Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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An improved Peroxide bleaching method
The bleaching of wood pulp with peroxides is an established technique in the
manufacture of bleached pulp grades, hydrogen peroxides being the by far the
most common reagent used for such purposes. In a large number of patents and
reports various details of this technique and its numerous ramifications are
described .
Combinations of hydrogen peroxide with certain organic acids stand out as an
especially efficient form of the peroxide bleaching technique. The treatment is
carried out within the acid range of the pH-scale, facilitating the formation ofperoxyacids which are the active chemical species in the bleaching process.
Contrary to this, the common way, of bleaching with peroxides is carried out
using slightly alkaline conditions enhancing the formation of peroxide anions H02--
which are the active ionic species. Higher alkalinities are to be avoided since
hydrogen peroxide then decomposes to oxygen.
The purpose of the present disclosure is to describe a simple method to enhance
the bleaching efficiency of hydrogen peroxide by adding certain inexpensive
organic acids to the bleaching solution. The acids in question are formic, acetic,
lactic, maleic acid and phtalic acid; they can be used separately or in various
combinations. Contrary to similar mixtures of hydrogen peroxide with such acids
which have been proposed to increase the efficiency of the bleaching process, cf.
for instance Austrian Patent 203.454, the mixtures of the present disclosure arebeing used within the alkaline range of the pH-scale and
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not on its acid side. This alkalinity of the treating
solution is the main characteristic feature of the present
invention relating to peroxide bleaching process in the
presence of the organic acids enumerated above, i.e.
formic, acetic, lactic, maleic acid and phthalic acid or
mixtures thereof.
Disclosures similar to those of Austrian Patent
203,454, relating to the use of hydrogen peroxide
containing certain organic acids at acidic conditions, can
also be found in a number of other patents, cf. for
instance U.S. Patent 2,720,441, French Patent 1,565,397 and
Japanese Published Application 77.120.104. It is therefore
even more surprising that a significant enhancement of the
bleaching efficiency of hydrogen peroxide can be attained
in the alkaline range of the pH-scale, where such an
enhancement normally would not be expected. The increased
bleaching efficiency of hydrogen peroxide in the presence
of certain organic acids under acidic conditions can be
related to the formation of peroxy acids. However, under
the conditions specified in the present invention peroxy
acids cannot be of any importance since the alkaline
environment does not favour reactions leading to such
peroxy acids.
It has according to the present invention been
found that the bleaching efficiency of peroxides can be
improved if the bleaching process is carried out in the
presence of low concentrations of one or more of the
enumerated acids while maintaining the pH-value within the
alkaline range 8-13, which pH-range is provided by the
addition of an alkali metal hydroxide, such as sodium
hydroxide or potassium hydroxide.
Thus, the invention provides a method of
bleaching wood pulp without the use of peroxy acids by
means of from 0.1 to 10%, preferably 0.25 to 10~, based on
the dry weight of the pulp, of hydrogen peroxide, wherein
an organic acid selected from formic acid, acetic acid,
lactic acid, maleic acid and phthalic acid and mixtures
B
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thereof is added to the pulp suspension to be bleached
together with such an amount of an alkali hydroxide as to
give a pH-value of the suspension within the alkaline range
of from 8.0 to 13, preferably from 9 to 10.
When using peroxide and alkali metal hydroxide
for bleaching of pulp with the addition of low
concentrations of the enumerated acids a considerably
improved bleaching is attained as compared to bleaching
carried out with peroxide and the alkali metal hydroxide
without the use of said acids.
B
The method according to the present invention allows bleaching with lower acid
and peroxide concentrations than when using common peroxy acid methods
while still obtaining the same bleaching effect.
Furthermore by using the method according to the present invention problems
associated with the use of common peroxide stabilizers are avoided. Among
these primarily silicates may be mentioned which when utilizing bleaching
processes based on such methods in a technical scale may give rise to
undesirable deposits (scaling).
Another surprising finding according to the present invention is the fact that
potassium hydroxide gives a significantly higher brightness of the bleached
material than does sodium hydroxide. The difference amounted to 2-8 SCAN
brightness units.
The enhancement of the bleaching efficiency of hydrogen peroxide by the acids
listed above, using KOH to adjust the pH-value, will now be illustrated by some
examples. The experiments were carried out with unbleached mechanical pulp
(pine). The chemicals (peroxide and acid) were added simultaneously to a 2%
suspension of the pulp in water, whereafter the pH-value was immediately
adjusted to 9-10 by KOH at 60C, 1 h stirring. After completing the treatment,
the pulp was formed into laboratory sheets using standardized procedures (SCAN
C11:75). The measurement of brightness was carried out according to SCAN
C11:75, the brightness being expressed in ISO-units. The brightness value of theunbleached pulp was 57-60.
The invention is illustrated by means of the following examples.
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Example 1
A bleaching mixture containing 4% H2O2, 1% acetic acid, and 1% formic acid
(based on the weight of the quantity of pulp present in the system) gave after
bleaching carried out as described above a brightness of 78%, the brightness
gain being 1 8 units.
Example 2
When carrying out the bleaching operation at lower peroxide and acid
concentrations, namely 1.5% H2O2, 0.75% acetic acid, and 0.75% formic acid a
brightness of 72% (gain 15 units) was recorded. Again, the concentration
figures are percentages of the amount of pulp used in the reaction. In all otherrespects, the conditions were similar to those used in Example 1.
Example 3
With a reaction mixture of 4% H202 and 4% lactic acid a brightness value of
78% was obtained. This value was increased to 81.4 when adding a magnesium
compound (0.4% Mg2+ = 4% MgSO4x7H20). Reducing the peroxide and lactic
acid concentrations to 1.5% resulted in a brightness reading of 72% which could
be increased to 75 in the presence of 0.4% Mg2+ (=4% MgSO4x7H20).
With H202/maleic acid mixtures similar results were obtained. With H2O2/phtalic
acid mixtures similar results were obtained.
The above results have been optimized with regard to the pH-value of the
reaction mixture, the pH-value being adjusted using KOH. As already mentioned,
the use of NaOH at the same pH resulted in lower brightness values. No
optimization was, however, carried out with regard to the pulp concentrations.
Using higher pulp concentrations, the brightness gain can be expected to increase
by at least 6 units (cf. "The Bleaching of Pulp", L.A. Beeman and J.S. Reichert,Tappi Monograph No. 10, 210 (1953)). The efficiency of the present method
thus appears to have been demonstrated with sufficient clarity.
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It may be mentioned that experiments where the acid was added to the pulp
suspension after the addition of alkali and peroxide produced similar results.
