"Electrostatic process and device for the separation of particles with equal electrical conductivity, applied to the purification coffee beans"

Electrostatic separation makes use of the differentiated behaviour of matter with respect to its ability to conduct electric charge in order to promote a selective separation of particles contained within a mixture.

The ability of a particle to accumulate electric charge depends on what kind of matter it is made up of, its weight and its geometrical form.

Electrostatic devices usually have a particle feeder which transports the mixture to be separated towards .a rotating metallic rotor. When the particles are placed on to the rotor, they gain electric charge through the ionic bombardment produced by a high voltage electrode. The more conductive particles, once in .contact with the rotor, rapidly lose the electric charge they had obtained, thereby being segregated to one of the extremities of the device as a combined result of centrifugal and gravitational forces. The less conductive particles take longer to lose their charge, remaining attached to the rotor and being afterwards removed at the other end of the device, with the help of a cleaning rotor. Collectors are placed under the rotors in order to selectively collect the segregated particles, which usually have a granulometry of less than ten millimeters.

The device created in the present invention uses the electrostatic separation principle in order to purify mixtures composed of different kinds of particles, with at least one of the materials presenting a granulometry equal to or above 12 millimeters.

The main elements of the device are the following: a) a feeder which supplies the mixture to be separated uniformly to the transportation troughs; b) a rotor with a conductive surface where the particles lose their charge; c) a feeding mechanism or device, responsible for the aligned placement of particles on the rotor's conductive surface; d) an ionizing electrode which ionizes particles individually by ionic bombardment; e) a static electrode with the same polarity as the ionizing electrode/ to which the particles are subjected when they are placed on to the separation rotor; f) a group of collectors which are individually adjustable by deflectors and fitters used for collecting and steering the particLes after separation; g) a cleaning rotor, jointly operated with the separation rotor, which removes from the latter the particles attaching to it.

In order to improve the final performance of the separation process, particles classified as mixture (collected in the collector placed between the two ends of the device) are again subjected to a similar device as the one described in a) to g) , the separation being thereby obtained by a sequential procedure.

PROBLEM PRESENTATION AND STATE OF THE ART

The goal of this invention is to provide a compact enough electrostatic separator which yields differentiated pathways for the flow of materials to be separated. This device is intended to work rapidly and efficiently and to be mainly applied to the purification of coffee beans after roasting, although it can be used in other applications.

The problem this invention intends to solve has to do, essentially, with an efficient separation of coffee beans from their contaminating elements - wooden sticks - that come from the coffee plant and are wrongly packed along with the coffee beans right after they are peeled.

This device also allows the separation of metallic contaminants proceeding from the equipment and tools for handling the coffee beans during their harvest, transportation and processing.

Nowadays, the separation of these impurities from the coffee beans is achieved through mechanical devices whtch use screening and visual inspection techniques. The difficulties in obtaining efficiency in these procedures are the following, listed by impurity type: 1. Wooden impurities

1.1. The procedures used nowadays are not efficient enough to remove these impurities because the wooden sticks' diameter is similar to that of the coffee beans, therefore being able to overcome the barrier created by screening process;

1.2. Visual inspection does not make possible the removal of all the impurities present amidst the coffee, due to difficulties in their detection and to the low number of impurities relative to the high number of processed coffee beans;

1.3. The use of screening procedures is efficient in the removal of impurities of great dimension. However, in industrial plants with a high production of roasted coffee, the impurities tend to block the production line if the screening holes are of small diameter.

For this reason, industrial plants usually use a screening mesh whose holes are much bigger than the coffee beans' diameter, thus allowing the flow of a great number of impurities along with the coffee beans.

2. Metallic impurities

2.1. Along with wooden impurities, coffee beans are also contaminated with a variety of metallic impurities which affect the food hygiene of the final product;

2 .2. Metallic impurities caii either be pure metallic elements or alloys of metals such as aluminium, copper or steel;

2.3. Some of these impurities are magnetic, being thus easily separated through high power permanent magnets;

2.4. Non-magnetic impurities can only be removed after the roasting process, in the final packing procedure, through metal detectors and auxiliary pneumatic equipments, and not through the above mentioned permanent magnets;

2.5. The detection of these impurities occurs, therefore, at the end of the production line, thus involving, with respect to the rejected material, packing, processing and labour costs, not being able to remove the impurities, but only withdrawing a certain fraction of the coffee beans contaminated with metallic particles; 2.6. If an efficient purification is required using metal detectors, a high percentage of pack rejection may occur because the use of a high sensitivity adjustment detector will be necessary to identify metallic impurities of small dimensions, as will be the case with the packing of grinded coffee.

Nowadays, there are a number of different Corona effect electrostatic devices used for particle separation. However, such devices are destined mainly to recycle materials and concentrate ores, such as those described in patents WO0209882, US2548771, US3058589, US2006081507 and DE19836349.

Both ore concentrating and material recycling devices can only be used to separate conductive from non-conductive materials and they are not suitable for separating particles above 12 mm in size. However, coffee purification requires the separation of particles of similar electric nature with size above 12 mm. in the food industry, there are other devices which use electrostatic technology for the separation of non- conductive particles - patents US6225587, EP1038583 and EP0752281. However, these devices apply only to very small particle separation or to the separation of beans from their peels.

Such devices cannot be used in the separation of wooden contaminants from coffee beans as their use is restricted to the separation of very fine materials or of particles with similar shapes but with a great difference in mass. Given the greater difference in shape but, on the other hand, the smaller difference in mass between the coffee beans and the contaminating wooden sticks in relation to the particles the said devices separate, the latter are not suitable for the purification of coffee beans.

The present invention uses electrostatic separation to remove all impurities from roasted coffee beans, thus achieving the separation of both conductive and non- conductive impurities, as well as the separation of particles with a diameter equal to or above 12 mm, which is not yet currently achievable by any other technique or group of techniques. MAIN FEATURES OF THE INVENTION

The present invention is based on two fundamental features: control of the mixture's humidity and the manner in which the particles are exposed to the electrostatic field.

Humidity control

Particles become more or less conductive according to their level of humidity, i.e. a sample of coffee with a higher humidity level presents a more electrically conductive behaviour than a sample with a lower humidity level. Therefore, the absence of a humidity level control may lead to inefficient results.

For electrostatic separation to take place, it is absolutely decisive that the particles have a constant pattern of electric conductivity when they reach the electrostatic field, in order to assure an effective separation procedure and constant regtilation.

After the roasting of the mixture, the particles lose almost all of their humidity. The coffee mixture should therefore be submitted to purification by electrostatic separation only after it has been roasted so that it reaches the electrostatic field with a low and repeatable humidity level.

Manner of exposure to the electrostatic field

Metallic particles are easily and efficiently separable from non-conductive particles through electrostatic separation.

However, until the present invention, non-conductive coffee impurities - wooden sticks - and coffee beans were not separable due to their similar electric nature.

The electrostatic separation of wooden sticks from coffee beans, both non-conductive,. is obtained due to the different amount of electrical charge that the particles can absorb, i.e. : a) Both the specific mass and geometrical dimensions of the wooden sticks are greater than those of the coffee beans. Consequently, the contaminating wooden sticks have a greater mass than the coffee beans, thus presenting a greater ability to retain electrical charge; b) The wooden sticks have a predominantly cylindrical geometry and present a greater adherence, when placed on the separation rotor, compared to the coffee beans. The adherence effect is more intense if the wooden sticks' main axis orientation is perpendicular to the rotor's rotating axle.

When exposed to an electrostatic field, the non-conductive mixture's particles acquire different levels of electrical charge according to the description expounded in the last two items, being selectively separated according to the time they take to discharge the said electrical charge. Therefore, as the wooden sticks initially acquire a greater charge, they present a longer discharge time, consequently being deposited in a different location from the coffee.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is explained in greater detail with the help of the following drawings:

Figure 1 - depicts the arrangement of the different components which constitute the fundamental separating unit (separation cell);

Figure 2, 3, 4 and 5 - depict examples of concave, triangular, convex and saw tooth profiles respectively, to be used in the transportation troughs so that these may adjust themselves to the geometry of the mixture's wooden impurities in order to allow them to be placed on the separation rotor with their main axis perpendicular to the said rotor's rotation axle;

Figure 6 - depicts an example of a cascade arrangement of separation cells, 30 that impurities' separation is achieved with a high degree of efficiency;

Figure 7 - depicts an example of an integrated system for the separation of metallic and wooden impurities;

Figure 8 - depicts a detail of an example of a damping profile to be used in the inner ducts of the device, so as to prevent breaking or deterioration of the coffee beans' geometrical shape during their flow inside the device;

Figure 9 - depicts the layout of two fundamental separation units (separation cells) in a mirror arrangement. The separation cell is composed of the following devices, depicted in Figure 1:

• a feeder 117 that supplies the mixture to be separated to the transporting troughs 116 uniformly throughout their width;

• a rotating cylinder 102 with a conductive surface 103 of low roughness/ called separation rotor, to which the particles lose their electric charge;

• a static electric brush 112 and respective holder that promote the connection of the separation rotor 102 to the source of electrostatic supply through the electric contact in its axle 111;

• a vibratory system composed of one or several vibrators 115 and transporting troughs 116 for the supply of the particle mixture to the separation rotor 102;

• an ionizing electrode 118, negatively charged with respect to the separation rotor 102, which ionizes the particles during their movement through the transporting troughs 116;

• a static electrode 101, negatively charged with respect to the separation rotor 102, to which the particles are exposed when deposited in the said rotor;

• a cleaning device consisting of a rotating cylinder 114, called cleaning rotor, composed of a cleaning brush or combination of cleaning brushes made of insulating material, whose axle of rotation 113 is driven by the axle 111 of the separation rotor 102, from which it removes adherent particles and, optionally, is aided by a neutralizer AC electrode 122;

• a set of collectors placed under the rotors, meant to collect, in collectors 105 and 110, the purified particles, less adherent to the separation rotor 102, and, in collector 107, the unsorted particles (mixture) , less conductive and more adherent to the separation rotor 102; .

• a set of deflectors 108 and 104 that can be horizontally, vertically and angularly positioned, through the regulation of tuners 109 and 106 respectively, for a better definition of the relative position of the collectors.

• a high voltage DC electrostatic source for the production of the electrostatic field.

The particle mixture is supplied by a feeder 117 to the transporting troughs 116, placed on top of a vibrator 115.

The feeder 117 can be vibrated through the coupling of a vibrator 119, should It be necessary to increase the mixture deposition speed.

When in contact with the transporting troughs 116, the particles are vibrated so that they can be evenly transported to the separation rotor 102. The vibration is achieved through a high frequency and low range vibrator 115, so that the particles can be placed on the transporting troughs 116 in a pellicular layer.

The transporting troughs can have different shapes, for which the use of a corrugated profile depicted in figures 2, 3, 4 and 5 is suggested.

The transporting troughs 116 are adapted to the geometry of the wooden impurities in the mixture of coffee beans, so that the said impurities are deposited onto the separation rotor 102 with their main axis perpendicular to the said rotor's rotation axle. This alignment is of paramount importance in order to charge the particles with greater electrical charge, due to the existence of a continuous surface of exposure to the ionizing electrode 118.

One of the main features of the present invention is, as mentioned above, the way the particles are exposed to the electrostatic field, through the combined effect of the placement of an ionizing electrode 118 above the transporting troughs 116, whose length must be enough to allow an increase of the particles' exposure time to the said ionizing electrode 118, with the perpendicular alignment of the wooden impurities in relation to the rotation axle of the separation rotor 102 at the moment of their deposition thereon, thus ensuring a longer time of the particles' exposure to the electrostatic field and a greater adherence of the wooden impurities to the rotor's surface 103. This effect allows a greater electric charge to be accumulated by the wooden impurities to be separated than by the coffee beans and the elimination of unwanted rotation torques. In order for the particles to keep maximum charge while being displaced along the transporting troughs 116, the material that constitutes or coats the transporting troughs 116 must be an insulator, such as wood, card, polymers or ceramics, preference being given to polymeric material.

In order to control the intensity of the electrostatic field to which the particles are exposed, the electrostatic voltage or the distance between the ionizing electrode 118 and transporting troughs 116 can be regulated.

After transportation, the particles are placed on the top part of the separation rotor 102. The materials of insulating nature lose their charge slowly, adhering to the separation rotor 102. Later, the action of the static electrode 101 continuously repels the particles and strengthens their adherence to the separation rotor 102. The particles only free themselves from the said rotor when either the electrostatic force that keeps them adhered tα it is overcome by the resultant of the gravitational and centrifugal forces or the particles are subject to the action the cleaning rotor 114.

The coffee beans after being roasted present a semi- ellipsoidal shape, with a curved top surface and a lower flat surface. This asymmetry gives the coffee beans different behaviours with respect to charge transfer when in contact with the separation rotor 102. The coffee beans deposited with the curved face in contact with separation rotor 102 acquire a low adherent effect due to reduced contact surface. In this case, depending on the angle formed between the tangent at the contact point of the particles with the separation rotor 102 and axle 121, the grains are projected from the said rotor by the conjugated action of centrifugal and gravitational forces which are opposite to and greater than the acquired electrostatic force, being collected in collectors 105 and 110, located in diametrically opposite places relative to axle 121.

Coffee beans deposited with the flat face on surface 103 of the separation rotor 102 present a high adherent effect, being therefore removed during the final part of the movement of rotor 102, either by effect of gravity or by action of the cleaning rotor 114, being collected in collector 107.

In order to improve the effectiveness of the cleaning rotor 114, a neutralizing AC electrode 122 may be optionally used. This neutralizing electrode has the function of reducing the electrical charge of the particles that are in contact with surface 103 of the separation rotor, consequently lessening the adherence of the said particles, thus aiding the removal of particles by the cleaning rotor 114.

In order to regulate the angle of contact of the said particles with surface 103 of the separation rotox 102, the distance between the transportation troughs 116 and axle 121 must be adjusted.

The wooden sticks are placed onto the separation rotor 102 with their main axis perpendicular to the rotation axle of the said rotor, thus preventing the existence of rotation torques and improving the adherent effect of the said wooden sticks avoiding their projection into collector 105. The wooden sticks remain adhered to the separation rotor 102 while the electrostatic force is greater than the sum of the centrifugal and gravitational forces, being removed only at an intermediate stage, after passing under deflector 104, by action of their weight or of the cleaning rotor 114. In this way, the wooden sticks are collected together with a portion of coffee beans in collector 107.

APPLICATION EXAMPLES

The detail shown in figure 1 corresponds to a fundamental separation cell, representing one basic separation unit of a final industrial device. This cell will have to be repeated and individually configured according to application requirements, its use being possible:

• In multi-stage systems, i.e. through the parallel association of separation cells;

• In cascade arrangement of separation cells;

• In high flow systems, with cascade association of two separation cells in mirror arrangement .

The amount of separation cells, as well as the regulating parameters of the process, may be configured differently for each device, being defined by the type of mixture to be separated and the efficiency intended for the said process. For the separation of roasted coffee beans, the use of a cascade configuration represented in figure 6 is suggested, in with which the three cells of separation are individually configured for an efficient removal of wooden particles. This system is organized in the following manner: a) The first cell 1 separates the purified coffee beans to collectors 105 and 110, later carried out of the device by ducts 11 and 12, respectively; b) The mixture that wasn't purified in the first cell 1, deposited in collector 107, is directed to feeder 217 of the second separation cell 2, being subjected to a second electrostatic separation process; c) Purified coffee in cell 2 is collected in collectors 205 and 210 and carried out of the device by ducts 11 and 12 respectively; d) The mixture that wasn't purified in the second cell 2 is directed to feeder 317 of the third separation cell 3 for a new electrostatic separation stage, after having been previously collected in collector 207; e) Purified coffee in cell 3 is initially gathered in collectors 305 and 310 and later added to the previously separated coffee in movement throughout ducts 11 and 12 respectively; f) The mixture that wasn't separated by this device is collected in collector 307 and sent out by duct 13; g) Ducts 11 and 12 are provided with damping surfaces 15, in order to prevent the breaking or deterioration of the coffee beans' geometric shape while being displaced within the device; h) In cascade arrangements, feeder 117 of the top separation cell of the assembly is provided with a level limit switch 120 which acts upon the drive of the external particles feeder in order to prevent overfeeding; i) Each separation cell may be independently configured, in order to improve the efficiency of the purification process.

The present invention may also be used in purification systems of coffee beans from other non-conductive impurities, such as coconut shells proceeding from coconut palms sometimes existing in coffee plantations, whieh are also present during the bagging process of coffee beans. This application is obtained through the regulation of the constituent parts of the separation cell as previously described.

The present invention may also be used in integrated systems for the separation of wooden and metal impurities (combined systems) . The configuration suggested for these systems is represented in figure 7.

The proposed device associates six cells in a cascade arrangement, the first three for the separation of metallic impurities and the remaining for removal of the wooden ones. An elevator 25 is incorporated between cells 3 and 4 in order to transport the not yet purified mixture.

Each cell of the combined device represented in figure 7 functions, individually, according to the previously described principle for the fundamental separation cell.

The combined device operates in a way similar to the device destined to separate wooden impurities, with the particularity that the separation of metallic impurities takes place with a different particle collection flow: a) in cells 1, 2 and 3, destined for the separation of metallic impurities, the said impurities are collected in collectors 105, 205 and 305 and carried out of the device through duct 11. The coffee which is free of metallic impurities but not of wooden sticks, is collected in collectors 110, 210 and 310, and redirected to cell 4 through duct 12 and transporters 24 and 25. The coffee mixture which has not been separated, βtill with metallic impurities, is successively purified, following the route defined by collectors 107, 207 and 307, being sent out of the device through duct 13; b) in cells 4, 5 and 6, destined for the separation of the contaminating wooden sticks, the collection of the purified coffee occurs according to the previously described flow for the case of the device presented in figure 5, i.e. the purified coffee is collected according to the route defined by collectors 405, 505, 605, 410, 510 and 610, being carried out of the device by ducts 21 and 22. The coffee mixture that has not been separated, still containing wooden impurities, is successively purified, following the route defined by collectors 407, 507 and 607, being carried out of the device through duct 23; c) Each separation cell may be independently configured, in order to improve the efficiency of. the purification process.

For high flow processing, the present invention may also be used in systems that form a cascade arrangement of the mirror arrangement represented in figure 9.

Each cell of the mirror arrangement represented in figure 9 functions, individually, according to the previously described principle for the fundamental separation cell, the only difference being that the purified particles in collectors 105 of both cells are collected in one single collector 105C.

In any one of the above described configurations, the parameters to be used in each one of the separation cells can individually be set according to desired functioning. In general, these parameters vary in the following way:

• the rotation speed of the separation rotor 102 usually lies within the interval of 10 rpm to 120 rpm;

• the electrostatic field is obtained using an electrostatic source that allows a voltage of up to 36 kV, applied by means of electrodes 118 and 101, electrodes which may have the same electric potential through the electric connection 100 or different potentials through the connection of each one to two independent voltage sources;

• for electrodes 118 and 101 the use of corona electrodes is recommended, such as: a) electrodes with the shape of a comb or metallic brush, made up of a set of lined up ionizing metallic needles, mounted onto a holder and electrically connected among themselves, a parallel arrangement of the said needles being possible in order to allow the construction of larger electrodes; b) commercial electrodes, traditionally used in the electrostatic separation of fine mixtures; c) a corona wire association, in parallel arrangement and electrically connected among themselves; d) an association of pipes or corona rods, in parallel arrangement and electrically connected among themselves;

• in order to improve the efficiency of the separation, static electrode 101 may be of metallic type or dielectric type, it being that with the latter type, electrode 101 may be placed closer to the separating rotor 102 in order to obtain more intense electrostatic fields and to prevent electric arcs between separating rotor 102 and the said static electrode;

• the said static electrode 101 of dielectric type, mentioned in the previous item, can be obtained through the use of a corona electrode whose ionizing metallic part of the wire, needle, pipe or rod is coated with an isolating coating; • in order for the electrostatic field produced by static electrode 101 to be as uniform as possible, it is recommended that the said static electrode have the same curvature as the separation rotor 102;

• the diameter of the separating rotor 102, not being critical due to the possibility of speed variation, normally lies within the 125 mm to 500 mm interval and, is preferably set between 200 mm and 320 mm;

• vibration of transporting troughs 116 can be adjusted between 1500 and 3000 beats per minute;

• the distance of electrodes 101 and 118 to rotor 102 and transporting troughs 116 respectively, is adjusted according to the previous parameters and the diameter of the particles to be separated. For use in systems of coffee beans purification, distances between 0 mm and 75 mm are suggested and, preferably, between 20 mm and 30 mm;

• the distance between transporting troughs 116 and axle 121 is adjustable between 0 mm and 40 mm.