NZ319564A - Powder cleaning process and device; powder projected against surface - Google Patents

Description

•k. . % New Zealand No. 319564 International No. PCT/EP95/04917 TO BE ENTERED AFTER ACCEPTANCE AND PUBLICATION Priority dates: 24.12.1994;01.12.1995; Complete Specification Filed: 13.12.1995 Classification:^) C01F7/46.02; B03B1/00; B02C13/00 Publication date: 27 May 1998 Journal No.: 1428 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: Powder cleaning process and device Name, address and nationality of applicant(s) as in international application form: ABB FLAKT AB, a Swedish company of Sickia Alle 13, Nacka, S-120 86 Stockholm, Sweden 319 5 6 4 PCT/EP 95/04917 1 18 November 1996 Description and device for the cle*nj"gr of powders ;The invention relates to a method and a device for the mechanical cleaning of a powder according to the precharacterizing clause of Patent Claims 1 and 2. ;which adhere to the surface of powder particles. Such methods are employed, for example, in the cleaning of powder-form aluminium oxide, which is intended for the production of aluminium. The powder-form primary aluminium oxide is, during aluminium production, first used to clean the flue gas produced during the smelting electrolysis. In this case the flue gas is directed past the primary aluminium oxide, the particles, in the form of fluoride, iron, phosphorus, carbon, silicon, vanadium and nickel, evolved from the smelting electrolysis, adhering to the surface of the powder -form aluminium oxide. Before this aluminium oxide is fed to the smelting electrolysis, the impurities in the form of iron, phosphorus, carbon, silicon, nickel and vanadium must be removed, since otherwise the process would be enriched with them. This would have a detrimental effect on the quality of the aluminium and the efficiency of the process. The fluorine required for carrying out the smelting electrolysis must be collected and fed back to the process. ;A method for the removal of impurities from the surface of powder-formed particles is disclosed by FR-A-2,499,057, which is identical to Norwegian patent 147,791 » corresponding to US Application No. 669045 and US Registration No. 4525181. In this method, an air stream which is loaded with the powder to be cleaned is directed against an impact plate. The effect of the impact is that the impurities adhering to the surface of the powder are detached. The impurities and the powder are then ;Such a method is suitable for removing impurities ;3195 ;PCT/EF 95/04917 ■ - 2 - 18 November 1996 ;separated from each other by screening. This method is very expensive to control and furthermore has an excessively low efficiency in the case of a throughput of several tonnes per hour. ;5 French Patent 7,732,072 discloses a method for the stripping of particulate impurities from the surface of powders. In this method, the powder to be cleaned is introduced into two crossed air jets. At the crossover point of these jets, the powder particles strike one 10 another. In this case the particles rub on one another, the impurities adhering to the surface being detached. Since the stripped impurities are lighter than the powder particles, they can be removed with the aid of an air current, while the heavier powder particles fall down-15 wards. This method is very difficult when it is used in a current large-scale process, since it is not designed for the deeming of large quantities of particles. ;DE-A-1,607,465 discloses an impact crusher for crushing hard and medium-hardness material. The impact 20 crusher consists of a housing with impact plates which are arranged opposite the beater arms of the rotor. The housing furthermore has a delivery guide for feeding the material, as well as an output opening. The impact plate is arranged below the delivery guide and has a roof-25 shaped design. The impact plate has from place to place a width which is greater than half the diameter of the rotor. ;US-A-4,361,290 describes a rotating beater mill with which material can be reduced to three different 30 sizes and separated. The beater mill is provided with a rotor, the arms of which have plates for reducing the material fastened to them. ;EP-A-337137 discloses a hammer mill for reducing the size of ore and similar materials. The device is i ;35 !bounded by a cylindrical housing. Inside the latter, ;j impact plates and a rotor are arranged, a plurality of r~ j hammers being fastened to the latter at a defined mutual ;; > ;::J j ^ '.separation and being used for the ore to have its size ;'• 'J } ;reduced or to be projected against the impact plates. ;PCT/EP 95/04917 r 2a - 18 November 1996^ ;The object of the invention is to provide a^ ^ , method with which it is possible to strip impurities ^ adhering to the surface of powders cost-effectively and ^ ;with a higher efficiency than hitherto, and to provide a ^ L, device, using which this method can be carried out. ^ * Thus, the invention provides a method for the mechanical cleaning of a powder (25) , in particular of primary aluminium oxide, which is directed against a surface (2P) in order to strip particulate impurities (35) adhering to its surface, characterized in that the aluminium oxide (25) to be cleaned is introduced into a stripping device (2) and, with the aid of the striking mechanism (2R) of this stripping device (2), is directed with a speed of 20 to 30 m/s several tens of times per second for a time period of < 1 to 10 seconds against impact blades (2P) rotating in the opposite direction to the striking mechanism (2R) , and in that the particles of the cleaned, powder-form aluminium oxide (30) having a size of > 10 fm are separated from the cleaned, powder-form aluminium oxide (31) having a size < 10 /zm and the impurities (35) with the aid of an air sifter (3) and/or a cyclone (4), and fed to the smelting electrolysis, and in that the powder-form aluminium oxide (31) having a size < 10 #un and the impurities (35) are fed to a tip or are further processed as raw material.

In a further aspect, the invention provides a cleaning device for the mechanical detachment of particulate impurities (35) from the surface of a powder (25) , in particular of primary aluminium oxide, having a stripping device (2) , within which the powder (25) can be directed against a surface (2P), characterized in that the stripping device (2) is designed as an impact mill having at least one striking mechanism (2R) , with the aid of which the powder (25) to be cleaned can be directed with a defined speed against impact blades (2P) rotating in the opposite direction to the striking mechanism (2R) , and in that the striking mechanism (2R) and the impact blades (2P) are made of a material in the form of hard metal, a ceramic or a polymer, and in that an air sifter (3) and/or a cyclone (4) are/is connected to the stripping device (2).

PCT/EP95/04917 - 2b - ^ ^564 When the method according to the Invention is carried out, the powder to be cleaned passes through a mechanically operating cleaning device. In one embodiment of the invention, this device comprises a stripping device, after which a cyclone and/or an air sifter are connected. The construction and mode of operation of the stripping device correspond to those of an impact crusher. In this device, the powder is directed, by a rotor or a striking mechanism, with a predetermined speed against impact bladas which rotate in the opposite direction to the striking mechanism. By means of the rotational speed of the rotor or of the striking mechanism and a fixed dwell time of the powder in the stripping device, it is possible to 319564 control the impact epeed of the powder and the number of impacts of the powder on the plates. The impact speed is less than 120 m/s. It is preferably set at 20 to 40 m/s. The setting of the impact speed is particularly 5 important, since it is only by this that it is possible to achieve the effect that all of the impurities are removed from the surface of the powder, but without the powder being broken down. After the cleaned powder and the impurities detached therefrom have been withdrawn 10 from the stripping device, the cleaned powder having a particle size of > 10 fm is separated from powder having a particle size of < 10 pm and the impurities, which are likewise no larger than 10 /im. This is done, for example, with the aid of a cyclone and/or an air sifter, which are 15 connected after the stripping device. Air sifters are very expensive to produce. Costs can be saved by connecting the air sifter after the cyclone. This has the advantage that a fraction of the powder having a particle size of > 20 to 30 /a is already stripped in the cyclone. 20 It is therefore possible to use a smaller air sifter. In favourable cases it is also sufficient to connect only a cyclone after the stripping device.

In a simplified embodiment of the cleaning device, the impact crusher and a cyclone are not 25 employed. The cleaning device consists only of an air sifter. In this case, however, the air sifter must be designed in such a way that the air routing in its interior causes a disintegration effect. The air sifter must be built in such a way that the powder to be cleaned 30 is directed with the above described speed against one or more surfaces, so that the impurities adhering to the powder are stripped. The advantage of using only an air sifter for the powder cleaning is that separation of the cleaned powder can be carried out simultaneously there-35 with.

All the abovedescribed cleaning devices are designed in such a way that it is possible to separate the cleaned powder according to size^in_j3uch a way that powder having a particltf~~5Tze:^tff~is -10" im_J^s fed to the \ i -j % - 4 319564 smelting electrolysis for the production of aluminium. The powder having a particle size < 10 fan, and the impurities, are stored in a tip or further processed as raw material. Using the method according to the inven-5 tion, it is possible to remove at least 25 % of the iron, more than 50 % of the phosphorus and 25 % of the carbon, which adheres as impurity to the powder-form aluminium oxide. Using this method it is also possible to recycle 60 % of the fluorine into the smelting electrolysis. The 10 quantity of stripped impurities 35, and the quantity of fluorine which can be recovered using the method according to the invention, represents a significant improvement over the results which are obtained using known methods. Since vanadium and nickel adhere to iron, 15 vanadium and nickel are also isolated from the smelting-electrolysis flue gas in accordance with the stripped quantity of iron.

Further essential features of the invention are identified in the subclaims.

The invention is explained in more detail below with the aid of schematic drawings, in which: Figure 1 shows a cleaning device according to the invention, Figure 2 shows a variant of the cleaning device 25 represented in Figure 1, Figure 3 shows a simplified embodiment of the cleaning device.

Figure 4 shows an air sifter as stripping and separating device.

Figure 1 shows a cleaning device 1 for the mechanical detachment of impurities 35 adhering to the surface of a powder 25. This cleaning device has a stripping device 2 and an air sifter 3. As can be seen from the figure, the air sifter 3 is connected directly 35 after the stripping device 2. The stripping device 2 is essentially designed in the manner of an impact crusher. It has at least one rotor or striking mechanism 2R, with the aid of which the powder 25 introduced into the stripping device 2 is directed against stationary impact 319D 64 plates or against impact blades 2P rotating in the opposite direction to the striking mechanism. The cleaning device 1 represented here is intended for the cleaning of powder-form primary aluminium oxide 25. However, it is also possible to use it to clean other powders. The striking mechanism 2R and the impact plates or blades are made of materials which are particularly suitable for this treatment of powder-form aluminium oxide. Preferably, those components of the stripping device 2 which come into contact with the aluminium oxide 25 are made of a hard metal, a ceramic or a polymer having the corresponding properties. With the aid of the cleaning device 1, the impurities 35 adhering to the surface of the powder 25 are detached. These are particles having a size of < 10 fim. In the primary aluminium oxide 25, the impurities essentially consist of fluorine, iron, phosphorus, carbon, silicon, nickel and vanadium. The powder-form primary aluminium oxide 25 is first used for cleaning the flue gas evolved by the smelting electrolysis. It is then itself cleaned and fad to the smelting electrolysis for the production of aluminium. Cleaning of the flue gas involves taking up the abovedescribed impurities. Cleaning of the flue gas involves accumulation of the above described impurities. It is necessary to. clean the aluminium oxide 25 so that the smelting electrolysis is not enriched with these impurities. If this cleaning is not carried out, then the phosphorus and vanadium lead to a decrease in the current efficiency during the smelting electrolysis. This means that the overall efficiency of the process decreases. The quality of the aluminium is impaired by the iron and the silicon. The powder-form primary aluminium oxide 25 is introduced into the stripping device 2 through a metering device 40. The throughput of the stripping device 2 shown here is chosen to be high enough for it to be possible to clean approximately 20 tonnes of aluminium oxide 25 per hour. The powder-form aluminium oxide 25 is introduced automatically. The speed of rotation of the striking mechanism 2R is in this case 8 <3 tJLn-euch a way that the 319564 aluminium oxide 25 strikes the impact plates or blades 2P with a speed o£ 20 to 30 m/s. The impurities 35 adhering to the surface of the powder-form aluminium oxide 25 are stripped in the process. Approximately 50 % of the 5 aluminium oxide 25 to be cleaned has a particle size of 50 fun. The remaining aluminium oxide 25 has a larger particle size. The speed with which the aluminium oxide 25 is directed against the impact blades or plates 2P is just large enough for the impurities 35 to be stripped, 10 but without the powder-form aluminium oxide 25 being broken down. After the aluminium oxide has been directed at least a few tens of times per second with a speed of [lacuna] 20 and 30 m/s onto the impact plates or blades, it is removed, together with the stripped impurities 35, 15 from the stripping device 2 and fed to the air sifter 3. Both in turn take place automatically. The impurities 35, which are < 10 ftm, and the aluminium oxide 31 having the same particle size are, as shown in Figure 1, diverted off to one side with the aid of an air current. The 20 deemed powder-form aluminium oxide 30, which has a particle size of > 10 pm, is diverted downwards out of the air sifter 3 under the effect of gravity, and fed to the smelting electrolysis (not represented here).

In the case of the cleaning device 1 shown in 25 Figure 2, which is essentially the same in structure as the cleaning device 1 according to Figure 1, a cyclone 4 is connected between the stripping device 2 and the air sifter 3. Cleaned aluminium oxide particles having a size of more than 20 pm to 30 fun. are isolated by the cyclone 30 4 and fed to the smelting electrolysis. The remaining powder, having a particle size < 20 to 30 pm, is fed to the air sifter for further separation. This cleaning device 1 has the advantage that, in contrast to the deeming device 1 according to Figure 1, a substantially 35 smaller air sifter 3 can be used, since the fraction of the cleaned aluminium oxide whose particles are larger than 20 to 30 fun is already fed out directly back to the smelting electrolysis. Since 50 % of the deemed aluminium oxide is larger .than -50tfm7~~tha amount of 3195 powder to be further treated in the air sifter 3 is greatly reduced by intermediate connection of the cyclone 4.

Figure 3 shows a cleaning device 1 which is 5 essentially the same in structure as the cleaning device 1 according to Figure 1. In this case, only a cyclone 4 is connected after the stripping device 2. It is suitable to use this cleaning device 1 whenever stripping of particles < 16 fm. is sufficient.1 10 In a further embodiment according to Figure 4, only an air sifter 3 is provided for stripping the ' impurities 35 from the powder 25 and for separating the powder according to size. The powder 25 to be cleaned is likewise fed to this sifter via a metering device 40. In 15 this case use is made of an air sifter 3 which has a disintegration zone (not represented here) . This zone makes it possible to feed the powder 25 to be cleaned with the required speed of 20 to 30 m/s against at least one surface (not represented here), so that the 20 impurities can be stripped. Using this air sifter 3 it is possible subsequently to carry out separation of the powder according to particle sizes. Separation down to a particle size < 8 fim is thereby possible.

With the abovedescribed devices it is possible to 25 remove at least 25 % of the iron, more than 50 % of the phosphorus and 25 % of the carbon, which adheres as impurity 35 to the powder-form aluminium oxide 25. Using this method it is also possible to recycle 60 % of the fluorine into the smelting electrolysis. The amount of 30 stripped impurities 35 and the amount of fluorine which cam be recovered using the method according to the invention represents a significant improvement over the results which are obtained with known methods.

Claims (4)

PCT/EP 95/04917 8 18 November 1996

1. Method for the mechanical cleaning Patent claims of a powder ~ (25), in particular of primary aluminium oxide, which is directed against a surface (2P) in order to strip particulate impurities (35) adhering to its surface, characterized in that the aluminium oxide (25) to be cleaned is introduced into a stripping device (2) and, with the aid of the striking mechanism (2R) of this stripping device (2), is directed with a speed of 20 to 30 m/s several tens of times per second for a time period of < 1 to 10 seconds against impact blades (2P) rotating in the opposite direction to the striking mechanism (2R) , and in that the particles of the cleaned, powder-form aluminium oxide (30) having a size of >10 /an are separated from the cleaned, powder-form aluminium oxide (31) having a size < 10 ftm and the impurities (35) with the aid of an air sifter (3) and/or a cyclone (4), and fed to the smelting electrolysis, and in that the powder-form aluminium oxide (31) having a size < 10 fm. and the impurities (35) are fed to a '.ip or are further processed as raw material.

2. Cleaning device for the mechanical detachment of particulate impurities (35) from the surface of a powder (25), in particular of primary aluminium oxide, having a stripping device (2), within which the powder (25) can be directed against a surface (2P), characterized in that the stripping device (2) is designed as an impact mill having at least one striking mechanism (2R), with the aid of which the powder (25) to be cleaned can be directed with a defined speed against impact blades (2P) rotating in the opposite direction to the striking mechanism (2R) , and in that the striking mechanism (2R) and the impact blades (2P) are made of a material in the form of hard metal, a ceramic or a polymer, and in that an air sifter (3) and/or a cyclone (4) are/is connected to the stripping device (2). PCT/EP95/04917 -9

3. - A method according to claim 1 substantially as described or exemplified.

4. A cleaning device according to claim 2, substantially as herein described or exemplified. END OF CLAIMS