WO2011018799A2

WO2011018799A2 - A process for preparing vanadium oxide from vanadate sludge

Info

Publication number: WO2011018799A2
Application number: PCT/IN2010/000531
Authority: WO
Inventors: Sanjaya Ranjana Mohapatra; Rahul Jadhav; Seema Salvi; Sadguru Kulkarni
Original assignee: Aditya Birla Science and Technology Co Ltd; Hindalco Industries Ltd
Current assignee: Aditya Birla Science and Technology Co Ltd; Hindalco Industries Ltd
Priority date: 2009-08-10
Filing date: 2010-08-10
Publication date: 2011-02-17
Anticipated expiration: 2012-02-10
Also published as: WO2011018799A3

Abstract

Vanadate sludge obtained as a by-product during Bayer's process contains fairly higher quantity of precious metal vanadium as vanadium oxide ( 5- 20% ) along with other impurities like sodium, aluminium and phosphorus salt. The present invention relates to a process for recovering vanadium oxide from vanadate sludge obtained from Bayer's process. The effluent generated during the process is recycled and the impurities such as sodium, aluminium and calcium are recovered as valuable by-product.

Description

A PROCESS FOR PREPARING VANADIUM OXIDE FROM VANADATE SLUDGE

FIELD OF THE INVENTION

The invention relates to a process for preparing vanadium oxide.

The present invention envisages a process for preparing vanadium oxide from vanadate sludge.

BACKGROUND OF THE INVENTION

The Bayer process is the principal industrial means of refining bauxite, which is the most important ore of aluminium to produce alumina. During the production of alumina from bauxite by the Bayer's process, a sludge containing fairly higher quantity of vanadium approximately 5-20% V₂O₅ along with other impurities like P₂O₅ and gallium compounds is generated.

The typical composition of vanadium sludge is as given below V2O5:8-15%; Na2O: 10-40%; A12O3.3H2O: 0.2-7%; Water: 47- 53%.

The vanadium sludge prepared as above is the source for the production of vanadium oxide, which is used as" such or which may be processed to metallic vanadium or vanadium ligature. Vanadium finds application as a catalyst and as a raw material for the production of ferrovanadium. Various methods to recover vanadium from vanadate sludge have been proposed. These methods include the neutralization of the alkaline sludge followed by precipitation of vanadate as ammonium or sodium salt or use of lime or other calcium salts to precipitate calcium vanadate.

GB600833 discloses a method of extracting vanadium oxide from vanadium sludge. The method consists in neutralizing the suspension of vanadiferous salts with sulphuric acid or phosphoric acid to isolate sodium sulphate or sodium phosphate and treating the liquor with sodium or ammonium salt to precipitate sodium or ammonium vanadate salt. The vanadate salt is calcined to yield vanadium oxide.

US 3472612 discloses a method for recovering vanadium from sodium vanadate solution by treating sodium vanadate solution with sulphuric acid at pH 6 to 8 to precipitate aluminum salts and. treating the filtrate with ammonium salts and sulphuric acid The resulting solution is held at an elevated temperature to precipitate vanadium oxide substantially free from sodium and aluminum.

US 4039582 describes a method of preparing vanadium oxide from sodium vanadate solution. The method consists in that the solution is treated at a pH of 4 to 6 with ammonium chloride or sulphate to prepare an ammonium- vanadate salt precipitate which is then dissolved in hot water and recrystallized by a mineral acid. The new precipitate of the ammonium vanadate salt is calcined to prepare vanadium oxide. In all abovementioned prior art processes, none of the documents describes a process to recover impurities of sodium or aluminium from sodium vanadate sludge. In the prior art processes sodium and aluminium are present as their hydroxides or carbonates and converted to corresponding sulphate or chloride salts depending on whether sulphuric or hydrochloric acid used. However, the sulfates or chloride salts of sodium and aluminium are low value products and go as waste/effluent along with other impurities.

During the recovery of vanadium based compounds, the liquid effluent generated has un-reacted ammonium salt due to addition of large amount of ammonium salt like chloride/sulphate/nitrate/hydroxide and also has high amount of dissolved un-precipitated vanadium, which is not economical and eco-friendly to dispose. The solid waste generated also has high amount vanadium along with other impurities such as aluminum, arsenic, phosphate & fluoride, etc., which is also not economical and eco-friendly to dispose.

Thus, there is a need for a process for recovering vanadium from vanadate sludge which is more economical and eco-friendly as compared to the prior art processes.

Object of the present invention

It is an object of the present invention to provide a process for preparing vanadium from vanadate sludge with minimum loss of vanadium salt during the impurity removal process, thereby making the process more eco-friendly & economical. Another object of the present invention is to provide a process wherein the vanadium oxide obtained is highly pure.

Yet, another object of the present invention is to provide a process for recovery of vanadium oxide wherein liquid effluent is recycled.

Yet, another object of the present invention is to recover the calcium impurities present in the vanadate sludge as calcium sulphate.

Yet,another object of the present invention is to recover the sodium present in the sludge as valuable salt of hydroxide, carbonates or oxide.

Yet, another object of the present invention is to recover the aluminum present in the sludge as valuable salt of ammonium hydroxide or sodium aluminate.

Further object of the present invention is to provide an eco-friendly process with zero discharge of soluble toxic (arsenic, vanadate, ammonium ion & other elements) elements in the effluent.

Summary of the Invention

In accordance with the present invention, a process for preparing vanadium from vanadate sludge, said process comprising the following steps:

i) heating a slurry containing water and an alkaline metal salt to a temperature of about 70 ⁰C to about 120 ⁰C; ii) adding to said slurry, vanadate sludge comprising optionally calcium, phosphate and aluminium salts, wherein the ratio of water to the vanadate sludge by weight is greater than 3, to form a reaction mixture;

iii) stirring the reaction mixture for a period of about 1 to about 5 hours to form a suspension containing alkaline vanadate;

iv) filtering the suspension to isolate an alkaline cake and obtain a first filtrate;

v) reacting the alkaline cake with a mineral acid at a temperature of about 5

⁰C to about 120 ⁰C, preferably 5⁰C to about 50⁰C and at a pH below 4 to form a second slurry comprising vanadic acid, insoluble alkaline salt and optionally soluble phosphates;

vi) filtering the second slurry to remove the insoluble alkaline salt and obtain a second filtrate comprising vanadic acid and optionally containing soluble salts of phosphate;

vii) treating, at pH above 3 the second filtrate with at least one additive selected from a group consisting of calcium vanadate sludge, sodium vanadate sludge, oxides, hydroxides, carbonates of alkali metals, alkaline earth metals and ammonium compounds and optionally in the presence of at least one metal salt selected from a group consisting of iron and aluminium to precipitate phosphate salts and to obtain vanadic acid in a third filtrate; viii) treating the third filtrate to obtain a fourth filtrate by at least one of the steps selected from the group of steps consisting of;

a) reacting the third filtrate with at least one fluoride salt at a temperature about 5 to about 120 ⁰C for about about 5 minutes to 10 hours to obtain a slurry comprising inorganic metal salts and vanadic acid, filtering the slurry to remove inorganic metal salts and to obtain a fourth filtrate; and b) passing the third filtrate through a column of cationic exchange resin to obtain a fourth filtrate;

ix) reacting, at pH below 4 the fourth filtrate with a mineral acid at a temperature of about 10⁰C to about 120 ⁰C , preferably 50⁰C to about 95 ⁰C to precipitate polyvanadate;

x) isolating the precipitated polyvandate by filtration to yield acidic liquor; and

xi) calcining the isolated polyvanadate at a temperature of about 400⁰C to about 1000 C for a period of about 10 to about 60 minutes to obtain vanadium oxide.

Typically, the alkaline metal salt in step (i) is selected from a group of metal salts consisting of calcium oxide, calcium carbonate, calcium sulfide, calcium hydroxide, magnesium carbonate, magnesium oxide, magnesium hydroxide, magnesium sulfide, barium oxide, barium carbonate, barium sulfide, barium hydroxide, strontium oxide, strontium carbonate, strontium sulfide and strontium hydroxide.

In accordance with the preferred embodiment of the present invention the molar ratio of alkaline metal content of the alkaline metal salt used in step (i) to the sodium content of the sludge is in the ratio of about 0.4 to about 2 .

Typically, the molar ratio of alkaline metal content of the alkaline metal salt used in step (i) to the sodium content of the sludge is in the ratio of about 0.5 to about 0.6. In preferred embodiment of the present invention, the alkaline metal salt used in step (i) is pretreated with water prior to the reaction.

In accordance with the process, more than 99% of vanadium in the vanadate sludge is reacted to form alkaline vanadate.

Typically, the alkaline vanadate is calcium vanadate. Typically, the mineral acid used in step (v) is sulphuric acid.

The iron salt used in step (vii) is selected from a group consisting of Red mud, iron chloride, iron sulphate, iron nitrate, iron oxide, iron hydroxide and iron carbonate.

The aluminium salt used in step (vii) is selected from a group consisting of Bauxite, aluminium chloride, aluminium sulphate, aluminium nitrate, aluminium oxide, aluminium hydroxide and aluminium carbonate.

Typically, the additive used in step (vii) is calcium vanadate sludge. Typically, the step vii) is carried out at a pH range of about 4 to about 8.

In accordance with the present invention the mole ratio of iron content in the iron salt used in step (vii) to the phosphate content in the second filtrate is in the range of about 0.05 to about 5. The mole ratio of aluminum content in the aluminium salt used in step (vii) to the phosphate content in the second filtrate is in the range of about 0.05 to about 5. The fluoride salt is selected from a group consisting of hydrogen fluoride, ammonium fluoride and alkali metal fluoride.

In another aspect of the present invention the process for preparing vanadium from vanadate sludge comprises introducing an ammonium salt selected from a group consisting of ammonia, ammonium chloride, ammonium sulphate, ammonium carbonate, ammonium nitrate and ammonium hydroxide during the process.

Typically, wherein the ammonium salt is introduced in step (v) to the mixture containing alkaline vanadate and water.

Typically, the ammonium salt is introduced in step (ix) in the third filtrate.

The mineral acid used in step (ix) selected from the group consisting of hydrochloric acid, sulphuric acid and nitric acid.

Typically, the mineral acid is sulphuric acid.

In preferred embodiment of the present invention the phosphate salts formed in step (vii), the inorganic metal salts formed in step (viii) and the acidic liquor isolated in step x) are recycled to step v). >

In accordance with the present invention, the polyvanadate is ammonium vanadate. The polyvanadate is calcined at about 800⁰C temperature to yield vanadium oxide. Typically, the purity of vanadium oxide is greater than 99%. DETAILED DESCRIPTION OF THE INVENTION

During the production of alumina from bauxite by Bayer's process, byproduct vanadate sludge containing 2% to 20% vanadium oxide is obtained. The vanadium sludge is the source for the production of vanadium oxide, which finds application as a catalyst and as a raw material for the production of ferrovanadium.

A typical composition of vanadate sludge as used in the present invention is V2O5: 8-15%, Na2O: 10-40%, A12O3.3H2O:3-7% and water: 47-53%.

Vanadate sludge referred herein is any source of vanadium comprising vanadium and sodium salts.

Sodium and Vanadium are the major constituents of vanadate sludge .This vanadate sludge is treated with a slurry of water and at least one alkaline earth compound selected from a group consisting of calcium oxide, calcium hydroxide, calcium , carbonate, calcium sulfides, magnesium carbonate, magnesium oxide, magnesium hydroxide, Barium carbonate, Barium hydroxide, Strontium carbonate and Strontium hydroxide, at a temperature in the range of about 50⁰C to HO⁰C to obtain a suspension containing alkaline vanadate and a filtrate (referred as first filtrate/ filtrate- l)containing soluble sodium and aluminium salts. The sodium and aluminium salts are further recovered from the filtrate, as sodium hydroxide and/or sodium aluminate. Preferably the reaction is carried out with calcium salt to yield calcium vanadate. Various calcium salts used are calcium hydroxide, calcium oxide and calcium carbonate Typically, the molar ratio of calcium metal to sodium in the sludge is in the range of 0.4 to 2, preferably 0.5 to 0.6.

After extensive experimentation, it was found that, the vanadium percentage in the first filtrate is less when the mass of the reaction water is at least 3-4 times higher than amount of vanadate sludge added. wt/wt. ratio (Reaction water amount to sodium vanadate % V in sludge) filtrate

4.00 0.008

3.33 0.042

2.86 0.084

2.50 . 0.127

2.22 0.1825

2.00 0.3205

Similarly, the sodium content in the final residue decreases with increase in the reaction time. For example, when the reaction is carried out for 1 hr, the sodium content in residue is 3%. On the other hand, when the reaction is carried out for 2 hrs the sodium content in residue is 0.3%.

When lime, water & sodium vanadate sludge are taken together and heated to 90-95 ⁰C for 2hr, the sodium content in the calcium vanadate residue was about 0.6% by weight, but when the slurry containing lime and water was heated to 90-95'⁰C and then vanadate sludge was added , the sodium content in calcium vanadate residue was about 0.3% by weight.

It was also found that, lime soaked in water prior to the reaction increased the efficiency of the reaction. It was also found that, efficiency of the reaction increases with the decrease in the particle size of lime. Therefore lime needs to be properly grinded.

The calcium vanadate thus obtained contained more than 99% of the total vanadium present initially in the vanadate sludge.

Flowchart of the Calcium vanadate process from vanadate sludge as described above is illustrated in figure 1.

Figure 2 illustrates the XRD of calcium vanadate prepared in accordance with the process as described in the flow chart.

Calcium vanadate as obtained above is reacted with sulphuric acid solution by slowly adding sulphuric acid to calcium vanadate solution over a period of 10 minutes, maintaining the pH below 5, preferably 2.5. The reaction temperature is maintained between 5⁰C to 120⁰C to obtain calcium sulfate and a second filtrate containing vanadic acid and impurities of Calcium, Iron, Aluminium and Phosphate elements.

The calcium sulfate recovered is a value added product.

The second filtrate containing vanadic acid and impurities of Calcium, Iron, Aluminium and Phosphate salts is treated with aluminum or iron salts in the presence of an additive such as calcium vanadate sludge, sodium vanadate sludge, oxide alkali metals, hydroxide of alkali metal, carbonates of alkali metals, oxide alkali earth metals, hydroxide of alkaline earth metal, carbonates of alkaline earth metal and ammonium compounds to precipitate phosphate salts. The phosphate salts are removed by filtration to obtain a third filtrate. The additive is added to maintain the pH above 5. Calcium vanadate sludge is used as additive.The iron and/or aluminium salts are selected from a group of salts consisting of bauxite ore, red mud, chloride/sulphate/nitrate/oxide/hydroxide/carbonate salt of iron and chloride/sulphate/nitrate/oxide/hydroxide/carbonate salt of aluminum. The mole ratio of iron or aluminum content in the respective salts to the phosphate content in the second filtrate is in the range of 0.05 to 5.

After phosphate removal, the third filtrate is treated with a fluoride salt. The fluoride salt is selected from a group consisting of hydrogen fluoride, ammonium fluoride, fluoride salts of alkali metals, preferably sodium fluoride. The fluoride salt is added to the third filtrate at room temperature and stirred for 30 minutes to obtain slurry. The slurry is then filtered to remove insoluble inorganic residue containing calcium and other multivalent metal ions as their fluoride salts and yield a filtrate containing vanadic acid . The residue containing fluoride salt can be used for the generation of hydrogen fluoride.

Alternatively, cationic resin is employed to remove calcium impurities from the third filtrate.

The separated filtrate, (referred as fourth filtrate) containing vanadic acid is optionally heated in the presence of at least one ammonium salt selected from a group consisting of ammonia, ammonium chloride, ammonium sulphate, ammonium carbonate, ammonium nitrate and ammonium hydroxide at a temperature above 60⁰C for a period of about 30 minutes and then the temperature is further raised to 80⁰C to 95⁰C. The pH of the vanadic acid solution is then lowered by slowly adding mineral acid. The reaction with the acid at pH below 4 is continued further to precipitate polyvanadate. The polyvanadate is then separated from the liquor, washed with water and then calcined at a temperature of about 400⁰C to 1000⁰C for about 20 to 50 minutes to obtain vanadium oxide having purity in the range of 98% to 100%.

The mineral acid is selected from a group consisting of hydrochloric acid, nitric acid and sulfuric acid. Preferably, the acid used in is 60% sulphuric acid solution.

The process for preparing vanadium oxide from vanadate sludge as illustrated in figure 3 consists of following steps: i) heating a slurry containing water and an alkaline salt to a temperature of about 70 ⁰C to about 120 ⁰C;

ii) adding vanadate sludge obtained from Bayer's process to the above slurry;

iii) stirring the above reaction mixture for a period of about 1 to about 5 hours to form a suspension containing alkaline vanadate;

iv) filtering the suspension to isolate the alkaline cake and obtain a first filtrate;

v) reacting the alkaline cake with a mineral acid at a temperature of about 5

⁰C to about 120 ⁰C , preferably 5 ⁰C to about 120 ⁰C and at a pH below 3 to form a second slurry;

vi) filtering the second slurry to obtain a second filtrate;

vii) treating the second filtrate at pH above 3 with at least one additive selected from a group consisting of calcium vanadate sludge, sodium vanadate sludge, oxides, hydroxides, carbonates of alkali metals, alkaline earth metals and ammonium compounds, and in the presence of at least one metal salt selected from a group consisting of iron and aluminium , to precipitate phosphate salts and obtain a third filtrate;

viii) treating the third filtrate to remove calcium impurities and obtain a fourth filtrate

ix) reacting the fourth filtrate with a mineral acid, at pH below 3 and at a temperature of about 10⁰C to about 120 ⁰C, preferably 50⁰C to about 95 ⁰C to precipitate polyvanadate;

xi) calcining the isolated polyvanadate at a temperature of about 400⁰C to about 1000°c for a period of about 10 to about 60 minutes to obtain vanadium oxide.

The acidic liquor obtained after filtering polyvanadate precipitation can be recycled to the sulphuric acid of step (v)and reused in the next batch.

The phosphate and the fluoride salts recovered during the process can be recycled to calcium vanadate of step (v) and reused in the next batch.

The first filtrate obtained in the process and containing sodium and aluminium elements can be processed further to recover sodium hydroxide or sodium aluminate. X-ray powder diffractogram of Calcium sulphate prepared in accordance with the process is shown in figure 4.

Figure 5 illustrates the XRD pattern of Vanadium Oxide prepared in accordance with the present invention.

The invention will now be described with respect to the following examples which do not limit the invention in any way and only exemplify the invention.

Example- Preparation of Calcium Vanadate

Composition of Vanadium sludge obtained from Bayer's alumina process, Na- 19%, Al-0.7%, V-8.3%, CaO- 0.3%, SiO2-4%, Water-47%, P- 0.6%

A mixture containing 30 gms lime (CaO, 92.5% pure) and 200ml water was taken in a reactor. After 20 minutes, the temperature of the mixture was raised to 90-95° C. 100 gms vanadate sludge obtained from Bayer's alumina process was added along with 200 gms of water. After the addition, the mixture was stirred at a temperature in the range of 90-95⁰C for 2 hrs to yield a suspension containing calcium vanadate. Calcium vanadate was filtered and washed with 80 gms warm water (70-80⁰C). Total filtrate obtained was 462gms including washing. The weight of alkaline cake (calcium vanadate) obtained was 139 gms.

The compositional analysis of calcium vanadate: Water- 55%, Na- 0.3%, V-5.94%, Al- 0.14%, P-0.43%.

The compositional analysis of first filtrate:

Na=4%, Al= 0.1%, V=0.01%

Example-2

A mixture containing 39.65 gms of slaked lime (Ca (OH) _2, 92.5% pure) and 190 gms water was taken in a reactor. After 5 minutes, the temperature was raised to 90-95 ⁰C. 100 gms vanadate sludge was added along with 200 gms of water to the above mixture. The mixture was stirred at a temperature of about 90-95⁰C for 2 hrs to yield a suspension containing calcium vanadate. Calcium vanadate was filtered and washed with 80 gms warm water (70- 80⁰C). Total filtrate including washing was 461.7gms. The weight of cake obtained was 139.5 gms.

The compositional analysis of the alkaline cake:

Water= 55.2% Na =0.27%, V=6%, Al= 0.1%, P =0.4%

The compositional analysis of filtrate:

Na= 4% Al= 0.1%, V=0.01%

The filtrate containing sodium and aluminum salts are further processed to recover the sodium hydroxide or sodium aluminate as a value added products.

Example-3

A mixture containing 161.2 gms of Barium hydroxide octahydrate (Ba (OH)₂₎ 8H2O and, Purity 97%) and 200 gms water was taken in a reactor. After 5 minutes, the temperature was raised to 90-95 ° C and then 100 gms vanadate sludge Was added along with 200 gms water. The mixture was stirred at 90-95⁰C for 2 hrs to yield a suspension containing Barium vanadate. Barium vanadate was filtered and washed with 80 gms warm water (70-80⁰C). Total filtrate obtained was 460 gms including washing. The weight of wet residue obtained was 275 gms.

The compositional analysis of Barium vanadate:

Water= 60%, Na = 0.2%, V= 3%, Al= 0.09%, P =0.15%

The compositional analysis of filtrate:

Na=4% Al= 0.08%, V=0.01%

Example-4

139 gms of wet calcium vanadate as obtained above was mixed with 171.2 ml water and to it 225.8 gms of 20% sulphuric acid solution was added slowly over 10 minutes to keeping p^'H of the mixture below 3. The reaction was further continued for a period of 40-50 minutes at a temperature maintained between 30⁰C to 50 ° C and at pH 2.5 to yield a slurry containing vanadic acid and insoluble alkaline salt .The slurry was filtered to remove

145.6 gms of insoluble alkaline salt. The filtered cake was washed with 125 gms water. The washings and the filtrate was combined to give 510.8gms of second filtrate.

The compositional analysis of insoluble salt:

Mostly CaSO4, Vanadium content less than 0.05% by wt and water-51%.

The compositional analysis of second filtrate:

V-1.6%, Al-0.04%, Na-0.05%, Ca-0.2%, As-0.05%, Fe-0.001%, P-0.1%. (Removal of calcium sulphate salt at different pH)

Therefore, the ideal pH for the calcium sulphate separation is below 3.0.

Filtrate-2 processing:

(a) Phosphate removal by adding iron sulphate

To 506 gms of second filtrate , was added 0.92 gms Fe as ferric sulphate. 14 gms calcium vanadate sludge was further added to the mixture to raise the pH to 5.0. The mixture was kept for^' ageing for 45 min at room temperature to precipitate phosphate salts. The phosphate salts(22gms) was filtered to obtain a third filtrate.

The compositional analysis of filtrate-3:

V- 1.63%, Ca-0.2%, Na-0.02%, As-0.04%, Al-0.002%, Fe-0.0004%, P-

0.015%.

It was found that phosphate removal efficiency of ferric as ferric sulphate is similar to aluminum as aluminum sulphate on molar basis. One can choose either of two salts or their combination for phosphate removal.

(b) Calcium removal using fluoride salt

2gms sodium fluoride was added to the third filtrate at room temperature and stirred for 30 minutes to precipitate calcium and other multivalent metal ions as their fluoride salts. 4.5gms of calcium fluoride was separated from 499 gms of fourth filtrate.

The calcium fluoride was used for HF generation by adding sulphuric acid and separating calcium sulphate salt. Then HF can be reused as such or as salt of ammonium or alkali metals. Also, the calcium fluoride obtained as above can be reused in next batch with calcium vanadate.

Chemical composition of filtrate -4:

V- 1.63%, Ca-0.08%, Na-0.14%, As-0.04%, Al-0.007%, Fe-0.0004%, P- 0.02%

(c) Precipitation of solid vanadium compound

10 gms of ammonium sulphate was added to filtrate-4 and the mixture was stirred for 30 minutes at room temperature, the temperature was increased to 60⁰C and the stirring continued for 30 minutes. After this, temperature was raised to 90⁰C. 10.5gms of 60% sulphuric acid was slowly added to the mixture to lower the pH below 2 and the reaction was carried out for 90 minutes to precipitate polyvanadate. The precipitate of polyvanadate was filtered to yield 482 gms acidic liquor. The weight of polyvanadate after drying was 15.6 gms.

The precipitate was washed with water and calcined at 800⁰C for 30 minutes to give 14 gms vanadium oxide having following chemical composition. V=55.9%, Na=O.1%, P=0.001%, ^' Ca=O.15%, Al=0.06%, Si=0.006%, As=0.1%, Fe: 0.02% Vanadium content in the acidic liquor was less than 0.1%. The acidic liquor can be used in the next batch or sent to effluent treatment plant. Sulphuric acid content of the liquor was 1.5%.

Example-5:

(Using Cationic resin after phosphate removal to remove calcium salt)

Third filtrate was passed through a (styrene based)sulphonic resin at room temperature to remove calcium ions from the filtrate. The chemical composition of fourth filtrate obtained after passing through cationic resin was:

V-1.63%, Ca-0.001%, Na-0.008%, As-0.04%, Al-0.001%, Fe-0.0001%, P-

0.015%

The precipitation of vanadium compound was carried out as mentioned in

Example-4 (c) and the vanadium oxide was obtained after calcination at

800⁰C for 30 minutes.

The chemical composition of the vanadium oxide so obtained.

V-56%, Na-0.05%, P-0.001%, Ca-0.1%, Al-0.04%, Si-0.002%, As-0.1%,

Fe-0.01%.

Example-6:

(Passing through cationic resin before phosphate removal)

Filtrate-3 having following composition

V-1.6%, Al-0.04%, Na-0.05%, Ca-0.2%, As-0.05%, Fe-0.001%, P-0.1% was directly passed through cationic resin and the filtrate with the following composition was obtained. V- 1.63%, Ca-0.001%, Na-0.04%, As-0.04%, Al-0.002%, Fe-0.0004%, P- 0.1%.

The precipitation of polyvanadate was carried out as mentioned in Example

4(c), and vanadium oxide was obtained after calcination of polyvanadate at

800⁰C for 30 minutes. The Vanadium oxide obtained had the following chemical composition.

V-55.8%, Na-0.05%, P-0.001%, Ca-0.1%, Al-0.06%, Si-0.002%, As-0.17%,

Fe-0.012%.

Example-7:

(Precipitation of vanadium oxide without adding ammonium salt)

The vanadium oxide precipitation was carried out without adding ammonium salt to filtrate-4 in example-4(c).

The polvanadate so obtained was calcined at 800⁰C for 30 minute to give 14.4 gm vanadium oxide having following chemical composition.

V=55.2%, Na=0.3%, P=0.001%, Ca=0.3%, Al=0.06%, Si=0.006%, As=0.2%, Fe: 0.02%

This reaction is attractable if metal content of liquor is very low, otherwise alkali or alkali earth metal vanadate such as sodium or calcium polyvanadate will be formed. To avoid sodium or calcium polyvanadate formation, it is preferable to use more ammonium salts.

(Phosphate removal using different salts)

The phosphate removal was carried out by adding aluminum sulphate and/or ferric sulphate to the second filtrate at different reaction sets, where pH was raised above 3 by the addition of sodium vanadate sludge or calcium vanadate residue. The result obtained is given below.

Results show that both iron salts and aluminum salts are effective for phosphate removal.

(Calcium salt removal by adding sodium fluoride at different pH)

Sodium fluoride was added to the third filtrate at different pH.

Result shows that multivalent metal ions can be effectively removed at pH more than 3 by adding monovalent fluoride salt.

Example-7 (b): (Calcium salt removal by adding ammonium fluoride

Similar to Example-4(b), 2.4gms ammonium fluoride was added to filtrate-3 at room temperature and stirred for 30 minutes to precipitate calcium and other multivalent metal ions as their fluoride salts. Calcium fluoride 4.9gms so obtained was separated by filtration.

Chemical composition of filtrate -4:

V= 1.63%, Ca=0.04%, Na=0.05%, Al=0.007%, Fe=0.0004%, P= Not detected, As= Not detected,

Example-8:

(Using acidic liquor of example 4 c in next batch) 139 gms of wet calcium vanadate was taken with 171.2 gms acidic liquor obtained in example 4-c, and to it 213 gms of 20% sulphuric acid made from acidic liquor obtained in example 4-c, was added slowly over 10 minutes to keep the pH of slurry at 2.5. The reaction was carried out as in example-4, the vanadium oxide with following composition was obtained,

V=55.92%, Na=0.07%, P=0.001%, Ca=0.11%, Al=0.06%, Si=0.006%, As=0.08%, Fe: 0.02%.

This shows that the liquid effluent generated in example-4 can be re used in the process.

Process carried out on large scale. Batch -1

Step-1

lOOOgms of calcium vanadate cake (50% moisture) & 90 gms ammonium sulphate was charged in a reactor. 1300gm water was added to it to get slurry. The slurry was analyzed,

V: 1.73%, Na: 0.027%, Al: 0.42%, As: 0.183%, Fe: 0.006%, P: 0.37%.

1910 gms of 22% sulphuric acid was added to the slurry over a period of 20 minutes to bring the reaction pH to 2.4. Temperature of the reaction was maintained below 50⁰C. Reaction was carried out for lhr. Calcium sulphate was separated and washed with 1400gms of hot water.

Analysis of Calcium Sulphate

V: 0.05%, Na: 0.013%, Al: 0.33%, As: 0.18%, Fe: 0.01% & P: 0.56%.

Analysis of the filtrate:

Na: 0.03%, Al: 0.14%, As: 0.044% & P: 0.03%, Ca: 0.06%, Fe: 0.0002%. Step-2

3800 gms of filtrate (obtained above) was charged to a stirring vessel, to this, 7 gms of ferric sulphate (Fe2 (SO4)3, nH20) and 45 gms of calcium vanadate cake was added. The mixture was stirred for lhr at room temperature (25-35° C) to precipitate 140 gms of phosphate salt and 3550gms of filtrate. Phosphate salt was kept aside to be reused in the 1^st step of next experiment.

Analysis of the filtrate:

Na: 0.03%, Al: 0.0.06%, Ca: 0.072%, Fe: 0.0002%, As: Not detected & P: Not detected.

Step-3

The filtrate as obtained above was charged into a stirring vessel, 7.9 gms ammonium fluoride was added and the mixture was stirred for lhr at 40-45° C to obtain a slurry. The slurry was filtered to yield 28 gms of wet residue and 3535 gms of filtrate. The wet residue was kept aside to be reused in 1^st step of next experiment.

Analysis of the filtrate

Na: 0.03%, Al: 0.0.06%, Ca: 0.04%, Fe: 0.0002%, As: -Not detected & P: Not detected.

Step-4

3435 gms of filtrate obtained in step 3 was taken in a stirring vessel and slowly heated to 90-100 ⁰C , 74.2 gms of 61.6% sulphuric acid was added slowly till the pH of the mixture was about 1.6. The stirring was further carried out for 2 hrs. 104 gms of ammonium polyvanadate obtained was separated by filtration and washed with 100 gms water. The filtered ammonium polyvanadate was dried at HO⁰C to yield 68.43 gms of dry ammonium polyvanadate. Yield of acidic liquour obtained after filtering polyvanadate was 3201 gms .

Analysis of acidic liquor

V: 0.07%, Na: 0.082%, Ca: 0.039%, Al: 0.054%, Fe: 0.0001%, NH4:

0.68%, P: Not detected & As: Not detected.

The filtrate was kept aside for its use. in 1^st step of next experiment.

Step-5

62.18 gms of ammonium polyvanadate (obtained above) was calcined at 800⁰C for 40 minutes in furnace to obtain 54.44 gms of vanadium oxide having following composition.

V2O5: 100.8% (Volumetric titration), Na: 0.15% (AAS), Ca: 0.2% (AAS), Al: 0.12% (AAS), Fe: 0.035%, As: Not detected (AAS) & P: Not detected (UV-ammonium- molybdate method).

Batch-2

(Reusing filtrate (effluent) of experiment-1)

Step-1

880gms of calcium vanadate cake (50% moisture) & 6 gms ammonium sulphate was charged in a reactor.. 139 gms filtered residue of step-2 & 27gms filtered residue of step-3 of experiment-1 were added to the above mixture. 1300gms filtrate of step-4 in experiment-1 was added to the above mixture to form a slurry having following composition,

V: 1.91%, Na: 0.03%, Al: 0.54%, As: 0.225%, Fe: 0.062%, P: 0.38%. ' 1680 gms of 22% sulphuric acid (prepared using 1310.4 gms filtrate of step-

4 in experiment- 1 ) was added to the reactant mass in 20 minutes to bring the reaction pH to 2.4. Temperature of reaction was maintained below 50° C.

Reaction was carried out for lhr. Calcium sulphate was separated and was washed with 1250 gms water containing 550 gms of filtrate from step-4 in experiment- 1. 1290 gms of of calcium sulphate (45.24% moisture) was obtained.

Analysis of Calcium Sulphate

V: 0.06%, Na: 0.022%, Al: 0.19%, As: 0.12%, Fe: 0.1% & P: 0.37%.

Step-2

3650gms of filtrate (obtained above) having following contents was charged in a stirring vessel,

Na: 0.025%, Al: 0.28%, As: 0.1% & P: 0.11%, Fe: 0.0046%, Ca: 0.07%. To this, 12gms of ferric sulphate (Fe2 (SO4)3, nH2O) was added followed by 80 gms of calcium vanadate cake to increase the reactant pH to 4.5. Reaction was carried out for lhr at room temperature (25-35⁰C). 250 gms of wet residue was separated and kept for its reuse in 1^st step of next batch.

Step-3

3310 gms of the filtrate obtained in above step and containing following contents was charged in a stirring vessel,

Na: 0.03%, Al: 0.15%, Ca: 0.085%, Fe: 0.0006%, As: 0.001 & P: Not detected. To this, 8.23 gms ammonium fluoride was added and stirred for lhr at 40-45 deg C. 28.4 gms of wet residue obtained after filtration was reused in 1^st step of next batch. Step-4

3240 gms of filtrate obtained in the above step-3 and having following composition was taken in stirring vessel.

Na: 0.03%, Al: 0.12%, Ca: 0.06%, Fe: 0.0006%, As: 0.003 & P: Not detected.

The solution was slowly heated to 90-100⁰C and 67.4 gms of 61.6% sulphuric acid was added slowly till pH was 1.7. The mixture was stirred for 2 hrs. 105 gms of wet ammonium polyvanadate was separated out by filtration and washed with water. The polyvanadate was dried to remove free moisture at 110 deg C. 71.44 gms of dry ammonium polyvanadate was obtained. The filtrate 3008gms with following composition was obtained: V: 0.04%, Na: 0.05%, Ca: 0.06%, Al: 0.1%, Fe: 0.0001%, NH4: 0.66%, P: Not detected & As: 0.001%.

The filtrate is reused in 1^st step of next batch.

Step-5

55.79 gm of ammonium polyvanadate (obtained above) was kept at 800⁰C for 40 minutes in furnace to obtain 49.88 gm of vanadium oxide having following composition.

V2O5: 100.4% (Volumetric titration), Na: 0.13% (AAS), Ca: 0.31% (AAS), Al: 0.096% (AAS), Fe: 0.06%, As: Not detected (AAS) & P: Not detected (UV-ammonium molybdate method)

Batch-3 (Reusing filtrate (effluent) of Batch-2)

Step-1

765gms of calcium vanadate cake (50% moisture) & 9 gms ammonium sulphate were charged in a reactor. 249 gms residue of step-2 & 27gms residue of step- 3 of experiment-2 were added. 1300gms of filtrate of step-4 in experiment-2 was added to it to get a slurry of following composition. V: 1.96%, Na: 0.134%, Al: 0.61%, As: 0.326%, Fe: 0.097%, P: 0.48%.

1570 gms of 22% sulphuric acid (prepared using 1224.6 gm filtrate of step-4 in experiment-2 ) was added to the reactant mass in 20 minutes to bring reaction pH to 2.4. Temperature of the reaction was maintained below 50° C. Reaction was carried out for lhr. Calcium sulphate was separated and was washed with 1250 gms water (400 gms filtrate of step-4 in experiment-2 & 850 gms of water). 1220gms of wet cake (41.88% moisture) of calcium sulphate having following composition was obtained. V: 0.06%, Na: 0.034%, Al: 0.25%, As: 0.37%, Fe: 0.16% & P: 0.46%.

Step-2

3620gms of filtrate (obtained above) containing following metal contents was charged in a stirring vessel.

Na: 0.098%, Al: 0.3%, As: 0.08% & P: 0.16%, Ca: 0.038%, Fe: 0.0072%. To this, 17.4 gms of ferric sulphate (Fe2 (SO4)3, nH2O) and 92.4 gms of calcium vanadate cake were added to increase the reactant pH to 4.5. Reaction was carried out for lhr at room temperature (25-35 deg C). 282.6 gms of wet residue was separated and kept aside for its reuse in 1^st step of next batch.

Step-3

3318 gms of filtrate (obtained) above containing having following analysis. Na: 0.1%, Al: 0.22%, Ca: 0.052%, Fe: 0.0009%, As: 0.003% & P: 0.011% was taken in stirring vessel. To this, 5 gms ammonium fluoride was added and stirred for lhr at 40-45 deg C. 10.8 gms of wet residue was separated by filtration and kept for its reuse in 1^st step of next batch. Step-4

3240 gm of filtrate (obtained) as above and containing Na: 0.102%, Al: 0.21%, Ca: 0.045%, Fe: 0.0009%, As: 0.002 & P: 0.008 was slowly heated to 90-100 deg C and 66.4 gms of 61.6% sulphuric acid was added slowly till pH was 1.65. The reaction was carried out for 2 hr. 115.4 gms of wet ammonium polyvanadate was separated out by filtration and 100 gms water washing was given to the filtered cake. The cake was dried to remove moisture at 110 ° C. 79.6gms of ammonium polyvanadate was obtained. The filtrate obtained (3014 gms) had following composition: V: 0.04%, Na: 0.11%, Ca: 0.04%, Al: 0.23%, Fe: 0.0002%, NH4: 0.65%, P: 0.009% & As: 0.002%.

Step-5

50gms of ammonium polyvanadate (obtained above) was calcined at 800⁰C for 40 minutes in furnace to obtain 45.86 gms of vanadium oxide containing V2O5-100.21% (Volumetric titration), Na-0.12% (AAS), Ca-0.26% (AAS), Al-0.1% (AAS), Fe-0.04%, As-0.014% (AAS) & P-Not detected (UV- ammonium molybdate method)

Batch-4 (Reusing filtrate (effluent) of batch-3)

Step-1

744gm of calcium vanadate cake (50% moisture) & 6 gm ammonium sulphate was taken in reactor. 281 gm residue of step-2 & 9.5 gm of residue of step-3 of experiment-3 was added. 1300gm of filtrate of step-4 in experiment-3 was added to it to get slurry. The analysis of slurry was given below. V: 1.9%, Na: 0.127%, Al: 0.6%, As: 0.27%, Fe: 0.15%, P: 0.38%.

1497 gm of 22% sulphuric acid (prepared using 1167.66 gm filtrate of step-4 in experiment-3 ) was added to reactant mass in 20 minutes to bring reaction pH to 2.4. Temperature of reaction maintained below 50 deg C. Reaction was carried out for lhr. Calcium sulphate was separated using centrifuge and 1250 gm of washing (400 gm filtrate of step-4 in experiment-3 & 850 gm of water) was given. 1172gm of wet cake (39.78% moisture) of calcium sulphate was obtained having following analysis. V: 0.05%, Na: 0.027%, Al: 0.27%, As: 0.25%, Fe: 0.28% & P: 0.39%.

Step-2

3650gm of filtrate (obtained above) was taken in a stirring vessel having following analysis,

Na: 0.1%, Al: 0.29%, As: 0.09% & P: 0.12%, Ca: 0.065%, Fe: 0.006%.

To this, 13.14 gm of ferric sulphate (Fe2 (SO4)3, nH2O) and 86.3 gm of calcium vanadate cake was added to increase reactant pH to 4.5. Reaction was carried out for lhr at room temperature (25-35 deg C). 275.4 gm of wet residue was separated and kept for its reuse in 1^st step of next experiment.

Step-3

3350 gm of filtrate (obtained) above was taken in stirring vessel having following analysis. Na: 0.1%, Al: 0.13%, Ca: 0.077%, Fe: 0.0006%, As: 0.004% & P: Not detected. To this, 7.55 gm ammonium fluoride was added and stirred for lhr at 40-45 deg C. 26.8 gm of wet residue separated by filtration and kept for its reuse in 1^st step of next experiment.

Step-4 following analysis. Na: 0.107%, Al: 0.11%, Ca: 0.05%, Fe: 0.0004%, As: 0.003 & P: Not detected.

This solution was slowly heated to 90-100 deg C and 65.8 gm of 61.6% sulphuric acid was added slowly for lowering pH to 1.7. The reaction was carried out for 2 hr. 118 gm of wet ammonium polyvanadate was separated out by filtration and 100 gm water washing was given to above residues. The residue was dried to remove free moisture at 110 deg C. 78.03 gm of ammonium polyvanadate was obtained. The filtrate obtained (3078 gm) has following analysis: V: 0.05%, Na: 0.11%, Ca: 0.05%, Al: 0.12%, Fe: 0.0004%, NH4: 0.65%, P: Not detected & As: 0.003%. The filtrate kept for its use in 1^st step of next experiment.

Step-5

50gm of ammonium polyvanadate (obtained above) was kept at 800degC for 40 minutes in furnace to obtain 43.91 gm of vanadium oxide having following composition.-

V2O5: 100.4% (Volumetric titration), Na: 0.12% (AAS), Ca: 0.27% (AAS), Al: 0.06% (AAS), Fe: 0.017%, As: 0.014% (AAS) & P: Not detected (UV- ammonium molybdate method).

Batch-5 (Reusing filtrate (effluent) of batch-4)

Step-1

748.3gms calcium vanadate cake (50% moisture) & 9 gm ammonium sulphate were taken in a reactor. 274.5 gms residue of step-2 & 26 gms of residue of step-3 of experiment-4 were added to the mixture. 1300gms of filtrate of step-4 in experiment-4 was added to it to get a slurry containing,

32 V: 1.95%, Na: 0.12%, Al: 0.66%, As: 0.29%, Fe: 0.12%, P: 0.46%.

1525 gms of 22% sulphuric acid (prepared using 1189.5 gm filtrate of step-4 in batch-4 ) was added to the reactant mass in 20 minutes to bring the reaction pH to 2.4. Temperature of the reaction was maintained below 50° C. Reaction was carried out for lhr. Calcium sulphate was separated and washed with 1250 gms of water (400 gm filtrate of step-4 in batcht-4 & 850 gm of water). 1179gms of calcium sulphate (40.06% moisture) was obtained containing following impurity profile V: 0.058%, Na: 0.024%, Al: 0.22%, As: 0.33%, Fe: 0.28% & P: 0.43%.

Step-2

3675gms of filtrate (obtained above) containing Na: 0.09%, Al: 0.35%, As: 0.08% & P: 0.16%, Ca: 0.06%, Fe: 0.006% was taken in a stirring vessel , 17.42 gm of ferric sulphate (Fe2 (SO4)3, nH2O) and 94 gm of calcium vanadate cake was then added to increase the reactant pH to 4.5. Reaction was carried out for lhr at room temperature (25-35 ⁰C). 265.7 gms of wet residue was separated and kept for its reuse in 1^st step of next batch.

Step-3

3400 gms of filtrate (obtained) above and containing Na: 0.087%, Al: 0.16%, Ca: 0.082%, Fe: 0.001%, As: 0.003% & P: 0.008, 8.15 gms ammonium fluoride were charged to a stirring vessel, the mixture was stirred for lhr at 40-45° C. The slurry obtained was filtered to yield 28.2 gms of wet residue and 3300gms filtrate.

Step-4 3300 gms of filtrate (obtained) above containing Na: 0.08%, Al: 0.13%, Ca: 0.05%, Fe: 0.0008%, As: 0.002 & P: 0.0072 was slowly heated to 90-100 ⁰C and 68.9 gms of 61.6% sulphuric acid was added slowly for lowering pH to 1.7. The reaction was carried out for 2 hrs. 124.5 gms of wet ammonium polyvanadate was separated out by filtration and washed with 100 gms water. The residue was dried to remove moisture at 110° C. 80.74 gms of dry ammonium polyvanadate was obtained. The filtrate containing V: 0.04%, Na: 0.08%, Ca: 0.05%, Al: 0.132%, Fe: 0.0002%, NH4: 0.67%, P: 0.008% & As: 0.002% was obtained.

Step-5

50gms of ammonium polyvanadate (obtained above) was calcined at 800⁰C for 40 minutes in furnace to obtain 43.985 gms of vanadium oxide containing V2O5- 100.22% (Volumetric titration), Na-0.12% (AAS), Ca- 0.24% (AAS), Al-0.11% (AAS), Fe-0.03%, As-Not detected (AAS) & P- Not detected (UV-ammonium molybdate method).

Batch-6 (Reusing filtrate (effluent) of Batch-5)

Step-1

728 gms of calcium vanadate cake (50% moisture) & 6 gms ammonium sulphate was charged in a reactor. 265 gms residue of step-2 & 27.5 gm of residue of step-3 of batch-5 was added. 1300gms filtrate of step-4 in batch- 5 was added to it to get a slurry _.conataining, V- 1.92%, Na-0.097%, Al- 0.86%, As- 0.26%, Fe-0.18%, P- 0.54%.

1504 gms of 22% sulphuric acid (prepared using 1173.12 gms filtrate of step-4 in batch-5 ) was added to the reactant mass over a period of 20 minutes to bring the reaction pH to 2.4. Temperature of the reaction was maintained below 50 ⁰C. Reaction was carried out for lhr. Calcium sulphate was separated ψiά washed with 1250 gms of washing (400 gm filtrate of step-4 in experiment-5 & 850 gm of water). 1197gms of wet cake (41.08% moisture) of calcium sulphate having following contents was obtained. V: 0.059%, Na: 0.02%, Al: 0.31%, As: 0.31%, Fe: 0.34% & P: 0.74%. Contents of the filtrate Na: 0.075%, Al: 0.44%, As: 0.065% & P: 0.11%, Ca: 0.06%, Fe: 0.002%.

Step-2

3600gms of filtrate (obtained above) was taken in a stirring vessel. To this, 11.56 gm of ferric sulphate (Fe2 (SO4)3, nH2O) and 85.6 gm of calcium vanadate cake was added to increase the reactant pH to 4.5. Reaction was carried out for lhr at room temperature (25-35° C). 255.2 gms of calcium sulphate was separated and kept for its reuse in 1 step of next batch.

Step-3

3350 gms of filtrate (obtained) above and containing Na: 0.07%, Al: 0.23%,

Ca: 0.086%, Fe: 0.0003% was taken in a stirring vessel, to this 8.43 gms ammonium fluoride was added and stirred for lhr at 40 -45 ° C. 25.6 gms of wet residue was separated by filtration and kept for its reuse in 1^st step of next batch. The filtrate 3250 gms obtained showed following profile,

Na: 0.07%, Al: 0.193%, Ca: 0.05%, Fe: 0.0003%, As: Not detected & P: Not detected.

Step-4 3250gms of filtrate (obtained) above was taken in a stirring vessel and was slowly heated to 90-100⁰C and 65.9 gms of 61.6% sulphuric acid was added slowly for lowering pH to 1.7. The reaction was carried out for 2 hr. 115.2 gms of wet ammonium polyvanadate was separated out by filtration and 100 gm water washing was given to the above residue. The residue was dried to remove moisture at HO⁰C. 75.33 gms of ammonium polyvanadate was obtained. The filtrate obtained (3081 gm) had following composition, V: 0.06%, Na: 0.07%, Ca: 0.05%, Al: 0.2%, Fe: 0.0002%, NH4: 0.674%, P: Not detected & As: Not detected. The filtrate was kept aside for its use in 1^st step of next batch.

Step-5

50gms of ammonium polyvanadate (obtained above) was calcined at 800⁰C for 40 minutes in furnace to obtain 44.45gms of vanadium oxide having following composition.

V2O5: 100.34% (Volumetric titration), Na: 0.1% (AAS),- Ca: 0.27% (AAS), Al: 0.08% (AAS), Fe: 0.01%, As: Not detected (AAS) & P: Not detected (UV-ammonium molybdate method).

This shows that the liquid effluent generated during the process can be recycled and reused in the process. While considerable emphasis has been placed herein on the specific steps of the preferred process, it will be appreciated that many steps can be made and that many changes can be made in the preferred steps without departing from the principles of the invention. These and other changes in the preferred steps of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

Claims

Claims:

1. A process for preparing vanadium from vanadate sludge, said process comprising the following steps: i) heating a slurry containing water and an alkaline metal salt to a temperature of about 70 ⁰C to about 120 ⁰C;

ii) adding to said slurry, vanadate sludge comprising optionally calcium, phosphate and aluminium salts, wherein the ratio of water to the vanadate sludge by weight is greater than 3, to form a reaction mixture;

v) reacting the alkaline cake with a mineral acid at a temperature of about 5

⁰C to about 120 ⁰C, preferably 5⁰C to about 50⁰C and at a pH below 4 to form a second slurry comprising vanadic acid, insoluble alkaline salt and optionally soluble phosphate salts;

vi) filtering the second slurry to remove the insoluble alkaline salt and obtain a second filtrate comprising vanadic acid and optionally containing soluble phosphates;

a) reacting the third filtrate with at least one fluoride salt at a temperature about 5 to about 120 ⁰C for about about 5 minutes to 10 hours to obtain a slurry comprising inorganic metal salts and vanadic acid, filtering the slurry to remove inorganic metal salts and to obtain a fourth filtrate; and

b) passing the third filtrate through a column of cationic exchange resin to obtain a fourth filtrate;

xi) calcining the isolated polyvanadate at a temperature of about 400⁰C to about 1000⁰C for a period of about 10 to about 60 minutes to obtain vanadium oxide.

2) The process as claimed in claim 1, wherein the alkaline metal salt in step (i) is selected from a group of metal salts consisting of calcium oxide, calcium carbonate, calcium sulfide, calcium hydroxide, magnesium carbonate, magnesium oxide, magnesium hydroxide, magnesium sulfide, barium oxide, barium carbonate, barium sulfide, barium hydroxide, strontium oxide, strontium carbonate, strontium sulfide and strontium hydroxide. 3) The process as claimed in claim 1, wherein the alkaline metal salt used in step (i) is selected from a group of metal salts consisting of calcium oxide, calcium carbonate and calcium hydroxide.

4) The process as claimed in claim 1, wherein the molar ratio of alkaline metal content of the alkaline metal salt used in step (i) to the sodium content « of the sludge is in the ratio of about 0.4 to about 2.0.

5) The process as claimed in claim 1, wherein the molar ratio of alkaline metal content of the alkaline metal salt used in step (i) to the sodium content of the sludge is in the ratio of about 0.5 to about 0.6.

6) The process as claimed in claim 1, wherein the alkaline metal salt used in step (i) is pretreated with water prior to the reaction.

7) The process as claimed in claim 1 , wherein more than 99% of vanadium in the vanadate sludge is reacted to form alkaline vanadate.

8) The process as claimed in claim 1, wherein the aikaline vanadate is calcium vanadate.

9) The process as claimed in claim 1, wherein the mineral acid used in step (v) is sulphuric acid.

10) The process as claimed in claim 1, wherein the iron salt used in step (vii) is selected from a group consisting of Red mud, iron chloride, iron sulphate, iron nitrate, iron oxide, iron hydroxide and iron carbonate. 11) The process as claimed in claim 1, wherein the aluminium salt used in step (vii) is selected -from a group consisting of Bauxite, aluminium chloride, aluminium sulphate, aluminium nitrate, aluminium oxide, aluminium hydroxide and aluminium carbonate.

12) The process as claimed in claim 1, wherein the additive used in step (vii) is calcium vanadate sludge.

13) The process as claimed in claim 1, wherein the step vii) is carried out at pH range of about 4 to about 10.

14) The process as claimed in claim 1, wherein the mole ratio of iron content in the iron salt used in step (vii) to the phosphate content in the second filtrate is in the range of about 0.05 to about 5.

15) The process as claimed in claim 1, wherein the mole ratio of aluminum content in the aluminium salt used in step (vii) to the phosphate content in the second filtrate is in the range of about 0.05 to about 5.

16) The process as claimed in claim 1, wherein the fluoride salt is selected from a group consisting of hydrogen fluoride, ammonium fluoride and alkali metal fluoride.

17) The process as claimed in claim 1 further comprises introducing an ammonium salt selected from a group consisting of ammonia, ammonium chloride, ammonium sulphate, ammonium carbonate, ammonium nitrate and ammonium hydroxide during the process.

18) The process as claimed in claim 17, wherein the ammonium salt is introduced in step (v) to the mixture containing alkaline vanadate and water.

19) The process as claimed in claim 17, wherein the ammonium salt is introduced in step (ix) in the third filtrate.

20) The process as claimed in claim 1, the mineral acid used in step (ix) selected from the group consisting of hydrochloric acid, sulphuric acid and nitric acid.

21) The process as claimed in claim 20, wherein the mineral acid is sulphuric acid.

22) The process as claimed in claim 1, wherein the phosphate salts formed in step (vii) in the process is recycled to step v).

23) The process as claimed in claim 1, wherein the inorganic metal salts formed in step (viii) in the process is^" recycled to step v).

24) The process as claimed in claim 1, wherein the acidic liquor isolated in step x) is further recycled to step (v).

25) The process as claimed in claim 1, wherein the polyvanadate is ammonium vanadate. 26) The process as claimed in claim 1, wherein the polyvanadate is calcined at a temperature of about 800⁰C to yield vanadium oxide.

27) The process as claimed in claim 1, the purity of vanadium oxide is greater than 99%.