SE534232C2 - Recycling chemicals used to dissolve cellulose in a cellulose pulp mill's chemical recycling system - Google Patents

Description

Dissolving pulp is a low-yield (30-40% on wood) bleached chemical wood pulp that has a high alpha-cellulose content (> 90%). This mass has special properties, such as a high level of brightness and high purity. Dissolving pulp is used for the production of regenerated cellulose products. The dominant process for the production of regenerated cellulose fibers, the viscose process¿ suffers from a high environmental impact and a high energy requirement. The viscose process uses large quantities of carbon disulfide, a chemical that is often contaminated with malodorous impurities such as carbonyl sulfide, hydrogen sulfide and organic sulfides. Even the best of current technologies are unable to suppress the odors emitted in a viscous plant. Furthermore, there is no efficient chemical recycling process for recycling spent solution and regeneration chemicals.

The NMMO (N-methylmorpholine oxide) process, a fairly new non-derivatizing process for the production of regenerated cellulosic fibers, appears as an alternative to the viscose process, but recovery of the NMMO solvent is complicated, energy-intensive and costly. Chinese patent application CN 101280476 is directed to a new process for recovering NMMO solvents using cationic and anionic resins.

European patent application EP 1900860 is directed to a process for dissolving cellulose in a sodium hydroxide urea mixture. Although this process may have an advantage over the viscose and NMMO processes, there is no suggestion on how to recover solvent / coagulation chemicals. Dissolving compositions, whether prepared by a pre-hydrolysis process or a sulphite process, are traditionally used in the manufacture of viscose or cellulose derivatives, such as cellulose esters, rayon fibers and cellophane.

Rayon is a soft textile material that is mostly used in tops, coats and jackets. Viscose materials can be made either from dissolving pulp or from cotton fibers.

The manufacturing process begins with treatment of the fibers with sodium hydroxide (mercerization). The mercerized mass is then mixed with carbon disulfide to form cellulose xanthogenate, a cellulose ester. The cellulose xanthogenate dissolves in sodium hydroxide to form a viscous cellulose solution. The cellulose solution or viscose is extruded into an acid bath either through a slit to make cellophane or through fine nozzles to make rayon. In the acidic environment, the xanthogenate ester is broken down into cellulose and sulfur-containing compounds. Some of the carbon disulfide is recycled and recycled to treat new cellulose.

The European patent EP1521873 is directed to a process for the production of solid regenerated viscose fiber, which describes certain new features in relation to the traditional viscose process. The viscose process was developed more than a hundred years ago and the process still has a dominant position in the market for the production of regenerated cellulose. For (and the NMMO procedure) reference is given to “Regenerated Cellulose Fibers” The more details on the viscose procedure Textile Institute, Ed. Calvin Woodings, Cambridge 200l. (ISBN 1 85573459 1) 10 15 20 25 30 534 232 Dissolving pulp can be produced by alkaline (kraft, soda) and acidic (sulphite, bisulphite) pulp production processes. In the power process, the cooking liquid consists of sodium hydroxide and sodium sulfide. In soda ash use, sodium sulfide is at least partially replaced by anthraquinone. In sulphite factories, sodium sulphite or magnesium sulphite / bisulphite are used as active cooking chemicals. The chemical recycling cycle in a pulp mill includes a recovery boiler, evaporation plant, sulfur dioxide recycling units (for sulphite plants) and a re-causticification plant (alkali pulp mills). For a detailed description of kraft, soda and sulphite chemical pulp cooking, reference is made to “Chemical Pulping” Book 6A, Ed. Johan Gullichsen, 2000. (ISBN 952-5216-06-3) and “Pulp and Paper Manufacture” Volume 4. Sulfite Science & Technology "Ed. By O.V.

Ingruber, 1985. (ISBN 1-919893-22-8).

Since the purpose of the cellulose production process is to produce a dissolving pulp, the physical target quality parameters for the product are different from the target quality parameters for pulp. Abrasion and tensile strength are no longer important while the cellulose pulp unit is important (low lignin content, low metal and ash content).

It is clear that there is a need for a new and more efficient cellulose dissolution process to replace the traditional viscose process.

Summary of the Invention The present invention is directed to a process for combining the production of cellulosic fiber products in a power, sulfite or soda AQ pulp mill with a process for dissolving cellulose using a novel solvent system. wherein at least a part of the spent cellulose solvent chemicals is recovered in one or more unit operations in the chemical recovery cycle in the pulp mill.

An object of the present invention is to provide an efficient cellulose dissolution process for producing molded cellulosic materials, which is integrated in a cellulosic pulp mill. Alkaline and acidic solvents comprising an amphiphilic additive are used to break up crystallinity and dissolve cellulose.

Another object of the present invention is to provide a low capital intensive and environmentally superior process for preparing a cellulosic solution suitable for molding into new cellulosic fibers, films or cellulose derivatives.

A further object is to provide a cellulose dissolution and cellulose forming process in which delignification chemical recovery and cellulose solution chemical recovery are performed at least in part in the same equipment.

Detailed Description of the Invention The process of the present invention is thus related to a cellulose pulp manufacturing and cellulose dissolution process with an integrated recycling system for recycling pulp coke chemicals and recycling chemicals for dissolving cellulose. The present process is carried out in several steps, the first step comprising physical and chemical treatment of lignocellulosic material, such as wood or annual plant material, to increase the availability of the lignocellulosic material. After the chemical and physical pretreatment, the material is boiled in an alkaline or acidic buffer solution, possibly in the presence of one or more active chemical reagents, to obtain a delignified brown stock. The brown stock is bleached by the use of environmentally friendly chemicals, such as ozone and hydrogen peroxide, to obtain a final dissolving pulp product with desired physical properties and brightness. The cellulose effluent generated in the process, including lignin components and spent chemical reagents, is concentrated followed by full or partial oxidation in a gas generator. The gas generator forms a stream of hot raw gas and a stream of alkaline chemicals and chemical reagents for subsequent recycling and reuse in the pulp manufacturing process. Alternatively or combined with combustion, the liquor is treated with an acid for recycling lignin.

The dissolving pulp prepared by the procedure referred to above is dissolved in a solvent to form a cellulosic solution. The pulp dissolution step is preferably performed directly in connection with a dissolving pulp plant.

In the following sections, the invention is described in more detail starting with the preparation of the raw material. i) Raw material preparation and removal of hexose-containing hemicelluloses Both hardwoods, such as eucalyptus, acacia, beech, birch and mixed tropical hardwoods, as well as softwoods, such as pine, spruce and hemlock spruce, can be used as raw materials for the manufacture of a dissolving pulp which 534 232 is suitable for dissolution in the cellulosic solvent system of the present invention.

Hemicelluloses must be at least partially removed from a dissolving pulp type. Removal of hemicelluloses can be carried out by an acidic or alkaline prehydrolysis step before boiling or by extraction of xylan from the pulp product. The traditional dissolving pulp feed to viscose plants or in the production of cellulose esters has a hemicellulose content below about 7% and preferably below about 3%.

To remove hemicelluloses from the lignocellulosic feed material, the cooking step can be preceded by a chip prehydrolysis step. Such treatment would, in addition to removing hemicelluloses, increase the availability of cooking chemicals to the interior of the wood structure and reduce the effective alkali requirement in subsequent pulping operations. A prehydrolysis step can be preferably used when the starting material for the process of the present invention contains a substantial amount of hexoses, such as glucomannans.

A variant of prehydrolysis in this context is autohydrolysis, which is essentially an anghydrolysis of the lignocellulosic material at temperatures of 175-225 ° C. Under autohydrolysis conditions, such as in prehydrolysis, the hemicellulose components are solubilized and the lignin is partially hydrolyzed by cleavage of phenolic and ether linkages.

In yet another variant of prehydrolysis, vapor explosion hydrolysis, the wood material is treated with steam at a temperature of 200-250 ° C for a few minutes. This treatment is followed by an explosively rapid expansion to decompose the cellulosic material. In this type of process, however, both chemical and mechanical attacks on the cellulosic material lead to extensive depolymerization of the carbohydrates.

The liquor resulting from the prehydrolysis treatment should preferably be removed from the cellulosic material before the pulp is subjected to further treatment. The liquor can be removed by a strainer by washing or by pressing the cellulosic material. After any recirculation, the effluent is emptied from the pre-treatment step. The pH during a prehydrolysis step can be adjusted (with temperature, to any suitable value in the range from about 0.5 to 7.0, depending on the desired degree of hemicellulose removal, time and additive), preferably to a level between 1.0 and 5.0, and in the case of autohydrolysis, a pH in the range of from 4 to 6. ii) Boiling / Delignification After the cellulosic material has been subjected to some pre-treatment, such as chipping, steaming or pre-hydrolysis, the material is boiled in the presence of alkaline or acidic boiling liquid based on a soluble alkali metal or alkaline earth metal compound.The base alkali metal is sodium and the alkaline base metal is magnesium or calcium.

The purpose of the boiling step is to separate cellulose and lignin by dissolving the lignin in the cooking liquid. In the case of a power or soda process, alkaline cooking liquid consists primarily of an alkali metal hydroxide or carbonate (and sodium hydrosulfide in effect). Acidic boiling liquid could be any sulfite cooking liquid having a capacity to sulfonate and dissolve lignin. In alkaline pulp processing processes, alkali metal phosphates and alkali metal boron compounds can be used as boiling liquid alkali. The most preferred cooking liquid comprises hydroxide, carbonate or borates of sodium or mixtures of these compounds.

The alkaline cooking liquid originates in the chemical recovery system of the pulp production plant from where it is recycled, with or without partial causticization, to the boiling step. As boron-based alkali is present in the cooking liquid, the need for causticization and mesa burning is reduced because boron chemicals are partially autocausified in the recovery boiler.

In a sodium sulfite or sodium bisulfite pulp mill, the sulfur must be separated from the sodium base to regenerate the coke acid. Sodium sulfate, sulfite and lignosulfonates form a melt comprising sodium sulfide in the recovery boiler. The melt dissolves and sodium and sulfur compounds are separated to form fresh coke acid. In magnesium-based pulp mills, magnesium and sulfur are separated in the recovery boiler. Solid magnesium oxide particles separate and dissolve to form a magnesium hydroxide solution. Gaseous sulfur oxides are scrubbed with magnesium hydroxide, which solution forms fresh cooking liquid.

Regardless of the pulp production process, the temperature in the cooking step of the present invention is maintained in the range of from about 110 ° C to about 200 ° C, preferably from about 120 to 160 ° C. At the higher boiling temperatures, a shorter residence time in the reaction vessel is required. A residence time of from 30 to 60 minutes may be sufficient at a temperature in the range of from 170 to 200 ° C, while from 60 to 360 minutes residence time may be necessary to obtain the desired result at lower boiling temperatures. than about 170 ° C.

Traditional types of continuous boilers with single or double vessels of the hydraulic or vapor liquid type as well as batch boilers where the wood material is retained in the reaction vessel throughout the cooking procedure can be used to contain the boiling reactions. The recovery of the effluent from these steps can be integrated in a known manner with the recovery of the effluent from an oxygen delignification step. The effluent can be concentrated by evaporation and incinerated in a separate boiler or carburetor or mixed with other effluents for further treatment.

Delignification catalysts and other additives can be added to the boiling step of the present process. Some of these additives are commonly used to increase the rate of delignification during alkaline boiling of cellulosic material.

Sulfur-free pulp preparation is of particular interest both for environmental reasons but also for the possibility of sulfur-free lignin recycling. In a sulfur-free alkaline mass production process, specific polyaromatic organic compounds may be added to be present in the boiling step, such compounds including anthraquinone and its derivatives, such as 1-methylanthraquinone, 2-methylanthraquinone, 2-ethylanthraquinone, 2-methoxyanthraquinone, 2,3-dimethylanthraquinone and 2,7-dimethylanthraquinone. Other additives with a potentially beneficial function in this step include carbohydrate protectors and radical scavengers. Such compounds include various amines, such as triethanolamine and ethylenediamine, and alcohols, such as methanol, ethanol, n-propanol, isobutyl alcohol, neopentyl alcohol and resorcinol and pyrogallol.

Anthraquinone and its derivatives are the preferred organic additives for use in the cooking step of a sulfur-free pulp production process. The anthraquinone additives are preferably used in quantities not exceeding 1% by weight of the dry cellulosic substances and more preferably below 0.5%.

Optimal operating conditions and chemical investments in the cooking step of the process depend on several parameters, including the source and origin of the cellulosic raw material, the end use of the product, etc. These specific conditions can be easily determined by the person skilled in the art for each individual case. iii) Oxygen delignification After boiling / delignification, the cellulosic material is subjected to a mechanical defibration treatment to release the fibers, which facilitates efficient contact between the reactants in a subsequent oxygen delignification step.

Defibration can be achieved, in its broadest sense, by introducing a fibrous accumulated material into a treatment device in which the fibers are at least partially detached from each other by breaking bonds between individual fibers and by leaving bonds affected by physical forces. essentially undisturbed. Further defibration of treated fiber clusters can be performed by subjecting materials to shear forces of sufficient strength to substantially and completely separate said fibers without cleavage or division of the solid chemically bonded particles within the fiber clusters. 10 15 20 25 30 534 232 12 Oxygen delignification and bleaching with oxygen-based molecules has become standard in connection with the production of bleached kraft and sulphite pulp and the cost of oxygen has decreased significantly.

In analogy with an alkaline boiling step, alkaline liquor is also present during the oxygen delignification. The alkaline liquor comprises alkali metal hydroxide and carbonate.

Other buffering chemicals may be used, such as alkali metal phosphates and alkali metal boron compounds. The alkaline lye used in the oxygen delignification step originates in the chemical recovery system, more particularly from a causticizing unit where the alkalinity of the lye is restored to a pH value above about 13. To eliminate reduced sulfur compounds in the alkaline lye solution, the lye may be oxidized. (with lute oxidation) through the use of air, oxygen and / or ozone.

The oxygen added to the oxygen delignification step can be either pure oxygen or an oxygen-containing gas, where oxygen cost and partial pressure required in the reactor are considered. The total pressure in the reactor consists of the partial pressure of steam, oxygen and other gases that are injected or developed as a result of the reactions in the oxygen delignification process. The partial pressure of oxygen should be kept in the range of from 0.1 to 2.5 MPa. iv) Bleaching and xylan recovery In order to obtain a high quality dissolving pulp, the oxygen delignified cellulose pulp is finally treated by alkali extraction and / or bleaching using known bleaching agents, such as chlorine dioxide, hypochlorite, peroxide and / or oxygen, ozone, cyanamide, peroxyacid, Nitrogen oxides or combinations of any of such bleaches, in one or more steps.

The lignin content of the cellulose pulp after bleaching is less than about 2% by weight, preferably less than about 0.5% by weight.

Apart from makeup alkali which is added upstream of the pulp flow, some or all of the alkali used in the bleaching plant and the extraction steps are recycled to the pulp mill's causticizing plant. All reduced sulfur in recycled alkali can be removed by oxidation.

The alkaline bleach filtrates are preferably recycled countercurrently back to the oxygen delignification step. Acidic bleach filtrates, especially those derived from chlorine dioxide, ozone, nitric oxide or other acidic treatment steps, can be recycled directly or indirectly to a prehydrolysis feed pretreatment step.

Since the feedstock for the pulp production process is a pentose-rich hardwood and partial removal of pentoses from the pulp is desirable, pentoses, such as xylans, can be advantageously removed from the cellulosic pulp at any position after the boiling / delignification step prior to cellulose dissolution. The extraction is carried out with an aqueous extractant, preferably an aqueous alkali metal hydroxide (cold extraction or hot extraction) under conditions selected for selective dissolution of xylans. Dissolved xylan precipitates from the extractant by acidifying with an acid, such as carbon dioxide, or by diluting the extractant with water or an organic solvent, such as a C1-C4 monohydric alcohol, such as ethanol or isopropanol. Precipitated xylan is removed and the extractant is recycled, and reconstituted if necessary, to treat new pulp. Recycled pentoses (exemplified herein above with xylan) can be exported from the pulp mill or upgraded on site to green chemicals and polymers, such as furfural, lactic acid, PLA, etc. Recovered pentoses can preferably be converted on site to furfural using an acidic process involving an acidification reactor followed by distillation or converted directly to furfural in a catalytic distillation unit. v) Chemical and energy recovery Cellulose effluent, with or without prior extraction of lignin and other organic materials, is withdrawn for further processing in an alkali metal compound or alkaline earth metal compound recovery process and energy values.

The cellulose liquor contains almost all the inorganic cooking chemicals together with lignin and other organic material separated from the lignocellulosic material. The initial concentration of weak effluent removed from the digester is about 15% dry solids in an aqueous solution. Weak liquor is concentrated to combustion conditions in evaporators and concentrators to a solids content in the range from about 65% to about 85%. Although a conventional recovery boiler can be used for processing cellulose effluent, a chemical recovery system based on gasification or partial oxidation of the cellulose effluent in a gas generator can also be used. 10 15 20 25 30 534 232 15 Gasification of carbonaceous materials for energy and chemical recycling is a well-established technology and appears as an alternative for recycling chemicals and energy in pulp mills. Cellulose liquors contain a large proportion of salty inorganic compounds with a low melting and agglomeration point, and although various fluidized bed processes have been proposed for the conversion of cellulose liquor, it is generally considered that a suspension or cocurrent carburettor is more suitable for gasification of cellulose liquor.

Several types of carburetors or gas generators can be used, with minor modifications, in the practice of the present invention, including, for example, the carburetors described in U.S. Patent No. 4,917,763, U.S. Patent No. 4,808,264 and U.S. Patent No. 4,692,209. These gasification systems are suitable for chemical and energy recovery from high sulphide effluents / power and sodium based sulphite cooking liquids. The sulfur chemicals are recycled as alkali sulphides, but a significant part of the sulfur will also accompany the crude fuel gas such as hydrogen sulphide and carbonyl sulphide. Co-entrained molten alkaline chemicals in the crude fuel gas are separated from the gas stream in a cooling and washing step and dissolved in an aqueous solution. The alkaline solution, called green liquor, is causticized with lime to obtain a white alkali with high alkalinity, the traditional cooking chemical used in kraft pulp processes. On the other hand, upstream carburetors with two reaction zones designed for gasification of heavy hydrocarbons and coal can also be used, with minor modifications, in the practice of the present invention, such carburetors being described in, for example, U.S. Patent No. 4,872,886 and U.S. Patent No. 4,060,397. . Another carburettor having a suitable design for use in the present invention is described in U.S. Patent No. 4,969.93l.

Regardless of the type and design of the carburetor and recovery boiler, inorganic molten droplets and aerosols formed in the unit are separated from the gas flow and dissolved in an aqueous solution. The solution comprises alkaline compounds in a form suitable, via causticization, for use as alkali in oxygen delignification, alkaline boiling steps and cellulose dissolution / cellulose forming steps of the present invention. Causticization is a well-known unit operation in the field of chemical pulp production and is not described herein.

If sulfur compounds are present in the cellulose effluent, these compounds will form alkali metal sulphides and alkali metal sulphates depending on the design of the recovery unit.

Alkali sulfide as such is an effective pulp cook chemical / catalyst in the power process. In sodium basulfite plants, sodium and sulfur must be separated to recover fresh coke acid. The design and operation of chemical recycling in power and sulphite factories are well known to those skilled in the art.

In addition to the chemicals that are recycled for pulp production and bleaching, the recycling system in the pulp factory is also used for recycling and transformation of cellulose solution, cellulose coagulation and shaped cellulose washing liquids. These aqueous liquids contain alkali metal value and need to be converted to active dissolution chemicals. Recovery of alkali metal hydroxide for use in a cellulose dissolution step may include one or more of concentration in an evaporator, partial or full oxidation combined with cellulose effluent in a recovery boiler, gas generator, causticization in a recoustic acid treatment plant and treatment with in a white liquor oxidation plant. If sulfuric acid or sulfur-containing acids are used in a cellulose coagulation step, such acids can be recovered from the division of sodium and sulfur (by gasification and / or by acidification of crude green liquor) over oxidation of reduced sulfur compounds to sulfur oxides, which, when dissolved in water, form acidic liquids. which are suitable for use in a cellulose coagulation step.

Alkaline liquors produced in the recovery system of the present invention may be subjected to an oxidative treatment with an oxygen-containing gas to eliminate traces of sulfide before the liquor is recycled and added to the desired cellulose dissolution / shaping, bleaching or oxygen delignification step of the invention. . vi) Sulfur-free lignin recovery In one embodiment of the present invention, the pulp production process is substantially sulfur-free. In this process configuration, a part of the lignin can be advantageously extracted and separated from an effluent stream or boiler circulation stream before final concentration and further transport to a recovery boiler or carburettor. A substantially sulfur-free lignin can be recovered in accordance with known lignin recovery techniques. The sulfur-free lignin can be used as a raw material or precursor for fine chemicals, carbon fibers, phenols and plastic products or used as a sulfur-free biofuel. Lignin can be precipitated from cellulose effluent liquids with a solids content in the range of 3-30%, under the influence of one or more acids, including sulfur oxide acid liquids and carbon dioxide. Sulfur oxide acidic liquids can be prepared on site as described herein. Carbon dioxide can be recovered from gaseous streams in the dissolving pulp mill. The total sulfur content of lignin recovered by the procedure described herein (including covalently bonded sulfur) is less than about 1% by weight of dry lignin, preferably less than 0.5% sulfur and most preferably less than about 0%. 1% by weight of sulfur. vii) Dissolution of pulp in alkaline or acidic solvents and cellulose formation A type of dissolving pulp imported into or produced in an integrated pulp mill is dissolved in an aqueous cellulose solvent, which forms a substantially homogeneous cellulose solution. The dissolved cellulose is formed into new fibers, films or cellulose derivatives in one or more processing steps after the cellulose dissolution. Cellulose formation can be performed by injecting the cellulose solution through nozzles, directly or through an air gap, into a coagulation bath comprising coagulation chemicals.

The coagulation chemicals are characterized by being poor cellulose solvents. Cellulose fibers are regenerated in the coagulation step into a filament or fiber line of regenerated cellulose. The filament or fiberline can preferably be further converted into a cellulosic staple fiber for export from the cellulose dissolution plant.

Cellulose staple fibers can be used in a wide range of end products, such as textiles and consumer hygiene products. Cellulose forming can also be performed by injecting the cellulose solution onto a moving bed, forming a non-woven cellulosic fiber network ("spunlaid nonwoven").

Coagulation fluid can be added to the moving bed, or thereafter, to strengthen the fiber network. Various forms of water treatment of fibrous web can be used to obtain the desired cellulose network structure and strength.

Cellulose formation can also be carried out by reacting the dissolved cellulose in homogeneous phase with a reactant, to produce cellulose derivatives such as cellulose esters.

We have discovered that it is a great technical, environmental and commercial advantage if the recycling of cellulose solution and / or coagulation chemicals can be recycled in connection with the recycling of boiling and oxygen delignification chemicals. Surprisingly, chemicals used in the dissolution or coagulation steps of the present invention, provided that the selected conditions and additives are used, can be recovered in recovery systems also used for boiling and / or oxygen identification.

In order to obtain a high cellulose concentration in the cellulose solution, the dissolving mass (obtained after boiling, bleaching and hemicellulose extraction) should be activated before dissolution primarily to increase the availability of cellulose solution chemicals. Activation can also partially decrystallize the cellulose and shorten the cellulose molecules from a typical dP value (degree of polymerization of anhydroglucose repeating units) in the range 700-1300 in the dissolving mass to a range of 200-700.

The mass activation can be performed by any of or a combination of swelling in alkali hydroxide, enzymatic treatment, electron beam treatment, hydrothermal treatment and steam explosion treatment. A hydrothermal treatment of the cellulosic material can be carried out in a closed vessel at from 100 to 200 ° C for from 30 minutes to 5 hours with or without the presence of additives (batch boilers) (such as weak organic acids). Microwaves can be used for energy supply to the dissolving mass activation step.

The most preferred cellulose solution chemical system is based on alkali hydroxide in the presence of an amphiphilic additive compound. Amphiphilic compounds are characterized by possessing both a hydrophilic and a lipophilic moiety. The alkali hydroxide is preferably sodium hydroxide prepared by causticizing sodium carbonate-rich liquor. Sodium hydroxide can be pretreated with oxygen or ozone to oxidize reduced sulfur. The concentration of alkali hydroxide in the cellulose solution is below 20% by weight, preferably adjusted to around 10% by weight.

The amphiphilic compound is a polyelectrolyte, surfactant (anionic, cationic, nonionic or zwitterionic), polyethylene glycol, urea, thiourea or guanidine. Specifically preferred amphiphilic compounds include SDS (sodium dodecyl sulfate), polyethylene glycol / polypropylene glycol copolymers and lecithin. The additive is used in a concentration below about 10% based on the weight of the cellulose in the solution, preferably below about 3%.

The dissolution of the cellulose is preferably carried out in the temperature range of from minus 15 ° C to plus 20 ° C.

Provided that the cellulose spinning system can handle highly viscous solutions, the concentration of cellulose in the cellulose solution may be as high as 25% by weight. new fibers with high tenacity. 10 15 20 25 30 534 232 21 The concentration of cellulose can also be kept lower (5-15% by weight) to have a low viscosity and suitable cellulose solution rheology for spinning with conventional spinning equipment.

The spinning solution comprising cellulose can be injected into a coagulation liquid bath through fine nozzles to form a cellulose filament or fiberline; alternatively, the spinning solution can be injected into a moving bed forming a nonwoven fabric. The coagulation fluid is any suitable fluid with a low or very low capacity to dissolve cellulose. Advantageously, the coagulation liquid comprises an alcohol, such as a monohydric alcohol (ethanol, methanol, propanol, isopropanol), acetone or a polyhydric alcohol being composed of acids (organic acids, such as acetic acid, (glycerol). sulfuric acid) or a phosphate and / or sulphate salt solution. Finally, the coagulation fluid may be a dilute cellulose solution. Various types of additives, including zinc compounds, may be added to the coagulation fluid to promote the formation of a cellulosic fiber having the desired physical properties and geometric shape. Consumed coagulation fluids are at least partially recycled to and recycled in the same equipment used for recycling chemicals for boiling, oxygen delignification or pulp bleaching.

Regardless of whether cellulose films, nonwoven fabrics, water-treated bonded fibrous web material, filaments or fiber ropes are produced, the cellulose product is normally washed in a multi-step process with washing liquids. The system for recirculation and recycling of washing liquids is preferably integrated in the pulp mill's energy and chemical recycling system.

In addition to the alkaline cellulose solvent system described herein, concentrated phosphoric acid, organosulfonic acid (methanesulfonic acid, ethanesulfonic acid or arylsulfonic acid) and / or molten salt hydrates may also preferably comprise zinc chloride and / or a lithium anion, 3H2O, used for cellulose dissolution and cellulose forming. Coagulation fluid in this configuration alcohol, alkali hydroxide. Although energy integration with a mass can be water, acetone, phosphate salts or factory is uncomplicated with these dissolution chemicals, the regeneration of acids and molten salt hydrate chemicals from the coagulation fluids is considerably more complicated than recovery of alkaline cellulose solvents. Fresh phosphoric acid for cellulose solution can be prepared by treating phosphate salts recovered from a coagulation bath with sulfuric acid (optionally generated on site) to form sulfate and the desired phosphoric acid.

Sulphate salts in spent chemicals (by-product from the production of phosphoric acid on site or present in a spent cellulose coagulation liquid) can be injected directly or indirectly into a recovery boiler or gas generator for reduction to sulphides. Sulfides can then in turn be separated, for example in the form of hydrogen sulphide gas, which gas is oxidized to sulfur oxides. Sulfur oxides prepared in this way can be dissolved in water to form sulfuric acid or an acidic sulfur oxide solution suitable for use in a cellulose coagulation step.

According to the present invention there is provided a process for the preparation of a molded cellulosic material from lignocellulose and for the recovery of chemicals used in said process as set out in independent claim 1.

Additional features and specific embodiments of the invention are set forth in the dependent claims.

Claims (1)

A process for producing shaped cellulosic material from lignocellulose by a sequence of separation and dissolution steps, characterized in that the process comprises the steps of: ê) Providing a starting material of finely divided lignocellulosic material comprising cellulose, hemic lignin; Separation of lignin from the lignocellulosic starting material by boiling the material at a temperature between 110 and 200 ° C for about from 1 hour to 6 hours in an aqueous solution comprising a soluble alkali metal compound or alkaline earth metal compound, thereby forming a first stream of solids enriched with cellulose and a second stream rich in dissolved lignin; Treating said first stream of cellulose from step b) by at least one of oxygen delignification, bleaching and alkali extraction to form a cellulose pulp having a lignin content below about 2%, preferably a lignin content below about 1%; Treatment of the second stream rich in dissolved lignin from step b) in a chemical recovery system comprising a cellulose effluent concentration unit, optionally a causticizing plant, and a gas generator or boiler, whereby alkali metal compounds or alkaline earth metal compounds are recovered without further treatment with metal compounds, , to step b); E 15 Activation of dissolving cellulose pulp to increase the availability of cellulose solution chemicals; f) Dissolving activated cellulose pulp from step e) in an aqueous alkaline or acidic solution or a molten salt, to form a substantially homogeneous solution containing dissolved cellulose and then forming the dissolved cellulose into fibers, films or cellulose derivatives; g) Recycling of spent cellulose solution chemicals extracted from at least one of; gl) step f) g2) a cellulose coagulation step g3) a washing step for shaped cellulose to one or more of the chemical recovery steps of step d). Process according to Claim 1, characterized in that an alkali metal compound in step b) is sodium. Process according to Claim 1, characterized in that an alkaline earth metal compound in step b) is magnesium. Process according to one of Claims 1 to 3, characterized in that the hemicelluloses have been separated from the lignocellulosic starting material before boiling in step b). Process according to one of Claims 1 to 3, characterized in that pentoses, eg xylans, have been separated from the pulp after the boiling step b) but before the pulp is fed to the activation step e). 10 15 20 25 10. ll. Process according to claim 5, characterized in that xylan is separated from cellulose pulp by cold or hot extraction with an aqueous extractant containing sodium hydroxide followed by precipitation of xylan from the extractant by diluting the extractant with water, an alcohol or by acidifying the extractant. the aunt with an acid. Process according to Claim 6, characterized in that an alcohol is a lower monovalent C 1 -C 4 alcohol. Process according to Claim 6, characterized in that precipitated xylan is converted to furfural by using an acidic process comprising one of an acidification reactor and a distillation step or by catalytic distillation. Process according to any one of the preceding claims, characterized in that activation of cellulose in step e) is carried out by one or more of swelling in alkali hydroxide, by electron beam treatment, by meadow explosion, by hydrothermal treatment and by enzymatic treatment. Process according to one of the preceding claims, characterized in that cellulose is activated in step e) by a hydrothermal treatment in which cellulose pulp is treated with water and optionally additives, at a temperature between 100 and 200 ° C for a period of from 0.5 to 5 hours. Process according to Claim 10, characterized in that an additive is an acidic compound. Process according to any one of the preceding claims, characterized in that spent cellulose solution chemicals in step g) are recycled directly or indirectly to a causticization plant, whereby alkali carbonate is converted to alkali hydroxide. Process according to any one of the preceding claims, characterized in that spent cellulose solution chemicals in step g) are recycled directly or indirectly to a recovery boiler or a gas generator for recycling alkali metal compounds or alkaline earth metal compounds. Process according to one of the preceding claims, characterized in that spent cellulose solution chemicals in step g) are recycled directly or indirectly to a recovery boiler or a gas generator for the recovery of sulfur compounds. Process according to one of the preceding claims, characterized in that spent cellulose solution chemicals in step g) are removed from a cellulose coagulation step. Process according to one of the preceding claims, characterized in that spent cellulose solution chemicals comprise one or more of alkali hydroxide, alkali carbonate, alkali metal sulphate, zinc or phosphate. Process according to one of the preceding claims, characterized in that spent cellulose solution chemicals in step g) are recovered from a washing step for shaped cellulose. Process according to any one of the preceding claims, characterized in that one or more additives are added in addition to alkaline or acidic solvents used for dissolving cellulose in step f). Process according to Claim 18, characterized in that an additive is an amphiphilic compound, preferably an ionic or nonionic surfactant, a polyethylene glycol compound, urea, thiourea, lecithin or guanidine. Process according to any one of the preceding claims, characterized in that an acidic liquid used in step f) is one or more of concentrated methanesulfonic acid, ethanesulfonic acid, arylsulfonic acid or phosphoric acid. Process according to any one of the preceding claims, characterized in that a molten salt used in step f) comprises one or more of zinc chloride and lithium salts. Process according to any one of the preceding claims, characterized in that sulfuric acid is produced from reduced sulfur compounds originating in a recovery boiler or gas generator, wherein at least a part of said sulfuric acid is used in a cellulose coagulation step or for the production of phosphoric acid. Process according to any one of the preceding claims, characterized in that reduced sulfur compounds, essentially in the form of hydrogen sulphide, are released from green liquor formed by dissolving a melt from a recovery boiler or separated from a gas generator gas, said hydrogen sulphide being oxidised to sulfur oxides and dissolved in water to form an acidic solution. 24. 25. 26. 534 232 28 A process according to claim 23, characterized in that an acidic solution is used in a cellulose coagulation step and / or for precipitating lignin from a cellulose liquor. Process according to any one of the preceding claims, characterized in that substantially no sulfur compounds are present during boiling in the boiling step b) according to claim Process according to one of the preceding claims, characterized in that substantially sulfur-free lignin is recovered from cellulose effluent before supplying the cooking liquid to a recovery boiler or gas generator.