WO2002042502A2

WO2002042502A2 - Method and equipment for handling dross and recovering the non-ferrous metals contained therein

Info

Publication number: WO2002042502A2
Application number: PCT/BR2001/000142
Authority: WO
Inventors: Antonio Carlos Da Cruz
Original assignee: Instituto De Pesquisas Tecnológicas Do Estado De São Paulo S.A. - Ipt
Priority date: 2000-11-21
Filing date: 2001-11-21
Publication date: 2002-05-30
Also published as: BR0013444A; WO2002042502A3; WO2002042502A8; AU2002223314A1; WO2002042502B1

Abstract

The present invention refers to a method and respective apparatus for the recovery of the free metal present in hot dross which is produced when non-ferrous metals are processed at temperatures above their melting point. The method consists of charging the hot dross collected from the liquid bath surface of holding or melting furnaces, directly into an equipment provided with thermal insulation and means for control of its internal atmosphere. This operation is followed by the transportation of such an equipment to a processing unit located not far from the site were the dross has been generated. The processing unit being provided with transfer devices and one or more sealed rotary furnaces provided with controlled atmosphere, which work in the accumulation and agitation of the dross, and the subsequent separation of the free metal contained therein. The equipment used in the recovery of non-ferrous metals present in dross, object of the present invention, consists of a recipient provided with thermally insulated walls, and a cover also thermally insulated that is well adjusted to the body of the recipient, and is provided with an inert gas injection port, a lifting and transfer device for removing the dross from the recipient into a feeding device well adjusted to the door of a sealed furnace that acts both as a dross accumulator and in the free metal separation from dross.

Description

"METHOD AND EQUIPMENT FOR HANDLING DROSS AND RECOVERING THE NON-FERROUS METALS CONTAINED THEREIN."

FIELD OF THE INVENTION

The present invention relates to a method and respective apparatus for the recovery of non-ferrous metals from hot dross, immediately after the dross has been removed from the furnace where it has been generated, without allowing the dross to cool.

BACKGROUND OF THE INVENTION

The world wide non-ferrous metal industry annually produces millions of tons of a large variety of materials. In several stages of the production process, these metals have to be kept as liquid baths, at temperatures above the melting point of the metal. If exposed to a reactive atmosphere, either during holding, re-melting or handling operations, a metal oxide skim forms on the surface of these baths, usually referred to as dross.

As a non-restrictive example, this situation occurs in the primary production and recycling operations of aluminium. Large volumes of molten aluminium are kept inside holding or re-melting furnaces, before the liquid metal can be transferred to casting units. In contact with oxygen usually present in the furnace atmosphere, a metal oxide skim continuously forms on the surface of the molten bath. To prevent oxide particles to entrain the bath, and subsequently the as cast product, the so formed dross must be periodically removed by skimming or similar practice. Besides the oxide, the dross generally contains metallic impurities, nitrides and, particularly, retains a considerable amount of free (non- reacted) aluminium. Because of economic reasons, it is desirable to recover as much as possible of the non-reacted metal that is contained in the dross.

When the dross is removed from the surface of the molten bath, its temperature may range from 700 to 1000 °C. In contact with atmospheric air, the free aluminium present in the dross continues to react with oxygen. Being strongly exothermic, this reaction results in the heating of the reaction region (which may reach temperatures of the order of 1600 °C), and a fast consumption of the free-metal.

Many different techniques have been proposed for recovering the free metal entrained in the dross. In a typical skimming operation, preceding all the methods described below, including the method presently disclosed, with the furnace door open, the dross and other impurities is manually or mechanically removed from the surface of the metal bath. This operation is carried out under as low turbulence as possible since too much turbulence causes accelerated consumption of metal through oxidation reaction. Still in the interior of the furnace, the dross is pulled near the furnace door and hold that way until a large enough amount of the free metal that is entrapped in it has been drained back to the molten metal bath. In the next step, the dross is charged into a collection recipient. According to the most simple practice, from this point on the dross is spread out over a floor. This action has the effect of cooling the dross, but do not totally avoid the loss of metal, consumed by reaction with oxygen from surrounding air, neither the evolution of fumes. In more elaborated methods, means are provided to cool the dross as quickly as possible. A more sophisticated technique makes use of a mechanical cooler, in general formed by a rotating perforated drum, slightly inclined, in which interior the dross is fed through one extreme and transported up to the other. Even though such coolers have reached a relative success mainly because of their simplicity of use and efficiency concerning the dross cooling operation itself, they are disadvantageous mainly when used in large capacity plants. This type of equipment requires a considerable investment and have a high maintenance cost. Also, this equipment is problematic when processing dross containing large amounts of liquid metal. In this case, it becomes necessary to pre treat the dross, or even to use alternative dross cooling means. Another disadvantage is the fact that this method generates a large amount of fines, which are difficult to process.

The Patent US4842255 describes a system intended to cool dross that employs a metallic open-top pan to receive hot dross removed from the surface of liquid aluminium bath. This recipient being divided into internal open-top partitions, which have thick metallic walls that are intended to provide sufficiently high thermal capacity to act as heat sinks that absorb energy from the dross and, consequently, accelerate dross cooling. When the pan has been fully charged with dross, it is transported to a station where a hood shaped cover is well adjusted over the pan forming a closed cavity. An inert gas such as argon is injected into this cavity to displace oxygen and reduce the extent of oxidation of residual metal present in the dross. The cooled dross is then transported, usually by long distances, to the recovering units. In these units, the dross is heated up to a temperature above the melting temperature of the metal, in general using rotary furnaces heated by an external heat source. Once the free metal is melted, it is separated from the solid phase by coalescing initially dispersed portions through the charge agitation effect which is provided by the rotation movement of the furnace.

In the method described in the Patent US5401294, once the dross is removed from inside the holding or melting furnace, it is charged into an open-top recipient that, when full with dross, is transported directly to a furnace in which temperature homogenisation is carried out. Following the temperature adjustment operation, the dross is transferred from the furnace to a centrifuging equipment. By imposing a determined radial acceleration, the process acts on the difference of density that exists between the liquid and solid phases present in the dross, separating them and thus promoting the liquid metal extraction.

The Patent US5599379 refers to a method for the recovery of free aluminium from dross that comprises the collection of the hot dross from the interior of a furnace into open-top recipients, and the placement of these recipients inside of a large storage container. The storage container is thermally insulated to maintain the dross at or near the temperature it had when it was removed from inside the furnace in which it has been generated. An inert gas is then introduced into the storage container to purge the storage container of all gases other than the inert gas. The storage container is also provided with an electronic controller that directs the movement of its cover and monitors its internal pressure and temperature. The controller also modulates the flow of the inert gas that is introduced in the storage container to prevent reaction of the free aluminium with oxygen from the surrounding atmosphere. After a certain amount of dross has been accumulated, the method includes the transference of the dross for batch processing operation into a rotary furnace.

The above processes that involve the cooling of the dross have the disadvantage of not making use of the energy which is already present in the dross when it is removed from the liquid bath in which it has been generated. Because the recovery plant is usually located far way from the site where the dross has been generated, these methods also have the disadvantage of requiring long distance transportation of the dross (and of recovery products back to the aluminium plant, which is usually the case). Both energy loss and transportation substantially add to the final cost of the recovery operation. Another important point is that the methods which have been proposed so far, either involving cold or hot dross processing, do not address to the problem of free metal loss while the dross is still being collected and handled towards a cooling and/or storage device. Particularly in case of aluminium, because its reaction with oxygen is highly exothermic, hot spots may easily arise (thermiting) while the dross is still in the collecting process, with the collection recipient placed aside the furnace. Typically, the temperature of the thermiting dross rises to approximately 1500 °C, resulting in a large consumption rate of aluminium which could otherwise be recovered. Because of the enhanced contact area then provided, these losses become even more dramatic if the free aluminium is finelly dispersed in the dross.

The present invention refers to a method and respective apparatus for the recovery of the free metal present in hot dross which is produced when non-ferrous metals are processed at temperatures above their melting point. The method consists of charging the hot dross directly into an equipment provided with thermal insulation and means for control of its internal atmosphere through the introduction of an inert gas into the collection recipient. In other words, according to the presently disclosed method, the dross is kept under well controlled atmosphere and conditions that provide the minimal heat loss right after it has been removed from the bath surface of holding or melting furnaces, independently whether the collection recipient is partially or totally full. In the next step, being the collection recipient partially or totally full of dross, it is transported to a free- metal separation unit located as close as possible to the local where the dross is generated.

The equipment used for the recovery of non-ferrous metals present in hot dross, object of the present invention, consists of a recipient for the collection of hot dross provided with thermally insulated walls and bottom, and a also thermally isolated cover that is well adjusted to the body of the recipient, a device for lifting and transferring the hot dross from the interior of the collection recipient into a feeding device well adjusted to the door of a sealed furnace that acts both as a dross accumulator and in the separation of the free metal separation from the dross. SUMMARY OF THE INVENTION

The method and equipment here disclosed avoids unnecessary handling of dross, which includes dross cooling, dross storage, transportation of dross for long distances up to the dross recovery centre, dross upgrading operations such as crushing of large blocks which form during the cooling operation, consequent formation of fines which are difficult to process, and all the additional costs that occur along with such operations. With respect to other methods proposed for the recovery of free metal that involves processing the dross while it is hot, the use of the method and equipment presently disclosed allows the recovery of the free metal according to simple operations, minimising losses of both metal and energy during the operations of collection of the dross and its transportation to the separation equipment, and without using sophisticated and expensive equipment like centrifuges, or storage centres provided with complex control devices which have to be heated up in order to maintain the dross at a temperature close to the temperature it had when removed from the liquid bath, which may required the use of external heating sources or, in order to provide extra energy, may even burn part of the metal which can be recovered. Above all, with respect to the methods which were proposed so far, the present method has the advantage of keeping the dross under a atmosphere with low oxygen activity and thermally insulated immediately after the dross has been removed from the furnace where it has been generated, allowing the maximum optimisation possible with respect to both metal recovery and energy efficiencies. Also to be considered is the fact that the skimming or equivalent operation is typically irregular, may occur in cycles that take few hours up to a complete working shift, the quantities of dross removed in each skimming operation are in general small and, therefore may require long term waiting periods of the collection recipient aside a single generation furnace, all of which profit of the fact that the collection recipient employed in the present invention is tightly sealed and well thermally insulated.

BRIEF DESCRIPTION OF THE DRAWINGS

As shown in Figure 1, which is a schematic diagram of the process, the operation of removing the dross (5) from the surface of the liquid bath (2) in the interior of the holding or melting furnace (1) is carried out according to the current state of the art of the skimming or equivalent operation, as carried out in the industrial units that include liquid metal processing. The dross (5) is then introduced into a collection recipient

(3) and transported by a convenient transportation means (4) to a dross treatment centre located as close as possible of the furnace in which the dross is generated, which contains one or more sealed rotary furnaces (6) that work both in the accumulation of dross (5) and separation of the dross into liquid metal (8) and dry solid non-metallic residue (9). The transfer of the dross (5) from the interior of the collection recipient (3) to the interior of a rotary furnace (6) is done by means of a lifting and tilting device (7), which may be supported by an ancillary transfer device well adjusted to the rotary furnace (6) door. Once an adequate amount of dross has been accumulated inside the rotary furnace (6), the temperature of the interior of this furnace is risen to a level which is higher than the melting point of the metal contained in the dross, and the furnace charge is agitated by means of the rotation movement of the furnace in order to separate the liquid and solid phase. Control and adjustment of the temperature in the interior of the rotary furnace (6), at a level which is higher than the melting point of the metal, is carried out by means of an external energy source, preferably using plasma or electric arc in a salt free metal recovery process. Alternatively, adjustment and control of the temperature inside the rotary furnace (6) can be carried out by the use of a flame produced by the combustion of oil or gas with oxygen form air, air mixed with extra oxygen, or pure oxygen, in a specific burning device, or by the use of the energy released by the partial oxidation of the free mental contained in the dross, obtained from controlled introduction of oxygen or any other oxidising agent into the rotary furnace (6), or by the combination of any of the possible heating means described above. After the operation involving the separation of the free metal from the dross has been concluded, the liquid phase (molten metal) is then tapped preferably into a thermally insulated ladle (10) and sent back to the melting/holding furnace (1), or alternatively tapped into a crucible or any equivalent device. The return of the liquid metal then recovered to the melting/holding furnace (1) is convenient because of the considerable increment in energy savings which are allowed by this procedure.

The recipient (3) for the collection and transport of the dross (5), shown in the Figures 2A, 2B, 2C and 2D, respectively a lateral view with main cover closed, a lateral view with main cover open, a lateral cross section, and a top view, is a recipient provided with one or more compartments (the top view of Figure 2D shows a recipient with two compartments), with side walls (20) and bottom (33) thermally insulated, and preferably with its walls (20) internally inclined at an angle (α) greater than 90° with respect to the bottom (33) of the recipient, to facilitate the discharging operation of the dross (5). The recipient (3) for the collection and transportation is provided with guiding holes (21) for lifting and transportation by an appropriate vehicle or, alternatively, provided with wheels (32) for free movement over floor or guided movement over rails, and with supports (22) for lifting and unloading the dross (5) directly into the interior of the rotary furnace (6), or over any ancillary transfer equipment, using an appropriate lifting device (7). To minimise surrounding air circulation into its cavity, the collection recipient (3) is provided with a top cover (23) that is also thermally insulated, and which may be provided with handles (24) to manually open the cover, provided or not with a pivot (25) and counterweight (26) to facilitate its opening, an inert gas injection port (27), and, alternatively, a smaller cover (29) provided with a handle (30) for the introduction of dross, using or not an ancillary device (34), adjusted to the same (28). The top cover (23) is preferentially well adjusted to the recipient by the immersion of its borders into a powder seal (31) which may be composed by an oxide of the metal being processed, like aluminium oxide powder in case of recovering aluminium from dross, or a fine powder extracted from the non-metallic residues (9) of the metal recovery process, or any other powder which might be convenient for the process, as for example a powder which presents capacity for oxygen absorption.

Claims

WHAT IS CLAIMED IS DEFINED AS FOLLOWS:

1. A method and equipment for recovering non-ferrous metals from hot dross containing the same, characterised by the charging of this hot dross (5) into a collection equipment (3) which, being totally or partially charged, is transported up to a processing unit that contains one or more rotary furnaces (6) which accumulate the hot dross and work in the separation of the liquid metal which is tapped preferentially into a thermally insulated ladle (8), and is returned to the holding or melting furnace (1), or is tapped into a crucible or any equivalent device.

2. A method and equipment for recovering non-ferrous metals from hot dross, according to claim 1, characterised by the use of a rotary furnace (6) as the accumulator of the dross (5).

3. A method and equipment for recovering non-ferrous metals from hot dross, according to claim 1, in which the temperature of the rotary furnace (6) is kept at a level equal or higher than the melting point of the metal to be recovered; characterised by the adjustment of the temperature of the rotary furnace (6) made by means of an external heat source, or by the use of the energy that is released by the partial oxidation of the free metal contained in the dross.

4. A method and equipment for recovering non-ferrous metals from hot dross, according to claims 1 and 3, characterised by the partial oxidation of the free metal contained in the dross which is carried out by the controlled introduction in the rotary furnace (6) of oxygen or any other oxidising agent, preferentially mixed with argon or any other inert gas.

5. A method and equipment for recovering non-ferrous metals from hot dross, characterised by the equipment used for the collection of hot dross to be a recipient provided with thermally insulated walls (20), with guiding holes (21) for lifting and transportation by an appropriate vehicle or equivalent means, or, alternatively, provided with wheels for free movement over the floor or guided movement over rails, and with supports (22) for lifting and unloading the dross (5) directly into the interior of the rotary furnace (6), or over any ancillary transfer device; and provided with a thermally insulated top cover (23), with handles (24), which may include or not pivots (25) and counterweights (26), a gas injection port (27), -and alternatively include a second smaller cover (29) with handles (30).

6. A method and equipment for recovering non-ferrous metals from hot dross, according to claim 5, characterised by side walls (20) of the equipment being preferably internally inclined at an angle (α) greater than 90° with respect to the bottom (33) of the recipient.

7 A method and equipment for recovering non-ferrous metals from hot dross, according to claim 5, characterised by the borders of the top cover being well adjusted to the recipient by the immersion of its borders into a powder seal (31) which may be composed by an oxide of the metal being processed, like aluminium oxide powder in case of recovering aluminium from dross, or a fine powder extracted from the non-metallic residues (9) of the metal recovery process, or by any other powder which might be convenient for the process, as for example a powder which presents capacity for oxygen absorption.

8. A method and equipment for recovering non-ferrous metals from hot dross, according to claim 5, characterised by the guide holes (21) for lifting and transportation, wheels (32) for free displacement of over rails, handle for manual opening (24), and counter weight (26), which are optionals that can be used depending on the degree of mechanisation of the process.