SK183692A3 - Method of elimination of trace elements, especially fe3+, from nonbleached pulp before its processing in oxygen step - Google Patents

Abstract

The process consists of whitewashed partial chlorination pulp with Ch gas or a Ch t CIO 2 mixture which produces a low pH of the solution and the acid formed hydrochloric acid dissolves trace elements, especially however, Fe3 * bound by whitewashed pulp.

Description

Title of the invention

Process for removing trace elements, in particular Fe, from unbleached pulp prior to its treatment in the oxygen stage.

Technical field

The invention relates to a process for removing trace elements, in particular Fe ⁺⁺⁺ , from unbleached pulp for bleaching in a subsequent oxygen bleaching step.

BACKGROUND OF THE INVENTION

The oxygen delignification stage (also referred to as the oxygen bleaching stage) has found widespread use in pulp production by the sulphate as well as sulphite (acidic and semi-acidic) processes in the last 20 years. The oxygen delignification stage consists of the pressure heating of the pulp in an oxygen reactor in the presence of alkali and a certain oxygen overpressure. For the operational application of the oxygen delignification stage, the following conditions are given: density of the substance 25 ~ 3O initial oxygen pressure of 0.4-1.0 MPa, temperature 80-120 ° C, reaction time of less than 1 hour, alkali used 2-5% NaOH per pulp and a protector amount of less than 0.05% Mg ⁺⁺ per pulp.

The oxygen delignification stage has found application in production because of its indisputable advantages, which can be summarized as follows:

- reduce the consumption of chlorine needed to pulp the pulp,

This also reduces production costs,

- reduce the amount of sorted spillage,

- increase the whiteness stability of the bleached pulp,

- reduce pollution of watercourses by toxic, chlorinated degradation products of lignin and also by chlorides,

- reduce water consumption due to recirculation of leaches after oxygen heating.

The oxygen delignification stage also has some disadvantages.

The major drawbacks include the degradation of the polysaccharide pulp fraction during oxygen heating associated with loss of mass. Degradation of the polysaccharide portion of the pulp takes place despite the application of Mg ⁺⁺ ions as a retarder of the degradation reaction. The degradation of cellulose is catalytically promoted by the trace elements, naoma Fe, present in the pulp. Mg ⁺⁺ ions form complexes with trace elements and delay the degradation of cellulose by oxygen in the form of radical reactions.

Pulp manufacturers are trying to reduce the extent of cellulose degradation that occurs during oxygen heating. One way to reduce the degradation of cellulose is to remove the trace elements present in the pulp by washing the pulp with dilute acids. For this purpose, a separate stage consists of a tower through which the pulp suspension with the addition of mineral acid gradually passes. The residence time of the pulp in the tower is about 1 hour. During this time, the trace elements present in the pulp dissolve, the pulp is washed on the scrubber after passing through the tower, and the washed pulp is fed into an oxygen reactor. Dilute sulfuric acid is used to dissolve the trace elements present in the pulp.

SUMMARY OF THE INVENTION

The principle of the novel process for removing trace elements, in particular Fe ⁺⁺⁺ ions, from unbleached pulp prior to treatment in the oxygen delignification stage according to the invention is that the pulp suspension prior to treatment in the oxygen stage is subjected to partial chlorination in the chlorination stage with Cl ₂ gas or a mixture of Cl ₂ + ClO 2 using 5-25% of the assets. Cl ₂ preferably 10-15% of the assets. Cl ₂ per kappa number of unbleached pulp. The stoichiometric equilibrium indicates that half of the chlorine dosed in the chlorination stage into the pulp suspension is converted to hydrochloric acid. Diluted HCl, formed during the chlorination, produces a low pH value of the pulp suspension and dissolves the trace elements present in the unbleached pulp as chlorides that are removed when the pulp is washed on the filter. It is therefore preferred that the pulp after partial chlorination is washed with softened and chemically treated water so that no trace elements, especially Fe ⁺⁺⁺ , are introduced into the pulp with the wash water. It is also preferred that the NaOH solution which is metered into the pulp suspension before it was stored in a concentrated solution in a fiberglass tank and not in an iron tank.

The advantage of removing the trace elements from the pulp according to the invention is that the plant does not have to buy and store the mineral acid used in the pulp de-ashing process applied hitherto. The addition of mineral acid used hitherto interfered with the balance and regeneration of chemicals in the regeneration system, respectively. in wastewater management. Bleached pulp plants have a sufficient supply of Clg gas and do not need to buy and store it separately.

A further advantage of the application of the removal of trace elements from the pulp according to the invention is that the lignin present in the unbleached pulp is chlorinated by partial chlorination and oxidized to the extent corresponding to the amount of chlorine applied. The lignin thus modified is v. the subsequent oxygen stage dissolves substantially better and its solubility depends on the chlorination intensity and the delignification intensity in the oxygen stage.

As shown in the attached table, the application of 15% of assets. Clg per kappa number was 1 hour under the oxygen stage conditions used. at 120 ° C with a loading of 5% NaOH per pulp dissolved up to 92% of the lignin present in unbleached pulp Pulp ashed by the process of the invention showed, after the oxygen stage, using the above Kappa experimental conditions of 1.7 and brightness. 67% MgO, with a decrease in PPS of 22%. The bleaching of the pulp so produced to a brightness of 85-86% MgO will require only a small amount of bleaching agents and can be carried out with hydrogen peroxide.

A further advantage of the bleaching process applying the partial chlorination process and the subsequent oxygen step of the invention is the formation of less toxic, chlorinated lignin degradation products. These are present only in wastewater after partial chlorination. Since the partial chlorination of unbleached pulp, according to the invention, is carried out with a low content of assets. The amount of toxic chlorinated lignin degradation products will be only 1/3 - 1/4 of the amount currently formed in pulp bleaching at present. The partially chlorinated pulp after pressurized oxygen heating in alkanes does not contain the cold chlorinated lignin fragments, since these are degraded at the oxygen stage and the chlorine bound to the lignin presented by the yellow to orange color of the chlorinated pulp.

DETAILED DESCRIPTION OF THE INVENTION

The pulp suspension at 3% density of the substance is partially chlorinated in the chlorination tower of the bleaching plant with the addition of 5-25% of the active, Cl 2 gas. The partial chlorination is preferably carried out with an addition of 10-15% of the assets. Cl ₂ · In plants that have C10 ₂ at their disposal, partial chlorination can be carried out with a mixture of Cl ₂ + ClO ₂ at the rate used by the plant in the conventional production of bleached pulp. The residence time of the pulp suspension in the chlorination tower is 45 min. The partially chlorinated pulp is washed in a further wash on a wash filter. The wash water used for the wash filter screens should not contain trace elements and should therefore be softened and chemically treated. The washed pulp is thickened to a desired density of 25-30% by means of a screw press and filled into an oxygen reactor through a filling device. The conditions of pressure oxygen heating are the same as those normally used in the plant and may be as follows: 2 - 5% HaOH alkali load on pulp, temperature 100 - 110 ° C

-5 oxygen pressure 0.6 - 0.0 MPa n pulp retention time in oxygen reactor 30 - 45 min. The pulp coming from the oxygen reactor is diluted with water and washed on a scrubber filter. The amount of lignin removed from the unbleached pulp after partial chlorination and the oxygen stage corresponds to the conditions of chlorination and oxygen pressurization used and may correspond to 75-95% of the amount of lignin present in the unbleached pulp. The bleaching of the pulp thus obtained is conditional on the purpose for which the pulp produced is to serve.

Industrial applicability of the invention

The invention finds application in the manufacture of bleached kraft and sulphite pulps, manufactured from various types of coniferous and deciduous trees, in which bleaching uses an oxygen bleaching step.

Claims (1)

Process for removing trace elements, in particular Fe, from unbleached pulp prior to treatment in the oxygen delignification step, characterized in that the pulp is subjected to partial chlorination in the chlorination stage by applying 5-25% active, preferably 10-15% active, prior to oxygen treatment. Cl 2 per kappa number of unbleached pulp, wherein the partial chlorination is carried out with a gaseous or a mixture of + CO 2 in varying proportions.