WO2014199193A1

WO2014199193A1 - Production of chromium oxide

Info

Publication number: WO2014199193A1
Application number: PCT/IB2013/054738
Authority: WO
Inventors: Oaitse Joyce JABANE-BAUWENS
Original assignee: Trivalent Chrome Cc
Priority date: 2013-06-10
Filing date: 2013-06-10
Publication date: 2014-12-18

Abstract

This invention relates to a method for recovering chromium from an aqueous feedstock containing Chromium III, to obtain a high purity chromium product suitable for use in the pigment industry. The method including the steps of regulating the pH of the aqueous feedstock to less than pH 5, and passing the solution through a weak acid ion exchange resin, for the Chromium III in the solution to be adsorbed onto the resin; and recovering Chromium from the resin.

Description

Production of Chromium Oxide

BACKGROUND TO THE INVENTION

Chromium is present in aqueous streams resulting from various industrial processes. The disposal of Chromium containing waste can be costly and it can have a negative impact on environment.

Conventional processes for removal of the Chromium ions from aqueous solution often include addition of alkali to precipitate the Chromium ion in the form of Chromium hydroxide. After filtration the resulting filtrate may be safely disposed of. Other processes involve the use ion exchange resins for removal of Chromium ions from the aqueous stream. Both, cation exchange resins and anion exchange resins have been used. Most of the past methods focus on disposal of the waste after Chromium ions removal, rather than on obtaining a Chromium product.

International patent publication no. WO2006/103713 discloses a process for treating tannery waste to obtain a chromium-oxide based product for use in other industrial fields and in particular in the ceramics field, refractory materials field, and for use in pigments for glass and colorants for metallic foil. This process requires washing tannery chromium hydroxide with an EDTA solution before calcination. This process however results in the final product of 54% Cr₂0₃ purity, which is below the pigment industry standards.

An object of the present invention is to provide the process of recovery of Chromium from aqueous stream that results in a high purity product that is suitable for use in the pigment industry. SUMMARY OF THE INVENTION

According to the present invention, there is provided a method for recovering chromium from an aqueous feedstock containing Chromium III, preferabiy to obtain a high purity chromium product suitable for use in the pigment industry; the method including the steps of:

regulating the pH of the aqueous feedstock to less than pH 5, typically pH 2 to less than 5, preferably pH 2 to 4.8, most preferably from pH 2.5 to 3, and passing the solution through a weak acid ion exchange resin, for the Chromium III in the solution to be adsorbed onto the resin; and

recovering Chromium from the resin,

Preferabiy, the concentration of Chromium in the aqueous feedstock solution is controlled to 2 to 20g per litre, preferably to 4 to 12g per litre, most preferably to 4 to 8g per litre.

The weak ion exchange resin has carboxyiic acid functional groups and is preferably a macroporous, acrylic based cation exchange resin, most preferably Lewatit CNP-80.

The ion-exchange step may be carried out at a temperature of between 40 and 60°C,

Preferably, the weak acid cation resin is conditioned prior to the adsorption step with baste aqueous solution, such as 2-6%, preferably 4% NaOH, with a contact time of at least 30 minutes.

After adsorption, Chromium may be recovered from the weak acid ion exchange resin by elution, by way of the following steps:

pre-treatment with a counter-current flow of basic aqueous solution such as 0.5-2M, preferably 1 M (mol/litre) concentration of Sodium hydroxide; and - elusion by subjecting the resin to the flow of acid, preferably sulphuric acid solution having 1 -3M, preferably 2 concentration, with a preferred flow rate of elusion varying between 1 and 3BV per hour (BV being defined as 1rn3 solution per m3 of resin), and to obtain an eluate comprising a chromium sulphate solution.

Chromium may be recovered as chromium hydroxide precipitate from the chromium sulphate solution by raising the pH to between 6 and 8 with addition of a hydroxide such as sodium or ammonium hydroxide.

Preferably, the chromium hydroxide is treated with a chelating agent such as EDTA to remove metals such as Fe, Ca, Mn, Cu, Al, and Ni, especially Fe.

The chromium hydroxide precipitate may be converted to a pigment grade chromium oxide product (containing more than 80% pure chromium oxide), by calcination conducted in temperature range of 600 to 1100°C, preferably in a reducing environment.

The aqueous feedstock containing Chromium III may be derived from waste, for example:

a chrome-rich effluent from a tannery;

a chrome-rich effluent from mine residues;

- ferrochrome sand; or

any combination thereof.

Typically, the waste is treated in a leaching step in the presence of an acid, preferably sulphuric acid, at pH 1 to 1.5, to obtain the aqueous feedstock containing Chromium ill.

If necessary, prior to passing the aqueous feedstock through the weak acid ion exchange resin, the feedstock is passed through a strong acid cation exchange resin, typically having sulfonic acid groups, which selectively adsorbs Aluminium III ions, but is not selective for Chromium Mi ions, such as Purolite C160.

BRIEF DESCRIPTION OF THE DRAWING

The Drawing is a flow diagram of a process of present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

With reference to in the Drawing, a feedstock 10 for the process of the present invention may be derived from any of the following waste feeds alone or in combination:

. Tannery Chromium waste 12

2. Chromium mine residue 14

3. Ferrochrome sand 16.

As the waste feed 12, 14 or 16 for the process of the present invention is generally available in a solid or a sludge form, it is necessary to subject it to pre-treatment 18 to obtain a homogenous feed 10, followed by an acid leaching process 20 to convert chromium in the waste stream to Chromium III (i.e. Cr³⁺, also known as tri-vaient Chromium).

Pre-treatment

Tannery Waste

Tannery waste 12 containing Cr III is first dried and calcined between 400°C and 650°C to drive off moisture and organic compounds, and to reduce the feed stock by at least 50%. The waste is then cooled, pulverised and milled to a size of to below 200 microns, preferably about 50 microns to increase its surface area in order to expedite the rate of reaction.

Chromium Mine Residue

Chromium mine residue 14 is pulverised and milled to about 50 microns, to increase a surface area. An oxidant is required to remove Chromium entrapped in the mine residue, the milled residue is mixed with sodium peroxide (Na₂0₂) or sodium carbonate (Na2C03) and sodium hydroxide (NaOH). About 10 and 30 % (w/w) sodium peroxide and sodium hydroxide relative to the residue are required to remove entrapped Chromium. The resultant mixture is then calcined at a temperature of 500°C to 650°C for at least 30 minutes. The subsequent reaction is illustrated by the following formula (NB this formula is not balanced):

4FeCr₂0₄ (s) + 8Na₂C0_3(s) + 70₂ = 8Na₂Cr0₄(s) + 2Fe₂0₃(s) + C0_2(s)

The calcined mixture is leached with hot water to remove soluble Chromium in the form of sodium chromate {Na₂CrC>4), The Chromium in the sodium chromate is in the form of Chromium VI and in order to convert it to Chromium III, the solution is reduced by addition of sodium bisulphite:

2H₂Cr0₄ + 3NaHS0₃ + 3H₂S0₄ = Cr₂(S0₄)_3(aq) + 5H₂0(aq) + 3NaHS0₄(aq)

Sulphuric acid is added to maintain the pH at 2.5 and below. To produce homogenous feed suitable to be used in the method of the present invention sodium hydroxide (NaOH) is added to precipitate Chromium in the form of hydroxide at a pH between 7 and 8:

Cr₂(S0₄) a + 6NaOH = Cr₂0₃.3H₂0 + 3Na₂S0₄

Moisture is then removed by running the chromium oxide solution through a filter press.

Ferrochrome sand

No pre-treatment is required on ferrochrome sand as the particle size are very small and all elements present in the ferrochrome sand 16 are soluble is low concentration sulphuric acid.

2FeCr_(s) + 5H₂S(¾_{aq) -» Cr₂(S0₄)_3(s) + FeS0_4(aq) + 5H_¾g) Acid Leach

A feedstock 10 prepared from a waste feed 12, 14 or 16, or any combination thereof, for example a dry, pulverised chromium hydroxide from the tannery waste 12 may be mixed with wet chromium hydroxide from mine residue sand 6; and subjected to the leaching step 20.

In the leaching step 20, the feedstock 10 is mixed with sulphuric acid 22, to dissolve Chromium in the form of chromium sulphate, to obtain Chromium III ions in solution. The optimum leaching pH for the leaching step is between 1 and 1.5. The leaching step 20 takes from 35 minutes to one hour.

Chromium sulphate obtained in the leaching step 20 is subjected to an oxygenation step 24, where oxygen 26 is sparged into the solution while stirring, to cause oxidation of metals such as Fe, Si, Ca, Mg and other elements. The compressed air 26 can also improve separation of any hair and fats present in tannery chromium feed.

The Chromium concentration in the leachate is reduced to from 4g/l to 8g/I to meet ion exchange requirements by addition of least five times dilution with water.

Thereafter, leachate solution is run through the filtration step 28 to remove un-dissolved solids and particles. Washing 30 with water to remove excess chromium sulphate and pH adjustment 34, to less than pH 5, typically pH 2 to less than 5, preferably pH 2 to 4.8 most preferably from pH 2.5 to 3, is conducted before the leachate is fed into the ion exchange columns.

Ion Exchange

The present invention makes use of ion exchange resins to extract metals from the aqueous solution of Chromium lit. An ion-exchange resin is an insoluble matrix (or support structure) normally in the form of small (1- 2 mm diameter) beads fabricated from an organic polymer substrate. The material has highly developed structure of pores on the surface of which are sites with easily adsorbed and released ions. The adsorption of ions takes place only with simultaneous releasing of other ions; thus the process is called ion-exchange. There are multiple different types of ion-exchange resin which are fabricated to selectively prefer one or several different types of ions.

The process of the present invention makes use of two types of ion exchange resins:

• strongly acidic cation (typically, sulfonic acid groups, e.g. sodium polystyrene sulfonate or polyA PS)

• weakly acidic cation (mostly, carboxylic acid groups)

If necessary, the stream is passed through a first ion exchange process 36 to remove Aluminium ions from the solution. Aluminium ions are removed by running the leachate solution through two ion exchange columns 38. The ion exchange columns are loaded with a strong acid cation resin, preferably Purolite C160 (available from Puroiite Ion Exchange Resins), which is found to have a high affinity for Aluminium ions at ambient operating conditions and a pH between 1 and 1.5, while having a poor affinity for Chromium III tons. The leachate solution is maintained at the pH of between 1 and 1.5, with a flow rate of 1 BV/hr to 4BV/hr.

Absorbed Al³⁺ ions are eluted with low concentration sulphuric acid solution 40 (pH 2 or below) according to the following reaction:

Al³⁺ + 3H₂S0₄ = AI₂(S0₄)₃ + 6H^*

Eluted Aluminium ions are extracted in the form of aluminium sulphate 42.

The leachate is then passed through a second ion exchange process 44 to extract Chromium III. The second ion exchange process comprises four ion exchange columns 46. Weak acid cation resins and chelating resins are efficient and effective resins for recovery of metal ions. Theoretically, chelating resins may be more effective, however the fact that they require a higher amount of elution reagents and they are much more expensive than the weak acid cation resins make them economically and practically less attractive. Weak acid cation resins also have a higher loading capacity in comparison to the chelating resins available on the market. According to the present invention, the ion exchange columns are loaded with Lewatit CNP-80 resin (available from Sybron Chemicals Inc.). CNP-80 is a weakly acidic, macroporous, acrylic based cation exchange resin of standard bead size distribution (>90% 0.3 - 1.6 mm). It is characterized by high total absorption capacity, excellent chemical and mechanical stability with high resistance to osmotic shock. The resin operates efficiently when regenerated with a slight excess of acid.

Other weak acid cation resins such as Purolite C104 (gel-type polyacrylic weak acid cation exchanger available from Purolite Ion Exchange Resins) or Lewatit SP 112 (a weak acidic, macroporous-type cation exchange resin available from Sybron Chemicals Inc.) may be used in the process of the present invention. However, CNP-80 has been found to be the best resin for the purposes of the present invention.

The extraction of Chromium Hi by Lewatit CNP-80 resin can be described as follows:

2(RCOO)₃Na⁺ + Cr_zS0₄ = 2(RCOO)₃Cr + Na_zS0₄ + 6(RCOO H⁺) where R is a resin matrix.

Chromium is efuted or stripped from the resin with sulphuric acid 48 (1 M to 2IV1, preferably 2M sulphuric acid solution):

2(RCOO)₃Cr +H₂S0₄ = Cr₂(SO)₄ + 6(RCOO H⁺) The po!y-vaient ions present in the tannery and ferrochrome ieachate solutions are Cr(lil), Ai(III) and Fe(IM). These elements are all present in a solid state at the neutral and alkaline pH. They are present in liquid or ionic form in the acidic pHs, especially pH below 3.

According to the manufacturer's specification, the CNP-80 resin should be operated at pH of between 5 and 14. However, the pH of the chromium Ieachate solution cannot be raised above 3 as chromium starts to solidify into chromium hydroxide at this pH level, making it impossible for the Chromium in this form to be fed onto the ion exchange resin. Therefore, according to the present invention, a new operating regime has been developed for extraction of Cr III from the Ieachate solution.

According to the present invention the CNP-80 resin is conditioned with 4% NaOH with a contact time of at least 30 minutes. The concentration of chromium in the chromium Ieachate solution to be subjected to the extraction process is maintained between 4 and 8g per litre. The pH of the solution is adjusted and maintained below pH 5, usually between 2 and 4.8, preferably from 2.5 to 3. Leachate solution is then introduced into the ion exchange columns 46 with conditioned CNP-80 resin at a flow rate of between 2 and 4BV/h and at a temperature between 40 and 60°C, preferably between 50 and 60°C. The temperature is maintained throughout the process of Chromium extraction in the ion exchange columns 46 until the resin is fully loaded. A Chromium-bare solution exits from the ion exchange columns 46.

Thereafter, an elution process is followed for removal of Cr III from the fully loaded resin. In order to start a removal process the loaded resin is conditioned with 1M solution of sodium hydroxide (NaOH) in a counter- current flow direction at a flow rate of between 1 and 2 BV/h and at the contact time of between 30 and 60 minutes. Pre-conditioned resin was rinsed with water with at least 2BV at the flow between 2 and 4 BV/h. The resin is then eluted with 2 to 4 BV of 2M solution of sulphuric acid 48 (pH 1 or less) at a flow rate between 1 and 3 BV/h. The purity of Chromium recovered at this stage is about 77%. Subsequently the resin is slowly rinsed with 2 to 3 BV of water at the rate of between 2 and 4 BV/h. This operation is followed by the fast rinse with 4 to 6 BV of water at the flow rate of 3 to 6 BV/h, After rinsing the resin, it is reconditioned with 4% solution of sodium hydroxide in the counter-current flow direction at the flow rate of 1 to 3 BV/h or at the contact time between 30 minutes and 60 minutes. After reconditioning, the CNP-80 resin is ready for reuse.

The Chromium is recovered in the form of chromium sulphate solution. The purity of the chromium recovered at this stage was about 77%.

Production of Chromium Oxide

To obtain a high purity Chromium product, the Chromium sulphate solution undergoes a precipitation process 52, where Cr III is precipitated as Chromium hydroxide by adding a hydroxide (sodium hydroxide) 54 and raising the pH of the solution to between 6 and 8. The precipitation reaction with sodium hydroxide may be expressed as follows:

Cr₂(S0₄)₃ + 6NaOH = Cr₂0₃. 3H₂0 + 3Na₂S0₄

The hydrous Chromium hydroxide, which is formed is filtered to remove water. The cake obtained after filtration is then undergoes a metal-removal process 56, where it is soaked in 0.1 Wl solution of EDTA chelating solution 58 to allow complexing of ferric ions and other elements with high affinity to chelation. EDTA forms complexes with metals like Fe, Ca, Mn, Cu, Al, and Ni to keep them in solution. As a consequence these complexes can be removed during filtration. The pH of the EDTA solution is increased to between 8 and 9 during EDTA soaking process. The contact time of the filter cake in the EDTA solution is at least one hour.

The Chromium hydroxide and EDTA mixture is filtered to remove the filtrate, and the generated filter cake is washed with deionised water to remove excess of EDTA solution. The Chromium hydroxide is fed into a kiln 60 operated between 600 and 1100°C, typically from 400°C to 650°C and calcined for 30 minutes to 1 hr {typically 650°C for 45 minutes) to produce Chromium oxide (Cr₂0₃). If desired, 10% borax can be added to the Chromium hydroxide before calcinations in order to improve a tinting strength and colour of the final product in case whereby the pigment grade of Chromium oxide is to be produced (the product will however then have to be washed in a step 62 to remove unwanted salts introduced by addition of borax).

The final product is milled in step 64 to reduce the particle size to between 0.2 and 0.3 microns and spray dried in a step 66 with a hot air inlet temperature of 350°C (max) to dry the product to a maximum moisture content of 0.2%, and provide pigment grade Chromium oxide (Cr₂0₃) 68 with at least 80%, preferably at least 95%, Chromium oxide content, which is ready for packaging.

The invention will now be described in more detail with reference to the following non-limiting Examples:

Example 1

Tannery Chromium waste was analysed and pre-treated prior to the acid teaching process. The analysis of tannery Chromium waste is shown in Table 1 below.

Table 1

At: 0.3-0.5% P: 0 - 0.2%

Fe: 0.2 - 0.4% Sb: 0 - 0.2% : 0.1 - 0.3% Si: 3 - 9%

Mg: 0.2 - 0.7% Ni: 0 - 0.2%

Cr(lll): 10 - 25% Moisture: 40-60

tn a pre-treatmertt step, Chromium hydroxide waste from tanneries was dried at 105°C and at 350°C respectively to reduce the amount of organic contaminants present in this feedstock. The resulting product was analysed for the presence of Cr VI and none was detected. However, at above 140°C to 1100^DC Chromium hydroxide converts to Chromium oxide, which is highly insoluble in mineral acids. Therefore it was established that with the product dried at 350^DC chromium recovery was very low with the extraction efficiency of about 67%. Chromium recovery achieved in the leachate solution of the un-dried tannery waste was about 99%. The recovery of the Chromium from the feedstock dried at 05°C was 87%.

Example 2

Acid leach tests were carried out to:

• Determine the characteristics of the tannery chromium waste and ferrochrome sand from ferrochrome smelters

« Extract Chromium HI ions from Chromium waste by leaching the waste with mineral acid, sulphuric acid

• Determine quantities of the reagent required for testing

• Determine the amount of contaminants that can be removed during the acid leach test

• Identify the reaction rate and Chromium recovery efficiency.

The acid leach tests were carried out by: addition of about 0.1 M sulphuric acid and continuous agitation to release hydrogen gas.

The results of the leach tests indicated that for an un-dried Chromium hydroxide sample: every 291.79g of tannery feedstock requires 25 ml of 98%w/w of H₂S0₄ and 500 ml of water to obtain leachate solution at a pH of 1 and Cr HI concentration of between 8 and 15g/l.

For the dried chromium hydroxide sample (dried at 105°C), 15 g of chromium hydroxide requires 3ml of 98%w/w of H₂S0₄ and 150 ml of water to achieve between 5 and 7g/i of chromium concentration.

Example 3

Equilibrium isotherm tests were conducted on several ion exchange resins to determine their performance in extraction of Aluminium ions and Chromium ili ions.

The main physical characteristics of the other resins investigated are provided in Table 2 below:

Typical physical properties of CNP-80 resin are provided in Table 3 below: Table 3

T ical h sical and chemical ro erties" US Units Imernationai Units

Puroiite C160 was tested for extraction of Al III, and Chromium III. Puroiite C106, CNP-80, Puroiite C104 and SP112, were tested for the extraction of Chromium 111.

Al III extraction on Puroiite C160 strong acid cation resin.

Puroiite C160, a strong acid cation resin, was tested for removal of aluminium ions (Al³⁺).

Equilibrium isotherm tests were carried out using 5ml of each of these resins to determine their affinity to metal ions of interest and their loading capacity. The test results indicated that Puroiite C160 has higher loading capacity to Aluminium ions at ambient operating conditions and at pH between 1 and 1.5. This resin showed poor affinity to Chromium III ions. Therefore this resin was selected for removal of Aluminium ions from the leachate solution.

Chromium III extraction Puroiite C106, CNP-80, Puroiite C104 and SP1 2 weak cation exchange resins

The following restns were selected for further resin loading tests for extraction of Chromium ill from leachate solution: Puroiite C106, Lewatit CNP-80, Puroiite C104 and S 112.

The results of the loading capacity of Puroiite C106 are shown in Table 4 below. Table 4

Purolite C106 resins indicated an average loading capacity of about 17g of Cr III per litre of feachate solution.

The results of the loading capacity of Lewatit CNP-80 resin is illustrated in Table 5 beiow.

Table 5

Lewatit CNP-80 resin showed loading capacity of 50g of Cr Ml per litre of leachate solution at ambient operating conditions.

Tables 6 and 7 below, respectively, illustrate a loading capacity of Purolite C104 and SP112. Table 6

Table 7

It was found that a Purolite C104 has an average Cr III loading capacity of 39g per litre and SP 112 resin has a loading capacity of 51 g per litre.

Due to the fact that the SP112 is not economically viable and Purolite C104 has at least 85% swelling capacity, hence a short life span compared to other resins of a similar functional group only Lewatit CNP-80 and Purolite C106 were subjected to further tests. As the Purolite C106 has shown a lower loading capacity Lewatit CNP-80 was selected for removal of Cr III from the solution obtained from which the Aluminium has been extracted.

The test performed on Lewatit CNP-80 resin is exemplified in Example 4.

Example 4 - Cr Ml extraction on CNP-80 weak acid cation resin.

CNP-80 resin was selected for test work using Dow column test procedure.

The ion exchange column used in the experiment had a diameter of 20mm and a height of 1000mm. A rubber stopper and liquid distributor were used at the top of the glass column to control the flow and the pressure within the column. A Sera dosing pump with 1.5 bar back pressure adapter, to ensure constant flow, was used for dosing of ieachate solution from leach vessel to the ion exchange column.

Tests were carried out with 250 mi and scaled up to 800 ml CNP 80 resins. The CNP-80 resin was loaded into the column and the Ieachate solution was fed at 3 BV/h, CNP-80 resin was operated at the pH lower than recommended by the manufacturer and at elevated temperature up to 60°C.

An alkaline, ammonium hydroxide or sodium hydroxide was added to precipitate chromium at pH between 7.5 and 8 from chromium sulphate eiuant from ion exchange process. The mixture was dewatered to remove ammonium sulphate. Chromium hydroxide produced was rinsed with deionised water and soaked in an 0.2 EDTA solution. The mixture was then dewatered and the cake was rinsed with deionised water. High purity chromium hydroxide generated was dried at 105°C and then calcined at 650°C between 45 minutes and an hour to produce chromium oxide green.

The pH of below 4.8 was used for the experiment. The chromium oxide product quality of this test indicated that of at least 95% of Cr₂0₃ was achieved.

Claims

1. A method for recovering chromium from an aqueous feedstock containing Chromium Hi; the method including the steps of:

- regulating the pH of the aqueous feedstock to less than pH 5, and passing the solution through a weak acid ion exchange resin, for the Chromium III in the solution to be adsorbed onto the resin; and recovering Chromium from the resin.

2. The method as claimed in claim 1 , for obtaining a high purity chromium product suitable for use in the pigment industry.

3. The method as claimed in claim 1 or 2, wherein the pH of the aqueous feedstock is regulated to pH 2 to less than 5.

4. The method as claimed in claim 3, wherein the pH of the aqueous feedstock is regulated to pH 2 to 4.8,

5. The method as claimed in claim 4, wherein the pH of the aqueous feedstock is regulated to pH 2.5 to 3.

6. The method as claimed in any one of the preceding claims, wherein the concentration of Chromium in the aqueous feedstock solution is controlled to 2 to 20g per litre.

7. The method as claimed in claim 6, wherein the concentration of Chromium in the aqueous feedstock solution is controlled to 4 to 12g per litre.

8. The method as claimed in claim 7, wherein the concentration of Chromium in the aqueous feedstock solution is controlled to 4 to 8g per litre.

9. The method as claimed in any one of the preceding claims, wherein the weak ion exchange resin has carboxylic acid functional groups.

10. The method as claimed in claim 9, wherein the weak acid cation exchange resin is a macroporous, acrylic based cation exchange resin.

11. The method as claimed in claim 10, wherein the weak acid cation exchange resin is Lewatit CNP-80.

12. The method as claimed in any one of the preceding claims, wherein the ion-exchange step is carried out at a temperature of between 40 and 60°C.

13. The method as claimed in any one of the preceding claims, wherein the weak acid ton exchange resin is conditioned prior to the adsorption step with basic aqueous solution, such as 2-6%, preferably 4% NaOH, with a contact time of at least 30 minutes.

14. The method as claimed in claim 13, wherein the basic aqueous solution is 2-6% NaOH.

15. The method as claimed in claim 14, wherein the basic aqueous solution is 4% NaOH,

16. The method as claimed in any one of the preceding claims, wherein, after adsorption, Chromium is recovered from the weak acid ion exchange restn by elution, by way of the following steps:

- pre-treatment with a counter-current flow of basic aqueous solution; and

- elusion by subjecting the resin to the flow of acid, to obtain an eluate comprising a chromium sulphate solution.

17. The method as claimed in claim 16, wherein the basic aqueous solution is 0.5 - 2M (mo!/litre) concentration of Sodium hydroxide.

18. The method as claimed in claim 17, wherein the basic aqueous solution is 1 (mol/litre) concentration of Sodium hydroxide

19. The method as claimed in any one of claims 16 to 18, wherein the acid is a sulphuric acid solution having 1-3M concentration.

20. The methid as claimed in claim 19, wherein the sulphuric acid solution has a 2M concentration.

21. The method as claimed in any one of claims 16 to 20, wherein the acid has a flow rate of elusion varying between 1 and 3BV per hour (BV being defined as 1m3 solution per m3 of resin).

22. The method as claimed in any one of the preceding claims, wherein Chromium is recovered as chromium hydroxide precipitate from the chromium sulphate solution by raising the pH to between 6 and 8 with addition of a hydroxide.

23. The method as claimed in claim 22, wherein the hydroxide is sodium or ammonium hydroxide.

24. The method as claimed in claim 22 or 23, wherein the chromium hydroxide is treated with a chelating to remove metals.

25. The method as claimed in claim 24, wherein the chelating agent is EDTA.

26. The method as claimed in claim 24 or 25, wherein the metals are selected from Fe, Ca, n, Cu, Al, and Ni.

27. The method as claimed in claim 26, wherein the metal is Fe.

28. The method as claimed in any one of claims 22 to 27, wherein the chromium hydroxide precipitate is converted to a pigment grade chromium oxide product (containing more than 80% pure chromium oxide), by calcination conducted in temperature range of 600 to 1 00X.

29. The method as claimed in claim 28, wherein the caicination is in a reducing environment.

30. The method as claimed in any one of the preceding claims, wherein the aqueous feedstock containing Chromium Ml is derived from waste.

31. The method as claimed in claim 30, wherein the waste is:

a chrome-rich effluent from a tannery;

- a chrome-rich effluent from mine residues;

- ferrochrome sand; or

any combination thereof.

32. A method for recovering chromium from an aqueous feedstock containing Chromium III substantially as herein described.