Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
1999-10-12
2002-04-02
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S641000, C210S729000, C210S806000, C210S912000
Reexamination Certificate
active
06365051
ABSTRACT:
BACKGROUND OF THE INVENTION
For instance, within the Department of Energy (DOE) complex, 335 underground tanks were employed to process and store radioactive and chemical mixed wastes generated from weapon materials production over the past 50 years. These tanks hold collectively over 90 million gallons of high-level-wastes (HLW) and low-level-wastes (LLW) in forms of sludge, saltcake, slurry, and supernate. Some of the tanks have exceeded their life expectancy and leaked to contaminate about 470 billion gallons of groundwater.
Most of these tanks contain waste with a diverse portfolio of mainly inorganic anions including nitrate, nitrite, hydroxide, sulfate, and phosphate. Temperature profiles of these tanks vary from near ambient to temperatures over 93° C. Much of the radioactivity arises mainly from strontium, cesium, and to a lesser extent from rubidium, yttrium, barium, technetium, and actinides. Table 1 presents concentrations profiles for some of the DOE aqueous waste streams.
Cesium is the primary radioactive component found in supernatants and salts cakes. Strontium and technetium tend to be concentrated in supernatants and sludge washing liquids. Cesium and strontium are the major source of radiation and heat, while technetium tends to be very mobile in the environment and persist for a long period of time (half-life: 210,000 years). Actinides, however, tend to be concentrated in the sludge portion of the waste, and thus in the soluble portion, their concentrations are very low.
The DOE economic waste minimization strategy was centered on expending separation methods to pretreat the radioactive waste. These methods provide a sequence of processes to partition the waste into a small volume of HLW for deep geologic disposal, and a large volume of LLW for disposal in near-surface facilities. Vitrification, the process of converting materials into a glass-like substance, is currently the preferred HLW immobilization in deep geologic repository. The estimated cost of the vitrification and the repository disposal is about $1 million per canister of glass produced. As such, vitrifying wastes directly is prohibited, and the process is limited to concentrated HLW. Thus, the DOE treatment methods are focused primarily on the separation of the small quantities of radioactive species from the waste bulk. This would result in a small volume of HLW for vitrification, and a larger volume of LLW which can be disposed at a much less expensive cost.
A number of aqueous streams are encountered in tank waste treatment. These streams may include tank waste supernatant, waste retrieval sluicing water, sludge wash solutions, and the like. Several separation techniques are considered for any given tank waste stream. Solvent extraction, particularly by crown ethers, has gained significant consideration for DOE applications in separation of radioactive alkali and alkaline earth cations from liquid streams. Factors such as ion speciation in strong mixed-cation mixtures, and sensitivity to the volume and grade (solid-free) of the liquid stream are under experimentation. Traditional pressure-driven membrane processes such as nanofiltration, ultrafiltration, and microfiltration enhanced with molecular recognition agents attached to flow through membranes have been proposed and tested for selective removal of cesium, strontium, and technetium from aqueous streams. Factors such as membrane applicability and stability, separation selectivity, and capacity are critical issues that are under evaluation. Ion exchangers are currently under development for the removal of cesium and strontium from liquid waste streams. Organic and inorganic exchange materials are employed in the ion exchange process. Although both exchange materials exhibit strong retention for the cesium and strontium, factors such as chemical stability and capacity are yet to be resolved.
After retrieval of liquid waste from storage in tanks, sludge dissolution is needed. One of the specific techniques that is used by the DOE program is the enhanced sludge washing (ESW) process. The objective of the ESW is to remove key non radioactive inorganic species such as sulfate, phosphate, aluminum, and in some cases chromium from the tank waste sludge portion, leaving the radioactive species, primarily strontium and actinides, in the solid phase for vitrification. The process involves first the addition of 3 molar of sodium hydroxide to dissolve aluminum from the sludge. The sludge is then washed with an aqueous solution containing 0.01 molar sodium hydroxide and 0.01 molar sodium nitrate to remove interstitial liquid and any remaining soluble solids. The efficiency of the ESW process is questionable. First, inadequate removal of the key non-radioactive sludge components always results in production of an unacceptably large volume of HLW. Second, substantial volumes of aqueous wash solutions as well as sludge are generated which both require treatment and disposal as LLW. Inspection of Table 1 indicates high concentrations of sodium in forms of hydroxide, nitrate, and nitrite as a result of the ESW process (sodium negatively interferes with vitrification). Sludge handling and disposal is becoming more expensive as burial requirements increase and approved burial sites become less available. Third, excessive settling times of suspended solids with gel formation.
Activities such as sludge retrieval and sludge washing have resulted in excess water in tanks wastes. To conserve tank storage space and reduce the volume of the final waste form, evaporator has been demonstrated in a small scale operation to remove the generated excess water. The objective is to evaporate the aqueous waste to a level approaching the solubility limit of the dissolved salts. Conventional evaporation, however, implies high capital and operating costs. Innovative evaporation techniques are thus of a prime interest.
The DOE is currently seeking processing technologies to treat waste streams directly, or after appropriate separation, to produce environmentally stable, and regulatory acceptable final waste forms. Of particular interest are technologies that can perform highly selective separation of calcium/strontium or complexed technetium from aqueous streams (e.g., groundwater, supernate, slurry). Also, of prime importance are processes that are either capable of directly and selectively separate: (1) radioactive species such as strontium and actinides; or (2) bulk non radioactive inorganic species such as sodium and aluminum from sludge dissolution activities. Processes are also sought for the removal of radionuclides from calcined waste streams (plutonium and other actinides) at high temperatures; particularly processes that: (1) separate inorganic species into concentrated product streams; (2) can withstand a radiolytic environment; (3) can be scaled to processing at rates of 2 to 30 gallons per minute; (4) are simple to construct and operate; and (5) are economically viable.
Although some of the DOE under development emerging technologies individually remove their target contaminants effectively, these technologies would presumably be employed in series, and would each entail separate process requirements, consumption and stripping of materials, effluent streams, and different impacts on vitrification. However, in many severe cases such as the DOE waste streams, a single type system may not be the best answer. Hybrid systems to improve productivity and achieve better separation would be the optimum solutions. As such, there are compelling advantages to a single hybrid processing system that could concentrate radioactive species for small HLW, and evaporate the aqueous phase to produce an ultrapure effluent stream, leaving low volume of concentrated LLW. This invention provides an innovative hybrid process based on combining precipitation with membrane distillation concepts. The precipitation step would perform the highly selective separation of alkaline earth cations (calcium, strontium, barium, and radium), alkali earth cations (rubidium and cesium), and other radioa
Fellers Snider Blankenship Bailey & Tippens, P.C.
Fortuna Ana
Ward Richard W.
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