How does electromagnetism get involved with the grinding of limestone, clinker or any other construction material? As a result of mechanical effects (such as blows, crushing and abrasion) on a comminuted material, a huge quantity of free electric charges are generated inside a cement mill. Blows produce cracks in the comminuted material. Electric charges emerge on the walls of the cracks. Some cracks will split the comminuted material pieces as intended, but other cracks will be tightly closed by the dissimilar electric charges attracting one another, and thus will not split the pieces. The most complicated situation is usually created in the last chamber of the mill. Here, the grinding takes place on the crystal level. There is also an aggregation of powder material in the outlet of the mill due to electrostatic attraction. When grinding aids are not applied, the metal surfaces contacting with comminuted material become covered with hard, dirty coats. These adhesion forces have an electrostatic nature too. Aggregation of particles is also observed during separation. Some parts of the fine powder fraction are carried away by bigger particles and is returned to the grinding circuit. This happens because, in this case, electrostatic attraction is stronger than centrifugal forces. Thus, free electric charges decrease the efficiency of grinding.
Special grinding aids are usually used for intensification of these processes. They help to decrease the adhesion of powdered material to the grinding surfaces, as well as to decrease the aggregation of comminuted particles to each other. Grinding aids have conductive properties. They partly neutralise electric charges on the material surface inside the mill.
But it is not enough for maximum realisation of the potential of a cement mill and separator. Grinding aids are fed to the input of the mill. Evidently, some part of them evaporates under high temperature and is taken away by the airflow without any positive results for grinding. Therefore, especially in the last chamber of the mill, where negative electrostatic forces are the strongest, the effect of grinding aids is not enough.
The Ecofor Device and the method for intensification of grinding have been invented and developed by The Ecofor Company. It is protected by the international application No PCT/RU98/00226. The device removes electric charges accumulated during the comminution more effectively. It is therefore possible to increase the capacity of a cement mill.
Many benefits claimed
Several properties are claimed for the Ecofor Device. It decreases the resistance of the material to grinding, which is the same as increasing the grindability of the material. It also prevents the aggregation of powder material in the last chamber of the mill and helps to segregate the fine particles from larger ones in the separator, thus decreasing the return flow of material to the mill. The Ecofor Device helps to avoid the charging-up of insulating material inside mills and separators.
Cement is not overground and the shape of particles becomes less spherical. Dispersion of granule sizes becomes narrower. In other words, there are less coarse and super-fine fractions, and there is an absence of spherical fine-particle conglomerates.
The work of the mill, separator and auxiliary equipment (such as a Fuller – pump) is improved. As a result of the process, grinding bodies and working surfaces become cleaned of hard dirty coats (Fig. 1). In mills working without grinding aids, application of the Ecofor Device may result in increasing the mill capacity by 12-25%. The percentage depends on the piezoactivity of the comminuted material and the grade of the manufacturing cement. The higher is the grade, the higher the capacity increase (see Table 1). The device can also be combined with grinding aids. It is possible to reduce the quantity of grinding aids to 25 to 30% and at the same time to increase the capacity of the mill by up to 15%.
The Ecofor Device can be applied to both opened and closed grinding systems. If the output of the mill is less than 40t/h, only one Ecofor Device, connected in parallel to the mill and separator, need be used. When output is more than 40t/h, it is necessary to use two devices: one for the mill and the other for the separator.
Data from official reports of the “ECOFOR” method and device demonstration.
|Tourah Portland Cement
Without grinding aids
|Navoi Cement Plant
|Without grinding aids
|Without grinding aids
Cement Plant “China”
|Without grinding aids, Slag 30%
|Cement Plant “Jun Nun”
|Without grinding aids, Slag 28%
|Cement Plant “Huai Ha”
|Without grinding aids
|Raysut Cement Co.
|raw meal OPC, ROC
Cement dust ejection was decreased by 15%. Grinding aids were excluded completely.
|Kunda Nordic Cement Corp.
|Grinding aids less 30%
|Oman Cement Co.
|Grinding aids less 12%
|Podolsky Cement Co.
|With grinding aids
Without grinding aids
|Vassiliko Cement Works Ltd.
|With grinding aids
With grinding aids
Installation and operation
The device uses single-phase electric power, 220-240V, 50-60Hz, 100W. After turning on the devices, the grinding and separation process becomes more active. At first the elevator load begins to increase because the powder material aggregation in the last chamber decreases. The return flow from the separator decreases too. The mill becomes emptier and the Blaine increases. It becomes possible to increase the feed to the mill. Here, a temptation to reach the highest capacity inevitably leads to mill overload if the operator does not stop the additional feeding in time. If the mill becomes overloaded, the operator has to stop the feeding for some minutes and thereafter keep the feeding at an acceptable level. The main principle is to keep the right quantity of material inside the mill. Application of the Ecofor Device allows more material than usual inside the mill.
The quality of grinding depends on several factors, including the quantity of material inside the mill (Q). The quality of grinding (Blain) has an optimal relation to Q. There is a maximum Blaine value for a definite optimal Q. When Q is more than the optimal level, the mill becomes fuller and the Blaine decreases. When the Blaine is below normal, the operator or the control system decreases feed to the mill and when the Blaine is above normal they increase the feed. Usually everybody is working in such an ‘optimum material-quantity corridor’.
After turning on the Ecofor Device for several hours, Q may decrease to a less-than-optimal level, and thus the mill becomes emptier and ‘hungry’. Abrasion of the material weakens and the Blaine drops accordingly. Q depends on the total feeding and grindability of the material. The grindability of the material depends on its strength and is increased by the Ecofor Device. After several hours of mill operation, without changing the feed rate, it is possible to obtain small Blaine values. This means that the mill is ‘hungry’, as described earlier. In order to get the right Blaine level, it is necessary to increase the feeding step by step, and to find a new material-quantity corridor – a new working regime, which will increase the capacity of the mill.
Closed grinding systems are very sensitive. A mill and a separator is a system with positive feedback, as some part of the material returns from the separator into the mill. But it is well known that negative feedback is used for improvement and stabilisation of complex control systems. The positive feedback system tends to be unstable. Both the mill and separator change their operation when connected to the Ecofor Device. If the feed of the mill is increased, a regime of undesired oscillation may be set up, during which increased oscillation of the system will be observed, or the material returning to the mill will constantly increase.
The grinding improvement effect of the device appears to be in 12 to 24 hours, depending on the mass of equipment, and reaches its greatest effect after several days. After the device is switched off, the mill continues to work with a high capacity for some time – it may be 2 days for closed system and more for an open one. It is not necessary to switch off or disconnect the device for the period of a technological or a maintenance stoppage of the mill. The device cannot do any harm to the operator or to the equipment. On the contrary, it influences positively the bearings and gears of the equipment. Friction losses decrease by about 12%, because friction is of an electrostatic nature too. The flowability of the powdered material is improved as a result of its electrostatic discharging. It is important not only to cement but to lime meal too.
Using the ECOFOR method and device gives the possibility to save energy or, in other words, to have a more profitable cement plant.