Note: Descriptions are shown in the official language in which they were submitted.
3 ~
The present invention relates to a process for the
separation of a gaseous mixture which contains the vapours of
metals and/or semimetals. Gaseous mixtures of this kind are
formed in various processes, for example ln the thermal decomp-
osition of hydrides, nitrides, sulphides and halides, and also
in the reduction of oxides with hydrogen, water gas, generator
gas, natural gas and other gaseous reducing agents, but also
with liquid or solid reducing agents, such as fuel oil, coal and
coke or petroleum coke.
In the interests of simplicity, processes such as these
will hereinafter be generically referred to as "reduction",
because they are always accompanied by the formation of elemental
metal or semimetal vapour, albeit in mechanical admixture with
other gaseous substances.
In addition, metals, including the metals of the First
and Second Group of the Periodic System, and semimetals, also
mixtures thereof, will be referred to in the interests of sim-
plicity as "metal" and their corresponding compounds and mixtures
of these compounds as "compound 1l .
Again in the interests of simplicity, the gaseous
substances mixed with -the vapours of elemental metal will be
referred to hereinafter as "gases". Gaseous substances of this
kind are, for example, CO, CO2, H2O, SO2, H2S, 2' N2~ gaseous
sulphur, gaseous low-grade halides, oxides and sulphides, also
saturated gaseous metal and semimetal compounds.
Hitherto, vain attempts have been made economically
to separate the vapour of elemental metal present for example
in the gaseous mixtures resulting from the reduction of oxides
with solid, liquid or gaseous carbon-containing and hydrogen-
containing substances, in order to recover metals. For example,
the quenching of a gaseous mixture of Mg-vapour and CO was only
partly successful and by no means economical because a large part
3~
of the Mg-vapour is reoxidised by the CO into magnesium oxide.
This is because, if a gaseous mixture formed, for example, by
reduction consisting of metal vapour and other reduction products,
such as CO, CO2 and H2O, is cooled, the temperature-dependent
thermodynamic vapour-gas equilbrium is reversed, metal vapour
being reoxidised into the metal oxide, in many cases even with
the simultaneous formation of carbon black. Accordingly, the
quenching of gaseous mixtures such as these always results in
the formation of metal powders which are contaminated by an
uneconomically large amount of oxide. This also applies in the
same way to any other reversible temperature-dependent gas-metal
vapour equilibrla. Accordingly, these processes have never been
successfully worked on an industrial scale both for technical
and Eor economic reasons.
In other known processes, hot gaseous mixtures contain-
ing metal vapours, such as are formed in carbothermal reduction,
are quenched with much cooler metal melts at a temperature below
the respective condensation level of the metal vapours to be
separated. In said process at first the metal vapours are
condensed, then the condensates (e.g. as a mist consisting of
small droplets) form liquid alloys with the molten quenching
metals whilst a part of the metals to be gained is again reformed
into the original metal oxides. The metal yield is not satis-
factory.
Attention is drawn to the fact that the term "absorption"
in many publications is not correctly used. In physics and
chemistry "absorption" means the taking up of gases by liquids
and solids, e.g. the taking up of CO2 by water or of Mg-vapour
by molten Sn. In contrast hereto, the taking up of a mist or
3Q an aerosol consisting of fine droplets by a liquid ~melt) is no
"absorp-tion" but a mixing or solving procedure.
In another known process for separating carbon monoxide
from a gaseous mlxture containing magnesium vapour, the mixture
is brought into contact with metal carbides, the carbon monoxide
oxidising the surface oE the carbide partlcles to form metal
oxides with deposition of carbon. The carbide particles are
then regenerated with the carbon deposited to form the carbides.
The formation of oxide-carbide mixtures, a residue oE CO left
for thermodynamic reasons in the magnesium vapour obtained and
considerable superheating and supercooling of the solicls make
this proposed process both technically and economically impract-
icable.
The recovery of metals by melt electrolysis was finallyleft as a possible method of obtaining numerous metals. The
disadvantages of these processes which, today, are almost exclus-
ively used the large-scale recovery of aluminium, magnesium and
many other metals, lie above all in the poor volume-time yield,
the expensive electrical installations and the consumption of
large quantities of electrical energy.
Accordingly, the present invention reduces and desirably
removes the disadvantages referred to above by providing a process
for the separation of a gaseous mixture (which is formed during
the reduction of a compound and which contains the vapour of
elemental metal and/or semimetal ? by contact with an absorbent
(metals and metal salts in solid or liquid form) which can be
carried out simply and economically and which in particular enables
electrical energy to be saved.
According to the invention the gaseous mixture is con-
tacted with an absorbent which is capable of absorbing the metal
vapour, preferably in the form of a melt, under such -thermodynamic
conditions that the absorbent directly absorbs the metal vapour
from the gas phase.
It has been found that the vapour of elemental metal
and/or semimetal is absorbed in the absorbent (metals and metal
-- 3 --
salts in solid or liquid form) to a large extent or completely,
depending upon the particular procedure adopted in practice,
whereas the other gaseous constituents are not.
In the application of the process according to the
invention, therefore, -the metal vapour contained in the gaseous
mixture must remain gaseous-en route to the absorbent and also
during absorption. In the application of the process according
to the invention, a gaseous mixture will generally arrive at the
absorption stage in the thermodynamic state which is assumed
lQ during its formation. The thermodynamic conditions have to be
taken into consideration in the event of any changes in temperature
or pressure by changing the pressure or temperature or the comp-
osition of the gaseous mixture in such a way that the gas phase
remains intact. Shortly, the absorption of the metal vapour
according to the invention always is effected at a temperature
(absorption temperature) above the respective condensation level
of the metal vapour to be separated. Condensation level means
that temperature at which a gas or vapour depending ~rom its
partial pressure condenses to a liquid or a solid. The normal
condensation level is only a special case, viz. that temperature
at which a gas or vapour condenses at a partial pressure of 1 at.
The necessary quantitative temperature and pressure conditions
may be thermodynamically calculated with the means known at the
present time for any reversible equilibrium and gaseous mixture,
and may be experimentally determined.
In contrast to the process according to the invention,
the hot metal vapour to be separated in conventional processes
condenses in the surroundings and in the vicinity of the surface
of the much cooler metal (for example in bubbles or in droplets),
with which the gaseous mixture is brought into contact, to form
the liquid or solid metal. In these processes, absorption o-f the
metal vapour to be separated from the gas phase directly into the
metal i5 by nature impossible because, due to cooling far below
the boiling point or below the melting point of the gaseous metal,
it always has to pass first into the liquid or solid phase before
it can be absorbed in the metal.
The absorbent used in accordance with the invention
should have as high an "absorption capacity" (alloying affinity,
chemical affinity) as possible for the metal to be absorbed,
coupled with an extremely low vapour pressure at the working
temperature, in addition to which, when it is used in the form
of a melt, it should have as low a melting point as possible for
technical reasons and, when it is additionally used for transport-
ing heat in endothermic reduction reactions, it should have as
high as possible a heat capacity and/or evaporation enthalpy and,
finally, it should incur the lowest possible production costs
for economic reasons. In many cases, these requirements give
rise to the need to combine with one another several metals and/or
semimetals according to their chemical, physical and thermodynamic
properties and also their prices.
The metals, metal and semimetal salts and semimetals
used in accordance with the invention for the absorption of metal
vapours, also every possible mixture, solution, alloy and compound
thereo, are hereinafter referred to simply as "absorbent". The
absorbent may be used in solid form, for example in the form of
small beads, Raschig rings or nests of tubes, in liquid form or
gaseous form.
It is known that chemical and physical reactions are
accelerated when the reactants, as far as possible in statu
nascendi, are intimately mixed with and whirled through one
another in finely distributed form. Accordingly, it is of
advantage vigorously to whirl the compound with the absorbent
during reduction, i.e. the gaseous mixture during its formation.
In this way, absorption of the metal vapours is accelerated and
the need for a separate absorption chamber is eliminated.
During the separation of a metal vapour from an already
formed gaseous mixture, which takes place in countercurrent with
an absorbent, the amount of metal vapour in the gaseous mixture
decreases. As a result, the tendency which the compound has to
reform is reduced, so that the temperature can decrease accordingly
during absorption or along the absorption path without the com-
pound being reformed. Accordingly, it is not absolutely essential
for the entire separation of the gaseous mixture to be carried
out at the reduction temperature. However, any reformation of
the compound in the gaseous mixture is reliably avoidecl in the .
temperature of the absorbent, on being brought into contact with
the gaseous mixture, is at least as high as the temperature of
the gaseous mixture during its formation.
In endothermic reduction reactions, the reactants and,
optionally, the absorbent as well are known to cool down when no
heat is conveyed to the system. Heat may be applied in known
manner, for example by means of electrical energy or by heating
with fuels. However, in order to simplify the apparatus and to
avoid problems of materials, it is preferred in accordance with
the invention to adopt a procedure in which, during its formation,
which is accompanied by reduction, the gaseous mixture is brought
into contact with the absorbent with a heat content which is
used at least partly for maintaining the reduction temperature.
According to the invention, it is even possible to
manage with a smaller quantity of absorbent providing its heat
of the evaporation is utilised. Accordingly, another feature of
the invention is that, during its formation accompanied by re-
duction, the gaseous mixture is brought into contact with at
least part of the absorbent in gaseous form which is condensed.
In order to heat the absorbent before it is used for
separation, heat may be conveyed to it in known manner indirectly,
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i.e. through a container wall, with smoke gases and flame gases
of burners, by radiation from electrical resistance heating
elements, by induction heating or by direct electrical resistance
heating or in any other manner.
So far as the transfer of heat and the outlay on
apparatus are concerned, it is of particu:Lar advantage in
accordance with the invention to heat the absorbent before sep-
aration by direct contact with smoke and flame gases and, during
combustion, to adjust the fuel/air or oxygen ratio in such a way
that the absorbent cannot be oxidised, or to add the necessary
amount of reducing gas to the smoke and flame gases.
In cases where it is desired to recover the absorbed
metal from the gaseous mixture, it is removed from the absorbent
in known manner preferably by desorption by reducing pressure
and/or increasing temperature or by rectification, and the result-
ing metal vapour is cooled so that either liquid or solid metal
is obtained.
However, the solution of absorbed metal and absorbent
may also be used otherwise as an alloy or as a chemical reagent
2~ (for example for deoxidising crude metal melts, as a blowing
agent for producing foamed concrete, etc.).
Following separation, it is possible in accordance with
the invention either to desorb the entire solution of absorbed
metal and absorbent or further to use only parts thereof (for
example as an alloy and/or chemical reagent) and to desorb the
rest and then to reuse the absorbent for the separation thereof
or of another gaseous mixture.
Most of the waste gases from reduction reactions contain
large quantities of CO and H2. According to the invention,
therefore, the eaonomy of the process is improved if, following
separation, the non-absorbed waste gas or at least a part of this
waste gas is used as a fuel and/or another part as chemical
reagent (for example as a reducing agent, for the synthesis of
NH3 of plastics).
If an absorbent can be chemically attacked by the gases
formed during reduction, this effect is avoided in accordance
with the invention by the addition of an adequate quantity of a
gas preventing this chemical attack to the gaseous mixture during
its formation either during or after reduction.
In the still process, the gases leaving the absorbent
take with them a small amount of the absorbed metal in vapour
form. According to the invention, this may be avoided by bringing
the gaseous mixture into contact with the absorbent in countercurrent
during its formation during and/or after reduction. The fresh
absorbent absorbs the last traces of the metal vapour in counter-
current, becomes enriched with metal from the gaseous mixture
along the absorption path and leaves the apparatus saturated with
the absorbed metal.
According to the invention, it is of particular advant-
age to combine the countercurrent process with the recycle process.
The gaseous mixture is continuously brought into contact with
the absorbent in countercurrent during its formation during and/or
after reduction. The absorbent is continuously desorbed, is
directly or indirectly heated in countercurrent, optionally
continuously, and is continuously reused for absorption.
In cases where solids such as coke or coal are used for
reduction and/or for directly heating the absorbent, the absorbent
takes up small quantities of metals, emanating from the mineral
constituents of the solid reducing agents and/or fuels, such as
iron, aluminium, silicon, alkali and alkaline earth metals, from
the fly dust entrained by the gaseous mixture or on contact with
the reduction reactants and/or smoke gases. In order to avoid
harmful accumulations or enrichments, especially in cases where
the absorben~ is recycled, the absorbent is in accordance with the
invention periodically or continuously freed from impurities in
known manner until only harmless residues are left. For example,
alkali and alkaline earth metals are removed by desorption,
aluminium and silicon are vaporized by the action of halogens
or halides and iron is oxidised with air and the solid iron
oxides separated from the absorbent.
The invention is illustrated by the following Examples.
EXAMPLE 1
In the reduction of Na2O with C at 1000C, a gaseous
mixture with the following composition is formed:
64.212 % by volume of Na-vapour
35.788 ~ by volume of CO
The reaction pressure is 3.5 atms. After reduction, this mixture
is brought into contact in countercurrent with molten lead heated
to 1030C as absorbent. It absorbs the Na-vapour whilst pure CO
escapes. A melt with the following composition:
83 % by weight of Pb and
17 % by weight of Na,
is obtained and is desorbed in a rectification column at 1050C/
2Q 0.1 atm (76 (Torr). The Na-vapour escaping from the rectification
colume is condensed and the lead melt containing a small residue
o~ sodium is reused as absorbent for separating the Na-vapour
from the CO.
EXAMPLE 2
50 t/h of calcined magnesite are continuously reduced
at 1650C with 33,000 Nm3/h of natural gas (85 % by volume of CH4
and 15 % by volume of N2) in a tower consisting of several
chambers.
Whilst the mixture of magnesite dust and the cracking
products of the natural gas flows upwards, a gaseous mixture of
27,630 Nm3/h of magnesium vapour, 27,640 Nm3/h of carbon monoxide,
55 ! 350 Nm3/h of hydrogen and 4950 Nm3/h of nitrogen i9 formed.
330 m3/h of absorbent consisting of 42.8 % by weight of lead
and 57.2 % by weight of tin, which has been heated to a temper-
ature of 1840C, are passed continuouslv through the chambers
in countercurrent (downwards). A further quantity of absorbent,
in the form of 106,700 Nm3/h of hot lead vapour distributed among
the individual chambers is introduced at a temperature of 1840C.
The reaction pressure during the reduction of MgO with
natural gas at a temperature of 1650C is approximately 0.5 atm.
A working pressure of approximately 1 atm is reached by the
introduction of lead vapour.
The lead vapour and the Pb/Sn melt cool down to the
reduction temperature, the lead vapour being condensed into liquid
lead. The heat of evaporation of the lead vapour and the sensible
heat o~ lead vapour and Pb/Sn melt supply the heat required ~or
reduction, the entire Pb/Sn melt dlrectly absorbing the magnesium
vapour from the gaseous mixture as it is formed; CO, H2 and N2
leave the tower at its uppermost part.
450 m3/h of a mel-t consisting of 4.4 % by volume of Mg,
48.5 ~ by volume of Sn and 47.1 % by volume of Pb, flow off
continuously from the lower end of the tower Whereas the vapour
pressure in the chambers of the tower during reduction and absorp-
tion amounts -to approximately 1 atm, the melt is desorbed in a
rectification column under a pressure of only 10 Torr, 30 t/h of
Mg-vapour escaping continuously from the melt. The Mg-vapour is
cooled to 720C, liquefying in the process.
During desorption, the melt is left with a residue of
approximately 0.01 % by weight of Mg which is continuously en-
trained in the recycle process. The melt is again continuously
heated to 1840C, 106,700 Nm3/h of lead evaporating again. As
already described, lead vapour and residual melt are again intro-
duced into the tower. For heating the melt and evaporating the
lead the 87,940 Nm /h of (CO + H2 + N2) continuously escaping
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during separation of the gaseous mixture and in addition 19,000
Nm3/h of natural gas are burnt with air in gas burners. Before
combustion, the air is heated by the smoke gas of the gas burner,
which has a temperature of 1900C, in a radiation recuperator.
Impurities emanating from the calcined magnesite, such
as iron, aluminium, silicon, calcium, sodium and potassium,
accumulate in the recycled (heating-absorption-desorption-heating)
Pb-Sn melt. The melt is periodically cooled as required to 1000C
and treated with airl resulting in the formation of mixed oxides
Of Fe3O4, Al2O3, SiO2, CaO, MgO, K2O and Na2O. They flow as
crusts on the liquid Pb-Sn melt and are separated off.
A gaseous mixture formed during the reduction of pure
MgO with natural gas can o course also be separated in accordance
with the invention, which affords the additional advantage that
no impurities accumulate in the absorbent so that the need for
purifying the absorbent according to Example 2 is eliminated.
Pure MgO is obtained for example in the reduction of
pure aluminium chloride, followed by burning of the MgCl2 formed
or in the reduction of pure Al2O3 with Mg to form pure aluminium.
Processes such as these are now of outstanding significance
because, in a recycle process comprising for example
l) reduction of Al2O3 with Mg (to form Al and MgO)
2) the reduction of MgO with natural gas (to form Mg-
vapour and CO)
3) the separation of Mg and CO in accordance with the
invention
l) the reduction of Al2O3 with Mg
the process according to the invention enables substantially non-
reducible metals, such as Mn, Cr, Al, Ti and Zr to be recovered
surprisingly economically.
If in accordance with Example 2 the heat required for
endothermic reduction of the MgO were to be conventionally supplied
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and if pure lead were to be used for separating the gaseous
mixture (Mg-vapour + H2 + Co + N2), 1200 m /h of Pb melt would
be required for absorbing the 30 t/h of Mg-vapour. If pure tin
were to be used, only 300 m3/h of Sn melt would need to be
introduced because under the Mg partial pressure prevailing tin
is able to absorb considerably more magnesium than lead. If the
heat required was to be supplied by means of a melt of absorbent,
as much as 8580 m3/h of lead (1730C) or 3150 m3/h o tin (1840C)
would be necessary (the boilin~ point of lead is 1753C).
However, if as in Example 2 condensing lead vapour is
used as heat source for the reduction of MgO and, in addition, if
molten tin is used as the principal absorbent, a Pb-Sn-melt is
formed as absorbent. From the kechnological point of view, the
problem of heat supply for the endothermic reduction of MgO is
elegantly solved in this way. I then following separation the
magnesium is removed from the melt and the melt reheated to
1840C, lead vapour is again formed, although on this occasion,
depending upon the temperature, pressure and activity conditions
prevailing, a proportion of lead remains behind in the melt and
is continuously circulated together with the tin in the recycle
process. Out o this there arises the optimum input of 330 m3/h
of Pb-Sn melt containing 42.8 % by weight of Pb and 57.2 % by
weight of Sn as liquid absorbent, and 10~,700 Nm3/h of condensed
lead vapour as the additional amount of absorbent and heat carrier,
coupled with elimination of the need for separate heating of the
reduction apparatus.
EXAMPLE 3
In the reduction of zinc oxide with carbon at 1000C,
a gaseous mixture of
3G 50.37 % by volume of ~n vapour r
48.89 % by volume of CO and
0.74 % by volume of CO2
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is formed. This mixture is brought into contact with steel
elements (as absorbent) in an absorption chamber. The steel
absorbs the zinc vapour, whilst the zinc-free waste gas consist-
ing of CO and CO2 leaves the absorption chamber. The zinc is
absorbed in the surface of -the steel elements up -to a content
of 70 % by weight which in a 1 mm thick surface layer decreases
inwards to 0 % of Zn.
EXAMPLE 4
In the reduction of MnO with low-sulphur petroleum coke
at 1750C, a gas mixture consis-ting of 50 % by volume of manganese
vapour and 50 % by volume of CO is formed. Since the reaction
pressure is only 0.3 atm, this pressure or a lower pressure as
the working pressure would have to be produced by evacuation.
Technically it is simpler to add nitrogen ln order to reach a
working pressure of 1 atm. In this case the gas mix-ture consists
of
15 % by volume of CO,
15 ~ by volume of Mn vapour and
70 % by volume of N2
The nitrogen is obtained from part of the smoke gas of a burner
freed from CO2, SO2 and steam by washing with water under pressure.
It is then heated to 1650C in a countercurrent heat exchanger by
means of the hot smoke gas and added to the CO/manganese vapour
mixture during its formation from the MnO/petroleum coke mixture.
The gaseous mixture is passed through a melt consisting
of 82 % by weight of Sb and 18 % by weight of Al. Whereas the
antimony has a particularly high solvent action on manganese, the
addition of aluminium prevents the boiling off of antimony whose
normal boiling point is 1635C.
The gaseous mixture is separated in the Sb-Al melt,
giving on the one hand a gas of CO and N2, which contains traces
of SO2, and on the other hand an Sb-Al melt containing 40 ~ by
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weight of manganese. The manganese is removed from the Sb-Al
melt in a rectification column.
EXAMPLE 5
By heating Bi2S3 to 90QC while argon is passed through,
a gas mixture is formed by thermal decomposition, containing in
addition to argon
57.1 ~ by volume of Bi vapour and
42.9 % by volume of S2 vapour
It is introduced into molten tellurium heated to 930C, resulting
in formation of the commercially significant intermetallic
compound Bi2Te3 (melting point 585C). The sulphur vapour sep-
arated from the Bi vapour escapes.
A reaction pressure which is above or below or equal to
1 atm is adjusted both in dependence upon the thermodynamic
properties of the reduction reactants and in dependence upon
the reduction temperature. Since the separation of gaseous
mixtures in accordance with the invention can be carried out
technically more simply when the working pressure is not signif-
icantly below 1 atm, it is of advantage in accordance with the
invention to add to the gaseous mixture during its formation
during and/or after reduction such a quantity of a gas which does
not have too adverse an effect upon the metal vapour/gas equili-
brium that the required working pressure is reached.
In Example 2, it was pointed out that the reaction
pressure of the gaseous mixture of Mg vapour + CO + ~2 + N2
amounts to approximately 0O5 atm and that this pressure is in-
creased by the introduction of lead vapour to a working pressure
of approximately 1 atm. If no lead vapour were to be introduced
and if separation of the gaseous mixture were to be carried out
at a working pressure of 1 atm for example, 1 Nm of for example
hydrogen, argon or zinc vapour would have to be introduced per
Nm3 of gaseous mixture.
- 14 -
3~
As expected, it has been found that, when absorbent
and gaseous mixture are brought into contact after reduction,
i.e. in the absence of the reduction reactants, metal vapour is
absorbed in a quantity which corresponds thermodynamically to its
partial pressure, to the vapour pressure of this metal in its
pure form and to its activity in the absorbent at the absorption
temperature.
However, if the absorbent is brought into contact with
the gaseous mixture as it is formed during reduction, i.e. in the
presence of the reduction reactants, and then removed as ~uickly
as possible, it contains a much larger quantity of absorbed metal
vapour than corresponds to the thermodynamic laws. Accordingly,
an important feature of the invention is that the gaseous mixture
is brought into contact with the absorbent as briefly as possible
during its formation during reduction.
In cases where an absorbent has a considerable vapour
pressure at the working temperature prevailing so that a signifi-
cant amount of its vapour is entrained by the non-absorbed waste
gases, the waste gas pipe is initially encrusted and finally
blocked through cooling of the waste gases and condensation of
absorbent vapour. This also applies correspondingly to the direct
heating of the absorption metal with smoke or -flame gases.
According to the invention, this danger is obviated by passing
the waste gas and the smoke gas after they have left the absorbent
through a condenser from which the condensate either flows back
to the absorption metal or from which is it removed mechanically,
physically or chemically ~for example by scrapers, melting off or
vaporization with chlorine).
In order as far as possible to recover for the process
the heat removed from the waste gas in the condenser and the heat
content of the smoke gas from the burners which heat the absorben-t
and also the heat given off during condensation of -the metal
- 15 -
evaporating off during desorption (collectively "waste heat"),
the air and/or the fuel for the burner and/or the reducing agent
and/or the compound to be reduced are in accordance with the
invention heated in known manner (for example in countercurrent
heat exchangers) by the waste heat before they are used for
combustion or for reduction. In this way, the need for additional
fuel is kept to a minimum which ensures maximum economy of the
process according to the invention.
The costs of the process according to the invention are
surprisingly low and the consumption of electrical energy minimal,
amounting solely to the handling costs of the process where the
absorbent and endothermically reacting reduction mixtures are not
completely or partly heated by electrical energy. Accordingly,
an improtant economic feature of the invention is that the metal
separated off is either directly used as a vapour after desorption
or rectification or cooled and liquefied into a melt or in solid
form for processes which are still extremely expensive or which
hitherto it has not been possible to work on a commercial scale
for reasons of poor economy. An acute example is the reduction
2Q of substantially difficultly reducible metal oxides or halides
with metals of the First and Second Group of the Periodic System.
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