Method and a system for isolating xylan from plant material, as well as xylan

A method for preparing xylan and calcium carbonate from cellulose fibres that contain xylan, the method comprising:

• extracting xylan from said cellulose fibres with sodium hydroxide to form a mixture comprising cellulose fibres and an extract solution that contains xylan,
• removing the cellulose fibres from said mixture comprising the cellulose fibres and the extract solution to isolate said extract solution,
• adding carbon dioxide to said isolated extract solution to form a mixture comprising a liquid brightener and precipitated xylan,
• separating said brightener from the mixture comprising the liquid brightener and the precipitated xylan to isolate the xylan,
• adding calcium hydroxide to said separated brightener to form a mixture comprising precipitated calcium carbonate and liquid sodium hydroxide, and
• removing the sodium hydroxide from said mixture comprising the sodium hydroxide and the precipitated calcium carbonate to isolate the calcium carbonate.