Dissolution of sintered thoria.

Thoria, prepared by calcining thorium oxalate, and compacted and sintered, became increasingly more difficult to dissolve in HNO3 HF mixture as the sintering temperature was increased. The temperature of calcination of the oxalate is an important factor. Low calcination temperatures produced a more reactive thoria with a greater surface area. This thoria sintered more readily, producing denser compacts which had a smaller B.E.T. surface area and were more difficult to dissolve. Thoria produced by calcination at 600°C showed "mottling" when sintered at 170°C. These dark-coloured mottles dissolved much more slowly than the white matrix in which they were embedded. No material other than thoria was detected in an X-Ray analysis of the mottled material. No extra lines or deformations were visible, and although interstitial carbon was suspected, no evidence for this was obtained