Silver Fact Sheet
|Minerals Downunder | Rock Files | Fact Sheets|
Silver's (Ag) relative scarcity, attractive appearance and malleability for easy shaping has made it suitable for use in jewellery, ornaments and silverware since before ancient Roman times. Its use has been extensive in coins throughout history, although this application has declined in recent times. In Australia, the 1966 50cent piece was the last coin in general use to contain silver (80% silver, 20% copper). Although silver is resistant to oxidation it readily forms a surface tarnish of silver sulphide. While its high electrical and thermal conductivity make it useful in the electronics industry, the largest use of silver is in photographic paper and film.
Silver is mined and produced mainly as a co-product of copper, lead, zinc and to a lesser extent, gold. Its main source is silver minerals in lead ore.
The main silver minerals are tetrahedrite (Cu,Fe,Zn,Ag12Sb4S13), freibergite (tetrahedrite with up to 30% Ag), pyragyrite (Ag3SbS3), argentite (Ag2S), proustite (Ag3AsS3), ceragyrite (AgCl). Australian silver occurs mainly as trace amounts of freibergite, tetrahedrite and pyragyrite in the lead mineral galena (PbS). Silver in gold ores occurs mainly as minor amounts of the natural gold-silver alloy called electrum.
Warm to hot (hydrothermal) fluids generated within the earth form copper, lead and zinc deposits which contain silver. These fluids may be trapped below the surface in cracks where galena containing silver and other minerals may precipitate to make vein deposits. Where limestones occur the fluids may fill cavities to form rich but patchy lead-zinc-silver deposits. Some fluids can reach the ocean floor in areas of underwater volcanic activity, to form 'volcanogenic' deposits. Some are forming today under the ocean off Papua New Guinea and Canada.
For thousands of millions of years, deposits have been forming in this manner and may eventually be exposed at the surface following weathering and erosion. Some are completely eroded away and may be recycled by natural processes into new deposits. Partially eroded deposits exposed at the surface were relatively easily discovered. Examples of surface exposure are the deposits at Broken Hill in New South Wales and Mt Isa in Queensland, both of which formed the basis of Australia's silver (lead-zinc) mining industry.
Exposed deposits are becoming harder to find in Australia, and exploration companies are looking beneath the surface for the deposits of the future. This is a more costly and difficult way to find orebodies although a series of successes have occurred since the late 1970s. Such discoveries include the Scuddles mine (140 metres deep) in Western Australia, the Cannington deposit (10 metres deep) in north Queensland, the Hellyer mine (90 metres deep) in Tasmania and the Wilga deposit in Victoria (50 metres below the surface).
In 1883, Charles Rasp discovered the Broken Hill heavy rocks which he thought may contain tin. Subsequent assays (analysis) of these rocks proved that he had located rich oxidised (weathered) silver and lead minerals. Ore is still being extracted from Broken Hill, which has been the largest producer of lead-zinc-silver in Australia.
The upper parts of the Broken Hill deposit and the Mt Isa deposit discovered in 1923 by John Campbell Miles were very rich in silver, the prime commodity sought in the early days. The wealth generated by mining the Broken Hill ore allowed The Broken Hill Proprietary Company Limited to prosper and, although it no longer has an interest in the deposit, its successor, BHP Billiton has become one of Australia's largest companies.
The rich lead-silver lodes discovered by John Campbell Miles at Mt Isa remain in production after more than 80 years of mining. Discovery of the nearby rich Hilton deposit occurred in the late 1940s but it was not developed until the mid 1980s. In the Mt Isa region, there are large mines at the George Fisher (adjacent to Hilton), Cannington (very rich), and Century deposits, while the Dugald River and Lady Loretta deposits are yet to be mined. In the Northern Territory, the huge McArthur River lead-zinc-silver deposit is a major producer.
Australia has the largest share of the world's economic silver resources, outstripping Mexico, Canada and the USA as a result of the discovery and development of the Cannington, Century and McArthur River lead-zinc-silver deposits. This position is further supported by resources in the many other base metal and gold deposits of various sizes in Australia.
Australia's silver production ranks after Mexico, Peru and the USA. About 25% of Australia's mine output is refined to silver metal and mainly sent to Japan. Most of the remainder is exported in lead bullion to the United Kingdom where it is extracted and refined.
Most of Australia's silver is produced from the silver-bearing lead mineral, called galena. Some also is produced from copper and gold mining.
Almost all of Australia's silver (lead-zinc and/or copper) mines are highly mechanised, underground operations. Ore is drilled and blasted in large volumes and transferred to underground rock crushers by large loaders and trucks. The crushed ore is then hoisted to the surface in skips or driven directly to the surface by truck via a spiral access tunnel (decline).
At the surface, the ore is subjected to additional crushing and fine grinding. A flotation process is used to separate the silver-bearing galena from the waste rock particles (tailings) to form a concentrate. Development of the flotation process occurred in the early days of mining at Broken Hill. Today, more efficient versions of this technology are used world-wide. Many mines around Australia and overseas use the improved Jameson flotation cell, which also was developed in Australia.
In this process, ground-up ore, water and special chemicals are mixed together and constantly agitated in banks of flotation cells. Air is blown through the mixture in each cell and fine silver bearing galena particles stick to the bubbles, which rise to form froth on the surface of the cell. The tailings sink to the bottom of the cell and are removed. The froth is skimmed off and the resulting silver-lead sulphide concentrate may assay 800 grams to one kilogram of silver per tonne of concentrate.
The concentrate is sintered (partly melted) to combine the fine particles into lumps and to remove some sulphur as sulphur dioxide. It is then smelted in a blast furnace and drossed (removal of trace copper and impurities) to produce crude lead metal that may contain more than two kilograms of silver per tonne of concentrate.
Silver is recovered when crude lead is refined to high purity. Crude lead is re-melted and antimony and other impurities removed. The molten lead is then poured into a large upright container called a 'kettle' where it plunges through a molten zinc metal layer floating on top of molten lead. The kettle is increasingly cooled towards its base so that as the incoming molten lead descends, the contained silver and any minor gold or copper combines with the molten zinc dragged down from the zinc layer and forms crystals of zinc-silver-gold-copper alloy. The crystals float to the surface and re-dissolve in the zinc layer. The zinc layer is periodically skimmed off and the silver-gold-copper alloy called dore is removed by further smelting then cast into plates for electrolytic removal of copper and separation of high purity silver and gold.
The Port Pirie lead smelter and refinery in South Australia is the most important Australian producer of refined silver and is where lead concentrates from the Broken Hill and other mines are processed. Most of the silver output from Mt Isa and Hilton mines is in lead concentrates, which are smelted to lead bullion at Mt Isa. The bullion is exported to the United Kingdom for refining and silver extraction. Lead bullion, containing silver, is also produced at the Cockle Creek smelter (NSW) and it is sent to Port Pirie for refining and extraction of silver.
Silver ingots are also produced at the Olympic Dam mine from residues generated during electrolytic copper refining.
Silver is extracted and refined by gold refineries in Perth, Kalgoorlie and Melbourne from gold dore or bullion sourced from mainly Australian gold mines.
The use of silver dates from the earliest historic records and was usually extracted by melting lead ore (galena). Silver ornaments and utensils have been used since the 4th century BC. The Romans produced silver from lead-silver mines, smelters and refineries in Britain, Sardinia, Spain and elsewhere and it became the basis of their coinage. In the 16th and 17th centuries, Spain explored and colonised South America and, as a result, mining from rich deposits discovered in Mexico, Peru and Bolivia caused a large increase in world silver production. The increased production reduced silver's perceived value and gold gradually replaced it as a monetary standard. Silver continued to be used in ornaments, tableware and some coins.
Today, photographic paper and film, followed by the electronics and jewellery/tableware industries are the most important users of silver. Other uses are in brazing alloys, solder, mirrors, medicines, tooth fillings, coins and medallions.