Novel methods in peroxide bleaching
Creators: name: Finnegan, Daniel James
Publisher: name: University Of Tasmania
Year: 2023

description: Hydrogen peroxide has been used by manufacturers of high yield mechanical pulps as the preferred bleaching chemical. For example, Australian Newsprint Mills(ANM) Ltd operates a hydrogen peroxide refiner bleaching Pine TMP plant at its Albury mill and a peroxide tower bleaching plant at the Boyer mill processing Eucalypt CCS. Hydrogen peroxide bleaching requires alkaline conditions to form the active bleaching species 001-1 - . However, under these conditions hydrogen peroxide is very susceptible to transition metal ion catalysed decomposition. The metals manganese, iron, and copper promote the greatest rate of peroxide decomposition. DTPA is added to chelate the transition metal ions in the process and prevent hydrogen peroxide decomposition. DTPA is a strong complexing agent and there are some concerns about its environmental impact. Alternatives to DTPA have been investigated. Recently a patent was published using Zeolite as an alternative chemical to DTPA. In this project the affect of zeolite and sodium citrate on peroxide decomposition and bleaching of Pine TMP and Eucalypt CCS pulp has been investigated. Manganese, a known decomposition catalyst, has been used to produce conditions which induce peroxide decomposition. Various factors including pH, zeolite type, zeolite concentration and sodium citrate concentrations were all found to affect the rate of peroxide decomposition and bleaching response. Zeolite-A was found to reduce decomposition more than the other two zeolites, types X and Y. Zeolite-A has a lower Si/A1 ratio which allows for greater cation exchange. The combined zeolite and citrate system had the greatest stabilising effect on decomposition of peroxide solutions in the presence of manganese under alkaline conditions. Zeolite-A also had a better bleaching response than the other zeolites. Combined zeolite-A and citrate systems achieved brightness results slightly less than DTPA though the residual peroxide concentrations were significantly lower for peroxide bleaching of Pine TMP. The order of chemical addition is important. If the alkali is added prior to zeolite, and manganese is oxidised from Mn2+ state to higher states forming Mn-O-Mn type complexes, zeolite was found to be ineffective. Consequently zeolite-A had no effect on manganese induce peroxide decomposition leading to a poor peroxide bleaching response with Eucalypt CCS pulps. Zeolite is believed to be acting as a chelating agent rather than as a catalyst for hydrogen peroxide oxidation reactions (bleaching). Zeolite is able to cation exchange Mn2+ ions but not Mn-O-Mn complexes. If Mn-O-Mn complexes are allowed to form, decomposition of peroxide occurs. Addition of citrate helps transfer Mn ions into the zeolite structure rather than providing an overall increase in chelating ability. Addition of zeolite improves optical properties of handsheets by increasing opacity and light scattering co-efficient which is beneficial to newsprint grades. No significant losses in strength of handsheets occurred with the addition of zeolite.