Preparation of metallic cobalt and cobalt-barium titanate spheres as high temperature media for magnetically stabilized fluidized bed reactors.

Abstract

Fluidized bed reactors are extremely efficient when mass transfer limitations dominate global reaction rates, such as in combustion or incineration [1, 2]. Because fluidized beds operate using a balance between direct fluid dynamic forces and the gravitational restoring force, the maximum mass transfer rates for a given reactor configuration are limited by gravity. This limitation has been overcome using magnetic fields acting upon magnetically susceptible fluidization media to augment the force of gravity [3–7]. However, the use of magnetically stabilized fluidized beds in high temperature reactions has been limited by the lack of suitable media. Three magnetic properties are required to obtain suitable media: high magnetic susceptibility, high Curie temperature, and low coercivity. Cobalt, with a Curie temperature of 1121 ◦C, meets these requirements. We have investigated the preparation of two forms of cobalt containing spherical fluidization media: metallic cobalt, and cobalt impregnated barium titanate. Novel ferromagnetic media including cobalt impregnated barium titanate and metallic cobalt spheres have been prepared for high temperature application in magnetically stabilized fluidized beds. Spherical polymeric beads were initially formed by gelation of an alginateprecursor oxide suspension. Water was then eliminated from the hydrous gel at low temperature and the organic polymer was removed by oxidation in air. The temperature was then raised and the firing atmosphere changed to a reducing mixture containing hydrogen. Under these conditions, cobalt oxide was first reduced to the metallic form, and at a sufficiently high temperature, densification occurred via sintering. Cobalt impregnated barium titanate was prepared by intimately mixing BaCO3, TiO2, MgSO4, SiO2, and Co3O4 in a ball mill using zirconium oxide milling media, with molar ratios corresponding to one mole of Ba0.95Mg0.05TiO3, 0.064 moles of TiO2, 0.311 moles of Co, and 0.0207 moles of SiO2. The excess TiO2 and SiO2 form a low melting eutectic, predominately BaTiSi2O7, which melts at 1245 ◦C and acts as a sintering aide. The cobalt content corresponds to 10% (w/w) of the resulting BaTiO3. The oxides, carbonates, and sulfates were initially added to 1.4 times their weight