BENCH SCALE SALTSTONE PROCESS DEVELOPMENT MIXING STUDY

Abstract

The Savannah River National Laboratory (SRNL) was requested to develop a bench scale test facility, using a mixer, transfer pump, and transfer line to determine the impact of conveying the grout through the transfer lines to the vault on grout properties. Bench scale testing focused on the effect the transfer line has on the rheological property of the grout as it was processed through the transfer line. Rheological and other physical properties of grout samples were obtained prior to and after pumping through a transfer line. The Bench Scale Mixing Rig (BSMR) consisted of two mixing tanks, grout feed tank, transfer pump and transfer hose. The mixing tanks were used to batch the grout which was then transferred into the grout feed tank. The contents of the feed tank were then pumped through the transfer line (hose) using a progressive cavity pump. The grout flow rate and pump discharge pressure were monitored. Four sampling stations were located along the length of the transfer line at the 5, 105 and 205 feet past the transfer pump and at 305 feet, the discharge of the hose. Scaling between the full scale piping at Saltstone to bench scale testing at SRNL was performed by maintaining the same shear rate and total shear at the wall of the transfer line. The results of scaling down resulted in a shorter transfer line, a lower average velocity, the same transfer time and similar pressure drops. The condition of flow in the bench scale transfer line is laminar. The flow in the full scale pipe is in the transition region, but is more laminar than turbulent. The resulting plug in laminar flow in the bench scale results in a region of no-mixing. Hence mixing, or shearing, at the bench scale should be less than that observed in the full scale, where this plug is non existent due to the turbulent flow. The bench scale tests should be considered to be conservative due to the highly laminar condition of flow that exists. Two BSMR runs were performed. In both cases, wall shearing was shown to reduce the rheological properties of the grout as it was processed through the transfer line. Samples taken at the static feed tank showed that gelling impacted the rheological properties of the grout before it was fed into the pump and transfer line. A comparison of the rheological properties of samples taken at the feed tank and transfer line discharge indicated shearing of the grout was occurring in the transfer line. Bench scale testing of different mixing methods with three different salt solutions showed that method of mixing influences the rheological properties of the grouts. The paddle blade mixing method of the salt solution used for the BMSR testing provided comparable rheological properties of the grout prepared in the BMSR after 14 minutes of processing, B3. The paddle blade mixing method can be used to represent BMSR results and mixing time can be adjusted to represent larger scale mixing.