AU675361B2 - Process for the treatment of bauxites containing alumina trihydrate and alumina monohydrate - Google Patents

Description

C d I C1 'L~b 1 Process for the Treatment of Bauxites Containing a Mixture of Alumina Trihydrate and Alumina Monohydrate Field of the Invention The invention concerns a process for alkaline digestion treatment using the Bayer process of mixed bauxites, ie., alumina trihydrate bauxites, or gibbsite, also containing alumina in its monohydrated state, or boehmite, in a proportion of at least 5% by weight of the total quantity of alumina in the bauxite.

State of the Art The Bayer process has been widely described in the specialist literature. It constitutes the main technique for the production of alumina for transformation into aluminium by dry electrolysis or for use as the hydrate, transition alumina, calcined alumina, or as sintered or fused alumina in numerous applications in the field of alumina technology.

In this process, the hot bauxite mineral is treated using an aqueous sodium is hydroxide solution at a concentration sufficient to dissolve the alumina and produce a supersaturated solution of sodium aluminate, Following separation of the solid phase constituting the undigested mineral residue (red mud), the supersaturated sodium aluminate solution is seeded with alumina trihydrate particles to induce precipitation of the alumina in the form of alumina trihydrate. The 20 alumina-depleted sodium aluminate liquor, the spent liquor, is recycled to the digestion stage after concentrating and replenishing with sodium hydroxide, if required, to constitute a liquor, the digestion ]liquor, at an appropriate concentration for the nature of the mineral to be digested.

The skilled person is well aware that the treatment conditions must be adapted to the 25 degree of hydration and the crystal structure of the alumina as well as the nature and amount of impurities present. Thus, bauxites containing alumina monohydrate (boehmite, diaspore) are treated in the Bayer process at temperatures above 200 0 C, generally between 220°C and 300 0 C, while bauxites containing alumina trihydrate (gibbsite) are treated at temperatures below 200°C, generally between 100°C and 170°C. In the two cases, the s30 soluble alumina extraction yield can be greater than 95% of the soluble alumina contained in the bauxite.

Effective operation is often difficult when working with trihydrate-'ich bauxites which also contain a certain amount of alumina monohydrate (at least 5% by weight of the total quantity of alumina in the bauxite), termed a mixed bauxite in the remainder of this disclosure. Effective operation means a treatment which produces a soluble alumina extraction yield of at least 95% without altering the purity of the alumina rihydrate which is subsequently precipitated, under conditions which remain economically acceptable particularly in terms of operating costs and productivity.

IN LIOCIO0443 JOC I of I~l L II

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A first possibility consists in treating the mixed bauxite under conditions for a monohydrate bauxite, ie,, by alkaline digestion at high temperatures generally between 220°C and 300°C, which requires the use of high pressure reactors and thus a high level of investment and high operating costs particularly as regards energy consumption and maintenance costs. While the use of very concentrated caustic soda liquors allows large quantities of alumina to be dissolved to obtain high productivities, this also increases the risk of certain impurities such as iron passing into a more or less colloidal state which then reappear in the precipitated alumina trihydrate.

A further known possibility is a two step digestion process for the mixed bauxite o1 comprising a first conventional digestion step at less than 200°C for the alumina trihydrate using a fraction of the spent liquor, ie., depleted in sodium aluminate, followed by a second digestion step for the insoluble residue from the first digestion step constituted by the red mud and undigested monohydrate, at higher temperature using the remaining fraction of the spent liquor. The sodium aluminate liquors from the two digestion steps are then filtered and mixed before being decomposed in the presence of a seed to precipitate alumina trihydrate. This production scheme is illustrated in Figure 1 and will be explained in the description by comparison with the process of the invention, Two step mixed bauxite digestion processes, which are apparently better adapted economically than direct high temperature digestion processes, are in fact often disadvantaged by the deleterious effects of two types of secondary reaction which occur during operation: 1) Significant reduction in the soluble alumina extraction yield which can be as high as 3% to This is due to untimely precipitation of alumina in the form of a fine suspension of trihydrate and perhaps boehmite from the supersaturated sodium 26 aluminate liquor during the red mud decantation-separation step following the first digestion step.

This precipitation, known also as retrograding, is triggered by spontaneous nucleation of the alumina trihydrate in solution when the degree of supersaturation in the sodium aluminate liquor is high, particularly when aiming for high S so productivity by increasing the weight ratio Rp (soluble A1 2 0 3 g/L caustic NazO, g/L) above 1.2 with Na20 concentrations which can reach 150g/L.

Retrograding, which can be triggered by the presence of fine particles of mud in suspension which act as seeds for decomposition, increases in iUinporlance with increasing decantation times. Moreover, retrograding cannot be corrected during the second digestion step carried out at a higher- temperature by redissolving the precipitated hydrate since, because of its fineness, it cannot be separated bec-sse it chokes the decanters and blocks the decantation-backup filtering installations for the supersaturated liquor to be decomposed.

2) Pollution of this supersaturated liquor by certain impurities contained in the mixed bauxite which can pass into solution or a colloidal state and remain in the liquor to IN Lt18Cj00443:JOC 2020 I- p 1 ~~1111 be decomposed even after backup filtering, This is especially the case for silica in the form of hydrated aluminosilicate which is soluble in alkaline medium such as kaolin, 2Si0 2 .Al 2 0 3 .2H 2 0, during the first low temperature digestion step for the mixed bauxite. During the second high temperature digestion step, certain iron oxides can pass into a colloidal ferrous hydroxide state or form a soluble complex such as ferrate ion Fe(OH)4' by a little known mechanism. During decomposition of the supersaturated liquor, these impurities reappear in part in the precipitated trihydrate.

A number of means of overcoming these deleterious effects is known to the skilled person.

In YAMADA J1 of Jap, Inst. of Light Metals vol.31 p.43-48 January 1981, retrograding of dissolved alumina in a supersaturated sodium aluminate liquor from low temperature digestion of the trihydrate-rich bauxite is significantly reduced and retarded by limiting the degree of supersaturation in the liquor for decomposition. This means that is in practice the weight ratio R, must remain 1.1 in the liquor for decomposition, as it is not possible to alter the caustic soda concentration in the liquor which must remain below 160g NazO/litre in order not to affect subsequent decomposition operations in the presence of a seed, The productivity measured by the variation in the concentration of dissolved alumina in a specific volume of liquor before and after decomposition (generally S 20 expressed as kg A1 2 0 3 /m 3 is affected and hardly rises above 70kg/m 3 Apart from altering the digestion conditions, red mud decantation can be facilitated to reduce the retrograding triggering effect of fine particles in suspension knowing that

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intense retrograding can still develop 5 or 6 hours after decantation is commenced. To Sachieve this, the skilled person has a number of flocculating agents at his disposal, generally anionic polyelectrolytes, which can advantageously be combined with a polysaccharide based clarifying agent as described in United States patent US-A-5 217 620. According to United States patent US-A-4 994 244 and European patent EP-A-0 422 893, a further solution consists in decanting the red mud at more than 100 0 C under pressure to reduce the viscosity of the liquor and above all increase the efficiency of the s3 flocculants.

Regarding the problem of pollution of the supersaturated liquor by silica and thus alumina trihydrate precipitated out during decomposition, the skilled person can select from a number of desilication processes, described in particular in United States patents US-A-3 413 087, US-A-4 426 363 and European patent EP-A-0 203 873, consisting in as precipitating at least a portion of the dissolved silica in the form of sodium silicoaluminate before or during the first low temperature digestion step.

Few processes, however, provide a satisfactory solution to alumina contamination by iron contained in mixed bauxites, in particular those originating from Guinea (BOKE) containing a ,ow iron content, since the majority of prior art processes are based ont IN 110=G443,AC 30120 either specific purification operations which increase production costs and reduce yields, such as United States patent US-A-3 607 140. This describes predecomposition or the supersaturated liquor in the presence of a seed during which the iron compounds, which are in a metastable or colloidal state, co-precipitate with fraction of the alumina trihydrate which is thus sacrificed as an impurity trap; or filtration techniques for the supersaturated liquors which are more selective but delicate to operate on an industrial scale, such as sand bed or bauxite bed filtration as described in United States patents US-A-3 792 542 and US-A-3 728 432, also US-A-4 446 117, where the effectiveness remains limited to trapping small ferrous hydroxide particles and cannot bring the iron concentration in the liquor to below following this enhanced backup filtration. This concentration is still too high to guarantee a weight ratio of iron Fe/A1 2 0 3 which is often required for the alumina trihydrate subsequently precipitated.

It is clear that the combination of different prior art methods described above would limit the deleterious effects of retrograding on the soluble alumina extraction yield and on pollution of the alumina trihydrate produced by reactive silica and iron. On the other hand, this combination of methods significantly increases operating costs firstly because of a loss in productivity due to limits on the Rp in the liquor for decomposition (Rp 1,1) and secondly on the costs of operating these specific complementary methods, in particular when accelerated decantations must be carried out on an industrial scale under pressure at more than 100°C, or when enhanced backup filtrations must be carried out.

The Problem The development of an effective process for the treatment of mixed bauxites is thus a desirable objective considering the increasing importance of this type of bauxite as a raw i" 25 material. It must be able to satisfy the following four particular characteristics: soluble]e alumina extraction yield decomposition productivity greater than or equal to 70kg AlOJ/m 3 of liquor; iron content 10mg/litre in the liquor before decomposition; SiO 2 /NazO weight ratio 50.8% in the liquor before decomposition, S o30 Aim of the Invention We have developed a process which simultaneously achieves these objectives. It consists principally in carrying out low temperature digestion of the mixed bauxite with a concentrated caustic soda liquor to produce a very high Rp, of the order of 1.20 to 1.35, in which retrograding of the supersaturated liquor during decantation is considerably as reduced as soon as the caustic soda concentration is at least equal to 160g NazOllitre.

The two problems arising from this practice, namely increasing the iron content in the liquor from the second high temperature Cgestion step of the mud and boehmite consecutively with the use of a concentrated caustic soda digestion liquor and the IN Ad0C100443 JOC 4 of ~I I- I difficulty in decomposing the supersaturated liquor even in the presence of a seed because of the high caustic soda concentration, are simultaneously resolved by recycling the decanted liquor from the second digestion step (where Rp 1) to the head of the process as the liquor for the first digestion step mixed with the spent liquor where, surprisingly, the dissolved or colloidal iron is trapped on contact with the bauxite. Thus the purity of the supersaturated liquor from the first digestion step is not affected at all, while the required caustic soda dilution (at least 160g Na20/litre) before decomposition is obtained by direct recycling of the wash water from the trihydrate production step, which is normally used as counter-current wash water for the red mud, to the head of the o1 decomposition step.

More precisely, the invention concerns a process for the treatmeni of bauxite containing alumina trihydrate and alumina monohydrate in a proportion of at least 5% by weight of the total quantity of alumina in said bauxite, by low temperature caustic soda digestion of the ground bauxite to form a suspension which is decanted and filtered to separate a supersaturated sodium aluminate liquor for decomposition in the presence of a seed and an insoluble residue containing red mud and alumina monohydrate, said insoluble residue then being digested in a caustic soda medium at a higher temperature to dissolve the alumina monohydrate in the form of a sodium aluminate liquor which is separated from the red mud by decantation, the process being characterised by the following combination of operations: a) low temperature digestion, or first digestion, of the ground bauxite by bringing it into contact with a liquor with a caustic soda concentration greater than or equal to 150g NazO/litre and Rp of between 0.6 and 0.9, said digestion liquor being constituted by a mixture of an aliquot of spent liquor from a previous decomposition operation, with a decanted liquor from the high temperature digestion step, or second digestion step, for the insoluble residue, b) after a contact time sufficient to dissolve the alumina trihydrate, decanting the suspension formed in the presence of a flocculent to separate a supersaturated sodium aluminate liquor with an R, of between 1.20 and 1.35 and caustic soda S 30 concentration of at least 160g NazO/litre from the insoluble residue, c) filtering then diluting the filtered supersaturated liquor with the wash water from the alumina trihydrate produced in the preceding cycle to reduce the concentration to less than 160g Na 2 O/litre, d) decomposition the supersaturated sodium aluminate liquor after dilution in the presence of a seed to precipitate alumina trihydrate where the wash water after filtration is preferably used as diluting water for the supersaturated liquor at the start of the decomposition step, e) recycling the decomposed liquor to the head of the process in three aliquots which are respectively reintroduced to the grinding and second digestion steps as well as to .L IOOC100443 JOC G z rthe first digestion step after mixing with the decanted liquor from the second digestion step, f) high temperature digestion, or second digestion, of the insoluble residue from decantation and filtration of the suspension previously formed by bringing it into contact with an aliquot of spent liquor for a sufficient time to dissolve the alumina monohydrate contained in the insoluble residue and form a suspension of red mud in a sodium aluminate liquor with an Rp of less than 1, g) decanting and counter-current washing the insoluble residue from the second digestion step constituted mainly by red mud which is eliminated while the sodium aluminate liquor with Rp 1 from the second digestion step is recycled to the head of the process to form the liquor for the first digestion step following mixture with an aliquot of spent liquor, During double digestion treatment of mixed bauxites, particularly those originating from Guinea and Australia, we have discovered a common solution to the problem of alumina retrograding which is particularly marked after the first digestion step and to the problem of iron pollution which indirectly follows when using liquors with high concentrations of caustic soda in the digestion steps to attempt to obtain a high alumina extraction yield from the bauxite along with high productivity.

The invention is based on a first observation, namely that retrograding of alumina in S 20 the form of trihydrate in the supersaturated liquor from the first digestion step from the start of the decantation step can be limited by reducing the level of supersaturation, not by using the conventional method of reducing the Rp to less than 1.1, but by considerably increasing the caustic soda concentration in the liquor, generally between 100 and 150g to at least 160g NazO/litre and preferably to between 160 and 180g NazO/litre, ie., to a concentration range which is considered incompatible with subsequent decomposition of the supersaturated liquor in the presence of a seed. This difficulty has been overcome by slightly diluting the supersaturated liquor at the start of the decomposition step with wash water from the alumina trihydrate production step.

Thus, in the normal range of temperatures in which decantation is carried out, the S 3o degree of supersaturation B, defined as the ratio Rp/ equilibrium Rp in the supersaturated liquor, it can be seen from Table 1 below that for a degree of supersaturation B :1.20, an S" Rp of at least 1.20 and even more than 1.30 is possible without retrograding in the form of the trihydrate at caustic soda concentrations S160g NaO0/litre.

IN !IBC100443 JOC 002 0 3_ *9 *L 9S 9..

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*I 9I Table 1 Test no B t°C Caustic Initial Rp AI(OH) 3 Evolution Na 2 O in mud of Rp g/litre 1 1.05 100 165 1.20 0 slow fall 2 1.10 103 165 1.30 0 slow fall 3 1.10 98 180 1.33 0 slow fall 4 1.20 95 150 1.20 0 slow fall 1,25 90.5 160 1.20 0 slow fall 6 1.30 90 160 1.31 rapid fall 7 1.30 94 150 1,30 rapid fall 8 1.30 93 140 1.25 rapid fall The influence of caustic soda concentration on the R, stability in the supersaturated liquer following digestion (10 minutes at 145°C using BOKE bauxite) and at the end of a very long decantation step (18 hours at 100°C) has been studied. The main results are E shown in Table 2 below.

Table 2 Test Liquor after Liquor after A Rp Backup no digestion 18h at 100°C retrograding AI(OH) 3 filtration rate Rx in of filtration ___nmd mn/m //h NazO Rp NazO R, g/L 9 142 1.27 142 1.17 0.10 0.3 10 142 1.33 145 1.12 0.21 0.2 11 163 1.32 166 1.26 0.06 0 12 165 1.32 168 1.26 0.06 0 1.4 13 155 1.27 158 1.22 0.06 trace Spectral line Very marked Spectral Line 0 No spectral line) It can be seen that in the high Rp range, 1,20 to 1.35, retrograding is slow and exclusively boehmitic when the caustic soda concentration is s160g Na20/litre. Near 140g Na20/litre, however, trihydrate retrograding is intense and manifests itself from the start of decomposition. This observation was confirmed after decantation by X ray detection of alumina trihydrate in the mud and a significant reduction in the filtration rate of the supersaturated liquor. While the use of digestion liquors with high concentrations of caustic soda has no deleterious effect on the purity of the supersaturated iiquor after is decanting and backup filtration, since the iron content is systematically below regardless of the mixed bauxite used, this is not the case for the sodium aluminate liquor from the second digestion step (between 2200C and 300°C) for the insoluble residue.

Depending on the origin of the bauxite and the temperature of the second digestion step, even after backup filtration the iron concentration can vary from 20 to o2 ruling out the possibility of any mixing with the supersaturated liquor from the first digestion step before decomposition if a concentration of less than 100ppm of iron in the tW IJ.ICICC44 JOC; t of 8 precipitated alumina trihydrate is required. These tests confirm that the dissolved iron can be in different forms: soluble iron in the form of ferrate ions Fe(OH) 4 colloidal iron in the form of non crystalline aggregates of very small dimensions (-50A) which do not interfere with light passing through the solution which therefore remains clear, and iron in the form of small hydroxide particles of between 0.1 and 5pti in size.

The problem of selective purification of the fraction of liquor from the second digestion step other than by prior art filtration techniques with limited efficacy and the problem of using this liquor, are resolved by recycling this liquor, which is simply decanted (with iron concentrations which can thus reach 500mg/L), to the head of the 1o process as the first digestion liquor mixed with an aliquot of spent liquor. Surprisingly, said mixture of liquors, where the iron concentration can still reach 300mg/litre, is purified on contact with the bauxite such that the purity of the resulting supersaturated liquor from the first digestion step is equivalent (Fe 8mg/L) to that of a supersaturated liquor from the first mixed bauxite digestion step of the prior art, simply using an aliquot of spent liquor, In addition to the observed purification effect, recycling the sodium aluminate liquor from the second digestion step to the head of the process immediately after decantation means that a delicate backup filtration operation can be cut out. Finally, since the risks of retrograding during decantation of this liquor from the second digestion step remain low because RP 1, the wash water from the trihydrate production step, still containing about of NazO/litre, is much more usefully employed as diluting water for the supersaturated liquor before decomposition than as counter-current wash water for the mud during decantation as generally recommended by the prior art, A considerably modified process including these new conditions produces soluble alumina extraction a2 yields of more than 95%, even 98% in certain embodiments, with very low silica and iron 499, contents in the supersaturated ]liquor for decomposition.

The addition of a small amount of lime facilitates dissolution of the alumina contained in the residue from the first digestion step which is mainly in the monohydrate form. Knowing that lime can be used as a filtration additive for the decanted liquor from 3o the first digestion step, the addition of lime to the residue before high temperature a* digestion is automatically carried out with recovery of the insoluble filtration cake, Further, in the relatively frequent case where the bauxite contains reactive silica, ie., silica which can pass into solution in the digestion liquor then reprecipitate at the wrong time during subsequent operations, polluting the products and silting up the heat exchangers, it is convenient to first desilicate the mixed bauxite by bringing it into contact with an aliquot of spent liquor at about 100°C for a suitable time of the order of 5 to hours, to dissolve the reactive silica and reprecipitate it as insoluble sodium aluminium silicate.

The invention will be better understood from the following detailed description with reference to the following Figures: IN LiOCtCo04 3 OC Figure I is a schematic diagram of the product streams in a two step digestion process of the prior art; Figure 2 is a schematic diagram of the product streams in a basic process in accordance with the invention; s Figure 3 is a schematic diagram of the product streams in a preferred embodiment of the process of the invention combining a desilication operation with diea basic process.

In Figure 1 representing the prior art, mixed bauxite 1, following wet grinding at A in the presence of a first aliquot of spent liquor 20a, is brought into contact at B with a second aliquot 20b of spent liquor to form a suspension whi b "I to a temperature o1 of less than 200°C for a period which is sufficient to dissolv( n *r trihydrate.

Suspension 3 resulting from this first digestion .step is cot. then decanted at C to separate the supersaturated liquor 5 from the sodium aluminate which is to be decomposed following filtration at G. At this stage liquor 16, with Rp 1.1, contins ]ess than 8mg of Fe per litre. The dilution the stream of decanted mud 4 is mixed with insoluble filtration cake 6. The resulting suspension 7, formed by the red mud, undigested alumina monohydrate and any filtration additive used in step G, is brought into contact at D at a temperature of between 220°C and 300°C with a third aliquot 20(c of spent liquor Suspension 8 from this second digestion step, after mixing with wash water 12 from the decanted mud from a previous operation, constitutes suspension which is decanted at F during which the decanted mud 10 is counter-current washed at F before discharge 14, Sodium aluminate liquor stream 11 from decantation step E, with an iron content which can be as high as 500mg/L, is filtered to reduce this content to between 15 and in the filtered liquor, Insoluble filtration cake 13 is added to the washed red mud from decantation step 14 as for discharge at 15 while filtered supersaturated liquor 17, with R, of about 1, is mixed :i with supersaturated liquor stream 16 from the first digestion step to constitute supersaturated liquor stream 18 which has an iron concentration of more than and which, after decomposition at H in the presence of recycled alumina trihydrate seed S22, forms an alumina trihydrate suspension 19 in the spent liquor.

0 so Filtration of this suspension at I separates spent liquor 20, with an iron content of less than 5mg/litre and which is recycled to the head of the process as first digestion liquor 20a, 20h and second digestion liquor 20c, while the solid phase of alumina trihydrate 21 is divided, with or without granulometric grading, into a first fraction which is recycled as seed 22 and a second fraction 23 for the production of alumina 24 after 3s washing with water at J.

Wash water 26, is reused at F as first counter-current wash water for the residue from the second digestion step before washing with pure water 27. it should be noted that the alumina trihydrate precipitated during the decomposition step contains a portion of the iron contained in supersaturated liquor 18 before decomposition such that the iron concentration in the trihydrate with respect to the weight of Alz03 systematically remains N iZ3 2o I- I- above 0.01%. The total extraction yield of soluble alumina in the mixed bauxite varies, depending on the origin of the mineral, from 94% to 96% while the productivity does not exceed 65kg A1 2 0 3 per m 3 of liquor since the Rp before and after decomposition is respectively of the order of 1,05 and 0,6 while the average concentration of caustic soda is about 140g Na 2 0/litre, The basic process of the invention shown in Figure 2 is operated as follows: The mixed bauxite I, with a monohydrate or boehmite alumina content which can vary from 5% to 30% of the total weight of alumina in said bauxite, is wet ground at A before digestion to less than 200utm) from a suspension containing a very high lo concentration o" dry matter (700 to 100g of mineral per litre of suspension) in a first aliquot 20a taken from stream 20 of spent liquor where the caustic soda concentration is between 140 and 180g NazO/litre, ratio Rp is between 0.5 and 0.75, preferably between 0.55 and 0.65, and the iron content is less than or equal to 3mg/litre. The ground suspension 2 is then diluted in 3 to 6 times its volum of digestion liquor 20d produced by 1s mixing a second aliquot 20b of spent liquor 20 with the decanted liquor 11 from second digestion step 8. The caustic soda concentration of liquor 11 after decanting and washing the mud from the second (digestion step is generally between 150 and 180g NaZ0/litre and ratio R, preferably between 0.8 and 0,95. The iron content depends on the origin of the bauxite and can vary fron 10 to 500mg/litre. Thus digestion liquor S 20 produced from this mixture is characterised by a caustic soda concentration l2150g NaOl/litre, preferably between 155 and 180g Na 2 O/litre and a ratio Rp between 0.6 and 0.9, preferably between 0.65 and 0,85, The dilutd suspension is then heated to between 130°C and 190 0 C, preferably between 135°C and 180'C for 5 minutes to 1 hour, preferably between 5 and 20 nainutes. Suspension 3 from first digestion step is cooled to 26 less than 100 0 C, then decanted at C to separate supersaturated liquor where R, is preferably between 1.25 and 1.30 and the caustic soda concentration is preferably between 160 and 180g NazO/litre. Surprisingly, after backup filtration at G, this filtered supersaturated liquor 16 contains less than 8mg of iron per litre. It is then diluted to Sreduce the caustic soda concentration to less than 160g NaO0/litre, preferably between 130 and 155g NazO/litre before decomposition at H in the presence of recycled seed 22. On filtration at I, spent liquor 20 is separated and recycled to the process as aliquots 20c while solid alumina trihydrate phase 21 is divided, with or without granulometric grading, into a first fraction which is recycled as seed 22 and a second fraction 23 which is sent to the alumina production step 24 after washing with water 25 at 3a J. Wash water 26, which in the prior art is recycled as counter-current wash water for the mud from the second digestion step, is in the present case recycled to the head of decomposition step H as dilution water for liquor 16. It should oe noted that the alumina trihydrate precipitated in the decomposition step from a supersaturated liquor 16 which contains few impurities (Fe 8mg/L) contains little iron since the concentration of iron with respect to A1 2 0j does not exceed 0.005%.

IN U0QC-ACC441 JQC 10 of A significant improvement in the total soluble alumina extraction yield can be obtained if lime 29 is added before the second digestion step to residue 7 from the first digestion step constituted by decanted mud 4 mixed with the insoluble cake from filtration at G. This addition of lime, which can be as high as 8% but is preferably 1% to 5% by weight of CaO with respect to the corresponding quantity of digested bauxite, is at least partially obtainea by using a lime based filtration additive 28 at filtration step G which is subsequently recovered in insoluble cake 6 and thus in residue 7 from the first digestion step which is brought into contact with a third aliquot 20c of spent liquor stream 20. This second digestion step is carried out at a temperature of between 220°C and 300 0

C,

io preferably between 240°C and 250 0 C for 10 minutes to 1 hour, preferably 15 to minutes. Suspension 8 resulting from this second digestion step, after mixing with wash water 12 from the decanted mud from a previous operation, constitutes suspension so.k 9 which is decanted at E during which the decanted mud 10 is counter-current washci at F before discharge is In order to ensure that the monohydrate contained in the residue dissolves and to avoid any risk of retrograding, ie,, any risk of reprecipitation in the form of alumina monohydrate and trihydrate in solution, ratio Ri, of the supersaturated solution 11 from decantation step E is kept below 1, preferably between 0,b and 0.95. Under these conditions, soluble alumina extraction yields of the order of 96% are normally obtained, these yields being capable of going above 98% if lime is added to the residue before the second digestion step. This controlled liquor 11 is then recycled directly as first digestion step liquor mixed with spent liquor 20b which has been deprived of its very high irnn concentration.

The preferred embodiment of the process of the invention shown in Figure 3 consists in preceding low temperature digestion of the mixed bauxite by desilicai'on.

After low temperature digestion of the mixed bauxites, the supersaturated liquor is enriched in silica. This enrichment, measured by the ratio between the weight of silica in solution to the weight of caustic soda expressed as NazO, can be greater than 1% in decanted liquor 5 while it should stay within the limits of solubility of the silica in the different liquors used in the process, ie,, between 0.5% and 0,8% depending on, among others, the temperature and the caustic soda concentration in these liquors, As indicated above, supersaturation of the soluble silica in the sodium aluminate liquors in circulation results in untimely precipitation of silica in the form of sodium aluminium silicate which appears as silt deposits in the circuits, in particular in the tube 3j heat exchangers, but also as silica pollution of the alumina trihydrate precipitated from the liquors containing abnormal amounts of silica, ie., with a ratio SiOIcaustic NazO of more than 0.8% by weight.

To overcome these problems, mineral suspension 2 from wet grinding step A is subjected to an intermediate destication step A' in the presence of spent liquor 20a with a soluble silica SiOtcaustic NazO content of less than 0.7% before low temperature IN iIO, ICC44 Mi3 tt 020 digestion at B with liquor 20d, Desilication consists in heating suspension 2, containing a very high dry matter content (700 to I l00g/litre of suspension in liquor 20a) to about 100°C, more precisely to between 95°C and 107 0 C, for 6 to 12 hours, preferably for 7 to 8 hours. Under these conditions the major portion of the reactive silica is dissolved in s liquor 20a to form a sodium aluminate solution which is supersaturated with silica; it then has the time to reprecipitate in the form of insoluble sodium aluminium silicate with slow reaction kinetics, Suspension 2' from desilication step A' is thus characterised by a soluble silica content of less than also by a caustic Na20 concentration of between 130 and 170g/L. During desilication a portion of the alumina trihydrate passes into o1 solution. This dissolution continues during low temperature digestion at B in the presence of liquor 20d under conditions which are identical to those described above for the basic process of Figure 2, This is also the case for the subsequent operations.

Example 1 This example concerned the batch treatment of 10t of Australian bauxite from Weipa with the following composition by weight: A1 2 0 3 57.7% Fe203 11.9% SiO 2 5.41% Ti02: 2.78% Loss on burning: The weight percentages of soluble alumina in the monohydrate form (boehmite) and in the trihydrate form (gibbsite) were respectively 80% and 20%. The treatment was carried out using the basic process described in Figure 2, ie, without desilication and with 20 a total CaO addition of 2% by weight.

Wet grinding at A of the high concentration suspension with a dry matter content of 1425g of mineral per litre of spent liquor 20a where the granulometry had been reduced to less than 200pm.

Analysis of spent liquor 20 divided into 3 aliquots 20a, 20b and 2a caustic Na 2 O g/L: 163.7 0.634 caustic SiO z /Na 2 0: 0.82 Iron mg/L: 2 First digestion B, at 135°C for 20 minutes, of the suspension from the grinding step 3o diluted in 4.6 times its volume of liquor from the first digestion step 20d. This first digestion liquor, produced by mixing spent liquor 20b and liquor 11 from the second digestion step, had the following analysis: caustic NazO g/L: 174 Rp: 0.82% 3s caustic SiOC!NazO: 0.74 Iron mg/lL After cooling and decanting suspension from the first digestion step at C in the presence of a flocculent of Flocgel with respect to the weight of insoluble residue), IN 03010441 IOC l ia of 1o 13 the analysis s of supersaturated liquor 5 from the decantation step at 100 0 C was as follows: caustic Na20 g/L: 163 Rp: 1.25 caustic SiO 2 /Na20: 0,79 Iron mg/L: This liquor, after filtration at G in the presence of 0.5% of CaO as a filtration additive with respect to the weight of dry bauxite used during the first digestion step, was diluted then decomposed in the presence of a seed. The iron concentration in the filtered liquor before decomposition was reduced to less than 4mg/L and the iron concentration in the alumina trihydrate precipitated during decomposition and intended for production after washing was 30ppm with respect to the weight of A12O.

Digestion at D at 250°C for 15 minutes using spent liquor 20d of insoluble residue 7 constituted by decanted mud 4 containing undigested boehmite and insoluble filtration cake 6 containing the CaO based filtration additive; 1,5% by weight of CaO with respect to the weight of dry bauxite used in the first digestion step was added to the decanted mud 4.

Decantation at E of suspension 9 from digestion step D, in the presence of a flocculent of Flocgel with respect to the weight of insoluble residue with the addition of counter-current wash water, and separation of the mud which was discharged after washing at F, while decanted liquor 11 was recycled as first digestion liquor mixed with aliquot 20 of spent liquor.

The liquor leaving the decantation step had the following analysis: caustic Na20 g/L: 177 Rp; 0.927 caustic SiO 2 /Na 2 O: S Iron mg/L: The total soluble alumina extraction yield was 97% and the productivity, determined S by the change in Rp of the supersaturated ].liquor before and after decomposition, was S 30 very close to 92kg A1 2 a0/m 3 Example 2 This exam le concerned the batch treatment of 10t of Guinean bauxite from BOKE with the following composition by weight.

A1 2 0: 60.5% Fe 2

O

3 4.75% SiO 2 1.36% TiO,: 3.65% Loss on burning: 3a 29.0%. The percentages by weight of soluble alumina in the monohydrate form (boehmite) and in the trihydrate form (gibbsite) were respectively 14% and 86%.

The treatment was carried out using the preferred embodiment of the process described in Figure 3, ie., with desilication but without addition of CaO at filtration step G or during second digestion step D.

INJ 11l 0(10044 1A t3 of Wet grinding at A of the highly concentrated suspension with a dry matter content of 1800g of mineral per litre of spent liquor.

Analysis of liquor 20 divided into 3 aliquots 20a, 20b and caustic Na20 g/l: 164 Rp: 0,63 caustic SiO2/Na20; 0.62 Iron mg/L: 2 Suspension 2 from the grinding step underwent desilication at A' by heating at 103°C for 8 hours.

Analysis of the liquo, in suspension 2' from the desilication step: caustic NzO g/L: 136 0.954 caustic SiOz/NazO: 0,61 First digestion B, at 150°C for 7 minutes, of suspension 2' diluted in 5,6 times its volume of liquor from the first digestion step 20d, This latter liquor, produced by mixing spent liquor 20b and liquor 11 from the second digestion step, had the following analysis: caustic NaO2 g/L: 171 Rp: 0.80 caustic SiO 2 /NazO: 0.40 20 Iron mg/L: 22 After cooling and decantating suspension 3 from the first digestion step, at C in the S presence of a flocculent of Flocgel with respect to the weight of insoluble residue), the analysis of supersaturated liquor 5 from the decantation step was as follows: caustic NazO g/L: 162 2s Rp: 1.31 caustic SiOz/Na 2 O: 0.63 SIron mg/L: This liquor, after filtration at G in the absence of lime, had an iron concentration which had been reduced to 6mg/L. The iron concentration in the alumina trihydrate 30 precipitated during decomposition for production after washing was 50ppm with respect to the weight of AlOj., Digestion at D at 245°C for 15 minutes using spent liquor 20d of insoluble residue 7 constituted by decanted mud 4 containing undigested biehmite and insoluble filtration cake 6.

as Decantation at E of suspension 9 from digestion step D, in the presence of a flocculent of Flocgel) with the addition of wash water, and separation of the mud which was discharged after washing at P, while decanted liquor 11 was recycled as first digestion liquor mixed with aliquot 20b of spent liquor.

Liquor 11 leaving the decantation step had the following analysis: IM &IOC100441 IOC U4 0 1O caustic Na 2 O g/L: 172 Rp: 0,885% caustic SiO 2 /NaO2: 0.4 Iron mg/L: S The total soluble alumina extraction yield was 95.8% without addition of lime and the productivity was of the order of 100kg A1 2 0/m 3 Example 3 This example concerned the batch treatment of 10t of Guinean bauxite from BOKE from Example 2 using the preferred embodiment of the process described in Figure 3, ie., with desilication and with a total addition of CaO of 3% by weight.

Wet grinding at A of the highly concentrated suspension with a dry matter content of 1780g of bauxite per litre of spent liquor.

Analysis of liquor 20 divided into 3 aliquots 20a, 20b and Caustic Na 2 O g/L: 164 1 Rp: 0.64 caustic SiO 2 /Na 2 0: 0.66 Iron mg/L: 2 Desilication at A' by heating at 103°C for 7 hours.

Analysis of the liquor in suspension 2' from the desilication step: 20 caustic Na 2 O g/L: 125 Rp: 095% caustic SiOz/Na20: 0.70 First digestion B, at 180 0 C for 10 minutes, of suspension 2' diluted in 6,2 times its volume of liquor from first digestion step 20d, This latter liquor, produced by mixing 25 spent liquor 20b and liquor 11 from the second digestion step, had the following analysis: caustic Na 2 O g/L: 162 0.80 caustic SiO 2 /Na 2 0.45 Iron mg/L: 14 After cooling and decanting suspension 3 from the first digestion step, in the presence of a flocculent of Flocgel) the analysis of supersaturated liquor 5 from the decantation step was as follows: caustic Na20 g/L: 161.7 Rp: 1,28 as caustic SiO2/Na20O 0.65 Iron mg/L: This liquor, after filtration at G in the presence of CaO as a filtration addttive with respect to the weight of bauxite digested, had an iron concentration which had been reduced to less than 4mg/L, was decomposed in the presence of a seed, The iron IN IBoCI43 JOC 16 concentration in the alumina trihydrate precipitated during decomposition for production after washing was 30ppm with respect to the weight of A1 2 0 3 Digestion at D at 240 0 C for 20 minutes using spent liquor 20d of insoluble residue 7 constituted by decanted mud 4 with 2.5% by weight of added CaO and by insoluble filtration cake 6, then decantation at E in the presence of a flocculent of Flocgel) of suspension 9 from digestion step D, with the addition of wash water, and separation of the mud and decanted liquor in accordance with Examples 1 and 2 above, Liquor 11 leaving the decantation step had the following analysis: caustic Na 2 O g/L: 158 Rp; 0.892 caustic SiO 2 /Na20: 0.41 Iron mg/L: The total soluble alumina extraction yield was 99% and the productivity was slightly over 95kg Al 2 0 3 /m 3 1s Importaince of the Process While it ensures soluble alumina extraction yields of at least 95 and productivities of the order of 90kg of A1 2 0 3 per m 3 of liquor for decomposition, the process also reduces the iron concentration in the alumina trihydrate to less than 0.005% with respect to the weight of A1 2 0 3 since the iron concentration in the liquor prior to decomposition is 20 always substantially less than 10mg/litre. In addition it substantially simplifies operation by removing the backup filtration step for the decanted liquor from the second digestion step which is directly recycled to the head of the process as digestion liquor mixed with an aliquot of spent liquor.

Finally, the efficiency of the process is not affected by the addition of a desl!ication 26 step before digestion which means that a SiOl/caustic NazO weight ratio of less than 0.8 can be maintained in the supersaturated liquor before decomposition. As a consequence, the content of silica in the precipitated alumina trihydrate remains below 150ppm by weight with respect to A1 2 0 3 (N LIOCIO443 10C toot

Claims (10)

1. A process for the treatment of bauxite containing alumina trihydrate and alumina monohydrate in a proportion of at least 5% by weight of the total quantity of alumina in said bauxite, by low temperature caustic soda digestion of the ground bauxite s to form a suspension which is decanted and filtered to separate a supersaturated sodium aluminate liquor for decomposition in the presence of a seed and an insoluble residue containing red mud and alumina monohydrate, said insoluble residue then being digested in a caustic soda medium at a higher temperature to dissolve the alumina monohydrate in the form of a sodium aluminate liquor which is separated from the red mud by decantation, the process being characterised by the following combination of operations: a) low temperature digestion, or first digestion, of the ground bauxite by bringing it into contact with a liquor with a caustic soda concentration greater than or equal to 150g Na 2 O/litre and R, of between 0.6 and 0.9, said digestion liquor being constituted by a mixture of an aliquot of spent liquor from a previous decomposition operation, with a decanted liquor from the high temperature digestion step, or second digestion step, for the insoluble residue, b) after a contact time sufficient to dissolve the alumina trihydrate, decanting the suspension formed in the presence of a flocculent to separate a supersaturated sodium aluminate liquor with an Rp of between 1 20 and 1,35 and caustic soda concentration of at least 160g NaO2/litre from the insoluble residue, c) filtering then diluting the filtered supersaturated liquor with the wash water for the alumina trihydrate produced in the preceding cycle to reduce the concentration to less than 160g NazO/litre, Sd) decomposing the supersaturated sodium aluminate liquor after dilution in the presence of a seed to precipitate alumina trihydrate where the wash water after filtration is preferably used as diluting water for the supersaturated liquor at the start of the decomposition step, e) recycling the decomposed liquor to the head of the process in three aliquots which are respectively reintroduced to the grinding and second digestion steps as well as 3o to the first digestion step after mixing with the decanted liquor from the second digestion step, Sf high temperature digestion, or second digestion, of the insoluble residue from decantation and filtration of the suspension previously formed by bringing it into contact with an aliquot of spent liquor for a sufficient time to dissolve the alumina monohydrate 3s contained in the insoluble residue and form a suspension of red mud in a sodium aluminate liquor with an R, of less than 1, g) decanting and counter-current washing the insoluble residue from the second digestion step constituted mainly by red mud which is eliminated while the sodium aluminate liquor with Rp 1 from the second digestion step is recycled to the head of the IN .EOCI10443 JOC il Ot 18 process to form the liquor for the first digestion step following mixture with an aliquot of spent liquor.

2. Process according to claim 1, characterised in that the monohydrate alumina content varies from 5% to 30% of the total weight of alumina contained in the mixed bauxite.

3. Process according to claim I or claim 2, characterised in that, prior to digestion, the bauxite is wet ground to a granulometry of less than 200.im from a suspension in which the concentration of dry matter is between 700 and 1100g of mineral per litre of suspension.

4. Process according to any one of claims 1 to 3, characterised in that the caustic soda concentration in the spent liquor is between 140 and 180g Na 2 O/litre and the ratio Rp is between 0.5 and 0,75, preferably between 0.55 and 0.65. Process according to claim 1, characterised in that the decanted liquor from the second digestion step has ratio Rp preferably between 0.8 and 0.95. 1i 6, Process according to claim 1, characterised in that the low temperature bauxite digestion liquor produced by mixing the spent liquor and the decanted liquor from the second digestion step preferably has a caustic soda concentration of between 155 and 180g Na2O/litre and a ratio R, preferably between 0.65 and 0.85,

7. Process according to any one of claims 1 to 6, character in that the low 20 temperature digestion step, or first digestion step, for the suspenrin ;round bauxite following dilution in the digestion liquor is carried out between C and 190°C, preferably between 135°C and 180°C for 5 minutes to 1 hour, preferably for 5 to minutes.

8. Process according to any one of claims I to 7, characterised in that the decanted supersaturated liquor from the first digestion step has an R, which is preferably between 1.25 and 1.30 and a caustic soda concentration preferably between 160 and 180g NazO/litre,

9. Process according to any one of claims 1 to 8, characterised in that wash water for the alumina trihydrate precipitated during decomposition of the supersaturated liquor S 30 and intended for the production step, is recycled to the head of the decomposition step as diluting water to reduce the caustic soda concentration in the filtered supersaturated liquor to between 130 and 155g NazO/litre. Process according to claim 1, characterised in that, prior to the second digestion step, lime is added to the residue from the first digestion step constituted by the decanted insoluble residue mixed with the insoluble filtration cake from the supersaturated sodium aluminate liquor for decomposition in a maximum proportion of preferably 1% to 5% by weight of CaO with respect to the quantity of bauxite digested.

11. Process according to claim 10, characterised in that lime addition to the residue from the first digestion step is partially achieved using a lime based filtration additive for filtering the supersaturated liquor. IN 0CO443 JOC to of a

12. Process according to any one of claims 1 to 11, characterised in that high temperature digestion of the residue, or second digestion, is carried out at a temperature between 220 0 C and 300*C, preferably between 240°C and 250°C for 10 minutes to I hour, preferably from 15 to 20 minutes, 13, Process according to any one of claims 1 to 12, characterised in that the suspension of ground mineral in an aliquot of spent liquor is desilicated before low temperature digestion by heating to between 90°C and 107"C for 6 to 12 hours, preferably between 7 and 8 hours.

14. A process for the treatment of bauxite containing alumina trihydrate and alumina monohydrate in a proportion of at least 5% by weight of the total quantity of alumina in said bauxite, by low temperature caustic soda digestion of the ground bauxite to form a suspension which is decanted and filtered to separate a supersaturated sodium aluminate liquor for decomposition in the presence of a seed and an insoluble residue containing red mud and alumina monohydrate, said insoluble residue then being digested in a caustic soda medium at a higher temperature to dissolve the alumina monohydrate in the form of a sodium aluminate liquor which is separated from the red mud by decantation, substantially as hereinbefore described with reference to any one of the examples. A process for the treatment of bauxite containing alumina trihydrate and of to 20 alumina monohydrate in a proportion of at least 5% by weight of the total quantity of alumina in said bauxite, by low temperature caustic soda digestion of the ground bauxite to form a suspension which is decanted and filtered to separate a supersaturated sodium aluminate liquor for decomposition in the presence of a seed and an insoluble residue containing red mud and alumina monohydrate, said insoluble residue then being digested 26 in a caustic soda medium at a higher temperature to dissolve the alumina monohydrate in the form of a sodium aluminate liquor which is separated from the red mud by S decantation, substantially as hereinbefore described with reference to figure 2 or figure 3 S of the accompanying drawings. 9 Dated 25 October, 1994 Aluminium Pechiney 00 Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON IN VlBCI00441 JOC I L- Process for the Treatment of Bauxites Containing a Mixture of Alumina Trihydrate and Alumina Monohydrate Abstract A process for treatment by alkaline digestion according to the Bayer process, of mixed bauxite or bauxite containing alumina trihydrate and alumina monohydrate in a proportion by weight of at least 5% of the total quantity of alumina in the bauxite is described. Liquors with a high concentration of caustic soda are used during two bauxite digestion steps at low and at high temperature. The liquor for the first digestion step is constituted by a mixture of an aliquot of spent liquor with a liquor which has o1 been simply decanted from the high temperature digestion step of the residues from the first digestion step containing the alumina monohydrate. Following decantation, filtration and dilution to reduce the caustic soda concentration, a high purity trihydrate is precipitated from the supersaturated liquor from the first digestion step by decomposition in the presence of a seed, The productivity is high and the soluble alumina is extraction yield is greater than o. e Figure 3 4J