BEHAVIOR OF MERCURY IN HIGH-TEMPERATURE VITRIFICATION PROCESSES

The Pacific Northwest Laboratory (PNL) has evaluated the waste processing behavior of mercury in simulated defense waste. A series of tests was performed under various operating conditions using an experimental-scale liquid-fed ceramic melter (LFCM). This solidification technology had no detectable capacity for incorporating mercury into its product, borosilicate glass. Essentially all the mercury fed to the melter was lost to the off-gas system as gaseous effluent. An ejector venturi scrubber condensed and collected 97% of the mercury evolved from the melter. Chemically, the condensed mercury effluent was composed almost entirely of chlorides, and except in a low-temperature test, Hg2Cl2 was the primary chloride formed. As a result, combined mercury accounted for most of the insoluble mass collected by the process quench scrubber. Although macroscopic quantities of elemental mercury were never observed in process secondary waste streams, finely divided and dispersed mercury that blackened all condensed Hg2Cl2 residues was capable of saturating the quenched process exhaust with mercury vapor. The vapor pressure of mercury, however, in the quenched melter exhaust was easily and predictably controlled with the off-gas stream chiller. These tests followed 12 earlier experiments performed at PNL to study the behavior of mercury during vitrification of simulated typical defense waste. The experiments were conducted using an experimental-scale spray calciner/in-can melter (SC/ICM) system plus a very similar off-gas system. Results with both the LFCM and SC/ICM technologies were essentially the same. Just as with the LFCM, the mercury was completely volatilized from the in-can melter system in all experiments. It reacted predominantly with halogens to form a fine particulate solid, most of which was deposited in the off-gas system piping.