trade market. The packing plant draws the cement from the silos, passes it through a screening and separation system that removes any foreign bodies and tramp metal, and feeds the rotary packer, the rotating machine with a number of filling spouts that fills the bags by weight at high speed, with the bag weight controlled to the legal accuracy and checked by a check weigher. The filled bags are loaded onto pallets by the palletiser or handled directly onto the trucks, and the bagged cement is dispatched in truckloads, with the dispatch documentation recording the cement type, the quantity and the destination.

The bulk dispatch is the dominant route in the modern market: the cement is loaded from the silos into the bulk tankers, typically through a loading spout with a dust collection system, at rates of several hundred tonnes per hour, and the tankers transport the cement to the ready-mix plants and the construction sites. The dispatch operation is integrated with the sales and logistics system, with the cement orders, the loading and the delivery documented in the plant information system, so that the cement is traceable from the production batch through the loading to the delivery. The packing plant also handles the export logistics for the plants that ship cement by sea, loading the bagged or bulk cement onto the vessels at the port terminal.

The environmental control of the packing and dispatch stage is the control of the cement dust, which is the most visible environmental issue of the plant: the silo vent filters, the loading spout dedusting and the cleaning of the loading areas all contribute to keeping the fugitive dust within the plant limits. The packing plant is operated on a shift schedule matched to the shipping demand, and its maintenance, particularly of the packer, the weighing systems and the dust collection, is planned so that the dispatch operation does not become the bottleneck of the plant. The packing and dispatch stage completes the process chain: the raw material from the quarry, transformed by the chemistry and the heat of the process, leaves the plant as the finished cement that builds the concrete structures of the market.

14. Process Control and the Plant Information System

The control of the modern cement plant is performed by a distributed control system (DCS) that integrates the control of every stage of the process, from the quarry through the packing plant, into a single control room. The DCS collects the measurements of the temperatures, the pressures, the flows, the levels, the weights, the speeds and the compositions, presents them to the operators on the control screens, and executes the control loops, the sequences and the interlocks that operate the plant. The control loops are organised hierarchically: the regulatory loops hold the individual process variables on their set points, the supervisory loops coordinate the stages, and the optimising functions, increasingly based on expert systems and model-predictive control, adjust the operating point to the economic optimum, particularly for the kiln, the most complex and the most energy-intensive unit of the plant.

The process laboratory is an integral part of the control system, providing the chemical analyses that the on-line instruments cannot provide. The x-ray fluorescence (XRF) analyser determines the oxide composition of the raw materials, the raw meal and the cement, and the x-ray diffraction (XRD) analyser determines the phase composition of the clinker, and the laboratory results are fed into the process control, the raw mix proportioning and the quality control. The quality management system of the plant holds the cement to the product specification, the national standards such as the ASTM, the EN and the national equivalents, and it documents the production, the testing and the dispatch so that the product quality is traceable and the plant certification is maintained. The plant information system integrates the process data, the laboratory data and the commercial data, providing the management reports, the energy and emission reports and the production statistics that drive the plant management.

The process control also includes the safety systems that protect the plant and its people. The interlock systems prevent the unsafe operation of the equipment, the burner management systems protect the kiln firing system, the gas analysis systems monitor the combustible gases in the process, and the alarm management and the emergency procedures define the response to the process upsets. The control of the plant is therefore the combination of the automation, the chemistry and the process knowledge, operated by a team of process engineers and operators who understand the process chain as a single, coupled system, and who use the control system to keep the plant at the optimum operating point within the safe envelope.

15. Energy, Emissions and Sustainability of the Process

The cement industry is one of the largest industrial consumers of energy and one of the largest industrial emitters of carbon dioxide, and the sustainability of the process is therefore a central concern of the modern plant. The specific heat consumption of the modern dry process, about 3.0 to 3.6 gigajoules per tonne of clinker, is close to the thermodynamic minimum of the process, and the further reduction comes from the waste heat recovery, the alternative fuels and the process optimisation. The alternative fuels, including the refuse-derived fuel, the tyres, the biomass and the industrial wastes, replace a portion of the fossil fuel and reduce the fossil carbon dioxide emissions, with modern plants reaching substitution rates of 50 percent and more. The waste heat recovery systems convert part of the preheater and cooler gas heat into electrical power, typically 20 to 30 kilowatt-hours per tonne of clinker, improving the electrical self-sufficiency of the plant.

The carbon dioxide emissions of the process have two sources: the calcination reaction, which releases about 535 kilograms of carbon dioxide per tonne of clinker from the limestone, and the combustion of the fuel, which releases a similar amount. The calcination carbon dioxide is inherent to the process, and its reduction requires either the capture of the carbon dioxide or the use of the alternative cementitious materials that avoid the calcination altogether. The carbon capture technologies, the oxyfuel process, the amine scrubbing and the calcium looping, are under development for the cement industry, and the first full-scale demonstration plants are in operation, but the commercial deployment of the carbon capture on a large scale remains a major technical and economic challenge. The industry is also reducing its carbon footprint through the blended cements, which replace a portion of the clinker with the supplementary materials that do not require the calcination, and which are the most cost-effective means of reducing the clinker factor and therefore the carbon intensity of the cement.

The environmental performance of the process extends to the air emissions, the water and the waste. The dust emissions are controlled by the bag filters and the electrostatic precipitators that clean the process gas and the vent air of every stage; the nitrogen oxide emissions are controlled by the combustion management, the staged combustion and the selective non-catalytic reduction; and the sulphur dioxide and the organic emissions are controlled by the process chemistry and the emission abatement systems. The process is essentially water-neutral in the dry process, because the water is used only for the cooling and the dust control, and the plant manages its water consumption and its effluent carefully. The waste materials of the process, the cement kiln dust, the filter dust and the bypass dust, are either returned to the process or disposed of in a controlled manner, and the goal of the modern plant is a process that closes its material loops, minimises its emissions and uses its energy efficiently, which is the definition of sustainability in the cement industry.

16. Frequently Asked Questions

What is the difference between the wet and the dry cement manufacturing process?

The wet process grinds the raw materials with water into a slurry and feeds it to a long wet kiln, consuming about 5.0 to 6.0 gigajoules per tonne of clinker, while the dry process grinds the raw materials dry and feeds a preheater and a precalciner, consuming about 3.0 to 3.6 gigajoules per tonne of clinker, and the dry process is the technology of every modern plant.

Why is the precalciner used?

The precalciner burns 55 to 65 percent of the kiln system fuel and carries out about 90 to 95 percent of the calcination before the kiln, so that the kiln is left with the clinkering task alone, which allows a much higher production rate for a given kiln size and reduces the heat consumption.

What is the clinkering temperature and what happens at the burning zone?

At the burning zone the material is held at 1400 to 1450 degrees Celsius, where the calcium oxide reacts with the silica to form the alite and the belite phases, and the melt phase binds the material into the nodules of clinker.

Why is gypsum added to the cement?

Gypsum is added at 3 to 5 percent to control the setting of the cement: it reacts with the tricalcium aluminate in the presence of water to form ettringite, which retards the setting and gives the concrete its workable handling time.

What are the main control variables of the rotary kiln?

The main control variables are the kiln fuel rate, which sets the heat input, the kiln speed, which sets the material residence time, and the burning zone temperature, which is controlled by the fuel rate, together with the kiln inlet gas temperature, the shell temperatures and the clinker free lime.

What are the two sources of carbon dioxide in cement manufacturing?

The calcination of the limestone releases about 535 kilograms of carbon dioxide per tonne of clinker, and the combustion of the fuel releases a similar amount; the calcination emission is inherent to the process and requires carbon capture or clinker substitution to reduce it.

How is the cement fineness controlled?

The cement fineness is controlled by the classifier in the closed grinding circuit and by the mill operation, and it is expressed by the Blaine specific surface, typically 300 to 400 square metres per kilogram, or by the residue on the 45 and 90 micrometre sieves.

17. Summary

The cement manufacturing process is a complete industrial chain that begins with the blasting of the limestone in the quarry and ends with the dispatch of the bagged and bulk cement to the market, and it is, at every stage, a precisely engineered combination of mechanics, chemistry and heat. The raw material preparation, from the quarry through the crushing, the pre-homogenisation, the proportioning and the raw grinding to the homogenisation silos, converts the naturally variable deposit into a constant-quality raw meal whose chemistry is designed by the lime saturation factor, the silica ratio and the alumina ratio for the target clinker phases; the thermal process, from the suspension preheater through the precalciner, the rotary kiln and the grate cooler, converts that raw meal into clinker, using the preheater to recover the kiln gas heat, the precalciner to carry out the bulk of the calcination, the kiln to complete the clinkering at 1450 degrees Celsius and the cooler to recover the heat and fix the clinker microstructure; and the finishing stages, the cement grinding with the gypsum and the additives, the storage and the packing and dispatch, convert the clinker into the finished cement with the fineness, the composition and the strength required by the market. Every stage of the chain has its process data, its control logic and its optimisation target, and the modern plant integrates them into a single, coupled system operated from a single control room, with the process laboratory, the quality management and the plant information system completing the control picture. The economics of the plant are set by the heat and the electricity consumption, which the modern dry process minimises, and its environmental performance is set by the emissions control and, increasingly, by the alternative fuels, the blended cements and the emerging carbon capture technologies that will define the sustainability of the industry in the decades ahead. For the process engineer, the understanding of this complete chain, from the quarry to the dispatch, is the foundation of every engineering decision, because no stage of the process can be optimised in isolation, and the best plant is the plant in which every link of the chain, from the raw meal chemistry to the cement fineness, is designed and operated as part of the single, integrated system that this article has described.

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.