CA1288906C - Pulp souring process enhancing conservation of specific bleaching agent residuals - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

There is disclosed a process for souring chemical or mechanical cellulosic pulp using carbon dioxide to obtain proper acidification of the pulp slurry. The souring is performed after a single or a multistage bleaching sequence ending with sodium hypochlorite, calcium hypochlorite or hydrogen peroxide or the like in a common process for the preparation of paper pulp. The use of CO2 permits bleaching to continue because CO2 does not destroy the bleaching residuals remaining from the single or last bleaching stages.

Description

TITLE OF THE lNVENTlOly Pulp souring process enhancing conservation of specific bleaching agent residuals .
FIELD OF Ttl~ INV~NTION
The present invention relates to the use of a pulp souring agent after a single or a multistage bleaching sequence ending with sodium hypochlorite or clacium hypochlorite (designated as a H stage) or hydrogen peroxide (designated as a P stage) or the like.
BACKGROUND OF THE INVENTION
In the pulp and paper industry, various processes are employed for bleaching pulps. Bleaching is a continuation of the cooking process in which the ligneous material and coloring matter remaining in the chemical pulp are removed selectively with as little degradation of the pulp fibers as possible.
Bleaching of pulp has advanced to a high degree of sophisticalion involving single s~age and multi-stage procedures. The choice of bleaching agent has traditionally been dependent on whether the pulp is a mechanical pulp or a chemical pulp.
Mechanical and groundwood pulps have many desirable characteristics for low-cost papers, e.g. high yield, good bulk, high opacity, and good printing properties. The natùral brightness of these pulps is, however, too low for the better grades of groundwood content papers. Also, the brightness of unbleached groundwood pulp varies with the species, the wood process, i1s age and its quality. Hence, ~he need for suitable bleaching processes arose in order to compensate for natural brightness variations of the wood; to obtain still brighter pulp to meet the ever-increasing demands for higher quality groundwood papers; and to improve the brighlness of the :, :

pulp. The most important characteristic of mechanical pulp is the low cost of manufacture and this is due to its high unbleached yield. Oxidizing agents, for example, hydrogen peroxide (P) and hypochlorites (H) and reducing agents, for example, hydrosulfites (Hs), borohydrides (B), amino-boranes, peracetic acid, and bisul~ites have been used since they raise the brightness of mechanical pulps without materially effecting the yield. Bleaching efficiency can sometimes be improved when combining these chemical in multistage systems, e.g. two stages P-Hs or Hs-P or three stage Hs-P-Hs systems.
0 The bleaching of chemical pulp is accomplished in several stages. A
stage constitutes a phase sSarting with addition and reaction of a chemical with a pulp, and ending with the washing of the pulp. Wi~hin each stage there are many process variables which are dictated by ~he type of reaction desired in that particular stage, and the operating conditions of the stage. These variables include: percent of chemical added and consumed, chemical concentration, consistency, temperature, time and pH. A series of such stages is called a bleaching sequence.
In such multistage bleaching process, the firs~ operation for the removal of lignin and olher encrustan~s (usually following the diges~ion s~age and the subsequent washing stage in a kraft, soda or sul~ite process) consists in treating the pulp aqueous suspension with elemental chlorine in aqueous solution. A b!eaching stage performed using elemental chlorine is designated as a C stage. Chlorination of the unbleached pulp so changes the ligneous impurities that they become in part soluble in wa~er while, of the portions not readily soluble in water, a part is soluble in alkaline solutions such as dilute sodium hydroxide.

~2889V6 Among the agents used to bleach chemical pulp are elemental chlorine, chlorine dioxide, hypochlorites, chlorites, peroxides, chlorates, bichromates, and permanganates, as weli as reducing agents, e.g. sulfurous acid, bisulfites, dithionites, and borohydrides. However, for chemical pulps 5 (e.g. those made by the kraft, sulfite or soda processes) the bleaching agents traditionally used are chlorine and chlorine dioxide (a bleaching stage performed using chlorine dioxide is designated as a D stage) usually used in a multis~age process. The fist step thus usually consists of treating the pulp in an aqueous suspension with chlorine in solution in the aqueous phase.
10 Chlorine dioxide may be used either in admixture with the chlorine, or in replacement of the chlorine, in this first stage. In some mills, hydrogen peroxide is used as a final bleaching treatment in high density storage chest.
The next stages usually consist of a washing stage, preferably an alkaline washing or extraction stage (designated as an E stage). The products 15 of pulp chlorination and o~ the oxidative bleaching stages are more soluble in an alkaline medium than in water, and they are generally taken out ot lhe system by an a1kaline extraction. For the attainment of brigh~ness wi~h strenglh preservation, for brightness stability and bleaching economy, the reaction products resulting from chlorina~ion and oxidative bleaching are 20 removed as they are formed in those operations by means of alkaline extraction. Caustic soda is the preterred agent, but other alkalis have been used. Some examples ot multistage bleaching sequences include CEH, CEDP, CEDEDP and CEHH.
After a single or a multistage bleaching, it is common practice to 25 treat or ~sour'` the bleached pulp with sulfur dioxide. This SO2 treatment serves many purposes.

~28~

For example, in the case of a multistage bleaching performed using sodium hypochlorite or calcium hypochlorite, hydrogen peroxide and chlorine dioxide among others, the SO2 treatment destroys the active bleaching agents remaining from the hypochlorite, peroxide and chlorine dioxide stages. Also, 5 the SO2 treatment is required to bring the pulp solution to non-alkaline conditions in order to avoid indesirable losses with regard to brightness. It is well known that alkaline conditions tend to darken the pulp.
Presently, the pulp and paper industry universally employs SO2 dissolved in water to perform acidification or neutralization in both chemical 10 and mechanical pulp bleaching. However, the use of sulfur dioxide has the consequence ot des~roying all the active bleaching agent residues remaining from bleaching stages. In the case of a bleaching stage performed with chlorine dioxide, it is desirable to treat the bleaching waters with SO2 lo destroy the remaining bleaching agents.
However, when it comes to bleaching agents such as sodium or calcium hypochlorite and hydrogen peroxide, their destruction is by no means necessary because these compounds presen~ low loxicities and do not interfere with subsequent paper making operations. As a matter of fact, the presence of such residuals would be exlremely desirable because i~ wou)d allow the 20 bleaching process of the pulp lo slowly continue during subsequent pulp and paper making operations. This prolongation of lhe bleaching process would likely yield a final product possessing enhanced brigh~ness properties without increasing operational costs. It is also to be noted that these bleaching agents and their residuals also possess mild biocide properties that 25 allow for the preven~ion of bac~erial growth in ~he pulp solution. The 12~8~

proliferation of bacteria in the pulp solution usually leads to undesirable darkening of the pulp itself.
Furthermore, the use of aqueous SO2 solutions may present serious health risks for the workers handling the treated pulp as well as potential S environmental hazards that could be caused by the ~isposal of corrosive and polluted SO2 solutions in natural effluents.

Therefore, a souring agent possessing the property to diminate alkaline conditions while maintaining desirable bleaching agen~ residuals in the pulp solution would be highly desirable.
10 SUMMARY OF THE INVEI\ITION

Thus, in a process for the preparation of paper pulp comprising a single or mul~istage bleaching of the pulp comprising at leas~ one single or last peroxide bleaching stage or treatment followed by treatment of the bleached pulp wi~h aqueous sulfur dioxide, the presenS inven~ion c~nsis~s o~
15 substiluting carbonic acid for sulfur dioxide. Carbonic acid may be introduced in the form o~ CO2 gas and/or CO2 liquid in the dilulion and/or washing waters.
The process of the present invention may be used on either chemical.
mechanical or recycled pulp, regard)ess of the bleaching process. In o~her 20 words, the process of the present invention may be used afler a mullistage or a single stage bleaching process provided that the lasl or single slep is performed using sodium hypochlorite or calcium hypochlorite or hydrogen peroxide or equivalents thereof.
The process of the present invention is also suitable for 25 thermornechanical and chemical thermomechanical pulps which usually 1 2~8~0fi require a single stage bleaching.
Numerous interesting advantages result from the use of the process of the present invention. Thus, both the use of carbon dioxide and sulfur dioxide eliminate pulp darkening alkaline conditions by acidifying the resulting pulp slurry. However, while the reducing action of sulfur dioxide destroys all the bleaching agent residuals present in the pulp solution, carbon dioxide wiil not destroy residuals from bleaching agent such as sodium hypochlorite and calcium hypochlorite and hydrogen peroxide. Thus, the fact that these residuals can be maintained in the pulp solution will lead to a continuation of the bleaching process without necessitating further stages or further addition of bleaching agen~s, thereby increasing the brightness of the resulting pulp while maintaining the viscosity of the pulp solution at the same level. Furlhermore, the quality of the resulling pulp product will be enhanced thus leading to a more valuable product.
Therefore, the replacement of SO2 by CO2 will lead to much more efficient bleaching stages without increasing operational costs and operational times. Furthermore, the cost ol carbon dioxide can be as much as 2 ~o 10 times lower than the cosl of sulfur dioxide, making CO2 a very economical souring and pulp and paper treating agen~.
Another important advantage of the process of the present invention may be found at the environmental level. At the present time, the quantities of residual sulfur dioxide souring solution dumped in various effluents are extremely important and present serious threats as far as ecologica1 equilibrium of the neighbouring ecosystems is concerned. On the other hand, the use of CO2 does not present such risks because CO2 is far less soluble in water and exhibits no toxicity a~ the levels ernployed. Furthermore, CO2 is odorless whereas SO2 has a strong, unpleasant odor.

Finally, it is to be noted that because carbon dioxide does not destroy desirable bleaching agent residuals, these residuals will not only continue 5 their bleaching action but will also act as mild biocides and prevent proliferation of undesirable bacteria possessing the ability to darken the pulp.
The novel process of the present invention is carried out in the following manner. The pulp is first bleached using single or multistage 10 bleaching process known in the art. After the last or single bleaching step of the pulp, carbon dioxide is introduced either in the pulp slurry, the dilution waters or the washing waters as carbonic acid either in gaseous, liquid or solution form. The quantity of CO2 to be added will vary but because the solubi)ity of CO2 in wa~er is ra~her low, CO2 will usual1y be added unli) the 15 pulp solution reaches a saturated stage.
It is also possible to oversaturate the pulp slurry by either injecling C2 at a precise location or by performing several sequential addi~ions to the pulp solution. In any event, the amount of CO2 to be added to the pulp solution is not important in the sense that one will add CO2 until the desired degree of 20 alkalinity is reached. Therefore, the amount of CO2 injections required will depend on the type of bleaching stage performed as well as on the targetted final degree of alkalinity~ In standard procedures, alkalinity is measured by a pH-meter and CO2 addition is automatically stopped when the solution has been stabilized at the desired alkalinity. Hence, the CO2 treatment will allow '1288~fi the pH of the pulp solution ~o be stabili~ed between 4.0 and 7.5. It is to be noted that there is virtually no upper limit as to the starting pH of the bleached pulp solu~ion before the CO2 treatment is effected. In other words, it is possible for example to use the process of the present invention ~o bring the pH of a pulp solution from 13.5 to 4. Therefore, the pH of the pulp solution before treatment may be 13.5 or less.
The temperature at which the CO2 treatment or souring of the pulp will be performed may vary widely but will usually range between 5 and 80C. As for the time required to effect the CO2 treatment or souring of the lû pu)p, it varies between 30 seconds and 120 minutes. It is also desirable to have a pulp consistency ranging between 0.1 and 25%.
The process of the present invention will be more readily illustrated by referring to the following examples which are introduced in order to ~ s~ra~e ra~her ~han )imit ~he scope o~ the slaims.

Example 1 A lhermomechanlcal pulp slurry was bleached following a PRP
(Peroxide, reducing agent and peroxide) sequence. Atler Ihe second P
~Pero~ide) stage, ~he pulp P-1 had a brightness o~ 70.65 ISO. The pulp slurry was then trea~ed wi~h a hydrogen peroxide solution containing 2%
H22 with respect ~o the pulp, 1.5% NaOH, 1% Na2SiO3 and 0.05% MgSO4.

The pulp consistency was 10%, the reaction temperature 65C and the retention time 2 hours. The pH of the resulting pulp solution was 8.3 and the residual hydrogen peroxide concentralion was 1.3% with respect to the dry pulp. The pulp slurry was then concentrated to a consistency of 27% and 1288~

divided in two samples. SO2 water was added to the first sample in order to adjust the pH to 5.5 and to bring the consistency back to 10%. No hydrogen peroxide residue was found in this first sample. After sheet making, the brightness was measured and determined to be 80.05% ISO. The second 5 sample was treated with water containing dissolved carbon dioxide. The pulp was also diluted to a consistency of 10% and its pH was adjusted to 5.7. The hydrogen peroxide residual concentration was found to be identical ~o the concentration measured before the souring treatment. Furthermore, the brightness of the resulting sheet was measured and found to be 81.17% ISO.

10 Example 2 An unbieached thermomechanical pulp slurry having a brightness of 57.43% ISO was bleached using a peroxide bleaching stage. The peroxide solution contained the fallawin~ compounds: 2% H20~. 2% ~laOH, 2%
Na2SiO3, 0.5% MgSO4 and 0.2% DTPA (Sodium diethylene Irimine 15 pentaacetate). The pulp consistency was 10%, the temperalure was 85C and the relenlion lime 30 minules. The pH of lhe ~inal pulp solulion was found to be 8.1 and the peroxide residual concenlralion was equivalen~ ~o 0.61% H22 wilh regard ~o ~he dry pulp. The bleached pulp solulion was then divided inlo two samples. To the firsl sample, SO2 waler was added ~o adjust the pH to 20 5.4. No peroxide residual could be found afler the SO2 injection. The pulp slurry was then drained and brightness sheets were made according lo s~andard procedures. The brigh~ness was then measured and de~ermined to be 72.14% ISO. To the second pulp sample, dissolved CO2 was injecled inlo the ~89~6 pulp slurry until !he pH reached 5.6, The peroxide residual concentration was then determined to be 0.58%. The pulp slurry was drained and brightness sheets were again made according to standard procedures, The brightness of ~he sheets was delermined to be 74.24% ISO.

5 Example 3 A chemical softwood kraft pulp slurry was bleached by following a CDEoDP sequence. After the P stage, the pH was found to be 9.9 and ~he pulp solution was divided into two samples, One sample was treated with water S2 in order to adjus~ lhe pH to 6.0, The pulp slurry was lhen drained and 10 brigh~ness shee~s were prepared following s1andard pr~cedures. The brightness was determined to be 86.50% ISO. These brightness sheets were ~hen heated for one hour at ta50c. The brightness was rneasured again and found to be 83.24% ISO. The second sample of the pulp solution was ~reated with an aqueous CO2 solution in order to àdjust the pH to 6,3. The pulp 15 slutry was then drained and brightness sheets were made according to slandard procedures. The brightness was determined to be 87,16% ISO.
Agaln, the brlghtness sheets were heated for one hour at 1 05C . The brighlness a~ter heating was measured and determined to be 85.30% ISO.

Claims (8)

1. A process for the preparation of paper pulp comprising a single or multistage bleaching of a pulp slurry said process being characterized in that at least one single or last calcium hypochlorite or sodium hypochlorite or hydrogen peroxide bleaching stage is followed by treatment of the bleached pulp suspension with a carbonic acid solution as a souring agent.

2. Process according to Claim 1 characterized in that it comprises a washing step performed between the bleaching and carbonic acid souring steps.

3. Process according 10 Claim 1 and 2 characterized in that carbonic acid is in the form of gaseous or liquid CO2 and is introduced in the dilution and/or washing waters.

4. Process according to Claim 1 wherein carbonic acid is in the form of gaseous or liquid CO2 and is introduced in the bleached pulp suspension.

5. Process according to Claim 1 wherein the carbonic acid treatment is performed at a temperature ranging between 5 and 80°C.

6. Process according to Claim 1 wherein the carbonic acid treatment is performed over a period of time ranging between 30 seconds and 120 minutes.

7. Process according to Claim 1 wherein the consistency of the pulp in the pulp slurry ranges between 0.1% and 25%.

8. Process according to Claim 1 wherein the pH of the pulp solution before the CO2 treatment is 13.5 or less.