NO314443B1 - Process for producing secondary aluminum - Google Patents

Abstract

The invention concerns a process for purifying waste furnace gases and for using the resultant filter residue. Alkaline aggregates, containing sodium or potassium as the cation, are added to the waste furnace gas. In a subsequent stage, dusts and droplets are filtered out of the waste furnace gas. The resultant filter residue is added to the cover salt (7) necessary when melting secondary aluminium.

Description

The invention relates to a method for the production of secondary aluminum where aluminum-containing waste is melted down in a furnace under a protective salt formed predominantly of NaCl and/or KCl, the furnace exhaust gases are supplied in an exhaust gas channel up to 5 g per Nm<3> alkaline aggregates containing sodium and/or potassium as cation, and the thus treated furnace exhaust gases are freed of dust and droplets by being passed through a cloth filter.

The cleaning of furnace exhaust gases in the method according to the invention belongs to the type of cleaning methods where aggregates are first added to the exhaust gas stream coming from the furnace and in the further process it is cleaned again from dust and droplets in a separator.

Typical furnace exhaust gases that can be cleaned are the exhaust gases from waste incineration plants, smelting furnaces and from furnaces in which scrap metal is melted down.

As a separator serves e.g. fabric filters, pipe filters, cyclones, electrostatic separators or wet separators.

The aggregates improve the separation of harmful substances such as e.g. metal oxides, acidic components, organic hydrocarbons, dioxins (PCDD) and furans {PCDF), respectively they make it difficult for certain harmful substances to form.

Aggregates work mostly through the adsorption of harmful substances, i.e. by depositing or binding harmful substances on a specific large surface. Many times the aggregates also cause a chemical transformation of harmful substances or a blocking of catalysts which leads to the formation of harmful substances.

The aggregates are separated again in the separator together with substances previously present in the exhaust gas.

Often, the furnace-heated exhaust gas is also cooled before entering the separator. For this, a liquid is injected into the exhaust duct, usually an aqueous basic solution. Aggregates can also be introduced with the liquid in loose or dispersed form. The liquid evaporates and thereby cools the exhaust gas very quickly below the critical temperature range in which PCDD/PCDF are formed. As the liquid is basic, acidic components in the exhaust gas stream are neutralized.

Known aggregates that are frequently added in combination with each other, either with water or as dust, also in connection with substances with which they usually occur together, are activated carbon, lime (CaC03), hydrated lime (Ca(OH)2), magnesium compounds (MgO,' MgC03 ...), sodium carbonate (Na2C03) and NH compounds.

With the known methods, it is problematic that the degree of separation of PCDD/PCDF is often not sufficient (e.g. due to the fact that copper, magnesium and calcium compounds can act as a catalyst in the formation of PCDD/PCDF). By adding a large proportion of activated carbon, it is true that PCDD/PCDF can be separated well, but the resulting filter residue cannot be simply recycled, but must be treated as a harmful substance at great expense or landfilled. Filters with a high proportion of activated carbon can also tend to self-ignite.

The task which forms the basis of the invention consists, in contrast to this, in providing a cleaning method for furnace exhaust gases with an advantageous utilization option for the resulting filter residue as raw material.

This task is solved by adding substances to the exhaust gas which contain sodium or potassium as cations and which are filtered out in the further process, and the resulting filter residue by melting down scrap aluminum is added to the protective salt used.

The present invention thus relates to a method for the production of secondary aluminum where aluminum-containing waste is melted down in a furnace under a protective salt formed predominantly of NaCl and/or KC1, the furnace exhaust gases are supplied in an exhaust gas channel up to 5 g per Nm<3> alkaline aggregates that contain sodium and/or potassium as cations, and the furnace exhaust gases treated in this way are freed from dust and droplets by being passed through a cloth filter, which is characterized by the fact that the filter residue collected at the cloth filter is fed to a later used filling with protective salt.

The alkaline additives can e.g. be sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, sodium sulphide (Nå2S), sodium sulphite (Na2S03) or analogous compounds on a potassium basis or mixtures of these components. They can be added in solid (dusty) form, or in the form of an aqueous solution or partly in solid form and partly in the form of an aqueous solution through nozzles into the exhaust duct.

Due to the alkaline nature of the additives introduced, at high temperatures organochlorine compounds such as e.g. PCDD/PCDF are broken down, respectively their formation is prevented.

The filter residue then consists of a high proportion of alkali halides, particularly of compounds of sodium or potassium with chlorine, to a lesser extent also with fluorine and of unreacted aggregates.

These are precisely the substances that are used as protective salt in the melting down of secondary aluminium, above all in rotary drum furnaces. The purpose of this protective salt is that, as a flux, it dissolves the oxide skin on the surface of the added aluminum pieces, inhibits the oxidation of the aluminum melt, and absorbs certain impurities. In a rotary drum furnace commonly used for melting down secondary aluminium, the weight ratio between added material and protective salt is approximately 4:1. The feed material consists of aluminum and impurities, and is composed of scrap aluminum, aluminum shavings and aluminum-containing furnace slag from aluminum casting furnaces.

It is therefore particularly obvious to advantageously use the method according to the invention when it comes to cleaning the furnace exhaust gas, which is formed by the melting down of secondary aluminum in rotary drum furnaces. The filter residue that results within the framework for refilling the oven can be re-added with the necessary protective salt at a later refill. The filter residue only needs to be transported in the immediate operating area. When the furnace gas from the rotary drum furnace in which secondary aluminum is melted using protective salt is purified in the context of the invention, approximately one-tenth to one-fifth of the weight of the protective salt results again as filter residue. This means that you save on the purchase of a tenth to a fifth of the necessary protective salt.

Under the combustion conditions in the rotary drum kiln, the organic compounds introduced with the filter residue are destroyed so that these do not have a negative effect. The other pollutants thus brought in such as e.g. heavy metal oxides are reduced by aluminum to metals, and when dissolved in aluminum then form normal, desired alloying elements.

The filter residue is thus given a meaningful use when used as a protective salt in the melting down of secondary aluminum and one does not have to bother with its disposal.

When the alkaline aggregates are at least partially injected into the exhaust gas duct with an aqueous solution, i.e. it is cooled with the solution, the exhaust gas is thus quickly cooled. Thereby, it can be quickly cooled below the temperature range in which PCDD/PCDF forms. When the water evaporates, the alkaline aggregates are formed as fine dust with a very high specific surface area (large surface area per mass). This specific surface area is larger than when the aggregates are already introduced in dust form into the exhaust duct. The concentration of the alkaline aggregates in the aqueous solution is between 1 and 50% of the saturation concentration. If the concentration is too high, there is a risk that the nozzles through which the solution is injected into the exhaust duct may become blocked.

The quantity of the alkaline aggregates in relation to the quantity of exhaust gas depends on the quantity and nature of the pollutants in the exhaust gas. In order for a good cleaning effect to be achieved and for the resulting filter residue to also have the desired composition, namely an excess of alkaline components, 0.01 to 5 g of alkaline aggregates per Nm<3>("normaIcubic meter") exhaust gas. For a longer period, in normal cases, an average of 0.5 to 0.6 g of aggregate is required per Nm<3> exhaust gas. Fig. 1 shows a process diagram of the method according to the invention whereby the exhaust gas from two drum furnaces is treated and the aggregates are injected both in aqueous solution and in dust form into the exhaust duct. Fig. 2 shows the method in fig. 1 whereby it further

emulsion is injected into the exhaust duct.

Fig. 3 shows the method in fig. l whereby it further

synthetic soot is injected into the exhaust duct.

In drum furnace 1, scrap aluminum (cans, packaging foils, tin waste...), furnace slag (floats in aluminum casting furnaces at the smelter) and aluminum shavings (contaminated with emulsions from metalworking machines) are melted down, in the presence of protective salt. From a container 2, by means of a pump 3, the alkaline aggregates in aqueous solution are injected through nozzles 4 into the exhaust duct leading from the drum furnace 1.

The temperature of the now-cooled exhaust gas is measured at a location in the exhaust gas channel just below it in the direction of flow. Depending on this temperature, the supply pump 3 is set to a larger or smaller supply capacity, so that the exhaust gas temperature at the measuring point is ideally approximately constant at 200°C to 220°C.

From a further container 11, the alkaline aggregates are blown into the exhaust duct in dust form.

In the further process, the exhaust gas enters the pipe filter 6 where the filter residue is separated. The resulting filter residue is mixed in a container 7 with protective salt and supplied together with this again to the drum furnace 1 in a further refill.

As the filter residue can be harmful to health due to its dust form, it should either only be transported and handled in systems that are isolated from the ambient air, or it should for the most part be pressed or briquetted.

In a typical application example corresponding to fig. l, two gas-heated drum furnaces are each filled with 12 tonnes of raw material, which is composed of approximately equal parts of furnace slag from aluminum casting furnaces, shavings and scrap aluminium, and the raw material is melted down. As protective salt, 3 tonnes of salt are supplied to each oven. The exhaust gases from the two furnaces are brought together in a line and cleaned

in a common pipe filter for dust and droplets. Before the tube filter, the exhaust gas temperature is regulated by suitably rapid injection of a 5% soda solution (water + Na2C03) from a total of three nozzles to 200 to 220°C. The need for soda solution averages 300 1 per hour. 2 kg/hour of sodium carbonate (Na2C03) in dust form is also supplied. After about four hours, the molten secondary aluminum can be drained off. The protective salt floating on the smelt is extracted separately.

This results in approximately 1,000 kg of filter residue. The filter residue is compacted by means of a transport screw and is mixed in a coarser form with a refill of protective salt to be used later, so that the mixture has the same weight as the originally added amount of protective salt. This protective salt filling, now composed of new salt and filter residue from the previous furnace filling, is added during a comparison filling, where the same amount of similar raw material is added.

Comparative measurements show that with regard to the composition of the resulting filter residue, the filtered off-gas ("clean gas") and finally the spent protective salt to be emptied, as well as with regard to the quality of the melted down secondary aluminium, no significant differences can be ascertained between the two oven fillings compared. It follows from this that the filter residue, which during the cleaning of the exhaust gas results from the melting down of secondary aluminium, can be added again in its entirety to the protective salt within the same facility.

If additionally supplied filter residue obtained x the context of the present invention is also mixed into the protective salt, the resulting composition of the actually used protective salt must necessarily be checked. The stronger the composition of the mixed filter residue deviates from the ideal composition of the protective salt, the less filter residue can be mixed into the protective salt.

At least 75% of the resulting protective salt actually used must actually be salt (NaCl or KC1).

The following table shows upper limits for some contaminants in protective salt:

If necessary, purification of exhaust gases can be further improved by introducing additional aggregates with a purifying effect before the filter stage in the exhaust gas channel. These aggregates must have. such a nature and are dosed so that they do not prevent the further applicability of the filter residue as a protective salt. Activated carbon must only be added to an extent of approximately 10% of the weight of the alkaline aggregates, as the filter residue may otherwise become combustible. The filter residue should also only contain a maximum of 5

% calcium compounds as pollutants, then

calcium compounds adversely affect the properties of the protective salt, above all with regard to its disposal, which was common at the time.

Very advantageous aggregates are emulsions of water with organic substances such as e.g. chipping and rolling emulsions, which at such temperatures are fed into the exhaust duct, where the water does evaporate, but the organic substances are neither burned nor evaporated. It follows from this that the temperature in the exhaust gas after injection of the emulsions and evaporation of the water contained in the emulsions should no longer be above approximately 250°C. The emulsion can be introduced together with the alkaline cooling solution or better injected from a separate container through a separate line (fig. 2). Thin layers of oil form on the dust particles in the exhaust gas. Organochlorine compounds are dissolved in these oil layers, such as e.g. PCDD/PCDF. In this way, in addition to adsorption, absorption is also used as a cleaning mechanism. The emulsions also usually also contain substances which suppress the catalytic properties of the dust in the exhaust gas to form PCDD/PCDF. Used oil emulsions, metalworking emulsions and rolling mill emulsions which have become unusable, especially from aluminum rolling mills, can be used. The weight of the organic substances introduced together with the emulsions into the exhaust gas duct can be approximately in the order of magnitude of the weight of the introduced alkaline substances. The organic components thus introduced into the filter residue do not interfere with the further use of the filter residue as protective salt, as these substances, due to the dust from the exhaust gas and due to the added new protective salt, only occur in such a small concentration that they do not form any ignitable dust.

A further advantageous additional aggregate is carbon black; which is advantageously produced shortly in front of the place where it is introduced into the exhaust duct (fig. 3). The exhaust gas temperature where the soot is advantageously introduced should no longer be above 250°C.

The soot can e.g. produced using a soot burner,

e.g. an acetylene soot burner. Carbon black produced in this way is almost pure carbon, has a very large specific surface area and is also reactive immediately after its production. Therefore, it has excellent adsorption properties. The advantage of soot is that, in contrast to activated carbon, it is difficult to ignite as, for a very short time after its production, it is graphite-like in terms of chemical properties.

Claims (6)

1. Process for the production of secondary aluminum where aluminum-containing waste is melted down in a furnace under a protective salt formed predominantly of NaCl and/or KC1, the furnace exhaust gases are supplied in an exhaust gas channel up to 5 g per Nm<3 >alkaline aggregates containing sodium and/or potassium as cations, and the thus treated furnace exhaust gases are freed of dust and droplets by being passed through a cloth filter, characterized by the fact that the filter residue collected at the cloth filter is added to a later used top-up with protective salt. 2. Method as stated in claim 1, characterized in that the alkaline aggregates are sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, sodium sulphide, sodium sulphite or analogous potassium-based compounds, which are at least partially injected into the exhaust gas channel in aqueous solution, as much is injected solution that the temperature in the exhaust gases is cooled to 150 to 300°C by evaporation of the water and that such a large quantity of alkaline aggregates is injected in total that the filter residue contains an excess of alkaline components. 3. Method as stated in claim 2, characterized in that, in addition to the alkaline aggregates, an emulsion (8) of water with organic substances is additionally injected into the exhaust duct. 4. Method as stated in claim 3, characterized in that as emulsion (8) respectively for the organic substances in the emulsion (8) emulsions originally used for another purpose or oils which have become unusable are used. 5. Method as stated in claim 2, characterized in that after the area in the exhaust gas channel in which the exhaust gas was cooled by injecting the aqueous alkaline solution, but before the filter (6), soot (9) is added. 6. Method as set forth in claim 5, characterized in that the soot (9) is produced by means of a soot burner (10) near the place of its introduction into the exhaust duct.