Inhibition of the Anatase—Rutile Phase Transformation with Addition of K2O, P2O5, and Li2O

Abstract

During the sulfate process of TiO2 pigment production, hydrated titanium dioxide is calcined. As a result of the polymorphous conversion, rutile is obtained. Roasting additives are introduced into the calciner, in order to achieve required process temperature, crystallite size of individual phases, optical properties, and the pigment photostability. Calcination process was investigated on the laboratory scale with the use of hydrated titanium dioxide containing rutile nuclei from the industrial installation. The influence of both temperature and calcination time on the anatase—rutile transformation ratio and the crystallites growth was determined. The obtained dependences were described using exponential equations. The ratio of anatase—rutile transformation was higher at elevated temperatures. It was noticed that the process temperature affects the conversion ratio considerably more than the calcination time. During the experiments alkaline metals (potassium, lithium) and phosphate were introduced into the calcination suspension. It was noted that the increase of phosphates content in the calcinated TiO2 · nH2O (0.1—0.5 mass % P2O5 in relation to TiO2) caused the anatase—rutile transformation ratio to drop 2 to 10 times. Moreover, the phosphates presence restricted an unfavourable anatase and rutile crystallites growth. In fact, their crystallites size was 2—3 times lower, compared to the crystallites size measured when the phosphates were not added. If the potassium content in hydrated TiO2 was increased (0.05—1.0 mass % K2O in relation to TiO2), during calcination the rutile formation passed through a maximum. Similar behaviour was observed when lithium (0.025— 0.5 mass % in relation to TiO2) was introduced instead of potassium. Neither anatase nor rutile average crystallites size was influenced by the potassium or lithium addition.