OA10034A - Treatment of bayer process red mud slurries - Google Patents

Description

-1- 1 0034

TREATMENT OF BAYER PROCESS RED MUD SLURRIES

Background of the Invention

This invention relates to a method of treating Bayer processred mud slurries to improve or facilitate the handling of suchslurries.

In the Bayer process for obtaining alumina from bauxite, the5 bauxite is initially digested with caustic soda to extract aluminavalues as dissolved sodium aluminate in an aqueous liquor whichalso contains, in suspension, undissolved particulate residues ofthe original bauxite. The particulate residues, known as red mud,are separated from the Bayer process liquor and collected in a 10 concentrated slurry for transport to a storage or disposai site,while the process liquor is subjected to décomposition to precipit-ate alumina trihydrate, the latter being calcined to recover thedesired alumina product.

Red mud is generally considered as a waste product, although 13 it has some présent or potential uses. Cheroically, it comprises,in varying amounts (depending upon the composition of the startingbauxite), oxides of iron and titanium, residual alumina, sodalite,silica, and minor quantities of other métal oxides.

In the Bayer process, after initial séparation from thetO process liquor, the red mud is typically washed in a countercurrentwashing circuit to remove most of the caustic soda carried with thered mud from the starting liquor. It is thereby made up into anaqueous slurry, which is thickened or filtered to a high solidstS concentration (high solids consistency, typically about 10-70% byweight solids) for delivery to a storage or disposai site. Thepercentage of solids dépends on the granulometry of the bauxite andof the red mud. For very finely divided red mud, such as obtainedfrom Jamaica bauxite, the solids content is between 15 and 30%; for 30 coarser bauxite and red mud, such as obtained from African bauxite,the solids content can range from 60 to 70%. The aqueous vehicleof this slurry still has significant alkalinity, e.g. a concentra- 1 0034 tion of NaOH (expressed as Na2CO3) of 1 to 40 g/L, but more usuallyfrom 4 to 25 g/L. Such high-solids, alkaline red mud slurriesunderflow the mud thickeners and mud filters of Bayer processplants, and are usually pumped therefrom (ordinarily with high S pressure positive displacement pumps) over relatively longdistances to storage or disposai sites. The term "Bayer processred mud slurry" will be used herein to refer to alkaline aqueousred mud slurries of this character, viz., slurries of red mud asseparated from a Bayer process line and prepared for discharge or 10 transport to a storage/disposal site.

One known technique for disposing of Bayer process red mud slurry is known as "stacking" or "stacking and drying." Thistechnique involves adjusting the solids concentration of the slurryto produce a flowability that will enable the slurry to stack, at 15 its angle of repose, at the disposai site. The angle of reposeneeds to be within fairly well defined limits in order to allowrainfall to drain off the stack rapidly, but not so rapidly as tocreate érosion channels in the stack, and to maximize the life ofthe stacking site. 20 Heretofore, the handling and pumping of Bayer process red mud slurries hâve presented problems, owing in particular to the highviscosity (and conséquent low flowability for transport) of suchslurries. To facilitate pumping and reduce pumping pressure or toobtain a lower consistency of the red mud slurry, water is often 25 added. The addition of water, however, is attended with disad-vantages, in that it is a diluent which must be removed (as byévaporation and/or drainage) to achieve proper consolidation of themud at the disposai site. The necessity for such removal slowsdown the consolidation of the mud. Thus, in the stacking and 50 drying technique, stacking is followed by solar drying of the stackto produce a dry, hard solid. In this case it is important tominimize, or eliminate completely, the addition of water toincrease flowability, as such addition increases the évaporationload at the disposai site. Furthermore, because of the unavoidable 1 0034 alkalinity of Bayer process red mud slurries, the water drained atthe disposai site cannot be simply discharged to the environmentbut must be pumped back to the plant.

Various expédients that hâve been proposed to increase the5 flowability of Bayer process red mud slurries without addition ofwater hâve included violent agitation or the addition of adispersant. Agitation, however, often has only limited effective-ness, and the addition of dispersants is frequently expensive (incost per unit of tailing solids, i.e. red mud, treated) . As 10 illustrative of such dispersants, U.S. patent No. 4,146,573describes adding a low molecular weight polymer such as acrylicacid polymer to enhance the fluidity of the mud, and U.S. patentNo. 4,464,479 describes admixing the red mud (e.g. directly afterthe red mud has been separated by filtration from the Bayer process 15 liquor) with coal dust, preferably very finely divided brown coalor lignite, in an amount equal to 10$-200% of the red mud, so asto reduce flowability and produce a grainy mixture which can beeasily handled or transported.

Additives hâve also been proposed to improve the consistency 20 of red mud as a useful commercial product. Thus, U.S. patent No.3,886,244 sets forth that the consistency of red mud when used tomake bricks can be adjusted by adding a water-fixing substance suchas coal dust, cork or peat.

Humic substances are naturally occurring complex organic 25 matériels always found in low to medium concentrations in soils,but which occur in deposits of high concentration in some géograph-ie areas. Humic substances hâve been described by Schnitzer, M.,and Khan, S.U., on page 3 of Humic Substances in the Environment.New York: Marcel Dekker Inc., 1972, as amorphous brown or black, 30 hydrophilic, acidic, polydisperse substances of molecular weightfrom several hundred to tens of thousands. Humic substances aregenerally classified according to their solubility in alkali andacid as follows: 1 0034 (1) Humic Acid: soluble in dilute alkali but is precipitatedon acidification of the alkaline extract. (2) Fulvic Acid: humic fraction that remains in the aqueousacidified solution, i.e. soluble in both acid and alkali. 5 (3) Humin: humic fraction that cannot be extracted by dilute alkali and acid.

High concentrations of humic acids can be found in lignite or browncoal. According to Kirk-Othmer Encvclopedia of Chemical Tech-nology. 3d ed., vol. 14, pp. 313-43, lignite and brown coal are 10 terms used to describe coals of the lowest degree of coalification.Typically, lignite deposits are from one to 60 million years old.Lignitic coals tend to oxidize more readily than older ones. Themoisture content of lignites is much higher than that of oldercoals, ranging up to 70%. The oxygen:carbon ratio ranges above 15 0.2. Low température carbonization produces oils for conversion to motor fuels.

Leonardite is a naturally oxidized form of lignite (browncoal) with a high content of humic acids, which may be describedas carboxylated phenolic polymers. It is described in Kirk-Othmer 20 Encvclopedia of Chemical Technology. 3d ed., vol. 17, p. 155(1982), and in U.S. patent No. 4,778,608. Leonardite is mined, inthe western United States, e.g., in Williston, North Dakota, andmanufactured into dry, free flowing powders. It has been used indrilling fluids as a thinner (to adjust the flowability of drilling 25 muds) , for filtration réduction and for oil émulsification and asa stabilizer, and to reduce the viscosity of coal slurries pumpedin long-distance pipelines.

There is no sharp démarcation line between lignite andleonardite. It is to be understood that the terms "lignite" and 50 "leonardite" are used herein interchangeably to dénoté a naturallyoccurring matériel that contains useful concentrations of humicacid or humâtes. Furthermore, the term "humâtes" is used hereinto mean the salts of humic acids, which either occur in nature or 1 0034 are formed when humic acids react with substances which convert theacids to another form.

Summarv of the Invention

The présent invention broadly contemplâtes a method oftreating a Bayer process red mud slurry by adding thereto a minorproportion, of material of the class consisting of humic acids andhumâtes, effective to reduce the viscosity of the slurry.

In accordance with the invention, in currently preferredembodiments, the humic acids may be added in the form of ligniteor leonardite, either as a dry powder or as a slurry, e.g. in anaqueous vehicle, in a proportion of about 0.1 to about 10% (ailproportions of lignite addition herein are expressed as percent byweight of lignite solids based on the weight of red mud solids).It is found that even small (0.1-5%) additions of lignite villreduce the slurry consistency and make the slurry advantageouslymore fluid. A currently particularly preferred range of ligniteaddition is 0.25-2%.

The lignite powder must be mixed intimately with the slurry.In one procédural embodiment of the invention, the lignite isintroduced to the slurry in a red mud thickener in a Bayer processplant. In another embodiment, currently preferred in at leastcertain instances, the Bayer process red mud slurry is divided intoa major stream and a minor stream; lignite powder is admixed withthe minor stream; and the minor stream is then recombined with themajor stream, advantageously ahead of a pump or agitator in a linefor delivering the slurry to a disposai site, the pump or agitatorserving to complété the intimate mixing of the powder into the fullvolume of the slurry. In the latter embodiment, the concentrationof lignite powder added to the minor stream (in weight percentbased on the red mud solids concentration of the minor stream) isequal to the desired lignite concentration in the full slurry timesthe ratio of the flow rate of the full slurry stream to the flowrate of the minor stream. 1 0034 - 6 -

As at présent believed, the reason that humic acids andhumâtes are effective as thinning agents for red mud is that theyare soluble in the caustic or alkali solution always présent inBayer process red mud slurries. The dissolved humâtes affect the 5 water-retaining properties of the red mud, probably by changing theelectric double layer around the particles (thereby affecting thesurface charge) or by removing the flocculant structure.

Further features and advantages of the invention will beapparent from the detailed description hereinbelow set forth, 10 together with the accompanying drawing.

Brief Description of the Drawing

Fig. 1 is a simplified schematic and partially diagrammatic view of the red-mud-handling portion of a conventional Bayerprocess plant, illustrating the incorporation therein of one 15 exemplary embodiment of the invention; and

Fig. 2 is a simplified schematic view in illustration of another embodiment of the invention.

Detailed Description

Referring first to Fig. 1, the Bayer process plant therein20 represented is of the usual character, including an extraction ordigester stage (not shown) wherein bauxite is digested in anaqueous caustic liquor to extract alumina values from the bauxiteas sodium aluminate dissolved in the liquor, leaving undissolvedthe residue known as red mud, forming a slurry with the liquor; a 25 séparation stage 10, to which this slurry is advanced from thedigester, for separating the red mud from the liquor; and adécomposer stage (also not shown) to which the clarified liquor isdelivered from the séparation stage for précipitation of aluminumtrihydroxide. The thickened red mud slurry passes from the 30 séparation stage 10 to a countercurrent washing circuit 11 shownas including a succession of vessels lia, 11b, 11c, and a finalthickener vessel 12. 1 0034 - 7 -

In the washing circuit, the red mud is slurried in an aqueouswash liquid, which is pumped upwardly from vessel to vessel asindicated by lines 14a, 14b, 14c, 14d and 14e while the slurriedred mud descends from vessel to vessel (i.e., underflowing each 5 vessel) as indicated by lines l-6a, 16b, 16c, 16d and 16e. The vashliquid progressively rénovés, from the red mud, caustic sodacarried with the red mud from the process liquor, and therebyacquires an increasing caustic concentration; thus, the wash liquidentering the washing circuit 11 through line 14a is dilute, and the 10 liquid leaving the circuit through line 14e has a relatively strongconcentration of caustic soda, which may be recovered for reuse inthe Bayer process line.

The red mud slurry underflowing the final thickener 12(through line 16e) nevertheless still has a substantial concentra-tion of caustic, e.g. a concentration of about 1 to about 40 g/LNaOH (expressed as Na2CO3). In and ahead of the thickener 12, ithas been thickened to a high consistency (solids concentration),typically on the order of 15 to 55%, depending on the propertiesof the mud. From line 16e it is delivered by means of a pump 18 20 and pipeline 20 to a disposai site 22, which may be remote from theBayer process plant. At the disposai site, the red mud slurry maybe deposited on a stack 24 for solar drying, in accordance with the"stacking" disposai technique.

In an alternative arrangement practiced at other locations, 15 the red mud leaving the last washing stage 12 is further thickenedby a filtration operation to give a residue that contains up to 45to 65% solids. This thickened residue on the filter may be furtherwashed and the wash filtrate is then returned to processstream 14a.

As thus far described, the Bayer process operation representedin Fig. 1 is wholly conventional and well-known to persons ofordinary skill in the art. The thickened, alkaline, aqueous redmud slurry underflowing thickener 12 (or after further thickeningby filtration) exemplifies the Bayer process red mud slurries with 1 0034 which the présent invention is concerned. In its conventionalcondition, it is characterized by high viscosity and corresponding-ly poor flowability, and accordingly requires use of a highpressure positive displacement pump as the pump 18, especially when S it must be transported over a substantiel distance to the disposaisite.

In accordance with the invention, there is added to the Bayerprocess red mud slurry a minor proportion of lignite (in finepowdered state, either dry or slurried e.g. in an aqueous vehicle) *0 effective to reduce the viscosity of the slurry and thereby toincrease its flowability. Lignite additions of about 0.1 to about10% (expressed, as stated, in terms of weight of dry leonardite asa percent of dry weight of red mud solids) are found to beeffective for this purpose, the amount of the addition being *5 dépendent on the solids concentration of the untreated slurry, theproperties of the particular red mud being treated, and the desiredviscosity or flowability of the treated slurry. Lignite additionsof about 0.1-5% (most preferably about 0.25-2%) are currentlypreferred in at least most instances. Î0 The lignite may be added at any convenient point in the red mud line between the séparation stage 10 and the disposai site 22.To optimize the benefit of enhanced flowability, it is advantageousto perform the addition at or ahead of the upstream end of thepipeline 20 (in the System of Fig. 1). Thus, for example, thelignite may be added to the red mud slurry in the thickener 12, asindicated at 28 in Fig. 1. Alternatively, the lignite can beinjected directly into the bed of red mud which has settled in thelower part of the thickener 12. Direct injection of the lignite,conveniently as a slurry suspended in a dilute solution of caustic, 50 into the bed reduces the possibility that the lignite may reportback to the supernatant liquor of the thickener, and from there tothe process streams. In yet another alternative, as shown in Fig.2, the lignite may be added between the thickener 12 and the pump18. In any event, the fine powdered lignite or a slurry of lignite 1 0034 - 9 - in caustic solution is added in such manner as to be thoroughly andintimately mixed with the slurry.

In the case where the mud is thickened by filtration, ratherthan by settling as described above, the lignite is added to thefiltered solids in a mud mixer before being sent to the disposaipumps. When the lignite is not added, these solids can be mixedonly in a high rate shear mixer, after the addition of water todecrease their viseosity.

Fig. 2 further illustrâtes a convenient process feature foraddition of lignite to a Bayer process red mud slurry (either atthe last-mentioned location or at some other point in the red mudline between the séparation stage and the disposai site), toachieve the requisite intimate mixture. As there shown, from amain stream of thick Bayer process red mud slurry (exemplified asadvancing through line 16e), a minor flow or sroall side stream isdiverted through a conduit 30 into an open tank or funnel 32, whilethe major flow of the slurry continues to advance along pipe 34.Fine dry powdered lignite is added to the tank or funnel 32 asindicated at 36, for intimité admixture with the side stream. Fromfunnel 32, the lignite-bearing side stream is reinjected (by a pumpor other suitable means, not shown) through a conduit 38 into themajor flow in pipe 34; preferably, this injection occurs at thesuction side of a pump (e.g. pump 18) or upstream of an in-lineagitator (not shown) so as to complété the intimate mixing of thepowder into the full volume of the slurry. Pump 18 then advancesthe slurry, reduced in viseosity or effectively "thinned” by theleonardite addition, e.g. along pipeline 20 to the disposai site.

The rate of addition of dry lignite powder to the funnel 32,in the embodiment illustrated in Fig. 2, is that which will producea lignite concentration C(S), in the minor flow or side stream,equal to C(M)(F(M)/F(S)}, where C(M) is the required ligniteconcentration in the main stream (to achieve a desired réductionin viseosity or "thinning" effect) and F(M)/F(S) is the ratio ofthe main stream flow rate F(M) (ahead of conduit 30) to the flow 1 0034 10 rate F(S) of the side stream in conduit 30. In this way, therequired main-stream concentration of lignite is readily andintimatelÿ incorporated into the full flow of the slurry beingtreated. 5 In the preferred embodiment of the process for adding the lignite, the minor flow stream is taken off after the pump 18, sentto the admixing tank 32, and from there is returned to the mainstream at a point before the pump 18.

As mentioned above, for pumping concentrated red mud slurriesto over long distances, at présent high pressure positive displacementpumps are required. With the addition of lignite, less energy isrequired and regular centrifugal pumps could be used, or fewer positive displacement pumps are required.

The addition of lignite may also enable the more liquid red15 mud slurry to be refiltered again to higher solids concentrationsthan hâve heretofore been obtained. In addition, lignite added tored mud in a Bayer process mud thickener (as shown in Fig. 1) mayresuit in higher underflow solids concentrations since thickeners,and in particular deep thickeners, are believed to be consistency 20 controlled.

In summary, the présent invention enables a Bayer process redmud slurry of any given solids concentration to be handled moreeasily, and pumped with less energy consumption, than has beenpossible in the past. Alternatively, the invention enables a 25 substantially higher solids concentration (with consequentlyreduced water removal requirements) to be handled with the samefacility as slurries of currently conventional solids content.

The invention will be further described, by way of spécifieexample, with reference to the treatment of Bayer process red mud 30 slurries at commercial Bayer process plants in Jamaica; atJonquiere, Quebec, Canada; and at Aughinish, Ireland. The red mudsproduced at these plants differ from each other because the bauxiteores they respectively process corne from different sites and hâvedifferent compositions. A comparison of typical physical proper- 1 0034 - il - ties of these three red muds is given in Table 1 below; and acomparison of their typical Chemical and minerai compositions isgiven in Table 2 below. TABLE 1

Jonguiere Jamaica Aughinish 10 average 3.2 3.35 3. range 3.1-3.3 3.2-3.8 3.2- Spécifie Surface Area Red Mud ! Solids im2/q) 12.0-13.0 39-50 17 Granulometrv qreater than particle size, microns) 200 microns 16 3 - 7 0 " 23 5 4 10 " 52 17 23 4 " 72 43 50 2 " 92 87 90 15 1 0034 12 TABLE 2

Jonguiere Jamaica AUghinlsh Chemical Composition (XRF)fweight percent) A12O3 24.1 16.5 17.2 Fe2O3 34.6 46.1 21.7 5 SiO2 14.04 5.09 9.01 TiO2 7.37 6.44 23.0 Cr2O3 0.12 0.20 0.58 CaO 0.73 7.27 11.91 P2O5 0.18 1.35 0.31 10 ZrO2 0.25 0.15 0.55 ZnO 0.002 0.059 0.005 MnO 0.09 1.36 0.09 V2O5 0.13 0.25 0.27 K2O 0.06 0.08 0.04 15 MgO 0.02 0.14 0.10 Ga2O3 0.012 0.002 0.003 Na2O 7.2 2.4 5.2 SO3 0.3 0.2 0.4 LOM 8.47 9.67 7.39 20 Minerai Composition fXRD) hématite major major minor Al. goethite major medium minor Bayer sodalite medium minor — anatase medium medium — 25 boehmite minor minor minor quartz minor — — rutile minor minor — gibbsite minor — minor 50 portlandite min. (poss.) — — lime — minor -- calcite — minor — perovskite — — major cancrinite — — ? 1 0034 13

EXAMPLE I A sériés of laboratory tests were conducted with red mud fromJamaica Bayer process plants of Alcan Jamaica Company, using a dryleonardite powder commercially available under the trade nane"Humite."

In the first of these tests, a small sample of the leonarditepowder was added to plastic red mud at 38% solids concentration(from the red mud pond at Alcan Jamaica Company’s Kirkvine Works)in a beaker. After about one minute of hard mixing, the mud wasthin énough to pour out of the beaker.

In a second test using the same Kirkvine red mud at 38% solidsconcentration, the addition of leonardite required to thin the mudto a torque reading of 60 (indicating stackable flowability) was0.7% of the red mud solids weight. The torque reading was obtainedwith a Brookfield digital Viscometer, Model RV Spindle No. 2, ata rotation speed of 100 r.p.m.

Several tests were then performed on samples of red mud fromAlcan Jamaica Company's Ewarton Works. In each case, the amountof leonardite addition (expressed as weight % of dry mud solids)required to achieve a stackable consistency (flowability) wasdetermined. Results were as follows:

Sample Solids Conc. (%) Leonardite addition needed (%) 1 40 1.6 2 33 0.9 3 33 1.2 4 31 1.0

Samples 1 and 2 were of the same red mud, taken from the surgetank, and were quite sandy. Sample 3 was a sample of deepthickener underflow. Sample 4 was the same material as sample 3but diluted to 31% solids concentration.

The indication from these tests is that about 1% of leonardite(weight percent, based on the weight of dry mud solids) isnecessary to thin a 30% solids Bayer process red mud slurry to astackable consistency. 1 0034 - 14 -

To test the effectiveness of introducing the leonardite powderto a side stream (as shown in Fig. 2), a 1/10 portion of a sampleof the abovementioned Kirkvine Works red mud in a beaker wasremoved from the beaker, and leonardite powder was added to this 3 removed 1/10 portion, in an amount equal to 7% of the weight of theremoved 1/10 portion. The latter portion, with the admixedleonardite, was returned to the balance of the sample in the beakerand thoroughly mixed therein. The resuiting concentration ofleonardite in the whole sample was 0.7% and a check on its fluidity 10 (by the abovedescribed torque measurement) showed that the thinning(viscosity réduction) effect was the same as when 0.7 weightpercent of leonardite was added directly to a full sample (i.e.,as in the second test described above).

In ail the above sériés of tests, the possibility that the 13 thinning effect was due not to the leonardite powder, but ratherto the agitation used to mix it, was elioinated by running blanktests with the same agitation but no powder addition.

In Jamaica, the Ewarton Works Bayer plant pumps out to the drymud stack (disposai site) a red mud slurry at a consistency vhich 20 is obtained as a solids concentration of 25%. This mud on thestack has to dry in a limited time period before a new layer of mudis laid out. With the addition of 0.5% of leonardite the sameconsistency can be obtained at 30% solids as at 25% solids withoutthe addition so that considerably less water has to be removed by 25 solar drying.

EXAMPLE II

Laboratory tests hâve been carried out on the effect ofdifferent dosages of lignite from Rossland, Alberta, Canada (whichhad a screen size of 47% >38 microns) on the rheology of Bayerprocess red mud slurries from the Aughinish Bayer process plant 30 (67.9% solids) and from the Jonquiere Bayer process plant (65.6% solids). The results of the rheological test work using a HaakeVT500 viscotester with a vane type spindle (FL100) are summarized 1 0034 15 below. From the shear sta shear rate at D=40/sec. ress/shear rate are as follows: curve the résulté for Lignite addition (%)Aughinish mud 0 0.25 0.5 1.0 2.0 5 shear stress (Pa)Jonguiere mud 100 20 10-12 19 12 shear stress (Pa) 110 100 10 10

The Aughinish mud ïs a Bayer process red mud slurry in asolution containing 4 g/L NaOH, expressed as Na2CO3. The 10 Jonquiere mud is a Bayer process red mudcontaining 25 g/L NaOH as Na2CO3. The screen size distribution of theexample was as follows: slurry in a solution lignite used in this Cumulative Wt.% Mesh Microns Wt % 15 +100 150 15.2 15.2 100 x 150 105 7.4 22.6 150 x 200 74 8.7 31.3 200 x 325 44 11.7 43.0 325 X 400 38 3.5 46.5 20 -400 38 EXAMPLE 53.5 III 100.0 This test was conducted to détermine : whether the hum.ate material used according to the invention would hâve a négativeaffect on the précipitation of alumina from the Bayer processprégnant liquor. 25 Three alumina rich liguons were prepared by dissolving under pressure in an autoclave at 143’C, sufficient purealumina hydrate in spent liquor to make an alumina rich liquorcontaining about 110 g/L of A12O3. Three different types ofspent liquor were used: 30 Liquor A- normal spent liquor, containing 155.5 g/L of caustic, and 56.0 g/L of alumina.

Liquor B- spent liquor made from the run off liquid of ared mud stack, containing 16.0 g/L of causticand 0.14 g/L of alumina, and concentrated byévaporation and causticized to contain 184 g/Lof caustic and 3.4 g/L of alumina before the 35 10034 16 dissolution of the additional alumina hydrate.

Liquor C- spent liquor made from the run off liquid of a red mud stack, prepared from a slurry of red mud containing 118 g of lignite/ton of mud. 5 This run off liquor was evaporated and then caustici2ed to contain 185.3 g/L of caustic and1.63 g/L of alumina before the dissolution ofthe additional alumina hydrate.

The aluroina rich liquor resulting front the digestions vas10 seeded with either fine (80% less than 44 micron) or coarse (about 70 micron médian) alumina seed, to give seed concentra-tions of 40 g/L and 200 g/L respectively. The resultingslurry of alumina rich liquor and seed was agitated at 70°Cfor 20 hours with fine seed, and at 65’C for 24 hours in the 15 case of coarse seed. The product alumina precipitated was separated by filtration, washed and dried and weighed and thescreen size distribution was determined. The weight ofalumina actually precipitated was calculated by making acorrection for the amount and type of seed added. The tests 20 were carried out in triplicate.

The results of the three tests are summarized in Tables 3 and 4, showing respectively the weight of the productprecipitated and the screen size distribution of the product.

Tt is concluded from these tests that the presence of any 25 lignite in the liquor leached out of the red mua did not decrease the yield of alumina (Table 3), nor did it affect thescreen size distribution (Table 4), with either fine or coarseseed. Thus the run off liquid from the red mud stackscontaining lignite can be returned to the Bayer process 30 without adversely affecting the operation. 17 1 0034 TABLE 3 PRECIPITATION YIELDSThe yields (in g/L) were as follows:-

First sériés of tests - Using fine seed (40 g/L) Liquor ARegular Stent Liguor Liquor B Run-off No Lignite Liquor CRun-off With Lignite Run 1 38.7 3 6.4 42.1 Run 2 38.9 37.2 41.7 Run 3 39.3 37.2 41.5 Second sériés of tests - Using ccarse seed (200 g/L) Liquor ARegular Spent Liguor Liquor BRun-off No Lignite Liquor CRun-off With Lignite Run 4 47.1 44.7 48.7 Run 5 49.7 47.6 48,6 Run 6 48.2 45.0 49.9 TABLE 4 PARTICLE SIZE DISTRIBUTIONAS WT%

The size distribution of the product from the above tests was measured in three size fractions: (a) +200 mesh; (b) +325-200 aesh; (c) 325 œesh. Liquor ARegular Spent Liguor Liquor BRun-off No Lignite Liquor C Run-off With Lignite a b c a b c â b C Run 1 58 36 6 51 42 7 54 39 7 Run 2 60 34 6 53 41 6 58 34 8 Run 3 59 34 7 52 41 7 58 35 7 1 0034 18

Second sériés of tests - Yield test using coarse seed Liquor ARegular Spent Liquor abc Liquor BRun-off No Lianite abc Liquor CRun-off With Lianite a b c Run 4 52 42 6 47 44 g 49 43 8 Run 5 53 41 6 47 44 9 48 45 7 Run 6 52 43 5 48 44 8 EXAMPLE IV 49 44 7

This example demonstrates the effect of the lignite onthe settling of the red mud solids in the presence of thecommonly used synthetic flocculants. The spent liguors usedin this example are identical to spent Liquors 3 and C ofExample ITT.

The slurry of red mud was prepared by diluting underflcw,from the penultimate thickener stage in the counter currentwashing train of an alumina plant, with twice the volume ofLiquor B or Liquor C, to sim.ulate the feed slurry to the laststage. This gave a slurry containing from 47 to 95.5 g/L redmud solids. These slurries were treated with different typesof synthetic flocculant as indicated in Table 3. The dosageof flocculant was 121 g/ton of red mud solids. The slurry wasplaced in a 1 L graduated cylinder held in a thermostattedbath at 45’C, the flocculant was added as a suspension inwater, and the mixture was toixed by mcving a perforated disevertically throughout the entire height of the column. Theheight of the interface of the slurry settling in the cylinderheld in the bath at 45’C at various times was then noted todétermine the settling rate.

The test results are summarized in Table 5, in which thesettling rate, expressed in métrés per hour, are comparedbetween slurries containing lignite, and those free oflignite. 19 1 0034 TABLE 5

SETTLING RATES

Settling Rates rn/hr 5 Run No. Solids Conc. aZi Flocculant Type Liquor BRun-off No Liqnite Liquor CRun-off With Liqnite 7 66.0 Nalco 7879 7.5 4.9 8 47.0 Nalco 7879 20.8 33.4 9 4 6.5 Nalco 7879 29.6 40.5 10 10 46.5 Nalco 7879 29.7 31.1 il 60.5 Alclar 405 6.4 10.7 12 47.0 Alclar 405 8.7 12.2 13 95.5 Nalco 7879 1.6 1.2 14 76.4 Nalco 7879 19.9 29.7 15 15 69.5 Nalco 7879 45-6 37.6

The results indicate that there is no significantdifférence in the settling rates in the presence or absence oflignite. The différences are within the experimental error.

EXAMPLE V 20 Thi.s example is to show the économie advantage of using lignite as additive over the use of a commercial dispersant,NALCO 8652.

For this example, slurries of 35% by weignt red mudsolids in an alkaline liquor containing about 35 g/L of NaOH,were taken from the mud thickener underflow of the washingcircuit of a Bayer Plant. The slurries were separatelytreated with varying amounts of additives which were: (a)powdered lignite; (b) granulated lignite; (c) Nalco 8652, acommercially available dispersant made by thé Nalco Chemical 30 company, Napierville, IL, U.S.A.

The dosages used ranged from 4.5 to 20.4 kg of additiveper ton of dry mud solids. The relative viscosity of theslurries was measured at 25’C with a Brookfield digitalviscometer, using a No. 2 spindle at 50 rpm. In addition, the 35 costs in US$ per ton to achieve the relative viscosities withthe lignite and NALCO 3865 were calculated.

The results of this experiment are shown in Table 6. Theresults obtained with the powdered and granulated lignite are 20 1 0034

essentially équivalent. Both Nalco 8652 and the lignitereduce the relative viscosity of the slurries, but a muchsmaller dosage of lignite is required, and conseguently thecosts of using lignite are about 5 times lower than with NALCO S 8652. TABLE 6

COMPARTSON OF DISPERSANTS

AdditiveDosageRg/ton mud Relative Viscositv at 25*C Cost USS/Ton of Mud Nalco 8652 Lignite Powder Granulated Lignite Nalco 8652 Lianite 0 82 82 82 0 0 4.5 92 95 100 6.2 1.20 9.1 93 88 84 12.4 2.40 13.6 83 57 48 18.6 3.6 18.1 60 35 23 24.8 4.80 20.4 50 29 22 27.9 5.40

Claims (10)

1. 21 1 0034 characterized ïn that characterized in that Clairos :

   1. A method of treating a red r.ud slurry produced in aBayer process for obtaining alumine from bauxite in which amaterial is added to the red mud slurry to reduce theviscosity thereof, 5 characterized in that the matériel which is added to the slurry to reduce the viscosity thereof is a humic acid orhumate, e.g. lignite.
2. 2. A method according to claim 1, characterized in thatsaid lignite is added as a dry powder.
3. 3. A method according to claim 1,said lignite is added as a slurry.
4. 4. A method according to claim 2, said lignite is added in a proportion of about 0.1 to about10%, preferably about 0.1 to about 5%, by weight.
5. 5. A method according to claim 1, characterized in thatsaid red mud slurry is a slurry of red mud in an agueoussolution containing at least about 1 g/L NaOH expressed asNa2CO3 and has a solids concentration of at least about 10%.
6. 6. A procedure fer recovering alumina values frombauxite by the Bayer process, wherein bauxite is digested incaustic liquor to extract alumina values as sodium aluminatedissolved in the liquor, leaving undissclved red mud residue,and wherein the red mud residue is successively separated fromthe liquor, washed to form a Bayer process red mud slurry, andtransported in said slurry to a disposai site, characterizedin that lignite is added to the red mud slurry in a minorproportion effective to reduce the viscosity of said slurry.
7. 7. Procedure according to claim 6, characterized in thatthe red mud slurry is thickened in a thickener before beingtransported to the disposai site, and wherein the lignite isadded to the slurry in the thickener.
8. 8. Procedure according to claim 7, characterized in thata bed of red mud settles in the lower part of the thickener,and wherein the lignite is injected as a slurry directly intosaid bed of mud settled in the lower part of the thickener.
9. 9. Procedure according to claim 6, characterized in thatthe red mud slurry is thickened by filtration before beingtransported to the disposai site, and is transported to the 22 1 0034 disposai site by one or more disposai puraps, and wherein thelignite is added to the thickened slurry after filtration in amixing device upstream of said pumps.
10. 10. A Bayer process red mud slurry characterized by5 having incorporated therein a minor proportion of a humic acid or humate effective to reduce the viscosity of said slurry.