Evaluation of melter performance: Behavior of noble metal sludge

Abstract

Velocities in a flat bottom, two electrode melter are 0.03 to 0.3 cm/s and are lowest on the cavity bottom where noble metals would tend to accumulate. Velocities in 45{degrees} sloped bottom, three-electrode design are estimated to be from 0.1 to 0.4 cm/s and are highest near the bottom of the cavity, The terminal velocity of 100 micron-sized Pd in molten glass is about 1 x 10{sup -3} cm/s, which is far slower than normal convection currents. Accumulation of noble metals is thus attributed to trapping inside boundary layers where fluid is nearly motionless. In a flat bottom melter, the boundary layer near the cavity floor could be large due to low bulk fluid motion. Once a sludge layer forms, it takes on distinct Theological properties having a higher viscosity with pseudo-plastic behavior. The critical velocity for resuspension, calculated from a correlation for flow in a pipe, is 1000 times higher than convection currents in the LFCM. It is thus important to keep noble metals suspended and prevent sludge formation. Gravity could play a role in directing sludge to a bottom drain. The calculated steady state velocity of a falling film of sludge is sufficient to keep a 45{degrees} sloped bottom free from major accumulations if it is a Newtonian fluid.