Effect of White Water Metal Ions on Whiteness of Bleached Mechanical Pulp

Metal ions in papermaking white water have a significant negative effect on the bleaching of peroxides (P) and the bleaching of mechanical pulps of dithionite (Y).

Researchers at the Pulp and Paper Research Center at Fredericton University in New Brunswich, Canada, showed that metal ions in papermaking white water are more sensitive to Y-bleached millstone (SGW) than P-bleached SGW, which is more sensitive to metal ions. In the middle, iron ions have the greatest effect on the whiteness of Y-bleached and P-bleached SGW. Copper (II) and manganese (II) ions also have a significant negative effect on Y-bleached SGW.

The metal ions contained in the paper white water of the Y-bleached and P-bleached SGW supercalendered paper also have a detrimental effect. The addition of DTPA (diethylenetriaminepentaacetic acid) to the bleaching stage can compensate for the loss of whiteness of the mechanical pulp as DTPA can interact with metal ions. Under the same operating conditions, Y-bleached SGW absorbs iron ions more easily than P-bleached SGW and can be confirmed by the formation of a colored metal complex with the iron lignin structure in the pulp.