US3586599A - Purification of fluffed or shredded pulp with chlorine dioxide - Google Patents

Purification of fluffed or shredded pulp with chlorine dioxide Download PDF

Info

Publication number
US3586599A
US3586599A US769879A US3586599DA US3586599A US 3586599 A US3586599 A US 3586599A US 769879 A US769879 A US 769879A US 3586599D A US3586599D A US 3586599DA US 3586599 A US3586599 A US 3586599A
Authority
US
United States
Prior art keywords
pulp
chlorine dioxide
brightness
shredded
consistency
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
US769879A
Other languages
English (en)
Inventor
Frederic Harrison Yorston
Norman Liebergott
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Pulp and Paper Research Institute of Canada
Original Assignee
Pulp Paper Res Inst
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Pulp Paper Res Inst filed Critical Pulp Paper Res Inst
Application granted granted Critical
Publication of US3586599A publication Critical patent/US3586599A/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

Links

Images

Classifications

    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
    • D21C9/10—Bleaching ; Apparatus therefor
    • D21C9/12—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds
    • D21C9/14—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites

Definitions

  • a process for bleaching chemical pulp which comprises adjusting the moisture content of the pulp to a level from 60 to 80%, preferably 60 to 70%, shredding or finding the pulp into fibres and fibre aggregates and then adding chlorine dioxide gas diluted with a non-reactive gaseous diluent such as nitrogen or steam to the shredded flulfed pulp mass.
  • the partial pressure of the chlorine dioxide is desirably of 100 millimeters of mercury or less, the bleaching time is of the order of to 30 minutes, and the temperature is desirably from 65 to 100 C.
  • the pH of the pulp is controlled suitably by the incorporation of a bulfering agent into the pulp before the pulp, before washing is in the range of 2 to 6.5.
  • the present invention relates to a multistage process for bleaching a chemical cellulosic pulp and in particular to a relatively high speed treatment of the pulp during such a multistage bleaching process with gaseous chlorine dioxide.
  • aqueous solution containing chlorine dioxide at a consistency from 6 to i.e. the pulp being in a form of a slurry containing 5.7 to 15.7 pounds of water per pound of dry pulp and at a temperature in the range of 65 to 85 C. to effect oxidation of said pulp.
  • Treatment with chlorine dioxide must be continued for a period of from 3 to 5 hours in order to obtain satisfactory brightness of the pulp.
  • an aqueous solution of chlorine dioxide is used since it is substantially impossible in practice to achieve uniform treatment of a pulp slurry by means of gaseous chlorine dioxide.
  • This oxidation treatment with chlorine dioxide is conventionally effected in the third stage of the multistage process on a partially delignified chemical pulp obtained from the extraction stage following a chlorinating delignification stage, and with a kraft pulp it may also be effected in the fifth stage of the multistage process following the fourth extraction stage.
  • the aforesaid treatment of chemical cellulosic pulp with chlorine dioxide according to the aforesaid conventional process involving the use of large volumes of water both in the pulp and in the aqueous solution of the chlorine dioxide, is a relatively slow and cumbersome process involving substantially high operating costs and large expenditures on capital equip- 3,586,599 Patented June 22., 1971 ment required to handle the process over substantial periods of time.
  • the present invention provides a process for the oxidative treatment of a chemical cellulosic pulp with chlorine dioxide which process is a relatively high speed process in which the pulp is contacted with chlorine dioxide over substantially reduced periods of time as compared with the conventional process and which involves the use of substantially reduced volumes of water for its operation.
  • the multistage bleaching of a chemical cellulosic pulp in which said pulp is subjected to an oxidative treatment with chlorine dioxide the improvement which comprises adjusting the moisture content of the pulp to a level of from 60 to 80% shredding or fluffing the pulp into fibres and fibre aggregates, and exposing the shredded or fluffed pulp to chlorine dioxide gas diluted with a nonreactive gaseous diluent at a temperature of at least 65 C. for a period of time sufficient to obtain the desired brightnessin said pulp.
  • Flufied chemical cellulosic pulp (sometimes referred to as shredded chemical cellulosic pulp) is the product obtained by the fragmentation of a chemical cellulosic pulp into fibre and fibre aggregates or flocks, substantially each discrete fibre aggregate or flock being spongy, compressible and readily gas permeable.
  • Each of said fibre aggregates or flocks is usually an average of A; to 1" in diameter and presents a fiuifed up dry appearance even with about to moisture in the cellulosic pulp.
  • fluffing or shredding of the chemical cellulosic pulp is directed to a process of producing this pulp, the fragmentation being effected for example by the use of disc mills.
  • the process of the present invention By adjusting the moisture content of the pulp to be in the range of 60 to 80% and preferably in the range 60 to 70% and by contacting the chlorine dioxide in gaseous form with the pulp, shredded or fiuifed into fibre or fibre aggregates, it is possible by the process of the present invention to obtain a fast intimate contact of the chlorine dioxide with the interior of the fibres and fibre bundles and as a result to reduce the time necessary for obtaining the extent of reaction necessary for high brightness in the pulp. Thus it is possible according to the process of the present invention to reduce this time for obtaining high brightness in the pulp from a period of 3 to 5 hours as is required in the conventional process to the period from 5 to 30 minutes and thus provide a relatively high speed process for the chlorine dioxide oxidation of the pulp.
  • the oxidized pulp produced by the process of the present invention is found to be similar in all essential respects to the oxidized pulp produced by the aforesaid conventional process and in particular; has the advantage of better resistance to colour reversion than the oxidized pulp produced in the conventional process.
  • the pulp prior to contact with the gaseous chlorine dioxide be fluifed or shredded into the form of fibres or fibre aggregates to obtain the aforesaid intimate contact with the gaseous chlorine dioxide resulting in fast reaction time and thus high brightness over a short treatment period with the gaseous chlorine dioxide.
  • the pulp have a consistency of at least 20% and preferably at least 30%; while the pulp may have a consistency of about 40% and still achieve the desirable results of the present invention, the obtention of a consistency of above 40% increases the cost of dewatering to an economically impractical extent.
  • the gaseous chlorine dioxide be diluted with a non-reactive gas as undiluted chlorine dioxide gas is subject to spontaneous explosion.
  • the gaseous chlorine dioxide is diluted with a non-reactive gas such as air, steam, nitrogen to a partial pressure not greater than 100 mm. of mercury and preferably about 30 mm. of mercury.
  • the temperature of the reaction is at least 65 C. to obtain a reasonably rapid reaction and the speed of reaction increases with increasing temperature; preferably the reaction temperature is in the range of 65 C. to 100 C. and more preferably 80 C. to 100 C. and even more preferably 90 C. to 100 C.
  • the treatment of the chemical cellulosic pulp according to the present invention may be effected in the third stage of the multistage bleaching process for both kraft and sulphite pulps and may also be effected in the fifth stage for kraft pulps.
  • the third stage of themultistage bleaching process where the plup is obtained from a caustic extraction stage following a chlorinating delignification stage, it has been found that, by using the process of the present invention, substantially higher brightness can be obtained under otherwise the same reaction conditions where the pH of the pulp is controlled in such a Way that the final pH of the oxidized pulp is in the range 3.5 to 6.5 and preferably in the range 4 to 5.5.
  • This control of the final pH of the oxidized chemical plup may suitably be effected by adding a buffering agent such as an alkaline metal salt of a weak acid, e.g. sodium carbonate, to the pulp before contact with the gaseous chlorine dioxide.
  • a buffering agent such as an alkaline metal salt of a weak acid, e.g. sodium carbonate
  • FIG. 1 is a graph of brightness against basis weight of handsheets of partly delignified sulphite pulp treated with chlorine dioxide according to the process of the present invention. It also compares the brightness levels obtainable on handsheets with those on shredded pulp at the same consistencies by the same process.
  • FIG. 2 is a graph of brightness and CED viscosity against final pH of a partially delignified sulphite pulp subjected to the process of the present invention
  • FIG. 3 is a graph of brightness and CED viscosity against final pH of a partially delignified Kraft pulp subjected to the process of the present invention.
  • EXAMPLE 1 An unbleached sulphite pulp from coniferous wood having a Roe chlorine number of 5.1 and a Kappa No. 24.4 was treated with gaseous chlorine at 30 C. The pulp was then washed, treated at 10% consistency with 2% by weight of sodium hydroxide for one hour at 60 C., and the pulp again washed. The semi-bleached pulp so obtained had a Kappa number of 0.8, a CED viscosity of 20.1 cps. and a brightness of 67.3 on the scale of the Elrepho meter. This pulp was divided into two equal portions and one portion of the pulp containing 60 g.
  • dry fibre was dewatered to a 30% consistency by pressing, and then passing through a disc mill so as to shred and fluff the pulp into fibres and fibre aggregates and the fluffed and shredded pulp was placed in a glass tower 17" in length and 3 /2" in diameter.
  • the tower was set in a bath maintained at 65 C. and gaseous chlorine dioxide diluted with nitrogen to a partial pressure of 30 mm. of mercury was passed into the tower until 0.6 gram of chlorine dioxide had been absorbed by the pulp, whence the pulp was held at 65 C. for 30 minutes. Unreacted chlorine dioxide was swept from the tower by a stream of air and absorbed in cold water.
  • the quantity of chlorine dioxide recovered was by analysis found to be 0.216 gram and accordingly 0.384 gram of chlorine dioxide equal to 0.64% by Weight of the pulp was found to have been consumed.
  • the bleached pulp was found to have a brightness of 92.7 and a CED viscosity of 20.0 cps.
  • the other portion of the pulp was treated with 1% by weight of chlorine dioxide at a consistency of 6% for two hours at C. according to conventional techniques. It was found that chlorine dioxide equal to 0.12% by weight of the pulp remained unreacted such that 0.8% had been consumed and the brightness of the pulp so obtained was 91.3 and a CED viscosity was 20.8 cps.
  • EXAMPLE 2 An unbleached sulphite pulp was chlorinated, treated with sodium hydroxide and washed as described in Example 1.
  • the semi-bleached pulp obtained had a Kappa number of 1.2, a CED viscosity of 28.0 cps., and a brightness of 66.2 on the scale of Elrepho meter.
  • This semi-bleached pulp was dewatered to 30% consistency as in Example 1, fluffed and shredded by passage through a disc mill as in Example 1 and placed in a tower in a similar manner to Example 1. Steam at atmospheric pressure was passed upwardly through the tower and the flow of steam was continued until the temperature of the pulp as shown by a thermometer embedded in it was 89 C.
  • EXAMPLE 3 An unbleached sulphite pulp was partially delignified in a similar manner as in Example 1 by the following sequence of treatment; chlorination, water washing, extraction with sodium hydroxide and washing with water to neutrality.
  • the partially delignified pulp was divided into four different portions.
  • the first portion of the pulp was treated with an aqueous solution of sodium carbonate (1% sodium carbonate on pulp) and shredded and flutfed as in Example 1 at 20% consistency and treated with gaseous chlorine dioxide.
  • the second portion was pretreated with an aqueous solution of sodium carbonate in a similar manner as the first portion and shredded and flufied at 30% consistency and treated with gaseous chlorine dioxide in a similar manner to the first portion.
  • the third portion and the fourth portion were made into sheets in accordance with standard hand sheet making procedure, one set of sheets having an initial consistency of 25%, the other set having an initial consistency of 35%.
  • the sheets were sprayed with an aqueous solution of sodium carbonate (1% sodium carbonate on pulp) to reduce the consistencies to 20% and 30% respectively, and the basic weight of the hand sheets was 44, 88 and 176 pounds per square foot.
  • the sheets were then cut into 1" wide strips and treated with chlorine dioxide. All the pulp samples, fluffed and in sheet form, were treated with gaseous chlorine dioxide in an amount of 1% on pulp the chlorine dioxide being diluted with nitrogen to a partial pressure of 30 tor-r for a bleaching time of 30 minutes at 65 C. The results obtained are shown in FIG.
  • Chlorine dioxide diluted with nitrogen to a partial pressure of 30 mm. of mercury was passed into the tower until the moist pulp had taken up chlorine dioxide equal to 1% by weight of the dry weight of the pulp when the tower and its contents were maintained at 65 C. for a period of 5 minutes. Excess chlorine dioxide 'was then swept out of the tower by aeration and the amount swept out determined analytically whereby the chlorine dioxide consumed was calculated to be 0.51% by weight of the pulp. The final pH of the pulp was 4.95
  • this example amply illustrates the criticality of the pH in the third stage of the multistage bleaching process as the increase in brightness by controlling the pH is of the order of 5 points.
  • EXAMPLE 5 Unbleached kraft pulp from coniferous wood was chlorinated and extracted with sodium hydroxide solution as in Example 1.
  • the extracted pulp had a brightness of 31.2 and Kappa number 7.3. Portions of the extracted pulp were fiutfed as in Example 1 at 20, 30 and 40% consistency and contacted with 1% by weight on the dry pulp of chlorine dioxide for a period of 30 minutes, the chlorine dioxide having a partial pressure of 30 mm. of mercury in nitrogen.
  • the oxidations were run at three temperatures of 65 C., C., and 92 C. respectively, and in some of the oxidations, sodium carbonate was added to the pulp in an amount to give a pH of about 4.5 after chlorine dioxide treatment. In other oxidations no buffer was employed and the final pH was 2.2.
  • the brightness of the treated pulps are set out in the following Table I.
  • EXAMPLE 6 An unbleached sulphite pulp from coniferous wood having a Roe chlorine number 7.9, Kappa number 33.0 and CED viscosity 35.6 cps. was chlorinated, washed, extracted with dilute sodium hydroxide and washed again in a similar manner to Example 1.
  • the partially de lignified pulp had Kappa number 1.0, CED viscosity 33.1 cps. and an Elrepho brightness of 57.7%
  • the final pH of the moist, bleached pulp was 5.2.
  • a portion of the bleached pulp was washed, air dried, and the brightness measured.
  • the brightness tabs were then placed in a hot air circulating oven at 105 C. for 1 hour, then conditioned in a constant atmosphere; relative humidity 50% at 70 F.
  • the brightness after accelerated aging was measured and the brightness loss calculated.
  • the brightness and the loss in brightness after aging are set out in the Table II below.
  • EXAMPLE 7 An unbleached kraft pulp from coniferous wood having a Roe chlorine number 4.3, Kappa number 26.8, and 0.5% CED viscosity after acid chloride treatment 25.0 cps. was chlorinated, washed, extracted with dilute sodium hydroxide and washed again as in Example 1. The partially delignified pulp had a Kappa number 7.3, Elrepho brightness 31.2%, and 0.5% CED viscosity of 25.1 cps.
  • the pH of the moist, bleached pulp was 5.1.
  • the bleached pulp was washed, air dried, and the brightness measured.
  • the brightness tabs were then placed in a hot air circulating oven at 105 C. for 1 hour, then conditioned in a constant-atmosphere, relative humidity 50% at F.
  • the brightness after accelerated aging was measured and the brightness loss calculated.
  • the bright ness and the loss in brightness after aging are set out in Table IV.
  • EXAMPLE 8 then the pulp was shredded into fibre and fibre aggregates. Each sample of shredded pulp was placed in a tower, the tower was set in a bath and temperature maintained at 80 C. Chlorine dioxide diluted with nitrogen to a partial pressure of 30 torr was passed into the tower until 0.6% C10 on pulp o.d. basis was passed into the tower. The pulp was held at 80 C. for 30 minutes. Unreacted chlorine dioxide was removed from the tower by aeration and the pH of the moist bleached pulp was measured for each of these samples. The bleached pulp was washed, air dried, the brightness and the CED viscosity determined on the dried pulp samples. The results obtained are given in Table V and in FIG.
  • the viscosity of solutions of pulp in suitable solvents such as cupriethylenediamine is a function of the degree of polymerization of the cellulose, and although the mechanical properties of the fibre are not directly correlated with the viscosity, it can be seen from the values of the mechanical properties of the pulp, presented in Table IV, that the tensile and bursting strength of the pulp are reduced, accompanying the drop in viscosity as the final pH is increased above 7.
  • the viscosity of solutions of pulp in suitable solvents such as cupriethylenediamine is a function of the degree of polymerization of the cellulose, and although the mechanical properties of the fibre are not directly correlated with the viscosity, it can be seen from the values of the mechanical properties of the pulp, presented in Table VI, that the tensile and bursting strength of the pulp are reduced, accompanying the drop in viscosity as the final pH is increased above 7.
  • the improvement which comprises adjusting the moisture content of the pulp to a level from 60 to shredding or flufiing the pulp into fibers and fiber aggregates and exposing the shredded or fluffed pulp to chlorine dioxide gas diluted with a non-reactive gaseous diluent at a temperature of at least 65 C. for a period of time sufficient to obtain the required brightness in said pulp, the consistency of the pulp during said exposure being maintained substantially constant in the range 20 to 40%, and the pH of the pulp being controlled so that the final pH of the thus-treated pulp, before washing, is in the range 2 to 6.5.
  • a process as claimed in claim 1 in which the pulp treated with the chlorine dioxide is obtained from a caustic extraction stage following a chlorinating delignification stage and the pH of the pulp is controlled such that the final pH thereof is in the range 3.5 to 6.5.

Landscapes

  • Life Sciences & Earth Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Wood Science & Technology (AREA)
  • Paper (AREA)
US769879A 1964-08-07 1968-10-23 Purification of fluffed or shredded pulp with chlorine dioxide Expired - Lifetime US3586599A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
CA908947 1964-08-07

Publications (1)

Publication Number Publication Date
US3586599A true US3586599A (en) 1971-06-22

Family

ID=4142055

Family Applications (1)

Application Number Title Priority Date Filing Date
US769879A Expired - Lifetime US3586599A (en) 1964-08-07 1968-10-23 Purification of fluffed or shredded pulp with chlorine dioxide

Country Status (4)

Country Link
US (1) US3586599A (pt)
DE (1) DE1546277A1 (pt)
FI (1) FI43526B (pt)
SE (1) SE314286B (pt)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3874992A (en) * 1972-09-29 1975-04-01 Pulp Paper Res Inst Press alkaline extraction of cellulosic pulp
US3907632A (en) * 1973-10-23 1975-09-23 Ppg Industries Inc Bleaching cellulosic pulps with gaseous chlorine at a pH of 7 to 10 in a multistage bleaching process
US4128454A (en) * 1977-12-07 1978-12-05 Ingersoll-Rand Co. System and method for gas phase pulp bleaching
US6153300A (en) 1994-04-18 2000-11-28 Ahlstrom Machinery, Inc. Bleaching cellulose pulp having cleanliness which varies significantly over time using at least two different bleaching stages and bleaching chemicals
CN105525463A (zh) * 2015-11-20 2016-04-27 北京鹏盛天纤科技有限公司 一种气相二氧化氯漂白废旧纺织品的装置以及气相二氧化氯漂白废旧纺织品的方法
CN105568659A (zh) * 2015-11-20 2016-05-11 北京鹏盛天纤科技有限公司 一种气相二氧化氯漂白絮状物的装置以及气相二氧化氯漂白絮状物的方法

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
AU655842B2 (en) * 1991-08-23 1995-01-12 Olin Corporation Delignifying wood pulp

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3874992A (en) * 1972-09-29 1975-04-01 Pulp Paper Res Inst Press alkaline extraction of cellulosic pulp
US3907632A (en) * 1973-10-23 1975-09-23 Ppg Industries Inc Bleaching cellulosic pulps with gaseous chlorine at a pH of 7 to 10 in a multistage bleaching process
US4128454A (en) * 1977-12-07 1978-12-05 Ingersoll-Rand Co. System and method for gas phase pulp bleaching
US6153300A (en) 1994-04-18 2000-11-28 Ahlstrom Machinery, Inc. Bleaching cellulose pulp having cleanliness which varies significantly over time using at least two different bleaching stages and bleaching chemicals
CN105525463A (zh) * 2015-11-20 2016-04-27 北京鹏盛天纤科技有限公司 一种气相二氧化氯漂白废旧纺织品的装置以及气相二氧化氯漂白废旧纺织品的方法
CN105568659A (zh) * 2015-11-20 2016-05-11 北京鹏盛天纤科技有限公司 一种气相二氧化氯漂白絮状物的装置以及气相二氧化氯漂白絮状物的方法
CN105525463B (zh) * 2015-11-20 2019-02-22 北京鹏盛天纤科技有限公司 一种气相二氧化氯漂白废旧纺织品的装置以及气相二氧化氯漂白废旧纺织品的方法
CN105568659B (zh) * 2015-11-20 2019-08-09 北京鹏盛天纤科技有限公司 一种气相二氧化氯漂白絮状物的装置以及气相二氧化氯漂白絮状物的方法

Also Published As

Publication number Publication date
SE314286B (pt) 1969-09-01
DE1546277A1 (de) 1970-02-26
FI43526B (pt) 1970-12-31

Similar Documents

Publication Publication Date Title
US4576609A (en) Process for the treatment of cellulosic materials with oxidizing agents and microwaves
US3663357A (en) Bleaching of mechanical cellulosic pulp with ozone in the presence of a peroxygen compound
US5384012A (en) Process for crosslinking of cellulosic fibers
US3652388A (en) Bleaching and delignification of partially delignified pulp with a mixture of chlorine and chlorine dioxide
US4076579A (en) Pulping of lignocellulosic material by sequential treatment thereof with nitric oxide and oxygen
CA2634202C (en) Wood-based lignocellulosic fibrous material
US3874992A (en) Press alkaline extraction of cellulosic pulp
US5656130A (en) Ambient temperature pulp bleaching with peroxyacid salts
US3586599A (en) Purification of fluffed or shredded pulp with chlorine dioxide
US3655505A (en) Two-stage purification of fibrous cellulose material employing gaseous chlorine dioxide in one stage and a peroxygen compound in the other
US3829357A (en) Oxidative manufacture of pulp with chlorine dioxide
US4227964A (en) Method of treating lignocellulosic or cellulosic pulp to promote the kinking of pulp fibres and/or to improve paper tear strength
US3472731A (en) Extraction of fibrous cellulosic material with ammonia following treatment with chlorine,chlorine dioxide or other reagents
US3532597A (en) Preparation of dissolving pulps from wood by hydrolysis and alkaline sulfite digestion
US3619349A (en) Bleaching of shredded or fluffed cellulosic pulp with gaseous chlorine monoxide
US3177111A (en) Hypochlorite sulfamic acid process of bleaching cellulosic materials
US5645687A (en) Process for manufacturing bleached pulp with reduced chloride production
US3284283A (en) Production of wood pulps including treatment of cellulosic fibers with bisulfite ion followed by alkali metal borohydride
US5217574A (en) Process for oxygen delignifying high consistency pulp by removing and recycling pressate from alkaline pulp
US1838326A (en) Refined wood pulp of low pentosan content
US3558428A (en) Method in the manufacture of chemomechanical pulps
US2779656A (en) Bleaching of kraft pulp
US3262839A (en) Neutral to weakly alkaline sulfite process for the extraction of cellulose from cellulosic material
US4244780A (en) Use of thiourea dioxide in pulp bleaching processes to preserve pulp strength and aid in brightness
US3740311A (en) Delignification of cellulosic pulp with ammonia gas followed by heating in the presence of oxygen