The Rule And Controlling Of Oxalate Formation During Pulp Clean Bleaching

Oxalate formation during pulping and bleaching process is one of the mostproblematic scale salts. In industrial process, oxalate related scale problems occurtypically in places like pipeline, evaporation, as well as in dewatering, screening, andfiltration stages. Numbers of studies have been done on the control and reduction ofoxalate scales. However, most of them have focused on oxalate formation andmechanism of chemimechanical pulp. There are few reports on the oxalate formationand control in the bleaching of chemical pulp and old newspaper pulp. Therefore, theformation and control of oxalate were investigated in this work, in order to controloxalate scales in the clean bleaching process of chemimechanical pulp and oldnewspaper pulp. The result is significant to deepen the recognition of oxalate scaleproblems, control the oxalate formation, improve energy efficiency and ensure thenormal operation of production.For these reasons, we studied the oxalate formation during chlorine dioxidebleaching of bamboo kraft pulp. It was found the amount of oxalate formation withchlorine dioxide dosage was a linear relationship. While increase bleaching time andtemperature, oxalate formation increased gradually and reached equilibrium at last,the oxalate content is about0.17g/kg. There was a good linear relationship betweenkappa number reduction and oxalate formation. Lignin model was selected to conductchlorine dioxide bleaching experiment, it obtained similar results to kraft pulp.A detailed investigation about oxalate formation and controlled methods inalkaline hydrogen peroxide bleaching of old newspaper pulp（ONP）and hard woodchemithermo-mechanical pulp（CTMP）was conducted, in combination with the novelspectrophotometric method for determination of hydrogen peroxide. The resultsshowed that oxalate content in the hydrogen peroxide bleaching effluent withperoxide and sodium hydroxide was linear. When increasing reaction time andtemperature, the formed oxalate content was also increased. In the same bleachingconditions, CTMP produced more oxalate than ONP. To decrease oxalate formation inperoxide bleaching of CTMP according to the Box-Benhnken central composite experimental design principles，the method of Response Surface Method with4factors and3levels was adopted. The studies showed that the optimum conditions ofoxalate formation during CTMP bleaching with hydrogen peroxide are: H2O2dosage6%, reaction time120min, temperature90°C, NaOH charge1.5%. At the optimalconditions, oxalate formation is0.90g/kg, pulp brightness can reach69.00%ISO.In addition, we selected several methods for controlling oxalate formation in theperoxide bleaching process. Oxalate related scaling could be minimized by partiallyreplacing NaOH with Mg(OH)2as the alkali source in the peroxide bleaching. Theamount of precipitated oxalate was20%-30%of the NaOH was replaced withMg(OH)2during bleaching. While the charge of xylanse to was15U/g, the totaloxalate content reached minimum value. Mixing NaBH4and NaHSO3before treatingthe pulp, and the mixed had a better effect on controlling the oxalate formation.Meanwhile, the rules of oxalate formationin oxygen delignification process wererevealed. The effects of process conditions on oxalate formation were also explored.Results shown that oxalate formation increased with temperature increase. Alkalicharge had a significance effect on oxalate formation during oxygen delignificationprocess. As increasing the reaction time, oxalate content increased firstly andstabilized then. Control methods of oxalate formation during oxygen delignificationwere magnesium hydroxide partially replaced by sodium hydroxide and the additionof magnesium sulfate.Oxalate formation during ECF and TCF bleaching sequences were studied. Insingle period of bleaching,(OP) and (PO) generated more oxalate than others.Compared the stage of O and (OP), it was found that hydrogen peroxide played acrucial role in the oxalate formation of oxygen delignification. DP and OO had asimilar oxalate content.