Method for producing a mat of cellulose fibres with a controlled level of mineral material for bitumen-impregnated roofing elements, and suitable device

A method for the continuous manufacturing of a mat of cellulosic fibres from a dilute slurry of cellulosic fibres (3) spread by a headbox (6) on a dewatering fabric (8) of a forming table (1) having a determined width, the dewatering fabric moving between an inlet (6) of the forming table (1) on the headbox side and an outlet (9) of the forming table, carrying along the dilute slurry of cellulosic fibres (3) towards the outlet, the liquids of said dilute slurry of cellulosic fibres being gradually removed through the dewatering fabric during said movement in order to obtain at the outlet said mat, the dilute slurry (3) having a certain level of pre-existing mineral materials and mineral material is added to the cellulosic fibres by spraying with sprays of an aqueous solution or dispersion of mineral fillers, over the width of the forming table, onto the surface of the dilute slurry having already lost a part of its initial liquids,

• wherein the mat of cellulosic fibres is intended for the making of roofing elements made of cellulose fibres impregnated with bitumen and has a thickness of at least 2 mm at the outlet of the forming table (1),
• the line or zone of impact of the drops of the sprays of aqueous solution or dispersion of mineral fillers on the dilute slurry is placed at such a level, along the forming table, that the dilute slurry has lost at least 25% in weight of its initial liquids as they were present at the outlet of the headbox or has a mass concentration of solid per liter of dilute slurry higher than 33% with respect to the initial one,
• the quantity of added mineral fillers is controlled in order to obtain in the roofing element a determined level of total mineral materials resulting from the addition of the mineral fillers to the pre-existing mineral materials that it was possible to preserve in the mat, said control leading in producing a dry mat, with at least 99% of dryness, including between 15% and 25% in weight of total mineral materials,
• the sprays formed by the spraying nozzles are inclined with respect to the vertical and oriented in the direction of, or opposite to the direction of, displacement of said dilute slurry of cellulosic fibres, from the top to the bottom, and
• the kinetic energy of the drops of the sprayed aqueous solution or dispersion of mineral fillers at their arrival on the surface of the dilute slurry having already lost a part of its initial liquids is reduced by implementation of mirror nozzles (5).