Mixing Effects on the Precipitation and Cross Flows Filtration of a Hanford Simulated Precipitated Radioactive Waste

As part of the River Protection Project at Hanford, Washington, Bechtel National, Inc. has been contracted by the United States Department of Energy to design a Waste Treatment and Immobilization Plant to stabilize liquid radioactive waste. Because of its experience with radioactive waste stabilization, the Savannah River Technology Center of the Westinghouse Savannah River Company is working with Bechtel National and Washington Group International, to help design and test certain parts of the Waste Treatment Plant. One part of the process is the separation of radioactive isotopes from the liquid waste by a precipitation reaction and cross-flow ultrafiltration. To better understand those combined processes an experiment was performed using a simulated radioactive waste, made to prototypically represent the chemical and physical characteristics of a Hanford waste in tank 241-AN-102 and precipitated under prototypic conditions. The resultant slurry was then filtered using a cross-flow filter prototypic in porosity, length, and diameter to the plant design. An important aspect of filtration for waste treatment is the rate at which permeate is produced. There are many factors that affect filtration rate and one of the most difficult to obtain is the effect of particles in the waste streams. The Waste Treatment Plant will filter many waste streams, with varying concentrations and types of dissolved and undissolved solids. An added complication is the need to precipitate organic complexants so they can be efficiently separated from the supernatant. Depending on how precipitation is performed, the newly created solids will add to the complicating factors that determine permeate flux rate. To investigate the effect of precipitated solids on filter flux a pilot-scale test was performed and two different mixing mechanisms were used for the precipitation reaction. A standard impeller type mixer, which created a homogeneous mixture, and a pulse jet mixer, which created a ''less than'' homogeneous precipitation environment. Pulse jet mixers are included in the design of the Waste Treatment Plant because they have no moving parts, which is advantageous for radioactive operations as no maintenance is required. A pulse jet mixer uses alternate pulses of pressurized air and vacuum to mix tank contents. This pulse jet mixer was not expected to establish a homogeneous mixture throughout the test tank, therefore, it was important to know its effect on filtration after a waste is precipitated. This paper discusses the experimental results of cross-flow filter flux after a simulated waste is precipitated under each of the two mixing methods. The main conclusion reached is that while the ''less than well mixed'' jet-pulsed waste gave a lower average filter flux it may still be sufficiently large enough to meet plant requirements