U.S. patent number 3,693,791 [Application Number 05/009,270] was granted by the patent office on 1972-09-26 for method of, and apparatus for, spiral air classification of solid particles in a gaseous carrier. This patent grant is currently assigned to Ingenieurbureau Dr. Brehm AG. Invention is credited to Erich Beck.
United States Patent | 3,693,791 |
Beck | September 26, 1972 |
A multi-stage spiral air classification method and apparatus for dividing into at least three fractions solid particles suspended in a gaseous carrier, in which a spiral flow of additional carrier gas is introduced to the flow of the suspension at the entry into each subsequent stage. This invention relates to a method of, and apparatus for, spiral air classification of solid particles in a gaseous carrier. For the classification of very finely divided solids, for example with particle sizes under about 50 microns and down to about 2 microns, screening methods are no longer suitable and for this purpose use is frequently made of so-called air classification processes. In these processes finely divided solids are selectively deposited in a gravitational field with the aid of flowing gaseous media, usually air. Depending on the mode of operation of the installation or the type of gravitational field, basic distinctions can be made between gravity air classifiers, screw air classifiers, cyclone air classifiers, and spiral air classifiers. In respect of the type of force applied to the particles to be sifted a sharp distinction is not always possible, since in some installations or in some methods, although use is mainly made of the force of gravity, nevertheless a certain rotation or centrifugal force may also be superimposed by suitably guiding the gaseous carrier. In the case of pure spiral air classifiers, the force of gravity plays practically no part, since the centrifugal forces to which the particles to be classified are subjected are here many times greater, for example from ten to one hundred times greater, than the force of gravity. For this reason it is possible to achieve accuracy of separation and throughputs which were quite impossible to obtain with earlier gravity classifiers. The development of spiral air classifiers is due to a considerable extent to the work of K. Wolf and H. Rumpf, and reference may be made to literature on this subject, for example Chemie-Ingenieur-Technik, 3 (1952) 129-135, "Weiterentwicklung des Spiralwindsichters (Wirbelsichters) (Further development of the spiral air classifier (whirl classifier))" by H. Rumpf and F. Kaiser, and also the passages of literature mentioned in this publication. The predominant majority of air classifiers introduced and tested industrially up to the present time permit separation of material into only two fractions. It is true that some proposals have already been made for dividing the material into more than two "fractions" by means of air classifiers, but this is not actually a type of classification that can be regarded as fractionation. In the case of genuine fractionation a series of fractions are obtained, that is to say portions of the material which in each case contain practically only constituents of a single particle range and only very few constituents of successive finer fractions. If, for example, a "fraction" consists of 60 - 70 percent of material with particle sizes of over 10 microns and 30 - 40 percent of components from the next finer fraction (also known as "undersize content"), for example with sizes of 3 - 10 microns, it is not possible to speak of fractions or fractionation but only of concentrations of particles of determined size ranges. Thus, for example, according to an earlier proposal the material introduced into a single annular disc type spiral chamber of an air classifier is split up into more than two portions of different grain sizes, but what is achieved in practice is only a concentration but not fractionation. Successful attempts to develop a multi-stage spiral air classifier capable of industrial use for sharp fractionation of fine to very fine material have hitherto not been known, apparently because the technical problems occurring in successively carrying out a series of stages of fractionation could not immediately be solved either with respect to the apparatus required or with respect to operational requirements. On the other hand, for the purpose of improving accuracy of separation in the dividing of a material into two fractions it has for a long time been known for two stages of a spiral air classifier to be operated in parallel. The present invention seeks to provide a spiral air classification method and an apparatus suitable for carrying out this method, for the purpose of dividing a material into three or more proper fractions, while avoiding the disadvantages of the prior art and without incurring unacceptable expense for apparatus or operation. According therefore to one aspect of the present invention, there is provided a method for the multi-stage spiral air classification of solid material in particle form for the purpose of dividing the material suspended in a gaseous carrier medium into at least three fractions of different particle sizes, said method including separating part of the material as a coarse fraction in each stage, passing the remainder of the material together with the carrier medium into a subsequent stage for further separation of a coarse fraction from the remainder of material, repeating the said separating and passing steps until after the last stage all the material still suspended in the carrier medium is separated practically as a whole, and feeding, at the or each subsequent stage, additional carrier medium in the form of a spiral flow to the flow of material and carrier medium entering said stage. It has been found that the method set forth above is capable of achieving sharp fractionation without excessive decrease of throughput with very fine separation limits. The number of fractions which can be obtained by the method set forth above will be seen to be one more than the number of stages in the process. The current of carrier medium additionally introduced into the stage can without difficulty be superimposed on the current of the material and carrier medium introduced into said stage if, as is generally preferred, the direction of the spiral flow of material and carrier medium is the same as the direction of the spiral flow of the additional carrier medium introduced. In principle however it is also possible to work with counter-current, in which case the direction of the spiral flow is reversed. The method set forth above is particularly suitable for dividing fine material, with particle sizes in the region from 40 microns down to 2 microns, into a number of sharp fractions, while satisfactorily sharp fractionation, even with a high throughput, can still be achieved particularly in the particle size range below 15 microns, and even below 10 microns. The finely divided material which can be treated in this manner includes various industrial materials, such as cement, powdered limestone, foamed or foamable quartz, polystyrene, gypsum, etc., that is to say the method is suitable both for processing inorganic or mineral materials and also organic materials from various sources. The method of the present invention offers particular advantages where economical and technically reliable fractionation of fine material is required, as is the case for example in the abrasives industry, particularly for the production of graduated very fine-grained abrasive powders. Provided that the material processed has no tendency to produce dust explosions, the gaseous carrier medium used may normally be air. It is however also possible to work with an inert gas, for example nitrogen, with an oxygen content of below about 1 percent, when there is a risk of explosion or of damage being caused to the material by oxygen. In respect of moisture content the carrier medium should correspond to the values usually applied to air classification processes, having regard to the material treated. A dry medium is generally preferred. In order to produce the flow in the various stages, a fan of the exhaust or blower type, with a suitably powerful drive, may be used in the usual manner. When working with air, it is possible to operate with an open or closed circuit for the carrier medium, whereas when working with inert gas only a closed circuit will normally be economically acceptable. The ratio of fine material to carrier medium in the first and subsequent stages depends on the kind of fine material, its particle shape, and its particle size distribution. Optimum operating values can be determined in individual cases by means of simple tests. The additional supply of carrier medium to the stages following the first stage - or in the preferred embodiment with pre-distribution also to the first stage - is preferably controlled in dependence on the decrease of particle size in each stage, that is to say the more the particle size decreases in a given stage the more additional medium preferably should be introduced into that stage. In the method of the present invention it is preferable to operate by imparting to the additional carrier medium introduced into a stage a peripheral speed component which is greater than the peripheral speed component of the flow of material and carrier medium introduced in the same direction into that stage. According to another aspect of the present invention, there is provided a multi-stage spiral air classifier for classifying solid particles suspended in a gaseous carrier medium into at least three fractions of different particle sizes, said classifier including separating means for separating part of the material as a coarse fraction in each stage, means for passing the remainder of the material together with the carrier medium into a subsequent stage for further separation of a coarse fraction from the remainder of the material, and means for feeding, at the or each subsequent stage, additional carrier medium in the form of a spiral flow to the flow of material and carrier medium entering said stage. The classification stages of the process are generally carried out in classification chambers which are in the form of disc rings or flat cylinders and which hereinbelow will also be referred to as classification chambers or simply as chambers. If a spiral flow is produced in a chamber of this kind, a given point in the flow moves with a speed component in the direction of rotation (peripheral speed component) and with a second speed component inwards in the radial direction. Although the peripheral speed at a given point in a stage of the process will practically always be higher than the speed in the radial direction, the flow effecting passage through the chamber will here be regarded as the main flow, that is to say in the classification chamber the radial direction inwards and, in the transitional region between two parallel adjoining classification chambers, the axial direction towards the downstream side. The movement sequence made possible by the method of the invention by the serial arrangement of a succession of stages is important to the attainable sharpness of the fractions. If the stages lie in superimposed parallel planes, at the transition from one stage to the next stage it is possible to obtain two changes of direction of the main flow by approximately 90.degree. each, that is to say at the transition from one stage to the next a given particle follows a step-shaped path, viewed radially. In respect of the total flow this means a transition from the spiral movement (in one stage) into a helical movement (in the region of transition to the next following stage), and then again a return to the spiral movement. This deflection, referred to as the "staircase effect," which in the method of the present invention can be controlled very effectively by the construction of the installation used to carry out the method, probably effects a retention or rebounding of the coarse fraction in a given stage, while the fraction of fine material in this stage passes into the following classification chamber with only a peripheral speed component, and can accordingly pass out with less disturbance on the coarse material side of this following stage. In order to make the best use of the above-described staircase effect or rebound effect, the transition from material and carrier medium from a preceding classification chamber, preferably in the form of a flat cylinder, into the following classification chamber, preferably likewise in the form of a flat cylinder, may be formed by an annular gap which is disposed coaxially to the appertaining classification chambers. Entry into the first stage and exit from the last stage are however also preferably effected in each case through a coaxial ring gap. As mentioned above, according to a preferred embodiment, the material to be treated is suspended in the carrier medium before it enters the first stage. This can be effected in an annular chamber into which a jet of the carrier medium is introduced tangentially while the material is fed in corresponding doses into the flow thus produced. If the connection between the predistributor and the classification chamber of the first stage, which is preferably in the form of a flat cylinder, is effected once again through a coaxial ring gap, here again there may be a deflection of the particles of material, that is to say already at the entry into the first stage. Similarly, the outlet from the last stage may be in the form of a coaxial ring gap and may effect a further deflection. The very fine material leaving the last stage can be separated in the usual manner from the carrier medium, for example by means of a cyclone, while any particles not retained by the cyclone can then be filtered out or recycled. The multi-stage spiral air classifier according to a preferred form of the invention is characterized by at least two classification spaces or classification chambers which are in the form of annular discs and disposed coaxially one above the other. These classification chambers are in each case connected to one another by an annular opening, which is disposed coaxially to the classification chambers and which forms the outlet aperture for the fine material and the carrier medium of the preceding stage, or the material inlet of the next following stage. At least the second classification chamber and each subsequent classification chamber have at the periphery an inlet for additional carrier medium. According to a first preferred embodiment, the first classification chamber has no inlet for additional carrier medium, but is fed only with the amount of carrier medium serving to suspend the charge material and with the charge material itself, that is to say in this case the distribution of the charge material in the carrier medium is also effected in the first stage. This classification chamber is preferably of spiral shape, while the second and every following classification chamber is in the form of a flat cylinder. According to a second preferred embodiment, the spiral air classifier of the invention has an annular predistributor or pre-mixing chamber, which is constructed for introduction of a tangential flow of carrier medium. The charge material is preferably fed or metered into the pre-mixing chamber in such a manner that on entering it, the material is deflected, for example by 90.degree.. The pre-mixing chamber may have its own blower. In this preferred embodiment with a pre-mixing chamber, the first classification chamber is also provided on its periphery with an inlet for additional carrier medium, so that in this embodiment all classification chambers can be supplied at their peripheries with additional carrier medium. It is advantageous for each inlet for additional carrier medium to be equipped with means of controlling the inflow of carried medium. A simple form of control for the admission of additional carrier medium and for feeding the same spirally into the corresponding classification chamber in the plant or the corresponding stage of the process is provided by a ring of guide blades which is disposed on the periphery of the respective classification chamber, these blades preferably being mounted so as to be adjustable. Spiral air classifiers according to the invention can be of simple construction, so that maintenance and operation present substantially no problems.
Inventors: | Beck; Erich (Worms-Weinsheim, DT) |
---|---|
Assignee: | Ingenieurbureau Dr. Brehm AG
(Zurich, CH) |
Family ID: | 21736630 |
Appl. No.: | 05/009,270 |
Filed: | February 6, 1970 |
Current U.S. Class: | 209/710; 209/722 |
Current CPC Class: | B04C 1/00 (20130101) |
Current International Class: | B04C 1/00 (20060101); B04c 001/00 () |
Field of Search: | ;209/144 |
3477569 | November 1969 | Klein et al. |
2694492 | November 1954 | Rumpf et al. |
3001727 | September 1961 | Block et al. |
2361758 | October 1944 | Fligue |
2846151 | August 1958 | Wehn et al. |
2999593 | September 1961 | Stern |
3433422 | March 1969 | Guenther |
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