Use of ferrous sulfide for the removal of selenium from gases

Abstract

A liquid suspension comprised of ferrous sulfide particles and method of using the same for the removal of selenium from industrial gas. Said liquid suspension may be (1) directly injected, or (2) coated onto or into a substrate and then injected into an industrial gas containing selenium to remove said selenium from the industrial gas stream in a dry scrubber system. Said liquid suspension of ferrous sulfide particles may also be used to remove selenium contained in industrial gases in a wet scrubber system.

Claims

1. A method of removing selenium from an industrial gas containing selenium which method comprises the steps of: providing an alkaline suspension of ferrous sulfide particles; contacting said industrial gas containing selenium with an alkaline liquid suspension of ferrous sulfide particles; contacting said industrial gas containing selenium with said alkaline liquid suspension of ferrous sulfide particles; and allowing the industrial gas containing selenium to react with said alkaline liquid suspension of ferrous sulfide particles and be removed by at least one of: i) adsorption onto said ferrous sulfide particles ii) adsorption onto iron (hydr)-oxides or green rusts that form in said alkaline liquid suspension of ferrous sulfide particles; and iii) absorption by reacting with sulfur formed in alkaline liquid suspension of ferrous sulfide particles and iron-selenium complexes or iron-selenium containing precipitates.

2. A method of removing selenium from an industrial gas containing selenium according to claim 1, wherein said step of providing alkaline liquid suspension of ferrous sulfide particles comprises combining together a ferrous ion source, a sulfide ion source and an alkalinity source to form said alkaline liquid suspension of ferrous sulfide particles.

3. A method of removing selenium from an industrial gas containing selenium according to claim 2, wherein said step of providing alkaline liquid suspension of ferrous sulfide particles comprises combining together ferrous chloride, sodium hydrosulfide and sodium hydroxide to form said alkaline liquid suspension of ferrous sulfide particles.

4. A method of removing selenium from an industrial gas containing selenium according to claim 3, further comprising controlling the amount of at least one of the ferrous chloride, sodium hydrosulfide and sodium hydroxide so as to control a ratio of selenium to ferrous sulfide in the alkaline liquid suspension of ferrous sulfide particles.

5. A method of removing selenium from an industrial gas containing selenium according to claim 2, further comprising controlling the amount of at least one of the ferrous ion source, the sulfide ion source and the alkalinity source so as to control a ratio of selenium to ferrous sulfide in the alkaline liquid suspension of ferrous sulfide particles.

6. A method of removing selenium from an industrial gas containing selenium according to claim 1, wherein the alkaline liquid suspension of ferrous sulfide particles inhibits the oxidation of lower valence states of selenium into selenate.

7. A method of removing selenium from an industrial gas containing selenium according to claim 1, wherein the step of contacting the industrial gas containing selenium with said alkaline liquid suspension of ferrous sulfide particles is performed in a wet scrubber system.

8. A method of removing selenium from an industrial gas containing selenium according to claim 1, wherein the step of contacting the industrial gas containing selenium with said alkaline liquid suspension of ferrous sulfide particles is conducted by either dispersing or injecting the industrial gas containing selenium into the alkaline liquid suspension of ferrous sulfide particles; spraying or injecting the alkaline liquid suspension of ferrous sulfide particles or ferrous sulfide particles into the industrial gas stream containing selenium; coating or impregnating the alkaline liquid suspension of ferrous sulfide particles onto or into a substrate and either injecting the substrate coated with the alkaline liquid suspension of ferrous sulfide particles into the industrial gas containing selenium or allowing the industrial gas containing selenium to pass through a vessel or container packed with a substrate that is coated or impregnated with the alkaline liquid suspension of ferrous sulfide.

9. A method of removing selenium from an industrial gas containing selenium according to claim 8, wherein said step of providing alkaline liquid suspension of ferrous sulfide particles comprises combining together a ferrous ion source, a sulfide ion source and an alkalinity source to form said alkaline liquid suspension of ferrous sulfide particles.

10. A method of removing selenium from an industrial gas containing selenium according to claim 9, wherein said step of providing alkaline liquid suspension of ferrous sulfide particles comprises combining together ferrous chloride, sodium hydrosulfide and sodium hydroxide to form said alkaline liquid suspension of ferrous sulfide particles.

11. A method of removing selenium from an industrial gas containing selenium according to claim 10, further comprising controlling the amount of at least one of the ferrous ion source, the sulfide ion source and the alkalinity source so as to control a ratio of selenium to ferrous sulfide in the alkaline liquid suspension of ferrous sulfide particles.

12. A method of removing selenium from an industrial gas containing selenium according to claim 11, further comprising controlling the amount of at least one of the ferrous chloride, sodium hydrosulfide and sodium hydroxide so as to control a ratio of selenium to ferrous sulfide in the alkaline liquid suspension of ferrous sulfide particles.

13. A method of removing selenium from an industrial gas containing selenium according to claim 8, wherein the alkaline liquid suspension of ferrous sulfide particles inhibits the oxidation of lower valence states of selenium into selenate.

14. A method of removing selenium from an industrial gas containing selenium according to claim 8, wherein the step of contacting the industrial gas containing selenium with said alkaline liquid suspension of ferrous sulfide particles is performed in a wet scrubber system.

15. In a method of using a wet gas scrubber to remove selenium from an industrial gas, said wet gas scrubber having a scrubber liquor and being configured for receiving and scrubbing said industrial gas with the scrubbing liquor, the improvement comprising providing as said scrubber liquor an alkaline liquid suspension of ferrous sulfide particles which removes selenium from the industrial gas by at least one of: i) adsorption onto said ferrous sulfide particles; ii) adsorption onto iron (hydr)-oxides or green rusts that form in said alkaline liquid suspension of ferrous sulfide particles; and iii) absorption by reacting with sulfur formed in alkaline liquid suspension of ferrous sulfide particles and iron-selenium complexes or iron-selenium containing precipitates.

16. A method of removing selenium from an industrial gas containing selenium according to claim 15, wherein the step of providing alkaline liquid suspension of ferrous sulfide particles comprises combining together a ferrous ion source, a sulfide ion source and an alkalinity source to form said alkaline liquid suspension of ferrous sulfide particles.

17. A method of removing selenium from an industrial gas containing selenium according to claim 16, further comprising controlling the amount of at least one of the ferrous ion source, the sulfide ion source and the alkalinity source so as to control a ratio of selenium to ferrous sulfide in the alkaline liquid suspension of ferrous sulfide particles.

18. A method of removing selenium from an industrial gas containing selenium according to claim 15, wherein the step of providing alkaline liquid suspension of ferrous sulfide particles comprises combining together ferrous chloride, sodium hydrosulfide and sodium hydroxide to form said alkaline liquid suspension of ferrous sulfide particles.

19. A method of removing selenium from an industrial gas containing selenium according to claim 18, further comprising controlling the amount of at least one of the ferrous chloride, sodium hydrosulfide and sodium hydroxide so as to control a ratio of selenium to ferrous sulfide in the alkaline liquid suspension of ferrous sulfide particles.

20. A method of using a wet gas scrubber to remove selenium from an industrial gas according to claim 18, wherein the alkaline liquid suspension of ferrous sulfide particles inhibits the oxidation of lower valence states of selenium into selenate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be described with reference to the attached drawings which are given as non-limiting examples only, in which:

(2) FIGS. 1a and 1b represent a single cell and a sheet of FeS.sub.m (mackinawite) respectively.

(3) FIG. 2 is a graph of the total activity of dissolved Fe.sup.2+ in equilibrium with FeS.sub.m at 25 C. Total activity of dissolved Fe(II) in equilibrium with FeS.sub.m

(4) FIG. 3 is a diagram of a process for removing selenium from an industrial gas according to one embodiment of the present invention using a wet scrubber system

(5) FIG. 4 is a diagram of a process for removing selenium from an industrial gas according to one embodiment of the present invention within a dry scrubber system

(6) FIG. 5 is a diagram of a process for removing selenium from an industrial gas according to one embodiment of the present invention within a dry scrubber system whereby the industrial gas is passed through the dry scrubber sorbent.

DETAILED DESCRIPTION OF THE DRAWINGS AND THE PRESENTLY PREFERRED EMBODIMENTS

(7) The present invention provides a ferrous sulfide suspension, a method for producing the ferrous sulfide suspension, and methods for using the ferrous sulfide suspension for the treatment and removal of selenium from industrial gases.

(8) The ferrous sulfide suspension of the present invention is a minimally soluble, colloidal suspension that can be used to enhance the selenium removal capabilities of wet scrubber systems, or if coated onto or impregnated into a substrate, used as a sorbent, used to enhance the selenium removal capabilities of dry scrubber systems.

(9) Through a combination of complex chemical reactions, precipitation, co-precipitation, and surface adsorption the ferrous sulfide suspension of the present invention can effectively remove selenium from industrial gases while preventing or minimizing the formation of selenate ions.

(10) The alkaline ferrous sulfide suspension of the present invention can be produced by combining together a ferrous ion source (e.g. FeCl.sub.2), a sulfide ion source (e.g. NaHS), and an alkalinity source (e.g. NaOH). According to different embodiments of the present invention the molar ratio of the ferrous ion source, sulfide ion source, and alkalinity source can be controlled/adjusted during the scrubbing of a flue gas so as to control/adjust the concentration of the insoluble ferrous sulfide (FeS).

(11) The alkaline ferrous sulfide suspension can be used in a wet scrubbing liquor in any type of wet gas scrubber system, including but not limited to venturi scrubbers, spray scrubbers, cyclone spray chambers, orifice scrubbers, impingement scrubbers, packed bed scrubbers, and jet bubbling reactors (JBR).

(12) During the course of the present invention the inventors discovered that a liquid suspension of containing minimally soluble ferrous sulfide (or a liquid suspension of containing minimally soluble ferrous sulfide coated or impregnated onto or into a substrate) efficiently and economically removes selenium by both absorption and adsorption mechanisms while simultaneously retarding or preventing the oxidation of lower valence states of selenium to selenate (Se.sup.6+).

(13) Though the combination of various molar ratios of a ferrous ion source (e.g. FeCl.sub.2), a sulfide ion source (e.g. NaHS), and an alkalinity source (e.g. NaOH), the resulting alkaline liquid suspension containing FeS particles provides an economical and efficient reagent for selenium removal from industrial gas streams.

(14) Ferrous sulfide, sometimes referred to as mackinawite, disordered mackinawite, and amorphous ferrous sulfide disassociates according to the following reaction:
FeSFe.sup.2++S.sup.2(1)

(15) Depending upon the environment in which ferrous sulfide is formed, the solubility product constant (Ksp) will be between 110.sup.3 and 110.sup.5. Since this is many orders of magnitude higher than the solubility product of FeSe (Ksp=310.sup.19), in the presence of selenide ions (Se.sup.2) that may be present (or formed) in the industrial gas or in the scrubber liquor of a wet scrubber system, the formation FeSe is favored and rapid. By providing the ferrous ion (Fe.sup.2+) ion in the form of a minimally soluble ferrous sulfide solid particle, only the stoichiometric amount of sulfide will enter the scrubber liquor that is necessary to precipitate any Se.sup.2. The advantage of the present invention when compared to the prior art, is the possibility of over or under dosing the required amount of ferrous iron necessary to precipitate the Se.sup.2+ is mitigated.

(16) The present invention also allows for the ability to adjust the molar ratios of the ferrous ion source, sulfide ion source, and alkalinity source to produce a ferrous sulfide suspension for selenium removal from industrial gases in a real-time, continuous basis. The ability to adjust the concentration of insoluble FeS in suspension, the ability to produce a ferrous sulfide suspension with specified concentrations of ferrous ions (or sulfide ions) by adjusting the stoichiometry of the feedstocks, pH, or combinations of both offers unique flexibility to produce a suspension of ferrous sulfide particles for removal of selenium from industrial gases that was not heretofore possible or foreseen.

(17) During the course of the present invention the inventors unexpectedly discovered that a liquid suspension containing minimally soluble ferrous sulfide (FeS) can efficiently and economically remove Se.sup.2, Se.sup.2+, and Se.sup.4+ ions by absorption and adsorption mechanisms while simultaneously minimizing or limiting formation of the selenate (Se.sup.6+) ion.

(18) Though the combination of various molar ratios of a ferrous ion source (e.g. FeCl.sub.2), a sulfide ion source (e.g. NaHS), and an alkalinity source (e.g. NaOH), the resulting alkaline liquid suspension containing FeS particles provides an economical and efficient wet scrubber liquor additive suitable for selenium removal from industrial gas streams, or when coated onto or impregnated into a substrate, used as a sorbent to enhance the selenium removal capabilities from industrial gases in dry scrubber systems.

(19) Ferrous sulfide, sometimes referred to as mackinawite, disordered mackinawite, amorphous ferrous sulfide disassociates by the following reaction:
FeSFe.sup.2++S.sup.2(2)

(20) FIGS. 1a and 1b represent a single cell and a sheet of FeS.sub.m (mackinawite). In these figures it is noted that each iron ion is four-way coordinated to each sulfur ion.

(21) Depending upon the environment containing the selenium, the selenium may be present as a selenide ion (Se.sup.2). The mechanisms for binding Se.sup.2 to FeS.sub.m (mackinawite) are believed to involve Se.sup.2 adsorption to the FeS.sub.m surface (3), precipitation as FeSe (4), or combination of both:
SFe+Se.sup.2ESFeSe.sup.2(3)
FeS.sub.(S)+Se.sup.2FeSe+S.sup.2(4)

(22) Herein adsorption is meant to encompass all processes responsible for Se.sup.2 accumulation at the FeS.sub.m-liquid interface (e.g., surface complexation at low surface coverage) and surface precipitation at high surface coverage.

(23) In other environmental conditions, selenium may be present as the selenite ion (Se.sup.4+). Research has demonstrated that Se.sup.4+ is reduced by FeS through intermediates such as Se.sup.0 to less soluble species such as Se.sup.2, followed by formation of a solid phase with a structure similar to FeSe or FeSe.sub.x. (XPS Analysis of Sorption of Selenium(IV) and Selenium(VI) to Mackinawite (FeS). Dong Suk Han, Bill Batchelor, and Ahmed Abdel-Wahab, (Environmental Progress & Sustainable Energy. 2013 (Vol. 32, No. 1), p. 84-93.)

(24) Other research indicates the mechanisms of absorption an adsorption of selenite was different depending upon pH. (Sorption of selenium(IV) and selenium(VI) to mackinawite (FeS): Effect of contact time, extent of removal, sorption envelopes. Dong Suk Han, Bill Batchelor, and Ahmed Abdel-Wahab. Journal of Hazardous Materials 2011 (186) 451-457. See also, Selenite Reduction by Mackinawite, Magnetite and Siderite: XAS Characterization of Nanosized Redox Products. Andreas C. Scheinots and Laurent Charlet. Environmnetal Science and Technology 2008 (42) 1984-1989). Since mackinawite solubility increases by about four (4) orders of magnitude when decreasing the pH from 6.6 to 4.4 (see FIG. 2), the final redox reaction products in presence of mackinawite at these two different pH values (FeSe at pH 4.4; Se.sup.0 at pH 6.3), suggests the final reaction product is significantly influenced by the solubility of mackinawite. Thus, the likely reaction pathways based on the assumption of heterogeneous surface reactions allowed these researchers to conclude the following:

(25) The reaction at pH 4.4:
6Fe.sup.2++HSeO.sub.3.sup.+FeS+6H.sup.+.fwdarw.6Fe.sup.3++FeSe+HS.sup.+3H.sub.2O (5)

(26) The reaction at pH 6.3:
4Fe.sup.2++HSeO.sub.3.sup.+5H.sup.+.fwdarw.4Fe.sup.3++Se.sub.(S)+3H.sub.2O. (6)

(27) At higher pHs (e.g. pH=7 to 8), Se.sup.4+ removal was better described by the BET isotherm (initially very fast followed by a relatively slower removal rate) it can be inferred that some multilayer sorption occurs at these higher pHs.

(28) Selenium may also be present as the selenate ion (Se.sup.6+), particularly in scrubber liquors. Although the aforementioned research indicated that greater than ten percent (10%) of Se.sup.6+ may be removed from solutions during the first hour irrespective of initial Se.sup.6+ concentrations, the additional removal of Se.sup.6+ rapidly slowed thereafter. The lower extent and slower rate of uptake of Se.sup.6+ compared to that of Se.sup.4+ may have be due to a lower affinity of Se.sup.6+ for the FeS surface. At pHs>7, this likely occurs since the FeS surfaces are negatively charged, and Se.sup.6+ would be present as an anion with two negative charges compared to Se.sup.4+ which would be present mostly as an anion with one negative charge.

(29) The effect of competitive ions such as sulfate (SO.sub.4.sup.2) which are prevalent in a majority of scrubber liquors at coal-fired boilers had a negligible effect on removal of Se.sup.4+ by FeS, however surprisingly, there was some indication that the higher level of sulfate resulted in increased Se.sup.6+ removal. The enhanced removal of Se.sup.6+ could have been caused by association of another intermediate solid-phase product such as green rust (GR).

(30) Green Rust (GR) is a mixed Fe.sup.2+/Fe.sup.3+ (oxy)hydroxide with the general formula;
[Fe.sup.2+.sub.(1-x) Fe.sup.3+.sub.x (OH).sub.2][x/n An.sup.:mH.sub.2O](7)
where An.sup. is a anion (e.g. SO.sub.4.sup.2, CO.sub.3.sup.2, Cl.sup.) located in the water interlayer to provide electron charge balance.

(31) Since Se.sup.4+ is more readily reduced than Se.sup.6+, the standard state redox potentials of Fe and Se species suggests that Fe.sup.2+ should be able reduce to Se.sup.6+ to Se.sup.0 or Se.sup.2. The overall reaction could be written as:
Se.sup.6+O.sub.4.sup.2+8Fe.sup.2++9H.sup.+.fwdarw.8Fe.sup.3++HSe.sup.+4H.sub.2O (8)

(32) The above equation however only describes the reaction in the aqueous phase as a function of pH, and more importantly, does not account for any solid surface reactions or kinetics (time required). Although Fe.sup.2+ is present primarily as the an aqua-ion [Fe(H.sub.2O).sub.6] in acidic solutions, it will precipitate as green rust in the presence of Fe.sup.3+ at pH>4.0, or as Fe(OH).sub.2 in the absence of Fe.sup.3+ at pH>8.0.

(33) In theory therefore, Se.sup.6+ reduction at pH 4.0 may occur by homogeneous reactions in the aqueous phase, by heterogeneous reactions either on GR surfaces by adsorption and reduction, in GR interlayers by co-precipitation and reduction, or any combination of these mechanisms.

(34) Selenate (Se.sup.6+) treated with mackinawite (FeS.sub.m) in an acidic solution (pH=3.5), where aqueous Fe.sup.2+ exists primarily as the aqua ion [Fe(H.sub.2O).sub.6] and without any GR precipitate, indicate that Se.sup.6+ was not reduced within 160 hours of reaction, and the Se.sup.6+ remained as uncomplexed SeO.sub.4.sup.2. In contrast however, aqueous Se.sup.6+ was unstable in samples prepared at pH>5.0. Under these conditions, the mixed valence GR precipitated initially and, over time, converted to Fe.sup.3+-oxides at pH 7.0 (magnetite and lepidocrocite) and to only magnetite at pH 9.5. Magnetite and lepidocrocite are formed when the Fe.sup.2+ present in the green rust becomes fully oxidized to Fe.sup.3+. The presence of GR and its oxidized products strongly influences the aqueous and solid-phase Se speciation, and therefore potential removal efficiency. When the selenium concentration is below the saturation limit for known Se solids, precipitation can be ruled out as the primary mechanism for Se removal. Selenate (Se.sup.6+) transformations may therefore occur within the interlayers, on the external surfaces of the GR, or both. The reduction of interlayer substituted Se.sup.6+ to Se.sup.0 promotes magnetite formation at the expense of GR, and the reduced Se.sup.0 atoms forms clusters on the GR particle surfaces. The rapid precipitation kinetics and the flexible crystal structure of GR may allow its formation under a variety of geochemical conditions. By way of example, GR with interlayers of sulfate can promote selenate (Se.sup.6+) reduction and be summarized by the following reaction:
HSeO.sup.4+4Fe.sup.(2+)Fe.sup.(3+)(OH).sub.12SO.sub.4:3H.sub.2OHSe.sup.+8Fe.sub.3O.sub.4+4SO.sub.4.sup.2+8H.sup.++32H.sub.2O (9)
(Abiotic Selenium Redox Transformations in the Presence of Fe(II, III) Oxides. S. C. B. Myneni, T. K. Tokunaga, G. E. Brown Jr. Science 1997 (278), 1106-1109.

(35) FIG. 3 is a diagram of a process for removing selenium from an industrial gas according to one embodiment of the present invention. As depicted in FIG. 3 a source of ferrous ions (e.g. FeCl.sub.2) 34, a source of sulfide ions (e.g. NaHS) 35, and an alkalinity source (e.g. NaOH) 36 are combined together to produce an alkaline liquid suspension of ferrous sulfide particles. The alkaline liquid suspension of ferrous sulfide particles is used as a wet scrubber liquor additive in a wet gas scrubber system 31 through which an industrial gas stream containing selenium 32 is passed to produce a selenium-free industrial gas stream 33.

(36) The process depicted in FIG. 3 allows for control/adjustment of the molar ratio of the ferrous ion source, sulfide ion source, and alkalinity source during processing allowing for a real time control/adjustment of the concentration of the minimally soluble ferrous sulfide (FeS) in the wet scrubbing liquor additive. In the alternative, the minimally soluble ferrous sulfide suspension can be made off-site, transported to the location of the site of the wet scrubber system and then injected into the wet scrubber liquor.

(37) FIG. 4 is a diagram of a process for removing selenium from an industrial gas according to another embodiment of the present invention. As depicted in FIG. 4 a source of ferrous ions (e.g. FeCl.sub.2) 44, a source of sulfide ions (e.g. NaHS) 45, and an alkalinity source (e.g. NaOH) 46 are combined together to produce an alkaline liquid suspension of ferrous sulfide particles. The alkaline liquid suspension of ferrous sulfide particles is then coated onto or impregnated into a substrate 42. The then coated or impregnated substrate 41 is then injected into an industrial gas containing selenium 43 to produce a selenium-free industrial gas stream 44.

(38) FIG. 5 is a diagram of a process for removing selenium from an industrial gas according to another embodiment of the present invention. As depicted in FIG. 5 a source of ferrous ions (e.g. FeCl.sub.2) 54, a source of sulfide ions (e.g. NaHS) 55, and an alkalinity source (e.g. NaOH) 56 are combined together to produce an alkaline liquid suspension of ferrous sulfide particles. The alkaline liquid suspension of ferrous sulfide particles is then coated onto or impregnated into a substrate 52. The then coated or impregnated substrate 51 is then placed into a cartridge or vessel 57 through which an industrial gas containing selenium 53 to produce a selenium-free industrial gas stream 58.

(39) Although the present invention has been described with reference to particular means, materials and embodiments, from the foregoing description, one skilled in the art can easily ascertain the essential characteristics of the present invention and various changes and modifications can be made to adapt the various uses and characteristics without departing from the spirit and scope of the present invention as described above and set forth in the attached claims.