DK2516730T3 - recirculation - Google Patents

Description

DESCRIPTION

TECHNICAL FIELD

[0001] The invention relates to the recycling of engineered wood materials such as fibre boards and particle boards. More specifically, the invention relates to recovery of the constituent wood fibres from waste engineered wood materials for re-use as a substitute for virgin wood fibre. This invention does not relate to pulping processes that recover fibre from lignocellulosic materials for paper or cardboard production.

BACKGROUND

[0002] Wood based composite fibre boards and particle boards (panels) are commonly used as raw materials for furniture manufacture, joinery, shop fitting and construction. They are mass produced in many parts of the world (upwards of 22 million tonnes annually). They typically comprise a matrix of refined timber fibres, wood chips or shavings bonded together under high pressure and temperature, usually with the addition of a resin or adhesive. Most prominent of these panel types are medium density fibre board (MDF) and high density fibre board (HDF).

[0003] Fibre boards and particle boards are fit for purpose but do not lend themselves readily to effective recycling. Over 170,000 tonnes of MDF is thought to enter the waste stream yearly in the UK from primary and secondary processing activity (commercial and industrial waste streams). Significantly greater amounts when post consumer waste is taken into account.

[0004] Handling of these waste streams is made more problematic by the commonly employed technique of applying a surface finish to the board during or after manufacture. Fixtures and fittings e.g. inserts and hinges that might be applied to products manufactured from fibre board also add complexity to the identification of an effective recycling process.

[0005] There is currently no viable economic process to recover wood fibre from these waste streams and consequently it is disposed of either to landfill or incineration. This contributes to the unsustainable use of landfill and loss of recycling potential.

[0006] EP-A-1 886 586 relates to devices for ohmic heating of food compositions, in particular soups.

STATEMENT OF INVENTION

[0007] The term "slurry" is used throughout this document to describe material that consists of a water and board mixture that ranges from small pieces of wet solid board through to individual wood fibres held in water.

[0008] The term "engineered wood material" is used to describe material such as fibre board and particle board that has been constituted by artificially bonding together fibres, chips or other small particles of timber.

[0009] The present invention is a method to recover the constituent wood fibres from waste and rejected engineered wood materials, preferably in a continuous process, such that the fibre recovered is of sufficient quality to be recycled into / reused within the fibre board manufacturing industry (as a substitute for virgin wood fibre) and other end use applications including but not limited to insulation materials, fillers for plastic / cement composites, spill absorbents and horticultural growth media.

[0010] The method employs (i) the addition of water with or without additives to a mass of waste composite engineered wood material and (ii) the application of ohmic (conductive) heating to that same mass. The benefit of applying ohmic heating is the uniform and penetrative heating effect generated by this technique without burning or charring the wood fibres. The waste material is preferentially mechanically broken down (shredded) before heat application or water addition to aid process efficiency. Further mechanical disruption of the board material is applied to the water board slurry before drying of the fibres takes place. Surface agents may be employed to accelerate the impregnation of the fibre board pieces by the added water. Electrolytes maybe added to the water to optimise conductivity and improve heating efficiency.

[0011] The method employs a use of high temperatures in the range of 30°C to 99°C (and preferably in the range of 80°C to 99°C) on a slurry (mix) of water and engineered wood material. The application of high temperature using ohmic heating reduces the adhesive (both added resin and natural lignin) bonding between the lignocellulosic elements thus allowing the constituent wood fibres to be readily separated from each other.

[0012] The method preferably further employs the use of a continuous system.

[0013] The method may further employ the use of elevated temperatures (above 100°C) in a pressurised ohmic heating module.

[0014] The method may further employ the use of rapid de-pressurisation, via a high pressure nozzle (or array of nozzles), as an aid to drying of the recovered fibres. A fibre separation effect is also induced during de-pressurisation which aids materials handling and facilitates more efficient supplementary drying if required. This drying method may operate either as a result of rapid de-pressurisation from a pressurised ohmic heating module or be employed as a subsequent independent pressure sub-system within a continuous process line that utilises ohmic heating at ambient pressure.

[0015] The invention is important from a sustainable development perspective as it offers the potential for reducing the environmental burden of an important element of the timber products supply chain whilst generating added value to, and prolonging the service life of, long length wood fibres.

[0016] The continuous nature of the processis a highly preferred factor that allows the process to be relevant to the high volume needs of the composite fibre board manufacturing sector, where raw material feed stock throughput rates can reach upwards of 40 tonnes per hour.

[0017] In addition, the method according to this invention is capable of separating commonly found surface finishes from the wood fibre element of the engineered wood product. These finishes are used during or after production as a means of adding value to 'commodity' products such as fibre board. These finishes typically include veneer, paper foil, paint or other type of 'laminate' and have to be removed before the recovered wood fibre can be reused. This invention addresses that requirement. Options exist to pre-treat the waste utilising mechanical methods such as sanding, high pressure water jetting or planing. However, where these methods are not practical, flotation separation (including froth separation) can be used after the application of ohmic heating. Alternative (or additional) separation can be achieved via the use of air separation technologies (such as cyclonic separators) during or after drying of the fibres has taken place.

[0018] This invention has global application in that composite boards are widely manufactured and used. It will provide a new source of raw material for the production of composite boards such as MDF, which are currently highly sensitive to the use of recycled timber feed stocks. The fibre recovered from this process will be of comparable 'quality' to that of the virgin wood fibres produced from chips or logs by the typically employed refining process. The fibres resultant from this inventive process can be applied in a flexible manner in terms of their physical characteristics; length, moisture content etc can be controlled. The effective manner in which this invention allows for wood fibre to be recycled is important at this time because it reduces the demand for virgin timber, a scarce natural resource.

ADVANTAGES

[0019] This technology allows for the recovery of wood fibres from waste engineered wood material such that the recovered fibres remain of suitable quality for the re-introduction into fibre board or other manufacture. This process is repeatable in that fibre board manufactured with recovered fibre content can itself be recycled when appropriate. This offers environmental and economic advantages over other recycling / disposal methods such as composting / incineration with energy recovery, whereby the fibres are 'lost' after a single cycle.

[0020] In one aspect, this invention allows for the continuous recovery of high volumes of wood fibre from waste engineered wood material thus allowing the recovered fibre to be introduced directly into the MDF manufacturing process. This is a significant advantage over 'batch' processing, which would be unable to supply recovered fibre in a manner suited to manufacturing throughputs ranging from 10 tonnes - 40 tonnes per hour. This invention also allows for flexibility and control of the moisture content of the recovered fibres, therefore offering multiple entry points into the fibre board manufacturing process line (which can vary between plants). Recovered fibres can also be dried and baled for more efficient storage and use in alternative application markets.

[0021] This invention differs from the invention disclosed in international patent application WO 03/026859 A1, whereby waste board material is processed in a low volume batch fashion using a chemo-thermo-mechanical process.

[0022] The use of ohmic heating is a highly efficient and cost effective means of heating in both capital and operational cost terms. This offers an advantage over the technology suggested by Jawaid (patent GB 2410746 B) whereby electromagnetic radiation is proposed as a means of assisting with the degradation and recovery of wood fibres from waste lignocellulosic board material.

[0023] Operational benefits from this ohmic heating technique would include a more controlled temperature profile and a more homogenous heating profile of the slurry during processing of the board and so a more predictable recovery cycle with no danger of burning or charring the material and reduced operational fire risk.

[0024] This invention incorporates an optional high pressure pumping unit into which the ohmic heating elements can be integrated. The unit may alternatively form a discrete module after the ohmic heating has been applied. The rapid release of pressure from this unit via a high pressure nozzle array designed to permit the passage of the water and suspended wood fibre allows for instant evaporation of a substantial portion of the inherent moisture contained within and between the wood fibres. A fibre separation effect is also witnessed. This drying effect is achieved at a relatively low energy and cost burden. Supplementary drying may also be required or desirable dependent on the application for the recovered fibre.

INTRODUCTION TO DRAWINGS

[0025] Figure 1 is a schematic of the preferred process for recovering wood fibre from waste fibre board.

DETAILED DESCRIPTION OF PROCESS

[0026] MDF and other engineered wood material waste can be categorised into two distinct types-that which is unfinished i.e. has no surface coating applied (raw board) or that which is finished i.e. has a veneer, foil, paint or other form of laminate applied to the surface and /or edges. This invention is capable of processing these wastes together i.e. mixed waste streams of both finished and unfinished material being treated simultaneously or by diverting finished waste material via an initial 'cleaning' module where the surface finish is removed via mechanical means [0027] Each waste recovery processing plant is likely to operate a specification for incoming waste material dependent on the commercial activity within its catchment area e.g. cluster of furniture manufacturers or a major retailer's distribution centre (where waste materials may be received after back hauling from multiple outlets).

[0028] If required, a mechanical breakdown module (1.1) will form the beginning of this invention with shredding the most likely operation to be employed. The removal of surface finishes is best achieved by shredding material to a relatively uniform size, each piece having approximate dimensions of 80mm x 80mm. Flowever, ohmic heating is most efficient when the average size of the shredded fibre board pieces is less than 20mm x 20mm. This allows for a more even distribution of interstitial water across the ohmic heater cavity. Taking into account these variations, the optimum waste piece size parameters have been established as between 25mm x 25mm and 45mm x 45mm. Removal of metallic or other solid contaminants will be addressed immediately after the shredder with an induction magnet (1,2a) for ferrous metal removal and an eddy current separator (1.2b) for the removal of non-ferrous contaminants (screws, nails, fixings, hinges and other inserts that might be found in part-worked or post consumer fibre board wastes).

[0029] After shredding the material is graded via an automated sieve (1.3). A mesh conveyor, which employs a degree of vibration, will remove dust and fines. Dust extraction (1.4) is used at this point to maintain a safe working environment.

[0030] The material is then fed into a dipping tank (1.5) which contains hot water at a temperature above 30°C with the optimum operational range of between 80°C and 99°C. The purpose of this module is to allow the fibre board waste to absorb sufficient volume of water so as to allow conductive heating to operate efficiently and predictably. The water may contain added surfactants to enhance or accelerate the wetting of the fibre board. The water may also contain added salts (for example NaCI in a 0.05M solution) to enhance the conductivity of the resulting slurry.

[0031] The initial shredding (1.1) allows for greater surface area of fibre board to be exposed to the water when immersed in the dipping tank therefore reducing the time required for sufficient absorption to take place. It is preferable to limit the water uptake to the minimum required so as to reduce the cost and energy involved in removing it again later in the process. Dependent on the mix of the waste streams being processed, moisture uptake will be optimised between the higher moisture content required to maximise ohmic heating efficiency and the lower moisture content to minimise the energy required to dry the processed fibres. In one example, the moisture content of the fibre board may be increased from approximately 8% wwb (wet weight basis / total weight) to approximately 70% wwb.

[0032] The waste material is then conveyed into a continuous feed hopper which introduces the fibre board pieces to the ohmic heating module (1.6). The ohmic heater will allow for rapid and homogenous heat absorption into the fibre board pieces. The feed into this module will be controlled by a plug screw, displacement pump or similar device which maintains a constant throughput. The plug screw also acts as a seal to ensure that no unwanted release occurs from the heating module which may be operating under raised (above atmospheric) pressure. A high pressure displacement pump may be employed to raise the internal pressure of this module preferably to between 1.0 atmosphere and 20 atmospheres (to prevent boiling) dependent on the process temperature requirements.

[0033] The heating module will typically take the form of an angled insulated cylindrical vessel between 0.2m and 20m in length with a diameter of between 50mm and 500mm. Based on a typical throughput rate of five tonnes per hour of waste fibre board, these dimensions would be approximately 15m in length with a diameter of 220mm. This would allow the slurry to remain in the heating module for a period of a few minutes: typically about 3 minutes depending on the operating temperature.

[0034] Electrodes are situated on the inside of the process vessel in a shaped array of two or more electrodes that is configured to maximise the heating and penetrative potential of the system. The shaped array is customised to flow rates, power requirements and conductivity of the slurry and can be controlled to optimise heater temperature for given throughputs, input temperatures and conductance of the slurry. The ohmic heater will typically operate in the range of 20KW to 1500KW. The slurry is maintained at a temperature of at least 50°C and preferably at least 90°C while it remains in the vessel. If the vessel is pressurized, the operating temperature may be increased up to 160°C.

[0035] The material may exit from the ohmic heater via a water column or tower in order to maintain the pressure within the heater.

[0036] After passing through the ohmic heater, the material may still contain loose 'clumps' (clusters or bundles) of un-separated fibres. The material may therefore require some additional agitation to reach a completely homogeneous state.

[0037] ' De-clumping' or de-agglomeration may be achieved by feeding the material into a rotating cylinder that may or may not be warmed. The rotating cylinder may also be used to selectively capture remaining pieces of waste laminate. An airflow system may also be used that introduces physical barriers to the free passage of fibres. This will achieve the effect of liberating the individual fibres from the remaining clumps at this stage.

[0038] The material post de-clumping is dried (1.7). Following drying any remaining pieces of waste laminate may be removed in an air cyclone prior to baling and packing of the recovered fibres (1.8). Alternatively, the recovered fibres may be fed immediately into a further manufacturing process at the same location.

[0039] In the case of more liquid slurries exiting the ohmic heater de-clumping of fibres is achieved via mechanical agitation, typically by the use of an in-line ribbon blender module. Flotation separation techniques may be applied to remove any residual waste laminate. This is followed by mechanical de-watering of the slurry. After mechanical de-watering the material is subject to de-clumping and drying. De-clumping or de-agglomeration and subsequent processes may then be carried out as previously described. Where the liquid slurry is still under pressure it may be pumped into one of two (or more) pressure vessels which are alternately isolated from the continuous system once full. From these pressurized holding tanks the slurry is pumped through a spray nozzle array where a sudden and explosive pressure release occurs which 'blow/s apart' the friable matrix of each waste fibre board piece such that individual constituent fibres are released. The material post spraying is further dried as required. Following drying any remaining pieces of waste laminate may be removed in an air cyclone.

REFERENCES CITED IN THE DESCRIPTION

This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description • EP1886586A [0006] . WQ030268S9A1 Γ00211 • GB2410748B [0022]

Claims (12)

A method of recovering wood fibers from engineered wood material comprising the steps of: mixing finely cut engineered wood material with water to form a sludge, and passing an electric current through the sludge to heat the fine wood material, thereby weakening the connections between the wood fibers in the wood material. . The process according to claim 1, wherein the sludge is heated to a temperature between 30 ° C and 99 ° C. The process of claim 2, wherein the sludge is heated to a temperature between 80 ° C and 99 ° C. The method of claim 1, wherein the heating is carried out over atmospheric pressure. The process of claim 4, wherein the heating is carried out at a pressure of between 1 and 20 atmospheres (0.1 and 2.0 MPa). The process of claim 4 or 5, wherein the sludge is heated to a temperature between 30 ° C and 160 ° C. A method according to any one of the preceding claims, wherein the step of mixing the sludge further comprises the addition of an electrolyte. A method according to any one of the preceding claims, wherein the step of mixing the sludge further comprises adding a surfactant. A method according to any one of the preceding claims, wherein the pieces of engineered fine-cut wood material have an average size in the range of 25 mm to 45 mm. A process according to any one of the preceding claims, further comprising a step of separating impurities from the sludge before or after the heating step. A process according to any one of the preceding claims, wherein the sludge is subjected to a rapid pressure drop after heating. A method according to any one of the preceding claims, further comprising transporting the sludge through a vessel wherein the heating step is carried out during a continuous process.