CA2160050A1 - Process for the production of a low soda content smelting grade alumina - Google Patents
Process for the production of a low soda content smelting grade aluminaInfo
- Publication number
- CA2160050A1 CA2160050A1 CA002160050A CA2160050A CA2160050A1 CA 2160050 A1 CA2160050 A1 CA 2160050A1 CA 002160050 A CA002160050 A CA 002160050A CA 2160050 A CA2160050 A CA 2160050A CA 2160050 A1 CA2160050 A1 CA 2160050A1
- Authority
- CA
- Canada
- Prior art keywords
- alumina
- leach
- process according
- carbon dioxide
- slurry
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F7/00—Compounds of aluminium
- C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
- C01F7/46—Purification of aluminium oxide, aluminium hydroxide or aluminates
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F7/00—Compounds of aluminium
- C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
- C01F7/44—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water
- C01F7/441—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water by calcination
Abstract
A process for the production of a low soda-content smelting grade alumina from alumina trihydrate, wherein the process comprises the steps of:
(a) calcining alumina trihydrate at a temperature between 700°C and 1200°C to produce alumina;
(b) leaching the alumina in an aqueous slurry to which carbon dioxide is added, the leaching occurring at a temperature less than 80°C;
(c) washing the leached alumina slurry and separating the washed alumina therefrom; and (d) heating the washed alumina slurry at a temperature between 300°C
and 1100°C to produce a smelting grade alumina with a low soda content.
(a) calcining alumina trihydrate at a temperature between 700°C and 1200°C to produce alumina;
(b) leaching the alumina in an aqueous slurry to which carbon dioxide is added, the leaching occurring at a temperature less than 80°C;
(c) washing the leached alumina slurry and separating the washed alumina therefrom; and (d) heating the washed alumina slurry at a temperature between 300°C
and 1100°C to produce a smelting grade alumina with a low soda content.
Description
_ - 2 --THE PRESENT INVENTION relates to a process for the production of alumina, in particular a low soda-content smelting grade alumina.
Throughout the aluminium industry there is a perception that most aluminium smelters will, at sometime in the future, require alumina refineries to produce alumina with a low soda content, typically as low as or lower than about 0.3%
(expressed as Na20). Indeed, some smelters require this level of soda at this time. Typical soda levels in smelting grade alumina (SGA) presently produced by most alumina refineries are in the order of 0.3 to 0.6% Na20.
Therefore, refineries which produce SGA at or towards the upper end of that range will be forced to develop techniques for reducing the soda content of their product.
The usual proposal for achieving reductions in soda levels is to increase the temperature at which the precipitation of alumina trihydrate is practised in theBayer process, or to reduce the specific precipitation rate in the early stages of the precipitation circuit. However, this technique may suffer from a number of disadvantages, such as reduced yield, increased energy costs, increased requirements for cooling in the alumina trihydrate precipitation circuit, and the production of weaker alumina trihydrate which is more prone to breakdown upon calcination or pneumatic handling.
An alternative approach to increasing the precipitation temperature that has been considered but never put into practise has been to adopt mineral acid leaching of non-SGA alumina (using for example, hydrochloric acid, sulphuric acid or nitric acid) to remove some of the sodium from the alumina. Although this has been documented and practised for use on a small scale for the production of chemicalgrade alumina and intermediate aluminas, it has not been practical for refineries producing smelting grade alumina on a large scale. Indeed, it suffers a number of disadvantages. In particular, this process introduces the need to treat the by-product salts (for example, sodium chloride, sodium nitrate or sodium sulphate) which are generally not environmentally suitable for disposal, and which also cause a degree of contamination of the SGA with chloride or sulphate.
Disposal of these salts can also lead to reduced SGA production due to elevated chloride, sulphate or nitrate levels in the refinery liquor stream. Also, the use of mineral acids poses a corrosion risk for process equipment.
With particular regard to prior documented techniques, US patent 4,260,585 discloses a process for producing low-soda alumina granules (typically 1-5 mm) suitable for porcelain and catalyst manufacture. This process requires alumina granules to be placed in a high temperature (up to 175C) autoclave with sufficient water to produce steam which condenses and leaches the soda from the granules. The present invention does not require the use of alumina granules. In fact these are unavailable and highly undesirable in the Bayer process for producing SGA. Furthermore, high temperatures such as 175C are very expensive to attain and thus are preferably avoided.
US patent 3,655,339 discloses a process for producing extremely low-soda alumina starting with an already low soda (<0.25%) alumina which contains at least 75% alpha alumina and then leaching in hot water or hot dilute acid. The preferred soda level of less than 0.25% stated in US 3,655,339 is extremely difficult and expensive to produce and is not generally available. Furthermore, alumina with alpha content greater than 75% would not be able to be sold as SGA since smelters could not subsequently use it in their processes.
US patent 4,120,942 discloses a process for producing shaped catalysts from a rehydratable alumina using a staged rehydration process which also includes a pH control step at high temperature (80 - 100C) and long times (greater than 1 hour). The present invention does not require the use of a rehydratable alumina as a feed stock. In fact, this material is not available in the Bayer process and would be very expensive to produce on the scale of manufacture of SGA.
Instead, the present invention is able to employ the alumina already produced byalumina refineries - smelting grade alumina - as a feed stock.
All of these prior art techniques disclose acceptable processes for producing small quantities of catalyst or similar grade aluminas. Such processes would be completely unacceptable for an alumina refinery desiring to produce quantities of low soda alumina of the order of 100,000 to 1,000,000 or more tonnes of alumina per year. Consequently, there is a need to develop a process which permits the very large scale production of a low soda smelting grade alumina.
It is an aim of the present invention to provide a process for the production of a low soda-content smelting grade alumina, the process being suitable to avoid, orat least partially alleviate, the above mentioned difficulties. The process of the invention may additionally produce an extremely low (typically less than 0.2 %) soda content smelting grade alumina, the production of such an extremely low soda-content smelting grade alumina also permitting the blending of normal SGA
with the extremely low soda-content SGA to produce whatever soda content is desired. Alternatively, the extremely low soda-content SGA afforded by this invention permits only a fraction of the output of a refinery or refinery system to be treated and then blended with the remainder to again provide a low soda content SGA.
The present invention is characterised by a process for the production of a low soda-content smelting grade alumina from alumina trihydrate, wherein the process comprises the steps of:
(a) calcining alumina trihydrate at a temperature between 700C and 1 200C to produce alumina;
(b) leaching the alumina in an aqueous slurry to which carbon dioxide is added, the leaching occurring at a temperature less than 80C;
(c) washing the leached alumina slurry and separating the washed alumina therefrom; and (d) heating the washed alumina at a temperature between 300C and 11 00C to produce a smelting grade alumina with a low soda content.
The calcination of the alumina trihydrate in step (a) preferably takes place in fluidised bed calciners or kilns that would normally be present in a Bayer plantcapable of producing a smelting grade alumina. Thus, and with regard to the adaptation of an existing Bayer plant to include the process of the present invention, it is the remaining steps, steps (b) to (d), which are added to the plant to produce the low soda-content SGA which the present invention aims to produce.
This first calcination will thus generally be conducted at temperatures in the order of 900 to 1100C and more preferably at about 1000C. Indeed, calcination at temperatures in the broadest range referred to above ensures that the alumina produced is not a rehydratable alumina, an alumina typically having a loss on ignition (LOI) greater than 3% which is too high to be acceptable to aluminium smelters (SGA typically having LOls of less than 1%), and typically being produced by calcination at temperatures in the order of 350 to 600C.
Furthermore, calcination at the high temperatures of the process of the present invention facilitates the conversion of entrapped soda into a form which is moreeasily removed by the subsequent leach of step (b).
The use of carbon dioxide as a leachant in the process of the invention is preferred over the mineral acids referred to above ffor example, hydrochloric, sulphuric and nitric acids) as the carbonic acid subsequently generated does notabsorb to any appreciable extent on leached alumina and thus does not contaminate the alumina. Indeed, sulphuric and hydrochloric acids can cause unacceptably high levels of sulphur and chlorine contamination respectively.
Furthermore, carbon dioxide is comparatively inexpensive and its use allows the waste products of the leaching reaction to be returned to the Bayer process, representing a recovery of soda values, without an adverse impact upon the production efficiency of the refining process. Further, by utilising carbon dioxide -as the leachant, the subsequently generated leach solution may be beneficially reacted with lime (at extremely high lime efficiencies) to produce a solution ofdilute sodium hydroxide. Of course, sodium hydroxide is the dissolving agent used in all Bayer process alumina refineries and the generation of any amount thereof is advantageous.
Also in relation to the leach step of the present invention, the rapid kinetics of the leach reaction makes it preferable to use an efficient carbon dioxide sparge design. In this respect, the soda leach reaction is fast (usually complete in less than one minute), while the dissolution of carbon dioxide is slow thus becoming the rate-limiting step in the reaction kinetics.
Preferably, the efficient carbon dioxide sparge design is provided by the injection of carbon dioxide close to the high shear region induced by the leach tank agitator.
However, it must be appreciated that the use of such a sparge design is merely preferred. Indeed, even with poor sparge design, and thus poor absorption efficiencies, the quality of the product of the leach may not be affected due to the comparative speed of the soda leach reaction, and the leach may nonetheless still be conducted comparatively rapidly, with leach times of less than 5 minutes being sufficient to ensure complete reaction.
With regard to other preferred operating conditions of the leach of the present invention, it will be appreciated that alumina possesses a surface charge when in contact with water, the surface charge being negative when the pH is above about 8 to 9 and being positive when below that. However, the most efficient leaching will generally occur at a pH between about 6 and 8; less than 8 to prevent the surface of the alumina absorbing cations, such as sodium; and greater than 6 to prevent the alumina absorbing anions, such as chloride.
Advantageously this pH range is easily achieved and controlled with carbon dioxide as a leachant.
However, to maintain the pH of the leach and wash solutions, it is also apparentthat some carbon dioxide is expended to keep the solution saturated with carbon dioxide. This, together with what can be significant losses of carbon dioxide atvarious preferred vacuum filtration stages within the process, may result in theloss of enough carbon dioxide during the leach to warrant carbon dioxide recovery from the spent solutions. Furthermore, excess CO2 that is not expended in the leach system may be recycled if necessary.
Interestingly, the control of the pH achievable in the leach step primarily results from the amount of sodium bicarbonate present, and is due to an equilibrium reaction between carbonic acid and the bicarbonate ion. Thus, in a preferred form of the invention, the sodium bicarbonate concentration in either the leach solution of step (b) or the wash solution of step (c) is kept below 1 g/L to achieve a pH of about 6 and to assist in ensuring a low soda content in the final product.
In order to reduce the sodium bicarbonate concentration, low solids densities may be used, a multiple stage counter-current leach system may be used, or the washing of the leached alumina slurry may be conducted as a counter-current wash on a filter. In a preferred form of the invention, the leach system adopted is a multiple-stage counter-current leaching system, preferably having two to sevenstages, although four stages would be most preferred. In a further preferred form, the carbon dioxide of the leach may be used at pressures above atmospheric, such as at pressures in the range of from 1 to 7 atmospheres. Further still, a single stage leach may be employed, with the leached alumina then being subjected to counter-current washes on a filter. Of course, any combination of these may also be a suitable alternative.
In the latter form of the invention, namely where a single stage leach and subsequent counter-current washes are conducted on a filter, the sodium bicarbonate concentration in the leach vessel may actually be allowed to substantially exceed 1 g/L since the sodium bicarbonate concentration will be reduced during the counter-current wash stages. This form of the invention -minimises the amount of wash solution that is used in the leaching and washing stages and thus is the preferred form of the invention.
In relation to the wash solution, water is the preferred medium, the aqueous slurry used for the leach thus being a water slurry. However, it will be appreciated that the water may contain impurities for the like, depending in part upon the most convenient source of the water.
In the preferred form of the invention, the alumina is preferably filtered after the leaching step, the filtration preferably being vacuum filtration. However, it has been found that some soda re-adsorbs on the alumina during vacuum filtration due to the rise in pH caused by thé reduction in CO2 partial pressure and by therelatively high concentration of sodium bicarbonate in the leach slurry solution. In the four-stage counter current process referred to above, this may contribute about 0.01% to the total soda in the final product, and may be reduced further by the use of a carbon dioxide shroud over the filter or by pressure filtration.
However, the same reduction may be achieved by the use of the series of counter-current washes on the filter wherein the filtrate from the final wash stage is used as the wash liquor for the preceding wash stage, the filtrate of which is used as the wash solution for the second preceding wash stage, and so on. The series of counter-current wash solutions may also be re-saturated with CO2 priorto their re-use as a wash solution for the preceding stage.
Preferably the final step of the process of the invention (step (d), the re-calcination of the washed and leached alumina slurry) is conducted in accordancewith the preferred embodiment in our co-pending Australian patent application PM8686 filed on 6 October 1994 and titled "Process for drying particulate material". The content of that specification is incorporated herein by reference.
Thus, the washed and leached alumina slurry of the present invention may be the wet alumina feed of the preferred embodiment of patent application PM8686. A
first portion of the wet alumina feed is firstly preferably subjected to pre-drying at a temperature between 90C and 300C to produce at least partially dried alumina and moisture. The at least partially dried alumina is separated from themoisture, and is then heated in a furnace to a temperature between 800 and 1200 C. Various products and streams are recirculated and separated and there are subsequent mixing, holding and cooling steps to produce a smelting grade alumina with the desired reduced soda content.
The present invention will now be described in relation to the following examples.
However, it must be appreciated that the examples are not to limit the generality of the above description. In all of the examples, the feed alumina was obtained directly from operating alumina refinery fluid bed calciners used for the production of smelting grade alumina. However, the alumina could have been obtained from other equipment such as a kiln.
Example 1 Carbon dioxide was sparged into a 3 litre stirred container held at 40C. When the pH had decreased to 6.0, 300 g of smelting grade alumina that had been calcined at 1 000C (soda content 0.51 %) was added. After five minutes a sub-sample was taken and filtered on a vacuum filter. After washing with five portions of hot deionised water (each portion being in the ratio of 2 litres of water to 1 kg of solid) and drying at 1100C, a smelting grade alumina with a soda content of 0.10% was formed.
Example 2 A 600g/L slurry of SGA that had been calcined at 1 000C (soda content 0.51 %) was formed and carbon dioxide was sparged through the well mixed slurry for 15 minutes. The temperature was approximately 25C. After this time, the sodium bicarbonate concentration had risen to 9 g/L. This slurry was filtered giving a cake of 35 to 40% moisture. The cake was successively washed on the filter under vacuum with a series of six carbon dioxide saturated sodium bicarbonate solutions of concentrations 8.3, 6.7, 5.0, 3.3, 1.7 and 0 g/L to simulate six stages of counter-current wash. The final product was dried and re-calcined at 1000C
to yield a smelting grade alumina with a soda content of 0.15%.
Example 3 In a continuous operation, SGA that had been calcined at 1000C (soda content 0.48%) was fed at 30 tonnes/hour into a 30 kL agitated leach tank equipped with a gas dispersion sparge through which was fed carbon dioxide at 275 kg/hour.
Water was also fed into the tank at 60 kL/hour to give a slurry density of 500 g/L.
The temperature in the leach tank was 40C. The mean residence time was 25 minutes. The slurry exiting from the leach tank was fed to a vacuum filter where it was filtered and then washed with 10 kL/hour of hot (60C) water (pH=6.9 throughcarbon dioxide addition). The leached and washed alumina was then re-calcined at 1000C to produce a smelting grade alumina with a soda content of 0.15%.
Example 4 ~n a continuous operation, smelting grade alumina that had been calcined at 1000C (soda content 0.47%) was fed at 108 tonnes/hour into two 120 kL
agitated tanks in series equipped with a gas dispersion sparge through which was fed carbon dioxide at 1000 kg/hour. Water was also fed into the tank at 150 kL/hour to give a slurry density of 500 g/L. The temperature in the leach tank was approximately 45C. The mean residence time was 80 minutes. The slurry exiting from the leach tank was fed to a belt vacuum filter equipped to wash thesolids in a counter-current manner. The first stage consisted of dewatering the slurry and the product of this stage was an alumina cake with 25% free moisture and a waste stream containing approximately 8 g/L sodium bicarbonate solution.
The final wash stage consisted of clean water and condensate, the pH of which was adjusted to between 6 and 8 with carbon dioxide. The first wash stage filtrate was fed in counter-current fashion to the leach tank. Wash water was dosed with carbon dioxide in between wash stages in order to maintain a pH
below 8 and preferably below 7. The leached and washed alumina was then flash dried at a temperature of about 120C and then re-calcined at a temperature of about 900C (the balance is dried at 300 to 400C) to produce a smelting grade alumina with a soda content in the range of 0.12 - 0 16%.
For an understanding of a preferred flow sheet for use with at least the process of Example 4, Figure 1 is attached hereto. However, it will be appreciated that theconfiguration of the process as illustrated in Figure 1 is not to limit the generality of the invention described above. Furthermore, and as mentioned above, it is envisaged that the heating of the final stage illustrated in Figure 1 may be conducted in accordance with the drying process as described in our co-pending Australian patent application PM8686.
In Figure 1, stream 10 consists of smelting grade alumina produced from the calciners of a normal SGA refinery and fed at rates in the range of from 80 to 120 tonnes/hour into two leach tanks 12 of about 120 kL capacity. The leach tank 12 preferably operates at atmospheric pressure and is fed carbon dioxide through a gas dispersion sparge 14 at a rate of 1000 kg/hour. A wash water stream 16 from the last stage of the counter-current filter 18 is fed into the leach tank 12 at a rate in the range of from 130 to 170 kL/hour to give a slurry density in the range offrom 400 to 600 g/L. The product slurry stream 20 is fed to the counter-current filter 18. In the first stage of filtration, the filtrate is disposed through line 22 to the residue ponds. At the other end of the filter, wash water which has been saturated with carbon dioxide is fed through line 24 at a rate in the range of from 160 to 200 kL/hour to wash the alumina cake on the filter 18. The filtrate is re-saturated with carbon dioxide and used in the preceding wash stage (and so on) to provide up to seven counter-current wash stages depending upon equipment availability. The moist alumina cake which now contains considerably reduced soda is fed to the heating stage 26 (preferably drying and recalcination) to produce an extremely low soda smelting grade alumina 28.
Example 5 In a continuous operation, smelting grade alumina that had been calcined at 1000C (soda content 0.47%) was leached, washed and re-calcined as in Example 4. This material was then blended with untreated SGA in a ratio of 1:2 to produce a smelting grade alumina with an average soda content of 0.36%.
Finally, there may be other modifications and improvements to the features described that are also within the scope of the present invention.
Throughout the aluminium industry there is a perception that most aluminium smelters will, at sometime in the future, require alumina refineries to produce alumina with a low soda content, typically as low as or lower than about 0.3%
(expressed as Na20). Indeed, some smelters require this level of soda at this time. Typical soda levels in smelting grade alumina (SGA) presently produced by most alumina refineries are in the order of 0.3 to 0.6% Na20.
Therefore, refineries which produce SGA at or towards the upper end of that range will be forced to develop techniques for reducing the soda content of their product.
The usual proposal for achieving reductions in soda levels is to increase the temperature at which the precipitation of alumina trihydrate is practised in theBayer process, or to reduce the specific precipitation rate in the early stages of the precipitation circuit. However, this technique may suffer from a number of disadvantages, such as reduced yield, increased energy costs, increased requirements for cooling in the alumina trihydrate precipitation circuit, and the production of weaker alumina trihydrate which is more prone to breakdown upon calcination or pneumatic handling.
An alternative approach to increasing the precipitation temperature that has been considered but never put into practise has been to adopt mineral acid leaching of non-SGA alumina (using for example, hydrochloric acid, sulphuric acid or nitric acid) to remove some of the sodium from the alumina. Although this has been documented and practised for use on a small scale for the production of chemicalgrade alumina and intermediate aluminas, it has not been practical for refineries producing smelting grade alumina on a large scale. Indeed, it suffers a number of disadvantages. In particular, this process introduces the need to treat the by-product salts (for example, sodium chloride, sodium nitrate or sodium sulphate) which are generally not environmentally suitable for disposal, and which also cause a degree of contamination of the SGA with chloride or sulphate.
Disposal of these salts can also lead to reduced SGA production due to elevated chloride, sulphate or nitrate levels in the refinery liquor stream. Also, the use of mineral acids poses a corrosion risk for process equipment.
With particular regard to prior documented techniques, US patent 4,260,585 discloses a process for producing low-soda alumina granules (typically 1-5 mm) suitable for porcelain and catalyst manufacture. This process requires alumina granules to be placed in a high temperature (up to 175C) autoclave with sufficient water to produce steam which condenses and leaches the soda from the granules. The present invention does not require the use of alumina granules. In fact these are unavailable and highly undesirable in the Bayer process for producing SGA. Furthermore, high temperatures such as 175C are very expensive to attain and thus are preferably avoided.
US patent 3,655,339 discloses a process for producing extremely low-soda alumina starting with an already low soda (<0.25%) alumina which contains at least 75% alpha alumina and then leaching in hot water or hot dilute acid. The preferred soda level of less than 0.25% stated in US 3,655,339 is extremely difficult and expensive to produce and is not generally available. Furthermore, alumina with alpha content greater than 75% would not be able to be sold as SGA since smelters could not subsequently use it in their processes.
US patent 4,120,942 discloses a process for producing shaped catalysts from a rehydratable alumina using a staged rehydration process which also includes a pH control step at high temperature (80 - 100C) and long times (greater than 1 hour). The present invention does not require the use of a rehydratable alumina as a feed stock. In fact, this material is not available in the Bayer process and would be very expensive to produce on the scale of manufacture of SGA.
Instead, the present invention is able to employ the alumina already produced byalumina refineries - smelting grade alumina - as a feed stock.
All of these prior art techniques disclose acceptable processes for producing small quantities of catalyst or similar grade aluminas. Such processes would be completely unacceptable for an alumina refinery desiring to produce quantities of low soda alumina of the order of 100,000 to 1,000,000 or more tonnes of alumina per year. Consequently, there is a need to develop a process which permits the very large scale production of a low soda smelting grade alumina.
It is an aim of the present invention to provide a process for the production of a low soda-content smelting grade alumina, the process being suitable to avoid, orat least partially alleviate, the above mentioned difficulties. The process of the invention may additionally produce an extremely low (typically less than 0.2 %) soda content smelting grade alumina, the production of such an extremely low soda-content smelting grade alumina also permitting the blending of normal SGA
with the extremely low soda-content SGA to produce whatever soda content is desired. Alternatively, the extremely low soda-content SGA afforded by this invention permits only a fraction of the output of a refinery or refinery system to be treated and then blended with the remainder to again provide a low soda content SGA.
The present invention is characterised by a process for the production of a low soda-content smelting grade alumina from alumina trihydrate, wherein the process comprises the steps of:
(a) calcining alumina trihydrate at a temperature between 700C and 1 200C to produce alumina;
(b) leaching the alumina in an aqueous slurry to which carbon dioxide is added, the leaching occurring at a temperature less than 80C;
(c) washing the leached alumina slurry and separating the washed alumina therefrom; and (d) heating the washed alumina at a temperature between 300C and 11 00C to produce a smelting grade alumina with a low soda content.
The calcination of the alumina trihydrate in step (a) preferably takes place in fluidised bed calciners or kilns that would normally be present in a Bayer plantcapable of producing a smelting grade alumina. Thus, and with regard to the adaptation of an existing Bayer plant to include the process of the present invention, it is the remaining steps, steps (b) to (d), which are added to the plant to produce the low soda-content SGA which the present invention aims to produce.
This first calcination will thus generally be conducted at temperatures in the order of 900 to 1100C and more preferably at about 1000C. Indeed, calcination at temperatures in the broadest range referred to above ensures that the alumina produced is not a rehydratable alumina, an alumina typically having a loss on ignition (LOI) greater than 3% which is too high to be acceptable to aluminium smelters (SGA typically having LOls of less than 1%), and typically being produced by calcination at temperatures in the order of 350 to 600C.
Furthermore, calcination at the high temperatures of the process of the present invention facilitates the conversion of entrapped soda into a form which is moreeasily removed by the subsequent leach of step (b).
The use of carbon dioxide as a leachant in the process of the invention is preferred over the mineral acids referred to above ffor example, hydrochloric, sulphuric and nitric acids) as the carbonic acid subsequently generated does notabsorb to any appreciable extent on leached alumina and thus does not contaminate the alumina. Indeed, sulphuric and hydrochloric acids can cause unacceptably high levels of sulphur and chlorine contamination respectively.
Furthermore, carbon dioxide is comparatively inexpensive and its use allows the waste products of the leaching reaction to be returned to the Bayer process, representing a recovery of soda values, without an adverse impact upon the production efficiency of the refining process. Further, by utilising carbon dioxide -as the leachant, the subsequently generated leach solution may be beneficially reacted with lime (at extremely high lime efficiencies) to produce a solution ofdilute sodium hydroxide. Of course, sodium hydroxide is the dissolving agent used in all Bayer process alumina refineries and the generation of any amount thereof is advantageous.
Also in relation to the leach step of the present invention, the rapid kinetics of the leach reaction makes it preferable to use an efficient carbon dioxide sparge design. In this respect, the soda leach reaction is fast (usually complete in less than one minute), while the dissolution of carbon dioxide is slow thus becoming the rate-limiting step in the reaction kinetics.
Preferably, the efficient carbon dioxide sparge design is provided by the injection of carbon dioxide close to the high shear region induced by the leach tank agitator.
However, it must be appreciated that the use of such a sparge design is merely preferred. Indeed, even with poor sparge design, and thus poor absorption efficiencies, the quality of the product of the leach may not be affected due to the comparative speed of the soda leach reaction, and the leach may nonetheless still be conducted comparatively rapidly, with leach times of less than 5 minutes being sufficient to ensure complete reaction.
With regard to other preferred operating conditions of the leach of the present invention, it will be appreciated that alumina possesses a surface charge when in contact with water, the surface charge being negative when the pH is above about 8 to 9 and being positive when below that. However, the most efficient leaching will generally occur at a pH between about 6 and 8; less than 8 to prevent the surface of the alumina absorbing cations, such as sodium; and greater than 6 to prevent the alumina absorbing anions, such as chloride.
Advantageously this pH range is easily achieved and controlled with carbon dioxide as a leachant.
However, to maintain the pH of the leach and wash solutions, it is also apparentthat some carbon dioxide is expended to keep the solution saturated with carbon dioxide. This, together with what can be significant losses of carbon dioxide atvarious preferred vacuum filtration stages within the process, may result in theloss of enough carbon dioxide during the leach to warrant carbon dioxide recovery from the spent solutions. Furthermore, excess CO2 that is not expended in the leach system may be recycled if necessary.
Interestingly, the control of the pH achievable in the leach step primarily results from the amount of sodium bicarbonate present, and is due to an equilibrium reaction between carbonic acid and the bicarbonate ion. Thus, in a preferred form of the invention, the sodium bicarbonate concentration in either the leach solution of step (b) or the wash solution of step (c) is kept below 1 g/L to achieve a pH of about 6 and to assist in ensuring a low soda content in the final product.
In order to reduce the sodium bicarbonate concentration, low solids densities may be used, a multiple stage counter-current leach system may be used, or the washing of the leached alumina slurry may be conducted as a counter-current wash on a filter. In a preferred form of the invention, the leach system adopted is a multiple-stage counter-current leaching system, preferably having two to sevenstages, although four stages would be most preferred. In a further preferred form, the carbon dioxide of the leach may be used at pressures above atmospheric, such as at pressures in the range of from 1 to 7 atmospheres. Further still, a single stage leach may be employed, with the leached alumina then being subjected to counter-current washes on a filter. Of course, any combination of these may also be a suitable alternative.
In the latter form of the invention, namely where a single stage leach and subsequent counter-current washes are conducted on a filter, the sodium bicarbonate concentration in the leach vessel may actually be allowed to substantially exceed 1 g/L since the sodium bicarbonate concentration will be reduced during the counter-current wash stages. This form of the invention -minimises the amount of wash solution that is used in the leaching and washing stages and thus is the preferred form of the invention.
In relation to the wash solution, water is the preferred medium, the aqueous slurry used for the leach thus being a water slurry. However, it will be appreciated that the water may contain impurities for the like, depending in part upon the most convenient source of the water.
In the preferred form of the invention, the alumina is preferably filtered after the leaching step, the filtration preferably being vacuum filtration. However, it has been found that some soda re-adsorbs on the alumina during vacuum filtration due to the rise in pH caused by thé reduction in CO2 partial pressure and by therelatively high concentration of sodium bicarbonate in the leach slurry solution. In the four-stage counter current process referred to above, this may contribute about 0.01% to the total soda in the final product, and may be reduced further by the use of a carbon dioxide shroud over the filter or by pressure filtration.
However, the same reduction may be achieved by the use of the series of counter-current washes on the filter wherein the filtrate from the final wash stage is used as the wash liquor for the preceding wash stage, the filtrate of which is used as the wash solution for the second preceding wash stage, and so on. The series of counter-current wash solutions may also be re-saturated with CO2 priorto their re-use as a wash solution for the preceding stage.
Preferably the final step of the process of the invention (step (d), the re-calcination of the washed and leached alumina slurry) is conducted in accordancewith the preferred embodiment in our co-pending Australian patent application PM8686 filed on 6 October 1994 and titled "Process for drying particulate material". The content of that specification is incorporated herein by reference.
Thus, the washed and leached alumina slurry of the present invention may be the wet alumina feed of the preferred embodiment of patent application PM8686. A
first portion of the wet alumina feed is firstly preferably subjected to pre-drying at a temperature between 90C and 300C to produce at least partially dried alumina and moisture. The at least partially dried alumina is separated from themoisture, and is then heated in a furnace to a temperature between 800 and 1200 C. Various products and streams are recirculated and separated and there are subsequent mixing, holding and cooling steps to produce a smelting grade alumina with the desired reduced soda content.
The present invention will now be described in relation to the following examples.
However, it must be appreciated that the examples are not to limit the generality of the above description. In all of the examples, the feed alumina was obtained directly from operating alumina refinery fluid bed calciners used for the production of smelting grade alumina. However, the alumina could have been obtained from other equipment such as a kiln.
Example 1 Carbon dioxide was sparged into a 3 litre stirred container held at 40C. When the pH had decreased to 6.0, 300 g of smelting grade alumina that had been calcined at 1 000C (soda content 0.51 %) was added. After five minutes a sub-sample was taken and filtered on a vacuum filter. After washing with five portions of hot deionised water (each portion being in the ratio of 2 litres of water to 1 kg of solid) and drying at 1100C, a smelting grade alumina with a soda content of 0.10% was formed.
Example 2 A 600g/L slurry of SGA that had been calcined at 1 000C (soda content 0.51 %) was formed and carbon dioxide was sparged through the well mixed slurry for 15 minutes. The temperature was approximately 25C. After this time, the sodium bicarbonate concentration had risen to 9 g/L. This slurry was filtered giving a cake of 35 to 40% moisture. The cake was successively washed on the filter under vacuum with a series of six carbon dioxide saturated sodium bicarbonate solutions of concentrations 8.3, 6.7, 5.0, 3.3, 1.7 and 0 g/L to simulate six stages of counter-current wash. The final product was dried and re-calcined at 1000C
to yield a smelting grade alumina with a soda content of 0.15%.
Example 3 In a continuous operation, SGA that had been calcined at 1000C (soda content 0.48%) was fed at 30 tonnes/hour into a 30 kL agitated leach tank equipped with a gas dispersion sparge through which was fed carbon dioxide at 275 kg/hour.
Water was also fed into the tank at 60 kL/hour to give a slurry density of 500 g/L.
The temperature in the leach tank was 40C. The mean residence time was 25 minutes. The slurry exiting from the leach tank was fed to a vacuum filter where it was filtered and then washed with 10 kL/hour of hot (60C) water (pH=6.9 throughcarbon dioxide addition). The leached and washed alumina was then re-calcined at 1000C to produce a smelting grade alumina with a soda content of 0.15%.
Example 4 ~n a continuous operation, smelting grade alumina that had been calcined at 1000C (soda content 0.47%) was fed at 108 tonnes/hour into two 120 kL
agitated tanks in series equipped with a gas dispersion sparge through which was fed carbon dioxide at 1000 kg/hour. Water was also fed into the tank at 150 kL/hour to give a slurry density of 500 g/L. The temperature in the leach tank was approximately 45C. The mean residence time was 80 minutes. The slurry exiting from the leach tank was fed to a belt vacuum filter equipped to wash thesolids in a counter-current manner. The first stage consisted of dewatering the slurry and the product of this stage was an alumina cake with 25% free moisture and a waste stream containing approximately 8 g/L sodium bicarbonate solution.
The final wash stage consisted of clean water and condensate, the pH of which was adjusted to between 6 and 8 with carbon dioxide. The first wash stage filtrate was fed in counter-current fashion to the leach tank. Wash water was dosed with carbon dioxide in between wash stages in order to maintain a pH
below 8 and preferably below 7. The leached and washed alumina was then flash dried at a temperature of about 120C and then re-calcined at a temperature of about 900C (the balance is dried at 300 to 400C) to produce a smelting grade alumina with a soda content in the range of 0.12 - 0 16%.
For an understanding of a preferred flow sheet for use with at least the process of Example 4, Figure 1 is attached hereto. However, it will be appreciated that theconfiguration of the process as illustrated in Figure 1 is not to limit the generality of the invention described above. Furthermore, and as mentioned above, it is envisaged that the heating of the final stage illustrated in Figure 1 may be conducted in accordance with the drying process as described in our co-pending Australian patent application PM8686.
In Figure 1, stream 10 consists of smelting grade alumina produced from the calciners of a normal SGA refinery and fed at rates in the range of from 80 to 120 tonnes/hour into two leach tanks 12 of about 120 kL capacity. The leach tank 12 preferably operates at atmospheric pressure and is fed carbon dioxide through a gas dispersion sparge 14 at a rate of 1000 kg/hour. A wash water stream 16 from the last stage of the counter-current filter 18 is fed into the leach tank 12 at a rate in the range of from 130 to 170 kL/hour to give a slurry density in the range offrom 400 to 600 g/L. The product slurry stream 20 is fed to the counter-current filter 18. In the first stage of filtration, the filtrate is disposed through line 22 to the residue ponds. At the other end of the filter, wash water which has been saturated with carbon dioxide is fed through line 24 at a rate in the range of from 160 to 200 kL/hour to wash the alumina cake on the filter 18. The filtrate is re-saturated with carbon dioxide and used in the preceding wash stage (and so on) to provide up to seven counter-current wash stages depending upon equipment availability. The moist alumina cake which now contains considerably reduced soda is fed to the heating stage 26 (preferably drying and recalcination) to produce an extremely low soda smelting grade alumina 28.
Example 5 In a continuous operation, smelting grade alumina that had been calcined at 1000C (soda content 0.47%) was leached, washed and re-calcined as in Example 4. This material was then blended with untreated SGA in a ratio of 1:2 to produce a smelting grade alumina with an average soda content of 0.36%.
Finally, there may be other modifications and improvements to the features described that are also within the scope of the present invention.
Claims (14)
1. A process for the production of a low soda-content smelting grade alumina from alumina trihydrate, wherein the process comprises the steps of:
(a) calcining alumina trihydrate at a temperature between 700°C and 1200°C to produce alumina;
(b) leaching the alumina in an aqueous slurry to which carbon dioxide is added, the leaching occurring at a temperature less than 80°C;
(c) washing the leached alumina slurry and separating the washed alumina therefrom; and (d) heating the washed alumina slurry at a temperature between 300°C
and 1100°C to produce a smelting grade alumina with a low soda content.
(a) calcining alumina trihydrate at a temperature between 700°C and 1200°C to produce alumina;
(b) leaching the alumina in an aqueous slurry to which carbon dioxide is added, the leaching occurring at a temperature less than 80°C;
(c) washing the leached alumina slurry and separating the washed alumina therefrom; and (d) heating the washed alumina slurry at a temperature between 300°C
and 1100°C to produce a smelting grade alumina with a low soda content.
2. A process according to claim 1, wherein the calcination in step (a) occurs in the fluidised bed calciners or kilns of a Bayer plant.
3. A process according to claim 1 or claim 2, wherein the calcination in step (a) is conducted at a temperature between 900°C and 1100°C.
4. A process according to any one of claims 1 to 3, wherein the leach of step (b) generates a leach solution which is reacted with lime to produce a solution of dilute sodium hydroxide.
5. A process according to any one of claims 1 to 4, wherein the carbon dioxide is introduced to the leach step via a sparger, the sparger taking advantage of the high shear region of an agitator.
6. A process according to any one of claims 1 to 5, wherein the leach step is conducted at a pH in the range of 6 to 8.
7. A process according to any one of claims 1 to 6, wherein carbon dioxide not expended in the leach is recycled.
8. A process according to any one of claims 1 to 7, wherein the amount of sodium bicarbonate in the leach of step (b) and/or the wash of step (c) is maintained below 1 gram per litre of solution.
9. A process according to any one of claims 1 to 8 wherein the leach is conducted with carbon dioxide at a pressure in the range of from one to seven atmospheres.
10. A process according to any one of claims 1 to 9 wherein the leach is conducted in a single stage, the wash being conducted as a series of counter-current washes on a filter.
11. A process according to any one of claims 1 to 9 wherein the leach is conducted in a multiple-stage counter-current system.
12. A process according to any one of claims 1 to 11 wherein the aqueous slurry is a water slurry.
13. A low soda content smelting grade alumina produced by the process of any one of claims 1 to 12.
13. A process according to claim 1 substantially as herein described in relationto any one of the examples.
13. A process according to claim 1 substantially as herein described in relationto any one of the examples.
14. A process according to claim 1 substantially as herein described in relationto the accompanying Figure.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AUPM8685A AUPM868594A0 (en) | 1994-10-06 | 1994-10-06 | Method of producing low soda content smelting grade alumina |
| AUPM8685 | 1994-10-06 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2160050A1 true CA2160050A1 (en) | 1996-04-07 |
Family
ID=3783204
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002160050A Abandoned CA2160050A1 (en) | 1994-10-06 | 1995-10-06 | Process for the production of a low soda content smelting grade alumina |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPH08225319A (en) |
| AU (1) | AUPM868594A0 (en) |
| CA (1) | CA2160050A1 (en) |
| FR (1) | FR2725440A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113716590A (en) * | 2021-10-25 | 2021-11-30 | 中铝山东工程技术有限公司 | Process for removing sodium oxide impurities in aluminum oxide |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE102006005400A1 (en) * | 2006-02-18 | 2007-08-23 | Nabaltec Ag | Preparing ceramic raw material on the basis of purified aluminum oxide comprises preparing an alumina raw material; calcinating; thermally treating; contacting a bubble-forming carbon dioxide; separating dealklylated alumina; and drying |
| CN109761254A (en) * | 2019-02-02 | 2019-05-17 | 中国铝业股份有限公司 | A method of reducing aluminium hydroxide or sodium oxide in aluminum oxide impurity content |
-
1994
- 1994-10-06 AU AUPM8685A patent/AUPM868594A0/en not_active Abandoned
-
1995
- 1995-10-06 CA CA002160050A patent/CA2160050A1/en not_active Abandoned
- 1995-10-06 FR FR9511962A patent/FR2725440A1/en not_active Withdrawn
- 1995-10-06 JP JP7286567A patent/JPH08225319A/en active Pending
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113716590A (en) * | 2021-10-25 | 2021-11-30 | 中铝山东工程技术有限公司 | Process for removing sodium oxide impurities in aluminum oxide |
Also Published As
| Publication number | Publication date |
|---|---|
| FR2725440A1 (en) | 1996-04-12 |
| AUPM868594A0 (en) | 1994-11-03 |
| JPH08225319A (en) | 1996-09-03 |
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