DESC:FIELD OF THE INVENTION
The present invention relates to recovery of selenium from industrial effluents in a suitable form having useful applications in photovoltaic and electronic products.

BACKGROUND OF THE INVENTION
Selenium is a precious non-metal having advantageous photovoltaic and conductive properties. Thus, selenium has applications in photovoltaic and electronic products. Selenium nanoparticles are a breakthrough in nanotechnology as they have applications in diagnostics and therapeutics. The unique features of such nanoparticles are the small size, high surface area, surface charge, surface chemistry, solubility and multi-functionality make them remarkably unique. They have huge potential as drug carriers in the delivery of the therapeutic molecules.

Selenium is available only in trace amounts in nature. Selenium is also found in soluble forms in waste generated by various industries, specifically copper smelters. Selenium is often extracted from anode slimes obtained during the copper electro refining process.

Several techniques have been reported to recover selenium from copper anode slimes. The commonly used techniques are pyro metallurgical process, in which anode slimes are subjected to sulphating and oxidation roasting, where selenium is oxidized and volatilized as SeO2. Another process employing soda roasting-alkaline leaching was proposed by Lu et al to recover selenium, SeO42.

Alternative hydrometallurgical processes were reported, which are relatively environment friendly. Under hydrometallurgical process, leaching agents were used for extracting selenium from copper anode slimes.

Yang et al. proposed a method to leach copper anode slimes with microwave assistance, and hydrogen peroxide to recover selenium. However, this method is not viable on an industrial scale.

US4163046A reports a process for recovery of selenium from anode slimes containing selenium, where copper anode slimes were subjected to a caustic oxidative leach to convert selenium values to the hexavalent form. Another patent, US3933635 recites a method for removing soluble selenium from acidic wastewater, by treating the wastewater with a metallic reducing agent. All the reported methods are applicable to recovery of selenium from anode slimes of copper smelters.

However, no methods have been reported for recovery of selenium metal in the form of colloid from dilute sulphuric acid solution generated during gas cleaning of sulphur dioxide containing gases generated in copper smelters.

Copper smelting processes generally include four steps of roasting, smelting, concentrating, and fire refining. Roasting is performed in copper smelters prior to charging reverberatory furnaces. In roasting, charge material of copper concentrate mixed with a siliceous flux (often a low-grade copper ore) is heated in air to about 650°C (1200°F), eliminating 20 to 50 percent of the sulphur as sulphur dioxide (SO2). Portions of impurities such as antimony, arsenic, and lead are driven off, and some iron is converted to iron oxide. In the smelting process, either hot calcine from the roaster or raw unroasted concentrate is melted with siliceous flux in a smelting furnace to produce copper matte. Converting produces blister copper by eliminating the remaining iron and sulphur present in the matte. Iron sulphide is oxidized to form iron oxide (FeO) and SO2. A final air blast ("final blow") oxidizes the copper sulphide to SO2, and blister copper forms, containing 98 to 99 percent coppers. The blister copper is removed from the converter for subsequent refining. The SO2 produced throughout the operation is vented as off gas into a collection system.

It has been noted that selenium if released into the wastewater and effluents, are toxic to nature. The toxicity mainly comes from related to the chemical speciation that selenium undergoes under changing redox conditions. Amongst its oxidation states, Se oxyanions, namely selenite (Se[IV])and selenate (Se[VI]), are water-soluble, bioavailable and toxic.

In contrast elemental selenium is less toxic and in view of the enormous potential of nano selenium in therapeutic, diagnostic, photovoltaic and electronic applications it would be of great value to extract selenium in a desired form from effluents and waste waters.

The present invention aims to extract elemental selenium from the off gas in the colloidal form.

OBJECT OF THE INVENTION
It is an object of the present invention to discloses a novel and inventive process of extraction of colloidal selenium.
It is the object of the invention to extract colloidal selenium directly from effluents coming out of copper smelters.
It is yet another object of the present invention to recover selenium as colloidal solution from dilute sulphuric acid.
It is yet another object of the present invention to optimise the process of selenium extraction thereby making it a viable treatment for wastewater for all trace elements.
Another object of the invention is to provide a process for simultaneous recovery and purification of colloidal selenium in an economical manner.

SUMMARY OF THE INVENTION
The present invention relates to a novel and inventive process of colloidal selenium extraction from effluents. The present invention further relates to an economical process of extracting selenium in colloid form from waste effluents of copper smelters.

A process for the recovery of selenium from effluents comprising the steps of collecting the SO2 rich off gas from the cleaning section by way of wash solution; allowing the suspended solids in the wash solution to settle for a period of about 2 hours; oxidation of the wash solution with hydrogen peroxide or sodium chlorite; separating the solids from the oxidised solution by centrifugation at a temperature range of 30°C to 50°C and at a speed of 800 to 1200 rpm; and purifying the obtained selenium by multiple washings with pure water

Additional objects and advantages of the invention will be set forth in the detailed description that follows, and in part will be obvious from the description, or maybe learned by practice of the invention. The objects and advantages of the invention will be attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

DESCRIPTION OF THE INVENTION
In describing and claiming the invention, the following terminology will be used in accordance with the definitions set forth below. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. As used herein, each of the following terms has the meaning associated with it in this section. Specific and preferred values listed below for individual process parameters, substituents, and ranges are for illustration only; they do not exclude other defined values or other values falling within the preferred defined ranges.

As used herein, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise.

The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful and is not intended to exclude other embodiments from the scope of the invention.

As used herein, the terms “comprising” “including,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to.

In copper smelters, selenium dioxide present in off-gas is reduced by sulphur dioxide gas to elemental selenium in gas cleaning scrubbers. During this process, dilute sulphuric acid solution is generated in gas cleaning systems of copper smelters. Such generated dilute sulphuric acid solution consists of trace elements, such as lead as lead sulphate (PbSO4), copper and selenium as copper selenides (Cu2Se), and also colloidal selenium (100 ppm to 200 ppm). The colloidal selenium is a metallic selenium of very fine particles.

The present invention discloses a process of extraction of selenium in a colloidal form, from effluents of copper smelters.
The process of extraction of selenium comprises the steps of taking the off gas from the copper smelter to refining the sulphur dioxide and also concentrating selenium from the wastewater in the colloidal form for re use.

The off-gas contains trace concentrations gases of SO3, As2O3, SeO2, ZnO, HgO, CdO, chlorides and fluorides etc along with about 10 – 15% SO2 gas rest being CO2, O2 and NOx gases. These gases are contacted with water at room temperature due to which condensation takes place in primary jet scrubbing, the gases are dissolved in water and forms water soluble compounds as per the following reactions:
SO3 + H2O = H2SO4 (l)
As2O3 + H2O = H3AsO3 (l)
ZnO + H2SO4 = ZnSO4 (l)
CuO + H2SO4 = CuSO4 (l)
PbO + H2SO4 = PbSO4 (s)
SeO2 + H2O = H2SeO3 (l)
H2SeO3 + SO2 = Se + H2SO4 + ½ O2
HgO + H2SO4 = HgSO4
HgSO4 + Se = HgSe
Chlorides form HCl
Fluorides form HF
But it is observed that the vapour pressure of elemental selenium is very high as compared to the rest of the elements like As, Zn, Cu, Pb, Hg etc. Hence, the fine sized selenium goes with the on-ward flowing off-gas and ultimately gets collected in the Secondary Reverse Jet Scrubber and Wet electrostatic precipitators.
The present invention discloses a method wherein the colloidal form of selenium is concentrated at each step and at the same time impurities such as arsenic is removed in a timely manner by oxidation.

In particular, the present invention is directed towards a process for the recovery of selenium from effluents comprising the steps of :
Collecting the SO2 rich offgas from the cleaning section by way of wash solution;
Allowing the suspended solids in the wash solution to settle for a period of about 2 hours;
Oxidation of the wash solution with hydrogen peroxide or sodium chlorite;
Separating the solids from the oxidised solution by centrifugation at a temperature in the range of 30°C to 50°C and at a speed in the range of 800- 1000 rpm;
Purifying the obtained selenium by washing with pure water to remove water soluble impurities, thereby producing highly pure colloidal selenium solution.

In an embodiment in accordance with the present invention, the effluent is resulting from the process of copper smelting.

In an embodiment in accordance with the present invention the offgas arising from the effluents is rich in elemental selenium.

In an embodiment in accordance with the present invention, the process for the recovery of selenium includes the offgas being put through a reverse jet scrubber and then a wet electrostatic precipitator in the cleaning section. The cleaning section of a copper smelter has multiple systems through which the off gas is made to go through. This includes a reverse jet scrubber and then a wet electrostatic precipitator. The final solution generated by the gas cleaning section is the wash solution.

In an embodiment of the process of the present invention the offgas is passed in a counter current direction to the water jet in the secondary reverse jet scrubber at a pressure of 15 to 20 psi, temperature of 50? C and at an efficiency of more than 90%, to remove particulate matter.
In an embodiment of the process of the present invention, the wet electrostatic precipitator removes the fine particles from the offgas at room temperature.

In accordance the process of the present invention, the gas stream from the cleaning section is made to undergo sedimentation for a time period in the range 1 to 3 hours.

In accordance the process of the present invention, the arsenic is completely removed by making it undergo oxidation either with hydrogen peroxide or sodium chlorite.

In accordance to the present invention, the solids containing selenium are separated from the oxidised solution by centrifugation of the solution, at a temperature in the range of 30°C to 50°C and at a speed in the range of 800- 1000 rpm.
With the sequential steps carried out as per the present invention, the selenium is concentrated in the solids that settle down on sedimentation. The colloidal selenium is recovered in high purity, after removing all trace impurities.

In an embodiment of the process in accordance to the present invention, said effluent is dilute sulphuric acid solution generated during gas cleaning of sulphur dioxide containing gases generated in copper smelters.

In an embodiment of the process in accordance to the present invention, said sedimentation is carried out by means of gravity.

In an embodiment of the process in accordance to the present invention, said oxidation is carried out with sodium chlorite. The oxidation of tetravalent arsenic to pentavalent arsenic is carried out after sedimentation.

In an embodiment of the process in accordance to the present invention the oxidation reaction of arsenic is represented by the chemical reaction as shown below.

In an embodiment the time required for completion of reaction can be instantaneous.
In another embodiment, it can also be completed in less than 60 minutes.
H3AsO3 + 1/2O2 ? H3AsO4
In yet another embodiment, the amount of sodium chlorite added during oxidation of arsenic is based on the concentration of trivalent arsenic present in the aqueous solution. The molar ratio of sodium chlorite to arsenious acid is 1:2 at room temperature and normal atmospheric pressure. The reactions involved are as below:

NaClO2 + 2H3AsO3 ? NaCl + 2H3AsO4

The method of present application further comprising of purification of colloidal selenium to reduce impurities.

The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to a person skilled in the art, the invention should be construed to include everything within the scope of the disclosure.

The present invention relates to the liquid discharge also called the wash solution, collected at wet electrostatic precipitator and secondary reverse jet scrubber, does not contain other elements apart from selenium. Since, turbidity is observed due to the presence of colloidal or nano-sized selenium particles, the turbidity can be a good parameter to find out the concentration of the selenium present in the liquid discharge also. The turbidity data is also useful to determine the efficiency of collection of the colloidal selenium extracted in the present invention.

Example 1:
5 litres of aqueous solution was collected from the collection system of a copper smelter. The collected solution was allowed to undergo sedimentation for a time period of 1 hour. The sulphuric acid concentration of the aqueous solutions has been determined and presented in Table 1. The sedimentation efficiency of the aqueous solution was confirmed by turbidity measurements as shown in Table 2. The turbidity was measured in terms of NTU (nephelometric turbidity unit).
Oxidation of the wash solution with hydrogen peroxide was done. Then the solids from the oxidised solution were separated by centrifugation at a temperature of 35°C and at a speed of 1000 rpm. Finally, the extracted colloidal selenium was purified by continuous washing with pure water.
The chemical analysis of the aqueous solution was measured at different stages as shown in Table 3. The chemical analysis of the colloidal solids collected at different stages is presented in Table 4. The chemical analysis of the colloidal selenium from the aqueous solution is presented in Table 5. The particle size analysis of the colloidal selenium was carried out by Zeta analyser. The size range of colloidal selenium varies from 150 nm to 230 nm. The concentration of colloidal selenium recovered was 0.025 wt% (0.25 grams per litre of aqueous solution).

Table 1: Sulphuric acid concentration of the aqueous solutions used for recovery of colloidal selenium
S.No Secondary Scrubber, gpl Wet Electrostatic Precipitator, gpl
Sample 1 41.65 120.05
Sample2 34.3 124.95
Sample 3 39.2 122.5
Sample 4 34.3 124.95

Table 2: Sedimentation and separation efficiency of colloidal selenium from aqueous solutions
Step Sample Turbidity, NTU
Step 1 Collected Wash Solution 28.9 NTU
Step 2 Solution after settling for 1 hour 19.0 NTU
Step 3 Aqueous solution after oxidation 18.0 NTU
Step 4 Aqueous solution after colloidal selenium separation 3.0 NTU

Table 3: Chemical analysis of aqueous solutions in different steps of colloidal selenium recovery
Step Sample As, ppm Se, ppm
Step 1 Collected Wash Solution 593 49.5
Step 2 Solution after settling for 1 hour 589 48
Step 3 Aqueous solution after oxidation 585 48
Step 4 Aqueous solution after colloidal selenium separation 584 2

Table 4: Chemical analysis of solids collected during different steps of colloidal selenium recovery
Step Solid Sample As, ppm Se, ppm
Step 2 Solids collected during sedimentation 20.62 257
Step 3 Solids collected after oxidation and filtration through G4 crucible 5.47 61.3
Step 4 Colloidal selenium recovered and completely dried ND 284

Table 5: Chemical analysis of the colloidal selenium recovered
S.No Element Concentration
1 Arsenic Non-detectable
2 Chloride 0.5 ppm
3 Fluoride 0.01 ppm
4 Iron 5 ppm
5 Sulphur 15 ppm
5 Selenium Remaining

Example 2:
10 litres of aqueous solution was collected from the collection system of a copper smelter. The collected solution was allowed to undergo sedimentation for 2 hours. The sulphuric acid concentration of the aqueous solutions was determined and presented in Table 6. The sedimentation efficiency of the aqueous solution was confirmed by turbidity measurements as shown in Table 7. The turbidity was measured in terms of NTU (nephelometric turbidity unit).
Oxidation of the wash solution with sodium chlorite was done.
The chemical analysis of the aqueous solution was measured at different stages as shown in Table 8. The chemical analysis of the solids collected in different stages is presented in Table 9. The chemical analysis of the colloidal selenium from the aqueous solution is presented in Table 10.
The solids were separated from the oxidised solution by centrifugation at a temperature of 30°C to 50°C and at a speed of 900 rpm. Post centrifugation, the purification was carried out by flushing with pure water to remove all the water soluble impurities.
The particle size analysis of the colloidal selenium was carried out by Zeta analyser. The size range of colloidal selenium varies from 180 nm to 240 nm. The concentration of colloidal selenium recovered was 0.020 wt% (0.20 grams per litre of aqueous solution).

Table 6: Sulphuric acid concentration in aqueous solutions collected for recovery of colloidal selenium
S.No Secondary Scrubber, gpl Wet Electrostatic Precipitator, gpl
Sample 1 26.95 117.6
Sample2 39.2 115.15
Sample 3 44.1 117.6

Table 7: Sedimentation and separation efficiency of colloidal selenium from aqueous solutions
Step Sample Turbidity, NTU
Step 1 Collected Wash Solution 30.1 NTU
Step 2 Solution after settling for 1 hour 22.8 NTU
Step 3 Aqueous solution after oxidation 20.9 NTU
Step 4 Aqueous solution after colloidal selenium separation 3.7 NTU

Table 8: Chemical analysis of aqueous solutions in different steps of colloidal selenium recovery
Step Sample As, ppm Se, ppm
Step 1 Collected Wash Solution 578 54.6
Step 2 Solution after settling for 1 hour 564 46
Step 3 Aqueous solution after oxidation 546 43
Step 4 Aqueous solution after colloidal selenium separation 567 1.7

Table 9: Chemical analysis of solids collected during different steps of colloidal selenium recovery
Step Solid Sample As, ppm Se, ppm
Step 2 Solids collected during sedimentation 25.9 356
Step 3 Solids collected after oxidation and filtration through G4 crucible 6.57 312
Step 4 Colloidal selenium recovered and completely dried ND 298

Table 10: Chemical analysis of the colloidal selenium recovered by complete drying
S.No Element Concentration
1 Arsenic Non-detectable
2 Chloride 0.3 ppm
3 Fluoride 0.03 ppm
4 Iron 9.6 ppm
5 Sulphur 18.2 ppm
5 Selenium Remaining

The advantages of recovering colloidal selenium from this process are numerous. The claimed process is an inexpensive alternative to produce commercial colloidal selenium as compared to other methods of producing colloidal selenium. The colloidal selenium produced through the proposed method does not increase any kind of pollution to air, water or soil. Therefore, it is a major addition to green chemistry due to energy efficiency, ability to stop toxic waste generation by extraction of selenium and using no hazardous substance in the process. The claimed process thus results in the efficient recovery of selenium from effluents and waste discharge of plants thereby reducing the selenium toxicity in the environment.
,CLAIMS:

1. A process for the recovery of selenium from effluents comprising the steps of :

Collecting the SO2 rich offgas from the cleaning section by way of wash solution;
Allowing the suspended solids in the wash solution to settle for a period of about 2 hours;
Oxidation of the wash solution with hydrogen peroxide or sodium chlorite;
Separating the solids from the oxidised solution by centrifugation at a temperature in the range of 30°C to 50°C and at a speed in the range of 800- 1000 rpm;
Purifying the obtained selenium by washing with pure water to remove water soluble impurities, thereby producing highly pure colloidal selenium solution.

2. A process for the recovery of selenium from effluents wherein the effluent is resulting from the process of copper smelting.
3. A process for the recovery of selenium from effluents wherein the offgas is rich in elemental selenium.
4. A process for the recovery of selenium wherein the offgas has been put through a secondary reverse jet scrubber and then a wet electrostatic precipitator in the cleaning section.
5. A process as claimed in claim 4, wherein the offgas is passed in a counter current direction to the water jet in the secondary reverse jet scrubber at a pressure of 15 to 20 psi, temperature of 50? C and at an efficiency of more than 90%, to remove particulate matter.
6. A process as claimed in claim 4, wherein the wet electrostatic precipitator removes the fine particles from the offgas at room temperature.
7. The process as claimed in claim 1, wherein the gas stream from the cleaning section is made to undergo sedimentation.
8. The process as claimed in claim 1, wherein the selenium is concentrated by the sequential process steps in the solids that settle down on sedimentation.
9. The process as claimed in claim 1, wherein the arsenic is removed completely by the end of the process steps by strategic oxidation by hydrogen peroxide or sodium chlorite.

10. The process as claimed in claim 1, wherein colloidal selenium is recovered in high purity, after removing all trace impurities.