Pressurized peroxide bleaching of kraft pulps employing organophosphonate-based bleach stabilizers.

Application: The performance of high temperature, pressurized peroxide stages ([P.sub.HT]) can be improved significantly and economically through the use of organophosphate-based bleach stabilizers.

Environmental concerns prompted our industry to switch from molecular chlorine-based bleaching to elemental chlorine-free (ECF) or totally chlorine-free (TCF) bleaching. Recent developments in peroxide bleaching have increased its efficiency for kraft pulps. Operating a peroxide stage at higher reaction temperatures (> 70[degrees]C) can increase peroxide consumption at a given charge, increase the overall brightness ceiling that can be obtained, and reduce the bleaching time when compared to conventional peroxide bleaching. Pressurization can enable the reactions to be run at even higher temperatures (>90[degrees]C). This type of enhanced (EOP) bleaching, designated as [P.sub.HT], is an attractive alternative to (PO) because it has a lower capital cost and it can be easily retrofitted into an existing bleaching sequence. Metals management is an important consideration for these aggressive peroxide stages to work effectively and selectively. The intent of this paper was to evaluate several organophosphonate-based bleach stabilizers as a metals management strategy for minimizing peroxide decomposition and maximizing peroxide brightening reactions during [P.sub.HT] bleaching of kraft pulps.

We conducted several [P.sub.HT] laboratory experiments on a semi-bleached kraft pulp to screen how effective diaminetriethylenepentaacetic acid (DTPA), magnesium sulfate (MgS[O.sub.4]), and the various bleach stabilizers performed against a DTPA pretreatment (i.e., Q stage) (Fig. 1). Brightness gains across the [P.sub.HT] stage varied from 4 to 6 points. DTPA and the aminocarboxylate-based stabilizers (Stabilizers A and B) generally yielded similar brightening responses; however, the organophosphonate-based stabilizers (Stabilizers C and D) provided ~ 1 point extra brightness. Somewhat similar trends were also observed with the peroxide residuals. Q stage pretreatment afforded a higher brightness and peroxide residual than adding DTPA, MgS[O.sub.4], or aminocarboxylate-based stabilizers directly into the [P.sub.HT] stage, whereas comparable results were obtained with organophosphonate-based stabilizer addition to the PHT stage.

[FIGURE 1 OMITTED]

This data indicated that organophosphonate-based stabilizers added directly to the [P.sub.HT] stage could work as well at selectively enhancing peroxide bleaching as pre-treating the stock with a Q stage. The semi-bleached pulp had a significant level (64 ppm) of iron (Fe) and a low level (2.5 ppm) of manganese (Mn). Due to this metals profile, the addition of a stabilizer directly into the [P.sub.HT] stage should work as well as a Q stage. The ability of organophosphonate-based stabilizers to further enhance [P.sub.HT] bleaching over the aminocarboxylate-based stabilizers is related to the capability of the chelant to complex with Fe. Organophosphonate chelants are known to form stronger complexes with Fe than aminocarboxylates, such as DTPA, at elevated pH.

Based on the above study, Stabilizer D was tested in the final [P.sub.HT] stage at the mill. Stabilizer A was already employed as a peroxide bleach stabilizer at the start-up of the [P.sub.HT] system several months earlier. The change from the aminocarboxylate-based Stabilizer A to the organophosphonate-based Stabilizer D resulted in ~ 1.3 point increase in final bleached brightness (i.e., from 86.3 to 87.6% ISO).

Additional experiments were performed during the trial to determine if the amount of peroxide used to reach a target brightness could be reduced. For a final target brightness of 86% ISO, the amount of peroxide used in the [P.sub.HT] stage could be reduced from 1.85% to 1.10% by employing Stabilizer D in place of Stabilizer A.

The results from this mill trial and laboratory study clearly indicated that an organophosphonate-based stabilizer can be employed either to improve the bleaching response of the final PHT stage or to reduce the chemical cost of the PHT stage to reach a given target brightness.

Brian N. Brogdon is senior research scientist at Vinings Industries, Inc., 1525 Church Street Ext. NE, Marietta, GA 30060, USA. Address correspondence to Brogdon by email at [e-mail].