Demonstration of the conditions conducive to agglomeration of zinc calcine in fluidized bed roasters

Agglomeration in the fluidized bed roasting of zinc sulphide concentrate was investigated in a continuous pilot-scale fluidized bed roaster. Alone or together, low stoichiometric excess oxygen (<10%) and high temperature (>940°C) contributed most significantly to agglomeration of zinc calcine particles. Only one test – with 0% excess oxygen at 940°C – led to severe agglomeration and subsequent de-fluidisation. Neither large initial particle size nor low gas superficial velocity promoted agglomeration as much as the combination of low stoichiometric excess oxygen and high temperature. Lead sulphide, a common impurity in zinc sulphide concentrate, also appeared to have played a key role in agglomeration. Both low excess oxygen and high temperature favoured a high partial pressure of sublimated lead sulphide. Upon contact with oxygen, lead sulphide oxidised to lead oxide, which then precipitated as a liquid onto the rather inert bed particles. As a liquid, lead oxide formed coatings that caused small particles to adhere to larger ones, as well as liquid bridges that bound particles together.