What methods can use in extracting metals from mineral ores?

What methods can use in extracting metals from mineral ores?

The Earth's crust contains many different rocks.

Rocks are a mixture of minerals and from some we can make useful substances.

A mineral can be a solid metallic or non–metallic element or a compound found naturally in the Earth's crust.

Mineral ores are naturally occurring rocks that provide an economic starting point for the extraction and manufacture of metals for a huge variety of purposes ie a metal ore is rock containing sufficient metal to be worth extracting the metal from it.

The simplest definition of an ore is a mixture of a metal containing mineral and other materials ('minerals') from the surrounding rocks, which can be described as impurities with respect to what you want from the ore.

Metal ores are obtained by mining and that this may involve digging up and processing large amounts of rock.

Most ores are mined have to be concentrated before the metal is extracted and purified. This often results in lots of waste material that must be dealt with from an environment of view.

The social, economic and environmental impacts of exploiting metal ores are discussed on a separate page.

The metal ore, a mineral or mixture of minerals from which economically viable amounts of metal can be extracted, i.e. its got to have enough of the metal, or one of its compounds, in it to be worth digging out!

Ores are often oxides, carbonates or sulphides.

These ores are all finite resources so we should use them wisely!

Any ore must contain enough of the metal to make it worthwhile to mine and then extract the metal.

High grade ores will therefore be the most economical to exploit BUT over time the better quality–richer sources will decrease, especially with the power of the global economy and powerful and growing economies of Brazil, China, India and other Asian economies.

The economics of metal extraction are not only dependent on the quality of the ore and the cost of extraction (ie richer ores lead to cheaper production), but also depend on the market price and demand. If demand is high the metal price rises and may off–set the price of mining lower grade ores, but if demand is low, the metal price falls and inefficient mines and smelters will go out of business ie its not worth extracting the metal. Of course it is possible to improve the technology of metal extraction and enable companies to produce more metal from the ore than was previously possible and even utilise low grade ores previously discarded and not considered worthwhile mining.

Since the majority of metals are found combined with non–metals like oxygen (oxide ion) or sulfur (sulfide ion) or the carbonate ion, chemical reactions are needed to free the metal from its mineral source.

 In order to extract a metal, the ore or compound of the metal must undergo a process called REDUCTION to free the metal i.e.

The oxide/sulfide loses oxygen/sulfur, to form the free metallic atoms, or

the positive metal ion gains negative electrons to form the neutral metal atom.

The chemical that removes the oxygen from an oxide is called the reducing agent i.e. carbon, carbon monoxide or sometimes hydrogen.

Detailed REDOX notes at the end of the metal reactivity series page, and on this page where appropriate.

Historically as technology and science have developed the methods of extraction have improved to the point were all metals can be produced. The reactivity is a measure of the ease of compound formation and stability (i.e. the more reactive the metal, the more readily the metal forms a stable compound eg with oxygen or sulfur, and therefore this greater compound stability, the more difficult it is to reduce to the compound to the metal).

The least reactive metals such as gold, silver and copper have been used for the past 10000 years because the pure metal was found naturally.

Moderately reactive metals like copper, iron, lead, tin have been extracted using carbon based smelting for the past 2000–3000 years.

BUT it is only in the last 200 years that very reactive metals like sodium or aluminium have been extracted by electrolysis.

In other words, our exploitation of metal mineral resources as developed and expanded as the technology of metal extraction has also developed and improved.

Therefore the economics of extraction may change over time with eg reduced costs by technological advances or increased by depletion of high grade ore reserves.

The depletion of high grade ore reserves has resulted in technology research  increasingly looking at ways of extracting metals from low grade ores which were previously uneconomic to use.

The crucial point is that generally speaking, the method of extraction depends on the metals position in the reactivity series

The reactivity series of metals can be presented to include two non–metals, carbon and hydrogen, to help predict which method could be used to extract the metal.

lower Pt Au Ag Cu (H) Pb Sn Fe Zn (C) Al Mg Ca Na K higher in series

RULE: Any element higher in the series can displace any other lower element



Notes on Reactivity Series of Metals & Metal Reactivity Experiments–Observations

Generally speaking, the more reactive a metal, the more difficult it is to extract.

This is because the more reactive a metal, the more strongly it combines with another non–metallic element like as oxygen or sulfur and therefore the oxide or sulfide is more difficult to reduce to the metal.

 Although most metals occur as compounds, some metals are so unreactive that they do not readily combine with oxygen in the air or any other element present in the Earth's crust, and so can be found as the metal itself (sometimes referred to as 'native' metal).

For example, a metal, most frequently found as the metal is gold (and sometimes copper and silver) and no chemical separation or extraction is needed.

In fact all the metals below hydrogen can be found as the 'free' or 'native' element, though they occur mainly as compounds combined with non–metals like oxygen (oxide ion) or sulfur (sulfide ion) or the carbonate ion in their ores.

Therefore, for most metals, their naturally occurring compounds require processing via chemical reactions to obtain the free metals.

Metals below carbon can be extracted by heating the oxide with carbon or carbon monoxide. The non–metallic elements carbon will displace the metals less reactive than carbon in a smelter or  blast furnace e.g. iron or zinc and metals lower in the series.

 Therefore metals like iron, copper, tin, lead, zinc can readily be extracted by reaction–reduction of their e.g. oxides using cheap carbon (i.e. coke made from coal).

Iron ore is used to make iron and steel and iron is produced in a blast furnace by reducing iron oxides with carbon and it is the carbon that removes the oxygen from the iron oxides – the carbon is known as the reducing agent.

The metal copper can be easily extracted BUT copper–rich ores are becoming scarce so new methods of extracting copper are being developed to exploit low grade ores.

A low grade ore is one with low concentrations of copper and research is going on to try and exploit waste material left over from processing high grade ores.

Metals below hydrogen will not displace hydrogen from acids. Their oxides are easily reduced to the metal by heating in a stream of hydrogen, though this is an extraction method rarely used in industry.

In fact most metal oxides below carbon can be reduced when heated in hydrogen, even if the metal reacts with acid.

Metals above carbon in the reactivity series cannot usually be extracted with carbon or carbon monoxide.

So, metals more reactive than carbon are usually extracted by electrolysis of the purified molten ore or other suitable compound.

Electrolysis is the process of breaking down a compound using electrical energy.

The process of electrolysis uses of large amounts of energy in the extraction of these reactive metals and makes them expensive to produce.

Aluminium is a very useful metal but expensive to produce.

e.g. aluminium from molten aluminium oxide or sodium from molten sodium chloride.

The ore or compound must be molten or dissolved in a solution in an electrolysis cell to allow free movement of ions (electrical current). The conducting melt or solution is called the electrolyte.  Theory given in the appropriate sections.

Because these reactive metals cannot be obtained by relatively cheap carbon reduction methods, their extraction tends to be more costly due to more specialised stages in the extraction process, more energy is needed (maybe costly electricity) and more costly specialist chemicals like a more reactive metal or chlorine (remember carbon–coke is relatively cheap e.g. as used in the blast furnace extraction of iron).

Other methods are used in special cases using the displacement rule.

A more reactive metal can be used to displace and extract a less reactive metal but these are costly processes since the more reactive metal also has to be produced in the first place!

Titanium is another very useful metal but expensive to produce.

See Titanium or see at the end of the section on copper extraction.

Sometimes electrolysis is used to purify less reactive metals which have previously been extracted using carbon or hydrogen (e.g. copper and zinc). Electrolysis is also used to plate one metal with another.

The demand for raw materials does have social, economic and environmental implications e.g. conservation of mineral resources by recycling metals, minimising pollution etc.

Metals can be mixed together to make alloys to improve the metal's properties to better suit a particular purpose.