NO334541B1 - Process and reactor for melting solid metal. - Google Patents

Abstract

The publication discloses a method of melting solid metal, such as scrap aluminum, wherein the solid metal is placed in a closed, refractory insulated container (12) with a bottom contact or electrode (26) and preferably packed at least on the bottom of the container (12). wherein an electrode (25) is lowered into contact with the preferably packed metal (11). Electrical current is connected to the bottom contact (26) and the electrode (25) so that an electrical current passes between the electrode (25) and the bottom contact (26) through the solid metal (11). As the electrode, a rotatable hollow cylinder (16) with a downwardly open end (18) is used, the electrode (25) itself being arranged inside the rotatable hollow cylinder (16) and the container (12) being further equipped with a cylinder (19). ), for example, of graphite lying around the rotatable electrode (25), that solid metal is at least disposed within the cylinder (19), that an arc (28) is established between the electrode (25) and the metal (11) contact with the bottom contact (26) and the electrode (25) being held stationary until the solid metal at least in the cylinder (19) is more or less melted, after which the electrode (25) is caused to rotate. The publication discloses a reactor (10) which uses an electrode contained in a hollow and rotatable rotor (15), the electrode (25) being disposed with a lower end inside the hollow rotatable rotor (15) and that the rotor (15) is arranged inside a stationary cylinder (19) which surrounds the rotor (15), the cylinder (19) being formed at its lower end with openings (20, 20 ') communicating with a space (13) in the container (12) which surrounds the cylinder (19).

Description

Technical field of the invention

The present invention relates to a method and reactor for melting solid metal, such as scrap aluminum, where the solid metal is placed in a closed, refractory insulated container with a bottom contact or electrode and is preferably packed together, at least at the bottom of the container, and where an electrode is lowered into contact with the preferably packed metal, after which electric current is connected to the bottom contact and the electrode, so that an electric current flows between the electrode and the bottom contact through the solid metal.

Background for the invention

As is known, recycling aluminum scrap requires only 5% of the energy required to produce primary metal. For both environmental and economic reasons, it is therefore desirable to recover as much scrap metal as possible in as efficient and energy-saving a way as possible. As the total amount of aluminum scrap increases, so does the need for efficient and energy-saving smelting reactors.

The metal to be melted can have thicknesses as low as 0.0006 mm, which means that the aluminum will easily oxidize if air is allowed into the melting process. A possible supply of oxygen will also have a negative effect on the melting of aluminium, as the degree of recovery of aluminum will be greatly reduced.

Another problem linked to the melting of scrap aluminium, and particularly aluminum used as packaging for beverages, consists in the fact that non-negligible volumes of liquid can be included in the waste aluminium. During heating of such cold metal in a melting reactor, steam can thus easily be created, which can cause an explosion inside the melting vessel and thereby easily make the process more unstable and dangerous. In this context, it should be stated that 1 mol of water (18 g) has a volume of 0.018 dm<3>and will, when it evaporates at 25 °C, increase its volume to 24.5 dm<3>at a pressure of 1 atm. After the scrap is heated to approx. 700 °C, the volume will be increased to 80 dm<3>. Water trapped in pores in the scrap can create very high pressures that cause the metal to burst. The water vapor can then cause an explosion.

It is previously known to use burner-fired furnaces for melting cold metal. These have a relatively low power concentration, kW/tonne melt, which conditions a disproportionately high energy consumption with correspondingly poor recycling economics. Furthermore, it is common for scrap to be compressed in packages that are filled in the oven. This means that heat from burners is transferred to the packages via the surface and heat is transported further into the packages by means of heat conduction in the compressed metal.

At the start, before melt is formed in the furnace, the thermal efficiency is low.

US 4,322,245 A describes a method for melting and circulating scrap aluminum in a container. The container consists of two chambers with openings in between, so that the molten metal can flow between the two chambers. One chamber is closed and has burners for heating, and the other chamber is open, with a supply of scrap aluminium, and with a rotor which helps to circulate the smelt. A half partition has been set up at the rotor to achieve the best circulation of the melt.

WO 2012/093843 A1 describes an apparatus for supplying heat to a metal melt by means of a rotor. The rotor accommodates an electrode for supplying electric current to form an arc against the surface of the molten metal. The rotor helps to move the heated melt away and create circulation. The container with the rotor is closed.

NO 329242 B1 describes a method for heating a fluid, in particular one that lacks electrical conductivity. A rotor body has an electrode arranged in the rotor chamber and an arc is formed between the electrode and the bottom of the rotor body, so that the fluid can be heated.

There is consequently a need to provide an improved process and an improved smelting reactor which gives a higher metal yield and which has a higher thermal efficiency, and in which disadvantages associated with burner-fired furnaces are eliminated or at least reduced.

Summary of the invention

The above-mentioned needs can be met with a method and a melting reactor according to the invention.

One purpose of the present invention is to provide a solution where the disadvantages associated with burner-fired ovens are overcome or at least reduced.

Another object of the invention is to provide a method and a reactor for melting scrap metal with a high thermal efficiency and preferably with an efficiency above 80%.

A further object of the present invention is to provide a method and a reactor in which the degree of recovery of aluminum is increased.

Yet another purpose of the invention is to provide a method and a reactor which enables the use of and which provides an efficient and energy-saving smelting process.

Yet a further object of the invention is to provide a method and a reactor which reduces the possibility of the formation of oxides in the melting process and/or which is suitable for melting aluminum cans and/or foils with a thickness down to 0.0006 mm.

Another object of the invention is to provide a method and a reactor which enable an efficiency increase in a safe way for the removal of volatile environmentally hazardous gases and/or particles from the melting process.

Another object of the invention is to provide a method and a melting reactor in which the exposure of the electrode to solid metal is eliminated or at least greatly reduced.

A further purpose of the invention is to provide a solution which reduces wear on the rotor and electrode and which protects the rotor and electrode against accidental contact with metal in solid form

The above-mentioned purpose is achieved with a method and a melting reactor as defined in the independent patent claims, while variants, different embodiments of the invention or equivalent solutions can be derived from the non-independent patent claims.

According to the invention, a method for melting solid metal, such as scrap aluminium, has been provided. The solid metal is placed in a closed, refractory, insulated container with a bottom contact or electrode and where the metal is preferably packed together, at least at the bottom of the container, and where an electrode is lowered into more or less contact with the preferably packed metal. Electrical voltage is applied to the contact/electrode system and electrical current flows through the system, so that an electrical current flows between the electrode and the bottom contact through the solid metal. According to the invention, the electrode itself is arranged inside the rotatable hollow cylinder. Furthermore, the container is equipped with a cylinder, for example of graphite, which lies around the rotatable electrode. According to one embodiment, the solid metal to be melted is placed at least inside the cylinder and the lower end of the rotor is preferably brought into physical contact with the solid metal arranged inside the cylinder. An arc is then established between the electrode and the metal in contact with the bottom contact. At least during significant parts of the heating period where the metal is in solid form, the electrode is held in a stationary position, and the rotation of the rotor with electrode is only initiated when the solid metal at least in the cylinder is more or less melted. Then the rotor with electrode can be made to rotate continuously.

According to the present invention, heat is transferred directly to the aluminum scrap, which is quickly converted into melt. Hot exhaust gas from the arc is led in around the lying scrap and provides effective preheating of the scrap.

According to one embodiment of the method, the melting can be carried out as a continuous melting process, where more solid metal can be added to the container through a feeding device as the originally introduced solid metal in the container is brought to melt. Alternatively, the melting process can be more or less discontinuous and carried out as a batch process.

The rotation speed and/or vertical position of the rotating cylinder and/or electrode can be regulated. The molten metal in the container can further be made to circulate inside the cylinder and possibly through openings in the cylinder's side walls to increase and spread heat energy to the metal outside the cylinder, whether this is in solid or liquid form. Thereby, a circulation system can be established which contributes to a more efficient distribution of heat and energy in the pouring container.

According to an advantageous variant, the height of the cold metal that is placed inside the cylinder prior to initiation of melting is lower than the corresponding metal height level of the metal waste placed on the outside of the cylinder. The purpose of such a level difference can be, among other things, to ensure that cold metal introduced into the container does not come into contact with the submersible rotor or electrode.

According to the invention, a device for melting cold metal, such as scrap aluminium, has also been provided. The device comprising a refractory insulated and gas tight container, where the container is equipped with an electrical bottom contact and an electrode projecting down into this. Electric current is connected to the bottom contact and the electrode to supply melting energy to the metal. The container is also equipped with devices to be able to remove exhaust gases that had to be formed during the melting process. According to the invention, the electrode is surrounded by a hollow and rotatable rotor, the electrode being placed with a lower end inside the hollow, rotatable rotor. The rotor with electrode is arranged inside a stationary cylinder which surrounds the rotor, the cylinder being designed at its lower end with openings which communicate with a space in the container which surrounds the cylinder.

According to one embodiment, the electrode and the bottom contact are configured such that an arc can be formed between one end of the electrode and the metal, which in turn is in electrical contact with the bottom contact.

Furthermore, the cylinder can advantageously be made of graphite and can be equipped with through holes in the cylinder wall, so that molten metal is allowed to circulate from a cavity inside the cylinder to the surrounding space and back again. The rotor, for its part, can advantageously be equipped with a smooth outer vertical surface, so that any solid particles or solid metal in a melt are not allowed to get stuck in the rotor and cause damage to this or the surrounding cylinder.

According to one embodiment, the container can further be equipped with a closable device for supplying cold metal to the container and a drain plug for draining metal in liquid form and/or that the container is equipped with an outlet channel for exhaust gases from the melting process which is preferably connected to a cleaning and recycling facilities.

According to the present invention, a solution has been provided in which the power concentration, kW/ton of melt, is significantly increased compared to burner-fired furnaces. Furthermore, the removal of volatile environmentally hazardous gases and particles from the melting of the scrap is made more efficient by the fact that the concentration in the exhaust gas due to the size of the furnace is high and much higher than for burner-fired furnaces. Per unit mass of scrap, the amount of volatile gases and particles is the same. As the gas chamber's melting level is low, the concentration (environmentally hazardous substances + particles//volume of exhaust gas) will be high and much higher than for burner-fired ovens.

The solution according to the present invention also provides the opportunity to increase total quantities! of recovered aluminum from the aluminum waste, at the same time that very thin aluminum waste, such as foils down to 0.0006 mm, can be recovered.

Furthermore, according to the invention, it is possible to achieve a thermal efficiency of over 80%, which makes the solution particularly suitable for melting aluminum cans and thin foils without oxidizing in the process.

As the heating process takes some time, the presence of water is also given the opportunity to gradually turn into steam, so that the melting process becomes more even and the possibilities for sudden furnace explosions are reduced, which also results in a more predictable process.

With a solution according to the invention, the power concentration, kW per tonne of melt, will be significantly higher than for burner-fired furnaces. Correspondingly, the removal of volatile environmentally hazardous gases and particles from the scrap metal will be made more efficient by the fact that it is possible to establish a high concentration of the exhaust gas inside the furnace. - much higher than corresponding concentrations in teak used for burner-fired ovens. Other benefits are:

<*>high and improved metal yield,

<*>high thermal efficiency, and

<*>good control of exhaust gases emitted from the scrap metal during the melting process. Some of these gases can be environmentally hazardous and can be treated in a closed system.

According to the invention, a cylinder is provided which surrounds the rotor and the electrode. As the cylinder is located around the rotor and electrode, the cylinder will act as a protective screen for the rotor and electrode against wear and tear caused by particles in the melt or larger or smaller solid particles or scrap items of solid material.

Brief description of the drawings

To better describe the invention, reference is made to the accompanying drawings which show a possible embodiment of the present invention, there

figure 1 schematically shows a side elevation in section through a smelting reactor according to the present invention, where solid metal is loaded into the reactor, right at the start of the smelting process;

Figure 2 schematically shows a side elevation, partly in section, of a rotor with a built-in electrode according to the present invention; and

figure 3 schematically shows an elevation which corresponds to that shown in figure 1, but where the melting process has progressed so far that all the metal has melted.

Detailed description of the invention

Figure 1 schematically shows a side elevation in section through a smelting reactor 10 according to the present invention, where solid metal 11 is loaded into the reactor 10. The figure shows a stage right at the start of the smelting process. Figure 1 further shows a refractory insulated container 12 which is equipped with a refractory insulated lid 14 which is removable to be able to fill scrap metal in a melting chamber 13 inside the container 12. The lid and the adjacent parts of the container 12 are configured with sealing and locking devices (not shown) which enables the sealing locking of the lid 14 to the container 12. The sealing and locking device (not shown) can be of any suitable type which gives the desired effect. These, together with the remaining parts and penetrations into the melting chamber 13, are of such a type and are configured in such a way that it is possible to create a negative pressure or vacuum inside the melting chamber 13.1 the bottom of the container 12, there is also arranged a bottom contact 26 which is connected to a power source (not shown).

The negative pressure or vacuum is created by means of a vacuum pump or a suction fan (not shown) connected (a hose connection) to the drain channel 22 which is connected to a purification plant (not shown). In the container 12, a rotor 15 is arranged which is driven by means of a motor 29 via a belt transmission 30 or the like to a pulley which is attached to a shaft on the rotor 15. The rotor 15 is hollow and at its lower end is equipped with an opening 18 which is in free communication with the metal 11 below and/or the surrounding melt 11'. The shaft is connected to the motor 29, which in turn is attached to a bracket. The bracket can be attached to the lid 14 of the container 11 or to a separate stand (not shown). The seal between the rotor shaft and the container is in the form of a suitable gasket. On the bracket there may be a bearing that guides the rotor shaft. An electrode 25 is placed centrally in the shaft. The upper end of the electrode 25 is connected to a power cable connection (not shown) a cable lug 27. If desired, a central hole can be drilled through the electrode 25 for the supply of gas. In that case, the hole can be connected to a spigot which is attached to the end of the electrode 25

The rotor 15 projects down through the lid 14 and into the melting space 13. Reference is made in this context to figure 2 which schematically shows a side elevation, partly in section of a rotor 15 with built-in electrode 25 according to the present invention. The rotor 15 is suspended and stored so that it is allowed to move up and down in an axial direction in relation to the bottom of the container 12. The rotor 15 is further stored so that it is allowed to rotate about a vertical axis of rotation in relation to the lid 14. To enable this, a sealing and rotation device (not shown) is arranged between the lid 14 and the rotor 15. As indicated in figure 2 the rotor comprises a hollow, cylindrical casing or casing 16 which at its lower end is equipped with a chamber 17 with a larger cross-sectional area than the remaining part of the casing or casing 16. At its lower end the chamber 17 is also equipped with an opening 18 towards the space outside the casing 16. The rotor 15 is of a type that is described in the applicant's own publication no. WO 2012/093943, the content according to this publication being hereby incorporated by reference as regards the operation and design of the rotating rotor with electrode and also other features of significance for the solution's operation, structure and function. Centrally and concentrically in the rotor 15, an electrode 25 is also arranged which can be configured to be able to be moved up and down in relation to the sheath 16, so that the lower end of the electrode can be moved up and down in the chamber 17.

In the melting chamber 13, there is also a cylinder 19, preferably made of graphite, which surrounds the rotor 15. The cylinder 19 rests on the bottom of the container 12 and is equipped at its lower end with a plurality of through holes 20' in the cylinder wall as well as openings or side holes 20 higher up on the wall of the cylinder 19, so that there is fluid communication between the interior of the cylinder 19 and the space in the container 12 outside the cylinder 19. The rotor 15 is preferably located centrally and coaxially inside the cylinder 19.

The smelting reactor 10 is further equipped with a pipe 21 which extends from the outside of the reactor 10, through the lid 14 and into the casting room 13. The pipe 21 is configured to be able to introduce further metal waste 11 in solid form as the smelting process needs to supply new, solid metal. The supply pipe 21 is for this purpose equipped with seals (not shown) which can be opened and kept tight without the vacuum or negative pressure inside the chamber 13 being affected to any particular extent.

Furthermore, the reactor is equipped with a drain channel 22 which is connected to a purification plant and extraction plant (not shown in the figure) for cleaning the gas from the process and for possible recovery of other metals or substances that evaporate during the process. The drainage channel 22 is also equipped with seals which enable temporary or continuous removal of the exhaust gases and at the same time maintain a vacuum or negative pressure inside the container.

At its lower end, the container 12 is equipped with a tapping channel and tapping plug 23 which is of a known type which can be opened and plugged again, and which is in contact with a tapping vessel 24 for receiving molten metal.

Figure 3 schematically shows an elevation corresponding to that shown in Figure 1, but where the melting process has progressed so far that all the metal 11' has melted and is in liquid form. As indicated above, the rotor 15 and the electrode 25 rotate in this phase, which helps to cause the molten metal 11' to circulate, first inside the cylinder 19, and then out through the openings 20 in the cylinder wall 19 and then back in through lower openings 20' in the cylinder wall 19. A possible circulation pattern is indicated by arrows in Figure 3. In this way, the temperature in the molten metal 11' is equalised.

In the following, the operation of the reactor will be described in more detail with reference to the drawings. The demountable lid 14, including rotor 15 with electrode 25 and supply pipe 21 and exhaust pipe 22, is removed from the top of container 12. Metal in solid form, for example in the form of compressed cans, shavings, thin foils and/or other metal waste 11 is placed on the bottom of the container 12 and is preferably pressed together. Such metal waste is placed and pressed together preferably up to one height outside the central cylinder 19, and to another height inside the cylinder 19, where the height inside the cylinder 19 is preferably lower than the height of scrap metal outside the cylinder 15. The solid metal scrap inside the cylinder is pressed together so much that an electrical contact is established with the bottom contact 26 in the container 12. The lid 14 with rotor 15 and electrode 25 is then placed in place on the top of the container 12 and locked tightly to it. Vacuum or negative pressure is established inside the closed, tight container 12. The rotor 15 with electrode 25 is then lowered to a desired level above the metal scrap 11 inside the cylinder 19.

Electric current is connected to the bottom contact 26 and the electrode 25 via the cable shoe 27, or by means of a tow contact towards 16 (not shown), while the rotor and electrode are kept stationary. The melting process starts with the electrode 25 forming an arc 28 inside the extended space 17 inside the rotor 15, against the metal scratch in 11 in the bottom of the cylinder 19, since this metal 11 is in electrical contact with the bottom contact

26. Eventually the metal 11 under the rotor 15 inside the cylinder and some of the metal outside the cylinder 19 will melt. The rotor 15 is kept still in order to thereby ensure that it cannot come into contact with solid metal parts 11 and thereby be damaged. The rotor 15 and the electrode will only be set in rotation when sufficient liquid melt 11' has been formed. After a time when all the metal has melted, as shown in Figure 3, the molten metal 11' will circulate through the side holes 20 and, as more energy is added, provide good temperature equalization in the molten metal 11'. Now scrap, possibly more metal 11 in solid form, can be added through the supply pipe 21, and the melt 11' can be transferred to the tapping vessel 24 by removing the tapping plug 23. The process can be both continuous and discontinuous. At start-up, the rotor is at rest and 25 is lowered until contact is made. When the molten mass surrounds the rotor, the rotation can begin. The height of the rotor is constant, but the electrode 25 is regulated so that maximum arc voltage is achieved. The rotation of the rotor 15 and the electrode 25 is provided in a known manner, for example as described in the applicant's own publication WO 2012/093943. The rotation of the rotor 15 causes the surface of the molten metal 11' inside the cylinder 19 to assume a shape like a hyperbola, which in turn contributes to an increased circulation effect of the molten material out of the openings 20 in the side wall of the cylinder 19 and back into the cylinder 19 through the openings 20' at the bottom for the supply of more heat energy from the arc 28. The cylinder 19 acts like a mammoth pump in that gas and melt flow upwards into the cylinder 19 and out through the side holes 20. When sufficient melt has been formed, the speed can be increased to the rotor so that the melt located in the cylinder forms a paraboloid of rotation which will increase the pumping speed through the side holes 20. This means that the turbulence in the melt 11 will increase, which increases the melting speed of the added scrap.

Claims (10)

1. Process for melting solid metal, such as scrap aluminum, where the solid metal is placed in a closed, refractory insulated container (12) with a bottom contact or electrode (26) and is preferably packed together at least at the bottom of the container (112) and where an electrode (25) is lowered into contact with the preferably packed metal (11), after which electric current is connected to the bottom contact (26) and the electrode (25), so that an electric current flows between the electrode (25) and the bottom contact (26) through the solid metal (11), characterized in that the electrode (25) itself is arranged inside a rotatable hollow cylinder (16) and that the container (12) is further equipped with a cylinder (19), for example of graphite, which lies around the rotatable electrode (25), that fixed metal is at least placed inside the cylinder (19), that an arc (28) is established between the electrode (25) and the metal (11) in contact with the bottom contact (26) and that the electrode (25) is kept stationary until the solid metal in the smallest in the cylinder (19) is more or less melted, after which the electrode (25) is made to rotate. 2. Method according to claim 1, where the melting can take place as a continuous process where more solid metal (11) can be added to the container through a feeding device (21) as the solid metal (11) in the container (12) is brought to melt. 3. Method according to claim 1 or claim 2, where the rotational speed and/or vertical position of the rotating cylinder (16) and/or the electrode (25) can be regulated. 4. Method according to one of claims 1 to 3, where the molten metal is made to circulate inside the cylinder (19) and through openings (20, 20') in the side walls of the cylinder (19) for increasing and dispersing heat energy to the metal (11 ) outside the cylinder (19). 5. Method according to one of claims 1 to 4, where the level of the amount of cold metal (11) introduced into the container (12) is such that the cold metal (11) does not come into contact with the submersible electrode (25). 6. Reactor (10) for melting cold metal (11), such as scrap aluminum, comprising a refractory insulated and gas-tight container (12) where the container (12) is equipped with an electrical bottom contact (26) and an electrode projecting down into this (25), where electric current can be connected to the bottom contact (26) and the electrode (25), and where the container (12) is equipped with devices (22) to be able to remove exhaust gases that may be formed during the melting process, characterized in that the electrode (25) is arranged with a lower end inside a hollow, rotatable rotor (15) and that the rotor (15) is arranged inside a stationary cylinder (19) which surrounds the rotor (15), the cylinder (19) at its lower end is designed with openings (20,20') which communicate with a space (13) in the container (12) which surrounds the cylinder (19). 7. Reactor (10) according to claim 6, where the electrode (25) and the bottom contact (26) are configured such that an arc (28) can be formed between one end of the electrode (25) and the metal (11), which in turn is in electrical contact with the bottom contact (26). 8. Reactor (10) according to claim 6 or 7, where the cylinder (19) is made of graphite and is equipped with through holes (20, 20') in the cylinder wall (19), so that molten metal 11') is allowed to circulate from a cavity inside the cylinder and to the surrounding space. 9. Reactor (10) according to one of claims 6 to 8, where the rotor (15) has a smooth outer vertical surface, so that any solid particles or solid metal in a melt (11') are not allowed to stick to the rotor (15 ) and damage this or the surrounding cylinder (19). 10. Reactor (10) according to one of claims 6 to 9, where the container (12) is equipped with a closable device (21) for supplying cold metal (11) to the container (12) and a drain plug (23) for draining metal in liquid form and/or that the container (12) is equipped with an outlet channel (22) for exhaust gases from the smelting process which is preferably connected to a cleaning and recycling facility.