Analysis of the Composition and Formation Mechanism of Fouling in the Concentration Process of Titanium White Waste Acid

Large quantities of titanium white waste acid (TWWA) are generated during TiO2 production by the sulfate process. Recycling by concentration is the most widely used method to treat TWWA. However, serious fouling occurs during this process. The chemical composition of fouling obtained from a commercial plant was analyzed. The phase equilibria of CaSO4-FeSO4-H2SO4-H2O at 373.15 and 378.15 K were studied to elucidate the fouling mechanism. The fouling was mainly composed of CaSO4 (anhydrite) (74.97 wt %) and TiO(OH)2 (anatase) (24.48 wt %). The mass fractions of CaSO4 and FeSO4 in the equilibrium liquid phase of the CaSO4-FeSO4-H2SO4-H2O system increased with an increase in temperature and a decrease in H2SO4 concentration. The Pitzer method was successfully employed to describe the equilibrium behavior. During the mixing of the recycled acid and the initial TWWA, some CaSO4 (anhydrite) precipitated from the liquid phase, while some FeSO4·H2O dissolved in the liquid phase during the mixing and heating process. In the heating process, CaSO4 (anhydrite) did not precipitate out from the liquid phase. It was thus concluded that CaSO4 (anhydrite) precipitation during the mixing process is the key factor for the fouling on the heat exchanger surface during the primary evaporation process.