CRYOLITE
Cryolite is a double fluoride of sodium and aluminium with chemical
composition Na3AlF6. Cryolite, an uncommon
mineral of very limited natural distribution was only found in large quantities
on west coast of Greenland. This natural deposit was exhausted in 1987. It is
an importantraw material for extraction of aluminium from alumina. It has a low
index of refraction close to that of water. Synthetic cryolite is used as
electrolyte in the reduction of alumina to aluminium due to non-availability of
natural cryolite all over the world. Composition and properties of synthetic
cryolite are the same as those of natural cryolite but synthetic cryolite is
often deficient in sodium fluoride. Chiolite is another sodium aluminium
fluoride mineral having the chemical composition 5NaF.3AlF3.
In the beginning, the only significant Cryolite mine was in Greenland where it
was mined until 1987. Man-made Cryolite rapidly emerged to replace the natural
mineral and now continues to meet worldwide Cryolite needs.Technically cryolite
is Na3AlF6, or Sodium Aluminum Fluoride, of the Halide group. Synthetic
cryolite is produced from fluorite. Cryolite’s alumino-fluoride chemical
structure loses fluoride ions in the presence of water. All those
fluoride ions running loose not only results in very effective pesticide
activity, but also leaves a toxic fluoride residue in our food supply.Cryolite
has several other commercial uses. For hundreds of years this mineral was used
as a flux in the smelting of ores, especially aluminum.It’s also used as a
pesticide applied in powder or liquid form, from ground or aerial applications,
protecting crops from insects. The most common uses of Cryolite are on
California grapes, potatoes and citrus.
Lars Ohrström
Popular
fiction villains often belong to sinister organisations with obscure names, and
one of the more enigmatic examples is the Cryolite Corporation of Denmark,
appearing in Peter Høeg's 1992 bestselling novel Miss Smilla's Feeling for Snow.
However, in addition to Høeg's protagonist Smilla Jaspersen, non-fictional
heroes also have connections to the very real and important chemical named
cryolite, with formula Na3AlF6. For example, Vernon Jones
landing his flake hit, petrol leaking, Flying Fortress on a bog in south west
Sweden in 1943, and Henry Larsen, commander of the St. Roch during a voyage
through the Northwest passage in 1940, the real purpose of which was not
revealed until 50 years later.
Cryolite,
also known as sodium hexafluoroaluminate, is a colourless compound forming
cube-like crystals consisting of aluminium 3+ cations binding six fluoride F-
anions, forming octahedral like AlF63-, with smaller
sodium + ions to balance the charge. Perhaps you have correctly guessed
that the importance of cryolite is related to aluminium, and that the World War
II connection has to do with aeroplane manufacturing. But if you think that
cryolite is an important source of aluminium, think again. The aluminium
content of cryolite is only 13 per cent, compared to around 50 per cent in
bauxite, the major source of aluminium since industrial production begun late
in the 19th century. Besides the low aluminium content, cryolite is extremely
rare, possibly the only mineral on Earth ever to be mined to extinction
Bauxite on
the other hand is relatively common, but to lure the metal out of the mineral
on an industrial scale turned out to be tricky. Three electrons need to be
added to the Al3+ ions to make them neutral and metallic, and
although it was recognised early on that the way to do this was to pass an
electric current through a solution of the ions - what we call electrolysis -
it took some 50 years of experimenting until this was achieved. The
problem is that you cannot electrolyse aluminium in water, as the electrons
would combine with H+ ions, producing hydrogen gas. If we circumvent
the problem by melting aluminium oxide directly, the very high melting point,
2072°C, turns out to be prohibitively expensive. This is where cryolite comes
in. In 1886, both Charles Hall in Ohio, US, and Paul Héroult in Normandy,
France, discovered that molten cryolite, with the moderate melting temperature
of only 1012°C, easily dissolves aluminium oxide. Thus the Hall-Héroult
process was born, still in use today. The name cryolite stems from the Greek
words for cold, "cryo" and stone, "lithos", and this brings
us to polar hero Henry Larsen's role in this story. Aluminium started to become
a major material for aeroplane construction in the 1930s, and the occupation of
Denmark by Germany in 1940 made the British and its allies nervous, as cryolite
was found in only one place on earth - the Ivittuu mine in southern Greenland.
Under the cover of a Northwest passage journey, the only Canadian government
vessel able to navigate the icy Greenlandic waters, the St. Roch under command
of Henry Larsen of the Royal Canadian Mounted Police, set out from Vancouver to
survey the situation, as there was fear of a German invasion. With the United
States entry into the war the cryolite question was resolved by Greenland
temporarily becoming a US protectorate, and the production of the Ivittuu mine
increased substantially. Whether there was ever a real German plan to capture
the cryolite mine, as hinted to in Peter Høeg's novel, I don't know, but the
only recorded Nazi attempt on Greenland was an effort to establish a weather
station with a humble invasion force of 17 that was soon discovered by the
Danish Hound Sledge Patrol.Instead, the Germans set up a factory for producing
synthetic cryolite next to the aluminium plant in Herøya in southern Norway.
This process was rather new at the time, but Nordische Aluminium never saw
full-scale production as it was the target of a successful bombing mission. Not
only were the factories destroyed: of the 180 B17s dispatched in the morning of
the 24th of July 1943, only one was lost. However, skilful navigation and
piloting of the damaged aircraft Georgia Rebel, safely landed 1st pilot Jones
and his crew on neutral ground. This was the first of over 200 such US Air
Force emergency landings in Sweden during world war II.
The Ivittuu
mine was depleted in 1987 and today only synthetic cryolite is used in the
production of aluminium. Most commonly this artificial cryolite is obtained
from calcium fluoride, also known as the mineral fluorspar, sodium carbonate
and aluminium hydroxide, in a multi step process. One wonders, had this mine
and rare mineral not been discovered, would chemists have been clever enough to
prepare it anyway and devise the Hall-Héroult process, or would there still be
aluminium plates and cutlery at French state dinners, just as in the times of
Napoléon III when aluminium was worth more than its weight in gold.
v Occurrence
A late-stage mineral in some granite pegmatites; in
tin-bearing alkalic granites; a vapor-phase mineral along fractures and in the
groundmass of some fluorine-rich, topaz-bearing rhyolites; in pods in a
carbonatite vein cutting fenitized biotite gneiss. Also as a rare authigenic component
of the marlstones and shales of the Green River Formation.
v Association
Pachnolite, thomsenolite, gearksutite,
cryolithionite, weberite, jarlite, prosopite, chiolite, microcline, quartz,
fluorite, siderite, topaz (Ivigtut, Greenland); sodalite, villiaumite, eudialyte,
lovozerite, natrolite, chabazite, aegirine (Mont St. Hilaire, Canada).
v Distribution
Formerly in a large deposit at Ivigtut, southwestern
Greenland. In the USA, from St. Peters Dome, near Pikes Peak, El Paso Co., in
the Goldie carbonatite, Fremont Co., and the Green River Formation, Colorado;
in the Round Top intrusion, Sierra Blanca Peaks, Hudspeth Co., Texas; from the
Zapot pegmatite, 25 km northeast of Hawthorne, Fitting district, Mineral Co.,
Nevada; in the Morefield pegmatite, Amelia, Amelia Co., Virginia. From the
Francon quarry,
Montreal
Island, Montreal, at Mont Saint-Hilaire, and from near Saint-Amable, Quebec,
Canada. From Miass, Ilmen Mountains, Southern Ural Mountains, the Khibiny
massif, Kola Peninsula, and elsewhere in Russia. In the Madeira and Agua Boa granites,
Pitinga region, Amazonas, Brazil.Several other minor occurrences are known.
Physical Properties and Crystallography of Cryolite |
|
Vitreous, Greasy, Pearly
|
|
Diaphaneity (Transparency):
|
Transparent, Translucent
|
Comment:
|
Pearly on {001}
|
Colour:
|
Colourless, white, brown, grey, black;
colourless in transmitted light.
|
Streak:
|
White
|
Hardness (Mohs):
|
2½
|
Tenacity:
|
Brittle
|
Cleavage:
|
None Observed
|
Parting:
|
On {001} and {110}, producing cuboidal
forms.
|
Fracture:
|
Irregular/Uneven
|
Density (measured):
|
2.96 - 2.98 g/cm3
|
Density (calculated):
|
2.973(2) g/cm3
|
Crystal System:
|
Monoclinic
|
Class (H-M):
|
2/m – Prismatic
|
Cell Parameters:
|
a = 7.7564(3) Å, b = 5.5959(2) Å, c = 5.4024(2)
Å
β = 90.18° |
Ratio:
|
a:b:c = 1.386 : 1 : 0.965
|
Unit Cell Volume:
|
V 234.48 ų (Calculated from Unit
Cell)
|
Morphology:
|
Crystals usually cuboidal with c, , or
modified by r, v, k; also short prismatic [001]. {110} faces striated [111], [111], or [110].
Massive, coarsely granular.
|
1.
CRYOLITEHIONITE
IN THE CRYOLITE DEPOSIT
In 1902 cryolithionite was found as inclusions in
cryolite in the foot of the western wall of the quarry 34 .5 m below sea level
. Fig . 1 shows the first sample sent from Ivigtut the 25th of October 1902 by
the mine manager Mr . E . F. Edwards . He accompanied the sample with the
following report (in translation) : "A block of cryolite from the NW
corner of the quarry 110' below sea level, which is peculiar because,in the
center of th e block occurs a cavity filled with water-clear cryolite suspended
in which the common minerals galena, siderite etc. are found. The box is marked
Mineral N o 1, 1902" .
This block, now in the Geological Museum of the
University of Copenhagen, is mentioned by Ussing (1904) who found that the
"water-clear cryolite " was indeed a new mineral . He named it
cryolithionite following a proposal by the famous Danish chemist Julius
Thomsen, then director of the Cryolite Company, who found that this name
emphasized the resemblance to cryolite and the considerable content of Li . In
his concluding remarks Ussing (1904) stressed, however, the crystallographic
dissimilarities between the two minerals and emphasized analogies between garnets
and the new mineral. Crystal structure determinations (Menzer 1927, 1930 and
Geller 1971 ) unambiguously demonstrated the garnet structure of cryolithionite
.
In the autumn of 1903 Mr . Edwards sent further
samples of the new minera l reporting that it could now be found all along the
SW border of the quarry " so far onl y in spots " . We have no
indications that further samples were recovered from these urroundings . The
existing samples show that cryolithionite appeared as rather large areas of
hexagonal outline in white cryolite, up to 17 cm across, see Fig . 1 .
Scattered in thi s cryolite, isolated grains of siderite and some sulphides
were present in small amounts . This and the location in the quarry taken
together with reports on the development of the mine in the years 1889, 1894,
1904 and 1911 indicate that cryolithionite occurred in the transition zone
between the siderite-quartz-sulphide bearing cryolite (siderite - cryolite) and
a body of pure, white cryolite which was being exposed in the bottom of the
western part of the pit in the first years of this century, 34 m below sea
level . Bøggild (1913) pointed out that cryolithionite in the intervening years
had bee n
found
also in black cryolite where the white crystals of the mineral stood out quite
conspicuously. These samples undoubtedly came from the mining of a peculiar
variety o fthe siderite-cryolite, exposed in 1889 in the bottom of the quarry,
about 28 m below se alevel .
Besides the usual impurity minerals of the
siderite-cryolite, cm-sized crystals o fred-brown fluorite constituted 5 to 10%
of the mass . According to a map and a profil edrawing, see Fig . 2, sent from
Ivigtut to the company in 1889, and some photographstaken in the period from
1890 to 1908, this peculiar variety of siderite-cryolite (in the following
named BCrbF for black cryolite with red-brown fluorite) constituted an
inclined,interrupted sheet through the siderite-cryolite, the only cryolite
type then exposed in the quarry . The sheet was several metres thick and about
30 to 40 metre swide and long . In the western wall it was exposed about 20 m
below sea level .The red-brown fluorite in the BCrbF had been observed in 1873
by Lieutenant S .Fritz (manager in Ivigtut 1866-79) in a shaft (started in 1867
from the surface) at a depth of 28 m where mining in 1889 eventually revealed
its nature as an inclined sheet ; Johnstrup (1880) mentioned the occurrence of
this fluorite .
Thomsen (1904) publishd a note on gases contained in
some Greenlandic minerals where he, i .a ., showed th epresence of He in this
fluorite . A few years later Jarl (1910) found that the fluorite was
radioactive . Pauly (1960) reported a content of nearly 0 .3% Th in the
red-brown fluorite . The black colour of the cryolite is attributed to the
irradiation (Pauly 1962) .Cryolithionite in BCrbF constitutes a significant
part of the total number of sample sof this rare mineral in the collections
kept in Denmark.
2. INDUSTRY
Synthetic cryolites are obtained by adopting several
processes. The selection of the process depends upon the availability and cost
of rawmaterials. The simplest and most common method of obtaining synthetic
cryolite is by reacting hydrofluoric acid with soda ash and alumina hydrate.
Hydrofluoric acid is produced by reacting acid grade fluorite with sulphuric
acid and by-product gypsum is obtained in this process. In the secondary
reaction between hydrofluoric acid and sodium chloride brine, sodium fluoride
and hydrochloric acid are produced. In the primary reaction, dry aluminium
hydroxide reacts with hydrofluoric acid to produce aluminium fluoride which
reacts with sodium fluoride produced earlier and forms synthetic cryolite.
Besides fluorite, by-product fluorine gas emanating
from plants of phosphatic fertilizer and phosphoric acid has emerged as an
important alternative source for hydrofluoric acid and other fluorine chemicals
including cryolite and aluminium fluoride. Rock phosphate usually contains 7-8%
CaF2. In terms of fluorine, it works out to 3-4% which is
liberated at the time of acidulation of rock phosphate with sulphuric acid. Fluorine
combines with silica to form silicon tetrafluoride which when scrubbed with
water forms fluorosilicic acid. By recycling, 18-24% fluorosilicic acid is
obtained, which serves as a raw material for manufacturing various
fluoro-chemicals including synthetic cryolite. From fluorosilicic acid,
fluorine values are precipitated as sodium fluorosilicate by treating it with
sodium salts. Sodium fluorosilicate becomes starting point for the production
of synthetic cryolite. For manufacture of synthetic cryolite from sodium
fluorosilicate, two routes are generally adopted in the country. They also
manufacture potassium cryolite (K3AlF6) which is a foundry
flux and used in welding chemicals and explosives. The total installed capacity
of aluminium fluoride in organised sector was 27,000 tonnes per annum. Production
of aluminium fluoride was 11,550 tonnes in 2009-10.
3. SPECIFICATIONS
The Indian Standard Specifications of cryolite for use
in aluminium industry defined vide IS - 5893 : 1989 (Second Revision;
reaffirmed 2008) are as follows:
Constituents (on dry
basis) Specification
1. F
53%
min
2. Na
31
to 34%
3. Al
13
to 15%
4. SiO2
0. 20%
max
5. Fe2O3
0. 10%
max
6. CaF2
0. 06%
max
7. Al2O3
1.00%
max
8. SO3
0. 50%
max
9. P2O5
0. 01%
max
10. Loss
on Ignition (LOI) 0.50%
max
11. NaF/AlF3(by
mass) 1.45 max
Note:
i
) LOI is to be determined at 550OC for 60 minutes.
ii) Moisture should not
be more than 0.20% when determined at 110-5OC.
Cryolite obtained as a by-product during phosphate
manufacture when utilised in the aluminium industry, necessary precautions are
observed as even 0.01% in the electrolyte could cause 1-1.5% reduction in
current efficiency in the production process of aluminium.
4. USES
AND TECHNOLOGY
The commercial application of cryolite is confined
mainly to aluminium metallurgy where it is used as electrolyte in the reduction
of alumina to aluminium metal by the Hall process. Alumina is a bad conductor
of electricity and its melting point is 2,348 oC. It is very expensive to carry
out electrolysis at this temperature. To facilitate electrolysis, alumina is
dissolved in molten cryolite as it lowers the melting point. Further, addition
of certain additives such as, aluminium fluoride improve the physical and
electrical properties of the electrolyte besides lowering the melting point.
The amount that is added is, however, limited as it also causes reduction in
electrical conductivity. Addition of fluorite (CaF2) further
depresses the melting point with less adverse effect on conductivity. In
contrast to this advantage, too much CaF2 raises the density of the melt closer to that of
liquid aluminium metal, thus inhibiting the separation of metal from
electrolyte. The substituent, sodium fluoride, though known to improve the density
and conductivity, also affects current efficiency. A compromise made on all
these factors has led to the following general composition of bath to be in use
– 80-85% cryolite, 5-7% AlF3, 5-7% CaF2, 0-7% LiF and 2-8% Al2O3. The
electrolyte bath tends to deplete AlF3 content of cryolite during the process.
Hence, the composition of the electrolyte has to be adjusted regularly by
addition of AlF3.
In aluminium refining, high density electrolyte capable
of floating aluminium is required. For this purpose, barium fluoride can also
be used to raise density. Aluminium fluoride can be used to improve current
efficiency of cryolite bath.
Other metallurgical uses of cryolite are in aluminizing
steel, in compounding of welding rod coatings and as fluxes. In glass, cryolite
functions as a powerful flux because of its excellent solvent power for oxides of
silicon, aluminium & calcium and for its ability to reduce melt viscosity
at lower melting temperatures. Cryolite is used as a filler for resin-bonded
grinding wheels in abrasive industry to give longer life. Sodium fluoride (NaF)
or fluorosilicic acid may also be used for this purpose. Cryolite is used in
certain nitrocellulosebased gun propellants required in small-calibre weapons,
cannons and small & large rockets.
The future of cryolite, as it may seem, is entirely
dependent upon its use in the aluminium industry. It is learnt that some US
firms have registered success in their research and pilot plant tests for
production of aluminium directly from the mineral bauxite without the
intermediate process of reduction cell. Viability of this may probably eliminate
the use of cryolite in the days to come.
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