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, 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 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. The limestone rock is crushed in the first crusher to reduce the rock to a maximum size of about 6 inches.
It is then fed into the second crusher with a mixing of clays to reduce particle size below 3 inches. The other raw materials that are used in cement manufacturing, called additives, are also stored in separate bins.
The raw mix and required additives are fed from their bins to the raw mill via blowers for drying and grinding. The raw mill contains two chambers - a drying chamber and a grinding chamber. The grinding chamber contains a certain quantity of ball charge in different sizes ranging from 30mm to 90mm that are used to grind the material.
It then feeds to a separator which separates the fine and coarse product. The latter, called reject, is sent to the mill inlet for regrinding.
Then, the hot gas and fine materials enter a multistage "cyclone". This is to separate the fine ground materials from the gases. The resulting raw meal, consisting of only the very fine raw meal materials, is conveyed to a concrete silo. From there the raw meal extracted from the silo, now called kiln feed, is fed to the top of the preheater kiln for sintering. The preheater kiln system consists of a multi-stage cyclone preheater, combustion chamber, riser duct, rotary kiln, and grate cooler.
In the preheater, the kiln feed is preheated by hot gas coming from the combustion chamber and rotary kiln.
It is then partially calcined in a combustion chamber and riser duct. The feed then moves into the rotary kiln where it is superheated to approximately C to form clinker components through a process called sintering. The heat is produced from the burning of fuel in the main burner rotary kiln and in the combustion chamber with the help of preheater exhaust Fans or Kiln ID Fans. Some typical materials used for calcium carbonate in cement manufacturing are limestone, chalks, marbles, marls, and oyster shell.
Some typical materials used for alumina in the cement manufacturing are shale, clay, slags, fly ash, bauxite, alumina process waste, and granite.
Some typical materials used for silica in cement manufacturing are sand, clay, claystone, shale, slag, and fly ash. Some typical materials used for iron in cement manufacturing are iron ores, blast furnace flue dusts, pyrite clinker, mill scale, and fly ash.
Ball Mill In A Cement Plant The general mining methods are surface mining, while some silicates, such as sand, are commonly mined using dredges, from lakes, rivers and waterways. There are a few underground limestone mines, but most are pits on the surface. Cement plants are typically located central to the minerals required to make the cement, which saves the transportation costs and reduces the price of the cement.
Sometimes, the raw materials are wet ground in the ball mill by adding water, and form a slurry. In either event, wet or dry, the ground powder is then blended, using the "Chemist's Secret Mixture", which can be a closely guarded "recipe", and are then transported to the rotary kiln for heat processing.
In the rotary kiln, first the carbon dioxide is driven off of the calcium carbonates, then the raw material is fused at a temperature somewhere around 2, degrees F.
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