Concentration

Like the three evaporators, the two concentrators are vertical‐tube, falling‐film design. Rather than using a vapor compressor to drive the system, the concentrator is operated with steam generated by the recovery boiler. The evaporation process in the concentrators is essentially the same as in the evaporators, but the effluent is concentrated further, to about 67% solids. The concentrated effluent is incinerated in the recovery boiler. The lead concentrator takes the liquor from 35 to 50%, while the lag concentrator goes from 50 to 67% solids.

Stripping

The foul condensate, only about 10% of the total condensate, is stripped of volatile organic compounds in a packed column stripper. The VOCs are selectively concentrated in the foul condensate because of the condensate segregation features built into the evaporator heating elements. Process steam from the concentrator is sent to a reboiler, which generates stripping steam from a portion of the stripped condensate. The stripped condensate is combined with the clean condensate and reused in the mill. The concentrated VOCs are incinerated in the recovery boiler as a concentrated vapor.

Incineration

At the recovery boiler, the organic components of the effluent are incinerated, a process that also generates steam to operate the concentrators. Inorganic chemicals in the effluent are recovered in the smelt from the boiler, which is cast into ingots and stored on site. The mill is considering recovering the sodium carbonate, which would then be converted to sodium hydroxide, a major chemical used in the BCTMP process.