Pulp mills kraft black liquor

A number of studies have evaluated the toxicity of pulping hquors, in particular the black liquors generated from Kraft mills. Table 8 shows a partial representation of toxicity data compiled by the NCASI (National Council of the Paper Industry for Air and Stream Improvement) and McKee and Wolf for Kraft mill pulping wastewaters [15,16]. The table indicates that hydrogen sulfide, methyl mercaptan, crude sulfate soap, and salts of fatty and resin acids are particularly... 

The process is conducted at the high pH in the presence of sodium sulfide, sodium hydrosulfide, and sodium hydroxide at 150-180°C for about 2 hours (Fig. 33.26). Along with the primary cellulose product, the process generates a solution containing dissolved lignin, most of the wood s hemicellu-loses, and spent pulping chemicals ( black liquor ). This solution is used in the kraft recovery boiler as a source of power for the mill and fuel for chemical recovery operations crucial to economic operation. The chemistry of pulping reactions has been the subject of much study, and summaries are available the literature.425 127... 

Kraft pulp mills Digesters batch and continuous Multiple-effect evaporators Recovery furnace Weak and strong black-liquor oxidation Smelt tanks Lime kiln Mercaptans, methanol (odors) H2S, other odors H2S, mercaptans, organic sulfides, and disulfides h2s Particulates (mist or dust) Particulates (dust), H2S Condensers and use of lime kiln, boiler, or furnaces as afterburners Caustic scrubbing and thermal oxidation of noncondensables Proper combustion controls for fluctuating load and unrestricted primary and secondary air flow to furnace and dry-bottom electrostatic precipitator noncontact evaporator Packed tower and cyclone Demisters, venturi, packed tower, or impingement-type scrubbers Venturi scrubbers... 

Black liquor from kraft or soda mills pulping hardwoods may present problems during its evaporation and combustion by conventional methods. These problems have been encountered with some eucalypt species, particularly with wood from trees of old age, and special measures may be needed to overcome the problems caused by extractives. Apart from the formation of deposits and scale, which reduces heat transfer in the evaporators, the extractives increase the viscosity (so that the evaporation process is affected) and reduce the swelling properties when incinerated, resulting in incomplete combustion (130). These difficulties with black liquor from the soda process can be avoided with the use of a wet oxidation process conducted under high pressure (97). 

In spite of these efforts to optimize the combustion of black liquor together with many improvements of the energy efficiency of pulp and paper mills, there have also been activities to improve significantly the efficiency of the energy (i.e., the evident production of electric power) and chemical recovery cycle in kraft pulping by one... 

In the kraft mill the wood is debarked and chipped, and the chips are pre-heated in a steaming vessel to remove air and assure good liquor permeation. The steamed chips are loaded into the top of the digester along with white liquor, which consists of sodium hydroxide plus sodium sulfide in water. After heating the chips to 170°C for 1 to 2 hours, the pulp is discharged into the blow tank. The pulp is washed free of black liquor and cleaned, then unbleached pulp is either sent to storage or to the bleach plant. 

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