EP0003704B1 - Method of and apparatus for continuously mixing powdery solids and liquids, especially plaster with water - Google Patents

Description

The present invention relates to the mixture of pulverulent solids and liquids and it finds an application in the preparation of hydraulic binders and in particular in the continuous preparation of a mixture of plaster and water.

It is known, to ensure the mixing of pulverulent solids and liquids, to use mixers of the type of coulter or paddle mixers, formed of a cylindrical tank in which rotates a vertical shaft provided with one or more levels of radial arms terminated by blades and / or plowshares. The blades or the coulters scrape the walls of the tank, turn the products over and thus carry out a stirring. But such mixers do not ensure sufficient dispersion of the pulverulent solid in the liquid, hence a heterogeneity of the fluidity of the outgoing mixture. On the other hand, turbine mixers are known which consist of a tank in which rotates at very high speed. a disc, a propeller or a turbine. The solid and the liquid are received on the turbine which disperses them instantly. In contrast to coulter mixers, turbine mixers achieve a high shear rate and intensive turbulence at all points in the system, so that the dispersion and homogenization of the products are satisfactory.

But if we study the behavior of such a mixer, by introducing a colored matter into it or in general any matter which can be easily detected, which is called a tracer, and by recording the concentration of said tracer at the outlet, we find that a variation in supply is reflected without modification at the output after a very short time of the order of a second. Thus, in a turbine mixer, the residence time of the products is very short, so brief that the irregularities in the feed cannot be erased by the mixing operation and are found entirely at the outlet. When one wishes a regular fluidity of the final product, impossible to obtain by a regularity of supply, the turbine mixers will therefore not be suitable.

It is also known from patent BE-A-556,314 to use in pastry making, a mixer operating continuously and comprising a tank with evacuation orifice equipped with an adjustment valve, in the bottom, in which rotates a rotor double substantially at mid-height, to create an upper vortex and a lower vortex of the products to be mixed.

Solid and liquid ingredients are continuously introduced and the mixture is drawn off from the bottom of the mixer so as to maintain a constant level in the tank. However, in such a mixer, products are liable to be recycled indefinitely by passing from one vortex to another, deposits may occur on the upper surface of the mixture and dead zones may exist between the vortices.

Such manifestations are without disadvantages in the case of stable products, but are unacceptable when the mixture is evolving.

The present invention overcomes the drawbacks of each of the known embodiments; it makes it possible to continuously supply a mixture of powdery solids and liquids with regular fluidity.

Furthermore, this invention prevents any parasitic deposition inside the mixer and, consequently, when working with evolutionary products, eliminates any premature build-up inside said mixer, a fluid evolutionary product being a liquid in which takes place a certain reaction giving rise to a physical or chemical transformation, capable in particular of causing a solid phase to appear or of modifying the characteristics of the solid phase initially charging the liquid. As an example of an evolving product, we can cite plaster which, as soon as it is mixed with water, begins to evolve rapidly until it is completely solidified.

• - à introduire en continu la phase liquide dans une cuve de mélangeage;
• - à introduire en continu la phase solide dans ladite cuve avec un débit tel que soient respectées les proportions désirées de solide et de liquide dans le mélange;
• - à assurer la rotation des produits contenus dans la cuve de façon à former un vortex;
• - à régler les débits d'alimentation en solide et en liquide pour que se maintienne constamment un certain état de remplissage de la cuve;
• - à évacuer en continu le mélange des produits pour que se conserve cet état de remplissage.

It offers a continuous mixing process for powdery solids and liquids, which consists of:

• - continuously introducing the liquid phase into a mixing tank;
• - Continuously introducing the solid phase into said tank at a rate such that the desired proportions of solid and liquid in the mixture are respected;
• - ensuring the rotation of the products contained in the tank so as to form a vortex;
• - to regulate the flow rates of solid and liquid so that a certain state of filling of the tank is constantly maintained;
• - to continuously evacuate the mixture of products so that this filling state remains.

Advantageously, the liquid is introduced along the wall of the mixer so as to form there a continuous curtain of liquid which prevents parasitic deposits.

Preferably, the mixed product is extracted through the bottom of the mixer so as to avoid any retention of solid in the tank of said mixer.

• - introduction dans une cuve de mélange du liquide et du solide dans les proportions du rapport pondérai de mélange préalablement choisi, jusqu'à atteindre un certain état de remplissage de ladite cuve de mélangeage;
• - mise en rotation dans la cuve, des produits introduits et maintien en rotation pendant un temps déterminé,

This permanent regime is obtained after a start-up period which includes the following stages:

• - introduction into a mixing tank of the liquid and the solid in the proportions of the weight ratio of mixing previously chosen, until reaching a certain filling state of said mixing tank;
• - rotation of the introduced products in the tank and maintenance of rotation for a determined time,

• - admission en continu dans la cuve, du liquide et du solide avec des débits tels que soit respecté le rapport mondéral de mélange;
• - évacuation en continu du mélange avec un débit tel que se conserve l'état de remplissage.

Then, simultaneously:

• - continuous admission into the tank, of liquid and solid with flow rates such that the global mixing ratio is respected;
• - continuous evacuation of the mixture at a rate such that the filling state remains.

The filling state of the mixing tank determines the average residence time of the products to be mixed in the mixing tank and said average residence time is at least 3 seconds and preferably between 15 and 30 seconds.

The invention also proposes an application of the method to the continuous mixing of plaster and water.

• - une cuve formée par un solide de révolution creux muni à son extrémité inférieure d'un orifice d'écoulement équipé de moyens de réglage du débit d'écoulement, ladite cuve possédant une paroi intérieure ajourée qui constitue un.fond intermédiaire;
• - une turbine défloculeuse disposée à l'intérieur de la cuve au-des-sus du fond intermédiaire et tournant autour d'un axe vertical situé suivant l'axe de-la cuve;
• - des moyens d'alimentation en liquide qui, avantageusement, distribuent le liquide en un rideau continu le long de la paroi de la cuve de mélangeage;
• - des moyens d'alimentation en solide.

The invention further provides a device for implementing the mixing process. Such a device is constituted by a mixer characterized in that it comprises:

• - A tank formed by a hollow solid of revolution provided at its lower end with a flow orifice equipped with means for adjusting the flow rate, said tank having an perforated inner wall which constitutes an intermediate bottom;
• - a deflocculating turbine placed inside the tank above the intermediate bottom and rotating around a vertical axis located along the axis of the tank;
• - liquid supply means which, advantageously, distribute the liquid in a continuous curtain along the wall of the mixing tank;
• - solid feed means.

• - figure 1: un schéma de la cuve de mélangeage;
• - Figure 2: une installation complète de mélange;
• - Figure 3: une section horizontale. de la cuve de mélange de la Figure 1, prise immédiatement au-dessus du fond intermédiaire;
• - Figure 4: une section horizontale du dispositif d'éjection prise immédiatement au-des- sus du fond dudit dispositif;
• - Figure 5: un schéma d'une partie du mélangeur dont le dispositif d'éjection est une couronne d'impact. ₁

The invention will now be described with reference to the drawings, which represent:

• - Figure 1: a diagram of the mixing tank;
• - Figure 2: a complete mixing installation;
• - Figure 3: a horizontal section. from the mixing tank of Figure 1, taken immediately above the intermediate bottom;
• - Figure 4: a horizontal section of the ejection device taken immediately above the bottom of said device;
• - Figure 5: a diagram of part of the mixer whose ejection device is an impact ring. ₁

FIG. 1 represents a mixer M of pulverulent products and of liquids, according to the invention. It consists of a vertical cylindrical tank 1 which narrows in its lower part 2 to converge towards a discharge orifice 2 '. A deflocculating turbine 3 is mounted inside the tank 1 on a vertical shaft 4 arranged along the axis of the tank and driven by a motor 5. An intermediate bottom 6 of the tank 1 is constituted by the upper surface of a core 7 forming an obstacle inside the converging lower part 2 of the tank 1. This obstacle is a solid of revolution whose shape converges downwards. It is centered on the axis of the tank and has dimensions smaller than the internal dimensions of the converging lower part 2 of the mixer, thus providing an annular orifice at its periphery. The upper surface 6 of the core 7 has a shape such that it follows the flow figures corresponding to the turbine 3. In a simple embodiment illustrated in FIG. 1, the core 7 is a cone arranged point downwards at the 'interior of the narrowed lower part 2, itself conical, of the tank, the intermediate bottom then being constituted by the flat base of said cone. As shown in FIG. 3, the cone 7 is held inside the lower part 2 of the tank 1 by fixing lugs 7A.

The lower part 2 of the mixer opens into an ejection device 8 in the form of an inverted cyclone, that is to say for example constituted by a conical envelope 9 arranged point at the top, with a flat base 10 and a collection pipe. 11 leaving flush with the base 10, tangentially to the conical casing 9 in the direction of rotation of the turbine 3. This collection pipe 11 extends vertically and is provided with a valve 12 for adjusting the flow rate which delivers the mixture by a pipe 13 to the installations for using the mixture. This valve 12 will be a pneumatic valve with direct passage constituted by an elastic sleeve enclosed in a rigid housing, controlled by a fluid sent under pressure between the housing and the elastic sleeve.

The upper edge of the tank 1 is provided with a covered annular weir 14, supplied with liquid by a flexible pipe 15. A bypass 16 connected to the flexible pipe 15 and equipped with an adjustment valve 17 takes part of the liquid and directs it on the shaft 4 of the turbine 3.

• vitesse de la turbine 1275 tours/mn n
• diamètre de la turbine 150 mm diamètre de la cuve de
• mélangeage 292 mm hauteur de la cuve au-dessus du
• fond intermédiaire 485 mm angle au sommet du cône qui
• constitue le noyau obstacle 145 degrés diamètre de la sortie du
• mélangeur 35 mm distance entre la turbine et le
• fond intermédiaire 15 mm angle au sommet de la partie
• basse du mélangeur environ 145 degrés.

For example, a mixer capable of supplying 30 to 65 kg of mixture per minute may have the following characteristics:

• turbine speed 1275 rpm n
• turbine diameter 150 mm tank diameter
• mixing 292 mm height of the tank above the
• intermediate bottom 485 mm angle at the top of the cone which
• constitutes the obstacle nucleus 145 degrees diameter of the outlet of the
• 35 mm mixer distance between the turbine and the
• intermediate bottom 15 mm angle at the top of the part
• mixer low about 145 degrees.

Figure 2 shows a complete mixing installation. We find the mixer M with its tank 1, its core 7 forming an obstacle inside the converging part 2 of the tank, its intermediate bottom 6 formed by the upper surface of the core 7, its annular flow orifice at the periphery the intermediate bottom 6, its ejection device 8 in the form of an inverted cycone, the collecting pipe 11 extended vertically and provided with the valve 12 for adjusting the flow rate of the outgoing mixture, the supply of liquid with the weir 14 and the bypass 16, the turbine 3 driven by the motor 5. This FIG. 2 gives an example of a device S for supplying powdered solid and a device L for supplying liquid, from the mixer M. The device S for supplying solid proposed here comprises a hopper 18 disposed above a conveyor belt 19 forming a balance, balanced on a knife shown diagrammatically at 20 when it is loaded with a determined weight of product. Such a device is said to be a constant weight carpet. This balance carpet 19 is associated with a hatch 21 for adjusting the thickness of the layer of product delivered by the hopper 18. A vibrating metal corridor 22 provided with a sieve is arranged below the end of the carpet. balance 19. This corridor 22 is inclined with respect to the horizontal by an angle which depends on the pulverulent product and which, in the case of plaster, will be of the order of 45 degrees. This corridor 22 is arranged so that its lowest end overhangs the tank 1 of the mixer M and the pulverulent solid which it delivers falls in the center of the tank 1 on the turbine 3.

In the liquid supply device L of this FIG. 2, the liquid supply is made from a reservoir 23 at constant level; the liquid flow is adjusted by a valve 24, a flow indicator 25 allows precise control of said flow.

the mixing installation operates in the manner which we will describe below. We will take plaster as an example of a powdery solid and water as an example of liquid.

est choisi, Eo et Po étant respectivement le débit massique d'eau et le débit massique de plâtre. Un rapport de 0,8 sera classiquement employé. Les débits de matière sont d'abord réglés.Before starting, a mixture weight ratio

is chosen, Eo and Po being respectively the mass flow of water and the mass flow of plaster. A ratio of 0.8 will be conventionally used. The material flow rates are first set.

The water flow rate is adjusted by the valve 24 to the desired Eo value. The plaster flow is then adjusted to the value Po: the plaster contained in the hopper 18 spreads on the constant weight balance carpet 19 adjusted in equilibrium on the knife 20 for a determined weight of product, then the flow Po is obtained by adjusting the running speed of the weighing belt 19. A residence time To of the plaster mixed in the mixer tank is chosen. The turbine 3 is rotated. The mixer tank is closed by obstruction of the pipe 13 or by closing the valve 12. The device L for supplying liquid, adjusted to provide a flow rate Eo, is open for the time To chosen. Water is introduced on the one hand by the weir 14, on the other hand by the pipe 16. The turbine 3 rotating at high speed creates agitation of the water. At the end of time To, the water supply is cut off. Then, the plaster supply device S, adjusted for a flow rate Po, is put into operation for a time To. At the end of the time To, the plaster supply is cut off.

The turbine 3 is allowed to mix the water and the plaster for a time of the order of To / 2 counted from the plaster supply cut-off. Then, after this mixing time To / 2, simultaneously, the water supply is always set for a flow rate Eo, the supply is always set for a flow rate Po, the mixture contained in the tank is allowed to flow of the mixer by opening the pipe 13 or by opening the valve 12 for adjusting the discharge rate of the mixture and by adjusting said valve 12 so that the quantity of product in the mixer remains constant and equal to the quantity present in the tank during starting during the mixing phase. The steady state is thus quickly reached. The water and plaster supply is continuous with respective flow rates Eo and Po, the mixing is permanent, a constant quantity of mixture remains in the mixer tank, the average residence time of the mixture in the mixer is constant and equal to To chosen at start-up, the flow of the mixture is also done continuously with a flow rate (Eo + Po).

The water introduced into the annular spillway 14 is distributed uniformly over its entire periphery and by overflow trickles along the inner wall of the tank 1. The water withdrawn by the pipe 16, metered by the valve 17 performs a sprinkling of the 'shaft 4 of the turbine 3. The plaster contained in the hopper 18 spreads on the constant weight balance carpet 19 in equilibrium on the knife 20. The balance carpet being adjusted for a flow Po, any overfeeding or underfeeding momentary plaster causes an imbalance which results in a change in position of the hatch 21 for adjusting the thickness of the plaster layer, a change which tends to restore balance.

At the end of the weighing carpet 19, the plaster falls on the sieve covering the vibrating metal corridor 22, the clusters break and the finely divided plaster flows in said metal corridor. This by its vibrations, spreads in the time the excesses and defects in the plaster supply, due to the landslide process; then it concentrates the plaster in a vein and causes the said vein to fall onto the turbine 3 which rotates at high speed inside the tank 1 of the mixer. The curtain of water formed on the wall of the tank 1 and the excess water created on the shaft of the turbine prevent any deposit of powdered plaster and any undesirable primer of solidification on the wall of the tank 1 and on the tree 4.

The turbine 3 rotating at high speed rotates the products contained in the tank. The speed of the turbine is determined so that a stable vortex with a vertical axis is formed, that is to say a single hollow vortex lining the walls of the tank internally.

The surface of the mixture then takes a conical shape centered on the shaft 4 of the turbine 3.

et de To.The depth of the vortex depends on the geometry of the tank of the mixer M and on the speed of rotation of the turbine which is adjusted so that the bottom of the vortex touches said turbine 3. This optimal speed depends on the fluidity of the mixture, which is a function of the ratio

and To.

An insufficient speed will produce an excessive covering of the turbine by the mixture and an excessively flat surface of the mixture on which may exist clumps of solid not dispersed in water.

On the contrary, too high a speed will tend to dig the vortex excessively until the entire turbine 3 is uncovered and to cause the mixture to rise too high along the wall of the tank 1, the latter then falling periodically onto the turbine, thus initiating an irregular rotational movement.

The powdery plaster delivered by the vibrating metal corridor 22 falls in the center of the vortex on the turbine 3 rotating at high speed.

. It is instantly dispersed and sprayed into the preexisting mixture in tank 1.

The rotation of the products ensures homogenization, and the inclination of the surface of the liquid avoids the stagnation of solid products in clusters. The intermediate bottom 6 of the mixer M constituted by the upper surface of the core 7 matching the flow figures of the turbine 3, that is to say the circulation lines of the mixing zone, and being for example flat and arranged in the immediate vicinity of the turbine, no deposit forms there. The plaster / water mixture is extracted regularly in the annular space located between the obstacle-core 7 and the converging wall of the lower part 2 of the mixer. The position of this core 7 with respect to the converging wall of the lower part 2 of the mixer defines the dimensions of this annular space and thus determines a certain limit flow rate. The mixture flows in this space with a sufficient speed which does not allow solidification. In the case where the core 7 is a cone and the outer wall of the converging part 2 of the mixer is itself conical, the speed of the plaster mixture measured along the generatrix of the cone core 7 will be at least 30 cm / second and generally around 1 m / second. The sections of the mixture extraction pipes located downstream will also be chosen so that this minimum speed is reached, thus avoiding deposits and premature caking.

The mixture converges at the tip 2 'of the lower part 2 of the mixer. The mixture still in rotational circulation enters the ejection device 8 in the form of an inverted cyclone. The mixture remains pressed along the conical walls of the ejection device 8 and descends along these walls to the base 10 of the device in a spiral. In this way, an uncontrolled vortex does not form, which can give rise to a dead zone where there is a risk of solidification. The mixture in rotational circulation is then captured by the collection pipe 11 and it then gives a full cylindrical stream whose flow will now be able to be adjusted precisely by the throttling valve located at the end of said collection pipe 11 .

The feed rates Po and Eo however not being perfectly stable and being able to undergo fluctuations which would cause fluctuations in the fluidity of the mixture, the passage section of this valve 22 is constantly adjusted to maintain a constant quantity of mixture in the tank. of the mixer and thereby a constant residence time of said mixture in said mixer. This residence time allows the homogenization of the mixture and the elimination of feed irregularities.

The adjustment of valve 12 can be obtained in various ways. It could be regular, but in the case of plaster, taking into account the rapid process of evolution of powdered plaster as soon as it is in contact with water, if one wants to have a mixture with constant fluidity, having so stayed a specific time in the mixer, it will be automatic. The regulation signal sent to valve 12 will be provided by a photoelectric cell reacting as soon as the mixture in the mixer tank has reached a certain level, or by a capacitive system measuring the electrical capacity of a determined height of the mixture, or by a pressure probe placed at the bottom of the tank, or even preferentially by weighing the mixer.

The valve 12 will be a direct passage valve constituted by a rigid body, by an elastic inner sleeve and by a fluid supply between the rigid body and the sleeve, said fluid being capable of compressing the elastic sleeve to reduce the flow rate of the valve. . To avoid deposits and solidification plaster at the throttle of valve 12, a modulation of the fluid pressure controlling the opening of valve 12 is produced, modulation which causes the sleeve to constantly deform.

Advantageously, to promote deformation, the sleeve will be made of an elastic, flexible impermeable material, such as air chamber rubber, and at rest it will have substantially the same dimensions as the interior of the rigid body which envelops it.

Advantageously, the valve 12 will be controlled using a pneumatic leakage control system varying the opening of said valve 12 as a function of the weight of the mixer and we will take advantage of the system's oscillations induced by the vibrations due to the movement of the mixture and the turbine in the mixer tank to modulate the pressure of the control fluid. More specifically in this case the mixer will be separated from the upstream and downstream installations to make weighing possible. Such a pneumatic leak system mainly comprises a pneumatic circuit and a force balance beam. The pneumatic circuit is supplied with a constant flow of compressed air; it has two branches, one going to the valve 12, the other opening into the open air near one end of the balance beam and thus determining a certain leak, variable when the beam oscillates. The balance beam weighs constantly on the mixer. It is equilibrated for a determined weight of the mixer thanks to a return spring and an adjustable calibration counterweight and it oscillates when this weight varies. It then increases or decreases the leakage from the pneumatic circuit and by backlash, respectively decreases or increases the pressure of the air directed on the valve, thus modifying the cross-section of the valve and consequently the discharge rate of the mixer. . In addition, the movements of the mixture and the vibrations of the turbine slightly oscillate the flail permanently; and it is these light oscillations which, detected by the pneumatic circuit, generate the pressure modulation of the valve control fluid. The elastic sleeve of the valve receiving in addition to its regulation signal a pulsating signal resulting from the vibrations induced by the turbine constantly deforms, thus preventing any plaster deposit.

The ejection device can be constituted by any conventional means in fluid mechanics, capable of transforming any flow, in particular a rotary flow, into a full vein. Thus, one can use an impact crown 26 formed by a pot with a bottom and a side ejection tube.

The time To must always remain lower than a value Tp corresponding to the time of start of setting of the mixture. Supply flows Po and Eo and therefore the discharge flow (Po + Ep) being determined, this average retention time To is determined by the filling state of the mixing tank and it is by the maintenance of this filling state that the average retention time is kept constant. The average retention time will be at least equal to three seconds and preferably between 15 and 30 seconds so that sufficient homogenization of the solid and liquid products is obtained.

The mixing of plaster and water has so far been described, but the process remains the same and the device operates in the same way if additives are introduced at any of the different stages of mixing, the additive word designating reactive or inert products, solid or liquid, preferably finely divided when they are solid. It will thus be possible to introduce solid additives with the pulverulent plaster, either because the addition has been made beforehand by the plaster manufacturer, or by distributing said additive in the hopper 18 or on the weighing carpet 19. It will also be possible introduce solid or liquid additives into the water, or even directly into the mixer. Said additives may consist of chemical additives or plaster reinforcing elements such as cut or finely divided fibers.

It is therefore necessary to take the words "pulverulent plaster" and "water" in a more general sense, and it is preferable to bet with solid phase or solid to designate both pulverulent plaster alone and the mixture of plaster with other solids , and to speak of liquid or liquid phase to designate water alone as well as water containing solid or liquid additives.

Claims (22)

1. Process for continuously mixing pulverulent solids and liquids, in particular primarily plaster and water, in which the solid phase and the liquid phase are continuously introduced into a mixing vat at the respective rates required to obtain the desired proportions of solid and liquid in the mixture, the products contained in the vat are swirled in the vat with the aid of a turbine to effect mixing, the mixture is continuously discharged, the rates of feed and of discharge are regulated to maintain constantly a certain state of filling, characterised In that the liquid is essentially introduced as a continuous film along the vertical wall of the mixer, the products are swirled inside a single vortex, the solid is introduced at the center of the vortex thus produced, and the mixture is extracted from the periphery of the bottom of the mixing vat

2. Mixing process according to claim 1, characterised in that the starting period comprises the following stages:

introduction of a predetermined quantity of water into the mixing vat.whose outlet is closed,

starting of the mixing turbine to produce the vortex,

addition of the pulverulent solid in the quantity required for the ratio by weight of the mixture,

waiting while mixing for a predetermined length of time,

and then simultaneously:

continuous admission of the water and solid into the vat at the rates required for the ratio by weight of the mixture,

opening of the outlet orifice of the vat and continuous discharge of the mixture at the flow rate required to maintain the level of filling.

3. Process according to one of the preceding claims, characterised in that the average dwell time of the products in the mixing vat, determined by the state of the filling of the said vat and the rate of discharge, is at least equal to 3 seconds and is preferably from 15 to 30 seconds.

4. Process according to any one of the preceding claims, characterised in that the speed of rotation of the turbine is regulated so that the bottom of the vortex touches the said turbine without covering it.

5. Process according to any one of the preceding claims, characterised in that liquid and solid additives are introduced into the mixing water or directly into the mixer.

6. Process according to any one of the preceding claims, characterised in that solid additives are introduced in a state of being mixed with the solid, in this case pulverulent plaster.

7. Process according to any one of the preceding claims, characterised in that the mixture is extracted from the mixer at a speed above 30 cm/second.

8. Vertical mixer of the vat type for pulverulent solids and liquids comprising a vat (1) formed by a hollow solid of revolution, provided at its lower end with a discharge orifice (2') equipped with means (12) for controlling the rate of discharge, a feed device (15) for liquid, a feed device (5) for solid, a turbine (3) rotating about a vertical axis inside the vat, characterised in that

the vat (1) is a vertical cylinder extended downwards by a part which narrows to converge to the discharge orifice (2'),

a lower wall (6) forming an intermediate base perforated at its periphery and situated just below the turbine (3),

the liquid feed device (15, 14) for distributing liquid as a continuous film over the whole side wall of the vat.

9. Mixer according to claim 8, characterised in that the turbine (3) is a deflocculating turbine.

10. Mixer according to any one of claims 8 or 9, characterised in that the intermediate base (6) has an upper surface conforming to the lines of circulation of the upper mixing zone.

11. Mixer according to any one of claims 8, 9 or 10, characterised in that the intermediate base (6) has an upper surface conforming to the lines of circulation of the upper mixing zone.

12. Mixer according to claim 11, characterised in that the intermediate base (6) has a circular wall centered on the axis of the vat (1) and with a diameter smaller than the internal diameter of the said vat, the openings of this intermediate base being formed by the gap between the said base and the said wall of the vat.

13. Mixer according to claim 12, characterised in that the intermediate base (6) is situated at the level of the junction of the cylinder with the lower, narrowing and converging part of the vat.

14. Mixer according to claim 13, characterised in that the intermediate base (6) is formed by the upper surface of a core (7) having the form of a downwardly converging solid of revolution situated inside the lower, converging part of the vat and centered on the axis of said vat, the said core having a smaller cross-section than the vat so that an annular gap is left between itself and the lower wall of the said vat.

15. Mixer according to claim 14, characterised in that the core (7) situated inside the lower part of the vat (1) of the mixer is a right circular cone arranged with its apex downwards, the plane base of which constitutes the intermediate base (6), the lower, converging part of the vat being thus itself conical.

16. Mixer according to any one of claims 8 to 15, characterised in that the means for regulating the outflow rate comprise a restrictor valve ( 12).

17. Mixer according to claim 16, characterised in that the restrictor valve (12) is preceded by an ejection device (8) situated under the discharge orifice (2') of the mixing vat and integral with said vat, the said device receiving the mixture and transforming its flow into a solid stream.

18. Mixer according to claim 17, characterised in that the ejection device (8) has the form of an inverted cyclone, that is to say it is formed by a conical envelope arranged with its point upwards with an inlet at the apex of the cone, with a plane base (10) and having a collecting pipe (11) leaving at the level of said base tangentially to the conical envelope in the sense of rotation of the turbine (3) of the mixer.

19. Mixer according to claim 18, characterised in that the ejection device (8) is an impact ring formed by a vertical pot with a base (10) and lateral outlet pipe (11).

20. Mixer according to any one of claims 16 to 19, characterised in that the restrictor valve (12) is automatic and is controlled by one of the following means: Photo-electric cell detecting the level of the mixture in the mixing vat, pressure probe situated on the intermediate base (6) of the vat (1 capacitive system, arm of balance beam permanently weighing the mixing vat, in which case the mixing vat is not fixed to the upstream and downstream installations.

21. Mixer according to any one of claims 8 to 20, characterised in that the supply means (5) for pulverulent solids comprise:

feed hopper (18) having a trap (21) to control the feed rate;

a constant weight weighing conveyor (19) on to which is poured the pulverulent solids from the hopper (18) and imbalance of which adjusts the position of the control trap;

a flow regulator for the pulverulent solids delivered by the constant weight conveyor, said regulator consisting of a vibrating chute.

22. Mixer according to any one of claims 8 to 21, characterised in that the liquid supply means (L) comprise:

an annular weir (14) extending over the upper edge of the vat (1) and distributing the liquid over the side walls of the vat;

a tube (16) directed at the shaft of the turbine;

a flow control valve (24).

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