The cement industry in India is a highly capital-intensive industry with several huge plants that have one million tonnes per annum capacity. There are about 97 of such plants making a total of 97 million tonnes per annum. Each of these huge plants cost as much as Rs 900 crores to install.
The government needs to do more to reduce the cost of setting up cement plants.
Cement production should be made such that common people in rural areas can produce it. But the consensus impression is that cement cannot be produced without setting up huge and very expensive cement plants.
But in reality, cement is a simple product that can be produced using cheap cement machinery with simple technology. For instance, a single one tonne per day plant producing about 20 bags per day would be suitable for rural dwellers. If you up the ante a bit, you could add solar energy to the plant and produce cement at Rs 50 per 60 kg bag.
How Cement Is Made
Let’s take a look at how cement is made. The main raw material for cement production is limestone. Limestone is primarily calcium carbonate which decomposes to yield calcium oxide and carbon dioxide at about 900 degrees Celsius. Clay, which contains silica, is added to the calcium oxide and reacts with it to produce calcium silicates and some other compounds. Cement is a by-product of some of those compounds.
How Portland Cement Is Made
Limestone is first pulverized into very fine powdered form. Clay and coal are also grounded into fine powdered forms. All these three raw materials – limestone, clay and coal are then thoroughly mixed to obtain a perfectly homogenous blend. For one tonne of cement, the mixing ratios should be 1400 kg limestone, 250 kg clay and 200 kg clay. The best quality of the cement produced would be a function of the three ingredients and the percentage of the calcium silicates.
In technical terms, you have to balance and control the following parameters: lime saturation factor (LSF), alumina modulus, silica modulus, free lime percentage, liquid content etc. to be able to get the best Portland cement quality.
The pulverized powdered forms of limestone, clay and coal can be mixed thoroughly in an inclined cylinder with a skewed shaft or a double cone mixture which are simple cement machinery, but they provide a homogenous mixture nonetheless. One they have been thoroughly mixed, they are then modulized into balls of 1 cm diameter by adding some water to the mixture and placing it in a nodulizer – an inclined revolving pan, which is another simple cement equipment.
When the nodules are ready, they are put into the vertical kiln for firing. There are two types of kilns: the batch type and the continuous type. The kiln is fired from the top, and a blast of cold air is added from the bottom to feed it with oxygen. Cement is obtained at about 1400 degrees Celsius. The nodules still contain coal which would have to be burnt off. So in the end, the clinker would have no coal but would contain several compounds ( mainly dicalcium silicates and tricalcium silicates).
When the clinker has cooled, it is pulverized again, and about 3% of gypsum is added to retard the processing of setting mortar. The cement sample is then taken to the lab to analyze it for the following: compressive strength, expansion property, fineness, magnesium content, free lime content, loss on ignition, and total sulphur content. This is how best quality Portland cement is made.
In making good quality Portland cement, the following simple cement machineries are used:
Pulverizer is common in the market or could be fabricated by a local blacksmith.
Double cone mixer or skewed shaft with horizontal cylinder
Readily available in local markets. Can also be fabricated.
A simple revolving inclined pan which can be fabricated locally.
Cement Burning Kiln
It should be designed properly with accurate dimensions. Then your local blacksmith can help fabricate it.
In conclusion, there are several benefits obtainable from decentralizing the cement production process. Limestone burning is an exothermic process which requires a lot of heat. If decentralized, the cost of cement production can be reduced by as much as 25%. If cement production is deployed to villages and rural areas, it would lead to development, boost the economy, arrest rural-urban drift and create jobs in these rural villages.