Process for removal of selenium from water by dithionite ions

Abstract

A method for efficiently removal of oxidised selenium from liquid, such as FGD wastewater. The method involves adding a non-iron-based reducing agent (e.g. sodium dithionite) and preferably Fe(II) ions to the liquid at a pH of above 7.5 or 8 and precipitating elemental selenium from the liquid.

Claims

1. A method of reducing the selenium content of wastewater, the method comprising contacting the wastewater with Fe(II) ions, and a dithionite ion (M), at a pH of from 7.5 to 10.5 and in the presence of lime; and heating the wastewater to a temperature of from 60 to 95° C., wherein the wastewater comprises selenite and selenate, wherein the wastewater further comprises from 4000 to 8000 ppm of sulfate, wherein the wastewater is purged with nitrogen.

2. The method according to claim 1, wherein sufficient Fe(II) ions are used so that the initial ratio Fe(II) ions to selenium in the water (Fe(II):Se) is in excess of 100.

3. The method according to claim 1, wherein the ratio of Fe(II):M is from 0.5 to 1.5.

4. The method according to claim 1, comprising heating the water to a temperature of from 70 to 95° C.

5. The method according to claim 1, comprising contacting the water with a pH modifier, wherein the pH modifier comprises calcium hydroxide.

6. The method according to claim 5, wherein the ratio of pH modifier to Fe(II) ions is from 0.6 to 1.4.

7. The method according to claim 1, comprising contacting the wastewater with a lime slurry.

8. The method according to claim 1, wherein the wastewater is flue gas desulfurization (FGD) wastewater.

9. The method according to claim 1, wherein the dithionite ion is added to the wastewater as a salt in an amount of 800 ppm.

10. The method according to claim 9, wherein the lime is added in an amount of 600 ppm, and the Fe(II) ions are added to the wastewater as a salt in an amount of 720 ppm.

11. A method of removing selenium from water, the method comprising contacting the water with a dithionite ion at a pH of 7.5 to 10.5; and heating the water to a temperature of from 70 to 95° C., wherein the wastewater comprises selenite and selenate, and from 4000 to 8000 ppm of sulfate, wherein the wastewater is purged with nitrogen.

12. The method according to claim 11, comprising contacting the water with Fe(II) ions.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The removal of selenium (VI) by reduction-precipitation to elemental selenium (0) with ferrous hydroxide has been disclosed in the literature.

(2) Selenocyanate, the major species present in oil refinery wastewaters at concentrations of 0.2-1 ppm, can easily be oxidized to selenite using oxidizing agent and pH control. An example of a process for the removal of selenocyanate ions is shown in US2012/0024798. Once most of the selenium is converted to selenite, it is then treated with ferric sulfate to bind the selenite and the resulting complex is subsequently precipitated with specialty Nalco metal removal product Nalmet 1689. This process is described in U.S. Pat. No. 8,282,835, which is owned by the current applicant. The disclosed process of treating refinery wastewater for selenium removal is currently practiced at a few refineries. It is believed that the major reason for success in this type of wastewater is the opportunity to carefully control the selenium speciation and the lower matrix interferences such as lower sulfate concentrations (<10 ppm) and hardness (CaCO.sub.3=400 ppm). This method of removal typically reduces residual total selenium concentrations to 50 ppb. Unfortunately, we have found (both in the lab and in a field trial) that this process is not effective on power plant FGD wastewater.

(3) Power plant wastewater generated from wet FGD scrubbers contains a significant amount of selenium (1 to 5 ppm) depending on the type of coal used for power generation. The selenium species present in the FGD wastewater are selenite (i.e. selenium (IV)) and selenate (i.e. selenium (VI)) at ratios that typically include selenate at more than 50%. Selenate is found to be the most stable, most soluble, and, consequently, the most difficult to remove. We believe, although we neither wish not intend to be bound by any theory, that one of the issues in the treatment of FGD wastewater is in relation to matrix interferences, which are significantly higher compared to oil refinery wastewater. For example, in FGD wastewater the concentration of sulfate ranges from 4000-8000 ppm (believed to be the major competing anion) and the hardness (i.e. CaCO.sub.3 concentration) is measured around 5,300 ppm.

(4) Selenium removal from FGD wastewater of coal fired power plants to meet the stringent environmental discharge norms (NPDES) is always very challenging. At present, the only option available is a biological method patented (e.g. U.S. Pat. No. 6,183,644) and commercialized by GE (sold under the trademark ABMet™). However this process is both capital intensive and requires significant operational management.

(5) A chemical method for selenium removal has been reported in the literature (e.g. Ind. Eng. Chem. Res 1988 27(1) pp. 187-191).

(6) The disclosed method for removal of selenium (VI) comprises reduction-precipitation to elemental selenium (0) with ferrous hydroxide, as shown in Reaction 1.
Na.sub.2SeO.sub.4+9Fe(OH).sub.2.fwdarw.Se+3Fe.sub.3O.sub.4+2NaOH+8H.sub.2O  Reaction 1

(7) The ferrous hydroxide, Fe(OH).sub.2 can be generated by reacting ferrous sulfate with lime. The inert reaction condition at 70° C. for 15 min at pH 9.0 was documented as the most favourable condition for selenium (VI) removal using ferrous hydroxide. It is believed that the precipitated selenium (Se) metal particles will be trapped within the Fe.sub.3O.sub.4 solid and a traditional solid-liquid separation method can be applied to furnish the selenium treated water.

(8) However, our experiments have shown that this methodology achieves only limited success, for example in relation to selenium removal from power plant FGD wastewater.

(9) Accordingly, it is an object of the invention to provide a chemical method of selenium removal which is more effective than prior art methods.

(10) It is a further non-limiting object of the invention, to provide a chemical method of selenium removal which is cheap, robust and scalable.

(11) The use of a non-iron-based reducing agent to remove selenium from a liquid, for example water, and compositions for use in removing selenium from a liquid is disclosed. In an embodiment, the water being treated is wastewater from an industrial process, such as a power plant, mining operation, or refinery. In a further embodiment, the water being treated is flue gas de-sulfurization (FGD) wastewater.

(12) The method preferably causes elemental selenium to precipitate from the liquid, to allow the selenium to be collected and, preferably, to render the liquid sufficiently selenium-free as to meet discharge standards. In this regard, a non-iron based reducing agent has been found to remove selenium from wastewater. In an embodiment, the method comprises contacting water with a non-iron based reducing agent to remove selenium from the wastewater.

(13) In another embodiment, the method comprises contacting water with a non-iron based reducing agent and iron to remove selenium from the wastewater. As shown in the examples, it has been discovered that the non-iron based reducing agent has a synergistic effect when used in combination with Fe (II) species. Without wishing to be bound by a particular theory, it is believed that the observed synergistic effect between iron and the non-iron based reducing agent occurs via, or because of, the formation of a sulphur dioxide radical. In an embodiment, the ratio of iron, such as Fe(II) species, and the non-iron reducing agent is from 0.5 to 1.5. In another embodiment, the ratio of iron, such as Fe(II) species, and the non-iron reducing agent is from 0.9 to 1.1.

(14) In embodiments, an iron species, such as Fe(II), is added to the water being treated such that the ratio of Fe(II) ions to selenium in the water being treated is in excess of 100. In an embodiment, the ratio of Fe(II) ions to selenium in the water being treated is more than 110. In another embodiment, the ratio of Fe(II) ions to selenium in the water being treated is more than 120. In yet another embodiment, the ratio of Fe(II) ions to selenium in the water being treated is more than 125, more than 130, more than 135, more than 140, more than 145, more than 150, more than 155, more than 160, or more than 165.

(15) In an embodiment, the reducing agent is a dithionite ion or salt thereof. It has been found that dithionite ions or salts thereof act synergistically with Fe(II) ions to remove selenium, and specifically selenium (VI) and/or selenium (IV) species. In an embodiment, the dithionite is an alkali metal dithionite. Examples of alkali metal dithionites include, but are not limited to, lithium, sodium, potassium, and rubidium dithionite.

(16) In an embodiment of reducing selenium (for example Se(VI) ions) in water, the method comprises contacting the water with dithionite ions or a salt thereof and optionally a ferric salt at an alkalinic pH and heating the water for a period of time to allow or cause elemental selenium to precipitate from the water. In an embodiment, the water being treated according to the method is heated to a temperature of at least 60° C. In another embodiment, the water being treated according to the method is heated to a temperature from 60 to 95° C. In yet another embodiment, the water being treated according to the method is heated to a temperature from 65 to 95° C.

(17) In an embodiment, the water being treated according to the method has a pH of about 8 or greater. In another embodiment, the water being treated according to the method has a pH from 7.5 to 10.5. In yet another embodiment, the water being treated according to the method has a pH from 8 to 10. It is believed that the reaction rate is fastest at a pH of 8 to 10.

(18) In further embodiments, a pH regulator can be added to the water to regulate the alkalinity of the water. Examples of a pH regulator include but are not limited to calcium hydroxide, calcium oxide, calcium carbonate, lime, and combinations thereof. In an embodiment, the water being treated is contacted with calcium hydroxide in the presence or absence of Fe(II) ions). In a further embodiment, the water being treated is contacted with calcium hydroxide at a ratio of 0.6 to 1.4 calcium hydroxide to Fe(II) ions.

(19) Another aspect of the invention is a composition for selenium removal. The composition generally comprises a non-iron reducing agent, such as a dithionite or salt thereof, and optionally iron, such as a ferric salt. The dithionite can be an alkali metal dithionite. Examples of alkali metal dithionite include but are not limited to, lithium, sodium, potassium, and rubidium dithionite. Examples of the ferric (Fe(II)) salt include iron sulphate. In an embodiment, the composition comprises an alkali metal dithionite located or dissolved in an aqueous solution of a ferric (Fe(II)) salt. In a preferred embodiment, the alkali metal dithionite is sodium dithionite. In an embodiment, the ratio of iron, such as Fe(II) species, and the non-iron reducing agent, such as dithionate, is from 0.5 to 1.5. In another embodiment, the ratio of iron, such as Fe(II) species, and the non-iron reducing agent, such as dithionite, is from 0.9 to 1.1.

(20) In embodiments, the composition contains an amount of iron, such as Fe(II), that the ratio of Fe(II) ions to selenium in the water being treated is in excess of 100. In an embodiment, the composition contains an amount of iron, such as Fe(II), such that the ratio of Fe(II) ions to selenium in the water being treated is more than 110. In another embodiment, the composition contains an amount of iron, such as Fe(II), such that the ratio of Fe(II) ions to selenium in the water being treated is more than 120. In yet another embodiment, the composition contains an amount of iron, such as Fe(II), such that the ratio of Fe(II) ions to selenium in the water being treated is more than 125, more than 130, more than 135, more than 140, more than 145, more than 150, more than 155, more than 160, or more than 165.

(21) The composition can optionally include a pH regulator to regulate the pH of the composition and/or to regulate the alkalinity of the water being treated. Examples of a pH regulator include but are not limited to calcium hydroxide, calcium oxide, calcium carbonate, lime, and combinations thereof. The composition generally comprises an alkalinic pH. In an embodiment, the composition comprises a pH of about 8 or greater. In another embodiment, the composition comprises a pH from 7.5 to 10.5. In yet another embodiment, the composition comprises a pH from 8 to 10.

(22) The pH regulator can be added to the composition to regulate the alkalinity of the water being treated. Examples of a pH regulator include but are not limited to calcium hydroxide, calcium oxide, calcium carbonate, lime, and combinations thereof. In an embodiment, the water being treated is contacted with calcium hydroxide in the presence or absence of an iron species, such as Fe(II) ions. In an embodiment, the composition comprises a ratio of 0.6 to 1.4 of pH regulator to Fe(II) ions. In a further embodiment, the composition comprises a ratio of 0.6 to 1.4 calcium hydroxide to Fe(II) ions.

EXAMPLES

(23) The following examples are illustrative and are provided to assist in a further understanding of the invention. Other embodiments are within the scope of the present invention. The particular materials and conditions employed are intended to be further illustrative of the invention.

(24) In order to test the efficacy of the prior art method we conducted the following experiments, as follows:

Comparative Example 1—Prior Art

(25) A power plant FGD wastewater (having a total selenium concentration of 0.98 ppm and a Se(VI):Se(IV) ratio of greater than 1) was placed in a round bottom flask equipped with a stirrer bar and placed on a heater. The wastewater was heated to 70-80° C. whilst purging with nitrogen. Purging continued for 30 to 60 minutes and then 1.5 ml of 10% lime slurry (equivalent to 600 ppm) was added to the reaction mixture. An aliquot of ferrous sulfate hexahydrate was added to the reaction mixture and the temperature maintained at 70-90° C. for 30 to 60 minutes and then allowed to cool to room temperature in an open atmosphere. The supernatant was filtered through a 0.45 micron syringe filer and the composition of the water was analysed by inductively coupled plasma (ICP) spectroscopy.

(26) As can be seen in the table of FIG. 1, contacting the FGD wastewater with a solution to provide a Fe(II)/Se ration of 73 led to a 20% reduction in the selenium.

Comparative Example 2—Prior Art

(27) A power plant FGD wastewater (having a total selenium concentration of 0.98 ppm) was placed in a round bottom flask equipped with a stirrer bar and placed on a heater. The wastewater was heated to 70-80° C. whilst purging with nitrogen. Purging continued for 30 to 60 minutes and then 1.5 ml of 10% lime slurry (equivalent to 600 ppm) was added to the reaction mixture. An aliquot of ferrous sulfate hexahydrate (200 mg) was added to the reaction mixture and the temperature maintained at 70-90° C. for 30 to 60 minutes and then allowed to cool to room temperature in an open atmosphere. The supernatant was filtered through a 0.45 micron syringe filer and the composition of the water was analysed by inductively coupled plasma (ICP) spectroscopy.

(28) As can be seen in the table of FIG. 2, contacting the FGD wastewater with a solution to provide a Fe(II)/Se ratio of 147 led to a 30% reduction in the selenium.

(29) Comparing Comparative Examples 1 and 2, it can be seen that doubling the amount of iron only leads to a modest (10%) increase in the amount of selenium removal.

(30) In order to improve selenium removal, we conducted experiments in accordance with the invention where Fe(II) is used in combination with a reducing agent.

Example 3

(31) A power plant FGD wastewater (having a total selenium concentration of 0.98 ppm) was placed in a round bottom flask equipped with a stirrer bar and placed on a heater. The wastewater was heated to 70-80° C. whilst purging with nitrogen. Purging continued for 30 to 60 minutes and then 1.5 ml of 10% lime slurry (equivalent to 600 ppm) was added to the reaction mixture. An aliquot of ferrous sulfate hexahydrate (200 mg) and solid sodium dithionite (200 mg) was added to the reaction mixture and the temperature maintained at 70-90° C. for 30 to 60 minutes and then allowed to cool to room temperature in an open atmosphere. A brownish to green solid settled out of the reaction mixture. The supernatant was filtered through a 0.45 micron syringe filer and the composition of the water was analysed by inductively coupled plasma (ICP) spectroscopy.

(32) In the above experiments the ratio of Fe(II):Se is 163.

(33) As can be seen in the table of FIG. 3, the reduction in the amount of selenium is markedly better when using iron and the reducing agent in combination that when using iron in isolation.

(34) In order to check the efficacy of the system, a further experiment in accordance with the invention was conducted using the reducing agent in isolation.

Example 4

(35) A power plant FGD wastewater (having a total selenium concentration of 0.98 ppm) was placed in a round bottom flask equipped with a stirrer bar and placed on a heater. The wastewater was heated to 70-80° C. whilst purging with nitrogen. Purging continued for 30 to 60 minutes and then 1.5 ml of 10% lime slurry (equivalent to 600 ppm) was added to the reaction mixture. An aliquot of solid sodium dithionite (200 mg) was added to the reaction mixture and the temperature maintained at 70-90° C. for 30 to 60 minutes and then allowed to cool to room temperature in an open atmosphere. The supernatant was filtered through a 0.45 micron syringe filer and the composition of the water was analysed by inductively coupled plasma (ICP) spectroscopy.

(36) As can be seen in the table of FIG. 4, the dithionite reaction was slightly better than the reaction of iron in isolation.

(37) We believe the dithionite (S.sub.2O.sub.4.sup.2−) reaction proceeds as:
Na.sub.2SeO.sub.4+X.sub.2S.sub.2O.sub.4.fwdarw.Se+Na.sub.2SO.sub.4  Reaction 2

(38) (Where X is an alkali metal, for example Li, Na, K, Rb)

(39) Whilst the dithionite reaction appears to be more able than iron to reduce the concentration of selenium, what is absolutely stark is the synergistic effect of the addition of iron and the reducing agent (e.g. Example 3). The results of Example 3 are markedly better than what one would expect from the addition of the reagents in isolation (e.g. from a comparison of Comparative Example 2 and Example 4).

(40) Clearly, not only is the reducing agent able to remove selenium when used in isolation but has a demonstrated synergistic effect when used in combination with Fe(II) species.

(41) Whilst we do not intend nor wish to be bound by any particular theory, we postulate that the synergistic effect between iron and dithionate ions may occur via, or because of, the formation of a sulphur dioxide radical. Therefore, we believe that any dithionite salt will be capable of reacting synergistically with Fe(II) ions to remove selenium from waste water.