CA1221548A

CA1221548A - Process for recovering metal values from alloy scraps

Info

Publication number: CA1221548A
Application number: CA000455726A
Authority: CA
Inventors: Osmo Vartiainen
Original assignee: Ov-Eng Oy
Current assignee: Ov-Eng Oy
Priority date: 1984-06-01
Filing date: 1984-06-01
Publication date: 1987-05-12

Abstract

ABSTRACT
A process for recovering metal values from Ni and Co based superalloy scraps containing Cr, W and Mo is disclosed. The process is a multistep pyrometallurgical oxidation process wherein the metal is successively oxidized by oxygen, Fe2O3 or both in a tiltable electric arc furnace with slag withdrawal after each oxidation step in order to obtain a substantially pure Ni/Co alloy melt.

Description

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The present inventioll relates to a process for recovering metal values from alloy scraps based on Ni, Co, or both, and, especially, superalloy scraps but also high alloy chromium bearing and corrosion resisting alloy scraps with considerable amounts of valuable metals. The scrap is becoming increasingly contaminated nowadays because of the higher content of alloys and tramp elements. The utility value of such scraps can be co~siderably increased by separating the valuable metals into marketable, techllically useful products.
Nickel and cobalt based superalloys typically contaill about 50 to 70 per cent nickel andtor cobalt, 15 to 30 per cent chromium, and the balance is made up of molybdenum, tungsten, columbium, titanium, aluminum, iron, manganese, silicon, and carbon. Contaminating metals may include copper, lead, zinc, and tin. Other contaminants include oil, moisture, rags, wood, earth, and corundum grinding debris.
Alloy scraps in the following forms are suitable for refining:
turnings, catalysts, uprunnings, grindings, sludges, muds, spills, off-grade multi-metals and ingots. Powder metallurgical products are normally not suitable. Table l gives the analyses of some typical superalloy scraps.
Table 1: Analysis of typical superalloy scraps Analysis Average Normal limits % %
Ni 39.3 1.5 - 63 Co 21.2 7 - 56 Cr 17.0 8 - 32 Mo 3.9 2 - 10 Ti 1.8 0.3 - 5 Al 1.7 0.1 - 6 Continued :~L2215~

Analysis Average Normal limits %
W 2.6 1 - 17.5 Cu 0.02 0.05 - 0.5 Ta 0.41 4.25 - 9 V 0.03 0.01 - 1.0 The recovery of metal values from the superalloy scraps contemplated herein has been tested so far mainly by using hydro- and pyrometallurgical methods.
The hydrometallurgical leaching and chemical purification methods are successful only with scrap in fine form such as drillings, turnings, etc.
Leaching of scrap in big pieces is slow, and is only possible on an industrial scale by the heap leaching method. This will not be practical for a long time.
In order to speed up the leaching, metallic superalloy scrap can be carburized at melting temperatures, and after cooling be crushed and ground after which it can be selectively leached so that the metals to be Tecovered can be hydrometallurgically refined by well-known methods.
Better results from the practical and economic points of view have been obtained by pyrometallurgical smelting methods in controlled conditions, but the practical industrial applications are very rare and they have been only partly successful.
The object of the present invention is to provide a process for refining the metal alloy product scraps to produce Ni/Co-alloys with the required purities and to recover the middle products for further purification by e.g.
well-known metallurgical and chemical methods choosing the most suitable one for each type of slag.

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According to the present invention there is no~ provided a process for recovering metal values from alloy scraps by a) first oxidizing the metal melt to such an extent that tungsten will remain substantially unoxidized, maintaining a slag basicity (ratio of the basic components to the said ones in slag) of 2-2.5, c~ld removing a chromium containing slag so formed;
b) further oxidizing the metal melt from step a) to such an extent that molybdenum will remain substantially unoxidized, a~d removing a chromium and tungsten containing slag so formed;
c) further oxidizing the metal melt from step b) at a slag basicity of 1.5 - 2.0 to bring the chromium content thereof to 0.5 - 1%, and removing the slag so formed;
d) repeatedly oxidizing the metal melt from step c) in admixture with a sodium containing slagging agent to obtain a slag basicity of 2.5 - 3, and remo-ving a molybdenum containing slag after each treatment.
The principal object of the above-mentioned refining steps is to bring the purity of the Ni/Co-melt to a required level of less than 0.05% of comp-onents such as Cr, ~lo, ~, etc.
In drawings illustrating the invention:-Figure 1 illustrates graphically the relative oxygen affinities of some important components of the superalloy scrap; and Figure 2 illustrates a phase diagram of the system Na20-CaO-SiO2.
The present process is based mainly on pyrometallurgical treatment of scrap resulting in the separation of Ni, Co, and Fe (also Cu) from other metals which, again, are obtained concentrated in intermediate products. This isa melt metallurgical process and the separation of metals is executed by slagging.

5~8 To separate metals selectively during the smelting process, the following factors have to be taken into consideration:
- oxidizing conditions, partial pressure of oxygen - reducing conditions - temperature - solubility of metals in iron - reactions betwecn metals, oxidizing/reducing reactions - partial pressure of metals - content and properties of slag Slag formation is based on the oxidation of the metals in accordance with the following general formula Me + O = MeO
the equilibrium constant being /MeO/
K = /Me/ . /O/
Becausc the oxidation order is affected by the proportional quantities of metals, some changes between elements in the affinity series can happen or the reactions do not go to completion if the ratio differences of metals are very great. The reactions between two metals then happen in principle as follows:
MelO + Me2 = Me2O Me K /Me2O/ . /Mel/
/Me10/ . /Me2/

The affinity of general alloying metals to oxygen in the temperature range 1500 - 1700C follows the order (Ca has the biggest affinity): Cu - Ni -Co - Mo - W - Fe - Cr - Nb - Ta - Mn - V - Si - Ti - Mg - Al - Ca.

~ ~iX1~48 Relative affinities of the constituents in question are also shown graphically in Figure 1.
Solubility of various metals in iron is a significant phenomenon Oll this occasion and the metals can thus be divided into the following groups:
1 completely soluble Al, Cu, Mn, Ni, Co, Si, Sb, Ti, Ce

2 partially soluble Cr, V, Mo, W, Sn, Pt

3 practically insoluble Pb, Ag, Bi

4 metalloids, partially soluble C, S, P, O, H, ~, As, Se gaseous st~te at temperatures of steelmaking Na, Li, Mg, Ca, Zn, Cd, Hg Aiming at clean Ni-Co-Fe alloys it is possible to oxidize such alloying metals as Cr, Mn, V, Ti, and to get them to the slag. W and Mo require much more closely controlled conditions and normally multistep purification treatment.
To control the copper content of the final product it is obvious that it has to be looked after selecting the raw materials to be refined.
The variation of free energy determines in which direction the processes occur in systems with constant temperature. Pressure is termed the isobaric isothermal potential, or thermodynamic potential (see Table 2 below).
One can note from Table 2 that the refining problems will be concentrated mainly on separating Cr, Mo, Ti, Al, and W from the alloy raw materials in question. Also Ta and V in certain steel scrap types have to be removed in addition to the aforesaid.

~'~,2~8 As has been shown theoretically, the affinities of Al and Ti as well as V and Cr to oxygen are considerable compared to those of ~lo and W.

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Oxidation of chromi Oxidation and reduction of Cr in melts has been researched very thoroughly in the literature. In the practice of the steelmaking technology the main problems are experienced frequently. In the following list there are given some features to remove Cr from melt by oxidizing:
- The effect of temperature is quite strong. At high temperatures a large amount of oxygen can coexist with chromium in the bath without reacting with it. Thus the process must be carried out at the least admissible temperature.
_ One has to take into account that silicon and manganese prevent chromium from being oxidized and have a reducing effect on chromium oxide.
- When the content of Cr203 of the slag exceeds about S% the slag viscosity increases and thus impairs the process of heat transfer. It is important to adjust slag conditions carefully during the process when removing the chromium.
- When removing chromium from metal to slag dephosphorization is also carried out at the same time if that comes in question.
Oxidation of molybdenum Because the affinity of molybdenum to oxygen is small compared to iron the elements like C, P, Mn, Si, Cr and Al have to be below certain limits before demolybdenization is possible. The oxygen content in molten steel has to exceed a certain level.
To get molybdenum oxidized sufficiently one has to take into account Mo-equilibrium between slag and molten steel. This means that the demolybdenization process with slag removal has to be repeated several times.
As an oxidizing medium not only oxygen can be used but also iron :12~ 8 oxides or alkalimetaloxides which form double salts of molybden~lm, thus ~3O
being in the slag as ~I2 and/or e.g. Na2O . 5~1oO3, Ca~1OO4, etc.
Oxidation_of tungsten Tungsten can be oxidized at the same time as molybden~un is oxidized step by step. There is a certain selectivity between ~lo and W ~hen the oxidation is made in several steps at relatively low temperatures.
Oxidation of vanadium Vanadium is easily oxidized to V2O3 and V2O5 with high recove~ies in the slag leaving to the steel normally less than 0.05%. Vanadium can then be chemically extracted from the slag.
Oxidation of titanium The affinity of titanium to oxygen is very intensive so slagging of titanium should not cause any difficulties when handling the metal compositions in question.
EXAMPLE: Refining of superalloy scraps The description is based on theoretical, laboratory and practical scale tests executed with superalloy scrap of various types. The scrap material used in the practical tests was taken directly from cases, barrels, etc. into which the scrap was sorted and packed by scrap receivers. The scrap was not pre-treated by, for instance, washing before melting.
3elting equipment Refining was done as a batch process in a conventional three-electrode (graphite) electric arc furnace which could be emptied by tilting it so that the slag and the metal was easily obtained separately. The furnace was able to reach a melting temperature of about 1800C.

_ g _ ~l ~Z~5~8 Treatment of metal melt and slags The process requires the treatment and removal of a number of different slags that nlust be kept apart. It must be possible to quickly remove the metal melt from the furnace and lead it into the casting machine after refining.
The tilting mechanism of the furnace was so constructed that the furnace coul-i easily and quickly be completely emptied or that the furnace could remain tilted at any angle for the removal of slag.
Chromium and tungsten removal _ Superalloy scraps, containing W and Mo in addition to Cr, were refined in two stages, the first concentrating on the removal of Cr and W and the other on Mo removal, each stage again having two or three different treatments.
In the first treatment of the first stage, after down smelting, the slagging material is added together with the oxidizer to the scrap (in a 5-tonne furnace 1 1/2 - 2 tonnes), Oxidization can be achieved using either oxygen above, iron oxide alone, or a combination of oxygen and iron oxide.
In the first treatment, the Cr content is decreased to such a level that W does not yet oxidize, i.e. some 10% when the initial content was 20 - 30%.
The amount of Cr oxidizer is calculated according to Fe203 + 2Cr = Cr203 + 2Fe at an oxygen utilisation ratio of 95 - 100%. If reduced iron is not desired in the metal product, the Fe203 can be exchanged for liquid oxygen. In the executed tests, the amount of oxygen gas was approximately 25% of the total oxygen amount. When the oxygen is fed under the surface of the melt with a lance, a good mix of melt and slagging material is achieved at the same time as ~L22~i4~

the temperature of the melt is ~ept sufficiently high Wit}l the aid of tile electrodes maintaining the electric arc.
Lime and silica sand are used as slag formers to create a slag basicity of 2 - 2.5. Fluorspar may be used to decrease the slag's viscosity. The slag's Cr content with tlle above treatment is 35 - 40%.
In the second treatment of the first stage, the Cr content of the metal phase is decreased to about 1/4 of the level of the first part, i.e.
2 - 3% by continuing the oxidation with oxygen and Fe203 for 5 - 7 minutes adding also slagging material as in the first part. The fluidity of the slag can be improved by a small amount of fluorspar.
In this treatment, the W content decreases to a level of some 2%, the initial content being some 8%. The W oxidation to W02 has to be calculated at an oxygen utilisation ratio of some 95% when defining the amount of oxidizer for this treatment. The Mo does not oxidize at this stage.
The Cr content can be expected to be some 20% and the W content some 7% in the slag of the second treatment.
In the third treatment of the first stage, the Cr content of the metal phase can be reduced to less than 0.05% by continuing the oxidation for some five minutes using both oxygen and Fe203 and adding slagging material, i.e. lime and silica sand and the basicity being some 1.5 - 2Ø The W
content of the metal phase is then 0.2 - 0.5% and the Cr content of the slag 12 - 15% and the W content 4 - 5% in this treatment. When the Cr content of the metal phase is less than one per cent, the ~lo starts oxidizing, e.g. in this work from the level of 1.5% to about 1.0%.
If a sufficiently low Cr content has not achieved with these three treatments, it will be achieved in connection with the Mo removal. In fact ~2Z~548 Mo removal can start wilen the Cr content is 0.5 - 1% which may be achieved with two treatments in the first stage when the process has been well tested.
W removal from the metal phase continues during the treatments of the second stage. In the executed tests a Cr content of 0.01% was achieved while the lowest ~ content achieved was 0.08%. The W content of the metal phase is not considerably dependent on the initial W level of the scrap.
Molybdenum removal ~ lo removal can thus be started when the Cr content is less than one per cent. Mo removal into the slag requires a slag containing sodium. The recommended refining slag composition is shown in a phase diagram of Na2O - CaO -SiO2 in Figure 2.
In order to remove Mo slagging components silica sand, lime and soda ash are added so that the basicity is 2.5 - 3 and Na2O : CaO = 2 :I.
Oxidization is made by using Fe2O3 and/or oxygen.
In the first treatment the ~lo content of the metal phase is decreased to 0.2 - 0.5% from some 1.5%. Slagging is carried out and the treatment repeated as before. After the second treatment, the Mo content of the metal phase could reach a level of less than 0.05%, even when the Mo content of the scrap initially was considerably higher than the above-mentioned 1.5%.
In the Mo removal stage the temperature of the melt should be as low as possible. It should, however, be sufficiently high to keep up a good fluidity, i.e. in practice 1450 - 1500C.
The total amount of slagging material components used in the Mo removal is 3 - 4% of the molten scrap amount, and the amount of soda ash is 5 - 6% accordingly9 while the Fe2O3 pellet requirement concerning oxygen 54~3 content is 75% of the total oxygen need. The remaining oxygen is fed in the form of gaseous oxygen.
In order to speed up the chemical reactions, the melt should be effectively mixed. This is achieved by feeding oxygen under the surface of the melt, and, with the aid of the electric arc in connection with regulation of the melt temperature. A more effective method of achieving a good mix and, at the same time, quick and complete chemical reactions, is to inject the additives directly into the melt while mixing mechanically or by using indirect agitating methods.
Results of the refining process -The following gives the results of, and the deductions based on, the tests on a practical scale made in a five-tonne electric arc furnace using scrap qualities the analyses of which varied as follows: Ni 8 - 21%, Co 19 - 49%, Cr 18 - 30%, W 2.5 - 8%, Mo 1.3 - 3%, Fe q - 30%. The scrap lots were either totally in the form of turnings or totally in the form of coarse cast solids.
The tests were concentrated only on refining the metal phase.
The refining of metals separated into the slag was not studied. The results well confirmed the preceding theoretical studies.
2~ 1 The Ni + Co content in the metal phase increased 1.40 - 1.95-fold from the initial level and to 92.8% at its best when, at the same time, the Fe content decreased from 19% to 6.8%. In the tests Fe203 pellets and oxygen were used as oxidizers.
2 The ~i recovery into the final metal phase varied between 81% and 95%.
3 The Co recovery into the final product was between 70 and 80%.

~L2~5~3 4 Cr losses in the final metal phase product varied between 0.02 -1.5%, the Cr content being 0.01 - 0.04%.
S 0.7 - 2.6% of the total W amowlt remained in the metal phase with about 0.08% W.
6 Mo losses in the metal phase were about 0.4 - 1%, the Mo content being 0.03 - 0.05%.
7 The total treatment time, from starting the charging the furnace to casting of the metal phase, was between two and three hours.
8 The Cr content of slags from the various treatments was - first slag 35 - 40% Cr - second slag 18 - 20% Cr - third slag 13 - 15% Cr 9 The W content of slags was:
- first slag about 0.3% W
- second slag 5 - 7% W
- third slag 3 - 4.5% W
- fourth slag 1.5 - 4% W
The Mo content of slags was:
- Cr ~ W removal slags very low - first Mo-treatment slag 1.5 - 1.8% ~lo - second Mo-treatment slag 2 - 3% Mo

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for recovering metal values from nickel or cobalt based alloy scraps containing at least one element selected from the group comprising chromium, tungsten, vanadium, iron and molybdenum by mixing a melt of the scrap with oxidizing and slagging agents and separating slags from molten metal, comprising:-(a) first oxidizing the metal melt to such an extent that tungsten will remain substantially unoxidized, maintaining a slag basicity of 2 - 2.5, and removing a chromium containing slag so formed;
(b) further oxidizing tile metal melt from step (a) to such an extent that molybdenum will remain substantially unoxidized, and removing a chromium and tungsten containing slag so formed;
(c) further oxidizing the metal melt from step (b) at a slag basicity of 1.5 - 2.0 to bring the chromium content thereof to 0.5 - 1%, and removing the slag so formed;
(d) repeatedly oxidizing the metal melt from step (c) in admixture with a sodium containing slagging agent to obtain a slag basicity of 2.5 - 3, and removing a molybdenum containing slag after each treatment in order to produce a substantially pure alloy melt of cobalt, nickel, or both.

2. The process of claim 1, wherein oxidation and mixing of the metal melt is effected by injecting oxygen under the surface of the melt.

3. The process of claim 1, wherein oxidation of the metal melt is effected by mixing ferric oxide with the melt.

4. The process of claim 1, or 2,or 3 wherein the smelting and oxidation of the scrap metal is carried out in an electric arc furnace which is tilted for removing the slag after each step.

5. The process of claim 1, or 2, or 3, wherein sufficient sodium containing slagging agent is added in step (d) that the ratio Na2O :
CaO in the slag is about 2 : 1.

6. The process of claim 1, or 2, or 3, comprising maintaining the temperature of the melt at 1450 - 1500°C.

7. The process of claim 1, or 2, or 3, comprising injecting the oxidizing and slagging agents into the melt and mechanically mixing the melt.

8. The process of claim 1, or 2, or 3, comprising repeatedly and selectively oxidizing and slagging a Cr and Ni based alloy scrap.

9. The process of claim 1, or 2, or 3, comprising oxidizing and slagging the alloy scrap to such an extent that the alloy melt from step (d) contains less than 0.05% of each of Cr, Mo, and W.

10. A process for selectively recovering metal values from nickel or cobalt based alloy scrap containing chromium and tungsten, by mixing a melt of the scrap with oxidizing and slagging agents and separating slag from molten metal, comprising:
(a) first oxidizing the metal melt to such an extent that tungsten will remain substantially unoxisized, maintaining a slag basicity of 2-2.5, and removing a chromium containing slag so formed;
(b) further oxidizing the metal melt from step (a) to such an extent that tungsten passes into a slag and removing a slag containing chromium and tungsten so formed;
(c) further oxidizing the metal melt from step (b) at a slag basicity of 1.5-2.0 to bring the chromium content thereof to 0.5-1% and removing the slag so formed;
(d) repeatedly oxidizing the metal melt from step (c) in admixture with a sodium containing slagging agent to obtain a slag basicity of 2.5-3, and removing a slag after each treatment in order to produce a substantially pure alloy melt of cobalt, nickel, or both.

11. The process of claim 10, wherein molybdenum is present in the alloy scrap, and wherein such molybdenum passes into the slags produced in the oxidizing process of step (d).

12. The process of claim 10, in which one or more of titanium, aluminum, tantalum and vanadium is also present in the alloy scrap and is removed as a component of the slag produced by oxidizing.