Packaging of polysilicon fragments

Abstract

Accurate and repeatable portioning of polycrystalline silicon chunks without excessive generation of fines is accomplished by separating silicon chunks into a multiplicity of weighed portions and combining at least four portions which together closely match a target weight into a plastic shipment bag which is then closed.

Claims

1. A method for packaging polysilicon chunks portioned from a supply of polysilicon chunks, comprising: a) defining a target weight and a target weight tolerance for polysilicon chunks to be contained in a plastic bag; b) removing a multiplicity of portions of polysilicon chunks having chunk sizes in the range of 20 to 200 mm from the supply of polysilicon chunks each of the portions having a weight less than the target weight: c) weighing each portion of the multiplicity of portions of polysilicon chunks, and storing each weighed portion in a buffer vessel of a buffer store comprising a multitude of buffer vessels, each buffer vessel containing a single weighed individual portion: d) selecting four or more buffer vessels from the multiplicity of buffer vessels of the buffer store, which together contain a total weight of polysilicon chunks within said target weight tolerance of the target weight; e) combining the polysilicon chunks from the buffer vessels selected in step (d); f) introducing the combined polysilicon chunks from step e) into the plastic bag and closing the bag; wherein the steps b), c), e) and f), are conducted manually, and wherein during steps b)-f) less than 0.5 wt. % of fine material consisting of particles having a size which could be sieved off by a mesh screen having a mesh size of 8 mm (square meshes) forms from the portions of polysilicon chunks.

2. The process of claim 1, wherein each buffer vessel contains a plurality of polysilicon chunks or a single polysilicon chunk.

3. The process of claim 1, where at least one buffer vessel contains a single polysilicon chunk having a size of 500 g to 2500 g.

4. The process of claim 1, wherein the target weight tolerance of polysilicon chunks in the closed bag produced in step f) is within 1 weight percent of the target weight.

5. The process of claim 1, wherein the supply of polysilicon chunks has been previously classified.

6. The process of claim 1, wherein prior to step b), fine material which could be sieved off by a mesh screen having a mesh size of 8 mm (square meshes) has been removed.

7. A method for packaging polysilicon chunks portioned from a supply of polysilicon chunks, comprising: a) defining a target weight and a target weight tolerance for polysilicon chunks to be contained in a plastic bag; b) removing a multiplicity of portions of polysilicon chunks having chunk sizes in the range of 20 to 200 mm from the supply of polysilicon chunks, each of the portions having a weight less than the target weight: c) weighing each portion of the multiplicity of portions of polysilicon chunks, and introducing each weighed portion into a single buffer vessel of a buffer store comprising a multitude of buffer vessels, each buffer vessel containing a single weighed portion: d) selecting four or more buffer vessels from the multiplicity of buffer vessels of the buffer store, which together contain a total weight of polysilicon chunks within said target tolerance of the target weight; e) combining the polysilicon chunks from the buffer vessels selected in step d) by introducing the contents of each selected buffer vessel into the plastic bag, and closing the bag; wherein the steps b), c), and e), are conducted manually, and wherein during steps b)-e) less than 0.5 wt. % of fine material consisting of particles having a size which could be sieved off by a mesh screen having a mesh size of 8 mm (square meshes) forms from the portions of polysilicon chunks.

8. The process of claim 7, wherein each buffer vessel contains a plurality of polysilicon chunks or a single polysilicon chunk.

9. The process of claim 7, where at least one buffer vessel contains a single polysilicon chunk having a size of 500 g to 2500 g.

10. The process of claim 7, wherein the target weight tolerance of polysilicon chunks in the closed bag produced in step f) is within 1 weight percent of the target weight.

11. The process of claim 7, wherein the supply of polysilicon chunks has been previously classified.

12. The process of claim 7, wherein prior to step b), fine material which could be sieved off by a mesh screen having a mesh size of 8 mm (square meshes) has been removed.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) This problem is solved by a method for packing polysilicon chunks, comprising providing an amount of polysilicon chunks, taking a multitude of portions from the amount of polysilicon chunks and weighing the portions, optionally classifying the portions of polysilicon chunks by size classes, defining a target weight for the silicon chunks to be packed in a plastic bag, combining the at least 4 portions from the multitude of portions of polysilicon chunks which have a total weight closest to the target weight, introducing the polysilicon chunks of the at least 4 portions into a plastic bag and welding the plastic bag.

(2) The amount of polysilicon chunks is preferably provided by means of polysilicon chunks moved on a conveyor channel. The size distribution of the polysilicon chunks in the starting material stream depends upon factors including the preceding comminution processes. The manner of division into coarse and fine chunks and the size of the coarse and fine chunks depend on the desired end product which is to be metered and packed.

(3) The portions of the polysilicon chunks may be a multitude of polysilicon chunks, but also individual chunks. The individual chunks are preferably chunks having a minimum weight of 500 g.

(4) The overall method of taking portions, weighing the portions, and combining the portions for metering to a target weight can run either manually or in automated form.

(5) It has been found that this method is particularly gentle on the product. The combination of at least 4 weighed portions can achieve a particularly exact metering outcome.

(6) It is thus envisaged that portions or individual chunks will be weighed and optionally classified. In the event of classification, more particularly, the portion is assigned to a particular size class.

(7) The size class is defined as the longest distance between two points on the surface of a silicon chunk (=max. length): Chunk size 0 [mm] 1 to 5 Chunk size 1 [mm] 4 to 15 Chunk size 2 [mm] 10 to 40

(8) As well as the aforementioned size classes, the classification and sorting of polycrystalline silicon into the following chunk sizes is likewise customary: Chunk size 3 [mm] 20 to 60 Chunk size 4 [mm] 45 to 120 Chunk size 5 [mm] 90 to 200

(9) In these cases, at least 90% by weight of each of the chunk fractions is within the size ranges mentioned.

(10) A multitude of portions can be taken from the amount of polysilicon chunks by means of shovels, grabs, vessels, or manually. When they are taken manually, preference is given to the use of suitable low-contamination gloves, for example an outer glove in combination with an inner glove, in which case the palm region area of the outer glove consists of LDPE and the backside region area of the outer glove consists of Tyvek® (DuPont), the two areas being welded to one another, and the inner glove consisting of Dyneema® (Royal DSM N.V.) coated with polyurethane.

(11) Tyvek® from DuPont is a fibrous functional textile resembling a sheet of paper and composed of thermally welded fibers of high-density polyethylene (HDPE).

(12) Dyneema® from Royal DSM N.V. is a high-strength polyethylene fiber (PE) having a tensile strength of 3 to 4 GPa (3000 to 4000 N/mm.sup.2).

(13) It is likewise possible to use a multilayer glove, in which case the outer layer of the glove consists of a palm region area composed of LDPE and a backside region area composed of Tyvek®, the two areas being welded to one another, and at least one of the inner layers of the glove consists of Dyneema® coated with polyurethane.

(14) The portions can be weighed volumetrically. Especially in the case of small chunk sizes, the bulk density of the polysilicon is known, and so, with the volume, the weight of the portions is also known or can be determined. The volume of the portion is known when vessels, shovels or the like with defined fill volume are used. This also applies to the manual taking of portions. If appropriate, an estimate can also be made.

(15) However, the method can also be automated. The portions can be weighed gravimetrically, for example by means of continuous weighing systems, or optically by means of camera systems with upstream metering channels for portioning. The material division is preferably effected by means of a regulation system which is controlled by means of volumetric or gravimetric input parameters. With this regulation system, it is possible to constantly feed a product conveying stream and to divide it into a multitude of portions.

(16) If a continuous conveying stream is involved, which is preferred, the conveying stream is controlled by the result from the weighing or classifying unit.

(17) Thereafter, the portions are preferably conducted into a buffer store in order to be combined at a later stage to give the desired packing amount. Such a buffer store comprises a multitude of buffer vessels each containing a portion of polysilicon chunks.

(18) This amount of polysilicon chunks, combined from the at least 4 portions, is subsequently packed into at least one plastic bag.

(19) Preference is given to packing into two plastic bags, inner bag and an outer bag, in which case the polysilicon chunks of the at least 4 portions are first filled into a first plastic bag, which is then closed, and the first plastic bag containing polysilicon chunks is subsequently introduced into a second plastic bag and the latter is likewise closed.

(20) It is likewise preferable to fill the polysilicon chunks of the at least 4 portions into a first plastic bag, which is inserted into a second plastic bag, and subsequently to close the two plastic bags. In this execution, one bag has already been inserted into the other before the polysilicon is introduced.

(21) If necessary, the polysilicon chunks are cleaned by wet-chemical means before packing.

(22) For dispensing, the device described in US 20100154357 A1 is suitable, comprising a freely suspended energy absorber which is introduced into the inner plastic bag before filling with the polycrystalline silicon. The polycrystalline silicon is filled into the plastic bag by way of the energy absorber. The freely suspended energy absorber is subsequently removed from the plastic bag filled with polycrystalline silicon, and the plastic bag is closed.

(23) The plastic bag can be closed, for example, by means of welding, bonding or form-fitting. It is preferably closed by means of welding.

(24) The bag ideally consists of a plastic film having a thickness of 100-500 μm.

(25) The plastic used is preferably a monofilm composed of PE-LD, PE-LLD or PE-HD. The use of multilayer films from blown film extrusion or casting processes is likewise possible.

(26) In the case of conventional metering, 1%-3% of the total amount of fine material has to be screened out again before the packing.

(27) In the method according to the invention, less than 0.5% of the total amount occurs in the form of fine material. Thus, much less fine material arises, which may have to be classified again.

(28) The fine material of polycrystalline silicon, in the context of the invention, for chunk sizes 3 to 5, comprises particles which can be sieved off by means of a mesh screen having a mesh size of 8 mm (square meshes) from the total amount of chunks combined from at least 4 portions. The fines fraction is quantified gravimetrically.

(29) In the case of chunk size 2, the fine material comprises particles of polycrystalline silicon having a size of less than 8.3 mm, preferably having a size of less than 9.5 mm.

(30) In the case of chunk size 1, the fine material comprises particles of polycrystalline silicon having a size of less than 3.5 mm, preferably having a size of less than 3.9 mm.

(31) In the case of chunk size 0, the fine material comprises particles of polycrystalline silicon having a size of less than 1 mm.

(32) The fines fraction of chunk sizes 0 to 2 is determined by means of a commercial particle size measuring instrument, for example a Camsizer® from Retsch. The measurement employed for the fines fraction was the 1% by weight quantile (=10000 ppmw).

(33) When metering greater chunk sizes having a piece weight from 500 g, a target weight of 10 kg (+−100 g) can only be attained through the combination of portions.

(34) In the case of a manual hand-packing process, the buffer vessels of the buffer store are marked visually in order to enable the combination of the portions to give the target weight.

(35) Through the combination of portions, it is also possible to establish a more economically viable process, especially since further work (for example because of under-/overweight outside the tolerance) is dispensed with. Process scatter is much smaller than with conventional net or gross balances.