A consultant suggested the following:

When Fe is present in refractories in a great enough quantitiy, it can combine with other impurities to cause low melting point eutectics with the Silica phase. An example is a combination of Fe, Ca and K that will lower the melting temperature below 1.000°F. I would speculate that the heavy metals would be fluxing agents (breaking chemical bonds in the Silica), and this phenomenon would be exacerbated by the higher iron content.

If there is a correlation between heavy metal content and sulfur content and sulfur levels are also elevated then the chemical reaction between the iron and the sulfur, forming iron sulfates, can occur. These materials are expansive and will destroy the refractory.