Publication: Pulp & Paper Magazine
Issue: December 1, 1994


Low Kappa Cooking, TCF Bleaching Affect Pulp Yield, Fiber Strength


Current low kappa pulping and bleaching techniques to produce TCF pulps indicate a decline in yield compared with ECF methods

RECENT REPORTS FROM SWEDEN and Finland1-3 have described pulping techniques used prior to TCF bleaching. Modified cooking procedures are used to obtain softwood kraft pulps with low kappa numbers, and reports claim good strength retention even down to a kappa number of 10 in some cases.1,2,4

Softwood pulp at kappa number 15 is produced routinely at the new Enocell mill in Finland.2 This low kappa number enables TCF bleaching sequences to reach high brightness levels.2

However, relatively few authors have addressed the subject of pulp yield from low kappa pulping and TCF bleaching. This is surprising because changes in pulp yield have both economic and environmental consequences. Evidence is beginning to suggest that low kappa number pulping and TCF bleaching produce lower yields of bleached pulp compared with ECF bleaching of pulps with kappa numbers of more than 20. Yield loss is of particular concern in the northwestern U.S. because of the anticipated shortage of woodchips in the region.


Table 1tab 1 shows data from a publication by Nordoeen and coworkers that illustrate the problem of low kappa pulping. The yield of bleached pine pulp decreases rapidly with kappa number so that 5 to 10% more wood is needed to produce bleached pulp via this method.

In a study of SuperBatch pulping of Scandinavian pine, Kovasin and Tikka1 recorded higher yields than Nordoeen et al., but noted a similar decline in bleached yields (from 45.5% to 43.7%) as the pulp was cooked down from 30.8 kappa number to 14 kappa number.

It is reported that when modified cooking is installed, any yield loss due to low kappa pulping is counterbalanced by lower rejects and better homogeneity.7

However, this advantage is independent of low kappa pulping because the yield gains obtained from decreased rejects can be obtained under conventional cooking conditions by chip-thickness screening and improvements to impregnation and liquor circulation.

Moreover, the yield-enhancing feature of displacement cooking is also available at 30 kappa, where the pulp yield usually exceeds that of conventional kraft cooks. For example, at 30 kappa number SuperBatch has an unbleached yield advantage of about 1.5% over conventional kraft cooking.6

However, when pulping is extended below 20 kappa number, this advantage is lost and the bleached yield from SuperBatch pulp at 20 kappa number is less than that obtained from 30 kappa number conventional kraft pulp.1

So within one type of cooking technology, it seems possible to conclude from the literature that pulping to kappa numbers lower than 20 always produces a measurable loss of bleached pulp yield.


Compared with conventional kraft cooking, modified continuous cooking (MCC) tends to consume about 1% more wood/ton of pulp at a given kappa number.10 This is because the countercurrent flow of the liquor strips hemicellulose from the nearly-cooked pulp.9 The new techniques of Isothermal Cooking and improved modified cooking may eliminate this yield disadvantage.10,11

As with batch cooking, the bleached yield from continuous cooking decreases if the kappa number is forced down. Figures quoted by Haas8 indicate a further 2.2% increase in wood consumption of bleached pulp when MCC pulping was extended from 30 kappa number to 20 kappa number.

Thus, continuous cooking to low kappas in order to use TCF bleaching would increase the pressure on wood resources.


Besides the yield losses caused by low kappa pulping, the literature indicates that TCF bleaching produces more yield loss than ECF bleaching. Foelkel, working with eucalyptus, reported a bleaching yield of 97.5% for a D-EO-D sequence and a 96.5% yield for the sequence A-ZQ-EO p.12

Malinen reported that for TCF sequences with ozone, "the bleaching yield tends to be 0.5 to 1% lower than in ECF bleaching."13

Dahlman et al., reported that bleach filtrates from TCF bleaching (O-Q-P) had higher carbohydrate contents than ECF filtrates.14


Several papers on Scandinavian softwood TCF pulps report a tear-strength deficit of 10 to 12% at constant tensile strength.13,15,16

Others showed either a smaller strength loss13,18

or no loss at all17 compared with ECF pulps. TCF bleaching of southern pine seems to be hard to achieve without strength loss.19

No TCF pulps have been reported to be stronger than ECF pulps made from the same brownstock.

Pulp strength also deteriorates as kappa number is decreased in cooking. Strength loss begins below 22 kappa number in the conventional cooking of softwood,17,20

while in Isothermal Cooking, the unbleached pulp reaches its maximum strength at about 18 kappa number, and 18 to 20 kappa number is considered the optimum operating region.17


If environmental considerations force digester kappa numbers and bleaching technology into regions where strength loss occurs, then a higher percentage of the low-yield TCF pulp must be added to the paper furnish to maintain the strength of the sheet. This decreases the amount of mineral filler, high-yield pulp, or secondary fiber that can be included and consequently further increases the amount of fresh wood that must be used to produce one ton of paper.


Low kappa pulping is a necessary step in the manufacture of TCF pulp. But there is growing evidence to suggest that pulping to kappa number 14 followed by TCF bleaching will increase wood consumption by 9 to 11% compared with pulping to 30 kappa number with the same digester configuration, followed by ECF bleaching. The lower yield also increases the load on the chemical recovery system in the pulp mill, and in some cases, low kappa pulping may jeopardize pulp strength.

ECF mills pulping to 25 to 30 kappa number and using biological treatment already have a high-quality effluent21

that is not likely to improve significantly from a change to low kappa pulping (i.e., below 20) and TCF bleaching. However, yield losses under these conditions will drive up the wood consumption/ton of pulp. Strength losses may increase the weight of kraft pulp needed to produce a ton of paper. Both factors increase the pressure on the wood supply. Consequently, North American mills should be cautious about adopting low kappa pulping and TCF bleaching.û


1. K.K. Kovasin and P.O. Tikka, Paperi Ja Puu, Vol. 75, No. 7, 1993, p. 491.

2. J. Pearson, Pulp & Paper International, June 1993, p. 24.

3. S. Nordoeen, et. al., reprints of the 1992 TAPPI Pulping Conference, p. 159.

4. R.G. Elliott and C.A. Walley, Proceedings of the International Pulp Bleaching Conference, Stockholm, Sweden, June 1991, Book 3, p. 253.

5. N.-G. Johansson, Proceedings of the 1994 International Non-Chlorine Bleaching Conference, Amelia Island, Fla.

6. P.O. Tikka and K.K. Kovasin, Paperi Ja Puu, Vol. 72, No. 8, 1990, p. 774.

7. S. Moldenius, Personal Communication, Jan. 1994.

8. M. Haas, Pacific Section of TAPPI, 43rd Annual Seminar, Seattle, Wash., Sept. 13-16, 1990.

9. P.O. Tikka, Pacific Section of TAPPI, 43rd Annual Seminar, Seattle, Wash., Sept. 13-16, 1990, p. 93.

10. B. Mao and N. Hartler, Nordic Pulp Paper Research Journal, No. 4, 1992, p. 173.

11. B. Dillner, American Papermaker, Feb. 1993, p. 35.

12. C. Foelkel, Proceedings of the 1994 International Non-Cchlorine Bleaching Conference, Mar. 1994, Paper 7.2, Question 21.

13. R. Malinen, T. Rantanen, R. Rautonen, and L. Toikkanen, Proceedings of the 1994 International Pulp Bleaching Conference, Vancouver, B.C., June 1994, p. 187.

14. O. Dahlman, A. Reimann, L. Stromberg, and R. Morck, Proceedings of the 1994 International Pulp Bleaching Conference, Vancouver, B.C., June 1994, p. 123.

15. P. Tibbling and B. Dillner, Proceedings of the 25th EUCEPA Conference, Vienna, Austria, 1993.

16. B. Dillner and W. Peter, Paperi ja Puu, No. 9, 1992, p. 724.

17. P. Tibbling and B. Dillner, Proceedings of the 1993 International Non-Chlorine Bleaching Conference, Hilton Head, S.C.

18. L. Ahlenius, C.-J. Alftan, L. Uhlin, and E. Wikberg, Proceedings of the 1994 International Pulp Bleaching Conference, Vancouver, B.C., June 1994, p. 195.

19. A. Rooks, PIMA Magazine, Nov. 1993, p. 39.

20. J.M. MacLeod, Paperi ja Puu, Vol. 73, No. 8, 1991, p. 773.

21. K. Solomon, H. Bergman, R. Huggett, D. MacKay, and B. McKague, Proceedings of the 1994 International Pulp Bleaching Conference, Vancouver, B.C., June 1994, p. 145.

BRUCE FLEMING and TOD SLOAN are senior research advisor and pulp lab research specialist, respectively, Boise Cascade Corp., Portland, Ore.



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