Fundamentals of Magnesium Alloy Metallurgy

Mihriban O Pekguleryuz, Karl Kainer, A. Arslan Kaya

• Publisher: Woodhead Publishing
• Year: 2013
• Print ISBN: 9780857090881
• eBook ISBN: 9780857097293
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

Magnesium and magnesium alloys offer a wealth of valuable properties, making them of great interest for use across a wide range of fields. This has led to extensive research focused on understanding the properties of magnesium and how these can be controlled during processing. Fundamentals of magnesium alloy metallurgy presents an authoritative overview of all aspects of magnesium alloy metallurgy, including physical metallurgy, deformation, corrosion and applications.Beginning with an introduction to the primary production of magnesium, the book goes on to discuss physical metallurgy of magnesium and thermodynamic properties of magnesium alloys. Further chapters focus on understanding precipitation processes of magnesium alloys, alloying behaviour of magnesium, and alloy design. The formation, corrosion and surface finishing of magnesium and its alloys are reviewed, before Fundamentals of magnesium alloy metallurgy concludes by exploring applications across a range of fields. Aerospace, automotive and other structural applications of magnesium are considered, followed by magnesium-based metal matrix composites and the use of magnesium in medical applications.With its distinguished editors and international team of expert contributors, Fundamentals of magnesium alloy metallurgy is a comprehensive tool for all those involved in the production and application of magnesium and its alloys, including manufacturers, welders, heat-treatment and coating companies, engineers, metallurgists, researchers, designers and scientists working with these important materials.- Overviews all aspects of magnesium alloy metallurgy- Discusses physical metallurgy of magnesium and thermodynamic properties of magnesium alloys- Reviews the formation, corrosion and surface finishing of magnesium and its alloys

1

Primary production of magnesium

R. Neelameggham, IND LLC, USA

Abstract:

This chapter reviews the production technology for a variety of magnesium processes developed over the past 150 years on a commercial scale. It discusses why processes vary considerably in the case of magnesium, unlike the case of aluminum production.

Key words

magnesium

light-weight structural metal

molten chloride electrolytic process

thermal reduction

Pidgeon process

electro-thermal

1.1 Introduction

Magnesium ion is the most abundant structural metal ion in the ocean; it is the fifth most abundant element in the hydrosphere (3.1 × 10¹⁵ tons). In the earth’s crust (lithosphere) magnesium is considered to be the eighth most abundant element. If we consider the topmost 3.8 km, magnesium is the third most abundant ‘structural metallic element’. It should be noted that the average depth of the ocean is 3.8 km – this is the hydrosphere, where magnesium is the only extractable structural metal. This makes magnesium a unique structural element, which can be extracted from either the hydrosphere or the lithosphere. Aluminum is sparse in the ocean, and is extracted from the lithosphere only.

As we all know, manmade materials are made by processes using the raw materials available, or which can be acquired at a low cost while converting them to a value-added material. Irrespective of the source of the raw material, additional energy matter is required to effect the conversion of the mineral into metal. The nature and cost of the energy and energy materials have been important factors in the choice of the process development of magnesium.

Since magnesium is available from the lithosphere and the hydrosphere, various routes are available for extraction into metal. This chapter is written so as to take us through the history of commercial processes in the nineteenth and twentieth centuries, before discussing the chemistry of process evolution, steps involved in production methods, and major equipment needed for different processes. Following this, future possibilities are discussed.

It took over 18 years of laboratory and pilot research, with personal attention given by Herbert H. Dow between 1896 and 1915, before a commercial line for producing magnesium came on line. It took another 16 years before reducing the cost of a pound of magnesium from 5 dollars to about 30 cents by the early 1930s. The process was further refined over the years in reducing operating costs (Campbell and Hatton, 1951). Dow Magnesium had a production of over 100 000 tons per year in its peak years during its 80 plus years of operation before being shut down in 1997.

The same period saw the development of magnesium for structural applications, both in the USA as well as in Germany. Dow Chemical is credited with introducing Dow-Metal pistons for the automotive sector in the 1920s, while the Germans developed a magnesium alloy engine for Volkswagen in the 1930s, helped by I.G. Farbenindustrie’s magnesium process. Herbert H. Dow also pioneered the introduction of magnesium into the construction of aircraft in the early 1920s (Campbell and Hatton, 1951), even though this pioneering effort was not able to compete with aluminum – which is 1.5 times heavier than magnesium. We still continue to revisit this subject time and again, even to the present day, in educating the public about the benefits of magnesium alloys as a structural metal and the fact that magnesium can be safely used (Gwynne, 2010). With the advent of higher strength magnesium alloys, magnesium composites can compete with fiber reinforced composites in alternative energy generation such as wind power, etc.

Unlike for other metals, the processes used in the production of magnesium have gone through several historic changes – almost following the changes in the economic dominance history on a global scale – whether it be the world wars, or the cold war through the 1980s, or the emergence of the global economy in the 1990s, and through the recent commodity rise and fall during 2006–9. In 1935, John A. Gann, Chief Metallurgist of The Dow Chemical Co., noted the following ‘… our light metals occur only in the form of compounds so stable that their discovery, isolation, commercial production, and use were forced to await some of the modern advances in chemistry and engineering. Under such conditions, the evolution of a new industry is often a romance in which scientific and industrial difficulties and near failures add to the thrill of success’ (Gann, 1935). The truth of this statement has been proved time and again in the production processes, even in recent times, and in the further development and uses of magnesium.

All magnesium metal production processes go through the following unit process steps (see Fig. 1.1):

1.1 General flow sheet for magnesium production.

i. Raw material upgrading

ii. Removal of unwanted and undesirable impurities

iii. Removal of impurities undesirable in the finished metal

iv. Converting the purified raw material into metal and separation from other component products – along with processing and or reuse of other raw material components

v. Melting, refining and casting metal and/or alloys

vi. Granular magnesium and alloys.

1.2 Raw materials and production methods

Most of the metallic elements are usually extracted or reduced from their respective oxides, or oxide compounds. The lithospheric compounds from which magnesium is extracted are: dolomite (CaCO₃·MgCO₃), magnesite (MgCO₃), periclase (magnesium oxide) (MgO), hydro-magnesite (3MgCO₃·Mg(OH)₂·3H₂O), brucite (MgO·H₂O), and silicates of magnesium (olivine(Mg,Fe)₂ SiO₄, serpentine 3MgO·2SiO₂·2H₂O with partial iron substitution of magnesium, fosterite, biotite micas, etc.). The lithospheric minerals magnesium sulfate (epsomite- MgSO₄·7H₂O), kieserite (MgSO₄ H₂O), langbeinite (K₂SO₄·2MgSO₄), and kainite (KCl·MgSO₄·3H₂O), carnallite (KCl·MgCl₂·6H₂O) are of hydrospheric origin found in evaporites.

The hydrosphere – oceans, and terminal lakes − has magnesium as the second most abundant metallic cation in the salinity. Sodium, present in a larger quantity, usually provides the ionic balance for the chloride ion in saline waters; sulfate is needed to provide ionic balance of magnesium along with chloride ions. Magnesium minerals found from the evaporites in the chloride form include carnallite (KCl·MgCl₂·6H₂O) and bischofite (MgCl₂·6H₂O). Many of these were identified initially in Stassfurt, Germany in the mid-nineteenth century. Most of the process variations have been caused by the choice of raw material, whether it is oxide or a chloride type material, as we will see in the forthcoming discussions.

1.2.1 Nineteenth century magnesium production processes

In 1808, Humphry Davy took moistened magnesium sulfate and electrolyzed it onto a mercury cathode. He also converted red hot magnesium oxide with potassium vapor, collecting the magnesium into mercury. Both processes produced magnesium amalgam, from which he made the metal by distilling out the mercury. In 1828, Bussy reduced magnesium chloride with potassium metal in a glass tube; when the potassium chloride was washed out, small globules of magnesium were present.

Faraday in 1833 electrolyzed impure magnesium chloride in a molten state to get magnesium metal; but it took two more decades before Robert Bunsen made a commercial quantity in a small laboratory cell using molten anhydrous magnesium chloride. He noted the need to dehydrate the magnesium chloride for improving the electrolysis by avoiding sludge formation. Bunsen demonstrated in 1852 that it is easier to dehydrate magnesium chloride in a potassium chloride bath – this later led to the use of naturally occurring carnallite as a source for making magnesium. Commercial production of magnesium on a larger scale was initiated in 1886 – about the same time as the beginnings of the Hall–Heroult cell for aluminum.

Since oxide magnesium ores, such as MgO, are found in high grade (90% plus purity), attempts were made to use this as feed material using a molten fluoride melt – similar to the Hall–Heroult cell during the late nineteenth century. But the high melting point of magnesium fluoride above 950 °C, along with the low solubility of magnesium oxide even in these fluorides, and the high vapor pressures of...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Chapter 1: Primary production of magnesium
• Chapter 2: Physical metallurgy of magnesium
• Chapter 3: Thermodynamic properties of magnesium alloys
• Chapter 4: Understanding precipitation processes in magnesium alloys
• Chapter 5: Alloying behavior of magnesium and alloy design
• Chapter 6: Forming of magnesium and its alloys
• Chapter 7: Corrosion and surface finishing of magnesium and its alloys
• Chapter 8: Applications: aerospace, automotive and other structural applications of magnesium
• Chapter 9: Applications: magnesium-based metal matrix composites (MMCs)
• Chapter 10: Applications: use of magnesium in medical applications
• Index