The Kraft cooking process is a common chemical pulping method for wood and non-wood sources to produce cellulosic fibers. Essentially, the Kraft process involves the chipping of raw woodstock and cooking it in a digester with sodium hydroxide and sodium sulfide (collectively known as white liquor) at a specified temperature and pressure. The resulting reaction product is separated into cellulosic fibers (generally called pulp) and spent cooking chemicals, together with most of the lignin, the organic material that binds the fibers together. During the cooking reaction, lignin is dissolved and becomes part of the liquor, along with the spent cooking chemicals. The spent cooking chemicals and dissolved lignin are collectively known as black liquor.
Kraft cooking can generally be separated into two categories: cooking for bleached products and cooking for unbleached products. The difference in the two categories is the amount of cooking chemicals (white liquor) used, the temperature at which the cook is carried out and the amount of time the chips are exposed to the cooking liquor. Depending on the desired grade of pulp to be produced, the cooking process is operated to achieve pulp of a specific degree of delignification, typically measured as a Kappa number.
The Kappa number test is used to determine the amount of lignin remaining on pulp after cooking. The Kappa number is defined as the number of milliliters of 0.1N potassium permanganate solution consumed by one gram of pulp and corrected for 50% consumption of the potassium permanganate initially added (TAPPI Test Method T236 cm-85; CPPA Standard G.18). Table 1, below, gives typical Kappa number values, % lignin and yield for pulps produced for various paper products.
TABLE 1 Pulp produced Bleached Unbleached Unbleached for Paper paper board Kappa Number 20-35 35-120 40-120 % Lignin on 2.9-5.1 5.1-18 6-18 Pulp Total Yield 44-46% 46-50% 50-58% Screened Yield 41-44% 45-56% 48-56%
The degree of cook is also indicative of the amount of lignin that is dissolved in the cooking liquor. This can be measured by taking the cooking liquor from a given Kappa cook, acidifying to a low pH (&lt;3) and recovering and measuring the weight of the resulting precipitate.
The Kraft cooking process recycles the spent cooking chemicals through a process known as the recovery cycle. The spent cooking chemicals and dissolved lignin are removed from the pulp product via counter-current washing with water. The washed pulp is recovered as solids and the diluted, spent cooking chemicals and dissolved lignin are recovered as a liquid known as weak black liquor. The weak black liquor is evaporated to high suspended solids concentration and is incinerated in a recovery boiler where some of the heat from burning lignin is recovered as power and steam and the spent cooking chemicals are recovered as a smelt. The spent cooking chemicals are then further processed to convert Na.sub.2 CO.sub.3 to NaOH together with a small amount of Na.sub.2 S, collectively known as white liquor.
Raw materials represent a substantial cost of any pulp. Improvements in pulp yield can dramatically affect the economics of the process. Therefore, even small improvements in pulp yield can translate into substantial economic benefits and increased production.
High yields can be achieved by various pulping methods, one of which is mechanical pulping that works by simply grinding the raw material into pulp. The Kraft process, however, has a relatively low yield but produces pulp having high strength. Yield is defined as the amount of pulp, by weight, that is produced from a given amount of raw material, expressed as a percentage of the given amount of raw material. For example, a yield of 70% means that 70 g of pulp are produced from 100 g of raw material.
One reason for the high strength of Kraft pulp is that the cellulose fibers are relatively unharmed by the cooking process--as opposed to being ground into smaller pieces as is done in mechanical pulping. On the other hand, the low yield of the Kraft process results from lignin being extracted from the wood, effectively reducing yield to between 41% and 44%.
Pulps produced for unbleached products are generally higher yield pulps than bleached pulps because less of the lignin is dissolved in the cooking liquor and washed away in the subsequent chemical recovery step. The difference between Total Yield and Screened Yield is the undercooked wood removed in screening (an operation performed to remove undercooked fiber bundles from the pulp stream). Increasing cooking severity increases Screened Yield at the expense of Total Yield.
There are many methods for improving Kraft pulp yield. Generally, yield improvements are achieved by one or more of three methods: process modifications, pulping additives, and method changes.
(a) One method of improving pulp yield involves the addition of additives to the cooking liquor at the digester in an attempt to protect the cellulosic pulp fibers from degradation. Such additives include anthraquinone (AQ) and polysulfide. The yield improvement results because the additives protect the cellulosic fibers from degradation.
(b) Slight modifications to the process can also improve yield. The most common process modification, called "high Kappa pulping", evolved from environmental requirements and the proliferation of oxygen delignification. It involves modifying the cooking conditions, as measured by H-factor, such that the lignin content of the final pulp product is higher than normal. H-factor is determined by plotting the relative reaction rate against the reaction time in hours, and measuring the area under the curve. Parsad demonstrated high Kappa pulping by modifying the H-factor of several Kraft cooks; his results show that as Kappa number increases, yield also increases. (See: Parsad, Brijender; et al. "High Kappa Pulping and Extended Oxygen Delignification Decreases Recovery Cycle Load." Tappi Journal, Vol. 77, No. 11 (November 1994)). This method of yield improvement occurs in the digester area. Furthermore, lignin is not precipitated onto the fibers, as is done by the invention described below. Rather, lignin is never broken down and dissolved in the cooking liquor for removal in washing. In addition, this process is intended to be used with oxygen delignification, which subsequently removes the lignin at a later process step by oxidizing and dissolving the lignin.
This method has the additional disadvantage in producing less Total Yield. If cooking is not carried out to a sufficient extent, all of the chips may not be broken down into individual fibers, leaving some fibers bundled together, known as shives. Shives can adversely affect the final product's appearance and physical properties due to the relatively poor fiber-to-fiber bonds. Shives are removed and recycled to the digester in a cleaning step known as screening, effectively reducing digester capacity.
(c) Another method of improving the yield of a Kraft cook is known as "sorption cooking" and has been investigated by Nils Hartler of the Swedish Forest Products Research Laboratory. (See: Hartler, "Sorption Cooking: Yield Increase for Unbleached Alkaline Pulps Through Sorption of Organic Substance from the Black Liquor." Svensk Papperstidn (October 1978); U.S. Pat. No. 3,937,647. This method involves a lowering of the pH of the black liquor at the end of the cooking process to precipitate lignin onto the fibers. An acid, preferably CO.sub.2, is used to lower the pH of the liquor to 8.0 with the result that yield is improved by 1% to 2%. Hartler uses an acid, preferably H.sub.2 SO.sub.4, to lower the pH to below 11.0 and to as much as 5.6.
This method of yield improvement is similar to the invention to be described below only so far as it involves precipitation of lignin with an acid. The acid is used to reduce the pH of the cooking liquor at the end of a Kraft cook where lignin concentrations are high, whereas the process of the present invention uses an acid to lower the pulp pH of a dilute lignin containing stream during washing.
(d) Although not a method designed to increase yield, in U.S. Pat. No. 5,429,717, Bokstrom addresses the problem of increasing washing efficiency by use of CO.sub.2 to lower the pH of the wash water to increase chemical recovery efficiency and to maintain dissolution of lignin. In the Bokstrom process, the pH of the pulp is lowered to between 6.8 and 9.4 during the washing step, resulting in a desorption of bound sodium and a decrease of dissolved lignin and spent cooking chemical carry over to the bleach plant.
Bokstrom alludes to problems that result when pulp pH is lowered too far, but fails to note the important benefits that can be gained by doing so. In fact, Bokstrom avoids certain pH conditions because of undesirable reactions with residual lignin (col. 2, line 14). Bokstrom balances sodium desorption with lignin removal to wash pulp with more efficient use of chemicals.
In a paper by White discussing Bokstrom's technique, White notes that CO.sub.2 addition must occur at the end of the wash line to avoid lignin precipitation (p. 54). (See: White, "Carbon Dioxide on Pulp During Washing in the Minimum Impact Mill." Pulp Washing '96, Tappi (October 1996).
In the above-described prior art, the yield improvement solutions require significant changes to existing equipment, e.g., use of additives to protect the cellulose, pulp cooking to retain lignin rather than precipitate and, in sorption cooking, lowering the pH of the black liquor at the end of cooking to precipitate lignin.
It is therefore an object of the invention to improve the pulp yield of unbleached pulp emerging from a Kraft cooking process.
It is a further object of the invention to provide an economic means for increasing pulp yield in unbleached pulp mills, without requiring substantial modifications to mill equipment.