Method for bleaching oxygen delignified cellulose-containing pulp with ozone

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85-06-17

79-10-05

79-04-02

79-04-02

Patent Office (62) National application number (86) International filing date (86) filing date for European patent application (30)

78-04-04 NO 781190 (11) Publication number 439 508

The application came as:

E Swedish patent application □ completed international patent application with number □ converted European patent application with number (71) Applicant Myrens Verksted AS. Oslo NO (72) Inventor B H. Fritzvoid, N. Soteland, Hosle, Oslo (74) Representative H Albihns patent agency AB (54) Designation Method for bleaching oxygen-suspended, cellulose-containing masses in water (56) Published publications: -

Other publications: Pulp and Paper Magazine of Canada, vol 75: 4 (1974), p 91-96 CA VoT.76 (1972) 61133m (57) Abstract:

In a method of bleaching water-suspended oxygen-delignified cellulosic pulp such as sulphate, soda and sulphite pulp having a dry matter content exceeding 15%, with ozone in acidic environment and peroxide, the ozone treatment being carried out in two stages, an ozone treatment is carried out with concomitant or subsequent peroxide treatment (ZP), after which the pulp is subjected to an intermediate washing step for pH adjustment, dewatering / pressing and fluffing, whereupon the pulp in the second stage is ozone and peroxide treated (ZP), whereby 55-85% of the total chemical amounts, ie ozone and peroxide used in the two bleaching stages, are added in the first stage, while utilizing a total amount of ozone, which does not exceed 1%.

IN

7902901-3

The present invention relates to a method for bleaching water-suspended oxygen-delignified cellulosic pulp such as sulfate, soda and sulfite pulp with a dry matter content exceeding 15% with ozone in acidic environment and peroxide, the ozone being treated in two stages.

Conventional bleaching processes include the preparation of fully bleached pulp, i.e., with a whiteness of 88% MGO, one or two chlorination steps with elemental chlorine and in most cases additionally one or two hypochlorite steps and one or two chlorine dioxide steps. In the chlorination step, 4-8% C1 is usually used₂ based on the amount of pulp, large amounts of hydrochloric acid are formed, which means that the backwater contains large amounts of this acid and, respectively, sodium chloride, where the acid is neutralized. In addition, the backwater will contain organic chlorine compounds. To a lesser extent, this also applies to hypochloride bleaching and chloride oxide bleaching.

These organic substances and, in particular, the organic substances, which dissolve during the bleaching of the pulp, constitute a substantial strain on the watercourses and lakes in which the backwater is discharged. Moreover, chlorine-containing liquids are highly corrosive and create major problems in a recycling system.

Due to this, a bleaching process has been sought, where the chlorine bleaching step has been replaced by an oxygen bleaching step.

However, there is as yet no way in which the use of chlorine-containing bleach such as chlorine dioxide can be avoided to achieve fully bleached qualities.

As examples of conventional bleaching sequences, the following processing steps may be cited: C / E / H, C / E / H / E / H, C / E / H / H, C / E / H / D, C / E / D / E / D, C / E / H / D / P, C / E / H / D / E / D /, C / E / H / E / D / and C / E / H / D / E / H.

More recently, attempts have been made to reduce the amount of chlorine-containing bleach by replacing chlorine (C) with oxygen (0).₂), but to achieve fully bleached qualities, chlorine dioxide must still be used. The following examples can be given of such treatment sequences: a / o₂/ d / e / d, o₂/ d / e / d, 0₂/ d / e / d, a / 0₂/ D / p / d.

7902901-3

In the above sequences C: chlorine, E: alkali, H: hypochlorite, D: chloride oxide, P: peroxide. 0₂: oxygen. A: oxygen washing, Z: ozone.

SE-PS 7305671-5 discloses a 4- to 5-step chlorine-free bleaching process with alternating use of peroxide and peracid. However, such a process is very expensive with today's prices of the chemicals that are part of the process and should therefore hardly be of practical use.

The object of the present invention is to provide a method for bleaching cellulose-containing pulp without the use of chlorine-containing bleach, so as to avoid the corrosion problems and environmental stresses caused by such bleaching processes.

Thus, the invention relates to a method of bleaching oxygen-slurried oxygen-delignified cellulosic pulp such as sulfate, soda and sulfite pulp with a dry matter content exceeding 15%, with ozone in acidic environment and peroxide, the ozone treatment being carried out in two steps, characterized in by conducting an ozone treatment in the first stage with simultaneous or subsequent peroxide treatment (ZP), after which the pulp is subjected to an intermediate washing step for pH adjustment, dewatering / pressing and fluffing, after which the pulp in the second stage is ozone and peroxide treated (ZP), and that 55-85% of the total chemical amounts, i.e. ozone and peroxide used in the two bleaching stages, are added in the first stage. , while utilizing it, a total amount of ozone not exceeding 1%.

The method according to the invention has the advantage that, in addition to being free from the above-mentioned disadvantages, it can also be directly included as part of an existing recycling system.

In Pulp and Paper Mag. Can.Vol. 75 No. 4 T153 (1974) describes in an article by N.Soteland a method of ozone and peroxide bleaching which forms the basis of the present invention. However, the known method does not give satisfactory results on several pulp types. This is due to the large quantities of chemicals, ie ozone, which must be used to obtain sufficient brightness, ie, fully bleached qualities, and these large quantities of chemicals led to an undesirably large reduction in strength of the cellulose pulp.

However, the present invention achieves fully bleached pulp qualities with satisfactory strength properties.

Another feature of the method of the present invention is that the mass of oxygen is delignified to a kappa number in the range 15-5 prior to two-stage bleaching with ozone and peroxide.

The invention will be described in more detail below

7902901-3 to the accompanying drawing, which is a general flow chart of the method according to the invention.

As shown in the drawing, boiled and washed cellulosic pulp is added to an oxygen delignification step 1, in which the pulp is known in the art to oxygen delignify to a coat number in the range 15-5.

After 0₂- the pulp, which is now alkaline, is added to a stage 2 where the pulp is washed, dewatered and fluffed. The combined washing and pH adjustment of the pulp is symbolized in the drawing by a smaller block 2a, and the pulp here gets its pH value adjusted to less than or equal to 7. For sulfate pulp this can preferably be carried out with sulfuric acid. For sulfite masses, the acidic condensates that come from the sulphite boiling itself are used. The pulp is washed to remove oxygenated lignin compounds from the 0, - step.

The block denoted by 2b symbolizes dewatering of the pulp to a dry content of about 25-60%, preferably 30-45%.

Block 2C stands for the fluffing step, where the pulp is given a finely divided and airy form to thereby achieve the largest possible specific surface for gas reaction.

The fluffed pulp is then treated with ozone in an ozone reactor 3, preferably of the type disclosed in Norwegian patent 137651, whereupon the ozone-treated pulp is treated with alkaline bleach, NaOH, DTPA, in an underlying high-consistency ripening reactor, preferably described by in the Norwegian patent specification 140771.

The ozone and peroxide-treated pulp is then fed to a wash and dewater step 4, where block 4a indicates washing and pH adjustment. The pH adjustment can be done by adding sulfuric acid to the wash water, as this results in a gain of 2-3 light units.

The block 4b in step 4 indicates dewatering / pressing to preferably 30-45% TS of the washed and pH-adjusted pulp, which after dewatering is fluffed to light and airy consistency.

In the drawing, the ozone and peroxide treatment step # 2 indicates, which is of the same type as described in connection with step 3 above.

55-85% of the total amount of chemical is added in step 3,

7902901-3 while the rest of the chemicals, i.e. 45-15%, are added to the ozone and peroxide treatment step 5 ·

Example 1

An oxygen delignified sulfate mass of eucalyptus having a kappa number 7 and a viscosity of 850 dm 2 / kg is pressed to 40% dry content, fluffased and ozone treated with 0.4% Oy. ⁺ -DTFA, ie diethylenetriamine pentaacetate, so that the consistency is 25% and. so that the quantities of chemicals calculated on the pulp are resp. 0.6%, 0.2% and 0.2%.

The pulp is treated with this mixture for one hour at 90 ° G. Then the pulp is pressed, pH adjusted to 2 with sulfuric acid and ozone treated again with 0.2% 0-.

After ozonation, a liquid consisting of extruded bleaching liquid from the first stage peroxide bleach plus NaOH + H 2 O 2 + DTPA is added to the pulp, so that the amounts of the chemical. calculated on the mass, respectively. 0.4%, 0.1% and 0.1%.

After a treatment time of 1 hour at 90 ° C, the pulp is washed and pH-adjusted with SO 2 ~ water. The results are shown in the table below

Example 2

An oxygen delignified sulphate pulp of softwood with a kappa number of 10 and a viscosity of 800 dm 2 Ag is treated as in Example 1 using 0.4% O-, in the first ozone stage and with the resp. 1.4% NaOH, 0.8% and 0.2% DTPA in the subsequent alkaline peroxide treatment.

The second step treatment consists of 0.2% 0.1% 0.6% NaOH, 0.2% H2O2 and 0.1 ° / e DTPA. The results are shown in the table below. Example 5

A pine sulphite pulp, oxygen delignified with MgCOH 3 as the base in the oxygen stage and with the kappa number and viscosity 780 dm 2 Ag treated as in Example 1.1 first step, 0.3% O, 0.5% NaOH, 0.2% H is used.₂0₂ and 0.2% DTPA.

In the second step, 0.2% Oy 0.3% NaOH, 0.1% H was used_n0 and 0.1% DTPA.

L ·. d.

The results are shown in the table below.

7902901-3 cz ✓

<td></td><td></td><td>Chart</td><td></td><td></td><td></td>

<td>Example No.</td><td>Oxygen delignified nasal type</td><td>Total ozone used%</td><td>Total peroxide used%</td><td>ISO -light- the%</td><td>Viscosity? dm³ / kg</td>

<td> 1</td><td>Eucalyptus sulfate</td><td> 0,6</td><td> 0,3</td><td> 90,5</td><td> 590</td>

<td> 7</td><td>Softwood sulphate</td><td> 0,6</td><td> 1,0</td><td> 89</td><td> 610</td>

<td> 3</td><td>Pine sulfite</td><td> 0,5</td><td> 0,3</td><td> 91</td><td> 650</td>

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