Does iron ore mining need water ? Why

Why does iron ore mining need water?

For magnetite mining, water is used in ore processing, to increase the quality and concentration of iron. After crushing, the dry ore is mixed with water and wet screened. This allows the ore to be directly fed into a steel pellet plant without further treatment. Some operations also use water to transport the ore via a slurry pipeline. Importantly, a significant amount of the water used in processing and transport is recycled.

Waste Characteristics

In certain mines with ores with high sulfur content, drainage from mine workings and waste heaps can become highly acidic and contain dissolved high concentrations of heavy metals. This Acid Mine Drainage (AMD) can have a pH of 3 or lower; sulfate levels of 800-1800 milligrams per liter (mg/L); copper levels up to 50 milligrams per liter (mg/L); iron levels up to 1000 milligrams per liter (mg/L); lead levels up to 12 milligrams per liter (mg/L); zinc levels up to 1700 mg/L, and cadmium levels of several milligrams per liter depending on the contents of the ore. The tailings pond effluent may also contain concentrations of chromium of several milligrams per liter. Base metal mining tailings decant may also contain high concentrations of thiosalts. Chemicals used in metal concentration (floatation and other) processes could also pose toxicity when released in effluents.

Surface runoffs may also pose significant environmental problems through erosion and carry-over of tailings and other mining residues. Explosives, such as ammonium nitrate, may also be present in surface runoff. Contamination of surface water can also occur from transport of mined material and from machinery maintenance and repair.

Significant levels of dust, above to 3 kilogram per ton (kg/t) (with a range of 0.003 to 27 kg/t) of ore mined, may be generated. This dust can be generated by extraction activities, crushing, ore beneficiation, transportation and traffic, and by wind borne losses. Significant releases of metal (including mercury) containing dust may result from the drying of the ore concentrate.

Process

Iron ore is converted into various types of iron through several processes. The most common process is the use of a blast furnace to produce pig iron which is about 92-94% iron and 3-5% carbon with smaller amounts of other elements. Pig iron has only limited uses, and most of this iron goes on to a steel mill where it is converted into various steel alloys by further reducing the carbon content and adding other elements such as manganese and nickel to give the steel specific properties.

Extraction

Refining

• The mined ore is crushed and sorted. The best grades of ore contain over 60% iron. Lesser grades are treated, or refined, to remove various contaminants before the ore is shipped to the blast furnace. Collectively, these refining methods are called beneficiation and include further crushing, washing with water to float sand and clay away, magnetic separation, pelletizing, and sintering. As more of the world's known supply of high iron content ore is depleted, these refining techniques have become increasingly important.
• The refined ore is then loaded on trains or ships and transported to the blast furnace site.

The Manufacturing Process

Charging the blast furnace

• After processing, the ore is blended with other ore and goes to the blast furnace. A blast furnace is a tower-shaped structure, made of steel, and lined with refractory, or heat-resistant bricks. The mixture of raw material, or charge, enters at the top of the blast furnace. At the bottom of the furnace, very hot air is blown, or blasted, in through nozzles called tuye'res. The coke burns in the presence of the hot air. The oxygen in the air reacts with the carbon in the coke to form carbon monoxide. The carbon monoxide then reacts with the iron ore to form carbon dioxide and pure iron.

Separating the iron from the slag

• 2 The melted iron sinks to the bottom of the furnace. The limestone combines with the rock and other impurities in the ore to form a slag which is lighter than the iron and floats on top. As the volume of the charge is reduced, more is continually added at the top of the furnace. The iron and slag are drawn off separately from the bottom of the furnace. The melted iron might go to a further alloying process, or might be cast into ingots called pigs. The slag is carried away for disposal.

Treating the gases

• 3 The hot gases produced in the chemical reactions are drawn off at the top and routed to a gas cleaning plant where they are cleaned, or scrubbed, and sent back into the furnace; the remaining carbon monoxide, in particular, is useful to the chemical reactions going on within the furnace.

A blast furnace normally runs day and night for several years. Eventually the brick lining begins to crumble, and the furnace is then shut down for maintenance.

Byproducts/Waste

There are a great many possible environmental effects from the iron industry. The first and most obvious is the process of open pit mining. Huge tracts of land are stripped to bare rock. Today, depleted mining sites are commonly used as landfills, then covered over and landscaped. Some of these landfills themselves become environmental problems, since in the recent past, some were used for the disposal of highly toxic substances which leached into soil and water.

The process of extracting iron from ore produces great quantities of poisonous and corrosive gases. In practice, these gases are scrubbed and recycled. Inevitably, however, some small amounts of toxic gases escape to the atmosphere.

A byproduct of iron purification is slag, which is produced in huge amounts. This material is largely inert, but must still be disposed of in landfills.

Iron making uses up huge amounts of coal. The coal is not used directly, but is first reduced to coke which consists of almost pure carbon. The many chemical by products of coking are almost all toxic, but they are also commercially useful. These products include ammonia, which is used in a vast number of products; phenol, which is used to make plastics, cutting oils, and antiseptics; cresols, which go into herbicides, pesticides, pharmaceuticals, and photographic chemicals; and toluene, which is an ingredient in many complex chemical products such as solvents and explosives.

Scrap iron and steel—in the form of old cars, appliances and even entire steel-girdered buildings—are also an environmental concern. Most of this material is recycled, however, since steel scrap is an essential resource in steel making. Scrap which isn't recycled eventually turns into iron oxide, or rust, and returns to the ground.