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FIELD OF THE INVENTION
The present invention relates to a method of removal of hexavalent chromium from the effluents of chromating and chromium plating industry.
More specifically the present invention relates to removal of hexavalent chromium from the said waste by simultaneous reduction and removal of hexavalent chromium.
BACKGROUND OF THE INVENTION
Chromium metal solution is widely used in industries such as in tanning, electroplating and metal finishing.
The effluents from the said industries are contaminated with hexavalent chromium.

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Hexavalent chromium compounds are highly toxic and can contaminate ground water and affect human and other living species of nature. The maximum tolerable concentration of hexavalent chromium compound as effluent is 0.05 mg/L as per EPA regulation.
For removal of hexavalent chromium compounds chemical precipitation treatment is widely used. Beside chemical treatment other ion exchange, activated carbon adsorption and electro chemical processes are also known in practice.
Electrochemical reduction of Hexa-valent Chromium methods are reported by:
3. A. Radwan, A. El-Kayer, H.A. Farag, G.H. Sedahmed. The role of mass
transfer in electrolytic reduction of hexavalent chromium at gas evolving
rotating cylinder electrodes in Journal of Applied Electrochemistry, 22
(1992) 116 and
4. D. Golub and Y. Oren, Removal of chromium from aqueous solutions by
treatment with porous carbon electrodes in Journal of applied
Electrochemistry 19(1989) 311.

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Reduction by Ion exchange and activated carbon adsorption methods are reported by:
1. A.R. Bowers, C.P. Haung. Activated carbon process for treatment of
automotive application for chromium waste waters containing hexavalent
Chromium, in Prog. Water Techno., 12 (1980) 629.
2. K. Muthukumaran, M. Bal Subramanian, Chromium removal from aqueous
wastes by reduction with Ferrous ion in Met. Finish, 93 (1995) 11.
3. K. Fetyanose, P. Fachantidis, Cr(III) and Cr (VI) precipitation in
environmental samples in Toxical Environ. Chem. 64(1997) 197.
Cr(VI) to Cr(IIII) by different reducing agents like FeS, FeSO4,SO2, NaHSO3, Na2S2O5 etc. methods are reported by:
1. M.Erdem, H. Altondogam, A. Ozer, F. TTAI/4men, Cr(VI) Reduction in aqueous solutions by using synthetic iron sulphide in 'Environ.Technol, 22 (2001) 22".

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2. T.E. Higgins and V. E. Sater, Combined removal of Cr, Cd and Ni from wastes
in "Environ, Prog., 3(1984) 12".
3. C.H. Hang and M.H. Wu. Chromium removal by activated carbon, in "J. water
Pollution Control Federation, 47 (1975) 2437.
4. Control and Treatment Technology for the metal Finishing Industry in US,
EPA (1980).
Reduction of Cr (VI) to Cr (III) Zn dust, aluminums and pure iron and iron scraps in a pH range 2-3 methods are reported by:
1. J.P. Gloud, The kinetics of hexavalent chromium reduction by metallic iron,
water Res. in 1691982) 871.
2. A.R. Bowers, C.A. Ortes and R.J. Cardozo, Hexavalent chromium reduction
using scrap iron filings in Pollution Control Metal Finishing, 37(1986).

All the above treatment methods involve with additions of additional chemical compounds to the waste which themselves contaminate ground water beside the fact that all those processes are expensive due to high power consumption. Moreover zinc and aluminium dust used in acid medium for test purpose further add to the contamination problem.

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The prior waste treatment of hexavalent chromium compounds therefore called for long time for an alternative eco-friendly treatment of Cr+6 waste management.
The proposed invention is aimed to solve the above mentioned disadvantages of prior art.
DESCRIPTION OF THE INVENTION
One of the objective of the invention is to simultaneous reduction and removal of Cr+6 compounds from waste of chrome plating industry through in situ reaction of solid ferrous sulfate to the waste solution at a pH range of 2-3.
Another objective of the invention is to recover trivalent chromium ions on treatment of the said waste solution.
A further objective of the invention is to treat the waste solution of Cr+6 by two phase reduction of Cr+6 to Cr+3 and 90% removal of trivalent chromium along with ferric ions.

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According to the invention there is provided a method of recovery of hexavalent chromium from waste of chromium plating and chromating industry comprising the steps of collecting a predetermined amount of chrome waste in the order of 4000-5000 ppm of chromium in 1000 liter of chrome waste on chromium plating, chromating containing hexavalent chromium and placing it in a reaction tank; adding solid ferrous sulphate on the said chrome waste in the reaction tank and stirring the whole mixture for 1 to 2 hours for completion of reaction of dissociated ferrous ion from ferrous sulphate to be oxidized to ferric ion as ferric sulphate and hexavalent chromium to be converted to trivalent chromium ions as chromium sulphate maintaining pH of the mixture at 2-3 through addition of acid, both the sulphates being precipitated out as co-precipitates and iron coagulation; pumping the mixture to a filter tank to filter out the precipitates in a pressure filter; the filtrate from the filter tank being treated with 20 % alkali in an alkali reaction tank and pumping out the whole mixture on completion of reaction and filtering out the precipitate in another filtering tank through a pressure filter and; draining out the filtrate after testing of hexavalent chromium content.
The removal of Cr+6 ion is carried through by adding ferrous sulphate in the waste in a reaction tank.

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Ferrous ions of ferrous sulphate solution are oxidized to ferric ions and hexavalent chromium ions are reduced to trivalent chromium ions. The resulted ferric sulphate and chromium sulphate are allowed to settle, the mixture is pumped to a filter tank and filtered. Chromium of the filtered cake is estimated and it is observed that 90 % of total chromium is removed from the waste along with iron.
Reduction of hexavalent chromiumin in chromic acid by ferrous ion of ferrous sulphate is an oxidation and reduction reaction which passes through following steps:
(i) Ferrous sulphate is dissociated in situ in chromic acid solution and generates ferrous and sulphate ions in solution. This unstable ferrous ion quickly oxidizes to ferric ion taking electron from hexavalent chromium and converts the same to trivalent chromium. As a result of which, chromium is converted chromium sulphate and the ferric ion is ferric sulphate. In high acidic condition (PH: 2-3), both the sulphates are precipitated out as to co precipitate and iron coagulation.
(ii) Fe+2 ions along with Cr+6 ions in solution interact where the following reaction takes place.


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(iii) No further addition of acid is required to perform the oxidation and reduction reaction because ferrous sulfate is easily dissociated in acid medium and produces sulphate and ferrous ion which impart acidity to the solution.
(iv) It is observed that chromium and ferric ion are removed based on iron coagulation with ferrous sulfate, which also reduces the hexavalent chromium to trivalent state. Studies on laboratory scale, containing 5000 ppm of chromium indicates that ferrous suphate is capable of reducing 99 % of chroimium with 90% removal of both trivalent chromium and ferric ion as a coagulate and co precipitate at PH: 2-3. No alkali addition is required.
(v) After removal of 90 % of trivalent chromium and ferric ions at PH: 2-3 addition of 20 % of NaOH to use the filtrate is required. It is observed that the remaining ferric and trivalent chromium ions are also removed as insoluble hydroxides.

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The reaction behaviour of NaoH with filtrate containing Cr+3 and Fe+3 ions are represented as follows:
Cr3+ and Fe 3+ ions react with OH-1 of NaOH where the following reaction takes place.

(vi) PH and hexavalent chromium contents of discharge water are 9.5 -10.50 and 0.01 - 0.05 mg/L respectively.
The operation of the inventive features of the invention is described with reference to the following accompanying drawings in which: -
Figure 1 represents a schematic diagram showing the steps of operation of the waste after chromium plating.

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In the figure numeral (1) represent a reaction tank in which 1000 liter of chrome waste of 4000-5000 ppm are collected and placed. 20 kg of solid ferrous sulphate (grade 97 %) is added in the tank and the whole content of the tank (1) is stirred constantly by a stirrer (5). After two hours of stirring the precipitates are allowed to settle in the tank and the whole mixture is pumped to the filter tank (2) by a pump (6) and is filtered and filtrate is collected in alkali reaction tank (3) wherein the filtrate is treated with 20 % alkali. The whole mixture of the tanks (3) after completion of reaction is pumped by a pump (7) to a filter tank (4) wherein the mixture is filtered to separate the precipitate and the filtrate is released to the drain after testing of hexavalent chromium. PH and hexavalent chromium contents of discharge water are to be maintained at 9.5 -10.50 and 0.01 - 0.05 mg/L respectively. If the content of hexavalent chromium content is found above the said limit, the filtrate is again retreated in the reaction tank (1).
It has been observed that in the filtered precipitate in the reaction tank (1) chromium is estimated to be found as 90 % of total chromium removal/recovery from the waste along with iron and chromium removal/recovery is found as approximately 10 % of the waste in the alkali reaction tank. The reaction tank (1), and filter tank (2) are provided with acid resistant coatings.

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In one illustrated embodiment the capacity of reaction tank (1) and filter tank (2) are maintained as 14.8 m3and 1.5 m3 respectively.
The invention is illustrated with bench optimization work through laboratory scale experiment as follows:
Experiments were conducted to find the optimum amount of ferrous sulphate needed to reduce chromium (IV) of a synthetic solution.
Accurately two liter of 5000 ppm of freshly prepared chromic acid (PH: 2-3) is taken for the experiment and placed in a 5 liter of glass breaker. As per stoichiometric requirement, 100 grams of ferrous sulphate (industrial grade 97%) is added slowly with constant stirring and allow to standing for 2 hours fter completion of addition, maintaining the PH of 2-3.
After completion of the reaction, this solution is filtered through Buckner funnel with a pump; the residue is collected and dried on an oven for chromium estimation.

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The filtrate is again treated with 5 % of NaOH and filtered through Buckner funnel; residue is dried and tested for chromium content. The addition of NaOH raises the PH value up to 7.5-8.5, where both the trivalent chromium and ferric ions were precipitated out as a hydroxides.
The filtrate is analyzed for hexavalent chromium by using UV-Visible spectrometer (Model UV5704SS, electronics Corporation of India). Diphenylcabazide is used to develop color. Absorbance at 540 nm is measured. Total chromium is estimated by the method of ICP-AES. (Spectra Analytical, German).
In this method, the ratio of iron to chromium is maintained at 3:1 for simultaneous reduction and removal of hexavalent chromium. Accordingly calculation is also made for Pilot Plant trial.
As the average chromium contents in the local chrome plating wastes are 4000-5000 ppm, laboratory experiments were carried out with a synthetic solution containing 5000 PPM of chromium.
Though the invention as herein described and illustrated with a specific embodiment, the invention should not be read and construed in a restrictive manner as various adaptations, alterations and modifications are possible within the scope and limit of the appended defined claims.

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WE CLAIM

1. A method of recovery of hexavalent chromium from waste of chromium plating and chromating industry comprising the steps:
- of collecting a predetermined amount of chrome waste in the order
of 4000-5000 ppm of chromium in 1000 liter of chrome waste on
chromium plating, chromating containing hexavalent chromium and
placing it in a reaction tank;
- adding solid ferrous sulphate on the said chrome waste in the
reaction tank and stirring the whole mixture for 1 to 2 hours for
completion of reaction of dissociated ferrous ion from ferrous
sulphate to be oxidized to ferric ion as ferric sulphate and hexavalent
chromium to be converted to trivalent chromium ions as chromium
sulphate by maintaining pH of the mixture at 2-3 through addition of
acid, both the sulphates being precipitated out as co-precipitates and
iron coagulation; pumping the mixture to a filter tank to filter out the
precipitates in a pressure filter;

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- the filtrate from the filter tank being treated with 20 % alkali in an alkali
reaction tank and pumping out the whole mixture on completion of
reaction and filtering out the precipitate in another filtering tank through
a pressure filter and;
- draining out the filtrate after testing of hexavalent chromium content.
2. A method of recovery of hexavalent chromium as claimed in claim 1,
wherein ferrous sulphate reduce upto 99 % of chromium in the chrome
waste with 90 % removal of both trivalent chromium and ferric ion as a
coagulate and co-precipitate at PH 2-3 in the reaction chamber and
approximately 10 % of trivalent chromium as insoluble hydroxides along
with ferric hydroxides in the alkali reaction tank.
3. A method of recovery of hexavalent chromium as claimed in claims 1 and 2,
wherein PH and hexavalent chromium contents of discharge water are
maintained within the ranges of 9.5 - 10.50 and 0.01 - 0.05 mg/L
respectively.

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4. A method of recovery of hexavalent chromium as claimed in the preceding claims wherein reduction of hexavalent chromiumin in chromic acid by ferrous ion of ferrous sulphate is an oxidation and reduction reaction which passes through steps of: ferrous sulphate being dissociated in situ in chromic acid solution and generating ferrous and sulphate ions in solution, this unstable ferrous ion being quickly oxidized to ferric ion taking electron from hexavalent chromium and converting the same to trivalent chromium, as a result of which, chromium being converted chromium sulphate and the ferric ion being ferric sulphate, in high acidic condition (PH: 2-3), both the sulphates being precipitated out as to co precipitate and iron coagulation, Fe+2 ions along with Cr+6 ions in solution interacting where the following reaction takes place.
FeSO4=Fe+2+SO4-2
Fe+2=Fe+3+3e
Cr+6+3e=Cr+3
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3Fe2++Cr+6=3Fe3++Cr+3

5. A method of recovery of hexavalent chromium as claimed in claim 1, wherein the acid used for conversion of Cr+6 to Cr+3 is chromic acid.

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6. A method of recovery of hexavalent chromium as claimed in claim 1, wherein alkali used with the filtrate from the first filter tank is NaOH and the precipitates in the second filter tank are removed as ferric and chromic insoluble hydroxides according to reactions of Cr+3 and Fe+3 with OH -1 of NaoH as follows:
6
7. A method of recovery of hexavalent chromium as claimed in the preceeding
claims wherein chromium and ferric ions are removed based on iron
coagulation with ferrous sulphate, which also reduces the hexavalent
chromium to trivalent state.
8. A method of recovery of hexavalent chromium from waste of chromium
plating and chromating industry as herein described and illustrated.

This invention relates to a method of recovery of hexavalent chromium from waste of chromium plating and chromating industry comprising the steps of collecting 1000 liter chrome waste of 4000-5000 ppm of chromium on chromium plating, chromating containing hexavalent chromium and placing it in a reaction tank; adding solid ferrous sulphate on the said chrome waste in the reaction tank and stirring the whole mixture for 1 to 2 hours for completion of reaction of dissociated ferrous ion from ferrous sulphate to be oxidized to ferric ion as ferric sulphate and hexavalent chromium to be converted to trivalent chromium ions as chromium sulphate maintaining pH of the mixture at 2-3 through addition of acid, both the sulphates being precipitated out as co-precipitates and iron coagulation; pumping the mixture to a filter tank to filter out the precipitates in a pressure filter; the filtrate from the filter tank being treated with 20 % alkali in an alkali reaction tank and pumping out the whole mixture on completion of reaction and filtering out the precipitate in another filtering tank through a pressure filter and; draining out the filtrate after testing of hexavalent chromium content.