A laboratory-scale process has been developed for the solidification of nitrate salt wastes in polyethylene. The process uses a commercially available single-screw extruder which continuously discharges prescribed polyethylene-waste mixtures from the hoppers to the output die, where it is extruded into a container while still in the molten form. The molten mixture (̃110 to 120°C) conforms to the shape of the container and solidifies upon cooling. Proportional feeders maintain waste-to-binder ratio and homogeneity of the waste form. Present studies use dry wastes, although wet solid wastes can be processed using vented extruders of the type used for the bitumen solidification process.

Tests were performed to determine leachability and mechanical stability. Emphasis is placed upon leaching of nitrates from the waste forms. Leach tests were performed according to ANS 16.1 as well as the EPA extraction procedure (EP). For polyethylene waste forms containing 30 to 70 weight percent sodium nitrate, ANS 16.1 leach indices range from 11 to 7.8, respectively. Compressive yield strengths range from 18 to 5 MPa (after 90 days' water immersion). The results of the EP indicate that the nitrate release levels for waste forms containing as much as 70 weight % sodium nitrate are not defined as characteristically hazardous waste.

Differential scanning calorimetry (DSC) was used to confirm the compatability of polyethylene and simulated salt wastes at elevated temperatures. Components of the polyethylene/sodium nitrate system, alone and in combination, were tested by DSC at temperatures to 400°C. At these temperatures, no chemical interactions were observed between polyethylene and nitrate waste compositions.