AU662969B2 - Alumina plant - Google Patents

Description

OFI DATE 13/09/93 APPLN. ID AOJP DATE 25/11/93 PCT NUMBER 36212/93 PCT/AU93/00071 II 11 11i I IAU933621 2 1 AU933621Z INTERNAI IUNAL At"'LILIIV iN tUu"LOA V*-r l -A. Y (PCT) (51) International Patent Classification 5 (11) International Publication Number: WO 93/16957 COIF 7/34 Al (43) International Publication Date: 2 September 1993 (02.09.93) (21) International Application Number: PCT/AU93/00071 (74) Agent: CARTER SMITH BEADLE; Qantas House, 2 Railway Parade, Camberwell, VIC 3124 (AU).

(22) International Filing Date: 18 February 1993 (18.02.93) (81) Designated States: AU, BR, CA, KR, US, European patent Priority data: (AT, BE, CH, DE, DK, ES, FR, GB, GR, IE, IT, LU, PL 0977 19 February 1992 (19.02.92) AU MC, NL, PT, SE).

(71) Applicant (for all desigi.ated States except US): COMALCO Published ALUMINIUM LIMITED [AU/AU]; 55 Collins Street, With internationalsearch report.

Melbourne, VIC 3000 (AU).

(72) Inventors; and Inventors/Applicants (for US only) BROWN, Gregory, Patrick [AU/AU]; 27A Ballinger Road, Buderim, QLD 4556 WOOD, David, Graham [AU/AU]; 164 Upper Brookfield Road, Brookfield, QLD 4069 (AU).

(54) Title: IMPROVEMENTS IN ALUMINA PLANTS (57) Abstract A process for the production of alumina by the Bayer process includes a precipitation process having an agglomeration stage and a growth stage. In the agglomeration stage, selected process parameters are controlled to limit the bound soda content of the slurry, whilst still maintaining conditions for high yield. In the growth stage, selected process parameters are controlled to controll the soda pick-up during growth so that a continuous decrease of soda content is obtained. Preferably, the growth stage is divided into a number of separate precipitation vessels and the temperature drop from one precipitator to the next is from I to 3 The temperature of the liquor leaving the final precipitator is preferably about 55 The agglomeration stage is preferably operated at 75 85 *C..Apparatus suitr.ble for use in the method is also described.

1 IMPROVEMENTS IN ALUMINA PLANTS Field of the Invention This invention relates to improvements in alumina plants, and particularly plants operating according to the Bayer process to extract alumina from bauxite.

Background of the Invention The Bayer process, and numerous improvements and modifications thereof, is widely used in the processing of bauxite to produce metal grade alumina. Most alumina plants producing metal grade alumina currently use a precipitation process which is a combination of agglomeration and growth to meet the requirements for crystal size and morphology. The precipitation process is normally carried out at the lowest possible temperature to maximize the process yield.

*15 Modern downstream alumina users now require that the alumina be provided with a low soda content, while maintaining the desired crystal size and morphology. To achieve a lower soda content, the precipitator start temperatures are usually increased, and this results in considerable loss of yield, and thus plant efficiency.

Summary of the Invention and Object It is the object of the present invention to provide alumina plant improvements which result in a high yield of loll soda alumina. In the present specification, the term "low 25 soda" should be understood to include soda contents of about 0.25 to 0.35 per cent Na 2 0.

In one aspect, the invention provides a process for the production of alumina using the Bayer process including a precipitation process having an agglomeration stage and a growth stage characterized in that the temperature of liquor in the agglomeration stage is within the range of about 75°C to about 85°C to limit the bound soda content of hydrate particles GWN:SM:IM604&SPC 17 July 1995 2 and the growth stage is divided into a multiplicity of separate stages and the temperature of the liquor in each of the multiplicity of growth stages is such that each successive downstream growth stage is operated at a temperature of about 1°C to about 3°C cooler than the adjacent upstream growth stage.

In the process of the present invention, controlling the temperature of the agglomeration stage such that the temperature of the liquor falls within the range of about to about 85°C limits the bound soda content of hydrate particles.

In the growth stage, controlling the temperature such that there is a temperature drop of about 1IC to about 3°C between successive stages (or precipitators) limits the supersaturation 15 of the liquor in each growth phase and this controls the degree of soda pick-up on the hydrate particles. As the liquor/hydrate particles slurry moves th-ough the successive stages of the growth stage, the soda content of the hydrate particles decreases. In this regard, the soda content is 20 calculated as a weight percentage of the hydrate.

The temperature of the liquor in each growth stage may be controlled by use of forced cooling in each growth stage. For example, heat exchange means may be associated with each growth stage to control the liquor temperature to the desired 25 temperature.

To further improve the results achieved, hydrate seed is added to both stages of the precipitation process. In a preferred form, a seed slurry is added to the agglomeration precipitator, or to the first of the agglomeration 30 precipitators where more than one is used, 0 GWN'SM:11604&SPC 17 uly 1995 WO 93/16957 PC/AU3/0007 1 3 the slurry having a solids content of about 10 to 20 per cent of particles less than about 44 microns with a d.

0 of about 60 to In the case of the growth stage or stages, a seed slurry is preferably added to the feed to the first growth stage, with the slurry preferably containing hydrate solids typically with about 5 to 8 per cent of particles less than abouc- 44 microns and with a d 5 of about 75 to 85 microns.

In another aspect, the invention provides an alumina plant operating according to the Bayer process including precipitation means having an agglomeration stage and a growth stage, characterized by means for controlling selected operating parameter(s) during said agglomeration stage to limit the bound soda content of the slurry while maintaining conditions necessary for a high yield, and means for controlling selected operating parameter(s) during said growth stage to control soda pick-up during the growth stage so that a continuous decrease of soda content is obtained.

In a preferred form of the invention, means are provided to control the temperature of the liquor in the agglomeration stage so that it substantially falls within the range 75 to 80 0 C. The apparatus is further preferably controlled so that the temperature of the liquor in the growth stage is gradually reduced in a step wise manner. For example, the growth stage is divided into a multiplicity of precipitation tanks, each of which has an associated heat exchanger which reduces the temperature of the liquor within each tank by about 1 to 3 0 C, and preferably about 2°C, so that an exit temperature of the order of 55°C is achieved.

The apparatus also preferably includes means for delivering hydrate seeding particles to each of the agglomeration stage and the growth stage, preferably as feed to each stage or to the first stage of each stage WO 93/16957 PCTI/AU93/00071 4if multi-stages are used.

Brief Description of the Drawing~s The invention will now be described in greater detail with reference to the accompanying drawings in s which a presently preferred embodiment of the invention is shown. In the drawings: Figi.ire 1 is a schematic process diagram of the Bayer process showing the modified precipitation stage; Figure 2 is a more detailed block diagram showing io the modified precipitation stages; Figure 3 shows graphs illustrating the improvements in soda pick-up and cumulative soda achieved by increasing the growth precipitation stages; Figure 4 is a graph showing the step wise reduction is in temperature achieved in the growth precipitation stages; Figure 5 shows a representation of the temperature profiles used to exemplify the present invention; Figure 6 shows the yield of alumina obtained from 2 bottle tests using the temperature profiles of Figure The alumina yields shown in Figure 6 have been calculated from liquor analysis; Figure 7 shows the yield of alumina (as calculated from solids analysis) obtained from bottle tests using the temperature profiles of Figure 5; and Figure 8 shows the soda pick-up in the precipitated alumina obtained from bottle tests using the temperature profiles of Figure Description of Preferred Embodiment The Bayer process shown schematically in Figure 1 of the drawings is widely used in the processing of bauxite to metal grade alumina and is therefore well understood by persons skilled in the art. No further description of the overall process is therefore required, In the preferred embodiment of the invention, the precipitation stage is modified to produce a high WO 93/16957 PCT/A U93/00071 5 yield of low soda alumina hydrate which is then processed ready for calcination.

The preferred precipitation process is shown in greater detail in Figure 2 of the drawings.

The precipitation area forms part of the overall white side which is designed to produce a hydrate: S Suitable for calcining in modern fluo-solids calciners with minimum breakdown of particle.

S Suitable for producing an alumina with a io consistent quality, as specified.

At a yield of 75 gAl 2 0 3 /1 (based on pregnant liquor).

To achieve the quality required, the precipitation process is separated into two stages: an agglomeration stage followed by a growth stage. The agglomeration stage is designed to maintain a balance of fine hydrate particles in the precipitation circuit by forming agglomerates. The growth stage consolidates and coarsens the agglomerates to form the strong hydrate by growth of Sthe hydrate particles.

In the agglomeration stage, (10) as shown in Fig.

2, hot pregnant liquor (11) is seeded with a washed tertiary seed The slurry is held in a number (eg.

two) of agitated precipitation vessels arranged in series to give time for agglomeration to proceed to the required level.

The resulting agglomeration slurry is then seeded in the growth stage with a more coarse secondary seed The slurry then passes through a series of large 3 agitated precipitators in the growth stage (15) and is progressively cooled to a low exit temperature. The size of the discrete cooling steps, exit temperature ar tankage volume is chosen to achieve high yield and low soda content.

The slurry (16) from the last of the growth precipitators is transferred for classification by WO 93/16957 PCr/A U93/0007 1 6 classification circuit (17) into product hydrate for calcination (18) and tertiary seed (12) and secondary seed (14) for recycle to the precipitation circuit. The overall precipitation process is advantageously arranged as shown in the process flow sheet shown as Figure 2 in our pending Australian Provisional Patent Application No. PL 0977, filed 19 February 1992, the entire contents of which are herein incorporated by reference.

The precipitation area may include two identical i0 parallel precipitation chains, with each chain capable of 0.5 mt/y production rate.

To produce a hydrate with the required qualities of strength, chemical composition and granulometry at the specified yield, requires consideration of a number of is variables of the precipitation process: liquor quality, seed charge/granulometry, temperature profile, repidence time, number of precipitators.

Based on computer modelling the process has been designed: to operate with a liquor having 225 g/B Na 2

CO

3 0.69 A/C and 90% C/S; with an agglomeration section in each chain which consists of two tanks in series, to give an agglomeration time of 4-6 hours operating at a temperature in the range of about 75-80*C, cooling the hot pregnant liquor from an inlet temperature of about 103*C; with a growth section in each chain which consists of about 10 tanks in series, to give an average time of 40-45 hrs based on final slurry volume; with temperature of the growth chain being reduced by small step cooling about 1* to 3"C, and preferably about 2*C, in each tank, to reach approximately 55°C in the last precipitator; and with seed charges to achieve up to 200 g/9 solids, WO 93/16957 PC/A U93/0007 1 7 preferably about 175 in the agglomeration slurry and up to 450 g/9 solids in the slurry from the last growth precipitator, preferably about 375 g/9.

Seed charges may be as low as about 100 g/9 in the agglomeration slurry and about 280 g/k in the growth slurry. In any event, the seeding is contained at a level which will achieve the alumina quality substantially as disclosed in Table 1.

The apparatus and process has potential for operating at a higher yield of 85-90 gAl 2 0 3 /1 while maintaining the quality of hydrate. This is achieved by increasing the caustic concentration to about 240-250 g/Q Na 2

CO

3 and A/C ratio of the fill liquor to about 0.71 to 0.73, and installation of an additional two precipitators on each chain, and/or producing hydrate which will produce an alumina with a soda content of 0.22-0.27%, which is lower than presently desired. This will be achieved by manipulating the supersaturation levels in the precipitation tanks by changing the cooling profile and/or the addition point of the pregnant liquor.

The design is required to give a process which is controllable and reasonably simple to operate.

Flexibility of the process is also a requirement, such that with minimum impact on product quality, the process is capable of absorbing changes which may occur as a result of variations from outside and/or inside the precipitation area e.g. plant flow rate, liquor quality change, fining excursion, etc.

It is also necessary that the operational aspects be considered. Precipitators are to be capable of being bypassed, emptied, cleaned and returned to service, without causing major disturbance to the process control and production of quality hydrate.

The agglomeration section of each precipitation WO 93/16957 IICT/A U93/00071 8 chain operates independently but typically with similar process conditions. Fifty percent of the pregnant liquor (at target temperature of about 78 0 C) and 50% of tertiary seed reslurry (from the tertiary seed slurry s tank) is added to the first agglomeration precipitator in each chain. The tertiary seed slurry will contain solids with typically 10-20% of particles less than 44 microns and with a d.

0 of 60 70 microns.

The agglomeration precipitators are typically of 2200 m' (average) volume, flat bottom, open top, straight sided vessels fitted with draft tube agitators to a standard design used in the alumina industry. The tanks are of constant diameter, but height will be varied to provide gravity overflow from first to last.

As the agglomeration section is located at the front of the precipitation process, the precipitator height must be sufficient to allow gravity overflow to the growth precipitators.

The precipitators are connected by a launder system which allows for flows to be maintained, when one or more adjacent precipitators is bypassed. Each agglomeration precipitator overflows to the transfer launder via an internal riser which allows sufficient depth below the riser to avoid bogging the riser pipe with settled hydrate when agitation is stopped. The launder system is fitted with a gate system which allows a precipitator to be readily bypassed and also allows safe isolation of the precipitator when bypassed. The agglomeration precipitators are not force-cooled but are 3o allowed to operate at temperature consistent with normal radiation and convection heat losses to achieve the target temperature.

The slurry from the agglomeration precipitators containing hydrate solids typically with 10 15% of particles less than 44 microns is transferred via overflow launders to the first of the growth WO 93/16957 PCr/A U93/00071 9 precipitators. A portion of the agglomeration slurry from each chain overflows to the Secondary Seed Reslurry Tanks for reslurry of secondary seed, to give a pumpable slurry of 800-850 g/9 solids.

s The growth section in each precipitation chain will be designed to provide up to 45 hours of nominal residence time for the slurry in the growth precipitators. The growth section in each chain will consist of about 11 precipitators arranged for series operation; normally 10 on line with one bypassed for cleaning/maintenance. Each growth precipitator has an associated cooling heat exchanger, such as an in-tank cooler of the type described in our corresponding Provisional Patent Application PK 4123 dated 7 January 1991, which operates to cool each stage by about 10 to 3 C, preferably about 2 0

C.

The major portion of the agglomeration slurry which overflows the last agglomeration precipitator on each chain flows directly to the first growth precipitator in each chain.

The secondary seed (reslurried in the Secondary Seed Reslurry Tanks with product wash filtrate and a fraction of the agglomeration slurry), is proportioned to each growth chain, normally 50% to each. The seed slurry is added to the launder after the last agglomeration precipitator and before the first growth precipitator. Secondary seed will contain hydrate solids typically with 5-8% of particles less than 44 microns.

The slurry from the first growth precipitator flows by gravity to the next on line growth precipitator and successively through the ten stages in series via an external launder system. As the slurry proceeds through the growth precipitators the agglomerates are strengthened by growth. Typically the slurry from the last of the growth precipitators contains hydrate with 4-8% less than 44 microns with d 50 of 75-85 microns.

WO 93/16957 PCTAU9300071 10 Each precipitator overflows via an internal riser pipe which allows sufficient depth below the riser for settled hydrate, to avoid bogginr during periods of lost agitation. The launder system connecting the s precipitators is arranged such that flows are maintained when one or more adjacent precipitators are bypassed.

The growth precipitators are built to a standard design used in the alumina Industry and are about 4400 m 3 average volume, flat bottom, open top, straight sided io vessels fitted with draft tube agitators. The tanks typically have a 2/1 height/diameter ratio to avoid excessive heights. The precipitators are arranged in an arrangement which best meets the requirements for continuous operation, gravity flow and bypassing.

The slurry from the last on-line growth precipitator of each chain is pumped from a bottom suction on the precipitator to the Classification area.

The level in the last precipitator is controlled by variable speed pump to maintain a submerged agitator and o full agitation of the precipitator. No riser is installed in the last precipitator. An emergency shut off valve is installed (if required) to avoid drain down on shutdown.

A single pump will be installed in each 2s precipitation chain and a third pump is installed which as a common spare for the two chains. The pump suction is piped and valved to each of the last two precipitators in the same chain, to allow bypassing of the last precipitator.

Graphs A and B of Figure 3 demonstrate the reduced soda pick-up (Graph A) and the reduced cumulative soda (Graph B) achieved as the number of growth precipitation stages increases, Graph C of Figure 4 shows the stepwise drop in temperature in the growth precipitation stages.

In order to demonstrate the advantages of the WO 93/16957 P~r/AU93/00071 11 present invention, a series of experimental runs were carried out to investigate soda impurity levels and hydrate yield for a range of precipitation temperature profiles and organic carbon levels. The trials were conducted as individual bottle tests in a rotating water bath. Three different temperature profiles were used.

Profile 1 is a temperature profile that falls within the scope of the present invention. Profiles 2 and 3 are indicative of temperature profiles currently used in i0 alumina production. The temperature profiles used in the experimental work are shown in Figure The experiments were carried out at low and high concentrations of organics in the liquor. The liquor used in the experimental runs had an initial caustic soda concentration in the range of 218 to 230 calculated as Na 2

CO

3 The alumina content of the liquor was around 155 g/Q. Initial A/C ratios were in the range of from 0.66 to 0.71, with C/S ratio ranging from 0.79 to 0.92.

The liquor and solids content of each bottle was sampled and analysed for alumina and caustic concentrations, solids mass, particle size distribution and soda content.

The yield of alumina obtained from the experimental runs is shown in Figures 6 and 7. Figure 6 shows the alumina yield as calculated from an analysis of the liquor, whilst Figure 7 shows the alumina yield calculated from the solids recovered from the tests.

As can be seen from Figures 6 and 7, there is no significant difference between the yield obtained from profile 1 (the profile that falls within the scope of the present invention) and the yields obtained from profiles 2 and 3.

Figure 8 shows the change in soda content of the precipitated alumina with time for each run. The soda content only represents the soda contained in the layer WO 93/16957 PC/A U93/00071 12 of newly precipitated alumina, not the existing soda in the fine and coarse seed. The results shown in Figure 8 clearly demonstrate that the profile that falls within the scope of the present invention (profile 1) produces hydrate with significantly lower soda content than profiles 2 and 3, which represent profiles currently used in the industry.

The soda levels recorded in the experiments are generally higher than those found in a refinery. This is due to the bottle experiments having a continuous change in the A/C ratio. However, in a refinery, the A/C ratio changes in a stepwise fashion as slurry from one tank overflows into the next tank. This is most important in the first tank of the precipitation train where pregnant liquor enters aet, say 0.69 A/C and immediately drops to, say, 0.62 as it mixes with the slurry contained in the first tank and so reduces the supersaturation. Although the soda level obtained in a refinery for all three profiles tested in the experimental runs will be lower than the results shown in Figure 8, the relativity between the profiles will still be present, with profile 1 exhibiting lower soda pick-up than profile 2 or profile 3.

The results shown in Figures 6, 7 and 8 show that the method of the present invention is capable of producing low soda alumina by the Bayer process whilst still maintaining a high yield.

WO 93/16957 PTA9/07 PCr/AU93/00071 13 TABLE 1 ALUMINA QUALITY TYPICAL SHIPMENT Fe 2

O

3 SiO 2 %0.0 12 0.015 max Na 2 O 0.30 0.35 max Cao% 0.020 TiO 2

V

2 0 5 %0.005 P205 %0.001 Gibbsite %0.10 0.20 max A lpha A1 2 0 3 %3 45 microns %8 12 max 150 microns %1 5 max S.S.A. M 2 /g 70 60 LOM (300-1100 0 C) %0.9 1.2 max 20 Microns max Attrition Index %15 25 max Bulk Density t/m 3 1.2 L- 1. Notes: No ESP dust in product Quality consistency imperative Capability of achieving 0.25% Na 2

O

Claims (8)

1. A process for the production of alumina using the Bayer process including a precipitation process having an agglomeration stage and a growth stage characterized in that the temperature of liquor in the agglomeration stage is within the range of about 75°C to about 85°C to limit the bound soda content of hydrate particles and the growth stage is divided into a multiplicity of separate stages and the temperature of the liquor in each of the multiplicity of growth stages is such that each successive downstream growth stage is operated at a temperature of about 1°C to about 3°C cooler than the adjacent upstream growth stage.

2. A process as claimed in claim 1 wherein each successive downstream growth stage is operated at a temperature 15 of about 2°C cooler than the adjacent upstream growth stage.

3. A process as claimed in claim 1 or claim 2 wherein the temperature of the liquor leaving the last growth stage is about 55"3.

4. A process as claimed in any one of claims 1 to 3 20 wherein the temperature is controlled by forced cooling of the liquor in each growth stage. S.

5. A process as claimed in any one of claims 1-4 which further comprises adding a seed slurry to the agglomeration stage, said seed slurry having a solids content of about 10 to 25 20% of particles less than 44 microns, with a dso of about to 70 microns.

6. A process as claimed in any one of claims 1 to which further comprises adding a seed slurry to the growth stage, said seed slurry having a solids content of about 5 to 8% of particles less than 44 microns and with a dso of about to 85 microns. 17 July 1995 15

7. A process as claimed in any one of the preceding claims wherein the precipitation process is carried out in a plurality of precipitators and the temperature in each precipitator is controlled such that the temperature drop between a precipitator and an adjacent downstream precipitator is from 1*C to 3°C.

8. A process for the production of alumina using the Bayer process including a precipitation process substantially as hereinbefore described with reference to the accompanying drawings. DATED: 17 July 1995 CARTER SMITH BEADLE Patent Attorneys for the Applicant: COMALCO ALUMINIUM LIMITED S 6 GWN:SM:#16046.SPC 17 Jule 1995