In the sulfate process for the production of TiO2 white pigments, gel-like metatitanic acid is calcined in a rotary kiln to microcrystals of the rutile polymorph. This process is monitored by the addition of alkali ions, Al3+, phosphate and rutile nuclei. During the last decade, a scientific understanding has developed how these additives influence the solid-state reactions of the material in the kiln, i.e., crystal growth, transformation of anatase to rutile, incorporation of dopants and aggregation of crystals. This review describes mechanisms and kinetics of the reactions, using modern concepts of solid state and surface chemistry and physics. It also describes the effects of the reactions on the quality of the calciner discharge in upgrading and for the customer, i.e., the grindability to pigment particles, their light-scattering properties and photocatalytic activity. The principles of the additive functions revealed here may be used in process engineering of other powder reactions as well as in particle chemistry and nanotechnology in general.