The fate of phosphorus in the bleaching of spruce TMP with the new bleaching agent - THPS

Tetrakis(hydroxymethyl)phosphonium sulfate (THPS) is a newly discovered, phosphorus-based bleaching agent that has been successfully tested in a pulp mill trial. The fate of phosphorus (P) and the chemistry in the course of THPS bleaching of a spruce TMP have been studied. The amount of P introduced into the THPS-bleached pulp depends on THPS dose and the bleaching conditions and it correlates well with the brightness gain. The P introduced into THPS-bleached pulp is covalently bonded to the pulp. The THPS-bleached and tetraethylphosphonium chloride-treated pulps have been additionally treated with calcium chloride before P analysis, and the results of these experiments support the covalent bonding hypothesis. Bleaching with 0.1 and 3.0% THPS (% based on oven-dried pulp) at 60 and 110 degrees C leads to pulps with 0.13 and 2.17 mg P g(-1) pulp, respectively, which corresponds to 0.08 and 1.4% THPS. More precisely, P is covalently bonded to lignin chromophores, such as lignin coniferaldehydes. This was found by P-content analysis, UV-visible spectroscopy, and P-31 {H-1} NMR studies of a crude milled wood lignin (crude MWL) and a partially purified MWL isolated from a THPS-bleached pulp.