Note: Descriptions are shown in the official language in which they were submitted.
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PAPER PLANT - METHOD AND APPARATUS FOR PRODUCING PULP FROM
CELLULOSIC FIBROUS RAW MATERIALS AND RECOVERING CHEMICALS
AND ENERGY FROM PULP LIQUORS
This invention relates to a paper plant having a small-scale process by which
cellulosic raw materials are converted into pulp for papermaking and black
liquor effluent generated from the pulping process is treated to recover
organic and inorganic chemicals and energy. It is also to be understood that
while the combined process has been designed for a paper plant, the
individual process steps and apparatus used may be used individually in other
suitable processes, not necessarily related to paper and pulp production.
Agricultural residues such as wheat and rice straw contain cellulose and can
be a good raw material for papermaking. However, as these raw materials are
bulky, transportation costs mean ~ that they are best pulped locally and
therefore on a relatively small scale of around 10-100 tonnes of pulp
production per day.
Pulp mills generate black liquor effluent which if discharged to watercourses
causes severe pollution. The technology currently used to treat black liquor
effluent is, depending on local economic conditions, only economically viable
on a scale of not less than 60,000 tonnes of pulp production per annum. The
typical scale of a modern wood pulp mill is over 360,000 tonnes of pulp
production per annum.
Lack of economically viable technology to deal with black liquor effluent
under
60,000 tonnes per annum of production has meant that many existing small
pulp mills have been forced to close to stop pollution of watercourses. This
lack of suitable technology has also prevented the establishment of new small
pulp mills, in particular new mills that might have used agricultural
residues.
The subsequent lack of demand for small pulp mills has meant that little
research and development of small pulp mill technology has been carried out.
Consequently small pulp mill technology and straw pulping in particular, has
CONFIRMATION COPY
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not advanced as far as large scale wood pulping technology during the latter
part of the 20t" century. Current small pulp mill technology is therefore
relatively resource inefFicient. Also there are particular properties of straw
as a
raw material, which can cause problems in processing and in the quality of the
finished pulp which have not been addressed by current technology. These
include partially pulped nodes leading to imperfections in the finished paper
and drainage problems on the paper machine caused by over processing of
the straw fibres. In addition processes currently used to pulp straw leave
silica, which is present in straw, in the black liquor which then forms glass
like
deposits when the effluent is concentrated for treatment, leading to
inefficiencies and down time for cleaning.
The present invention seeks to reduce or obviate one or more of these
problems.
According to a first aspect of the invention, there is provided a treatment
apparatus for treating fibrous cellulosic raw material suitable for use in a
paper making plant comprising extracting means for extracting contrary
material from the raw material, crushing means for crushing the raw material
to remove nodes therefrom and splitting means for splitting the crushed raw
material lengthways.
The extracting means may comprise a conveyor belt provided with means for
enabling the removal of contrary material.
The crushing means may comprise a pair of counter rotating knurled rollers
between which the raw material passes.
The splitting means may comprise a pair of counter-rotating pinned rollers and
between which the crushed material passes.
Between the crushing means and the splitting means may be provided means
for further removing contrary material present in the crushed material.
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According to a second aspect of the invention, there is provided a method for
treating fibrous cellulosic raw material suitable for use in a paper making
plant
comprising extracting contrary material from the raw material, crushing the
raw material from which contrary material has been removed to remove nodes
therefrom and splitting the crushed raw material lengthways.
The extraction of contrary material may take place on a conveyor belt
provided with means for enabling the removal of contrary material.
The crushing of the raw material may take place between a pair of counter
rotating knurled rollers between which the raw material passes.
The splitting of the crushed material may take place between a pair of counter
rotating pinned rollers and between which the crushed material passes.
Between the steps of crushing the raw material and splitting the crushed
material, further removal of contrary material present in the crushed material
may be carried out.
According to a third aspect of the invention, there is provided an apparatus
for
pulping fibrous cellulosic raw material comprising a co-rotating twin screw
conveyor, that is the screws both turn in the same direction, the conveyor
being divided up into a plurality of zones, means for inserting treatment
materials such as chemicals to assist in the pulping process such as sodium
hydroxide into at least one zone and means for controlling the temperature
and/or pressure in at least one of the zones.
The conveyor may comprise at least three zones comprising a feed zone, a
treatment zone to which treatment material is added and a pressure zone
maintained at a pressure above atmospheric.
The conveyor may comprise five zones comprising a feed zone, a treatment
zone to which treatment material is added, a first pressure zone at a pressure
greater than atmospheric to which treatment material is added, a second
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pressure zone at a pressure higher than the first pressure zone and a third
pressure zone at a lower pressure than the second pressure zone.
The feed zone may be enlarged compared to the other zones to allow the raw
material to be fed freely thereinto to increase the throughput of the
conveyer.
As the raw material moves forward into the treatment zone and the first and
second pressure zone, the area within the co-rotating twin screw conveyor
may be continually decreased which has the effect of continually increasing
the pressure within the zones.
The pressure and temperature of the first and third pressure zones may be
the same.
Steam may be inserted into the treatment zone and pulping agents may be
inserted into the first pressure zone.
The feed zone and the treatment zone may be maintained at atmospheric
pressure.
The raw material may be passed through five zones comprising a feed zone,
a treatment zone to which treatment material is added, a first pressure zone
at
a pressure greater than atmospheric to which treatment material is added, a
second pressure zone at a pressure higher than the first pressure zone and a
third pressure zone at a lower pressure than the second pressure zone.
The addition of heat, steam or any other treatment materials may take place
at any point along the co-rotating twin screw conveyor.
Extraction of any lipuids or volatiles may take place at any point along the
co-
rotating twin screw conveyor.
Pressure can be increased or decreased at any point within the co-rotating
twin screw conveyor.
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According to a fourth aspect of the invention, there is provided a method of
pulping pre-treated or untreated raw material comprising passing the raw
material through a plurality of zones, inserting treatment material into at
least
one zone and controlling the temperature and/or pressure of at least one of
the zones.
The raw material may be passed through at least three zones comprising a
feed zone, a treatment zone to which treatment material is added and a
pressure zone maintained at a pressure above atmospheric.
The raw material may be passed through five zones comprising a feed zone,
a treatment zone to which treatment material is added, a first pressure zone
at
a pressure greater than atmospheric to which treatment material is added, a
second pressure zone at a pressure higher than the first pressure zone and a
third pressure zone at a lower pressure than the second pressure zone.
The method may comprise controlling the pressure and temperature of the
first and third pressure zones to be the same.
The method may comprise inserting steam into the treatment zone and
inserting pulping agents into the first pressure zone.
The method may comprise maintaining the feed zone and the treatment zone
at atmospheric pressure.
The method may comprise the addition of heat, steam or any other treatment
materials at any point along the co-rotating twin screw conveyor.
The method may comprise extraction of any liquids or volatiles at any point
along the co-rotating twin screw conveyor.
The method may comprise an increase or decrease of pressure at any point
within the co-rotating twin screw conveyor.
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According to a fifth aspect of the invention, there is provided an apparatus
for
treatment of black liquor effluent produced in a paper pulp manufacturing
plant comprising, a processing vessel for treating the liquor at a
concentration
of 10 to 70 percent solids at a temperature between 300°C and
650°C, and a
closed conveyor for transporting the concentrated liquor from the evaporator
to the processing vessel.
The apparatus may comprise an evaporator for concentrating the liquor to 30-
70% solids,
The apparatus may further comprise a tank for dissolving the product
produced in the reaction vessel, a means of filtering the non-dissolved solids
from the dissolved reaction product, a dryer to dry the undissolved reaction
product and a boiler to recover energy from hot gases.
The solids content of the black liquor effluent arising from the co-rotating
twin
screw conveyor may be over 30% and so may mean that an evaporator is not
required as part of the apparatus if used in conjunction with pulping from the
co-rotating twin screw conveyor.
If required the processing vessel may also treat black liquor at 10-30%
solids.
The processing vessel may comprise the chamber of a toroidal fluidised bed
into which the concentrated black liquor is sprayed, the fluidised bed
containing an earth oxide at a ratio of between 0.2:1 and 1.3:1 earth oxide to
black liquor effluent dry solids, operating at stoichiometric or sub-
stoichiometric conditions.
The closed conveyor may be a twin screw conveyor with an earth oxide, the
ratio of earth oxide to black liquor being between 0.2:1 and 1.3:1 earth oxide
to black liquor effluent dry solids so that it becomes a granular friable
material.
The apparatus may include means for chemically converting the material in
the fluidised bed into sodium hydroxide and/or sodium carbonate and a gas
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and liquids with a combustible component, which can be utilised for energy
production.
According to a sixth aspect of the invention, there is provided a method of
treatment of black liquor produced in a paper manufacturing plant comprising,
passing the liquor at a concentration of 10 - 70°l° solids to a
processing vessel
and treating the concentrated liquor therein at a temperature of between 300-
650°C.
The method may comprise concentrating the liquor to 30-70% solids before
passing it to the processing vessel.
The concentrated black liquor may be sprayed into the chamber of a toroidal
fluidised bed containing an earth oxide at a ratio of between 0.2:1 and 1.3:1
earth oxide to black liquor effluent dry solids and set up under
stoichiometric
conditions or sub-stoichiometric conditions.
The concentrated black liquor may be fed into a twin screw conveyor with an
earth oxide, the ratio of earth oxide to black liquor being between 0.2:1 and
1.3:1 earth oxide to black liquor effluent dry solids so that it becomes a
granular friable material.
The output of the twin screw conveyor may be fed to a toroidal fluidised bed
reactor operating under stoichiometric conditions or sub-stoichiometric
conditions.
The method may include means for chemically converting the material in the
fluidised bed into sodium hydroxide and/or sodium carbonate and a gas and
liquids with a combustible component that may be utilised for energy
production.
The invention will now be described in greater detail, by way of example, with
reference to the drawings in which:
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Figure 1 is a block diagram of the entire process
Figure 2 is a schematic view of a roller arrangement for use in the raw
material pre-treatment process
Figure 3 is a schematic view of the construction of a self cleaning pin roller
which may be used in the roller arrangement of figure 2;
Figure 4 is a schematic view of a possible embodiment of the co-rotating twin
screw conveyor and
Figure 5 is a block diagram of a preferred embodiment of the black liquor
effluent treatment apparatus.
Wheat straw is usually chopped before pulping. Wheat straw contains nodes
within the stem which usually remain intact if straw is chopped before
pulping.
This is a serious drawback in the production of quality paper pulp with the
resulting poor quality paper being produced.
In accordance with the invention, a new method is employed which crushes
the nodes, opens out the straw stem lengthways in a gentle fashion and feeds
the raw material into the pulper in a positive, metered and continuous
process.
The entire process will now be described in general terms in relation to
figure
1:-
Firstly the material to be treated is fed to a conveyor belt 9 which has
included
in it means for the extraction of contrary material from the raw material. The
conveyor feeds the resulting raw material to a series of pairs of rollers 10.
A
first pair crushes the raw material to remove nodes and a second pair splits
the raw material lengthwise. A third pair, between the first and second pairs,
removes any contrary material produced in the crushing operation.
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Next, the pre treated raw material is fed to a co-rotating twin screw conveyor
12 which is divided into five zones comprising a feed zone, a treatment zone
to which treatment material is added, a first pressure zone at a pressure
greater than atmospheric to which further treatment material is added, a
second pressure zone at a pressure higher than the first pressure zone and a
third pressure zone at a lower pressure than the second pressure zone.
The pulped material output from this conveyor may then be further treated in
dependence on the quality of paper to be produced.
Black liquor produced by the paper manufacturing plant is treated in a
treatment plant 14 comprising an evaporator for concentrating the liquor to 30-
70% solids, or used directly from the co-rotating twin screw conveyor if
already of this concentration and passed by a closed conveyor at a
temperature above 90°C to a processing vessel where it processed at a
temperature between 300 and 650°C.
The processing vessel is a toroidal fluidised bed reactor into which the
concentrated black liquor is sprayed, the fluidised bed containing an earth
oxide at a ratio of between 0.2:1 and 1.3:1 earth oxide to black liquor
effluent
dry solids. This process converts the material into sodium hydroxide and/or
sodium carbonate and a gas and liquids with a combustible component which
can be used for energy production. The energy component may be recovered
using a standard boiler.
The individual parts of the process will now be described:-
Referring now to Figure 2, after the bales of straw have been opened, straw is
passed along a conveyor belt 101 where heavy items such as stones and
other contraries such as plastic string are removed. The straw is then passed
in to a feed hopper 103 which feeds the straw into to an arrangement of
knurled rollers 105 and 107 which crush the nodes in the straw stem and
rollers with pins which open the straw stem out lengthways in a gentle
fashion. Thus straw is fed between first and second counter-rotating knurled
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crushing rollers 105 and 107 to crush the straw nodes. The crushed material
then passes through two counter-rotating intermediate rollers 109 and 111
which prevent any contrary materials from damaging the rollers below.
The straw then passes through two more rollers 113 and 115, this time
rotating in the same direction. These latter rollers are provided with pins
which
open and shred the straw lengthways and act in co-operation with a feed
shoe.
The action of this system leaves the straw as shortened and opened
out/shredded material without nodes. This will facilitate quicker chemical and
steam penetration and so faster and more uniform pulping, whilst treating the
fibres gently so preserving their length. This results in the production of an
improved quality of pulp including a very significant reduction in visible
"shiners" in the paper sheet, due to dispersion of parenchyma cells, improved
drainage, a higher tensile and tear strength, a higher pulp yield and a
reduced
demand for pulping chemicals.
The treated straw then drops from the pinned rollers 113 and 115 into a feed
hopper 117 leading to either a conveyor or blower system (not shown) which
feeds the treated straw into a live bottom bin for buffer storage of the
prepared
material prior to pulping.
The above discussed pinned and knurled or fluted roller opening and feeding
system is specifically designed for straw but, with minor modifications, could
be used for any other suitable raw materials including flax, hemp, bagasse
and wood.
The pinned rollers can also be constructed to be self cleaning when used with
longer fibred cellulosic raw materials such as hemp and flax. This is to
prevent
the material wrapping around the rollers and fouling the apparatus. A
schematic of the functioning part of a pin roller is shown in Figure 3.
The pin roller 20 has an outer surface having a large number of radially
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extending pins 22. This is used with a matching perForated or woven belt 24
on which the material 26 being treated is carried. The pins 22 pick up the
material 26 and as the belt 24 leaves the pins, it takes off the material
keeping
the pin roller 20 free from tangled fibres.
The above described process has been tested and developed with straw, flax
and hemp through pilot scale laboratory trials.
The raw material from the buffer storage is thereafter pulped. To this end,
the
raw material (straw, flax, hemp, bagasse, wood chips or any other cellulosic
raw material) is drawn into a specially designed co-rotating twin screw
extrusion unit 31 figure 4. In this unit the screw profiles are specially
designed.
The two outer sections 33 and 34 have flights going in a first direction while
in
the middle section 35 the flight direction is reversed The screw flights are
manufactured from hardened steel with a deep cut flight and are specially
designed to minimise fibre damage. This particular design results in a reduced
energy demand, which means that a smaller drive shaft and gearbox can be
used, which also reduces capital cost. The design of the screw profile and the
reduced drive shaft size also allows throughput of raw material to be
increased by an anticipated 400% over conventional co-rotating twin screws.
As can be seen schematically in figure 4, one embodiment of the conveyor
has a first zone 37 to which the raw material is fed through a feed hopper 39
and a second zone 41 to which cooking liquor can be added through an inlet
43 and steam can be added to the conveyor jacket at 45. A third zone 47 is
constituted by the reversed flight region 35 in which some liquor is squeezed
out through a perforated wall 49. A fourth zone 51 is constituted by the
remainder of the conveyer, namely the part 34 from which the pulp emerges
through an output 53. It will be appreciated that while the screw conveyor
shown is set up with four zones, any number of zones, suitably from 3
upwards can be used and for whatever treatment set up is required.
The unit is suitably of modular construction which facilitates making changes
to both screw and barrel configurations. This could be a very cost-effective
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way to make use of one standard twin screw unit to process many difFerent
types of cellulosic raw materials and/or to produce different grades of pulp
simply by changing the screw and barrel configurations. Machine speeds of
between 50-500 rpm could be used. A speed of 50-250 rpm has been used in
practice. The speed needs to be adjusted for the raw material used and the
pulp quality required.
The twin screw can be built in such a way that chemicals and liquids can be
injected and liquids or steam can be vented or removed in each zone. This is
a standard feature of twin screw extruders.
It has further been discovered that a sophisticated gearbox and drive of the
type conventionally used in twin screw extruders is not necessary to suitably
pulp fibres. A simple gearbox and drive can be used, reducing the capital cost
and energy consumption. It is anticipated that the pulping system will
consume less than half the energy of a conventional twin screw used for this
purpose.
One method of pulping cellulosic raw materials such as straw using the new
system is as follows. Using a co-rotating twin screw with a barrel size of 100
millimetres, the co-rotating intermeshing twin screw extruder is set up with
five
zones as described below.
Zone 1 2 3 4 5
Action of zone Feed zone Steam zone Pressure zone Pressure zone Pressure zone
Introduce Introduce
steam NaOH
+ other pulping
agents
Temp °C 65 100 130 150 130
Pressure (bar) 0 0 2-3 4-5 2-3
Cellulosic raw material such as straw is positively fed into Zone 1 using an
Auger.
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Zone 1 is designed to be as open as possible in order to accept the material
into the unit. The temperature is 65 °C in Zone 1.
In Zone 2 saturated steam is introduced to prepare the material for pulping.
Temperature increases to 100 °C
In Zone 3 temperature and pressure are raised to 130 °C, 2-3 bar
pressure.
Sodium hydroxide is added at a rate of 12-14% to dry raw material using a
15% solution. Other materials may be added here as will be referred to
hereafter
In Zone 4 temperature and pressure are increased to 150 °C and 4-5
bar
pressure.
In Zone 5 temperature and pressure are reduced to 130 °C and 2-3
bar
pressure in preparation for the material leaving the twin screw system. The
material travels through the twin screw unit in between 2-3 minutes. The
screw speed is around 200 rpm.
In another embodiment of conveyor (not shown) one form of the conveyor, the
feed zone is enlarged compared to the other zones to allow the raw material
to be fed freely thereinto to increase the throughput of the conveyer. As the
raw material moves forward into the treatment zone and the first and second
pressure zone, the area within the co-rotating twin screw conveyor may be
continually decreased which has the effect of continually increasing the
pressure within the zones.
The pulp exiting from the twin screw would have approximately 50% moisture
content and would be expected to have a Kappa Number of 50. This is a
semi-chemical pulp suitable for use in fluted packaging for example. This
result is a function of the rpm and the flight design or time spent in the
twin
screw extruder together with the pressure, temperature and amount of pulping
chemicals used.
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To go on and produce a full chemical pulp it will then be necessary for the
pulp to be further digested in a single screw pulper using steam at 1-2 bar
pressure (120 °C) for a further 20-40 minutes. A Kappa Number of 14-20
is
achieved after this further processing. The pulp is then ready to be bleached
using conventional methods.
The invention also provides a method of precipitating silica present in straw
onto cellulosic fibres when pulping straw to make paper and so prevent it
entering the black liquor effluent and causing scaling of evaporators or
chemical recovery system.
Calcium hydroxide can be added in Zone 3 of the twin screw extruder at a
rate of 4% to dry raw material (straw) with 8% sodium hydroxide when pulping
straw as described previously. This method could be used in any alkaline
based pulping system. This has the effect of precipitating sodium silicate
onto
the cellulosic fibres as calcium silicate. This prevents silica from entering
the
black liquor effluent and causing scaling of the evaporators or chemical
recovery system.
The above system, in common with all traditional chemical pulping methods,
produces an effluent liquor, usually called black liquor which must be treated
if
environmental and health hazards are to be avoided. Typically the black liquor
is evaporated and conveyed to a separate processing stage for recovery of
the digestion chemicals and energy content. The most common process for
the treatment of the black liquor is high temperature combustion in an
apparatus known as a Tomlinson recovery boiler. The disadvantages of the
Tomlinson process include the requirement to use large and complicated
furnaces making them uneconomic at small throughputs (<60000 tpa), the
corrosive nature of the recovered smelt product and the risk of explosions
between the smelt and water. To date there are no economically viable small-
scale black liquor effluent treatment processes available.
The present invention provides a treatment process to recover organic and
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inorganic chemicals and energy from black liquor effluent arising from the
pulping of cellulosic raw materials to make paper. It is specifically intended
to
be used with the above-described pulping process but could be used alone to
treat black liquor from other pulping processes. It is designed to be
economically viable at small throughputs.
Referring to figure 5, a preferred embodiment of the effluent treatment
process will now be described.
Black liquor effluent arising from the pulping process is collected in a
digestion
liquor storage tank 301 and concentrated to 30-70% solids using a standard
evaporator 302 designed for concentration purposes. If the black liquor
effluent comes from the co-rotating twin screw conveyor at a solids
concentration of 30% or above it may be treated directly in the processing
vessel eliminating the evaporation step. The concentrated black liquor is
moved to a reactor vessel 304 at a temperature in excess of 90°C using
an
enclosed twin screw transport system 303. The enclosed transport system is
used to minimise the loss of organic components through vaporisation. A
temperature in excess of 90°C is required to decrease the viscosity of
the
black liquor so that it will transport without resistance. The black liquor is
treated in the reactor vessel 304 in either of two methods.
In a first method, the black liquor is introduced into a toroidal fluidised
bed
reactor 304 by spraying the concentrated liquor into the chamber of the
reactor in which a bed of fluidised material is supported. The material may be
an earth oxide such as lime at a ratio of 0.3:1 of lime to black liquor dry
solids.
The mean particle sire of the earth oxide may be between 1 and 4 mm. The
reactor may operate under stoichiometric or sub-stoichiometric conditions.
In a second method, black liquor effluent is pre-mixed in the twin screw
conveyor 303 with an earth oxide such as lime (Ca0) in the ratio 0.3:1 lime to
black liquor dry solids to become a granular friable material. The granular
friable material is then screw fed into a toroidal fluidised bed reactor 304.
The
reactor may operate under stoichiometric or sub-stoichiometric conditions.
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In a variation of both methods, the ratio of earth oxide, e.g. lime to black
liquor
dry solids may be in the range 0.2 to 1.3:1 lime to black liquor dry solids.
The
earth oxide may be supplied by a standard calciner 308.
In both cases the chamber of the toroidal fluidised bed reactor 304 is
maintained within the .temperature range 300 to 650 °C where the
necessary
chemical reaction takes place in the space of seconds.
In a further possible embodiment of the process a portion of the solids within
the toroidal fluidised bed reactor 304 may be recycled via the screw feeder
303 back to the reactor 304.
The material is converted by a chemical reaction to;
1. Sodium hydroxide and sodium carbonate and lime within the fluidised
bed reactor 304. The bed will overflow through a central discharge point and
is then dissolved in a dissolving tank 305 to recover sodium hydroxide as
green liquor in the traditional manner known as re-causticisation. The green
liquor is then filtered using a standard fitter 306 to make a calcium
carbonate
sludge and white liquor (containing sodium hydroxide) for re-use in the
pulping process.
However, in a variation of the process, if the temperature is carefully
controlled, re-causticisation can take place in the reactor. In this case,
sodium
carbonate is not formed and the sodium hydroxide can be recovered without
the use of the dissolving tank (306)
2. A gas and liquids with a combustible component which can be utilised
for energy production. The gas is collected to power a boiler 309 that will
produce energy and steam for use in the pulp mill process line. In a further
possible embodiment of the process the gas containing combustible
components may be recycled to the fluidised bed reactor to provide heat for
the chemical recovery reaction.
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The calcium carbonate sludge may be dried to remove some water and sent
to a second calciner reactor 308, which may be a toroidal fluidised bed
reactor. This reactor may operate at a temperature of around 1100°C
where
calcium carbonate CaC03 is converted back to calcium oxide Ca0 for re-use
in the black liquor effluent chemical recovery process.
Approximately 10% of the fluidised bed material generated may need to be
removed from the process continuously in order to prevent the build up of
heavy metals and other materials in the process.
If required, black liquor effluent below 30% solids can also be processed
using this method (and has been tested). However, energy consumption is
greater and so this is not the preferred method.
It will be appreciated that individual elements of the above described process
can be replaced by suitable equivalents without departing from the scope of
the invention. Also any of the individual processes and apparatus may be
used individually in other processes where they are suitable, not necessarily
related to paper making.
