The preparation of cement involves mining; crushing, and grinding of raw materials (principally limestone and clay); calcining the materials in a rotary kiln; cooling the resulting clinker; mixing the clinker with gypsum; and milling, storing, and bagging the finished cement. The process generates a variety of wastes, including dust, which is captured and recycled to the process. The process is very energy-intensive and there are strong incentives for energy conservation. Gases from clinker cooler are used as secondary combustion air.
There are two types of Cement Manufacturing:
The dry process, using pre heaters and pre-calciners, is both economically and environmentally preferable to the wet process because the energy consumption—200 joules per kilogram (J/kg)—is approximately half that for the wet process. Wet process is employed where raw materials contain high amount of moisture. In this process the kiln is a continuous stream process vessel in which the feed and fuel are held in dynamic balance.
Certain solid waste products from other industries, such as pulverized fly ash (PFA) from power stations, slag, roasted pyrite residues, and foundry sand, can be used as additives in cement production.
The various process involved in Cement Manufacturing can be summarized as under.
Limestone is the primary raw material in cement plants. Most plants would have their captive mines adjacent to the plant. The limestone has to be cement grade limestone, which is having all necessary oxides sufficient to produce cement clinker. The stone is blasted by explosives loaded into dumpers with the help of excavators and shovels. The Dumpers carry the limestone and dump into the hoppers of the crusher. The size of blasted limestone is preferably maintained at 1.0 Mtr x 1.5 Mtr. Higher size stone if any, is broken by Impactor.
Crushing and Stacking
This consists of hopper in which limestone is dumped. At the bottom of hopper is a grizzly feeder, which feeds stone above 150 mm size to the crusher. The crusher breaks the stone to less than 80 mm size and delivers it to a belt conveyor which takes the stone up to the stacker/reclaimer. The stone below 150mm size in the hopper goes to the screen house where the fraction of stones greater than 80 mm size is returned to the crusher and fraction of stone less than 20mm size is thrown away as reject.
The limestone is extracted from the pile by reclaimer and sent to the blended limestone hoppers, additives like high grade limestone, low grade limestone can be taken through a separate system of belts into their respective hoppers after weighment. These materials in the required proportion are extracted by weighing feeders and send to the vertical roller mills (VRM) where they are ground to a fine powder and the powder is collected in cyclone separators and the bag house. Through a conveyor system material is transferred to a continuous flow silo (CF Silo).
In this process homogenizing of the material is done and material is sent to the kiln at a constant feed rate. The powdered raw material is filled in blending silos and mixed thoroughly by air agitation to form a uniform material called Raw Meal. After chemical examination this is fed into storage silos. Chemical examination ensures that mixture is properly controlled with minimum variation.
Coal is used in the firing of pre-heater kiln. For this coal has to be pulverized. This is done using conventional ball mills to produce the desired fineness of fine coal.
This is done in multi-stage pre-heater kilns. The material comes in contract of hot gas coming out of kiln and temperature of material goes on increasing from first stage in the calciner. The temperature of calciner is maintained in the range of 1000 C to 1200 C.
The material from the pre heater kiln is transferred to the Rotary Kiln. After calcination the oxides slowly reach burning zone at about 1450 C and then react together to form Cement Clinker. The temperature is maintained constant by burning pulverized coal. The clinker is then cooled by blowing air to about 100 C and then shifted to storage yard.
Clinkers are sent to the cement mill where clinkers are mixed with gypsum and ground to a fine state called Ordinary Portland Cement and conveyed to storage silos. Cooled clinker is also mixed with gypsum and granulated blast furnace slag to form Portland Slag Cement.
Crushing equipments often shows signs of heavy wear, such as scratches and cracks in the breaker bars and impact plates. Pieces of rock that are too large may block the crusher rotor, which frequently results in rubbing damage to the jacket ribs and damage to the gears and shafts.
Tube mills and clinker grinding areas used for grinding the raw mix and clinker respectively are highly exposed to damage to the end walls (trunnions) at the entry and discharge ends. The high bearing loads and the effects of reversed bending forces may cause cracking in the transition radius area of the trunnions and the bearing journals.
In respect of rotary kilns, damage frequently occurs to the kiln jacket the riding rings and the support rolls If a riding ring is fitted too tightly, leaving too little clearance, the jacket sheathing may be constricted in operation. As a result cracks may form on the jacket and riding rings and the kiln lining may be damaged.
Explosion in Pre-heater and Cement Kilns though not common, can cause severe damages. The sheer size of a rotary kiln gives the enormity of the situation. A rotary kiln in a cement plant is the largest and hottest moving equipment in any manufacturing process in the world. The typical size of a rotary kiln is about 70 m with temperatures about 2000 C and hold up quantity of 6000 MT. Fortunately, it operates at less than atmospheric pressure. However in case of moisture content beyond permissible limits in the coal or feed explosion could be triggered.
Reduction gears and Rim Gears, Pinion shafts, gearwheels, and gear rims are highly susceptible to the effects of overloading during the start up phase. Also inadequate lubrication or lack of oil could cause failures. Teeth may be damaged at the flanks causing extended interruption of operations.
Where electrostatic filters are used special care should be taken to avoid the build-up of an explosive carbon-monoxide and air mixture so that explosions can be prevented.
Cement plants produce a great deal of dust and are frequently located close to the quarries supplying their raw material A build-up of dust can block the ventilation slits of the stator windings in electric motors resulting in overheating and breakdown of the machinery. Electric drives are used throughout the plant.
Cement Plant uses coal, lignite and coke for the process which are materials that are susceptible to spontaneous combustion. Proper ventilation should be provided for the material so that heat does not accumulate inside the material causing spontaneous combustion. For this heap of coal/coke/lignite should be overturned regularly. A provision to cool the heap during summer is recommended.
Lignite/Coal Crushing is done prior to using the material in the process. This generates considerable amount of dust which in combination with air can cause explosive mixtures causing dust explosion. There is also possibility of dust explosion in ESP's and cyclones used in the various mills. To mitigate dust explosions, proper study of the dust chemistry and properties is required. Some of the properties that need close study include, dust composition, dust particle size, moisture content, dust concentration, oxygen content, temperature, pressure, dust turbulence etc. A detailed analysis of the dust explosion scenario is suggested and suitable preventive action taken.
This should be properly attended for roller condition and for discharge of static charge accumulation. Jammed rollers can cause friction resulting in ignition of conveyor belts and spread of fire.
There is also probability of kiln fire and explosion since the process uses kilns in the process where fuels like lignite and coke are used. One of the main reasons for kiln explosion is the accumulation of dangerous fuel air mixtures inside the kiln and air ducting. To reduce the chances of explosion in furnaces and such other heating devices, it is strongly recommended that the safety aspects of the furnace operation are thoroughly analysed and suitable action taken. Entry of moisture content in the kiln should be prevented. There should be constant monitoring of kiln parameters like fuel, feed, kiln speed and gas flow.
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