FIELD CF THE INVENTION

The present invention relates to a process for producing lignocellulosic pulp in accordance with tne preamble to the attached Claim 1.

BACKGROUND OF THE INVENTION

The pulp and paper industry uses many processes to produce pulp from lignocellulosic materials such as softwoods, hardwoods or annual plants . These are used for the manufacture of several paper grades sucn as newsprint, speciality papers (LWC, SC, ... ) and fine papers (Art, MFC, ... } .

Chemical pulps used m fine papers are prepared by cooking the wood chips at elevated temperatures and pressures with various chemical agents such as Na ₂ S and NaOH at high pH (Kraft pulping) and Na^SO _j /NaHSO _j for sulfite/bisulfite pulping at neutral pH. In these processes, the purpose of the chemical agents is to degrade and dissolve the lignin and the hemicellulose material m wood to leave pure cellulose fibers m the 40-45% yield level. Following this process, chemical fibers are bleached to high levels such as 85-90% ISO- brightness. The fibers are flexible and high strength papers can be produced from them. Chemical pulps are therefore also used in producing newsprint and speciality papers to provide dry strength to the sheet to ensure good runabiiity in the press room. There are several problems associated with chemical pulping, however, with decreasing forest resources for instance, the resulting low yield from the process implies that a substantial amount of trees must be harvested. Furthermore, chemical puloing processes require high levels of fresh water and toxic chlorinated by-products (dioxm and AOX) are found in the effluent. Odour emissions are also noteworthy. Sulfur

CONFIRMATION COP/

emissions contribute to acid ram, wnich is responsible for forest deterioration and depletion of tne soil

Mechanical pulps of inferior strεngtn are also used m tne manufacture of newsprint and speciality papers because of their high lignt scattering coefficient or opacity as well as high oil absoraancy whicn is required for ink acceptance during printing. In the production of these pulps, mecnanical means are used For instance, grindstones are used to defiber logs the stone groundwooα process (ΞG) . Disc refiners are also used to defi er wood chips into pulp. Wood chips can be refined without any specific treatment such as m refiner mechanical pulping (RMP ¹ The wood chips can also oe thermally treated at temperatures oetween 100 to 130°C prior to ce g refmeα such as in tne tnermomecnanical process (TMP) In these processes, approximately five percent cf tne wood suostancε tnat s water soluble is lost for a net yield of approximately 95%. The strengtn of the resulting pulps is directly related to the degree cf mechanical energy applied to the fibers the form of shear stresses in the refining zone The energy transferred to fibers is used to increase their flexibility The internal cell wall dislocation or fibrillation resulting from oendmg and tcrsional stresses imposed onto fibers tne refining zone enhances tneir flexibility, bat often this increased flexibility s ootamed with reduction fiber length. It is generally recognized that there are three problems in the manufacture of refiner mechanical pulp. One is tne reduced strength of the paper formed from the pulp because the fibers are chopped and abraded. The second is tne nign electrical demand cf the refiner or hign specific refining energy The third is the low brightness of tne pulp produced.

Mechanical pulps lac the desirable strength properties to replace, whole or m part, low-yield cnemical pulps, e.g raft or sulphite pulps, Imerooard, newsprint, tissue, printing graαes and coated-base grade of paper Consequently,

it has been an aim of the art to improve the pr.ysicai and optical properties of mechanical pulps, so that such improved pulps would be used to replace low-yield chemical pulps. The major problem has been the significant quantity of electrical energy to comminute the wood chips into a useable pulp.

Impregnating wood chips with chemicals such as sulfite has been used to improve the strength and optical properties of refiner mechanical pulps. For instance, U.S. Patent 3,446,699 issued May 27, 1965 to Asplund et al . provides a method for producing mechanical and chemimechanical or semi-chemical pulps from lignoceilulose-containing material, in order to provide lnrcroved quality of the fibers with improved defloration. U.S. Patent 3,558,428 issued Jan. 25, 1971 to Asplund et al . provided a method for manufacturing chemimechanical pulps involving heating and defibrat g the same in an atmosphere of vapour at elevated temperature and under corresponding pressure of the impregnated chips to provide a more rapid and effective impregnation. In this chemi- hermomechanical pulping processes (CTMP) m which the yield range is between 88-92%, the wood chips are treated with relatively small amounts of sulfite and bisulfite which modifies the lignin by sulfonat g it sufficiently to produce a marked change its physical and chemical properties which give rise to brightness and strength improvement. However, the lignin is not made completely soluble water or in the cooking liquor so that relatively high yields are obtained. Chemical impregnation of chips also results lower debris but usually increases the long fiber content and reduces the scattering coefficien . Increases in energy requirement of 15-25% are observed to reach a given drainage or CSF level .

Falling midway between the chemical and mechanical processes are the sem -chemical and chemi-mechanical pulping processes (CMP ¹ . The yield range of the resulting pulps is between 60-85% so that good pulp properties are achieved at low refining

energy by cooking the wood chips (or other celiuiosic material) under severe conditions. Typical temperatures are tne 160- 24C°C range with high chemical charges to degrade a great deal of the lignin and hemicellulose . Cooking times range between one to four hours. As an example, U. S. Patent 4,115,758 issued Sept. 26, 1978 to M.J. Ford provided a process for producing high-yield chemimechanical pulps from woody lignoceii lose material by treatment with an aqueous solution of a mixture of sulfite and bisulfite, to provide a pulp which can be readily defibered by customary mechanical means. The resulting puio has different characteristics depending on the yield and refining energy applied as demonstrated by M. Barbe et al . , Journal of Pulp and Paper Science, 12(5) : J141-146 (September 1986) .

An alternate approach to improve the quality of mechanical pulps has been to treat the refiner pulp fibers. Two processes are currently used commercially. In the first process, described by A. Barnet ec al . , Pulp and Paper Canada 81, T255- T260 (1980; , TMP pulp fibers are being treated at 160°C for 45- 60 minutes in a reactor with sodium sulfite charges of 12-14% at pK of 9.8. The yield obtained is in the range of 65-88%. The refining energy to a given freeness is reduced while the strength properties are improved. The high chemical charges, low yield and therefore high effluent loads combined with the high cost for a reactor of appreciable size, as well as the dewater g or washing equipment required, offsets the benefits achieved. Therefore, this process alternative has not received wide commercial acceptance in the industry. In the second process, described by M. Barbe et al . , World Pulp and Paper Technology 1991, Frank Roberts, editor, Sterling Publications International Limited, London (1991) , CTMP pulp fibers are being treated at medium consistency (12%) and at 60°C for 120 minutes with an alkaline peroxide liquor at pH of 10-11.

The yield obtained with this process is in the range of 80-85%. Savings in refining energy to reach a given freeness are

obtained. Tne drawoac s of this latter process are similar to those of tne first process alternative The low yield, high effluent loads and cnemical charges combined witn the nigh costs for bleaching towers with two nours retention t me, with their discharge mecnanisms as well as the cost for the dewater g or washing equipment required, offsets the oenefits achieved. Therefore, this process alternative has not received wide commercial acceptance in tne industry.

The low or ghtness of mechanical pulps is clearly a major problem which has limited their use m different paper grades Several processes are currently employed m the pulp and paper industry to bleach mechanical or cnemithermomecnanicai pulps. One process uses a reducing agent sucn as dithionite or sodium ana zinc nvdrosulfite (Y) to orignten or bleach the pulps. With this bleacnmg chemical moderate gams of 4 to 10 points are obtained. Maximum orightness levels of 68 to 70% ISO can be reached with the addition of approximately 1% to 1.25% on o.d. pulp of sodium hydrosulfite. The process is usually carried out in an aqueous phase at 3 to 5% consistency, a pH of 4.5 to 6.0, a temperature of about 60°C and a retention time of up to one hour. The use of a chelating or sequestering agent such as sodium tπpolypnospnate (STPP) to remove naturally occurring trace metals is recommended. Peroxiαe (P) is alsc employed for bleaching. The process is carried cut a single tower or in two bleaching towers. In both cases, the bleaching is done at a pulp consistency of 15 to 35%, moderate temperatures of 50 to 70°C, and retention times of two to three nours for each stage. Stabilizers such as sodium silicate and magnesium sulfate are added to the bleacn liquor to prevent peroxide decomposition. Sodium hydroxide is also used to maintain an alkaline pH of 9.5 to 11 so as to increase the concentration of the perhydroxyl ion OOH which is believed to be tne active bleacn g agent . Furthermore, pulps are normally pretreated at low consistency with organic cneiat g agents such as sodium dietnylenetnamme penta-acetate (DTPA) to remove trace metals.

All of tne aoove bleaching processes involve tne addition of high chemical charges and hign costs m order to acnieve the desired oπgntness levels. However, these permit to extend the use of the mechanical or chemimechanical pulps m a wide range of paper products. High capital costs n equipment are required.

It is well known that the physical properties of wood pulps are strongly influenced by tne flexibility of tne individual fibers. In the papermakmg process, the fibers are brougnt into close contact with each other oy surface tension forces the water removal process or during tne pressing stages of tne process FlexiDie fibers tnerefore lead to oetter ponding and improved strength Natural wood fibers are flexmle due to the presence of large amounts (20-30% by weight) cf lign and crystalline cellulose (30-40% by weight) All of tne prior art indicate that fiber flexioility has been improved conventional mecnanical pulping by applying mechanical energy to the fibers and chemical or se ichemical or cne i- mechanical pulping processes by removing part, or nearly all, of the lignin or some other cases by modifying its cnemical nature with sulfonation in addition to tne mecnanical energy. However, mechanical pulping the increased fiPer flexioility and sneet strength is limited as the increase flexibility is done with cell wall dislocations that reduce fiber strength and fiber length. In chemimechanical pulping, or with tne existing pulp fiber treatment process, the increased fiber flexibility and sheet strength is obtained by reducing tne yield of the pulp.

It is therefore an object of the present invention to provide a process whicn permits the treatment of lignocellulosic materials so as to render the fibers flexible and conformable with low refining energy while improving the strength and brightness cf tne resulting pulp addition to maintaining the materials yield for tne production of pulp from weed chips and

other lignocellulosic materials. In addition to tne savings obtained with low refining energy, saving can also be obtained in capital cost requirements and bleaching chemicals when the trea _t ed pulps are bleached to high levels with the current state of the art bleaching technology. Conversely, high brightness levels can be reached at a given bleaching chemical charge, therefore permitting to improve the quality of mechanical pulps to extend their use in the production of high quality paper grades m replacement of chemical fibers.

This ob _j ect is obtained by the following process cf the type described the preamble to Claim 1 which is characterized by the following steps:

a) mixing the lignocellulosic material with a ^p uffer or chemical solution in a high shear mixer-reactor so as to adjust the pH of the pulp, while maintaining the pulp at high consistency,

b) submitting the pulp to high temperature for times and chemical charges sufficient to reduce the long range crys _t alline structure of the cellulose com ^p onent of wood, and

c) subsequent by subjecting the lignocellulosic pulp fibers to refining to produce a pulp with the desired end- product properties.

The present invention will be explained in more detail with reference to the drawings of which:

Fig. 1 shows the energy reductions obtained when pulps are trea _t ed in accordance with the present invention as a function of the final pH of the pulp,

Fig 2 shows the energy reductions obtained as a function of the total ionic content or yield,

Fig 3 shows the energy reduction/increase to oe expectεα upon changes or variation m the sulfonate content of the fibers obtained with the present state of the art CTMP or CMP technology,

Figs 4 and 5 are process flow diagrams illustrating the various steps of a continuous operation m which pulps are treated according to the present invention

While the invention is not limited to any tneory, _t is believed that tne process proviαes tne savings refining energy and improveα properties by reducing the long range crystaiiinity of cellulose and by the cnemical degradation of the lignm, hemicellulose and cellulose components of wooα Figure 1, showing energy reductions obtained following the treatment of the pulps according to the present invention, as a function of the final pH of the pulp, as well as Figure 2 showing energy reductions obtained as a function cf the total ionic content or yield, indicate that savings can be ootameα at low ionic content or high yield with treatments at low pH as well as at high total ionic content and hign pri Under o pH and ionic content conditions, it is believed that changes m the long range crystaiiinity of the crystalline structure of the cellulose prevails to give the savings refining energy The cnemicals or buffers used inhibit the acid hydrolysis of the poiysaccnarides and therefore preserves fiber strength. At high pH or high ionic content, tne chemical degradation of tne lignm, hemicellulose and cellulose may prevail addition to the cnanges in the crystalline structure of cellulose Figure 3 shows tne energy reduction/increase to be expected upon cnanges or variations tne sulfonate content of the fioers obtained with tne present state of the art CTMP or CMP technology

Figure 4 describes an application of the process by which a refiπeα pulp obtained directly at the outlet of a refiner ill is blown a high shear mixer (2) n wnicn a buffer or a chemical solution is added to adjust the pH of tne pulp. The conditions for the treatment temperature or pressure as well as the desired retention time and chemical charge in the high shear mixer (2) are to be selected according to the initial pulp pH, wood species and desired benefits either strength, optical or refining energy reduction to be sought. Subsequently, the treated pulp is mecnanically defiberized in a second refiner (3) which blows the pulp to a pressurized or atmospneric cyclone (4) .

Figure 5 describes an application of the process by which a refiner pulp obtained at the discharge of a dewater g device such as a screw press (5) , is fed with a pulp plug screw (6) into the high shear mixer (7) . Buffers or a chemical solution is added to adjust the pH of the pulp while the treatment temperature or pressure as well as the desired retention time and chemical charge is set to achieve the desired benefits. Subsequently, the treated pulp is mechanically defibered in a refiner (8) and blown into a pressurized or atmospheric cyclone (9) .

THE FOLLOWING EXAMPLES ARE GIVEN TO ILLUSTRATE MORE CLEARLY VARIOUS EMBODIMENTS OF THE INVENTION.

Example 1

In this example, as well as subsequent examples, the following experimental procedure was used for the treatment of pulps . The different chemical agents were mixed with a refined pulp m a commercial high shear mixer, treated in a digester and subsequently refined on a 30 cm single disc refiner The control pulps coded CB3N were only thermally treated m the digester prior to refining The control pulps coded C-6N were

simply refined the single disc refiner without tnermal treatment .

This example is intended to illustrate that when pulp fibers are treated in the presence of acids or bases to adjust tne pH, as described for aspects of this invention, refining energy is reduced appreciably while the strength properties are improved.

Table 1 gives the quality of pulps treated compared to the treated pulp without buffers or chemicals (coded C3-3N) and the untreated pulp (coded C-6N) .

The pulps coded C-6N give the properties of a pulp as produced by the current state cf the art. The pulp coded CB-3N gives the properties of a pulp submitted to a thermal treatment without the addition of a buffer. The pulps coded A1-4N, A2-4N and 3-4N give tne properties of a pulp submitted to a thermal treatment with the addition of NaOH or H ₂ S0 ₄ , to adjust the pH as per the precept of this invention. It can be observed that reduction of refining energy of 25 to 64% are obtained compared to the untreated pulp. Higher energy reductions are obtained at high and low pH adjustment. It is preferable to treat the pulp at high pH levels to obtain high strength properties such as tensile index and breaking length. However, the tear index remains low. The pH adjustment allows the optimization of different pulp quality under conditions m order to obtain high yield pulps . Table 2 shows tne effect of conventional peroxide bleaching on the pulp and paper properties for the pulps treated with H ₂ S0 ₄ and the pulp treated without buffers or chemicals . The pulps treated at low pH show higher brightness level compared to the pulp treated without buffers or chemicals at a given chemical charge and higner brightness level s also reached following a peroxide bleacning stage. Therefore, reductions m peroxide consumption are expected to be reached at a given brightness level wnen the pulps are treated at low

pH with buffers, compared to pulps treated without buffers or chemicals .

Example 2

This example illustrates the effect of pH of treatment upon adjustment with C0 ₂ and calcium hydroxide (Ca(OH),) .

Table 3 gives the quality of pulps treated compared to the treated pulp without buffers or chemicals (coded C3-3N) and the untreated pulp (coded C-6N) .

The pulps coded C-6N give the properties of a pulp produced by the current state of the art. The pulp coded C3-3N gives the properties cf a pulp submitted to a thermal treatment without the addition of a buffer. The pulps coded H2-5N and H3-6N give the properties of a pulp submitted to a thermal treatment with the addition of C0 ₂ and C0 ₂ /Ca<OH) ₂ _ to adjust the pH as per the precept of this invention. It can be observed that refining energy reductions of 13 to 28% are obtained compared to the untreated pulps while maintaining a high yield. The strength properties of the treated pulps are lower compared to the untreated pulps. However, with the heat treatment, reductions in energy consumption are obtained with the use of C0 ₂ or C0 ₂ /Ca(OH) _; together with improved strength properties and brightness of the pulps. In addition to the above benefits, the combined use cf the C0 ₂ or C0 ₂ /Ca(OH) ₂ give rise to the formation cf CaC0 ₃ which is used as a filter in the pulp and paper industry to confer high opacity to paper.

Example 3

This example illustrates the effect of sodium sulfite (Na ₂ S0 ₃ ) liquor used to control the pH of the heat treatment . In this example, pulps were heat treated with H ₂ S0 ₄ and NaOH in combination with the Na ₂ S0 ₃ to control the pH.

Table 4 gives the quality cf pulps treated compared to the treated pulp without buffers or chemicals icoded C3-3 ; and the untreated pulp (coded C-6N) .

The pulps coded C-6N give the properties of a pulp produced by the current state of the art. The pulp coded C3-3N gives the properties of a pulp submitted to a thermal treatment without the addition of a buffer. The pulps coded D1-3N, D2-7N, and D3- 7N give the properties of a pulp submitted to a thermal treatment with the addition of Na ₂ S0 ₃ and H ₂ S0 ₄ or NaOH, to adjust tne pH as per the precept of this invention.

It can be observed that refining energy reductions of 30 to 45% are obtained compared to the untreated pulps . The maximum energy reductions are obtained at low and high pK. It is preferable to treat the pulp at high pH to reach high strength properties such as tensile index or breaking length. However, the tear index is lower compared tc untreated pulps . The pK adjustment allows the optimization of different pulp quality under different conditions while the high yield of the pulps is maintained. Table 5 shows the effect of conventional peroxide bleaching en the pulp and paper properties for the pulps treated with Na,S0 ₂ at low pK and the pulp treated without buffers or chemicals. The pulps treated at low pH show higher brightness level compared to the pulp treated without buffers or chemicals. Higher brightness level is also reached following a peroxide bleaching stage. Therefore, reductions in peroxide consumption are also to be expected to reach a given brightness level when the pulps are treated with Na ₂ S0 ₃ at low pH compared to pulp treated without buffers or chemicals .

Example 4

This example illustrates the effect of reducing agents such as sodium nvdrosulfite (Na ₂ S ₂ 0 ₄ ) , sodium borohydride (NaBH.) and

formamidine suifinic acid (FAS; to control the pH of the pulp during heat treatment

Table 6 gives the quality of pulps treated compared to tne treated pulp without buffers or chemicals icoded C3-3N) and the untreated pulp (coded C-6N)

The pulps coded C-6N give the properties of a pulp proαuced py the current state of the art. The pulp coded CB-3N gives the properties of a pulp supmitted to a tnermal treatment without the addition of a buffer. The pulps coded C-1-5N, G2-4N, G3-5N, and H1-5N give tne properties of a pul submitted to a thermal treatment with the addition of Na ₂ S ₂ 0 ₄ and NaBH ₄ or FAS, to adjust tne pH as per the precept of this invention

It can be observed that refining energy reductions of 30 to 35% are obtained compared to the untreated pulps while maintaining a high yield. The strength properties of the treated pulps are lower compared to the untreated pulps. However, as a heat treatment reduces the energy consumption, it is shown that the addition cf reducing agents may also improve tne strength properties of the pulps. Under acidic conditions, tne brightness of the pulps is also improved compared to treated pulps without puffers or cnemicais Table 7 snows the effect of conventional peroxide bleaching en the pulp and paper properties for the pulps treated with NaBH ₄ at high pH and tne pulp treated without buffers or chemicals. The pulps treated at high pH show similar bπgntness level compared to tne pulp treated without buffers or chemicals. However, higher brightness levels (approximately 10 points) are reached following a peroxide bleaching stage. The pulps treated with a reducing agent have a better brightness response to peroxide bleaching compared to pulp treated with heat only. Therefore, reductions peroxide are also expected to reacn a given brightness level when tne pulps are treated witn a reducing

agent at high pH compared to pulps treated witnout buffers or chemicals .

Example 5

This example illustrates the effect of oxidizing agents such as peroxide (H ₂ 0 ₂ ) , oxygen (0 ₂ ) and a mixture of peroxide and oxygen. In this example, the pH of the pulps were adjusted with NaOH used with the H ₂ 0 ₂ solution.

Table 8 gives the quality of pulps treated compared to tne treated pulp without buffers or chemicals (coded CB-3N; and tne untreated pulp (coded C-6N) .

The pulps coded C-6N give the properties of a pulp produced by the current state of the art The pulp coαed C3-3N gives tne properties of a pulp submitted to a thermal treatment without the addition of a puffer. The pulps coded D1-5N, F2-4N, and F3-4N give the properties of a pulp submitted to a thermal treatment with the addition of oxidizing agent and NaOH.

It can be observed that refining energy reductions of 11 to 27% are oota ed compared to the untreated pulps The maximum energy reductions are ootamed at h gh pH. It is preferaole to treat the pulp at hign pH levels to obtain high strength properties such as tensile index and breaking length. The pH adjustment allows the optimization of different pulp quality under conditions while high yield pulps are produced. In order to reduce the refining energy and to improve the strength properties as well as the brightness of the pulps, it is preferable to como e the thermal treatment with the addition of oxidizing agents. However, the tear index remains lower. Table 9 shows the effect of conventional peroxide bleaching on the pulp and paper properties for the pulps treated with oxidizing agents and tne pulp treated without buffers or chemicals . The pulps treated with oxidizing agents show higner

brigntness level compared to the pulp treated witncut puffers or chemicals. Higher brightness levels are also reached following a peroxide bleaching stage. Therefore, reductions in peroxide are also expected at a given brightness level when the pulps are treated with a reducing agent compared to the pulps treated without buffers cr chemicals.

Example 5

This example illustrates the effect of oxidizing agents such as peracids made from a mixture of peroxymonosulfuric acid or Caro's acid (H ₂ S0 ₅ and peracetic acid (CH ₃ C0 ₃ H) . In this example, pulps were treated with adjustment cf the pH with NaOH m ccmomacion with tne mixture of H ₂ S0 ₅ :CH ₃ C0 ₃ H.

Table 10 gives the quality of pulps treated compared to tne treated pulp without buffers or cnemicals (coded C3-3N1 and the untreated pulp (coded C-6N) .

The pulps coded C-6N give the properties of a pulp produced by the current state of the art. The pulp coded C3-3N gives the properties of a pulp submitted to a thermal treatment without the addition of a buffer. The pulps coded I1-4N, I2-3N, and I3-4N give the properties of a pulp submitted to a tnermal treatment with the addition of oxidizing sucn as a mixture of H ₂ S0 ₅ , NaOH, and NaOH to adjust the pH as per the precept of this invention.

It can be observed that refining energy reductions of approximately 20% are obtained compared to the untreated pulps. To obtain full benefits m energy reduction, addition to strengtn and optical properties, it is preferable to combine the heat treatment with the addition of oxidizing agents such as a mixture of Caro's acid and peracetic acid under acidic conditions. Table 11 shows the effect of conventional peroxide bleaching en the pulp and paper properties for the pulps

treated with oxidizing agents and the pulp treated without buffers or chemicals. The pulps treated show lower brightness level compared to the pulp treated without buffers or cnemicals but higher brightness level is reached following a peroxide bleaching stage. Therefore, reductions in peroxide are also expected at a given brightness level when the pulps are treated with peracid agents compared to the pulps treated without buffers and chemicals. The pH adjustment allows the optimization of different pulp quality under selected conditions while retaining their yield pulps.

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