Arrangement for mixing a liquid with a flowing solid substance, comprising a reaction chamber (1), a connection (2) for feeding the flowing solid substance to the reaction chamber (1), a connection (3) for removing the mixed substance from the reaction chamber (1), and a connection (13) for feeding the liquid to the reaction chamber (1). For mixing the liquid there are elements (10) for feeding the liquid to the reaction chamber (1) in the form of droplets and elements (11,12,15) for feeding a compressed gas to the reaction chamber (1). On the central axis of the reaction chamber (1) is preferably arranged a mixer shaft (6), which comprises elements (9-12) for feeding the liquid and the gas to the reaction chamber from the outer surface of the shaft (6).