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Reconnaissance Surveys and Site Studies
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Quantitative and Qualitative assessment
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Engineering and Process Design Review
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Project Monitoring and Implementation
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Auditing, Optimization and Up-gradation
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CEMENT PROCESS
What is Cement?
Cement which is commonly used in construction industry is a binder, which sets and hardens after combining with water and gains strength and stability.
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Cement can bind together bricks, stones etc. in building construction.
It is generally known as Portland cement because of its resemblance with naturally occurring stone in Portland, south of England where it was developed in the 19th century. It is now the most popular and widely used construction material in the world.
How is it produced?
Portland cement is produced primarily from limestone, certain clay minerals and gypsum in a high temperature process
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that drives off carbon dioxide and chemically combines the primary ingredients into new compounds. These compounds are hydraulic in nature i.e. they set on coming into contact with water and form a hard rock like, mass.
PROCESS OF CEMENT MANUFACTURE
Raw Material Quarries and Quarrying
Economics of cement production dictates that the cement plant should be located in proximity to the limestone deposit, which is the main raw material; the other raw material is clay/shale. Read More
Approximate proportion of these two components for producing cement is about 75% limestone and 25% shale/clay.
The other components such as iron ore, silica sand etc. can be used in smaller proportions, incase there is a deficiency of any mineral in the main components.
The limestone, which generally exists in the form of rocks and boulders, is fragmented by drilling holes and using high explosives for blasting. The softer limestone rock is generally quarried by ripping. The smaller size stones are then loaded into dump trucks by shovels, excavators or loaders and unloaded into the crusher.
Crushing of Raw Material
There are several types of crushers, such as hammer crusher, impact breakers etc. The crusher reduces the size of stone to 25mm or 75mm, depending upon the type of raw grinding mill. In certain cases, crushing is carried out in two stages i.e. primary and secondary.
Pre-blending of Raw Material
The crushed raw material is discharged from the crusher on to a belt conveyor to storage yard, where it is stacked by a stacker in a longitudinal or circular pattern
in material layers of known quality and quantity for obtaining a homogenized bed of pre-designed uniform chemical composition.
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This is an important requirement to maintain normal process conditions in the kiln and for quality assurance of cement.
Reclamation of the material from the pile is done by means of a re-claimer which in the reclaiming process cuts sections of the material from pile, which represents the homogenized quality according to quality control requirement.
Raw Material Proportioning and Grinding
The reclaimed raw materials are transported from the pre-blending yard to the feed bins of raw grinding mill for proportioning.
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The raw materials are then discharged from the individual feed bins, weighed to the required proportion and conveyed by a belt conveyor into the raw grinding mill. The raw grinding mills are generally of two types, the conventional “Ball Mill” with grinding media balls of sizes varying from 20mm to 100mm, the mill is lined with wear resisting alloy steel plates. The other type is the “Vertical Raw Mill”, which is generally the preferred choice now because of energy efficiency, ease of installation and maintenance.
The raw materials are pulverized in the raw mill to form a fine powder, known as raw meal which is fed to the storage and blending silos. The raw meal is further homogenized/blended by aeration in the silos to ensure uniformity in quality, within the limits of “Standard Deviation”.
Pyro-processing – Rotary Kiln, Pre-heater/Pre-Calciner
The modern kilns in order to improve fuel efficiency and optimize the size of equipment, are equipped with pre-heating and pre-calcination system. The raw meal is extracted from the silo and is transported to the pre-heater cyclones consisting of a string of cyclones. In the pre-heater it is preheated by the hot gases from the kiln, according to the counter current principle of heat transfer.
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The raw meal traveling down the pre-heater cyclones then enters into the pre-calciner, where the raw meal temperature is raised to about 1000 °C by a set of burners. The heated air is supplied directly from the kiln and also by a tertiary duct coming from the kiln cooler or kiln hood; about 90% calcination is achieved in the pre-calciner.
The calcined raw meal now enters the kiln which is a long rotating tube with burner at the outlet end. The temperature in the burning zone is about 1500 °C; chemical changes take place at this stage resulting in formation of the compounds of silica, alumina and iron with calcium. The product formed is gray in color, nodular in shape and is known as “Clinker” in cement terminology.
The clinker from the kiln outlet, which has a temperature of about 1200 °C drops into a cooler where it is cooled by ambient air supplied from the cooling fans to achieve a temperature of about 70°C, above the ambient air temperature. It is then transported from the cooler by a conveyor to the clinker Storage.
Clinker Grinding
A conveyor belt from the clinker storage conveys the clinker to the bin of the grinding mill, known as “Cement Mill”. This is a ball mill of the same design as the raw grinding mill, mentioned earlier.
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The clinker is pulverized along with about 5% Gypsum to a fine powder, known as “Cement”; gypsum is added to retard the setting process. The cement is then transported pneumatically or by elevator into the cement storage silos.
Packing
The cement from cement silos is fed to the packing machines known as packers, which are generally of two types i.e. stationary and rotary.
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The rotary packers are now more commonly used because of their higher capacity and efficiency. Cement is packed into bags and conveyed by belt conveyor into trucks or any other means of transport.
Bulk Loading
Cement, to bulk consumers is generally transported in loose form in bulk carriers to economize on cost of bags, bagging and transportation.
Quality Control
The traditional method for control of quality of cement is by wet analysis. In order to eliminate the human factor and improve the efficiency and accuracy, the cement analysis is now generally carried out by x-ray fluorescence method. Read More
The results achieved are accurate and take fraction of time, as compared to wet analysis. The wet analysis is however still required for the purpose of R&D and standardization of samples. The modern system involves offline and online analysis with simultaneous corrections in the composition at various stages of the process.
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