CA2128408A1 - Gas phase delignification of lignocellulosic material - Google Patents
Gas phase delignification of lignocellulosic materialInfo
- Publication number
- CA2128408A1 CA2128408A1 CA002128408A CA2128408A CA2128408A1 CA 2128408 A1 CA2128408 A1 CA 2128408A1 CA 002128408 A CA002128408 A CA 002128408A CA 2128408 A CA2128408 A CA 2128408A CA 2128408 A1 CA2128408 A1 CA 2128408A1
- Authority
- CA
- Canada
- Prior art keywords
- pulp
- ozone
- chlorine
- lignocellulosic material
- chlorine dioxide
- 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.)
- Abandoned
Links
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
- D21C9/147—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications
- D21C9/153—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications with ozone
-
- 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
- D21C9/144—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites with ClO2/Cl2 and other bleaching agents in a multistage process
Landscapes
- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Wood Science & Technology (AREA)
- Paper (AREA)
- Polysaccharides And Polysaccharide Derivatives (AREA)
Abstract
A process for delignifying lignocellulosic material, such as wood pulp, which comprises treating the lignocellulosic material sequentially or simultaneously with gaseous chlorine dioxide and ozone which produces delignified cellulosic material and discharge effluents having reduced absorbed organic halides (AOX).
Description
W~3/1~2~ P~T/US~3/0~0~
2128~8 ~AS PHASE DELIGNIFICATION OF ~-LIGNOCELLULOSIC MATERIAL
The present inv~ntion relates to a process for sequential or simultaneous gas phase delignifica~ion of lignocellulosic material for use in paper making which ~:~
produces delignified cellulosic material having reduced s absorbed organic halides (AOX~.
Chlorine-based chemicals, such as chlorine, chlorine divxide and hypochl~rite have been used in pulp ~:
bleaching for several decades and continue to be used for removing lignin and ~leat,hing the pulp to high 10 brightness. In seneral terms, the extent of ~
delignification and bleaching (the degree o bri~htness ~-obtained) is ~etermined b~ th~ typ~ of pulp being :
delignifi~d and bleached and~the proposed end use of the paper. In all:instances, however, where chlori~e-based 15 delignifying and bleaching agents:have been employed in ~
the prior art, there are produced c~lorinated: organics. -:
These compounds ar~ ~o~ic~and are present:in the -:
discharge e~fluents from one or more of the sta~es ~f the paper making process ~eg. bleaching stage).
relati~ely smaller percentage of such chlorinated organics remain in the pulp and eventually appear in the paper product.
In the past decade, there has been a ~rowing concern about ~he environmental impact of the 25 chlorina~ed orsanic compounds pre ent in paper and ln ~`
discharg~ efluents. U~desirable chlorinated org~nics such ~s diogin have been detected in the e~haust gases W~93/tS2~ PCT/U~93/~
2128~
of incinerators burning municipal wastes includin~ for e~ample paper products made from chlorinated bleached pulps. Thus, it is desirable to limit the absorbed organic halides (AOX~ discharged in the efluent per ton S of wood pulp to less than 2.0 kg.
Several options have be~n proposed or practiced to reduce or eliminate chlorinated organics in the b~eaching process. The most straigh~forward method i5 to substitute non-chlorine bleaching chemicals such as o~sen9 pero~ides, oæone, perace~ic acid, etc. for chlorine-based bleaching chemial~ O~ygen delignification and bleaching is now a well-established commercial reality in pulp bleaching technology. Its use is restricted to partially remove lignin under alkaline 1~ conditions - further bleachiLng must be done to remove the residual liqnin color b~aring groupsr While whitening the pulp, great caution should be exercised to a~oid ~he degradation of th~ carbohydrate frac~ions.
When the complete remo~al of lignin is ac~omplished by 20 oxygen, the pulp strength suff~rs bec~use of ~:
unacceptable destruction ~f the celllllose.
Hyaro~en pero2ide is widely used to reinforce the alkaline e~traction stages which are carried out after the chlorination and ~hlorine dioxide stayes, but so ~ar 2~ neither o~ygen nor a combination of o~ygen and hydrogen pero~ide could match the efficiency of chlorination.
Ozone because of its high o~ida~ion potential should offer a better alternative, but its uses are limited because it must be used in the gaseous state and 3~ ozone generators produ~e only low concentrations of ~.~
ozone in oxyyen or air. Ozone has been used alone as a :.
sin~le stage ble~chin~ agent, or in a multi-stage bleaching process which may include treatment with other ~leaching agent~ such as oxygen, pero~ides, chlorine WO~3/152~ ~. 2 3~ CT/US93/0~08 dio~ide and hypochlorite. The fundamental bleaching variables affecting the use of ozone include (a) the type of pulp (hardwood or softwood), (b~ the pulpiIlg process used to produce the pulp, ~c) pre-treatment of the pulp p~ior to ozsnization, (d) sequential treatme~t of ozone and other bleaching chemicals, (e) pulp consistency, (f) ozone concentration and charge, (~) temperature, (h) time, and (i~ pH. ~ut it was ound `~
that o~on2, as a stron~ oxidant, tend~d to attack and 10 degrade cellulose as is the case with o~yg~n. In l:he later instance, the degradatiorl of cell~los~ is minimized l:)y using a magnesium salt as a protector.
However, it was determined that the amount of the ozone charge and the position of the ozone treatment in the 15 bleaching process were imporl~ant.
Another option to reduce the dis harge of chlorina~ed organic compounds is to reduce the c:hlorine usage in the first stage of ~he bleaching process. Two ~
alter2latives that produc:e no signif icant degFadàtion of ~:
20 pulp properties have b~en co~unercialized for this purpose. These are (a) extended delignification in the cooking stage and (b) o~l:ygen delignificatiorl. These alternatives, with proper e~traction, reduce the chlorirlated organic compounds in the bIeached pulp and 25 ensuin~ effluent to sufficiently low lev~ls.
A third option to reduce the generation of chlorinated organics in a bleaching process is to su~stitute chlorine dio~ide for chlorine. Chlorine dioxide.is a relatively strong o~idant compared to 30 chlorine. To ~chieve the same degree of delignification, it requires only about thirty eight weight percent chlorine dio~ide on th~ pulp compared with one hundred weight p~rcent of chlorine. Howev~r, th~3se pro~esses are of the DEDE~ type wherein the ~-W0~3/~52~ PCT/US93/~M~
~ 1 2 8 ~ 8 chlorine dio~ide stage (D) is followed by ~he conventional e~traction (E~ and additional chlorine dio~ide (D) stages. The pulp and discharge effluents resulting from the processes of this option contain higher concentra~ions of chlorinated organics than are acceptable and/or desira~le. Processes using both o~ygen delignification and chlorine dio~ide substitution have been suggest~d, but do not achieve the desired and/or regulated concentrations o chlorine containing re~idues in either the pulp or the effluent.
Chlorine dio~ide and ozone have found wide uses as disinfe~tants in water treatment~purification applications, delignifyin9 and~or as a ble`aching agent in pulp and paper production, and in a number of other u~es because of their high o:~idizing power.
Ozone is used as a delignifyin~ chemical to remove or reduce the lignin from the lignocellulosic material 9 but because of its high o~idizing power, it is indiscriminatory in its tendlency to attack both lignin and cellulose thereby producing inferior pulp.
Chlorine dio~ide in solution has been used to brighten ~nd produce a cl an pulp in pap r manufacture.
However, it has generally been felt that the use of ~:~
liquid chlorine dioxide in the first stage of a multiple stage bleaching process is not as effective as is using chlorine as a delignifying ~gent. However, the use of elemental chlorine has the undesirable e~fect of producing organo-chlorine compounds, which are ~
measur~le as adsorbable or~anic halides (AOX), in the :;
3~ effluent steams of processing facilities. M~nc~, as efforts have continued to focus o.n ways to ~liminate or substan~ially reduce AO~ discharge, chlorine dio~ide has continued ~o be a popular agent in pulp processing, but as a brightening agent.
3/1S264 PCr/US93/0~408 2128~0~
There are a number of chlorine dio~ide generator systems and processes avai lable in the marketplace, Most of the very large scale general:ors utilize a chlorate salt, a chloride ion source or reducing agent, 5 and a strong acid. In the presence of chloride ion and acid, chlorate ion reac:ts. to produce a mixture of chlorine and chlorine dio~cide. The chlorine present in these processes is an undesired by-produrt whic:h adds to the P~OX concentrations in discharge f low streams .
Many proces~es have been developed to produce chlorine dioxide with lower clhlorine coIlcentrations by adding a reducing agent. Some reducing agents which have been used in these applications include nnethanol or other organis compounds, sulfur, sulfur dioa~ide or other 15 sulfur-o~ygen species havin~ a sulfur valence of less than +6, and carbon mono~id~. When organic co~npou~ds are used, unreacted volatile organics, ir~cluding organic acids, are present in the product gas. Using sulfur containing reducing a~ents, the sulf ate or sulfuric acid produced accumulates as a waste product. When gaseous reducing agerlts, such as sulfur dîoacide or carbon mono~ide, are elrployed, reactor designs and process control systems must protect against unreacted reducing agent leaving ~he system with ~he ~hlorine dioxide gas.
Chlorine dio~ide has been produced in prior art :~
pros::esses rom chlorate salts by the addition of an excess of the acid used. While this acid is slowly neLItralized by the accumulation of alkali metal ions that e~t-er the process with the rhlorate salt, the accumulation of salts must be removed as a waste stream. This waste stream is either liquid or solid in every process currently practiced conunercially.
Th~ pr~paration of chlorin~ dio:~ide f rom chloric ~-:
acid has been accomplished to avoid the formation of an WO ~3/15~64 PCl[/US93/D0408 ~12~
acidic alkali metal salt. Chloric acid is, hc~wever, not commercially available, although its preparation has been taught in U. S~ Patent 3, 810, 969 issued May 14, 1974 to A . A . Schlumber~er . Schlumbergex teaches a process for producing chloric acid by passing an aqueous solution containing f rom O . 2 gram mole to 11 gram moles per liter of an alkali metal chlorate such as sodium chlorate through a selected cationic e~change resin at a temperature from SD to 40v C:. The proc:ess produces an aqueous solution containing ~rom O . 2 gxam mole to about 4 . O gram moles of HC103 per liter.
K.I,. Hardee et al, in U.S. Pat nt No. 4,798,715 issued Jan~ 17 ,1~89, describe a process to produce chlorine dioxide by electrolyzing a chloric acid 501Ut:Lon produ~ed by p~sing an aqu~3ous ~olution of ~n alkali metal chlorate through an ion ~:~cchange resin.
The electrolysis is c~rried ou~ using an electrocatalytic c:athode wh~sre the catalyst is, for e~ample, one or more valve metal o~ides which may be 2~ combined with a platinum groulp m~tal o~ide, or a :-platinum group metal, or oxides of a platinum group me~al, ma~rletite, ferrite, or mi:~ed metal o:~:ides.
The electrolyzed solution contains a mi~ture of chlorirle dio~:ide and chlori~ acid, which is fed to an extractor in which the chlorine dio;~:ide is stripped ~:.
of f . The ion e;~c~ange resin is r~gener~ted with -~
hydrochloric acid and an acidic solution of an alkali metal chloride formed. Such processes require the regener~tion Qf the ion e~change resin with acid to remove the alkali metal ions and the use or treatment and disposal of the acidic salt solution.. A150 the concentration of chloric acid that can be produced by an ion ~:cchange process is limited since more concentra'ced chlori~ acid solutions attack the ion exchange resins . ...
W~ ~3/15264 PCr/US93/~
2 1 2 ~ J ,~
used in the process. Lastly, the production of chloric acid by means of a cation e~change resin is not economically attractive.
Commercial ozone generators produce ozone by a S continuous high voltaqe electric discharge in presence of an o~ygen containing feed gas. The ozone generator contains a stainless stesl tube and a shell heat e~changer. High volta~e alternating current is imposed on a hîgh voltage ele~trode consisting of a permanent 10 stainless steel tub2 on one side of a dielectric .
material. The diel~ctric glass tube c~ntered in the stainless steel tube which has a ground electrode allows the electric discharge to occur as corona discharge across the.dischar~e gap between the dielectric and the ground electrode. As the o:gygen containing feed gas flows through the "corona", a portion of the oxygen is convert~d to ozone. Since ~much of the electrical ~nergy supplied to orm ozone is lost as heat, an efficient method of heat removal is incorpvrated into the desiqn o the generator.
G~s phase bleaching with chlorine or chlorine dio~ide has been proposed over the years in various p~tents and publications as a method of reducing the bleaching time whi~e cutting chemical costs. Gas phase 25 bleaching is carried out on higher consistency pulp ~;
using mi~tures of chlorine diogide and steam and/or inert gases such as air or nitrogen.
U,S. Patent No. 3,725,1~3, issued April 3, 1973 to R~; DeMonti9ny et al., describes a process for bleachi~g hi~h consistency pulps, which includes preheatin~ the pulp by direct steaming. A gaseous mixture vf chlorine dio~ide diluted with st~am or a non-reactive gas is then passed through the pulp. The contact period is in the order of a fraction of a WO g3/t5264 PCI/U~i93/0~ 8 3 l~ Q (~3 -8-second. The bleached pulp was then held in a retention vessel for 30 minutes. Vnreacted chlorine dio~ide was removed from a bleaching tower by aeration. The final pH of the bleached pulp was ~.2.
Ozone treatment of pulp is practiced under three pulp consistencies. They are ultra low consistency treatment, medium consistency treatme~t and high - consistency treatment. Hi~h co~sistency treatment of pulp inv~lves th~ use of gaseous 020ne applications.
Advantag~s alleged or gas phase bleaching of high consistency wood pulps includ~ superior control of bleaching because of the short retention times employed;
reduced chemical usage for t:he ~ame brightness; and low water usage and effluent volume. The g3s phase 15 bleaching of pulp i5 well documented, but no in~ormation :;
is available on the use of 51aseous chlori~e dioxide and ozone either s~quentially or simultan~ously for the deligniication of pulps. Both chlorine dio~ide and oæone are str~ng ogidizing agents, but their ability to ;~
complement one another in the d~liynifi ation process has been not e~plored.
The lack of a process for g~nerating chlorine dioxide gas instantanevusly or by a process having inconsequential start-up and shut-down times has ~loc~ed its successful use commercially of gas phase bleaching.
A process has be~n found that produces chlorine dio~idP
gas which can be used directly from the 9Bnerator without requiring the formation and storage of dilute aqueous solutions of chlorine dio~ide which are 30 subsequ~ntly stripped or which do not have significan~ :
concentrations of elemental chlorine ~as that damage the celiulose. After deli~nification there is substantially no residual chlorine dio~ide re~uiring r~coYery or disposal. Also, unknown in the industry is the method WO g3/1~26~ ~Cr/US93/00~8 212~
g of mi~ing the chlorine dioxide with another gas like o~one to get the powerful gaseous mi~ure having ~-superior and selectiYe o~idation ability to delignify pulp .
A recent process to reduce the AOX discharges from pulp mills has been disclosed by ~he Amcor Research and Technology Center in ~ustralia which uses an ogygen pretreatment step to passivate the pulp and enhance the reactivity of liquid chlorine dioxide applied in a first ~:
stage delignification operation, followed imm~diately by an alkaline e~traction stage withou~ an intermediate washing step. This process substantially reduces the AOX discharge, but still requires the pretr~atment step. There e~ists the need for a simplified process to directly delignify lignocellulosic material.
Canadian patents number 2,032,31S; 2,031,84B; and 2,031,850 teach the sequential addition of aqueous chlorine dio~ide and ozone at poînt~ s~parated from one another and each being at at least one place in the line and this stage being free ~om intermediate washing ~etween places for addition. Heterogenous sequential treatment of pulp with a~ueous solutions of ozone and chlorine dio~ide require repeated filtering and thickening of pulp stvck to adjust for the various ~s consistency requirements of each treatment stepO ~:
These probl~ms are solved in the proeess o~ the present invention by using gaseous chlo~ine dio~ide and ozone sequentiall~ or simultaneously to delignify and bleach- pulp and ther~by reduce the adsorbed organic -~
30 halides (AOX) in ~he delignif ied and ~leached pulp and discharge efflu~nts. A further reduc'cion of AOX is obtained by conducting a direct alkaline e~tr~ction without intermediate washing stage after the blea~hing st~p .
W0~3/152~ PC~/US93/~0~
~1~8'10S
It is an object of the present invention to provide an improved process for delignifying lignocellulosic material.
Another object of the present invention is to mix the chlorin~ dio~ide gas from the state of the art generation processes with an ozone o~ygen gas mixture to get a highly reactive chlorine dioxide and ozone ga~
mi~ture.
I~ is another obj~ct of the present inve~tion t~
provide a first stag~ tr~atment with a gaseou~ mixt~re of chlorine dio~ide and oæon~ which dramatically reduces ~:
the AOX discharges.
Another feature of the present invention is that any residual elemental chlo:rine formed in the chlorine dioxide generation is o~idized by mi~ing and reacti~g it with ozone, thereby ~liminating th~ chance~ of chlorine to react with pulp to generiate AOX~
Ano~her object of thle pre~ent inventio~ is that any residual elementa1 chlorine formed in the reaction ~o between chlorine dio~ide and lignocellulosic material is destroyed by the reaction of liqnocellulosic material with ozone, thereby reducing ~he chances for the formation of ab~or~able or~anic halidPs (AOX~.
Another object of the present invention is that 2s gaseous chlorine dioxide added to the lignocellulosic material acts as an acidic pH buffer of pH 2 - 5, the pH
range conducive for ozone delignification.
Yet another object of the invention is that chlor~ dio~ide addition to the pulp inhibits the 3~ carbohydrate desradation reactions of ozo~e which are responsible for the strength loss of the pulp.
It is~an ad~an~age of the present inv~ntion that lower AOX concentrations are achieved with pulps having higher pH ~alues.
WO 93~15264 ~ 1 2 ~ ~ O ~ PCr/USg3~0~08 It is still another adYantage of the present invention that a clean pulp material is o~tained in less ``
time and at lower cost in n~eded equipment when compared with conventional processes.
These and other objects, features and advantages are obtained by an improved process for d~lignifying lignocellulosic material which comprises tre~tin~ the lignocellulosic material se~u~ntially or simul aneously with gaseous chlorine dio~ide and ozone and then transportin~ it into a reaction chamb~r where it is retained for a short period of time-~o complet~ the reaction and delignification of the lignocellulosic material. Then an alkaline extraction is performed.
The delignified material is then separated from th~
effluent. This effluent contains less tha~ about 1.0 kg absorbable organic halides ~AOX) per ton of lignocellulosic material. ~-Suitable p~lps which can be bleached incl~de any of those which are commonly used such as chemical kraft, sulfite or mechanical and recycle pulps. Pulp having any suitable consistencies may be delignified including those of about lS percent or higher, for e~ample, from about 25 to about 60 percent can be treated by the process of thP present inventionO The pulps are 25 preferably shredded or fluffed.
Chlorine dioxide gas used as a reactant in the process of the prese~t invention preferably contains low concentrations of elemental chlorine and prefera~ly small amounts of inert gases, such as nitrogen or air.
30 Suitable c~lorine dio~ide gas used in delignifica~ion of pulp is a mi~ture o~ chlorine dio~ide, gaseous oxygen, - and water vapor. Partial pressure of chlorine dio~ide in the gas stream i~ between 10 to 100 mm of Hg. The chlorine dio~ide gas has a low elemental hlorine W0~3/~52~ P~T/~S93/~08 ~ 8 -12-content of less than about 18 percent. The chlorine dioxide gas can also ~e substantially free of chlorine, preferably less than about 2 percent by weight, and more preferably less than a~out 1.5 percent by weight based on the weight of chlorin~ dioxide in the gas. The gaseous mi~ture contains varying concentrations of oxygen and water vapor. The amount of o~ygen can be independently varied to provide molar ratios Qf 2 to ClO by volume of from about 1:4 to about 50:1. Wat~r :~
vapor or steam is preferably the primary diluent in the gaseous mi~ture and replaces other.inert gases which are normally used. As a diluent, steam supplies heat to the pulp and minimizes cooling by evaporation which occurs when, for e~ample, air is used as the diluent.
Gaseous mixtures containing chlorine dio~ide suitable for use in the novel process of the present invention preferably can be produced from co~centrated solutions of chloric acid.
Conversion of the chl~ric acid to chlorine dio~ide can be accomplished~ for example, by cathodic reduction in an electrolytic cell or by contac~ing the chloric acid with a suitable catalyst in the pres~nce of heat. In a~ alternative proce~s, highly acidic aqueous solutions containing chlorate ions, including chloric acid, perchlorate ion~ and hydrogen ions are reacted to produce a gaseous mixture of chlorine dioxide, oxygen and water vapor (steam). The gasev:us mixture has low concentrations of inert gases, such as air or nitrogen, and yet provides reduced risks to spontaneous explosions 3~ or "pufs". The aqueous solutions have an acid concentration of at least 2 molar, preferably at least 3 molar, and a concen~ration of chlorate ions of at least 0.02 molar and pref~rably from about 0.1 to ~bout 3 molar. These acidic solutions preferably are ~93/1~264 PCT/US93~0040~
2l2s4as substantially free of ionic impurities such as chloride ions, alkali metal and alkaline earth metal ions.
The hydrogen ion concentration from acid in the aqueous solutions may be provid~d by non-o~iaizable 5 inorganic acids such as sulfuric acid, phosphoric acid, chloric acid or perchlori.c acid. Preferably, the acidic solutions are reacted in the presence of a catalyst ~:
which promotes o~ygen evolution. ~:
Suitable as o~yg`en-evolYin~ catalysts are, for 10 example, metals and oxides of the elements of Group :~
VIIIA of the Periodic Table of Elements. Thus metals such as the platinum group metals including platinum, palladium, iridium~ rhodium or ruthenium; and mi~tures or alloys of these platinum group metals may be ~:
employed. Additionally o~ides of platinu~ group metals such as iridium, rhodium or ruthenium, as well as mixtures of these o~ides with platinum group metals or alloys of these precious m~tals could b~ suitably employed. Likewise, iron alloys such as stainless steel, nickel or ni~kel bas~d alloys, and cobalt based alloys can be used as v~ygen-eYolving catalysts in the process of the invention. Qther o~yg n-evolving catalysts include semiconductiv ceramics known as perovskites. To suppress or minimize the auto-oxidation of chloric acid to perchloric acid, for e~ample where an oxygen-evolving catalyst is employed, it is preferred to use, as the source of chlorate ions, a mi~ture of chloric acid and a non-o~idizable inorganic acid in which t~e concentration of chloric acid is low, for example~ less than about 20 percent by weight of the aqueous solution providing chlorate ions. These processes for generatin~ chlorine dio~ide can be started up in a few minutes and similarly stopped in a short W093/152~ P~T/USg3/~08 OY!
time so that storage o C10~ gas or dissolution in water or a solvent is not required.
High purity concentrated c~loric acid solutions are produced by the o~idation of hiyh purity hypochlorous acid solutions. One process suitable for producing the chloric acid solutions heats a hypochlorous acid solution, containing from about 35 to about 60 percen~ by weight of HOCl, at a temperature in the rang~ of from about Z5 to about 120~ C. Another process for producing the high purity chloric acid solution utilizes anodic o~idation of the high purity concentrated hyp~chlorous acid solution in an electrolytic cell having an anode compartment, a cathode compartment, and a cation e~change membrane separating the anode compartment from t:he cathode compartment.
In operation, this pr.ocess includes feeding an aqueous solution of hypochlorous acid to the anod~
compartment, and electrolyzing the aqueous solution of hypochlorous solution at a temperature of from about 0 to about 40C~ to produc~ the chloric acid solution.
Chloric acid solutions can be produced by these processes in any concentrations desired up to about 45%
by weight of HC103. ~owever, preferred con entrations are those in the range of from about 30 to about 40% by ~eight o HC103.
Generation of t~e chlorine dio~ide gas from the reaction mi~ture containing chloric acid and the non-oxidizable inorganic acid is preferably conducted at atmospheric pressure in the presence of a diluent gas, such as oxygen, nitrogen or air. The temperature of the reactor mi~ture should not e~ceed 120C, more preferably not more than about 90~C, to avoid d~compositiQn of the chlorine dio~ide in th~ gas phase. Where subatmospheric pressures are utillzed, for e~ample, pressures can be in WO931152~ PCT/US93/~OB
212~0~
the range of from about 15 to about 750, preferably from about 100 to about 500, and more preferably from about 150 to about 300 mm of Hg. At these pressures the temperature of the reaction mixture is in the range of from about 40C to about 90C, and preferably from about 50C to about 80C. The temperatures and pressures for subatmospheric pressures are selected to maintain continuous boiling o the reaction mi~ture.
The chlorine dio~ide gas generator must be in water ba~.ance to operate in steady state conditions.
The rate of heat addition to the ge~erator determines the guantity o~ water that is evaporated to maintain steady state concentration, it is necessary to evaporate water continuously at about the same rate at which water 5 i5 added~ with the chloric alcid feed solution adjusted for water formed by r~actiorl of the chloric acid.
Alternately, chlorine~ dio~ide gas can be obtained from any commercially available generators directly producinq gaseuus chlorine, or by removing the gas prior 20 to going to an adsorber for liquifaction, such as tho~e available from Albright and Wilson Americas under the model names R-8, R 9 and R-10, or from other suppliers of similar commercial equipment.
Ozone gas can be obtained from any commercially 25 available generators producing ozone by direct electric discharge th~ough air or o~ygen. The concentration of ozone from the reactor should be not less than 6 percent by weight on the o~y~en feed, but not limited to or not exceeding 13 percent b~ weight of o~ygen feed, the point 30 of self detonation of ozone. PreferabIe concentrations are from about 6 to 8 percent. The ozone gas ~rom the reactor may be taken to a mi~ing chamber where it may be used to mix the chlorine dio~ide gas from the novel generation process d~scribed above. Chlorine dio~ide ~93/152~ P~T/~S93/~08 2~2~0~
can also be obtained from state of the art genera~ors such as the R-8 generator or f rom other generators supplied by similar co~mercial equipment vendors.
The process of the invention can be practiced using any suitable prior art apparatus such as a conventional bleaching tower.
Delignification of pulp may be affected in the :~
initial stages of a multi-stage bleaching process, as well as in any D or DZ stag~s. The prvcess of the invention requires a rapid mixing and short retention in an apparatus similar to the existin~ first stage equipment capable of mi~ing a mi~ture of gaseous chlorine dio~ide and ozone w:ith lignocellulosic material. The delignification of unbleached lignocellulosic material or ]pulp is accomplished with the use of a gas~ous mi~ture of chlorine dio~ide and ozone produced by one of the aforementioned processes and simultaneously charging the ~a~eous mi~ture to the pulp or sequentially charging the individual gases to the pulp. The gaseous chlorine dio~ide has low concentrations, or is substantially ree, of elemental chlorine and is combined with a direct alkaline e2traction st~p in the absence of an intermedia~e washing ~o redu~e the AOX concentrations in the discharge stream. The delignification of unbleached lignocellulosic material or pulp may be carried out in any suitable eguipm~nt in whi~h the unbleach~d pulp can be contacted with the chlorine dio~ide gas. For pr~ctieal reasons any ~e~ctor should be gas tight~
Suitable gas tight equipment includes, for e~a~ple, agitated mixers, stati mi~ers, ri~bon bl~nders, steam chests, high consistency shear mi~ers, ~ pumps, MC
mixers, high v~locity pipe lin~s, fluffers, etc. ~.
WO93/lS2~ P~T/~Sg3J~8 212~8 Pressure requirements for the feed of the gaseous mixture of chlorine dioxide and ozone to ensure stability ar~ ~ritical, especially if the gaseous mixture is being introduced into a vacuum chamber. For example, partial pressure of the chlorine dio~ide in the chlorine dio~ide gas must be limited to about 100 mm of Hg tn insure stability. If the chlorine dio~ide is generated under a vacuum in the generator, i~ must be maintained under a vacuum to ensure introduction into the reaction chamber.
The pulp material should be in the range of about 20 to about 50 percent consiste~cy, and more preferably about 30 percent consistency. The pulp material can spend from about 5 seconds to about 45 minutes in the reaction chamber, preferably from about 5 seconds to about 5 minutes, and more pr.eferably from about 5 seconds to about 1 mi~ute contact time in the reaction chamber before e~iting.
The feed rate of the chlorine dioxide and ozone gas mi~ture to the lignocellulosic material or pulp can be controlled so that it can be passed through at least one time, ~nd possibly as many as two or three times, such as by the ~se of pressure differential between the upper inlet side and the lower outlet side hefore the 2s gas e~its through the reaction chamber for r~clamation~
The li~nocellulosic material or pulp can then be passed directly to an alkaline e~tractor apparatus to undergo the alkaline treatment. Although it i5 .;
prefeEred that the alkaline e~traction occur without any intermediate washing in the process of the instant invention, it is possible to use an intermediate wash prior to an alkaline treatment.
The eIeme~tal chlorine-ree C102 and oz~ne mi~ture is admi~ed with the pulp in the rea~tor which is . .
WO93/lS~ PCT/US93/~M08 at any suitable pressure including atmospheric, subatmospheric or superatmospheric pressures. In a preferred embodiment, the reactor is at subatmospheric pressures which are preferably slightly below those employed in the chlorine dioxide or ozone generator.
Optionally at superatmospheric pressures an eductor can be used which employs ozone and o~ygen gas mi~ture to dilute and to remove chlorine dio~ide from the generator and supply it to the pulp reactor. The chlorine dioxide and ozone gas mixture is used in amounts which provide a concentration or ratio of gas mi~ture to pulp that is suitable for delignifying the unbleached chemical pulp to the desired degree, as indicated, for e~ample, by the selected Kappa number as determined by TAPPI test method T236 cm-85 and accepted by the industry.
The delignification of the pulp using the process of the invention is accomplished in surprisingly short contact times, with the del:i~nification rate bein~
independent of pulp temperatures. The temperature of ~:
the d~lignificatio~ reaction is not critical and delignification can be carried out at surprisingly low ;-temperatures, from about ambient temperatureO
Temperatures in the range o~ from about ambient to about 90~C, preferably ambient to~about 80C, ~nd more preferably, from about 20C to about 60C can be employed. At these temperatures minimal time is ;~
required.
To maximize the efficiency of the gaseous mixture ~:
of ch~orine dioxide and ozone, deligni:fication of the pulp normally is carried out u~der acidic conditions:and the acidity can be derived from~the chlorine dio~ide gas. For e~ample, in an~ delignification stage following a caustic e~traction stage, the pH of the pulp to be treated normally is adjusted so that the final pH
WOg3/152~ PCT/US93/~M08 2 1 ~
of the delignified pulp before e~traction is in the range from about 1 to about 6. Residual chlorine dioxide and ozone gas mixture can be suitably removed, such as by passing an inert gas through the pulp and the gaseous mi~ture can then be passed through a catalytic converter to reclaim oxygen which can be sent to the chlorine dio~ide genera~or or to the ozone generator or to both for further use..
An e2traction stage (E) is then carried out without an interv~nin~ washing 5tep with, for e~ample, caustic soda to solubilize the higher molecul~r weight ~:
oxidized lignins, tv-hydrolyze organic chlorides to salts, etc. Where chlorine dio~ide has not been removed prior to extractin~, a suitable reducing ayent can be lS added with or prior to adding th~ e~tracting agents, such as sodium thiosulfate, sodium hydrosulfite, sodium sulfide, sodium sul~it~, hydrogen p~ro~ide, or csmbinations thereof.
The extraction is conducted at temperatures of 6~ to about 90~C., and a residence time of about 60 minutes where the final:pH is 10.5 or higher. The :~
e~traction stage may include the addition of o~ygen, a pero~ide such as hydrogen pero~ide, or hypochlorite, or a combination thereof for enhanced performance. The extracted pulp is filtered and washed to remove ~ater soluble lignins and low molecular weight organic chlorides, among others.
After the initial deligniication and extraction using the process of the present invention, the 30 delignified chemical pulp has a Xa~pa number in the ~:
range of from about 1 to about 10, and preferably from - about 2 to about 8.
The e~tracted pulp may be subsequ~ntly further delignified, for e~ample, using the process of the W093/152~ PCTJ~S93/~0~
~ 1 ~ 3 ~ ~ 8 zo--invention or any known bleaching stage to achieve the final brightness desired.
It has been discovered that the novel process of the present invention reduces the formation of toxic by-products organic halides. These undesirable by-products, collectivel~ identified a5 adsorbable organic halides (AOX), reguire effluents from pulp bleaching processes to undergo expensive treatment m~thod~ to eliminate AOX prior to discharge to sewers.
1~ The efflu~nt dischar~e from the bleachin~ process-of the present invention has less than about 2.0 kg adsorbable organic halides (AOX~ per ton of wood pulp.
AOX, as the term is used herein refers to the sum of the AOX, i.e. adsorbed organic halides, of the several stages of bleaching and alk~line e~traction with or without washing~ The pulp produced by the present in~ention has a brightness and visco~ity that meet or exceed the requirements of :industrial papermaking. ~:
To further illustrate the invention the following ~0 e~amples are ~rovided without any intention of being limited thereby. All parts and percentages are by wei~ht unless otherwise speciied and temperatures are in degrees Celsius.
~.XA~LPLE 1 -SIMU~T~NEQU~ AD~ITIQ~ OF ~ASEOUS MIXTURE
CHLORIN DIÇ2~IDE AND O~ONE
~. . , To a jacketed reaction vessel containing a gas distribution plate is added lOO gram$ of oven dried fluffed softwood pulp having a consistency of 35%. The 3~ initial Kappa No. of the softwood pulp is 30~0, but not certainly confin~d to the Kappa number mentioned. The W0~3/1~264 PCT/US93/~8 2~ 2,'~4~8 temperature of the pulp is maintained between 25C to 65C. The pulp reaction vessel is connected to two gas scrubbers, each of which contains a 10% svlution of potassium iodide. The chlorine dio~ide generator is initially filled with about 1500 grams of perchloric acid containing about 40 percen~ by weight of HC104, the perchloric acid contains about 15 grams of ruthenium dioxide, Ru02. A gaseous mi~ture of chlorine dioxide, o~ygen and water vapor is generated by fe~ding a chloric acid solution containing about 37.16 percen~ by weight of HC103~ During chlorine dioxide generation, the generator is maint~ined at a ~emperature between about 70 and 73C by regulating the feed rate of the chloric acid, a~d by providing sufficient heat of reaction to evaporate the water present in the chloric acid solution and that f~rmed durlng the cJeneration of chlorine dio~ide. The ozone reactor is a commercial reac~or using o~ygen as the f~ed gas for ozone generation. The ozone generator is maintained at temperatures between 10C and 25c. Two separate gas lines, independently carrying ozone and chlorine dioxide from the respectiYe generators are connected to a small mixing chamber prior :~
to entering the bottom of the reaction vessel. Th~n the - gaseous mixture is distributed into the pulp through a perforated sparger. The softwood pulp is treated for a time between 1 ~ 5 tninutes with the gaseous mi~ture.
Each delignified pulp sample is equally divided, one `~
half to be washed and the other not washed prior to -e~traction (E) . Where washing is employed the filtrate and the wash water are collected. E~traction o the - deli~nified pulp is carried out by adding the indicated amount of about 3.5% NaOH b~ wei~ht on pulp of a 10 percent sodium hydro~ide solution. The operating ccnsistency of pulp is abo~t 10 percent. The pulp is WO~3/152~ PCT/~S93~0408 212g~8 held at about 70C. for about 60 minutes. The extracted pulp is dried, washed and the Kappa Number is determined to be less than 10. The absorbable organic halides (AOX) in the combine~ solution mixture of the bleach filtrate, wash water, e~traction liquor and wash water is determined to be less than 1.0 kg/metric ton of pulp.
EX~M~LE 2 S~n~E~TIA~_~DDIrIO~ OF
GASEOU~ G~O~INE DIOXIDE ANP OZONE
A continuously stirred hot water jacketed reaction vessel containing ~ gas distribution device is loaded with 100 gm of oven dried shredded and fluffed sotwood pulp having a consistency of about 35 percent.
~he reaction vessél can act as an equipment vessel for ;~
rapid mi~ing of gas with pulp as well as a retention vessel for the continuation of the delignifying process after mi~ing the gases with~the pulp. The initial Kappa number of the pulp is about 30Ø The initial pulp temperature is at ambient conditions, but warmed up to about 80C and preferentially maintained between about 40C and about 6QC.
The outlet of the reactor i5 connected to a series of potassium iodide traps containing 10 percent KI which are acting as gas scrubbers for both the chlorine dioxide and ozone exiting the reaction vessel.
Betwee~ the reaction vessel and the gas~scrubbers, the catalytic convertor is employed to remove the or~anics carried over by the e~iting gases and also to remo~e the moisture from the oxygsn which was the carrier gas for both chlorine dio~ide and ozone. Two sepaxate gas lines, independently ca~rying ozone and chlorine dioxide W~93/1S2~ 2 1 ~ PCT/U~g3/~M08 from the respective generators are connected to the bottom of the reaction vessel and distributed into the pulp throu~h a perforated sparger. ~ach line has a gas metering device to measure accurately the gas flow to the reactor. The chlorine dio~ide gas line is also using a spectrophotometric device to measure the concentration of chlorine dio~ide flowing to the reactor. The ozone supply line has a concentrati~n measurement and monitoring unit utilizing W
spectrophotometr~ to measure the conc@ntration o ozone~
in the carrier yas. A feedback co~trol unit is used to adjust the o~ygen feed to the ozone generator.
The gases can be fed sequentially either by charging ozone first and the~: chlorine dioxide and vice 15 versa. In the event of chl~.ine dioxide being fed first ~-to react with pulp, the initial pulp pH has a range from 2 to about 8. On the other hand, if ozone is the gas first to rea~t with the pu1p, then the pH of the pulp is adjusted by using an acid, preerably sulfuric acid.
The pH of the pulp ranges from ~ to 5, but preferably less than 3. Also in cas~ of chlorine dioxide being charged first to the pulp, the exit gas is taken directly to the ~I traps by-passing the catalytic ~onvertor. The chlorine dio~ide feed to the pulp is 2~ calculated on the basis of the Kappa facto:r, a measure of chemical charqe deter~ined by the Rappa number of the pulp. The ~appa factor is in the range of 0.5 to ~Ø
The time for the reaction is varied from one minute to five mi~utes, but preferably less than about 2 minutes.
The ozone charge on the pulp is varied from 0.l pereent to 2.0 percent. The reaction time is varied from one minute to five mi~utes0 but pr~ferably less than about two minutes. The reactio~ can be carried out in subatmQspheric, atmospheric, or superatmospheric WO93/lS~ ~ PCT/~S93/~
-2~-pressure. The pressure range is 10 mm of Hg to 760 mm Hg to 1520 mm Hg, prefer3bly between 100 to 1520 mmHg.
Each delignified pulp sample is equally divided, one half to be washed and the other not washed prior to e~traction ~E). Where washing is employed, the filtrate and the wash water are collected. Extraction of the deliynified pulp is carried out by adding the indicated amount of about 3 . 5 percent sodium hydro~ide by weight on pulp o a 10 percent sodium hydroxid~ solution. The 10 op~rating consistency of pulp is about 10 p~rcent. The pulp is held at about 70C ~or about 60 minutes. The e~tracl~ed pulp is dried, washed and the Kappa numb~r is determined to be less than LOo The adsorbed organic halides ~OX~ in the combined solution mi:~:ture o the 15 bleach filtrate, wash water, e~traction liquor and wash water is determined to be less than 1. 0 kg~metric ton of pulp.
The pulp f rom the ~traction st~p is treated with one or more bleaching se~uences involving chlorine ;~G dio;ll:ide in aqueous or gaseous form, alkal~ne e~tr~ctior with sodium hydro:~ide reinforced by o~ygen and~or hydrogen pero2cide- and a gaseous mixture of chlorine ~:
dio~ide and ozone and combinations thereof to obtain pulp of required brightness and: strength. The adsorbed 25 organic halides (AOX) in the later bleach plant e~f luent f rom the bleaching sequences are fourld to be less than 0 . 3 . ky~metric ton of pulp .
While the invention has been described above with refereFlces to specific embodiments thereof, it is 30 apparent that many challges, modifications and variations ~:
in the materials, arrangements of parts and æteps can be made without departing f rom the inventive concept disclo~d herein. Accordingl~r, the spirit and broad W~ 93/15264 PCr/lJ~ig3/Ot~8 2~
scope of the appended claims is intended to embrace all suc:h c:hanges, modif ications and var$ations that may occur to one of skill in the art upon a reading of the di sc losure .
2128~8 ~AS PHASE DELIGNIFICATION OF ~-LIGNOCELLULOSIC MATERIAL
The present inv~ntion relates to a process for sequential or simultaneous gas phase delignifica~ion of lignocellulosic material for use in paper making which ~:~
produces delignified cellulosic material having reduced s absorbed organic halides (AOX~.
Chlorine-based chemicals, such as chlorine, chlorine divxide and hypochl~rite have been used in pulp ~:
bleaching for several decades and continue to be used for removing lignin and ~leat,hing the pulp to high 10 brightness. In seneral terms, the extent of ~
delignification and bleaching (the degree o bri~htness ~-obtained) is ~etermined b~ th~ typ~ of pulp being :
delignifi~d and bleached and~the proposed end use of the paper. In all:instances, however, where chlori~e-based 15 delignifying and bleaching agents:have been employed in ~
the prior art, there are produced c~lorinated: organics. -:
These compounds ar~ ~o~ic~and are present:in the -:
discharge e~fluents from one or more of the sta~es ~f the paper making process ~eg. bleaching stage).
relati~ely smaller percentage of such chlorinated organics remain in the pulp and eventually appear in the paper product.
In the past decade, there has been a ~rowing concern about ~he environmental impact of the 25 chlorina~ed orsanic compounds pre ent in paper and ln ~`
discharg~ efluents. U~desirable chlorinated org~nics such ~s diogin have been detected in the e~haust gases W~93/tS2~ PCT/U~93/~
2128~
of incinerators burning municipal wastes includin~ for e~ample paper products made from chlorinated bleached pulps. Thus, it is desirable to limit the absorbed organic halides (AOX~ discharged in the efluent per ton S of wood pulp to less than 2.0 kg.
Several options have be~n proposed or practiced to reduce or eliminate chlorinated organics in the b~eaching process. The most straigh~forward method i5 to substitute non-chlorine bleaching chemicals such as o~sen9 pero~ides, oæone, perace~ic acid, etc. for chlorine-based bleaching chemial~ O~ygen delignification and bleaching is now a well-established commercial reality in pulp bleaching technology. Its use is restricted to partially remove lignin under alkaline 1~ conditions - further bleachiLng must be done to remove the residual liqnin color b~aring groupsr While whitening the pulp, great caution should be exercised to a~oid ~he degradation of th~ carbohydrate frac~ions.
When the complete remo~al of lignin is ac~omplished by 20 oxygen, the pulp strength suff~rs bec~use of ~:
unacceptable destruction ~f the celllllose.
Hyaro~en pero2ide is widely used to reinforce the alkaline e~traction stages which are carried out after the chlorination and ~hlorine dioxide stayes, but so ~ar 2~ neither o~ygen nor a combination of o~ygen and hydrogen pero~ide could match the efficiency of chlorination.
Ozone because of its high o~ida~ion potential should offer a better alternative, but its uses are limited because it must be used in the gaseous state and 3~ ozone generators produ~e only low concentrations of ~.~
ozone in oxyyen or air. Ozone has been used alone as a :.
sin~le stage ble~chin~ agent, or in a multi-stage bleaching process which may include treatment with other ~leaching agent~ such as oxygen, pero~ides, chlorine WO~3/152~ ~. 2 3~ CT/US93/0~08 dio~ide and hypochlorite. The fundamental bleaching variables affecting the use of ozone include (a) the type of pulp (hardwood or softwood), (b~ the pulpiIlg process used to produce the pulp, ~c) pre-treatment of the pulp p~ior to ozsnization, (d) sequential treatme~t of ozone and other bleaching chemicals, (e) pulp consistency, (f) ozone concentration and charge, (~) temperature, (h) time, and (i~ pH. ~ut it was ound `~
that o~on2, as a stron~ oxidant, tend~d to attack and 10 degrade cellulose as is the case with o~yg~n. In l:he later instance, the degradatiorl of cell~los~ is minimized l:)y using a magnesium salt as a protector.
However, it was determined that the amount of the ozone charge and the position of the ozone treatment in the 15 bleaching process were imporl~ant.
Another option to reduce the dis harge of chlorina~ed organic compounds is to reduce the c:hlorine usage in the first stage of ~he bleaching process. Two ~
alter2latives that produc:e no signif icant degFadàtion of ~:
20 pulp properties have b~en co~unercialized for this purpose. These are (a) extended delignification in the cooking stage and (b) o~l:ygen delignificatiorl. These alternatives, with proper e~traction, reduce the chlorirlated organic compounds in the bIeached pulp and 25 ensuin~ effluent to sufficiently low lev~ls.
A third option to reduce the generation of chlorinated organics in a bleaching process is to su~stitute chlorine dio~ide for chlorine. Chlorine dioxide.is a relatively strong o~idant compared to 30 chlorine. To ~chieve the same degree of delignification, it requires only about thirty eight weight percent chlorine dio~ide on th~ pulp compared with one hundred weight p~rcent of chlorine. Howev~r, th~3se pro~esses are of the DEDE~ type wherein the ~-W0~3/~52~ PCT/US93/~M~
~ 1 2 8 ~ 8 chlorine dio~ide stage (D) is followed by ~he conventional e~traction (E~ and additional chlorine dio~ide (D) stages. The pulp and discharge effluents resulting from the processes of this option contain higher concentra~ions of chlorinated organics than are acceptable and/or desira~le. Processes using both o~ygen delignification and chlorine dio~ide substitution have been suggest~d, but do not achieve the desired and/or regulated concentrations o chlorine containing re~idues in either the pulp or the effluent.
Chlorine dio~ide and ozone have found wide uses as disinfe~tants in water treatment~purification applications, delignifyin9 and~or as a ble`aching agent in pulp and paper production, and in a number of other u~es because of their high o:~idizing power.
Ozone is used as a delignifyin~ chemical to remove or reduce the lignin from the lignocellulosic material 9 but because of its high o~idizing power, it is indiscriminatory in its tendlency to attack both lignin and cellulose thereby producing inferior pulp.
Chlorine dio~ide in solution has been used to brighten ~nd produce a cl an pulp in pap r manufacture.
However, it has generally been felt that the use of ~:~
liquid chlorine dioxide in the first stage of a multiple stage bleaching process is not as effective as is using chlorine as a delignifying ~gent. However, the use of elemental chlorine has the undesirable e~fect of producing organo-chlorine compounds, which are ~
measur~le as adsorbable or~anic halides (AOX), in the :;
3~ effluent steams of processing facilities. M~nc~, as efforts have continued to focus o.n ways to ~liminate or substan~ially reduce AO~ discharge, chlorine dio~ide has continued ~o be a popular agent in pulp processing, but as a brightening agent.
3/1S264 PCr/US93/0~408 2128~0~
There are a number of chlorine dio~ide generator systems and processes avai lable in the marketplace, Most of the very large scale general:ors utilize a chlorate salt, a chloride ion source or reducing agent, 5 and a strong acid. In the presence of chloride ion and acid, chlorate ion reac:ts. to produce a mixture of chlorine and chlorine dio~cide. The chlorine present in these processes is an undesired by-produrt whic:h adds to the P~OX concentrations in discharge f low streams .
Many proces~es have been developed to produce chlorine dioxide with lower clhlorine coIlcentrations by adding a reducing agent. Some reducing agents which have been used in these applications include nnethanol or other organis compounds, sulfur, sulfur dioa~ide or other 15 sulfur-o~ygen species havin~ a sulfur valence of less than +6, and carbon mono~id~. When organic co~npou~ds are used, unreacted volatile organics, ir~cluding organic acids, are present in the product gas. Using sulfur containing reducing a~ents, the sulf ate or sulfuric acid produced accumulates as a waste product. When gaseous reducing agerlts, such as sulfur dîoacide or carbon mono~ide, are elrployed, reactor designs and process control systems must protect against unreacted reducing agent leaving ~he system with ~he ~hlorine dioxide gas.
Chlorine dio~ide has been produced in prior art :~
pros::esses rom chlorate salts by the addition of an excess of the acid used. While this acid is slowly neLItralized by the accumulation of alkali metal ions that e~t-er the process with the rhlorate salt, the accumulation of salts must be removed as a waste stream. This waste stream is either liquid or solid in every process currently practiced conunercially.
Th~ pr~paration of chlorin~ dio:~ide f rom chloric ~-:
acid has been accomplished to avoid the formation of an WO ~3/15~64 PCl[/US93/D0408 ~12~
acidic alkali metal salt. Chloric acid is, hc~wever, not commercially available, although its preparation has been taught in U. S~ Patent 3, 810, 969 issued May 14, 1974 to A . A . Schlumber~er . Schlumbergex teaches a process for producing chloric acid by passing an aqueous solution containing f rom O . 2 gram mole to 11 gram moles per liter of an alkali metal chlorate such as sodium chlorate through a selected cationic e~change resin at a temperature from SD to 40v C:. The proc:ess produces an aqueous solution containing ~rom O . 2 gxam mole to about 4 . O gram moles of HC103 per liter.
K.I,. Hardee et al, in U.S. Pat nt No. 4,798,715 issued Jan~ 17 ,1~89, describe a process to produce chlorine dioxide by electrolyzing a chloric acid 501Ut:Lon produ~ed by p~sing an aqu~3ous ~olution of ~n alkali metal chlorate through an ion ~:~cchange resin.
The electrolysis is c~rried ou~ using an electrocatalytic c:athode wh~sre the catalyst is, for e~ample, one or more valve metal o~ides which may be 2~ combined with a platinum groulp m~tal o~ide, or a :-platinum group metal, or oxides of a platinum group me~al, ma~rletite, ferrite, or mi:~ed metal o:~:ides.
The electrolyzed solution contains a mi~ture of chlorirle dio~:ide and chlori~ acid, which is fed to an extractor in which the chlorine dio;~:ide is stripped ~:.
of f . The ion e;~c~ange resin is r~gener~ted with -~
hydrochloric acid and an acidic solution of an alkali metal chloride formed. Such processes require the regener~tion Qf the ion e~change resin with acid to remove the alkali metal ions and the use or treatment and disposal of the acidic salt solution.. A150 the concentration of chloric acid that can be produced by an ion ~:cchange process is limited since more concentra'ced chlori~ acid solutions attack the ion exchange resins . ...
W~ ~3/15264 PCr/US93/~
2 1 2 ~ J ,~
used in the process. Lastly, the production of chloric acid by means of a cation e~change resin is not economically attractive.
Commercial ozone generators produce ozone by a S continuous high voltaqe electric discharge in presence of an o~ygen containing feed gas. The ozone generator contains a stainless stesl tube and a shell heat e~changer. High volta~e alternating current is imposed on a hîgh voltage ele~trode consisting of a permanent 10 stainless steel tub2 on one side of a dielectric .
material. The diel~ctric glass tube c~ntered in the stainless steel tube which has a ground electrode allows the electric discharge to occur as corona discharge across the.dischar~e gap between the dielectric and the ground electrode. As the o:gygen containing feed gas flows through the "corona", a portion of the oxygen is convert~d to ozone. Since ~much of the electrical ~nergy supplied to orm ozone is lost as heat, an efficient method of heat removal is incorpvrated into the desiqn o the generator.
G~s phase bleaching with chlorine or chlorine dio~ide has been proposed over the years in various p~tents and publications as a method of reducing the bleaching time whi~e cutting chemical costs. Gas phase 25 bleaching is carried out on higher consistency pulp ~;
using mi~tures of chlorine diogide and steam and/or inert gases such as air or nitrogen.
U,S. Patent No. 3,725,1~3, issued April 3, 1973 to R~; DeMonti9ny et al., describes a process for bleachi~g hi~h consistency pulps, which includes preheatin~ the pulp by direct steaming. A gaseous mixture vf chlorine dio~ide diluted with st~am or a non-reactive gas is then passed through the pulp. The contact period is in the order of a fraction of a WO g3/t5264 PCI/U~i93/0~ 8 3 l~ Q (~3 -8-second. The bleached pulp was then held in a retention vessel for 30 minutes. Vnreacted chlorine dio~ide was removed from a bleaching tower by aeration. The final pH of the bleached pulp was ~.2.
Ozone treatment of pulp is practiced under three pulp consistencies. They are ultra low consistency treatment, medium consistency treatme~t and high - consistency treatment. Hi~h co~sistency treatment of pulp inv~lves th~ use of gaseous 020ne applications.
Advantag~s alleged or gas phase bleaching of high consistency wood pulps includ~ superior control of bleaching because of the short retention times employed;
reduced chemical usage for t:he ~ame brightness; and low water usage and effluent volume. The g3s phase 15 bleaching of pulp i5 well documented, but no in~ormation :;
is available on the use of 51aseous chlori~e dioxide and ozone either s~quentially or simultan~ously for the deligniication of pulps. Both chlorine dio~ide and oæone are str~ng ogidizing agents, but their ability to ;~
complement one another in the d~liynifi ation process has been not e~plored.
The lack of a process for g~nerating chlorine dioxide gas instantanevusly or by a process having inconsequential start-up and shut-down times has ~loc~ed its successful use commercially of gas phase bleaching.
A process has be~n found that produces chlorine dio~idP
gas which can be used directly from the 9Bnerator without requiring the formation and storage of dilute aqueous solutions of chlorine dio~ide which are 30 subsequ~ntly stripped or which do not have significan~ :
concentrations of elemental chlorine ~as that damage the celiulose. After deli~nification there is substantially no residual chlorine dio~ide re~uiring r~coYery or disposal. Also, unknown in the industry is the method WO g3/1~26~ ~Cr/US93/00~8 212~
g of mi~ing the chlorine dioxide with another gas like o~one to get the powerful gaseous mi~ure having ~-superior and selectiYe o~idation ability to delignify pulp .
A recent process to reduce the AOX discharges from pulp mills has been disclosed by ~he Amcor Research and Technology Center in ~ustralia which uses an ogygen pretreatment step to passivate the pulp and enhance the reactivity of liquid chlorine dioxide applied in a first ~:
stage delignification operation, followed imm~diately by an alkaline e~traction stage withou~ an intermediate washing step. This process substantially reduces the AOX discharge, but still requires the pretr~atment step. There e~ists the need for a simplified process to directly delignify lignocellulosic material.
Canadian patents number 2,032,31S; 2,031,84B; and 2,031,850 teach the sequential addition of aqueous chlorine dio~ide and ozone at poînt~ s~parated from one another and each being at at least one place in the line and this stage being free ~om intermediate washing ~etween places for addition. Heterogenous sequential treatment of pulp with a~ueous solutions of ozone and chlorine dio~ide require repeated filtering and thickening of pulp stvck to adjust for the various ~s consistency requirements of each treatment stepO ~:
These probl~ms are solved in the proeess o~ the present invention by using gaseous chlo~ine dio~ide and ozone sequentiall~ or simultaneously to delignify and bleach- pulp and ther~by reduce the adsorbed organic -~
30 halides (AOX) in ~he delignif ied and ~leached pulp and discharge efflu~nts. A further reduc'cion of AOX is obtained by conducting a direct alkaline e~tr~ction without intermediate washing stage after the blea~hing st~p .
W0~3/152~ PC~/US93/~0~
~1~8'10S
It is an object of the present invention to provide an improved process for delignifying lignocellulosic material.
Another object of the present invention is to mix the chlorin~ dio~ide gas from the state of the art generation processes with an ozone o~ygen gas mixture to get a highly reactive chlorine dioxide and ozone ga~
mi~ture.
I~ is another obj~ct of the present inve~tion t~
provide a first stag~ tr~atment with a gaseou~ mixt~re of chlorine dio~ide and oæon~ which dramatically reduces ~:
the AOX discharges.
Another feature of the present invention is that any residual elemental chlo:rine formed in the chlorine dioxide generation is o~idized by mi~ing and reacti~g it with ozone, thereby ~liminating th~ chance~ of chlorine to react with pulp to generiate AOX~
Ano~her object of thle pre~ent inventio~ is that any residual elementa1 chlorine formed in the reaction ~o between chlorine dio~ide and lignocellulosic material is destroyed by the reaction of liqnocellulosic material with ozone, thereby reducing ~he chances for the formation of ab~or~able or~anic halidPs (AOX~.
Another object of the present invention is that 2s gaseous chlorine dioxide added to the lignocellulosic material acts as an acidic pH buffer of pH 2 - 5, the pH
range conducive for ozone delignification.
Yet another object of the invention is that chlor~ dio~ide addition to the pulp inhibits the 3~ carbohydrate desradation reactions of ozo~e which are responsible for the strength loss of the pulp.
It is~an ad~an~age of the present inv~ntion that lower AOX concentrations are achieved with pulps having higher pH ~alues.
WO 93~15264 ~ 1 2 ~ ~ O ~ PCr/USg3~0~08 It is still another adYantage of the present invention that a clean pulp material is o~tained in less ``
time and at lower cost in n~eded equipment when compared with conventional processes.
These and other objects, features and advantages are obtained by an improved process for d~lignifying lignocellulosic material which comprises tre~tin~ the lignocellulosic material se~u~ntially or simul aneously with gaseous chlorine dio~ide and ozone and then transportin~ it into a reaction chamb~r where it is retained for a short period of time-~o complet~ the reaction and delignification of the lignocellulosic material. Then an alkaline extraction is performed.
The delignified material is then separated from th~
effluent. This effluent contains less tha~ about 1.0 kg absorbable organic halides ~AOX) per ton of lignocellulosic material. ~-Suitable p~lps which can be bleached incl~de any of those which are commonly used such as chemical kraft, sulfite or mechanical and recycle pulps. Pulp having any suitable consistencies may be delignified including those of about lS percent or higher, for e~ample, from about 25 to about 60 percent can be treated by the process of thP present inventionO The pulps are 25 preferably shredded or fluffed.
Chlorine dioxide gas used as a reactant in the process of the prese~t invention preferably contains low concentrations of elemental chlorine and prefera~ly small amounts of inert gases, such as nitrogen or air.
30 Suitable c~lorine dio~ide gas used in delignifica~ion of pulp is a mi~ture o~ chlorine dio~ide, gaseous oxygen, - and water vapor. Partial pressure of chlorine dio~ide in the gas stream i~ between 10 to 100 mm of Hg. The chlorine dio~ide gas has a low elemental hlorine W0~3/~52~ P~T/~S93/~08 ~ 8 -12-content of less than about 18 percent. The chlorine dioxide gas can also ~e substantially free of chlorine, preferably less than about 2 percent by weight, and more preferably less than a~out 1.5 percent by weight based on the weight of chlorin~ dioxide in the gas. The gaseous mi~ture contains varying concentrations of oxygen and water vapor. The amount of o~ygen can be independently varied to provide molar ratios Qf 2 to ClO by volume of from about 1:4 to about 50:1. Wat~r :~
vapor or steam is preferably the primary diluent in the gaseous mi~ture and replaces other.inert gases which are normally used. As a diluent, steam supplies heat to the pulp and minimizes cooling by evaporation which occurs when, for e~ample, air is used as the diluent.
Gaseous mixtures containing chlorine dio~ide suitable for use in the novel process of the present invention preferably can be produced from co~centrated solutions of chloric acid.
Conversion of the chl~ric acid to chlorine dio~ide can be accomplished~ for example, by cathodic reduction in an electrolytic cell or by contac~ing the chloric acid with a suitable catalyst in the pres~nce of heat. In a~ alternative proce~s, highly acidic aqueous solutions containing chlorate ions, including chloric acid, perchlorate ion~ and hydrogen ions are reacted to produce a gaseous mixture of chlorine dioxide, oxygen and water vapor (steam). The gasev:us mixture has low concentrations of inert gases, such as air or nitrogen, and yet provides reduced risks to spontaneous explosions 3~ or "pufs". The aqueous solutions have an acid concentration of at least 2 molar, preferably at least 3 molar, and a concen~ration of chlorate ions of at least 0.02 molar and pref~rably from about 0.1 to ~bout 3 molar. These acidic solutions preferably are ~93/1~264 PCT/US93~0040~
2l2s4as substantially free of ionic impurities such as chloride ions, alkali metal and alkaline earth metal ions.
The hydrogen ion concentration from acid in the aqueous solutions may be provid~d by non-o~iaizable 5 inorganic acids such as sulfuric acid, phosphoric acid, chloric acid or perchlori.c acid. Preferably, the acidic solutions are reacted in the presence of a catalyst ~:
which promotes o~ygen evolution. ~:
Suitable as o~yg`en-evolYin~ catalysts are, for 10 example, metals and oxides of the elements of Group :~
VIIIA of the Periodic Table of Elements. Thus metals such as the platinum group metals including platinum, palladium, iridium~ rhodium or ruthenium; and mi~tures or alloys of these platinum group metals may be ~:
employed. Additionally o~ides of platinu~ group metals such as iridium, rhodium or ruthenium, as well as mixtures of these o~ides with platinum group metals or alloys of these precious m~tals could b~ suitably employed. Likewise, iron alloys such as stainless steel, nickel or ni~kel bas~d alloys, and cobalt based alloys can be used as v~ygen-eYolving catalysts in the process of the invention. Qther o~yg n-evolving catalysts include semiconductiv ceramics known as perovskites. To suppress or minimize the auto-oxidation of chloric acid to perchloric acid, for e~ample where an oxygen-evolving catalyst is employed, it is preferred to use, as the source of chlorate ions, a mi~ture of chloric acid and a non-o~idizable inorganic acid in which t~e concentration of chloric acid is low, for example~ less than about 20 percent by weight of the aqueous solution providing chlorate ions. These processes for generatin~ chlorine dio~ide can be started up in a few minutes and similarly stopped in a short W093/152~ P~T/USg3/~08 OY!
time so that storage o C10~ gas or dissolution in water or a solvent is not required.
High purity concentrated c~loric acid solutions are produced by the o~idation of hiyh purity hypochlorous acid solutions. One process suitable for producing the chloric acid solutions heats a hypochlorous acid solution, containing from about 35 to about 60 percen~ by weight of HOCl, at a temperature in the rang~ of from about Z5 to about 120~ C. Another process for producing the high purity chloric acid solution utilizes anodic o~idation of the high purity concentrated hyp~chlorous acid solution in an electrolytic cell having an anode compartment, a cathode compartment, and a cation e~change membrane separating the anode compartment from t:he cathode compartment.
In operation, this pr.ocess includes feeding an aqueous solution of hypochlorous acid to the anod~
compartment, and electrolyzing the aqueous solution of hypochlorous solution at a temperature of from about 0 to about 40C~ to produc~ the chloric acid solution.
Chloric acid solutions can be produced by these processes in any concentrations desired up to about 45%
by weight of HC103. ~owever, preferred con entrations are those in the range of from about 30 to about 40% by ~eight o HC103.
Generation of t~e chlorine dio~ide gas from the reaction mi~ture containing chloric acid and the non-oxidizable inorganic acid is preferably conducted at atmospheric pressure in the presence of a diluent gas, such as oxygen, nitrogen or air. The temperature of the reactor mi~ture should not e~ceed 120C, more preferably not more than about 90~C, to avoid d~compositiQn of the chlorine dio~ide in th~ gas phase. Where subatmospheric pressures are utillzed, for e~ample, pressures can be in WO931152~ PCT/US93/~OB
212~0~
the range of from about 15 to about 750, preferably from about 100 to about 500, and more preferably from about 150 to about 300 mm of Hg. At these pressures the temperature of the reaction mixture is in the range of from about 40C to about 90C, and preferably from about 50C to about 80C. The temperatures and pressures for subatmospheric pressures are selected to maintain continuous boiling o the reaction mi~ture.
The chlorine dio~ide gas generator must be in water ba~.ance to operate in steady state conditions.
The rate of heat addition to the ge~erator determines the guantity o~ water that is evaporated to maintain steady state concentration, it is necessary to evaporate water continuously at about the same rate at which water 5 i5 added~ with the chloric alcid feed solution adjusted for water formed by r~actiorl of the chloric acid.
Alternately, chlorine~ dio~ide gas can be obtained from any commercially available generators directly producinq gaseuus chlorine, or by removing the gas prior 20 to going to an adsorber for liquifaction, such as tho~e available from Albright and Wilson Americas under the model names R-8, R 9 and R-10, or from other suppliers of similar commercial equipment.
Ozone gas can be obtained from any commercially 25 available generators producing ozone by direct electric discharge th~ough air or o~ygen. The concentration of ozone from the reactor should be not less than 6 percent by weight on the o~y~en feed, but not limited to or not exceeding 13 percent b~ weight of o~ygen feed, the point 30 of self detonation of ozone. PreferabIe concentrations are from about 6 to 8 percent. The ozone gas ~rom the reactor may be taken to a mi~ing chamber where it may be used to mix the chlorine dio~ide gas from the novel generation process d~scribed above. Chlorine dio~ide ~93/152~ P~T/~S93/~08 2~2~0~
can also be obtained from state of the art genera~ors such as the R-8 generator or f rom other generators supplied by similar co~mercial equipment vendors.
The process of the invention can be practiced using any suitable prior art apparatus such as a conventional bleaching tower.
Delignification of pulp may be affected in the :~
initial stages of a multi-stage bleaching process, as well as in any D or DZ stag~s. The prvcess of the invention requires a rapid mixing and short retention in an apparatus similar to the existin~ first stage equipment capable of mi~ing a mi~ture of gaseous chlorine dio~ide and ozone w:ith lignocellulosic material. The delignification of unbleached lignocellulosic material or ]pulp is accomplished with the use of a gas~ous mi~ture of chlorine dio~ide and ozone produced by one of the aforementioned processes and simultaneously charging the ~a~eous mi~ture to the pulp or sequentially charging the individual gases to the pulp. The gaseous chlorine dio~ide has low concentrations, or is substantially ree, of elemental chlorine and is combined with a direct alkaline e2traction st~p in the absence of an intermedia~e washing ~o redu~e the AOX concentrations in the discharge stream. The delignification of unbleached lignocellulosic material or pulp may be carried out in any suitable eguipm~nt in whi~h the unbleach~d pulp can be contacted with the chlorine dio~ide gas. For pr~ctieal reasons any ~e~ctor should be gas tight~
Suitable gas tight equipment includes, for e~a~ple, agitated mixers, stati mi~ers, ri~bon bl~nders, steam chests, high consistency shear mi~ers, ~ pumps, MC
mixers, high v~locity pipe lin~s, fluffers, etc. ~.
WO93/lS2~ P~T/~Sg3J~8 212~8 Pressure requirements for the feed of the gaseous mixture of chlorine dioxide and ozone to ensure stability ar~ ~ritical, especially if the gaseous mixture is being introduced into a vacuum chamber. For example, partial pressure of the chlorine dio~ide in the chlorine dio~ide gas must be limited to about 100 mm of Hg tn insure stability. If the chlorine dio~ide is generated under a vacuum in the generator, i~ must be maintained under a vacuum to ensure introduction into the reaction chamber.
The pulp material should be in the range of about 20 to about 50 percent consiste~cy, and more preferably about 30 percent consistency. The pulp material can spend from about 5 seconds to about 45 minutes in the reaction chamber, preferably from about 5 seconds to about 5 minutes, and more pr.eferably from about 5 seconds to about 1 mi~ute contact time in the reaction chamber before e~iting.
The feed rate of the chlorine dioxide and ozone gas mi~ture to the lignocellulosic material or pulp can be controlled so that it can be passed through at least one time, ~nd possibly as many as two or three times, such as by the ~se of pressure differential between the upper inlet side and the lower outlet side hefore the 2s gas e~its through the reaction chamber for r~clamation~
The li~nocellulosic material or pulp can then be passed directly to an alkaline e~tractor apparatus to undergo the alkaline treatment. Although it i5 .;
prefeEred that the alkaline e~traction occur without any intermediate washing in the process of the instant invention, it is possible to use an intermediate wash prior to an alkaline treatment.
The eIeme~tal chlorine-ree C102 and oz~ne mi~ture is admi~ed with the pulp in the rea~tor which is . .
WO93/lS~ PCT/US93/~M08 at any suitable pressure including atmospheric, subatmospheric or superatmospheric pressures. In a preferred embodiment, the reactor is at subatmospheric pressures which are preferably slightly below those employed in the chlorine dioxide or ozone generator.
Optionally at superatmospheric pressures an eductor can be used which employs ozone and o~ygen gas mi~ture to dilute and to remove chlorine dio~ide from the generator and supply it to the pulp reactor. The chlorine dioxide and ozone gas mixture is used in amounts which provide a concentration or ratio of gas mi~ture to pulp that is suitable for delignifying the unbleached chemical pulp to the desired degree, as indicated, for e~ample, by the selected Kappa number as determined by TAPPI test method T236 cm-85 and accepted by the industry.
The delignification of the pulp using the process of the invention is accomplished in surprisingly short contact times, with the del:i~nification rate bein~
independent of pulp temperatures. The temperature of ~:
the d~lignificatio~ reaction is not critical and delignification can be carried out at surprisingly low ;-temperatures, from about ambient temperatureO
Temperatures in the range o~ from about ambient to about 90~C, preferably ambient to~about 80C, ~nd more preferably, from about 20C to about 60C can be employed. At these temperatures minimal time is ;~
required.
To maximize the efficiency of the gaseous mixture ~:
of ch~orine dioxide and ozone, deligni:fication of the pulp normally is carried out u~der acidic conditions:and the acidity can be derived from~the chlorine dio~ide gas. For e~ample, in an~ delignification stage following a caustic e~traction stage, the pH of the pulp to be treated normally is adjusted so that the final pH
WOg3/152~ PCT/US93/~M08 2 1 ~
of the delignified pulp before e~traction is in the range from about 1 to about 6. Residual chlorine dioxide and ozone gas mixture can be suitably removed, such as by passing an inert gas through the pulp and the gaseous mi~ture can then be passed through a catalytic converter to reclaim oxygen which can be sent to the chlorine dio~ide genera~or or to the ozone generator or to both for further use..
An e2traction stage (E) is then carried out without an interv~nin~ washing 5tep with, for e~ample, caustic soda to solubilize the higher molecul~r weight ~:
oxidized lignins, tv-hydrolyze organic chlorides to salts, etc. Where chlorine dio~ide has not been removed prior to extractin~, a suitable reducing ayent can be lS added with or prior to adding th~ e~tracting agents, such as sodium thiosulfate, sodium hydrosulfite, sodium sulfide, sodium sul~it~, hydrogen p~ro~ide, or csmbinations thereof.
The extraction is conducted at temperatures of 6~ to about 90~C., and a residence time of about 60 minutes where the final:pH is 10.5 or higher. The :~
e~traction stage may include the addition of o~ygen, a pero~ide such as hydrogen pero~ide, or hypochlorite, or a combination thereof for enhanced performance. The extracted pulp is filtered and washed to remove ~ater soluble lignins and low molecular weight organic chlorides, among others.
After the initial deligniication and extraction using the process of the present invention, the 30 delignified chemical pulp has a Xa~pa number in the ~:
range of from about 1 to about 10, and preferably from - about 2 to about 8.
The e~tracted pulp may be subsequ~ntly further delignified, for e~ample, using the process of the W093/152~ PCTJ~S93/~0~
~ 1 ~ 3 ~ ~ 8 zo--invention or any known bleaching stage to achieve the final brightness desired.
It has been discovered that the novel process of the present invention reduces the formation of toxic by-products organic halides. These undesirable by-products, collectivel~ identified a5 adsorbable organic halides (AOX), reguire effluents from pulp bleaching processes to undergo expensive treatment m~thod~ to eliminate AOX prior to discharge to sewers.
1~ The efflu~nt dischar~e from the bleachin~ process-of the present invention has less than about 2.0 kg adsorbable organic halides (AOX~ per ton of wood pulp.
AOX, as the term is used herein refers to the sum of the AOX, i.e. adsorbed organic halides, of the several stages of bleaching and alk~line e~traction with or without washing~ The pulp produced by the present in~ention has a brightness and visco~ity that meet or exceed the requirements of :industrial papermaking. ~:
To further illustrate the invention the following ~0 e~amples are ~rovided without any intention of being limited thereby. All parts and percentages are by wei~ht unless otherwise speciied and temperatures are in degrees Celsius.
~.XA~LPLE 1 -SIMU~T~NEQU~ AD~ITIQ~ OF ~ASEOUS MIXTURE
CHLORIN DIÇ2~IDE AND O~ONE
~. . , To a jacketed reaction vessel containing a gas distribution plate is added lOO gram$ of oven dried fluffed softwood pulp having a consistency of 35%. The 3~ initial Kappa No. of the softwood pulp is 30~0, but not certainly confin~d to the Kappa number mentioned. The W0~3/1~264 PCT/US93/~8 2~ 2,'~4~8 temperature of the pulp is maintained between 25C to 65C. The pulp reaction vessel is connected to two gas scrubbers, each of which contains a 10% svlution of potassium iodide. The chlorine dio~ide generator is initially filled with about 1500 grams of perchloric acid containing about 40 percen~ by weight of HC104, the perchloric acid contains about 15 grams of ruthenium dioxide, Ru02. A gaseous mi~ture of chlorine dioxide, o~ygen and water vapor is generated by fe~ding a chloric acid solution containing about 37.16 percen~ by weight of HC103~ During chlorine dioxide generation, the generator is maint~ined at a ~emperature between about 70 and 73C by regulating the feed rate of the chloric acid, a~d by providing sufficient heat of reaction to evaporate the water present in the chloric acid solution and that f~rmed durlng the cJeneration of chlorine dio~ide. The ozone reactor is a commercial reac~or using o~ygen as the f~ed gas for ozone generation. The ozone generator is maintained at temperatures between 10C and 25c. Two separate gas lines, independently carrying ozone and chlorine dioxide from the respectiYe generators are connected to a small mixing chamber prior :~
to entering the bottom of the reaction vessel. Th~n the - gaseous mixture is distributed into the pulp through a perforated sparger. The softwood pulp is treated for a time between 1 ~ 5 tninutes with the gaseous mi~ture.
Each delignified pulp sample is equally divided, one `~
half to be washed and the other not washed prior to -e~traction (E) . Where washing is employed the filtrate and the wash water are collected. E~traction o the - deli~nified pulp is carried out by adding the indicated amount of about 3.5% NaOH b~ wei~ht on pulp of a 10 percent sodium hydro~ide solution. The operating ccnsistency of pulp is abo~t 10 percent. The pulp is WO~3/152~ PCT/~S93~0408 212g~8 held at about 70C. for about 60 minutes. The extracted pulp is dried, washed and the Kappa Number is determined to be less than 10. The absorbable organic halides (AOX) in the combine~ solution mixture of the bleach filtrate, wash water, e~traction liquor and wash water is determined to be less than 1.0 kg/metric ton of pulp.
EX~M~LE 2 S~n~E~TIA~_~DDIrIO~ OF
GASEOU~ G~O~INE DIOXIDE ANP OZONE
A continuously stirred hot water jacketed reaction vessel containing ~ gas distribution device is loaded with 100 gm of oven dried shredded and fluffed sotwood pulp having a consistency of about 35 percent.
~he reaction vessél can act as an equipment vessel for ;~
rapid mi~ing of gas with pulp as well as a retention vessel for the continuation of the delignifying process after mi~ing the gases with~the pulp. The initial Kappa number of the pulp is about 30Ø The initial pulp temperature is at ambient conditions, but warmed up to about 80C and preferentially maintained between about 40C and about 6QC.
The outlet of the reactor i5 connected to a series of potassium iodide traps containing 10 percent KI which are acting as gas scrubbers for both the chlorine dioxide and ozone exiting the reaction vessel.
Betwee~ the reaction vessel and the gas~scrubbers, the catalytic convertor is employed to remove the or~anics carried over by the e~iting gases and also to remo~e the moisture from the oxygsn which was the carrier gas for both chlorine dio~ide and ozone. Two sepaxate gas lines, independently ca~rying ozone and chlorine dioxide W~93/1S2~ 2 1 ~ PCT/U~g3/~M08 from the respective generators are connected to the bottom of the reaction vessel and distributed into the pulp throu~h a perforated sparger. ~ach line has a gas metering device to measure accurately the gas flow to the reactor. The chlorine dio~ide gas line is also using a spectrophotometric device to measure the concentration of chlorine dio~ide flowing to the reactor. The ozone supply line has a concentrati~n measurement and monitoring unit utilizing W
spectrophotometr~ to measure the conc@ntration o ozone~
in the carrier yas. A feedback co~trol unit is used to adjust the o~ygen feed to the ozone generator.
The gases can be fed sequentially either by charging ozone first and the~: chlorine dioxide and vice 15 versa. In the event of chl~.ine dioxide being fed first ~-to react with pulp, the initial pulp pH has a range from 2 to about 8. On the other hand, if ozone is the gas first to rea~t with the pu1p, then the pH of the pulp is adjusted by using an acid, preerably sulfuric acid.
The pH of the pulp ranges from ~ to 5, but preferably less than 3. Also in cas~ of chlorine dioxide being charged first to the pulp, the exit gas is taken directly to the ~I traps by-passing the catalytic ~onvertor. The chlorine dio~ide feed to the pulp is 2~ calculated on the basis of the Kappa facto:r, a measure of chemical charqe deter~ined by the Rappa number of the pulp. The ~appa factor is in the range of 0.5 to ~Ø
The time for the reaction is varied from one minute to five mi~utes, but preferably less than about 2 minutes.
The ozone charge on the pulp is varied from 0.l pereent to 2.0 percent. The reaction time is varied from one minute to five mi~utes0 but pr~ferably less than about two minutes. The reactio~ can be carried out in subatmQspheric, atmospheric, or superatmospheric WO93/lS~ ~ PCT/~S93/~
-2~-pressure. The pressure range is 10 mm of Hg to 760 mm Hg to 1520 mm Hg, prefer3bly between 100 to 1520 mmHg.
Each delignified pulp sample is equally divided, one half to be washed and the other not washed prior to e~traction ~E). Where washing is employed, the filtrate and the wash water are collected. Extraction of the deliynified pulp is carried out by adding the indicated amount of about 3 . 5 percent sodium hydro~ide by weight on pulp o a 10 percent sodium hydroxid~ solution. The 10 op~rating consistency of pulp is about 10 p~rcent. The pulp is held at about 70C ~or about 60 minutes. The e~tracl~ed pulp is dried, washed and the Kappa numb~r is determined to be less than LOo The adsorbed organic halides ~OX~ in the combined solution mi:~:ture o the 15 bleach filtrate, wash water, e~traction liquor and wash water is determined to be less than 1. 0 kg~metric ton of pulp.
The pulp f rom the ~traction st~p is treated with one or more bleaching se~uences involving chlorine ;~G dio;ll:ide in aqueous or gaseous form, alkal~ne e~tr~ctior with sodium hydro:~ide reinforced by o~ygen and~or hydrogen pero2cide- and a gaseous mixture of chlorine ~:
dio~ide and ozone and combinations thereof to obtain pulp of required brightness and: strength. The adsorbed 25 organic halides (AOX) in the later bleach plant e~f luent f rom the bleaching sequences are fourld to be less than 0 . 3 . ky~metric ton of pulp .
While the invention has been described above with refereFlces to specific embodiments thereof, it is 30 apparent that many challges, modifications and variations ~:
in the materials, arrangements of parts and æteps can be made without departing f rom the inventive concept disclo~d herein. Accordingl~r, the spirit and broad W~ 93/15264 PCr/lJ~ig3/Ot~8 2~
scope of the appended claims is intended to embrace all suc:h c:hanges, modif ications and var$ations that may occur to one of skill in the art upon a reading of the di sc losure .
Claims (23)
1. A process for delignification of lignocellulosic material characterized by contacting lignocellulosic material sequentially or simultaneously with a gaseous mixture of chlorine dioxide and ozone.
2. The process of claim 1 characterized in that the acidity during delignification is controlled such that the delignified material will have a pH ranging from about 2 to about 5.
3. The process of claim 1 characterized in that the treatment with said gaseous mixture is effected at subatmospheric pressure.
4. The process of claim 1 characterized in that said process is carried out at a temperature ranging from about 50° C to about 80° C.
5. The process of claim 1 characterized in that the gaseous chlorine dioxide contains from 0 to about 20 percent by weight of chlorine.
6. The process of claim 5 characterized in that the chlorine dioxide is produced by admixing an aqueous solution of chloric acid containing at least about 30 percent by weight of HClO3 with a non oxidizable inorganic acid.
7. The process of claim 1 characterized in that said lignocellulosic material is wood pulp.
8. The process of claim 7 characterized in that said wood pulp has a consistency from about 20 to about 50 percent.
9. The process of claim 1 characterized in that the delignified material is subjected to alkaline extraction with an alkali metal hydroxide directly following the delignification step.
10. The process of claim 1 characterized in that the delignification is effected at a pressure selected from the group consisting of subatmospheric, atmospheric, and superatmospheric.
11. The process of claim 10 characterized in that the temperature is from about 50°C to about 80°C.
12. The process of claim 11 characterized in that said lignocellulosic material is wood pulp having a consistency of at least about 15 percent.
13. The process of claim 12 characterized in that said chlorine dioxide is produced by admixing chloric acid containing at least 30 percent by weight of HClO3 with an inorganic acid selected from the group consisting of sulfuric acid, phosphoric acid and perchloric acid.
14. The process of claim 13 characterized in that the delignified pulp is subjected to alkaline extraction with an alkali metal hydroxide directly following the delignification step.
15. The process of claim 1 characterized in that the process is carried out simultaneously using a mixture of gaseous chlorine dioxide and ozone.
16. The process of claim 15 characterized in that said lignocellulosic material is wood pulp having a consistency of at least 15 percent.
17. The process of claim 15 characterized in that said chlorine dioxide is produced by admixing chloric acid containing at least 30 percent by weight of HClO3 with an inorganic acid selected from the group consisting of sulfuric acid, phosphoric acid and perchloric acid.
18. The process of claim 15 characterized in that the delignified pulp is subjected to alkaline extraction with an alkali metal hydroxide directly following the delignification step.
19. The process of claim 1 characterized in that the delignified material is subjected to a subsequent bleaching step which comprises treating the lignocellulosic material sequentially or simultaneously with gaseous chlorine dioxide and ozone.
20. The process of claim 1 characterized in that the sequential treatment of the lignocellulosic material is first with gaseous chlorine dioxide and then with ozone.
21. The process of claim 1 characterized in that the sequential treatment of the lignocellulosic material is first with ozone and then gaseous chlorine dioxide.
22. The process of claim 1 characterized in that the gases are fed to a continuously stirred reactor vessel through a concentration measurement means and a metering means.
23. The process of claim 1 characterized in that the continuously stirred reactor vessel is used as a rapid mixing means and/or a retention means to continue and complete the delignification of the lignocellulosic material.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US82379592A | 1992-01-22 | 1992-01-22 | |
| US823,795 | 1992-01-22 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2128408A1 true CA2128408A1 (en) | 1993-08-05 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002128408A Abandoned CA2128408A1 (en) | 1992-01-22 | 1993-01-19 | Gas phase delignification of lignocellulosic material |
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| Country | Link |
|---|---|
| EP (1) | EP0705361A1 (en) |
| JP (1) | JPH07502082A (en) |
| AU (1) | AU656658B2 (en) |
| BR (1) | BR9305760A (en) |
| CA (1) | CA2128408A1 (en) |
| FI (1) | FI943466A0 (en) |
| NZ (1) | NZ245712A (en) |
| RU (1) | RU94036007A (en) |
| WO (1) | WO1993015264A1 (en) |
| ZA (1) | ZA93224B (en) |
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| FR2743094B1 (en) * | 1995-12-27 | 1998-02-13 | Centre Tech Ind Papier | PROCESS FOR BLEACHING CHEMICAL PULP |
| US6773547B2 (en) * | 1998-05-08 | 2004-08-10 | American Air Liquide, Inc. | Process for the bleaching of low consistency pulp using high partial pressure ozone |
| US6579412B2 (en) * | 1998-05-08 | 2003-06-17 | L'air Liquide - Societe' Anonyme A' Directoire Et Conseil De Surveillance Pour L'etude Et L'exploitation Des Procedes Georges Claude | Process for ozone bleaching of low consistency pulp |
| WO2002016691A1 (en) * | 2000-08-25 | 2002-02-28 | Akzo Nobel N.V. | Chemical method |
| FI122626B (en) | 2006-03-31 | 2012-04-30 | Laennen Tutkimus Western Res Inc Oy | Chemical pulp bleaching process |
| EP3536694B1 (en) | 2014-08-06 | 2020-12-02 | California Institute of Technology | Silylation of aromatic heterocycles by earth abundant transition-metal-free catalysts |
| US10787475B2 (en) * | 2017-11-07 | 2020-09-29 | Ingevity South Carolina, Llc | Methods of making low color lignin |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| US1957937A (en) * | 1930-10-10 | 1934-05-08 | Int Paper Co | Process of bleaching fibrous cellulose material |
| CA902859A (en) * | 1969-11-26 | 1972-06-20 | Pulp And Paper Research Institute Of Canada | Chemical reaction between a solid and a gas |
| US3810969A (en) * | 1971-06-22 | 1974-05-14 | Hooker Chemical Corp | Process for the production of chlorine dioxide |
| US4216054A (en) * | 1977-09-26 | 1980-08-05 | Weyerhaeuser Company | Low-consistency ozone delignification |
| US4902381A (en) * | 1988-12-09 | 1990-02-20 | Kamyr, Inc. | Method of bleaching pulp with ozone-chlorine mixtures |
| US4959124A (en) * | 1989-05-05 | 1990-09-25 | International Paper Company | Method of bleaching kraft pulp in a DZED sequence |
| US5034095A (en) * | 1989-06-01 | 1991-07-23 | Oji Paper Co., Ltd. | Apparatus and process for the delignification of cellulose pulp |
| EP0561786B1 (en) * | 1990-08-14 | 1997-11-26 | International Paper Company | Bleaching chemical pulp using chlorine/chlorine dioxide then ozone |
-
1993
- 1993-01-13 ZA ZA93224A patent/ZA93224B/en unknown
- 1993-01-19 WO PCT/US1993/000408 patent/WO1993015264A1/en not_active Application Discontinuation
- 1993-01-19 JP JP5513284A patent/JPH07502082A/en active Pending
- 1993-01-19 BR BR9305760A patent/BR9305760A/en not_active Application Discontinuation
- 1993-01-19 CA CA002128408A patent/CA2128408A1/en not_active Abandoned
- 1993-01-19 AU AU34769/93A patent/AU656658B2/en not_active Ceased
- 1993-01-19 RU RU94036007/12A patent/RU94036007A/en unknown
- 1993-01-19 EP EP93903543A patent/EP0705361A1/en not_active Withdrawn
- 1993-01-20 NZ NZ245712A patent/NZ245712A/en unknown
-
1994
- 1994-07-21 FI FI943466A patent/FI943466A0/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| ZA93224B (en) | 1993-08-18 |
| EP0705361A4 (en) | 1995-09-27 |
| WO1993015264A1 (en) | 1993-08-05 |
| FI943466A (en) | 1994-07-21 |
| FI943466A0 (en) | 1994-07-21 |
| NZ245712A (en) | 1994-09-27 |
| AU3476993A (en) | 1993-09-01 |
| JPH07502082A (en) | 1995-03-02 |
| BR9305760A (en) | 1997-01-28 |
| EP0705361A1 (en) | 1996-04-10 |
| AU656658B2 (en) | 1995-02-09 |
| RU94036007A (en) | 1996-06-10 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Dead |