Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Treatment with particular chemical
Patent
1997-02-27
2000-12-26
Alvo, Steve
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Treatment with particular chemical
162 78, 162 79, 162 80, D21C 916
Patent
active
061653187
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the delignification of a chemical pulp, in which process the pulp is treated with a peroxide or a peracid in the presence of an activating Ti-, V- or Cr-group transition metal. The said transition metals include Mo, V, Nb, Ta, Ti, Zr, Hf and W.
After the cooking, chemical pulp is brown, owing to residual lignin present in it. The pulp to be used for higher-grade papers is bleached after cooking in order to remove the lignin.
The bleaching chemical used has conventionally been chlorine, by means of which an effective bleaching is achieved and the quality of the paper obtained is high. However, owing to the environmental problems caused by chlorine, there has recently been to an increasing degree a shift to other bleaching chemicals, such as chlorine dioxide, oxygen, ozone, peroxides, and peracids. The overall objective has been to shift to bleaching which is completely free of chlorine chemicals in order to avoid the environmental hazards caused by chlorine chemicals, and chlorine residues in completed paper.
The bleaching process usually comprises a bleaching sequence made up of successive treatment steps, wherein oxidative steps which decompose lignin and alkaline washing steps alternate. By bleaching without chlorine chemicals, wherein the oxidants used are oxygen and alkaline peroxide, usually a pulp has been obtained which in its brightness, 83-87% ISO, and in its strength is not of the level of pulp bleached with chlorine chemicals. When ozone has been used as the oxidant, a brightness above 88% ISO has been achieved, but there has been the problem of the proneness of the process to disturbances. Thus there has been a need to find a system by means of which, without the use of chlorine chemicals, a fully bleached pulp stronger than previously and corresponding in quality to conventional pulps bleached with chlorine chemicals could be obtained through a process reliable in operation.
It is known that the delignification of chemical pulps can be promoted by treating the pulp with hydrogen peroxide in the presence of certain metals, such as Sn, Ti, V, W, Mo, Cr, Nb, Os and Se, or compounds thereof (1, 2, 3, 4, 5, 6, 7, 8).
Metal compounds which have been used in organic chemistry to activate hydrogen peroxide are listed in, for example, the book Catalytic Oxidations with Hydrogen Peroxide as Oxidant (G. Strukul, Kluwer Academic Publishers 1992), Chapter 1, "Introduction and Activation Principles," page 9.
In the said references, the above-mentioned metallic activators have been used mainly in the peroxide step after the cooking or after the oxygen step.
On the other hand, Weinstock et al. (5) have disclosed a delignification process which is based on the exploitation of heteropolyacids formed by Mo. Heteropolyacid is used in the process as a stoichiometric bleaching chemical. Mo is first oxidized with oxygen, whereafter it is reduced in the bleaching, and the Mo is re-oxidized with oxygen gas after use. However, the process has disadvantages in the shield gas necessary for the reactions and the very high rates of Mo. This method is also not based on the use of hydrogen peroxide.
According to the present invention it has now been observed that the efficacy of peroxide and/or peracid delignification activated with a Ti-, V- or Cr-group transition metal can be increased by adding to the pulp a compound which contains at least one heteroatom, such as Si, P or B, which is capable of forming a heteropolyacid with the activating transition metal.
The chemistry of polyacids formed by transition metals, in particular molybdenum-and tungsten, has been discussed in, for example, the publication Pope, M. T., Heteropoly and Isopoly Oxometalates, Springer-Verlag 1983. Polyacids formed in mildly acidic solutions are classified into isopolyacids, which contain only Mo or W in addition to oxygen and hydrogen, and heteropolyacids, which contain one or two other elements in addition to the above-mentioned atom types.
Heteropolyacids form spontaneously when water-soluble compou
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Kubelka V., Francis R.C., Dence C.W., "Delignification with acidic hydrogen peroxide activated by molydate," Journal of Pulp and Paper Science: vol. 18, No. 3, May 1992, pp. J108-J114.
Weinstock I.A., Springer E.L., Minor J.L., Atalla R.H., "Alternative pathaways in non-chlorine bleaching," Non-chlorine bleaching conference, Mar. 14-18, 1993. S. Carolina, USA.
Sundman G.I., Ph.D. dissertation, SUNY College Environment Science and Forestry, Syracuse, USA, 1988.
Jakara Jukka
Paren Aarto
Patola Juha
Alvo Steve
Kemira Chemicals Oy
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