An Introduction to Clinker Grinding and Cement Storage
The production of cement clinker in the kiln is, as we have seen, a continuous process; all-day operation in which the production rate can be varied only within a limited range and then only gradually.
The production of finished cement, on the opposite, must closely follow seasonal and short-term fluctuations in the delivery of cement from the plants: the storage of the finished cement is expensive, in capital costs and the cement storage life is to a limited extent; in fact, the stocks of finished cement available at any time are rarely more than a week of production. The clinker, in contrast, can be stored conveniently and economically to meet maximum demands.
Cement grinding is, therefore, a flexible and generally intermittent operation, with mills that have sufficient capacity to grind the clinker considerably faster than kilns produce. This allows them to meet the maximum demands when necessary: at other times, they can be run at a capacity less than full or they can be stopped completely.
Energy considerations are equally important: having mills with a capacity greater than that of clinker production, grinding can be done during periods that offer the most favorable energy rates. The power supply and charges vary from plant to plant and also the arrangements for programming the grinding.
Normally, storage is established for several weeks of clinker production. Operating the industry as a whole, clinker stocks are normally equal between four and six weeks of production, although they may be lower at times of high demand or more at times of low demand.
Although the clinker is much more stable than the finished cement, it deteriorates to some extent with exposure and, for this reason, it is standard for clinker warehouses to be completely closed: dosed clinker warehouses also avoid the nuisance of dust from the fine particles of the clinker when stored outdoors.
Depending on the layout of the plant, two or more kilns can supply a common clinker or silo warehouse, or a single kiln can supply the warehouse or silo, especially when special clinkers such as white Portland cement or sulfate resistant cement are produced. Additionally, separate storage should be provided for the gypsum that is ground with the clinker to control the setting time, and for any other material that can be incorporated during grinding, such as slag for Portland-bulastfurnance cement or additives for masonry cement.
Due to its long storage life and the fact that it can be transported by normal methods of bulk granular material, the cement clinker does not need to be ground in the plant where it is produced. It can be sent between plants to match production or meet localized demand, or it can be sent to a milling facility that does not have its own kilns, usually where there is substantial local demand but not the raw materials necessary for clinker production. A substantial part of the international export and import trade of cement is in the form of a clinker instead of the finished product.
The cement clinker is ground as finished cement, in a vertical roller mill for cement or in ball mills similar to those normally used for grinding hard raw materials in dry and semi-dry processes. However, the cement clinker is more difficult to grind than raw materials and must also be finer ground. Therefore, finishing grinding ball mills are generally larger than those of raw materials and the grinding time is longer.
The typical finishing grinding ball mill is a rotating cylinder divided into two or three chambers containing steel grinding balls of classified size that reduce the clinker to the required fineness by a combination of impact and friction. The clinker, gypsum and other materials enter the mill through a rotating trunnion and leave the mill through the trunnion at the opposite end.
The typical dimensions of a mill composed of two or three compartments
would be a length of 13.5 m and a diameter of 2.5 m, with a speed of approximately 20 revolutions per minute.
Such a mill would normally have a load of approximately 90 tons of grinding media, graduated from 90 mm to 60 mm in the first chamber, 50 mm to 15 mm in the second chamber and 15 mm down in the final chamber. The chambers are separated by perforated diaphragms that allow the clinker to pass when it has reached the correct fineness, but at the same time they can retain the grinding media in their compartments. A mill of this type would be driven by a 1200 hp motor, but the mills can vary in size from 500 hp to 6000 hp with cement outlets that can vary from 10 to 140 tons per hour.
As with the grinding of raw materials, both the open circuit mill and the closed-circuit mill are used for clinker grinding. The grinding in open circuit, all the material is ground to the specified fineness in a single step through the mill. In closed circuit milling, part of the material that leaves the mill is still large: this is removed by a separator system and returned to the mill for additional crushing, while material that is already sufficiently fine passes to the cement silos.
Some composite mills have only two compartments, the first one is loaded with the larger ball as in a three-chamber mill and the second, the larger compartment is equipped with a separation liner that maintains the gradation of the size of the balls from an end of the camera to the other.
The mill speed is chosen during the design stage to provide the most effective grinding action. As the mill rotates, the grinding means are brought approximately to the “ten o’clock” position on the internal circumference of the mill before falling: if the mill speed were too high, the means would be transported around the drum by the centrifugal force, without grinding action, while, if it were too low, the balls would not be transported high enough to provide a crushing impact. The larger the diameter of the mill, the lower the speed should be: speeds of 15 to 20 revolutions per minute are normal, depending on the size of the mill.
During continuous grinding, the grinding media wear out and must be replaced. The mill is periodically ‘recharged’ with larger media in each chamber and, at longer intervals, the entire load is emptied and reclassified, discarding any media that has worn out or deformed excessively. At the same time, the opportunity is taken to carry out any other maintenance or repair, such as inspection or replacement of the lining of the mill and the diaphragms of the chamber.
Looking back, ball mill systems were used for the three grinding stages, but the development of more energy efficient vertical roller mills (VRMs) has led to their replacement.
Initially, vertical roller mills focused on grinding coal and raw materials, with the adoption of vertical roller mills for grinding cement products, with their finer grinding requirements, which were launched more recently at the end from the nineties. The main reason for the delay in the use of VRM technology for cement grinding was the concern of producers that the qualities of their products would not meet market requirements, specifically in three key areas: water demand, resistance development and setting times. However, during the last 20 years, it has been possible to demonstrate that these concerns are unfounded, and that the quality of the cement obtained from VRM grinding is as good or, in some cases, better than that produced in a ball mill. Consequently, most of the world’s leading cement producers now use vertical roller mills for grinding cement without hesitation. There is no doubt that vertical roller mills offer significant advantages over ball mills in terms of their energy efficiency. The specific energy consumption of a ball mill is greater than that of a vertical mill (VRM) that performs the same operations by a factor of between 1.5 and 2, depending on the degree of optimization of the ball mill.
Today, cement producers have the option of using a wide variety of different systems for cement grinding. A complete list of all available options would undoubtedly include traditional ball mill systems, high-pressure grinding rollers in all types of design and their various combinations with ball mills and, of course, VRM vertical roller mills. All these systems treat the material to be ground differently, since in reality one varies from the other.
Comparative performance parameters for the two systems when used to grind cement
In the case of ball mills the electrical energy consumed in the milling is mainly converted into heat. Excessive heating of the cement during grinding can lead to setting anomalies, so it is necessary to disperse a proportion of this heat by cooling the mill and its contents.
This cooling can be achieved by extracting air through the mill or by injecting a spray of water into the mill to cool the evaporation content.
When using the water injection method, care should be taken to ensure that all moisture evaporates and that none of the resulting vapors condenses again in the mill chambers: continuous automatic control circuits are used to regulate the rate of water spray, so that the temperature is maintained.
The advantage of the vertical roller mill for cement (compared to the ball mill) is that it produces little heat for grinding and quality problems are less likely to occur due to the excessive increase in the temperature of the cement, anyway, injection of water is applied to the table to stabilize the mill and reduce vibrations.
Cement Handling and Storage
From the mills, the finished cement is transported to the storage silos before being sent or packaged in bags for shipment.
Many different types of conveyors are used to handle the finished cement, but extensive use of pneumatic systems is established, taking advantage of the fact that the cement (like other dry bulk powders) behaves like a fluid when activated by air that goes through it. In a widely used type of pneumatic conveyor, air is used only to measure the size of the cement fluid, so that it flows by gravity. In the second type, the air fluidizes the cement and blows it through a pipe; such systems can transport the cement both up and level.
Cement storage silos are reinforced concrete construction, weather resistant and as tight as possible. The storage time is kept as short as possible, and the storage capacity of the silo is normally equal to only about two weeks of production. The cement in the storage settles and, therefore, the air is used to fluidize it for the extraction of the silos in the hoppers from where it is loaded for transport in bulk or packed for shipment in bags.
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