A remarkable 90 years of lead smelting at Mount Isa Mines

As Mount Isa Mines marks 90 years of lead smelting, we delve into the archives to look at Mount Isa’s rich lead smelting history and the phenomenal achievement of producing in excess of 9.2 million tonnes of crude lead bullion, mined from rock and processed to lead bullion at a single site.

The Mount Isa Lead Smelter is one of just a handful of advanced minerals processing facilities left in Australia. It was commissioned in June 1931 and has gone on to achieve significant tonnages.

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.

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 Lead Smelter, 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.

Construction of Mount Isa’s Lead Smelter began in 1929 and 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.

Following more than a decade of successful lead smelting from 1931 to 1943, and 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 to 23 per cent copper with a recovery of 80 to 85 per cent could be expected. These recovery figures were later improved to 93 to 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 excess of 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 to 1978.

The Lead Stack

The current Lead Stack, iconic for being the tallest stack, standing at 270 metres high, took two full years to construct from 1977 to 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 700 millimetres of concrete at the base, and a thickness of 235 millimetres 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.

The role of the Lead Smelter

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 our 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 1,250 degrees celsius.

This process melts the sinter into liquid, which flows from the bottom of the furnace into the crucibles in front of the furnace 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 4-tonne blocks, railed to Townsville then shipped to Britannia Refined Metals in England for further refining and sale on the London Metal Exchange.

As we mark the 90-year anniversary, our photo gallery on this web page takes you on a historical journey to pay tribute to the accomplished people who built it and who continue to operate it today.