Mount Isa Mines clocks up 90 years of lead smelting

When Mount Isa Mines was formed in 1924, and with no processing plants in the region or 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.

Mount Isa ore bodies were different to most other lead mines, with both sulphide and carbonate ores of high and low grades and commonly used method of processing and smelting lead did not deliver high returns for Mount Isa's uncommon ores.

Mount Isa Mines built an experimental plant at Mineside in 1928, which 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.

Lead Smelter construction began in 1929 and was commissioned in June 1931, at a total cost of £3,500,000 pounds.

Mount Isa's first tonne of lead bullion was cast and shipped overseas and began a decade of successful lead smelting from 1931 to 1943.

As the war intensified in 1942 Australia needed to expand production of armaments, and copper demand increased.

At the request of the Commonwealth Government, Mount Isa Mines transitioned from lead smelting to copper smelting to produce copper for war munitions.

During the transition, tests were conducted on one section of the Lead Mill, and demonstrated a concentrate grade of 22-23% copper with a recovery of 80-85%. These figures were improved to 93-95% by 1946.

After the war copper mining and smelting continued with parallel production of zinc-lead-silver.

Mount Isa Mines has produced in 9.2 million tonnes of crude lead bullion over 90 years, mined from rock and processed to lead bullion at the site.

From 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 three million tonnes, reached in March 1980.

Since commissioning 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 stands at 270 metres 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 22 metres at the base, a diameter of 12.4 metres at the top, a thickness of 700mm of concrete at the base, and a thickness of 235mm of concrete at the top.

Around 5000 tonnes of ice was used 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.

The Lead Smelter produces crude lead. Lead concentrate is mixed with fluxes (high grade silica, limestone and copper slag) in order to maintain the heat and chemical reactivity required to separate the valuable lead content from the waste material.

The concentrate mixture is fed into a 30-metre long, three-metre wide sinter plant where the conditioned concentrate is mixed with coke and then baked to remove sulphur and other impurities. This sinter, along with the coke, is then fed into the top of the blast furnace.

Then the blast furnace heats the output from the sinter plant to 1250C.

This process melts the sinter into liquid, which flows from the bottom of the furnace into the crucibles in front separating and drawing out impurities (slag), leaving only crude lead.

Liquid crude lead is transferred and left to settle in one of six 100-tonne kettles for further treatment. As it cools the heavy molten lead, with its relatively low melting point, sinks to the bottom of the kettle.

The lighter impurities float to the surface and begin to solidify, where they are scooped off. What remains is 99.6 per cent pure lead bullion.

Lead bullion is cast into four tonne blocks, railed to Townsville then shipped to Britannia Refined Metals in England for further refining and sale on the London Metal Exchange.

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