Method and apparatus for removing aluminum residues from an extrusion die

Abstract

A method for removing aluminum residues from at least one extrusion die, comprising the steps of: (i) introducing an extrusion die containing aluminum residues into a reaction vessel of an autoclave; (ii) adding to the extrusion die an aqueous solution in the vessel, preheated to a temperature greater than or equal to 80°C, of sodium hydroxide (NaOH) at a concentration comprised between 12% by weight and 30% by weight; wherein the aqueous solution is in a quantity sufficient to allow complete immersion of the extrusion die in the solution; (iii) maintaining in the autoclave vessel a temperature comprised between 85°C and 165°C and a pressure comprised between 100 and 400 kPa, obtaining the conversion of the aluminum residues into sodium aluminate and hydrogen. The invention also relates to an apparatus (1) for removing aluminum residues from at least one extrusion die, comprising: at least one reaction vessel (2) for immersing at least one extrusion die (M) comprising aluminum residues in a bath containing an aqueous solution of NaOH to obtain the conversion of the aluminum residues to sodium aluminate and hydrogen, means (3) for feeding the aqueous solution of NaOH into the reaction vessel (2), and means (4) for heating the aqueous solution of NaOH, which are functionally associated with at least one of the feeding means (3) and the reaction vessel (2).

Description

[0001] METHOD AND APPARATUS FOR REMOVING ALUMINUM RESIDUES FROM AN EXTRUSION DIE

[0002] The present invention relates to a method and an apparatus for removing aluminum residues from extrusion dies.

[0003] Extrusion is an industrial production process of plastic deformation which allows to produce parts having a constant cross-section (for example, tubes, bars, profiles, sheets). This process basically consists in forcing, by compression, metallic material of the type of aluminum in the “pasty” state to pass through a shape (“die” or “drawplate”) that reproduces the external shape of the part that one wishes to obtain. The extrusion die is the component that, within the process, determines the design of the profile obtained and is obviously of utmost importance as regards the quality of the surface of the extruded profiles. To maintain the quality standards required by the market, the extrusion die must be replaced frequently. Whenever the die is replaced for quality reasons (to be “corrected” or repaired) or for a production change (different design), it must be treated to remove the aluminum residue that solidifies inside it. Indicatively, each extrusion line may change as many as two dies per production hour, necessitating an efficient process of removing residual aluminum.

[0004] Removal of residual aluminum from the extrusion die is performed by immersing the dies in a NaOH solution at high temperatures. However, this procedure has the following problems:

[0005] - high consumption of highly corrosive chemical products (NaOH): each extrusion line can consume even more than 2000 liters / day of NaOH solution at 30% concentration (equivalent to 398 g / 1);

[0006] - high production of spent corrosive solutions that must be disposed of;

[0007] - high energy consumption for heating the solutions. In fact, the temperature is essential to make the process fast enough and allow the die to be reused as soon as possible; - aluminum removal times vary widely: from a minimum of two hours for small dies with a “simple” design , also known as flat dies, to more than 24 hours for large dies of complex design, such as for example hollow dies;

[0008] - risk of fire / explosion due to the generation of gaseous hydrogen during the immersion step, since the reaction between aluminum and NaOH generates hydrogen:

[0009] 2A1°(S) + 2NaOH(soi)

[0010] - hazardous environment for the operators in charge of supervising the process.

[0011] Automated apparatuses are commercially available which partially solve the above-mentioned problems, reducing the risk of fire / explosion and making the environment dedicated to the die cleaning process safer by using extraction systems that ensure a sufficient “dilution” of the hydrogen in air, increasing however the energy consumption for maintaining temperature. Indicatively, hydrogen at atmospheric pressure is flammable at concentrations between 4% and 17% in air and explosive at concentrations between 17% and 56% in air.

[0012] However, the proposed apparatuses do not solve the problem of the high consumption of caustic soda and, more importantly, do not solve the problem of disposing the large amount of spent solutions produced, which entails substantial costs.

[0013] There are some more recent technologies that allow partial or complete recycling of the solution. Unfortunately, these technologies are not applicable in the majority of apparatuses because of the all too obvious slowing down of the die cleaning process: in fact, these processes require the use of more dilute caustic soda solutions and need a rather complicated sequence of chemical steps that must be supervised by experienced and qualified personnel in order to avoid further problems that would compromise the production chain. The aim of the present invention is to provide a safe method that allows to speed up the process of removing residual aluminum from extrusion dies, reducing the consumption of chemical products and the energy costs.

[0014] Within the scope of this aim, an object of the invention is to provide a method with reduced emissions of pollutants and in particular reduced emissions of climate- changing gases into the environment.

[0015] Another object of the invention is to provide an apparatus for the removal of aluminum residues from extrusion dies that allows to perform the method according to the invention efficiently and safely, optimizing the results that can be obtained.

[0016] Not least object of the invention is to provide an apparatus that is highly reliable, relatively easy to provide, and at competitive costs.

[0017] This aim, as well as these and other objects that will become better apparent hereinafter, are achieved by a method for removing aluminum residues from at least one extrusion die, comprising the steps of:

[0018] (i) introducing an extrusion die comprising aluminum residues into a reaction vessel of an autoclave;

[0019] (ii) adding to the extrusion die an aqueous solution in the vessel, preheated to a temperature greater than or equal to 80°C, of NaOH at a concentration comprised between 12% by weight and 30% by weight; wherein said aqueous solution is in a quantity sufficient to allow complete immersion of the extrusion die in said solution;

[0020] (iii) maintaining in the autoclave vessel a temperature comprised between 85°C and 165°C, preferably between 130°C and 150°C, more preferably 140°C, and a pressure comprised between 100 and 400 kPa, obtaining the conversion of said aluminum residues into sodium aluminate and hydrogen.

[0021] The aim and objects of the present invention are also achieved by an apparatus for removing aluminum residues from at least one extrusion die, comprising: at least one reaction vessel for immersing at least one extrusion die comprising aluminum residues in a bath containing an aqueous solution of NaOH to obtain the conversion of said aluminum residues to sodium aluminate and hydrogen, means for feeding said aqueous solution of NaOH into said reaction vessel, and means for heating said aqueous solution, which are functionally associated with said feeding means and / or said reaction vessel, characterized in that it comprises at least one autoclave comprising said at least one reaction vessel, means for sensing and controlling the pressure inside said autoclave being furthermore provided.

[0022] Further characteristics and advantages of the invention will become better apparent from the detailed description that follows and from the accompanying Figure 1, which is a schematic view of a possible embodiment of an apparatus according to the invention for removing aluminum residues from at least one extrusion die.

[0023] In a first aspect, the present invention relates to a method for removing aluminum residues from at least one extrusion die, comprising the steps of:

[0024] (i) introducing an extrusion die comprising aluminum residues into a reaction vessel of an autoclave; once the die to be processed is loaded into the autoclave by means of adapted baskets, the hermetic lid is closed, keeping adapted vent valves open;

[0025] (ii) adding to the extrusion die an aqueous solution in the vessel, preheated to a temperature greater than or equal to 80°C, of NaOH at a concentration comprised between 12% by weight and 30% by weight; wherein said aqueous solution is in a quantity sufficient to allow complete immersion of the extrusion die in said solution. It is noted that the solution is loaded into the autoclave after the insertion of the die to avoid the safety- related risks that would occur if the die were placed in a hot solution of NaOH. Preferably, the aqueous solution of NaOH is added in the vessel in an amount such as to maintain a residual free volume of 15% or less. This refinement in fact allows to further reduce the risk of explosion.

[0026] The reaction begins as soon as the solution comes into contact with the aluminum present in the die, therefore also during the step of loading the NaOH solution. The vent valves are then closed, thus activating an automatic pressure control;

[0027] (iii) maintaining in the autoclave vessel a temperature comprised between 85°C and 165°C, preferably between 130°C and 150°C, more preferably 140°C, and a pressure comprised between 100 and 400 kPa, obtaining the conversion of said aluminum residues to sodium aluminate and hydrogen.

[0028] In a preferred embodiment, the method according to the invention further comprises the steps of:

[0029] (iv) extracting the hydrogen produced in step (iii);

[0030] (v) optionally, filtering the hydrogen extracted in step (iv), removing residues of NaOH, sodium aluminate and water vapor;

[0031] (vi) conveying the hydrogen obtained in step (iv) or (v) to one or more hybrid burners together with other combustible gas.

[0032] In a preferred embodiment, the hydrogen produced is conveyed to a hybrid burner together with methane or LPG.

[0033] Maintenance of the reaction temperature is achieved by means of the balancing between the hydrogen and the second combustible gas fed toward the hybrid burner.

[0034] In a preferred embodiment, the method according to the invention further comprises the steps of:

[0035] (vii) continuously monitoring the pressure in the autoclave until a pressure comprised between 100 and 400 kPa, preferably comprised between 200 and 250 kPa, is reached. Reaching the stall point within said range indicates successful removal of the aluminum residues of the die.

[0036] (viii) cooling the autoclave to a temperature lower than or equal to 100°C.

[0037] After cooling, one or more washing cycles are preferably carried out by means of a washing liquid, preferably water or an aqueous solution of NaOH up to 4% by weight.

[0038] In a preferred embodiment, the method according to the invention thus comprises the further steps of:

[0039] (ix) removing the aqueous NaOH solution from the vessel;

[0040] (x) filling the vessel with a washing liquid selected from water and an aqueous solution of NaOH up to 4% by weight;

[0041] (xi) removing the washing liquid from the vessel;

[0042] (xii) optionally, repeating steps (ix) to (xi).

[0043] After completing one or more washing cycles, the autoclave is opened and the vessel containing the dies from which the aluminum residue has been removed is extracted. Optionally, the baskets can be subjected to further washing by means of high-pressure hot water (steam cleaner).

[0044] In a second aspect, the invention relates to an apparatus for removing aluminum from at least one extrusion die, generally designated by the reference numeral 1 in the accompanying figure.

[0045] The apparatus 1 allows to apply the method according to the invention described above to at least one die M contaminated with aluminum residues and, preferably, to a plurality of dies M collected in conventional open baskets or the like. The dies M may have different shapes and sizes depending on their intended use. Typically, the dies M are made of metallic material.

[0046] The apparatus 1 comprises at least one reaction vessel 2 for immersing one or more dies M contaminated by aluminum residues within a bath containing an aqueous NaOH solution to achieve conversion of the aluminum residues to sodium aluminate and hydrogen, means 3 for feeding the aqueous NaOH solution into the reaction vessel 2, and means 4 for heating the aqueous NaOH solution which are functionally associated with at least one between the feeding means 3 and the reaction vessel 2.

[0047] The apparatus 1 comprises at least one autoclave 5, i.e., a tank with a cover provided with a hermetic closure system so as to form an inner chamber isolated from the external environment, comprising the reaction vessel 2.

[0048] More specifically, the reaction vessel 2 is formed by the inner chamber of autoclave 5.

[0049] When the autoclave 5 is closed hermetically, the reaction vessel 2 is isolated from the external environment.

[0050] It is not excluded that the apparatus 1 might have a plurality of autoclaves 5 comprising respective reaction vessels 2.

[0051] Moreover, the apparatus 1 comprises means 6 for sensing and controlling the pressure inside the autoclave 5.

[0052] Said sensing and control means 6 preferably comprise at least one pressure sensor, which is adapted to measure the pressure inside the autoclave 5 and is functionally connected to an electronic management unit, not shown, for processing the signal sent by the sensor.

[0053] The electronic management unit may be of the type of a conventional programmed or programmable electronic board.

[0054] The sensing and control means 6 can be configured to perform a continuous or periodic pressure measurement at preset or settable time intervals.

[0055] Moreover, the electronic management unit can be programmed to detect, during the operation of the apparatus 1, any overpressures with respect to a preset or settable threshold value, or to signal the end of the treatment when a preset or settable stable pressure value (stall point) comprised between 100 and 400 kPa, preferably between 200 and 250kPa, is reached.

[0056] It should be noted that the amount of hydrogen produced inside the autoclave 5 substantially depends on the amount of aluminum residues present on the dies M, the extent of the surface exposed to the aqueous NaOH solution of said dies, and the reaction temperature.

[0057] Preferably, the autoclave 5 is also provided with means for sensing the inside temperature and / or means for sensing the level of liquid present inside the reaction vessel 2. Such sensing means are of the type of conventional sensors or transducers functionally connected in input to the electronic management unit.

[0058] Advantageously, the apparatus 1 comprises means 7 for recovering at least part of the hydrogen produced inside the autoclave 5 during the operation of the apparatus 1. Said recovery means 7 comprise at least one hydrogen extraction line 8 in fluid communication with the inside of the autoclave 5 and associated with a valve 9 for adjusting the hydrogen flow in output from said autoclave.

[0059] The adjustment valve 9 may be, for example, of the type of a conventional electrically or electromagnetically operated modulating valve, actuated to open / close by the electronic management unit.

[0060] Moreover, the recovery means 7 may provide for at least one device 10 for filtering the hydrogen in output from the autoclave 5 to remove water vapor and any residues of sodium aluminate.

[0061] In a preferred embodiment, the heating means 4 comprise at least one hybrid burner adapted to be fueled with hydrogen and with a second combustible gas, for example methane or LPG. The recovery means 7 are associated in input to the at least one hybrid burner.

[0062] Preferably, the heating means 4 comprise at least two hybrid burners I la and 11b, of which a first burner I la is functionally associated with the autoclave 5 and a second burner 11b is functionally associated with the feeding means 3.

[0063] Each burner 1 la or 11b may be paired with a conventional fan 31 for evacuation of the output gases toward a disposal stack, which is not represented. Usefully, the apparatus 1 is also provided with means 12 for evacuating the hydrogen from the autoclave 5, which are normally closed and are actuated to open in the event of overpressures inside the autoclave itself.

[0064] Said evacuation means 12 comprise at least one vent valve 13, the intake and discharge ports of which are in fluid communication, respectively, with the interior of the autoclave 5 and with a delivery line 14. Preferably, the delivery line 14 is connected to a hood or stack 15.

[0065] The vent valve 13 may be of the type of a conventional electrically or electromagnetically operated safety valve, actuated to be opened by the electronic management unit if the sensing and control means 6 measure a pressure inside the autoclave 5 that is above the threshold value.

[0066] Preferably, the evacuation means 12 comprise at least two vent valves 13 arranged in parallel and associated with respective delivery lines 14 toward the hood or stack 15.

[0067] Advantageously, the apparatus 1 may be provided with means 16 for cooling the aqueous NaOH solution present inside the reaction vessel 2 at the end of each treatment cycle.

[0068] Said cooling means 16 comprise, for example, a conventional heat exchanger 17 associated in input to a feed line 18 and in output to one or more discharge lines 19 of a cooling fluid, said lines being controlled by respective flow control valves 32. In a preferred embodiment, the heat exchanger 17 is of the type of a conventional coil and the cooling fluid is water.

[0069] One of the discharge lines 19 may be connected, for example, to a conventional high-pressure hot- water washing device 30 (so-called steam cleaner), in which the baskets for collecting the dies M may be further cleaned at the end of the treatment.

[0070] The feeding means 3 comprise at least one first tank 20 for storing the aqueous NaOH solution, which is functionally associated with means, not detailed in the figures, for sensing at least one of the following chemical / physical quantities: temperature of the aqueous NaOH solution, pressure inside the first tank, minimum and / or maximum filling level of the first tank, density or conductivity of the aqueous NaOH solution.

[0071] Said sensing means are constituted by conventional sensors or transducers and are preferably connected in input to the electronic management unit.

[0072] In particular, the measurement made by the means, if any, for sensing the density or conductivity of the aqueous NaOH solution allows to detect the reaching of a threshold value that indicates the need to replace at least partially the aqueous NaOH solution present in the first tank 20.

[0073] In a variation, the feeding means 3 may comprise two or more first tanks 20 connected in parallel to increase the aqueous NaOH solution reserves.

[0074] The second burner 11b is functionally associated with the first tank 20 to heat the aqueous NaOH solution inside it.

[0075] Moreover, the feeding means 3 comprise a first connection circuit 21 which is interposed between the autoclave 5 and the first tank 20, which comprises pumping means 22 and first valve means 23 which are normally closed and can be actuated to open for loading / discharging the aqueous NaOH solution into / from the reaction vessel 2.

[0076] The first valve means 23 are of the type of conventional electrically or electromechanically operated flow control valves, actuated to open / close by the electronic management unit.

[0077] Finally, the apparatus 1 may comprise means 24 for washing the reaction vessel 2 after discharging the aqueous NaOH solution at the end of the treatment, comprising at least one second tank 25 for storing a washing liquid in fluid communication with the reaction vessel 2 of the autoclave 5 that is functionally associated with means, not detailed, for sensing at least one of the following chemical / physical quantities: temperature of the washing liquid, pressure inside the second tank, minimum or maximum filling level of the second tank, conductivity of the washing liquid.

[0078] The washing liquid can be water or, in case of repeated washes, an aqueous solution of NaOH up to 4% by weight.

[0079] Said sensing means are constituted by conventional sensors or transducers and are preferably connected in input to the electronic management unit.

[0080] In particular, the measurement made by the means, if any, for sensing the conductivity of the washing liquid allows to detect the reaching of a threshold value that indicates the need to replace at least partially the washing liquid present in the second tank 25.

[0081] In a variation, the washing means 24 may comprise two or more second tanks 25 connected in parallel to increase the washing liquid reserves.

[0082] The washing means 24 may provide for a third hybrid burner, not shown, which is functionally associated with the second tank 25, connected in input to the recovery means 7, and possibly cooperating with a corresponding fan, for heating the washing liquid present in said second tank.

[0083] Moreover, the washing means 24 comprise a second connection circuit 26 interposed between the autoclave 5 and the second tank 25, which comprises second valve means 27 which are normally closed and can be actuated to open for the delivery / return of the washing liquid toward / from the reaction vessel 2.

[0084] The second valve means 27 are of the type of conventional electrically or electromechanically operated flow control valves, operated to close / open by the electronic management unit.

[0085] The washing liquid in output from the reaction vessel 2 can be sent back to the second tank 25, be used elsewhere, or discharged.

[0086] In the embodiment shown, the second connection circuit 26 is in fluid communication with the first connection circuit 21 and, by means of the opening / closing of the first and second valve means 23 and 27, uses the pumping means 22 to move the washing liquid in input and in output toward / from the reaction vessel 2.

[0087] It is not excluded that the second connection circuit 26 may be independent of the first connection circuit 21 and may have its own pumping means for circulating the washing liquid.

[0088] Usefully, a return branch 28 may be provided from the discharge line 19 of the cooling means 16 toward the second tank 25, also controlled by third valve means 29 of the type described above.

[0089] The electronic management unit is adapted to manage the flows of aqueous NaOH solution, cooling fluid, washing fluid and hydrogen in the various operating steps of the apparatus 1 by means of the actuation of the valves described above as a function of the measurements made by the various sensors or transducers in the apparatus itself.

[0090] The following operating steps are provided for each operating cycle of the apparatus 1 :

[0091] - insertion of the dies M to be processed to remove aluminum residues, possibly collected in baskets, into the reaction vessel 2 inside the autoclave 5;

[0092] - hermetic closure of the autoclave 5, keeping the vent valves 13 open;

[0093] - turning on the pumping means 22 for delivery of the preheated aqueous NaOH solution from the first tank 20 to the reaction vessel 2 until a set fill level is reached. The dies M are completely submerged in the aqueous NaOH solution bath;

[0094] - closing of the vent valves 13 and initiation of control of the pressure measured by the sensing and control means 6 for possible evacuation of excess hydrogen through the vent valves 13;

[0095] - ignition of the first burner 1 la to reach and maintain the set reaction temperature;

[0096] - opening of the adjustment valve 9 of the recover means 7 during the reaction to send hydrogen toward the hybrid burners I la and 11b. The amount of second combustible gas fed to the hybrid burners must be such as to compensate for any shortage of hydrogen with respect to the power required by the hybrid burners.

[0097] The treatment continues until a stable pressure comprised between 100 and 400 kPa, preferably between 200 and 250 kPa, is reached inside the autoclave 5, indicating the exhaustion of the aluminum residues present on the dies M to be cleaned and thus the end of the reaction.

[0098] At this point:

[0099] - the cooling means are activated to lower the temperature of the NaOH solution inside the reaction vessel 2 and allow its safe discharge and return toward the first tank 21 ;

[0100] - the washing liquid is sent from the second tank 25 into the reaction vessel 2 and then fed back into said second tank. This washing step can be repeated several times if necessary;

[0101] - the autoclave 5 is then opened under completely safe conditions to extract the processed dies M. The collection baskets can be further cleaned by using the washing device, if any.

[0102] The invention will now be described with reference to the following non-limiting examples.

[0103] Example 1: Use of the hydrogen generated by the die cleaning reaction as fuel

[0104] A pilot apparatus constituted by an autoclave with a 200-liter volume complete with means for temperature and pressure control was used.

[0105] Pieces of aluminum (cylinders and parallelepipeds) weighing a total of 3.6 kg were immersed in a 15% NaOH solution by weight present in the autoclave, starting the reaction, setting a working temperature of 120°C and a maximum pressure limit of 150 kPa. The produced gas was collected by means of an adapted circuit for feeding a hybrid burner, said feeding circuit being provided with a filter to remove residues of NaOH and steam. The hybrid burner used is provided with a dual feed line, where both lines are managed by independent valves that can balance the ratio of the gases or exclude either one.

[0106] The burner was initially started on methane, then operated for 30 minutes with a mixture of methane and hydrogen in different concentration ratios: from 90% methane and 10% hydrogen to 70% methane and 30% hydrogen produced by the aluminum residue removal reaction. After this initial time, the methane supply failed (due to a pressure surge) but, surprisingly, the burner continued to operate with only gas from the autoclave for more than two hours. To prove this, the temperature values of the radiant tube, of the flue gases along with the emission values were collected and recorded on film (by means of a Seitron model “Chemist 500” combustion analyzer), showing a substantial absence of CO2, CO, NO and NOx. The hydrogen burner operated normally for more than an hour, generating only a small amount of condensate (water) inside the radiant tube. Starting with 3.6 kg of aluminum, the reaction generated 400 g of hydrogen (equivalent to 4.9 m3) at ambient pressure. Hydrogen combustion generates only water as per the reaction:

[0107] 2H2+ O2A 2H2O

[0108] In the radiant tube, 900 ml of water free of NaOH residues were collected, thus confirming that the filtration system used, installed upstream of the burner, is effective.

[0109] Conclusions:

[0110] The experiment demonstrated that, surprisingly, it is possible to use the hydrogen produced by the reaction as fuel for the energy needs of the process, thus advantageously reducing CO2, CO, NO, and NOx emissions while offering savings in terms of consumption of combustible gas (methane or LPG). For each kg of aluminum dissolved in the autoclave, 111 g of hydrogen are generated which, by burning, produce 3160 kcal of energy.

[0111] Example 2: Influence of temperature on the degree of aluminum removal

[0112] A pilot apparatus constituted by an autoclave with a 200-liter volume complete with temperature control means, constituted by a heating system (8-kW resistance heating elements) and cooling system with steel coil, and pressure control means, was used.

[0113] A typical die cleaning solution, composed of 375 g / 1 of NaOH and 42 g / 1 of dissolved aluminum, was placed in the autoclave and heated to 100°C. The goal of the experiment was to validate the increase in the efficiency of the reaction at temperatures higher than the boiling point (at ambient pressure).

[0114] Example 2a (for comparison): validation of the degree of removal under ambient pressure conditions at 100°C

[0115] A sample consisting of an aluminum cylinder with a diameter of 64.9 mm, a height of 10L5 mm and an initial weight of 914.0 grams was used.

[0116] This sample was immersed in the autoclave containing the solution at 100°C. After 3 minutes of immersion, the cylinder was taken out, washed, and then weighed and measured, obtaining the following data: diameter = 64.7 mm, height 10E1 mm and final weight 900.7 grams. Removal per minute was thus calculated as follows: obtaining a removal value of 1.62 g / dm2per minute of immersion.

[0117] Example 2b (invention): evaluation of the degree of removal achievable using an autoclave at a temperature of 125°C

[0118] The method of the invention was used to collect data on removal at a pressure higher than standard pressure.

[0119] An aluminum sample identical to the one used in Example 2a (64.7 mm diameter x 101.3 mm length, with an initial weight of 900.7 g) was placed in equilibrium on a stand not immersed in the solution from which it could be dropped with a small vibration. This allowed to make the piece come into contact with the solution (reacting) only when a temperature of 125°C was reached.

[0120] The cylinder was left immersed for 50 minutes (a deliberately long time, to minimize the influence of cooling time). The solution was cooled to 105°C in 3 minutes. The cylinder was then removed, washed, weighed and measured, obtaining the following values: diameter = 51.3 mm, height = 85.2 mm and final weight = 478.1 grams.

[0121] The removal value per minute was then calculated as follows: obtaining a removal value of 3.42 g / dm2per minute of immersion.

[0122] In this case, the variation in exposed surface of the cylinder cannot be ignored, and therefore the average surface area between the initial and final surfaces was considered: initial surface area = 2.72 dm2, final surface area = 1.79 dm2, average surface area = 2.25 dm2.

[0123] Conclusion: reaction efficiency increased from 1.62 [g / dm2per minute] to 3.42 [g / dm2per minute], demonstrating that the method according to the invention results in a 2.1 -fold increase in process speed.

[0124] In practice it has been found that the method and apparatus according to the invention fully achieve the intended aim, since they advantageously allow to obtain an increase in reaction speed for an equal NaOH concentration and a reduction in energy consumption. Moreover, the method and the apparatus according to the invention allow to reduce pollutant emissions and to reduce the solutions to be disposed of, since due to the increased efficiency the solutions can be recycled several times.

[0125] The method and apparatus thus conceived are susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept; all the details may furthermore be replaced with other technically equivalent elements.

[0126] In practice, the materials used, as well as the dimensions, may be any according to the requirements and the state of the art.

[0127] The disclosures in Italian Patent Application No. 102023000017679 from which this application claims priority are incorporated herein by reference.

[0128] Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

CLAIMS1. A method for removing aluminum residues from at least one extrusion die, comprising the steps of:(i) introducing an extrusion die comprising aluminum residues into a reaction vessel of an autoclave;(ii) adding to the extrusion die an aqueous solution in the vessel, preheated to a temperature greater than or equal to 80 °C, of sodium hydroxide (NaOH) at a concentration comprised between 12% by weight and 30% by weight; wherein said aqueous solution of NaOH is in a quantity sufficient to allow complete immersion of the extrusion die in said solution;(iii) maintaining in the autoclave vessel a temperature comprised between 85°C and 165°C, preferably between 130°C and 150°C, more preferably 140°C, and a pressure comprised between 100 and 400 kPa, obtaining the conversion of said aluminum residues into sodium aluminate and hydrogen.

2. The method according to claim 1, further comprising the steps of:(iv) extracting the hydrogen produced in step (iii);(v) optionally, filtering the hydrogen extracted in step (iv), removing residues of sodium aluminate and water vapor;(vi) conveying the hydrogen obtained in step (iv) or (v) to one or more hybrid burners for hydrogen and other combustible gas.

3. The method according to claim 1 or 2, wherein the aqueous solution of NaOH is added in such a quantity as to maintain a free volume in the vessel smaller than or equal to 15% of the total volume.

4. The method according to any one of the preceding claims, further comprising the steps of:(vii) continuously monitoring the pressure in the autoclave until a constant pressure comprised between 100 and 400 kPa is reached;(viii) cooling the autoclave to a temperature lower than or equal to 100°C.

5. The method according to claim 4, further comprising the steps of:(ix) removing the aqueous NaOH solution from the vessel;(x) filling the vessel with a washing liquid selected from water and an aqueous solution of NaOH up to 4% by weight;(xi) removing the washing liquid from the vessel;(xii) optionally, repeating steps (ix) to (xi).

6. An apparatus (1) for removing aluminum residues from at least one extrusion die, comprising: at least one reaction vessel (2) for immersing at least one extrusion die (M) comprising aluminum residues in a bath containing an aqueous solution of NaOH to obtain the conversion of said aluminum residues to sodium aluminate and hydrogen, means (3) for feeding said aqueous solution of NaOH into said reaction vessel (2), and means (4) for heating said aqueous solution of NaOH, which are functionally associated with at least one of said feeding means (3) and said reaction vessel (2), characterized in that it comprises at least one autoclave (5) comprising said at least one reaction vessel (2), means (6) for sensing and controlling the pressure inside said autoclave being furthermore provided.

7. The apparatus (1) according to claim 6, characterized in that it comprises means (7) for recovering at least part of the hydrogen produced inside said at least one autoclave (5).

8. The apparatus (1) according to claim 7, characterized in that said recovery means (7) comprise at least one device (10) for filtering the hydrogen leaving said at least one autoclave (5) in order to remove residues of sodium aluminate and water vapor.

9. The apparatus (1) according to one or more of claims 6-8, characterized in that said heating means (4) comprise at least one hybrid burner (I la, 1 lb) adapted to be fueled with hydrogen and other combustiblegas, the recovery means (7) being associated in input to the at least one hybrid burner (I la, 11b).

10. The apparatus (1) according to one or more of claims 6-9, characterized in that it comprises cooling means (16) for cooling the aqueous solution of NaOH present inside said reaction vessel (2).

11. The apparatus (1) according to one or more of claims 6-10, characterized in that it comprises means (12) for evacuating hydrogen from the inside of said at least one autoclave (5), which are normally kept closed and are adapted to be actuated to open in the event of overpressures inside said autoclave.

12. The apparatus (1) according to one or more of claims 6-11, characterized in that said feeding means (3) comprise at least one first tank (20) for storing said aqueous NaOH solution in fluid communication with said reaction vessel (2) functionally associated with means for detecting at least one of the following chemical-physical quantities: temperature of the aqueous NaOH solution, pressure inside the first tank (20), minimum and / or maximum filling level of the first tank (20), density or conductivity of the aqueous NaOH solution.

13. The apparatus (1) according to claim 12, characterized in that said feeding means (3) comprise a first connection circuit (21) interposed between said first tank (20) and the reaction vessel (2) of said at least one autoclave (5), which comprises pumping means (22) and first valve means (23) which are normally closed and can be actuated to be opened for loading / unloading the aqueous NaOH solution into / from said reaction vessel.

14. The apparatus (1) according to one or more of claims 6-13, characterized in that it comprises means (24) for washing said reaction vessel (2) which comprise at least one second tank (25) for storing a washing liquid in fluid communication with the reaction vessel (2) of said at least one autoclave (5), the second tank (25) being functionally associatedwith means for detecting at least one of the following chemical / physical quantities: temperature of the washing liquid, internal pressure of the second tank (25), minimum or maximum filling level of the second tank (25), conductivity of the washing liquid.

15. The apparatus (1) according to claim 14, characterized in that said washing means (24) comprise a second connection circuit (26), which is interposed between said at least one autoclave (5) and said second tank (25), comprising second valve means (27), which are normally closed and can be actuated to be opened for the delivery / return of the washing liquid to / from the reaction vessel itself.

16. The apparatus (1) according to one or more of claims 6-15, characterized in that it comprises means for detecting at least one physical quantity between internal temperature and filling level of said reaction vessel (2), which are functionally associated with said at least one autoclave (5).

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