EP2430215B1 - Device for collecting the solid debris present in the bath and the liquid metal of an electrolysis cell intended for aluminium production, by scraping the bottom of said cell - Google Patents

Description

The invention relates to the production of aluminum by igneous electrolysis according to the Hall-Héroult method. It relates more particularly to a device for collecting solid debris immersed or floating in the electrolysis bath and the liquid metal, in particular sludge from the electrolytic bath and which accumulate on the bottom of the tank, as well as the remains of carbon and crust debris that originate in particular from the various operations performed before and during the removal of spent anodes.

Aluminum is produced industrially by igneous electrolysis, according to the well-known Hall-Héroult process, in electrolysis cells. The plants contain a large number of electrolysis cells arranged in line, in buildings called halls or electrolysis rooms, and connected electrically in series using connecting conductors, so as to optimize the ground occupation of the cells. factories. The cells are generally arranged to form two or more parallel lines that are electrically bonded together by end conductors. In each cell, the electrolyte bath and the liquid metal are contained in a tank, called an "electrolysis cell", comprising a steel box, which is lined internally with refractory and / or insulating materials, and a cathode assembly located at the bottom of the tank. Anodes, typically of carbonaceous material, are partially immersed in the electrolyte bath.

In operation, an electrolysis plant requires interventions on the electrolysis cells including, in particular, the replacement of spent anodes with new anodes, the removal of liquid metal and the additions or withdrawals of electrolyte. In order to carry out these interventions, the factories are generally equipped with one or more service units comprising a movable bridge that can be translated over and along the series of electrolysis cells, and one or more service modules. , each comprising a carriage, adapted to be moved on the movable bridge, and handling and intervention devices, such as shovels and hoists, commonly called "tools". These service units are often referred to as "electrolysis service machines" or "MSE"("PTA" or "Pot Tending Assembly" or "PTM" or "Pot Tending Machine" in English). The service module generally comprises, attached to the carriage, a rotary frame, called tool turret, which is adapted to rotate about a vertical axis and is integral with said tools. Each tool can be attached to the end of a cable operated by a winch attached to said turret, or at the end of an arm, the latter being telescopic and / or articulated.

One of the necessary interventions during the anode replacement is the cleaning of the area occupied by the spent anode and to be occupied by the new anode. This zone is essentially constituted by the bath and the liquid metal but can contain many solid debris that it is necessary to remove before setting up the new anode. During the electrolysis, a hard crust of fluorinated cryolite and alumina is formed on the upper surface of the bath. This crust has the advantage of retaining heat within the bath and therefore constitutes an effective heat-insulating envelope. But it is extremely hard and adheres to the wall of the anode block, so it is necessary to break it around the worn anode, to allow the extraction thereof. Typically, rupturing of the crust is carried out using tools such as piercers, called "crust breakers". Then, during the removal of the spent anode, an orifice in the crust is formed, which orifice is left vacant until the installation of the new anode and which we will call "anodic hole". Crust failure and handling of the spent anodic block inevitably lead to the formation of solid pieces or parts which float or remain suspended in the electrolysis bath, or which fall to the bottom of the tank. It is then necessary to collect them by means of a collection tool, commonly called "crust".

The European patent application EP-A-0 440 488 describes an example of a crust scoop associated with a particular vehicle, distinct from a service machine. The European patent application EP-A-0 618 313 describes, but in a little detail, an example of a service machine equipped with a device to ensure the rupture of the crust in the vicinity of a worn anode and the cleaning of the anode hole. The crust scoop commonly used is a clamp consisting of two buckets arranged symmetrically with respect to a substantially vertical plane and hinged, pivoting about two substantially horizontal axes, possibly merged. Each bucket has an edge drive, also called "blade", vis-à-vis the leading edge of the other bucket. To collect the debris, the crust scoop is immersed in the open position in the bath and then the crust scoop is passed from an open position to a closed position, using at least one actuator that acts either directly on a scoop. , or preferably on a linkage designed to bring the cups in a rotational movement substantially symmetrical with respect to each other, the solid debris between the two cups being thus trapped, while the liquid medium mixture of Electrolyte bath and molten metal can still escape, in particular through openings in the walls of the buckets.

Traditionally, the opening and closing movement of the crustal scoop is driven by the actuation of a pneumatic cylinder which acts on a linkage designed to transform the translation movement of the cylinder in two symmetrical rotation movements of the buckets. Before placing the new anode in the cell, ensure that all crust and carbon debris in the anode hole has been removed. As some of them can rest on the bottom of the tank, it is necessary to plunge the scoop in the liquid medium consisting of the bath and the metal so that its leading edges are flush with the bottom of the tank. But as the leading edges of the buckets describe circular trajectories when the crust shovel closes to pick up the debris, the maneuvering of the shovel is very delicate, because the cathodic assembly which constitutes the bottom of the tank risks to undergo a significant damage during this operation. To avoid such deterioration, it would be necessary to impose on the axis or pivot axes of the buckets a position in altitude such that the leading edges of the buckets never touch the bottom of the tank during the maneuver, while being the most possible close to this bottom so that the cleaning is effective. However, this position is difficult to evaluate because there is no visual access to the bottom of the tank. On the other hand, this theoretical position makes, because of the circular circular blades, the crust scoop inefficient in the phases where they are furthest from the bottom of the tank, a part of the debris resting on the bottom of the tank risk of not being collected.

The German patent application DE3131624 proposes a mechanism making it possible to give the end of the blades of the shells of a crust scoop a substantially horizontal overall trajectory during their closing, preventing the blades from scraping the ground.

Thus, this cleaning operation of the anode hole using the crust shovel encounters two antagonistic difficulties: either it is too close to the bottom of the tank and, in this case, there is a risk of damaging it, or is too far away and cleaning is insufficient. What whatever the procedure, there remains a significant risk of general instability of electrical and magnetic order in the operation of the tank, leading to a decline in efficiency of the installation.

The European patent application EP-A-1 178 004 proposes a solution likely to solve the problem explained in the preceding paragraph. This solution consists of using a bucket scoop mounted on a vertical arm but not to directly attach the frame secured to the axes of the buckets on said vertical arm. To this end, the chassis is split into a part called "excavator frame", which remains attached to the arm attached to the turret and a part called "bucket support frame", vertically movable relative to the door frame so that, since the instantaneous center of rotation of the buckets can move while the arm remains stationary relative to the bottom of the tank, the leading edges of the buckets can be given a substantially rectilinear trajectory. The shovel can be placed so that its leading edges are flush with the bottom of the tank during the entire operation of closing the shovel. However, such a solution results in a major complication of the excavator mechanism, with a complex bucket closure assembly comprising "a transmission rod of forces, one of whose ends is articulated [...] on the buckets [...] and whose other end is articulated on a rotating actuating rod, itself hinged to the support frame of the buckets, said rotating rod being mechanically connected to the excavator frame by means of a connecting rod compensation means articulated on the excavator frame, said rotary rod being moreover actuated in rotation by means of an actuating cylinder, whose point of application is integral with the support frame of the buckets ". Such a solution requires the introduction into the crust with numerous intermediate pieces intended to operate in a hostile environment and to undergo vibrations of high amplitude, in particular because of the stresses associated with the breakdown of the buckets. This involves frequent replacement of these parts subject to rapid wear.

1. a) un mât vertical actionné par un premier actionneur, appelé également actionneur de levage, qui permet de déplacer ledit mât vertical suivant la direction verticale;
2. b) un châssis fixé sur ledit mât vertical;
3. c) au moins un godet articulé, pivotant autour d'un axe sensiblement horizontal, monté sur ledit châssis, présentant un bord d'attaque sensiblement horizontal et actionné par un deuxième actionneur, appelé également actionneur de fermeture, solidaire dudit châssis, imposant audit godet un mouvement de rotation autour dudit axe sensiblement horizontal,

caractérisé en ce que ledit premier actionneur est composé d'au moins un vérin hydraulique qui comprend un corps, un piston associé à une tige et une chambre côté tige, et qui est alimenté par un circuit hydraulique agencé de telle sorte que, au moins lorsque le deuxième actionneur est activé, la pression d'huile dans la chambre côté tige est maintenue à une valeur sensiblement constante permettant audit vérin de supporter une charge correspondant au poids de ladite unité de collecte, diminué d'un effort de valeur prédéterminée, de préférence inférieure à 1000 daN, typiquement comprise entre 200 et 600 daN. Pour ce faire, la portion de circuit alimentant ladite chambre côté tige est avantageusement munie d'un régulateur de pression, appelé également compensateur de pression.The aim that the applicant has set was to achieve, without risk of damage to the bottom of tank, effective cleaning of the anode hole during the replacement of the anodes, while using a simple tool, inexpensive, easy to clean, maintain and maintain.
A first object according to the invention is a collection unit intended to collect solid debris and sludge present in the liquid media of an aluminum production cell, such as the electrolysis bath and the liquid metal, in particular for cleaning anode holes, comprising:

1. a) a vertical mast actuated by a first actuator, also called lifting actuator, for moving said vertical mast in the vertical direction;
2. b) a frame fixed on said vertical mast;
3. c) at least one articulated bucket, pivoting about a substantially horizontal axis, mounted on said frame, having a substantially horizontal leading edge and actuated by a second actuator, also called closing actuator, secured to said frame, imposing said bucket a rotational movement about said substantially horizontal axis,

characterized in that said first actuator is composed of at least one hydraulic ram which comprises a body, a piston associated with a rod and a rod-side chamber, and which is fed by a hydraulic circuit arranged such that, at least when the second actuator is activated, the oil pressure in the rod-side chamber is maintained at a substantially constant value enabling said cylinder to support a load corresponding to the weight of said collection unit, reduced by a predetermined value of effort, preferably less than 1000 daN, typically between 200 and 600 daN. To do this, the circuit portion supplying said rod-side chamber is advantageously provided with a pressure regulator, also called a pressure compensator.

Said first actuator comprises at least one hydraulic jack which makes it possible to vertically move said vertical mast to which the remainder of the collection unit is attached. Preferably, the manipulator arm of the collection unit is telescopic, so that the rod of said hydraulic cylinder is integral with said vertical mast and the body of said hydraulic cylinder also moves vertically, actuated by an actuator integral with a movable frame which moves in a horizontal plane, for example which is fixed on a carriage able to follow the beam of a traveling crane. In this way, said collection unit can be moved rapidly and positioned above the working area and then down at the anode hole to perform the collection operation.

Of course, a "symmetrical" solution, consisting of making the mobile vertical mast integral with the body of the jack and the rod integral with the movable frame, is also possible. According to the invention, it acts on the chamber that is active during the rise of the load. As in both cases the rise of the collection unit is done by imposing a pressure in the chamber side rod, room that we will call later also "chamber-rod" is on the hydraulic circuit feeding said chamber side rod that is introduced a pressure regulator.

According to the invention, it imposes on the lifting actuator a mode of operation, called compensation, which consists in maintaining the pressure in the chamber on the rod side to a value that allows to hold, at least during the collection of debris, almost the entire weight of said lifting unit. This is in a state close to the hydraulic suspension, with an "apparent weight" limited to said predetermined value, preferably less than 1000 daN, typically between 200 and 600 daN. As the risks of shocks are substantially related to the substantially vertical movements of the collection unit when it is close to the bottom of the tank, the significant reduction in its apparent weight makes it possible to limit the intensity of the force generated by contacting the leading edge of the bucket against the bottom of the tank when the bucket pivots to collect debris.

The first actuator may include a plurality of hydraulic cylinders. It is understood that, in this case, all the rod-side chambers can be fed with the same circuit and that they must be subjected, at least when the second actuator is activated, to a substantially constant pressure allowing the assembly said jacks support a load corresponding to the weight of said collection unit, reduced by a predetermined value of effort, preferably less than 1000 daN, typically between 200 and 600 daN.

Such a device makes it possible in particular to collect debris by scraping the bottom of the tank, that is to say leaving the leading edge of the bucket to move on the bottom of the tank while remaining continuously in contact with the latter: the collection unit, in quasi-hydraulic suspension, can almost freely move up or down depending on the opening of the bucket. The support force, much lower than the unit's own weight collection, must however have a certain positive value, preferably limited to 1000 daN, typically between 200 and 600 daN, to prevent said collection unit is too easy to go back, in case of difficulty to lift some debris.

Advantageously, to ensure with the same device all of the descent and rise functions, said first actuator also comprises a piston-side chamber that can be powered by the hydraulic circuit: it is a double-acting cylinder.

Of course, the collection of debris is carried out more easily than the collection unit comprises a frame and two buckets mounted on said frame, arranged symmetrically with respect to a substantially vertical plane and articulated, pivoting about two substantially horizontal axes. , possibly confused, each bucket having a leading edge vis-à-vis the leading edge of the other bucket. Preferably, the second actuator, integral with said frame, imposes on each of said buckets a substantially symmetrical rotational movement relative to said substantially vertical plane, so that the solid debris between the two buckets are trapped by said buckets. With such a device, the debris can be collected by scraping the bottom of the tank without damaging it.

According to the invention, the hydraulic circuit supplying the hydraulic cylinder acting as a first actuator comprises, at least at the moment when the second actuator is actuated, a pressure regulator, also called a pressure compensator. The pressure regulator is a device designed in such a way that it lets oil flow from the circuit to the reservoir when the pressure in the circuit reaches a certain critical upper value, and that it provides a supply of oil from of the hydraulic power plant when the pressure in the circuit reaches a certain critical lower value.

1. a) un schéma différentiel, où la chambre côté tige et la chambre côté piston sont connectées à la la ligne pression de la centrale hydraulique, permettant d'assurer la descente à grande vitesse du mât;
2. b) un schéma correspondant au repos, l'unité de collecte restant suspendue: le circuit est aménagé de telle sorte que ladite unité de collecte reste soumise à des efforts limités en cas de rencontre d'obstacle lors de ses déplacements verticaux vers le haut ou vers le bas;
3. c) un schéma, où la chambre côté tige est connectée à la ligne pression de la centrale hydraulique, correspondant à la montée de l'unité de collecte;
4. d) un schéma "en compensation", correspondant à la phase de raclage de l'unité de collecte sur le fond de cuve, où la pression dans la portion de circuit alimentant la chambre côté tige est régulée pour être maintenue aux environs d'une valeur correspondant au poids de ladite unité de collecte, diminué d'un effort de valeur prédéterminée, de préférence inférieure à 1000 daN, typiquement comprise entre 200 daN et 600 daN.

Advantageously, the first actuator which makes it possible to move the vertical mast is a double-acting cylinder whose rod is integral with said collection unit, with a lower-side chamber, called lower, able to impose at any moment on said vertical mast a vertical movement. upwards and a piston-side chamber, said upper, able to impose at any time said vertical mast vertical movement downward. Advantageously, the two chambers can be connected, via a distributor, to the "pressure line" or the "return line" of a hydraulic power unit, the supply circuit comprising several circuit portions that allow the following hydraulic supply schemes to be realized:

1. a) a differential diagram, in which the rod-side chamber and the piston-side chamber are connected to the pressure line of the hydraulic power unit, making it possible to ensure the high-speed descent of the mast;
2. b) a diagram corresponding to the rest, the collection unit remaining suspended: the circuit is arranged such that said collection unit remains subject to limited efforts in case of obstacle encounter during its upward vertical movements or down;
3. c) a diagram, where the rod side chamber is connected to the pressure line of the hydraulic unit, corresponding to the rise of the collection unit;
4. d) a "compensation" pattern, corresponding to the scraping phase of the collection unit on the bottom of the tank, where the pressure in the circuit portion feeding the rod-side chamber is regulated to be maintained around a value corresponding to the weight of said collection unit, reduced by a predetermined value of effort, preferably less than 1000 daN, typically between 200 daN and 600 daN.

In the example presented below, we describe in more detail these different operating phases of the collection unit.

Another object according to the invention is a service module intended to be used in an igneous electrolysis aluminum production plant comprising a trolley and handling and intervention devices, characterized in that it also comprises a collection unit according to the invention, as described above.

Another object according to the invention is a service unit of an igneous electrolysis aluminum production plant comprising a traveling crane and characterized in that it also comprises at least one service module according to the invention, such as previously described.

Another object according to the invention is the use of a service module according to the invention for the interventions on electrolytic cells intended for the production of aluminum by igneous electrolysis, in particular for the cleaning of the anode holes.

The figure 1 schematically illustrates a service machine in a typical electrolysis room for the production of aluminum, seen in section.

The figure 2 illustrates a particular embodiment of a collection unit, which is a crust scoop, mounted on a telescopic vertical guide mast.

The figure 3 illustrates in perspective, the linkage and bucket bucket of the embodiment of the figure 2 .

The Figures 4 to 7 illustrate, in four different configurations, the diagram of a hydraulic circuit feeding the first actuator (lifting jack) of a collection unit according to the invention. These configurations correspond to the following operating modes: figure 4 ), fast descent ( figure 5 ), compensation (phase corresponding to scraping) ( figure 6 ) and climb ( figure 7 ).

Electrolysis plants for aluminum production include a liquid aluminum production area that includes one or more electrolysis rooms. The electrolysis room (1) illustrated on the figure 1 comprises electrolysis cells (2) and a service machine (5). The electrolysis cells (2) are normally arranged in rows or rows, each row or line typically having more than one hundred cells. The cells (2) are arranged so as to clear a circulation aisle along the electrolysis room (1). The cells (2) comprise a series of anodes (3) provided with a metal rod (4) for fixing and electrically connecting the anodes to a metal anode frame (not shown).

The service unit (5) is used to carry out operations on the cells (2) such as the anode changes or the filling of the ground bath supply hoppers and aluminum fluoride (AlF3). It can also be used to handle various loads, such as tank elements, pockets of liquid metal that are used during casting ("ladles") or anodes. It can also be used to clean the anode hole after removing a worn anode and before installing a new anode.

The service unit (5) comprises a movable bridge (6) which can be translated over the electrolysis cells (2), and at least one service module (7) comprising a movable carriage (8), said "Tool holder", adapted to be moved on the movable bridge (6) and equipped with several handling and intervention devices (10), such as tools, among which may be the crust scoop (100 '). The tools are here mounted on vertical telescopic poles (9) attached to the movable carriage (8). As we have already seen, for example in the European patent application EP-A-0 440 488 a crust scoop may also be moved and operated from a vehicle other than a service machine. The invention applies to any collection unit, regardless of its mode of movement and placement above the work area.

The figures 2 and 3 illustrate a particular embodiment of a collection unit (100), which is a crust scoop (100 ') attached to the end of a telescopic arm, at the end of the movable arm here called "shovel barrel" ( 11). The shovel drum is a movable vertical mast (9 ") sliding in a vertical mast (9 '), which also moves vertically, actuated by an actuator (not shown) integral with the tool turret of the movable carriage (8) of a service module (7) The crustal scoop comprises a frame (110) provided with two buckets (120a and 120b) placed facing each other, substantially symmetrically with respect to a substantially vertical plane and articulated pivoting about two substantially horizontal axes (115a and 115b). Each bucket (120a, 120b) has a leading edge (128a, 128b) facing the leading edge (128b, 128a) of the another bucket (120b, 120a) The second actuator is here in the form of two cylinders (200, 201) integral with the frame (110), operating simultaneously, by imposing on each bucket a substantially symmetrical rotation movement relative to in a substantially vertical plane, so that solid debris located between the two buckets are trapped by said buckets.

EXEMPLARY EMBODIMENT (FIGURES 2 TO 7)

The Figures 4 to 7 illustrate, in four different configurations, the diagram of a hydraulic circuit supplying the first actuator (50) of a collection unit according to the invention, which moreover possesses the characteristics described above ( figures 2 and 3 ).

The first actuator, or lift cylinder, (50) is a double acting cylinder (51) with a body (55) and a piston (56) associated with a rod (52). The rod (52) is integral with the collection unit (not shown on the Figures 4 to 7 ). The double-acting cylinder (51) has a stem-like chamber (53), called the lower one, capable of imposing at any moment on the vertical mobile mast (9 ") a vertical upward movement and a piston-like chamber (54), said upper , capable of imposing a vertical downward movement on the vertical mobile mast at any moment, The hydraulic circuit comprises two portions (63) and (64) which feed the two chambers (53) and (54) of the double-acting cylinder (51). The circuit can be connected via a three-position distributor, which we will call later "direction distributor" (80), to the "pressure line" (P) and to the "return line" (R) d. The directional distributor (80) is naturally in the position (802) which corresponds to the rest and can be excited to be put into one of the other two possible positions: the position (803) where the rod ( 52) of the jack moves the collection unit down a differential speed and the position (801) where the stem of u cylinder raises said collection unit.

The first circuit portion (64) includes a main branch (640) having one end connected to the directional distributor (80) and the other end branching into two branches, the first branch (641) being connected to the chamber - piston (54) of the cylinder (51), the second leg (642) being connected to the return port (73) of the pressure regulator (70).

The second circuit portion (63) comprises a main branch (630), one end of which is connected to the direction distributor (80) and whose other end branches into two sub-branches, each of which is equipped with a distributor. two positions (81, 82), the first sub-branch (631, 631 ', 631 ") being associated with a holding valve (90), the second sub-branch (632, 632', 632") being associated with the pressure regulating device (70). The two sub-branches meet at their other ends to constitute the end portion (633), which feeds the shaft side chamber (53) of the cylinder (51). The nonreturned check valve (90) serves two functions: a load holding function (the collection unit) and a safety function, by limiting the pressure inside the the chamber-stem in case of shock. These two functions could be fulfilled by other separate devices, for example a clapper dispenser, used as a load-retaining means, associated with a pressure limiter providing safety.

The figure 4 illustrates the circuit when the lift cylinder is at rest. The direction distributor (80) is naturally in the position (802), which puts the two circuit portions (63) and (64) in connection with each other through their respective main branches (630) and (640). The distributor (82) is in the position (821) which blocks the circulation in the second sub-branch and makes the pressure regulator (70) inoperative. Isolated by the distributor (82) in position (821) and by the non-conducting valve (90) (the piloting pressures of the connections (92) and (93) are insufficient to make it flow), the The stem chamber (53) is maintained, apart from any shock, at a substantially constant pressure, associated with the weight of the collection unit. The circuit branch (633) is equipped with a safety device, integrated into the function of the check valve (90), to limit the pressure in the stem chamber in the event of an impact.

The figure 5 illustrates the circuit when the lift cylinder is in rapid descent. The direction distributor (80) is energized to occupy the position (803), which places the two circuit portions (63) and (64) in communication with the pressure line (P) of the hydraulic power unit, the two circuit portions (63) and (64) also communicating with each other via their respective main branches (630) and (640) at the direction distributor (80) when in this position (803). The dispenser (82) is in the position (821) which makes the pressure regulator (70) inoperative. The distributor (81) is in the position (812) and allows the operation of the check valve (90): as soon as the resultant of the forces due to the piloting pressures coming from the one side of the branch (92) (" external control ") and on the other hand the branch (93) (via the valve (91) -" internal control ") is greater than a certain value, the check valve (90) becomes" pass ".

The check valve (90) is set to a critical value, typically close to 180 bar, so that, as soon as its piloting has sufficient pressure, it becomes free-running and the oil can flow from the stem-chamber ( 53) to the plunger chamber (54), via the branches (630) and (640), which communicate with each other at the direction distributor (80), placed in position (803). In this way, the flow of oil from the hydraulic unit is increased by the flow of oil from the piston chamber. If the x is the ratio (section of the piston chamber (54)) / (section of the shank (52)), the flow from the hydraulic unit is multiplied by x, so that with such a differential arrangement, the piston rod can descend with a speed x times faster than with a conventional assembly.

The figure 6 illustrates the circuit when the lifting cylinder is in a so-called compensation mode, activated during the collection of debris by scraping the bottom of the tank. The direction distributor (80) is energized to occupy the position (801), which places the main branch (630) in connection with the pressure line (P) of the hydraulic power unit and the main branch (640) in connection with the reservoir from the hydraulic power plant, via the return line (R). The distributor (82) is in the position (822) and the distributor (81) is in the position (811), which makes the pressure regulator (70) operational: if the pressure inside the chamber -tige (53) is greater than a given value, typically chosen to around 58 bar, the oil flows from the rod-chamber (53) to the piston chamber (54). If, on the other hand, the pressure inside the rod-chamber (53) is lower than this given value, the oil coming from the hydraulic power unit supplies said rod-chamber.

If the collection unit, during scraping, is no longer in contact with the bottom of the tank, the pressure in the shaft-side chamber (53) increases because the jack has to bear a load corresponding to the weight of the tank unit. collection, greater than the predetermined load. The pilot of the pressure regulator compares the force provided by the setting spring (71) with the force due to the pressure prevailing in the branch (72) which is connected to the stem-side chamber (53) via the part ( 632 ') of the second sub-branch, the distributor (82) in position (822) and the end portion (633). If the tare force is lower, the controller communicates, via the output (73), the portion (632 ') of the second sub-branch with the branch (642) of the first circuit portion (64) , so that the regulator is rendered "passing" between the chamber-rod (53) and the tank of the hydraulic unit, via the return line (R). In this way, the pressure in the rod chamber decreases and the apparent weight of the collection unit re-increases.

Conversely, if the collection unit, during scraping, is pressing too much on the bottom of the tank, the pressure in the stem-side chamber (53) is lower than the predetermined load. The pilot of the pressure regulator, which compares the force provided by the setting spring (71) with the force due to the pressure prevailing in the branch (72) which communicates with the stem-side chamber (53), then sets communicating, via the output (74), the portion (632 ') of the second sub-branch with the other portion (632) of the second sub-branch, so that the regulator is "switched" between the rod (53) and the pressure line (P) of the hydraulic power unit. In this way, the pressure in the rod chamber (53) increases and the apparent weight of the collection unit decreases.

The figure 7 illustrates the circuit when the lift cylinder raises the rod (52). The direction distributor (80) is energized to occupy the position (801), which places the main branch (630) in connection with the pressure line (P) of the hydraulic power unit and the main branch (640) in communication with the reservoir from the hydraulic power plant, via the return line (R). The distributor (82) is in the position (821) and the distributor (81) is in the position (812), which makes the pressure regulator (70) inoperative. The pressurized oil passes through the main branch (630), passes through the distributor (81) in position (812) and joins the portions (631 ") and (633) via the non-return valve (91), to supply the stem chamber (53) As the piston rises, the oil in the piston chamber (54) is discharged to the return line (R) of the hydraulic unit via the main branch (640). ).

Claims (11)

1. A collection unit (100) designed to collect the solid debris and mud in the liquid media of a cell for producing aluminum, such as the electrolytic bath and the molten metal, in particular a crust shovel (100') designed for deaning anode holes, comprising:

   a) a vertical mast (9") actuated by a first actuator (50), used to move said vertical mast in the vertical direction;

   b) a frame (110) fixed onto said vertical mast;

   c) at least one articulated bucket (120a, 120b), swiveling around a substantially horizontal axis (115a, 115b), fitted to said frame, with a substantially horizontal leading edge (128a, 128b) actuated by a second actuator (200, 201), interdependent with said frame, causing said bucket to adopt a rotary movement around saièi substantially horizontal axis,

   characterized in that said first actuator (50) is composed of at least one hydraulic jack (51) which indudes a body (55), a piston (56) together with a rod (52) and a chamber on the rod sicle (53), and which is powered by a hydraulic system arranged so that, at least when the second actuator is activated, oil pressure in the chamber on the rod side (53) is maintained at a substantially constant value allowing said jack to support a load corresponding to the weight of said collection unit, less a Joad of predetermined value, preferably lower than 1000 daN, and typically ranging between 200 and 600 daN.
2. Collection unit (100, 100') according to daim 1 characterized in that the portion of circuit (63) which feeds said chamber on the rod sicle is provided with a pressure regulator (70) which makes it possible to maintain the pressure within said chamber on the rod sicle in the vicinity of said substantially constant value.
3. Collection unit (100, 100') according to daim 1 or 2 characterized in that said rad (52) of said hydraulic actuating cylinder (51) is interdependent with said vertical mast (9").
4. Collection unit (100, 100') according to any of daims 1 to 3 characterized in that said first actuator is a double-acting jack.
5. Coll_ection unit (100, 100') according to any of daims 1 to 4, characterized in that it includes a frame (110) and two buckets (120a, 120b) fitted said frame, laid out symmetrically in relation to a substantially vertical plane and articulated, swiveling around two substantially horizontal axes (115a, 115b), each bucket having a leading edge (128a, 128b) opposite the leading edge of the other bucket.
6. Collection unit (100, 100') according to daim 5, in which the second actuator, interdependent with said frame, causes each of said buckets to make a substantially symmetrical rotation movement in relation to the substantially vertical plane, so that the solid debris located between the two buckets is trapped by said buckets.
7. Collection unit (100, 100') according to any of daims 1 to 6, in which said first actuator is a double-acting jack whose two chambers may be connccted, via a distributor (80), to the pressure line (P) or the return line (R) of a hydraulic unit, the feeding circuit including several portions of circuits which make it possible to provide the following hydraulic supply configurations:

   a) a differential configuration, in which both chambers are connected to the hydraulic unit, allowing the mast to descend at high speed;

   b) a configuration corresponding to the rest position;

   c) a configuration in which the chamber on the rod sicle is connected to the hydraulic unit, corresponding to the upward movement of the collection unit;

   d) a "compensation" configuration, corresponding to the phase when the collection unit scrapes the bottom f the pot, in which the pressure in the portion of circuit feeding the chamber on the rod sicle is controlled so as to be maintained around a value corresponding to the weight of said collection unit, less a Joad of predetênnined value, preferably lower than 1000 daN, and typically ranging between 200 daN and 600 daN.
8. Collection unit (100, 100') according to daim 7, in which said hydraulic system includes two portions (63) and (64) which feed the chamber on the rod side (53) and the chamber on the piston sicle (54) of the double-acting jack,

   a) the first portion of circuit (64) indudes a main branch (640) one end of which is connected to said distributor (80) and whose other end splits into two branches, the first branch (641) being oMected to the piston-chamber (54) of jack (51), the second branch (642) being used to feed the pressure regulator (70);

   b) the second portion of circuit (63) including a main branch (630) one end of whichis connected to the direction distributor (80) and whose other end splits into two sub-branches, cach of which is provided with a two-position distributor (81, 82), the fust sub-branch (631, 631', 631") being associated with a load-retaining means, typically a poppet valve or a shut-off valve (90), the second sub-branch (632, 632', 632") being associated with a pressure regulating device (70), the two sub-branches meeting at their other ends to make up the end portion (633), which feeds the rod -chamber (53) of said jack (51).
9. Service module (J) designed to be used in a plant for the production of aluminum by igneous electrolysis and comprising a carriage (8) and handling and servicing devices (10), characterized in that it also includes a collection unit (100) according to any of daims 1 to 8.
10. Pot Tending Assembly (5) for a plant producing aluminum by igneous electrolysis including an overhead travelling crane (6) characterized in that it includes also at least one service module (7) according to daim 9.
11. Use of a service module (7) according to daim 9 for servicing work on electrolytic cells (2) designed for the production of aluminum by igneous electrolysis, in particular for cleaning the anode holes.

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