Recycling of wood dust-laden atmospheres in workshops
Résumé
Wood dusts are included in CRAM (National Insurance Institution) Table 47 because of their allergenic (favouring asthma) and probably carcinogenic natures. These are not only recognised for solid wood, but also suspected for treated (e.g. heat treated) or reconstituted wood (e.g. MDF or particle board often containing urea-formol or phenolic glue featuring constituents). In this study, initially, we will characterize the wood dust sizes produced by different manufacturing processes on solid wood or reconstituted wood. Then, we will use these results for dimensioning an air filtration system. The air to be filtrated comes from dust extraction system of the machines and will be recycled into the workshops. We carried out the evaluation of the total production of dust and chips produced by usual machines: circular saw, single sided planer and sander. The selected conditions of machining correspond to current industrial practices (0.1 mm< chip theoretical thickness< 0.5 mm). The studied materials included solid woods (beech and spruce), particleboard, MDF (Medium Density Fibreboard) and heat treated poplar. For each material and machinery, we recovered all particles produced ranging from the coarsest (2 -3 cm) to the finest size (lower than the micrometer). We took into account the particles deposition on the caps of the machine, into the ventilation pipe, and into the finishing filter. Dust particles size distributions were measured by sieving for the particles sizes larger than 400 μm, and by laser-based size grading for the finest. Our results showed that in single sided planing and sawing, the type of tool and the type of material machined are determining factors for the particle size distribution. In sanding, it would seem that the material is not an influencing factor. The originality of our system of filtration is that it presents in cascade a cyclone followed by an electrostatic precipitator which dimensions are compatible with the sheath ventilation diameter. With a good electrostatic precipitator dimensioning, we recommend a direct recycling of chip extraction air while respecting the maximum wood dust content imposed by the actual legislation.