Description:FIELD OF THE INVENTION
[001] The present invention relates generally to a process for producing low sodium alumina from an aluminium feedstock. The instant process involves preparation of an aqueous acidic solution from the aluminium feed stock followed by adjusting the pH to between 4-5 and then using an organic reductant to precipitate the aluminium to obtain the low sodium alumina. The developed process also allows the recovery and recycling of organic reductant.

BACKGROUND OF THE INVENTION
[002] Alumina is economically produced from the Bayer process route and is used for various applications. In general, this alumina contains sodium, Na, in the range of 0.1-0.4% and is called smelter grade which is widely used for aluminium metal production.
[003] By exerting precise control on quality of alumina with respect to sodium content and particle size distribution, it is also used for ceramic and refractory applications. Alumina with low sodium content (Na<100 ppm) finds specialty applications such as in electronic ceramic, thermal conductive fillers, spark plugs etc. Few processes like acid washing of aluminium hydroxide (using acids such as hydro fluoric acid, acetic acid, sulfuric acid, chloric acid etc.) before calcination to alumina have been explored to reduce the sodium content (US 2411807). However, these processes have the limitation that the sodium present in the lattice of aluminium hydroxide is not removed. For instance, when aluminium hydroxide feedstock is washed with acetic acid a product with 500-1000 ppm sodium is obtained. Further reduction of sodium content to less than 100 ppm is not possible with this method. Another conventional method of arriving at low sodium alumina utilizes the alkoxide method wherein aluminium metal is used to arrive at low sodium alumina. This is a very cost prohibitive method.
[004] Therefore, there is a need for a low cost process that enables production of low sodium alumina from an aluminium feedstock where the sodium content is reduced to less than 100ppm.

SUMMARY OF THE INVENTION
[005] According to one embodiment, the present invention is a process for producing low sodium alumina, said process comprising:
(a) digesting an aluminium feedstock with an acid at a temperature selected from 150-250?;
(b) treating the digested feedstock with a pH modifier to adjust the pH to a value between 4-5;
(c) adding a quinoline based organic reactant to the pH adjusted feedstock to obtain a precipitate of an aluminium quinoline derivative;
(d) filtering out the aluminium quinoline derivative product; and
(e) subjecting the product of step (d) to calcination to obtain the low sodium alumina.
[006] According to another embodiment, the present invention is a process for producing low sodium alumina wherein, the aluminium quinoline derivative of step (d) of the process disclosed in the preceding paragraph is redissolved in hydrochloric acid that is then purged with anhydrous hydrogen chloride gas resulting in crystallization of aluminium chloride hexahydrate product, said product is filtered out and then subject to calcination to obtain the low sodium alumina.

BRIEF DESCRIPTION OF THE DRAWINGS
[007] Reference will be made to embodiments of the invention, examples of which may be illustrated in accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.
[008] Figure 1 shows a flowchart of an embodiment of the present process to produce alumina with low sodium content from an aluminium feedstock.
[009] Figure 2 shows a process flowchart according to a second embodiment where some of the chemicals are regenerated and recycled.

DETAILED DESCRIPTION OF THE INVENTION
[0010] In the following detailed description, the embodiments are described in sufficient detail to enable those skilled in the art to practice the invention and it is understood that other embodiments may be utilized and that changes may be made without departing from the scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the embodiments described herein, the description may omit certain information known to those skilled in the art. Accordingly, the description and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present teachings. It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
[0011] The present invention meets the afore stated needs by providing a process for producing low sodium alumina, said process comprising:
(a) digesting an aluminium feedstock with an acid at a temperature selected from 150-250?;
(b) treating the digested feedstock with a pH modifier to adjust the pH to a value between 4-5;
(c) adding a quinoline based organic reactant to the pH adjusted feedstock to obtain a precipitate of an aluminium quinoline derivative;
(d) filtering out the aluminium quinoline derivative product; and
(e) subjecting the product of step (d) to calcination to obtain the low sodium alumina.
[0012] In another embodiment of the process as disclosed in the preceding paragraph, the aluminium quinoline derivative of step (d) is redissolved in hydrochloric acid that is then purged with anhydrous hydrogen chloride gas resulting in crystallization of aluminium chloride hexahydrate product, said product is filtered out and then subject to calcination to obtain the low sodium alumina. The calcination releases hydrogen chloride fumes, said fumes are recycled for use in purging. Further, a solution comprising the quinoline based organic reactant is obtained after filtering out the aluminium chloride hexa hydrate product, said solution is recycled back into step (c) of the afore process.
[0013] In one embodiment of the present process, the pH modifier is selected from an alkali, ammonium hydroxide, ammonium acetate, and any other acetate compound.
[0014] In one embodiment of the present process, the ammonium hydroxide, ammonium acetate, and any other acetate compound is of a concentration selected from 1-3M.
[0015] In one embodiment of the present process, the quinoline based organic reactant is 8-hydroxyquinoline of a concentration selected from 10 – 20 % (weight by volume).
[0016] In one embodiment of the present process, the acid is selected from dilute hydrochloric acid, nitric acid and sulfuric acid, said acid having a concentration selected from 15 to 20 % (weight by volume).
[0017] In one embodiment of the present process, the low sodium alumina contains less than 100 ppm sodium.
[0018] In one embodiment of the present process, the aluminium feedstock is selected from aluminium hydrate feed, aluminium chloride feed stock, aluminium hydroxide product generated in the Bayer process, bauxite feedstock, aluminium dross, red mud, aluminium sulphate feed stock and any other aluminous feed stock. The feedstock may be from any aluminium containing solutions in acidic medium, such as chlorides, sulphates, nitrides, aluminium salt/ complexes with ammonia etc. The aluminium chloride feed stock maybe either in solid or liquid form, as anhydrous aluminium chloride, and poly aluminium chloride of any grade.
[0019] The novelty of the proposed process is to reduce sodium content drastically from the initial content of 1000-4000 ppm (0.1-0.4%) to <100 ppm in a single stage process based on selective precipitation of aluminium using an organic reductant such as a quinoline derivative. Precipitation is by complexation of the aluminium from the aluminium hydrate feed with hydroxy derivatives of quinoline. Controlling sodium impurity level in alumina is critical for its specialty applications. This is not possible in the conventional Bayer process. The instant invention helps the alumina refinery to sustainably generate low sodium alumina product at lower cost.
[0020] In another embodiment the present process allows regeneration of organic reductant for its reuse for selective precipitation of aluminium to produce alumina with low sodium (Na <10 ppm) content. The process step developed to recover the organic reductant involves a crystallization step which helps in further improving the purity of alumina (Na < 10 ppm) and also reduces the carbon emissions.
[0021] In general, the production of specialty grade or low sodium alumina requires large number of purification steps to achieve desired quality specifications. Novelty of this new process lies in its selectivity towards aluminium ions over other impurities (Na, Si, As, Ca, K etc.) when the complex derivative is formed.
[0022] Experiments showed that the order of adding reagents affects purity of the end product alumina. As seen in Table 1 below adjusting pH before addition of 8-hydroxyquinoline affects the purity of the alumina, the sodium content is lower.
[0023] Table1:
Concentration (ppm) pH Adjusted After addition of 8 hydroxy quinoline pH Adjusted Before addition of 8 hydroxy quinoline

Ca 3 - 7 3 - 5
Cr < 1 < 1
Fe 75 - 100 60 - 90
Na 30 - 40 10 - 20
Sb < 1 < 1
Ti 5 - 7 5 - 7
As < 1 < 1
Si 100 - 200 25 - 30

[0024] Figure 1 shows a process flowchart according to one embodiment of the present process to produce alumina with low sodium content (Na <100 ppm) from an aluminium feedstock. Aluminium hydroxide feed or a feed containing aluminium containing 0.1-0.4% Na is digested in dilute HCl, nitric acid or sulfuric acid by heating to ~150-250? for about 1 hour at high pressure of 3- 30 bar to release the lattice bound Na. This process is for selective extraction of aluminium from an acidic solution prepared from aluminium feed stocks such as aluminium hydrate, aluminium chloride, poly aluminium chloride, aluminium sulfates etc. These solutions in general are acidic in nature with pH<1. Therefore, pH needs to be increased above 4 with the pH modifier, followed by addition of the organic reductant, i.e. hydroxy quinoline for selective precipitation of its aluminium derivative. The pH of the solution is increased from <1 to pH 4-5 by adding ammonium hydroxide, ammonium acetate or any other acetate. The solution of organic reductant is then added to the solution for selective precipitation of aluminium. The organic reductant is a quinoline derivative in one embodiment. The precipitate is a solid aluminium complex with organic quinoline. This is then filtered and calcined at 1000 to 1200? to obtain low sodium (Na2O <100 ppm) alumina and the organic compound that burns and reports to the gaseous stream. These organic fumes generated are sent to a scrubber or a gas treatment plant to meet environmental norms.
[0025] In this process, any efforts to recover and reuse the organic reductant helps to reduce carbon foot print as well as improves the process economics greatly. Therefore, Figure 2 shows a process flowchart according to a second embodiment where some of the chemicals are regenerated and recycled. The process of this embodiment also involves a crystallization step. Aluminium hydroxide feed or a feed containing aluminium is digested in dilute HCl or nitric acid and sulfuric acid by heating to ~150-250? for 1 hour. The pH of the solution is increased from <1 to pH 4-5 by adding ammonium hydroxide or ammonium acetate or any other acetate. The organic reductant is then added to the solution for selective precipitation of aluminium as an aluminium quinoline derivative. The organic reductant is a quinoline derivative in one embodiment. In this embodiment, the solid aluminium complex precipitate is redissolved in hydrochloric acid, this solution is then purged with anhydrous hydrogen chloride gas resulting in crystallization of aluminum chloride hexahydrate product, said product is filtered out and then subject to calcination at 1000-1200? to produce the low sodium (<10 ppm) alumina. The calcination releases hydrogen chloride fumes, said fumes are recycled for use in purging.
[0026] In this process, excessive purging of HCl results in the precipitation of aluminium as aluminium chloride hexa-hydrate whereas organic compound remains in the acidic solution which is recycled directly. The recycled organic reductant solution has about 36-40% HCl and is recycled back into the precipitation step of the process. The recovery of organic reductant with the recycle stream is about 90-92%. Scope of invention extends to recovery of organic reductant by means of other process steps such as solvent extraction etc. To the recycled solution additionally make-up organic reductant may be added as shown in Figure 2.
[0027] In Figures 1 and 2 aluminium chloride solution is obtained after acidic digestion of the aluminium feedstock using the acid e.g. HCl.
[0028] In one embodiment, the present process may be combined with any pre-processing of aluminium hydroxide such as washing with acetic acid, hot water etc.
[0029] The following experimental examples are illustrative of the invention but not limitative of the scope thereof:
[0030] Example 1: Aluminium hydroxide product generated in Bayer process is used as feed stock. Typically, aluminium hydroxide used is of purity ~99.7% along with the associated critical impurities such as, Na: 1100-1300ppm, Si: 100-2000ppm, Fe: 100-200ppm, As: 700-1000, Cr: 100-200 ppm, Ti: 50-80 ppm, Sb: 10-30 ppm. Presence of high sodium content is mainly due to the hydrate crystallization in sodium aluminate solution (/liquor) in presence of seeds along with controlled cooling. The detailed analysis of hydrate feed (industrial sample) chosen for experimental investigation is mentioned in Table 2.
[0031] The product generated after selective precipitation using quinoline based organic reductant followed by its calcination at 1000 °C is also given in Table 2. Theoretically, 1.53-1.6 times of aluminium hydroxide is used to produce a unit of alumina. Therefore, the concentration of impurities that will report to alumina will be 1.53 times to that of those present in hydrate feed, if not adopted any purification process. However, with the use of the present novel selective precipitation process, using quinoline based derivatives using ammonium hydroxide (28-30% pure, AR grade) as pH modifier we have successfully minimized all critical impurities present in the alumina to the level of Na~34 ppm, Si~100-200 ppm, Fe~67 ppm, Ca~7ppm and all other impurities As, Sb, Cr < 1ppm. Thus, the produced material is of purity greater than 99.95% and can used for specialty glass or ceramic applications which requires low sodium content (<100 ppm) alumina.
[0032] Table 2: Purity of alumina produced with various embodiments of the present process as generally shown in Figures 1 and 2
Impurities (ppm) Raw Material
Al(OH)3 Low sodium alumina product Purity of alumina
Example-1
Ammonium hydroxide as pH modifier
(Figure 1) Example-2
Ammonium Acetate as pH modifier
(Figure1) Example-3
Organic precipitation followed by crystallization in acidic medium (Figure 2)
Na 1100-1300 10-35 34 12 4
Fe 100-200 50-90 67 90 54
Si 100-200 20-200 190 28 <10
As 700-1000 <1 <1 <1 <1
Cr 100-200 <1 <1 <1 <1
Ti 50-80 6-7 6-7 6-7 <1
Ca 45-50 6-7 6-7 <5 <1
Sb 10-30 <1 <1 <1 <1

[0033] Example 2: Here ammonium acetate (NH4CH3CO2, 98% pure, AR grade) is used as pH modifier in place of ammonium hydroxide in example 1, keeping all other steps same, such as preparation of aluminium chloride solution from aluminium hydrate and addition of hydroxy quinoline derivative etc. The sodium content of the final product was reduced to 12 ppm and a drastic reduction of silica content to 28ppm was reported in the final alumina product. As pH is adjusted to greater than 4, any impurities such as Fe and Si present in the feed solution/ reagents would report to aluminium compounds/ precipitates. Therefore, the use of high purity reagents would also improve the alumina quality.
[0034] Example 3: This is the process of Figure 2 where organic derivative of aluminium is dissolved by decreasing pH<1 and then purged with anhydrous hydrochloric acid. Then, aluminium is selectively precipitated as aluminium chloride hexa hydrate (AlCl3 6H2O). The final product obtained is calcined in single or two stages to 1000? to produce high purity alumina of 99.99% purity with specific impurities Na-4 ppm, Fe - 54 ppm, Ti<1ppm, Si<10 ppm
[0035] Example 4: The recovery & quality of organic reductant is tested with the recycling liquor and is mentioned in Table 3 below. The quality of organic reductant recovered is similar to as is sample except in the aluminium content which helps in improving alumina recoveries in the entire process. The recovery of organic reductant by weight is reported as 90-92%.
[0036] Table 3: Purity of organic reductant recovered is compared with the as received sample
Impurities (ppm) Purity of Organic reductant
Example -4
As received Recovered from recycling steam
Na 2 11
Fe 6 15
Al 2 9960-13015 (0.9 -1.3%)

[0037] Thus, the present process is for producing low sodium alumina from an aluminium feedstock. The present process is low cost and results in a low sodium alumina where the sodium content is below 100ppm.
[0038] While the present invention has been described with respect to certain embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.
, Claims:We Claim:
1. A process for producing low sodium alumina, said process comprising:
a. digesting an aluminium feedstock with an acid at a temperature selected from 150-250?;
b. treating the digested feedstock with a pH modifier to adjust the pH to a value between 4-5;
c. adding a quinoline based organic reactant to the pH adjusted feedstock to obtain a precipitate of an aluminium quinoline derivative;
d. filtering out the aluminium quinoline derivative product; and
e. subjecting the product of step (d) to calcination to obtain the low sodium alumina.
2. The process as claimed in claim 1, wherein the aluminium quinoline derivative of step (d) is redissolved in hydrochloric acid that is then purged with anhydrous hydrogen chloride gas resulting in crystallization of aluminium chloride hexahydrate product, said product is filtered out and then subject to calcination to obtain the low sodium alumina.
3. The process as claimed in claim 2, wherein the calcination releases hydrogen chloride fumes, said fumes are recycled for use in purging.
4. The process as claimed in claim 2, wherein a solution comprising the quinoline based organic reactant is obtained after filtering out the aluminium chloride hexa hydrate product, said solution is recycled back into step (c) of the process.
5. The process as claimed in claim 1, wherein the pH modifier is selected from an alkali, ammonium hydroxide, ammonium acetate, and any other acetate compound.
6. The process as claimed in claim 5, wherein the ammonium hydroxide, ammonium acetate, and any other acetate compound is of a concentration selected from 1-3M.
7. The process as claimed in claim 1, wherein the quinoline based organic reactant is 8-hydroxyquinoline of a concentration selected from 10 – 20 % (weight by volume).
8. The process as claimed in claim 1, wherein the acid is selected from dilute hydrochloric acid, nitric acid and sulfuric acid, said acid having a concentration selected from 15 to 20 % (weight by volume).
9. The process as claimed in claim 1, wherein the low sodium alumina contains less than 100 ppm sodium.
10. The process as claimed in claim 1, wherein the aluminium feedstock is selected from aluminium hydrate feed, aluminium chloride feed stock, aluminium hydroxide product generated in the Bayer process, bauxite feedstock, aluminium dross, red mud, aluminium sulphate feed stock and any other aluminous feed stock.