MXPA99011178A - Device for mixing and dissolving solid granules in a liquid, in particular for producing nitrophosphate fertilisers - Google Patents

Abstract

The invention concerns a device for mixing and dissolving in a liquid (L) solid prills or particles (G) of a substance soluble in said liquid but having a density lower than that of the liquid, and in particular for preparing a sulpho-ureic reagent corroding natural phosphates to produce phosphonitrate fertilisers. The device comprises a vat (2) containing first stirring means (A1) for driving the solid prills towards the vat (2) lower part, and second stirring means (A2) for generating in the reaction mixture a movement for dissolving the prills, in particular a shearing motion.

Description

DEVICE FOR MIXING AND DISSOLVING SOLID GRANULES IN A PARTICULAR LIQUID FOR THE PRODUCTION OF PHOSPHONITROGENATED FERTILIZERS DESCRIPTIVE MEMORY The invention relates to a device for mixing and dissolving, in a liquid, granules or solid particles of a substance soluble in that liquid, but whose density is less than that of the liquid. The device in question is of the type that includes a container containing stirring means and has at least one inlet for the liquid, an inlet for the solid granules and an outlet for the solution. The invention relates more particularly, but not exclusively, to said device for the production of phosphono-nitrogen fertilizers, especially in accordance with the process of EP-A-0 560 882 (WO 92 10443). This process employs the digestion of a natural phosphate, such as tricalcium phosphate, with reactable sulfoureic material obtained by mixing and dissolving solid granules or urea grains in a solution of sulfuric acid. This reactable material for sulfoureic digestion is a well-defined eutectic composition whose solution, in certain proportions, is important for continuous preparation.

The problem becomes difficult because the solid urea in the form of granules has a relative density of 0.6, while the sulfuric acid solution in which it is appropriate to dissolve the urea granules has a relative density of 1.4; therefore, solid granules have a strong tendency to float, which does not promote dissolution or homogenization. Furthermore, it is appropriate that this dissolution of the solid granules be carried out as quickly as possible, in particular so that the volume of the container is not very large for a certain flow velocity of the solution. The device of the invention has the object of satisfying these contradictory requirements created by the density of the solid to be dissolved, which is clearly less than, in particular less than half, the density of the liquid. According to the invention, a device for mixing and dissolving, in a liquid, granules or solid particles of a substance soluble in that liquid, but having a density lower than that of the liquid, of the type defined above, is characterized in that the entry of the granules into the container is located in a high portion, above the level of the reaction mixture in the container, and because the agitation means includes a first agitation means that can create a drag of the solid granules towards the portion bottom of the vessel in the reaction mixture, and a second agitation means, located in the vessel at a level lower than that of the first agitation means, and can create a movement in the reaction mixture that promotes the dissolution of the granules, in particular a cutting movement. The first and second agitation means preferably are mechanical agitation means. For convenience, the first agitation means consists of a turbine with inclined straight blades, also known as "saber turbine", in particular with four blades. Conveniently, the second agitation means consists of a radial turbine with straight blades, also known as the "Rushton turbine", in particular with 6 blades. The Rushton turbine is located below the saber turbine at an appropriate distance. In general, the container has a cylindrical shape with a vertical axis, and the first and second agitation means are conveniently placed on the same vertical arrow, preferably in a position coaxial with the container. In particular, said device is suitable for the preparation of a reactive sulfoureic material for the digestion of a natural phosphate in a plant for the production of phosphono-nitrogen fertilizers, according to the procedure of EP-A-0 560 882, in which case, the Granules are solid urea grains or granules and the liquid is a solution of sulfuric acid.

Apart from the placements explained above, the invention consists of a certain number of other placements which will be explained more explicitly below in relation to a preferred example of the embodiment described with reference to the accompanying drawings, but which does not imply any limitation . Figure 1 of these drawings is a diagram of a device according to the invention. Figure 2 is a vertical axial section, with an external portion, on a large scale, of the container of the device of Figure 1. Figure 3 is a large scale flat view of the saber turbine. Figure 4 is a large-scale elevation view of the Rushton turbine with 6 blades. Finally, Figure 5 is a top view in relation to Figure 4. When referring to Figure 1 of the drawings, a device 1 can be observed to mix and dissolve in a liquid L solid granules G of a substance soluble in the liquid, but that has a lower density than the liquid. In the example considered, the solid granules G are granules of urea of relative density 0.6, while the liquid L consists of a solution of sulfuric acid of relative density 1.4. The device 1 includes a container 2, generally cylindrical in shape, with a vertical axis and whose lower part faces outwards in a curved convex shape. The container 2 is closed with a removable lid 3, in the upper portion, in which are: an inlet 4 for the solution of sulfuric acid L, an inlet 5 for the granules of urea G, with water, and an inlet 6 for recycling a fraction of the reaction mixture M produced in the container 2. Since the solution of urea in sulfuric acid is exothermic, two concentric cooling coils 7a, 7b are located in the container 2 (forming a continuous operation reactor) ). As can be seen in Figure 2, the coils include turns that extend substantially over half the height of the container 2 and surround the agitation means A. The turns of the coils are located radially within counterbalances or divisions 8 consisting of four vertical blades, 90 ° apart, located in planes passing through the vertical axes of the container 2. The vertical external spine of the counterbalances 8 is close to the internal surface of the container 2. The cooling provided with the aid of the coils 7a, 7b is sufficient to prevent the temperature of 90 ° C from being exceeded in the container, a threshold over which the urea incorporated in the mixture M tends to decompose, as indicated in EP-A-0 560 882 The agitating means A includes a first agitating means A1 which can create a drag of the granules G towards the lower portion of the container 2 in the reaction mixture M, and a second medium of agitation. A2 located at a lower level than the first agitation means A1. This second means A2 has the ability to create in the mixture M a movement that promotes the dissolution of the granules G, in particular a cutting movement. The first and second agitation means A1, A2 are mechanical and consist of two turbines of different types, secured to the same pulse arrow 9 coaxial with the container 2. The pulse in the rotation of the arrow 9 is guaranteed by means of impulse 10 provided on the cover 3, on the outside of the container. The agitating means A1 consists of a turbine 11 with straight blades 12, whose middle plane is inclined with respect to the vertical axis of the arrow 9. As can be seen in figure 3, the width of the blades 12 can gradually decrease in the radial direction that moves away from the axis. The turbine 11 is located perceptibly to the middle of the container 2. The normal level N of the reaction mixture in the container 2 is located at a distance h1 on the middle plane of the turbine 11. This distance h1 is smaller at distance H1 from this same median plane towards the lower part of container 2. The inclination of blades 12 and the direction of rotation of arrow 9 are such that an essentially axial and radial flow is created in the reaction mixture M, as shown in the diagrams by the arrows F in figure 2. The reaction mixture is propelled from the bottom upwards in the outer radial zones and from the top downwards in the zones located in radial position inwards . This movement essentially makes it possible to attract the solid urea granules that arrive via the inlet 5 in the upper part of the container and drag them into the reaction mixture.; likewise, this movement makes possible the mixing of the added liquids and the reactable material already present in the container. In the example considered, the turbine 11 or saber turbine includes four blades 12 spaced at 90 ° angles. The base of these blades is close to the external surface of the arrow 9 and is secured to a center 13 wedged in the arrow. The flow created by the turbine 11 can occur following a direction parallel to the arrow 9, in the vicinity of the arrow. The second agitating means A2 includes a radial pipe 14 with straight blades which are secured to the lower end of the arrow 9 in a position coaxial with the turbine 11. The radial turbine 14 also referred to as the Rushton turbine includes six uniformly spaced blades 15. which consist of small plates located in planes passing through the axis of the arrow 9. These small plates are secured in half on a disk 16 that radially overlap outward perceptibly over half its length. A connector 17 is secured on the disk 16 in the center to mount the turbine 14 at the end of the arrow 9. The middle plane of the turbine 14, which corresponds to the middle plane of the disk 16, is closer to the bottom of the container 2 of the N level. The middle plane of the turbine 14 is preferably located at a distance H2 from the bottom of the container that is between one third and one half of the total height H of the N level on the lower part of the container (1/3 H < H2 < H / 2). The distance E to which the radial turbine 14 is located under the turbine 11 is chosen to obtain the best efficiency in dissolution and homogenization. Given its rotation, the turbine 14 creates a cutting movement that intercepts the currents of the flow created by the upper turbine 11, currents that drag the urea solid granules. This cutting movement allows the urea granules to dissolve effectively in the reactable material. The movement created by the turbine 14 also allows the composition of the reactable material to be maintained by the intrinsic homogenization of the liquid and the solid products added to the container 2. When the diameter of the disc 16 (figure 4) is identified with a D, it is chosen conveniently a length I of the order of D / 4 and a height b of the order of D / 5 for the blades 15. The lower part of the container 2 includes an exit orifice 18 equipped with a sieve 19 (figure 2). The pumping means 20 are connected to the orifice 18 to extract from the container 2 the reactable sulfoureic digestion material consisting of a solution of urea in a solution of sulfuric acid. A conduit 21 (Figure 1) directs this reactable material to another unit of the mixer type (not shown) in which the reactable material digests a natural phosphate according to the process of EP-A-0 560 882. A fraction of the reactable material pumped by the means 20, a fraction whose flow rate is controlled by a valve 22, is recycled in the container 2, and is returned by a pipe 23 connected to the inlet 6. As illustrated in figure 2, the input of recycling 6 includes a tube 24 equipped with a dipping tube without a tip. Input 4 for the sulfuric acid solution L is also equipped with a dip tube. The container 2 is equipped with pressure gauges or equivalent means (not shown) for the purpose of controlling the process parameters, such as temperature, composition of the reaction mixture and the like. The drive means 10 includes a reduction gear, the external volume of which can be seen in FIG. 2, while the pulse by a motor and belts is illustrated symbolically in 10a.

In the same figure 2, an opening V can be seen in the upper part of the container. In order to secure it, the container 2 includes supports 25 placed on top of the outer cylindrical wall, below the lid 3 and spaced at regular intervals. The external diameter of the blades of the radial turbine 14 is smaller than the external diameter of the blades 12 of the saber turbine 11, and in particular is almost equal to 2/3 of this diameter.

By way of non-limiting example of the embodiment, the outer diameter of the blades of the turbine 14 is about 1 m, while that of the blades of the pipe 12 is about 1.5 m. The internal diameter of the container 2 is about 3 m. The counterbalances 8 project inwardly over a radial distance of approximately 0.3 m, and the two coils 7b, 7a have a coil turn diameter of approximately 1.8 m and 2 m. The height of the container from the bottom to the base of the lid is about 3.7 m, and the normal level N is about 3.25 m above the bottom. The operation of the device appears next starting from the previous explanations. In normal operation, the sulfuric acid is administered via the inlet 4 and the solid urea granules, with water, are administered via the inlet 5 into the container 2, from the top, in specific proportions. A fraction of the reactable material pumped to the outlet 18 is reinjected via the inlet 6 into the upper portion of the container. The turbine blade 11 driven by the arrow 9 creates an axial movement, already explained above, of the reactable material, and this makes it possible to remove the solid granules in the reaction mixture and prevent these granules from remaining on the surface due to their markedly lower relative density; this movement also makes it possible to mix the added liquids and the reaction mixture already present.

In case of some incident in the turbine 11, the solid urea granules float on the surface of the reactable material much denser and there is no longer any production of reactable material with the desired composition. A first consequence of said incident is an increase in the level N in the container with an interruption of production. The lower turbine 14 with radial blades creates a cutting movement that allows to effectively dissolve, in the reaction mixture, the solid granules dragged downwards by movement, thanks to the turbine 11; this turbine 14 also allows the composition of the reactable material to be maintained by the intrinsic homogenization of the liquid in the solid products added to the container 2. In case of some incident in this turbine 14, the solid urea granules no longer completely dissolve and they then form thick, viscous blocks which can plug the suction silk 19 of the pump 22 and / or cause the level N in the container to rise, which also causes an interruption of production. In both cases of incidents in the turbine 11 and / or in the turbine 14, there is a loss of equilibrium of the heat equilibria, as well as a change in the activity of the reactable material, with very serious consequences in the finished product (interruption of production, a product that does not meet the desired test, etc. .) The example and the counterexamples that appear below help to make the previous comments more precise.

EXAMPLE 1100 liters of 92% sulfuric acid are introduced into a reactor according to the above description, of 10,000 liters capacity; then, the stirring starts at 50 rev / min; 4.0685 tons of urea are introduced while the temperature of the mixture is kept below 80 ° C by cooling (so as not to convert the urea to biuret). When the operation concludes, the temperature control starts at 65 ° C (at an interval of 2 ° C). Then 178.3 liters of water are introduced to obtain a "base product" consisting of reactable material, as defined above: 3.6 moles of urea-1 mole of sulfuric acid-1 mole of water. Then, in the same reactor, 6.7315 t / h of urea granules, 1820 l / h of sulfuric acid and 295.38373 l / h of water are introduced. As soon as a filling level of 7500 liters is achieved in the container, 7,312 m3 / h of reactable material can be exported to the phosphate digestion vessel. In an Israeli phosphate ("ZIN containing 31.1% of P2O5"), then it is possible to produce 15.35 tons / hour of phosphonitrogenated fertilizer (the finished product is also called by the acronym USP), mixing 5 t / h of phosphate with 7,312 m3 / h of reactable material (ie, 10.35 t / h of reactable material at 65 ° C) according to the teaching of patent application EP-A-0 560 882 or WO 92 10443.

CONTRAEJEMPLOS 1. - The previous reactor is equipped with a single saber blade placed at a distance H1 from the bottom of the container equal to 1/3 (one third) of the height of the container. This blade guarantees a movement of the liquid from the bottom upwards; this movement allows the granules (grains) of urea to circulate from the surface of the mixture towards the interior of the container. However, keeping in mind the kinetics of dissolution of these same granules in a reactive sulfoureic material, as defined above, it becomes necessary to increase the residence time of these granules in the container, and therefore the reduction of the production costs (and, consequently, the introduction of raw materials) of the finished product (USP). Pilot experiments have shown that if this speed reduction operation is not carried out, the undissolved granules enter the reactable material / phosphate mixture feed liquid and / or block the suction of the pump. In all cases, since the reactable material is no longer in the conditions of chemical equilibrium, as described in EP-A-0 560 882 (WO 92 10443), there is a change in the degree of digestion of the phosphate and, in Consequently, the finished product will not be in accordance with the subject of the patent application.

It should be noted that it is impossible to increase the size of the container (heat balance, pump size, agitator size, etc.). 2.- The previous reactor is equipped with a single Rushton turbine that has a height H2 equal to half the height of the reactor. Do not create enough vertical movement to cause the urea granules to flow into the mixture; therefore, the operation stops at the "base product" stage preventing the reactable material from forming. The urea becomes moistened and then agglomerates in a ball that dissolves only after a prolonged period (when the suction of the pump is not blocked first). The above explanations show that the combination of the two turbines 11 and 14 mounted in a coaxial position on the same arrow, in accordance with the special arrangement of the invention, allows to obtain results that are particularly convenient for the dissolution of a solid product in a density liquid that is markedly greater than that of the solid product.

Claims (10)

NOVELTY OF THE INVENTION CLAIMS

1. - The device for preparing a reactive sulfoureic material for the digestion of a natural phosphate, in a plant for the production of phosphono-nitrogen fertilizers, by mixing and dissolving in a liquid L which is a solution of sulfuric acid, solid granules of urea G, whose density is less than that of the liquid; which includes a container 2 of cylindrical shape with a vertical axis and a lower part, including stirring means A and provided with at least one inlet 4 for the liquid L, an inlet 5 for the solid granules, and an outlet 18 for the solution, characterized in that the inlet 5 of the granules in the container 2 is located in a high portion, above the level N of the reaction mixture M in this container, and in that the agitation means A includes a first agitation means A1 with the ability to create a drag of the solid granules G towards the bottom of the container 2, and a second agitation means A2 located in the container at a level lower than that of the first agitation means A1 and with the ability to create in the mixture of reaction a movement that promotes the dissolution of the granules, in particular a cutting movement.

2. The device according to claim 1, further characterized in that the first and second agitation means A1, A2 are mechanical agitation means.

3. - The device according to claim 2, further characterized in that the first agitating means A1 consists of a turbine 1 1 with inclined straight blades 12, also known by the name of "saber turbine".

4. The device according to claim 3, further characterized in that the turbine 11 includes four blades.

5. The device according to claim 3 or 4, further characterized in that the second agitation means A2 consists of a radial turbine 14 with straight blades 15, also known as the "Rushton turbine".

6. The device according to claim 5, further characterized in that the radial turbine 14 with straight blades includes six blades.

7. The device according to claims 3 and 5, further characterized in that the two turbines 1 1, 14 are mounted on the same vertical arrow 9, the radial turbine 14 which is located under the saber turbine 1 1 at a distance E. The device according to claim 7, further characterized in that the external diameter of the blades of the radial turbine 14 is smaller than the external diameter of the blades 12 of the saber turbine 1 1, in particular equal to about of 2/3 of this diameter. 9. The device according to claim 7, further characterized in that the middle plane of the radial turbine 14 is located at a distance H2 from the bottom of the container between one-third and one-half of the total height H of the level N on the bottom of the container (1/3 H <H2 <H / 2). 10. The device according to one of the preceding claims, further characterized in that an inlet 6 is provided in the lid of the container 3 to recycle a fraction of the reaction mixture M produced in the container.