Process for eliminating metallic impurities from magnesium by injecting a halogenated derivative or boron

The invention concerns a process for eliminating metallic impurities from magnesium, in particular iron, silicon and manganese. For that purpose, a halogenated derivative of boron such as BCl.sub.3 is injected into the molten magnesium, in a proportion of from 0.05 to 5 kg of pure boron per tonne of magnesium. The magnesium may first be subjected to a refining operation using TiCl.sub.4 or a mixture comprising TiCl.sub.4 +FeCl.sub.3. The magnesium produced contains less than 1 ppm of residual boron, less than 20 ppm of Fe and Si and less than 30 ppm of Mn.

The present invention concerns a process for eliminating metallic 
impurities contained in magnesium, by injecting a halogenated derivative 
of boron. 
Magnesium which is produced by a metallothermal reaction and in particular 
by the method of silicothermal reduction of dolomite or magnesia under 
reduced pressure in a molten slag at a temperature of about 1600.degree. 
C., based on CaO, SiO.sub.2, Al.sub.2 O.sub.3 and MgO (Magnetherm process, 
French patent No 1 194 556=U.S. No. Pat. 2,971,833 in the name of SOFREM), 
may contain proportions of metallic elements such as silicon, iron or 
manganese, that in some cases are considered to be too high in relation to 
various specifications, depending on the use thereof, in particular in 
regard to the aspects referred to as high-purity qualities. 
In order to purify "Magnetherm" magnesium, use has been made of the 
sublimation method which is generally suitable for the so-called 
high-purity qualities, but the sublimation operation is a slow and 
expensive one and suffers from the disadvantage of consuming a substantial 
amount of electrical energy. 
As regards to refining by segregation, as disclosed in French patent No 1 
594 154 (=U.S. Pat. No. 3,671,229) in the name of ALUMINIUM PECHINEY, that 
process is not suitable for the metallic impurities which are referred to 
as "peritectic" impurities, such as manganese. 
It is known for some of the impurities in magnesium, and more particularly 
manganese, iron and silicon, to be removed by injecting titanium 
tetrachloride TiCl.sub.4. That process is described in U.S. Pat. No. 
2,779,672 to DOW CHEMICAL CO. However, the amount of impurities removed by 
carrying out that process is still highly unsatisfactory, with the 
residual manganese content being of the order of 200 to 500 ppm. 
The present invention concerns a process for eliminating metallic 
impurities from liquid magnesium, which can be carried out either as an 
operation that is complementary to a preliminary treatment using any 
refining agent such as TiCl.sub.4 or halogenated fluxes, or as a single 
refining treatment, and which produces a level of purity that is 
compatible with the requirements of all the known uses of high-purity 
magnesium. 
The above-indicated process comprises injecting into the liquid magnesium, 
by a suitable means, a halogenated derivative of boron such as the 
trichloride BCl.sub.3, the trifluoride BF.sub.3 or alkali metal, 
alkaline-earth or magnesium fluoborates. 
Surprisingly, it was found that the refining process using a boron 
derivative left only a residual amount of boron in the magnesium, of the 
order of 1 ppm or even less, which does not give rise to any disadvantage, 
irrespective of the uses to which the metal which is thus purified is put. 
Among the boron derivatives which can be used for carrying the invention 
into effect, the trichloride is particularly suitable by virtue of its 
boiling point which is +12.5.degree. C., which permits it to be stored in 
a liquid condition and under a moderate pressure in a steel cylinder. In 
order to ensure that the BCl.sub.3 is regularly fed into and properly 
distributed in the mass of liquid magnesium to be refined, it is 
preferable for it to be entrained in a flow of dry inert gas such as 
argon. The amount of halogenated derivative injected into the Mg 
corresponds to an amount of pure boron which is between 0.05 and 5 kg and 
preferably 0.1 and 1 kg per tonne of crude magnesium. When using BCl.sub.3 
for example which contains 9.2% of pure B, that corresponds to an amount 
of 0.54 to 54 kg and preferably from 1.09 to 10.9 kg of BCl.sub.3 per 
tonne of magnesium. 
Finely powdered potassium fluoborate can also be entrained by means of an 
inert gas flow, using any known process. 
The actual operation of injecting the halogenated derivative into the mass 
of liquid magnesium may be effected by one of the procedures which are 
well known to the man skilled in the art, in particular by means of a pipe 
of suitable material, which dips to the bottom of the ladle containing the 
liquid magnesium, or a pipe which passes through the wall of the ladle 
adjacent to the bottom thereof, or by means of any other equivalent 
process. 
The reaction of the halogenated boron derivative with the magnesium 
probably results in the formation of "nascent" boron which reacts with the 
elements such as Fe, Mn, Si, giving stable borides, with a specific 
gravity which is much higher than that of the liquid magnesium at a 
temperature of 700.degree. to 750.degree. C., which rapidly settle. The 
settling operation may be facilitated by the addition of halogenated 
fluxes which are usually employed in casting magnesium. 
EMBODIMENTS 
In the various examples, the liquid magnesium was disposed in a steel ladle 
and raised to a temperature of from 700.degree. C. to 750.degree. C. 
The cylinder containing the BCl.sub.3 was suspended from a high-precision 
weighing means and provided with a heating means for raising it to a 
temperature of from 20.degree. to 60.degree. C. The flow rate was 
controlled by a control valve actuated in dependence on the variation in 
the weight of the cylinder.

EXAMPLE 1 
7140 kg of magnesium, which had been previously refined using TiCl.sub.4, 
was treated by injecting 35 kg of BCl.sub.3, which was entrained by a flow 
of dry argon, at a rate of 20 kg per hour, by means of a steel lance, at a 
temperature of 730.degree. C. The results obtained were as follows: 
______________________________________ 
Impurities in ppm 
Fe Si Mn 
______________________________________ 
Before injection of BCl.sub.3 
28 18 530 
After injection of BCl.sub.3 
&lt;20 10 &lt;30 
______________________________________ 
EXAMPLE 2 
In the same manner, and at the same rate, 9405 kg of magnesium which had 
been previously refined using TiCl.sub.4 was treated by injecting 33 kg of 
BCl.sub.3, in argon. The results obtained were as follows: 
______________________________________ 
Impurities in ppm 
Fe Si Mn 
______________________________________ 
Before injection 
30 &lt;20 450 
After injection 
&lt;20 &lt;20 &lt;30 
______________________________________ 
EXAMPLE 3 
2700 kg of crude thermal magnesium was directly treated by means of 21.5 kg 
of BCl.sub.3, at a temperature of 750.degree. C., at a rate corresponding 
to 12 kg/h of BCl.sub.3. The results obtained were as follows: 
______________________________________ 
Impurities in ppm Fe Mn 
______________________________________ 
Before injection 80 970 
After injection &lt;10 &lt;50 
______________________________________ 
The preliminary refining operation using TiCl.sub.4, as described in 
above-mentioned U.S. Pat. No. 2,779,672, is found to be effective in 
reducing the proportion of silicon, but highly unsatisfactory as regards 
magnesium and iron. 
The applicants found that, in the preliminary refining operation, the 
TiCl.sub.4 could be totally or partially replaced by ferric chloride 
FeCl.sub.3, which is sublimated and entrained by a flow of dry argon. 
Mixtures formed by from 10 to 100% of FeCl.sub.3 and from 0 to 90% of 
TiCl.sub.4 are satisfactory and permit the proportion of silicon to be 
reduced for example from 1500/1000 ppm to about 100 ppm. 
Moreover, it was found that certain metallic impurities which are not 
generally encountered in magnesium but which may accidentally occur 
therein such as chromium and nickel are also removed by the treatment 
employing BCl.sub.3, down to a proportion of from 10 to 30 ppm. 
Operations of quantitatively determining the amounts of boron, which were 
carried out on the magnesium after treatment with the BCl.sub.3, were 
performed by spectrophotocolorimetry of the complex formed with methylene 
blue. 
The residual boron content is from 0.5 to 1.5 ppm, which in particular 
permits that metal to be used for the production of high-purity titanium 
or zirconium using the Kroll process for reducing TiCl.sub.4 or ZrCl.sub.4 
by means of magnesium.