09-11-2012, 01:32 PM
Cement Manufacturing Process
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Ordinary Portland cement is produced by grinding cement clinker in association with gypsum (3-5%) to specified fineness depending on the requirements of the cement consumers. Cement clinker is produced on large scale by heating finely pulverised Calcareous and Argillaceous materials at very high temperature upto 1450oC in rotary kilns. The Calcarious and Argillaceous materials obtained from the earth are properly proportioned to get a suitable ratio of lime (CaO), Silica (SiO2), Alumina (Al2O3) and Iron (Fe2O3) present in the mixture. As the raw materials are obtained directly from limestone and clay mines, minor constituents like Magnesia (MgO), Sodium, Potassium, Sulphur, Chlorine compounds etc., may also be present in the raw materials upto limited extent which do not adversely affect either the manufacturing process or the quality of cement produced.
MINING OF LIMESTONE
As mentioned above the major quantity of limestone is obtained from the captive limestone mines of the plant. However, depending upon the proportions of different cement clinker phase forming components, the additive materials including high grade / low grade limestone are purchased from outside parties in required quantities in order to obtain the desired quality of cement grade raw meal. The methods of mining are discussed as under.
MINING METHODS
Limestone mining operation traditionally is carried out generally as Opencast mining in India. However, newly developed Eco-friendly Surface Miners also find use in limestone mines today in India cement plants.
DRILLING / BLASTING METHODS OF MINING
Out of the eight cement plants of India Cements Limited, seven plants are raising the limestone from their captive limestone mines by using drilling and blasting techniques. Sequential Timer Blasting Machines are used for blasting operations. This timer machine reduces the problem of noise, ground vibrations and improves fragmentation by introducing inter row delays. Noise, dust and illumination surveys are conducted at regular intervals in our mines to keep the same under control. Instruments used for noise and surveys include Noise Dosimeter and Gravimetric Dust Sampler respectively. Equipments used in this method of Open cast Mining includes HM-25 dumpers, Poclain & Demag shovels, Hindustan and Beml loaders, Beml dozers etc. The limestone produced by this method are in the form of big boulders having their physical dimensions may be from 1.5 meter to small fragment size upto few millimeters. Therefore before taking the limestone to grinding machines boulder crushing in one or two stages is required for reducing the size of limestone for preparing suitable feed size to grinding mills.
MINING WITH SURFACE MINER
Surface miners were firstly developed for road making and are now used for limestone mining operation also. Being ecofriendly machine, Surface Miners eliminate the problems associated with drilling, blasting, crushing & loading and the complaints associated with their activities. There are three surface miner machines working in Adanakurichi Limestone Mine of ICL’s Dalavoi works. The capacity of each machine is 200 tonnes per hour limestone.
The surface miner is a track-mounted machine with a powerful diesel engine and hydraulic pumps for delivering the power to the cutting drum. The cutting drum is made up of a special alloy steel with replaceable Tungsten Carbide cutting tools (spikes), which can be quickly detached or fixed. The drum can be lowered or lifted by hydraulic system with powerful hydraulic motors thereby varying the depth of cut. Dust is suppressed at the source itself by water spraying on to the milling drum thereby making it an environment friendly machine. As the cut materials are of uniform size and the impact on the crusher while crushing is reduced. Whenever, there is an interstitial reject band encountered, the same is eliminated by loading and dumping it separately in the specified dump thereby facilitating Selective Mining with this machine.
CRUSHING OF LIMESTONE
The big boulders produced during drilling and blasting methods of limestone mining are crushed in suitable type of crushers. The crushing is carried out either in single or double stages by using Primary crusher and Secondary crusher, or in a single stage crushing machine depending upon the size of the boulder produced while mining. This also depends on the type of grinding mills used for grinding of raw materials for preparation of finally pulverized raw meal. Jaw crushers as well as impact crushers / hammer crushers of different capacities are employed in India Cements Ltd., for reduction of size of limestone boulders to a suitable feed size acceptable to the different types of grinding machines installed in plants. The crushers are mainly installed at the plant site. The limestone produced in the mine is transported to crusher site with the help of dumpers and tippers of different capacities. In some mines the crushers are installed at mine site and crushed limestone is transported to plant stackpile with the help of Belt conveyor / Ropeway. Figure below illustrates the raw limestone mining and crushing at a limestone mine of a cement plant.
PREHOMOGENISATION
The crushed limestone is then transported to stacker reclaimer site with the help of belt conveyor / rope ways installed at different plant site. Finally the crushed limestone is pre-blended with the help of stacker and reclaimer systems. The crushed limestone travelling on the belt conveyors is stacked in layers with the help of stacker machine, which moves to and fro along the side of stacking yard. The stacked materials is then cut in slices with the help of a reclaiming machine which mixes the layers of stacked limestone which reduces the variation in quality of limestone as compared to the large variations obtained in the limestone obtained from mines. The full fledge stack pile ready for reclaiming and feeding to raw mill hoppers can be seen in the above figure.
GRINDING OF RAW MATERIALS
The pre-blended limestone from stack pile is transported to raw mill hoppers. More than one hoppers are used for proportioning of raw mix incase the limestone is obtained from more than one sources or purchased sweetner or additive materials are required to be mixed with captive mines limestone. Presently Raw mill hoppers are provided with continuous weighing machines known as weigh feeders in order to produce a suitable raw meal proportioned appropriately for production of desired good quality of cement clinker. Vertical Roller Mill and Tube Mill Grinding machines are used for production of pulverized raw meal at India Cements Plants.