DE102016104979A1 - Process for melting and cleaning metals, in particular metal waste - Google Patents

Abstract

The present invention relates to a method and an apparatus for melting metal and metalloid particles (1) by means of microwaves and purification with a filter medium (2), which can preferably absorb microwaves.

Description

The present invention relates to a method for melting and simultaneous cleaning of metal particles, as often incurred as metal waste.

Metal particles such as metal dusts, granules and fractions are recovered as waste in industrial processes. Typically, these particulate metal wastes are either used as an aggregate for, for example, alloys or refractory materials or melted down and optionally purified.

For example, the blending of solar grade silicon that results when cutting wafers is about 40% of the amount of silicon used. To date, however, the resulting silicon powder is not recovered. For example, powdered silicon powder can not be economically melted using conventional methods.

It is known to remove solid impurities in molten metal by filtration. As filter media so-called open-cell foam ceramics are usually used, which either retain the solids or bind the impurities contained in the melt on the surface.

Suitable ceramic filters and methods for their preparation are described, for example, in the US patents US 3,893,917 and US 4,708,740 described. US 3,893,917 discloses an open-pore ceramic filter, which is designed in particular for the purification of aluminum melts from solid impurities. US 4,708,740 relates to an open-pore ceramic filter, in particular for the filtration of a copper melt. The filter described here consists of a skeleton coated with silicon carbide (SiC).

The melting of metals is usually carried out in electric arc furnaces, resistance furnaces or induction furnaces whose energy requirement is very high.

An alternative type of heating, which is more energy-efficient than the methods described above is in EP 1 446 624 B1 proposed. In the method described here, the metals are indirectly melted by means of microwaves. The metal is for this purpose placed in a container having a microwave-absorbing material (microwave susceptor). When irradiated with microwaves, the container heats up and the heat is transferred to the metal. The resulting molten metal is then poured into a mold. The disadvantage, however, is that this method can only be used for small quantities and is not continuously feasible.

DE 10 2009 014 683 A1 describes a method in which a conventionally produced molten metal can be maintained at microwave temperature or even further heated by microwave radiation. The melting itself takes place by means of gas heating, heating elements or induction.

However, it is known that metallic or semi-metallic materials can not be heated and melted with microwaves. The reason for this is that solid metallic or semi-metallic materials generally reflect and hardly absorb microwaves. Thus, the penetration depth of the microwaves into these materials is only a few micrometers, whereby at most the surfaces can easily be heated.

It was an object of the present invention to provide a method and an apparatus with which inexpensive and effective metals, including semi-metals, in particular metal waste, can be melted and purified.

The present invention is based on the finding that particulate metallic or semi-metallic materials with microwaves can be heated to very high temperatures and melted. The reason is presumed in the above-mentioned heating of the surface, which has a much larger percentage of particulate matter than compact masses of the same mass. As a result, the effective penetration depth of the microwaves compared to a compact solid is significantly increased.

The object according to the invention is achieved by a method for melting particulate metals and semimetals by irradiation with microwaves and purification of the resulting melt by filtration with a filter medium.

According to a further aspect, the present invention relates to the melting of particulate metals and semi-metals by means of a filter medium by heating the filter medium by irradiation with microwaves and transferring the heat to the particulate metal or metal to be melted.

Furthermore, the present invention relates to an apparatus for carrying out the method.

For the present invention, the term "metals" or "metallic materials" refers equally to metals as to semimetals, such as silicon.

Since these metallic materials can sometimes be very expensive and / or the proportion of waste can be comparatively large, as shown above using the example of solar silicon, it is desirable to recycle this waste as inexpensively and efficiently as possible. The recycled material can be recycled back to the original process, dramatically increasing yields.

According to the invention, metal particles are used which are small enough and thus have a sufficient surface area to be melted by means of microwaves.

For example, the particle diameter may be 20 mm or less. It may be 10 mm or less, and more preferably 500 μm or less. For the present invention particles with a diameter> 10 mm are classified as granules, with a diameter between 100 microns to 10 mm as a powder and with a diameter of less than 100 microns to a few microns as dusts. The term "diameter" as used herein refers to the largest diameter of a particle.

As microwaves radiation is designated having a frequency of about 300 MHz to about 300 GHz. For example, the operating frequency of household standard microwave ovens is 2.455 GHz.

According to a first embodiment of the present invention, in a first step, the metal particles are melted by application of microwaves and then, in a second step, the resulting melt is allowed to pass through a filter medium ("filter").

As a filter medium filters can be used, as they are known for the filtration of molten metal. Examples of these are the open-cell foam ceramic filters mentioned above in connection with the prior art.

According to an advantageous embodiment, a filter medium can be used, which absorbs microwaves. In this case, microwaves may also be applied to the filter medium so that the filter medium is heated and heat is transferred to the metal.

The microwave absorbing filter medium, which may be an open cell foam ceramic filter, in this case consists of or contains a microwave absorbing material.

An example of a suitable filter is a filter made of SiC with an admixture of Al ₂ O ₃ , wherein the proportion of Al ₂ O _{3 may} preferably be 25-35 parts by weight.

Materials that can absorb and convert microwaves to heat are often referred to as microwave susceptors. Examples of susceptor materials are carbon, graphite or silicon carbide.

If a microwave-absorbing filter medium is used, the melting of the metal particles can take place directly by heat transfer from the filter medium heated by microwave irradiation.

As a result of the heating, the microwave susceptibility of the metal particles and the filter medium increases, which promotes the melting process.

The method according to the invention can advantageously be used for the reprocessing of blends of solar silicon, as they are obtained, for example, when cutting wafers.

For carrying out the method according to the invention, a device may be used which has a reaction space for melting and filtering the metal particles. For this purpose, the reaction space is equipped with a filter medium.

The microwaves are generated by a microwave generator and transmitted for example via waveguides in the reaction space. The position of the waveguide is selected as needed in each case so that the metal particles are melted and / or the filter medium is heated.

By simultaneously heating the filter medium, the temperature of the molten metal can be kept readily and also a better wetting of the filter can be obtained. The heating and melting is supported by the typical high thermal conductivity of metals or semi-metals.

In addition, heating of the filter medium by microwaves enables the indirect heating of the metal by the filter medium. This is particularly advantageous if the metal can not be heated sufficiently over the microwaves. Thus, with this embodiment, metal particles can be melted that absorb microwave energy of their own, as well as metals that have a very low own microwave absorption, since they can be heated via the filter medium.

The shape of the device and the reaction space is not particularly limited. The cross-sectional shape can be round, square or rectangular. The device is equipped with a feeder for feeding the metal waste. Further, the device is connected to a microwave generator for generating the microwaves, wherein the microwaves can be transmitted for example via waveguides in the reaction space.

The process according to the invention can be carried out batchwise or continuously as required. In this case, both the supply of metal particles and the removal can be automated.

The continuous implementation with automated supply and removal is particularly suitable for the integration of the recycling process according to the invention in large-scale plants. The resulting metal waste is melted and cleaned so that the recycled material can be returned to the original process. This can achieve a significant increase in yields.

Another advantage of the method according to the invention is that it can also be carried out economically with filters whose cross-sectional area is comparatively low. Usually, filters with a small cross-sectional area are considered unsuitable for industrial purposes, since the small cross-sectional area only allows a low throughput. According to the invention, however, the required temperatures and the associated required power generated by the use of microwaves directly and / or indirectly in the material to be heated, which can shorten the process times. Thus, filters with a small cross-sectional area can also be used according to the invention, since they can be compensated for by the shortened process times.

According to a further embodiment, the process is carried out under inert gas in order to avoid a reaction of the melt with atmospheric oxygen. For this purpose, the reaction space is flowed through with an inert gas. An example of a suitable inert gas is argon. However, other inert gases which do not react with the melt, e.g. Nitrogen. The device may thus be provided with a supply and a withdrawal for the inert gas.

The inert gas supplied, as well as possible exhaust gases which may be formed by impurities in the melt, are preferably actively removed, for example by means of a pump, via the exhaust from the reaction space, and can be recycled by recycling. The process can also be carried out under vacuum.

Hereinafter, the present invention will be explained in more detail with reference to the accompanying figures.

Show it:

1a , b, c, d schematically show the different types of heating according to the method of the invention, and

2 schematically an embodiment of an apparatus for continuously carrying out the method according to the invention.

In 1a , b, c, d are the metal particles 1 indicated by dots. reference numeral 2 denotes a filter. The heat propagation due to microwave radiation (not shown) in the metal particles 1 and / or the filter 2 is indicated by arrows. "Q" symbolizes the heat flow.

To 1a become the metal particles 1 directly heated by microwave radiation, with the heat in the metal particles 1 spreads.

In 1b is the heat propagation due to microwave irradiation (not shown) in the filter 2 shown, with the filter 2 here consists of a microwave-absorbing material.

In 1c is sketchy the heat propagation from the filter 2 in the metal particles 1 represented by the upward pointing arrows. Due to the high thermal conductivity of the metal particles 1 This results in excellent heat transfer from the filter into the metal particles 1 , That is, in this variant, the metal particles 1 indirectly heated and melted by heat transfer from the filter material.

In 1d is a combination illustrated by indirect heating of the metal particles 1 by heat transfer from the filter 2 corresponding 1c and direct heating of the metal particles 1 corresponding 1a ,

2 schematically shows a plant 3 for a continuous implementation of the method according to the invention. The attachment 3 includes a housing 4 , In the case 4 is the actual reaction space 5 housed a coat 6 which has the interior of the reaction space 5 opposite the surrounding housing 4 separates.

The upper end of the reaction space 5 opens into an upper connection unit 7 and the lower end of the reaction space 5 in a lower connection unit 8th ,

In the embodiment shown here, the reaction space extends 5 over the entire height of the housing 4 and opens at the top of the case 4 in the upper connection unit 7 and on the bottom surface of the housing 4 in the lower connection unit 8th ,

In the embodiment shown here, the filter is 2 in the lower part of the reaction space 5 arranged. Above the filter 2 become the metal particles 1 applied. On the bottom of the filter 2 The purified filtered melt exits (not shown).

The material to be melted and cleaned becomes the reaction space 5 over the upper connection unit 7 fed and the filtered melt through the lower connection unit 8th dissipated.

The housing 4 is here via waveguide 9 with a microwave generator (not shown) connected to the irradiation of microwaves 10 ,

In 2 is only a waveguide 9 shown. However, it should be understood that, as needed, one or more microwave generators having one or more waveguides at one or more positions of the housing 4 can couple.

The housing 4 may be made of a material as is typically used in engineering, such as metals, alloys, etc., for example a steel such as container steel.

The coat 6 may be formed of a microwave transparent or microwave absorbing material or a combination thereof. Examples of absorbing materials are silicon nitride, silicon carbide, graphite. Examples of permeable materials are corresponding permeable ceramics and silica. To avoid a reaction of the melt with the atmospheric oxygen is in the in 2 shown plant the reaction space 5 perfused with an inert gas. This is located in the system shown on the lower connection unit 8th a supply line 11 for the supply of the inert gas and at the upper connection unit 7 a deduction 12 , via which the inert gas and possible exhaust gases, which may be caused by impurities in the material, for example, can be dissipated. In order to support the removal of the inert gas and other exhaust gases, the trigger 12 be connected to a pump or the like.

Preferably, the cavity 13 between housing 4 and reaction space 5 be wholly or partially filled with ceramic and / or heat-insulating materials (not shown in the figure), on the one hand to make the process as effective as possible, but on the other hand also to protect the environment against the high process temperatures.

For further cooling of the housing walls and the microwave components used, cooling, for example water cooling, may be connected.

As filler for the cavity 13 Preferably, a material is used, which is microwave transparent. Examples of microwave transmissive materials are nitride or oxide based microwaveable ceramics. Also, silicon nitride has been found to be suitable.

However, a microwave absorbing material or a combination of microwave transparent and microwave absorbing materials may also be used. Widely used today are microwaves with an operating frequency of 2.455 GHz.

The inventive method allows an efficient and energy-saving purification of metal waste by melting the metal waste and filtering with a filter medium. In particular, the recycled material can be returned to the original process, which not only significantly increases yields, but also represents environmental progress.

LIST OF REFERENCE NUMBERS

1

 particulate metal, semi-metal

2

 filter

3

 investment

4

 casing

5

 reaction chamber

6

 coat

7

 upper connection unit

8th

 lower connection unit

9

 waveguide

11

 supply

12

 deduction

13

 cavity

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 3893917 [0005, 0005]
• US 4708740 [0005, 0005]
• EP 1446624 B1 [0007]
• DE 102009014683 A1 [0008]

Claims (11)

Process for melting and purifying particulate metal and metalloid ( 1 ), wherein the particulate metal or semimetal ( 1 ) melted by microwaves and the melt with a filter medium ( 2 ) is filtered. Method according to claim 1, wherein a filter medium ( 2 ) is used, which absorbs microwaves. Method according to claim 2, wherein the filter medium ( 2 ) is heated by microwave irradiation. Process according to claim 2 or 3, wherein the particulate metal or semimetal ( 1 ) by heat transfer via the filter medium ( 2 ) and / or absorption of microwaves is melted. Method according to one of the preceding claims, wherein the method is carried out under inert gas. Method according to one of the preceding claims, wherein the method is carried out continuously. Method according to one of the preceding claims, wherein the particulate metal or metalloid ( 1 ) is metallic waste. Method according to one of the preceding claims, wherein the particulate semimetal ( 1 ) Is silicon. Device for melting and purifying particulate metals and semimetal, the device ( 3 ) a housing ( 4 ), in whose interior a reaction space ( 5 ), and the reaction space ( 5 ) a coat ( 6 ) having the interior of the reaction space ( 5 ) opposite the housing ( 4 ), the reaction space ( 5 ) a filter medium ( 2 ) and above the filter medium ( 2 ) an area for receiving particulate metals or semimetals ( 1 ), wherein the upper end of the reaction space ( 5 ) in an upper connection unit ( 7 ) and the lower end of the reaction space ( 5 ) in a lower connection unit ( 8th ), and wherein the device ( 3 ) is connected to a microwave generator. Apparatus according to claim 9, wherein the device ( 3 ) is connected to more than one microwave generator. Apparatus according to claim 8 or 9, wherein the filter medium ( 2 ) is microwave absorbing.

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