Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Reexamination Certificate
2001-01-11
2004-03-02
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
C162S078000, C162S080000, C162S090000
Reexamination Certificate
active
06699358
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to the brightening of chemical pulps and deinked, mixed office waste pulps, and particularly to the brightening of such pulps in a peroxide or peroxide/oxygen stage of brightening.
BACKGROUND OF THE INVENTION
The process of making paper from wood involves the following general stages: (1) bark removal; (2) wood chipping; (3) pulping; (4) brightening; and (5) forming a sheet of paper on a machine. During pulping, wood is reduced to its fibrous state, with a portion of the lignin content in the wood being removed. Pulps can be divided into two main categories—chemical and mechanical pulps—depending on how they are made from wood. Chemical pulping involves the use of chemical reagents to effect a separation of the cellulose fibers from the other wood components, such as lignin and other extraneous compounds. In the process, most of the hemicelluloses are also dissolved. Thus, the yield for chemical pulping is typically 40-50% on wood.
Mechanical pulping involves the reduction of wood to the fibrous state by mechanical means, such as by grinding logs into pulp by large revolving grindstones. These pulps are called “mechanical” because a significant amount of mechanical energy (grinding or refining) is required to break down the wood chips. Except for a few water soluble components, all of the constituents of the wood are present in the ground wood pulp. Thus, mechanical pulps are characterized by their high yield and high lignin content. For example, although chemical pulps contain only about 5% lignin (weight basis on pulp) after pulping, mechanical pulps typically contain greater than 20% lignin for hardwoods and 25% for softwoods after pulping.
In general, the brightening of chemical and mechanical pulps occur by different mechanisms. This difference in approach is due, in part, to the difference in lignin content between the chemical and mechanical pulps and to the different nature of the lignin in chemical pulps than that in mechanical pulps. The remaining lignin in chemical pulps is typically more difficult to degrade than the majority of the lignin remaining in mechanical pulps. For example, chemical pulps, such as kraft pulps, are more difficult to brighten by H
2
O
2
. Thus, instead of the 60° C. bleaching temperature used for mechanical pulps, values in the range of 110° C. are used for kraft pulps (which use sodium hydroxide and sodium sulfide as the primary chemical reagents). As another example of the differences in brightening the two different types of pulp, chlorine dioxide (ClO
2
), a stable free radical, is involved in the brightening of more than 90% of bleached kraft pulp produced in the U.S.A. per year. On the other hand, it has been reported that chlorine dioxide actually darkens mechanical pulps. In sum, results obtained with mechanical pulps cannot be considered as being predictive for chemical pulps.
Various additives have been proposed to improve the brightening process of mechanical pulps or mixed chemical/mechanical pulps having a high lignin content (i.e., above 20% by weight on pulp). Sodium silicate is widely used for hydrogen peroxide brightening of such pulps. It acts as a peroxide stabilizer and as a buffer during the bleaching reaction. Sodium silicate, in combination with a small dose of MgSO
4
, has long been known to improve the brightness of mechanical pulps. The two chemicals are known to form an intermediate that adsorbs or complexes transition metal species which would otherwise undesirably catalyze the decomposition of H
2
O
2
. The two chemical combination is widely used in installations that bleach mechanical pulps. When magnesium sulfate has been used for such pulps, however, no increase in brightness has been observed when the MgSO
4
.7H
2
O dose is increased above 0.05% on mechanical pulp (2.0 mmoles/kg). Accordingly, previous researchers had no need to add more magnesium to their pulps.
Turning to the brightening of chemical pulps, some results have suggested that silicate has a negative effect on H
2
O
2
bleaching of kraft pulps. In some cases, the inclusion of silicate lowered the brightness by 4.6 percentage points (measured as % GE brightness in accordance with TAPPI Standard T452 om-92) and by 2-7 percentage points.
In peroxide brightening of mechanical pulps, the key is nucleophilic degradation of carbonyl compounds in the native lignin without the oxidation of phenols to o- and p-quinones, as shown below in reactions [A] and [B], respectively.
Reaction [A] is the desirable degradation of carbonyl compounds by the perhydroxyl anion, OOH
−
. This reaction breaks down the carbonyl compounds, which absorb light in the visible range, to a more soluble form to be washed away by water. On the other hand, reaction [B] represents the undesirable formation of o- and p-quinones, which are less soluble in water and also absorb light in the visible range.
The perhydroxyl anion is generated by the dissociation of H
2
O
2
as shown by equation [1] below. Mild conditions have to be used to prevent the oxidation of phenolics.
H
2
O
2
+H
2
O
H
3
O
+
+OOH
−
pKa=11.6 [1]
Mechanical pulps contain high concentrations of lignin and extractives whose negatively charged sites may complex transition metals. Also, some transition metal catalysis can be tolerated because lignin is a very good radical scavenger. It will scavenge the superoxide anion (.O
2
−
) and prevent wasteful decomposition to O
2
(equation [2]).
.O
2
−
+M
(m+1)+
→O
2
+M
n+
[2]
Actually, .O
2
−
is nucleophilic and its reaction with lignin is reported to result in increased brightness. In sum, transition metal deactivation by magnesium silicates is important but not critical to the brightening of mechanical pulps.
Unlike softwood mechanical pulps which typically contain about 25% lignin (by weight on pulp), chemical pulps enter the final brightening stage with typically less than 2% lignin, which is typically colored and very difficult to oxidize. The approach with H
2
O
2
is nucleophilic degradation by OOH
−
. However, equation [2] becomes favorable because there is not a large amount of reactive lignin to scavenge .O
2
−
. Equation [2], in conjunction with equations [3] and [4], results in wasteful decomposition of H
2
O
2
.
M
n+
+H
2
O
2
→M
(m+1)+
+.OH+
−
OH [
3
]
.OH+
−
OOH→.O
2
−
+H
2
O [
4
]
Equation [3] is favorable for Cu(I) and Fe(II) but not for Mn(II). A more probable mechanism for Mn(II) is outlined below. The subscripted s indicates soluble Mn(IV) and Mn(III).
Mn
2+
(s)
+H
2
O
2
→Mn
4+
(s)
+2OH
−
Mn
4+
(s)
+
−
OOH→Mn
3+
(s)
+H
+
+.O
2
−
Mn
3+
(s)
+
−
OOH→Mn
2+
(s)
+H
+
+.O
2
−
Lignin degradation by OOH
−
requires a high temperature because of the unreactive nature of the lignin in chemical pulps. At 110° C., transition metal deactivation is extremely important.
With respect to O
2
delignification, a simple and well-accepted scheme for free radical generation is provided below. RH is a reactive structure in the solution phase.
RH+O
2
→R.+.OOH [5]
.OOH+H.→HOOH (H
2
O
2
) [6]
(Abstraction of H atom from lignin)
R.+O
2
→ROO. [7]
ROO.+H.→ROOH [8]
H
2
O
2
and ROOH will be affected by transition metals in the same manner as in a peroxide/oxygen stage. Thus, the oxygen stage is similar to the peroxide/oxygen stage in that they both involve heterolytic (i.e., ionic) reactions and homolytic (i.e., free radical) reactions. In addition, they are both similar in that both rely on hydrogen peroxide, either as added in
Evans Timothy D.
Francis Raymond C.
Alvo Steve
National Silicates Partnership
RatnerPrestia
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