Metal extraction prior to chelation in chemical pulp production

Abstract

The amount of metals that can be removed from comminuted cellulosic fibrous material (such as wood chips) in the production of chemical pulp, prior to bleaching, is enhanced by extracting metals during an early stage of digestion, and prior to the addition of a chelating agent. After the material is steamed and slurried it is impregnated with cooking liquor at a temperature of 90° C. or more, and during or after impregnation metals are removed by an extraction (e.g. in a continuous digester), which typically removes about 30% or more of the Mn. Prior to this extraction it is not necessary to introduce chelating agents, and undesirable to do so since they may compete or interfere with the natural removal of metals. After the extraction, such as during the first part of cooking, about 0.05-10 kg/dry ton of material of chelant (such as EDTA) is added, and the chelant combines with released metal ions in the slurry to produce metal complexes. The metal complexes are substantially removed before bleaching. In this way more than 93% of Mn (and other undesirable metal ions) may be removed prior to bleaching with a minimal amount of chelating agent used.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of application Ser. No. 08/659,682 filed Jun. 5, 1996, the disclosure of which is hereby incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] It is known that the removal of metals from chemical pulp before it reaches the bleach line increases the efficiency of the bleaching process, especially when bleaching chemicals, such as peroxide, which are very adversely affected by the presence of metal ions (particularly transition metal ions) are present. In the present application, it has been recognized that the addition of a chelating agent to a digester (such as a continuous digester) during the digestion process enhances transition metal ion removal since the chelant combines with released transition metal ions in the slurry to produce metal complexes, which are subsequently removed during the cooking process. It has now been found that one of the particular embodiments disclosed in the parent application which is the most advantageous for removing the maximum amount of metal ions with a minimum amount of chelating agent is to add the chelating agent (metal-complex former) downstream of a liquor extraction, or after or during the process of extracting and diluting the cooking liquor. This chelate treatment may be practiced in a co-current or counter-current cooking mode, in a single or multiple vessel digester system, in a hydraulic or dual phase digester, or in batch or continuous fashion.

[0003] According to the present invention, the disclosure in the parent application with respect to adding the chelating agent after an extraction has been amplified upon and optimized, while still retaining the same basic concept. That is, during the digestion process, but after an extraction of metals that was obtained without the use of a chelating agent, the chelating agent is added. It has been found according to the present invention that if a wood chips slurry is simply treated by steaming and alkali impregnation (the first stage in the digestion process) at a temperature of about 90° C. or more, without the addition of chelants, a significant amount of the undesirable metals, such as Mn, may be removed. For example according to several procedures performed according to the invention, more than 37% of the Mn originally present in the chips can be removed just by conventional steaming and alkali impregnation. If—as according to the present invention—the chelating agent is then added after an extraction of these metals (typically with black liquor through a conventional extraction screen), then the total metal removal capability is improved. That is, it has been found that it is not necessary to introduce chelants prior to impregnation, or during impregnation, where they may compete or interfere with the natural removal of metals; but rather the chelants may be reduced later during the digestion process, typically after impregnation and at the start of cooking, so that a more efficient, less costly use of chelants is provided.

[0004] According to one aspect of the present invention there is provided a method of producing chemical pulp from comminuted cellulosic fibrous material containing metal containing compounds comprising: (a) Pressurizing and slurrying the comminuted cellulosic fibrous material containing metal containing compounds, and heating the slurry. (b) Digesting the material in the slurry, including in different stages, and by impregnating the material with cooking liquor, and cooking the material to produce chemical pulp. (c) At an early stage during digestion, extracting liquid containing some of the metal containing compounds therein, and removing the metal containing compounds from the slurry. (d) After (c), adding a chelating agent to the slurry to combine with released metal containing compounds in the slurry to produce metal complexes; (e) cooking the material; and (f) removing at least some of the metal complexes formed in (d). Metal containing compounds include metal ions, metal oxides, and any other metal-containing compound that is preferably removed from the slurry.

[0005] The method further comprises, after (f), bleaching the chemical pulp with at least one bleaching chemical adversely effected by at least some of the metal ions removed in (c) and (f), such as peroxide. Typically (c) is practiced to remove at least about 30% of the Mn in the material, from the material, and (c) and (f) are practiced to remove at least about 90% of the Mn, e.g. at least about 93%, as well as the majority of at least the other transition metals originally present in the material, such as wood chips.

[0006] Typically (c) is practiced after impregnation and just before cooking, at a temperature of between about 105-145° C. (e.g. about 110-130° C.), for a time period of between about 30-120 minutes, preferably between about 60-120 minutes (e.g. about 90 minutes), and with a charge of chelating agent of between about 0.05-10 kg/dry ton of material, preferably about 1-5 kg/dry ton of material, most preferably about 1-2 kg/dry ton of material. Typically (d) is practiced where the pH is 9 or more, although the invention can be practiced at a wide variety of pHs. The chelating agent may be selected from the group consisting essentially of EDTA, DTPA, derivatives and equivalents to EDTA and DTPA, oxalic acid, tartaric acid, and furoic acid; however almost any metal complex former may be utilized, preferably one that is temperature resistant (preferably one that can even withstand the typical cooking temperatures of about 150° C.), and one that can withstand high pH (that is a pH of greater than 9, typically greater than 10, preferably greater than about 12). In any event, enough chelating agent is used to remove at least 10% of the transition metal ions in the material, preferably at least the majority of the transition metal ions in the material, from the material.

[0007] According to another aspect of the present invention there is provided a method of producing chemical pulp from comminuted cellulosic fibrous material containing metal ions comprising: (a) Pressurizing and slurrying the comminuted cellulosic fibrous material containing metal compounds, and heating the slurry. (b) Impregnating the material with cooking liquor at a temperature of about 90° C. or more. (c) After (b), extracting liquid containing some of the metal compounds therein, and removing the metal ions from the slurry. (d) After (c) adding a chelating agent to the slurry to combine with released metal compounds in the slurry to produce metal complexes; (e) cooking the material; and, (f) removing at least some of the metal complexes formed in (d). The details of the method may be as described above.

[0008] In practicing the method (a)-(f) may be practiced in an upright vessel having an upper screen and a black liquor extraction screen; and wherein (d) is practiced by adding chelant in a first conduit at a first distance below the upper screen, and above the extraction screen, so that the chelant at least primarily flows with liquid up through the slurry to the upper screen, and wherein (e) is practiced in part by introducing cooking liquor in a second conduit at a second distance below the upper screen, greater than the first distance, so that the cooking liquor flows with the slurry downwardly toward the extraction screen.

[0009] The methods according to the present invention are preferably practiced substantially continuously, e.g. using conventional continuous digesters including all different types (one vessel, two vessels, hydraulic filled, gas phase, etc.). However, the invention is also applicable to batch digestion in the production of pulp. The invention is particularly suitable for use in kraft pulping, although other types of chemical pulping, including sulfite and soda pulping, also benefit significantly from the practice of the invention.

[0010] The invention also relates to the chemical pulp produced by the practice of the method as described above.

[0011] There is the possibility of hexenuronic acid (HexA) formation during the alkaline pulping process and there is the potential for these acids to act as charged sites on the cellulose to which charged metal ions can attach. According to one aspect of the present invention disclosed herein, this metal ion attachment to the pulp via charged HexA can be minimized by treating the pulp toward the end of the cook with a highly alkaline liquor. However, there is another aspect of the present invention that also pertains to when in the process the HexA is generated.

[0012] It is understood that HexAs are generated in the alkaline pulping process at temperatures at or above 100° C., and the problem is most significant at or above 150° C., for example, at or above 160° C. These electrically charged HexAs can attract charged metal ions. However, according to the preferred embodiment of this invention, at least some, if not most of the charged metal ions are removed, for example, via post-impregnation extraction or via chelate treatment, prior to the pulp being exposed to temperatures which generate HexAs. That is, another benefit of the present invention, in which metal ions are removed after impregnation or during chelate treatment, is that these metals are removed before they can be exposed to HexAs generated later in the cook.

[0013] Thus the invention also relates to a method as described above wherein the chelant is introduced prior to the slurry achieving a temperature at which the generation of an adversely significant amount of hexenuronic acids beings, i.e. prior to achieving a temperature of about 140° C. or more (or under some circumstances about 120° C., or even about 100° C., or more).

[0014] It is the primary object of the present invention to provide effective removal of transition metal ions from comminuted cellulosic fibrous material, such as wood chips, prior to bleaching, and with a minimum amount of chelant. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 is a schematic view illustrating an exemplary method according to the present invention;

[0016] FIG. 2 is a schematic view showing an exemplary continuous digester system which may be utilized for the practice of the present invention; and

[0017] FIG. 3 is a view like that of FIG. 2 showing another exemplary system which may be utilized in the practice of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 schematically illustrates one embodiment of a method according to the present invention. Cellulosic fibrous material, such as softwood or hardwood chips in line 1, is steamed at 2 to remove excess air and to begin heating the material. The steaming at 2 may be performed using any conventional techniques, preferably with a DIAMONDBACK® steaming vessel, as described in U.S. Pat. Nos. 5,500,083, 5,617,975, and 5,628,873, and such as sold by Ahlstrom Machinery. After steaming, cooking liquor (e.g., white liquor, black liquor, sulfite cooking liquor, etc.) 3 is added to the material to form a slurry, and the slurry is pressurized, as schematically illustrated at 4. Slurrying and pressurizing may be performed in a conventional high pressure feeder, or by a slurry pump and feeder as described in U.S. Pat. No. 5,476,572, or by one or more pumps as described in U.S. Pat. No. 5,753,075, all sold by Ahlstrom Machinery.

[0019] The cellulose material slurry is then heated at 5 to a temperature at which transition metal compounds (e.g. ions) dissociate from the material. This temperature is at least about 90° C., preferably at least about 100° C., typically at least about 110° C., which in the embodiment illustrated is in the impregnation stage of the digestion process. With some of the transition metal ions released from the material (typically in solution), extraction of the metal ions occurs as illustrated schematically at 6 in FIG. 1, producing a liquid flow 7 containing the metals, which is typically removed entirely from the slurry (that is, not recirculated, although after metal ion removal from the liquid, the liquid itself can be returned, or any desirable metal ions, such as Mg or Ca removed from the liquid, can be added back to the material). The extraction at 6 can be accomplished utilizing the conventional screens and like structures that are common in both batch and continuous digesters.

[0020] After the liquid containing the metals in line 7 is removed—which typically takes with it at least about 30% of the Mn, and also significant amounts of other undesirable metal ions—the slurry is subjected to a chelant treatment, as illustrated at 8 in FIG. 1. The chelant treatment 8 typically takes place at the same consistency as the rest of the pulping process, that is preferably medium consistency, e.g. about 8-20%. The relationship of the relative amount of material and liquid present is typically expressed by the liquid-to-material, or liquid-to-wood (UW) ratio. The UW ratio for the chelate treatment 8 typically ranges between 1.0-5.0, preferably about 2.0 to 4.0.

[0021] A chelating agent is added at one or more points 9 to the slurry, and typically the slurry is maintained at a relatively low temperature, for a significant period of time, so that the chelating agent combines with released metal ions in the slurry to produce metal complexes. Preferably the chelating agent treatment 8 takes place for a time period of between about 30-120 minutes, preferably about 60-120 minutes, and most preferably about 90 minutes, at a temperature of about 105-145° C., preferably between about 105-125° C., e.g. about 110° C. While this relatively low temperature range is preferred, a higher temperature range may be used if it suits the process, all the way up to the temperature at which the chelating agent may no longer be operable. Typically the pH during the treatment 8 is nine or more, normally ten or more, and usually at least about twelve, although the metal complexing action will take place even at much lower pHs.

[0022] The amount of chelant added at 9 is whatever is effective to remove the desired amount of metal ions. Typically the charge is between about 0.05-10 kg/dry ton of material, particularly when EDTA or its equivalent is utilized. Normally the range is between about 1-5, and most preferably is relatively low, e.g. about 1-2 kg/dry ton of material. Although any agent which complexes metal-containing compounds may be used in the practice of this invention, it is preferred that the agent used be stable under the conditions described, that is, the agent is preferably stable under alkaline conditions at temperatures of 100° C. and above. For example, preferred metal-complexing agents include nitrogen-containing polycarboxyl compounds, such as ethylenediaminetetraacetic acid (EDTA), and diethylenetriaminepentaacetic acid (DTPA), or their salts and derivatives, as well as hydroxycarboxyl compounds, such as citric and tartaric acids, or their salts and derivatives.

[0023] Cooking liquor, such as white liquor, may be added in one or both of the lines 10 to the slurry, and after the chelating treatment, the material is heated to cooking temperature, of at least about 140° C. and typically between 160-170° C., and digestion is completed by one or more cooking stages illustrated schematically at 11 in FIG. 1. During the cooking stage 11, and in fact even during the chelant treatment 8, liquid may be extracted containing high levels of dissolved organic material, and replacement liquors containing lower amounts of dissolved organic material (such as white liquor, water, filtrate, or the like) added, as illustrated schematically at 12 in FIG. 1.

[0024] At least some of the formed metal complexes are removed, schematically illustrated at 13 in FIG. 1, with black liquor during the cooking at 11, or in the subsequent wash 14, or at any other suitable point prior to bleaching at 15. Preferably the vast majority of metal complexes are removed, e.g. more than 90%.

[0025] After cooking, the chemical pulp produced is washed, as illustrated schematically at 14 in FIG. 1, and subsequently (typically after storage and screening and perhaps additional washing) is bleached, as schematically illustrated at 15 in FIG. 1 preferably TCF (totally chlorine free) bleaching, or ECF (elemental chlorine free) bleaching, typically including at least one stage of peroxide or ozone.

[0026] All of the stages 5 through 14 illustrated in FIG. 1 may take place in a conventional continuous digester, utilizing conventional processes such as EMCC® as disclosed in European patent 476,230 B1, Lo-Solids® treatment, such as disclosed in U.S. Pat. Nos. 5,489,363, 5,547,012, 5,620,562, and 5,849,150 (the disclosures of which are hereby incorporated by reference herein), or other suitable pulping processes may be utilized.

[0027] By practicing the invention as described above, an enhanced amount of metal ions can be removed, for a given amount of chelating agent—than in known processes. For example in the practice of the invention compared to the procedure set forth in U.S. Pat. No. 5,593,544 where the chelating agent (called a sequestering agent there) is added in a pre-impregnation stage, prior to the actual digestion process (that is prior to impregnation and cooking), according to the invention 2% or more of the amount of Mn, and enhanced amounts of other adverse metal ions, can be removed with a given amount of chelating agent.

[0028] FIG. 2 schematically illustrates a continuous digester system that may be utilized to practice the method of FIG. 1. In FIG. 2, wood chips, or like comminuted cellulosic fibrous material, are fed to a chip bin 20 (such as a DIAMONDBACK® chin bin) where preferably steaming takes place (although steaming may be in a steaming vessel located downstream of the chip bin 20), and then the chips are pressurized and slurried as in a conventional chip tube 21 or like conventional equipment. Then a high pressure feeder or slurry pump, the high pressure feeder being illustrated in FIG. 2 schematically at 22, feeds the pressurized slurry in line 23 to the inlet 24 of a continuous digester 25. Cooking liquor has already been added, as indicated at 25 in FIG. 2, and the liquid in line 23, which is recirculated in line 26, is hot, over 90° C. At the top of the digester 25 impregnation occurs, typically at a temperature of between about 105-145° C., and then utilizing a conventional screen 27 or the like, metals extraction occurs, as illustrated schematically at 28. Then a chelant is added, as illustrated schematically at 29 in FIG. 2, and approximately in zone 30 chelant treatment occurs, typically for a time period of about 60-120 minutes at a temperature of about 105-145° C. with a chelant charge of between about 0.05-10 kg/dry ton of pulp, and at a pH of over 9. A recirculation line 31, and heater 32, may be provided, and some of the liquid (typically high DOM liquid) may be withdrawn, as schematically illustrated at 33 in FIG. 2.

[0029] Black liquor, and metal complexes, can be removed as illustrated at 34 in FIG. 2, using a conventional extraction screen 35 or the like, and one or more cooking stages, illustrated schematically at 36 in FIG. 2, are provided, as well as a conventional wash/cook stage 37. Cooking stages 36 may also be omitted, if desired. Conventional recirculations 38, 39 are also provided, along with conventional heaters 40, 41, and some liquor may be extracted (such as high DOM liquor) as illustrated schematically at 42 and 43, and replaced with low DOM liquor. White liquor and/or low DOM dilution liquor may also be added at various points, as also illustrated schematically at 44 and 45 in FIG. 2.

[0030] According to the present invention actual cooks were made utilizing both softwood and hardwood chips to demonstrate the advantages of the invention.

[0031] In these tests, the softwood furnish used was Northern softwood from an Ontario mill (see Table 1). The Southern hardwood furnish came from a Southeastern U.S. mill. Treatment of the softwood was completed in five liter electrically heated vessels that swung through a 270° arc to maintain mixing and uniform temperature. For each treatment 750 grams of oven-dry chips were steamed for 15 minutes at 100° C. and then placed in the vessel. A synthetic white liquor charge with an Effective Alkali (EA) of 9.0% on wood and a sulfidity of 30% based on Active Alkali (AA) nominal was added at a liquid to wood ratio of 3.5 Ukg. There was a 15 minute rise to impregnation temperature of either 95° C. or 110° C. Impregnation was maintained for 30 minutes. 850 mL of free liquor was drained from the digester at the end of impregnation. A white liquor charge with an EA of 3.0% on wood equal in volume to the drained liquor was added. This charge did or did not include 1.0% on wood EDTA charge as required. The time from end of impregnation to the restarting of the heating was about 18 minutes. Heating was continued to bring the vessel back to the impregnation temperature. The time to reheat, about nine minutes, plus the time at temperature was either 45 or 90 minutes.

[0032] The liquor was rained after the treatment stage and the treated chips washed in the vessel for ten minutes with deionized water. The chips then underwent a displacement wash overnight with deionized water. The chips were drained for about two hours and a sample taken for analysis. The end of impregnation and treatment stage black liquors were analyzed for residual effective alkali. The liquors and the treated chips were analyzed for the metals calcium, copper, iron, magnesium, and manganese. The treatment conditions are listed in Table 2.

[0033] Conventional kraft cooking of the softwood chips, following Pruyn's Island Technical Center (PITC) standard operating procedure PITC-P2, then took place. One reference and one cook with 1.0% on wood EDTA added at impregnation were completed. The black liquors were analyzed for residual effective alkali. The liquors and pulp were analyzed for the metals calcium (Ca), copper (Cu), iron (Fe), magnesium (Mg), and manganese (Mn). The cook conditions are given in Table 3.

[0034] Laboratory Lo-Solids® cooking was then practiced, following procedure PITC-P27. One reference cook and one with a 1.0% on wood EDTA charge at impregnation were completed.

[0035] A third cook used a modification of the Lo-Solids® procedure, according to the invention. After impregnation, 3966 mL of black liquor was drained from the digester. An equal volume of white liquor with an EA of 3.0% on wood was added along with 1% on wood EDTA. This process took 17 minutes. The treatment stage was 90 minutes at 110° C. The cook continued with the standard Lo-Solids® procedure at the end of the treatment stage.

[0036] The black liquors were analyzed for residual effective alkali. The liquors and pulp were analyzed for the metals calcium, copper, iron, magnesium, and manganese. The cook conditions are given in Table 3.

[0037] A laboratory Extended Modified Continuous Cook (EMCC®) was completed with an EDTA charge of 1.0% on wood added to the impregnation stage. This cook followed procedure PITC-P3. The black liquors were analyzed for residual effective alkali. The liquors and pulp were analyzed for the metals calcium, copper, iron, magnesium, and manganese. The cook conditions are given in Table 3.

[0038] Thus, as shown in Table 2, in these trials the effect of the presence of 1% EDTA chelating agent (which is approximately 10.0 kg/ton of dry softwood chips), and of varying impregnation and treatment temperatures and times were compared to a conventional kraft cook (CK) without any chelant added, that is cook S1159. Note that these trials compared the results for impregnation and chelate treatment only. The chips were not subsequently cooked. The chips were first impregnated with alkali at time and temperature and then treated with chelant and alkali at time and temperature.

[0039] The resulting metal content of the chips treated in these tests appear in Table 5. As shown therein, two samples were analyzed for metal content for each treatment, and the mean and standard deviation for the metal content were calculated from the two sets of sample data.

[0040] The treatment conditions listed in the upper sections of Table 2 and the mean metal content that appears in Table 5 are summarized in Table 7. The metal content of the untreated chips (“Orig. Chips”) appears in the first column. Cook S1159 corresponds to the impregnation according to a conventional kraft cook without any chelate added. If the metal content is limited to Mn, which is known to exhibit the most significant cellulose damage, especially during non-chlorine bleaching, Table 7 shows that the conventional impregnation reduces the Mn content by about 37%. This shows that it is not necessary to introduce chelants early in the process where they may compete or interfere with the natural removal of metals.

[0041] In Cook S1160, the same conditions as Cook S1159 were used except that S1160 includes a treatment with chelating agent during impregnation. This resulted in Mn removal of 69%. In cook S1161, the treatment time with the chelant was extended from 45 to 90 minutes. As shown, the Mn removal only increased from 69 to 76% with the extra treatment time. In cook S1162, the chelant treatment time was again reduced to 45 minutes, but the treatment temperature for both the impregnation and the chelant treatment was increased from 95 to 110° C. However, compared to the lower temperature treatment, cook S1160, the metal removal actually decreased to 66%. In cook S1163, both the treatment time was increased from 45 to 90 minutes and the impregnation and chelant treatment temperature was increased from 95 to 110° C. This produced a markedly increased removal of Mn, 89%, and also a markedly increased removal of calcium (Ca) and magnesium (Mg) compared to the other treatments.

[0042] The result of these impregnation and chelate treatment trials was the finding that increased metal removal can be achieved when treating the chips at a higher temperature, for example, 110° C. instead of 95° C., for a longer treatment time, for example, 90 minutes instead of 45 minutes. This increased metal removal at higher temperature and longer time during pretreatment provided a basis for the further investigation of the impact of the presence of chelants on the metal content of the fully cooked pulp.

[0043] The cooking test conditions for the Northern softwood cooks appear in Table 3. Employing the results of the earlier treatment tests, the impregnation and chelate treatment, when applicable, were performed at the higher temperature, that is, at 110° C.

[0044] Cook S2222 corresponds to a conventional kraft cook without chelate addition. Cook S1182 corresponds to a conventional kraft cook with chelate addition, that is, the chips are treated with 1% EDTA on wood in a treatment stage after impregnation with alkali at time and temperature. Cook S1182 is similar to the process disclosed in U.S. Pat. No. 5,593,544 in which the chelant is added during a pretreatment without any liquor removal before chelant addition. Cook AL622 corresponds to a Lo-Solids® cook without chelate addition. This laboratory batch simulation includes a small liquor purge, or removal, (about 4 liters of liquor) during the counter-current (or displacement 1) stage to simulate a removal of liquor containing dissolved organic material and dissolved metals as is characteristic of Lo-Solids cooking. Cook AL624 is a Lo-Solids® cooking simulation similar to cook AL622 but with the addition of 1% EDTA during the impregnation stage. Cook AL624 also included a small liquor purge to simulate Lo-Solids®) cooking as in cook AL622. Cook AL625 is also a Lo-Solids-type cook with the addition of 1% EDTA, but according to the present invention, in cook AL625, after impregnation (without the presence of chelant), black liquor was drained from the vessel. After this liquor removal, alkali and EDTA were introduced to the digester. That is, test AL625 corresponds to the invention, and includes a post-impregnation extraction after which the chips are treated with a chelant and alkali for 90 minutes at 110° C.

[0045] Cook AE647 in Table 3 is a simulation of a conventional EMCC®-type cook with chelant added to the impregnation stage. Cook AE647 with chelant added during impregnation and no liquor removal before formal cooking is commenced is similar to Test 9 of U.S. Pat. No. 5,593,544.

[0046] The results of the testing shown in Table 3 are also shown in Table 5. Table 5 also includes the results of metal analysis for the pulp. The metal content of the untreated chips for the tests performed according to Table 3 is the same as the metal content shown for the chips. The testing conditions shown in Table 3, and the results shown in Table 5, are summarized in Table 8. As shown, again with reference to Mn removal, the cook performed according to the present invention (that is, cook AL625) removed the most Mn, 94%. The cook according the present invention removed more Mn than the cooks that corresponded generally to what is disclosed in U.S. Pat. No. 5,583,544, that is, more than cook S1182 (only 81%) and cook AE647 (91%).

[0047] Other tests were performed on Southern hardwood (see the bottom of Table 1). Treatment of the hardwood was completed in five liter electrically heated vessels that swung through a 270° arc to maintain mixing and uniform temperature. For each treatment 750 g of oven-dry chips were steamed for 15 minutes at 100° C. and then placed in the vessel. A synthetic white liquor charge with an EA of 9.0% on wood and a sulfidity of 30% based on AA nominal was added at a liquid to wood ratio of 3.5 Ukg. There was a 15 minute rise to impregnation temperature of 110° C. Impregnation was maintained for 30 minutes. 850 mL of free liquor was drained from the digester at the end of impregnation. A white liquor charge with an EA of 3.0% on wood equal in volume to the drained liquor was added. This charge did or did not include a 0.5 or 1.0% on wood EDTA charge as required. The time from the end of impregnation to the restarting of the heating was about 18 minutes. Heating was continued to bring the vessel to 110° C. or 130° C. The time to heat, about nine minutes, plus the time at temperature was 90 minutes.

[0048] The liquor was drained after the treatment stage and the treated chips washed in the vessel for ten minutes with deionized water. The chips then underwent a displacement wash overnight with deionized water. The chips were drained for about two hours and a sample taken for analysis. The end of impregnation and treatment stage black liquors were analyzed for residual effective alkali. The liquors and the treated chips were analyzed for the metals calcium, copper, iron, magnesium, and manganese. The treatment conditions are listed in Table 4.

[0049] That is, the treatment of hardwood chips was essentially identical to the treatment of the softwood chips discussed earlier. Again, the treatment conditions listed in Table 4 are for an impregnation and chelate treatment only, not a cook. The resulting metal content of the treated chips are shown in Table 6. The treatment conditions and resulting metal content are summarized in Table 9.

[0050] The conditions shown in Table 9 employ the findings of the testing performed on the softwood chips shown in Table 8. That is, the chelant treatment time is longer, 90 minutes, and the treatment temperature is higher, 110° C. Furthermore, in the last two trials (S1170 and S1171) the treatment temperature was even higher, that is, 130° C. These trials also included a reduction in the chelant concentration from 1.0% to 0.5% on wood.

[0051] Cook S1166 is the reference cook with no chelate addition during pretreatment. Again, as in the earlier tests on softwood, this test indicates that as much as 32% of the Mn can be removed from the chips without the use of a chelant. Cook S1168 is similar to cook S1166 but with the introduction of 0.5% EDTA in the treatment after impregnation. The Mn removal increased dramatically to 80%. In cook S1169 the EDTA charge was increased to 1.0% while maintaining the same time and temperature as in cooks S1166 and S1168; however, as a result of cook S1169 the Mn content was essentially the same as the treatment with 0.5% EDTA, though the Ca removal was markedly greater with the higher concentration of EDTA. This suggests that under these conditions, for hardwood chips, an increase in EDTA charge does not increase the Mn removal.

[0052] In cook S1170, the EDTA charge, based upon the results of cook S1169, was reduced to 0.5%, but the chelate treatment temperature was increased to 130° C. from 110° C. However, surprisingly, this increased temperature resulted in a lower Mn removal compared to the test at the lower temperature and same chelate charge (cook S1168). The reasons for this are not apparent at this time.

[0053] Finally, a chelate charge of 1.0% and a chelate treatment temperature of 130° C. were used in cook S1171. The Mn content of the chips treated in this fashion was the highest in this set of trials, 91% Mn removal. The Ca and Mg removal in the resulting chips was also the highest in cook S1171.

[0054] The data summarized in Table 9 again confirm that metals can be removed during pretreatment of the chips with chelating agents. Specifically, the data in Table 9 suggests that the metal removal can be more effective at higher temperature, for example, 130° C., using an EDTA charge of about 1% on wood, for a treatment time of about 90 minutes. Complete cooking trials for hardwood employing chelate treatment according to the present invention have yet to be completed. 1 TABLE 1 Chip Size and Chip Thickness Distributions Size (mm) % Thickness (mm) % Northern softwood 99-000473 0.0-3.0 0.1 0-2 1.6 3.0-7.0 2.1 2-4 24.3  7.0-12.7 16.9 4-6 34.7 12.7-25.4 57.0 6-8 22.0 25.4-45.0 21.6  8-10 12.0 greater than 45.0 2.4 greater than 10 5.5 Southern hardwood 99-001620 0.0-3.0 0.3 0-2 1.1 3.0-7.0 1.9 2-4 17.3  7.0-12.7 15.6 4-6 34.5 12.7-25.4 54 6-8 28.3 25.4-45.0 26.6  8-10 12.2 greater than 45.0 1.7 greater than 10 6.5

[0055] 2 TABLE 2 Summary of treatment conditions and treatment results for metals removal with EDTA for Northern pine. Furnish: Northern softwood 99-000473 Cook ID S1159 S1160 S1161 S1162 S1163 Cook Type Special CK Special CK Special CK Special CK Special CK EA Charge Impregnation (% on wood NaOH) 9.0 9.0 9.0 9.0 9.0 Treatment (% on wood NaOH) 3.0 3.0 3.0 3.0 3.0 WL Sulfidity (% AA) 30.0 30.0 30.0 30.0 30.0 EDTA Charge (% on wood EDTA) 0 1.0 1.0 1.0 1.0 Temperature (° C.) Impregnation 95.0 95.0 95.0 110.0 110.0 Treatment 95.0 95.0 95.0 110.0 110.0 Time (min) Impregnation 30.0 30.0 30.0 30.0 30.0 Treatment 45.0 45.0 90.0 45.0 90.0 EA Consumed (% on wood NaOH) Total 7.0 7.8 8.6 9.2 10.1 Impregnation Stage 4.1 4.2 4.5 6.5 6.7 Treatment Stage 2.9 3.6 4.1 2.7 3.4 Residual EA (g/L NaOH) End of Impregnation 9.9 9.7 9.0 5.1 4.7 End of Treatment 10.2 8.6 6.9 5.7 3.9

[0056] 3 TABLE 3 Summary of pulping conditions and pulping results for metals removal with EDTA cooks for Northern pine. Furnish: Northern softwood 99-000473 Cook ID S2222 S1182 AL622 AL624 AL625 AE647 Cook Type CK CK LS LS LS EMCC EA Charge Impregnation (% on wood NaOH) 19.2 19.2 9.5 9.5 9.5 11.0 Treatment 0.0 0.0 0.0 0.0 3.0 Displacement 1 (g/L NaOH) NA NA 92.5 92.5 92.5 97.8 Displacement 2 (g/L NaOH) NA NA 43.0 43.0 43.0 29.0 WL Sulfidity (% AA) NA NA 29.2 29.2 29.2 28.2 EDTA Charge (% on wood EDTA) NA 1.0% NA 1.0% 1.0% 1.0% at Imprg. at Imprg. at Treat. at Imprg. Temperature (° C.) Impregnation 110.0 110.0 110.0 110.0 95.0 110.0 Treatment NA 110.0 NA NA 110.0 NA Cooking 170.0 170.0 158.5 158.5 158.5 158.5 EA Consumed (% on wood NaOH) Total 16.1 16.9 17.4 17.9 18.9 16.4 Impregnation Stage NA NA 7.2 7.6 5.7 8.3 Treatment Stage NA NA NA NA 2.4 NA Displacement 1 Stage NA NA 6.5 6.6 7.4 — Co-current Stage NA NA 1.2 1.2 1.2 6.0 Displacement 2 Stage NA NA 2.6 2.5 2.2 2.1 Residual EA (g/L NaOH) End of Impregnation NA NA 6.7 5.3 10.8 8.9 End of Treatment NA NA NA NA 4.1 NA End of Displacement 1 NA NA 15.6 14.5 17.0 0.0 End of Co-current NA NA 12.3 11.1 13.6 6.1 End of Cook 8.9 6.7 17.4 16.8 19.1 18.5 H-factor 1440 1410 1717 1717 1716 1793 Kappa Number 24.9 24.4 25.5 26.1 22.0 23.6 Viscosity (mPa · s) 32.3 32.1 44.6 51.4 44.5 54.9 Viscosity/Kappa Number Ratio 1.3 1.3 1.7 2.0 2.0 2.3 Total Yield (% on wood) 45.7 46.7 44.9 45.5 44.8 45.4 Rejects (% on wood) 0.20 0.50 0.03 0.02 0.01 0.12 Screened Yield (% on wood) 45.5 46.3 44.8 45.5 44.7 45.3 Knots >2.5 cm (% on wood) 0.10 0.40 0.02 0.01 0.00 0.1

[0057] 4 TABLE 4 Summary of treatment conditions and treatment results for Southern hardwood. Furnish: Southern hardwood 99-001620 Cook ID S1166 S1168 S1169 S1170 S1171 Cook Type Special CK Special CK Special CK Special CK Special CK EA Charge Impregnation (% on wood NaOH) 9.0 9.0 9.0 9.0 9.0 Treatment (% on wood NaOH) 3.0 3.0 3.0 3.0 3.0 WL Sulfidity (% AA) 30.0 30.0 30.0 30.0 30.0 EDTA Charge (% on wood EDTA) 0.0 0.5 1.0 0.5 1.0 Temperature (° C.) Impregnation 110.0 110.0 110.0 110.0 110.0 Treatment 110.0 110.0 110.0 130.0 130.0 Time (min) Impregnation 30.0 30.0 30.0 30.0 30.0 Treatment 90.0 90.0 90.0 90.0 90.0 EA Consumed (% on wood NaOH) Total 7.4 7.5 7.7 8.4 8.7 Impregnation Stage 7.8 7.7 7.4 7.6 7.6 Treatment Stage −0.4 −0.2 0.4 0.9 1.0 Residual EA (g/L NaOH) End of Impregnation 3.5 3.6 4.6 4.1 3.9 End of Treatment 4.7 4.2 3.6 1.6 1.0

[0058] 5 TABLE 5 Metals in chips, brownstock, and black liquor for Northern softwood. Treatment Treatment Ca Cu Fe Mg Mn Ca Cu Fe Mg Mn EDTA Temperature Time Descriptive Repli- (mg/ (mg/ (mg/ (mg/ (mg/ (mg/ (mg/ (mg/ (mg/ (mg/ LIMS ID (% on wood) (° C.) (min) ID cate L) L) L) L) L) kg) kg) kg) kg) kg) 99-001208 — — — S1159 1 798.0 0.7 6.9 149.0 105.0 CHIPS 2 730.0 0.6 4.3 139.0 89.5 Mean 764.0 0.6 5.6 144.0 97.3 S.D. 48.1 0.0 1.8 7.1 11.0 99-001206 0.0 95 45 S1159 AI 1 42.9 0.3 2.1 6.0 4.9 2 39.6 0.3 1.7 4.9 4.7 Mean 41.3 0.3 1.9 5.4 4.8 S.D. 2.3 0.0 0.3 0.7 0.1 99-001207 S1159 ET 1 41.2 0.3 1.3 4.5 4.9 99-001209 S1159 1 695.0 0.4 13.4 95.9 57.0 TREATED 2 743.0 0.4 17.5 101.0 66.0 CHIPS Mean 719.0 0.4 15.4 98.5 61.5 S.D. 33.9 0.0 2.9 3.6 6.4 99-001246 1.0 95 45 S1160 AI 1 37.6 0.3 1.5 5.2 4.6 99-001246 2 36.2 0.3 1.4 4.1 4.5 Mean 36.9 0.3 1.5 4.7 4.6 S.D. 1.0 0.0 0.0 0.8 0.1 99-001247 S1160 ET 1 363.9 0.4 2.2 12.9 12.9 2 363.0 0.3 2.0 12.3 13.0 Mean 363.5 0.4 2.1 12.6 13.0 S.D. 0.6 0.1 0.1 0.4 0.1 99-001248 S1160 1 316.0 0.3 6.6 92.1 25.6 TREATED 2 362.0 0.3 7.4 95.1 35.0 CHIPS Mean 339.0 0.3 7.0 93.6 30.3 S.D. 32.5 0.1 0.6 2.1 6.6 99-001258 1.0 95 90 S1161 AI 1 40.3 0.3 2.2 6.1 5.1 2 38.9 0.3 2.1 5.2 6.5 3 39.4 0.3 2.3 5.2 5.1 Mean 39.5 0.3 2.2 5.5 5.5 S.D. 0.7 0.0 0.1 0.5 0.8 99-001259 S1161 ET 1 330.7 0.4 2.9 13.3 15.0 2 336.0 0.3 2.7 12.8 15.0 Mean 333.4 0.4 2.8 13.1 15.0 S.D. 3.7 0.0 0.2 0.4 0.0 99-001260 S1161 1 435.0 0.3 6.2 107.0 27.1 TREATED 2 245.0 0.3 8.1 78.5 19.8 CHIPS Mean 340.0 0.3 7.2 92.8 23.4 S.D. 134.4 0.0 1.4 20.2 5.2 99-001277 1.0 110 45 S1162 AI 1 46.0 0.3 3.0 7.0 5.3 99-001278 S1162 ET 1 300.0 0.4 3.1 16.4 13.5 99-001279 S1162 1 349.0 0.6 47.0 99.9 44.3 TREATED 2 281.0 0.5 36.7 77.8 21.0 CHIPS Mean 315.0 0.5 41.9 88.9 32.7 S.D. 48.1 0.1 7.3 15.6 16.5 99-001321 1.0 110 90 S1163 AI 1 51.0 0.3 3.4 7.1 5.7 2 51.0 0.3 3.3 7.1 5.6 Mean 51.0 0.3 3.4 7.1 5.6 S.D. 0.0 0.0 0.0 0.0 0.1 99-001322 S1163 ET 1 328.0 0.4 4.2 20.5 16.4 99-001324 S1163 1 163.0 0.4 19.9 71.6 11.3 TREATED 2 144.0 0.4 19.7 66.6 10.0 CHIPS Mean 153.5 0.4 19.8 69.1 10.6 S.D. 13.4 0.0 0.1 3.5 0.9 99-002417 1.0 — — S1182 EOC 1 225.0 0.3 47.5 17.6 10.0 99-002421 normal swing CK with S1182 SP 1 183.0 3.1 82.2 130.0 18.7 EDTA at impreg. 2 182.0 3.9 83.8 127.0 18.1 Mean 182.5 3.5 83.0 128.5 18.4 S.D. 0.7 0.6 1.1 2.1 0.4 99-002418 0.0 — — S2222 EOC 1 36.8 0.3 11.4 10.0 8.0 99-002422 normal S2222 SP 1 1184.0 4.7 31.3 140.0 44.9 swing 2 1199.0 10.1 32.4 140.0 45.7 CK Mean 1191.5 7.4 31.8 140.0 45.3 S.D. 10.6 3.8 0.8 0.0 0.5 99-002434 0.0 — — AL622 AI 1 55.4 0.4 1.6 6.7 7.4 99-002435 normal Lo-Solids ® AL622 EDI 1 55.6 0.3 1.9 6.2 5.4 99-002437 AL622 EOC 1 53.6 0.4 2.2 8.5 7.9 99-002441 AL622 SP 1 837.0 0.8 13.6 120.0 43.9 2 843.0 1.4 81.3 127.0 44.1 Mean 840.0 1.1 47.4 123.5 44.0 S.D. 4.2 0.4 47.9 4.9 0.1 99-002455 1.0 — — AL624 AI 1 298.0 0.3 2.4 18.3 17.5 99-002456 normal Lo-Solids ® AL624 EDI 1 199.0 0.3 2.4 13.1 9.5 99-002458 with EDTA at impreg. AL624 EOC 1 150.0 0.3 3.3 11.0 6.9 99-002462 AL624 SP 1 287.0 1.2 10.9 90.0 7.5 2 277.0 1.5 11.0 90.0 7.6 Mean 282.0 1.3 11.0 90.0 7.5 S.D. 7.1 0.2 0.1 0.0 0.0 99-002490 1.0 110 90 AL625 AI 1 44.2 0.3 1.9 5.3 6.4 99-002491 special Lo-Solids ® with EDTA at AL625 ETI 1 287.0 0.6 2.5 16.3 20.2 99-002492 treatment after impregnation AL625 EDI 1 192.0 0.3 2.1 12.2 9.3 99-002494 AL625 EOC 1 142.0 0.4 4.2 10.2 6.9 2 142.0 0.4 4.4 10.1 7.0 Mean 142.0 0.4 4.3 10.2 7.0 S.D. 79.4 0.1 1.2 3.6 5.4 99-002498 AL625 SP 1 218.0 0.9 15.4 73.0 5.9 2 220.0 1.9 23.6 72.0 6.0 Mean 219.0 1.4 19.5 72.5 5.9 S.D. 1.4 0.7 5.8 0.7 0.0 99-003958 1.0 — — AE647SP 1 186.2 0.6 12.7 90.3 8.9 EMCC ® with EDTA at 2 183.8 2.8 13.9 91.5 9.1 impreg. Mean 185.0 1.7 13.3 90.9 9.0 S.D. 1.7 1.6 0.8 0.8 0.1

[0059] 6 TABLE 6 Metals in chips, brownstock, and black liquor for Southern hardwood. Treatment Treatment Ca Cu Fe Mg Mn Ca Cu Fe Mg Mn EDTA Temperature Time Descriptive Repli- (mg/ (mg/ (mg/ (mg/ (mg/ (mg/ (mg/ (mg/ (mg/ (mg/ LIMS ID (% on wood) (° C.) (min) ID cate L) L) L) L) L) kg) kg) kg) kg) kg) 99-001723 — — — S1166 1 1074.0 1.2 9.4 275.3 140.7 CHIPS 2 2208.0 1.2 10.2 377.0 88.8 Mean 1641.0 1.2 9.8 326.2 114.8 S.D. 801.9 0.0 0.5 71.9 36.7 99-001721 0.0 110 90 S1166 AI 1 26.5 0.4 1.0 4.0 1.4 99-001722 S1166 ET 1 39.5 0.5 1.1 8.0 2.6 99-001724 S1166 1 1673.0 0.4 6.6 282.9 49.4 TREATED 2 926.0 0.5 6.4 351.7 106.1 CHIPS Mean 1299.5 0.5 6.5 317.3 77.8 S.D. 528.2 0.1 0.2 48.6 40.1 99-001746 0.5 110 90 S1168 AI 1 28.9 0.4 1.4 4.4 1.6 2 28.8 0.4 1.4 4.4 1.7 Mean 28.9 0.4 1.4 4.4 1.6 S.D. 0.1 0.0 0.0 0.0 0.0 99-001747 S1168 ET 1 95.7 0.4 1.8 8.1 3.0 99-001748 S1168 1 1857.0 0.4 13.2 223.1 21.1 TREATED 2 798.6 0.3 9.5 228.6 25.7 CHIPS Mean 1327.8 0.3 11.3 225.9 23.4 S.D. 748.4 0.1 2.6 3.9 3.2 99-001750 1.0 110 90 S1169 AI 1 32.6 0.3 1.4 5.9 1.9 99-001751 S1169 ET 1 134.0 0.3 2.0 4.2 5.2 99-001752 S1169 1 390.2 0.4 6.4 242.7 25.6 TREATED 2 279.6 0.3 6.3 185.1 20.7 CHIPS Mean 334.9 0.3 6.4 213.9 23.2 S.D. 78.2 0.1 0.1 40.7 3.5 99-001774 0.5 130 90 S1170 AI 1 31.4 0.4 1.3 5.5 1.9 99-001775 S1170 ET 1 100.8 0.3 2.1 11.6 4.0 99-001776 S1170 1 1196.0 1.0 11.3 245.0 29.7 TREATED 2 952.5 1.0 11.7 296.8 34.4 CHIPS Mean 1074.3 1.0 11.5 270.9 32.0 S.D. 172.2 0.0 0.3 36.6 3.3 99-001792 1.0 130 90 S1171 AI 1 27.4 0.3 1.1 4.2 1.5 99-001793 S1171 ET 1 125.0 0.3 2.5 17.7 5.3 99-001794 S1171 1 320.4 0.5 12.9 178.7 13.4 TREATED 2 201.6 0.4 9.6 139.7 6.4 CHIPS Mean 261.0 0.5 11.3 159.2 9.9 S.D. 84.0 0.1 2.3 27.6 5.0

[0060] 7 TABLE 7 Metal Removal During Kraft Cooking Northern Canadian SWD Effect of EDTA Addition to Post-impregnation Treatment Stage on Metal Removal. Sample Orig. Chips S1159 S1160 S1161 S1162 S1163 EA Charge (% NaOH) Impregnation 9.0 9.0 9.0 9.0 9.0 Treatment 3.0 3.0 3.0 3.0 3.0 WL Sulfidity (% AA) 30 30 30 30 30 EDTA Charge (%) 0 1.0 1.0 1.0 1.0 Temperature (° C.) Impregnation 95 95 95 110 110 Treatment 95 95 95 110 110 Time (min) Impregnation 30 30 30 30 30 Treatment 45 45 90 45 90 Metal Content (mg/kg) Ca 764 719 339 340 315 154 Cu 0.6 0.4 0.3 0.3 0.5 0.4 Fe 6 15 7 7 42 20 Mg 144 98 94 93 89 69 Mn 97 61 30 23 33 11 % Mn Removal 37% 69% 76% 66% 89%

[0061] 8 TABLE 8 Metal Removal During Kraft Cooking Northern Canadian SWD Effect of EDTA Addition to Impregnation and Post-impegnation Treatment Stages. Cook ID S2222 S1182 AL622 AL624 AE647 AL625 Cook Type CK Ref. CK LS Ref. LS EMCC LS EDTA Charge (%) NA 1.0% NA 1.0% 1.0% 1.0% at Imprg. at Imprg. at Imprg. at Treat. Temperature (° C.) Impregnation 110 110 110 110 110 95 Treatment NA 110.0 NA NA NA 110.0 Cooking 170.0 170.0 158.5 158.5 2.3 158.5 Kappa Number 24.9 24.4 25.5 26.1 23.6 22.0 Viscosity (mPa · s) 32.3 32.1 44.6 51.4 54.9 68.5 Viscosity/Kappa Ratio 1.30 1.32 1.75 1.97 2.33 3.11 Total Yield (% on wood) 45.7 46.7 44.9 45.5 45.4 44.8 Rejects (% on wood) 0.20 0.50 0.03 0.02 0.12 0.01 Metal Content (mg/kg) Ca 1192 183 840 282 185 219 Cu 7.4 3.5 1.1 1.3 1.7 1.4 Fe 32 83 47 11 13 19 Mg 140 129 124 90 91 73 Mn 45 18 44 7.5 9.0 5.9 % Mn Removal 53% 81% 55% 92% 91% 94%

[0062] 9 TABLE 9 Metal Removal During Kraft Cooking Southern USA HWD Table. Summary of treatment conditions and treatment results for Southern hardwood. Sample ID S1166 S1168 S1169 S1170 S1171 Org. Chips Impregnation Time, min. 30 30 30 30 30 Impregnation Temperature, ° C. 110 110 110 110 110 EDTA Treatment Time, min. 90 90 90 90 90 EDTA Treatment Temperature, ° C. 110 110 110 130 130 EDTA Charge, % on wood 0.0 0.5 1.0 0.5 1.0 Metal Content, mg/kg Ca 1300 1328 335 1074 261 1641 Cu 0.5 0.3 0.3 1.0 0.5 1.2 Fe 6.5 11.3 6.4 11.5 11.3 9.8 Mg 317 226 214 271 159 326 Mn 77.8 23.4 23.2 32.0 9.9 115 % Mn Removal 32% 80% 80% 72% 91% Notes: Drained off ⅓ BL after Impregnation. WL sulfidity 30%; 9% and 3% to Impreg. and treatment, respectively.

[0063] FIG. 3 illustrates a modified version of the FIG. 2 embodiment in which the vessel 25 is modified. In FIG. 3, the optional cooking circulation 36 has been deleted. A novelty of the FIG. 3 embodiment is the dual center-pipe discharge associated with the upper cooking circulation. In the upper circulation of FIG. 2, when the chelant containing liquor is introduced via conduit 31, some of the chelant passes counter-currently upward and complexes metal-containing compounds as desired, but some of the chelant passes downward with the downward flowing slurry, and may be removed via extraction screen 35. It is preferred that the chelant pass upward and treat the slurry before being removed via screen 27. In the embodiment shown in FIG. 3, the recirculation conduit 31 is split into at least two liquor introduction conduits 131 and 132. Chelating agent is introduced to conduit 132 via conduit 129. Optionally, at least some chelating agent, may also be introduced to conduit 131 via conduit 130, though typically, little or no chelant is introduced via conduit 130. Conduit 131, containing little or no chelating agent, discharges as is conventional in the vicinity of screen 133. Conduit 132, containing at least some chelating agent discharges at a point above the discharge of conduit 131. As a result, the chelating agent introduced via the upper outlet of conduit 132 passes upward with the counter-current flow of liquid, treats the down-flowing slurry, and is removed via screen 27 and conduit 28. The liquid in conduit 131, again, containing little or no chelant, is introduced in the vicinity of screen 133 and preferably passes downward into the cooking process that takes place below screen 133. According to this embodiment, little or no chelant introduced via conduit 129 is “lost” to the pulping process, and as much chelant as possible is present in the counter-current flow between screens 133 and 27 to sequester metal-containing compounds.

[0064] Thus the FIG. 3 embodiment teaches introducing two different liquor flows into the zone 30, an upper flow which contains chelant and primarily or substantially exclusively moves upwardly in vessel 25 with upflowing liquid to be removed through screen 27, while the liquor introduced at 131 primarily or substantially exclusively moves downwardly with slurry into lower zones of vessel 25.

[0065] Although the present invention is described in relationship to the kraft process, it is to be understood that it is also applicable to all conventional chemical pulping processes. These include the alkaline processes, such as the kraft process (also known as the sulfate process), soda process, or soda-AQ process. It is also applicable to acidic processes, such as the sulfite process and its derivatives. The present invention is also applicable to processes employing yield or strength-enhancing additives, such as AQ, polysulfide, and their equivalents and derivatives, and surfactants and penetrants. The present invention is also applicable to batch pulping processes as well as the continuous processes described in the drawings.

[0066] In all of the descriptions above, all narrower ranges within each broad range are also specifically disclosed herein. For example a consistency of 8-20% means 8-12%, 9-16%, 10-18%, and all other narrower ranges within the broad range.

[0067] It will thus be seen that according to the present invention by effecting an extraction of metals at an early stage in the digestion process, preferably right after impregnation, without any chelating agent treatment, and subsequently performing the chelating agent treatment, the amount of metals removed from the wood chip slurry during chemical pulping can be maximized for a given amount of chelant. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent methods and products.

Claims

1. A method of producing chemical pulp from comminuted cellulosic fibrous material containing metal-containing compounds, comprising:

(a) pressurizing and slurrying the comminuted cellulosic fibrous material containing metal containing compounds, and heating the slurry;

(b) digesting the material in the slurry, including in different stages, impregnating the material with cooking liquor and cooking the material to produce chemical pulp;

(c) at an early stage during digestion, extracting liquid containing some of the metal containing compounds therein, and removing the metal containing compounds from the slurry;

(d) after (c), adding a chelating agent to the slurry to combine with released metal containing compounds in the slurry to produce metal complexes;

(e) cooking the material; and

(f) removing at least some of the metal complexes formed in (d).

2. A method as recited in claim 1 further comprising, after (e), bleaching the chemical pulp with at least one bleaching chemical adversely affected by at least some of the metal-containing compounds removed in (c) and (f).

3. A method as recited in claim 2 wherein (c) is practiced to remove at least about 30% of the Mn in the material from the material.

4. A method as recited in claim 3 wherein (c) and (f) are practiced to remove at least about 90% of the Mn originally in the material from the material.

5. A method as recited in claim 1 wherein (c) and (f) are practiced to remove at least about 93% of the Mn originally in the material from the material.

6. A method as recited in claim 1 wherein (c) is practiced after impregnation, and just before cooking, at a temperature of between about 105-145° C.

7. A method as recited in claim 6 wherein (d) is practiced for a time period of between about 30-120 minutes, and with a charge of chelating agent of between about 0.05-10 kg/dry ton of material.

8. A method as recited in claim 7 wherein (d) is practiced at a pH of 9 or more.

9. A method as recited in claim 1 wherein (d) is practiced after impregnation, and just before cooking, at a temperature of about 110° C. and for a time period of about 90 minutes, and with a charge of chelant of between about 1-2 kg/dry ton of material.

10. A method as recited in claim 1 wherein the chelating agent is selected from the group consisting essentially of EDTA, DTPA, derivatives and equivalents to EDTA and DTPA, oxalic acid, tartaric acid, and furoic acid.

11. A method as recited in claim 1 wherein (d) is practiced with a charge of chelating agent of between about 0.05-10 kg/dry ton of material.

12. A method as recited in claim 11, wherein (a) through (f) are practiced substantially continuously; and wherein (c) and (f) are practiced to remove at least about 90% of the Mn originally in the material, from the material.

13. A method as recited in claim 1 wherein (d) is practiced to add chelating agent to the slurry prior to the slurry achieving a temperature of about 120° C. or more.

14. A method of producing chemical pulp from comminuted cellulosic fibrous material containing metal-containing compounds comprising:

(a) pressurizing and slurrying the comminuted cellulosic fibrous material containing metal-containing compounds, and heating the slurry;

(b) impregnating the material with cooking liquor at a temperature of about 90° C. or more;

(c) after (b), extracting liquid containing some of the metal-containing compounds therein, and removing the metal containing compounds from the slurry;

(d) after (c) adding a chelating agent to the slurry to combine with released metal-containing compounds in the slurry to produce metal complexes;

(e) cooking the material; and

(f) removing at least some of the metal complexes formed in (d) and (e).

15. A method as recited in claim 14 further comprising, after (e), bleaching the chemical pulp with at least one bleaching chemical adversely affected by at least some of the metal ions removed in (c) and (f).

16. A method as recited in claim 14 wherein (c) and (f) are practiced to remove at least about 93% of the Mn originally in the material from the material.

17. A method as recited in claim 14 wherein (b) is practiced at a temperature of about 95° C. or more, and wherein (d) is practiced at a temperature of between about 105-145° C.

18. A method as recited in claim 17 wherein (d) is practiced for a time period of between about 30-120 minutes, and with a charge of chelating agent of between about 0.05-10 kg/dry ton of material.

19. A method as recited in claim 15 wherein (b) is practiced at a temperature of about 95° C. or more, and wherein (d) is practiced at a temperature of between about 105-145°.

20. A method as recited in claim 19 wherein (d) is practiced for a time period of between about 30-120 minutes, and with a charge of chelating agent of between about 0.05-10 kg/dry ton of material.

21. A method as recited in claim 14 wherein (b) is practiced utilizing alkaline cooking liquor, and wherein (d) is practiced by alkaline cooking.

22. A method as recited in claim 14 wherein (d) is practiced to add chelating agent to the slurry prior to the slurry achieving a temperature of about 120° C. or more.

23. A method as recited in claim 14 wherein (a)-(f) are practiced in an upright vessel having an upper screen and a black liquor extraction screen; and wherein (d) is practiced by adding chelant in a first conduit at a first distance below the upper screen, and above the extraction screen, so that the chelant at least primarily flows with liquid up through the slurry to the upper screen, and wherein (e) is practiced in part by introducing cooking liquor in a second conduit at a second distance below the upper screen, greater than the first distance, so that the cooking liquor flows with the slurry downwardly toward the extraction screen.

24. A chemical pulp made by practicing the method of claim 21.