Title:
PULP BLEACHING PROCESS
United States Patent 3622444
Abstract:
A process for bleaching sulfate or sulfite wood pulp having as the first three steps; chlorination, caustic extraction and chlorination; the pulp in the chlorination steps being pretreated with ammonia or an ammonium salt. Additional process steps may follow. The process provides wood pulp bleached to a high-brightness level at relatively low chemical cost while avoiding severe degradation of the pulp.


Inventors:
ANDREWS DOUGLAS HAIG
Application Number:
05/022681
Publication Date:
11/23/1971
Filing Date:
03/25/1970
Assignee:
Canadian Industries Limited (Montreal, Quebec, CA)
Primary Class:
Other Classes:
162/63, 162/86, 162/89
International Classes:
D21C9/14; (IPC1-7): D21C3/18; D21C3/02; D21C3/26
Field of Search:
162/88,89,85,86
View Patent Images:
US Patent References:
Primary Examiner:
Caine, Howard R.
Claims:
What we claim is

1. A process for the bleaching of sulphate or sulphite wood pulp which comprises subjecting the wood pulp to the steps of

2. treating the wood pulp in aqueous suspension initially with 0.03 percent to 0.05 percent by weight of ammonia or an ammonium salt and sequentially with sufficient chlorine to provide residual chlorine of at least 0.1 percent on the pulp;

3. treating the chlorine-treated wood pulp in aqueous suspension with alkali base; and

4. treating the alkali-treated wood pulp in aqueous suspension initially with 0.3 percent to 0.5 percent by weight of ammonia or an ammonium salt and sequentially with sufficient chlorine to provide residual chlorine of at least 0.1 percent on the pulp.

5. A process as claimed in claim 1 wherein the wood pulp is washed with water at the completion of each step.

6. A process as claimed in claim 1 wherein the three initial steps are followed by additional steps comprising

7. A process as claimed in claim 3 wherein the wood pulp is washed with water at the completion of steps (a) and (b).

8. A process as claimed in claim 3 wherein alkali base is employed as an additional reagent in step (b).

9. A process as claimed in claim 1 wherein the treatment with chlorine in step (1) is carried out at 10° C. to 40° C. temperature at a wood pulp consistency of 2 percent to 5 percent.

10. A process as claimed in claim 1 wherein the treatment with chlorine in step (3) is carried out at 20° C. to 60° C. temperature at a wood pulp consistency of 2 percent to 15 percent.

11. A process as claimed in claim 1 wherein the three initial steps are followed by additional steps comprising

12. A process as claimed in claim 1 wherein the three initial steps are followed by additional steps comprising

Description:
This invention relates to a method for bleaching wood pulp.

Wood pulp produced by the sulphate or sulphite processes is too dark in color for many applications and requires to be bleached. The bleaching process operates through the solubilization of the dark colored lignin component of the wood pulp. However, reagents which act to solubilize the lignin also attach to a lesser extent the cellulose component of the wood pulp resulting in its degradation. A commonly employed reagent for bleaching sulphate and sulphite wood pulp is chlorine. The chlorination treatment is normally followed by caustic extraction which removes the chlorine-modified lignin. Many modifications of the sequence chlorine-caustic extraction have been devised with the view of increasing the brightness and decreasing the degradation of the cellulose. For example, chlorine dioxide can be substituted for part or all of the chlorine employed in the bleaching sequence or the initial chlorine-caustic extraction sequence may be followed by one or more chlorine dioxide treatments. Alternatively, the post-extraction steps may be with hypochlorites or peroxides. However, due to the ready availability and relatively low cost of chlorine, it is of economic advantage to base wood pulp bleaching processes upon this reagent, provided the process can impart high brightness levels accompanied by minimal degradation of the wood pulp.

It has now been discovered that treatment of sulfate or sulphite wood pulp in a three step process of sulphite caustic extraction, chlorination wherein the wood pulp in the chlorination steps is subjected to pretreatment with ammonia or an ammonium salt provides a treated pulp having the residual lignin in such condition that the pulp may be readily bleached to the desired brightness level without attendant degradation of the pulp. For example, when followed by a fourth step caustic extraction and fifth step chlorine dioxide treatment, a sulphate wood pulp can be bleached to a brightness of about 90, while maintaining the viscosity value at about 20 c.p.s. The novel process permits use of the relatively lower cost chlorine reagent in the third stage without attendant degradation of the wood pulp.

The primary object of the invention thus is to provide a process for bleaching wood pulp to high brightness level at relatively low chemical cost while avoiding severe degradation of the pulp. Additional objects will appear hereinafter.

The process of the invention for bleaching sulphate or sulphite wood pulp comprises the steps of:

1. TREATING THE WOOD PULP IN AQUEOUS SUSPENSION INITIALLY WITH 0.03 PERCENT TO 0.5 PERCENT BY WEIGHT OF AMMONIA OR AN AMMONIUM SALT FOLLOWED BY TREATMENT WITH SUFFICIENT CHLORINE TO PROVIDE RESIDUAL CHLORINE OF AT LEAST 0.1 PERCENT ON THE PULP;

2. TREATING THE CHLORINE-TREATED WOOD PULP IN AQUEOUS SUSPENSION WITH ALKALI BASE; AND

3. TREATING THE ALKALI-TREATED WOOD PULP IN AQUEOUS SUSPENSION INITIALLY WITH 0.03 PERCENT TO 0.5 PERCENT BY WEIGHT OF AMMONIA OR AN AMMONIUM SALT FOLLOWED BY TREATMENT WITH SUFFICIENT CHLORINE TO PROVIDE RESIDUAL CHLORINE OF AT LEAST 0.1 PERCENT ON THE PULP.

Preferably the wood pulp is washed with water at the completion of each step.

For certain applications the wood pulp at the completion of the third step of the process will be bleached to a sufficient level of brightness. However, it usually will be necessary to employ additional steps. Suitably a fourth step comprising caustic extraction and a fifth step comprising chlorine dioxide treatment will be sufficient to bring the brightness value of sulphate wood pulp up to the 90 level. Alternatively, the fourth step may be treatment with alkaline hypochlorite followed by a fifth step caustic extraction and a sixth step treatment with chlorine dioxide. Again alternatively, the fourth step may be treatment with hydrogen peroxide or alkali peroxide followed by fifth step treatment with chlorine dioxide. It is a characteristic of the process that the 90 brightness level can be attained while avoiding undue degradation of the pulp as indicated by the maintenance of the viscosity in the range of 15 to 20 c.p.s.

In the first step of the process, sulphate wood pulp in aqueous suspension at 2 percent to 5 percent consistency is treated first with 0.03 percent to 0.5 percent ammonia or ammonium salt, and then with 2 percent to 8 percent chlorine at 10° C. to 40° C. temperature for a period of 10 to 90 minutes. The chlorine treated wood pulp is preferably washed with water before commencement of the next process step.

In the second step of the process, the pulp at 3 percent to 30 percent consistency is treated with 1 percent to 6 percent alkali base usually sodium hydroxide, for 10 to 180 minutes at a temperature in the range 30° C. to 90° C. The caustic extracted wood pulp preferably is then washed with water.

In the third step of the process, the wood pulp in aqueous suspension is treated at 2 percent to 15 percent consistency first with 0.03 percent to 0.5 percent ammonia or ammonium salt and then with 1 percent to 3 percent chlorine at 20° C. to 60° C. temperature. The chlorine treated wood pulp is then washed with water.

When sulphate wood pulp is subjected to the three basic steps of the process, the product will have a brightness value in the range 50 to 70 and a viscosity in the range 15 to 25 c.p.s.

If a higher brightness level is required, additional bleaching steps will be necessary. For example, a fourth step comprising caustic extraction and a fifth step comprising chlorine dioxide treatment will provide a bleached pulp with a brightness at the 90 level and viscosity of 20 c.p.s. standard.

In the above fourth step, the pulp at 3 percent to 30 percent consistency is treated with 0.2 percent to 3.0 percent alkali base such as sodium hydroxide at a temperature in the range 30° C. to 90° C. for a period of 10 to 180 minutes. The wood pulp is then washed with water.

In the fifth step, the caustic extracted wood pulp is treated in aqueous suspension at 3 percent to 30 percent consistency with 0.2 percent to 1.0 percent chlorine dioxide. The treatment takes place at a temperature in the range 50° C. to 95° C. for a period of 60 to 300 minutes. It may be of advantage to buffer the reaction mixture by adding alkali base thus raising the pH. The wood pulp is finally washed with water.

In an alternative sequence having fourth step treatment with hypochlorite, fifth step caustic extraction and sixth step treatment with chlorine dioxide the following are the process conditions:

Step 4. Pulp at 3 percent to 15 percent consistency, 0.2 percent to 2.0 percent hypochlorite, treatment at 20 C. to 45° C. for a period of 30 to 180 minutes.

Step 5. Pulp at 3 percent to 30 percent consistency, 0.2 percent to 3.0 percent alkali base such as sodium hydroxide, treatment at 30° C. to 90° C. for a period of 10 to 180 minutes.

Step 6. Pulp at 3 percent to 30 percent consistency, 0.2 percent to 1.0 percent chlorine dioxide, treatment at 50° C. to 95° C. for a period of 60 to 300 minutes.

In an alternative sequence having fourth step treatment with alkaline hydrogen peroxide or alkali peroxide and fifth step treatment with chlorine dioxide the following are the process conditions:

Step 4. Pulp at 6 percent to 30 percent consistency, 0.1 percent to 1.0 percent peroxide (alkaline hydrogen peroxide or alkali peroxide) treatment at 50° C. to 90° C. for a period of 30 to 180 minutes.

Step 5. Pulp at 3 percent to 30 percent consistency, 0.2 percent to 1.0 percent chlorine dioxide, treatment at 50° C. to 95° C. for period of 60 to 300 minutes.

The ammonia or ammonium salt is mixed with the wood pulp prior to treatment with chlorine. It thus follows that at the commencement of the chlorine treatment there will be ammonium ion on the wood pulp. However, the ratio of chlorine to ammonia is such that at completion of chlorination there will be a residual of at least 0.10 percent chlorine on the wood pulp. It has been found that the sequence of adding the ammonia and chlorine is significant. If the ammonia and chlorine are mixed first in aqueous solution and then applied to the wood pulp the inhibition of pulp degradation is not observed.

The ammonia ingredient can be introduced into the aqueous suspension of the wood pulp as concentrated ammonium hydroxide, as gaseous ammonia, or as an ammonium salt such as ammonium chloride or ammonium sulfate.

The alkali base may be sodium hydroxide, potassium hydroxide, sodium carbonate or potassium carbonate.

In this specification percentages are by weight based upon the over dried weight of the wood pulp.

In this specification the wood pulp characterizing values of brightness, permanganate number, kappa number and viscosity refer to values obtained by the following procedures.

Brightness: TAPPI Standard T 217m, 218m Permanganate number: TAPPI Standard T 214m Kappa number: TAPPI Standard T 236 Viscosity: TAPPI Standard T 230 sm

The process of this invention possesses the advantage that it provides a bleached wood pulp of high brightness level at relatively low cost while at the same time avoiding undue degradation of the wood pulp.

The invention is illustrated by the following examples but the scope thereof is not limited to the embodiments shown in the examples.

EXAMPLE 1

A sulphate wood pulp derived from Eastern Canadian coniferous wood (mainly spruce and balsam fir) having a Kappa number of 27.3 was bleached by the following five step process. 1. The wood pulp in aqueous suspension at 3.0 percent consistency was treated at ambient temperature first with 0.1 percent ammonia and then with 5.2 percent chlorine for a period of 30 minutes. The treated pulp was then washed with water. 2. The chlorine treated wood pulp was next subjected to a caustic extraction step. The wood pulp at 12 percent consistency was treated with 4.7 percent sodium hydroxide for a period of 90 minutes at a temperature of 60° C. The wood pulp was then washed with water. At the completion of this step the wood pulp had a permanganate number of 4.3 and a viscosity of 28.3 c.p.s. 3. The wood pulp in aqueous suspension at a consistency of 5.0 percent was next treated first with 0.1 percent ammonia followed by 2.6 percent chlorine. The chlorine treatment was continued for 120 minutes at 40° C. The wood pulp was then washed with water. The wood pulp at the completion of this step had a brightness of 68.3 and a viscosity of 20.7.

4. The wood pulp in aqueous suspension at a consistency of 12.0 percent was next treated with 1.0 percent sodium hydroxide for a period of 90 minutes at 60° C. The wood pulp was then washed with water. The wood pulp at the completion of this step had a permanganate number of 1.85 and a viscosity of 19.9 c.p.s. 5. The wood pulp in aqueous suspension at a consistency of 12.0 percent was treated with 0.45 percent chlorine dioxide at 70° C. for a period of 180 minutes. The wood pulp was then washed with water. The viscosity of the bleached product was 20.2 c.p.s. and brightness was 89.4. Brightness reversion, measured after 1 hour at 105° C. in a forced air circulating oven was 4.6 brightness units.

EXAMPLE 2

The same wood pulp employed in example 1 was subjected to bleaching sequences analogous to that of example 1 but employing 3 percent sodium hydroxide in step 2 and a range of concentrations of reagents in steps 3, 4 and 5.

In step 3 three different concentrations of ammonia and chlorine were employed and the viscosity of the wood pulp was measured at the completion of the step.

In step 4 two concentrations of sodium hydroxide were employed and the permanganate number was measured at the completion of the step.

In step 5 the chlorine dioxide treatment was carried out at a temperature of 85° C. with and without the addition of sodium hydroxide. At the completion of the bleaching sequence the brightness, brightness reversion and viscosity of the product were measured. The sequences are illustrated in the following Table. ##SPC1##

It can be seen that the proportion of chlorine can be reduced in step 3 and that the use of sodium hydroxide in the chlorine dioxide step improves the brightness and reduces the degradation of the wood pulp.