Mineralizing Effect of Li ₂ B ₄ O ₇ and Na ₂ B ₄ O ₇ on Magnesium Aluminate Spinel Formation

Abstract

Lithium borate (Li 2 B 4 O 7 ) and sodium borate (Na 2 B 4 O 7 ) mineralize spinel formation from stoichiometric MgO and Al 2 O 3 between 1000° and 1100°C. Mineralization with both compounds is shown to be mediated by B‐containing liquids which form glass on cooling. However, the liquid compositions depend on the type of mineralizer and temperature, suggesting that templated grain growth or dissolution–precipitation mechanisms are operating, one dominating over the other under certain conditions. Na 2 B 4 O 7 ‐mineralized compositions show predominantly templated grain growth at 1000°C, which changes to dissolution–precipitation at 1100°C, whereas Li 2 B 4 O 7 ‐mineralized compositions show dissolution–precipitation from 1000°C. Li 2 B 4 O 7 is a stronger mineralizer as spinel formation is complete with 3 wt% Li 2 B 4 O 7 at 1000°C and with ≥1.5 wt% addition at 1100°C, whereas Na 2 B 4 O 7 ‐mineralized compositions are found to retain some unreacted corundum even at 1100°C.