AU471663B2 - Kraft pulp bleaching and recovery process - Google Patents

Description

67,290174

The present invention relates genlerally to a method of

bleaching Kraft pulp. More particularly the invention relates

to a process for bleaching Kraft pulp comprising from one to

about three treatments with ozone and a final treatment with an

alkaline peroxide solution, the treatments being separated by a-

alkaline extraction. The fresh water requirement is reduced by

utilizing effluent from the bleach treatments in subsequent

bleaching operations and stream and air pollution is eliminated I utilizing the final effluent from the bleaching operation in the

chemical recovery system of a Kraft pulping process.

Wood is composed of two main parts a fibrous carbo-

hydrate or cellulosic portion and a non-fibrous portion compris-

ing phenolic-type compounds,this latter portion being commonly

referred to as lignin.

In the Kraft, or sulfate, pulping process wood chips

are treated with a solution of sodium hydroxide and sodium

sulfide which attacks the non-fibrous (lignin) portion of the

wood so the individual fibers can be separated. The resulting p

is then transferred to a brown stock washing system wherein

chemicals are removed by washing with water. The effluent

from the washing operation plus the spent cooking liquor

from the digesters is commonly referred to as black liquor.

The black liquor contains practically all the alkali originally

added, plus about half the original weight of the wood chips.

Weak black liquor is either recycled to the digester where it

acts as a diluent for the fresh liquor, or it is treated as

follows:

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The weak black liquor (at approximately 15-20% solids) is treated in a series of evaporators, commonly referred to as

a multiple effect evaporator, and is discharged to a direct contact

evaporator where further water is evaporated until the solids

content is approximately 65-70%. The evaporated liquor is then screened and passed to a recovery unit where make-up chemical or salt cake (sodium sulfate) is added; the remaining water is removed; sodium salts are recovered mostly as sodium carbonate and sodium sulfide; and heat is obtained by burning the organic compounds in the liquor.

The molten ash, or smelt, from the recovery unit, which consists of sodiun carbonate and sodium sulfide, is dissolved

in water to form green liquor. The green color is due to iron im- purities which, along with silica and other suspended solids,

are removed by treatment with coagulating agents and settlement in a clarifier. The clarification step is followed by a causticizing step in which the green liquor is treated with

lime (calcium oxide) to convert the sodiumparbonate to sodium

hydroxide, the sodium sulfide remaining unchanged. The sladge from the lime treatment is removed, and the resulting

solution is referred to as white liquor which can be reused as cooling liquor in the digester. The sludge is washed in a lime mud washer and the filtrate used to dissolve addititional smelt from the recovery unit. The lime mud is treated in a kiln to yield reburned lime.

During the burning operation some calcium oxide and sulfur

compounds are collected in the kiln stack and removed by a

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67, 290 174

scrubber in said stack.

Unbleached Kraft, or sulfate, pulps are generally

bleached to an acceptable brightness by a multistage process em- ploying chlorine or chlorine-containing compounds such as calciur

or sodium hypochlorite and chlorine dioxide. Although chlorine and chlorine-containing compounds have proven to be effective

bleaching agents, these compounds are difficult to handle, intro- duce the problem of corrosion of the paper-making equipment, and render the effluents from the bleach plant incapable of being re-

covered andicycled. In addition waste liquors and wash water from the bleach plant incorporating such compounds can cause a

serious pollution problem. The stream pollution problems asso- ciated witheffluents from chlorine and/or chlorine-containing

bleaching processes are well recognized,and it has been

suggested that these effluents be concentrated and burned in the

pulping liquor recovery systemdescribed above. However, this Smethod of disposing of such effluents has several majoi

disadvantages which have discouraged the use of such a recovery

and disposal system for chlorine-containing bleach plant

effluents. The chloride content of the effluent introduces the

problems of corrosion, solid build up, volatilization of toxic

compounds, etc. in the recovery system.

Ozone is a well known bleaching agent and has been sug- gested for use in bleaching wood pulp. In United States Patent No. 396,325 to Brin et al. and in United States Patent No.

1,957,937 to Campbell et al. ozone is used in combination with chlorine, in a bleaching process. Because of the chloride con- tent or the effluents these processes would not eliminate the

pollution problems discussed above. Other systems which have

employed ozone in a bleaching process, 447t663

67,290/74

either alone or after the fibers were pretreated by exposure

to U.V. light or with various chemical solutions, are also

unacceptable, primarily because of the low brightness values

obtained when compared to those obtained from chlorine-

containing bleaching processes available in theart.

67,290174 In accordance with the present invention there is provided a method of Kraft wood pulping employing a Kraft chemical recovery system and a Kraft wood pulp bleaching operation, said method being free of any treatments employing chlorine or chlorine-containing compounds and whereby the need for treating effluent being introduced into the Kraft pulp chemical recovery system for the removal of chlorine- containing salts is eliminated, said method comprising the steps of bleaching the Kraft wood pulp by a sequence of treatments of from one to about three treatments with ozone and a final treatment with an alkaline peroxide solution, an: alkaline extraction following each ozone treatment or only following the last ozone treatmenf and preceding the alkaline peroxide treatment, recycling effluent from the alkaline peroxide treatment to the bleaching operation to reduce the fresh water requirements, and introducing effluent from the bleaching operation into the Kraft pulp chemical recovery system. In practising the process of this invention, the ozone

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treatments are best carried out by treating the pulp with a

stream of oxygen containing from about 2% to about 4% ozone.

Following each of the ozonation steps the pulp may be washed

if desired. The ozonated is then subjected to an alkaline extract-

ion which preferably comprises treatment with an aqueous solution of sodium hydroxide at an increased temperature for a period of

time ranging from about 1/2 to about 2 hours and after this time

the pulp is diluted with fresh water or, preferably, with effluent

from previous bleaching operations. The pulp is finally treated

with an alkaline solution of hydrogen peroxide such as is common- ly used in the bleaching of wood pulp and washed.

While it is preferred to subject the pulp to from 1 to

about 3 treatments with ozone, the following bleaching sequence

is especially preferred in carrying out this invention: ozona-

tion, alkaline extraction, ozonation, alkaline extraction,

peroxide treatment, and wash.

It has now been found that the amount of fresh water

required for the bleaching operation can be substantially reduced

if effluent from the various bleaching treatments is recycled and

used in sbsequent bleaching oprations. It is especially pre-

ferred to'introduce fresh water into the sytem at the final wash following the alkaline peroxide treatment and to utilize

the effluent from one stage at an earlier stage in a subsequent bleaching operation. In the especially preferred bleaching sequence

effluent from the peroxide treatment and wash is used in the

second alkaline extraction in a subsequent bleaching operation, and effluent from the second alkaline extraction is used in the

first alkaline extraction in a subsequent operation. It is

preferred not to reuse effluent from the first alkaline

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67,290174

extraction, but to treat it as below in the chemical recovery

system.

To eliminate stream and air pollution, effluent from

the first alkaline extraction, and, if desired, from other

bleaching treatments is introduced into the chemical recovery sys-

tem associated with the Kraft pulping operation. The effluent

can be introduced into this system at any point where fresh

water is normally employed. It is preferred to introduce the

effluent at the brown stock washer, the smelt dissolving operation, the lime mud washer or the kiln stack scrubber. If

the volume of effluent is large, it may be separated and introduced at more than one of these points. It is especially

preferred to introduce the effluent at the brown stock washer.

In order to describe the present invention so that it

may be more clearly understood, the following examples are set

forth. These examples are set forth primarily for the purpose

of Masutration~and any specific enumeration of detail contained

therein should not be interpreted as a limitation on the

concept of this invention.

Example I

A mixture of hardwood (beech, birch and maple) chips

was pulped via the Kraft process. Handsheets were prepared from

the resulting pulp in accordance with TAPPI Standard T 281m.

Sheets produced in this manner had a brightness, as measured on a General Electric Photometer and expressed in terms of G.E.

units, of- 30 when measured in accordance with TAPPI

Standard T 217 m.

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The pulp was dewatered to 35% consistency in a basket centrifuge and the product was fluffed in a high speed fiber sep- arator. The fluffed material was treated, in a countercurrent

stream, with oxygen containing from 2% to 4% ozone. The rate of feed of the pulp was adjusted so that at a residence time in the ozonation'equipment of 30 seconds and a temperature of 770 F. the amount of ozone consumed was 0.96% of the bone dry weight

of the fibers.

The pulp waswashed and, at a consistency of 11%, treat- ed with an aqueous solution of 2% sodium hydroxide based on the 0 bone dry weight of the pulp at 1500 to 160 F. for 96 minutes. The pulp was diluted with water to a consistency of The con- sistency was thickened to 35% and the pulp was again treated with

ozone, as above, so that the amount of ozone consumed was 0.87%

of the bone dry weight of the pulp. The pulp was again treated with an aqueous solution of

sodium hydroxide containing 1% sodium hydroxide based on the bone dry weight of the pulp as above. The pulp was washed and again treated with ozone so that the amount of ozone

consumed was 0.62% of the bone dry weight of the pulp. A sample of the pulp was washed with water and hand-

sheets were prepared as with the unbleached pulp. The brightness of this sheet was equal to 89.0 G.E. when measured as above for

sheets from the unbleached pulp.

A second sample of the pulp was washed with an alkaline

solution, thickened to 12% consistency and treated with an aqueous bleach solution comprising

hydrogen peroxide 0.4% sodium hydroxide 0.48%

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sodium silicate

The bleach solution was applied so that 0.4% hydrogen peroxide

was added based on the bone dry weight of the pulp. After 3-1/2 hours at 112 0 and a pH of between 9 and 10.2 the pulp was

washed with water containing sulfurous acid. Sheets prepared

as above had a brightness of from 90 to 91 G.E. and after one hour

at 2210 F. remained at between 89 and 90 G.E. The effluent from the several bleaching operations

is collected and introduced into the chemical recovery system of the Kraft pulping process.

Example II

Midwestern poplar chips were pulped via the Kraft

process. Handsheets prepared as in Example I had a brightness of

G.E. units.

A sample of the pulp was fluffed and subjected to a

first ozone treatment as in Example I. The amount of ozone con-

sumed was equal to 1.28% of the bone dry weight of the fibers.

After washing the pulp was treated with an aqueous

67, 290 in

solution of 1.5% sodium hydroxide based on the bone dry weight of

the pulp as in Example I.

Following a second ozonation in which the ozone con-

sumed was equal to 0.90% of the bone dry weight of the fibers

and a second alkali treatment, this time with an aqueous solu- tion containing 0.5% sodium hydroxide based on the bone dry

weight of the pulp, the pulp was treated with an aqueous solu-

tion comprising:

hydrogen peroxide 0.2% sodium hydroxide 0.25% sodium silicate All of these treatments were carried out as in Example 1

except for the amounts of reagents employed.

Handsheets prepared from the pulp had a brightness of 88.5 G.E. and after one hour at 221 0 F. retained a brightness of

86.5 G.E.

Example III A second sample of the unbleached pulp prepared in Example II was bleached by a method identical with that disclosed

in Example II except that the fresh water utilized in the second

alkali extraction in that example wasreplaced by the effluent from the peroxide treatment in Example II.

Handsheets prepared from pulp bleached in this manner

had a brightness of 90.0 G.E. and after 1 hour at 221 0F. re- tained a brightness of 88.5 G.E.

Example IV A third sample of the unble ached pulp prepared

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in Example II was bleached by a method identical with that discl4

ed in Example II except for two modifications. The fresh water in the second alkali extraction was replaced by the wash water

from the peroxide treatment in Example III.- Also the fresh

water in the first alkali extraction was replaced by the

effluent from the second alkali extraction in Example III.

Handaheeta prepared from pulp bleached in this manner had a brightness of 89.3 G.E. and after 1 hour at 2210 F. re-

tained a brightness of 87.9 G.E.

The effluent from the first alkali extraction is intro- duced into the chemical recovery system of the Kraft pulping pro-

cess at the brown stock washer.

Example V A mixture of hardwood chips (beech, birch and maple) were pulped via the Kraft process. Handsheets prepared as in Example I had a brightness of 30 G.e. units.

The pulp was dewatered to 35% consistency in a basket centrifuge and the product was fluffed in a high spee

fiber separator. The fluffed material was treated, in a counter- current stream with oxygen containing 2.53% ozone based on the bo dry weight of the pulp. The amount of ozone consumed was equal to 0.96% of the bone dry weight of the fibers.

The ozonated pulp was treated with an alkaline solution and thereafter bleached at 12% consistency with an aqueous solu-

tion comprising 0.27% hydrogen peroxide, 5% sodium silicate and 0.75% sodium hydroxide for 4-1/2 hours at 1130 F. Handsheets, prepared and tested as in Example I, had an average brightness

of between 84 and 85 G.E. units.

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67.290

Effluent from the bleach treatments is collected and

utilized in subsequent bleaching operations or in the

chemical recovery system of a Kraft pulping process.

471663

67,290 THE CL IMS DEFINING THE INVENTION ARE AS FOLLOWS.

1. A method of Kraft wood pulping employing a Kraft chemical recovery system and a Kraft wood pulp bleaching operatic said method being free of any treatments employing chlorine or chlorine-containing compounds and whereby the need for treating effluent being introduced into the Kraft pulp chemical recovery

system for the removal of chlorine-containing salts is eliminated

said method conprising the steps of bleaching the Kraft wood

pulp by a sequence of treatments of from one to about three

treatments with ozone and a final treatment with an alkaline

peroxide solution, an alkaline extraction following each ozone treatment or only following the last ozone treatment and precedin the alkaline peroxide treatment, recycling effluent from the alkaline peroxide treatment to the bleaching operation to reduce the fresh water requirements, and introducing effluent

from the bleaching operation into the Kraft pulp chemical

recovery system. 2. A method as recited in claim 1, in which the effluent from the alkaline peroxide treatment is recycled to an alkaline

extraction step in the bleaching operation. 3. A method as recited in claim.i, in which the effluent from a second or third alkaline extraction step is recycled to an earlier alkaline extraction step in the bleaching operation.

4. A method as recited in any one of claims 1, 2 or 3,

in which the number of ozone treatments is 2.

A method according to any one of the preceding claims, wherein effluent from the bleaching operation is

introduced into the Kraft recovery system at the brown stock washer in said recovery system. 6. A method according to any one of claims 1 to 4, wherein effluent from the bleaching operation is introduced

into the Kraft recovery system at the smelt dissolv.ing operation

in said recovery system. 14 47166 3

Claims (1)

1. 67, 290 /74 7. A method according to any one of claims 1 to 4, wherein

   effluent from the bleaching operation is introduced into the Kraft recovery system at the lime mud washer in said recovery system.

   8. A method according to any one of claims 1 to 4, wherein effluent from the bleaching operation is introduced into the Kraft recovery system at the kiln stack scrubber in said recovery system. 9. A method of Kraft wood pulping according to claim 1, substantially as hereinbefore described with reference to the

   Examples.

   DATED this 2nd day of April, 1976.

   SCOTT PAPER COMPANY

   EDWD. WATERS SONS,

   (9- Patent Attorneys, 50 Queen Street, Melbourne, Victoria, AUSTRALIA.

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