[en] One of the major programs in the Department of Energy (DOE) waste management operations at Hanford is the volume reduction and solidification of Hanford Defense Residual Liquor (HDRL) wastes. These wastes are neutralized radioactive wastes that have been concentrated and stored in single-shell underground tanks. Two production vacuum evaporator-crystallizers were built and are operating to reduce the liquid volume and solidify these wastes. The process involves evaporating water under vacuum and thus concentrating and crystallizing the salt waste. The high caustic residual liquor is composed primarily of nitrate, nitrite, aluminate, and carbonate salts. Past evaporator-crystallizer operation was limited to crystallizing nitrate, nitrite, and carbonate salts. These salts formed a drainable salt cake that was acceptable for storage in the original single-shell tanks. The need for additional volume reduction and further concentration necessitated this process development work. Further concentration forms aluminate salts which pose unique processing problems. The aluminate salts are very fine crystals, non-drainable, and suitable only for storage in new double-shell tanks where the fluid waste can be continuously monitored. A pilot scale vacuum evaporator-crystallizer system was built and operated by Rockwell Hanford Operations to support flowsheet development for the production evaporator-crystallizers. The process developed was the concentration of residual liquor to form aluminate salts. The pilot plant tests demonstrated that residual liquors with high aluminum concentrations could be concentrated and handled in a vacuum evaporator-crystallizer system. The dense slurry with high solids content and concentrated liquor was successfully pumped in the insulated heated piping system. The most frequent problem encountered in the pilot plant was the failure of mechanical pump seals due to the abrasive slurry