US2461105A

US2461105A - Bleaching of cellulosic matter

Info

Publication number: US2461105A
Application number: US724696A
Authority: US
Inventors: Bloch Rudolf; Goldschmid Kurt; Schnerb Isaac; Goldschmidt Paul
Original assignee: Individual
Current assignee: Individual
Priority date: 1946-01-01
Filing date: 1946-12-17
Publication date: 1949-02-08
Anticipated expiration: 1966-02-08
Also published as: NL65914C; NL129513C

Description

Patented Feb. 8, 1949 BLEACHING OF CELLULOSIC MATTER Rudolf Bloch, Jerusalem, Kurt Goldschmid, Tel Aviv, and Isaac Schnerb and Paul Goldschmidt,

Jerusalem, Palestine No Drawing. Application December 17, 1946, Se-

rial No. 724,696. In Great Britain January 1,

Claims.

This invent-ion relates to the bleaching and refining of cellulose fibres, and materials containing cellulose fibres, such as textile fibres, yarns of fabrics, paper, cotton wool, wood pulp, straw or the like. The invention is more particularly concerned with the treatment carried out with aqueous solutions of alkali or alkaline earth, hypochlorites. The invention also applies to the treatment of crude cellulosic materials with elementary chlorine or oxidizing chlorine compounds with a view to producing a fibre pulp.

Hypochlorite bleaching as hitherto practised has several drawbacks, The operation is slow unless the Vet has. a very high initial concentration of availablechlorine, or the temperature is raise'd,'but this would injure the fibre.

A further considerable decrease of the speed of the operation occurs towards the end, in consequence of which the available chlorine can in practice not be utilized exhaustively but the operation has to be broken oil when about one third 'to 9, practically hampers the action of the chlorine. Thissensibility of the operation to the pH'value of the vat is very inconvenient for the pH of the vat tends to decrease during the operation which would make it desirable to start the operation with a rather high pH value. Thus, a careful observation and regulation of the reaction is required during the operation.

Many attempts have been made at improving thehypo'chlorite bleaching in order to obtain acceleration of the process, complete whiteness without second bleaching, and full utilisation of the available chlorine, but no satisfactory solution of these problems has so far been found.

This invention is based on the discovery that the drawbacks referred to above can be overcome if the hypochlorite treatment is carried out in the presence of bromine ion.

.By the presence 'of bromide; the available chlorine is made more active, which entails two main consequences. Firstly, the. bleaching op-.

eration is markedly accelerated, that is, the consumption of available chlorine, proceeds at a quicker pace. Secondly, rather low concentrations can be made to perform the bleaching in a short time. Of this second feature, use can be made either thereby that the available chlorine of a vat is almost completely exhausted in one bath, or a spent liquor containing still some available chlorine is used for a subsequent bath until complete exhaustion of the chlorine, whereafter a fresh amount of hypochlorite, may be, added.

Both features aforesaid, that is, acceleration of the process and exhaustive utilization of available chlorine, make it possible to. start the operation at a lower concentration of available chlorine than is used heretofore, with the result that the danger of affecting the tensile strength of the fibres is diminished.

The usually desired high degree of whiteness can be obtained in one single operation. In this manner, and in view of the acceleration aforesaid, the time spent for the bleaching operation proper is decreased to almost half of what it has been hitherto.

The operation is less sensitive to the alkalinity of the vat, and an initial pH value of about 10 to 11 may be used without inconvenience. This facilitates the operation, for on the one hand there is no need for a very careful removal of the alkali adhering to the fibres when they come from the preparatory treatment in the kier, on the other hand the vat may have so high an initial alkalinity that in spite of the gradual drop of the pH, the reaction of the vat remains "suificiently alkaline. This greater ins-ensibility of the operation to the alkalinity makes it possible to dispense with the careful observation and regulation of the reaction.

The bromide contents of the vat may be of the order of 0.5% of bromine ion calculated to the weight of the goods to be bleached, or about 0.25 g. of bromine ion per litre of the bleaching vat.

The addition of bromide to the vat may be effected at the beginning of the bleaching operation. In this case, much of the available chlorine is consumed at once, about during the first quarter of an hour, and further amounts of hypochlorite are suitably then added to the vat in order that the initial concentration of available chlorine be substantially restored. As compared with ordinary hypochlorite bleaching the initial concentration of available chlorine may be lower, about 0.7 g. per litre.

On the other hand. the bleachin operation may be started with hypochlorite alone and the bromide may be added later on, for example, after one half up to two thirds of the initial amount of available chlorine have been used up.

According to a modification of the invention aforesaid the addition of the bromide to the bleaching liquor is spread over a great part or the whole of the time required for the bleaching.

The term addition comprises the introduo- The total amount of bromine in the bleaching liquor should not exceed about one third of the weight of available chlorine, or about one sixth of a mole of bromine for each mole of available chlorine.

The addition of the bromine may be effected in a continuous flow or intermittently in small portions, and it may be started either at the beginning of the bleaching or after some bleaching with hypochlorine alone.

It has further been observed that while the bleaching of cellulose with hypochlorite alone is preferably and usually carried out at a pH above 9, the adjustment of the hydrogen ion concentration of a bromide-containing hypochlorite liquor to a pH value of 7 to 8.2 entails two unexpected major advantages: the speed of the 1 bleaching reaction is greatly increased, and the quality of the product, expressed in terms of fluidity (determined in accordance with the pre* scriptions of Clibbens and Geake, Shirley Inst.

Memoirs, 1927, No. 6, page 117) is much improved.

The invention, therefore, also comprises the adjustment of the pH value of the bromide-containing bleaching liquor to Within the range stated.

It has been observed that by thus spreading the addition of bromide over a greater part of the bleaching time the same advantages, as compared with hypochlorite bleaching without bromide, can be obtained as in the embodiments of the invention described hereinbefore, but with a considerably smaller amount of bromide, This offers the additional advantages that the process becomes cheaper and the fibre is virtually not attacked by the small amount of hypobromite present in the liquor at any time.

Like the aforesaid embodiments of the invention, this embodiment also is applicable to cellulosic material of various description. It has proved particularly valuable for the bleaching of wood pulp. In addition this embodiment is very suitable for bleaching textile fibres, yarns or fabrics in a continuous operation by running them through a vat, or preferably through several vats, or several compartments of one and the same vat, in, succession, which they leave ready for the usual after-treatment. This can be done owing to the acceleration of the bleaching by the addi-- tion of bromide to the bleaching liquor and owing, moreover, to the surprising fact established by the inventors that the combined action of hypochlorite and bromide on the cellulosic material allows the kiering treatment of the latter to be shortened.

The invention is illustrated by the following examples to which it is not limited.

- Example 1 100 kg. of cotton cloth are bleached in a vat of 2000 litres contained in a bleaching machine provided with usual winches over which the cloth runs as an endless band, a section of the latter hanging down into the vat. To the vat, so much of a concentrated aqueous solution of sodium hypochlorite is added that the concentration of available chlorine in the vat is about 0.7 to 0.8 g. per litre. Moreover, 750 g. of potassium bromide are dissolved in the vat before the bleaching operation is started. During the first 15 minutes of the bleaching operation some more sodium hypochlorite solution, in the total amount of 800 g. of available chlorine, is gradually added. This amount corresponds to the consumption of chlorine during the first fixteen minutes so that the initial concentration is substantially restored. During the subsequent hour the chlorine concentration drops to about 0.4 to 0.5 g. chlorine per litre, which marks the end of the operation.

The cloth is then withdrawn from the bleaching vat, after-treated in the usualmanner by washing in bisulphite and in soap solutions and rinsing in water, and is obtained completely white without requiring after-bleaching.

If the same cloth were to be bleached in the usual manner with hypochlorite alone, the initial chlorine concentration would have to be of the order of 1.0 to 1.5 per litre, the operation would require at least about two hours, and the end concentration of chlorine would be of about 0.5 to 0.9 g. of chlorine per litre. After the usual aftertreatment, an after-bleaching operation would be necessary since the goods are not obtained completely white after the first bleaching.

Example 2 With the same cloth as in Examplev 1, the bleaching operation is started with 2000 litres of a vat containing 1 to 1.2 g. of available chlorine per litre. After about 30 to 45 minutes the chlorine contents have dropped to about 0.7 to 0.8 g. per litre. At this stage, 750 g. of potassium bromide are dissolved in the vat, The bleaching operation takes altogether about to 105 min utes.

Example 3 kg. of cotton cloth are bleached with the aid of a jigger in a vat of 200 litres containing sodium hypochlorite in an amount corresponding to a concentration of available chlorine of about 5 to 6 g. per litre. After about half an hour the concentration of chlorine has dropped to about one third its initial value, and 600 g. of potassium bromide are then added to the vat. Complete whiteness is reached after about 1% hours from the beginning of the operation, and the available chlorine is then practically exhausted.

Example 4 The bleaching operation is carried out as in Example 1, the only difference being that instead of potassium bromide, a concentrated aqueous solution of sodium hypobromite-bromide prepared by the addition of bromine water containing 500 g. of bromine to a concentrated aqueous solution of about 250 g. of N aOH, is added to the vat.

Example 5 The vat is prepared thereby that into a dilute aqueous solution of sodium hydroxide containing about 4750 g. of NaOH, 4000 g. of gaseous chlorine and bromine water containing 500 g. tit-bromine are gradually introduced whereafter water is added up to a total volume of 2000 liters. The bleaching operationis then carried out as in EX.- ample 1.

I Example 6 In the manner known from the preparation of bleaching powder (chloride of lime), a bleaching mixture is prepared by the introduction of 4000 g. of elementary chlorine and bromine water containing 500 g. of bromine into an aqueous suspensionof 6000' g. of calcium hydroxide, and this mixture is diluted with so much water that a clear solution of a total volume of 2000 liters is obtained which is used as vat in a bleaching operation carried out as described in Example'i.

Example I Dry commercial wood-pulp is mixed with somuch water that a wet pulpy mass is formed. To this, a concentrated aqueous solution of sodium hypochlorite is added in the amount necessary for obtaining the desired degree of bleaching, which is previously determined with a small portion of-the pulp. As arule, an amount of from 6 to 12 g. of available chlorine is required for each 100 g. of dry matter of the pulp, To this, potassium bromide is added in aqueous solution,

the amount of Br being 3 g. for each 100 g. of dry matter of the pulp. The vat is then allowed to stand for some-hoursuntil the desired degree of bleaching. has been reached.

- Example I 2000 li res of wood-pulp having a dry-matter rat o of'5% (i. e. 5 kgs. of dry matter in each 100 litres of pulp) are added to with 280 litres of an aqueous solution of bleaching powder (chloride of lime) containing a total amount of 7 kgs.

of available chlorine. i. e. 7% by weight of dry cellulosic matter. The temperatureof the liquor is ke t at; about 35 C. After one hour, when about 60 to 70% oi. the available chlorine have been used up. 300 litres of an aqueous solution of potassium bromide of 0.5% strength by weight are allowed to flow in during about one further hour. The total amount of'bromine ion thereby introduced is about 1% by weight of the dry cellulosic matter. While the potassium bromide solution flows in, the liquor is stirred. When the addition or bromide begins, the hydrogen ion concentration of the liquor is initially brought to pI-I=7.5 by the addition ofacid, e. g. hydrochloric or formic acid, and during the further operation the hydrogen ion concentration is kept under observation, and whenever necessary, adjusted by the addition of alkali hydroxide or milk of lime so as to remain more or less constant at about pl-I=7.5. At the end of about two hours from the beginning of the bleaching, the available chlorine has been fully consumed. The liquor is then diluted with about one hundred times its volume of water, neutralized with sillphuric acid, and filtered, and the residue is washed with water.

Example 9 bromine thus introduced being about 0.75% by weight of the dry cellulosicmatter,

Example 10' .j

2000 litres of wood-pulp with a dry-matter ratio of 5% are added to with 280 litres of. an aqueous solution of chloride of lime containing altogether 7 kgs. of available chlorine. The liquor is kept at 45 C. At once with the addition of the chloride of lime, one starts with. the addition to the vat of 600 litres of an aqueous solution of potassium bromide of 0.5% strength by weight, which is spread over 45 minutes. The total amount of bromine thus introduced is 2 kgs, or 2% of the dry cellulosic matter. The whole of the available chlorine has been consumed after about minutes, when the liquor is further treated as described in Example 1.

Example 11 v To a wood-pulp-hypochlorite mixture composed as described in Example 1 and kept at about 25 0., a total amount of 3'75 litres of an aqueous potassium bromide of 0.5% strength by weight is added, the addition being started about one hour after the beginning of the bleaching and being spread over minutes. The total amount of bromine thus introduced corresponds to 1.25% of the dry cellulosic matter. The con sumption of available chlorine has been completed after about 250 minutes from the beginning of the bleaching, whereafter the mixture is Worked up as described'in Example 1.

posed as described in Example 1 and kept at about 45 0., 165 litres of an aqueous solution of potassium bromide of 05% by weight are gradually added during minutes, starting with the beginning of the bleaching operation, altogether 550 grs. of bromine, or 0.55% of the Weight of. thecellulosic dry matter. At the end of this time, the whole of the available chic-- rine has been consumed. The mixture is then further treated as described in Example 1.

Example 13 A web of cotton cloth is continuously run through a vat containing a. boiling solution of caustic soda with a content of 15 grs. of NaOH per litre, the rate of advance of the web through the bath being such that each part thereofro mains in contact with the liquor for about 20 minutes. The web then passes through water for washing and thereafter through a succession of bleaching vats containing hypochlorite liquor with 0.4 gr. of available chlorine per litre and kept at a pH of 7.0-8.0. To each vat an aqueous potassium bromide solution is supplied continuously or in small portions at such a rate that the bromide concentration in the bath is kept at about 0.2 gr. of bromine per litre. Makeup hypochlorite solution is also added to the vatscontinually or in small portions so as' to keep the contents of available chlorine in the vats substantially constant. The make-up hypochlorite may be added either in mixture with washing bath,thereafter through another bath of boiling caustic soda (containing 10 grs. of NaOH per litre), where each part thereof is again kept for about 20 minutes, then through another series of bleaching vats containing a hypochlorite solution (withoutbromide) kept at 'a-con-' centration of 0.2 gr. of available chlorine 'pe'r' 7 litre and a pH of about 9, to which each part of the web is again exposed for about 20 minutes.

The web is then subjected to a usual aftertreatment.

Example 14 Kraft pulp is treated with hypochlor' e solution containing per cent, chlorine calculated on the pulp to be bleached, to which 0.2 to 0.5 per cent. of bromine in the form of bromide calculated on the dry pulp is added in three portions in the course of 30 minutes.

After 50 to 70 minutes the free chlorine is exhausted, then the pulp is washed with a 2 per cent. soduim hydroxide solution calculated on the pulp at temperatures from the normal up to 100, filtered, washed, and again bleached with 2.5 per cent. free chlorine, calculated on the dry pulp, with the addition of a small amount of bromide. This stage takes another 50 to 120 minutes. The Kraft pulp treated in this way is of a satisfactory full white. The strength of the pulp is not impaired and the copper figure is low.

It is, of course, possible to have more chlorine used in the second stage, and less in the first stage according to the quality of the pulp.

Example 15 Strong sulphite pulp is first chlorinated with chlorine water (the total amount of chlorine'used being 3.5% of the weight of the dry pulp), and washed with an aqueous sodium hydroxide solution (2% by weight of NaOH calculated ondry pulp). Then the pulp is mixed with'water to form a pulpy mass having a dry-matter ratio of 5%. Then so much of an aqueous solution of bleaching powder (chloride of lime) is added that altogether 0.7% of chlorine (calculated on the dry matter of the pulp) is introduced. The temperature is kept at 37 C., and the pH at 9. .After 45 minutes stirring, about 55% of the available chlorine have been used up, and the pH is changed to 7.0-7.5 by the addition of some hydrochloric acid. Then an amount of an aqueous solution of potassium bromide corresponding to 0.5% Br of the dry matter of the pulp is allowed to flow in during half an hour. After 90 minutes from the beginning of the bleaching, 80% of the available chlorine have been consumed. The pulp is then washed with water and treated during 30 minutes at ordinary temperature with a solution of sodium hydroxide (2% NaOH calculated on the dry matter of the pump). After washing with water the pulp is after-bleached with 0.1% C1 (in the form of a solution of bleaching powder) at a temperature of 37 and pH of 9. After 30 minutes the pulp is after-treated with S02. The fluidity of the bleached pulp is 13.3.

Bleaching of the pulp with hypochloride alone, without adding bromide, to the same whiteness and with the same consumption of available chlorine, takes five hours and the bleached pulp has a fluidity of 16.0.

Example 16 Easy bleaching sulphite-pulp with a dry-matter content of 5%, is treated with 6.5% Cl, calculated on the dry matter, in the form of a solution of bleaching powder, at a temperature of 37 C. and a pH of about 9. After one hours stirring 66% of the available chlorine have'been consumed. Then the pH is changed to 7.0-7.5 by theaddition of some hydrochloricacid. A solution of potassium bromide, corresponding to 0.5% B1 tit calculated on the dry matter of the pulp, is allowed to flow in during 30 minutes. After two hours from the beginning of the bleaching, of the available chlorine have been consumed. The pulp is then washed and after-treated with S02. The fluidity of the bleached pulp is 16.7.

Bleaching of the same pulp to the same whiteness with the same consumption of available chlorine with hypochlorite alone, without adding bromide, takes four hours and the bleached pulp has a fluidity of 22.2.

This application is a continuation-in-part of copending application, Serial No. 610,676, filed August 13, 1945, now abandoned.

We claim:

1. Process oi bleaching cellulosic material which comprises treating it with an aqueous hypochlorite liquor having a pH of at least 7 and containing water -soluble bromide in an equivalent amount smaller than the available chlorine content of the liquor.

2. Process of bleaching cellulosic material which comprises treating it with an aqueous hypochlorite liquor having a, pH of at least 7 and containing water-soluble bromide initially added thereto in an equivalent amount smaller than the available chlorine content of said liquor.

3. Process of bleaching cellulosic material which comprises treating it with an aqueous hypochlorite liquor having a pH of at least 7 and containing water-soluble bromide added thereto in the course of the bleaching operation in an equivalent amount smaller than the available chlorine content of said liquor at the time of the beginning of such addition.

, Process of bleaching cellulosic material which comprises treating it with an aqueous hypochlorite liquor having a pH of at least 7 and adding to said liquor in the course of the bleaching operation and before the exhaustion of the available chlorine of the hypochlorite, water-soluble bromide in an equivalent amount smaller than the content of available chlorine of said liquor at the time of the beginning of such addition, said addition of bromide being spread over at least a substantial part of the total bleaching time.

5. Process of bleaching cellulosic material which comprises subjecting it to an initial bleaching with an alkaline reacting aqueous hypochlorite liquor, then, and before exhaustion of the available chlorine of the hypochlorite, adding to said liquor water-soluble bromide in an equivalent amount smaller than th content of available chlorine of said liquor at the time of such addition, while adjusting the pH of the liquor to a value between 7 and 8.2, and continuing the bleaching operation after such addition.

6. Process of bleaching cellulosic material which comprises subjecting it to an initial bleaching with aqueous hypochlorite liquor at a pH of about 9, then, and before exhaustion of the available chlorine of the hypochlorite, changing the pH to a value between 7 and 8.2, adding to said liquor water-soluble bromide in an equivalent amount smaller than the content of available chlorine of said liquor at the time of the beginning of such addition, and continuing the bleaching operation, the addition of the bromide being spread over at least part of such continued bleaching while the pH value of the bleaching liquor is maintained between 7 and 8.2.

'7. Process of bleaching cellulosic material which comprises treating it with an aqueous hypochlorite liquor having a pH of at least 7 and containing a water-soluble bromide-hypobromite mixture with an equivalent bromine content smaller than the available chlorine content of said liquor.

8 Process of bleaching cellulosic material which comprises treating it with an aqueous hypochlorite liquor having a pH of at least '7, and adding to said liquor in the course of the bleaching operation and before exhaustion of the available chlorine of the hypochlorite, at least one alkaline reacting compound selected from the group consisting of alkali metal hydroxides and carbonates, and elementary bromine in an equivalent amount smaller than the available chlorine content of said liquor at the time of the beginning of the addition of bromine to said liquor.

9. Process of bleaching cellulosic material which comprises subjecting it to an initial bleaching with aqueous hypochlorite liquor at a pH of about 9, then, and before exhaustion of the available chlorine of the hypochlorite, changing the pH to a value between 7 and 8.2, adding to said liquor at least one alkaline reacting substance selected from the group consisting of alkali metal hydroxides and carbonates, and elementary bromine in an equivalent amount smaller than the available chlorine content of said liquor at the timeof beginning the addition of bromine, and continuing the bleaching operation, the addition of said alkaline reacting substance and bromine being spread over at least part of such continued 10 bleaching while the pH value of the bleaching liquor is maintained between 7 and 8.2.

10. Process of treating cellulosic material in a continuous process comprising kiering and bleaching said material by running it through a succession of baths, bleaching it in at least one of said baths with an aqueous hypochlorite liquor having a pH of at least '7 and containing bromide in an equivalent amount smaller than the available chlorine content of said liquor, and maintaining the contents of available chlorine and bromine in said liquor substantially constant by the continued addition thereto of hypochlorite and bromide.

RUDOLF BLOCH. KURT GOLDSCHMID. ISAAC SCHNERB. PAUL GOLDSCHMIDT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 125,658 Campbell Apr. 16, 1872 1,522,560 Weber Jan. 13, 1925 1,843,465 Traquair Feb. 2, 1932 1,883,193 Wells Oct. 18, 1932 2,001,268 Rue May 14, 1935 2,265,033 Fryer Dec. 2, 1941