Process Technology of Cement Industries [PDF]

Zitiervorschau

The Process Technology of cement Industries By Mulatu Betsegaw

HiOT Chemical Engineering

Background 

Cement is an important ingredient in concrete locking together the sand and gravel constituents in an inert matrix.

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It is one of the principal building and construction materials in building and construction sectors.

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It is therefore essential to meet society’s needs for housing and basic infrastructure such as bridges, roads, water treatment facilities, schools and hospitals.

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Currently, the country has more than 16 cement plants (other than 2 new entrants).

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In terms of size and installed production capacity: four are large and integrated firm with more than 2 Mta installed cement production capacity; four are mid-sized cement plants with combined installed cement production capacity of 2.3 Mta; while the balance are small cement plant with vertical shift kiln technologyincluding clinker grinding facilities.

Cement definition 

Cement is a manmade mineral structure created at high temperatures, mainly comprising lime (CaO), Silica (SiO2) and oxides of aluminum and iron (Al2O3 and Fe2O3.

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Cement is a hydraulic powder material, which reacts with water to produce strength-bearing lattices.

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The mixture of aggregates, cement and water is concrete. The strength and durability of concrete makes it one of the most useful materials developed by man.

Raw materials used to produce cement The raw materials are naturally occurring mineral and basically all CaO, SiO2 Al2O3 and, Fe2O3 bearing minerals can be used to manufacture cement . Plants generally rely on nearby quarries for limestone to minimise transport 1.Primary rawmaterials  limestone (CaCO3) & limestone containing minerals, 2.Secondary rawmaterials  clay ( Al2O3 ),sand (silica, aluminium and iron dxides). 3.Tertiary rawmaterials  Gypsum Other "alternative" raw materials slag, mill scale, fly ash

Types of processes 

There are two main process routes for the manufacture of cement. These are:

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Dry process:-In the dry process, the raw materials are ground and dried to raw meal in the form of a flow able powder. The dry raw meal is fed to the preheater or precalciner kiln or, more rarely, to a long dry kiln.

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Wet process: - the raw materials (often with high moisture content) are ground in water to form pumpable slurry. The slurry is either fed directly into the kiln or first to slurry dryer.

Major unit operations 1.

Quarrying

2.

Cushing

3.

Proportionating / Prehomogenization

4.

Raw mill Grinding

5.

Homogenization /Silo

6.

Raw meal preheating

7.

Kiln operation

8.

Clinker cooling/storage

9.

Cement mill and gypsum addition/storage

Quarrying Rock blasted from the quarry is transported to the primary crusher where large "run of mine" rocks are broken into pieces of approximately 100mm.

Crushing and Grinding 

Cement plant raw materials blasted in the quarry requires size reduction for further processing, size reduction is performed in crushers and grinding mills. Crushing is communition in the coarse range, where as grinding refers to communition in the fine range.

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Primary crushing involves limestone rock fed through large capacity crushers. This reduces the rock to a maximum size of approximately 150 mm. Secondary crushing further reduces this to 75mm or under. Residue material is stacked and reclaimed with a bridge reclaimer in a cross section so that the homogeneous limestone is fed further in the process.

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The grinding media (steel ball) and the feed material to be ground are brought together in a rotating tubular or drum-shaped compartment. The media and material rises to an optimum height, necessary for grinding operation, and come tumbling down (cascading and/or contracting).

The actual height to which they are lifted depends on a number of factors: the speed of mill, the type of lining, the filing ratio (mill loading percentage), and the properties of mill feed material (like moisture content.)

proportionating homogenisation 

The raw materials are then proportioned to the correct chemical balance and milled together to a fine powder, "rawmeal. To ensure high quality of cement, the chemistry of the raw materials and rawmeal is very carefully controlled.

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Materials are also homogenised to ensure consistency of product quality.

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Kiln exhaust gases are used in the rawmill to dry the raw materials. In some gases with wet materials, additional heat sources are required for drying.

Preheater/Cyclones 

A pre-heater is a series of vertical cyclones. Where the raw meal is passed down through these cyclones it comes into contact with the swirling hot kiln exhaust gases moving in the opposite direction and as a result heat is transferred from the gas to material.

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This pre-heats the material before it enters the kiln so that the necessary chemical reactions will occur more quickly and efficiently. By retaining energy from the exhaust gases, energy is saved.

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Depending on the raw material moisture, a kiln may have 3 to 6 stages of cyclones with increasing heat recovery with each extra stage.

Calcination 

The calciner is a combustion chamber at the bottom of the preheater above the kiln back-end. Up to 65% of the total energy needs of the kiln system can be supplied to the calciner.

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Calciners allow for shorter rotary kilns and for the use of lower grade alternative fuels. Calcination is the decomposition of CaCO3 to CaO and CO2. CaCO3

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CaO + CO2.

These process emissions comprise 60% of the total emission from a cement kiln. The combustion of the fuel generates the rest.

Kiln operatoion 

Raw meal, more accurately termed "hot meal" at this stage then enters the rotary kiln.

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The kiln is the world's largest piece of industrial equipment.

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As the kiln rotates at about 3-5 revolutions per minute, the material slides and tumbles down through progressively hotter zones towards the flame

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Fuel is fired directly into the rotary kiln and the heat is absorbed into the material being processed.

Burning zone : The burning zone is where combination takes place between the lime ( CaO), and the silica (SiO2 ), Alumina ( Al2O3 ) and ferric oxide (Fe2O3) to form the four basic clinker components (i.e. C3S,C2S,C4AF and C3A). The basic chemical reactions taking place in the burning zone are the following:

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Burning zone : The burning zone is where combination takes place between the lime ( CaO), and the silica (SiO2 ), Alumina ( Al2O3 ) and ferric oxide (Fe2O3) to form the four basic clinker components (i.e. C3S,C2S,C4AF and C3A).

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The basic chemical reactions taking place in the burning zone are the following:  Alite formation: -

3CaO + SiO2 ------> 3 CaO. SiO2 (C3S)  Belite formation:-

2CaO+ SiO2 ------> 2 CaO. SiO2 (C2S)  Aluminates formation:-

CaO+ Al2O3 -----> 3 CaO. Al2O3 (C3A)

Ferrite formation:CaO+ Al2O3+ Fe2O3 -----> 4CaO. Al2O3. Fe2O3 (C4AF )

Energy source 

Fuel oil, Coal, natural gas, pet coke and other alternative fuels are energy sources

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plastic, rubbers, solvents, waste oil or meat and bone meal are burned to feed the flame which can reach as high as 2000oC.

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The high temperature allows the materials to become partially molten as the intense heat causes the chemical and physical changes that transform the raw feedstock into a material called clinker

Clinker formation 

Expressed at its simplest, the series of chemical reactions converts the calcium and silicon oxides into calcium silicates, cement primary constituent. At the lower end of the kiln, the raw materials emerge as a new substance red-hot particles called clinker.

Clinker

clinker cooler 

The clinker tumbles onto a grate cooled by forced air. Once cooled the clinker is ready to be ground into the grey powder known as Portland cement. To save energy, heat recovered from this cooling process is re circulated back to the kiln or preheater tower.

Cement mill 

The clinker is then ground with other mineral components to cement

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Gypsum is used to control the setting time of the product;

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Slag and fly ash can also be used to control other properties of the cement.

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Traditionally, ball mills is used for cement milling.

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Portland cement is produced by intergrinding cement clinker with a few percent of natural or industrial gypsum (or anhydrite) in a cement mill.

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Blended cements (or “composite” cements) contain other constituents in addition such as granulated blast-furnace slag, natural or industrial pozzolana (for example, volcanic tuffs or fly ash from thermal power plants), or inert fillers such as limestone.

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In recent years technologies with better energy efficiency have been developed. Compound mill systems include pre-crushing and sophisticated separator systems to reduce electricity consumption.

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Vertical cement mills mill the material in a roller mill with reduced electricity consumption also.

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From the grinding mills, the cement is conveyed to silos for shipment. Most cement is shipped in bulk by trucks, rail or barge. A small percentage of the cement is bagged for customers who need only small amounts or who have special needs.

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CO2 and Climate Protection

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Globally, the cement industry produces about 5% of man-made CO2 emissions. This breaks down to:

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Clinker production: 50%

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Burning fuel: 40%

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Electricity use and transportation: 10%

…End

Thank you!!?? By Mulatu Betsegaw, A.lectuer @ HiOT Chemical Engineering

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