Lithium and rubidium extraction from zinnwaldite by alkali digestion process: Sintering mechanism and leaching kinetics

Lithium and rubidium were extracted from zinnwaldite [KLiFe2 +Al(AlSi3)O10(F,OH)2] by (1) its sintering with and CaCO3 powders and (2) water leaching the obtained sinters-the alkali digestion process. The experimental results showed that sintering proceeded in three partly overlapping stages: (1) decomposition of zinnwaldite at temperature up to 800 °C, (2) formation of new phases in the temperature range between 750 and 835 °C, and (3) formation of amorphous glassy phase at temperature above 835 °C. Densification of the reaction mixture occurred via a liquid phase sintering at temperatures above 750 °C and diffusion of calcium, potassium, silicon and rubidium resulted in the formation of the new phases. The decomposition of zinnwaldite and the formation of the new phases increased extraction of lithium and rubidium. The formation of glassy phase probably hindered extraction of lithium but did not affect that of rubidium because of its outward diffusion to sinter's surface. The optimal extraction efficiencies of 84% of lithium and 91% of rubidium were achieved at sintering temperature of 825 °C and leaching temperature of 95 °C. The good fit of the hyperbolic and uniform reaction models to the leaching data indicated that dissolution of lithium and rubidium proceeded through two stages. Application of the shrinking core model showed that dissolution of lithium was controlled by diffusion. The formation of a layer of Ca(OH)2 on surface of sinters apparently slowed and then terminated dissolution of lithium and rubidium in the later stage of leaching.