SE501309C2 - Pulp production by digestion of a mixture of cellulose-based recycled material and raw wood - Google Patents

Abstract

A process for the production of a waste-based cellulosic pulp useful for the manufacture of paper and paperboard whether used in admixture with virgin pulp or as the sole pulp component, comprising the steps: a) digesting a waste cellulosic material in an aqueous cooking liquor having a pH lower than about neutral and containing free and/or bound SO2 and a base; b) carrying out digestion with the exclusion of atmospheric oxygen at an elevated temperature, such as within the range about 120 to 160 DEG C and at a correspondingly increased pressure; and c) recovering the resulting cellulosic pulp.

Description

501 309 2 and fractionation techniques. The main disadvantage of the said decolourisation technique is the relatively high consumption of decolorizing chemicals, high installation costs for a plant, relatively high energy consumption, high organ weight in the discharge and the need for dumping of contaminant sludge on tips, such as landscape filling masses, etc.

Fibers recycled using the technology described above are often, in terms of strength and optical properties, significantly inferior to pulp fibers and can generally only be used in admixture with high-quality new fibers in the production of paper products. Furthermore, said recycled discolored fibers can cause severe production disruptions in the paper / board machine in the form of sticky thermoplastic residues; insufficient fiber length; microbiological infection of the paper machine's backwater system: entrapment of foreign materials, such as metal shavings, paper clips, etc. Paper products made using a high proportion of said recycled fibers usually have lower brightness, are dirtier in spots and less uniform in formation (transparency) than their new fiber-based counterparts. It would therefore be difficult to market such papers as high-quality fine papers.

Other technology that can be used for recycling secondary cellulosic materials involves high-consistency dispersion, whereby the paint particles and contaminants are reduced to such a size that the human eye can no longer distinguish them. Such recycled pulps are usually matt and of poor strength properties and contain relatively high amounts of residual filler, organic waste and foreign materials.

SUMMARY AND OBJECT OF THE INVENTION The main object of the invention is to provide a process for producing a cellulosic pulp using cellulosic materials as starting materials, such as printed or unprinted papers, for example newsprint, tissue, coated or uncoated paper. , packaging paper / board comprising corrugated board, solid fiber board, millbroke, raw machine board, sawmill waste, etc. or mixtures of any of these cellulosic materials in suitable proportions.

Another object of the invention is to provide a process for the production of bleached or unbleached recycled pulp, which can be used in admixture with or as the only pulp component in the production of paper and board. Such products include writing and printing paper, newsprint, tissue, packaging paper, uncoated graphic paper, coated graphic paper, etc.

A further object of the invention is to provide a process in which energy and chemical constituents can be recovered and reused within the process and in which any excess energy resulting from the process is converted to heat energy for use in the process or for example as drying energy in e.g. an integrated paper machine or the like. Yet another object of the invention is to provide a process for the production of a cellulosic pulp which contains practically no foreign organic substances, such as ink carrier vehicles, polymeric coatings, adhesives, pastes, etc.

For these and other purposes, which will become apparent from the following description of the present invention, there is provided a process for producing a cellulosic pulp useful for making paper and board, whether used in admixture with fresh pulp or as a single pulp component. The process of the invention comprises the following steps: a) digesting a cellulose-based recycled material in an aqueous cooking liquid having a pH lower than about neutral and containing free and / or bound SO 2 and a base; b) performing digestion excluding atmospheric oxygen at elevated temperature, such as in the range of about 120 to 160 ° C and at a correspondingly elevated 501,309 pressure; and c) recovering formed cellulose pulp.

In such a process, a cellulosic recycled material of arbitrary origin and in any mixture is digested in a cooking liquid having a pH of from acidic to approximately neutral containing free and / or bound SO 2 and a base involving magnesium, sodium, calcium or ammonium.

After completion of delignification and / or dissolution of organic substances derived from printing inks, adhesives, coatings, etc., a brown base fiber pulp and liquor is produced. The cooking time depends on the composition of the recycled material and the cooking temperature used, the degree of delignification, the hemicellulose retained, as well as the desired strength and optical properties of the pulp, and the cooking time can vary from between about 1 and about 10 hours.

In accordance with a preferred embodiment of the process according to the invention, a mixture of cellulose-based recycled material and raw wood in a suitable digestible form is used as starting material. Such raw wood is preferably of softwood origin.

Although the ratio between cellulose-based recycled material and raw wood in such a mixture is not critical, it is preferable to work in the range from about 5 to about 90% by weight of raw wood. It is especially preferred that the raw wood constitutes a smaller part of said mixture, ie constitutes less than 50% of the mixture. It is especially preferred that said mixture contains from about 10 to about 30% by weight of raw wood based on the total weight of raw wood and cellulose-based recycled material.

The addition of raw wood to the cellulose-based recycled material used as a starting material results in a cellulose pulp which is delignified to a greater degree at given process parameters than the same recycled material without the addition of wood chips. A kappa number of from about 10 to about 30 can thus be easily achieved compared to kappa numbers of from about 30 to about 60 without the addition of raw wood. The raw wood is preferably 501 309 in the form of chips, conceivable, types, such as sawdust, planer shavings, etc. although other materials are such as abrasive wood or sawmill waste of various types. The cellulosic pulp produced by the process according to the invention is washed in a conventional manner with water to form brown bast washing liquid, which contains organic substances formed during digestion and inorganic chemicals partly derived from the digestible raw materials and partly from the cooking liquid. directly in the production of paper products or over- The washed brown base pulp can be used is transported to a bleaching plant, where bleaching with a bleaching agent or a combination of bleaching agents commonly used in the pulp industry for fresh fiber pulp is carried out.

A major portion of the brown bast wash liquor is concentrated by evaporation to form a concentrate containing combustible organic substances and non-combustible inorganic chemical substances. The organic substances are burned for the production of heat energy that is recovered and the organic chemicals are recycled to form new cooking liquid for the process.

According to another aspect of the invention, a smaller amount of the cooking liquid or brown bast washing liquid may be recycled in the process instead of or in addition to the added raw wood. This smaller amount of liquid is preferably from about 1 to about 10% of the total amount thereof. The process of the invention provides a cellulose pulp on recycle base which has been delignified to a kappa number which is lower than the average kappa number for the mixture of cellulose-based recyclable recyclable materials. The kappa number is preferably in the range of about 10 to about 30, which makes the pulp fully bleachable. The bleaching of the produced pulp, however, is a non-mandatory step.

As stated above, it is preferred according to the present invention to use a mixture of cellulose-based recycled material and raw wood in a digestible form.

Such addition of raw wood, such as in the form of chips, results in a cellulose pulp on the return base which can be delignified to a greater extent at given process parameters than when using the same return material without the addition of raw wood. Although the present invention is not limited to any particular theory or mechanism, it appears that some extracts from the crude wood increase the condensation of lignin, while others act as protective compounds against the decomposition of hemicellulose, which means that the pulp produced in this way retains a higher degree of its strength properties. It should also be noted that without the addition of raw wood to the starting material, the mass produced in the same way is not bleachable to the same brightness range, about 65 to about 85, using a simple, two-step hydrogen peroxide bleaching sequence compared to the situation when added raw wood.

The present invention thus offers the possibility of producing a cellulose pulp from cellulose-based recycled materials mixed with raw wood, in a simple cooking operation. Depending on the strength / optical properties required for a paper product produced with such pulp, it is possible to use the pulp as the sole fiber component in a variety of paper and board grades ranging from tissue, via unbleached packaging paper to highly bleached, spotless fine paper. When using the additional step of bleaching, pulp capacity below about 30 generally results in a brightness of about 65 or significantly higher.

In the present presentation, all brightness values are given in ISO brightness, according to ISO 2470, coinciding with SCAN-P 3:75, AFNOR Q 03-039, ATICEL-CA MC 12-68, BS 4432: 2, CPPA E.IP and cSN 50 0241. refers to the weight unless otherwise stated.

TOTAL PERCENTAGES 501 309 7 DESCRIPTION OF PREFERRED EMBODIMENTS The accompanying drawing schematically shows a plant useful in the production of cellulosic pulp in accordance with the process of the present invention.

The illustrated plant is briefly described in the following.

The plant shown in the drawing comprises a server 1 for cellulose-based return material, further comprising a cleaning unit 5 and a press 7. The plant also comprises a reactor or reboiler 11, a pulp vessel 19 and a twig strainer 14, 15, 17, 17, a brown base washing and a fine sieving apparatus 23 to 30. The plant also comprises a recovery system consisting of an evaporator 107 and a recovery boiler for burning thick liquor to form steam which can be used in the process.

Boiling liquid is supplied to the digester 1, so that together with the cellulose-based return material a suitable concentration of fibrous material is obtained, such as from 2 to 4%. A line 4 connects the digester 1 to the cleaning plant 5, which is not mandatory, and a line 6 further connects to the press 7, in which the solids content is increased to 25 to 30% depending on the quality of the starting materials. A line 8 returns the excess liquid from the press 7 to the digester 1. Through the line 9 the press 7 is connected to the reactor or the boiler 11. Wood chips and / or brown bast washing liquid are added to the reactor or the boiler 11 via lines 10 and 64 respectively. A line 101 connects a of the liquid vessels 94,96,98, 10 0 to the digester or reactor depending on the boiling liquid quality desired in the process and used to fill the reactor 11 with acid boiling liquid before the start of the cooking procedure. After closing the filled reactor, the boiling liquid is heated, preferably by indirect heating via a steam heat exchanger, to a normal between 120 and 160 ° C, preferably 125 to 135 ° C. suitable temperature, 501 309 8 A line 66 connects the reactor to the primary liquid vessel 67 and is used to dump the cooking liquid so that the digestion procedure is terminated. The pulp is flushed from the reactor 11 to the brown base vessel 12 with brown base wash liquid (flushing liquid) taken from the flushing liquid vessel 63 using line 64.

A conduit 13 connects the brown base vessel 12 to the primary and secondary twig strainers 14,15. The reject from the primary twig strainer 14 is an injection into the secondary twig strainer 15. The acceptance from the secondary strainer 15 is collected in a vessel 17 intended for acceptance from the secondary strainer and is in turn connected via a line 18 to the injection of the primary twig strainer. The line 16 carries the acceptance from the primary twig strainer 14 to the brown bast wash 19. The wash water used in the brown bast wash originates from the thickener 33 as eluate, the connection being indicated by the line 71. The brown bast wash 19 is connected to a buffer vessel 21 connected to the primary fine sieve 23 via line 22. Line 24 connects the primary fine screen 23 to the reject screen 25, from which the acceptance follows the line 26 back to the buffer vessel 21. The reject is deposited for further transport to another production unit. A line 27 carries the acceptance from the primary fine screen 23 to the buffer vessel 28, which is connected to the secondary fine screen 30 via the line 29. The reject from the secondary fine screen 30 is transferred to the reject screen via a line 31. After screening, the pulp passes through a additional batch of thickener 33,39,4l. The number of thickeners depends on how accurate the washing of the pulp must be with a given amount of water determined by the evaporator capacity, which is limiting for the amount of water used for washing.

After washing and screening the pulp, it can, if desired, be bleached in any bleaching sequence, such as the sequence P / E, P / E. Hydrogen peroxide and sodium hydroxide are added to the pulp in a high-temperature mixer 43. The pulp and the chemical mixture are transported through a steam mixer and are thereby heated to a temperature in the range 65 to 90 ° C, preferably 75 to 85 ° C. The mixture moves downwards from the steam mixer to a downstream tower 45, and after a retention time of 2 to 4 hours the pulp leaves the tower and is pumped to a filter 47, where it is washed. After washing the pulp, it is diluted in slightly acidified water and pumped to a neutralization tower 49, at the end of which the pH is readjusted with SO 2 water to about pH 7. After the neutralization in the tower 49, the pulp is washed and dewatered on a filter. dose of hydrogen peroxide and sodium hydroxide and the pulp To the dewatered pulp is added an outer pump to the bleaching tower 54.

Brown bast washing liquid is pumped through a line 62 to a vessel called flushing liquid vessel 63. Excess liquid from this vessel is mixed with the primary liquid in the primary liquid vessel 67. The mixture of primary liquid and brown base liquid is called barrel liquor. The barrel liquor is neutralized in a vessel 69, in which it is mixed with magnesium oxide from the chemical recovery system. The purpose of this neutralization is to prevent volatile organic acids from evaporating in the evaporator 107. The neutralized barrel liquor is concentrated in the evaporator to a solids content of 50 to 60%, preferably 56 to 60%. The concentrated liquid is called thick liquor and is burned in the recovery digester 86 to produce about 2.3 to 2.9 tons of steam per ton of thick liquor. In the ash collected on the electric filter in the recycling plant 86 after the thick liquor is incinerated, the main component is magnesium oxide.

This magnesium oxide is washed in lukewarm water to dissolve salts of sodium and potassium. After washing, the magnesium oxide is mixed in hot water, where it reacts to form magnesium hydroxide, which in turn is pumped into a gas absorption system, in which the sulfur dioxide-rich exhaust gases from the thick liquor combustion are washed with the magnesium hydroxide slurry to form new crude acid.

This crude acid is pumped from the recovery plant 86 through a sedimentation tank 88, a sand filter 90 and a heat exchanger 92 into the crude acid vessel 94. The pressure in the crude acid vessel is close to atmospheric pressure, while the pressure in the other vessels is higher than atmospheric pressure. Excess gas from the reactor II enters a countercurrent relative to the acid in the acid vessels.

EXAMPLES The following examples illustrate the invention in more detail but may not be construed as limiting the scope of the invention except as defined in the appended claims.

EXAMPLE 1 Commercial recycled corrugated board (kappa number 87) is boiled with an acidic boiling liquid containing a total SO 2 of 50 g / l and one of 24 g / l. The pH is 4.5 and the cation (base) used is bound SO 2 of 26 g / l, resulting in free SO 2 magnesium. Boiling is carried out for 6 hours at a temperature of 228 ° C. The obtained washed pulp has a kappa number of 75.1 and a brightness of 28.7. Mass yield calculated on dry raw material is 70.7%.

EXAMPLE 2 Pulp produced according to Example 1 is bleached in a 2-step P / EP / E bleaching sequence. The hydrogen peroxide dosage is 2% in step 1 and 1% in step 2. The bleaching time is 2.5 hours in step 1 and 3 hours in step 2 at a common temperature of 75 ° C. The bleached and washed pulp has a brightness of 46.9.

EXAMPLE 3 Commercial recycled corrugated board according to Example 1 mixed with raw wood is boiled with an acidic cooking liquid as in Example 1 but the cooking time is increased to 7 hours at a temperature of 228 ° C. The pulp has a kappa number of 47.3, a brightness of 29.4 and a yield calculated on the return material of 67.6%. EXAMPLE 4 Pulp obtained according to Example 3 and bleached as in Example 2. The pulp has a brightness of 47.3.

EXAMPLE 5 The recycled material used in Example 3 is boiled with an acidic boiling liquid containing total SO 2 of 80 g / l and a machine combined SO 2 of 22 g / l resulting in free SO 2 of 58 g / l. The pH value is 1.5 and the cation (base) used is magnesium. Boiling is carried out for 4 hours at a temperature of 228 ° C. The resulting pulp has a kappa number of 33.7, a brightness of 29.1 and the yield is 59.0%.

EXAMPLE 6 Pulp produced according to Example 5 is bleached as in Examples 2 and 4. The bleached pulp has a brightness of 46.2.

EXAMPLE 7 A mixture of 50% commercial recycled corrugated board according to Example 1 and 50% spruce chips is boiled with an acidic cooking liquid according to Example 1. Boiling is carried out for 6 hours at a temperature of 228 ° C as in Example 1. The resulting mass has a kappa number of 60.8, a brightness of 38.0 and the yield is 57.8%.

EXAMPLE 8 Pulp produced according to Example 7 is bleached as in Example 2. The bleached pulp has a brightness of 43.2.

EXAMPLE 9 The mixture of 50% recycled material and 50% spruce chips as used in Example 7 is boiled with the boiling liquid described in Example 5. Boiling is carried out for 6 hours at a temperature of 228 ° C. The pulp has a kappa number of 24.3, a brightness of 39.7 and the yield is 48.7%. EXAMPLE 10 Pulp produced according to Example 9 is bleached as in Example 2. The bleached pulp has a brightness of 73.6.

EXAMPLE 11 The mixture of recycled material / chips described in Example 7 is boiled according to Example 5. The boiling is carried out for 7 hours at a temperature of 128 ° C. The pulp has a kappa number of 20.5, a brightness of 34.7 and the yield is 42.3%.

EXAMPLE 12 The pulp produced in Example 11 is bleached as described in Example 2. The pulp obtains a brightness of 75.7.

EXAMPLE 13 The recycled material / chip mixture is changed from 50/50% according to Example 7 to 67% recycled corrugated board and 33% spruce chips.

The mixture is boiled according to Example 11 with a boiling liquid according to Example 5. The mass has a kappa number of 13.1, a hot number of 38.1 and the yield is 53.7%. a light EXAMPLE 14 Pulp produced according to Example 13 is subjected to bleaching according to Example 2. The mass obtains a brightness of 71.4.

EXAMPLE 15 A mixture of 50% spruce chips, 25% recycled corrugated cardboard and% unsorted printed recycled newsprint is boiled according to Example 11 with a cooking liquid according to Example 5. The pulp has a coat number of 13.9, a brightness of 44.7 and the yield is 37.6%. .

EXAMPLE 16 The pulp produced in Example 15 is subjected to bleaching according to Example 2. The pulp obtains a brightness of 77.2. EXAMPLE 17 The mixture of raw materials according to Example 15 is supplemented with additional plastic materials (polyethylene and polyvinyl chloride) and metal staples and metal filings (iron, copper and bronze). This composition is boiled according to Example 11 with a cooking liquid according to Example 5. The metal contaminants decompose completely and the plastic materials form larger or smaller agglomerates, which can be easily filtered off in the primary and secondary screening equipment commonly used in conventional cleaning technology. The kappa number of the pulp is 17.6, the brightness 42.8 and the yield is 34.0% calculated on total weighed fiber material plus contaminants.

EXAMPLE 18 A mixture of 25% spruce chips, 37.5% recycled corrugated cardboard and 37.5% sorted printed recycled newsprint is boiled according to Example 11 with a cooking liquid according to Example 5. The pulp has a coating number of 10.7, a brightness of 70.2 and the yield is 38.1%.

EXAMPLE 19 The pulp produced in Example 18 is bleached according to Examples 2, 4, 6, 8, 10, 12, 14, 16 and 18. The pulp has a brightness of 70.2.

EXAMPLE 20 The bleached pulp from Example 4 is formed into standard sheets according to SCAN-C 26:76 R and the following strength properties are tested: tensile index is 75 and tear index is 6.8.

EXAMPLE 21 The bleached pulp from Example 6 is tested according to Example 20. The tensile index is 51 and the tear index is 3.2. EXAMPLE 22 The bleached pulp from Example 12 is tested according to Example 20. The tensile index is 44 and the tear index is 9.3.

EXAMPLE 23 The bleached pulp from Example 16 is tested according to Example 20. The tensile index is 46 and the tear index is 7.3.

EXAMPLE 24 The bleached pulp from Example 19 is tested according to Example 20. The tensile index is 61 and the tear index is 6.7.

EXAMPLE 25 A commercial decolorized return-based pulp having a brightness of 54.4 is tested according to Example 20. The tensile index is 42 and the tear index is 5.9.

Claims (10)

10 15 20 25 30 35 501 309 15 PATENT CLAIMS A process for the production of a cellulosic pulp usable for the production of paper and board whether used in admixture with fresh pulp or as a single pulp component, comprising the steps of: a) digesting a mixture of cellulosic-based recycled material and raw wood in a digestible form in an aqueous boiling liquid having a pH lower than about neutral and containing free and / or bound SO 2 and a base; b) performing digestion excluding atmospheric oxygen at elevated temperature, such as in the range of about 120 to 160 ° C and at a correspondingly elevated pressure; and c) recovering formed cellulose pulp. claim 1, wherein said about 5 to about 90 2. Process according to mixture consists of from percent by weight of raw wood. wherein said A method according to claim 2, raw wood constitutes a minor part of the mixture. content 30% by weight The method of claim 3, wherein the raw wood in said mixture is from about 10 to about percent. A method according to claim 1, wherein a small amount of the cooking liquid resulting from the digestion is recycled to step a). The method of claim 5, wherein said minor amount of cooking liquid constitutes about 1 to 10% of the total amount thereof. A method according to any one of the preceding claims, wherein the cooking liquid resulting from the digestion after concentration to a content solids of about 30 to 60% by weight is burned to produce energy. A method according to any one of the preceding claims, wherein the cellulosic pulp resulting from step c) is subjected to a chemical bleaching step to form a bleached, delignified pulp for the production of bleached paper or board. A method according to any one of the preceding claims, wherein the kappa number of the produced pulp is lower than the kappa number of both the recycled cellulose-based recycled material and the raw wood. Cellulose pulp prepared by the process according to any one of the preceding claims. 10 15 20 25 30 35