FIELD OF THE INVENTION
[0001] The present invention relates to a process for bleaching pulps, such as wood pulp
and regenerated pulp, which can be realized at lower temperatures within a short period
of time.
BACKGROUND OF THE INVENTION
[0002] For realizing bleaching of wood pulps, namely, mechanical pulps (high efficiency
pups), such as ground pulp (GP), refiner ground pulp (RGP), thermo-mechanical pulp
(TMP), chemical ground pulp (CGP) and semi-chemical pulp (SCP); chemical pups, such
as craft pulp (KP) and sulfite pulp (SP); and deinked waste paper pulp (DIP), namely
regenerated pulp, there have been in wide use peroxides, such as hydrogen peroxide,
sodium percarbonate and so on.
[0003] Peroxidic bleaching liquors conventionally employed have, in general, additives inclusive
of sequestering agent, for example, a metal chelating agent, such as sodium diethylenetriaminepentaacetate
(DTPA), tetra- and disodium ethylenediaminetetraacetate (EDTA), nitrilotriacetic acid
(NTA) and so on, and magnesium sulfate, sodium silicate of commercial grade No. 3
and so on, for preventing useless decomposition of the peroxide under the bleaching
condition due to the presence of contaminant trace metals of natural occurrence or
delivered from waste paper.
[0004] It has been known that sodium silicate of commercial grade No. 3 functions also as
a buffer and stabilizes the pH of the bleaching solution, in addition to the function
of preventing useless decomposition of the peroxide. It has been confirmed further
that sodium silicate of commercial grade No. 3 causes the formation of a protective
coating film on the surface of metals, thus preventing corrosion of the metal.
[0005] Caustic soda is also employed in general, due to its low price pH regulator, since
peroxides exhibit their bleaching action in an alkaline condition.
[0006] It is also known that treatment by these chemicals can be imparted to the pulp not
only during but also before, the oxidative bleaching takes place.
[0007] In terms of the typical parameters of conventional tower bleaching of pulp with a
bleaching solution in which hydrogen peroxide is used as the peroxide, the conditions
for bleaching include, in general, a longer bleaching duration of 1-4 hours at 40-70°C
and a solution pH of 10-11. For higher temperature bleaching, the conditions include,
in general, a bleaching duration of 20-30 minutes at 75-85°C. For attaining the maximum
whiteness, a bleaching duration of 4-3 hours at 40-60°C is required at each concentration
of the peroxide used.
SUMMARY OF THE INVENTION
[0008] The object of the present invention is to provide a process for bleaching pulps that
is able to be carried out at a lower temperature within a shorter operation duration
as compared with the conventional technique.
[0009] Thus, the above object is attained according to the present invention by a process
for bleaching pulps with a peroxide, which method comprises use of, as an additional
component, an activator which liberates active or nascent oxygen upon reaction with
the peroxide.
[0010] By this method, maximum whiteness at each concentration of the peroxide can be attained
within a very short period of time, say, 5-60 minutes at a relatively low temperature
of 5-70°C, referably 20-60°C.
BRIEF DESCRIPTION OF THE DRAWING
[0011] Fig. 1 is a graph showing the relationship between the whiteness of the bleached
pulp and the bleaching duration based on the experimental results of Example according
to the present invention and Comparison Example according to the prior technique.
DETAILED DESCRIPTION OF THE INVENTION
[0012] Pulps to be bleached by the method according to the present invention include every
sort of pulp, such as those enumerated previously.
[0013] For the peroxide to be employed in the method according to the present invention,
there may be enumerated, for example, hydrogen peroxide and various inorganic peroxides,
such as sodium peroxide, sodium and potassium salts of percarbonate, perborate, persulfate
and perphosphate.
[0014] The activator to be employed in the method according to the present invention encompasses
various known compounds. It includes N-acyl compounds, such as tetraacetylethylenediamine
(TAED), tetraacetylmethylmethylenediamine, tetraacetylglycol uril (TAGU) and so on,
and activated esters, such as pentaacetylglucose (PAG), sodium acetoxybenzenesulfonate,
sodium nonanoyloxybenzenesulfonate (NOBS), sodium benzoyloxybenzenesulfonate (BOBS)
and so on. Above all, tetraacetylethylenediamine (TAED) is particularly advantageous.
[0015] In the method according to the present invention, additional components other than
the peroxide and the activator can be employed, such as for example, conventional
additives employed in the prior bleaching technique, i. e., additives for preventing
the useless decomposition of the peroxide under the bleaching condition in the presence
of contaminant metals of natural occurrence or originated from waste papers to be
regenerated including a sequestering agent, such as, a metal chelating agent, for
example, sodium diethylenetriaminepentaacetate (DTPA), tetra- and disodium ethylenediaminetetraacetates
(EDTA), nitrilotriacetic acid (NTA) and so on, and magnesium sulfate, sodium silicate
of commercial grade No. 3 and so on, as well as a pH regulator, corrosion inhibitors
for various metals and so on.
[0016] The pH value of the bleaching solution at the start of bleaching by the method according
to the present invention may preferably be in the range from 8 to 12, the highest
bleaching effect being attained, in particular, in the pH range of 9-10. For the pH
regulator, there may be employed various inorganic alkaline compounds, such as caustic
soda, sodium silicate of commercial grade No. 3, sodium sulfite and so on.
[0017] The activator may be added to the bleaching solution either before the addition of
the peroxide and during the step of adding the metal chelating agent, pH regulator
and so on, or at the same time with or after the addition of the peroxide.
[0018] The reaction scheme of liberating the nascent oxygen upon reaction of the activator
with the peroxide employed in the method according to the present invention exemplified
for the case of using hydrogen peroxide and TAED is represented as follows:

[0019] In the above reaction scheme, (O) represents the active oxygen nascent oxygen, which
brings about an efficient bleaching.
[0020] Theoretically calculated, one mole of TAED will be required to react with 2 moles
of hydrogen peroxide.
[0021] Since the molecular weight of hydrogen peroxide is 34 and that of TAED is 228, 3.35
grams of TAED will react with 1 gram, calculated as pure substance, of hydrogen peroxide
employed for bleaching the pulp.
[0022] In the widely realized conventional practice for bleaching pulps, the amount of hydrogen
peroxide, calculated as pure substance, is, in general, in the range from 0.3 to 10
%.
[0023] In the method according to the present invention, however, the lower limit of the
amount of hydrogen peroxide can be reduced to 0.05%; Therefore, the amount of TAED
to be employed in the method according to the present invention will, theoretically,
be in the range from 3.35 times 0.05% to 3.35 times 10 %.
[0024] Practically, however, it may be reasonable to employ hydrogen peroxide, which has
a lower price than TAED, in an excess amount, in order to further increase the rate
of reaction of hydrogen peroxide with TAED and to improve the bleaching effect. In
addition, more hydrogen peroxide becomes effectively active in the bleaching of pulps
by the addition of TAED, so that the amount of hydrogen peroxide actually employed
can be reduced.
[0025] While the advantageous effects of the method according to the present invention is
explained for the case of using hydrogen peroxide and TAED, corresponding effects
will be achieved using other peroxides and activators.
[0026] Below, the method according to the present invention is explained for the case of
application to a pulp product used for papers for domestic uses.
[0027] A pulp used for papers for domestic uses is obtained from waste papers of high quality
and medium quality papers and of simili paper. For producing pulp from such waste
papers, the so-called globe kettle method may be employed. This method comprises treating
the raw stock of waste papers in the kettle by cooking under the condition of high
temperature (130-140°C) and high pressure using a digesting agent and a bleaching
agent at a pulp concentration of 15-25% for 4-5 hours.
[0028] According to Report No. 36 of the Paper Manufacturing Testing Laboratory of Shizuoka
Prefecture Japan, effects of preventing useless decomposition of sodium percarbonate
and of deinking of pulp were attained by the concurrent use of sodium percarbonate
and sodium silicate of commercial grade No. 3, with a simultaneous improvement in
the whiteness, a decrease in the COD load of waste water, a lowering of the digestion
temperature and an increase in the paper quality and so on, as compared with the conventional
practice of digestion with sodium sulfite. By the concurrent use of TAED with sodium
percarbonate or further with sodium silicate of commercial grade No. 3, it becomes
possible to lower the bleaching temperature in the globe kettle from the conventionally
employed temperature of 130-140°C to 5-70°C and to reduce the bleaching time to the
range of 5-30 minutes. In the modified globe kettle method in which the pulp is digested
beforehand by the solution of another alkaline compound, such as sodium sulfite, before
bleaching it with the addition of sodium percarbonate, it is made possible according
to the present invention to reduce the bleaching time and to economize energy for
effecting the digestion and bleaching, as compared to the conventional technique.
[0029] In the method according to the present invention, the activator does not serve for
any acceleration of decomposition of the peroxide, but does serve for the generation
of active oxygen from the peroxide efficiently for realizing the bleaching and, thus,
reducing the bleaching time.
[0030] The advantages realized by the present invention exist in that the energy cost can
be reduced, the pulp bleaching apparatus can be designed compactly and the bleaching
performance per unit operation time can be increased.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0031] Below, the present invention is described in more detail by an Example and a Comparison
Example.
Example
[0032] Bleaching tests were carried out using a ground pulp in accordance with the following
procedures:
1) dispersing the ground pulp (GP), preparing an absolutely dried pulp on a hot plate,
collecting a sample of 25 g of this absolutely dried pulp in a bag of polyethylene
film and sealing the bag in a gas-tight condition so as to exclude absorption of moisture;
2) dissolving 1 gram of tetraacetylethylenediamine (TAED) in a warm water of 55°C
to prepare a 1% aqueous solution thereof and maintaining it at a temperature of 50-55°C;
3) formulating a 4% aqueous solution of sodium diethylenetriaminepentaacetate (DTPA)
to be used as the chelating agent;
4) preparing a 1% aqueous solution of sodium silicate of commercial grade No. 3;
5) preparing a 1% aqueous solution of sodium hydroxide and
6) preparing an aqueous solution of hydrogen peroxide having an effective component
concentration of 3%.
[0033] To the polyethylene bag containing 25 g of GP collected as 1) above, there was added
an amount of the DTPA solution prepared as 3) above so as to reach a DTPA concentration
of 0.4%, based on the dry weight of the pulp, whereto the TAED solution prepared as
2) above was admixed so as to adjust a TAED concentration of 3.35%, based on the dry
weight of the pulp. This dispersion was supplemented with warm water so as to reach
a final pulp concentration of 20%, that is, an amount of the warm water was added
to the dispersion after the addition of hydrogen peroxide so as to adjust the pulp
concentration of 20%. Then, the pulp dispersion was homogeneously mixed by rubbing
with hand in order to reach a uniform distribution of GP, DTPA and TAED, whereupon
the dispersion was left standing for 30 minutes.
[0034] Thereto was then added the aqueous solution of sodium silicate prepared as 4) above
so as to reach a sodium silicate concentration of 3%, based on the dry weight of the
pulp, whereto was further added the solution of sodium hydroxide prepared as 5) above
so as to reach a concentration thereof of 1% and the dispersion was then homogeneously
mixed by rubbing with hand in order to reach a uniform distribution of each component
among the pulp dispersion. The polyethylene bag containing the GP dispersion with
the additives was placed in a hot bath and warmed to a temperature of 55°C, whereupon
the bag was held at this temperature for 15 minutes.
[0035] Three bags of the same pulp dispersion were tested (Test lots A, C and E).
[0036] To each of these bags of Lots A, C and E, the hydrogen peroxide solution prepared
as 6) above was added so as to reach a concentration of hydrogen peroxide of 3.53%,
1.76% and 0.88% respectively, based on the dry weight of the pulp, whereupon each
pulp dispersion was supplemented with an amount of warm water so as to reach a pulp
concentration of 20%. The so treated bags were again placed in the hot bath and were
observed for their whiteness at 55°C according to Hunter, after 5, 10, 15, 30, 60
and 120 minutes respectively. Results are summarized in the Table given below and
are also illustrated graphically in Fig. 1, in which the whiteness values (%) were
based on Hunter-Whiteness and the original whiteness of the pulp before bleaching
was 41.6%.
Comparison Example
[0037] Three samples of pulp dispersion were prepared and tested as in the above Example,
except that the employment of TAED was omitted. Results obtained are also recited
in the Table below (Test Lots B, D and F) and illustrated in Fig. 1.
[0038] As seen from these experimental results, the present invention provides a marked
contribution to the industry by the attainment of capability of realizing bleaching
of pulps at lower bleaching temperature within a very short bleaching time.
