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 degrees C) contributed most significantly to agglomeration of zinc calcine particles. Only one test - with 0% excess oxygen at 940 degrees C - led to severe agglomeration and subsequent defluidisation. 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. (C) 2011 Elsevier Ltd. All rights reserved.