Risdon technology

The production of electrolytic zinc at Risdon dates from 1917, just two years after the first commercial production by this process simultaneously in Montana and British Columbia. The activity in Australia was spurred by the need to meet the zinc requirements of the British munitions industries in the First World War. Broken Hill zinc concentrates had been largely smelted in Europe and that outlet was then closed. The decision to adopt the new electrolytic method was bold and farsighted. Early experimental laboratory work in Broken Hill had revealed the presence of cobalt as a minor but toxic impurity in the zinc concentrates, and there were other warning signs that purification of plant solutions before electrolysis would need to be of the same order of efficiency as in the analytical laboratory. Methods were devised for reduction of the contents of cobalt, cadmium, copper, iron, antimony, arsenic and other constituents prior to electrolysis, and in the electrolytic cell itself modifications were introduced including the use of glue and other additional agents for maximum current efficiency.

In the calcining and leaching stages prior to solution purification Risdon practice has fully maintained equality in efficiencies and costs with the industries overseas. Calcining in particular has graduated through all stages of hearth roasting, flash roasting and fluid bed roasting; the current two fluid bed units provide calcine sufficient for production of 200,000 t/yr of refined zinc and will be supplemented for the planned 320,000 t/yr.

An important feature of the leaching practice at Risdon has been the zinc ferrite remaining in current and stockpiled insoluble residues from leaching which may carry over 10 per cent of the total zinc content of the original concentrates. The iron content of the marmatite mineral in the Broken Hill ore makes the zinc ferrite more significant in the Risdon operations than elsewhere. The Risdon metallurgists have developed and refined a 'jarosite process' for the leach residues which produces a discardable jarosite precipitate whilst recovering most of the zinc content in solution, together with a lead-silver residue which is shipped to the Cockle Creek plant and incorporated in the feed to the ISF blast furnace.

Associated operations within the Risdon complex include recovery of 350,000 t/yr of sulphuric acid from the fluid bed roaster gases and production of 150,000 t/yr of phosphatic fertiliser, also some production of special zinc alloys and zinc dust. Cadmium in refined form and cobalt oxide are by-products of the solution purification prior to electrolysis.

Mount Isa technology

The extraordinary saga of the discovery and development of the Mount Isa lead-silver-zinc-copper field, commencing with the discovery in 1923 and reaching production of lead-zinc-silver in 1931 and copper in 1943 has been related in fascinating detail by Blainey in 'Mines in the Spinifex' and by others who have dwelt on the daunting physical necessities of the establishment of a major mining and metallurgical operation 1000 km inland and 1500 km from a source of fuel, calling for a massive investment in power production, transport, water supply, engineering maintenance facilities and community development. The record of realization of a viable industry under these conditions with a present annual capacity of 150,000 t copper, 160,000 t lead, 200,0001 zinc and 5001 silver, and reserves to support this level for 30 years or more, is accordingly impressive, especially when other technical considerations are understood. Whilst Mount Isa had some general problems in common with Broken Hill inasmuch as a combination of oxidised and sulphide ores was involved, the character of the ores differed greatly. By contrast with the massive high grade and coarse grained Broken Hill orebody which responded so successfully to flotation the Black Star, Black Rock and Racecourse orebodies of Mount Isa were relatively low grade, extremely fine grained and complicated by dissemination of the lead and zinc sulphides throughout a matrix of pyrite-pyrrhotite in a shale bed containing also a carbon-bearing gangue material which floated as readily as the sulphides. The unfortunate combination of features led the first mill superintendent and metallurgist, the American L. K. Jacobsen, to observe that the situation called for two mills: one to treat the ore and the other to treat the metallurgists. The comment was valid. After years of effort the grade of lead flotation concentrate was 53 per cent Pb with a recovery therein of 86 per cent of the lead and 87 per cent of the zilver. The zinc concentrate grade was 52 per cent Zn carrying 72 per cent of the zinc values. Comparable figures for Broken Hill would be 76 per cent Pb containing 97 per cent of the lead and 94 per cent of the silver, and 53 per cent Zn with 90 per cent of the zinc.

Australian Academy of Technological Sciences and Engineering