The present invention relates to a procedure for prebleaching cellulose pulp that has been obtained by a chemical process.

Conventional prebleaching of cellulose pulps cαooprises a treatment with gaseous chlorine. Hcwever, no one has been able to solve the recovery and sensible use of the chlorine con-pounds which are produced in the bleaching process, and therefore the chlorine has unavoidably been emitted as wastes into the environment. Thus evidently remarkable need exists to substitute for chlorine bleach¬ ing other methods which impose less stress on the environment. The known replacing methods inclυde oxygen bleaching, which presents the drawback of high oxygen consuπption, and nitrogen bleaching, which is hampered by the fact that nitrogen c πpour-ds are emitted into the envi-ranment.

in the present invention it has been unexpectedly found that the chlorine treatment, as the first step in the bleaching process for chemical pulps, can be replaced with a phenol treatment. The phenol treatment affords numerous advantages over the use of chlorine. Firstly, the phenol can be recovered and reused, whereby there will be no emission of bleaching residues det-dmeπcal to the en¬ vironment. The procedure of the invention renders possible also recovery of tine residual lignin that has been detached from the cellulose in the bleaching process, in addition to the phenol.

The use of phenol furtheπi-ore endows the cellulose pulp with useful extra properties. No one of these advantages can be gained when using conventional chlorine prebleaching.

The use of phenol in treating substances ∞ntaining cellulose is not novel in itself. For instance, cellulose pulp has been made from wood by treating wood chips with a phenol solution containing

a strong acid, e.g. hydrochloric acid, acting as a catalyst. As examples may be mentioned the DE Patents No. 2229673 and 326705. Hcwever, said patents relate to solubilizing lignocellu- lose, such as wood, not to cellulose bleaching. The use of phenol in prebleaching cellulose manufactured by chemical methods is not known in the art. In contrast, it has been found with the aid of NMR examinations that when wood is treated with a mixture of phenol and hydrochloric acid there occurs condensation between the native lignin in the wood and phenol and, consequently, darkening of the pulp; this suggests that phenol would not be suitable at all for use as a celluloseHaleach-Lng chemical.

The procedure of the invention is characterized in that said cellu¬ lose pulp is treated with phenol at elevated temperature, whereafter said cellulose pulp, possibly after washing, is bleached by single or multiple-step bleaching kncwn in itself in the art.

The other characteristic features of the procedure of the invention are stated in claims 2-6.

In the procedure of the invention a phenol treatment is applied in the prebleaching of chemical pulp. In this case pulp is concerned which has already been defibrated and in which the --residual lignin that is present has changed under effect of the digesting chemicals so that condensation with phenol no longer occurs. Therefore a substantially lower catalyst content and lcwer temperature suffice for deteching the lignin frcm the defibrated cellulose. In addition, the digesting process proper, a sulphate process for instance, may be discontinued earlier than conventionally usual without detriment to the quality ch-_-racfteristics of the ultimate pulp, and hereby the digesting capacity can be substantially increased.

It is a further advantage of the prebleaching procedure of the invention that phenol also dissolves extractible substances con¬ tained in the cellulose pulp. This is of significance particularly regarding the quality of hardwood cellulose because extractible

1 substances cause yellow discolouration of the cellulose and impair¬ ment of the absortivity, which is i-πportant in view of the fluff pulp, and quality inpairment of the soluble cellulose.

The procedure of the inventicn may be applied in prebleaching chemical cellulose pulps. Chemical pulps are here understood to be those pulps which have been obtained by chemical digestion methods. These methods include e.g. the sulphate process, various sulphite processes, and the soda process.

10

In practice, prebleaching as taught by the invention is simply carried cut in that the chemical cellulose pulp from the digesting process is treated with a phenol solution a sufficiently long time and at high enough tenperature, whereafter -the pulp is conducted,

15 after removal of the treat--meπt liquid, to further bleaching steps.

The phenol content in -the aqueous solution may vary so that the water/phenol proportion is in the range from 3:7 to 1:1. The phenol solution preferably also contains a small quantity of an acid,

20 advantageously hydrochloric acid, of which the quantity may vary in the range of 0.1-1.0%, preferably 0.2-0.5%, referred to the water quantity in the solution. Excessive acid quantity causes hydrolysis of the cellulose and hemicelluloses.

25 The treatment tenperature is not critical from the viewpoint of the invention, but since phenol forms an azeotrope with water, with boiling point about 100°C, it is advantageous to perform the treatment at this tenperature. The apprcprlate treatment tenperature is thus e.g. 9δ-102°C. The treatment time naturally depends on the

30 chemical cx_ncent_ratiαns and tenperatures employed, but it is nor¬ mally enough if the temperature is raised to treatment temperature within 30-60 min., and the treatment time at normal tenperature may vary from 1 to 60 min.

35 According to an advantageous en-bodi ent of the invention, the phenol treatment solution is removed from the cellulose pulp after

its tenperature has been raised to treatment tenperature, and it is replaced with fresh solution. The prebleaching will then be enhanced because a greater amount of lignin can be removed without substantially affecting the viscosity characteristics.

The procedure of the invention is applicable both in the case of hardwood and softwood celluloses. Especially in the case of hardwood cellulose, the procedure of the invention has turned out to be advantageous because it eliminates the problems which extractible substances cause regarding the cellulose quality.

Subsequent to treatment, the cellulose is washed in standard manner to be free of phenol cxatpcunds,. these being returned to the begin¬ ning of the process for reuse. The cellulose is further conducted, possibly after washing, to furtherbleaching steps, which thus need not be any chlorine or hypochlorite bleaching processes: they may consist for instance of chlorine dioxide or peroxide bleaching processes. The washing is advantageously done with water contcdning phenol and with hot water.

The invention is described more closely in the following with the aid of examples.

Example 1

Unbleached birch cellulose with characteristics as presented in Table I was treated with phenol under the following conditions:

Cellulose:liquid 1:8 Water:phenol 3:7

HCl in the water (%) 0.2

Treatment t-s-i-perature (°C) 98-100 liaised to t-reatment tenperature in min. 45

Treatment time (min.) 10

In Table I is shown a cα-parison between the characteristics of

the cellulose before and after the phenol treatment mentioned above.

Tabte 1

Initial Treated cellulose cellulose

Chlorine number (K&) 4.57 2.46

Viscosity (SCftN), dm ³ kg 1190 1180

DKM extract (%) 0.46 0.03

Ash content (%) 0.90 0.55

Yield (%) 94.9

Yield from wood (%) 52.8 50.1

Lignin removed in prebleaching (%) - 46.6

Fibre length, average (mm) 0.93 0.86

Table I reveals, among other things, that the viscosity of the cellulose is almost unchanged, implying that the lew-molecular hartcellulose has not been dissolved and no cellulose chains have been cleaved. The average fibre length is unchanged, indicating that acid hydrolysis has not cleaved any fibres.

Example 2

Example 1 was repeated, but the hydrochloric acid content was increased to 1.0%. The quantity of removed lignin rose then to 79%, but the viscosity went down to 480, indicating that there has been substantial cleaving of cai chydrates.

Example 3

Example 1 was repeated, changing the water:phenol proportion to be 1:1. Only 32% of the lignin could then be removed from the pulp. This indicates that a greater amount of water impairs the solu- bilization of lignin and the efficiency of the prebleaching. The viscosity of the cellulose retained the value 1330.

Example 4

Example 1 was repeated with the modification that the acid content was 0.12% of the water quantity. A cellulose product was then obtained which has chlorine number 3.27, viscosity 1330. The lignin quantity removed was only 29%.

Example 5

Unbleached birch sulphate cellulose having the characteristics shewn in Table II was treated with phenol under the following conditions:

Cellulose:liquid 1:12 Water:phenol 3:7

HC1 in the water (%) 0.2

Treatment temperature (°C) 98-100

Raised to treatment temperature in min. 45

Treatment time (min.) 60

After the treatment, the characteristics of the cellulose were compared with those of the -Initial cellulose. The results are presented in Table H.

Table II

I-nitial Treated cellulose cellulose

Chlorine number (KC1) 5.23 3.31

Viscosity (SC-AN), dm kg) 1340 1330

DKM extract (%) 0.41 -

Yield (%) 96.6

Yield from the wood (%) 53.2 51.4

Lignin removed in prebleaching (%) - 37

Fibre length, average (mm) 0.92 0.90

Example 6

Example 5 was repeated, replacing the treatment solution with fresh solvent after the temperature had been raised to treatment tenperature. When the treatment time was 30 min., the chlorine number was 2.46 and viscosity 1030, and removed lignin quantity 52%. When the same experiment was repeated with treatment time 60 min., the chlorine number 1.85 and viscosity 910 were obtained, and removed lignin quantity 65%. The results indicate that it is advan- tageous to replace the treatment solution.

Example 7

Pine sulphate cellulose with characteristics as shown in Table III was -treated with phenol solution under the following conditions:

Cellulose:liquid 1:10 ater:phenol 3:7

HCl in the water (%) 0.16 Treatment temperature (°C) 98-100

Raised to t-reatment temperature in min. 45

After treatment, the characteristics of the cellulose were cαipared with those of the initial cellulose. The results are presented in Table IH.

Table III

Initial Treated cellulose cellulose

Chlorine number (KC1) 6.2 4.50

Viscosity (SCBN), dmP kg 1070 850

Yield (%) 97.8

Yield from the wood (%) 46.5 45.5

Lignin removed in prebleaching (%) - 24.5

Fibre length, average (mm) 1.94 1.97

The example indicates that the procedure of the invention is also apprcprlate to be used in prebleaching softwood cellulose.