Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
2000-04-14
2001-05-22
Simmons, David A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C210S721000, C210S723000, C210S724000, C210S726000, C210S757000, C210S758000, C210S767000, C210S806000
Reexamination Certificate
active
06235204
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to methods and apparatus for removing undesirable metallic ions from aqueous streams. More specifically the invention relates to a process and system for the chemical reduction and co-precipitation of selenium and the stabilization of the resultant selenium solids generated during the treatment of purge waters that are produced in a limestone-based flue gas desulfurization (FGD) scrubber typical of most coal-fired electric power utilities.
BACKGROUND OF THE INVENTION
In the limestone-based flue gas desulfurization (FGD) scrubber typical of most coal-fired electric power utilities, the limestone is used in the scrubbing process to remove combustion by-products during steam generation. Sulfur dioxide is the primary target of this scrubbing process and is produced when coal with high sulfur content is burned in the boilers. The selenium that is derived from the particular coal used as fuel to the boilers will be found in fly ash, flue gas emissions, scrubber solids, and in the scrubber liquids. Additionally other heavy metals will accumulate in the scrubber water. The selenium concentrations found in FGD scrubber liquids will vary from 0.5 to several parts per million (ppm).
The present invention is a process and system that will remove selenium from water that contains high concentrations of dissolved solids, and is an improvement upon the method disclosed in U.S. Pat. No. 4,806,264 (Feb. 21, 1989). The method described in U.S. Pat. No. 4,806,264 uses the ferrous ion to reduce the oxy-anions of selenium (i.e., selenate [SeO
4
2−
] and selenite [SeO
3
2−
]) to elemental selenium (Se°) and co-precipitates the selenium with the resultant ferrous and ferric hydroxide sludges produced during the reaction. However, the application of the method described in U.S. Pat. No. 4,806,264 has not been found satisfactory by the present inventors for the removal of the selenium present in FGD scrubber purge water. Typical FGD scrubber liquids contain high concentrations of calcium, magnesium, sulfate, and total dissolved solids (TDS). With certain coals, the TDS will contain high concentrations of chlorides, on the order of 15,000 to 20,000 ppm, as chloride. In addition, this purge water may contain reduced forms of sulfur such as sulfite (SO
3
2−
), thiosulfate (S
2
O
3
2−
), bisulfite (HSO
3
−
), metabisulfite (S
2
O
5
2−
), dithionate (S
2
O
6
2−
), trithionate (S
3
O
6
2−
), and tetrathionate (S
4
O
6
2−
); organic acids such as adipic, glutaric, and succinic acids; and sulfur-nitrogen compounds such as hydroxylamine disulfonate (HADS), hydroxylamine sulfonate, and amine disulfonate. The sulfur-nitrogen compounds are formed through a reaction between the NO
2
and SO
2
absorbed from boiler flue gas.
SUMMARY OF THE INVENTION
After extensive study of the treatment method described in U.S. Pat. No. 4,806,264 and its application to FGD scrubber purge water, it was determined that the presence of certain reduced sulfur species, selenium-thionate complexes, sulfur-nitrogen compounds, and the concentration of ferrous ion available for reaction impacted the treatment performance. Specifically, the reduced sulfur species present in FGD scrubber purge water that most inhibit ferrous ion reduction of the oxy-anions of selenium are sulfite, bisulfite, and thiosulfate. To a lesser degree, inhibition also results from the formation of selenium-thionate complexes such as the selenopentathionate [Se(S
2
O
3
)
2
2−
] and/or selenotrithionate ions, which serve to increase the concentration of dissolved selenium unavailable for reduction by the ferrous ion. Selenium will also complex directly with thiosulfate to form selenothiosulfate (SeS
2
O
3
2−
). However, the sulfur-nitrogen compounds were found to be the most inhibiting constituents in FGD scrubber purge water. Specifically, the sulfur-nitrogen compound, HADS, was found to severely reduce the rate at which the oxy-anions of selenium could be reduced with the ferrous ion, resulting in incomplete treatment and noncompliance with established effluent discharge limitations. In all cases, the inhibiting constituents impacted the apparent rate of selenate/selenite reduction in FGD purge water and resulted in an increase in the required dosage of ferrous ion needed to achieve the target effluent selenium concentration, as defined by the government regulatory agency. This apparent retardant effect on the rate of selenium oxy-anion reduction was believed to be indicative of a competitive inhibition.
The inhibiting constituents (O
2
, NO
3
−
, HCO
3
−
, Ca
2+
, Mg
2+
) cited by Moody and Murphy (1989) in their paper on treating toxic substances in agricultural water supply and drainage, Selenium Removal with Ferrous Hydroxide: Identification of Competing and Interfering Solutes, (Pan American Regional Conference—1989) were determined to either not be present or to be a minor influence on the rate of selenium oxy-anion reduction when actual FGD scrubber purge waters were treated with the present invention. More specifically, it was verified that the ferrous ion chemical reduction reaction described above must be conducted under strict anaerobic conditions, without nitrate present, and at optimum ferrous ion dose levels for the reactions to proceed to completion in a reasonable time period (e.g., less than 6.0 hours). Air will stop the reduction of selenate and selenite to elemental selenium by oxidizing the ferrous ion reducing agent, and nitrate will be reduced to ammonia and nitrite preferentially over the reduction of the selenium oxy-anions. Fortunately neither oxygen nor nitrate is present at significant concentrations in typical FGD scrubber purge waters. Moreover, the influence of these inhibitors is easily overcome by the addition of excess ferrous ion, providing that the sulfur-nitrogen compounds and/or reduced sulfur species are removed to acceptable levels by the appropriate pretreatment. The components of hardness (i.e., HCO
3
−
, Ca
2+
, Mg
2+
), albeit present at high concentrations in typical limestone-based FGD scrubber purge waters, were found to have minimal impact on the removal of the oxy-anions of selenium with the present invention. Experimental investigations conducted by the inventors produced data which indicated that the ferrous ion reactions were most optimum when the measured oxidation-reduction potential (ORP) of the process was more negative (usually between −650 and −750 mV). It also was determined that low to medium levels of sulfite, sulfide, thiosulfate, and possibly bisulfite would retard the selenium removal reaction rate. Moreover, when any of these reduced sulfur species were present at high concentrations in the FGD scrubber purge water and HADS was also present, the removal of selenium, primarily as selenate, would stop completely.
Experimental investigations also determined that it was possible to effect improved removal of selenium from the FGD scrubber purge waters at a short hydraulic retention time by using a plug-flow reaction vessel coupled with continuous sludge separation. It also was determined that once the selenium-iron solids were separated from the treated wastewater, the collected solids (i.e., sludges) could be stabilized in a separate reaction vessel to prevent the desorption of reduced forms of selenium back into the liquid stream. This stabilization reaction vessel is designed to complete the reduction of all oxidized forms of the selenium (e.g., selenate and selenite) in the purge water to elemental selenium and to guarantee that the selenium co-precipitated with iron oxyhydroxide would remain stable. This lead to the development of the reduction/co-precipitation-stabilization process described in the invention.
Thus the present invention represents a substantial improvement upon the process of U.S. Pat. No. 4,806,264, and optimizes treatment of limestone-based FGD scrubber purge waters to remove the oxy-
Behrens Gregory P.
Castaldi Frank J.
Hargrove, Jr. Oliver W.
Hoey Betsey Morrison
Klauber & Jackson
Radian International LLC
Simmons David A.
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