CA2023904C - Deinking and bleaching secondary fiber - Google Patents

Abstract

The brightness and bleachability of recycled secondary fiber is enhanced by subjecting the pulped fiber to washing in aqueous surfactant before and/or after treatment with oxygen-containing gas dispersed in an aqueous alkaline medium.

Description

2~23~04 DEINKING AND BLEACHING SECONDARY FIBER

TECHNICAL FIELD
The present invention relates to the deinking and bleaching of secondary cellulosic fibers and is particularly concerned with improvements in the methods for treatment of such fibers whereby desired brightness levels are attained with zero to minimal production of harmful chlorinated organics.

BACKGROUND OF THE INVENTION
In existing conventional plants for deinking and bleaching of secondary fibers, such as salvaged old papers and other reused or recycled fiber-con-taining materials, these materials are collected and segregated in piles according to type (ledger, newspaper, cardboard, etc.). The fiber materials from these piles are blended in a hydropulper, pulped and cleaned, followed when required by a deinking step carried out by water washing or by flota-tion. The thus treated pulp is then directed to the bleach plant where it is subjected to a series of delignifying/bleaching and extraction steps, 15 each stage involving distinctively different treating chemicals and process conditions utilized. The initial pulping and washing is to eliminate ink present in the fiber (deink) and the subsequent bleaching process serves to whiten the pulp and to get rid of any residual ink and/or lignin that may have been left in the initial fiber stock. The effectiveness of the pulp 20 bleaching is measured and designated by parameters of brightness and vis-cos~ty.
For the deinking and bleaching of recycled waste paper and other secondary fiber materials, the most common practices employ chlorine-based chemicals (generally for fine papers) or hydrosulfite (generally for news-25 paper stock). Various treating sequences are utilized in the bleaching ofthe recycled pulp to attain desired brightness levels, typically above 76 brightness units (ISO) and commonly in the range of 78-82. Over 80X of the the fine paper (ledger) bleaching is done by either a CEH type sequence or a ,~

~23~4 CH type. As commonly employed in the art these letter designations respec-tively stand for:

C = Chlorination with chlorine (C12) E = Alkali extraction with NaOH
H = Alkaline hypochlorite (NaOCl) Concern over the negative impact on the environment of chlorine-based bleach plant effluents has accelerated in recent years, particularly since the discovery of the highly toxic chlorinated dioxins and furans in some bleach plant effluents, sludge, and pulp products. Today it is generally accepted that it is critical to reduce the amount of chloro-organics in the pulp and in the plant effluent.
Since formation of chlorinated organics is strongly related to the use and consumption level of molecular chlorine in the chlorination and hypo-chlorite stages of the conventional bleaching sequence, it is of greater importance to minimize formation of chloro-organics through a cost-effec-tive means to reduce the amount of chlorine utilized in the chlorination and hypochlorite stages, rather than rely on post-treatment technologies such as advanced wastewater treatment systems. Unl~ke the Kraft Paper Industry (virgin wood processing), the recylced paper industry is ~ust starting to deal with the issue of chlor~nated tox~ns including the ini-tiation of pro~ects involving the reduction of chlorine and hypochlorite.

PRIOR ART
All of the more common treating sequences employed or proposed for - bleaching and delignifying of kraft pulp as well as those concerned with bleaching of secondary fibers are chlorine based. While some experimental research has been directed to the use of molecular oxygen in processes for bleaching of secondary fiber (Markham, L. D., et al., TAPPI 1988 PULPING
CONFERENCE PROCEEDINGS, pp 189-196), these attempts have failed to produce a pulp having acceptable high brightness without also using comparatively large amounts of chlorinated chemicals in the bleaching sequence.

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Among the objects of the present invention are:
A. To avoid or minimize the production of harmful chlorinated products in the deinking and bleaching of secondary fiber materials.
B. To enable deinking and bleaching of secondary celluosic fiber to target brightness at least as high or even higher than that heretofore attainable, without necessitating resort to the extensive use of chlorinated chemicals.
C. To provide a process for deinking and oxygen bleaching of secondary fiber wherein redeposit of residual ink and/or other contaminants during washing is avoided.
D. To enable effective bleaching of secondary fiber to desired brightness notwithstanding high amounts of residual lignin present in the initial pulp.

SUMMARY OF THE INVENTION
In accordance with the present invention pulped secondary fiber, after deinking including conventional washing and cleaning, is treated with an oxygen-containing gas in the presence of added alkali, and by washing in an aqueous medium containing dissolved surfactant, whlch may be followed if desired by further treatment in a final bleach stage. It has been found that such proposed presence of surfactant during the wash prevents redeposition of residual ink and other contaminants on the fiber. Moreover, the use of the surfactant wash was found to enhance the bleachability of the pulp in a subsequent or final chemical bleaching stage.
In accordance with a preferred embodiment of the invention, surfactant-containtng alkaline liquid washed out from the pulp is recycled to a selected stage of the deinking sequence, with consequent important savings in operation and chemicals costs.

BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more clearly understood and its several advan-tages appreciated from the description which follows read in connection with the accompanying drawings, wherein:

~239~4 -Figure 1 is a process diagram depicting a system for the oxygen stage and the surfactant wash applied to the secondary fiber pulp in practice of an embodiment of the invention.
Figure 2 is a graph showing the effect of surfactant wash on attained br~ghtness under conditions corresponding to Example 1 of the description.
Figure 3 is a series of bar graphs showing the effect of process variations on attained pulp brightness at various stages of the bleaching sequence.
Figure 4 is a graph showing the effect of sodium hydroxide dosage on attained pulp brightness before and after peroxide bleach.
Figure 5 comprises graphs showing the effect on attained brightness by variations in retention time in the treating sequence at two different temperatures.

DETAILED DESCRIPTION
In general, practice of the present invention involves the use of oxy-gen, alkali and surfactant to bleach and deink secondary fiber. As shown in Figure 1, after the secondary fiber material has been pulped, cleaned, deinked and washed by conventional procedures it is subjected to an initial oxygen bleaching stage (10). The treatment with molecular oxygen is conducted in an aqueous alkaline medium under pressure. The selected process conditions depend upon the type of recycled or salvaged fiber starting material supplied. In general, the oxygen reaction is conducted at a temperature in the range of 60-130C, sodium hydroxide dosage in the range of 1 to 7% (by weight of air dried pulp), during a retention time of 0.1 to 2 hours at a pressure of 20-150 psig. Following the oxygen treatment the pulp is diluted (12) and washed (14) on a vacuum filter in water containing 0.1 to 1.0% (by weight of pulp) surfactant. The surfactant wash serves to keep residual ink and other contaminants from redepositing. The thus washed pulp may then be subjected to a final bleaching stage (15) and again washed and dried on a vacuum filter as indicated at (16).
The preferred operating conditions to be employed in individual cases will depend largely on the particular makeup of the starting recycled fiber, as to lignin content, amount and type of ink present, content of coatings 202 3g~

and fillers, etc. Treatment at lower than required severity limits on pulp having high amounts of any of the contaminants above mentioned would result in smaller than desired increase in brightness. Operation at condit~ons much beyond the listed upper limits would result in large loss in strength and viscosity and would entail large capital constraints on the equipment needed for effective bleaching and deinking of the pulp.
Without relying on any particular theory to account for the unexpected high improvements in pulp brightness achleved in practice of the inventlon, it now appears that at least two contributory factors are present. First, during the oxygen treatment stage delignification of any contained ground wood (unprocessed f~ber) takes place, thus removing lignin color bod~es by breaking these down and washing them out of the pulp. Also, residual ~nk that has not been earlier removed from the fiber, is oxidized and released.
Without the inclusion of surfactant in the washing step the ink and residual lS color bodies would normally redeposit onto the pulp. The surfactant, on the other hand, removes or prevents such deposition.
Among the other benefits obtained by the invention is the decrease or total avoidance in the final bleach of the otherwise needed use of chem-icals, such as hydrosulfite andlor chlorinated compounds, that are harmful 20 to the environment. Accordingly, it becomes possible that a typical f~ve stage bleaching sequence can be reduced to a two or three stage sequence with resulting reduction in capital equipment costs.
A number of experiments were carried out in the laboratory to determine the effect of washing the pulp in aqueous surfactant solution follow~ng the 25 oxygen bleaching step. Two different procedures were followed. In Pro-cedure I the sequence designated sP was employed in Procedure II (desig-nated (CEoP) a moderate chlorination stage was employed preceding the oxygen treating step.
The operating sequence employed in Procedure I is substantially that illustrated in Figure 1 of the accompanying drawings.

~23~a4 . _ Procedure I. - The OSP Process (Oxygen-surfactant, Peroxide) s A. Standard Oxygen Bleaching Stage (O):

The cleaned and washed pulp was placed in a laboratory oxygen reactor which consists of a pressurized reactor, mixer, flow controls, and condition monitors. An amount of water and sodium hydroxide was added to the pulp at a 10% consistency of pulp to water (sodium hydroxide amounts varied). Then the reactor was sealed and heated with both steam and an electric mantle (to 60-130C). Next, the reactor was pressurized with oxygen to 30-150 psig and the hlgh shear mixer was turned on for one minute. The pulp was retained in the reactor at these conditions anywhere from thirty minutes to two hours with peri-odic mixing at ten-minute intervals.

B. Surfactant Wash (S):

The pulp from the oxygen reactor was diluted and poured into a beaker where it was mixed w~th .25-2% by weight of pulp of a surfactant. This mixture was then poured into a vacuum filter and rinsed with approximately 4000 mls of water per 60 grams of air dried pulp.

C. Peroxide Stage (P):

After being washed and filtered the pulp was placed in a polyester bag with 1% sodium hydroxide dosage and 1% hydrogen peroxide dosage. Water was added to make the pulp a 12% consistency. The bag was heat sealed and placed in a constant temperature bath at 150-180F, and 60-120 minutes. The pulp was then removed from the bath and bag, put into the vacuum filter, and dried to approximately 25Z consistency.

2~23~

Viscosity (T230), handsheets (T218), and brightness (T217) determina-tions were made in accordance with the respective TAPPI Standard Test Procedures. Chemical charges are on a weight percent basis, pulp weight is air dry basis. The listed test procedures are described in 5TAPPI: Standard Test Methods 1989.

The operating conditions employed in the runs using Procedure I are set out ~n Table 1 below and the measured brightness of the pulp at the several treating stages is reported in Table 2.

Table Oxygen Conditions Peroxide Conditions Temp., C 130 -65 Alkali dosage (%) 5 1.0 Retention Time (hrs) 1 2 Pressure (psig) 150 Surfactant (%) 0.5 Peroxide (%) - 1.0 Table 2 Stage Brightness (ISO units) With Without Surfactant Surfactant Delta Initial 56.9 56.9 Oxygen 69.0 69.0 Wash 71.4 70.3 1.1 Peroxide 78.0 74.1 3.9 Although the brightness of the pulp showed a measured increase of about 1.56X following the surfactant wash step, the increase ~n brightness measured 30 after the final peroxide bleach was surprisingly over 5%.

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Procedure II - The CEoSP Process s A. Chlorination Stage (C):

Sixty oven-dried grams of pulp at 4% consistency to water was placed in a polyester bag and an amount of chlorine water added to make a charge of 2.0% active chlorine. The bag was heat sealed and the chlorination proceeded at ambient temperature for one hour. The pulp was filtered and washed with water.

B. Alkali Oxygen Extraction Stage (Eo) Procedure for (IA) was followed with different conditions within the reactor (as reported below) being observed. Temperature was set at 65-100C. Pressure was set 30-50 psig, and for a l0-minute retention time to simulate the upflow tube of an Eo reactor. Total retention time was set at 60 minutes and the sodium hydroxide dosage was held at 1.5%.

C. Surfactant Wash (S): Same as (IB).

D. Peroxide Stage (P): Same as (IC).
E. Other: Same as (ID).

The data compiled using procedure II (CEoSP) is reported in Table 3 below and the attained Brightness at the several stages is set out in Table 4.

2~23904 Table 3 Conditions: C12_ E ~ P

Temperature, C 45 65 65 C12 Dosage, (%) 2.0 * *
Consistency (%) 4.0 10.0 12.0 Retention Time (hrs) 0.5 0.83 2.0 Pressure (in upflow tower) psig * 30 *
Retention Time (in upflow tower) (min) * 10 *
Peroxide Dosage (%) * * 1.0 NaOH Dosage (%) * 1.5 1.0 Surfactant Dosage (in washer * 1.0 *
after Eo) (%) NOTE: Oxygen was mixed in the reactor for the first ten minutes of the Eo run, at two one-minute intervals every five minutes.
Table 4 BRIGHTNESS (ISO) With Without Stage Surfactant Surfactant Delta Initial 56.9 56.0 --C12 61.0 61.0 __ Eo 66.8 66.8 --Wash 69.2 67.9 1.3 Peroxide 77.6 73.4 4.2 All of the runs in Examples 1 and 2, as well as those hereinafter reported, were made on commercial, unbleached, recycled paper pulp. The surfactant used in the runs of Examples 1 and 2 was a nonionic blend of nonylphenoxy(ethyleneoxy)ethanol, ethylene glycol and water.
From the data observed in the foregoing examples it appears that the surfactant treatment enhanced the bleachability of the pulp in the final treating stage thereby accounting for the unexpectedly high increase in brightness.

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In following the procedure of Example 2 wherein an initial chlorine bleaching step is employed, only comparat~vely low to moderate concentra-tions of act~ve chlorine are required, in the preferred range of about 0.5 to 3.0Z active chlorine in the aqueous medium, and generally at less than 5X.
s EXAMPLES 3 to 10 A series of experiments were conducted with variations in process conditions to determine at what stage or stages in the bleaching sequence the washing with surfactant solution would be most beneficial. The results are plotted in Figure 2 of the drawings.
Eight experiments were run in all with four being run at (A) set of conditions (100C, 4% NaOH, RT-60 min, 130 psig), another four exper~ments at (B) set of the conditions (130C, 5% NaOH, RT-45 min, 130 psig). One exper~ment from each set was kept as a control, one pair was treated prior to the oxygen run with a surfactant wash, one pa~r treated after the oxygen run with a surfactant wash, one pair treated prior and after with a sur-factant wash. Then brightness was measured, the results of which are reported in Table 5.

~A) Brightness (B) Brightness Oxygen 1) 62.2 2) 67.9 Surfactant-Oxygen 3) 64.5 4) 68.8 Oxygen-Surfactant 5) 65.1 6) 71.8 Surfactant-Oxygen- 7) 69.0 8) 72.0 Surfactant From the data shown in Table 5 it appears that surfactant wash~ng followed by an oxygen reaction, at B conditions, in turn followed by a further surfactant wash gives the best brightness results (72). What also must be taken into account ~s oxygen-surfactant conditions do basically as ~Z3 gO~

good a job (71.8) with one less washer and only half the amount of sur-factant. If one wanted to achieve high brightness at a milder set of conditions (A), the surfactant-oxygen-surfactant sequence affords the best route.
As show in Table 5, treatment with surfactant before the oxygen treat-ment does obtain some improvement in brightness but less than that achieved by application of surfactant in the washing step following oxygen treatment.

EXAMPLES 11-14 The effect of different types of surfactants There are basically three different types of surfactants: nonionic, anionic, and cationic. In the runs reported below one oxygen reaction was performed and the pulp from that reaction was broken up into four samples;
the tested brightness of each is set out in Table 6.

Table 6 Brightness 11.Washed with DI water (1000 mls/), then bleached 74.5 with peroxide.
12.Washed with nonionic surfactant in a 1000 mls 77.3 of DI water, then bleached with peroxide.
13.Washed with anionic surfactant in a 1000 mls 76.2 of DI water, then bleached with peroxide.
14.Washed with cationic surfactant in a 1000 mls 72.3 of DI water, then bleached with peroxide.
The surfactants employed at 0.5% by weight of pulp, were Run 11 none Run 12 nonylphenoxy(ethyleneoxy)ethanol in aqueous ethylene glycol Run 13 Adogen 464 (Ashland Chem.) Methyltrialkyl C8-C10-ammonium chloride Run 14 Sodiumdioctyl sulfosuccinate (American Cyanamide) 2~23~

EXAMPLES 15-17 Process Conditions Numerous experiments were run to determine what range of condit~ons (temperature, pressure, retention time, caustic dosage) were most appro-priate for the oxygen stage. Also examined was the effect shifting certain conditions while holding others constant, would have on the final br~ghtness of the pulp.
Each of the experiments of these examples addresses a particular condi-tion or conditions.

The effect of sodium hydroxide dosage on pulp brightness.
Temperature - 130C, Pressure - 150 psig, Retention Time - 45 m~n.

Table 7 Brightness NaOH % Dosage After Oxygen StageAfter Oxygen & Peroxide Sta~e 1.5 70.2 74.0 3.0 70.1 74.6 4.0 70.0 75.1 5.0 69.9 78.0 5.5 71.8 79.1 6.0 72.3 80.6 7.0 73-4 82.2 The results are plotted in Figure 3.

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The effect of retention time and temperature. Pressure held at 130 psig, NaOH at 4%.

Table 8 Brightness Retention Time 100C 130C
45 min 65.5 70.0 90 min 67.6 70.1 135 min 69.7 70.8 The results are plotted in Figure 4.

The effect of pressure on brightness results. Temperature held constant at 100C, NaOH at 4%, and retention time at 1 hour.

Table 9 Pressure PSIG Brightness 63.4 65.2 67.1 120 68.3 150 69.6 In practice of the present invention desired brightness of the treated pulp often may be obtained without necessitating a final chemical bleaching step after the oxygen gas treatment and washing with surfactant. If such chemical bleaching is used, it is preferred to employ peroxide. The use of controlled small amounts of hypochlorite in a final bleach sequence is not ruled out, however, particularly in instances that chlorine or chlorine compounds have not been earlier employed in the involved pulp treating sequence as to result in a total amount having a negative impact on the 35environment. !

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As indicated in Figure 1 of the drawings, the surfactant may be in-troduced at any one ore more points of the pulp treating sequence, par-ticularly as indicated by the arrows 20, 21 and 22. In addltion, part of the surfactants, if desired, may be introduced into the oxygen reactor (10) as shown at 23. The pulp, after rinsing on the vacuum filter (14), will be substantially free of applied surfactant.
In accordance with a preferred embodiment of the invention at least part of the alkaline filtrate from the oxygen stage washer 14 is recycled to a selected stage of the deinking sequence, whereby costs of energy supplied and chemical consumption costs are greatly reduced. As shown, by the dotted lines in Figure 1, the wash liquor filtrate 25 is dischargewd from washer 14 and a controlled portion is recycled to a selected stage of the deinking sequence. Thus, as shown in the illustrated embodiment (Figure 1), the withdrawn portion of the filtrate 25 may be returned to the hydropulping step (line 26) or to the deinking step at the flotation stage (line 27).
Optionally a part of the liquor recovered from line 25 may be returned and included as part of the wash water employed in the oxygen stage washer 14 as shown at 28, thus providing useful surfactant at this stage.
The oxygen stage filtrate 25, it should be noted, contains heat, water, caustlc and surfactant (washed out from the pulp). These resources would probably be sent to sewer (as waste) in normal operation of a secondary fiber bleach plant. By the proposed recycling step important savings in mill operation are attained without sacrifice in product quality.

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Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The method of deinking and bleaching of pulp comprising secondary cellulosic fiber which includes the steps of subjecting an aqueous alkaline slurry of such pulp to reaction with an oxygen-containing gas, then washing the thus bleached pulp in an aqueous bath containing nonionic or anionic surfactant, added to the pulp slurry prior to or during said washing.

2. The method as defined in Claim 1 wherein the pulp is subjected to washing with surfactant following said reacting with oxygen-containing gas.

3. The method as defined in Claim 1 wherein the pulp is subjected to chlorination prior to said reacting with oxygen-containing gas.

4. The method as defined in Claim 1 wherein the pulp is washed with a nonionic surfactant before and after said reacting with oxygen-containing gas.

5. The method as defined in Claim 4 wherein the reacting with oxygen-containing gas is carried out at superatmospheric pressure at a temperature in the range of 60 to 130°C and for a retention time of 0.1 to 2 hours.

6. The method as defined in Claim 1 wherein prior to said reacting with oxygen-containing gas the pulp is subjected to chlorination in an aqueous medium comprising 0.5-5% active chlorine for about an hour at am-bient temperature.

7. The method as defined in Claim 6 wherein the washing of the pulp following said reacting with oxygen-containing gas is performed with non-ionic surfactant at a dosage of 0.1 to 2 percent by oven dried weight of pulp.

8. The method as defined in Claim 1 wherein said surfactant is of the nonionic type.

9. The method as defined in Claim 1 wherein the recited surfactant wash of the pulp is followed by at least one chemical bleaching stage.

10. The method as defined in Claim 1 wherein the surfactant-containing liquid recovered from washing of the bleached pulp, is recycled to a selected stage of a deinking sequence previously applied to the pulp.

11. The method as defined in Claim 10 wherein said deinking sequence includes a flotation stage and wherein at least part of said surfactant-containing wash liquor is recycled to said flotation stage of the deinking sequence.

12. The method as defined in Claim 10 wherein at least part of said recovered surfactant-containing wash liquor is utilized in the initial pulping of said secondary cellulosic fiber.

13. The method of treating secondary cellulosic fiber wherein these fibers are pulped and then subjected to a deinking treatment followed by one or more bleaching stages, the improvement wherein at least one of said applied bleaching stages comprises subjecting the deinked pulp to treatment in an aqueous alkaline bath in the presence of molecular oxygen, washing the oxygen treated pulp and recovering the wash filtrate, and recycling at least part of the wash filtrate to the deinking treatment.