A method and apparatus for conversion of paper residue into a mineral product uses a fluidized bed device with a distribution plate for securing an even distribution and supply of at least combustion air to the bed material and to the paper residue. An air box below the distribution plate supplies combustion air to the bed material and paper residue above the distribution plate. A heat exchanging section receives in separate parts ambient air and flue gases from the fluidized bed device for exchanging heat between flue gases and ambient air. The heat exchanging section is connected to the air box for supplying the heated ambient air to the air box for use as combustion air. A control system is employed for controlling the amount of bed material and the dimension of its particles and is arranged to monitor and maintain a process parameter within a predefined range.