Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
2003-12-01
2004-12-14
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C071S042000, C162S189000, C210S724000, C210S725000, C210S727000, C210S928000
Reexamination Certificate
active
06830696
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
FIELD OF THE INVENTION
This invention relates to the field of paper production, more specifically in the treatment of spent digestion liquor resulting from the production of paper pulp.
BACKGROUND
Paper manufacture requires a source of cellulose fibers. Common raw materials as cellulose fiber sources are hardwoods and softwoods as well as those of annual vegetable origin, such as wheat and rice straw, bagasse (sugar cane stalks after processing), hemp, and jute. Rag materials, as well as recycled fibers, can also be used. However, wood has been, and continues to be, a primary source of cellulose fibers for paper production.
Before use, the wood or other cellulose containing raw material must be processed to release the cellulose fibers. This operation is referred to as “pulping”. At present, commercial pulping operations are of three principal types: mechanical, full chemical, and semichemical. The processes with which the invention is concerned are full chemical and semichemical pulping based on alkali, typically soda, breakdown of the fiber source. Another method of pulping, known as sulfite pulping, involves chemical transformation of the lignin component of the fiber source resulting in chemically modified lignin to be solubilized in water. Sulfite pulping is generally excluded from the inventive process to be described herein.
Full chemical and semichemical pulping employ chemical reagents to effect separation of the cellulosic fibers from other components. Wood chips or other raw materials are cooked with suitable chemicals in aqueous solution, usually at elevated temperatures and pressures. The object is to dissolve the organic binders holding the cellulosic fibers, termed “lignins”, which comprise up to 26% of raw wood, for example, along with other types of organic molecules, such as saccharide molecules, and other extraneous compounds, leaving the cellulose fibers intact. Although there is generally some resulting cellulose degradation, the objective can be realized to a commercially satisfactory degree through the use of a variety of chemical reagents. Pulp yields from wood using such processes are usually about 50% of the wood weight.
Lignins have been studied extensively and are believed to consist of the noncarbohydrate portion of the cell walls of plant materials. Originally, the lignin content of plant materials was defined as the residue after hydrolysis with strong acid following removal of waxes, tannins, and other extractives, including resins and tall oils. Lignins are amorphous, have high molecular weight, and are predominantly aromatic in structure. In general, the monomeric units comprising lignins can be referred to as p-hydrocycinnamyl alcohols. More specifically, according to The Merck Index, lignins comprise coniferyl, p-coumouryl, and sinapyl alcohols. Their precise composition vary with several factors including the method of isolation, and the species, age, and growing conditions, of the plant. Lignins are more or less completely removed from fiber by the digestion process of chemical pulping and enter the water phase of digestion liquor in oxidized colloidal form.
Digestion liquors obtained from alkaline pulping usually contain not only all the lignins in the source material, but substantial amounts of cellulose or carbohydrate monomers, and other carbohydrates. When using annual plant materials as a cellulose source, such as rice and wheat straw, a significant percent by weight of the material is silica. Such used or spent digestion liquors, normally waste, pose problems that are unique in alkaline pulping operations. Waste liquor streams from other operations during the paper making process pose different problems, such as removal of the resins and tall oils found in gymnosperm trees.
The lignin solids precipitate when the spent alkaline digestion liquid is acidified. This process poses a particular problem, because the lignin solids are polymerized by acidification to produce an amorphous gum.
Soda and sulfate pulping are both known in the art as being alkaline pulping processes. The soda process employs caustic soda (sodium hydroxide), whereas the sulfate process employs sodium sulfide in addition to caustic soda. The sodium sulfide used in the sulfate process results in a stronger cooking liquor and accounts for stronger pulp and faster cooking in the sulfate process as compared with soda pulping. The term “kraft pulping” is an alternative to the term sulfate pulping. For the purposes of this invention, there is no practical difference between the lignin-laden, spent liquors that result from either the sulfate or the soda process or from semichemical or other pulping processes which make use of alkaline agents in conjunction with mechanical means to make pulp, except there is often a relatively great amount of silica in soda process digestion liquor as compared with sulfate (kraft) process digestion liquor. The common link between pulp-making processes with which the invention is concerned is that spent digestion liquors employing alkali, whether buffered or not, become laden with solids and with organic matter, usually referenced to as total organic carbon [TOC], primarily lignins, and that both the inorganic and organic constituents must be recovered or otherwise processed to accommodate environmental concerns as well as to recycle inorganic digestive chemicals. All cooking or pulping reagents employing alkali chemicals, especially caustic soda and sodium sulfide, are expensive. Moreover, the inorganic waste materials are usually too toxic to release the spent liquor to the environment.
The sulfate, (e.g. kraft process) and the semikraft process are generally used when wood is the raw material. The active pulping ingredients, sodium hydroxide and sodium sulfide, make up a strongly alkaline solution. Standard in the kraft pulping process is the provision of a liquor-recovery cycle in which the organic constituents in the spent digestion liquor (primarily residual lignins and carbohydrates) are burned for steam generation and for recovery of the inorganic, alkaline, pulping chemicals in molten form, they being then solubilized by the addition of water to form so-called “green” liquor, which is further processed for reuse.
The traditional waste digestion liquor recovery cycle applied most frequently to kraft or semikraft process digestion liquors comprises the step of evaporating digestion waste liquor, the so-called “black liquor”, to a high concentration, to so-called “concentrated black liquor” or “black kraft liquor”, which is usually (up to) 70% solids, or greater, by weight. Organic sulfur compounds are found in the black liquor from the sulfate process in association with sodium sulfide (NaS). Sodium carbonate (Na
2
CO
2
), sodium sulfate (Na
2
SO4), and silica (SiO
2
) are also present. Total solids are usually about 15 percent by weight in black liquor after separation from fiber pulp following digestion.
The term “black liquor” is often also applied to other lignin-laden, used or spent digestion liquors, the compositions of which vary with the reagent chemicals used, the raw material, and the particular mill concerned.
The soda process is normally applied to raw materials of annual vegetable origin, such as cereal (e.g. wheat and rice, straw). Such materials normally contain a relatively high percentage of silica, which is solubilized in the digestion liquor. This poses additional separation problems, because, as well known in the art, separating out silica by acidification of the alkaline spent digestion liquor produces a gelatinous or gummy mass that cannot be separated from the liquor in a practical manner. The elevated silica content of liquor derived by pulping such vegetable fiber sources, as much as one percent by weight, as compared to the relatively low silica content from wood fiber sources, generally precludes practical application of separation and recovery methods presently
Abdel-Aal El-Sayed A.
El-Shall Hassan E.
Akerman & Senterfitt
Hruskoci Peter A.
University of Florida Research Foundation Inc.
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