WO2013177487A2

WO2013177487A2 - Method of producing pulp using oxalic acid

Info

Publication number: WO2013177487A2
Application number: PCT/US2013/042588
Authority: WO
Inventors: Masood Akhtar; Eric Gene HORN; Ross Edward SWANEY; Zhiyong Cai
Original assignee: Biopulping International, Inc.
Priority date: 2012-05-25
Filing date: 2013-05-24
Publication date: 2013-11-28
Also published as: WO2013177487A3

Abstract

The present disclosure is related to methods of producing pulp from wood using an application of oxalic acid prior to mechanical defibration/refining.

Description

METHOD OF PRODUCING PULP USING OXALIC ACID

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority to United States Provisional Patent Application No. 61/651,782, filed on May 25, 2012, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] In manufacturing paper and other fiber composite products from wood, the wood is first reduced to an intermediate stage in which the fibers in the wood are separated from their natural environment and transformed into a thick liquid suspension called pulp.

[0003] Pulp may be produced from various types of woods using any one of several pulping techniques. The techniques are generally categorized as either chemical pulping (kraft or sulfite pulping), or mechanical pulping. In chemical pulping methods, a chemical/water solution is generally used to dissolve the lignin portion of the wood that binds the fibers together, allowing the separation of the fibers with little or no mechanical action. In mechanical pulping methods, a mechanical operation grinds or abrades the wood in water until a desired state of freeness is achieved between its constituent fibers and the fibers have been reduced to the desired size.

[0004] Mechanical pulp methods generally include ground wood, thermomechanical pulping (TMP), refiner mechanical pulping (RMP), and hybrid techniques based on applying mechanical pulping as a subsequent stage of conversion to fiber applied after a prior stage of chemical "cooking" treatment such as in chemimechanical or chemithermomechanical pulping

(CMP,CTMP). The ground wood process grinds the whole wood against abrasive stones to produce fibers. The mechanical method most favored in industrial production is TMP, wherein wood chips are preheated and then fed to a specialized rotary mechanical device known as a refiner or defibrator (hereinafter known as "defibrator/refiner"). In the TMP refiner, the feed material is destructured and defibrated/refined at elevated temperature in the presence of pressurized steam. The RMP method is similar except that the refiner is not operated under pressurized conditions. Most commonly, the feed material is subjected to a sequence of two or more stages (or passes) of mechanical refining, using either TMP, or RMP, or a sequence of both. The pressure, temperature, number of stages, and severity of mechanical action is selected to achieve the desired state of freeness between the fibers and obtain fibers of the desired properties.

[0005] The operation of mechanical pulping refiners uses large amounts of electrical power. This operating cost is a major factor in the production economics of mechanical pulp. Early efforts to reduce this energy consumption lead to the CTMP process, which in some cases uses 10-20% less energy than TMP. However, the CTMP process has several drawbacks, including large amounts of waste effluents and darkening of the pulp product, and is infrequently employed. A chemical pretreatment process employing oxalic acid used in a cooking process stage prior to refining described in U.S. Pat. No. 7,306,698 achieves energy savings with fewer drawbacks than CTMP. By cooking the feed material at elevated temperature for a period of time, a chemical reaction is effected on the material such that the fibers separate more easily, allowing reduced power input during defibrating/refining. However, the reaction uses a cooking time of at least several minutes at an elevated temperature and uses a reaction vessel of sufficient volume to accomplish the cooking step prior to refining. The investment cost of this equipment is a financial drawback.

[0006] There is a need for an alternative method for producing mechanical pulp in an energy efficient manner, which preserves or improves pulp properties while not increasing waste effluents, and which does not have a major investment cost in order to implement the method in industrial pulp production.

SUMMARY OF THE INVENTION

[0007] The present invention relates to a method for producing a pulp from wood feed material wherein an oxalic acid-containing solution either is applied to the wood material just before being fed to the mechanical action zone of the defibrator/refiner, or is co-fed into the defibrator/refiner, where the step of mechanical defibration/refining is performed on the material.

[0008] In one embodiment, the wood may be reduced to a coarse size acceptable for mechanical defibration/refining, for example the wood may be reduced to wood chips. An oxalic acid-containing solution is applied to the size-reduced material just before it is fed to the mechanical action zone of a defibrator/refiner. The material is defibrated/refmed in this primary pass to produce pulp. The oxalic acid containing solution includes oxalic acid in a concentration of between 0.05% and 3% by weight in water. The oxalic acid-containing solution may be applied at a rate of solution between 5% and 70% of the dry weight of the wood feed material and may be applied at a temperature between 20°C and less than 100°C.

[0009] In another embodiment, the oxalic acid-containing solution is applied to the coarser pulp produced by previous refining passes just before it is fed to the mechanical action zone of a defibrator/refiner in a subsequent pass. The oxalic acid containing solution includes oxalic acid in a concentration of between 0.05% and 3% by weight in water. The oxalic acid-containing solution may be applied at a rate of solution between 5% and 70%> of the dry weight of the wood feed material, and may be applied at a temperature between 20°C and less than 100°C.

[0010] The major advantage of the present invention is that the electrical power input used by the refiners to obtain a pulp of the desired freeness is reduced from the power input used without application of the oxalic acid-containing solution. The strength properties of the paper made from the pulp are retained or enhanced relative to pulp produced without application of the oxalic acid-containing solution.

[0011] Another advantage of the present invention is that the pulp produced with the application of the oxalic acid-containing solution may have improved brightness and color properties and energy savings in mechanical refining with strength retention/improvement relative to pulp produced without application of the oxalic acid-containing solution.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention relates to a method for producing pulp from wood using mechanical pulping in which an oxalic acid-containing solution is applied to the wood feed material just before or during the process of its being fed to the refiner. This method is distinguished from prior methods that perform pretreatment of the feed material in a cooking process step. Whereas the cooking pretreatments have chemical temperatures above 100°C and cooking treatment times of more than 3 minutes to achieve efficacy, the present invention does not involve application of the oxalic acid-containing solution at an elevated temperature and does not involve any discernible pretreatment time transpiring before the mechanical refining proceeds. This avoids the use of expensive capital equipment, while reducing energy costs by typically 5-15%. [0013] The wood feed material may be any wood suitable for pulp production, including, for example, hardwoods (i.e., broad-leafed species) and softwoods (i.e., conifers). More specifically, these materials may include the Southern Yellow Pines, Spruces, Western Hemlock, Aspens, and other smaller diameter trees. The material may also originate from either round wood (e.g., whole trees), residue (e.g., wood scraps left behind from forest and sawmill operations), or recovered paper. Recovered paper may include both pre-consumer recovered paper, such as trimmings and scraps from printing, carton manufacturing, or other converting processes which are reused to make pulp without reaching the final consumer, or post-consumer paper, such as corrugated boxes, newspapers, magazines, and office paper which has been recycled.

[0014] In general, the wood material is first reduced to a size appropriate for pulping, as is well known in the art. In the preferred embodiment, the material to be treated is reduced to wood chips. The present method may also be effective with materials not reduced to wood chips, such as those materials derived from recovered paper or wood residues. The present method may also be effective in treating pulp and screening rejects. The present method may also be effective in treating fibrous material obtained from nonwoody plants.

[0015] Mechanical pulping defibrators/refmers comprise a class of devices known in the art that can employ various mechanical arrangements, for examples the single disc, double disc, and cone types, and may be of various sizes, and may employ various internal plate patterns. The present invention may use mechanical refiners of various types and sizes and may be effective using any of the various mechanical specifics of the refiner.

[0016] In one embodiment, wood of a reduced size acceptable for mechanical

defibration/refining is fed to the inlet of a refiner. In proximity to the point where the wood enters the mechanical action zone of the refiner, an oxalic acid-containing solution is added to the wood. The solution contains oxalic acid in a concentration in the range of 0.05-3% by weight. The solution is added at a rate in the range of 5-70% weight of solution per dry weight of wood material. The added solution is at a temperature between 20°C and less than 100°C. The solution is either directly poured or sprayed/dispersed onto the moving wood material. The optimal solution concentration, application rate, and application temperature may be varied to suit the particular wood material being processed, the mechanical equipment employed, and the pulp properties desired in a given situation. The wood and the applied oxalic acid-containing solution are then processed through the mechanical refiner, the operating settings of which are adjusted to obtain desired pulp properties as is known in the art.

[0017] In another embodiment, the oxalic acid-containing solution is injected directly into the entry point of the mechanical action zone of the refiner, for example into the eye of the refiner disc, allowing the contacting of the oxalic acid-containing solution and the wood material to occur as a result of the mechanical action of the refiner.

[0018] In another embodiment, the oxalic acid-containing solution is applied to the wood material at a point of convenient access along the path of wood conveyance from its source to the refiner inlet. The transit time between the point of oxalic acid-containing solution application and the refiner inlet may be less than 3 minutes.

[0019] Mechanical pulping of wood material may be carried out through a sequence of individual refiner passes. The refiner pass may be carried out in different refiner devices or through repeated use of one or more devices, with or without recycle of some or all of the intermediate pulps produced in prior passes. The invention may be applied to one or more of the passes in any combination.

[0020] The overall energy efficiency of the process of the present invention may be compared with the overall energy efficiency of a conventional process (a control) by pulping identical material in the same apparatus while at the same time monitoring the energy consumption of the refiner. As shown in the Examples below, the process of the present invention uses less total energy input to the refiner passes to achieve the same level of freeness in the resulting pulps.

[0021] The pulps made through this procedure may then be made into paper using standard papermaking techniques. Standard techniques (as described by the Technical Association of the Pulp and Paper Industry, TAPPI) known to work with refined pulps work equally well with pulps created by the process described herein.

[0022] Paper made from the pulp prepared according to the present invention may be compared in quality, strength and texture to paper created from pulp not prepared using the present invention, each paper produced using otherwise standard methods. Paper made from the new process pulp may exhibit similar or increased strength properties, indicating that the process of the present invention does not sacrifice the quality or strength of the paper in order to achieve the highly desirable energy savings. Additionally, paper produced from the new process pulp may exhibit improved brightness properties, which are also highly desirable. The present invention provides a unique combination of reduction in energy use and brightness improvement while maintaining strength properties of the resulting paper.

[0023] The present invention is illustrated in the following Examples which describe the laboratory-scale utilization of the present process and the results achieved thereby.

EXAMPLES

Example 1

[0024] Spruce/fir wood chips were refined using the present invention to make pulp, and the pulp was used to prepare handsheets for testing. Identical chips were processed using the corresponding conventional process (control) for direct comparison.

[0025] Wood source: Wood chips produced from a combination of spruce and fir were obtained from a Wisconsin paper mill. These were produced using standard production methods and had a nominal size of 6-14 mm. They were stored in barrels and frozen to prevent the growth of contaminating microorganisms. The chips were thawed at room temperature prior to use. The chips had a moisture content of 56%.

[0026] Mechanical Refining Procedure: Approximately 45 kg oven dry basis of wood chips were hydrated to 39% solids by impregnation with water. 3.5 kg of the wood chips were placed into the steaming tube of the TMP refiner and presteamed at 40 psig. Feed flow of the chips into the refiner was then begun after 30 sec of presteaming at a chip feed rate of 1.0 kg/min. Exiting pulp was discharged through a blowline and cyclone to atmospheric pressure. The TMP refiner was a 12-inch single disc pressurized Sprout-Bauer Model #12-1 CP.

[0027] Chips were fed continuously into the refiner and pulp was collected for a duration of 2.5 minutes. To ensure uniformity of operation during batch sample collection, pulp from the first and last 0.5 kg of feed chips was discarded, allowing the collection of 2.5 kg of pulp for the batch.

[0028] During TMP refining, either water or an oxalic acid-containing solution at

approximately 70°C was injected at a rate of 0.35 kg/min, corresponding to an application rate of 0.35 (kg solution) / (kg dry wood). Concentrations of the applied solutions are listed in Table I. The solution was injected into the feed conveyor of the refiner at a point approximately 6 inches before the eye of the refiner disc, the eye being the location where the feed chips enter the mechanical action zone of the refiner. Transit time of the chips from the solution injection point to the zone of mechanical action was approximately 1-4 seconds.

[0029] After the TMP primary pass, all pulps produced measured at 48% solids. Each pulp was then hydrated in the normal manner with hot water in order to get the pulp to 20% solids for RMP secondary pass refining. The RMP refiner was a Sprout- Waldron Model D2202 300 mm diameter single rotating disc atmospheric refiner. The RMP refining was conducted at 16% solids by running water into the RMP and further hydrating the pulp. Feed rates through the RMP refiner corresponded to motor loads between 10 kW and 15 kW. Two RMP passes were performed, a secondary and a tertiary pass, for a total of (1 TMP + 2 RMP) passes. Pulp was collected after each pass and samples of the pulp were taken and tested for the Canadian

Standard Freeness (CSF). Samples were refined to a final freeness of approximately 100 CSF. Samples of the 100 CSF pulps were taken for brightness testing and bleaching operations.

Handsheets were prepared and tested using TAPPI standard testing methods.

[0030] Energy Consumption and Handsheet Properties: TMP refiner energy consumption was measured using an Ohio Semitronic Model WH 30-11195 integrating wattmeter attached to the power input of the TMP 44.8 kW electric motor. RMP energy consumption was measured using an Ohio Semitronic Model WH 30-11195 integrating wattmeter attached to the power input of the RMP 44.8 kW electric motor. The energy data so obtained were summed over the three passes and normalized based on dry weight of feed wood to obtain total specific energy values. These values are listed in Table I and reported as percent energy savings compared to the conventional process (control). Hand sheets were also prepared and tested using TAPPI standard testing methods. The results from this testing are also reported in Table I.

[0031] Table I for the spruce/fir feed shows energy savings relative to the 2,720 W-h/kg specific energy measured for the control pulp (i.e., water injection only). An increase in brightness was observed, and the strength properties were retained or improved, taking into consideration the normal range of variability in the values of standard strength property measurements. TABLE I.

Solution Energy Brightness Tear Tensile

Concentration Savings ISO Index Index

(% wt) (%) (%) (mN-m²/g) (N-m g)

Control — 55.1 7.15 40.5

0.114 1.8 56.0 7.05 40.2

0.229 2.9 56.2 7.12 41.0

0.714 7.8 57.1 7.15 41.1

2.290 8.3 57.5 7.20 41.1

Example 2

[0032] Loblolly pine wood chips were obtained from a paper mill in the southern US. These were produced using standard production methods and had a nominal size of 6-14 mm. They were stored in barrels and frozen to prevent the growth of contaminating microorganisms. The chips were thawed at room temperature prior to use. The chips had a moisture content of 53%.

[0033] The pine chips were processed and resulting pulp was tested in the same manner as in Example 1. Energy savings and pulp properties results are reported in Table II. Energy savings were seen relative to the 2,588 W-h/kg specific energy measured for the control pulp. An increase in brightness was observed, and the strength properties were retained or improved.

TABLE II.

Solution Energy Brightness Tear Tensile

Concentration Savings ISO Index Index

(% wt) (%) (%) (mN-m²/g) (N-m/g)

Control — 52.1 6.69 32.2

0.114 2.5 53.6 6.75 32.1

0.229 3.6 54.0 6.72 32.0

0.714 8.6 55.8 6.79 32.5

2.290 10.0 56.1 6.80 32.2 Example 3

[0034] Aspen wood chips were obtained from a Wisconsin paper mill. These were produced using standard production methods and had a nominal size of 6-14 mm. They were stored in barrels and frozen to prevent the growth of contaminating microorganisms. The chips were thawed at room temperature prior to use. The chips had a moisture content of 62%.

[0035] The aspen chips were processed and the resulting pulp was tested in the same manner as in Example 1. Energy savings and pulp properties results are reported in Table III. Energy savings were seen relative to the 2,895 W-h/kg specific energy measured for the control pulp. An increase in brightness was observed, and the strength properties were retained or improved.

TABLE III.

Solution Energy Brightness Tear Tensile

Concentration Savings ISO Index Index

(% wt) (%) (%) (mN-m²/g) (N-m/g)

Control — 60.5 2.11 18.5

0.114 2.0 60.6 2.15 18.5

0.229 4.0 61.4 2.14 18.2

0.714 8.0 62.0 2.15 18.5

2.290 9.3 62.2 2.18 18.9

Example 4

[0036] Eucalyptus wood chips were obtained from a Brazilian paper mill. These were produced using standard production methods and had a nominal size of 6-14 mm. They were stored in barrels and frozen to prevent the growth of contaminating microorganisms. The chips were thawed at room temperature prior to use. The chips had a moisture content of 58%.

[0037] The aspen chips were processed and resulting pulp was tested in the same manner as in Example 1. Energy savings and pulp properties results are reported in Table IV. Energy savings were seen relative to the 2,749 W-h/kg specific energy measured for the control pulp. An increase in brightness was observed, and the strength properties were retained or improved. TABLE IV.

Solution Energy Brightness Tear Tensile

Concentration Savings ISO Index Index

(% wt) (%) (%) (mN-m²/g) (N-m g)

Control — 57.8 2.08 17.1

0.114 1.6 58.5 2.08 17.5

0.229 2.7 58.9 2.03 17.5

0.714 7.5 59.5 2.05 17.8

2.290 9.5 60.3 2.08 18.0

Example 5

[0038] Spruce/fir wood chips as used in Example 1 were processed and the resulting pulp was tested in the same manner as in Example 1 except that the temperature of the applied oxalic acid- containing solution was 23°C. Energy savings and pulp properties results are reported Table V. Application of the oxalic acid-containing solution at a cold temperature produced essentially the same results as were obtained in Example 1 where the applied oxalic acid-containing solution was warm. Energy savings were seen relative to the 2,715 W-h/kg specific energy measured for the control pulp. An increase in brightness was observed, and the strength properties were retained or improved. The efficacy of the present invention may not result from or use of elevated temperatures of the applied oxalic acid-containing solution.

TABLE V.

Solution Energy Brightness Tear Tensile

Concentration Savings ISO Index Index

(% wt) (%) (%) (mN-m²/g) (N-m/g)

Control — 55.0 7.06 40.7

0.114 0.8 55.7 7.01 40.5

0.229 2.1 56.0 7.05 41.2

0.714 7.6 57.0 7.00 41.0

2.290 8.2 57.3 7.09 41.0 Example 6

[0039] Spruce/fir, loblolly pine, and aspen wood chips as used in Examples 1-3 were processed and the resulting pulp was tested in the same manner as in Example 1 , except that no solution was applied in the primary TMP refining pass. Instead, oxalic acid-containing solution at a concentration of 0.714 wt% and at a rate of 0.35 kg/min was injected into the eye of the RMP refiner during the secondary pass. The tertiary RMP pass was performed without applying solution. Energy savings and pulp properties results for each of the three types of wood are reported Table VI, along with the corresponding control values in brackets for comparison. Some energy savings were seen relative to the values measured for the control pulps. An increase in brightness was observed, and the strength properties were retained or improved. Efficacy of the invention applied in the secondary RMP pass alone was observed, even if the benefits were not as great as those seen in Examples 1-3 with TMP primary pass application of the oxalic acid-containing solution.

TABLE VI.

Wood Type Energy Brightness Tear Tensile

Savings ISO Index Index

(%) (%) (mN-m²/g) (N-m/g)

Spruce/fir 1.7 55.9 [55.3] 7.10 [7.10] 41.0 [40.6]

Loblolly pine 1.3 53.2 [52.2] 6.69 [6.57] 32.6 [32.0]

Aspen 1.4 61.2 [60.8] 2.11 [2.15] 18.5 [18.4]

Example 7

[0040] Spruce/fir wood chips as used in Example 1 were processed and the resulting pulp was tested in the same manner as in Example 1, except that in addition to the solution injection into the TMP primary pass, oxalic acid-containing solution at a rate of 0.35 kg/min was injected additionally into the eye of the RMP refiner during the secondary pass. The same solution concentration was injected into both the TMP pass and the secondary RMP pass for each individual trial, using the concentration values indicated in Table VII. The tertiary RMP pass was performed without applying solution.

[0041] Energy savings and pulp properties results are reported Table VII. Energy savings were seen relative to the 2,738 W-h/kg specific energy measured for the control pulp. An increase in brightness was observed, and the strength properties were retained or improved. This demonstrates efficacy of the invention when applied multiple times in a sequence of refining passes.

TABLE VII.

Solution Energy Brightness Tear Tensile

Concentration Savings ISO Index Index

(% wt) (%) (%) (mN-m²/g) (N-m/g)

Control — 55.3 7.01 40.2

0.114 1,6 56.2 7.02 40.1

0.229 2.5 56.5 7.10 40.5

0.714 8.2 57.8 7.11 40.8

2.290 8.9 58.1 7.18 41.0

Example 8

[0042] Pulp samples produced in Examples 1-4 using spruce/fir, loblolly pine, aspen, and eucalyptus wood chips and using an applied oxalic acid solution concentration of 0.714 wt% were then bleached with: (1) Hydrogen peroxide [2.0% H₂0₂, 1.8% NaOH, and 0.3% DTP A at 5%> pulp consistency and 66°C water bath temperature]; (2) Hydrosulfite [0.3%> hydrosulfite and 0.5%) DTPA at 10%> pulp consistency and 66°C water bath temperature]; or (3) a sequence of (1), followed by rinsing in water, followed by (2). Pulp samples were prepared for brightness properties testing using TAPPI Method 218. Brightness test results are reported in Table VIII, along with the corresponding control values in brackets for comparison. Improvements in brightness were obtained for all four wood types both before and after bleaching, indicating that the invention affords brightness increases in addition to those provided by bleaching. TABLE VIII.

Wood Type Brightness ISO (%)

Hydrogen Hydrogen

Hydrosulfite

Unbleached Peroxide Peroxide &

Bleaching

Bleaching Hydrosulfite

Spruce/fir 57.1 [55.1] 68.9 [67.0] 65.5 [63.7] 74.1 [72.4]

Loblolly pine 55.8 [52.1] 65.0 [62.3] 61.8 [59.4] 71.3 [69.0]

Aspen 62.0 [60.5] 71.1 [69.7] 69.0 [68.1] 76.2 [74.8]

Eucalyptus 59.5 [57.8] 68.9 [67.8] 65.2 [64.0] 70.2 [69.8]

Example 9

[0043] Spruce/fir wood chips as used in Example 1 were processed and the resulting pulp was tested in the same manner as in Example 1 , except that solutions including chemicals other than oxalic acid were injected into the TMP primary pass. Solutions of H₂SO₄, NaOH, and H₂0₂ at the concentrations listed in Table IX were injected at a rate of 0.35 kg/min in separate trials.

[0044] Energy savings and pulp properties results are reported Table IX. Also shown for comparison are the values from Example 1 produced using the present invention using oxalic acid solution. H₂S0₄ provided only slight energy savings and no brightness improvement.

NaOH provided some modest energy savings and strength improvement, but with a severe loss in brightness. H₂0₂ provided only slight energy savings but noticeable brightness improvement. The present invention provided more energy savings along with improved brightness. This differentiates the efficacy of the application of oxalic acid- containing solution from common acids, alkaline, and peroxide.

TABLE IX.

Solution Energy Brightness Tear Tensile

Chemical Concentration Savings ISO Index Index

(% wt) (%) (%) (mN-m²/g) (N-m/g)

0.229 2.6 55.1 7.10 41.0

H₂S0₄

0.714 2.1 55.3 7.04 41.2

0.229 4.1 48.9 7.45 45.8

NaOH

0.714 4.4 47.9 7.53 46.3

0.714 2.6 56.0 7.10 42.0

H₂0₂

2.286 1.9 56.2 7.12 41.4

0.229 2.9 56.2 7.12 41.0

Oxalic Acid

0.714 7.8 57.1 7.15 41.1

Claims

CLAIMS What is claimed is:

1. A process for producing pulp, the process comprising:

- reducing a wood feed material to produce a size-reduced material, wherein the size- reduced material is a size acceptable for mechanical defibration/refining;

- applying an oxalic acid-containing solution to the size-reduced material;

- feeding the size-reduced material to the mechanical action zone of a defibrator/refmer; and

- defibrating/refining the size-reduced material in a primary pass mechanical pulping method to produce a primary pass pulp, wherein the oxalic acid-containing solution is applied to the size-reduced material before feeding the size-reduced material to the mechanical action zone of a defibrator/refmer, and wherein the time between oxalic acid-containing solution application and size-reduced material entry to the mechanical action zone is less than 3 minutes.

2. The process of claim 1, wherein the oxalic acid-containing solution is injected into or onto the size-reduced material at a location within close proximity to the mechanical action zone of the defibrator/refmer.

3. The process of claim 1 or 2, wherein the oxalic acid-containing solution is injected into the eye of the disc or cone mechanical defibrator/refmer near the chip feed entry point.

4. The process of any one of the preceding claims, wherein the primary pass mechanical pulping method is thermomechanical pulping (TMP).

5. The process of any one of claims 1-3, wherein the primary pass mechanical pulping method is refiner mechanical pulping (RMP).

6. The process of any one of the preceding claims, further comprising processing the primary pass pulp with one or more subsequent mechanical refining passes and producing a feed pulp.

7. The process of claim 6, further comprising applying the oxalic acid-containing solution to the feed pulp of the subsequent pass before the feed pulp is fed to the mechanical action zone of the refiner, wherein the time between oxalic acid-containing solution application and feed pulp entry to the mechanical action zone is less than 3 minutes.

8. The process of claim 6 or 7, wherein the subsequent passes may individually be RMP and/or TMP.

9. A process for producing pulp, the process comprising:

- feeding the size-reduced material to the mechanical action zone of a defibrator/refmer;

- defibrating/refining the size-reduced material in a primary pass mechanical pulping method to produce a primary pass pulp;

- processing the primary pass pulp with one or more subsequent mechanical refining passes and producing a feed pulp; and

- applying a oxalic acid-containing solution to the feed pulp of the subsequent pass before the feed pulp is fed to the mechanical action zone of the refiner, wherein the time between oxalic acid-containing solution application and feed pulp entry to the mechanical action zone is less than 3 minutes

10. The process of claim 9, wherein the oxalic acid application is applied in one or more of the subsequence mechanical refining passes.

11. The process of claim 9 or 10, wherein the subsequence passes may individually be RMP and/or TMP.

12. The process of any one of the preceding claims, wherein the oxalic acid-containing solution contains oxalic acid in a concentration of between 0.05% and 3% by weight in water.

13. The process of any one of the preceding claims, wherein the oxalic acid-containing solution is at a temperature between 20°C and less than 100°C.

14. The process of any one of the preceding claims, wherein the oxalic acid-containing solution is applied at a rate between 5% and 70% of the dry weight of the wood feed material.

15. The process of any one of the preceding claims, wherein the wood feed material comprises wood chips, pulp screening rejects, recovered paper material, softwood, hardwood or nonwoody plant material.