The present invention is directed to a specific process for gas phase delignification of cellulosic material with chlorine dioxide. In particular, the present invention is directed to a process for gas phase delignification of cellulosic material whereby the cellulosic material is contacted with a gas stream containing a limited quantity of chlorine dioxide so that substantially all of the chlorine dioxide in the gas stream will react with the cellulosic material and the effluent gas stream from the contacting zone will be substantially free of chlorine dioxide.

Chlorine dioxide has found wide use as a disinfectant in water treatment/purification, as a bleaching agent in pulp and paper production, and a number of other uses because of its high oxidizing power.

Gas phase bleaching with chlorine dioxide has been proposed over the years in various patents and publications as a method of reducing the bleaching time while cutting chemical costs. Gas phase bleaching is carried out on higher consistency pulp using mixtures of chlorine dioxide and steam and/or inert gases such as air or nitrogen. U.S. Patent No. 3,725,193, issued April 3, 1973 to DeMontigny et al., describes a process for bleaching high consistency pulps, which includes preheating the pulp by direct steaming. A gaseous mixture of chlorine dioxide diluted with steam or a nonreactive gas to a partial pressure of chlorine dioxide of not more than 100 mm of Hg is then passed through the pulp. The contact period is in the

order of a fraction of a second. The bleached pulp was then held in a retention vessel for 30 minutes. Unreacted chlorine dioxide was removed from a bleaching tower by aeration. The final pH of the bleached pulp was 5.2.

U.S. Patent No. 3,655,505, which issued to Yorston et al. on April 11, 1972, teaches a two- stage process for bleaching an unbleached chemical cellulosic pulp in which the first stage comprises fluffing a pulp that has been adjusted in moisture content to a consistency of from about 20% to 60%, contacting the pulp with gaseous chlorine dioxide diluted with a nonreactive gas at a temperature of between 15° and 100°C for a period of 20 seconds to 60 minutes, the pulp then being washed with water; and finally in the second stage adjusting the pulp moisture content to 5% and contacting the pulp with a peroxygen compound having a peroxygen content of from 0.025-1.5% by weight based on the weight of the dry pulp at a temperature up to 100°C for a period of from . to 5 hours.

Reaction of gaseous chlorine dioxide with lignin in high consistency pulp is faster than reaction of a chlorine dioxide solution with lignin in low consistency pulp because a higher concentration of chlorine dioxide can contact the lignin and individual pulp fibers. Gas phase chlorine dioxide bleaching results in lower AOX formation than either gas phase chlorine bleaching or liquid phase chlorine dioxide or chlorine bleaching. However, commercial processes that utilize gas phase chlorine dioxide for continuous bleaching or delignification of cellulosic material

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have been designed so that the concentration of gaseous chlorine dioxide in the contacting zone is relatively high and that only a portion of the chlorine dioxide is consumed on each pass through the contacting zone. Also, these commercial processes employ chlorine dioxide-containing gas streams that are at much lower temperatures than the pulp being treated.

This approach has several disadvantages. First, large volumes of chlorine dioxide-containing gas must be recycled. If water is used to strip the chlorine dioxide from the effluent gas stream, it must be later treated or disposed of. During the recycling operation, some chlorine dioxide is lost due to slow decomposition. Thus, valuable quantities of chlorine dioxide may be simply wasted. Also, the potential hazard of a rapid and explosive chlorine dioxide decomposition is created by the recycling of these gases. Accordingly, there is a need in the part to reduce the complexity and cost of gas phase chlorine dioxide bleaching. The present invention provides a solution to this need.

One aspect of the present invention is directed to a process for delignifying high consistency cellulosic material having a kappa number greater than about 5 characterized by contacting said cellulosic material with a gas phase containing chlorine dioxide for a sufficient amount of time to at least partially delignify said cellulosic material, wherein substantially all of said chlorine dioxide in said gas phase reacts with said cellulosic material during said contacting time; and

then separating the spent gas phase from said delignified cellulosic material, wherein said separated spent gas phase is substantially free of chlorine dioxide. Another aspect of the present invention is directed to a process for delignifying high consistency cellulosic material having a kappa number greater than about 5 characterized by contacting said cellulosic material with a gas phase containing chlorine dioxide for a sufficient amount of time to at least partially delignify said cellulosic material, wherein the delignification efficiency as defined by the equation:

Delignification Efficiency =* Initial Kappa No. (K )-Extracted Kappa No. (K _e ) x 100

Initial Kappa No. (K _L )

is greater than about 50 and less than about 90; and then separating the spent gas phase from said delignified cellulosic material, wherein said separated spent gas phase is substantially free of chlorine dioxide.

The gas phase containing chlorine dioxide used in the present invention may be made by any conventional process for generating chlorine dioxide. There are a number of chlorine dioxide generator systems available in the marketplace. Most of the very large scale generators utilize an alkali metal chlorate salt, a reducing agent, and a strong acid. If sodium chloride is employed as a reducing agent or if hydrogen chloride is employed

as the acid, then a mixture of chlorine and chlorine dioxide is produced.

Generally, the additional presence of chlorine in a chlorine dioxide product is not desired and, for that reason, many processes have been developed to produce chlorine dioxide having little or no chlorine concentrations therein. These processes use nonchlorine-containing acids such as sulfuric acid and reducing agents such as hydrogen peroxide, methanol or other organic compounds, sulfur dioxide, other sulfur-oxygen species having a sulfur valence of less than +6, or carbon monoxide and the like. Processes for generating chlorine dioxide by reacting an alkali metal chlorate, a mineral acid, and a reducing agent are shown include U.S. Patents Nos. 4,938,943 (Norell) ; 4,978,517 (Norell et al.); 4,986,973 (Svedin et al.); 5,002,746 (Norell); 5,091,166 (Engstrom et al.); 5,091,167 (Engstrom et al.); and 5,093,097 (Engstrom et al.). Alternatively, chlorine dioxide may be generated by reacting a mineral acid such as sulfuric acid or phosphoric acid in the presence of selected catalysts. See U.S. Patent Nos. 4,362,707 (Hardee et al.); 4,381,290 (Hardee et al.); and 4,501,824 (Hardee et al.). Still further, chlorine dioxide may be generated electrochemically from an aqueous feedstock solution of an alkali metal chlorate and a mineral acid. See U.S. Patent Nos. 4,426,263 (Hardee et al.) and 4,767,510 (Lipsztajn) . To avoid the formation of some or all of the above-noted byproducts, it is preferred to use chloric acid instead of all or part of the alkali

metal chlorate salt precursor for chlorine dioxide generating systems.

For example, see U.S. Patent Nos. 5,084,148 (Kaczur et al.); 5,174,868 (Lipsztajn et al.); 5,223,103 (Kaczur et al.); 5,242,553 (Kaczur et al.); 5,242,554 (Kaczur et al.); 5,248,397 (Cawlfield et al.); 5,258,105 (Kaczur et al.); 5,264,089 (Kaczur et al.); 5,284,443 (Lipsztajn et al.); 5,296,108 (Kaczur et al.); 5,322,598 (Kaczur et al.); 5,348,683 (Kaczur et al.); and 5,354,435 (Kaczur et al.) .

Also, it may be preferred to electrolyze a chloric acid solution to produce chlorine dioxide. See U.S. Patent No. 5,089,095 (Cawlfield et al.). And further, it is preferred to produce chlorine dioxide by heating a reaction mixture comprising an aqueous solution containing hydrogen ions, chlorate ions, and perchlorate ions in the presence of an oxygen-evolving catalyst in solid form in the absence of an added reducing agent. See U.S. Patent No. 5,342,601 (Cawlfield et al.). This process uses chloric acid as a chemical precursor. Most preferably, the gas containing chlorine dioxide is generated by a process that is capable of producing relatively chlorine-free chlorine dioxide under subatmospheric pressures. One example of such a generator is described in U.S. Patent No. 5,342,601. The chlorine dioxide containing gaseous mixture can be transported to the contacting devices by a vacuum system, or any other device capable of creating the necessary pressure differential between the chlorine dioxide generator and the contacting device to allow gaseous flow at a reasonable rate.

A reasonable gas flow rate is defined such that a continuously flowing pulp can be delignified to the desired kappa number while consuming substantially all (i.e., more than 95% by moles) of the chlorine dioxide in the gas.

The chlorine dioxide-containing gas added to the mixing zone or delignification contacting zone is preferably added with a diluent to keep the feed partial pressure of chlorine dioxide below about 100 mm Hg. This is preferably accomplished by main¬ taining a total gas phase pressure of about 100 to 300 mm Hg in the feed and using water vapor as a diluent, while the partial pressure of chlorine dioxide is held at about 5 to 50 mm Hg in the feed. This allows a substantial safety factor while allowing the chlorine dioxide concentration to rise when water vapor condenses on cool pulp. Specifically, once inside the pulp interstices the local partial pressures of chlorine dioxide may be much higher because of the condensation of the water vapor. Higher partial pressures of chlorine dioxide in such interstices of pulp are advantageous since they cause an increased delignification rate. Since the chlorine dioxide is inside the pulp, higher local C10 ₂ partial pressures can be used safely because the pulp acts as a barrier to C10 ₂ decomposition.

Any high consistency cellulosic material may be used in the process of this invention. The term "high consistency" means consistencies of at least 15. Preferably consistencies of from about 20-50 are employed. The preferred cellulosic material is wood pulp.

Low consistency pulp can be dewatered to make high consistency pulp that is suitable for purposes of this invention. That pulp can be dewatered by any device capable of increasing pulp consistency to 20 to 50. Examples of such devices are presses, filters, centrifuges and the like.

To achieve complete absorption of chlorine dioxide in the pulp, it is necessary to limit the amount of chlorine dioxide fed to that amount which participates in a very rapid reaction with the lignin on the pulp. This amount fed may be calculated based on a well-known parameter called the kappa factor. This factor represents the ratio of oxidant added to the oxidizable lignin components in the pulp. In this invention, kappa factors are limited to 0.01 to 0.15, more preferably between 0.01. to 0.10. At higher kappa factors, we have found that much greater residence time is required to completely consume the chlorine dioxide. The kappa number of the pulp, a measure of the lignin content, can preferably range from about 10 to about 200 entering the chlorine dioxide treatment process. The kappa number may be calculated by TAPPI Test Method T-236 "Kappa Number of Pulp" Tappi Press, Atlanta, Georgia (1994) . When the initial lignin content is much lower than 10, the rate of reaction between chlorine dioxide and pulp is proportionately lower. Complete reaction of chlorine dioxide with the pulp in the contractor cannot be achieved when a pulp with low lignin content (i.e., kappa number = 5) is treated with a relatively high chlorine dioxide dose.

The ratio of kappa number to kappa factor (K _n /K _f ) is related to the kinetics or rate of reaction of chlorine dioxide to the wood pulp. The higher this ratio is, the faster is the reaction. Preferably, this ratio should be at least about 50:1, more preferably, at least about 100:1.

The novel delignification process of the present invention may use any conventional gas phase bleaching or delignification apparatus, as well as any suitable mixers and the like. Suitable gas tight mixing equipment includes, for example, agitated mixers, static mixers, ribbon blenders, steam chests, high consistency shear mixers, MC pumps, MC mixers, high velocity pipe lines, blowers, static beds, and the like. The high consistency pulp is preferably fluffed before or simultaneously with chlorine dioxide gas contacting to allow thorough mixing of the pulp fibers and liquid with the gas. Also, fluffed pulp suppresses unwanted chlorine dioxide auto-decomposition. Delignifi¬ cation of pulp may be affected in the initial stage of a multi-stage bleaching process, as well as in any "D" stage, for example, in the third stage for both kraft and sulfite pulps, as well as in the fifth stage for kraft pulps. The initial deligni¬ fication step of unbleached pulp may be carried out in any suitable mixing equipment in which thorough admixing of the unbleached pulp and the chlorine dioxide gas can be accomplished. The series of processes that collectively react and/or remove color bodies from cellulosic material are known generically as bleaching. The first steps of a bleaching process following chemical pulping of

wood remove the majority of oxidizable species in the pulp are normally referred to as delignification steps, while the later stages are often referred to as brightening steps. It is the delignification steps to which this invention applies. Where the term bleaching is used in connection with this invention, it is in the generic sense for which delignification is a more specific term.

Flow of chlorine dioxide between the mixing zones can be either cocurrent or counter-current with the pulp. Counter-current flow provides greater concentrations of chlorine dioxide in the pulp that has already been partially delignified and serves to reduce the total residence time required for complete reaction of chlorine dioxide with the pulp.

Preferably the mixing or contacting zone is made of a fluffer, a blower, a contactor, or a moving bed or combinations thereof. A fluffer is a device which separates the matted pulp into small clumps of fibers and ultimately to individual cellulosic fibers. If only the fluffer is used, it also becomes a device where the chlorine dioxide and pulp is reacted. A blower is a device used to transport pulp to a contactor. If only the fluffer and blower combination is used, the reaction of the chlorine dioxide and pulp can occur in both devices. A contactor is a device where pulp and gaseous chlorine dioxide mixture react. It can be used alone or with a fluffer or blower or both. A moving bed is a type of contactor that provides a longer residence time for the gas phase so that slower lignin reactions occur. The moving bed may be used

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to separate the spent gas phase from the delignified pulp after the reaction occurs in one or more of the other devices.

The exhaust gas stream leaving the process contains less than about 5% of the chlorine dioxide fed to the mixing zone. Accordingly, the term "substantially free of chlorine dioxide", as applied to the gas phase leaving the contacting zone, means that the gas phase contains less than 5% of the chlorine dioxide fed into that zone. This exhaust gas may be absorbed in a scrubber to remove any residual chlorine dioxide before releasing it to the atmosphere or using it in other parts of the mill. Where the chlorine dioxide feed gas contains oxygen, this oxygen will be uneffected by the delignifi- ation step and may be advantageously used in later alkaline extraction steps used by the bleaching sequence. In this latter case, no scrubbing of spent gas phase is needed because any small amounts of residual chlorine dioxide will be fully absorbed in alkaline pulp slurry.

The conditions inside the contactor are, therefore, preferably at a pressure below one atmosphere, and at least slightly below those of the source of the chlorine dioxide-containing gas. The temperatures of the pulp entering the contactor are preferably below about 180°F, more preferably less than about 120°F. These relatively low temperatures of the pulp versus the usual 180°F temperature of chlorine dioxide generator act to condense a greater fraction of water vapor in the chlorine dioxide and, therefore, increase the partial pressure of chlorine dioxide initially contacting the pulp.

The mixing compartment, where chlorine dioxide contact the pulp, is needed to optimize the uniformity of bleaching. The residence time of pulp in the process and the kinetics of reaction with the lignin components of the pulp determines the quantity of chlorine dioxide that can be absorbed from a given concentration of chlorine dioxide gas. In a well mixed contactor, the concentration of chlorine dioxide in the contactor can be assumed to be approximately equal to the concentration in the gas leaving the contractor.

In order to achieve complete reaction between chlorine dioxide and pulp, a contactor may preferably contain at least two and preferably between 3 and 5 mixing compartments in series so that chlorine dioxide not reacted in the first mixing compartment can react in later mixing compartment. The total residence time of pulp in all of these mixing compartments in a contacting or mixing zone is preferably between 5 seconds and 60 minutes and, more preferably, between 10 seconds and 10 minutes.

After the initial delignification and extraction using the process of the present invention, the delignified chemical pulp preferably has a kappa number in the range of from about 1 to about 10 and more preferably from about 1 to about 5. A better measure of effectiveness of the delignification process can be shown by measuring the "delignification efficiency". This is defined as the percentage difference of the initial kappa number minus the extracted kappa number divided by

initial kappa number (DE = K _^ -K _g x 100) . As

mentioned above, this is preferably greater than about 50 and less than about 90. Following extraction, the viscosity of the treated pulp remains high indicating minimal degradation of the pulp.

The extracted pulp may be subsequently further delignified, for example, using the process of the invention or any known bleaching stage to achieve the final brightness desired.

To maximize the efficiency of chlorine dioxide use, delignification of the pulp is carried out under acidic conditions. For example, in any delignification stage following a caustic extraction stage, the pH of the pulp is adjusted so that the final pH of the delignified pulp is in the range of from about 1 to about 6.

An extraction stage (E) is then carried out with, for example, caustic soda to solubilize the higher molecular weight oxidized lignins, to hydrolyze organic chlorides to salts, and the like. The extraction is conducted at temperatures of 40-90°C, and a residence time of about 60 minutes where the final pH is 9.5 or higher. The extrac¬ tion stage may include the addition of oxygen, a peroxide such as hydrogen peroxide, or hypochlorite for enhanced performance. The extracted pulp is filtered and washed to remove water soluble lignins and low molecular weight organic chlorides, among others.

The following examples further illustrate the present invention. All parts and percentages are by

weight and all temperatures are degree Celsius unless explicitly stated otherwise.

EXAMPLE 1

About 100 g of cellulosic material, such as soft wood kraft pulp of 30.7 kappa number, was pH adjusted to about 5 at about 3% consistency and dewatered to about 26% consistency, fluffed, and placed in the contactor. The temperature of the pulp was at about 80°F. The chlorine dioxide was generated by the methods described in U.S. Patent

No. 5,342,601. The temperature of the generator was at 180°F, and the operating pressure of the gener¬ ator was 215 mm Hg. The gaseous mixture temperature was 150°F. The chlorine dioxide generation rate was measured by potassium iodide titration techniques.

The chlorine concentration of the gaseous stream was measured to be <3% of the total chlorine dioxide present in the gaseous stream. The pressure of the pulp was reduced to subatmospheric conditions to about 215 mm Hg absolute prior to contacting with the gaseous chlorine dioxide and oxygen enriched mixture also at subatmospheric pressure. The gaseous mixture containing chlorine dioxide was applied. The unreacted gases were collected in series of scrubbers containing potassium iodide solution to collected any unreacted chlorine dioxide gas. The amount of chlorine dioxide to be added was predetermined. After 0.95 g of chlorine dioxide had been applied, the chlorine dioxide supply was cut off. The pulp and gaseous chlorine dioxide were contacted for 28 seconds. The potassium iodide

scrubber solution was analyzed and contained 0.08 g of chlorine dioxide. About 92% of the applied chlorine dioxide reacted with the pulp. (The pulp was optionally washed in some experiments.) The reacted pulp was then extracted at about 11% consistency by adding sodium hydroxide, oxygen (optional) , and/or hydrogen peroxide (optional) to the pulp; and maintaining the mixture at 90°C for about 1-2 hours. After the extraction, the pulp was washed and its kappa number was measured to be 7.17 indicted the delignification efficiency of about 77%. The wash water was collected and its AOX (absorbable organic halides) content was measured to be 0.36 kg per 1,000 kg of dried pulp.

EXAMPLE 2

About 110 g of cellulosic material, such as soft wood kraft pulp of 30.7 kappa number, was pH adjusted to about 5 at about 3% consistency and dewatered to about 26% consistency, fluffed, and placed in the contactor. The temperature of the pulp was at about 80°F. The chlorine dioxide was generated by ,the methods described in U.S. Patent No. 5,342,601. The temperature of the generator was at 182°F, and the operating pressure of the genera- tor was 203 mm Hg. The gaseous mixture temperature was 154°F. The chlorine dioxide generation rate was measured by potassium iodide titration techniques. The chlorine concentration of the gaseous stream was measured to be <3% of the total chlorine dioxide present in the gaseous stream. The pressure of the pulp was reduced to subatmospheric conditions to

about 203 mm Hg absolute prior to contacting with the gaseous chlorine dioxide and oxygen enriched mixture also at subatmospheric pressure. The gaseous mixture containing chlorine dioxide was applied. The unreacted gases were collected in series of scrubbers containing potassium iodide solution to collected any unreacted chlorine dioxide gas. The amount of chlorine dioxide to be added was predetermined. After 0.84 g of chlorine dioxide had been applied the chlorine dioxide supply was cut off. The pulp and gaseous chlorine dioxide were contacted for 23 seconds. The potassium iodide scrubber solution was analyzed and contained 0.0 g of chlorine dioxide. About 100% of the applied chlorine dioxide reacted with the pulp. The reacted pulp was then extracted at about 11% consistency by adding sodium hydroxide, oxygen (optional) , and/or hydrogen peroxide (optional) to the pulp; and maintaining the mixture at 90°C for about 1-2 hours. After the extraction, the pulp was washed and its kappa number was measured to be 8.1 indicating the delignification efficiency of about 74%. The wash water was collected, and its AOX (absorbable organic halides) content was measured to be 0.29 kg per 1,000 kg of dried pulp.

EXAMPLE 3

About 110 g of cellulosic material, such as soft wood kraft pulp of 30.6 kappa number, was pH adjusted to about 5 at about 3% consistency and dewatered to about 26% consistency, fluffed, and placed in the contactor. The temperature of the

pulp was at about 80°F. The chlorine dioxide was generated by the methods described in U.S. Patent No. 5,342,601. The temperature of the generator was at 186°F, and the operating pressure of the genera- tor was 228 mm Hg. The gaseous mixture temperature was 155°F. The chlorine dioxide generation rate was measured by potassium iodide titration techniques. The chlorine concentration of the gaseous stream was measured to be <3% of the total chlorine dioxide present in the gaseous stream. The pressure of the pulp was reduced to subatmospheric conditions to about 228 mm Hg absolute prior to contacting with the gaseous chlorine dioxide and oxygen enriched mixture also at subatmospheric pressure. The gaseous mixture containing chlorine dioxide was applied. The unreacted gases were collected in series of scrubbers containing potassium iodide solution to collected any unreacted chlorine dioxide gas. The amount of chlorine dioxide to be added was predetermined. After 1.1 g of chlorine dioxide had been applied, the chlorine dioxide supply was cut off. The pulp and gaseous chlorine dioxide were contacted for about 30 seconds. The potassium iodide scrubber solution was analyzed and contained 0.043 g of chlorine dioxide. About 95% of the applied chlorine dioxide reacted with the pulp. The reacted pulp was then extracted at about 11% consistency by adding sodium hydroxide, oxygen (optional) , and/or hydrogen peroxide (optional) to the pulp; and maintaining the mixture at 90°C for about 1-2 hours. After the extraction, the pulp was washed and its kappa number was measured to be 9.1 indicating the delignification efficiency of about

70%. The wash water was collected, and its AOX (absorbable organic halides) content was measured to be 0.38 kg per 1,000 kg of dried pulp.

EXAMPLE 4

About 100 g of cellulosic material, such as soft wood kraft pulp of 25.6 kappa number, was pH adjusted to about 5 at about 3% consistency and dewatered to about 26% consistency, fluffed, and placed in the contactor. The temperature of the pulp was at about 80°F. The chlorine dioxide was generated by the methods described in U.S. Patent No. 5,342,601. The temperature of the generator was at 186°F, and the operating pressure of the genera¬ tor was 152 mm Hg. The gaseous mixture temperature was 142°F. The chlorine dioxide generation rate was measured by potassium iodide titration techniques. The chlorine concentration of the gaseous stream was measured to be <3% of the total chlorine dioxide present in the gaseous stream. The pressure of the pulp was reduced to subatmospheric conditions to about 152 mm Hg absolute prior to contacting with the gaseous chlorine dioxide and oxygen enriched mixture also at subatmospheric pressure. The gaseous mixture containing chlorine dioxide was applied. The unreacted gases were collected in series of scrubbers containing potassium iodide solution to collected any unreacted chlorine dioxide gas. The amount of chlorine dioxide to be added was predetermined. After 0.63 g of chlorine dioxide had been applied, the chlorine dioxide supply was cut off. The pulp and gaseous chlorine dioxide were

contacted for about 17 seconds. The potassium iodide scrubber solution was analyzed and contained 0.02 g of chlorine dioxide. About 97% of the applied chlorine dioxide reacted with the pulp. (The pulp was optionally washed in some experiments.) The reacted pulp was then extracted at about 11% consistency by adding sodium hydroxide, oxygen (optional) , and/or hydrogen peroxide (optional) to the pulp; and maintaining the mixture at 90°C for about 1-2 hours. After the extraction, the pulp was washed and its kappa number was measured to be 5.8 indicating the delignification efficiency of about 77%. The wash water was collected, and its AOX (absorbable organic halides) content was measured to be 0.15 kg per 1,000 kg of dried pulp.

EXAMPLE 5

About 100 g of cellulosic material, such as soft wood kraft pulp of 25.6 kappa number, was pH adjusted to about 5 at about 3% consistency and dewatered to about 26% consistency, fluffed, and placed in the contactor. The temperature of the pulp was at about 80°F. The chlorine dioxide was generated by the methods described in U.S. Patent No. 5,342,601. The temperature of the generator was at 183°F, and the operating pressure of the genera¬ tor was 228 mm Hg. The gaseous mixture temperature was 129°F. The chlorine dioxide generation rate was measured by potassium iodide titration techniques. The chlorine concentration of the gaseous stream was measured to be <3% of the total chlorine dioxide

present in the gaseous stream. The pressure of the pulp was reduced to subatmospheric conditions to about 228 mm Hg absolute prior to contacting with the gaseous chlorine dioxide and oxygen enriched mixture also at subatmospheric pressure. The gaseous mixture containing chlorine dioxide was applied. The unreacted gases were collected in series of scrubbers containing potassium iodide solution to collected any unreacted chlorine dioxide gas. The amount of chlorine dioxide to be added was predetermined. After 0.68 g of chlorine dioxide had been applied, the chlorine dioxide supply was cut off. The pulp and gaseous chlorine dioxide were contacted for 18 seconds. The potassium iodide scrubber solution was analyzed and contained 0.029 g of chlorine dioxide. About 97% of the applied chlorine dioxide reacted with the pulp. The reacted pulp was then extracted at about 11% consistency by adding sodium hydroxide, oxygen (optional) , and/or hydrogen peroxide (optional) to the pulp; and maintaining the mixture at 90°C for about 1-2 hours. After the extraction, the pulp was washed and its kappa number was measured to be 5.3 indicating the delignification efficiency of about 79%. The wash water was collected, and its AOX (absorbable organic halides) content was measured to be 0.26 kg per 1,000 kg of dried pulp.

While the invention has been described above with reference to specific embodiments thereof, it is apparent that many changes, modifications, and variations can be made without departing from the inventive concept disclosed herein. Accordingly, it is intended to embrace all such changes, modifica-

tions, and variations that fall within the spirit and broad scope of the appended claims.