Ultra pure aluminas
High purity alumina is defined as at least 99.99% Al2O3, The size and morphology of crystallites are the other key traits. It is estimated that nearly half of the annaul production is used to manufacture sapphire. Other uses are tubes for high-pressure sodium vapour lamps and a number of polishing applications. These include for tape or magnetic discs for videos or computers, where they play the role of cleaning and polishing agent for recording heads. Metallographical polishing and the polishing of optical or electronic materials can also be mentioned.
THE MANUFACTURE OF ULTRA PURE ALUMINA
Four manufacturing processes for ultra pure aluminas are currently in use, either
using Bayer gibbsite or using aluminium.
1 - The most common process is that of using ammonium alum and consists in dissolving Bayer gibbsite in an excess of sulphuric acid, neutralizing it in ammonia in the form of NH4Al (SO4)2, 12H2O, crystallizing it by cooling, then calcining it at more than 1 000°C leaving a very friable residue of Al2O3.
2 - The alkoxide route (see manufacture of gels) that produces an even purer alumina when aluminium isopropylate is obtained by attack of pure aluminium, then purified by distillation.
3 - The chloride process, which consists in dissolving pure alumina in hydrochloric acid, precipitating the hexahydrated aluminium chloride and then calcining to produce alumina as a residue.