Selective reclamation of waste paper products

Abstract

Waste corrugated paperboard is reclaimed by a selective pulping process which separates the charge into two fractions, one containing the major portion of the neutral sulphite semi-chemical pulp and other short fibered constituents, and the other containing the major portion of the kraft pulp and a relatively small amount of short fibered pulp. The process includes a separating step following a pulping treatment controlled to a sufficiently short interval to defiber the short fibered constituents while leaving the long fibered constituents substantially undefibered. The process is applicable to other mixtures of waste paper sheets having different properties of defiberability.

Description

BACKGROUND OF THE INVENTION

The paperboard industry produces vast quantities of corrugated paperboard for use in packaging, especially for quantity packaging of food products for delivery to retail outlets, and also as cartons for a wide variety of products. Corrugated paperboard is commonly composed of a fluted layer having a layer of linerboard on one or both sides thereof, the double-sided board being almost universally used for the production of cartons.

The fluted layer of corrugated paperboard is usually composed of a sheet known as 9-point corrugating medium, which is made from relatively low grade and short-fibered pulp, the most commonly used materials being neutral sulphite semichemical (NSSC) pulp and short-fibered kraft pulp known as "bogus". Frequently one or both of these pulps are used together with some waste paper stock. The liners used on corrugated board, however, are commonly made of a good grade of kraft pulp, and on the average they constitute of the order of 70% by weight of conventional three-ply corrugated board.

Containers formed of corrugated board are usually used only once and then discarded, especially by the food industry. They therefore constitute a major source of waste paper available for reuse by a waste paper mill. When corrugated board is pulped in the usual way for use as waste paper stock, the high grade kraft constituents of the liners and the relatively lower grade fiber constituents of the corrugating medium are thoroughly mixed, and the resulting furnish is no longer suitable as liner stock. It is therefore commonly used to make lower grade products, such as boxboard, fillers for other types of paperboard, and in corrugating medium. Such downgrading of the high quality kraft fiber used in corrugated board liner is economically wasteful, but up to the present time, no practical procedure for reclaiming corrugated board has been proposed by which the kraft fiber could be separated from the lower grade fibers in the corrugating medium.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide an effective and economical process whereby used corrugated board can be separated into two fractions, one of which is composed so predominantly of kraft fiber that it is suitable for reuse as a substitute for virgin kraft pulp. The other fraction will contain the majority of the NSSC and other short fibered constituents of corrugating medium and will be suitable for reuse for such purposes as the production of additional corrugating medium as well as paper-board of various types.

The invention is based on the discovery that when corrugated board is initially subjected to a pulping action in water suspension, the sheet of corrugating medium formed of short fibered pulp has a higher degree of wettability than the layers of linerboard formed of long fibered pulp, and therefore the corrugating medium tends to defiber considerably more quickly than the kraft liners. The invention therefore proposes to subject used corrugated board to a carefully controlled pulping action which will be effective to defiber the corrugating medium to a range of relatively small particles sizes while leaving the liner in substantially larger pieces, and then to separate the two kinds of pulp by screening through screen perforations which will pass the small particles but reject the larger pieces of predominantly kraft sheet.

The invention is applicable to both continuous and batch operation. Continuous operation can be carried out in a pulper equiped with an extraction plate having relatively large extraction holes, e.g., of the order of 1.0 to 2.0 inches in diameter. If such a pulper is operated at a high throughput rate which will establish a short retention time for the corrugated board, namely of the order of one minute, the corrugating medium will be reduced to the desired small particle size range while the kraft liner is still in relatively large pieces. The extracted stock is then screened through screen perforations sized to pass only the relatively small defibered particles, e.g., 1/8 inch in diameter. The resulting two fractions can then be separately treated to prepare them for use.

It has been established by test that the fraction accepted by 1/8 inch screen perforations as described will contain the majority of the NSSC and other short fibered constituents of the corrugating medium but will also include substantially more kraft fiber than the original medium and is therefore suitable for reuse not only in the production of additional corrugating medium but for other purposes as well. The fraction which is rejected by the screen will consist so predominantly of long fibered kraft pulp that it can be used for many purposes normally requiring high quality kraft fiber, and it is particularly suitable for blending with virgin kraft for many uses.

Batch operation in accordance with the invention may be carried out by means of a pulper of similar construction except that its extraction plate is provided with relatively small perforations e.g., 1/8 inch in diameter. In this case, the pulper should be operated for only a brief interval, with extraction starting promptly and continuing until a substantial portion , e.g., one third, of the original charge has been removed. The fraction remaining in the pulper is then dumped separately, and both fractions can be treated as required to prepare the fiber therein for reuse.

While the invention was developed with the primary purpose of salvaging the maximum value from used corrugated board, its principles are applicable to selective fractionation of other mixtures of waste paper sheets having different properties of defiberability, such for example as the following:

1. Hard wood and soft wood paper sheets.

2. Separation of long fiber from short fiber in paper mixtures.

3. Separation of the components of high density papers from the components of low density papers.

4. Separation of wet strength fiber from non-wet strength fiber in paper mixtures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a system for practicing the invention as a continuous process; and

FIG. 2 is a similar view of a system for practicing the invention as a batch process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the pulper 10 is preferably of the construction shown in Felton et al U.S. Pat. No. 3,339,851 and comprises a rotor 11 rotating above an extraction plate 12 having perforations therethrough which are of substantial size, namely of the order of 1.0-2.0 inches in diameter. These perforations communicate with an outlet chamber 13 at the bottom of the pulper from which a discharge line 14 leads to the rest of the system. Since waste paper usually includes varying amounts of solid contaminants, the pulper 10 is preferably equipped with a junk remover 15 connected by an outlet 16 with the bottom of the pulper tub at a position radially outwardly of the extraction plate 12, and the pulper will usually also be provided with a ragger 17.

In the operation of the system, the pulper 10 is continuously supplied with used corrugated board as indicated at 20, and extraction through plate 12 is carried out continuously by the pump 21 at a sufficiently rapid rate to limit the retention time of the board in the pulper tub to a short interval sufficient to effect the desired initial disintegration of the board and defibering of the corrugating medium while leaving the kraft liners in as large pieces as can be extracted through the perforations in the plate 12. Tests indicate that under normal conditions, a retention time of the order of one minute is satisfactory for the purposes of the invention.

The slurry of defibered corrugating medium and pieces of kraft liner extracted from the tub is supplied by the pump 21 to a liquid cyclone 22 for removing heavy reject materials and then to a screening station 23 comprising one or more screens capable of separating the small particles of corrugating medium from the larger pieces of kraft sheet. Satisfactory results for this purpose have been obtained by the use of a vibrating screen wherein the screen plate has screening openings of about 1/8 inch in diameter. The stock transmitted by the openings in the screen or screens 23 is delivered to a series of processing stations indicated as a cleaning station 24, a dewatering station 25 and a baling station 26. The equipment installed at each of these stations may be of any type conventionally used for these purposes in the treatment of reclaimed waste paper pulp, and wherein the screening openings may be round holes, slots, or other shaped openings.

Test results indicated that the stock transmitted by the screening station 23 will constitute approximately one third by weight of the initial charge. The material too large for transmission by the screening station 23 is delivered to a second pulper 30 which may be of the same construction as the pulper 10, including a similar rotor 31 and extraction plate 32, except that the holes in the extraction plate 32 should be relatively small, e.g., 1/8 inch in diameter. The pulper 30 does not require a junk remover, but it is preferably equipped with an overflow device 33 for eliminating floating trash such as plastic sheets, tape and strings. The discharge from the device 33 is shown as delivered by a pump 34 to a screening and defibering station 35 from which accepted stock is recycled at 36 and returned to the pulper 30, and the reject material is discharged at 37 as trash. Satisfactory results have been obtained utilizing apparatus of the type disclosed in Herbert U.S. Pat. 3,698,649 at the station 35.

The stock extracted from the pulper 30 is delivered by a pump 40 to a series of cleaning, screening and other processing stations which may comprise any conventional equipment suitable for eliminating fines and high specific gravity contaminants from the stock and for otherwise improving its characteristics in conventional manner in preparation for its reuse. This part of the system is accordingly represented diagrammatically as a cleaning station 41, screening station 42, dewatering station 43 and baling station 44. Since the pulp fraction delivered to the baling station 26 includes the majority of the shorter fibers, it may be practical to increase the yield of that fraction by recycling rejected fines from the screening station 42 to the other side of the system, as indicated by the line 45.

As an example of the efficacy of the practice of the invention, in one test run, it was determined that each 100 tons of waste corrugated board included approximately 10 tons of light and heavy trash, 16 tons of NSSC fiber and 74 tons of kraft fiber, and it was further determined that the pulps included 60 tons of fibers long enough for retention on 35-mesh screen and 17 tons of fines capable of passage through 100-mesh screen. Treatment of this material in accordance with the process of the invention as described resulted in a yield of the following two fractions from each 100 tons of initial charge:

A. A mixture of 21 tons of kraft fiber and 9 tons of NSSC fiber, out of which total, 18 tons would be retained on 35-mesh screen and 7 tons would pass through 100-mesh screen.

B. A mixture of 48 tons of kraft fiber and 6 tons of NSSC fiber, out of which total, 44 tons would be retained on 35-mesh screen and 4 tons would pass through 100-mesh screen.

It will thus be seen that fraction A contained the majority of the NSSC fiber originally present in the charge but only a minor portion of the kraft fiber originally delivered to the pulper, and it will further be seen that fraction B contained by far the larger portion of long fiber pulp and only less than 10% fines. Fraction B is accordingly of sufficiently high grade for many uses for which repulped corrugated board would be unacceptable. For example, it can be mixed with other pulps, or substituted for some virgin kraft for products normally made entirely of virgin kraft, such as liner board. It can be used unmixed as the surface layer of liner board and other multi-layer products, and its quality is such as to warrant additional upgrading process steps such, for example, as digesting and bleaching.

Fraction A is also suitable for many purposes, having particularly in mind that it has a higher kraft content and a higher percentage of long fibered pulp than the original corrugating medium. It is therefore particularly well suited for reuse in corrugating medium, and it may have additional NSSC fiber mixed therewith for that purpose. If it is to be so used, minimal cleaning will be required because any particles of bark and shives which may be present are not objectionable in corrugating medium. For other uses, the extent to which it is upgraded by cleaning or other treatment depends upon the appearance desired for the finished product.

In the system for batch operation illustrated by FIG. 2, the pulper 50 includes a rotor 51, extraction plate 52, outlet chamber 53, discharge line 54 and junk remover 55 similar to the elements 11-15 in FIG. 1. but the perforations in the extraction plate 52 should be of relatively small size, e.g., 1/8 inch in diameter. In the operation of this system, the pulper 50 is initially charged with sufficient water to fill the pulper tub to the normal level for conventional pulping, and with the rotor operating, the batch of corrugated board is added as rapidly as pulping conditions permit. As soon as pulping commences, extraction is started to remove the particles of sufficiently small size to pass through the small holes in the extraction plate 52, and water is added to maintain an effectively constant level.

Extraction is continued as the balance of the batch is added together with sufficient water to maintain the liquid level in the pulper. When all of the charge has been delivered to the pulper, extraction continues until the level in the pulper has dropped as far as possible and approximately one-third of the charge has been extracted. During this interval, water may be added at a reduced rate to prevent clogging of the extraction plate, and operation of the rotor is also continued. The slurry extracted through the plate 52 is shown as delivered by discharge line 54 and pump 58 through a liquid cyclone 60 and shut-off valve 61 to a storage chest 62, and from there it is indicated as supplied at 63 to a process line which may be of the same characteristics shown in FIG. 1 commencing with the cleaning station 24.

After extraction of the short fibered stock from pulper 50 has been terminated, as by closing the valve 61, the balance of the undefibered material which was not extracted from the pulper 50 requires defibering in substantially the same manner as was described in connection with the pulper 30 in FIG. 1. This can be done in the pulper 50 after extraction of the short fibered portion of the batch, by refilling the pulper with water and continuing pulping operations until substantially all the sheet in the pulper has been defibered sufficiently for extraction through the extraction plate 52. In this case, the valve 61 should be a two-way valve having a connection 64 leading to a chest 65 from which the stock may be supplied at 66 to the appropriate process line, which may be of the same characteristics shown in FIG. 1 commencing with the station 41. Alternatively, the pulper 50 may have the balance of its contents dumped through a separate outlet 70 and valve 71 to a supply line 72 and pump 73 connected through a liquid cyclone 74 with the storage chest 65. In this case, the connection 66 from chest 65 should lead to a second defibering station corresponding with the pulper 30 in FIG. 1.

It appears to be a matter of choice whether to practice the invention on a batch or continuous basis. In either case, application of the practice of the invention as disclosed above results in effective separation of the more readily defiberable sheet constituents from the less easily defibered sheet, even when the mixture is as intimate as in corrugating board. The invention thus makes it possible and practical to effect selective fractionation of waste paper mixtures in accordance with the relative values of the major components of the mixture. The invention is accordingly of particular value for the selective reclamation of waste corrugated board because of the great quantities of that material available as waste paper, but it is also applicable to other mixtures of waste paper sheets which have distinctively different degrees of defiberability as are typified by the other mixtures listed hereinabove.

While the methods herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise methods, and that changes may be made therein without departing from the scope of the invention.

Claims

What is claimed is:

1. The process of selectively recovering two different fibrous constituents of waste paper products which have different properties of defiberability from a mixture of such products, comprising:

a. subjecting said mixed products to a primary pulping action in water suspension,

b. terminating said pulping action on said suspension as a whole as soon as the majority of the more readily defibered of said products is defibered sufficiently for passage through screening openings of a predetermined small size and while the majority of the less readily defibered of said products has not defibered sufficiently for passage through said holes,

c. selectively extracting said defibered product from said suspension by effecting passage thereof through screening holes of said predetermined size, and thereafter

d. subjecting the remaining said suspension to further defibering treatment to defiber the paper products remaining therein.

2. A continuous process as defined in claim 1 wherein said primary pulping action is carried out at a first station, said extracting step is carried out at a screening station separate from said first station, and further comprising the step of transporting said suspension from said first station to said screen station at a sufficiently rapid rate to terminate said pulping action at said first station as defined in claim 1.

3. A batch process as defined in claim 1 wherein said primary pulping action is carried out at a first station, and wherein said extracting step is carried out by effecting passage of said defibered product through screening openings of said predetermined size constituting outlet means from said first station.

4. A process as defined in claim 1 wherein said waste paper products consist essentially of corrugated paperboard comprising at least one outer layer of a substantially lower degree of defiberability than the corrugated layer thereof.

5. A process as defined in claim 1 comprising the further step of subjecting at least one of said defibered products to further treatment.