U.S. patent application number 10/504806 was filed with the patent office on 2005-07-21 for method for ozone treatment of used paper.
Invention is credited to Trichet, Alain.
Application Number | 20050155729 10/504806 |
Document ID | / |
Family ID | 27636293 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050155729 |
Kind Code |
A1 |
Trichet, Alain |
July 21, 2005 |
Method for ozone treatment of used paper
Abstract
The invention concerns a novel method for ozone treatment of
used paper which consists in transforming used paper into pulp in
an aqueous medium, screening then de-inking said paper by liquid
flotation, the method further comprising an ozone treatment step
carried out at latest prior to the liquid flotation step.
Inventors: |
Trichet, Alain; (Montelier,
FR) |
Correspondence
Address: |
Air Liquide
Intellectual Property Department
Suite1800
2700 Post Oak Boulevard
Houston
TX
77056
US
|
Family ID: |
27636293 |
Appl. No.: |
10/504806 |
Filed: |
August 16, 2004 |
PCT Filed: |
February 13, 2003 |
PCT NO: |
PCT/FR03/00460 |
Current U.S.
Class: |
162/4 ; 162/55;
162/6; 162/7; 162/8 |
Current CPC
Class: |
D21C 5/02 20130101; Y02W
30/64 20150501; Y02W 30/648 20150501; D21C 9/153 20130101 |
Class at
Publication: |
162/004 ;
162/006; 162/007; 162/008; 162/055 |
International
Class: |
D21B 001/32; D21C
005/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 18, 2002 |
FR |
02/02062 |
Claims
1-12. (canceled)
13. A method for treating used paper comprising: a) pulping said
paper in an aqueous medium; b) screening said pulp at least once to
remove coarse particles; c) deinking said pulp with at least on
flotation; and d) performing an ozonization step prior to said
flotation.
14. The method of claim 13, wherein said ozonization step is
performed at a used paper pulp consistency of between about 0.5%
and about 5%.
15. The method of claim 14, wherein said consistency is between
about 0.5% and about 3%.
16. The method of claim 13, further comprising performing said
method in water, without adding chemicals other than ozone.
17. The method of claim 13, further comprising performing said
method in the presence of caustic soda NaOH, hydrogen peroxide
H.sub.2O.sub.2, and sodium silicate Na.sub.2SiO.sub.3.
18. The method of claim 13, wherein said ozonization is performed
after said screening.
19. The method of claim 13, wherein said ozonization further
comprises being performed in a two-phase tubular contactor, wherein
said contactor comprises a gas-liquid type.
20. The method of claim 19, further comprising performing said
ozonization in said contactor in wave mode.
21. The method of claim 19, further comprising performing said
ozonization in said contactor in plug mode.
22. The method of claim 13, further comprising performing said
ozonization with a gas mixture wherein said mixture comprises
between about 50 g and about 200 g of ozone per m.sup.3 of gas
mixture.
23. The method of claim 13, further comprising performing said
ozonization with a gas mixture wherein said mixture comprises at
least one member selected from the group consisting of: a) an
air-ozone gas mixture; and b) an oxygen-ozone gas mixture.
24. The method of claim 19, further comprising: a) introducing said
ozone in a gas mixture at a rate greater than about 0.5 m/s and
less than or equal to about 10 m/s; b) introducing said pulp at a
rate greater than about 0.5 m/s and less than or equal to about 10
m/s.
25. The method of claim 24, wherein: a) said gas mixture rate is
between about 0.5 m/s and about 2 m/s; and b) said pulp rate is
between about 0.5 m/s and about 2 m/s.
26. The method of claim 25, wherein said gas mixture comprises at
least one member selected from the group consisting of: a) an
air-ozone mixture; b) an oxygen-ozone gas mixture; and c) a
nitrogen-ozone mixture.
Description
[0001] The invention relates to a method for treating used
paper.
[0002] There is a strong world demand today for recycling used
paper, in particular to limit deforestation. Used paper is
recovered and treated to produce new paper sheets or new paper or
cardboard packaging materials, for example.
[0003] The present method for treating used paper for the recycling
thereof begins with pulping of the paper recovered for
recycling.
[0004] This pulp is screened one or more times to remove the coarse
particles such as paperclips, staples, coarse ink particles,
etc.
[0005] This is followed by one or a plurality of flotation steps to
further complete the removal of the ink which served to print the
waste paper.
[0006] In fact, used paper is generally printed. The printing ink
consists of organic or inorganic dyes and organic binders which, on
drying, imprison the dye particles and maintain them on the
paper.
[0007] Moreover, used paper is often coated with glues or
adhesives, for example binding glues and adhesives for sealing
envelopes.
[0008] Used paper is also soiled, by food and other materials; the
pulp obtained therefore contains microorganisms such as bacteria,
fungi, yeasts, and enzymes such as the enzyme catalase.
[0009] To permit waste paper recycling, all the foreign bodies must
be removed, such as paperclips, staples, traces of food and other
materials, as well as the printing ink and glues and adhesives, and
the microorganisms and, in particular, catalase enzyme, because the
latter pollute the circuits of the installation in which the paper
is treated.
[0010] For this purpose, at the present time, various chemicals are
added during or immediately after the paper is pulped. This serves
to improve the results of the various steps.
[0011] Caustic soda is introduced in particular to swell the paper
fibers and to permit a better separation of the solids. Hydrogen
peroxide and/or oxidizing chlorine products are also added to
destroy the microorganisms and to improve deinking. Chelating
agents, such as sodium silicate, of which the action mechanism is
poorly known, are also added to further facilitate deinking. They
form complexes with the binders and dyes which are then more easily
removed.
[0012] Hydrogen peroxide and/or chlorine products have the function
not only of destroying the microorganisms present in the pulp but
also of purifying the circuits of the installation. However, since
hydrogen peroxide is destroyed by the catalase enzyme present in
the pulp, this causes high overconsumption of this compound during
bleaching.
[0013] Once these pulping, screening and flotation steps have been
completed, very fine particles of ink may still remain in the
pulp.
[0014] A series of mixings is accordingly carried out, followed by
flotations of the pulp, to remove the fine ink particles
mechanically or at least to coat them in the mass of the pulp in
order to obtain a uniform "whiteness".
[0015] Used paper to be recycled generally consists of a mixture of
papers of different types: newspapers, lined papers, magazines or
higher grade papers.
[0016] The differences in quality of the papers mean that, once the
preceding treatment steps have been completed, the paper pulp
obtained may yield a paper of which the "whiteness" may not be
satisfactory for its final use.
[0017] In this case, the pulp obtained can be subjected to a
bleaching step. This bleaching step is carried out, as for new
paper pulps, by treatment with oxidants such as hydrogen peroxide
or chlorine products, or with reducing agents such as sodium
dithionite.
[0018] Used paper may also contain paper that has been treated with
optical brighteners (optical brighteners are fluorescent compounds
that make the paper appear whiter via an optical mechanism), and
these fluorescent compounds have been suspected of being
carcinogenic; it may therefore be desirable to proceed with a
defluorescence of the pulp, and, here also, hydrogen peroxide is
used.
[0019] The pulp is then converted into paper sheets by known
methods.
[0020] It is known that ozone has been used for treating used
paper. However, ozone treatments are performed after the flotation
steps, that is during the subsequent bleaching and defluorescence
steps. Moreover, ozone is always used as a supplement to other
chemicals, that is hydrogen peroxide, chlorine products, caustic
soda, and the chelating agents already present in the method.
[0021] The method of the prior art, in these different variants,
has a number of drawbacks.
[0022] Firstly, this method is highly pollutant because of the many
chemicals used, which are found in the effluents discharged by the
used paper treatment plants.
[0023] Moreover, this process incurs high overconsumption of
hydrogen peroxide due to the presence of catalase enzyme, and
sometimes of metals, which degrade the hydrogen peroxide.
[0024] Finally, the flotation step for deinking purposes may not be
sufficient and, as it has been observed, it then becomes necessary
to carry out a plurality of mixing steps followed by flotation
steps to complete the deinking.
[0025] It is necessary here to emphasize the distinction between:
the deinking step, which is aimed at removing the printing ink
present on the used paper to be treated and serves to obtain a
corresponding whiteness close to that of the original paper and the
actual bleaching step, which is practised both on pulps produced
from used paper and on new paper pulps, that is obtained directly
from cellulose. The bleaching as such consists in removing the
lignin present in the pulp (new or recycled). The deinking step is
unnecessary in the case of new paper pulps.
[0026] It is an object of the invention to overcome the drawbacks
of the method of the prior art by proposing a method for treating
used paper that serves to reduce or indeed eliminate the
overconsumption of hydrogen peroxide and to improve deinking. This
allows for a shorter treatment method comprising fewer or no mixing
steps, followed by flotation steps aimed at completing the
deinking.
[0027] In a variant, the invention further proposes a method for
treating waste paper which serves to significantly reduce the
pollution of the effluents discharged by the treatment plants.
[0028] For this purpose, the invention proposes a method for
treating used paper of the type comprising the successive steps of:
a) pulping of the used paper in an aqueous medium, b) at least one
screening to remove coarse particles, c) at least one flotation to
deink the pulp, characterized in that it further comprises an
ozonization step d) carried out at the latest before the flotation
step c).
[0029] Advantageously, the ozonization step d) is carried out at a
used-paper pulp consistency of between 0.5 and 5%.
[0030] Preferably, the ozonization step d) is carried out at a
used-paper pulp consistency of 0.5 to 3%.
[0031] According to a preferred embodiment of the method of the
invention, the steps a), b), c) and d) are carried out in water,
without the addition of chemicals other than ozone in step d).
[0032] According to another embodiment of the method according to
the invention, the steps a), b), c) and d) are carried out in the
presence of caustic soda, hydrogen peroxide and/or chlorine
products, sodium silicate and/or chelating agents.
[0033] In these two embodiments, the step d) is preferably carried
out after the screening step b).
[0034] Advantageously, the ozonization step d) is carried out in a
two-phase tubular contactor of the gas-liquid type.
[0035] In this case, according to a first variant, the ozonization
step d) is carried out in the tubular contactor operating in wave
mode.
[0036] According to a second variant, the ozonization step d) is
carried out in the tubular contactor operating in plug mode.
[0037] Preferably, the ozonization step d) is carried out with an
air-ozone or oxygen-ozone gas mixture comprising between 50 and 200
g of ozone per m.sup.3 of gas mixture.
[0038] Advantageously, the ozonization step d) is carried out in a
two-phase gas-liquid tubular contactor in which the ozone is
introduced in the form of an air-ozone or oxygen-ozone gas mixture
at a rate above 0.5 m/s and less than or equal to 10 m/s while the
used-paper pulp is introduced at a rate above 0.5 m/s and less than
or equal to 10 m/s.
[0039] However, most advantageously, the ozonization step d) is
carried out in a two-phase tubular contactor in which the ozone is
introduced in the form of an air-ozone or oxygen-ozone gas mixture
at a rate between 0.5 and 2 m/s while the used-paper pulp is
introduced at a rate between 0.5 and 2 m/s.
[0040] The invention will be better understood and other details,
advantages and features thereof will appear more clearly from a
reading of the description that follows, made with reference to the
figures appended hereto in which:
[0041] FIG. 1 shows an enlarged view of a particle of printing ink
bonded to the surface of a used paper,
[0042] FIG. 2 shows an enlarged view of the ink particle of FIG. 1,
on the same scale, after the deinking step of the prior art
method,
[0043] FIG. 3 shows an enlarged view of the ink particle of FIG. 1,
on the same scale, after the deinking step of the method of the
invention,
[0044] FIG. 4 shows a cross section of a two-phase gas-liquid
tubular contactor in segregated mode,
[0045] FIG. 5 shows a cross section of a two-phase liquid-gas
tubular contactor in wave mode,
[0046] FIG. 6 shows a cross section of a two-phase gas-liquid
tubular contactor in plug mode,
[0047] FIG. 7 shows a cross section of a two-phase gas-liquid
tubular contactor in dispersed bubble mode,
[0048] FIG. 8 shows, in the form of bar graphs, the number of
points of ink remaining, as a function of their size, in a
used-paper pulp after the screening step (FIG. 8A), the ozonization
step (FIG. 8B), and the flotation step (FIG. 8C), according to a
first embodiment of the method of the invention, and
[0049] FIG. 9 shows, in the form of bar graphs, the number and size
of the points of ink remaining in a used-paper pulp after the
screening step (FIG. 9A), the flotation step (FIG. 9B) and the
ozonization step (FIG. 9C), carried out according to the prior art
method.
[0050] The method for treating used paper to be recycled according
to the invention comprises, like the method for treating used paper
of the prior art, a step of pulping of the mass of paper to be
recycled in an aqueous medium. The pulp then undergoes a plurality
of solid-solid separation steps in order to remove the undesirable
elements such as paperclips, staples and coarse ink particles.
[0051] For this purpose, at least one screening step is carried
out.
[0052] This is followed by one or a plurality of flotation steps in
order to remove the very fine ink particles.
[0053] In the prior art method, these steps are put into practice
in an aqueous medium in the presence of caustic soda to adjust the
pH and to swell the paper fibers in order to permit an easier
detachment of the ink particles and other impurities, in the
presence of hydrogen peroxide and/or sodium hypochlorite in order
to facilitate removal of the ink particles, eliminate the
microorganisms, such as bacteria, fungi and yeasts, as well as the
enzymes such as catalase enzyme, so as to clean up not only the
pulp itself but also the circuits of the installation in which the
method is put into practice. Chelating agents such as sodium
silicate are also added to complex the metals, originating for
example from the dyes of the printing inks, in order to be able to
subsequently remove them with hydrogen peroxide or the chlorine
products already mentioned. Any other chemical may be added, as
known in the art.
[0054] The method for treating used paper of the invention differs
from the method of the prior art in that, at the latest before the
flotation steps, the paper pulp is subjected to an ozonization
step.
[0055] This ozonization step serves to improve the effectiveness of
the flotation steps for deinking the paper pulp and, moreover, it
destroys the microorganisms and, above all, the catalase
enzyme.
[0056] Thus, by making the deinking step by flotation more
effective, the number and duration of the subsequent treatment
steps intended to further complete the deinking, for example by
carrying out a series of mixings followed by flotations, are
reduced in comparison with the prior art or indeed eliminated.
[0057] Furthermore, since ozone destroys the catalase enzyme which
reduces the activity of the hydrogen peroxide, this eliminates the
overconsumption of hydrogen peroxide in the event that the hydrogen
peroxide treatment step is maintained.
[0058] The high reactivity of ozone enables it to degrade the
unsaturated products with which the waste paper is impregnated.
[0059] The following products are thereby found, rich in
unsaturated and aromatic groups:
[0060] dyes, particularly printing inks,
[0061] optical brighteners to improve the whiteness of the
paper,
[0062] glues, commonly called "stickies" in the art, which are
present, for example, as glue for binding, for example, of the
reams of paper, or as an adhesive for envelopes, and
[0063] binders of the inks which serve to fix the printing dyes on
the paper, when the paper is dried.
[0064] All these products are easily degraded by ozone in aqueous
medium or in another solvent.
[0065] Although the most important unsaturated product in the
products to be treated is usually the lignin in different forms, it
is important to note here that the ozone used in the method of the
invention is used to react with the unsaturated products
impregnating the used paper and less with the lignin present,
because the latter is "coated" by these products and others such as
mineral fillers.
[0066] The destruction by ozone of these products impregnated on
the surface of the fibers automatically causes changes in the
wettability properties of the paper fibers. This gives rise to
substantial modifications during the flotation of the inks and
fillers.
[0067] In fact, in the liquid state, printing inks consist of
particles of carbon or another dye dispersed in a liquid binder.
When the binder dries, it coats the carbon particles and other
particles and fixes them to the surface of the paper sheet.
[0068] FIG. 1 shows an enlarged cross section of a particle of
dried printing ink on the paper surface. The carbon particle 2 is
enveloped and bonded to the paper surface 3 by the dried binder
1.
[0069] FIG. 2 shows a cross section of the ink particle of FIG. 1,
on the same scale, after the pulping, screening and flotation steps
according to the prior art method. As may be observed in FIG. 2,
the carbon particle 2 is still coated and bonded to the paper
surface 3 by the binder 1' which has decreased in thickness.
[0070] Thus, in the prior art, it is necessary to separate, by
mechanical action, that is by mixing, the binder 1' from the paper
surface 3 to release the carbon particle 2, and then carry out a
flotation step to remove the particles of binder and carbon from
the paper pulp.
[0071] When the method according to the invention is applied, that
is when the ozonization step has been put into practice before the
flotation step, fewer particles of ink remain adhering to the paper
surface. However, if one considers the ink particles which still
adhere to the paper surface, for each of these ink particles, as
shown in FIG. 3, which is a cross section of the same ink particle
as the one shown in FIG. 1, on the same scale, but after the
flotation steps for deinking according to the method of the
invention, the binder 1" is more strongly degraded and coats the
carbon particle 2 "less well".
[0072] The mechanical action necessary to detach this ink particle
2 from the paper surface 3 is less than that required to separate
the ink particle 2 shown in FIG. 2. The number of mixing-flotation
steps necessary to completely deink the paper to be recycled
according to the method of the invention is smaller.
[0073] Furthermore, the ozone destroys the microorganisms and,
above all, the catalase enzyme. Since catalase enzyme destroys the
activity of the hydrogen peroxide used in the prior art method,
this allows a more effective use of the hydrogen peroxide. Its
overconsumption is decreased, and it can even be eliminated for
certain end uses of the paper.
[0074] The ozone also serves to clean up the circuits of the
installation to which the method for treating used paper is put
into practice.
[0075] It has also been discovered surprisingly that the ozone can
be used without adding any of the reagents commonly used in the
prior art method.
[0076] The method of the invention, in a particularly advantageous
embodiment, hence consists in pulping the mass of paper to be
recycled, in water, in proceeding with the solid-solid separation
steps by screening followed by the flotation steps, always
exclusively in water and without the addition of any reagent,
provided that an ozonization step is put into practice at the
latest before the flotation steps.
[0077] This method, which uses ozone only without other chemical
reagents, offers considerable advantages, its simplicity and its
economy, and, furthermore, it is a method for treating used paper
from which the effluents have a very low level of pollution.
[0078] In the two embodiments of the method of the invention, the
ozonization step must be carried out at the latest before the
flotation step for deinking. It can be carried out before, during
or just after the pulping step or after the screening step.
[0079] Preferably, it is carried out after the screening step in
order to permit good contact between the ozone and the pulp to be
treated, for greater effectiveness of the ozone; the coarse
particles and undesirable elements can be removed easily by the
screening, and the ozone is therefore used more efficiently and
without overconsumption to remove the fine ink particles, purify
the installation circuits and/or react with certain additives.
[0080] The effectiveness and extent of the degradation of the
products to be removed in the used paper for the same contact time
depend on the extent and effectiveness of the contacting of the
ozone with the products to be degraded. In other words, the
effectiveness of the ozonization reaction depends on the rate of
dissolution of the ozone in the pulp.
[0081] As previously stated, ozone has already been used in methods
for producing paper pulps to bleach virgin pulps. At the present
time, these ozonization methods take place at a solids
concentration between 8 and 40 g of solids per 100 g of pulp.
[0082] The paper pulps are treated as a liquid medium, generally an
aqueous medium, in which the solid particles are in suspension.
[0083] The ozonization reaction therefore produces a situation of a
two-phase mixture: a gas phase and a liquid phase containing
solids. Since the chemical ozonization reaction is fast, it is
necessary to have a very rapid gas dissolution rate in the liquid
phase containing the solids so that the ozone reacts with the
solids.
[0084] When operating at atmospheric pressure, because of the high
solids concentration of the pulp and the high reactivity of the
ozone, risks of heterogeneous treatment exist. In order to increase
the dissolution rate of the ozone in the liquid-solid phase,
another method accordingly proposes to carry out the ozonization
reaction under pressure in units of the centrifugal pump type,
which requires compressing the ozone.
[0085] This raises problems already discussed, that is the fact
that the paper fiber is subjected to high mechanical stresses which
can damage it, and the need to compress the ozone, which entails
the use of compressors which are costly to purchase and
maintain.
[0086] It has now been discovered that all these drawbacks are
overcome by carrying out the ozonization step on paper pulps with
low or very low consistencies.
[0087] In the above discussion and in what follows, the expression
"low consistency paper pulp" means a paper pulp in which the paper
concentration is between 0.5 and 5% by weight of the total weight
of the pulp.
[0088] In the above discussion and in what follows, the expression
"very low consistency paper pulp" means a paper pulp in which the
paper concentration is about 1% by weight of the total weight of
the pulp. For such pulps, it is possible to use two-phase
gas-liquid tubular contactors which serve to obtain very high gas
dissolution rates in the paper pulp.
[0089] The invention proposes to put the ozonization step into
practice in a two-phase tubular contactor of the gas-liquid type
and to introduce the paper pulp at low or very low consistency.
[0090] The tubular contactor for putting into practice the method
of the invention can be a horizontal or vertical tubular contactor
operating in cocurrent flow of the liquid phase containing the
solids.
[0091] The gas can be introduced via a simple pipe or via a more
sophisticated system, for example a static mixer.
[0092] The ozone dissolution rate in the paper pulp depends on the
gas-liquid mass transfer coefficients and, more particularly, on
the interfacial area between the two phases.
[0093] The aim here is to obtain a good gas-pulp emulsion.
[0094] Depending on the respective rates of introduction of the gas
and the pulp, the reactor will operate in segregated mode, in wave
mode, in plug mode or in dispersed bubble mode.
[0095] FIG. 4 shows a cross section of a horizontal tubular
contactor T operating in segregated mode in which the gas phase has
the numeral 4 and the pulp has the numeral 6. In FIG. 4, the
interfacial area is shown by the contact surface 5 between the gas
phase 4 and the pulp 6.
[0096] This contact surface 5, when the tubular contactor T is in
segregated mode, as shown in FIG. 4, can be treated as a plane
surface.
[0097] In this case, the interfacial area corresponding to the
contact surface 5 is smaller than in the cases shown in FIGS. 5 to
7, which are discussed below. For this reason, the ozone
dissolution rate is less favorable.
[0098] Such a segregated mode is obtained when the respective rates
of introduction of the gas phase and the pulp are lower than 1 m/s,
more particularly higher than 0.5 m/s and lower than 1 m/s.
[0099] The contact surface between the gas phase and the liquid
phase containing solids is improved when the contactor T operates
in wave mode, as shown in FIG. 5.
[0100] As may be observed in FIG. 5, the contact surface with the
numeral 5' between the gas phase 4 and the pulp forms waves. It is
increased in this domain. Hence the gas is dissolved more rapidly
in this case than in the case shown in FIG. 4, that is in
segregated mode.
[0101] The ozone dissolution rate, and hence the reaction rate of
the ozone with the pulp, is further improved when, as shown in FIG.
6, the tubular contactor T operates in plug mode. As may be
observed in FIG. 6, when the tubular contactor T operates in plug
mode, the pulp 6 is actuated with a movement which makes it touch
the two inside horizontal walls of the contactor.
[0102] The gas phase 4 therefore has a large contact surface with
the numeral 5" for contact with the pulp 6 when the contactor
operates in such a plug mode.
[0103] The contactor T operates in wave mode or in plug mode when
the respective gas phase and pulp introduction rates are between 1
and 2 m/s.
[0104] The best results are obtained when the contactor is in
dispersed bubble flow mode, as shown in FIG. 7. In this case, the
pulp 6 forms bubbles which are dispersed in the gas phase 1 and the
contact surface 5'" for contact between the gas phase 4 and the
pulp 6 is very large.
[0105] Such a dispersed bubble mode is obtained for respective gas
and pulp introduction rates above 2 m/s and up to 10 m/s.
[0106] However, to obtain such an operation in dispersed bubble
mode, it is necessary to use very high gas pressures and this is
not preferable in the method of the invention, because it entails
the use of a gas compressor with the attendant purchase and
maintenance costs. Furthermore, the drawback associated with the
mechanical stresses applied to the pulp, and in particular to the
paper fiber, is observed here.
[0107] Hence, for the reasons stated above, although the
ozonization step can be put into practice in the contactor T
operating in segregated mode, in wave mode, in plug mode or in
dispersed bubble mode, it is preferably put into practice in wave
mode or in plug mode, that is with gas introduction rates between
0.5 and 2 m/s and pulp introduction rates between 0.5 and 2
m/s.
[0108] The introduction rates mentioned above and below are rates
obtained when the contactor T is empty. In other words, the gas
introduction rate is calculated as a function of the internal cross
section of the contactor used, in order to obtain a gas
introduction rate between 0.5 and 2 m/s when the contactor contains
no pulp.
[0109] Similarly, the pulp introduction rate is calculated as a
function of its introduction rate in the empty contactor and as a
function of the internal cross section of the contactor.
[0110] To obtain a high reaction rate, it is advantageous to use a
gas mixture containing ozone in a high concentration in a carrier
gas such as air, for example. In fact, as already stated, ozone is
highly reactive and the quantities of ozone to be consumed are
generally small. By way of example, in the case of paper pulps,
they are lower than 20 kg/t of pulp.
[0111] Thus it has been determined that an ozone-air gas mixture
containing between 50 and 200 g of ozone per m.sup.3 of gas mixture
is perfectly suitable.
[0112] However, an oxygen-ozone or nitrogen-ozone gas mixture or
any other mixture of ozone in a carrier gas compatible with ozone
can also be used.
[0113] In order to better understand the invention, several
examples of application will now be described. These examples are
given purely for illustration and are not limiting.
EXAMPLE 1
Treatment of Used Paper According to the Method of the Invention in
the Presence of the Usual Reagents
[0114] Pulping Step
[0115] 1.5 kg of waste papers consisting of a mixture of old
newspapers and magazines are mixed with 13.5 l of water and the
usual prior art quantities of hydrogen peroxide, caustic soda and
sodium silicate.
[0116] The mixture is stirred in a pulper marketed by Lamort for 20
min. The pH is close to 10.5.
[0117] Screening Step
[0118] The paper pulp obtained is then screened through screens,
following the usual methods of the profession.
[0119] Ozonization Step
[0120] The ozonization is carried out at a consistency of 3% in a
glass reactor, the ozone is introduced in the form of an
oxygen-ozone gas mixture containing 150 g of ozone per m.sup.3 of
gas mixture, until 10 kg of ozone are consumed per metric ton of
dry paper.
[0121] Flotation Step
[0122] The pulp obtained is diluted to a consistency of 1% and
transferred to a Lamort type flotation cell. The flotation is
carried out for 6 min and prolonged for 4 min. A foaming agent is
added.
[0123] In all these steps, the temperature has remained close to
40.degree. C.
[0124] After each step, a sample of pulp is taken and converted
into a sheet of paper. The whiteness of this sheet of paper is
measured with UV according to standard ISO (2420).
[0125] The fluorescence of the pulps is measured by an identical
method, by inserting a UV blocking filter in the optical path
(application of the operating mode described in "TAPPI T 452
appendix C"). The results are given in Table 1 below:
1 TABLE 1 WH iso % WH iso % Fluorescence Step with UV without UV %
After screening 61.9 57.3 4.6 After ozonization 59.2 59.7 -0.4
After flotation 65 65.2 -0.2
[0126] The results given in Table 1 show that when the method of
the invention in its first variant, that is comprising an
ozonization step, is put into practice with the usual chemical
reagents of the profession, the sheet of paper finally obtained has
an improved whiteness and a decreased fluorescence.
[0127] The number and size of the black spots was also measured on
each sheet per m.sup.2 of surface area.
[0128] The results are shown in FIG. 8.
[0129] As shown in FIG. 8, with the method of the invention in its
first variant, the number of black spots remaining after flotation
is very small.
[0130] This means that the ozonization step, when introduced before
flotation, improves the effectiveness of the flotation step itself,
that is of the deinking.
[0131] The number of black spots per m.sup.2 and the size in
microns of these black spots were determined by image analysis.
EXAMPLE 2
Application of the Method of the Invention without Addition of any
Chemical Reagent Other than Ozone
[0132] The method was followed as in example 1 but without adding
hydrogen peroxide, caustic soda and sodium silicate. The pH was
close to 7.5.
[0133] The same measurements of whiteness with UV and without UV
and of fluorescence were carried out on the sheets of paper
obtained after the screening, ozonization and flotation operations
respectively.
[0134] The results are given in Table 2 below:
2 TABLE 2 WH iso % WH iso % Fluorescence Step with UV without UV %
After screening 58 54 4 After ozonization 54 53.4 -0.2 After
flotation 58.6 56.5 0.7
[0135] Here also, a significant decrease in fluorescence and a
significant increase in whiteness are observed.
COMPARATIVE EXAMPLE 3
[0136] By way of comparison, the prior art method was put into
practice in which the ozonization is carried out after the
flotation step. To do this, the method was followed in the same way
as in example 1, but by carrying out the ozonization step only
after the flotation step.
[0137] Measurements were taken of the whiteness with and without
UV, of the fluorescence and the number of black spots per m.sup.2
of sheets of paper obtained by this method, as well as their size
in microns, on each sheet of paper obtained after the screening
step, after the flotation step and after the ozonization step.
[0138] The results of the measurements of whiteness with and
without UV and of fluorescence are given in Table 3 below:
3 TABLE 3 WH iso % WH iso % Fluorescence Step with UV without UV %
After screening 61.9 57.3 4.6 After flotation 67.1 62 5.1 After
ozonization 66.9 62.1 4.8
[0139] The results in Table 3 show, when compared with the results
of the measurements of whiteness with and without UV and of
fluorescence carried out on the sheets of paper obtained in example
1, that with the method of the invention the sheets of paper
finally obtained have a higher whiteness and a lower fluorescence
in the case of the sheets treated by the method of the invention
than in the case of the prior art method.
[0140] The results of the measurements of the number of black spots
per m.sup.2 present on the sheets of paper obtained according to
the prior art method as described in the present example, and of
their sizes, are given in FIG. 9.
[0141] By comparing the results of these measurements with those
obtained with the method of the invention as described in example
1, it is observed that the method of the invention permits a better
deinking than the prior art method.
COMPARATIVE EXAMPLE 4
[0142] The method is followed as in example 2, without the addition
of any chemical reagent other than ozone but by carrying out the
ozonization step after the flotation step.
[0143] The whiteness with and without UV and the fluorescence of
the sheets of paper obtained with the pulps obtained after the
screening, after the flotation and after the ozonization are given
in Table 4 below:
4 TABLE 4 WH iso % WH iso % Fluorescence Step with UV without UV %
After screening 57.9 54 3.9 After flotation 60.4 56.1 4.3 After
ozonization 54.7 53.6 1.3
[0144] By comparing the results of the measurements given in Table
4 above with those of the measurements given in Table 2 of example
2, it may be observed that by putting into practice the method of
the invention without the addition of any reagent other than ozone,
with the method of the invention, a better whiteness with UV is
obtained and a better defluorescence of the paper pulp obtained
from the UV papers.
EXAMPLE 5
[0145] The test in example 2 was reproduced using a horizontal
stainless steel tubular contactor having an inside diameter of 4.5
cm and a length of 100 m supplied with a used-paper pulp with a
consistency of 2.5%.
[0146] The paper pulp introduction rate was selected in order to
obtain pulp rates (in empty contactor) between 1 and 2 m/s.
[0147] The same rates were used for the gas mixture consisting of a
mixture of oxygen and ozone having an ozone content of 100
g/m.sup.3 of mixture.
[0148] Under these conditions, the pressure drops are between 2 and
3 bar.
[0149] The ozone transfer capacity is accordingly 1.5 to 3 kg/t of
pulp for a residence time of one-and-a-half minutes.
[0150] The same results were obtained as in example 2.
[0151] All these measurements show that the method of the invention
serves to obtain sheets of recycled paper presenting a better
whiteness, a better defluorescence and a better deinking, with
fewer subsequent treatment steps, without overconsumption of
hydrogen peroxide, and even without the addition of any reagent
other than ozone, than the sheets obtained according to the prior
art.
[0152] Evidently, the invention is not at all limited to the
embodiments described and shown which are only given as
examples.
[0153] On the contrary, the invention comprises all the technical
equivalents of the means described as well as combinations thereof
if these are carried out in its spirit.
* * * * *