PROCESS FOR PREPARING A MESOPOROUS ALUMINA BY SHAPING A MIXTURE OF DRIED POWDER AND FILTER CAKE

Abstract

The present invention relates to the preparation of a mesoporous alumina. In particular, the present invention relates to an amorphous mesoporous alumina having a specific pore distribution and in particular a high pore volume compared to the prior art. More specifically, the present invention relates to a process for the preparation of an alumina comprising the preparation by precipitation under very specific operating conditions, of a boehmite gel or boehmite having a very high dispersibility index and preferably greater than 70%, said process being characterized by the implementation of a specific shaping step in which a mixture of at least a part of atomized boehmite gel powder and at least a part of the boehmite gel filter cake is introduced into the extrusion kneading step, making it possible to reduce the energy consumption of said process.

Description

Title of the invention: PROCESS FOR PREPARING A MESOPOROUS ALUMINA BY SHAPING A MIXTURE OF DRIED POWDER AND FILTER CAKE Technical field

[0001] The present invention relates to the preparation of a mesoporous alumina. In particular, the present invention relates to an amorphous mesoporous alumina having a specific pore distribution and in particular a high pore volume compared to the prior art. More specifically, the present invention relates to a process for preparing an alumina comprising the preparation by precipitation under very specific operating conditions, of a boehmite gel or boehmite having a very high dispersibility index and preferably greater than 70%, said process being characterized by the implementation of a specific shaping step in which a mixture of at least a part of atomized boehmite gel powder and at least a part of the boehmite gel filter cake is introduced into the extrusion kneading step, making it possible to reduce the energy consumption of said process.

[0002] The alumina prepared according to the invention, due to its interesting properties, particularly in terms of porous distribution, can be used as a catalyst support in all refining processes as well as an adsorbent. PREVIOUS ART

[0003] Numerous patents describe the preparation of alumina. Methods for preparing alumina gels by coprecipitation of an acidic source and a basic source, at least one of said sources comprising aluminum, to obtain a boehmite gel are known from the prior art.

[0004] Generally, the preparation of alumina consists of filtering, washing, and drying the boehmite gel obtained until a powder is obtained before adding a certain quantity of water to obtain a paste which can be shaped and then calcined to obtain the alumina.

[0005] For example, patent application FR3022238 A1 describes the preparation of a mesoporous alumina from a specific boehmite gel having a high dispersibility rate, prepared according to a process comprising the preparation of a boehmite gel by at least one precipitation step in which at least 40% by weight of alumina in A12O3 equivalent relative to the total quantity of alumina formed at at the end of said gel preparation process, are formed from the first precipitation step, followed by a heat treatment, filtration and washing step followed by a drying step carried out at a temperature between 20 and 50°C and for a period between 1 day and 3 weeks or by atomization then shaping according to methods known to those skilled in the art.

[0006] The only example according to the invention describes an alumina having a total pore volume of 0.69 ml / g.

[0007] For example, US patent 6,589,908 describes a process for preparing an alumina characterized by an absence of macropores, less than 5% of the total pore volume consisting of pores with a diameter greater than 35 nm, a high pore volume greater than 0.8 ml / g, and a bimodal pore distribution in which the two modes are separated by 1 to 20 nm and the primary pore mode is larger than the median pore diameter.

[0008] For this purpose, the described method implements two stages of precipitation of alumina precursors under well-controlled temperature, pH and flow rate conditions. The first stage operates at a temperature between 25 and 60°C, and at a pH between 3 and 10. The suspension is then heated to a temperature between 50 and 90°C. Reagents are again added to the suspension, which is then washed, dried, shaped and calcined to form a catalyst support.

[0009] No information is given on the dispersibility of the boehmite gel, nor on a particular selection of the rate of progress of the first stage, which varies according to the examples given between 14 and 34%.

[0010] Furthermore, US patent 7,790,652 describes the preparation by precipitation of an alumina support having a very specific porous distribution, which can be used as a catalyst support in a process for hydroconversion of heavy hydrocarbon feedstocks.

[0011] The alumina support is prepared according to a method comprising a first step of forming an alumina dispersion by mixing, in a controlled manner, a first alkaline aqueous solution and a first acidic aqueous solution, at least one of said acidic and basic solutions, or both comprising an aluminum compound. The acidic and basic solutions are mixed in proportions such that the pH of the resulting dispersion is between 8 and 11. The acidic and basic solutions are also mixed in quantities making it possible to obtain a dispersion containing the desired quantity of alumina, in particular, the first step makes it possible to obtain 25 to 35% by weight of alumina relative to the total quantity of alumina formed at the end of the two precipitation steps. The first step operates at a temperature between 20 and 40°C. When the desired quantity of alumina is formed, the temperature of the suspension is increased to a temperature between 。between 45 and 70°C, and then the heated suspension is then subjected to a second precipitation step by bringing said suspension into contact with a second alkaline aqueous solution and a second acidic aqueous solution, at least one of the two solutions or both comprising an aluminum compound. Similarly, the pH is adjusted between 8 and 10.5 by the proportions of the acid and basic solutions added and the remaining quantity of alumina to be formed in the second step is provided by the quantities of the second acid and basic solutions added. The second step operates at a temperature between 20 and 40°C. The boehmite gel thus formed comprises at least 95% boehmite. The dispersibility of the boehmite gel thus obtained is not mentioned.The boehmite gel is then filtered, washed and optionally dried according to methods known to those skilled in the art, to produce an alumina powder which is then shaped according to methods known to those skilled in the art, then calcined to produce the final alumina support.

[0012] The first precipitation step of the preparation process of US patent 7,790,652 is limited to an alumina production of between 25 and 35% by weight, a higher alumina production at the end of the first step not allowing optimal filtration of the gel obtained. The material obtained is an alumina-based support having a high specific surface area, a total pore volume greater than or equal to 0.75 ml / g and a median pore diameter of between 10 and 14 nm. Less than 5% of the pore volume is made up of pores with a median mesoporous diameter greater than 21 nm.

[0013] The processes for shaping alumina gels generally contain at least one drying step. The drying techniques conventionally used are listed in engineering techniques. In the case of a boehmite gel or boehmite obtained by precipitation, the cake is atomized after resuspension of the cake to obtain a powder.

[0014] For drying colloidal solutions, we will cite for example spray drying which is the most classically used. These dryers are composed of a generally vertical cylindrical-conical tower. This chamber is connected to the liquid product supply circuit via a spraying member and a hot air circuit. During this drying, at atmospheric pressure the temperatures are generally between 160°C and 900°C (preferably around 300°C at the top of the dryer). We will also cite fluidized bed dryers and cylinder dryers. For pastes or cakes we will cite cylinder dryers for drying a cake or a suspension, heating mixers or even flash and spin flash dryers.

[0015] As regards the shaping by direct drying of the filter cake, we can cite application FR3067021A1 which describes a process for preparing a mesoporous alumina from a dispersible gel prepared continuously and without drying. pushed.

[0016] It should be noted that the operability of this type of unit operation is difficult in terms of controlling loss on ignition (LOI) as well as in terms of homogeneity of the boehmite gel after drying.

[0017] Surprisingly, it has been demonstrated in the present invention that a hybrid process mixing at the input of a shaping step by extrusion mixing, a specific portion of undried filter cake (loss on ignition noted PAF1) with a specific portion of powder dried preferably by atomization (loss on ignition noted PAF2) advantageously makes it possible to overcome the control problems mentioned above.

[0018] An advantage of the present invention relates to a hybrid process in which a mixture of at least a portion of atomized powder and at least a portion of the filter cake is shaped, which makes it possible to achieve a substantial reduction in energy consumption mainly linked to evaporation in the dryer compared to the conventional shaping scheme where the entire filter cake is redispersed and dried by atomization.

[0019] An objective of the present invention is therefore to provide an improved process for producing, from a boehmite gel and a series of specific steps, an amorphous mesoporous alumina with an energy gain and a reduced environmental footprint. Summary of the invention

[0020] The subject of the present invention is a process for preparing a mesoporous alumina having a total pore volume greater than 0.75 ml / g, said process comprising at least the following steps:

[0021] a) at least a first step of precipitation of a boehmite gel, in an aqueous reaction medium, of at least one basic precursor chosen from sodium aluminate, potassium aluminate, ammonia, sodium hydroxide and potassium hydroxide and of at least one acid precursor chosen from aluminum sulfate, aluminum chloride, aluminum nitrate, sulfuric acid, hydrochloric acid, and nitric acid, in which at least one of the basic or acid precursors comprises aluminum, the relative flow rate of the acid and basic precursors is chosen so as to obtain a pH of the reaction medium of between 8.5 and 10.5 and the flow rate of the acid and basic precursor(s) containing aluminum is adjusted so as to obtain a progress rate of said first step of between 20 and 100%,the rate of progress being defined as being the proportion of alumina formed in A12O3 equivalent during said first precipitation step compared to the total quantity of alumina formed at the end of the preci- step(s), pitation, said first precipitation step operating at a temperature of between 10 and 50°C, and for a duration of between 2 minutes and 30 minutes,

[0022] b) a step of heat treatment of the suspension obtained at the end of step a) at a temperature of between 50 and 200°C for a duration of between 30 minutes and 5 hours allowing a boehmite gel to be obtained,

[0023] c) a step of filtration of the suspension obtained at the end of step b) of heat treatment, followed by at least one step of washing the boehmite gel obtained, to obtain a filter cake, said filter cake being separated into two streams, a first stream representing 24% by weight to 90% by weight of the entire filter cake from step c) and feeding a step d) of spray drying and a second stream of filter cake directly feeding a step e) of shaping by extrusion kneading,

[0024] d) a step of spray drying said first stream representing 24% by weight to 90% by weight of the entire filter cake from step c) to obtain a dried filter cake or a dried powder,

[0025] e) a step of shaping by kneading-extrusion of the dried filter cake or a dried powder from step d) mixed with said second flow of filter cake obtained in step c), the dried filter cake or a dried powder from step d) representing 5 to 65% by weight of the mixture entering said step e), and said second flow of filter cake obtained in step c) representing 35 to 95% by weight of the mixture entering said step e), to obtain the raw material or boehmite paste,

[0026] said step e) being carried out with a total acid level, expressed as a percentage by weight relative to the mass of dried gel introduced in step e) of between 0 and 4% (defined relative to the quantity of boehmite expressed as A12O3) and a neutralization level expressed as a percentage by weight of base relative to the quantity of acid introduced in said step e) of between 0 and 200%,

[0027] f) a step of heat treatment of the raw material obtained at the end of step e) at a temperature between 500 and 1000°C, in the presence or absence of an air flow containing up to 60% by volume of water.

[0028] An advantage of the present invention is to provide a process for producing an amorphous mesoporous alumina from a boehmite gel and a sequence of specific steps, with a reduced environmental footprint, due to the reduction in energy consumption and thus the reduction in CO2 emissions and the quantity of water used, generated by the reduction in the flow rate used in the spray drying step.

[0029] Another advantage of the present invention is to provide a method for producing an amorphous mesoporous alumina from a boehmite gel and an en- chain of specific steps, allowing to limit the size of the drying unit implemented and preferably of the atomization unit. Characterization techniques.

[0030] The definitions and characterization techniques below are given for the entire description.

[0031] The dispersibility index of the alumina or boehmite gels obtained is defined as the percentage by weight of peptized boehmite gel that can be dispersed by centrifugation in a polypropylene tube at 3600G for 10 min.

[0032] Dispersion is measured by dispersing 10% boehmite or boehmite gel in a water suspension also containing 10% nitric acid relative to the mass of boehmite. The suspension is then centrifuged at 3600G for 10 min. The collected sediments are dried at 100°C overnight and then weighed.

[0033] The dispersibility index, noted ID, is obtained by the following calculation: ID(%)=100%-mass of dried sediments(%).

[0034] The loss on ignition of a PAF sample is obtained by the difference between the mass of the sample before and after passing through the muffle furnace for 3 h at 1000°C.

[0035] Within the meaning of the present invention, the various embodiments presented can be used alone or in combination with each other, without limitation of combination.

[0036] In the sense of the present invention, the different parameter ranges for a given step such as pressure ranges and temperature ranges may be used alone or in combination. For example, in the sense of the present invention, a preferred range of pressure values ​​may be combined with a more preferred range of temperature values.

[0037] In the remainder of the text, the expressions "between ... and..." and "between .... and ..." are equivalent and mean that the limit values ​​of the interval are included in the range of values ​​described. If this were not the case and the limit values ​​were not included in the range described, such precision will be provided by the present invention.

[0038] In the present description, the expression "greater than..." is understood as strictly greater than, and symbolized by the sign ">", and the expression "less than" as strictly less than, and symbolized by the sign "<". Detailed description of the invention

[0039] According to the invention, said preparation process comprises at least a first step a) of precipitation of a boehmite gel, in an aqueous reaction medium, of at least one basic precursor chosen from sodium aluminate, potassium aluminate, ammonia, sodium hydroxide and potassium hydroxide and at least one basic precursor chosen from sodium aluminate, potassium aluminate, ammonia, sodium hydroxide and potassium hydroxide and at least one basic precursor chosen from sodium aluminate, potassium aluminate, ammonia, sodium hydroxide and potassium hydroxide and at least one basic precursor chosen from sodium aluminate, potassium aluminate, potassium hydroxide, sodium hydroxide ... at least one acid precursor selected from aluminum sulfate, aluminum chloride, aluminum nitrate, sulfuric acid, hydrochloric acid, and nitric acid, wherein at least one of the basic or acid precursors comprises aluminum, the relative flow rate of the acid and basic precursors is chosen so as to obtain a pH of the reaction medium of between 8.5 and 10.5 and the flow rate of the acid and basic precursor(s) containing aluminum is adjusted so as to obtain a progress rate of the first stage of between 20 and 100%, the progress rate being defined as being the proportion of alumina formed in A12O3 equivalent during said precipitation stage a) relative to the total quantity of alumina formed at the end of the precipitation stage(s) and more generally at the end of the stages of preparation of the boehmite gel, said stage a) operating at a temperature of between 10 and 50 °C,and for a duration of between 2 minutes and 30 minutes.

[0040] Generally speaking, the term "progress rate" of the nth precipitation step means the percentage of alumina formed in A12O3 equivalent in said nth step, relative to the total quantity of alumina formed at the end of all the precipitation steps and more generally at the end of the steps of preparing the boehmite gel.

[0041] In the case where the progress rate of said precipitation step a) is 100%, said precipitation step a) generally makes it possible to obtain an alumina suspension having an A12O3 concentration of between 20 and 100 g / L, preferably between 20 and 80 g / l, more preferably between 20 and 50 g / l. Step a) of precipitation

[0042] The mixing in the aqueous reaction medium of at least one basic precursor and at least one acid precursor requires either that at least the basic precursor or the acid precursor comprises aluminum, or that both the basic and acid precursors comprise aluminum.

[0043] The basic precursors comprising aluminum are sodium aluminate and potassium aluminate. The preferred basic precursor is sodium aluminate.

[0044] The acid precursors comprising aluminum are aluminum sulfate, aluminum chloride and aluminum nitrate. The preferred acid precursor is aluminum sulfate.

[0045] Preferably, the basic and acidic precursor(s) are added in said first precipitation step a) in aqueous solutions.

[0046] Preferably, the reaction medium is water.

[0047] Preferably, said step a) operates with stirring, in a stirred reactor.

[0048] Preferably, said step a) is carried out in the absence of organic additive.

[0049] The acid and basic precursors, whether they contain aluminum or not, are mixed, preferably in solution, in the aqueous reaction medium, in proportions such that the pH of the resulting suspension is between 8.5 and 10.5.

[0050] The precipitation of boehmite gel in step a) also boehmite is obtained by controlling the pH. During the precipitation in step a), a suspension of boehmite gel is formed.

[0051] In accordance with the invention, it is the relative flow rate of the acid and basic precursors, whether they contain aluminum or not, which is chosen so as to obtain a pH of the reaction medium of between 8.5 and 10.5.

[0052] In the preferred case where the basic and acid precursors are respectively sodium aluminate and aluminum sulfate, the mass ratio of said basic precursor to said acid precursor is advantageously between 1.6 and 2.05.

[0053] For the other basic and acid precursors, whether they contain aluminum or not, the base / acid mass ratios are established by a curve of neutralization of the base by the acid. Such a curve is easily obtained by a person skilled in the art.

[0054] Preferably, said precipitation step a) is carried out at a pH between 8.5 and 10 and very preferably between 8.7 and 9.9.

[0055] The acid and basic precursors are also mixed in quantities making it possible to obtain a suspension containing the desired quantity of alumina, depending on the final alumina concentration to be achieved. In particular, said step a) makes it possible to obtain 20 to 100% by weight of alumina in A12O3 equivalent relative to the total quantity of alumina formed at the end of the precipitation step(s). In accordance with the invention, it is the flow rate of the acid and basic precursor(s) containing aluminum which is adjusted so as to obtain a progress rate of the first step of between 20 and 100%.

[0056] Preferably, the progress rate of said precipitation step a) is between 25 and 99%, preferably between 30 and 90% and more preferably between 30 and 85%. In the case where the progress rate obtained at the end of precipitation step a) is less than 100%, a second precipitation step is necessary so as to increase the quantity of alumina formed. In the case where a second precipitation step is implemented, the progress rate is defined as being the proportion of alumina formed in A12O3 equivalent during said precipitation step a) relative to the total quantity of alumina formed at the end of the two precipitation steps of the preparation process according to the invention and more generally at the end of the steps of preparing the boehmite gel.

[0057] Thus, depending on the alumina concentration targeted at the end of the precipitation step(s), preferably between 20 and 100 g / l, the quantities of aluminium to be provided by the acid and / or basic precursors are calculated and the flow rate of the precursors is adjusted according to the concentration of said precursors in aluminum added, the amount of water added to the reaction medium and the rate of progress required for the precipitation step(s).

[0058] The flow rates of the acid and / or basic precursor(s) containing aluminium depend on the size of the reactor used and thus on the quantity of water added to the reaction medium.

[0059] Preferably, said precipitation step a) is carried out at a temperature between 10 and 45°C, preferably between 15 and 45°C, more preferably between 20 and 45°C and very preferably between 20 and 40°C.

[0060] It is important that said precipitation step a) operates at low temperature. In the case where said preparation process according to the invention comprises two precipitation steps, precipitation step a) is advantageously carried out at a temperature lower than the temperature of the second precipitation step.

[0061] Preferably, said precipitation step a) is carried out for a duration of between 5 and 20 minutes, and preferably 5 to 15 minutes. Step b) heat treatment

[0062] According to the invention, said preparation process comprises a step b) of heat treatment of the suspension obtained at the end of step a) of precipitation, said heat treatment step operating at a temperature between 50 and 200°C for a duration between 30 minutes and 5 hours, to obtain the boehmite gel.

[0063] Preferably, said heat treatment step b) is a ripening step.

[0064] Preferably, said heat treatment step b) operates at a temperature between 65 and 150°C, preferably between 65 and 130°C, preferably between 70 and 110°C, very preferably between 70 and 95°C.

[0065] Preferably, said heat treatment step b) is carried out for a duration of between 40 minutes and 5 hours, preferably between 40 minutes and 3 hours and more preferably between 45 minutes and 2 hours. Second optional precipitation step

[0066] According to a preferred embodiment, in the case where the progress rate obtained at the end of precipitation step a) is less than 100%, said preparation method preferably comprises a second precipitation step a') after the first precipitation step.

[0067] Said second precipitation step makes it possible to increase the proportion of alumina produced.

[0068] Said second precipitation step a') is advantageously implemented between said first precipitation step a) and heat treatment step b).

[0069] In the case where a second precipitation step is implemented, a step of heating of the suspension obtained at the end of precipitation step a) is advantageously carried out between the two precipitation steps a) and a').

[0070] Preferably, said step of heating the suspension obtained at the end of step a), carried out between said step a) and the second precipitation step a') operates at a temperature between 20 and 90°C, preferably between 30 and 80°C, preferably between 30 and 70°C and very preferably between 40 and 65°C.

[0071] Preferably, said heating step is carried out for a duration of between 7 and 45 minutes and preferably between 7 and 35 minutes.

[0072] Said heating step is advantageously implemented according to all the heating methods known to those skilled in the art.

[0073] According to said preferred embodiment, said preparation method comprises a second step of precipitation of the suspension obtained at the end of the heating step, said second step operating by adding to said suspension at least one basic precursor chosen from sodium aluminate, potassium aluminate, ammonia, sodium hydroxide and potassium hydroxide and at least one acid precursor chosen from aluminum sulfate, aluminum chloride, aluminum nitrate, sulfuric acid, hydrochloric acid, and nitric acid, in which at least one of the basic or acid precursors comprises aluminum, the relative flow rate of the acid and basic precursors is chosen so as to obtain a pH of the reaction medium of between 8.5 and 10.5 and the flow rate of the acid and basic precursor(s) containing aluminum is adjusted so as to obtain a progress rate of the second step of between 0 and 80%,the rate of progress being defined as being the proportion of alumina formed in A12O3 equivalent during said second precipitation step relative to the total quantity of alumina formed at the end of the two precipitation steps, more generally at the end of the steps of preparation of the boehmite gel and preferably at the end of step a') of the preparation process according to the invention, said step operating at a temperature of between 40 and 90°C, and for a duration of between 2 minutes and 50 minutes.

[0074] As in the first precipitation step a), the addition to the heated suspension of at least one basic precursor and at least one acidic precursor requires either that at least the basic precursor or the acidic precursor comprises aluminium, or that both the basic and acidic precursors comprise aluminium.

[0075] The basic precursors comprising aluminum are sodium aluminate and potassium aluminate. The preferred basic precursor is sodium aluminate.

[0076] The acid precursors comprising aluminum are aluminum sulfate, aluminum chloride and aluminum nitrate. The preferred acid precursor is aluminum sulfate.

[0077] Preferably, said second precipitation step operates with stirring.

[0078] Preferably, said second step is carried out in the absence of organic additive.

[0079] The acidic and basic precursors, whether they contain aluminum or not, are mixed, preferably in solution, in the aqueous reaction medium, in proportions such that the pH of the resulting suspension is between 8.5 and 10.5.

[0080] Preferably, the basic and acidic precursor(s) are added in said second precipitation step a') in aqueous solutions.

[0081] As in precipitation step a), it is the relative flow rate of the acidic and basic precursors, whether they contain aluminum or not, which is chosen so as to obtain a pH of the reaction medium between 8.5 and 10.5.

[0082] In the preferred case where the basic and acid precursors are respectively sodium aluminate and aluminum sulfate, the mass ratio of said basic precursor to said acid precursor is advantageously between 1.6 and 2.05.

[0083] For the other basic and acid precursors, whether they contain aluminum or not, the base / acid mass ratios are established by a curve of neutralization of the base by the acid. Such a curve is easily obtained by a person skilled in the art.

[0084] Preferably, said second precipitation step is carried out at a pH between 8.5 and 10 and preferably between 8.7 and 9.9.

[0085] The acid and basic precursors are also mixed in quantities making it possible to obtain a suspension containing the desired quantity of alumina, depending on the final alumina concentration to be achieved. In particular, said second precipitation step makes it possible to obtain 0 to 60% by weight of alumina in A12O3 equivalent relative to the total quantity of alumina formed at the end of the two precipitation steps and preferably at the end of step a').

[0086] As in precipitation step a), it is the flow rate of the acidic and basic precursor(s) containing aluminium which is adjusted so as to obtain a progress rate of the second step of between 0 and 80%, the progress rate being defined as the proportion of alumina formed in A12O3 equivalent during said second precipitation step relative to the total quantity of alumina formed at the end of the two precipitation steps of the process according to the invention and preferably at the end of step a').

[0087] Preferably, the progress rate of said second precipitation step a) is between 1 and 75, preferably between 10 and 70% and more preferably between 15 and 70%.

[0088] Thus, depending on the alumina concentration targeted at the end of the precipitation step(s), preferably between 20 and 100 g / l, the quantities of aluminium to be provided by the acid and / or basic precursors are calculated and the flow rate of the precursors is adjusted according to the concentration of said precursors in aluminum added, the amount of water added to the reaction medium and the rate of progress required for each of the precipitation stages.

[0089] As in precipitation step a), the flow rates of the acidic and / or basic precursor(s) containing aluminium depend on the size of the reactor used and thus on the quantity of water added to the reaction medium.

[0090] For example, if we work in a 3 1 reactor and we aim for 11 of alumina suspension with a final concentration of 50 g / 1 in Al2O3, the targeted rate of progress is 50% in Al2O3 equivalent for the first precipitation step. Thus, 50% of the total alumina must be provided during precipitation step a). The alumina precursors are sodium aluminate at a concentration of 155 g / 1 in Al2O3 and aluminum sulfate at a concentration of 102 g / 1 in Al2O3. The precipitation pH of the first step is set at 9.5 and the second at 9. The quantity of water added to the reactor is 622 ml.

[0091] For the first precipitation step a) operating at 30°C and for 8 minutes, the flow rate of aluminum sulfate must be 10.5 ml / min and the flow rate of sodium aluminate is 13.2 ml / min. The mass ratio of sodium aluminate to aluminum sulfate is therefore 1.91.

[0092] For the second precipitation step, operating at 70°C, for 30 minutes, the flow rate of aluminum sulfate must be 2.9 ml / min and the flow rate of sodium aluminate is 3.5 ml / min. The mass ratio of sodium aluminate to aluminum sulfate is therefore 1.84.

[0093] Preferably, the second precipitation step is carried out at a temperature between 40 and 80°C, preferably between 45 and 70°C and very preferably between 50 and 70°C.

[0094] Preferably, the second precipitation step is carried out for a duration of between 5 and 45 minutes, and preferably 7 to 40 minutes.

[0095] The second precipitation step generally allows the production of an alumina suspension having an A12O3 concentration of between 20 and 100 g / L, preferably between 20 and 80 g / l, more preferably between 20 and 50 g / l.

[0096] In the case where said second precipitation step is implemented, said preparation method also advantageously comprises a second step of heating the suspension obtained at the end of said second precipitation step to a temperature between 50 and 95°C and preferably between 60 and 90°C.

[0097] Preferably, said second heating step is carried out for a duration of between 7 and 45 minutes.

[0098] Said second heating step is advantageously implemented according to all the heating methods known to those skilled in the art.

[0099] Said second heating step makes it possible to increase the temperature of the medium re actionable before subjecting the suspension obtained to step b) of heat treatment. Step c) filtration

[0100] In accordance with the invention, the process for preparing alumina according to the invention also comprises a step c) of filtration of the suspension obtained at the end of step b) of heat treatment, followed by at least one step of washing the boehmite gel obtained, to obtain a filter cake, said filter cake being separated into two streams, a first stream representing 24% by weight to 90% by weight of the entire filter cake resulting from step c) and feeding a step d) of spray drying and a second stream of filter cake directly feeding a step e) of shaping by extrusion kneading.

[0101] Said second filter cake stream directly feeding step e) is not dried in drying step d). Said second stream is therefore an undried cake stream.

[0102] It advantageously represents 10 to 66% by weight of the total filtration cake resulting from step c).

[0103] Said filtration step is carried out according to methods known to those skilled in the art.

[0104] The filterability of the suspension obtained at the end of precipitation step a) or of the two precipitation steps is improved by the presence of said final heat treatment step b) of the suspension obtained, said heat treatment step promoting the productivity of the process according to the invention as well as an extrapolation of the process to the industrial level.

[0105] Additives such as flocculants may also be added to the suspension to improve filterability.

[0106] Said filtration step is advantageously followed by at least one water washing step and preferably one to three washing steps, with a quantity of water equal to the quantity of filtered precipitate.

[0107] The sequence of steps a) and c) and possibly the second precipitation step, the second heating step and the optional filtration step, makes it possible to obtain a specific boehmite gel having a dispersibility index greater than 50%, a crystallite size of between 1 and 35 nm, as well as a sulfur content of between 0.001% and 2% by weight and a sodium content of between 0.001% and 2% by weight, the weight percentages being expressed relative to the total mass of filter cake.

[0108] The filter cake thus obtained has a dispersibility index of between 50 and 100%, preferably between 55 and 100%, very preferably between 60 and 100% and even more preferably between 65 and 100%.

[0109] The dispersibility index is defined as the percentage by weight of peptized boehmite gel that can be dispersed by centrifugation in a polypropylene tube at 3600G for 10 min.

[0110] Dispersion is measured by dispersing 10% boehmite or boehmite gel in a water suspension also containing 10% nitric acid relative to the mass of boehmite. The suspension is then centrifuged at 3600G rpm for 10 min. The collected sediments are dried at 100°C overnight and then weighed.

[0111] The dispersibility index, noted ID, is obtained by the following calculation: ID(%)=100%-mass of dried sediments(%).

[0112] The loss on ignition is measured before the shaping step e) by taking a portion of the boehmite filter cake obtained at the end of the filtration step c) and then by placing it in a muffle furnace for 3 hours at 1000°C. The loss on ignition is obtained by the difference between the mass of the sample before and after passing through the muffle furnace.

[0113] PAF1 is the loss on ignition of the undried filter cake from filtration step c).

[0114] The loss on ignition PAF1 of the boehmite filter cake obtained in step c) is generally between 75% and 90%, preferably between 80 and 90%. Step d) drying part of the cake

[0115] According to the invention, the method comprises a step d) of drying said first stream representing 24% by weight to 90% by weight of the entire filter cake resulting from step c) to obtain a dried filter cake or a dried powder.

[0116] The preparation process according to the invention advantageously comprises, prior to said drying step, a step of resuspending said first stream representing 24% by weight to 90% by weight of the entire filter cake to be dried obtained at the end of step c) by bringing said first stream of filter cake into contact with water having an acid content of between 0 and 10%, preferably between 0.5 and 6%, more preferably between 3 and 6% by weight relative to the total weight of boehmite expressed in A12O3 equivalent, to obtain a suspension of acidified boehmite.

[0117] The quantity of water is advantageously added so as to obtain a dry solid content in the suspension of between 10 and 40% by weight and preferably between 15 and 25% by weight of alumina A12O3.

[0118] Preferably, the acid is chosen from nitric acid and carboxylic acids preferably chosen from acetic acid, citric acid and butyric acid and preferably nitric acid.

[0119] Said drying step is advantageously carried out by any means known to those skilled in the art, and preferably by freeze-drying, fluidized bed drying, or spray drying.

[0120] In a preferred embodiment, said drying step d) is carried out by atomization.

[0121] In this case, said first flow representing 24% by weight to 90% by weight of the entire filter cake obtained at the end of filtration step c) and resuspended, is advantageously dried by atomization to obtain a powder.

[0122] In this case, the acidified boehmite suspension is prepared from atomized powder.

[0123] Spray drying is advantageously carried out according to techniques known to those skilled in the art.

[0124] In spray drying processes, the dryers are generally composed of a cylindrical-conical drying chamber generally in the form of a vertical tower. The liquid product to be treated (colloidal solution or suspension) is conveyed to the drying chamber by means of a dedicated feed circuit. At the inlet of the chamber, the product is introduced and then sprayed in the form of a cloud of fine droplets by means of a spraying device ("atomization" carried out according to methods known to those skilled in the art, i.e. by hydraulic or pneumatic nozzles or by centrifugal turbine). Within the drying chamber, the product droplets are instantly brought into contact with a flow of hot gas, generally air, which makes it possible to induce the evaporation of the solvent, water, and to generate the progressive drying of the product to form solid particles or granules.During this drying, at atmospheric pressure the temperatures of the hot gas are generally between 120°C and 900°C (preferably around 300°C). Other examples include fluidized bed dryers, rotating cylinder or rotating tube dryers, tunnel or belt dryers, flash or spin-flash dryers, and fixed bed ovens or dryers. For pastes or cakes, cylinder dryers are used to dry a cake or a suspension.

[0125] PAF2 is the loss on ignition of the dried or powdered filter cake, preferably dried by atomization.

[0126] The loss on ignition of the dried filter cake or dried powder obtained in step d) is generally between 18% and 30%, preferably between 18 and 25%.

[0127] The drying step causes a loss of mass due to the evaporation of the water contained within the load (reduction of the loss on ignition from PAF1 to PAF2).

[0128] According to the invention, the dried filter cake or dried powder from step d) represents 5 to 65% by weight of the mixture entering said step e).

[0129] According to the invention, the second flow of filter cake obtained in step c), not dried, is used directly in step e) of extrusion mixing in a mixture with the dried filter cake or dried powder from step d).

[0130] The feed mixture entering said step e) comprises and is preferably constituted by the dried filter cake or dried powder from step d) and the second filter cake stream obtained in step c), not dried. Step e) of shaping

[0131] According to the invention, the method comprises a step e) of shaping by kneading-extrusion of the dried filter cake or a dried powder from step d) mixed with said second filter cake flow obtained in step c), the dried filter cake or a dried powder from step d) representing 5 to 65% by weight of the mixture entering said step e), and said second filter cake flow obtained in step c) representing 35 to 95% by weight of the mixture entering said step e), to obtain the raw material or boehmite paste.

[0132] The raw material or boehmite paste thus obtained has a loss on ignition noted PAF. The loss on ignition of the boehmite paste obtained in shaping step e) is generally between 55% and 70%, preferably between 58 and 68%.

[0133] The proportions in the different streams constituting the mixture at the inlet of step e) are calculated in accordance with the ranges of ignition losses envisaged for the process, i.e. a loss on ignition PAF1 of between 75 and 90% for the filter cake from c) and not dried, a loss on ignition PAF2 of between 18 and 30% for the dried cake from d) and a loss on ignition PAF of between 55 and 70% for the boehmite paste.

[0134] According to the invention, the raw material or boehmite paste of said step e) is produced with a total acid level, expressed as a weight percentage relative to the mass of alumina oxide equivalent introduced in step e) of between 0 and 4% and a neutralization level expressed as a weight percentage of base relative to the quantity of acid introduced in said step e) of between 0 and 200%.

[0135] Raw material or boehmite paste means the shaped material which has not undergone any heat treatment steps.

[0136] Preferably, said step e) is carried out with a total acid level, expressed as a percentage relative to the mass of alumina oxide equivalent introduced in step e), of between 0 and 3%, preferably between 0 and 2%, and very preferably between 0.5 and 1%, and a neutralization level expressed as a percentage by weight of base relative to the quantity of acid introduced in said step e), of between 0 and 150%, preferably between 0 and 130%, preferably between 0 and 60%, very preferably between 0 and 50% and more preferably between 5 and 35%.

[0137] Preferably, the acid used in step e) is chosen from nitric acid and carboxylic acids preferably chosen from acetic acid, citric acid and butyric acid and preferably nitric acid.

[0138] Preferably the base used in step e) is chosen from inorganic bases chosen from sodium hydroxide, potassium hydroxide, and ammonia, and organic bases in solution chosen from amines and quaternary ammonium compounds. Preferably, the organic bases in solution are chosen from alkylethanol amines and ethoxylated alkylamines. The organic bases are preferably used in solution in water.

[0139] Very preferably, said base is ammonia and preferably ammonia in aqueous solution (NH4OH+ H2O). Step f) heat treatment

[0140] In accordance with the invention, the raw material obtained at the end of shaping step e) then undergoes a heat treatment step f) at a temperature of between 500 and 1000°C, for a duration of between 2 and 10 h, in the presence or absence of a flow of air containing up to 60% by volume of water.

[0141] Preferably, said heat treatment step f) operates at a temperature between 500°C and 850°C.

[0142] Preferably, said heat treatment step f) operates for a duration of between 2 hours and 10 hours.

[0143] Said heat treatment step f) allows the transition of boehmite to the final alumina.

[0144] Preferably, said step f) is carried out in the presence of an air flow containing a water content of between 5 and 50% by mass, and preferably between 5 and 45% by mass, and preferably between 5 and 40% by mass.

[0145] The heat treatment step may be preceded by drying at a temperature between 50°C and 120°C, according to any technique known to those skilled in the art.

[0146] The loss on ignition of the alumina support obtained in step f) of heat treatment generally lies between 1% and 5%, preferably between 1 and 4%.

[0147] The present invention also relates to the mesoporous alumina capable of being obtained by the preparation process according to the invention.

[0148] The invention is illustrated by the following examples which are in no way limiting. LIST OF FIGURES

[0149] [Fig.l]

[0150] [Fig.l] illustrates a process diagram for manufacturing the aluminum catalytic support according to the non-compliant example.

[0151] [Fig.2]

[0152] [Fig.2] illustrates a process diagram for manufacturing the aluminum catalytic support according to the example in accordance with EXAMPLES Non-compliant example

[0153] According to a process not in accordance with the invention, the manufacture of the alumina catalytic support comprises the following steps: Precipitation of the alumina gel (step a)

[0154] The synthesis of a boehmite gel is carried out in a stirred reactor with a useful volume of 22 m3: a final suspension of 16 m3 in two precipitation stages is obtained by the protocol indicated below. The final alumina concentration targeted is 45g / L.

[0155] The initial quantity of water injected into the reactor is 10.5 m3. Stirring is maintained at 250 W / m3 throughout the synthesis.

[0156] A first step of co-precipitation in water, aluminum sulfate A12(SO4) and sodium aluminate NaAlOO is carried out at 30°C and pH=9.5 for a period of 8 minutes. The precursors are diluted so as to reach the following target concentrations: A12(SO4) at 102g / L in A12O3 and NaAlOO at 155g / L in A12O3.

[0157] A solution of aluminum sulfate A12(SO4) is added continuously for 8 minutes at a flow rate of 13.4 m3 / h to a solution of sodium aluminate NaAlOO at a flow rate of 16.3 m3 / h according to a mass ratio of base / acid flow = 1.84 so as to adjust the pH to a value of 9.5. The temperature of the reaction medium is maintained at 30°C.

[0158] A suspension containing an alumina precipitate is obtained.

[0159] The alumina concentration obtained at the end of the first precipitation step is 36 g / L, the flow rate of the precursors aluminum sulfate A12(SO4) and sodium aluminate NaAlOO containing aluminum introduced in the first precipitation step are respectively 13.4 m3 / h and 16.3 m3 / h.

[0160] These flow rates of acid and basic precursors containing aluminum make it possible to obtain a progress rate of 72% at the end of the first precipitation stage.

[0161] The suspension obtained is then subjected to a temperature increase from 30 to 68°C.

[0162] A second step of co-precipitation of the suspension obtained is then carried out by adding aluminum sulfate A12(SO4) diluted to a concentration of 102g / L in A12O3 and sodium aluminate NaAlOO diluted to a concentration of 155g / L in A12O3. A solution of aluminum sulfate A12(SO4) is therefore continuously added to the heated suspension obtained at the end of the first precipitation step for 30 minutes at a flow rate of 1.4 m3 / h to a solution of sodium aluminate NaAlOO at a flow rate of 1.7 m3 / h according to a mass ratio of base / acid flow = 1.86 so as to adjust the pH to a value of 9. The temperature of the reaction medium in the second step is maintained at 68°C.

[0163] A suspension containing an alumina precipitate is obtained.

[0164] The final alumina concentration targeted being 45g / L, the flow rate of the precursors aluminum sulfate A12(SO4) and sodium aluminate NaAlOO containing aluminum introduced in the second precipitation stage are respectively 1.4 m3 / h and 1.7 m3 / h.

[0165] These flow rates of acid and basic precursors containing aluminum make it possible to obtain a progress rate of 28% at the end of the second precipitation stage.

[0166] The suspension obtained is then subjected to a temperature increase from 68 to 90°C. Heat treatment of the suspension (step b)

[0167] The suspension obtained in step a) undergoes a heat treatment step (maintained at 90°C for 60 minutes) allowing the aluminum gel to mature. This step is carried out successively to the precipitation step a), i.e. in the same stirred reactor. Filtration and washing of the suspension (step c)

[0168] The alumina gel from the maturing step b) is then sent to a filtration step to extract the water from the suspension. This step is carried out using a belt filter. Within the same belt filter, the filter cake is washed by injecting water to reduce the salt content (Na, SO4). The loss on ignition (PAF1) obtained is around 83%. Drying the filter cake (step d)

[0169] A preliminary repulping step (addition of a small quantity of water and nitric acid, 3% acid content HNO3 relative to the alumina A12O3) of the cake obtained in step c) is carried out. The entire washed / repulped filter cake is then sent to a spray drying step. The spray drying is carried out at atmospheric pressure, air inlet temperature below 450°C and allows the production of boehmite gel in the form of dried powder having a loss on ignition of the order of 26% (PAF2). Formatting (step e)

[0170] The dried powder obtained in step d) is sent to a mixing / extrusion step allowing the production of catalytic support extrudates. The atomized gel is introduced into the mixer mixed with nitric acid (3% acid content HNO3 relative to alumina A12O3), ammonia (neutralization rate defined according to a base / acid molar ratio of 40% acid) and water. The boehmite paste extrudates have a targeted loss on ignition of PAF = 62% (target achieved by adapting the quantity of water added in the shaping step e).

[0171] Heat treatment of the support extrudates (step f)

[0172] The boehmite paste produced during step e) undergoes a heat treatment step aimed at drying and then calcining the catalytic support extrudates. This step is carried out using an industrial annular furnace, supplied by an air flow with an inlet temperature above 450°C. The loss on ignition of the material obtained is 1.68%.

[0173] [Fig.l] illustrates such an industrial manufacturing method. Only the main unit operations have been reported there; in particular the intermediate and ancillary equipment dedicated to the transfer of flows between stages and known to those skilled in the art (pumps, buffer tanks, pneumatic transport, dosing or feed hoppers, solid conveying, etc.) have not all been indicated.

[0174] The flow rates indicated correspond to equivalent continuous flow rates allowing the ultimate production of a “1000 kg / h” base of catalyst support; but the operation of the equipment can be indifferently batch or continuous.

[0175] Table 1 lists the equipment and its main characteristics according to the non-compliant example:

[0176] [Tableauxl] No. Equipment description Sizing capacity (kg / h) Heat / process power absorbed (kW) Steps a and b: precipitation of alumina gel and heat treatment (ripening) El Sodium aluminate tank 2,724 E2 Aluminum sulfate tank 4,213 E3 Precipitation and ripening tank 23,791 Step c: suspension filtration and cake washing E4 Filter feed pump 23,791 E5 Belt filter 23,791 Step d: filter cake drying E6 Repulping tank 8,936 E7 Spray feed pump 8,936 16.8 E8 Spray drying 8,936 4,774 Step e: shaping by mixing E9 Mixer-Extruder 2,587 Eli Nitric acid tank 86 E12 Ammonia tank 21 Step f: heat treatment of the support E10 Drying and calcination oven 2,587,952 Example conforms

[0177] According to a process in accordance with the invention, the manufacture of the alumina catalytic support comprises the following steps: Precipitation of alumina gel (step a)

[0178] This step is identical in operation to step a) of the method not in accordance with the invention. Heat treatment of the suspension (step b)

[0179] This step is identical in operation to step b) of the method not in accordance with the invention. Filtration and washing of the suspension (step c)

[0180] This step is identical in operation to step c) of the method not in accordance with the invention. At the end of step c), the output flow is split into two parts.

[0181] A first part of the cake, equal to 73% by weight of the entire filter cake, is directed towards step d) of spray drying.

[0182] A second part of the filter cake, equal to 27% by weight of the entire filter cake, is used directly in shaping step e).

[0183] Drying of a portion of the filter cake (step d)

[0184] A preliminary repulping step (= addition of a small quantity of water and nitric acid) of the first filter cake stream produced in step c) is carried out. The repulped filter cake stream is then sent to a spray drying step. The conditions used are identical to those of the non-compliant example, the loss on ignition obtained for the dried powder being PAF2 = 26%. Shaping (step e)

[0185] The dried powder obtained in step d) is mixed with said second filter cake stream obtained in step c), within the mixing / extrusion step e) aimed at producing catalytic support extrudates. In the range of ignition losses retained for the compliant example, the proportion of the second undried filter cake stream obtained in step c) represents 61% by weight of the mixture entering said step e). The proportion of dried powder from step d) represents 39% by weight of the mixture entering said step e).

[0186] Nitric acid (3% acid content HNO3 relative to alumina A12O3) and ammonia (neutralization rate defined according to a base / acid molar ratio of 40% acid) are also introduced so as to optimize the mixing conditions. The boehmite paste extrudates have a targeted loss on ignition of PAF = 62%. Unlike the non-compliant example, the addition of water is not necessary in the shaping step e): it is the adjustment of the proportions of the second undried filter cake stream obtained in step c) and the dried powder stream from step d) which allows compliance with the PAF target.

[0187] Heat treatment of support extrudates (step f)

[0188] This step is identical in operation to step a) of the process not in accordance with the invention. The loss on ignition of the material obtained is 1.68%.

[0189] [Fig.2] illustrates such an industrial manufacturing method. Only the main unit operations have been reported; in particular, the intermediate and ancillary equipment dedicated to the transfer of flows between stages and known to those skilled in the art (pumps, buffer tanks, pneumatic transport, dosing or feed hoppers, solid conveying, etc.) have not all been indicated.

[0190] The flow rates indicated correspond to equivalent continuous flow rates allowing the ultimate production of a “1000 kg / h” base of catalyst support; but the operation of the equipment can be indifferently batch or continuous.

[0191] Table 2 lists the equipment and its main characteristics according to the compliant example:

[0192] [Tables2] No. Equipment description Sizing capacity (kg / h) Heat / process power absorbed (kW) Steps a and b: precipitation of alumina gel and heat treatment (ripening) El Sodium aluminate tank 2,724 E2 Aluminum sulfate tank 4,213 E3 Precipitation and ripening tank 23,791 Step c: suspension filtration and cake washing E4 Filter feed pump 23,791 E5 Belt filter 23,791 Step d: filter cake drying E6 Repulping tank 6,561 (-27%) E7 Spray feed pump 6,561 (-27%) 12.3 (-27%) E8 Spray drying 6,561 (-27%) 3,505 (-27%) Step e: shaping by mixing E9 Mixer-Extruder 2 587 Eli Nitric acid tank 86 E12 Ammonia tank 21 Stage f: heat treatment of the support E10 Drying and calcination oven 2 587 952

[0193] Comparison of the compliant and non-compliant examples shows the gains made possible by the method according to the invention:

[0194] A significant reduction in the size of the equipment in step d) of drying the filter cake (-27% on the sizing capacity of equipment E6 to E8) as well as energy costs (-27% of the process heat / power absorbed by the E7 and E8 equipment). This results in a significant reduction in the process's CO2 emissions and therefore an improvement in its environmental footprint.

[0195] A reduction in the overall quantity of water consumed: 35,848 kg / h for the compliant example versus 37,870 kg / h for the non-compliant example, i.e. a reduction of 5%.

Claims

Claims

1. A process for preparing a mesoporous alumina having a total pore volume greater than 0.75 ml / g, said process comprising at least the following steps: a) at least a first step of precipitation of a boehmite gel, in an aqueous reaction medium, of at least one basic precursor chosen from sodium aluminate, potassium aluminate, ammonia, sodium hydroxide and potassium hydroxide and of at least one acid precursor chosen from aluminum sulfate, aluminum chloride, aluminum nitrate, sulfuric acid, hydrochloric acid, and nitric acid, in which at least one of the basic or acid precursors comprises aluminum, the relative flow rate of the acid and basic precursors is chosen so as to obtain a pH of the reaction medium of between 8.5 and 10,5 and the flow rate of the acidic and basic precursor(s) containing aluminium is adjusted so as to obtain a rate of progress of said first step of between 20 and 100%, the rate of progress being defined as being the proportion of alumina formed in A12O3 equivalent during said first precipitation step relative to the total quantity of alumina formed at the end of the precipitation step(s), said first precipitation step operating at a temperature of between 10 and 50°C, and for a duration of between 2 minutes and 30 minutes, b) a step of heat treatment of the suspension obtained at the end of step a) at a temperature of between 50 and 200°C for a duration of between 30 minutes and 5 hours allowing a boehmite gel to be obtained, c) a step of filtration of the suspension obtained at the end of step b) of heat treatment, followed by at least one step of washing the boehmite gel obtained, to obtain a filter cake, said filter cake being separated into two streams, a first stream representing 24% by weight to 90% by weight of the entire filter cake from step c) and feeding a step d) of spray drying and a second stream of filter cake directly feeding a step e) of shaping by extrusion kneading, d) a step of spray drying said first stream representing 24% by weight to 90% by weight of the entire filter cake from step c) to obtain a dried filter cake or a powder dried, e) a shaping step by kneading-extrusion of the dried filter cake or a dried powder from step d) mixed with said second filter cake stream obtained in step c), the dried filter cake or a dried powder from step d) representing 5 to 65% by weight of the mixture entering said step e), and said second filter cake stream obtained in step c) representing 35 to 95% by weight of the mixture entering said step e), to obtain the raw material or boehmite paste, said step e) being carried out with a total acid level, expressed as a weight percentage relative to the mass of dried gel introduced in step e) of between 0 and 4% (defined relative to the quantity of boehmite expressed as A12O3) and a neutralization level expressed as a base weight percentage relative to the quantity of acid introduced in said step e) of between 0 and 200%,f) a step of heat treatment of the raw material obtained at the end of step e) at a temperature between 500 and 1000°C, in the presence or absence of an air flow containing up to 60% by volume of water.,

2. Preparation process according to claim 1 wherein the basic precursor is sodium aluminate.

3. Preparation process according to one of claims 1 or 2 in which the acid precursor is aluminum sulfate.

4. Preparation process according to one of claims 1 and 3 in which the progress rate of said precipitation step a) is between 30 and 85%.

5. Preparation process according to one of claims 1 to 4 wherein in the case where the progress rate obtained at the end of the first precipitation step a) is less than 100%, said preparation process comprises a second precipitation step a') after the first precipitation step.

6. Preparation process according to claim 5 in which a step of heating the suspension obtained at the end of precipitation step a) is carried out between the two precipitation steps a) and a'), said heating step operating at a temperature between 20 and 90°C and for a duration between 7 and 45 minutes.

7. Preparation process according to one of claims 5 or 6 in which said second precipitation step a') of the suspension obtained at the end of the heating step, operates by adding to said suspension of at least one basic precursor chosen from sodium aluminate, potassium aluminate, ammonia, sodium hydroxide and potassium hydroxide and of at least one acid precursor chosen from aluminum sulfate, aluminum chloride, aluminum nitrate, sulfuric acid, hydrochloric acid, and nitric acid, wherein at least one of the basic or acid precursors comprises aluminum, the relative flow rate of the acid and basic precursors is chosen so as to obtain a pH of the reaction medium of between 8.5 and 10.5 and the flow rate of the acid and basic precursor(s) containing aluminum is adjusted so as to obtain a progress rate of the second stage of between 0 and 80%, said progress rate of the second stage being defined as being the proportion of alumina formed in equivalent A12O3 during said second precipitation stage a') relative to the total quantity of alumina formed at the end of step a'),said second step a') operating at a temperature between 40 and 90°C, and for a duration between 2 minutes and 50 minutes.,

8. Preparation process according to one of claims 1 to 7 comprising, prior to said drying step, a step of resuspending said first stream representing 24% by weight to 90% by weight of the entire filter cake to be dried obtained at the end of step c) by bringing said first stream of filter cake into contact with water having an acid content of between 0 and 10%, preferably between 0.5 and 6%, more preferably between 3 and 6% by weight relative to the total weight of boehmite expressed in A12O3 equivalent, to obtain a suspension of acidified boehmite.

9. Preparation process according to claim 8 wherein said drying step d) is carried out by atomization.

10. Preparation process according to one of claims 1 to 9 in which said step e) is carried out with a total acid level, expressed relative to the mass of dried gel introduced in step e) of between 0 and 3% and a neutralization level expressed in base weight relative to the quantity of acid introduced in said step e) of between 0 and 60%.

11. Preparation process according to claim 10 in which said step e) is carried out with a total acid level, expressed relative to the mass of dried gel introduced in step e) of between 0 and 2% and a neutralization level expressed in base weight relative to the quantity of acid introduced in said step e) of between 5 and 35%.

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