Partial autocausticizing of kraft smelt with sodium borates--Part I: effects on recovery boiler performance.

Application: Borate can partially causticize sodium carbonate in a recovery boiler and reduce lime requirement in the slaker and the resulting lime mud flow in the causticizing and lime kiln.

This paper reports a full-scale mill trial on partial borate-autocausticizing up to 30% conducted at Caribou Pulp and Paper Company, Quesnel, B.C., for more than six months. Borate introduction to the liquor cycle used addition of sodium tetraborate pentahydrate to the black liquor mixing tank initially and then to the green liquor surge tank to obtain a maximum B/Na weight ratio of 0.03. The objective was to examine if borate can partially causticize sodium carbonate in the recovery boiler and therefore reduce lime requirement in the slaker and the resulting lime mud flow in the causticizing plant and lime kiln. Results showed that the lime consumption during the trial was lower and that borate had no adverse effect on pulp quality, effluent quality, process equipment, and operations. The only major problem encountered was difficulty in obtaining accurate liquor data with the conventional ABC titration method because of the interference of borate with titration endpoints. The recovery boiler had no difficulty in burning the borate-containing black liquor. Boiler inspection at the end of the trial showed no indication of corrosion and tube cracking that could relate to the borate trial. The two electrostatic precipitators were much cleaner than usual. This is presumably due to the suppression of fume generation by borate.

Trial results

Figure 1 shows the B/Na weight ratio of precipitator dust and that of black liquor, smelt, and carry-over deposits collected using an air-cooled probe inserted in the superheater at the bullnose elevation. The B/Na ratios of superheater deposits, smelt, and black liquor were the same at approximately 0.022. The B/Na ratio of the dust was much lower at approximately 0.005. This suggests that borate did not enrich in the dust. Figure 2 clearly shows the non-enrichment of borate. The figure compares the enrichment factors of chloride (Cl), potassium (K), and borate (B) in the precipitator dust.

[FIGURE 2 OMITTED]

Conclusions

A full-scale mill trial on partial borate-autocausticizing using up to 30% was successful at Caribou Pulp for more than six months. The results showed the following:

* The lime consumption was lower as anticipated.

* The recovery boiler had no difficulty burning the borate-containing black liquor. The trial seemed to generate less fume and dust. The dust contained little borate.

* No indication of corrosion and tube cracking occurred in the recovery boiler that could relate to borate.

* No adverse effects on pulp quality, effluent quality, process equipment, and operations occurred. The only major problem was the difficulty obtaining accurate liquor data due to the interference of borate with titration endpoints of the ABC titration method.

* The borate loss was about 2.5% of the total input.

1. B/Na weight ratios

Dust                n = 28
Carryover deposit   n = 5
Smelt               n = 2
Black liquor        n = 7

Note: Table made from bar graph.

Tran is at Pulp & Paper Center, University of Toronto, Toronto, ON, Canada. Blair and McBroom are at US Borax Inc., Valencia, CA, USA. Strang and Morgan are at Caribou Pulp and Paper Co., Quesnel, BC, Canada. Address correspondence to Tran at [e-mail].