Patent Publication Number: US-2002003032-A1

Title: Method for pre-processing and processing pulp

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Technical Field  
       [0002] This invention relates to a method for pre-processing and processing lignocellulosic material into pulp.  
       [0003] 2. Background Information  
       [0004] There is increasing interest in the incorporation of non-wood cellulosic fibers into paper products. There is a tremendous amount of non-wood fibers available for pulp production. It has been estimated that the world straw production is approximately 500,000,000 metric tons per year. Only about half of that amount is used for low value purposes, such as building materials, fuel, and cattle feed. Most of the remaining amount is wasted by burning, without energy recovery, or plowing back into the ground. However, among the limitations in making straw into paper is incorporating the lower quality fiber derived from straw into a product with sufficient strength, durability, and brightness for a marketable end product.  
       [0005] U.S. Pat. No. 4,040,899, issued in 1977 to Emerson, attempts to address the use of straw pulp in paper. Emerson teaches intertwining and crimping fibers into a paper web.  
       [0006] According to Pulp and Paper, published 1952, 1960, by Interscience Publishers, Inc.,  
       [0007] “Straw is pulped by chemical processes and by a combination mechanical and chemical process known as mechano-chemical process. Among the different chemicals used for pulping straw are: (1) sodium hydroxide, (2) lime alone or in combination with other alkalies, (3) sodium sulfite plus other alkali, (4) chlorine, and (5) sodium hydroxide plus sodium sulfide (sulfite process). Other chemicals suggested for the pulping of straw are nitric acid and sodium chlorite. Sodium carbonate plus sulfur has been suggested for the preparation of a coarse pulp (straw Kraft) used for corrugated papers. There are four principal processes for making high grade bleachable straw pulp: (1) soda process, (2) sulfate process, (3) monosulfite process, and (4) the Pomilio chlorine process. The soda and sulfate processes produce good pulp, but in rather low yield.” 
       DISCLOSURE OF THE INVENTION  
       [0008] The present invention provides a method for pulping straw chips and forming a usable byproduct. According to the method, straw chips are mixed with steam and water to form a mixture. The mixture is exposed to temperatures and pressures sufficient to significantly soften the straw chips to form straw pulp, lignins, and hemicellulose when exposed to a rapid decrease in pressure. The mixture is washed to separate a portion of the lignins and hemicellulose from the straw pulp such that a usable byproduct of water, lignins and hemicellulose, referred to as black liquor, is formed and passed through a mechanical refiner.  
       [0009] The straw chips may be mixed with a caustic before the mixture of straw chips, steam and water is exposed to temperatures and pressures sufficient to significantly soften the straw chips to form straw pulp, lignins and hemicellulose in the mixture when exposed to a rapid decrease in pressure. The caustic may be present in the mixture in about less than two percent by weight. It is more preferred that the caustic is present in the mixture in about less than one-half percent by weight. The caustic may comprise sodium hydroxide. Also, the caustic may comprise potassium hydroxide.  
       [0010] In one form of the method, the mixture may be exposed to temperatures and pressures sufficient to significantly soften the mixture when exposed to a rapid decrease in pressure for between about two and one-half minutes to about eight minutes.  
       [0011] In yet another form of the invention, at least about seventy percent by weight of the straw chips are formed into straw pulp.  
       [0012] In yet another form of the invention, the pressure sufficient to significantly soften the straw chips when exposed to a rapid decrease in pressure is between about 140 psig and about 200 psig.  
       [0013] In another form of the invention, the fiber chips may be selected from non-wood fibers such as ryegrass, wheat and a mixture of grain and cereal straws.  
       [0014] Straw chips for use in the method may be formed by chopping straw to form a mixture of straw chips, meal, ash and grit and screening the mixture to separate a substantial portion of the straw chips from the meal, ash and grit. The meal, ash, and grit may be used as animal feed. In a preferred form of the invention, the meal, ash, and grit may be mixed with the black liquor byproduct to form animal feed.  
       [0015] The straw pulp formed by this method has a Canadian Standard Freeness between about 200 and about 600.  
       [0016] The present invention also provides a paper product made from wood pulp and straw pulp wherein the straw pulp is produced by mixing straw chips with steam and water to form a mixture of straw chips, steam and water. The mixture is exposed to temperatures and pressures sufficient to significantly soften the straw chips into straw pulp when exposed to decreased pressures. The mixture is then refined and a substantial portion of the straw pulp is separated from the mixture. A portion of the wood pulp and the paper product may be derived from processed post-consumer waste.  
       [0017] The straw chips may be formed by chopping straw to form a mixture of straw chips, meal, ash and grit and screening the mixture to separate a substantial portion of the straw chips from the meal, ash and grit.  
       [0018] In yet another form of the invention, the straw pulp has a Canadian Standard Freeness (CSF) of between about 200 and about 600 and an STFI between about 14 and about 21.  
       [0019] These and other advantages and features will become apparent from the detailed description of the best mode for carrying out the invention that follows. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0020] In the drawings, like element designations refer to like parts throughout the several views, and:  
     [0021]FIG. 1 is an elevational view of a rye grass plant;  
     [0022]FIG. 2 is a flow diagram of a pre-processing plant constructed according to the method of the present invention; and  
     [0023]FIG. 3 is a flow diagram of a pulping plant constructed according to the method of the present invention.  
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
     [0024] The present invention provides an improved method for pre-processing and processing various lignocellulosic source materials. Suitable lignocellulosic materials include non-wood fibers such as grain and cereal straw, and corn stover. However, the methods of the present invention have been found to be particularly suited for lignocellulosic material derived from grass and straw, such as annual ryegrass, fescue, and wheat. As used herein, straw chips may refer to chips derived from any non-wood fibers such as grain and cereal straw, grass and corn stover. However, for simplicity, all such chips shall be referred to as straw chips.  
     [0025] Referring first to FIG. 1, by way of example, a ryegrass plant  10  is shown growing in soil  12 . The ryegrass plant  10  has seeds or kernels  14  held within ears  16 , hollow stems or internodes  18  between joints or nodes  20 , and an extensively branching fibrous root system  22 . Sheath portions  24  are formed on the stems  18  and attach leaves  26  to the stems  18 .  
     [0026] During harvest, the ryegrass plants  10  are cut or swathed about three to six inches from the ground, generally below the first of the joints or nodes  20 . Then, the cut ryegrass plants are left to cure in the fields for a few weeks. Seeds or kernels  14  may be harvested from the cut plants by use of a combine. After the combine has removed the seeds or kernels  14 , the remainder of the ryegrass plants, generally known as straw  30 , is dropped to the ground. The straw  30  may be recovered from the field and baled for storage prior to further processing.  
     [0027] Referring to FIG. 2, for pre-processing, straw  30  is fed into a tub grinder  28 . The straw  30  may be fed into the tub grinder  28  in bales. The tub grinder  28  breaks up the bales and chops the straw into pieces averaging roughly one inch in length. As the chopped straw  32  exits the mill  34 , the chopped straw  32  includes nodes  20 , leaves  26 , sheath portion  24 , and internodes  18 , as well as ash, silica, and grit.  
     [0028] After exiting the tub grinder  28 , the chopped straw  32  is sent into a mill  34  to reduce the chopped straw  32  to pieces averaging roughly one-half inches in length. The mill  34  grinds most of the nodes  20 , leaves  26 , and sheath portion  24  into powder.  
     [0029] The chopped straw  32  may be stored in a storage tank  35  before being sent to the mill  34  if the capacity of the mill  34  does not permit the chopped straw  32  to be fed directly from the tub  28  grinder to the mill  34 .  
     [0030] After exiting the mill  34 , milled mixture  36  is passed through multilayer screens  37 . Meal  38  is dislodged and removed from straw chips  40  by passing the milled mixture  36  through the multilayer screens  37 . The meal  38  includes non-fibrous particles and ash, enriched with silica and grit. The meal  38  may be used as an agricultural byproduct for feeding livestock.  
     [0031] The straw chips  40  are then ready for processing into pulp. The straw chips  40  may be stored before further processing. In addition, if the pulp processing is to be carried out at a place remote from where the straw chips  40  have been preprocessed, the straw chips  40  may be transported by any manner used to transport bulk materials, such as by the use of containers.  
     [0032] Referring to FIG. 3, for processing the straw chips  40  into pulp, the straw chips  40  are fed into a mixer  42  where they are mixed with steam and water  44  and any caustic or digestive additives  46 . Typically, the amount of digestive additives  46  utilized is calculated on a weight basis per amount of dry straw chips entering the system. Scales may be incorporated into the pulping process so that the straw chips  40  may be weighed as they enter the system and the amount of digestive additives  46  may be administered at the appropriate rate.  
     [0033] Preferably, the amount of digestive additives  46  mixed with steam and water  44  and straw chips  40  is between about zero and about 2.0 percent by weight. However, it is more preferable that the amount of digestive additives  46  is between about zero and about 0.5 percent by weight. Suitable digestive additives  46  for use in the present invention include sodium hydroxide and potassium hydroxide.  
     [0034] From mixer  42 , a stream  48  composed of straw chips  40 , steam and water  44 , and any digestive additives  46  is then fed into reactor  50 . In reactor  50 , stream  48  is exposed to temperatures and pressures sufficient for the steam  44  and any digestive additives  46  to penetrate the straw chips  40  such that when exposed to a rapid decrease in pressure, a mixture of softened chips, cellulose, lignins, hemicellulose and water is formed.  
     [0035] Suitable reactors for use in the present invention include continuous feed reactors such as those manufactured by Stake Technology, Ltd. of Oakville, Canada, and described in U.S. Pat. Nos. 4,798,651 and 4,947,743, the contents of which are incorporated herein by reference.  
     [0036] In the present invention, reactor products  52  may be exposed to a rapid decrease in pressure by passing the reactor products  52  into a blow tank  56 . The reactor products  52  may be metered from the reactor  50  into the blow tank  56  by a blow valve  54 . The reactor products  52  include softened straw pulp mixed with lignins, hemicellulose and water. In other words, straw chips are steam exploded as they pass through the blow valve  54  when exiting the reactor  50  to form straw pulp, lignins and hemicellulose.  
     [0037] Stream  58  from the blow tank  56  is sent into a screw press  59  where stream  126  of a straw pulp mixture is separated from the steam exploded straw chips. Stream  126  is composed substantially of black liquor and undissolved solids. Generally, stream  126  is substantially composed of water, reacted lignins and hemicellulose. Stream  126  is sent to a byproduct chest  142 . Stream  144  from byproduct chest  142  is passed through filtration system  128 . Solids  130  removed at filtration system  128  are sent to blow tank  56 . Liquid stream  138  from filtration system  128  are collected in storage tank  140 .  
     [0038] The black liquor collected in storage tank  140  constitutes a usable byproduct. This byproduct has usefulness as an animal feed additive. In a preferred form of the invention, the byproduct may be mixed with meal  38  from preprocessing to form animal feed. The animal feed may be formed into a mash or pellets by methods known in the art.  
     [0039] Stream  61  from screw press  59  is then defiberized. The defiberization may be carried out by a mechanical refiner  60 , such as an Ahlstrom MDR (frotopulper).  
     [0040] The pulp is then ready to be washed. The pulp may be washed by any method, such as dilution and extraction. For example, the refined pulp mixture  62  may be sent to a dilution tank  64  where it is diluted with water  73 . Then, diluted pulp mixture  66  from tank  64  may be thickened by use of a wash press  68 . Weak black liquor in stream  69  from wash press  68  is sent to blow tank  56 .  
     [0041] Thickened pulp  70  may then be screened to remove unwanted particles. In preparation for the screening, the thickened pulp  70  may be sent to a prescreening dilution tank  72  where it is mixed with water  73 . Diluted pulp  74  from the dilution tank  72  may then be passed into a primary screening  76 . Accepts  78  from the primary screening  76  may be collected in accepts tank  84 .  
     [0042] Rejects  86  from primary screening  76  may be passed to a primary screen rejects tank  90 . Rejects  92  from rejects tank  90  may be passed to a secondary screening  80 . Accepts  82  from secondary screening  80  may be collected in accepts tank  84 . Rejects  88  from secondary screening  80  are sent to drainer  146 .  
     [0043] Drained stream  148  is sent to a secondary rejects tank  150 . Stream  152  from secondary rejects tank  150  is sent to a rejects defiberizer  154 . Stream  156  from the rejects defiberizer  154  is collected in a tertiary screen feed tank  158 . Stream  160  from the tertiary screen feed tank  158  is passed through a tertiary screening  162 . Accepts  164  from tertiary screening  162  are collected in the rejects tank  90 . Rejects  166  from tertiary screening  162  are sent to the drainer  146 .  
     [0044] The nature and substance of the instant invention as well as its objects and advantages will be more clearly understood by referring to the following specific examples.  
     Example 1  
     [0045] Annual rye grass straw is obtained. The protein level of the straw is determined to be about three to four percent (3%-4%). The moisture content of the rye grass straw is determined to be approximately ten percent (10%). The annual rye grass straw is preprocessed and pulped according to the method described in the best mode of this application and as set forth herein.  
     [0046] Bales of the rye grass straw are fed into an agricultural tub grinder where the bales are broken up and the straw chopped into pieces averaging roughly one (1) inch in length. The agricultural tub grinder has grate plates with round holes sized between about one half (0.5) inches and about two (2) inches.  
     [0047] After exiting the agricultural tub grinder, the chopped straw is sent into a disc mill. The disc mill is a United Milling Systems mill which has milling gaps of between about 0.6 mm and about 1.0 mm. In the disc mill, the chopped straw is reduced into approximately one-half inch pieces. Also, the disc mill grinds most of the hard node, leaf and sheath of the straw into a powder. Ash and grit are determined to compose approximately two to five percent (2%-5%) of the straw fed into the tub grinder.  
     [0048] After leaving the milling machine, the straw is passed into a vibrating six-deck screen of  35  mesh. The screen is a gyratory screen equipped with fine mesh and ball decks for cleaning fines and dirt from the surfaces of the screen. Approximately eighteen percent (18%) of the straw mixture is removed at the screen as meal. Part of the meal removed is the powder formed in the disc mill by the hard node, leaf and sheath of the straw. The remainder of the straw chips leaving the screen averages approximately one half (½) inch in size.  
     [0049] The meal has a protein level of about eight to thirteen percent (8%-13%). The silica content is between about 8-15%. It is determined that the meal removed at the screen is usable as animal feed.  
     [0050] The preprocessed straw chips are then stored prior to pulping. The straw chips are preprocessed at a site remote from the pulping system. The preprocessed straw chips are transported to the pulping system in container trucks.  
     [0051] The preprocessed straw chips are fed into a pulping system at a rate consistent with the capacity of the pulping system. The preprocessed straw chips are fed into a mixer where they are mixed with steam and water to achieve a moisture content of about 40-50% based on the charge of preprocessed straw chips to the mixer. The preprocessed straw chips mixed with steam and water are fed into a Stake Digester steam explosion reactor. In the reactor, the preprocessed straw chips are exposed to pressure of about 160 psig for about five and a half (5½) minutes.  
     [0052] Pulp from the reactor is metered into a blow tank by a blow valve. Pulp from the blow tank is passed through a screwpress where black liquor is separated from the pulp. Then the pulp is defiberized or refined in an Ahlstrom frotopulper mechanical refiner. The refined pulp is sent to a dilution tank where it is mixed with liquid at a dilution factor of about 2.5. The diluted pulp is then thickened by use of a wash press. The thickened pulp is screened in a two-stage pressure screen with about 0.010 inch slotted plates.  
     [0053] It is determined that at least about seventy percent (70%) by weight of the straw chips are formed into straw pulp.  
     [0054] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI (Swedish Technical Forestry Institute) between about 14 and 21. As a comparison, OCC (pulp derived from recycled corrugated cardboard) is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 500 and STFI of between about 15 and about 22. Kraft pulp is determined to have a Canadian Standard Freeness (CSF) of between about 450 and about 750 and STFI of between about 21 and about 26.  
     [0055] Straw pulp with a Canadian Standard Freeness (CSF) between about 200 and about 600 and STFI between about 14 and about 21 is suitable to be blended with Kraft pulp and OCC to make liner board in standard mill grades. The straw pulp comprises about less than twenty percent (20%) of the liner board furnish.  
     Example 2  
     [0056] Example 1 is repeated using about one-half percent (0.5%) by weight sodium hydroxide in the mixer.  
     [0057] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     Example 3  
     [0058] Example 1 is repeated using about one percent (1%) by weight sodium hydroxide in the mixer.  
     [0059] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     Example 4  
     [0060] Example 1 is repeated using two percent (2%) by weight sodium hydroxide in the mixer.  
     [0061] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     Example 5  
     [0062] Example 1 is repeated using about one-half percent (0.5%) by weight potassium hydroxide in place of the sodium hydroxide in the mixer.  
     [0063] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     Example 6  
     [0064] Example 1 is repeated using about one percent (1%) by weight potassium hydroxide in place of the sodium hydroxide in the mixer.  
     [0065] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     Example 7  
     [0066] Example 1 is repeated using about two percent (2%) by weight potassium hydroxide in place of the sodium hydroxide in the mixer.  
     [0067] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     Example 8  
     [0068] example 1 is repeated allowing the preprocessed straw chips mixed water to remain in the reactor for about two and one-half minutes.  
     [0069] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     Example 9  
     [0070] Example 1 is repeated allowing the preprocessed straw chips mixed water to remain in the reactor for about 4 minutes.  
     [0071] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     Example 10  
     [0072] Example 1 is repeated allowing the preprocessed straw chips mixed with water to remain in the reactor for about six (6) minutes.  
     [0073] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     Example 11  
     [0074] Example 1 is repeated allowing the preprocessed straw chips mixed water to remain in the reactor for about eight (8) minutes.  
     [0075] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     Example 12  
     [0076] Example 1 is repeated with the preprocessed straw chips mixed with water to be exposed to a pressure of about 140 psig in the reactor.  
     [0077] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     Example 13  
     [0078] Example 1 is repeated with the preprocessed straw chips mixed with water to be exposed to a pressure of about 180 psig in the reactor.  
     [0079] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     Example 14  
     [0080] Example 1 is repeated with the preprocessed straw chips mixed with water to be exposed to a pressure of about 200 psig in the reactor.  
     [0081] After exiting the process, the straw pulp is determined to have a Canadian Standard Freeness (CSF) of between about 200 and about 600 and STFI between about 14 and about 21.  
     [0082] While specific embodiments of the present invention have been shown and described in detail to illustrate the utilization of the inventive principles, it is to be understood that such showing and description have been offered only by way of example, and not by way of limitation. Protection by Letters Patent of this invention in all its aspects is set forth in the appended claims and is sought to the broadest extent that the prior art allows.