U.S. patent application number 11/416944 was filed with the patent office on 2006-09-14 for methods for producing pulp and treating black liquor.
This patent application is currently assigned to Bioregional Minimills (UK) Limited. Invention is credited to Trevor William Ridgley Dean, Andrew Timothy Harris.
Application Number | 20060201641 11/416944 |
Document ID | / |
Family ID | 36969588 |
Filed Date | 2006-09-14 |
United States Patent
Application |
20060201641 |
Kind Code |
A1 |
Harris; Andrew Timothy ; et
al. |
September 14, 2006 |
Methods for producing pulp and treating black liquor
Abstract
A method is provided for treating black liquor particularly
derived from non-wood pulp, by heating with an alkaline earth metal
oxide in a toroidal fluidised bed reactor at a temperature of above
650.degree. C. The method may be used alone or as part of a method
of converting graminaceous raw material to pulp for paper or board,
said method comprising (a) digesting said raw material with a white
liquor based on sodium hydroxide and further comprising calcium
hydroxide in an amount effective to substantially convert silica of
said raw material to calcium silicate; (b) recovering pulp and
black liquor substantially free of uncombined silica; (c) heating
the black liquor in a fluidized bed reactor containing calcium
oxide for catalysing conversion of organic content of said black
liquor to gas and for providing recovered solids including sodium
values of said white liquor and calcium oxide; and regenerating
said white liquor using said recovered solids. The use of the above
mentioned white liquor permits treatment of wheat straw, rice straw
and other high-silica materials without resulting in a black liquor
that is difficult to treat.
Inventors: |
Harris; Andrew Timothy;
(Sydney, AU) ; Dean; Trevor William Ridgley; (High
Wycombe, GB) |
Correspondence
Address: |
BAKER & BOTTS
30 ROCKEFELLER PLAZA
44TH FLOOR
NEW YORK
NY
10112
US
|
Assignee: |
Bioregional Minimills (UK)
Limited
|
Family ID: |
36969588 |
Appl. No.: |
11/416944 |
Filed: |
May 3, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10773870 |
Feb 6, 2004 |
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11416944 |
May 3, 2006 |
|
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PCT/GB02/03641 |
Aug 7, 2002 |
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10773870 |
Feb 6, 2004 |
|
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Current U.S.
Class: |
162/37 ; 162/18;
162/239; 162/30.1; 162/47; 162/68 |
Current CPC
Class: |
D21C 5/00 20130101; Y02P
40/40 20151101; D21C 11/125 20130101; Y02P 40/44 20151101 |
Class at
Publication: |
162/037 ;
162/018; 162/047; 162/030.1; 162/068; 162/239 |
International
Class: |
D21C 3/26 20060101
D21C003/26; D21C 11/12 20060101 D21C011/12; D21C 9/10 20060101
D21C009/10; D21C 11/06 20060101 D21C011/06; D21C 1/02 20060101
D21C001/02; D21C 7/00 20060101 D21C007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 3, 2003 |
GB |
03 255 78.3 |
Aug 7, 2001 |
GB |
01 192 37.6 |
Claims
1. A method for treating black liquor to produce inorganic material
and a synthesis gas having as components CO2, CO, H2O, and H2
together with methane and, C2+ components, said process comprising:
providing a reactor having a processing region provided with a mass
of particulate material consisting of or containing an alkaline
earth metal oxide; supplying heated fluidizing gas to the
processing region so as to induce a swirling flow of the
particulate material within said processing region, the particulate
material to assuming a compact band and circulating about an axis
of said processing region in a turbulent manner, the fluidizing gas
including a sub-stochiometric quantity of oxygen for partially
oxidizing the organic material of the black liquor and for
converting other organic material of the black liquor to said
synthesis gas; feeding the black liquor into said compact band of
particulate material so that it is heated at a temperature of
650-725.degree. C. and becomes gasified; recovering said synthesis
gas as off-gas from said bed containing <1% by volume of oxygen;
and recovering inorganic material from said black liquor as solids
from said bed.
2. The method of claim 1, wherein the black liquor is from the
pulping of a graminaceous material selected from straw of wheat,
rice, barley, oats, flax, rye, bagasse, sorghum or corn.
3. The method of claim 2, wherein the black liquor is from a
soda-type white liquor containing calcium hydroxide in an amount
effective to convert harmful silica in said graminaceous material
to calcium silicate.
4. The method of claim 1, wherein the black liquor comprises 15-30%
solids.
5. The method of claim 1, which comprises heating the black liquor
at 675-700.degree. C.
6. The method of claim 1, wherein the particulate material consists
of or contains calcium oxide of mean particle size of the calcium
oxide between 1 and 4 mm, the black liquor and calcium oxide being
supplied to the fluidized bed so as to maintain a ratio of calcium
oxide to dry solids of the black liquor of 0.2:1 to 0.4:1.
7. The method of claim 6, in which the black liquor is pre-mixed
with calcium oxide to become a granular friable material and the
granular friable material is then fed into the fluidised bed
reactor.
8. The method of claim 7, in which sodium hydroxide and/or sodium
carbonate and calcium hydroxide and/or calcium carbonate are
produced within the fluidised bed reactor.
9. The method of claim 1, in which the black liquor is introduced
into the fluidised bed reactor by spraying the concentrated liquor
into the chamber of the reactor in which a bed of fluidised
material is supported.
10. The method of claim 9, wherein the nominal fluidizing velocity
of the gases supplied to said bed is >2 m/s.
11. The method of claim 1, in which the fluidised bed overflows
through a central discharge point, overflowing material is then
dissolved in a dissolving tank to recover sodium hydroxide as green
liquor and the green liquor is filtered to make a calcium carbonate
sludge and white liquor containing sodium hydroxide for re-use in
the pulping process, substantially no sodium carbonate being
formed.
12. A method of converting graminaceous raw material to pulp for
paper or board, said method comprising: digesting said raw material
with a white liquor based on sodium hydroxide and further
comprising calcium hydroxide in an amount effective to
substantially convert silica of said raw material to calcium
silicate; recovering pulp from said digestion step; recovering
black liquor from said digestion step by washing said digested pulp
and optionally also by recovering black liquor direct from said
digestion step, said recovered black liquor being substantially
free of soluble silicate; heating the black liquor in a fluidized
bed reactor containing calcium oxide for catalysing conversion of
organic content of said black liquor to gas and for providing
recovered solids including sodium values of said white liquor and
calcium oxide; and regenerating said white liquor using said
recovered solids.
13. The method of claim 12, wherein said graminaceous raw material
is wheat straw, rice straw or bagasse.
14. The method of claim 12, wherein recovery of black liquor
includes combining a black liquor stream from the digestion step
with a black liquor stream from the pulp washing step, and the
black liquor is concentrated by evaporation to a solids content of
20-40 wt % before heating in said fluidized bed reactor.
15. The method of claim 12, wherein the fluidizing gases contain at
least stochiometric quantities of free oxygen for completely
oxidizing the organic material of the black liquor.
16. The method of claim 12, wherein the fluidizing gases contain a
sub-stochiometric quantity of oxygen for partially oxidizing the
organic material of the black liquor and for converting other
organic material of the black liquor to a combustible off-gas and
further comprising supplying fluidizing gases and black liquor so
as to produce an off-gas above said bed containing <1%
oxygen.
17. The method of claim 12, comprising: providing a reactor having
a processing region provided with a mass of particulate material
consisting of or containing calcium oxide; supplying heated
fluidizing gas to the processing region so as to generate a
swirling flow of fluid within said processing region, the fluid of
said swirling flow of fluid causing the particulate material to
assume a compact band and circulate about an axis of said
processing region in a turbulent manner, the fluidizing gas
including oxygen for at least partially combusting organic material
in the black liquor; feeding the black liquor into said compact
band of particulate material and treating the black liquor in said
bed so as to gasify organic materials in said black liquor;
recovering organic material from said black liquor as off-gas from
said bed; and recovering inorganic material from said black liquor
as solids from said bed.
18. The method of claim 17, wherein the raw material supplied for
digestion has been crushed to remove nodes therefrom and split
lengthways, crushing being by means of a pair of counter rotating
knurled rollers between which the raw material passes, and
splitting being by means of a pair of counter rotating pinned
rollers between which the crushed material passes.
19. Use of calcium hydroxide as an additive in the soda process
digestion of a graminaceous starting material to form pulp for
inhibiting scaling during black liquor concentration and recovery
when processing black liquor at least partly from pulp washing.
20. The use of claim 19, wherein the black liquor is partly from a
digester and partly from pulp washing, silica in said black liquor
in the form of calcium silicate to the substantial exclusion of
sodium silicate, a flocculating agent is added during washing to
inhibit dispersion of calcium silicate said flocculating agent
being polyacrylamide.
Description
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 10/773,870, filed Feb. 6, 2004, which claims
priority to PCT International Application No. PCT/GB2002/003641,
filed Aug. 7, 2002, published as WO 2003/014467 on Feb. 20, 2003,
which claims priority to GB 0119237.6, filed Aug. 7, 2001, to each
of which priority is claimed, and each of which is incorporated by
reference in its entirety herein. This application is also a
continuation of PCT International Application No.
PCT/GB2004/050023, filed Nov. 3, 2004, published as WO 2005/045126,
which claims priority to UK Patent Application No. 03 25578.3,
filed Nov. 3, 2003, to each of which priority is claimed, and each
of which is incorporated by reference in its entirety herein.
FIELD OF THE INVENTION
[0002] The present invention relates to a method for producing pulp
from graminaceous fibres and also to a method of treatment of black
liquor that may be a by-product of said pulp production method or
may have arisen otherwise e.g. Kraft black liquor or a mixture of
soda black liquor with Kraft black liquor.
BACKGROUND TO THE INVENTION
The Kraft process
[0003] The principal component of wood is long straight translucent
cellulose fibres based on chains of glucose molecules that make up
about 42 wt % of softwoods and 45 wt % of hardwoods. Hemicelluloses
form a further component of wood and are short, branched chains of
glucose and other sugar molecules that are relatively soluble in
water and are removed during the pulping process. The cellulose
fibres are held together by lignin which is a three-dimensional
phenolic polymer network that holds the cellulose fibres together
and imparts rigidity. Lignin comprises about 28 wt % of softwood
and about 20 wt % hardwood. It is selectively removed during
chemical pulping and subsequent bleaching without significantly
degrading the cellulose fibres. Extractives account for about 3 wt
% of softwoods and about 5 wt % of hardwoods. They include plant
hormones, resin and fatty acids.
[0004] The Kraft or sulphate process is preferred for the chemical
pulping of wood because it can deal effectively with the resin
component of many woods. It uses sodium hydroxide as the main
cooking chemical and sodium sulphide as catalyst, and it gives
stronger final pulp than the soda process, which employs sodium
hydroxide alone. Anthraquinone is often used as an auxiliary
catalyst in both the Kraft and soda processes. In the Kraft
process, chips are cooked in a digester under heat and pressure
with "white liquor" (in this case aqueous sodium hydroxide and
sodium sulphide) to dissolve the lignin selectively. After 2 to 4
hours, the cooked mixture of pulp, spent pulping chemicals and wood
waste is discharged from the digester. The resulting pulp is
separated form a mixture of pulping chemicals and waste referred to
as "black liquor".
[0005] The treatment chemicals (sodium sulphide and sodium
hydroxide) are then regenerated from the black liquor by a process
whose main piece of equipment is a so-called Tomlinson furnace.
Black liquor at about 65% dry solids content is sprayed into the
furnace. During their descent, the black liquor droplets lose the
remaining water by evaporation and the solids undergo pyrolysis to
form a char bed at the bottom of the furnace. The char bed bums
under reducing conditions at a temperature of
750.degree.-1050.degree. C. and the recovered chemicals, mainly
Na.sub.2CO.sub.3 and Na.sub.2S, are drained from the furnace as a
smelt which is dissolved in water to produce so-called green
liquor, the precursor of the white liquor. The gases generated
during pyrolysis and burning of the char are fully combusted at a
higher location in the furnace. Flue gases must be thoroughly
scrubbed to remove mercaptans that form under the process
conditions. The furnace is provided with a suitable heat exchanger
to recover heat from the hot combustion gases for steam and
electricity generation.
[0006] Although useful recovery of chemicals and energy can be
achieved in commercial operation, the use of a Tomlinson furnace
presents a number of problems. For example, inadvertent contact
between water or dilute black liquor and the inorganic smelt may
result in an explosion. Also, high char bed temperatures lead to
increasing emission of sodium salts and excessive fouling of the
steam pipes in the upper part of the furnace. Furthermore, the
technology currently used to treat black liquor effluent is,
depending on local economic conditions, only viable on a scale of
not less than 60,000 tonnes of pulp production per annum, which may
be compared with the typical scale of a modern wood pulp mill which
is over 360,000 tonnes of pulp production per annum. Treatment of
straw and other gramminaceous materials is, of course, on a much
smaller scale inter alia because long-distance transport of bulky
agricultural residues such as straw is uneconomic.
Fluidised Bed Recovery in the Kraft Process
[0007] To solve these problems, and also to reduce capital
investment and increase the energy efficiency of the recovery
operation, a number of Kraft recovery process have been described
in which the smelt-water explosion hazard is eliminated and the
emission of sodium salts reduced by maintaining the inorganic
chemicals in solid rather than molten form.
[0008] This principle was disclosed in U.S. Pat. No. 3,309,262
(Copeland et al) which discloses a process for treating black
liquor in a reaction vessel containing a fluidized bed of solid
particles consisting substantially entirely of residual inorganic
materials derived from the black liquor. The process comprises:
[0009] (a) concentrating the black liquor by evaporation to a
solids content of 20-45 wt %, said liquor having a combustible
content sufficient to support autogenous combustion;
[0010] (b) spraying the concentrated black liquor into free space
above the bed so that substantial evaporation is achieved within
said free space, and the remaining further concentrated atomised
black liquor flows into the fluidised bed;
[0011] (c) maintaining fluidity of the bed by introducing at a
speed of 30-150 cm/sec (1-5 ft/sec) oxygen-containing fluidizing
gas in an amount sufficient to effect complete elimination of
organic material as off-gas by substantially total autogenous
combustion within the fluidised bed;
[0012] (d) maintaining the bed at a non-smelting temperature below
the eutectic temperature of the residual chemical mixture within
the bed but in a temperature range of about 540-982.degree. C. to
form gaseous combustion products above the bed and agglomerates
within the bed from the residual inorganic materials of the black
liquor that are of sufficient weight to prevent their entrainment
in the fluidizing gas;
[0013] (e) discharging the agglomerates from the fluidized bed;
and
[0014] (f) discharging the off-gas from above the bed.
[0015] For sodium-based waste liquor the maximum recommended bed
temperature is 760.degree. C. (although the inventors are aware
that this value was exceeded in practical operation). Introduction
of the black liquor as a mixture of coarse and fine droplets is
recommended in order to combine rapid evaporation of water, an
efficient scrubbing action that reduces dust loading, and promotion
and control of agglomeration of the bed particles. Oxidising
conditions within the reactor are maintained to prevent the
formation of hydrogen sulphide gas, and conversion of organic
material into combustible gas is not disclosed. The end products
are Na.sub.2CO.sub.3 and Na.sub.2SO.sub.4 which have to be
subjected to recaustication to regenerate white liquor. Although it
has been reported in the patent literature that there have been
attempts to commercialise the Copeland process, the inventors are
aware that it is prone to severe bed agglomeration, especially when
treating black liquor of relatively low calorific value from the
cooking of straw, and that the process has since fallen into disuse
for lack of technical and commercial viability. The experience of
the inventors is that simple fluidized beds of the kind disclosed
by Copeland are subject to unacceptable agglomeration, which makes
operation impractical for anything beyond a short start-up
period.
[0016] U.S. Pat. No. 3,523,864 (Osterman) discloses a recovery
process for Kraft black liquor based on a reaction vessel having
lower, intermediate and upper fluidized beds disposed one above
another and each formed by pellets of CaO. The lower bed operates
at 704-760.degree. C. (1300-1400.degree. F.) and contains solid
reaction products in which Na.sub.2SO.sub.4 becomes reduced to
Na.sub.2S. The intermediate bed is at 648-704.degree. C.
(1200-1300.degree. F.) and is fed with black liquor and preheated
air in an amount of about 30% of that required for complete
combustion to produce Na.sub.2CO.sub.3 and Na.sub.2SO.sub.4 which
become deposited on the surface of the CaO pellets together with
combustion gases and organics. The upper bed receives recycled
CaCO.sub.3 which becomes calcined to regenerate CaO and provide the
material for the fluidised beds which descends progressively from
upper to lower beds. Overhead combustion gases are partly recycled
as fluidising gas and after cyclone treatment are partly fed to a
steam generator. Again all three beds are of the simple bubbling
type, and the intermediate bed is subject to unacceptable
agglomeration for the reasons already given.
[0017] There are two further reasons for the absence of commercial
utilization of these low-temperature fluidized bed processes:
firstly the relatively high temperature required for fast and
complete conversion of Na.sub.2SO.sub.4 to Na.sub.2S and secondly
the ease of formation of H.sub.2S when Na.sub.2S is contacted with
combustion gases below the melting point of the inorganic salts.
So, while high temperatures favour the reduction, the above
alternative processes require a relatively low temperature just
below the melting point of the inorganic salt mixture. The
consequence is that in fluid bed processes operating in the
reducing mode, most of the Na.sub.2S formed is rapidly converted to
H.sub.2S (and some COS) according to the overall reaction
Na.sub.2S+CO.sub.2+H.sub.2O.fwdarw.. Na.sub.2CO.sub.3+H.sub.2S
resulting in a low recovery of solid Na.sub.2S.
[0018] For the sake of completeness, there should be mentioned U.S.
Pat. No. 4,011,129 (Tomlinson) which discloses a method for
increasing the chemical recovery capacity of a Kraft recovery
furnace by injecting solid pellets of sodium sulphate and sodium
carbonate directly onto the char bed in the reducing zone of the
furnace while maintaining the temperature and reducing atmosphere
in that zone, thereby forming a smelt containing sodium sulphide
and sodium carbonate from the injected pellets. These pellets may
be produced from a further quantity of black liquor in an auxiliary
incinerator such as a fluid bed combustion unit, which permits
recovery capacity to be increased without needing the construction
of a further recovery furnace.
Production of Non-Wood Cellulosic Pulp
[0019] The use of agricultural residues from graminaceous annual
crops could provide a solution to many problems of concern to the
pulp and paper industry including fibre supply, farmers' concerns
over the cost and availability of disposal alternatives, and
consumer concerns over limited forest resources.
[0020] Broadly defined, graminaceous crop residues are the
materials left over after annual agricultural crops have been
harvested for their primary or intended purpose. They include
cereal straws, such as wheat, rice, barley and oats; seed grass
straws such as flax and rye; the crushed stalks of sugar cane known
as bagasse; sorghum and corn stalks; and other agricultural
residues e.g. cotton linters, the short fibres adhering to cotton
seed after cotton ginning In countries where there is little or no
supply of wood, pulp from straw and bagasse is being used in high
proportions for paper-making--up to 90% for high quality printing
and writing paper. For example, in China, over 85% of papermaking
pulp has been reported to come from non-wood raw materials,
predominantly straw. In India, approximately 55% of papermaking
pulp has been reported to come from non-wood sources with about
half from agricultural-residues. As legislation increasingly
prohibits the burning of agricultural wastes, there is new
incentive to develop alternative uses for this resource. With
proper soil management, farmers can supply small scale pulping
mills with a continuous source of fibre while sustaining crop
production.
[0021] Agricultural residues such as wheat and rice straw contain
cellulose and can be a good raw material for papermaking. As
previously mentioned, these raw materials are bulky, so that
transportation costs and logistics 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 that if discharged to watercourses causes severe
pollution. 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 not advanced as far
as large-scale wood pulping technology.
[0022] Straw can be pulped by chemical processes and by a
combination mechanical and chemical process (chemi-mechanical
pulping). For the cooking of non-wood raw material sodium hydroxide
alone is recommended as the active chemical because most non-wood
fibre does not contain sticky resins and the sodium sulphide
catalyst is unnecessary. For this reason the major part of chemical
pulp production from this class of raw materials is performed with
a process called the soda process in which raw material is heated
together with a highly alkaline cooking liquor containing sodium
hydroxide to a temperature of 140-170.degree. C. under pressure.
Under these conditions, the main portion of lignin dissolves.
Sodium hydroxide can be recovered from the resulting black liquor
and organic substances in the black liquor can be used as fuel for
energy generation. Contrary to the Kraft process, in which recovery
requires reduction of sulphate to sulphide, black liquor from the
soda process can be burnt even under highly oxidising conditions.
Chemical recovery therefore involves evaporating the black liquor
to a suitable content of dry matter and burning the evaporated
liquor by means of excess oxygen. The inorganic combustion residue,
consisting mainly of sodium carbonate, is dissolved in water and
re-causticised with burnt lime to regenerate sodium hydroxide,
which is recycled. In a variant, slaked lime Ca(OH).sub.2 has been
used in admixture with NaOH as active chemicals in white liquor
because it also serves as a digesting agent and is of lower cost.
However, a process for recycling NaOH/Ca(OH).sub.2 black liquors
has not been described and such black liquors have in the past
merely been discharged untreated.
Silica in Pulp from Graminaceous Starting Materials
[0023] The relatively high content of silica in straw and other
non-wood cellulosic agricultural products presents difficulties for
chemical recovery. Wheat straw contains 4-10 w % silica as small
crystals embedded in the straw. Rice straw has an even higher
silica content, 9-14 wt %. Other cereals such as barley, oat and
rye straw have 1-6 wt % silica. Wood on the other hand has a silica
content of less than 1 wt %. In the soda process as applied to
straw pulping, most of the silica in the straw reacts with the
sodium hydroxide to form water-soluble sodium silicate, which
remains in the black liquor in addition to lignin and other organic
compounds. Black liquor of high silica content gives rise to
scaling (coating equipment with a glass like substance) especially
in an evaporative process. A modified wood-based recovery system
may be used if the silica content of the cereal straw is less than
5 - 6 wt %, but at higher capital and operating costs. However, for
products of higher silica content, especially for rice straw, there
has been up to now no process that is technically and commercially
viable.
[0024] Our WO 03/014467 (the disclosure of which is incorporated
herein by reference) discloses a method for treating raw elongate
material suitable for use in a paper making plant comprising:
[0025] extracting contrary material from the raw material;
[0026] crushing the raw material from which contrary material has
been removed to remove nodes;
[0027] splitting the crushed raw material lengthways;
[0028] supplying the split raw material to a co-rotating screw
conveyor divided into a plurality of zones and processing said
material in said conveyor to produce pulp and a black liquor
effluent;
[0029] supplying treatment material to at least one zone;
[0030] controlling the temperature and/or pressure of at least one
zone; and
[0031] spraying concentrated black liquor into a processing vessel
in the form of a fluidised bed reactor for treatment of said black
liquor, said processing vessel being part of treatment material and
energy recovery means. The alkali supplied to the co-rotating screw
conveyor to bring about pulping may include sodium hydroxide and
additionally calcium hydroxide, which has the effect of
precipitating silica onto the cellulosic fibres and preventing
silica from entering the black liquor as calcium silicate.
[0032] WO 03/014467 further describes a process for treating black
liquor in which black liquor effluent arising from the pulping
process is collected in a digestion liquor storage tank and
concentrated to 30-70% solids using a standard evaporator designed
for concentration purposes. If the black liquor effluent has 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 at a
temperature in excess of 90.degree. C. using a pipe or an enclosed
twin-screw transport system. The enclosed transport system is used
to minimise the loss of organic components through vaporisation. A
temperature in excess of 90.degree. C. is required to decrease the
viscosity of the black liquor so that it will transport without
resistance. The black liquor is treated in a toroidal fluidised bed
reactor vessel. Although a limit of 650.degree. C. is specified for
the upper temperature of the fluidised bed, in practice a maximum
temperature of 610.degree. C. has only ever been used. This is
because at temperatures above 600.degree. C., volatilisation of the
inorganic alkali metal species present in the black liquor (e.g. Na
and K) has been demonstrated to occur in other processes. When
these species are in the vapour phase, additional process equipment
is required to recover them at increased overall cost.
SUMMARY OF THE INVENTION
[0033] The advantage of a higher reaction temperature is the
enhanced rate of reaction for black liquor processing and hence
throughput may be increased, whilst maintaining output quality. We
have now discovered that a temperature of above 650.degree. C. can
be used in the above mentioned black liquor recovery process.
Recent experiments have shown that the loss of inorganic species
from the black liquor when heated to between 650 and 700.degree. C.
or even 725.degree. C. in the fluidised bed, was minimal, i.e. the
losses were not economically significant, and hence no additional
equipment was required to ensure their recovery.
[0034] According to the invention there is provided a method for
treating black liquor, which comprises heating the black liquor at
a temperature above 650.degree. C. in a reactor containing an
alkaline earth metal oxide, e.g. calcium oxide.
[0035] In the reactor the black liquor may react with the alkaline
earth metal oxide to form a mixture of sodium hydroxide and sodium
carbonate and alkaline earth metal carbonate and a volatile gas and
liquid component which contains a combustible component and can be
used as a fuel as in conventional treatment processes, e.g. a
boiler.
[0036] In an alternative aspect, the invention provides a method of
treating graminaceous materials which reduces or overcomes the
problems associated with a high silica content in the resulting
black liquor.
[0037] The invention further provides a method of converting
graminaceous raw material to pulp for paper or board, said method
comprising:
[0038] digesting said raw material with a white liquor based on
sodium hydroxide and further comprising calcium hydroxide in an
amount effective to substantially convert silica of said raw
material to calcium silicate;
[0039] recovering pulp and black liquor substantially free of
soluble silicate;
[0040] heating the black liquor in a fluidized bed reactor
containing calcium oxide for catalysing conversion of organic
content of said black liquor to gas and for providing recovered
solids including sodium values of said white liquor and calcium
oxide; and
[0041] regenerating said white liquor using said recovered
solids.
[0042] Under some conditions, there is a risk that significant
quantities of silicate may pass into black liquor streams during
the pulp washing process. However, the inclusion of calcium
hydroxide in alkali liquor used for pulping causes the silicate to
pass into the black liquor as calcium silicate in preference to, or
to the substantial exclusion of, sodium silicate. Calcium silicate
is significantly less likely than sodium silicate to give rise to
downstream processing problems. In a further aspect, the invention
relates to the use of calcium hydroxide as an additive in the soda
process digestion of a graminaceous starting material to form pulp
for inhibiting scaling during black liquor concentration and
recovery when processing black liquor at least partly from pulp
washing.
DESCRIPTION OF THE DRAWINGS
[0043] The invention will now be described in greater detail, by
way of example, with reference to the drawings in which:
[0044] FIG. 1 is an overall block diagram of a process for making
pulp from wheat straw according to the invention;
[0045] FIG. 2 is a schematic view of a roller arrangement for use
in a raw material pre-treatment process forming part of the pulp
manufacturing process of FIG. 1;
[0046] FIG. 3 is a schematic view of the construction of a
self-cleaning pin roller that may be used in the roller arrangement
of FIG. 2;
[0047] FIG. 4 is a schematic view of a possible embodiment of a
co-rotating twin screw conveyor that may be used for converting
straw to pulp in the process of FIG. 1; and
[0048] FIG. 5 is a block diagram of a preferred black liquor
effluent treatment apparatus that may be used in the process of
FIG. 1.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Overview of the Treatment of Wheat or Rice Straw
[0049] The present process will be described, by way of
illustration with reference to the treatment of wheat straw, which
is usually chopped before pulping and which contains nodes within
the stem that 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. A
method is therefore preferably employed which crushes the nodes,
opens out the straw stem lengthways in a gentle fashion and feeds
the raw material into a digester in a positive, metered and
continuous process. The straw to be treated passes from a delivery
conveyor 9 to a straw pre-treatment station 10 where the stems are
crushed between rollers, contrary material is extracted, and the
stems are split longitudinally. The conditioned straw is then
supplied to a digesting or pulping station 12 where it is subjected
to mechanical work in the presence of aqueous alkali (white liquor)
whilst subject to elevated temperatures and pressures. The
resulting black liquor is then passed to an effluent treatment
station 14 where it is heat treated to provide solids that can form
a so-called green liquor That liquor in turn is contacted with lime
derived from a CaCO.sub.3 feed and regenerated to white liquor for
recycling to the pulping station 12. Off-gas recovered from
heat-treating the black liquor can be used for generating steam and
for process heat. A solids bleed of CaCO.sub.3 sludge is removed to
avoid excessive build-up of trace metals in the white liquor.
Pre-Treatment
[0050] Where straw is the raw material from which pulp is to be
made, it may take the form of chopped straw, straw that has been
subjected to a longitudinal splitting or shredding operation, or
straw that has been both longitudinally split and/or shredded and
chopped.
[0051] Referring now to FIG. 2, according to a preferred
pre-treatment method, after the bales of straw have been opened,
the straw is passed along a conveyor belt 101 where dust, heavy
items such as stones and other contraries such as plastic string
are removed. The straw is then passed into 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
crushing rollers 105 and 107 to crush the straw nodes. The crushed
material then passes through two counter-rotating intermediate
rollers 109 and 111 that 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 that open and shred the
straw lengthways and act in co-operation with a feed shoe.
[0052] 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.
[0053] The treated straw then drops from the pinned rollers 113 and
115 into feed hopper 117 leading to either a conveyor or blower
system (not shown) that 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 chips or sawdust.
[0054] The pinned rollers can also be constructed to be
self-cleaning when used with longer fibered 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 FIG. 3. The pin
roller 120 has an outer surface having a large number of radially
extending pins 122. This is used with a matching perforated or
woven belt 124 on which the material 126 being treated is carried.
The pins 122 pick up the material 126 and as the belt 124 leaves
the pins, it takes off the material keeping the pin roller 120 free
from tangled fibres.
[0055] The above process has been tested and developed with straw,
flax and hemp through pilot scale laboratory trials. Furthermore,
the above-described pinning treatment for raw materials can be used
with wood if the logs are flaked rather than chipped, as has been
demonstrated using pinned wood wool instead of chips. This method
of raw material preparation is particularly useful if the wood has
a short fibre because the combination of flaking and pinning
protects fibre length.
Digestion of Graminaceous and Other Cellulosic Starting
Materials
[0056] Although chopped straw can be used as a feed, the preferred
feed for the digestion stage of the present process is straw or
other graminaceous plant stem material that has been longitudinally
split and/or shredded. In such material the white liquor used for
digestion makes easy contact with the graminaceous plant stem
material and with any remaining node material, dissolving silica
therein and digesting lignin and other alkali-sensitive
materials.
[0057] For the cooking of non-wood raw material, sodium hydroxide
alone is required as an active chemical, and for this reason the
major part of chemical pulp production from these raw materials is
performed with the process called the soda process. In that process
the raw material is heated together with an alkaline cooking liquor
containing sodium hydroxide to a temperature in the range from 140
to 170.degree. C. or in some instances up to 180.degree. C. under
pressure. The cooking liquor should have a high alkaline
concentration. Under these conditions, the main portion of lignin
will be dissolved from the raw material; however, also the main
portion of any silicon in the raw material will react with the
sodium hydroxide, forming water-soluble sodium silicate. Thus the
black liquor produced in cooking will contain silicate ions in
addition to lignin and other organic compounds. The preferred white
liquor used is therefore of the soda type (i.e. no sodium sulphide)
and additionally contains calcium hydroxide in an amount effective
to precipitate the silica. Either calcium hydroxide is present in
the white liquor with which the graminaceous feed is treated, or
calcium hydroxide is added shortly after the sodium hydroxide and
in either case the amount of calcium hydroxide should be sufficient
to precipitate silica onto the straw fibres as calcium silicate to
reduce the soluble silicate content of the resulting black liquor.
An effective amount of calcium hydroxide should be present to
convert substantially all the silica or a desired portion thereof
into insoluble silicate, much of which precipitates on the straw
fibres, before extraction of black liquor from partially or
completely digested straw is initiated.
[0058] The straw or other non-wood cellulosic material may be
digested using a continuous digester, e.g. a single tube or
multi-tube screw-fed digester, for example a Pandia digester
available from Lenzing Technik GmbH & Co KG. The use of fast
cooking of the plant fibre in a horizontal tube continuous digester
with screw feeder is reviewed in Atchison, J. E., Rapid Cooking
Horizontal Tube Continuous Digester with Screw Feeder--Now the
World Standard for Pulping Non-Wood Plant Fibers, 1990 Pulping
Conference Proceedings. It is explained that this technology was
developed initially for the pulping of bagasse, but is also
applicable to other forms of non-wood plant material including
wheat straw and rice straw. It is stated to permit a cooking time
of only 10-15 minutes for bagasse, straw and most other non-wood
plant fibres, in contrast, to earlier cooking methods using rotary
batch digesters, which required cooking cycles of four hours or
more. In the experience of the present inventors, this performance
is over-stated, and it is impossible to produce a semi-chemical
pulp in less than 20 minutes unless uneconomic quantities of
cooking chemicals are used. The use of a screw feed permits the
density of the feed material to be increased, and thereby the
processing capacity of the digester, and also promotes continuous
mixing of the white liquor and feed material. Again, to the
inventors' knowledge, the digester normally comprises 2, 3 or 4
series connected tubes each of diameter about 1 metre, so that the
installation is of considerable size and capital cost.
[0059] Because of the rapid absorbency of straw or bagasse when
these materials are subjected to pressure and elevated temperature
in a horizontal tube digester, especially after the pinning
operation described above, pulping starts immediately and proceeds
rapidly. The white liquor may be of the soda or Kraft type, and for
straw or bagasse pulping typically 12-14 wt % NaOH or 6-7 wt % NaOH
and 6-7 wt % Na.sub.2S are used based on the weight of the dry
straw, with pulping temperatures of 170-180.degree. C., pulping
pressures of 7-9 bar and cooking times stated to be 10-15 minutes
in order to produce a chemical pulp, and 3-5 minutes for a
semi-chemical pulp. As explained above these figures are in the
inventors' experience not obtainable in practice.
[0060] A preferred pulping process for bagasse or straw (including
rice straw) uses a horizontal tubular digester in which straw
transport is by a conveyor based on a co-rotating and intermeshing
double screw system. Such a digester can be made physically small
for its processing capacity and consequently carries a lower
capital cost than competing technologies. A significant advantage
is that pulping can be conducted with low amounts of water,
permitting black liquor to be produced at higher concentration and
reducing or removing the need for subsequent black liquor
evaporation. Shredded and/or chopped straw is continuously fed from
storage into the barrel of the digester where white liquor and
steam are injected through ports in the barrel or instead of steam
injection electrical heating external to the barrel is provided. As
the intermeshing twin screws rotate, the straw and black liquor are
intermixed and the straw is worked mechanically and digested. The
twin screw used is co-rotating, which reduces the mechanical
treatment given to the fibres and thus minimises fibre damage to
the fibres. In a twin screw arrangement transport along the barrel
is primarily by the intermeshing flights of the two screws, whereas
in a single screw conveyor transport is because the material is
trapped between the advancing screw flights and the static barrel
wall. The single screw arrangement is therefore less efficient
because of friction and it can give rise to slippage (pressure in
the barrel causes the straw to slip between the screw and the
barrel wall) and surging. The material in the twin screw digester
barrel with co-rotating screws travels in a figure-of-eight shaped
path and thus takes a longer route than if the screws were counter
rotating which gives better mixing of the straw and black liquor.
The state of the art in relation to twin screw digesters for making
paper pulp is shown in U.S. Pat. No. 4,088,528 (Berger et al) and
U.S. Pat. No. 4,214,947 (Berger), the disclosures of which are
incorporated herein by reference. It will be noted that both of
these references are concerned with the treatment of wood chips
rather than graminaceous material.
[0061] Trials have confirmed that it is possible to produce using a
single digester of the twin screw type to convert pinned shredded
straw of length about 25 mm into a chemical or semi-chemical pulp
having a kappa number in the range 30-70 and useful e.g. for use in
box or carton manufacture, corrugated packaging or the like. In a
series of experiments, straw was fed into the inlet of a 40 mm
diameter twin-screw digester, the pitch of the screw varying along
its length to define five treatment zones of temperature ranging
from 90.degree. C. at the inlet to 165.degree. C. in the
penultimate zone. Alkali was added using a metering pump, with
sufficient water to achieve the indicated liquid to solids
digestion ratio. A temperature in the downstream cooking zones of
the digester of not less than 165.degree. C. was found preferable
in order to achieve a well-disintegrated pulp. It was concluded
that it is possible to achieve Kappa numbers in the low forties
from a relatively short 40mm diameter twin screw, in less than one
minute and with caustic additions below 10%, which represents a
quality consistent with a semi-chemical pulp. In a full-scale
machine it is envisaged that the cooking temperature would be
170.degree. C. which is usual for making semi-chemical pulp using a
continuous screw digester. TABLE-US-00001 Run No. 1 2 3 4 5 6 7
Temperature 165 165 165 165 165 165 165 Screw Speed 120 120 120 120
120 120 50 Straw Feed Rate 6 3.75 6 6 6.3 6 3.75 Liquid to fibre
1.6 2.5 1.6 1.6 1.5 3.2 3.36 ratio (to 1) Caustic to fibre 9.4 15
9.4 9.4 9 9.4 8.3 ratio % Kappa No. 45.8 45.8 43.7 43.9 39.9 60.5
42.3
[0062] It is predicted that using a commercial scale twin-screw
extruder, it may be possible to achieve fully digested chemical
pulp suitable for printing or writing papers without an additional
cooking stage with a retention time of e.g. about 1 minute and a
cooking pressure of 7 bar. Alternatively the twin-screw extruder
may be used to produce semi-chemical pulp in a first stage, which
pulp may be converted into chemical pulp in a second cooking stage
e.g. with 30 minutes additional cooking at a pressure of 7 bar
before pulp de-watering.
[0063] An embodiment of a twin-screw co-rotating digester 131
appears in FIG. 4. Graminaceous raw material (straw, flax, hemp,
bagasse), wood chips or any other cellulosic raw material from
buffer storage can be pulped. To this end the raw material is drawn
into the digester 131 in which the screw profiles are specially
designed with two outer sections 133 and 134 having flights going
in a first direction while in a middle section 135 the flight
direction is reversed. The flights of the conveyor screws are
manufactured from hardened steel with a deep cut flight to improve
the size of the region where raw material is positively conveyed as
explained above and are specially designed to minimize 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.
[0064] As can be seen schematically in FIG. 4, one embodiment of
the conveyor has a first zone 137 to which the raw material is fed
through a feed hopper 139. The flights of the conveyor screws in
zone 1 are designed to be as open as possible in order to accept
the material into the unit. In a second zone 141 cooking liquor
which is preferably a soda process white liquor not containing
significant sulphide but containing Ca(OH).sub.2 can be added
through an inlet 143 and steam can be introduced into the second
zone of the digester barrel through inlet 145. The length of the
region 141 and the dwell time of the raw material in that region
vary depending upon the nature of the material, but are sufficient
for a digestion of significant quantities of lignin located in
regions of the starting material that are readily accessible to the
pulping liquor (easy lignin) and dissolving other readily soluble
material.
[0065] As explained above, it is desirable to convert silica
present in straw or other graminaceous starting materials into
insoluble silicate, much of which becomes precipitated onto the
cellulosic fibres when pulping straw to prevent harmful amounts of
silica from entering the black liquor effluent as soluble silicates
which can give rise to scaling of evaporators or other parts of the
downstream chemical recovery system. For that purpose when pulping
straw or other graminaceous material calcium hydroxide can be added
in the second zone 141 at a rate of 4% to dry raw material (straw)
with 8% sodium hydroxide. In general, there is used about one part
by weight of calcium hydroxide for two parts by weight of sodium
hydroxide. This method is applicable in any alkaline based pulping
system and has the effect of re-precipitating sodium silicate onto
the cellulosic fibres as calcium silicate some, most or
substantially all of which can remain in situ on those fibres
(depending upon the subsequent pulp treatment conditions) when they
have separated as pulp and during subsequent washing, bleaching and
conversion into paper. In a mixed alkali system in which both NaOH
and Ca(OH).sub.2 are present, reaction to form insoluble
CaSiO.sub.3 is highly favoured over the competitive reaction to
form soluble Na.sub.2SiO.sub.3, so that the silica is retained on
the fibres as calcium silicate or enters the black liquor as
insoluble calcium silicate. In consequence there is no significant
difference as regards soluble silica content between the black
liquor from graminaceous materials treated with NaOH/Ca(OH).sub.2
and the black liquor from wood pulping, which is amendable to
treatment by known methods. The precipitated calcium silicate, once
formed, is not prone to re-dissolve under the conditions
encountered in subsequent pulp processing operations including
bleaching, de-watering and paper or board manufacture none of which
employ low pH values (pH<4), and it simply provides an innocuous
part of the ash content of the pulp, being chemically similar to
Wollastonite which can be used as a filler in paper-making and also
being similar to china clay which is a complex silicate. The only
difference, so far as subsequent processing is concerned is that
the pulp from graminaceous materials may have an ash content of
e.g. 3-4% whereas the ash content of wood pulp is usually about 1%.
It will be noted that the effluent treatment station 14 described
below which gasifies the black liquor using a fluidized bed of CaO
naturally produces a white liquor containing Ca(OH).sub.2 as well
as regenerated NaOH, this black liquor both digesting the feed and
precipitating calcium silicate on the cellulosic fibres so that it
does not enter the black liquor in harmful quantities.
[0066] The partially digested raw material passes to a third zone
147 where the conveyor screws have a reversed flight region 135
which acts as a braking zone for the advancing raw material which
forms a plug of material being processed, with a high pressure zone
thus being generated upstream of the plug. In this zone the barrel
wall has a perforated region 149 through which some of the cooking
liquor becomes squeezed out. The action of the white liquor on the
raw material is to rapidly solubilize all or much of the readily
accessible lignin content of the raw material which dissolves with
soluble hemicellulose and other dissolvable organic solids. In the
third region a portion of the white liquor which may typically
correspond to about half of the white liquor initially added exits
the region 149 as a black liquor stream of high solids content,
typically about 30 wt % solids. Removal of the easy lignin, soluble
hemicellulose and other soluble organic materials in this black
liquor stream means that these materials are no longer present to
impede alkali attack on lignin remaining in the partly digested raw
material in subsequent zones, assists the later stages of
digestion, and also provides a solids-rich black liquor stream that
contributes to recovery of a final combined black liquor stream of
relatively high solids content. A remainder of the barrel and
conveyor screws define a fourth zone 151 and a fifth zone 132
leading to an outlet 153 for pulp. In Zone 4 temperature and
pressure are increased as indicated so that digestion continues
through residual white liquor on the pulp which also serves to
lubricate the partly digested starting material as it progresses
along the digester barrel. In Zone 5 temperature and pressure are
reduced in preparation for the material leaving the twin-screw
digester. The material travels through the twin-screw unit in
between 2-3 minutes. The screw speed may be around 200 rpm. It will
be appreciated that while the twin-screw digester shown is set up
with four/five zones, any number of zones, suitably from 3 upwards
can be used and for whatever treatment regime is required.
[0067] The digester is suitably of modular construction which
facilitates making changes to both screw and barrel configurations.
This should be a cost-effective 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 may 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 digester 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, which is a
standard feature of twin-screw extruders. It has further been
realized 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. In another
embodiment of the digester (not shown), the feed zone into the
conveyor screws is enlarged compared to the other zones to allow
the raw material to be fed freely into said zone to increase the
throughput of the conveyer. As the raw material moves forward into
the treatment zone and the first and second pressure zones, 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.
[0068] Using a co-rotating twin screw with a barrel size of 100
millimeters, the co-rotating intermeshing-twin screw extruder is
set up with five zones as described below. TABLE-US-00002 Zone 1 2
3 4 5 Type Straw Treatment Initial Digestion Digestion/ intake
steam/ liquor discharge alkali recovery T .degree. C. 65 100 130
150 130 P (bar) 0 0 2-3 4-5 2-3
[0069] The pulp exiting from the twin screw may have approximately
50% moisture content and would be expected to have a Kappa Number
of 30-40. This is an unbleached chemical pulp ready for bleaching
using standard methods and suitable for printing and writing
papers. A semi-chemical pulp with a higher kappa number suitable
for use in fluted packaging can also be produced if required. The
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. It is expected that
kappa-numbers as low as 20 may be achievable using a single
screw.
[0070] If production of a full chemical pulp is not achieved in the
twin-screw digester 131 as expected, it may then be necessary for
the pulp to be further digested e.g. in a further twin-screw
digester or in a single screw digester of the kind disclosed by
Atchison above e.g. using steam at 1-2 bar pressure (120.degree.)
for a further 20-40 minutes, and optionally adding further white
liquor. Atchison discloses the use of a two horizontal tube
digesters disposed one above another with the discharge outlet of
the upper digester feeding partly digested material into the inlet
of the second digester. In practice, in the prior art, it has often
been necessary to use three such digesters arranged in series.
However, the twin-screw digester disclosed above produces
significant breakdown of the graminaceous or other starting
material, and it is expected that only a single further twin- or
single-screw digester stage will be needed to achieve a chemical
pulp ready for bleaching so that installation size and capital cost
may be reduced. A Kappa Number of 14-20 may be achieved after this
further digestion.
[0071] Pulp from the twin-screw digester or from a subsequent
further digestion stage is then washed for further recovery of
black liquor before further treatment e.g. bleaching in the case of
a chemical pulp. Washing is normally a multi-stage operation in
which e.g. the pulp is successively contacted with wash liquid and
passed through a plurality of dewatering stages arranged in series
e.g. 2-4 such stages with 3 stages being usual. Some of the calcium
silicate on the fibres may become dispersed in the wash liquor, and
at the end of the multi-stage washing operation under come
conditions e.g. about 50% of the calcium silicate may become
re-dispersed into the wash liquor. Re-dispersion of calcium
silicate may be inhibited by incorporation of a flocculating agent
e.g. polyacrylamide into the wash liquor. However, under the
conditions used in evaporation of the black liquor, any
re-dispersed calcium silicate is significantly less prone to form
harmful deposits than sodium silicate. Furthermore, calcium
silicate is relatively water-insoluble and high melting,
Wollastonite melting at 1 540.degree. C., well above the bed
temperatures for fluidized bed gasification of black liquor and
well above the melting temperature of sodium silicate. Under the
process conditions contemplated herein, it is expected that any
calcium silicate in the black liquor will remain as discrete
particles, and will not either volatilise or promote agglomentation
of the fluidized bed used in the recovery process.
[0072] Each dewatering stage may take place in a screw-type press
in which an elongated rotating screw fits within a foraminous
sleeve that in turn is contained within a housing forming means for
collecting wash liquor that has passed through the sleeve, pulp
being advanced longitudinally of the screen by the rotating screw
and being subjected to a squeezing action e.g. with the
cross-sectional area of the channel defined by the screw thread or
the spacing between adjacent screw threads diminishing from the
inlet end of the screen towards the outlet, so that the pulp
collected becomes compressed and liquid is progressively squeezed
from the pulp. U.S. Pat. No. 6,792,850, U.S. Pat. No. 6,393,728,
U.S. Pat. No. 6,736,054 and U.S. Pat. No. 3,256,808 illustrate some
of the common features of this type of press. Wash liquid normally
progresses in the opposite sense to the pulp so that if there are
first, second and third screw presses connected in series, water is
supplied to a mixing tank for washing the pulp to be fed to the
third screw press, recovered wash liquor from the third press is
supplied to a further mixing tank to wash the pulp to be fed to the
second screw press, and recovered wash liquor from the second screw
press fed to a mixing tank for washing the chemical pulp supplied
to the first screw press, recovered liquor from the first screw
press which may by now have a solids concentration of above 10 wt %
e.g. about 12-15 wt % providing a relatively concentrated black
liquor stream which can be combined with the still more
concentrated black liquor stream at 149 and passed for alkali
recovery. Some, or even a major part of the silica that has become
converted into calcium silicate will find its way into the black
liquor. However, the calcium silicate will not tend to dissolve
under the recovery process conditions employed and in contrast to
sodium silicate which is soluble will give rise to no or to reduced
harmful glassy deposits when the black liquor is concentrated by
evaporation. It will be appreciated that in-line mixers may be used
instead of mixing tanks, and that other known ways of dewatering
pulp may be used in place of screw presses. Screw presses are
relatively small compared to drum washers that could also be used,
and are preferable for the smaller-scale operations involved in
pulping of graminaceous materials.
Black Liquor Recovery
[0073] The present invention provides a treatment process to
recover organic and 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.
[0074] Owing to the absence of sulphides, the black liquor may be
treated to volatilise the organic component thereof in a fluidized
bed reactor under oxidizing conditions in the presence of a
stochiometric amount of oxygen or oxygen-containing gas such as
air. Preferably, however, the black liquor is gasified (partially
oxidixed) to a synthesis gas having as components inter alia
CO.sub.2, CO, H.sub.2O, and H.sub.2 usually together with methane
and, C.sub.2+ components under pyrolysing or partial oxidizing
conditions in the presence of a sub-stochiometric amount of oxygen
or free-oxygen-containing gas. Such gas may be a mixture of steam
and combustion gas from a natural gas boiler, or a mixture of steam
and combustion gas from a boiler supplied with cleaned recycled
synthesis gas supplemented with natural gas as required. The gases
act as fluidising medium for the bed, and the bed material consists
of or comprises CaO which catalyses the gasification process and
also the gasification of any char which may form as by-product
within the bed. In such a process, the amount of oxygen in the gas
mixture supplied to the bed should be sufficient to support partial
oxidation and maintain bed temperature, but insufficient to convert
the sodium and/or calcium hydroxide content of the black liquor
entirely to carbonate, it being believed possible to conduct the
reaction so that at least some of the NaOH remains as such in the
bed. Thus the oxygen content of the fluidizing gas may be <5%
oxygen and usually about 1.5-2% oxygen, giving an oxygen content in
the off-gas above the bed of <1%, typically about 0.8%, all by
volume.
[0075] Thermal degradation of the organic matter in black liquor
begins above 200.degree. C. producing water vapor, CO.sub.2, CO,
H.sub.2, light hydrocarbons, tar and in the case of Kraft and other
sulfur containing liquors, light sulfurous compounds (e.g.
mercaptans). By 600.degree. C. devolatilisation is essentially
complete with the char residue containing fixed carbon, some
hydrogen and most of the inorganic matter. Char composition can
vary widely and depends upon both processing conditions (e.g.
temperature) and fuel characteristics. Straw black liquor has a
lower calorific value than wood black liquor, which should be taken
into account when designing the fluidized bed reactor.
[0076] A series of non-isothermal experiments was conducted using
temperature ramp thermogravimetric analysis (TGA). This technique
allows rapid measurement of the temperature decomposition profile
of a material and the subsequent determination of its thermal
decomposition kinetics. For the straw black liquor, heated under
N.sub.2 at a rate of 20.degree. C./min, five separate peaks were
identified: (a) 25 to 105.degree. C. loss of moisture, (b) 105 to
250.degree. C. main volatiles peak, (c) 300 to 350.degree. C.
smaller volatiles peak, (d) 425 to 500.degree. C. smaller volatiles
peak and (e) 650.degree. C. devolatilisation of inorganic species
(e.g. Na and K). It was not possible to identify which organic
components of the liquor were associated with specific peaks from
these tests although it is likely the three main organic
components, lignin, hemicellulose and carboxylic acids can be
attributed to the three main volatiles peaks. These tests also
indicate that the operating temperature of the industrial reactor
should preferably not exceed 750.degree. C. in order to allow the
majority of pulping chemicals (i.e. Na and K) to be recovered in
the preferred solid form, reaction temperatures of e.g.
675-725.degree. C. being preferred, e.g. 675-700.degree. C.
[0077] Batch fluidized bed experiments using a conventional or
bubbling fluidized bed of silica sand of 0.1 m internal diameter
were conducted to determine typical off-gas compositions and yields
of char, off-gas and tars from black liquor pyrolysis, gasification
and combustion experiments. Bed mixing and agglomeration tendency
was also investigated. Black liquor concentrated to 29% and 45%
solids was fed to the bed operating at 500-700.degree. C. with
U/U.sub.mf=4 (i.e. vigorously fluidized), supplied with mixtures of
N.sub.2 and O.sub.2. Analysis of the off-gas showed typical
compositions of the main product gases ranged between 0-5% for
H.sub.2, 7-12.5% for CH.sub.4, 7.5-15% for CO, 55-89% for CO.sub.2
and 0-8% for C.sub.2+ species. Bed agglomeration was severe in all
cases despite the high U/U.sub.mf and typically resulted in the
experiment being terminated in less than 20 minutes, due to loss of
fluidization. Gas yields were typically very low (around 9% at
550.degree. C. increasing to 25% at 700.degree. C.). Together the
low gas yields and gas compositions (i.e. high [CO.sub.2] and low
[CO] and [H.sub.2]) suggest that the steam reforming reactions of
the char were not occurring as they should under gasifying
conditions. This is most likely due to the poor gas-solid contact
occurring in an agglomerated fluidized bed. The increase in gas
yield with temperature is important however and suggests an
industrial reactor (which is designed to have a significantly
higher tolerance for bed agglomeration) should be run as close to
the melting point of K and Na as possible in order to maximize the
production of synthesis gas. For pyrolysis experiments (with
N.sub.2 only), when a condenser was added to the off-gas line, the
yield of tar (the condensable fraction) was measured to be 30 and
38% at 550 and 700 .degree. C. respectively. Char yields were 45%
at 550.degree. C. decreasing to 39% at 600.degree. C. and 31% at
700.degree. C., again suggesting temperature is an important
process variable.
[0078] A spouted fluidized bed differs from a bubbling fluidized
bed in that it is designed with a central jet that forces material
from the base of the bed along the central axis before it is
allowed to settle back down along the walls of the vessel. The bed
is typically conically shaped rather than cylindrical to assist
with the bulk circulation movement. It may exhibit greater bed
stability and fewer tendencies to agglomerate than a bubbling
fluidized bed.
[0079] The reactor is preferably a toroidal fluidized bed reactor;
such reactors are described in U.S. Pat. Nos. 4,479,920 and
4,559,719 (the disclosures of which are incorporated herein by
reference) and are available from Torftech Limited of Reading, UK
(www.torftech.com). Such a reactor is intended to overcome the
problems of conventional fluidised bed reactors as regards control
of temperature and rate of heat transfer within the bed resulting
from the random nature of lateral movement of the bed particles in
a bed that remains essentially static and is fluidised by a
vertical flow of gas/air mixture. Both a bubbling fluidized bed and
a spouting fluidized bed suffer from these problems. In the
experience, of the inventors, agglomeration occurs when there are
hot spots in the bed. A toroidal fluidised bed minimises this risk
and the active CaO bed will also help.
[0080] The solution proposed by Torftech and which is followed
according to a preferred aspect of the invention, as applied to the
treatment of black liquor, comprises providing a reactor having a
processing region provided with a mass of particulate material
consisting of or containing calcium oxide;
[0081] supplying heated fluidizing gas to the processing region so
as to generate a swirling flow of fluid within said processing
region, the fluid of said swirling flow of fluid causing the
particulate material to assume a compact band and circulate about
an axis of said processing region in a turbulent manner, the
fluidizing gas including oxygen for at least partially combusting
organic material in the black liquor;
[0082] feeding the black liquor into said compact band of
particulate material and treating the black liquor in said bed so
as to gasify organic materials in said black liquor;
[0083] recovering organic material from said black liquor as
off-gas from said bed; and
[0084] recovering inorganic material from said black liquor as
solids from said bed.
[0085] It is believed that each particle travels to and fro inside
the processing region along the full periphery of the compact band
so that uniform processing conditions may be obtained. The motions
of the particles within the particulate mass are determined by the
combined effects of the fluid flow, gravity, and the centrifugal
forces created by the swirling of the fluid, and the result is a
thorough and continuous mixing of these particles and matter to be
processed, on the supply of such matter into the band of particles.
Consequently, a very efficient processing operation may be achieved
using only a shallow band of particles. Furthermore, the process
gas stream impacts on and minimises the insulating microscopic gas
layer around each particle. As a result, the heat and mass transfer
rate is greater than in other types of reactor, which should permit
faster and more effective processing.
[0086] Tests have been carried out using a semi-industrial scale
reactor of the above toroidal fluidized bed type. Where the black
liquor to be treated is converted into solids form, the solids were
fed through a two-stage screw feeder (one dosing screw, one feeding
screw). Where the black liquor to be treated was in the form of a
liquid, it was fed using a centifugal pump either through the top
of the reactor or through a two-phase nozzle (supplied with Ar or
N.sub.2) directly above and perpendicular to the bed surface. The
off-gas and some solid material exited through the top of the
reactor and passed through a length of duct into a cyclone and
venturi scrubber before being passed to an afterburner chamber
operating at 850.degree. C. Bed solids were removed from the bed
through a central discharge orifice. Solid and gas samples were
taken from different locations around the plant. The reactor
chamber was of a high temperature ceramic (not strictly required
for this work) and was of 400 mm at the base rising to 500 mm at
the top and was of height 850 mm. The distributor through which
fluidizing gas entered the reactor chamber from beneath the
fluidized bed comprised a number of parallel plates aligned at an
angle, between which plates the fluidizing gases passed. The free
surface area of the distributor was 30%. The effect of the
distributor was to impart a severe swirling motion within the bed,
which caused high local gas velocities and high turbulence without
blowing particles out of the top of the reactor. The nominal
fluidizing velocity used for all experiments was 10 m/s, which
adequately supported a calcium oxide bed of mean particle size 1.5
mm.
[0087] Two different processing modes were considered for black
liquor treatment in these experiments: (a) preparation of a premix
containing black liquor and calcium oxide in a specified ratio and
(b) direct spraying of black liquor (of various solids contents)
onto a resident bed of quicklime in the reactor. Operationally,
feeding a dried solid to the toroidal fluidized bed is the simplest
alternative, but there are engineering issues that prevent this
from being preferred. The reaction product with CaO is a plastic
mass with thermoplastic properties (even using a substantial excess
of lime) that requires further heating to achieve complete dryness
and then cooling to achieve solidification before it can be crushed
and fed through a gas-tight seal (e.g. a screw feeder or rotary
valve) into the reactor. This is expensive and inconvenient.
Furthermore, equilibrium modeling shows that an excess of CaO
prevents the formation of hydrocarbons (particularly CH.sub.4) by
reaction to form CaCO.sub.3. The alternative is to pump the black
liquor directly onto a resident bed of CaO (mixed with inert
material; either silica sand or alumina) inside the gasifier using
a conventional black liquor spray nozzle. Swelling of black liquor
as it reaches 200.degree. C. is a difficulty at the semi-industrial
scale, but is not expected to be a problem with the larger
equipment used on the industrial scale.
[0088] The bed solids containing the inorganic components from the
black liquor, as well as the calcium from the catalyst, exit the
bed via a central discharge weir where they can be dissolved to
give a mixture of Na.sup.+, Ca.sup.2+, CO.sub.3.sup.2- and
(OH).sup.- ions in solution according to the equation:
CaO+H.sub.2O+Na.sub.2CO.sub.3.fwdarw.2NaOH+CaCO.sub.3. This
solution is then treated as described above.
[0089] The processing temperature used in these experiments ranged
between 550 and 725.degree. C. In general an increase in
temperature increased the rate of conversion of the black liquor to
reaction products. The majority of the inorganic content of the
black liquor could be recovered in solid form at higher
temperatures than previously believed possible. Typical recoveries
were in excess of 90% where experimental data was measured within
95% confidence limits.
[0090] The influence of temperature upon the off-gas quality was
indiscernible. The black liquor solids concentration also did not
have a discernable influence upon either sodium recovery or off-gas
composition but as expected the off-gas contained more water when
the more dilute liquors (16% solids) were processed. It is expected
that the optimum solids content for feeding to the black liquor
process will be dictated by upstream processing conditions and
economic constraints, i.e. the solids content produced by the
pulping process and an analysis of the cost of evaporating water in
the toroidal fluidized bed reactor compared with doing so in
advance in a standard multi effect evaporator.
[0091] As regards oxygen concentration in the fluidizing gas, early
experiments with 5-20% [O2] in the produced off-gas mixtures of
N.sub.2, CO.sub.2 and H.sub.2O as expected under these oxidizing
conditions. Inorganic species recovery, i.e. Na, C and K, was good.
Bed and cyclone underflow samples ranged from 39-60 wt % Ca and
2.5-13 wt % Na. Experiments conducted with <2% [O2] produced gas
mixtures containing predominantly N.sub.2, CO.sub.2, H.sub.2O and
hydrocarbons, H.sub.2 or CO being difficult to detect
chromatographically as they were obscured by the nitrogen peak.
[0092] Equilibrium modeling has shown that CaO is important for the
gasification reaction. It also prevents solid carbon from forming
(i.e. char and tars) and forms complexes with sulfur (specifically
CaS), although this may been seen as very much a beneficial side
reaction. It is preferably added in the preferred amount CaO:DS
(dry solids) of 0.2:1 to 0.4:1, most preferably about 0.35:1. At a
ratio of 1.2:1 it lowers the amount of hydrocarbons formed
considerably by tying up carbon as CaCO.sub.3
[0093] Thus, in a preferred embodiment, black liquor of solids
content 10-40% e.g. 15-30% may be supplied directly to a toroidal
fluidized bed reactor containing either calcium oxide alone or
calcium oxide and an inert material, and supplied with steam and
combustion gas from a burner supplied with recycled synthesis gas
from the reactor supplemented with natural gas as required. An
evaporation plant and a calciner to recover CaO from CaCO.sub.3 are
desirable, and a small quantity (about 10%) of the CaCO.sub.3
stream will need to go to waste to prevent build-up of heavy
elements which are naturally drawn up form the soil by the plants.
This material could however be sent for local re-processing and
used to make bricks or in cement works.
[0094] Referring to FIG. 5, a preferred embodiment of the effluent
treatment process will now be described.
[0095] 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.degree. C. using an enclosed twin-screw transport system 303.
The enclosed transport system is used to minimize the loss of
organic components through vaporization. A temperature in excess of
90.degree. C. is required to decrease the viscosity of the black
liquor so that it becomes easy to transport. The black liquor is
treated in the reactor vessel 304 in either of two methods.
[0096] In a first method, the black liquor is introduced into a
toroidal fluidized bed reactor 304 by spraying the concentrated
liquor into the chamber of the reactor in which a bed of fluidized
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 size of the earth oxide may be between 1 and 4 mm. As
previously explained, 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 (CaO) in the ratio e.g. 0.3:1 lime to black
liquor dry solids to convert the black liquor a granular friable
material which may then be screw fed into a toroidal fluidized bed
reactor 304. Again when the black liquor is converted to a dry
solid before it is supplied to the bed, the reactor may operate
under stoichiometric or sub-stoichiometric conditions. 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
fluidized bed reactor 304 is maintained within the temperature
range 300 to 750.degree. C. and preferably 650-750.degree.. 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 fluidized bed reactor 304 may be
recycled via the screw feeder 303 back to the reactor 304.
[0097] The black liquor is converted by a chemical reaction to:
[0098] (1) Sodium hydroxide and sodium carbonate and lime within
the fluidized bed reactor 304. The bed will overflow through a
central discharge point and the overflowing material 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 known filter 306 to form
a calcium carbonate sludge and white liquor (containing sodium
hydroxide and calcium 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).
[0099] (2.) A gas and liquids with a combustible component which
can be utilized 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 fluidized bed reactor to provide heat for the chemical
recovery reaction.
[0100] The calcium carbonate sludge may be dried to remove some
water and sent to a second calciner reactor 308, which may be a
toroidal fluidized bed reactor. This reactor may operate at a
temperature of around 1100.degree. C. where calcium carbonate
CaCO.sub.3 is converted back to calcium oxide CaO for re-use in the
black liquor effluent chemical recovery process. Approximately 10%
of the fluidized 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 preferred.
[0101] It will be appreciated that various changes may be made to
the embodiment described above without departing from the
invention. For example, the black liquor treated in the fluidized
bed could be a Kraft liquor or a mixture of the soda/Ca(OH).sub.2
black liquor and Kraft liquor or black liquor from the
soda/anthraquinone process.
* * * * *