June 2019

A century of zinc production

  • By R J Sinclair FAusIMM

This article is an excerpt of an ebook titled ‘A century of zinc production in Australia’, which is available to download for free here.

Australia has been in the business of producing zinc for well over one hundred years and for many of those years has been the world’s largest source of zinc. It has also been a dominant player in zinc smelting and in the development of zinc smelting technology. However, as in many other areas of endeavour, today’s dominance of world production of zinc now lies with China, which accounts for 38 per cent of world mine output and 50 per cent of world smelter production. With recent closures of several large mining operations, Australia is now in fourth place in zinc output behind Peru and India. India is a fast emerging power and has doubled its metal output in the last few years.

The commencement of Australian industry’s role in the zinc business was at the transition of a raw materials change from oxide ores such as Calamine to sulfide ores. Particularly in Europe, but also in North America, oxide ores used as feed to the zinc retort process were fast being depleted. Zinc sulfide ores were available, but needed to be roasted to separate sulfur, and this created environmental problems with emission of relatively large quantities of sulfur dioxide. At the turn of the 20th century, some smelters in Belgium and Germany were able to do this, although not in Britain where the zinc smelting industry declined as local Calamine deposits were exhausted.

Early mining operations in Australia containing zinc were largely centred on Broken Hill and the west coast of Tasmania, and initially targeted the recovery of lead and silver. At first the ores were heavily oxidised and contained only small amounts of zinc, but as they moved to depth into the sulfides, the relative amount of zinc increased considerably. Lead-silver concentrates were produced by gravity methods and separation of zinc was difficult, particularly for finer grained ores such as those in Tasmania. Zinc contained in the lead concentrate created problems during smelting and the need to keep within limits saw recovery of lead and silver decline to the point where the smelters at Zeehan in Tasmania had to close. Large dumps of concentrator tailings containing high levels of zinc as well as lead and silver were accumulating at Broken Hill. The ‘Zinc Problem’ encouraged a great deal of investigation, particularly at Broken Hill where the incentive to recover these metals was very high. These investigations led to the development of the flotation process for mineral separation which did indeed revolutionise mineral processing, and greatly improved the performance of the mining industry at that time. As a result, the recovery of lead and silver and the quality of feed to the lead smelters was greatly improved, and a new zinc sulphide concentrate was produced. At this time, the zinc concentrates were mainly shipped to European smelters in Belgium and Germany. 

New companies were floated at Broken Hill to acquire and treat the tailings dumps and included the Zinc Corporation and Amalgamated Zinc (de Bavays) Limited. Mining consultant Herbert Hoover – later President of the USA – was one of those instrumental in these developments and was for a time joint managing director of Zinc Corporation.

In parallel with mineral processing developments, attempts were also being made to produce zinc metal in Australia from sulfide materials. The first of these was Edgar Ashcroft, an electrical engineer working at Broken Hill, who attempted to develop an electrolytic process for zinc-based on leaching roasted zinc concentrates. His first trials using a sulfate electrolyte failed, so he turned to a chloride system which had some success. With some confidence, he floated the Sulphide Corporation (Ashcroft Process) Ltd in London and constructed a ten ton per day demonstration plant at Cockle Creek near Newcastle in NSW in 1897. This was a complete failure and the Sulphide Corporation converted the plant to the zinc retort process, but that in turn was uneconomic and the site again converted, this time to lead smelting operations. BHP also installed a zinc retort plant at their Port Pirie lead smelter in 1908. That operated until 1921, but recovery of zinc was poor and it was never profitable.

Another proponent of the electrolytic concept was James Gillies, who had worked at Broken Hill and thought he could overcome the problems experienced by Ashcroft. He operated a pilot plant using a sulfate electrolyte and developed an idea with Professor Alexander McAuley of the University of Tasmania to construct a hydroelectric scheme to provide low cost power to a zinc smelter and calcium carbide plant to be located in an industrial complex near Snug in southern Tasmania. The Hydro-Electric Power and Metallurgical Company was formed in 1910 and commenced construction of the Great Lakes Power Scheme. Unfortunately, the electrolytic process was unsuccessful and due to global turmoil at the time, the company was unable to secure funds to complete the power scheme. That was acquired by the Tasmanian Government in 1914 and the scheme was completed under the Hydro-Electric Department – later the Hydro-Electric Commission.

The advent of World War I at that time saw the outlets for zinc concentrate to the Belgian and German smelters closed and Britain lost a major part of its supply of zinc metal to produce armaments. Britain turned to the United States for supply and encouraged further development of zinc smelting in both Canada and Australia, particularly for high-grade metal which required the electrolytic process. Supply contracts for Australian zinc were negotiated with the British Government involving the Zinc Producers Association and the Australian Government, led by the Prime Minister of the time – William Morris Hughes. A successful electrolytic process had just been developed by the Anaconda Company at Butte in Montana, USA, and a team from the Amalgamated Zinc Company inspected the new process and commenced negotiations with the Tasmanian Government for power supply and a site for a new zinc smelter at Risdon on the Derwent River close to Hobart. This scheme was selected for development in preference to the original proposals by Gillies, in large part due to the fact that roasting would not be done on site, but closer to mining operations with the zinc calcines shipped to Hobart.

‘There is an incredibly rich history in the development of mining and smelting technologies associated with the zinc industry in Australia.’

The choice of roasting elsewhere provided the opportunity to produce sulfuric acid and secure a direct outlet to fertiliser operations. At that time, acid was produced by the chamber process at a strength of around 70 per cent. At this strength, it is highly corrosive and cannot be shipped, so is best produced where it is used. A number of fertiliser plants in South Australia and at Cockle Creek were used for this purpose, where roasters and acid plant were installed, and the zinc calcines then shipped to Hobart. Fertilizer production was also introduced at Risdon to permit some roasting on site. With the development of the catalytic conversion process, acid of 98 per cent strength could be produced and as it is not corrosive to mild steel, it could be readily stored and shipped in large quantities. Subsequently, all roasting was shifted to the smelter site. The production of sulfuric acid is a significant adjunct to zinc smelting with around two tonnes of acid produced for each tonne of metal.

The Electrolytic Zinc Company of Australasia Limited (EZ) was formed to construct and operate the Risdon plant and the first zinc metal was successfully produced in a pilot facility in 1916. The technology of zinc electrolysis was difficult due to the presence of many minor impurities in solution and considerable research work to understand the role of impurities and how to remove them was initially undertaken by a cooperative effort between EZ and the fledgling CSIRO. The Risdon plant pioneered many technological improvements and expanded virtually continuously from its original shape in 1920 at around 30 000 tonnes per year to over 200 000 tonnes per year over a period of at least 50 years. A significant development near the end of that growth phase was the jarosite process to recover zinc from zinc ferrite leach residues and raise zinc recovery from 85 per cent to 95 per cent plus. Other similar developments occurred elsewhere, which saw significant improvements in the performance of zinc smelters. Due to environmental issues, that process has now been replaced.

‘The development of some of these large deposits lifted Australian zinc output to be the largest in the world.’

Following on from the arrangements between the British and Australian governments for zinc metal supply, the zinc industry in the UK was dominated by the Consolidated Zinc Corporation in the years following World War I through its subsidiary The Imperial Smelting Corporation. That group operated zinc retort plants at Swansea Vale and Avonmouth and in the late 1940s developed the Imperial Smelting Process as a blast furnace method for the simultaneous production of zinc and lead. One of the first of these furnaces was constructed at the Cockle Creek site in 1961 with a capacity of around 50 000 tonnes per year of slab zinc. This plant set the scene in improvements, and in time lifted output to 70 000 tonnes per year. Many similar plants were constructed around the world and subsequently a much larger furnace in Avonmouth in the UK.   

The metal produced from the Imperial Smelting Furnace was not high-grade zinc.  It contained lead, was suitable for galvanising applications, but had to be refined by distillation to achieve high grade specifications. This added considerable cost and eventually the technology was uncompetitive and many plants around the world had to close, including Cockle Creek. 

In 1967, the Port Pirie lead smelter installed a slag fuming plant to recover zinc oxide fume from lead blast furnace slag. This was aimed at treating current slag production as well as a large slag stockpile. A small electrolytic plant with a capacity of 40 000 tonnes per year was also installed to produce zinc metal from the recovered oxide. The technology was old as a batch operation and eventually with the Port Pirie and Risdon plants under the same management and spurred on by changes to electricity tariffs in South Australia, the electrolytic operation was closed and the oxide from the slag fumer shipped to the larger plants for conversion to metal. 

Mount Isa, in north west Queensland, was a major source of zinc concentrates from the 1940s. Although Mount Isa Mines commenced with lead smelting operations in the 1930s and added copper smelting in 1953, it sold all its zinc concentrates, particularly to Japanese smelters. From the 1960s onwards, several large zinc deposits were discovered in the region from around Mount Isa up to the Gulf of Carpentaria. These include Cannington, Dugald River, Lady Loretta, Century and McArthur River – all major zinc resources. Most were very fine grained, making the separation of zinc and lead difficult. It took many years before improved flotation methods were developed to address this problem and the McArthur River mine took forty years from the point of discovery to commencement of commercial mining operations. The development of some of these large deposits lifted Australian zinc output to be the largest in the world. Subsequently, there have been closures, particularly the completion of the Century Mine, which have recently reduced that status.

The availability of zinc concentrates in that region in the 1990s encouraged the idea of developing a large-scale smelting operation in North Queensland – focused on Townsville as the rail head for the region and export port. A number of proposals were developed for a new smelter by the major Australian zinc producers, but all were discouraged by an economic downturn at the time. It was the Korea Zinc Company that made the bold decision and constructed the Townsville Smelter through its subsidiary Sun Metals Limited in 1999. The plant was designed with the latest technology from their own smelter in Korea and with tankhouse technology from the Belgian company Umicore. The Townsville smelter has since expanded from its original capacity of 170 000 tonnes per year to 250 000 tonnes per year and is now of comparable size to the much older Risdon plant.

Today, only the Risdon (Hobart) and Townsville zinc smelters remain in operation. The Broken Hill mines, which were the powerhouse of mining, are a shadow of their past. Tasmanian mines on the West Coast, which once saw themselves as rivals of the great Broken Hill field, are mainly history with only the Rosebery field producing zinc today. Old mining fields in New South Wales, South Australia and Western Australia are also mainly history and north Queensland remains the major zinc mining area at this time.

Many great companies grew out of mining operations centred on the lead and zinc industry, such as BHP. The wealth created in the early days was reinvested broadly throughout the economy and contributed to a large extent in the industrial development of Australia. It is however sobering to think how few of those great Australian corporations have survived to this day.

There is an incredibly rich history in the development of mining and smelting technologies associated with the zinc industry in Australia which does need to be remembered and recorded, and I hope the associated text may do that in an interesting and stimulating way. 

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