Hard gray scale

Abstract

The dry scrubbing and secondary alumina handling systems for pre-bake cell technologies are hampered by accumulations of hard gray scale, or HGS. HGS is an amorphous reaction product formed from collision-induced reactions of bath fines, alumina and water. HGS formation is greatest in high attrition areas owing to the high surface energy of new alumina surfaces created by the fracture of alumina particles. If not systematically addressed with cleaning and maintenance procedures HGS can: greatly reduce the removal efficiency of gaseous fluoride in dry scrubbers, reduce the capability to convey alumina in sufficient quantities, reduce filter life and alter the concentration of fluoride on secondary alumina. By moving to higher line load and/or pot hooding flow rates an acceleration of HGS formation in dry scrubbers and reacted alumina transport lines is likely, especially when existing equipment is utilized without upgrades. This accentuates the need for counter-measures to reduce or control HGS formation rates. This paper defines the chemistry of HGS, proposes a mechanism for HGS formation, discusses the conditions that are necessary to form HGS, identifies the key areas of concern in a smelter, and discusses practical alternatives to reduce rates of HGS formation.