BE1031715A1 - Dry and room temperature process for recycling wood fibre materials - Google Patents

Abstract

The invention provides a method for recycling sheet material, such as rigid panels or flexible sheets, based on natural fibers comprising the grinding of the sheet at room temperature and atmospheric pressure, by exclusively mechanical action to obtain bulk fibers. In certain cases, the sheet to be recycled may undergo a mechanical destructuring pretreatment step prior to grinding.

Description

' BE2023/5496

Dry, room temperature process for recycling wood fibre materials.

The invention concerns the material recycling of materials, and more particularly pre- or post-consumer wood fiber panels to manufacture new objects meeting the standards of the circular economy.

The manufacture of panels based on glued wood fibres was invented and developed in the 1920s. Fresh wood chips are cooked for 3 to 7 minutes in a pressure cooker with 6 to 10 bars of saturated steam at temperatures of 175 to 195°C, then defibrated while still wet in an attrition defibrator to form fibres 0.5 to 10 mm long and 1 to 100 micrometres in diameter. The fibres are then dispersed in a tank of water, collected on a conveyor belt in the form of a mattress, drained, then pressed and cooked at high temperature to form soft- or hardboard fibre panels. This so-called "wet" manufacturing process is still used, but has the disadvantages of a low-quality final product (mechanical and water resistance), of being very energy-intensive, but above all polluting in terms of effluents and atmospheric emissions.

In 1965, the so-called "dry" process was introduced. The preparation of the fibres remains the same but they are glued and dried (hot air dryer 250 to 300°C) before forming the mattress (this time dry), before pressing at high temperature (150 to 225°C).

This type of line does not produce liquid effluents after defibering, but remains very energy-intensive and emissive in terms of gases released into the environment. The use of synthetic glues has enabled the development of panels with much superior mechanical and aesthetic qualities compared to the first generation: flexible panels, low, medium or high density panels. It is important to note that fiberboard panels

? BE2023/5496 pressed woods, although renowned for their anisotropic quality, are in fact quite inhomogeneous. Indeed, hot and unidirectional pressing induces the formation of two very dense external faces (up to 1.2 g/cm3) and from these a decreasing density towards the centre of the panel (0.5 to 0.6 g/cm3). The density of the material is directly correlated to its internal cohesion.

A modelling study of energy consumption on a "dry" MDF (medium density fibreboard) production line shows that the stages of log preparation up to defibration consume 30%, drying consumes 50% and finally pressing 20% of all the energy consumed on the line (Li and Pang, 2006).

Wood defibering units, whether followed by a “wet” or “dry” mat-forming process, allow the production of flexible panels used for insulation or rigid panels with various mechanical properties, fire resistance and humidity resistance for use in furniture, fittings and construction. It should be noted that another product that can be manufactured on such lines is bulk wood fiber used mainly for insulation by blowing into cavity walls, attics or by wet spraying on the inside of building walls. These are the same fibers as those produced for panels. Once dried, compacted into bales and packaged, they are sold on the bio-based insulation market. This product is quite similar to cellulose wadding obtained mainly from recycled paper processed in an attrition mill. However, wood fiber has a superior capacity to absorb and release moisture, thanks to the presence of lignin, in particular.

It is estimated that the energy required to produce one tonne of bulk fibre is 20% less than the energy required to produce one tonne of MDF or other fibreboard because there is no

* BE2023/5496 of pressing energy requirement. In the study of Li et al of 2006, the total estimated electricity consumption is 420 kWh/T and 1300 kWh/T of heat for each ton of panel produced. On such a line, the production of bulk fibers is expected to consume 330 kWh/T of electricity and 1040 kWh/T of heat. These data coincide with the generally recognized amount of gray energy for insulating bulk wood fiber of 1000 to 1400 kWh/T. Most of the heat is needed to evaporate the water present in the fresh fibers (50 to 55% water). Another part of the heat is used for cooking the wood chips, in order to soften them and reduce the electrical energy required for the defibrator.

In all grinding processes, six types of mechanical actions are distinguished: grinding by impact of the particle on the tool, grinding by movement of the tool on the particle, grinding by impact between particles, grinding by crushing with the tool, grinding by shearing and grinding by attrition with the tool.

The defibrator is mainly based on the principle of attrition grinding to obtain a sufficient fraction of long fibres with insulating qualities or good mechanical strength for the future panel, but at the cost of numerous energy losses due to friction. The energy consumption per tonne of fibre produced can be estimated at 100 and 150 kWh/T for attrition defibration (Hua et al, 2012). The defibrator represents a significant investment and generally determines the maximum production capacity of a line.

The aesthetic and anisotropic qualities of wood fibre panels have allowed this industry to grow to reach an overall annual production volume of over 100 million tonnes. The product life span depends on the sector concerned: short for fittings, medium for furniture, and the longest for construction. An average life span of around 15 to 20 years is generally accepted. These volumes form the post-consumer part

° BE2023/5496 of fibreboard waste. Pre-consumer waste comes from production and trading scraps and offcuts from panel processing.

In Europe, pre- and post-consumer waste is collected and forms a share of 5 to 15% (Nguyen et al., 2023) of grade wood.

B or 2 (non-toxic treated wood). There are currently two recycling routes: Wood-Energy or Wood-Material. The high specific surface area of the fibers requires the use of large quantities of glue, i.e. 4 to 15% of the weight of the dry fibers.

Aminoplast glues (Urea-Formaldehyde and Urea-Melamine-

Traditional wood (formaldehyde) is very rich in nitrogen and induces nitrogen oxide (NOx) emissions 30 to 40 times higher than untreated wood during combustion (Sartor et al, 2014). Wood-Material recycling currently relies very largely on the segment of the wood panel industry: the production of chipboard panels. A sometimes very significant portion of the fresh wood used can be substituted by class B wood, particularly for low-quality chipboard. The presence of fiberboard waste requires a proportional increase in the quantity of glue, because the fibers absorb much more glue than other wood chips and pieces (higher specific surface area). A marginal fraction of class B wood is composted or transformed into litter or other absorbents.

The number of industrial players is decreasing as this industry intensifies and concentrates on a highly competitive mature market. The number of production sites is decreasing, but the size of the units is increasing (minimum 300,000 T/year) and is concentrated in a few areas where the pressure on forest resources is the least tense. This leads to an increase in transport distances for the finished product to reach customers.

° BE2023/5496

Given the reduction in forest resources and the responsibility of the sector to manage the volume of waste generated, various research projects have been undertaken to find new alternatives for recycling fibreboard and, if possible, as a raw material for manufacturing new fibreboard as was done for chipboard with OSB waste, chipboard, etc.

All attempts at simple processing by grinding the panels generated too many fines and too few long fibres that could be reused for panel manufacturing.

This is why all the processes studied or patented more recently are of the hydro-thermo-mechanical type to minimize fines and increase the share of long fibers. At the same time, since the 1920s, the evolution of techniques tends to minimize the quantity of water used in order to reduce the volume of effluents and their toxicity, but also the waste of energy to evaporate water from the fibers, a reduction in temperature and pressure to limit the energy applied (and reduce gas emissions) and the reduction of the addition of expensive, even toxic, chemical reagents.

It is commonly accepted that fibers recycled by these processes can only replace a minority share, well below 25%, of virgin fibers obtained from fresh wood without negatively impacting the mechanical qualities of the panels (Nguyen et al, 2023). Indeed, recycled fibers are shorter and glue residues reduce compatibility with fresh glue, impacting the mechanical strength of the panels. On the other hand, the clogging of the pores of the recycled fiber by glue residues helps to increase water resistance and reduces formaldehyde emissions for panels containing it.

The previous disclosures therefore relate to a recycling process based on hydro-thermo-mechanical:

° BE2023/5496 — US3741863A targets the recycling of landfill waste and uses steam. - WO1996033309A1 and WO2021225491A1 refer to the use of water vapor and an attrition defibrator. - WO2021176326A1 requires wetting with liquid water, possible reagents, high-pressure steam cooking and possible passage through an attrition defibrator. - WO2021112749A1 and WO2003026859A1 avoid the wetting step and recommend mechanical treatment before steam explosion — WO2005007968A1 requires soaking in water and the use of electromagnetic radiation followed by mixing. - EP2516730A1 proposes grinding, followed by ohmic soaking/heating before a steam explosion. - WO2001039946A1 and DE4224629A1 consist of modifying the operating parameters of the cooker and the defiberer of a conventional fiberboard production plant.

Other documents relate only to the design of defibrators, all based on the principle of attrition, the use of which can relate to the defibration of fresh wood chips or various recycled materials: WO2019035754A1, RU2665100C1, EP2559808B1,

CN111910460B, CN104343034B.

EP3778220A1 relates to an MDF panel whose fibres from fresh wood would constitute the surfaces of the panel and the hydro-thermo-mechanically recycled wood chips would only be used for the core of the panel to limit the reduction in mechanical and aesthetic performance on the surface thereof.

The problem is therefore that the crushing of panel waste destroys too many fibres, and that hydro-thermo-mechanical treatments are very energy-intensive and can only replace a minority of virgin fibres.

It was therefore deemed necessary to propose new, more energy-efficient solutions for recycling wood fibre panels.

Solution of the invention

The present invention provides a new, eco-efficient and unexpected solution for the material recycling of wood fibre panels such as soft- and hardboard, LDF, MDF, HDF and for obtaining free wood fibres usable as raw material for the production of new materials with an incorporation rate of up to 100%. The invention is however not limited to wood panels, whether rigid or not, but also applies to thin sheet materials such as paper and cardboard, also made of fibres, in particular cellulose fibres.

Surprisingly, it has been observed that it is not necessary to implement one or a combination of the following techniques to obtain recyclable free fibres: — a solvent or liquid reagents, aqueous or otherwise, alone or in mixture or containing solutes and other compounds; — water vapour or other compounds, alone or in mixture, in gas phase; — heat, whether supplied in the form of ohmic resistance, electromagnetic radiation, steam or other and higher than the ambient heat of the production premises (0 to 40°C); — pressure conditions lower or higher than atmospheric pressure; — mechanical treatment based on the principle of attrition with the tool.

The invention relates to a method for recycling sheet material based on natural fibres comprising the grinding of the sheet at room temperature and atmospheric pressure, by exclusively mechanical action to obtain bulk fibres.

The invention thus consists in applying to pre- or post-consumer waste of wood fibre panels or sheets of paper or cardboard, at ambient temperature and pressure, without any addition of liquid, solid or gaseous products, a grinding based on one or more of the six types of mechanical action, but without the need to necessarily resort to the attrition technique.

The natural fibre sheet material preferably refers to a panel (i.e. rigid, and for example having a thickness of between a few millimetres and a few centimetres) or a sheet (i.e. flexible, and for example having a maximum thickness of a few millimetres), such as for example paper or cardboard. A textile or fabric is not considered to be a sheet within the meaning of the invention.

The term sheet or panel in the context of the invention means any piece of sheet or panel which is either industrial waste from manufacturing or post-use scrap. It is therefore not necessarily a whole panel or a whole sheet.

The natural fibers here are preferably fibers comprising lignin and/or cellulose. These may be wood fibers, or other natural fibers such as flax, hemp, cotton, rice, corn, all straws or a mixture of fibers.

Loose fibers are unbound fibers, separated from each other. Loose fibers are known for their properties

° BE2023/5496 insulating and can advantageously be used either in bulk or to form insulating materials, or new objects.

The grinding of the sheet, by exclusively mechanical action, is preferably non-shearing, that is to say it does not include a tool or step consisting of cutting the fibers.

The grinding of the sheet, by exclusively mechanical action, preferably includes an impact grinding step.

Grinding is typically performed using at least one grinder or crusher. For example, it may include the use of a slow roller mill, with or without pins or teeth, and/or a fast mill such as a hammer mill which provides unexpectedly higher throughput and fiber quality.

This involves, for example, passing the sheet through a hammer mill or crusher. Hammer crushers are well known to those skilled in the art. Depending on the scale of work, the initial material, and the type of crusher may vary. Generally, hammers are operated in a drum so as to impact the pieces of the sheet until they have reached a size allowing them to pass through a grid arranged on the periphery of the drum. The mesh size of this grid defines the size of the bulk fiber network desired at the outlet of the process.

Preferably, depending on the characteristics of the fibers desired at the process outlet, the grinding is carried out in one or more stages, i.e. several successive crushing stages can be applied, with grids having different mesh sizes, i.e. increasingly fine, to obtain finer fibers at each stage.

Room temperature means that no heating is applied. Typically, room temperature is between 0°C and 30°C, preferably between 5°C and 25°C.

The process is based on the use of one or more crushers, possibly of different types in series or in parallel to progressively reduce the sheets/fiber panels to the desired characteristics.

The invention may also include a mechanical pre-treatment to deconstruct the natural fiber sheet material. This pre-treatment may be considered as a pre-grinding of the material.

For example, when the material is a rigid wood fibre panel, it is not homogeneous, the core of the panel is generally softer than the external faces.

It may be advantageous to deplanarize the material before grinding, i.e. to apply stresses to it to crack it, delaminate it, and thus create “scales”. For example, a 2 cm thick panel can be cracked into scales with a thickness of about 1 to 2 mm.

Such pre-treatment makes it possible to considerably increase the grinding flow rate. It also makes it possible to limit “fines” or wood dust.

The invention also relates to the bulk fibers obtained according to the process of the invention. These fibers are of very high quality, which gives them excellent insulating properties.

The bulk fibers obtained by the process of the invention preferably have a thermal conductivity measured by fluxmetry according to standard EN 12667 of between 0.010 and 0.055 W/m.K, preferably between 0.020 and 0.049 W/m.K.

The invention finally covers the use of the bulk fibers of the invention to manufacture a new material or a new object.

This includes any object made from wood fiber such as flexible or rigid panels used for insulation, structure, decoration (furniture, layout, construction, packaging, toys), any other object pressed and/or glued hot or cold with any type of binder, resin glue, mineral or organic, single-fiber or composite.

As a result, the grey energy content of the fibres produced is very low compared to equivalents on the market, and impossible to achieve using the techniques already described.

The invention will be better understood with the aid of the examples below, which however do not intend to limit the scope of the invention, with reference to the drawings in which: - figure 1 is a diagram of the method of the invention.

Referring to Figure 1, the method of recycling a panel

MDF 1 (or panel portion) comprises a step A of mechanical destructuring pretreatment, here the passage between two rollers 2 causes the deplanarization of the panel 1 to form flakes 3. These flakes 3 are then introduced, in a step B, into a crusher 4 comprising hammers 7 rotating in a cylinder surrounded by a grid 6, having a certain mesh allowing the fibers 8 to escape which are small enough to pass through the mesh. The panel 1 is thus converted into fibers 8 at room temperature and atmospheric pressure, by exclusively mechanical action to obtain bulk fibers.

Alternatively, when recycling flexible sheets, only step B can be implemented, as destructuring is not necessarily necessary.

Example 1

Post-consumer MDF recycling into bulk wood fibers

Pieces of end-of-life MDF panels were treated using the method of the invention.

They were first subjected to a mechanical deplanarization constraint by passing between two 20 cm diameter metal rollers, fitted with teeth, and causing a curved trajectory resulting in the formation of cracks in and on the surface of the material and the formation of MDF scales.

The scales obtained are then defibrated with a hammer mill, fitted with a 4 mm grid, of the type used for defibrating straw and other agricultural materials. The measured processing flow rate is of the order of 1,000 kg/h. The following materials were used:

Supplier | Reference Thickness and | Glue type [ma | aaa 1 Unilin MDF Pure 18 mm, p MDI

Fa Pe 2 Unilin MDF 38 mm, Urea

Standard 600 kg/m3 Formaldehyde

Fibrabel 3 Valchromat MDF stained - | 30 mm, Urea black 740 kg/m3 formaldehyde melamine 4 Valchromat MDF stained - | 8 mm, 850 | Urea black kg/m3 formaldehyde melamine

Example 2 - Thermal conductivity

The thermal conductivity of the fibres obtained from MDF of line 1 of example 1 was measured according to the method described in EN 12667 (2001). Briefly, the fibres are contained in a wooden frame with internal dimensions of 566 mm x 566 mm and a height of 101 mm. This constitutes the test piece.

The specimen was dried in a ventilated oven at 70 °C until constant mass, after which it was conditioned at 23 °C + 2 and 50%rh + 5 until constant mass. Before testing, the specimen was completely surrounded by plastic sheets, (0.15 mm total thickness).

The equipment used (CSTC No.: HFM1) is a flowmeter type device, with a symmetrical configuration with a single test tube.

The dimensions of the device are 600 mm x 600 mm. The measurements are carried out with the test piece in a horizontal position, it being placed between the two flow meters (hot side below and cold side above).

The equipment is calibrated using a certified reference material of the IRMM-440 type. This is a glass wool plate (identification number 4) measuring 600 mm x 600 mm and 34.35 mm thick.

The thermal conductivity is 0.03994 W/(m.K) for an average temperature of 9.98°C for a dry density of 50.3 kg/m.

Bibliographic References

K. Sartor, Y. Restivo, P. Ngendakumana, P. Dewallef: Prediction of

SOx and NOx emissions {from a medium size biomass boiler., vol.25, June 2014, Pages 91-100.

Nguyen, D., Luedtke, J., Nopens, M. et al. Production of wood-based panel from recycled wood resource: a literature review. Eur. J.Wood

Prod. 81, 557-570 (2023).

Li, J., Pang, S. (2006) Modeling of Energy Demand in an MDF Plant.

Auckland, New Zealand: CHEMECA 2006, 17-20 Sep 2006. Conference

Proceedings of CHEMECA 2006: Knowledge and Innovation, 6pp.

Energy demand in wood processing plants e J. Li, M. McCurdy, S. Pang e Published 2006 e Materials Science, Engineering

Hua, J., Chen, G., Xu, D., and Shi, S. OQO. (2012). “Impact of thermomechanical refining conditions on fiber quality and energy consumption by mill trial,” BioRes. 7(2), 1919-1930.

Claims (15)

Claims 1. Method of recycling sheet material based on natural fibres comprising the crushing of the sheet at room temperature and atmospheric pressure, by exclusively mechanical action to obtain bulk fibres. 2. Method according to claim 1, in which the grinding is non-shearing. 3. Method according to one of claims 1 and 2, according to which the grinding comprises impact grinding. 4. A method according to claim 3, wherein the impact crushing is carried out using a hammer crusher. 5. A method according to any one of claims 1 to 4, wherein the natural fibre sheet material is a wood fibre board or a sheet of paper or cardboard. 6. Method according to one of claims 1 to 5, according to which the grinding comprises several successive crushing stages. 7. Method according to one of claims 1 to 6 in which the natural fibers are fibers comprising lignin and/or cellulose. 8. Method according to one of claims 1 to 7 further comprising a mechanical destructuring pretreatment step prior to grinding. 9. Bulk fibers obtained by the method according to one of claims 1 to 8. 10. Bulk fibers according to claim 39, having a thermal conductivity measured by fluxmetry according to standard EN 12667 of between 0.010 and 0.055 W/m.K. 11. Insulation material comprising the loose fibres according to claims 9 or 10. 12. Use of the bulk fibers according to one of claims 9 and 10 for manufacturing a new material or a new object. 13. Use according to claim 12 of the bulk fibers mixed with other fibers. 14. Use according to claim 12 of bulk fibers mixed with other wood fibers. 15. Use according to one of claims 12 to 14, in which the new material or the new object is a flexible or rigid insulation panel, a decorative element, a pressed and/or hot or cold glued object optionally comprising a binder, resin glue, mineral or organic, single-fibre or composite.