Note: Descriptions are shown in the official language in which they were submitted.
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
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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.
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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.
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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
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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
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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
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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
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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
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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.
