U.S. patent application number 10/292321 was filed with the patent office on 2003-07-24 for process for electrochemical decomposition of superalloys.
Invention is credited to Erb, Michael, Gille, Gerhard, Mathy, Wolfgang, Meese-Marktscheffel, Juliane, Nietfeld, Georg, Olbrich, Armin, Stoller, Viktor.
Application Number | 20030136685 10/292321 |
Document ID | / |
Family ID | 7705648 |
Filed Date | 2003-07-24 |
United States Patent
Application |
20030136685 |
Kind Code |
A1 |
Stoller, Viktor ; et
al. |
July 24, 2003 |
Process for electrochemical decomposition of superalloys
Abstract
A process for recovery of valuable metals from superalloys by
electrochemical decomposition is described, both electrodes being
formed by the superalloy and the polarity of the electrolysis
current being reversed with a frequency of from 0.005 to 5 Hz.
Inventors: |
Stoller, Viktor; (Bad
Harzburg, DE) ; Olbrich, Armin; (Seesen, DE) ;
Meese-Marktscheffel, Juliane; (Goslar, DE) ; Mathy,
Wolfgang; (Langelsheim, DE) ; Erb, Michael;
(Salzgitter, DE) ; Nietfeld, Georg; (Bad Harzburg,
DE) ; Gille, Gerhard; (Goslar, DE) |
Correspondence
Address: |
BAYER CHEMICALS CORPORATION
100 BAYER ROAD
PITTSBURGH
PA
15205
US
|
Family ID: |
7705648 |
Appl. No.: |
10/292321 |
Filed: |
November 12, 2002 |
Current U.S.
Class: |
205/717 ;
205/771 |
Current CPC
Class: |
C22B 11/046 20130101;
Y02P 10/20 20151101; C22B 7/007 20130101; C22B 3/045 20130101 |
Class at
Publication: |
205/717 ;
205/771 |
International
Class: |
C25C 001/24 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 14, 2001 |
DE |
10155791.4 |
Claims
What is claimed is:
1. Process for recovery of valuable metals from superalloys
comprising decomposing the superalloys electrochemically, wherein
the superalloy are used both as the anode and as the cathode, and
the polarity of the electrolysis current is reversed with a
frequency of from 0.005 to 5 Hz.
2. Process according to claim 1, wherein the superalloy contains
one or more of the metals Co, Ni, Cr or Al as major constituents
and one or more of the elements Ta, Re, W, Mo, Hf or Pt as minor
constituents.
3. Process according to claim 2, wherein the superalloy contains
from 1 to 10 wt. % of Re.
4. Process according to claim 1, wherein the electrochemical
decomposition is carried out with an electrolysis voltage of from 2
to 6 volts at a constant electrolysis current.
5. Process according to claim 1, wherein an inorganic acid is used
as the electrolyte.
6. Process according to claim 5, wherein an oxygen-free inorganic
acid is used.
7. Process according to claim 6, wherein as a result of the
electrochemical decomposition, the elements Co, Ni, Cr and Al are
obtained as salts dissolved in the electrolysis brine and the
elements Ta, W, Hf and Pt are obtained as filterable oxides, so
that essentially quantitative separation of the two element groups
can be carried out by filtration of the electrolysis brine.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a process for
electrochemical decomposition of superalloys, in particular
superalloy scrap, with the particular purpose of recovering rare
and valuable metals such as rhenium, platinum, tantalum and
hafnium.
[0003] 2. Brief Description of the Prior Art
[0004] Superalloys are high melting, high strength and extremely
wear-resistant alloys of a comparatively large number of metals,
which are used predominantly in turbine construction, especially
aircraft turbines. Their special properties are owed in part to the
addition of very rare and expensive elements, for example tantalum,
hafnium or even rhenium/platinum. Because of their tough nature, it
is difficult to recycle them, and to date they are not recycled
economically, after their service life.
[0005] This has led to irretrievable loss of these high value,
strategic raw materials with limited availability, owing to fusion
of the said superalloy scrap into normal steels, on the order of,
for example, up to 10 t/a (tons per year) of rhenium and 30 t/a of
tantalum. Just by itself, the rhenium quantity of 10 t/a
corresponds to approximately one third of the world's primary
production per year. Hence the lack of recycling constitutes a
waste of resources not only in economic terms but also in terms of
the "responsible care" concept adopted by the chemical industry. As
to tantalum, although it is not itself as rare as rhenium, it
nevertheless occurs naturally only to a very limited extent in the
form of workable ores. Additionally, appreciable quantities have
been obtained from Sn slag containing Ta, which originate
predominantly in Thailand and Malaysia. At any rate, because of the
explosive development of the electronics industry, requiring
constantly increasing demand for tantalum, the supply of this
raw-material is becoming ever weaker. Therefore, it is economically
and strategically sensible to recycle superalloys with Ta contents
of up to 8%.
[0006] While there are a number of pyro- and hydrometallurgical
approaches for recovering the metallic constituents of superalloys,
they are not economically suitable because of their cost-intensive
complexity or their time-consuming nature. For example, it is known
to melt superalloys under a protective-gas atmosphere, subsequently
spray the melt to form a finely divided powder, and then decompose
the powder by using a time-consuming treatment with inorganic
acids. It would also be conceivable to comminute superalloy-part
scrap after prior suitable embrittlement by elaborate grinding
processes, but this would be just as expensive. For, because of its
high strength, toughness and extraordinary wear resistance, such
superalloy scrap necessitates preparation processes/grinding
specially tailored/developed for these material classes. The actual
decomposition of the alloy is in turn carried out wet-chemically by
heat treatment in mineral acids of suitable concentration and
composition (see, for example, Potter et al., Eff. Technol.
Recycling Metal 1971, 35). In order to separate the Re from the
solutions containing multiple metals, solvent extraction combined
with sulphide precipitation reactions and electrodeposition
reactions may, for example, be used (see, for example, Churchwood
et al., J. Metals, September, 1963, 648).
[0007] A good summary of oxidative, pyrometallurgical and
hydrometallurgical is provided in an article by Kenworthy et al.
(Experimental Extraction of Strategic Components from S-816 Alloy
Scrap, Report of Investigations 5786, United States Department Of
The Interior, Bureau Of Mines, 1976), in which electrolytic
corrosion studies are also described. The report described
decomposition tests on the special example of S-816 scrap, a
Re/Ta-free Co-based alloy (40+%) with high proportions of Cr (20%)
and Ni (20%) as well as, inter alia, Fe, Nb, W and Mo in the 4%
range. The use of sulphuric acid as a corrosive electrolyte medium
at 7.times.10.sup.-5 Hz (polarity reversal every 4 hours) is in
this case presented as being best suited to this type of scrap.
Hydrated (Co, Ni, Fe) sulphate mixtures are subsequently
crystallised from the electrolyte solution at -20.degree. C. and
subjected to intermediate consecutive processing operations.
[0008] Further processes which relate to the decomposition of alloy
scrap by using electrochemical processes are:
[0009] U.S. Pat. No. 3,649,487: the high-melting metals (Cr/Mo/W)
contained in scrap of an Fe/Ni/Co/Cu-based alloy are first
thermally converted (by a melting process) into carbides, borides,
silicides, nitrides or phosphides by adding non-metallic compounds
of group III, IV or V, fused to form anodes, or connected as
anodes, and then subjected to anodic oxidation. In this case, Ni,
Co and Cu are cathodically deposited, whereas the high-melting
metals remain in the anode slime as, for example, carbides.
Regarding this recycling of Ni, Co or copper, there is a lack of
any information about current, current density, anodic/cathodic
current efficiency, precise electrolyte composition, completeness
of the separation, as well as estimable space-time yields or
information about economic viability.
[0010] An article by Venkatachalam et al. (J. Electrochem. Soc.
India, 1986, 35-2, 127) studies the effect of current density,
electrolyte concentration, electrolysis time and
alternating-current frequency on the effectiveness of dissolving Ni
when electrolysing nickel-based superalloy scrap in acids. In this
case, however, the lowest selected alternating-current frequency
was 25 Hz (screened range: 25-150 Hz).
[0011] According to WO 96/14440, a decomposition process, which is
based on anodic oxidation of the alloy in an electrolysis bath with
a protic, organic solvent component, is used in order to recover
the metallic constituents from superalloys. This patent
specification states that at most 10% water may be added to the
electrolyte solution, so that the process still functions according
to the invention (otherwise formation of a gel which is difficult
to process and passivation of the anode surface and therefore
termination of the electrolysis). The electrolytically obtained
filtration residue is processed, for example, thermally by
calcination after mixing into milk of lime, the calcination
product, for its part, being subsequently processed further by
customary hydrometallurgical separating operations.
SUMMARY OF THE INVENTION
[0012] The present invention therefore relates to a process for
decomposition of superalloys, wherein both electrodes of an
electrochemical cell are formed by the superalloy to be decomposed,
and the polarity of the electrolysis current is reversed with a
frequency of from 0.005 to 5 Hz, preferably from 0.08 to 2 Hz, and
particularly preferably from 0.01 to 1 Hz. In the context of the
present invention, superalloys are alloys which contain from 50 to
75 wt. % of nickel as the major component, respectively from 3 to
15 wt. % of at least one of the elements cobalt, chromium and
optionally aluminium, as well as from 1 to 10 wt. % of one or more
of the elements tantalum, niobium, tungsten, molybdenum, rhenium,
platinum and hafnium.
[0013] Such superalloys are not susceptible to decomposition by
means of direct-current electrolysis in aqueous solutions, since a
superficial passivation layer is formed after only a short
electrolysis time, which then brings the electrolysis current to a
standstill.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a schematic representation of the process from the
identification of the superalloy scrap (1), electrolytic dissolving
(2), suspension (3), filtration/washing (4) and further processing
variants (5) and (6) respectively in order to recover valuable
materials, in particular rhenium, platinum, tantalum and
hafnium.
[0015] FIG. 2 is a schematic representation of three variants for
the processing (6) of the filtrate (4.2) containing
Ni/Co/Cr/Al/part of the Re/majority of the Mo.
DETAILED DESCRIPTION OF THE INVENTION
[0016] It has been found that electrolytic decomposition can be
carried out very favourably and effectively if a very low-frequency
electrolysis current is used. Surprisingly, current efficiencies of
up to 150%, in general between 120 and 140%, have been found in
this case to be effective, which lead to the conclusion that a
chemical dissolving process is also taking place besides the
electrolytic dissolving. While the underlying mechanism of this
additional chemical dissolving process is not fully understood, it
is conceivable that, by evolution of gas, the passivation layer
becomes detached with the inclusion of metallic constituents, which
are then exposed to oxidation by acid attack, or that
boundary-layer effects or effects in conjunction with the build-up
and breakdown of boundary layers lead to the increased current
efficiency.
[0017] According to the invention, an inorganic acid is used as the
electrolyte, preferably hydrochloric acid, and particularly
preferably a hydrochloric acid solution with an HCl content of from
15 to 25 wt. %. Nevertheless, mixtures of hydrochloric acid and
sulphuric acid may also be used advantageously if subsequent stages
and refluxes are taken into account.
[0018] The electrolysis is advantageously conducted with an
electrolysis-current density of from 80 to 600 mA/cm.sup.2 of
cross-sectional area of the electrolysis cell. In this case, the
electrolysis voltage between the electrodes is between 2 and 6
volts, depending on the electrolyte conductivity, the current
density and the spacing of the electrodes. According to the
invention, the electrochemical decomposition is preferably carried
out at a constant electrolysis current. Preferably, the temperature
in the electrolysis cell is from 20 to 100.degree. C., and
particularly preferably from 60 to 80.degree. C.
[0019] The superalloy electrochemically decomposed according to the
invention is subsequently processed, in a manner which is known per
se, in order to recover the valuable materials, in particular
rhenium, platinum, tantalum and hafnium. This aspect of the
invention is further described with particular reference to the
drawings. This is represented schematically in the appended FIGS. 1
and 2. According to FIG. 1, the superalloy, which may contain the
elements rhenium, tantalum, hafnium, platinum, chromium,
molybdenum, tungsten, nickel and cobalt, is electrochemically
decomposed according to the invention (2); a suspension (3) is
produced, from which a filter residue (4.1), which contains the
elements tantalum, hafnium and platinum, as well as part of the
rhenium and a little molybdenum, is obtained after filtration and
optionally washing the filter residue (4). The elements nickel,
cobalt, chromium and aluminium, part of the rhenium and the
majority of the molybdenum are contained in the filtrate (4.2).
[0020] For further processing, the filter residue (4.1) is further
decomposed of oxidising leaching (5) by suspending in fully
deionised water, adding sodium hydroxide solution, heating to a
temperature of from 65 to 90.degree. C. supplementing with hydrogen
peroxide while stirring. The cooled suspension is filtered (5.1)
and the filter residue is washed. The filtrate (5.3), which
contains the tungsten, molybdenum and part of the rhenium and a
little Pt, can be separated further, in a manner which is known per
se, by means of strongly basic ion exchangers. The filter residue
(5.2) containing the valuable tantalum, hafnium and platinum is, if
platinum is present, processed further via hydrofluoric acid
decomposition (5.4) to solubilize the valuable tantalum/hafnium.
The residue of the HF decomposition (5.4) contains the valuable
platinum (5.5). The filtrate (5.6) contains the valuable
tantalum/hafnium, which can be separated further by extraction with
MIBK.
[0021] Three variants, which are explained in FIG. 2, are available
for the processing (6) of the filtrate (4.2) which has been
obtained from the filtration (4). According to variant 1 (6.1), the
filtrate (4.2) from FIG. 1 is sent through an ion exchanger (7.1)
and the rhenium is obtained as an eluate (8.1). From the raffinate
(9.1), the nickel/cobalt units can be separated (10.1) via a
solvent-extraction (SX) system.
[0022] According to variant 2 (6.2), the filtrate (4.2) is
subjected to fractional hydroxide precipitation (7.2); after
filtration (8.2), a residue (10.2) containing aluminium and
chromium is obtained and a filtrate (9.2), from which rhenium is
separated by means of an ion exchanger (11.2) and is recovered by
elution (12.2). The raffinate (13.2) consists of a nickel/cobalt
solution.
[0023] According to variant 3 (6.3) complete hydroxide
precipitation (7.3) is carried out; after filtration (8.3), the
hydroxide slime (10.3) which is obtained also contains nickel and
cobalt. The hydroxide slime can be reprocessed in the usual way
(11.3). From the filtrate (9.3) of the filtration (8.3), rhenium is
adsorbed by means of an ion exchanger (12.3) and is recovered by
elution (12.4).
EXAMPLE
[0024] 10.4 kg of dilute hydrochloric acid solution (18.5 wt. %)
are placed in a 15-litre electrolysis cell made of polypropylene.
Two titanium baskets filled with superalloy scrap, with a total
scrap content of 8.0 kg (composition, wt. %: 8.5 Ta, 3.1 Re, 5.8 W,
9.8 Co, 60.9 Ni, 4.9 Cr, 5.1 Al, 1.9 Mo) are used as the
electrodes. The electrode spacing is approximately 2 cm. The
electrolytic dissolving is carried out at 70.degree. C. by means of
a square-wave current at a frequency of 0.5 Hz, a current of 50
amperes and a resulting voltage of approximately 3 to 4 volts.
After an electrolysis time of 25 hours, the amount of scrap
detached or dissolved is 1.6 kg. The resulting suspension is
filtered and the residue (1) is washed with 0.63 kg of fully
deionised water.
[0025] The 0.422 kg of filtration residue (1) contains wt. %: 39.5
Ta.sub.2O.sub.5, 6.2 ReO.sub.2, 27.8 WO.sub.3, 1.6 MoO.sub.3 and 25
H.sub.2O. The filtrate is purified with the wash water and wt. %:
0.3 HReO.sub.4, 0.4 H.sub.2MoO.sub.4, 2.8 CoCl.sub.2, 17.6
NiCl.sub.2, 1.9 CrCl.sub.3, 3.3 AlCl.sub.3 and 0.2 HCl are found in
solution (1).
[0026] Processing of the Filtration Residue (1)
[0027] The wet filtration residue is suspended in 195 g of fully
deionised water in a 2-litre beaker while stirring, supplemented
with 160 g of 50% strength sodium hydroxide solution and heated to
80.degree. C. 41 g of 30% strength hydrogen peroxide solution are
then introduced. After 2 hours of stirring at 80.degree. C., the
suspension is cooled, filtered and the residue is washed with 0.370
kg of fully deionised water. The 0.222 kg of filtration residue (2)
contains wt. %: 74.9 Ta.sub.2O.sub.5, 0.1 ReO.sub.2, 1.0 WO.sub.3
and 23.0 H.sub.2O. The filtrate is purified with the wash water and
wt. %: 2.3 NaReO.sub.4, 10.6 Na.sub.2Wo.sub.4, 0.7
Na.sub.2MoO.sub.4 and 2.2 NaOH are found in solution (2).
[0028] Processing of the Filtration Residue (2)
[0029] Tungsten and rhenium are separated in a known manner by
means of strongly basic ion-exchange resins, and can thereafter be
sent to the further value chain as precursors for the production of
tungsten and rhenium products.
[0030] Processing of the Solution (1)
[0031] 4.3 kg of 50% strength sodium hydroxide solution are added
to the solution in a 20-litre stirred rector and thermally
regulated to 80.degree. C. After a reaction time of 2 hours, the
suspension is filtered and the residue is washed with 6.5 kg of
fully deionised water. The resulting 3.96 kg of filtration residue
(3) contains wt. %: 6 Al(OH).sub.3, 6.2 Co(OH).sub.2, 38.9
Ni(OH).sub.2, 3.9 Cr(OH).sub.3, 45 H.sub.2O. The filtrate is
purified with the wash water and wt. %: 6 Al(OH).sub.3, 6.2
Co(OH).sub.2, 38.9 Ni(OH).sub.2, 38.9 Ni(OH).sub.2, 3.9
Cr(OH).sub.3, 45 H.sub.2O are found in solution (3). The filtrate
is purified with the wash water and %: 0.2 NaReO.sub.4 and 0.3
Na.sub.2MoO.sub.4 are found in solution (3).
[0032] Processing of the Solution (3)
[0033] Molybdenum and rhenium are separated in a known manner by
means of strongly basic ion-exchange resins, and can thereafter be
used as precursors for the production of molybdenum and rhenium
products.
[0034] Processing of the Filtration Residue (3)
[0035] The filtration residue can be reprocessed in a known manner,
for example reducing melt to form Ni--Co alloys.
[0036] Although the invention has been described in detail in the
foregoing for the purpose of illustration, it is to be understood
that such detail is solely for that purpose and that variations can
be made therein by those skilled in the art without departing from
the spirit and scope of the invention except as it may be limited
by the claims.
* * * * *