CA2328389C - Device and procedure for separating liquid products from a suspension produced by a fischer-tropsch reactor and in the presence of a diluent - Google Patents

Abstract

The invention relates to a method for separating liquid products from a suspension from a Fischer-Tropsch reactor and containing at least one Fischer-Tropsch catalyst, and a device for separating said liquid products comprising at least one means allowing extracting from a Fischer-Tropsch reactor, a part of a suspension containing at least one Fischer-Tropsch catalyst, at least one means for adding a diluent to all or part of the suspension extracted via the means, to minus a primary separation means provided with at least one means for bringing the diluted suspension to said at least one separation means, at least one means for collecting the separated liquid, and at least one means for collecting a more concentrated suspension, and at least one secondary separation means for separating and then collecting via at least one means a substantially purified liquid.

Description

DEVICE AND METHOD FOR SEPARATING LIQUID PRODUCTS
FROM A SUSPENSION FROM A FISCHER REACTOR-TROPSCH AND IN THE PRESENCE OF A DILUENT

The present invention relates to a device and a method for separation of a liquid phase from a suspension containing a Fischer-Tropsch.
Fischer-Tropsch catalysts can be operated in a Fischer-Multiphasic tropsch (slurry reactor), the catalyst being in suspension in liquid phase, also sometimes called slurry.

This liquid phase contains in particular a liquid sometimes called solvent slurry, which makes it possible to use said catalyst in suspension, as well as least one fraction of the products resulting from the Fischer-Tropsch synthesis reaction. The reactor Fischer-Tropsch multiphase most commonly consists of a column at bubble (bubble column reactor according to the English terminology).

PRIOR ART:

The Fischer-Tropsch synthesis process is a well-known industrial process for the production of essentially paraffinic hydrocarbons. In step resulting from, for example, isomerizing hydrocracking, these hydrocarbons can be converted into fuels (gas oil, kerosene), and / or lubricants.

In this process, hydrocarbons are produced by means of a conversion chemical synthesis gas, that is to say a gas comprising hydrogen of carbon monoxide and carbon dioxide), obtained from coal, naphtha or most often from natural gas.

The chemical conversion of synthesis gas into hydrocarbon by the process Fischer-Tropsch is carried out in the presence of a catalyst based on at least one metal of the group VIII. The catalysts employed can be of various kinds, but most often contain iron or cobalt mass or supported. The supports used are generally based on silica, alumina, or titanium oxide, optionally mixed with other oxides.

the Fischer-Tropsch synthesis is generally performed at temperatures between 200 C and 300 C and at pressures between 1 MPa and 8 MPa preferably between 2 MPa and 4 MPa.

2 The reactors used for these syntheses can be of several types. The Fischer-Tropsch catalyst can be arranged in fixed bed, or operated in bed driven. It is also possible to use a bubble column type reactor (bubble column reactor or siurry bubble column) in which the finely divided catalyst is suspension in a liquid phase to constitute a suspension also called siurry. -The synthesis gas is brought into contact with the liquid / solid mixture in specific hydrodynamic conditions in order to obtain productivities high in essentially paraffinic hydrocarbons. An internal exchanger and / or external to reactor allows to evacuate the calories generated by the exothermic reaction of Fischer-Tropsch synthesis. The presence of a liquid phase also favors the evacuation of these calories.

The Fischer-Tropsch synthesis facilities also include means separation of liquid hydrocarbons, on the one hand, of products gaseous residuals including inerts, light gaseous hydrocarbons, water formed in the reaction in significant amount, as well as the fraction of the gas of synthesis that did not reacted.

These products must be substantially completely separated from the catalyst Fischer-Tropsch, for example up to residual catalyst levels lower than a few dozen parts per million or even around 1 to a few parties by million (ppm) so that they can be used or processed in stages later.

This separation of products is delicate, complex and expensive because of service conditions (high pressures and high temperatures) and quantities often very significant amounts of fine catalyst particles, micron or submicron, present in the products.
This difficulty is further increased in the case of a reactor operated in the presence of gas and a suspension (slurry), such as for example a reactor type column to bubble, because of the very high concentration of fine particles of Fischer catalyst Tropsch in the suspension.

3 Typically it is indeed possible to have in a suspension (siurry) a amount of solid Fischer-Tropsch catalyst particles representing generally from 10% to 65% by weight relative to the suspension weight. These particles usually have a mean diameter of less than 100 microns, and contain significant amounts of very fine particles, or ultra fine particles, with a size less than 10 microns, or even submicron.
Several technical means are known that make it possible to carry out the separation liquid / solid: either in the reactor or in an outdoor slurry loop to the reactor.
These technical means are, however, proving very difficult to implement, and require significant investment when it comes to separating suspensions very concentrated, containing in particular up to 20 or 30% weight of solid, see more.
Thus, US-A-5,527,473 describes a method for carrying out reactions in a liquid-solid siurry through which passes continuously a gas ascendingly, and operated at high temperature and high pressure. of the items filters with openings between 0.5 and 100 microns are willing in the reactor and surrounded by the slurry suspension. They allow maintain the solid catalyst in the reactor while discharging the formed liquid.
The patent application EP-A-0 609 079 describes a method and an installation of slurry type to produce liquid and possibly gas from reagents gaseous. This installation includes a filtration means included in the reactor. These filters can be cleaned or unclogged, after interruption of the filtration, using a fluid flowing in the opposite direction (backfiush type system).

US-A-5,770,629 discloses a process for the synthesis of hydrocarbon slurry in which the slurry is sent to an area of disengagement from the gas and solid located in the reactor, then in a zone of disengagement and filtration situated at the outside of the reactor. Filtration means is disposed in contact with the siurry in this zone of external disengagement. This disengagement zone allows removing some of the gas and the solid from the suspension before filtration, and to increase the efficiency of filters by avoiding in particular their clogging.

4 The patent application WO 97/31693 claims a method of separating a liquid from a liquid slurry / gaslsolide. This method includes a degassing of the suspension, and the passage through a cross-flow filter (cross-to fiiter) located outside the reactor and which collects liquid and a siurry more concentrated in solid.

Patent application WO 98/27181 discloses a device for separating paraffin catalyst particles (waxes) formed in the reaction of Fischer-Tropsch synthesis. This device comprises a dynamic decanter and requires no pump.

SUMMARY OF THE INVENTION:

The invention relates to a device for separating liquid products from from a suspension containing a Fischer-Tropsch catalyst comprising:
= at least one first means for extracting a Fischer-Tropsch, a part of a suspension containing at least the Fischer-Tropsch = at least one second means for adding a diluent to all or part of the suspension extracted via the first means, = at least one primary separation means having at least one third means for bringing all or part of the diluted suspension to said means of primary separation, of at least a fourth means collect the separated liquid, and at least one fifth means for collect a more concentrated suspension, = at least one secondary separation means for separating then collect via at least one sixth means a substantially purified liquid, = at least one means for condensing the gas from the Fischer-Tropsch = at least one means for separating the aqueous phase, the phase organic liquid containing hydrocarbons, and possibly gas condensed, and recycling means in said reactor of said suspension more concentrated;
and in which the diluent is constituted by said liquid organic phase.
The invention also relates to a method using the device according to the invention, that is to say a process for separating liquid products from go of a suspension containing a Fischer-Tropsch catalyst, to which is added a diluent.

This process, in its most general form, includes at least following steps:
a1) take in a Fischer-Tropsch reactor, operating with at least a Fischer-Tropsch catalyst suspended in a liquid phase, a part of the suspension containing said catalyst, a3) condensing and separating a hydrocarbon fraction constituted by the compounds entrained by vapor pressure in the gaseous effluents of the Fischer-Tropsch reactor, b) mixing the suspension taken in step a1) with another hydrocarbon cut called diluent, (c) primary separation of at least part of the suspension diluted in step b), by means of at least one separation technique, in order to produce a liquid having a solid content of less than 5% by weight, and a more concentrated suspension, d) secondarily separating the liquid obtained in step c) and most of the residual solid contained in it, using at least one technical separation, and e) recycling in said reactor said more concentrated suspension, and wherein the diluent consists of the hydrocarbon compounds of step a3).

5a SUMMARY DESCRIPTION OF THE FIGURE

FIG. 1 schematically illustrates a separation device according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The subject of the present invention relates to a device and a method of separation of a suspension of slurry type containing a Fischer-Tropsch.
Said device and process ledits are more reliable, more efficient and represent a lower investment than the devices or methods of the prior art.

The method and the device according to the invention are particularly well adapted to use in combination with a Fischer-Tropsch type slurry reactor (That is, say in which a Fischer-Tropsch catalyst is suspended in a phase liquid), for example a bubble column reactor (bubbie slurry column).

Such bubble columns have been described for example in the patent US-A-5,252,613, as well as in patent applications EP-A-820,806 and E PA-823 470.
The method according to the invention comprises several steps, and in particular at least the next steps (ai) Sampling in a Fischer-Tropsch reactor, operating with at least one Fischer-Tropsch catalyst suspended in a liquid phase, part of the suspension containing said catalyst.
a2) optionally degassing of the suspension taken in step a1.
s b) mixing the suspension taken in step a1 or degassed in step a2 with another cut of hydrocarbons called diluent, c) Primary Separation of at Least One Part of the Diluted Suspension at Step b using at least one preferably chosen separation technique in the group consisting of: filtration, decantation, separation by hydrocyclone, and centrifugal separation, to produce a liquid having a solid content of less than 5% by weight, preferably less than 2% by weight, more preferably less than 1% by weight, and is very preferably less than 0.5% weight ..

d) Secondary Separation of the Liquid Obtained in Step c and the Essence of solid residual content it contains by means of at least one separation technique, preferably selected from the group consisting of: filtration, separation Centrifugal and magnetic separation. This step provides a liquid essentially free of catalyst, containing only traces said catalyst, a few tens of ppm, or even only a few ppm or of the order of 1 ppm (parts per million expressed by weight).

More preferably, the separation technique of step c is a decantation or filtration, and very preferably decantation.

More preferably, the separation technique of step d is a filtration or magnetic separation, and very preferably filtration.

The diluent used in step b consists of any hydrocarbon cutting of low or medium boiling point. For example, a C3 cut will be used.
or a C4 section or a C4-C5 section, that is to say a section containing respectively three-carbon hydrocarbons, four carbon atoms, carbons, or 4 to 5 carbon atoms, or a naphtha cut, or gas oil or a mixed of naphtha and gas oil.

Preferably, the diluent used in step b is a broad cut hydrocarbons.
According to a preferred variant of the process according to the invention, a wide cut hydrocarbons, most of which are included in the group consisting of C3 to C22 hydrocarbons (i.e. hydrocarbons having from 3 to 22 atoms of carbon) will be used as a diluent.

Indeed, such cuts can be easily prepared from cuts oil, or Fischer-Tropsch effluents. Preferably at least 80% by weight, and more preferably at least 90% by weight of the hydrocarbons of the cut used as diluent are C3 to C22 hydrocarbons. More preferred, said cut consists essentially of paraffins. Said cut contains very preferred way at least 60% by weight of paraffins, and still more more preferred at least 70% by weight of paraffins.

For example, a section comprising the compounds driven by vapor in the gaseous effluent of a Fischer-Tropsch reactor can be easily condensed. This cut is an effective diluent for the process according to the invention.
During this condensation, the water entrained in the form of steam is also condensed, but the aqueous phase and the organic phase containing the hydrocarbons can be easily separated, for example by racking at levels different.
Such a diluent can thus be obtained without splitting the products of the reaction of Fischer-Tropsch synthesis.

This section, obtained by simple condensation of the gaseous effluents, can advantageously be reheated and stabilized by elimination of the compounds too much volatile, before being mixed with the suspension to be separated. The mixture of suspension taken in step a1 with this diluent makes it possible to obtain a suspension diluted product having a reduced viscosity and solid concentration, therefore more easily manipulated. Moreover, it appears unexpectedly that use of the process according to the invention makes it possible to significantly improve the efficiency of the separation of the liquid products contained in the suspension.

The Fischer-Tropsch catalyst or the concentrated suspension recovered at the stages c and d can be returned to the Fischer-Tropsch reactor, eventually after dilution or resuspension in a hydrocarbon fraction. However, when the Recovered Fischer-Tropsch catalyst has a too small particle size, said The catalyst is not reintroduced into said reactor.

In general, the amount of diluent that can be used in the process according to the invention is not limited. Preferably have used a quantity of diluent at least about 5% by volume, or about 10% by volume report the suspension volume taken in step a1. More preferably, the The suspension is mixed with a volume of diluent which represents between 20% and 300%
in volume relative to the volume of suspension taken in step al.

Any separation device known to those skilled in the art may be used for is steps c and d of the method according to the invention.

Among these means we can mention among others:
= filtration, frontal or tangential, on filter media such as of the sintered or fibrous or woven porous materials, of metal type, ceramic or Polymer.
= settling = centrifugal separation by means of hydrocyclones or centrifuge.
= magnetic separation Among the separation means that can be used in the process according to the invention, can be mentioned, for example, the separation means that the decanters dynamic or cross-flow filtration systems manufactured in particular by the MOTT company or filtration systems comprising a mesh structure type) tel than those manufactured by PALL FILTER Corp., or 30 filtration described in the Kirk-Othmer encyclopedia (Kirk-Othmer Encyclopedia of Chemical Technology (1993), Vol. 10, pp. 841-847).

It is also possible to use the hydrocyclones described in the encyclopedia Uflmann (Ullmann's Encyclopedia of Industrial Chemistry, 1988, Fifth edition, volume B2, pages 11-19 to 11-23).

The separation device according to the invention comprises in its most general:
= At least one means (4) for extracting a Fischer-Tropsch reactor, part of a suspension, containing at least one Fischer-Tropsch = At least one means (27) for adding a diluent to all or part of the suspension extracted via the means (s) (4), = At least one primary separation means (29) provided with at least one means (28) for bringing all or part of the diluted suspension to said or said means for separating, at least one means (33) collect the separated liquid, and at least one means (30) for collect a more concentrated suspension, = At least one secondary separation means (34) for separating then collect via at least one means (36) a substantially purified liquid, that is, at-say essentially free of catalyst or containing only traces said catalyst.

According to a particular form, the device according to the invention comprises in in addition to least a means for separating (6) the gas contained in the suspension leaving the Fischer-Tropsch reactor by means (4).

Said means (s) is (are) preferably located (s) between the reactor Fischer-Tropsch (2) and the primary means (s) (29).

In this particular form of the device, the separated gas is preferably collected via at least one means (5) and the liquid from the column and degassed is of preferably collected via at least one means (7), and preferably sent to the OR
the means (s) of primary separation, preferably via at least one means (28), and of preferably after adding thinner via the means (27).

According to another preferred variant of this form of the device according to the invention, the device comprises two particular means for sending the suspension leaving the Fischer-Tropsch reactor by means (4) towards the separation Primary (29):

= A pipe (9) that opens into the pipe (28) after adding thinner via the driving (27), A line (8) in which no diluent is added.

This variant makes it possible to add a diluent only to a fraction of the liquid outgoing Fischer-Tropsch reactor.

it is also possible, without departing from the scope of this invention, to use the previous preferred variant by degassing the liquid exiting the Fischer-Tropsch by means for example of a degassing balloon or any other known means of the skilled person. -According to another more preferred variant, the device according to the invention includes in in addition to a pump (31) for returning to the Fischer-Tropsch reactor (2) the concentrated suspension from the primary separation (29).

The synthesis gas is fed via the pipe (1) into the Fischer-2o Tropsch (2) of bubble-column type, filled in part by means of a hydrocarbon fraction in which a Fischer-Tropsch catalyst has been set suspension (slurry).

The Fischer-Tropsch reactor is operated under the operating conditions usual of the Fischer-Tropsch synthesis, for example at a temperature of 235 C and a pressure of 2.5 MPa to synthesize liquid products essentially paraffinic.

This part of the device is an example of a reaction section to which he 3o is possible to connect the separation device according to the invention.
Any other Fischer-Tropsch reaction section known to those skilled in the art, such as by example a bubble column described in patent applications EP-A-820 806 and EP-A-823 470 or US-A-5,252,613 may be readily substituted for the section reaction presented in Figure 1, with some adjustments within reach of the skilled person.

The gas formed or unreacted during the reaction is separated into the disengagement zone above the liquid level in the reactor Fischer-Tropsch (2), then leaves the Fischer-Tropsch reactor by driving (3). A
fraction of the suspension is sent via line (4) to a balloon degassing

(6), in order to separate-on the one hand the gas that is sent via the pipe (5) to the driving (3) on the other hand the degassed suspension which is sent to the clarifier (29) via the driving (7).

It is possible to add a diluent to all or part of the suspension degassed. Thus, the pipe (7) can optionally be separated into two pipes (8) and (9), only the liquid arriving via the pipe (9) and then diluted by a chopped off hydrocarbon coming via the pipe (27), before reaching the decanter (29) via the driving (28). The suspension arriving via the pipe (8) is introduced directly in decanter (without dilution) at a level of preference lower than the level introducing the diluted suspension (28).

However, it is possible to send the entire degassed suspension of the line

(7) by line (9), so that it is fully diluted before arriving to the decanter.
Non-condensed gas from the Fischer-Tropsch reactor via lines (3) and (5) is mixed, then passes via the line (10) into a refrigerant (11) which allows to lower the temperature, for example at 100 C under 2.45 MPa of total pressure. Via the line (12) the cooled mixture is introduced into a separation flask (13) allowing to collect the gas in the line (16), an aqueous phase containing essentially from water via line (14) and an organic phase containing hydrocarbons liquids via the line (15).
The non-condensed gas can again possibly be sent to a refrigerant (17) in order to cool it to 40 C under 2.40 MPa, then via the line (18) in a 3o new separation stage (19) for collecting again a aqueous phase in (20) and a hydrocarbon liquid phase in (21). The latter can, when she is present, be combined with the liquid recovered via line (15), then sent by the pipe (22) to the heater (23), in order to warm the entire preference to temperature of the Fischer-Tropsch reaction (for example 235 C), and then via line (24) to a separator (25). This separation makes it possible to obtain gas (26) and liquid stabilized diluent that is sent to all or part of the suspension via the line (27).

The decantation in the decanter (29) of the undiluted suspension (8) and the diluted suspension (28) provides a more concentrated suspension via the line (30) which is recycled to the Fischer-Tropsch reactor via line (30), by means of pump (31) and line (32). At the top of the decanter a liquid is obtained (33) which contains a a small amount of Fischer-Tropsch catalyst, easily separated by means of a separation technique known to those skilled in the art (for example: filtration, centrifugation, magnetic separation) in the separator (34). Liquid essentially free of catalyst or containing only traces of said catalyst, a few tens of ppm, or even only a few ppm or of the order of 1 ppm is obtained continuously in the line (36). Fischer-Tropsch catalyst is recovered via the outlet (35), continuously or discontinuously according to the technique separation employee.

Another preferred device according to the invention consists for example in substitute the decanter (29) either by filtration or separation equipment centrifugal, either by a hydrocyclone.

In summary, the device according to the invention therefore consists in separating products from a suspension containing a Fischer-Tropsch catalyst. This device in its most general form includes:

= At least one means (4) for extracting a Fischer-Tropsch reactor, part of a suspension, containing at least one Fischer-Tropsch = At least one means (27) for adding a diluent to all or part of the suspension extracted via the means (s) (4), = At least one primary separation means (29) provided with at least one means (28) for bringing all or part of the diluted suspension to said or said means for separating, at least one means (33) collect the separated liquid, and at least one means (30) for collect a more concentrated suspension, = At least one secondary separation means (34) for separating then collecting via at least one means (36) a substantially purified liquid.

The device according to the invention may optionally further comprise less means for separating (6) the gas contained in the suspension leaving the reactor Fischer-Tropsch by means (4). Said means is preferably located between reactor Fischer-Tropsch (2) and the primary separation means (29).

According to a variant, the device according to the invention can optionally understand 1o two means for sending the suspension leaving the Fischer-Tropsch by means (4) to primary separation (29):
= A pipe (9) that opens into the pipe (28) after adding thinner via the pipe (27), A line (8) in which no diluent is added.
Preferably, the device further comprises a pump (31) for return to the Fischer-Tropsch reactor (2) the concentrated suspension of the primary separation (29).

The invention also relates to a separation process. In summary, said process is a process for separating liquid products from a suspension containing a Fischer-Tropsch catalyst, said process comprising at least following steps :
a1) Sampling in a Fischer-Tropsch reactor, operating with at least one Fischer-Tropsch catalyst suspended in a liquid phase, part of the suspension containing said catalyst.
b) Mixing the suspension taken in step a1 with another cut of hydrocarbons called diluent.
c) Primary Separation of at Least One Part of the Diluted Suspension at Step b by at least one separation technique, to produce a liquid having a solid content of less than 5% by weight.
d) Secondary Separation of the Liquid Obtained in Step c and the Essence of solid residual content it contains, using at least one separation technique.

The method according to the invention may optionally further comprise before step b), a step a2 degassing the suspension taken in step a1.
Of preferably, the diluent used in the process according to the invention is a cut hydrocarbon derived from Fischer-Tropsch synthesis. More preferably, at less than 80% by weight of the hydrocarbons of the cut used as diluent are of the C3 to C22 hydrocarbons.

In a very preferred variant of the process according to the invention, the diluent is constituted by a section comprising the compounds formed by tension of steam in the gaseous effluents of the Fischer-Tropsch reactor, and then condensed. After condensation, said cut can optionally be warmed and stabilized by elimination of too volatile compounds.

In another variant of the method according to the invention, the suspension can possibly be split into two fractions of which only one is mixed with the diluent.

Claims (13)

1. Device for separating liquid products from a suspension containing a Fischer-Tropsch catalyst comprising:
.cndot. at least one first means (4) for extracting from a reactor Fischer Tropsch, a part of a suspension containing at least the Fischer-Tropsch .cndot. at least one second means (27) for adding a diluent to any or part of the suspension extracted via the first means (4), .cndot. at least one primary separation means (29) provided with at least one third means (28) for bringing all or part of the suspension diluted to said primary separation means, at least a fourth means (33) for collecting the separated liquid, and at least one fifth means (30) for collecting a more concentrated suspension, .cndot. at least one secondary separation means (34) for separating then collect via at least one sixth means (36) a liquid essentially sleek, .cndot. at least one means (11) for condensing the gas from the Fischer-Tropsch .cndot. at least one means (13) for separating the aqueous phase, the phase organic liquid containing hydrocarbons, and possibly gas condensed, and .cndot. recycling means in said reactor of said suspension more concentrated;
.cndot. and wherein the diluent is constituted by said organic phase liquid. The device of claim 1, further comprising at least one means (23, 25) for stabilizing said liquid organic phase. 3. Device according to claim 1 or 2, further comprising at least one seventh means for separating (6) the gas contained in the outgoing suspension of the Fischer-Tropsch reactor by the first means (4). 4. Device according to claim 3, wherein said seventh means is between the Fischer-Tropsch reactor (2) and the primary separation means (29). 5. Device according to claim 1 or 2, comprising two pipes to send the suspension leaving the Fischer-Tropsch reactor by the first means (4) to the primary separation means (29), with:
.cndot. a first pipe (9) which opens into the separation means primary (29) after addition of thinner, and .cndot. a second pipe (8) which opens into the separating means primary (29) and in which no diluent is added. 6. Device according to claim 1 or 2, further comprising a pump (31) for returning to the Fischer-Tropsch reactor (2) the suspension concentrate from the primary separation means (29). 7. Process for separating liquid products from a suspension containing a Fischer-Tropsch catalyst, said process comprising at least the following steps:
a1) take in a Fischer-Tropsch reactor, operating with at least a Fischer-Tropsch catalyst suspended in a liquid phase, a part of the suspension containing said catalyst, a3) condensing and separating a hydrocarbon fraction constituted by the compounds entrained by vapor pressure in the gaseous effluents of the Fischer-Tropsch reactor, b) mixing the suspension taken in step a1) with another hydrocarbon cut called diluent, (c) primary separation of at least part of the suspension diluted in step b), by means of at least one separation technique, in order to produce a liquid having a solid content of less than 5% by weight, and a more concentrated suspension, d) secondarily separating the liquid obtained in step c) and most of the residual solid contained in it, using at least one technical separation, and e) recycling in said reactor said more concentrated suspension, and wherein the diluent consists of the hydrocarbon compounds of step a3). The method of claim 7, further comprising before step b), a step a2) degassing the suspension taken in step a1). The method of claim 7 or 8, wherein at least 80%
The weight of the hydrocarbons in the cut used as diluent are C3 to C22 hydrocarbons. The method of claim 7, wherein the cut obtained is reheated and stabilized by removal of the highly volatile compounds. The method of any one of claims 7 to 10, wherein step c) is carried out by filtration, decantation, separation by hydrocyclone or centrifugal separation. The method of any one of claims 7 to 11, wherein step d) is carried out by filtration, centrifugal separation or separation magnetic. The method of any one of claims 7 to 12, wherein the suspension taken in step a1) is separated into two fractions of which only one is mixed in step b) with the diluent.