US3337139A - Treatment of hardwood chips for bark and wood separation - Google Patents

Description

g. 2 1967 w. D. LLOYD ETAL TREATMENT OF HARDWOOD CHIPS FOR BARK AND WOOD SEPARATION Filed Jan. 15, 1965 H S T. S C M E C E E R R 4 E w w E --IY FIG. I

TO CHIP BIN $0. SCREENS United States Patent Ofiice 3,337,139 Patented Aug. 22, 1967 3,337,139 TREATMENT OF HARDWOOD CHIPS FOR BARK AND WOOD SEPARATION William D. Lloyd, Kapuskasing, Ontario, Canada, and

Robert E. Lee, Neenah, Wis., assignors to Kimberly- Clark Corporation, Neenah, Wis., a corporation of Delaware Filed Jan. 13, 1965, Ser. No. 425,323 6 Claims. (Cl. 24-120) ABSTRACT OF THE DISCLOSURE The separation of the outer bark and inner bark of hardwood chips from white wood by sequential screening and sink-float procedures. Two successive sink-float procedures on media of increasing density improves the inner bark and white wood separation. Compression wood, tension wood, knots, heartwood and the like are removable with the inner bark.

This invention relates to improvements in the preparation of chips from hardwoods for making wood pulp. The improvements specifically relate to freeing a mass of wood chips from bark, so that hardwood pulpwood or sawmill residues can be chipped with the bark on and the bark subsequently removed by the process described below. Our process permits us to bypass the more expensive and cumbersome drum barking or other mechanical debarking systems customarily employed in the pulp industry for debarking pulpwood prior to the chipping step.

A known method for the separation of bark from wood chips is described in Patent 3,070,318 which utilizes the nip action of a series of spaced rolls to remove bark from the chips. We have found that, for hardwood species particularly, this process is only partly effective. We find a sharply diminishing effectiveness of bark removal by successive crushing stages. The final bark content is too high for producing quality pulp; the successive crushing stages grind or force bark particles into the wood chips; and the repeated crushing exerts too much damage to the chips, which results in excessive yield losses in the subsequent chip handling and pulping steps as well as strength losses in the pulp product. We, therefore, have investigated other means of providing for more complete bark removal while still retaining acceptable chip characteristics. The purpose of this invention is to provide for more complete bark removal and, additionally, to permit simultaneous removal of objectionable stained and abnormal wood if such are present in the bark-chip mixture.

We have found that if a bark-chip mixture is first subjected to crushing action followed by screening, then the resultant chips and contaminating inner bark may be separated by a simple sink-float step. Our invention relates to the utilization of the differing structure and physical properties of inner and outer bark. The outer bark of hardwoods in nature is composed of dead, corky material with entrained air. By itself, this material will float in an aqueous solution. The inner bark, on the other hand, is composed of living, resilient, conductive tissue which normally contains high moisture content. This material, by itself, will rapidly absorb additional moisture and sink in an aqueous solution. The crushing action mentioned cracks and crumbles the outer bark free of the inner bark thereby permitting removal of outer bark by oscillating screens. The agitation action of the screens also aids in bark removal as the chips contact with each other on the screens. The inner bark is concentrated by the screening action with a small quantity of chips for more eflicient removal in the previously mentioned flotation system.

We have further found that, while a sink-float step in which water is the liquid medium is suitable, in some instances the white wood yield tends to be reduced because of excessive usable white wood in the sink fraction; such difficulty may be overcome by a second sink-float procedure in a medium of greater density which causes white wood chips to float and the inner bark to remain in the sunken condition. Most white wood will float in an aqueous medium of specific gravity of l.081.l.

The high sinkage in aqueous media of the inner bark of hardwoods is attributed to its capacity for water takeup. This is apparently due to the anatomical differences between the inner and outer bark of hardwoods. The inner bark of coniferous woods, by contrast, is reported to float in an aqueous medium of a density of about 1.08.

A particular allied benefit of our procedure is that wood containing defects, abnormalities due to growth conditions, and stained heartwood of these hardwoods may be eliminated from the white wood by the sink-float procedure of this invention. The defects or abnormalities may include, for example, compression wood, tension wood, knots, and the like. Such woods have a higher avea-ge density than normal wood and sink in an aqueous medium. Stained heartwood similarly has a higher average density than norm-a1 wood because such wood contains undesirable dark colored mineral depositions, gums and resins. Such wood, when pulped, contributes to low pulp brightness and increased bleaching costs. Therefore, it is desirably removed where high brightness is an important consideration.

A further finding has been that in our procedures appropriate preliminary treatment of the chips may make it unnecessary to subject all of the wood undergoing processing to the sink-float step. This preliminary treatment may involve a suitable screening so that substantially all or a major portion of the inner bark will pass with a small fraction of the smaller dimensioned white wood. Such screening leaves the chips of larger size essentially free of the bark and avoids the necessity of sink-floating all of the chips.

The invention will be more fully understood by reference to the following detailed description and accompanying drawings wherein:

FIG. 1 illustrates schematically an equipment arrangement for the practice of the process of the invention; and

FIG. 2 illustrates an optimum situation with respect to inner and outer bark removal from a chip.

As in the. prior art, Patent 3,070,318, best results are attained when the rough chips have a moisture content of about 45-50% by weight. This is a normal moisture con tent for fresh cut hardwoods. Also, as in the prior art, the removal of bark by roll action is improved by tempering the chips by treatment in hot water, if the barktends to be dry.

Referring now to the drawing, the numeral 1 designates a hopper with rotary chip feeder for feeding unbarked chips to an inclined screen system 2. The system 2 includes a series of oscillating screens of decreasing mesh indicated at 3, 4 and 5. The screen 3 is effective to remove oversize chips of about 1 /2" and above and which chips may be returned to the chipper device (not shown). The screen 4 is effective to retain chips and bark of about /3" size and larger. The screen 5 is effective to retain chips and bark of about A1 size and larger but material below about A" and fines are screened out.

Two sets of rolls are positioned below the screen system 2. These sets are in parallel and are designated generally at 7 and 8. Set 7 receives material retained on in- 3 clined screen 4 and set 8 receives material retained on inclined screen 5.

Screen 4 through chute l communicates with nip 11 formed by rolls 12, 13 of set 7. A scraper 14 bears against roll 12 and a similar scraper 15 bears against roll 13. The nip 11 clearance between rolls 12, 13 for the noted maximum chip and bark size of f/s" is suitably between 0.010" and 0.020". The rolls include one smooth and one knurled steel press roll. Rolls of the nature Set forth in Patent 3,070,318 are effective for the purpose.

Similarly, screen through chute 6 communicates with nip 17 formed by similar rolls 18, 19 or set 8; scrapers 20, 21 respectively bear against rolls 18, 19. Nip 17 for the maximum chip and bark size of A" preferably has a spacing of 0.0050.010".

The nips 11 and 17 communicate with a common chute 23 through which chips crushed by the rolls to effect bark removal are directed to a screen system generally designated at 24.

The rolls rotate in the direction indicated by the arrows and function to compress the chips breaking the inner and outer bark layers free as well as cracking the bark loose from the white wood at the cambium. This effect is shown in FIG. 2.

As illustrated in FIG. 2 the chip includes white wood 55, outer bark S6, inner bark 57, and the cambium is designated at 58. Such a chip, when passed to the rolls designated at 18', 19', is compressed to some extent. Ideally, the eflect of such crushing is to separate the chip into the white wood designated at 55', the inner bark designated at 57 and to crush the outer bark into particles designated at 56'. In practice the outer bark to a large extent does crumble and in the moist condition will adhere to the rolls as at 18, 19 in FIG. 1; some inner bark and some small amount of white wood may also be retained by the rolls. However, some outer bark and a detrimental amount of inner bark pass with the white wood to the chute 23. The inner bark may be adhered to some extent to the outer bark and also to the white wood. For this reason some agitation is necessary in the step following the roll pressing to further crumble and eliminate the outer bark, to further remove any inner bark from the outer bark, and to minimize any bark remaining on the chips. We have found that oscillating screens suitably provide the necessary agitation.

The scrapers 14-, 15, 20 and 21 function to remove material adhering to the rolls and to divert it to guides 26, 27 and 28. Guide 28 functions as a common guide for material from scrapers I5, 20 and slopes rearwardly from the chute 23 and the plane of the drawing.

In the specific embodiment illustrated, oscillating screen 30 of screen system 2d functions to retain chips of /3 and larger. Such chips usually are substantially completely free of bark and may be directed to a chip bin. In some instances the chips may include abnormal wood, stained heartwood and the like which it is desirable to remove from the usable white wood. In this event the material retained by screen 30 may be directed over chute 31 (shown in dotted lines) to sink-float equipment described hereinafter. The inner bark and crumbled outer bark will commonly pass screen 30 to screen 32.

Oscillating screen 32 functions to eliminate fragments of less than A", crumbled outer bark, fines and the like whether of white wood or bark. Inner bark, together with some white wood, is retained by the screen 32; such material, in the specific embodiment described, has a size of about A" and more up to /8" and is fed from screen 32 to sink-float equipment generally designated at 33.

Rejects of screen 32. pass to guide 29*.

The sink-float equipment comprises a tank 35 having a sloping bottom 36, chip removal equipment of conventional sweep type 37, and a launder 38. An inlet 39 provides for introduction of chips through head tank and pump 40' to a suitable separation medium, for example, water. A bottom sweep system and outlet conduit 41 hav- -3 ing a valve 42 provides for removal of rejects together with medium A. The water requirement for medium A is small since the liquid is collected and recycled by means of head tank and pump 43. The head tank is shown at 34.

The chips of hardwood, and which are white wood, are of a density, at 50% moisture, such that they float in water. The inner bark, however, sinks almost immediately.

The equipment may be operated on a batch basis by closing off valve 42 and suitably closing a slide trap 44 in hopper l. The equipment, however, is preferably operated on a continuous basis with medium A inflow through headtank and pump 40, an outflow through valve 4-2, of water and the sink portion and removal of the float portion by the sweep.

The float portion, if all material is subjected to sinkfloat, is passed to storage for pulping; if only a portion of the material is passed to sink-float, then the dry accepts are combined with the float portion for pulping.

The sink portion in many instances is profitably subjected to a second heavy density media sink-float operation to refloat usable white wood. Occasionally some chips are cut at an excessive angle; this influences the amount of water takeup by the chips. Too much end grain surface exposure promotes water absorbing and good white wood may sink in excessive quantity such that a secondary reclaim system is needed. For this purpose the outfiow through valve 42 and conduit 41 is directed to an inclined screen 45. Screen 45 is effective to dewater the sink solids which pass to the secondary sink-float tank 46. This equipment setup is like tank 35 but usually smaller and includes a sloping bottom 47, chip sweep removal equipment 48, a launder 49 and an outlet 50. Valve 51 is provided in conduit 50; a headbox and pump system 52 similar to the pump and headbox 43, 43a is provided to recycle heavy density media back to the tank; a simple screen system 53 is provided to dewater the reject material. In this instance, however, the separation medium is of greater density, for example, waste neutral sulfite liquor of a density of about l.081.l. Accordingly, some of the material which had sunk in the first sink-float step will now float. A degree of selectivity in reclaiming usable chips can be exercised by adjusting the density of the media; thus, only the desirable white wood can be reclaimed, whereas the inner bark, stained wood and other undesirable chips remain in the sunken condition.

The extent to which the second sink-float step is practiced is dependent upon the nature of the hardwood to some degree and on the quality of the chips required for a specific pulping procedure. We have found that in many instances the second sink-float step is unnecessary. In others, however, efficiency of wood recovery will be improved by the second step without material detriment to the final pulp.

The material which sinks in the second float-sink step is commonly rejected for pulp purposes. This is because the medium A and medium B for commercial purposes are selected to have a significantly different density within the confines of this process. While additional sink-float steps at graduated densities might be employed, such tends to be expensive and is economically unnecessary.

The procedure is attractive commercially because it may be practiced with little loss in White wood while removing substantially all of the inner bark and while eliminating stained heartwood and abnormal wood. For the purpose of this invention these categories may be classed as contaminants of the chips. We have found that in many instances the white wood loss may be as little as 3 to 5% by weight of the white wood available to the system; by weight in many such instances the sinkfloat procedure has been effective to remove inner bark by weight to the extent of five times or more that of the white wood loss on the overall process; also, six or seven times the white wood loss overall of abnormal wood has been eliminated. Thus, in most cases, the weight of the contaminants in the sink portion far exceeds the white wood loss overall and the white wood in the sink portion. This is particularly true of hardwoods such as aspen, red maple, silver maple, white birch, yellow birch and the like.

Hard maple, we have found, tends to have a greater quantity of wood lost, particularly in the sink-float step with water as the medium. Accordingly, with such woods a second sink-float with a medium of greater density is desirable. Spent neutral sulfite liquor in the second sinkfioat and at a density of about 1.08 will materially reduce the white wood in the sink portion without affecting the other contaminants.

The procedure may also be affected to some extent by variations in the natural moisture content of the wood. A wood having greater than normal moisture content may tend to sink. In this event the use of a medium of a greater density as medium A or the use of the two step sink-float with liquors of varying specific gravity such as 1.04 for medium A and 1.08 for medium B is effective. In practically all instances the contaminants in aqueous media will remain submerged at densities at which the white wood will float.

Outer bark which is not screened out may remain with the chips even through the float-sink process. Such is usually minor in amount and will ordinarily be removed in the pulping processes in conventional cleaning steps. For example, outer bark which has not been appropriately screened out and has been entrained with the chips is usually in the form of independent grains or specks and they are readily removed from an ultimate pulp in usual centri-cleaning operations. In general, however, we consider that if less than about 2 to 2.5% by weight of outer bark is carried with the accepted chips, the product is useful for pulping by kraft or semichemical processes. For sulfite processes less than 1% residual bark is desirable and for this purpose, and depending upon the nature of the wood, only some or all of the chips are subjected to sink-float.

It will be understood that this invention is susceptible to modification in order to adapt to diflerent usages and conditions and, accordingly, it is desired to comprehend such modifications with the invention as may fall within the scope of the appended claims.

What is claimed is:

1. A process in which chips of hardwood are crushed to debark the chips and to provide an admixture of outer bark, inner bark and white wood chips, the improvement comprising the steps of agitating the admixture and screening out outer bark, and then separating white wood chips from inner bark by a sink-float process step in an aqueous medium in which white wood chips float and inner bark sinks.

2. The process as claimed in claim 1 and in which the sink portion of the sink-float process step including the inner bark is subjected to a second sink-float process step in an aqueous medium of greater density in which the inner bark sinks and white wood chips present with the inner bark of the sink portion of the first float-sink process step float.

3. A process in which chips of hardwood are crushed to debark the chips and to provide an admixture of outer bark, inner bark and white wood chips, the improvement comprising the steps of agitating and simultaneously screening the admixture in successive steps to screen out finely divided outer bark and to divide large white wood chips from both smaller white wood chips and inner bark, and then separating said smaller white wood chips from inner bark by a sink-float process step in an aqueous medium in which the White wood chips float and the inner bark sinks.

4. The process as claimed in claim 3 and in which the white wood chips which float are recombined with the larger white wood chips.

5. A process in which chips of hardwood are crushed to debark the chips and to provide an admixture of outer bark, inner bark and white wood chips, the improvement comprising the steps of agitating and simultaneously size screening the admixture in successive steps to screen out outer bark and to divide large white wood chips from both smaller white wood chips and inner bark, and recombining the larger white wood chips with the smaller white wood chips and inner bark in a sink-float process in an aqueous medium in which the white wood chips float and the inner bark sinks.

6. A process in which chips of hardwood including as contaminants chips of stained heartwood, compression wood, tension wood and the like are crushed to debark the chips and provide an admixture of outer bark, inner bark, white wood chips and contaminant chips, the improvement comprising the steps of agitating the admixture and screening out outer bark, then separating white wood chips from inner bark by a sink-float process step in an aqueous medium in which white wood chips float and inner bark and contaminants sink.

References Cited UNITED STATES PATENTS 2,764,289 9/1956 Scheid 209173 2,874,908 2/ 1959 Heritage 241-24 3,042,198 7/1962 Slavich 20944 3,254,847 6/1966 Brandts 241-24 WILLIAM W. DYER, IR., Primary Examiner.

HARRY F. PEPPER, JR., Examiner.

Claims (1)

1. A PROCESS IN WHICH CHIPS OF HARDWOOD ARE CRUSHED TO DEBARK THE CLIPS AND TO PROVIDE AN AMIXTURE OF OUTER BARK, INNER BARK WHITE WOOD CHIPS, THE IMPROVEMENT COMPRISING THE STEPS OF AGITATING THE ADMIXTURE AND SCREENING OUT OUTER BARK, AND THEN SEPARATING WHITE WOOD CHIPS FROM INNER BARK BY A SINK-FLOAT PROCESS STEP IN AN AQUEOUS MEDIUM IN WHICH WHITE WOOD CHIPS FLOAT AND INNER BARK SINKS.

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