This work presents the development of a unknown input observer for on-line estimation of density and viscosity of a mineral slurry flowing through a pipes and fittings assembly (PFA). A phenomenological based semi-physical model (PBSM) is developed based on the conservation principle applied to mass and momentum transfer. Model equations include variables directly linked with density and viscosity of mineral slurries through frictional losses considerations. The theory for state observers for processes with unknown inputs was used to develop the observer for estimating those properties of a mineral slurry from the mentioned PBSM.