Pulp and Paper 3135 



easily bleached filler in such items as food board, white linerboard, milk carton, 

 and cup stock. For most uses of bleached hardwood pulp the amount of hard- 

 wood fiber in the product is between 25 and 75 percent. 



Preferred species. — Schroeder's (1976) survey indicated that the oaks, ex- 

 cept for live oak {Quercus virginiana Mill.) avoided because of its high density, 

 were preferred hardwood species for kraft pulping; they accounted for 53 per- 

 cent of all hardwood pulped. Sweetgum and tupelo sp. were next in importance, 

 comprising 33 percent. All other species accounted for only 14 percent of 

 hardwood pulped. Some mills, in addition to reluctance to accept live oak, also 

 restricted use of hickory, dogwood {Cornus florida L.) and locust Gleditsia 

 sp.) — probably because of their high density. 



KRAFT PROCESS 



Pulping cycle and chemical recovery furnace. — The kraft pulping cycle 

 (fig. 25-23) requires wood, water, fuel (usually fossil fuel for the lime kiln plus 

 bark residue for steam and electricity generation), calcium carbonate (CaC03) in 

 the form of lime rock, and sodium sulfate (Na2S04) in the form of salt cake. 

 Manufacture of 1 ton of ovendry unbleached southern red oak kraft pulp at 54 

 percent yield might consume the following raw-materials. 



Raw material Quantity 



Southern red oak wood, pounds (ovendry) 3.700 



Water makeup, gallons (see text below) 9,000 - 13,000 



Electrical energy, horsepower-days (see text below) 5-10 



Process heat, million Btu (see text below) 9,000 - 13.000 



Lime rock makeup, pounds (see text below) 55 



Salt cake makeup, pounds (see text below) 125 



The water requirement for producing a ton of unbleached kraft pulp is difficult to 

 determine. A significant and highly variable amount of water which is not 

 considered as makeup water is the moisture in the green chips. Water makeup 

 value given is for all process water, including that for steam generation, brown 

 stock washing, dissolving smelt, lime mud washing, and employee needs. The 

 trend is toward efficient recovery and recycling of water; makeup water needed 

 will therefore continue to decrease. 



About 700 pounds of bark (ovendry) would accompany 3,700 pounds of 

 southern red oak (ovendry); in an energy efficient mill this bark, together with 

 the black liquor, can provide about 68 percent of the total electrical and heat 

 energy tabulated above. As mills continue to adopt more energy conservation 

 measures they will approach self sufficiency. If furnace and generating capacity 

 is adequate, the shortfall can be supplied by forest residues (except possibly for 

 the lime kiln). 



The lime rock and salt cake makeup quantities indicated are for a mill which is 

 capable of recycling 90 percent of both lime for the lime cycle, and chemicals for 

 pulping. Here also mills are becoming more efficient and the ability to recycle a 

 higher percentage is being realized. At high recycling rates of the pulping 

 chemicals there tends to be a reduction in sulfidity. To correct this situation it 



