EP0581039B1 - Method of recycling wood materials - Google Patents

Abstract

A method is specified for recycling elements comprising wood-based materials glued with urea-formaldehyde binders, with or without plastic coating, in which method, in a pretreatment step, the elements are broken into small fragments and any metal parts, such as fittings or the like, are separated out, then, in a pulping step, the fragments are subjected to a, preferably saturated, steam atmosphere under high pressure and at high temperature for a specific pulping time and then the material - secondary material - which has then already disintegrated into its fibrous etc. basic components, is glued to form new elements, chemically adapted urea-formaldehyde binders, namely those condensing with the urea residues still adhering to the fibres etc., being used for gluing. There is also the possibility, after the pulping step, of washing, in a cleaning step, the material which has then already disintegrated into its fibrous etc. basic components, under normal pressure, intensively with a cleaning liquid, in particular with clean water, and thus producing a secondary material which is free of adhering urea residues and can be glued using binders suitable for virgin material to form new elements.

Description

The invention relates to a method for recycling generally plate-shaped elements from wood materials glued with urea-formaldehyde binders.

Wood materials of the type in question are in particular wood fiber boards, chipboard and plywood boards, which do not have to be coated, but are often used with a coating of plastic as worktops and furniture elements and as elements for interior construction (coating with HPL boards, ie high-pressure laminate boards or the like.). The professional world dealt with the recycling of such elements many years ago (early 1970s), since it was recognized early on that valuable material would be destroyed senselessly and in an environmentally hazardous manner when it was burned or deposited. In a method proposed at the time, the elements should be broken up into larger parts so that they fit into the pressure chamber of an autoclave. For the technology of thermal-hydrolytic digestion in autoclaves, reference may be made to the generally known prior art, for example in Lueger "Lexicon der Technik", DVA Stuttgart, keyword "autoclaves" or "pressure vessel".

The previously described process had to be carried out at a relatively high pressure in the autoclave, the chip-shaped basic components present after this disintegration step were then re-glued with urea-formaldehyde binders to new elements. However, the quality of the new elements made from this secondary raw material was so poor that they could not really be used seriously. Despite the diverse efforts of the experts since then, it has not yet been possible to find a process which leads to new glued elements made of secondary material with properties comparable to new goods.

The teaching of the invention is based on the object of specifying how to use a recycling method of the type in question to create new elements satisfactory properties that can be compared with those made from new goods.

A first alternative of the method according to the invention is the subject of claim 1, a second alternative is the subject of claim 2.

In both alternatives of the method according to the invention, the elements are first broken into small fragments in the pretreatment step, i. H. Fragments with dimensions of a few centimeters. This has two results, on the one hand a significantly increased surface for the interaction with the saturated water vapor in the digestion step, i.e. a significantly improved effectiveness of the thermal-hydrolytic digestion, on the other hand a significantly better use of space in the pressure vessel, especially in the autoclave. This makes the breakdown into the fibrous, chip-shaped or platelet-shaped basic components much more effective.

In the first alternative of the method according to the invention, the essential essence of the invention lies in the choice of the correct binders for the re-gluing of the basic components to new elements. It has been recognized here that it is not possible to work with the binders that have been tried and tested for new goods (molar ratio of urea: formaldehyde 1: 1.0 to 1: 1.2), but that other condensed urea-formaldehyde binders should be used, namely those with a higher one Molar ratio of formaldehyde, in particular with a molar ratio of 1: 1.3 to 1: 2.0. This finding is based on the fact that the thermal-hydrolytic disruption of the fragments restores the fibrous basic structure of the basic components by reversing the crosslinking provided by the gluing, but that there is still so much urea residue on the individual fibers, chips or platelets adhere to the fact that an excess of urea occurs when normal binders are used, which greatly impairs the properties of the resulting elements. According to the invention, the excess of urea detected in this way is compensated for by an excess of formaldehyde in the binder, so that excess formaldehyde present there with the residual urea present on the fibers ultimately leads again to the correct setting conditions.

The alternative described in claim 2 is based initially on the same knowledge of the urea residues on the fibrous basic components after the disruption step. This alternative includes an additional process step, namely that of cleaning the basic components with a cleaning liquid, which should preferably be pure water. As a result, the urea residues are rinsed off the fibers and the fibers are returned to their initial state. This measure is based on the knowledge that the urea residues have become water-soluble as a result of the digestion step and thus accumulate in the cleaning liquid. This ultimately ensures that the fibrous, chip-like, platelet-like or similar basic components can be processed further as is done with new goods, in particular with the usual urea-formaldehyde binders or also with other binders, for example based on phenolic resin . You do not have to work with the special or chemically specially adjusted binders required in the first alternative. If there are still urea residues on the fibers despite washing, slightly higher molar ratios can also be used.

In both alternatives, as tests have shown, new, generally plate-shaped elements made of wood-based materials are produced as end products, the properties of which are practically inferior to those made of new goods.

As has already been explained above, the digestion step is preferably carried out using saturated steam, that is to say in a saturated steam atmosphere. The preferred temperature / pressure ranges are given in claim 4. The correlations result from the state diagram of water, which can be found in the literature. Experiments have shown that the disruption time described in claim 5 is sufficient.

For procedural reasons and in particular for reasons of optimizing the energy balance of the entire process, it is recommended that the hot steam from the digestion step is used in a preheating step at normal pressure to preheat and dampen the fragments. This can happen in connection with an autoclave with both batch and continuous loading, one works in a countercurrent, so that the warming up of the fragments in the autoclave takes less time and the disruption is a little faster.

In order to prevent or at least reduce the development of an odor, it is recommended that the steam be sucked out of the autoclave before opening and emptying the autoclave and preferably replaced by fresh air.

It is also recommended that after the digestion step, preferably only after the cleaning step, contaminating material residues such as non-ferrous metals, plastic parts, coating parts etc. are separated from the secondary material in a separation step. This measure is of course particularly recommended in the case of elements provided with coatings of plastic as the starting material, for example kitchen worktops or the like. It would be unfavorable for the new element made from the secondary material if a large number of impurities are contained in the starting material. A small amount of relatively small-volume impurities, however, does not matter. The material is expediently screened and sifted in the separation step. For the construction of suitable sifters, reference may be made to the literature (see again Lueger "Lexicon der Technik" loc. Cit. For the keyword "sifter").

For the method according to the second alternative according to claim 2, i.e. the method including the essential cleaning step of the basic components, it is recommended that the rinsing takes place in countercurrent in a cascade system, i.e. the already cleaned basic components with the cleanest cleaning liquid and even more so basic components not cleaned with the most contaminated cleaning liquid. be rinsed.

Urea compounds, essentially methylol urea, dimethylol urea etc. collect in the cleaning liquid, in particular in water. These chemical products can also be used for further use. For this purpose, it is recommended that the used (dirty) cleaning liquid condenses, thickened in particular by evaporation or ultrafiltration, and a residue concentrate which essentially contains urea compounds is thus obtained.

This concentrate essentially consists of the urea compounds already given. Possible uses for such a concentrate are mentioned optionally in claim 10. In particular, the use for the reduction of nitrogen oxides in incineration plants appears very interesting according to the current state of knowledge, since the materials concerned are particularly effective here.

Claims (10)

1. A process for recycling elements, generally in the form of boards, comprising wood materials bonded with urea-formaldehyde binders with or without a coating made of plastic, particularly wood fibre, wood chip or plywood boards,
      wherein in a pretreatment step the elements are broken into small fragments and any metal components which are present, such as metal fittings or the like, are separated,
      wherein in a digestion step the fragments are then subjected for a fixed digestion period to a water vapour atmosphere, which is preferably saturated, at high pressure and at high temperature, and
      wherein the material, which has then already disintegrated into its fibrous, chip-like, plate-like or similar basic constituents - the secondary material - is then bonded to form new elements, generally in the form of boards,
      wherein chemically adjusted urea-formaldehyde binders, namely those in which residues of urea which are still adhering to the fibres, chips, platelets or the like are incorporated by condensation, are used with a superproportionally high mole ratio of formaldehyde for bonding.
2. A process for recycling elements, generally in the form of boards, comprising wood materials bonded with urea-formaldehyde binders with or without a coating made of plastic, particularly wood fibre, wood chip or plywood boards,
      wherein in a pretreatment step the elements are broken into small fragments and any metal components which are present, such as metal fittings or the like, are separated,
      wherein in a digestion step the fragments are then subjected for a fixed digestion period to a water vapour atmosphere, which is preferably saturated, at high pressure and at high temperature, and wherein in a cleansing step the material, which has then already disintegrated into its fibrous, chip-like, plate-like or similar basic constituents is intensively washed at normal pressure with a cleansing liquid, particularly with pure water, and a secondary material freed from urea residues is thus produced which can be bonded using binders suitable for new materials to form new elements.
3. A process according to claim 1 or 2, characterised in that urea-formaldehyde binders with a (urea:formaldehyde) mole ratio of 1:1.3 to 1:2.0 are used.
4. A process according to any one of claims 1 to 3, characterised in that a temperature between 120°C and 180°C (390 to 450 K), preferably a temperature between 140°C and 160°C (410 to 430 K), and a pressure between 2 and 11 bar, .pa preferably between 4 and 5 bar, are employed in the digestion step.
5. A process according to any one of claims 1 to 4, characterised in that a digestion period of 2 to 5 minutes, preferably of about 3 minutes, is adhered to, with a lead time (heat-up time) of 0.5 to 2 minutes, preferably about 1 minute.
6. A process according to any one of claims 1 to 5, characterised in that the hot water vapour from the digestion step is used in a preheating step at normal pressure for preheating and steaming the fragments.
7. A process according to any one of claims 1 to 6, characterised in that the digestion step is conducted in an autoclave which is fed batch-wise or continuously, and preferably that the steam is extracted from the autoclave before the autoclave is opened and emptied and is preferably replaced by fresh air.
8. A process according to any one of claims 1 to 7, characterised in that contaminant residual materials such as nonferrous metals, plastic components, parts of coatings, etc., are separated from the secondary material in a separation step after the digestion step, preferably not until after the cleansing step, and preferably that the separation is effected by screening and/or sorting.
9. A process according to claim 2 and optionally according to any one of claims 3 to 8, characterised in that washing is effected as a counter-current operation in a cascade system, and therefore the basic constituents which have already been cleaned the best are washed with the cleanest cleansing liquid, and the basic constituents which have not yet been cleaned at all are washed with the cleansing liquid which is already the most contaminated.
10. A process according to claim 2 and optionally according to any one of claims 3 to 9, characterised in that the spent (contaminated) cleansing liquid is condensed, thickened, particularly by means of evaporation or ultrafiltration, and a recyclings concentrate is thus obtained which essentially contains urea compounds, and preferably that the recyclings concentrate is used as a secondary raw material for the production of adhesives based on urea resin, and in particular is re-used for the production of corresponding wood elements, or that the recyclings concentrate is used for the resin condensation of urea-formaldehyde resins or that the recyclings concentrate is used for the selective, non-catalytic reduction of nitrogen oxides (NO_X) in combustion installations.