Cement grinding in wet production method
The cement plants using wet method generally produce portland cement clinker using soft clayey and hard limestone components as raw materials.
The initial process operation for clinker production is grinding of the raw materials. The necessity of fine grinding of the raw materials is determined by the fact that the compositionally uniform clinker can only be produced using thoroughly mixed raw mix consisting of the finest particles of its components.
The lumps of the source raw materials are often up to 1200 mm in size. Such lumps can only be used to produce the material as the finest grains in stages. First the lumps are subjected to coarse comminution — crushing, then to fine comminution — grinding. Different crushers are used for coarse comminution, and the fine comminution is carried out in mills with large amount of water, depending on the source material properties.
When chalk is used as a limestone component, it is ground in washing plants. If a hard clayey component is used, then it is fed into the mill after crushing.
From the washing plant the clay slurry is pumped into the mill where the limestone is ground. The joint comminution of two components allows producing a more compositionally uniform raw slurry.
The limestone and clay slurry are fed into the mill in a definite ratio corresponding to the required chemical composition of clinker. However, even in case of the most thorough batching of the source materials it is not possible to get a slurry of the required chemical composition from the mill due to variations of the chemical composition of the raw material produced at the same mine field. To obtain the slurry of the set chemical composition, it is corrected in tanks.
To that end, the slurry with knowingly low or high CaCO3 content (referred to as titre) is prepared in one of several mills, and such a slurry is added into the adjustment slurry tank in a definite ratio.
The slurry prepared in such a way is a thick creamy mass with up to 35 to 45% water content. It is pumped into the supply tank from which it is uniformly discharged into the kiln.
Rotary kilns are used for clinker calcination when wet production method is applied. A rotary kin is a 150 to 230 m long steel drum up to 7 m in diameter. It is lined with refractory bricks inside; capacity of such kilns is 1000 to 3000 tons of clinker per day.
The kiln drum is set at 3 to 40 degr. slope, the slurry is fed from the raised part of the kiln, the cold end, and the gas, coal dust of fuel oil residue fuel is blown into the kiln from the opposite side (hot end). As a result of the tilted drum rotation, the materials contained in the drum move in the kiln towards its hot end. The maximum temperature is developed in the fuel combustion area: material – up to 15000 С, gases – up to 17000 С, and chemical reactions resulting in clinker formation are completed.
Furnace gases move along the kiln drum towards the material being calcinated. Coming across the cold materials, the furnace gases heat them up while cooling down themselves. As a result, starting from the calcination zone, the gas temperature along the kiln decreases from 1700 to 150–2000 С.
From the kiln the clinker is fed into the cooler where it is cooled by cold air moving towards it.
The cooled clinker is delivered to the storage. In some cases the clinker from the cooler is fed directly to cement mills for grinding.
Before grinding the clinker is crushed to 8–10 mm grains so that to facilitate the mill operation. The clinker is comminuted with gypsum, hydraulic and other additives. The joint grinding ensures thorough mixing of all materials, and high uniformity of cement is one of important guarantees of its quality.
Hydraulic additives, being highly porous materials, are generally high-moisture materials (up to 20–30% and more). Therefore before grinding they are dried to about 1% moisture content after crushing to 8–10 mm grains. Gypsum is only crushed, as it is added in small amounts, and moisture contained in it is easily evaporated due to the heat emitted in the mill as a result of mutual collisions and abrasion of milling bodies with each other and with the mill material.
From the mill the cement is transported to the bin type storage equipped with mechanical (elevators, screw conveyors), pneumatic (pneumatic pumps, air slide conveyors) or pneumatic transport.
The cement is shipped to the consumer in packaged state — in multi-wall paper bags, 50 kg each, or in containers in bulk, truck or railway cement cement carriers, in specially equipped ships. Each cement batch is provided with a certificate.
Various devices are used for execution of all portland cement production process steps – cement crushers, cement mills, cement kilns, etc., which are integrated into an in-line system. The unit layout is shown in the master plan of the cement plant operating based on lamellated materials and gaseous fuel.
The Samara Strommashina plant offers both turn-key cement production process systems, and separate equipment items. We will help you in selecting all necessary equipment and will provide technical support.