The efficient use of papermaking potential of secondary fiber by gentle refining of recycled pulp is one of the basic principles for the recovered paper stock preparation. The use of mild refining modes for recycled fiber is more important in the case of wet -strength paper than for conventional waste paper. This is due to the fact that recycled fiber is weakened by hard conditions of wet-strength paper repulping, in particular, long repulping time, elevated temperature, the use of alkali and oxidizers. The study aims at evaluating the changes in the morphological characteristics of secondary fiber during gentle low-consistency refining of recycled pulp. The analysis of the secondary fiber morphological characteristics was carried out in two complementary directions, i.e. the analysis of the fibers (part 1) and the analysis of the fines (part 2). Recycled pulp (RP) was obtained from commercial wet -strength tissue paper consisting of 100 % bleached softwood kraft pulp (BSKP) made of pine (Pinus sylvestris L.). The wet-strength paper repulping was performed at 60 degrees C in three ways, namely, repulping with sodium hydroxide (indicated as RP(NaOH)), repulping with sodium hydroxide and sodium persulfate (RP(NaOH + Na2S2O8)), and finally repulping with sodium hydroxide and potassium monopersulfate (RP(NaOH + KHSO5)). Secondary fibers as well as primary fibers of BSKP had similar length distribution before and after gentle refining. Mean fiber length after first recycle reduced by 5-6 %. The calculated values of the fiber -shortening index showed that fiber cutting practically does not occur during gentle refining of the recycled pulps. The increase in the refining degree of the pulps is mainly explained by the fibrillation of secondary fibers, rather than the formation of a large amount of fines. Secondary fibers obtained from the wet-strength paper have a high slenderness ratio ((L/W)l from 66.3 to 66.5 and (L/W)(w) from 83.6 to 84.3; where L - length, W - width) corresponding to pulp with good papermaking potential. The results showed that low-consistency gentle refining of the recycled pulp obtained by accelerated persulfate and monopersulfate repulping of the wet-strength paper leads to fiber straightening. The overall increase in the shape factor of secondary fibers was achieved due to a combination of accelerated repulping of the wet -strength waste paper (by similar to 60 %) and gentle refining of the pulp (by similar to 40 %).
