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The Magnesium Home Page
 Magnesium Information

Introduction

Uses of Magnesium

Magnesium Alloys

Properties of Pure Mg

Ores of Magnesium

Production of Magnesium Metal

Refining of Magnesium

Handling of Molten Magnesium

Cast Magnesium Components

Sources of further information

Further reading

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Introduction

Magnesium is the lightest of the structural metals with a density of only 1.74 g.cm-3. However, magnesium is used as a structural metal in an alloyed form and most magnesium alloys have a density slightly higher than this. Magnesium is a reactive metal and is usually found in nature in the form of and oxide, carbonate or silicate, often in combination with calcium. This reactivity is one of the reasons why the production of magnesium metal requires large amounts of energy.

Uses of magnesium

The uses of magnesium largely centre on 3 properties of the metal:

The world production of magnesium is small compared to the other structural metals such as steel and aluminium at only about 300,000 tonnes per annum. About half of this is used directly in aluminium alloys to harden and strengthen them. For example, the body of an aluminium can contains approximately 1.5% Mg and the can top about 4.5% Mg.

Other significant uses are:

Magnesium alloys

Magnesium components are generally in the form of magnesium alloys. The addition of other alloying elements can strengthen and harden magnesium as well as alter its chemical reactivity. The common magnesium alloys are shown in Table 1.

Table 1 - Common magnesium alloys

Alloy Designation

Alloying Additives

Uses

Reasons for use

AZ91

9.0 % Al 0.7 % Zn 0.13 % Mn

General casting alloy

Good castability, good mechanical properties at T<150ºC.

AM60

6.0 % Al 0.15% Mn

High pressure die casting alloy

Greater toughness and ductility than AZ91, slightly lower strength. Often preferred for automotive structural applications.

AZ31

3.0 % Al 1.0 % Zn 0.2 % Mn

Wrought magnesium products

Good extrusion alloy.

ZE41

4.2 % Zn 1.2 % RE 0.7 % Zr

Specialist casting alloy

Rare earth addition improves creep strength at elevated temperatures. Pressure tight.

AS41

4.2 % Al 1.0 % Si

General casting alloy

Better creep resistance than AZ91 at elevated temperatures but lower strength.

In addition to Table 1, there are many specialist alloys. For critical low weight applications, alloys can be used that contain zirconium, expensive rare earths and even silver. Alloy development remains a strong area for research, particularly with a view to improving the corrosion resistance and high temperature creep resistance of cast components.

 

Properties of pure Mg

Atomic number

12

Atomic mass

24.31

Colour

Silvery grey metal

Density

1.738 g.cm -3 at 20ºC

1.58 g.cm-3 at 650RC (1)

Melting point

650ºC

Boiling point

1103ºC

Crystal structure

close-packed hexagonal

Heat of combustion

25020 kJ.kg -1

Flame temperature

~2800ºC

Heat of fusion

368 kJ.kg-1

Heat of vaporisation

5272 kJ.kg-1

Specific heat

1025 J.K -1.kg at 20ºC

Vapour pressure

20 Pa at 527ºC(s)

360 Pa at 650ºC (1)

1400 Pa at 727ºC

Valence states

Mg2+

Viscosity

1.25 cp at 650ºC (1)

 

Ores of magnesium

Magnesium is the 6th most abundant element metal on the surface of the earth at about 2.5% of its composition. However, its high chemical reactivity means that it is not found in the metallic state in nature. Sea water contains 0.13% Mg and some production facilities use the magnesium content of sea water for the production of magnesium metal. This is often after the precipitation of other salts which leave a magnesium-rich brine.

Other common commercial sources are dolomite ((CaMg)CO3) and magnesite (MgCO3). The calcining of magnesite also produces magnesium oxide, large tonnages of which are used annually for the making of refractories. This tonnage far exceeds the annual production of magnesium metal.

There are many magnesium silicate minerals known to geologists but they are rarely pure enough to warrant processing to metallic magnesium. The exception is the Magnola Project, conducted by Noranda in Canada, which is seeking to establish a 60,000 tonnes per year magnesium smelter based on the leaching of asbestos mine tailings rich in magnesium minerals.

Production of magnesium metal

Production of magnesium metal is an interesting example of an industry where little information is shared. Unlike for example the aluminium industry where almost all of the world production is made by the Hall-Héroult process, there are many production processes used commercially to produce magnesium metal. They do however fall into 2 basic groups:

1. Silicothermic production routes

Includes the Pidgeon process, Magnetherm process, Bolzano process

These rely on the use of ferrosilicon to reduce magnesium oxide in a molten slag at temperatures of 1200ºC-1600ºC under a reduced gas pressure above the slag to produce a magnesium vapour. This vapour is condensed at a location removed from the main furnace. The crowns of condensed magnesium are then re-melted, refined and cast.

These production routes are capable of producing magnesium of purity as high as 99.95%.

2. Electrolytic processes

Commercial magnesium electrolysis is conducted in a chloride melt of mixed alkali metals at temperatures usually below 700C. The feed to the electrolysis process is either anhydrous magnesium chloride, MgCl2.KCl produced from dehydration of carnallite or partially dehydrated magnesium chloride.

Pure anhydrous magnesium chloride is probably the preferred feed material. However, the production of magnesium chloride with low levels of magnesium oxide is quite difficult, due the hygroscopic nature of magnesium chloride.

Unlike the high temperature production routes, the electrolysis processes have difficulty achieving metal purity greater than about 99.8%. This is due largely to the presence of 300-400 ppm of iron due to contact with steel components in the electrolysis cell. However, for production of greater than 10,000 tonnes per annum, the electrolysis process develops cost benefits over the high temperature processes.

Refining of magnesium

Magnesium is one of the metals like aluminium where very little post refining of the metal is conducted, other than the removal of physical impurities such as electrolyte and oxide (although some processes are available). The high temperature processes rely on the high vapour pressure of magnesium to transport high purity magnesium vapour. The electrolysis processes all rely on chemically clean feed materials and the operation of the electrolysis cells in a regime that favours magnesium reduction over the reduction of other alkali metal chlorides.

Handling of molten magnesium

Molten magnesium burns in air to produce a brilliant white light and large amounts of heat. Many people who did high school chemistry remember burning magnesium ribbon. This propensity to rapidly oxidise is one of the reasons that magnesium has not had the wider acceptance as a structural material as aluminium.

Molten magnesium has been traditionally protected by the use of alkali metal fluxes which melt and form a liquid covering that restricts access of the air to the metal. Alternatively, sulphur dioxide has been used. Whilst both of these methods are effective, both result in significant corrosion of surrounding equipment and present a health hazard to operators.

In recent years, the use of dilute mixtures of sulphur hexafluoride in either dry air and/or carbon dioxide have become the industry norm. Sulphur hexafluoride is colourless, odourless and non-toxic gas. It does result in minor corrosion problems if used in the wrong concentrations and in recent years has come under attention as a greenhouse gas.

One redeeming feature of magnesium is the ability to contain it in molten form in steel containers. The low melting point of 650ºC (even lower for most commercial alloys) and the limited solubility of iron in magnesium at temperatures below 700ºC (about 400 ppm) means that magnesium can be contained in heated steel vessels and pumped in a manner like water, using heated steel pipes.

Cast magnesium components

The largest growth market for magnesium is for cast (die cast or sand cast) components for automotive applications, particularly in the US market. The reason for this is the US Government's CAFE regulations which require fuel economy targets to be met by automobile manufacturers annually. One way to achieve these targets is to make cars of lighter mass.

Hence, there is a large interest in the substitution of steel components in cars by the lighter structural metals, magnesium and aluminium. The aluminium industry has been more successful at achieving this substitution, due in part to the good corrosion resistance of aluminium and familiarity with its use.

However, in recent years magnesium has been gaining popularity as the chemical purity of the alloys has been improved, resulting in a significant increase in corrosion resistance. The use of sulphur hexafluoride as a cover gas instead of flux has also resulted in an increase in physical purity and an easing of the difficulty of molten magnesium oxidation protection.

The full benefits of the high pressure die casting of magnesium are also being realised as that process is steadily improved, resulting in more dense castings. Combined with the excellent castability of the common magnesium-aluminium alloys, large structural components such as seat frames, steering wheels, support brackets and instrument panels can now be successfully cast, often replacing complex multi-piece steel stampings. This has resulted in significant production cost and weight savings in some instances. Other potential advantages of magnesium castings are:

There is a problem with this increased use of magnesium. Demand for magnesium die cast components is growing at about 15% per year and is scheduled to outstrip supply of available primary metal by the end of the decade. This is keeping the price of magnesium metal high and providing a disincentive for its use. New production capacity from a number of new smelter projects and expansions of existing smelters is expected to ease this tight supply situation towards the end of the decade.

Sources of further information

The editors of this home page have found very little information on magnesium on the Internet other than that related to the importance of magnesium to the health of humans. This was one of the driving forces for the formation of this Home Page. The magnesium industry is also somewhat secretive and good published information is a little difficult to find. Try looking through the Links Page at the Internet information we have uncovered.

The Annual International Magnesium Association (IMA) Conference Proceedings provides a good place to start and gives good information on the uses of magnesium and the supply and demand situation each year. The IMA also produces an excellent Buyer's Guide ever year of suppliers to and within the industry. They can be contacted by e-mail.

Magnesium Monthly Review, published each month by Bob Brown provides up to date news of developments in the industry.

The 3 International Magnesium Conferences (the latest was held in Wolfsburg in Germany in April 1998) organised by The Institute of Metals provide some up to date scientific research findings, although of somewhat academic interest.

The Annual TMS Light Metals Conference usually contains a few magnesium papers. The Feb. 2000 Conference in Nashville will have a parallel magnesium conference. TMS also has a Reactive Metals Committee which has interests in magnesium.

The ASM Handbook Series, formerly known as Metals Handbook also has some good general information on the use and properties of magnesium alloys.

The CRC for Alloy and Solidification Technology (CAST) in Australia and the Institute of Magnesium Technology in Québec (ITM) both have large research programs in magnesium, in particular cast magnesium products, that are non-aligned to any magnesium producers and could be useful sources of information. The CAST Design and Prototyping Service can supply more information about designing cast components in magnesium alloys.

For further information on the Statistics of metal production and recycling in the United States of America (USA) try looking at the Minerals Information section of the US Geological Survey.

Natural Resources Canada now publish a Nonferrous Metals Outlook on the Internet with a magnesium section which can be download as a .pdf file.

For more specialised information, the following guide may help:

a. General properties of magnesium

Webelements contains some useful information about the physical properties of magnesium but is of more academic interest. Busk [1] provides more useful information of use to people wanting to use magnesium alloys for a structural component and also contains useful tables of properties of both pure magnesium and magnesium alloys

b. Uses of magnesium

The IMA Annual Meeting proceedings provide a good introduction in this area but with an automotive bias.

c.. Production of magnesium metal

Øye [2] and Høy-Petersen et al [3] give good summaries of the range of production processes. Their references provide sources of information on the various production routes.

d. The handling of magnesium alloys

Good safety information is available through The International Magnesium Association [4,5] and includes both written and audio-visual material. All of the major magnesium suppliers will supply general handling information for their products and physical property data on their alloys.

Further Reading

1. Busk, R.S., Magnesium products design, Marcel Dekker, New York, 1987.

2. Øye, H., Proceedings of the Elliot Symposium on Chemical Process Metallurgy, P.J. Koros and G.R. St. Pierre Eds., Iron and Steel Society, 1990, 33-50.

3. Høy-Petersen, N. et al, "Magnesium" in Ullmanns Encyclopedia, 559-580, 1989.

4. Lockwood, L.L., Recommended practice for melting high purity magnesium alloys, International Magnesium Association.

5. Magnesium: Safe handling for manufacturing, storage and use, Videorecording, International Magnesium Association.

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For more information and comments about this page please contact Nigel Ricketts