| Kraft paper is paper produced by what is called the | | | | Woodchips are fed into vessels called digesters that |
| kraft process from wood pulp. It is very strong and | | | | are capable of withstanding high pressures. Some |
| relatively coarse. Kraft paper is usually brown in | | | | digesters operate in a batch manner and some in a |
| colour. It is used for paper grocery bags, wrapping, | | | | continuous process, such as the Kamyr digester. |
| envelopes and other packaging. Kraft paper is widely | | | | Digesters producing 1,000 tonnes of pulp per day and |
| available in both sheet and roll form but rolls are most | | | | more are common with the largest producing more |
| commonly used. | | | | than 3,500 tonnes of pulp per day. Wood chips are |
| Kraft paper is manufactured using a technique called | | | | impregnated with the cooking liquors. The cooking |
| the kraft process, kraft pulping or sulfate process | | | | liquors consist of warm black liquor and white liquor. |
| and describes the technology for converting wood | | | | The warm black liquor is the spent cooking liquor |
| into wood pulp consisting of almost pure cellulose | | | | coming from the blowing. White liquor is a mixture of |
| fibers. The process involves treatment of wood chips | | | | sodium hydroxide and sodium sulfide, produced in the |
| with a mixture of sodium hydroxide and sodium | | | | recovery process. In a continuous digester the |
| sulfide that break the bonds that link lignin to the | | | | materials are fed at a rate which allows the pulping |
| cellulose. The process name is derived from German | | | | reaction to be complete by the time the materials |
| word kraft which means strength or power. | | | | exit the reactor. Typically delignification requires |
| The kraft process was developed by Carl Dahl in | | | | several hours at 130 to 180 °C. Under these |
| 1879 and a pulp paper mill using this technology | | | | conditions lignin and some hemicellulose degrade to |
| started in Sweden in 1890. The invention of the | | | | give fragments that are soluble in the strongly basic |
| recovery boiler by G.H. Tomlinson in the early 1930s, | | | | liquid. The solid pulp (about 50% by weight based on |
| was a milestone in the advancement of the kraft | | | | the dry wood chips) is collected and washed. At this |
| process. It enabled the recovery and reuse of the | | | | point the pulp is quite brown and is known as brown |
| inorganic pulping chemicals such that a kraft mill is | | | | stock. The combined liquids, known as black liquor, |
| almost closed-cycle with respect to inorganic | | | | contain lignin fragments, carbohydrates from the |
| chemicals, apart from those used in the bleaching | | | | breakdown of hemicellulose, sodium carbonate, |
| process. For this reason, in the 1940s, the kraft | | | | sodium sulfate and other inorganic salts. |
| process surpassed the sulfite process as the | | | | Kraft paper is nowadays not only used in roll and |
| dominant method for producing wood pulp. | | | | sheet form but also in paper bags. |