Mineral Synthetic Cryolite
Description:
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.
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 aluminum 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 operational 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.
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