Formation of titanium phosphate composites during phosphoric acid decomposition of natural sphene

Decomposition of mineral sphene, CaTiOSiO4, by H3PO4 is investigated in detail. During the dissolution process, simultaneous calcium leaching and formation of titanium phosphate (TiP) take place. The main product of decomposition is a solid titanium phosphate–silica composite. The XRD, solid-sate NMR, IR, TGA, SEM and BET data were used to identify and characterize the composite as a mixture of crystalline Ti(HPO4)2·H2O and silica. When 80% phosphoric acid is used the decomposition degree is higher than 98% and calcium is completely transferred into the liquid phase. Formation of Ti(HPO4)2·H2O proceeds via formation of meta-stable titanium phosphate phases, Ti(H2PO4)(PO4)·2H2O and Ti(H2PO4)(PO4). The sorption affinities of TiP composites were examined in relation to caesium and strontium ions. A decrease of H3PO4 concentration leads to formation of composites with greater sorption properties. The maximum sorption capacity of TiP is observed when 60% H3PO4 is used in sphene decomposition. The work demonstrates a valuable option within the Ti(HPO4)2·H2O–SiO2 composite synthesis scheme, to use phosphoric acid flows for isolation of CaHPO4·2H2O fertilizer.