3 1 36 Chapter 25 



may be necessary to add elemental sulfur as a makeup chemical. The cost of lime 

 rock and salt cake makeup, tabulated above, is approximately $3.50 (1981) per 

 ton of pulp. 



The chemical recovery furnace is the heart of a kraft pulp mill. Mayer' ^ 

 summarized its function as follows: 



"The furnace is fired with spent cooking Uquor (black liquor) after concentrating it to 65- 

 percent soUds. Some combustion takes place immediately in the furnace, but liquor 

 droplet size is regulated to assure its falling on the hearth, where it forms a pile of char. 

 Primary air is introduced under this bed sufficient only to bum the carbon to monoxide. 

 The CO then reduces the salt cake in the char to Na2S. The char bed is at 1 ,700°F, enough 

 to melt all the salts and cause them to puddle on the bottom on the hearth. This puddle [of 

 green smelt] is allowed to overflow to the outside into a dissolving tank. The non-sulfur 

 salts are all converted to sodium carbonate by the furnace. 



"The CO above the hearth is oxidized to CO2 by secondary air. Its heat content is then 

 extracted by superheaters and the water-cooled walls. Maximum steam temperatures and 

 pressures are 900°F and 1500 psi. Ruptured water wall tubes often cause serious damage if 

 sufficient water is released to react with the hot hearth materials. The superheater tubes 

 must be cleaned on a regular schedule to remove fume deposits that may slag. Heavy 

 entrained fume falls out at the rear of the furnace and is returned to the incoming liquor. 

 Light fume is recovered by electrostatic precipitators with guaranteed 99.5 percent effi- 

 ciency. This fume is also returned to the feed liquor along with make-up chemical. To 

 minimize the generation of obnoxious mercaptans in the furnace, the black liquor is 

 oxidized with air or oxygen to convert residual sulfides to thiosulfide or sulfites" 



Green liquor from the dissolver, primarily NaC03 and Na2S, is added to 

 calcium hydroxide (formed by water-slaking calcium oxide from the lime kiln) 

 yielding white liquor comprised chiefly of NaOH, Na2S, and Na2C03. This 

 white liquor is used to digest the wood chips, yielding pulp fibers and the spent 

 black liquor which goes to the chemical recovery furnace. 



Process references, southern pine. — Texts listed at the beginning of this 

 chapter contain descriptions of the kraft pulping process. Readers needing a 

 succinct review of southern pine kraft pulping will find useful Kleppe's (1970) 

 work summarized in Koch (1972, p. 1431-1446). Kleppe (1978) supplemented 

 this description with a review of high-yield kraft pulping published in the 

 proceedings of the symposium "Complete-tree utilization of southern pine." For 

 process details, readers are referred to these reviews and the literature cited 

 therein. 



Process references, hardwoods. — Forman and Niemeyer (1946) made the 

 first comprehensive review of kraft pulping southern hardwoods. Another study 

 published nearly 20 years later, included data on bleaching kraft pulps from 

 southern hardwoods (MacLaurin and Peckham 1955). Bibliographies of hard- 

 wood kraft pulping were assembled by Weiner and Roth (1967) and Weiner and 

 Pollock ( 1 972). Other papers on kraft pulping of southern hardwoods include the 

 following: 



'^Mayer, B. K. 1979. Pulping — A state-of-the-art review. Paper presented at 1979 Fall Meeting, 

 TAPPI/API Coll. Relations Comm. Conf., Houston. 12 p. 



