CA1089157A - Bleach plant operation - Google Patents
Bleach plant operationInfo
- Publication number
- CA1089157A CA1089157A CA279,885A CA279885A CA1089157A CA 1089157 A CA1089157 A CA 1089157A CA 279885 A CA279885 A CA 279885A CA 1089157 A CA1089157 A CA 1089157A
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- Canada
- Prior art keywords
- pulp
- washer
- seal tank
- seal
- washing
- Prior art date
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Abstract
BLEACH PLANT OPERATION
ABSTRACT OF THE DISCLOSURE
A pulp mill bleach plant operation having a low effluent volume, a low consumption of water, energy and chemicals, and yet provides efficient bleaching, caustic extraction and washing is described. Water conservation is practised by controlling the use of wash water in the bleach plant, controlling the design and operation of washers, deckers and other mechanical devices used in the bleach plant and controlling the inflow of water with chemicals.
ABSTRACT OF THE DISCLOSURE
A pulp mill bleach plant operation having a low effluent volume, a low consumption of water, energy and chemicals, and yet provides efficient bleaching, caustic extraction and washing is described. Water conservation is practised by controlling the use of wash water in the bleach plant, controlling the design and operation of washers, deckers and other mechanical devices used in the bleach plant and controlling the inflow of water with chemicals.
Description
:
This invention relates to a pulp mill bleach plant, and in particular, relates to a bleach plant construction and operation for use in a liquid effluent free bleached pulp mill.
In a liquid effluent free ~leached pulp mill, in which bleached pulp is formed by digesting cellulosic fibrous material and bleaching and purifying the pulp and in which spent pulping liquors are subjected to recovery and ~egenera-tion to form fresh pulping liquor, liquid effluents from the bleaching and purification operations (bleach plant effluent) --are discharged into the recovery and regeneration operation.
The organic materials content of the bleach plant effluent is burned off in the recovery furnace of the recovery and regeneration operation and the aqueous phase is evaporated in the recovery and regeneration operation.
Owing to the high cost of evaporating water in a pulp mill, in the interests of minimizing operating costs, it is desirable to decrease the total volume of bleach plant ~ effluent which must be discharged into the pulp miIl recovery ; 20 and regeneration operation and hence minimize the total ~; evaporation load. It is also desirable that any bleach plant ~ ;
effluent volume decrease not significantly adversely affect `
the pulp ~uality obtained. ~~
~ , - S
In accordance with the present invention, there is provided a bleach plant operation in which water conservation is practised by controlling the use of wash water in the bleach plant, controlling the~design and operation of washers, .
deckers and other mechanical devices used in the bleach plant, and controlling the inflow of water with chemicals.
' 3~ ' The invention is particularly applicable to a bleach plant operation using a D/CEDED sequence, in which D/C means ~ ;
bleaching with an aqueous solution of chlorine dioxide and chlorine wherein the chlorine dioxide provides the majority of the available chlorine of the solution, D means bleaching with an aqueous solution of chlorine dioxide and E means caustic extraction with aqueous sodium hydroxide solution.
The bleach plant operation of the invention produces two liquid effluents from the bleach plant, one acid and the other alkaline. These effluents then pass to ;
the recovery operation, preferably in accordance with the teachings of our Canadian Patent No. 1,070,908. ~-~
The inyention is described further, by way of illustration, with reference to the accompanying arawings, in which:
Figure 1 is a schematic flow sheet of a liquid ~ -effluent free pulp mill; and Figure 2 is a schematic flow sheet of a bleach ~-plant in accordance with one embodiment of the invention. -~
. . :. ~ . . .
Referring first to Figure 1, wood chips are digested -`
in pulping liquor in a digester 1 and pass to a brown stock ~-washer 2 wherein the pulp is freed from entrained pulping liquor. The pulp then passes to the bleach plant 3 for ~-;~ bleaching and purification with intermediate washing operations, using chlorine dioxide and chlorine, chlorine .
dioxide and sodium hydroxide solutions and water, and bleached pulp is recovered by line 4.
Spent pulping liquor from the brown stock washer
This invention relates to a pulp mill bleach plant, and in particular, relates to a bleach plant construction and operation for use in a liquid effluent free bleached pulp mill.
In a liquid effluent free ~leached pulp mill, in which bleached pulp is formed by digesting cellulosic fibrous material and bleaching and purifying the pulp and in which spent pulping liquors are subjected to recovery and ~egenera-tion to form fresh pulping liquor, liquid effluents from the bleaching and purification operations (bleach plant effluent) --are discharged into the recovery and regeneration operation.
The organic materials content of the bleach plant effluent is burned off in the recovery furnace of the recovery and regeneration operation and the aqueous phase is evaporated in the recovery and regeneration operation.
Owing to the high cost of evaporating water in a pulp mill, in the interests of minimizing operating costs, it is desirable to decrease the total volume of bleach plant ~ effluent which must be discharged into the pulp miIl recovery ; 20 and regeneration operation and hence minimize the total ~; evaporation load. It is also desirable that any bleach plant ~ ;
effluent volume decrease not significantly adversely affect `
the pulp ~uality obtained. ~~
~ , - S
In accordance with the present invention, there is provided a bleach plant operation in which water conservation is practised by controlling the use of wash water in the bleach plant, controlling the~design and operation of washers, .
deckers and other mechanical devices used in the bleach plant, and controlling the inflow of water with chemicals.
' 3~ ' The invention is particularly applicable to a bleach plant operation using a D/CEDED sequence, in which D/C means ~ ;
bleaching with an aqueous solution of chlorine dioxide and chlorine wherein the chlorine dioxide provides the majority of the available chlorine of the solution, D means bleaching with an aqueous solution of chlorine dioxide and E means caustic extraction with aqueous sodium hydroxide solution.
The bleach plant operation of the invention produces two liquid effluents from the bleach plant, one acid and the other alkaline. These effluents then pass to ;
the recovery operation, preferably in accordance with the teachings of our Canadian Patent No. 1,070,908. ~-~
The inyention is described further, by way of illustration, with reference to the accompanying arawings, in which:
Figure 1 is a schematic flow sheet of a liquid ~ -effluent free pulp mill; and Figure 2 is a schematic flow sheet of a bleach ~-plant in accordance with one embodiment of the invention. -~
. . :. ~ . . .
Referring first to Figure 1, wood chips are digested -`
in pulping liquor in a digester 1 and pass to a brown stock ~-washer 2 wherein the pulp is freed from entrained pulping liquor. The pulp then passes to the bleach plant 3 for ~-;~ bleaching and purification with intermediate washing operations, using chlorine dioxide and chlorine, chlorine .
dioxide and sodium hydroxide solutions and water, and bleached pulp is recovered by line 4.
Spent pulping liquor from the brown stock washer
2 in line 5 and two aqueous effluents from the bleach plant 3 in line 6 pass to a recovery and regeneration operation 7 wherein combustible organic materials are combusted, pulping liquor is regenerated and sodium chloride, arising from the sodium atoms and the chlorine atoms in the bleach plant effluents, is removed by line 8 to prevent its build up in the system. Regenerated pulping liquor is recycled to the digester 1 by line 9. ~ ~:
In the aqueous effluent-free pulp mill as illustra-ted in Figure 1, noxious aqueous effluents from the pulp mill are eliminated by discharge of the bleach plant effluents `
into the recovery and regeneration operation 7. The organic ~ .
. materials content of the bleach plant effluent is burned off -.
in the recovery furnace of the recovery and regeneration operation 7 while the aqueous phase is evaporated.
''.' ~;:
In Figure 2, there is illustrated a detailed flow :` ;
~ sheet which includes a bleach plant 3 constructed and operated :~: - in accordance with the present invention for use in the ~ ~
effluent-free pulp mill illustrated in Figure 1. ~.
Referring to Figure 2, a pulp treatment operation - ~ lQ, incorporating a bleach plant designed to use a D~CEDED
b~leaching and caustic extraction sequence, includes a DjC
bleaching tower 12, an El caustic extraction tower 14, a D
bleaching tower 16, an E2 caustic extraction tower 18 and a ` .
D2 bleaohin~ tower 20.
Drum washers are provided for each bleaching and caustic extraction stage including a D/C washer 22, an El :
washer 24, a Dl washer 26, an E2 washer 28 and D2 washer -~:
: ~ 30. Seal tan~s are associated with each of the washers, ~ - 4 ~
`: :
including a D/C seal tank 32, an El seal tank 34, a Dl ~
seal tank 36, an E2 seal tank 38 and a D2 seal tank 40. ~:.
An -unbleached decker 42 in the form of a displacement washer is provided along with an associated seal tank 44. A similar bleached decker 46 is provided with ; an associated seal tank 48. Unbleached pulp and bleached ~:
pulp storage towers 50 and 52 respectively are provided. . :~ :
Screens and cleaners 54 and 56 are provided for unbleached pulp and bleached pulp respectively.
Unbleached pulp from the brown stock washing is ~ .
passed by line 58 through the screens and cleaners 54 and via line 60 to the unbleached decker 42 and thence by line 62 to the storage tank 50. The pulp is passed from .
: the storage tank 50 by line 64 to a sensor 66 which senses chlorine dioxide and chlorine required by the pulp. From the sensor the pulp passes by line 68 to the D/C bleaching tower 12. ~
.After D/C bleadhing, the pulp passes by line 70 `
to the D/C washer 22. From the D/C washer the pulp passes ~ by line 72~through an El mixer 73 and by line 74 to the El ~ -:~ extraction tower 14. The pulp passes from the El tower 14 by line 76 to the El washer 24 and thence by line 78 to a :~ : . Dl mixer 80 and by line 82 to the Dl bleaching tower 16.
, ~ Following Dl bleaching, the pulp next passes by f~ line 84 to the Dl washer 26 and by line 86 to the E2 extraction ~ ~ tower 18. From E2 extraction, the pulp passes by line 88 to i ~ the E2 washer 28, by line 90 to a D2 mixer 92 and by line 94 to the D2 bleaching tower 20.
:: : The pulp next passes by line 96 to the D2 washer 30 before passage by line 98 to the screens and cleaners 56, q by line 100 to the bleached decker 46 and by line 102 to ,. -` ~ bleached pulp storage 52. From the bleached pulp storage ~ ~
_ 5 _ .. ..
52 the pulp is passed forward by line 104, such as, to a pulp drying machine, not shown.
Each of the washers 22, 24, 26, 28 and 30 and each of the de~kers 42 and 46 is a rotating foraminous drum type having wash water shower bars 106 arranged adjacent the periphery thereof for the application of wash water to the pulp mat as it is transported on the drum surfàce, the water displaced from or passing through the mat passing to the appropriate seal tank by lines 108, 110, 112, 114, 116, 118 and 120 respectively. Wire cleaners 122 also are provided for each of the washers and deckers. ~- :
A complete countercurrent flow wash water system is utilized. Thus, fresh water passes to the pulp drying machine ~`
and pulp drying machine white water or other relatively fresh water is passed by line 124 to the showers 106 on the bleached ,, - :
decker 46 to displacement wash the pulp mat on the bleached `
decker. Part of the displaced liquor collected in the seal tank 48 is passed by lines 126 and 128 to the showers 106 .
of the D2 washer 30, with the remainder passing by lines 126 and 130 for dilution of the pulp in line 98 before passage of the diluted pulp to the screens and cleaners 56.
Part of the liquor collected in the D2 seal tank 40 passes by lines 132 and 134 to the washing showers 106 -~
on the E2 washer 2B. Part of this liquor passes by lines 132 `~
` and 136 to pulp~passing from the D2 ~leaching tower 20 to the `~
, ~ - .
D2 washer 30 in line 96. Part of the collected liquor passes from the seal tank 40 by line 138 to the D `~
` 2 bleachlng tower 20.
The liquor collected in the E2 seal tank 38 partially passes by lines 140, 142 and 144 to the washing ~;
shower 106 of the Dl washer 26. Another portion of this liquor passes by lines 140 and 146 to the pulp passing in ~089157 line 88 from ~he E2 extr~ction tower 18 to the E2 washer 28. ~ ~!
Further quan~ities of the collected liquor pass by line 148 to the E2 extraction tower 18. -~
A small quantity of the liquor collected in the E2 seal tank is used to dilute caustic extraction chemical :~
and is passed to the caustic extraction chemical inlet feed ~:
line for this purpose by lines 140, 142 and 150.
The Dl seal tank liquor is partially passed by line 152 to the Dl bleaching tower 16. A portion of the liquor passes by lines 154, 156 and 158 to the showers 106 .
on the El washer 24, while another portion of the liquor :-:
passes by lines 154, 156 and 160 to the first showers 106 on the D/C washer 22. Part of the Dl seal tank liquor ~:
passes by lines 154 and 162 to the pulp in line 84 passing from the Dl bleaching tower to the Dl washer 26.
:~ . From the El seal tank 34, part of the liquor ,~
passes by lines 163, 164 and 166 to the last showers 106 :
on the D/C washer 22 while another part passes by lines 163/ -^ ;
164 and 168 to the pulp passing in line 76 from the El ~ extraction tower 14 to the El washer 24. Another part :; passes by line 170 to the El extraction tower 14 while the :~ remainder is discharged from the bleach plant by lines 163 and 172 for passage to the pulp mill recovery system. :
Liguor from the D/C seal tank 32 partly passes by lines 174 and 176 to dilute the pulp passing by line 64 ;~
from the storage 50 to the sensor 66 with another part passing . ~. , ; . :
by lines 174 and 178 to the pulp passing by line 70 from the D/C bleaching tower 12 to the D/C washer 22. Another part of the llquor~is passed by lines 180, 182 and 184 to the '~
showers 106 of the un~leached dec~er 42. The liquor passing ;~
:~ ~ in this way to the showers 106 of the unbleached decker .
~: 42 is neutrali2ed by sodium hydroxide solution fed by line -: - 7 -.~ .
186. The remainder of the liquor passes out of the bleach plant 10 by line 188 for passage to the pulp mill recovery system.
The bleach plant effluents in lines 172 and 188 and the unbleached decker seal tank liquor in line 190 may be utilized as described in our aforemen.tioned Canadian Patent No. 1,070,908.
From the unbleached decker seal tank 44 liquor .
passes by line 190 for use as wash water in the brown . ~
stock washer of the pulp mill and liquor passes by line 192 ~ .
to the unbleached pulp entering the unbleached screens and cleaners 54 by line 58. ~ ~;
It will be seen, therefore, that thére is a general -eountercurrent flow of wash water and pulp through the pulp ~:
treatment'operation 10.
. Each of the cleaner showers 122 on the washers and deckers is fed by hot water. An inlet feed in line 194 feeds the cleaner 122 on the decker 42 by line 196, the ~;~
cleaner 122 on the D/C washer 22 by lines 198 and 200, the -:
cleaner 122 on the El washer 24 by lines 198~ 202 and 204, ;~ .
the cleaner 122 on the Dl washer 26 by lines 198, 202, 206 ..
~ and 208 the cleaner 122 on the E2 washer 28 by lines 198, . -: 202, 206, 210 and 212, the cleaner 122 on the D2 washer 30 by lines lg8, 202, 206, 210, 214 and 216 and the cleaner 122 ~: on the bleached decker 46 by lines 198, 202, 206, 210 214 and ~: 218.
. Steam for heatlng purposes also is used. Thus, : .
~ steam is fed by lines 220 and 222 to the El mixer 72, by ~.
.~ lines 220, 224 and 226 to the Dl mixer 80, by lines 220, 224, ~ :
228 and 230 to an injection ring 232, and by lines 220, 224, ~: 228 and ~234 to the D2 mixer 92. :~
~' .
s -; . , . , . , . , . . ,:
:,: - .. . . . ~
To accommodate emergency overflow conditions, the seal tanks are connected in a countercurrent flow overflow arrangement. Thus, overflow from the bleached decker seal tank 48 passes by line 236 to the D2 seal tank 40, the overflow from the D2 seal tank 40 passes by line 238 to the ..
E2 seal tank 38, the overflow from the E2 seal tank 38 passes .
by line 240 to the D~ seal tank 36 and the overflow from the ~.
Dl seal ~ank 36 passes by line 242 to the El seal tank 34. .-From the E1 seal tank 34, the overflow passes by line 244 to a first spill storage.tank 246 whi~e overflow from the D/C seal tank 32 and the unbleached decker seal tank 44 pass to a second spill tank 248 by lines 249 and 250 and lines 252 and 250 respectively.
- The overflow collected in the second spill tank 248 under emergency conditions may be returned to the system, ~ 6 ...~. , after neutralization with sodium hydroxide solution, at a.
convenient time by lines 254, 256 and 258 while the overflow ;~
collected in the first spill tank 246 passes to the recovery system for use in brown stock washing or white liquor ,. ..
20 dilution.
With this seal tank overflow arrangement, when there is a temporary discharge of a pulp mat of lower than normal consistency from one washer to another, the excess :~ ~
water is returned to the preceding stage by seal tank overflow....... `
~: .
High density pumps are used.for p~mping ~ the pulp through the bleach plant and such pumps use ~ater-:~: fed seal glands. Hot water fed from llne 194 is used for ~ such seal glands. Thus, seal glands of a pump 260 -: :~
- ~ :
conveying the pulp by line 74 are fed by hot water in line ~ 30 262, seal glands of a pump 264 conveying the pulp by line ;......... ~82 are fed by hot water in line 266, seal glands of a pu~p _ g _ .
268 conveying the pulp by lin~ 86 are fed by hot water in line 270, and seal glands of a pump 272 conveying the pulp by line 94 are fed by hot water in line 274. The ~ .
pressure on the pump glands is controlled to minimize the flow of fresh water into the pulp passing through the pump.
Chemical feed for the bleaching and caustic extraction operations is provided by dilute sodium hydroxide solution, aqueous solutions of chlorine dioxide and chlorine .
and sodium hypochlorite solution. Sodium hydroxide solution in concentrated form (.typically 50~- ~y wt~2 i5 fed by line 276 to the system and is diluted by the E2 seal tank liquor ~:
in line 140 to the concentration required. The diluted sodium :
hydroxide solution then is passed by lines 278 and 280 to the pulp leaving the D/C washer 22. Sodium hydroxide ~:
so~ution also is passed by lines 278, 282 and 28~ to the pulp .
leaving the El washer 24 by lines 278, 282, 286 and 288 to the pulp leaving the Dl washer 26, lines 278, 282, 286, 290 ~ .
and 291 to the pulp leaving the E2 washer 28. Dilute sodium hydroxide solution also passes by line 292 to the sodium : :
hydroxide neutralization feed 186,~provision a~so being made for emergency flow to the second spill tank 248 in line 293.
. . .
An aqueous solution of chlorine dioxide and chlorine ~:
is fed by line 294 to the pulp passing by line 64 ;
from the pulp to storage 50 to the sensor 66. Sodium :~
~:: hypochlorite solution is fed by iine 296 to the pulp in :
- line 64.
An aqueous chlorine dioxide solution is fed by ::
~. ~
lines 298 and 300 to the pulp in line 82 before passage thereof into the Dl bleaching tower 16. .A mixer, not shown, ~.
is located immediately after the injection point of the chlorine dioxide solution to ensure even mixing of the . -~
.. "~ - .. . . . .
1089~57 solution with the pulp.
Chlorine dioxide solution is also fed by lines 298 and 302 to the pulp in line 94 before passage thereof into the D2 bleaching tower 20. A mixer, not shown~ is located immediately after the injection point of the chlorine dioxide solution to ensure even mixing of the solution with the pulp.
The chlorine dioxide solution fed by line 298 is one having a low concentration of dissolved chlorine whereby ~
over 90% of the available chlorine content of the chlorine ~ ::
dioxide solution is provided by chlorine dioxide. A typical -solution is one having a chlorine dioxide concentration of ~-10 gpl and chlorine concentration of 2 gpl. ~:~
The chlorine dioxide and chlorine solution fed by ~ :
line 294 is one having a higher dissolved chlorine : concentration than the chlorine dioxide solution in line 298, ~ :~
:~ whereby about 70~ of the available chlorine content of the :~
solution is pro~ided by chlorine dioxide and the remainder of the available ch~orine is provided by the chlorine. A ~ .
suitable solution contains about 10 gpl C102 and about - 6 gpl C12 ~.
. The sodium hypochlorite solution in line 296 ... :
breaks~down under the acid condition of the pulp in line 64 to produce chlorine for tke bleaching of the pulp in the : D/C bleaching tower 12.
~: : . . .
The chlorine dioxide solution in line 298, the .
~; ~ chlorlne dioxide and chlorine solution in line 294 and the -~: `
. , . ~ . . ~
sodium hypochlorite solution in line 296 all may be produced :: ~ from a single chlorine dioxide and chlorine generator, for .
~ u example, using the procedure outlined in U.S. Patent No. :~
: - 4,OIO,112. By the use of the latter chlorine dioxide , ~ ' :
~._, . - .
,1,, , ~ . . , ~ .
~89157 generation system, the most efficient use of bleaching chemicals is obtained while ~he volume of water entering --the bleach plant with chlorine dioxide and chlorine is mini-mized.
In operating the pulp mill system described above, steps are taken to ensure optimum bleaching, caustic :
extraction and washing, minimal consumption of water, energy and chemicals and the discharge of a minimal volume of liquid effluent, in the region of about 4000 U.S. gallons/air dried : ~
ton (USG/ADT) of pulp, as compared with the liquid effluent :-discharge from a conventional bleach plant operation of up to about 25,000 USG/ADT while the bleached pulp produced has -properties at least comparable to those of pulp produced in conventional operations. :~
The fresh water consumption in the bleach plant is very small, with the principal inputs of fresh water to the bleach plant being pulp machine white water, chlorine dioxide solutions and the water in the unbleached pulp.
Thus, not only does this bleach plant operation decrease the . effluent volume to a level which is suitable for feed to the recovery system, but also decreases fresh water and hot water :
~ use from typically 20,000 USG/ADT to a negligible value. ~;
.~ Steam consumption is also decreased considerably from the conventional 5,000 to 7000 1bs/ADT to less than 1000 ~;
lbs/ADT, a considerable saving. -~ .-I~ the~ bleach plant operation, the pulp leaving line : 64 is diluted with D/C seal tank filtrate in line 176 to the .
~ required consistency, typically about 4%, before mixing with `.-:~
. .
the chlorine dioxide and chlorine ~olution in line 294, with : 30 additional chlorine being supplied by the sodium hypochlorite ;~
~ 12 -~085~157 solution in line 296. In this way, the necessity for conventional chlorine gas injection is eliminated. While the use of sodium hypochlorite solution to provide part of the first stage chlorine requirement raises the pH of the D/C ~-bleaching, it has been found that efficient bleaching chemical utilization is maintained even up to 1~ of sodium hypochlor-ite (determined as available C12 on the pulp).
Owing to the heat in the filtrate used in the dilution and arising from the elimination of conventional dis-ch~e of unbleached decker filtrate, the pulp fed to the bleac~ing tower 12 has a temperature of about 120 to 140F (50 to 60C).
As is well known, pulp chlorination is usually carried out at temperatures less than 90F (30C) and higher temperatures increase the rate of reaction of the chlorine with the pulp.
To control against overchlorination of the pulp with conseqùent strength losses, the chlorine dosage to the pulp is controlled in accordance with the sensor 66 which senses the Kappa number of the pulp passing therethrough. This control, which may be by an optical or oxidation-reduction potential sensor, a~so results in no residual chlorine `; `-values and effective chemical usage.
The D/C bleaching tower 12 may be a conventional upflow tower having about 30 to 60 minutes retention time or a two-stage upflow-downflow tower with a retention timè
of about 20 minutes in the upflow and 0 to 25 minutes in . ~
the downflow. The variation in retention time achieved by an upflow-downflow tower allows ready compensation for variations ~; ~ in temperature and ~ClO2 substitution.
~ The substituti~n of chlorine ~ ide for about seventy ; 30 percent of the chlorine in the first stage bleaching - operation is an important feature of the bleach plant 1o89ls7 operation. Since all the sodium ~nd chlorine atoms in the recovered effluent must be matched, providing part of the oxidizing power with chlorine dioxide decreases the total chlorine atoms and hence the required matching sodium atoms, enabling the overall quantity of sodium hydroxide ~:
required to be decreased.
It follows, therefore, that the use of 70/30 D/C
~leaching decreases the total quantity of sodium chloride :~
discharged to the recovery system as compared with 100%C.
Hence, any detrimental effec.ts which may result from the discharge of sodium chloride containing liquors into the recovery system are decreased.
The use of 70/30 D/C first stage bleaching also :~
produces brighter, stronger pulp with greater stability to i yellowing with age as compared with 100~ C first stage . .~ s:~
bleaching and results in an improved yield of bleached pulp. ~ :
In each of the other stages, conventional towers are use~ operating at conventional temperatures of about ~ .
. 160F (70C), while the consistency is about 13%, which is .~- :
higher than the conventionally achieved value of 10 to 12%.
; The use of the E2 seal tank filtrate to dilute the 50% NaOH solution for use in the system results in water and - ~ ~
steam savings, the resulting solution being hotter than is -usually used.~ It is also preferred to use a concentration of about 10 to 13~ NaOH, which is more concentrated than conventionally used.
In the countercurrent washing operation, the wash water for a given stage is obtained from the seal tank of the following stage. Good washing is required on the E
washer to minimize the carry over of El stage solids which .
:, .
, , 1~)89~57 would result in increased chlorine dioxide consumption in the Dl stage. Ilence a dilution factor of at least three is used in the ~1 washer and on all other washers a dilution factor of at least 2 is used.
The showers 106 on each of the washers are placed and oriented for optimum wash water distribution on the pulp mat and hence most efficient washing on each of the washers. The washer size is such as to provide a consistency of at least 13~ on each washer for greatest washing efficiency.
The washing on the D/C washer is split between first ~ ~
Dl filtrate and then El filtrate with the El filtrate ~ -application being controlled to about 75% of the water contained in the pulp mat in order to prevent passage of E
filtrate through the mat and into the D/C filtrate. The presence of El filtrate in the D/C filtrate increases j~ -chemical consumption in the D/C stage and operation in the described manner avoids this problem.
On each of the washers an air doctor is used in place of a conventional external water-fed hydraulic doctor to ~remove the pulp mat from the screen after washing, although it may be possible to use a hydraulic doctor which uses filtrate recycled within the particular stage.
The wire cleaning shawers 122 which are used in place of conventional hydraulic doctors which also remove the pulp mat from the washer screen are high pressure low volume ~-~ wire cleaning showers. Each of the wire cleaning showers 122 is timer controlled so that they operate for only a small percentage of the time in order to decrease fresh hot water usage, typically to an overall volume of about 10 USG/min ~- 30 on the D/C and El washers and to an overall volume of about 5 USG/min on the Dl, E2 and D2 and bleached decker washers.
~ 15 ~
Conventional hydraulic doctors use about 100 USG/min.
The use of hot water on the showers 122 decreases ~, the thermal shock of conventional cold water hydraulic docto~s, thereby improving the effective washer life and decreasing the overall steam heating requirement.
The liquid level in the bleached decker seal tank 48 is controlled by the addition of pulp machine white water in line 304 while the levels in the seal tanks 32, 34, 36, 38 and 40 are controlled by level control valves which feed the shower water to the preceding stage, providing a positive con-trol on these levels, in place of the conventional overflow system. The seal tanks only overflow under emergency conditions.
The seal tanks have a tangential drop leg entry.
The use of tangential entry releases entrained air and minimizes foaming tendencies. A parallel tangential entry of -overflow from the following seal tank may be used to prevent reverse overflow.
The seal tanks are sized to allow sufficient time for entrained air separation, the sizing typically being such as 2D to provide a filtrate retention time of about 120 seconds for the `
unbleached aecker, D/C and El seal tanks and of about 60 seconds for the Dl, E2, D2 and bleached decker seal tanks.
A large freeboard space also is provided in each seal tank to allow for air separation, typically about 8 feet.
The present invention, therefore, provides a bleach plant process which results in a low efficient volume and yet produces good pulp quality~ Modifications are possible within the scope of the invention.
. .
- 16 - ~
B~ :`
...
:: ~
....... ~ . .. ` . ' . . .. ... . ~..... ; . .
In the aqueous effluent-free pulp mill as illustra-ted in Figure 1, noxious aqueous effluents from the pulp mill are eliminated by discharge of the bleach plant effluents `
into the recovery and regeneration operation 7. The organic ~ .
. materials content of the bleach plant effluent is burned off -.
in the recovery furnace of the recovery and regeneration operation 7 while the aqueous phase is evaporated.
''.' ~;:
In Figure 2, there is illustrated a detailed flow :` ;
~ sheet which includes a bleach plant 3 constructed and operated :~: - in accordance with the present invention for use in the ~ ~
effluent-free pulp mill illustrated in Figure 1. ~.
Referring to Figure 2, a pulp treatment operation - ~ lQ, incorporating a bleach plant designed to use a D~CEDED
b~leaching and caustic extraction sequence, includes a DjC
bleaching tower 12, an El caustic extraction tower 14, a D
bleaching tower 16, an E2 caustic extraction tower 18 and a ` .
D2 bleaohin~ tower 20.
Drum washers are provided for each bleaching and caustic extraction stage including a D/C washer 22, an El :
washer 24, a Dl washer 26, an E2 washer 28 and D2 washer -~:
: ~ 30. Seal tan~s are associated with each of the washers, ~ - 4 ~
`: :
including a D/C seal tank 32, an El seal tank 34, a Dl ~
seal tank 36, an E2 seal tank 38 and a D2 seal tank 40. ~:.
An -unbleached decker 42 in the form of a displacement washer is provided along with an associated seal tank 44. A similar bleached decker 46 is provided with ; an associated seal tank 48. Unbleached pulp and bleached ~:
pulp storage towers 50 and 52 respectively are provided. . :~ :
Screens and cleaners 54 and 56 are provided for unbleached pulp and bleached pulp respectively.
Unbleached pulp from the brown stock washing is ~ .
passed by line 58 through the screens and cleaners 54 and via line 60 to the unbleached decker 42 and thence by line 62 to the storage tank 50. The pulp is passed from .
: the storage tank 50 by line 64 to a sensor 66 which senses chlorine dioxide and chlorine required by the pulp. From the sensor the pulp passes by line 68 to the D/C bleaching tower 12. ~
.After D/C bleadhing, the pulp passes by line 70 `
to the D/C washer 22. From the D/C washer the pulp passes ~ by line 72~through an El mixer 73 and by line 74 to the El ~ -:~ extraction tower 14. The pulp passes from the El tower 14 by line 76 to the El washer 24 and thence by line 78 to a :~ : . Dl mixer 80 and by line 82 to the Dl bleaching tower 16.
, ~ Following Dl bleaching, the pulp next passes by f~ line 84 to the Dl washer 26 and by line 86 to the E2 extraction ~ ~ tower 18. From E2 extraction, the pulp passes by line 88 to i ~ the E2 washer 28, by line 90 to a D2 mixer 92 and by line 94 to the D2 bleaching tower 20.
:: : The pulp next passes by line 96 to the D2 washer 30 before passage by line 98 to the screens and cleaners 56, q by line 100 to the bleached decker 46 and by line 102 to ,. -` ~ bleached pulp storage 52. From the bleached pulp storage ~ ~
_ 5 _ .. ..
52 the pulp is passed forward by line 104, such as, to a pulp drying machine, not shown.
Each of the washers 22, 24, 26, 28 and 30 and each of the de~kers 42 and 46 is a rotating foraminous drum type having wash water shower bars 106 arranged adjacent the periphery thereof for the application of wash water to the pulp mat as it is transported on the drum surfàce, the water displaced from or passing through the mat passing to the appropriate seal tank by lines 108, 110, 112, 114, 116, 118 and 120 respectively. Wire cleaners 122 also are provided for each of the washers and deckers. ~- :
A complete countercurrent flow wash water system is utilized. Thus, fresh water passes to the pulp drying machine ~`
and pulp drying machine white water or other relatively fresh water is passed by line 124 to the showers 106 on the bleached ,, - :
decker 46 to displacement wash the pulp mat on the bleached `
decker. Part of the displaced liquor collected in the seal tank 48 is passed by lines 126 and 128 to the showers 106 .
of the D2 washer 30, with the remainder passing by lines 126 and 130 for dilution of the pulp in line 98 before passage of the diluted pulp to the screens and cleaners 56.
Part of the liquor collected in the D2 seal tank 40 passes by lines 132 and 134 to the washing showers 106 -~
on the E2 washer 2B. Part of this liquor passes by lines 132 `~
` and 136 to pulp~passing from the D2 ~leaching tower 20 to the `~
, ~ - .
D2 washer 30 in line 96. Part of the collected liquor passes from the seal tank 40 by line 138 to the D `~
` 2 bleachlng tower 20.
The liquor collected in the E2 seal tank 38 partially passes by lines 140, 142 and 144 to the washing ~;
shower 106 of the Dl washer 26. Another portion of this liquor passes by lines 140 and 146 to the pulp passing in ~089157 line 88 from ~he E2 extr~ction tower 18 to the E2 washer 28. ~ ~!
Further quan~ities of the collected liquor pass by line 148 to the E2 extraction tower 18. -~
A small quantity of the liquor collected in the E2 seal tank is used to dilute caustic extraction chemical :~
and is passed to the caustic extraction chemical inlet feed ~:
line for this purpose by lines 140, 142 and 150.
The Dl seal tank liquor is partially passed by line 152 to the Dl bleaching tower 16. A portion of the liquor passes by lines 154, 156 and 158 to the showers 106 .
on the El washer 24, while another portion of the liquor :-:
passes by lines 154, 156 and 160 to the first showers 106 on the D/C washer 22. Part of the Dl seal tank liquor ~:
passes by lines 154 and 162 to the pulp in line 84 passing from the Dl bleaching tower to the Dl washer 26.
:~ . From the El seal tank 34, part of the liquor ,~
passes by lines 163, 164 and 166 to the last showers 106 :
on the D/C washer 22 while another part passes by lines 163/ -^ ;
164 and 168 to the pulp passing in line 76 from the El ~ extraction tower 14 to the El washer 24. Another part :; passes by line 170 to the El extraction tower 14 while the :~ remainder is discharged from the bleach plant by lines 163 and 172 for passage to the pulp mill recovery system. :
Liguor from the D/C seal tank 32 partly passes by lines 174 and 176 to dilute the pulp passing by line 64 ;~
from the storage 50 to the sensor 66 with another part passing . ~. , ; . :
by lines 174 and 178 to the pulp passing by line 70 from the D/C bleaching tower 12 to the D/C washer 22. Another part of the llquor~is passed by lines 180, 182 and 184 to the '~
showers 106 of the un~leached dec~er 42. The liquor passing ;~
:~ ~ in this way to the showers 106 of the unbleached decker .
~: 42 is neutrali2ed by sodium hydroxide solution fed by line -: - 7 -.~ .
186. The remainder of the liquor passes out of the bleach plant 10 by line 188 for passage to the pulp mill recovery system.
The bleach plant effluents in lines 172 and 188 and the unbleached decker seal tank liquor in line 190 may be utilized as described in our aforemen.tioned Canadian Patent No. 1,070,908.
From the unbleached decker seal tank 44 liquor .
passes by line 190 for use as wash water in the brown . ~
stock washer of the pulp mill and liquor passes by line 192 ~ .
to the unbleached pulp entering the unbleached screens and cleaners 54 by line 58. ~ ~;
It will be seen, therefore, that thére is a general -eountercurrent flow of wash water and pulp through the pulp ~:
treatment'operation 10.
. Each of the cleaner showers 122 on the washers and deckers is fed by hot water. An inlet feed in line 194 feeds the cleaner 122 on the decker 42 by line 196, the ~;~
cleaner 122 on the D/C washer 22 by lines 198 and 200, the -:
cleaner 122 on the El washer 24 by lines 198~ 202 and 204, ;~ .
the cleaner 122 on the Dl washer 26 by lines 198, 202, 206 ..
~ and 208 the cleaner 122 on the E2 washer 28 by lines 198, . -: 202, 206, 210 and 212, the cleaner 122 on the D2 washer 30 by lines lg8, 202, 206, 210, 214 and 216 and the cleaner 122 ~: on the bleached decker 46 by lines 198, 202, 206, 210 214 and ~: 218.
. Steam for heatlng purposes also is used. Thus, : .
~ steam is fed by lines 220 and 222 to the El mixer 72, by ~.
.~ lines 220, 224 and 226 to the Dl mixer 80, by lines 220, 224, ~ :
228 and 230 to an injection ring 232, and by lines 220, 224, ~: 228 and ~234 to the D2 mixer 92. :~
~' .
s -; . , . , . , . , . . ,:
:,: - .. . . . ~
To accommodate emergency overflow conditions, the seal tanks are connected in a countercurrent flow overflow arrangement. Thus, overflow from the bleached decker seal tank 48 passes by line 236 to the D2 seal tank 40, the overflow from the D2 seal tank 40 passes by line 238 to the ..
E2 seal tank 38, the overflow from the E2 seal tank 38 passes .
by line 240 to the D~ seal tank 36 and the overflow from the ~.
Dl seal ~ank 36 passes by line 242 to the El seal tank 34. .-From the E1 seal tank 34, the overflow passes by line 244 to a first spill storage.tank 246 whi~e overflow from the D/C seal tank 32 and the unbleached decker seal tank 44 pass to a second spill tank 248 by lines 249 and 250 and lines 252 and 250 respectively.
- The overflow collected in the second spill tank 248 under emergency conditions may be returned to the system, ~ 6 ...~. , after neutralization with sodium hydroxide solution, at a.
convenient time by lines 254, 256 and 258 while the overflow ;~
collected in the first spill tank 246 passes to the recovery system for use in brown stock washing or white liquor ,. ..
20 dilution.
With this seal tank overflow arrangement, when there is a temporary discharge of a pulp mat of lower than normal consistency from one washer to another, the excess :~ ~
water is returned to the preceding stage by seal tank overflow....... `
~: .
High density pumps are used.for p~mping ~ the pulp through the bleach plant and such pumps use ~ater-:~: fed seal glands. Hot water fed from llne 194 is used for ~ such seal glands. Thus, seal glands of a pump 260 -: :~
- ~ :
conveying the pulp by line 74 are fed by hot water in line ~ 30 262, seal glands of a pump 264 conveying the pulp by line ;......... ~82 are fed by hot water in line 266, seal glands of a pu~p _ g _ .
268 conveying the pulp by lin~ 86 are fed by hot water in line 270, and seal glands of a pump 272 conveying the pulp by line 94 are fed by hot water in line 274. The ~ .
pressure on the pump glands is controlled to minimize the flow of fresh water into the pulp passing through the pump.
Chemical feed for the bleaching and caustic extraction operations is provided by dilute sodium hydroxide solution, aqueous solutions of chlorine dioxide and chlorine .
and sodium hypochlorite solution. Sodium hydroxide solution in concentrated form (.typically 50~- ~y wt~2 i5 fed by line 276 to the system and is diluted by the E2 seal tank liquor ~:
in line 140 to the concentration required. The diluted sodium :
hydroxide solution then is passed by lines 278 and 280 to the pulp leaving the D/C washer 22. Sodium hydroxide ~:
so~ution also is passed by lines 278, 282 and 28~ to the pulp .
leaving the El washer 24 by lines 278, 282, 286 and 288 to the pulp leaving the Dl washer 26, lines 278, 282, 286, 290 ~ .
and 291 to the pulp leaving the E2 washer 28. Dilute sodium hydroxide solution also passes by line 292 to the sodium : :
hydroxide neutralization feed 186,~provision a~so being made for emergency flow to the second spill tank 248 in line 293.
. . .
An aqueous solution of chlorine dioxide and chlorine ~:
is fed by line 294 to the pulp passing by line 64 ;
from the pulp to storage 50 to the sensor 66. Sodium :~
~:: hypochlorite solution is fed by iine 296 to the pulp in :
- line 64.
An aqueous chlorine dioxide solution is fed by ::
~. ~
lines 298 and 300 to the pulp in line 82 before passage thereof into the Dl bleaching tower 16. .A mixer, not shown, ~.
is located immediately after the injection point of the chlorine dioxide solution to ensure even mixing of the . -~
.. "~ - .. . . . .
1089~57 solution with the pulp.
Chlorine dioxide solution is also fed by lines 298 and 302 to the pulp in line 94 before passage thereof into the D2 bleaching tower 20. A mixer, not shown~ is located immediately after the injection point of the chlorine dioxide solution to ensure even mixing of the solution with the pulp.
The chlorine dioxide solution fed by line 298 is one having a low concentration of dissolved chlorine whereby ~
over 90% of the available chlorine content of the chlorine ~ ::
dioxide solution is provided by chlorine dioxide. A typical -solution is one having a chlorine dioxide concentration of ~-10 gpl and chlorine concentration of 2 gpl. ~:~
The chlorine dioxide and chlorine solution fed by ~ :
line 294 is one having a higher dissolved chlorine : concentration than the chlorine dioxide solution in line 298, ~ :~
:~ whereby about 70~ of the available chlorine content of the :~
solution is pro~ided by chlorine dioxide and the remainder of the available ch~orine is provided by the chlorine. A ~ .
suitable solution contains about 10 gpl C102 and about - 6 gpl C12 ~.
. The sodium hypochlorite solution in line 296 ... :
breaks~down under the acid condition of the pulp in line 64 to produce chlorine for tke bleaching of the pulp in the : D/C bleaching tower 12.
~: : . . .
The chlorine dioxide solution in line 298, the .
~; ~ chlorlne dioxide and chlorine solution in line 294 and the -~: `
. , . ~ . . ~
sodium hypochlorite solution in line 296 all may be produced :: ~ from a single chlorine dioxide and chlorine generator, for .
~ u example, using the procedure outlined in U.S. Patent No. :~
: - 4,OIO,112. By the use of the latter chlorine dioxide , ~ ' :
~._, . - .
,1,, , ~ . . , ~ .
~89157 generation system, the most efficient use of bleaching chemicals is obtained while ~he volume of water entering --the bleach plant with chlorine dioxide and chlorine is mini-mized.
In operating the pulp mill system described above, steps are taken to ensure optimum bleaching, caustic :
extraction and washing, minimal consumption of water, energy and chemicals and the discharge of a minimal volume of liquid effluent, in the region of about 4000 U.S. gallons/air dried : ~
ton (USG/ADT) of pulp, as compared with the liquid effluent :-discharge from a conventional bleach plant operation of up to about 25,000 USG/ADT while the bleached pulp produced has -properties at least comparable to those of pulp produced in conventional operations. :~
The fresh water consumption in the bleach plant is very small, with the principal inputs of fresh water to the bleach plant being pulp machine white water, chlorine dioxide solutions and the water in the unbleached pulp.
Thus, not only does this bleach plant operation decrease the . effluent volume to a level which is suitable for feed to the recovery system, but also decreases fresh water and hot water :
~ use from typically 20,000 USG/ADT to a negligible value. ~;
.~ Steam consumption is also decreased considerably from the conventional 5,000 to 7000 1bs/ADT to less than 1000 ~;
lbs/ADT, a considerable saving. -~ .-I~ the~ bleach plant operation, the pulp leaving line : 64 is diluted with D/C seal tank filtrate in line 176 to the .
~ required consistency, typically about 4%, before mixing with `.-:~
. .
the chlorine dioxide and chlorine ~olution in line 294, with : 30 additional chlorine being supplied by the sodium hypochlorite ;~
~ 12 -~085~157 solution in line 296. In this way, the necessity for conventional chlorine gas injection is eliminated. While the use of sodium hypochlorite solution to provide part of the first stage chlorine requirement raises the pH of the D/C ~-bleaching, it has been found that efficient bleaching chemical utilization is maintained even up to 1~ of sodium hypochlor-ite (determined as available C12 on the pulp).
Owing to the heat in the filtrate used in the dilution and arising from the elimination of conventional dis-ch~e of unbleached decker filtrate, the pulp fed to the bleac~ing tower 12 has a temperature of about 120 to 140F (50 to 60C).
As is well known, pulp chlorination is usually carried out at temperatures less than 90F (30C) and higher temperatures increase the rate of reaction of the chlorine with the pulp.
To control against overchlorination of the pulp with conseqùent strength losses, the chlorine dosage to the pulp is controlled in accordance with the sensor 66 which senses the Kappa number of the pulp passing therethrough. This control, which may be by an optical or oxidation-reduction potential sensor, a~so results in no residual chlorine `; `-values and effective chemical usage.
The D/C bleaching tower 12 may be a conventional upflow tower having about 30 to 60 minutes retention time or a two-stage upflow-downflow tower with a retention timè
of about 20 minutes in the upflow and 0 to 25 minutes in . ~
the downflow. The variation in retention time achieved by an upflow-downflow tower allows ready compensation for variations ~; ~ in temperature and ~ClO2 substitution.
~ The substituti~n of chlorine ~ ide for about seventy ; 30 percent of the chlorine in the first stage bleaching - operation is an important feature of the bleach plant 1o89ls7 operation. Since all the sodium ~nd chlorine atoms in the recovered effluent must be matched, providing part of the oxidizing power with chlorine dioxide decreases the total chlorine atoms and hence the required matching sodium atoms, enabling the overall quantity of sodium hydroxide ~:
required to be decreased.
It follows, therefore, that the use of 70/30 D/C
~leaching decreases the total quantity of sodium chloride :~
discharged to the recovery system as compared with 100%C.
Hence, any detrimental effec.ts which may result from the discharge of sodium chloride containing liquors into the recovery system are decreased.
The use of 70/30 D/C first stage bleaching also :~
produces brighter, stronger pulp with greater stability to i yellowing with age as compared with 100~ C first stage . .~ s:~
bleaching and results in an improved yield of bleached pulp. ~ :
In each of the other stages, conventional towers are use~ operating at conventional temperatures of about ~ .
. 160F (70C), while the consistency is about 13%, which is .~- :
higher than the conventionally achieved value of 10 to 12%.
; The use of the E2 seal tank filtrate to dilute the 50% NaOH solution for use in the system results in water and - ~ ~
steam savings, the resulting solution being hotter than is -usually used.~ It is also preferred to use a concentration of about 10 to 13~ NaOH, which is more concentrated than conventionally used.
In the countercurrent washing operation, the wash water for a given stage is obtained from the seal tank of the following stage. Good washing is required on the E
washer to minimize the carry over of El stage solids which .
:, .
, , 1~)89~57 would result in increased chlorine dioxide consumption in the Dl stage. Ilence a dilution factor of at least three is used in the ~1 washer and on all other washers a dilution factor of at least 2 is used.
The showers 106 on each of the washers are placed and oriented for optimum wash water distribution on the pulp mat and hence most efficient washing on each of the washers. The washer size is such as to provide a consistency of at least 13~ on each washer for greatest washing efficiency.
The washing on the D/C washer is split between first ~ ~
Dl filtrate and then El filtrate with the El filtrate ~ -application being controlled to about 75% of the water contained in the pulp mat in order to prevent passage of E
filtrate through the mat and into the D/C filtrate. The presence of El filtrate in the D/C filtrate increases j~ -chemical consumption in the D/C stage and operation in the described manner avoids this problem.
On each of the washers an air doctor is used in place of a conventional external water-fed hydraulic doctor to ~remove the pulp mat from the screen after washing, although it may be possible to use a hydraulic doctor which uses filtrate recycled within the particular stage.
The wire cleaning shawers 122 which are used in place of conventional hydraulic doctors which also remove the pulp mat from the washer screen are high pressure low volume ~-~ wire cleaning showers. Each of the wire cleaning showers 122 is timer controlled so that they operate for only a small percentage of the time in order to decrease fresh hot water usage, typically to an overall volume of about 10 USG/min ~- 30 on the D/C and El washers and to an overall volume of about 5 USG/min on the Dl, E2 and D2 and bleached decker washers.
~ 15 ~
Conventional hydraulic doctors use about 100 USG/min.
The use of hot water on the showers 122 decreases ~, the thermal shock of conventional cold water hydraulic docto~s, thereby improving the effective washer life and decreasing the overall steam heating requirement.
The liquid level in the bleached decker seal tank 48 is controlled by the addition of pulp machine white water in line 304 while the levels in the seal tanks 32, 34, 36, 38 and 40 are controlled by level control valves which feed the shower water to the preceding stage, providing a positive con-trol on these levels, in place of the conventional overflow system. The seal tanks only overflow under emergency conditions.
The seal tanks have a tangential drop leg entry.
The use of tangential entry releases entrained air and minimizes foaming tendencies. A parallel tangential entry of -overflow from the following seal tank may be used to prevent reverse overflow.
The seal tanks are sized to allow sufficient time for entrained air separation, the sizing typically being such as 2D to provide a filtrate retention time of about 120 seconds for the `
unbleached aecker, D/C and El seal tanks and of about 60 seconds for the Dl, E2, D2 and bleached decker seal tanks.
A large freeboard space also is provided in each seal tank to allow for air separation, typically about 8 feet.
The present invention, therefore, provides a bleach plant process which results in a low efficient volume and yet produces good pulp quality~ Modifications are possible within the scope of the invention.
. .
- 16 - ~
B~ :`
...
:: ~
....... ~ . .. ` . ' . . .. ... . ~..... ; . .
Claims (16)
1. A bleach plant process including a combination of steps to achieve a low volume of effluent, a low consump-tion of water, energy and chemicals and efficient bleaching, caustic extraction and washing of cellulosic fibrous material pulp in a D/CEDED sequence, the steps comprising:
(a) passing unbleached pulp sequentially to a D/C bleaching tower for initial bleaching with an aqueous solution of chlorine dioxide and chlorine mixed therewith, to a D/C washer for washing the initially-bleached pulp, to an E1 caustic extraction tower for initial extraction with an aqueous sodium hydroxide solution mixed therewith, to an E1 washer for washing the initially-extracted pulp, to a D1 bleaching tower for further bleaching with an aqueous chlorine dioxide solution mixed therewith, to a D1 washer for washing the further-bleached pulp, to an E2 extraction tower for further extraction with aqueous sodium hydroxide solution mixed therewith, to an E2 washer for washing the further-extracted pulp, to a D2 bleaching tower for final bleaching with an aqueous chlorine dioxide solution mixed therewith, to a D2 washer for washing the finally-bleached pulp, to a bleached pulp decker for final washing of the bleached pulp and to bleached pulp storage;
(b) controlling the quantity of chlorine fed to the initial bleaching operation to minimize overchlorination and strength losses and to result in a substantial absence of residual chlorine in the initially-bleached pulp;
(c) passing filtrate from each of said washers and said bleached decker to a seal tank associated with the respective washer or decker;
(d) removing two effluents from the process consisting of an acid effluent from the D/C seal tank and an alkaline effluent from the E1 seal tank;
(e) providing a countercurrent flow of wash water with respect to the flow of pulp through the bleach plant consisting of passing relatively fresh water to the bleached pulp decker, passing filtrate from the bleached decker seal tank to the D2 washer, passing filtrate from the D2 seal tank to the E2 washer, passing filtrate from the E2 seal tank to the D1 washer, passing filtrate from the D1 seal tank to the E1 washer and to an initial portion of the washing on the D/C washer, and passing liquor from the E1 seal tank to a latter portion of the washing on the D/C
washer while avoiding the presence of El seal tank filtrate in the D/C filtrate;
(f) conducting said washing on said E1 washer at a dilution factor of at least about 3 while conducting said washing on said D/C washer, D1 washer, E2 washer, D2 washer and bleached decker washer at a dilution factor of at least about 2;
(g) individually controlling the liquid level in each of said seal tanks below a maximum level except under emergency conditions by cycling filtrate at a controlled rate from the respective seal tank to the appropriate washer;
and (h) connecting the seal tanks in countercurrent overflow arrangement for the accommodation of emergency conditions in which overflow from the bleached decker seal tank passes to the D2 seal tank, overflow from the D2 seal.
tank passes to the E2 seal tank, overflow from the E2 seal tank passes to the D1 seal tank, overflow from the D1 seal tank passes to the E1 seal tank, overflow from the E1 seal tank passes to a first spill storage tank and overflow from the D/C seal tank passes to a second spill storage tank.
(a) passing unbleached pulp sequentially to a D/C bleaching tower for initial bleaching with an aqueous solution of chlorine dioxide and chlorine mixed therewith, to a D/C washer for washing the initially-bleached pulp, to an E1 caustic extraction tower for initial extraction with an aqueous sodium hydroxide solution mixed therewith, to an E1 washer for washing the initially-extracted pulp, to a D1 bleaching tower for further bleaching with an aqueous chlorine dioxide solution mixed therewith, to a D1 washer for washing the further-bleached pulp, to an E2 extraction tower for further extraction with aqueous sodium hydroxide solution mixed therewith, to an E2 washer for washing the further-extracted pulp, to a D2 bleaching tower for final bleaching with an aqueous chlorine dioxide solution mixed therewith, to a D2 washer for washing the finally-bleached pulp, to a bleached pulp decker for final washing of the bleached pulp and to bleached pulp storage;
(b) controlling the quantity of chlorine fed to the initial bleaching operation to minimize overchlorination and strength losses and to result in a substantial absence of residual chlorine in the initially-bleached pulp;
(c) passing filtrate from each of said washers and said bleached decker to a seal tank associated with the respective washer or decker;
(d) removing two effluents from the process consisting of an acid effluent from the D/C seal tank and an alkaline effluent from the E1 seal tank;
(e) providing a countercurrent flow of wash water with respect to the flow of pulp through the bleach plant consisting of passing relatively fresh water to the bleached pulp decker, passing filtrate from the bleached decker seal tank to the D2 washer, passing filtrate from the D2 seal tank to the E2 washer, passing filtrate from the E2 seal tank to the D1 washer, passing filtrate from the D1 seal tank to the E1 washer and to an initial portion of the washing on the D/C washer, and passing liquor from the E1 seal tank to a latter portion of the washing on the D/C
washer while avoiding the presence of El seal tank filtrate in the D/C filtrate;
(f) conducting said washing on said E1 washer at a dilution factor of at least about 3 while conducting said washing on said D/C washer, D1 washer, E2 washer, D2 washer and bleached decker washer at a dilution factor of at least about 2;
(g) individually controlling the liquid level in each of said seal tanks below a maximum level except under emergency conditions by cycling filtrate at a controlled rate from the respective seal tank to the appropriate washer;
and (h) connecting the seal tanks in countercurrent overflow arrangement for the accommodation of emergency conditions in which overflow from the bleached decker seal tank passes to the D2 seal tank, overflow from the D2 seal.
tank passes to the E2 seal tank, overflow from the E2 seal tank passes to the D1 seal tank, overflow from the D1 seal tank passes to the E1 seal tank, overflow from the E1 seal tank passes to a first spill storage tank and overflow from the D/C seal tank passes to a second spill storage tank.
2. The process of claim 1 wherein sodium hypochlorite solution also is used as a source of chlorine in said initial bleaching.
3. The process of claim 1 wherein said aqueous solution of chlorine dioxide and chlorine has a chlorine dioxide concentration of about 10 gpl and a chlorine concen-tration of about 6 gpl and said aqueous chlorine dioxide solutions both have a chlorine dioxide concentration of about 10 gpl and a chlorine concentration of about 2 gpl.
4. The process of claim 1 wherein said D/C bleaching is carried out at a temperature of about 50° to 60°C and said D1 and D2 bleachings and said E1 and E2 extractions are carried out at a temperature of about 70°C.
5. The process of claim 1 wherein said D/C bleaching tower is a two-stage upflow-downflow tower having a retention time of about 20 minutes in the upflow and 0 to 25 minutes in the downflow.
6. The process of claim 1 wherein said aqueous sodium hydroxide solutions have a concentration of about 10 to 13%
NaOH.
NaOH.
7. The process of claim 1 wherein said aqueous sodium hydroxide solutions are provided from a single source of relatively concentrated sodium hydroxide solution which is diluted to the desired concentration for said caustic extraction operations by the use of filtrate from the E2 seal tank.
8. The process of claim 1 wherein each of said washers and said bleached decker is a rotating foraminous drum type having wash water shower bars arranged adjacent the periphery thereof for the application of wash water to the pulp mat transported on the drum surface and oriented for optimum wash water distribution on the pulp mat.
9. The process of claim 8 wherein each washer is sized to provide a pulp consistency of at least about 13%
on each washer.
on each washer.
10. The process of claim 8 wherein a high pressure and low volume wire cleaning shower is provided for each washer or decker, hot water is passed to said wire cleaning shower, and the cleaning operation of said wire cleaning shower is controlled for intermittent operation only.
11. The process of claim 8 including removing pulp mat from the foraminous drum after completion of said washing by the use of an air doctor or a hydraulic doctor which utilizes filtrate recycled from the respective seal tank.
12. The process of claim 1 including pumping said pulp through the bleach plant using high density pumps having hot water-fed seal glands and controlling the hot water pressure on the seal glands to minimize the flow of such water into the pulp passing through the pump.
13. The process of claim 1 wherein each of said seal tanks has a tangential drop leg entry and is sized to permit entrained air separation.
14. The process of claim 1 wherein overflow collected in said second spill tank is neutralized and returned to the bleach plant.
15. The process of claim 1 including heating said pulp between each washer and subsequent tower by the use of steam.
16. The process of claim 1 wherein said unbleached pulp is provided from an unbleached pulp storage tank to which washed pulp is passed from an unbleached decker having an associated seal tank, overflow from the unbleached decker seal tank passes to said second spill tank and washing is effected on said unbleached decker using neutralized filtrate from the D/C seal tank.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA279,885A CA1089157A (en) | 1977-06-06 | 1977-06-06 | Bleach plant operation |
| CA349,126A CA1089161A (en) | 1977-06-06 | 1980-04-02 | Bleached kraft pulp mill operation |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA279,885A CA1089157A (en) | 1977-06-06 | 1977-06-06 | Bleach plant operation |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1089157A true CA1089157A (en) | 1980-11-11 |
Family
ID=4108814
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA279,885A Expired CA1089157A (en) | 1977-06-06 | 1977-06-06 | Bleach plant operation |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1089157A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115491916A (en) * | 2022-09-22 | 2022-12-20 | 牡丹江恒丰纸业股份有限公司 | A non-powered equipment conveying system and conveying method for high-concentration slurry |
-
1977
- 1977-06-06 CA CA279,885A patent/CA1089157A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115491916A (en) * | 2022-09-22 | 2022-12-20 | 牡丹江恒丰纸业股份有限公司 | A non-powered equipment conveying system and conveying method for high-concentration slurry |
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