Method and apparatus for pre-treatment of fibre material to be used in the manufacture of paper, board or the like

2A method and apparatus for pretreating fiber material to be used in manufacturing paper or the like, for example, by precipitating mineral substances to the fibers. During precipitation, fiber material, which is advantageously pretreated in a through-flow mixer operating on the principle of an impact mill, is fed to the precipitation reactor. Gas is led to the precipitation reactor in order to generate a gas space in the reactor. The gas contains a substance, which precipitates the mineral substance such as carbon dioxide (CO). The fiber material is fed to the gas space of the precipitation reactor as small liquid and solid substance fractions such as drops and/or particles.