Zinc

Facts about Zinc

Was used in Rome and China more than two thousand years ago
Was first smelted from zinc ore in India in about 1200.
Commercial production started in Europe in the 18 th century.
The main zinc mineral is sphalerite, containing up to 67% zinc.
Commonly occurs as a mixed ore with lead, silver and copper.
Is formed from hydrothermal fluids generated from deep within the earth.
Is alloyed with copper to make brass.
Is primarily used for galvanizing iron and steel to protect against rust, and as an alloy with aluminium for roofing, cladding and fences.
Is also used in sun protective products and skin care, in agriculture, in plastics, rubber products, paints, and for use in die casting parts.
Broken Hill is the largest producer of lead-silver-zinc in Australia
Zinc ore is mined in all states and the Northern Territory ranking Australia as first in the world for zinc resources, third in world mine production and second in exports.
Most zinc-silver-lead ore mines are underground, though Century in Queensland is a large open cut.

Zinc in Tasmania

The Nyrstar Hobart Smelter, is situated on the Derwent River on the outskirts of Hobart. Commercial production of zinc began in 1921 in the operation known as the Electrolytic Zinc Company or the ‘Zinc Works’ as it was locally known. The site was selected for a number of reasons. In 1916 the government of the day wanted to develop hydro electric resources. Operations such as a zinc smelter require large amounts of energy. As well, the site offered deep water for shipping and a stable workforce in close proximity.

Before the electrolytic process was invented zinc was produced using large amounts of coal, and required a large labour force working in very poor conditions. The zinc ore was roasted, mixed with charcoal and smelted to make a vapour which was then condensed. However, very little zinc was recovered as the process was inefficient and very expensive. Germany and Belgium were the main producers. Ore containing silver, lead and zinc was found at Broken Hill in New South Wales and at Rosebery on Tasmania’s West Coast, but after extraction of the silver and lead, the zinc was left in slag heaps until technology improved the process and world demand increased due to shortages as a result of World War 1. At that time, as it is now, zinc was widely used for the purpose of galvanizing, as an anti corrosive and alloyed with copper to make brass for munitions, especially shell and cartridge casings.

In the early 1900s a proposal for processing zinc electrolytically in Tasmania was put forward by inventor and metallurgist James Gillies. In 1908 he set up his own power scheme, but ran out of money, after which the government bought the power station with the view to encourage big industries. Subsequently Gillies’ process for smelting was deemed unsuitable for a number of reasons, including environmentally, though the process that was eventually used came with its own environmental and health concerns. Zinc smelting produces liquid by- products, poisonous fumes and dust containing chemicals such as sulphur, cadmium, mercury and lead. In those days these were not well contained or treated, a far cry from how they are managed today.

In 1916 an Australian company called Amalgamated Zinc was encouraged to develop a smelter in Tasmania, lured by the cheap hydro power, a willing labour force, good rail and shipping facilities, and the encouragement of the British government for facilities to be developed in allied territories. A floatation plant was already operating in Broken Hill to produce zinc concentrate, the material needed to further refine to make zinc metal.

In 1918 the smelter produced 1,582 tons (1,744 tonnes). Employment at the site in 1920 was 1300, though this figure rose and fell by hundreds when times were tough such as in the Great Depression of the 1930s or when production was high during the Second World War. Employment peaked in 1967 at 2800, gradually reducing due to technological change. Today the operation employs around 700 and produces 250,000 tonnes annually of zinc metal.

Setting up the Risdon site was not without its difficulties. The electrolytical process turned out to be more difficult than anticipated due to impurities, but over time these metallurgical problems were solved. Financial problems came with a drop in world prices, as well as transportation and technological inadequacies.

Later industrial unrest would cause havoc, bringing with it its own set of problems and reducing production. The cyclical nature of the metals industry influences production and capital works decisions, and this has been evident throughout the operation’s life. Despite this the operation entered into a major construction phase which included a 100 ton plant, built to increase capacity and improve profitability. It began operating in 1921, processing over 100,000 tons of concentrate a year resulting in the production over 50,000 tons of zinc metal by the end of the decade. The plant set the scene for production at Risdon for the rest of century. Other capital works to improve and increase production since have included an Ammonium Sulphate Plant,( no longer in operation) a residue treatment plant, a contact acid plant and much later calcine storage, lead residue filtration plants, and an effluent treatment plant. The Roasting process produced sulphur dioxide. A sulphuric acid plant was built for this. The acid was used in the leaching process and also in the production of the fertilizer Superphosphate which is used in the agricultural industry in Tasmania.

In the 1920s the production line looked like this -

Concentrate arrived on the wharf where three electric traveling jib cranes unloaded.
A belt conveyer took the raw material to a storage shed, where an electric shovel loaded it on to side-tipping trucks.
A rope haulage system with a winch pulled the trucks up the hill and transferred the concentrate to the plant.
Material went into the Roasting Division, where furnaces removed the sulphur, then by another conveyor to the Leach, where it was dissolved in sulphuric acid.
The insoluble portion containing lead and silver and some zinc was filtered off and sent back to Port Pirie for treatment.
The solution was stripped of impurities such as cobalt and cadmium, and then sent back to the Cell Room.
An electrical current was passed through the solution and zinc was deposited on the cathodes.
These were lifted out and strippers removed the sheets of zinc, which were melted and cast into slabs in the Casting Division.
A chain conveyor took the slabs to the wharf, where they were weighed and stacked in piles.
The cranes were equipped with cradles which were fitted over the stacks of zinc and lifted then into the ships.
The zinc was transported to Europe, Japan, India and other Australian States.

In the 1920s zinc ore was sourced from the Rosebery mine and mixed with ore from the Broken Hill mine, thus ensuring supply to the smelter. Today concentrates come from Rosebery and the Century mine in Queensland.

Zinc was in high demand again during World War 11. The metal was used for shells and cartridges, galvanizing iron, making paint, tyres, smoke mixtures and flares. Because of the vital nature of zinc, the Zinc Works became a security target. Zinc was in high demand and access to supply was difficult. Security was implemented that meant workers had to produce identity discs, the site boundary was patrolled with armed personnel, a barbed wire fence was erected and public traffic was prohibited. In 1942, fears were realised when a Japanese plane from a submarine making reconnaissance flights flew up the Derwent River, over the Zinc Works.

Despite the excellent opportunity for production, a shortage of labour restricted the operation’s ability to fulfill the demand. A female research technician was the first skilled woman to be employed at the site during this period, though women had been employed in the administrative area from the start. During this phase a construction program was begun to improve the process, particularly with the aim to achieve more pure zinc which was being used for die-casting in weapons. Clydesdale horses and carts, picks and shovels were the common methods used for transportation and construction labour. It wasn’t until the late 1940s that trucks were introduced, bringing in a more mechanical age to Risdon.

Later in the 1970s advances in technology, a world glut of zinc, low prices and newer smelters around the world dictated a whole new set of operational decisions. It was during this time environmental practices began to gain some attention.

Hobart had expanded quite significantly since the early part of the century, with increased human activity, housing and several industries along the river influencing the flow and quality. Effluent containing heavy metals such as zinc, mercury, cadmium and lead from the Zinc Works was discharged into the River Derwent, sulphur fumes and zinc dust into the air. Increasingly attention, not only in Tasmania, but all around the world, was being placed on improving environmental quality. Monitoring of the River began, assisted by the availability of a measuring device called the Atomic Absorption Spectrophotometer which could measure metals in parts per million. In 1973 the government of the day set up the Department of Environment and the Environmental Protection Act was introduced. An Environmental Services section was established at EZ and over time treatment plants for contaminated water and effluent were built as well as the implementation of other measures to reduce the pollution.

Jarosite, a by product occurring from the extracting of metals from the residue dump, also caused problems. The solution to this was for it to be dumped out at sea and this continued until the 1990s. Today due to improvements in metal recovery it is sent to Port Pirie, another smelter in South Australia, where it is further treated. Rehabilitation continues today on the old dumps at Risdon.

Keeping up in an international market meant further capital works to modernise the plant. Aligned with this was, and still is, access to well priced power. In 1984 the EZ Company became part of North Broken Hill group and the Pasminco group in 1989. These changes brought with them a new era. In 2003 subsequent to Pasminco going into Administration, Nyrstar was formed. The Hobart Smelter forms part of the group which includes the Rosebery Mine in Western Tasmania, the Century Mine in Queensland, smelters in Port Pirie, the Netherlands and Tennessee USA. High quality zinc metal and alloys are sent mostly to Asia and in particular China, where Nyrstar alloy is the market leader.

In 2004 at the Hobart smelter the process of converting the zinc concentrate into zinc ingots ready for use in manufacturing is more comprehensive due to improved technology, safer and better environmental practices. The sequence still involves roasting, leaching, purification and electrolysis in a cyclical process.

Roasting – Zinc concentrate is roasted to calcine to make it more readily soluble for further purification. A by-product of this step is the production of sulphur dioxide.

Leaching – the calcine is leached in a five stage process using acidic solution which is the spent electrolyte from the electrolysis step. This produces an impure zinc sulphate solution, and leaves a lead/silver residue. Iron is also removed as paragoethite, which requires further treatment off site.

Purification – The zinc sulphate solution is purified removing elements such as copper, cadmium, cobalt and nickel by their displacement through the addition of zinc dust. Copper is recovered as an important copper-sulphate by product, and cadmium metal is also recovered for sale.

Electrolysis – the solution undergoes electrolysis whereby it is depleted of a portion of its zinc with the regeneration of equivalent sulphuric acid giving spent electrolyte for use in the leaching process.

Casting – Cathode zinc is cast into ingots and blocks, and added to alloys.

Transportation is by train to Bell Bay for shipments to markets.

Effluent treatment system – Removes remaining heavy metals form waste liquid. The system is made up of three main sections -

A rearrangement of plant drains to collect all spillage and contaminated water flows and pumps this to a contaminated water storage pond.
A Mercury Removal Plant to remove mercury from liquid wastes and transform this into a marketable product, and
A lime Neutralisation Plant to remove other valuable metals, including zinc and cadmium.

Ongoing environmental works include the elimination as well as rehabilitation of stockpiles, upgrading of equipment to meet high environmental standards, such as scrubbers in the chimney stack to eliminate poisons. (The white plume commonly seen rising from the operation is steam).

Community Involvement involves ongoing consultation and communication.

Zinifex Rosebery Mine

Zinifex Rosebery Mine is a medium sized underground zinc-lead-silver-gold-copper mine situated on the West Coast of Tasmania. Mining activities in this rugged, mountainous area go back as far as the 1890′s, and the site has operated continuously since 1936 (previously known as Electrolytic Zinc Company of Australasia Limited & Pasminco Australia Limited.

Ore production has increased substantially in recent years – from around 600,000 tonnes per annum in 2000, to approximately 710,000 tpa and 760,000 tpa in 2001 and 2002 respectively. The site is now operating at a rate in excess of 800,000 tpa. The operation produces more than 140,000 tonnes of zinc concentrate per year (grading approx. 57% zinc) and more than 40,000 tpa of lead concentrate (grading 65% lead and 1,300 g/t silver). Approximately 5,500 tonnes of copper/silver/gold concentrate is also produced (copper grade 4,300 g/t silver, 150 g/t gold, 23 % copper), including just under 400 kg pa of gold ore.

Discovery and successful exploitation of deeper, narrower orebodies in recent years has extended the life of the mine. At current levels of production, Rosebery’s resource is expected to last at least eight years.

Mining

The Rosebery orebody is a stratabound massive sulphide in a pyroclastic and sedimentary setting. Ore occurs over a 1700 metre strike in a series of lenses up to 40 metres wide, dipping at 45 degrees. Fine-grained sphalerite and galena are the most important minerals, with some zinc as tetrahedrite and some chalcopyrite.

Rosebery uses mechanised underground mining methods, notably bench stoping, and sub-level open stoping with some cut-and-fill. Remnant mining is carried out above 17 Level. A network of rail haul-ways, internal shafts and declines was progressively developed as ore was discovered at greater depths. As a result, multiple handling of ore and materials made mining expensive to date.

The surface decline was completed in October 2002, the haulage shaft, rail system, underground crusher and all underground loading facilities have been decommissioned and all ore is trucked directly to the surface, eliminating the need for multiple handling. Apart from significantly reducing mining costs, the decline has given access to additional pillar ore in old mining areas.

The challenge of mining at depth has necessitated a number of other key capital projects in recent years. A $7.9 million Northern Upcast Ventilation shaft was introduced in 2000 to provide sufficient air to support employees and machinery at deeper levels. The underground mining truck fleet was boosted to counteract increased haulage distances at a cost of $1.2 million. The operation has also recently taken delivery of one new production drill and two new development drill rigs, while a new pump station and rising main are currently being installed

Overall, with significant investments and work practice changes which have unlocked the potential of the deeper orebodies, Rosebery is well placed to economically mine the resource until at least the end of the decade.

Milling

Milling capacity is around 850,000 tonnes per annum. In past years, additional ore was purchased to use the concentrator’s excess capacity. However, the boost in mining of ore at Rosebery in recent years means that very little ore is now externally purchased.

Relatively soft Rosebery ore is subjected to two stages of crushing and two or three stages of ball-milling to 80 microns before beneficiation. Around 30 percent of all gold is recovered by gravity separation and smelted on site before being sold as doré bullion. Copper/gold/silver, lead/silver and finally, zinc concentrates are progressively produced using sequential froth flotation. Tailings can be used for underground fill with the remainder going to dam storages.

Metallurgical recoveries to target concentrates are typically 91 percent, 80 percent and 55 percent for zinc, lead and copper respectively. Recovery of precious metals is very high, with 85 percent of silver and gold being recovered into bullion and lead or copper concentrates respectively. Zinc and lead recoveries have continued to improve with changes in mineralisation and improvements within the concentrator.

A $1 million fine grinding sand mill for lead regrind is currently being installed to further increase zinc concentrate grades and recoveries.

Logistics

Concentrates are railed via the Tasrail-owned Emu Bay Railway to the port of Burnie on the north west coast of Tasmania. Product is loaded onto ocean going bulk carriers, with all zinc and lead concentrates transferred to the respective Nyrstar smelters in Hobart and Port Pirie. All copper concentrate is sold externally.