Cement Manufacturing Process
When Portland Cement, a fine powder is mixed with about half its own weight of water, it forms Cement Paste, an adhesive mud like mixture. This paste is brought into contact with an aggregate such as sand, gravel or crushed stone to form the artificial rock, concrete. Following paragraphs explain how Portland Cement is produced, packaged and shipped for use in making concrete.
Typically, cement process combines a calcium-bearing material such as limestone, shell or chalk with a silicon bearing material such as shale, slate or clay to produce a material containing silicates of calcium and compounds of aluminum and iron. The total cement making process may be readily categorized by the following functions:
1. Quarrying : Excavating rock from the limestone or Cement rock quarry.
Limestone, usually quarried by the open pit method is blasted from the quarry wall with explosives. The rock, some fragments of boulder size is loaded directly into railroad cars or trucks by diesel or electric shovels and transported to the cement plant for crushing.
2. Crushing : Breaking the large rock into a size suitable for controlled feeding.
The primary crusher reduces the rock from shovel-size pieces to 6-10” in diameter. A Second crusher again reduces the limestone to pieces 1” in size suitable for mill feeding.
3. Proportioning or blending: mixing the correct amount of silica, alumina and iron oxide with the limestone. The proper proportioning of the raw material is accomplished through the use of Weigh feeders for the Raw Grinding Mills.
4. Raw Mix Grinding : Grinding the raw mix to small particle size before Kiln burning.
It is process is to grind the above particles to a size of 90 microns or less which is done in a raw mill, a closed circuit ball mill equipped with classifier.
5. Homogenizing : Blending the raw materials to a uniform mixture. After achieving the 90 microns size the fine grinded material also known as raw meal is sent to the continuous blending silos (CFC) for homogenization & extracted by means of load cell hopper for the next step which is feeding to the kiln pre heaters. If there is no stacker reclaimer, we cannot go in for continuous blending of the raw meal and will have to go in for batch blending.
6. Pyro Processing
The raw material is heated to exceeding 1,450 °C (2,700 degrees F) in 4 or 5 or 6 stage pre-heater tower (about 110 metres in height) in the suspended form, than through a pre-calciner. The kiln using suspension type pre-heaters to recover heat from the kiln exhaust gases, have the following normal operation : The finely ground dry raw meal is dropped into the stream of hot kiln gases and then separated from the gases by means of cyclones. This pre-heating increases the efficiency of the kiln system. Some fuel is fired at the lowest cyclone or near the inlet end of the rotary kiln, so the raw mix is almost completely dissociated or decarbonated before it enters the kiln proper and hence the thermal load in the rotary kiln proper is reduced substantially..
7. Fuel: Coal, oil or Gas can be used as fuel. In case of Coal, it is has to be ground dry.
Raw coal always contains 2%-10% moisture. During rainy season, it may even go up to 20%. Therefore coal has to be dried suitably before grinding. Coal could be dried and ground simultaneously with the help of hot air in air swept ball mill.
8. Kiln Burning & cooling: Heating the raw mix to produce the necessary chemical reactions for cement clinker formation.
Ground coal is fired in Kiln burner which is basically a Vernier Burner, located in the throat of the cooler. Its primary function is to provide regulating heat (up or down) to maintain a constant secondary air temperature. The secondary additive function is to alter the main flame ignition point in response to change in burning zone temperature.
Refractory Lining is provided in a Cement Kiln to retain the heat and to protect the steel shell. Burning is a continuous process and once the kiln is lighted up it runs continuously for a few months without any interruption or stoppage.
As the material moves through the kiln, certain elements are driven off in the form of gases. The remaining elements unite to form a new substance with new physical and chemical characteristics. The new substance, called clinker, is formed in pieces about the size of marbles.
Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in Grate coolers. The heated air from the coolers is returned to the pre-heater thus recovering the heat which saves fuel and increases burning efficiency.
The cooled clinker is conveyed to the clinker pile in the storage area, where it is stored until needed for clinker grinding operation.
9. Cement milling : Fine grinding of Clinker with Gypsum.
From the storage area, the clinker is combined with gypsum and fed to the finish mill. Weigh feeders under laboratory control, automatically proportion the Clinker and Gypsum for the finish mill. The gypsum ultimately retards the setting time of cement. Clinker grinding is done in Closed circuit Ball Mill equipped with Classifier, to maintain consistency in fineness. Since finished grinding generates heat which is detrimental to cement quality, the shell is cooled by external water sprays to dissipate the heat generated.
10. Cement Storage, Packing and Loading :
Finished cement is storage in large silos, adjacent to packing and loading plant. Extraction from the silos is by pneumatic conveyors/screw conveyors loading to an elevator, which feeds to the Packing machine. Roto packer, capable of filling bags at a higher rate are employed. Filled bags are automatically discharged into the trucks by Truck loader.
11. Dust Emission :
In making of Cement, dust emission is controlled by Bag dust collectors / ESP or Bag house, through these equipment the dust emission & gas emission is maintained to International standard of 50 mgm/Nm3
12. Process water :
Since this is a dry process, no water is required for the process, thereby no water pollution. For cooling purpose, certain quantity of water is required, which is recyclable.
13. Instrumentation and Automation:
Proper instrumentation is provided to achieve the desired quality of Clinker and reduce power and fuel consumption also to check the emission level. The Plant can also be automized to level required and can be operated from the control room.
14. Laboratory:
A well designed and equipped Modern Laboratory is housed in a Cement Plant for performing verities of duties such as examination of the raw material components, the decision as to which individual components are to be quarried and in what quantities and the determination and monitoring of the raw meal composition. One of the most important tasks is the monitoring of the cement production process to ensure unvarying product quality, conformity to the relevant standards etc. Various testing sequences can proceed in accordance with a logical pattern. Before releasing to the market, the finished product is tested both physically and chemically in laboratory as per relevant standards.
