Patent Publication Number: US-5158239-A

Title: Dispersing process and stirred ball mill for carrying out this process

Description:
This application is a division of application Ser. No. 518,273, filed as PCT/EP85/00526, Oct. 9, 1985 and now abandoned. 
    
    
     The invention relates to a dispersing process according to the precharacterising clause of Claim 1. 
     It is known that solids can be dispersed in a liquid phase, for example pigments and fillers in a solution of a binder, in stirred ball mills by supplying mechanical energy. The stirred ball mills contain a grinding medium, for example sand, and the energy is supplied by the movement of rotors located in the stirred ball mill. In the dispersing processes used to date, 70 to 90% by volume of the grinding chamber of the stirred ball mill is filled with sand. The mill base flows through the grinding chamber in an axial direction. The throughput of the mill base through the container is in general chosen so that the prescribed desired fineness is achieved after one or more passes. This procedure is frequently referred to as the one-pass or multi-pass procedure. 
     The level of production achievable by this procedure, that is to say the amount of finished mill base produced per hour, can be increased substantially if the procedure described in German Patent Specification 2,230,766 or German Laid-Open Application 1,902,152 is used. In this circulatory procedure, the throughput of mill base through the mill is high, and, after leaving the mill, the mill base flows back into a container, from which it is conveyed again into the mill by means of a pump. The same effect can be achieved if, using the so-called pendulum procedure, the mill base flows through the stirred ball mill with high throughputs from one container into a second container. This process is repeated until the desired fineness is achieved. 
     Furthermore, it is known that the level of production can be increased when finer grinding media are used. In the circulatory procedure or pendulum procedure described, the high throughput of mill base causes relatively high drag forces to act on the fine grinding medium, which is the conveyed with the flow towards the grinding medium separating system of the stirred ball mill. 
     In these procedures, achieving a very hard-wearing seal for the moving parts of the stirred ball mill and separating off the grinding medium from the mill base leaving the stirred ball mill present problems. For the latter purpose, sieves are employed, these being exposed to a great deal of wear due to friction with the grinding medium. 
     It is an object of the invention to provide a dispersing process which overcomes the disadvantages of the prior art and permits rapid and effective dispersing. 
     The invention therefore relates to a dispersing process according to Claim 1. 
     Surprisingly, it has been found that this object can be achieved by reducing the relative charge of grinding medium in the container, and choosing the speed of the rotors so that the grinding medium charge forms a hollow rotating cylinder in the stirred ball mill as a result of the centrifugal force. 
     Because the mill base is fed in radially, the path of the mill base through the grinding medium charge is shorter than in the prior art procedure. This is compensated by virtue of the fact that the mill base has to pass the grinding medium charge more frequently. The drag forces acting radially from the outside towards the inside cause fluidisation of the grinding medium in the centrifugal field. In general, it is advantageous to choose a high radial flow velocity. Surprisingly, in spite of this high flow velocity, very effective dispersing is achieved, the total dispersing time and the cost of monitoring the process being reduced in the circulatory procedure and the pendulum procedure. In this procedure, dispersing of temperature-sensitive goods can also be carried out without difficulty, since only a small increase in the temperature of the mill base can be observed per passage through the stirred ball mill. This supplied heat can readily be removed from the mill base again in an external cooler. In addition, substantial reduction in the dispersing energy employed is achieved with this procedure compared with the procedure involving passes. 
     In this dispersing process, fine grinding media can be employed with high throughputs through the mill; these grinding media cannot be used in machines corresponding to the prior art because they are carried onto the separation sieve at high throughputs. 
     Advantageous embodiments of the process according to the invention are described in the subclaims. 
     The invention also relates to a stirred ball mill for carrying out the dispersing process. 
     Advantageous embodiments of the stirred ball mill according to the invention are described in Claims 12 to 15. 
    
    
     The invention is illustrated below with reference to the drawings. 
     In the drawings, 
     FIG. 1 shows a vertical longitudinal section through a stirred ball mill, 
     FIG. 2 shows a vertical transverse section through a stirred ball mill, 
     FIG. 3 shows the variation of the fineness of a suspension as a function of time, 
     FIG. 4 shows a section corresponding to FIG. 1 but with a stationary cylindrical separating sieve, 
     FIG. 5 shows a section through a stirred ball mill according to FIG. 1 but with sieves arranged along part of a circle, 
     FIG. 6 shows a section according to FIG. 2 through the stirred ball mill according to FIG. 5, 
     FIG. 7 shows a stirred ball mill with stationary sieves, 
     FIG. 8 shows a stirred ball mill with a stationary syphon tube and 
     FIG. 9 shows a stirred ball mill with a sieve rotating in the space free of grinding medium. 
    
    
     In the drawings, 1 denotes a container in which rotors 2 in the form of paddles are located. The mill base is fed in at 3, and 4 represents a sieve. 5 denotes a stock vessel. The rotors 2 are driven via a hollow shaft 6, which also serves for removal of the mill base. 7 represents a gland, and 8 denotes the required pump. 9 denotes a manometer and 10 denotes the grinding medium charge inside the container 1. 
     11 represents a discharge sieve for residual products, and the outflow of mill base is illustrated by arrow 12, while the inflow of mill base is illustrated by arrow 13. 14 represents the inflow of cooling water, and 15 represents the outflow of cooling water. 
     In FIG. 2, 16 represents an idealised mill base path, while the arrows 17 and 18 indicate, respectively, the radial velocity of the mill base and the peripheral velocity of the mill base. 
     Illustrative example: 
     
         ______________________________________                                    
Mill base consisting of                                                   
               30.5% by weight of an alkyd resin                          
               60.5% by weight of titanium                                
               dioxide                                                    
                8.0% by weight of an aromatic                             
               solvent                                                    
                1.5% by weight of additives                               
______________________________________                                    
 
    
     90 kg of this mill base are predispersed in a dissolver. Thereafter, dispersing is carried out by means of the stirred ball mill shown in the figure. 
     Machine conditions: 
     Throughput: 900 kg/h 
     Rotary speed: 650 rpm 
     Effective power consumption: 10.8 kW 
     Grinding medium volume: 15 L 
     Type of grinding medium: Silicon/zirconium oxide grinding medium (diameter 0.6-2.5 mm) 
     Measurement of the maximum sizes of the solid particles according to Hegman gave a value of 100 μm after predispersing in the dissolver, and a value of 6 μm after dispersing for 30 minutes in the stirred ball mill. This gives a production rate of 180 kg/h. 
     It is known that grinding medium charges in stirred ball mills are subject to wear, the resulting loss of grinding medium must be replaced from time to time to ensure optimum operation, and the amount of grinding medium to be replaced is most advantageously determined via the power consumption of the rotating rotor of the stirred ball mill; however, in the case of stirred ball mills based on present-day technology, this is possible only with very expensive topping-up apparatuses, but is achieved in a surprisingly simple manner by the concept according to the invention, in which the required supplementary amounts of grinding medium can be metered, with the rotor running, freely via a feed pipe into the centre which is free of grinding medium, until a prescribed set value of the power consumption of the rotor is achieved. 
     In the graph shown in FIG. 3, the fineness (Xmax Hegman) of a suspension is plotted along the ordinate, as a function of time. The curve 19 shows that the suspension has reached a Hegman fineness of 35 μm after a pass corresponding to 400 minutes, and has reached a fineness of 19 μm after two passes corresponding to 780 minutes. The curve 20 shows that this result can be achieved in a substantially shorter time using a circulatory procedure. 
     FIG. 4 shows, as in FIG. 1, a cross-section through a stirred ball mill according to the invention, but in this case with a stationary cylindrical separating sieve 4. 
     FIGS. 5 and 6 show a section through a stirred ball mill, but in this case with sieves 4 arranged along part of a circle and rotating together with the drive shaft. 
     FIG. 7 shows sieves 4 which are arranged along part of a circle in a manner corresponding to FIG. 5 but in this case are in the form of stationary, i.e. non-rotating, sieves. 
     FIG. 8 shown, as the apparatus for separating off grinding medium, a stationary syphon tube 21 which projects into the space which is free of grinding medium. A sieve is not provided. 
     FIG. 9 shows, as the apparatus for separating off grinding medium, a sieve 22 which rotates in the space free of grinding medium and the speed of which is independent of the speed of the rotor drive.