Surface precipitation of MgCO3 on MnCO3 in aqueous solution at 90°C

Data presented in this work are used to suggest that after oxygen delignification, Mg(II) and Mn(II) are present in kraft pulps as carbonate solids, where the Mn(II)CO,(s) is coated by a layer of MgCO3(s). This can explain the role of Mg(II) as a redox stabiliser for Mn(II) in situations where hydroxide phases are thermodynamically unstable but where metal carbonates could form. The coating of MnCO3(s) by MgCO3;(s) has been studied with aqueous Mg(II):Mn(II) molar ratios ranging from 1:3 to 31:1 and at a temperature of 90(degrees)C. The formation of the various precipitates has been characterised by means of optical and energy dispersive SEM, x-ray powder diffractometry and x-ray photoelectron spectroscopy. To ascertain the inertness of the Mg(II)-coated Mn(II) carbonate particles, hydrogen peroxide stability tests were performed. The results showed that particles generated from Mg(II):Mn(II)-solutions of a molar ratio of l or above were significantly less active against the hydrogen peroxide in alkaline solutions. To prove that this mechanism also occurs under mill conditions, it is shown in the article that the corresponding type of particles can be precipitated from an oxygen-stage filtrate, i.e. a solution rich in carbonate ions and dissolved organic material. Potentiometric measurements have been applied with a setup that allows for titration and oxygen-free sampling at temperatures of up to 90degreesC. The free metal concentrations were analysed using the ICP-OES method. These data were used to thermodynamically characterise the solids forming and they gave, in contrast to the corresponding hydroxide system previously studied, no support for the formation of a solid (Mg,Mn-l,)CO3(s) solution.