DE2721402A1 - PROCESS FOR BLEACHING PULP - Google Patents

Description

₀ 7157 BL

272U02

German gold and silver refinery formerly Roessler 6000 Frankfurt am Main, Weißfrauenstrasse 9

Process for bleaching pulp

The invention relates to a method for bleaching pulp in an aqueous liquor, consisting of an acidic chlorination stage and an alkaline peroxide stage, which is optionally used known hypochlorite, chlorine dioxide or peroxide stages can follow in any order.

It is known to bleach cellulose in aqueous liquor first in an acidic chlorination stage with elemental chlorine and then in an alkaline peroxide stage with HO. A series of known bleaching stages with chlorine dioxide, hypochlorite or H ₂ as bleaching agent can be connected to these stages in any sequence.

However, this process has the disadvantage that after the chlorination stage, a very intensive washing has to be carried out in order to remove all the substances dissolved by the chlorination from the pulp, because these substances dissolved out of the pulp primarily with the H ₂ O _{2 of} the alkaline bleaching stage and secondarily only react with the pulp. As a result, the consumption of H ₂ O _{2 increases} sharply and only low purity values and degrees of whiteness are achieved.

The object of the invention is to carry out a method for bleaching pulp in aqueous liquor, in which the H_0 ₂ used can be used completely for the bleaching of the pulp in the alkaline peroxide stage after the acidic chlorination stage and disruptive side reactions are avoided.

The invention relates to a process for bleaching cellulose in an aqueous liquor, consisting of an acidic chlorination stage and an alkaline peroxide stage, which one

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809846/0371

7157 BL

272U02

possibly known hypochlorite, chlorine dioxide or peroxide can connect stages in any order, which is characterized in that before performing the acidic chlorination stage of the bleaching liquor a magnesium salt, or a magnesium compound added. Magnesium oxide can be used as the magnesium compound. The magnesium compound can be used in amounts of 0.2 to 1.5 wt .- ^, preferably 0.5 to 1.0 wt .- ^, based on dry pulp can be added.

Due to the addition of magnesium salts, it is possible to supply all of the H 0p used to the cellular material and higher To obtain degrees of whiteness and purity values.

The method of the present invention is illustrated by the following Examples explained and described in more detail:

Example 1 :

Unbleached sulfite pulp with a permanganate number of is bleached with 3 wt .- ^ o elemental chlorine (based on dry cellulose) at room temperature and a consistency of 2.5 wt .- ^ Then the bleaching liquor is thickened to a consistency of 10 $ and without Wash immediately with 1.25 wt .- ^ H _? 0 ₃ (100% and based on dry cellulose) at an NaOH concentration of 3 wt .- ^ (based on dry cellulose) and at a temperature of 70 ° C. for 90 minutes.

The pulp obtained in this way has a whiteness of $ 75 MgO on. (Determined according to the German standard methods (Zellcheming— Leaflets) with Zeiss-Elrepho whiteness meter filter R ^ 6)

The purity of the pulp obtained, rated according to a rating scale from 1 to 6, in which 1 means absolute purity, i.e. no dirt spots, and 6 means unbleached pulp with a very high proportion of bast and bark, is 3 to h,

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272U02

The remaining percentage of unused peroxide is $ 0 based on the dry cellulose.

The pulp is further bleached in a further stage with sodium hypochlorite (1 wt .- $ calculated as active chlorine and 1 wt .- ^ NaOH based on dry cellulose) at a consistency of 10 ^ and a temperature of 35 C.

The whiteness achieved is $ 89.5 MgO and the purity 1.75.

Example 2:

Unbleached sulfite pulp with a permanganate number of is bleached with 3 wt .- ^ elemental chlorine (based on dry cellulose) at room temperature and a consistency of 2.5 wt .- ^. The pulp is then washed intensively and the bleaching liquor is thickened to a consistency of 10% by weight. The thickened bleach liquor is then concentrated at 1.25 wt. 56 H_0p (100% and based on dry pulp) at an NaOH concentration of 3 wt Leave for 90 minutes.

The pulp obtained in this way has a whiteness of 81 MgO, measured as indicated in Example 1. The purity, evaluated as in Example 1, is 2.

The remaining proportion of unused peroxide is 0.05 % by weight (based on dry cellulose),

In a further stage, the pulp is covered with sodium hypochlorite (1% by weight, calculated as active chlorine and 1% by weight NaOH on dry cellulose) with a consistency of JO $ and one Bleached at a temperature of 35 C.

The whiteness achieved is $ 91.0 MgO (measured as in Example 1). The purity, evaluated as in Example 1, is 1.20.

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7157 BL

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Example 3;

To an aqueous liquor with a consistency of 2.5 wt. -56, which still contains completely unbleached sulfite pulp with a permanganate number of 3I, 1 wt .- $ MgO (based on dry cellulose) is added. The pulp is then bleached with 3 wt .- ^ elemental chlorine (based on dry pulp) at room temperature. The bleaching liquor is thickened without washing to a consistency of 10 wt .- ^ and with 1.25 wt -. ^ 2 ° ^H 2 ⁽¹⁰ ° ^ S ^and based on absolutely dry pulp) at a NaOH concentration of 3 wt .- ^ (based on dry cellulose) and leave at a temperature of 70 C for 90 minutes.

The pulp obtained in this way has a whiteness of $ 81> MgO, measured as indicated in Example 1. The purity, evaluated as in Example 1, is 2.

The remaining proportion of unused peroxide is 0.2 % by weight (based on dry cellulose).

In a further stage, the pulp is covered with sodium hypochlorite (1% by weight calculated as active chlorine and 1% by weight NaOH on dry cellulose) with a consistency of 10 # and bleached at a temperature of 35 C.

The degree of whiteness achieved is 91.0 # MgO (measured as in Example 1). The purity, evaluated as in Example 1, is 1.20.

809846/0371

Claims (1)

7157 BL - * - 272U02 Claims Process for bleaching cellulose in aqueous liquor, consisting of an acidic chlorination stage and an alkaline peroxide stage, which can optionally be followed by known hypochlorite, chlorine dioxide or peroxide stages in any order, characterized in that the bleaching liquor is carried out before the acidic chlorination stage a magnesium salt or a magnesium compound is added. 2. The method according to claim 1, characterized in that magnesium oxide is added to the bleaching liquor as the magnesium compound. 3. Process according to claims 1 or 2, characterized in that the magnesium compound is added in amounts of 0.2 to 1.5% by weight, preferably 0.5 to 1.0% by weight, based on dry cellulose. May 10, 1977 PAT / Dr.We-Eh 809846/0371