MXPA00002492A - Process for the gas phase polymerisation of olefins - Google Patents

Abstract

The present invention provides an improved chromium oxide based catalyst supported on a granular or microspherical refractory oxide for the gas phase polymerisation of at least one alpha olefin containing from 2 to 12 carbon atoms, characterised in that the supported chromium oxide based catalyst has been subjected to a sole calcination/activationstep consisting of a single thermal treatment performed at a temperature ranging from 200 to 450°C under an oxygen- containing atmosphere.

Description

PROCESS FOR THE POLYWERIZACIÓW OF THE OLEFINAS GAS PHASE The present invention relates to a process for the polymerization of the gas phase of olefins with the aid of a supported catalyst based on chromium oxide. The preparation of the olefin polymerization catalysts by impregnating an oxide refractory carrier with a titanium compound, subjecting the resulting product to a calcination step by heating at a temperature T1 of between 150 and 1200 ° C, adding a chromium compound and subjecting the resulting product to an activation step, by heating to a temperature T2 of between 100 and 1200 ° C, is known from the British patent GB 1, 429,174 so, according to the patent GB 1 , 429,174, two separate thermal treatments are required (T1, T2), to which we will refer hereinafter as a calcination process and an activation procedure, to obtain the desired catalyst. The use of modified catalysts based on chromium oxide for the polymerization of ethylene in a paste is known from US Pat. No. 3,622,521. This patent describes a final catalyst activation step, which can be carried out in dry air for from 1 to 50 hours using a temperature in the range of 350 ° to 2,000 ° F (176 ° to 1 093 °). C). All of the catalysts of US Pat. No. 3,622,521 are prepared using an activation procedure of 5 hours in dry air at a temperature of 1, 300 ° F (704 ° C).

The preparation of ethylene polymers by a polymerization process of the gas phase with the aid of a catalyst based on titanium modified chromium oxide, is known from US Pat. No. 4,011,382. This patent describes a final step of activation of the catalyst which can be carried out by heating the catalyst in air or oxygen at a temperature of 300 ° to 900 ° C, and preferably at a temperature of 700 ° to 850 ° C. All of the catalysts of US Pat. No. 4,011,382 are prepared using an 8 hour activation procedure in dry air at a temperature of either 750 ° C or 825 ° C.

It is also of general knowledge that there is an almost linear relationship of 850 ° C. All of the catalysts of US Pat. No. 4,011,382 are prepared using an 8 hour activation procedure in dry air at a temperature of either 750 ° C or 825 ° C.

The use of a chromium supported catalyst for the polymerization of olefins is also known from EPO Patent Application 055 863. All the catalysts described in said patent application are supported on a base containing aluminum phosphate.

Furthermore, it is generally known that there is an almost linear relationship between the activation temperature and both, the activity of the catalyst and the melt index of the ethylene polymer prepared from said chromium oxide catalyst. If the activation temperature is higher, the activity and fusion rates are higher. Therefore, those skilled in the art know that in order to properly activate the chromium oxide-based catalyst at catalyst activation temperatures, temperatures in excess of at least 500 ° C are used.

These calcination / activation procedures applied to the modified support are long and expensive. However, since these procedures are necessary for the good activity of the final catalyst and the high melt index of a resulting ethylene polymer, the person skilled in the art continues to proceed in the same way.

Therefore, it is an object of the present invention to provide a process for the preparation of an ethylene (co-) polymer that exhibits a high melt index in the presence of the supported catalyst based on chromium oxide, which has a reasonably good activity and that does not require these too long and / or activation / calcination procedures at high temperatures. The Applicants have unexpectedly discovered that, while the aforementioned quasi-linear relationship between the activation temperature and the catalyst activity and the fusion of the resulting (co) polymer, could be systematically verified in the polymerization of pastes, this is not the case, when using the supported catalyst based on chromium oxide in the (co) polymerization of the gas phase of ethylene.

The present invention makes it possible to avoid or at least mitigate the disadvantages to which we referred above. In particular, if it consists of a polymerization process of the gas phase, which makes it possible to manufacture polymers that have a high melting rate with a support which has been subjected to a single step of calcination / activation consisting of a heat treatment Unique made at a temperature in a range of from 200 to 450 ° C under an oxygen-containing atmosphere. According to the present invention, a polymerization reaction of at least one alpha-olefin is carried out with the aid of said supported catalyst based on chromium oxide. Another object of the present invention is a method of preparing a catalyst based on chromium oxide supported on a granular or microspherical refractory oxide for the polymerization of the olefin gas phase (s), characterized in that the supported catalyst based on Chromium is subjected to a single calcination / activation step of a single heat treatment performed at a temperature in a range of from 200 to 450 ° C under an oxygen-containing atmosphere. A further object of the present invention is to provide an improved chromium oxide-based catalyst supported on a granular or microspherical refractory oxide for the polymerization of the olefin gas phase (s), wherein the catalyst can be obtained by a preparation which is characterized in that the supported catalyst based on chromium oxide is subjected to a single calcination / activation step consisting of a single heat treatment, carried out in a temperature range of from 200 to 450 ° C under an oxygen-containing atmosphere . The supported catalyst based on chromium oxide contains in most cases 0.1 to 3% chromium. According to a preferred embodiment of the present invention, the catalyst is a supported catalyst based on chromium oxide advantageously modified with titanium or aluminum, more preferably a supported catalyst based on titanium-modified chromium oxide. For example, the catalyst can be modified with from 0.1 to 8% by weight of titanium or from 0.1 to 6% by weight of aluminum. The catalyst is supported on a granular or microspherical refractory oxide, such as silica oxide, alumina, zirconium or a mixture or a coprecipitate of these oxides. The support can be obtained by various known processes, especially by the precipitation of silicone compounds such as, for example, silica, from an alkali metal silicate solution, (or in addition, by the co-precipitation of a gel or refractory oxide hydrogel from solutions containing at least two compounds selected from silicone, titanium, zirconium or aluminum compounds). The granular support advantageously has a specific surface area (BET) of between 200 and 1200m2 / g, a pore volume in a range of from 1 to 3.5 ml / g and may consist of particles which have a diameter of between 20 and 250 μm, preferably between 30 and 150 μm. It advantageously contains hydroxyl functional groups and is preferably free of water at the time of its use during the preparation of the catalyst. For this purpose, it can be heated to a temperature in a range of for example, 100 to 200 ° C. The catalyst is preferably prepared by a process comprising a first stage during which the support is impregnated with a chromium compound, and an optional second stage during which the product originating from the first stage is impregnated with a compound, since be it titanium or aluminum. The chromium compound used can be an oxide to be converted to chromium oxide by calcination, such as, for example, a chromium nitrate or sulfate, an ammonium chromate, a chromium carbonate, acetate or acetylacetonate or in addition, i ^ i ^ oThettobuthane. The titanium compounds which can be used advantageously, are titanium alcoholate such as, for example, titanium tetraisopropylate or titanium tetrabutylate. The aluminum compounds which can be used advantageously are, for example, acetyl acetate, acetylacetonate of alkoxy or alkyl types. The impregnation of the support with the titanium or aluminum compound can advantageously be carried out, just before or during, the single calcination / activation step applied to the catalyst. The catalyst can also be prepared by a process consisting of a co-precipitation of a gel or hydrogel, such as the one referred to above, in the presence of a chromium compound and a titanium compound, so as to form a co-gel, which comprises, on the one hand, at least one refractory oxide such as silica or alumina and, on the other hand, a chromium compound and a titanium compound. Before use, the supported catalyst must be subjected to a single calcination / activation step, which consists of a single heat treatment performed at a temperature in a range of from 200 to 450 ° C under an oxygen-containing atmosphere. In accordance with the present invention said unique calcination / activation treatment of the catalyst must be carried out at a low temperature, in particular at a temperature of between 200 ° C and 450 ° C, and preferably from 300 to 400 ° C. According to the present invention said unique calcination / activation treatment of the catalyst must be carried out under an oxygen-containing atmosphere, preferably under dry air. Said single treatment has an average duration of between 10 minutes and 12 hours and more particularly between 30 minutes and 8 hours. This treatment can be carried out by known means, using a non-reducing atmosphere. For example, it can be performed in a fluidized bed activator. The obtained catalyst can be injected directly into the gas phase polymerization reactor. It can also be introduced in the form of a prepolymer previously prepared during a prepolymerization step. The prepolymerization step consists in contacting the catalyst, for example, with ethylene optionally mixed with an alpha-olefin and optionally, in the presence of hydrogen. The prepolymerization step can be carried out in the presence of an organometallic metal compound of groups 2 and 13, and optionally, 1 and 12 of the periodic classification of the elements. The polymerization of the gas phase of the alpha-olefin can be carried out in a fluidized and / or mechanically stirred bed reactor, according to any of the methods known in the art. The polymerization reaction can be carried out at a temperature between 0 and 120 ° C, preferably between 50 and 110 ° C, and at a total pressure in a range of from 0.1 to 5 Mpa. The process according to the present invention is particularly suitable for the manufacture of ethylene polymers such as, for example, ethylene homopolymers or ethylene copolymers containing at least one alpha-olefin containing from 3 to 12 carbon atoms such as, for example, 1-butene, 1-hexene or 1-octene. In general, the ethylene copolymers prepared by the process according to the present invention contain, in addition to ethylene, less than 10% and in most cases less than 4% and preferably less than 1% by weight of another olefin. -alpha containing 3 to 12 carbon atoms. The polymers obtained by the process according to the present invention can have a relative density in a range of from 0.915 to 0.970, preferably in a range of from 0.935 to 0.965 and more particularly from 0.940 to 0.960. In most cases, they have a molecular weight distribution, (MWD), measured according to the ratio of the average weight of the molecular weight, Mw to the number average molecular weight, MN, of between 5 and 55. In most of the cases, has an average molecular weight between 50,000 and 500,000. In general, they contain less than 5 ppm of chromium due to the good activity of the catalyst. In addition, they have a resistance to breakage by stress greater than 10 hours and in most cases greater than 15 hours. They also generally have a critical cut-off range greater than 800s "1 and in most cases greater than 1000s" 1. The unexpected advantage obtained according to the present invention are the relatively high values of the melt index of the polymers. As already explained above and will be further illustrated in the following (comparative) examples, it is quite surprising that supported catalysts based on chromium oxide, which have been subjected to a single calcination / activation treatment at low temperature, when they are used in the gas phase processes, they produce polymers that have high values of the melt index. When the catalyst is activated at a temperature of from 200 to 450 ° C, it is possible to prepare polymers having a high melt index, for example, a melt index higher than 1.5 g / 10 minutes. In these ranges the melting rate of the polymer increases when the activation temperature decreases. Such polymers, particularly, in the polymer having an Mls greater than 1.5g / 10 minutes, the critical cutting range may be greater than 1200s.1 The polymer also contains a very low proportion of volatile substances.These substances generally represent, on a weight basis, less than 800 and more particularly, less than 500 and in most cases less than 400 parts per million (ppm) of the polymer.In addition, they generally have a resistance drop greater than 2 m and in most of cases greater than 2.5 m They have a low swelling in the die, in particular less than 35 g The polymers obtained according to the process of the present invention are particularly suitable for the manufacture of die-cut objects or by blow extrusion. The following examples illustrate the present invention: The properties of the polymer have been measured according to the following procedures: Method of determination of resistance to ro The stress resistance is measured in the polymer bottles according to the method of M.J. Cawood and T.J.C. Sleeman (BP Chemicals Ltd., Great Britain), which is described in the journal Polymer Testing 1 (1980) pages from 191 to 199, except for the fact that the bottles are kept at a temperature of 50 ° C in instead of 60 ° C. According to this method, the resistance to stress fracture is expressed in hours. Method of determining the content of volatile material of a polymer. According to the present invention the volatile content of a polymer is determined by measuring the loss in weight of the polymer after it has been kept in a furnace at a temperature of 100 ° C for 17 hours. The weight loss is expressed in ppm. Method of determining the critical cutting range. The critical cutoff range is determined from a graph, which provides the stress imposed on the polymer as a function of the cutoff range to which the polymer is subjected, which has been established with the help of a capillary rheometer, the which has a die in which the proportion of its length to its diameter is 30. The critical range of cut is determined as the lower value of the cut range, in which the stability to the effort is observed. In this value the graph exhibits a point of inflection. The effort and cutting range are defined in ASTM Standard D 3835. A cutoff range is expressed in s "1. Fine particle content According to the present invention the fine particle content is the proportion of particles in the polymer having a diameter of less than 125 μm.The content is expressed as a percentage of weight Flow parameter The flow parameter "n" is calculated by means of the formula n = log (Ml2? 6 / MI5) / log (21.6 / 5) according to the method ASTM D 1238 0> The index Fusion is expressed in s / t0 * minutes.

Examples 1. 2. C3 a) Preparation of the catalyst The catalyst is activated by thermal activation, which is carried out in a quartz tube heated from the outside by a surrounding electric furnace. A sintered quartz disc fused to the tube serves to support the fluidized bed. The temperature of the bed is measured by a thermoelectric battery placed near the center. The gases used in the flow of ascending activation through the disk, produce the fluidification. 25 grams of a catalyst composition based on chromium oxide sold under the trademark "EP30" (by Joseph Crosfield and Sons, Warrington, United Kingdom) are introduced into the fluidized quartz tube with a stream of dry nitrogen at room temperature . This solid composition contains about 1% by weight of chromium in the form of trivalent chromium acetate supported on silica. The quartz tube is then heated from room temperature to 150 ° C in a range of 57 ° C / h and is maintained at a temperature of 150 ° C. Later, a mixture of titanium tetraisopropylate and tetra-n-butylate sold under the trademark "Tilcom BIP" (by Titanium Intermediates Limited, Billingham, United Kingdom) in an amount corresponding to 23.44 millimole of titanium is added to the reactor. The catalytic solid formed in this way is maintained at a temperature of 150 ° C for 3 hours. The reactor is then heated to 300 ° C in a range of 57 ° C / h and is maintained at this temperature for one hour. The fluidized nitrogen is subsequently replaced by a dry air stream and the catalytic solid is successively heated from 300 to the final activation temperature (400, 450 and 500 ° C respectively, producing catalyst 1a, 2a and C3a.), Maintained in this temperature for 5 hours, and then it is finally cooled in a range of 57 ° C / h to 325 ° C. The fluidizing air is then replaced by a stream of dry nitrogen and the catalytic solid is slowly cooled to room temperature. 24.5 grams of an activated catalyst containing 4.5% by weight of titanium and 0.98% by weight of chromium are recovered; This catalyst is composed of particles with a weight average diameter of 100 μm. It is stored in an atmosphere of dry nitrogen, b) Phase polymerization of ethylene gas They are introduced into a stainless steel reactor with a capacity of 2.6 liters equipped with a power stirrer that rotates at 200 rpm and is maintained under an atmosphere of nitrogen, 400 grams of polyethylene pellets prepared in a previous polymerization. Temperature control is provided by circulating silicone oil, which it cools and which is heated by means of water and steam in the surrounding jacket. The temperature is maintained within +/- 0.5 ° C of the desired temperature. The reactor is heated to 104 ° C and about 50 milligrams of one of the above catalysts are introduced (see catalysts 1a, 2a and C3a mentioned above). 0.5 grams of silica containing 1.5 millimoles of triethylaluminum per gram are then introduced into the reactor to sweep the poisons. Hydrogen is then introduced to pressurize the reactor to 0.1 Mpa. Finally, ethylene is introduced until a total pressure of 0.9 Mpa is obtained. The introduction of ethylene is continued to maintain the reactor pressure at 0.9 Mpa. The reaction continues until 2500 grams of catalyst polymer are formed. The content of the reactor is then recovered and the polyethylene powder formed in the reaction is separated from the pellets by sifting. The activity of the catalyst and the properties of the polymer produced are shown in Table I. Examples C4 to C6 (comparative) a) Preparation of the Catalyst The same catalysts were used as in Examples 1 to C3. b) Ethylene paste phase polymerization Paste polymerization experiments are conducted in a stainless steel reactor with a capacity of 2 liters equipped with a three-blade ascending marine propeller rotating at a speed of 400 rpm and maintained under one atmosphere of nitrogen. The reactor is heated to 104 ° C and about 250 milligrams of one of the above catalysts (for example, catalysts 1 a, 2 a, and C3a) are introduced into the reactor. Then 1 liter of dry sobutane is introduced. Ethylene is supplied at a pressure of 4 MPa, which is kept constant during the polymerization reactions. The induction time is taken from the moment when the reactor pressure reaches 4 MPa until the ethylene flow starts again. The reaction is timed from the moment the ethylene flow starts again. The ethylene flow range increases gradually during the polymerization from 0 to 600 g / h. The reaction is stopped when the productivity reaches 2500 grams of polymer per gram of catalyst. The activity of the catalyst and the properties of the polymer produced are those shown in Table 1 Table 1 M.B. = very low N.D. = not determined Example 7 a) Preparation of the catalyst 30 grams of a chromium oxide-based catalyst composition sold under the registered trademark "EP211 B" (by Joseph Crosfield and Sons, Warrington, United Kingdom) in a heated reactor are introduced. at 93 ° C and fluidized with a stream of dry nitrogen. This solid composition contains about 1% by weight of chromium in the form of trivalent chromium acetate and about 2.1% by weight of aluminum supported on silica. Subsequently, the reactor is heated from 93 to 150 ° C in a range of 90 ° C / h and is maintained at 150 ° C for 30 minutes. The reactor is then heated to 300 ° C in a range of 90 ° C / h and is maintained at this temperature for 4 hours. The fluidizing nitrogen is then replaced by a stream of dry air and the catalytic solid is successively heated from 300 to 450 ° C, maintained at 450 ° C for 4 hours, then finally cooled in a range of 90 ° C / ha to 300 ° C. ° C. The fluidizing air is then replaced by a stream of dry nitrogen and the catalytic solid is slowly cooled to room temperature. 25 grams of an activated catalyst containing 2.1% by weight of aluminum and 1% by weight of chromium are recovered; This catalyst is composed of particles with a weight average diameter of 100 μm. It is stored in an atmosphere of dry nitrogen. b) Polymerization of the Ethylene Gas Phase 200 grams of polyethylene powder prepared in a previous polymerization are introduced in a stainless steel reactor with a capacity of 2.6 liters equipped with a powder stirrer rotating at 350 rpm and kept under an atmosphere of nitrogen. The reactor is heated to 100 ° C and 210 milligrams of the above catalyst (7a) are introduced. Then 0.2 millimoles of triethylaluminum are introduced into the reactor to sweep the poisons. Hydrogen is then introduced to pressurize the reactor to a pressure of 0.3 Mpa. Finally ethylene is introduced until obtaining a total pressure of 1.5 Mpa. The introduction of ethylene continues in a range of 183 g / mM.h.b. 600 grams of polyethylene are recovered from the reactor, whose properties are: • Volume density 370 kg / m3 • High load melt index (ASTM method D-1238, measured at 190 ° C) 138 g / 10 minutes • Average size of particle 660 μm • Density 940.6 kg / m3 • Polydispersity 10.7 Example 8 (comparative) a) Preparation of the Catalyst 30 grams of a catalyst composition based on chromium oxide sold under the registered trademark "EP211 B" (by Joseph Crosfield and Sons, Warrington, United Kingdom) in a reactor heated to 93 ° C and fluidized with a stream of dry nitrogen. This solid composition contains about 1% by weight of chromium in the form of trivalent chromium acetate and about 2.1% by weight of aluminum supported on silica. The reactor is then heated to 93 to 150 ° C in a range of 90 ° C / h and is maintained at 150 ° C for 30 minutes. The reactor is then heated to 300 ° C in a range of 90 ° C / h and is maintained at this temperature for 4 hours. The fluidizing nitrogen is then replaced by a dry air stream and the catalytic solid is successively heated from 300 to 600 ° C, maintained at 600 ° C for 4 hours, then finally cooled in a range of 90 ° C / ha to 300 ° C. ° C. The fluidizing air is then replaced by a stream of dry nitrogen and the catalytic solid is cooled slowly to room temperature. 25 grams of an activated catalyst containing 2.1% by weight of aluminum and 1% by weight of chromium are recovered; This catalyst is composed of particles with a weight average diameter of 100 μm. It is stored in an atmosphere of dry nitrogen. b) Polymerization of the Ethylene Gas Phase 200 grams of polyethylene powder prepared in a previous polymerization are introduced, in a stainless steel reactor with a capacity of 2.6 liters equipped with a pofvo agitator rotating at 350 rpm and maintained under a nitrogen atmosphere. The reactor is heated to 100 ° C and 210 milligrams of the above catalyst (8a) are introduced. Then 0.2 millimoles of triethylaluminum are introduced into the reactor to sweep the poisons. Hydrogen is then introduced to pressurize the reactor to 0.3 MPa. Finally ethylene is introduced until obtaining a total pressure of 1.5 MPa. The introduction of ethylene in a range of 183 g / h is continued for two hours and 18 minutes. The average activity during this period is 680 g / mM.h.b. 600 grams of polyethylene are recovered from the reactor; whose properties are: • Volume density 290 kg / m3 • High load melt index (ASTM method D-1238, measured at 190 ° C) 8.6 g / 10 minutes • Average particle size 1010 μm • Density 945.8 kg / m3 • Polydispersity 14.7 Example 90 a) Preparation of the Catalyst 30 kilograms of a chromium oxide-based catalyst composition sold under the registered trademark "EP30" (by Joseph Crosfield and Sons, Warrington, United Kingdom) in a heated reactor were introduced. at 93 ° C and fluidized with a stream of dry nitrogen. This solid composition contains about 1% by weight of chromium in the form of trivalent chromium acetate supported on silica. The reactor is then heated to 93 to 150 ° C in a range of 90 ° C / h and is maintained at 150 ° C for 30 minutes. A mixture of titanium tetraisopropylate and tetra-n -butylate sold under the registered trademark "Tilcom BU" (by Titanium Intermediates Limited, Billingham, United Kingdom) in an amount corresponding to 23.75 moles of titanium is then added to the reactor. The catalytic solid formed in this way is maintained at a temperature of 150 ° C for 2 hours. The reactor is then heated to 300 ° C in a range of 90 ° C / h and is maintained at this temperature for 4 hours. The fluidising nitrogen is then replaced by a stream of dry air and the catalytic solid is successively heated from 300 to 400 ° C, maintained at 400 ° C for 4 hours, then finally cooled in a range of 90 ° C / h to 300 ° C. C. The fluidizing air is then replaced by a stream of dry nitrogen and the catalytic solid is slowly cooled to room temperature. 30 kilograms of an activated catalyst with a content of 3.8% by weight of titanium and 1% by weight of chromium were recovered; this catalyst is composed of particles with an average weight diameter of 103 μm. It is stored in an atmosphere of dry nitrogen, b) Preparation of the Prepolymer A pre-polymerization operation is carried out in a reactor with a capacity of 1 m3 equipped with a heating jacket and a stirrer that rotates at 140 rpm. The prepolymerization is carried out at a temperature of 75 ° C in 450 liters of n-hexane in the presence of 15 grams of an antistatic agent, sold under the registered trademark "ASA-3" (by Shell, The Netherlands), the which contains 0.55 weight percent of both chromium and calcium, and 10 kilograms (1.92 moles of chromium) of the catalyst described above (9 a). The reactor is prepared with ethylene fed from a pressurized line at 1.0 MPa. A constant feeding range of 15 kilograms / h is established.

After 4 hours and 32 minutes of prepolymerization, 82.8 kilograms of prepolymer are formed. The liquid float is drained and the prepolymer is then dried in a stream recycled from dry nitrogen. The dry prepolymer obtained in this way has excellent flow properties and a volume density of 420 kg / m3. It is composed of particles with an average size of 169 μm. c) Phase polymerization of fluidised bed gas of ethylene 100 kilograms of a well-dried polyethylene powder prepared in a previous polymerization are charged to a fluidized bed reactor of 45 cm in diameter. This is fluidized at 96 ° C by a gas mixture composed of hydrogen, ethylene and nitrogen flowing up to 35 cm / s. In the gas mixture, the partial pressures of the constituents are: • hydrogen 0.30 MPa. • ethylene 1.22 MPa. • Nitrogen 0.48 MPa. 2 liters of a 0.1 molar solution of triethylaluminum in h-hexane are fed to the reactor and these conditions are maintained for one hour to initiate the reaction. The prepolymer prepared above (Example 9b) is then fed to the reactor in a range of 99 grams / h. After a period of stabilization of the polymerization conditions, 20 kilograms / h of polymer are produced with the following properties: • Volume density 3.45 kg / m3 • Melt index 0.90 g / 10 min • Flow parameter (n) 2.09 • Density 955 kg / m3 • Average particle size 1140 μm • Fine (<125 μm) 1.4% by weight • Residual chromium 5.5 ppm Example 10 a) Preparation of the Catalyst 280 kg of a catalyst composition based on the oxide was introduced. chromium sold under the registered trademark "EP30" (by Joseph Crosfield and Sons, Warrington, United Kingdom) in a reactor heated to 60 ° C and fluidized with a stream of dry nitrogen flowing up to 12 cm / s. This solid composition contains about 1% by weight of chromium in the form of trivalent chromium acetate supported on silica. The reactor is then heated to 60 to 150 ° C in a range of 90 ° C / h and is maintained at 150 ° C for 30 minutes. Subsequently, 80 kilograms of a mixture of titanium tetraisopropylate and tetra-butylate sold under the registered trademark "Tilcom BIP" (by Titanium Intermediates Limited, Billingham, United Kingdom) are added to the reactor. The catalytic solid formed in this way is maintained at a temperature of 150 ° C for 2 hours. The reactor is then heated to 300 ° C in a range of 90 ° C / h and is maintained at this temperature for 4 hours. The fluidifying nitrogen is then replaced by a dry air stream and the catalytic solid is successively heated from 300 to 450 ° C, maintained at 450 ° C for 4 hours, then finally cooled in a range of 90 ° C / h to 200 ° C. C. The fluidizing air is then replaced by a stream of dry nitrogen and the catalytic solid is cooled slowly to room temperature. b) Phase polymerization of fluidised bed gas of ethylene 40 tons of a very dry polyethylene powder prepared in a previous polymerization are introduced into a fluidized bed reactor of 3.5 m in diameter. This is fluidized at 94.5 ° C by a gas mixture composed of hydrogen, ethylene, ethane, pentane and nitrogen flowing up to 49 cm / s. In the gas mixture, the partial pressures of the constituents are: hydrogen 0.30 MPa. ethylene 0.39 MPa. Ethane 0.20 MPa. pentane 0.10 MPa. Nitrogen 1.21 MPa. Pentane is a mixture of n-pentane and isopentane. The catalyst prepared above (Example 10a) is then fed to the reactor in a range of 2 kilograms / h. After a period of stabilization of the polymerization conditions, 5 tons / h of polymer were produced with the following properties: • Bulk density 380 kg / m3 • Melt index (5 kg load) 1.2 g / 10 min • Parameter of flow (n) 2.26 • Density 956.1 kg / m3 • Average particle size 1080 μm • Fine (<125 μm) 1.4% by weight • Residual chromium 4.0 ppm • Resistance to stress cracks 8.9 hours • Critical cut-off range 1390s-1 Example 11 (comparative) a) Catalyst preparation 400 kilograms of a catalyst composition based on chromium oxide sold under the registered trademark "EP30" (by Joseph Crosfield and Sons, Warrington, United Kingdom) in a reactor heated to 60 ° C and fluidized with a stream of dry nitrogen flowing up to 12 cm / sec. This solid composition contains about 1% by weight of chromium in the form of trivalent chromium acetate supported on silica. The reactor is then heated to 60 to 150 ° C in a range of 90 ° C / h and is maintained at 150 ° C for 30 minutes. Subsequently, 114 kilograms of a mixture of titanium tetraisopropylate and tetra-butylate sold under the registered trademark "Tilcom BIP" (by Titanium Intermediates Limited, Billingham, United Kingdom) were added to the reactor. The catalytic solid formed in this way is maintained at a temperature of 150 ° C for 2 hours. The reactor is then heated to 300 ° C in a range of 90 ° C / h and is maintained at this temperature for 4 hours. The fluidifying nitrogen is then replaced by a stream of dry air and the catalytic solid is successively heated to 300 to 550 ° C, maintained at 550CC for 4 hours, then finally cooled in a range of 90 ° C / h to 200 ° C. The fluidizing air is then replaced by a stream of dry nitrogen and the catalytic solid is slowly cooled to room temperature. b) Phase polymerization of fluidized bed gas of ethylene 40 tons of a very dry polyethylene powder prepared in a previous polymerization are introduced into a fluidized bed reactor of 3.5 m in diameter. This is fluidized at 101.5 ° C by a gas mixture composed of hydrogen, ethylene, ethapplirithane and nitrogen flowing up to 49 cm / s. In the gas mixture, the partial pressures of the constituents are: hydrogen 0.30 MPa. ethylene 0.32 MPa. Ethane 0.20 MPa. pentane 0.10 MPa. Nitrogen 1.28 MPa. Pentane is a mixture of n-pentane and isopentane. The catalyst prepared above (Example 11a) is then fed to the reactor in a range of 2 kilograms / h. After a period of stabilization of the polymerization conditions, 5 tons / h of polymer was produced with the following properties: • Bulk density 380 kg / m3 • Melt index (5 kg load) 1.2 g / 10 min • Parameter flow rate (n) 2.13 • Density 954.8 kg / m3 • Average particle size 1190 μm • Fine (<125 μm) 1.6% by weight • Residual chromium 3.5 ppm • Resistance to stress fractures 7.0 hours • Critical cutting range 910s -1

Claims (6)

1. Process for the preparation of a supported catalyst based on chromium oxide for the polymerization of olefin (s) characterized in that the supported catalyst based on chromium oxide is subjected to a single stage of calcination / activation which consists of a single thermal treatment in a temperature in a range of 200 to 450 ° C under an oxygen-containing atmosphere, with the proviso that the supported catalyst does not comprise an aluminum phosphate support.

The process as described in Claim 1, further characterized in that the single term treatment is carried out at a temperature in a range of from 300 to 400 ° C.

3. The process as described in the preceding Claims further characterized in that the oxygen-containing atmosphere is air.

4. The process as described in any of the foregoing Claims further characterized in that the supported catalyst based on chromium oxide is a supported catalyst based on chromium oxide with titanium or modified aluminum.

The process as described in Claim 4, further characterized in that the supported catalyst based on chromium oxide is a supported catalyst based on chromium oxide with modified titanium.

6. The supported catalyst based on chromium oxide for the polymerization of the olefin gas phase (s) obtainable by means of the preceding claims. Process for the polymerization of the gas phase of at least one alpha-olefin containing from 2 to 12 carbon atoms, further characterized in that the polymerization is carried out with the aid of a supported catalyst based on chromium oxide which has been subjected to a single calcination / activation stage, consisting of a single heat treatment carried out in a temperature range from 200 to 450 ° C under an oxygen-containing atmosphere, with the proviso that the supported catalyst does not comprise catalysts on a support of aluminum phosphate.