Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: PROCESS FOR ALUMINA RECOVERY
BACKGROUND OF THE INVENTION
The instant invention is directed to a process of
alumina manufactur2 via the Bayer processO The Bayer pro-
cess is the almost universally used process for the manufac-
ture of alumina. In its broadest aspects, this method iscarried out almost exclusively in aqueous solution, and is
achieved by reaction of bauxite and a strong base such as
caustic soda or lime in steam heated autoclaves whereby the
alumina is transformed into a soluble aluminate formO In
this step, a considera~le a unt of insoluble impurities re-
sults or is released from the bauxite, which recrement must
be separated rom the desired alumina constituent. These
residues commonly known as red muds include iron oxides,
sodium aluminosilicate, titanium oxide and other materialsO
Generally these muds appear as very fine particles which are
difficult to separate out. Yet the red muds which usually
constitute about 10 to 50 by weight of the ore must be rapid-
ly and cleanly separated from the solubilized alumina
liquor in order to make this particular step economically
efficient. If the rate of separation is too slow, output is
materially diminished and overall process efficiency im-
paired. Likewise, if the separation is not clean, the re-
sultant alumina in the form of aluminate is somewhat crude
and undesirable for a number of end-uses. The insoluble im-
purities present in the alumina as carry-through from the
manufacturing process tend to add extraneous non-active
matter into the specific media, such as water, being treated
with aluminate for a variety of purposesO For example, low
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gradc sodiu~ lwllinate containillg relatively large amounts of mud im-
purities when used to treat water resultsin a situation of increased
tendency to form slime masses as the direct result of the insoluble im-
purities present, which masses tend to foul feeding equipment. Also, if
the crude sodium aluminate contains substantial amounts of impurities as an
admixture, solution problems are quite difficult to overcome if the aluminate
is fed in the form of a solid.
One method of overcoming the above problems, and materially speed-
ing up separation of red muds from alumina as well as effecting a cleaner
separation of the constituents is disclosed in U.S. Patent No. 3,390,959
wherein it is disclosed that the use of homo- or copolymers of acrylic acid
and acrylates which contain not more than 20% of other ethylenically unsat-
urated polymerizable polar monomers as red mud flocculants improves the
overall efficiency of the Bayer process.
The '959 patent teaches the use of copolymers of acrylic acid or
acrylates with ethylenically unsaturated monomers as red mud flocculants,
it likewise discloses, however, that when the ethylenically unsaturated
monomers constitute more than 5 molar percent the separation rate dramatically
drops and at more than 20 molar percent no significant separation is obtained.
Although this teaching is accurate for the initial or "head" stage of the
caustic recovery circuit, it has surprisingly been found not to apply to
the results obtained at the latter, more dilute stages of the caustic re-
covery circuit.
S~MMARY OF THE INVENTION
The present invention provides for a novel process for recovering
alumina via the Bayer process wherein red muds are flocculated from the
caustic recovery circuit by adding to at least the first stage of the caustic
recovery circuit an effective amount of a flocculant selected from the group
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consistillg of starch, holnopolylllors of acrylic acid or acrylate, copolymers of
acr~lic acid or acrylates contailling at least 80 mole percent acrylic acid or
acrylate monomers and combinations thereof and thereafter adding to a subsequent
stage or stages an effective amount of a copolymer comprising from about 35 to
75 molar percent of acrylic acid or acrylate monomers and from about 65 to 25
molar percent of acrylamide monomers. The employment of a secondJ distinct
polymer in the latter stages of the caustic recovery circuit where the system is
more dilute and the conditions are less harsh has surprisingly shown effective
flocculation of the red muds.
DETAILED DESCRIPTION OF TilE INVENTION
In accordance with the present invention there is provided a novel
process for flocculating red muds produced as a byproduct in the Bayer process
of recovering alumina from bauxite. This process comprising utilizing a
conventional red mud flocculant in at least the first or "head" stage of the
caustic recovery circuit and at some subsequent stage employing a copolymer of
about 35 to 75 mole percent acrylic acid or acrylate and about 65 to 25 mole
percent acrylamide as the red mud flocculant.
The conventional flocculant to be employed in at least the first
stage includes starch, homopolymers of acrylic acid or acrylate, copolymers
of acrylic acid or acry]a-te wherein the copolymer contains at least 80 mole
percent acrylic acid o:r acrylate monomers, hydrolyzed acrylamide monomers or
polymers and combinations thereof. By acrylate what is meant is the salt
of an acrylic acid wherein such salts are either alkali metal, or ammonium
salts. When copolymers of acrylic acid or acrylate are employed a wide
variety of comonomers may be employed in amounts up to about 20 mole percent.
Typical comonomers include acrylamide, methacrylamide, acrylonitrile, the
lower alkyl esters of acrylic and methacrylic acids9 vinyl methyl ether, meth-
acrylic acid salts, maleic anhydride and salts thereof, isopropenyl acetate,
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itaconic acid, vinyl acetate, alpha-methyl styrene, styrene, fumaric acid,
aconitricacid,citracollic acid, amides of any of the foregoing acids, alkali
metal derivatives (e.g., sodium, potassium and lithium), and ammonium salts
of any of the above monomeric acids or others, the partial alkyl ester
amides and salts of various polycarboxylic acids, vinyl toluene, chloro-
styrene vinyl chloride, vinyl formate, ethylene, propylene, isobutylene,
etc. Of the just-mentioned comonomers, greatly preferred materials among
these contain a hydrophilic group in a side chain off the ethylenically unsat-
urated hydrocarbon group. Those monomers which do not contain such hydro-
philic solubilizing group should be used in lesser amounts of say about 1-5%
by weight based on total weight of monomer present.
Still other monomeric substances which may be employed in conjunction
with the acrylic acid or acrylic acid salt constituent include materials such
as sulfoethyl acrylate, carboxyethyl acrylate, diethyl vinyl phosphonate,
crotonic acid or salts thereof, vinyl sulfonate, or salts thereof, vinyl
alcohol and vinyl aryl hydrocarbons containing solubilizing groups such as
sulfonates, etc.
Particularly useful homopolymers or copolymcrs of the type described
above should preferably have a molecular weight in excess of 50,000 and more
preferably in excess of 100,000. Excellent additive polymers have molecular
weights even as high as ten million.
Starch materials useful in the instant invention incude potato, corn,
tapioca, amylose, sorghum and other readily available starches.
The effe~ive amount of the conventional flocculant employed in at
least the first stage of the caustic recovery circuit will vary depending upon
the specific bauxite composition being processed, the conditions present in
the recovery stage i.e., temperature, pH, solids concentration and the like,
and the red mud flocculant or flocculants employed. Generally, however, when
starch or combinations containing starch is employed, the effective amount will
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range from 0.05 to 2.Q percent by weight of the dry mud residue. ~len
synthetic polymers or copolymers are employed, the effective amount will
generally be between Q.Oland 2,0 pounds of chemical per ton of dry mud residue.
The copolymer added to some of the subsequent stages of the caustic
recovery circuit is specifically a copolymer comprising from about 35 to 75
mole percent of acrylic acid or acrylate (with acrylate being defined as set
forth above) and from about 65 to 25 mole percent of acrylamide. Although it
is believed that ethylenically unsaturated monomers besides acrylamide may
be effectively employed in this copolymer, for considerations such as economy,
availability and performance the acrylamide comonomer is the preferred
comonomer for employment in the instant inventions. These subsequently
added copolymers should have a molecular weight such that the Brookfield
viscosity of a 0.15% solution of the polymer in lM NaCl at pH 8, UL adaptor
at 60 r.p.m. is at least 2.0 centipoise, preferably at least 3.0 centipoise.
The effective amount for these subsequently added copolymers will also
vary depending upon the specific bauxite composition being processed and
the conditions present in the recovery stage. Generally, however, the effect-
ive amount will be between 0.01 and 1.0 pound of copolymer per ton of dry
red mud solids, preferably about 0.05 to 0.5 pound of copolymer per ton of
dry red mud solids.
At what specific stage subsequent to the initial or "head" stage in
the caustic recovery circuit the copolymer comprising from about 35 to 75
mole percent acrylic acid or acrylate and from about 65 to 25 mole percent
acrylamide will be effective depends upon a number of variables within the
recovery system. Stage conditions such as lemperature, pH, dilution of
liquor, and concentration of red mud solids are believed to play an important
role. The type of bauxite ore itself may also contribute to the effective
location. Insofar as the exact mechanism of the interaction between the
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~locculallt and the red mud solids is as yet unknown, the predictability as
to which stage or stage variable controls the copolymer's effectiveness re-
mains outside of simple categorization. Generally, the copolymer has been
observed to be highly effective in the fourth or latter stages when the
grams of NaO~ and Na2CO3 in solution are less than about 100 per liter.
Since this figure is extremely approximate and may rest more upon coin-
cidence than the controlling stage variable it is recommended that the fol-
lowing test be undertaken to determine which stage in a given caustic recov-
ery circuit is the effective location at which to add the copolymer. This
test entails:
GENERAL TEST PROCEDURE
The stage in the caustic recovery circuit to be tested, which stage
is not the "llead" stage, is referred to as the "nth stage" for the purposes
of this test. The (n-l)th stage washer underflow is diluted to 1:4 with
nth stage washer overflow to produce a simulated nth stage washer feed. This
high dilution level is necessary to produce a level of reproducible free
settling in a test cylinder, preferably a 500 to 1000 ml. graduated cylinder.
To this simu]ated nth stage washer feed there is added to flocculant
to be tested in a 0.05 weight percent solution. The solution can be either
water or diluted spent liquor (NaOII). The tested dosage of flocculant is
added by syringe and mixed into the simulated washer feed by five strokes
of a perforated plunger. The descent of the liquid/solid interface is
timed in feed per hour to determine the utility of the flocculant to the
stage being tested.
Whereas the exact scope of the instant invention is set forth in the
appended claims, the following specific examples illustrate certain aspects
of the present invention, and more particularly, point out methods of evalu-
ating the same. However, the examples are set forth for illustration only
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and are not to be construed as limitations on the present invention except
as set forth in the appended claims. All parts and percentages are by weight
mless otherwise specified.
EXAMPLE 1
Following the General ~est Procedure set forth above, a red mud
flocculant copolymer containing 60 weight percent sodium acrylate and 40
weight percent acrylamide having a molecular weight between 5-10 million and
added to a simulated washer feed of an Australian red mud circuit wherein
the initial stage was treated with a 95 weight percent sodium acrylate, 5
weight percent acrylamide copolymer. Dosages employed and settling rates
obtained for the individual stages are set forth in Table I below.
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CC1MPARATIVE E~X~MPLE A
The procedure of Example 1 is followed in every
material detail except for the employment of a 95 weight
percent sodium acrylate, 5 weight percent acrylamide copoly-
mer as the red mud flocculant in the test stage. Test re-
sults are set forth in Table I belo~.
COMPARATIVE EXAMPLE B
The procedure of Example 1 is followed in every
material detail except that the red mud flocculant employed
in Comparative Example A is used in addition to the red mud
flocculant of Example 1. Test results are set forth in Table
I below.
COMPARATIVE EXAMPLE C
The procedure of Example 1 is followed in every
material detail except that there is now employed a sodium
acrylate emulsion having a molecular weight of 5-10 million
as the red mud flocculant. Test results are set forth in
Table I below.
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A sample from a Jamaican red mud circuit is taken
from the washer stage underflow and diluted according to the
General Test ProcedureO To this is added a flocculant in an
O.OS weight percent solution of NaOH. The flocculant tested
is a 5-10 million molecular weight copolymer of sodium
acrylate and acrylamide in the proportions set forth in
Table IIo Following the General Test Procedure in every
material detail, the results listed in Table II are obtained.
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I\S can ~eadily be seen flom the oregoing data, employment of
copolymers of sodium acrylate and acrylamides produce dramatic increases in
flocculation at later stages in tlle caustic recovery circuit. Although a
copolymer of 40 weight percent sodium acrylate and 60 weight percent acrylamide
did not show any noticeable effect in the early stages of the test, the same
copolymer produced surprisingly excellent results in a later stage, thus
emphasi~ing the i~portance of evaluating the appropriateness of the individual
stages in the relation to the copolymer employed.
