IE48227B1 - Process and apparatus for continuously mixing pulverulent solids and liquids - Google Patents

Abstract

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 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, characterized 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.

Description

The present invention deals with the mixing of pulverized solids and liquids and can be applied, for example, to hydraulic binders preparation and specifically to the continuous process preparation of a plaster powder and water mixture.

It is a widespread practice, in mixing pulverized solids and liquids, to use mixers having teeth or blades and consisting of a cylindrical container having a vertical shaft fitted with one or several sets of revolving radial arms bearing blades or teeth. These blades or teeth scrape the container's walls, thereby mixing the products. But such mixers do not provide a satisfactory dispersion of pulverized solid into the liquid; hence there is a lack of homogeneity in the fluidity of the discharged mixture. On the other hand, there are turbine mixers consisting of a container in which a disc, a propeller or a turbine is revolving at very high speed. The solid and the liquid reach the turbine which disperses them instantly. Contrary to teeth mixers, turbine mixers achieve a high shearing rate and an intensive turbulence in all points of the system, so that product dispersion and homogenization are 1"^satisfactory. But the study of such a mixer's performance, through the introduction of some colored substance, or generally any tracer substance which can easily be detected, reveals that a variation in the supply is reflected without any change at the outlet after a very brief delay of the order of one second. Thus, in a turbine mixer, the time during which the product remains in the mixer is very short, indeed, so short that irregularities in supply are not suppressed by the mixing operation and still exist unchanged at outlet.

When an even fluidity of final product is desired, which is impossible to obtain through evenness of supply, turbine mixers will not be satisfactory.

The present invention obviates the drawbacks of both known systems; it makes it possible to provide a continuous process mixing of pulverized solids and liquids with an even fluidity.

In addition this invention overcomes the problem of premature setting of a fluid evolutive product in the mixer. A fluid evolutive product is a liquid in which a reaction resulting in a physical or chemical transformation takes place producing a solid phase or modifying the characteristics of a solid phase initially carried by the liquid. A plaster powder water mixture is exemplary of such a product. - 3 4 8 2 27 The invention provides a 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, and the rates of feed and cf discharge are regulated to maintain constantly a certain state of filling, in which 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. _ 4 4822? The invention also provides a vertical mixer of the vat type for pulverulent solids and liquids comprising a vat formed by a hollow solid of revolution, provided at its lower er.d with a drainage opening equipped with means for controlling the rate of discharge, a feed device for liquid, a feed device for solid, and a turbine rotating about a vertical axis inside the vat, in which; the vat is a vertical cylinder extended downwards by a part which narrows to converge to the discharge orifice, a lower wall forming an intermediate base perforated at its periphery is situated just below the turbine, and the liquid feed device is for distributing liquid as a continuous film over the whole side wall of the vat.

The invention will now be described more particularly by way of example with reference to the accompanying drawings, in which: Figure 1 is a sketch of the mixing mechanism; Figure 2 is a schematic showing of the supply and mixing mechanism; Figure 3 is a horizontal section taken immediately above the cone in the mechanism of Figure 1; Figure 4 is a horizontal section taken immediately _ 5 _ 8 2 27 above the bottom of the ejection device of the mechanism of Ficure 1; Figure 5 is a schematic drawing of part of a mixer having an impact crown ejector.

Referring to Figure 1, a mixer M for pulverized products and liquids has a vertical cylindrical container or tank 1 tapered in its lower section 2 to lead to a drainage opening 2'. A deflocculating turbine 3 is mounted on the side of tank 1 with its impeller inside of container 1 on a '2 vertical shaft 4 placed along the container's axis and driven by a motor 5. There is a container intermediate bottom 6 constituted by the top surface of a cone 7 designed to create an obstruction inside lower tapered section 2 of container 1. This obstruction is a solid of revolution whose lower section is tapered. It is centered along the container's axis and its dimensions are smaller than the inner dimensions of lower tapered section 2 of the mixer, thus providing a ring shaped opening between it and the lower section 2 of container 2( i . Ccr.e 7 is a cone with its point down inside of lower tapered section 2 of the container which also is in the shape of a cone, with the intermediate bottom 6 constituted by the flat base of said inverted cone. Cone 7 is supported by bars 7 A which appear in Figure 3. 2 The lower section 2 of the mixer leads to an inverted cyclone shaped ejection device 8, i.e. constituted by conical casing 9 mounted with its point up, with flat base 10 and a collector pipe 11 whose end is flush with base 10, tangential to conical casing 9 and extending in the direction of rotation of turbine 3.

This collector pipe 11, then runs vertically downward and is equipped with flow rate regulating valve 12 which brings the mixture to a pipe 13 leading to mixture utilization facilities (not shown). Valve 12 may be, for example, a modulated pressure controlled elastic sleeve valve of the type disclosed in our copending patent application No. 224/79 (Patent Nc. 47824 ) entitled Process and Mechanism for Evolutive Pulp Flow Regulation and filed concurrently with this application. The top rim of container 1 is fitted with a covered ring-shaped pouring spout 14, supplied with liqiud throuqh a flexible hose 15. A pipe 16 connected to flexible hose 15 and equipped with a regulating valve 17 directs liquid at shaft 4 of turbine 3 to keep it clean.

By way of specific illustration a mixer with the following specifications can deliver from 30 to 65 kg/min of mixture: _ 7 _ 150 nm 292 mm 485 nm 145 degrees 35 mm. 15 ram 145 degrees Speed of the turbine 1275 revolutions/min Diameter of the turbine impeller blade Diameter of the mixer impeller blade Height of the mixer top above the intermediate bottom Angle of the base of the cone Diameter of the outlet of the mixer Distance between turbine blade and intermediate bottom Angle at the bottom of the mixer, about ...

Figure 2 shows a complete mixing installation.

Elements already described, such as mixer M with its container 1, its obstructing cone 7 inside of the container tapered section 2, intermediate bottom 6 constituted by the top part of cone 7, ring shaped opening between cone 7 ana container lower section 2, inverted cone shaped 15 ejection device 8, collector pipe 11 equipped with outcoming mixture flow rate regulating valve 12, liquid supplied through pouring spout 14 and pipe 16, and turbine 3 driven by motor 5 are there.

Pulverized solid supply system S and a liquid 20 supply system L for mixer M are shown in Figure 2. The solid supply system S includes a hopper 18 mounted above a weight sensitive conveying belt 19, balanced on a knife 20 when loaded with a selected weight of product. Such a device is known as a constant weight weight-sensitive 25 conveyor. This v/eight-sensitive conveyor 19 is combined with a trap 21 for regulating the thickness of the layer of powder supplied by hopper 18. A vibrating metallic channel 22 equipped with an overlying screen is mounted below the discharge end of the constant weight-sensitive conveyor 19. This channel is inclined in relation to the horizontal direction at an angle which depends on the pulverized product and which for plaster powder, will be preferably approximately 45 degrees. Channel 22 is mounted in such a way that its lower end hangs over container 1 of mixer M and that the powder brought by it falls into the center of container 1 on turbine 3.

The solid supply system S is known to the art so it need not be further detailed.

In the liquid supply mechanism L of Figure 2, liquid supply is effected from a constant level tank 23; a regulation of liquid flow rate is insured by a valve 24, a flow-meter 25 making it possible to control the rate of flow into hose 15 with precision.

The mixing facility operates as follows.

Plaster powder (P) will be taken as an examole of the pulverized solid and water (W) as an example of the liquid.

Wo Prior to the start, a ratio is selected, Wo and Po being respectively the mass flow rate of water (Wo) and the mass flow rate of plaster powder (Po) which flow rates are first set. The water flow rate is regulated _ 9 _ by valve 24 to selected Wo value. Then the plaster flow rate is regulated at Po value: plaster powder contained in hopper 18 spreads on constant weight weight-sensitive conveyor 19 set in equilibrium on knife 20 for a selected weight of product in tank 1, and then flow rate Po is obtained by regulating the translation speed of the weight-sensitive conveyor 19. A length of stay To of mixed plaster in mixer container 1 is selected. Rotation of turbine 3 is started. Mixer container 1 is closed by plugging pipe 13 or by closing valve 12.

Liquid supply mechanism L, adjusted to supply a flow rate Wo is opened for the time To selected. Water is introduced through pouring spout 14, and through pipe 16. By turning at high speed, turbine 3 stirs up the water.

At the end of time To, the water supply is shut off.

Then the plaster powder supply mechanism S, adjusted for a flow rate Po, is put in operation for a period of time Tc. At the end of time To, the plaster powder supply is shut off.

Turbine 3 is allowed to mix water and plaster powder for a period of time of approximately To/2 starting with plaster powder supply shutoff. Then, after this mixing time To/2, at the same time, the water supply, still adjusted for a flow rate Wo, is opened, the plaster supply still adjusted for a flow rate Po is opened, mixture contained in the mixer container is allowed to run out either by opening pipe 13 or by opening valve 12, and by adjusting valve 12 so that the amount of product in the mixer M remains constant and equal to the amount present in the container at startup. Thus, a permanent running condition is quickly reached. Water and plaster powder supply is in continuous process with respective flow rates Wo and Po, mixing is continuous, a constant amount of mixture remains in the mixer container, the average time of stay of the mixture in the mixer is constant and equal to time To selected at start, and running off of mixture is also a continuous process with a (Wo + Po) flow rate.

Water introduced into ring-shaped pouring spout 14 is uniformly distributed all around it and overflows along the inner wall of container 1. Water from pipe 16, controlled by valve 17, sprays and cleans shaft 4 of turbine 3. Plaster powder contained in hopper 18 spreads on constant weight weight-sensitive conveyor 19 balanced on knife 20. The weight-sensitive conveyor 19 being set for a flow rate Po, any temporary oversupply or undersupply of plaster powder results in an unbalance which leads to a change of position of trap 21 for regulation of the thickness of the plaster powder layer, a change which tends to reestablish balance. - 11 48227 At the end of weight-sensitive conveyor 19, plaster powder falls on the screen that covers vibrating metallic channel 22, breaking into blocks and plaster powder runs into channel 22. The channel 22, through its vibrations, spreads the plaster powder and then forces it through a spout formed in its end to fall into turbine 3 revolving at high speed inside of container 1 of mixer M. The sheet of water formed on the wall of container 1 and the water sprayed on turbine shaft 4 prevent any 10 deposit of plaster and any unwanted beginning of setting of plaster on container 1 wall and on shaft 4.

High speed revolving turbine 3 moves the powder and water inside container 1. The turbine speed is set so that a single vertical axis vortex becomes established, i.e. a hollow whirlpool covering the inside of the tank walls. Then the outer surface of the mixture assumes a conical form, centered on shaft 4 of turbine 3.

Vortex depth depends on geometric data of mixer M 20 container 1 and on the revolving speed of turbine 3 which is adjusted so that bottom of the vortex touches turbine 3 and eliminates any dead spots of mixture on bottom 6. This optimal speed depends on mixture fluidity, which is a function of ratio: Wo . , _ =t- and of To 8227 With too low a speed there is an excessive covering of the turbine blade by the mixture and an excessively flat surface of the mixture on which blocks of solid powder may remain because the powder is not dispersed.

On the other hand, too high a speed tends to hollow out the vortex excessively so as to uncover all of turbine 3 and to cause the mixture to climb too high along the container 1 wall, the latter falling back periodically on the turbine 3 and thus causing an irregular revolving motion.

Plaster powder supplied by vibrating channel 22 falls in the center of the vortex upon turbine 3 revolving at high speed. It is instantly dispersed and projected outwardly into the preexisting mixture In container 1.

Rotation of the mixture insures homogenization, and the slope of the liquid surface prevents stagnation of solid products in agglomerates. The plaster/water mixture assumes turbines 3 flow lines, i.e. mixing zone circulation lines adjacent intermediate bottom 6. Thus, there is no deposit formed on the intermediate bottom 6 because the mixture sweeps across it. The plaster/water mixture flows out of the tank in an even manner through the ring-shaped space between obstructing cone 7 and mixer lower section 2 wall without leaving any solid residue. The position of cone 7 in relation to mixer lower tapering section 2 wall defines the dimensions of this ring-shaped space and thus determines a limit on the flowing-off of the contents of the container 1. The mixture flows off through this space with a sufficient speed so as not to induce setting of the mixture. When cone 7 is a cone and when mixer tapered section 2 outer wall is itself conical, plaster mixture speed as measured along cone 7 is preferably at least 30 cm (11.81 in.) /second and generally about 1m (3.28 ft.) /second. Cross-sections of mixture run off piping located downstream are selected so that this minimal speed can be reached, thus preventing premature deposits and mass setting of the mixture.

The mixture gathers toward opening 2' of mixer lower section 2 with the mixture still revolving as it flows into inverted cyclone-shaped ejection device 8.

The mixture hugs the conical walls of the device 8 and flows down along these walls as far as base 10 in a spiral descent. This way, no uncontrolled vortex is likely to create a motionless zone where mass setting could occur. Then the mixture in rotating flow is received in collector pipe 11 and forms a full cylindrical stream whose rate of flow can be accurately regulated by flow regulating valve 12 located at the end of collector pipe 11.

However, since supply flow rates Po ana Wo are not 4-8 22 7 perfectly stable and may be subject to fluctuations which would result in mixture fluidity fluctuations, valve 12 is constantly adjusted to maintain a constant amount of mixture in mixture container 1 and hence a constant time of stay of the mixture in the mixer. This time of stay makes it possible for the mixture to be homogenized and for the supply unevenness to be suppressed.

The setting of valve 12 can be arrived at in several ways. It may be a manually set one, but in the case of plaster, taking into account pulverized plaster's rapid evolution process as soon as it mixes with water, if a constant fluidity mixture must be available, consistent with a very precise time of stay in the mixer, it preferably is automatically controlled by an arm of a balance beam permanently weighing the mixing vat in which case the mixing vat is not fixed to the upstream and downstream installations.

Valve 12 may, for example, be a direct channel valve having a rigid housing, an elastic inner sleeve, and a fluid intake between rigid housing and sleeve, said fluid being capable of compressing the elastic sleeve to decrease the valve flow rate. In order to prevent any plaster deposit or mass setting in such a valve it is advantageous to undulate the control fluid pressure as disclosed in our aforesaid co-opending Patent Application No. 224/79 (Patent No. 47824 ).

It is beneficial to control the valve 12 with 5 a pneumatic escape type regulating mechanism that causes a variation of valve 12 opening as a function of the weight of the mixture, and to use oscillations induced by vibrations resulting from mixture and turbine motion in the mixer container. Such an escape pneumatic mechanism includes essentially a pneumatic circuit and a force balance beam. The pneumatic circuit is supplied with a constant compressed air flow: it includes two branches, one of them leading to valve 12, the other one having a nozzle for which the balance beam acts as a flapper plate, thus providing a certain escape of air that varies with the position of the beam. Thus, the balance beat, is constantly monitoring the mixer's weight. Its equilibrium is set for a determined weight of the mixer and is disturbed when this weight varies. It then causes an increase or a decrease of pneumatic circuit fluid escape and consequently, causes a decrease or an increase of the pressure of air directed toward the valve, thus modifying the valve aperture and consequently the flow rate from the mixer. In addition, the turbine motions make the beam vibrate and faintly oscillate continuously and these faint oscillations are picked up by the pneumatic circuit and create valve control fluid pressure modulation to change the shape of the elastic sleeve and vibrate it. Since the valve elastic sleeve is constantly changing shape, no plaster deposit whatsoever can form there. Such a pneumatic escape type regulating mechanism is disclosed in our aforesaid patent application No. 224/79 (Patent No. 47824 ).

The ejection device 8 can be any standard fluid mechanics means which has the capacity of transforming any flux whatsoever, and particularly a revolving flux, into a full stream. Thus, a cylindrical impact crown 26 in Figure 5, made from a cam with a bottom and a lateral discharge pipe can be used.

Retention time To must always remain shorter than a value Tp corresponding to the start of setting of the mixture. Once supply flow rates Po and Wo and hence running off rate (Po + VJo) are set, this mean retention time To is determined by the mixing container filling level, and it is by maintaining this filling level that mean retention time is kept constant. Mean retention time is at least three seconds and preferably between 15 and 30 seconds in order for a satisfactory homogenization of solid and liquid products to be achieved.

So far a plaster powder and water mixture has been described, but the process remains the same and the mechanism operates in the same manner if additives are added at one of the various mixing steps, additive being understood as reactive or inert, solid or liquid products, preferably finely powdered for the solid ones. Thus, it is possible to introduce solid additives with pulverized plaster, either with addition made beforehand by the plaster powder manufacturer, or by spreading the additive in hopper 18 or on weight-sensitive conveyor belt 19.

It is also possible to introduce solid or liquid additives into the water, or else directly into the mixer. Said additives can be chemical catalysts, or plaster reinforcing elements such as chopped or finely divided fibres.

Thus, it is necessary to take the words plaster powder and water in a broader sense, and to use terms such as solid phase or solid to designate plaster powder by itself and mixtures of plaster with other solids, and to use terms such as liquid phase or liquid to designate water by itself as well as water containing solid or liquid additives.

Claims (24)

1. A process for continuously mixing pulverulent solids and liquids, in particular primarily plaster and water, in which the solid phase and the liquid phase are 5 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 10 is continuously discharged, and the rates of feed and of discharge are regulated to maintain constantly a certain state of filling, in which the liquid is essentially introduced as a continuous film along the vertical wall of the mixer, the products are swirled inside a single 15 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. A mixing process according to Claim 1, in which the starting period comprises the following stages: 20 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 25 required for the ratio by weight of the mixture, waiting while mixing for a predetermined length of time. - 19 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, 5 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. A process according to one of the preceding claims, in which the average dwell time of the products in the 10 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. A process according to any one of the preceding 15 claims, in which the speed of rotation of the turbine is regulated so that the bottom of the vortex touches the said turbine without covering it.

5. A process according to any one of the preceding claims, in which liquid and solid additives are 20 introduced into the mixing water or directly into the mixer.

6. A process according to any one Of the preceding claims, in which solid additives are introduced in a state of being mixed with the solid, in this case 25 pulverulent plaster.

7. A process according to any one of the preceding claims, in which the mixture is extracted from the mixer - 3o 48227 at a speed above 30 cm/second.

8. A vertical mixer of the vat type for pulverulent solids and liquids comprising a vat formed by a hollow solid of revolution, provided at its lower end with a drainage opening equipped with means for controlling the rate of discharge, a feed device for liquid, a feed device for solid, and a turbine rotating about a vertical axis inside the vat, in which the vat is a vertical cylinder extended downwards by a part which narrows to converge to the discharge orifice, a lower wall forming an intermediate base perforated at its periphery is situated just below the turbine, and the liquid feed device is for distributing liquid as a continuous film over the whole side wall of the vat.

9. A mixer according to Claim 8, in which the turbine is a deflocculating turbine.

10. A mixer according to any one of Claims 8 or 9, in which the intermediate base has an upper surface conforming to the lines of circulation of the upper mixing zone.

11. A mixer according to any one of Claims 8, 9 or 10, in which the intermediate base has an upper surface perpendicular to the axis of the vat.

12. A mixer according to Claim 11, in which the 21 227 intermediate base has a circular wall centered on the axis of the vat and with a diameter smaller than the internal diameter of the said vat, the openings of this intermediate base being formed by the qap between the said base and the said wall of the vat.

13. A mixer according to Claim 12, in which the intermediate base is situated at the level of the •ijnction of the cylinder with the lower, narrowing and concerning part of the vat.

14. A mixer accordinq to Claim 13, in which the intermediate base is formed by the upper surface of a core 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 ir.-.jlar cap is left between itself and the lower wall of the sai-d vat.

15. A -ixer according to Claim 14, in which the cone situated inside the lower part of the vat of the mixer is a n.vt circular cone arranged with its apex downwards, the plane base of which constitutes the ; •'termediate rase, the lower, converging part of the vat bei.nc thus itself conical.

16. A fixer according to any one of Claims 8 to 15, in which the means for regulating the outflow rate comprise u flew rate receipting valve. 1'. A m.?‘r according to claim 16, in which the 48327 flow rate regulating valve is preceded by an ejection device situated under the difecharge orifice of the mixing vat rnd integral with said vat, the said device receivirg the mixture and transforming its flow into a solid 5 stream.

17. 18. A mixer according to Claim 17, in which the ejection device 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 10 apex of the cone, with a plane base and having a collecting pipe leaving at the level of said base tangentially to the conical envelope in the sense cf rotation of the turbine of the mixer.

18. 19. A mixer according to Claim 18, in which the 15 ejection device is an impact ring formed by a vertical pot with a base and lateral outlet pipe.

19. 20. A mixer according to any one of Claims 16 to 19, in which the flow rate regulating valve is automatically controlled by an arm of a balance beam permanently weighing 20 the mixing vat in which case the mixing vat is not fixed to the upstream and downstream installations.

20. 21. A mixer according to any one of Claims 8 to 20, in which the supply means for pulverulent solids comprise: feed hopper having a trap to control the feed rate? a constant weight weighing conveyor on to which is poured the pulverulent solids from the hopper and imbalance of which adjusts the position of the control 5 trap; a flow regulator for the pulverulent solids delivered by the constant weight conveyor, said regulator consisting of a vibrating chute.

21. 22. A mixer according to any one of Claims 8 to 21, in 10 which the liquid supply means comprise: a ring shaped pouring spout extending over the upper edge of the vat and distributing the liquid over the side walls of the vat; a tube directed at the shaft of the turbine; 15 a flow control valve.

22. 23. A mixing process, substantially as herein described with reference to the accompanying drawings.

23.

24. A mixing apparatus, substantially as herein described with reference to and as shown in the 20 accompanying drawings.