The phenomena of swelling and growth-expansion during the vitrification of MTR type waste material

A study has been carried out on the foaming effect which occurs during the calcination of MTR type waste materials, in order to improve the in-pot vitrification technology. Foaming is accompanied by swelling and growth-expansion phenomena, and is influenced by the initial composition of the waste and by the operating parameters in the applied vitrification process (in particular the heating conditions). The waste studied is made of a mixture of alkaline sludges and zeolites coming from the treatment of simulated high aluminum content nitric liquid waste (the so-called 'alkaline declassification' process), and the vitrifying additives. During the heat treatment of the samples, attention has been focused on: - softening temperature, - thermal decomposition, examined by differential thermogravimetric analysis, - residual nitrate ion and released gas composition, - iron oxidation state after heat treatment in air and in nitrogen. The foaming effect on the calcined material occurred in the temperature range 923/1023 K and it is due to gas release at temperatures higher than the softening temperature of the calcined material. The most effective remedy in minimizing foaming was found to be the addition of organic substances, such as sugar to the material to be calcined. A weight ratio between sugar and the nitrate content of 1.5 : 1 was found to be sufficient to eliminate the foaming effect. During the waste heating process in presence of sugar, nitrates are totally destroyed at temperatures much lower than the bulk softening point of the material. In addition, within 1273 K in nitrogen, only a partial reduction of iron occurs. (author)