Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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REMOVING SUSPENDED SOLIDS BY ADDITION OF HYDROXAMATED POLYMERS
IN THE BAYER PROCESS
The ~r~:se"l invention is cli.~c~ed to a p,ucess of alumina manufacture via the Bayer
5 r,~,cess. More particularly it is col~ce",ed with improvements in the Bayeralumina p,ucess
by the improved removal of sus~e"ded solids from the settler overflow and/or the washer
train feed stream by CGI ,lacting the settler feed stream or the stream to the first stages of
the washer train first with a) a lower molecular weight polymer which contains hyd~o~~
acid groups or salts thereof and then with b) a blend of an anionic polymer flocculant and
10 a second higher mc ~ J~--weight polymerwhich contains hydlux~lli~ acid groups orsalts
thereof.
BACKGROUND OF 1-HE INVENTION
The almost universally used p,~cess for the manufacture of alumina is the Bayer
Process. In a typical commercial Bayer Process raw bauxite is pulverized to a finely
divided state. The pulverized ore is then fed to a slurry mixer where a 50% solids slurry
i5 prepared using spent liquor and added caustic. This bauxite slurry is then diluted and
sent through a series of digesters where at about 300 800~~. and 100-2000 p.s.i. 98% of
20 the total available alumina is extracted from the ore which may contain both trihydrate and
monohydrate forrrls of alumina. The effluent from the digesters passes through a series of
flash or blow-off tanks or 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 co,llai.1s about 1-20% solids which consist of the insoluble
25 residues that remain after reaction between the bauxite ore and basic material used to
digest the ore and the insoluble co",poner,l~ which precipitate during digestion. The
coa,aer solid particles are generally removed with a "sand trap" cyclone. To separate the
finer solid particles from the liquor the slurry is nommally fed to the center well of a mud
settler (also called a decanter. a residue thickener or a raking ll~ e,) where it is treated
30 with a flocculant such as a polyacrylate polymer. As the mud settles clarified sodium
aluminate solution referred tc as "green" or "~,~y"a"l liquor overflows a weir at the top
of the mud settling tank and is passed to subsequent processing steps. The settled solids
("red mud") are withdrawn from the bottom of the mud settler and passed through a
countercurrent washing circuit (called "the washer train") for further recovery of sodium
35 aluminate and soda. Aluminate liquor overFlowing the settler still coll~ s typically 50 to
-
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aiuminate and soda. Aluminate liquor ove,~lo.1 ;. ,g the settler still conL~. ,s typically 50 to
200 mg of sus~n~eci solids per liter. This liquor is then generally further claliried by
filtration to give a filtrate with 10 mg. suspel-d~d solids per liter of liquor. Alumina, in
relatively pure fomm, is then pl~-;p ~ d from the filtrate as alumina trihydrate crystals. The
5 remaining liquid phase or spent liquor is retumed to the initiai digestion step and e."r'~yed
as a dige~lant of additionai ore after being recorctitln~d with aJ-lilional caustic. After
pA-~sas~e through the fill-dlio-- step, the level of sus~ended solids should be suri-ic;onlly low
to provide an aiumina product from the p,~;~ un step which meets ail of the industry
standards.
The aforementioned insoluble cc""ponerlt~ are p~2re~dbly separated from the slurry at
a relatively fast rate so as to make the overall Bayer Process efficient. This is generally
accomplished in large settlers, decanters, residue ll.:~k~ne.~ etc, as mer,lioned above. The
separdlion itself should be clean and COIl F!~ ~ with but minimal amounts of residue
remaining as a ~ pe,~ed phase in the solubilized aluminate liquor. The settlers, decante~s,
residue (or raking) II. ':ene,~- (all of which are her~,:.-drler referred to as "setter(s)" may
exceed 49m in diameter. Some are of a multideck configuration, ho.Ycvcr, single-deck units
are almost exclusively used at the present time. In operdlion, the settler(s) feed liquor is
fed at the center of the settler(s) and clarified solution overflows at the perimeter. The
settler feed liquor is added to the settler(s) via a feedpipe which exits into a ree~w_ll
centrally located at the top of the settler. As the solution flows radially across the seKler,
the l1orku.,lal and vertical velocities become very low and the solids i.e. red mud, sink to
the bottom because the specific gravity thereof is higher than the solution. The faster the
settling rate, the more material which can be processed in the settler(s). The solution
overflowing the seKler(s) contains very few solids whereas the unde,~lu./ can contain up to
35% solids. However, the overflow contains the majority of the Al203 to be recovered as
product. A rotating mechan;s", with plows mounted at an angle, slowly moves the settled
solids across the bottom of the settler(s) to a discharge cone usu~lly located in the bottom
center thereof. The fine solids of the seKler feed liquor settle very slowly unless accele. dl.eli
by the addition of flocculants which act to bind the fine solids into flocs that often are
several millimeters in diarneter. The ratio of mass to drag forces is thereby i"~;,based
causing the flocs to settle more rapidly. The settled solids from the settler(s) are treated
in the counter-current washing circuit (washer train) discussed above, to further remove the
red mud II~e,~r~o", by washing. Decanlaliol1 systems are employed for this purpose using
washing thickeners similar in design to the settler(s). The washing operation isaccol"~' hed in as many as ten (10) stages, the solids moving counter-current to the wash
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stream to recover solubles and the overflow being recycled back from each stage of the
washer train to its p~dec~sor. Flocculant addition to the stages of the washer train
i, ~l~ases the solids separ~iùn, it being generally known to utilize different floccu'~nts in the
- earlier stages from those used in the latter stages see for example U.S. Pat. No.
4,678,585 hereby inco"Jordled herein by ~~f~ ce. The settler~s) overflow must becla,i~ied in order to recover as pure a product as possible. The overFlow is SU~8 H '~ to
what is called polish fill,dlion (so",eli",es l~felled to as clear pr~ssi,lg) whereby the solids
r~maining are removed. Slationary filters are also used for this purpose. It is nommal to
allt~ l to reduce the solids to below about 0.5 mg/L to enable product purity to be as high
as possible since the ultimate product is used for applications where purity is ess~nlial
e.g.toc~tl "~asle.
The reduction of the amount of sl,spendedl solids in the stream co",prising the Bayer
Process settler(s) overfiow and that which proceeds to the initial stages of the washer train
ie. the first separalion stages to which the red mud slurry from the last settler are
introduced has been a majo! pn~t e " for many years.
Among the methods of overcoming the above p,.t ~ ~,s and materially speeding up
sepa,i~lion of sus~nded solids from Bayer Process ~lr~"s as well as effecting a cleaner
separation of the constituents, is that di~closed in U.S. Patent No. 4 767,~40, issued on
August 30 1988. In this patent there is ~lisc~osed the addition of a water-soluble polymer
colllai.lil,g ~endanl h~dluA~ll;c acid or salt groups to Bayer Process ~ a",s alone or in
conjunction with an anionic flocculant. The polymer containing hyd,u,~a",ic acid groups may
be added into the Bayer Process caustic aluminate process stream alone subsequent to
followed by or in A~soc:~liol1 with the anionic flocculent e.g. a conventional polyacrylate
polymen This treatment is shown to ~educe levels of suspended solids in the prucess
stream as compared to then existing state-of-the-art prucesses.
Said U.S. 4 767 540 ho.vevcr does not di~l~e the use of a lower m~P~ r weight
hyd,u,~"ated polymer followed by a blend of an anionic flocculant and a highe m"o-ec~ r
weight h~nl~u,ca",ated polymer in acco,dance with the present invention.
SUMMARY OF THF INVFNTION
~ It has now been found to be une)t~,ecledly advant~geou-s when treating the feed to
frt~m the primary seKler~s) (or digester blow-off) of the Bayer Process and/or the feed to the
J initial stages of the washer train to contact said feed liquor first with A) a lower mc e l~r
weight polymer containing the pendant h~dlu,cdll,ic acid groups and/or hyclro,~dl"ic acid salt
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groups f~"~ued by B) a blend of an anionic flwculant and a second, higher ",c~e~ nr
weight polymer containing hyd~uAd-- ~~ acid or sait groups.
This sequence of aduii~ion of additives results in a clearer settler overflow as indicated
by a higher rillldliun rate and/or more ~Ire~t;~,c flocc-~'-'ion of the red mud in the washer
5 train.
D~TAII FD DF~CRIPTION OF THF INVFNTION
According to the present invention, there is provided a p~cess for improving thefiltration rate of the settler overflow and/or the floccu'~lon in the initiai stages of the washer
10 train in the Bayer Process u,he,~by there is added to the feed to the settler or to the feed
to the initiai stages of the washer train, a) a water-soluble, lowem~ cl~ r weight polymer
containing hyci~uAdr ,ic acid groups or saits thereof followed by b) a blend of an anionic
flocculant and a second, higher lll~2cUI~'r weight, water-soluble polymer containing
hydl.~xd",ic acid or sait groups and removing the resultant filtered solids.
1~ The anionic flocculant used in the prucess of the present invention is preferably a
ho",opolymer of an acryiic acid or a copolymer of an acrylic acid conLai,l ~g at least 80
molar percent of the acrylic acid or the aikaii metal, aikaiine earth metal or a~ on:urn sait
thereof, or a combination of any of the foregoing. Examples of acrylic acids which may be
used include acrylic acid, methacrylic acid etc. Copolyrners and terpolymers of said acrylic
20 acids with copolymerizable monoethylenically unsaturated co",ono",e,s such as acrylamide,
methacrylamide etc. may be used. Pr~:~erdbly, the anionic flocculant is a ho",opolymer or
copolymer of an acrylic acid salt.
Both the water-soluble anionic, flocculant and water-solubie, hydroxamated polymers
to be e" F'cycd in the present invention should be sufficiently stable to be effective under
25 the Bayer Process conditions used, e.g. high temperatures and strong caustic conditions
typically, 185-225~F, and 80~0009./1. total alkali content (~A~u,~:ssed as sodium ca,l~onale
equivalent).
Any water-soluble h)~dluAdlll c acid or hyd,u,~d-, c acid salt group-conlai";.,g polymer
may be used in the p~cess of the present invention. The useful polymers can best be
30 eAe~ll F' 'ied by those collld;.. ~9 pendanl groups of the Fommula (I);
O
Il
- C - NH- OR (I)
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~ :.er~:., R is hydrogen or a cation. These polyiners are well known in the art and can be
derived from polymers containing pend~,l estel; amide, anhydride, nitrile, etc., groups by
the ~~action thereof with a hydroxylamine or its salt.
E~e,..~,lary of the polymers which may be hy(J~uAdllldled for use in the ~n,cess of the
5 prese.,l invention are acrylic, methyacrylic, ~-ul(n- etc., acid ester polymers such as
polymers produced from the poly...e.i,dlion of methyl acrylate, ethyl acrylate, t-butyl
acrylate, methyl ~--eU,acrylate, ethyl ~I,eU-aclylate, cyclohexyl ~,--:II,acrylate, dimethyl
~-.;.,oetl,yl methacrylate, dimethyl aminoethyl "-~:ll,acrylate, dimethyl aminoethyl acrylate,
methyl c~londle, etc.; polymers of maleic anhydlride and esters thereof, and the like; nitrile
10 polymers such as those pr~duced from acrylon-- ile etc; amide polymers such as those
produced from acrylamide, m~tl ,acrylamide and the like.
H~d~u~d",ated polymers are well known to those skilled in the art and are specifically
di;,closed, as are methods for their production, in the above U.S. Patent No. 4,767,540,
hereby inco,,uordled he~ein by r~er~nce. Generally, these hy~-uxd..,aled polymers may be
15 produced by reacting the polymer containing the pen~,l reactive group, in solution, with
a hydroxylamine or its salt at a te",perdlure ranging from about 10~C to 100~C for several
hours. In addition to ,~actiùn of hydroxylamine or its salt with a polymer solution, it has
been found that a polymer latex may be reacted directly with the hydroxylamine or its salt.
Tl1e latex may be, e.g. a copolymer of acrylarnide and methyl acrylate, a copolymer of
20 acrylic acid and methyl acrylate etc. In these cases, the hydroxylamine or its salt reacts
primarily with the ester groups to fomm h~ dll c acid groups.
Also, it has been found that inverse emulsions made of, for ~,~cd~ e, aqueous
polyacrylamide or acr~amide/acrylic acid copolymers dispersed in oil can be reacted
directly with hydroxylamine or its salt to give very I ;igh molecular weight polymers containing
25 hy~l~u~ lic acid groups, all of which function efflciently in the prwess of the present
invention.
The degree of hyd.u,~"alion, i.e., the concenl,dlion of Fommula I units in the polymers
useful herein, may range from about 1 to about 100 mole percent, plt:~erdbly from about
15 to about 90 mole percent and, most pr~ferdbly, from about 20 to about 80 mole percent
30 of the polymer.
Suitable hydroxylamine salts include the su~f~tes, sulfites, phospl1ales, perchlorates,
hyd,~l,'ondes, ~e~les, prl r ~nales and the like. The pH of the solution of the polymer
- to be h~d,o,~dn,ated is adjusted to over about 6.0, pr~fe,dbly over about 10.0, beforehand.
Any water-soluble polymer may be used in the present pr~ess which, after
35 hyd,u~"ation, perfomms to settle suspended solids. Thus, homopolymers, copolymers,
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terpolymers, etc. of the above eAe"" ~ unumel~ may be used. .SI~ -~'e co,-,ono",e,:,
which, by copol~",eri~dlion, may form, for exarnple, up to about 95 mole percent of the
polymers useful herein can include acylic acid, sodium acrylate""~lhac~ylic acid, maleic
anhydride, vinyi ~r-et~ts, vinyl pyrrolidone, butadiene, styrene as well as others of the above
enl""erdled esters, amides an/or nitriles and the like as is known in the art and is set forth
in the above-i"co"~,aled patents as long as such copolymers, terpolymers etc., are water- ''
soluble after hydl xdlll-dlion. The weight average ",c'e~ r weight of the hycl,u~,,aled
polymers useful in the p,ucess of the present invention range from about 1 x 10~ to about
3x10'. Thema'e~u'~-weightofthehy~l~o~dledpolymerisbestcc",l,."o~'bycontrolling
the molecular weight of the polymer to be hyd,u~,,ated, such by the use of chain-length
regulators, e.g. ~"erudplans, during pol~,",e,i~dlion.
The water-soluble polymer containing h~,dlu~ll;c acid and/or salt groups which is first
added to the settler(s) feed and/or the feed to the initial washer stages of the Bayer process
in accor~ance with the present invention must have a Ill~ r weight lower than that of
the hyd,uxd",ic acid and/or s~t group containing polymer which fomms part of the blend
which is added thereafter. Although any polymer having a molecular weight within the
above range may be used, it is preferred that the first added h)~d,u,~d,,,aled polymer have
a molecular weight of below about 7 million, most pr~fer~bly below about 3 million.
Preferably, the hyd,u,td,,,ated polymer which constitutes part of the blend with the anionic
flocculant will have a mslecu'~r weight over about 7 million, most pr~t7rably over about 10
million, hu.vevcr, the specific m~ r ~ r weight of either additive is critical only to the extent
that the h~n~uxa~aled polymer of the blend has a m~tec,J'~r weight higher than that of the
hydroxamated polymer added individually, i.e., the lower molecular weight polymer.
The blend of the anionic flocculant and the higher ",c'e ~ weight polymer containing
hydroxamic acid and/or salt groups should be co",p,ised of from about 10 to about 90
weight percent of the higher molecular weight polymer and about 90 to about 10 weight
percent of the anionic floca~' nt, p,~r~r~bly about 80 to about 20 c.nd about 20 to about 80,
same basis, l~spe-;lively, most pr~fer~l~ about 70 to about 30 and about 30 to about 70,
same basis, respectively.
The blend can be added to the stream being treated as a single dose or via a series
of dos~ges, it being pr~ei,~ that the blend be added in at least 2 dos~ges for purposes L'
of insuring co,-lr'--te admixture with the stream.
The anionic flocculant and the water-soluble, h~d,u,tdl,,ated polymers used in the
present invention are e",, '~yed by adding them, i.e., the lower molecular weight polymer
and/or the blend, in the fomm of aqueous solutions, to the feed to the settler(s) containing
-
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solubilized alumina and s~Jspended solids di~pel:jed throughout, and/or the feeds to the
initial stages of the washer train, in an amount at least sufficient to settle suspended solids
Il ,er~,u"~ Generally, for best results, at least about 0.1 mg, of the lower m- 'e u~- r weight
hyd~A~.,-dled polymer, per liter of the settler(s) and/or washer train stage feeds should be
5 employed. More prt:tt:ld~ly, at least 1.0 mg, of the lower ,- ~ 'e~ r weight hy.JIuAdlllated
polymer, per liter of the settler(s) andlor washer stage feeds should be e" ~'~ycd. The
blend of the anionic flocculant hydluAdllldled polymer may be added in amounts ranging
fnDm about 0.01 to about 40 Ibs. of blend solids per ton of solids to be settled.
It is unde,:,lood, that higher amounts than those above stated may be ell f ' jcd
10 without depa,li"~ from the scope of the invention, although generally a point is reached in
which ad.Jilional amounts of blend and/or h~-J~x~lldled lower m~e ~ weight polymer
do not improve the separation rate over already achieved maximum rates. Thus, it is
uneconomical to use eAcessive amounts of either additive when this point is reached.
The addition of the water-solub!e, lower m~e ul~r weight hy~rùA0~aled polymer
1~ should precede the ad~lilion of the blend ~y sufficient time such as to enable the lower
molecular weight polymer to at least begin to perfomm. Thus, for ~example, when both
additives are added to the settler fccd there must be sufficient time allowed between the
individual additions to enable the lower molecular weight h~,u~d",ated polymer to at least
begin to flocculate the susperlded solids. The same applies to diffe,~-"l addition points of
20 ths initial washer train stages.
The r~ ..,;. ,9 examples are set forth for purposes of illu:,l,dlion only and are not to be
construed as lir"i~alions on the present invention except as set forth in the ap,vended claims,
All parts are by weight unless otherwise specified.
r YS-m~7les 1-2~
To a vessel containing a settler feed from a commercial Bayer Process alumina plant
is added, as set forth in Table 1, below, an 80 mole percent hy~JIuAdlllaled polymer
containing 10 mole percent of acrylic acid units and 10 mole percent of acrylamide units
and having a weight average molecular weight of about 350,000 (identified as A). A
commercially available ammonium polyacrylate flocculant having a nurnber average" ~'e-u!~r weight of about 10-15 million (ide,ltified as B) is added alone (cc""pardlive) and
as a 70/30 blend, ~~spe~,1ively, with a second polymer containing 18 mole percent
hyd,ux~"ate groups, ~2 mole percent of acry~ic acid units and 30 mole percent acrylamide
units and having a weight average molecular weight of 12-20 million (ider,lified as C).
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The additives are added as indicated with mixing betwecn each addilion such as to
simulate the mixing in the plant. The results are set forth in Table 1, below.
All additives are made up to 0.1% with 10 gpl caustic in water. The till-~lion volume
is 100 ml.
Exd",r'es 1-14 employ one settler feed stream, Exdll, 'es 15-17 employ another
dirr~,enl settler feed stream and Examples 18-29 employ a third settler feed strearn.
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TABI Fl
r;itl dliOn
FXAMPI F ~nD. #1 ~nD. #? ~nD #3 mls #1 mls#2 mls #3 ~
a~ /hr/rrl~)
51 X - B B - 3.0 3.04.386
2X - B B - 2.0 2.02,819
3X - B B - 2.5 2.56,579
4X A B B 0.5 2.0 2.05,639
5X A B B 1.0 2.0 2.05.639
106X A B B 2.0 2.0 2.05,263
7X A B B 0.25 2.0 2.0 7,894
8X - B B - 2.0 2.04,644
9X - C C - 3.0 3.07,177
1 OX - C C - 2.5 2.56,C73
1511 A C C 0.5 2.0 2.08,772
12 A C C 0.5 2.0 2.011,287
13 A C C 0.5 2.0 2.09,288
1 4X - C C - 2.0 2.04,644
1 5X - C C - 4.0 4.01,338
2016 A C C 2.0 4.0 4.01,925
17 A C C 2.0 4.0 4.01,925
1 8X - C C - 3.0 3.01,462
19 A C C 0.5 3.0 3.03,036
A C C 1.0 3.0 3.03,759
2521 A C C 1.5 3.0 3.03,588
22X - B B - 3.0 3.00,897
23X A B B 0.5 3.0 3.01,974
24X A B B 1.0 3.0 3.03,289
25X A B B 1.5 3.0 3.03,759
3026 A~ C C 0.05 3.0 3.0 3,588
27 A C C 5.0 3.0 3.03,588
28X - C C - 3.0 3.01,925
29 A~ C C 0.02 3.0 3.0 3.432
ADD = additive
35~12% solution
X = comparative
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As can be readily app,~ciale~ the results of Table I show that an overall superior effect
is obtained dS cv;denced by the higher fill,alion rate when the blend of the anionic flooal'~nt
and the higher ",~ weight h~ Adlll~ted polymer is used suhsequent to the ac~-~ition
of the lower molecular weight h~lUA0lldl~ polyrner.
FYI~MPI FS 30-39
F_ r,i.l~ the testing procedure of Examples 1-29, a series of lower molecular weight
hy(J~w~dllldled polymers are e" ~'cyed in the treatment of a settlcr feed stream from a
ccj"""er~ial Bayer Process plant. The same blend of EA~IIII, '~S 1-29, i.e., that desiy"aled
as C, is then added. The results are shown in Table ll, below.
Polymer D = 80% hydluAdl~laled, M.W. 220,000
Polymer E = 60% h~nlluAdlllale~J M.W. 350 000
Polymer F = 100% h~J,uAa.,,ated; M.W. 350 000
Polymer G = 80% h~luAdlllated; M.W. 700,000
Polymer H = 20% h~ ùAdlllated; M.W. 350,000
Polymer I = 25% h~ll UAdl I Id~ M.W. 10 million
Polyrner J = 80% h~ uAdrl lated; M.W. 100 000
TARI F 11
rlll, dliUn
FXAMpl F ~nD. #1 ADD. #~ ~nD. #3 mls #1 mls #2 mls #3 ~lhrtrr~)
30X - C C - 3.0 3.0 1,650
31 A C C 1.0 3.0 3.0 1,925
32 D C C 1.0 3.0 3.0 1,754
33 E C C 1.0 3.0 3.0 2 134
34X - C C - 3.0 3.0 1 645
F C C 1.0 3.0 3.0 2,392
36 G C C 1.0 3.0 3.0 3,158
37 H C C 1.0 3.0 3.0 2,256
38 I C C 1.0 3.0 3.0 3,298
39 J C C 1.0 3.0 3.0 2,392
X = comparative
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These data clea~y show that variation of the degre!e of h~ uAdilldlion and the
",c ecu'~- weight of the lower m~'e l-' r weight hyd~Ad",ated polymer still results in a
superior i"c,ease in the overflow lill,dlion rate.
FY~Mpl ~ 40
Fc ~ ing the procedure of Example 12 except that the anionic floccu'-nt of the blend
is a ~ re:~l co---~er~;ia',ly available polyacrylate having a number average l-l~ ~cu'~
weight of about 1~15 million similar results are ach;eved.
FY~MPI 1- 41
The anionic flocculant of Example 40 is replaced by a 90/10 sodium acrylate/acry~amide
copolymer. Similar results are achieved.
FY~MPI F 42
Replacement of the h~d,uA~",ated polymer of blend C of Example 13 with a polymerwhich is 25% hycdloAdillated and has a weight average m~ o ul~r weight of a~out 12 million
results in a similar incf~ased filtration rate.
t5 FYI~MPI 1~ 43
Again fo~ ;ny the procedure of Example 12 except that the anionic flocculant is a
ho",l)polymer of sodium acry,ate similar results are obse,-ved.
FXAMpl F~ 44-72
When Exd",Pec 1-29 are again followed individually using the additives of Table 1
except that a feed to the first stage of the washer train of a commercial Bayer Prwess
a,umina plant is used as the charge medium an overall improvement in floccl~'~tion is
shown by those eAa",r'es representative of the instant ir,vention as compared to the other
comparative exd",r es
