Process for Bleaching Wood Pulp With Ozone in the Presence of a Cationic Surfactant

1. Field of the Invention

This invention relates to a process for bleaching pulp with ozone. More particularly, it relates to bleaching pulp at low consistencies with ozone in the presence of a cationic surfactant.

2. The Prior Art

It is known that ozone is an effective bleaching chemical when applied to pulps at consistencies of say from 25 to 65%. High consistency slurries have been used because the bleaching rate of ozone at low consistencies is extremely slow. Brabender et al. in U.S. Pat. No. 2,466,633 were among the first to teach high consistency bleaching pulp with ozone. However, it was felt that early attempts to bleach with ozone were detrimental to the cellulose fibers. Because of the strong oxidizing power of ozone, there is an indiscriminate ozone oxidation attacking carbohydrate and lignin concurrently producing significant degradation of fibers over extended reaction times.

Wade in U.S. Pat. No. 3,318,657 claimed to have overcome this problem by treating cellulose fibers prior to contact with ozone with an alkali metal borohydride. Later, Ancelle et al. in U.S. Pat. No. 3,451,888 disclosed the use of wetted ozonized air as a bleaching agent.

It is evident that the development of a commercially acceptable ozone-based bleaching system would eliminate chlorine containing bleach effluents and reduce chemical costs. It is, therefore, the general object of this invention to provide a commercially acceptable ozone pulp-bleaching process.

Another object of this invention is to provide a process for bleaching pulp with ozone at low consistencies.

Other objects, features and advantages of the invention will be evident from the following detailed description.

It has been found that the presence of 0.01 to 1.0%, based on oven dried pulp, of a cationic surfactant improves the ozone reaction rate dramatically at low consistencies, i.e., 0.1% to about 20.0%, as well as the bleaching efficiency. The bleaching efficiency is the quantity of ozone required for a given increase in brightness or decrease in Kappa Number.

In the process of this invention, an aqueous slurry of wood pulp from a kraft, sulfite or soda process is treated, at a consistency between about 0.1% and 20.0%, preferably between 1% and 10% and most preferably between 1% and 5%, by weight based on oven dried (O.D.) pulp, with ozone.

To the aqueous slurry of cellulose pulp is added from 0.01% to 1.0%, preferably 0.1% to 0.5%, based on oven dried pulp, of a cationic surfactant. The cationic surfactant may be added prior to or co-currently with the ozone. The cationic surfactants are of the following type ##STR1## wherein R is CH.sub.3 or CH.sub.2 CH.sub.3, R.sub.1 is an alkyl radical containing from 3 to 15 carbon atoms, an aryl radical or an alkylaryl radical, and X is an inorganic anion, such as a halogen. Preferred surfactants include hexadecyl trimethyl ammonium bromide, nonyl trimethyl ammonium bromide and dodecyl trimethyl ammonium chloride.

In solution, the surfactant apparently performs a number of functions which increase the ozone reaction rate. The cationic surfactant decreases the surface tension of water and this, in turn, decreases the bubble size of ozone being passed through the pulp slurry. The coalescence of the bubbles is diminished as the result of a surfactant coating forming about each bubble to provide an ionic film that repulses the approach of other bubbles. From these two effects, it follows that at a given volumetric flow rate the total surface area of all bubbles passing through the pulp slurry is greater when the cationic surfactant is present in the liquid phase.

If the rate controlling step in ozone bleaching is transfer of ozone across the gas-liquid interface, an increase of surface between these two phases increases the rate of ozone transfer to the liquid phase. When the surfactant is added to water, the transfer of ozone from gas phase to liquid phase is increased even in the case of ozonizing water in the absence of pulp. When the cationic surfactant is added to water plus pulp, a significant increase in ozone consumption and pulp bleaching occurs.

The rate enhancing effect of surfactant is believed due to at least three phenomena: (1) the surfactant produces a greater number of smaller bubbles which better disperse the gaseous reagent directly to the fiber, (2) the mass transfer area is increased because of the smaller bubbles, and (3) in the case of a cationic surfactant, the bubbles are surrounded by a positively charged film which causes a greater attraction between bubbles and fibers. Cationic bubbles may then cling to fibers and transfer ozone directly through the immobilized water layer on the fiber to the reaction sites, thereby circumventing the diffusion of ozone through the bulk liquid where it will be more susceptible to destruction by its known decomposition reaction with water.

The ozonation is carried out by passing ozone gas through the aqueous pulp slurry in an amount of say 10-80 mg./l. of oxygen or air. The time of exposure to give the desired bleaching must be determined for each fiber and is related to ozone concentration and temperature. These parameters should be adjusted so that about 0.1% to 3.0% of ozone based on the weight of the dried pulp is consumed during the process. Exposure time may require 2 hours at room temperature to 2 minutes or less at 100.degree. C. Again, the exact conditions for each pulp must be tested individually. After ozone treatment, the pulp is washed and, if desired, adjusted to a pH required for further processing or finishing.

However, to maintain low effective surfactant applications, it is preferable in a batch type reactor to add acid to the pulp slurry before surfactant is added because the cationic surfactant may be strongly attracted to negative sites on pulp fibers and thus removed from the surrounding liquid. The ozone treatment can be carried out in an aqueous solution having a pH between 1 and 7 and preferably between 2 and 4. This may be accomplished by treating the fibers with a solution of an acid, such as hydrochloric acid, sulfuric acid, acetic acid, and the like. Acid neutralizes the anionic sites on the fiber, thereby decreasing this attraction so that more surfactant remains in solution. Nevertheless, there is always some attraction between cationic surfactant and pulp fibers.

Using the process of this invention, semi-bleached or bleached cellulosic fibers are obtained which are 5-10% brighter than fibers treated with ozone alone; and the bleaching efficiency is increased.

The process of the present invention is best illustrated by the following specific examples:

A 24-gram sample, sample "A", of unbleached hardwood pulp was diluted with water to a consistency of 0.5% by weight based on O.D. pulp. An ozonized oxygen stream containing 52 mg./l. ozone was passed through the pulp slurry at room temperature for 2 hours.

For comparison, a second 24-gram sample of the same hardwood pulp, sample "B", was diluted to a 0.5% consistency treated at the same conditions, except 0.4% by weight O.D. pulp of hexadecyl trimethyl ammonium bromide was added.

A third 24-gram sample of the same hardwood pulp, sample "C", was diluted to 0.5% consistency treated at the same conditions as sample B, except 1.0% by weight of O.D. pulp of sulfuric acid was added and the time was 1 hour.

A fourth identical sample, sample "D", was treated at the same conditions as sample A (no surfactant added), except 1.0% by weight on O.D. pulp of sulfuric acid was added and the time was 1 hour.

The main characteristics of the ozone bleached pulps are compared in the following table.

The increased G.E. brightness of samples B and C shows clearly the desirable effects of the cationic surfactant over the use of ozone without the surfactant. Sample C with the addition of sulfuric acid likewise also illustrates the bleaching efficiency and increased reaction rate.

A second unbleached hardwood pulp, 50.0-gram O.D. sample "E", at a consistency of 1.0% by weight based on O.D. pulp, was treated with 47 mg./l. of ozone in oxygen for 1.5 (no surfactant added).

Another sample "F", identical to sample E, was treated at the same conditions as sample E, except 44 mg./l. ozone in oxygen was used; 0.4% by weight O.D. pulp of hexadecyl trimethyl ammonium bromide and 5.0% by weight O.D. pulp of sulfuric acid were also added.

The results are compared in the following table.

The results show that with the addition of the surfactant to an acidified slurry a 1% consistency dramatically increased the G.E. brightness from 38.1 to 56.3 and the bleaching efficiency over 1.5 hours (0.83% to 1.71% ozone being consumed during the same time).

This example illustrates the utility of various cationic surfactants at different pulp consistencies.

Another portion of the unbleached hardwood pulp of Example 2, sample "G", was diluted with water and treated with ozone for 1.5 hours (no surfactant added).

Sample "H", identical to sample G, was diluted to a consistency of 1.0%; and 0.4% nonyl trimethyl ammonium bromide and 5.0% sulfuric acid were added during ozonation lowering the initial pH to about 2.

Sample "I" was identical to sample H, except dodecyl trimethyl ammonium chloride was substituted as the surfactant.

Sample "J" was a 50.0-gram sample at 3.3% consistency treated like sample G, except dodecyl trimethyl ammonium chloride was used as the surfactant.

The results are shown in the table below.

These results clearly show the effectiveness of various cationic surfactants.

While the invention has been described and illustrated herein by references to various specific materials, procedures and examples, it is understood that the invention is not restricted to the particular materials, combinations of materials, and procedures selected for that purpose. Numerous variations of such details can be employed, as will be appreciated by those skilled in the art.