CA1192322A

CA1192322A - Process for alumina recovery

Info

Publication number: CA1192322A
Application number: CA000415406A
Authority: CA
Inventors: Neville J. Brownrigg
Original assignee: American Cyanamid Co
Current assignee: Wyeth Holdings LLC
Priority date: 1981-12-30
Filing date: 1982-11-12
Publication date: 1985-08-20
Also published as: ES518556A0; FR2518985A1; BR8207566A; IT8249718A0; AU8060982A; GR78426B; YU243882A; GB2112366A; JPS58120517A; AU536264B2; DE3245806A1; FR2518985B1; ES8405641A1; IT1158038B

Abstract

PROCESS FOR ALUMINA RECOVERY

Abstract of the Disclosure Aqueous suspensions of red mud are effectively removed from the Bayer process for making alumina by the addition to at least the first stage of the caustic recovery circuit of a flocculant selected from the group consisting of starch, homopolymers of acrylic acid or acrylates, co-polymers of acrylic acid or acrylates containing at least 80 molar percent acrylic acid or acrylate monomers and com-binations thereof and subsequent addition to later, more dilute stages in the caustic recovery circuit of a copolymer containing from about 35 to 75 molar percent of acrylic acid of acrylate and from about 65 to 25 molar percent of ethylenically unsaturated polymerizable monomers.

Description

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PROCESS FOR ALUMINA RECOVE:RY

5~ I n ven C i o n The instatlt invet~ion is directed to a process o alumina manufacture via the Bayer process. The Bayer proces~ i5 che almost universally used process for the manufacture of alumina~ In its broadest aspec~s ! this 5 method is carried out almost exclusively in aqueous s31u tion, and is achieved by r~action of bauxite and a strong base such as caus~ic soda or lime in s~eam hea~ed auto-cla~res wher~l~y ~he alumirla is trarlsformed in~o a soluble aluminat~ form. In this si:ep9 a considerable amoun~ of ~ inso~ubïe impuriti~s resul~s or is released from the baL~xi~e/ which recrement mu~t be s~parated from the desired slusDi~a c~nstituent. Tbese residues commonly known as red muds i~clude iron oxide~9 sodium alumin~silicate, eitanium oxide and o~her materials. Generally these muds 15 appe~r a~ very fine pa~ticles which are difficult tO
separa~e cut. Yet the red muds whic:h usually consti~ute about 10 to 50 by w~ig~t of the ore must be rapidly and cle~rlly separated from the solubilized alumina liquor in order ~o alak e th i s par ~: i cu 1 ar s ~ ep ec onomi c al 1 y e f i c i en t .
20 If the rate o separation i~ ~oo slow, output is materially dimi~ished and overall p~oc~ss ef f icieney impaired . Like wi~e, if the separation is not clean, the resultant alumlna i~ ~he orm of aluminate is ~omewha~ crude and undesirable for a number of end-uses. The insoluble impuriti~s pre-se~lt in the alumi~a as carry-through from th~ manuf2~ur~
;ng process tend to add extraneous non~3c~ive mat~er in~o ~' t ~ ~r~-the speciic media, such as water, being treated wi~h aluminate for a varie~y of purposes. For example, low grade sodium alumina~e containing relatively large amounts of mud impurities when used to treat water results in a situation of increased tendency to form slime masses as the direc~ resul~ of the inxoluble impuri~ies presenc, whie~
ma~es tend to foul fee~ing equipment. Also, if the crude sodi~m aluminate con~ains substantial amounts of i~puri-ties as a~ admixture, solution problems are quice diffi-~0 cult to overcome i the aluminate is fed in the form of asolid.
One me~hod of overcoming the above problems, and materially spe~ding up separation of red muds from alumina as well as efecting a cleaner separation of the constitu-e~ts is disclosed in U. S. Patent ~o. 3,390,959 w~erein itis disclosed ~hat the use of homo- or copolymers of acryl-ic acid and acrylates which contain not more than 20% of other ethylenically unsa~urated poly~erizable polar mono-mers as red mud flocculants improves the overall effi-cieney of ~he Bayer process.
The 1 959 patcnt teaches the use of copolymers ofacrylic acid cr acryla~es W? th ethylenically unsaeurated monomers as red mud flocculan~s, it likewise discloses, however, that when the ethylealcally unsa~urated monomers co~stl~ute more than 5 molar percent ~he separation rate dramaticalIy drops and at more tha~ 20 molar percent no significan~ separation is obeained. Although this teach-ing is accurate for the ini~ial or "~ead" stage of th~
recovery circuit9 it has surprisingly been ound not ~o apply ~o the resul~s obtained at the latter 9 more dilute s~ages of the caustic recovery circuie.
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The present invention provide~ for a novel pro-cess for recovering alumina via the Bayer process wherein red muds are flocculated from the caustic rccovery circuit by adding to at least the firsc stage of the caustic recov-ery circui~ an efective amount ot a flocculant selec~ed from the group consisting of starch, homopolymers of acrylic acid or acrylate, copolymers of acrylic acid or acrylates ccntaining at least 80 mole percent acryl ic acid or acrylates con~aining at least 80 mole percent acrylic acid or acrylate monome~s and combinations thereof and thereafter adding to some subsequent stage or stages an effective amoun~ of a copolymer comprising from about ~G 75 mole percen~ of acryiic acid or acrylate and from about 65 to 25 mole percent of acrylamide. The employment of a ~econd, distinct pclymer in the latter s~age~ of ehe caustic recovery circuit w~ere ~he system is more dilute and thc conditions are less harsh has surprisi~gly s~own effective flocculation of thc red muds.
Detailed Description of the Invention In accordance with ~he present invention, there is provided a novel process for flocculating red mud.s produced as a byprodu e in the 8ayer process of recovering alumina from bauxite. This process comprising utilizing ~0 a conveational red mud flocculant in at lease ~he first or "head" stage of the caustic recovery circuit and at some subsequent stage employing a copolymcr o about 35 to 75 mole pcrcent acrylic acid or acrylate and about 65 to 2S mole percent acrylamide as the red mud flocculant.
~5 The conventlonal flocculant ~o be employed in at least the first ~tage includes s~aroh, homopolymcrs of acrylic acid or acrylate, copolymers of acrylic acid or acrylate wherein the copolymeE contains at lea t 80 mole percent acrylic acid or acrylate monomers, hydrolyzed aorylamide monomers or polymers and combinations thereof.
By acrylate what is meant is the salt of an acrylic acid whe~ein such salts are either alkali metal, or a~monium salts. When copolymers of acrylic acid or acryla~e are employed a wide variety of comonomers may be employed in amounts up to about 20 mole ~ercent/ Typical comonomers include acrylamide, methacrylamide9 acrylonitrile9 the lower alkyl esters of acrylic and methacrylic acids, vinyl methyl ether, methacrylic acid salts, maleic anhy-dride and salts thereof, isopropenyl aceta~e, itaconic acid, vinyl acetate, alpha-methyl styrene, styrene, fumaric acid, aconitric acid, ci~raconic acid, amides of any of the foregoiag acids, alkali metal derivatives (e.g., sodium, potassium and li~hium), a~d ammonium salts o~ any of the above monomeric acids or othcrs, the partial alkyl escer amides and salts of various polycarboxylic acids, ~inyl ~oluene, chlorostyrenc, vinyl chloride, vinyl formate, ethylene, propylene, isobutylene, cec. Of the just-mentioned comonomers, gr@atly preferred materials a~ong these contain a hydrophilic group in a side chain off the ethylenically unsatura~ed hydrocarbon group. Those monomers which do no~ contain such hydrophilic solubilizing group should be used in lesse~ amounes of say about 1-5% by weight based on total weight of monomer present.
St;ll other monomeric substances ~hich may be empl~yed in conjunction wi~h the acrylic acid or acrylic acid salt constitu nt include materials such as sulfoethyl acrylate, carboxyethyl acrylate, diethyl vinyl phosphonate, crotonic acid or sal~s thereo, vinyl sulfonate, or salts thereof, vinyl alcohol and vinyl aryl hydrocarbons con-taîning solubiliz.ing groups such as sulfonates, etc.
Particularly ~seful homopolymers or cop~-ly~ers of ~he typ~ described above should preferably have a molecular weight in excess o 50,000, and more preferably in excess of lOQ,000. Excellent addi~ive polymers have molecular weights evcn a~ high as ten million.
Starch materials useful in the instant invention include potato7 corn~ tapioca~ amylose, sorgh~m and other readily available starches~
The effective amount of the conventional floccu-lan~ employed in at least the first stage of the caustic recovery circuit will vary depending upon ~he s~ecific bauxite composi~ion being processed, the conditions present in the recovery stage, i.e., temperature, pH, solids con centration and the like, and the red mud flocc~lant or ~1 ,9 5~r3r~ ~rgs flocculants employed. Generally, however, ~Ihen starc~ or combinations con~aining starc~ is employed, the efEeccive amount will range from 0.05 to 2.0 percent by weiOhc of the dry mud residue. When synehetic polymers or copolymers are employed, the effective amount will generally be between 0.01 and 2.0 pounds of chemical per ton of dry mud residue.
The copolymer added to scme of the subsequent sta~es 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 ~e~ orth above) and from about 65 to 25 ~ole percenr of acrylamide. AlthQugh it is believed ehat ethylenically unsaturated monomers besides acrylamide may be effectively employed in this copolymer, for considerations such as lS economy, availability and performance, the acrylamide como~omer is the preferred comonomer for employment in the instant inveneions. These subsequently added copolymers r-hould have a molecular weight such that t~e Brookfield viscosity of a 0.15% solu~ion of ~he polymer in lM NaCl at pH 8, UL adaptor at 60 r~p.m. is at leas~ 2.0 cencipoise, precrably at least 3.0 centipoiser The effective amount for these subsequently added copoly~ers will also vary depending upon the specific bauxite composi~ion being processed and the conditions present in the reco~ery stage. Generally, ~owever, che effectivc amount will be bctween 0.01 and l.0 pound of copolymer pcr ton of dry red mud solids, preferably about 0.05 to 0.5 pound of copolymer per ton o dry red mud ~olids.
At what specific stage subsequent to the ini~ial or "head" s~age in the caustic recoYery oircwit the copoly-mer comprising from about 35 to 75 mole percent acrylic acid or acrylate and from about 6S to 25 mole percent acrylamide r~ill be eEfective depQnds wpon a number or variables wit~ he recovery sys~em Sta!,e conditions such as temperature, pH, dilu~ion of liquor, and concen~ra-tion o~ red mud solids are believed ~o play an important rol~. The ~ype of bauxite ore itselE may also contribute to the effective loca~ion. Insofar as the exact mechanism of eh~ interaeeion bet~een the flocculant and the red mud solids i5 as yet unknown, the predietability as to which stage or stag~ variable controls the cop~lymer's efE2ctive-ness remains ou~sid of simple ca~egoriza~ion. Generally, the copolymer ~las been observed to be highly effective in the fourth or latter stages where the grams of ~aOH plus NaC03 in the solueion are less than about 100 per liter.
Since this Eigure is extremely approximate and may rest more upon coinciden e than the concrolling stage variable it is re~ommended that the following test be undertaken to deter-mine which s~age in a giYen caustlc recovery circuit is the effec~ive location at which ~o add the copolymer. This test entails:
General Test Procedure _ _ The stagc in ~he caus~ic recovery circuit to be te ted, which stage is not the ~Ihead~ stage, is referred ~o as the "~th stage" for the purpose of this test. The ~n-l)th stage washer underflow is diluted to 1.4 with nth tage washe~ overflow to produc@ a simulated n~h ~tage wa~her feedO This high dilution level is necessary to produce a level of reproducible ree settling in a test cylinder, preferably a 500 to 1000 ml. graduated cylinder.
To this simulated nth stage washer feed there is added the ~locculant to be tested in a 0.C5 weight p~rcent solution. The solution can be either w2ter or diluted spent liquor 5NaOH). The tested dosage of flocculant is added by syrin~c and mixed into the simulated washer feed by five strokes of a perforated plunger. The descent of the liquid/solid interace is timed in reet per ~our co determine the utili~y of the flocculant to the stage being testcd.
Whereas the exact scopc of the instant invention 35 i5 se~ forth in the appended claims, the ollowirlg specific examples illustrate cer~ain aspects of the presen~ inven-tion, and more particularly, point out methods of e~7aluat-ting ~he same. However, the examples are sec forth forillus~ration only and are not to be eonstrued as limita-tions on the present invention exeept as set for~h in the appended claims. All parts and percentages are by weight S unle~s otherwise specified.

Following the General Test Procedure set forth ab~ve, a red mud 10cculant copolymer containing 60 weight percent ~odium acrylate and 40 weight percent acrylamide having a molecular weight between 5-lO million and added ~o a simula~ed wa.sher feed of an Australian red mud caustic recovery ci~cuit wherein the initial stage was treaced with a 95 weight pereent so~ium acrylate, 5 weight percent acrylamide copol~mer. Dosages employed and se~tling rates obtained for the individual sta~es are set forth in Table I below.
Com~arat~ve Example A
The procedure of Example l is ollowed in every material detail except or the employment of a 95 weig~t ~0 percent sodium acrylate, 5 weigh~ percent acrylamide opolymer as the red mud floceulant in the test stage. Test results are set forth in T~ble I below.
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The procedure of Example l is followed in every 25 material detail except that the red mud flocculant employed in Compara~lve Example A is used in additio~ to ~he red mud floceulant of Example lo Test re~ul~s are set forth in Table I below.
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The procedure of E:xample 1 is followed in every material de~ail exeept ~hat there is now employed a sodium acryla~e emulsion havin~ a molecular weight of 5-10 million aæ the red ~ud flocculant. Te~t results are set forth in Table I below.

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a~
U~

O

JJ
:~ Q ~ cl ~"C ~ ~¢ C) O ~ ~ ~ o ,,, , o , .
O X ~ ~ ~ ~
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m ~ e~ CJ e~ e~ u e~ cJ

Li S.~ ~ h al a) a JJ
O
~n ~ ~ i A
al ~ e ~:
.~
.~ o ~ r~ O
~3 ~ r~ s~, o ~: tn . ~ ,_, .. , Ci s ~ J.J

3 en e ~1 ,~
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~ ~ z o~ ~ *

A sample from a Jamaican red mud circuit is taken from che washer stage underflow and diluted according to t~e G~neral Test Procedure. To this is added a floc S culant in a 0.05 weight percent solution of ~aOH. The flocculane ~ested is a 5-10 million molecular weighc co-pol~mer of sodium acrylate and acrylamide in the propor-tions set forth in Table II. ~ollowing the General Test Procedure in every material detail, the resulcs listed in Ta~le II are obeained.

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z z z ~ z ~ . o . . ~ . .
r~ o al ~
u~
-a3 c ~ o Ul ~ ~ oD ~ ~ ~ O
O U~~ O C~ 0 3 C3 C~

~ j .,_j _i 3 o ~1 0 0 0~ ~ O C:l O U~ O C~ 5 1~- 0 C C~

l~j v-l ~ s~ ~ o a c ~ o o o ~ u~ o o o o u~ o c ~
C~ U G~ a ~n~ r- ~ -n ~ cr~ r~ ~ ~ ~r c~ ~ ~n c ~ U
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a~
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~ co c~ r ~ ~e ~ ~ c ~r ~r a) G ~
u~

sa a:
:5 a ~ a3 u~

As can readily be seen ~rom the foregoing data, employment of copolymers of sodium acryla~e and acryl-amides produce dramatic increases in flocculation ac later stages in the caustic recovery circuic. Although a copoly-S mer of 40 weight percent sodium acrylate and 60 ~eightpercent acrylamide did not show any noticeable efect in the early stages of the test, the same copolymer produced ~urprisingly excellent results in a later s~age, thus emphasizing the importance of evaluating the appropriate-1~ ness of the individual stages in relation to the copolymeremployed.

2a ~5

Claims

WHAT IS CLAIMED IS:

1. A process for flocculating red muds from the Bayer alumina recovery circuit which comprises adding to at least a first stage of the caustic recovery circuit an effective amount of a flocculant selected from the group consisting of starch, homopolymers of acrylic acid or acrylates, copolymers of acrylic acid or acrylates con-taining at least 80 molar percent acrylic acid or acrylate monomers and combinations thereof and thereafter adding to some subsequent stage or stages of the caustic recovery circuit an effective amount of a copolymer containing from about 35 to 75 molar percent of acrylic acid or acrylate monomers and from about 65 to 25 molar percent of acryl-amide monomers.

2. The process of Claim 1 wherein the flocculant Added to at least a first stage of the caustic recovery circuit is a copolymer of acrylic acid or acrylate con-taining at least 90 molar percent acrylic acid or acrylate monomers and no more than 10 molar percent acrylamide monomers.

3. The process of Claim 1 wherein the flocculant added to some subsequent stage or stages of the caustic recovery circuit is a copolymer containing 50 to 70 molar percent of acrylic acid or acrylate monomers and from 50 to 30 molar percent acrylamide monomers.

4. The process of Claim 1 wherein the flocculant added to at least a first stage of the caustic recovery circuit is a copolymer of acrylamide acid or acrylate con-taining at least 90 molar percent acrylic acid or acrylate monomers and no more than 10 molar percent acrylamide monomers and wherein the flocculant added to some subse-quent stage or stages of the caustic recovery circuit is a copolymer containing 50 to 70 molar percent of acrylic acid or acrylate monomers and from 50 to 30 molar percent acrylamide monomers.

5. The process of Claim 1 wherein the effective amount of the first added flocculant is between 0.01 and 2.0 percent by pounds of flocculant per ton of dry mud residue and the effective amount of the subsequently added flocculant is between 0.01 and 1.0 pound of flocculant per ton of dry mud residue.

6. The process of Claim 1 wherein the subse-quently added flocculant is added to a stage or stages wherein the NaOH plus Na2CO3 concentration of the solution is less than about 100 grams per liter.

7. The process of Claim 1 wherein the subse-quently added flocculant is added to a fourth or latter stages of the caustic recovery circuit.

8. The process of Claim 1 wherein the subse-quently added flocculant is a copolymer containing 70 molar percent sodium acrylate and 30 molar percent acrylamide with a molecular weight in the range of 5-10 million.