Method and apparatus for stabilizing liquid elemental mercury

Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iib metal

Reexamination Certificate

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Details

C423S566100, C588S256000, C210S914000

Reexamination Certificate

active

06403044

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed generally to techniques for stabilizing mercury-containing materials and specifically to techniques for stabilizing elemental and speciated mercury in liquid and solid wastes.
BACKGROUND OF THE INVENTION
Because of the low melting point of elemental mercury and the toxicity of elemental mercury and environmentally unstable forms of speciated mercury to animals and humans, many environmental regulatory agencies, such as the U.S. Environmental Protection Agency, restrict the disposal of elemental mercury and unstable forms of speciated mercury. Unstable forms of speciated mercury include mercury oxide and water soluble mercury compounds such as mercuric chloride, methylated mercury, and organomercury compounds. While the disposal of such forms of mercury remains a problem, hazardous waste materials containing mercury are being generated daily by many sources, such as laboratories that use unstable forms of mercury in testing procedures, manufacturers that build products containing unstable forms of mercury, and scrap products containing unstable forms of mercury such as thermometers, vacuum tubes, x-ray tubes, thermostats, and the like. The U.S. Department of Energy alone has over 5,000 kilograms of radioactively contaminated, liquid elemental mercury awaiting disposal.
A common method for stabilizing mercury-containing compounds is amalgamation with an amalgamating agent, such as lead, copper, silver, zinc, magnesium, aluminum, and sulfur. Methods using amalgamation commonly (a) are unable to stabilize adequately unstable forms of mercury in the hazardous waste material, particularly liquid elemental mercury waste materials or waste materials containing both elemental and unstable forms of speciated mercury, to pass applicable environmental regulations, (b) use expensive additives, and/or (c) operate on only a small scale and are difficult to scale up economically to handle large quantities of mercury-containing wastes.
There is therefore a need for a process for stabilizing mercury that effectively stabilizes both elemental and speciated mercury such that the treated waste material complies with pertinent environmental regulations, uses relatively inexpensive additives, and/or operates economically both on small and large scales.
SUMMARY OF THE INVENTION
The method and apparatus of the present invention provide a simple, scalable, inexpensive methodology for converting hazardous forms of mercury in waste materials into environmentally stable and nonhazardous waste materials. In one embodiment, the method includes the steps of:
combining the mercury-containing feed material with a polysulfide and a reactive sulfur-containing compound other than the polysulfide to form a composite feed material; and
mixing the composite feed material to react the reactive sulfur-containing compound with the elemental and/or environmentally unstable forms of speciated mercury to form mercuric sulfide. The process effectively stabilizes both elemental and environmentally unstable forms of speciated mercury such that the treated waste material complies with pertinent environmental regulations (i.e., can pass the Toxic Characterization Leach Procedure or TCLP, or contains preferably no more than about 1,000 ppm and more preferably no more than about 200 ppm elemental mercury and/or environmentally unstable forms of speciated mercury).
The feed material can include high or low concentrations of either environmentally unstable forms of speciated mercury such as mercury oxide and water soluble mercury compounds (e.g., mercuric chloride, methylated mercury and organomercury compounds), and/or elemental mercury. Typically, the feed material has a total content of elemental and unstable forms of speciated mercury broadly ranging from about 0.01 to about 100% by weight. The process can be highly effective in applications where the feed material contains small or large amounts of elemental mercury and little or no speciated mercury or small or large amounts of speciated mercury and little or no elemental mercury. For liquid elemental mercury in particular, the feed material typically contains at least about 50 wt % elemental mercury and commonly is radioactive.
Because many mercury contaminated wastes contain water, the reactive sulfur-containing compound is any sulfur-containing compound (other than a polysulfide) that reacts with mercury in aqueous solutions. More preferably, the compound is elemental sulfur, an inorganic sulfide, and/or mixtures thereof, and even more preferably elemental sulfur, an alkali metal hydrogen sulfide, a mercaptan, an alkali metal sulfide, or mixtures thereof, with elemental sulfur being even more preferred. Elemental sulfur is reactive with mercury and is readily available and inexpensive. The preferred molar ratio between the reactive sulfur-containing compound on the one hand and the elemental mercury and/or unstable forms of speciated mercury on the other ranges from about 1:1 to about 30,000:1 and more preferably from about 2:1 to about 100:1. The reactive sulfur-containing compound typically ranges from about 10 to about 50 wt % and more typically from about 20 to about 30 wt % of the composite feed material.
The reactive sulfur-containing compound is preferably added to the feed material in the form of a powder. The average particle size of the powder preferably ranges from about 10 to about 500 micrometers and more preferably from about 50 to about 100 micrometers.
The polysulfide acts as an activator of the reaction between the reactive sulfur-containing compound and the unstable forms of mercury and is preferably selected from the group consisting of calcium polysulfide, sodium polysulfide, and other alkaline earth polysulfides and mixtures thereof. Such forms of polysulfide are readily soluble/suspendable in water and are readily available and inexpensive. The composite feed material preferably includes from about 0.5 to about 20 wt % and more preferably from about 2 wt % to about 10 wt % of the polysulfide. The preferred molar ratio between the reactive sulfur-containing compound on the one hand and the polysulfide on the other ranges from about 3:1 to about 650:1 and more preferably from about 10:1 to about 150:1 and between the polysulfide on the one hand and the elemental mercury on the other and unstable forms of speciated mercury preferably range from about 2000:1 to about 0.01:1 and more preferably from about 100:1 to about 0.1:1.
The polysulfide is typically dissolved or suspended in a liquid carrier or solvent. The preferred liquid carriers/solvents is water. A particularly preferred solution comprises from about 5 to about 29 wt % polysulfide. The polysulfide additive includes preferably from about 70 to about 95% and more preferably from about 75 to about 95% by weight of the liquid carrier/solvent.
The composite feed material is typically in the form of a flowable liquid (e.g., a nonviscous liquid or a viscous liquid such as a paste or slurry) and has a pH preferably at least about pH 7 and more preferably from about pH 9 to about pH 12 to inhibit the release of H
2
S gas during the mixing step.
The duration of the mixing step is important to realize the substantially complete conversion of the elemental mercury and speciated mercury to mercuric sulfide. Preferably, the mixing step has a duration of at least about 30 minutes and more preferably from about 60 to about 120 minutes.
The mixing step is preferably performed by intrusive mixing techniques. The mixing step can be performed by any suitable mixing device capable of mixing a viscous material and expelling (or venting) any vaporized liquid, such as by blending, beating, grinding, and the like, with stirred vessels including a plurality of rotating mixing blades being more preferred. The mixer is preferably at least one of the following: a pug mill, a screw-type mixer, a planetary mixer, and a ribbon blender. In such mixers, the blades preferably rotate at least about 5 rpm and most preferably at from about 10 to about 200 rpm. The mixer can

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