Calcination of hydrated alumina in a rotary kiln - has kiln fired by stoichiometric mixt. of natural gas and oxygen@ burner, provides alumina for electrical insulators

Process for the calcination of hydrated alumina in a kiln at a mean temp. of 1000-1700 deg.C, opt. in the presence of calcination aid, operates kiln using a burner supplied with mixt. of oxygen and natural gas. The oxygen-natural gas mixt. is stoichiometric, the kiln is a rotary kiln, and the hydrated alumina which is calcined, has a sodium oxide content of 0.07-0.6 wt.% and a water content of 0.5 to 35 wt.%, based on the hydrated alumina. Pref., 0.1-3 wt.% of a calcination aid is added. USE/ADVANTAGE - Used in the mfr. of high grade alumina for electrical insulators. The disadvantages of previous processes, viz. a high level of entrainment of alumina dust in the off-gas and the discharge of nitrogen oxides, are prevented and the process is economical to operate. In an example, hydrated alumina (throughput: 165 kg/hour) with a average grain size of 80 microns, a water content of 2 wt.%, a specific surface of 150 m2/g, and a sodium oxide content of 0.25 wt.%, was mixed with a small amt. of boric acid (throughput: 850 g/hour) as a calcination aid and calcined in a rotary kiln for 18 minutes at a average temp. of 1450 deg.C. The kiln burner was fed with 14 Nm3/hour natural gas and 28 Nm3/hour oxygen. The amt. of dust produced was 3.8 wt.%, and the nitrogen oxides content of the off-gas was less than 0.02 g/rai