AU615001B2 - Pulp bleaching process - Google Patents

Description

.4 ZkXMAnms~doNWIrIH0'V 'Id 01, 11.25 1. COMMONWEALTH OF AUTS PATENTS ACT 1952 COMFLETEhSPBfQ NAME ADDRESS OF APPLICANT: A-tochem.

La Defense 4 8 Cours Michelet Puteaux 92800 France NAME(S) OF INVENTOR(S): 0.0 o 0 0 o0 00 o 0 0 00 0 0 000 00V00 0 0 0 0 0 0 00 0 0 0 0 Jacques KERVENNAL Francoise CHOSSON Domninique LACHENAL ADDRESS FOR SERVICE: DAVIES COLLISON Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

COMPLETE SPECIFCATION FOR THE INVENTION ENTTED: Pulp bleaching process The following statement is a full description of this invention, including the best miethod of performing it known to me/us:- 2 The present invention relates to the bleaching of mechanical, thermomechanical, chemomechanical or chemothermomechanical pulps in the papermaking and related fields to high brightness, with the aid of hydrogen peroxide. In everything that follows, these pulps are referred to by the expression "pulps of mechanical origin".

They are produced industrially, in most cases from wood, generally in the form of chips, by mechanical grinding of the ligno-cellulose raterial, for example in a millstone grinder or a disc grinder or refiner, in combination or otherwise, depending on the intended pulp, with a 0ooo o o° preliminary treatment using steam and/or a chemical reactant °.0oo. such as sodium sulphite.

O 0 0000 00.

0 0 The yield of an oFeration of this kind is always 00 0 oo 15 very high and can reach and frequently even exceed 90% by weight relative to the dry material.

Pulps of mechanical origin, also called high yield 0 00 o0.o pulps, obtained in the unbleached state, must be bleached 0oo0 effectively and with a minimum loss of zerial, so that the 0 o 00 S20 manufacture of the products derived fterefrom can meet 000000 0 0 industrial quality and economy re1urements.

000000 o o It is kno;n to perform the bleaching of pulps of 0 0 0 mechanical origin using hydrogen peroxide in alkaline medium.

3 Bleaching of this type is being increasingly widely adopted by the industry for reasons of effectiveness, but also because of a reduction in the pollution load. The industry is making continuous efforts to improve a technique of this kind.

For example, D. Lachenal, C. de Choudens and L. Bourson, TAPPI Proceedings of the 19861 Pulping Conference, 569-573, propose a two-step process in which it is possible to do without the presence of sodium silicate in the second step, but not in the first.

o" in fact, in the bleaching of pulps using hydrogen 0 peroxide in alkaline medium, when considered as a whole, it is accepted that sodium silicate is indispensable for obtaining the best results from the standpoint of bleaching, S, 1 5 as well as from that of econony. This is continually recognized, for example by W. G. Strunk, Pulp and Paper, June 1980, 156-141, R. W. Allison, Appita Vol. 36, No. t o0 .0 Marca 1983, 362-370, C. W. Kutney, Pulp and Paper Canada, o00Oo 86:12, (1985), 182-189, and finally confirmed by oa000o 20 C. W. Dence, S. Omori, TAPPI Journal, October 1986, 120-125.

0 a Today it is also known that diethylenetriamine- 00000 pentamethylenephosphonic acid (DTMPA) is capable of 00 0 o0 behaving on a par with sodium silicate. According to R. W. Allison, in the paper referred to, this equivalence can be observed when a mechanical pulp from a particular kind of wood is bleached under well-controlled conventio:nal '1 4 conditions, but is no longer confirmed when the same pulp is bleached in a refiner. In any case, the use of DTMPA is not economical.

Thus, only a result inferior to that obtained from a process involving sodium silicate or a compound capable of behaving on a par with sodium silicate would normally be expected from a process making no use whatsoever of sodium silicate.

The process of the invention makes use of neither and yet results in a pulp with brightness on the same level 00o°° 10 as that which can be obtained with their use. Thus by 0 00 0 o virtue of the process of the invention the industry has at 0 ooo0oo its disposal a bleached pulp of mechanical origin obtained 0000 ooo in an economical manner and presenting none of the 0 0 disadvantages linked with the inevitable presence of sodiu silicate, such as those recalled, for example, in the 0oo abovementioned paper by G. W. Kutney.

0 o According to the present invention there is 0oo, provided a process for bleaching a pulp of mechanical origin 0 0 o using hydrogen peroxide in alkaline medium but without 0o0900 0 0 sodium silicate or a compound capable of equalling the "ooo.0 effectiveness of sodium silicate in two consecutive steps 00 0 00 separated by a washing step such that the quantity of hydrogen peroxide used in the first step is from 0t.5% to 3%

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n by weight relative to the pulp solids and the quaitity of hydrogen peroxide used in the second step is from 2% to by weight relative to the pulp solids.

In all that follows, unless stated or demonstrated otherwise, the quantities are expressed in percent by wpight relative to the dry pulp.

The bleaching effectiveness of a compound relative to sodium silicate is understood here to be that formulated as in the publication by C. W. Kutney, already referred to.

Setting aside the special features of the process "0o0o ofi the invention, the bleaching in two steps can be carried out under conditions which are generally adopted currently.

4 o The bleaching changes as a function of variations in the principal parameters, as indicated in the paper by S0o 15 C. D. Dence and S. Omori, already referred to.

The alkalinity in both steps is obtained in most cases by using sodium hydroxide, and the quantity of 00o0* alkaline agent used in each of the two steps is that needed o to ensure an initial pH of, say, 10.5 to 12, in both of these steps.

The preferred weight ratio of sodium hydroxide to 0 o a 0 0 hydrogen peroxide in the first step, as in the second step, So00 is from 0.6:1 to 1.2:1.

The temperature may be identical or different in 1~ h f :il

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0 -f Yi=YPiiP~ SP1 ID*XleuiUI-~Wo0 oo 0 o 0 oooo 0000 0 0 o 0 0 0000 00o 0o 0 0oo 0 00 0 0 0 00 0 00 0 00 0 0 0 0 000 .0 6 the two steps. In most cases it is from 400 to 70°C, most frequently 600 to 700C.

The consistency, the concentration of dry pulp in the bleaching medium in percent by weight, is generally from 10% to 30%, frequently from 10% to The compounds which are commonly used in combination with sodium silicate but which, nevertheless, exhibit a bleaching effectiveness which is greatly inferior to that of the former, for example diethylenetriaminepentaacetic acid (DTPA), or magnesium introduced, for example, in the form of a soluble salt such as the sulphate, are advantageously present in the first step, as well as in the second.

These products can be employed in a quantity 15 similar to that which usually accompanies sodium silicate, for example 0.1% to especially 0.25% to of an aqueous solution containing 40% by weight of DTPA sodium salt, or less than 0.1% and frequently 0.01% or 0.05% of magnesium.

20 The duration of each of the two steps depends on the choice of the remaining parameters adopted. When the latter lie within the preferred range, the duration of the first step is generally from 0,5 to 2 hours, and that of the second step from 2 to 4 hours.

Washing between the two steps means an operation which makes it possible to remove at least a part of the 7 liquid present in the pulp at the ;nd of the first step.

This washing can be performed in known manner, for example by pressir7 the pulp in combination or otherwise with its liquid pht te, generally with water.

A washing eficiency, or degree of removal of liquid, which is equal to or higher than, say, 80% ensures the production of a pulp of high brightness at the end of the second step, but a lower efficiency, for example 50%, is still sufficient for this purpose.

The Examples which follow further illustrate the present invention.

.ooo .In these Examples: 0 0 .0 0 the quantities of material are expressed as already 9 indicated, in by weight relative to the dry pulp, 0006 o0 I 15 DTPA is employed in the form of its sodium salt in solution at a concentration of 40% by weight, and the quantity So indicated is that of this solution, o 0 0 silicate denotes an aqueous solution of sodium silicate at 0 0 a concentration of 35% by weight, and the quantity indicated o 0 20 is that of this solution, QQoco magnesium is employed in the form of a soluble salt, "oo sulphate in this case, but expressed as Mg, 00 0 o o hydrogen peroxide HC 2 is calculated on a 100% basis, washing efficiency, which is 80% or more, unless stated -8otherwise, is the same in the case of a comparison as in the illustration of the invention, relative to which the comparison is made, and brightness is measureed (457 nm) in with the aid of a Carl Zeiss spectrophotometer of the Elrepho type.

Example 1: A stone ground softwood pulp with a brightness of 57.2% is subjected in a first step, at a consistency of for 1.5 hours, at a temperature of 60 0 C, t~o the simul~taneous actioi of the following compounds: NaOH: H 2 0 2 2%, DTPA: and is t' tn washed before being subjected in a second step, for 3i hours, at a consistency and a temperature which are still 15% and 600 C respectively, to the simultaneous action of the following compounds: NaOH: 3%, 20 Mg: 0.05%.

2 0 The brightness of the pulp obtained at the end of 4 10 the second step is 81.1%, whereas it was only 71.5% at the 940 end of the first step.

By way of comparison, Example 1 is repeated using 0 4 0 pulp finally obtained is 81.5%, and therefore very close to 444440 that obtained when the silicate is left out, although at the 4 0 end of the fizst step the brightness was 76% on this occasion, however.

Example 2: The unbleached pulp of Example, 1 is bleached as -9 in Example 1 but using 3.4% of H 2 02 in the second step, instead of When proceeding according to the invention, the brightness of the pulp obtained at the end of the first step is only 72%, whereas that of the pulp obtained at the end of the second step is 82%.

By way of comparison, when proceeding as hereinbefore but in the presence, in the first step, of a quantity of silicate equal to 1% which produces a brightness of 72.3% at the end of this first step, the brightness of the pulp at the end of the second step is only 79.3%.

0. o In the illustration of the invention which is given above, the efficiency of washing between the two steps is The brightness indicated in the case of the pulp at 00% 15 the end of the second step is maintained when this o o illustration is repeated both with 50% and 80% efficiency of o 0 mpwashing between the two steps.

o0 Example 3: 0 .0 A stone ground softwood pulp with a brightness of S 20 53.7% is subjected in a first step, at a consistency oi o 0 for 1 hour, at a temperature of 600C, to the simultaneous a action of NaOH: H202 0.85%, DTPA: before being 2 2 o washed and being subjected in a second step, for 4 hours, at a temperature of 60 0 C and a consistency of 20%, to the acLion of NaOH: H202: 3.4% Mg: 0.03%.

The brightness of the pulp at the end of the -j bw 10 second step reaches 80.9%, Example 4: A softwood chemotheriomechanical pulp with a brightness of 59.6% is treated in a first step with the aid of NaOH: H22 DTPA: for 1.5 hours at a temperature of 60 0 C and at a consistency of 15%, before being washed and being treated in a second step with th aid of NaOH: H02 Mg: 0.05% for 3 hours at a temperature of 60°C and at a consistency of The pulp resulting from the second step 1:as a 0oo0 brightness of 80.5%.

0 00 oo o By way of comparison, the same chemothermo- 0 0 0 o mechanical pulp as above is treated as above, but DTMPA oo000 00o o 0 9 replaces the DTPA in kind and in quantity. The quantity of 0000 1 1; DTMPA thus used should be equivalent to the presence of the Q 0o order of 4 to 5% of silicate and should therefore produce a o o result superior to that obtained in its absence. However o a a 0 oo the brightness found in actual fact at the end of the second 0 0 step is only 79.6%.

0000 0 0 o000 00*0000 0 0 0o 000o00 00 Q u ii- ;1

Claims (7)

1. 3. Process aording bleaching a pulp of mewhch anical usratio of ngodium hydroxide to ydrogehydrogen peroxide in alkaline medium butthe without sodium silicate or a compound capable of equalling12. S5. Process according to silicate in two consecutive i S in h ich th e temperature in the teon steps is from 402 to
2. 6. Process according to any one of Claims ic to 5, in which one or more compounds whose bleaching effectiveness 0.o 3. Process according to Claim 2, in which tne initial pf is achieved with the aid of /iodium hydroxide. 4. Process according to Claim 3, in which the s i ieight ratio f sodium hydroxide to hydrogen peexide in the S i S two steps is from 0.6:1 to 1.2:l. 0. 5. Process according to any one of Claims 1 to 4, in which the temperature in the two steps is from 40C to 0 C. 6. Process according to any one of Claims 1 to in which one or more compounds whose bleaching effectiveness is inferior to that of sodium silicate are present in either or in both steps. I 12
3. 7. Process according to Claim 6, in which sodium diethylenetriaminepentaacetate is present in the first step.
4. 8. Process according to Claim 6, in which a soluble magnesium salt is present.
5. 9. Process according to any one of Claims 1 to 8, in which the efficiency of wasning between the two steps is et least Process according to Claim 9, in which the washing efficiency is at least
6. 11. Process according to Claim 1 substantially as described in any one of the Examp 1s.
7. 12. Pulp whenever bleached by a process as claimed in any one of the preceding claims. It S I 1 f d 13 I I I I I_ disclosed herein or referred to or indicated i specification and/or clairsnf--lsapliaton individualy--r o lectively, and any and all combinations Mf i.x oz~rp o f ricl ftr'. 0000 0 00 00 00 0 0 0 0 0000 0 0~0 6003 0 0 00 DATED this TWENTY FIRST day of OCTOBER 1988 Atochem by DAVIES COLLISON Patent Attorne.- for the applicant(s) 0 00 0 40 0 0 0 940 0 a0 0 0