Daryl Roberts, Robin Stewart, Tom Broderick
ADA Technologies, Englewood, CO 80112

John Litz
Colorado Minerals Research Institute, Golden, CO 80403

Cliff Brown, Andrea Faucette
Advanced Integrated Management Services, Inc., Arvada, CO 80002

ADA Technologies and its subcontractors, Colorado Minerals Research Institute (CMRI) and Advanced Integrated Management Services, Inc. (AIMSI), have demonstrated the amalgamation of ordinary elemental mercury and radioactively contaminated elemental mercury in batch sizes up to 75 pounds (2.5 liquid liters) using sulfur in a conventional pug mill mixer. The process satisfies the Environmental Protection Agency’s definition of amalgamation as given in 40 CFR 268.42, Table I. After developing the technology with ordinary mercury, we have demonstrated the technology by conducting treatibility studies on wastes provided by the Los Alamos National Laboratory (LANL). To date, three separate batches of LANL waste have been processed, a total of 185 pounds. The extent of conversion of mercury to HgS was over 99.92% for each batch, and each batch passed the Toxic Characterization Leach Procedure (TCLP) test for leachable mercury.

The reaction of mercury with sulfur is exothermic at room temperature. However, mixing these reagents is difficult. In one extreme, it is possible to ignite a spontaneous flame, and in another extreme, it is possible to coat the mercury with a layer of HgS, thwarting further reaction. It was our goal to conduct the amalgamation at a large enough scale to be of use in the DOE complex and at the same time to use conventional mixing equipment that could be easily scaled up to larger sizes and could be easily operated. We accomplished this goal by successfully using a pug mill mixer, a rotating blade device commonly used in metallurgical and chemical operations, as opposed to ultrasonic mixers, paint can shakers, and the like that had been used by previous workers in this field.

We estimate the cost to process one gallon of liquid mercury to be $10,000, assuming that the facility processes fifty batches of one gallon each per year for 10 years. This cost includes analyses required for rad safety, for following the extent of reaction, and for determining leachability. This figure excludes the cost of paperwork for handling and shipping radioactive waste and the cost of final disposal. Based on the legacy waste inventory of about 4,500 gallons of liquid mercury, the total cost to treat and dispose of DOE’s legacy waste mercury is estimated to be between $50 million and $100 million.

Mercury and mercury-contaminated wastes are some of the more pervasive and troublesome wastes in the inventory of DOE legacy waste materials. Most of the larger DOE sites have radioactively-contaminated liquid, elemental mercury in their mixed waste inventories. Complex-wide, there are approximately 16.5 m³ (500,000 pounds) of elemental mercury in the legacy waste and about 0.2 m³ (5,700 pounds) per year generated at the Savannah River Site (Petersell, 1998).

The Environmental Protection Agency (EPA) specifies amalgamation as the treatment method for radioactively contaminated elemental mercury. Although the chemistry of amalgamation is known, the practical engineering of a sizable amalgamation process is not known (e.g., Gorin, et al., 1984; Tyson, 1992). A process that will serve the DOE complex must process at least approximately two to three liters of mercury (60 to 90 pounds) per batch since even at this scale, treating the entire DOE inventory would require approximately 5,000 batches (one batch per day for over ten years, seven days per week).

Before funding work in this area, DOE’s Mixed Waste Focus Area (MWFA) established a Technology Deficiency Requirement Document (TDRD) with the following criteria for a successful mercury amalgamation process:

1. the process must meet EPA’s definition of an amalgam given in 40 CFR 268.42, Table I;
2. the waste must pass EPA’s 0.2 mg/L treatment standard based on TCLP so as to allow the waste to be disposed in a subtitle D landfill;
3. the mercury vapor concentration above the waste must be less than 50 µg/m³;
4. the process must be readily scalable; and
5. the process must be economically viable.

ADA Technologies, Inc., and its subcontractors, Colorado Minerals Research Institute (CMRI) and Advanced Integrated Management Services, Inc. (AIMSI) have demonstrated the amalgamation of both ordinary elemental mercury and radioactively-contaminated elemental mercury in batch sizes up to 75 pounds (2.5 liquid liters) using sulfur in a conventional pug mill mixer. After developing the technology with ordinary mercury, we have demonstrated the technology by conducting treatibility studies on wastes provided by the Los Alamos National Laboratory. To date, three separate batches of LANL waste have been processed, a total of 185 pounds. The extent of conversion of mercury to HgS was over 99.92% for each batch, and each batch passed the Toxic Characterization Leach Procedure (TCLP) test for leachable mercury. We are confident that the process meets the first two, and the only regulatory, requirements set by the MWFA.

Because we have used conventional mixing equipment to accomplish the amalgamation, the process is both scaleable and economical (requirements number 4 and 5). We have yet to perform the mercury vapor pressure tests, but we expect that our process will meet this requirement also.

The following sections describe the process, testing with ordinary elemental mercury, the treatibility tests with the LANL waste, and an approximate economic evaluation of the process.

We mix the liquid mercury with sulfur in a conventional mixer known as a pug mill. Pug mill mixers are commonly used in metallurgical and chemical operations where intense mixing of pasty material is required. Tens of thousands of such systems are utilized industrially today. Examples of common pug mills are the Rietz thermal screw, the Holoflite dryer, and the Bethlehem Porcupine Processor. The Holoflite dryer has been used to make sulfur polymer cement in tests at DOE’s Idaho National Laboratory (Darnell, et al., 1992). Others have been used to stabilize RCRA wastes (e.g., Barth, 1990; Trezek, 1992). Manufacturers of pug mills for chemical stabilization of contaminated soils and sludges include Marion Mixers, Inc. (Marion, IA), Pugmill Systems, Inc. (Columbia, TN), and Scott Equipment Company (New Prague, MN). A brief description of these types of mixers and their relationship to other industrial mixers can be found in Kirk-Othmer’s encyclopedia (Faulkner and Rimmer, 1995).

We used a small, dual shaft mixer that accommodates approximately 2 ft³ of material (Figures 1 and 2). This mill is three feet long and has a one-foot square cross section. Its blades are 5.5" long. A liner was placed in the pug mill to reduce the dead volume beneath the blades. The typical rotation speed of the pug mill blades is 50 RPM. This size of pug mill accommodates the desired full-scale processing rate of 100 lb in an eight-hour shift with no difficulty.

The basic process involves adding sulfur to the pug mill first, then pouring in the mercury. The mixing and reaction are followed by monitoring the mixture temperature and periodically taking samples for analysis. Mixing is concluded when the reaction exotherm subsides and the free elemental mercury analysis indicates that over 99.9% of the mercury has reacted. The details of the process are the subject of a patent application.

Fig. 2. Top View of Blades of Pug Mill

The reactions of mercury with a variety of amalgamating agents are exothermic and, in principle, should proceed at room temperature. In practice, the mixing of the mercury with the amalgamating agent is the principle difficulty to overcome. Nearly 100% extent of reaction can be achieved when small quantities of mercury, approximately 10 cm³ or less, are reacted in the laboratory with conventional shakers or manual stirring. However, with the quantities of liquid elemental mercury waste in the DOE complex, batch sizes of approximately two to three liters will be required to allow the DOE inventory to be processed in a reasonable time.

Therefore, our goal in working with "surrogate waste" (that is, ordinary elemental mercury) was to learn to conduct the mixing of two to three liters of mercury with sulfur in conventional mixing equipment. Working with a conventional mixer was an important part of our process development strategy so that we would be confident in our ability to scale up the process.

In the laboratory, we found it difficult to achieve greater than 50% extent of conversion of the mercury when reacting sulfur with mercury. However, when we added a sulfur-containing liquid to the mixture, we could achieve up to 98.8% extent of reaction of the mercury.

In working with the pug mill and 30 pounds of mercury, we were able to achieve 99.9% extent of conversion of the mercury, but the TCLP results were in the range of 1.2 mg/L to 2.6 mg/L, well above the statuatory limit of 0.2 mg/L. Only when we further added sand to the mixture were we able to achieve more than 99.9% extent of reaction, and then the TCLP results were consistently below 0.1 mg/L. During this work with the pug mill, we found physical forms of sulfur that worked and did not work, quantities of sulfur-containing liquid that worked and did not work, and quantities of sand that worked and did not work. The resulting formulation and processing conditions form the basis of our patent application.

The key results, however, of the ordinary mercury tests are that we achieve in the pug mill more than 99.9% extent of reaction of the mercury with the sulfur, and we achieve a leachable mercury concentration below the TCLP limit of 0.2 mg/L.

We received 242 pounds (110 kg) of contaminated, waste mercury from Los Alamos National Lab. The shipment came from Los Alamos in a 40-gallon carbon steel drum. Inside the drum were five two-liter steel flasks, each of which were roughly full with mercury. The radioactivity level of the mercury was quite low, and in fact no radioactivity was detected with a standard gamma scan.

To date we have treated 185 pounds of this waste in three separate batches weighing 50 pounds, 62 pounds, and 73 pounds. At the end of each treatment, we determined the mass concentration of the unreacted mercury. The extents of reaction (1 minus the free mercury, expressed as a percentage) in these batches were 99.963%, 99.951%, and 99.922%, respectively. The TCLP testing with each batch showed less than 0.1 mg/L of leachable mercury.

We have one 57-pound batch remaining of the Los Alamos waste in the treatibility study. We have also received a 55-pound batch of waste mercury from the Fernald site. The last Los Alamos batch and the Fernald batch will finish our treatibility study.

The basis for the cost evaluation was a facility designed to treat 50 gallons of waste mercury per year for 10 years. The figure of 50 gallons per year is the quantity currently generated at the Savannah River Site. Waste transportation and disposal costs were not included in the cost evaluation.

We assume that one gallon of mercury is processed in each batch, and we also assume that the equipment must be cleaned between every other batch. Besides the pug mill itself, the installed equipment for the process includes safety equipment, such as radiation and mercury monitors, eyewash/shower station, and scales to weigh process materials and drums with treated waste. Laboratory equipment is also required to perform analytical procedures during the waste processing operation.

The total installed facility cost was estimated from the cost of the individual components by multiplying by a factor of three (Perry and Chilton, 1973). Annual maintenance costs were also estimated using recommendations from Perry and Chilton. In general, maintenance should be a minimum of four percent per year of the capital equipment cost for chemical processing equipment. Maintenance costs include the material, installation, and overhead costs.

Labor costs were estimated based on a three person staff. The staff would be required to perform the operational, analytical, and clerical functions associated with the process. Direct labor, materials and supplies, subcontracted analytical costs, and the indirect costs were included in the operating costs for this process. The annual analytical costs were determined based on the number of waste streams. These cost included characterization tests of the wastes to document that the waste passed TCLP for mercury and cost of swipe tests to certify cleanliness of the process equipment.

Operating costs for the process were escalated at a rate of 3.5% per year over the life of the project. The capital costs of the facility were amortized over the ten year life of the facility. When these costs were combined with the operating costs, the overall treatment cost for the stabilization process was estimated to be $10,000 per gallon of waste mercury, or $190 per kilogram of mercury.

We have demonstrated amalgamation of radioactively contaminated liquid elemental mercury with sulfur in a conventional pug mill mixer. The process satisfies EPA’s definition of an amalgamation process, and the resulting waste satisfies TCLP leaching criteria, allowing disposal of the waste in a subtitle D landfill. The pug mill equipment is easy to scale up and easy to operate, and consequently, this technology represents a viable path for the treatment of DOE’s mercury wastes. The total cost of disposing of DOE’s legacy mercury wastes with this technology is estimated to be $50 million to $100 million.

The tests of the radioactively-contaminated mercury were funded by Lockheed Martin Energy Systems under subcontract 1GX-05496. The tests of the surrogate mercury wastes were funded by ADA Technologies.

The authors wish to acknowledge the active participation of the Mercury Working Group headed by Mr. Tom Conley of the Oak Ridge National Lab in pushing hard to develop technologies that will treat real wastes in the DOE complex. We also wish to thank Mr. Chris Duy at the Los Alamos National Lab and Mr. Al Schmidt at the Fernald site for their diligent assistance in coordinating the transfer of the radioactively-contaminated mercury to CMRI for the treatibility studies.

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