Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Reexamination Certificate
1998-11-24
2001-07-24
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
C162S072000, C162S076000, C162S078000
Reexamination Certificate
active
06264790
ABSTRACT:
The present invention relates to a process for the bleaching of chemical pulp, in which process the pulp is delignified with a peracid and additionally the pulp is chelated in order to bind heavy metals, such as Fe, Mn and/or Cu, into a chelate complex.
In the production of chemical pulp, a cellulose-containing material is cooked with suitable cooking chemicals, and the raw pulp thus obtained is delignified and bleached with oxidizing chemicals. The purpose of the conventional bleaching of chemical pulp is to complete the removal of lignin from the raw pulp obtained from the cook. Chlorine or chlorine dioxide has conventionally been used for bleaching, but recently a shift has increasingly been made to other, replacing bleaching chemicals. The bleaching takes place in a plurality of successive steps, for example so that the first step is an oxygen delignification, whereafter the delignification may be continued by using, for example, ozone, peracetic acid or hydrogen peroxide in acidic or alkaline conditions.
When oxygen-containing bleaching chemicals, such as oxygen, ozone, hydrogen peroxide and peracids, are used, problems are caused by heavy metals present in the pulp; these include in particular iron, manganese and copper, but also, for example, chromium and nickel. These heavy metals enter the raw pulp along with wood, process waters or cooking chemicals, and they catalyze the decomposition of the carbohydrates in the presence of oxygen chemicals, thereby increasing the consumption of the bleaching chemical and deteriorating the quality of the pulp. Furthermore, they cause after-darkening of the pulp. It is possible to remove the heavy metals, for example, by an acid wash preceding the bleaching step, but since the subsequent bleaching step in the bleaching sequence is usually alkaline, an acid wash performed at a low pH will increase the amount of alkali required for the adjustment of the pH. There is the further disadvantage that the acid wash may reduce the strength of the pulp and that earth-alkali metal ions which are regarded as advantageous for bleaching are removed in the wash. Earth-alkali metals, such as magnesium and calcium, stabilize peroxide in peroxide bleaching and protect the carbohydrates of the pulp from decomposition in the oxygen step; for this reason magnesium sulfate is often added to the bleaching steps.
An alternative method for mitigating the disadvantages due to heavy metals is chelating, wherein complexing agents are used as chelating agents. It is assumed that chelating has the advantage over acid wash that calcium and magnesium will better remain in the pulp. Known chelating agents which bind heavy metals include polycarboxylic acids, of which ethylenediaminetentaacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA), and salts thereof, are the ones most commonly used.
The chelating of chemical pulp will in general not succeed satisfactorily in alkaline conditions, since iron forms very poorly soluble compounds by precipitating in the form of hydroxides, oxides and oxyhydroxides. When the pH rises above 7, manganese also begins to bind very strongly to the pulp. For this reason, chelating in bleaching has been carried out as a separate step in acidic conditions. Such a separate chelating step is very effective in binding heavy metals, but it neither bleaches the pulp nor delignifies it; thus it serves only as a preliminary step for the subsequent oxygen chemical step. If the separate chelating step could be replaced by some sufficiently selective delignifying step, for example the kappa number in the cook could be raised owing to the improved delignification and the yield could be improved, or respectively, the delignification could be brought to a further point and higher degrees of brightness could be reached without a reduction of the strength of the pulp.
One delignification process which has been found to be highly selective is based on the use of peracids. In peracid delignification it is possible to use organic or inorganic peracids, of which peracetic acid and Caro's acid are the most commonly used. It has been observed that peracetic acid (PAA) is an especially selective delignification chemical by means of which the strength properties of the pulp can be maintained good. The PAA step is in fact itself not very sensitive to heavy metals, and even moderately high Fe or Mn concentrations do not affect the selectivity of PAA. In the PAA step the iron remains in the form of poorly soluble Fe(III), whereas a considerable amount of manganese dissolves. This released manganese will pass together with the pulp to the subsequent final bleaching with alkaline peroxide, in which manganese has a detrimental effect and before which the pulp must be chelated in order to avoid the said disadvantage. Furthermore, the PAA step removes from the pulp earth-alkali metals, mainly magnesium and calcium, which are useful in further bleaching.
In delignification with peracetic acid, the optimum pH is very close to that (4-6.5) used in the chelating step. However, chelating which binds heavy metals has not been successful in connection with a PAA step, since DTPA and EDTA, the most commonly used chelating agents, form with manganese a complex which decomposes peracetic acid very effectively. Furthermore, if DTPA decomposes in the reactions, large amounts of iron and manganese are left in the pulp; this is detrimental for the subsequent bleaching steps. In the PAA step the final pH is often rather low, approximately 4, in which case large amounts of Mg and Ca dissolve and, as a consequence, the Mg:Mn ratio in the pulp passing to the subsequent step is poor. The wrong metal profile is in general not seen in the pulp properties after the actual PAA steps; the viscosity is at a good level. However, in the subsequent alkaline peroxide step the wrong metal profile causes extra consumption of peroxide and a lowering of the viscosity of the pulp. Thus a separate chelating step between the PAA and peroxide steps has been required; such a step is futile in terms of delignification, but without it the strength of the pulp could be reduced and the consumption of peroxide could increase considerably in the peroxide step, which is the actual bleaching step.
The object of the present invention is to provide a process in which a chelating which binds heavy metals can be combined with peracid delignification, so that the process will be simpler than previously. The process is based on the use of new complexing agents, and it is characterized in that the chelating is carried out using a chemical selected from a group consisting of N-bis-((1,2-dicarboxy-ethoxy)-ethyl)-amine, N-bis-((1,2-dicarboxy-ethoxy)-ethyl)-aspartic acid and N-tris-((1,2-dicarboxy-ethoxy)-ethyl)-amine and their alkali metal salts and earth-alkali metal salts, and that the peracid and chelating treatments are performed simultaneously by combining the peracid and the said chelating chemical in the same solution phase.
The formulae of the four- and six-branch complexing agents (A, B, C) used as chelating agents in the process are:
N-bis-[(1,2-dicarboxy-ethoxy)-ethyl]-amine (A) is hereinafter referred to by using the acronym BCEEA, N-bis-[1,2-dicarboxy-ethoxy)-ethyl]-aspartic acid (B) by using the acronym BCEEAA and N-tris-[(1,2-dicarboxy-ethoxy)-ethyl]-amine (C) by using the acronym TCEEA.
The achievement of the invention is that peracid delignification, chelating and bleaching, which previously constituted three separate steps in the bleaching process, can be performed during two steps. As the separate chelating step is eliminated, the resources (e.g. a bleaching tower) required by it can be used, for example, for increasing the retention time in some other bleaching step.
The use of the new chelating agents has the additional advantage that, while they chelate iron and manganese sufficiently effectively, they hardly seem to chelate magnesium and calcium.
The peracid used as the delignification chemical may be peracetic acid, performic acid, perpropionic acid or
Aksela Reijo
Jakara Jukka
Paren Aarto
Renvall Ilkka
Alvo Steve
Kemira Chemicals Oy
Morgan & Finnegan L.L.P.
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