Method and installation for manufacturing a starting material for producing rare earth magnets

Abstract

A method for producing a powdered starting material, which is provided for production of rare earth magnets, including includes the following steps: pulverizing an alloy, including at least one rare earth metal, wherein a powdered intermediate product is formed from the alloy including the at least one rare earth metal, and carrying out at least one classification aimed at particle size and/or particle density for the powdered intermediate product. A fraction of the powdered intermediate product, which is formed by the at least one classification, is used for fabrication of rare earth magnets. Furthermore, at least one dynamic classifier is provided, implementing at least one classification directed at particle size and/or particle density for the powdered intermediate product and thereby separates the fraction from the powdered intermediate product, which forms the starting material for manufacturing rare earth magnets.

Claims

1. A method for producing a powdered starting material provided for production of rare earth magnets, comprising the following steps: pulverizing an alloy that includes at least one rare earth metal to form a powdered intermediate product, said powdered intermediate product includes said at least one rare earth metal; carrying out at least one classification directed at particle size and/or particle density using at least one dynamic classifier to separate a fraction from the powdered intermediate product; dispersing the fraction using the at least one dynamic classifier for establishing a homogenous distribution of particles in the fraction; and carrying out a renewed classification using the at least one dynamic classifier to separate a further fraction from the dispersed fraction; wherein the further fraction forms the starting material for manufacturing rare earth magnets, wherein the at least one dynamic classifier comprises a classifying rotor, wherein said at least one classification comprises at least two classifications that follow one another in time, each directed at particle size and/or particle density, wherein as part of a first classification of the at least one dynamic classifier, directed at particle size and/or particle density, coarse material is separated from the powdered intermediate product, and wherein as part of a second classification of the at least one dynamic classifier, directed at particle size and/or density, fine material is separated from the powdered intermediate product.

2. A method for producing a powdered starting material provided for production of rare earth magnets, comprising the following steps: pulverizing an alloy that includes at least one rare earth metal to form a powdered intermediate product, said powdered intermediate product includes said at least one rare earth metal; carrying out a classification using at least one static classifier to separate a portion of the powdered intermediate product; carrying out at least one classification directed at particle size and/or particle density using at least one dynamic classifier to separate a fraction from the portion of the powdered intermediate product; dispersing the fraction using the at least one dynamic classifier to establishing a homogenous distribution of particles in the fraction; and carrying out a renewed classification using the at least one dynamic classifier to separate a further fraction from the dispersed fraction; wherein the further fraction forms the starting material for manufacturing rare earth magnets, wherein the at least one dynamic classifier comprises a classifying rotor, and the at least one static classifier comprises a cyclone classifier, wherein said at least one classification comprises at least two classifications that follow one another in time, each directed at particle size and/or particle density, wherein as part of a first classification of the at least one dynamic classifier, directed at particle size and/or particle density, coarse material is separated from the portion of the powdered intermediate product, and wherein as part of a second classification of the at least one dynamic classifier, directed at particle size and/or density, fine material is separated from the portion of the powdered intermediate product.

3. The method according to claim 2, in which the first classification and the second classification are performed by exactly one dynamic classifier.

4. The method according to claim 2, wherein the alloy including at least one rare earth metal is pulverized, preferably mechanically, into separate steps, wherein the powdered intermediate product is formed from the pulverized material in separate steps.

5. The method according to claim 2, wherein the at least one dynamic classifier performs the at least one classification directed at particle size and/or particle density for the portion of the powdered intermediate product under a protective gas atmosphere.

6. A method for manufacturing rare earth magnets including the following steps: producing a starting material by the method according to claim 2, introducing the starting material into molds and pressing the starting material into the molds, forming blanks from the starting material, sintering the blanks and exposing the sintered blanks to a magnetization pulse so that as a result the sintered blanks that have been exposed to the magnetization pulse are formed as rare earth magnets.

7. The method for manufacturing rare earth magnets according to claim 6, wherein the starting material includes a fraction of particles >8 μm in an amount volume percent and/or a fraction of particles <2 μm in an amount volume percent.

8. The method according to claim 7, wherein the fraction of particles >8 μm is in a range between 0.1 volume percent and 1 volume percent.

9. The method according to claim 7, wherein the fraction of particles <2 μm is in a range between 0.05 volume percent and 2 volume percent.

10. The method according to claim 2, further comprising: entraining the portion of the powdered intermediate product with a gas and supplying the entrained portion to the at least one dynamic classifier, passing the entrained portion through a static guide vane cage of the at least one dynamic classifier to disperse the entrained portion, and passing the dispersed portion over the classifying rotor to separate the fraction from the portion of the powdered intermediate product.

11. The method according to claim 2, wherein the at least one dynamic classifier provides for the starting material to include a fraction of particles >μm in an amount ≤2 volume percent and/or a fraction of particles <μm in an amount ≤2 volume percent.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments below should illustrate the invention and its advantages in greater detail on the basis of the accompanying figures. The size ratios of the individual elements to one another in the figures do not always correspond to the actual size ratios because some shapes have been simplified and other shapes have been illustrated as enlarged diagrams in relation to the other elements for the sake of a better illustration. The features described below are not closely associated with the respective embodiment but instead may be used in the general context.

(2) FIG. 1 shows schematically the method steps for manufacturing a starting material for manufacturing rare earth magnets such as those that may be provided individually or in the combination shown.

(3) FIG. 2 shows a cross section through a dynamic classifier such as that which may be provided in various embodiments of the method according to the invention as well as in various embodiments of the installation according to the invention.

(4) FIG. 3 shows a lateral cross section through the dynamic classifier according to FIG. 2.

(5) FIG. 4 shows a conceivable particle size distribution of a powdered intermediate product for various embodiments of the method and/or installation according to the invention as compared with a conceivable particle size distribution of a starting material provided for manufacturing rare earth magnets.

(6) FIG. 5 shows a scanning electron micrograph, such as that which can be produced for the powdered intermediate product.

(7) FIG. 6 shows a scanning electron micrograph of a starting material, such as that which can be produced by means of the method according to the invention and/or the installation according to the invention in various embodiments.

DETAILED DESCRIPTION

(8) Identical reference numerals are used for the same elements of the invention or those having the same effect. Furthermore, for the sake of an, overview only reference numerals that are needed for the description of the respective figure are shown in the individual figures. The embodiments illustrated here represent only examples of how the invention may be embodied and do not constitute a conclusive delineation.

(9) FIG. 1 shows schematically process steps for producing a starting material AM for manufacturing rare earth magnets. A suitable RFeB alloy containing the ingredients R=rare earth metal, Fe=iron and B=boron in the desired quantity ratios is used as the basis for this starting material. For example, an NdFeB alloy is used to manufacture a so-called neodymium magnet. Under some circumstances, an alloy must first be produced from the elements in the desired quantity ratios. This alloy is then subjected to a coarse milling in a first working step, for example, in a mechanical milling system or by embrittlement with hydrogen. Particles with a size of up to a few mm are produced in this way. Then the coarse powder fraction gPF obtained as part of the coarse milling process is subjected to a fine milling, in which particles with an average particle size between d50=2 μm and 5 μm are produced or should be produced. In other words, the d50 value of the fine powder fraction fPF is between 2 μm and 5 μm, with a broad particle distribution accordingly in terms of finer as well as coarser particles, with the corresponding amounts of superfine particles (d10=approx. 1-2 μm) and coarse fractions (d90=approx. 8-15 μm). The coarse particles gP are chemically stable, in contrast with the particles fP of the superfine fraction described below and they can also be oriented well in magnetic fields, but they have negative effects on the opposing field stability of the magnet because these coarse particles gP already undergo remagnetization in low magnetic opposing fields and thus exacerbate the opposing field stability (and/or the coercitive field strength) of the entire magnet. For this reason, it is advantageous to further reduce the amount of coarse particles gP in the staring mixture for the production of sintered permanent magnets.

(10) The particles fP of the superfine fraction are highly reactive chemically because of their fineness, and they react with the oxygen or even with the nitrogen from the environment at even the lowest oxygen concentrations. These superfine particles fP can cause spontaneous powder fires in further processing of the powders. Another disadvantage of the superfine particles fP is that it is very difficult to orient these fine particles in the magnetic fields and press devices that are usually available (order of magnitude approx. 10-20 kOe) and therefore have a negative effect on the remanence of the magnets produced from these particles. For this reason, in a fourth or additional method step, superfine fractions, in particular particles with a diameter of 1-2 μm are removed from the fine powder fraction fPF. To do so, the mixture is passed through a cyclone, for example, following the coarse milling and fine milling according to points 101 and 102, so that the superfine fraction is entrained by means of a suitable gas stream and thereby separated from the mixture. This forms the intermediate product ZP, but this still contains a not insignificant amount of up to 10% superfine particles smaller than approx. 1 μm to 2 μm.

(11) To remove these remaining fractions of superfine particles fP μm to 2 μm and/or coarse particles gP between 10 μm and 15 μm as thoroughly as possible, the intermediate product ZP is subjected to at least one additional classification operation to remove unwanted superfine particles fP or coarse particles gP or superfine particles fP and coarse particles gP and thereby further improve the homogeneity of the particles in the target size ZG, in particular to obtain a powder mixture, which essentially comprises only particles with particle sizes in the target range between approx. 2 μm and 8 μm as the starting material AM, because these particles are the best powder fraction from a magnetic standpoint. All the additional steps that follow with regard to the step according to point 104 are carried out with the help of a dynamic classifier 10 (cf. FIGS. 2 and 3) and/or a high-performance classification device.

(12) The particles with the target size ZG between 2 μm and 8 μm are sufficiently stable chemically so that they do not cause any additional oxidation in the normal production process. Furthermore they can be oriented well with the usual magnetic fields. They thus make a substantial contribution toward achieving a high remanence of the magnets thereby produced and are therefore desirable, necessary and beneficial. The more powder particles of this target size ZG are present, the better are the magnetic values (remanence Br and opposing field stability HcJ) of the magnets produced from these particles.

(13) In another, i.e., the present fifth method step, the powdered intermediate product ZP is dispersed to establish the most homogenous possible distribution of the different particles of the intermediate product ZP. In doing so, in particular molecular and magnetic attractive forces between the particles are overcome and a renewed classification and separation of particles of the superfine fraction or particles of the coarse fraction is possible following the dispersion. For this process step a dynamic classifier 10 (cf. FIGS. 2 and 3) or high-performance classifying device is also used.

(14) The dispersed, powdered, intermediate product ZP is classified again and particles of the superfine fraction and/or particles of the coarse fraction are removed. This establishes an optimum separation of superfine fraction and coarse fraction for the desired particle target size ZG. The superfine fraction of particles the size of which is μm is reduced to a fraction of less than 1%. Alternatively or additionally, the coarse fraction of particles, the size of which is greater than 10 μm, can also be reduced to an amount of less than 1%.

(15) This at least one additional classification process is preferably carried out under a protective gas atmosphere, for example, under helium, argon or nitrogen, but this should not be a final list of options. The protective gas atmosphere prevents spontaneous powder fires due to the superfine particles fP in particular.

(16) The fifth and sixth method steps and/or the last two method steps, i.e., dispersion and separation of superfine particles fP and/or separation of coarse particles gP, may preferably take place in particular in a dynamic classifier 10 according to FIGS. 2 and 3.

(17) With regard to the embodiment of a dynamic classifier 1 according to FIGS. 2 and 3, the powdered intermediate product ZP is supplied to the classifying device and/or to the dynamic classifier 10 from above through the product feed 1. The necessary process air VL, which entrains the powdered intermediate product ZP, is supplied through the product feed 1 via the air inlet 2 of the classifying device, and passes through a plurality of adjustable guide vane gaps in the static guide vane cage 3, so that the intermediate product ZP is dispersed. In the present case, a protective gas is used as the process air VL.

(18) The intermediate product ZP, which is dispersed in this way, is passed over a classifier wheel 4, the rotational speed of which is continuously adjustable, so that the particle sizes are separated either into the target material and coarse material or into target material and superfine material.

(19) The optimized classifier wheel design ensures that a very great fineness can also be achieved with high throughputs by using only one classifier wheel 4. The superfine particles fP leave the classifying device 10 at the center of the classifier wheel 4 and/or the dynamic classifier 10, which is installed with a horizontal shaft 8. The coarse particles gP are rejected by the classifier wheel 4 and are discharged through the coarse material outlet 6 on the bottom of the machine housing 9 at the rear side by the machine housing 9, which is designed in a helical shape and provided with a partition 5. The discharge of the coarse particles gP in difficult separation jobs can be regulated by the position of the coarse material valve 7, so that the cleanliness of the coarse particles gP is influenced in this way. The particles of the target size ZP leave the dynamic classifier 10 together with the coarse material through the coarse material outlet 6. The superfine particles fP are separated from the particles of the target size ZP and therefore do not form a component of the fraction leaving the dynamic classifier 10 through the coarse material outlet 6.

(20) The desired target particle size ZG is regulated here in particular by regulating the gas stream of process air VL and/or the rotational speed of the classifier wheel 4. A higher gas stream and/or a lower rotational speed result in a coarser product while a lower gas stream and/or a higher rotational speed lead to a finer product.

(21) In addition, FIG. 3 shows the at least two cracked gas feeds (11), which are necessary to flush the gap between the fine material outlet and the classifier wheel (4) with so-called cracked gas. However, embodiments with just one cracked gas feed (11) are also possible. This flushing prevents particles in the classifier wheel (4) and/or in the gap between the outlet for fine material and the classifier wheel (4) from being deposited and clogging the gap. The flushing takes place by means of a fluid suitable for this purpose, namely by means of protective gas in a preferred embodiment.

(22) FIG. 4 shows the particle size distribution in the intermediate product ZP and in the starting material AM. The diagram shows in particular the particle size in μm, plotted as a function of the amount of the volume density of the respective mixture in percentage. This shows clearly that a more homogeneous particle mixture in the starting material AM, in which the amount of superfine particles fP constitutes ≤1% of the volume density, and in which the fraction of coarse particles gP also constitutes ≤1% of the volume density, can be achieved by the additional method step of dispersing the intermediate product ZP and classification, with subsequent separation of superfine particles fP ≤1 μm and/or coarse particles gP ≥10 μm through a dynamic classifier 10. In particular, the fractions of superfine particles fP and coarse particles gP illustrated with hatching are removed from the powdered intermediate product ZP.

(23) The starting material AM prepared in this way is suitable in particular for production of sintered rare earth magnets because of the particle size between 1 μm and 10 μm, preferably between 2 μm and 8 μm, because with these particle sizes of the starting material AM especially good magnet values can be achieved. In particular, high (improved) remanence values BR and a good (improved) opposing field stability HcJ as well as a definite improvement in the square-wave property of the demagnetization curve are achieved with the starting material AM for the production of permanent magnets.

(24) FIG. 5 shows a scanning electron micrograph of the powdered intermediate product ZP, and FIG. 6 shows a scanning electron micrograph of the starting material AM, as produced in various embodiments of the method according to the invention, and as can be used to produce rare earth magnets. Whereas the intermediate product ZP is a highly heterogeneous mixture of different particle sizes, and contains in particular a large amount of superfine particles fP, FIG. 6 shows clearly that the twice-classified starting material AM contains mainly only particles of a target size ZG between 1 μm and 10 μm, preferably between 2 μm and 8 μm.

(25) The embodiments, examples and variants of the preceding paragraphs, claims or the following description and figures including their various views or respective individual features may be used independently of one another or in any combination. Features, which are described in conjunction with one embodiment, may be used for all embodiments if the features are not inconsistent. The invention has been described with reference to preferred specific embodiments. It is conceivable for those skilled in the art that modifications or amendments to the invention can be made without going beyond the scope of protection of the following claims. It is possible to use some of the components or features of one of the examples in combination with features or components of another example.