Title of Invention	"AN IMPROVED EXTRACTION PROCESS FOR THE ROMOVAL OF HEXAVALENT CHROMIUM FROM WASTE SOLUTIONS"
Abstract	The present invention relates to an extraction process for the quantitative removal of hexavalent chromium from waste solution containing Cr(VI). This invention particularly relates to the back extraction of chromium by a dilute NaOH solution for the removal and reuse of the chromate solution in a concentrated form from the acidic industrial waste solutions containing hexavalent chromium, by solvent extraction using tris 2-ethylhexyl amine (TEHA) mixed with 2% iso-decanol as phase modifier and diluted with distilled kerosene. This concentrated strip chromate solution can be used in electroplating bath or can be sold as a pure chromate salt. The present invention utilizes very dilute solution of tris 2-ethylhexyl amine(TEHA) for the recovery of pure hexavalent chromium from industrial waste solutions containing Cr(VI), leaving all the impurities in the raffinate. The raffinate solution free from hexavalent chromium can be used as wash or rinse water in the process. The process not only meets the strict environmental regulations but also helps in the removal and reuse of Cr(VI) in a concentrated usable form, from industrial waste solution.

Title of Invention

"AN IMPROVED EXTRACTION PROCESS FOR THE ROMOVAL OF HEXAVALENT CHROMIUM FROM WASTE SOLUTIONS"

Abstract

The present invention relates to an extraction process for the quantitative removal of hexavalent chromium from waste solution containing Cr(VI). This invention particularly relates to the back extraction of chromium by a dilute NaOH solution for the removal and reuse of the chromate solution in a concentrated form from the acidic industrial waste solutions containing hexavalent chromium, by solvent extraction using tris 2-ethylhexyl amine (TEHA) mixed with 2% iso-decanol as phase modifier and diluted with distilled kerosene. This concentrated strip chromate solution can be used in electroplating bath or can be sold as a pure chromate salt. The present invention utilizes very dilute solution of tris 2-ethylhexyl amine(TEHA) for the recovery of pure hexavalent chromium from industrial waste solutions containing Cr(VI), leaving all the impurities in the raffinate. The raffinate solution free from hexavalent chromium can be used as wash or rinse water in the process. The process not only meets the strict environmental regulations but also helps in the removal and reuse of Cr(VI) in a concentrated usable form, from industrial waste solution.

Full Text	Field of the Invention The present invention relates to an improved extraction process for the removal of hexavalent chromium from waste solutions. This invention particularly relates to the removal of chromium(VI) from industrial waste solution by solvent extraction process using tris 2-ethylhexyl amine under various experimental conditions such as number of contacts of organic solvent with acid containing aqueous feed, solvent concentration, time of extraction and organic to aqueous ratio [O/A ratio], back extracting the chromium from the chromium-loaded organic with dilute NaOH solution and producing pure chromate, which can be used in plating industries or can be sold as chromate salt. The raffinate can either be used as wash or rinse water or can be safely disposed off. Background of the Invention: Chromium Cr (VI) enters the environment through its use as a corrosion inhibitor, especially the chemical passivating component of stainless steels. It also emerges from various plant operations such as the chromic acid anhydride production, the chromium electroplating, cooling towers, and leather tanning wastes. The effluents from these operations can contain significant quantities of Cr (VI) endangering the quality of surface water. The presence of Cr (VI) even in trace quantities, known to be a strong oxidant, a mutagen and a carcinogen on human health, produces a toxic influence on aquatic life. Thus, removal of Cr (VI) from industrial wastewater to its permissible limit before its disposal into surface waters is of deep concern, as well as a challenging problem to the industry. The total chromium maximum contaminant level (MCL) and the World Health Organization (WHO) level represent a guideline value that can be found in "Guidelines for Drinking-Water Quality, Vol. 1, pp. 80-85, WHO, Geneva, 1984" yielding a limit of 10.6 mol/L Cr (VI) in drinking water [J.A. Salvato, P.E. Dee, Environmental Engineering and Sanitation fourth ed (1992) Wiley New York]. However, according to the US EPA standards Cr(VI) contents in fresh water should not exceed 4x10~4 mol/dm3 required for the protection of aquatic life [J.A. Salvato, P.E. Dee, Environmental Engineering and Sanitation fourth ed (1992) Wiley New York ]. Actually, Sadaoui et al. [Z. Sadaoui, C. Azoug, G. Charbit, F. Charbit, J. Chem. Eng. Jpn. 30 (1997) 799-805.] define the waste-water norm of Cr(VI) to be restricted strongly about (1.9x10.6 mol/dm3) that will become more severe in the near future. Excessive concentrations of chromium and other heavy metals in soluble form are thought of as a toxicant group which strictly reduces the activity of microorganisms. It is well known that the reduced oxidation state of chromium, Cr(lll), is much less toxic and soluble at neutral pH values than its oxidized state of Cr(VI) [J.N. Veenstra, D.A. Sanders, A. Seyoung, J. Environ. Eng. June (1999) 522-531, P.N. Cheremisinoff, Y.H. Habib, Water Sewage Works July (1972) 73-86]. because chromium is basically present in the Cr(VI) form in aerobic biological systems. The deleterious effects of toxic Cr(VI) compounds on biological processes are complex, and generally depend on the influent matrix such as pH, the concentration of toxicant, solubility, the toxicity composition limit of other incorporated ions or molecules present in the bulk [Z. Lewandowski, Water Res. 19 (1985) 589-596]. Hence, emphasis has been given on the removal, recovery and reuse of chromium from such waste waters and effluents. Various processes are available such as membrane technology, ion exchange, electro dialysis etc., A project of extensive equilibrium studies by Cr(VI) based on a nondispersive technique in hollow-fiber modules has been fulfilled by Ortiz and coworkers [M.I. Ortiz, B. Galan, J.A. Irabien, Ind. Eng. Chem. Res. 35 (1996) 1369-1377, A.I. Alonso, A.M. Urtiaga, J.A. Irabien, M.I. Ortiz, Chem. Eng. Sci. 49 (1994) 901-909], Sirkar and co-workers [C.H. Yun, R. Prasad, A.K. Guha, K.K. Sirkar, Ind. Eng. Chem. Res. 32 (1993) 1186-1195, Z.F. Yang, A.K. Guha, K.K. Sirkar, Ind. Eng. Chem. Res. 35 (1996) 4214-4220]. At last, significant progress has been made in the use of neutral crown ethers, not only for the extraction of a metallic cation and its salts, but also for the extraction of more complex ion pairs appearing as a complex anion [Y. Takada, Top. Curr. Chem. 121 (1984) 1-38, C. Musikas, W.W. Schultz, in: J. Rydberg, C. Musikas, G. Choppin (Eds.), Principles and Practices of Solvent Extraction, Marcel Dekker, New York, 1992 (Chapter 11), A.H. Bond, M.L. Dietz, B. Chiarizia, Ind. Eng. Chem. Res. 39 (2000) 3442-3464]. Another conventional method of treating wastewater containing Cr(VI) includes its chemical reduction to convert Cr(VI) into the Cr(lll) cations by S02, Na2S03, or a ferrous ion compound (FeS04) as reducing agents [L.E. Eary, D. Rai, Environ. Sci. Technol. 22 (1988) 972-977, M.R. Unnithan, T.S. Anirudhan, Ind. Eng. Chem. Res. 40 (2001) 2693-2701], and the following precipitation of cations as Cr(OH)3. Permeable-reactive redox walls (reactive barriers) placed below the ground surface in the path of flowing groundwater could provide an alternative remediation approach for removing Cr(VI) chemicals from contaminated waters [D.W. Blowes, C.J. Ptacek, J.L. Jambor, Environ. Sci. Technol. 31 (1997) 3348-3357]. However, particularly frustrating aspects of the reduction method are the significant sludge production and the long term environmental consequences. Most widely used process to treat chromate solutions is the reduction of Cr(VI) to Cr(lll) and precipitation of Cr(ll) as hydroxide. One such process is described in U.S. Pat. No. 3,371,034 which, illustrates a direct precipitation process utilizing great quantities of barium carbonate and aqueous solutions acidified with strong acids, such as nitric or hydrochloric acid, or their salts. Such procedures encountered difficulties in separation of the chromium solids which are precipitated from the liquid waste media, thereby necessitating the use of one or more settling tanks which can be required to handle the excessive amounts of sludge produced. Moreover, the requirement for large amounts of barium carbonate increases the amount of sludge generated. Reference may be made to U.S. Pat. No. 3,869,386 which provides a method for the direct removal of hexavalent chromium by adding aqueous barium acetate. Izdebski states that this precipitation can be accomplished from neutral or slightly acid solutions in cases of precipitating both chromic acid and dichromates. But the disadvantage encountered with this process was that with the use of barium acetate the amount of acetate becomes more than is required. Thus, barium chromate so precipitated is discarded. No reuse of the chrome is contemplated by Izdebski. Reference may also be made to U.S. application Ser. No. 917,506 which discloses a process for the purification of chrome-containing waste water by treating the waste water with a reducing agent suitable for converting Cr(VI) to Cr(lll) followed by raising the pH of the acidic solution containing Cr(lll) with caustic to form a precipitate of chromic hydroxide. Reference may also be made to a Japanese Pat. No. 47-17697, which discloses the precipitation of chromic hydroxide from waste water by the process of (a) reducing hexavalent chromium in the waste water to trivalent chromium at a pH of about 2 using a reducing agent; (b) neutralizing with alkali at a pH of at least 8; and (c) adding magnesium ions (either prior to or after the reduction and neutralization steps) to precipitate the chromic hydroxide. In another Japanese Pat. No. 71-16389, a process for treating chromium containing waste water comprises of reduction of Cr(VI) to Cr(lll) and addition of a mixture of aluminum sulfate and sodium hydroxide or calcium hydroxide to the resulting solution of reduced chromium in order to precipitate chromic hydroxide. But the generation of a huge amount of chromium oxide and alkaline solution are the associated disadvantages. Further, methods such as micellar enhanced ultrafiltration have been applied to bind a heavy metal ion, especially Cr(VI) to a large species which is easily retained by an ultrafiltration membrane [Z. Sadaoui, C. Azoug, G. Charbit, F. Charbit, J. Chem. Eng. Jpn. 30 (1997) 799-805]. Recently, various natural and biological adsorbent materials, such as activated carbon, leaf mould, peat moss, silicagel, a natural clinoptilolite, and green algae [S.K. Ouki, R.D. Neufeld, J. Chem. Technol. Biotechnol. 70 (1997) 3-8, L. Monser, N. Adhoum, Sep. Purif. Technol. 26 (2002) 137-146, D.C. Sharma, C.F. Foster, Biores. Technol. 49 (1994) 31-40] have been reported to be potentially useful in the removal of Cr(VI) and heavy metals from aqueous solution through adsorption. A comparison of low-cost sorbents and their applications have well documented by Polland et al. [S.J.J. Polland, G.D. Fowler, C.G. Sollars, R. Perry, Sci. Total Environ. 116 (1992) 31-52]. However, the adsorption process is generally costly. Microorganisms are also effective at removing many heavy metals from waters including chromium. Cr(VI) is an essential cellular micronutrient, and can be actively and temporarily accumulated in the bacterial cells, as long as toxic concentrations are not reached. Additionally, many types of bacteria manufacture extracellular polymers, which also act to bind metals. Currently, to detoxify or remove various heavy metals activated sludge or other suspended growth processes have been used[F.B. Dilek, C.F. Gokcay, U. Yetis, Water Res. 32 (1998)303-312]. Nevertheless, amongst the possible alternatives to the treatment of industrial chromium plating baths or contaminated wastewaters, solvent extraction combining with membrane technology offers advantages [R. Molinari, E. Drioli, G. Pantano, Sep. Sci. Technol. 24 (1989) 1015-1032, M.E. Vallejo, F. Persin, C. Innocent, P. Sistat, G. Pourcelly, Sep. Purif. Technol. 21 (2000) 61-69]. However, the literature revealed very little insight relating to the validity of a generalized method for the treatment of Cr(VI) effluents at high concentration levels (over than 1.92x10.3 mol/dm3) and the evaluation of the effectiveness regeneration capabilities for recycling or recovery purposes [B. Gupta, A. Deep, S.N. Tandon, Ind. Eng. Chem. Res. 41 (2002) 2948-2952, F.J. Algucil, A.G. Coedo, M.T. Donato, A.M. Sastre, Hydrometallurgy 61 (2001) 13-19]. In this context, Liquid-liquid extraction is found to be the most effective conventional method to be imperative to significantly reduce the Cr(VI) discharge levels and to promote recycling and reuse. Within the limited number of potential extractants, tributyl phosphate (TBP), tertiary (Alamine) or quaternary (Aliquat 336) amines, alkylphosphoric acid, and phosphine oxide (Cyanex 921, 923) and oxime derivatives (LIX 84) are of critical importance due to their favorable qualities such as coordination ability and stability of the complex strength. However, process considerations dealing with the competition between various solvent extraction methods for Cr(VI), such as an amine, TBP, ethers or alkylphosphoric acid solvents still remain a challenging problem since such systems show extremely nonideal behavior of a complex aggregation. Therefore, the aim of the present invention is to obviate the disadvantages associated with the prior art. The present invention recites an improved process to treat an industrial waste solution containing 15-1500 ppm of hexavalent chromium for chromium removal by solvent extraction method using a very dilute solution of the solvent, wherein the loaded chromium is back extracted by dilute NaOH solution. More than 99% of chromium was removed and back extracted to give a pure and a concentrated chromate solution which can be used in electroplating industry or sold as chromate salt. The solvent after back extracting the chromium is washed with water and a very dilute acid and is reused for extraction experiment. This extraction and back extraction steps can be done continuously in a mixer settler unit which is very simple to operate. The main aspect of the present invention is the production of pure and concentrated sodium chromate solution from the chromium containing waste solutions from various industries using tris 2 ethylhexyl amine(TEHA) leaving all the other impurities in the raffinate as compared to other available processes. The produced concentrated sodium chromate solution can be recycled back into the plating bath or can be sold as chromate salt. In this method a large volume of waste water can be treated with a small amount of dilute solution of the extratant mixture at room temperature i.e. 30°C. Objects of the Invention The main object of this invention is to provide an improved solvent extraction process and a solvent for the removal of hexavalent chromium present in waste solution from various industries. Another object of the invention is to provide a process for the removal and subsequent recovery of chromium in an aqueous solution which generates no chromium related hazardous waste, as in the case of precipitation and other processes, thereby eliminating the need to store chromium waste in landfills and the subsequent liabilities attended thereto. Yet another object of this invention is to provide a process for the removal and recovery of chromium from chromium containing industrial waste solution, which produces pure and concentrated solutions that can be recycled back into the plating bath in the electroplating industry. Still another object of the invention to provide a process for the removal and recovery of chromium from chromium containing industrial waste solution which produces any usable chromate salt which can be sold in the market. Summary of the invention In the process of the present invention, the removal and-recovery of chromate from industrial waste waters containing chromium is carried out by a selective solvent extraction of the said waste waters by an organic solvent mixture containing tris-2-ethylhexylamine and iso-decanol diluted in kerosene. Under the optimized conditions an organic phase loaded with Cr(VI) is produces leaving the aqueous phase eventually free from Cr(VI). The chromium loaded organic phase is then treated with 0.5M NaOH to strip out the loaded chromium. The strip solution which contains chromate in a concentrated and pure form can be used by electroplating industry or various salts of chromates can be made and sold. The Cr free raffinate can either be used by the industry or can be disposed off. The solvent generated after the back extraction of chromium can be washed and reused in the extraction cycle again. Accordingly, the present invention provides an improved process for the extraction of chromium present in waste solution, wherein the steps comprising: [a] preparing an organic extractant by mixing 0.0112 M to 0.224 M tris 2 ethylhexyl amine [TEHA] in kerosene oil with 2% of iso-decanol as phase modifier diluted in kerosene; [b] simultaneously, varying the pH of the chromium containing waste solution in the range of 0 to 10, wherein the concentration of the Cr(VI) in the solution ranges from 50ppm to 2000ppm ; [c] mixing the organic extractant as obtained in step [a] with chromium containing waste solution as obtained in step [b] at an organic to aqueous ratio [O/A ratio] in the range of 1: 1 to 1:20 (vol/vol), followed by separating the chromium loaded organic from the raffinate by known methods and washing thereof; [d] mixing the chromium loaded organic as obtained in step [c] with 0.5M NaOH solution at a liquid to liquid ratio in the range of 1:10 to 2:1 to get a concentrated and pure sodium chromium solution back in the aqueous phase, Detailed description of the invention In the extraction process of the present invention, the solution containing hexavalent chromium is subjected, preferably at room temperature (30°C), to a process for extraction by means of solvents. An organic solvent is preferably used, such as one which is a mixture of a dilute solution of tris 2 ethylhexyl amine and iso decanol in kerosene. The process of the present invention can be described as a reaction for selective extraction between the chromate present in the industrial waste solutions/waters and tris 2 ethylhexyl amine as an extractant leaving other metals and impurities in the raffinate. Tris 2 ethylhexyl amine was procured from Fluka and was used as delivered by the producer, i.e., without further purification, however isodecanol was added to it as a phase modifier and was diluted with distilled kerosene(160 to 200°C) fraction to make the solvent of different concentration for this work since a concentrated or undiluted solvent is not required to treat such solutions. The average molecular weight of this solvent is 353.68 and the density is 0.817. The mechanism for the extraction and stripping of the loaded chromium was also studied. Thus a logarithmic plot of the distribution coefficient of chromium extraction against the organic phase concentration and simultaneously slope analysis of the plot gives an idea of the requirement of number of moles of extractant for the extraction of one mole of the Cr (VI). The extraction of Cr (VI) by tris 2 ethylhexyl amine under the conditions used in this investigation can be represented by the general reaction described as HCrO;aq + //; + (R3N)2org -> (tf3N)2 H.H2Cr04 1 And the stripping of loaded chromium(VI) with dilute solution of an alkali (OH") can be represented as (R3N)2 H.HCrO, + OH~aq -> (R3N)2 + Cr04~ + H20 2 Thus, tris 2 ethylhexyl amine selectively extracted chromium(VI) leaving other metal in the raffinate. The extracted chromium was stripped with NaOH solution at 30°C without affecting the stripping efficiency and extracting properties of the extractant. The solvent is found suitable for the extraction and enrichment of chromium from waste solutions from plating solutions and rinse waters of electroplating industries and the stripped solution obtained could be used to make chromium trioxide, alkali salts or insoluble chromate salts that can be recycled back into the market place. In an embodiment of the present invention, the waste rinse waters of electroplating industries used may be selected from composition in range of; Cr(VI) : 15-800ppm , Mg : 5 - 50ppm, Ca : 15 -100ppm, Fe : 1-15ppm, Cr(lll) : 0.5 -25ppm, Chloride: 250-1500ppm, Sulphate : 10- 900ppm, pH : 2.5-6. In another embodiment of present invention, the used tris 2 ethylhexyl amine extractants may also be used without further purification and in different concentration. In still another embodiment of the present invention the solvent extraction is carried out in one stage to four-stages to extract chromium leaving all the impurities in the raffinate In still another embodiment of the present invention loaded chromate on the solvent is stripped with 0.1 to 1.0M NaOH to get pure sodium chromate. In yet another embodiment of the present invention, the molarity of TE-HA used is preferably 0.112MTEHA. In another embodiment of the present invention, the O/A ratio is preferably 1:1. In a further embodiment of the present invention, the pH of the chromium containing waste solution is preferably 0.34. EXAMPLES The following examples are given by way of illustration of the present invention and therefore should not be construed to limit the scope of the present invention. Example-1 50 ml of the Cr(VI) solution containing 615ppm of chromium was mixed with 50 ml of 0.112M tris 2 ethylhexyl amine at different pH of the aqueous phase(adjusted with HCI in the acidic range and NaOH in the basic range) under stirring for 5 minutes at room temperature (30 C). The results presented in Table-1 indicate that extraction of Cr(VI) increases from 0(no extraction) to 97.2% with the decrease in pH of the solution from 10 to 0.3 in a single contact at an organic to aqueous ratio [O/A ratio] of 1:10. Table-1 (Table Removed) Example-2 Aqueous solution containing 600ppm Cr(VI) was mixed with 0.112M tris-2-ethylhexyl amine (v/v) with 2% iso-decanol diluted in kerosene at different organic to aqueous ratio [O/A ratio] varying from 1:1 to 1:20. The results given in Table-2 indicate that 99.9% Cr(VI) is extracted at an 0/A ratio of 1:1 which decreases marginally to 98% at O/A ratio of 1:5. It becomes 85% at an O/A ratio of 1:10 and decreases to 47% at an O/A ratio of 1:20. Thus, TEHA is found to be a very effective reagent for the extraction and removal of hexavalent chromium from industrial effluents from various industries. McCabe-Thiele plot shows that at O/A of 1:9, 99.9% Cr(VI) can be extracted in two stages at a pH of 0.5. Table-2 (Table Removed) Example-3 Tris 2 ethylhexyl amine(TEHA) was initially studied for the extraction and separation of Cr(VI) present in waste solution dealing with chrome plating and other related industries spent acidic industrial effluents of steel and electroplating industry. It was then found very effective for Cr(VI) extraction hence 0.0112M to 0.224M of tris 2 ethylhexyl amine with 2% iso-decanol diluted with distilled fraction(160-200°C) of the kerosene was mixed with 50 ml aqueous feed solution containing 443 ppm of Cr(VI) for 5 minutes in a 250ml glass beaker with mechanical stirring at organic to aqueous ratio [O/A ratio] of 1:1 at room temperature. The extraction of Cr(VI) in the organic phase increased from 85.64 to 99.316% (Table-3). The solvent, tris 2 ethylhexyl amine can thus be used in a very dilute form for the extraction of Cr(VI) from electroplating waste streams or any other industrial waste solution containing Cr(VI). Table-3 (Table Removed) Example-4 50 ml of the aqueous feed containing 25ppm to 1995ppm Cr(VI) usually present in the electroplating effluents was mixed with 50 ml of 0.112M tris 2 ethylhexyl amine using a magnetic stirrer, at a pH of 0.5 and temperature of 30°C. Table-4 shows that the extraction of chromium decreases with the increase in Cr (VI) concentration in the aqueous feed from 99.8 to 24.7% in a single contact at an organic to aqueous ratio [O/A ratio] of 1: 10. Thus this solvent can function in a wide range of chromium (VI) in the waste streams/solutions. Table-4 (Table Removed) Example-5 Stripping of loaded Cr(VI) is the major concern in order to recover the loaded chromate as a value added product and to reuse the solvent back to the system, hence stripping of the loaded chromium from 0.112M TEHA containing 2.56g/L of Cr(VI) was performed with varying concentration of NaOH at organic to aqueous ratio [O/A ratio] of 1:2 at 30°C. The results given in Table-5 indicate the percent stripping of Cr(VI) in a single contact. Thus with the increase in the cone of NaOH from 0.1 to 1% there is an increase in the percent Cr stripping from 54.6% to about 92.2%. Table-5 (Table Removed) Example-6 Stripping of loaded Cr (VI) from 0.112M TEHA was performed at different organic to aqueous ratio [O/A ratio] using 0.5M NaOH. Table 6 indicate that the percent stripping of chromium in a single contact increased from 24% to 98.8% with the variation of organic to aqueous ratio [O/A ratio] from 2:1 to 1:10. Based on these results McCabe-Thiele plot was drawn showing the requirement of three stages for complete recovery of loaded Cr at O/A ratio of 1:2.4 at 30°C. Table-6 (Table Removed) Advantages of the present invention: 1. The tris 2 ethylhexyl amine (TEHA) a tertiary amine is found to have a great affinity for hexavalent chromium. 2. The complete Cr(VI) is recovered leaving all the other impurities in the aqueous phase as found in plating and rinse water used in the electroplating industry. 3. No need of high temperature and capital intensive equipment as the extraction could be done at room temperature and stripping of the loaded acid could be done with NaOH solution. 4. This extractant tris 2 ethylhexyl amine could directly be reused after the stripping of Cr(VI) and washing with acid solution. 5. The recovered chromium which is pure and concentrated can be recycled back into the plating bath or can be used to produces either chromium trioxide, alkali salts or insoluble chromate salts that can be recycled back into the market place. We claim: 1. An improved process for the extraction of chromium present in waste solution, wherein the steps comprising: [a] preparing an organic extractant by mixing 0.0112 M to 0.224 M tris 2 ethylhexyl amine [TEHA] in kerosene oil with 2% of iso-decanol as phase modifier diluted in kerosene; [b] simultaneously, varying the pH of the chromium containing waste solution in the range of 0 to 10, wherein the concentration of the Cr(VI) in the solution ranges from 50ppm to 2000ppm ; [c] mixing the organic extractant as obtained in step [a] with chromium containing waste solution as obtained in step [b] at an organic to aqueous ratio [O/A ratio] in the range of 1: 1 to 1:20 (vol/vol), followed by separating the chromium loaded organic from the raffinate by known methods and washing thereof; [d] mixing the chromium loaded organic as obtained in step [c] with 0.5M NaOH solution at a liquid to liquid ratio in the range of 1:10 to 2:1 to get a concentrated and pure sodium chromium solution back in the aqueous phase. 2. A process as claimed in claim 1, wherein the waste solution containing chromium has a composition in range of; Cr(VI) : 15-800ppm , Mg : 5 - 50ppm, Ca : 15 -100ppm, Fe : 1-15ppm, Cr(lll) : 0.5 - 25 ppm, Chloride : 250-1500ppm, Sulphate : 10- 900ppm, pH : 2.5-6. 3. A process as claimed in claim 1, wherein the molarity of TEHA used is preferably 0.112M TEHA. 4. A process as claimed in claim 1, wherein the O/A ratio is preferably 1:1. 5. A process as claimed in claim 1, wherein the pH of the chromium containing waste solution is preferably 0.34. 6. A process as claimed in claim 1, wherein the steps [a] to [d] for chromium extraction are repeated one to four times to extract chromium leaving all the impurities in the raffinate. 7. A process as claimed in claim 1, wherein the loaded chromate on the solvent is stripped with 0.25 to 1.5M NaOH to get pure sodium chromate. 8. An improved process for the extraction of chromium present in waste solution substantially as herein described with reference to the foregoing examples.

Full Text

Field of the Invention
The present invention relates to an improved extraction process for the removal of hexavalent chromium from waste solutions. This invention particularly relates to the removal of chromium(VI) from industrial waste solution by solvent extraction process using tris 2-ethylhexyl amine under various experimental conditions such as number of contacts of organic solvent with acid containing aqueous feed, solvent concentration, time of extraction and organic to aqueous ratio [O/A ratio], back extracting the chromium from the chromium-loaded organic with dilute NaOH solution and producing pure chromate, which can be used in plating industries or can be sold as chromate salt. The raffinate can either be used as wash or rinse water or can be safely disposed off.
Background of the Invention:
Chromium Cr (VI) enters the environment through its use as a corrosion inhibitor, especially the chemical passivating component of stainless steels. It also emerges from various plant operations such as the chromic acid anhydride production, the chromium electroplating, cooling towers, and leather tanning wastes. The effluents from these operations can contain significant quantities of Cr (VI) endangering the quality of surface water. The presence of Cr (VI) even in trace quantities, known to be a strong oxidant, a mutagen and a carcinogen on human health, produces a toxic influence on aquatic life. Thus, removal of Cr (VI) from industrial wastewater to its permissible limit before its disposal into surface waters is of deep concern, as well as a challenging problem to the industry.
The total chromium maximum contaminant level (MCL) and the World Health Organization (WHO) level represent a guideline value that can be found in "Guidelines for Drinking-Water Quality, Vol. 1, pp. 80-85, WHO, Geneva, 1984" yielding a limit of 10.6 mol/L Cr (VI) in drinking water [J.A. Salvato, P.E. Dee, Environmental Engineering and Sanitation fourth ed (1992) Wiley New York]. However, according to the US EPA standards Cr(VI) contents in fresh water should
not exceed 4x10~4 mol/dm3 required for the protection of aquatic life [J.A. Salvato, P.E. Dee, Environmental Engineering and Sanitation fourth ed (1992) Wiley New York ]. Actually, Sadaoui et al. [Z. Sadaoui, C. Azoug, G. Charbit, F. Charbit, J. Chem. Eng. Jpn. 30 (1997) 799-805.] define the waste-water norm of Cr(VI) to be restricted strongly about (1.9x10.6 mol/dm3) that will become more severe in the near future. Excessive concentrations of chromium and other heavy metals in soluble form are thought of as a toxicant group which strictly reduces the activity of microorganisms.
It is well known that the reduced oxidation state of chromium, Cr(lll), is much less toxic and soluble at neutral pH values than its oxidized state of Cr(VI) [J.N. Veenstra, D.A. Sanders, A. Seyoung, J. Environ. Eng. June (1999) 522-531, P.N. Cheremisinoff, Y.H. Habib, Water Sewage Works July (1972) 73-86]. because chromium is basically present in the Cr(VI) form in aerobic biological systems. The deleterious effects of toxic Cr(VI) compounds on biological processes are complex, and generally depend on the influent matrix such as pH, the concentration of toxicant, solubility, the toxicity composition limit of other incorporated ions or molecules present in the bulk [Z. Lewandowski, Water Res. 19 (1985) 589-596]. Hence, emphasis has been given on the removal, recovery and reuse of chromium from such waste waters and effluents.
Various processes are available such as membrane technology, ion exchange, electro dialysis etc., A project of extensive equilibrium studies by Cr(VI) based on a nondispersive technique in hollow-fiber modules has been fulfilled by Ortiz and coworkers [M.I. Ortiz, B. Galan, J.A. Irabien, Ind. Eng. Chem. Res. 35 (1996) 1369-1377, A.I. Alonso, A.M. Urtiaga, J.A. Irabien, M.I. Ortiz, Chem. Eng. Sci. 49 (1994) 901-909], Sirkar and co-workers [C.H. Yun, R. Prasad, A.K. Guha, K.K. Sirkar, Ind. Eng. Chem. Res. 32 (1993) 1186-1195, Z.F. Yang, A.K. Guha, K.K. Sirkar, Ind. Eng. Chem. Res. 35 (1996) 4214-4220]. At last, significant progress has been made in the use of neutral crown ethers, not only for the extraction of a metallic
cation and its salts, but also for the extraction of more complex ion pairs appearing as a complex anion [Y. Takada, Top. Curr. Chem. 121 (1984) 1-38, C. Musikas, W.W. Schultz, in: J. Rydberg, C. Musikas, G. Choppin (Eds.), Principles and Practices of Solvent Extraction, Marcel Dekker, New York, 1992 (Chapter 11), A.H. Bond, M.L. Dietz, B. Chiarizia, Ind. Eng. Chem. Res. 39 (2000) 3442-3464].
Another conventional method of treating wastewater containing Cr(VI) includes its chemical reduction to convert Cr(VI) into the Cr(lll) cations by S02, Na2S03, or a ferrous ion compound (FeS04) as reducing agents [L.E. Eary, D. Rai, Environ. Sci. Technol. 22 (1988) 972-977, M.R. Unnithan, T.S. Anirudhan, Ind. Eng. Chem. Res. 40 (2001) 2693-2701], and the following precipitation of cations as Cr(OH)3. Permeable-reactive redox walls (reactive barriers) placed below the ground surface in the path of flowing groundwater could provide an alternative remediation approach for removing Cr(VI) chemicals from contaminated waters [D.W. Blowes, C.J. Ptacek, J.L. Jambor, Environ. Sci. Technol. 31 (1997) 3348-3357]. However, particularly frustrating aspects of the reduction method are the significant sludge production and the long term environmental consequences.
Most widely used process to treat chromate solutions is the reduction of Cr(VI) to Cr(lll) and precipitation of Cr(ll) as hydroxide. One such process is described in U.S. Pat. No. 3,371,034 which, illustrates a direct precipitation process utilizing great quantities of barium carbonate and aqueous solutions acidified with strong acids, such as nitric or hydrochloric acid, or their salts. Such procedures encountered difficulties in separation of the chromium solids which are precipitated from the liquid waste media, thereby necessitating the use of one or more settling tanks which can be required to handle the excessive amounts of sludge produced. Moreover, the requirement for large amounts of barium carbonate increases the amount of sludge generated.
Reference may be made to U.S. Pat. No. 3,869,386 which provides a method for the direct removal of hexavalent chromium by adding aqueous barium acetate. Izdebski states that this precipitation can be accomplished from neutral or slightly acid solutions in cases of precipitating both chromic acid and dichromates. But the disadvantage encountered with this process was that with the use of barium acetate the amount of acetate becomes more than is required. Thus, barium chromate so precipitated is discarded. No reuse of the chrome is contemplated by Izdebski.
Reference may also be made to U.S. application Ser. No. 917,506 which discloses a process for the purification of chrome-containing waste water by treating the waste water with a reducing agent suitable for converting Cr(VI) to Cr(lll) followed by raising the pH of the acidic solution containing Cr(lll) with caustic to form a precipitate of chromic hydroxide.
Reference may also be made to a Japanese Pat. No. 47-17697, which discloses the precipitation of chromic hydroxide from waste water by the process of (a) reducing hexavalent chromium in the waste water to trivalent chromium at a pH of about 2 using a reducing agent; (b) neutralizing with alkali at a pH of at least 8; and (c) adding magnesium ions (either prior to or after the reduction and neutralization steps) to precipitate the chromic hydroxide.
In another Japanese Pat. No. 71-16389, a process for treating chromium containing waste water comprises of reduction of Cr(VI) to Cr(lll) and addition of a mixture of aluminum sulfate and sodium hydroxide or calcium hydroxide to the resulting solution of reduced chromium in order to precipitate chromic hydroxide. But the generation of a huge amount of chromium oxide and alkaline solution are the associated disadvantages.
Further, methods such as micellar enhanced ultrafiltration have been applied to bind a heavy metal ion, especially Cr(VI) to a large species which is easily retained by an ultrafiltration membrane [Z. Sadaoui, C. Azoug, G. Charbit, F. Charbit, J. Chem. Eng. Jpn. 30 (1997) 799-805]. Recently, various natural and biological adsorbent materials, such as activated carbon, leaf mould, peat moss, silicagel, a natural clinoptilolite, and green algae [S.K. Ouki, R.D. Neufeld, J. Chem. Technol. Biotechnol. 70 (1997) 3-8, L. Monser, N. Adhoum, Sep. Purif. Technol. 26 (2002) 137-146, D.C. Sharma, C.F. Foster, Biores. Technol. 49 (1994) 31-40] have been reported to be potentially useful in the removal of Cr(VI) and heavy metals from aqueous solution through adsorption.
A comparison of low-cost sorbents and their applications have well documented by Polland et al. [S.J.J. Polland, G.D. Fowler, C.G. Sollars, R. Perry, Sci. Total Environ. 116 (1992) 31-52]. However, the adsorption process is generally costly.
Microorganisms are also effective at removing many heavy metals from waters including chromium. Cr(VI) is an essential cellular micronutrient, and can be actively and temporarily accumulated in the bacterial cells, as long as toxic concentrations are not reached. Additionally, many types of bacteria manufacture extracellular polymers, which also act to bind metals. Currently, to detoxify or remove various heavy metals activated sludge or other suspended growth processes have been used[F.B. Dilek, C.F. Gokcay, U. Yetis, Water Res. 32 (1998)303-312].
Nevertheless, amongst the possible alternatives to the treatment of industrial chromium plating baths or contaminated wastewaters, solvent extraction combining with membrane technology offers advantages [R. Molinari, E. Drioli, G. Pantano, Sep. Sci. Technol. 24 (1989) 1015-1032, M.E. Vallejo, F. Persin, C. Innocent, P. Sistat, G. Pourcelly, Sep. Purif. Technol. 21 (2000) 61-69]. However, the literature revealed very little insight relating to the validity of a generalized method for the
treatment of Cr(VI) effluents at high concentration levels (over than 1.92x10.3 mol/dm3) and the evaluation of the effectiveness regeneration capabilities for recycling or recovery purposes [B. Gupta, A. Deep, S.N. Tandon, Ind. Eng. Chem. Res. 41 (2002) 2948-2952, F.J. Algucil, A.G. Coedo, M.T. Donato, A.M. Sastre, Hydrometallurgy 61 (2001) 13-19].
In this context, Liquid-liquid extraction is found to be the most effective conventional method to be imperative to significantly reduce the Cr(VI) discharge levels and to promote recycling and reuse. Within the limited number of potential extractants, tributyl phosphate (TBP), tertiary (Alamine) or quaternary (Aliquat 336) amines, alkylphosphoric acid, and phosphine oxide (Cyanex 921, 923) and oxime derivatives (LIX 84) are of critical importance due to their favorable qualities such as coordination ability and stability of the complex strength. However, process considerations dealing with the competition between various solvent extraction methods for Cr(VI), such as an amine, TBP, ethers or alkylphosphoric acid solvents still remain a challenging problem since such systems show extremely nonideal behavior of a complex aggregation.
Therefore, the aim of the present invention is to obviate the disadvantages associated with the prior art. The present invention recites an improved process to treat an industrial waste solution containing 15-1500 ppm of hexavalent chromium for chromium removal by solvent extraction method using a very dilute solution of the solvent, wherein the loaded chromium is back extracted by dilute NaOH solution. More than 99% of chromium was removed and back extracted to give a pure and a concentrated chromate solution which can be used in electroplating industry or sold as chromate salt. The solvent after back extracting the chromium is washed with water and a very dilute acid and is reused for extraction experiment. This extraction and back extraction steps can be done continuously in a mixer settler unit which is very simple to operate.
The main aspect of the present invention is the production of pure and concentrated sodium chromate solution from the chromium containing waste solutions from various industries using tris 2 ethylhexyl amine(TEHA) leaving all the other impurities in the raffinate as compared to other available processes. The produced concentrated sodium chromate solution can be recycled back into the plating bath or can be sold as chromate salt. In this method a large volume of waste water can be treated with a small amount of dilute solution of the extratant mixture at room temperature i.e. 30°C.
Objects of the Invention
The main object of this invention is to provide an improved solvent extraction process and a solvent for the removal of hexavalent chromium present in waste solution from various industries.
Another object of the invention is to provide a process for the removal and subsequent recovery of chromium in an aqueous solution which generates no chromium related hazardous waste, as in the case of precipitation and other processes, thereby eliminating the need to store chromium waste in landfills and the subsequent liabilities attended thereto.
Yet another object of this invention is to provide a process for the removal and recovery of chromium from chromium containing industrial waste solution, which produces pure and concentrated solutions that can be recycled back into the plating bath in the electroplating industry.
Still another object of the invention to provide a process for the removal and recovery of chromium from chromium containing industrial waste solution which produces any usable chromate salt which can be sold in the market.
Summary of the invention
In the process of the present invention, the removal and-recovery of chromate from industrial waste waters containing chromium is carried out by a selective solvent extraction of the said waste waters by an organic solvent mixture containing tris-2-ethylhexylamine and iso-decanol diluted in kerosene. Under the optimized conditions an organic phase loaded with Cr(VI) is produces leaving the aqueous phase eventually free from Cr(VI). The chromium loaded organic phase is then treated with 0.5M NaOH to strip out the loaded chromium. The strip solution which contains chromate in a concentrated and pure form can be used by electroplating industry or various salts of chromates can be made and sold. The Cr free raffinate can either be used by the industry or can be disposed off. The solvent generated after the back extraction of chromium can be washed and reused in the extraction cycle again.
Accordingly, the present invention provides an improved process for the extraction of chromium present in waste solution, wherein the steps comprising:
[a] preparing an organic extractant by mixing 0.0112 M to 0.224 M tris 2 ethylhexyl amine [TEHA] in kerosene oil with 2% of iso-decanol as phase modifier diluted in kerosene;
[b] simultaneously, varying the pH of the chromium containing waste solution in the range of 0 to 10, wherein the concentration of the Cr(VI) in the solution ranges from 50ppm to 2000ppm ;
[c] mixing the organic extractant as obtained in step [a] with chromium containing waste solution as obtained in step [b] at an organic to aqueous ratio [O/A ratio] in the range of 1: 1 to 1:20 (vol/vol), followed by separating the chromium loaded organic from the raffinate by known methods and washing thereof;
[d] mixing the chromium loaded organic as obtained in step [c] with 0.5M NaOH solution at a liquid to liquid ratio in the range of 1:10 to 2:1 to get a concentrated and pure sodium chromium solution back in the aqueous phase,
Detailed description of the invention
In the extraction process of the present invention, the solution containing hexavalent chromium is subjected, preferably at room temperature (30°C), to a process for extraction by means of solvents. An organic solvent is preferably used, such as one which is a mixture of a dilute solution of tris 2 ethylhexyl amine and iso decanol in kerosene.
The process of the present invention can be described as a reaction for selective extraction between the chromate present in the industrial waste solutions/waters and tris 2 ethylhexyl amine as an extractant leaving other metals and impurities in the raffinate. Tris 2 ethylhexyl amine was procured from Fluka and was used as delivered by the producer, i.e., without further purification, however isodecanol was added to it as a phase modifier and was diluted with distilled kerosene(160 to 200°C) fraction to make the solvent of different concentration for this work since a concentrated or undiluted solvent is not required to treat such solutions. The average molecular weight of this solvent is 353.68 and the density is 0.817.
The mechanism for the extraction and stripping of the loaded chromium was also studied. Thus a logarithmic plot of the distribution coefficient of chromium extraction against the organic phase concentration and simultaneously slope analysis of the plot gives an idea of the requirement of number of moles of extractant for the extraction of one mole of the Cr (VI). The extraction of Cr (VI) by tris 2 ethylhexyl amine under the conditions used in this investigation can be represented by the general reaction described as
HCrO;aq + //; + (R3N)2org -> (tf3N)2 H.H2Cr04 1
And the stripping of loaded chromium(VI) with dilute solution of an alkali (OH") can be represented as
(R3N)2 H.HCrO, + OH~aq -> (R3N)2 + Cr04~ + H20 2
Thus, tris 2 ethylhexyl amine selectively extracted chromium(VI) leaving other metal in the raffinate. The extracted chromium was stripped with NaOH solution at 30°C without affecting the stripping efficiency and extracting properties of the extractant. The solvent is found suitable for the extraction and enrichment of chromium from waste solutions from plating solutions and rinse waters of electroplating industries and the stripped solution obtained could be used to make chromium trioxide, alkali salts or insoluble chromate salts that can be recycled back into the market place.
In an embodiment of the present invention, the waste rinse waters of electroplating industries used may be selected from composition in range of; Cr(VI) : 15-800ppm , Mg : 5 - 50ppm, Ca : 15 -100ppm, Fe : 1-15ppm, Cr(lll) : 0.5 -25ppm, Chloride: 250-1500ppm, Sulphate : 10- 900ppm, pH : 2.5-6.
In another embodiment of present invention, the used tris 2 ethylhexyl amine extractants may also be used without further purification and in different concentration.
In still another embodiment of the present invention the solvent extraction is carried out in one stage to four-stages to extract chromium leaving all the impurities in the raffinate
In still another embodiment of the present invention loaded chromate on the solvent is stripped with 0.1 to 1.0M NaOH to get pure sodium chromate.
In yet another embodiment of the present invention, the molarity of TE-HA used is preferably 0.112MTEHA.
In another embodiment of the present invention, the O/A ratio is preferably 1:1.
In a further embodiment of the present invention, the pH of the chromium containing waste solution is preferably 0.34.
EXAMPLES
The following examples are given by way of illustration of the present invention and therefore should not be construed to limit the scope of the present invention.
Example-1
50 ml of the Cr(VI) solution containing 615ppm of chromium was mixed with 50 ml of 0.112M tris 2 ethylhexyl amine at different pH of the aqueous phase(adjusted with HCI in the acidic range and NaOH in the basic range) under stirring for 5
minutes at room temperature (30 C). The results presented in Table-1 indicate that extraction of Cr(VI) increases from 0(no extraction) to 97.2% with the decrease in pH of the solution from 10 to 0.3 in a single contact at an organic to aqueous ratio [O/A ratio] of 1:10.
Table-1
(Table Removed)
Example-2
Aqueous solution containing 600ppm Cr(VI) was mixed with 0.112M tris-2-ethylhexyl amine (v/v) with 2% iso-decanol diluted in kerosene at different organic to aqueous ratio [O/A ratio] varying from 1:1 to 1:20. The results given in Table-2
indicate that 99.9% Cr(VI) is extracted at an 0/A ratio of 1:1 which decreases marginally to 98% at O/A ratio of 1:5. It becomes 85% at an O/A ratio of 1:10 and decreases to 47% at an O/A ratio of 1:20. Thus, TEHA is found to be a very effective reagent for the extraction and removal of hexavalent chromium from industrial effluents from various industries. McCabe-Thiele plot shows that at O/A of 1:9, 99.9% Cr(VI) can be extracted in two stages at a pH of 0.5.
Table-2
(Table Removed)
Example-3
Tris 2 ethylhexyl amine(TEHA) was initially studied for the extraction and separation of Cr(VI) present in waste solution dealing with chrome plating and other related industries spent acidic industrial effluents of steel and electroplating industry. It was then found very effective for Cr(VI) extraction hence 0.0112M to 0.224M of tris 2 ethylhexyl amine with 2% iso-decanol diluted with distilled fraction(160-200°C) of the kerosene was mixed with 50 ml aqueous feed solution containing 443 ppm of Cr(VI) for 5 minutes in a 250ml glass beaker with mechanical stirring at organic to aqueous ratio [O/A ratio] of 1:1 at room temperature. The extraction of Cr(VI) in the organic phase increased from 85.64 to 99.316% (Table-3). The solvent, tris 2 ethylhexyl amine can thus be used in a very dilute form for the extraction of Cr(VI) from electroplating waste streams or any other industrial waste solution containing Cr(VI).
Table-3
(Table Removed)
Example-4
50 ml of the aqueous feed containing 25ppm to 1995ppm Cr(VI) usually present in the electroplating effluents was mixed with 50 ml of 0.112M tris 2 ethylhexyl amine using a magnetic stirrer, at a pH of 0.5 and temperature of 30°C. Table-4 shows that the extraction of chromium decreases with the increase in Cr (VI) concentration in the aqueous feed from 99.8 to 24.7% in a single contact at an organic to aqueous ratio [O/A ratio] of 1: 10. Thus this solvent can function in a wide range of chromium (VI) in the waste streams/solutions.
Table-4
(Table Removed)
Example-5
Stripping of loaded Cr(VI) is the major concern in order to recover the loaded chromate as a value added product and to reuse the solvent back to the system, hence stripping of the loaded chromium from 0.112M TEHA containing 2.56g/L of Cr(VI) was performed with varying concentration of NaOH at organic to aqueous
ratio [O/A ratio] of 1:2 at 30°C. The results given in Table-5 indicate the percent stripping of Cr(VI) in a single contact. Thus with the increase in the cone of NaOH from 0.1 to 1% there is an increase in the percent Cr stripping from 54.6% to about 92.2%.
Table-5
(Table Removed)
Example-6
Stripping of loaded Cr (VI) from 0.112M TEHA was performed at different organic to aqueous ratio [O/A ratio] using 0.5M NaOH. Table 6 indicate that the percent stripping of chromium in a single contact increased from 24% to 98.8% with the variation of organic to aqueous ratio [O/A ratio] from 2:1 to 1:10. Based on these results McCabe-Thiele plot was drawn showing the requirement of three stages for complete recovery of loaded Cr at O/A ratio of 1:2.4 at 30°C.
Table-6
(Table Removed)
Advantages of the present invention:
1. The tris 2 ethylhexyl amine (TEHA) a tertiary amine is found to have a great affinity for hexavalent chromium.
2. The complete Cr(VI) is recovered leaving all the other impurities in the aqueous phase as found in plating and rinse water used in the electroplating industry.
3. No need of high temperature and capital intensive equipment as the extraction could be done at room temperature and stripping of the loaded acid could be done with NaOH solution.
4. This extractant tris 2 ethylhexyl amine could directly be reused after the stripping of Cr(VI) and washing with acid solution.
5. The recovered chromium which is pure and concentrated can be recycled back into the plating bath or can be used to produces either chromium trioxide, alkali salts or insoluble chromate salts that can be recycled back into the market place.

We claim:
1. An improved process for the extraction of chromium present in waste solution,
wherein the steps comprising:
[a] preparing an organic extractant by mixing 0.0112 M to 0.224 M tris 2 ethylhexyl amine [TEHA] in kerosene oil with 2% of iso-decanol as phase modifier diluted in kerosene;
[b] simultaneously, varying the pH of the chromium containing waste solution in the range of 0 to 10, wherein the concentration of the Cr(VI) in the solution ranges from 50ppm to 2000ppm ;
[c] mixing the organic extractant as obtained in step [a] with chromium containing waste solution as obtained in step [b] at an organic to aqueous ratio [O/A ratio] in the range of 1: 1 to 1:20 (vol/vol), followed by separating the chromium loaded organic from the raffinate by known methods and washing thereof;
[d] mixing the chromium loaded organic as obtained in step [c] with 0.5M NaOH solution at a liquid to liquid ratio in the range of 1:10 to 2:1 to get a concentrated and pure sodium chromium solution back in the aqueous phase.
2. A process as claimed in claim 1, wherein the waste solution containing
chromium has a composition in range of; Cr(VI) : 15-800ppm , Mg : 5 -
50ppm, Ca : 15 -100ppm, Fe : 1-15ppm, Cr(lll) : 0.5 - 25 ppm, Chloride :
250-1500ppm, Sulphate : 10- 900ppm, pH : 2.5-6.
3. A process as claimed in claim 1, wherein the molarity of TEHA used is preferably 0.112M TEHA.
4. A process as claimed in claim 1, wherein the O/A ratio is preferably 1:1.
5. A process as claimed in claim 1, wherein the pH of the chromium containing waste solution is preferably 0.34.
6. A process as claimed in claim 1, wherein the steps [a] to [d] for chromium extraction are repeated one to four times to extract chromium leaving all the impurities in the raffinate.
7. A process as claimed in claim 1, wherein the loaded chromate on the solvent is stripped with 0.25 to 1.5M NaOH to get pure sodium chromate.
8. An improved process for the extraction of chromium present in waste solution substantially as herein described with reference to the foregoing examples.

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Patent Number

270476

Indian Patent Application Number

315/DEL/2009

PG Journal Number

01/2016

Publication Date

01-Jan-2016

Grant Date

23-Dec-2015

Date of Filing

18-Feb-2009

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL, RESEARCH

Applicant Address

ANUSANDHAN BHAWAN, FAFI MARG, NEW DELHI-110 001, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	ARCHANA AGRAWAL	NATIONAL METALLURGICAL LABORATORY, JAMSHEDPUR-831007 JHARKHAND, INDIA
2	CHANDANA PAL	NATIONAL METALLURGICAL LABORATORY, JAMSHEDPUR-831007 JHARKHAND, INDIA
3	KAMALA KANTA SAHU	NATIONAL METALLURGICAL LABORATORY, JAMSHEDPUR-831007 JHARKHAND, INDIA

PCT International Classification Number

C23C

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA