CH708039B1 - Mixing of raw material for metallurgy of powders. - Google Patents

Abstract

The subject of the invention is a ceramic granulator mixer with a tank (2), a mixing means (3), and heat exchange means (4) comprising cooling means (42) for cooling the contents of the tank. the tank (2). It comprises control means arranged to control the heat exchange means (4) which comprise heating means (41) arranged for heating the contents of the tank (2) to a temperature between a lower temperature (TINF) and a higher temperature (TSUP) stored for a particular mixture, and the heating means (41) are arranged to exchange the energy with a heat exchange circuit (8) maintaining the mixing temperature, external to the tank (2). ), and whose thermal inertia of the circuit (8) is greater than that of the tank (2) at full load. A kneading process is also claimed.

Description

Description

Field of the Invention [0001] The invention relates to a kneader for the manufacture of ceramic-type granules comprising at least one oxide powder and at least one binder, said kneader comprising at least one tank in which at least one mixing means, and comprising heat exchange means comprising cooling means for cooling said tank and / or its contents.

The invention also relates to a process for mixing raw material for powder metallurgy, in particular for producing granules of a given type of ceramic from a mixture comprising at least one oxide powder and at least one less a binder.

The invention relates to the field of powder metallurgy for obtaining ceramics, and in particular the kneading process of the mixture of raw materials to form an intermediate material for feeding an injection press for shaping of the component to be produced.

BACKGROUND OF THE INVENTION [0004] In the manufacture of hard materials for jewelery and the watch industry, or even electronics or telephony, particularly hard materials generally referred to as the generic name "ceramics", uses powder metallurgy techniques. We will call here "ceramic" the synthetic material obtained, whatever the nature of this material, sapphire, ruby, artificial diamond, sapphire crystal, ceramic, micro-magnet, or other.

The basic raw materials are of different kinds, some are kept secret for the protection of production. In general, the raw materials used comprise at least, on the one hand, ceramic powder, and on the other hand binders such as resins or plastics or the like which allow injection and good resistance of the component produced with the mixture of all the raw materials; other additives may be included in the mixture. It is understood that the raw materials can be of different textures: solid, powdery, liquid, or pasty. The mixture may change structure during its preparation, in particular, and not limitatively, when complementary components of a resin undergo a polymerization reaction.

The overall method of manufacturing a ceramic component comprises at least the following steps: - preparation of raw materials; - mixture (s) of the raw materials, or / and pre-mix two to two (or more) if necessary; homogenization mixing; - Pressing, in particular in a molding chamber, a quantity of powder or granules from the mixing, for producing a component blank. This pressing can be carried out by injection, under pressure, in particular in a screw injector comprising means for heating this quantity of powder or granules resulting from mixing; - debinding stoving for the combustion and / or dissolution of certain components of the binder mixture heat treatment of the component blank, or sintering; heat treatment of the blank at the debinding end, for the sintering giving its final coherence to the finished component. This heat treatment causes a dimensional shrinkage, which makes it possible to obtain a component in finished dimensions; - appearance finishing treatment of the component.

This simplified explanation of the process hides the actual development complexity, which is specific to each composition of raw material mixture, and to each type of finished component according to its physical characteristics, in particular wear resistance and durability. aspect, and according to its mechanical and chemical characteristics.

The conduct of each step is tricky, and requires compliance with specific parameters, otherwise irreversible changes in the characteristics of the mixture, the blank, the debonded blank, or the sintered component.

[0009] In particular, the homogenization kneading step is crucial for the rest of the process. This mixing step can in certain cases be combined with the preliminary step of mixing the raw materials, which can be done directly in the manufacturing station that is called here "kneader".

Indeed, during the mixing occur reactions between certain raw materials, and these reactions immediately change the physical conditions in which the mixture is being kneaded. In particular, uncontrolled or uncompensated exothermic reactions can lead to complete alteration of the mixture, which is then unusable for the manufacture of the final component provided. The parameters of temperature, speed, and torque are to be closely monitored. The repetitiveness of the physical characteristics obtained in fine is an obligatory condition, which imposes a perfect regulation of the mixing, the anticipation and the control of the reactions which take place there.

In particular, when kneading such a mixture with rotary knives in a kneader, the elements of the mixture rise very quickly in temperature, under the effect of friction, to exceed their melting temperature and s' to amalgamate in pasty form. The problem lies in the extremely high temperature gradient in the mixture when it arrives in the vicinity of this or these melting temperatures, with a value of the order of several ° C. per second, in particular 10 ° C. per second. It is then very difficult to apply effective cooling to prevent runaway and deterioration of the mixture. SUMMARY OF THE INVENTION [0012] The invention proposes to improve the kneading in metallurgy of powders for obtaining ceramics, so as to obtain a production of very reproducible quality, with a controlled withdrawal coefficient, with an amplitude of relative dispersion less than 1 per thousand.

For this purpose, the invention relates to a kneader for the manufacture of granules of the ceramic type comprising at least one oxide powder and at least one binder, said kneader comprising at least one tank in which at least one means is movable. kneading machine, and comprising heat exchange means comprising cooling means for cooling said tank and / or its contents, characterized in that said kneader comprises control means connected to measuring means and means storage of temperature parameters according to the type of ceramic to be produced, and characterized in that said control means control said heat exchange means which comprise heating means arranged for heating said tank and / or its contents at a temperature between a lower temperature from which a mixture corresponding to a given type of ceramic reaches in the pasty state, and an upper temperature below which must remain said mixture corresponding to said given ceramic type, said lower temperature and said upper temperature being stored, for said mixture corresponding to a given type of ceramics, in said means storage device, and further characterized in that said heating means exchange the energy, in a first connection, with a first heat exchange circuit maintaining the mixing temperature, external to said tank, and whose thermal inertia said first circuit is greater than that of said tank full load said mixture in a first factor.

According to one characteristic of the invention, the cooling means exchange the energy, in a second connection, with a second circuit at ambient temperature, external to said tank, and whose thermal inertia is much greater than that of said vessel at full load of said mixture in a second factor.

The invention also relates to a process for mixing raw material for metallurgy of powders, in particular for the production of granules of a given type of ceramic from a mixture comprising at least one oxide powder and at least one less a binder, according to which: - said mixture is introduced into the tank of a kneader comprising at least one kneading means; - Temperature stabilization, by connection of heat exchange means to a first heat exchange circuit maintaining the mixing temperature said tank and its contents in the vicinity of a mixing temperature between a lower temperature from which said mixture reaches the pasty state, and at an upper temperature below which said mixture must remain; - is set to move at a speed less than or equal to 700 revolutions per minute said mixing means; the mixture is kneaded until a homogeneous compact mass is obtained; stopping the stabilization at high temperature, greater than or equal to a temperature specific to the mixture considered and characteristic of a homogeneous compact mass, of said tank and of its contents, the temperature of which is allowed to be lowered, either naturally or by connecting said heat exchange means to a second circuit (9) at ambient temperature of 20 ° C; - Said compact mass, or in said tank at a temperature below 100 ° C and at a speed greater than or equal to 700 revolutions per minute of said mixing means, or in a crushing station annex to said mixer.

BRIEF DESCRIPTION OF THE DRAWINGS [0016] Other features and advantages of the invention will appear on reading the detailed description which follows, with reference to the appended drawings, in which: FIG. 1 represents, schematically, the driving system of a kneader according to the invention; fig. 2 shows, schematically, partially (the connections with the control system being only trimmed) and in section passing through the axis of a kneading shaft, a kneader according to the invention; fig. 3 and FIG. 4 show, schematically and in side view, two kneading trees assembled in different compositions; fig. 5 shows, schematically, partially (the links with the control system not being shown) and in top view, a mixer according to the invention; fig. 6 shows schematically, partially (the connections with the control system not being shown) and in view from above, a mixer according to the invention, the mixing shaft was extracted, and replaced by a set mobile crushing shutters of a slab, in combination with an endless screw housed in a groove of the bottom of the tank; fig. 7 represents, in the form of a block diagram, a mixing process sequence according to the invention in a mixer according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The so-called ceramic powder more particularly used in the context of the invention is a powder of oxides, for example oxides of zirconia or alumina, carbides, nitrides, or the like, especially in oxide mixtures.

These oxides make it possible to guarantee a high hardness, a high resistance to wear, a high resistance to mechanical stresses, and exceptional durability without alteration.

Binders used, resins or plastics or the like, used to bring the oxide powder to the pressing operation and / or injection, with a sufficient viscosity for its flow in a mold, while having sufficient strength to prevent deformation.

Kneading is intended to coat the powder grains with the binder or binders, so as to obtain a homogeneous paste.

This homogeneous paste becomes a compact mass after cooling the output of the kneading. This compact mass is then fractionated by crushing to obtain granules of homogeneous composition and calibrated dimensions, which are called "feedstocks" and which are ready to be used in the state of supply of an injection press, for example .

In a particular implementation, and not limiting, of the invention, using a kneader 1 for the manufacture of ceramic type granules comprising at least one oxide powder and at least one binder, with at least one tank 2. This mixer 1 comprises at least one mixing means 3 immersed in a corresponding tank 2.

The kneader 1 comprises heat exchange means 4, which may comprise at least one circuit in which a fluid flows in a double wall of a tank 2, or circulates in coils immersed in a tank 2, or other .

In particular, these heat exchange means 4 comprise cooling means 42 for cooling the tank 2 and / or its contents.

According to the invention, the mixer 1 comprises control means 5 connected to measuring means 6 and storage means 7 of temperature parameters depending on the type of ceramic to be produced.

These control means 5 are arranged for the temperature regulation of the tank 2 and the heat exchange between the tank 2 and at least one external medium to the tank 2, by the heat exchange means 4.

These control means 5 control in particular these heat exchange means 4, in correlation with values of tree speed and / or compacted mass flow measured, compacted mass or / and tank temperatures measured. , and with threshold values, in particular of temperature, imposed for the manufacture of a given product. The set of parameters relating to a given product, stored in the storage means 7, advantageously makes it possible to control the whole of its manufacturing cycle, including all the desired timings.

According to the invention, the heat exchange means 4 further comprise heating means 41 arranged for heating the vessel 2 and / or its contents to a temperature between a lower temperature TINF from which a Mixture corresponding to a given type of ceramic reaches the pasty state, and at a higher temperature TSUP below which must remain the mixture corresponding to the given ceramic type.

This lower temperature TINF and this upper temperature TSUP are stored, for the mixture corresponding to a given type of ceramics, in the storage means 7.

These heating means 41 exchange the energy, in a first connection, with a first heat exchange circuit 8 for maintaining the mixing temperature, external to the tank 2. And the thermal inertia of the first circuit 8 is greater than that of the tank 2 at full load of the mixture. Preferably it is higher in a factor K1 greater than 2.

The fact of heating the tank 2 and its contents goes against prejudices of the prior art. The heating makes it possible to have less deviation from an average temperature, and to completely control the temperature gradient. It is no longer necessary to rotate the kneading shaft at high speed to achieve the melting temperatures of the components. The production obtained is more homogeneous, which is crucial in powder metallurgy, because it is a question of perfectly controlling the shrinkage coefficient during sintering, and this coefficient depends on the quality of the kneading. For example, for a production of ceramic based on mineral raw materials, the prior art made it possible to obtain for a production in 5 daily batches of 20 kg each, a withdrawal coefficient in the range 1.2850 to 1.2920, while that the implementation of the kneader according to the invention and the kneading process associated allows, all things being equal, to reduce this coefficient in the range 1.2880 to 1.2890, which is excellent, since the amplitude of the relative dispersion is now 0.8 per thousand against 5.4 per thousand in the prior art, a division by a factor of about 7. Production is thus very reproducible.

In a preferred version, the cooling means 42 are distinct from the heating means 41, as shown in FIGS. 2 and 5.

Advantageously, the cooling means 42 exchange the energy, in a second connection, with a second circuit 9 at ambient temperature, external to the tank 2, and whose thermal inertia is much greater than that of the tank 2 at full load of the mixture, and preferably higher in a second factor K2 greater than 2.

Preferably, the control means 5 control the heat exchange means 4 so as to activate the heat exchange with the tank 2, at a given instant, the only cooling means 42 or only heating means 41 .

It is nevertheless possible to use a single heat exchange circuit at the tank, alternatively put in a heat exchange situation with a hot source or with a cold source. But the solution of two separate circuits, on which one can connect the tank instantaneously, overcomes the effects of the thermal inertia specific to the tank 2 by its instantaneous communication with a circuit of inertia thermal superior to his, allowing to stabilize very quickly in a range of temperatures consistent with the proper conduct of the process.

The mixing means 3 preferably comprises, and not limited to, a shaft 30 in rotation carrying blades 33 and / or knives in the tank 2. Each mixing shaft 30 is preferably driven by a motor 31 equipped with a continuous drive connected to control means 5 which control this drive. The shaft 30 is preferably equipped with a tachometer dynamo 63 which communicates the real speed of rotation of the shaft 30 to the control means 5.

Figs. 1 and 2 illustrate the action of the control means 5, on the basis of at least one speed information of the mixing means 3 or a mass of product being processed by the mixer 1, and at least a temperature information of the tank 2 or of this mass of product, read by sensors included in the measuring means 6, for controlling the speed of the motor 31 driving the mixing shaft 30, and a heat exchange rate, in particular by a first pump 81 on the first circuit 8 of the heating means 41.

When, preferably, the cooling means 42 exchange the energy, in a second connection, with a second circuit 9, the control means 5 also act on a second pump 91 on the second circuit 9 means of cooling 42.

The control means 5 comprise a clock 51, making it possible to respect the process parameters introduced in the storage means 7.

The measuring means 6 may, in particular and not limited to, include all or part of the following sensors: - a temperature sensor 61 in the first circuit 8 of the heating means 41, preferably in the tank 2 or the most near the tank; a temperature sensor 62 in the second circuit 9 of the cooling means 42, preferably in the tank 2 or as close as possible to the tank; a tachometric dynamo 63 for measuring the speed of rotation of the mixing shaft 30; a motion sensor 64 characterizing the movement of the dough in the tank, in particular a rotational speed sensor of a worm or a freewheel mounted on an axis at the bottom of the tank, or the like; a temperature sensor 65 at the heart of the mixture or of the paste, in particular coupled with the preceding motion sensor 64; a temperature sensor 66 on an inner surface of the tank 2, preferably in the vicinity of the bottom of the tank; a temperature sensor 67 of the mixing shaft 30, preferably towards the end of the latter in the vicinity of the bottom of the tank 2; a temperature sensor 68 in a large reservoir of the first exchange circuit 8; a temperature sensor 69 in a large reservoir of the second exchange circuit 9.

The control means 5 may, again, act on a first regulator 82 making a contribution (or removal) of heat to the first circuit 8, and / or act on a second regulator 92 performing a removal (or a contribution) heat of the second circuit 8, these first 82 and second 92 controllers may include a resistor or / and a cold group. Preferably, the first circuit 8 carries oil, while the second circuit 9 conveys brine, or the like.

In a particular embodiment, the mixer 1 comprises a plurality of tanks 2 and equipped, communicating with each other from an upstream tank where the raw materials are discharged by a feed 21 such as a hopper or the like, up to a downstream tank used in particular for final crushing of the compacted kneaded mass. This downstream tank can have a dual function of mixing tank and crushing tank: the raw materials in mixture are poured therein by the upstream tank, at least one mixing shaft performs the mixing itself with blades or and shaped knives adapted to the turning of a pasty mass in the mixing tank and its separation by slicing, the final crushing can be performed, as the case, by such a mixing shaft 30, or by at least one shaft crusher equipped with knives 22 more precisely adapted to the fragmentation of a solidified solid mass. If necessary an auxiliary crusher can be used downstream to achieve the desired particle size.

FIG. 2 illustrates the case of a single vessel 2, in which the entire mixing process is carried out, from the introduction of raw materials, to the crushing of the cooled kneaded mass called slab.

More particularly, as shown in FIG. 2, the shaft 30 is vertical, and comprises in particular blades 33, preferably distributed in several parallel planes.

The staggering of the blades and / or knives is preferably adjustable, so as to be effective both for small loads and for large ones: or else the entire kneading shaft is interchangeable, at a coupling 32, or it comprises a succession of bushes carrying blades or knives, resting on each other and strung on a common shaft, and separated if necessary by spacers 35 to obtain a particular configuration, as visible on the fig. 3 and 4, where the shaft 30 is thus equipped with three sets of lower blades 33A, 33B, 33C, surmounted by upper blades 34. If the arrangement in substantially plane levels of the blades or knives is the most common, one can also use , especially for the upper level suitable for large loads, blades 34 or knives inscribed in a substantially conical envelope relative to the axis of the shaft. By blades is meant substantially radial fins having a shape to give a particular movement, the mixture of raw materials first, and the paste afterwards. By knives are meant fins of similar shape and finer section, and having a sharpened leading edge, in particular for slicing the pasty mass while driving it in motion.

In known manner, the blades 33 and / or knives preferably have a slight incidence relative to the plane perpendicular to the axis of the shaft 30. This incidence can be adjusted, either very simply by exchange of a floor cutter mounted on a sleeve as presented above, either in larger installations with a return mechanism which however is more sensitive to wear generated by the movement of the dough. Depending on the case, the incidence can be adjusted, depending on the direction of rotation of the shaft, either to push the compact mass to the bottom of the tank, or on the contrary to tend to take off: a mixed embodiment certainly consumes more than power, but an upper stage tending to take off the dough from the bottom of the tank facilitates its amalgam, while a lower cutting stage tends to fold the dough to the bottom of the tank is advantageous especially in the final stages of the process and in the fractionation of the slab obtained after cooling of the pasty compact mass.

Crushing of the wafer may also be performed, after a vertical clearance of the mixing shaft 30, by the combined action of a worm 37 embedded in a groove 39 at the bottom of the tank 2, and flaps 36 articulated on a vertical axis, the extraction of crushed pellets being by reversing the direction of the worm and conveying to a service station 38.

In another variant, the crushing is conducted until obtaining a flour. This flour is transformed downstream, in an auxiliary granulation station where it is first compressed to form an extruded pudding, cut into pellets as and when it advances.

Each tank 2 is preferably provided with a closure means comprising at least one valve or a pressure relief outlet.

The temperature exchange, at the level of the tank in which the mixing is carried out, allows: by a rise in temperature, a softening of certain binders, under a maximum maximum threshold softening temperature; by maintaining at a temperature kept constant a first heat exchange circuit 8 maintaining the mixing temperature, whose thermal inertia is much greater than that of the tank at full load, to control the thermal gradient of the compact mass in the tank to a value below 3 ° C per minute during the friction of the components of the mixture, to be compared with a gradient of the order of 10 ° C per second in kneaders of the prior art only provided with cooling means, this low gradient value obtained by the implementation of the invention allowing mixing at a lower speed; by a decrease in temperature, maintaining the temperature of the dough under a maximum limit specific to the mixture defined to prevent any alteration of its properties, and in particular when exothermic reactions occur between certain constituents of the binder, and / or when the kneading speed is too high, and / or when the friction in the mixture or with the blades / knives or with the tank is too great; - By a rapid decrease in temperature following the disengagement of the heat exchange circuit maintaining the mixing temperature, the cooling of the compact mass previously kneaded for its solidification; this rapid decrease in temperature can be obtained by connecting the heat exchange means 4 to a second circuit 9 at room temperature, whose thermal inertia is much greater than that of the tank at full load.

The speed control of rotation of at least one mixing shaft allows: - by a decrease in speed, the lowering of the friction described above; - By a decrease in speed in the final phase, a progressive intake of the compact mass until it solidifies in the form of a slab; by an increase in speed, the improvement of the agglomeration of the components of the binder around the grains of oxide powder; by an increase in speed, the fragmentation in the form of granules of the compact mass previously solidified in the form of a slab.

The conduct, as a function of time, the temperature exchange means and the mixing speed therefore conditions the quality of the final product, along with the parameters specific to the intermediate product, in particular its viscosity. The good management of this pipe naturally conditions the cycle time in the mixer, and therefore the cost of production and the depreciation of the installation.

In general, efforts are made to maintain both the temperature and the mixing speed under threshold values specific to each mixture.

The mixer 1 may, again, be equipped with means for measuring the speed of circulation of the compact mass in the tank, for example at a mobile 60 such as a worm or a wet wheel immersed in the mass inside the vessel, the rotational speed of which is measured by a dough movement sensor 64, and, advantageously, the core temperature of the pasty compact mass by a dough temperature sensor 65.

The means for measuring the temperature of the compact mass are located, or at such a mobile 60, and / or at the bottom of the tank 2 by a temperature sensor 66 on an inner surface of the tank 2, or and at the periphery of the mixing shaft 30, by a temperature sensor 67 preferably at its lower part near the bottom of the tank 2.

The process for mixing raw material for metallurgy of powders, according to the invention, in particular for the production of granules of a given type of ceramic from a mixture comprising at least one oxide powder and at least one less a binder, is carried out comprising at least the following steps: - the mixture is introduced into the tank 2 of a kneader 1 comprising at least one kneading means 3; - Temperature stabilization, by connecting heat exchange means 4 to a first heat exchange circuit 8 maintaining the mixing temperature tank 2 and its contents in the vicinity of a mixing temperature between a lower temperature TINF specific to the mixture in question and from which the mixture reaches the pasty state, and at a higher TSUP temperature specific to the mixture under consideration and below which the mixture must remain; the kneading means 3 is set at a speed of less than or equal to 700 revolutions per minute; the mixture is kneaded until a homogeneous compact mass is obtained; stopping the stabilization at high temperature, greater than or equal to a temperature T5 proper to the mixture considered and characteristic of a homogeneous compact mass, of the tank 2 and its contents, which is allowed to fall in temperature, either in a natural way or by connecting the heat exchange means 4 to a second circuit 9 at an ambient temperature of 20 ° C; the compact mass is crushed, either in the tank 2 at a temperature below 100 ° C. and at a speed greater than or equal to 700 rpm of the kneading means 3, or else in a crushing station attached to the kneader 1.

A particular sequence of use of the kneader 1 is presented below, for an example of production batches with a mass of about 5 kg, with the following steps, which specify in particular what is meant by slow speed , high speed, high temperature: - 100: loading at the feed 21 of a first part of a charge of powder and structuring agents, launching of the tank temperature regulator at the maximum temperature of TO = 180 ° C. by activation of the heating means 41 and deactivation of the cooling means 42, start of rotation of the mixing shaft 30 at VO = 300 rpm; - 110: after reaching a temperature of T1 = 145 ° C and a rotation speed of V1 = 300 revolutions per minute, loading a second part constituting the rest of the charge of powder and structuring agents; - 120: after reaching a temperature of T2 = 160 ° C, stop rotation of the shaft 30, opening of the tank 2, inspection, scraping of the walls and blades / knives if necessary (this phase of inspection can be assisted by a camera, however the protection against pollution is difficult, the best fouling control of the tank 2 and blades 33 and 34 can be performed by measuring the torque or the power absorbed at the motor 31 , with reference to reference values of a reference production stored in the storage means 7); - 130: reversion, after reaching a temperature of T3 = 168 ° C and a rotation speed of V3 = 700 revolutions per minute, stopping the rotation of the shaft 30, opening the tank , stage 135 inspection, scraping of the walls and blades / knives step 136 if necessary; - 140: reversion, after reaching a temperature of T4 = 170 ° C and a rotation speed of V4 = 700 rpm, mixing for a pre-defined time D4; - 150: measurement of the temperature of the compact mass, which must be between T5 = 180 ° C and T6 = 200 ° C (test step 155), further mixing until reaching this temperature range; - 160: stopping the rotation of the shaft 30, cooling by deactivating the heating means 41 and activation of the cooling means 42;

Claims (13)

- 170: after reaching a temperature between T7 = 150 ° C and T8 = 180 ° C (test step 175), rotating the compact mass to unlock the blades / knives and / or improve shear; 180: point rotations at V9 = 300 revolutions per minute to constitute a slab, and cooling at a temperature between T9 = 95 ° C. and T10 = 150 ° C. by deactivation of the heating means 41 and activation of the cooling means 42; - 190: verification of the absence of pollution, and stop of any rotation in case of pollution and then step 195: manual finishing of the cut of the slab; 200: crushing at V11 = 700 rpm; 210: stopping the rotation of the shaft 30; - 220: evacuation to less than V12 = 2000 rpm and less than T12 = 85 ° C. In this example, the total engine cycle time is between 20 and 30 minutes, the total cooling time is between 20 and 30 minutes, the total evacuation time is between 10 and 15 minutes. Such a sequence is particularly suitable for a mixture of raw materials comprising 7.74% by weight of a binder which itself comprises 25% by volume of a constituent material of a structuring matrix, 8% by volume of a material giving resistance to bending and cold break, 4% of a material giving resistance to bending at high temperature, 45% of a thickening and stiffening material, 3% of a constituent material a fluidizing matrix, and 15% of a demolding and antioxidant material. This range of work, used with a mixer 1 as defined above, prevents many hazards of the prior art: - all temperature gradients are controlled and accurate; the rise in temperature of the mixture and of the compact mass is strictly limited to a predefined maximum threshold, here equal to T6 = 200 ° C .; - The cooling time is reduced by means of heat exchange which allows cooling of the compact mass, either directly or through the vessel; the temperature of the compact mass can be maintained at a given value when the rotation of the shaft is stopped, thanks to the heat exchange means which allows a heating or cooling of the compact mass, either directly or by means of intermediate of the tank; - the temperature of the compact mass is correctly approximated by the temperature of the tank, it is even better determined with a submerged sensor; the powder of the mixture no longer adheres to the walls of the tank during the rise in temperature; the regulation makes it possible to limit the wear of the wall of the tank and that of the blades / knives, and as a result the pollution is very much reduced, and the equipment is used much less quickly; - The process control can be performed with the closed vessel; optical monitoring, in particular by camera, can make it possible to determine a possible need for scraping the walls of the tank, which theoretically is less fouled than in the prior art by a gradual rise in temperature; the homogenization of the mixture of the compact mass is satisfactory, and as a result the granules or feedstocks have identical and repetitive behavior at the level of the injection pressing; - The energy consumption of drive, heating, and cooling, are reduced. claims 1. Mixer (1) for producing ceramic granules comprising at least one oxide powder and at least one binder, said mixer (1) comprising at least one tank (2) in which at least one mixing means is movable (3), and comprising heat exchange means (4) comprising cooling means (42) for cooling said tank (2) and / or its contents, characterized in that said mixer (1) comprises control means (5) connected to measuring means (6) and to storage means (7) of temperature parameters depending on the type of ceramic to be produced, and characterized in that said control means (5) are arranged to drive said heat exchange means (4) which comprise heating means (41) arranged for heating said tank (2) and / or its contents to a temperature between a lower temperature (TINF) from of which a mixture corresponding to a given type of ceramic reaches the pasty state, and a higher temperature (TSUP) below which must remain said mixture corresponding to said given ceramic type, said lower temperature (TINF) and said upper temperature (TSUP) being stored , for said mixture corresponding to a given type of ceramic, in said storage means (7), and further characterized in that said heating means (41) are arranged to exchange the energy, in a first connection, with a first circuit (8) heat exchange maintaining the mixing temperature, external to said tank (2), and whose thermal inertia of said first circuit (8) is greater than that of said tank (2) at full load of said mixture . 2. Mixer (1) according to claim 1, characterized in that the thermal inertia of said first circuit (8) is greater than that of said tank (2) at full load of said mixture in a factor (K1) greater than 2. 3. Mixer (1) according to claim 1 or 2, characterized in that said cooling means (42) are arranged to exchange the energy, in a second connection, with a second circuit (9) at room temperature, external to said tank (2), and whose thermal inertia of said second circuit (9) is greater than that of said tank (2) at full load of said mixture in a factor (K2) greater than 2. 4. Mixer (1) according to claim 1 or 2, characterized in that said control means (5) are arranged to control said heat exchange means (4) so as to activate the heat exchange with said tank (2). ), at a given time, the only cooling means (42) or only said heating means (41). 5. Mixer (1) according to one of claims 1 to 4, characterized in that said control means (5), on the basis of at least one speed information of said mixing means (3) or a mass of product being processed by said mixer (1), and at least one temperature information of said tank (2) or of said product mass, read by sensors that comprise said measuring means (6), are arranged to control, on the one hand the speed of a motor (31) arranged to drive a mixing shaft (30) that comprises said mixing means (30), and on the other hand a heat exchange rate by a first pump (81) on said first circuit (8) of said heating means (41). 6. Kneader (1) according to claims 3 and 5, characterized in that said control means (5) are arranged to control a heat exchange rate by a second pump (91) on said second circuit (9) of said means cooling circuit (42) during energy exchange between said cooling means (42) and the second circuit (9). 7. Mixer (1) according to claim 3 or 4, characterized in that said control means (5) are arranged to control a first regulator (82) arranged to perform a supply or removal of heat to said first circuit (8) , or / and a second regulator (92) arranged to effect a removal or a supply of heat to said second circuit (8). 8. Mixer (1) according to one of claims 3 to 6, characterized in that said first circuit (8) conveys oil, and in that said second circuit (9) conveys brine. 9. Mixer (1) according to one of claims 1 to 8, characterized in that said measuring means (6) comprise a temperature sensor (61) in said first circuit (8) of said heating means (41), in said tank (2) or as close as possible to the tank (2), a temperature sensor (62) in said second circuit (9) of said cooling means (42), in said tank (2) or as closely as possible possible from the tank (2), and a tachogenerator (63) for measuring the speed of rotation of a mixing shaft (30) that comprises said mixing means (3). 10. Mixer (1) according to one of claims 1 to 9, characterized in that said measuring means (6) comprise a movement sensor (64) characterizing the movement of the dough in said tank (2), in the form of a rotational speed sensor of a mobile (60) such as a worm or idler immersed in the mass inside the tank, and a temperature sensor (65) at the heart of the mixture or paste, coupled with said motion sensor (64). 11. Mixer (1) according to one of claims 1 to 10, characterized in that said measuring means (6) comprise a temperature sensor (66) on an inner surface of said vessel (2), near the bottom of said tank (2). 12. Mixer (1) according to one of claims 1 to 11, characterized in that said measuring means (6) comprise a temperature sensor (67) of a mixing shaft (30) that comprises said mixing means (3), in the vicinity of the bottom of said tank (2). 13. Process for mixing raw material for powder metallurgy, in particular for producing granules of a given type of ceramic from a mixture comprising at least one oxide powder and at least one binder, according to which: said mixture is introduced into the tank (2) of a kneader (1) comprising at least one kneading means (3); - temperature stabilization, by connecting heat exchange means (4) to a first heat exchange circuit (8) for maintaining the mixing temperature said tank (2) and its contents in the vicinity of a mixing temperature comprised between a lower temperature (TINF) from which said mixture reaches the pasty state, and a higher temperature (TSUP) below which said mixture must remain; - The stirring means (3) is set at a speed of less than or equal to 700 revolutions per minute; the mixture is kneaded until a homogeneous compact mass is obtained; stopping the stabilization at high temperature, greater than or equal to a temperature (T5) proper to the mixture considered and characteristic of a homogeneous compact mass, of said tank (2) and of its contents, the temperature of which is allowed to be lowered, or in a natural way, either by connecting said heat exchange means (4) to a second circuit (9) at an ambient temperature of 20 ° C; said compact mass is crushed, or in said tank (2) at a temperature below 100 ° C. and at a speed greater than or equal to 700 revolutions per minute of said mixing means (3), or else in an auxiliary crushing station mixer audit (1).