Mixing process development needs

In the tributyl phosphate extraction process developed at the Ames Laboratory, Iowa State University (46—48), a solution of tributyl phosphate (TBP) in heptane is used to extract zirconium preferentially from an acid solution (mixed hydrochloric—nitric or nitric acid) of zirconium and hafnium (45). Most other impurity elements remain with the hafnium in the aqueous acid layer. Zirconium recovered from the organic phase can be precipitated by neutralization without need for further purification. 

Table 4.21 lists the mixing process variables that are easy to measure using readily available instrumentation. Interfacing the controls with an alarm system allows the operator to be alerted to any out-of-specification condition. This needs to be corrected by cleaning, maintenance and recalibration before a major problem develops. 

The choice of a plasticizer is mainly ruled hy its compatibility with the polymer material involved. Indeed, as explained by the plasticization theories, plasticizer molecules have to deeply penetrate into the macromolecules network and remain stable inside. Thereby, the initial emulsion during the mixing process using an extruder or roll mixers requires a thermodynamically favourable plasticizer/polymer pair. In order to quantify the compatibility, several approaches have been developed. The most complex ones are the QSAR (Quantitative Structure-Activity Relationship) or UNIFAP (Universal Functional Activity coefficient for Polymers),which are highly effective, but need extensive amounts of data and consequently might be uncomfortable to employ in a first approach. 

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