EP1958699A1 - Method and device for perfecting the operation of hydrocyclones - Google Patents

Abstract

The method involves removing centrifugal particles from a wall such that the particles are subjected to a radial flow (5) of fluid e.g. water, controlling segregation i.e. granulometric segregation, of the particles. The fluid is continuously injected into a centrifugal pulp (4) via a slit parallel to an axis of a centrifugal particle segregation apparatus e.g. hydrocyclone, where injection speed of the fluid has a radial component that drives pulp forming particles towards interior of the apparatus.

Description

Field of the invention

The invention relates to the field of cyclones in general and hydrocyclones in particular. They are called hydrocyclones in the text.

The invention relates more particularly to a method and apparatus implementing this method, which make it possible to reduce the proportion of particles poorly classified in the underflow.

A particular form of the invention makes it possible to recycle unclassified grainy particles heading towards the overflow.

This particular form of the invention makes it possible to increase the dilution of the pulp without adding additional fluid.

These applications of the invention lead to a decrease in imperfection and d50.

State of the art

Hydrocyclones are well known in the industry for thickening pulps, eliminating slimes and, in general, for making granulometric separations for fine granulometry where sieving becomes problematic.

The pulp is a mixture of a fluid and particles of material of varied size. The fluid can be water, this is the case of hydrocyclones, it can be air or any other fluid.

Thus, the hydrocyclone performs a granulometric cut to form two fractions, one of finer particles, the other more granular.

This operation uses centrifugation to increase sedimentation rates. This separation uses differentiation of sedimentation rates as a function of particle size.

The pulp is injected tangentially inside a cylindrical cavity forming the upper part of the hydrocyclone. This entry is usually made in a volute-shaped pipe.

The particles constituting the pulp are then centrifuged thanks to the rotation speed generated by this injection which requires an adequate pressure.

A tube descends axially in this cylinder. It is open outside at the pressure lower than that existing in the cyclone which is most often the atmospheric pressure. The pressure difference between the inside of the cylinder where the pulp is rotated fast and the output of this tube creates a flow. This tube being located axially to the cylinder, the flow caused in this tube creates a radial flow in the cylinder, from the outside to the inside where the outlet is located.

The particles contained in the pulp are then subjected to two opposing forces: a centrifugal force generated by their rotation and a centripetal force generated by the drive in the radial stream.

Particles are carried on the side where the resultant of the forces is predominant, the fine particles are entrained in the tube and the grids are centrifuged on the cylindrical surface. The fraction of the pulp discharged into the axial tube is called "overflow" (Anglo-Saxon term commonly used in the technical literature).

Grainy particles continue their spiraling in a conical cavity that emerges at the same pressure as the overflow (often atmospheric pressure) through an evacuation called apex. This fraction of pulp is called "underflow" (Anglo-Saxon term commonly used in the technical literature). The conicity of this part constitutes a parameter of operation of the hydrocyclones. This taper can go decreasing towards the apex by assembling elements of decreasing taper.

This cavity is conical in order to better maintain the speed of rotation of the pulp. Indeed, the extraction a fraction of its flow to the overflow results in downstream flow reduction. The speed of rotation is then maintained by reducing the diameter of the cylindrical cavity downstream, that is to say towards the apex.

This pressure difference generates the helical flow of the pulp centrifuged on the wall towards this outlet. The adjustment of the pressure drop at this apex is a setting parameter of the desired particle size breaking mesh since it regulates the flow of the underflow.

It happens that the vortex is traversed by an upward flow of air. This phenomenon can be avoided.

The particle size corresponding to an equal probability for which the larger particles in this dimension are moving towards the underflow and the smaller particles moving towards the overflow is called "d50".

Ideally, all smaller particles at d50 should go to overflow and all others to underflow. It would be perfection. This is not so, and the further away from the d50, the greater the probability that a particle will start from the right side. The more this probability increases for a given difference in particle size, the closer we get to perfection and the more the index called "imperfection" decreases.

The graph of this probability as a function of particle size is a curve, called the sharing curve which looks like an integration of the Gaussian curve of a normal probability.

This curve, which represents the probability that particles of a given dimension will go into the underflow, should start from scratch. Indeed, the particles of dimension close to zero should all be driven by the radial water flow and end up in the overflow.

When we look at the separation curves of the separation of all hydrocyclones, we notice that they do not start from zero as logic would suggest. This sharing curve is always shifted upwards.

This means that a certain percentage of particle is moving towards the underflow whatever their size. This percentage can reach a value of 30%. We call this phenomenon a "dysfunction" of hydrocyclones.

The explanation of this dysfunction comes from the fact that the radial velocity of the current is zero at the wall and the fine particles are not therefore driven inwards; these particles then accompany the grainy particles towards the underflow.

This dysfunction is very detrimental to the applications of hydrocyclones.

In the context of grinding for example, the hydrocyclones are arranged at the outlet of the mill to classify the pulp that comes out. Particles greater than the desired size (d50) are returned to the mill while the particles that have reached the correct size are removed from the circuit.

This malfunction of the hydrocyclones then causes the return to the mill of particles that are smaller than the required size and thus undergo overgrinding. This causes excessive energy consumption and overproduction of very fine particles that may hinder the process downstream. These fine particles increase the viscosity of the pulp in the mill which is responsible for a lower grinding efficiency.

Summary of the invention

The invention aims to remedy the problem of poorly classified fine particles present in all known hydrocyclones.

The invention also aims to reduce the imperfection of hydrocyclones on the fine particles side, but also on the side of the grainy particles.

Especially on the side of fine particles:

The invention consists in causing a tangential fluid stream with a radial component at the cylindrical wall on which the particles are centrifuged. In this way, the fine particles that there are dislodged to escape the trap that leads them to the underflow. In fact, at the level of the wall where all the centrifuged particles are located, there is no radial fluid flow which subjects these particles to the laws of segregation. This injected fluid provides this desired flow at the surface. In addition, this fluid injection provides additional energy to maintain the rapid rotation movement responsible for segregation.

Especially on the side of grainy particles:

The injected fluid used to dislodge the particles is taken from the periphery of the overflow where poorly sized particles of particles are likely to be centrifuged. The actual overflow is extracted by a pipe centered on this first pipe. The sampling of fluid for injection is taken from the annular space between these two pipes. The poorly graded particles of particles return to the cyclone for further segregation. The injection slot feed design will be executed so that the particles are oriented towards the underflow side of the slot so as to give them an additional chance to head to the right side.

This particular form of the invention makes it possible to artificially increase the dilution of the pulp without the addition of fluid. This increase in dilution is sometimes necessary to reduce the correction factor of the formula calculating the D50. In other words, this dilution increase will decrease the value of the D50 for any other constant parameter.

The configuration of the output of this fraction must favor its rapid rotation to centrifuge and recover the badly classified particles. A volute exit is required. The circulation pump causes the axial movement of its high-speed feed pipe which communicates with the radial velocity in the annular space via the volute.

A pump rotor can be installed on the inner axis of the annular sampling space which acts by accelerating the rotational speed of the fluid while increasing its head to be injected into the cyclone in closed circuit.

The invention is inspired by the method and apparatus described and claimed in European Patent Application 05020997.2 / EP05020997 (GENIMIN). It uses the principle of injecting a tangential water slide. As stated in document 05020997.2 / EP05020997 , the water injection slot can be placed indifferently. In the particular form of the invention mentioned in this text, the injection slot is placed parallel to the axis of the cyclone. It can be inclined.

The centrifuged pulp overlaps, at each turn, the ply of injected fluid.

The direction of tangential fluid injection may comprise an axial component upwards or downwards. low since a portion of the injection fluid will accompany the underflow (downward) or the overflow (upward). In the particular form of the invention described, this direction of injection has no axial component so as not to favor any of the outlets. Having this injection without axial component slows the opening of the pitch of the helix to the underflow.

The most granular particles are sedimented before having made a complete revolution while the finest particles are not sedimented and are therefore driven by the radial current caused by this fluid injection.

This particular shape has the advantage of simplicity and ease of adjustment of the injection opening. Indeed, this adjustment of the thickness of the injected water blade can be done by the deformation of the outer wall of the rectangular injection pipe. (see description of the figures).

The radial velocity expressed in m / sec of this injection is equal to its thickness expressed in meters multiplied by the number of passes per second. Thus, for a rotation speed of 3000 revolutions per minute and a blade thickness of 5 mm, the radial velocity is 0.25 m / sec.

The invention relates in particular to a high-performance method for the granulometric classification of a granular material, according to which a suspension of said granular material in a dispersive medium is subjected to centrifugation in a cyclone from which a sample is withdrawn. coarse fraction of the suspension and a fine fraction of the suspension. The method is characterized in that a ply of fluid is injected at the wall so that the centrifuged suspension overlaps this sheet at each turn.

In one embodiment of this efficient method, the aforesaid direction of injection of the fluid sheet has a tangential component to the centrifugation which maintains it and a radial component which causes the finely divided particles towards the interior of the cyclone. The injection direction may also have an axial component to the cylinder.

In the aforementioned high performance process, the fluid is preferably injected substantially continuously into the suspension.
In the aforementioned efficient process, the dispersive medium of the suspension can be any fluid, most often this fluid is water or air.
In a particular embodiment of said high-performance method, a portion of the abovementioned fine fraction of the suspension is taken to form the injected fluid, making it possible to recycle particles that would be poorly classified in the overflow.

Brief description of the figures

• La figure 1 est une vue, en section axiale, d'un hydrocyclone antérieur à l'invention ;
• La figure 2 est une vue analogue à la figure 1, d'une première forme de réalisation du cyclone selon l'invention ;
• Les figures 3a et 3b montrent, en section transversale, le cyclone de la figure 2 dans deux réglages distincts ;
• Les figures 4a et 4b sont des figures analogues à celles des figures 3a et 3b et montrent une variante de réalisation du cyclone de la figure 2 ;
• La figure 5 montre, en plan, un détail de l'hydrocyclone de la figure 2 ;
• La figure 6 est une vue analogue à la figure 1 d'une autre forme de réalisation de l'hydrocyclone selon l'invention.
• Les figures ne sont pas dessinées à l'échelle.

Généralement, les mêmes numéros de référence désignent les mêmes éléments.Particularities and details of the invention will appear during the following description of the annexed figures, which represent some particular embodiments of the invention.

• Figure 1 is a view, in axial section, of a hydrocyclone prior to the invention;
• Figure 2 is a view similar to Figure 1, a first embodiment of the cyclone according to the invention;
• Figures 3a and 3b show, in cross section, the cyclone of Figure 2 in two separate settings;
• Figures 4a and 4b are similar to those of Figures 3a and 3b and show an alternative embodiment of the cyclone of Figure 2;
• Figure 5 shows, in plan, a detail of the hydrocyclone of Figure 2;
• Figure 6 is a view similar to Figure 1 of another embodiment of the hydrocyclone according to the invention.
• The figures are not drawn to scale.

Generally, the same reference numbers designate the same elements.

Detailed description of particular embodiments

Figure 1 shows a conventional hydrocyclone that is on the market. The realization is often modular and the assembly of subassemblies is often carried out by means of flanges. From top to bottom, we thus find the extraction tube (1) of the fine fraction (overflow) fixed in the center of the element (2) which ensures the tangential injection which centrifuge the pulp to be separated.

A cylindrical section (3) is fixed on this injection element on which the pulp is centrifuged. The movement of the pulp (4) induced by its rotation is helical since an axial component is necessary to allow its evacuation. This cylindrical section makes it possible to give the particles time to be centrifuged against the wall. The tube (1) penetrates at this cylinder (3) and opens at a lower pressure than that present in the cyclone (usually atmospheric pressure). An evacuation stream is created which causes a radial flow (5) to this outlet and a decrease in the pulp flow downstream of the hydrocyclone. In order to maintain the rotation of the pulp and keep the particles centrifuged on the wall, its section decreases. This element is then cone-shaped which may consist of several elements with decreasing taper (6-7). The conical section opens at the same pressure as the outlet of the tube (1) above (usually atmospheric pressure). This orifice of (8) is called the apex which allows the evacuation of the underflow.

The rotating pulp follows a helical movement up and down the periphery of the cyclone towards the underflow and from bottom to top at its center towards the overflow. This central flow is called a vortex. It happens that this vortex is hollow and traversed by an air core from the apex to the overflow. Known techniques make it possible to eliminate this air core.

The hydrocyclone represented in FIG. 2 is in accordance with the invention. It comprises for this purpose an additional subassembly (9) which is fixed between the cylinder and the cone or between two successive cylindrical sections.

In the particular form described, the inner surface (10) of this subset is cylindrical corresponding to the cylinder diameter of the hydrocyclones. The injection is done by a slot (11) which is outside the template of the cylinder. The outer surface of this slot joins the cylindrical surface by a volute (12).

In the particular form described a simple way of adjusting the thickness of the water blade is by changing the profile of the outer surface of the supply line (Figs 3 and 4) which opens and closes the injection slot. The direction of the injection defines a downstream side (11) and an upstream side (14) of the injection pipe (15).

The sedimentation surface (10) of the centrifuged particles consists of an abrasion-resistant curved plate (16). This curved wear plate protects the structure of the subassembly (16) and is fixed by external screw (17) so as to keep the inner surface rigorously smooth.

These fasteners are interrupted at the level of the inlet volute of the water injection allowing the deformation of this curved plate.

On the upstream side, a sheet (18) closes a cavity (19) which allows the linear movement of the plate between this sheet and the inner surface of the subassembly. This flexibility of the sheet allows it not to oppose the low rotation of the plate. This sheet is fixed on a piece (20), itself attached to the subassembly serving as a range to the injection fluid supply line.

The adjustment of the thickness of the slot is done at each end by a screw (25). The force of these two adjusting screws is distributed over the entire width of the plate by means of a rigid reinforcement (26) integral with the wear plate (10).

On the opposite side of the supply cavity is fixed a wear part (27) which ensures the junction between the ply of injected fluid and the rotating pulp in the cylinder. The curved wear plate (10) must be sufficiently elastic to allow its deformation, (the polyurethane lends itself well to this application). The joining piece (27) must, on the contrary, be very rigid to prevent any deformation. This is a reason for these two pieces (the plate and the junction) to be distinct and made of different materials.

This type of assembly allows to extract this part without disassembly of the subassembly and replace it in case of wear. A guide groove (28) is formed in the top and bottom closure plates (Fig 5) which close the subassembly. These plates laterally close the installation plane of the injection piping and secures the adjacent sub-assemblies by screws in the threaded holes provided in these plates (29). These plates are attached to the subassembly by countersunk screws so as to keep the laying surface flat with the adjacent subassemblies (position of the holes (30)).

The deformation can be caused, not by the elastic deformation of the coating as exposed, but by the movement of two rigid curved plates (31,32), articulated on linear ball joints (33) (Figure 4). The adjustment of the opening of the slot is always by thrust screw (25) accessible from the outside of the subassembly.

The linear ball joint is achieved by the cylindrical milling (convex and concave) of the adjacent sides of these plates. The opposite side of the downstream plate (34) is also articulated on the same linear ball whose concave side is milled in the protective coating of the cylindrical surface of the subassembly (10), of the same thickness as these articulated plates. The convex side of the ball joints is located upstream so that the edge of the ball is oriented in the direction of flow. The upstream side (14) of the upstream plate slides on the inner face of the subassembly. The plate is pushed by a spring (35) and held against the inner wall by a deformable sheet fixed to a part (36), itself fixed on the subassembly. This part serves as a laying plane to the supply pipe of the injection fluid.

This second particular form of the invention has the disadvantage compared to the first one of concentrating the curvature modification at the patellae. This is less favorable for flows that must avoid any one-off directional variation.

In a particular and advantageous form of the invention, the injection fluid is taken from the overflow (see FIG. 6) at its periphery, in the annular space between the outlet tube of the overflow (37) and the outer tube (38) which descends into the hydrocyclone to collect the overflow.

This sample undergoes an increase in its head in a pump (39) to be injected at the required pressure. This particular form of the invention makes it possible to perform the injection without additional fluid supply. The shape of the outlet of this sample is in volute (40) so that the velocity of the fluid communicated by the circulation pump can best maintain the speed of rotation in the annular sampling space and allow the particles badly classified to be captured in this peripheral annular space.

Claims (13)

Process for the improvement of the operation of any centrifugal particle segregation apparatus and in particular the cyclones and hydrocyclones which consists in dislodging from the wall (10) all the particles centrifuged on it so that they are subjected to a radial flow (5) of fluid responsible for their segregation. Process according to Claim 1, characterized in that the term segregation relates to any segregation, the most common of which is granulometric segregation. Process according to the preceding claims, characterized in that the fluid (5) is injected in a substantially continuous manner into a pulp (4) centrifuged Method according to the preceding claim, characterized in that the fluid (5) is injected by a slot (11) arranged in the most appropriate manner and, in particular, parallel to the axis of the hydrocyclone. Process according to the preceding claims, in which the injection speed has a radial component which causes the particles constituting the pulp (4) to be driven into the interior of the apparatus. Method according to the preceding claims, wherein the energy provided by the injected fluid (5) maintains the centrifugation speed of the pulp (4). A method according to the preceding claims, wherein the injection fluid (5) is taken from the overflow (22) allowing circulation in a closed circuit to artificially increase the dilution of the pulp (4) at the level where the segregation. Method according to the preceding claims, wherein the injection fluid (5) is taken from the periphery of the overflow (22) allowing the misclassified particles to undergo a new segregation A method according to the preceding claims, wherein the sampling outlet is in a volute cavity allowing a transition of the rotary helical motion towards an axial flow directing it towards the rotary pressurizing pump. Method according to the preceding claims, which may be a subset (9) adapted to be assembled to each type of hydrocyclone or cyclone modular manufacturing or other segregation apparatus. A method according to the preceding claims, for adjusting the thickness of the injected fluid plate by deformation of the outer surface of the supply line of the injected fluid. Method according to the preceding claims, which uses the anti-abrasion protection coating as a deformation surface for closing the injection slot. Method according to the preceding claims, wherein a rigid part, removable from the outside, makes the junction of the two streams (injection fluid and the pulp in rotation).

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