KRAFT PULPING

Kraft pulping is based on the soda process, which employs the use of sodium hydroxide (NaOH). Originally, the soda process couldn't compete with the sulfite process due to the cost associated with NaOH and the need for chemical recovery. Around 1879, a fellow in Germany was looking for alternatives to reduce costs associated with the soda process. The makeup chemical used in the soda process was sodium carbonate (Na2CO3), and he replaced it with the cheapter sodium sulfate (Na2SO4). Upon heating sodium sulfate, one forms sodium sulfide (Na2S). Mixing sodium sulfide with water provides a mixture of sodium hydroxide (NaOH) and sodium hydrosulfide (Na2S). With this innovation, the kraft process was born. The first mill was in Sweden in 1890 and the first mill in the US was up and running in Roanoke Rapids, NC (Champion) in 1909.

The development of the kraft process allowed access to the So. pines which typically didn't fare well in the sulfite process, due to their high resin content. Kraft pulps initially had two distinct disadvantages: a dark pulp and chemical recovery was required. However, the pulp is stronger and has better dimensional stability. Chemical recovery systems rapidly became quite advanced and most advanced mills are now shooting for 98-99% chemical recovery. The final nail in the coffin of the sulfite process was the development of chlorine dioxide bleaching, which we will get to shortly.

General Aspects of the Process. The heart of the kraft pulping operation is actually chemical recovery, or liquor recycling. Sodium hydroxide and sodium hydrosulfide (see bottom equation above) are the reagents that delignify. Typically solution pH >12 and cooking times range from 0.5-3 hours at temperatures between 160-180 C. There are a number of variables used to control the process, such as total alkali, active alkali, sulfidity, and H-factor. We will just concern ourselves with the H-factor, which describes the rate of delignification. Pulp mills shoot for a certain H-factor, and this will provide a pulp of a specific lignin content. Lignin content is estimated by determination of the Kappa number, which is a measure of the pulps ability to reduce potassium permanganate (KMnO4) to MnO2. The remaining lignin is oxidized in the permanganate reaction, the higher the lignin content, the higher the kappa number. As a rough estimate Lignin Content = (0.15)(Kappa number).

Kraft Digesters. Lignin removal can occur in batch digesters or continuous digesters. As most modern mills have installed batch processes, we will concentrate on this methodology. There are four major stages in a kraft continuous pulping process:

1. Steaming stage. The objective here is to remove air and heat the chips. This will improve subsequent chip impregnation.
2. Chip impregnation. Usually occurs between 115 and 120 C.
3. Delignification stage. Usually at 170 C. The time of delignification depends on how much lignin is to be removed.
4. Cooling/Washing stage. Separate the soluble lignin from the pulp. The lignin soluble fraction is termed black liquor and the pulp is brownstock. Must be careful to avoid precipitation of the lignin back onto the fibers.

Delignification Chemistry. The removal of both lignin and polysaccharides occurs in the process. Brownstock to be bleached will typically have lignin contents on the order of 3% for hardwods and 4-5% for softwoods. About 50% of the hemicelluloses have been removed as well as 10% of the cellulose. As you may recall, polysaccharides are lost due to peeling. During the course of delignification, the reaction pH drops due to mostly acetate cleavage and peeling byproducts. All of the pulping reagents as well as the byproducts and degraded lignin are in the black liquor which is sent to chemical recovery for processing. There are 3 distinct phases of delignification:

1. Lignin Extraction. This is the fast phase of delignification and involves solubilization of wood extractives and low molecular weight lignin. The extractives are typically collected and separated into volatile (turpentine) and tall oil fractions.
2. Bulk Delignification. During bulk delignification, the majority of lignin is removed. This is a first order reaction.
3. Residual Delignification. Removal of the last vestiges of lignin is difficult and this phase is slower than the bulk phase. Care must be taken when entering into this phase as the soluble lignin can also undergo condensation reactions as described below.

Major side reactions that can occur during kraft delignification are lignin condensation, lignin precipitation onto pulp fibers, formation of smelly mercaptans and disulfide, and formation of methanol and formaldehyde. Lignin condensation will increase lignin molecular weight, and if left unchecked, this can lead to lignin precipitation onto the pulp fibers. This precipitation can also occur if the pH in the digester drops too much. Lignin solubility is higher at high pH, and too large of a decrease in pH can lead to lignin precipitation.

Recent Modifications

Comparison of Kraft and Sulfite Pulps. As far as strength properties are concerned kraft and sulfite have equivalent tensile strengths but kraft has a much higher tear. As mentioned previously, kraft pulps require more bleaching. Sulfite pulps are much more flexible and have better swelling characteristics, and it is generally said that sulfite pulps are "softer." Kraft pulps usually have to be refined somewhat to soften them. Kraft pulps have better ageing resistance, partially due to the acid involved with the sulfite process, but this resistance is also due to the use of specific chemicals during the sheet forming process.

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