Preparation of Raw Materials

Limestone and other reoccurring raw substances are normally received into the plant in the size range of 1 to 2 mm from the quarry. Production of a mixture of raw materials with a maximum size of about 0.2 mm is done by breaking down the sizes of the raw materials derived from the quarry. The ratio of reduction to which these raw materials need to be 1000-2000: 0.2 (Innovations in Portland Cement Manufacturing,2009). This breaking down of raw materials is difficult to be achieved by one stage operation. Therefore, after the reduction stage it should be ensured that raw mix has a good homogeneity to achieve superior quality product and increase in the efficiency of the plant. Stocks need to be regularly built to keep the continuity of the manufacturing process at various points.

The process flow for the preparation of raw materials is a multi-stage process and involves the following steps

Crushing is the basic process which involves reduction in the size of raw materials and this process is carried out by a mechanical equipment inside the plant. Crushing is done in three stages:

1. Name
2. Product
3. Size

Size Range (mm)

• Coarse Highest 200-150
• Intermediate Medium 70-40
• Fine Small 25-5

Compression, attrition, impact combined or individually affect the crushing process in a combination or each individually. Primary reduction of raw materials in a cement plant is done by crushers namely single or twin rotor or impact crushers. In case of raw materials which are hard other type of crushers are used named as Jaw crushers, to break down abrasive materials the jaw crushers are used along with roll crushers.

The necessity of pre-homogenizing the raw materials in a cement production after the secondary crushing is very important as it evens out the materials as their chemical composition varies. Limestone and component of aluminosilicate are the main raw materials for which this unit operation is carried out.

Another name for the pre -homogenizing systems is “blending beds”. The ratio between the input significant chemical parameter of a given stockpile (Sin) to the standard deviation of the output parameter for the same stockpile (Sout) is known as the Blending Effect (H).Blending Effect (H) = Sin/Sout Measuring the homogenizing effects which are greater than 1: 10 is not possible as inaccuracies take place during the analysis and sampling of the lumpy materials. However, in practice, the efficiency of blending of these systems is within the required range (Innovations in Portland Cement Manufacturing, 2009).Two types of grinding namely dry and wet grinding further crush the preblended raw materials into a fine mixture. Most common type of grinding used is dry grinding as wet grinding process is energy intensive which leads to excessive cost of energy usage.

Balls Mills are used majorly used all over the world for grinding process. These mills consist of rotating cylinders which are full of steel balls, as the material to be grinded enters these cylinders due to the attrition from the grinding balls present in it there is an impact on the material and the grinding takes place. The peak usage of the ball charge energy for fine grinding determines the output and efficiency of the ball mills (Innovations in Portland Cement Manufacturing, 2009).The major step in the clinker production process is the homogenization of the raw meal before the calcination process. All over the world there are 4 methods which are widely known in the cement industry for the raw meal homogenization. Firstly, in the wet process comes the slurry mixing which is the most traditional method, all the other methods that include gravity systems, mechanical systems come under the dry process plants (Innovations in Portland Cement Manufacturing, 2009).In the homogenization of slurry, the slurry from the is transported to the slurry tanks, the tracking of the average composition of the slurry is done by chemical analysis of on-spot samples. As the slurry tank is filled about 2/3th it is compresses with air for the homogenization of slurry. Before entering the kiln, the slurry from the tank is again sent into a basin for further homogenization which is done by using mechanical and pneumatic components. Working of the basin is related to the working of an ideal blender.

Multiple storage slurry tanks are installed with regulated facilities of withdrawal and are all present in a mechanical system. Regular withdrawal of the material from the tank or silos at variable rates is done to achieve blending. Usage of this type of mixing uses less power, but the power consumption increases as these systems require high material handling. Adding to the information, the number of silos required in this system is more compared to other systems. Summing up all these shortcomings this type of homogenization system is not widely used in the cement industry (Innovations in Portland Cement Manufacturing, 2009). Air fluidization method based on pneumatic unit is the most common type of homogenization method used over the past several decades. In this system, the raw meal almost behaves like a fluid due to the air which is introduced through a permeable medium in the bottom of the silo. This method is done to increase the blending efficiency for a dry material, in some cases this method is also the highest power consumer (Innovations in Portland Cement Manufacturing,2009).

Reduction in power consumption is only way which paved for the homogenization through the gravity approach.2Onsite and offsite equipment control almost all off the above unit operations. An important part of the raw materials preparation facility is the control laboratory which needs to be equipped with proper software and hardware. (Innovations in Portland Cement Manufacturing,2009).The quality of the finished product is dependent on the preparation of a proper and uniform kiln feed and requires good monitoring about the quality of the raw materials entering the kiln, the preblending stockpile composition, size distribution of the particles and the variation in the chemical composition of the raw milk output and the consistency and the efficacy of the homogenization system in terms of the output which forms the kiln feed. Sampling arrangements are used to control and monitor the system with the help of computer software and specialized instruments.

After the process which includes the selection, blending, homogenization and grounding the raw materials into a fine uniformed kiln feed and for the clinkering reactions to take place this feed is subjected to tremendous heat. Starting from the extraction of the kiln feed from the storage which is then weighed and moved to the kiln, and the extraction of the clinker on its way to the clinker storage from the cooler, this entire process is known as the Pyro-Processing stage and schematic diagram given below represents the process.