THE RECOVERY OF ALUMINA TRIHYDRATE DURING THE BAYER PROCESS
USING CROSS-LINKED POLYSACCHARIDES
Cross-Reference to Related Applications
None.
Statement Regarding Federally Sponsored Research or Development
Not Applicable.
Background of the Invention
The invention relates to a method for improving the Bayer process for the production
of alumina from bauxite ore. The invention concerns the use of cross-linked polysaccharides,
specifically cross-linked dextran or cross-linked dihydroxypropyl cellulose to improve the
performance of unit operations within the Bayer process, in particular to enhance the settling of fine
alumina trihydrate crystals.
In the typical Bayer process for the production of alumina trihydrate, bauxite ore is
pulverized, slurried with caustic solution, and then digested at elevated temperatures and pressures.
The caustic solution dissolves oxides of aluminum, forming an aqueous sodium aluminate solution.
The caustic-insoluble constituents of bauxite ore are then separated from the aqueous phase
containing the dissolved sodium aluminate. Solid alumina trihydrate product is precipitated out of
the solution and collected as product.
As described at least in part, among other places, in US Patent 6,814,873, the Bayer
process is constantly evolving and the specific techniques employed in industry for the various steps
of the process not only vary from plant to plant, but also are often held as trade secrets. As a more
detailed, but not comprehensive, example of a Bayer process, the pulverized bauxite ore may be fed
to a slurry mixer where an aqueous slurry is prepared. The slurry makeup solution is typically spent
liquor (described below) and added caustic solution. This bauxite ore slurry is then passed through a
digester or a series of digesters where the available alumina is released from the ore as causticsoluble
sodium aluminate. The digested slurry is then cooled, for instance to about 220° F,
employing a series of flash tanks wherein heat and condensate are recovered. The aluminate liquor
leaving the flashing operation contains insoluble solids, which sol ids consist of the insoluble residue
that remains after, or are precipitated during, digestion. The coarser solid particles may be removed
from the aluminate liquor with a "sand trap", cyclone or other means. The finer solid particles may
be separated from the liquor first by settling and then by filtration, if necessary.
The clarified sodium aluminate liquor is then further cooled and seeded with alumina
trihydrate crystals to induce precipitation of alumina in the form of alumina trihydrate, Al(OH)3. The
alumina trihydrate particles or crystals are then classified into various size fractions and separated
from the caustic liquor. The remaining liquid phase, the spent liquor, is returned to the initial
digestion step and employed as a digestant after reconstitution with caustic.
Within the overall process one of the key steps is that of precipitation of the alumina
trihydrate from the clarified sodium aluminate liquor. After the insoluble solids are removed to give
the clarified sodium aluminate liquor, also referred to as "green liquor", it is generally charged to a
suitable precipitation tank, or series of precipitation tanks, and seeded with recirculated fine alumina
trihydrate crystals. In the precipitation tank(s) it is cooled under agitation to induce the precipitation
of alumina from solution as alumina trihydrate. The fine particle alumina trihydrate acts as seed
crystals which provide nucleation sites and agglomerate together and grow as part of this
precipitation process .
Alumina trihydrate crystal formation (the nucleation, agglomeration and growth of
alumina trihydrate crystals), and the precipitation and collection thereof, are critical steps in the
economic recovery of aluminum values by the Bayer process. Bayer process operators strive to
optimize their crystal formation and precipitation methods so as to produce the greatest possible
product yield from the Bayer process while producing crystals of a given particle size distribution. A
relatively large particle size is beneficial to subsequent processing steps required to recover
aluminum metal. Undersized alumina trihydrate crystals, or fines, generally are not used in the
production of aluminum metal, but instead are recycled for use as fine particle alumina trihydrate
crystal seed. As a consequence, the particle size of the precipitated trihydrate crystals determines
whether the material is to be ultimately utilized as product (larger crystals) of as seed (smaller
crystals). The classification and capture of the different sized trihydrate particles is therefore an
important step in the Bayer process.
This separation or recovery of alumina trihydrate crystals as product in the Bayer
process, or for use as precipitation seed, is generally achieved by settling, cyclones, filtration and/or a
combination of these techniques. Coarse particles settle easily, but fine particles settle slowly.
Typically, plants will use two or three steps of settling in order to classify the trihydrate particles into
different size distributions corresponding to product and seed. In particular, in the final step of
classification a settling vessel is often used to capture and settle the fine seed particles. Within the
settling steps of the classification system, flocculants can be used to enhance particle capture and
settling rate.
The overflow of the last classification stage is returned to the process as spent liquor.
This spent liquor will go through heat exchangers and evaporation and eventually be used back in
digestion. As a result, any trihydrate particles reporting to the overflow in this final settling stage
will not be utilized within the process for either seed or product. Effectively such material is
recirculated within the process, creating inefficiencies. Therefore, it is important to achieve the
lowest possible concentration of solids in the overflow of the last stage of classification to maximize
the efficiency of the process.
As described for example in US Patent 5,041 ,269, conventional technology employs
the addition of synthetic water soluble polyacrylate flocculants and/or dextran flocculants to improve
the settling characteristics of the alumina trihydrate particles in the classification process and reduce
the amount of solids in the spent liquor. While various flocculants are often used in the trihydrate
classification systems of Bayer plants, it is highly desirable to reduce as far as possible, the loss of
solids with the spent liquor.
Thus there is clear need and utility for a method of improving the classification and
flocculation of precipitated alumina trihydrate in the Bayer process. Such improvements would
enhance the efficiency of the production of alumina from bauxite ore.
The art described in this section is not intended to constitute an admission that any
patent, publication or other information referred to herein is "prior art" with respect to this invention,
unless specifically designated as such. In addition, this section should not be construed to mean that
a search has been made or that no other pertinent information as defined in 37 CFR § 1.56(a) exists.
Brief Summary of the Invention
At least one embodiment of the invention is directed towards a method for settling
alumina trihydrate in the Bayer process. The process comprises adding to the system an effective
amount of cross-linked dextran or cross-linked dihydroxypropyl cellulose. The cross-linking is the
result of reacting the dextran/dihydroxypropyl cellulose or dextran/dihydroxypropyl cellulosecontaining
substance with a bifunctional cross-linking agent under appropriate reaction conditions.
The use of such a cross-linked dextran or cross-linked dihydroxypropyl cellulose tlocculants results
in improved settling of alumina trihydrate when compared to the use of conventional tlocculants
employed in this process. At least one embodiment of the invention is directed towards a method for
producing alumina comprising the addition of a composition containing one or more
polysaccharides, one of which is cross-linked dextran or cross-linked dihydroxypropyl cellulose to
liquor of a Bayer process fluid stream. The composition may be added to said liquor in a trihydrate
classification circuit of said alumina production process. The composition may be added to said
liquor at one or more locations in said process where solid-liquid separation occurs. The addition
locations may facilitate inhibiting the rate of nucleation of one or more alumina trihydrate crystals in
said process. The addition location may facilitate reducing the rate of scale formation in said
process. The composition may improve the yield of alumina trihydrate sequestration.
Detailed Description of the Invention
For purposes of this application the defini tion of these terms is as follows:
"Dextran" is an ct-D-1,6 glucose-linked glucan with side chains 1-3 linked to the
backbone units of the biopolymer.
"Dihydroxypropyl cellulose" is a cellulose derivative with the addition of 1,2-
dihydroxypropyl ether group to the cellulose backbone.
"Liquor" or "Bayer liquor" is liquid medium that has run through a Bayer process in
an industrial facility.
In the event that the above definitions or a description stated elsewhere in this
application is inconsistent with a meaning (explicit or implicit) which is commonly used, in a
dictionary, or stated in a source incorporated by reference into this application, the application and
the claim terms in particular are understood to be construed according to the definition or description
in this application, and not according to the common definition, dictionary definition, or the
definition that was incorporated by reference. In light of the above, in the event that a term can only
be understood if it is construed by a dictionary, if the term is defined by the Kirk-Othmer
Encyclopedia of Chemical Technology, 5th Edition, (2005), (Published by Wiley, John & Sons, Inc.)
this definition shall control how the term is to be defined in the claims.
In at least one embodiment, a process for extracting alumina trihydrate comprises the
digestion of pretreated bauxite ore in an alkaline liquor to produce a slurry of red mud solids and
aluminate in suspension in the alkaline liquor then decanting the red mud solids from the alkaline
liquor suspension to produce the decanting liquor; the passing of said decanting liquor through
security filtration to remove all solids, precipitation and production of a slurry containing alumina
trihydrate solids which then are flocculated and settled with the addition of a cross-linked
polysaccharide. Larger trihydrate particles are put through the calcination process to produce
purified alumina while finer particles are re-used as seed for the precipitation process.
In at least one embodiment the preferred flocculant of the trihydrate solids in the
process is a crosslinked polysaccharide and the preferred polysaccharides are dextran and
dihydroxypropyl cellulose. The flocculant is added in the range of 0.1 to 100 ppm. The most
preferred dose range for the flocculant is 0.3 to 20 ppm.
In at least one embodiment a cross-linked dextran or cross-linked dihydroxypropyl
cellulose is produced by addition of dextran or dihydroxypropyl cellulose to an alkaline solution
containing sodium hydroxide, potassium hydroxide, or other alkali metals or water soluble alkaline
earth metal hydroxide, to provide a causticized polymer solution having a pH in the range of 11 to
14. The causticized polysaccharide is then reacted with an appropriate Afunctional cross-linking
agent. Suitable cross-linking agents able to react with two or more hydroxy! groups include but are
not limited to epichlorohydrin, dichloroglycero!s, divinyi sulfone, bisepoxide, phosphorus
oxychloride, trimetaphosphates, dicarboxylic acid anhydride, N,N ' -methylenebisacrylamide; 2,4,6-
trichloro-s-triazine and the like. The cross-linking with one of the above reagents results in the
causticized polymer solution becoming a highly viscous solution or paste. When an optimum desired
solution viscosity is reached, the reaction can be terminated via pH neutralization of the solution
with an appropriate acidic solution examples of which are acetic acid, sulfuric acid, hydrochloric
acid and the like.
As described at least in US Patents 6,726,845, 6,740,249, 3,085,853, 5,008,089,
5,041,269, 5,091,159, 5,106,599, 5,346,628 and 5,71 6,530 and Australian Patents 5310690 and
737191, dextran itself has previously been used in the Bayer Process.
However, by cross-linking the dextran or dihydroxypropyl cellulose chains (or for that
matter, other suitable polysaccharides), superior and unexpected improvements are observed in the
activity of cross-linked material when compared to conventional polysaccharides or the uncrosslinked
analog. Prior art uses of polysaccharides are impaired by the fact that increasing dosages of
polysaccharides in Bayer liquor result in superior flocculation only up to a maximum dosage. After
the maximum dosage has been reached, further addition of such polysaccharide material typically
produces no furtlier performance improvement. When the cross-linked polysaccharides are used and
in particular when cross-linked dextran is used, superior performance (not possible at any dose rate
using conventional polysaccharides) can be achieved. Surprisingly the maximum performance of
cross-linked dextran is superior to the maximum performance using conventional dextran at any
dose. Additionally, for cross- linked polysaccharides, the dose at which continued addition results in
no further performance benefits is increased.
Furthermore, when the polysaccharide is cross-linked an unexpected 50% increase in
effectiveness has been observed. For example, a composition comprising 5% cross-linked dextran
will perform at least as well as a 10% composition of dextran, and in some cases better.
US Patents 5,049,612 and 4,339,331 teach that in mining applications such as sulfide
ore flotation, it was found that the performance of starch, a traditional flotation depressant, can be
improved after cross-linking. So while it is true that cross-linked polysaccharides have been used in
mining applications such as in US patents 5,049,6 12 and 4,339,331, it is quite unexpected that in
Bayer process applications, the activity of dextran would be significantly improved after crosslinking.
Furthermore, the ability of cross-linked polysaccharides to have up to or at least a 50%
improvement in performance or to increase the maximum effective dosage of polysaccharides is
unexpected and novel. In at least one embodiment the mass ratio of a general cross linking
reagent/polysaccharide can be varied between, but is not limited to, about 0 to 0.2. Specifically, for
epichlorohydrin as the cross linking reagent, the ratio can be varied between, but is not limited to, 0
to 0.1, most preferably 0.005 to 0.08. Appropriate cross-linking is achieved as measured by an
increase in the solution viscosity of at least 10% above the original solution viscosity.
In at least one embodiment the composition is added to liquor in a trihydrate
classification circuit of said alumina trihydrate production process. The composition can be added to
said liquor at one or more locations in a Bayer process where solid-liquid separation occurs.
In at least one embodiment the composition can be added to said liquor at one or more
locations in a Bayer process where it inhibits the rate of nucleation of one or more alumina hydrate
crystals in said process.
In at least one embodiment the composition can be added to said liquor at one or more
locations in a Bayer process where it reduces the rate of scale formation in said process.
In at least one embodiment the composition can be added to said liquor at one or more
locations In a Bayer process where it facilitates red mud clarification in the process.
In at least one embodiment the composition can be added in combination with or
according to any of the compositions and methods disclosed in commonly owned and at least
partially co-invented co-pending patent application having an attorney docket number of 7987 and a
title of "THE RECOVERY OF ALUMINA TRIHYDRATE DURING THE BAYER PROCESS
USING SCLEROGLUCAN."
EXAMPLES
The foregoing may be better understood by reference to the following
examples, which are presented for purposes of illustration and are not intended to limit the
scope of the invention.
Example 1:
A series of cross-linked dextran products were produced using a conventional
cross-linking process familiar to those skilled in the art where dextran (commercially
available from Sigma-Aldrich) was added to a caustic solution and subsequently cross linked
by reacting with epichlorohydrin. Within this method a variety of epichlorohydrin / dextran
ratios varying from 0.030 to 0.055 were used to produce a range of materials with different
levels of cross-linking, which were monitored through the increase of solution viscosity.
These were denoted as products A - D. The performance of these cross-linked dextran
products was compared to the performance of dextran in a series of settling tests using the
following method.
A series of 200mL samples of a Bayer process slurry were prepared each
comprising 5()g/L aluminum trihydrate solids (DF225 aluminum trihydrate, commercially
available from RJ Marshall Co, USA) and Bayer process liquor (with total caustic 233.6g/l as
Na^CCh). The Bayer process liquor samples were each equilibrated at 60°C in 250ml
Nalgene bottles for 1 hour. Then the aluminum trihydrate solids were added to the bottles and
mixed for 30 seconds. Dextran or its cross-linked analogs were then added as appropriate to
individual bottles containing the hot slurry and the bottles were mixed for 1 minute and then
left to settle for 3 minutes. The unsettled solids from each bottle, (and hence an indication of
flocculation performance) was measured by filtering a 60ml aliquot of slurry taken from the
top of the liquor after the 3 minute settling period. Each sub-sample was filtered through a
pre-weighed No. 934 AH filter paper and washed with hot deionized water. The filter paper
and contents were then dried at 100°C and reweighed. Solids content of the 60mL subsample
were then calculated in g/L. From the results listed in ' Fable 1, it is evident that,
compared to the use of dextran, the flocculation performance was significantly improved for
all cross-linked dextran products. This was evident across the whole range of cross-linking
ratios.
Table 1: Settling tests of standard aluminum trihydrale with addition of dextran and crosslinked
dextrans
Example 2 :
The same flocculation test method as that detailed in example 1 was used in this
example. However, the performance of the products at two separate dose rates was assessed in this
test. Additionally, another cross-linked dextran product (from a reaction where a
epichlorohydrin/dextran ratio of 0.0575 was used) was also assessed across the two dose rates. This
product was denoted as product E. Results of are listed in Table 2. With only one exception, all
cross-linked analogs outperformed dextran at both dose rates (from 1 to 2ppm).
Table 2: Settling tests of standard aluminum trihydrate with addition of dextran and crosslinked
dextrans
Example 3:
The same flocculation test method as that detailed in examples 1 and 2 was used in
this example. However, a series of cross-linked dextran products, denoted G-J were used in
addition to product E. In the manufacture of these products a fixed ratio of epichlorohydrin to
dextran was used but the reaction time was varied in the range from 4 hours to 16 hours. Results of
are listed in Table 3. Those products with a shorter reaction time (denoted as products G, H and I),
which have substantially less cross-linliing of the dextran molecules, show no performance benefit
versus dextran. However, those products where substantial cross-linking has taken place due to a
longer reaction time (J and E), demonstrate superior flocculation performance versus dextran.
Table 3. Settl ing tests of standard aluminum trihydrate with addition of dextran and crosslinked
dextran samples.
Example 4 :
In this example, a series of one litre samples of fresh Bayer plant Secondary Overflow
slurry (containing about 140 g/L solids) were collected from an operating plant in individual one litre
bottles. These were then stored in an oven at 75°C. After equilibration at temperature, the samples
were transferred to individual one litre cylinders and conventional settling tests using a gang plunger
were conducted on the slurry samples. Treatments using dextran and cross-linked dextran (product
E) were compared at a variety of doses as detailed in Table 4. After mixing of the flocculant, the
slurry was allowed to settle for 4 minutes before removal of a 60ml sub-sample from the top of the
cylinder. Samples from each treated slurry were filtered using a 0.45 micron glass microfibre filter
paper, washed with hot deionized water and then dried. The mass of solids collected in the samples
were then determined and recorded as an indication of flocculation performance. Table 4 shows the
results of each treatment as the unsettled solids (reported in g/L). Again it is apparent that compared
to dextran, the addition of cross-linked dextran can significantly reduce the amount of unsettled
solids that would normally report to the overflow of the settling vessel. At all doses, the crosslinked
product reduces the amount of unsettled solids. This example also surprisingly shows that,
while increasing doses of dextran beyond 1.6ppm results in no further reduction in solids, increasing
doses of the cross-linked product (E) results in a reduced amount of solids across the whole dose
range. This result surprisingly indicates that while the maximum benefit of the dextran is achieved
within this dose range, further improvements in fiocculation across this entire dose range are
achieved using the cross-linked product.
Table 4 : Settling tests of seed secondary overflow with addition of dextran and cross-linked
dextran at different dosages
Example 5.
Fresh Bayer plant Secondary Overflow slurry (containing approximately 67g/L solids)
was collected from an operating plant and placed into a series of one litre measuring cylinders.
These were then equilibrated and stored in a waterbath at 65°C. A conventional settling lest using a
gang plunger was then conducted on the slurry samples. After mixing of the treatments, settling rates
were determined by measuring the time taken for the solid/liquor interface to pass the 600ml mark
on each cylinder. Samples were allowed to settle for 4 minutes then a 50ml sub-sample of slurry was
taken from the top of each cylinder and the soli ds content determined as outlined in example 4. in
this example, a cross-linked dextran (denoted as product Q) was used. It was produced from a
reaction using an epich!orohydrin/dextran mass ratio of 0.02. Treatments using dextran and crosslinked
dextran (product Q) were compared at a variety of doses as detailed in Table 5.
Table 5: Settling tests of seed secondary overflow with addition of dextran and cross-linked dextran
at different dosages
The data in table 5 indicates that even when cross-linked dextran was used at
significantly lower dosages compared to dextran, superior settling rates and residual solids levels at
or close to that observed with dextran treatment (at much higher dosage rates) were achieved.
Example 6.
A similar method to that used in example 5 was also employed in this example. The
slurry sample collected from the plant contained 67g/L solids. In this example a series of products
containing a mixture of dextran and cross-linked dextran were assessed. A product containing both
dextran and cross-linked dextran together was formulated. The dextran/cross-linked dextran ratio of
this product was approximately 10:1. The settl ing performance of this product (denoted Z) was
assessed in a settling test and compared to the activity of dextran and cross-linked dextran (product
Q) alone. Results are shown in Table 6.
Table 6: Settling tests of seed secondary overflow with addition of dextran, cross-linked dextran (Q)
and dextran/cross-linked dextran combinations.
A combination of dextran and cross-linked dextran together (product Z) improves
both settling rate and overflow solids content when compared to the individual components when
used alone.
Example 7.
A similar method to that used in examples 5 and 6 was also employed in this example. The solids
content of the slurry used was 84g/L. Dextran at various doses was compared to a cross-linked
dextran (product Q) applied at much lower doses. Results are shown in Table 7.
Fable 7: Settling tests and overflow solids of seed secondary overflow with addition of dextran or
cross-linked dextran.
Cross-linked dextran, when applied at low doses, shows faster settling rate and reduced overflow
solids compared to dextran.
Example 8.
A similar method to that used in examples 5,6 and 7 was also employed in this
example. The solids content of the slurry used was 67g/L. Two products containing combinations
of dextran and cross-linked dextran (denoted S and T) were compared to the use of cross-linked
dextran (product Q). Results are shown in Table 8.
Table 8 : Settling tests of seed secondary overflow with addition of dextran, cross-linked dextran (Q)
and dextran/cross-linked dextran combinations.
The addition of dextran to cross-linked dextran in products S and T improves perfomiance in both
settling rate and clarity.
Example 9:
The same flocculation test method as that detailed in example 1 was used in this
example. However, the products tested were dextran, dihydroxypropylcellulose (W) and its crosslinked
analogs (X, Y). The results are listed in "Fable 9. from the results, it is evident that, compared
to the use of dihydroxypropylcellulose, the flocculation performance was significantly improved
after cross-linking. Furthermore, the cross-linked dihydroxypropyl cellulose outperformed dextran in
overflow solids reduction.
Table 9: Settling tests of standard aluminum trihydrate with addition of dihydroxypropyl
cellulose (W) and cross-linked dihydroxypropyl cellulose (X, Y)
While this invention may be embodied in many different forms, there are shown in
the drawings and described in detail herein specific preferred embodiments of the invention. The
present disclosure is an exemplification of the background and principles of the invention and is not
intended to limit the invention to the particular embodiments illustrated. All patents, patent
applications, scientific papers, and any other referenced materials mentioned anywhere herein are
incorporated by reference in their entirety. Furthermore, the invention encompasses any possible
combination of some or all of the various embodiments described herein and incorporated herein.
The above disclosure is intended to be illustrative and not exhaustive. This
description will suggest many variations and alternatives to one of ordinary skill in this art. All these
alternatives and variations are intended to be inciuded within the scope of the claims where the term
"comprising" means "including, but not limited to". Those familiar with the art may recognize other
equivalents to the specific embodiments described herein which equivalents are also intended to be
encompassed by the claims.
All ranges and parameters disclosed herein are understood to encompass any and all
subranges subsumed therein, and every number between the endpoints. For example, a stated range
of " 1 to 10" should be considered to include any and all subranges between (and inclusive of) the
minimum value of 1 and the maximum value of 10; that is, all subranges beginning with a minimum
value of 1 or more, (e.g. 1 to 6.1), and ending with a maximum value of 10 or less, (e.g. 2.3 to 9.4, 3
to 8, 4 to 7), and finally to each number 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 contained within the range.
Thi s completes the description of the preferred and alternate embodiments of the
invention. Those skilled in the art may recognize other equivalents to the specific embodiment
described herein which equivalents are intended to be encompassed by the claims attached hereto.
Claims
1. A method of producing alumina comprising the addition of a composition containing one or
more cross-linked polysaccharides to a liquor or slurry of said alumina production process.
2. The method of claim 1 wherein the polysaccharides contains at least one composition made
by cross-linking dextran or dihydroxypropyl cellulose to form a cross-linked dextran or
dihydroxypropyl cellulose molecule.
3. The method of claim 2 wherein the cross-linked polysaccharides are one selected from the list
consisting of: cross-linked scleroglucan, cross-linked pullulan, cross-linked zooglan, cross-linked
lactan, cross-linked rhamsan, and any combination thereof.
4. The The method of claim 1 further comprising the addition of another polymer, which can be
dextran or a synthetic polymer, to said process.
5. The method of claim 1 wherein the composition is added to said liquor in a trihydrate
classification circuit of said alumina production process.
6. The method of c laim 1 wherein the composition is added to said liquor at one or more
locations in said process where solid-liquid separation occurs.
7. The method of claim 1 wherein the composition is added to said liquor at one or more
locations and thereby inhibits the rate of nucleation of one or more alumina trihydrate crystals in said
process.
8. The method of claim 1 wherein the composition is added to said liquor at one or more
locations and thereby reduces the rate of scale formation in said process.
9. The method of claim 1 wherein the composition is added to said liquor at one or more
locations to facilitate red mud clarification in said process.
10. The method of c!aim 1 wherein the composition addition improves the yield of alumina
trihydrate sequestration from an alumina trihydrate process by adding the composition of claim 1 to
said liquor of said process.
11. rhe method of claim 1 wherein the polysaccharide is cross linked with a suitable crosslinking
agent capable of reacting with at least two hydroxy1groups.
12. The method of claim 11 wherein the suitable crosslinking agent is selected from the list
consisting of: epichlorohydrin, dichloroglycerols, divinyl sulfone, bisepoxide, phosphorus
oxychloride, trimetaphosphates, dicarboxylic acid anhydride, N,N'-methylenebisacrylamide; 2,4,6-
trichloro-s-triazine, and any combination thereof.
13. A composition comprising Bayer liquor and a cross-linked polysaccharide.
14. The composition of claim 14 wherein the polysaccharide is dextran or dihydroxypropyl
cellulose.