Method for treating chemical pulps by treatment with ozone in the presence of magnesium ions

Abstract

This invention relates to a method for treating chemical paper pulp comprising a step of treating the pulp with ozone followed by a step of alkaline extraction, in which alkaline extraction takes place in the presence of magnesium ions (Mg.sup.2+).

Claims

1. Method for treating chemical paper pulp comprising: (a) a step of treating the pulp with ozone, followed by (b) an alkaline extraction step, comprising carrying out alkaline extraction of the treated pulp in the presence of magnesium ions (Mg2+) in an amount equal to or greater than 0.01% weight of the dry weight of the pulp, and in the absence of any of added oxygen and hydrogen peroxide.

2. Method for treating chemical paper pulp according to claim 1 wherein the magnesium ions appear in the form of at least one magnesium salt.

3. Method for treating chemical paper pulp according to claim 2, wherein the magnesium salt is selected from the group consisting of magnesium sulfate, magnesium carbonate, magnesium hydroxide, magnesium oxide, magnesium gluconate, and mixtures thereof.

4. Method for treating chemical paper pulp according to claim 1, wherein the magnesium ions represent between 0.01% and 0.5% by weight of the dry weight of the pulp.

5. Method for treating chemical paper pulp according to claim 1, comprising adding the magnesium ions at a neutral or acidic pH.

6. Method for treating chemical paper pulp according to claim 5, comprising adding the magnesium ions at a neutral or acidic pH between the step of treating the pulp with ozone and the step of alkaline extraction.

7. Method for treating chemical paper pulp according to claim 1, comprising washing between the step of treating the pulp with ozone and the step of alkaline extraction.

8. Method for treating chemical paper pulp according to claim 7, comprising adding the magnesium ions between washing and the alkaline extraction step.

9. Method for treating chemical paper pulp according to claim 7, wherein said washing comprises washing in water.

10. Method for treating chemical paper pulp according to claim 1, comprising carrying out the alkaline extraction step using a soda solution.

11. Method for treating chemical paper pulp according to claim 1, wherein the chemical pulp is a kraft pulp or sulphite pulp.

12. Method for treating chemical paper pulp according to claim 1, further comprising, upstream of the step of treating the pulp with ozone, or downstream of the step of alkaline extraction, at least one step of pulp processing with hydrogen peroxide.

13. Method for treating chemical paper pulp according to claim 1, wherein the magnesium ions represent between 0.02% and 0.1% by weight of the dry weight of the pulp.

Description

EXAMPLES

(1) The invention and its resultant advantages will be understood more clearly from the following. However, these are not to be considered as limiting cases in any way.

Example 1 (According to the Prior Art)

(2) In a known way, eucalyptus kraft pulp is processed in two successive stages using oxygen (OO) to produce a pre-bleached pulp, having the following properties: Residual Lignin Factor Represented by the Kappa Number: 10; Brightness: 51.2%; Degree of Polymerization (DP) of the Cellulose: 1630.

(3) Viscosimetry measures the degree of polymerization of the cellulose according to the ISO standard No. 5351/1-1981.

(4) Following a conventional operation known as fluffing, which consists in mechanically opening the pulp structure to facilitate the reaction of the ozone with the fibres, the pulp is then processed with ozone (Z) at high consistency (40%) at 25? C., with a quantity of ozone corresponding to 0.6% of the weight of dry pulp at a pH of 2.5 for the time needed to add the ozone (which reacts instantly), that is approximately 3 minutes. Following this treatment, the pulp is washed copiously in pure water then treated with soda (E) at a consistency of 10%, with a quantity of soda of 2% compared to the pulp, at 70? C. for 1 h. The pH of the pulp is then greater than 11.

(5) Following this treatment, the pulp has the following characteristics: Kappa Number: 6.1; Brightness: 75.8%; DP of Cellulose: 1203.

(6) It appears that the delignification and bleaching that occur during this process are accompanied by the severe depolymerization of the cellulose.

Example 2 (According to the Invention)

(7) The same sequence as described in example 1 is repeated but this time by adding 0.3% by weight of MgSO.sub.47H.sub.2O (i.e. approximately 0.03% of Mg) with respect to the weight of dry pulp, after washing and before the addition of soda, that is at a moment when the pH of the pulp is not yet alkaline.

(8) Following this treatment according to the invention, the pulp has the following characteristics: Kappa Number: 6.0; Brightness: 75.7%; DP of Cellulose: 1327.

(9) This example indicates the advantages of the invention which, while maintaining the same level of performance regarding delignification and increased brightness of the conventional ZE process, substantially reduces the depolymerization of the cellulose.

Example 3 (According to the Invention)

(10) The same experiment as described in example 2 is reproduced except that the quantity of MgSO.sub.47H.sub.2O, in this case, is 2% (that is 0.2% of Mg) by weight compared to the weight of the dry pulp.

(11) Following this treatment, the pulp has the following characteristics: Kappa Number: 6; Brightness of the Pulp: 73%; DP of Cellulose: 1340.

(12) In this example, it is evident that using excessive loads of MgSO4 tends to reduce the performance of the delignification and bleaching process for a very slight additional improvement of the DP of the cellulose.

Example 4

(13) To illustrate the advantage of the ZE process according to the invention in different bleaching sequences from those demonstrated previously, the same pulp as in example 1, already put through OO treatment, is first treated by acidolysis (treatment A) at pH 3, for 3 h at 90? C., and in the presence of EDTA, a conventional sequestering agent for transition metal cations, so as to remove part of the hexenuronic acids that appear.

(14) It is then treated by a stage using hydrogen peroxide (P) at a consistency of 10%, with 2% of hydrogen peroxide at 90? C. for 2 h.

(15) Following this sequence, the pulp has the following characteristics: Brightness: 68%; DP of Cellulose: 1328.

(16) Also, analyses reveal that it is almost free of transition metal cations, considered to be an impediment in the bleaching process.

(17) The pulp prepared in this manner is put through ZE treatment under conventional conditions illustrated in example 1, in particular with 0.6% ozone by weight compared to the weight of the pulp measured dry.

(18) Following this OOAPZE treatment, the pulp has the following characteristics: Brightness: 84.8% DP of Cellulose: 1165.

(19) Thus, an increase of the pulp brightness is observed but with a drop in the cellulose polymerisation degree.

(20) On the other hand, if magnesium is added under the same conditions as in example 2 (0.3% by weight of MgSO.sub.47H.sub.2O, i.e. approximately 0.03% of Mg compared to the measured dry weight of the pulp, after washing and before the addition of soda), the pulp has the following properties: Brightness: 84.8% DP of Cellulose: 1250.

(21) Accordingly, for unchanged brightness there is less degradation of the cellulose. This example shows that the ZE process according to the invention maintains its full advantage when it is placed further down the bleaching sequence and on a pulp that has already been purified.

Example 5

(22) In another embodiment of the invention, the pulp of example 1 is treated with ozone as indicated in example 1 then, without any transition, the consistency of the pulp is reduced to 10% by the soda solution and the alkaline extraction (E) continues under the conditions of example 1.

(23) Following this [ZE] treatment, the pulp has the following characteristics: Kappa Number: 6.4; Brightness: 74.2% DP of Cellulose: 1140.

(24) This process that no longer includes the washing stage between Z and E evidently offers a lower performance than the process of example 1 (higher kappa number and lower brightness). The reduction in the polymerization degree of the cellulose remains considerable.

(25) If the process is modified according to the invention, that is by adding the load of MgSO4 of example 2 as a solution in water (0.3% by weight of MgSO.sub.47H.sub.2O, that is, approximately 0.03% of Mg compared to the weight of dry pulp), directly after the Z phase and before the addition of the soda solution, the pulp has the following characteristics: Kappa Number: 6.5; Brightness: 74%; DP of Cellulose: 1260.

(26) The improvements to the degree of polymerization of the cellulose, due to the presence of the magnesium cations, is again high in this embodiment.