Pulp production

A method of manufacturing chemical pulp out of comminuted cellulosic fiber material comprising digesting the fiber material with digestion liquid without preceding peroxide stage. According to the invention the comminuted fiber material is treated in at least one stage prior to said digestion, in the presence of a liquid containing at least one compound having the ability to form complexes with metals existing naturally in the fiber material.

FIELD OF THE INVENTION 
The present invention relates to a method of manufacturing chemical pulp 
out of comminuted cellulosic fiber material, comprising digesting the 
fiber material with digestion liquid, said method excluding any peroxide 
stage before said digesting. The invention relates particularly to a 
method of the kind described, that gives improved properties with respect 
primarily to tearing resistance, viscosity and yield. 
The object of the invention is to produce a chemical pulp which, already 
after the digestion process, has considerably reduced content of 
transition metals and at the same time considerably improved properties 
with regard especially but not exclusively to tearing resistance, 
viscosity, yield, kappa number and brightness. 
BACKGROUND AND SUMMARY OF THE INVENTION 
The method according to the invention is substantially characterised in 
that the comminuted fiber material is treated in at least one stage prior 
to said digestion, in the presence of a liquid containing at least one 
compound having the ability to form complexes together with metals 
existing naturally in the fiber material. Thus, the treatment with 
sequestering agent is carried out immediately prior to a pre-impregnation 
of the chips, for instance, or alternatively during, i.e. simultaneously 
with the pre-impregnation usually performed before digestion. Treatment 
with the sequestering agent added is performed so that a pulp is obtained 
after said digestion process which pulp, besides having a lower content of 
metals, primarily manganese, has a tearing resistance at least 10% higher, 
a viscosity at least 5% higher, and produces a yield at least 1% higher 
than corresponding parameters for a pulp manufactured without said 
pre-treatment with sequestering agent, calculated within the same kappa 
number interval. 
The invention is applicable to any method whatsoever for manufacturing 
chemical pulp. A chemical pulp is defined as a pulp having a kappa number 
below about 100. Such pulps include sulphite and bisulphite pulps based on 
sodium, potassium or magnesium, alkaline neutral sulphite pulp, pulps of 
anthraquinone plus hydroxide (NaOH/KOH) or carbonate (Na.sub.2 CO.sub.3 
/K.sub.2 CO.sub.3) plus possibly oxygen gas, polysulphide pulp, sulphate 
pulp and pulp produced by pre-impregnating wood with hydrogen sulphide 
before alkaline delignification, and also pulps produced by 
delignification of wood with organic solvent such as methanol, ethanol, 
possibly in the presence of inorganic solvent. 
The compound able to form complexes with metals in the fiber material is 
suitably selected from the group consisting of non-nitrogenous 
polycarboxylic acids, nitrogenous polycarboxylic acids and phosphonic 
acids. Diethylene triamine pentacetic acid (DTPA), ethylene diamine 
tetracetic acid (EDTA) or nitrilo triacetic acid (NTA) are preferred from 
the first category, oxalic, citric or tartaric acid from the second 
category, and diethylene triamine pentaphosphoric acid from the third 
category. Most preferred are EDTA and DTPA. Two or more of the compounds 
may also be used, and in any combination whatsoever. 
The treatment with sequestering agent is suitably performed at a pH value 
above about 5.0 and at a liquid/fiber material ratio greater than 2:1, 
preferably greater than 3:1. According to a suitable embodiment said 
treatment is performed at a temperature of at least 80.degree. C., 
preferably at least 100.degree. C., a pressure of at least 2 bar, 
preferably at least 5 bar, most preferably at least 10 bar, and over a 
period of at least 20 minutes, preferably at least 40 minutes, most 
preferably at least 60 minutes. 
The sequestering agent is supplied in a quantity suitably within the 
interval 0.5-10 kg per ton of dry fiber material, preferably 1.5-5 kg and 
most preferably 2-4 kg per ton of dry fiber material. 
A separate treatment vessel may be used for the treatment of the wood with 
sequestering agent, said vessel being located before, i.e. upstream of the 
digester tank. The treatment according to the invention may be included 
with the digestion in a continuous process or a discontinuous process for 
pulp production. The invention is applicable to all types of continuous 
and discontinuous digestion methods for the manufacture of chemical pulp. 
According to one embodiment of the invention at least a considerable 
portion of free liquid containing metal complexes formed by said treatment 
is removed from the wood upon completion of the treatment with 
sequestering agent. This can be achieved by draining, i.e. thickening, and 
subsequent washing of the wood with a liquid free from metals or having 
low metal content. The liquid containing metal complexes is preferably 
removed by being displaced by cleaner liquid of the type described. The 
liquid removed is transferred directly to an evaporation system. 
Alternatively the formed metal complexes are permitted to accompany the 
fiber material into the digestion process. 
At least a part of said liquid present during treatment of the fiber 
material and containing the sequestering agent, consists of spent liquor, 
fresh digestion liquid, effluent from bleaching processes, condensation, 
mains water or lake water, or mixtures thereof. The spent liquor used is 
suitably the spent liquor having reduced, low content of metals that is 
obtained at said digestion following said treatment with sequestering 
agent. 
Generally the digestion process includes a pre-impregnation of the wood 
with digestion liquid and/or spent liquor and according to one embodiment 
of the invention, the treatment with sequestering agent is performed prior 
to said pre-impregnation and is followed by a washing stage of suitable 
type as described above. According to another embodiment the treatment 
with sequestering agent is performed in combination with the actual 
pre-impregnation as an integrated treatment, in which case the 
sequestering agent is preferably added together with the impregnation 
liquid. In this case the metal complexes formed, together with any excess 
of sequestering agent remaining, accompany the wood to the digestion 
zone(s) and are not therefore removed before digestion, but at a later 
stage when the spent liquor is withdrawn. In certain cases the 
impregnation phase may be relatively short, such as down to about 1 
minute, during which brief period treatment with sequestering agent is 
performed before the digestion phase is started in the continuous process. 
Said spent liquor may be black liquor received from the digestion of wood 
that has been treated with sequestering agent in accordance with one of 
the alternatives described above. The cooking liquid may be fresh white 
liquor. 
The treatment with sequestering agent may be most advantageously performed 
in conjunction with an isothermal cooking process that includes a final 
extended displacement step in which the operating conditions correspond, 
or substantially correspond, to those prevailing in the preceding 
digestion zone(s). 
The pulp is delignified with oxygen gas after the digestion process. The 
pulp is suitably treated with sequestering agent immediately prior to the 
delignification with oxygen gas. The pulp delignified with oxygen gas may 
then suitably be bleached with a bleaching agent containing hydrogen 
peroxide, possibly in combination with ozone and/or peracetic acid.

DETAILED DESCRIPTION OF THE INVENTION 
In the diagrams shown in the drawings the numbers 1-9 indicate the plotted 
values from the experiments with the same numbering that are described in 
the following examples, i.e. the number 1 in the diagram according to FIG. 
1 indicates the yield and kappa number values from Experiment 1. The four 
different symbols are explained in FIG. 1. ITC stands for isothermal 
cooking which is explained further below. 
EXAMPLE 
Test 1 
Moist chips equivalent to 2.5 kg absolutely dry chips of Scandinavian 
softwood were treated with steam in a digester with circulation for 5 min. 
at 110.degree. C. and a pressure of 1.0 bar. The chips contained 220 ppm 
manganese calculated on the digested pulp at a yield of 45%. 
In accordance with the present invention the steamed chips were treated 
with a sequestering agent dissolved in a liquid. The liquid used was 
de-ionized water and the sequestering agent used was EDTA in a quantity of 
0.005 kg, corresponding to 2.0 kg EDTA per ton of wood. The liquid/wood 
ratio was 5.5:1. The pH value of the liquid containing EDTA was 6.7. The 
treatment with EDTA was performed in a digester with circulation for 60 
min. at 110.degree. C. and a pressure of 10 bar, the liquid being 
circulated the whole time. Free liquid was then emptied from the digester 
in an amount corresponding to 65% of the total content of free and bound 
liquid. Hot, de-ionized water (without EDTA) was added and allowed to 
circulate through the digester under steam pressure for 60 min. at a 
temperature of 110.degree. C. Free liquid is then again emptied from the 
digester in an amount corresponding to 65% of the total content of liquid. 
The chips pre-treated in this way were then subjected to a digestion 
process of the isothermal cooking type (ITC), preceded by impregnation 
with digestion liquid in the form of white liquor. The digestion process 
comprised concurrent digestion, countercurrent digestion displacing black 
liquor with white liquor, and then an extended displacement phase with 
white liquor corresponding to the conditions in a "Hi-heat" zone. The 
white liquor had a sulphidity of 33.2%. At the starting impregnation 140 
kg white liquor was used, calculated as effective alkali (EA) per ton of 
wood. The impregnation was carried out for 30 min. at 125.degree. C. and a 
pressure of 10 bar (nitrogen gas). At the end of the impregnation the 
temperature was increased to a digestion temperature of 164.degree. C. and 
the pressure was gradually reduced to steam pressure. Concurrent digestion 
was started at said digestion temperature and pressure, the free digestion 
liquid being caused to circulate through the circulation digester from the 
top and down for a period of 60 min. Additionally 40 kg white liquor (EA) 
per ton of wood was added initially during the concurrent digestion. The 
countercurrent digestion was started upon completion of the concurrent 
digestion, whereupon 10 liter digester liquid was gradually pumped in and 
allowed to displace the same amount of black liquor for 60 min. The 
temperature was maintained constant at 164.degree. C., as well as the 
liquid/wood ratio, during the time of 60 min. that the countercurrent 
digestion was in progress. The concentration of white liquor was 
calculated so that approximately 12 g effective alkali (EA) per liter 
remained at the end of the countercurrent digestion. The extended 
displacement phase then followed and took place at the same temperature 
(164.degree. C.). It commenced with white liquor having a concentration of 
10 g effective alkali per liter being added to displace spent liquor out 
of the circulation digester. 14.4 liter spent liquor was displaced in this 
way over a period of 180 min. The digested chips were then transferred to 
a propeller-operated disintegrator to be defibered for 15 min. The yield 
was determined after washing and thickening the unscreened pulp thus 
obtained. 
Test 2 
Test 1 was repeated, the only difference being that the temperature during 
the digestion process was increased 2.degree. to 166.degree. C. and the 
amount of white liquor added during the concurrent digestion was increased 
to 50 kg per ton calculated as effective alkali. The pH value of the 
liquid containing EDTA was 6.2. 
Test 3 
Test 1 was repeated for comparison, but the steamed chips were not 
subjected to any treatment with EDTA. Instead they were digested 
immediately under the same conditions. The impregnated chips had an 
effective alkali content of 11.8 g/l. 
Test 4 
Test 3 was repeated for further comparison, the only difference being that 
the temperature during the digestion process was lowered 2.degree. to 
162.degree. C. The impregnated chips had an effective alkali content of 
12.1 g/l. 
Test 5 
Test 3 was repeated, with the difference that the chips were impregnated 
with black liquor instead of white liquor, the amount of white liquor 
being increased to an equivalent extent during the concurrent digestion in 
order to achieve the necessary content of effective alkali, and that the 
temperature during the digestion process was lowered 2.degree. to 
162.degree. C. The chips impregnated with black liquor contained no 
effective alkali (pH 10.8). 
The results of the five experiments are given in the following Table 1. 
"Alkali consumption" refers to the totally consumed effective alkali (EA) 
in kg per ton of wood calculated as absolutely dry. 
TABLE 1 
______________________________________ 
Invention Reference 
Test 1 
Test 2 Test 3 Test 4 
Test 5 
______________________________________ 
EDTA, kg/ton wood 
2.0 2.0 0 0 0 
Digestion temp., .degree.C. 
164 166 164 162 162 
Alkali consumption 
172 182 181 171 168 
Yield, % of wood 
46.3 45.0 44.6 45.4 45.6 
Kappa number 13.7 10.8 16.8 20.1 20.7 
Viscosity, dm.sup.3 /kg 
1120 1010 1087 1164 1160 
Brightness, ISO 
36.5 38.1 33.5 32.1 -- 
Mn, ppm 31 30 92 107 -- 
Mg, ppm 79 50 377 405 -- 
Ca, ppm 1043 1003 1688 1805 -- 
Cu, ppm 1 3 54 27 -- 
Fe, ppm 41 25 22 58 -- 
Tensile index, 
80 80 -- 80 80 
kNm/kg 
Beat revolutions, 
1100 1200 -- 1350 1000 
PFI 
Drainage resistance, 
15.5 15.5 -- 15.5 15.0 
.degree.SR 
Density, kg/dm.sup.3 
630 640 -- 640 630 
Air resistance, 
2.3 2.6 -- 3.5 3.3 
sec/100 ml 
Burst index, MN/kg 
5.6 5.4 -- 6.1 5.9 
Tear index, Nm.sup.2 /kg 
26.5 25.6 -- 19.1 19.7 
______________________________________ 
A high tear index is obtained per se with the digestion process including a 
final extended displacement phase at digestion temperature, known as the 
ITC technique, used in the tests. This can be seen from the reference 
Tests 4 and 5. A lower tear index, normally at the level 15-16 Nm.sup.2 
/kg, is obtained without this ITC technique. The pulps produced according 
to the invention have tear indexes of 26.5 and 25.6 Nm.sup.2 /kg at a 
tensile index of 80 kNm/kg, as compared with 19.1 and 19.7 Nm.sup.2 /kg 
for the reference pulps. This result is very surprising. The difference is 
in itself surprising but even more surprising is that the difference is so 
great. Such high tear index values have not previously been measured for 
pulp made of Scandinavian softwood. Not even Douglas firs, which have the 
strongest fiber, produce pulps with such high tear index values. 
The experiments also show that the pulps according to the invention are 
just as easily beaten as the reference pulps, and they have the same 
density despite considerably lower kappa number. The high permeability to 
air (low air resistance) which indicates good drainage properties in 
washing equipment for the pulp, is also remarkable. This was confirmed 
both visually and sensorially since the pulps according to the invention 
were dewatered extremely easily when being further processed and had the 
same rugged character as a high yield pulp. This may possibly be the 
explanation for the negligibly lower burst resistance. 
Extrapolation of the yield values obtained to the kappa number interval 
12-16 indicates that the pulps according to the invention give 2.5-3.0% 
higher yield than the reference pulps. 6-7% more pulp can thus be produced 
from the same quantity of raw material irrespective of whether the pulp is 
bleached or unbleached. 
Extrapolation of the viscosities obtained to the kappa number interval 
12-16 indicates that pulps according to the invention show viscosities 
150-200 SCAN units (dm.sup.3 /kg) higher than the reference pulps. 
Normally a lower viscosity indicates poorer strength properties. The pulps 
according to the invention surprisingly show a different and higher level 
for this relationship. The pulp according to Test 2 has a viscosity of 
1010 dm.sup.3 /kg and a tear index of 25.6 Nm.sup.2 /kg, as compared with 
the reference pulps according to Tests 4 and 5 for which the mean value of 
the viscosity is 1162 dm.sup.3 /kg, but the tear index is 19.4 Nm.sup.2 
/kg, i.e. the tear index is 32% higher for the invention than for the 
references, despite lower viscosity. The reject percentage upon screening 
through 0.15 mm slits was also determined in Test 2 and proved to be below 
a level of 0.1% of the pulp. For an ITC-pulp this value is usually just 
below 0.5%. 
Extrapolation of the brightness values obtained to the same kappa number 
shows that the pulps according to the invention are 1.5-2.0 ISO units 
brighter than the reference pulps. 
The mechanisms causing these surprising results are not fully explained. 
Without being tied to any explanations, however, a decrease in the 
manganese content probably has at least a certain significance. According 
to Tests 1 and 2 treatment with sequestering agent (EDTA) enabled a 
reduction in the manganese content from about 100 ppm (calculated on 
absolutely dry pulp) to 30 ppm. Manganese reciprocates or alternates 
between the valency levels +4 (MnO.sub.2, pyrolusite) and +6 (MnO.sub.4 
--, green-coloured ion) in a redox cycle continuously generating free 
radicals (OH.multidot.) which break down the carbohydrates in accordance 
with a known pattern. The process is known as the Haber Weiss cycle and is 
described in Trieselt W., "Chemistry of catalytic degradation during 
hydrogen peroxide bleaching", Melliand Textilberichte V51 (1970), page 
1094. 
EXAMPLE 1 
Test 6 
Steamed chips according to Example 1 were treated with EDTA dissolved in a 
liquid, in accordance with the present invention. The liquid used was 
black liquor obtained from Experiment 2 in Example 1, and was therefore 
partially freed from manganese. The quantity of EDTA was 0.005 kg, and 
this was mixed with about 9 liter black liquor. The liquid/wood ratio was 
5.5:1. The pH value of the black liquor containing EDTA was 10.3. The 
treatment with EDTA was performed in a circulation digester for 60 min. at 
110.degree. C. and a pressure of 10 bar, the black liquor being circulated 
the whole time. Free liquid was then emptied from the digester in an 
amount corresponding to 65% of the total content of free and bound liquid. 
9 liter of the same black liquor was added and allowed to circulate for 
another 60 min. at an increased temperature of 125.degree. C. and a 
pressure of 10 bar. Free liquid was then emptied from the digester in a 
quantity corresponding to 65% of the total amount of liquid. 
After the pre-treatment with EDTA 120 kg white liquor per ton wood was 
added, calculated as effective alkali, after which the chips were digested 
in accordance with Test 1 in Example 1. 
Test 7 
Test 6 was repeated, the only difference being that the temperature during 
the digestion process was increased 3.degree. to 167.degree. C. The pH 
value of the black liquor containing EDTA was 10.7. 
Test 8 
Test 6 was repeated with the difference that the treatment with EDTA in 
black liquor was continued for 25 min. instead of 60 min. and subsequent 
washing with black liquor for 20 min. instead of 60 min., and that the 
temperature during the digestion process was 165.degree. C. The pH value 
of the black liquor containing EDTA was 11.3. 
The results of the three experiments are given in the following Table 2. 
TABLE 2 
______________________________________ 
Test 6 Test 7 Test 8 
______________________________________ 
EDTA, kg/ton wood 
2.0 2.0 2.0 
Digestion temp., .degree.C. 
164 167 165 
Alkali consumption 
185 184 183 
Yield, % of wood 
45.3 43.9 45.0 
Kappa number 12.4 9.1 11.9 
Viscosity, dm.sup.3 /kg 
1067 886 1017 
Brightness, ISO 38.5 41.4 38.9 
Mn, ppm 49 54 73 
Mg, ppm 80 92 149 
Ca, ppm 587 783 838 
Cu, ppm 10 13 17 
Fe, ppm 28 26 27 
Tensile index, kNm/kg 
80 80 80 
Beat revolutions, PFI 
1100 1500 800 
Drainage resistance, .degree.SR 
15 16 15 
Density, kg/dm.sup.3 
620 630 630 
Air resistance, 2.2 2.7 3.0 
sec/100 ml 
Burst index, MN/kg 
5.4 5.3 5.8 
Tear index, Nm.sup.2 /kg 
27.3 26.6 21.4 
______________________________________ 
Although black liquor (with metals partially removed) was used as liquid in 
the EDTA treatment and the digestion was carried out so that still lower 
kappa numbers were obtained, the tearing resistance in Tests 6 and 7 was 
increased even more than in the pulps according to Tests 1 and 2. A 
tendency towards slightly greater need for beating of a pulp with kappa 
number 9.1 according to Test 7 can possibly be discerned. Other mechanical 
properties of the pulps according to this example are substantially the 
same as for the pulps according to Tests 1 and 2. 
At the same kappa number Tests 6-8 gave digestion pulps with almost the 
same high yield as Tests 1 and 2. 
The viscosity and brightness were also on the same levels as for the pulps 
according to Tests 1 and 2. 
It is remarkable that, despite the higher content of manganese in Test 6, 
namely 49 ppm, a somewhat higher tearing resistance was obtained than with 
the digestion pulps according to Tests 1 and 2, the latter having 
manganese contents of 31 and 30 ppm, respectively. This indicates that 
treatment with a sequestering agent in accordance with the present 
invention has a surprising effect in addition to that derived from the 
formation of complexes and displacement to reduce the metal content in the 
wood. 
Test 9 
Steamed chips according to Example 1 were treated with 2.0 kg EDTA per ton 
of wood, in accordance with the present invention. EDTA was mixed with 140 
kg white liquor, calculated as effective alkali, per ton of wood and the 
white liquor containing EDTA was supplied to the circulation digester for 
pre-impregnation of the chips under the same conditions as in Test 1, 
except that at the end of the impregnation the temperature was increased 
to 167.degree. C. Thereafter digestion of the ITC type was performed in 
accordance with Test 1, but at said higher digestion temperature of 
167.degree. C. Thus in this experiment no EDTA metal complexes were 
removed before the digestion. 
The results are given in the following Table 3. 
TABLE 3 
______________________________________ 
Test 9 
______________________________________ 
EDTA, kg/ton wood 2.0 
Digestion temp., .degree.C. 
167 
Alkali consumption 184 
Yield, % of wood 43.8 
Kappa number 10.2 
Viscosity, dm.sup.3 /kg 
886 
Brightness, ISO 38.3 
Mn, ppm 50 
Mg, ppm 245 
Ca, ppm 1290 
Cu, ppm 38 
Fe, ppm 20 
Tensile index, kNm/kg 
80 
Beat revolutions, PFI 
2300 
Drainage resistance, .degree.SR 
15.5 
Density, kg/dm.sup.3 
660 
Air resistance, sec/100 ml 
3.5 
Burst index, MN/kg 6.1 
Tear index, Nm.sup.2 /kg 
24.5 
______________________________________ 
As is clear from the above results, the tearing resistance of also this 
pulp shows a considerable improvement over the reference pulps according 
to Tests 4 and 5, as well as being clearly better than the reference pulps 
in other respects, within the same kappa number interval. The results must 
be deemed surprising also in view of the fact that no withdrawal of liquid 
containing metals was performed. 
As is evident, the manganese content in the pulp has been halved as 
compared with the reference experiments. 
Test 10 
The pulp obtained from Test 1 was subjected to delignification with oxygen 
gas supplied in excess. In each delignification 100 g pulp, calculated as 
absolutely dry, was supplied to an autoclave and varying quantities of 
NaOH were added. The pulp had a consistency of 10%. Delignification was 
carried out at a temperature of 105.degree. C. and a pressure of 5 bar 
over a period of 60 min. 
Test 11 
Test 10 was repeated with the exception that treatment with EDTA was 
performed before the oxygen gas treatment. 2.0 kg EDTA per ton dry pulp 
was allowed to act on the pulp with a consistency of 10% for 60 min. at a 
temperature of 70.degree. C. The final pH value was 5.0. The pulp was then 
treated with oxygen gas as in Experiment 10. 
Test 12 
The pulp obtained from Test 4 was subjected to delignification with oxygen 
gas in the same way as in Test 10. 
The results of the three experiments are given in the following Table 4. 
TABLE 4 
__________________________________________________________________________ 
Invention Reference 
Test 10 Test 11 Test 12 
A B C A B C A B C 
__________________________________________________________________________ 
Kappa number 
13.7 
13.7 
13.7 
13.7 
13.7 
13.7 
20.1 
20.1 
20.1 
Viscosity, dm.sup.3 /kg 
1120 
1120 
1120 
1120 
1120 
1120 
1164 
1164 
1164 
O-stage 0 0 0 2.0 2.0 2.0 0 0 0 
EDTA, kg/ton wood 
O.sub.2 -stage 
NaOH, kg/ton wood 
15 20 25 15 20 25 15 20 25 
Final pH 11.2 
11.6 
11.8 
11.1 
11.3 
11.7 
-- -- -- 
Kappa number 
7.6 7.1 6.9 7.3 5.6 5.7 8.6 7.7 6.8 
Viscosity, dm.sup.3 /kg 
975 961 944 1029 
972 966 980 959 920 
Brightness, % ISO 
44.7 
45.7 
48.3 
49.7 
54.1 
54.3 
-- -- -- 
__________________________________________________________________________ 
As is clear from the above results, pulps with kappa number 6 and viscosity 
1000 dm.sup.3 /kg can be manufactured from chips that have been 
EDTA-treated in accordance with the invention. Kappa number 9 is reached 
with the same viscosity for the reference pulp according to Test 12. 
This reduction of the kappa number by 35% enables the production of finally 
bleached sulphate pulps of softwood with correspondingly reduced 
quantities of bleaching agent such as chlorine dioxide, ozone and/or 
hydrogen peroxide. 
Thus, the expression "prior to said digestion" means that no treatment with 
any other chemical such as peroxide is performed after the wood has been 
treated with sequestering agent. The method according to the invention is 
thus free from such peroxide treatment before the digestion process, i.e. 
also before said treatment with sequestering agent. The only additional 
treatment is that a second stage with sequestering agent may be performed, 
as well as impregnation of the wood with digestion liquid if the digestion 
forms part of a process that also includes such impregnation.