METHOD FOR RECOVERING CHEMICALS FROM SPENT PULP LIQUORS

PROCEDE POUR LA RECUPERATION D'AGENTS CHIMIQUES PRESENTS DANS UNE LIQUEUR DE PULPE EPUISEE

English Abstract

A b s t r a c t
In order to recover chemicals from spent pulp liquors
while at the same time making use of energy liberated in
the process, the spent liquors are supplied to the
reaction zone of a reactor while external thermal energy
independent of the combustion, is simultaneously supplied,
for instance, in the form of an energy-rich gas heated
in a plasma generator, the temperature and oxygen potential
in the zone being governed independently of each other by
the controlled supply of said thermal energy, and possibly
also the supply of carbonaceous material and/or gas
containing oxygen, so that substantially all alkali and
sulphur, i.e. 95 to 100% thereof, is bound in a molten
phase which is separated from the gas phase and withdrawn
via an outlet connected to the reactor, the organic portion
of the spent liquor being withdrawn in the form of a gas.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method for recovering chemicals from spent pulp
liquors while simultaneously utilizing energy liberated in
the process, wherein the spent pulp liquors are fed into a
reaction zone of a reactor under simultaneous supply of
external energy, said external energy being independent of
combustion, wherein temperature and oxygen potential in the
zone are controlled independently of each other by controlled
supply of said thermal energy and/or carbonaceous material,
c h a r a c t e r i z e d in that a temperature of 900° to
1100° C is maintained in the reactor and that the oxygen
potential is controlled by a controlled supply of the oxygen-
containing gas and/or the carbonaceous material, whereby
sulfur contained in the spent pulp liquors is reduced and
essentially all alkali and sulfur is bound in a molten phase,
which molten phase is separated from the gas phase.
2. A method according to claim 1, c h a r a c t e r -
i z e d in that the supplied gas is air and acts as carrier
for external thermal energy supplied to the reactor.
3. A method according to claim 2, c h a r a c t e r -
i z e d in that the external energy is supplied in form of a
energy-rich gas heated in a plasma generator.
4. A method according to any of claims 1 - 3,
c h a r a c t e r i z e d in that 95 to 100 % of all alkali
and sulfur is bound in a molten phase.

Description

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The present invention relates to a method for recovering
chemicals from spent pulp liquors while at the same
time making use of energy liberated in the process,
the spent liquors being supplied to the reaction zone
S of a reactor while external thermal energy, independent
of the combustion, is simultaneously supplied.
Within the paper and pulp industry the aim is to re-use
chemicals and energy as far as possible, for both
economic and environmental reasons. In principle the
recovery processes for this are four part-processes,
i.e. a sulphur-reduction process, a process for sepa-
ration of inorganic products, an oxidation process for
the organic substance with generation of energy and the
processing of alkali to a usable form. These processes
can be performed as separate part-processes or several
may be performed in the same step. In a modern soda
recovery boiler, the Tomlinson boiler, the first three
processes are performed in one step whereas the alkali
processing is performed in a subsequent causticizing step.
It is generally the soda recovery boiler which limits
the possibility of expanding and/or increasing the
capacity in an existing pulp factory. The capacity of
the soda recovery boiler is usually limited by the
volume of gas which can pass through its primary air
zone without taking within too much of the solid and
floating particles. Another limitation may be the thermal
load of the steam part.
The soda recovery boiler may also be limited by the
difficulty of optimizing both chemical recovery and
combustion at the same time. This means that both
alkali and sulphur will to a certain extent be released
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in gaseous phase. One solution to this problem is offered in
Swedish patent 83 02 245-0 in which the whole recovery
process is divided into three steps. The process starts with
a high-temperature step where sulphur is reduced and
withdrawn in the form of a melt, both alkali and organic
material being gasified at the same time. Alkali is condensed
in a second step, at the same time being converted to desired
form, and finally a third step in which the gas from the
organic portion is combusted and thereby generates energy.
The aim of the above-mentioned invention is thus to eliminate
subsequent alkali processing~
Another process for recovering chemicals from pulp liquors is
described in s~ 72 04 304-5. Here, melt and gas are separated
in a pre-reactor and the process is entirely dependent on the
energy developed at the partial combustion to obtain a
sufficiently high tèmperature. In practice this entails
considerable difficulties. If the partial combustion is
insufficient the temperature will be too low to guarantee a
flame whereas if it is too much, both sulphur and alkali will
depart with the gas leaving.
In one aspect the invention provides a method for recovering
chemicals and energy from spent pulp liquors, comprising: (a)
feeding the pulp liquors to a reaction zone of a reactor; (b)
supplying said reaction zone with thermal energy; (c)
supplying said reaction zone with an oxygen containing gas, a
carbonaceous material or a mixture thereof; and (d)
controlling, independently, the temperature and oxygen
potential of said reaction zone by the supply of thermal
energy; wherein: (i) the temperature of said reaction zone is
controlled at between 900 and 1,000C; (ii) by the controlled
supply of components (c) reducing and binding in the melt
essentially all sulfur contained in the pulp liquors,
converting to sodium carbonate and binding in the melt most
of the alkali contained in the spent liquors,
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and removing from the spent liquors the sulphur and alkali
products; (iii) recovering gases, including organic
components of the spent liquors, liberated in the process,
said gases, when combusted, being the source of the energy
recovered in the process; and (iv) said thermal energy source
being independent of the energy recovered in (iii).
Thanks to a great number of experiments, we have now found
that within a temperature interval of 900 - llOO~C and at a
given oxygen potential in the liquor evaporation unit, all
sulphur and alkali will be bound in a molten phase.
According to the present invention the recovery process is
divided into three separate steps. In the first step the
sulphur is reduced and withdrawn in a molten phase together
with the inorganic portion of the spent liquor, the organic
portion being si~ultaneously gaqified. This
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step is optimized in order to obtain substantially
total separation of sulphur and alkali. In the second
step the gas is burned to generate energy. This can be
achieved in many ways and for many purposes but since
it is entirely separate from the recovery step it can
easily be sub-optimized. The third step is conventional
processing of alkali to the desired form.
( When performing the present invention the spent liquors
are supplied to the reaction zone of a reactor while
external thermal ener~y, independent of the combustion,
is also supplied. Characteristic of the method according
to the invention is that the temperature and oxygen
potential in the zone are governed independently of each
other and carbonaceous material and/or gas containing
oxygen are possibly supplied so that substantially all
alkali and sulphur is bound in a molten phase which is
separated from the gas phase and withdrawn via an outlet
connected to the reactor, the organic portion of the
spent liquor being simultaneously withdrawn in the form
of a gas.
( The external energy may advantageously be supplied in
the form of an energy-rich gas heated in a plasma
generator.
A temperature of 900C to 1100C is preferably main-
tained in the liquor gasifier.
The method is preferably performed in such a manner that
the melt withdrawn consists primarily of Na2S, NaOH and
Na2CO3, 95~ to 100~ of the sulphur and alkali content
of the liquor being in a molten phase within the stated
temperature interval and at a given oxygen potential
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In the method according to the invention, as with the
concept described in SE 83 02 245-9, a source of energy
independent from the combustion is utilized, thus enabling
the problems mentioned earlier which appear with a method
according to SE 72 04 304-5 to be eliminated.
In the specific embodiment using a plasma generator, it
has unexpectedly been discovered that the reaction rate
for gasification of heavy black liquor is extremely high
and the reactor can therefore be designed specifically
to give maximum separation of the molten phase from the
gas.
The possibility of accurately controlling the temperature
according to the invention also allows optimum composition
of the chemicals with respect to the digestion process
without the melting point and fluidity of the molten phase
necessarily giving rise to problems in the recovery step.
Example
In experimental equipment 9 kg heavy black liquor was
( supplied to a cylindrical reactor before a plasma generator
20 B located tangentially on the cylinder~ 270 m3N air~a~
conducted through the plasma generator per hour. A further
( 120 m3N~;eupplied to the process through another
tanqential inlet.
The heavy black liquor had the following composition:
Na 20.5% dry material
S 4.5~ " "
H2 3.7~
2 37 ~ ~ .
C 34 ~ " "
Cl 0.3~ " ~
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65~ dry substance and calorimetric thermal value
13.89 MJ/kg DS.
The temperature in the reactor was maintained at 1000C.
Additionally 1260 kW energy was required to maintain
this temperature, partly to cover losses in the reaction
vessel. By keeping the swirl number higher than 0.6 and
~- the Reynolds number higher than 18000, as well as
selecting suitable reactor dimensions, a substantially
100% separation of the melt can be obtained.
The melt was tapped continuously throughout the experiment
and the quantity was 147.5 kg per hour. Analysi~s gave
the following:
NaOH 2.1%
2 3 64.7
NaCl 0.7%
Na2S 31.5~
A gas was also obtained having the following dry gas
composition:
CO 16.8% Na 0.03%
CO2 13.0% NaOH 0.04%
H2 23.5~ NaCl 0.02%
N2 46.6%
If the quantity of air were to be increased in order
to decrease the energy addition, Na2S would to a great
extent be oxidized - which is to be avoided at all costs.
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