Delignification of lignocellulosic material with a soda pulping liquor containing a Diels Alder adduct of benzoquinone or naphthoquinone in admixture with a nitro aromatic compound

Delignification of lignocellulosic material, such as wood, straw or bagasse, with a soda pulping liquor containing a diketo hydroanthracene selected from the unsubstituted and lower alkyl-substituted Diels Alder adducts of naphthoquinone and benzoquinone and a nitro aromatic compound selected from mono and di-nitrobenzenes and the amino, carboxy, hydroxy and methyl derivatives of said nitrobenzenes.

This invention relates to a process for the delignification of 
lignocellulosic material such as wood, straw, bagasse, etc. 
The processing of lignocellulosic material to produce cellulose suitable 
for the manufacture of paper products involves the removal of lignin and 
other non-cellulosic components such as gums. Reagents that attack lignin 
without affecting appreciably the cellulose component are preferred for 
this purpose. In the sulphate or kraft process, lignocellulosic material 
is cooked with a mixture of sodium hydroxide and sodium sulphide. In the 
soda process the cooking is carried out with sodium hydroxide alone. In 
Canadian Pat. No. 895,756, issued on Mar. 21, 1972 to H. E. Worster and M. 
F. Pudek, there is described a two stage soda-oxygen pulping process 
comprising a first stage sodium hydroxide digestion, followed by 
defiberization of the product of the sodium hydroxide digestion, and a 
second stage digestion with sodium hydroxide in the presence of excess 
oxygen. This process produces pulp in yield comparable to the yield of a 
conventional kraft process. Although these processes are effective in the 
removal of lignin from lignocellulosic material such as wood, the 
cellulose component of the material is attacked also to a certain degree, 
resulting in a lowering of yields and degradation of the product. 
The contribution to air pollution of volatile mercaptans and hydrogen 
sulphide is a serious disadvantage of the kraft process. The soda process 
is superior in this respect; however, the soda process is unsuitable for 
pulping coniferous woods because of long cooking times and low yields. 
Even in the case of hardwoods, yields are inferior to those achieved using 
the kraft process. A recent publication (B. Bach and G. Fiehn, Zellstoff 
Papier 21, No. 1,3-7, January 1972) and a related East German Pat. No. 
98,549 of June 20, 1973 disclose the use of anthraquinone-2-monosulphonic 
acid (AMS) as a means of improving yields in the soda process. More 
recently, U.S. Pat. No. 3,888,727) this additive was employed in the first 
stage of a soda-oxygen process, resulting in yields superior to those of a 
conventional kraft process; the pulp possessed strength properties 
comparable to kraft. Unfortunately, the soda-AMS pulping process does not 
eliminate the odour problem, since sulphur derived from the additive is 
converted to sulphide in the pulping chemicals recovery systems and thence 
to mercaptans or hydrogen sulphide during the next cooking cycle. The 
economic advantages resulting from higher yields are largely offset by the 
relatively high cost of AMS. Other derivatives previously evaluated in 
soda cooking (Bach and Fiehn, above), not containing sulphur, were 
substantially less effective than AMS. 
In U.S. patent application Ser. No. 718,980 filed on Aug. 30, 1976, now 
U.S. Pat. No. 4,012,280, it is proposed to use, instead of AMS as an 
additive in the soda process, a sulphurfree cyclic keto compound such as, 
among others, naphthoquinone, anthraquinone, anthrone, phenanthrenequinone 
and the alkyl, alkoxy and amino derivatives of said quinones. Compared to 
AMS, these quinone additives have the very great advantages that they do 
not contribute to pollution and that for a given concentration and under 
comparable pulping conditions, they are more effective. 
The Pulp and Paper Research Institute of Canada has reported [Sv. Pappers. 
71 (23) 857-863 (1968)] the effects of several nitro aromatic compounds in 
accelerating and improving the yields from the soda pulping of softwood. 
While not the most effective, nitrobenzene is identified in this 
publication as the only additive of commercial significance. Large amounts 
of nitrobenzene are used (1-10%) resulting in yields equivalent to that of 
the kraft process. However, the process is not felt to be commercially 
practicable due to severe deficiencies in cooking time and poor strength 
properties when compared to the kraft process. 
It has now been found that lignocellulosic material can be delignified in 
higher yield than heretofore attained by a process which comprises a 
digestion with a soda pulping liquor in the presence of a Diels Alder 
adduct of naphthoquinone or benzoquinone together with a nitro aromatic 
compound. Optionally, the digestion with the soda pulping liquor may be 
followed by a second stage digestion in alkaline medium with oxygen or an 
oxygen-containing gas under pressure. Compared to the above prior 
processes wherein a cyclic keto compound or an aromatic nitro compound is 
used alone as an additive, the novel process provides a pulp in a much 
higher yield at a given kappa number with a comparable rate of 
delignification and comparable strength properties. When used in 
combination with Diels Alder adducts as in the novel process of this 
invention, nitro aromatic compounds have been found to exert negligible 
negative effects on pulp properties (viscosity) and key paper making 
parameters whereas when used alone, they are not commercially practicable 
as is indicated in the above publication of the The Pulp and Paper 
Research Institute of Canada. 
Thus the main object of the invention is to provide a soda pulping process 
for the efficient digestion of softwood. Another object is to provide a 
soda pulping process that gives an increased yield of cellulosic pulp as 
compared to that of the kraft process. A further object is to provide a 
pulping process that has a low pollution potential. Additional objects 
will appear hereinafter. 
The process of the invention comprises the steps of 
1. treating lignocellulosic material in a closed reaction vessel with a 
pulping liquor containing alkali metal base and, as additives, from 0.001% 
to 10.0% by weight, based on the lignocellulosic material, of a diketo 
hydroanthracene selected from the unsubstituted and the lower 
alkyl-substituted Diels Alder adducts of naphthoquinone and benzoquinone, 
and from 0.01% to 10.0% by weight, based on the lignocellulosic material, 
of a nitro aromatic compound selected from the group consisting of mono- 
and di-nitrobenzenes and the amino, carboxy, hydroxy and methyl 
derivatives of said nitrobenzenes, the treatment taking place at a maximum 
temperature in the range of 150.degree. C. to 200.degree. C. for a period 
of 0.5- 480 minutes, and 
2. displacing the pulping liquor from the lignocellulosic material with 
water or an aqueous liquor inert to the lignocellulosic material to obtain 
delignified lignocellulosic material. 
The delignified lignocellulosic material produced by the above two steps 
may be used without further treatment or may be subjected to conventional 
bleaching steps. 
Optionally, the delignified lignocellulosic material may be subjected to 
the following additional treatment steps: 
3. treatment of the delignified lignocellulosic material in aqueous 
suspension at a consistency of 2% to 40% by weight for 0.5 to 60 minutes 
at 20.degree. C. to 90.degree. C. with 2% to 20% by weight of an alkali 
metal base, and 
4. treatment of the alkaline material in aqueous medium at a consistency of 
from 3% to 40% by weight with oxygen or an oxygen-containing gas for 0.5 
to 120 minutes at a temperature of 80.degree. C. to 150.degree. C. and a 
partial pressure of oxygen of 20 to 200 pounds per square inch. 
When the lignocellulosic material employed is wood, this is first converted 
into the form of chips. This step will not be required when the 
lignocellulosic material is of fibrous form. 
The lignocellulosic material may be refined between steps (1) and (2) or 
between steps (2) and (3). Refining can be carried out with known 
equipment such as a single disc or double disc refiner. 
The process of this invention can be used to delignify either coniferous or 
deciduous species of wood. By coniferous is meant species such as pine, 
spruce and balsam fir. By deciduous is meant species such as birch, aspen, 
eastern cottonwood, maple, beech and oak. When employed with a high 
density deciduous wood such as birch, it is preferable to employ a longer 
time to reach maximum cooking temperature in the first step. In spite of 
this, however, the overall cooling time is still greatly reduced in 
comparison with that of the conventional soda process. In the case of high 
density deciduous wood, it is also preferable that the alkali base added 
in the optional third step be so added while the pulp is at a low 
consistency, e.g. 2% to 6%. 
The soda liquor employed in the first step of the process contains from 8% 
to 20% by weight of alkali metal base expressed as percent effective 
alkali, based on the weight of the lignocellulosic material, and normally 
also contains alkali metal carbonate. 
Since the first step treatment of the process is carried out in a closed 
reaction vessel at a temperature in the range of from 150.degree. C. to 
200.degree. C. in the presence of water, the reaction will take place 
under supra atmospheric pressure. 
As mentioned above, the compounds which are suitable for use as additives 
in the process of the invention in combination with the nitro aromatic 
compounds are diketo hydroanthracenes selected from the group consisting 
of the unsubstituted and lower alkyl-substituted Diels Alder adducts of 
naphthoquinone and benzoquinone. These compounds, which are not quinones, 
have surprisingly been found to afford pulping results at least as good 
as, and when used in combination with nitro aromatic compounds much better 
than those obtained with the quinones of the above-identified patent 
application. 
More particularly, the unsubstituted Diels Alder adducts are those obtained 
by reacting 1 or 2 moles of butadiene with naphthoquinone and benzoquinone 
respectively and the lower alkyl-substituted adducts are those obtained 
where, in the above reaction, either one or both of the reactants are 
substituted with the appropriate lower alkyl groups. The alkyl groups in 
the lower alkyl-substituted Diels Alder adducts may range from 1 to 4 in 
number, may each contain from 1 to 4 carbon atoms and may be the same or 
different. Examples of the above defined diketo hydroanthracenes are 
1,4,4a, 9a-tetrahydro-9,10-diketo anthracene, 
2-ethyl-1,4,4a9a-tetrahydro-9,10-diketo anthracene, 2,3-dimethyl-1,4,4a, 
9a-tetrahydro-9,10-diketo anthracene, 1,3-diemthyl-1,4,4a, 
9a-tetrahydro-9,10-diketo anthracene, 1,4,4a, 5,8,8a, 9a, 
10a-octahydro-9,10-diketo anthracene, 2,3,6,7-tetramethyl-1,4,4a, 5,8,8a, 
9a, 10a-octahydro-9,10-diketo anthracene and a mixture of 2,6 and 
2,7-diethyl-1,4,4a,5,8,8a, 9a, 10a-octahydro-9,10-diketo antracene. The 
diketo hydroanthracene additive is employed in proportions of from 0.001% 
to 10.0%, preferably 0.01% to 1.0%, by weight based on the lignocellulosic 
material. 
As is also mentioned above, the nitro aromatic compounds which are suitable 
for use as additives in the process of the invention in combination with 
the diketo anthracenes are selected from the group consisting of mono- and 
dinitrobenzenes and the amino, carboxy, hydroxy and methyl derivatives of 
said nitrobenzenes. Examples of these compounds are nitrobenzene, 
2-nitroaniline, 4-nitroaniline, 4-nitrobenzaldehyde, 4-nitrobenzoic acid, 
2-nitroresorcinol, 4-nitrostyrene, 2-nitrotoluene, 4-nitrotoluene, 
1,2-dinitrobenzene, 1,3-dinitrobenzene, 1,4-dinitrobenzene, 
2,4-dinitrotoluene, 3,5-dinitrobenzoic acid, 4,6-dinitro-o-cresol and 
2,4-dinitroresorcinol. Among the above compounds, nitrobenzene is 
particularly preferred because of its favourable cost:benefit ratio. The 
nitro aromatic compound is employed in proportions of from 0.01% to 10.0%, 
preferably 0.10% to 2.0%, by weight based on the lignocellulosic material. 
It is understood that all the combinations of additives formed from any one 
of the above defined diketo anthracenes with any one of the above defined 
nitro aromatic compounds are suitable for use in the process of the 
invention. Preferred, however, are the combinations comprised of 
nitrobenzene with any one of the diketo hydroanthracenes selected from 
1,4,4a, 9a,-tetrahydro-9,10-diketo anthracene, 2-ethyl-1,4,4a 
9a-tetrahydro-9,10-diketo anthracene, 2,3-dimethyl-1,4,4a, 
9a-tetrahydro-9,10-diketo anthracene, 1,3-dimethyl-1,4,4a, 
9a-tetrahydro-9,10-diketo anthracene, 1,4,4a, 5,8,8a, 9a, 
10a-octahydro-9,10-diketo anthracene and 2,3,6,7-tetramethyl-1,4,4a 
5,8,8a-9a, 10a-octahydro-9,10-diketo anthracene. Especially preferred are 
the combinations of nitrobenzene with 1,4,4a-9a-tetrahydro-9,10-diketo 
anthracene or 1,4,4a, 5,8,8a, 9a, 10a-octahydro-9,10-diketo anthracene. 
After the first step treatment with pulping liquor, the resulting pulp 
yield will be 40% to 70%, by weight, based on the lignocellulosic 
material. The kappa number of the material at completion of the first step 
will lie in the range 10 to 150 for coniferous woods and in the range 5 to 
100 for deciduous woods. 
The partially delignified material resulting from the first treatment step 
is discharged from the pulping vessel and the spent liquor displaced by 
fresh water or optionally by an aqueous liquior inert to lignocellulosic 
material such as the spent liquor from the alkaline oxygen treatment step 
or "white water" from a later stage of a papermaking process. 
Optionally, the delignified lignocellulosic material may then be subjected 
to an alkaline oxygen treatment. To the material is added alkali metal 
base. The alkali metal base may be provided in the form of pulping liquor 
such as used in the first step of the process. This liquor therefore may 
contain carbonate in addition to alkali metal base. Preferably, there is 
also added 0.1% to 1.0%, by weight of the pulp, of a magnesium salt such 
as magnesium chloride or magnesium sulphate calculated as magnesium ion. 
The magnesium salt may be added directly as the salt or as a complex 
formed with the spent liquor from the alkaline oxygen treatment step. 
The alkaline treated material is then fed into an oxygen treatment vessel. 
The material is then treated with oxygen or an oxygen-containing gas under 
a partial pressure of oxygen of from 20 to 200 pounds per square inch. The 
product of the oxygen treatment is separated from the spent liquor and 
washed with water. It will have a residual lignin content of 1% to 6%, 
preferably 1.5% to 4.5% of the weight of the original cellulosic material 
corresponding to a yield of 80% to 98% by weight. 
The alkali metal base employed as reagent in the process of this invention 
may be sodium hydroxide, potassium hydroxide, sodium carbonate or 
potassium carbonate. 
The material resulting from step (2) may be bleached by any conventional 
bleaching process. A conventional sequence comprising chlorination, 
alkaline extraction, chlorine dioxide treatment, alkaline extraction, 
chlorine dioxide treatment (C-E-D-E-D) when applied to the material 
resulting from step (2), will provide a product having a brightness of 
approximately 85-90 units (Elrepho). The material resulting from step (4) 
may be bleached by the sequence chlorination, alkaline extraction, 
chlorine dioxide treatment (C-E-D) or any other conventional sequence. 
When applied to the material resulting from step (4), the sequence C-E-D 
will provide a product having a brightness of approximately 85-90 units 
(Elrepho). 
The process of this invention has the advantage that the absence of 
sulphur-containing reagents results in lessened pollution potential as 
compared to the process of East German Pat. No. 98,549. The process also 
provides a pulp in much higher yield at a given kappa number, than has 
been heretofore attainable by any of the prior art pulping processes 
mentioned hereinabove. 
The invention is illustrated by the following examples but its scope is not 
limited to the embodiment shown therein. 
In the Examples, kappa number and viscosity determinations were carried out 
by the following methods. 
Kappa Number: TAPPI Method T-236 M-60 
Viscosity: TAPPI Method T-230 SU-66 
In all the following Examples, pulping was carried out in stainless steel 
pressure vessels of either one of the following two types; (1) a set of 
three such vessels each containing a rotatable horizontal basket, and (2) 
an assembly of eight such vessels (hereinafter called the microdigester 
assembly) each of which is itself horizontally rotatable. Large size 
samples of chips of 300, 600 or 2400 grams (oven dried weight) were pulped 
in any one of the three vessels of the first type while small size samples 
of 75 grams were pulped eight at a time in the second type of vessels, 
i.e. in the microdigester assembly. The chips were dried to approximately 
90% consistency divided into appropriate portions in consideration of the 
number and size of the pulping runs to be carried out and stored at 
4.degree. C. Exact amounts of chips of accurately known consistency were 
weighed out and soaked 24 hours in water prior to pulping. Soaked chips 
were placed inside the pressure vessel and optionally pre-steamed for 10 
minutes. Pulping liquor and dilution water were then added in the amounts 
required to give the desired effective alkali and to obtain a liquor to 
wood ratio of 4:1. Indirect electrical heating was used in both types of 
vessels. In the case of the microdigester assembly water under pressure 
was employed as a heat transfer medium. Heating was controlled to linearly 
raise the temperature to a preset maximum in a given time and to maintain 
it within .+-. 2.degree. C. of said maximum to the end of the cooking 
period. 
After completion of the cooking, the pressure was released and the pulp 
together with the used cooking liquor was transferred to a mixer such as a 
Cowless dissolver, diluted to 2% consistency and stirred for 5 minutes to 
simulate the blowdown of pulp that occurs in a commercial scale digester. 
The pulp was then washed twice by dilution to 2% consistency with water 
and filtered and pressed to 25% consistency. The pulp was then crumbed in 
a Hobart mixer, weighed and samples were taken for yield, kappa number and 
viscosity measurements.