SK1196A3 - Vegetable material bleaching method - Google Patents

Abstract

Process for bleaching a vegetable material in the form of a micronised powder using H2O2 in the presence of a base, comprising (a) if necessary drying the micronised powder to a moisture content of less than 30 wt.%, pref. 4-15 wt.% w.r.t. total wt., and heating to a temp. ta of 20-100 degrees C, (b) impregnating the powder simultaneously or successively (in either order) with separate solns. A and B in which soln. A is an aq. H2O2 soln. contg. an alkaline agent, in relative amts. such that the two solns. are completely absorbed by the powder so that it remains in a free flowing solid powder state with a water content of 10-50 wt.% w.r.t. total wt., and the impregnated powder has a temp. tb of 40-100 degrees C, and (c) maintaining the impregnated powder at a temp. tc of 40-100 degrees C until at least 75% of the initial amt. of H2O2 has been consumed.

Description

Method of bleaching plant material

Technical field

The invention relates to a process for bleaching plant material in the form of a micronized powder with hydrogen peroxide in the presence of a base.

BACKGROUND OF THE INVENTION

EP 0433413B1 discloses a porous and micronised plant batch having a residual moisture content of less than 20% and preferably less than 15%:

- a particle size dg men of less than 200 microns (which indicates that at least 95% by weight of the particles of said plant batch pass through a sieve with a square mesh of 200 x 200 microns),

- a physical surface area of less than 2 m / g,

a water wettable surface area of less than 2 m / g, and

a density of less than 500 kg / m and preferably less than a

or equal to 300 kg / m, and in addition provided that the batch is obtained by milling / micronizing at a temperature of less than 150 ° C and preferably at a temperature of less than or equal to 100 ° C.

All plant sources, in particular softwood species such as fir, pine or spruce, hardwood such as birch, beech, hornbeam or chestnut, and others are suitable for obtaining this micronized plant batch. For essentially economic reasons, the plant source is preferably vegetable waste and in particular wood waste. The wood waste may come from, for example, the log industry, the coarse and semi-coarse woodworking industry, or the industry where sawing, planing or veneering. The wood waste may also come from an industry in which wood products are used or processed, in particular light or heavy wood packaging. The wood waste may also come from chemical pulp factories.

The source of the plant material may also come from the harvest of cereals, in particular corn cobs.

The micronised plant batch may be subjected to a conventional bleaching process in the paper industry to obtain a whiteness between 60 and 90 degrees of whiteness (measured according to French Standard Q03039), said whiteness being expressed as a percentage relative to a 100% control.

The aforementioned patent describes a bleached plant batch, made of wood waste from the coastal pine from Landes, having the following characteristics:

d d <150 micrometers

- physical surface area = 0,65 m / g

- water wettable surface area = 0.52 m / g

- whiteness = 70%.

This whiteness is achieved by bleaching with 4% hydrogen peroxide, 2% sodium hydroxide, 3% silicate, and 0.25% DTPA (sodium diethylenetriamine pentaacetate). Thus, this bleaching is performed using a single aqueous solution containing a dilute mixture of hydrogen peroxide, sodium hydroxide, silicate, and DTPA and comprising washing the plant batch with water.

These unbleached and bleached plant batches can be used in the manufacture of pulp, paper, cardboard and non-woven materials on the one hand and in the manufacture of plastics, composites, paints, coatings and construction materials on the other.

In addition, patent EP 0419385 B1 discloses a process for the production of bleached and dried plant fibers in which bleaching is carried out using hydrogen peroxide and drying by evaporating water from the pulp using a dry gas atmosphere circulating in contact with the pulp, whitening is carried out simultaneously with drying . This method employs a single bleach solution or liquor containing hydrogen peroxide and optional products such as basic reagents, complexing agents and stabilizing products for hydrogen peroxide. The liquor is mixed with the plant fiber so that the initial consistency of the mixture thus obtained is at least 10% and preferably between about 20% and 35%, the fiber consistency being the dry matter content expressed as a percentage by weight relative to its total weight.

The plant fiber used in this method is characterized by a high content of food fiber. They are selected from beet pulp, citrus fruit, fruit, oilseeds, cereals or vegetables, after the product which normally increases the value of each such as sugar, fruit juice, pectin, oil, starch, flour or where appropriate, extracted, extracted or separated the grain. The production of pulp from sugar beet is described, for example, in Sucrerie Francaise (French Sugar Refining), October 1985, 439-454. The dry mass of these pulps does not have several micrometers in the micronized state.

Conventional bleaching in the paper industry, as recommended by EP 0433413 B1 (above) for high yield paper pulp, is difficult to apply to micronized plant batches.

In fact, this type of bleaching, which is used at an average consistency of 10 to 20% in general and up to 30% in extreme cases, is followed by acid neutralization and washing with water. These procedures are intended to remove excess alkaline reagent prior to drying and thereby prevent irreversible discoloration that interferes with bleaching.

This type of conventional bleaching in the paper industry has been described by V.G. Strunk at Work Pulp and Paper Manufacture, Peroxide Bleaching, 3rd Edition, Volume 2, Chapter XX, pp. 238-251.

In such conventional bleaching, it is necessary to substantially dilute the wood pulp with water during bleaching in a basic medium and then during neutralization and washing. These neutralization and wash operations, which utilize filtration and / or compression, become difficult and expensive when applied to the micronised material due to the fineness of the micronised particles having a particle size dg of less than 300 microns.

Moreover, industrial technical means for filtration and / or compression do not allow a consistency of 20 to 30% to be exceeded. The amount of water that must be removed by drying is very large and results in high energy consumption.

In addition, the aqueous effluents have a large volume and a high organic matter content. These effluents cause high costs for the purification process before they are released.

It is an object of the present invention to find a method of bleaching micronized plant material that does not have the disadvantages of conventional bleaching methods mentioned above.

SUMMARY OF THE INVENTION

This object is achieved by a method of bleaching plant material in the form of a micronized powder with hydrogen peroxide in the presence of a base, characterized in that:

(a) take the micronised powder, optionally subjected to drying, so that the water content is less than 30% by weight with respect to its total weight, and then bring it to a temperature of t _and 20 to 100 ° C;

(b) the powder is impregnated, simultaneously or sequentially, with two separate solutions A and B, whereby it is possible for A to precede B or vice versa, each in an amount relative to the amount of powder to be impregnated, such that it completely absorbs the solutions so that it remains in the form of a solid powder with a water content of 10% to 50% by weight with respect to its total weight and, on the other hand, such that the powder has a temperature t = 40-100 ° C. hydrogen peroxide containing 50 to 500 g H2O2 per liter and B consists of an aqueous basic solution containing a basic reagent,

c) the thus impregnated powder is maintained at a temperature T _c of 40 to 100 DEG C. for a time sufficient to be consumed at least 75% of the supplied amount of hydrogen peroxide.

The plant material in the micronized powder state can come from the same sources as the materials of patent EP 0433413 B1 mentioned above. In addition, the beet pulp can also be a source of micronized powder after removal of the sugar.

By micronised powder is meant a powder having a particle size of less than 300 microns (micrometers) and preferably less than 200 microns. Advantageously, the plant material may be subjected to a complexing treatment with metals known to catalyze the decomposition of hydrogen peroxide prior to micronizing. This treatment with the complexing agent followed by water washing, as is known per se, is carried out using complexing agents selected from DTPA (sodium diethylenetriaminepentaacetate), EDTA (sodium ethylenediaminetetraacetate), poly (α-hydroxyacrylic acid) salts and phosphinic acid salts.

It is also possible to carry out a pre-complexation treatment of the micronised powder, but this leads to washing and pressing, which are difficult to classify on an industrial level.

In step a), the moisture content of the micronized powder is preferably that of the plant material when exiting the micronizing operation.

This is done with a plant material that is as dry as possible but with a certain level of moisture to reduce the risk of ignition during micronizing operations. Therefore, the initial moisture content of the micronised powder prior to bringing it to the temperature t of the inlet a) is preferably between 4 and 15% by weight of water relative to. its total weight.

The temperature t in step a) is preferably from 20 to 40 ° C, which is the temperature of the micronized powder obtained during its preparation, at the end of the milling and sieving operations. A temperature t of the order of 90 to 100 ° C is preferred if the micronized vegetable powder contains catalase. This enzyme is secreted by microorganisms that can grow on finely divided plant material. This enzyme, which decomposes hydrogen peroxide, can be inactivated by heating.

In step b), the powder is impregnated on the one hand with an aqueous solution A containing hydrogen peroxide and on the other hand with an aqueous solution B containing a basic agent and optionally a stabilizing agent for hydrogen peroxide and / or a complexing agent for metals with metal salts. The A-impregnation preferably precedes the B-impregnation, although the B-impregnation may precede the A-impregnation in one variant.

Very preferably, A and B are applied to the powder simultaneously.

The powder is preferably impregnated by spraying with solution A and solution B. In this way, the fine droplets A and B meet only in or near the powder grains. This makes it possible to use solutions A and B having a high concentration on the one hand of hydrogen peroxide and on the other hand of the basic agent and / or the stabilizing agent and / or the complexing agent. In fact, a single solution C, in which A and B had previously been mixed with the same active ingredient content, would be unstable, hydrogen peroxide would decompose and the stabilizing agent, especially sodium silicate, would precipitate by gel formation.

The volume ratio of the B / A solutions is preferably in the range of 0.5 to 2 and more preferably is approximately equal to 1.

The amount of H2O2 (calculated as pure and therefore expressed as 100%) used for impregnation is from 1 to 10% by weight based on the weight of the powder obtained at the end of step a).

This amount depends on the bleachability of the plant material. In general, an amount of from 2 to 6% for micronized wood is sufficient to achieve a high degree of whitening. The concentration of H2O2 ⁱⁿ solution A is selected so that solutions A and B are completely absorbed by the micronised powder, which therefore retains the appearance and characteristics of the powder. Preferably, solution A containing 100 to 350 g H2O2, expressed in pure form, is used, i.e. solution A containing 10 to 35% by weight H2O2 by volume.

Solutions A with a concentration greater than 35% are more difficult to use due to the exothermicity of the reaction they induce and there is a risk of ignition in a normal atmosphere. This danger can be partially controlled by carrying out steps b) and c) and drying in an inert atmosphere.

The temperature t _b of the powder during the impregnation step b) is preferably between 60 and 90 ° C.

This temperature is obtained by heating the reactor containing the powder or otherwise due to the exothermic reaction when the powder, solution A and solution B come into contact. The duration of step b) is preferably from 5 minutes to 2 hours and preferably from 10 to 30 minutes. The basic reagent of solution B is preferably selected from NaOH, KOH, Na 2 CO 4, K 2 CO 3, sodium silicate, DTPA or any other reactant which allows a pH greater than 7 to be obtained for solution B. The amount to be impregnated varies depending on the amount of ^2θ2 used ^{and the <} ^ nature of the plant material. This amount is selected so that the final pH of the powder at the end of step c) is from 6.5 to 8.5.

When NaOH is used, an amount of from 0.5 to 5% by weight of NaOH is suitable, based on the weight of the plant powder to be impregnated.

The amount of water used to form solution B is selected such that the active ingredient (s) of the solution are (are) completely dissolved. The total amount of both solutions A and B is preferably calculated as a function of the moisture level of the powder at the end of step a) to obtain a water content of 30 to 40% by weight based on the total weight of the powder after impregnation in step b).

The stabilizing agent for hydrogen peroxide is selected from sodium silicate, magnesium salts, disodium phosphate, sodium polyphosphates, sodium pyrophosphate and phosphonates. Preferably, sodium silicate and more preferably an aqueous solution of sodium and silicate having a density of 1.33 g / cm @ 2 is used. The amount of stabilizing agent (s) is preferably in the range of from 1 to 10% by weight based on the dry weight of the powder to be impregnated in step b). Preferred amounts range from 4 to 8%. The complexing agent is selected from DTPA (sodium diethylenetriaminepentaacetate), EDTA (sodium ethylenediaminetetraacetate), poly (α-hydroxyacrylic acid) salts, and phosphonic acid salts.

The preferred reagent is for reasons of cost and efficiency of DTPA as a 40% solution.

The amount used is preferably from 0.1 to 1% by weight of 40% DTPA based on the weight of the plant material prior to impregnation.

Solution B further preferably comprises a fluorescent whitening agent to improve the shade of the resulting white. This fluorescent bleach can be selected from those commonly used in papermaking.

The amount of fluorescent bleach added is preferably between 0.01 and 0.5% by weight relative to the dry weight of the powder and delivered to the reaction in step b).

In step c), known as the latency degree, bleaching is completed. During this step, the impregnated micronized powder is stirred for d _Q at t. Conveniently, the temperature T _c from 60 to 90 ° C.

For d "it depends on the nature of the plant material, and the temperature T _c. Generally, this time is from 5 to 120 minutes and preferably between 15 and 60 minutes.

Certain uses of bleached micronised powder require a product that is as dry as possible. In this case, the drying is then carried out after step c). This drying is carried out according to conventional procedures used in the wood and plant materials industry, taking into account the specific risks of ignition and / or explosion in an oxidizing atmosphere. To avoid this risk, drying can be carried out in an inert atmosphere or in an atmosphere that is very low in oxygen.

Sodium silicate, if present in solution B, acts as a flame retardant by its presence in the final powder, since no washing is required from step a).

The powder obtained at the end of step c) is preferably dried to a water content of from 5 to 20% by weight with respect to its total weight.

In addition to the foregoing description, the following application examples will enable the invention to be more fully understood.

DETAILED DESCRIPTION OF THE INVENTION

The process may be carried out in a batch or continuous manner.

1) Batch whitening

The micronized vegetable powder is placed in a powder mixer preheated to approximately 40 ° C and then sprayed with a 35% H 2 O 2 solution on the one hand and a basic solution on the other.

the% H2O2 solution and the basic solution have a volume ratio in the region of about 1.

The temperature of the powder rises to 80 C. After completion of the addition of the solutions, stirring is maintained for an additional 30 to 60 minutes at a powder temperature of 70 ° C, then the powder is drained, optionally dried and packaged in crates.

The above mixer may be:

- cone mixer, blended screw,

- screw-type mixer,

- concrete mixer,

a mixer with a bed, a fluidized gas stream or mechanical mixing.

The materials used for the mixer must be compatible and resistant to the use of Η ₂ 0 ₂ and, for example, selected from stainless steel, enamelled steel, steel coated with durable paint or plastic.

2) Continuous bleaching

This embodiment is advantageous for high performance industrial production and allows better control of the exothermicity of the bleaching reaction.

The micronized plant material is fed continuously through a dosing screw into a continuous mixer into which a solution of H 2 O 2 is continuously injected on one side and a basic solution on the other.

The residence time in the mixer is of the order of 1 to 5 seconds and can vary up to 1 to 3 minutes.

At the outlet of the continuous mixer, the micronised powder passes continuously through a holding vessel in which the residence time is sufficient for consumption and thus at least 75% of the H2O2 introduced to disappear.

The optimum bleach temperature is from 70 to 80 ° C. This temperature is preferably achieved due to the exothermic of the oxidation reaction with H2O2.

The maximum temperature reached t 1 depends on the amount of H 2 O 2 delivered, the moisture level of the plant material after mixing the reactants, the initial temperature of the plant material and the temperature of each of the solutions A and B injected into the mixer.

If the exothermic of the bleaching reaction is insufficient to allow an optimum bleaching temperature, whether due to the use of a small amount of H2O2 or because the whitening of the plant material having a water content of less than 20%, the plant material then heat before or during the addition of the reactants. In this case, the heating must be homogeneous and must not cause local overheating. Preferred means for heating are circulating hot water in the jacket around the mixer and / or around the vessel to hold or blow steam into the mixer.

Heating of basic solution B is also a suitable means for introducing heat into the reaction mixture. The basic solution B is preferably brought to a temperature of 40 to 90 ° C.

Preheating of the micronized plant material prior to introduction into the continuous mixer may be accomplished in a metering screw or in a feed container for the metering screw, by vapor blowing, or by circulating hot water in the sheath around the metering screw and / or around the feed supply container for the metering screw.

The devices that can be used to ensure continuous impregnation of the micronized vegetable powder with solutions A and B are selected from the following:

- screw-type powder mixer,

- rotary dispersion mixer of different types: containing a horizontal blade shaft or containing a rotating plate of paper pulp cleaner type,

- turbo-mixer,

- mixer with bed, fluidised gas stream or mechanical mixing,

- a rotating cylinder with an internal screw hoop of a cement drier type.

Apparatus that can be used to provide a continuous impregnation of the micronized vegetable powder with residence time solutions A and B are selected from the following:

- High performance Archimed screw

- conical mixer,

- a rotating cylinder of the type of cement drier.

It should be noted that the impregnation (step b) and residence time (step c) can be realized in the same plant.

The following micronised wood masses were used in the following examples:

I - Pine wood / spruce

Whiteness = 49% ISO pH = 5.1

Moisture content = 5.6% by weight based on the total weight

II - Micronized timber

Whiteness = 50% ISO pH = 4.4

Moisture content = 10.7% by weight

III - Micronized spruce wood

Whiteness = 51% ISO pH = 6.2

Moisture content = 6.1% by weight

IV - Beech micro-ground wood

Whiteness = 41% ISO pH = 5.7

Moisture content = 7% by weight

These micronised wood masses having a particle size d50 of less than 150 μιτι originate from Société Parisienne des

Sciures (Paris sawdust company).

The degree of whiteness was measured with a Hunterlab spectrophotometer according to the ISO whiteness standard used in the paper industry at 457 nm to measure the whiteness of the paper.

Laboratory procedure

(a) Used equipment:

The jacketed 1 liter glass reactor is heated with hot water circulating in the jacket, and is equipped with a central speed propeller and two slow wiping blades to stir the contents.

Solution A (35% H 2 O 2) is sprayed by pumping the solution using a H.P.L.C. and then spraying it using a nozzle above the reactor.

The basic solution spraying means is a copy of the above-mentioned means.

(b) General application example:

g of micronized plant material selected from materials I, II, III and IV is charged to the reactor. Solution A and solution B are then sprayed simultaneously for 15 minutes. Stirring is maintained for a residence time of 30 to 60 minutes to complete bleaching, and the material is then discharged from the reactor and dried in a ventilated furnace at 90 ° C to a residual moisture level of approximately 5% by weight based on dry weight of powder.

The whiteness, pH and H2O2 consumption are then measured

The various examples 1 to 14 and their results are shown in Tables I and II below.

The amounts of hydrogen peroxide and NaOH are expressed as a percentage by mass of the pure product relative to the dry weight of the micronized plant material.

The amounts of DTPA are expressed as a percentage by weight of the commercial solution (about 40%) based on the dry weight of the micronized plant material.

The amounts of sodium silicate are expressed as a percentage by weight of an aqueous sodium silicate solution having a density of 1.33 g / cm2, based on the dry weight of the plant material.

In certain tests, DTPA pretreatment was performed with 0.5% DTPA, 10% consistency, at 90 ° C and for 15 minutes. At the end of this treatment, the micronized plant material was filtered, washed with water and then dried to the initial moisture level.

Example 15

The method is the same as in Example 3, with another micronized spruce wood with an initial whiteness of 44.7% ISO, a consistency of 94% and an initial pH of 5.

At the same time, 50 g of this wood preheated to 70 ° C were sprayed with the following:

- 11,4 g of 35% H ₂ 0 ₂ (8%),

16.9 g of solution B containing 1.5 g of NaOH, 8% sodium silicate, 0.5% DTPA and 0.3% liquid BLANCOPHOR P fluorescent bleach, manufactured by Bayer, expressed by the weight of each commercial solution.

After a residence time of 45 minutes at 70 ° C, the pH was 6.9 and the consumption of H ₂ O ₂ 84.3%.

The whiteness was 62.5% ISO.

Color measurement using Hunterlab spectrophotometer according to CIE standard D 65/10 provided:

L * = 87.49 and * = -1.01 b * = 8.77.

table

Moisture content after impregnation % 28 28 38 31 CO 't 42 x at about about about about about about <o • H c P about r- r- ' Γ ' Γ ' r- D 'P /-with < IN) IN) <n > (/) x cx about about * about H about about about cx d > d υ Q D cx P 3 'd P Fri D ΙΛ • H 00 «2 00 y? F "i D • H > 0 '3 en WHAT x IN) IN) v> IN) Ό about - * * N - * D N H r- < M WHAT FROM WHAT ABOUT & N m m 00 and 00 00 X ϋ l · Η 0 P Ό d (J about about about about ABOUT about ϋ -H 0 · t ' t'- r- r- R r- E 0 Ή x c P P CX x 1 ctf Ό d P -p U> 7) P (D h 'd CX -H E CX /-"with <λ d 0 0 Θ 0 Θ υ o d D u d < D D • H D • H • H p d x • H Ό E U > d 0 cx For DTP 'd 'd D 'd D D P 3 1 d n. P 0 > (Λ P d d after P Fri • H X cx> w E ' Ή 'Ctf D h- < > - ( P P d d hH CX cx The nature of the lower wood T3 D R-1 Fri H N m wed IN) about Ή P ex

Table I - continued

about C about about about about r- Ov IN) IN) m m m m (N m about about about about about about about about VO VO r- Ό t '' and m m IN) m * * * · about about *> about about about about about about 00 00 00 00 00 00 T 00 IN) oo 00 00 vr> IN) » * about ·> * * · * · * (N M r-i H m H H M 00 00 00 00 WHAT 00 Ό 00 ABOUT about about about about about ABOUT ABOUT VO OV VO VO MO > 0 0 4) 4) 0 Θ 0 U • rl Ή • H • H C • H C • H No. C C C '0 $ c: 'd C ►- < > J-H h- ( ) - < Γ 00 σν about rh (N m M ^- r ^- < Ή H N- ( r "<

Table II

Latent period step c) The results Example C ^x C The residence time in step (c) in minutes or hours Consumed ^H 2 ° 2% Final pH % ISO brightness 1 70 30 min. 100 10.3 61 2 70 30 min. 95.5 8 68 3 70 45 min. 92.3 7 68 4 70 45 min. 88 7.6 72 5 70 20 min. 77.4 8.1 74 6 70 45 min. 84.7 7.8 75 7 60 14 h 100 9.5 63 8 60 5 h 98.6 9.8 67 9 60 16 h 92.2 7.1 72 10 60 20 h 89 6.4 73 11 60 16 h 99.5 9.8 67 12 60 6 h 93 8 73 13 70 15 min. 84.1 6 68 14 70 45 min. 98.5 7 64.5

Example 16 - Test Experiment No. 1 kg of micronized wood III was placed in a stainless steel cement mixer preheated to 40 ° C with a working volume of 40 L (volume that can be mixed in an inclined position).

Then simultaneously for 15 min. using two metering, piston pumps sprayed the following:

- on the one hand, 1,6 kg of 35% hydrogen peroxide,

- on the other hand, 2,37 kg of a basic solution consisting of 126 g NaOH, 560 g aqueous sodium silicate solution, and

and a density of 1.33 g / cm @ 3, 35 g of 40% DTPA, and 1.65 kg of water.

During these spraying operations, the temperature reached t ≥ 78 ° C and the final water content of the impregnated micronised wood was 40%.

The impregnated powder was stirred for an additional 45 minutes at 70 ° C.

10.5 kg of bleached micronised wood having a pH of 9.1 and containing 0.7% residual (expressed as a percentage by weight of pure H2O2 relative to the weight of the micronised wood in the dry state) were obtained. The whiteness was 62.7% ISO (dried sample).

After holding for 5 hours at 70 ° C, the pH was 7 and the amount of residual H 2 O 2 was 0.18%, the whiteness was 63% ISO (dried sample).

Example 17 - Test no. 2

This example is identical to Example 16 except that the amount of NaOH is lower: 105 g instead of 126 g (i.e. 1.6% instead of 1.8% relative to dry wood).

The maximum temperature reached during two simultaneous spraying operations was 70 ° C. After stirring for 45 minutes at 70 ° C, the pH was 8.5, the amount of residual H2O2 was 1.25% and the whiteness was 62.3% ISO.

After 5 hours at 70 ° C, the pH was 6.1, the amount of residual H 2 O 2 was 0.5%, and the whiteness was 63% ISO.

Examples 18, 19 and 20. 3, 4 and 5 with other micronised materials.

The following micronised materials were used.

V - Corn bran in the form of flour

Whiteness = 32% ISO pH = 5

Moisture content = 8%

VI - Sunflower husks in the form of flour

Whiteness = 26% ISO pH = 6.5

Moisture content = 7.6%

VII - Sugar beet pulp in the form of flour

Whiteness = 30.5% ISO pH = 6

Moisture content = 4%

These micronised powders have a particle size of 180 μπι.

The micronized plant material V or VI or VII is placed in a 100 liter S-screw mixer-stainless steel dryer (GUEDU trademark) with heating of the jacket with hot water circulation and the batch preheated.

A 35% H2O2 solution on one side and a basic solution on the other side are simultaneously sprayed into the mixer. The reaction is allowed to proceed until at least 75% of the added hydrogen peroxide is consumed and drying is then carried out in vacuo.

Examples 18 to 20 and their results are shown in Tables III and IV.

- 19 Table III

Final moisture content % 36 30 30 <u ni n. 3 Ή > E m bi X tr in r- < m m 0 N 3 P 0 about Ή and Temp ^x b ° C σ \ σ \ σ \ X /-with m FROM"\ Z "X s £ about x * X x 3 about H about bl about bl FROM 3 II 3 II 3 II bO r- FROM 3 FROM 3 FROM 3 X 44 b X about x X X 0 * · N - N - N P> bi T3 WHAT Ό WHAT TJ υ z-a N about '3 about '3 about '3 • H X ABOUT rh IN) Ή ΙΛ Ή cfl C Pi s-out > X-Z > p-Z > honor vr ¹ IN (D > what Q. n z-a ό Z-X '3 3 about bl * bl bl about ä P b) ABOUT rh ABOUT b ABOUT H honor m X Ό b) II bi II b) II FROM BFL x X 3 about X honor rP X 3 44 44 Ή X * 44 r. X 0 * N WHAT N x ÍČ N P> X TJ TJ TJ N ABOUT '3 <n 'honors x '3 ABOUT m cfl m WHAT (Ό 10 Pi x-z > \-from *> x-z > ABOUT I · Η 0 P 13 3 U bl ABOUT about 0) -H o P P CU X b- 00 b 3 0 -H • p H X -H CO 3 3 Ό 0 bl x O -H c 44 bO ABOUT * b C P 3 3 X b) Ό r- P U X <U rh 00 0 P -H p E 3 P X E ΛΧ > rP 1 0 P honor 44 • H 3 E H U 0> 3 > > - ( and 3 X -H > »-1 3 X Ό P > P 3 P (D 00 > Θ P O H (Ij CU E E TJ 3 rh 44 00 σ \ about Ή t < rP bl P CU

Table IV

Latent period step c) The results Example temperature ° C Dwell time in min. Final pH Residual Η ₂ θ2 in% ISO whiteness in % 18 90 15 6 0.18 46 19 90 65 5.6 0.21 54 20 70 60 5.4 1.5 50

Ρι /

Claims (14)

A method for bleaching plant material in the form of a micronized powder with hydrogen peroxide in the presence of a base, characterized in that: (a) take the micronised powder, optionally subjected to drying, so that the water content is less than 30% by weight with respect to its total weight, and then bring it to a temperature of t _and 20 to 100 ° C; (b) the powder is impregnated, simultaneously or sequentially, with two separate solutions A and B, whereby it is possible for A to precede B or vice versa, each in an amount relative to the amount of powder to be impregnated, such that it completely absorbs the solutions so that it remains in the form of a solid powder with a water content of 10% to 50% by weight with respect to its total weight and, on the other hand, such that the powder has a temperature t = 40-100 ° C. hydrogen peroxide containing 50 to 500 g H2O2 per liter and B consists of an aqueous basic solution containing a basic reagent, c) the thus impregnated powder is maintained at a temperature T _c of 40 to 100 DEG C. for a time sufficient to be consumed at least 75% of the supplied amount of hydrogen peroxide. Method according to claim 1, characterized in that the micronised powder has a water content of between 4 and 15% by weight with respect to its total weight before it is brought to the temperature t _and step a). Method according to claim 1 or 2, characterized in that the volume ratio of the B / A solutions is in the range of 0.5 to 2. The method according to claims 1 to 3, characterized in that solution B comprises a stabilizing agent for hydrogen peroxide. A process according to claim 4, wherein the stabilizing agent is sodium silicate. The process according to claim 5, characterized in that the sodium silicate is present in an amount of 1 to 10% by weight relative to the dry weight of the powder to be impregnated in step b). The process according to any one of claims 1 to 6, wherein solution B additionally comprises a complexing or masking agent for metals of metal salts known to catalyze the decomposition of hydrogen peroxide. Process according to any one of claims 1 to 7, characterized in that the powder obtained at the end of step c) is dried to a water content of 5 to 20% by weight with respect to its total weight. Process according to any one of claims 1 to 8, characterized in that the temperature t in step a) is from 20 to 40 ° C. The method according to any one of claims 1 to 9, characterized in that the temperature t v in step b) is from 60 to 90 ° C. Process according to any one of claims 1 to 10, characterized in that the temperature t in step c) is from 60 to 90 ° C. Method according to any one of claims 1 to 11, characterized in that in step b) the powder is impregnated by spraying with solution A and spraying with solution B. Method according to any one of claims 1 to 12, characterized in that in step b), the amount of hydrogen peroxide (calculated in pure form) used for the melting is from 1 to 10% by weight, based on the weight of the powder obtained. at the end of step a). Method according to any one of claims 1 to 13, characterized in that solution B additionally comprises a fluorescent whitening agent.