Similarly, the evaluation of the metallurgical processing has been determined by factors peculiar to the mineral content of the ore and the mineral texture, which is unusually coarse grained. The galena and marmatite are substantially liberated from each other and from the gangue minerals by relatively coarse grinding, and the substantial absence of iron sulphides simplifies the process of segregation into separate lead and zinc concentrates. Also, the association of the silver values primarily with the galena is a fortunate circumstance. However, with the cessation of smelting of the oxidized capping in 1897 the mines were confronted by the problem of the sulphide ore from which only 70 per cent of the lead and 50 per cent of the silver were recoverable using conventional gravity and magnetic separation methods, whilst the zinc values were completely discarded in dumped residues containing 20 per cent zinc.

Intensive studies in mill laboratories and trial plants over the next 15 years resulted in the world-shaking development of the froth flotation process, which was successfully applied in the first instance (1905) for recovery of zinc concentrates from accumulated gravity residues, and later for selective separation of lead and zinc concentrates from current gravity tailings, and later again (1912) for selective separation from the ore as mined. This bald summary of events cannot do justice to the ingenuity and persistence of the managers, the chemists and metallurgists and the practical mill operators whose efforts were ultimately responsible for the successful development of a process which was destined to become the most important single mineral concentrating technology in the world. The story has been told in detail by Woodward and Blainey in particular and by many other authors, and the names of Potter and Delprat in the initial stages and Bradford, Lyster and Owen in the final phase and many others in between are enshrined in the histories.

Broken Hill's gift of the flotation process to the world's mineral industries was essentially the fruit of trial and error plant scale research under dauntingly difficult conditions, and the overall result was a classic case of practice outrunning theory. The physical chemistry behind the process is complicated and the mechanism is still only partly understood, despite having engaged the attention of many eminent researchers throughout the world. The Broken Hill companies have contributed technically and financially to such researches, notably those of Wark and his co-workers which uncovered some of the most basic principles involved and inspired further researches which continue to this day in Australian and overseas research establishments and universities. The companies also contributed to the mineragraphic investigations of Stillwell and Edwards which included major studies of the mineralogy of Broken Hill.

Australian Academy of Technological Sciences and Engineering