CHEMICAL TREATEMENT TO IMPROVE RED MUD SEPARATION AND
WASHING IN THE BAYER PROCESS
Background of the Invention
The invention relates to compositions, methods, and apparatuses for
improving the performance of primary settlers and washer circuits for red mud in a
Bayer Process. As described for example in US Patents 6,814,873, 6,033,579,
6,048,463, and US Published Patent Application 2008/0257827, in the Bayer
Process alumina trihydrate is extracted from bauxite ore. Bauxite ore is pulverized
then goes through a digestion stage (where it is slurried in a caustic liquor medium
and is heated under pressure), a clarification stage, a precipitation stage, a
classification stage, and finally a calcination stage from which the final product is
recovered.
In the clarification stage red mud is separated from the alumina in the
ore. Red mud is the large percentage of the ore that remains insoluble after
digestion with caustic medium. This insoluble fraction must be removed prior to the
alumina trihydrate recovery step in the process so as to avoid contaminating the final
Bayer Process product. The digestion slurry consists of finely suspended mud
particles which are commonly removed by the addition of flocculants in large
separation vessels called thickeners or settlers. The flocculant acts to bind the mud
particles increasing their rate of settling in the thickener. The overflow liquor then
typically reports to filters, often referred to as Security Filtration, to remove any
remaining insoluble material prior to alumina trihydrate recovery. The red mud
slurry from the thickener underflow contains valuable alumina and caustic in
solution that is recovered in the Red Mud Washer Circuit otherwise known as a
counter-current decantation (CCD) circuit. Over several stages, mud slurry is mixed
with progressively more dilute (lower in valuable alumina and caustic) wash water.
The advancing wash water progressively increases in caustic and alumina content as
it recovers the valuables from the mud slurry, then ultimately exits as a dilution
stream for use back into the process at an appropriate point. The resulting red mud
is sent to disposal. Some examples of red mud clarification are described in US
Patents 3,085,853, 3,397,953, 3,445,187, 3,541,009, 3,681,012, 4,767,540, and
5,008,089.
Because of the cost associated with each additional filtration and
dilution step, large savings and efficiencies can be realized from reducing the
number or intensity of the required filtration and dilution steps. As a result, there is
clear utility in novel methods of more effectively flocculating and separating red
mud from the red mud-containing liquors.
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
improving the process of separating and washing insoluble red mud solids in the
Bayer process by addition of cross-linked polysaccharides in combination with
flocculants in the primary settling and washing stages of the process.
Detailed Description of the Invention
The following definitions are provided to determine how terms used
in this application, and in particular how the claims, are to be construed. The
organization of the definitions is for convenience only and is not intended to limit
any of the definitions to any particular category.
"Clarifier" means a separation device used for red mud clarification
including: a thickener, a settler, or a washer.
"Consisting Essentially of means that the methods and compositions
may include additional steps, components, ingredients or the like, but only if the
additional steps, components and/or ingredients do not materially alter the basic and
novel characteristics of the claimed methods and compositions.
"Dextran" is a polysaccharide characterized as being an a-D-1,6
glucose-linked glucan with side chains 1-3 linked to the backbone units of the
polysaccharide.
"Distal" is the opposite of "Proximal" and means subsequent to a
particular step in a sequential process.
"Flocculant" means a composition of matter which when added to a
liquid containing finely divided suspended particles, destabilizes and aggregates the
solids through the mechanism of interparticle bridging, it may have a low charge
density and a high molecular weight (in excess of 1,000,000).
"Green Liquor" means the alumina containing liquor that has passed
through a Security Filtration stage and no longer contains red mud.
"Liquor" or "Bayer liquor" means a caustic, liquid medium that has
run through a Bayer process in an industrial facility.
"Polysaccharide" means a polymeric carbohydrate having a plurality
of repeating units comprised of simple sugars, the C-O-C linkage formed between
two such joined simple sugar units in a polysaccharide chain is called a glycosidic
linkage, and continued condensation of monosaccharide units will result in
polysaccharides, common polysaccharides are amylose and cellulose, both made up
of glucose monomers, polysaccharides can have a straight chain or branched
polymer backbone including one or more sugar monomers, common sugar
monomers in polysaccharides include glucose, galactose, arabinose, mannose,
fructose, rahmnose, and xylose.
"Primary Settler Feed" means Bayer process digested slurry as
charged to the first solids/liquid separation stage, it may be an admixture of the
digested slurry plus dilution liquor, and the dilution liquor is routinely the counter
current technique wash water from the red mud washing stages, the primary settler
feed differs from the liquors or slurries subjected to clarification and/or separation in
the subsequent clarification stage or the red mud washing stages by composition as
to the solids content, dissolved sodium aluminate content, and total alkalinity. In
addition, the primary settler feed also differs from the liquors or slurries subjected to
clarification and/or separation in the subsequent clarification stage or the red mud
washing stages in that no insoluble fraction thereof has received an earlier
flocculation treatment.
"Proximal" is the opposite of "Distal" and means prior to a particular
step in a sequential process.
"Red Mud" means the insoluble solid material which is a residual
product from the Bayer liquor that does not freely dissolve during the digestion
stage, or which precipitates as part of the digestion process. Red mud can comprise
ferric oxide (from which the mud typically derives its red-brown color), alumina,
silica, silicon oxide, calcium oxide, sodium alumino-silicates, and/or titanium oxides
(depending on the specific compositions of the ore input into the Bayer process) as
well as caustic and other materials from the digestion liquor that may be part of the
liquid phase of the Bayer process red mud slurry.
"Rheology" means the interrelatedness of the flow rate and the
elastic, viscous and/or plastic properties of flowing matter.
"Slurry" means a mixture comprising a liquid medium within which
fine solid particles are dispersed or suspended.
"Thickener" or "Settler" means a vessel used to effect a solid-liquid
separation of a slurry, often with the addition of flocculants, the vessel constructed
and arranged to receive a slurry, retain the slurry for a period of time sufficient to
allow solid portions of the slurry to settle downward (underflow) away from a more
liquid portion of the slurry (overflow), decant the overflow, and remove the
underflow. Thickener underflow and thickener overflow are often passed on to
filters to further separate solids from liquids.
"Washer" means a vessel used to effect a solid-liquid separation of
red mud from liquid by utilizing counter current decantation separating the material
into an underflow (a highly concentrated suspension typically at the bottom of the
washer), and an overflow (a clarified liquid stream typically at the top of the
equipment), the liquid used to effect the counter current may be water, liquor a
mixture of water and liquor or overflow from elsewhere in the Bayer Process.
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.
At least one embodiment of the invention is directed towards a
method of treating Bayer Process red mud slurry to enhance the recovery of caustic
and alumina. The treatment can occur in settlers and in washer circuits. The
method comprises contacting primary settler feed or washer feed slurry with a
flocculant in combination with a modified polysaccharide. The method modulates
the rheology of thickened red mud allowing higher underflow densities to be
targeted in each vessel of the Wash Circuit or in the primary settlers.
Red mud slurry flow in the clarification stage suffers from a number
of contradictory properties. Ideally the underflow of a settler or washer should have
a very high density. Such a high density results from highly effective removal of
soluble caustic and alumina from the red mud underflow, so effective in fact that the
red mud comprises only insoluble materials. However as solids density in the
underflow increases, the yield stress of the slurry will also increase and
consequently this reduces the ability of the underflow to actually flow through the
equipment used in the clarification process. As a result, in practice the underflow
density must be kept below a degree of thickness even though this means that
valuable alumina and caustic remain within the underflow. This is a costly practice
which reduces the amount of actual alumina that can be recovered from a given
sample of ore to an amount significantly below its theoretical yield.
Additionally, by reducing the level of suspended solids that remain in
the supernatant above the flocculated slurry formed in the primary settler liquor, the
solids to be removed during the subsequent clarification of the overflow by filtration
are diminished.
In at least one embodiment the method alters the rheology (viscosity,
elasticity, and/or plasticity) of the underflow that passes through washer(s) and/or
settler(s) such that the underflow has properties that would otherwise be associated
with an underflow slurry of lower solids density. This allows operators to increase
the underflow density in one, some, or all of the vessels of the circuit, resulting in
better extraction efficiency of alumina and caustic and increased throughput.
In at least one embodiment the underflow passing through at least a
portion of the circuit has a density so high that but for the method, it would not be
capable of passing through the washer(s) and/or settler(s) or would only be able to
do so with the input of a significant amount of energy, effort, and/or cost.
Suitable flocculants generally have molecular weights in excess of
1,000,000 and often in excess of 5,000,000. The flocculants may be anionic,
cationic and/or zwitterionic. The dose of flocculant depends on the properties of
the slurry being treated and can be empirically determined by one of skill in the art.
In general, the flocculant polymer dose required depends on both the nature of the
feed slurry (settler or washer) and the type of flocculant used. However, typically
dose rates are generally within the range of 10-500g/T based on polymer solids, per
ton of red mud and more preferably 30-400g/T.
In at least one embodiment the flocculant is selected from the group
consisting of: (i) homopolymers of acrylic acid, (ii) copolymers of acrylic acid and
acrylamide, (iii) copolymers of acrylic acid and acrylamide modified to contain a
hydroxamic acid moiety; and (iv) copolymers of acrylic acid and acrylamide
modified to contain and comprise ammonium acrylate. In at least one embodiment
the flocculant has a molecular weight greater than 10 million.
Similarly, while anionic polymer flocculants may be formed using
anionic monomers, it is also possible to modify certain nonionic vinyl addition
polymers to form anionically charged polymers. Polymers of this type include, for
example, those prepared by the hydrolysis of polyacrylamide.
The flocculant may be prepared in the solid form, as an aqueous
solution, as a water-in-oil emulsion, or as a dispersion in water. Representative
anionic polymers include copolymers of acrylamide with sodium acrylate and/or 2-
acrylamido 2-methylpropane sulfonic acid (AMPS) or an acrylamide homopolymer
that has been hydrolyzed to convert a portion of the acrylamide groups to acrylic
acid.
In at least one embodiment the polysaccharide is cross-linked. The
cross linking may be achieved with a crosslinking agent. In at least one embodiment
the cross-linking is accomplished by an the interaction with a cross linking agent on
an ethylenically unsaturated monomer either containing at least two sites of
ethylenic unsaturation or containing one site of ethylenic unsaturation and one site
of a reactive group such as an epoxide or an aldehyde. Representative Cross-Linking
Agents include N,N-methylenebisacrylamide, N,N-methylenebismethacrylamide,
polyethylene glycol diacrylate, ethylene glycol dimethacrylate, diethylene glycol
dimethacrylate, triethylene glycol dimethacrylate, polyethylene glycol
dimethacrylate, polypropylene glycol dimethacrylate, N-vinyl acrylamide, divinyl
benzene, triallyl ammonium salts, N-methyl allylacrylamide, glycidyl acrylate,
acrolein, methylolacrylamide, glyoxal, epihalohydrins, dialdehydes diglycidyl
ethers, and the like, and any combination thereof. In at least one embodiment the
crosslinking agent is added to the cross-linked polymers at a dosage of from about
0.0001 to about 10, preferably from about 0.0001 to about 0.2 weight percent based
on the weight of the polymers.
In at least one embodiment, upon flocculation of a primary settler
feed, a clear liquor/mud interface will form, and will gradually settle, providing a
clear liquor supernatant layer overlying a mud layer. The lower mud layer contains
the flocculated material. The overlying supernatant is the liquor that is separated for
subsequent filtration and contains only a minimal amount of mud solids. The
invention diminishes the amount of suspended solids in such supernatant, and hence
decreases the extent of filtration required to obtain a given purity of sodium
aluminate solution.
In at least one embodiment, the solids captured by the combination of
the flocculant and modified polysaccharide contain more of the minerals and solids
comprised of silica and potassium such that the solids reporting to the overflow of
the clarifier contain a lower amount of such materials, resulting in selective
flocculation and settling of such materials.
In at least one embodiment, the polymeric flocculant and the
polysaccharide are both added to the primary settler feed as aqueous solutions to
facilitate rapid dispersion of each agent within the primary settler feed.
In at least one embodiment the method is conducted according to
and/or in conjunction with some or all of one or more of the: compositions,
apparatuses, and methods used in the Bayer Process described in one or more of: US
Patents 6,365,116, 6,726,845, 5,217,620, 5,478,477, 5,387,405, International Patent
Application WO 99/29626, and scientific papers: Step change improvements in
underflow rheology by BergerA. et al., Proceedings of the 14th International
Seminar on Paste and Thickened Tailings, pp. 135-141 (201 1), and Effect of
Surfactants on Bauxite Residues Suspensions Viscosity by Frost, R, et al, Colloids
and Surfaces A: Physicochemical and Engineering Aspects, 292(1), pp. 2 1 - 26
(2007).
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.
Red mud settling tests were performed in order to evaluate the effect
on liquor overflow clarity and settling rate with and without addition of a crosslinked
polysaccharide to a standard anionic flocculant treatment. These tests
involved mixing a known amount of flocculant solution (and polysaccharide where
appropriate) into a cylinder containing Bayer process red mud slurry. After mixing,
the cylinders were left to settle and after a given period of time the liquor at the
surface of the cylinder was sampled, filtered and the filtered residue weighed. The
clarity of the liquor is described in terms of overflow solids with units of gram per
litre (essentially the lower the overflow solids the more effective the chemical
treatment strategy).
Example 1
Samples of plant settler feed slurry were treated using a 0.1%
solution of a commercially available 100% anionic latex flocculant and a 13%
solution of a modified (cross-linked) polysaccharide. In this case the modified
polysaccharide was dextran. Table 1 details the overflow solids for red mud slurry
flocculated with (i) anionic flocculant and (ii) a combination of anionic flocculant
and the cross-linked polysaccharide. Addition of modified polysaccharide to the
slurry, at a dose rate in the order of 30 ppm, resulted in approximately 50%
reduction in overflow solids. A substantial reduction in overflow solids was
achieved over the flocculant dose range of 40 - 130 g/T.
Table 1. Overflow solids measured from red mud settling tests using a standard
anionic flocculant with and without additional treatement with modified
polysaccharide.
AF 85 0 1.19
AF + MPS 85 4.2 0.45
AF 128 0 0.71
AF + MPS 128 4.4 0.44
AF = Anionic Flocculant
MPS = Modified polysaccharide (cross-linked dextran)
Example 2
The method of application of the modified polysaccharide was
assessed by addition of a modified polysaccharide prior to, with or after the
flocculant addition. The anionic flocculant and modified polysaccharide were the
same as those used in example 1 as was the test method. Slurry used was again
plant settler feed. Table 2 details the overflow solids when the modified
polysaccharide was pre-dosed, and co-dosed with the anionic flocculant in one set of
tests and and when it was co-dosed and post-dosed with the anionic flocculant in a
second set of tests. The modified polysaccharide was effective in all methods of
application, reducing the overflow solids when compared to the same dose of
anionic flocculant applied without additional modified polysaccharide.
Table 2. Overflow data from settling tests comparing the method of application of a
modified polysaccharide when applied with a conventional anionic flocculant.
Example 3
The use of polysaccharides such as dextran has previously been
identified and used in combination with anionic flocculants as a clarity aid in red
mud settlers (as described, for example, in US 3,085,853). Red mud settling tests, as
described above, were conducted to assess the relative efficacy of a modified (crosslinked)
polysaccharide compared to the same unmodified polysaccharide. The
anionic flocculant and modified polysaccharide were the same as those used in
example 1. The modified and unmodified polysaccharide solutionss contained the
same amount of polysaccharide in each treatment. Table 3 details the overflow
solids of red mud treated with a combination of (i) anionic flocculant and crosslinked
dextran and (ii) anionic flocculant with dextran. In this example a
commercially available, 100% anionic flocculant was again used but it was a
different product to that used in examples 1 and 2.
Table 3 : Overflow solids from red mud testing with modified polysaccharide
and unmodified polysaccharide combinations with anionic flocculant.
The data demonstrates that addition of modified polysaccharide is surprisingly more
effective at reducing overflow solids compared with the use of unmodified
polysaccharide.
Example 4
Analysis of the solids obtained from settling tests conducted as described in the
previous examples was completed to determine the nature of the solids removed by
the application of modified polysaccharide. Red mud settling tests were completed
using a combination of commercially available hydroxamated anionic flocculant
both with and without additional application of a cross linked polysaccharide. The
treatment and overflow solids obtained is listed in table 4.
Table 4. Overflow solids from red mud settling tests using a hydroxamated
flocculant and modified polysaccharide.
Additionally, samples of the overflow solids were collected, dried and subjected to
X-ray fluorescence spectroscopy (XRF) to determine the elemental components
present in the overflow. Table 5 shows the amount and relative change in a range of
elements analyzed in the treated samples. Surprisingly treatment with the crosslinked
polysaccharide results in significantly less silica and potassium containing
materials reporting in the overflow solids. When compared to the reduction in the
relative amounts of other elements such as iron, titanium and calcium, there is a lot
less silica and potassium in the treated samples. This indicates that the modified
polysaccharide is selective in removing insoluble minerals which may be high in
silica and potassium content. Examples of such minerals would be muscovite or
mica.
Table 5. Concentration of product (expressed as ppm or mg of analyte per litre of
overflow)
Example 5
Raked settling tests conducted in an Imhoff cone were performed to
evaluate the effect of a flocculant / modified polysaccharide combination on the
flow characteristics and rheology of flocculated red mud. These tests involved
mixing one litre of Bayer plant final washer feed red mud slurry together with a
known amount of conventional flocculant solution and modified polysaccharide
solution in a cylinder. After mixing, the slurry was immediately transferred to an
Imhoff cone and the slurry was then raked to a specific mud bed volume (bed
underflow density was calculated from the bed volume and known feed solids
concentration). The Imhoff cone plug was then released and the time taken for the
thickened slurry to discharge from the cone was measured. The time required for
discharge of the entire slurry is a measure of mud flow-ability (or rheology). This
discharge time was compared with feed slurry treated with flocculant alone (the
'normal' process for the settler and washer circuit). The faster the discharge rate the
better the mud underflow rheology and ultimately properties for pumping from the
thickener underflow.
Table 6 details the average discharge rate of the thickened red mud
slurry from the raked Imhoff cone after treatment with flocculant and after treatment
with the combination of flocculant and modified polysaccharide. The flocculant
dose was kept constant at 100 g/T while the dose of modified polysaccharide (when
applied) was 3.75ppm. In this case, the modified polysaccharide used was a crosslinked
dextran. The flocculant used was a conventional polyacrylate/polyacrylamide
flocculant commercially available and typically used for red mud settling in
washers.
Table 6 : Average time taken to discharge consolidated red mud slurry from
an Imhoff cone across a range of average bed underflow densities.
Flocculant dose was constant at 100 g/T. Polysaccharide dose was 3.75ppm.
Raking of slurry treated with conventional flocculant to an average
underflow density of greater than 21%, resulted in effectively no flow when the plug
was removed. Effectively the slurry did not discharge from the Imhoff cone due to
the poor flow characteristics of the raked, consolidated mud at the base of the cone.
This was compared to the results obtained after raking slurry that was
treated with the same dose of conventional flocculant together with modified
polysaccharide (3.75ppm). Across a similar range of high average underflow
densities, slurry discharged from the cone without restriction (-100 mL/sec). This
is a significant increase in the flow properties of the red mud when compared to the
control test (flocculant only treatment).
In a separate test mud was treated in a similar manner (the same
flocculant at the same dose) but the modified polysaccharide dose was reduced to
0.75ppm. When consolidated to an average underflow density of 23.8g/100mL a
discharge time of 19 seconds was recorded (average discharge rate of 53ml/sec).
Comparing this to the data from Table 1, consolidation of mud to this concentration
without addition of modified polysaccharide would be expected to result in no flow,
while a higher dose of modified polysaccharide (3.75) clearly results in substantially
higher flow rates.
The results of these tests indicate that addition of flocculant and
polysaccharide in combination to a red mud washer circuit or primary settler would
enable the targeted underflow density to be substantially increased while fluid
characteristics of the underflow solids could be maintained or enhanced.
While this invention may be embodied in many different forms, there
are described in detail herein specific preferred embodiments of the invention. The
present disclosure is an exemplification of the 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
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/or incorporated herein. In addition the invention encompasses
any possible combination that also specifically excludes any one or some of the
various embodiments described herein and/or 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 included
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. All percentages, ratios and proportions herein are by
weight unless otherwise specified.
This 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 for treating Bayer Process liquor containing red mud comprising:
(a) adding to the slurry a flocculant and a cross-linked polysaccharide and (b)
removing flocculated red mud contained in the Bayer process liquor by at least one
of: sedimentation, centrifugation, filtration the use of a washer, and the use of a
settler.
2. The method of claim 1 in which the cross-linked polysaccharide is added to
the slurry prior to the addition of the flocculant.
3. The method of claim 1 in which the cross-linked polysaccharide is added to
the slurry after the addition of the flocculant.
4. The method of claim 1 in which the cross-linked polysaccharide is added to
the liquor at the same time as the addition of the flocculant.
5. The method of claim 1 in which the cross-linked polysaccharide and the
flocculant are added to liquor containing red mud which is primary settler feed or
feed slurry of a red mud washer.
6. The method of claim 5 further comprising the step of increasing the density
of the flocculated red mud to an amount such that but for the presence of the
flocculant with the cross-linked polysaccharide, the flocculated red mud would have
a rheology that would prevent its free flow as an underflow through a settler or
washer.
7. The method of claim 1 in which the flocculant is selected from the list
consisting of: (i) homopolymers of acrylic acid, (ii) copolymers of acrylic acid and
acrylamide, (iii) copolymers of acrylic acid and acrylamide modified to contain a
hydroxamic acid moiety, and (iv) copolymers of acrylic acid and acrylamide
modified to contain an comprises ammonium acrylate, and (v) any combination
thereof.
8. The method of claim 1 in which the cross-linked polysaccharide comprises
two or more different types of polysaccharide polymers.
9. The method of claim 1 in which the cross-linked polysaccharide is crosslinked
dextran.
10. The method of claim 1 in which the flocculant has a molecular weight of
greater than 10 million.
11. The method of claim 1 in which the method further comprises the step of
raking the liquor after the addition of the flocculant and a cross-linked
polysaccharide.
12. The method of claim 1 comprising the step of reducing the overflow solids in
a clarifying vessel.
13. The method of claim 1 comprising the step of selectively reducing silicon
and potassium containing minerals in the overflow solids.