Cement Manufacturing: wet process and dry process PDF

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Summary

Cement is manufactured using two methods based upon the mixing method i.e., wet process and dry process...

Description

CONCRETE TECHNOLOGY CEMENT Cement is a greenish grey coloured powder, made of calcined mixtures of clay and limestone. When mixed with water becomes a hard and strong building material. The function of the cement is fist to bind the sand and coarse aggregates together, and second to fill the voids in between sand and coarse aggregates particles to form a compact mass. It is the only scientifically controlled ingredient of concrete. The history of cement goes back into Roman Empire. The modern-day cement that is Portland cement was first produced by a British stone mason, Joseph Aspdin in 1824, who cooked cement in his kitchen. He heated a mixture of limestone and clay powder in his kitchen, and grind the mixture into powder creating cement, that hardens when mixed with water. The name Portland was given by the inventor as it resembles a stone quarried on the Isle of Portland. The first use of modern-day Portland cement was in the tunnel construction in the Thames River in 1828.

1. Portland Cement Portland cement is the most common type of cement which is a composition of an extremely fine-grained material obtained by the burning material(calcareous and argillaceous) together at high temperature, Gypsum is also added in raw material.

1.1.

Manufacture of Portland Cement

The process 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 a kiln at a temperature of about 1300 to 1500°C, at which temperature, the material sinters and partially fuses to form nodular shaped clinker. The clinker is cooled and ground to fine powder with addition of about 3 to 5% of gypsum. The product formed by using this procedure is Portland cement. The manufacturing procedures of Portland cement is done in four distinct steps described below: a) b) c) d)

Mixing of raw Material Burning Grinding Storage and Packaging

1.1.1. Mixing of raw material The major raw materials used in the manufacture of cement are calcareous materials and argillaceous materials which provides minerals like Calcium, Silicon, Iron and Aluminum. These minerals are used in different form as per the availability of the minerals.

Table 01. Raw materials for Portland Cement

The mixing procedure of the manufacture of cement is done in 2 ways depending upon whether the mixing and grinding of raw materials is done in wet or dry condition. • •

Wet Process Dry Process

Wet Process 1. In the wet process, Calcareous materials are crushed using crushers and argillaceous material is washed with water in the container. 2. After crushing, the lime stones are stored in silos and similarly after washing, the clay is stored in basins 3. The crushed materials from different silos and basins are drawn in correct proportions in a channel called wet grinding mills. Both the materials are intimately mixed in the presence of water and to from a fine thin paste known as slurry. 4. Slurry is then stored in another silo where the composition of raw materials is checked and can be adjusted if required. 5. The slurry contains around 38-40% water that is stored in storage tanks and kept ready for the rotary kiln. Dry Process 1. In dry and semi dry processes for manufacturing of cement, the raw materials are crushed and fed in the correct proportions into a grinding mill. 2. The raw materials are dried and reduced in size to a fine powder in to grinding mill. The dry powder is called the raw meal. 3. The raw meal is then further led to blending silo and blended and corrected for its right composition and mixed by means of compressed air. 4. The aerated powder tends to behave almost like liquid and in about one hour of aeration a uniform mixture is obtained and is termed as blended meal. 5. The blended meal is further sieved and fed into a rotating disc called granulator. 6. A quantity of water about 12 percent by weight is added to make the blended meal into pellets. In this manner hard pellet about 15 mm in diameter are formed. (This is necessary, as cold powder fed direct into kiln would not permit the air flow and

exchange of heat necessary for the chemical reactions of formation of cement clinker in the rotary kiln). 7. The pellets are fed into rotary kiln for further chemical reactions and conversion of the same into clinker. 1.1.2. Burning of Raw materials The burning process is carried out in the rotary kiln while the raw materials are rotated at 1-2rpm at its longitudinal axis. The rotary kiln is made up of steel tubes having the diameter of 2.5-3.0 meter and the length differs from 90-120meter. The inner side of the kiln is lined with refractory bricks. The kiln is supported on the columns of masonry or concrete and rested on roller bearing in slightly inclined position at the gradient of 1 in 25 to 1 in 30. The raw mix of dry process and corrected slurry of wet process are injected into the kiln from the upper end. The kiln is heated with the help of powdered coal or oil or hot gases from the lower end of the kiln so that the long hot flames is produced. As the kiln position is inclined and it rotates slowly, the material charged from upper end moves towards lower end at the speed of 15m/hr. In the upper part, water or moisture in the material is evaporated at 400oC temp, so this process is known as Drying Zone. The central part i.e. calcination zone, the temperature is around 10000C, where decomposition of lime stone takes place. The remaining material is in the form of small lumps known as nodules after the CO2 is released. 𝐶𝑎𝐶𝑂3 = 𝐶𝑎𝑂 + 𝐶𝑂2

The lower part (clinkering zone) have temperature in between 1500-17000C where lime and clay are reacts to yielding calcium aluminates and calcium silicates. These aluminates and silicates of calcium fuse to gather to form small and hard stones are known as clinkers. The size of the clinker varies from 5-10mm. The lower part i.e. clinkering zone has the temperature around 1500-1700C. In the region lime and clay reacts to yield calcium aluminates and calcium silicates. This products of aluminates and silicates of calcium fuses together to form hard and small stones known as clinkers. The size of the small and hard clinkers varies from 5 to 10mm.

2CaO + SiO2 = Ca2SiO4 (dicalcium silicate (C2S)) 3CaO + SiO2 = Ca3SiO5 (tricalcium silicate (C3S)) 3CaO + Al2O3 = Ca3Al2O6 (tricalcium aluminate (C3A)) 4CaO + Al2O3 + Fe2O3 = Ca4Al2Fe2O10 (tetracalcium aluminoferrite(C4AF)) The clinker coming from the burning zone are very hot. To bring down the temperature of clinkers, air is admitted in counter current direction at the base of the rotary kiln. The cooled clinkers are collected in small trolleys. Note: - The rate of cooling influences the minerology of clinker, i.e., the degree of crystallization, the size of crystals and the amount of amorphous materials. The minerology of clinker influences the hydration and strength of properties of cement considerably. A moderate rate of cooling results in high strength cements.

1.1.3. Grinding of Clinkers The cooled clinkers are finely ground in ball mills or tube mills.Also, the gypsum is added during grinding about 2-4%. The gypsum acts as a retarder and so allows the cement to mix with sand or aggregate and to be placed in position. i.e. it increases the initial setting time of cement. 1.1.4. Storage and Packing As cement comes out from grinding mills, it is collected in a hopper and taken in bucket elevator for storage in silos.The cement from silos is packed by machines in bags. Each bag of cement contains 50 kg or 0.035 m3 of cement.

Figure 1. Flow Chart of Wet Process for manufacturing of cement

1.2.

Difference between Dry Process and Wet Process

If we consider the quality and rate then wet process is better and if we consider fuel consumption and time of process then dry process is better.

D r y p ro ces s

1. Mixing of raw material in dry state in blenders using compressed air. 2. The dry materials exiting the mill are called “kiln feed”.

We t pro ces s

1. Mixing of Raw materials in wash mill with 35 to 50% water. 2. Materials exiting the mill are called “slurry” and have flowability characteristics. 3. Fuel consumption is low i.e., 100 kg of 3. Fuel consumption is high i.e., 350 kg of coal per tonne of cement produced coal per tonne of cement produced 4. Cost of production is less. 4. Cost of production is high 5. Capital cost is high due to blenders. 5. Capital cost (Cost of establishment) is comparatively less 6. Size of the kiln needed for 6. Size of the kiln needed for manufacturing of cement is smaller. manufacturing of cement is bigger. 7. Difficult to control mixing of Raw 7. Raw material can be mixed easily, so a materials, so it is difficult to obtain a better better homogeneous material can be homogeneous material. obtained

Summary Portland Cement is manufactured using two processes i.e., Wet Process and Dry Process....