Note: Descriptions are shown in the official language in which they were submitted.
CA 02258387 1999-O1-06
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
CA 02258387 1999-O1-06
1
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
CA 02258387 1999-O1-06
2
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
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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.
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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
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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
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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%.
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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
CA 02258387 1999-O1-06
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
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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;
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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
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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
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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,
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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
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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.
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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
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16
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.
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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.
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18
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.
CA 02258387 1999-O1-06
19
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
CA 02258387 1999-O1-06
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.
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21
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.
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22
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
CA 02258387 1999-O1-06
23
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.
