Hydrometallurgical production of zinc oxide from roasted zinc concentrates

#This invention relates to the production of high quality zinc oxide from roasted zinc sulphide concentrates.

#By "roasted zinc sulphide concentrates" is meant the product obtained by roasting a zinc sulphide bearing concentrate. The term zinc calcine is also used to refer to such products. Typically, roasted zinc sulphide concentrates contain zinc oxide and other components, including zinc sulphate, zinc sulphide, mixtures of metal oxides such as copper, lead, calcium, cadmium, and magnesium, zinc ferrite, magnetite, possibly some haemetite, and usually some metal silicates, sulphate and sulphides.

#Zinc sulphide bearing concentrates are commonly roasted in a fluid bed roaster. In the fluid bed roasting of zinc concentrates, the concentrates are continuously fed into the roaster chamber, wherein air blown up through the chamber converts sulphides to oxides releasing sulphur into the gas stream chiefly as sulphur dioxide, with the development of heat. Smaller and lighter particles in the feed concentrate tend to be carried out of the chamber with the gas stream and are collected by downstream equipment. Larger and heavier particles tend to remain in the roaster fluidized bed through which the moving air is blown. Since the bed is full during continuous operations, a bed overflow stream outlet is provided by which further accumulation of coarse solids is prevented by allowing bed material to overflow out of the chamber. This material is referred to as "bed overflow calcine". In some zinc refineries, calcine is composed of an approximately 50:50 by weight mixture of bed overflow calcine and fine material that is recovered from the gas stream, as described above. This mixture is then used as a feed material in the production of zinc metal and is referred to as normal calcine at least in some cases, such as at the zinc refining from which samples were used in the U.S. Pat. No. 5,028,410 hereafter referred to below.

#FIG. 1 provides a description of the process in the form of a block diagram .

#In this example a specimen of normal zinc calcine and a specimen of bed overflow zinc calcine were obtained and analyzed. The results of the analysis are reported in Table 1. The bed overflow calcine contained lesser amounts of arsenic and sulphur compared to the normal calcine. The bed overflow calcine contained 61.2% extractable zinc, comprising 61.0% in an ammonia-soluble form and 0.2% in a water-soluble form. The normal calcine contained 57.7% extractable zinc, comprising 54.9% in an ammoniasoluble form and 2.8% in a water-soluble form.

#A specimen of normal and bed overflow calcine were separately processed in accordance with the process described below.

#The calcines were leached in a 28% aqueous ammoniacal ammonium carbonate solution which was prepared by feeding gaseous carbon dioxide into a concentrated ammonium hydroxide solution with vigorous stirring. The final pH of this solution was 11.0. An amount of 450 g of calcine was leached in 1.5 liters of this aqueous ammoniacal ammonium carbonate solution with agitation at a temperature of 50.degree. C. Leaching was conducted for 40 minutes with the occasional addition of gaseous carbon dioxide gas to maintain the pH at approximately 11.0. The resulting slurry was filtered through a filter and the resulting filtrate and residue were analyzed. The assay of the resultant filtrate and residue is reported in Table 2.