AU606930B2

AU606930B2 - Pulp bleaching process

Info

Publication number: AU606930B2
Application number: AU14067/88A
Authority: AU
Inventors: Michel Devic; Jacques Kervennal; Jean-Pierre Schirmann
Original assignee: Atochem SA
Current assignee: Arkema France SA
Priority date: 1987-04-02
Filing date: 1988-03-31
Publication date: 1991-02-21
Anticipated expiration: 2008-03-31
Also published as: NO881404D0; ES2004846A4; DE3877403T2; EP0285530A1; AU1406788A; DE285530T1; EP0285530B1; ATE84581T1; JPH0156197B2; ES2004846T3; NZ224144A; NO173886B; DE3877403D1; FR2613388A1; JPS63256786A; CA1310797C; NO881404L; FI93749C; NO173886C; FI881544A0

Abstract

Bleaching of pulps of mechanical origin in which the pulp is treated with a metal-complexing or sequestering agent and is then washed with an efficiency higher than 95 % before being subjected to the action of hydrogen peroxide at a temperature above 100 DEG C and not exceeding 250 DEG C at the pressure of steam at the said temperature for 1 to 30 minutes, not conjointly with a mechanical beating action.

Description

N/43875 6" 06 9 30 FORM COM MON WE A LT H O F AU ST RA L XA, PATENTS ACT 1952-1973 (original) FOR OFF CC USE".

class Int.. Class Applicati.on Num'ber-.

Lodged,; Complete Specification Lodged., Accepted: Publi shed; P~r i o r i tyt :L I' .V ccc~

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'Related Art.- ,Name of Applicant: Address of Applicant Actual Inventor(SUL Address :for Service': .La Defense 4 8 Cours Michelet, 92800 PUTEAUX;, France Michel DEVIC, Jacques YERVENNAL and Jean-Pierre SCHIRMANN Davies Collison, Patent Attorneys, 1 Little Collins Street, MELBOURNE 3000, Australia.

ATOCHEM

Complete Specification for the invention entitled: "PULP BLEACHING PROCESS" The following statement is a full description of this invention, including the best method of performing it known to us: To: THE COMMISSIONER OF PATENTS v (a member of the firm of DAVIES COLLISON for and on behalf of the Applicant), Davies Collison, Melbourne and Canberra.

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2 c O 000 0 C 0 000 o 0 ©000 C C 300 000 00 o0 C a0 3 0 0 3 The present invention relates to the bleaching by means of hydrogen peroxide of mechanical, thermomechanical, chemicomechanical and chemicothermomechanical pulps in the papermaking and associated fields.

These pulps are hereafter referred to as "pulps of mechanical origin".

In most cases such pulps are produced on an industrial scale from wood, generally in the form of chips, by mechanical grinding of the lignocellulosic material, for 10 example in a stone grinder or a disc grinder or refiner, which is combined or otherwise, depending on the intended pulp, with a preliminary treatment with steam and/or a chemical reactant such as sodium sulphite.

The solids yield of an operation of this kind is 15 always very high and can reach or even exceed Pulps of mechanical origin, which are also known as "high-yield pulps" must be bleached efficiently and with a minimum loss of material in order that the manufacture of the products derived therefrom may meet the industry's 20 quality and economy requirements.

It is known to perform the bleaching of pulps of mechanical origin with the aid o2 an oxidizing agent such as hydrogen peroxide in an alkaline medium.

It is generally accepted that the optimum temperature for performing such a traditional bleaching operation using hydrogen peroxide is 60 0 C-70 0 C, as -3indicated, for example, by C.W. Dence and S. Omori 1 Tappi Journal,. October 1986, p. 120-125, and French patent specification No. 2,371,544. It is impossible to obtain efficient bleaching at 100"C and above.

Above 100 0 C, the bleaching effect'may even be non-existent, both because of the extremely rapid decomposition of hydrogen peroxide and. of the specific detrimental effect of steam, as referred to, for example, in the abovementioned French patent and by W.E. Lunan, K.B.

10 Miles and W.D. May, Proceedings Tappi, 1983 Pulping

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0 Conference (Book p. 239-253.

According to the abovementioned French patent, 0000 acceptable bleaching can be obtained at a temperature of 0 0 1000C to 150 0 C in a steam atmosphere using hydrogen peroxide in the presence of an alkaline agent, but only if the action 001 of hydrogen peroxide on the material to be bleached takes 0 3C place conjointly with a fibre-disintegrating action by means

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0 of mechanical grinding or refining for a period of time C0CC which is only of the order of a fraction of a second.

The process of the present invention provides 0 CC o c efficient bleaching despite the use of conditions which are C CC known to have a contrary effect.

According to the present invention there is provided a process for bleaching a pulp of mechanical origin (as hereinafter defined), which comprises treating the pulp with a metal-complexing or sequestering agent at a pH below or 4 equal to 9, washing it with an efficiency (as hereinbefore defined) of at least 95% and then subjecting it to hydrogen peroxide at a temperature from 100° C to 250° C in the presence of an alkaline agent and saturated steam at the pressure of saturated steam at the said temperature for 1 to minutes independently of any mechanical grinding action.

In the treatment using the ecmplexing or

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0 sequestering agent, the latter is typically employed in a a proportion of 0.1% to 1% by weight based on the weight of dry pulp. Except where otherwise stated, the quantities of products in all that follows are also expressed in per cent by weight based on the weight of dry pulp.

oJ 0 )c o 0 The complexing or sequestering agent is suitably sodium tripolyphosphate, sodium tetrapyrophosphate, or a S 15 sodium salt of citric, nitrilotriacetic, c ethylenediilminetetraacetic or diethylenetriaminepentaacetic 0 acid.

CCS In the great majority of cases the treatment with the complexing or sequestering agent is carried out at a pH 20 value of 4 to 8, although such a pH is not essential to C success, -It is naturally carried out at a temperature which is preferably below 100°C, but is generally higher than for example 50°C to 95°C, in order to ensure a rate of complexing or sequestration which is rapid without pressure 5 r i ,I .i being necessary.

In the treatment with the complexing or sequestering agent, the consistency, the weight percentage of pulp in the dry form in the treatment medium, can vary within fairly wide limits, generally from 5% to 30%. In most cases it is from 10% to 15% to give the best compromise between economy and efficiency of the subsequent washing.

The duration of the treatment with the complexing or sequestering agent depends on other parameters. It is generally from 5 minutes to 2 hours.

Washing means the operation which consists in removing, more or less completely, and in this case efficiently, the liquid which is present in the pulp. This removal can be effected, for example, by pressing the pulp on a filter or by a pulp dilution and pressing sequence, repeated if necessary, in which the dilution is most generally with water.

The washing which follows the treatment with a complexing or sequestering agent is in most cases carried out at a temperature from 20 0 C to 90 0 C, frequently from 20 0

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S to 60 0 C, for reasons which &re essentially economic.

"Efficiency", expressed in per cent, means the degree of removal of the liquid phase present in the pulp before washing. Thus draining the pulp from 10% to consistency would be interpreted as a 55.5% washing

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6efficiency; draining the pulp from 10% to 20% consistency then diluting the pulp to 10% consistency and draining it Sagain to 20% consistency would be interpreted as a washing efficiency.

Likewise, for economic reasons, but also for a better guarantee of bleaching efficiency, the action of hydrogen peroxide takes place in the process of the 0 invention preferably at a temperature above 1200C, for example 140 0 C and higher.

The quantity of hydrogen peroxide which is used is that generally accepted in the traditional bleaching of pulps of mechanical origin and it is consequently generally from 0.5% to 10%, in most cases not less than about 2%.

The alkaline agent which is present with the hydrogen peroxide and which is suitable for performing the invention is typically sodium hydroxide which is in most I cases employed in a proportion from 0.25% to with a weight ratio sodium hydroxide: hydrogen peroxide which is preferably from 0.2:1 to 1:1.

The invention allows the action of the hydrogen peroxide to take place for a time which is both sufficiently long to enable this product to develop its effectiveness to its full extent, and is advantageously reduced relative to that of a traditional bleaching operation. It generally does L, .I i 7 not exceed 15 minutes and frequentiy about 5 minutes is sufficient.

As already indicated, the action of the hydrogen peroxide takes place in a medium at a pressure which is essentially equal to the pressure of saturated steam at the chosen temperature. This pressure is in most cases from 2 bars absolute to 40 bars absolute.

The consistency of the pulp brought into contact with hydrogen peroxide is preferably chosen to be as high as possible while remaining compatible with the production of a satisfactorily homogeneous mixture of the pulp with the other products present. This means that the consistency is generally limited to a value not exceeding about 30%, or even 25%. On the other hand, it is preferably not lower than about The present invention enables one to produce high bleaching results with pulps of mechanical origin without the presence of compounds which have a stabilizing effect on hydrogen peroxide being essential for the effectiveness of its action. The latte:. may, nevertheless, be improved by i the presence of compounds such as sodium silicate, employed in most cases in an amount from 2% to magnesium salts such as magnesium sulphate, employed in a quantity which is generally from 0.05% to and derivatives of poly(a-hydroxyacrylic) acid such as the Clarene® products L "I r.

8 which are generally employed in a proportion of 0.05% to 2%.

The process of the invention may be carried out discontinuously or continuously in any plant comprising mixing equipment, generally of the pug mill type, in which the treatment using the complexing or sequestering agent and the mixing of the original pulp or pulp which has been washed with hydrogen peroxide and the alkaline agent can be carried out, washing equipment such as filters and presses, and, finally, for the hydrogen peroxide reaction stage, pressure-resistant and steamtight equipment such as a tubular reactor equipped, for example, with a pulp-propelling system of the screw type, or an autoclave.

The following Examples further illustrate the present invention. Some of them are given by way of comparison.

In these examples: the quantities of reactants are given, as indicated earlier, in by weight relative to the pulp in the dry state, diethylenetriaminepentaacetic acid (DTPA) is employed in the form of its sodium salt in an aqueous t solution at a concentration of 40% by weight and the quantity indicated is that of this solution, the term "silicate" denotes an aqueous solution of sodium silicate with a relative density of 1.33 and the quantity indicated is that of this solution, 4 nS LILj--;; i ;tii 9

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the abbreviation LS denotes ClareneR, the hydrogen peroxide is employed in the form of an aqueous solution at a concentration of 35% by weight of peroxide and the quantity indicated is that of 100% hydrogen peroxide, as is also the quantity of hydrogen peroxide consumed, the pulp to be bleached is or is not subjected to the complexing treatment and washing before the action of hydrogen peroxide. When this is not carried out, the example is given by way of comparison and its number is followed by C. In the former case, corresponding to thi process according to the invention, the pulp treatment carried out in a pug mill using 0.5% of DTPA, for minutes, at 600C in the case of Examples 1 to 11ii, 900C in the case of Examples 12 to 14, at a pH from 4 to 8 in each case and which is the same in a given example and in the comparative example, with a consistency of 10%. The washing which follows, with an efficiency which is greater than in each case, is carried out at 600C by dilution with water and pressing on a filter the pulp which has been treated with DTPA, the original pulp or the pulp which has been washed after treatment with DTPA is mixed with hydrogen peroxide, sodium hydroxide, the quantity of water which is sufficient to ensure the desired consistency, equal to _j 10 in this case, and, where appropriate, with the silicate or Clarene® by milling for less than 5 minutes at 20-25 0

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before being subjected to the action of hydrogen peroxide in a reactor of the pressure-resistant autoclave type, equipped with a lock chamber for introducing the pulp and leaktight to the saturated steam which ensures the pressure at the chosen temperature T, and 00 the degree of whiteness of the pulp collected r ;after decompression of the autoclave and rapid cooling is 10 measured (457 nm) in with the aid of a Carl Zeiss model Elrepho spectrophotometer.

EXAMPLES 1 to 11 Each of the examples relates to the bleaching of 100 g of stone- groundwood pulp of resinous wood with a degree of whiteness of 53.7% and a weight of 38.2 g in the dry state.

For each example, the conditions in which the action of hydrogen peroxide takes place, together with the results obtained, are assembled in Table I below: 11 c' o o 0' 000k 00, :0 0 0 S 1' TABLE I Test T Time H 0 Na0H SiLicate LS egree H 0 No. .t m i. 22'of22 1~l con- .hiteness sumed 1 145 5 5 2 78.5 2.55 2 145 5 -5 2 4 ISO 2.3 3 .145 5 5 40.5 81 3.2 4 145 15 5 2 77 3 'i80 2 3 2 75.5 1.8 5c 180 2 3 2 70 2.45 6 180 2. 5 2 '17 2.4 7 1,80 2 10 2 78.5 4 7 c 180 2 10 2 75.5 8 180 2 10 4 79 8 9 180 2 10 4 0.5 82.5 4.7 245 1 5 2 77.5 3.05 11 245 1 5 4. 0.5 79 32 From the above Examples it can be seen that the process according to the invention makes it possible to reach a much improved degree of whiteness which can be in the region of 80% and which can even be appreciably in of this value in the preferred conditions.

This improvement is accompanied by a major decrease in the usage of hydrogen peroxide or, at least, entails practically no increase in the latter.

A given degree of whiteness is attained with a substantial saving in hydrogen peroxide.

~L U-CII-aasrurr~LC-*Yrr 12 There is no appreciable change in the weight of pulp in the dry state in the examples according to the invention.

EXAMPLES 12 and 13 Each of' the examples relates to the bleaching of 100 g; of chemicothermomechanical sulphite pulp with a degree of whiteness of 59.6% and a weight of.40 g-in the dry state.

For each example, the conditions in which the action of hydrogen peroxide takes place, together with the results obtained, are assembled in Table II below:

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TABLE II Test T Time H 0 NaOH LS Degree- H 0 2 2 of 2 2 No. *C min whiteness consumed 12 150 2 5 4 0,5 78 3 13 180 2 5 4 0.5 79 3.3 13c 180 2 5 4 0.5 74.5 4 The comments made in relation to examples 1 to 11 remain valid.

Claims

1. Process for bleaching a pulp of mechanical origin (as hereinbefore defined), which comprises treating the pulp with a metal-complexing or sequestering agent at a pH below or equal to 9, washing it with an efficiency (as hereinbefore defined) of at least 95% and then subjecting it to hydrogen peroxide at a temperature from 100°C to 250 0 C in t the presence of an alkaline agent and saturated steam at the pressure of saturated steam at the said temperature for 1 to minutes independently of any mechanical grinding action. o 10

2. Process according to Claim 1, in which the quantity of complexing or sequestering agent is from 0.1% to 1% by weight based on the weight of dry pulp.

3. Process according to Claim 1 or 2 in which the treatment with the complexing or sequestering agent is 15 carried out at a pH of 6 to 8.

4. Process according to any one of Claims 1 to 3, in which the treatment with the complexing or sequestering agent is carried out at a temperature of 500C to 950C.

5. Process according to any one of Claims 1 to 4 20 in which the washing is carried out at a temperature of 20 0 C to 90 0 C.

6. Process according to any one of Claims 1 to in which the pulp is subjected to hydrogen peroxide at a temperature of at least 120 0 C. 14

7. Process according to any one of Claims 1 to 6, in which the quantity of hydrogen peroxide is from 2% to by weight based on the weight of dry pulp.

8. Process according to any one of Claims 1 to 7, in which the pulp is subjected to hydrogen peroxide in the presence of sodium hydroxide in an amount from 0.25% to by weight based on the weight of dry pulp and from 0.2 to 1% by weight relative to the hydrogen peroxide.

9. Process according to any one of Claims 1 to 8 in which the pulp is subjected to hydrogen peroxide for not more than 15 minutes.

Process according to any one of Claims 1 to 9 in which the pulp is subjected to the hydrogen peroxide in the presence of a compound which has a stabilizing effect on the latter.

11. Process according to Claim 10, in which the compound with a stabilizing effect is sodium silicate, a magnesium salt, or a derivative of poly (a-hydroxyacrylic) acid.

12. Process according to Claim 1 substantially as described in any one of Examples 1 to 13.

13. Pulp of mechanical origin whenever bleached by a process as claimed in any one of the preceding claims. I

14-~ stp Dated this 20th day of April 1988 ATOCHEM By its Patent Attorneys DAVIES COLLISON cGc