CA2098144A1 - Method for improving quality of deinked high grade waste paper - Google Patents

Abstract

Abstract Quality of deinked/bleached high grade waste paper is improved by bleaching the deinked pulp with thiourea dioxide, said deinked pulp having been obtained by subjecting high grade waste paper to a digesting and aging step with a sulfite, alkali or alkaline peroxide, a disaggregation step, a deinking step and a dust removing step.

Description

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Method for Improving Quality Or Deinked High Grade Waste Paper Field of the Invention This invention relates to a chlorine-free bleaching method which does not use any chlorine-based compound when waste paper is deinked and bleached for producing deinked/bleached pulp.
Background Or the Invention -With the view to ensure an effective utilization of forest resources, the deinked/bleached pulp has conventionally been produced by means Or deinking and bleaching waste paper.
Deinked/bleached pulp has been utilized for various endusages reflecting nature of the raw materials, its physical property, easiness to attain its deinking effect or its brightness. For example, deinked/bleached pulp containing higher lignin content recovered from waste newspaper and magazine etc. is used for low and medium grade printing paper or paperboard, and deinked/bleached pulp containing less lignin content recovered from high grade waste paper such as printed woodfree, ruled lines shavings and OA waste paper can be utilized, as the substitute for high and medium grade printing paper or BKP
(bleached kraft pulp) for board surface but majority of them have been utilized as toilet paper or tissue paper and the like.
On the other hand, most of materials for hiBh Brade printing paper is prepared firstly by processing wood into kraft pulp(KP) and then, bleaching them with a large quantity of chlorine based bleaching agent, and various by-.
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products such as dioxinejand AOX (absorbable orgainc halogens) are generated during those processes, for which technical development has been made for the purpose of controlling their generation since they are considered as environmental pollutants. Furthermore, in the case of bleaching hiBh grade waste paper, chlorine and hypochlorite are adopted since its lignin content is low.
There are several types of hiBh grade waste paper such as printed woodfree, colored paper, OA (printed woodfree paper for typewriting (CPO) and copying paper (PPC)), and they are generally subjected to a high temperature/pressure digesting (globe digester) or low temperature digesting (pulper and tower), followed by deinking and bleaching. Since the above waste paper does primarily consist of BKP as its main component, hiBh brightness is expected to be obtainable if separation, dust removal and deinking were conducted satisfactorily.
However, the waste paper also includes, in addition to high grade one, white wood-containing shavings and magazine paper that contain lignin, which undoubtedly results in much more difficulty to prevent it from mixing since distinguishment of those types of paper has not been an easy task as the endusage of papers have been diversified in these years.
In addition to this, recent trend shows that volume of multi-color printed paper have been increasing and printed ink has been gettin8 more difficult to be removed reflecting the improvement in printing technology, and those waste papers now require a bleaching treatment since they contain inorganic or organic dyestuff, pi~ment, filler and the like other than wood, which affect brightness(bleached) or ' : :

~0981~4 deinked products, and nowadays, it is an established practice to apply additional bleaching with hypochlorite to deinked pulp. However, in some cases, hypochlorite has caused not only changes in the color shade but also deteriorations to the brightness of paper through discoloration upon reacting on lignin and cause damages, especially when it is used exce~sively, to the fibre structure which in turn affects strength of products as well as deterioration of production yield. Futhermore, it also increases COD in waste water and could cause an increase of AOX and dioxine which has lately brought about a serious environmental issue.
On the other hand, it is well known fact that the chlorine based bleaching agent, particularly sodium hypochlorite or calcium hypochlorite is effective in the case of bleaching method of high grade waste paper. A
method by digesting with use of hydrosulfite salt and alkaline agent or thiourea dioxide and alkaline a8ent has been proposed in JP2-4717A which adopts the process Or low temperature digesting using the said agent instead of conventional high temperature/pressure digesting in the globe digester with sodium sulfite, sodium hypochlorite being adopted ln the bleachlng thereof. Also, decoloration of pulp made from colored paper and pressure sensitive copying paper by feeding formamidine sulfinic acid (thiourea dioxide) as an additive has been proposed in JP62-276094A.
In spite of these proposals, a satisfactory effects has not been yet obtained as a system on the whole.
In consideration of problems prevailing in the conventional method which requires hypochlorite in _ 3 _ 2~9~14~

bleaching for high grade waste paper and increase in the quantity of hypochlorite in order to achieve better brightness invites a resultant deteriorations both in the product strength and yield of the deinked/bleached pulp production and a resultant increase of COD and AOX content, and further cause the generation of dioxine during the course of the treatment, this invention now provide a chlorine-free bleaching method without use of hypochlorite In addition to this, additional effect of this inventlon has been observed in the reduction of paper cut loss or (breaks) as well as of contamination on felt, wire and drier in a paper making using the obtained deinKed/bleached pulp.
Summary of the Invention The present invention have established, through the development of said method comprises bleaching a deinked pulp with thiourea dioxide, said deinked pulp having been obtained by subjecting high Brade waste paper to a digesting and aging with a sulfite, alkali or alkaline peroxide, a hi~h concentration disaggregation step 1 a deinking step and a dust removing step. According to this invention, the desired quality of deinked/bleached pulp made from high grade materials as a main feed as experienced in the past can be maintained even in case some medium Rrade materials are involved in part.
Brief Description of the Drawings Fi8 1 is a graph illustrating the difference in the brightness of several types of deinked/bleached pulp processed by different bleaching agents.
Fig. 2 is a graph illustrating the difference in the color tone of several types of deinked/bleached pulp :~ ~ : ... ..

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processed by different bleaching agents.
Preferred Embodiments of the Invention Examples of high grade waste paper (generally including printed portions with carbonaceous ink) high grade white printed woodfree, ruled lines shavings, OA waste paper and the like, and a mixture of such high grade-waste paper with white wood-containing shavings and or high 8rade wood-containing wastes.
The waste paper is diarged into a digesting and aging apparatus such as a pulper/tower or globe digester and an alkali, alkaline hydrogen peroxide or sulfite and deinking agent (such as surfactant) are added thereto and facilitated their chemical function in order to make pulp and useless substances such as ink, clay and talc ready to be separated, and then ink and other useless substances are separated completely from the pulp with the hiBh concentration disaggregating apparatus such as kneader of disperser and, at the same time, wax and bitumen etc. are dispersed, and then, deinkinB is conducted through floatation and cleaning steps, and finally, all the above useless substances are removed by means of dust removal apparatus such as cleaner and screen. The pulp purified thereof has an acceptable quality but its brightness is still on an insufficient level, and no satisfactory result is expected even if sulfite or hydrogen peroxide were applied at the digesting step.
Pulp purified thereof is bleached with thiourea dioxide. Usually this pulp bleaching step is performed firstly by concentrating i~ down to 8 to 25wt% concentration and secondly by addinB thiourea dioxide in the ranBe Or 209~

to 5.0wt% at a temperature Or 30C to 100C and finally by adjusting pH in the range of 5 to 10 with alkali or acid.
In this invention, it is essential that high Brade waste paper is fed to a digesting and aging step using a sulfide, alkali or alkaline hydrogen peroxide, a disaggregation step, deinking step and dust removal step prior to the bleaching step using thiourea dioxide. While there is no specific requirement regarding the sequence of steps, it is recommendable to apply the same sequence as above mentioned. The conditions applicable to each step shall be the one which is well known conventionally. For the ordinary digesting and aging step, waste paper is charged to a globular tank (globe digester) then, fill it with a cool or hot water containing sulfite such as sodium sulfite (prererably 2 to 5wt%), alkali such as soda ash and/or caustic soda (preferably 0 to 2wt%) and deinkinB
agent such as surfactant agent (preferably 0.1 to 0.5wt%) upto approximately 50% of the mixkure on mass basis, then apply it with steam and the tank is rotated under high temperature and pressure (preferably 50 to 180C and 2 to 10kg/cm2 steam). In the case of using a pulper/tower process, a cool or hot water containing waste paper, alkali, deinking agent and preferably hydrogen peroxide is fed to a pulper and mixed therein at the temperature in the range of room temperature to 90C, and then aged in a chest or tower.
In the disaggregation step, the mixture thus digested and aged which usually has a pulp content of more than 15wt%, preferably more than 20wt% is disaggregated in a disaggregating apparatus such as kneader or dispensor to fully separate pulp and other components such as ink.

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Deinking step comprises cleaning, floatation or combination thereof.
Dust removal step is conducted with use Or cleaner and screen and others.
Adoption of thiourea dioxide in this invention has proved remarkable bleaching effect in combined application of series of purifing steps above mentioned, which is much superior to those attained with use of conventional reducing agent such as hydrosulfite, boron hydride and sulfite. Also, it is better than hypochlorite of chlorine based bleaching agent in attaining stable brightness as the specific feature. In particular, conventional hypochlorite is able to improve the brightness of pulp to some extent when waste paper contains lower rate of lignin, while, in case lignin content BetS higher namely increase in the ratio of white wood-containing shavines among the feed material, improvement on brightness is quite limited and, moreover, the pulp processed thereby indicates a tendency of.getting discolorated under an influence of chiorous lignin. In contrast, thiourea dioxide maintains higher brightness of pulp on a constant level irrespective of lignin content without causing any discoloration effect. Thus, a bluish brightness can be obtained in the finished pulp which is cleaner than the pulp bleached with hypochlorite. In addition to this, the treatment with thiourea dioxide provides no yield loss to fragility of fiber structure in comparison with hypochlorite method and generation of breaks and paper dust is reduced during a paper makinB proc~sses using the deinked/bleached pulp for producing various paper products such as printing paper, toilet and tissue paper 20981~ ~

since strength of pulp can be maintained at higher level, which reduces containation of felt, wire and drier and, as the result, enables to improve overall productivity.
Furthermore, the major advantage in the utilization of thiourea dioxide not only comes from the reduction of COD in waste water but also from the control of generation of dioxin and AOX, in fact, the latter can be cut back down to less than one tenth of the process depending upon hypochlorite.
This invention adopt chlorine free bleaching process which ensures an improvement environrnental problem and, at the same time, enables to obtain higher quality products.
The bleaching agent adopted in the conventional process such as hydrosulfite is unable to improve brightness substantially in the treatment of hiBh grade waste paper -and its effect on improvement of brightness is limited even with the additional use of the aBentS~ which in turn affects overall economy. Boron hydride is also a similar ~ype of reducin~ agent and the trend in its bleaching effect is all the same. Hydrogen peroxide is considered as advantageous in case the pulp contains higher lignin content since bleaching takes place in alkaline condition, which, however, limits the improvement of brightness but undeniably tends to cause discoloration. In the case of hypochlorite, improvement in its effect on brightness of pulp is somehow observed but yellow is dominant in the color shade (high b value) after bleaching, of which value gets higher whenever the lignin content in the feed materials increases. On the contrary, thiourea dioxide improves brightness significantly with the addition of small quantity and blue . . -. . -.

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(low b value) is predominant in its color shade thus, exhibiting a remarkable difference against other bleachin~
agents. These advantages are itemized as followsj (1) No additional generation of AOX which is environmental pollutant and AOX can be drastically reduced in comparison with the conventional method which relies on the application of chlorine based bleaching agents thus, capable of controlling ~eneration of dioxine incidental to such process.
(2) Ability to prevent deinked/bleached pulp from 8etting fragile during the process, which ensures reduction of COD in waste water to eliminate possible environmental problem.

(3) Improvement in the strength Or deinked/bleached pulp to ensure prevention of breaks during paper machine process, and resultant reduction in paper dust prevent contamination on felt, wire and drier etc., thus productivity can be signiricantly improved.

(4) It assures attainment of stable hiBh level Or brightness ror pulp product irrespective of variation in the quality of feed materials and improvement of product quality since thiourea dioxide is capable or bleaching pulp without causing any discoloration of fiber and lignin and other components.

(5) Further remarkable improvement in the bleaching process is possible with use of thiourea dioxide by a combination use of the respective steps (digesting) and aging, high concentration disaggregation, deinkinB and dust removal in advance of the bleaching step with thiourea dioxide.

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Further, this invention ensures reutilization of natural resources by utilizing waste paper and obtainment of better quality pulp without any adverse effect on environment, which is economically advantageous and extremely useful and effective industrial point of view.
Examples of this invention are mentioned together with comparative examples. The following method and analytical apparatus were used for measurement of data.
Brightness JIS P-8123 Kumagaya Riki Color shade (L.a.b) Nihon Denshoku Bursting strength JIS P-8112 Kumagaya Riki AOX Mitsubishi Kasei TOX-10~
Meantime, quantity Or agent used (%) in the examples shows weight percentage as against feed materials.
[Comparative Example 1 and Examples 1 and 2]
In comparative Example 1, feed waste paper comprising 40% of printed woodfree paper, 30% of ruled lines shavings 25% of high Brade OA paper, and 5% Or white wood-containing shaving was charged to a globe digester as the digesting and aging step, and after aginB for 5 hours with pulp concentration Or 30% upon putting sodium sulfite 0.5%, soda ash 0.3% into it and then, after processing them through the steps Or hiBh concentration disaggregation, deinkinB and dust removal, bleaching was conducted with pulp concentration of 20% and at 30C for 6 hours upon addinB 10%
of sodium hypochlorite (12%). In Examples 1 and 2, thiourea dioxide, 1.5% and 3% respectively was used instead Or sodium hypochlorite in the Comparative Example. The results are - I O -, , .. . - . , . ~ , .
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:. ,, : . ~ ,. , 2~98144 shown in Table 1. Meantime, brightness at the intake of bleaching stage was 68%.
Table 1 Comparative _ Example 1 Example 2 Example 1 Sodium 10% _ hypochlorite(l2%`
(100%) 1.5% .3.0%
(intial pH) (9.0) (9.2) Brightness 73.1 74.2 75.8 . (Hunter) L 86.3 87.3 88.5 a -2 1 -0.2 -0.1 b 3.6 1.6 1.1 . . _~
COD in waste water, ppm . __ . .. _ _ AOX kg/
0.30 0.05 -0.06 productton ._ .. _ _ . _ [Comparative Example 2 and Examples 3 and 4]
Pulper and aging tower were employed at the digesting and aging step using the same waste paper, the f`eed materials. as in the case of Examples 1 and 2, and 0.5% of caustic soda, 0.5% of hydrogen peroxide (35%) and 0.3% of deinking agent were fed to the pulper. After processing the mixture with pulp concentration of' 4%, at 40C f`or 30 minutes in the pulper, the mixture was processed in the tower with pulp concentration of` 15% at 50~C 5 hours, then, af`ter coursing through the steps of' high concentration disaeereestlon; deinkine and dust removal in - ~ ' ~ .. ' - .

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accordance with standard procedure, 7% of sodium hypochlorite (12%) being used in conventional method was added, and the mixture was bleached with pulp density of 20%
at 30C for 8 hours, of which product was adopted as the Comparative Example. I`he product which was processed at the bleaching step with pulp concentration of 15% at 40~C for 8 hours, with an addition of 0.25% and 0.5% of thiourea dioxide, upon adjustin~ its pH 9.0 with caustic soda, instead of the corresponding step in the process for the Comparative Example was adopted as Examples l and 2 respectively, which are shown in the following Table.
Meantime, brightness prior to the intake of bleaching tower was 70.2.

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Table 2 Comparative Example 3 Example 4 Example 2 .
Sodium 7% _ hypochlorite (12%) Thiourea dioxide (100%) 0.25% 0.5%
(initial pH) (8.9) (9.0) _ ._ _ Brightness (Hunter) 79.2 81.2 83.2 L 88.5 88.2 88.8 a -0.5 0.0 0.0 b 2.0 0.3 -0.3 _ . _ .
Bursting strength kg/cm2 1.04 1.35 1.29 ._ . . _ _ Pulp yield % 72.6 74.B 75.2 _ _ . _. _ . _ _ , COD in waste 108 43 .47 water, ppm . __ . _ _ . .. _ _ AOX kg/product ton 0.28 0.02 0.01 .. ._ , ._ _ [Comparative Examples 3, 4, 5 and 6 and Example 5]
Feed ~naterials comprising 45% of printed woodfree paper, 38% Or ruled lines shavings, 14% Or high 8rade OA
paper and 3% of white wood-containing shavings was charged to pulper at the actual plant, and upon adding 0.5% of H202 (35%), 0.5% o~ NaOH (100%) and 0.3% Or deinking agent, the mixture was disintegrated at 40~C for 30 minutes followed by processing with pulp concentration 20% ror 8 hours in aging tower, then ink was separated from fiber using hi8h concentration disag~regation equipment. After deinking at - .

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the following step comprising floatation, extractor anddecker, product pulp was sampled upon removing heavy and light foreign matters by centrifugal cleaner, slit screen and fine screen etc. The brightness of this pulp sample was 71.1 and L, a and b value were 86.0, -0.4 and 1.8 respectively. To this pulp, sodium hypochlorite (NaC10), hydro sulfite (NaS204), boron hydride (NaBH~) and hydrogen peroxide were added - in the various volume used in the process for Comparative Examples as specified in the following table - for bleachin~ purpose and the results were compared with that of Example 3, i.e. bleaching with thiourea dioxide, of which comparison is shown in Table 3, Figure 1 and 2.

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Table 3 Bleaching Vol.of Whiteness L a b -agent additive %
Com. Ex. 3 NaC10(12%) 1 73.0 86.5 -0.6 1.6 3 75.5 87.6 -o.9 1.3 76.4 87.9 -0.9 1.4 7 77.3 88.5 -1.1 1.6 . .. _ .
Com. Ex. 4 NaS2 4 1 72.8 86.3 -0.7 1.8 3 73.3 86.5 -0.6 1.4 73.9 86.6 -0.6 0.8 _ . ._ Com. Ex. 5 NaBH4 (3 1 73.2 86.5 -0.7 1-7 times of ! 2 73.6 86.6 -0.6 1.6 NaHSO3 was used ._ ._ together) ._ _ _ Com. Ex. 6 H202 (35%) 1 73.8 86.9 -0.5 1.2 2 74.4 87.1 -0.5 1.1 3 _ 74.2 87;0 -0.5 0.9 NaOH(100%),0.5~ 1.0% and Na2 SiO3 ( 42~ 43 Bé), 3~ 5%, was used together.
Ex. No.3 TUD 0.5 76.3 87.2 -0.4 0.4 1.0 77.7 87.7 -0.3 -0.2 2.0 78.8 88.0 -0.3 -0.4 _ _ 3.0 79.1 88.2 -0.3 -o.5 _ pH adjusted to 9.5~ 10 with NaOH

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Claims (4)

1. A method for improving quality of a deinked/bleached pulp which comprises bleaching the deinked pulp with thiourea dioxide, said deinked pulp having been obtained by subjecting high grade waste paper to a digesting and aging step with a sulfite, alkali or alkaline peroxide, a disaggregation step, a deinking step and a dust removing step.

2. A method of claim 1, wherein the bleaching step using thiourea dioxide is carried out at a pulp concentration of 8 to 25wt%, a thiourea dioxide concentration of 0.1 to 5.0wt%, a temperature of 30°C to 100°C and pH of 5 to 10.

3. A method of claim 1, wherein the sulfite is sodium sulfite.

4. A method of claim 3, wherein the digesting and aging step is carried out in the presence of 2 to 5wt% of the sodium sulfite, 0 to 2wt% of alkali such as NaOH and or soda ash and 0.1 to 0.5% of a surfactant at a temperature of 50 to 180°C and a pressure of 2 to 10kg/cm2 steam.