CEMENT PROCESS

Cement Process

- QUARY
Basic Elements

For its raw materials, cement uses minerals containing the four essential elements for its creation: calcium, silicon, aluminum and iron.

Raw Materials

Most plants rely on a nearby quarry for limestone. The most common combination of ingredients is limestone (for calcium) coupled with much smaller quantities of clay and sand (as sources of silica, aluminum and iron). Other raw materials, such as mill scale, shale, bauxite and fly ash are brought in from outside source when necessary.

Crusher

Rock blasted from the quarry face is transported to the primary crusher, where chair sized rocks are broken into pieces the size of baseballs. A secondary crusher reduces them to the size of gravel. Some plants now crush materials in a single stage

- PROPORTIONING, BLENDING & GRINDING

The raw materials are now analyzed in the plant laboratory, blended in the proper proportion, and then ground even finer. Plant grind the raw materials into powder against a rotating table. After grinding, the material is now ready for the kiln or preheater, depending on plant type.

- PREHEATER TOWER

The preheater tower supports a series of vertical cyclone chambers through which the raw materials pass on their way to the kiln. To save energy, modern cement plants preheat the materials before they enter the kiln. Rising more than 200 feet, hot exit gases from the kiln heat the raw materials as they swirl throught the cyclones

- KILN

Raw materials now enter the huge rotatign furnae called kiln. It's the heart of the cement making process - a horizontally sloped steel cylinder, lined with firebrick, turning from about one to three revolutions per minute. The kiln is the world's largest piece of moving industrial equipment.

From the preheater, the raw material enters the kiln at the upper end. It slides and tumbles down the kiln through progressively hotter zones toward the flame. At the lower end of the kiln, fuels such as powdered coal and natural gas feed a flame that reaches 3400'F (1870 'C) - one-third of the temperature of the sun's surface. Here in the hottest part of the kiln, the raw materials reach about 2700 F (1480C) and become partially molten.

This intense heat triggers chemical and physical changes. Expressed at its simplest, the series of chemical reactions converts the calcium and silicon oxides into calcium silicates, cement's primary constituent. At the lower end of the kiln, the raw materials emerge as a new substance; red hot particles called clinker.

- CLINKER COOLER & FINISH GRINDING

The clinker tumbles onto a grate cooled by forced air. Once cooled the clinker is ready to be ground into the gray powder known as portland cement.

To save energy, heat recovered from this cooling process is recirculated back to the kiln and preheater tower.

The clinker is ground in a ball mill - a horizontal steel tube filled with steel balls. As the tube rotates, the steel balls tumble and crush the clinker into a super-fine powder. It can now be considered portland cement. The cement is so fine it will easily pass through a sieve that is fine enough to hold water. A small amount of gypsum is added during final grinding to control the set.

- BAGGING & SHIPPING

From the grinding mills, the cement is conveyed to silos where it awaits shipment. Most cement is shipped in bulk by trucks, rail or barge. A small percentage of the cement is bagged for customes who need only small amounts or for special uses such as mortar. Most cement is shipped to ready-mixed concrete producers. There, it's combined with water, sand and gravel to make concrete delivered in the familiar truks with revolving drums. Cement is also used for a wide array of precast concrete products.



Raw Materials

The main raw materials used in the cement manufacturing process are limestone, sand, shale, clay, and iron ore. The main material, limestone, is usually mined on site while the other minor materials may be mined either on site or in nearby quarries. Another source of raw materials is industrial by-products. The use of by-product materials to replace natural raw materials is a key element in achieving sustainable development.


Raw Material Preparation


Mining of limestone requires the use of drilling and blasting techniques. The blasting techniques use the latest technology to insure vibration, dust, and noise emissions are kept at a minimum. Blasting produces materials in a wide range of sizes from approximately 1.5 meters in diameter to small particles less than a few millimeters in diameter.


Material is loaded at the blasting face into trucks for transportation to the crushing plant. Through a series of crushers and screens, the limestone is reduced to a size less than 100 mm and stored until required.


Depending on size, the minor materials (sand, shale, clay, and iron ore) may or may not be crushed before being stored in separate areas until required.


Raw Grinding

In the wet process, each raw material is proportioned to meet a desired chemical composition and fed to a rotating ball mill with water. The raw materials are ground to a size where the majority of the materials are less than 75 microns. Materials exiting the mill are called "slurry" and have flowability characteristics. This slurry is pumped to blending tanks and homogenized to insure the chemical composition of the slurry is correct. Following the homogenization process, the slurry is stored in tanks until required.


In the dry process, each raw material is proportioned to meet a desired chemical composition and fed to either a rotating ball mill or vertical roller mill. The raw materials are dried with waste process gases and ground to a size where the majority of the materials are less than 75 microns. The dry materials exiting either type of mill are called "kiln feed". The kiln feed is pneumatically blended to insure the chemical composition of the kiln feed is well homogenized and then stored in silos until required.


Pyroprocessing

Whether the process is wet or dry, the same chemical reactions take place. Basic chemical reactions are: evaporating all moisture, calcining the limestone to produce free calcium oxide, and reacting the calcium oxide with the minor materials (sand, shale, clay, and iron). This results in a final black, nodular product known as "clinker" which has the desired hydraulic properties.


In the wet process, the slurry is fed to a rotary kiln, which can be from 3.0 m to 5.0 m in diameter and from 120.0 m to 165.0 m in length. The rotary kiln is made of steel and lined with special refractory materials to protect it from the high process temperatures. Process temperatures can reach as high as 1450oC during the clinker making process.


In the dry process, kiln feed is fed to a preheater tower, which can be as high as 150.0 meters. Material from the preheater tower is discharged to a rotary kiln with can have the same diameter as a wet process kiln but the length is much shorter at approximately 45.0 m. The preheater tower and rotary kiln are made of steel and lined with special refractory materials to protect it from the high process temperatures.


Regardless of the process, the rotary kiln is fired with an intense flame, produced by burning coal, coke, oil, gas or waste fuels. Preheater towers can be equipped with firing as well.


The rotary kiln discharges the red-hot clinker under the intense flame into a clinker cooler. The clinker cooler recovers heat from the clinker and returns the heat to the pyroprocessing system thus reducing fuel consumption and improving energy efficiency. Clinker leaving the clinker cooler is at a temperature conducive to being handled on standard conveying equipment.


Finish Grinding and Distribution

The black, nodular clinker is stored on site in silos or clinker domes until needed for cement production. Clinker, gypsum, and other process additions are ground together in ball mills to form the final cement products. Fineness of the final products, amount of gypsum added, and the amount of process additions added are all varied to develop a desired performance in each of the final cement products.


Each cement product is stored in an individual bulk silo until needed by the customer. Bulk cement can be distributed in bulk by truck, rail, or water depending on the customer's needs. Cement can also be packaged with or without color addition and distributed by truck or rail.