Yıldıray YANIK
General Manager at DS Makina Sanayi ve Ticaret Limited Şirketi
In general, cement is described as a hydraulic binding material that is obtained by grinding a mixture of natural limestones and other components heated at high temperatures. After creating a solid mass following a reaction with water, hydraulic binding materials turn into something that is not dissolved in water and that maintains/increases its rigidity and durability.
The first stage in cement production is to extract the suitable raw materials from the earth’s crust and prepare them for reactions to be carried out at high temperatures. Among the main raw materials used in cement production are CaO (calcium oxide), SiO2 (silicon dioxide), A1203 (alumina), and FeO3 (ferric oxide). Raw materials extracted from the earth’s crust are divided into micron-sized particles so that they can better enter heat and mass transfer processes. Such raw materials subject to physical change enter into series of chemical reactions at temperatures of up to 1450°C and produce the basic cement component known as clinker.
And the process of mixing and grinding the produced clinker with gypsum (CaSO4.2H2O) provides cement.
Normally the cement compounds include the following:
CaO + SiO2 …………………………………………………….……………………. 84%
Al2O3 + MgO + Fe2O3 ………………………………………………….……….. 12%
SO3 + Na2O + K2O + NaO2 + TiO2 + P2O5 ……………………….…………. 4%
Total…………………………………………………………………………………. 100%
Plants are generally built according to the nearest limestone quarries so as to minimize transportation needs. Some materials having the hydraulic binding features used in cement production can also be beneficial in producing mixed cement types. For example, “alternative” raw materials such as fly ash and slag are brought from other sectors.
- Klinker ve dolayısıyla kireçtaşı yataklarının tasarrufu,
- Diğer sanayideki “atık malzemeleri” kullanmakla çevre temizliğini sağlanabilir.
Mixed cement not only meets the requirements of normal Portland cement, it also offers an additional advantage as it discharges less heat during hydration – a feature that makes it suitable for “mass structures” such as dams.
The basic stages of cement production can be listed as follows:
- Raw Meal preparation
- Clinker baking
- Cement grinding
RAW MEAL PREPARATION
Crusher Unit
Crushing plants can be built in or near a quarry. The first crushing process generally uses hammer crushers and, sometimes, jaw crushers. Roller crushers and cone crushers can also be preferred for materials that are in proper rigidity and moisture. Hammer crushers and jaw crushers are preferred for rigid and abrasive materials. Hammer crushers, in particular, can crush very big particles to under a fineness of 30 mm. In general, double-rotor big hammer crushers are chosen to act as the first crusher. The crusher capacity in cement production is generally chosen in 8 operation hours, in other words, in a manner to make one shift meet the 24-hour raw material need of a plant. Limestones, marls, or clays to be extracted from quarries are fed into crusher bunkers. The ratio of limestones and marls is arranged via steel belts in accordance with desired chemical composition content and fed into a material crusher. Materials passing through a double-rotor crusher are forwarded to a pre-blending unit with the help of transfer systems. Dusts appearing during material crushing are contained using a bag filter and, as a result, de-dusting is ensured.
General equipment available in a crusher unit:
– Limestone or other components feeding bunkers
– Below bunker crusher feeding steel belts
– Double-rotor hammer crusher
– Rubber belt conveyor systems
– De-dusting filters
Blending Stockyard
Materials, which are in a desired chemical composition and reduced in size in a crusher, are piled in a circular blending plant using a stacker.
Homogeneous materials obtained from each point of the cross-section surface of a pile using a rake are taken to a raw mill feeding bunker on rubber belts using a reclaimer.
General equipment available in a blending unit:
– Stacker
– Reclaimer
– Rubber Belt Conveyor Systems
– De-dusting Filters
Raw Meal Grinding and Storage
A raw material mixture is ground first so as to be efficiently heated and calcined. The thin material, which appears after the grinding process and whose main components are lime and silica, is called raw meal. Lime enters the environment more from calcium carbonate-including rocks such as limestones or marls. As for the silica, clay is the main resource. These are followed by alumina and ferric oxide. Other materials such as magnesium and alkali oxides are also available, however in small quantities.
Grinding process is carried out in ball or vertical mills. Homogeneous materials are transferred to raw mills from bunkers in order to be ground, and they are ground together until they turn into fine dust particles. Raw material and raw chemistry is of great importance to ensure top quality cement. For a more efficient grinding process, kiln exhaust gases are transferred to the raw mill and the raw material is dried. But this procedure needs to be done using a hot gas producing generator when the kiln is out of use. The product taken from the raw mill is forwarded to raw meal silos to be storage. Fluid silo systems are used in order to benefit from the fluid characteristic of raw. The silo is homogenized by providing air in a pressure that is sufficient enough to ensure a fast mixing. The mixing and percentage rate adjusting system where the produced raw material does not show a chemical or physical change but does show a constant flow is called the homogenization system. Fluidity begins when the weight of the unit areas of the in-silo particles is equal to the pressure drop deep in the silo. Such system ensures a very efficient mixing process and helps particles in different size, shape, and density mix into each other within certain limits. The raw obtained from the raw silo is first taken to the scale bunker and then to the weight feeder via elevators. The raw, which is automatically weigh with belt weight feeders, is finally transferred to pre-heaters via airlift.
General equipment available in a raw mill and storage silos unit:
– Feeding bunkers; premix, limestone, and iron ore
– Dosing Belts
– Rubber Belt Conveyor Systems
– De-dusting Filters
– Mill Feeding System
– Raw Mill
– Raw Cyclones
– System Filter and Fan
– Gas Pipes
– Raw Transfer Air Slides
– Raw Silo
Preheater-Rotary Kiln-Clinker Cooling
Raw baking is the most important stage of cement production. The ground raw material is transferred from silos to the preheater system in order to be baked at a high temperature. In modern cement factories, raw is subject to pre-heating before it goes into the kiln. This procedure is carried out to save energy. Pre-heating is the first part of the baking system. A pre-heater consists of vertical cyclones lined one after another. As raw goes down from these cyclones, it contacts the turbulent hot gases discharging from the kiln and moving in the opposite direction and, as a result, the heat is transferred from the gas to the materials.
The pre-heater heats the raw material before it goes into the kiln and, thus, the necessary chemical reactions develop in a more efficient and faster manner. Energy is also saved by keeping the energy from exhaust gases. 3 to 6-staged cyclones may be required for a kiln depending upon the moisture of the raw material. Calciner (pre-calcination system) is a combustion chamber located on the kiln edge at the bottom of the pre-heater. Up to 65 percent of the total energy need of the kiln system can be supplied for the calciner. Using calciners makes it possible to use short rotary kilns and low-grade alternative fuels. Calcination is the separation of CaCO3 and CaO and the discharge of CO2. The heat must be provided according to the following chemical reaction in order for the calcination process to complete partially in the pre-calcination system and in the kiln.
CaC03 = CaO + CO2 – 420 kcals
The rate of the calcination process of the raw entering into the pre-calcination unit from the final pre-heating cyclone stage along with hot air and fuel is based on the rate of the fuel consumed. In general, when 60 percent of fuel is ignited in the calciner, the obtained calcination rate becomes 90-95 percent. The hot air required for the ignition is taken to the cooling unit via the kiln or the tertiary duct. The substances produced and the CO2 separated throughout the calcination are drawn towards the smokestack using the pre-heating fan, and the partially calcined raw is separated from gases and fed into the kiln. The raw material called the raw or, in this stage, more correctly the “hot raw” enters into the rotary kiln after the pre-heating process. Rotary kilns are considered the largest process elements available in industrial plants in the world. A rotary kiln is an equipment with refractory brick lining. Rotary kilns shell are made from steel with a thickness of 40 to 80 mm and can go up to 3-7 meters in diameter and 50-75 meters in length (generally 70 percent of alumina or magnesite brick is used in the combustion area, and the remaining parts of the kiln are covered with alumina brick at a level of about 35 percent). The kiln mounted with a slope of about 3-4 percent rotates with a cycle of 1.5-4 per minute. The partially (at a level of about 90 percent) calcined material coming from the pre-heating enters the kiln from the upper edge, and rotates, rolls, and moves together with the kiln to proceed to hotter areas where the lower edge burner is found. In the meantime the remaining CO2 is also removed from the material to complete the calcination process. Fuel is directly pulverized into the rotary kiln and absorbed into the material, as is the case with the ash calciner. Alternative fuels such as coal, petroleum coke, gas and alike, plastics, solvent, waste oil are ignited to feed the flames that can reach a high temperature of up to 2000°C. This high temperature causes chemical and physical changes and helps the materials in the kiln melt in part and turns the raw into a substance called clinker. In the simplest term, a series of chemical reactions turn calcium and silicon oxides into calcium silicates, which are the primary components of cement. At the lower point of the kiln,raw materials appear as hot particles called clinker. Components such as alkali and chloride which are found in small amounts in raw begin to evaporate in the combustion area of the kiln but they are cooled off and intensified again at the kiln entrance and the kiln hood. Unless a balance is provided, inner circulation increases due to the repeating cycles of evaporation and concentration. The main compounds consist of substances such as potassium, sodium, chloride, and sulfur, and if they are not taken out of the process they affect the characteristics of clinker. If alkalis continue to intensify, serious residues can appear on the pre-heater cyclone walls, in the channel in between the two below cyclones, and inside the kiln entrance channel and, thus, gas passage can be blocked on the system. Bypassing a certain part of the kiln gases at the kiln entrance (a certain part of the kiln exit gas is taken out of the system) is a way to overcome this problem, however, bypassing increases fuel consumption. The temperature of the gases requiring a bypass process here is at about 1000C and those gases need to be cooled off through air or at a cooling tower before they enter into a dust catcher. Problematic situations are very less in a system with a pre-calcined unit as only 40 percent of gases passes from a kiln in such system. Therefore, such system allows for the usage of materials that are high in alkali and chloride. Cement production processes have two main types and different kiln types. Cement production can be “wet” or “dry” depending upon the water content of the raw material. The wet process requires a much higher energy due to the amount of the slurry water necessary to be evaporated before the calcination process can be carried out. As a result, the dry process requires less energy. The clinker taken out of the kiln is carried to the cooling unit. Clinker gets out of a rotary kiln at about 1300 C. After this stage it is proceeded to cool off the clinker and gain the heat in the environment. The most frequently used method here is to blow pressurized free air into the clinker particles slowly moving on grille sheets. It is of great importance to have a controlled cooling rate as it will affect the internal structure of the clinker. After the clinker is cooled off it becomes ready for the grinding process, which will help it turn into a gray dust known as Portland cement. The produced clinker is kept in clinker stockyard.
General equipment available in a clinker baking unit:
– Pre-heating Group
– Rotary Kiln
– Clinker Cooling
– Ignition System
– De-dusting System
– Coal Mill
CEMENT GRINDING
Traditionally, ball mills are used for cement grinding. Recently, mills have been developed to boast better energy efficiency and technology. Combined mill systems known as vertical mills are a different grinding equipment used to reduce electricity consumption. Modern ball cement mills are manufactured to have two or three cabins. In a two-cabin mill, the first cabin is used to grind to a sufficient fineness and the second cabin is used for the final grinding process. In order to ensure such process, utmost attention should be paid to the selection of mill sheet designs and ball sizes.
Clinker taken from clinker stackhalls is transferred to cement mill feeding bunkers through rubber belts. Feeding bunkers consist of clinker, plaster, and additive bunkers. Materials taken from below bunker are dosed and transferred to a cement mill. Materials ground in a mill come out as cement and are then transferred to a cement silo. It is the cement silos where cement brought to desired sizes in cement mills is stocked as a ready-for-sale product. The ready-for-sale cement is later transferred from silos to packaging units.
Finally the produced cement is sent to the market as bulk or packaged cement.
Bulk cement should be stocked in a clean, closed, and damp-free place. Bulk cement are directly loaded on vehicles by opening the discharging doors available under cement silos.
Packaged cement, on the other hand, is given to bunkers from cement silos through air slides or elevators. The cement is poured into a scale reservoir from bunkers and the poured cement is automatically put in packages with the help of filling nozzles, and the packages are arranged to weight 50 kg.
General equipment available in a cement grinding unit:
– Feeding Bunkers
– Dosing Equipment
– Cement Mill
– Separation and Filtering System
– Cement Stock Silos
– Bulking and Packaging System
Source: Many thanks to ESA-GROUP’s Bülent Tuncer for his support.
