Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Reexamination Certificate
1998-09-23
2001-12-04
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
C162S080000, C162S083000, C162S090000
Reexamination Certificate
active
06325892
ABSTRACT:
FIELD OF THE INVENTION
The present invention is concerned with a single stage process for decreasing carbohydrate degradation of sulphite pulps during the O
MgO
process by the addition of a catalytic amount of sodium borohydride in situ to provide a pulp with enhanced strength properties and increased viscosity.
BACKGROUND OF THE INVENTION
Because of incensing environmental concerns worldwide, pulp and paper mills discharge effluents are constantly under scrutiny to ensure that environmental regulations are followed. Because of the high costs involved in the treatment of effluents before their release in the environment, a great deal of research is directed to the modification of current pulp and paper production processes. The research concentrates its efforts in replacing toxic reagents with more environmentally friendly products. A further benefit sought with such changes is that effluents will hopefully require fewer costly conditioning treatments before their release in the environment.
In the various processes proposed in the literature, the oxygen delignification technology is one of the available options towards this direction. Conventionally, oxygen delignification technology uses sodium hydroxide as the alkaline source and the resulting effluent produced can therefore be incorporated into the chemical recovery system of the process for preparing kraft pulps because the same reagent, namely sodium hydroxide, is used, and therefore, there is no reagent interference. On the other hand, the effluent from the sodium hydroxide-based oxygen delignification process (referred to as the O
NaOH
technology herein) cannot be sent to the recovery system of the magnesium-based sulphite process because, obviously, the sodium salts are not compatible with the magnesium-based sulphite recovery process. Several publications have therefore concluded that magnesium oxide-based oxygen delignification technology, referred to as O
MgO
herein, is preferred for magnesium-based sulphite pulping processes. (see for example Bokstrom et al.,
Pulp and Paper Canada,
1992, 92 (11), 38; and Luo et al.,
Tappi Journal,
1992, 75 (6), 183).
Sodium hydroxide has been replaced lately as a base with magnesium oxide (MgO) or magnesium hydroxide (Mg(OH)
2
) for the oxygen delignification of sulphite pulps. However, because of the low alkalinity of MgO or Mg(OH)
2
, the temperature of delignification with MgO or Mg(OH)
2
must be about 30° C. higher than for the same process using NaOH as the delignification agent (see Luo et al., supra). Alternatively, the delignification rate can be increased in the O
MgO
process by the addition of a very limited amount of NaOH, since small concentrations of sodium salts can be tolerated in the recovery system of magnesium-based sulphite process. However, the risk of contamination in the long run is such that this alternative does not represent a desirable selection.
Changing the alkali source in the oxygen delignification process from sodium hydroxide to magnesium oxide or magnesium hydroxide, as taught by Bokstrom et al. supra, decreases the selectivity of lignin to carbohydrate degradation. Moreover, the strength properties also decrease, as illustrated in the relationship between tear index versus tensile index of FIG. 6, by Luo et al. supra. For a given type of wood chips used as starting material, it is well known that sulphite pulps usually have strength properties inferior to that of kraft pulp, and a further decrease in strength properties during the delignification process is therefore unacceptable for commercial operations.
It is known that a post treatment stage with sodium borohydride on an oxidized pulp, such as ozone delignified pulp, leads to increased pulp viscosity. For example, it was reported by Chirat et al. in Holzforschung, 1994, 48 Suppl. 133, that a reduction treatment stage with 0.1% sodium borohydride increases the viscosity of ozone bleached pulp from DP
v
of 710 to 920. The chemistry of sodium borohydride reduction is well understood: carbonyl groups present in carbohydrates are reduced to alcohol functionalities (B. Browning, Methods of Wood Chemistry, Vol. 2, P. 685, Interscience Publishers).
In addition, it is proposed by S. Beharic in Papir Dec. 20, 1992, 3(4) pp. 11-15 to add sodium borohydride either before ozone bleaching or after peroxide bleaching to limit the reduction in pulp viscosity. Again, two stages are involved for this pulp treatment.
Accordingly, there is therefore a great need to develop an oxygen delignification process providing pulps with enhanced strength properties and increased viscosity. Preferably, a single stage bleaching process should be considered, wherein a reducing agent would be added in situ. This would represent a significant advance in pulp bleaching, and bring significant benefits to the industry, because the elimination of one treatment stage of pulp represents a significant capital cost reduction.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is now provided an improvement to oxygen delignification process of pulps. More specifically, the present invention comprises the conventional steps of oxygen delignification of pulp, namely treating a pulp slurry in a reactor under oxygen overpressure in the presence of an alkali source, over a period of time sufficient to complete delignification of the pulp, with the improvement comprising adding in situ an effective amount of a reducing agent substantially non-reactive with the oxygen, to provide delignified pulps with enhanced strength properties and increased viscosity.
The present oxygen delignification process is particularly advantageous for sulphite pulps when MgO or Mg(OH)
2
are used as the alkali source. Other possible alkali sources include Ca(OH)
2
, NH
4
OH, NaOH and the like. Reducing agents include sodium borohydride, sodium hydrosulphite and the like, with sodium borohydride being the most preferred.
In another aspect of the present invention, the process comprises a first stage wherein the pulp is treated with the reducing agent, and then washed and pressed if necessary, and a second conventional oxygen delignification stage. Although good results are obtained with the two-stage process, the single stage process is much preferred because of the elimination of washing and pressing operations required after treatment with the reducing agent in the two-stage process.
REFERENCES:
patent: 3759783 (1973-09-01), Samuelson et al.
patent: 4248662 (1981-02-01), Wallick
patent: 4248663 (1981-02-01), Kubes et al.
patent: 4384921 (1983-05-01), Pihlajamaki et al.
patent: 4574032 (1986-03-01), Ringley
patent: 4915785 (1990-04-01), Siminoski et al.
patent: 4938842 (1990-07-01), Whiting et al.
patent: 5139617 (1992-08-01), Tikka et al.
patent: 5391261 (1995-02-01), Van Den Bergh
patent: 5449436 (1995-09-01), Bourson et al.
patent: 5464501 (1995-11-01), Kogan et al.
patent: 5534115 (1996-07-01), Hoyos et al.
patent: 5611889 (1997-03-01), Leary et al.
patent: 611510 (1960-12-01), None
patent: 2157154 (1996-03-01), None
patent: 38 09 332 (1989-10-01), None
patent: 2692917 (1993-12-01), None
patent: 0010451 (1980-04-01), None
patent: 0262988 (1988-04-01), None
patent: 2692917 (1993-12-01), None
patent: 96053654 (1996-02-01), None
Michell et al, “Bleaching and Yellowing of Eucalyptus Chemimechanical Pulps. Diffuse Reflectance Spectra of Oxygen Bleached Pulps”, Appita, vol. 44, No. 5, Sep. 1991.*
Chirat et al., Paperi Ja Puu, 1994, 76 (6-7), 417-422.
Beharic S., Papir 20, 1992, 20 (3-4), 11-15 (English Abstract Only).
Bokstrom et al., Pulp & Paper Canada, 1991, 92(11) 38-43.
1997 Pulping Conference—Tappi Proceedings, Oct. 19, 1997.
Kang Guo Jun
Ni Yonghao
Skothos Anastasios
van Heiningen Adriaan R. P.
Alvo Steve
University of New Brunswick
Zavis Katten Muchin
LandOfFree
Method of delignifying sulphite pulp with oxygen and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of delignifying sulphite pulp with oxygen and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of delignifying sulphite pulp with oxygen and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2592530