Chipping, Screening and Cleaning-f
Content
CHIPPING, SCREENING AND AND CLEANING
Debarked
logs intended for c refiner pulping need to be chipped inlD pieces of uniform size, and so do sdiII residuals 'K except those produced by a headrig). In the case of chemical wood would otherwise take too long ID impregnated with chemicals and hca digester, and its outside would be (i.e. the lignindissolving chemicals started to attack the cellulose and fibers) while its inside was still und was still a chunk of unseparated fibcnt case of refiner mechanical pulping.
Debarked
logs intended for c refiner pulping need to be chipped inlD pieces of uniform size, and so do sdiII residuals 'K except those produced by a headrig). In the case of chemical wood would otherwise take too long ID impregnated with chemicals and hca digester, and its outside would be (i.e. the lignindissolving chemicals started to attack the cellulose and fibers) while its inside was still und was still a chunk of unseparated fibcnt case of refiner mechanical pulping.
The
chip size used in practice is of magnitude roughly midway between extremes. most common common range sions for mill-cut chips chips is: The most length al between % and 11,4 inches;thickness. inch; and width, variable. This go to have developed for the cooking by acid sulfite liquors, in which penetration in the grain direction is times faster than in the cross-grain . This may be due uncertainty about all the factors include the moisture content and fissures in the chip), in part to the producing chips controlled in diInelDlii. than length, and in part to advantage of having but one size of pulping operations.
Disc Chippers The
means used to subdivide logs must be lpid and cheap - for very large quantities are .nvolved - and yet must produce a relatively .
Uniform-sized
product with minimum damage and debris (see Process Summary 8).
The
principle universally used today is to produce each piece with a single blow of a sharp steel knife - i.e. to chip the log. There are two basic types of chipping action which differ in the degree to which they can control chip dimenions.
The
first type exerts positive control only ver length. The knife blade, travelling in a :lfection inclined at 30 ° to 40° from the long zxis, strikes across this axis at a fixed advance 'om the last cut. This fixed advance :dines the length of the chip, i.e. in the grain : erection.
Chip thickness, though not directly controlled, can be manipulated to a certain extent.
Chip also splits at intervals along planes at right angles to the plane of the blade but perpendicular to its edge. The distance between these splits is the width of the chip. ahe width is normally greater than the thickness, but it is extremely variable.
•
•
The commonest equipment using this kind of action is the disc chipper. This consists basically of a rotating disc, 4 to 10 feet in diameter, on which some 4 to 20 knives are mounted radially. Several types of knife arrangement and mounting are used, of which the most common are : Norman design : in which the wear (face) plates of the disc
are made so that at any given radius, the slope of the wear plate is a constant from one knife to the next chip slot. Therefore the slope will be larger at a small radius than a large one. The chipper knives are ground so that the clearance
Process Summary 8 Operation
Chipping
Reduce stems, logs and log segments, wood product residuals and other pulpable wood waste into pieces of relatively uniform size called "chips" Equipment Description Size Flat disc with knives mounted radially Disc dia. 3-12 ft; 3options 15 knives; Disc 50-1000 HP drive Drum" made from two conical segments Dia, 2-10 ft; 6-20 V-drum knives; 50-1000 HP drive Drum Function
Gouge-shaped knives mounted on cylindrical drum
Dia. 2-4 ft; 12-50 knives; 50500 HP drive
Size class
Retained on screen
Overlength
+ 45 mm
Overthick
Chips from sawmill residuals
Chips from whole logs
2% 4%
Green veneer waste
1%
+ 8 mm (hwds), + 10 mm (sftwds)
5
7
0
Accepts
+ 7 mm
81
71
75
Pin chips
+ 3 mm
11
14
21
Fines
- 3 mm (i.e. caught in pan)
1
4
3
(a) Sketches of chipping action
(b) Assemblage
of chips, showing range of dimensions (Carthage).
(c) Chip, with "pin chip" that has broken off it. (Photo courtesy PAPRICAN).
Disc chipper
a) Feed side of 12-knife 8S-inch disc chipper Carthage Norman). The heavy belt-driven pulley '"1ounted behind the disc acts also as a flywheel. -he upper half of the casing has been removed and stands in the background.
(c) Discharge side of disc chipper fitted with cardbreaking pins, for gravity discharge (pulley removed). (Carthage) (d)
Discharge side of disc chipper fitted with vanes for blowing (top) discharge (pulley removed). (Carthage)
Knife arrangements used in disc chipper
(a) Contoured face (Norman design) (b) Slot (clamp)-mounted knife
(c) Face-mounted knife.
•
•
Slot mounted (tclamp-mounted', " " knives slide into the chipper
6.....:: angle of about 38 °, and can be' from above or below. This type C sharpened by removing material F ace-mounted knif e : the chipper are placed on the surface of the . disc, and held in place by bolts which pass through the disc (wear) plates on this style of cb:;:, flat and of constant thickness ",-nos .
V-Drum Chippers •
Another equipment configuration same type of action is the V-drum chi 108), formed by joining two truncaiec together at their small ends. The mounted on the cone surfaces V-shaped space. The log is fed endwise space at right angles to the axis of the chipper.
Drum Chippers Increased attention to the role thickness in pulping has led to an control this dimension also. One equipment designed to do so is a drum where the cutting element is a gouge, bottom and straight sides, all edged, gouges are mounted in rows on the surf drum, and pointed in the direction rotation.. The log is presented parallal drum axis. Each gouge strikes the log angles to the grain and removes a chip the length is the width of the gouge. thickness is the depth to which the strikes. Such equipment does not appear "Lrc general use, possibly because of required to sharpen its multiplicity of gouges and the lower capacity of drum •
Chipper Operation
Chipping is evidently a precision operation. in vhich the choice of chipper size, configuration nd adjustment for getting chips of a given size at given rate from a given raw material involves ne consideration of a good many variables Table 21).
Some of the commoner chipper applications are shown in Table 22. Typical settings of a drop-feed disc log chipper in conifer .nd hardwood pulpwood respectively are:
Wood Species Shear strength parallel to grain Wood specific gravity Tree growth rate Wood grain orientation Piece size Length Diameter Shape Dryness Shear strength parallel to grain increases with wood dryness
Season Frozen wood requires different chipper settings
Chipper
Chipper drive power Cutting speed Chip cut length target (knife projection) Spout configuration (designed to keep wood stable while being chipped) Knife grind angle (face-mounted knife) Knife clearance angle (slot-mounted knife) Knife-toanvil clearance
Table 22. Wood Chipper Applications Material
Chipper Type
Large cia. (12-24 in) logs long (10-30 ft) lengths
Multiple (4-15) knife disc 6- Drop or horizontal feed with 10ft dia. with Norman & bottom (gravity) discharge slot-mounted knife arrangement
Large cia. (4-12 in) logs long (4-20 ft) lengths
Multiple (4-12) knife disc 48 ft dia. with Norman. slot& face-mounted knife arrangement
Sawmill slabs, edge & end .nrn, & reject lumber
Multiple (4-16) knife disc 48 ft dia, with Norman, slotor face-mounted knives
Green & dry plywood venMultiple (6-8) knife disc 6-7 ft eer waste (without glue) dia. with face-mounted knives
Multiple (6-8) knife disc 6-7 ft eer, with face-mounted knives
Oversized chips rejected from chip screens
Hammerhog; small dia. (3-4 ft) multiple (4-6) knife disc chipper; small dia. drum or
Additional data
Configuration
A few mills have used Vdrum chippers. Some have installed rear & overhead (blowing) discharge Drop feed for mixed lengths Rather than try to & shorter material; precision-cut, some mills horizontal feed for reduce with knife or consistently longer logs hammer hogs Vertical drop feed with bottom discharge Horizontal feed with Good chip size bottom or overhead distributions hard to get discharge; OR. V-shaped because short, oddlength, infeed chute to keep wood variable-sized pieces hard to stable while being chipped keep stable Horizontal feed with feed Potentially the best chips, rollers & hold-downs to because thickness permit clean cuts without controlled. But higher pin shattering chips can result if feed system does not hold veneer securely Drop feed with overhead or Chip slicers normally used bottom discharge only on overthick chips rejected by disc thickness
Chipper types
(a) Disc chipper with horizontal feed for large roundwood.
(c) V-drum chipper (Soderhamn).
(b) Disc chipper with drop-feed residuals (top of casing removed)
Slicing rechipper (Rader) (f)
(g) Diagram of slicing rechipper (Rader)
CHIP SCREENING
Chips as made whether in the pulpmill or elsewhere, usually vary in their dimensions over a considerable range. Since a digester load of chips will be cooked under the conditions which will give the desired result for average-sized chips.
the wider this range of dimensions is, the higher the proportions of overcooked and undercooked chips will be. Moreover, in some types of pulping equipment, oversized and undersized chips cause mechanical problems.
Finally, the chips usually contain undesirable foreign matter eg. particles of bark. sand and dirt - a good proportion of which is substantially smaller than the desired chip size.
An operation which would narrow the size distribution by removing oversized and undersized chips along with some of the foreign matter is therefore technically attractive.
However, before embarking on such an operation. a mill should know the characteristics that its chip supply possesses. the economic benefits of improving these characteristics to specified targets (which largely depend on the pulping process and equipment which the mill uses, and the nature of its products), and the cost of doing so, Not all mills need chip screens.
In screening (see Process Summary 9), the chips are normally fed first to a screen surface with holes of a size which will retain the oversized and pass the acceptable and undersized, and then to subsequent screen surfaces with holes of a smaller size which will retain the acceptable and pass the undersized.
Fig..Chipping headrig in small-log sawmill (Chip-N-Saw). Top, the machine; bottom, its profi lr ripping patterns.
Fig. 1 Chip screens - flat and drum
ui u z ui a: a: :::: > u o o LL o >u z w :::: > aw a: LL PARTICLE SIZE
Fig. Chip size distribution before and after screening
(b) Diagram of gyratory
(a) Gyratory suspended flat screen (CarthageKone).
(c) Vibratory flat screen - fixed
Fig. Chip screens, disc
(a) Diagram of chip screening/slicing system, V-orientation (Rader)
(b) Chip screening/slicing system, V-orientation (Rader)
(d) Disc scalping screen (c) Disc thickness screen, flat orientation (Black Clawson)
(Rader)
Flat Inclined Gyratory Screen
There are several types of screening equipment which differ in the above regards (Fig. 114). The most widely used is the flat inclined gyratory screen. This has one to three screen decks, each 4 to 10 feet wide and 6 to 20 feet long, inclined at about 4 0, and base-mounted or suspended by cables from above. A gyratory motion in the horizontal plane is imparted to these decks by an eccentric drive. The commonest configuration is two screen decks, with openings of 11/2 to 2 inches in the upper deck and to Ys inches in the lower one. Sometimes the upper deck is made of a perforated plate, which does not upend and accept overlong chips as much as wire mesh, and a lower deck of wire mesh, which has a greater open area than a perforated plate.
Flat Inclined Vibratory Screen •
•
•
A flat inclined vibratory screen is the same, except that the agitation imparted to the screens is vibration in the vertical plane. Compare horizontal gyration, this vibration is violent, produces a greater variety of orientations, and is believed to be more eff 'IC' in disengaging undersized material.
Drum Screen
In
this type, the screen is in the form of a inclined and rotated, so that the tumbling a:-xa imparts a succession of orientations to the ; as it passes through. Drum diameter can be 4 to 8 feet, and drum length from 10 to The punched plate openings range in size 3/16 to V2 inch.
Disc Thickness Screen
The aim of this type of screen is to pass •. retain chips only on the basis of their srnJ::II: dimension, normally their thickness. Its element is a shaft 4 to 8 feet long on whicc llZ" mounted a series of discs spaced at a disul., equal to the desired maximum chip thickness Some 6 to 12 of these shafts are mountec :
The
chips are fed into the top of this array and agitated by rotating the disks, which are toothed.
This
agitation, combined with the sharpness of the slot edges, causes each chip to upend so one of its longer dimensions is oriented perpendicular to the slot opening and the other one lengthwise to it.
Process Summary 9 Operation
Function Equipment Options Flat gyratory with fixed mount Suspended
gyratory
Screening
Narrow the size distribution of the chip flow by removing largest and smallest 5 ze fractions. (After removal, overlength and overthick are reprocessed and recycle chips are metered back into flow to control variability or sent to separate pulpir ; system; fines are normally conveyed to wood waste fuel) Description Flat and slightly inclined screen with wire mesh or punched plate surfaces Same as Fixed Gyratory, but
Tumbling drum Disc (scalping) suspended by cables Disc (thickness)
Size Rectangular, 4-12 ft wide, 8-20 ft 0 Openings: overlength 11/2-3 in; fines :. 1/2 in Rectangular: dimensions same as Gyratory Circular: dia. up to 15 tt, pieSeqrne, screen plates Openings same as Fixed Gyratory
Slightly inclined rotating drum with punched plate or dia 4-12ft. , 8-24 ft wire mesh screen surfaces Discs arranged in staggered pattern on parallel shafts all
Quality parameter
rotating in same direction Same as Disc (scalping)
No. of shafts, 8-24; array 4-8 ft Wide Disc interface opening, 1-4 in
Same shaft no. and array width as C s: (scalping)
Screen removal efficiency: A function of feed composition, feed rate, screen surface area , screen openings, screen motion. Evaluated by measuring chip size distribut tr' before and afer screening. Removal efficiency of a size fraction = wt. of fraction before - wt. of fraction a"3" wt. of fraction before
Disc Scalping Screen
•
A more rugged version of the disc thickness screen can be used ahead of the screening system to protect it by removing rocks, metal, and wood chunks from purchased or outside-stored chips. In this case the slot width is larger, e.g. from 1 to 5 inches.
Processing Chip Screen Rejects
Overlong chips are usually reprocessed in a smaller version of the disc-type roundwood chipper, and overthick chips in a chip slicer, the product in both cases being returned to the main chip flow ahead of the screens.
The fines are sent to fuel, or else rescreened. Pin chips are metered back into the main chip flow, or sent to a fine particle pulping system.
CHIP CLEANING
If fragments of non-wood contaminants such as rocks, metal and plastics are allowed to accompany the flow of chips to the pulp mill, they will cause wear and damage to the processing equipment, particularly to valves, pumps, and refiners. They may also reduce product quality, the risk of their doing so usually becoming greater as they break up.
A large part of these contaminants (though never all) can be kept out of the chip flow in the first place by impressing supplier and mill staffs with the seriousness of these problems.
Of the contaminant fragments which nevertheless get in, the fragments which are above the size range of accepted chips are removed by the screening step, and so are most of the fragments below this range.
However, screening does not remove those contaminant fragments which are within the size range for accepted chips, nor those which are below this range but adhere to the accepted chips. Contaminant fragments of these two kinds can only be removed by taking advantage of other differences in physical properties between them and the chips.
Magnets A magnet will remove ferrous-metal objects from the chips. It must however be designed and positioned so that it can pull such objects up through the chip flow when the latter is moving at full capacity, in terms both of belt speed and depth of chips on the belt. •
Chip Washers
A chip washer will remove contaminants that sink in water, provided that enough dwell time is allowed for them to separate after chips and water have been mixed. The last materials to separate are fine grit and sand, because of their tendency to adhere to the rough surfaces of the chips, and their slower settling rates. Fine particles of wood and bark will also be removed by a chip washer. However, these will not settle out of the wash water, and must be filtered out. Hence to maintain high efficiency in the wash water recycling system, as much of the fines as possible should be removed by screening first.
Pneumatic Cleaners
A pneumatic cleaner separates chips from denser material by suspending the chip flow on a fluidized bed or in a vertical column of air. Its efficiency depends on the difference between the terminal velocity of the chips and that of the contaminant particles; the dwell time in suspension; and the air velocity and turbulence. Hence a pneumatic cleaner is most efficient on larger rocks and pieces of metal, and least efficient on sand and grit. In pneumatic systems. particularly those in areas with large seasonal changes in air temperature and humidity. Centrifugal air cleaners can be designed into pneumatic systems to remove fine particles of wood. sand and grit. Their removal efficiency depends on screen hole size. flow rate. proportion of fines. and chip moisture content.
Fig. Self-cleaning magnet suspended over a belt conveyor carrying chips (Eriez)
Contamination by Plastics Plastics constitute the one important form contamination for
which no removal technique is yet available.
Among the most cornmor; sources are sandwich and other food wrappers, the packages in which equipment parts are received, broken plastic parts, and plastic strapping.
Education is therefore the most effective tool for avoiding this product quality problem.
Pneumatic chip cleaning devices
(c) Fluidized-bed pneumatic separator (Triple-S Dynamics) (a) Pneumatic cyclone chip cleaner (Rader)
(b) Vertical column pneumatic separator (Rader ).
Cleaning Full-tree Chips
Full-tree chips present a special problem.
They contain a high percentage of foliage, dirt and bark, and much of this bark is still attached so firmly to the chip that it is not removed merely by screening and washing.
Without further treatment, therefore, full-tree chips can only be used in large proportion for pulps in which bark is no problem or for pulps made by processes which effectively decolorize bark fragments, e.g. fully-bleached kraft.
For many other pulps e.g. semi-bleached kraft for newsprint, unbleached sulphite, and refiner pulps - their use is limited.
Fig. Chip washing system (Defibrator). Inset: closeup of screw drainer.
Debarked
logs intended for c refiner pulping need to be chipped inlD pieces of uniform size, and so do sdiII residuals 'K except those produced by a headrig). In the case of chemical wood would otherwise take too long ID impregnated with chemicals and hca digester, and its outside would be (i.e. the lignindissolving chemicals started to attack the cellulose and fibers) while its inside was still und was still a chunk of unseparated fibcnt case of refiner mechanical pulping.
Debarked
logs intended for c refiner pulping need to be chipped inlD pieces of uniform size, and so do sdiII residuals 'K except those produced by a headrig). In the case of chemical wood would otherwise take too long ID impregnated with chemicals and hca digester, and its outside would be (i.e. the lignindissolving chemicals started to attack the cellulose and fibers) while its inside was still und was still a chunk of unseparated fibcnt case of refiner mechanical pulping.
The
chip size used in practice is of magnitude roughly midway between extremes. most common common range sions for mill-cut chips chips is: The most length al between % and 11,4 inches;thickness. inch; and width, variable. This go to have developed for the cooking by acid sulfite liquors, in which penetration in the grain direction is times faster than in the cross-grain . This may be due uncertainty about all the factors include the moisture content and fissures in the chip), in part to the producing chips controlled in diInelDlii. than length, and in part to advantage of having but one size of pulping operations.
Disc Chippers The
means used to subdivide logs must be lpid and cheap - for very large quantities are .nvolved - and yet must produce a relatively .
Uniform-sized
product with minimum damage and debris (see Process Summary 8).
The
principle universally used today is to produce each piece with a single blow of a sharp steel knife - i.e. to chip the log. There are two basic types of chipping action which differ in the degree to which they can control chip dimenions.
The
first type exerts positive control only ver length. The knife blade, travelling in a :lfection inclined at 30 ° to 40° from the long zxis, strikes across this axis at a fixed advance 'om the last cut. This fixed advance :dines the length of the chip, i.e. in the grain : erection.
Chip thickness, though not directly controlled, can be manipulated to a certain extent.
Chip also splits at intervals along planes at right angles to the plane of the blade but perpendicular to its edge. The distance between these splits is the width of the chip. ahe width is normally greater than the thickness, but it is extremely variable.
•
•
The commonest equipment using this kind of action is the disc chipper. This consists basically of a rotating disc, 4 to 10 feet in diameter, on which some 4 to 20 knives are mounted radially. Several types of knife arrangement and mounting are used, of which the most common are : Norman design : in which the wear (face) plates of the disc
are made so that at any given radius, the slope of the wear plate is a constant from one knife to the next chip slot. Therefore the slope will be larger at a small radius than a large one. The chipper knives are ground so that the clearance
Process Summary 8 Operation
Chipping
Reduce stems, logs and log segments, wood product residuals and other pulpable wood waste into pieces of relatively uniform size called "chips" Equipment Description Size Flat disc with knives mounted radially Disc dia. 3-12 ft; 3options 15 knives; Disc 50-1000 HP drive Drum" made from two conical segments Dia, 2-10 ft; 6-20 V-drum knives; 50-1000 HP drive Drum Function
Gouge-shaped knives mounted on cylindrical drum
Dia. 2-4 ft; 12-50 knives; 50500 HP drive
Size class
Retained on screen
Overlength
+ 45 mm
Overthick
Chips from sawmill residuals
Chips from whole logs
2% 4%
Green veneer waste
1%
+ 8 mm (hwds), + 10 mm (sftwds)
5
7
0
Accepts
+ 7 mm
81
71
75
Pin chips
+ 3 mm
11
14
21
Fines
- 3 mm (i.e. caught in pan)
1
4
3
(a) Sketches of chipping action
(b) Assemblage
of chips, showing range of dimensions (Carthage).
(c) Chip, with "pin chip" that has broken off it. (Photo courtesy PAPRICAN).
Disc chipper
a) Feed side of 12-knife 8S-inch disc chipper Carthage Norman). The heavy belt-driven pulley '"1ounted behind the disc acts also as a flywheel. -he upper half of the casing has been removed and stands in the background.
(c) Discharge side of disc chipper fitted with cardbreaking pins, for gravity discharge (pulley removed). (Carthage) (d)
Discharge side of disc chipper fitted with vanes for blowing (top) discharge (pulley removed). (Carthage)
Knife arrangements used in disc chipper
(a) Contoured face (Norman design) (b) Slot (clamp)-mounted knife
(c) Face-mounted knife.
•
•
Slot mounted (tclamp-mounted', " " knives slide into the chipper
6.....:: angle of about 38 °, and can be' from above or below. This type C sharpened by removing material F ace-mounted knif e : the chipper are placed on the surface of the . disc, and held in place by bolts which pass through the disc (wear) plates on this style of cb:;:, flat and of constant thickness ",-nos .
V-Drum Chippers •
Another equipment configuration same type of action is the V-drum chi 108), formed by joining two truncaiec together at their small ends. The mounted on the cone surfaces V-shaped space. The log is fed endwise space at right angles to the axis of the chipper.
Drum Chippers Increased attention to the role thickness in pulping has led to an control this dimension also. One equipment designed to do so is a drum where the cutting element is a gouge, bottom and straight sides, all edged, gouges are mounted in rows on the surf drum, and pointed in the direction rotation.. The log is presented parallal drum axis. Each gouge strikes the log angles to the grain and removes a chip the length is the width of the gouge. thickness is the depth to which the strikes. Such equipment does not appear "Lrc general use, possibly because of required to sharpen its multiplicity of gouges and the lower capacity of drum •
Chipper Operation
Chipping is evidently a precision operation. in vhich the choice of chipper size, configuration nd adjustment for getting chips of a given size at given rate from a given raw material involves ne consideration of a good many variables Table 21).
Some of the commoner chipper applications are shown in Table 22. Typical settings of a drop-feed disc log chipper in conifer .nd hardwood pulpwood respectively are:
Wood Species Shear strength parallel to grain Wood specific gravity Tree growth rate Wood grain orientation Piece size Length Diameter Shape Dryness Shear strength parallel to grain increases with wood dryness
Season Frozen wood requires different chipper settings
Chipper
Chipper drive power Cutting speed Chip cut length target (knife projection) Spout configuration (designed to keep wood stable while being chipped) Knife grind angle (face-mounted knife) Knife clearance angle (slot-mounted knife) Knife-toanvil clearance
Table 22. Wood Chipper Applications Material
Chipper Type
Large cia. (12-24 in) logs long (10-30 ft) lengths
Multiple (4-15) knife disc 6- Drop or horizontal feed with 10ft dia. with Norman & bottom (gravity) discharge slot-mounted knife arrangement
Large cia. (4-12 in) logs long (4-20 ft) lengths
Multiple (4-12) knife disc 48 ft dia. with Norman. slot& face-mounted knife arrangement
Sawmill slabs, edge & end .nrn, & reject lumber
Multiple (4-16) knife disc 48 ft dia, with Norman, slotor face-mounted knives
Green & dry plywood venMultiple (6-8) knife disc 6-7 ft eer waste (without glue) dia. with face-mounted knives
Multiple (6-8) knife disc 6-7 ft eer, with face-mounted knives
Oversized chips rejected from chip screens
Hammerhog; small dia. (3-4 ft) multiple (4-6) knife disc chipper; small dia. drum or
Additional data
Configuration
A few mills have used Vdrum chippers. Some have installed rear & overhead (blowing) discharge Drop feed for mixed lengths Rather than try to & shorter material; precision-cut, some mills horizontal feed for reduce with knife or consistently longer logs hammer hogs Vertical drop feed with bottom discharge Horizontal feed with Good chip size bottom or overhead distributions hard to get discharge; OR. V-shaped because short, oddlength, infeed chute to keep wood variable-sized pieces hard to stable while being chipped keep stable Horizontal feed with feed Potentially the best chips, rollers & hold-downs to because thickness permit clean cuts without controlled. But higher pin shattering chips can result if feed system does not hold veneer securely Drop feed with overhead or Chip slicers normally used bottom discharge only on overthick chips rejected by disc thickness
Chipper types
(a) Disc chipper with horizontal feed for large roundwood.
(c) V-drum chipper (Soderhamn).
(b) Disc chipper with drop-feed residuals (top of casing removed)
Slicing rechipper (Rader) (f)
(g) Diagram of slicing rechipper (Rader)
CHIP SCREENING
Chips as made whether in the pulpmill or elsewhere, usually vary in their dimensions over a considerable range. Since a digester load of chips will be cooked under the conditions which will give the desired result for average-sized chips.
the wider this range of dimensions is, the higher the proportions of overcooked and undercooked chips will be. Moreover, in some types of pulping equipment, oversized and undersized chips cause mechanical problems.
Finally, the chips usually contain undesirable foreign matter eg. particles of bark. sand and dirt - a good proportion of which is substantially smaller than the desired chip size.
An operation which would narrow the size distribution by removing oversized and undersized chips along with some of the foreign matter is therefore technically attractive.
However, before embarking on such an operation. a mill should know the characteristics that its chip supply possesses. the economic benefits of improving these characteristics to specified targets (which largely depend on the pulping process and equipment which the mill uses, and the nature of its products), and the cost of doing so, Not all mills need chip screens.
In screening (see Process Summary 9), the chips are normally fed first to a screen surface with holes of a size which will retain the oversized and pass the acceptable and undersized, and then to subsequent screen surfaces with holes of a smaller size which will retain the acceptable and pass the undersized.
Fig..Chipping headrig in small-log sawmill (Chip-N-Saw). Top, the machine; bottom, its profi lr ripping patterns.
Fig. 1 Chip screens - flat and drum
ui u z ui a: a: :::: > u o o LL o >u z w :::: > aw a: LL PARTICLE SIZE
Fig. Chip size distribution before and after screening
(b) Diagram of gyratory
(a) Gyratory suspended flat screen (CarthageKone).
(c) Vibratory flat screen - fixed
Fig. Chip screens, disc
(a) Diagram of chip screening/slicing system, V-orientation (Rader)
(b) Chip screening/slicing system, V-orientation (Rader)
(d) Disc scalping screen (c) Disc thickness screen, flat orientation (Black Clawson)
(Rader)
Flat Inclined Gyratory Screen
There are several types of screening equipment which differ in the above regards (Fig. 114). The most widely used is the flat inclined gyratory screen. This has one to three screen decks, each 4 to 10 feet wide and 6 to 20 feet long, inclined at about 4 0, and base-mounted or suspended by cables from above. A gyratory motion in the horizontal plane is imparted to these decks by an eccentric drive. The commonest configuration is two screen decks, with openings of 11/2 to 2 inches in the upper deck and to Ys inches in the lower one. Sometimes the upper deck is made of a perforated plate, which does not upend and accept overlong chips as much as wire mesh, and a lower deck of wire mesh, which has a greater open area than a perforated plate.
Flat Inclined Vibratory Screen •
•
•
A flat inclined vibratory screen is the same, except that the agitation imparted to the screens is vibration in the vertical plane. Compare horizontal gyration, this vibration is violent, produces a greater variety of orientations, and is believed to be more eff 'IC' in disengaging undersized material.
Drum Screen
In
this type, the screen is in the form of a inclined and rotated, so that the tumbling a:-xa imparts a succession of orientations to the ; as it passes through. Drum diameter can be 4 to 8 feet, and drum length from 10 to The punched plate openings range in size 3/16 to V2 inch.
Disc Thickness Screen
The aim of this type of screen is to pass •. retain chips only on the basis of their srnJ::II: dimension, normally their thickness. Its element is a shaft 4 to 8 feet long on whicc llZ" mounted a series of discs spaced at a disul., equal to the desired maximum chip thickness Some 6 to 12 of these shafts are mountec :
The
chips are fed into the top of this array and agitated by rotating the disks, which are toothed.
This
agitation, combined with the sharpness of the slot edges, causes each chip to upend so one of its longer dimensions is oriented perpendicular to the slot opening and the other one lengthwise to it.
Process Summary 9 Operation
Function Equipment Options Flat gyratory with fixed mount Suspended
gyratory
Screening
Narrow the size distribution of the chip flow by removing largest and smallest 5 ze fractions. (After removal, overlength and overthick are reprocessed and recycle chips are metered back into flow to control variability or sent to separate pulpir ; system; fines are normally conveyed to wood waste fuel) Description Flat and slightly inclined screen with wire mesh or punched plate surfaces Same as Fixed Gyratory, but
Tumbling drum Disc (scalping) suspended by cables Disc (thickness)
Size Rectangular, 4-12 ft wide, 8-20 ft 0 Openings: overlength 11/2-3 in; fines :. 1/2 in Rectangular: dimensions same as Gyratory Circular: dia. up to 15 tt, pieSeqrne, screen plates Openings same as Fixed Gyratory
Slightly inclined rotating drum with punched plate or dia 4-12ft. , 8-24 ft wire mesh screen surfaces Discs arranged in staggered pattern on parallel shafts all
Quality parameter
rotating in same direction Same as Disc (scalping)
No. of shafts, 8-24; array 4-8 ft Wide Disc interface opening, 1-4 in
Same shaft no. and array width as C s: (scalping)
Screen removal efficiency: A function of feed composition, feed rate, screen surface area , screen openings, screen motion. Evaluated by measuring chip size distribut tr' before and afer screening. Removal efficiency of a size fraction = wt. of fraction before - wt. of fraction a"3" wt. of fraction before
Disc Scalping Screen
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A more rugged version of the disc thickness screen can be used ahead of the screening system to protect it by removing rocks, metal, and wood chunks from purchased or outside-stored chips. In this case the slot width is larger, e.g. from 1 to 5 inches.
Processing Chip Screen Rejects
Overlong chips are usually reprocessed in a smaller version of the disc-type roundwood chipper, and overthick chips in a chip slicer, the product in both cases being returned to the main chip flow ahead of the screens.
The fines are sent to fuel, or else rescreened. Pin chips are metered back into the main chip flow, or sent to a fine particle pulping system.
CHIP CLEANING
If fragments of non-wood contaminants such as rocks, metal and plastics are allowed to accompany the flow of chips to the pulp mill, they will cause wear and damage to the processing equipment, particularly to valves, pumps, and refiners. They may also reduce product quality, the risk of their doing so usually becoming greater as they break up.
A large part of these contaminants (though never all) can be kept out of the chip flow in the first place by impressing supplier and mill staffs with the seriousness of these problems.
Of the contaminant fragments which nevertheless get in, the fragments which are above the size range of accepted chips are removed by the screening step, and so are most of the fragments below this range.
However, screening does not remove those contaminant fragments which are within the size range for accepted chips, nor those which are below this range but adhere to the accepted chips. Contaminant fragments of these two kinds can only be removed by taking advantage of other differences in physical properties between them and the chips.
Magnets A magnet will remove ferrous-metal objects from the chips. It must however be designed and positioned so that it can pull such objects up through the chip flow when the latter is moving at full capacity, in terms both of belt speed and depth of chips on the belt. •
Chip Washers
A chip washer will remove contaminants that sink in water, provided that enough dwell time is allowed for them to separate after chips and water have been mixed. The last materials to separate are fine grit and sand, because of their tendency to adhere to the rough surfaces of the chips, and their slower settling rates. Fine particles of wood and bark will also be removed by a chip washer. However, these will not settle out of the wash water, and must be filtered out. Hence to maintain high efficiency in the wash water recycling system, as much of the fines as possible should be removed by screening first.
Pneumatic Cleaners
A pneumatic cleaner separates chips from denser material by suspending the chip flow on a fluidized bed or in a vertical column of air. Its efficiency depends on the difference between the terminal velocity of the chips and that of the contaminant particles; the dwell time in suspension; and the air velocity and turbulence. Hence a pneumatic cleaner is most efficient on larger rocks and pieces of metal, and least efficient on sand and grit. In pneumatic systems. particularly those in areas with large seasonal changes in air temperature and humidity. Centrifugal air cleaners can be designed into pneumatic systems to remove fine particles of wood. sand and grit. Their removal efficiency depends on screen hole size. flow rate. proportion of fines. and chip moisture content.
Fig. Self-cleaning magnet suspended over a belt conveyor carrying chips (Eriez)
Contamination by Plastics Plastics constitute the one important form contamination for
which no removal technique is yet available.
Among the most cornmor; sources are sandwich and other food wrappers, the packages in which equipment parts are received, broken plastic parts, and plastic strapping.
Education is therefore the most effective tool for avoiding this product quality problem.
Pneumatic chip cleaning devices
(c) Fluidized-bed pneumatic separator (Triple-S Dynamics) (a) Pneumatic cyclone chip cleaner (Rader)
(b) Vertical column pneumatic separator (Rader ).
Cleaning Full-tree Chips
Full-tree chips present a special problem.
They contain a high percentage of foliage, dirt and bark, and much of this bark is still attached so firmly to the chip that it is not removed merely by screening and washing.
Without further treatment, therefore, full-tree chips can only be used in large proportion for pulps in which bark is no problem or for pulps made by processes which effectively decolorize bark fragments, e.g. fully-bleached kraft.
For many other pulps e.g. semi-bleached kraft for newsprint, unbleached sulphite, and refiner pulps - their use is limited.
Fig. Chip washing system (Defibrator). Inset: closeup of screw drainer.
