The present invention relates to coarsely crystalline alumina, hereinafter also called technical grade aluminum oxide, as well as to a method for producing it.
Technical grade aluminum oxide is usually produced by heating (calcination) of an aluminum hydroxide, such as a hydrargillite, in a rotary drum furnace or fluidized bed furnace at temperatures around 1100.degree. C. The thus-produced aluminum oxide with a red heat loss of 0.5 to 1% is composed of a mixture of transition oxides of the gamma Al.sub.2 O.sub.3 series and of alpha Al.sub.2 O.sub.3, and is characterized by a specific surface of 5 to 30 m.sup.2 /g. The average primary particle size, determined according to the Eppendorf method in which after grinding of the aluminum oxide the optical density of an aluminum oxide sample is measured in water in dependence on time by means of the Eppendorf photometer and the resulting grain distribution of the aluminum oxide is determined, is about 0.5 to 1.mu..
If the calcination temperature is increased to more than 1200.degree. C., aluminum oxide forms almost exclusively in alpha modification, the specific surface is reduced to values of &lt; 1 m.sup.2 /g, and the average primary particle size grows to about 3.mu..
It is also known that in the presence of about 0.001 to 0.5 percent by weight of a mineralizer in the starting aluminum hydroxide, e.g. in the presence of fluorides such as AlF.sub.3, CaF.sub.2 or Na.sub.3 AlF.sub.6, aluminum oxides of the alpha modification are produced already at temperatures of about 1100.degree. C. with a specific surface of 0.5 m.sup.2 /g which is also small. The average primary crystal size of such aluminum oxides lies at about 6.mu.. Higher temperatures and/or greater quantities of the exemplary mineralizers mentioned above have only insignificant influence on the above-mentioned parameters such as alumina modification, specific surface and average primary crystal size.
However, for certain applications, such as for use during grinding and lapping operations, aluminum oxides are required which have average primary crystal sizes of more than 6.mu..
The well-known Bayer process for producing aluminum oxide (alumina) is universally operated on a continuous basis and involves pressure digestion of aluminous ores, such as bauxites and laterites, in caustic aluminate liquor of varying caustic soda concentration and at elevated temperatures depending upon the type of ore to extract the available alumina. The caustic liquor enriched in sodium aluminate is then subjected to clarification by settling, washing, and filtering to remove the so-called red mud residue of the ore which consists primarily of hydrated ferric oxide and a desilication product. Aluminum hydroxide (alumina hydrate) is then auto-precipitated from the clarified green liquor by decomposition of sodium aluminate through seeding with previously-precipitated alumina hydrate. The precipitated aluminum hydroxide is then separated as by filtration from the liquor and is then washed and calcined to form the desired aluminum oxide. The spent caustic liquor is recycled to the digestion phase, usually after concentration as by evaporation and addition of required amounts of make-up caustic liquor. The sodium aluminate liquor which has been concentrated as by evaporation contains a number of contaminants in the form of salts which must be removed from the liquor from time to time. For this purpose, part of the concentrated liquor can be cooled to precipitate the salts which can then be separated from the liquor in a centrifuge. A mixture of salts is obtained, which mixture is often referred to as foreign salts. The most valuable component of these foreign salts is vanadium and, as a result, the mixture of foreign salts is also known as "vanadium salt".