Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the bleaching of a cellulosic pulp. More particularly, it relates to the treatment of groundwood or mechanical pulp in a moist comminuted form with gaseous ozone or ozone-bearing gases.
2. Description of the Prior Art
In the conventional bleaching of mechanical pulp, the bleaching is effected to decolorize the pulp without rendering the lignin contained therein soluble. To effect such bleaching, two classes of bleaching chemicals are employed namely reducing agents and oxidizing agents.
The most commonly used reducing agents are hydrosulphides whence bleaching is conducted in the aqueous phase at 3 to 4 percent consistency, at a pH from 4.5 to 6.0, at a temperature of about 60° C. with a 1-hour retention time and normally chelating or sequestering agents are also present.
The most commonly used oxidizing agents for the bleaching of mechanical pulps are peroxides which in contrast to hydrosulphides involve an alkaline pH, a relatively high pulp consistency of 15 to 25 percent, moderate temperatures of about 60° C., and a retention time of 2 to 3 hours. Further, in such peroxide bleaching, it is necessary to have present stabilizers such as sodium silicate and magnesium sulphate in the bleach liquor to prevent decomposition thereof and further the mechanical pulp is normally pretreated at low consistency with organic chelating agents, such as sodium diethylenetriamine penta-acetate (DPTA), to remove naturally occurring trace metals in the mechanical pulps.
It has also been proposed (see Canadian Pat. No. 769,631 issued Oct. 17, 1967 to G. A. Wigren) to bleach cellulose or cellulosic compounds by adding ozone to the mixture of the hypochlorite and/or hypochlorous acid and/or chlorine solution containing free chlorine used as bleaching agent and the cellulose to be bleached. Thus, while hypochlorites have been proposed for the bleaching of mechanical pulps, it has been found that an excessive quantity of the hypochlorite is required and further that the hypochlorite attacks the lignin and carbohydrate in the mechanical pulp resulting in a substantial loss of yield of bleached pulp.
It has already been taught, in U.S. Pat. No. 2,466,633 issued Apr. 5, 1949 to G. J. Brabender et al., that shredded chemical pulp containing from 45 to 75 percent moisture may be bleached by treatment with air containing 0.5 percent ozone at a pH of 5 to 6 at room temperature.
However, it is also known (see Scoteland and Kringstad "Norsk Skogindustri" Vol. 2, pages 49 and 51, 1968) that attempts to extend this treatment to the bleaching of mechanical pulp have not been entirely successful. Thus, when ozone is passed into a solution of mechanical pulp or when the ozonization is carried out with the dry matter content in the range of 30 to 40 percent, the brightness of the mechanical pulp becomes yellow and lower than the untreated pulp. To reduce this brightness loss it was proposed that the mechanical pulp be either reduced by preacetylation, or that the reaction with ozone be carried out with pulp in an acetone-water medium. Moreover, it was found that the attempts to obtain a more specific ozonization, i.e. to raise the pulp brightness and to remove the yellowing effect without preacetylation of the pulp were not successful.
SUMMARY OF THE INVENTION
Advantages of the Invention
An object of a broad aspect of this invention is the provision of a process for the bleaching of a mechanical pulp with ozone whereby the brightness and/or the strength properties of the pulp are enhanced.
Broad Statement of Invention
By a broad concept of this invention, a process is provided for the bleaching of pulp, the process comprising carrying out the bleaching on moist pulp, having a consistency of 15 to 60 per cent by weight solids in the form of fiber and fiber aggregates by treating with a gaseous ozone-oxygen or ozone-air mixture, in the presence of a peroxygen compound containing a peroxygen equivalent of 0.025-1.0 per cent by weight, at a temperature of from 20° to 80° C. for a period of from 1 to 60 minutes at a pH of from 3.5 to 11.5.
According to one aspect of the present invention, the process comprises adding a small quantity of peroxygen compound containing a peroxygen equivalent of 0.025 - 1.0 per cent by weight of the dry pulp, then comminuting the pulp to the form of fiber and fiber aggregates having a consistency of 15 to 60 per cent by weight solids and then treating the pulp with a gaseous ozone-oxygen or ozone-air mixture at a temperature of from 20° to 80° C. for a period of from 1 to 60 minutes at a pH of from 3.5 to 11.5.
By other aspects of the present invention, the peroxygen compound may be added during or after the pulp comminution stage. The essential feature of the invention is that the bleaching with ozone takes place in the presence of the peroxygen compound while the pulp is in the form of fiber and fiber aggregates. In other words, the peroxygen compound is added to the unbleached mechanical pulp, either prior to comminution or during comminution, e.g. by spraying the solution on the fiber and fiber aggregates. The comminuted pulp, in the form of fiber and fiber aggregates, is then passed into a reactor in which it is treated with an ozone-bearing gas for a period of from 1 - 60 minutes at a pH of from 3.5 to 11.5.
The peroxygen compound may be any conventional inorganic peroxide, including hydrogen peroxide and sodium peroxide or organic peroxide including benzoyl peroxide and ditertiarybutyl peroxide.
The ozone may be prepared in concentration of 4 percent on a weight/weight basis O3 /O2 using a commercial laboratory ozonizer, or may be obtained and used (with proper precautions) from pressurized tanks.
It has also been found that the improvement in the strength properties of the mechanical pulp is a function of contact time and pH when a constant fixed dosage of ozone (on o.d. pulp basis) is supplied to the pulp.
Fluffed mechanical cellulosic pulp (sometimes referred to as shredded mechanical cellulosic pulp) is the product obtained by fragmentation of a mechanical cellulosic pulp into fiber and fiber aggregates or flocks. Each discrete fiber aggregate or flock is spongy, compressible and readily gas permeable. Each of the fiber aggregates or flocks is usually of an average size of about 1/4 - 1 inch in diameter and presents a fluffed-up, dry appearance even with about 40 - 85 percent by weight moisture in the cellulosic pulp. One preferred manner of fluffing or shredding the mechanical pulp to the form above-described is provided in Canadian Pat. application Ser. No. 068,411 filed Nov. 26, 1969, now Canadian Pat. No. 869,267 issued Aug. 27, 1971.
In one embodiment, the mechanical pulp may be treated with a peroxide solution of inorganic or organic nature, then comminuted or shredded as described in the above-identified Canadian patent and finally treated with an ozone-bearing gas. The resulting oxidized pulp will then have reached a suitable brightness with the strength properties suitably enhanced.
Description of Embodiments of the Invention
The following are comparative examples showing the unexpected advance over the prior art obtainable by the practice of aspects of this invention.
EXAMPLE I
A commercial spruce-balsam mechanical pulp, brightness 57.3 was pressed to 35 percent consistency. The pressed pulp was sprayed with a solution containing hydrogen peroxide (0.2 percent H2 O2 on o.d. pulp), sodium silicate (2.5 percent Na2 SiO3 on o.d. pulp), sodium hydroxide (1 percent NaOH on o.d. pulp), magnesium sulphate (0.05 percent MgSO4 on o.d. pulp) and water sufficient to reduce the pulp consistency to 30 percent. The pulp was then comminuted into fiber and fiber aggregates.
a. A portion of the peroxide treated pulp was placed into a vessel which was set in a bath and the temperature maintained at 40° C. for 2 hours. The final pH of the pulp was 10.9. A portion of the bleached pulp was washed and air-dried and the brightness measured. The brightness tabs were then placed in a hot air circulatory oven at 105° C. for 1 hour, then conditioned in a constant atmosphere, relative humidity 50 percent at 70° F. The brightness and loss of brightness were measured after aging, and the strength properties of the pulp tested on standard handsheets are set out in Table I below.
b. A portion of the peroxide treated pulp was placed in a rotary vessel which was set in a bath and the temperature maintained at 40° C. A gaseous ozone-oxygen mixture was passed into the rotary vessel so that within 5 minutes time 1.0 percent ozone on o.d. pulp basis was passed into the vessel. The pH of the moist comminuted pulp was 10.5. A portion of the bleached pulp was washed and air-dried and the brightness measured. The brightness tabs were then placed in a hot air circulatory oven at 105° C. for 1 hour, then conditioned in a constant atmosphere, relative humidity 50 percent at 70° F. The brightness and loss of brightness were measured after aging, and the strength properties of the pulp tested on standard handsheets are set out in Table I below.
c. A portion of the peroxide treated pulp was placed in a rotary vessel which was set in a bath and the temperature maintained at 40° C. A gaseous ozone-oxygen mixture was passed into the rotary vessel so that within 30 minutes time 1 percent ozone on o.d. pulp basis was passed into the vessel. The pH of the moist comminuted pulp was 10.3. A portion of the bleached pulp was washed and air-dried and the brightness measured. The brightness tabs were then placed in a hot air circulatory oven at 105° C. for 1 hour, then conditioned in a constant atmosphere, relative humidity 50 percent at 70° F. The brightness and loss of brightness were measured after aging, and the strength properties of the pulp tested on standard handsheets are set out in Table I below.
d. A commercial spruce-balsam mechanical pulp, brightness 57.3, was pressed to 35 percent consistency. The pressed pulp was sprayed with a solution containing sodium hydroxide (1.5 percent NaOH on o.d. pulp and water sufficient to reduce the pulp consistency to 30 percent. The pulp was then comminuted into fiber and fiber aggregates.
The shredded pulp was then placed into a rotary vessel which was set into a bath and the temperature maintained at 40° C. A gaseous ozone-oxygen mixture was passed into the vessel so that within 30 minutes 1 percent ozone on o.d. pulp basis was passed into the vessel. The pH of the moist comminuted pulp was 10.0. The pulp was then washed, air-dried and the brightness measured. The brightness is set out in Table I.
EXAMPLE II
A commercial spruce-balsam mechanical pulp, brightness 57.3 was pressed to 35 percent consistency. The pressed pulp was sprayed with a solution containing hydrogen peroxide (0.2 percent H2 O2 on o.d. pulp), sodium silicate (0.5 percent Na2 SiO3 on o.d. pulp), sodium hydroxide (0.25 percent NaOH on o.d. pulp), and magnesium sulphate (0.05 percent MgSO4 on o.d. pulp) and water sufficient to reduce the pulp consistency to 30 percent. The pulp was then comminuted into fiber and fiber aggregates.
a. A portion of the peroxide treated pulp was placed into a vessel which was set in a bath and the temperature maintained at 40° C. for 2 hours. The final pH of the pulp was 3.8. A portion of the bleached pulp was washed and air-dried and the brightness measured. The brightness tabs were then placed in a hot air circulatory oven at 105° C. for 1 hour, then conditioned in a constant atmosphere, relative humidity 50 percent at 70° F. The brightness and loss of brightness were measured after aging, and the strength properties of the pulp tested on standard handsheets are set out in Table I below.
b. A portion of the peroxide treated pulp was placed in a rotary vessel which was set in a bath and the temperature maintained at 40° C. A gaseous ozone-oxygen mixture was passed into the rotary vessel so that within 5 minutes time 1.0 percent ozone on o.d. pulp basis was passed into the vessel. The pH of the moist comminuted pulp was 3.6. A portion of the bleached pulp was washed and air-dried and the brightness measured. The brightness tabs were then placed in a hot air circulatory oven at 105° C. for 1 hour, then conditioned in a constant atmosphere, relative humidity 50 percent at 70° F. The brightness and loss of brightness were measured after aging, and the strength properties of the pulp tested on standard handsheets are set out in Table I below.
c. A portion of the peroxide treated pulp was placed in a rotary vessel which was set in a bath and the temperature maintained at 40° C. A gaseous ozone-oxygen mixture was passed into the rotary vessel so that within 30 minutes time 1 percent ozone on o.d. pulp basis was passed into the vessel. The pH of the moist comminuted pulp was 3.5. A portion of the bleached pulp was washed and air-dried and the brightness measured. The brightness tabs were then placed in a hot air circulatory oven at 105° C. for 1 hour, then conditioned in a constant atmosphere, relative humidity 50 percent at 70° F. The brightness and loss of brightness were measured after aging, and the strength properties of the pulp tested on standard handsheets are set out in Table I below.
d. A commercial spruce-balsam mechanical pulp, brightness 57.3 was pressed to 30 percent consistency and then comminuted into fiber and fiber aggregates.
The shredded pulp was then placed into a rotary vessel which was set into a bath and the temperature maintained at 40° C. A gaseous ozone-oxygen mixture was passed into the vessel so that within 30 minutes 1 percent ozone on o.d. pulp basis was passed into the vessel. The pH of the moist comminuted pulp was 4.1. The pulp was then washed, air-dried and the brightness measured. The brightness is set out in Table I. ##SPC1##
The unexpected synergistic effect on the brightness is evident on comparing the brightness of pulp bleached with hydrogen peroxide alone, with ozone alone and with the combination of the peroxygen compound and ozone.
The preceding examples can be repeated with similar success by substituting the generically and specifically described reactants and operating conditions of this invention for those used in the preceding examples.
From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Consequently, such changes and modifications are properly, equitably, and "intended" to be, within the full range of equivalence of the following claims.