This is a post refereed version of the paper published in:

Separation and Purification Techniques

Samuel K Mamo, Mathieu Elie, Mark G. Baron, Andrew M Simons, Jose

Gonzalez-Rodriguez* (2019) Leaching kinetics, separation, and recovery of

rhenium and component metals from CMSX-4 superalloys using

hydrometallurgical processes, Separation and Purification Techniques 212,

150-160.

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Leaching kinetics, separation, and recovery of rhenium and component metals from CMSX-4 superalloys using hydrometallurgical processes

Samuel K Mamo, Mathieu Elie, Mark G. Baron, Andrew M Simons, Jose Gonzalez-Rodriguez*

School of Chemistry, University of Lincoln, Joseph Banks Laboratories, Green Lane, Lincoln, Lincolnshire LN6 7DL, United Kingdom, email: jgonzalezrodriguez@lincoln.ac.uk, Telf: +441522886878.

Abstract

A study to leach and recycle rhenium from CMSX-4, a second generation ultrahigh-strength single

crystal nickel-based superalloy, containing 3% rhenium is presented. Experimental factors involved

in the leaching process of rhenium from CMSX-4 using aqua-regia solution have been investigated

and reported. Experimental factors such as concentration of aqua-regia, the use of sonication,

leaching period, solid-to-liquid ratio and stirring speed have been optimised and the leaching

kinetics of rhenium and other component metals of superalloy CMSX-4 have been studied.

Increasing aqua-regia concentration to 100% (v/v) resulted in more than 30% gain in recovery

which can be attributed to the increased availability of leaching reagents. The leaching kinetics of

rhenium from superalloy CMSX-4 in 100% aqua-regia solution fit into a chemical reaction

controlled kinetics model for the first period of leaching. For the leaching period after 480

minutes, the leaching kinetics of rhenium fits a diffusion through the product layer model.

Application of ultrasonic waves to the leaching process has proven useful to slightly increase yield.

Given the small increase, however, sonication may not be economically feasible for recovery of

rhenium on an industrial scale. Precipitation of other metals away from the rhenium rich solution

has also been explored to prepare the solution for a downstream rhenium recovery process. It was

found that that two unique precipitates can be recovered from the solution. The first precipitate,

recovered at pH 5.05, has been determined to be mainly composed of the oxides of aluminium,

chromium, molybdenum, and titanium. A green precipitate of mixed hydroxides of cobalt and

nickel (MHP) which has commercial value can then be recovered at pH 7.0 leaving a rhenium

enriched solution for further processing.

Keywords: CMSX-4; superalloy; leaching kinetics; rhenium; recycling

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1. Introduction

Because of its high melting point, rhenium is added to superalloys to raise the operating

temperature of turbine blades in aircraft and gas turbine engines (1–3). Superalloys exhibit high

mechanical strength, good surface stability, and resistance to thermal creep deformation and

oxidation. They are categorised as nickel based, iron based, and cobalt based (4). Nickel-based

superalloys typically constitute 40–50% of the total weight of an aircraft engine and are used most

extensively in the combustion and turbine sections of the engine where elevated temperatures are

maintained during operation. These superalloys contain up to 40% of the total weight of a

combination of other elements such as rhenium, chromium, cobalt, tungsten, tantalum,

molybdenum, hafnium, titanium, and aluminum (5). The superalloy sector accounts for 80%

rhenium use with an average annual rise in demand of 5% expected in the coming decades (1,6).

In addition to its production as by product of molybdenite smelters, rhenium can be recycled from

superalloy scraps and recycled rhenium has the same quality as the mined rhenium (7–9). The high

rhenium content (3 – 6% (w/w)) of second and third generation superalloys makes them highly

attractive rhenium recycling sources (4,6). CMSX-4 is a second generation ultrahigh-strength single

crystal nickel-based superalloy which contains 3% rhenium added as a solid solution strengthening

component (10). CMSX-4 superalloy has been successfully used in numerous aerospace and

industrial gas turbine applications and its scarps are widely available in the market (4,11).

Recycling rate of rhenium reached more than 50% due to its scarce availability and the growing

demand for rhenium containing superalloys in high-tech engines and turbines (1,12). This interest

is aided by the fact that many governments consider rhenium as an element of strategic military

importance, and are investing in recycling research to create a secure rhenium resource (13).

Modern methods which are under development intended for the isolation of rhenium from scraps

suggest increasing the purity and range of the compounds thereof in comparison with the

production from primary raw materials (8,14).

From an operational point of view the industrial recycling method chosen to reprocess this metal

can be critical for obtaining the desired purity but also from an energy-saving and environmental

point of view. Pyrometallurgical recycling methods have been previously employed for recycling

rhenium from spent platinum-rhenium catalyst (15), molybdenum-rhenium alloys (16), superalloys

(17), and rhenium-tungsten wire scrap (1). Nonetheless, pyrometallurgical recycling processes are

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complex and expensive to maintain, and have several drawbacks including the emission of gases,

high energy demand, low purity of final product, and up to 20% loss of the critical alloying

elements during the re-melting process (18,19). As an alternative to this solution,

hydrometallurgical processes are generally considered to be more environmentally friendly and

consume less energy (20).

Hydrometallurgical processes for rhenium recovery from copper industrial waste (21,22), spent

reforming catalysts (9), superalloys (11), and other multicomponent refractory alloys (8) have been

reported in literature. In general, hydrometallurgical rhenium recovery processes involve alkaline

or acid leaching and/or electro-decomposition of the scrap or waste followed by separation using

solvent extraction (6,14,23), ion exchange (12,24), adsorption (25), and precipitation (26).

Srivastava et al., reported HCl leaching of the base and thermal barrier alloying elements from

CMSX-4 superalloy followed by extraction of Re from the residue using electro-generated Cl2 in HCl

solution. Rhenium was quantitatively recovered from the resulting HCl solution using mixed-

phosphine oxide solvents (6). Truong et al., also employed tributyl phosphate (TBP) for selective

recovery of Re from a synthetic HCl solution containing rhenium, molybdenum and vanadium (19).

However, both methods were unable to quantitatively separate Re and Mo from the same leach

solution. However, these methods are in laboratory scale with limited scalability due to

complicated optimisation of the solvent extraction procedures, environmental concerns from the

waste solvent, fouling and degradation of ion-exchange columns, and inefficient trapping of

rhenium by the sorbents.

One of the main drawbacks of using hydrometallurgical techniques to extract rhenium from

superalloys is the long leaching times required to dissolve rhenium. This is due to the monocrystal

nature of the superalloy coupled with the inability to grind the scrap as would usually be done in

hydrometallurgical plants. Sonication is a potential way to accelerate leaching rates using

ultrasound waves (27). During the process of sonication, agitation of particles through ultrasound

waves of high frequency is used to speed up the dissolution of solids. The strength of agitation

increases as the frequency of ultrasound waves increases leading to the continuous formation and

implosion of vacuum bubbles, a process known as cavitation. These cavitation process together

with micro-streaming and degassing processes lead to the disruption of molecular interactions and

dismantling of clumps of particles resulting in an improved dissolution of solids (28). Water bath

sonicators or probe sonicators can be used based on the scale of dissolution aimed. Some of the

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advantages industrial applications of sonication claimed by some authors include reduced reagent

consumption, improved metal recoveries, and enhanced dissolution (27).

Here in we present a simple, scalable, organic solvent free, and cheap alternative for industrial

scale separation and recovery of rhenium from CMSX-4 superalloy. Different experimental factors

involved in the dissolution process of CMSX-4 with aqua-regia and the leaching of rhenium have

been reported. These experimental factors such as concentration of aqua-regia, sonication,

leaching period, solid-to-liquid ratio and stirring speed have been optimised and the leaching

kinetics of rhenium and the other component metals of superalloy CMSX-4 have been studied.

Precipitation of the other component metals away from the rhenium rich solution has also been

explored to prepare the solution for a downstream rhenium recovery process.

2. Experimental

2.1. Chemicals and Reagents

Aqua-regia leach solutions (20%-100%) were prepared using deionised water, laboratory reagent

grade S.G 1.18 (~37%) hydrochloric acid, and laboratory reagent grade nitric acid S.G 1.42 (70%).

Both acids were purchased from Fisher Scientific (UK) and used as leaching solution for pure

metals and CMSX-4 superalloy scrap. For precipitation separation of the superalloy component

metals from the leach solution, a 50% sodium hydroxide (w/w) was prepared. Sodium hydroxide

was purchased from Fisher Scientific (Fisher Scientific, UK).

For ICP-OES measurements, a periodic table mix certified reference material of elemental

standards was purchased from Fluka Analytical (Sigma-Aldrich GmbH, Switzerland).

2.2. Apparatus and Instruments

During aqua-regia leaching of CMSX-4 superalloy scrap (Select Alloys and Materials Ltd, UK), and

during precipitation separation experiments, continuous stirring of these reaction mixtures was

performed using a stirring bar on a magnetic stirrer hot plates (Stuart Scientific, UK). For leaching

experiments involving continuous and timed sonication the samples were kept in an ultrasonic

bath (VWR international, Belgium). All of the leaching and precipitation reactions studies were

conducted in KIMBLE™ value ware KIMAX® boro 3.3 glass beakers of various volumes. For

continuous follow up of the pH of the leach solutions PH-100ATC pH meter (Voltcraft, Taiwan) was

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used, and for monitoring the pH of the leach solution during precipitation separation experiments

the more robust and accurate Mettler Toledo® InLab-Pro pH probe and meter (Mettler Toledo ,

UK) was used for pH monitoring. PTFE membrane filters of 0.45 μm pore size (VWR international,

USA) were used to filter the leach and pregnant solutions before being diluted for elemental

analysis.

Inductively coupled plasma-optical emission spectrometer Thermo Scientific™ iCAP™ 7000 ICP-OES

Analyser (Thermo Scientific, UK) was used for quantitative elemental analysis of the leach and

precipitation.

The emission wavelength and the slit width was set up for multi-element analysis in radial

measure mode for each element as follows; Na (589.592 nm, H), Ti (334.941 nm, H), Hf (339.980

nm, H), Ta (268.517 nm, H), Mo (202.030 nm, L), Re (227.525 nm, L), Co (228.616 nm, L), Ni

(221.647 nm, L), and Al (167.079 nm, L). The radial measure mode was selected because it is not

as sensitive to matrix interference as the highly sensitive axial mode. Multi element calibration

standards of concentration 0.1, 1.0, 10.0, and 100 mg L–1 were prepared from periodic table mix

certified reference material of elemental standards.

2.3. Optimisation of aqua-regia leaching of superalloy CMSX-4

To optimise the leaching process the concentration of aqua-regia solution, leaching time, solid-to-

liquid ratio, stirring speed, and sonication time have been investigated as experimental factors

that can influence the recovery of rhenium from superalloy CMSX-4. The CMSX-4 superalloy

turbine blade scraps used in all of the superalloy leaching optimisation experiments were obtained

in smaller sized scraps and their composition is presented in Table 1.

Table 1. Composition of superalloy CMSX-4

Metal Cr Co Mo W Ta Al Ti Re Hf Ni

%(w/w) 5.7 11.0 0.4 5.2 5.6 5.2 0.7 3.0 0.10 Bal

Leaching experiments were performed in a partially closed 500 mL PYREX® glass beakers in a well-

ventilated fume hood. Aqua-regia solutions at 100%, 60%, and 20% (v/v) in deionised water were

used to leach superalloy scraps. Timed samples were taken throughout the leaching period where

100 μL aliquots were taken from the leach liquor, diluted to 100 mL, and analysed using ICP-OES.

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Leach curves were plotted using percentage recoveries of rhenium and the other component

metals calculated using equation 1 below.

%Recovery= (Concentrationof metal (g L−1 ))×(LeachingVolume (L ))

(Amount ofmetal∈the superalloy (% ww ))×(massof the superalloy (g ))

×100

(1)

Leaching of the superalloy scrap samples were performed with and without sonication to

investigate if an increased rhenium leaching rate and recovery can be gained by application of

external ultrasonic waves. The recovery of rhenium has been monitored until no further significant

increase in recovery was observed.

The leaching of rhenium from superalloy scraps in 100% (v/v) aqua-regia solution at solid-to-liquid

ratios of 25 g L–1, 65 g L–1, and 100 g L–1 were investigated to study effect of solid-to-liquid ratio on

leaching rate and recovery of rhenium. All leaching without sonication experiments have been

performed at constant stirring throughout the leach period in order to harmonise transport effects

that might alter results.

To investigate the effect of agitation speed on the recovery of rhenium the stirring speed was

varied from 100 to 800 rpm (revolutions per minute) while keeping the leaching period, the solid-

to-liquid ratio, and the concentration of aqua-regia constant.

2.4. Precipitation separation of component metals of superalloy CMSX-4

The optimised leaching conditions were used to leach the superalloy CMSX-4 turbine blade scraps

to generate the leach liquor for the precipitation separation experiments. 1000 mL of the 100%

aqua-regia solution in a partially closed 2000 mL PYREX® glass conical flask was used to leach the

superalloy scraps for 6000 minutes of leaching without sonication and with constant stirring. At

the end of the leaching time insoluble solids were removed by filtration. All bulk leaching

experiments and filtration of the resulting leach liquor were carried out in a well-ventilated fume

hood to prevent emission of effluents from aqua-regia solutions.

Precipitation of the superalloy component metals from the leach liquor was performed by raising

the pH of the stirred solution through continuous addition of 50% (w/v) sodium hydroxide. The pH

of the solution was constantly monitored using a pH meter and sampled constantly as the pH was

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raised. Samples were withdrawn using 5.0 mL plastic syringe at selected pH points in the range of

0.0 to 8.0. The sampled solutions were filtered through 0.45 μm filter and 1.0 mL portion of it was

diluted to 25 mL and analysed using ICP-OES. At each sample point the amount of caustic solution

consumed was monitored, and the dilution factor was calculated to plot the pH versus percentage

of metal species in solution for each of the component metals of superalloy CMSX-4. To

characterise the composition of the precipitates observed at each precipitation stage, the

precipitates were separated from the pregnant solution at particular pH levels. The solution was

filtered through a Whatmann 3 filter paper to separate the precipitates at each stage of

precipitation, and the rhenium rich filtrate was kept for further clean up and separation

procedures. The composition of the precipitates from the caustic precipitation separation were

also analysed using ICP-OES by re-dissolving them either in deionised water or in acidified water

depending on the pH of their precipitation. The residue obtained at the end of the leaching period

was filtered and air dried before analysis. Only XRF analysis was conducted on this residue as it

was insoluble over a wide pH range making the ICP-OES analysis impractical.

3. Results and discussion

3.1. Effect of aqua regia concentration and sonication on rhenium recovery

The rate of rhenium leaching from superalloy CMSX-4 can be accelerated with the application of

ultrasonic waves with a 7.8% increase in rhenium recovery compared to same results without

sonication in 100% aqua-regia. Figure 1 shows the increase in recovery of rhenium when CMSX-4 is

leached in all concentrations of aqua-regia leach solution with continuous sonication compared to

the leaching without the application of sonication.

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Figure 1. Effect of continuous sonication and aqua regia concentration (inset) on the recovery of rhenium

over time from superalloy CMSX-4, using solid-to-liquid ratio of 65 g L-1 and steering speed of 200 rpm for

leaching without sonication.

These increases in rhenium recoveries can be attributed to the micro-streaming of the reagents

through the pores created between the crystals of CMSX-4 by the cavitation effect of sonication. In

addition to the cavitation effect, sonication continuously removes the patina that could deposit on

the leach surface. This removal of surface layer improves the extent of leaching by exposing the

unleached surfaces to fresh leach reagent.

The histograms in the inset of Figure 1 show the extent of the enhanced rhenium recoveries with

the application of continuous sonication for the all studied concentrations of aqua regia. While

sonication has an effect in the rhenium leaching at all times, this seems to increase with the

dissolution time. These results also demonstrate that the main observed increase in the rate of

rhenium leaching was predominantly due to the increasing aqua-regia concentration with a

smaller increase due to the application of sonication.

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3.2. Effect of aqua regia concentration and leaching time on rhenium recovery

As previously discussed, the concentration of aqua-regia solution plays a significant role in the

recovery of rhenium from the superalloy CMSX-4. To study the kinetics of the leaching process

optimum leaching conditions were selected and the leaching period was extended. Considering

the economic cost of using sonication and the low increase in percentage recovery of rhenium by

using sonication (i.e., less than 10% after 420 minutes) the optimum leaching conditions did not

involve the use of sonication. As shown in Figure 2, the slope of the rhenium leach curve changes

for all aqua regia concentrations as the leaching time progresses suggesting a change in the rate

limiting mechanism during leaching. For the first 480 minutes of leaching when using a 100% aqua

regia solution, a rapid rise in rhenium recovery was observed compared to the other two leaching

solutions. This demonstrates that the availability of the acids present in the leaching solution

increases the rate of leaching at the initial stages of the leaching process. After this fast dissolution

rate, the slope becomes shallow as the reaction continues at a significantly slower rate. The slope

of the leach curve for the 60% aqua regia solution shows a steady rate compared to the leach

curves for 100% and 20% aqua-regia solution. The slow rate of rhenium dissolution shown by the

20% aqua regia solution can be explained due to the limited availability of reagents for speeding

up the leaching reaction.

An extended leaching period enhances rhenium recovery at lower aqua-regia concentrations.

Therefore a compromise has to be made between the amount of acid used and the time spent

during the dissolution process in order to optimize cost-effectiveness and time efficiency.

10

Figure 2. Effect of aqua-regia concentration on rhenium recovery over time from superalloy CMSX-4 with

no sonication, 200 rpm steering speed, and 65 g L-1 solid-to-liquid ratio.

3.3. Effect of solid-to-liquid ratio on rhenium recovery

The change in the leaching rate and recovery of rhenium with changing solid-to-liquid ratio was

investigated using 5, 15, 25, 65, and 105 g L–1 ratios of the superalloy turbine blade scrap in 100%

aqua regia leaching solutions. As shown in Figure 2, the leaching rate slowed significantly after 480

minutes of leaching in 100% aqua-regia solution and hence the leaching was conducted over a

shorter period. Nonetheless, a leaching time of 2880 minutes was selected to guarantee the

completion of the leach reactions at the surface of the superalloy scrap. Figure 3 shows that the

rate of leaching and the maximum percentage recovery of rhenium increase with decreasing solid-

liquid ratio. The leach curves in Figure 3 also reached the plateau recovery value at faster rate with

decreasing solid-to-liquid ratio. The inset in Figure 3 also shows the increase in leaching rate and

percentage recovery of rhenium with decreasing solid-to-liquid ratios at 2880 minutes. This shows

that decreasing the solid-to-liquid ratio favours a fast mass transfer rate between the superalloy

surface and the aqua-regia solution. The point at which the leaching reaction slows significantly

remains similar for the different solid to liquid ratios used at approximately 500 minutes into the

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experiments. The fact that this change occurs at a similar point in time suggests that the change is

caused by the acid concentration becoming low enough, through its consumption in leaching, that

the rate limiting steps changes from chemical control to another mechanism. This is opposed to

the mechanism changing due to the build up of a porous product layer which would be indicated

by the point occurring at a similar product recovery in all cases.

Figure 3. Effect of solid-to-liquid ratio on the recovery of rhenium over time for a 100% aqua regia solution

and without sonication and 200 rpm steering speed. The inset shows the effect of solid-to-liquid ratio on

the total recovery of rhenium at 2880 minutes.

The lower the solid-to-liquid ratio the larger the reagent volume required to leach the same

amount of superalloy scrap. Therefore, a compromise has to be made between the maximum

tolerable amount of reagent consumption and the time cost of the superalloy leaching process. To

find the optimum between the amount of reagent that can be used and the leaching period, the

experimental results from section 3.3 were used to calculate the optimum solid-to-liquid ratio for

the stirring speed optimisation experiments. The optimisation was calculated using the GRG

nonlinear algorithm of Microsoft® Excel solver. Based on this optimisation calculation, the effect of

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stirring speed on the recovery of rhenium has been studied in 100% aqua-regia solution at 53.8 g

L–1 solid-liquid ratio.

3.4. Effect of stirring speed

The kinetics of the superalloy leaching can be affected by the rate of diffusion through the bulk of

the solution, which in turn is affected by the rate of agitation applied to the reaction mixture. The

stirring speed was changed from 100 rpm to 800 rpm to study the influence of agitation speed on

the rate of leaching. Figure 4 shows how increasing the stirring speed affects the rate of rhenium

leaching from superalloy scrap. When stirring speed is over 100 rpm, the leach curves do not

show significant variation in leaching efficiency with change in the agitation speed. The inset in

Figure 4 shows that the speed of agitation has no significant effect on the rate of rhenium

recovery.

Figure 4. Effect of stirring speed on the recovery of rhenium over time from superalloy CMSX-4 using 100%

aqua-regia solution, solid-to-liquid ratio of 53.8 g L–1, and no sonication.

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A reaction that is controlled by the process of diffusion through the bulk is dependent on the mass

transfer coefficient which in turn is dependent on the degree of agitation in the system. Hence

increased stirring speed or agitation should increase the rate of a reaction that is dependent on

mass transfer coefficient. However, reactions that are dependent on diffusion through a porous

product layer and chemically controlled reactions are independent of the degree of agitation (29).

This experiment showed that the leaching kinetics of rhenium from superalloy CMSX-4 is not

controlled by the diffusion through the bulk as the recovery of rhenium has not been improved

significantly by increasing the degree of agitation. The experimental results indicated that the

effect of diffusion through the bulk could be ignored when the stirring speed exceeded 100 rpm.

Therefore, a stirring speed of greater that 100 rpm was chosen for the subsequent experiments to

ensure sufficient agitation without causing splashing of the leach liquor at high stirring speed.

3.5. Leaching kinetics of rhenium from superalloy CMSX-4

In order to establish the reaction kinetics and rate controlling step for the aqua-regia leaching of

superalloy CMSX-4, the experimental data was analysed against the shrinking core model. This

model describes the leaching process as the reaction is taking place on the surface of a spherical

solid that shrinks along the course of the reaction leaving an unleached porous product layer. The

leach reactions occurring in the fluid-solid heterogeneous system generally have the following

steps:

1. Diffusion of the leaching reagent through the bulk of the reagent solution to the surface of the

solid.

2. Diffusion of the leaching reagent through a porous product layer of unleached material.

3. Reaction of the leaching reagent with the solid being leached at the unleached central core.

4. Diffusion of the leach product back through the porus product layer.

5. Diffusion of the leach products through the product film layer back to the bulk of the reagent

solution.

The leaching rate can be limited by any of the steps above, but given that stirring speed had no

impact of leaching rate of rhenium from the superalloy (Section 3.4) steps 1 and 5 can be excluded

as potential rate controlling steps.

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Equations 2 and 3 describe the equations for product layer diffusion controlled and chemical

reaction controlled leaching rates, respectively. Where x is the fraction reacted, MB the molecular

mass of the solid material, C A concentration of the reagent, D the diffusion coefficient of the

reagent, ρB the density of the solid material, σ the stoichiometric coefficient of the reagent, r0the

initial radius of the solid, K r the reaction rate constant, Kd the diffusion constant, and t the

reaction time. Based on these equations a linear relationship is expected between the left side of

the equations and time of reaction (29).

1−23 x−

(1−x )23=

2MBDCA

ρBσ r02 t=Kd t (2)

1− (1−x )13 =

K M BC A

ρBσ r 0t=K r t (3)

To determine the rate-limiting step of the leach process, data from the leach curve were

transformed and fitted into Equations 2 and 3, and the fitting degree was evaluated by the

correlation coefficient (R2) values. The better fitting data indicates the rate controlling step.

The rhenium leaching from superalloy scrap demonstrates two types of leach kinetics as shown in

Figure 5. The early stage of the leach reaction, up to 420 minutes, is governed by the rate of

chemical reaction at the superalloy surface. This is likely due to the availability of fresh leaching

reagent and the fact the leach product did not form enough layers to affect the rate of the leach

reaction. After 480 minutes of leaching, the rate controlling step changes to diffusion through a

product layer. This demonstrates the product layer accumulated enough to limit the leaching of

rhenium from the superalloy scrap surface back to the bulk solution.

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a

0 100 200 300 400 5000

0.07

0.14

R² = 0.967363059157517

Time (min)

1 - (

1 - x

)1/3

b

0 2000 4000 60000.00

0.02

0.03

0.05

R² = 0.979198371547358

Time (min)

1-2x

/3 -

(1-X

)2/3

Figure 5. The leaching kinetics of rhenium from superalloy CMSX-4 as calculated by the shrinking core model

when the leaching kinetics was (a) chemical reaction controlled and (b) diffusion through a product layer

controlled. For leaching in 100% aqua-regia solution using 53.8 g L–1 solid-to-liquid ratio, and 200 rpm steering

speed.

This outcome explains why the rhenium leach curves showed a rapid rate of rhenium recovery

before 480 minutes of leaching and the drop in recovery rates thereafter.

In order to evaluate whether the chemical reaction controlled model fits under all experimental

conditions and can be used to model the leaching process, the model was fitted to the leach

curves obtained during the optimisation of the concentration of aqua-regia and solid-to-liquid

ratio. The linear fit curves in Figure 6 show correlation coefficients greater than 0.9 (except for

20% aqua-regia leaching) which indicates that the leach process can be expressed by this model in

both experimental conditions. The leach kinetics curves also demonstrated that faster reaction

rates are expected within 420 minutes of superalloy leaching in 100% aqua-regia solution at 5 g L–1

solid-to-liquid ratio.

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a b

Figure 6. Chemical reaction controlled leaching kinetics model of rhenium with respect to variation in a) solid-to-

liquid ratio and b) concentration of aqua-regia solution for constant steering speed of 200 rpm.

3.6. Leaching kinetics for component metals of a superalloy CMSX-4

The leaching kinetics of the individual component metals present in a superalloy CMSX-4 were

individually studied in 100% aqua-regia leach solutions for a period of 6000 minutes.

As shown in Figure 7, a rising and plateauing leach curves were observed for the recovery of

rhenium, chromium, nickel, and cobalt in 100% aqua regia solution. The recovery of the

component metals increases rapidly up to 600 minutes of leaching and then slows significantly

after 1800 minutes. The other component metals of CMSX-4 also showed faster rate of leaching at

earlier periods of leaching, between 120 and 420 minutes in 100% aqua-regia. However, the

recoveries of the component metals other than rhenium, chromium, nickel, and cobalt showed a

decrease or remain unchanged after 420 minutes. The recoveries of tantalum and hafnium shows

a drop as these metals remain largely in the final residue as unleached metal precipitates. The

recoveries of titanium and molybdenum remain unchanged after 420 minutes as the oxides of

these metals can precipitate even at low pH values of the leach solution which may lead to the

inhibition of their recovery to the bulk solution.

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0 2000 4000 60000

20

40

60

80

Re Cr

Ni Co

Ta Hf

Ti Mo

Time (min)

% R

ecov

ery

Figure 7. Leaching curve of superalloy CMSX-4 component metals for 6000 minutes of leaching in 100%

aqua-regia solution, 53.8 g L–1 solid-to-liquid ratio, without sonication, and 200 rpm steering speed.

Tungsten did not leach from the superalloy scraps in 100% aqua-regia leaching solutions.

Molybdenum and hafnium showed an increased recovery with decreasing concentration of aqua-

regia (from 100% to 60%). This can be attributed to the increased tendency of formation of the

oxides of these metals at very high concentrations of aqua-regia which will passivate the surface of

the metals from further reaction with the reagents leading to their decreased recovery. For nickel,

chromium, cobalt, and rhenium higher concentrations of the aqua-regia using the 100% solution

led to the leaching reaction of these metals proceed at faster rate and reach plateau state earlier

than the more diluted aqua-regia solutions. It can also be seen from Figure 7 that the leaching

kinetics of these component metals in 100% aqua-regia solution is faster in the period of leaching

where the kinetics is dictated by the rate of the chemical reaction.

The recovery of rhenium in 100% aqua-regia is controlled by the rate of chemical reaction at the

surface of the superalloy scrap for the first 420 minutes of leaching. To study whether the leaching

kinetics of the other component metals fit into the same kinetics model as rhenium, the leaching

curves of the component metals for 420 minutes of leaching in 100% aqua-regia solution was

modeled using the chemical reaction controlled model. The linear curves in Figure S1 of 18

supplementary materials show that the correlation coefficient is greater than 0.9 for all the

component metals considered except hafnium and tantalum. An explanation of this deviation of

the chemical reaction controlled kinetics model might be because of the corrosion resistant nature

of these two metals. Rhenium, chromium, nickel and cobalt show similar fit to the model with

higher leaching rates than the other component metals.

3.7. Single stage precipitation of superalloy CMSX-4 component metals

In order to explore the differential precipitation of the component metals with pH, the leach

solution from a piece of a superalloy leaching experiment was obtained and the pH was raised to

precipitate out the component metals with samples being taken at various pH points. It is

apparent from Figure 8 that around a pH of approximately 5.0 all of the aluminium, chromium,

molybdenum, titanium, and tantalum have precipitated from the solution. Rhenium, cobalt, and

nickel, however, are largely left in solution. By pH 8.0 nickel and cobalt have also precipitated out

leaving rhenium as the only metal in solution. Slight re-dissolution of molybdenum was observed

as the pH was raised above 7.0.

-2.50 -0.50 1.50 3.50 5.50 7.50 9.500.00

0.25

0.50

0.75

1.00

1.25

Re

Ti

Ta

Mo

Cr

Ni

Al

Co

pH

Frac

tion

of m

etal

spe

cies

in s

olut

ion

Figure 8. The pH gradient precipitation separation for component metals of superalloy CMSX-4.

To gain a deeper understanding of the results from the precipitation experiments thermodynamic

analysis was also used. Even though no kinetic information can be discerned from them, the Eh-pH

diagrams of each of the component metals can be used to analyse the thermodynamics of these 19

metals in aqueous systems in order to understand their precipitation behaviour (30). These

diagrams are constructed for a specified temperature, pressure, and dissolved ion concentration

of each metal component. The dissolved ion concentration of the metal affects the predominance

area of each species of the metal in the Eh-pH diagram. The solid (Crystalline) species

predominance area will increase with concentration while the dissolved species predominance

area decreases with increasing concentration (31).

The Eh-pH diagrams in Figures S2 and S3 in supplementary material have been calculated using

MEDUSA® software with settings similar to the experimental conditions; 25 0C reaction

temperature, 1 atm pressure, pH range of -2.0 to 12.0, and concentration ratio of the components

used in the actual experiment.

Molybdic acid (H2MoO4) will co-precipitate with crystalline MoO2 at lower pH ranges and this

precipitate will dissolve at slightly alkaline pH to form MoO42- which will polymerise into Mo7O24

6- in

acidic solutions (32). Huang et al. reported the tendency of MoO42- to form complex with other

cations in the solution in acidic conditions (33). This can explain the plateau observed between pH

2.35 - 3.53 in the pH gradient precipitation graph in Figure 8. The pH gradient precipitation

separation graph in Figure 8 shows that molybdenum re-dissolves when pH > 6.5. It has been

reported that molybdenum will re-dissolve at pH > 6.0 due to the formation of the species MoO42-

in alkaline solutions (34). Introduction of a filtration step in the pH gradient precipitation

separation will help remove molybdenum species from the system during its precipitation at lower

pH ranges preventing redissolution from occuring in solution at higher pH values due to its

absence.

Under strong acid conditions chromium leaches to form Cr3+ and unstable Cr2+ which will

subsequently oxidise to Cr3+ (Figure S2a in supplementary material). With increasing pH of the

solution the soluble Cr3(OH)45+ will form which will result in the formation of Cr2O3 on further

increase of the solution pH as per the following equations (31).

3Cr (aq )3+¿+8H 2O( l )⇌Cr 3(OH)4( aq )

5+¿+4H3 O(aq)+¿ ¿¿

¿

2Cr3(OH )4(aq )5+¿+10O H( aq)

−¿→3 Cr2O 3( S)+9H2 O(l) ¿ ¿

The only two predominant species in aqueous system of aluminium are the aqueous soluble Al3+ in

acidic conditions and the crystalline precipitate of Al(OH)3 which forms when the solution pH is

20

raised from strongly acidic conditions to alkaline (Fig S2b in supplementary material). The pH at

which precipitation of Al(OH)3 begins in aqueous solutions mainly depends on the amount of

aluminium dissolved (35).

Al(aq)3+¿+3(OH )(aq)

−¿→Al (OH )3( s) ¿ ¿

The Eh- pH diagrams in Figure S2 in supplementary material show the predominance area for the

precipitates of of Cr2O3 and Al(OH)3 begin at pH > 3.0 which is in agreement with the results of pH

the experiment. Precipitates of titanium are also predicted to form at pH > -1.0 with complete

precipitation expected at pH > 5.0 (Fig S2d in supplementary material). The precipitation of

titanium in these ranges also agrees with the experimental results.

Cobalt exists as Co2+ in acidic and weakly alkaline aqueous solutions depending on the amount

cobalt in the system. With increasing pH Co2+ reacts with OH- ions in the solution to form an

insoluble precipitate of Co(OH)2 within the pH range of 5.0 – 7.0 (36). The result of the experiment

agrees with this finding and examination of an Eh-pH diagram for cobalt system (Figure S3b in

supplementary material) also agrees with the result. Nickel also precipitates as nickel hydroxide

(Ni(OH)2) (Figure S3a in supplementary material) over a similar pH range to cobalt (Figure S3b in

supplementary material), which is again in line with the experimental results.

The Eh-pH diagram in Figure S3c in supplementary material shows the predominant oxide of

rhenium ReO4- being soluble over the pH range considered in this study. Also shown in Figure S3c

in supplementary material are the oxides of rhenium which are expected to co-precipitate as ReO3

and ReO2 at lower pH ranges which likely explains the slight loss of rhenium from solution as the

pH is increased in the experiment.

Tantalum forms corrosion resistant and thermodynamically stable oxide at all pH values in

aqueous systems (Figure S3d in supplementary material) which explains the recovery of tantalum

in the experiment.

3.8. Two stage precipitation of superalloy CMSX-4 component metalsThe single stage precipitation of metals showed that by increasing pH of the solution most of the

component metals could be precipitated from the leach liquor leaving a rhenium rich solution.

Whilst many metals precipitated over similar pH ranges, it was observed that it may be possible to

21

yield a separate nickel cobalt product from the other metals. This would be useful as mixed Nickel-

Cobalt hydroxide precipitates (MHP) are industrially important in the production of high grade

nickel and cobalt (37) and hence a marketable product in their own right.

Given this, an experiment was performed where the pH of a leach solution was raised to pH 5.0

before the solids were filtered away (Figure 9a). The filtrate pH was then raised to pH 8.0 and the

solids filtered away (Figure 9b). Figure 9a shows that by adjusting the pH to 5.0 nearly all of the

tantalum, titanium, molybdenum, chromium, and aluminium is precipitated with only 0.1%, 9.4%,

and 1.2% losses of rhenium, nickel, and cobalt respectively.

The precipitate filtered out at pH 5.0 can be used for recovery of molybdenum as it can be

separated from the other metals by selectively re-dissolving it at pH > 6.5. The molybdenum in

solution can be subsequently re-precipitated by conventional molybdenum recovery techniques

(38). After leaching out the molybdenum from precipitate obtained the remaining precipitate will

be mostly composed of the oxides of titanium, aluminium, and chromium. Considering the fact

that titanium, aluminium, and chromium are the three most important alloying metals in super-

critical water-cooled reactors, due to their highly corrosion resistant properties, this precipitate

can also be of economic importance (30).

a

-3.00 -1.50 0.00 1.50 3.00 4.50 6.000

20

40

60

80

100

Re

Ti

Ta

Mo

Cr

Ni

Al

Co

pH

Leac

hing

ratio

(%)

22

b

4.5 5.5 6.5 7.5 8.50

20

40

60

80

100

Re

Ni

Co

pH

Leac

hing

ratio

(%)

Figure 9. The pH gradient precipitation separation for component metals of superalloy CMSX-4 for a pH

range from a) -2.5 to 4.7 and b) 4.7 to 8.0.

The process flow chart in Figure 10 summarises the separation and recovery of the rhenium value

from CMSX-4 superalloy scraps, and Table 2 lists the solution concentration (mg L -1) of each

component metal at all stages of the separation process. The residue filtered off the initial leach

solution mainly contains tungsten and hafnium. The precipitate at the first stage precipitation

separation contains most of the component metals except nickel, cobalt, and rhenium. The second

stage of the precipitation separation removes nickel and cobalt as nickel/cobalt mixed hydroxide

precipitate(MHP) leaving a purified final leach solution of sodium perrhenate (NaReO4) which can

be fed for downstream clean-up to be processed for production of high grade perrhenate salts and

other rhenium products.

23

Figure 10. Process flow chart for separation of rhenium value from superalloy CMSX-4

Table 2. Solution concentration (mg L-1) of component metals of CMSX-4 superalloy at all stages the separation processes.

Re Ti Ta Mo Cr Ni Al Co Hf W

Leach solution 595.2 157 68.6 129.4 1906 12754 1043 2414 2 ND*

First stage separation 507.2 0.0 0.0 0.6 0.4 8525 1.2 1482 0.0 ND

Purified solution 534.2 0.0 0.0 0.6 0.4 1 0.0 0.4 0.0 ND*ND = Not detected

24

Feed 1CMSX-4 superalloy scrap100% Aqua-regia

Leach Solution

Second stage Precipitation liquor

First stage Precipitation liquor

Feed 250% (w/w) NaOH

Feed 350% (w/w) NaOH

Unleached residue (mainly W and Hf)

Solid precipitate (predominantly Al, Cr, Mo, Ta and Ti) to be processed

Ni/Co hydroxide precipitate

NaReO4 purified solution for downstream clean up

4. Conclusions

Hydrometallurgical recycling of rhenium from nickel based superalloy CMSX-4 using aqua-regia

solution is a cost effective alternative compared to the traditional pyrometallurgic rhenium

recycling. The optimum conditions were: 100% aqua regia solution (37% HCl and 70% nitric acid

solutions in 3:1 volume ratio), sonication for 480 min (an increase in recovery of 7.83%), solid-to-

liquid ratio of 53.8 g L-1 and stirring speed of 200 rpm.

Application of ultrasonic waves to the leaching process has proven useful to slightly increase the

recovery; however this may not be economically feasible to significantly impact the recovery of

rhenium. The main gain in recovery of rhenium is predominantly due to increase in aqua-regia

concentration. Increasing aqua-regia concentration to 100% (v/v) resulted in more than 30% gain

in recovery which can be attributed to the increased availability of leaching reagents. In addition

to the concentration of aqua-regia, the solid-to-liquid ratio of the leach system has a significant

effect on the rate and percentage recovery of rhenium from CMSX-4. The recovery of rhenium

from superalloy CMSX-4 in 100% aqua-regia solution was observed not to be dependent on the

speed of agitation. Furthermore, aqua-regia enabled to achieve the maximum leaching of rhenium

from CMSX-4 superalloy scraps.

The leaching kinetics of rhenium from superalloy CMSX-4 in 100% aqua-regia solution fit into a

chemical reaction controlled kinetics model for the first period of leaching. For leaching period

after 480 minutes, the leaching kinetics of rhenium fits into the diffusion through the product layer

controlled kinetics model.

Logarithmic leach curves have been produced for rhenium, chromium, nickel, cobalt,

molybdenum, and titanium after 6000 minutes of leaching in 100% aqua-regia solution with a

significant reduction in the leach rate observed after 1800 minutes. The leaching kinetics of

rhenium, chromium, nickel, cobalt, and molybdenum fit into the chemical reaction rate controlled

kinetics model for 420 minutes of leaching in 100% aqua-regia solution.

Precipitation of metals from the leach solution by increasing the pH showed that two distinct

precipitates can be recovered, one at pH 5.05 and the other around pH 7.0. Thermodynamic Eh-pH

curves, calculated using MEDUSA® software, were employed to help explain the precipitates

observed and the results were largely consistent. The precipitate at pH= 5.05 is mainly composed

of the oxides of aluminium, chromium, molybdenum, and titanium. A green precipitate of mixed 25

hydroxides of cobalt and nickel (MHP), which has commercial value, is precipitated out at pH 7.0

leaving a purified sodium perrhenate solution. This purified sodium perrhenate solution can be fed

for downstream clean-up processes to produce high grade perrhenate salts.

Industrial fume scrubbers can be used to treat the effluents of the aqua-regia solution and prevent

the emission of the corrosive fumes to the atmosphere. Considering the current market value of

rhenium (around 2300 USD per kg price November 2018) and ammonium perrhenate, the reagent

cost for the proposed rhenium recycling process is low, and costs of facility can be quickly

returned. Moreover, the present hydrometallurgical processes can be extended to the recycling of

rhenium from other second and third generation superalloys with higher rhenium content.

5. References

1. Singh Gaur RP, Wolfe TA, Braymiller SA. Recycling of rhenium-containing wire scrap. Int J

Refract Met Hard Mater. 2015;50:79–85.

2. Hori H, Yoshimura Y, Otsu T, Kume K, Mitsumori Y, Kutsuna S, Koike K. Efficient

photochemical recovery of rhenium from aqueous solutions. Sep Purif Technol.

2015;156:242–8.

3. Anderson CD, Taylor PR, Anderson CG. Extractive metallurgy of rhenium: A review. Miner

Metall Process. 2013;30(1):59–73.

4. Pollock TM, Tin S. Nickel-based superalloys for advanced turbine engines: Chemistry,

microstructure, and properties. J Propuls Power. 2006;22(2):361–74.

5. Xu BX, Wang XM, Zhao B, Yue ZF. Study of crystallographic creep parameters of nickel-based

single crystal superalloys by indentation method. Mater Sci Eng A. 2008;478(1–2):187–94.

6. Srivastava RR, Kim MS, Lee JC. Novel Aqueous Processing of the reverted turbine-blade

superalloy for rhenium recovery. Ind Eng Chem Res. 2016;55(29):8191–9.

7. Cheema HA, Ilyas S, Masud S, Muhsan MA, Mahmood I, Lee J. Selective recovery of rhenium

from molybdenite flue-dust leach liquor using solvent extraction with TBP. Sep Purif

Technol. 2018;191:116–21.

26

8. Shipachev VA. Some processing techniques for rhenium isolation and purification from

refractory alloys. Chem Sustain Dev. 2012;20:323–6.

9. Angelidis TN, Rosopoulou D, Tzitzios V. Selective rhenium recovery from spent reforming

catalysts. Ind Eng Chem Resour. 1999;(114):1830–6.

10. Harris K, Wahl JB. Improved single crystal superalloys. Superalloys. 2004:45–52.

11. Srivastava RR, Kim MS, Lee JC, Jha MK, Kim BS. Resource recycling of superalloys and

hydrometallurgical challenges. J Mater Sci. 2014;49(14):4671–86.

12. Laatikainen M, Virolainen S, Paatero E, Sainio T. Recovery of ReO4- by weakly basic anion

exchangers: Modeling of sorption equilibrium and rate. Sep Purif Technol. 2015;153:19–28.

13. Huang M, Zhu J. An overview of rhenium effect in single-crystal superalloys. Rare Met.

Nonferrous Metals Society of China; 2016;35(2):127–39.

14. Nguyen TH, Truong HT, Lee MS. Separation of rhenium (VII) and vanadium (V) from

hydrochloric acid solutions by solvent extraction. J Korean Inst Met Mater.

2017;55(10):724–31.

15. Kasikov L, Petrova A. Processing of deactivated platinum-rhenium catalysts. Theor Found

Chem Eng. 2009;43(4):544–52.

16. Abisheva ZS, Zagorodnyaya AN, Bekturganov NS. Review of technologies for rhenium

recovery from mineral raw materials in Kazakhstan. Hydrometallurgy. 2011;109(1–2):1–8.

17. Mishra B, Anderson CD, Taylor PR, Anderson CG, Apelian D, Blanpain B. CR3 Update:

Recycling of strategic metals. JOM. 2012;64(4):441–3.

18. deBarbadillo JJ. Nickel-based superalloys: Physical metallurgy of recycling. Metall Trans A,

Phys Metall Mater Sci. 1983;14A(3):329–41.

19. Truong HT, Lee MS. Separation of rhenium(VII), molybdenum(VI), and vanadium(V) from

hydrochloric acid solution by solvent extraction with TBP. Geosystem Eng. 2017;20(4):224–

30.

27

20. Srivastava RR, Kim M, Lee J, Jha MK, Kim B-S. Resource recycling of superalloys and

hydrometallurgical challenges. J Mater Sci. 2014;49(14):4671–86.

21. Amer A. The hydrometallurgical extraction of rhenium from copper industrial wastes. JOM.

2008;60(8):52–4.

22. Xiong Y, Song Y, Tong Q, Zhang P, Wang Y, Lou Z, Zhang F, Shan W. Adsorption-controlled

preparation of anionic imprinted amino-functionalization chitosan for recognizing

rhenium(VII). Sep Purif Technol. 2017;177:142–51.

23. Srivastava RR, Kim MS, Lee JC, Ilyas S. Liquid-liquid extraction of rhenium(VII) from an acidic

chloride solution using Cyanex 923. Hydrometallurgy. 2015;157:33–8.

24. Virolainen S, Laatikainen M, Sainio T. Hydrometallurgy Ion exchange recovery of rhenium

from industrially relevant sulfate solutions: Single column separations and modeling.

Hydrometallurgy. 2015;158:74–82.

25. Markovi BM, Spasojevi VV, Kusigerski VB, Pavlovi VB, Onjia AE, Nastasovi AB. Selective

magnetic GMA based potential sorbents for molybdenum and rhenium sorption. J Alloy

Compd J. 2017;705:38–50.

26. Joo S, Kim Y, Kang J, Yoon H, Kim D, Shin SM. Recovery of molybdenum and rhenium using

selective precipitation method from molybdenite roasting dust in alkali leaching solution.

Mater Trans. 2012;53(11):2038–42.

27. Swamy KM, Narayana KL. Intensification of leaching process by dual-frequency ultrasound.

Ultrason Sonochem. 2001;8(4):341–6.

28. Thompson LH, Doraiswamy LK. Sonochemistry: Science and Engineering. Ind Eng Chem Res.

1999;1215–49.

29. Sun Y, Fu G, Jiang L. Kinetic study of the leaching of low-grade manganese ores by using

pretreated sawdust as reductant. Minerals. 2017;7(6):83.

30. Cook WG, Olive RP. Pourbaix diagrams for chromium, aluminum and titanium extended to

high-subcritical and low-supercritical conditions. Corros Sci. 2012;58:291–8.

28

31. Beverskog B, Puigdomenech I. Revised pourbaix diagrams for chromium at 25–300 °C.

Corros Sci. 1997;39(1):43–57.

32. Zeng L, Yong Cheng C. A literature review of the recovery of molybdenum and vanadium

from spent hydrodesulphurisation catalysts. Part II: Separation and purification.

Hydrometallurgy. 2009;98(1–2):10–20.

33. Huang JH, Oliazadeh M, Alfantazi AM. pH-Controlled precipitation of cobalt and

molybdenum from industrial waste effluents of a cobalt electrodeposition process.

Hydrometallurgy. 2004;75:77–90.

34. Ryzhenko BN. Technology of groundwater quality prediction: 1. Eh-pH diagram and

detention coefficient of molybdenum and tungsten in aqueous solutions. Geochemistry Int.

2010;48(4):407–14.

35. Ferreira DA, Prados LMZ, Majuste D, Mansur MB. Hydrometallurgical separation of

aluminium, cobalt, copper and lithium from spent Li-ion batteries. J Power Sources.

2009;187(1):238–46.

36. Huang JH, Kargl-Simard C, Oliazadeh M, Alfantazi AM. pH-controlled precipitation of cobalt

and molybdenum from industrial waste effluents of a cobalt electrodeposition process.

Hydrometallurgy. 2004;75(1–4):77–90.

37. Williams C, Hawker W, Vaughan JW. Selective leaching of nickel from mixed nickel cobalt

hydroxide precipitate. Hydrometallurgy. 2013;138:84–92.

38. An JW, Lee YH, Kim SJ, Tran T, Lee SO, Kim MJ. Production of high purity molybdenum

compounds from a Cu-Mo acid-washed liquor using solvent extraction. Part 1: Laboratory

studies. Miner Eng. 2009;22(12):1020–5.

29

Supplementary material

Figure S1. Chemical reaction controlled leaching kinetics model for component metals of superalloy CMSX-4 for

420 minutes of leaching in 100% aqua-regia solution, 58.3 g L-1 solid-to-liquid ratio, and 200 rpm steering speed.

a

2 4 6 8 1 0 1 2- 2

- 1

0

1

ES

HE

/ V

p H

C r O 42

C r 3 +

C r 2 O 72

C r 2 +

C r 3 ( O H ) 45 +

C r ( c )

C r 2 O 3 ( c r )

C r O 2 ( c )

[ T i O 2 + ] T O T = 1 0 . 0 0 m M[ T a ( O H ) 5 ] T O T = 6 5 . 0 0 m M[ M o O 4

2 ] T O T = 6 0 . 0 0 m M

[ C r O 42 ] T O T = 6 5 . 0 0 m M

[ A l 3 + ] T O T = 5 6 . 0 0 m M

t = 2 5 C

b

- 2 0 2 4 6 8 1 0 1 2- 2

- 1

0

1

ES

HE

/ V

p H

A l 3 +

A l ( O H ) 3 ( c r )

A l ( s )

[ T i O 2 + ] T O T = 1 0 . 0 0 m M[ T a ( O H ) 5 ] T O T = 6 5 . 0 0 m M[ M o O 4

2 ] T O T = 6 0 . 0 0 m M

[ C r O 42 ] T O T = 6 5 . 0 0 m M

[ A l 3 + ] T O T = 5 6 . 0 0 m M

t = 2 5 C

30

c

- 2 0 2 4 6 8 1 0 1 2- 1 . 0

- 0 . 5

0 . 0

0 . 5

1 . 0

ES

HE

/ V

p H

M o O 42

H 2 M o 7 O 2 4 4

H M o 7 O 2 4 5

M o 3 +

H 2 M o O 4 ( c )

M o O 2 ( c r )

M o ( c )

[ T i O 2 + ] T O T = 1 0 . 0 0 m M[ T a ( O H ) 5 ] T O T = 6 5 . 0 0 m M[ M o O 4

2 ] T O T = 6 0 . 0 0 m M

[ C r O 42 ] T O T = 6 5 . 0 0 m M

[ A l 3 + ] T O T = 5 6 . 0 0 m M

t = 2 5 C

d

- 2 0 2 4 6 8 1 0 1 2- 1 . 0

- 0 . 5

0 . 0

0 . 5

1 . 0

ES

HE

/ V

p H

T i O 2 +

T i 2 +

T i 3 +

T i O 2 ( c r )

T i ( O H ) 3 ( c )

[ T i O 2 + ] T O T = 1 0 . 0 0 m M[ T a ( O H ) 5 ] T O T = 6 5 . 0 0 m M[ M o O 4

2 ] T O T = 6 0 . 0 0 m M

[ C r O 42 ] T O T = 6 5 . 0 0 m M

[ A l 3 + ] T O T = 5 6 . 0 0 m M

t = 2 5 C

Figure S2. Potential-pH (Eh-pH) diagrams for the aqueous systems of a) chromium, b) aluminium,

c) molybdenum, and d) titanium.

a

- 2 0 2 4 6 8 1 0 1 2

- 1 . 0

- 0 . 5

0 . 0

0 . 5

1 . 0

ES

HE

/ V

p H

N i 2 +

b - N i O O H ( c )g N i O 2 ( s )

N i ( c )

N i ( O H ) 2 ( c )

[ N i 2 + ] T O T = 6 5 0 . 0 0 m M[ C o 3 + ] T O T = 9 6 . 0 0 m M

[ R e O 4 ] T O T = 3 0 . 0 0 m M

t = 2 5 C

b

- 2 0 2 4 6 8 1 0 1 2

- 1 . 0

- 0 . 5

0 . 0

0 . 5

1 . 0

ES

HE

/ V

p H

C o 2 +

C o ( O H ) 3 ( c )

C o O 2 ( c )

C o ( c )

C o ( O H ) 2 ( c )

[ N i 2 + ] T O T = 6 5 0 . 0 0 m M[ C o 3 + ] T O T = 9 6 . 0 0 m M

[ R e O 4 ] T O T = 3 0 . 0 0 m M

t = 2 5 C

31

c

- 2 0 2 4 6 8 1 0 1 2

- 1 . 0

- 0 . 5

0 . 0

0 . 5

1 . 0

ES

HE

/ V

p H

R e O 4

R e

R e O 2 ( c )

R e O 3 ( c )

R e ( c )

[ N i 2 + ] T O T = 6 5 0 . 0 0 m M[ C o 3 + ] T O T = 9 6 . 0 0 m M

[ R e O 4 ] T O T = 3 0 . 0 0 m M

t = 2 5 C

d

- 2 0 2 4 6 8 1 0 1 2

- 1 . 0

- 0 . 5

0 . 0

0 . 5

1 . 0

ES

HE

/ V

p H

T a ( c )

T a 2 O 5 ( c )

[ T i O 2 + ] T O T = 1 0 . 0 0 m M[ T a ( O H ) 5 ] T O T = 6 5 . 0 0 m M[ M o O 4

2 ] T O T = 6 0 . 0 0 m M

[ C r O 42 ] T O T = 6 5 . 0 0 m M

[ A l 3 + ] T O T = 5 6 . 0 0 m M

t = 2 5 C

Figure S3. Potential-pH (Eh-pH) diagrams for aqueous systems of a) nickel, b) cobalt, c) rhenium, and d) tantalum.

Table S1. % of recovery from superalloy CMSX-4 accelerated with the application of ultrasonic

waves in 100% aqua-regia solution and solid-to liquid ratio of 65 g L-1.

Time (min) %recovery with sonication % recovery without Sonication Difference

480 38.89 31.06 7.83420 31.97 28.30 3.67360 28.01 25.63 2.39300 23.37 22.19 1.18240 18.99 18.27 0.72180 15.05 11.63 3.42120 13.20 6.63 6.5790 7.28 3.34 3.9460 2.84 1.60 1.2430 0.92 0.74 0.1815 0.18 0.30 -0.12

32