Process and Device for Nelting Light Metal~
The invention concerns a process for the preferably continuous melting of light metal parts which are small and/or have a small diameter, in particular scraps contaminated with organic substances, for example, metal cuttings resulting from a machining of workpieces or crushed beverage cans made of aluminum and aluminum alloys.
Furthermore, the invention comprises a device, in particular for carrying out the above process, essentially having a heatable metallurgical melting crucible and equipment for feeding the light metal parts below the melting surface by means of a conveyor pipe.
Structural parts, small elements, vessels, containers and similar parts are increasingly made of light metals, in particular aluminum and aluminum alloys. Light metals have the advantages of good machinability and workability, are resistant to corrosion and splinterproof, neutral with respect to taste and advantageously have a low specific weight.
During manufacture of the parts, scrap pieces are produced naturally or cutting waste is produced during turning, milling, boring and the like. Light metal scrap, which also includes empty beverage cans, preserving cans, containers as well as cut pieces among other things, represents an important and ,economic base material for liquid metal recovery because recycling of these materials is extremely energy-saving.
Furt]hermore, recovery of light metals from scrap is associated with ecological relief, advantages in waste management and largl_ly avoids waste disposal costs for the metal.
CA 02227126 1998-01-1~
When melting scrap, in particular with a high ratio of surface to weight, i.e. fine or small scrap pieces, however, high oxidation losses occur when exposed to air because light meta:Ls have a very hiqh affinity for oxygen. However, not only the metal yield is reduced by the oxidation, as a result of which it deteriorates, but it also adds to the occurrence of slag or waste scrapings which brings about an increase in disposal problems for this material.
To minimize oxidation, it is known (US Patent No. 4702768 and Submerge Melt Technique for Aluminum Scrap, Light Metals 1992.
San Diego, California, USA, 1-5 Mar. 1992, The Minerals, Metals & Materials Society, 420 Commonwealth Dr., Warrendale, Pennsylvania 15086, USA, 1992, 9206-72-273, 877-879), to press aluminum cuttings through tubular insertion means below the surface in a liquid melt and thus largely avoid cinder losses, whereby a high metal yield is attained when melting the light scrap.
Light metal scrap, whether cutting scrap with machining oils or tin scrap with partially decorative lacquer coatings or content residues, is in most cases contaminated with organic compounds. These substances adhering to the scrap form problematic and/or toxic volatile reaction products when being heateld to the melting temperature of the metal and can result in considerable environmental pollution. To avoid these environmental pollutions, it may be necessary, especially when melting scrap in the metallurgical plant, to collect and purify the waste gases produced which, due to the considerable amounts of gas, does not take place completely in most cases and/or is associated with a high expenditure.
For melting beverage cans, it has already been proposed and CA 02227126 1998-01-1~
proven, as is known from the literature (Aluminium Beverage Can Recycling in Sweden by Lage Knuttson and Gosta Sjoberg, presented at The Recycling of Metals, D~sseldorf, 13 - 15 May 1992~l, in a first step, to remove the lacquer in a separate, essentially closed furnace by annealing the charge and to melt the scrap treated in this way in a second step. The contaminants produced thereby in the waste gases are partially burned, then neutralized by chemical reactions, separated and disposed of.
By means of an additional step in scrap processing or purification prior to melting, together with an intensive treat;ment of the waste gases by the treatment and melt aggregates, the environmental pollutions can, in particular during utilization of contaminated scrap, be quite effectively counteracted, however, this is very time-consuming and requires great technical and financial expenditures because high waste gas volumes must be treated or purified.
It ic the object of the present invention to now create a process and a device of the aforementioned type which make it possible to melt light metal parts, in particular contaminated light metal scrap, in an econmic and environmentally safe manner.
In a generic process of this type, this object is solved there]by that the light metals are introduced into an overheated and flowing molten metal from a sealable or closed housing of a feed device via conveying means and melted while air o:r oxygen is prevented from entering, whereby the gases and vapours, produced during heating until the parts are melted, are discharged from the conveyor pipe and sealable housing, inserted into a waste gas purification unit or CA 02227126 1998-01-1~
separator and the components of the gases and vapours are at least: partially separated and/or conveyed to a high-temperature unit and burned.
The a,dvantages of the new process lie, in particular, therein that a high melting speed can be attained by the overheated molten metal flowing about the scrap parts and thus also correspondingly high feed or performance rates of scrap through the conveyor pipe can also be attained for technical reasc,ns associated with melting. When heating the compact scrap, pieces being moved to the molten metal in the conveyor pipe, the volatile components formed and reaction products of the contamination substances are, deespite the high performance rates, led in the opposite direction, introduced into the housing, then removed therefrom and conveyed to a separator. These gases and vapours thereby protect the components or the portions remaining after a condensation of the gases and vapours produced heating the light metal parts are burned, in particular flared.
To safely separate environmental poisons, it may also be advantageous if the atmospheric pressure in the conveyor pipe is set and regulated, in particular by means of a pressure governor in the tube connection to the waste gas combustion instaLlation, in such a way that no gas and vapours formed durinq heating of the light metal parts escape from the conveyor pipe at the melting end.
In an installation of the aforementioned type, the further object: of the invention is solved thereby that, to prevent environmental pollution or, when separating problematic and/or toxic substances produced during heating, the conveyor pipe coming into contact with the molten metal or projecting into -CA 02227126 1998-01-1~
it i'3 connected in a gastight manner with a sealable housing of a feed device for small light metal parts, said housing having at least one inlet sluice for a charging of the feed device and at least one connection for a waste gas discharge pipe, whereby the connected discharge pipe opens into at least one w~aste gas purifier or separator with a substance discharge and the purifier(s) or separator(s) has (have) a pipe connection with a waste gas combustion plant at the outlet end via a pressure governor or the connected discharge pipe is connected directly to a waste gas combustion plant.
The advantages of the facility designed in this way according to the invention can essentially be seen therein that, even for a high efficiency of the separation of the environmental poisons or of the emission which disadvantageously affects the environment during melting of light metal scrap, the overall dimensions and expenditures of the installation can be kept small. This also results in a high flexibility, low costs for a retrofitting of existing installations and, on the whole, especially advantageous aspects for an energy-saving utilization of light metal scrap with a further competency increase in waste management.
It is thereby essential to the invention that the connection of the conveyor pipe to the housing, the feed device or the inlet sluice for the light metal scrap and the separator are connected to one another so as to be gastight and/or exhibit a lower inside gas pressure in comparison with the atmospheric pressure. The equipment parts are thereby made of materials which withstand the respective temperature and/or corrosion demancils. A neutralization of the separated discharge substances and a burning or flaring of the residual gases are known from chemical and system engineering.
CA 02227126 1998-01-1~
An especially effective embodiment of the facility is attained when the separator for the components of the gases and vapours formed during heating of the light metal parts is a condenser.
In this case, the gas can be cooled at selective temperatures and pressure, if necessary, in a cascade-like manner, and result in desired fractions.
With respect to a safe and continuous supply of the light metal scrap, it can finally be advantageous if the device for feeding the light metal parts is made essentially in the form of a screw conveyor.
The invention shall be described in greater detail in the following with reference to a drawing showing only one embodiment.
Fig. 1 schematically illustrates a device for continuously melting light metal scrap.
Liqui~l light metal 11 is situated in a melting crucible 1 which has, at least in the area A of a feed for solid light metal scrap, a relative movement 12 to the latter. A finely reduced, perhaps compact scrap is introduced under the surface of the melt 11 through a conveyor pipe 3 or the like, whereby pipe 3 has, on the one hand, an insoluble insert piece, on the other hand, is connected with a sealable housing 2 so as to be gastight. The sealable housing 2 is connected with a sluice 21 or a sluice system which enable or enables scrap to be brought in, for example aluminum cutting scrap, without environmentally harmful volatile substances escaping from the housing 2. A known feed device is located in the housing 2, perhaps sealed, in which the light metal scrap is inserted and by means of which the latter is also discharged through a conveyor 3 or pipe, perhaps by means of a screw conveyor or CA 02227126 1998-01-1~
the like. The housing 2 is also connected with a separator 5 via a waste gas discharge pipe 4, said separator 5 comprising a substance discharge 6 for e.g. condensates or similar harmful substances. Following the path of the gas, a pressure governor 7, by means of which a gas pressure can be set in the marshalled system, is situated between separator 5 and a waste gas combustion plant 8. For example, the gas pressure in the sealed housing 2 can be set with aid of the pressure governor 7 and subsequently at the outlet of the conveyor pipe 3 in such a way that no volatile pollutants which result from a breakdown of the contaminations of the scrap, can escape into the environment. A pressure governor 7 can also be placed in the area of the discharge pipe 4, so that specific waste gas fractions can be separated by condensation in a coolable separator 5. Due to the low waste gas volumes which can be attained with the device of the invention or with the process of the invention during melting of contaminated fine scrap, it can also be possible to connect a discharge pipe 4 from the sealable housing 2 directly to a pressure governor 7 and to make these harmless for the environment directly by flaring, without separation or partial separation of pollutants.