Title of Invention

ARSENIC RECOVERY FROM ITS CONTAMINATED WASTE BY DIRECT PRECIPITATION.

Abstract The present invention relates to a process for recovery of arsenic as arsenic sulphide from waste to provide the waste substantially free of arsenic. The process comprises the steps of treating the solid waste with HCL, boiling the acid treated filtrate of the waste with NaHS03 until SO2 evolution ceases and providing the solution preferably as 8-9 (M) with respect to HCL. The solution is then saturated with H2S Gas in closed vessel to obtain crude AS2S3 which is treated with NaOH solution followed by acidification with HCI and passing H2S gas to reprecipitate the As2S3. The present invention also relates to a process for producing a treatment solution suitable for treating arsenic contaminated water and a process for producing solid crystalline hydrous ferric oxide suitable as an arsenic adsorbent and use thereof in treatment of arsenic contamination.
Full Text FIELD OF THE INVENTION
The present invention relates to a method of recovery of arsenic from arsenic rich waste and to provide the waste substantially free of arsenic and in particular to an environmental friendly process for recovery of arsenic from arsenic rich waste as arsenic sulphide (HI). The invention also relates to provisions for treatment of arsenic contaminated water and preparation of crystalline hydrous ferric oxide for arsenic adsorption,
BACKGROUND ART
Arsenic contamination (above 10 ppb) in ground water is a serious global problem to the public health. The contamination is highest in Bengal basin of the river Ganga. The worst suffered countries with the problem are specifically India especially West Bengal and Bangladesh, improper disposal of arsenic-rich waste will increase soil arsenic level above the maximum allowed levels.
Due to environmental and health problems associated with arsenic contamination various methods for removal of arsenic from waste has been developed to favour reducing arsenic from the contaminated ground water below permissible level. The method usually used include coagulation-filtration using alum and ferric salts, adsorption using activated alumina and granular ferric oxide, nano-filtration, reverse osmosis and ion exchange. Most widely accepted industrial methods include coagulation -filtration and surface adsorption. Attempts have also been made to manage waste by stabilization through cementation and leaching techniques.
Recovery of arsenic from waste in the form of sodium arsenate, ferric arsenate, zinc arsenate and arsenic trioxide is known.
US 5344479 and US 4610723 disclose recovery of arsenic as arsenic (HI) sulphide. However, such methods are not eco-friendly and are therefore not utilized/use for waste management.

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OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide for a method for recovery of arsenic as arsenic (III) sulphide from waste to provide the same substantially free of arsenic contamination which would be simple and eco-friendly and thereby can be advantageously used for waste management.
Another object of the present invention is directed to providing for recovery of arsenic from waste material especially solid waste as arsenic (III) sulphide following a simple and ecofriendly method.
Yet further object of the present invention is to provide for a simple and cost-effective waste management which would enable environmentally safe disposal of waste and effective utilization of the treatment solution /its derivatives in the process of treatment of arsenic contaminated water.
Yet further object of the present invention is directed to favour arsenic recovery from waste wherein the solution after the arsenic recovery can advantageously be used for reducing arsenic content from contaminated water by precipitation of the metal hydroxide.
Yet further object of the present invention is to generate crystalline hydrous ferric oxide which would have improved capacity for arsenic adsorption.
SUMMARY OF THE INVENTION
Thus according to basic aspect of the present invention there is provided a
process for recovery of arsenic as arsenic sulphide from waste to provide the
waste substantially free of arsenic comprising:
i) treating the solid waste with HCL;
ii) boiling the acid treated filtrate of the waste with NaHSO3 until SO2
evolution ceases and providing the solution preferably as 8-9 (M) with
respect to HCL ; iii) saturating the solution with H2S Gas in closed vessel to obtain crude
As2S3 ;

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iv) treating the crude As2S3 with NaOH splution followed by acidification with HCI and passing H2S gas to reprecipitate the AS2S3 .
According to a preferred aspect of the present invention in the above process for the recovery of arsenic as arsenic sulphide in step iii above the unreacted H2S is adsorbed in NaOH solution .
In accordance with a preferred aspect the process for recovery of arsenic as
arsenic sulphide from waste to provide the waste substantially free of arsenic
comprise :
i) treating the solid waste with HCL;
ii) boiling the acid treated filtrate of the waste with NaHSO3 until SO2
evolution ceases and providing the solution preferably as 8-9 (M) with
respect to HCL ; iii) saturating the solution with H2S Gas in closed vessel to obtain crude
As2S3; iv) treating the crude AS2S3 with NaOH solution followed by acidification
with HCl and passing H2S gas to reprecipitate the AS2S3 ;and v) filtering the precipitated AS2S3 through sintered glass crucible washing
with carbon disulphide to free it from.sulfur and drying at 120-130°C.
According to another aspect of the present, invention there is provided a
process for generating a treatment solution for treatment of arsenic containing
water comprising :
i) treating solid waste with HCL;
ii) boiling the acid treated filtrate of the waste with NaHSO3 until SO2
evolution ceases and providing the solution preferably as 8-9 (M) with
respect to HCL ; iii) saturating the solution with H2S Gas in closed vessel to obtain crude
As2S3 ; iv) subjecting the filtrate from step (iii) above to treatment with NaOCI
solution to precipitate metal hydroxide ; v) dissolving the precipitate of metal hydroxide in HCI to obtain the
treatment solution suitable for treating arsenic contaminated water.

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In accordance with yet further aspect of the present invention it is possible to convert the acidic ferric solution of step (v) above to solid crystalline hydrous ferric oxide suitable as arsenic adsorbent.
DETAILED DESCRIPTION OF THE INVENTION
The above process of the invention therefore provides for a simple, eco-friendly process for the treatment of waste to substantially free the same of arsenic. Importantly, in the above process the acid concentration 8.0 to 9.0 (M) with respect to HCI is required for selective and quantitative separation of arsenic sulphide from the solution. This would be clearly apparent from the variation in the percentage purity of the arsenic (III) sulphide based on the variation in the acid conc, as identified by way of the invention and noted hereunder in Table I.

Below the acid concentration of 8-9 (M), antimony with some other metal sulphide is found to be precipitated with arsenic sulphide. Also by increasing the acid cone. Beyond 9(M) ,it is found not possible to improve further the purity of the product. The above clearly and sufficiently bring out the preferred selective range of 8 to 9.0(M) of the acid cone, desired in the above process of the invention. Moreover, the solution left after recovery of the arsenic as arsenic sulphide in the above process can be reused for water treatment after treatment of the same with sodium hypochloride solution in weakly basic medium.

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The basic materials required for the above treatment thus include cone. HCI, NaHSO3, H2S (g), NaOCI, NaOH and carbon disulphide.
The waste solution preferably acidified is boiled with 20ml of 5% NaHSC>3 solution and then treated with H2S (g).
The precipitate formed was separated from the liquid by simple filtering followed by washing with H2S-saturated water. The filtrate with washing was made weakly basic and treated with NaOCI solution. Here, sulphide becomes sulfate on boiling and metal hydroxide precipitate appears. The precipitate was next dissolved in HCL while the solution Was reused for water treatment to reduce the arsenic content below toxic level. The volume of solution to be added 10-15 ml per litre of water to reduce arsenic Arsenic sulphide precipitate was next dissolved in hot NaOH, solution, made acidic again to 8.0-9.0 M with respect to HCI and reprecipitated by H2S(g) as mentioned earlier followed by filtration through a known weight sintered glass crucible and washing with carbon disulphide, drying in an air oven at 120°C for 3 hours. Importantly, in the above process the step of treatment with NaOCI in weakly alkaline medium avoids environmental hazards converting sulphide to sulfate and hence, the process is eco-friendly. Also, the step of the process involving the reduction of arsenic(V) or arsenic(lll) by boiling with sodium bisulfite solution(10%) and then, the acid concentration 8.0 to 9.0 M with respect to HCI before adding H2S(g) are found to be critical for effective removal of arsenic as arsenic sulphide.
The crystalline hydrous ferric oxide can also be prepared from the acidic ferric solution. This solid crystalline hydrous ferric oxide was found to be useful for arsenic (III) adsorption. Adsorption capacity was found to be 60-65mg/g at pH 6.0 to 7.0. Thus, synthetic crystalline hydrous ferric oxide could be used as an adsorbent for arsenic removal filters.

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The details of the invention, its objects and advantages are explained hereunder in greater detail in relation to non-limiting exemplary illustrations as hereunder:
Examples
Material:
Hazardous arsenic-rich waste collected for the study were of five types as detailed hereunder in TABLE II

Method for Recovery of Arsenic involving.the process of the invention from the above varieties of wastes was carried out as detailed hereunder:
20.0 gm of each dried waste was dissolved in 50ml of 6(M) HCL, evaporated nearly to pasty mass, then extracted with water and filtered. The filtrate was acidified with HCl and boiled with 1.0g NaHSO3 till SO2 evolution ceased. It was made 8.0 to 9.0 (M) with respect to HCI, which was then saturated with H2S(g) at hot condition into a closed vessel, fitted with an outlet. The open end of the outlet was kept into a 0.1 (M) NaOH solution to adsorbed

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unreacted H2S (g). Then, the crude, As2S3 was dissolved in 25ml of 1 (M) NaOH solution. After that this solution was acidified with 9(M) HCI solution. Now H2S gas passed through the solution in hot condition and the yellow AS2S3 is reprecipitated. The yellow precipitate of As2S3 was filtered through a known weighed sintered glass crucible; washed with carbon disulphide to make free from sulfur and finally dried at 120-130°C. The dried product was reweigh to have the amount of As2S3. The filtrate and H2S adsorber were boiled separately with NaOCI solution to convert unreacted H2S to sulfate in alkaline medium.
The recovery of arsenic as arsenic (III) sulphide from the above waste varieties are represented in Table III hereunder:

Finally, arsenic in the filtrate was analyzed. Crystalline hydrous ferric oxide (CHFO) was prepared from the iron-rich filtrate solution. It was found that arsenic adsorption capacity of this CHFO is comparable to the product obtained from a fresh ferric chloride solution.
After recovery of As2S3 precipitate, the filtrate of volume 500 ml was analyzed for arsenic and found arsenic content was always less than 0.05mg/l. Thus the solution can be disposed off on surface soil safely, since the average arsenic content in surface soil is in the range of 5.5 to 13 mg/Kg.

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Furthermore, as disclosed above the waste variants such as FASW, FCSW and GFOW contain high amount of iron in'the filtrate. The filtrate, after converting dissolved sulphide into sulfate by boiling with NaOCI in alkaline medium, gives precipitate of Fe(OH)3. After drying of the precipitate, it was used systematically in removing arsenic from contaminated water. It was to be found that this Fe(OH)3 has good arsenic sorption capacity, which is comparable to the Fe(OH)3 prepared from pure ferric Chloride solution at pH of 6.0 to 7.0.

9 WE CLAIM:
1. A process for recovery of arsenic as arsenic sulphide from waste to provide
the waste substantially free of arsenic comprising:
i) treating the solid waste with HCL;
ii) boiling the acid treated filtrate of the waste with NaHSO3 until SO2
evolution ceases and providing the solution preferably as 8-9 (M) with
respect to HCL ; iii) saturating the solution with H2S Gas in closed vessel to obtain crude
As2S3; iv) treating the crude AS2S3 with NaOH solution followed by acidification
with HCI and passing H2S gas to reprecipitate the AS2S3 .
2. A process for recovery of arsenic as arsenic sulphide from waste as
claimed in claim 1 wherein in step iii above the unreacted H2S is adsorbed in
NaOH solution .
3; A process for recovery of arsenic as arsenic sulphide from waste as
claimed in anyone of claims 1 to 2 comprising :
i) treating the solid waste with HCL;
ii) boiling the acid treated filtrate of the waste with NaHS03 until SO2
evolution ceases and providing the solution preferably as 8-9 (M) with
respect to HCL ; iii) saturating the solution with H2S Gas in closed vessel to obtain crude
As2S3; iv) treating the crude As2S3 with NaOH solution followed by acidification
with HCI and passing H2S gas to reprecipitate the AS2S3 ;and v) filtering the precipitated As2S3 through sintered glass crucible washing
with carbon disulphtde to free it from sulfur and drying at 120-130°C.

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4. A process as claimed in anyone of claims 1 to 3 wherein said waste solution preferably acidified is boiled with 5% NaHSO3 solution and then treated with H2S (g).
5. A process for recovery of arsenic from waste as claimed in anyone of claims 1 to 4 wherein the percentage recovery is above 95%.
6.A process for the recovery of arsenic from waste as claimed in anyone of claims 1 to 5 wherein after recovery of the arsenic the arsenic content in the waste was less than 0.05mg/l.
7. A process for the recovery of arsenic from waste as claimed in anyone of claims 1 to 6 wherein the sodium hydroxide solution used is 0.1 (M) NaOH.
8.A process for producing a treatment solution suitable for treating arsenic
contaminated water comprising :
i) treating solid waste with HCL;
ii) boiling the acid treated filtrate of the waste with NaHSO3 until SO2
evolution ceases and providing the solution preferably as 8-9 (M) with
respect to HCL ; iii) saturating the solution with H2S Gas in closed vessel to obtain crude
AS2S3; iv) subjecting the filtrate from step (iii) above to treatment with NaOCI
solution to precipitate metal hydroxide ;
v) dissolving the precipitate of metal hydroxide in HCI to obtain the treatment solution suitable for treating arsenic contaminated water.
9. A process for producing solid crystalline hydrous ferric oxide suitable as an
arsenic adsorbent comprising :
i) treating solid waste with HCL;
ii) boiling the acid treated filtrate of the waste with NaHSO3 until SO2
evolution ceases and providing the solution preferably as 8-9 (M) with
respect to HCL ;

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iii) saturating the solution with H2S Gas in closed vessel to obtain crude As2S3; iv) subjecting the filtrate from step (iii) above to treatment with NaOCl solution to
precipitate metal hydroxide ;
v) dissolving the precipitate of metal hydroxide in HCI;
vii) converting the acidic ferric solution of step (v) above to solid crystalline hydrous ferric oxide suitable as arsenic adsorbent.
10, A method of treatment of arsenic contaminated water comprising the step of adding
the treatment solution as prepared in claim 8 to the water to be treated.
11, A method of treatment of arsenic contaminated water as claimed in claim 10
wherein the volume of solution to be added is 10-15 ml per litre of water to reduce
arsenic
12, A method as claimed in claimed 9 wherein the crystalline hydrous ferric oxide thus
formed has an adsorption capacity of 60-65 mg/g at pH 6.0 to 7.0.
13, A method of adsorption of arsenic from arsenic contaminated source comprising the
step of using the crystalline hydrous ferric oxide as produced in claims 8 or 12.
14, A process for recovery of arsenic as arsenic sulphide from waste, a treatment
solution and crystalline hydrous ferric oxide and use thereof in treatment of arsenic
contamination substantially as hereindescribed and illustrated with reference to the
accompanying examples.
The present invention relates to a process for recovery of arsenic as arsenic sulphide from waste to provide the waste substantially free of arsenic. The process comprises the steps of treating the solid waste with HCL, boiling the acid treated filtrate of the waste with NaHS03 until SO2 evolution ceases and providing the solution preferably as 8-9 (M) with respect to HCL. The solution is then saturated with H2S Gas in closed vessel to obtain crude AS2S3 which is treated with NaOH solution followed by acidification with HCI and passing H2S gas to reprecipitate the As2S3. The present invention also relates to a process for producing a treatment solution suitable for treating arsenic contaminated water and a process for producing solid crystalline hydrous ferric oxide suitable as an arsenic adsorbent and use thereof in treatment of arsenic contamination.


Documents:

00090-kol-2004-abstract.pdf

00090-kol-2004-claims.pdf

00090-kol-2004-correspondence.pdf

00090-kol-2004-description(complete).pdf

00090-kol-2004-form-1.pdf

00090-kol-2004-form-18.pdf

00090-kol-2004-form-2.pdf

00090-kol-2004-form-3.pdf

00090-kol-2004-letters patent.pdf

00090-kol-2004-p.a.pdf


Patent Number 206913
Indian Patent Application Number 90/KOL/2004
PG Journal Number 20/2007
Publication Date 18-May-2007
Grant Date 16-May-2007
Date of Filing 04-Mar-2004
Name of Patentee PRINCIPAL, PRESIDENCY COLLEGE.
Applicant Address PRINCIPAL, PRESIDENCY COLLEGE, AN INSTITUTION UNDER THE DEPT, OF HIGHER EDUCATION, MINISTRY OF EDUCATION, GOVERNMENT OF WEST BENGAL, (B)OF 86/1, COLLEGE STREET,KOLKATA-700 073,(C) AN INDIAN NATIONAL
Inventors:
# Inventor's Name Inventor's Address
1 GHOSH,UDAY CHAND DEPARTMENT FO CHEMISTRY,PRESIDENCY COLLEGE,86/1,COLLEGE STREET,KOLKATA-700073
2 HOSSAIN,JAKIR DEPARTMENT FO CHEMISTRY,PRESIDENCY COLLEGE,86/1,COLLEGE STREET,KOLKATA-700073
PCT International Classification Number CO1B27/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA