Patent Publication Number: US-2016221140-A1

Title: Method for creating a grit pattern on a grindstone

Description:
TECHNICAL FIELD 
     The present invention discloses a method to create and optimize a grit pattern on a grindstone. 
     PRIOR ART 
     In mechanical pulping, such as ground wood (GW), stone ground wood (SGW), pressure ground wood (PGW), modified or developed versions of the above such as super pressurized ground wood (SPGW), or chemical pressurized ground wood (CPGW), a grindstone (or grinding stone) is used to defibrate or liberate fibers from the wood matrix. The main principle in grinding as well as in all mechanical defibration processes is to bring the wood raw material into a cyclic oscillating stress field whereby the absorbed mechanical energy breaks down the structure of the fibrous raw material. 
     By using different conditions during the grinding, pulps with different properties can be obtained. The grinding conditions such as wood type, wood feeding rate, stone composition and characteristics (amplitude, frequency), temperature, pressure, water etc., affect the yield, energy consumption as well as obtained fiber and furnish properties. Typical for mechanical pulps are the high yield that is obtained, &gt;95%, which is of course a benefit when aiming for efficient usage of raw materials. Normal pulping stone tools are usually quite large and contain a plurality of abrasive segments assembled about a cylindrical concrete core see for example U.S. Pat. No. 5,243,789. Critical aspects in the grinding process are the pulp stone quality and particularly the surface being in contact with the wood. Protruding active grits, and the physical dimension and pattern of the grits affect not only the fiber or furnish quality but also yield and energy consumption. In a ceramic pulp stone, the grits have typically a particle size of 0.2-0.5 mm. The grits in a traditional new paper pulp stone is on an average of 0.6 mm apart cyclically load the fibers at frequencies in the range of 40-50 kHz. 
     Without bound to any specific theory, the heat, pressure and mechanical impact in the grinding zone affects the release of fibers and fibrils. The pulp stone usually comprise different ceramic materials such as silicon carbide, aluminum oxide, whereas the inner core is cement based material. The grit pattern and grind exposure is crucial for the performance of the stone. Sharpening can be made with different mechanical tools whereas condition of the surface can be made with a water jet. The water jet made at high pressure (500-2400 bar) can be made during the production and affects therefore not the production. Besides removing impurities, this also improves the quality of the pulp and production capacity. The sharpening, on the other hand, provides sharped edges in the pattern which is important for water channels and enables removal of fibers. Typical groove depth is 0.2-0.5 mm. 
     By sharpening the grinding stone surface, the quality of the pulp can be maintained or improved. The dressing tool, also referred as a burr, is normally rolled over the surface under sufficient pressure. However, the features of the burr itself might affect the life time of the stone and particularly the grinding performance and the subsequent pulp quality. This will not only affect grinding efficiency, but also the papermaking process and end product performance. 
     US 2003/0145842 A1 discloses a mechanical method or tool (burr) for sharpening or dressing a grinding stone comprising a cylindrical body containing plurality of teeth. 
     US 2008/0250725 A1 discloses a method to use abrasive round particles in a grinding surface for making longer fibers. Abrasive proppant grit is formed by attaching proppant to a grinding stone surface using an adhesive. Salmi, A., “What should the grindstone surface look like to produce pulp with least energy, Dissertation”, University of Helsinki, 2012 
     One negative aspect of mechanical pulp is the high energy consumption required in the pulping process, which can be as much high as 3.5 MWh/metric ton. Only 1-10% of the consumed energy goes to defibrating of fibers. In the publication by Salmi, A. “What should the grindstone surface look like to produce pulp with least energy, Dissertation”, University of Helsinki, 2012, the role of the grinding stone pattern on energy consumption is revealed. By introduction a sinusoidal curve to the surface, it was possible to reduce grinding energy by 30%. US Patent 2009/0308549 (Tuovinen, O., Device and method for defibration of wood) and Finnish Patent FI 98148 (Björkvist, T., Menetelmä ja laite puun mekaaniseksi kuiduttamiseksi, 1997) a two zone grindstone surface is proposed. The inventors called this a “kneading part” and a “loosening part” alternating in quick sessions. 
     However, the solutions described above to improve and manufacture a grinding stone surface are based on mechanical or chemical treatment technologies. The dressing results obtained with mechanical dressing is highly dependent on the dressing tool sharpness, pressure, stone composition and other factors. The surface treatment based on the use of proppants, on the other hand, requires also continuous control of surface topography and that the interface between adhesive and stone remains strong enough. By using a mechanical tool, the surface is treated regardless it is in bad condition or not. 
    
    
     A purpose with the present invention is to improve the method to prepare a grinding stone surface. By using a more flexible technique, the pattern can be created or fixed according to the current conditions and needs. 
     The present invention is related to a process based on using a laser beam in making a grit pattern on a surface of a grindstone. Moreover, the use of laser beam enables either etching or sintering or both simultaneously for preparing an optimal grinding stone surface. In contradiction to conventional mechanical sharpening techniques, this enables a faster and more precise solution to upgrade the pattern. The disclosed invention can also be equipped with an imaging system, which hence repairs and upgrades only selected areas. 
     The treatment of the laser beam can be operated in a normal mode to engrave the surface and create a sharp pattern for optimizing the grinding performance (pulp quality) or to reduce the grinding energy. By adjusting the energy used in the laser, or the laser source, the grooves can be selectively clean from fibers or resins, and also remove part of the resins to get grit more protruding. The laser can also be used to smoothening lands or to create microstructure on the lands. 
     Laser beam can further be used in order to fuse a coating or proppant on the grinding core stone. According to prior art, the grits are normally adhered with an adhesive such as a resin. However, the use of laser beam enables sintering of abrasive particles onto the stone surface by focusing the beam on the proppant which then after fuses into the resin matrix. 
     Laser technology can further be used in a smart-variable mode, such that, the grindstone surface first is imagined and then a specific area is repaired or treated. Such a repair might improve the grinding performance but also facilitate cleaning, and exposure of grains, grooves, cleaning and opening of pores, improving the water flow on stone surface. It might also enable more variable pattern optimization on the surface, which might be optimal due to seasonal effects etc. 
     The laser technology can be combined with existing technologies, thus offering both mechanical patterning with e.g. laser sharpening of grooves and smoothening of the lands. 
     The laser technology can also be used to sharpen the mechanical tools and to provide maintenance of the tools during the grindstone preparation and maintenance. 
     A benefit with the present invention is that laser beam method more efficiently makes and optimizes the grit pattern on the grindstone in order to reduce refining energy and to better tailor-made the fibers and pulp properties.