Semichemical

Semichemical or chemimechanical pulping procedures involve a mild chemical treatment aimed at weakening the lignin-carbohydrate complex of the fiber bond. The mechanical stage achieves the fiber separation to produce the pulp, which is followed by refining to develop improved properties. The yields of pulp obtained are between 65% and 90% depending on the extent of chemical treatment and on the wood species. 

Digestion of wood chips under pressure and temperatures around 150 C with various chemicals reduces the amount of mechanical energy needed to fiberize the chips. The chemicals also remove varying amounts of lignins and hemicelluloses so that subsequent processes can improve strength, as well as color and brightness. 

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CODESSA (calculation of a series of difFerent descriptors including quantum chemical descriptors) Semichem Inc. - 7204 Mullen, Shawnee, KS 66216, USA http //www.semichem.com... 

Price category production, departmental, institutional Platforms PC (Windows Linux), SGI, RS6000, Alpha, Sun, HP-UX Contact information Semichem P.O. Box 1649 Shawnee Mission, KS 66222... 

GaussView (we used Version 2.08) is a graphic interface for use with the Gaussian ah initio program. It can be used to build molecules, set up the options in the input hie, run a calculation, and display results. GaussView uses the molecule builder that was written by SemiChem, but has screens for setting up calculations that are different from those in the AMPAC GUI sold by SemiChem. 

Semibulk containers Semicarb azones Semichemical pulps Semichemicals Semicoke... 

A high yield chemical pulp, eg, 52—53% bleached yield from softwoods, can be obtained, but strength properties ate inferior to those obtained from the kraft process. If a protector, eg, potassium iodide, is added, an additional 2—3% yield is obtained, as is an improvement in all strength properties. The gas penetration problem can be minimized if ftbetization is accompHshed before treatment with oxygen. Oxygen treatment of virtually all types of semichemical and mechanical pulps has been explored (55). Caustic, sodium bicarbonate, and sodium carbonate have been used as the source of base (56,57). In all cases, the replacement of the kraft by these other processes has not been justified over the alternative of pollution abatement procedures. 

Chemical recovery ia sodium-based sulfite pulpiag is more complicated, and a large number of processes have been proposed. The most common process iavolves liquor iaciaeration under reduciag conditions to give a smelt, which is dissolved to produce a kraft-type green liquor. Sulfide is stripped from the liquor as H2S after the pH is lowered by CO2. The H2S is oxidized to sulfur ia a separate stream by reaction with SO2, and the sulfur is subsequendy burned to reform SO2. Alternatively, ia a pyrolysis process such as SCA-Bidemd, the H2S gas is burned direcdy to SO2. A rather novel approach is the Sonoco process, ia which alumina is added to the spent liquors which are then burned ia a kiln to form sodium aluminate. In anther method, used particulady ia neutral sulfite semichemical processes, fluidized-bed combustion is employed to give a mixture of sodium carbonate and sodium sulfate, which can be sold to kraft mills as makeup chemical. 

NSSC Pulping. The characteristics of semichemical pulps are especially suited to the production of cormgated medium, which is the raw material for the fluted center ply of cormgated boxes. Neutral sulfite semichemical pulping was developed specifically as a semichemical process for cormgation medium and lends itself to small mills having minimal capital investment. For many years, this was the only semichemical pulping process. 

Semichemical pulping can be accompHshed by a sulfur-free system of sodium hydroxide and sodium carbonate. The sodium carbonate is recovered by simple incineration, and sodium hydroxide is added as makeup. Advantages in recovery operation are obtained if potassium hydroxide is added occasionally to maintain ca 20 mol % potassium carbonate (66). Lastiy, semichemical pulping can be accompHshed by using kraft white Hquor of low strength. 

J. L. Minor and N. Sanyer, "Oxygen Pulpiag of Wood Chips with Sodium Carboaate," ia Ref. 55 J. P. Hansoa and S. I. KokoUch, "Semichemical Pulpiag With Na CO and NaOH Combiaations," ia Ref. 55. 

Pulp Manufacture. Sodium sulfite is utilized in neutral semichemical pulping, acid sulfite pulping, high yield sulfite cooling, and some kraft pulping processes (339). Many pulp mills prepare their own sulfite and recycle as much as possible, but use of merchant sodium sulfite by pulp mills is substantial. Much of the by-product sodium sulfite from resorcinol manufacture goes into pulp appHcations as well as a substantial fraction of the lower assay manufactured sodium sulfite. 

Pulp mills. These separate the fibers of wood or other materials, such as rags, Enters, waste-paper, and straw, in order to create pulp. Mills may use chemical, semichemical, or mechanical processes, and may create coproducts such as turpentine and tall oil. Most pulp mills bleach the pulp they produce, and, when wastepaper is converted into secondary fiber, it is deinked. The output of some pulp mills is not used to make paper, but to produce cellulose acetate or to be dissolved and regenerated in the form of viscose fibers or cellophane. 

Semichemical Combination of impure fibers Intermediate pulp refiner mechanical pulp High-yield kraft, high- 6... 

Chemical, semichemical, or mechanical breakdown of pulping material into fibers Removal of pulp impurities, cleaning and thickening of pulp fiber mixture Addition of chemicals in a staged process of reaction and washing increases whiteness and brightness of pulp, if necessary At nonintegrated pulp mills, pulp is dried and bundled into bales for transport to a paper mill... 

Semichemical pulping comprised 6% of U.S. pulp production in 1993.1 Semichemical pulp is often very stiff, making this process common in corrugated container manufacture. This process primarily uses hardwood as furnish. 

Semichemical pulps are typically bleached with hydrogen peroxide in a bleach tower. 

U.S. EPA, Pulp, Paper and Paperboard Industry—Background Information for Proposed Air Emission Standards Manufacturing Processes at Kraft, Sulfite, Soda, and SemiChemical Mills (NESHAP), U.S. EPA, Washington, 1993. 

NSSC [Neutral sulfite semichemical pulping] A papermaking process in which wood chips are digested in an aqueous solution of sodium sulfite and sodium carbonate at 140 to 170°C for several hours. 

SemiChem products are available at http //www.semichem.com/prods.html. AMPAC , available as a stand-alone product with Windows-based and work-station-level interfaces, is a semiempirical quantum mechanical program featuring SAMI, AMI, MNDO, MNDO/d, PM3, MNDO/C, and MINDO/3 semiempirical... 

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