An Encyclopedia of the History of Technology part 14 ppsx

PART ONE: MATERIALS
112
Deville, at Salindres, found that only after very carefully purifying his
alumina was he able to produce aluminium containing less than 2 per cent
impurities. The technique he employed, the Deville-Pechiney process, was
developed by his associate Paul Morin; it is still occasionally used for the
treatment of bauxites which contain a lot of iron although it was not too
effective in removing large quantities of silica.
The Deville process was, however, superseded in 1887 by a cheaper and
simpler approach devised by the Austrian chemist Karl Joseph Bayer which is
now almost universally employed. Unlike the expensive Deville technique, it
depended entirely upon wet chemistry and involves no fusion process. Bauxite
was digested under pressure by a caustic soda solution in an autoclave at
temperatures between 150° and 160°C. This reaction produced a soluble
solution of sodium aluminate, the major impurities, such as iron oxide, titania
and most of the silica, being left behind as a red mud. Alumina was precipitated
from the caustic soda solution when it was cooled and diluted with water. After
calcination it was then suitable as a feedstock for the electrolytic cells.
The British Aluminium Company (BAC) Limited was floated on 18
December 1894 to acquire the British rights to the Bayer and Héroult
processes and others including the patents and factory site of the Cowles
Syndicate at Milton in Staffordshire where a rolling mill was installed. Lord
Kelvin was appointed as Scientific Adviser to the company and in 1898 he
joined the board of directors. The progress of this company, however, is
inevitably associated with William Murray Morrison, who was appointed as
chief engineer at the beginning of 1895 and served BAC for half a century.
Over this time the world output of aluminium increased from 200 tonnes per
annum in 1894, to 5000 tonnes in 1900, and in 1945 to well over two million
tonnes. The world output of primary aluminium in 1980 reached a peak in the
vicinity of 16 million tonnes per annum.
The first BAC plant was established at Foyers, close to the Falls of Foyers on

the southern side of Loch Ness, in 1895. It produced about 3.7MW (5000hp)
of hydroelectric power and by June 1896 it was extracting about 200 tonnes
per year of aluminium, most of which could not be sold. In 1896 about half
the power generated at Foyers was sold to the Acetylene Illuminating
Company which made calcium carbide by fusing lime and carbon in an
electric furnace, a process which had been invented by Moissan in 1892.
When after the turn of the century the world demand for aluminium began to
increase significantly, BAC started to build an additional hydroelectric plant at
Kinlochleven. This took about five years to build.
The Lochaber scheme, which commenced in the mid-1920s, was far more
ambitious, since it took water from a catchment area covering more than 300
square miles. Waters draining from the Ben Nevis mountain range and the
waters of Lochs Treig and Laggan were collected and fed to a powerhouse
situated only a mile from Fort William. Further hydroelectric plants were
NON-FERROUS METALS
113
established in other regions of the Western Highlands as the demand for
aluminium increased after the 1930 period.
Magnesium
Magnesium was first isolated in 1808 by Humphry Davy, who electrolysed a
mixture of magnesia and cinnabar in naphtha. The magnesium liberated was
absorbed into a mercury cathode to form an amalgam. The first chemical
reduction was accomplished in 1828 by Bussy, who used electrochemically
produced potassium to reduce anhydrous magnesium chloride. Magnesium,
however, remained a chemical curiosity until 1852 when Bunsen, at
Heidelberg, devised a method of producing it continuously by the electrolysis
of fused anhydrous magnesium chloride. In 1857 the metal was produced for
the first time in quantities large enough to allow its properties to be evaluated
by Deville and his colleague Caron. Using the technique he had already
perfected for aluminium production (see p. 103), Deville reacted sodium with

fused magnesium chloride to obtain magnesium, which was found to be a very
light reactive metal. It was also volatile, and could readily be separated, by
distillation in hydrogen, from the mixture of fused sodium and magnesium
chloride left behind in the reaction vessel.
By this time it was also known that magnesium was a metal which burned
readily to produce a very intense white light. Bunsen, who studied this effect,
found that the light emitted had powerful actinic qualities which rivalled
those of sunlight, so that the metal could therefore be of value to the new
science of photography. In 1854, Bunsen and a former pupil of his, Dr
Henry Roscoe, published a paper on the actinic properties of magnesium
light in the Proceedings of the Royal Society and this stimulated a great deal of
interest in the metal. In 1859, Roscoe became Professor of Chemistry at
Owens College, Manchester. The paper stimulated the inventive genius of
Edward Sonstadt, a young English chemist of Swedish descent who is known
today largely because he was the first analyst to determine accurately the
concentration of gold in sea water. Between November 1862 and May 1863,
Sonstadt applied for patents which covered an improved process for
producing magnesium, and for purifying it by distillation. By the summer of
1863 Sonstadt was able to claim that his ‘labourer and boy’ had produced
several pounds of magnesium metal.
During 1863, Sonstadt met Samuel Mellor, from Manchester, who made his
living in the cotton industry. Mellor was an enthusiastic amateur chemist, who
had, as a part-time student at Owens College, worked under Roscoe and had
established close personal contact with him. Sonstadt and Mellor became
partners, and Sonstadt moved his laboratory from Loughborough to Salford.
In Manchester, Mellor introduced Sonstadt to the pharmaceutical chemist
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114
William White, who had developed an improved method of making sodium
metal, needed by Sonstadt for his magnesium reduction process.

On 31 August 1864 the Magnesium Metal Company was incorporated
with a working capital of £20,000. Land for a production plant was acquired
at Springfields Lane at Patricroft, with a frontage on the River Irwell. One of
the directors appointed at this time was William Matther who also acted as
chief engineer of the company. At a later stage, as Sir William Matther, he
headed the well-known firm of Matther and Platt. At Salford he developed
improved methods of drawing magnesium wire which was then rolled into
ribbon. Magnesium ribbon was a major innovation, since it was simpler and
cheaper to produce than fine round wire. A very important advantage from the
photographic viewpoint was that it burned far more consistently than wire of
circular crosssection.
Magnesium production at Salford commenced in 1864 and in 1865 the
plant produced 6895 ounces (195.5kg) of the metal. A peak output of 7582
ounces (215kg) of metal was reached in 1887. In 1890 magnesium was being
imported into Britain from Germany at 1s 6d per lb: Salford could no longer
compete and the works were closed.
Just before the First World War, Britain had no indigenous source for the
magnesium it required, and in 1914 Johnson Matthey, in association with
Vickers, set up a magnesium plant at Greenwich. Using the sodium reduction
process originally developed by Sonstadt, this plant produced most of the
magnesium needed by the Allies for pyrotechnical purposes. After it was closed
down in 1919, no more magnesium was produced in the United Kingdom
until 1936.
German magnesium
Bunsen had appreciated as early as 1852 that the cheapest and most favourable
production route would involve the electrolysis of a fused anhydrous
magnesium salt. Large deposits of carnallite were found in the salt beds of
Stassfurt, and most of the earliest German magnesium production operations
attempted to utilize this readily available compound.
Carnallite, which is found in many evaporite deposits is extremely

hygroscopic. Magnesium metal was first obtained on a production scale by
the electrolysis of fused carnallite in 1882 by the German scientists
Groetzel and Fischer. In 1886 the Aluminium- und Magnesiumfabrik
Hemelingen established a plant for the dehydration and electrolysis of
molten carnallite using cell designs based almost completely on Bunsen’s
original conceptions. Much of the magnesium originally produced at the
Hemelingen plant was used for the manufacture of aluminium by a variant
of Deville’s process (see p. 106).
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115
Shortly after the turn of the century, the Hemelingen plant was acquired by
Chemische Fabrik, Griesheim Elektron, who transferred it to Bitterfeld in
Saxony which soon became the centre of the magnesium world. Before this
time magnesium had been regarded merely as an inflammable metal which
had applications in photography and pyrotechnics. Griesheim Elektron,
however, was led by the energetic and far sighted Gustav Pistor who was one
of the first to appreciate the engineering possibilities of a metal with a density
of only 1.74g/cm
3
(0.063lb/in
3
). Aluminium, the only comparable alternative,
had a density of 2.70g/cm
3
(0.098lb/in
3
). Magnesium could be of great value, it
was felt, particularly in the aeronautical field. The first hurdle to be overcome
was to develop a magnesium free from the corrosion difficulties which had
hitherto inhibited its industrial application.

Pistor, supported by the very able chemist Wilhelm Moschel, concentrated
initially upon the existing carnallite process and evolved an electrolytic bath
based on the chlorides of sodium, calcium and magnesium, mixed in such
proportions that the eutectic mixture formed melted at approximately 700°C.
This type of bath, although melting at a lower temperature than the cryolite/
bauxite mixture used for aluminium production, was and still remains far more
difficult and expensive to operate. The main difficulty is that the magnesium
metal obtained, being lighter than the electrolyte, floats to the surface of the
bath where it must be collected without shorting the electrodes. Furthermore,
the gas liberated at the anode is chlorine rather than the oxygen given off in
the aluminium cell. The chlorine liberated in magnesium production is
collected in bells surrounding the anode and utilized in the production of fresh
magnesium chloride.
These disadvantages were soon appreciated and after 1905 determined
attempts were made to evolve a magnesium bath using an analogy of the
Hall/ Héroult method in which magnesia was dissolved in a fused salt which,
like cryolite, did not participate in the electrolytic process. A process
developed in 1908 by Seward and Kügelgen utilized magnesia which was
dissolved in a mixture of magnesium and lithium fluorides. A cathode of
molten aluminium was used and the cell produced a magnesium-aluminium
alloy. A variation of this approach was introduced by Seward in 1920 (see
Figure 1.9) and was used in the 1920s by the American Magnesium
Corporation: magnesia was dissolved in a bath of fused sodium, barium and
magnesium fluorides and electrolysed to obtain magnesium of 99.99 per cent
purity. The fundamental obstacle in such processes is the low solubility of
magnesia in fluoride baths, so that it is difficult to ensure its constant
replenishment.
A problem encountered at Bitterfeld was caused by the reluctance of the
magnesium globules produced to coalesce. To improve this, small quantities of
calcium fluoride were added to the bath. This addition had no beneficial effect,

however, upon the hygroscopic tendencies of the carnallite-containing baths,
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116
and special techniques were required to dehydrate the fused salts so that
magnesium oxychloride was not produced.
It was against this difficult electrochemical background that Pistor and
Moschel developed the first practical magnesium alloys, which contained
aluminium and zinc for strengthening purposes and were introduced by
GriesheimElektron to the general public at the International Aircraft Exhibition
at Frankfurt in 1909. These alloys were used in considerable quantities by
Germany during the First World War, by which time Griesheim Elektron had
merged with IG Farben Industrie.
After struggling with the carnallite process until 1924, Pistor and Moschel
decided to synthesize a truly anhydrous magnesium chloride using the chlorine
which was readily available from IG Farben. Chlorination was accomplished
in large retorts where magnesite was reacted with chlorine in the presence of
carbon at temperatures above 708°C, the melting point of the chloride. This
accumulated as a liquid in the base of the reactor, and was transferred, still in
the molten state, to the electrolytic cells. Since 1930 most of the world’s
magnesium has been produced by the electrolytic route of Pistor and Moschel,
now known as the IG electrolytic process. Although some technical
improvements have been introduced, the general philosophy of this approach
Figure 1.9: Electrolytic magnesium. A sectional view of the cell subsequently
introduced by Seward in 1920 (US Pat. 1408141, 1920). In this process, operated
by the American Magnesium Corporation, pure magnesia was dissolved in an
electrolyte consisting of the fluorides of magnesium, barium and sodium. The
cell shown used currents of 9000–13,000 amperes at emfs of 9–16 volts. The
Dow process by then was producing purer and cheaper magnesium, and the
American Magnesium Corporation ceased production in 1927.
NON-FERROUS METALS

117
has never been seriously challenged in situations where chlorine and cheap
electric power are readily available and continuous magnesium production
over a long timescale is envisaged.
Revival of magnesium production in Britain
After the First World War, British interest in the future of magnesium was kept
alive by two firms, F.A.Hughes and Co. and British Maxium, later Magnesium
Castings and Products.
F.A.Hughes was led by Major Charles Ball, who originally established
contact with IG Farben and Griesheim Elektron while he was still in the post-
war Control Commission. F.A.Hughes and Co. gradually developed the
market for magnesium, although the demand was not great until the mid-
1930s when industry began to revive. By that time it was obvious that Britain
would soon require an indigenous source of magnesium, and in 1935
F.A.Hughes, supported by the British government, acquired the British and
Commonwealth rights to the IG patents covering the Pistor/Moschel process of
magnesium production, and then, in partnership with IG and ICI, set up
Magnesium Elektron Ltd (MEL) as an operating base.
The site of the works at Clifton Junction, near Manchester, was determined
by its proximity to the ICI works from which the large quantities of chlorine
needed for the production of anhydrous magnesium chloride could be
obtained. The Clifton Junction plant began production in December 1936. It
was initially intended to produce 1500 tons of magnesium a year, although by
government intervention in 1938 this capacity was increased to 4000 tons per
annum. In 1940 another unit of 5000 tons per annum was added, and a
further plant, capable of producing 10,000 tons per annum of magnesium
commenced production at Burnley in 1942.
The MEL production process soon began to use sea-water-derived
magnesia as a raw material. Before 1938 it had been dependent upon imported
magnesite. The sea-water magnesia was produced by the British Periclase

Company at Hartlepool, by treating sea water with calcined dolomite so that
both constituents precipitated magnesia. During the period 1939–45, 40,000
tonnes per annum of magnesia were produced in this way for the UK.
Magnesium in North America
Magnesium appears to have first been manufactured in the United States by an
offshoot of Edward Sonstadt’s Magnesium Metal Company in Salford. The
American Magnesium Company (AMC) of Boston, Massachusetts, was
incorporated by Sonstadt on 28 April 1865, shortly after the grant of his US
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118
Patent, to which the company was given an exclusive licence. Sonstadt himself
appears to have been the head of this company, which continued to produce
magnesium by the sodium reduction process until it ceased operations in 1892,
two years after the demise of the parent company in Lancashire.
Magnesium production in the United States then ceased until 1915 when a
number of prominent American companies, such as General Electric, Norton
Laboratories and the Electric Reduction Company, were persuaded to produce
some part of the magnesium requirements of the Allies. American production
in 1915 totalled 39 tons. Three producers were involved in the production of
magnesium stick and ingots which sold for $5 per lb.
The Dow Chemical Company started to produce magnesium in 1916. By
1920 only Dow and the American Magnesium Corporation remained in
operation. The AMC, then a subsidiary of the Aluminium Company of
America, operated the oxide/fluoride process developed by Seward and
Kügelgen. AMC went out of business in 1927, leaving Dow as the sole primary
producer of magnesium in the United States.
The Dow magnesium process
Dr Herbert Dow initially established his company to extract the alkali metals
sodium and calcium, together with the gases chlorine and bromine, from the
brine wells of Michigan. No outlet was found for the magnesium chloride

solutions which were originally run to waste. As the demand for magnesium
began to increase, however, Dow devised a very elegant method of making
anhydrous magnesium chloride in which the hydrated chloride was dried
partially in air and then in hydrochloric acid gas, which inhibited the
formation of the oxychloride. The anhydrous chloride thus obtained was then
electrolysed in a fused salt bath. This magnesium extraction process, which ran
as an integral part of the existing brine treatment operation, produced
magnesium so cheaply that Dow soon emerged as the major US manufacturer.
In 1940, Dow moved his magnesium plant from Michigan to Freeport in
Texas where anhydrous magnesium chloride was obtained from sea water by
processes similar to those used by MEL in Britain. The Texas site had the
advantage of unlimited supplies of natural gas, which allowed it to produce
magnesium very cheaply indeed.
The strategic significance of magnesium as a war material had become very
evident by 1938, and between 1939 and 1943 the United States Government
financed the construction of seven electrolytic and five ferrosilicon plants so
that any foreseeable demands for the metal could be satisfied. The biggest
plant, Basic Magnesium Inc., was jointly built and managed by Magnesium
Elektron from Clifton Junction and by the US Company Basic Refractories.
This plant, in the Nevada Desert, used power from the Boulder Dam, and had
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119
a rated capacity of 50,000 tonnes per annum of magnesium. All these
government-financed plants ceased production in 1945, leaving the Dow
Chemical Company unit at Freeport as the sole US manufacturer, although
some were reactivated for short periods during the Korean war.
Thermochemical methods of magnesium production
Magnesium, unlike aluminium, is a fairly volatile metal which at high
temperatures behaves more like a gas than a liquid. It can, therefore, be
obtained from some of its compounds by thermochemical reduction processes

to which alumina, for example, would not respond. Because of its volatility,
magnesium is able to vacate the vicinity of a high temperature reaction zone as
soon as it is liberated, and this provides the reduction mechanism with a
driving force which is additional to that provided by the reduction capabilities
of carbon or of a reactive metal. Some of these processes are capable of
producing high quality magnesium directly from magnesite without consuming
vast quantities of electric power, although the total energy requirements of such
processes are usually greater than the electrolytic technique, and they tend also
to be highly labour intensive. During the war years, when magnesium was
urgently required, the economics of thermal reduction processes were of less
importance than the simplicity of the plant required and its ability to produce
metal quickly, using indigenous resources without the benefit of hydroelectric
power. Several processes were pressed into service and some proved
surprisingly effective, although few survived into the post war era.
The ferro-silicon reduction process was developed at Bitterfeld by Pistor and
his co-workers in parallel with electrolytic routes to magnesium production. As
with the electrolytic process, British and Commonwealth rights to the IG
Ferrosilicon process were acquired by MEL at Clifton Junction in 1935.
The process depended upon the reaction which occurs between dolomite
and silicon, in which, as indicated below:
2 Mg O Ca O+Si → 2 Mg+(CaO)2 Si O
2
In this reaction, the tendency of silicon to reduce magnesia is assisted by the
high affinity of lime for silica. Because the product of the reaction, calcium
silicate, is solid, it has little tendency to reoxidize the liberated magnesium,
which escapes from the reaction zone as a superheated vapour and is then
rapidly condensed directly to the solid state.
Rocking resistor furnaces were used at Bitterfeld to obtain magnesium from
dolomite in the 1930s. The coaxial resistance element provided temperatures
in the reaction zone of 1400°C and the furnace was run 1n a vacuum. Ferro-

silicon was found to be the cheapest and most effective reduction agent, and
the magnesium liberated from the reduction zone condensed directly to the
solid state in a cooled receiver at the end of the furnace. Furnaces of this type,
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120
which produced about a tonne of magnesium a day, were built by IG Farben
for the Italian government and used at Aosta until 1945. Magnesium is still
produced from dolomite in Italy by variants of the ferrosilicon process.
The ferrosilicon process was also used, very successfully, by the Allies during
the Second World War, more particularly in North America. The process
introduced by M.Pidgeon in Canada in 1941 produced high purity magnesium
from dolomite by reacting it with ferrosilicon in horizontal tubular metal retorts
which in the first pilot plant were only about 10cm in diameter. These retorts
were disposed horizontally, as shown in Figure 1.10, in a manner which
resembled closely that of the Belgian zinc retort process (see p. 93). A plant built
by the Dominion Magnesium Company at Haley, Ontario, began to produce
magnesium at a rate of about 5000 tonnes per annum in August 1942, and five
similar plants were subsequently established in the United States.
Figure 1.10: One of the earlier experimental retorts used by M.Pidgeon in
Canada in 1941 for his thermo-chemical magnesium production process. In this
approach dolomite was reduced by ferrosilicon, in nickel-chromium retorts, in
vacuo, at temperatures around 1100°C. The magnesium vapour which distilled
from the reaction zone condensed, as indicated, on the inside of a tube
maintained at 450°C. Gas pressure inside the retorts were kept below 0.2 mm of
Hg. From L.M.Pidgeon and W.A.Alexander, Magnesium, American Society for
Metals, 1946, p. 318. With permission.
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121
Magnesium can be obtained by the direct reduction of magnesite with
carbon, although at atmospheric pressures the reduction does not commence at

temperatures below about 2000°C. The magnesium escapes from the reaction
zone as a superheated vapour, and to prevent reoxidation it must then be
rapidly cooled, when a fine pyrophoric powder is obtained.
The reaction involved in the carbo-thermic process is as indicated by the
equation below completely reversible.
MgO+C Mg+CO
If, therefore, the magnesium vapour released from the reaction zone is allowed to
establish contact with carbon monoxide at a lower temperature, it will reoxidize.
In this respect, therefore, the vapour of magnesium resembles that of zinc. After
carbo-thermic reduction the magnesium vapour must be rapidly cooled and a
variety of approaches to this difficult problem have been explored.
Most of the plants which attempted to operate the carbo-thermic process
used variants of the approach patented by Hansgirg in Austria in the 1930s.
This involved the reaction of magnesite with carbon in a carbon arc furnace at
temperatures which exceeded 2000°C. The magnesium vapour leaving the
furnace was rapidly cooled by a recirculating curtain of gas or by an
appropriate organic liquid, and was obtained in metallic form as a fine
pyrophoric powder.
A good deal of magnesium was made this way in Austria by the
OsterrAmerick Magnesit AG before and during the Second World War. In
1938 the Magnesium Metal Corporation was jointly established by Imperial
Smelting and the British Aluminium Company to produce magnesium by this
approach. The factory, at Swansea in Wales, produced magnesium at a high
cost and was shut down in 1945. The magnesium powder produced was
handled under oil to avoid pyrophoric combustion, and required pelleting
before it could be melted down. A large carbo-thermic plant built privately by
Henry Kaiser during the Second World War at Permanente, California, had a
rated capacity of nearly 11,000 tonnes of magnesium per annum. The
magnesium vapour leaving the reaction furnaces at this plant was condensed in
a curtain of oil. The lethal potentialities of this type of mixture were soon

appreciated. Under the trade name of Napalm it was soon in great demand.
Although the bulk of the world’s requirement of magnesium is still
produced electrochemically, thermochemical reduction processes continue to
attract a good deal of research effort and it is possible that a cheaper and more
viable alternative to electrolysis will eventually emerge.
AGE HARDENING ALLOYS
At the beginning of the twentieth century, steels were the only alloys which
were intentionally strengthened by heat treatment. In 1909, however, Dr Alfred
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