Technology:
Polyethylene Macroencapsulation:
- Davis-Standard 4.5-in single-screw extruder feed hopper, two-stage rotating augerlike screw, heat-controlled barrel, and output die assembly:
- Extruder equipped with five electric clamshell-type barrel heating zones and two die heating zones with thermocouple controllers and cooling loop
- Output capacity of 2000 lb/hr
- Temperature of melted polyethylene exiting extruder - 300-350°F
- Virgin polymer (LDPE) with a melt index of 2 g/min initially used for demonstration; changed to LDPE with melt index of 9 g/min
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Cost Factors:
- Costs were shared between Envirocare and DOE under the terms of the cooperative agreement. Envirocare paid for equipment and supplies, facility construction and modification, permitting and personnel training, and provided facilities for the treatment and disposal of wastes. DOE paid for the treatment and disposal of the encapsuated waste. DOE's cost for disposal of about $1 million for 500,000 lb or $1.92/lb
- An estimate of current costs for polymer macroencapsulation are $90 to $100/cubic foot. Polyethylene macroencapsulation operating costs at DOE sites average about $800/55-gal drum. |
Description:
Envirocare of Utah, Inc. (Envirocare) located in Salt Lake City, Utah, is licensed and RCRA-permitted to treat and dispose of low-level radioactive and mixed waste. Under a cooperative agreement between the DOE Idaho Operations Office (DOE-ID) and Envirocare , a demonstration of a polyethylene macroencapsulation extrusion process, developed by DOE at Brookhaven National Laboratory, was conducted at Envirocare's Utah facility to evaluate the technology for mixed waste lead and debris. The company obtained the required RCRA-permit modification to operate this technology, and, under the cooperative agreement, waste streams from 23 DOE sites were shipped to Envirocare.
The polyethylene macroencapsulation extrusion process heats, mixes, and extrudes the polyethylene into the waste container in one operation. The four basic components of the extruder are the feed hopper, rotating auger-like screw, heat-controlled barrel, and output die assembly. The polyethylene is masticated by the rotating screw, heated gradually, and mixed. The melted polyethylene is conveyed from the extruder at 300-350 °F and poured directly into the waste container where it flows around and into the waste matrix voids to encapsulate the waste. The polyethylene melt has sufficient heat capacity to provide a fusion bond at the cold polyethylene interface resulting in a continuous monolithic pour. For the demonstration, Envirocare used a Davis-Standard 4.5 inch single-screw extruder with an output capacity of 2000 lb/hr. A virgin polymer (LDPE) with a relatively low melt index of 2 g/min was chosen for this demonstration because Envirocare planned to augment the polymer feed with recycled plastics. During the demonstration, Envirocare determined that the use of this polymer was not well suited for production-scale operations for two reasons: (1) the extrudate was overly viscous and would not flow around the waste without manual assistance and (2) the recycled plastics had inconsistent properties from batch to batch, and therefore would not be efficient for production-scale operations. Envirocare experimented with composite LDPE mixtures with varying melt indexes before determining that LDPE with a melt index of 9 g/min (blend of materials with melt indexes of 2 and 60 g/min) provided the optimum feed stock for production-scale operations. (Envirocare found that using LDPE with high melt indexes ranging from 24 to 60 g/min were prone to cracking.) During the demonstration and throughout the cooperative agreement, Envirocare has continued to expand its process capabilities; the process has been proven effective for package sizes ranging from 5-gal buckets to 55-gal drums in 110-gal overpacks. Based on the results of the demonstration, Utah state regulators have developed specific waste acceptance criteria for the macroencapsulation process. Details of these criteria are presented in the report, along with an analysis of technology applicability and alternatives.
Through the cooperative agreement, Envirocare paid for equipment and supplies, facility construction and modification, permitting and personnel training, and provided facilities for the treatment and disposal of wastes. DOE paid for the treatment and disposal of approximately 500,00 lb of mixed waste lead and debris (lead bricks) that had been macroencapsulated using this process. The cost for this disposal was about $1 million or $1.92/lb. This amount includes substantial treatability study activities and costs for Envirocare to experiment with scale-up and process improvements. An estimate of current costs for polymer macroencapsulation are $90 to $100/cubic foot. Polyethylene macroencapsulation operating costs at DOE sites average about $800/55-gal drum. |
