Process for refining scrap aluminum

A salt melt of alkali and/or alkali earth chlorides and fluorides, heated to 50.degree.-100.degree. C. above the melting point is employed to purify heavily contaminated scrap aluminum. The scrap aluminum, preheated to 400.degree.-500.degree. C. is added to a melting or holding furnace containing the less dense salt melt, the mixture stirred if necessary and then held at the above mentioned melt temperature for at least one hour. First the sedimented metal phase, then the salt melt are filtered through a resistant open pore ceramic filter and thus freed of solid impurities. The aluminum is transferred to an electrolytic cell for purification, and the molten salt melt recycled.

BACKGROUND OF THE INVENTION 
The invention relates to a process for refining heavily contaminated scrap 
aluminum using a melt made up of salts of alkali or alkaline earth 
chlorides and fluorides. 
Particles of glass, oxides and stones are in general very harmful when 
refining heavily contaminated scrap aluminum, especially when the 
concentration of aluminum is below 50 wt %. The removal of such foreign 
matter has caused considerable difficulty when employing the conventional 
technology for this purpose. Before electrolytic purification however, 
whether by three layer electrolysis, bipolar electrolysis or fractional 
crystallisation, the contaminating particles of solid material must be 
removed. 
The salts employed by the secondary aluminum industry today in conventional 
processes are consumed to an excessive degree because of the high 
concentration of oxides. 
The object of the present invention is therefore to develop a process for 
purifying heavily contaminated scrap aluminum, using a melt of molten 
salts of alkali and alkali earth chlorides and fluorides, and this such 
that the said process is economical and operates with a high degree of 
efficiency. 
SUMMARY OF THE INVENTION 
This object is achieved by way of the invention in that 
the scrap aluminum preheated to 400.degree.-500.degree. C., is fed to a 
melt of molten salts which is heated to 50.degree.-100.degree. C. above 
the melting point and has a lower density than aluminum at the operating 
temperature, and the molten mixture is held for at least one hour at this 
temperature in a melting or holding furnace, 
then first the molten aluminum which has sedimented during holding, 
followed by the molten salt melt, is separated from the solid impurity 
matter by passing the aluminum and salt melt in that order through an open 
pore ceramic filter which is resistant to both filtrates, and 
finally the filtered molten aluminum is fed to an electrolytic cell for 
purification and the salt melt to a melting or holding furnace for re-use. 
The use of the ceramic filter makes it possible, after melting the scrap 
aluminum in a molten salt bath and sedimenting out the molten aluminum in 
a holding furnace, to separate the molten aluminum with its metallic 
impurities from solid particulate material, mainly ceramic particles which 
are caught up in the scrap metal. Aluminum scrap from car dump for example 
usually contain more than 50 wt % of impurities. 
In the subsequent electrolyte purification process the metallic impurities, 
such as iron, silicon and copper for example are separated selectively 
from the scrap metal. The electrolytic purification is carried out 
preferably in the form of the well known three layer electrolyte process 
which has been employed by the aluminum industry for a very long time, or 
by means of the bipolar electrolyte cell described in the U.S. patent 
application Ser. No. 630,289. A bipolar cell is employed in particular if 
the scrap aluminum has a high concentration of silicon and/or iron, as the 
large losses in copper occuring in the three layer process can be avoided. 
The amount of salt required for carrying out the process according to the 
invention is calculated such that it is preferably 1.2-2 times, in 
particular 1.5 times, as large as the weight of scrap to be purified. The 
charges of salts employed comprise, usefully, of 30-50 wt % NaCl, 30-50 wt 
% KCl and 15-25 wt % cryolite. A melt which has proved particularly 
advantageous contains about 40 wt % NaCl, 40 wt % KCl and 20 wt % 
cryolite, said melt being held at a temperature of about 750.degree. C. 
before the pre-heated scrap aluminum is added to it. Especially when the 
scrap is in small pieces it can be advantageous, before holding, to stir 
the molten mixture of salts and scrap aluminum. The main effect of this is 
to lower the concentration of magnesium in the aluminum scrap. The 
stirring is achieved by known mechanical means, for example using a 
magnetic stirrer or by injection of inert gases. 
The purifying action of the salt melt can be increased by employing a 
conventional scrubbing gas such as chlorine before holding. 
During holding, which lasts preferably 1-2 hours, droplets of aluminum 
sediment to the bottom of the holding vessel, for which the melting 
furnace also usefully serves. 
After holding, the molten aluminum and the contaminated salt are present as 
two separate phases, the heavier aluminum being at the bottom and ready 
for filtering. 
The filterchamber, which can be heated, holds the ceramic filter which, for 
example, is a version of that described in the German patent publication 
DE-OS No. 26 13 023 and is in the form of a slab. The relatively coarse 
pored filter made in particular of MgO, Al.sub.2 O.sub.3, MgOAl.sub.2 
O.sub.3 or ZrO.sub.2 has a porosity of 20-50 ppi (pores per inch), in 
particular 40 ppi. 
The molten aluminum can flow unhindered through this exchangeable ceramic 
filter, while all the solid particles are held back. The aluminum thus 
freed of solid impurities is passed continuously or in charges from the 
filter chamber to a holding furnace or directly to the cell for 
electrolytic purification. 
After all of the aluminum has been passed through the suspension in it is 
allowed to pass through the filter. Whereas the molten aluminum that has 
to be filtered contains only few solid particles, the main fraction of 
solid contaminants is presented to the ceramic filter by the molten salt. 
There the particles form a cake which, after passage of the charge, can be 
removed either together with or separate from the ceramic filter. 
The molten salt purified of solid particulate matter is returned to the 
melting furnace where it can be fed the next charge of scrap aluminum. 
The invention is explained in greater detail with the help of a schematic 
drawing.

DETAILED DESCRIPTION 
The device for purifying heavily contaminated scrap aluminum comprises 
essentially a melting furnace 10 which at the same time serves as a 
holding furnace, and a filter chamber 12 which can be heated, both vessels 
being connected by a channel 14 featuring a drainage valve 16. The whole 
device, apart from the insulating lid 18 which can be raised, is 
surrounded by refractory brickwork 20 which is lined with magnesite bricks 
22. 
The preheated scrap aluminum 24 is poured into the furnace via an opening 
which for simplification is omitted here. Shown in the figure are the 
phases which have separated after holding, the lighter molten salt melt 26 
containing the main fraction of the solid impurities 28 being at the top. 
The lower, molten aluminum contains only a small amount of such impurities 
28. 
The ceramic filter 30 situated in the filter chamber 12 at a level lower 
than the connecting channel 14 is plate or slab shaped. 
If the molten aluminum, freed of solid impurities, is drained as a charge, 
then the volume below the ceramic filter is at least sufficiently large 
that it can hold the whole metal fraction of the scrap aluminum charged to 
the furnace. 
The molten aluminum freed of the solid impurities 28 is removed from the 
filter chamber 12 through a channel 40 which can be closed off by valve 
42. 
The molten salt bath scrubbed of solid impurities 28 is returned to the 
melting furnace 10 by means of a pump 32 via pipeline 34 which features 
two closing-off devices 36, 38. 
For simplicity the conventional means for heating the melting furnace and 
the filter chamber are not known here; likewise the optional means for 
mechanical stirring or introducing inert or chemically active gases are 
omitted here. 
Further, the filter chamber is fitted with a drainage pipe, not shown here, 
for draining off the salt bath if it is so heavily contaminated that it is 
not to be returned to the furnace 10, but instead is to be replaced. 
1ST EXAMPLE 
Scrap aluminum containing 50-60 wt % impurities in particular in the form 
of oxide phases, was melted in a mixture of 40 wt % sodium chloride, 40 wt 
% potassium chloride and 20 wt % cryolite and filtered through a 40 ppi 
MgO filter plate. The metal leaving the filter was found to contain 9 wt % 
Si, 0.7 wt % Fe and 3.3 wt % Cu. The aluminum yield (extraction 
coefficient) was 90%. 
The aluminum, freed of solid impurity matter, was then fed to a bipolar 
cell for purification according to the U.S. patent application Ser. No. 
630,289. The separation factors for Si and Fe were 99%, for Cu 98% and for 
Mg 95%. These separation factors designate a purified scrap aluminum of 
99.99% purity. 
2ND EXAMPLE 
During metal transfer, alloy preparation and gas treatments in various 
holding and melting furnaces, considerable quantities of aluminum dross 
are produced. Conventional treatment of such low grade, finely divided 
metal contaminated with non-metallic substances is not possible without 
some kind of preliminary purification. Preheated aluminum dross with a 
metal content of 60-80 wt % was melted in a salt bath comprising 45 wt % 
KCl, 45 wt % NaCl and 10 wt % AlF.sub.3 and filtered through a highly 
sintered 45 ppi Al.sub.2 O.sub.3 filter plate. The extraction coefficient 
for the metal lay between 85 and 90%. The aluminum obtained had a purity 
of 90-99%. This was purified further in a conventional three layer 
electrolyte cell. The resultant separation factor corresponded to those 
achieved today using smelter aluminum (approx. 89.5%) i.e. the purified 
aluminum corresponds to conventional 99.99% metal. 
3RD EXAMPLE 
Extremely fine granular aluminum or aluminum dust for which direct melting 
is out of the question was mixed with an anhydrous salt mixture of 45 wt % 
NaCl, 45 wt % KCl and 10 wt % NaF and melted under a protective atmosphere 
of nitrogen. The dross is filtered off by a filter plate such as is 
mentioned in the second example. 
4TH EXAMPLE 
Dross containing large amounts of salts can cause considerable 
environmental problems. Such aluminum dross containing salts (up to 48 wt 
% of salts) was melted in a salt mixture, comprising 45 wt % NaCl, 45 wt % 
KCl and 10 wt % cryolite, at 750.degree. C., allowed to sediment and then 
filtered through a 45 ppi MgO filter. The average extraction coefficient 
was 70-80%. 
5TH EXAMPLE 
Aluminum dross containing only 20 wt % of metallic aluminum was treated as 
in example No. 4. The extraction coefficient was 60-75%. The aluminum, 
freed of solid impurities was, as in examples 3 and 4, purified further by 
means of a bipolar cell as in the last example.