Note: Descriptions are shown in the official language in which they were submitted.
5~
.1
Method for the pr~paration of pulp and for the
recovery of chemicals
The invention is concerned with an alkaline
pulp cooking in which sodium aluminate and sodium ~ulphide
are used as cooking chemicals. More specifically, the
invention is concerned with a method for the preparation
of pulp and for the recovery of chemicals, whereby
hardwood or softwood chips are cooked by means of a sodium-
sulphide-containing aqueous solution of sodium aluminate
in a conventional sulphate pulp cooker under standard
conditions. The method is characterized in that the
spent liquor separated from ~he cooking is, after evapo-
xation, fed into a gasifier, wherein the temperature is
t5 700 to 1250C, the hot inorganic matter is r moved out
of the gasifier, cooled by means of the air required for
the gasification, crushed and dissolved in water, and
the alkaline aluminate solution obtained is used as an
absorption solution for the H2S contained in the gas
generated in the gasifier, whereby a solution is obtained
which contains 1 to 4 percent by weight of Na~S and NaHS
as well as 5 to 20 percent by weight of ~aAlO2 ancl
which is as such, without separate causticizing, suita~le
for the cooking of pulp, and the obtained gas, free from
hydrogen sulphide, is burned in a suitable boiler.
In the method in accord~nce with the invention,
the cooking chemicals can be recirculated to the cooking
in the regenerated form without a separate causticizing
step. The use of aluminate as an auxiliary chemical in
autocausticizinoisin itself known in the sulphite pro-
cess, in the so-called Sonoco process (Cook, W.R.,
Tappi 57 (1974), 9, pp. 94-96), as well as in the two-
stage burning according to the Finnish Patent No. 62,562.
Aluminate has also been suggested to be added to sulphur-
free alkaline cooking Iso-called soda cook) (see U.S.
Pa ent 2,601,110), whereby, however, about one half of
the alkali must be c~dded in the form of sodium hydroxide r
t which makes autocausticizing impossible.
In the paper Paperi ja puu No. 2, 1979,
pp. 98-103, it is stated that sodium aluminate is
not a sufficiently strong base in order that it could
5 be substituted for sodium hydroxide in cooking.
In the paper Paperi_~a puu No. 3, 1978, pp.
129-132, it is stated that sodium-aluminate auto-
causticizing can be used only ~n sulphur-free cooking.
In the following, reference will be made to
10 the attached Figure 1, which shows the circulation of
Al~OH)3 and the gasification of the spent li~uor in the
method in acoordance with t.he inven~ion.
The evaporated black liquor 2 obtained from
the cooking 1 i5 granulated with circulated AltOH)3
15 and NaAlO2 into granules of 10 to 20 mm in a granulating
drum 3, in a way known per se, whereafter the gran-
ules are fed into the gasifier 4 for spent liquor,
wherein the tempexature is 700 to 1250C. Because of
the high melting point of the sodium aluminate formed,
20 the operation is all the time carried DUt in the solid
phase. Gasification of spent liquor is known, e.g.,
from the S Q -Billerud process (STFI Meddelanden, serie
D7, 1976, pp. 132-138~. The hot granules, which no
more contain ~rganic material or sulphur, are discharged
25 from the bottom of the gasifier and cooled, for example,
by means of a fluidized bed cooler 5. The cooling is
effected by means of the air needed for the gasifica~ion~
Hereafter, the granules are crushed and, if desired, a
part of the sodium aluminate formed is circulated to gran-
30 ulation 1. The rest of the sodium aluminate is dissol-
ved in water 6, and, if desired, the material insoluble
in water can be separated and recirculated.
The gas mixture obtained from the gasii-
cation of the spent liquor is cooled in a waste heat
35 boiler 7 and possibly in addition by means of a spray
cooler. The H2S is absorbed straight into the alkaline
aluminate solution. This is a well-known technique,
which can be carried out with Yery small H~S losses.
In the absorption devlce 8, H2S reacts with NaOH~ whereby
a solution is obtained which contains Na2S, NaHS, and
NaAlO2; the H~S-free gas is burned in a suita~le boiler.
Unexpectedly it has been found that the
solution obtained from absorption of H~S is as such,
without separate causticizing, suitable as a cooking li
quor for pulp. In this way, it is possible to simplify
the chemical circulation o~ the sulphate pulp process
without deterioration o the cooking efficiency, whereby
the high investment costs of traditional causticizing
and of a soda boiler as well as thP operating diffi~
culties and risks related ~o the soda boiler are avoided.
As compared with known methods, ~he method
in accordance with the invention has the difference and
the advantage that formation of melt in the spent liquor
gasifier is avoided. Because of the high melting point
of sodium aluminate, the operation is all the time
carried out in the solid phase. In addition to this,
the investment costs involved in the sulphate process
can be reduced signif~ an~y when using an aluminate 501u-
tion by com~letely avoiding causticizing and by simpli-
fying the heat recovery from the black liquor.
The invention is illustrated by the
following examples.
Example 1 (comparative example~
Conventional sulphate cellulose cooking was
carried out under laboratory conditions as follows:
To a 1 litre autoclave provided with a
rotating mixer were added
100 g air-dried Finnish pine chips ~moisture 10 %)
12.6 g NaOH
6.3 g Na2S
3.5 g Na2cO3
35 350 g H2O
The autoclave was closed and cooking was
carried out by raising the temperature by means of
thermos~ated oil firing from 20~C to 80C during 0.5
hours and from B0C ~o 170C during ~.0 hours. The
reactor was maintained at the final cooking temperature
170C for 1.5 hours. The autoclave was cooled in on~
hour to 60C.
From the autoclave, the reaction mixture was
transferred to a 2 l decanter to which was added 300 g
cold water, whereupon the fibres were separated from the
cooking liquor by filtration. The separated cellulose
fraction was washed twice with 5 litres water, where-
after the fibres were separated by filtration and dried
for 12 hours at 105C. The so washed and dried cellulose
fraction was weighed, and a yield of 48.3 g cellulose
was noted in the exemplary cooking. Th~ lignin content
of the obtained cellulose was determined on the basis of
the so-called kappa number (TAPPI standard3. Lignin
content = 7.9 ~.
Example 2
-
A test cooking was carried out under laboratory
conditions where the alkali addition was dosed as
sodium aluminate only. Both the sulphidity and the total
alkali in the cooking liquor were the same as in the
comparative cooXing lExample 1).
To a heatable 1 litre autoclave provided with
a rotating mixer were thus added:
100 g air-dried Finnish pine chips (moisture 10 ~)
54 g NaAlO2
12.6 g Na2S
3.5 g Na2CO3
30750 g H~O
The cooking was carried out by using the same
temperatures as in Example 1.
Upon reaction, the reaction mixture was
transferred into a 2 l decanter as in the preceding
example, 300 g cold water was added and the fibxes
were separated from the cooking liquor by filtration.
The separated black :Liquor was recovered for later
o~
r
test~, and the cellulose fraction was washed and dried
as in Example 1. The yield of washed and dried cellu-
lose was 55.6 g~ the kappa number 66.2, and the corres-
ponding lignin content was 9.9 ~.
Example 3
A test cooking according to Example 2 was
carxied out by using 100 g air-aried birch chips
(moisture 9 %) in stead of pine chips. After washing
and drying, the yield of cellulose was found to be
54.0 g, the kappa number was 3~.9, and the corresponding
lignin content was 5.5 %.
The black liquor separated after cooking w~s
recovered for further tests.
Exam~le 4
The black liquor sample separated in the two
preceding examples 2 and 3 was concentrated and treated
for regeneration of the cooking chemicals in the
following manner.
After concentration, the composition of the
black liquor was~
43.2 g organic matter of which 9 % w~s sodium (3,9 g)
15 g Al(OH)3
3.2 g Na2~O4
0.56 g NaO~
0.2 g Na2S
g H2C~
The above-mentioned black liquor sample was
dried at 110~C, whereby 61.5 g dry substance was obtai-
ned. 50 g of this dry substance was crushed and heated
ln a gasification furnace under reducing conditions
(CO gas flow 30 ml/min) by raising the temperature to
800C in 2 hours and by keeping the product for 1/2
hour at 800C before cooling.
The gas discharged from the gasification
reactor was washed by passing it through a 2-n NaO~
solution (100 ml) to collect possibly liberated H2S
gas. The residue of 29 g obtained from the heating
, .
;,"
5~
was dissolved in 500 ml distilled water, and the
undi~solved carbon-containing residue was separated
by filtration, dried and weighed. This undissolved
residue (20 g~ was heated in a porcelain crucible at
800C to burn the carbon; the water-insoluble residue
(4.9 g) obtained from the heating was assumed to be
aluminium oxide iA1203).
The water-~luble portion of the residue
obtained from the heating was analysed. It was found
that ~he aqueous solution contained no carbonates and
that the Na~SQ4 quantity dissolved therein was 0~16 g,
corresponding to about 6 % of the original sulphate
quantity of the sample.
The Na2S content of the solution was
determin~d by titration. The solution was found to
contain 1.2 g Na2S,which corresponds to 2.2 g Na~SO~
~of the oriyinal quantity of 2 o6 y ~a2S04). A
hydrogen sulphide quantity corresponding to O.2 g
soaium sulphate was found in the sodium hydroxide
washing solution. The missing sulphur obviously had
been discharged as hydrogen sulphide along with the
gas flow.
The total al~ali of the solution was titrated.
From this was deducted the share o~ sodium sulphide,
whereby the alkalinity was found to correspond to
O.087 mol NaOH. According to this, 7.1 g Na Al 2 was
dissolved in the ~olution (the excess aluminium hydrox~de
was present, as was already stated previously, in the
insoluble residue after heating as A1203).
According to the foregoing, the composition of
the part dissolved in water after gasification was:
7.1 g NaA102 5~ g
0.16 g Na2S04 x 7.6 1.2
1.3 g Na2S 1004
which almost exactly corresponds to the cooking
chemical composition used in Examples 2 and 3.
..... . . . ..
o~
Example 5
An experiment in accordance with Example 4
was carried out, in which the gas di~charged from the
reactor was washed for the recovery of H2S, by irst
passing the gas through an aluminate solution (200 ml~,
whose composition was
14 g Na AlO2
0.4 g Na2SO4
200 g ~2
and hereafter through a 2N NaOH solution (100 ml).
The solution~ were analysed after the exper~
ment, whereat it was noticed that the sodium aluminate
solution con~ained a quantity of sodium s~p~ide ~hydrogen
sulphide) corresponding to 0,3 g sodium sulphate. In
the other washing solution ~2N NaOH~, no sulphur was
detected.
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