In-situ delignification of wood pulp by electrochemically generated Mn(III)CyDTA
Creators: name: Gyenge, Elod Lajos
Publisher: name: University of British Columbia
Year: 2009

description: The present work consisted of three parts, i.e. cyclic voltammetric investigation of the electrochemical behavior of Mn(III)CyDTA, development of a new, electrochemical method of Mn(IH)CyDTA synthesis in alkaline media and in-situ electrochemically mediated oxygen bleaching of softwood kraft pulp with the Mn(III)CyDTA complex. The cyclic voltammograms of Mn(III)CyDTA were recorded on three electrodes, i.e. graphite disk, stainless steel wire and platinum wire, as a function of pH (5 to 13.5) and scan rate (jo : 33.34 mV/s to 166.67 mV/s) at 25 °C. The presence of sodium bicarbonate (NaHC03 0.1 M) was found to be essential in bringing about the cyclic voltammetry response of Mn(m)CyDTA. The cyclic voltammetry investigations revealed the existence of Mn(II) adsorption on all of the electrodes. Increasing the pH shifted the cathodic peak potential toward more negative values with simultaneous decrease of the cathodic and anodic peak currents. At pH above 13, the cathodic peak vanishes, due to fast disproportionate of the Mn(IH) ion. The actual reduction potential of Mn(m)CyDTA is in the range of+30 to -200 mV/SCE, at pH 10 to 11 and 25 °C. The electrosynthesis of Mn(III)CyDTA was investigated in a divided batch electrochemical reactor, at 25 °C, in alkaline media, pH 9.0 and 10.5, for two CyDTA/Mn molar ratios, 1/1 and 2/1 respectively. The explored Mn(II) concentration range was between 6 mM and 54 mM. The supporting electrolyte was a 0.1 M NaHC03 solution, which contributes to the stabilization of the Mn(III) ion as well. Six anodes were tested. i.e. stainless steel screen, plus stainless steel, graphite, nickel, lead and platinized titanium plates. It was found that the electrosynthesis is favored by low CyDTA/Mn molar ratio (1/1) and high pH (10.5). The five anode plates were operated at current densities ranging from 13 A/m2 to 102 A/m2 , while on the stainless steel screen electrode the current density ranged from 2.6 A/m2 to 15.5 A/m2 . Over the entire domain of explored current densities and Mn(IT) concentrations, the anodic oxidation of Mn(II)CyDTA is under different forms of electrode kinetic control. Furthermore, in the case of an equimolar ligand to metal ratio, on the stainless steel and lead anodes a visible, anodic adsorption layer is formed, which modifies the surface and electrochemical properties of the anode and reduces the current efficiency for Mn(m)CyDTA formation. The graphite and platinized titanium anodes allow for much higher current densities without a visible anodic film formation (the highest current density employed in the present work being 102 A/m2). The current efficiencies for Mn(U) oxidation at pH 10.5 were between 78 and 40 %, for a 18 mM Mn(II) concentration. In the case of CyDTA/Mn = 2/1 (molar ratio), there was no visible build up of anodic film on any of the anodes. In the investigated range of current densities and Mn(II) concentrations, the current efficiency for Mn(III) electrosynthesis on the graphite anode was below 20 % at pH 10.5 and below 10 % at pH 9.0. The anolyte obtained after one hour of electrosynthesis, containing various amounts of Mn(III)CyDTA, was employed without further processing in the in-siiu electrochemically mediated oxygen bleaching of chemical pulp. Two anodes, i.e. stainless steel screen and graphite, were tested: The same pulp consistency (1 %) and oxygen pressure (0.1 MPa) were employed in all experiments. Factorial and parametric experiments were performed in order to determine the influence of different process variables on both the Kappa number and pulp viscosity. In the in-situ electrochemically mediated oxygen bleaching employing a graphite plate anode at a current density of 102 A/m2 , with an initial Mn(III)CyDTA cone, of 1.4 mM, under milder conditions than in the conventional oxygen bleaching, i.e. pH 9.0, atmospheric oxygen pressure, 80 °C, a 50 % delignification was obtained in 3 hr. (Kappa number dropped from 30.0 to 15.0), while the viscosity decreased from 34.6 to 20.5 cP. Under similar conditions the control experiment (i.e. with oxygen at atmospheric pressure but without catalyst and current) generated only a 3.2 points decrease in the Kappa number. The half factorial experimental design with six process variables, i.e. oxygen catalyst, current, temperature, pH and CyDTA/Mn molar ratio, at two levels and one centerpoint (26"1 + 1 design) showed that the delignification was mainly due to high second and third order synergistic effects between : catalyst - current, catalyst - oxygen, oxygen - temperature, catalyst - oxygen - temperature and catalyst - current - oxygen. Also, it was found that carbohydrate protection in the present system was achieved by both, high CyDTA/Mn molar ratio ( 2 to 4) and by high third order interaction effect between catalyst, pH and temperature. The third order interaction effect actually means that under conditions that cause intensive decomposition of Mn(IU)CyDTA (high pH and temperature) the carbohydrates are protected from degradation. Consequently, the Mn(II) species e.g. Mn(OH)2, Mn(II)CyDTA, are effective cellulose protecting agents. The same conclusion was strengthened by the parametric experiments. A small, but statistically significant, curvature effect was found proving the existence of a true catalytic interaction between Mn(III)CyDTA, oxygen and temperature. The stainless steel screen was employed as anode in the in-situ electrochemically mediated oxygen bleaching under conditions where the adsorption does not significantly affect the processi(i.e. at a current density of 2.6 A/m2 and with an initial Mn(HI)CyDTA cone, of 0.5 mM). A full factorial experimental design was performed with three process variables, oxygen, catalyst and current, at two levels. The pH and temperature were held constant at 10.5 and 25 °C respectively. Under these conditions the Kappa number dropped from 30.0 to 22.0 in 3 hr. while the viscosity was 24.9 cP. The delignification was mainly due to the additive effect of the catalyst, rather than synergistic effects with other process variables. The presence of 10 % v o i dimethylsulfoxide brought about a considerable improvement of selectivity, the final Kappa number after 3 hr. was 22.7 while the viscosity dropped from 34.6 cP to only 27.0 cP.