This paper presents a dynamic model for a chemical precipitation and dissolution process in an aqueous solution. The precipitation process is a part of the electrolyte purification section at the Falconbridge Nikkelverk in Kristiansand, Norway. The model is partly mechanistic, and partly based on first principles. The state variables in the model are chosen as a combination of linear reaction invariants and non-linear variants. The effect of dispersion and radial mixing in the non-ideal plug flow reactor (PFR), where the precipitation takes place, is modelled as a combination of ideal continuous stirred tank reactors (CSTR). The CSTR-elements can be viewed as general building blocks or objects, which may also form a larger and more complex reactor system. Easier maintenance and better chance for reuse are some of the benefits of using only one central building block when modelling reactor systems. The chemical precipitation and dissolution rates in the model are controlled by the pH value and the reduction potential. The simulation results shows a high degree of correspondence with actual measurements at the refinery, and therefore form a promising basis for design of control and operator support systems.