CA2258387A1 - Removal of transition metal ions from wood pulp fibers - Google Patents

Abstract

A process for the removal of transition metal ions from wood fiber slurries (cellulose pulp, including mechanical and chemical pulps), such as manganese, applies a pulse of solution containing beneficial metal ions, such as magnesium, chelant, or ion exchange chemical (or acid at a pH between about 1-4) through a pulp mat or pad. This leads to an efficient removal of manganese and a good retention of magnesium in pulp fibers at a magnesium sulfate charge as low as about 1 % (on pulp).
Results from subsequent peroxide bleaching show that the MgSO4 eluted pulp has a similar bleaching performance as that of the conventional DTPA chelated pulp. More efficient removal of manganese from pulp fibers can be achieved when DTPA chelation is performed by applying a pulse of DTPA solution through a pulp pad or mat compared to that of a conventional chelation stage.

Description

BP # 9470-27 BERESKIN & PARK Canada Title: REMOVAL OF TRANSITION METAL IONS FROM WOOD PULP
FIBERS
Inventor(s): Y. Ni ~!. Li A. R. P. van Heiningen REMOVAL OF TRANSITION METAL IONS FROM WOOD PULP
FIBERS
BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates to a process for the removal of harmful metal ions from wood pulps so that subsequent bleaching with metal-sensitive bleaching agents, such as hydrogen peroxide, becomes more efficient.
The production of mechanical pulp, in particular that of thermomechanical (TMP) and chemithermomechanical pulp (CTMP), has grown rapidly in recent years. A large fraction of these mechanical and so-called ultra high yield pulps, is bleached with either hydrogen peroxide or hydrosulfite, or both, to increase the brightness. Hydrogen peroxide is favored over sodium hydrosulfite because the former is more effective in improving the pulp brightness.
Generally, mechanical pulps can be bleached with an alkaline hydrogen peroxide solution containing hydrogen peroxide stabilizers, most commonly, sodium silicate and magnesium sulphate, to brightness levels of 10 - 12, 13 - 15 and 16 - 18 points higher that their initial brightness of 50 - 60% ISO using a peroxide charge of 1 %, 2% and 3%
(by weight on pulp) respectively.
Hydrogen peroxide may be decomposed to form oxygen under conditions typically encountered in the alkaline hydrogen peroxide bleaching. This is particularly so when transition metal ions, such as manganese, are present. The hydrogen peroxide decomposition represents a loss of the potential bleaching power of the hydrogen peroxide charged. Therefore, prior to an alkaline hydrogen peroxide stage, it is common practice to remove the metal ions by a chelation stage.
Conventionally, transition metal ions are removed by chelating with sequesters, such as DTPA and EDTA. A typical chelation stage is performed under the following conditions: DTPA charge 0.3 - 0.6%, temperature 25 - 60°C) pH 5 - 7) 1 - 3% pulp consistency, 10 - 45 minutes. Following the chelating stage, the treated pulp is thickened to about 25 to 35 % pulp consistency. The pressate is usually recycled to retain the fines.
In a U.S. Appl. 966,745 and PCT Int. Appl. WO 94 10,375 Prough, J. R., Greenwood, B. F., Wiley, W. E.) Bilodeau, V. L., and Stromberg, C. B.
disclosed a displacement chelate treatment of pulp for metal removal for non chlorine bleaching in two stages. In the first stage, the pulp is washed at medium consistency with a liquor containing a chelating agent.
In the second stage, the pulp is washed with a liquor to displace the chelate and to produce a washed pulp having the majority of metals removed from it. The PCT application) for example, clearly discloses a treatment with chelating agents in one device) retention) and then removal in a second device. The concentration of chelating agent and the retention time between addition and removal are clearly distinct from the present invention.
Brelid et al, in a series of papers (Proceedings, the 8th ISWPC, Helsinki, p. 277; Nordic Pulp and Paper Research J., 9(4):222 (1995) and 10(2):105 (1996); Proceedings; 1996 Intl. Pulp Bleaching Conf.) reported that ion exchange of softwood kraft pulp with calcium and magnesium in a polypropylene beaker at 50°C and 3% pulp consistency for 2 h can remove manganese. However, the concentration of either magnesium or calcium acetate must be very high, for example, higher than 0.05 mol/L in order to be effective. This is equivalent to a magnesium sulfate charge of about 20% (on oven dried pulp fibers). Such a high chemical charge may represent a significant disadvantage for potential commercialization of such a technique.
Hydrogen peroxide was traditionally not used to bleach chemical pulps. However) environmental and market pressures have recently focused attention on Totally Chlorine Free (TCF) bleaching of chemical pulps, and hydrogen peroxide is now one of the key chemicals in a TCF
sequence. Analogous to bleaching of mechanical pulps with alkaline hydrogen peroxide, removal of transition metal ions is vital for an efficient peroxide stage in a TCF sequence. This is again achieved mostly by chelating with sequesters.
Magnesium sulfate is usually added to an alkaline peroxide stage to stabilize the bleaching chemicals. A typical magnesium sulfate charge varies between 0. 1 to 0.5% (on oven dried pulp).
The present invention relates to a process that removes transition metal ions from pulp fibers so that their residual transition metal ion content is decreased and the subsequent peroxide bleaching is improved.
Furthermore, the present invention provides a process wherein the harmful metal ions are efficiently removed by applying a pulse of a solution containing magnesium and/or other beneficial metal ions through a pulp fiber pad at a reasonable chemical charge.
The present invention also shows that when a chelation stage to remove transition metal ions from pulp fibers is performed by sending a pulse of DTPA solution through a pulp fiber bed, the residual metal ion content in the chelated pulps is much lower and furthermore can be achieved with less chelating chemical than in a conventional manner.

The present invention also shows that the residual metal ion content of a pulp can be reduced by applying a pulse of an acidic solution through the pulp fiber bed.
The process according to the present invention may be carried out in commercial washers, such as chain or belt washers) or like washers which form a pulp rnat of relatively small thickness (e.g. about 4 cm or less), and only a single washer need be utilized to remove the majority of transition metal ions (e.g. at least about 80% of the manganese).
The invention has been developed on the basis of a recognition by the inventors that the removal of metal ions with chelants is a thermodynamically controlled process, that is the contact time between the fibers and the chelant does not significantly affect the chelation efficiency, that is the chelation reaction, unlike other reactants typically encountered in pulping or bleaching, is not controlled by time of exposure of the pulp to the chelants or the temperature at which the contact takes place. Rather it has been found according to the invention that the chelation reaction is a very rapid reaction and that only the presence or absence of reactants (i.e. metal ions and chelating agents) affects the efficacy of the treatment. Thus) it has been found that it is possible to treat metal ion containing pulp with a high concentration of chelants over a short period of time and achieve improved metal removal, and improved bleaching using metal-sensitive bleaching chemicals.
According to one aspect of the present invention a method of removing transition metal ions from cellulose pulp (including mechanical pulp such as TMP) CTMP) and CMP as well as chemical pulp such as kraft or sulfite pulp) is provided comprising the steps of: (a) bringing a solution of chelant, ion exchange chemical, or both chelant and ion exchange chemical, into contact with a mat of a cellulose pulp liquid slurry to displace most of the liquid in the pulp liquid slurry; and (b) displacing the solution introduced in step (a) with another liquid to remove transition metal ions from the pulp; and wherein step (a) is practiced for less than five minutes.

5 Step (a) may be practiced using a charge of chelant, such as DTPA, ion exchange chemical, such as MgSO,, or both, that is at least 20% less (typically at least 50% less and possibly only one-fifth of the amount) than the charge that would be required for the same pulp in a conventional chelation manner (such a conventional chelation process is disclosed in igerud, L., "Mill Experiences of Lignox Bleaching", Non-Chlorine Bleaching Conference Notes, Miller Freeman, San Francisco, 1993, and Bouchard, J, Nugent, H.M., and Berry, R.M., "A Comparison Between Acid Treatment and Chelation Prior to Hydrogen Peroxide Bleaching of Kraft Pulps", J. Pulp Paper Science 21 (8):J 268 (1995)) while effecting the removal of at least as many transition metal ions (and typically more) as treatment in a conventional chelation vessel or process.
For example, step (a) may be practiced with a DTPA charge of 0.01-0.3%
(as pure DTPA on pulp), preferably 0.03-0.2% (as pure DTPA on pulp)) or other equivalent chelant or ion exchange chemicals. That is, if some other chelant or ion exchange chemical is used, it is adjusted so that it has the same effective metal ion removal as the DTPA within the charge range as discussed above. This is a reduced chelant charge as compared to that of conventional chelation. The invention is not limited to this range of charges. Higher charges can also be used and a more thorough metal ion removal can be achieved than when conventional methods are used.
Step (a) is typically practiced using a DTPA concentration that is relatively high. That is, the DTPA solution typically contains 0.02-15 g/L

DTPA, preferably about 0.1-2 g/L DTPA. At the same DTPA charge the DTPA concentration during pulse chelation is always much higher (at least ten times higher) than that during conventional chelation. Step (a) is also practiced so that the DTPA solution moves through the pulp (which typically is in mat form) as a relatively narrow band. That is the DTPA
solution is 0.08-3 bed volumes, preferably 0.2-0.8 bed volumes, of the liquid volume contained in the pulp mat. Or another equivalent chelant or ion exchange chemical charge or band is utilized. For example, if the method is practiced in a conventional vacuum drum washer, having a pulp mat with a thickness of about 25-50 mm (i.e. about 1-2 inches) the band of chelant-container solution passing through the mat preferably has a width ranging from approximately 5.0 to 40 mm (that is, 0.2 times 25 mm to 0.8 times 50 mm).
Step (a) may also be practiced using an acid in combination with a chelant or other ion exchange chemical, such as MgS04. The solution containing the acid typically has a pH less than 6, typically between 1 and 4. Typical acids that may be used include sulfuric acid, hydrochloric acid and oxalic acid, among others.
Step (a) is typically practiced for about a minute or less, and steps (a) and (b) are practiced for less than about 10 minutes. Steps (a) and (b) may be practiced by forming a substantially undisturbed fiber mat in a single drum (e.g. vacuum drum) or belt washer and effect the removal of more than' 50% of the transition metal ions, and preferably more than about 90% of these ions, from the pulp. Steps (a) and (b) may be practiced with a pulp consistency between about 1-45%, preferably between about 6-20% and most preferably between about 8-15%, or other consistency ranges between 1-45%.

According to another aspect of the present invention, a method of removing transition metal ions from cellulose pulp, using a drum, belt, or other washer forming the pulp mat, is provided. The method comprises the steps of: (a) Forming a substantially undisturbed liquid-containing cellulose pulp mat in the washer. (b) In the washer, bringing a solution of chelant, ion exchange chemical, or both chelant and ion exchange chemical, into contact with the cellulose pulp mat. And (c) in the washer, substantially immediately after the practice of step (b), removing the liquid solution added in step (b) to remove transition metal ions from the pulp along with the removed solution.
Step (b) is typically practiced using a charge of chelant, acid, ion exchange chemical, or a combination thereof) or a charge of DTPA or the equivalent, or a concentration of DTPA and a relatively narrow band) as discussed above with respect to the previous aspect of the invention.
Step (c) is typically practiced by introducing a wash liquid into the pulp which displaces the solution added in step (b), and wherein during the practice of steps (b) and (c) the pulp mat has a thickness of less than about 4 cm (e.g. less than about 3 cm).
According to another aspect of the present invention a method of removing transition metal ions from cellulose pulp is provided comprising the following steps: (a) bringing a solution of chelant, exchange chemical, or both chelant and exchange chemical, into contact with a mat of a cellulose pulp liquid slurry using a charge of chelant, ion exchange chemical, or both, that is at least 20% less than the charge that would be required for the same pulp in a conventional chelation vessel, while effecting removal of at least as many transition metal ions as treatment in the conventional chelation vessel; and (b) removing the solution from the pulp. Step (a) is preferably practiced for 60 seconds or less, e.g. 30-50 seconds, and is practiced with the pulp in a moving mat having a thickness of less than 10 cm, e.g. between about 1-2 inches, or about 5 or 6 cm or less. After the treatment as set forth in any of the aspects of the invention described the pulp is typically bleached, such as with peroxide or other non-chlorine bleaching chemical.
According to yet another aspect of the present invention a method of treating a liquid slurry cellulose pulp to remove transition ions therefrom and subsequently bleaching the pulp, is provided by: (a) Forming a moving mat of the cellulose pulp having a thickness of 10 cm or less, moving in a first direction. (b) while the mat is moving in the first direction, causing a solution of chelant, exchange chemical, or both chelant and exchange chemical) to flow through the mat in a second direction, substantially transverse to the first direction. Then (c) while the mat is moving in the first direction, removing the solution from the mat, along with at least 50% of the transition metal ions. And then (d) non-chlorine bleaching the pulp. Step (d) is preferably practiced using peroxide, such as hydrogen peroxide, and steps (a) through (d) may be practiced using mechanical pulp (TMP, CTMP or CMP), or a chemical pulp (such as kraft pulp or sulfite pulp). Step (b) is typically practiced using a solution having a volume of water less than the volume of water in the mat during the practice of step (b).
Steps (b) and (c) may be practiced in a single drum or belt washer, and to remove at least 80% of the manganese from the pulp.
Step (b) may be practiced using DTPA or MgS04 or their equivalents. For example) step (b) may be practiced using a MgS04 solution with a charge of 0.5-2.0 (% on pulp), a concentration of 2-10 g/L, and a bed volume of 0.2-0.8 times the pulp bed volume. Step (b) may be practiced with a pulp consistency of about 8-15%. Step (b) may also be practiced using an acid solution (pH 1-4).
According to the invention transition metal ions are effectively removed from pulp using a relatively high concentration solution of a chelation andlor ion exchange chemical which moves as a well defined and relatively narrow band through a pulp mat. The solution displaces the original liquid inside the fiber mat and, is itself displaced by another liquid, such as wash water, introduced into the mat. Preferably the volume of the solution containing the chelant and/or ion exchange chemical is less than that of the water originally present in the fiber mat.
It is a primary goal of the present invention to provide an effective method of removing metal ions from cellulose pulp which uses much less chelant or ion exchange chemical than conventional treatments, can effect treatment much more quickly than the conventional treatments, and can use simple commercially available washing equipment, for example, only a single conventional washer being utilizable to practice the invention. The invention will become more apparent from an inspection of the detailed description of the invention and from the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic illustration of the utilization of a conventional vacuum drum washer of the practice and method according to the present invention;
FIGURE 2 is a schematic illustration showing the action of the liquids added to the pulp mat using the apparatus of FIGURE 1, and the liquids that are extracted;

FIGURE 3 is a schematic view like that of FIGURE 1 only showing the utilization of a conventional belt washer for the practice of the present invention; and FIGURE 4 is a graphical representation of the improved 5 brightness/peroxide consumption ratio that can be achieved by practicing the method of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
FIGURE 1 schematically illustrates the utilization of a conventional vacuum drum washer in the practice of the method according to the 10 present invention. The pulp stock, for example having a consistency of about 0.05-16% (e.g. about 1-5%), is introduced at inlet 11 to the drum washer 12, and first flows into the formation zone 13 where a cellulose pulp mat is created. The consistency of the mat is between 1-45%, preferably between about 6-20% and most preferably between about 8-15%. It typically has a thickness of between about 1.5 cm - 6 cm, for example between about 1-2 inches, and in many cases less than 4 cm, and even less than 3 cm.
As the drum 12 rotates the pulp mat then moves to the treatment zone 14 where a chelant or ion exchange solution is added as indicated by inlet 15. Almost any chelant (such as DTPA), or ion exchange chemical (such as MgS04), or an acid, rnay be utilized. A relatively high concentration solution is added so that the required volume of chelant used is less than that of the water originally present in the fiber mat. A
"relatively high concentration of DTPA solution" means about 0.02-15 g/L
DTPA preferably 0.1-2 g/L DTPA. It should be noted that at the same DTPA charge the DTPA concentration during the pulse chelation of the invention is always much higher (at least 10 times) than that during a conventional chelation. The chelant solution moves through the fiber mat as a relatively narrow band. That is) the chelant solution is between about 0.08-3 bed volumes, preferably 0.2-0.8 bed volumes, of the liquid volume contained in the pulp mat. For example, if the pulp mat has a thickness of about 25-50 mm the band of chelant-containing solution passes through the mat with a width ranging from approximately 5.0-40 mm (that is 0.2 times 25 mm to 0.8 times 50 mm).
Compared to conventional chelation the "pulse" addition of chelant solution according to the invention results in a reduced chelant charge and also makes it possible to practice the invention in a single washer 12.
A "reduced chelant charge" means a DTPA charge of 0.01-0.3% (as pure DTPA on pulp), preferably 0.03-0.2% (as pure DTPA on pulp). Of course, the exact charge will vary depending upon the wood species and its metal ion content. For example, a pulp from coastal New Brunswick, Canada may have a high metal content so that when using conventional practice a 0.5% charge is used, whereas a pulp produced from wood from Quebec, Canada may have a lower metal ion content, and conventionally require only a 0.2% charge. Practicing the invention) however, the charge may be reduced to one-fifth of the conventional charge, that is to about 0.1 % for the New Brunswick pulp, and .04% for the Quebec pulp.
The required chelate charge according to the present invention is typically at least 20% less than for a conventional treatment, and preferably at least 50% less, and can be only one-fifth that of the conventional treatment.
The parameters described above are for DTPA. However, equivalent parameters are provided for other chelants or ion exchange chemicals. For MgSO, chelation, for example, parameters would be as follows:
Parameters Broad Range Preferable Range MgS04 solution (bed 0.05-3 0.2-0.8 volume) MgSO, charge (% on pulp)0.2-5 0.5-2.0 MgS04 concentration 0.5-25 2-10 (glL) After the addition of the chelant solution at 15, the treatment of the pulp with the chelant typically lasts for less than a minute depending upon the speed and geometry of the drum. The treatment may last for less than about 30 seconds or even less than about ten seconds, and the chelant solution displaces the liquid in the pulp, which is withdrawn in filtrate tine 16 by, for example, a vacuum source associated with the drum 12. After that the pulp mat continues in the direction of rotation of the drum 12 to the wash zone 17 where washing liquid is added, such as schematically illustrated by inlet 18 in FIGURE 1, to displace the chelant solution from the pulp. Washing typically lasts less than one minute (typically 30 seconds or less) and the displaced chelant solution, as well as more than 50% of the transition metal ions (typically more than 80%) e.g. 90% or even more, of the manganese), is withdrawn from the pulp in the filtrate line 19. The vastly metal depleted pulp then leaves the washer at outlet 20 of the drum 12 and is passed (with or without further treatments to one or more bleaching stages 21, typically a hydrogen peroxide bleaching stage or stages.
FIGURE 2 schematically illustrates the practice of a method of the invention utilizing a single washer. The pulp mat is shown schematically by reference numeral 22 in FIGURE 2 and has a thickness 23, which typically is between about 1-10 cm) typically between about 1-2 inches, and in many situations less than 6 cm, or 5 cm or less. The chelant, or ion exchange chemical (including acid), solution - indicated by arrows 24 - passes in a narrow band (that is, a narrow band having a thickness of about 0.2-0.8 times the thickness 23) inta the pulp mat 22, displacing the liquid in the pulp, which is indicated by arrows 25. The pulp then continues to the wash stage where wash liquid -- indicated by the arrows 26 - is applied over a relatively large area to completely displace the solution and the transition metal ions; the displaced solution and ions are indicated by the arrows 27 in FIGURE 2.
FIGURE 3 illustrates the utilization of a conventional belt washer, shown generally by reference numeral 30, in the practice of the method of the invention. The pulp stock is fed at inlet 31 and the pulp mat is formed in the formation zone 32. The moving pulp mat is then subjected to the chelant/ion exchange chemical solution, introduced at 33 in FIGURE 3 in the chelation zone 34, with displaced liquid from the pulp removed in filtrate line 35. The pulp continues to move on the belt of the belt washer 30 to the washing zone 36 where wash liquid is introduced as indicated by inlet 37, and displaces the chelant solution and the removed metal ions, which are removed in filtrate line 38. The metal-depleted pulp then is discharged from the outlet 39 and bleached andlor otherwise treated.
While conventional drum and belt washers are shown in FIGURES
1 and 3, they are shown only schematically and it should be understood that a wide variety of washers might be utilized. For example, washers as disclosed in the following U.S. patents may be utilized for practice of the present invention since they all form a pulp mat which can be subjected to the chelation solution and then the washing liquid: 4,266,413, 4,769,986, 4,795,170, 4,919,158, 5,116,423, 5,120,398, 5) 139,671 and 5,264) 138. It is also noted that, according to the present invention) the chelant solution and metal ions may be removed by pressing instead of or in addition to displacement washing.
The method of the present invention is illustrated by the following examples:
Example 1 A TMP pulp (174 ppm Mn) 22 ppm Fe, 128 ppm Mg, 855 ppm Ca, 18.56 g oven-dried pulp) was disintegrated with a blender in deionized water at a pulp consistency of 0.05% and then poured into a displacement cell while draining to form a pulp fiber pad. (The cell was made of stainless steel and has a cylindrical shape of 200 mm height and 75 mm inside diameter). After drainage the fiber pad was compressed to the desired mat thickness (40 mm) so that the required pulp consistency was obtained. It was found that over 90% of the metal ions remain with the pulp fiber after the pad formation under the present conditions. The desired temperature of 70°C was achieved by circulating hot water in the shell around the cylindrical cell. Subsequently, a MgS04 solution containing 4.8 gIL (as MgSO,) was pumped through the pulp pad with a peristaltic pump at a flow rate of 52 mUmin. After 48 seconds, which is equivalent to a pulse of I cm in thickness of the magnesium sulfate solution, and to a magnesium sulfate charge of 1.02% (on oven-dried pulp), deionized water was pumped through the fiber pad at 52 mUmin for about 9 min. The pulp pad was then taken out from the cell, air dried and analyzed for metal ion contents with an Atomic Absorption Spectrophotometer in accordance with Tappi Testing methods. The contents of manganese, iron, magnesium and calcium of the resulting pulp are 5.4 ppm, 8 ppm, 425 ppm and 44 ppm) respectively. This indicates that with about 1% MgSO,, the harmful manganese present in the pulp fibers can be effectively removed.

Example 2 For comparison, it will be shown that the residual manganese content after a pretreatment with MgS04 in a static system is much higher than that obtained with the above described process.
5 The same TMP Spruce pulp (10 g oven-dried pulp) was disintegrated with a blender in deionized water. Equivalent to 5.4%
MgS04 (by weight on pulp) was added to the pulp slurry. The chelation was performed in a plastic bag at 60°C, 10% pulp consistency for 60 minutes. After this time period, the pulp slurry was transferred to a 10 buchner funnel, and pressed to about 30% pulp consistency. One portion of the pressed pulp was air dried, and analyzed for its metal ion content.
Another portion of the pressed pulp was thoroughly washed with about 200 mL deionized water, air dried and analyzed for its metal ion content.
The results are listed in Table 1. Compared to the results in Example 1 15 (also shown in Table 1), one can conclude that although the MgS04 charge is about 5 times higher in this example, the residual manganese content is substantially higher. It is known, for example, that peroxide bleaching can be very sensitive to the presence of manganese and minimum manganese content is preferred.
Table 1. Transition Metal Content of the TMP Pulp Samples Metal ion contents in pulp (ppm) Mn Fe Mg Ca Original Pulp 174 22 128 855 MgS04 (5.4%) treated and 36.6 6 1327 168 pressed MgS04 (5.4%) treated and 24.4 8 341 63 washed MgSO, pulse (1.02%) treated5.4 8 425 44 and washed Examale 3 In this example, it will be shown that a higher brightness can be achieved for the pulps treated in Example 1 (the sample is denoted as "1.02% MgS04 (eluted)" in the following discussion) during the subsequent peroxide bleaching compared to the pulps treated in Example 2 (the samples are denoted as "5.4% MgS04 (+pressed)" and "5.4%
MgS04 (+ washed)") respectively) under the same hydrogen peroxide bleaching conditions. The alkaline peroxide bleaching was carried out with 3% or 5% H202 charge. The other conditions are: 10% pulp consistency) 3.0% NaZSi03, 70°C, 120 min. The results are presented in Table 2.
Table 2.
Bleaching Responses of the Subsequent Peroxide Stage H~OZ NaOH Hz02 charge Sample charge consumptionBrightness (%) (%) (%) (% ISO) 3 1.02% MgS04 (eluted)2.4 1.95 74.7 3 5.4% MgSO, (+ pressed)2.4 2.50 65.5 3 5.4% MgSO, (+ washed)2.4 2.20 72.8 5 1.02% MgS04 (eluted)4.0 2.69 78.5 5 5.4% MgSO, (+ pressed)4.0 4.12 72.2 5 5.4% MgSO, (+ washed)4.0 3.08 77.8 Table 2 clearly shows that the magnesium sulfate eluted sample, according to the present invention, responds the best to the subsequent peroxide bleaching, leading to the production of bleached TMP with the highest brightness compared to the other two pulp samples at the same hydrogen peroxide charge. Furthermore, the hydrogen peroxide consumption is the least.

Example 4 In this example, it will be shown that similar bleaching results can be obtained from peroxide bleaching of the manganese depleted pulps with either DTPA in a conventional manner or MgS04 elution, following the procedure in Example 1.
The same TMP pulp as in Example 1 was used in this example.
The removal of manganese with DTPA in a conventional manner was performed in a plastic bag under the conditions of: 0.5% DTPA charge, 60°C, 3% pulp consistency, 30 min. Subsequently, the chelated pulp was.
washed thoroughly with deionized water) and subjected to a peroxide bleaching under the conditions of: 3% H202, 2.4 % NaOH, 3% Na2Si03, 0.10% MgS04, 70°C, 10 % pulp consistency and 2 hours. The resulting pulp has a brightness of 73.8% ISO and b* of 12.35.
The manganese depleted pulp with MgS04 elution was obtained in Example 1 with a total MgS04 charge of 1.02%. The subsequent peroxide bleaching was performed under exactly the same conditions specified in the previous paragraph except that no MgS04 was added.
The resulting pulp has a brightness of 74.4% (SO and b* of 12.16.
Example 5 The process of Example 1 wherein the MgS04 concentration was changed from 0.83 to 12 g/L and the thickness of the MgS04 pulse varied from 0.4 to 4 cm. All the other conditions and procedures were the same as those ih Example 1. The residual metal ion contents in the resulting pulp are shown in Table 3.

Table 3.
Removal of Manganese by Elution with MgSO, Containing Solution MgSO, Thickness of MgSO, Residual Run concentrationthe charge metal (g~~) MgS04 pulse (% on pulp)ion (cm) content (ppm) Mn Mg Ca 1 0.83 4 0.70 11 493 61 2 1.66 2 0.70 14 499 69 4 3.3 1 0.70 21 454 86 12 0.4 1.02 19 428 48 6 12 1 2.55 4 456 21 7 12 2 5.10 1 599 1 Again these results confirm that the removal of harmful metal lon by elution with MgS04 solution is efficient, although the best conditions 5 appear to be those specified in Example 1.
Example 6 In this example) it will show that elution with a pulse of DTPA
containing solution through a pulp fiber pad can remove more harmful metal ions, such as manganese with less DTPA charge than a conventional DTPA chelation stage. A CTMP Spruce pulp (36.5 ppm Mn.
32.0 ppm Fe, 18.5 g oven dried pulp) was used. The pulp fiber pad was prepared following the procedure in Example 1. Instead of MgSO,, a pH 6 DTPA solution containing 0.1 g/L DTPA was pumped through the fiber pad a# a flow rate of 45 mUmin. The temperature was operated at 70°C.
After 90 seconds, which is equivalent to a pulse of 1.7 cm in thickness of the DTPA solution and to a DTPA charge of 0.036% (on oven dried pulp) deionized water was pumped through the fiber bed at 45 mUmin for about 3.5 minutes. The residual contents of manganese and iron of such chelated pulp is 1 ppm and 14 ppm respectively.

For comparison, it will be shown that the residual contents of iron and manganese after a conventional chelation stage are higher than those obtained with the DTPA elution technique described above.
The same CTMP Spruce pulp (10 g oven-dried pulp) was disintegrated with a blender in deionized water. Equivalent to 0.5% DTPA
(by weight on pulp) was added to the pulp slurry and its pH was adjusted to about 6 by the addition of sulfuric acid. The chelation was performed in a plastic bag at 70°C, 3% pulp consistency for 30 minutes. After this time period, the pulp slurry was transferred to a buchner funnel, and pressed to about 30% pulp consistency. One portion of the pressed pulp was air dried, and analyzed for its content of iron and manganese. Another portion of the pressed pulp was thoroughly washed with about 200 mL
deionized water, air dried and analyzed for the contents of iron and manganese. The results are listed in Table 4. One can conclude that although the DTPA charge is about 14 times higher in the conventional treatment. The residual Mn content is substantially higher than that present in the DTPA eluted sample.
Table 4. Transition Metal Content of the CTMP Pulp Residual transition Samples metal ion contents in pulp (ppm) Mn Fe Original pulp 36.5 32.0 Chelated (0.5%) 13.5 22.2 and pressed Chelated (0.5%) 5.5 17.0 and washed Chelated eluted 1.0 14.0 (0.036%) and washed Example 7 In this example, it will be shown that a higher brightness can be achieved for the DTPA eluted pulp during the subsequent peroxide bleaching compared to the conventionally chelated pulp under the same 5 hydrogen peroxide bleaching conditions.
A TMP pulp from a mill in Eastern Canada (54.8% ISO, 144 ppm Mn, 18 ppm Fe, 21.20 g o.d. pulp) was used. The procedures for the DTPA elution were the same as in Example 6. The conditions were:
70°C, mUmin. 12% pulp consistency, pH of the DTPA solution of 6, DTPA
10 concentration of 0.4 gIL. After 2.6 minutes, which is equivalent to a pulse of 1. 9 cm in thickness of the DTPA solution and a DTPA charge of 0.17%
(by weight on pulp), deionized water was pumped through the fiber bed at 30 mUmin for about 5.2 min. The residual content of manganese and iron of the chelated pulp is 1.0 ppm and 19 ppm respectively (the sample 15 is denoted as "0. 17% DTPA (eluted)" in the following discussion). This indicates that with 0.17% DTPA charge) the manganese present in the pulp can be efficiently removed.
The same TMP pulp was chelated in a plastic bag under the conditions of: 3% pulp consistency, 0.5% DTPA charge, 70°C, 60 20 minutes, pH of about 6. After the required residence time. the pulp slurry was transferred to a buchner funnel and pressed to about 30% pulp consistency. One portion of the pressed pulp (the sample denoted as "0.
5% DTPA' (+ pressed)" in the following discussion) was air dried and analyzed for its content of iron and manganese. Another portion of the 25 pressed pulp was thoroughly washed with deionized water (the sample is denoted as "0.5% DTPA (+ washed)" in the following discussion)) air dried and analyzed for its content of iron and manganese. The results, are listed in Table 5.

Table 5. Transition Metal Content of the TMP Pulp Samples Mn Fe Unchelated pulp 144 18 0.5% DTPA (+ washed) 11 19 0.5% DTPA (+ pressed) 40 21 0.2% DTPA (+ washed) 20 19 0.17% DTPA (eluted) 1.0 19 The same TMP was chelated in a plastic bag under the same conditions as above except that the DTPA charge is now 0.2% (by weight on pulp). After a residence time of 60 minutes. the pulp slurry was transferred to a buchner funnel and pressed to about 30% pulp consistency. Subsequently, the pressed pulp was thoroughly washed with deionized water (the sample is denoted as "0.2% DTPA (+ washed)"
in the following discussion)) air dried and analyzed for its content of iron and manganese. The results are included in Table 5.
Subsequently, four chelated pulps in accordance with the above techniques, i.e. 0.17% DTPA (eluted), 0.5% DTPA (+ washed), 0.5%
DTPA (+ pressed), 0.2% (+ washed), were subjected to alkaline peroxide bleaching with various hydrogen peroxide charges. The results are presented in Table 6 and Figure 4. Other conditions for the H202 bleaching are: 10% pulp consistency, 3.0% Na2Si03, 0.1 % MgS04, 70°C, 120 min. Figure 4 clearly shows that the DTPA eluted pulp responds the best in the subsequent peroxide bleaching, leading to the production of bleached TMP with the highest brightness compared to the conventional chelated pulps at the same hydrogen peroxide charge. Furthermore, the hydrogen peroxide consumption is the least.

Table 6.
Bleaching Responses of Different Chelated Pulps NAOH HzOz HzOZ Sample charge consumptionBrightness % % (% ISO) 1 0.17% DTPA (eluted) 0.8 0.89 66.74 1 0.5% DTPA (+ washed) 0.8 0.92 63.52 1 0.5% DTPA (+ pressed)0.8 0.96 62.13 1 0.2% DTPA (+ washed) 0.8 0.92 63.36 2 0.17% DTPA (eluted) 1.6 1.43 72.48 2 0.5% DTPA (+ washed) 1.6 1.64 70.47 2 0.5% DTPA (+ pressed)1.6 1.87 67.99 2 0.2% DTPA (+ washed) 1.6 1.74 68.77 3 0.17% DTPA (eluted) 2.4 1.99 76.25 3 0.5% DTPA (+ washed) 2.4 2.30 74.08 3 0.5% DTPA (+ pressed)2.4 2.77 71.63 3 0.2% DTPA (+ washed) 2.4 2.54 72.39 4 0.17% DTPA (eluted) 3.2 2.49 78.12 4 0.5% DTPA (+ washed) 3.2 3.06 76.20 4 0.5% DTPA (+ pressed)3.2 3.81 72.10 4 0.2% DTPA (+ washed) 3,2 3.64 73.10 Example 8 A Western Canadian softwood kraft pulp (30 kappa no.) 142 ppm manganese, 18.56 g oven-dried pulp) was disintegrated with a blender in deionized water at a pulp consistency of 0.1 % and then poured into a displacement cell while draining to form a pulp fiber pad. (The cell was made of stainless steel and has a cylindrical shape of 200 mm height and 75 mm inside diameter.) After drainage the fiber pad was compressed to the desired mat thickness (40 mm) so that the required pulp consistency was obtained. The desired temperature of 25°C was achieved by circulating water in the shell around the cylindrical cell. Subsequently, a sulfuric acid solution having a pH of 1.5 was pumped through the pulp pad with a peristaltic pulp at a flow rate of 51.3 mUmin. After 78 seconds, which is equivalent to a pulse of 1.6 cm in thickness of acidic solution, or 0.4 bed volume, deionized water was pumped through the fiber pad at 51.3 mUmin for about 3.5 min. The pulp pad was then taken out from the cell, air dried and analyzed for the residual Mn content with an Atomic Absorption Spectrophotometer in accordance with Tappi Testing methods. The manganese content of the pulp is 2.7 ppm. This indicates that the pulse chelation technique with an acid as an ion exchange chemical can effectively remove the harmful manganese present in the pulp fibers.
In practicing the invention using acid, as set forth above, preferably the pH is between 1-4, with at least 20% (e.g. greater than 35%) less acid solution necessary than in conventional acid treatment per unit of pulp treated.
It has thus been shown that according to the present invention various advantageous methods for removing metal ions from cellulose pulp are provided, which reduces the amount of chelant or ion exchange chemical necessary, more effectively removes metal ions) in a shorter period of time than is conventional, and utilizing a single conventional washer. While the invention has been shown and described in what is presently conceived to be the most practical preferred embodiments 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 processes.

Claims (31)

1. A method of removing transition metal ions from cellulose pulp, comprising the steps of:
(a) bringing a solution of chelant, ion exchange chemical, or both chelant and ion exchange chemical, into contact with a mat of a cellulose pulp liquid slurry to displace most of the liquid in the pulp liquid slurry;
and (b) displacing the solution introduced in step (a) with another liquid to remove transition metal ions from the pulp; and wherein step (a) is practiced for less than five minutes.

2. A method as recited in claim 1 wherein step (a) is practiced using a charge of chelant, ion exchange chemical, or both, that is at least 20% less than the charge that would be required for the same pulp in a conventional chelation vessel, while effecting removal of at least as many transition metal ions as treatment in the conventional chelation vessel.

3. A method as recited in claim 2 wherein step (a) is practiced with a DTPA charge of 0.01-0.3% (as pure DTPA on pulp), or other equivalent chelant or ion exchange chemical.

4. A method as recited in claim 1 wherein step (a) is practiced using-a DTPA concentration of 0.02-15 g/L DTPA, and of 0.08-3 bed volume of the liquid volume contained in the pulp mat, or other equivalent chelant or ion exchange chemical.

5. A method as recited in claim 1 wherein step (b) is further practiced so that the concentration of DTPA, or equivalent, is at least ten times that used in a conventional chelation vessel treatment.

6. A method as recited in claim 1 wherein steps (a) and (b) combined are practiced for less than about ten minutes.

7. A method as recited in claim 1 wherein steps (a) and (b) are practiced by forming a substantially undisturbed fiber mat in a single drum or belt washer, and effect removal of more than about 80% of the manganese from the pulp.

8. A method as recited in claim 1 wherein steps (a) and (b) are practiced at a consistency of about 6-20%.

9. A method of removing transition metal ions from cellulose pulp, using a drum, belt, or other washer forming a pulp mat, comprising the steps of:
(a) forming a substantially undisturbed liquid-containing cellulose pulp mat in the washer;
(b) in the washer, bringing a solution of chelant, ion exchange chemical, acid, or a combination of two or more of acid, chelant and ion exchange chemical, into contact with the cellulose pulp mat; and (c) in the washer, substantially immediately after the practice of step (b), removing the liquid solution added in step (b) to remove transition metal ions from the pulp along with the removed solution.

10. A method as recited in claim 9 wherein step (b) is practiced using a charge of chelant, ion exchange chemical, or both, that is at least 20% less than the charge that would be required for the same pulp in a conventional chelation vessel, while effecting removal of at least as many transition metal ions as treatment in the conventional chelation vessel.

11. A method as recited in claim g wherein step (b) is practiced with a DTPA charge of 0.03-0.2% (as pure DTPA on pulp), or equivalent other chelant or ion exchange chemical.

12. A method as recited in claim g wherein step (b) is practiced using a DTPA concentration of 0.1-2 g/L DTPA, and of 0.2-0.8 bed volume of the liquid volume contained in the pulp mat, or equivalent other chelant or ion exchange chemical, and wherein steps (b) and (c) are practiced at a consistency of between about 6-20%.

13. A method as recited in claim 9 wherein step (b) is further practiced so that the concentration of DTPA, or equivalent, is at least ten times that used in a conventional chelation vessel treatment.

14. A method as recited in claim 9 wherein step (c) is practiced by introducing a wash liquid into the pulp which displaces the solution added in step (b), and wherein during the practice of steps (b) and (c) the pulp mat has a thickness of less than about 6 cm.

15. A method as recited in claim 9 wherein step (b) is practiced using sulfuric, oxalic, or hydrochloric acid at a pH of between about 1-4.

16. A method of removing transition metal ions from cellulose pulp, comprising the steps of:

(a) bringing a solution of chelant, exchange chemical, or both chelant and exchange chemical, into contact with a mat of a cellulose pulp liquid slurry using a charge of chelant, ion exchange chemical, or both, that is at least 20% less than the charge that would be required for the same pulp in a conventional chelation vessel, while effecting removal of at least as many transition metal ions as treatment in the conventional chelation vessel; and (b) removing the solution from the pulp.

17. A method as recited in claim 16 wherein step (a) is practiced with a DTPA charge of 0.01-0.3% (as pure DTPA on pulp), or equivalent other chelant or ion exchange chemical.

18. A method as recited in claim 16 wherein step (a) is practiced using a DTPA concentration of 0.02-15 g/L DTPA, and of 0.08-3 bed volume of the liquid volume contained in the pulp mat, or equivalent other chelant or ion exchange chemical.

19. A method as recited in claim 16 wherein step (a) is further practiced so that the concentration of DTPA, or equivalent, is at least ten times that used in a conventional chelation vessel treatment.

20. A method as recited in claim 16 wherein step (a) is practiced for about 60 seconds or less.

21. A method as recited in claim 16 wherein step (a) is practiced with the pulp in a moving mat having a thickness of less than 10 cm.

22. A method of treating a liquid slurry of cellulose pulp to remove transition metal ions therefrom, and then subsequently bleaching the pulp, by:
(a) forming a moving mat of the cellulose pulp having a thickness of 6 cm or less, moving in a first direction;
(b) while the mat is moving in the first direction, causing a solution of chelant) ion exchange chemical, or both chelant and ion exchange chemical, to flow through the mat in a second direction, substantially transverse to the first direction; then (c) while the mat is moving in the first direction, removing the solution from the mat, along with at least 50% of the transition metal ions;
and then (d) non-chlorine bleaching the pulp.

23. A method as recited in claim 22 wherein step (d) is practiced using peroxide.

24. A method as recited in claim 22 wherein steps (a)-(d) are practiced using TMP, CTMP, or CMP.

25. A method as recited in claim 22 wherein steps (a)-(d) are practiced using chemical pulp.

26. A method as recited in claim 22 wherein step (b) is practiced by using a solution having a volume of water less than the volume of water in the mat during the practice of step (b).

27. A method as recited in claim 22 wherein steps (b) and (c) are practiced in a single drum or belt washer, and to remove at least about 80% of the manganese from the pulp.

28. A method as recited in claim 22 wherein step (b) is practiced using DTPA, or MgSO4.

29. A method as recited in claim 22 wherein step (b) is practiced using a MgSO4 solution with a charge of 0.5-2.0 (% on pulp), a concentration of 2-10 g/L, and a bed volume of 0.2-0.8.

30. A method as recited in claim 22 wherein step (b) is practiced with the pulp having a consistency of between about 8-15%.

31. A method of removing transition metal ions from cellulose pulp, comprising the steps of:
(a) bringing a solution of acid at a pH between about 1-4, into contact with a mat of a cellulose pulp liquid slurry to displace most of the liquid in the pulp liquid slurry; and (b) displacing the solution introduced in step (a) with another liquid to remove transition metal ions from the pulp; and wherein step (a) is practiced for less than five minutes.