Abstract: A thermal reactor apparatus and method of pyrolyticaly decomposing silane gas into liquid silicon product and hydrogen by-product gas is disclosed. The thermal reactor (1) has a reaction chamber (21) which is heated well above the decomposition temperature of silane. An injecter probe (100) introduces the silane gas tangentially into the reaction chamber (21) to form a first, outer, forwardly moving vortex (22) containing the liquid silicon product and a second, inner, rearwardly moving vortex (23) containing the by-product hydrogen gas. The liquid silicon in the first outer vortex (22) deposits onto the interior walls (28) of the reaction chamber (21) to form an equilibrium skull layer (26) which flows to the forward or bottom end of the reaction chamber where it is removed. The by-product hydrogen gas in the second inner vortex (23) is removed from the top or rear of the reaction chamber by a vortex finder (30).