Roasting of mixed sulphide ores or concentrates

In the roasting (oxidation) of mixed zinc sulphide/lead sulphide concentrates, to give an oxidic product for reduction, improved sulphur elimination and reduced lead volatilization are achieved by mixing 2-20% by weight of zinc oxide with the mixed sulphide materials, forming pellets of 2-15 mm diameter and roasting these under controlled oxidation conditions, firstly at a lower temperature and then at a higher temperature; preferably at 850.degree.-950.degree. C. for the first stage and at 950.degree.-1050.degree. C. for the last stage.

This invention relates to the roasting (oxidation) of sulphide ores or 
concentrates and more particularly to the roasting of raw materials 
containing both zinc sulphide and lead sulphide. 
The usual method of roasting such materials to give an oxidic feed, for a 
smelting process, is by sinter roasting on a conventional Dwight-Lloyd 
sinter machine with updraught of air to support combustion (see for 
example "Updraught zinc/lead sintering" by R. M. Sellwood, Mining Journal 
1960, 254, 15th April, 434-435). Using this technique concentrates 
containing up to 25% by weight of lead sulphide can be roasted. 
There is however a need to develop a roasting process for concentrates 
containing zinc sulphide and lead sulphide, which is independent of this 
sintering technique. It is known that if such materials can be calcined to 
particulate oxide form then they can be densified into briquettes which 
are suitable for feeding to a blast-furnace, e.g. by the techniques 
described in our British patent specification Nos. 1,302,864 and 
1,394,609. 
One approach to this problem, by forming pellets having a lead sulphide 
core and a zinc sulphide shell, for subsequent roasting, is set out in our 
U.K. patent specification No. 2,024,792 published Jan. 16, 1980. This 
approach is aimed at preventing molten phases causing pellet 
agglomeration. However certain residual problems remain with such a 
roasting technique and these problems are even more marked when 
homogeneously mixed zinc sulphide/lead sulphide concentrates are to be 
roasted. The problems are principally: 
(a) loss of lead by volatilization (probably as sulphide) during roasting, 
(b) incomplete or inadequate sulphur elimination from the roasted pellet. 
We have now found that by addition of controlled amounts of zinc oxide to 
zinc and/or lead sulphides to be roasted, coupled with the use of 
carefully controlled oxidation conditions these problems can be greatly 
ameliorated or largely overcome. 
This invention consists in a process for roasting a material containing 
both zinc sulphide and lead sulphide, to give an oxidic product, 
comprising forming the material into pellets of between 2 and 15 mm 
diameter, with the incorporation of between 2 and 20% by weight of zinc 
oxide powder, and roasting these pellets in an oxygen-containing gas in at 
least two stages, the first being at a lower temperature and the last 
being at a higher temperature. 
Preferably the first oxidation stage is conducted at 
850.degree.-950.degree. C. and the last is conducted at 
950.degree.-1050.degree. C. 
Preferably the amount of zinc oxide incorporated in the pellets is between 
2% and 10% based on the pellet weight. 
Preferably the pellets are formed from mixed zinc sulphide and lead 
sulphide concentrates. However, the pellets may be formed having a 
substantially lead sulphide core and a substantially zinc sulphide shell 
according to the technique described in U.K. patent specification No. 
2,024,792. 
Preferably the oxygen-containing gas is air or a roaster gas containing 
SO.sub.2 and oxygen. 
In this way it is possible to obtain roasted pellets containing less than 
1% of residual sulphur coupled with 25% or less lead volatilization 
(equivalent to a sinter roasting process).