WO2003022737A1

WO2003022737A1 - Method for the preparation of an aqueous hydrogen peroxide solution from a direct synthesis

Info

Publication number: WO2003022737A1
Application number: PCT/EP2002/009998
Authority: WO
Inventors: Achim Stammer; Martin Fischer; Stefan Quaiser; Karin Pickenäcker
Original assignee: Basf Aktiengesellschaft
Priority date: 2001-09-07
Filing date: 2002-09-06
Publication date: 2003-03-20
Also published as: DE10144013A1

Abstract

The invention relates to a method for the preparation of an aqueous hydrogen peroxide solution obtained according to a direct synthesis method from hydrogen and oxygen. Difficultly volatile inorganic impurities are separated into an n-stage evaporator cascade. The vapors are concentrated in order to increase the hydrogen peroxide content.

Description

Process for working up an aqueous hydrogen peroxide solution from a direct synthesis

description

The present invention relates to a process for working up an aqueous hydrogen peroxide solution, which was obtained by a direct synthesis process from hydrogen and oxygen.

Hydrogen peroxide has many technical areas, such as. B. in chemical synthesis, for the bleaching of paper and pulp, for the treatment of waste water and as a component of chemical polishing fluids found a wide application. The correspondingly large market need is currently mainly covered by hydrogen peroxide from the so-called anthraquinone process. According to this process, an anthraquinone derivative is reduced in an organic solvent mixture on a hydrogenation catalyst to the corresponding anthrahydroquinone. In the subsequent oxidation of this hydroquinone solution with air or oxygen, hydrogen peroxide results in recovery of the anthraquinone derivative originally used. After extraction with water, a crude solution results which has a large number of inorganic and organic impurities. Depending on the intended use, the aqueous hydrogen peroxide solutions must therefore be subjected to various cleaning and / or concentration steps. However, the quality of the hydrogen peroxide solutions obtained in this way is generally only sufficient for technical applications which do not place high demands on the removal of inorganic and / or organic contaminants.

Some of the disadvantages inherent in the anthraquinone process can be avoided by the direct synthesis of hydrogen peroxide by reacting hydrogen with oxygen. The reaction is usually carried out in purely aqueous reaction media, to which a strong acid, such as H ₂ SO ₄ , H ₃ PO ₄ and / or HC1, is added for the purpose of inhibiting the decomposition of hydrogen peroxide formed. So-called promoters, for example chlorides or bromides, can also be used to increase the selectivity. For example, noble metal catalysts based on palladium and / or platinum are used as catalysts for the direct synthesis of hydrogen peroxide, and these are preferably heterogeneous catalysts. The after Hydrogen peroxide solutions obtained in direct synthesis processes usually contain a sufficiently low content of organic impurities. In order to meet the requirements of the market, however, it is generally necessary to increase the hydrogen peroxide content by concentrating and to reduce the content of inorganic contaminants which have been introduced in the form of the aforementioned stabilizers and / or promoters.

US Pat. No. 5,296,104 describes a process for the purification of aqueous hydrogen peroxide solutions, in which a crude hydrogen peroxide solution which contains organic and / or inorganic impurities is evaporated, a vapor being obtained which comprises a mixture of water, hydrogen peroxide and volatile and contains cracked contaminants. This steam is fed into a distillation column with a washing zone and rectification zone and worked up, a purified hydrogen peroxide solution being obtained. A disadvantage of this process is that only a maximum of 90% by weight of the hydrogen peroxide contained in the raw hydrogen peroxide solution is converted into the vapor phase by the evaporation.

The US 5,705,040 describes a method for purifying an aqueous hydrogen peroxide solution by partial evaporation to form an H ₂ 0 ₂ containing steam and a H ₂ 0 ₂ and impurity-containing liquid phase and subsequent single- or multistage partial condensation of the steam to give an H ₂ 0 ₂ -enriched condensate and an H ₂ 0 ₂ -enriched residual steam. A disadvantage of this process is that the partial evaporation and condensation steps always leave some of the hydrogen peroxide behind and the yield of the product of value is thus reduced.

EP 0 419 406 describes a distillation plant for concentrating hydrogen peroxide. The problem of the purification of aqueous H ₂ 0 ₂ solutions is not mentioned in this document.

DE-A-198 17 794 describes a method for producing highly pure aqueous hydrogen peroxide solutions for the electronics industry. In addition to the production of these special solutions, suitable measures for their cleaning and / or concentration are also described. Thus, according to embodiment 3, a hydrogen peroxide solution can be subjected to continuous evaporation in a falling film evaporator, the evaporation achieved being described as being almost complete without specifying a degree of evaporation. Because the evaporation is single-stage in a circulation type evaporator and in the presence of sulfuric acid and phosphoric acid takes place, if this type of work-up, as will be explained in more detail below, does not succeed, the hydrogen peroxide contained in the aqueous solution used is essentially completely (ie at least 95% by weight) and without losses due to decomposition into the vapor phase to convict.

The present invention is based on the object of specifying a process for working up an aqueous hydrogen peroxide solution from direct synthesis which allows the hydrogen peroxide to be isolated as completely as possible.

Surprisingly, it has now been found that a multi-stage evaporator cascade can advantageously be used for working up aqueous hydrogen peroxide solutions from direct synthesis.

The invention therefore relates to a process for working up an aqueous hydrogen peroxide solution from a direct synthesis, in which

a) feeds the hydrogen peroxide solution to remove the low-volatile inorganic impurities contained therein into an n-stage evaporator cascade and thermally separates vapors containing hydrogen peroxide and a residue containing the impurities, where n is an integer of at least 2 and at least in the n -th stage a continuous evaporator is used.

b) subjecting the vapors to a concentration to increase the hydrogen peroxide content.

The inventors have found that the mineral acids contained in aqueous hydrogen peroxide solutions from direct synthesis as stabilizers and / or promoters, such as phosphoric acid, sulfuric acid and hydrohalic acids, eg. As hydrogen chloride and hydrogen bromide, surprisingly lead to decomposition of the hydrogen peroxide contained in highly concentrated solutions. Processes known from the prior art, in which an aqueous hydrogen peroxide solution from direct synthesis are subjected to a one-stage evaporation step for processing, thus necessarily lead to a loss of valuable material, since either high evaporation rates are avoided to avoid H ₂ 0 ₂ decomposition or a decomposition in the increasingly concentrated solution has to be accepted. Processes are particularly disadvantageous in which an evaporator operating according to the recirculation principle is used for evaporation, since long residence times of the hydrogen peroxide solution solutions occur. Thus, methods known from the prior art generally cannot convert more than 90% by weight of the hydrogen peroxide and water contained in a synthesis discharge from the direct synthesis into steam. 5

In general, aqueous hydrogen peroxide solutions obtained by a conventional direct synthesis process can be used as the starting material for the work-up process according to the invention. They contain e.g. B. 2.5 to

10 25 wt .-%, preferably 5 to 20 wt .-%, hydrogen peroxide. They also generally contain 0.001 to 1% by weight, for example 0.01 to 0.05% by weight, of low-volatile inorganic impurities. In the context of the present application, the term low-volatile inorganic impurities includes such contaminations

15 cleanings that do not essentially pass into the vapor phase under the conditions used for the evaporation of H ₂ 0 ₂ / water mixtures. These include, for example, the non-volatile inorganic acids, such as sulfuric acid, used to stabilize the hydrogen peroxide in direct synthesis.

20 phosphoric acid and the salts thereof. The aqueous hydrogen peroxide solution used as the starting material for the process according to the invention can furthermore contain up to 0.1% by weight, for example 0.0001 to 0.1% by weight, of volatile inorganic impurities. Within the scope of the present invention

25 the term volatile inorganic impurities encompasses those impurities which essentially pass into the gas phase under the conditions used for the evaporation of H ₂ 0 ₂ / water mixtures. These include, for example, those in aqueous hydrogen peroxide solutions from direct synthesis as stabilized

30 lisators and / or promoters used hydrohalic acids and / or their salts, such as. B. hydrogen chloride, hydrogen bromide, chlorides and bromides.

A hydrogen peroxide solution from direct synthesis is preferably used as the starting material

2.5 to 25% by weight of hydrogen peroxide,

0.01 to 1% by weight of low volatile inorganic impurities,

0 to 0.1% by weight of volatile inorganic impurities and

45 - water ad 100% by weight contains.

Suitable direct synthesis methods are described without claim to completeness, for example in US 5,500,202, to which reference is made here. Preferred catalysts are those described in German patent application P 100 48 844.7, to which reference is made here.

According to the invention, an n-stage evaporator cascade is used in step a), n preferably representing an integer of at least 2, particularly preferably 3 to 6, in particular 3 or 4. In a preferred embodiment, n is 3.

The evaporator cascade used according to the invention can be constructed from the same or different evaporator types. If several evaporators of one type are used, they can differ both in terms of their dimensions and the operating parameters under which they are operated. Suitable evaporators are described, for example, in Ullmann's Encyclopedia of Technical Chemistry, Volume 1, pp. 532-542, 3rd edition (1951) and in Perry's, Chemical Engineers' Handbook, 11-107 to 11-118, 7th edition (1997) , In the present invention, a distinction is made between the evaporator types of the circulation evaporator and the continuous evaporator. Circulation evaporators are characterized by a circulation of the solution to be evaporated, which is repeatedly guided past a heating means for heating. They generally have effective heat transfer. The evaporated solution only flows once through the evaporator. They generally have shorter residence times for the solution to be evaporated than circulation evaporators.

Both circulation evaporators with natural circulation and forced circulation evaporators are suitable for the process according to the invention. Suitable evaporators with natural circulation are, for example, tube evaporators with natural circulation, etc. Suitable forced circulation evaporators are, for example, those in which the liquid circulation is brought about by a pump switched into the circulation line. Film evaporators, such as falling film evaporators and wiper blade evaporators, are preferably used for the process according to the invention. Suitable continuous evaporators are, for example, tube evaporators.

According to a particularly preferred embodiment of the process according to the invention, a circulation evaporator is used in the first to (nl) -th stage of the evaporator cascade and in the n-th Stage a continuous evaporator. Advantageously, long residence times of the solution concentrated in inorganic impurities and hydrogen peroxide can be avoided in the nth stage.

A series connection of the individual evaporators is advantageous, a liquid phase enriched with low-volatile inorganic impurities and vapors containing hydrogen peroxide being obtained at each stage. A liquid discharge is taken from the liquid phase of the first to (n-1) -th stage and fed to the next evaporator stage. The nth stage results in a highly concentrated aqueous solution of inorganic components of the hydrogen peroxide solution used, which, if appropriate after working up, can be used again in the direct hydrogen peroxide synthesis. The vapors containing hydrogen peroxide are passed on for further processing, preferably after being combined.

The total degree of evaporation is understood as the percentage by weight of the volatile (volatile) components (water, H ₂ 0 ₂ , optionally volatile inorganic compounds) contained in the aqueous hydrogen peroxide solution used, which is converted into the vapor phase (vapors). The degree of evaporation of an evaporator stage means the percentage by weight of the evaporable components contained in the feed supplied to the stage, which is converted into the vapor phase in this stage. The degree of evaporation of a stage (and thus also the total degree of evaporation) depends on reactor-specific parameters such as the volume and the specific heating output. It can be set to a desired value by suitable selection of the operating parameters, especially the pressure, the temperature and the flow rate.

In the first to (nl) -th evaporation stage, the degree of evaporation per stage is preferably in a range from about 30 to 85%, particularly preferably 40 to 80%. The degrees of evaporation of the individual stages can in each case be the same or different for the first to (nl) th stage. The degrees of evaporation of the first to (nl) -th stage preferably have a gradient with decreasing values from the first to the (nl) -th stage. For example, in a three-stage evaporator cascade, the degree of evaporation of the first stage can be in a range of approximately 70 to 80% and the degree of evaporation of the second stage can be in a range of approximately 45 to 55%. The total degree of evaporation, based on the nth to (nl) th stage, is preferably 75 to 92%, particularly preferably 80 to 90% and especially 85 to 90%. With these total degrees of evaporation, it is advantageous essentially no decomposition of the hydrogen peroxide was observed at the nth to (nl) th stage. The degree of evaporation of the nth stage is preferably in a range from approximately 90 to 99%, particularly preferably 95 to 98%. The total degree of evaporation of the evaporator cascade used according to the invention is preferably at least 95%, particularly preferably at least 97% and especially at least 98%. In general, even higher levels of total evaporation of 99% and beyond can be achieved.

The temperature during the evaporation is preferably in a range from about 20 to 150 ° C., preferably 40 to 100 ° C., for all evaporator stages. A temperature gradient with increasing values from the first to the nth stage is preferred.

The pressure during evaporation is preferably in a range from approximately 10 mbar to ambient pressure, particularly preferably 20 to 500 mbar, in particular 30 to 100 mbar. A pressure gradient with decreasing values from the first to the nth evaporator is possible.

The vapors obtained in step a) preferably contain at least 95% by weight, particularly preferably at least 97% by weight, in particular about 99% by weight of the hydrogen peroxide and the water of the hydrogen peroxide solution used.

To concentrate in step b), the vapors obtained in step a) can be subjected to thermal fractionation by condensation and / or distillation. For this purpose, the vapors obtained in the individual evaporator stages are expediently combined beforehand. However, it is also possible to feed vapors from one or more evaporator stages to a separate preparation.

According to a first suitable process variant, the vapors are concentrated by distillation. The distillation can be carried out continuously or batchwise, preference being given to continuous distillation. Suitable distillation apparatuses for concentrating aqueous hydrogen peroxide solutions are known from the prior art. They comprise one or more distillation columns with a sufficient number of plates. Distillation columns with 1 to 30 plates, particularly preferably 2 to 10 plates, are preferred for concentration. Columns made of stainless steel, aluminum or coated columns are preferably used which have a coating with inert materials, such as polyvinylidene fluoride or perfluoroalkoxy polymers (PFA), on the surfaces which come into contact with H ₂ O ₂ . The columns used for concentration can have column internals, such as fixed, adjustable or movable column trays, packing, liquid distributors, ordered packings, for example made of wire mesh, reflux distributors, supporting gratings, etc. Structured packings, eg. B. from sheet metal or expanded metal as well as fabric and packing. The internals preferably have a geometric surface area of 50 m ² / m ³ to 5000 m ² / m ³ . Particularly high yields are achieved when using column evaporators with short residence times. These include falling film evaporators, thin film evaporators and climbing film evaporators.

The vapors obtained in step a) are preferably used without prior condensation for concentration by distillation. The feed into the distillation column is preferably carried out at the side of the column base, ie. H. z. B. on the zero to third separation stage of a distillation column comprising at least 2 to 10 separation stages.

A concentration process is preferred in which the vapors are fed laterally into a column for thermal fractionation, separation into a gaseous top product depleted of hydrogen peroxide and a liquid bottom product enriched with hydrogen peroxide.

In the case of concentration by distillation, the temperature at the foot of the distillation column is preferably 20 to 100 ° C., preferably 30 to 80 ° C. The pressure used for the distillation is preferably in a range from about 30 to 150 mbar.

A suitable distillation plant for the production of concentrated hydrogen peroxide is that described in EP-A-0 419 406.

In a preferred embodiment of the distillative concentration of the vapors, heat is withdrawn from the gaseous top product by contacting it with a heat exchanger, and the extracted heat is used in a heat-consuming step of the work-up process or another process.

The temperature of the gaseous top product can be raised in a conventional vapor compressor before contacting the heat exchanger in order to ensure better heat recovery. Suitable vapor compressors are, for example, mechanical compressors and propellant jet compressors, with only a part of the vapors possibly being used in the latter for compression. For example, the temperature of the top product, which is usually in a range from about 30 to 100 ° C, are increased to values from about 150 to 250 ° C.

If desired, the temperature of the compacted head product can be reduced to temperatures of about 70 to 120 ° C. by so-called quenching.

The gaseous top product of the working up by distillation is condensed, optionally after compression and / or quenching. In order to make use of the amount of heat generated, the condensation is preferably carried out by bringing it into contact with a heat exchanger. The heat is removed, for example, by bringing it into contact with a cooling medium, such as water. It is particularly expedient not to use a secondary heat exchanger, but rather to feed the gaseous top product directly to a heat-consuming step. Suitable heat exchangers are the usual heat transfer devices in which heat is transported from one place or medium to another. You can be carried out in conventional designs, such as. B. plate and tube bundle exchanger. The heat extracted from the gaseous overhead is used in a heat consuming step of the work-up process or other process. This includes in particular heating the evaporators in step a).

The hydrogen peroxide content in the top product of the working up by distillation and the condensate is preferably at most 1% by weight, particularly preferably at most 0.5% by weight and in particular at most 0.1% by weight. The hydrogen peroxide concentration in the bottom product of the working up by distillation is preferably 30 to 70% by weight.

According to a further preferred embodiment, the vapors can be concentrated in step b) by fractional condensation. For this purpose, the combined vapors can be subjected to a one- or multi-stage partial condensation, with a separation into a condensate, which essentially contains the hydrogen peroxide contained in the vapors, and a vapor, which essentially consists of water, takes place.

The vapors obtained in step a) of the work-up process according to the invention can contain volatile inorganic compounds. As described above, these are essentially additives from the direct synthesis of the hydrogen peroxide solution to be worked up, for example hydrohalic acids and / or their salts. The vapors obtained in step a) are preferably used to remove at least some of them Before the concentration in step b), impurities are washed with a suitable washing liquid (vapor wash). Suitable washing liquids for separating hydrohalic acids are, for example, aqueous solutions of nonvolatile inorganic bases. Suitable bases are alkaline earth or alkali metal hydroxides, e.g. B. NaOH, KOH, Ca (OH) ₂ . The vapors containing water / hydrogen peroxide vapor are brought into contact with the washing solution in customary apparatuses known to those skilled in the art, for example in a washing column. The pressure in the vapor washing is preferably in a range from 0.03 bar to 0.2 bar, particularly preferably 0.05 bar to 0.15 bar. The liquid load is preferably in the range of 0.1 m ³ m ^-2 h ^_1 and 60 m ³ m ^-2 h ^-1 , particularly preferably between 1 and 50 m ³ m ^-2 h ^_1 . The gas load (F factor) is preferably 0.2 to 30, particularly preferably 0.3 to 6 Pa ⁰ ' ⁵ .

After the vapor wash, the content of volatile inorganic impurities in the resulting hydrogen peroxide is at most 10 ppm by weight, particularly preferably at most 1 ppm by weight.

As an alternative to a vapor wash or in combination therewith, the aqueous hydrogen peroxide solutions used can be contained before they are fed into the evaporator cascade (step a)) and / or the hydrogen peroxide solutions obtained after the concentration (step b)) to remove at least some of them volatile inorganic contaminants are brought into contact with an ion exchanger. Suitable ion exchangers for removing hydrohalic acids and their salts are, for example, Amberlite® IRA 92 from Rohm & Haas. Since hydrogen peroxide tends to decompose when it comes into contact with basic components, it is advantageous to pre-charge the ion exchanger with an acid whose acid strength is lower than that of the hydrohalic acid to be removed. For example, to remove chloride or bromide, the ion exchanger can be preloaded with phosphoric acid.

The aqueous hydrogen peroxide solutions obtained by the process according to the invention preferably have a hydrogen peroxide content of at least 30% by weight, particularly preferably at least 35% by weight. Aqueous hydrogen peroxide solutions with concentrations of up to about 70% by weight, such as 30 to 55% by weight, can be obtained by the process according to the invention. If desired, the hydrogen peroxide solution obtained after concentration can be a stabilizer, e.g. B. an aqueous sodium pyrophosphate solution can be added.

The aqueous hydrogen peroxide solutions obtained by the process according to the invention preferably have an inorganic impurity content of at most 0.001% by weight, particularly preferably at most 0.0001% by weight.

The process according to the invention is generally suitable for working up aqueous hydrogen peroxide solutions from direct synthesis. The invention thus also relates to a process for the preparation of aqueous hydrogen peroxide solutions which contain at most 0.001% by weight of inorganic impurities, in which

i) hydrogen and oxygen in the presence of at least one catalyst and at least one inorganic protonic acid in an aqueous medium and

ii) subjecting the reaction mixture obtained in step i) to working up as described above.

Suitable direct synthesis methods of stage i) are those described and referred to at the beginning.

The process according to the invention is advantageously suitable for working up aqueous hydrogen peroxide solutions from direct synthesis. In this case, a valuable substance is avoided by incomplete evaporation of the hydrogen peroxide contained in the raw solution. Furthermore, decomposition of the hydrogen peroxide, as occurs with prolonged contact with highly concentrated inorganic impurities, especially with high acid concentrations, can also be advantageously avoided.

The invention is illustrated by the following, non-limiting examples.

Examples

Comparative example (analogous to example 3 of DE-A-198 17 794)

A 10% aqueous hydrogen peroxide solution containing 0.5% by weight phosphate and 100 ppm bromide was subjected to continuous evaporation in a falling film evaporator made of metal (diameter 30 mm, length 1000 mm) with liquid circulation (100 - 150 1 / h) subjected. At an evaporator temperature of 90 ° C. and a pressure of 75 mbar, 95% by weight of the evaporable components (water, hydrogen peroxide, bromide) contained in the hydrogen peroxide solution used were evaporated. A hydrogen peroxide decomposition of about 40% by weight, based on the hydrogen peroxide content of the solution used, was observed.

Example 1

The hydrogen peroxide solution also used in Comparative Example 1 was subjected to continuous evaporation in a cascade of two falling-film evaporators and one continuous evaporator. The two falling film evaporators (metal, diameter 30 mm, length 1000 mm) with liquid circulation (100-150 l / h) were operated at a temperature of 90 ° C. and a pressure of 75 mbar. The degree of evaporation in the first falling film evaporator was 75%, the degree of evaporation in the second evaporator was 50%. The continuous evaporator was operated at a temperature of 90 ° C and a pressure of 75 mbar, the degree of evaporation was 97%. A total degree of evaporation of 99% was achieved, with no hydrogen peroxide decomposition being observed.

Claims

claims

1. Process for working up an aqueous hydrogen peroxide solution from a direct synthesis, in which one

a) feeds the hydrogen peroxide solution to remove the low-volatile inorganic impurities contained therein into an n-stage evaporator cascade and thermally separates vapors containing hydrogen peroxide and a residue containing the impurities, where n is an integer of at least 2 and at least in a continuous evaporator is used in the nth stage.

2. The method of claim 1, wherein n is an integer of at least 3.

3. The method according to any one of the preceding claims, wherein the hydrogen peroxide solution used from the direct synthesis

2.5 to 25% by weight of hydrogen peroxide,

0.01 to 1% by weight of low volatile inorganic impurities,

- 0 to 0.1% by weight of volatile inorganic impurities and

Water ad 100% by weight

contains.

4. The method of claim 3, wherein the low volatility inorganic contaminants comprise sulfuric acid, phosphoric acid and / or a salt thereof.

5. The method of claim 3, wherein the volatile inorganic contaminants comprise a hydrohalic acid and / or a salt thereof.

6. The method according to any one of the preceding claims, in which a circulation evaporator is used in the first to (n-l) th stage of the evaporator cascade and a continuous evaporator in the nth stage.

7. The method according to any one of the preceding claims, wherein the vapors obtained in step a) at least 95 wt .-%, preferably at least 97 wt .-%, in particular 99 wt .-% of the hydrogen peroxide and the water of the hydrogen peroxide used Solution included.

8. The method according to any one of claims 1 to 7, in which the hydrogen peroxide solution used in the direct synthesis additionally contains volatile inorganic impurities and one removes at least a part of these

Contaminates the vapors obtained in step a) before washing in step b) with a washing liquid.

9. The method according to any one of claims 1 to 7, in which the hydrogen peroxide solution used from the direct synthesis additionally contains volatile inorganic impurities and to remove at least some of these impurities, the hydrogen peroxide solution before feeding into the evaporator cascade and / or in contact with an ion exchanger after concentration.

10. The method according to any one of the preceding claims, in which the vapors are subjected to a thermal fractionation by condensation and / or distillation in step b).

11. The method according to claim 10, in which the vapors are fed laterally into a column for thermal fractionation, wherein a separation into a gaseous top product consisting essentially of water and a liquid bottom product essentially containing the hydrogen peroxide contained in the vapors , he follows.

12. The method according to claim 11, in which heat is withdrawn from the gaseous top product by contacting it with a heat exchanger and the extracted heat is used in a heat-consuming step of the work-up process or another process.

13. The method of claim 12, wherein the temperature of the top product is increased by contacting before contacting the heat exchanger.

14. The method according to any one of the preceding claims, in which an aqueous hydrogen peroxide solution having a hydrogen peroxide content of at least 30 wt .-% is obtained.

15. The method according to any one of the preceding claims, in which an aqueous hydrogen peroxide solution with an inorganic impurity content of at most 0.001 wt .-% is obtained.

16. A process for the preparation of aqueous hydrogen peroxide solutions which contain at most 0.001% by weight of inorganic impurities, in which

ii) subjecting the reaction mixture obtained in step i) to a workup as described in one of claims 1 to 15.