ORE TREATMENT BROKEN HILL MINES The initial oxidised ore mined at Broken Hill was rich in silver and lead, and was smelted direct.  BHP erected the first smelters at Broken Hill in 1886 and a refinery at Port Pirie in 1889.  The first mill was erected in 1889 to treat lower grade lead carbonate ores using gravity methods. By 1893, the oxidised ore was nearly exhausted and the field became increasingly dependent on the outlying sulphide ore.  Concentration mills for the treatment of sulphide ore commenced in 1894.  The method consisted of dry crushing in Cornish rolls and concentration in jigs, tables, vanners and buddles.  This separates  the galena and sphalerite to such an extent that the lead (and silver)could be smeltered.  The process recovered only about 50% of the lead and 67% of the silver in the original ore but left a zinc – rich residue, which was dumped around the mines.  This inability to treat the zinc residue became known as the sulphide problem and led to the closure of many mines.  By 1904, more than 6.5 million tons of zinc – rich tailings had been dumped. The solution to the problem was eventually  found by metallurgists at Broken Hill with the advent of the flotation process.  This process involves the mixing of finely crushed ore and liquid in a container, through which bubbles of air or gas are passed.  Chemical agents are added to the container which enables the sulphide minerals to float upwards on the bubbles for collection in overspill troughs. The Potter – Delprat method was the first commercial flotation process.  It was introduced by BHP in 1904 and operated with very little change in their mill until closure in 1927.  Initially, flotation was used to produce a zinc concentrate from the zinc – rich tailings.  In 1905, Zinc Corporation was formed to treat accumulated tailings.  Selective flotation was formed to treat accumulated tailings.  Selective flotation was discovered in 1912, and produces a differential concentration of lead and zinc sulphide. After 1930, there was a gradual change from combined gravity and flotation mills to plant recovering lead, zinc and silver by flotation only.  Modern all – flotation mills were erected at the South Mine in the early 1930s, Zinc Mine in 1936, North Mine in 1939, and NBHC Mine in 1952.  Concentration mills at North and NBHC  Mines are in operation at present, and the selective flotation process remains essentially the same as that introduced at Broken Hill during the early 1900s.  Today, flotation produces a silver – lead concentrate containing 97% of the lead and 94% of the silver, and zinc in the original ore. Lead concentrate is railed to the Pasminco owned refinery at Port Pirie, South Australia, and mixed concentrate is smeltered at Cockle Creek, NSW.  A portion of the zinc concentrate is exported and the remainder is smelted at Cockle Creek, N.S.W or Risdon, Tasmania.