Production of Alumina Monohydrate From Aluminum Nitrate Solutions

This invention relates to an improvement in process for production of alumina from clays, such as kaolin. Such processes conventionally involve an initial leach of the calcined clay with a mineral acid, generally nitric acid, to dissolve the aluminum as aluminum nitrate. Iron, which is also leached, must be separated from the leach liquor prior to recovery of alumina. A variety of prior art processes have been developed for this purpose, including those of U.S. Pat. No. 3,221,521 and U.S. application Ser. No. 639,024.

Al (N0.sub.3).sub.3.9H.sub.2 O crystals are recovered from the purified leach liquor by removing enough water to cause crystallization when the solution is cooled. Al (N0.sub.3).sub.3.9H.sub.2 O can be decomposed to A1.sub.2 O.sub.3 either by direct roasting or in a two step process where a basic aluminum nitrate is formed at a low temperature (around 150.degree. C) and the basic nitrate is then roasted to A1.sub.2 O.sub.3. The disadvantage of crystallizing A1(NO.sub.3).sub.3.9H.sub.2 O and roasting it is that energy consumption is very large, a large heat input being required for vaporization of the excess water and for roasting the A1(N0.sub.3).sub.3.9H.sub.2 O.

It has now been found, according to the process of the invention, that alumina monohydrated, A100H, may be produced directly from the purified aluminum nirate leach liquor without evaporation of water, thereby substantially reducing energy consumption. In addition, the AlOOH may be roasted to Al.sub.2 O.sub.3 with considerably less energy consumption than that required for roasting of Al (NO.sub.3).sub.3.9H.sub.2 O to Al.sub.2 O.sub.3. The process of the invention consists of treatment of the leach liquor, or other aluminum nitrate solution, at a temperature of about 250.degree. to 350.degree. C and a pressure of about 500 to 2500 psig for a time sufficient to convert a major proportion of the aluminum nitrate to alumina monohydrate according to the reaction

Leach liquors suitable for treatment according to the process of the invention will typically consist essentially of aqueous solutions of aluminum nitrate nonahydrate containing about 0.5 to 7.2 weight percent of aluminum. The process is preferably carried out in an autoclave by means of which the required pressure is produced autogenously when the temperature is adjusted to the required value. Optimum reaction time will vary with temperature and pressure, as well as concentration of the solution, but generally a period between about 1 and 6 hours will be sufficient.

The reaction mixture is then cooled to room temperature and the granular precipitate of AlOOH separated by conventional means such as filtration. The AlOOH may be readily converted to Al.sub.2 O.sub.3 by roasting at a temperature of about 900.degree. to 1300.degree. C. The recovered HNO.sub.3 can be recycled to leach additional clay.

The following example will serve to more specifically illustrate the invention.

A charge of 150g Al (NO.sub.3).sub.3.9H.sub.2 O and 300g H.sub.2 O was put in an autoclave. The temperature was raised to 275.degree. C and held at that value for four hours, an autogenous pressure of 950 psig being generated. After cooling, 16.9g of precipitate was recovered and identified by X-ray diffraction as primarily .gamma.AlOOH, representing 70 percent Al recovery.