Sample pretreatment for oxalate analysis and the effect of peroxide bleaching parameters on oxalate formation

Calcium oxalate scaling can cause major problems in papermaking in connection to peroxide bleaching of mechanical pulps. Reliable oxalate analysis methods are needed for monitoring the oxalate balance in the process or for performance evaluation of a possible chemical anti-scale treatment. This is challenging due to the low solubility of calcium oxalate. In this study, the sample pretreatment and storage methods both for soluble and total oxalate analysis in mechanical pulp samples have been investigated. In addition, the effects of dissolved and colloidal substances in the dilution water, bleaching consistency, and replacement of NaOH with Mg(OH)(2) as alkali source in alkaline peroxide bleaching of groundwood pulp were studied. All experiments were carried out on groundwood pulp of Norway Spruce (Picea abies). Suitable and simple pretreatment methods were established both for analysis of soluble oxalate and for the acidification of pulp samples prior to total oxalate analysis by ion chromatography. The results from the peroxide bleaching experiments showed that a higher concentration of dissolved and colloidal substances in the peroxide bleaching will result in increased oxalate formation. Replacing NaOH with Mg(OH)(2) as the alkali source in peroxide bleaching to the same brightness level decreased the total amount of oxalate formed and increased its solubility. Increased bleaching consistency led to higher brightness gain and higher oxalate concentration in the filtrate, but the oxalate formation based on dry pulp was lower. The results indicate that Mg(OH)(2)-based peroxide bleaching at high consistency combined with good pulp washing would be a good way to minimize oxalate formation.