Cryolite

Sodium Aluminum Fluoride
Na₃AlF₆

A curious note about cryolite is the fact that it has a low index of refraction close to that of water. This means that if emersed in water, a perfectly clear colorless crystal of cryolite or powdered cryolite will essentially disappear.

The History Says

Mining for cryolite was started in 1854 and continued until 1987 at Ivittuut. The most important mine in recent times was the Maarmorilik mine, which produced zinc, lead and silver during the period from 1973 to 1990. In total more than 20 different mining operations are known from the last 150 years.

The Present Scenario

The commercial application of cryolite is confined mainly to aluminium metallurgy, where it is used as an electrolyte in the reduction of alumina to aluminium metal by the Hall process.

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The mineral cryolite is a double fluoride of sodium and aluminium (3NaF, ALF₃). It crystallizes in momoclinic system. It is snow-white in colour and can be distinguished easily from any identical mineral by its colour. Theoretically, it contains 54.4% fluorine, 12.8% aluminium and 32.8% sodium. Its sp. gr. Is 2.96 and hardness 2.5. the refractive index is about 1.339 so close to water that this mineral becomes invisible when immersed in water. Its snow-white colour gives one distinct advantage in identifying it megascopically.

Hardness	Associated Minerals	Chemical/Typical composition	Colour	characteristics	Luster	Field Indicators
2.5 - 3	siderite quartz topaz fluorite chalcopyrite galena cassiterite molybdenite columbite wolframite	Na2O44.28 % Al2O324.28 % F54.30 %	clear or white to yellowish, but can also be black or purple	there is no salty taste which is helpful in distinguishing cryolite from the mineral halite	vitreous	lack of salty taste, density, index of refraction, locallity and crystal habit

World Resources

Commercial deposit of cryolite was known to occur only at one place in the world viz., Ivigtut, South Greenland. It was worked by open quarry and about 70,000 tons a year were minded. The mineral lying as stocks at the mine-site is mainly shipped to the USA and Denmark for processing. The deposit is held under lease by Kryolit Seleskabet Oresund A/D Copenhagen, Denmark. Other occurrences, though unimportant, have been noted in the St. Peter's Dome district near Pike's Peak in Colorado and at a few places in Russia, Spain and Canada.

In South Greenland, cryolite was found in pegmatite in porphyritic granite associated with fluorite, siderite, pyrites, arsenopyrite, galena, topaz, molybdenite, etc.

Industrial Applications

The commercial application of cryolite is confined mainly to aluminium metallurgy, where it is used as an electrolyte in the reduction of alumina to aluminuum metal by the Hall process. Here, reduction cells consist of mild-steel boxes with carbon lining that serves as the negative electrode, the anode's being carbon rods suspended from overhead bus bars or maybe a single 'Soderberg' carbon electrode. Cryolite is maintained at a temperature of 950 - 1000°C in these cells. Fluorite is added to the molten cryolite which acts as a flux. It also helps in balancing the fluorine content of the bath. From time to time, aluminium fluoride and occasionally sodium fluoride are added to balance aluminium and sodium contents of the electrolytic bath.

When alumina is added to the molten cryolite and current is passed, alumina decomposes and aluminium metal in the molten form sinks to the bottom from where it is tapped. Cryolite does not undergo any change when current is passed but there is an operational loss and this rate of opertional loss or rate of consumption is variable depending upon the efficiency of the plant and the grade of cryolite. The reat of consumption varies from 50 kg. to 90 kg. per tonne of metal produced. Withe advancement of technology, the upper limit rate has been gradually brought down and is being maintained at 60 kg. per tonne of metal produced. In addition when a new smelter is set up, there is an ad hoc requirement of cryolite at the rate of 80 tonnes per 1000 tonnes of installed capacity.

The consumption of cryolite is affected by the condition of cathode lining in the cells. When the lining is new some quantity of cryolite is absorbed. After a certain period, when saturation point is reached, cryolite consumption per tonne of metal produced is considerably reduced. Another factor effecting cryolite consumption is the composition of alumina and the extent of impurities contained therin. When the cathode lining is replaced with new lining, the cryolite consumption per tonne of metal produced increases again.

Other uses of cryolite of minor importance are as a whitener for enamel and an opacifier in glass. It also finds use as one of the bonding agents in grinding wheels and abrasives, and as an ingredient in welding rod coatings. Very finely ground cryolite is the active agent in some insecticide mixtures.

Synthetic Cryolite

Synthetic cryolite is used for aluminium metal production. It is generally manufactured by reacting hydrofluoric acid with soda ash and aluminium hydrate. There are also other processes known for the manufacture of synthetic cryolite. Hydrofluoric acid is obtained from acid grade fluorspar.

Market Specifications

Generally, cryolite is sold in three grades. The first grade containing a minimum of 98% 3NaF, ALF? is used mainly in aluminium industry. The maximum permissible limit of impurities in this grade is 1.5% Sio2; 0.25% Fe₂O₃ and below 1% CaO. The other two grades carry 93-94% of cryolite with Fe2O3 not exceeding 0.75%. Arc-welding rod manufacturers also prefer first quality cryolite. Iron and silica are considered undesirable impurities.

Future Outlook

The future of cryolite, it seems, is entirely dependent upon its use in the aluminium industry. It is learnt that some US firms are doing research and pilot plant tests have been carried out with success for the production of aluminium directly from the mineral bauxite without employing the intermediate process of reduction cell, whereby it will be possible to completely eliminate the use of cryolite. But nothing can be said at present about the success, achievement and economics of this process and it will take too long to shift from the conventional process to the new one, if the experiments turn out to be economically feasible.