Source: http://www.docstoc.com/docs/45718671/Process-For-The-Chlorine-free-Bleaching-Of-Cellulosic-Materials-With-Ozone---Patent-5346588
Timestamp: 2014-03-09 21:42:56
Document Index: 797378335

Matched Legal Cases: ['art 3', 'art 4', 'art 2', 'art 4', 'art 2', 'art\n3']

Process For The Chlorine-free Bleaching Of Cellulosic Materials With Ozone - Patent 5346588
United States Patent: 5346588
Process for the chlorine-free bleaching of cellulosic materials with
A process for the chlorine-free bleaching of pulps in an aqueous suspension
which comprises forming the suspension with a consistency of 3 to 20 mass
percent; introducing into the suspension an ozone-containing gas having an
ozone content of 20 to 300 g/m.sup.3 in an amount corresponding to at most
2 mass percent ozone calculated on dry pulp of the suspension during
vigorous agitation of the suspension to form a reaction mixture;
maintaining a pressure of the ozone-containing gas at a pressure of 1 to
15 bar during introduction into the suspension; and controlling reaction
conditions during contact of the ozone-containing gas with the suspension
to maintain a reaction temperature of 15.degree. to 80.degree. and a pH
value of 1 to 8.
Sixta; Herbert (Vocklabruck, AT), Gotzinger; Gerhard (Fistersdorf, AT), Hoglinger; Anton (Timelkam, AT), Hendel; Peter (Vocklabruck, AT), Ruckl; Wilfried (Vocklabruck, AT), Peter; Walter (Vocklabruck, AT), Kurz; Friedrich (Attnang-Puchheim, AT), Schrittwieser; Alfred (Vocklabruck, AT), Schneeweisz; Manfred (Vocklabruck, AT)
(Lenzing,
07/859,236
605744Oct., 1990
162/65  ; 162/78; 162/89
D21C 9/153&amp;nbsp(20060101); D21C 9/147&amp;nbsp(20060101); D21H 17/00&amp;nbsp(20060101); D21C 009/153&amp;nbsp()
162/65,78,88,29,89
4902381
0397308A2
Osawa et al, &quot;The Action of Gaxous Reagents on Cellulosic Materials&quot;, TAPPI, vol. 46, No. 2 pp. 84-89.
Liebergott et al, &quot;The Use of Ozone in Bleaching and Brightening Wood Pulps&quot;, Pulping &amp; Bleaching Seminar, New Orleans (Nov. 1978).
Douglas W. Reeve and Paul F. Earl; &quot;Mixing gases, water, and pulp in bleaching&quot;; Jul. 1986 Tappi Journal; pp. 84-88 (Pulp Mixing).
&quot;Effect of Pulp Consistency and pH Inozone Bleaching&quot; Part 3., Carl-Anders Lindholm, Helsinki Univ. of Technology, Espoo, Finland, pp. 44-50.
&quot;Effect of Pulp Consistency and pH in Ozone Bleaching&quot; Part 4, Carl-Anders Lindholm, Paper &amp; Timber Feb. 1989, pp. 145, 147, 149, 150, 152, 154.
&quot;Effect of Pulp Consistency and pH in Ozone Bleaching&quot; Part 2., Carl-Anders Lindholm, 1987 Int. Oxygen Delignification Conf., pp. 155-165..
Myers; Jonathan
This is a continuation of co-pending application Ser. No. 07/605,744 filed
on Oct. 30, 1990 , now abandoned.
1.  A process for the chlorine free bleaching of pulps in an aqueous suspension which comprises:
(b) introducing into said suspension an ozone-containing gas having an ozone content of 20 to 300 g/m.sup.3 in an amount corresponding to at most 2 mass percent ozone calculated on dry pulps of said suspension during vigorous agitation in a high
shear mixer of the suspension to react with the suspension forming a reaction mixture;
(d) transferring the suspension of the ozone-containing gas from the high shear mixer to a tube reactor and continuing the reaction of the suspension and the ozone-containing gas, while maintaining the pressure of the ozone-containing gas at 1.1
to 15 bar;  and
(e) controlling reaction conditions during contact of the ozone-containing gas with the suspension during steps (b), (c) and (d) to maintain a reaction temperature of 15.degree.  to 80.degree.  C., a pH of 1 to 8, and a volume ratio of gas:liquid
in said reaction mixture of 1:2.5 to 1:6.
2.  The process for the chlorine-free bleaching of pulps defined in claim 1 wherein according to step (d) the reaction mixture has a residence time in the reactor of about 2 minutes.
3.  The process defined in claim 1 wherein:
said temperature is 40.degree.  to 70.degree.  C.;
said ozone-containing gas introduced into said suspension contains 50 to 150 g/m.sup.3 ozone;
said suspension has a consistency of 5 to 20 mass percent;  and
4.  The process defined in claim 3 wherein:
5.  The process defined in claim 3 wherein said ozone-containing gas is compressed in a cooled compressor before introduction into said suspension.
6.  The process defined in claim 5 wherein said ozone-containing gas is compressed in a water-ring pump forming said cooled compressor before introduction into said suspension.
7.  The process defined in claim 3 wherein the bleaching is repeated in a subsequent bleaching stage following a prior bleaching stage.
8.  The process defined in claim 7, further comprising the step of effecting an alkali extraction on said mixture between said bleaching stages.
9.  The process defined in claim 3 wherein the ozone bleaching is carried out after an oxygen-reinforced or peroxide-reinforced extraction of the suspension.
10.  The process defined in claim 3 wherein the ozone bleaching is carried out after an oxygen-reinforced or peroxide-reinforced extraction of the suspension followed by an alkaline peroxide treatment step.
11.  The process defined in claim 5 wherein the ozone bleaching is followed by a peroxide stage or an alkali extraction.
12.  The process defined in claim 3 wherein at least part of the suspension is treated prior to contact with said ozone-containing gas with a waste-water filtrate from ozone-treated waste water and, together therewith, with acid for adjusting the
pH of said suspension.
13.  The process defined in claim 12 wherein said acid is sulfuric acid.  Description
Our present invention relates to a process for the bleaching of ligno-cellulosic materials, for example, dissolving pulps, for example hardwood (dissolving) pulps with initial kappa values of 15 to 1, usually 4 to 1, or paper pulps, for example
softwood pulps with initial kappa values up to 30 and generally up to 10, using ozone.
It has already been proposed to utilize ozone as a bleaching agent for ligno-cellulosic materials of the type described in order to enable the bleaching to be effected as much as possible in a chlorine-free manner and thus with greater
Typical of such processes is a process in which the pulp suspension is treated with an ozone-containing gas under vigorous agitation.  The term &quot;vigorous agitation&quot; is used herein to include vigorous mixing.
Indeed, chlorine-free and thus ecologically harmless bleaching of pulps, which can be worked up to paper or fibers, utilizing ozone, is described in numerous patents and publications.  The various processes described differ primarily in the
parameters of the process and the reaction conditions.  An important parameter is the consistency, i.e. the percent by weight which is essentially equivalent to the mass percent of the solid pulp in the aqueous suspension.
Since ozone bleaching normally has not been carried out as the exclusive bleaching operation but is generally provided in combination with other bleaching steps and conventional bleaching can scarcely be carried out at such high consistency
ranges, expensive dewatering units must be provided to prepare the pulp suspension for the ozone bleaching.  The reaction of ozone with pulp is a two phase reaction which proceeds rapidly to completion.
Aside from the high capital cost of equipment for carrying out HC bleaching for the reason given above, i.e. the cost of the dewatering units, a disadvantage of the HC process is a nonhomogeneous cellulose-damaging ozone attack which appears to
be most pronounced at low initial kappa starting values.
(The significance and definition of kappa can be found in col.  2 of U.S.  Pat.  No. 4,229,252).
In the literature, therefore, it has been suggested that HC ozone bleaching should not be used at kappa values below 10 (Lindholm C. -A. &quot;Effect of pulp consistency and pH in Ozonbleaching&quot; Part 4 Paperi ja Puu--Paper and Timber 2/1989;
Lindholm C. -A. &quot;Effect of pulp consistency and pH in Ozonbleaching&quot; Part 2 1987 Int.  Oxygen Delignification Conference, San Diego, Jun.  7-11, 1987, Proceedings, p. 155; Lindholm C. -A. &quot;Effect of pulp consistency and pH in Ozonbleaching&quot; Part
3 Nordic Pulp and Paper Research Journal, No. 1/1988.)
The only alternative according to the state of the art is the LC ozone bleaching process if one wishes to avoid the use of chlorine containing environmentally hazardous compounds.  The LC ozone bleaching process by comparison to the HC process
utilizes more ozone, is more complicated to carry out and requires a greater amount of mixing energy.  Furthermore, the reaction volumes are greater and the danger of importing dirt into the process is increased.
For example, U.S.  Pat.  No. 4,216,054 emphasizes a consistency range of up to 0.7%.  Such a consistency range means that the equipment must include a significant investment for a closed water circulation system.  This patent describes systematic
investigations of LC technology for kraft pulp and concludes that the reaction of ozone with the cellulose is limited by two barriers, namely, the transfer of the ozone from the gas phase to the liquid phase and the transfer from the liquid phase to the
solid phase i.e. to the fibers from a minimum mixing power of 11 kW/m.sup.3 the second transfer remains rate determinative according to this patent.
An LC bleaching process is also described in U.S.  Pat.  No. 4,080,249.  It is suggested that the agitation energy should amount here preferably to at most 18 kWh/t of the pulp suspension.  The bubbles of the ozone containing gas should have a
size of at most 3 millimeters.  In all of the examples of this patent, consistencies of between 1 and 2% are described, thereby clearly indicating that the document refers to an LC process.
AS part of a broadcast disclosure, apparently to foreclose circumvention of the patent, mention is made of consistencies up to 10% although it is clear in any case that consistencies below 3% are preferred, thereby providing an equally clear
indication that consistencies above 3% are not preferred or are detrimental.
Substantially the same can be said for U.S.  Pat.  No. 4,372,812.  Here there is an equally broadcast disclosure of between 1 and 40% although the example only operates in the LC range, namely, with a consistency of 1% (see table 1 of this
patent).  This document also deals with a multistage bleaching process in which ozone is introduced into one or more stages but not with an ozone bleaching stage per se.
It is, therefore, the principal object of the present invention to provide an improved method of or process for the bleaching of ligno-cellulosic materials, particularly the materials described above, whereby the aforementioned drawbacks of both
HC and LC ozone bleaching processes are avoided and the overall process can be carried out more economically and efficiently while remaining ecologically harmless.
We have discovered that the prior art drawbacks can be obviated most surprisingly by providing a middle consistency or middle-consistency range operation which has heretofore not been found to be economical or possible with ozone bleaching by
utilizing a pulp suspension having a consistency of 3 to 20 mass percent preferably 5 to 20 mass percent, and even more advantageously 7 to 15 mass percent, by injecting the gas at a superatmospheric pressure of 1 to 15 bar and preferably 1.1 to 10 bar
into the pulp suspension.
The process is carried out utilizing ligno-cellulosic materials derived from hardwood (dissolving) pulps with a deciduous wood cellulose with a initial kappa value of 15 to 1, preferably 4 to 1 or with paper pulps or softwood pulps with an
initial kappa value up to 30 and preferably up to 10, by bleaching the pulp suspension at a temperature of 15.degree.  to 80.degree.  C., preferably 40.degree.  to 70.degree.  C., at a pH value of the suspension and the mixture of 1 to 8, preferably 2 to
3, utilizing the ozone containing gas injected at the superatmospheric pressure with various agitation of the mixture.  The ozone containing gas can contain 20 to 300 g/m.sup.3 ozone, preferably 50 to 150 g/m.sup.3 ozone, and the ozone containing gas is
supplied to the suspension in an amount corresponding to at most 2 mass percent based upon the dry pulp content of the suspension treated and preferably 0.05 to 0.5 mass percent of the dry pulp treated.  Throughout this description, mass percent can be
considered interchangeable with weight percent.
We have found that operating in the middle consistency range described has the advantage over the LC bleaching technique that the reaction vessel can be significantly smaller and the important advantage over the HC technique that in spite of the
small volume treated, no expensive dewatering units of the type required by the HC technique are necessary.,
By injecting the ozone containing gas under pressure simultaneously with vigorous agitation or mixing we are able most surprisingly to obtain excellent bleaching results in the middle-consistency range.  More specifically we obtain a homogeneous
and uniform efficient reaction of the cellulose with ozone.  The mixing energy required is less than in the case of LC bleaching and the reaction of the ozone with the cellulose is carried out more homogeneously than in the HC bleaching technique.
The specific ozone consumption (O.sub.3 consumption per eliminated kappa point) is significantly lower than in the case of the LC process.
Existing apparatus can be readily retrofitted or converted to the MC process since apart from the pH-controlled acidification (which is required also for LC and HC processes) it is merely necessary to provide an middle-consistency range pump and
an MC mixer.  Waste water recycling and reuse of reaction waste gas which may have a residual ozone content is possible so that the system can operate in an ecologically harmless manner taken as a whole, even considering mixing energy, ozone quantities
used and the requisite equipment, the process is highly economical.
A further advantage of the invention can be obtained when the bleaching process is used in a multistage bleaching of pulp as the ozone stage.  In this system the ozone stage can be utilized together with oxygen bleaching and all operations can be
carried out in the middle consistency range with the advantage that a change in the pulp consistency by dewatering or the addition of liquid is not necessary.  The overall process is therefore highly economical.
It is known from Austrian Patent 380 496 to carry out an ozone bleaching with pressure.  In this process, however, the pulp suspension in the LC range (2.5 to 4.5% consistency) is intensively contacted with an ozone containing gas under pressure
(4 bar in the example).  Thereafter, the pulp is dewatered to a consistency of 10 to 30% and must be held during the dewatering for at least 20 minutes at the same pressure and the same temperature.  According to this patent there is an after reaction
which involves an intimate contact of the LC pulp with the ozone containing gas (page 3, lines 41 to 45 of the patent).
By contrast with this disclosure, the present invention has discovered that middle-consistency range pulp can be directly treated with ozone containing gas provided that the gas is under pressure and the process is carried out with simultaneous
vigorous agitation.  A dilution and dewatering of the pulp suspension as is required by Austrian Patent 380 496 (see page 3, lines 19-20 and 35-36) is unnecessary.
High-shear mixers are known and have been used for various purposes.  For example we may use the high-shear mixer utilized for the dispersion of pigments or dyestuffs in German Patent Document 24 06 430, the high-shear mixer used in the
production of PVC powder in U.S.  Pat.  No. 3,775,359, the high-shear mixer used for the production of semisolid emulsions in U.S.  Pat.  No. 3,635,834, or the high-shear mixer used in conjunction with pulp suspensions in Japanese Patent 63099389.
It has been found to be advantageous to repeat the ozone bleaching, i.e. to carry out the bleaching process as described in a plurality of successive stages, between which an alkali extraction can be optionally effected.  The alkali extraction
can be carried out with the use of oxygen or peroxide.  This multistage operation can be carried out in a simplified manner in practice by recovering a portion of the pulp downstream of the reactor and recirculating it to the high-shear mixer.
In addition or alternatively, the ozone bleaching step or steps can be followed by a peroxide bleaching stage and/or an alkali extraction.  In peroxide bleaching stages oxygen can also be included.
It has also been found to be advantageous to bring the waste water filtrate resulting at the O.sub.3 treatment into contact with at least part of the pulp suspension before the latter is contacted with the ozone containing gas.  Together with the
waste water filtrate, we may feed to the suspension the acid required to establish the required pH value, preferably sulfuric acid.  Since the waste water filtrate is acidic, this method allows a saving in acid.  Furthermore, the waste water filtrate can
be reused so that it need not be discharged to become a burden to the environment.
If softwood pulp with an initial kappa value of 30 to 10 is used in the process, kappa values below 10 and as low as 5 can be reached by the bleaching operation.  If hardwood pulp with initial kappa values of 15 to 1 is used and preferably kappa
values of 4 to 1, the product will have kappa values of 12 to 0.5 or 1.5 to 0.5.  Initial brightness of 50 to 80%, generally 70 to 80%, can be raised to at least 65 to 90% and usually 75 to 90%.
With the process of the invention it is advantageous to prescribe the molecular weight distribution of the dissolving pulp to obtain best results.  For a given pulp, by variation of pH value, the charge of ozone used and the temperature, within
the ranges specified, the desired viscosity, DP distribution and reactivity, measured at the filter value can be maintained.
The pump 3 pumps the suspension into the middle-consistency range mixer 4 which is a high-shear mixer as described.  Ozone containing gas is fed at 7 through the cooled water ring compressor 8 to the mixer 4 where it enters the mixer and
maintains it under pressure.  In the middle-consistency range mixer 4 an intimate rapid pressurized mixing of the suspension and the ozone containing gas is effected.
The reaction continues in a reactor 5 which can be a tube reactor and which is maintained under the pressure of the ozone containing gas.  At the end of the reactor 5 a feedback 9 is provided in the form of a pipe and pump to return a portion of
the pulp suspension to a location upstream of the mixer so that the pulp suspension is repeatedly subjected to bleaching operation.  In both FIGS. 1a and 1b, the binding of the gas treated solid suspension is carried out in a conventional bleaching tower
10 which has been illustrated although its use is not absolutely necessary.  FIG. 1a shows the tower to be traversed upwardly and FIG. 1b shows an embodiment wherein the tower 10 is traversed downwardly.
In the use of the downwardly flowing tower (FIG. 1b), the pulp suspension from the reaction tube 5 is passed through the throttle 6 to a degassing unit 16 wherein the pressure is relieved to atmospheric pressure.  The suspension then passes by
gravity through the tower 10 and is transferred to the washing filter 12.  Diluting water can be added to the tower if desired.
In both embodiments, the waste gas which may still contain small quantities of ozone may be subjected to treatment by an ozone gas removal process, for example, catalytic or thermal ozone destruction.  The oxygen resulting from the waste gas
ozone destruction can be fed to an oxygen bleaching stage and the oxygen excess, after appropriate cleaning can be returned to the ozone generator.  To the extent that the oxygen is not fed to an oxygen bleaching stage, it can be completely recycled to
the ozone generator after any required cleaning steps.
______________________________________ Kappa unwashed 2.1  Kappa washed 1.9  brightness 76% (Elrepho)  viscosity 255 mP (cuoxam)  COD accompanying waste water  5 g/kg dry pulp  ______________________________________
______________________________________ Pressure 5.2 bar  consistency 10%  temperature 47.degree. C.  pH 2.3  spec. O.sub.3 -change 1.82 g O.sub.3 /kg  spec. O.sub.3 -consumption  1.69 g/kg  Ozone conc. in fresh gas  76.8 mg/l (STP)  Ozone conc.
in waste gas  5.2 mg/l (STP)  Reaction Time 120 s  mixing time 20 s  V.sub.g /V.sub.l 1/3.2 (at 5.2 bar)  speed of high-shear mixer  1700 RPM  ______________________________________
______________________________________ Kappa 0.9  delta Kappa 1.85  O.sub.3 consumption/delta Kappa  0.91  brightness 83.5%  delta brightness 7.5%  viscosity 214 mP  delta viscosity 41 mP  ______________________________________
______________________________________ Kappa unwashed 2.9  Ozone bleaching parameters  Pressure 5.0 bar  Consistency 9.5%  Temperature 50.degree. C.  pH 2.5  spec. O.sub.3 -change 1.60 g/kg  spec. O.sub.3 -consumption  1.60 g/kg  Ozone conc. in
fresh gas  79.7 mg/l (STP)  Ozone conc. in waste gas  1.3 mg/l (STP)  Reaction Time 120 s  mxing time 20 s  V.sub.g /V.sub.l 1/2.6 (at 5.0 bar)  speed of high-shear mixer  3,200 RPM  ______________________________________
______________________________________ Kappa 1.25  delta Kappa 1.65  O.sub.3 consumption/delta Kappa  0.95  brightness 82.5%  delta brightness 6.5%  viscosity 227 mP  delta viscosity 28 mP  ______________________________________
______________________________________ Kappa 1.9  Viscosity 255 mP  Brightness 76%  ______________________________________
______________________________________ Pressure 5 bar  Stock density 10%  Temperature 50.degree. C.  pH 5.0  spec. O.sub.3 -change 1.5 g/kg  spec. O.sub.3 -consumption  1.13 g/kg  Ozone conc. in fresh gas  78. mg/l (STP)  Ozone conc. in waste gas 17. mg/l (STP)  Reaction Time 120 s  mixing time 120 s  V.sub.g /V.sub.l 1/2.6 (at 5 bar)  speed of high-shear mixer  3200 RPM  ______________________________________
______________________________________ Kappa 1.1  delta Kappa 0.95  O.sub.3 consumption/delta Kappa  1.25  brightness 82.0%  delta brightness 6.0%  viscosity 218 mP  delta viscosity 37 mP  ______________________________________
______________________________________ Pressure 5.0 bar  Consistency 10.7%  Temperature 23.degree. C.  pH 2.5  spec. O.sub.3 -change 1.6 g/kg  spec. O.sub.3 -consumption  1.2 g/kg  Ozone conc. in fresh gas  83.2 mg/l (STP)  Ozone conc. in waste
gas  21 mg/l (STP)  Reaction Time 120 s  mixing time 120 s  V.sub.g /V.sub.l 1:2.6 (at 5 bar)  speed of high-shear mixer  3200 RPM  ______________________________________
______________________________________ Kappa 0.60  delta Kappa 1.3  O.sub.3 consumption/delta Kappa  0.91  brightness 86.3%  delta brightness 10.3%  viscosity 228 mP  delta viscosity 27 mP  ______________________________________
The difference in the solids characteristics between Examples 3 and 4 thus appear to be exclusively a consequence of the different pH values and temperatures.  The pH value also can serve to adjust the viscosity.
The following Examples 5 and 6 relate to spruce sulfite pulp.  The following test standards for the pulp parameters were used.
______________________________________ Breaking length  Austrian Standard  results in m  ONORM L 1114  WRA = Further  German Industrial  results in mNm/m  tearing strength  DIN 53 115  Viscosity Zellcheming results in mPas .multidot. 10
______________________________________ Kappa (Tappi 236 os-76  20.4  Viscosity 1500 mPa s .multidot. 10  Brightness (Elrepho) 49.7%  Breaking length (24 oSR)  8900 m  Breaking length (41 oSR)  9200 m  WRA (24 oSR) 1143 mNm/m  WRA (41 oSR) 1010
mNm/m  Bursting (24.degree. SR)  4.4 kg/cm.sup.2  Bursting (41.degree. SR)  4.2 kg/cm.sup.2  ______________________________________
Bleaching is carried out by the sequence: EOP-Z.sub.1 -PE.sub.1 -Z.sub.2 -PE.sub.2 (EOP=peroxide-reinforced alkali oxygen treatment;
______________________________________ NaOH-supplied 2.0%/dry solids  H.sub.2 O.sub.2 -supplied  2.0%/dry solids  O.sub.2 -supplied 2 bar  Consistency 10%  Residence Time 3 h  Temperature 80.degree. C.  ______________________________________
______________________________________ Kappa 6.6  Brightness 75.5%  Viscosity 1498 mPas .multidot. 10  Breaking length  7800 m (24 oSR); 8300 m (37 oSR)  WRA 810 mNm/m (24 oSR); 1507 mNm/m 37 oSR)  Bursting strength  3.3 kg/cm.sup.2 (24 oSR);
3.5 kg/cm.sup.2 (37 oSR)  ______________________________________
With this EOP-prebleached cellulose the remainder of the Sequence Z.sub.1 -PE.sub.1 -Z.sub.2 -PE.sub.2 was carried out in three different ways V.sub.1, V.sub.2, V.sub.3.
b) O.sub.3 the stage--1 (Z.sub.1)
______________________________________ Parameter V1 V2 V3  ______________________________________ Consistency (%)  8.5 8.2 9  Pressure (bar)  5.6 5.6 5.6  Temperature (.degree.C.)  20 31 44  pH 2.5 2.5 2.5  mixing time (s)  15 15 15  Reaction
time (s)  120 120 120  Speed (RPM) 3200 3200 1500  Spec. O.sub.3 -change  1.85 1.78 1.94  (kg/t)  Spec. O.sub.3 -consumption  1.80 1.70 1.86  V.sub.1 /V.sub.g (at 5.6 bar)  3.1 2.87 2.61  Kappa 4.9 4.5 4.0  delta Kappa/O.sub.3  0.94 1.2 1.40  consumption Brightness (%)  73.0 73.4 73.2  Viscosity (mPas .multidot. 10)  1048 971 976  ______________________________________
c) PE.sub.1 -stage
______________________________________ Parameter V1 V2 V3  ______________________________________ NaOH-supplied (% based  1.0 1.0 1.0  upon dry cellulose  H.sub.2 O.sub.2 -supplied  0.7 0.7 0.7  (% based upon dry cellulose)  Consistency (%) 10 10
10  Residence time (h)  2 2 2  Temperature (.degree.C.)  65 65 65  Kappa 3.2 3.2 2.7  Brightness (%) 83.5 84.3 85.2  Viscosity (mPas .multidot. 10)  1047 981 972  ______________________________________
d) Ozone stage--2 (Z.sub.2)
______________________________________ Parameter V1 V2 V3  ______________________________________ Consistency (%) 8 8 8  Pressure (bar) 5.6 5.6 5.6  Temperature (.degree.C.)  21 33 45  pH 2.5 2.5 2.5  Mixing time (s) 15 15 15  Reaction time (s)
120 120 120  Mixer-Speed (RPM)  3200 1800 3200  Spec. Ozone change  2.70 2.38 2.34  (kg/t)  Spec. Ozone consumption  2.06 1.85 1.92  (kg/t)  V.sub.1 /V.sub.g (at 5.6 bar)  2.5 2.6 2.5  Kappa delta 1.24 1.19 1.19  Kappa/Ozone-consumption  0.95 1.08 0.79
Brightness (%) 82.3 83.9 83.5  Viscosity (mPas .multidot. 10)  679 581 631  ______________________________________
e) PE.sub.2 -stage
______________________________________ Parameter V1 V2 V3  ______________________________________ NaOH-supplied (% based  0.7 0.7 0.7  upon dry pulp  H.sub.2 O.sub.2 -supplied  0.5 0.5 0.5  (% based upon dry pulp  Consistency (%) 10 10 10
Temperature (.degree.C.)  65 65 65  Kappa 0.6 0.6 0.6  Brightness (%) 90.6 90.0 90.0  Viscosity (mPas .multidot. 10)  650 583 577  Tear Length  (oSR) m 7600 (20) 7900 (21)  7500 (20)  (oSR) m 8000 (34) 8200 (36)  8000 (35)  WRA  (oSR) mNm/m 1043 (20)
1080 (21)  1060 (20)  (oSR) mNm/m 1100 (34) 1040 (36)  1047 (35)  Bursting strength  (oSR)kg/cm.sup.2  3.13 (20)  3.30 (21)  3.27 (20)  (oSR)kg/cm.sup.2  3.50 (34)  3.37 (36)  3.43 (35)  ______________________________________
The strength values correspond, in spite of the exceptionally high degree of brightness (greater than 90% and the low viscosity), to those of standard bleached pulp.  By standard bleaching we refer to the sequence C-PE-H-H wherein C refers to
chlorine bleaching and H to hypochlorite bleaching.
The same raw material is used as in Example 5, i.e. spruce sulphite pulp and is subjected to the bleaching sequence EOP-Z-PE the conditions V4, V5 of the final bleaching stage PE were varied with the goal of obtaining a degree of brightness
greater than 85% with the highest possible strength values.
______________________________________ Parameter  ______________________________________ Consistency (%) 12  Pressure (bar) 6.2  Temperature (.degree.C.)  24  pH-Value 2.5  Mixing time (s) 15  Reaction time (s) 120  MC-Mixer-Speed (RPM) 1700
spec. Ozone change (kg/t)  2.62  spec. Ozone consumption (kg/t)  2.37  V.sub.1 /V.sub.g 2.56  Kappa 3.7  delta Kappa/ozone-consumption  1.22  Viscosity (mPas .multidot. 10)  771  Brightness (%) 75.7  ______________________________________
______________________________________ Parameter V4 V5  ______________________________________ NaOH-supplied (% based  2.5 2.5  upon dry pulp  H.sub.2 O.sub.2 -supplied  1.0 1.5  (% based upon dry pulp  Consistency (%) 10 10  Residence Time 3 3
Temperature (.degree.C.)  65 65  Mg-Salt (%) 0.2 0.2  Kappa 86.2 87.1  Brightness (%) 86.2 87.1  Viscosity (mPas .multidot. 10)  904 713  Breaking Length (oSR) m  7900 (23 7800 (21)  m 8200 (23)  8100 (35)  WRA (oSR) mNm/m 1020 (23)  1030 (21)  Bursting
Strength (oSR) kg/cm.sup.2  3.40 (23)  3.3 (21)  ______________________________________
The strength values of the pulp resulting from this three-stage bleaching corresponded substantially to those of the five-stage bleached pulp.  With sequential use of lesser specific ozone quantities, the strength characteristics of the pulp are
not effected but a much higher degree of brightness can be obtained.
Process for the chlorine-free bleaching of cellulosic materials with ozone, Sixta, et al., Herbert Sixta, Gerhard Gotzinger, Anton Hoglinger, Peter Hendel, Wilfried Ruckl, Walter Peter, Friedrich Kurz, Alfred Schrittwieser, Manfred Schneeweisz, Application number 07 859-236, Paper Making And Fiber Liberation, Expiration Date, patent applications, E. globulus, aqueous suspension, gaseous components, thin membrane, oxygen sensor, conductive coating, parylene coating, waste tank
Our present invention relates to a process for the bleaching of ligno-cellulosic materials, for example, dissolving pulps, for example hardwood (dissolving) pulps with initial kappa values of 15 to 1, usually 4 to 1, or paper pulps, for examplesoftwood pulps with initial kappa values up to 30 and generally up to 10, using ozone.BACKGROUND OF THE INVENTIONIt has already been proposed to utilize ozone as a bleaching agent for ligno-cellulosic materials of the type described in order to enable the bleaching to be effected as much as possible in a chlorine-free manner and thus with greaterenvironmental protection.Typical of such processes is a process in which the pulp suspension is treated with an ozone-containing gas under vigorous agitation. The term "vigorous agitation" is used herein to include vigorous mixing.Indeed, chlorine-free and thus ecologically harmless bleaching of pulps, which can be worked up to paper or fibers, utilizing ozone, is described in numerous patents and publications. The various processes described differ primarily in theparameters of the process and the reaction conditions. An important parameter is the consistency, i.e. the percent by weight which is essentially equivalent to the mass percent of the solid pulp in the aqueous suspension.In principle, these processes can be considered to be in either of two categories, namely, the high consistency (HC) or the low consistency (LC) techniques.HC ozone bleaching is carried out with consistencies in excess of 25% and generally around 35 to 40%.Since ozone bleaching normally has not been carried out as the exclusive bleaching operation but is generally provided in combination with other bleaching steps and conventional bleaching can scarcely be carried out at such high consistencyranges, expensive dewatering units must be provided to prepare the pulp suspension for the ozone bleaching. The reaction of ozone with pulp is a two phase reaction which proceeds rapidly to completion.Aside from the
Chemistry of Chlorine Dioxide Pulp Bleaching
CHLORINE DIOXIDE BLEACHING OF SODA-ANTHRAQUINONE JUTE PULP TO A VERY
Chlorite-permanganate cellulosic bleaching process
Chinese Markets for Bleaching Agent
Generate chlorine dioxide for pulp bleaching
BENEFITS OF OZONE BLEACHING
Bleaching Technology - PDF