MIXER FOR COHESIVE POWDER MATERIALS

Abstract: Method for a mixer for especially cohesive powder materials by rubbing the cohesive materials against the mixer's wall (6, 8) with at least one rubbing vane on each rotor so that the shearing forces are applied to the material during the rubbing process between the rubbing vanes and the wall (6, 8). The mixer comprises two rotors with several vanes (3) spaced along the rotor with at least one rubbing vane (1, 4, 5, 7) provided on each rotor in addition to the mixer's mixing vanes (3), and the rubbing vanes are arranged such that the materials are rubbed between the rubbing vanes and the wall (6, 8) in the mixer's housing.

Description: Mixer for cohesive powder materials The present invention relates to a method and a mixer for mixing of especially cohesive powder materials. In various industrial branches, many industrial processes involve the mixing of different materials in one way or another, be it raw materials or intermediate products. Such industrial mixing processes can vary significantly in complexity, from the quite simple process where desired homogeneity in the mixture is reached with simple means, through the mor difficult processes which put greater demands on the equipment and the process cycles. Mixing processes can generally be divided into two categories, namely mixing of liquids, gases, solids in liquids or a combination of such on one hand, and a mixture of parti- culate materials, that is powder in dry form or possibly with a moistened content that is lower than the saturation limit. In mixing of powders, there is no motion of particles without the particles being affected, in contrary to fluids, where motion can happen by diffusion or also as Brownian movement. Powder affects in a large degree the mixing process itself by the particles physical properties as size, density, form, surface conditions, and so on. Further, the individual particle in a mass of powder at a size that is several orders of magnitude higher than the particles in a fluid. Many of the problems that arise in mixing processes of various types, are due to differences in the above mentioned properties which are not known, or that have not in sufficient degree been considered in the selection of equipment or mixing cycle. The problems that arise for this reason, result in for example a series of failed attempts to use equipment that is designed for mixing of liquids or the mixing of dry powder materials. Poorly performed mixing processes can result in great losses for the user, losses that can manifest themselves in the form of unnecessary high consumption of raw materials, large production of rejects, problems in the following equipment in the production and dissatisfied costumers and possible loss of market shares . The primary function in the mixture of one or more cohesive powders in the same mixture requires a velocity gradient that varies from zero and up to peripheral velocity, and at the same time a shearing force larger than the slip force in the micro plane. This slip force is the force that the individual particle must have in order to escape the particle lump or heap or agglomerate, that is a collection of single particles that works as a collected particle, so that it works alone and therefore also so that the entire surface, and not only a small side surface becomes available for the mixture's ultimate target, which may be the colouring by pigments. This can be better illustrated by considering on one hand that the pigment particles consist of a large ball in a football field, which still will look green. If the ball is divided into particles with for example red colour and with the size of 2 μm, the whole field will look red. I practice the situation is often that the pigment particles work as a collection of particles, consisting of for example ten particles of 2 μm, and the colour yield will then only make one half red, because the pigments are lying on the top of each other and not beside each other. We understand then, that for pigment processes the trick is to convert collections of particles to single particles. For example, in asphalt production the stone material must show up in their smallest single particles to obtain the desired reactivity with the emulsion. What is important here, is the specific surface because the efficiency of the reactivity relates to the area of the available surface that is exposed to reaction. It is usual that the filler appears as collections of 1 mm in dimension, it is evident that the end product will be totally different than if the particles appear by themselves in the size of about 10 μm. Cohesive particles are particles that because of some internal exchange of forces have tendency to appear as a collection of single particles because the mass is not large enough to dissolve the lump. In for example medicines, the quantity of filling materials with the present invention might be reduced because less solution is required. The mass is dissolved into single particles so that one can produce much smaller tablets and in fact make the medicines safer and easier to swallow. Generally speaking, the mixture of powders is more difficult than the mixture of liquids and gases, especially mixture of dry powders. It is often merely impossible to reach theoretically optimum conditions within the frame of the available process techniques, that is use of time, production volume and costs. Choice of proper equipment that could be used for various assignments is essential for an economically favourable result for the users of the mixing equipment. On the other hand, there are many different types of equipment that at first sight seem to be the same, on account of the many problems around mixing processes. Mixing of solids are carried out by applied diffusion, applied convection or applied shearing force. Most processes and equipment will work with a combination of these mechanisms, but for one base mechanism. The physical bulk properties of the powder materials change in a significant degree, depending on the size of the particles. If the mass has an average particle size of about 100 μm, the powder mass will usually behave as a free flowing mass with a defined slide angle, without tendency to cohesiveness. The mass will not acquire an inner strength after outside pressure, nor will it withstand greater pressure without break down. In order to obtain a successful mixture of such mass, it should be diffusive or convective. If the particle size on the other hand is smaller, that is the main part of particles are substantially less than 100 μm, the powder mass will change character from a free flowing mass to a cohesive mass. It will then get an indefinite, but very large slide angle, which could also be negative, and it will acquire a relatively large strength after outside strain. Changes in the physical properties also show up in the fact that the powder get a significantly greater ability to form agglomerates, that is there will be large lumps that each consists of many small particles. The cause of this change in mass properties is that the interparticular forces that contribute to the tying together of the particles will be the dominating forces working in the powder when the particle size is reduced. Interparticular forces could be Van der Waal forces, electrostatic forces, chemical, magnetic or capillary forces. This means that the effects of the force of gravity would be less compared to these forces. As a result, the force of gravity will not by itself be able to break down the powder to single particles, which is necessary in order to obtain a good mixture at microlevels. The powder or agglomerates must thus be exposed to shearing .forces in the form of rubbing or high kinetic energy. Use of high kinetic energy, as for example may be obtained by fast rotating knives, is well known and used in many connections. It is an effective method, but may, however damage the powder material if this is of such a nature that it is easily crushed or in other ways is unable to withstand rough treatment. Further, other problems may arise, for example pollution of the powder in that the crushing tool itself wears down. If the powder is of an adhesive nature, the result could easily be that the crushing tool soon be covered by powder and therefore stop functioning satisfactorily. The present invention supplies a possibility to supply to the powder mass shearing forces without the above mentioned draw backs. This is achieved by the method and mixing device according to the invention as they are defined with features set forth in the claims. The present invention achieves a number of advantages compared to the known art. Segregation properties are signi- ficantly improved on account of a fine distribution in the micro plane. Environmental improvements are achieved by the fact that the process can be trimmed very precisely so that smaller quantities of harmful materials are being used. Significant reduction in costs are achieved as a result of small consumption of expensive component. In this connection one could mention pigment processes, such as production of powdered lacquers, in which the present invention can achieve savings in the order of 15% of the consumption of pigment, which cost significantly more than filler materials. Other effects are increased repetition reliability, simplified and at the same time improved process control, better planning tool and more reliable delivery. The drawing shows schematically embodiments of the invention, where Fig. 1 shows a cross section of a mixer with two rotors in an embodiment with rubbing wanes, Fig. 2 shows a corresponding partial section with another embodiment of the rubbing wane, Fig. 3 shows, corresponding to Fig. 2, a third embodiment of rubbing wanes, Fig. 4 shows a plane view of the mixer in Fig. 1, and Fig. 5 shows an embodiment of a mixer where the rubbing process takes place against the mixer ' s end wall . With special rubbing wanes 1, 4, 5 provided between regular wanes 3 which takes care of the transport and blending in the mixer's housing 6, it is achieved that the powder is broken down in several shearing planes, and that it therefore takes place a crushing of agglomerates at the same time as the mixing takes place at the micro plane. Rubbing wanes 1, 4, 5 rub, by rotation by the rotor 2, materials against the bottom wall against the housing 6, and thereby applies sufficient shearing forces that the bonds in the agglomerates are broken. The rubbing process can also be provided by the rubbing of the materials by rubbing wanes 7 against the end wall 8 of the housing 6, as shown in Fig. 5. Such a mixing process can be performed at low velocities, and therefore it is gentle to the powder mass. The method and the mixer device can be combined with other mixing prin- ciples, so that the three mechanisms for mixing can be utilised at the same time, since the mixing will take place by means of shearing in the micro plane while the entire mass of particles is stirred and mixed by means of diffusion and convection. A mixer ' s housing 6 for the embodiment of the method according to the present invention can make use of rubbing wanes 1, 4, 5 mounted on two parallel rotor 2 with opposite rotation direction, where the rubbing wanes press against a part of the powder mass and rub this against the stationary housing 6. This causes, because of the applied stress, that a number of breaks will occur in the powder mass. In the break plane it will, because of the shearing stresses, be a relative motion between each side of the break plane, and because of this motion, it will be a relative mass transport in the break zone itself. Agglomerates that happen to be in the break zone will therefore be broken down because of the shearing stresses, and the various parts of agglomerates will immediately after the break down be transported away from each other because of the relative motion than happens between the two sides of the shearing plane. This results in a mixing process on micro level that can be performed at low velocity. The method and mixing devices according to the present invention can be utilized in all processes comprising mixing of fine, cohesive powders where an active medium is to be evenly distributed in a mass of powder. Such tasks exist within several branches of industry, for example, in production of medicines, for mixing of pigments in raw materials for paint, chemical reactants and catalyzers, flourmixtures, coloured chalk, coal or binders, powders for toners and copying machines, etc. The method and the mixing device may also advantageously be used for mixing of small quantities of liquid in dry powders or for mixing of products in the paste form. The rubbing wanes can have the shape shown with the rubbing wane 1 in Fig. 1, which consists of a pipe which is bent in the form as shown in the Figure. Such bent rubbing wanes 1 in the form of pipes are fastened to the rotors 2, distributed over its length between the mixer' s mixing wanes 3. Fig. 2 shows another embodiment where the rubbing wanes 4 are bent so that the products are rubbed along rubbing wanes 4 while they are thrown radially outwardly. Fig. 3 shows on the other hand an embodiment of a rubbing wane 5 that has the form of an elliptic disc, also shaped such that the products will rub against the wall 6. The width of the rubbing wanes 1, 4, 5, 7 against the wall 6, 8 are chosen, depending on the materials to be mixed, and otherwise adapted to the special conditions. The rubbing wanes may be produced in the form of plates or pipes. The present invention has shown to provide specially good results for gentle mixers with low peripheral velocities, for example in the range from 0 to 3 m/s.

Claims: P a t e n t C l a i m s 5 1. Method for a mixer for especially cohesive powder materials, CHARACTERIZED IN that the cohesive materials are rubbed against the mixer's wall (6, 8) with at least one rubbing wane on each rotor, so that the materials are exposed to shearing forces during the rubbing process between the rubbing wanes and o the wall ( 6 , 8 ) . 2. Mixer for blending of especially cohesive powder materials, including rotors with several wanes (3) provided in a space along the rotor, CHARACTERIZED IN that at least one rubbing wane (1, 4, 5, 7) is provided on each rotor in addition s to the mixer ' s mixing wanes ( 3 ) , and in that the rubbing wanes are arranged so that the materials are rubbed between the rubbing wanes and the wall (6, 8) in the mixer's housing. 3. Mixer according to claim 2, CHARACTERIZED IN that a number of rubbing wanes (1, 4, 5) are mounted on the rotor (2) 0 between the rotor ' s mixing wanes ( 3 ) , and in that the rubbing wanes are shaped radially outward extending rods with preferably a rounded surface against the particles, and in that at least the outer parts of the rubbing wanes are bent to ascertain that the particles are rubbed against the rubbing wanes both against each 5 side of each rubbing wane and against the periphery, so that they provide shearing forces that break down the agglomerates and the particles are mixed. 4. Mixer according to claim 3, CHARACTERIZED IN that the rubbing wanes ( 5 ) are radially arranged disks with rounded o edges with an axial direction elliptical or similar shape. 5. Mixer according to claims 2-4, CHARACTERIZED IN that the rubbing wanes (1, 4) are pipes that are bent in the radially outer portions. 6. Mixer according to claim 2, CHARACTERIZED IN that 5 each rotor (2) has one rubbing wane (7) at each end, adapted to rub the material against the mixer ' s end wall ( 8 ) .

Inventors: NORDAHL GEIR, NORDAHL, GEIR
Applicants: NORDAHL GEIR, NORDAHL, GEIR
Application year: 1998
Main classification: B01F7
IPC codes: B01F3, B01F7