Stacks of Isa history in lead smelter milestone
It is a mighty milestone for the mining city, with a skyline marked by the towering presence of the operation’s current lead stack, standing at 270m.
When Mount Isa Mines was formed in 1924, with no processing plants in Queensland, ore was sent the enormous distance to either Port Pirie or Newcastle for processing.
For the company to be profitable, given the huge expense that came with handling, brokerage, freight and processing, the ore needed to contain a minimum of 45 per cent lead and 20 ounces of silver per tonne.
It quickly became apparent that for mining to be economically viable, the processing of ore to metal needed to take place in the heart of the Mount Isa mining lease.
To compound matters, Mount Isa ore bodies were different to most other lead mines, with Mount Isa having both sulphide and carbonate ores of high and low grades.
Therefore, the commonly used method of processing and smelting lead did not deliver high returns for Mount Isa’s uncommon ores.
Prior to the construction of the city’s lead smelter in 1931, Mount Isa Mines built an experimental plant at Mineside in 1928.
This included crushers, grinders, concentrating tables, flotation banks, rollers for drying the concentrate, sinter plant and a blast furnace. At the time, it was reported as being the most up-to-date testing plant in the world.
Construction of Mount Isa’s lead smelter began in 1929 and it was commissioned in June 1931, at a total cost of 3,500,000 pounds, including all site infrastructure.
By 1931, Mount Isa’s very first tonne of lead bullion was cast and shipped overseas, which marked the start of a remarkable 90 years of lead smelting at Mount Isa Mines.
During the intensification of the war effort in 1942 it became necessary for Australia to expand production of armaments, and consequently copper demand increased.
During World War II and at the request of the Commonwealth Government, Mount Isa Mines transitioned from lead smelting to copper smelting to produce copper for war munitions, in an extraordinary display of Australian nationalism and ingenuity.
During the transition, tests were conducted on one section of the lead mill, and demonstrated that a concentrate grade of 22-23 per cent copper with a recovery of 80-85 per cent could be expected. These recovery figures were later improved to 93-95 per cent by 1946.
What was to follow was another remarkable period for Mount Isa Mines, this time for copper mining and smelting with parallel production of zinc-lead-silver.
In an impressive production achievement, Mount Isa Mines has produced in 9.2 million tonnes of crude lead bullion over the 90 years, mined from rock and processed to lead bullion at the one single site.
From the first tonne of lead cast in 1931, it took 28 years to produce one million tonnes. The second million tonnes was produced 13 years later in May 1972. It took a further eight years to reach the remarkable record of three million tonnes of crude lead bullion, reached in March 1980.
Since its commissioning in 1931, the lead smelter has been home to three separate lead stacks.
The first one was located near the Urquhart Shaft, the second was constructed along with a new baghouse in 1953 and was located just east of the current lead stack. The third, and current lead stack, was built from 1977-1978.
The current lead stack, standing at 270m high, took two full years to construct from 1977 – 1978, with around five metres of concrete poured each day.
The total weight of concrete used was 17,400 tonnes, which doesn’t include the reinforcing steel.
During the planning phase, load testing of the ground that would support the stack was carried out using large tonnes of lead ingots.
The lead stack has a diameter of 22m at the base, a diameter of 12.4m at the top, a thickness of 700mm of concrete at the base, and a thickness of 235mm of concrete at the top.
Interestingly, 5,000 tonnes of ice was used during construction, to keep the concrete cool while it was being poured in Mount Isa’s searing heat.
The portion of the flue which protrudes from the stack is insulated and clad with stainless steel. The stack is tapered to 205 metres and then continues parallel to the top.
New Zealand-based Chatham Rock Phosphate has made a $12.3 million deal to acquire the Korella phosphate mine, 150km south-east of Mount Isa.The compan...