Along with chemicals deliberately added in the process, the process water quality is affected by species dissolved from the minerals and the air. In the case of the separation of calcite and apatite by flotation, Ca2+, CO32- and HCO3- are such species, and the variations of their aqueous concentrations can be expected to influence the process performance. In this study, both the long-term and short-term variations of pH, as well as the above-mentioned concentrations in two process water streams at the Siilinjarvi apatite concentrating plant, were monitored on-line. Obvious seasonal and diurnal variations in these concentrations were observed in the recirculated water reclaimed from the tailings ponds. Such variations are probably due to changes in the biological activity in the ponds. Further, the commercial process titrator OMT 20 DX delivered by Murtac GmbH, and sampling using laboratory-scale CERAMEC filters delivered by Outokumpu Mintec, proved to be a reliable concept in the demanding conditions of a concentrating plant. (C) 2003 Elsevier Science Ltd. All rights reserved.