CA1053604A - Production of chlorine dioxide and integration with effluent-free pulp mill - Google Patents
Production of chlorine dioxide and integration with effluent-free pulp millInfo
- Publication number
- CA1053604A CA1053604A CA219,729A CA219729A CA1053604A CA 1053604 A CA1053604 A CA 1053604A CA 219729 A CA219729 A CA 219729A CA 1053604 A CA1053604 A CA 1053604A
- Authority
- CA
- Canada
- Prior art keywords
- chlorine
- sodium chloride
- chlorine dioxide
- sodium
- reaction
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B1/00—Electrolytic production of inorganic compounds or non-metals
- C25B1/01—Products
- C25B1/24—Halogens or compounds thereof
- C25B1/26—Chlorine; Compounds thereof
-
- 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
Abstract
ABSTRACT OF THE DISCLOSURE
A continuous cyclic process for the production of chlorine dioxide includes the formation of a gaseous mixture of chlorine dioxide, chlorine and steam from an aqueous reaction medium containing sodium chlorate and hydrochloric acid at an activity below about 1N. Chlorine dioxide is recovered from the gaseous mixture. Sodium chloride formed in the reaction is deposited in the reac-tion vessel and is removed therefrom. The continuous cyclic process forms part of an effluent dioxide is used as a bleaching chemical and from which sodium chloride is removed. Sodium hydroxide solution used in caustic extrac-tion of the pulp in the bleach plant is formed by electro-lysis of an aqueous solution sodium chloride, which may be formed from that removed from the chlorine dioxide genera-tor and/or recovered from the pulp mill. Sodium chlorate feed for the chlorine dioxide generator is formed by electrolysis of aqueous sodium chloride solution, which may be formed from that removed from the chlorine dioxide generator and/or recovered from the pulp mill and not used to form sodium hydroxide solution. Inefficiencies in the production of chlorine dioxide and losses from the system are made up by the use of additional chlorine and sodium chloride.
A continuous cyclic process for the production of chlorine dioxide includes the formation of a gaseous mixture of chlorine dioxide, chlorine and steam from an aqueous reaction medium containing sodium chlorate and hydrochloric acid at an activity below about 1N. Chlorine dioxide is recovered from the gaseous mixture. Sodium chloride formed in the reaction is deposited in the reac-tion vessel and is removed therefrom. The continuous cyclic process forms part of an effluent dioxide is used as a bleaching chemical and from which sodium chloride is removed. Sodium hydroxide solution used in caustic extrac-tion of the pulp in the bleach plant is formed by electro-lysis of an aqueous solution sodium chloride, which may be formed from that removed from the chlorine dioxide genera-tor and/or recovered from the pulp mill. Sodium chlorate feed for the chlorine dioxide generator is formed by electrolysis of aqueous sodium chloride solution, which may be formed from that removed from the chlorine dioxide generator and/or recovered from the pulp mill and not used to form sodium hydroxide solution. Inefficiencies in the production of chlorine dioxide and losses from the system are made up by the use of additional chlorine and sodium chloride.
Description
The present invention relates to the production of chlorine dioxide using hydrochloric acid and to the integration of the production of chlorine dioxide with an effluent-free bleached pulp mill.
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- Chlorine dioxide is known to be prepared by the reduction of an alkali metal chlorate, generally sodium chlorate, using hydrochloric acid as the reducing agent and to provide acidity to the reaction medium.
Two competing reactions are known to occur in ; 10~ the reactior medi~m which may be represented by the following equation8 (1) and (2), in the case where the alkali metal i8 sodium:
NaC103 + 2HCl~ C102 + ~C12 + NaCl + H20 NaCI03 + 6HCl ~ NaCl + 3C12 + 3H20 ..(2) Since tha reaction according to equation (2) ; fails to produce chlorine dioxide, the extent to which equation (1) predomlnates is~ the efficiency of conversion of the sodium chlorate to chlorine dioxide.
Chlorine dioxide is produced commercially from 20~ hydrochloric acid by the so-called Day-KçYting process di-c1osed in Canadian Patent No. 461,586 is8ued December s~
:,~ .
.... , ................. , :....... ... . . ... .
.. . .. .. ...... ...
6, 1947 to Brown Company. In this process the chlorine dioxide generator consists of a plurality of, generally six, reaction vessels arranged in series. Feed liquor containing sodium chlorate, hydrochloric acid and some sodium chloride is fed into the uppermost of the six reactors, from which the solution cascades by gravity through the remaining reactors. The two bottommost reactors are operated at boiling temperature and the steam strips them of chlorine dioxide while the remaining hydro-chloric acid is oxidized. Air is introduced into the gas stream from the last two reactors, mixes with the vapour and is bubbled countercurrently through the solution in the reactors three to six.
It has previously been proposed in Canadian Patent No. 782,574 issued April 9, 1968 to Chemech ~ Engineering Limited, to produce chlorine dioxide and- ~ chlorine from sodium chloride and hydroohloric acid in a single reaction vessel in place of the cascade six-vessel system used in the Day-Kesting process. In the process ~ described in~Canadian Patent No.~782,574, gaseous hydrogen is passed through the reaction vessel to agitate the reaction liquor and as gaseous diluent for the chlorine dioxide and chlorine.
~ . . -- Chlorine dioxide gas in concentrated gaseous form spontaneously decomposes and in processes of producing d lorine dioxide it is necessary to make some provision for diluting the gases produced in the reaction. Air has ¢ommonly been used, as in the commercial Day-Xesting opera-tion described above, and in the Canadian Patent No.
782,574, hydrogen is the diluent gas.
In a liquid-effluent-free pulp mill, bleach plant effluent, containing sodium chloride values from chlorine-containing spent bleaching chemicals including chlorine dioxide, along wi~;h some chlorine, and soda values from spent caustic extraction chemicals, typically sodium hydroxide, the bleaching and caustic extraction chemicals being used in the bleaching and purification procedures of the bleach plant, is discharged to the spent pulping liquor recovery and regeneration operation of the pulping process producing the pulp for bleaching and puri-fication.
Typically such recovery and regeneration opera-tion involves burning of the liquors to remove carbona-ceous materials and form a mixture of chemicals reusable as or convertible into pulping chemicals for recycle.
Since sodium chloride is unaffected chemically by the recovery and regeneration operation, it would remain as a dead loan in the effluent-free pulp mill and build up in the system unless steps are taken to remove the same. The sodium chloride may be removed from the recovery and re-generation operation, usinq a variety of procedures, for example, that described in Canadian Patent No. 915,361, wherein the sodium chloride is removed by evaporating the pulping liquor prior to recycle.
In accordance with the present invention chlorine dioxide is formed in a continuous process, by reduction of an alkali metal chlorate, preferably sodium chlorate, using hydrochloric acid at an acidity of about 0~05 to about 1 normal. The reaction medium is maintained at its boiling point at the prevailing absolute pressure thereon to cause evaporation of water from the reaction medium to form a gaseous mixture .~s :
IOS3 ~~1 of chlorine dioxide, chlorine and steam which is removed from the reaction zone and from which an aqueous chlorine dioxide solution is formed. The reaction zone is main-tained under a reduced, i.e. subatmospheric, pressure in order to maintain the boiling temperature below the temperature above which substantial decomposition of chlorine dioxide occurs, preferably substantially below.
The sodium chloride which is formed as a by-product of the chlorine dioxide-producin~ process, once saturation of the reaction medium is reached, precipitates in the reac-tion zone and is removed therefrom in continuous or inter-mittent manner.
The chlorine dioxide-producing reaction is integrated with an ef~luent-free bleached pulp mill. The SD~ it ~
chlorine dioxide~along with chlorine is used to bleach the pulp in a bleach plant of the mill and sodium hydroxide solution is used for caustic extraction of the pulp in the bleach plant. All the liquids introduced to the bleach plant are discharged from the bleach plant as a single effluent containing sodium chloride. The sodium chloride is separated from the mill after the waste liquor consisting of spent pulpi~g liquor and the single effluent has been subjected to the recovery and regeneration operation of the pulp mill.
Regenerated pulping liquor is recycled to form new pulp.
The sodium chloride separated from the pulp mill and that separated from the chlorine dioxide-producing zone are used to form sodium chlorate for the chlorine dioxide producing reaction and to form the sodium hydroxide and chlorine. The chlorine which is not forwarded to the bleach plant is combined with that separated from the chlorine dioxide and is used to form hydrogen chloride for formation of the hydrochloric acid feed for the chlorine dioxide generation by reaction with part of the hydrogen off-~as from the ~odium chlorate form~tion.
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The chlorine dioxide-producing process used in the present invention is carried out at the boiling tempera-ture of the reaction liquor at the prevailing absolute pressure. Operating in this manner ensures maximum vaporization of water from the reaction solution in the reaction zone or vessel, whereby chlorine dioxide and chlorine are rapidly educted from the vessel and water is removed from the reaction medium. The temperature of the reaction solution should be below the temperature above which substantial decomposition of chlorine dioxide occurs and this is achieved by maintaining the reaction zone under subatmospheric pressure, the magnitude of which is dependent upon the reaction temperature. It is preferred to carry out the chlorine dioxide-producing reaction at temperatures of approximately 60 to 80C.
Any convenient equipment may be utilized to sub- t-~
ject the reaction vessel to a reduced i.e. subatmospheric pressures, for example, a water stream jet eductor may be used. The water fed to the eductor may serve not ~ only to produce the required reduced pressure, but also to condense the water vapour and to dissolve the chlorine dioxide and part of the chlorine produced.
During continuous operation, the liquid level in the reaction zone is maintained substantially constant by balancing the quantity of water removed from the reaction zone as steam with the quantity of water formed by the chlorine dioxide producing reaction and the quan-tity of water introduced to the reaction zone with the sodium chlorate and hydrochloric acid solut~ons.
- Some spent reaction liquor may overflow from the reaction vessel and is recycled to the make-up feed streams.
. . .
, The acidity of the chlorine dioxide-producing reacting solution is generally below lN, preferably below about 0.5N. In the first embodiment of the invention the acidity preferably is from about 0.05 to about 0.5 normal, more preferably from about 0.1 to about 0.2N. The effec-tive range of acidities in the process of the present invention is generally about 0.05 to about lN.
It is preferred to operate the process of the present invention at molar concentrations where the ratio of Cl : ClQ3 is approximately 2:1 in the feed. It is possible to use lower ratios, say 1:1, but all of the chlorate present is not reacted under these circumstances.
It also is possible to use higher ratios of chloride to chlorate, say 5:1, but at these concentrations, equation
. ~ ..
:.
- Chlorine dioxide is known to be prepared by the reduction of an alkali metal chlorate, generally sodium chlorate, using hydrochloric acid as the reducing agent and to provide acidity to the reaction medium.
Two competing reactions are known to occur in ; 10~ the reactior medi~m which may be represented by the following equation8 (1) and (2), in the case where the alkali metal i8 sodium:
NaC103 + 2HCl~ C102 + ~C12 + NaCl + H20 NaCI03 + 6HCl ~ NaCl + 3C12 + 3H20 ..(2) Since tha reaction according to equation (2) ; fails to produce chlorine dioxide, the extent to which equation (1) predomlnates is~ the efficiency of conversion of the sodium chlorate to chlorine dioxide.
Chlorine dioxide is produced commercially from 20~ hydrochloric acid by the so-called Day-KçYting process di-c1osed in Canadian Patent No. 461,586 is8ued December s~
:,~ .
.... , ................. , :....... ... . . ... .
.. . .. .. ...... ...
6, 1947 to Brown Company. In this process the chlorine dioxide generator consists of a plurality of, generally six, reaction vessels arranged in series. Feed liquor containing sodium chlorate, hydrochloric acid and some sodium chloride is fed into the uppermost of the six reactors, from which the solution cascades by gravity through the remaining reactors. The two bottommost reactors are operated at boiling temperature and the steam strips them of chlorine dioxide while the remaining hydro-chloric acid is oxidized. Air is introduced into the gas stream from the last two reactors, mixes with the vapour and is bubbled countercurrently through the solution in the reactors three to six.
It has previously been proposed in Canadian Patent No. 782,574 issued April 9, 1968 to Chemech ~ Engineering Limited, to produce chlorine dioxide and- ~ chlorine from sodium chloride and hydroohloric acid in a single reaction vessel in place of the cascade six-vessel system used in the Day-Kesting process. In the process ~ described in~Canadian Patent No.~782,574, gaseous hydrogen is passed through the reaction vessel to agitate the reaction liquor and as gaseous diluent for the chlorine dioxide and chlorine.
~ . . -- Chlorine dioxide gas in concentrated gaseous form spontaneously decomposes and in processes of producing d lorine dioxide it is necessary to make some provision for diluting the gases produced in the reaction. Air has ¢ommonly been used, as in the commercial Day-Xesting opera-tion described above, and in the Canadian Patent No.
782,574, hydrogen is the diluent gas.
In a liquid-effluent-free pulp mill, bleach plant effluent, containing sodium chloride values from chlorine-containing spent bleaching chemicals including chlorine dioxide, along wi~;h some chlorine, and soda values from spent caustic extraction chemicals, typically sodium hydroxide, the bleaching and caustic extraction chemicals being used in the bleaching and purification procedures of the bleach plant, is discharged to the spent pulping liquor recovery and regeneration operation of the pulping process producing the pulp for bleaching and puri-fication.
Typically such recovery and regeneration opera-tion involves burning of the liquors to remove carbona-ceous materials and form a mixture of chemicals reusable as or convertible into pulping chemicals for recycle.
Since sodium chloride is unaffected chemically by the recovery and regeneration operation, it would remain as a dead loan in the effluent-free pulp mill and build up in the system unless steps are taken to remove the same. The sodium chloride may be removed from the recovery and re-generation operation, usinq a variety of procedures, for example, that described in Canadian Patent No. 915,361, wherein the sodium chloride is removed by evaporating the pulping liquor prior to recycle.
In accordance with the present invention chlorine dioxide is formed in a continuous process, by reduction of an alkali metal chlorate, preferably sodium chlorate, using hydrochloric acid at an acidity of about 0~05 to about 1 normal. The reaction medium is maintained at its boiling point at the prevailing absolute pressure thereon to cause evaporation of water from the reaction medium to form a gaseous mixture .~s :
IOS3 ~~1 of chlorine dioxide, chlorine and steam which is removed from the reaction zone and from which an aqueous chlorine dioxide solution is formed. The reaction zone is main-tained under a reduced, i.e. subatmospheric, pressure in order to maintain the boiling temperature below the temperature above which substantial decomposition of chlorine dioxide occurs, preferably substantially below.
The sodium chloride which is formed as a by-product of the chlorine dioxide-producin~ process, once saturation of the reaction medium is reached, precipitates in the reac-tion zone and is removed therefrom in continuous or inter-mittent manner.
The chlorine dioxide-producing reaction is integrated with an ef~luent-free bleached pulp mill. The SD~ it ~
chlorine dioxide~along with chlorine is used to bleach the pulp in a bleach plant of the mill and sodium hydroxide solution is used for caustic extraction of the pulp in the bleach plant. All the liquids introduced to the bleach plant are discharged from the bleach plant as a single effluent containing sodium chloride. The sodium chloride is separated from the mill after the waste liquor consisting of spent pulpi~g liquor and the single effluent has been subjected to the recovery and regeneration operation of the pulp mill.
Regenerated pulping liquor is recycled to form new pulp.
The sodium chloride separated from the pulp mill and that separated from the chlorine dioxide-producing zone are used to form sodium chlorate for the chlorine dioxide producing reaction and to form the sodium hydroxide and chlorine. The chlorine which is not forwarded to the bleach plant is combined with that separated from the chlorine dioxide and is used to form hydrogen chloride for formation of the hydrochloric acid feed for the chlorine dioxide generation by reaction with part of the hydrogen off-~as from the ~odium chlorate form~tion.
- :, ' . - -, , ~ .
.. .. , ~
The chlorine dioxide-producing process used in the present invention is carried out at the boiling tempera-ture of the reaction liquor at the prevailing absolute pressure. Operating in this manner ensures maximum vaporization of water from the reaction solution in the reaction zone or vessel, whereby chlorine dioxide and chlorine are rapidly educted from the vessel and water is removed from the reaction medium. The temperature of the reaction solution should be below the temperature above which substantial decomposition of chlorine dioxide occurs and this is achieved by maintaining the reaction zone under subatmospheric pressure, the magnitude of which is dependent upon the reaction temperature. It is preferred to carry out the chlorine dioxide-producing reaction at temperatures of approximately 60 to 80C.
Any convenient equipment may be utilized to sub- t-~
ject the reaction vessel to a reduced i.e. subatmospheric pressures, for example, a water stream jet eductor may be used. The water fed to the eductor may serve not ~ only to produce the required reduced pressure, but also to condense the water vapour and to dissolve the chlorine dioxide and part of the chlorine produced.
During continuous operation, the liquid level in the reaction zone is maintained substantially constant by balancing the quantity of water removed from the reaction zone as steam with the quantity of water formed by the chlorine dioxide producing reaction and the quan-tity of water introduced to the reaction zone with the sodium chlorate and hydrochloric acid solut~ons.
- Some spent reaction liquor may overflow from the reaction vessel and is recycled to the make-up feed streams.
. . .
, The acidity of the chlorine dioxide-producing reacting solution is generally below lN, preferably below about 0.5N. In the first embodiment of the invention the acidity preferably is from about 0.05 to about 0.5 normal, more preferably from about 0.1 to about 0.2N. The effec-tive range of acidities in the process of the present invention is generally about 0.05 to about lN.
It is preferred to operate the process of the present invention at molar concentrations where the ratio of Cl : ClQ3 is approximately 2:1 in the feed. It is possible to use lower ratios, say 1:1, but all of the chlorate present is not reacted under these circumstances.
It also is possible to use higher ratios of chloride to chlorate, say 5:1, but at these concentrations, equation
(2) above becomes predominant, producing large quantities of chlorine in preference to chlorine dioxide.
A variety of reaction ~essels for chlorine dioxide generation may be utilized in the practice of the present invention. A suitable one is described in Canadian Pate~t No. 825,084 issued October 14, 1969 to Electric Reduction Company of Canada, Ltd. modified as required. Such Canadian patent describes the produc-tion of chlorine dioxide from an alkali metal chlorate, an alkali metal chloride and sulphuric acid in a single vessel generator-evaporator-crystallizer. An alternative form of reaction vessel together with a recycle and feed system is described in our copending Canadian application Serial No. 175.282.
The crystalline sodium chloride precipitated from the reaction solution is removed from the reaction zone, continuously or intermittently, and washed with water to remove last traces of trapped reaction medium.
The spent wash water may be recycled to the reaction zone.
- \ -The sodium chloride thereby is recovered in a ~ubstantially pure state.
Sodium chloride also is removed from the effluent-free pulp mill. The sodium chlorate feed for the reaction zone may be provided by electrolysis of a sodium chloride solution formed from sodium chloride removed from the chlorine dioxide-producing reaction zone or from the effluent-free mill.
Mixtures of sodium chloride from both sources may be used.
The electrolysis reaction results in an aqueous solution containing sodium chlorate and sodium chloride which may be fed directly to the reaction zone, the sodium chloride content recycling as a dead load.
The hydrochloric acid feed for the generator is formed by reaction of chlorine with part of the hydrogen off-gas from the chlorate cell. The chlorine is provided partly by that recovered from the gases formed in the chlorine dioxide-producing zone and partly by part of that formed in the caustic-chlorine cell.
The reaction occurring in the chlorate~cell is:
NaCl + 3H2O C o3 3H2 and the reaction involved in producing hydrogen chloride by combustion is: ~
H2 - C12 3 2HCl ~ -It will be seen from the above equation 1 for the production of chlorlne dioxide that only two moles of hydrogen chloride ---(as hydrochloric acid in the reaction medium) is required for each mole of sodium chlorate. Therefore, only approximately one-third of the mole quantity of hydrogen produced in the chlorate cell need be combusted with the same molar quantity of chlorine to produce the required mole amount of hydrogen chloride, the required amount depending on the efficiency of the chlorine dioxide producing reaction.
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The excess hydrogen from the cell off-gases may be burned in air or oxygen to produce heat which enables the heat balance of the operation to be maintained, and water which may be used as part of the water require-ment.
The chlorine dioxide solution recovered from the gaseous mixture produced in the generator is used in the bleach plant of the pulp mill, wherein the pulp is subjected to bleaching and caustic extraction oper-ations.
Sodium hydroxide solution for use in the causticextraetion operations is formed by electrolysis of sodium chloride solution, which also. produces chlorine. Part of - this chlorine is used in the bleach plant as bleaching chemical and the remainder of the chlorine is used to form . . hydrochloric acid for feed to the chlorine dioxide-producing reaction.
The pulp is washed in the bleach plant, usually after each of the bleaching and caustic extraction steps.
:20 : :In the.effluent-free pulp mill, the spent liquids of the bleach plant are mixed to form a bleach plant effluent which is discharged from the bleach plant to the spent . pulping liquor fed to the recovery and regeneration opera-., ::
;~ : tions resulting in fresh pulping liquor for recycle to ~ the pulping stage.
.~ AS set forth in Canadian.Patent No. 956,754 the - bleach plant effluent may be formed by a countercurrent flow of.wash water with respect to the flow of pulp through the bleach plant, the bleach plant effluent being~
used to wash the pulp free from entrained spent pulping liquor and passing to the spent pulping liquor.
The mixing of the bleach plant chemicals to g .. . : . , . :
. .
form the bleach plant effluent results in the formation of sodium chloride by reaction between the sodium values of the sodium hydroxide and the chlorine values of the chlorine dioxide and chlorine. The quantities of sodium and chlorine values fed to the bleach plant are balanced to provide all such values as sodium chloride in the bleach plant effluent.
By passing the bleach plant effluent to the pulp mill spent pulping liquor recovery and regeneration operations, preferably by utiiizing at least part thereof to wash the pulp prior to its passage to the bleach plant to decrease the overall mill water requirement, the dis-'charge of polluting liquid effluents from the mill isavoided. The furnacing step of the recovery and regen-eration operations burns off the carbonaceous materials introduced to the spent pulping liquor'with the bleach plant effluent.
The sodium chloride introduced with the bleach ~ -plant effluent remains unaffected by the recovery and ... .
20 regeneration steps and is removed from the pulping `'' components prior to their recycle'to the pulping step, typically using the pulping liquor evaporation procedure ` outlined in Canadian Patent No. 915,361 mentioned above.
The separated sodium chloride along with the sodium chloride removed from the chlorine dioxide-producing reaction is used to provide the sodium chlorate feed for - the chlorine dioxide-producing reaction and the sodium hydroxide and chlorine used in the bleach plant.
Additional quantities of sodium chloride to those introduced by the bleach plant effluent may be pre-sent in the pulping liquor and removed during the removal step. Such additional quantities of sodium chloride may arise from sea-borne logs in coastal regions, or from -~
~ 053604 the use of brackish water or contaminated chemicals. The additional sodium chloride, along with additional chlorine, may be used as required to make-up for losses from the system.
The invention is described further, by way of illustration, with reference to the accompanying drawing, which is a schematic flow sheet for a pulp mill system in accordance with the present invention.
Referring to the drawing, a pulp mill 110 includes a pulping zone 112 to which wood chips 114 or other cellu-losic fibrous material, such as sawdust, are fed by line 114. Pulping liquor containing at least one active pulping chemical is fed to the pulping zone 112 by line 116. The active pulping chemical(s) may be any de-sired material, for example, sodium sulphide and sodium hydroxide in a Kraft mill process, sodium hydroxide in a soda mill process or sodium sulphide in an alkafide pro-~ cess. -Spent pulping liquor from the pulping operation is passed by lines 118 and 120 to a recovery and regenera-tion Operation 122 wherein the spent pulping liquor is converted into fresh pulping liquor in line 124.
The steps involved in the recovery and regenera-tion operation depend on the particular pulping process used but generally include a concentration step to decrease the volume of liquor and a furnacing step to remove carbonaceous materialsand to form a smelt contain-ing one or more chemicals usable as and/or convertible into pulping chemicals.
The pulp formed in the pulping zone 112 after washing is passed by line 126 to a bleach plant 128 where-in the pulp is subjected to brightening and purification steps to form a bleached pulp of the required brightness : ' : ' '. , ' : ' ' ;: ' ' . - : , .
.
and purity which is removed from the bleach plant by line 130.
. . The brightening and purification steps typically may comprise an initial bleaching using a mixture of chlorine dioxide and chlorine using a chlorine dioxide solution fed by line 132 and a chlorine feed in line 134, a caustic extraction using an aqueous sodium hydroxide solution fed by line 136, a second bleaching using the aqueous chlorine dioxide solution in line 132, a second : 10 caustic extraction using the aqueous sodium hydroxide solu-tion in line 136 and a final bleaching using the aqueous -chlorine dioxide solution in line 132. Washing usual}y is carried out intermediate each chemical treatment using water fed by.line 138.
The used chemicals and wash water are combined to form a single effluent, namely, bleach plant effluent, - . in line 140. The mi.xing of the used chemicals and wash water to form the bleach plant effluent may be carried .~ out using the countercurrent procedure outlined in the ab~ve-mentioned Canadian Patent No. 956,754.
.~ ~ Owing to the presence of sodium and chlorine values in the used bleach plant chemicals, sodium chloride .
is formed, and is present in the bleach plant effluent 140 together with the carbonaceous materials removed from -.
the pulp in the brightening and purification steps. The process is operated so that the sodium values fed to the bleach plant in line 136 balance the chlorine values fed to the bleach plant in lines 132 and 134, whereby all the sodium and all the chlorine values introduced to the bleach plant 128 emerge as sodium chloride in the bleach plant effluent 140.
. - 12 ,, ~05360~ - /3 The bleach plant effluent in line 140 is intro-duced to the spent pulping liquor in line 118 to form a combined waste liquor feed in line 120 to the recovery and regeneration operation 122. Preferakly, the bleach plant effluent in line 140 first is used to wash the pulp in line 126 before the passage of the washed pulp to the bleach plant 128, in accordance with the procedure out-lined in the above-mentioned ~ahadian Patent No. 956,754.
The recovery and regeneration operation evapor-ates water from the bleach plant effluent and removes the carbonaceous materials. Water removed from the waste liquor.in line 120 in the recovery and regeneration operation 122 may be recovered and used, after cleaning, if necessary, elsewhere in the system, such as in the water supply to the bleach piant in line 138.
The sodium chloride content of the bleach plant effluent, however, is unaffected chemically by the .
recovery and regeneration operation and hence is present in the smelt.
? Sodium chloride is removed from the pulp mill 110 in a sodium chloride removal zone 142. The sodium chloride removal may be carried ou~-by evaporating the regenerated liquor in line 124 in accordance with the pro-cedure outlined in the above-mentioned Canadian Patent No.915,361tO deposit sodium chloride therefrom, substan-tially pure crystalline sodium chloride being removed from the sodium chloride removal zo~e 142 by line 144.
The removal of sodium chloride from the pulp mill may be carried out by any other convenient procedure, before or after conversion of all the reusable components of the smelt to pulping chemicals.
The pulping liquor having a decreased sodium chloride concentration may be recycled by line 116 to the pulping zone 112, after dilution, if desired.
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The quantity of pure sodium chloride removed from the pulp mill 110 is substantially equal to that intro-duced to or formed within the pulp mill 110. Generally, the quantity of pure sodium chloride removed from the pulp mill 110 by line 144 is equal to that formed in the bleach plant effluent 140 from the sodium hydroxide, chlorine dioxide and chlorine fed to the bleach plant by lines 136, 132 and 134 respectively.
However, additional quantities of sodium chloride may be introduced to the pulp mill 110 with the wood chips in line 114 where they are formed from sea-borne logs, or from the use of brackish water. Such additional quantities of sodium chloride are removed from the pulp mill 110 in the sodium chloride removal zone 142 to prevent their build up in the pulp mill 110.
- While the pulping liquor in line 116 contains residual quantities of sodium chloride, under steady state conditions these residual quantities recycle through the system as a dead load and do not build up when the quantity Of pure crystalline sodium chloride removed by line 144 equals that introduced to and formed in the pulp mill 110. ~ -A chlorine dioxide generator 146 is provided to which aqueous sodium chloride solution is fed by line 148 and hydrochloric acid is fed by line 150, the chlorine dioxide-producing reaction medium within the generator 146 being maintained at its boiling point at the absolute pressure thereon and the generator 146 being maintained under a subatmospheric pressure, whereby the boiling tem-perature of the chlorine dioxide-producing reaction medium - 30 is main~ained below the temperature above which substan-tial decomposition of chlorine dioxide occurs.
The chlorine dioxide-producing reaction medium under continuoussteady stat; operation is saturated with "'''~ ' ' " ' ' ~, sodium chloride and hence solid crystalline sodium chloride ' is precipitated from the reaction medium in the generator 146.
Chlorine dioxide, chlorine and steam are formed in the generator 146 and are removed as a gaseous mixture by line 152, the steam maintaining the partial pressure of chlorine dioixde in the gaseous phase well below that of chlorine dioxide decomposition.
The quantity of steam removed from.the genera-tor 152 balanc~ the water formed in the chlorine dioxide-producing reaction and that introduced with the sodium chlorate solution in line 148 and the hydrochloric acid 150 ,to maintain the liquid level in the generator 146 at a sub'stantially constant value. At the same time, the feed of hydrochloric acid in line 150 maintains the acidity of the chlorine dioxide-producing reaction medium in the generator 146 at about 0.05 to about 0.2 N.
, . The mixture of chlorine dioxide, chlorine and steam in line 152 is passed to an adsorption tower 154 wherein the mixture is contacted with water fed by line 156, causing condensation of the steam and the formation of an aqueous solution of chlorine dioxide. The aqueous . solution of chlorine dioxide contains some dissolved : chlorine, the proportion of which may be decreased by air stripping prior to passage of the chlorine dioxide solution by line 132 to the bleach plant 128.
The gaseous chlorine resulting from the adsorp-tion tower 154 passes therefrom by line 158. A vacuum pump (not shown~, or other suitable means, may be provided in line 158 to maintain the vacuum in the generator 146.
The chlorine in line 158 forms parts of a com-bined chlorine feed stream 160 to a hydrogen chloride reactor and adsorber 162. In the hydrogen chloride .. . .. .
~053604 reactor-adsorber 162, the chlorine fed by line 160 is reacted with hydrogen to form hydrogen chloride which then is adsorbed in water fed by line 164 to form hydrochloric acid for feed to the chlorine dioxide generator 146.
While a single hydrogen chloride reactor-adsor-ber-is shown this is for ease of illustration and normally the two separate operations are carried out in separate apparatus.
The hydrogen reacted with the chlorine to form the hydrogen chloride is fed by line 166 from a chlorate cell 168 wherein aqueous sodium chloride solution fed by line 170 is electrolysed to form sodium chlorate solution for feed to the chlorine dioxide generator by line 148 and the hydrogen in line 166. The quantity of hydrogen in line 166 contains an excess to the stoichiometric requirement of hydrogen chloride and the excess may be vented by line 172.
. The crystalline sodium chloride deposited from the reaction medium in the chlorine dioxide generator 146 is removed by line 174 and, after washing to remove en-trained liquor, the wash water being returned to the generator 146, typically with the sodium chlorate solution in line 148, is passed to a sodium chloride dissolver 176 to which also is passed the pure crystalline sodium chloride removed from the pulp mill 110 by line 144.
In the sodium chloride dissolver 176, the sodium chloride is formed into an aqueous solution with water fed by line 178, the resulting sodium chloride solution in line 180 being passed partly to the chlorate cell 168 . by line 170 and partly to a caustic-chlorine cell 182 by . line 184. In the caustic-chlorine cell 182, the sodium chloride solution in line 184 is electrolyzed to form sodium hydroxide solution which is passed by line 136 to - 16 - .-' the bleach plant 128 and chlorine in line 186 which partly is fed by line 134 to the bleach plant 128 and partly by line 188 to join with the chlorine in line 158 to form the chlorine feed in line 160. Hydrogen by-product also is formed and is vented by line 189.
While the solid sodium chloride separated from the generator 146 and removed from the pulp mill 110 is illustrated as being forwarded to a common dissolver 176 for formation of the sodium chloride solution fed to the chlorate cell 168 and the caustic-chlorine cell 182, it is possi~le to operate in other alternate manners.
For example, the sodium chloride removed from the chlorine dioxide generator 146 may be formed into the aqueous solution fed to the chlorate cell 168 and the sodium chloride recovered from the pulp mill 110 may be formed into the aqueous solution for feed to the caustic- i chlorine cell 182.
Assuming no losses from the system, chlorine dioxide i~ produced ~holly by equation (1) and 100% separation of chlorine dioxide from chlorine, the following stoichio-metry applies:
Chlorine dioxide generator 146:
` NaC103 + 2HCl ~ C12 + ~C12; + H20 + NaCl Chlorate cell 168:
NaCl + 3H20 ~ NaC103 + 3H2 HCl reactor 162:
. H2 + C12 ~ 2HCl Caustic-chlorine cell 182:
2NaCl + 2H20 ----3 2NaOH + C12 + H2 Bleach Plant:
2NaOH + ~C12 + C102 ) 2NaCl + H20 + 3rO7 The system thus is in balance with regard to sodium and chlorine and the system requires only water . . , .
~053604 which results in a by-product hydrogen. Apart from an initial change of sodium chloride, the integrated system is self-balancing with the exception of the water requirement.
Taking into account the effect of equation (2) in the chlorine dioxide-producing process:
Generator 168:
(l-x)NaClO3 + 2(1-x)HCl ) 2 (12x)C12 + (1-x)H2O + (1-x)NaCl x NaC103-~6xHCl --~ 3xC12 + 3xH20 + xNaCl where x is the mol~r proportion of sodium chlorate react-. .
ing by equation 2.
i.e. NaC103 + 2(1-2x)HCl --~(l-x)C102 + (l+Sx)C12 +
(1+2x)H2O + NaCl As a result of this, the quantity of chlorine required by line 188 is (1-x)C12 in view of the amount of ~ -(1+5x) C12 in line 158. The quantity of chlorine present in line 134, assuming no excess, therefore is (l+x) C12 which then forms sodium chloride with the excess sodium . 20 hydroxide resulting from the presence of (1-x)ClO2 in line .-132.
If, however, it is aesired to keep in the bleach plant the relative proportions of chlorine dioxide and chlorine and the same molar quantity of chlorine dioxide, as discussed above in the ideal system extra chlorine - dioxide must be formed by reaction in the chlorine dioxide generator 146 to compensate for the losses due to equation 2.
- Since, overall, the chlorine dioxide system operates according to the equation:
~C12 + 2H20 ~ C12 + 2H2 the introduction of an additional quantity of chlorine, . . . .
10536¢4 such as by line 190 results in the required compensation.
The extra chlorine required in line 190 may be provided by electrolysis of.additional sodium chlori~e in the caustic~
chlorine cell 182, the extra soda being used to make up any soda losses in the pulp mill 110. The additional sodium chloride may arise from additional quantities recovered from the pulp mill 110 from the sources men-tioned above, or may be introduced from external sources by line 192. .
The inefficiency of separation of chlorine from chlorine dioxide in the adsorption tower 154 resulting in the presence of dissolved.chlorine in the chlorine dioxide solution in line 132 may be compensated for by feeding a decreased amount of chlorine by line 134 so that the total quantity of chlorine fed to the bleach plant !' 128 by lines 132 and 134 remains constant, the additional - -chlorine in line 188 then compensating for the quantity .
absent from line 158. Any other losses of sodium or chlorine values may be compensated for by appropriate feed of additional quantities of sodium chloride and/or chlorine :.
:~ by lines 192 and 190 respectively.
The process of the invention therefore.represents i an economic integration of.a hydrochloric acid-based chlorine dioxide-generating system and an effluent-free , pulp mill requiring, after an initial charge of sodium and chlorine values, only water and make up quantities of sodium and chlorine values to maintain the system in ~ balance.
. Modifications are possible within the scope of the invention.
... . .
,~ . .
A variety of reaction ~essels for chlorine dioxide generation may be utilized in the practice of the present invention. A suitable one is described in Canadian Pate~t No. 825,084 issued October 14, 1969 to Electric Reduction Company of Canada, Ltd. modified as required. Such Canadian patent describes the produc-tion of chlorine dioxide from an alkali metal chlorate, an alkali metal chloride and sulphuric acid in a single vessel generator-evaporator-crystallizer. An alternative form of reaction vessel together with a recycle and feed system is described in our copending Canadian application Serial No. 175.282.
The crystalline sodium chloride precipitated from the reaction solution is removed from the reaction zone, continuously or intermittently, and washed with water to remove last traces of trapped reaction medium.
The spent wash water may be recycled to the reaction zone.
- \ -The sodium chloride thereby is recovered in a ~ubstantially pure state.
Sodium chloride also is removed from the effluent-free pulp mill. The sodium chlorate feed for the reaction zone may be provided by electrolysis of a sodium chloride solution formed from sodium chloride removed from the chlorine dioxide-producing reaction zone or from the effluent-free mill.
Mixtures of sodium chloride from both sources may be used.
The electrolysis reaction results in an aqueous solution containing sodium chlorate and sodium chloride which may be fed directly to the reaction zone, the sodium chloride content recycling as a dead load.
The hydrochloric acid feed for the generator is formed by reaction of chlorine with part of the hydrogen off-gas from the chlorate cell. The chlorine is provided partly by that recovered from the gases formed in the chlorine dioxide-producing zone and partly by part of that formed in the caustic-chlorine cell.
The reaction occurring in the chlorate~cell is:
NaCl + 3H2O C o3 3H2 and the reaction involved in producing hydrogen chloride by combustion is: ~
H2 - C12 3 2HCl ~ -It will be seen from the above equation 1 for the production of chlorlne dioxide that only two moles of hydrogen chloride ---(as hydrochloric acid in the reaction medium) is required for each mole of sodium chlorate. Therefore, only approximately one-third of the mole quantity of hydrogen produced in the chlorate cell need be combusted with the same molar quantity of chlorine to produce the required mole amount of hydrogen chloride, the required amount depending on the efficiency of the chlorine dioxide producing reaction.
.: ' ' - ' : ' :,. ' .
The excess hydrogen from the cell off-gases may be burned in air or oxygen to produce heat which enables the heat balance of the operation to be maintained, and water which may be used as part of the water require-ment.
The chlorine dioxide solution recovered from the gaseous mixture produced in the generator is used in the bleach plant of the pulp mill, wherein the pulp is subjected to bleaching and caustic extraction oper-ations.
Sodium hydroxide solution for use in the causticextraetion operations is formed by electrolysis of sodium chloride solution, which also. produces chlorine. Part of - this chlorine is used in the bleach plant as bleaching chemical and the remainder of the chlorine is used to form . . hydrochloric acid for feed to the chlorine dioxide-producing reaction.
The pulp is washed in the bleach plant, usually after each of the bleaching and caustic extraction steps.
:20 : :In the.effluent-free pulp mill, the spent liquids of the bleach plant are mixed to form a bleach plant effluent which is discharged from the bleach plant to the spent . pulping liquor fed to the recovery and regeneration opera-., ::
;~ : tions resulting in fresh pulping liquor for recycle to ~ the pulping stage.
.~ AS set forth in Canadian.Patent No. 956,754 the - bleach plant effluent may be formed by a countercurrent flow of.wash water with respect to the flow of pulp through the bleach plant, the bleach plant effluent being~
used to wash the pulp free from entrained spent pulping liquor and passing to the spent pulping liquor.
The mixing of the bleach plant chemicals to g .. . : . , . :
. .
form the bleach plant effluent results in the formation of sodium chloride by reaction between the sodium values of the sodium hydroxide and the chlorine values of the chlorine dioxide and chlorine. The quantities of sodium and chlorine values fed to the bleach plant are balanced to provide all such values as sodium chloride in the bleach plant effluent.
By passing the bleach plant effluent to the pulp mill spent pulping liquor recovery and regeneration operations, preferably by utiiizing at least part thereof to wash the pulp prior to its passage to the bleach plant to decrease the overall mill water requirement, the dis-'charge of polluting liquid effluents from the mill isavoided. The furnacing step of the recovery and regen-eration operations burns off the carbonaceous materials introduced to the spent pulping liquor'with the bleach plant effluent.
The sodium chloride introduced with the bleach ~ -plant effluent remains unaffected by the recovery and ... .
20 regeneration steps and is removed from the pulping `'' components prior to their recycle'to the pulping step, typically using the pulping liquor evaporation procedure ` outlined in Canadian Patent No. 915,361 mentioned above.
The separated sodium chloride along with the sodium chloride removed from the chlorine dioxide-producing reaction is used to provide the sodium chlorate feed for - the chlorine dioxide-producing reaction and the sodium hydroxide and chlorine used in the bleach plant.
Additional quantities of sodium chloride to those introduced by the bleach plant effluent may be pre-sent in the pulping liquor and removed during the removal step. Such additional quantities of sodium chloride may arise from sea-borne logs in coastal regions, or from -~
~ 053604 the use of brackish water or contaminated chemicals. The additional sodium chloride, along with additional chlorine, may be used as required to make-up for losses from the system.
The invention is described further, by way of illustration, with reference to the accompanying drawing, which is a schematic flow sheet for a pulp mill system in accordance with the present invention.
Referring to the drawing, a pulp mill 110 includes a pulping zone 112 to which wood chips 114 or other cellu-losic fibrous material, such as sawdust, are fed by line 114. Pulping liquor containing at least one active pulping chemical is fed to the pulping zone 112 by line 116. The active pulping chemical(s) may be any de-sired material, for example, sodium sulphide and sodium hydroxide in a Kraft mill process, sodium hydroxide in a soda mill process or sodium sulphide in an alkafide pro-~ cess. -Spent pulping liquor from the pulping operation is passed by lines 118 and 120 to a recovery and regenera-tion Operation 122 wherein the spent pulping liquor is converted into fresh pulping liquor in line 124.
The steps involved in the recovery and regenera-tion operation depend on the particular pulping process used but generally include a concentration step to decrease the volume of liquor and a furnacing step to remove carbonaceous materialsand to form a smelt contain-ing one or more chemicals usable as and/or convertible into pulping chemicals.
The pulp formed in the pulping zone 112 after washing is passed by line 126 to a bleach plant 128 where-in the pulp is subjected to brightening and purification steps to form a bleached pulp of the required brightness : ' : ' '. , ' : ' ' ;: ' ' . - : , .
.
and purity which is removed from the bleach plant by line 130.
. . The brightening and purification steps typically may comprise an initial bleaching using a mixture of chlorine dioxide and chlorine using a chlorine dioxide solution fed by line 132 and a chlorine feed in line 134, a caustic extraction using an aqueous sodium hydroxide solution fed by line 136, a second bleaching using the aqueous chlorine dioxide solution in line 132, a second : 10 caustic extraction using the aqueous sodium hydroxide solu-tion in line 136 and a final bleaching using the aqueous -chlorine dioxide solution in line 132. Washing usual}y is carried out intermediate each chemical treatment using water fed by.line 138.
The used chemicals and wash water are combined to form a single effluent, namely, bleach plant effluent, - . in line 140. The mi.xing of the used chemicals and wash water to form the bleach plant effluent may be carried .~ out using the countercurrent procedure outlined in the ab~ve-mentioned Canadian Patent No. 956,754.
.~ ~ Owing to the presence of sodium and chlorine values in the used bleach plant chemicals, sodium chloride .
is formed, and is present in the bleach plant effluent 140 together with the carbonaceous materials removed from -.
the pulp in the brightening and purification steps. The process is operated so that the sodium values fed to the bleach plant in line 136 balance the chlorine values fed to the bleach plant in lines 132 and 134, whereby all the sodium and all the chlorine values introduced to the bleach plant 128 emerge as sodium chloride in the bleach plant effluent 140.
. - 12 ,, ~05360~ - /3 The bleach plant effluent in line 140 is intro-duced to the spent pulping liquor in line 118 to form a combined waste liquor feed in line 120 to the recovery and regeneration operation 122. Preferakly, the bleach plant effluent in line 140 first is used to wash the pulp in line 126 before the passage of the washed pulp to the bleach plant 128, in accordance with the procedure out-lined in the above-mentioned ~ahadian Patent No. 956,754.
The recovery and regeneration operation evapor-ates water from the bleach plant effluent and removes the carbonaceous materials. Water removed from the waste liquor.in line 120 in the recovery and regeneration operation 122 may be recovered and used, after cleaning, if necessary, elsewhere in the system, such as in the water supply to the bleach piant in line 138.
The sodium chloride content of the bleach plant effluent, however, is unaffected chemically by the .
recovery and regeneration operation and hence is present in the smelt.
? Sodium chloride is removed from the pulp mill 110 in a sodium chloride removal zone 142. The sodium chloride removal may be carried ou~-by evaporating the regenerated liquor in line 124 in accordance with the pro-cedure outlined in the above-mentioned Canadian Patent No.915,361tO deposit sodium chloride therefrom, substan-tially pure crystalline sodium chloride being removed from the sodium chloride removal zo~e 142 by line 144.
The removal of sodium chloride from the pulp mill may be carried out by any other convenient procedure, before or after conversion of all the reusable components of the smelt to pulping chemicals.
The pulping liquor having a decreased sodium chloride concentration may be recycled by line 116 to the pulping zone 112, after dilution, if desired.
~ ;
-, . , . :
The quantity of pure sodium chloride removed from the pulp mill 110 is substantially equal to that intro-duced to or formed within the pulp mill 110. Generally, the quantity of pure sodium chloride removed from the pulp mill 110 by line 144 is equal to that formed in the bleach plant effluent 140 from the sodium hydroxide, chlorine dioxide and chlorine fed to the bleach plant by lines 136, 132 and 134 respectively.
However, additional quantities of sodium chloride may be introduced to the pulp mill 110 with the wood chips in line 114 where they are formed from sea-borne logs, or from the use of brackish water. Such additional quantities of sodium chloride are removed from the pulp mill 110 in the sodium chloride removal zone 142 to prevent their build up in the pulp mill 110.
- While the pulping liquor in line 116 contains residual quantities of sodium chloride, under steady state conditions these residual quantities recycle through the system as a dead load and do not build up when the quantity Of pure crystalline sodium chloride removed by line 144 equals that introduced to and formed in the pulp mill 110. ~ -A chlorine dioxide generator 146 is provided to which aqueous sodium chloride solution is fed by line 148 and hydrochloric acid is fed by line 150, the chlorine dioxide-producing reaction medium within the generator 146 being maintained at its boiling point at the absolute pressure thereon and the generator 146 being maintained under a subatmospheric pressure, whereby the boiling tem-perature of the chlorine dioxide-producing reaction medium - 30 is main~ained below the temperature above which substan-tial decomposition of chlorine dioxide occurs.
The chlorine dioxide-producing reaction medium under continuoussteady stat; operation is saturated with "'''~ ' ' " ' ' ~, sodium chloride and hence solid crystalline sodium chloride ' is precipitated from the reaction medium in the generator 146.
Chlorine dioxide, chlorine and steam are formed in the generator 146 and are removed as a gaseous mixture by line 152, the steam maintaining the partial pressure of chlorine dioixde in the gaseous phase well below that of chlorine dioxide decomposition.
The quantity of steam removed from.the genera-tor 152 balanc~ the water formed in the chlorine dioxide-producing reaction and that introduced with the sodium chlorate solution in line 148 and the hydrochloric acid 150 ,to maintain the liquid level in the generator 146 at a sub'stantially constant value. At the same time, the feed of hydrochloric acid in line 150 maintains the acidity of the chlorine dioxide-producing reaction medium in the generator 146 at about 0.05 to about 0.2 N.
, . The mixture of chlorine dioxide, chlorine and steam in line 152 is passed to an adsorption tower 154 wherein the mixture is contacted with water fed by line 156, causing condensation of the steam and the formation of an aqueous solution of chlorine dioxide. The aqueous . solution of chlorine dioxide contains some dissolved : chlorine, the proportion of which may be decreased by air stripping prior to passage of the chlorine dioxide solution by line 132 to the bleach plant 128.
The gaseous chlorine resulting from the adsorp-tion tower 154 passes therefrom by line 158. A vacuum pump (not shown~, or other suitable means, may be provided in line 158 to maintain the vacuum in the generator 146.
The chlorine in line 158 forms parts of a com-bined chlorine feed stream 160 to a hydrogen chloride reactor and adsorber 162. In the hydrogen chloride .. . .. .
~053604 reactor-adsorber 162, the chlorine fed by line 160 is reacted with hydrogen to form hydrogen chloride which then is adsorbed in water fed by line 164 to form hydrochloric acid for feed to the chlorine dioxide generator 146.
While a single hydrogen chloride reactor-adsor-ber-is shown this is for ease of illustration and normally the two separate operations are carried out in separate apparatus.
The hydrogen reacted with the chlorine to form the hydrogen chloride is fed by line 166 from a chlorate cell 168 wherein aqueous sodium chloride solution fed by line 170 is electrolysed to form sodium chlorate solution for feed to the chlorine dioxide generator by line 148 and the hydrogen in line 166. The quantity of hydrogen in line 166 contains an excess to the stoichiometric requirement of hydrogen chloride and the excess may be vented by line 172.
. The crystalline sodium chloride deposited from the reaction medium in the chlorine dioxide generator 146 is removed by line 174 and, after washing to remove en-trained liquor, the wash water being returned to the generator 146, typically with the sodium chlorate solution in line 148, is passed to a sodium chloride dissolver 176 to which also is passed the pure crystalline sodium chloride removed from the pulp mill 110 by line 144.
In the sodium chloride dissolver 176, the sodium chloride is formed into an aqueous solution with water fed by line 178, the resulting sodium chloride solution in line 180 being passed partly to the chlorate cell 168 . by line 170 and partly to a caustic-chlorine cell 182 by . line 184. In the caustic-chlorine cell 182, the sodium chloride solution in line 184 is electrolyzed to form sodium hydroxide solution which is passed by line 136 to - 16 - .-' the bleach plant 128 and chlorine in line 186 which partly is fed by line 134 to the bleach plant 128 and partly by line 188 to join with the chlorine in line 158 to form the chlorine feed in line 160. Hydrogen by-product also is formed and is vented by line 189.
While the solid sodium chloride separated from the generator 146 and removed from the pulp mill 110 is illustrated as being forwarded to a common dissolver 176 for formation of the sodium chloride solution fed to the chlorate cell 168 and the caustic-chlorine cell 182, it is possi~le to operate in other alternate manners.
For example, the sodium chloride removed from the chlorine dioxide generator 146 may be formed into the aqueous solution fed to the chlorate cell 168 and the sodium chloride recovered from the pulp mill 110 may be formed into the aqueous solution for feed to the caustic- i chlorine cell 182.
Assuming no losses from the system, chlorine dioxide i~ produced ~holly by equation (1) and 100% separation of chlorine dioxide from chlorine, the following stoichio-metry applies:
Chlorine dioxide generator 146:
` NaC103 + 2HCl ~ C12 + ~C12; + H20 + NaCl Chlorate cell 168:
NaCl + 3H20 ~ NaC103 + 3H2 HCl reactor 162:
. H2 + C12 ~ 2HCl Caustic-chlorine cell 182:
2NaCl + 2H20 ----3 2NaOH + C12 + H2 Bleach Plant:
2NaOH + ~C12 + C102 ) 2NaCl + H20 + 3rO7 The system thus is in balance with regard to sodium and chlorine and the system requires only water . . , .
~053604 which results in a by-product hydrogen. Apart from an initial change of sodium chloride, the integrated system is self-balancing with the exception of the water requirement.
Taking into account the effect of equation (2) in the chlorine dioxide-producing process:
Generator 168:
(l-x)NaClO3 + 2(1-x)HCl ) 2 (12x)C12 + (1-x)H2O + (1-x)NaCl x NaC103-~6xHCl --~ 3xC12 + 3xH20 + xNaCl where x is the mol~r proportion of sodium chlorate react-. .
ing by equation 2.
i.e. NaC103 + 2(1-2x)HCl --~(l-x)C102 + (l+Sx)C12 +
(1+2x)H2O + NaCl As a result of this, the quantity of chlorine required by line 188 is (1-x)C12 in view of the amount of ~ -(1+5x) C12 in line 158. The quantity of chlorine present in line 134, assuming no excess, therefore is (l+x) C12 which then forms sodium chloride with the excess sodium . 20 hydroxide resulting from the presence of (1-x)ClO2 in line .-132.
If, however, it is aesired to keep in the bleach plant the relative proportions of chlorine dioxide and chlorine and the same molar quantity of chlorine dioxide, as discussed above in the ideal system extra chlorine - dioxide must be formed by reaction in the chlorine dioxide generator 146 to compensate for the losses due to equation 2.
- Since, overall, the chlorine dioxide system operates according to the equation:
~C12 + 2H20 ~ C12 + 2H2 the introduction of an additional quantity of chlorine, . . . .
10536¢4 such as by line 190 results in the required compensation.
The extra chlorine required in line 190 may be provided by electrolysis of.additional sodium chlori~e in the caustic~
chlorine cell 182, the extra soda being used to make up any soda losses in the pulp mill 110. The additional sodium chloride may arise from additional quantities recovered from the pulp mill 110 from the sources men-tioned above, or may be introduced from external sources by line 192. .
The inefficiency of separation of chlorine from chlorine dioxide in the adsorption tower 154 resulting in the presence of dissolved.chlorine in the chlorine dioxide solution in line 132 may be compensated for by feeding a decreased amount of chlorine by line 134 so that the total quantity of chlorine fed to the bleach plant !' 128 by lines 132 and 134 remains constant, the additional - -chlorine in line 188 then compensating for the quantity .
absent from line 158. Any other losses of sodium or chlorine values may be compensated for by appropriate feed of additional quantities of sodium chloride and/or chlorine :.
:~ by lines 192 and 190 respectively.
The process of the invention therefore.represents i an economic integration of.a hydrochloric acid-based chlorine dioxide-generating system and an effluent-free , pulp mill requiring, after an initial charge of sodium and chlorine values, only water and make up quantities of sodium and chlorine values to maintain the system in ~ balance.
. Modifications are possible within the scope of the invention.
... . .
,~ . .
Claims (8)
1. A chlorine dioxide producing and utilization process for bleaching cellulosic pulp which comprises:
continuously forming an aqueous solution of chlorine dioxide by the steps of: continuously feeding hydrochloric acid and an aqueous solution of sodium chlorate to a reaction zone while maintaining an aqueous reaction medium in said reaction zone having an acidity of about 0.05 to about 0.5 normal, continuously forming chlorine dioxide and chlorine by reaction between said sodium chlorate and said hydrochloric acid in said reaction medium, continuously maintaining said reaction medium at its boiling point and at a temperature below that above which substantial decomposition of chlorine dioxide occurs to boil water from said reaction medium to form continuously a gaseous phase in said reaction zone con-sisting essentially of a mixture of chlorine dioxide, chlorine and water vapor and to maintain the liquid level in said reaction zone substantially constant, continuously maintaining said reaction zone under a subatmospheric pressure, con-tinuously removing said gaseous mixture of chlorine dioxide, chlorine and water vapor from said reaction zone, con-tinuously depositing sodium chloride from said reaction medium in said reaction zone, removing said deposited sodium chloride from said reaction zone, and continuously forming said aqueous chlorine dioxide solution from said removed gaseous mixture;
continuously forming an aqueous solution of sodium chlorate and hydrogen gas by electrolysis of a first aqueous solution of sodium chloride and continuously forwarding the resulting sodium chlorate solution to said reaction zone as said continuous feed of aqueous sodium chlorate solution;
continuously forming hydrochloric acid by reacting hydrogen and chlorine to form hydrogen chloride and absorbing the hydrogen chloride in water, and continuously forwarding the hydrochloric acid to said reaction zone as said continuous feed of hydrochloric acid;
continuously forming chlorine gas and an aqueous solution of sodium hydroxide by electrolysis of a second aqueous solution of sodium chloride;
continuously forming a pulp of a cellulosic fibrous material by digesting raw cellulosic fibrous material with a pulping liquor containing at least one active pulping chemical;
continuously washing said pulp substantially free from entrained pulping chemicals and forming a waste liquor consisting of spent pulping liquor from said digestion and spent wash water from said washing;
subjecting said washed pulp to a series of brighten-ing and purification steps to form continuously a bleached and purified pulp wherein said brightening step(s) is (are) carried out utilizing said continuously-formed solution of chlorine dioxide and part of the chlorine formed in said electrolysis of sodium chloride solution to form sodium hydroxide solution and said purification step(s) is (are) carried out using said aqueous sodium hydroxide solution, the quantity of sodium values in said sodium hydroxide solution used in said purification step(s) and the quantity of chlorine values in said chlorine dioxide and chlorine used in said brightening step(s) being controlled to provide one atom of sodium for each atom of chlorine, and washing said pulp at least after said series of brightening and purification steps and optionally after each individual step of said series;
continuously forming a single aqueous effluent from said series of brightening and purification steps and said latt?r washing, said single aqueous effluent containing used wash water and chemicals with the sodium and chlorine values having been combined to provide sodium chloride therein, and utilizing said single aqueous effluent in said washing of said pulp free from entrained pulping chemicals so that said waste liquor contains said single aqueous effluent;
subjecting said waste liquor to a recovery and re-generation operation to form a smelt containing sodium chloride and at least one material reusable as or convertible into said at least one pulping chemical;
continuously removing solid substantially pure sodium chloride from the components of said smelt in an amount at least equal to the amount introduced to said waste liquor in said single aqueous effluent;
continuously forming pulping liquor containing said at least one pulping chemical from said at least one material and recycling said pulping liquor to said digestion step; and continuously utilizing the sodium chloride removed from the components of said smelt and the sodium chloride removed from said reaction zone to form said aqueous solutions of sodium chloride which are separately electrolyzed to form said sodium chlorate solution and said sodium hydroxide solution.
continuously forming an aqueous solution of chlorine dioxide by the steps of: continuously feeding hydrochloric acid and an aqueous solution of sodium chlorate to a reaction zone while maintaining an aqueous reaction medium in said reaction zone having an acidity of about 0.05 to about 0.5 normal, continuously forming chlorine dioxide and chlorine by reaction between said sodium chlorate and said hydrochloric acid in said reaction medium, continuously maintaining said reaction medium at its boiling point and at a temperature below that above which substantial decomposition of chlorine dioxide occurs to boil water from said reaction medium to form continuously a gaseous phase in said reaction zone con-sisting essentially of a mixture of chlorine dioxide, chlorine and water vapor and to maintain the liquid level in said reaction zone substantially constant, continuously maintaining said reaction zone under a subatmospheric pressure, con-tinuously removing said gaseous mixture of chlorine dioxide, chlorine and water vapor from said reaction zone, con-tinuously depositing sodium chloride from said reaction medium in said reaction zone, removing said deposited sodium chloride from said reaction zone, and continuously forming said aqueous chlorine dioxide solution from said removed gaseous mixture;
continuously forming an aqueous solution of sodium chlorate and hydrogen gas by electrolysis of a first aqueous solution of sodium chloride and continuously forwarding the resulting sodium chlorate solution to said reaction zone as said continuous feed of aqueous sodium chlorate solution;
continuously forming hydrochloric acid by reacting hydrogen and chlorine to form hydrogen chloride and absorbing the hydrogen chloride in water, and continuously forwarding the hydrochloric acid to said reaction zone as said continuous feed of hydrochloric acid;
continuously forming chlorine gas and an aqueous solution of sodium hydroxide by electrolysis of a second aqueous solution of sodium chloride;
continuously forming a pulp of a cellulosic fibrous material by digesting raw cellulosic fibrous material with a pulping liquor containing at least one active pulping chemical;
continuously washing said pulp substantially free from entrained pulping chemicals and forming a waste liquor consisting of spent pulping liquor from said digestion and spent wash water from said washing;
subjecting said washed pulp to a series of brighten-ing and purification steps to form continuously a bleached and purified pulp wherein said brightening step(s) is (are) carried out utilizing said continuously-formed solution of chlorine dioxide and part of the chlorine formed in said electrolysis of sodium chloride solution to form sodium hydroxide solution and said purification step(s) is (are) carried out using said aqueous sodium hydroxide solution, the quantity of sodium values in said sodium hydroxide solution used in said purification step(s) and the quantity of chlorine values in said chlorine dioxide and chlorine used in said brightening step(s) being controlled to provide one atom of sodium for each atom of chlorine, and washing said pulp at least after said series of brightening and purification steps and optionally after each individual step of said series;
continuously forming a single aqueous effluent from said series of brightening and purification steps and said latt?r washing, said single aqueous effluent containing used wash water and chemicals with the sodium and chlorine values having been combined to provide sodium chloride therein, and utilizing said single aqueous effluent in said washing of said pulp free from entrained pulping chemicals so that said waste liquor contains said single aqueous effluent;
subjecting said waste liquor to a recovery and re-generation operation to form a smelt containing sodium chloride and at least one material reusable as or convertible into said at least one pulping chemical;
continuously removing solid substantially pure sodium chloride from the components of said smelt in an amount at least equal to the amount introduced to said waste liquor in said single aqueous effluent;
continuously forming pulping liquor containing said at least one pulping chemical from said at least one material and recycling said pulping liquor to said digestion step; and continuously utilizing the sodium chloride removed from the components of said smelt and the sodium chloride removed from said reaction zone to form said aqueous solutions of sodium chloride which are separately electrolyzed to form said sodium chlorate solution and said sodium hydroxide solution.
2. The process of claim 1 wherein said hydrogen reacted with chlorine to form hydrogen chloride is provided by part of said hydrogen gas formed in said electrolysis to form aqueous sodium chlorate solution.
3. The process of claim 1 wherein said chlorine reacted with hydrogen to form hydrogen chloride is provided at least in part by chlorine recovered from said gaseous mixture of chlorine dioxide, chlorine and water vapor and by chlorine formed in said electrolysis to form sodium hydroxide solution and not used in said brightening step(s).
4. The process of claim 1 wherein said removal of sub-stantially pure sodium chloride from said components of said smelt is carried out by forming said smelt into an aqueous solution thereof, if necessary converting said at least one material to an active pulping form to form a pulping liquor containing said at least one pulping chemical, evaporating the pulping liquor to deposit sodium chloride therefrom and separating substantially pure solid sodium chloride from the deposited materials.
5. The process of claim 1 wherein said sodium chloride removed from said reaction zone and said sodium chloride removed from the components of said smelt continuously are formed into a common aqueous sodium chloride solution which is divided into the separate feeds for the two electrolysis reactions.
6. The process of claim 1 wherein said sodium chloride removed from said reaction zone is formed into an aqueous solution thereof and said latter aqueous solution is electrolyzed to form said aqueous sodium chlorate solution, and said sodium chloride removed from the components of said smelt is formed into an aqueous solution thereof which is electrolyzed to form said aqueous sodium hydroxide solution.
7. The process of claim 1 wherein said chlorine dioxide and chlorine are continuously generated by the simultaneously-occurring reactions represented by the equations:
(1-x)NaClO3 + 2(1-x)HCl?(1-x)ClO2 + Cl2 +
(1-x)H2O + (1-x) NaCl .(1) xNaClO3 + 6xHCl ? 3xCL2 + 3xH2O + xNaCL .(2) where x is the molar proportion of sodium chlorate reacting by equation (2), sodium chlorate for said reactions of equations (1) and (2) is continuously formed by electrolysis in accordance with the equation:
NaCl + 3H2O ? NaClO3 + 3H2 .(3) hydrochloric acid for said reactionsof equations (1) and (2) is continuously formed by absorbing in water hydrogen chloride formed by reacting hydrogen and chlorine in accordance with the equation:
(1+2x) H2 + (1+2x) Cl2? 2(1+2x) HCl .(4) where said (1+2x) H2 is provided from said 3H2 formed by the reaction of equation (3), said sodium hydroxide solu-tion is formed by electrolysis of sodium chloride solution in accordance with the equation:
2NaCl + 2 H2O? 2NaOH + Cl2 + H2 .(5) said sodium chloride present in said single effluent, removed from said smelt and used in the reaction of equation (5) is formed by reaction according to the par-tial equation:
2NaOH + (1-x) ClO2 + Cl2? 2NaCl .(6) the said Cl2 being provided at least in part from said chlorine formed by the reaction of equation (5), the remainder of said Cl2 being provided by a minor proportion of the formed in the reaction of equations (1) and (2) and chlorine dissolved in said chlorine dioxide solution, said (1+2x) C12 used in the reaction of equation (4) being provided by the major pro-portion of said and the proportion of chlorine formed by the reaction of equation (5) and not used to provide said Cl2 used in the reaction of equation (6).
(1-x)NaClO3 + 2(1-x)HCl?(1-x)ClO2 + Cl2 +
(1-x)H2O + (1-x) NaCl .(1) xNaClO3 + 6xHCl ? 3xCL2 + 3xH2O + xNaCL .(2) where x is the molar proportion of sodium chlorate reacting by equation (2), sodium chlorate for said reactions of equations (1) and (2) is continuously formed by electrolysis in accordance with the equation:
NaCl + 3H2O ? NaClO3 + 3H2 .(3) hydrochloric acid for said reactionsof equations (1) and (2) is continuously formed by absorbing in water hydrogen chloride formed by reacting hydrogen and chlorine in accordance with the equation:
(1+2x) H2 + (1+2x) Cl2? 2(1+2x) HCl .(4) where said (1+2x) H2 is provided from said 3H2 formed by the reaction of equation (3), said sodium hydroxide solu-tion is formed by electrolysis of sodium chloride solution in accordance with the equation:
2NaCl + 2 H2O? 2NaOH + Cl2 + H2 .(5) said sodium chloride present in said single effluent, removed from said smelt and used in the reaction of equation (5) is formed by reaction according to the par-tial equation:
2NaOH + (1-x) ClO2 + Cl2? 2NaCl .(6) the said Cl2 being provided at least in part from said chlorine formed by the reaction of equation (5), the remainder of said Cl2 being provided by a minor proportion of the formed in the reaction of equations (1) and (2) and chlorine dissolved in said chlorine dioxide solution, said (1+2x) C12 used in the reaction of equation (4) being provided by the major pro-portion of said and the proportion of chlorine formed by the reaction of equation (5) and not used to provide said Cl2 used in the reaction of equation (6).
8. The process of claim 7 including increasing the quantity of chlorine dioxide used in said brightening step(s) from (1-x) to 1 by increasing the quantity thereof formed by the reaction of equation (1) at the prevailing value of x, said increase being provided by reaction of additional chlorine in accordance with the overall equation:
xCl2 + 2xH2O ? xClO2 + 2xH2 .(7)
xCl2 + 2xH2O ? xClO2 + 2xH2 .(7)
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA219,729A CA1053604A (en) | 1975-02-10 | 1975-02-10 | Production of chlorine dioxide and integration with effluent-free pulp mill |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA219,729A CA1053604A (en) | 1975-02-10 | 1975-02-10 | Production of chlorine dioxide and integration with effluent-free pulp mill |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1053604A true CA1053604A (en) | 1979-05-01 |
Family
ID=4102249
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA219,729A Expired CA1053604A (en) | 1975-02-10 | 1975-02-10 | Production of chlorine dioxide and integration with effluent-free pulp mill |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1053604A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5324497A (en) * | 1992-02-26 | 1994-06-28 | Westerlund G Oscar | Integrated procedure for high yield production of chlorine dioxide and apparatus used therefor |
-
1975
- 1975-02-10 CA CA219,729A patent/CA1053604A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5324497A (en) * | 1992-02-26 | 1994-06-28 | Westerlund G Oscar | Integrated procedure for high yield production of chlorine dioxide and apparatus used therefor |
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