1. Field of the Invention
The present invention relates to a ball mill for grinding materials such as cement clinker and, more particularly, to a partition diaphragm for use in a ball mill, the partition diaphragm being formed inside a body of the mill and having multi-grinding chambers.
2. Prior Arts
Generally, plural grinding chambers, i.e., multi-grinding chambers are provided in a ball mill for the purpose of improving productivity while reducing energy cost per material thereby improving grinding efficiency as a whole, and in which a series of grinding processes are performed comprising the steps of primarily grinding a bulky material using a ball of large diameter (serving as a grinding medium) in a primary grinding chamber; filtering the material ground to a certain grain size; feeding the filtered material to a secondary grinding chamber; and grinding the fed material finely.
For that purpose, the partition diaphragm placed between one grinding chamber and the other performs a filtering action through a screen plate which is stretched over the partition diaphragm and permits only materials ground to a cetain grain size to pass through. In this filtering, if only one screen plate is employed, it is essential that this screen plate is resistant to the impact of the ball in the grinding chamber and that the clearance of the slits provided in the screen plate be about 4 mm. From the viewpoint of casting technique, it is not always easy to secure such a small clearance, and moreover if trying to secure a certain thickness considering sufficient resistance to the impact, there arise further disadvantages such as deflection or blinding of the slits.
Several partition diaphragm have been developed and proposed to solve the mentioned disadvantages as disclosed in Japanese Laid-Open Publication (unexamined) No. 54-127060, the contents of which were further improved and disclosed in Japanese Patent Publication (examined) No. 60-8867. In this improved proposal, a primary screen plate of a partition diaphragm facing primary grinding chamber is formed of a relatively thick member provided with slits each having 20 mm in clearance so as to be resistant to dynamic contact with a ball of large diameter and secure void area. On the other hand, a secondary screen plate disposed in parallel to the mentioned primary screen plate with a rib therebetween is provided with slits each having 6 mm in clearance. Thus, a connecting chamber for communicating with the entire circumferential direction is formed by the mentioned two screen plates. The secondary screen plate can be formed to be relatively thin because this screen plate is not put in contact with the ball. Furthermore, the secondary screen plate can be provided with a large number of punched small holes in view of ventilation.
In the actual operation of the ball mill, it is an essential requirement to balance the level of material in the primary grinding chamber with the level of material in the secondary grinding chamber. In case of the mentioned prior art, the partition diaphragm is formed into a double structure, in which the slit clearance of the primary screen plate is 20 mm and that of the secondary screen plate is 6 mm. As a result of such an arrangement, the problem pertinent to the partition diaphragm comprised of only one screen plate having a slit clearance of 6 mm in clearance of slits are certainly solved, but from the viewpoint of practical operation, it is not always optimum to establish the grain size of the material passing from the primary grinding chamber to the secondary grinding chamber to be smaller than 6 mm in diameter. To be more specific, when carefully observed, it is found that the amount of material fed from the primary grinding chamber to the secondary grinding chamber passing through the filtering slits is excessive over the grinding capacity of the secondary grinding chamber, and that a number of relatively large size (approximately 6 mm) grains are mixed and left in the material fed to the secondary grinding chamber and discharged therefrom as they are without further grinding, which results in the application of a further load to the secondary grinding chamber. Such a disadvantage can be obviously recognized by checking the lowest level of the material presented in a vertical section of the first and second grinding chambers as shown in FIG. 5. That is, referring to FIG. 5, the level of the moving material immediately before the partition diaphragm of the primary grinding chamber is largely lowered, thereby the grinding efficiency at such a portion is also largely lowered. This negative influence is carried over to the next secondary grinding chamber, and such unbalanced movement of the material will eventually bring about a decline in the grinding performance of the entire ball mill. Such an essential problem as mentioned above still remains unsolved and has not been overcome by the mentioned proposal of a mere double structure of a partition diaphragm.
It may be an obvious improvement to establish a slit clearance of the secondary screen plate to be smaller than 6 mm. It is, however, not always easy to manufacture by casting a screen plate the slit clearance of which is smaller than 4 mm. Under the present state of casting technique, disadvantages such as blinding, deflection occur easily and it will be an excessive demand beyond the state of the art in casting to establish and maintain a constant slit clearance smaller than 4 mm. It is certainly possible to provide by punching an iron plate with a large number of circular holes the diameter of which is smaller than 4 mm. But in the case of such punched holes, being quite different from the screen plate manufactured by casting, there remain sharp edges on the punched holes. And it may be sometimes the case that grains of material are caught by such sharp edges and sticked thereto resulting in blinding of the holes thereby causing the operation of the entire plant to stop. This kind of problem occurs easily when the hole size (i.e., hole diameter) is smaller, and the stuck material may bring about corrosion of the screen plate which, in turn, needs troublesome works such as disassembly or replacement of the partition diaphragm. Frequent occurrence of such operation stoppage has a grave influence not only on the grinding process but also on the production activities of the entire cement plant as a matter of course.