What is Iron Ore?
Iron Ore Introduction
Iron (Fe) is one of the most abundant rock-forming elements, constituting about 5% of the Earth's crust. It is the fourth most abundant element after oxygen, silicon and aluminium and, after aluminium, the most abundant and widely distributed metal. Iron is indispensable to modern civilisation and people have been skilled in its use for more than 3,000 years. However, its use only became widespread in the 14th century, when smelting furnaces (the forerunner of blast furnaces) began to replace forges.
Iron ores are rocks from which metallic iron can be economically extracted. These rocks are usually found in the form of hematite (Fe2O3) or magnetite (Fe3O4). About 98% of world iron ore production is used to make iron in the form of steel. Iron ore provides the foundation for one of Australia’s major export industries.
Most iron ores mined today comprise the iron oxide minerals hematite, Fe2O3 (70% Fe); goethite, Fe2O3s H2O, (63% Fe); limonite, a mixture of hydrated iron oxides (up to 60% Fe); and magnetite, Fe3O4 (72% Fe).
Most of the world's important iron ore resources occur in iron-rich sedimentary rocks known as banded iron formations (BIFs), which are almost exclusively of Precambrian age (i.e. greater than 541 million years old). BIFs occur on all continents. In many instances they are mined as iron ores but, most importantly, they are the source rocks for most of the large high-grade concentrations of iron ore currently mined throughout the world.
In the Hamersley Province in the Pilbara district of Western Australia there are three main types of deposit: iron oxide enrichments within BIFs; iron oxides deposited along ancient, mainly Tertiary age river channels (palaeochannels); and iron oxide deposits formed from the erosion of existing orebodies (detrital iron ore deposits).
The BIF enrichment deposits comprising hematite and hematite goethite are the most important in regard to resources and production. The iron content of these ores varies widely and until recently most deposits needed to have an average grade of more than 60% Fe for mining to be commercially viable. However, some deposits can now have iron grades between 56%-59% Fe and be commercially viable.
The palaeochannel deposits composed of pisolitic limonite are the next in importance and are prized for their low impurities such as phosphorus. They are not as rich in iron as the BIF enrichment ores. Those mined usually contain 57%-59% Fe.
Detrital iron ore deposits, including scree and canga deposits, are found downhill of the BIF enrichment deposits from which they have been eroded. They are usually easily recovered and have a grade of between 40%-55% Fe. BIF enrichment deposits also occur elsewhere in Western Australia in the Pilbara and the Yilgarn Block and in South Australia
Numerous magnetite deposits of igneous origin or association occur in most States and uneconomic sedimentary sideritic (those containing the mineral siderite, FeCO3) iron ore deposits occur in Queensland and the Northern Territory.
Although iron ore resources occur in all the Australian States and Territories, almost 90% of identified resources (totalling 54 billion tonnes) occur in Western Australia, including almost 80% in the Hamersley Province, one of the world's major iron ore provinces.
In 2014 Australia's economic demonstrated resources (i.e. those that have been sufficiently tested by drilling and that could be economically extracted at current prices with existing technology) totalled 54 billion tonnes and ranked Australia first (with 29%) in the world followed by Brazil (16%).
As with most iron ore mines throughout the world, all the major Australian iron ore mines are open cut. The ores from the major mines in Western Australia's Pilbara region are hauled from working faces to crushing and screening plants using trucks that can carry over 300 tonnes. The ore is then transported for further treatment and blending to port sites in trains consisting of up to three locomotives and over 250 wagons. Trains of this size are over 2 kilometres long and contain loads in excess of 25,000 tonnes. There are three major Pilbara iron ore producers: BHP Billiton Ltd (BHP), Rio Tinto Ltd (Rio) and Fortescue Metals Group Ltd (FMG)
Concentration includes all the processes that will increase (upgrade) the iron content of an ore by removing impurities. Beneficiation, a slightly broader term, includes these processes as well as those that make an ore more usable by improving its physical properties (e.g. pelletising and sintering). Many of the iron ore mines employ some form of beneficiation to improve the grade and properties of their products. At many operating mines, including Mount Tom Price, Paraburdoo, Mount Whaleback and Christmas Creek, ore processing facilities (OPF) have been constructed to enable beneficiation of low-grade iron ores, including ores which have been contaminated with shale, to be mined and, after upgrading, sold as high-grade products. The operation of the OPFs has also increased the iron ore resources available at these mines.
Pelletising is a treatment process used for very fine or powdery ores. Pellets are an ideal blast furnace feed because they are hard and of regular size and shape. In Australia, concentrates pumped from Savage River are pelletised at Port Latta for shipment to domestic and overseas markets and fine Middleback Range ores are pelletised prior to smelting in the Whyalla blast furnace.
Sintering is a process used to agglomerate iron ore fines in preparation for blast-furnace smelting and is usually carried out at iron and steelmaking centres. It involves the incorporation of crushed limestone, coke and other additives available from iron and steelmaking operations. These additives include wastes extracted from furnace exhaust gases, scale produced during rolling mill operations, and coke fines produced during coke screening.
Pig iron is an intermediate step in the production of steel and is produced by smelting iron ore (commonly in lump, pellet or sinter form) in blast furnaces. Blast furnaces in Australia are located at Port Kembla and Whyalla. The removal, by oxidation, of impurities in pig iron such as silicon, phosphorus and sulfur and the reduction in the carbon content, results in the production of steel.
Steel is produced at Port Kembla and Whyalla. In 2014-15 financial year, Australian iron and steel production, including recovery from scrap, totalled 4.3 million tonnes which, although locally significant, is small on a world scale.
Although iron in cast form has many specific uses (e.g. pipes, fittings, engine blocks) its main use is to make steel. Steel is the most useful metal known being used 20 times more than all other metals put together. Steel is strong, durable and extremely versatile. The many different kinds of steel consist almost entirely of iron with the addition of small amounts of carbon (usually less than 1%) and of other metals to form different alloys (e.g. stainless steel). Pure iron is quite soft, but adding a small amount of carbon makes it significantly harder and stronger. Most of the additional elements in steel are added deliberately in the steelmaking process (e.g. chromium, manganese, nickel, molybdenum). By changing the proportions of these additional elements, it is possible to make steels suitable for a great variety of uses.
Steel's desirable properties and its relatively low cost make it the main structural metal in engineering and building projects, accounting for about 90% of all metal used each year. About 60% of iron and steel products are used in transportation and construction, 20% in machinery manufacture, and most of the remainder in cans and containers, in the oil and gas industries, and in various appliances and other equipment.