Abstract: A process and apparatus for replacing intra-fiber liquid in fibers by a replacement liquid, includes compressing the fiber cake to express a substantial quantity of intra-fiber liquid to the space between the fibers; forcibly supplying replacement liquid to the fiber cake during the compression thereby replacing the intra-fiber liquid from the space between the fibers and letting the fibers expand while supplying additional replacement liquid. The invention gets rid of or significantly reduces the content of contaminating material in not only the areas between the fibers in a fiber cake but also in the fiber wall pores and the lumen and replaces this liquid by a replacement liquid.