JPH08232184A - How to bleach ultra-fine ground plant powder - Google Patents

Abstract

(57) Abstract: A method of bleaching a plant material in the form of ultrafine powder with hydrogen peroxide in the presence of a base is provided. SOLUTION: Two separate solutions of A consisting of a hydrogen peroxide solution and B consisting of a basic aqueous solution containing an alkaline substance are completely absorbed by the powder so that the powder remains in a solid powder state. The micronized plant powder is impregnated with these solutions.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method of bleaching plant material in the form of ultrafine powder with hydrogen peroxide in the presence of a base.

[0002]

2. Description of the Related Art European Patent No. 0433413
Residual water content less than 20%, preferably less than 15%
, D₉₅The particle size is less than 200 μm (that is, the amount of plant material particles is small.
95% by weight at least, opening size is 200 × 200μ
m is passed through a square mesh sieve) and the physics
Specific surface area is 2m²/ G, less than water surface specific surface area
Is 2m²/ G and the density is 500 kg / m³Less than,
Preferably 300 kg / m ³Below is a porous ultra-fine grinding
Plant material at a temperature below 150 ° C., preferably
Obtained by crushing / ultrafine crushing at a temperature below 100 ° C
Disclosed materials.

In order to obtain such ultrafinely ground plant material,
All plant sources are suitable, in particular species such as conifers such as fir, pine or spruce, and hardwoods such as birch, beech, hornbeam or chestnut. For economic reasons only, it is preferable to use plant waste, particularly wood waste, as the plant source.
Wood chips can be obtained, for example, in the wood cutting, rough or intermediate wood processing industry, or in the sawing, planing or veneering industry. Wood chips can also occur in the industry of using or processing wood products, especially in light or heavy wood packaging. Wood chips can also occur in chemical pulp mills.

The plant source can also be obtained from the plant debris produced in the harvest of grains, especially corn cobs.

This ultrafinely ground plant material can be subjected to a bleaching treatment customary in the paper industry, to obtain a whiteness of 60 to 90% (measured according to French standard Q03039). This whiteness is expressed as a percentage of the control with a value of 100%.

The above-mentioned patents have ad ⁹⁵ of less than 150 μm, physical specific surface area of 0.65 m ² / g, water surface specific surface area of 0.52 m ² / g and whiteness of 70%.
Describes bleached plant material produced from pine wood scraps of the French coast of Rand.

This whiteness is 4% hydrogen peroxide, 2% sodium hydroxide, 3% silicate and 0.25% D.
TPA (sodium diethylenetriaminepentaacetate)
It is obtained by bleaching with. Therefore, this bleaching is
Hydrogen peroxide, sodium hydroxide, silicate, DT
Performed using a single aqueous solution containing a dilute mixture with PA, including rinsing the plant material with water.

These unbleached or bleached plant materials are used for the production of pulp, paper, cardboard, nonwovens, as well as plastics,
It can be used in the manufacture of composites, paints, paints and building materials.

Furthermore, European patent 0419385
U.S. Pat. No. 4,968,049 discloses a process for making bleach-dried plant pulp which comprises bleaching with hydrogen peroxide and using a dry gas atmosphere which circulates in contact with the pulp to evaporate and dry the water from the pulp. This bleaching is carried out simultaneously with the drying. This method uses a single bleaching solution or liquid containing hydrogen peroxide and optionally substances such as alkaline substances, complexing agents and stabilizers for hydrogen peroxide. This liquid is mixed with vegetable pulp and the initial consistency of the mixture thus obtained is at least 10%, preferably about 20% to 35%.
%. Pulp consistency is the dry matter content expressed as weight percent of the total weight.

The plant pulp used in the process is characterized by a high dietary fiber content. Plant pulp is beet,
Substances selected from the pulp of citrus fruits, fruits, oil plants, cereals or vegetables, which usually enhance the value of each of these foods (eg sugar, fruit juices, pectins, oils, starches, flours or kernels if appropriate) ) Is used after extraction or separation. The production of sugar beet pulp is described, for example, in the document "Sucrerie Francais [Frenc.
h Sugar Refinery] ”(1985, 1
0, 439-454). The dry matter from these pulps is not in the finely ground state of a few hundred μm.

[0011]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention Regarding papermaking pulp having a large production volume, European Patent No. 0433413 (mentioned above)
It is difficult to apply conventional bleaching in the paper industry, such as that recommended in US Pat.

In fact, in this type of bleaching, the average consistency of which is generally from 10 to 20%, and in extreme cases below 30%, it is subsequently neutralized with acid and washed with water. These treatments are intended to remove excess alkaline material prior to drying to avoid irreversible color shifts that negate the bleaching effect.

Conventional bleaching of this type in the paper industry is described in W. G. Strunk's article "Pulp and
Paper Manufacture, Peroxid
eBleaching ”(3rd edition, Volume 2, Chapter XX, 2
38-251).

Such conventional bleaching requires a considerable dilution of the wood pulp with water during bleaching in basic medium and subsequent neutralization and washing. These neutralization and washing operations of filtering and / or pressing the micronized material are difficult and costly because the ultrafine particles are fine with ad ₉₅ particle size of less than 300 μm.

Moreover, technical means for filtration and / or pressing cannot exceed a consistency of 20-30%. Since the amount of water to be removed by drying is very large, energy consumption becomes large.

Furthermore, the aqueous effluent is large in volume and contains a large amount of organic substances. These effluents add to the cost of the refining process before discharge.

It is an object of the present invention to find a method of bleaching ultrafine milled plant material which does not have the disadvantages of conventional bleaching mentioned above.

[0018]

This object is achieved by a method of bleaching plant material in the form of ultrafine powder with hydrogen peroxide in the presence of a base, which method comprises: a) a water content of 30% by weight relative to the total weight. Optionally dried to less than 100% to an ultrafine powder, then 20-100
B) heated to _a temperature ta of b, b) two separate solutions, A consisting of an aqueous hydrogen peroxide solution containing 50-500 g / L H ₂ O ₂ and B consisting of a basic aqueous solution containing an alkaline substance, are powdered The powder remains in the state of a solid powder having a water content of 10 to 50% by weight and the temperature t _{b of the} powder is 4
Determining the amount of each solution to be 0-100 ° C. relative to the amount of powder to be impregnated, the powder is treated with two solutions simultaneously or A then B or B then A. Impregnating, and c) maintaining the powder thus impregnated at a temperature t _c of 40-100 ° C. for a time sufficient to consume at least 75% of the amount of hydrogen peroxide charged. Characterize.

[0019]

DETAILED DESCRIPTION OF THE INVENTION The ultrafine powdered plant material can be obtained from the same sources as described in the above mentioned EP 0433413. Furthermore, sugar beet pulp after sugar extraction can also be a source of ultrafine powder.

By ultrafine powder is meant a powder having a d ₉₅ particle size of less than 300 μm (micrometer), preferably less than 200 μm. It may be advantageous to subject the plant material prior to micronization to a complexing treatment of metals known to catalyze the decomposition of hydrogen peroxide. This treatment with complexing agents and subsequent washing with water is known and is selected from among DTPA (sodium diethylenetriaminepentaacetate), EDTA (sodium ethylenediaminetetraacetate), poly (α-hydroxyacrylic acid) salts and phosphates. Is carried out with a complexing agent.

It is also possible to carry out the complexing pretreatment of the ultrafine powder, but this makes it difficult to carry out the washing and pressing on an industrial scale.

In step a), the water content of the ultrafine powder is preferably the water content of the plant material obtained by the ultrafine grinding operation.

The ultrafine grinding operation is carried out with the plant material as dry as possible, but with some water content to reduce the risk of fire during the ultrafine grinding operation. Therefore, the initial water content relative to the total weight before the ultrafine powder is heated to a temperature t _a of the step a) is preferably 4 to 15 wt%.

The temperature t _a is 20 to 40 ° C., i.e. the temperature of the ultrafine obtained during the grinding and production after the end of the sieving working step a) is preferable. When micronized plant material contains catalase is advantageous temperature t _a of approximately 90 to 100 ° C.. This enzyme is secreted by microorganisms that can grow on finely ground plant material. This enzyme that decomposes hydrogen peroxide can be inactivated by heating.

In step b), the powder is impregnated with an aqueous solution A containing hydrogen peroxide and an aqueous solution B containing an alkaline substance and optionally a stabilizer for hydrogen peroxide and / or a complexing agent for metals of metal salts. . In a modification, after impregnating B, A
Can be impregnated, but it is preferable to impregnate A and then B.

It is highly preferred to apply A and B to the powder simultaneously.

It is preferable to spray solution A and solution B to impregnate the powder. Therefore, only A and B microdroplets contact or approach the powder particles. Depending on the concentration of hydrogen peroxide, alkaline substance and /
Alternatively, solutions A, B having a high concentration of stabilizer and / or complexing agent can be used. In fact, the single solution C in which A and B are premixed with the same amount of active ingredient is unstable, hydrogen peroxide decomposes and the stabilizer, especially sodium silicate, precipitates by gel formation.

It is preferred that the volume ratio of the solution B / A is 0.5 to 2, and it is even better if it is equal to about 1.

The amount of H ₂ O ₂ used for impregnation (calculated in pure state and thus expressed as 100%) is 1 to 10% by weight, based on the weight of the powder obtained after the end of step a).

This amount depends on the degree of bleaching of the plant material.
In general, with ultra-fine ground wood, 2
An amount of ~ 6% is sufficient. The H ₂ O ₂ concentration of the solution A is selected so that the solutions A and B are completely absorbed by the ultrafine powder and the powder retains the appearance and characteristics of the powder. 100-350g /
It is preferred to use a solution A containing L H ₂ O ₂ (expressed in a pure state), that is, a solution having a weight ratio of H ₂ O _{2 to the} volume of 10 to 35% by weight.

When the concentration of the solution A is 35% or more, the reaction caused by the solution is exothermic, and there is a risk of fire in normal atmosphere, which makes it difficult to use.
These risks can be controlled in part by carrying out steps b), c) and drying under an inert atmosphere, especially under a nitrogen atmosphere.

The powder temperature t _b during impregnation in step b) is 60
~ 90 ° C is advantageous.

This temperature is obtained by heating the reactor containing the powder or is caused by the exothermicity of the reaction when the powder, solution A and solution B are contacted. The time of step b) is advantageously from 5 minutes to 2 hours, from 10 minutes to 3
0 minutes is preferred. The alkaline substance of the solution B is NaO.
It is advantageous to choose among H, KOH, Na ₂ CO ₃ , K ₂ CO ₃ , sodium silicate, DTPA or other reactants which can obtain a pH of 7 or higher for solution B. The weight of the alkaline substance used with respect to the weight of the powder to be impregnated depends on the amount of H ₂ O ₂ used and the type of plant material. This amount is chosen such that the final pH of the powder after step c) is 6.5-8.5.

If NaOH is used, it is suitable if the weight of NaOH is 0.5 to 5% by weight, based on the weight of the plant powder to be impregnated.

The amount of water used to form solution B is chosen so that the active ingredients of this solution are completely dissolved. The total amount of the two solutions A, B is such that the water content of the powder after the end of step a) is such that a water content of 30-40% by weight relative to the total weight of the powder is obtained after the impregnation of step b). It is preferable to calculate according to

The stabilizer for hydrogen peroxide is selected from sodium silicate, magnesium salt, disodium phosphate, sodium polyphosphate, sodium pyrophosphate and phosphonate. It is preferred to use sodium silicate, and even better if sodium silicate is used as an aqueous solution with a density of 1.33. Advantageously, the amount of stabilizer is from 1 to 10% by weight, based on the weight of the powder to be impregnated in step b) expressed dry. The preferred amount is 4-8%. The complexing agent is selected from among DTPA (sodium diethylenetriaminepentaacetate), EDTA (sodium ethylenediaminetetraacetate), poly (α-hydroxyacrylic acid) salts and phosphates.

For reasons of cost and effectiveness, the preferred material is DTPA as a 40% solution.

The amount of 40% DTPA used is advantageously 0.1 to 1% by weight, based on the weight of the plant material before impregnation.

It is advantageous if solution B additionally contains an optical brightener in order to improve the final white shade. This optical brightener can be selected from those commonly used in the paper industry.

The amount of optical brightener added is advantageously 0.01 to 0.5% by weight, expressed in dry form, relative to the weight of the powder charged to the reaction mass in step b). .

Bleaching is completed in step c), known as the latency step. During this process, the impregnated micronized plant powder is stirred for a time of d _c at a temperature of t _c . A preferable temperature t _c is 60 ° C to 90 ° C.

The time d _c depends on the type of plant material and the temperature t _c . Generally, this time is 5 minutes to 120 minutes, preferably 15 minutes to 60 minutes.

Depending on the application of the bleached ultrafine powder, a material as dry as possible is required. In this case, drying is carried out after step c). This drying is carried out according to the prior art used in the wood and plant materials industry, taking into account the special risks of ignition and / or explosion under oxidizing atmosphere. To avoid this danger, this drying can be carried out under an inert gas atmosphere or under a very oxygen-deficient atmosphere.

Sodium silicate is present in the final powder and acts as a flame retardant, since washing from step a) is unnecessary if it is present in solution B.

The powder obtained after step c) is dried and
It is preferable that the water content with respect to the total weight be 5 to 20% by weight.

In addition to the above description, the following embodiments will provide a better understanding of the present invention.

The process can be carried out batchwise or continuously.

1) Batch bleaching Ultrafine milled plant powder was charged into a powder mixer preheated to about 40 ° C., then 35% H ₂ O ₂ solution on the one hand and alkaline solution on the other hand were simultaneously sprayed onto the powder. .

The volume ratio of the 35% H ₂ O ₂ solution to the alkaline solution is about 1.

The temperature t _b of the powder rose to 80 ° C. Stirring was maintained at a powder temperature of 70 ° C. for a further 30-60 minutes after the solution was added, then the powder was discharged, optionally dried and then placed in a container.

The mixer can be a scraper screw agitated conical mixer, a spiral ribbon mixer, a concrete mixer, a gas flow or a mechanical agitated fluid bed mixer.

The material used in the mixer must be compatible with and resistant to the use of H ₂ O ₂ , such as stainless steel, enamel steel, steel coated with resistant paint or plastic. To be selected.

2) Continuous bleaching This mode is preferable for industrial production with a large amount of treatment, and the exothermic control of the bleaching reaction is improved.

The micronized plant material was continuously charged into the continuous mixer by means of a metering screw. In the mixer,
H ₂ O ₂ on the one hand and a basic solution on the other hand were continuously injected.

The residence time in the mixer is about 1-5 seconds,
It may be up to 1 to 3 minutes.

At the outlet of the continuous mixer, the ultrafine powder continuously passes through the retention tank, and the retention time in the tank consumes at least 75% of the introduced H ₂ O ₂ and therefore disappears. Is enough for

The optimum bleaching temperature t _b was 70-80 ° C. This temperature was preferably obtained because the oxidation reaction with H ₂ O ₂ was exothermic.

The maximum temperature t _{b that} can be achieved is the amount of H ₂ O ₂ introduced, the water content of the plant material after mixing the reactants, the initial temperature of the plant material, and the temperature of each solution A, B injected into the mixer. Depends on.

If the exothermicity of the bleaching reaction is insufficient to reach the optimum bleaching temperature, the water content is less than 20%, due to the small amount of H ₂ O ₂ used, The plant material is heated before or during the addition of the reactants, whether due to bleaching plant material close to this value. in this case,
The heating should be uniform and should not cause localized overheating. A preferred heating means is to circulate hot water in the jacket around the mixer and / or around the holding tank or inject steam into the mixer.

Heating the basic solution B is also a convenient means of applying heat to the reaction mixture. 40 ~ basic solution B
It is preferable to heat to a temperature of 90 ° C.

Prior to introducing the superfinely ground plant material into the continuous mixer, by steam injection or by circulating hot water in the jacket around the metering screw and / or around the screw supply tank, the metering screw or this metering screw The feed tank for the mold screw can preheat the micronized plant material.

Apparatuses which can be used for continuously impregnating the solutions A, B into micronized plant powders include various types including spiral ribbon powder mixers, horizontal shafts with knives or rotating disks of the paper pulp refiner type. Type of rotary dispersive mixer, turbo mixer, gas flow or mechanically stirred fluidized bed mixer, cement furnace type rotary cylinder with internal hopper.

The equipment which can be used to secure the residence time of the ultrafinely ground plant material impregnated with the solutions A and B is a high-throughput spiral Archimedean screw, a conical mixer, or a cement furnace type rotary cylinder. To be selected.

It should be noted that the impregnation (step b) and the residence time (step c) can be carried out in the same device.

[0065]

EXAMPLES In the following examples the following ultra-fine ground wood was used: I-Ultra-fine ground pine / spruce wood Whiteness = 49% ISO pH = 5.1 Moisture content = 5. 6 wt% (w / w _t) II- micronized French maritime pine wood whiteness = 50% ISO pH = 4.4 water content = 10.7% w / w _t III- micronized spruce wood whiteness = 51% ISO pH = 6.2 water content = 6.1% w / w _t IV- micronized beech wood whiteness = 41% ISO pH = 5.7 water content _{= 7% w / w t d} 95 particle size The above ultra-finely ground wood of less than 150 μm is
ciete Parisienne des des Sciu
res [Paris Sounddusts Compan
y].

The whiteness is 457 for measuring the whiteness of paper.
It was measured with a Hunterlab spectrophotometer according to the "ISO brightness" standard used in the paper industry at a wavelength of nm.

Laboratory procedure a) Equipment used: A jacket type glass reactor having a volume of 1 L was heated by circulating hot water in the jacket. This reactor is equipped with a central high speed propeller and two
It was equipped with two low speed scraper blades.

Solution A (35% H ₂ O ₂ ) was added to H. P. L.
C. The solution was atomized by injecting using a mold high pressure pump and then atomizing using a nozzle above the reactor.

The means for spraying the basic solution B is the same as the above means.

B) General Examples: Materials I, II, II
50 g of micronized plant material selected from I and IV were charged to the reactor. Then, Solution A and Solution B were simultaneously stirred for 15 minutes. Bleaching is terminated by maintaining the agitation for 30-60 minutes, then discharging the material from the reactor and drying in a ventilated oven at 90 ° C. to obtain the residual water content relative to the powder weight expressed in dry conditions. The ratio was about 5% by weight.

Then, the whiteness, pH and H ₂ O ₂ consumption were measured.

The various Examples 1-14 and the results are shown in Tables I and II below.

The amount of hydrogen peroxide and NaOH is% by weight of pure product relative to the weight of dry ultrafine milled plant material.
It is represented by.

The amount of DTPA is expressed as% by weight of the commercial solution (about 40%) with respect to the weight of the dry micronized plant material.

The amount of sodium silicate is expressed as% by weight of an aqueous sodium silicate solution having a density of 1.33, based on the weight of the plant material in a dry state.

In some tests, 0.5% DTPA is pretreated for 15 minutes at a consistency of 10% and a temperature of 90 ° C. At the end of this treatment, the micronized plant material was filtered, washed with water and dried to the indicated initial water content.

Example 15 The procedure is the same as in Example 3, with an initial whiteness of 44.7%.
Another micronized spruce wood with an ISO, 94% consistency and an initial pH of 5 was used.

50 g of this wood preheated to 70 ° C. was simultaneously sprayed with the following ingredients.

35% H ₂ O ₂ (8%) 11.4 g, 1.5
Solution B containing 16.9 g of NaOH, 8% sodium silicate, 0.5% DTPA, and 0.3 from Bayer.
% Liquid "BLANCOPHOR P" optical brightener (expressed by weight ratio of each commercial solution).

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

The whiteness was 62.5% ISO.

Hun in accordance with CIE standard D 65/10
Color measured with a terlab spectrophotometer: L * = 87.49 a * =-1.01 b * = 8.7
7.

[0083]

[Table 1]

[0084]

[Table 2]

Example 16-Pilot Test 1 A stainless steel cement mixer with a processing capacity of 40 L (capacity to stir in a tilted position) preheated to 40 ° C. was charged with 7 kg of ultrafine ground wood III.

The following components were then simultaneously sprayed in over 15 minutes by means of two metering piston pumps: 35%
1.6 kg hydrogen peroxide, 126 g NaOH, 560 g sodium silicate aqueous solution with a density equal to 1.33,
2.37 kg of an alkaline solution consisting of 35 g of 40% DTPA and 1.65 kg of water.

During these spraying operations, the temperature t _b was 78 ° C.
And the final water content of the impregnated ultra-fine ground wood was 40%.

The impregnated powder was stirred at a temperature of 70 ° C. for a further 45 minutes.

10.5 kg of bleached ultrafine ground wood was obtained. The wood had a pH of 9.1 and had 0.7% residual H ₂ O ₂ (pure H based on the weight of dry micronized wood).
₂ O ₂ expressed as wt.%). Whiteness is 6
It was 2.7% ISO (dry sample).

After holding at 70 ° C. for 5 hours, the pH is equal to 7, the residual H ₂ O ₂ percentage is 0.18% and the whiteness is 6
It was 3% ISO (dry sample).

Example 17-Pilot Test 2 This example is identical to Example 16. However, NaOH
Was low, 105 g instead of 126 g (ie 1.6% instead of 1.8% on dry wood).

The maximum temperature reached during the two simultaneous spraying operations was 70 ° C. After stirring for 45 minutes at 70 ° C, the pH
Is 8.5, the residual H ₂ O ₂ ratio is 1.25%, and the whiteness is 62.
It was 3% ISO.

After treatment at 70 ° C. for 5 hours, the pH is 6.1.
And the residual H ₂ O ₂ ratio is 0.5% and the whiteness is 63% IS
It was O.

Examples 18, 19 and 20-Other micronizations
Pilot tests with plant material Ultrafine milled material of 3, 4, 5 or less was used.

V-Wheat flour maize bran Whiteness = 32% ISO pH = 5 Moisture content = 8% VI-Wheat flour sunflower whiteness = 26% ISO pH = 6.5 Moisture content = 7.6% VII- Sugar beet pulp in the form of wheat flour Whiteness = 30.5% ISO pH = 6 Moisture content = 4% The d ₉₅ particle size of these ultrafine powders was 180 μm.

A 100 L stainless steel S spiral mixer-dryer (trademark GUEDU), which is circulated with warm water and heated by a jacket, is charged with the micronized plant material V, VI or VII and then the charge. Preheated.

The 35% H ₂ O ₂ solution and the alkaline solution were simultaneously sprayed into the mixer. The reaction was allowed to proceed until at least 75% of the hydrogen peroxide charged to the reactor was consumed, then vacuum dried.

Examples 18 to 20 and the results thereof are shown in Table III.
And IV.

[0099]

[Table 3]

[0100]

[Table 4]

Claims (14)

[Claims] 1. A method of bleaching a plant material in the form of ultrafine powder with hydrogen peroxide in the presence of a base, which comprises: a) optionally drying to a water content of less than 30% by weight relative to the total weight. , Ultrafine powder, then 20-100
Heated to _a temperature ta of 0 ° C., b) _two separate powders, A, consisting of an aqueous hydrogen peroxide solution containing 50-500 g / L H ₂ O ₂ and B, consisting of a basic aqueous solution containing an alkaline substance. Of the solution is impregnated simultaneously or successively with A then B or B then A, the amount of each solution being relative to the amount of powder to be impregnated, even if the two solutions are completely absorbed by the powder. It remains in the form of a solid powder having a water content of 10 to 50% by weight,
And the temperature t _{b of the} powder is between 40 and 100 ° C., and c) the powder impregnated in this way has a time sufficient to consume at least 75% of the amount of hydrogen peroxide charged, Said method characterized in that it is maintained at a temperature t _c of 40-100 ° C. 2. A method according to claim 1, the water content relative to the total weight before the ultrafine reach temperatures t _a step a) is characterized in that 4 to 15% by weight. 3. The method according to claim 1, wherein the volume ratio of the solution B / A is 0.5 to 2. 4. The method according to claim 1, wherein the solution B contains a stabilizer for hydrogen peroxide. 5. The method of claim 4, wherein the stabilizer is sodium silicate. 6. Sodium silicate is 1 to 1 based on the dry weight of the powder to be impregnated in step b).
Process according to claim 5, characterized in that it is present in an amount of 0% by weight. 7. The solution B further comprises a metal complexing agent or a sequestering agent of a metal salt known to catalyze the decomposition of hydrogen peroxide.
7. The method according to any one of items 6 to 6. 8. The powder according to claim 1, wherein the powder obtained after the completion of step c) is dried to a water content of 5 to 20% by weight relative to the total weight. the method of. 9. The method according to any one of claims 1 to 8, characterized in that the temperature t _a is 20 to 40 ° C. in step a). 10. The method according to claim 1, wherein the temperature t _{b in} step b) is 60 to 90 ° C. 11. The method according to claim 1, wherein the temperature t _{c in} step c) is 60 to 90 ° C. 12. In step b), the solution A is sprayed and the solution B is
3. The powder is impregnated by spraying with.
12. The method according to any one of 1 to 11. 13. In step b), the amount of hydrogen peroxide used for impregnation (calculated in pure state) is between 1 and 10% by weight, based on the weight of the powder obtained after the end of step a). 13. The method according to any one of claims 1 to 12, characterized in that 14. The method according to claim 1, wherein solution B further comprises an optical brightener.