An introduction to Portland Cement Manufacturing

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The Cement Institute
Reliability cement plant

An introduction to Portland Cement Manufacturing

The details of the cement manufacturing process are very broad. No two plants are the same and there may be differences within the same plant in particular, since new kiln and their associated equipment can be added to existing ones: for example, wet and dry processes can operate side by side using the same raw materials and producing the same final product and having completely different process conditions:

  • The coarse size of the raw material is reduced for further processing. The size reduction is done through crushers and mills. Crushing is the transformation of material in the coarse range, while grinding refers to crushing in the fine range. A variety of processes and equipment are available for the selection of the right type of size reduction machinery.
  • The raw materials are mixed and combined to produce a mixture of raw material of uniform chemical composition containing calcium carbonate, silica, alumina, iron oxide and other compounds in the required proportions. Depending on the process used, the mixing and composition may take place partially during the grinding stage or it may be a completely separate operation.
  • The combined raw meal mixture is heated to the point where all moisture is removed as water vapor or steam.
  • The dried mixture is heated until decarbonization of the calcination temperature, approximately 800 °C. At this temperature, the calcium carbonate in the mixture dissociates into calcium oxide (free lime), which remains in the mixture and carbon dioxide that is removed as a gas.
  • The mix is further heated and, as the temperature rises, the oxides of calcium, silicon, aluminum and iron react to form the calcium silicates, aluminate and aluminoferrite, which are the main active compounds of Portland cement. This process is completed at a temperature of around 1400 °C; the resulting product is Portland cement clinker.
  • The clinker is cooled to a temperature at which it can be conveniently handled, 60 -150 °C. The clinker can be sent directly to the cement grinding mills, but it is usually stored. The clinker can be stored for relatively long periods without deterioration and, for this reason, the supply of cement to places far from the plants can be done by sending clinker instead of finished cement, since clinker grinding can be done in grinding stations closer to the point of use.
  • The clinker is ground to the specified fineness, with the addition of a small proportion of plaster to control the setting time of the finished cement. Masonry additives and other special types of cement are incorporated during the grinding stage; where Portland cement is manufactured, the slag component can also be incorporated during grinding.
  • The finished cement is stored in silos for a relatively short time before being sent to the customer in bags or in large quantities. Bulk supply, using specially designed bulk transport.
Ball Mill

Types of Manufacturing Process

Manufacturing methods can be divided into two broad categories, wet and dry processes, which differ in the way materials are treated to the state of calcination.

In the wet process, the raw materials are reduced to the required fineness in water and mixed, stored and fed into the kiln as a fluid slurry. The water in the slurry (typically 30 – 40 % by weight) is removed in the initial stage of the kiln processing.

In the dry process, the moisture in the raw materials is partially removed by heating in the initial processing stage, generally in the case of hard materials during the milling stage itself. The relatively dry “material” is mixed and normally passed through a preheating system that completes the drying and in the preheater zone the material is raised to a temperature where it is partially calcined. The preheating stage uses the heat remaining in the exhaust gases once they have passed through the kiln.

The two main variants of wet and dry processes are semi-wet and semi-dry methods. In both cases the raw material, prepared by wet or dry methods, depending on the nature of the raw materials, pallets or nodules with an average moisture content are formed. The pallets or modules are introduced into the tray by means of a grid preheater in which the mobile bed of nodulized materials is dried and brought to the heat calcining temperature of the kiln.

The choice of the process to be used depends on a complex combination of various factors, which must be balanced when choosing the methods that will be used in modern plants, and when deciding whether, or with what process, to expand or modernize existing plants. These include the nature of raw materials, thermal efficiency or different processes and their detailed variations, fuel prices and other types of energy and many other considerations.

Given the recent global increases in the price of fuels, the wet process of higher fuel consumption, which was once widely used for raw materials of all kinds, has been largely replaced in new or modernized plants by processes dry or semi-dry processes wherever the raw materials are suitable.

Cement Manufacturing Wet Process

The thermal efficiency of the wet process is much lower than the dry process, the wet process has been almost universally replaced by dry or semi-dry process.

The wet process of cement manufacturing differs from the dry process only in that the materials are ground and burned wet. No drying of raw materials is necessary. On the other hand, water must be added to raw materials (except in the case of marl and shells dredged below water and alkaline waste) at some point before they are ground, usually after crushing and just before that raw materials are fed.

When limestone and shale represent the raw materials, the wet process does not differ from the dry process until grinding is reached, water is added here, the proportion is 40 to 60 percent of the dry raw materials, and the mixture It is pulverized. The resulting slurry will contain 30 to 40 percent water depending on the amount added. The slurry has the consistency of a thin mud. Only enough water is used to make the mixture fluid enough to handle it easily. Any excess over this is undesirable as it will increase the fuel consumption in the kiln. The same machinery that is operated for crushing is used in the dry process and generally the same storage of crushed material is used. The mixture can be produced at any convenient point, as in the dry process.

When clay is used, limestone is crushed and handled as in the dry process, but the general practice is to dump the clay into a washing mill, adding water in a “slip” or fine mud, which is introduced into the system grinding at a set speed along with the limestone. In some plants, the clay is mixed with the limestone, since it comes from the pits, either before or after the limestone is crushed.

Usually, the material is produced with enough water to make it fluid in a washing mill, or a ball mill; the clay receives the same treatment and procedure as the marl or in a separate machine. In the latter case, the two suspensions are mixed in the required quantities.

Wet Process Cement Plant

Cement Manufacturing Dry Process

Stacker and relcaimer

As the name implies, the raw materials in the dry process are ground, mixed and fed to the kiln in a dry state, instead of slurry, as is the case with the wet process. For this reason, the dry process is more suitable for raw materials that have a relatively low natural moisture content.
In general, the dry process is used with hard materials (limestone and shale), but in recent years they have also been used for certain softer materials that were previously handled with the wet process. With the right materials, the dry process consumes much less fuel than the wet process and, for that reason, it is now favored where the material is suitable.

Raw Materials Processing

The raw materials for the dry process are reduced to the fineness needed to feed the kiln through a combination of crushing and grinding.
Where hard materials are used, primary crushing in the quarry is often performed to reduce the stone to a more manageable size, especially where conveyor belts are used for transport. Depending on the equipment used and the nature of the raw materials, several crushing stages may be necessary; in the case of soft materials, such as clay chalk, little or no crushing may be required.
From the crushers, the raw material is transported to the limestone warehouse where it is stored, there are several storage and recovery systems, but all are designed to minimize variations in the composition of the raw material supplied to the mills. The quarry material is analyzed at frequent intervals; the batches of different chemical compositions can be stored separately to be able to extract them in controlled quantities.
Alternatively, they can be stored and recovered, so that the recovered materials are a mixture of materials from different parts of the quarry face; one means of doing this is to transport the material to the warehouse in horizontal layers and then claim it as a vertical cross-section of all layers.

Drying

Raw materials may require drying before they can be handled as feedstock. This can be achieved separately in rotary dryers but is generally achieved by passing the gases from the kiln or hot air through the mill itself. The heat of the dry process kiln gases is sufficient to dry the material having a moisture content of approximately 8 %, but for a higher moisture content additional heat is required.

Grinding

Various types of mills (ball mill, roller press or vertical mill) can be used to grind the raw material. The most common for hard materials is the ball mill, however, over the years, new modernized plants are using vertical mills for grinding. The raw grinding ball mill is similar in construction and operation to the mills used to grind the cement clinker. However, for raw milling, in the ball mills the material is swept with air to dry and grind. The raw material enters the mill already crushed to a reasonably small size; In general, 90% of the limestone is less than 20 mm and 90% of the shale is less than 50 mm.
When wet and sticky materials are used that present crushing problems, the Aerofall mill can be used. This is an air swept mill with a large diameter in relation to its length, capable of handling stones up to a maximum size of approximately 225 mm. At the Aerofall mill, large stones or rocks are ground with each other with the help of large steel balls to grind. The gases that pass through the mill sweep the ground material in a system of classifiers, cyclones and filters that returns the largest particles to the mill for later crushing, passes the intermediate size material to a second finishing mill in order to achieve the required fineness.

Grinding with vertical roller mill

This class of mills reaches many variants that, however, have certain fundamental characteristics in common.
The roller mills used in the cement industry have grinding elements in various ways. Thus, in some mills they are cylindrical rollers, in others, the rollers are conical-shaped or have flat lateral faces and convex circumferential surface.
The reduction of the size of the material is affected by the rollers or comparable grinding elements that runs on a circular table of material and that the material, after passing under the rollers, is subjected to a preliminary sorting action by a stream or air sweep through the mill. Depending on the speed of the air flow, a certain proportion of the pulverized material is transported to a classifier (air separator) that normally forms an integral characteristic of the upper part of the mill housing. Large particles rejected by the classifier return to the grinding table, while the fines are swept with the air from the mill and is received by a filter or a set of cyclones. As the pneumatic transport of the material in the mill passes to the separator, it requires considerable air flow rates, and when the material that leaves the grinding table and is taken to the sorter comes into intimate contact with the air, the roller mills are especially suitable for drying moist deed material in combination with grinding.

Homogenization

After passing through the mill, the raw meal passes to the homogenization and storage silos. As with the wet process, in the raw meal mixture samples are taken regularly and frequently in the successive stages of its preparation, so that the desired composition can be maintained homogeneously. Several methods are used to mix dry raw meal, but all have the final results of producing a kiln feed that is homogeneous and consistent in its composition.

Preheating

Raw meal passes through a suspension preheater before entering the kiln. The preheater consists of a series of cyclones in which the raw meal is exposed to the hot gases that come out of the kiln. In a complete preheater installation of four or more stages, the raw meal is heated to a temperature of approximately 800 °C during its passage through the preheater tower, so that when it enters the kiln it is already 40 % calcined. However, in some plants, fewer preheating stages are used, the raw meal dries completely instead of partially calcining before it enters the kiln.

Ball Mill

The operation of the kiln represents the most critical part of the entire cement manufacturing process, not only for the purpose of ensuring the constant quality of the cement, but also to prevent damage to the kiln itself.

Kiln firing

Firing of the kiln can be by means of pulverized coal, gas or fuel oil, as well as alternative fuels such as paper, cardboard, vegetable matter, tires, etc. Coal for firing is stored in sufficient quantities to ensure an uninterrupted supply to the kiln. From the warehouse, coal is transported by belt conveyors to the mills where it is dried and ground. You must first go through the magnetic separators to remove any metal that could harm the mill. Various types of mills can be used to pulverize coal, air-swept roller mills are particularly suitable and are widely used.
Hot air to dry the coal during milling can be supplied by a heat generator, but whenever possible it is extracted from the kiln hood or clinker cooler to keep the energy reduction to a minimum.
The pulverized coal can be stored in a hopper or silo but is most often transported directly to the kiln where it is injected by air through a tube that extends to the ignition end of the kiln.
Ashes from pulverized coal are incorporated into the mixture in the kiln, and this must be taken into account when establishing the proportions of the mixture: the chemical composition of coal ashes is rich in silica, alumina and iron oxide and low in lime. Although the composition of coal does not vary much, there is considerable variation in the proportion of ashes produced by the different coals, and coals with a variable ash content create significant problems with the clinker prepared from a constant and carefully proportioned feed.

The rotary kiln operates continuously and reaches operating times that must exceed 94% of the available hours throughout the year. A planned stop occurs once or twice a year to perform basic maintenance and service the lining of refractory bricks.

In good operating conditions, the hottest part of the refractory lining is protected by a layer of molten clinker, and maintaining this protection is essential for a good service life of the refractory material. Under certain operating conditions, the lining may come off with the consequent risk of hot spots developing in the kiln shell. To avoid stops, then it is necessary to adjust the operating conditions and review the refractory conditions.

On the other hand, the coating may build up locally or can accumulate locally to an unacceptable degree, creating clinker rings, which will prevent the flow of material and gases through the kiln. Without the proper chemical composition, ring formation can occur. To avoid stops under these conditions, clinker rings can be removed by shooting them with a lead head gun specially designed for these conditions. You can also use a water jet system that uses pumps that develop extremely high pressures.

To avoid problems in the operations of the rotary kiln, therefore, it is essential to maintain very precise control over all aspects of the kiln process, especially during the start and stop of the kiln. This will necessarily require a high standard of mechanical and electrical reliability, as well as precision in the chemical control of the raw material feed and in the fuel supply.

 

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Kiln control

In times past, the control of the kiln operation was essentially an art rather than a science, largely depending on the expert operator or “burner”. The skill and experience of the burner remain of crucial importance, but modern instrumentation and control systems have made their work easier and more scientific.

Kiln burner firing

To monitor the temperatures in the combustion zone, for example, it is now a standard practice to use optical pyrometers that look through the inspection ports at the ignition end of the kiln; the temperature of the gas passing through the kiln is recorded by thermocouples in the kiln lining, and an optical scanning pyrometer can be located next to the kiln, opposite the combustion zone to detect possible “hot spots”.
The conditions in the kiln are controlled by adjusting a series of variables such as the amount of raw material and the speed at which the kiln is fed, the amounts and proportions of fuel and air injected through the firing end. These factors are monitored by weighers and flow meters, and the readings are displayed next to the kiln controls.
The kiln control itself is, in fact, only a couple, although an important part, of controlling a complete flow process that extends from the extraction of raw materials from storage to the clinker discharge in the clinker warehouse: an interruption or imbalance in any part of the process can create problems along the line. For this reason, in modern plants, the instrumentation screens and master controls that govern the entire process are duplicated together with the kiln controls so that the operator in charge can monitor and control the entire system and continuously scan a significant number of variables and notify the operator discrepancies and necessary corrective actions.
However, even with the most sophisticated instrumentation and data processing systems, the operation of a successful kiln still depends largely on the skill and experience of the operator in charge and its assistants.

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