Process for producing polycrystalline silicon

Abstract

Polycrystalline silicon chunks are packed for further processing and/or for shipment with minimal contamination by introducing the chunks into a cardboard container lined with a non-contaminating plastic. The process is economical, avoids the problems associated with packaging in plastic bags, and provides far less contamination and spillage during transport.

Claims

1. A process for producing polycrystalline silicon, comprising providing polycrystalline silicon rods, comminuting the polycrystalline silicon rods into polycrystalline silicon chunks, and introducing the polycrystalline silicon chunks into a cardboard packing container comprising: a base, a wall and an opening, wherein at least the inner surfaces of the base and wall which come into contact with the polycrystalline silicon chunks, have a plastic coating.

2. The process of claim 1, wherein the plastic contains less than 100 ppbw of boron, less than 100 ppbw of phosphorus and less than 10 ppbw of arsenic.

3. The process of claim 1, wherein the plastic is selected from the group consisting of polypropylene, polyethylene, polyurethane, polyvinylidene fluoride, polyvinylidene chloride and polysiloxane.

4. The process of claim 1, wherein inner surfaces of the base and wall have been coated with plastic of a thickness of from 50 m to 500 m.

5. The process of claim 1, wherein the polycrystalline silicon chunks are introduced manually by a worker into the cardboard container, the worker wearing gloves made of PE or PU containing less than 100 ppbw of boron, less than 100 ppbw of phosphorus and less than 10 ppbw of arsenic.

6. The process of claim 1, further comprising sorting and classifying the polycrystalline silicon chunks into particular size classes prior to introducing the polycrystalline silicon into the container.

7. The process of claim 6, further comprising cleaning of the sorted and classified polycrystalline silicon chunks prior to introducing the chunks into the container.

8. The process of claims 1, wherein the polycrystalline silicon chunks, in each case after comminuting, after an optional sorting and classifying step, and after an optional cleaning step, are packed in a cardboard container with plastic coating on the inner surfaces thereof and transported to a further processing step.

9. The process of claim 1, wherein the polycrystalline silicon chunks are packed in a cardboard container with plastic coating on the inner surfaces thereof and transported to a customer.

10. The process of claim 9, wherein several cardboard containers are fixed on a pallet and transported to the customer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates one embodiment of the process of the invention.

(2) FIG. 2 illustrates in larger detail, the walls, base, and plastic lining of the box of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) The polycrystalline silicon is preferably deposited on heated thin silicon rods, using a silicon-containing component and hydrogen as reaction gas (Siemens process). Preferably, the silicon-containing component is a chlorosilane, more preferably trichlorosilane. The deposition is effected according to the prior art, reference being made, for example, to WO 2009/047107 A2.

(4) After the deposition, the polycrystalline silicon rods are comminuted. Preferably, there is first a preliminary comminution of the polysilicon rods. For this purpose, a hammer made from a low-abrasion material, for example cemented carbide, is used. The preliminary comminution is effected on a workbench having a surface that preferably consists of wear-resistant plastic or of silicon.

(5) This is followed by comminution of the pre-comminuted polysilicon to the desired target size of chunk size 0, 1, 2, 3, 4 or 5. The chunk size is defined as the longest distance between two points on the surface of a silicon chunk (=max. length) as follows: Chunk size (CS) 0 [mm] 0.1 to 5 Chunk size 1 [mm] 3 to 15 Chunk size 2 [mm] 10 to 40 Chunk size 3 [mm] 20 to 60 Chunk size 4 [mm] 45 to 120 Chunk size 5 [mm] 100 to 250

(6) In this context, at least 90% by weight of the chunk fraction in each case is within the size ranges mentioned.

(7) The comminution is effected by means of a crusher, for example with a jaw crusher. Such a crusher is described, for example, in EP 338 682 A2.

(8) Subsequently, the crushed silicon is optionally classified into the abovementioned chunk sizes by means of a mechanical screen. Before that, it is preferably packed in a cardboard container and transported from the comminution apparatus to an apparatus that sorts and classifies it.

(9) Optionally, the chunks are cleaned before the final packing. For this purpose, the classified silicon is packed in a cardboard container and transported to the cleaning apparatus. Preference is given to using the same cardboard container in which the chunks were packed after the comminution.

(10) In the cleaning operation, preference is given to using a cleaning solution comprising HNO.sub.3 and HF.

(11) Preferably, the polysilicon chunks are washed with an oxidizing cleaning solution in a preliminary cleaning operation in at least one stage, washed with a cleaning solution comprising HNO.sub.3 and HF in a main cleaning operation in a further stage and, in the case of hydrophilization, washed with an oxidizing cleaning fluid in yet a further stage. The preliminary cleaning is preferably effected by means of HF/HCl/H.sub.2O.sub.2. The hydrophilization of the silicon surface is preferably effected by means of HCl/H.sub.2O.sub.2.

(12) After the cleaning or directly after the comminution (if no cleaning is effected), the polysilicon chunks are packed in a cardboard container. This is preferably a cardboard container comprising a base, a wall and an opening, wherein at least the inner surfaces of the base and wall which come into contact with the polycrystalline silicon chunks have been coated with plastic. Preferably, the cardboard containers used in the packing after the cleaning are the same as those in which the chunks were packed after the comminution.

(13) Cardboard is a material produced from cellulose, wood pulp and used paper, the use of which is customary in the packing industry for protection of packed goods. It is essentially paper having a greater thickness. Cardboard generally has multiple plies, i.e. consists of several plies of paper of different thickness, which are pressed together without using adhesive.

(14) Preferably, the cardboard container is closed after being filled with the polycrystalline silicon chunks. This is preferably done using a cover which likewise consists of cardboard (cardboard box lid). More preferably, the inner surfaces of the cover facing the polycrystalline silicon chunks have likewise been coated with plastic.

(15) The plastic coating preferably has a thickness of 50 m to 500 m.

(16) The plastic used contains preferably less than 100 ppbw of boron, less than 100 ppbw of phosphorus and less than 10 ppbw of arsenic.

(17) The plastic is preferably selected from the group consisting of polypropylene (PP), polyethylene (PE), polyurethane (PU), polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC) and polysiloxane (silicone).

(18) The cardboard container may have been lined, laminated, sprayed or vapor-coated with plastic.

(19) The coating can be effected, for example, with aqueous polymer dispersions or in an extrusion process. In the case of extrusion coating, polymer pellets are used, which are converted to a liquid thermoplastic melt. The liquid melt is applied to the carrier web.

(20) Preferably, the polycrystalline silicon chunks, in each case after the comminution, after an optional sorting and classifying step and after an optional cleaning step, are packed in an aforementioned cardboard container with plastic coating and transported to the next processing step.

(21) Preferably, the polycrystalline silicon chunks, after the last processing step, are packed in an aforementioned cardboard container with plastic coating and transported to the customer.

(22) The cardboard container is more preferably used both for transport of the chunks between apparatuses in the manufacturing process and as the final packing container which is transported to the customer.

(23) This leads to a lower space requirement and to an increase in productivity.

(24) This constitutes a considerable advantage in the production process. No plastic bags are used. It is therefore possible to avoid the problems known from the prior art.

(25) The cardboard box size can be varied as desired by the customer.

(26) A preferable net weight is from 5 kg to 1200 kg.

(27) Particular preference is given to a net weight of 5 kg to 500 kg, more particularly to a net weight of 10 kg to 40 kg.

(28) Because of the flexible cardboard box sizes, it is also possible to retrofit existing manufacturing plants without any great cost and inconvenience.

(29) After filling, the cardboard boxes can be automatically welded or sealed.

(30) Preferably, the cardboard containers have a rectangular shape. This enables easy automation of further processing.

(31) The use of commercial pallet systems is possible and preferred. The cardboard containers are preferably secured on a pallet, more preferably lashed down.

(32) Especially in the case of packing of chunks of chunk sizes 0-3, no adverse effects at all on the product properties (contamination, puncture, formation of fines) have been found.

(33) In the case of packing of chunks of chunk sizes 4 and 5, it has been found to be advantageous to provide a layer thickness of the plastic coating of at least 250 m on the inside of the cardboard box.

(34) In the course of packing, the chunks can be dosed directly into the container. It is possible to use standard packing machines or robots with grab arms. A relatively low level of fines arises in the course of filling of the container.

(35) If the container is filled manually, preference is given to using gloves made from high-purity polyethylene or made from PU. The material of which the gloves consist should contain less than 100 ppbw of boron, less than 100 ppbw of phosphorus and less than 10 ppbw of arsenic.

(36) In the case of the prior art plastic bags, it was generally necessary to pre-shape the bags, for example by means of a shaping tube, or by pulling the bag over a collar. There is no need to do this in the process according to the invention since a cardboard container is being used. The problems with punctures known from the prior art do not occur.

(37) There is no need for the visual monitoring for damage to the packing material, which is required in the prior art.

(38) The cardboard container preferably comprises operating elements secured to the outer wall of the container in order to enable gripping and holding of the cardboard container.

(39) FIGS. 1 and 2 illustrate one embodiment in a process for producing polycrystalline silicon, comprising providing polycrystalline silicon rods 1, comminuting 3 the polycrystalline silicon rods into polycrystalline silicon chunks 4, and packing the polycrystalline silicon chunks by introducing the polycrystalline silicon chunks into a cardboard container 5 having a base 6, a wall 7, and an opening 8, wherein at least the inner surfaces 9 of the base and wall which come into contact with the polycrystalline silicon chunks have been coated with plastic 10.