1. Mixing of Raw Materials: – The raw materials such as limestone or chalk and shale or clay may be mixed either in dry condition or in wet condition. The process is accordingly known as the dry process or the wet process of mixing. (I) Dry Process (Modern Technology): In this process, the raw materials are first reduced in size of about 25 mm in crushers.
A current of dry air is then passed over these dried materials. These dried materials are then pulverised into fine powder in ball mills and tube mills. All these operations are done separately for each raw material and they are stored in hoppers. They are then mixed in correct proportions and made ready for the feed of rotary kiln.
This finely ground powder of raw materials is known as the raw mix and it is stored in storage tank. Fig.6-1 shows the flow diagram of mixing of raw materials by dry process.
- The dry process has been modernised and it is widely used at present because of the following reasons:
- (i) Competition:
At present, several dry process cement plants are vying or competing with each other. The cement consumers in general and the practicing civil engineers in particular are greatly benefited by such competition.
- (ii) Power:
- The blending of dry powders has now perfected and the wet process, which required much higher consumption of power, can be replaced with confidence.
- (iii) Quality of Cement:
- It is found that the quality of the production no longer depended on the skilled operators and workmen because temperature control and proportioning can be done automatically through a centralised control room.
- (iv) Technology:
There has been several advances in instrumentation, computerisation and quality control. The application of the modern technology has made the production of cement by dry process more economical and of superior quality. Following is the procedure of manufacture of cement by the dry process using modern technology: (a) Most of the cement factories are located very close to the limestone quarries.
The boulders upto 1.2 m size are transported in huge dumpers upto 300 kN capacity and dumped into the hopper of the crusher. (b) The hammer mill crushers of single stage are now used for crushing as against the time consuming two stage crushers used in earlier plants. The crushed limestone now of 75 mm size is moved from the crusher by a series of conveyors for stacking.
The modern stacker-reclaimer system is now in use in most of the modern plants. The stacker helps in spreading the crushed materials in horizontal layers and the reclaimer restricts the variation of calcium carbonate in crushed limestone to less than 1% thereby minimizing quality variation in the materials.
- C) The argillaceous or clay materials found in the quarry are also dumped into the crusher and stacked along with the limestone.
- D) The crushed materials are checked for calcium carbonate, lime, alumina, ferrous oxide and silica contents.
- Any component found short in the quarried materials is added separately.
For instance, if silica content is less, the crushed sandstone is separately transported to the raw material hopper. In a similar way, if limestone is found to contain less content of lime, the high grade limestone is crushed and stored separately in the raw material hopper.
- E) The additive material and crushed limestone are conveyed to the storage hoppers.
- The raw materials are fed to the raw mill by means of a conveyor and proportioned by use of weigh feeders which are adjusted as per the chemical analysis done on the raw materials taken from the hoppers from time to time.
(f) The materials are ground to the desired fineness in the raw mill. In some of the modern plants, the high efficiency vertical grinding mills are installed. The fine powder which emerges as a result of the grinding in the raw mill is blown upwards, collected in cyclones and fed to the giant sized continuous blending and storage silo by use of aeropole.
The advantage of these silos is that one stage of pumping is eliminated which was inevitable in the traditional pattern of different silos for blending and storage. (g) The material is dropped merely by gravity from the blending to the storage silo thereby conserving power. (h) The material is then once again pumped using an aeropole into the preheater.
The most modern preheaters have five stages. The temperature of the material fed from the top is increased in stages from 60°C to 850°C as hot gas at temperature of 1000°C is blown against the falling gradient. (i) The material from the bottom of the preheater is fed to the rotary kiln.
- Due to the use of multi-stage preheaters in the modern plants, the length of rotary kilns is considerably reduced thereby resulting in saving of maintenance cost and power requirements.
- II) Wet Process (Old Technology): In the earlier part of the century i.e., from 1913 to 1960, the wet process was used for the manufacture of cement.
From 1913 onwards, the cement industry underwent a number of changes mainly to suit the requirements of the manufacturers and the govt. policies till early 1982. All the cement plants set up after 1980 use the dry process for the manufacture of cement.
- In this process, the calcareous materials such as limestone are crushed and stored in silos or storage tanks.
- The argillaceous material such as clay is thoroughly mixed with water in a container known as the wash mill.
- This washed clay is stored in basins.
- Now, the crushed limestone from silos and wet clay from basins are allowed to fall in a channel in correct proportions.
This channel leads the materials to grinding mills where they are brought into intimate contact to form what is known as the slurry. The grinding is carried out either in ball mill or tube mill or both. The slurry is led to correcting basin where it is constantly stirred.
At this stage, the chemical composition is adjusted as necessary. The corrected slurry is stored in storage tanks and kept ready to serve as feed for rotary kiln. Fig.6-2 shows the flow diagram of mixing of raw materials by the wet process. It is thus seen that in case of mixing of raw materials by dry process, the raw mix is formed and in case of mixing of raw materials by wet process, the slurry is formed.
The remaining two operations namely, burning and grinding, are the same for both the processes.
Which process is the best for manufacturing of cement?
Different minerals need to be mined in order to make cement. Limestone (containing the mineral calcite), clay, and gypsum make up most of it. The US Geological Survey notes that cement raw materials, especially limestone, are geologically widespread and (luckily) abundant.
- Domestic cement production has been increasing steadily, from 66.4 million tons in 2010 to about 80.5 million tons of Portland cement in 2014 according to the U.S.
- Geological Survey 2015 Cement Mineral Commodity Summary,
- The overall value of sales of cement was about $8.9 billion, most of which was used to make an estimated $48 billion worth of concrete.
Most construction projects involve some form of concrete. There are more than twenty types of cement used to make various specialty concrete, however the most common is Portland cement. Cement manufacturing is a complex process that begins with mining and then grinding raw materials that include limestone and clay, to a fine powder, called raw meal, which is then heated to a sintering temperature as high as 1450 °C in a cement kiln.
In this process, the chemical bonds of the raw materials are broken down and then they are recombined into new compounds. The result is called clinker, which are rounded nodules between 1mm and 25mm across. The clinker is ground to a fine powder in a cement mill and mixed with gypsum to create cement.
The powdered cement is then mixed with water and aggregates to form concrete that is used in construction. Clinker quality depends on raw material composition, which has to be closely monitored to ensure the quality of the cement. Excess free lime, for example, results in undesirable effects such as volume expansion, increased setting time or reduced strength.
Several laboratory and online systems can be employed to ensure process control in each step of the cement manufacturing process, including clinker formation. Several laboratory and online systems can be employed to ensure process control Laboratory X-Ray Fluorescence (XRF) systems are used by cement QC laboratories to determine major and minor oxides in clinker, cement and raw materials such as limestone, sand and bauxite.
Read Analysis of Clinker and Cement with Thermo Scientific ARL OPTIM’X WDXRF Sequential Spectrometer to learn why XRF is the technique of choice for elemental analysis in cement industry. Combination X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD) systems accomplish both chemical phase analysis for a more complete characterization of the sample.
- Clinker phase analysis ensures consistent clinker quality.
- Such instrumentation can be fitted with several XRF monochromators for major oxides analysis and a compact diffraction (XRD) system which has the capability of measuring quartz in raw meal, free lime (CaO) and clinker phases as well as calcite (CaCO 3 ) in cement.
Read XRF/XRD Combined Instrumentation Can Provide Complete Quality Control of Clinker and Cement to learn more about technology that combines the advantages of both XRF and XRD together. Cross Belt Analyzers based on Prompt Gamma Neutron Activation Analysis (PGNAA) technology are installed directly on the conveyor belt to measure the entire material stream continuously and in real time to troubleshoot issues in pre-blending stockpile control and quarry management, raw mix proportioning control, and material sorting.
Read PGNAA Improves Process and Quality Control in Cement Production to learn what makes PGNAA particularly suited for cement analysis. Accurate cement production also depends on belt scale systems to monitor output and inventory or regulate product loadout, as well as tramp metal detectors to protect equipment and keep the operation running smoothly.
The Cement Manufacturing Process flow chart sums up where in the process each type of technology is making a difference. NOTE: Need a Belt scale system for your bulk material handling? To help you decide which belt scale system is best for your mining operation, we’ve outlined the options in an easy-to-read belt scale system selection guide so you can decide which belt scale system is right for you. Click on the image, take a look at the chart, and see if it helps you decide.
Which cement manufacturing process is widely used?
Visit ShapedbyConcrete.com to learn more about how cement and concrete shape the world around us. Portland cement is the basic ingredient of concrete. Concrete is formed when portland cement creates a paste with water that binds with sand and rock to harden.
Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients. Common materials used to manufacture cement include limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore.
These ingredients, when heated at high temperatures form a rock-like substance that is ground into the fine powder that we commonly think of as cement. Bricklayer Joseph Aspdin of Leeds, England first made portland cement early in the 19th century by burning powdered limestone and clay in his kitchen stove.
- With this crude method, he laid the foundation for an industry that annually processes literally mountains of limestone, clay, cement rock, and other materials into a powder so fine it will pass through a sieve capable of holding water.
- Cement plant laboratories check each step in the manufacture of portland cement by frequent chemical and physical tests.
The labs also analyze and test the finished product to ensure that it complies with all industry specifications. The most common way to manufacture portland cement is through a dry method. The first step is to quarry the principal raw materials, mainly limestone, clay, and other materials.
- After quarrying the rock is crushed.
- This involves several stages.
- The first crushing reduces the rock to a maximum size of about 6 inches.
- The rock then goes to secondary crushers or hammer mills for reduction to about 3 inches or smaller.
- The crushed rock is combined with other ingredients such as iron ore or fly ash and ground, mixed, and fed to a cement kiln.
The cement kiln heats all the ingredients to about 2,700 degrees Fahrenheit in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as 12 feet in diameter—large enough to accommodate an automobile and longer in many instances than the height of a 40-story building.
- The large kilns are mounted with the axis inclined slightly from the horizontal.
- The finely ground raw material or the slurry is fed into the higher end.
- At the lower end is a roaring blast of flame, produced by precisely controlled burning of powdered coal, oil, alternative fuels, or gas under forced draft.
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 called clinker. Clinker comes out of the kiln as grey balls, 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 various types of coolers.
The heated air from the coolers is returned to the kilns, a process that saves fuel and increases burning efficiency. After the clinker is cooled, cement plants grind it and mix it with small amounts of gypsum and limestone. Cement is so fine that 1 pound of cement contains 150 billion grains. The cement is now ready for transport to ready-mix concrete companies to be used in a variety of construction projects.
Although the dry process is the most modern and popular way to manufacture cement, some kilns in the United States use a wet process. The two processes are essentially alike except in the wet process, the raw materials are ground with water before being fed into the kiln.
What is the dry process of cement?
2. Cement Plants – Portland cement manufacture accounts for about 98% of the cement production in the United States. The raw materials are crushed, processed, proportioned, ground, and blended before going to the final process, which may be either wet or dry.
- In the dry process, the moisture content of the raw material is reduced to less than 1% before the blending process occurs.
- The dry material is pulverized and fed to the rotary kiln.
- Further drying, decarbonating, and calcining take place as the material passes through the rotary kiln.
- The material leaves the kiln as clinker, which is cooled, ground, packaged, and shipped.
For the wet process, a slurry is made by adding water during the initial grinding. The homogeneous wet mixture is fed to the kiln as a wet slurry (30–40% water) or as a wet filtrate (20% water). The burning, cooling, grinding, packaging, and shipping are the same as for the dry process.
|Pollutant||Emissions (kg metric ton −1 )|
|Dry process||Wet process|
|Kilns||Dryers, grinders, etc.||Kilns||Dryers, grinders, etc.|
|Sulfur dioxide a||5.1|
|Gas combustion||2.1 × S b||—||Neg||—|
|Oil combustion||3.4 × S||—||2.1 × S b||—|
|Coal combustion||—||3.4 × S||—|
a If a baghouse is used as control device, reduce SO 2 by 50% because of reactions with an alkaline filter cake. b S is the percent of sulfur in the fuel. Source: Ref. Control of particulate matter emissions from the kilns, dryers, grinders, etc. is by means of standard devices and systems: (1) multiple cyclones (80% efficiency), (2) ESPs (95% + efficiency), (3) multiple cyclones followed by ESPs (97.5% efficiency), and (4) baghouses (99.8% efficiency).
Which cement manufacturing method is best wet or dry Why?
Difference between Wet and Dry process of cement
Mixing of Raw materials in wash mill with 35 to 50% water. Materials exiting the mill are called “slurry” and have flow-ability characteristics. Size of the kiln needed for manufacturing of cement is bigger. Raw material can be mixed easily, so a better homogeneous material can be obtained Fuel consumption is high i.e., 350 kg of coal per tonne of cement produced. Cost of production is high. Capital cost (Cost of establishment) is comparatively less.
Mixing of raw material in dry state in blenders. The dry materials exiting the mill are called “kiln feed”. Size of the kiln needed for manufacturing of cement is smaller. Difficult to control mixing of Raw materials, so it is difficult to obtain a better homogeneous material. Fuel consumption is low i.e., 100 kg of coal per tonne of cement produced. Cost of production is less. Capital cost is high due to blenders.
If we consider the quality and rate then wet process is better and if we consider fuel consumption and time of process then dry process is better. Difference between dry and wet process in table form:-
|1. Mixing of raw material in dry state in blenders.||1. Mixing of Raw materials in wash mill with 35 to 50% water.|
|2. The dry materials exiting the mill are called “kiln feed”.||2. Materials exiting the mill are called “slurry” and have flowability characteristics.|
|3. Fuel consumption is low i.e., 100 kg of coal per tonne of cement produced||3. Fuel consumption is high i.e., 350 kg of coal per tonne of cement produced|
|4. Cost of production is less.||4. Cost of production is high|
|5. Capital cost is high due to blenders.||5. Capital cost (Cost of establishment) is comparatively less|
|6. Size of the kiln needed for manufacturing of cement is smaller.||6. Size of the kiln needed for manufacturing of cement is bigger.|
|7. Difficult to control mixing of Raw materials, so it is difficult to obtain a better homogeneous material.||7. Raw material can be mixed easily, so a better homogeneous material can be obtained|
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