U.S. patent application number 13/125466 was filed with the patent office on 2012-05-17 for method for manufacturing papermaking pulp.
Invention is credited to Christophe Calais, Jean-Christophe Hostachy.
Application Number | 20120118518 13/125466 |
Document ID | / |
Family ID | 40677697 |
Filed Date | 2012-05-17 |
United States Patent
Application |
20120118518 |
Kind Code |
A1 |
Calais; Christophe ; et
al. |
May 17, 2012 |
METHOD FOR MANUFACTURING PAPERMAKING PULP
Abstract
The invention relates to a method for manufacturing papermaking
pulp, including: a step of refining wood, including an ozone
treatment, so as to obtain a mechanical pulp; a deacidification
step including placing the mechanical pulp in contact with a
deacidification composition including at least one alkaline agent
for more than one hour; and a bleaching step including placing the
deacidified pulp in contact with a bleaching composition; wherein
the pH varies by less than 0.2 point during the last 30 minutes of
the deacidification step, and the pH is 6. to 10 at the end of the
deacidification step.
Inventors: |
Calais; Christophe;
(Meyzieu, FR) ; Hostachy; Jean-Christophe;
(Taluyers, FR) |
Family ID: |
40677697 |
Appl. No.: |
13/125466 |
Filed: |
October 23, 2009 |
PCT Filed: |
October 23, 2009 |
PCT NO: |
PCT/IB2009/007208 |
371 Date: |
May 24, 2011 |
Current U.S.
Class: |
162/65 |
Current CPC
Class: |
D21C 9/10 20130101; D21C
9/147 20130101; D21B 1/16 20130101; D21C 9/153 20130101 |
Class at
Publication: |
162/65 |
International
Class: |
D21C 1/00 20060101
D21C001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 24, 2008 |
FR |
08 05907 |
Claims
1. A process for manufacturing a paper pulp, comprising: refining
wood, comprising an ozone treatment, so as to obtain a mechanical
pulp; contacting the mechanical with a deacidification composition
comprising at least one alkaline agent for more than 1 hour to form
a deacidified pulp; and contacting the deacidified pulp with a
bleaching composition, wherein the improvement comprises the pH
varies by less than 0.2 units during the last 30 minutes of the
contacting with the deacidification composition and the pH is
between 6 and 10 at the end of the contacting with the
deacidification composition.
2. The process as claimed in claim 1, in which the duration of the
contacting with the deacidification composition is equal to or
greater than 1 hour 30 minutes.
3. The process as claimed in claim 1, in which the pH at the end of
the contacting with deacidification composition is between 7 and
8.
4. The process as claimed in claim 1, in which the pH varies by
less than 0.1 units during the last 30 minutes of the contacting
with the deacidification composition.
5. The process as claimed in claim 1, in which the alkaline agent
of the deacidification composition: is selected from the group
consisting of oxides, hydroxides, silicates and carbonates of
alkaline-earth metals and alkali metals, ammonia, aqueous ammonia
and mixtures thereof.
6. The process as claimed in claim 1, in which the bleaching
composition comprises a bleaching agent and at least one alkaline
agent selected from the group consisting of oxides, hydroxides,
silicates and carbonates of alkaline-earth metals and alkali
metals, ammonia, aqueous ammonia and mixtures thereof.
7. The process as claimed in claim 1, which further comprisies
contacting the deacidified pulp with a chelation composition
comprising a chelating agent.
8. The process as claimed in claim 1, in which 20 kg or less of
alkaline agent per tonne of mechanical pulp is used.
9. The process as claimed in claim 1, in which the bleaching
composition comprises: 5 to 100 kg of bleaching agent per tonne of
mechanical pulp; and 5 to 100 kg of alkaline agent per tonne of
mechanical pulp.
10. The process as claimed in claim 1, in which the loss of wood
yield after contacting with the bleaching composition is less than
or equal to 20%.
11. The process as claimed in claim 1, in which the whiteness of
the pulp after contacting with the bleaching composition is equal
to or greater than 65.
12. The process as claimed in claim 1, producing effluents having a
chemical oxygen demand less than or equal to 210 kg per tonne of
pulp.
13. The process as claimed in claim 1, in which the mechanical pulp
is selected from the group consisting of SGW, PGW, RMP, TMP, HTMP
and CTMP pulp.
14. The process as claimed in claim 1, in which the wood is
selected from the group consisting of resinous wood, deciduous tree
wood and a mixture thereof, preferably a resinous wood and more
particularly preferably a pinewood or spruce wood.
15. (canceled)
16. The process as claimed in claim 1, in which the duration of the
contacting with the deacidification composition is equal to or
greater than 3 hours.
17. The process as claimed in claim 1, in which the pH varies by
less than 0.05 units during the last 30 minutes of the contacting
with the deacidification composition.
18. The process as claimed in claim 1, in which the alkaline agent
of the deacidification composition is selected from the group
consisting of potassium hydroxide, sodium hydroxide, magnesium
hydroxide, calcium hydroxide, sodium carbonate, magnesium
carbonate, sodium silicate and mixtures thereof.
19. The process as claimed in claim 1, in which the bleaching
composition comprises a bleaching agent and at least one alkaline
agent selected from the group consisting of sodium hydroxide,
magnesium hydroxide and mixtures thereof.
20. The process of claim 7 wherein said chelating agen is selected
from the group consisting of ethylenediaminetetraacetic acid and
one of its sodium salts and diethylenetriaminepentaacetic acid and
one of its sodium salts.
21. The process as claimed in claim 1, in which the bleaching
composition comprises: 20 to 40 kg of bleaching agent per tonne of
mechanical pulp; and 10 to 70 kg, of alkaline agent per tonne of
mechanical pulp.
22. The process as claimed in claim 1, in which the loss of wood
yield after contacting with the bleaching composition is less than
or equal to 10%.
23. The process as claimed in claim 1, in which the whiteness of
the pulp after the contacting with the bleaching composition is
equal to or greater than 70%.
24. The process as claimed claim 1, producing effluents having a
chemical oxygen demand less than or equal to 145 kg per tonne of
pulp.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a paper pulp manufacturing
process.
TECHNICAL BACKGROUND
[0002] Paper pulps called "mechanical pulps" or "high-yield pulps"
or "wood pulps" are obtained directly from wood by a series of
mechanical (defibering and refining) treatments carried out by
means of grinders and/or refiners. The pulp may then undergo a
bleaching phase in one or more steps.
[0003] It is known that an ozone treatment during pulp refining
helps to improve the quality of the pulp produced, in particular in
terms of physical properties (especially the mechanical strength of
the pulp) and to reduce the energy consumption of the process.
[0004] However, the ozone treatment raises particular problems,
namely a reduction in the whiteness of the pulp, a subsequent
bleaching difficulty, a loss of yield relative to the wood and an
increase in the chemical oxygen demand of the bleaching
effluents.
[0005] An ozone treatment as mentioned above is described in
document DE 2444475. Subsequent bleaching of the pulp is
mentioned.
[0006] Document FR 2329794 also describes an ozone treatment.
[0007] After the treatment, the treated pulp is mixed with a lye
(caustic soda) in order to stabilize the properties conferred on
the pulp by the ozone treatment. The pulp is then stored and
subsequently used directly without being bleached in order to
manufacture paper.
[0008] A mechanical pulp ozone treatment is described in document
FR 2406023. The treatment is followed by a maturing step lasting
less than 30 minutes, preferably less than 10 minutes, during which
the ozonated pulp remains in alkaline medium, optionally being
mixed with bleaching chemicals.
[0009] In documents FR 2388933 and FR 2388934, just as in the above
documents, a maturing step follows the ozonation, in the presence
of a lye (caustic soda) and optionally a bleaching agent (hydrogen
peroxide), for the purpose of stabilizing the pulp and optionally
bleaching it at the same time. Adding caustic soda to the ozonation
stage itself in order to swell the pulp is also envisaged.
[0010] Document EP 0276608 provides an illustration of an ozone
treatment method. The examples in said document show that the ozone
treatment causes a loss of whiteness of the pulp.
[0011] The document entitled "The effect of ozone on mechanical
pulps" by N. Soteland, Can. Wood Chem. Symp. Extended Abstracts,
pages 13-20 (1976) identifies a loss of whiteness problem in paper
pulps obtained from ozone-treated resinous wood. The author
indicates that by adjusting the pH of the ozonated pulp with
caustic soda and by adding DTPA it is possible to increase the
whiteness of the pulp. A loss of yield problem due to the ozonation
is also mentioned.
[0012] The document entitled "Ozone treatment of mechanical pulp,
Part III: influence on optical properties" by C. A. Lindholm, in
the journal Paperi ja puu-Papper och Tra{umlaut over ( )} (No. 4a,
1977, pages 217-232) describes the negative effect of ozonation on
the whiteness, whiteness stability and opacity of the paper pulp.
Various approaches are tested to solve the problem: modification of
the ozone treatment itself; caustic soda neutralization; and
heating of the ozonated pulp or ozonation accompanied by a
treatment with a mixture of hydrogen peroxide and caustic soda.
None of these approaches is deemed to be satisfactory both as
regards the mechanical strength of the pulp, and the optical
properties thereof.
[0013] The document entitled "Effect of ozone on high-temperature
thermomechanical pulp" by R. W. Allison, Appita Vol. 32, No. 4,
pages 279-284 (1979) studies the bleaching of ozonated paper pulp
(of the HTMP type) and notes the influence of the pH of the
treatments after ozonation on the yield of the process: more
precisely, subsequent bleaching under alkaline conditions leads to
a significant loss of yield. Extraction with caustic soda for a
short time of 30 minutes after the ozonation and before the
hydrogen peroxide bleaching is proposed. It should be noted that
with this process the observed pulp yields and/or the whiteness
levels achieved are low despite the use of a very large amount of
chemicals.
[0014] The document entitled "Effect of neutralization on the
bonding ability of ozone-treated mechanical pulp fibers", by C. A.
Lindholm, Cellulose Chem. Technol., Vol. 17, pages 647-653 (1983)
studies in detail in what way neutralization of ozonated mechanical
pulp with caustic soda is liable to impart advantageous mechanical
properties on said pulp.
[0015] The document entitled "Ozone in the production of softwood
and hardwood high-yield pulps to save energy and improve quality"
by M. Petit-Conil, C. de Choudens and T. Espilit, Nordic Pulp and
Paper Research Journal, Vol. 13, No. 1, pages 16-22 (1998), studies
the influence of an ozone treatment followed by hydrogen peroxide
bleaching in the presence of caustic soda on pulps prepared from
various types of wood essence. This document does not address the
loss of yield observed during bleaching.
[0016] The document entitled "Use of ozone in mechanical pulping
processes" by M. Petit-Conil, ATTP, Vol. 57, No. 2, pages 17-26
(2003) compares the effect of various ways of introducing ozone.
That document too does not address the loss of yield observed
during bleaching.
[0017] Document WO 2008/081078 describes the use of caustic soda
and other bases during the ozone treatment itself, so as to limit
the deleterious effect of the organic acids produced during
ozonation on the subsequent bleaching and on corrosion.
[0018] However, the methods used in the prior art are not
completely satisfactory.
[0019] In particular, there is still a need to obtain a better
yield and/or achieve high whiteness levels and/or to reduce the
amount of chemicals necessary for the paper pulp treatment and/or
to reduce the amount of effluents generated by the bleaching.
SUMMARY OF THE INVENTION
[0020] A first subject of the invention is a process for
manufacturing a paper pulp, comprising: [0021] a wood refining
step, comprising an ozone treatment, so as to obtain a mechanical
pulp; [0022] a deacidification step, in which the mechanical pulp
is brought into contact with a deacidification composition
comprising at least one alkaline agent for more than 1 hour; and
[0023] a bleaching step in which the deacidified pulp is brought
into contact with a bleaching composition, in which the pH varies
by less than 0.2 units during the last 30 minutes of the
deacidification step and the pH is between 6 and 10 at the end of
the deacidification step.
[0024] According to one method of implementation, the
deacidification step is equal to or greater than 1 hour 30 minutes,
preferably equal to or greater than 2 hours and ideally equal to or
greater than 3 hours.
[0025] According to one method of implementation, the pH at the end
of the deacidification step is between 7 and 8.
[0026] According to one method of implementation, the pH varies by
less than 0.1 units during the last 30 minutes of the
deacidification step, preferably by less than 0.05 units.
[0027] According to one method of implementation, the alkaline
agent of the deacidification composition: [0028] is chosen from
oxides, hydroxides, silicates and carbonates of alkaline-earth
metals and alkali metals, ammonia, aqueous ammonia and mixtures
thereof; [0029] is preferably chosen from potassium hydroxide,
sodium hydroxide, magnesium hydroxide, calcium hydroxide, sodium
carbonate, magnesium carbonate, sodium silicate and mixtures
thereof; and [0030] is ideally magnesium hydroxide.
[0031] According to one method of implementation, the bleaching
composition comprises a bleaching agent and at least one alkaline
agent: [0032] said alkaline agent being chosen from the oxides,
hydroxides, silicates and carbonates of alkaline-earth metals and
alkali metals, ammonia, aqueous ammonia and mixtures thereof;
[0033] said alkaline agent preferably being chosen from potassium
hydroxide, sodium hydroxide, magnesium hydroxide, calcium
hydroxide, sodium silicate, sodium carbonate, magnesium carbonate
and mixtures thereof; and [0034] said alkaline agent ideally
comprising sodium hydroxide and/or magnesium hydroxide.
[0035] According to one method of implementation, the process
includes a chelation step, preferably between the deacidification
step and the bleaching step, in which the deacidified pulp is
brought into contact with a chelation composition comprising a
chelating agent, preferably ethylenediaminetetraacetic acid or one
of its sodium salts or diethylenetriaminepentaacetic acid or one of
its sodium salts.
[0036] According to one method of implementation, 20 kg or less but
preferably less than 15 kg or less and more particularly preferably
10 kg or less of alkaline agent per tonne of mechanical pulp is
used during the deacidification step.
[0037] According to one method of implementation: [0038] 5 to 100
kg, preferably 10 to 50 kg and more particularly preferably 20 to
40 kg of bleaching agent per tonne of mechanical pulp; and [0039] 5
to 100 kg but preferably 10 to 70 kg, of alkaline agent per tonne
of mechanical pulp are used during the bleaching step.
[0040] According to one method of implementation, the loss of wood
yield after the bleaching step is less than or equal to 20%,
preferably less than or equal to 16%, ideally less than or equal to
13% or even less than or equal to 10%.
[0041] According to one method of implementation, the whiteness of
the pulp after the bleaching step is equal to or greater than 65%,
preferably equal to or greater than 66.3%, ideally equal to or
greater than 68% or even equal to or greater than 70%.
[0042] According to one method of implementation, the process
produces effluents having a chemical oxygen demand less than or
equal to 210 kg per tonne of pulp, preferably less than or equal to
180 kg per tonne of pulp, or particularly preferably less than or
equal to 160 kg per tonne of pulp or even less than or equal to 145
kg per tonne of pulp.
[0043] According to one method of implementation, the mechanical
pulp is an SGW, PGW, RMP, TMP, HTMP or CTMP pulp.
[0044] According to one method of implementation, the wood is a
resinous wood, a deciduous tree wood or a mixture thereof,
preferably a resinous wood and more particularly preferably a
pinewood or spruce wood.
[0045] Another subject of the invention is a paper manufacturing
process, comprising the manufacture of paper pulp according to the
process described above, and the use of this paper pulp for
producing paper.
[0046] The present invention makes it possible to overcome the
drawbacks of the prior art. More particularly, it provides a more
effective paper pulp bleaching process having a better yield and/or
using a small amount of chemicals and/or generating less effluent
to be treated.
[0047] This is accomplished thanks to a deacidification step
between the ozone treatment and the bleaching, carried out for a
long enough time until the pH is sufficiently stabilized.
[0048] Specifically, after the ozonation, the acids within the
fibers tend to be progressively released. The nature and the
duration of the deacidification according to the invention are
therefore adapted so as to obtain a stable pH before the bleaching
step, thereby making it possible to neutralize all the acid species
and obtain better bleaching.
[0049] Depending on certain particular methods of implementation,
the invention also has one or more of the advantageous
characteristics mentioned below: [0050] the invention makes it
possible to obtain a paper pulp bleaching efficiency equivalent to
or greater than that of the prior art (with a whiteness parameter
of the same order or even higher); [0051] using a mild base
(especially magnesium hydroxide, calcium hydroxide, sodium
carbonate, magnesium carbonate, sodium silicate or mixtures
thereof, and preferably magnesium hydroxide) as the alkaline agent
of the deacidification composition, the yield and/or the reduction
in the amount of chemicals necessary and the amount of effluent to
be treated and/or the whiteness are further improved; [0052] also
using a mild base during the bleaching step, the yield, the
reduction of the amount of chemicals necessary and the amount of
effluent to be treated, or even the whiteness, are further
improved; and [0053] the mechanical properties of the pulps
obtained using the process of the invention are as satisfactory as
those obtained in the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] FIG. 1 shows an example of the variation in pH during a
deacidification step according to the invention (experimental
measurement), in which the time in minutes is plotted on the x-axis
and the pH is plotted on the y-axis.
DESCRIPTION OF METHODS OF IMPLEMENTATION OF THE INVENTION
[0055] The invention will now be described in greater detail, but
in a nonlimiting manner, in the following description.
[0056] Schematically, the paper pulp manufacturing process
according to the invention comprises four successive steps: [0057]
wood refining, comprising an ozone treatment, so as to obtain a
mechanical pulp; [0058] deacidification, in which the mechanical
pulp is brought into contact with a deacidification composition
comprising at least one alkaline agent; [0059] optionally,
chelation, in which the pulp is brought into contact with a
chelation composition comprising a chelating agent; and [0060]
bleaching, in which the pulp is brought into contact with a
bleaching composition.
[0061] The expression "the pH varies by less than X units during
the last 30 minutes of the deacidification step" means that the
absolute value of the difference between the measured pH at the end
of the deacidification step and the pH measured 30 minutes before
the end of the deacidification step is less than X units.
[0062] The "end of the deacidification step" coincides with the
start of the next step. The start of the next step may denote the
start of the chelation step or the start of the bleaching step, but
it may also denote the start of an intermediate washing step or the
start of an intermediate change-of-consistency step. In general,
"the next step" corresponds to a new action that differs from the
deacidification action and comes immediately thereafter.
[0063] First step: Refining and Ozone Treatment
[0064] The wood refining and ozone treatment are carried out in the
manner conventionally employed in the field.
[0065] The raw material is any kind of suitable wood, especially
resinous wood, deciduous tree wood or a mixture of the two.
Preferably, it is resinous wood. Examples of appropriate wood
essences include pine and spruce.
[0066] Before refining, the wood may undergo one or more
preliminary treatments, for example debarking, removal of wood
knots, heat treatment, pressurization, cutting into chips, shives,
logs or the like.
[0067] The refining comprises grinding (defibering) in a grinder
(under a stream of water) or in a disk refiner.
[0068] The refining may comprise several stages. For example, after
a first grinding/defibering, the product may be separated into an
accepted fraction and a rejected fraction, the rejected fraction
then being refined anew before being mixed with the accepted
fraction. It is possible to carry out such intermediate separations
several times.
[0069] During refining, an ozone treatment (ozonation) is carried
out to make it easier to separate the wood fibers. The ozone
treatment therefore helps to reduce the specific energy needed to
separate the fibers and to increase the mechanical properties of
the paper pulp thus treated. The ozone treatment is carried out
according to the methods known to those skilled in the art, for
example according to the methods described in the documents of the
prior art mentioned in the introduction of the present application.
The ozone loading is typically 0.5 to 8%, preferably 1 to 4%. The
ozone treatment may relate to all of the pulp or only a fraction
thereof, for example the rejected fraction from the separation
after the first refining step.
[0070] Optionally, the pH may be adjusted during the ozonation or
before the ozonation, for example by adding an alkaline agent,
especially sodium hydroxide. Thus, it is possible to adjust the pH
of the entire pulp after a first refining step or the pH of the
rejected fraction after the separation, for example between 9 and
11. The reader may refer on this subject to the document WO
2008/081078.
[0071] The addition of an alkaline agent at this stage serves to
minimize the drop in pH during the ozone treatment. This drop in pH
results from the appearance and accumulation of acid organic
by-products that are generated by the oxidizing action of ozone on
the constituents of the wood. It should be noted that, even in the
case in which the pH is adjusted before ozonation, the pulp is
highly acid after the ozonation: the pH is between 5 and 7
immediately after the ozonation step. After the second successive
ozonation refining step, the acid organic by-products are released
into the stock, which has a very acid pH (generally between 3 and
4).
[0072] After the ozone refining/treatment, a mechanical pulp is
obtained that may be of any of the types known to those skilled in
the art, namely in particular: [0073] a stone groundwood (SGW) pulp
obtained from logs or blocks treated at atmospheric pressure by
defibrator grinders; [0074] a pressure groundwood (PGW) pulp
obtained from logs or blocks treated under pressure using
defibrator grinders; [0075] a refiner mechanical pulp (RMP)
obtained from chips or shives in refiners working at atmospheric
pressure; [0076] a thermomechanical pulp (TMP) or high-temperature
thermomechanical pulp (HTMP) obtained from chips or shives in
refiners after heat treatment of the wood at high vapor pressure;
and [0077] a chemithermomechanical pulp (CTMP) obtained by
defibering under pressure followed by chemical impregnation in the
presence of caustic soda and sodium bisulphite at a temperature
above 100.degree. C.
[0078] Second Step: Deacidification
[0079] The deacidification preferably takes place after the ozone
refining/treatment, that is to say once the ozone
refining/treatment has been completed. It consists in neutralizing
the acid by-products generated by the action of ozone on the
constituents of the wood, which acid by-products accumulate within
and on the outside (immediate environment) of the wood fibers. Even
if a base were to be used before or during the ozonation, the
deacidification involved here is in any case useful in order to
neutralize the acid species, since the acids within the fibers are
released after the ozonation, and the stock tends to become ever
more acid over the course of time.
[0080] The deacidification comprises, preferably consists of,
bringing the mechanical pulp from the first step into contact with
a deacidification composition.
[0081] The contacting operation preferably takes place by simple
mixing. Typically, before the deacidification the pH lies within a
3 to 5 pH range, depending on the ozone treatment (amount of ozone,
optional addition of caustic soda before or during the ozonation).
At the end of deacidification, the pH lies in the 6 to 12,
preferably 7 to 8, range.
[0082] The deacidification composition is preferably an aqueous
solution comprising an alkaline agent. This alkaline agent may be
chosen from the oxides, hydroxides, silicates and carbonates of
alkali metals and alkaline-earth metals, ammonia, aqueous ammonia
and mixtures thereof. Among the preferred basic species to be
chosen as alkaline agent, mention may be made of potassium
hydroxide, sodium hydroxide, magnesium hydroxide, calcium
hydroxide, sodium carbonate, sodium silicate, magnesium carbonate
and mixtures thereof.
[0083] According to a preferred method of implementation, the
alkaline agent comprises a mild base. The term "mild base" is
understood to mean a base that dissociates only partially in water
(pKb>0) and/or has a low solubility. In particular, the alkaline
agent may comprise a mild base chosen from magnesium hydroxide,
calcium hydroxide, sodium carbonate, sodium silicate, magnesium
carbonate and mixtures thereof, preferably magnesium hydroxide.
[0084] According to one method of implementation, the alkaline
agent comprises a mild base as mentioned above in combination with
another base, for example sodium (or potassium) hydroxide. However,
it is preferable for the deacidification composition not to contain
sodium (or potassium) hydroxide and for the alkaline agent to
consist of a mild base as defined above.
[0085] Compared with sodium or potassium hydroxide, the use of a
mild base such as magnesium hydroxide minimizes the extraction and
dissolution of certain oxidized constituents of the wood after the
ozone action. Therefore, the loss of yield is smaller and the
polluting load resulting from the extraction is reduced.
[0086] The amount of alkaline agent used during the deacidification
step is preferably less than or equal to 20 kg, more particularly
preferably less than or equal to 15 kg and ideally less than or
equal to 10 kg per tonne of mechanical pulp.
[0087] The duration of the deacidification step is preferably
greater than 1 hour, more particularly preferably greater than or
equal to 1 hour 30 minutes, ideally greater than or equal to 2
hours or even greater than or equal to 3 hours, so as to optimize
the positive effect of this step on the total yield, on the
production of effluents and on the whiteness.
[0088] The deacidification step is preferably carried out at a
temperature between 4 and 95.degree. C., more particularly
preferably between 25 and 85.degree. C. and ideally between 35 and
80.degree. C. A temperature of about 70.degree. C. is particularly
appropriate.
[0089] The consistency during the deacidification step is
preferably between 0.5 and 10%, more particularly preferably
between 1 and 5% and ideally between 2 and 3%. The consistency or
dryness value of the pulp corresponds to the percentage of dry
matter expressed in relation to water.
[0090] The consistency is the ratio of the mass of pulp to the mass
of stock which comprises the pulp, water and added chemical
compounds. In other words, this is the mass concentration of pulp
in the aqueous medium.
[0091] Third Step (Optional): Chelation
[0092] The chelation preferably takes place after the
deacidification, i.e. once the deacidification step has been
completed.
[0093] The chelation comprises, preferably consists of, bringing
the mechanical pulp from the second step into contact with a
chelation composition.
[0094] The chelation composition is preferably an aqueous solution
comprising a chelating agent. The chelating agent may be any
chemical compound conventionally used for this purpose in the
field. Preferably, it is ethylenediaminetetraacetic acid or one of
its sodium salts or diethylenetriaminepentaacetic acid or one of
its sodium salts.
[0095] The chelating agent has a particular affinity for the metal
cations present in trace amounts in the paper pulp. The objective
of the chelation treatment is to neutralize these cations by
sequestering them and removing them from the pulp by washing the
latter. Implementing the chelation step helps to improve the
performance of the hydrogen peroxide bleaching treatment.
[0096] The amount of chelating agent used in the chelation step is
typically between 0.5 and 30 kg, preferably between 1 and 20 kg,
more particularly preferably between 2 and 10 kg and ideally
between 3 and 5 kg per tonne of mechanical pulp.
[0097] The duration of the chelation step is preferably equal to or
greater than about 30 minutes.
[0098] The chelation step is preferably carried out at a
temperature between 4 and 95.degree. C., more particularly
preferably between 25 and 85.degree. C. and ideally between 35 and
80.degree. C. A temperature of about 70.degree. C. is particularly
appropriate.
[0099] The consistency during the chelation step is preferably
between 0.5 and 10%, more particularly preferably between 1 and 5%
and ideally between 2 and 3%.
[0100] It should be noted that the chelation step may also be
carried out at the same time as the deacidification (and not
thereafter) in order to limit the number and volume of the
equipment items necessary for implementing the process. In this
case, all that is required is to bring the pulp into contact with
the chelating agent at the deacidification stage, either by adding
the latter separately or by including it in the deacidification
composition, preferably in the amounts indicated above.
[0101] Fourth Step: Bleaching
[0102] The bleaching is preferably carried out after the chelation
(or after the deacidification if there is no chelation), i.e. once
the chelation step (or the deacidification step if there is no
chelation) has been completed.
[0103] The bleaching comprises, preferably consists of, bringing
the mechanical pulp from the third step into contact with a
bleaching composition.
[0104] The consistency during the bleaching step is preferably
between 5 and 50%, more particularly preferably between 10 and 45%
and ideally between 20 and 40%.
[0105] The bleaching rate is more rapid at high consistency
(whereas the chelation rate is rapid even at low consistency). The
consistency of the pulp may be increased, for example by pressing
it and eliminating filtrates, comprising especially chelated
metals.
[0106] Preferably, the contacting step takes place by the bleaching
composition being simply mixed with the pulp. The type of apparatus
used for the mixing operation is adapted according to the
consistency of the pulp: direct mixing by means of an injection
pump if the consistency is low or moderate (less than 10%), while a
mixer is used for higher consistency (up to about 40%).
[0107] The bleaching composition is preferably an aqueous solution
comprising a bleaching agent and an alkaline agent.
[0108] The bleaching agent may be any chemical compound
conventionally used for this purpose in the field. Preferably, it
is hydrogen peroxide, but sodium hydrosulphite may also be
used.
[0109] According to one method of implementation, the alkaline
agent may comprise (or consist of) a mild base as defined in the
second step.
[0110] According to one method of implementation, the alkaline
agent may comprise (or consist of) a base different from a mild
base as defined in the second step, for example it may comprise (or
consist of) sodium (or potassium) hydroxide.
[0111] According to a preferred method of implementation, the
alkaline agent may comprise (or consist of) a mixture of a mild
base, as defined in the second step, and a base different from a
mild base (for example sodium or potassium hydroxide). A mixture of
sodium hydroxide and magnesium hydroxide constitutes a preferred
alkaline agent in the bleaching composition.
[0112] In this case, according to one method of implementation, the
ratio by weight of the mild base to the base different from a mild
base (for example the ratio by weight of sodium hydroxide to
magnesium hydroxide) is between 0.001 and 1000, preferably between
0.01 and 100, more particularly preferably between 0.1 and 10 and
ideally between 0.2 and 5.
[0113] The presence of a mild base in the bleaching composition
minimizes the extraction and dissolution of certain oxidized
constituents of the wood after the action of the ozone. As a
consequence, the use of a mild base makes it possible to further
improve the wood yield, to reduce the effluents and the consumption
of chemicals or even to improve the whiteness of the pulp. In
practice, the ratio by weight of the mild base to the base
different from a mild base generally results from a compromise
between whiteness and yield.
[0114] Preferably, the alkaline agent of the bleaching composition
comprises sodium silicate. Sodium silicate has the additional
function of stabilizing the bleaching agent (especially hydrogen
peroxide). It is also possible to provide in the bleaching
composition another stabilizing agent in addition to or in place of
the sodium silicate. Polyhydroxyacrylate-type compounds constitute
possible stabilizing agents.
[0115] The amount of bleaching agent used is typically between 5
and 100 kg, preferably between 10 and 50 kg and more particularly
preferably between 20 and 40 kg per tonne of mechanical pulp.
[0116] The amount of alkaline agent used is typically between 5 and
100 kg, preferably between 10 and 70 kg per tonne of mechanical
pulp.
[0117] The bleaching composition may also comprise a chelating
agent as defined above, especially if the chelation step is absent
or has resulted in incomplete chelation.
[0118] It should be noted that the bleaching composition may be
prepared separately and then brought into contact with the pulp,
but it may also be prepared directly upon contact with the pulp. In
the second case, the various compounds of the bleaching composition
are added in succession directly to the pulp.
[0119] The duration of the bleaching step varies depending on the
type of agent used.
[0120] In the case of hydrogen peroxide, this duration is typically
between 10 minutes and 8 hours, preferably between 30 minutes and 6
hours and more particularly preferably between 2 hours and 4
hours.
[0121] The bleaching step is preferably carried out at a
temperature between 4 and 95.degree. C., more particularly
preferably between 25 and 85.degree. C. and ideally between 35 and
80.degree. C. A temperature of about 70.degree. C. is particularly
appropriate.
[0122] After the process, a paper pulp is obtained that preferably
has one or more of the following characteristics: [0123] the
whiteness is equal to or greater than 65%, preferably equal or
greater than 66.5%, ideally equal to or greater than 68% or even
equal to or greater than 70%; [0124] the loss of wood yield of the
process is less than or equal to 20%, preferably less than or equal
to 16%, ideally less than or equal to 13% or even less than or
equal to 10%; [0125] the chemical oxygen demand (COD) of the
effluents is less than or equal to 210 kg per tonne of pulp,
preferably less than or equal to 180 kg per tonne of pulp, more
particularly preferably less than or equal to 160 kg per tonne of
pulp or even less than or equal to 145 kg per tonne of pulp.
[0126] In the context of the present application, the whiteness of
the pulp is defined according to the ISO 2470 standard.
[0127] In the context of the present invention, the loss of wood
yield of the process is defined as follows: after each step
(deacidification, chelation, bleaching), a 25% fraction of the
aqueous filtrate is recovered by pressing the stock. The water is
evaporated by heating the filtrate at 50.degree. C. until a dry
residue of constant mass (corresponding to the residue dissolved in
the filtrate and therefore lost) is obtained. The loss of yield of
the step in question is calculated in the following manner:
[0128] Loss of yield step=(100/25).times.(mass of residue at the
end of the step)/mass of initial pulp [after the refining
step].
[0129] The loss of yield of the process is the sum of the losses of
yield observed for each of the deacidification (when implemented),
chelation and bleaching steps. This loss of yield does not take
into account the ozonation step.
[0130] In the context of the present application, the chemical
oxygen demand of the effluents is defined according to the ISO
15705 standard, the measurement being carried out using a Hach
DR/2000 spectrophotometer.
EXAMPLES
[0131] The following examples illustrate the invention, but without
limiting it.
Example 1
[0132] A pine pulp was prepared using the TMP process. The pulp
underwent an ozone treatment during the refining phase. More
precisely, the reject from the primary refiner was treated with 2%
ozone, the pH being adjusted by adding 5 kg of caustic soda per
tonne of pulp, and then the ozone-treated reject was mixed with the
accepted fraction from the secondary refiner. The whiteness of the
pulp after the ozone refining/treatment was 47.3%.
[0133] After the ozone refining/treatment, a chelation step was
provided, during which the pulp was treated with 4 kg of DTPA per
tonne of pulp, with a consistency of 4% and at a temperature of
70.degree. C. and a pH of 6-7 for 1 hour.
[0134] After the chelation step, the pulp underwent a bleaching
step, with a consistency of 20%, at a temperature of 70.degree. C.
for a time of 2 hours.
[0135] To prepare pulp A (comparative example), the following
parameters were also used: [0136] no deacidification step between
the ozone refining/treatment and the chelation; and [0137]
bleaching composition: [0138] hydrogen peroxide: sufficient amount
for 30 kg/tonne of pulp, [0139] sodium silicate: sufficient amount
for 30 kg/tonne of pulp and [0140] sodium hydroxide: sufficient
amount for 42.5 kg/tonne of pulp.
[0141] To prepare pulp B, the following parameters were also used:
[0142] a deacidification step lasting 3 hours between the ozone
refining/treatment and the chelation, by means of kg of sodium
hydroxide per tonne of pulp (7.3 pH at the end of the treatment);
and [0143] a bleaching composition identical to that used for pulp
A except for the amount of sodium hydroxide: 32.5 kg per tonne of
pulp instead of 42.5 kg.
[0144] To prepare pulp C, the following parameters were also used:
[0145] a deacidification step lasting 3 hours between the ozone
refining/treatment and the chelation, by means of 7 kg of magnesium
hydroxide per tonne of pulp (7.2 pH at the end of the treatment);
and [0146] a bleaching composition identical to that used for pulp
B.
[0147] To prepare pulp D, the following parameters were also used:
[0148] a deacidification step identical to that used for pulp C
(7.2 pH at the end of the treatment); and [0149] a bleaching
composition identical to that used for pulp A except for the amount
of sodium hydroxide (8 kg instead of 42.5 kg per tonne of pulp) and
the addition of 13.5 kg of magnesium hydroxide per tonne of
pulp.
[0150] The whiteness of the pulp at the end of the process, the
loss of yield relative to the initial wood and the total chemical
oxygen demand (COD) generated were measured on each pulp. The
results are given Table 1 below.
TABLE-US-00001 TABLE 1 Parameters measured on pulps A to D COD per
tonne of Pulp Whiteness Loss of yield pulp A .sup. 68% 13.2% 185 kg
B 70.7% 12.7% 178 kg C 70.2% 12.2% 171 kg D 65.9% .sup. 10% 140
kg
[0151] The comparison between pulp A on the one hand and pulp B, C
and D on the other, shows that the presence of the deacidification
step improves the whiteness of the pulp and the wood yield of the
process and reduces the COD.
[0152] The comparison between pulp B and pulp C shows that the use
of magnesium hydroxide in place of sodium hydroxide during the
deacidification step, with a reduction of 30% in the amount of
chemical necessary, improves the wood yield of the process and
reduces the COD, with only a slightly lower whiteness.
[0153] The comparison between pulp C and pulp D shows that a
reduction of about 75% in the mass of sodium hydroxide used in the
bleaching step, and replacing about 41% of this sodium hydroxide
with magnesium hydroxide enables the loss of yield and the COB to
be reduced by 20%.
[0154] As regards the manufacture of pulp B, an experiment was also
carried out to measure the pH during the deacidification step, by
prolonging the latter beyond 3 hours. The result of this experiment
is given in Table 2 below, and also in FIG. 1. Time t=0 corresponds
to the start of the deacidification.
TABLE-US-00002 TABLE 2 Change in pH during the deacidification step
(pulp B) Time (in min) pH 0 4.06 1 10.25 4 9.93 9 9.33 19 8.59 30
8.27 60 7.81 90 7.56 120 7.37 150 7.28 220 7.27 260 7.25
[0155] From this example it may be seen that the initially acid pH
becomes highly basic in the first minute of deacidification.
Subsequently, the progressive release of acids by the fibers leads
to a slow reduction in the pH, which stabilizes only after 2 to 3
hours.
Example 2
[0156] A pine pulp was prepared using the TMP process. The pulp
underwent an ozone treatment during the refining phase. More
precisely, the reject from the primary refiner was treated with 2%
ozone, the pH not being adjusted, and then the ozone-treated reject
was mixed with the accepted fraction from the secondary refiner.
The whiteness of the pulp after the ozone refining/treatment was
48.3%.
[0157] After the ozone refining/treatment, a chelation step was
provided, during which the pulp was treated with 4 kg of DTPA per
tonne of pulp, with a consistency of 2-3% and at a temperature of
70.degree. C. and a pH of 6-7 for 1 hour.
[0158] After the chelation step, the pulp underwent a bleaching
step, with a consistency of 20%, at a temperature of 70.degree. C.
for a time of 2 hours.
[0159] To prepare pulp E, the following parameters were also used:
[0160] a deacidification step lasting 3 hours between the ozone
refining/treatment and the chelation, by means of 20 kg of sodium
hydroxide per tonne of pulp (7.4 pH at the end of the treatment);
and [0161] bleaching composition: [0162] hydrogen peroxide:
sufficient amount for 30 kg/tonne of pulp, [0163] sodium silicate:
sufficient amount for 30 kg/tonne or pulp and [0164] sodium
hydroxide: sufficient amount for 22.5 kg/tonne of pulp.
[0165] To prepare pulp F, the following parameters were also used:
[0166] a deacidification step lasting 3 hours between the ozone
refining/treatment and the chelation, by means of 9 kg of magnesium
hydroxide per tonne of pulp (7.3 pH at the end of the treatment);
and [0167] a bleaching composition identical to that used for pulp
E.
[0168] To prepare pulp G, the following parameters were also used:
[0169] a deacidification step lasting 3 hours between the ozone
refining/treatment and the chelation, by means of 12 kg of
magnesium hydroxide per tonne of pulp (7. 6 pH at the end of the
treatment); and [0170] a bleaching composition identical to that
used for pulp E.
[0171] To prepare pulp H, the following parameters were also used:
[0172] a deacidification step identical to that used for pulp G
(7.6 pH at the end of the treatment); and [0173] a bleaching
composition identical to that used for pulp E except for the amount
of sodium hydroxide (5.6 kg per tonne of pulp instead of 22.5 kg)
and the addition of 9 kg of magnesium hydroxide per tonne of
pulp.
[0174] The whiteness of the pulp at the end of the process, the
loss of yield relative to the initial wood and the total chemical
oxygen demand (COD) generated were measured on each pulp. The
results are given in Table 3 below.
TABLE-US-00003 TABLE 3 Parameters measured on pulp E to H COD per
tonne of Pulp Whiteness Loss of yield pulp E 68.0% 15.9% 222 kg F
68.1% 15.1% 212 kg G 68.2% 14.6% 205 kg H 68.4% 12.8% 179 kg
[0175] Comparing pulp E on the one hand and pulp F and G on the
other shows that the use of magnesium hydroxide in place of sodium
hydroxide, with a reduction of 55% or 40% in the amount of chemical
necessary, improves the wood yield of the process and markedly
reduces the COD, but also slightly improves the whiteness.
[0176] Comparing pulp G with pulp H shows that the approximately
75% reduction in the weight of caustic soda used in the bleaching
step and the replacement of about 40% of the caustic soda with
magnesium hydroxide also markedly improves all the parameters of
interest, namely the whiteness of the pulp, the yield and the
COD.
Example 3
[0177] A spruce pulp was prepared using the TMP process. The pulp
underwent an ozone treatment during the refining phase. More
precisely, the reject from the primary refiner was treated with 2%
ozone, the pH not being adjusted, and then the ozone-treated reject
was mixed with the accepted fraction from the secondary refiner.
The whiteness of the pulp after the ozone refining/treatment was
51.2%.
[0178] After the ozone refining/treatment, a chelation step was
provided, during which the pulp was treated with 4 kg of DTPA per
tonne of pulp, with a consistency of 2-3% and at a temperature of
70.degree. C. and a pH of 6-7 for 1 hour.
[0179] After the chelation step, the pulp underwent a bleaching
step, with a consistency of 20%, at a temperature of 70.degree. C.
and for a time of 2 hours.
[0180] To prepare pulp I (comparative example) the following
parameters were also used: [0181] a deacidification step lasting 5
to 10 minutes between the ozone refining/treatment and the
chelation, by means of 10 kg of sodium hydroxide per tonne of pulp;
and [0182] bleaching composition: [0183] hydrogen peroxide:
sufficient amount for 30 kg/tonne of pulp, [0184] sodium silicate:
sufficient amount for 30 kg/tonne of pulp and [0185] sodium
hydroxide: sufficient amount for 27.5 kg/tonne of pulp.
[0186] To prepare pulp J, the following parameters were also used:
[0187] a deacidification step identical to that for pulp I, except
that it lasted 3 hours; and [0188] a bleaching composition
identical to that used for pulp I, except that the amount of sodium
hydroxide used was 22.5 kg/tonne of pulp.
[0189] To prepare pulp K (comparative example) the same parameters
as for pulp I were used, except that there was no deacidification
step and the amount of sodium hydroxide used in the bleaching
composition was 37.5 kg/tonne of pulp.
[0190] To prepare pulp L (comparative example), the same parameters
as for pulp J were used, except that the duration of the
deacidification step was only 30 minutes.
[0191] To prepare pulp M, the same parameters as for pulp J were
used, except that the duration of the deacidification step was 90
minutes.
[0192] The whiteness of the pulp at the end of the process, the
loss of yield in relation to the initial wood and the total
chemical oxygen demand (COD) generated were measured on each pulp.
The results are given in Table 4 below.
TABLE-US-00004 TABLE 4 Parameters measured on pulps I to M COD per
tonne of Pulp Whiteness Loss of yield pulp I 72.4% 9.6% 134 kg J
73.0% 9.2% 128 kg K .sup. 70% 9.6% 134 kg L 71.2% 9.1% 127 kg M
72.7% 9.2% 128 kg
[0193] Comparing pulp I or pulp K on the one hand with pulp J or
pulp M on the other shows that by extending the duration of the
deacidification step sufficiently to stabilize the pH it is
possible to improve the final whiteness while reducing the amount
of reagents used in the bleaching step. In contrast, the test
carried out on pulp L, compared with the tests on pulps J and M,
demonstrate that, for the same amount of reactants, a relatively
short deacidification step (lasting 30 minutes) gives a lower
whiteness than a long deacidification step (lasting 90 minutes or
more) for a similar loss of yield.
* * * * *