AU2014408624A1 - Assisted-vortex cyclone - Google Patents

Abstract

The invention relates to a complemented cyclone for optimising classification by size of suspended particles in pulps or gases. It involves the introduction of energy into the cyclone through an impeller actuated externally with a variable-speed motor, for accentuating the vortex effect, thereby increasing the efficiency thereof. A significant advantage over current cyclones is that variable frequency drive technology allows the process to be controlled with great precision and allows in-line control to be established so as to adjust the r.p.m of the impeller according to the deviations of the product as a result of fluctuations in the power supply.

Description

CYCLON WITH ASSISTED VORTEX

BACKGROUND

It is related with cyclones, which have been used extensively for over half a century in the classification by size of particles in Comminution circuits. The cyclones are used both in primary and secondary grinding circuits as well as in the regrinding circuits, of solids in liquids suspension (pulp).

The main problem of this equipment is the low efficiency in the cutting of sizes classification. That is that both products of the cyclone: the overflow that is the finer product and the underflow (the thickest), both drag important quantities of particles, which, because of their size should exit, from the cyclone, in the opposite flow. The above-mentioned means a less metallurgical recovery and in excess of energy consumption in the mill circuit

From the control viewpoint, we can define the cyclones as equipment eminently passive, that is to say, the variables controlling their operation are external to the equipment. These variables are: the density, the viscosity, the flow rate and the feeding pressure of the pulp. All of them are variables of very rigid tune up, which impede making precise adjustments, during operation, to improve their performance

Cyclone functioning.

The centrifuge force is a function of the turning radius, the greater the radio the greater is the driving force that maintains the particles of higher mass circulating, adhered to the wall of the cyclone, until these go outside the cyclone for the lower discharge outlet, also in the form of pulp, with higher density than the pulp fed to the cyclone.

It is exactly the lower pressure, produced near the axis of the vortex, which induces the exit by the upper overflow pipe of a flow of pulp that contains the finer particles material and is of less density than the feed to cyclone.

The cyclone is used to separate the coarse fraction from the fines fraction. Fines contain, principally, the liberated - from waste - valuable ore species in the closed circuits of wet grinding.

The solid particles that enter the cyclone with the fluid vein can be considered as moving substantially parallel to the axis of the entry tube, with a speed the same as that of the fluid. When the fluid turns due to the pressure reflected from the curved wall, the solid particles tend to continue in straight lines to the wall and particularly the thicker ones. The finer particles however are maintained in suspense by the whirlwind in the fluid and tend to follow the general direction of the fluid.

Cyclones: Types and functionality.

In the ore concentration processes, the cyclones normally are a main part of the closed wet grinding circuit. They are fed by means of pumps with a mixture of solids and water, known as pulp, coming from the ball mills. The pumps inject the pulp inside the cyclone at a certain speed, in the range of 3.5 to 6.0 m/s with the purpose of inducing a centrifuge force, that generates a forced vortex capable of causing the classification of the material in coarse and finer grains.

The function of the cyclone in the copper mineral processing plants, for example, is to separate the finer material - normally under 74 microns (mesh 200 Tyler), which will feed the next regrinding stage or will go directly to the flotation circuit - from the thicker material, which will return to the mill (circulating load) to liberate by milling the useful particles once they acquire the fineness required.

In the cyclones it is favored the cutting efficiency of the finest fractions with the purpose of achieving a good recovery of the valuable particles in the flotation cells. In fact the thicker particles do not permit a total release of the useful species so that if they are neither recovered or contaminate the final product: the CONCENTRATE.

The thicker fraction that leaves the cyclone as underflow is, in the wet grinding circuits the CIRCULATING LOAD and usually drags several fine particles that should come through the overflow. This inefficiency is translated into a significant increase in the mass of the circulating load, which remarkably affect the efficiency of the mill in the aspects of productivity and energy consumption.

The productivity is affected because the fine material that returns can reach figures of up to 20% of the circulating load and if we take into account that the circulating load is of the order of 300% of the fresh feeding to the mill, it is clear then that it’s bearing on the reduction of productivity is not a minor issue.

It is obvious that thee effect of the additional circulating load also had a very significant bearing in the extra consumption of energy in the grinding process, which represents around 70% of the total consumption of energy in the process of concentration of minerals by flotation

Another important problem of these equipments is the difficulty to exceed the high threshold of the entering pressure to the cyclone. This limits the tangential speed to enter to cyclone and consequently the centrifugal speed, which is the most important of the parameters that determine the classification’s efficiency. In general with a higher speed a remarkable increase is generated in the abrasion of the equipment’s and the circulating and distribution facilities of the pulps, with the consequent increase of the maintenance costs. As design criterion the maximum speed of the pulp in the transportation by pipes is 4 m/s. With this it is possible to maintain a level of wear out by abrasion in economic levels. In order that the cyclone may generate the vortex effect, it is required to increase that speed between 8 and 12 m/s. This is achieved reducing the transversal section of the admission to the cyclone to half or less of the feeding pipeline's section.

Solution of the problems

There has been intense research in this area and a significant development endeavoring to optimize the efficiency of these equipments, with special emphasis in the search to reduce the circulating load and the grinding circuits. Among the most important we can mention the following developments: a) Designs of the cyclones. “Two large groups are classified: conical and cylindrical. Within the first group would be included the pronounced cone and those of extended cone. The second group would pick up the cylindrical ones with a flat bottom and the cylindrical ones with central discharge. a.1.- Pronounced cone. This group draws the cyclones with an angle of less than 20Q characterized by a relatively long body due to its conical shape. This type of design is accompanied with cylindrical parts of great length (greater than the diameter once more) and tewels of feeding and outlet in small dimensions to increase the residence time. This due to the great altitude of vortex (distant between the lower border of the overflow and the vertex of the conic part) and its influence inversely proportional to the size of the cut makes them the most adequate for fine classifications as required in operations of clarification and thickening. a.2.- Extended or wide cone- They are of an angle over 20Q and are used principally to classify both for size and density. The angle of the conical part varies between 20° and 45° although exceptionally hydroclones can be found with up to 160°. They are constructed in diameters between 250 mm and 1250 mm, though some manufacturers construct models of up to 2,000 mm. As is logical, when reducing the residential time of the pulp inside the cyclone, because it is shorter, increases the size of separation. This brings as a consequence that these cyclones do not reach a high recovery of solids (referring to the underflow) but do present a best selectivity. a3 - Cylindrical hydro cyclones with periphery discharge. They consist basically in a conventional cyclone of which its conic zone has been eliminated, replacing it by a cylindrical part of similar length. The bottom of the cyclone is flat and the extraction of the thick product is carried out tangentially by the low zone of the cylindrical wall. Regardless of the fact that this design provokes a much diluted discharge, due to the short-circuit (fine articles in suspense in the liquid together with the discharge), the efficiency of separation is reduced. Some 30 years ago one of the main manufacturers of hdrocyclones, Krebs, commercialized a type of cyclones known as EE. Based on this design of tangential discharge connected two units, in series; the first of large diameter and cylindrical wall and a second a conventional one, of fewer diameters. This design was not too well accepted at least at industrial scale, perhaps due to the high wear and tear that could be expected in the zone of transition due to the high concentration of solids. Around the year 1987 this “old idea” reappeared in the market with the variation of introducing a water injection in the chamber of connection between the two cyclones. In this way, the water acted diluting the feed to the second cyclone, thus improving the efficacy of the classification. Summarizing, this new type of design used for practice by the firm Larox under the name of TWIN VORTEX CYCLONE presents the advantages of a double stage of classification, but without the need of an intermediate pump. a.4.- Cylindric cyclones with central discharge. Flat bottom. “This design differs from the previous one in that the discharge is done conventionally, that is to say through a central hole. In order to expand the work area of the cyclones towards larger cut sizes, over the 150 microns, the development of the so-called fit bottom cyclones was created, based on the bed of solids that is created in the cyclones of obtuse cone, the development of the so-called flat bottom cyclones, better called by its inventor Prof Dr. Helmut Trawinski de Amberger Kaolinwerker GmbH, CBC Cyclones (Circulating Bed Cyclone or Circulating bed classifier). It was explained before that the “fluid” bed in the lower zone of the cyclones of wide cone is not a stationary bed, only it has convection movement around the central nucleus which favors the reclassification of light or small particles, poorly classified, which in their constant movements are at some time dragged by the interior or principal whirlwind, finally being emptied with the overflow. b) The problem of wear and tear. This is perhaps one of the main problems of these equipments. For many years the market of the cyclones has been dominated by the North American company KREBS. At present it has had a strong competitor represented by the firm WARMAN which holds as principal strength having significantly reduced the wear and tear in the zone of admission of the cyclone. This is obtained through a redesign of the geometry of the admittance chamber. According to WARMAN the maintenance time was reduced between 60 to 70% there were significant economies in replacement elements, less stock of spares was required in the warehouse and less labor. All this translated into more time of operation due to the reduction of changes of wear and tear elements. According to WARMAN it was also achieved to reduce the thick particles in the overflow which increased the recovery of mineral in the following stages of the process.

Disadvantages of the art technique. a.- Elevated energy consumption and wear and tear of the equipments.- These matters are very much related between them. In fact, a poor classification substantially increases the consumption of energy of the grinding circuit, which in itself is quite high. In addition, as the cyclones are equipments whose efficiency is directly proportional to the kinetic energy that is developed by the pulp inside, it is required that this pulp be fed to the cyclone by powerful pumps at high pressures and high speed. The wear and tear generated in the equipment and elements for the handling of pulps is directly proportional to that energy and it is a direct function of the abrasivity of the material that is handled. b.- Classification's efficiency.- The progressive and inexorable exhaustion of the high grade ore resources makes it necessary to try to increase by metallurgic ways the recovery of the useful mineral species. This implies increasing the degree of grinding of the mineral to release the finer mineral particles. Summarizing, every day it is required to grind finer, and therefore it is necessary to improve the classification of the grounded ore in much narrow ranges and smaller size.

We saw previously that to the lower cutoff size, the cyclones applicable necessarily have a small diameter. This implies that for large volumes of production many cyclones are required, the which, to be fed require extensive pipeline circuits. The combined effect of these factors demands increasingly more power to absorb the higher losses by friction of the system and therefore bigger investments.

The disadvantages detailed above constitute a kind of vicious circle as, with lower size cuts much more energy and more equipment is required, which causes a greater wear and tear of the equipment and facilities and therefore higher costs, reducing progressively their efficiency also.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 schematically shows the cyclone and the elements that constitute the complement to optimize their performance; here the following can be highlighted. 1 Typical cyclone 2 Impeller (schematic) 3 Impeller’s shaft 4 Motor of the impeller (schematic) 5 Impeller depth regulator (schematic)

DESCRIPTION

Purpose.- To complement the actual cyclone configuration, in order to increase its efficiency.

The invention consists in increasing the “vortex” effect incorporating inside of the cyclone 1 an impeller 2 connected to a shaft 3 that juts out of the upper part of the cyclone. The impeller is driven by a motor 4 (symbolically represented by two arrows) connected to the shaft and located outside of the cyclone.

The depth position of the impeller will be regulated 5 in function of the particular characteristics of the pulp, but normally it is situated below the overflow's tewel. The optimum location of the impeller, as well as its shape and dimensions are going to be determined experimentally.

During its operation, by means of torque sensors, the r. p. m., and the depth position of the impeller can be modified in order to improve its performance or to compensate their wear and tear. In any case, the impeller will operate in the nucleus of the pulp, particularly in the zone near the overflow, which is characterized by its lower density and being constituted by the smaller particles and therefore the less abrasives.

The impeller will turn in the same sense as the fluid injected to the cyclone, with which when increasing the angular speed of the pulp mass, will increase the radial pressure. The r.p.m. of the impeller are determined experimentally for each type of cyclone and for each specific pulp.

Invention technical back up.- Any increase in the r.p.m. of the pulp inside the cyclone implies, automatically, an increment of the radial pressure, proportional to the 2nd power of the tangential speed.

The greater radial pressure directly bears on the efficiency of the classification by sizes of particles in suspension in a fluid.

With the modern technologies, represented by the variable speed motors, will be possible to operate the impeller to that r. p. m. that optimize the classification process achieving an efficacious classification by sizes.

The impeller depth regulation allows modifying the momentum of the pulp inside the cyclone.

Main Advantages of the Invention A brief enumeration of the advantages of the invention it is as follows: a) The main advantage will be the reduction of the energy consumption in the wet grinding processes by effect of the best classification achieved in the product discharged by the cyclone. b) The impeller action will avoid the partial short circuit, of thick material, directly to the overflow. c) The best classification by sizes in the wet grinding processes, as well also as the greater treatment capacity by reducing the volume of the circulating load will be very important. . d) The introduction of the impeller represents the addition of dosed energy according to the results sought, as well as the application of that energy in the precise place. It will be inserted in a fluid medium characterized by having a predominance of particles with less mass and of less abrasivity and of a lower density than that of the flow that enters the cyclone. e) With the enhancement of the vortex effect it will be possible to increase the size, of the cyclones to process ultra fine material or also reduce significantly the number of cyclones to process a certain amount of flow. f) It offers the possibility of establishing a control on line, by means of instruments that register the size of the material that abandons the cyclone and, in case variations are detected, with this information adjust the cyclone to correct deviations, acting on the motor of the impeller, the which for these purposes must be of variable speed. g) . It will be possible to determine experimentally, for each type of pulp, the size, the design and location of the impeller, as well its optimal rpm. h) It will be possible to increase the speed of the pulp just only inside the cyclone, without subjecting to the pumping system, neither the net of pipes to an undue wear and tear.

REFERENCES 1. - ELEMENTS OF ORE DRESSING; Arthur F. Taggart; edited by John Wiley & Sons, Inc., 1.951. pages. 110 to 125. 2. - Bulletin N° 333-C; Fisher - Klosterman Inc, EE.UU., 1.993. 3. - Magazine CANTERAS Y EXPLOTACIONES - May 1990. (Spain). Article: TRATAMIENTO Y CONCENTRACION.

Claims (2)

1. Claims
2. 1. Cyclones are used to classify particles suspended in pulps according’s size in order to recover, by flotation, the free valuable mineral species liberated in the wet milling closed circuits. An impeller is incorporated inside the cyclones to improve its classification efficiency and to reduce the energy consumption by reducing the circulating load between the cyclones and mills; characterized by: a) The impeller will rotate partially or totally immersed in the pulp’s vortex. b) The depth of the impeller in the pulp is regulated by displacing its shaft axially from outside the cyclone. c) The impeller is start up through a variable speed motor located outside the cyclone d) The momentum of the pulp can be modified, either by varying the impeller rpm and/or changing the depth of the impeller in the pulp, until getting the optimum cutting sizes of particles.