Disc refiner with increased gap between fiberizing and fibrillating bands
Plate elements, a plate configuration, and associated system for thermomechanical refining of wood chips wherein destructured and partially defibrated chips are fed to a rotating disc primary refiner, where opposed discs each have an inner band pattern of bars and grooves and outer band pattern of bars and grooves, such that substantially complete fiberization (defibration) of the chips is achieved in the inner band and the resulting fibers are fibrillated in the outer band. One embodiment is directed to a pair of opposed co-operating refining plate elements for a flat disc refiner wherein the bars and grooves on each of the inner bands form an inner feed region followed by an outer working region, the bars and groove on each of the outer bands form an inner feed region followed by an outer working region, and the gap and/or material flow area formed when the plates are placed in front of each other increases between the inner working region and the outer feed region.
1. In a primary wood chip refiner having flat, relatively rotating discs, each having a working plate thereon, the working plates being arranged in confronting coaxial relation thereby defining a refiner gap which extends substantially radially outward from the inner diameter of the discs to the outer diameter of the discs and defines a flow area for the chip material to be refined, the improvement comprising: each plate having a radially inner fiberizing band and a radially outer fibrillating band, each band having an inner feeding region and an outer working region, the radial width of the inner band being less than about 35% of the total radius of the plate, wherein the working region of the inner band is defined by a first pattern of alternating bars and grooves, and the feeding region of the outer band is defined by a second pattern of alternating bars and grooves, and said flow area increases immediately after the inner working region into the outer feed region.
2. The refiner of claim 1 , wherein
the refiner gap includes an outer gap defined between the confronting outer bands and an inner gap defined between the confronting inner bands,
the refiner is a thermomechanical pulp refiner that generates steam in the outer gap between the working regions of the outer bands; and
at least one of the confronting plates includes a steam backflow channel for directing some of said steam from the outer gap to or upstream of the inner gap at the inner feed region while bypassing the inner gap at the inner working region.
3. The refiner of claim 2 , wherein the steam backflow channel includes a passageway through the plate beneath the inner working region of the plate.
4. The refiner of claim 2 , wherein one of the plates is mounted on a stator disc and the backflow channel includes a passageway through the stator disc.
5. The refiner of claim 2 , including an annular space between the inner band and the outer band.
6. The refiner of claim 5 , wherein the steam backflow channel has an inlet at the annular space between the inner and outer bands.
7. The refiner of claim 1 , wherein the inner band and the outer band are distinct members attached to a common refiner disc.
8. The refiner of claim 1 , wherein the inner band and the outer band are integrally formed on a common base.
9. The refiner of claim 1 , wherein
the radial width of the feed region of the inner band is greater than the radial width of the working region of the inner band, and
the radial width of the feed region in the outer band is less than the radial width of the working region of the outer band.
10. The refiner of claim 1 , wherein
the pattern of bars and grooves in the working region of the outer band has at least two zones, one of said zones contiguous with the feed region of the outer band and another of said zones contiguous with the outer circumference of said outer band; and
the pattern of bars and grooves in said one zone is less dense than the pattern of bars and grooves in said other zone.
11. The refiner of claim 10 , wherein the pattern of bars and grooves throughout the working region of the inner band has a uniform density.
12. The refiner of claim 1 , wherein the grooves throughout the working region of the inner band are narrower than the grooves throughout the feed region of the outer band.
13. The refiner of claim 1 , wherein
the relatively rotating discs comprise a rotor disc and an opposed stator;
the outer band of the rotor has curved feeding bars in the feeding region; and
the feeding region in the outer band on the stator has a substantially flat portion defining a recess for accommodating the curved feeding bars.
14. The refiner of claim 1 , wherein
the refiner gap includes an inner feed gap between the opposed inner feed regions, an inner working gap between the opposed inner working regions, an outer feed gap between the opposed outer feed regions, and an outer working gap between the opposed outer working regions, and
the inner working gap has a minimum immediately before a transition to a larger outer feed gap.
15. The refiner of claim 14 , wherein
the radial width of the feed region of the inner band is larger than the radial width of the working region of the inner band, and
the radial width of the feed region in the outer band is less than the radial width of the working region of the outer band.
16. In a disc refiner for lignocellulosic material, a pair of confronting, relatively rotating flat discs having respective refining plates, comprising:
a first plate having inner and outer radii R i , R o and a working face including a pattern of raised and relief structures and an intermediate transition portion having an inner radius R a and an outer radius R b ;
a second plate having inner and outer radii R i , R o and a working face including a pattern of raised and relief structures and an intermediate transition portion having an inner radius R a and an outer radius R b ;
said plates having a radially extending gap between their working faces;
whereby a radially dependent flow area A for the lignocellulosic material extends from the inner radius R i to the outer radius R o , such that
dA/dr< 0 from R i to R a
dA/dr> 0 from R a to R b
dA/dr< 0 from R b to R o
where R i <R a <R b <R o .
17. The disc refiner of claim 16 , wherein the diameter of the plates is at least about 36 inches, A is defined in part by a radially extending gap between the opposed plates, and the dA/dr at a radius immediately outside R a is defined by the combination of said gap and a transition from a relatively fine pattern of bars and grooves on each of the opposed plates at R a , to a coarse pattern of bars, grooves or recesses immediately outside R a .
18. In a primary wood chip refiner having flat, relatively rotating discs, each having a working plate thereon, the working plates being arranged in confronting coaxial relation thereby defining a refiner gap which extends substantially radially outward from the inner diameter of the discs to the outer diameter of the discs and defines a flow area for the chip material to be refined, the improvement comprising: each plate having a radially inner fiberizing band and a radially outer fibrillating band, each band having an inner feeding region and an outer working region, wherein the working region of the inner band is defined by a first pattern of alternating bars and grooves and the feeding region of the outer band is defined by a second pattern of alternating bars and grooves, and a steam backflow channel being formed as a groove on the surface of the working region of the inner band, oriented diagonally through the bars and grooves of said pattern of bars and grooves.