FORM 2
THE PATENTS ACT 1970
(39 of 1970)
AND
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10 and rulel3)
1. TITLE OF THE INVENTION:
"PROCESS FOR RECOVERY OF SILVER CHLORIDE AND MERCURY SULPHIDE NANOPARTICLES FROM CHEMICAL OXYGEN DEMAND TEST
WASTEWATER"
2. APPLICANT:
(1) (a) NAME: Enviro Tech Limited
(b) NATIONALITY: Indian
(c) ADDRESS: Plot No. 2413/14, GIDC Industrial Estate, Ankleshwar,
Gujarat 393002, India.
(2) (a) NAME: Shroff S.R. Rotary Institute Of Chemical Technology (SRICT)
(b) NATIONALITY: Indian
(c) ADDRESS: Block No: 402, Ankleshwar-Valia Road, Tal:Valia,
Dist: Bharuch, Gujarat, 393135, India.
3. PREAMBLE TO THE DESCRIPTION:
The following specification particularly describes the invention and the manner in which it is to be performed:

Technical filed:
The invention relates to a process for recovering silver chloride and mercury sulphide nanoparticles from a laboratory wastewater. More particularly, the invention relates to a process for recovering silver chloride and mercury sulphide nanoparticles from chemical oxygen demand (COD) test wastewater.
Background and prior art:
The chemical oxygen demand (COD) test is commonly used to indirectly measure the amount of organic compounds in water. The COD test is performed to evaluate the biochemical oxygen requirement of organic compounds present in water to convert into CO2 and H2O. The COD test is applied to determine the amount of organic pollutants found in surface and ground waters thereby making the COD test method useful for the measurement of quality of water in any given sample that includes domestic and industrial samples. COD test method also determines the amount of oxygen required for the chemical oxidation of pollutants and also used to measure the amount of oxygen in water consumed for chemical oxidation of organic pollutants.
The basic principle of COD test is to completely oxidize the organic matter present in the waste water. There are different reagents added to the test solution viz., potassium dichromate is added as oxidizing agent, silver sulphate is added as a catalyst and mercury sulphate is added to reduce the interference of chloride ions. The method includes refluxing the sample with a known amount of potassium dichromate in presence of sulfuric acid, silver sulphate and mercury sulphate. After digestion of the test sample, the sample is analyzed and titrated with ferrous ammonium sulphate to measure unreacted dichromate. The dichromate consumed by the sample is considered as equivalent to the amount of oxygen required to oxidize the organic matter. The wastewater generated from this test generally contains silver sulphate, mercury chloride and unreacted mercury sulphate. However, the prior art fails to address the recovery of these

essential and expensive reagents from COD waste water, which can be recycled and reused for any other industrial purpose.
Therefore, the present inventors felt a need to address this issue by recovering the silver and mercury from the COD test waste water in the present invention, for which protection is sought.
Summary of the invention:
In line with the above objective, the present invention provides a cost-effective process for recovering silver chloride and mercury sulphide nanoparticles from a laboratory wastewater, specifically, chemical oxygen demand (COD) test waste water which comprises;
a) Treating the COD test waste water with sodium chloride solution to precipitate silver chloride; and
b) Isolating the silver chloride precipitate prior to treating the silver removed waste water with sodium sulphide solution to precipitate mercury sulphide.
Brief Description of drawing:
Figure 1 describes flow sheet of the process of the recovery of the silver chloride
and mercury sulphide nanoparticles from chemical oxygen demand test
wastewater
Figure 2 depicts SEM image of silver chloride nanparticles
Figure 3 depicts SEM image of mercury sulphide nanoparticles.
Detailed description of the invention:
The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be fully understood and appreciated.
The COD waste water generally comprises silver sulphate, mercury chloride and mercury sulphate in addition to other metal ions such as iron and chromium.

It is observed by the present inventors that one liter of COD waste water comprises an amount of 3100 mg/1 of silver and 2600 mg/1 of mercury along with iron 500 mg/1 and 450 mg/1 chromium.
According to the process of the present invention, silver present in wastewater is precipitated in the first stage as fine grain powder in the form of silver chloride spherical nanoparticles by employing reagent A. After recovering silver chloride, the mercury present in the left over waste water is precipitated in the form of mercury sulphide nanoparticles, by employing reagent B.
In a preferred embodiment, the reagent A is sodium chloride and the reagent B is sodium sulphide, both are easily available and cheaper reagents.
Accordingly, the present invention provides a cost-effective process for recovering silver chloride and mercury sulphide nanoparticles from a laboratory wastewater specifically chemical oxygen demand (COD) test wastewater which comprises;
a) Treating the COD test waste water with sodium chloride solution to precipitate silver chloride; and
b) Isolating the silver chloride precipitate prior to treating the silver removed waste water with sodium sulphide solution to precipitate mercury sulphide.
In a preferred embodiment, COD waste water is treated with IN solution of sodium chloride in the first stage to precipitate silver chloride in the form of nanoparticles. Accordingly, 1 N solution of NaCl is prepared and 1 liter of COD waste water is treated with 40 ml of IN NaCl solution to recover 99.9 % of silver from the COD waste water in the form of silver chloride.
After the recovery of the precipitated silver chloride by filtration, the left over COD waste water is treated with Sodium sulphide. Accordingly, 1 N solution of Sodium sulphide is prepared and used to precipitate mercury sulphide in the form

of nanoparticles. 4.7 gm of mercury sulphide per litre of COD waste water was recovered which is equivalent to 99.9% mercury sulphide removal from wastewater.
The treatment of waste water according to the invention is conducted at ambient temperature.
In a specific embodiment, the treatment of both the stages is conducted at ambient temperature. Ambient temperature for the purpose the present invention may be considered as the temperature in the range of 25 to 35°C.
In a specific embodiment, the treatment is conducted under stirring at 100 to 500 rpm.
The size of the silver chloride and mercury sulphide nanoparticles are in the range of 50-80nm and 30-50 nm, respectively. The size of the particles is confirmed with SEM images of the particles as shown in figures 2 and 3 respectively.
Other features and embodiments of the invention will become apparent by the following examples which are given for illustration of the invention rather than limiting its intended scope. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art.
Examples
Example: 1
Silver chloride recovery
To recover silver chloride, 1 N solution of NaCl was prepared and 40 ml of NaCl was added to 1 liter of waste water under stirring at 200 rpm at room temperature to recover 99.9 % of silver in the form of silver chloride nano particles.

The amount of NaCl present in 40ml of IN solution was about 2.32 gm, which is used to recover 99.9% silver present in one liter of COD wastewater. The chemical equation is shown below:
Ag2S04 + 2NaCl ► 2AgCl + Na2S04
Silver Sulphate Sodium chloride Silver chloride Sodium sulphate
By mole balance 1 mole of silver sulphate reacts with 2 moles of sodium chloride and produces 2 moles of silver chloride. 1 mole of silver sulphate has 311.79 gm and will react with 2 moles of sodium chloride (116.88gm) and will produce (286 gm) 2 moles of silver chloride. This is a theoretical calculation.
In the current experiment, 2.32 gm of NaCl was used in one liter of wastewater and 2.6 gm of silver chloride was recovered. It is quite in agreement with theoretical analysis.
Mercury sulphide recovery
After recovery of silver chloride, the wastewater was treated for mercury sulphide recovery. Sodium sulphide of 1 N solution was prepared and used to precipitate mercury sulphide. 40 ml of 1 N solution of sodium sulphide was used to precipitate mercury sulphide in one liter of wastewaterunder stirring at 200 rpm at room temperature to recover 99.9 % of mercury in the form of mercury sulphide nano particles.
The amount of sodium sulphide present in 40 ml of 1 N solution is about 3.2gm. The treatment of 1 liter COD waste water with 40 ml of 1 N solution of sodium sulphide results in precipitation of 4.7 gm of mercury sulphide and by quantitative analysis it is 99.9 % recovery.
4.7gm of mercury sulphide per litre was recovered that results in 99.9% mercury removal from wastewater.

HgS04 + Na2S ► HgS + Na2S04
Mercury sulphate Sodium sulphide Mercury sulphide Sodium sulphate
According to chemical reaction 1 mole of mercury sulphate reacts with 1 mole of sodium sulphide and equal mole of mercury sulphide is generated. 1 mole of mercury sulphate carries 296 gm of mercury sulphate and makes 232 gm of mercury sulphide with 78 gm of sodium sulphide in equal moles. The experimental result are in agreement with theoretical prediction.
Recovery of both Reagents was carried out at room temperature under 200 rpm stirring. The pH of the wastewater was observed 0.01 before and after recovery of reagents. COD of the wastewater was 3000 mg/1 before recovery and 1300 mg/1 after recovery of reagents.
After precipitation, both the reagents were washed with distilled water to remove excess acidic reagent and dried separately in air and further dried in oven at 60 °C for 2 hrs.
Industrial advantage of the present invention
The process for recovery of silver chloride and mercury sulphide from chemical oxygen demand test generated wastewater to recover silver chloride and mercury sulphide is simple and cost-effective. The recovery of these two valuable laboratory reagents provides great industrial advantage as both the reagents are expensive and can be put to reuse.

We claim,
1. A cost-effective process for recovering silver chloride and mercury
sulphide nanoparticles from a laboratory wastewater specifically chemical
oxygen demand (COD) test wastewater comprising;
c) Treating the COD test waste water with sodium chloride solution to precipitate silver chloride; and
d) Isolating the silver chloride prior to treating the silver removed waste water with sodium sulphide solution to precipitate mercury sulphide.

2. The process according to claim 1, wherein, the treatment of waste water in step a) is conducted at ambient temperature.
3. The process according to claim 1, wherein, the treatment of waste water in step a) is conducted at ambient temperature.
4. The process according to claim 1, wherein, thetreatment is conducted under stirring at 100 to 500 rpm.
5. The process according to claim 1, wherein, the silver chloride is obtained in spherical nanoparticles.
6. The process according to claim 1, wherein, the mercury sulphide is obtained in nanoparticles.
7. The process according to claim 1, wherein, the size of the silver chloride and mercury sulphide nanoparticles are in the range of 50-80nm and 30-50 nm, respectively.
8. The process according to claim 1, wherein, the solution of sodium chloride and sodium sulphide is used as IN solution.

9. The process according to claim 1, wherein, the pH of the waste water is the same before and after recovery of sodium chloride and mercury sulphide.
10. The process according to any one of the preceding claims, COD test waste water comprises silver sulphate, mercury chloride and mercury sulphate.