Sodium sulfate kraft wood pulping

Kraft Process. The dominant chemical wood pulping process is the kraft or sulfate process. The alkaline pulping Hquor or digesting solution contains about a three-to-one ratio of sodium hydroxide, NaOH, and sodium sulfide, Na2S. The name kraft, which means strength in German,... 

The principle uses of Na2S04 are in the manufacture of paper, soaps, and detergents. These accounted for 65% of U.S. consumption from 1990 to 1995, representing a significant shift from 1980 when paper production alone consumed 67%. Pulp (qv) and paper consume only 25% (2). The kraft paper process uses a mixture of sodium sulfide and sodium hydroxide to digest wood chips. Both the sulfide and hydroxide are generated, starting with sodium sulfate as the raw material. 

Manufacture. The oldest method for producing Na2S is by the reduction of sodium sulfate with carbon in a refractory oven at 900—1000°C. Whereas this method is no longer used commercially in the United States, a variation is used to produce sodium sulfide captively during kraft pulp processing to replace lost sodium and sulfur values that were initiated into the system by merchant-suppHed sodium sulfide. In this method, sodium sulfate is added to the system in the recovery furnace, where it is reduced by carbon from the wood pulp to produce sodium sulfide. 

The Important chemistry in the kraft method is divided into inorganic and organic parts. Figure 22.2 summarizes the inorganic chemistry. The inorganic loop is a closed system with the exception of sodium sulfate being added periodically. Only wood enters the loop and only pulp leaves. 

Sodium Lignosulfonate occurs as a brown, amorphous polymer. It is obtained from the spent sulfite and sulfate pulping liquor of wood or from the sulfate (kraft) pulping process. It may contain up to 30% reducing sugars. It is soluble in water, but not in any of the common organic solvents. The pH of a 1 100 aqueous solution is approximately between 3 and 11. 

Fourdrinier machine—The machine that forms paper from pulp, named after the English family that financed its development in the early 1 800s. Furnish—Specific combination of pulp and other ingredients used to make a particular kind of paper. Kraft process—process in which sodium sulfate is reduced by heating with carbonaceous matter in a furnace to form sodium sulfide, which is then used in a water solution with sodium hydroxide as a cooking liquor. The wood pulp is then cooked under pressure and at high temperatures. The kraft process, also known as the sulfate process, has a less corrosive influence on iron and steel than the sulfite process. 

Commercially, most cellulose is extracted from wood by one of two methods, the kraft (sulfate) process or the steam explosion process. The product of these reactions is wood pulp, which consists primarily of cellulose. In the kraft process, wood chips are treated with a solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S) at temperatures of about 175°C (35o°F) for two to six hours. This process usually results in a yield of about 40 to 45 percent wood pulp. The pulp is then treated with a bleaching agent, such as calcium or sodium hypochlorite (Ca(0Cl)2 or NaCIO) or chlorine dioxide (C102) to remove the color of lignin and other impurities. 

KRAFT PROCESS - A wood-pulping process in which sodium sulfate is used in the caustic soda pulp-digestion liquor. Also called Kraft pulping or sulfate pulping. 

In wood processing, in addition to mechanical methods, sulfite and sulfate processes are used. In the sulfite process, used for around 10% of wood pulping, gaseous SO2 reacts with calcium carbonate to produce calcium hydrogen sulfite. Wood chips are boiled in this solution at a pH-value of 3 to 5 at 150 to 170 °C. This causes the lignin and hemicellulose to dissolve, while the cellulose remains as undissolved pulp. In the sulfate process (Kraft process), by which some 75% of wood is processed, the wood is treated with sodium hydroxide/sodium sulfate to separate cellulose from lignin and hemicellulose. 

A primary objective of wood pulping is to separate lignin from cellulose. Two chemical processes are commonly used to achieve this, namely the sulfite and the sulfate methods (Kraft-process). In the sulfite process, the lignin in wood is sulfo-nated with aqueous solutions of sulfites or hydrogen sulfites, with added SO2, at high temperatures, so that soluble lignin sulfonic acids are produced, which can be dissolved out of the wood. The sulfate method uses sodium hydroxide, sodium sulfide and sodium carbonate as basic pulping chemicals. It derived its name from the small amounts of sodium sulfate which are added to balance out alkali losses. 

For chemical delignification and defibration of wood several basic chemicals are used, depending on the process (see Volume 1). In kraft pulping, which is the dominant alkaline process, the main active compounds are sodium hydroxide (NaOH) and sodium sulfide (Na2S). In addition, soda (Na2COs), sodium sulfate... 

Sulfate Pulp n Paper pulp made from wood chips cooked under pressure in a solution of caustic soda and sodium sulfide. Known as Kraft. 

When coniferous wood is pulped in the strongly alkaline Kraft pulping process, also called the sulfate pulping process , the predominant chemical pulping process today, the resin and fatty acids are converted into sodium soaps, and are after cooking recovered as so-called sulfate soap from the surface of the pulping liquor, the black liquor. The sulfate soap is a lamellar crystalline phase where the resin and fatty acids form the lamellae, and neutral components, such as sterols, are included between the lamellae [7],... 

---