In this paper the ZnS precipitation in a continuously stirred tank reactor (CSTR) is modelled using mass balances. The dynamics analysis of the model reveals that the ZnS precipitation shows a two time-scales behaviour with inherent numerical stability problems, which therefore needs special attention during implementation. The mass balance model allows the design of an on-line estimator of the zinc concentration and precipitation rate, given only wet-chemical measurements of the sulphide concentration and of the pH in the reactor. The on-line estimation algorithm, also called observer or software sensor, is tested in a simulation experiment with a block disturbance showing numerically stable results. Furthermore, on the basis of real-world experimental data, the observer shows an appropriate performance. The estimation procedure offers good possibilities to be applied for other metal removal processes. (C) 2008 Elsevier B. V. All rights reserved.