a process for producing fibrous material which includes the following : preparing a solution of chemicals that includes less than 25 % of sulphite , based on the oven - dry amount of the lignocellulosic raw material ; mixing the solution of chemicals with wood in a specified liquor material ; heating the solution of chemicals and the wood to a temperature above room temperature ; and then either of the following alternatives : removing the free - flowing solution of chemicals and digestion of the wood in the vapor phase ; having the wood digested in the liquid phase and separating the free - flowing solution of chemicals and the wood .