Delignification of lignocellulosic material with an alkaline pulping liquor containing a Diels Alder adduct of benzoquinone or naphthoquinone

Delignification of lignocellulosic material, such as wood, straw or bagasse, by treatment with an alkaline pulping liquor containing a diketo hydroanthracene selected from the unsubstituted and lower alkyl substituted Diels Alder adducts of benzoquinone and naphthoquinone.

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 usually 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 
sulphur-free derivatives were tested by Bach and Fiehn (above recent 
publication) and were found to be substantially less effective than AMS. 
In the United States 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 an alkaline pulping liquor, a sulphur-free quinone 
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. 
It has now been found that lignocellulosic material can be delignified in 
high yield by a process which comprises a digestion with an alkaline 
pulping liquor in the presence of a diketo hydroanthracene selected from 
the group consisting of the unsubstituted and the lower alkyl-substituted 
Diels Alder adducts of naphthoquinone and benzoquinone. The compounds of 
this group, which are not quinones, have surprisingly been found to afford 
pulping results at least as good as those obtained with the quinones of 
the above-identified patent application while offering the major advantage 
that they can be prepared more readily and more economically. Optionally, 
the digestion with alkaline pulping liquor may be followed by a second 
stage digestion in alkaline medium with oxygen or an oxygen-containing gas 
under pressure. The novel process provides a pulp in higher yield at an 
increased rate of delignification in comparison to similar processes 
without additive. In addition, the diketo hydroanthracene additives 
proposed in this invention are free of sulphur and hence have the marked 
advantage over the anthraquinone monosulphonic acid proposed in East 
German Pat. No. 98,549 of producing no polluting sulphur compounds. 
Furthermore, the concentrations of the diketo anthracene additives 
required are at an economically advantageous level and often are less than 
that required with the known quinone additives. 
Thus the main object of the invention is to provide a pulping process which 
gives a high yield of cellulosic pulp. Another object is to provide a 
pulping process having a high rate of delignification, thus achieving a 
lower energy consumption and a higher throughput. A further object is to 
provide a pulping process which has a low pollution potential. A still 
further object is to provide a pulping process employing additives more 
readily and more economically prepared than AMS and the quinone additives. 
The process of this invention comprises the steps of 
1. treating lignocellulosic material in a closed reaction vessel with an 
alkaline pulping liquor containing from 0.001% to 10.0% by weight, based 
on the lignocellulosic material, of a diketo hydroanthracene selected from 
the group consisting of the unsubstituted and lower alkyl-substituted 
Diels Alder adducts of naphthoquinone or benzoquinone, the treatment 
taking place of maximum temperature in the range of 150.degree. C. to 
200.degree. C. for a period of 0.5 to 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 
a 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 3.0% 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, it is first converted 
into the form of chips. This step is not 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 treating 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 cooking time is still greatly reduced in comparison 
with that of the conventional processes using an alkaline liquor without 
additives. 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%. 
For the reasons given above, the alkaline pulping liquor ideally suitable 
for use in the first step of the process of the invention is the soda 
liquor. However, other conventional alkaline pulping liquors can be used, 
e.g. the kraft or polysulphide liquor, in which case environmental effects 
are still present but, due to the presence of the additives of the 
invention, the pulping action is accelerated and yields are increased. 
The soda liquor 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. Digestion with this liquor in the presence of the diketo 
anthracene additives according to the invention results in certain cases, 
in the cooking time being lessened by a factor of four. 
The kraft or sulphate liquor contains from 8% to 15% by weight of alkali 
metal base expressed as percent effective alkali (TAPPI T-1203 S-61) and 
from 5% to 40% by weight of alkali metal sulphide expressed as percent 
sulphidity (TAPPI T-1203 OS-61), based on lignocellulosic material. This 
pulping liquor will normally contain alkali metal sulphate and alkali 
metal carbonate. The pulping liquor may contain excess sulphur, i.e. 
polysulphides. The presence of polysulphides results in an improved yield 
and an amount of 1.0% to 5.0%, preferably 2.0% thereof (expressed as 
sulphur and based on weight of lignocellulosic material) in the liquor is 
therefore a definite advantage. 
Effective alkali is the sum of all alkali hydroxide in solution expressed 
as Na.sub.2 O including that formed by hydrolysis of the alkali sulphide, 
also expressed as Na.sub.2 O. 
Sulphidity is the total sulphide, expressed as Na.sub.2 O, calculated as a 
percentage of total titrable alkali, including that formed by hydrolysis 
of the sulphide, also expressed as Na.sub.2 O. 
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 are the diketo hydroanthracenes selected 
from the group consisting of the unsubstituted and lower alkyl-substituted 
Diels Alder adducts of naphthoquinone or benzoquinone. More particularly, 
the unsubstituted Diels Alder adducts are those obtained by reacting 1 or 
2 mols 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 in number from 1 to 4, may each 
contain from 1 to 4 carbon atoms and may be the same or different. 
Examples of the above diketo anthracenes are 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, 
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 and 
1,3-dimethyl-1,4,4a,9a-tetrahydro-9,10-diketo anthracene. The 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. 
After the first step treatment with pulping liquor the resulting pulp yield 
will be about 40% to about 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 liquor inert to lignocellulosic 
material such as the spent liquor from the alkaline oxygen treatment step 
or "white water" from a later stage of a paper making 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, depending upon 
whether it is a soda liquor or a kraft liquor, may therefore contain 
carbonate or sulphide, sulphate and carbonate in addition to alkali metal 
base. Where the pulping liquor is a kraft liquor, it may be of advantage 
to oxidize the liquor by aeration with an oxygen-containing gas prior to 
its addition to the delignified lignocellulosic material. Preferably there 
is also added from 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 there 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 
from 1% to 6%, preferably from 1.5% to 4.5% of the weight of the original 
lignocellulosic material corresponding to a yield of from 80% to 98% by 
weight of the pulp entering the oxygen treatment. 
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 alkali metal sulphide may be sodium sulphide or 
potassium sulphide. 
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 additives are 
effective at concentrations that are economically favourable and that it 
also requires a lower amount of pulping chemicals. Compared with the 
conventional pulping processes wherein no additives are used, the process 
of this invention provides a pulp in higher yield at an increased rate of 
delignification, thus permitting lower raw material cost, lower energy 
consumption and higher throughput. Another advantage of the process is 
that it results in lessened pollution potential as compared with the 
process of East German Pat. No. 98,549. This latter advantage is only 
significant if soda pulping is used as against kraft or polysulphide 
pulping. 
Still another advantage of the present invention over the process described 
in said East German patent is that, for a given concentration and under 
comparable pulping conditions, the present additives and especially the 
alkyl derivatives are more effective than AMS. 
The invention is illustrated by the following Examples but its scope is not 
limited to the embodiments shown therein. 
In the Examples the 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 than 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 
Cowles dissolver, diluted to 2% consistency and stirred for 5 minutes to 
simulate the blowdown of pulp that occurs in a commercial scale digesters. 
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.