Note: Descriptions are shown in the official language in which they were submitted.
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Reduction of Impurities in Bayer Process Alumina T,il.y.l
Field of the Invention
-
The present invention is directed to a pr~,cess of alumina manufacture via the Bayer
10 process. More particularly, it is concer,-ed with reducing the level of impuriUes, especially
soda, in alumina trihydrate produced in the Bayer alumina process by contacting an alumina
trihydrate-containing process stream with a polymer which contains hyd,oxa",-c acid groups
or salts thereof, to thereby floccl~'-te alumina trihydrate, and s~bjectirlg the resultant
floccu'oted alumina trihydrate to centrifugation.
B~ckground of the Invention
The almost universally used ploces~ for the manufacture of alumina is the Bayer
prucess. In a typical commercial ~3ayer process, raw bauxite ore is pulverized to a finely
20 divided state. The pulverized ore is then fed to a slurry mixer where a slurry is ~ ~epan:d
using water, spent liquor and added caustic. This bauxite slurry is then diluted and sent
through a series of digesters where, at about 300-- 800' F. and 100-2000 p.s.i., most of
the total available alumina is extracted from the ore which may contain both trihydrate and
monohydrate forms of alumina. The effluent from the digesters passes through a series of
25 flash tanks wherein heat and condensate are recovered as the digested slurry is cooled to
about 230' F. and brought to atmospheric pressure. The aluminate liquor leaving the
flashing operation (blow-off discharge) contains about 1-20% solids, which consist of the
insoluble residues that remain after reaction between the bauxite ore and basic material
used to digest the ore and the insoluble products which precipitate during digestion. Herein,
30 all percentages are by weight, based on total weight, unless otherwise stated. The coarser
solid particles are then generally removed with a "sand trap" cyclone. To separate the finer
solid particles from the liquor, the slurry is normally fed to the center well of a primary mud
settler where it is treated with a flocculant such as a polyacrylamide polymer, polyacrylate
polymer, hydroxamated polymer, flour and/or starch. As the mud settles, clarified sodium
35 aluminate solution, referred to as Ugreen" or "pregnant" liquor, overflows a weir at the top
of the mud settling tank and is passed to the subsequent process steps. The settled solids
(Ured mud") are will-dl~w,l as underflow from the bottom of the primary mud settler and
passed through a countercurrent washing circuit, generally co, ~ ri~ed of a series of
washers, for recovery of sodium aluminate and soda. Aluminate liquor OV~IO~L~9 the
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primary settler still contains typically 50 to 200 milligrams (mg) of suspended solids per liter.
This liquor is then generally further clarified by filtration to give a filtrate with less than about
10 mg suspended solids per liter of liquor.
The aluminate liquor filtrate is typically cooled and routed to a precipitator, where
seeding of the liquor may take place. A series of precipitators may be used. Alumina, in
relatively pure form, is then precipitated from the filtrate as alumina trihydrate crystals. The
alumina trihydrate suspension, or slurry, may then be fed to a series of decanters, or
classifiers, which classify the trihydrate according to particle size. Ordinarily, some of the
classifier exit streams are product streams and some are seed streams. For instance,
underflow from the primary classifier is typically a product stream. Overflow from the
primary classifier may be a feed stream for a secondary classifier, the underflow of which
may be a product stream or a seed stream, or both. Secondary classifier overflow is
typically a feed stream for a tertiary classifier, or tertiary thickener, the underflow of which
is usually a seed stream, and the overflow of which is generally routed back to the ore
digester. The trihydrate crystals suspended in the cl~sifier product streams are generally
washed in a hydrate tank and filtered to remove soda (sodium salts e.g Na2O and NaOH)
and other impurities. The resulting trihydrate filter cake is then dried and calcined to give
alumina trihydrate product that is suit~hlQ for commercial purposes. The seed streams from
the classifiers, which tend to contain smaller trihydrate crystals than the product streams,
are usually routed back to the precipitators to supply seed crystals for subsequent
precipitations. The remaining liquid phase or spent liquor is returned to the initial ore
digestion step and employed as a digestant of additional ore after being reconstituted with
additional caustic.
U.S. Patent No. 4,614,642 discloses a method of producing alumina trihydrate in
which an alumina trihydrate suspension is subjected to a separating method, which may be
chosen from the group comprising decanting, cycloning, filtering, and/or centrifuging, to
produce a fraction containing fine particles. The fraction containing the fine particles is then
sl~bject~d to a known type of treatment. including partially dissolvil1g the fine particles or
chen,ical agglomeration of the fine particles, so as to reduce their number by at least 50%.
Because of the rheological characteristics of the classifier streams, centrifugation is
not typically used to separate suspended trihydrate from product or seed streams. Instead,
separation of the solids is generally accomplished by the use of filters and/or other
classi~ier~i. Classifiers rely on the gravitational settling of the solids to achieve separation.
Polymer flocculants may be added to some streams to increase the efficiency of separation.
~ Flocculstion of the solids aids in the settling process by tending to agglomerate smaller
<)~1~ . L ~'A/L,~ iw i j h ~ s ;~ t . (-- t ' l l / T~ I .>-~
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~articles .nto larger ones, which tend to settle faster. Floccu~ati~n also aid In ~ne filterin~a
proceSs because larger a~lomerates are easier to filter th.an ~maller ones, and lecs li~:e)y to
plug the tiltering rneans. Dewatenng aids such as those desctibed in U.S. Patent No
~,451,32g rnay also De added to the classifier streams to reduGe the water lavel-in the iilter
5 cal~e.
Because the alumlna is forrned in a sodiurr hydroxide environn)ent, it ~enerallycontair,s a significant am~unt of soda ~typically 0.3% to 0.4~,'0 for product alurnina) as well as
other impurities. The total soda is ptesent as leachable soda, which can be rem~ved ~y
wa~hin~, and n~nleachable sod~, which cannot normally b~ removed by washing beoause it
10 is contained with~n th~ alurnina trihydrate eryst~is. Fcr rr~st alumina use~. such as electrolytic
prodL~ction of aluminum rnetal or formation of ordinar~ ceramic products, the alumina is usa~e
even with these hi~h levels of impufities. For a number of othe~ applicattons. however, these
impurity levels (particuiarly hlgh s~da anr~ siJica leYeis) are unaccept2bl~. T~ese applications
ir~cllJcle products inlended for such uses as synthetic sapphir~ and as tfanslucent bodies~
15 Also, since the soda in ~e alurnina is not available to be recycled b2clc into the Bayer process,
it mus~ be replaced in the ptocess at significant additional cost.
i iuch e~fort has beetl devoted to producing alumfna with r~uce~ levels of impurities,
includin~ soda. In IJ,S, Patent No. ~,560,541, a process is descrii~ed which involves, inter alia.
reac~r,~ alumina with hydrochloric acid and adding water to dis~olve tt~e alurninum-containing
20 rzaction product, then separatin~ the solution from the insolut)l6 impurities by such methods
as centrifugation and f~ltration. The removal of iron-conta~ning impuritles is taught in U.S.
Patent No 3.607,140, which process involves the separat~on of iron-containing ~lumina
hydrate i~y runnlng the moving stream through a liquia cyc~one, centri~uge, ~ilter, or ~he iike.
The removal of organic impurities from SPent liqtJor b~ concentr~ting a soluti~n to preCipitatQ
25 the or~anic resrdues, then filtering or centrifuging to remove ~he pre~ipitates is revealed in U.
S. Patent No. 2,~81.600. In none of these three patents ~as a polymer flocclJlant utilized.
A reduction in the turbrdity of Bayer pr~cess liqu~rs contalning a cationic p~lyrner-
humate comPIex was achieved in U S. Patent No. ~,133,874 by adding a sec~Jn~ cationic
polyrner to fl~~ t3 Ihe complex and separating the fiocc:~lated polymer-h~Jmate con~plex by
3Q filtration. Gentrifugation or li~e. The flocculatl~n and separation tal~e plaoe before the alumina
trihy~rate is precl~it~led. so the polyrners are remo~ed by the separation process and are not
available t~ flocculate the su~sequentlv p, ~ipi:ated alurnina trihyrlrate.
NDED S~
lPEA/EP
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Polymers containing hydroxamic acid groups for flocculation of suspended solids in
Bayer process streams are described in U.S. Patent No. 4,767,540, which is hereby
incorporated herein by reference. This patent does not disciose the use of centrifugation
in combination with the use of these polymers.
SU~Jr;S;~IYIY~ it has now been discovered that the level of soda in alumina trihydrate
is lower when polymer flocculants which contain hydroxamic acid groups are used in
combination with centrifugation to dewater alumina trihydrate suspensions, than when
centrifugation alone is used.
The processes of the present invention are designed to reduce the level of soda and
other impurities in alumina trihydrate made by the Bayer process. The improvement
forming the basis of the present invention lies in the centrifugation of alumina trihydrate
crystals that have been floccuiated using polymers that contain hydroxamic acid groups, as
compared to centrifugation of trihydrate crystals that were not flocculated with such
polymers.
Detailed Description of the Preferred Embodiments
According to the present invention, there is provided a process for reducing thelevels of impurities, particularly soda, in alumina trihydrate made by the Bayer process
whereby a polymer contz,ning hydroxamic acid groups or salts thereof is added to a
suspension of alumina trihydrate to flocculate the trihydrate, and the resulting flocculated
solids are dewatered by centrifugation.
The hydroxamated polymer to be employed in the present invention can vary ratherbroadly in type. It should be sufficiently stable to be effective under the process conditions
used, e.g., high temperatures and strong caustic conditions, typically, 185' - 225' F., and
80-400 grams per liter total alkali content (expressed as sodium carbonate equivalent).
Thus, for example. any water-soluble hydroxa"~ic acid or salt group-containing
polymer may be used in the process of the present invention. The useful polymers can
best be exemplified by those containing pendant groups of the Formula (1):
(1) --C Nll OR
3~?
,
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wherein R is hydrogen or a cation. These polymers are well known in the art and can be
derived from polymers containing pendant ester, amide, anhydride, nitrile, etc., groups by
the reaction thereof with a hydroxylamine or its salt, or by polymerization of a monomer
which contains a hydroxamic acid or salt group. Hydroxamated polymers derived from
polymers containing amide groups e.g polyacrylamides are preferred.
Exemplary of the polymers which may be hydroxamated for use in the process of
the present invention are acrylic, methacrylic. crotonic etc., acid ester polymers such as
polymers produced from the polymerization of methyl acrylate, ethyl acrylate, t-butyl
acrylate, methyl methacrylate, ethyl methacrylate, cyclohexyl methacrylate, dimethyl
aminoethyl methacrylate, dimethyl aminoethyl acrylate, methyl crotonate, etc., polymers of
maleic anhydride and esters thereof, and the like; nitrile polymers such as those produced
from acrylonitrile etc; amide polymers such as those produced from acrylamide,
methacrylamide and the like.
Hydroxamated polymers are well known to those skilled in the art and are
specifically di;,closed, as are methods for their production, in U.K. Patent ~ tion
2171127 and U.S. Pat. Nos. 3,345,344; 4,480,067, 4,532,046; 4,536,296; 4,587,306;
4,767,540; 4,902,751; and 5,128,420; all of which are hereby incorporated herein by
reference. Generally, these hydroxamated polymers may be produced by reacting the
polymer containing the pendant reactive group, in solution, with a hydroxylamine or its salt
at a temperature ranging from about 10' C to 100 C, p~rer~bly below about 50' C, for
several hours, more preferably at a pH over about 10. From about 1-90% of the available
pendant reactive groups of the polymer may be replaced by hydroxamic groups in
accordance with said procedures.
In addition to reaction of a hydroxylamine or its salt with a polymer solution, it is
known that a polymer latex may be reacted directly with a hydroxylamine or its salt. The
latex may be, e.g., a copolymer of acrylamide and methyl acrylate or a copolymer of acrylic
acid and methyl acrylate. In these cases, the hydroxylamine or its salt reacts primarily with
the ester groups to form hydroxamic acid groups.
Also, it is known that aqueous solutions of polymers derived from inverse emulsions
and inverse microemulsions (herein referred to also as emulsions and microemulsions)
function ~rricienlly in the process of the present invention. These emulsions and
microemulsions are made of, for example, aqueous polyacrylamide, or acrylamide/acrylic
acid copolymer solutions dispersed in oil and reacted directly with a hydroxylamine or its
salt to give very high molecular weight polymers containing hydroxamic acid groups. Dilute
aqueous solutions of these polymers, useful in the instant invention, are derived from
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emulsions and microemulsions by "breaking"; e.g. adding the emulsions and
microemulsions to waterS optionally in the presence of a breaker surfactant, and agitating
to dissolve the polymer.
The~degree of hydroxamation, i.e., the concentration of Formula I units in the
polymers useful herein, may range from about 1 to about 90 mole percent, preferably from
about 5 to about 85 mole percent, more preferably from about 20 mole percent to about 80
mole percent, and most preferably from about 25 to about 70 mole percent. The degree
of hydroxamation may be determined by nuclear magnetic resonance spe~l~oscopy
techniques well known to those skilled in the art.
Suitable hydroxylamine salts include the su~f?~tes, sulfites, phosphates, perchlorates,
hydrochlorides, acetates propionates and the like. The pH of the solution is adjusted to be
in the range of about 3 to about 14, preferably over about 7, and more preferably over
about 10, by means of acid or base addition to the solution.
Any water-soluble polymer may be used in the present process which, after
hydroxamation, performs to settle suspended alumina trihydrate solids. Thus,
homopolymers, copolymers, terpolymers, etc. of the above exemplified monomers may be
used. Suitable comonomers which, by copolymerization with the above monomers, may
form, for example, up to about 95 mole percent of the polymers useful herein can include
acrylic acid, sodium acrylate, methacrylic acid, maleic anhydride, vinyl acetate, vinyl
pyrrolidone, butadiene, styrene as well as others of the above enumerated esters, amides
and/or nitriles and the like as is known in the art and is set forth in the above-incorporated
patents as long as such copolymers, terpolymers etc., are water-soluble after
hydroxamation. The weight average molccLIl~r weight of the polymers useful in the process
of the present invention range from about lX10~ to about 1 X 10B, preferably from about 3
X 10~ to about 5 X 107. Weight average molecular weight may be determined by light
scattering techniques well known to those skilled in the art.
The polymers used in the present invention are employed by adding them, usually
in the form of a dilute aqueous solution, to the liquor containing suspended alumina
trihydrate solids in an amount at least sufficient to settle said suspended solids. Generally,
for best results, at least about 0.1 milligram (mg) of hydroxamated polymer per liter of
alumina trihydrate suspension is added.
It is understood, that higher amounts than those just stated may be employed
without departing from the scope of the invention, although generally a point is reached in
which additional amounts of hydroxamated polymer do not improve the separation rate over
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already achieved maximum rates. Thus, it is uneconomical to use excessive amounts when
this point is reached.
The technology of centrifugation is well known to those skilled in the art and adetailed description may be found in e.g Ullman's Encyclopedia of Industrial Chemistry,
Volume B2, pp. 11-1 to 11-27, which is hereby incorporated herein by reference. Any
centrifuge, including filter centrifuges. screen centrifuges, sedimentation centrifuges,
decanting centrifuges, etc. may be used in the present invention. Sedimentation and
decanting centrifuges are preferred, and classifying decanter centrifuges are most pref~ d.
The oplil"i~ation of centrifuge performance is well known in the art e.g. D.E. Sullivan
10 and P.A. Vesiland, "Centrifuge Trade-Offs~, Operations Forum, pp. 24-27 (1986), which is
hereby incorporated herein by reference. Feed volume depends on the size of the
centrifuge and type of centrifuge. For a horizontal classifying decanter centrifuge with a
bowl diameter of about 20 inshes and a length of about 80 inches, a feed volume of about
4 to about 250 gallons per minute may be used, preferably about 20 to about 200 gallons
15 per minute. Feed solids may range from about 1% to about 40%, preferably from about 5%
to about 15%. The G-force is generally in the range of about 300 to about 2000 X G,
preferably in the range of about 500 to about 1000 X G. The differential between the scroll
and the bowl is generally less than 150 revolutions per minute (rpm), preferably from about
1 to about 100 rpm, more preferably from about 5 to about 50 rpm, most preferably from
20 about 10 to about 40 rpm.
Water-soluble polymers containing pendant hydroxamic acid or salt groups are
generally mixed with the alumina trihydrate suspension in a holding tank prior to being
introduced to the centrifuge, or pumped into the process stream feed line, or added via a
feed tube directly inside the centrifuge. The alumina trihydrate suspension may be any
Bayer process stream which contains prec;~ilated trihydrate e.g precipitator stream, primary
classifier underflow, primary classifier overflow, secondary classifier underflow, secondary
classifier overflow, tertiary thickener underflow, tertiary thickener overflow, hydrate tank
stream, etc. Preferred process streams are underflow from the primary classifier, hydrate
tank stream, and overflow from the secondary classifier. Preferably, the polymer is added
in the form of a dilute solution, e.g. from about 0.01% to about 3%, directly inside the
centrifuge. Those skilled in the art recognize that the optimum polymer concentration in
the dilute solution depends on the alumina solids level in the Bayer process stream, and
can be ascertained by routine experimentation.
When hydroxamated polymers are used, preferably within the ranges specified
above, to flocculate suspended trihydrate solids, the flocculated solids may be centrifuged
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to pr<~duce centrifuged solids (cake) and aqueous liquid, preferably when the operation of
the centrifuge is optimized according to principles well known in the art_ The cake solids
(the weight percent solids of the centrifuged solids) is greater than the feed solids,
preferably greater than about 40%, more preferably greater than about 50%, most
5 preferably greater than about 60%. Although is it generally p~efened for the solids level to
be as high as possible, plugging of the centrifuge may occur at very high solids levels e.g
90%. For obvious reasons plugging of the centrifuge is to be avoided.
It is desilablE for the levels of impurities in the centrifuged alumina trihydrate to be
as low as possible. Centrifugation of flocculated trihydrate crystals may be performed at
10 any staye of the process, so the impurity level in the centrifuged trihydrate depends on the
process stream being centrifuged. For instance, a seed stream from the underflow of a
tertiary thickener may be centrifuged to reduce the level of soda and other impurities in the
trihydrate crystals before the crystals are returned to the precipitator. In this case,
leachable soda levels below 10% are desirable and are generally obtained by the proce.sses
15 of this invention, preferably below 8%, most preferably below 5%.
In the case of product streams, the alumina trihydrate is typically washed in a
hydrate tank before filtration and calcining. Floccul~tion and centrifugation of the hydrate
tank output stream lowers the level of impurities in the alumina trihydrate. It is often
desirable that the centrifuged alumina trihydrate solids be further purified by washing with
20 wash water or another Bayer process stream to further reduce the levels of impurities. The
centrifuged alumina trihydrate solids may be washed during and/or after centrifugation.
Pr~7~erably, the wash water is introduced at a feed manifold of the centrifuge. The wash
water may be added at any rate. Preferably, the wash water is added at a rate of from
about 5% to about 100% of the feed volume, most preferably from about 10% to about 50%
25 of the feed volume.
In practical terms, the centrifugation of flocculated alumina trihydrate solids to
achieve purified, dewatered trihydrate and aqueous spent liquor is most often opti"li~ed in
the context of other plant operations. For instance, the level of impurities in the dewatered
trihydrate may be adjusted up or down to achieve other desirable outcomes such as lower
30 power consumption, reduced waste disposal costs, increased rates of production, increased
product purity, reduced consumption of raw materials, etc. Flocculation and centrifugation
may be used in place of, or in addition to, or in combination with, the usual means of
solids/liquids separation employed in the Bayer process e.g settlers, decanters, thickeners,
classifiers, and filters. A consecutive or intermittent series of centrifuges may also be
35 employed, with the output of one as the input for another.
,
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Other polymer flocculants that are added earlier in the Bayer process for any
reason e.g. for red mud flocculation are not typically effective for_ alumina trihydrate
floccu~tion. However, such other flocculants may be added in conjunction with the
hydroxamated polymer flocculant to help flocculate the alumina trihydrate so that it may be
5 centrifuged effectively.
The following examples are set forth for illustration purposes only and are not to be
construed as limits on the present invention.
In the following Examples, % leachable soda in alumina trihydrate is determined by
adding 50 grams of dry alumina trihydrate to 100 t " " ' ~ of standardized 0.10 M
10 hydrochloric acid, stirring to form a slurry, filtering, and back-titrating the filtrate with
standardized 0.1M sodium hydroxide solution. Leachable soda is expressed as the wt. %
sodium carbonate equivalent in the trihydrate.
EXAMPLE A
Polymer A is prepared as follows: 230 Parts of 30% aqueous hydroxylamine sulfatesolution are combined with 2 parts of sodium thiosulfate stabilizer and 88 parts of water,
and the mixture is neutralized with 160 parts of 50% aqueous sodium hydroxide. The
resulting solution is added to a mixture of 179 parts aliphatic oil, 1 part ethoxylated amine
20 surfactant, and 293 parts of a polyacrylamide microdispersion having a molecular weight
of about 20,000,000 and having about 25% polymer solids. The mixture is stirred while
maintaining the temperature below 3~' C for 24 hours or more. The resultant
hydroxamated polymer microdispersion contains 7% polymer solids and is shown by nuclear
magnetic resonance spectroscopy to contain about 65 mole % hydroxamic acid groups.
EXAMPLES 1-6
The alumina trihydrate suspension, or slurry, used in these examples is tertiary classifier
underflow. The slurry is fed with a variable speed pump to a horizontal classifying decanter
30 centrifuge having a bowl diameter of about 20 inches and a length of about 80 inches. The
polymer, as a 0.125% solution, is fed directly into the feed chamber of the centrifuge via
a feed tube along with wash water (where used). The G-force ranges from about 740 X G
to about 790 X G. The differential speed ranges from about 13 rpm to about 35 rpm. Table
1 shows that the wt % leachable soda in the floccu~t~d and centrifuged trihydrate is less
35 than the wt % leachable soda in trihydrate that has been centrifuged without being
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flocculated with the polymer, with or without wash water. Table 1 also shows the wt % -
solids of the trihydrate slurry feed, the slurry feed rate in gallons per rninute (gpm), the
identity of the flocculant and the dosage of the flocculant, in units of grams of real polymer
per dry ton of flocculated solids (g/T). The feed rate of the wash water (where used) is
5about 18 gpm.
Table 1
Hydrate
Hydrate Slurry Flocculant Leachable
Slurry Feed Polymer Dosage, WashSoda,
Example Solids, Rate,Flocculant g/T WaterWt.%
No. Wt. % gpm
8.1 165 None 0 None9.05
2 7.9 175 Polymer 25 None6.19
A
3 8.4 50 None 0 None8.51
4 10.1 50 Polymer 26 None5.20
A
7.8 50 None 0 Yes 4.74
6 12.1 50 Polymer 22 Yes 4.35
A
EXAMPLES 7-8
The alumina trihydrate suspension used in these examples is secondary classifier overflow
having a solids level of about 3% to about 4%. The trihydrate is flocculated and centrifuged
as in Examples 1-6, with appropriate adjustment for the lower feed solids. The wt %
leachable soda in the flocculated, centrifuged trihydrate is less than the wt % leachable soda
30 in trihydrate that has been centrifuged under substantially the same conditions, but without
being flocculated with the polymer (Comparative Example 8).
EXAMPLE 9-10
~5
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The alumina trihydrate suspension used in these examples is output from a hydrate tank
having a solids ievel of about 40%. The trihydrate is flocculated and centrifuged as in
Examples 1-6, with appr~riate adjustment for the higher feed solids. The wt % leachable
soda in the floccul~ted, centrifuged trihydrate is less than the wt % leachable soda .n
5 trihydrate that has been centrifuged under substantially the same conditions, but without
being floccl-~ ted with the polymer (Comparative Example 10).
-
