PNNL

Abstract

High level waste melter feed is converted into glass in a joule heated melter, where it forms a floating layer of reacting feed, called the cold cap. After the glass-forming phase becomes connected, evolving gases produce bubbles that form a foam layer under the cold cap. The bubbles coalesce into cavities that escape around the edges of the cold cap. The foam layer insulates the cold cap from the heat transferred from the molten glass below. More information is needed about the formation and behavior of the foam layer to control, limit and possibly avoid foaming, thus allowing for a higher rate of melting. The cold cap behavior was investigated in a laboratory scale assembly with a sealed silica-glass crucible. A high alumina waste simulant was fed into the crucible and the feed charging rate was varied from 3 to 7 mL min-1. After a fixed amount of time (35 min), feed charging was stopped and the crucible was removed from the furnace and quenched on a copper block to preserve the structure of the cold cap and foam during cooling. During the rapid quenching, thermal cracking of the glass and cold cap allowed it to be broken up into sections for analysis. The effect of the charging rate on the height, area and volume of the cold cap was determined. The size of the bubbles collected in the foam layer under the cold cap increased as the cold cap expanded. Under the cold cap, the bubbles coalesced into oblong cavities. These cavities allowed the evolved gases to escape around the edges of the cold cap through the molten glass into the melter plenum.