Manufacture of Portland Cement

Cement Factory

The raw materials required for the manufacture of cement are calcareous materials, like limestone or chalk, and argillaceous materials like shale or clay. Cement factories are established where these raw materials are available in plenty. Cement factories have come up in many regions in India, eliminating the inconvenience of long-distance transportation of raw and finished materials. the method of manufacture of cement consists of grinding the raw materials, mixing them intimately in certain proportions depending upon their purity and composition, and burning them in an exceeding kiln at a temperature of about 1300 to 1500°C, at which temperature, the material sinters and partially fuses to make nodular shaped clinker. The clinker is cooled and ground to a fine powder with the addition of about 3 to 5  gypsum. the merchandise formed by using this procedure is Portland cement. There are two processes referred to as “wet” and “dry” processes depending upon whether the mixing and grinding of raw materials are worn out in wet or dry conditions. With a touch change within the above process, we've got the semi-dry process also where the raw materials are ground dry so mixed with about 10-14 percent of water and further burnt to clinkering temperature. for several years the wet process remained popular due to the chance of more accurate control within the mixing of raw materials. The techniques of intimate mixing of raw materials in powder form weren't available then. Later, the dry process gained momentum with the trendy development of the technique of dry mixing of powdered materials using compressed gas. The dry process requires much less fuel because the materials are already in an exceedingly dry state, whereas within the wet process the slurry contains about 35 to 50 percent water. To dry the slurry we thus require more fuel. In India, most of the cement factories used the wet process. Recently some factories are commissioned to employ the dry process method. Within the next few years, most of the cement factories will adopt a dry process system.

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Wet Process

In the wet process, the limestone brought from the quarries is first crushed into smaller fragments. Then it is taken to a ball or tube mill where it is mixed with clay or shale as the case may be and ground to a fine consistency of slurry with the addition of water. The slurry is a liquid of creamy consistency with a water content of about 35 to 50 percent, wherein particles, crushed to the fineness of Indian Standard Sieve number 9, are held in suspension. The slurry is pumped to slurry tanks or basins where it is kept in an agitated condition through rotating arms with chains or blowing compressed air from the bottom to prevent the settling of limestone and clay particles. The composition of the slurry is tested to give the required chemical composition and corrected periodically in the tube mill and also in the slurry tank by blending slurry from different storage tanks. Finally, the corrected slurry is stored in the final storage tanks and kept in a homogeneous condition by the agitation of the slurry. The corrected slurry is sprayed onto the upper end of a rotary kiln against hot heavy hanging chains. The rotary kiln is an important component of a cement factory. It is a thick steel cylinder of diameter anything from 3 meters to 8 meters, lined with refractory materials, mounted on roller bearings, and capable of rotating about its own axis at a specified speed. The length of the rotary kiln may vary anything from 30 meters to 200 meters. The slurry on being sprayed against a hot surface of a flexible chain loses moisture and becomes flakes. These flakes peel off and fall on the floor. The rotation of the rotary kiln causes the flakes to move from the upper end towards the lower end of the kiln subjecting itself to higher and higher temperatures. The kiln is fired from the lower end. The fuel is either powered by coal, oil, or natural gas. By the time the material rolls down to the lower end of the rotary kiln, the dry material undergoes a series of chemical reactions until finally, in the hottest part of the kiln, where the temperature is in the order of 1500°C, about 20 to 30 percent of the materials get fused. Lime, silica, and alumina get recombined. This is where the oxides in raw materials will be combined to form compounds in the clinker. The fused mass turns into the nodular form of size 3 mm to 20 mm known as clinker. The clinker drops into a rotary cooler where it is cooled under controlled conditions The clinker is stored in silos or bins. The clinker weighs about 1100 to 1300 gms per liter. The liter weight of the clinker indicates the quality of the clinker. The cooled clinker is then ground in a ball mill with the addition of 3 to 5 percent of gypsum to prevent flash-setting of the cement. A ball mill consists of several compartments charged with progressively smaller hardened steel balls. The particles crushed to the required fineness are separated by currents of air and taken to storage silos from where the cement is bagged or filled into barrels for bulk supply to dams or other large work sites. In the modern process of grinding, the particle size distribution of cement particles is maintained in such a way as to give a desirable grading pattern. Just as the good grading of aggregates is essential for making good concrete, it is now recognized that a good grading pattern of the cement particles is also important.

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Dry Process

Dry Process In the dry and semi-dry process the raw materials are crushed dry and fed in correct proportions into a grinding mill where they are dried and reduced to a very fine powder. The dry powder called the raw meal is then further blended and corrected for its right composition and mixed using compressed air. The aerated powder tends to behave almost like liquid and in about one hour of aeration, a uniform mixture is obtained. The blended meal is further sieved and fed into a rotating disc called a granulator. A quantity of water about 12 percent by weight is added to make the blended meal into pellets. This is done to permit airflow for the exchange of heat for further chemical reactions and conversion of the same into clinker further in the rotary kiln. The equipment used in the dry process kiln is comparatively smaller. The process is quite economical. The total consumption of coal in this method is only about 100 kg when compared to the requirement of about 350 kg for producing a ton of cement in the wet process. In March 1998, in India, 173 large plants were operating, out of which 49 plants used wet process, 115 plants used dry process and 9 plants used semi-dry process. Since the time of partial liberalization of the cement industry in India (1982), there has been an up gradation in the quality of cement. Many cement companies upgraded their plants both in respect of capacity and quality. Many of the recent plants employed the best equipment, such as a cross belt analyzer manufactured by Gamma-Metrics of USA to find the composition of limestone at the conveyor belts, high-pressure twin roller press, six-stage preheater, precalciner, and vertical roller mill. The latest process includes stacker and reclaimer, on-line X-ray analyzer, Fuzzy Logic kiln control system, and other modern process control. In one of the recently built cement plants at Reddypalayam near Trichy, Grasim Industries, employed a Robot for the automatic collection of hourly samples from 5 different places on the process line and help analyze the ame, throughout 24 hours, untouched by men, to avoid human errors in quality control. With all the above-sophisticated equipment and controls, consistent quality of clinker is produced. The methods are commonly employed for direct control of the quality of clinker. The first method involves reflected light optical microscopy of polished and etched sections of clinker, followed by point count of areas occupied by various compounds. The second method, which is also applicable to powdered cement, involves X-ray diffraction of powder specimen. Calibration curves based on known mixtures of pure compounds, help to estimate the compound composition. As a rough and ready method, liter weight (bulk density) of clinker is made use of to ascertain the quality. A liter weight of about 1200 gms. is found to be satisfactory. It is important to note that the strength properties of cement are considerably influenced by the cooling rate of the clinker. This fact has of late attracted the attention of both the cement manufacturers and machinery producers. It can be seen from Tables 1-2 that a moderate rate of cooling of clinker in the rotary cooler will result in higher strength. By moderate cooling, it is implied that from about 1200°C, the clinker is brought to about 500°C in about 15 minutes, and from the 500°C, the temperature is brought down to normal atmospheric temperature in about 10 minutes. The rate of cooling influences the degree of crystallization, the size of the crystal, and the number of amorphous materials present in the clinker. The properties of this amorphous material for similar chemical composition will be different from the crystalline one.