METHOD AND APPARATUS FOR IMPROVING THE GRINDING RESULT OF A PRESSURE CHAMBER GRINDER

Abstract: Method and apparatus for improving the grinding result of a pressure chamber grinder. Finely divided material is fed into a pressurized equalizing tank (2) by a mechanical feeder device (1). The fed material is then transferred into a pre-grinder (3) where the material is fluidized by grinding-gas jets. The fluidized material-gas flow is divided by a bisecting device (6) into two component flows and accelerated through two accelerating nozzles (8) directed towards the centre point of the main grinding chamber (9). The invention is characterized in that the outlet end of main grinding-chamber (9), via an acceleration tube (10), is connected to a free-flow grinder (11) provided with tangentially directed grinding-gas nozzles (12) and wherefrom the ready-ground final product is being removed constantly through a centrally located exhaust pipe (13).

Description: Method and apparatus for improving the grinding result of a pressure chamber grinder The present invention is concerned with a method and an apparatus for improving the grinding result of a pressure chamber grinder. A pressure chamber grinder is described in the . ' US Patent No. 4,586,661. Therein the finely divided material to be ground is fed by means of a mechanical feeder device into a pressurized equalizing tank, the material, which may have been clodded in the equalizing tank, is made loose by means of a rotor, and the material that was made loose in this way is transferred into a pre-grinder, wherein several grinding-gas jets are applied to the material to be ground so that the material to be ground is fluidized, the fluidized material-gas flow is passed into a bisecting device, wherein it is divided into two component flows of equivalent magnitude and composition, each component flow is passed into the main grinding chamber through a long accelerating nozzle of its own, which said nozzle is directed so that a collision zone for the two component flows is formed in the centre point of the said main grinding chamber. It is an advantage of such a pressure chamber grinder that, as regards its energy economy, it is by far superior to conventional jet grinders, wherein ejectors are usually used as the feeder device. Since in principle, in a pressure chamber grinder, the material particles to be ground are sub- jected to the grinding effect only once, as a rule, depending on the material to be ground, a very little proportion of the particles can pass through or by-pass the grinding zone without being crushed. Even though the proportion of this coarser material fraction in the whole material flow is, as a rule, very little, e.g. less than 1 per cent by weight, in the case of many products there is a necessity to remove these coarse particles from the ground product. In such a case, it is necessary to resort to a separate classifier, from which the coarse particles are returned, in one way or another, into the main grinding chamber for regrinding. In practice, it has, however, been noticed that when an extremely finely divided final product is aimed at, such as in the preparation of pigments, a qualita¬ tively and/or economically fully satisfactory final result cannot be achieved by means of the classifiers in use. This is due to the fact that the particle size of the material to be classified is at the maximum a few micrones. For example, the primary crystal size of titanium dioxide pigments is of the order of 0.2 micro¬ meters, and the average particle size of finely divided titanium dioxide pigment grades is only slightly larger than that. In the jet grinders in common use, in par¬ ticular in the so-called disk-jet grinders, one of which is described, e.g., in the US Patent 2,032,827, a gas suspension of solid material ends up in a circulatory movement, whereby the centrifugal force prevents the coarse particles from escaping from the grinder until they have been ground sufficiently finely. Further developments of this basic jet grinder are described in several patents, e.g. in the US Patent 3,178,121. Attempts have been made to improve the ability of the basic grinder to classify and to grind the coarser and less readily grindable material fraction included in the material to be ground by to the basic grinder connecting various supplementary grinders and circulation systems for coarse material. Such methods and systems are described, e.g., in the US Patents 4,189,102 and 4,238,387. The improvements have given increased efficiency for the grinding of the coarse material, but the solutions are not energy-economically satisfactory. In many cases, the consumption of energy has been increased further. After the apparatuses have become even more complicated. their reliability in operation has suffered at the same time, in particular in the most extensive fine-grindings (pigments) , because the narrow pipe systems and uneven flows result in rapid clogging of the equipment. With reduced homogeneity of the gas suspension of the solid material subject of grinding, the ability of classifi¬ cation of the grinder equipments has been deteriorated even if the grinding capacity has been increased. This is seen as a necessity to separate the unground fraction in order to return it to the primary grinding. The subject of the present invention is a grinding method and an equipment combination which com¬ bine the high grinding efficiency of the pressure chamber grinder described above and the good ability of classifi- cation of a free-flow grinder so that the combination becomes free from the various drawbacks of the two apparatus types at the same time. It has been noticed surprisingly that this can be achieved with an overall energy consumption that is of an order of only 1/2 to 1/3 of the energy required by the conventional jet grinders. This has been achieved by means of a method which is characterized in that a solids-gas mixture ground in a pressure chamber grinder is passed through an acceler¬ ation tube into a free-flow grinder so as to reach a final product of steeper particle distribution, whereby grinding gas is passed into the said free-flow grinder through substantially tangentially directed grinding-gas nozzles, whereby the material-gas flow fed at a high velocity into the free-flow grinder is brought into a rapid circulatory movement so that, by the effect of centrifugal force, substantially only the large particles are subjected to an efficient grinding. By using such a solution, the desired final result is obtained without a separate classifier and substantially with the same good energy economy as in the conventional pressure chamber grinder technique, for in a free-flow grinder the grinding conditions are chosen so that only the oversize particles are ground and the finer particles pass through this after-grinder almost without delay. In such a case, in the after-grinder no more energy is required than in a conventional classifi- cation process. In the solution in accordance with the present invention, it has been possible to reduce the energy consumption even to one third of the energy con¬ sumption of apparatuses using an ejector feeder. The characteristics of the invention come out from the attached claims 1 to 13. In the following, the invention will be de¬ scribed in more detail with reference to the attached drawings, wherein Figure 1 is a schematical illustration of the particle distribution of the final product when a pressure chamber grinder alone is used as well as when a solution in accordance with the present invention is used, Figure 2 is a side view of an exemplifying embodiment of the apparatus of the present invention, and Figure 3 is a top view of the apparatus partly in section. The apparatus in accordance with the invention comprises a mechanical feeder 1 , which may be either a plug feeder, by means of which the finely divided material to be ground is fed into a pressurized equalizing tank 2 as a gas-tight plug by means of a push piston, as is described in the US Patent No. 4,586,661, or a valve feeder, as is illustrated in Figures 2 and 3. The use of such a valve feeder is described, e.g., in the International Patent Specification WO86/02287, so that its operation will not be described in further detail in this connection. The material, which may have been clodded in the equalizing tank, is made loose by means of a rotor (not shown) and is transferred at a preset rate into a pre-grinder 3 by means of a screw conveyor 4. In the equalizing tank 2, an approximately equal pressure is maintained as compared with the pre-grinder 3. In the pre-grinder 3, several strong grinding-gas jets are applied to the material to be ground, so that the material to be ground is fluidized. Grinding gas is passed into the pre-grinder through a gas pipe 5. The fluidized material-gas mixture is made to rush from the pre-grinder 3 into a bisecting device 6, where the said material-gas jet is divided into two component flows of equivalent magnitude and composition. The two outlet pipes 7 of the bisecting device 6 are connected to the two long accelerating nozzles 8 of the pressure chamber grinder, which said nozzles are prefer¬ ably shaped like venturi tubes. The accelerating nozzles 8 are directed so that the component flows rushing through them at an increasing velocity collide with each other in a collision zone formed in the middle point of the main grinding chamber 9. A highly efficient grinding of the material particles takes place in this collision zone. If, by chance, the coarsest particles in the material-gas mixture collide in the main grinding chamber 9 only against particles of a considerably smaller size, the grinding remains incomplete in respect of these coarser particles. When the material-gas flow coming from the main grinding chamber 9 is passed through the acceler¬ ating tube 10 into the free-flow grinder 11, into which grinding gas is passed through substantially tangentially directed grinding-gas nozzles 12, the solids-gas mixture rushing into this grinder 11 at a high velocity is forced into a rapid circulatory movement so that, by the effect of centrifugal force, the coarsest particles stay in this grinder 11 longer and become groun-d more thoroughly than the finer particles, which escape from the free-flow grinder 11 almost immediately, through its exhaust pipe 13, which is placed centrally. Such an apparatus is excellently suitable for the grinding of various pigments, in particular for the grinding of titanium dioxide pigments. In the case of pigments, e.g. titanium dioxide, the basic grinding in the pressure chamber grinder part of the equipment is already so efficient that the major part of the material becomes ground therein sufficiently fine (almost to primary crystals) , and the proportion of an excessively coarse material fraction in the product flow is very little, often lower than one per cent by weight in the whole material quantity. Since these excessively coarse particles are also of very small size, in the latter grinder a very good classification efficiency and only little grinding power are required. The grinding conditions should preferably be chosen so that the sufficiently fine material passes through the free-flow grinder rapidly and that only the excessively large particles become ground. By adjusting the grinding-gas feeds so that a positive pressure of about 0.5 to 1.0 bar prevails in the grinding chamber of the pressure chamber grinder, the flow velocity of the solids-gas suspension at the final end of the accelerating tube 10 becomes higher than 250 m/s. Thereby, highly advantageous grinding conditions are obtained in the free-flow grinder 11. According to the present invention, it is possible to use compressed air as the grinding gas both in the pressure chamber grinder part and in the free-flow grinder, but it is also possible to use, e.g., compressed air in the pressure chamber grinder part and steam in the free-flow grinder, or the other way round. As the free-flow grinder 11, it is possible to use, e.g., a conventional disk grinder, into which the homogeneous pre-ground gas suspension is passed at a high velocity through the accelerating tube 10 without a conventional ejector feed. The grinding-gas nozzles 12 terminate at the mantle face of the grinding chamber. The feed through the accelerating tube 10 is guided so close to the outer circumference of the grinding chamber that an efficient collision with the gas flows discharged out of the nozzles 12 is produced. Thus, the feed point is preferably outside the circle that is contacted by the gas flows discharged out of the nozzles 12 tangen- tially. This location as well as the high velocity in the accelerating tube 10 also guarantee an efficient classification in the grinder chamber. One end wall of the disk grinder is provided with an exhaust pipe 13, which terminates in a gas separator, where the finished product is separated from the grinding gas. In order to reduce the strain on the gas separator, it is possible to install a closing feeder at the opposite end wall of the disk grinder 11, through which said feeder part of the final product is removed. The gas pipe 5 is provided with a control valve 15 for the control of the pressure prevailing in the disk grinder and of its grinding efficiency. On the accelerating tube 10 , whose shape is preferably that of a venturi tube, a manometer may be installed in order to permit observation of the pressure prevailing in the tube 10. In stead of a disk grinder, it is also possible to use a so-called tube grinder as the free-flow grinder, in which said tube grinder the material to be ground is circulated along a closed path and the final product is removed through a centrally placed exhaust opening into the gas separator. From the graph of Fig. 1 it is seen clearly how much steeper the particle distribution is that is obtained by means of a solution in accordance with the present invention as compared with the use of a pressure chamber grinder alone. The vertical parameter is the percentage of penetration of the final product, and the horizontal parameter is the particle size of the par- tides. Since both curves intersect each other at the penetration value of 50 %, the average particle size with both of the methods is the same. In the case of pigments, in particular of titanium dioxide pigments, the change produced by the after-grinder in the particle-size distribution curve is not equally clear, because, out of the whole material quantity, the proportion to be ground in the after-grinder is little. From the point of view of the quality and usability of the product, the improvement that can be achieved is, however, of great importance. Pigments are used most of all in paint industry, and considerable quantities also in plastics and fibre industries. A minute proportion by weight of coarse particles is enough to produce deLrimental nubs or holes in thin paint or plastic films.

Claims: WHAT IS CLAIMED IS: 1. Method for improving the grinding result of a pressure chamber grinder, wherein the finely divided material to be ground is fed by means of a mechanical feeder device (1) into a pressurized equalizing tank (2), the material, which may have been clodded in the equal¬ izing tank, is made loose by means of a rotor, and the material that was made loose in this way is transferred into a pre-grinder (3) , wherein several grinding-gas jets are applied to the material to be ground so that the material to be ground is fluidized, the fluidized material- gas flow is passed into a bisecting device (6) , wherein it is divided into two component flows of equivalent magni- tude and composition, each component flow is passed into the main grinding chamber (9) through a long accelerating nozzle (8) of its own, which said nozzle is directed so that a collision zone for the two component flows is formed in the centre point of the said main grinding chamber, c h a r a c t e r i z e d in that a solids- gas mixture ground in the main grinding chamber (9) is passed through an acceleration tube (10) into a free¬ flow grinder (11) so as to reach a final product of steeper particle distribution, whereby grinding gas is passed into the said free-flow grinder (11) through substantially tangentially directed grinding-gas nozzles (12), whereby the material-gas flow fed at a high velocity into the free-flow grinder (11) is brought into a rapid circulatory movement so that, by the effect of centri- fugal force, coarser material particles stay in this grinder (11) longer and are ground more thoroughly than finer particles. 2. Method as claimed in claim 1 , c h a r ¬ a c t e r i z e d in that the grinding conditions are chosen so that only the oversize particles are ground in the free-flow grinder. 3. Method as claimed in claim 1 or 2, c h a r a c t e r i z e d in that a positive pressure of about 0.5 to 1.0 bar prevails in the main grinding chamber. 4. Method as claimed in claim 3, c h a r ¬ a c t e r i z e d in that the homogeneous solids-gas suspension formed and ground in the pressure chamber grinder is passed as such, accelerated by means of a difference in pressure, into the grinding and classifi- cation zone of the after-grinder. 5. Method as claimed in claim 3 or 4, c h a r a c t e r i z e d in that compressed air is used as the grinding gas both in the pressure chamber grinder part and in the free-flow grinder. 6. Method as claimed in claim 3 or 4, c h a r a c t e r i z e d in that, as the grinding gas, compressed air is used in the pressure " chamber grinder part and steam is used in the free-flow grinder. 7. Apparatus for improving the grinding result of a pressure chamber grinder, which said apparatus com¬ prises a mechanical feeder device (1), a pressurized equalizing tank (2) jointly operative with the feeder, which said equalizing tank is provided with a rotor and with a screw conveyor (4) for carrying the material to be ground into the pre-grinder (3) , into which grinding gas is passed through a gas pipe (5) , as well as a bisecting device (6) provided at the outlet side of the pre-grinder (3) , both of whose outlet pipes (7) are connected to long accelerating nozzles (8) of their own, terminating in the main grinding chamber (9) and being directed so that the material-gas jets rushing out of them collide against each other in the centre point of the grinding chamber (9), c h a r a c t e r i z e d in that to the outlet end of the main grinding chamber (9), via an acceleration tube (10), a free-flow grinder (11) is connected, into which grinding gas is passed through tangentially directed grinding-gas nozzles (12), and that from the material-gas flow, which is in a rapid circulatory movement, the ready-ground final product is being removed constantly through a centrally located exhaust pipe (13). 8. Apparatus as claimed in claim 7, c h a r ¬ a c t e r i z e d in that the free-flow grinder (11) consists of a conventional disk grinder, in whose mantle face the grinding-gas nozzles (12) terminate and in one of whose end walls, at the centre axis, the exhaust pipe (13) is arranged, which said exhaust pipe terminates in a gas separator. 9. Apparatus as claimed in claim 8, c h a r ¬ a c t e r i z e d in that the acceleration tube (10) terminates in the efficient grinding and classification zone in the disk grinder so that the feed point is out¬ side the circle that is contacted by the gas jets dis¬ charged from the grinding gas nozzles (12) tangentially. 10. Apparatus as claimed in claim 8 or 9, c h a r a c t e r i z e d in that a closing feeder (14) is provided centrally in the opposite end wall, part of the final product being removed out of the disk grinder (11) through the said closing feeder. 11. Apparatus as claimed in claim 10, c h a r - a c t e r i z e d in that the acceleration tube (10) has the shape of a venturi tube and is provided with a mano¬ meter for indication of the pressure prevailing in the tube (10). 12. Apparatus as claimed in claim 7, c h a r ¬ a c t e r i z e d in that the free-flow grinder is of the tube grinder type, wherein the material to be ground is circulated along a closed path. 13. The use of the method and the apparatus as claimed in claims 1 to 12 in the preparation of titanium dioxide pigments.

Inventors: KORHONEN HEIKKI, NIEMI JOUKO, PYOERIAE KAARLO, KORHONEN, HEIKKI, NIEMI, JOUKO, PYOERIAE, KAARLO
Applicants: FINNPULVA AB OY, KEMIRA OY, OY FINNPULVA AB
Application year: 1986
Main classification: B02C19
IPC codes: B02C19