Basic lead sulfates as agglomerating agents during the fluid-bed roasting of zinc concentrates
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bonding was stronger and the core consisted of a shell with an internal fibrous structure. At 850 and 900 ~ the core was hollow apart from a small amount of debris. The alumina and silica particles were originally white in color, but when the agglomerates formed it was possible to discern a yellow glaze on the surface of the particles. X-ray diffraction analyses of the agglomerates formed at 850 ~ with all three bed materials are shown in Table I. The analyses show that lead sulfates were present in all of the agglomerates and that there was no crystalline lead silicate in the agglomerate formed with the silica bed. The results obtained with the different bed materials were marked by their similarity. The only notable difference was in the sizes of the agglomerates--at any of the experimental temperatures, the order of increasing size was zinc calcine, alumina, and silica. This may have been caused by variations in the distribution of particle sizes rather than by variations in the wetting
During the fluid-bed roasting of zinc concentrates, agglomeration and fusion of the bed sometimes occur. Lead compounds, particularly lead silicate,~,2 have long been suspected as the source of the trouble, and in some industrial plants 2,3 precautions are taken to limit the amount of lead in the feed and to lower the temperature of the bed as the proportion of lead increases. An earlier investigation 4 had failed to reveal the presence of lead silicate in fused bed materials from an industrial plant, Table I. XRD Analyses of Agglomerates Formed at 850 ~ so the present work was undertaken to determine the role of lead compounds, particularly lead silicate, as Bed Material Major Phases Present agglomerating and setting agents. Alumina Alumina, monobasic lead sulfate Small pellets of galena were put in a stainless-steel (PbO 9 PbSO4) Silica Silica, monobasic lead sulfate wire mesh basket and suspended in a laboratory-scale Calcine Zincite (ZnO) with traces of franklinite fluid-bed furnace (see Fig. 1). By choosing appropriate (ZnO 9 Fe203), monobasic lead sulfate sizes for the galena pellets, the basket mesh, and the bed Note: Minor amounts of anglesite (PbSO4) and of dibasic and material, it was possible to contain the pellets in the tetrabasic lead sulfate (2PbO 9 PbSO4and 4PbO 9 PbSO4)were also basket for later recovery and yet allow the bed material detected. to circulate freely through the mesh and around the fluidized pellets. The sizes used were: galena pellets, - 1550/+ 1150/~m; basket-mesh aperture, 1212/xm; and bed material, - 4 8 8 / + 60/xm. Vent The galena pellets, which were aggregates of smaller particles, were screened from a lead concentrate containing 74.8 pct Pb. Three bed materials were used: high-purity tabular alumina, silica sand, and zinc /// calcine. Tabular alumina is not a normal component of ,/// ~ ~ ~ - Basket zinc concentrates, but it was included in the present work to demonstrate that the same basic agglomerating Thermocouples mechanism occurs regardless of the kind of bed material. The zinc calcine
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