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.