Process for the chemical refining of cellulose pulp

A process is provided for the chemical refining of cellulose pulp which comprises, in sequence, the steps of: PA1 (1) impregnating the cellulose pulp with refining chemicals in an amount selected to effect chemical modification of the pulp; PA1 (2) adjusting the pulp consistency to within the range from about 30 to about 70%; and PA1 (3) passing the pulp in turbulent flow through an elongated reaction zone from one end to another end thereof in a gaseous atmosphere consisting essentially of steam and preferably containing less than 1% by volume of oxygen at a superatmospheric pressure within the range from about 5 to about 400 kPa and a temperature within the range from about 100 to about 150.degree. C. at which the chemical modification proceeds without a mechanical working sufficient to change the degree of beating of the pulp by more than about 2.degree. (Schopper-Riegler) and the freeness of the pulp by more than about 10 ml, and with less than an 8% change in the pulp dry solids content, at a flow rate such that the impregnated chemicals are substantially completely consumed by the time the pulp reaches the end of the zone.

The bleaching of chemical, semichemical and mechanical pulps with bleaching 
agents such as chlorine, chlorine dioxide, hypochlorite, and 
lignin-preserving bleaching agents such as peroxides and dithionite is 
usually carried out by impregnating the pulp with the bleaching chemicals 
and then effecting the bleaching reaction at a pulp consistency below 
about 20% for several hours at temperatures seldom exceeding 85.degree. C. 
The extraction of cellulose pulps with aqueous alkaline solutions in order 
to remove hemicellulose and other alkali-soluble materials such as resins, 
fatty acids and unsaponifiable substances is normally effected by 
impregnating the pulp with aqueous alkali, such as sodium hydroxide, and 
then allowing the alkali to react with the pulp for several hours at 
temperatures generally below 85.degree. C. However, hot alkali-refining 
can also be used at temperatures above 85.degree. C., when producing 
dissolving cellulose pulp. 
Svensk Papperstidning No. 15 pp 480-482 (1977) shows that in experiments 
with peroxide bleaching of unbleached sulfate pulp at 110.degree. C. in a 
digester, a very rapid and complete reaction between the peroxide and the 
pulp is established. The experiments indicate that the reaction mechanism 
depends upon temperature, and that there are different reaction mechanisms 
at high temperatures and at low temperatures. In spite of this, for 
reasons of economy, a two-stage bleaching sequence was proposed utilizing 
oxygen, followed by peroxide, at 70.degree. C., instead of a single stage 
peroxide bleaching at above 100.degree. C., in order to obtain a 
sufficient increase in brightness in the bleaching of sulfate pulp, even 
though the selectivity may be lower than in a single stage high 
temperature peroxide bleaching. The experiments were carried out with the 
pulp in a stationary bed at a pulp consistency of 30% for bleaching times 
as short as 5 minutes. 
U.S. Pat. No. 3,492,199, patented Jan. 27, 1970, discloses a process for 
simultaneously bleaching and drying mechanical pulp in order to obtain 
rapid drying of the pulp while obtaining a high brightness. The 
finely-divided pulp is impregnated with hydrogen peroxide at a pulp 
consistency of from 20 to 50%, and is then dried in an air stream at a 
temperature of from 260.degree. to 538.degree. C. at atmospheric pressure 
in a transit time of from 2 seconds to 10 minutes to a solids content of 
from 65 to 95%. However, in this process the consumption of bleaching 
chemicals is high, and energy consumption is excessive, while the content 
of fiber notes is unacceptable. Moreover, the pulp cannot be treated with 
reducing sulfur compounds added in the drying gas, and bleaching with 
reducing bleaching agents such as dithionite is impossible, since these 
decompose in the presence of oxygen at the high drying temperatures. 
In accordance with the invention, a process is provided for chemically 
refining cellulose pulps using, for example, bleaching agents and/or 
alkaline extracting agents, which gives a cellulose pulp having good pulp 
characteristics in a short processing time with a low consumption of 
chemicals and a low energy consumption. The process in accordance with the 
invention comprises, in sequence, the steps of: 
(1) impregnating the cellulose pulp with refining chemicals in an amount 
selected to effect chemical modification of the pulp; 
(2) adjusting the pulp consistency to within the range from about 30 to 
about 70%; and 
(3) passing the pulp in turbulent flow through an elongated reaction zone 
from one end to another end thereof in a gaseous atmosphere consisting 
essentially of steam and preferably containing less than 1% by volume of 
oxygen at a superatmospheric pressure within the range from about 5 to 
about 400 kPa and at a temperature within the range from about 100.degree. 
to about 150.degree. C. at which the chemical modification proceeds 
without a mechanical working sufficient to change the degree of beating of 
the pulp by more than about 2.degree. (Schopper-Riegler) and the freeness 
of the pulp by more than about 10 ml, and with less than an 8% change in 
the pulp dry solids content, at a flow rate such that the impregnated 
chemicals are substantially completely consumed by the time the pulp 
reaches the end of the zone. 
In the course of step (3), the fibers should not be either shortened or 
fibrillated by mechanical working. The shortness is defined by the degree 
of beating of the pulp, and the fibrillation by the freeness of the pulp. 
Consequently, a change of .+-. about 2.degree. (Schopper-Riegler) in the 
degree of beating and of .+-. about 10 ml in the freeness is undesirable, 
and is to be avoided. 
The process of the invention is applicable to cellulose pulps of all kinds, 
prepared by any chemical or mechanical pulping process or mixture of 
chemical and mechanical pulping processes from any kind of lignocellulosic 
material such as straw, bagasse, or wood. Thus, the invention is 
applicable to chemical pulps, such as sulfate pulps, soda pulps, and 
sulfite pulps, semichemical pulps, chemimechanical pulps, and to 
mechanical pulps, such as groundwood pulps produced at normal pressure or 
superatmospheric pressure, refiner mechanical pulps, and thermomechanical 
pulps. 
The invention is especially applicable to cellulose pulps derived from 
wood, such as spruce pulp, pine pulp, hemlock pulp, birch pulp, fir pulp, 
cherry pulp, sycamore pulp, hickory pulp, ash pulp, beech pulp, poplar 
pulp, oak pulp, and chestnut pulp. The invention is particularly 
advantageous in the preparation of any pulp in which it is especially 
desired to avoid degradation of the cellulose during processing, such as 
most grades of paper pulp, and when it is desired to obtain a uniform 
controlled degradation, such as in the manufacture of viscose pulp of a 
desired viscosity. 
In most cases where the starting cellulose pulp is free of lignin, or where 
the lignin content is low, either naturally so, or because it has been 
delignified, the process of the invention can be applied to remove 
hemicellulose, and/or cause oxidation of the cellulose, with a regulated 
diminution of the pulp viscosity. 
The method has shown particularly favorable results with hardwood pulps, 
such as pulp from birch and/or aspen, but good results have also been 
obtained with pulps from softwood, e.g., spruce and/or pine. 
The term "chemical refining" is used herein to refer to a modification of 
the cellulose pulp by chemical treatment, of which bleaching and alkali 
extraction are preferred embodiments, and are illustrative. 
In bleaching, any chemical bleaching agents can be used, such as, for 
example, chlorine, chlorine dioxide, hypochlorous acid, sodium 
hypochlorite, calcium hypochlorite, peroxide compounds such as hydrogen 
peroxide, sodium peroxide, sodium perborate, barium peroxide, peracetic 
acid, performic acid, perpropionic acid, and sodium dithionite. Additional 
peroxide bleaching chemicals can be added, such as stabilizers and pH 
modifiers, for example, sulfuric acid, sodium hydroxide, sodium silicate, 
sodium phosphate, and magnesium sulfate. Preferred bleaching agents are 
hypochlorite and lignin-preserving bleaching agents such as peroxides and 
sodium dithionite. 
Alkali extraction can be carried out with any aqueous alkali solution. 
Sodium hydroxide and magnesium hydroxide are preferred, but potassium 
hydroxide, calcium hydroxide, sodium carbonate, sodium bicarbonate, 
potassium carbonate and potassium bicarbonate can also be used. 
Bleaching in the process of the invention is enhanced if the pulp prior to 
the treatment is impregnated with a complexing agent. The pulp should have 
a low consistency within the range of 1 to 10%, for uniform distribution 
of the agent throughout the pulp. The complexing agent is capable of 
chelating with or sequestering heavy metal or polyvalent metal cations. 
The complexing agent is effective however in inhibiting degradation of the 
cellulose even if no polyvalent metal cations are present. Preferred 
complexing agents are hydroxy carboxylic acids, amino carboxylic acids, 
and polyphosphates, for example, nitrilotriamino acetic acid, diethylene 
triamine pentaacetic acid, ethylene diamine tetraacetic acid, citric acid, 
tartaric acid, pentasodium tripolyphosphate, and tetrasodium 
pyrophosphate. 
The complexing amino polycarboxylic acids have the formula: 
##STR1## 
and the alkali metal salts thereof, in which A is the group --CH.sub.2 
COOH or --CH.sub.2 CH.sub.2 OH, where n is an integer from zero to five. 
The mono, di, tri, tetra, penta and higher alkali metal salts are useful, 
according to the number of acid groups available and converted to alkali 
metal salt form. 
Examples of such aminopolycarboxylic acids are ethylene diamine tetraacetic 
acid, nitrilotriacetic acid, diethylene triaminopentaacetic acid, ethylene 
diamine triacetic acid, tetraethylene pentaamine heptaacetic acid, and 
hydroxy ethyl ethylene diamine triacetic acid, and their alkali metal 
salts, including the mono, di, tri, tetra and penta sodium, potassium and 
lithium salts thereof. Other types of amino carboxylic acids which can be 
used to advantage are imino diacetic acid, 2-hydroxy ethyl imino diacetic 
acid, cyclohexane diamine tetraacetic acid, anthranil-N,N-diacetic acid, 
and 2-picolylamine-N,N-diacetic acid. 
Also effective complexing agents are the aliphatic alpha-hydroxy carboxylic 
acids of the type RCHOHCOOH and the corresponding beta-hydroxy carboxylic 
acids RCHOHCH.sub.2 COOH; having the formula: 
##STR2## 
In the above formula, n is zero or one. When n is zero, the acid is an 
alpha-hydroxy acid, and when n is one, the acid is a beta-hydroxy acid. 
R in the above formula is hydrogen or an aliphatic radical, which may be a 
hydrocarbon radical having from one to about ten carbon atoms, or a 
hydroxy-substituted hydrocarbon radical having from one to nine hydroxyl 
groups, and from one to about ten carbon atoms. 
Exemplary alpha- and beta-hydroxy carboxylic acids are glycolic acid, 
lactic acid, glyceric acid, .alpha.,.beta.-dihydroxy butyric acid, 
.alpha.-hydroxy-butyric acid, .alpha.-hydroxy-isobutyric acid, 
.alpha.-hydroxy-n-valeric acid, .alpha.-hydroxy-isovaleric acid, 
.beta.-hydroxy-butyric acid, .beta.-hydroxy-isobutyric acid, 
.beta.-hydroxy-n-valeric acid, .beta.-hydroxy-isovaleric acid, erythronic 
acid, threonic acid, trihydroxy-isobutyric acid, and sugar acids and 
aldonic acids, such as gluconic acid, galactonic acid, talonic acid, 
mannonic acid, arabonic acid, ribonic acid, xylonic acid, lyxonic acid, 
gulonic acid, idonic acid, altronic acid, allonic acid, ethenyl glycolic 
acid, and .beta.-hydroxy-isocrotonic acid. 
Also useful are organic acids having two or more carboxylic groups, and no 
or from one to ten hydroxyl groups, such as oxalic acid, malonic acid, 
tartaric acid, malic acid, and citric acid, ethyl malonic acid, succinic 
acid, isosuccinic acid, glutaric acid, adipic acid, suberic acid, azelaic 
acid, maleic acid, fumaric acid, glutaconic acid, citramalic acid, 
trihydroxy glutaric acid, tetrahydroxy adipic acid, dihydroxy maleic acid, 
mucic acid, mannosaccharic acid, idosaccharic acid, talomucic acid, 
tricarballylic acid, aconitic acid, and dihydroxy tartaric acid. 
The polyphosphoric acids are also good complexing agents, and the alkali 
metal salts of these acids are useful, alone or in combinations with the 
complexing amino polycarboxylic acid salts. Exemplary are tetrasodium 
pyrophosphate, pentasodium tripolyphosphate and sodium polymetaphosphate. 
Especially advantageous complexing agents from the standpoint of cost are 
the acids naturally present in waste liquors obtained from the alkaline 
treatment of cellulosic materials. These acids represent the alkali- or 
water-soluble degradation products of polysaccharides which are dissolved 
in such liquors, as well as alkali- or water-soluble degradation products 
of cellulose and hemicellulose. The chemical nature of these degradation 
products are complex, and they have not been fully identified. However, it 
is known that saccharinic and lactic acids are present in such liquors, 
and that other hydroxy acids are also present. The presence of C.sub.6 
-isosaccharinic and C.sub.6 -metasaccharinic acids has been demonstrated, 
as well as C.sub.4 - and C.sub.5 -metasaccharinic acids. Glycolic acid and 
lactic acid are also probable degradation products derived from the 
hemicelluloses, together with beta-gammadihydroxy butyric acid. 
Carbohydrate acid-containing cellulose waste liquors which can be used 
include the liquors obtained from the hot alkali treatment of cellulose, 
liquors from sulfite digestion processes, and liquors from sulfate 
digestion processes, i.e., kraft waste liquor. The waste liquors obtained 
in alkaline oxygen gas bleaching or digestion processes and alkaline 
peroxide bleaching processes can also be used. In this instance, the 
alkaline liquor can be taken out from the process subsequent to completing 
the oxygen gas treatment stage, or during the actual treatment process. 
The impregnation with bleaching agent is then carried out, usually simply 
by mixing an aqueous solution of the bleaching chemicals with the pulp 
suspension. 
The aqueous bleaching solution is then uniformly distributed in the pulp 
suspension using, for example, agitation, such as in a blade or propeller 
mixer of conventional type. 
The amount of bleaching agent can be within the range from about 0.2% to 
about 6% by weight of the dry weight of the pulp, and is preferably within 
the range from about 0.5 to about 5%. 
The consistency of the pulp during the bleaching should be within the range 
from about 30 to about 70%, although consistencies within the range from 
about 45 to about 65% are preferred. The pulp can be dewatered or diluted, 
according to the consistency of the starting pulp, so that a consistency 
within the stated range is obtained. A press is preferably used for 
dewatering. The excess of the impregnating chemical solution is recovered. 
In alkali extraction, the alkali-soluble materials, such as hemicellulose 
and resins as well as alkali-hydrolyzable substances such as 
carbohydrates, are removed and dissolved in the alkali extraction 
solution. Alkali extraction is very suitable, for example, in producing 
dissolving pulp, i.e., alkali-soluble pulp. 
Any alkali can be used, such as, for example, sodium hydroxide, potassium 
hydroxide, magnesium hydroxide, calcium hydroxide and cuprammonium 
solutions. The alkali concentration is not critical, and can be within the 
range from about 1 to about 50% in the extracting solution. The amount of 
alkali is adjusted according to the materials to be removed, and is within 
the range from about 0.5 to about 10% by weight based on the solids 
content of the pulp, preferably within the range from about 1 to about 6% 
based on the solids content of the pulp. 
For optimum chemical modification in the refining treatment of the 
invention, the pulp fibers should be in finely divided form. If the pulp 
is then not sufficiently finely divided, it can be further defibrated in a 
disc refiner. 
The pulp suspension is then ready to be passed through the elongated 
reaction zone, from one end to the other end thereof, during which the 
reaction between the impregnated chemicals and the pulp is carried out. 
In its passage through the zone, the pulp suspension is bathed in a steam 
atmosphere and also is heated by the steam which is at superatmospheric 
pressure. The temperature is within the range from about 100.degree. to 
about 150.degree. C., and the steam superatmospheric pressure is within 
the range from about 5 to about 400 kPa, preferably from about 50 to about 
300 kPa, and still more preferably at from about 100 to about 200 kPa. The 
atmosphere contains less than 1% oxygen and other gases, if the chemical 
refining agent is reactive with oxygen or other gases, at the refining 
process temperature. 
The steam and pulp are thoroughly blended in turbulent flow, to ensure 
adequate mixing in transit through the zone. This can be done mechanically 
using, for example, fans or agitators, or a pump, or a helical screw 
conveyor. Pneumatic turbulence can be achieved by fans, or by bubbling the 
steam into the pulp using sparging apparatus or similar conventional 
equipment. 
The rate of transit through the reaction zone is dependent upon the 
temperature and the type of chemical refining being carried out, and is 
normally at least 10 meters per second, so as to ensure a traverse time 
within which the impregnating chemicals are substantially completely 
consumed and the chemical modification completed to the required extent. 
The reaction is surprisingly fast under the proper turbulent flow 
conditions, and can be completed within as little as 5 seconds, and at 
most in a matter of minutes, but usually not in excess of about 10 
minutes. A preferred transit time is within the range from about 5 seconds 
to about 60 seconds. 
During transit through the reaction zone, the dry solids content of the 
pulp should not change appreciably. A maximum change increment of 8% is 
acceptable, but preferably the change is less than 6%, and still more 
preferably, there is no substantial change at all. 
If the treated pulp has been bleached, the dry solids content of the pulp 
at the delivery end of the reaction zone should be at least 40%, and if 
the pulp has been alkali-refined or extracted, the pulp dry solids content 
should be at least 30%. 
After transit through the reaction zone, the steam is separated from the 
pulp. If the chemical refining is an alkaline extraction, the pulp is 
washed after steam separation. If the chemical refining is a bleaching, 
washing is optional, and not essential. The materials dissolved out in the 
alkali extraction liquor or bleaching liquor are of course separated with 
the liquor. 
Steam separation can be carried out in any conventional steam-separating 
equipment, such as, for example, a cyclone or hydrocyclone, or a 
centrifuge. 
Thereafter, the pulp can be dried, or further treated as required. A pulp 
which has been extracted with alkali can be bleached, and likewise a pulp 
that has been bleached can be bleached in another stage, using the same or 
another bleaching agent, if desired in a repetition of the process of the 
invention, by recycling the pulp to the apparatus, or passing it on to a 
second apparatus in series with the first. 
After drying, and with or without further treatment, the pulp can be used 
in the production of paper, and in other ways. 
The drying of the pulp following the separation of steam and washing, if 
applied, can be carried out using the usual drying apparatus. A flash 
drying is particularly suitable, the pulp being suspended in a turbulent 
gas stream such as steam or air, at a temperature within the range from 
about 110.degree. to about 500.degree. C. The transfer of heat from the 
carrier gas to the pulp is thereby facilitated. 
A preferred drying carrier gas is superheated steam at a superatmospheric 
pressure within the range from about 20 to about 400 kPa. Very good 
economy can be achieved by using the excess steam after the drying for 
heating and other purposes, such as a source of heat in the refining of 
the invention. The excess steam can also be recycled to the drying, after 
reheating. 
A preferred drying apparatus is the so-called "counter pressure" dryer 
described in U.S. Pat. No. 4,043,049, patented Aug. 23, 1977, the 
disclosure of which is hereby incorporated by reference. In this drying 
apparatus, the pulp is dried in the form of particles or flakes which flow 
through vertical towers under a superatmospheric pressure of steam at a 
high rate, for example, 21 meters per second. The pulp particles or flakes 
and steam are driven at high speed by means of fans. The carrier steam is 
heated indirectly by pressurized steam pipes, the temperature of which can 
be considerably higher than that of the carrier steam. The carrier steam 
heats the moist pulp instantaneously, which leads to a rapid evaporation 
of the moisture in the pulp. In this flash drying process, a dried pulp is 
obtained in from 10 to 20 seconds. 
During drying, the pump can be treated with pH adjusting substances, such 
as sulfur dioxide gas, which can be supplied to the pulp with the carrier 
steam, or calcium oxide in finely divided powder form. 
The excess steam is recovered from the dried pulp in a steam separator, 
such as a cyclone.

The following Examples in the opinion of the inventors represent preferred 
embodiments of the invention: 
EXAMPLE 1 
Chemimechanical washed birch cellulose pulp, obtained by partial 
delignification with sodium bisulfite followed by defibration in a disc 
refiner, and having a brightness of 66% SCAN, was processed in the 
apparatus shown in FIG. 1. First, the pulp was mixed in mixer 1 with hot 
water and 0.2% diethylenetriamine pentaacetic acid based on the dry weight 
of the pulp to a pulp consistency of 4% at a temperature of 62.degree. C. 
The pulp was allowed to stand for thirty minutes, and then dewatered in 
the press 2 to a 35% solids content. The dewatered pulp was then shredded 
to pieces approximately 1 cm.sup.2, and mixed in mixer 3 with an aqueous 
bleaching solution of 28 g/l hydrogen peroxide, 50 g/l sodium silicate, 18 
g/l sodium hydroxide and 0.2 g/l magnesium sulfate. After mixing in the 
bleaching solution, the pulp was dewatered in a screw press 4 to a 50% 
solids content, in order to remove excess bleaching chemicals. The 
dewatered pulp was found by analysis to contain 3% hydrogen peroxide, 5 % 
sodium silicate, 1.5% sodium hydroxide and 0.02% magnesium sulfate, based 
on the dry weight of the pulp. 
The pulp was then ground in disc refiner 5 to individual fibers and fiber 
bundles, and then continuously fed by way of a sluice or rotary vane 
feeder 6 into a stream of recycled steam as a carrier gas from line 7 and 
then in the first stage 8, a flash dryer of U.S. Pat. No. 4,043,049, 
modified as shown in FIG. 1 to form a processing apparatus in accordance 
with the invention. 
In the first stage 8 of this apparatus, the pulp was carried in a stream of 
saturated steam at a superatmospheric pressure of 70 kPa at a temperature 
of 115.degree. C. The steam was obtained as saturated excess steam from 
the cyclone 12 at the end of stage 8, via line 7. The stream of steam and 
pulp was introduced into the first stage 8 in such a way that a turbulent 
flow of pulp through the stage was obtained. Fans 9 were used to aid in 
transport of the pulp through the stage 8. The pulp proceeded through the 
stage 8 at a rate of about 10 meters per second, and the total traverse 
time through the stage 8 was eight seconds. The pulp solids content on 
leaving stage 8 was 45%. Before the pulp left stage 8, steam was separated 
from the pulp in the cyclone 12 and this steam recycled through line 20 to 
steam the wood material supplied to the digestion, in part, and as 
recycled carrier steam through line 7. 
The chemically bleached pulp was delivered from stage 8 by way of a sluice 
or rotary vane feeder 13, washed with water, and analyzed. The water 
obtained for washing had only traces of peroxide. 
The pulp analysis results are given in Table I, which follows Example 2. 
EXAMPLE 2 
Chemimechanical washed birch cellulose pulp, obtained by partial 
delignification with sodium bisulfite followed by defibration in a disc 
refiner, and having a brightness of 66% SCAN, was processed in the 
apparatus shown in FIG. 1. First, the pulp was mixed in mixer 1 with hot 
water and 0.2% diethylenetriamine pentaacetic acid based on the dry weight 
of the pulp to a pulp consistency of 4% at a temperature of 62.degree. C. 
The pulp was allowed to stand for thirty minutes, and then dewatered to a 
35% solids content. The dewatered pulp was then shredded to pieces 
approximately 1 cm.sup.2, and mixed in mixer 3 with an aqueous bleaching 
solution of 28 g/l hydrogen peroxide, 50 g/l sodium silicate, 18 g/l 
sodium hydroxide and 0.2 g/l magnesium sulfate. After mixing in the 
bleaching solution, the pulp was dewatered in a press to a 50% solids 
content, in order to remove excess bleaching chemicals. The dewatered pulp 
was found by analysis to contain 3% hydrogen peroxide, 5% sodium silicate, 
1.5% sodium hydroxide and 0.02% magnesium sulfate, based on the dry weight 
of the pulp. 
The pulp was then ground in disc refiner 5 to individual fibers and fiber 
bundles, and then continuously fed by way of sluice feeder 6 into stage 8 
of FIG. 1. 
In this apparatus, the pulp was carried on a stream of saturated steam at a 
superatmospheric pressure of 70 kPa at a temperature of 115.degree. C. The 
steam was obtained as saturated excess steam from the end of stage 8 and 
was introduced into stage 8 in such a way that a turbulent flow of pulp 
through the stage was obtained. Fans 9 were used to aid in transport of 
the pulp through the stage 8. The pulp proceeded through stage 8 at a rate 
of about 10 meters per second, and the total traverse time through the 
stage was 8 seconds. The pulp solids content on leaving stage 8 was 45%. 
Before the pulp left the stage, steam was separated from the pulp in a 
cyclone, and this steam recycled to steam the wood material supplied to 
the digestion, in part. 
The chemically bleached pulp was delivered from stage 8 by way of a rotary 
vane feeder 13. After leaving feeder 13, the pulp was continuously 
introduced at substantially constant solids content, without washing, into 
a counter-pressure drying unit 15 of U.S. Pat. No. 4,043,049, equipped 
with circulating fans 14, and in which the drying agent was superheated 
steam at a superatmospheric pressure of 300 kPa and a temperature of 
150.degree. C. The heat exchangers (not shown) for heating the carrier 
steam were heated with steam at 160.degree. C., with the result that the 
carrier steam was rapidly superheated, with a rapid transfer of moisture 
from the pulp to the carrier steam. Both pulp and steam were then led to a 
cyclone 16, in which the steam was separated from the pulp, and recycled 
via line 17 to the beginning of the drying stage. The pulp was delivered 
via the rotary vane feeder 19 from the cyclone. The solids content of the 
dried pulp was 91.2%, and it had a pH of 7.7. The pulp was analyzed, and 
the results are given in Table I below. 
Two controls were run, for comparison with Examples 1 and 2 of the 
invention. 
In Control 1, chemimechanical washed birch cellulose pulp, obtained by 
partial delignification with sodium bisulfite followed by defibration in a 
disc refiner, and having a brightness of 66% SCAN, was mixed in mixer 1 
with hot water and 0.2% diethylenetriamine pentaacetic acid based on the 
dry weight of the pulp to a pulp consistency of 4% at a temperature of 
62.degree. C. The pulp was allowed to stand for thirty minutes, then 
dewatered to a 35% solids content. The dewatered pulp was then shredded to 
pieces approximately 1 cm.sup.2 and mixed in mixer 3 with an aqueous 
bleaching solution of 28 g/l hydrogen peroxide, 50 g/l sodium silicate, 18 
g/l sodium hydroxide and 0.2 g/l magnesium sulfate. After mixing in the 
bleaching solution, the pulp was dewatered in a press to a 50% solids 
content, in order to remove excess chemicals. The dewatered pulp was found 
by analysis to contain 3% hydrogen peroxide, 5% sodium silicate, 1.5% 
sodium hydroxide and 0.02% magnesium sulfate, based on the dry weight of 
the pulp. 
The pulp was then ground in disc refiner 5 to individual fibers and fiber 
bundles and then continuously fed by way of the sluice feeder into stage 
15 of the counter-pressure dryer shown in FIG. 1, as used in Example 2, in 
which the drying agent was superheated steam at a superatmospheric 
pressure of 300 kPa and a temperature of 150.degree. C. The pulp was thus 
simultaneously subjected to bleaching and drying while passing through the 
counter-pressure dryer, and the solids content increased from 50% to 
91.5%. This pulp (Control 1) was analyzed, and the analytical results are 
given in Table I. 
As Control 2, chemimechanical washed birch cellulose pulp, obtained by 
partial delignification with sodium bisulfite followed by defibration in a 
disc refiner, and having a brightness of 66% SCAN, was mixed in mixer 1 
with hot water and 0.2% diethylenetriamine pentaacetic acid based on the 
dry weight of the pulp to a pulp consistency of 4% at a temperature of 
62.degree. C. The pulp was allowed to stand for thirty minutes and then 
dewatered to a 35% solids content. The dewatered pulp was then shredded to 
pieces approximately 1 cm.sup.2 and mixed in mixer 3 with a bleaching 
aqueous solution of 28 g/l hydrogen peroxide, 50 g/l sodium silicate, 18 
g/l sodium hydroxide and 0.2 g/l magnesium sulfate. After mixing in the 
bleaching solution, the pulp was dewatered in a press to a 50% solids 
content, in order to remove excess bleaching chemicals. The dewatered pulp 
was found by analysis to contain 3% hydrogen peroxide, 5% sodium silicate, 
1.5% sodium hydroxide and 0.02% magnesium sulfate based on the dry weight 
of the pulp. 
The pulp was then ground in disc refiner 5 to individual fibers and fiber 
bundles, and then continuously fed by way of a sluice feeder into the 
flash dryer described in U.S. Pat. No. 3,492,199. The drying air was 
heated with the aid of an oil burner to a temperature of 450.degree. C. At 
the end of the drying operation, the drying air temperature was 
120.degree. C. This pulp (Control 2) was analyzed and the analytical 
results are given in Table I. 
TABLE I 
__________________________________________________________________________ 
Method according to 
Comparison with simultaneous 
the invention bleaching and drying of the prior art 
Example 2 
Control 1 
Control 2 
Bleaching 
Drying Drying 
Starting 
Example 1 
followed 
in flash dryer of 
in flash dryer of 
Treatment 
pulp Bleaching 
by drying 
No. 4,043,049 
No. 3,492,199 
__________________________________________________________________________ 
Solids content 
50 45.0 91.2 91.5 91.1 
Brightness 
SCAN % 66.0 85.3 85.5 73.0 72.5 
Increase in 
-- 19.3 19.5 7.0 6.5 
brightness 
SCAN % 
Fiber bundles 
0 60 250 280 1050 
number per 
100 g pulp 
pH 5.6 8.2 7.7 7.8 6.2 
__________________________________________________________________________ 
The results in Table I show that, quite surprisingly, when the process of 
the invention is used it is possible to bleach chemimechanical birch pulp 
in an extremely short time to a very high brightness, and thereafter to 
dry the pulp without intermediate treatment to a solids content of about 
91%, while maintaining an acceptable number of fiber bundles. 
As each of Control 1 and Control 2 show, the bleaching result is 
considerably worse if the bleaching and drying are carried out at the same 
time. In this case, the poor bleaching result possibly can be explained by 
the fact that the bleaching solution evaporates before it has had time to 
have any substantial bleaching effect, whereas at the superatmospheric 
pressure of the invention, no evaporation can take place. 
The brightness results show that only about one-third of the optimum 
bleaching effect is obtained when simultaneously bleaching and drying, as 
in the Controls. The process of the invention is furthermore very 
economical in energy consumption. 
EXAMPLE 3 
Chemimechanical birch pulp produced by delignification with sodium 
bisulfite and defibration in a disc refiner, followed by washing, and 
having a brightness of 66% SCAN, was processed in the apparatus shown in 
FIG. 1. First, the pulp was mixed in mixer 1 with hot water and 0.2% 
diethylenetriamine pentaacetic acid based on the dry weight of the pulp to 
a pulp consistency of 4% at a temperature of 62.degree. C. The pulp was 
allowed to stand for thirty minutes, and then dewatered to a 35% solids 
content. The dewatered pulp was then shredded to pieces approximately 1 
cm.sup.2, and mixed in mixer 3 with an aqueous bleaching solution of 28 
g/l hydrogen peroxide, 50 g/l sodium silicate, 18 g/l sodium hydroxide and 
0.2 g/l magnesium sulfate. After mixing in the bleaching solution, the 
pulp was dewatered in press 4 to a 50% solids content, in order to remove 
excess bleaching chemicals. The dewatered pulp was found by analysis to 
contain 3% hydrogen peroxide, 5% sodium silicate, 1.5% sodium hydroxide 
and 0.02% magnesium sulfate, based on the dry weight of the pulp. 
The pulp was then ground in disc refiner 5 to individual fibers and fiber 
bundles, and then continuously fed by way of sluice feeder 6 into stage 8 
of FIG. 1. 
In stage 8, the carrier steam was saturated steam at 105.degree. C., under 
a superatmospheric pressure of 20 kPa. Transit time was seven seconds. 
Upon leaving stage 8, the steam and pulp were run through cyclone 12, 
where the steam was separated and used for steaming the lignocellulosic 
material fed to the digestion process. 
The chemically treated pulp was delivered from stage 8 by way of rotary 
vane feeder 13, and brought to a storage tank, where it was stored for 
fifteen minutes at a solids content of 47%. The temperature of the pulp at 
the end of the storage time was 90.degree. C. Analysis of the pulp showed 
a hydrogen peroxide content of 0.1%, with a pulp brightness of 84.9% SCAN. 
The results show that in the process of the invention, a milder chemical 
treatment at a lower temperature can be combined with a short 
after-treatment, such as a residence period in the storage tank, for 
completing the bleaching process, and so reach a high brightness in the 
bleached pulp. 
EXAMPLE 4 
Chemimechanical birch pulp produced by delignification with sodium 
bisulfite and defibration in a disc refiner, followed by washing, and 
having a brightness of 66% SCAN, was processed in the apparatus shown in 
FIG. 1. First, the pulp was mixed in mixer 1 with hot water and 0.2% 
diethylenetriamine pentaacetic acid based on the dry weight of the pulp to 
a pulp consistency of 4% at a temperature of 62.degree. C. The pulp was 
allowed to stand for thirty minutes, and then dewatered to a 35% solids 
content. The dewatered pulp was then shredded to pieces approximately 1 
cm.sup.2, and mixed in mixer 3 with an aqueous bleaching solution of 28 
g/l hydrogen peroxide, 50 g/l sodium silicate, 18 g/l sodium hydroxide and 
0.2 magnesium sulfate. After mixing in the bleaching solution, the pulp 
was dewatered in a press 4 to a 50% solids content, in order to remove 
excess bleaching chemicals. The dewatered pulp was found by analysis to 
contain 3% hydrogen peroxide, 5% sodium silicate, 1.5% sodium hydroxide 
and 0.02% magnesium sulfate, based on the dry weight of the pulp. 
The pulp was then ground in disc refiner 5 to individual fibers and fiber 
bundles, and then continuously fed by way of sluice feeder 6 into stage 8 
of FIG. 1. 
In stage 8, the carrier steam was saturated steam at 105.degree. C., under 
a superatmospheric pressure of 20 kPa. Transit time was seven seconds. 
Upon leaving stage 8, the steam and pulp were run through cyclone 12, 
where the steam was separated and used for steaming the lignocellulosic 
material fed to the digestion process. 
The chemically treated pulp was delivered from stage 8 by way of rotary 
vane feeder 13 and brought to a storage tank, where it was diluted to a 
concentration of 4%, using a hot aqueous sodium dithionite solution, so 
that the temperature after mixing with the pulp was 76.degree. C. The 
amount of sodium dithionite charged was 0.4% based on the weight of dry 
pulp. The pulp residence time in the storage tank was 10 minutes. 
Analysis of the pulp at the end of this time showed that it had a 
brightness of 88.3% SCAN, an extremely high brightness for a 
chemimechanical pulp, and comparable to the brightness of a fully bleached 
chemical pulp. 
EXAMPLE 5 
Chemimechanical washed birch cellulose pulp, obtained by partial 
delignification with sodium bisulfite followed by defibration in a disc 
refiner, and having a brightness of 66% SCAN, was processed in the 
apparatus shown in FIG. 1. First, the pulp was mixed in mixer 1 with hot 
water and 0.2% diethylenetriamine pentaacetic acid based on the dry weight 
of the pulp to a pulp consistency of 4% at a temperature of 62.degree. C. 
The pulp was allowed to stand for thirty minutes, and then dewatered in 
the press 2 to a 35% solids content. The dewatered pulp was then shredded 
to pieces approximately 1 cm.sup.2, and mixed in mixer 3 with an aqueous 
bleaching solution of 28 g/l hydrogen peroxide, 50 g/l sodium silicate, 18 
g/l sodium hydroxide and 0.2 g/l magnesium sulfate. After mixing in the 
bleaching solution, the pulp was dewatered in screw press 4 to a 50% 
solids content, in order to remove excess bleaching chemicals. The 
dewatered pulp was found by analysis to contain 3% hydrogen peroxide, 5% 
sodium silicate, 1.5% sodium hydroxide and 0.02% magnesium sulfate, based 
on the dry weight of the pulp. 
The pulp was then ground in disc refiner 5 to individual fibers and fiber 
bundles, and then continuously fed by way of sluice feeder 6 into stage 8 
of FIG. 1. 
In this stage, the pulp was carried on a stream of saturated steam at a 
superatmospheric pressure of 70 kPa at a temperature of 115.degree. C. The 
pulp proceeded through the stage at a rate of about 10 meters per second, 
and the total traverse time through the stage was eight seconds. The pulp 
solids content on leaving the stage was 45%. When the pulp left stage 8, 
steam was separated from the pulp in a cyclone 12 and some steam recycled 
to steam the wood material supplied to the digestion. The chemically 
processed pulp was delivered from stage 8 by way of rotary vane feeder 13 
and then gaseous sulfur dioxide was added, in an amount corresponding to 
0.3% based on the dry weight of the pulp. The SO.sub.2 -impregnated pulp 
was then continuously introduced at substantially constant solids content 
without washing into stage 15 of the counter-pressure drying unit of U.S. 
Pat. No. 4,043,049, in which the drying agent was superheated steam at a 
superatmospheric pressure of 300 kPa and a temperature of 150.degree. C. 
The heat exchangers for heating the carrier steam were supplied with steam 
at 160.degree. C., with the result that the carrier steam was rapidly 
superheated, with a rapid transfer of moisture from the pulp to the 
carrier steam. Both pulp and steam were then led to cyclone 16, in which 
the steam was separated from the pulp. 
The solids content of the pulp after passing through the counter-pressure 
dryer was 91.8%, brightness was 85.2% SCAN, and pH was 7.0. 
Thus, by addition of sulfur dioxide it is possible in the process of the 
invention to bleach and dry the pulp, as well as adjust the pH to a 
desired level. 
EXAMPLE 6 
Washed spruce groundwood pulp, obtained by grinding spruce chips on a 
conventional wood chip grinder and having a brightness of 62% SCAN, was 
passed through a disc refiner and fed by way of the sluice feeder into a 
modified flashed dryer of FIG. 1, immediately after passing through the 
sluice feeder, being sprayed with an aqueous bleaching solution containing 
sodium dithionite and ethylenediamine tetraacetic acid as a complexing 
agent, in amounts such that the pulp contained 0.8% sodium dithionite and 
0.15% of the EDTA complexing agent, based on the dry weight of the pulp. 
In stage 8, the pulp was carried on a stream of saturated steam at a 
superatmospheric pressure of 70 kPa at a temperature of 115.degree. C. The 
pulp proceeded through stage 8 at a rate of about 10 meters per second, 
and the total traverse time through the stage was eight seconds. The pulp 
solids content on leaving stage 8 was 45%. Steam was separated from the 
pulp in cyclone 12, and some steam recycled to steam the wood material 
supplied to the digestion. 
The bleached pulp was delivered by way of rotary vane feeder 13 at 
substantially constant solids content, without washing, into stage 15 the 
counter-pressure drying unit of U.S. Pat. No. 4,043,049, in which the 
drying agent was superheated steam at a superatmospheric pressure of 300 
kPa at a temperature of 150.degree. C. The heat exchangers for heating the 
carrier steam were supplied with steam at 160.degree. C., with the result 
that the carrier steam was rapidly superheated, with a rapid transfer of 
moisture from the pulp to the carrier steam. Both pulp and steam were then 
led to cyclone 16, in which the steam was separated from the pulp. 
The bleached pulp had a solids content of 91.9%, and a brightness of 73% 
SCAN, a very high brightness, considering that sodium dithionite was used 
as the bleaching agent. 
As a control, another batch of the same spruce groundwood pulp having a 
brightness of 62% SCAN, was passed through a disc refiner and then 
continuous fed by way of sluice feeder into the conventional flash dryer 
as described in U.S. Pat. No. 3,492,199, immediately after passing through 
the sluice feeder being sprayed with an aqueous bleaching solution 
containing sodium dithionite and ethylenediamine tetraacetic acid as a 
complexing agent, in amounts such that the pulp contained 0.8% sodium 
dithionite and 0.15% of the EDTA complexing agent, based on the dry weight 
of the pulp. 
The flash dryer was heated with the aid of an oil burner to a temperature 
of 450.degree. C. At the end of the drying operation, the drying air 
temperature was 120.degree. C. The resulting bleached pulp had a solids 
content of 91.5%, while its brightness was only 63% SCAN. 
These results show that the bleaching and drying process of the invention 
gives a very good bleaching effect, while simultaneous bleaching and 
drying in a conventional flash dryer gives only a small improvement in 
brightness. A possible explanation may be that sodium dithionite 
decomposes in a conventional flash dryer, because of the presence of 
oxygen in the drying air. When bleaching in a steam atmosphere under 
superatmospheric pressure in accordance with the invention, in the absence 
of oxygen in the carrier steam, the sodium dithionite is not lost, and so 
the bleaching is not disturbed. 
According to the technical literature, the maximum increase in brightness 
as % SCAN obtainable with sodium dithionite bleaching for a period of 60 
minutes at a 4% pulp consistency is about 10 to 11%. The process of the 
invention results in an increase in brightness of 11%, which shows that 
the maximum increase in brightness was obtained. 
EXAMPLE 7 
Thermomechanical pulp produced from 50% spruce and 50% aspen wood, with a 
brightness of 56.1% SCAN, was mixed with an aqueous solution of 0.2% 
diethylene triamine pentaacetic acid in hot water in a mixer, to a pulp 
consistency of 4% at a temperature 62.degree. C. The pulp was then 
dewatered to a 35% solids content. The dewatered pulp was mixed in a mixer 
with an aqueous bleaching solution containing 22 g/l hydrogen peroxide, 40 
g/l sodium silicate, 12 g/l sodium hydroxide and 0.1 g/l magnesium 
sulfate, and then pressed in a press to a 50% solids content. The 
dewatered pulp contained 2% hydrogen peroxide, 4% sodium silicate, 1% 
sodium hydroxide and 0.01% magnesium sulfate based on the dry weight of 
the pulp. 
The pulp thus impregnated with bleaching agents was taken through disc 
refiner 5, and then fed into stage 8 of the apparatus of FIG. 1. The 
carrier steam temperature in stage 8 was 114.degree. C., under a 
superatmospheric pressure of 64 kPa. This steam was composed of saturated 
excess steam coming partly from the steam separator after stage 8 and 
partly from the steam separator after stage 15, and introduced via fan 9 
into stage 8, so that a turbulent flow of pulp through stage 8 was 
obtained. The residence time of the pulp in stage 8 was nine seconds, and 
in stage 15 twelve seconds, and the pulp was dried to a solids content of 
90.5% by the time it had completed its passage through the dryer. 
The brightness of the bleached and dried pulp was 79.2% SCAN, which is a 
very high brightness for thermomechanical pulp. Usual tower bleaching of 
such pulp would have required a charge of 3% hydrogen peroxide, and a 
bleaching time of two hours. 
EXAMPLE 8 
Spruce wood sulfite pulp, bleached in one step with chlorine dioxide, and 
neutralized with sodium hydroxide, having a viscosity of 1150 dm.sup.3 /kg 
according to SCAN, an extractives content of 0.42% SCAN, a brightness of 
69% SCAN, and a solids content of 30%, was mixed with a dilute aqueous 
bleaching solution of sodium hypochlorite and sodium hydroxide to a 10% 
pulp consistency, and then dewatered to a solids content of 52%. The 
dewatered pulp contained 0.7% sodium hypochlorite calculated as active 
chlorine, and 0.5% sodium hydroxide based on the dry weight of the pulp. 
The pulp was shredded into flakes in disc refiner 5, and then introduced 
into stage 8 of the apparatus of FIG. 1. Carrier steam at 120.degree. C. 
entered stage 8, corresponding to a superatmospheric pressure of 100 kPa. 
The residence time of the pulp in stage 8 was eight seconds and in stage 
15 twelve seconds. 
The bleached pulp had a solids content of 90.1%, a viscosity of 1105 
dm.sup.3 /kg, an extractives content of 0.42%, and a brightness of 89.5%. 
It is apparent from these results that, using the process of the invention, 
it is possible to bleach sulfite spruce pulp in a very short time, without 
noticeable decomposition of the carbohydrates, in comparison with 
conventional tower bleaching, which would have required a bleaching time 
of several hours. 
EXAMPLE 9 
A semi-bleached pine sulfate pulp having a brightness of 76% SCAN and a 
viscosity of 945 dm.sup.3 /kg was mixed with an aqueous bleaching solution 
of diethylenetriamine pentaacetic acid, hydrogen peroxide, sodium 
hydroxide, and water, such that the pulp consistency was 8%. The 
suspension was then thickened to a solids content of 45%. The dewatered 
pulp contained 0.8% hydrogen peroxide, 0.2% diethylenetriamine pentaacetic 
acid, and 0.6% sodium hydroxide. 
The pulp was shredded to flakes in disc refiner 5 and then, introduced into 
stage 8 of the apparatus of FIG. 1. Carrier steam at 120.degree. C. 
entered stage 8, corresponding to a superatmospheric pressure of 100 kPa. 
The residence time of the pulp in stage 8 was nine seconds and in stage 15 
twelve seconds. 
The bleached pulp had a solids content of 91.3%, a viscosity of 922 
dm.sup.3 /kg, and a brightness of 85% SCAN. This viscosity is surprisingly 
high, considering that the brightness was increased by 9%. 
The results show that by the process of the invention it is possible to 
bleach pine sulfate pulp without noticeable degradation of the 
carbohydrates in a very short time, in comparison with conventional tower 
bleaching, which would have required a bleaching time of several hours. 
EXAMPLE 10 
Screened spruce sulfite pulp having a brightness of 62% SCAN, a viscosity 
of 1140 dm.sup.3 /kg, and an extractives content of 1.88% SCAN, was mixed 
with sodium hydroxide and water to a pulp consistency of 10%, and then 
thickened to a solids content of 42%. The resulting pulp contained 2% 
sodium hydroxide. It was then introduced into stage 8 of the apparatus of 
FIG. 1. The carrier was saturated steam at a temperature of 115.degree. 
C., corresponding to a superatmospheric pressure of 69 kPa. The transit 
time for the pulp through stage 8 was twelve seconds. The steam was then 
separated from the pulp by passing the mixture into cyclone 12. The pulp 
was fed out by way of rotary vane feeder 13 and then brought to a storage 
tank, where it was diluted with hot water. The pulp exiting from the 
feeder had a solids content of 39.5%, a viscosity of 1055 dm.sup.3 /kg, 
and an extractives content of 0.38% SCAN. 
The results show that the process in accordance with the invention makes it 
possible by alkaline extraction to effectively deresinate sulfite pulp in 
a very short time. Conventional alkaline extraction for deresination in a 
tower requires a time of at least one hour. 
EXAMPLE 11 
In this Example, pulp was bleached under superatmospheric pressure in a 
closed apparatus provided with a screw conveyor. A turbulent flow of pulp 
through the apparatus was provided mechanically, by way of the screw 
conveyor. The screw conveyor was placed at the bottom of the apparatus, 
and disposed horizontally, conveying the pulp through the apparatus at a 
rate of about 1 meter/second to a pressure cyclone directly connected to 
the discharge end of the screw. This cyclone in turn was provided with a 
screw feeder, for controlling the transit time of the pulp through the 
cyclone. 
Thermomechanical spruce pulp having a solids content of 33% was taken out 
directly from the disc refiner in which the pulp had been defibrated and 
brought to a chemical mixer under the same superatmospheric prressure as 
the disc refiner, in this case 150 kPa. An aqueous solution of bleaching 
chemicals was then mixed into the pulp in an amount to give, by weight of 
the dry pulp, 3% hydrogen peroxide, 5% sodium silicate, 1.5% sodium 
hydroxide, 0.02% MgSO.sub.4.7H.sub.2 O, and 0.2% diethylenetriamine 
pentaacetic acid. The pulp was then dewatered to a solids content of 32% 
at a temperature of 110.degree. C. 
This pulp impregnated with the bleaching chemicals was then carried by way 
of a high consistency pump into the processing apparatus described above. 
While passing through the apparatus, the pulp was under an oxygen-free 
steam atmosphere at a temperature of 107.degree. C. and a superatmospheric 
pressure of 30 kPa. The transit time for the pulp through the apparatus 
was 6 seconds. The pulp was then taken to the pressure cyclone, where the 
steam was separated from the pulp, and the pulp allowed to fall down into 
the screw discharger. The time of passage through the cyclone and the 
screw discharger was about 3 seconds. 
The pulp delivered at the end of the screw disc discharger had a 
temperature of 96.degree. C. and a pulp consistency of 32%, and contained 
0.06% residual peroxide. After diluting with cold water to a 4% pulp 
consistency, the pH of the resulting pulp suspension was 8.1. The diluted 
pulp was then dewatered to a pulp concentration of about 30% in a 
centrifuge, and dried to a solids content of about 92.4%. The brightness 
of the pulp was then measured, and found to be 74.3% ISO, which is 
surprisingly high, considering the short bleaching time, and the 
relatively simple bleaching installation employed. 
EXAMPLE 12 
In this Example, pulp was bleached under superatmospheric pressure in a 
closed apparatus provided with a screw conveyor. A tubulent flow of pulp 
through the apparatus was provided mechanically, by way of the screw of 
the conveyor. The screw conveyor was placed at the bottom of the 
apparatus, and disposed horizontally, conveying the pulp through the 
apparatus at a rate of about 1 meter/second to a pressure cyclone directly 
connected to the discharge end of the screw. This cyclone in turn was 
provided with a screw feeder for controlling the transit time of the pulp 
through the cyclone. 
Thermomechanical spruce pulp having a solids content of 33% in the last 
defibration stage was taken through a disc refiner in which during the 
defibration there was mixed into the pulp aqueous bleaching solutions of 
hydrogen peroxide, sodium silicate, sodium hydroxide, magnesium sulfate 
and diethylenetriamine pentaacetic acid, while the pulp was under 
superatmospheric steam pressure of 120 kPa, corresponding to a temperature 
of 123.degree. C. During the refining, the aqueous solutions bleaching 
chemicals were added at different places along the radius of the grinding 
discs. An aqueous solution of diethylenetriamine pentaacetic acid and 
hydrogen peroxide was applied in a stream close to the center of the 
grinding discs, while an aqueous sodium hydroxide solution was applied at 
a point half way along the radius of the discs, and an aqueous sodium 
silicate solution was applied at a point about 5 cm from the outer edge of 
the discs. The pulp emerging from the disc refiner contained 0.15% DTPA, 
3% H.sub.2 O.sub.2, 1% NaOH and 3% Na.sub.2 SiO.sub.3 based on the weight 
of the dry pulp. 
The defibrated pulp was blown to a pressure cyclone connected to the 
processing apparatus provided with a screw conveyor. During the passage 
through the pressure cyclone and the screw conveyor, the steam pressure 
was reduced from 120 kPa to 50 kPa, and thereby the temperature was also 
reduced from 123.degree. C. to 111.degree. C. The transit time for the 
pulp through the processing apparatus screw conveyor was 4 seconds. The 
pulp was then blown to a second cyclone for separating steam from the 
pulp. 
The pulp emerging from the cyclone was at 95.degree. C. and contained 0.14% 
H.sub.2 O.sub.2. The pulp consistency was 36%. After dilution with cold 
water to a pulp consistency of 4%, the pH of the pulp suspension was 8.2. 
The diluted pulp suspension was dewatered to a pulp consistency of about 
30% in a centrifuge, and dried to a solids content of 91.8%. The 
brightness of the pulp thus obtained was 74.6% ISO. 
As this Example shows, in the manufacture of thermomechanical pulp it is 
possible to add the bleaching chemicals in the disc refiner, and then 
apply the method in accordance with the invention, with the result that a 
surprisingly bright pulp is obtained, in a short time, and with simple 
bleaching apparatus. 
This Example shows that the process of the invention makes possible the 
bleaching of both mechanical and chemical pulp while the pulp is being 
transported with steam under superatmospheric pressure and with subsequent 
drying. Excess steam is obtained as a by-product of the process, apart 
from the bleaching effect, and this steam can be utilized elsewhere in a 
pulp mill, for example, for preheating the wood chips before they are 
digested, giving the process good heating economy. The bleaching chemical 
cost is also low, using the process of the invention. 
When applied to the extraction of materials from the pulp, it becomes 
possible to carry out an effective extraction within a very short time, 
while the pulp is being conveyed by steam at superatmospheric pressure. 
In both bleaching and extraction, the rapid reaction time means that plant 
requirements are low, with a resulting low investment cost for both 
apparatus and plant buildings. The use of high bleaching and extraction 
concentrations in the process of the invention also reduces pollution of 
the atmosphere. 
The process can be carried out continuously by continuously feeding 
cellulose pulp into the reaction zone at one end, and continuously 
withdrawing the modified cellulose pulp at the other end of the zone. This 
is the preferred mode of operation, since control of the reaction time is 
easy to accomplish, in terms of transit time through the zone at a 
selected flow rate. However, it is also possible to carry out the process 
in a batch or semibatch procedure, in which a batch of cellulose pulp is 
passed through the zone. Whereas the continuous flow approach is best 
carried out by entraining the pulp in a flow of carrier steam, thus 
carrying it through the zone, the batch or semibatch approach is more 
easily carried out using a screw conveyor, or a train of buckets, or other 
mechanical means arranged to transport one entire batch of material at a 
time through the zone. Other variations will be apparent to those skilled 
in this art, and any continuous or batch flow reaction zone apparatus can 
be used. A continuous flow flash dryer is illustrated in the Examples, but 
elongated continuous flow reactors can of course be used as well.