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 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.

Graphical abstractPlot showing the magnitude of the effects of 5 operator-controllable factors on agglomeration, with temperature and excess oxygen being the most influential, and gas velocity having the opposite effect, causing elutriation.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Low excess oxygen (<10%) and temperature above 940 °C contribute to agglomeration of zinc calcine particles during roasting. ► Lead sulphide, an impurity in zinc sulphide concentrate, plays a key role in agglomeration of zinc calcine particles. ► Gas velocity, oxygen content, and initial size of the bed material do not significantly affect agglomeration.