After upgrading its bleach plant in 1987 to accommodate a two-stage elemental chlorine-free (ECF) bleach sequence, Cross Pointe Paper (now Flambeau River Papers LLC), Flambeau mill in Park Falls, Wis. subsequently added a totally chlorinefree (TCF) sequence in response to market demand. The TCF sequence, however, required heating, rather than cooling, of the mill's mediumconsistency stock during transfer from the first-stage O2 washer to the second-stage retention tower. After careful study, Flambeau installed the more costeffective heating system.
This new inline direct steam injection system--reported to be the first of its kind--has reduced heating costs at the mill by two-thirds. The mill had originally heated the stock with O2 washwater, which is a convenient but costly method.
With a capacity range from 170 to 210 tons/day, Flambeau pulps 100% white birch timber. The sulfite mill, which produces uncoated free-sheet printing/ writing papers, operates three batch digesters. The mill runs its TCF bleaching sequence, depending on customer specification, about four times/month, and runs the ECF sequence the remainder of the time. Both sequences produce bleached pulp with a brightness level of 87.
In Flambeau's TCF sequence, pulp passes through the first O2 delignification stage to the O2 washer, with 50% delignification. In the past, temperature adjustment to the stock, which is at 10% consistency, was made prior to leaving the washer. Hot water showers on the washer were used to raise stock temperature to 190deg F.
From there, the heated stock is transferred by the mill's medium consistency pump to a second-stage, upflow retention tower, where it undergoes a 90-min residence time. At this point, it was becoming clear to the mill that heating the stock using steam would be less expensive than heating it with the washwater.