CA2197457A1 - Improved process for the extraction of alumina from bauxite - Google Patents

Abstract

A process for the extraction of alumina from bauxite having a relatively high boehmite content including the steps of digestion of the gibbsite fraction of the bauxite over a period of about 2 to 2.5 minutes at a temperature of about 135~ to 145 ~C and at an A/C ratio of about 0.76, to achieve negligible boehmite reversion, substantially complete kaolinite dissolution and substantially no precipitation of DSP, subjecting the liquid fraction from the gibbsite digestion stage to post-desilication in the presence of hydroxysodalite seed to reduce the silica concentration close to the solubility of hydroxysodalite, subjecting the liquid fraction to secondary desilication using tricalcium aluminate produced externally by the addition of lime to spent liquor to minimise alumina losses from the pregnant liquor and to reduce the silica content in the spent liquor to reduce scaling, subjecting the residue from the biggsite digestion to an additional digestion step to digest the boehmite, said digestion of the boehmite fraction being formed at lower temperature and/or lower residence time than is normally the case.

Description

2197~
W096/06~3 PCTIAU95100~26 IM:PROVF,D PROCESS FOR THE 13XTRACTION OF ALUMINA
FROM BAUXITE
Field of the Invention This invention relates to processes for the eYtraction of alumina from bauYite, and particularl~ to improvements in double digestion processes for the extraction of alumina from bauxite.
Back~round of the Invenbion Although there have been previous disclosures and patents (for example U.S.
Patent 4994244 assigned to Alcan lnl~orn~fionsll Inc and Internatiorlal Patent 10 Publication WO 94~'074167 Comalco Aluminium Limited~ for processes based on multi-stage lower t~UI~J~ldlu.c digestion of boehmite cont~ining bauxites, none have yet been developed through to a Gul~ull~l-,ia] operation. The additional processsteps necessary compared to a conventional high ~ single stage circuit make these processes unattractive unless there are major gains in processing 15 efficiency. Tlle targets for such a process therefore have to be major cost items such as caustic usage, high IIIA;II~ C costs due to the high ~ alulc~a and scaling, improved liquor productivity and lower energy usage.
Digestion of g;bbsite at t~lul).lalul~a around 140~C is standard technology for many bau.Yite deposits. However, digestion at this hl~ I a~ul c; is not commonl-y 20 practised for boehmit- containing deposits because the boehmite is not attacked and needs subsequent digcstion at higher t~ la~ulc for recover,. In this case there is little advantage in attacking the hvo minerals separately. There is also a potential problem known in the industry as boehmite reversion which can lead to unwanted precipitation cf boehmite bacli out of the digestion liquor leading to losses of25 alumina and reduced productivit~.
This reversion issue has beell addressed in the patent literature by Alcan ~U.S. Patent No 499~244) by developing a pressure ~l~r~nt~tion system to enable faster separation of the pregnant liquor from the solids residue without the need i'or cooling. This step is largely the basis of the above U.S. Patent for countercurrent 30 digestion at klwer t~lu~J~alu~a. However, im/estigation of this process usingconventional feed preparation and residence times for the digestion conditions W096/06043 2~ ~7 7~7 PCTlb,119:5/0~526 disclosed in the ~.S. Patent. revealed that the digestion step as proposed did not give liquor of as high as desired alumina content due either to insuf'ficient solubility (at the lo~rer temperatllres) or due to significant losses of alumina from re-~ersicm at the higher tempcratures.
In a recent International Patent Publication WO 94/l~l22 Alcan have disclosed an aluminll extraction process in which a stream of ~7round bauxite containing gibbsitic a umina is f'ormed with a small portion of Ba~77er process speni caustic aluminate liquor used for digestion; the stream is mixed with the remainder of the spent liquor stream previously preheated to form a preheated slurr~-liquor IO mixture; the prehe7lted liquor-slurry mixture is passed throug}l one or more parallel reactic~n tuhes sized such that the slurrv remains in the reaction tube for a resideIlce time just suf~icient to ex~ract essentially all of the gibbsite from the slurr~ and no more than about four minutes, red mud solids, still containing undissolved boehmite and70r diaspore alunnina, are separated from the pregnant liquor in a solid-liquid separator operating at cllhshlntiqily the same ~ tulc and pressure as in the reaction tube; and dissolved silica is removed from the pregnant liquor b~ seeding the liquor ~ith Bayer process ~ sili~7Atinn product.
In International Patent Publication WO 93120251 Sumitomo disclose a similar Bayer process which pre-empts much of the subject matter ol the abo~e Alcan publication.
Each of the abo- e prior art dis.losul~ suffers from the disadvanta7~7e that thefinal silica le~el ia either too high andlor the loss of caustic during ~l~ci~ qtinn is too great l'or the pn~cess to be rco~ lly viable for bauxite v.7ith signif;cant levels of reac~ive silica.
Summar-7 of Iuventioll and Object It is an object of the present ins7ention to provide improYements in double digestion processes ol~ the type described abo~e to provide impro-/ed e~traction of alumina Irom mixed gibbsiticiboehmitic bauxite using acceptable processing conditions. wllilst minimicing7 the ~ trim~ntql effects of DSP lormation, including losses of caustic, and uncontrolled scale for7nation ~ ~74~7 ~'0 96~0C0~i3 PCTIAU9~/00526 In one aspect, the in- ention provides a process for the extraction of alumina from baw;ite havin~ a relati~ely high boehmite content including the steps of digestion ot'the~ gibbsite fraction of the bau~;ite, solidlliquid separation and digestion of the boehmite fractiol1~ said process being characterised by relatively f'ast lou 5 t..llp,ldlule digestion of the gibbsite ~raction to give a high A/C ratio liquor, negligible boehmite reversion, cllbst~nti~lly complete l~aolil1ite dissolution and sllhct~nti l~ly no precipitation of desilication product (DSP) in digestion~ and a two stage post-desilication including the remo~al of some of the silica in a first stage followed by remo~al oi' sllbst~nti1~lly all of the remailling silica in a second stage 10 bv the use oftrical-ium aluminate to reduce losses oi'alumina and lo-vering ofthe A/C ratio of tl1e pregllant liquor In a presently preferred form of the invention, digestion of the gibbsite fraction is conducted over a period of about 0 5 minutes to 5 minutes7 preferably about 2 to 5 minutes at a t~ul~lalulc sllhsl~nti1llly falling within the range 120~C
15 to 160~C, preferabl\ about 135 to 145~C7 to an AIC ratio of 070 to 0807 preferably about 0 75 The process can operate at AIC ratios less than 0 70 or over a wider t~ul~ alul~ range but ef~;ciency is reduced The process is preferably carried out at caustic soda levels (e7~pressed as Na~CO3) from 200 g~L to 450 g,/L, and most preferably from 300 g~ to 350 g,'L
20 The use of higller caustic soda le~els allows higher pregnant liquor A/C's to be achieved withc)ut reachillg the solubilit~ limit of the alumina This assists in pre~enting &ibbsite precipitation during the solid liquid separation and desilication stages particularly ~here these are carried out at alllm~iJlle~ic pressure Use of lo~er l~ J.lalui~ for the l)SP seeded post desilication can be advantageous in that 25 the DSP formed at these t~ ,iul~ has a lower caustic soda content and this reduces the loss of caustic soda firom the circuit This high caustic soda level also increases the solubilih~ of the hydroYysodalite which allows more silica to remain in the liquor wl1ich can be detrimental to subsequent operations where the dilution needed to reduce the caustic soda level prior to precipitation also destabilises the 30 silica The use of the two stage post-desilication with TCA treatment overcomes this problem.

. . . . .. .. .. . .... . .. . . . .. .. . . . . . . .. . . . . . . .

WO '96/06W3 2 ~ 9 7 4 ~ 7 PCTIAlr9S/00526 Preferabl,v a mpid s(3]id/liquid separation process is applied to the product ofthe gibbsite digeslion process to ~nh~t~nti~lly prevent prec;pit.ation of OSP and gibbsite. This .step Cclll either be carried out at the digestion t~-".l,. ,,.n"c ~as tauyht in Alcan's patent~ or after cooling to boiling point in cases where the residue has 5 sufficiently good settling characteristics to allo~ rapid separation.
While the ba-lxite processed according to the above definitions can be subjected to grinding to e~pose the gibbsite, it is preferred that the bauxite is onl~
crushed or lightl~ grourld if necessary and is not subjected to a i;ne grindillg step~
essential to c thr r processes, such as that proposed by Alcan in WO 94/18122. The 10 avoidance oi' fine grinding assists in the solid/liquid separation and is most beneficial ~~here this separation is carried out at atmospheric pressure. The absence of the need for ~rindiny can also simplify the equipment used and improve heat exchange eff;ciency particularly where the bauxite is not slurried prior to addition to the digester. as required by the Alcan process just referred to. In many 15 instances. the bauxite as mined, particularly the highly reactive ~h'eipa bauxite, ;s often suitable tor processing witnout even a crushing step. It has been found that particle sizes Up to .~bout !Otnm are capable of being subjected to satisfactoryextraction processes. In the preferred t~vo stage process small amounts of unreacted gibbsite~ can he extracted during the ~.,h~l" .ll boehmite digestion and are 20 recovered.
If the bauxit- is subjected to crushing or grinding prior to the initial digestiom then a particle size of greater than 20011m and preferably between about 200~1111 and about lmlll has been found to be preferable to assist in subsequentremo~al of quartz prior to boehmite extraction if this step is desired. 'I'he slurty, 25 residue trom the in;tial digestion stage may also be lightly g~round tu assist in pro~ iding a more ef'tlcient separation of the quartz and to assist in impro~ ing the boehmite digestion step.
The sholt residellce times and relatively IOW temperatures stipulated above also have the advalltage that none of the quartz present in the bauxite is attacked 30 while the l~aolin presellt is digested to give a high silica liquor. ~Nith the short trme used this silica is retained in solution urtil after the boehmite cnnt~ining solid ~ 97~l57 ,~ wo 96/060~3 PCr/.~llg~100~26 s residue is separated alld thus can be treated separately allo~ing the potential for reducing the caustic losses. Aluter- and Sumitomo have disclosed in U.S. Patent No 5122349 and h1 Internatiollal Patent Publication WO 93/20251, operation of the digestion process such as to a~oid part of this kaolin dissolution. This has limited S advantages for bauxite ~here the boehmite is to be leached subsequently unless the residual kaolin can be readily separated. There is no obvious commercially attractive teclmologv for such a separation step at present.
The residue ~rom the gibbsite digestion is treated to reco~er the alumina contained as boehmite. and an~ inr~-m~ ly digested gibbsite, ~hich has not been attacllced. The exact process used depends upon the amount of boehmite present and the level and physical nature of an~ quartz present.
Il1 another aspect. the invention provides a process for the extraction oi' alumina from bauxite having a gibbsite fraction and a boehmite fraction, hlcluding the steps of digestion of the gibbsite fraction, solidlliquid separation and digestion of the boehmite fraction, said process being ~Lala~t~ ,d by digestion of the boehmitic fraction by digestion of a boehmite containing mud slurry at a loi~
tell,l,e,d~ùlc using spent liquor having a low AJC ratio and a caustic soda content of from '~50-i5() g!l and preferably from 300-350 g/L.
In a preferred folm, the moderate A/C ratio liquor separated from the ~0 boehmite digestion stage is used for the low t~ aLwci digestion of the gibbsite fraction defined above.
The boelmlite dige.stion stage is preferably- conducted for a period of about 2 to 12() minutes at a tCllllJ.ldLulc of about 150~C' to 200~C to yield an AIC ratio of about 0.~ to 0.6.
It has been determined that by using specialist equipment ~hich improves the solid/liquid contac~ and increases the exposure of the boehmite to the liquor the reco~,ery cau become acceptable at tt;llll~/CldLUlC:~ as low as 150~C. One example of the equipment suitable is the stirred mill such as provided by Metprotech.
Alternati~ely very short residence times can be used at higher t~-"p r~ CS~
The tc~ .laiulc chosel~ tc~r the step is preferably set by the solubility requirement rather than the reac~ion kinetics throu~h having Lhe colrect combination of .. . .. .. . . . . .. . .. .. . ... . . .. .. . ....

3 21 ~ 7 ~ 5 7 PCr/AU9~100526 equipment and physical properties of the boehmite containing residue to bP
digested. The solubilit~ needed in turn depends upon the alumina level in the spent liqllor bein~ used and Ihe total amount of alumina desired to be extracted from the solids. The use of the spent liquor to prepare TCA, as described further belo-5 further helps in lo~erhlg the t~ atulr needed.
To minimise the quart~. dixsolution in this step even at the lowertemperatures it n~ay be preferable to remove it via physical separation f'rom the residue. The preferred metllod is to use hvdrocyclones or similar separat(lrs tc~ take out tlle quartz particles ~vhich are coarser than the boehmite minerals. This step is 10 preferably carried out w1der pressure to avoid the need to cool the liquor. Part of the spent liquol can bc mixed with the residue from the pressure decanter to give a slurry more suilable for the separation. As to whether this step is included or not depends upon the 1e~ e l of' quartz present, its reactivity under the conditions chosen for the boehmite digestiom and the efficiency possible in the separation step. 'I he 15 quartz attacl~ does not interfere with the process and therefore treatment to remo~ e it is purel~ an ~conomic consideration.
As alread~ mentioiled above~ the pregnant liquol produced by the gibbsite extraction sta,~e is much too high in silica~ being already ~u~ u~ to be fed to precipitation for recovery of the alumina. Removal of this silica through 2V formation of a desilication product such as sodalite by seeding with d~ ti-~nproduct is conventionally practised and calT,ving this step out either at atmc~spheric pressure or under pressure at elevated l~ alu~ is disclosed in the Alcan U.S.
Patent. The limitatioll of the prior art technology is that the solubilit- of the sil;ca is relativel! high and is dependant upon the levels of caustic and alumina in the 25 liquor. It has heell found that at the caustic and alumina levels preferred to give the desired productivity from the digestion the residual silica levels from conventional desilication are ll"~rc~l,t~l-ly high. These silica levels can lead to excessive scalin, and cu~ ";"~ n in the precipitation circuit and during sllh~P(IllPnt reheating of the spent liquor for use in digestion. Previous workers 30 have employed lime additions for silica removal, for example, Crisp in W0 94/02416 propose.s a t~o stage post-desilication treatment using desilication product ~ ~74~7 WO ~)61060~3 PCTIAU95100S26 seed and lime at the tligestion temperature. The use of fime is unattractive as the hydrogarnels formed contain alumina and thus reduce the alumina contellt of the liquor as evidenced by a drop hl .~IC and hence the ~ du~ti-~ity.
In a further aspect, the invention provides a process for the extraction of 5 alumina from bauxite containhlg gibbsite and boehmite by digestion of the gibbsite and boehmite fractions of the bauxite, characterised by one or more desilicationsteps in which tricalcium aluminate is added to pregnant liquor to remove silicatherefrom, said process being characterised by the step of produchlg tricalcium aluminate f'rom spent liquor by the addition of a suitable calcium cnn~ining 10 compound such as lime to the spent liquor.
In a prel'erred rorm of this aspect, the gibbsite and boehmite fractio~s are separated in separate process steps, and the pregnant liquor produced by the gibbsite extraction step is subjccted to post-desilication in accordance v~ith the above definition.
ln a particularlv preferred forrn of the invention, the pregnant liquor is subjected to prima~ ~md secondary post-desilication. the primary desilication step preferably being achicved by subjecting the pregnant liquor to hydroxysodalite seed to reduce the silica concentration close to the solubility of hydroxysodalite. The secondary desilication step preferably involves the use of tricalcium aluminate to remove the majc rity of the remaining silica as hydrogarnet.
In a related a~pect, the invention further provides a process for removing silica from Bayer process liquors comprising the step of introducing tricalciumaluminate into the liquor at a c~mf f ntr~tion necessary to convert at least some of the silica h~ the liquor to hydrogarnet.
In a prefèrred forrn of the invention, the liquor to wllich tricalcillmqlnnnin~tf~
is introduced is to pre nant liquor, the trif~ ,.qhl.~.i"~tf preferably being produced in thc spenl liquor to reduce the amount of alumina in the spent liquorand to reduce the armount of silica in the spent liquor.
In a still iurther aspect of the invention, the invention provides a process forthe extraction of alurnina from bau.~cite including the steps of digestion of the gibbsite fractioll, solidiliquid separation and digestion of the boehmite ~raction, said , . ... .. ..... .. . . . .

2~4~7 WO 961060 U PCIIA~195/00~2li ~
. ~ ~

process being characterised by subjecting the liquid fraction from the gibbsite digestion stage to post~ i licatirn in the presence of hydroxysodalite seed to reduce the silica conccntration close to the solubility of hydroxysodalite.
In a prefèrred form the liquid fraction is subjected to a secc)ndary 5 desilication process in which trir:~lcillmalnmin ~tr (TCA) is used to rermove the majority of the remaining silica as hydrogarnet. Ille l'C'A is uscd to chanye the solubi]ity of the silica species :from being controlled by that of thc hydro~:ysodalite to that of the hydrogan1et. Other calcium containing species can also achieve this (e.g. I;me Grossular. Dolomite) and can also be employed but do not appear to be10 as commerciall~ attractive as TCA. The post-d~ r~tinn stages are preferably carried out at a tt:ln~ Lulc~ s~hstantially falling within the range 8()~ to 160~C.
Tempenitures hl thc higher range are preferred to maintain gibbsite solubilit~, and to improve the kh1etics of the rircilir?~tir,n steps but may be detrimental in increasing the amoullt of caustic soda lost.
1~ The TCA is preferably prepared externally and in tbis way the alumina and calcium needed are being fed to the liquor so only the silica is being removed.
rcA can be prep_re~ in a number of wa~ s all of which invol~e reacting lime Wi~l alumina gellerally in a caustic sohltion. In the industr~ this has most commonly beeu done using pregnant liquor (for example IJ.S. Patellt 4~18571 20 assigned to ~v AMI) wl1ich has a high alumina content and the reaction readily goe~s to completion.
As defined abo~e~ it is preferred that spent liquor is used to producc the l C.P~. This a~oids ioss of potential product alumina in the pregnant liquor and also lowers the alumina level in the spent liquor to allou~ it to dissolve more alumina 2~ from the boehmite in the gibbsite digestion residue. A further advantage ot' preparing TCA in spent liquor is that residual silica can be removed thus reducing scaling during reheating of the liquor for the boehmite digestion stage. In certain configurations this procedure of adding lime to spent liquor may be applied solel v to reduce this scaling rather than to pro~ide TCA for fl~cilinating pregnant liquor 30 In that applicatioll the resultant tl~il;. atirn product may be allowed to remain ~/ith the liquor and retum to the digestion rather than be separated out.

-~ wosc/06w3 ~1~7A~ PCTI~U9slons2h A complexity in using spent liquor to produce TCA is that the ratio of lime to liquor has to be carefully controlled to ensure complete reaction as if the liquor is depleted too much in alumina the TCA species become soluble and the tr2~nsfnrm~tion is incomplele. The situ Ltion is further comrlieSItl~d by the f'act that 5 high dissolved Na2CO3 concentrations in the liquor can cause formation of CaC(~3 rather than TCA.
I'his use of 'I'C'A prepared extemally with spent liquor for control of the fhlal residual silica le~el in pregnant liquor is applicable to any circuit but is particularly uscful for operations running ~vitll high caustic and alumina levels 10 ~here the silica solubility f'or the nomlal sodalite is high. In these circuits the solubility change acrvss the precipitation stage is suf~lcientl~ high to cause problems of siiica scaling and c~,l,n~"~i"~ n The addition point of this step will depend upon the plant arrangement but would most probably be during the final stages of the liquor cooling ~vhere the s,vstem is above boiling point and under~5 pressure. The desilication reactions proceed more rapidly at higher te~ clatulcs.
It has been found that the prei'erred limelspent liquor ratio depends upon the tcl~ dlulc and liquor composition with 10 grams of CaO per one litre of spent liquor being preferred at 100~C for liquor of CS - 240 glL and A/C of 0.40. Tileupper limit is dependant upon the solubility of TCA and avoiding excess residual~0 lime.
In the preferred embodiment of the im~ention around 75-80~~o of the silica is remo~ ed sls .sodalite ~vith the remainder then beillg p, c~ J;~lt:d out as hydrog~rLrnet using the TCA. The t~vo silica containing residues are separated out using either decantation or f il~ers and can be further treated to recover v aluable conctitllPntc such 25 as caustic sod a i f desired. Processes for this have been disclosed by other ~A~ori~ers and include treatment with sulphur dioxide andlor carbon dioxide.
In a pret'crred arrangement, the tricSllcillnn~ minslt. is separately produced from spent ]iqu(ir alld lime, although other processes may be used.
It is preferred that all of the above defined aspects of the inventioll are 30 included in the extraction process to ma~imise the gains in processing efficiency.
In t~is form the invention provides an improved ~ ,C~ lg method for boehmite .... . .. . ... . . . ... . . . . . . . . . . . .

::

WO 96/U60~3 21 9 7 4 5 7 PcrlAtl95lllos26 ~

.

containing bau~ites ~hich does allou k~r operalion at lower tennperahlre whilst giving higller producti~ ity reduction in flle caustic losses due to silica auld reduced scaling due to silica throughout the entire circuit Brief l)escription of the Drawin~s In order that the in~ention may be more readil~ understood reference is made to the rollowing description which should be read in conj~mction with the drawillos hl ~hich:
Figure 1 is a process flowstleet wLlich conceptuall~ details the process steps of the prefe~Ted embodiment of the invention:
I() Fic~ure ~ is a graph of the liquor A/C achieved in G~ hllGIIlal trial runs testing the process ermbodying the invention;
Figure 3 details the liquor caustic after digestion for the experimental trial runs referred to above;
Figure 4 details the gibbsite extraction achieved in the GXp~ tal trial runs refèrrecl to above;
Figure 5 details the boehmite mass in bau~ite and in mwd for the G~ lnll trial runs referred to above;
F igure 6 de~a;ls the kaolinite fli~iolilfit~n achieved in the ~ trial rurcs referrecl to above;
Figure 7 details the liquor SiO~/CS ratio after digestion for the experimelltal trial runs referred to above;
Figure 8 is a graph showing liquor A/C liquor SiO2/CS and Na.O/SiQ mud ratio achieved in different G,X~ trials o~er the indicated time period;
Figure 9 is a graph showing the distribution of red mud components aRer digestiom and Figure L0 is a graph showing changes in soda~silica ratio with temperature for varh~us Na.CO~ ievels D~s~ Jtion of Preferred ~ 1-Referring to th- preferred flowsheet of Figure 1~ it will be noted that the notable sleps of the flow-sheet are the short residence time narrow t~ tUlG
range gibbsite digestion at high caustic soda leveis the use of' t~o stage pc~st-WO96~0~1)43 ~ 4 5 7 PCTIAU95/005~6 desilication ~ith the second step employing tricaleillmAlllminAtP and the silicacontaining residue being separated for further treatment if desired; the preparation of the TCA~f'rom spent liquor to minimise the alumina losses from the pregnant liquor~ to pro~dde a lower alumina content spent liquor for the boehmite digestion 5 and to reduce the silica content of spent liquor and reduce scaling boehmite digestion at lower temperature and/or low-er residence times than is nonnally achieved; and separation of the quart~ from the gibbsite digestion residue to avoid attack in the boehmite digestion The no~el steps can be applied either as a complete pacl~slge or \vhere ~il.u~ Ld.,ces are appropriate indi~!idual parts can be I () incorporated to improve an existing operation Most of the steps in the flowsheet can be carried out under pressure to avoid the need to cool the liquor except where there are adis~ntages in decreasing caustic soda losses by operating at lower temperatures belo~ the boiling point The operation and advantQges of the preferred flo~vsheet ~ ill be better understood from the follo~ing description Gibbsite Extraction Cibbsite to Boehmite Tr " . r Based purel~ on thermodynamic calculations, the gibbsite to boehmite transition temperature is predicted to be around 100~C Ho~ever, the rate of conversion is e~;ceedirlgly slov at this ttlll~).,alu~ A number of l~al~h~l~ claim that the upper temperature limit for the existence of gibbsite in aqueous or caustic solutions lies in the range 125-155~C Results of our e ~ hlle~ are in general agreement ~ith tshesc ihldings ~vith the transformation occurring at 140-150~C in ~vater and 12()-130~C in caustic solutions (seeded w-ith boehmite) l~or shorter contact Limes little transformation v~ras observed belo~ 140~C
l he exrerimentsil data presented in Figwre 3 supports the v iabiliti- of a fastlo-~ U~5diUI- digestion of the gibbsite fraction to give a high AIC ratio liquor, 5ll~ t5~ntiAlly c~lmplete kaolinite dissolution, and prevention of DSP precipitation These digesticll1 tests were conducted at 130~C in spent liquor of caustic c~,nc~ulldLion 'oOg,i~ and A,~C ratio 0 32 The AIC ratio (diamonds~ reaches a maximum at ~2 minutes digestion time as does dissolved silica (stars) Precipitation oi' DSP i~circles), characterised hy the Na~OlSiO2 ratio in mud is onl-WO 960G043 ~ 1 9 7 ~ 5 7 PCTfAlJ9~/00526 ~20~b complete o~ er the same period. Similar data is produced at 145~C digest;on Lu~.
Digestion A compretlensive study ~has undertaken tc~ identify conditions under uhich S boehmite reversioll w as negligible during low le~ tulc extraction of gibhsite to high liquol- A,C' ratios. The parameter ranges u-e}e:
T~".P~L~ C : 130-180~C
Time : 0-30 minutes Margin : 0-0.25, from gibbsite solubility Results hldicated that boehmite reversion uas minimiced if digestion was conducted at as lo~! as possible a te~u~ alule, for a short time and maintaining as large as possible a margin ~vith the e~quilibrium gibbsite solubility. M~ints~ininE a margin ~as less important at lower digestion t~ lalul~. The minimum digestion t~ .dlule, however, was also governed by the need to achie~e high alumina 15 Gvllc~,lLL~Iions in the liquor. A set of successful conditions were identified as the follo~ illg:
Temper~lhlre : 140~C
Time : 2 minutes Caustic : 300 ~L as Na~COj Aim ~inal A/C : 0.76 ~e~lllilil.. ;.l,.l AIC 0.79) Results ohlained from trials employing the abc~ve conditions (Figmres 2-71 concluded that gibbsite extraction was complete and re~ersion of dissolved gibbsite to boehmite ~as negligible as the amounts of boetlmite enterillg and leavillg digestioil werc similar. Virtually all of the kaolin present dissolved.
rhe practicality of suppressing kaolinite dissolution during gibbsite extractio is not appealing fc1r mixed gibbsitic,boehmitic bauxites. ~uring extraction oi boehmite in a subsequent digestion stage, any remaining kaolinite wollld almo.stcertainly dissolve. It is therefore pre~erable to operate the gibbsite~ extraction stage under COll(iitiOllS ~here all the kaolinite dissol-~es. The only iikely exceptiol1 would be in cases ~here this gibbsite digestion step is applied without any attempt to wo 96~ 043 ~ PCTIAI~gS/00~26 reduce soda consw~lption where a predesilication step is included to transform the reactive silica to DSP prior to digestion.
One of the major advantages of full silica dissolution but minimal l)SP
formation in the reactor is that this allo~s the initial post-desilication to be carried 5 out at sufficiently lo~v tc.llp~aLule ~C100~) to enable formation of a lower sodium level DSP and hence reduce the caustic losses. The difi'erences in soda/silica ratios in DSP as a lunction of te~ atul~ is shown in Figure 10. The secondarS
desilication step USillg TCA could then be carried out at either 100CC t'or simplicity, or after reheatillg to 135~C+ to improie the liinetics ofthe reaction.
Silica Removal Digestioll of bauxite ~mder conditions specified above is sufficient to dissolvethe majoritv of' kaolinite present but in~nfficient to allo~v ~ignifir~nt formation of DSP provided a rapid solidlliquid separation is possible. Hence, bound soda losses to the red mud are minimi.~ed As a co~ e howe~er, the resulting liquor is 15 very high hl silicâ and requires post-desilication prior to precipitation.
E'ost-desilication of the liquor at 140~C in the presence of DSP
(hydroxysodalite) seed has been found to be successful in reducing the SiO2!CS
ratio from 27.Y10 3 (~8 g~) to 5x10-3 ~1.5 g/L). This ratio remains too high forthe liquor to be acceptable for preririt:~tion. This is particularly the case when the 20 pregnant liqucr has higù caustic and AIC ratios and where the solubility of hydroxysodalite is si~lificantly higher than under the conditions described by Alcan in U.S. Patent No 4994244. This means that single stage post desilication with hydroxysodalite seed would be ull~u~e~;,rul in reducing the silica concentration to acceptable le~els. I)ilution of this liquor prior to desilication would assist in 25 lowering the silica levels but would also destahilise the gibbsite fraction. For this reason, the level of dilution ~hich can be used to assist desilication is limited and is not suff cient to allo~ adequate desilication whilst mAintRinin~ hi~h productivity.
Even if excess silica was tolerable in precipitation. additional scaling in spent liquor heaters upon reheating prior to being used in digestion ~ould be 1~ c~

.. , ... . ...... .. . ..... . ... . . . _ ... . . .. .. . .. . . . ..

2~ S7 WO 96/~ 3 PCT/AU9~1Oû526 liydroxysodalite Solnbility A study was undertaken to confirm the expected solubilih trends in high caustic and A/C liquors and resuited in development of the following equation:
[SiO2]eq = 1.7 x 10-~ ICS][A~
'l'his equation is in good agreement Witil data extrapolated from previous work. It will be appreciated that increasing the caust;c and A/C ratio to improve liquor producti~itv will also result in increased dissolved silica levels. T}le conclusion is that increased liquor productivity and single stage post-desilication using h~ droxysodalite seeding are not compatible.
I() ~ Tllislimitatiol-ledtotheapproachofusingt~(>stagepc~st-desilicatioll where as much silica as possible is removed in a conventional DSP seeded post-desilication and then using lime additions to remove most of the remaining silica from solution as hydrogarnet species (less soluble than hydrox~sodalite~ as disclosed in WO 94/02416.
l'wo Stage Post~ n Lime was found to be very effective in removing silica. Mowever, uuaco~ LLl~le reductions in the pregnant liquor A~C ratio were also observed due to the large amoul1ts of alumina incorporated in hy~ Jt,dl~ ,. To counter this the preferred approacll of using externaily prepared TCA to provide the calcium and 20 alumina was de~eloped. Alternatively ~lf ciiir~tinn can be achieved usino species such as Grossular (3CaO.AI2O3.3SiO2) or the analogous MgO systems buL these do not appear to offèr any marked technical advantages and are not as ecnnomir~lly attractive.
IL has been shown that it is possibie to produce an almost pure sample of ~5 TCA in synt}letic liquor, however the fornation of 4CaO.AI2O3.CO2.11H2O is preferred whell carbonate is present.
In many plant liquors. carbonate will always be present in large~ amounts due to the reaction of caustic soda with carbon dioxide in the atmosphere:
?NaOH + CO2 ~ Na2CO3 + H20 E.~ were performed to deterrnine whether (i) TCA can be formed when lime is added to plant liquor, and (ii) the species forrned as a function of ~ g~7 WO 96t01i043 ~ PCTIA1195100526 temperature Oll addition o~ lime to spent liquor and the results are detailed belo~h~
In each ~2,t~ C.lL, lOg of lime was added to spent liquor (240 g,'L CS7 0 65 ~'L
SiO2 AIC 0 40) and reacted at the desired t~ lulc for one hour The spent liquor may typicall~ ha-~e an A/C ratio of 0 30 to 0 40, more palli~ulall~ 0 35 to ~ , 5 0,40 and this provides sufficient alumina for the production of the required amounts of'l'CA
Table 1: Liquor results Trial #Temperature CS g/L A/C SjO2 SiO2/CS
60~C~ 237 9 (~ 376 0 44 0 00185 2 100~C 233 5 0 381 0,13 0 0()()56 3 140~C 239 5 0 373 0, I l 0 00046 4 180~C 261 2 0388 0 10 000038 220~C~ 256 0 0 362 0 08 0 00031 6 250~C 235 9 0 368 0 09 0 00038 The X~l i analysis of the solid products show that there are var,~ing amounts of impurities in each of the trials (Table 2) The major phase in each of the samples is Ca3AI2(0H),..
Table 2: XR:D Analysis rial# Ca~AI2~OM),2 Ca,Al,(O~4CaOAI2O,CO2llH2O ca(O~1)2 CaCO2 Tricalcium TricalciomCalcium AluminiurnPortlandite Calcite Alulllblate AluminateOYide Carbonate H~drate 21) 1 M ajor Reference Trace Trace '['race - 2 M,ljor Reference Nl) ND Trace 3 Major Reference Trace ND 1'race 4 1' lajor Reference ND Trace Trace Major Reference Trace Minor Trace 6 Major Refe~rence Trace Minor Trace WO 9~i~1)60'13 21 9 7 4 5 7 pCr/A n~- rr -~f' The XRD r~sults sho w that TCA was the ma.ior phase in the solid produced at all t~ e.aLulcj investigated. Only trace amounts of 4CaO.AI,03.C0I.l]H,O
were present. Tb. XRI:~ pattern iadicated that e7~cess lime was also present in the solid product.
S From the experimellts coaducted it caa be concluded that a high puri~ CA
can be formed in spent liquor. The main impurities are Ca(OH)2. C'aCO} auld 4CaO.AI~03.CO..l IH,O. T~ alul~ has no significant iniluence over the formation of TCA.
The formation of TCA also concurrently removes the silica from the liquor 10 e~tèctivel~ desilicating the spent liquor.
Quartz Removal It will be noted from the preferred flowsheet of Fi~ure 1 that quartz removal preferabb, takes place prior to the boehmite digestion stage. This is preferabl~achieved by thc use of a conventional physical separation step. This is possible15 because the size distrihutions of the quartz and other mud cv~ ,u~ lL~ after tbe gibbsite digestion stage, as detailed in the graph ol' Figure 9, indicates that quartz remains in the course fraction. Therefore size separation using standard physical separation teclmiques~ such as cyclones and screens~ allows betweell 80~~o aad 90~o of tae quartz to be removed with much less loss of the boehmite.
Boehmite Extraction ~lechallo Leaching Extractioll of boehmite from gibbsite free mud was first evaluated using a pressurised stin-d ball mill as the reactor. Results were very encouraging in that boehmite could he extracted (95%) at 1 50~C oi er 15 minutes to an A~C. ratio of 0.48 (initial A/C' ().3 ], equilibrium AJ'C ().48) and final caustic strength of 340 giL, In those samples tile quarL!7 W.15 not removed prior to this step and ~ iihlldtely 7()U~o of the qu~rtz uas attacked as a result ol'the milling where the meaa particle size of the mud i~ilS rcduct,d to CI()~utn Con~entional Autoclaves Under tbc same digestion conditions use-d for the stirred mill trialst quartz attacl~ was negli(Jible in con~ eutional autoclaves, however, boehmite extraction w as ~'0 9610G0~3 219 7 45 7 PCTIAU9~100~26 only 64% and the final ~IC ratio 0.42. Over a series of comparative trials, the final C, was typically 0.05 units lo ~er, boehmite extraction 25-30% lower and S;Oa inIiquor 35~o higher in the autoclave compared to the stirred mill. On the positive side, quartz, attack ~as negligible in autoclaves at these lo ~ temperatures.
A further stlldy in autoclaves was conducted where hlcreasing the digestion residence time irom 15 to 60 minutes increased the final AIC ratio k~ 0.45 and boehmite extraction to 78~/o. This was very similar to a result obtained at 30 minutes residence time so it was felt that no further gains would be made by increasing residcnce time be~yond 6() minutes.
It has been found that A/C ratios approaching equilibrium could best be obtained by overcharging the system (i.e. Iow extraction). Conversely. high extractions uere possible vihen charging was done ~ith a large marghl (i.e. Io~
final AIC1, for ~ hich the TCA production from the spent liquor prior to its use in the desilication is therefore very advantageous. Increasing the lr~ ,...m~c also15 assists as it increases the equilibrium solubility and therefore the ~/C margin available as ~~ell as increashlg the reaction rates.
By careful selection of conditions digestion times as low as 2 minutes were found to extract virtually all of the boehmite at 200~C without significant quartz attack. In this case as ~~ith the gibbsite digestion a simple pipe system would be 20 effective.
The pref'erred equipmenVt~ J..alu.c~ combination is therefore largely an economic decision ~hich ~ill depend upon the properties of the bauxite being treated and the cost of energy at the operating location.
Preferre(l Flo~shcet Based Oll the h1tormatiol1 presented abo-e, it is possible to specify a flowsheet which represents a pret'erred sequence of operations for a dual stage/lo~ temperature digestion circuit for processing mixed gibbsitic/boehmitic bauxite and this is provided in FigLIre 1. A sult1mary of the main features of the flowsheet is given belo~h:
~ A fast (~2 minutes) low tellll,.l.. lulc (~140~C) digestion of bauxite is carried out to e~itract the gibbsite ~iraction. The resulting liquor has an A/C ratio of WO 96/i\6043 ;~ ~ ~ 7 ~5 7 PCT/AU9~/00526 about ().7~ or boehmite reversion is negligible and kaolinite dissolution is virtLIall~
complete. Insufficient time is allowed for DSP formation so the liquor is high in silica.
~ A mpid solid}li4uid separation at the digestion t~ alure is et'fected 5 to prevent hoehmitc re~ ersion and precipitation of DSP onto the mud.
~ Thc Iiquor is subjected to post-desilication in the presencc of hydro,~;ysodalite seed to reduce the silica c-",~-~ ,l,,.l;on ciose to the soiubility of hydroxysodalite.
~ 'Ihe boehmitic mud slurny is digested at low lt~ J.,Ialul~: in spent 10 liquor of lou A/C ratio to a moderate final AIC' raticl. lhe moderate A'C' ratio liquor is used for the gibbsite extraction operation.
~ 1'he portion of DSP not recvcled for liquor post-desilication is transtèrred to a soda recovery stcge.
~ L,iquor from post-desilication is sub3ected to a secondar~v ~lPcilic~3tion 15 using rcA to remc7ve the majority of remaining silica as hydrogarnet.
~ 'I'CA to be used for secondary dl ciliaQtinn is produced by reaction of spent liquor witll lime. Traces of silica in spent liquor are removed and the A/C.
ratio is lowered ~vhich facilitates lo~Y tclll~lc~atulc extraction of boehmite.
The percei-/ed advantages and novelties of the flowsheet described abo~e are 20 as follous:
~ Collducting the gibbsite digestion under conditions sufficient to e.~dract both the gibbsite and liaolinite but insuffilcient to allow significant precipitation o f DSP onto the red mud or re~ersion of gibbsite dissolYed in the pregnant liquor tc boehmite.
2~ ~ Rapid separation of the solid from the liquid to minimise the t'orrnatioll of DSP during this step t'.~ilitated by control of the size distribution ot' the initial bauxite charge.
~ T\~(l stage post-desilication of the pregnant liquor7 where the~ second polishing stage is carried out using TCA to avoid reductions in pregnant liquor ~'C
30 ratio normaliy associated uith lime and at t~,~up~lalul~.~ high enough to a~oid precipitation of alumina trihydrate.

~7~57 wo 9u06043 PCT/~Il95l00526 ~ (.ontrol of the A/C of the pregnant liquor to allow the initial desilication to be carried out below boiling point such that a lower soda content DSP
- is obtained in the seeded desilication ~vithout concurrent gibbsite phase precipitation occurring ~ Prcparalioll of TCA by reaction of lime with spent liquor where any residual silica in the spent liquor is removed which acts to reduce scaling during sl~bc~PqllPnt heating of the spent liquor and the A/C ratio of the spent liquor is reduced to f:acilit3te subsequent boehmite digestion This preparation of TCA andsilica reduction from the addition of lime to spent liquor and the subsequent use of ]0 the TCA ~Vit]lill the circuit can also bc used in other ~rr~ngPmPntc of the Bayer process as well ~s bl the prefèrred two stage digestion process ~ E~lraction of boehmite at a tpmp~pr~tllre similar to that of the gibbsite extraction ~here the Icl~ler spent liquor A/C ratio caused by TCA production iacilitates lou t~lUIJCIalUl~ boehmite extraction ~ Ph~sical separation of quartz after the gibbsite digestion if e~ollo~.licLIlly attractive to allow either a stirred mill or high ~ ul~ to be used to further enhallce bochmite extraction

Claims (31)

CLAIMS:

1. A process for the extraction of alumina from bauxite having a relatively highboehmite content including the steps of digestion of the gibbsite fraction of the bauxite, solid/liquid separation and digestion of the boehmite fraction, said process being characterised by relatively fast low temperature digestion of the gibbsitefraction to give a high A/C ratio liquor, negligible boehmite reversion, substantially complete kaolinite dissolution and substantially no precipitation of desilication product (DSP) in digestion, and a two stage post-desilication including the removal of some of the silica as DSP in a first stage followed by additional removal of part of the remaining silica in a second stage by the use of tricalcium aluminate (TCA).

2. The process of claim l, wherein the first post-desilication stage is performed using hyroxysodalite seed to reduce the silica concentration close to the solubility of hydroxysodalite.

3. The process of claim 1 or 2, wherein said TCA is prepared externally of the pregnant liquor, the alumina and calcium needed to prepare the TCA are fed to the liquor, if necessary, so that only silica is removed from the pregnant liquor.

4. The process of claim 3, wherein said TCA is prepared using spent liquor thereby avoiding the loss of potential product alumina in the pregnant liquor and lowering the alumina level in the spent liquor to allow it to dissolve more alumina from the boehmite fraction in the gibbsite digestion residue and removing residual silica from the spent liquor.

5. The process of claim 4. wherein said TCA is produced by adding lime to the spent liquor in a concentration substantially falling within the range 1 g/L to 150 g/L.

6. The process of claim 5, wherein the ratio of lime to spent liquor is about 10 g/L.

7. The process of any preceding claim, wherein digestion of the gibbsite fraction is conducted over a period substantially falling within the range of 0.5 minutes to 5 minutes, at a temperature substantially falling within the range 120°C to 160°C, and at an A/C ratio substantially falling within the range 0.70 to 0.80.

8. The process of claim 7, herein digestion of the gibbsite fraction is conducted over a period of about 2 to 2.5 minutes, at a temperature substantially falling within the range 135°C to 145°C, and at an A/C ratio of about 0.75 to 0.76.

9. The process of any preceding claim, wherein said solid/liquid separation stepis performed over a short time period and at a temperature substantially the same as the boehmite digestion temperature to substantially prevent boehmite reversion and precipitation of DSP onto the mud.

10. The process of any preceding claim, wherein said bauxite includes particle sizes up to about 10mm and said bauxite is not subjected to grinding.

11. The process of claim 5, wherein said bauxite is subjected to crushing to achieve a particle size no greater than about 2 to 10mm.

12. A process for the extraction of alumina from bauxite having a gibbsite fraction and a boehmite fraction including the steps of digestion of said gibbsite fraction, solid/liquid separation and digestion of said boehmite fraction, said process being characterised by digestion of said boehmite fraction by digesting a boehmite containing mud slurry at a low temperature using liquor having a low A/C ratio.

13. The process of claim 12, wherein said liquor is spent liquor.

14. The process of any preceding claim, wherein moderate A/C ratio liquor separated from the boehmite digestion stage is used for the low temperature digestion of the gibbsite fraction.

15. The process of any preceding claim, wherein said boehmite digestion stage is conducted for a period substantially falling within the range 2 to 120 minutes at a temperature substantially falling within the range 150°C to 200°C to yield an A/C
ratio substantially falling, within the range 0.40 to 0.60.

16. The process of any preceding claim, further comprising the step of physically separating quartz from the residue of the gibbsite digestion stage.

17. The process of claim 16, wherein said physical separation step is carried out under pressure to avoid the need to cool the liquor.

18. The process of claim 16 or 17, further comprising the step of mixing part ofthe spent liquor with the residue to produce a slurry more suitable for the physical separation step.

19. A process for the extraction of alumina from bauxite containing a gibbsite fraction and a boehmite fraction by digestion of the gibbsite and boehmite fractions of the bauxite, characterised by one or more desilication steps in which tricalcium aluminate (TCA) is introduced to the pregnant liquor to remove silica therefrom,said process further being characterised by production of the tricalcium aluminate for this purpose using spent liquor.

20. The process of claim 19. wherein said TCA is produced by the addition of lime to said spent liquor to produce a lime/spent liquor concentration substantially falling within the range 1 g/L to 150 g/L.

21. The process of claim 22. wherein said lime/spent liquor concentration is about 10 g/L.

22. The process of claims 19, 20 or 21, wherein said desilication step is a post-desilication step.

23. The process of claim 22 wherein said post-desilication step is a secondary post-desilication step.

24. The process of claim 23, wherein said secondary post-desilication step is performed after a primary post-desilication step involving subjecting the liquidfraction from the gibbsite digestion stage to hydroxysodalite seed to reduce the silica concentration close to the solubility of hydroxysodalite.

25. A process for removing silica from Bayer process liquors. comprising the step of adding tricalciumaluminate produced externally of the pregnant liquor to the pregnant liquor at a concentration sufficient to convert at least some of the silica in the liquor to hydrogarnet.

26. The process of claim 25 wherein said liquor is spent liquor.

27. The process of claim 25 or 26 further comprising the step of producing said tricalciumaluminate in spent liquor to reduce the amount of alumina in the spentliquor and to reduce the amount of silica in the spent liquor.

28. The process of any one of claims 19 to 27 wherein the or each post-desilication step is carried out to produce a low soda DSP at a temperature less than the boiling point of the liquor.

29. A process for the extraction of alumina from bauxite containing gibbsite andboehmite by digestion of the gibbsite and boehmite fractions of the bauxite, characterised by the physical separation of quartz from the residue of the digested gibbsite fraction.

30. A process for the extraction of alumina from bauxite containing a gibbsite fraction and a boehmite fraction by digestion of the gibbsite and boehmite fractions of the bauxite, characterised by said bauxite not being subjected to grinding and having particle sizes of up to about 1 to 10 mm.

31. A process for the extraction of alumina from bauxite having a gibbsite fraction and a boehmite fraction and having a relatively high boehmite content including the steps of digestion of the gibbsite fraction of the bauxite, solid/liquid separation and digestion of the boehmite fraction, said process being characterised by relatively fast low temperature digestion of the gibbsite fraction to give a high A/C ratio liquor, negligible boehmite reversion, substantially complete kaolinite dissolution and substantially no precipitation of desilication product (DSP), and by said bauxite not being subjected to grinding and having particle sizes of up to about 1 to 10mm.