Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
1999-10-18
2001-01-23
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C210S717000, C210S719000, C210S724000, C210S737000, C210S738000, C210S911000, C210S912000, C210S913000, C423S508000, C423S602000
Reexamination Certificate
active
06177015
ABSTRACT:
TECHNICAL FIELD
The present invention relates to treatment of aqueous solutions in general, and, more particularly, to processes for reducing the concentration of dissolved metals and metalloids present in aqueous solutions, e.g., pyrometallurgical plant scrubber solutions.
BACKGROUND ART
Modem base metal smelters capture the bulk of sulfur dioxide gas (SO
2
) and flue dusts from off-gases produced by smelting. Off-gas cleaning by aqueous scrubbing typically produces acidic solutions rich in SO
2
and dissolved metals such as chromium, cobalt, copper, iron, lead, nickel, zinc, etc., and dissolved metalloids such as selenium, tellurium, arsenic, etc. The concentrations of some of the dissolved species in the scrubber solution may be above governmental water management guidelines (see, e.g., “Water Management Policies, Guidelines, Provincial Water Quality Objectives”, Ministry of Environment and Energy, Ontario, Canada, July 1994). Removal of these species from scrubber solution is therefore desirable. Removal of many of these species may be accomplished by precipitation via neutralization using, e.g., lime (see, e.g., Jackson, E., Hydrometallurgical Extraction and Reclamation, Wiley, Toronto, 1986). However, neutralization alone is typically not effective for the removal of dissolved selenium, tellurium and arsenic. Smelter scrubber solutions, for example, can contain 1 to 60 milligrams per liter (mg/L) of dissolved selenium or more in the form of selenite (Se(IV)) ions and/or selenate (Se(VI)) ions, 1 to 5 mg/L or more of dissolved tellurium in the form of tellurite (Te(IV)) ions and/or tellurate (Te(VI)) ions, and 50 to 230 mg/L or more of dissolved arsenic in the form of arsenite (As(III)) ions and/or arsenate (As(V)) ions.
Procedures for the removal of dissolved arsenic from scrubber solutions are disclosed in U.S. Pat. No. 5,820,966 to Krause et al. This process involves oxidizing the components of the solution by addition of, e.g., air, followed by neutralization by addition of, e.g., slaked lime in the presence of a sufficient quantity of dissolved ferric (Fe(III)) ion. The Fe(III) ion co-precipitates with dissolved arsenic to produce an environmentally preferred solid iron-arsenic compound, thus reducing the concentration of dissolved arsenic in the solution. A reduction in the concentration of many dissolved metals in solution also occurs. However, this process by itself is not effective for removal of selenium from solutions containing significant concentrations of dissolved selenium, and Se(VI) ions in particular.
A known process for removing selenium ions from solution involves providing SO
2
and heat to a solution containing Se(IV) ions to reduce the Se(IV) ions to elemental selenium (see, e.g., Kudryavtsev, A. A., The Chemistry and Technology of Selenium and Tellurium, Collet's, London, 1974). The process requires long residence times and is not effective for the removal of Se(VI) ions from solution.
U.S. Pat. No. 4,405,464 to Baldwin et al. discloses a process for the removal of Se(VI) ions from aqueous solution by contact with metallic iron at a solution pH adjusted to below about 6.0. The examples in the Baldwin et al. patent pertain to the treatment of solutions containing less than about 0.5 mg/L of Se(VI). This process is not effective for reducing high concentrations of Se(VI).
U.S. Pat. No. 4,806,264 to Murphy discloses a method of removing Se(IV) and Se(VI) ions from an aqueous solution. The method includes contacting the solution with an amount of ferrous hydroxide solids at a pH of about 8 to 10 and a preferred temperature of from about 10° C. to 35° C. The examples in the Murphy patent pertain to the treatment of solutions containing less than about 1 mg/L of Se(VI). However, this process is not effective for reducing high concentrations of Se(VI).
The effectiveness of the above prior art processes for the removal of dissolved tellurium from solution is not known.
There is a need for new, efficient methods that are capable of removing dissolved metals and metalloids from aqueous solutions, particularly those methods capable of removing relatively high concentrations (e.g., greater than 1 mg/L) of dissolved metalloids such as selenium, tellurium and arsenic.
SUMMARY OF THE INVENTION
Accordingly, there is provided a process for reducing the concentration of metalloid ions in an aqueous solution which includes introducing an aqueous solution containing an initial concentration of metalloid ions into a reaction zone; reacting ferrous (Fe(II)) ions with hydroxyl (OH
−
) ions in the reaction zone to precipitate Fe(II) ions in the presence of the metalloid ions to precipitate metalloid ions, resulting in a reduction in the concentration of metalloid ions in the solution. Particularly suitable metalloid ions herein include Se(IV), Se(VI), Te(IV), Te(VI), As(III) and As(V) ions. In one embodiment, the aqueous solution also contains an initial concentration of metal ions, at least a portion of which is reacted with OH
−
ions in the reaction zone to precipitate metal ions and cause a reduction in the concentration of metal ions in the aqueous solution.
There is also provided a process for reducing the concentration of metalloid ions in an aqueous feed solution containing an initial concentration of metalloid ions and SO
2
which includes introducing the feed solution into a reaction zone, reacting Fe(II) ions with OH
−
ions in the reaction zone to precipitate Fe(II) ions in the presence of the metalloid ions to form a slurry containing precipitated metalloid and a solution having a reduced concentration of metalloid ions as compared to the initial concentration of metalloid ions; removing the slurry from the reaction zone; and separating from the slurry at least a portion of the solution having a reduced concentration of metalloid ions. In one embodiment, the process also involves reducing an initial concentration of metal ions in the feed solution wherein metal ions in the feed solution are reacted with OH
−
ions in the reaction zone to precipitate metal ions in the slurry and the solution separated from the slurry contains a reduced concentration of metal ions as compared to the initial concentration of metal ions in the feed solution.
The processes described herein can advantageously reduce the concentration of metal ions and metalloid ions such as selenium ions, tellurium ions and arsenic ions to a level typically below 1.0 mg/L, respectively.
REFERENCES:
patent: 4405464 (1983-09-01), Baldwin et al.
patent: 4566975 (1986-01-01), Allgulin
patent: 4724084 (1988-02-01), Pahmeier et al.
patent: 4806264 (1989-02-01), Murphy
patent: 5045214 (1991-09-01), Walker
patent: 5330658 (1994-07-01), Grant et al.
patent: 5820966 (1998-10-01), Krause et al.
Kolthoff, I.M., Elving, P.J., eds., Wiley,Treatise on Analytical Chemistry Part II: Analytical Chemistry of the Elements, vol. 7, Wiley, (1961), pp. 155-156.
“Water Management-Policies, Guidelines Provincial Water Quality Objectives of the Ministry of Environment and Energy”, Ontario, Canada, Jul. 1994 (Jul. 1995 reprint) Ont. 152.0-155.6.
E. Jackson,Hydrometallurgical Extraction and Reclamation, Wiley, Canada, 1986.
A.A. Kudryavtsev,The Chemistry & Technology of Selenium and Tellurium, translated by E.M. Elkin, Collects, London, 1974.
Blakey Brian Charles
Raskauskas Justin
Hruskoci Peter A.
Inco Limited
Steen Edward A.
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