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Lixiviation kinetics of cobalt from cemented carbides
International Journal of Refractory Metals & Hard Materials 70 (2018) 239–245
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Lixiviation kinetics of cobalt from cemented carbides a,⁎
a
b
Gregor Kücher , Stefan Luidold , Christoph Czettl , Christian Storf a b
MARK
b
CD Laboratory for Extractive Metallurgy of Technological Metals, Nonferrous Metallurgy, Montanuniversitaet Leoben, Franz-Josef-Str. 18, A-8700 Leoben, Austria CERATIZIT Austria GmbH, Metallwerk-Plansee-Str. 71, A-6600 Reutte, Austria
A R T I C L E I N F O
A B S T R A C T
Keywords: Semi-direct recycling Reclamation Cemented carbides Leaching Hard metals Chemical kinetics investigation Rate law Lixiviation
This report aims to disclose interactions between semi-direct recycling concepts, corrosion analysis of hard metals and possible leaching mechanisms. Accordingly, an examination reveals chemical kinetic dependencies via leaching a cutting insert in formic acid inclusive an oxidant. Currently, contradictions and issues in literature slow down the progress of semi-direct technologies in comparison to direct and indirect recycling concepts. Additionally, corrosion investigations show helpful explanations for leaching mechanisms of cobalt from cemented carbides, which so far are not connected to reclamation reviews. In order to obtain reliable data from experiments, the statistical software Modde 11 assists to apply a design of experiments and to perform a multiple regression analysis. Nevertheless, the experimental setup decides the result of the rate law investigation. The sample geometry and leaching conditions in the reactor affect the outcome in the form of a kinetic model equation. Finally, an empirical kinetic law with its dependencies inside the experimental boundaries presents potential ways to favorably influence cobalt leaching out of cemented carbides.
1. Introduction Indirect recycling dominates today's recycling technologies for cemented carbide scrap. About 85% of the total recycled hard metal amount in Germany [1] and at least half in the USA [2] are treated accordingly. The zinc process, a direct method, handles the residual quantity [2,3]. Consequently, the semi-direct techniques only play a minor role caused by technological obstacles, but offer advantages in comparison to the prevalent processes. The semi-direct recycling of hard metals rests upon the idea of leaching the binder metal (mostly cobalt) as well as coatings without affecting the hard phases (commonly WC, tungsten carbide). Subsequently, the remaining brittle WC can be mechanically crushed and reused without further processing. Thus, the produced tungsten carbide powders facilitate a direct application for the production of new cemented carbide commodities. In addition, common hydrometallurgical methods retrieve the lixiviated binder out of the solution. Hence, semi-direct methods potentially omit process steps like solution purification, SX, crystallization, reduction and carburation in contrast to primary or indirectly recycled powder [1,4,5]. Naturally, there exist reasons why no considerable industrial process is currently in operation. Numerous authors [4–7] reveal that one main issue - the partial oxidation of WC - contaminates the recyclate and hinders the leaching reaction through limited diffusion of the reactants. Also chemically resistant coating materials represent a
⁎
diffusion barrier and a potential impurity [7]. Generally, the dissolution of the binder metal slows down, depending on the grain size as well as binder content if bulky material is leached without mechanical support [4,6]. Schiesser [4] for instance discloses leaching times of approximately 30–160 days for a cube of 1 cm3 depending on material grade and conditions in acetic acid. Therefore, some studies examine hard metals with high cobalt content, for example Kojima et al. [8], or pulverized scraps like Gürmen et al. [9] to achieve a more reasonable extraction rate. Especially the inefficient mechanical disintegration of untreated hard metals seems not feasible as a result of high wear rates. A lack of knowledge remains in literature concerning the very bases of a potential technique. Beside the stated issues in literature, only Edtmaier et al. [10] reported the reaction rate of semi-direct leaching of a bulky substrate once. Others like Kojima et al. [8] inform roughly about the time dependency. The report of Gürmen et al. [9] deals with pulverized fractions and further available information results from patents. Normally, these did not comprise details about reaction rate relations. Additionally, a variety of available publications concerns the corrosion properties of hard metals as well as possible dissolution mechanisms [11–14], beneficial regions for the selective cobalt leaching [15,16] and the formation of a carbide skeleton [15,17,18]. Generally, scientific information about semi-direct methods is hardly available and therefore a necessity for an accurate experimental design and specimen preparation persists for a proper assessment. Side effects caused by the geometry should be excluded, while more care has
Corresponding author. E-mail addresses: [email protected] (G. Kücher), [email protected] (S. Luidold).
http://dx.doi.org/10.1016/j.ijrmhm.2017.10.004 Received 6 July 2017; Received in revised form 12 October 2017; Accepted 14 October 2017 Available online 16 October 2017 0263-4368/ © 2017 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 70 (2018) 239–245
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Table 1 Individual experiments with their parameters of the compiled CCF design. No.
T [K]
cCH2O2 [mol/l]
cH2O2 [mol/l]
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17
353.15 303.15 353.15 328.15 353.15 303.15 328.15 303.15 328.15 328.15 328.15 353.15 328.15 303.15 303.15 353.15 328.15
6.00 6.00 6.00 3.75 1.50 3.75 3.75 1.50 3.75 3.75 6.00 3.75 1.50 1.50 6.00 1.50 3.75
1.00 1.00 4.00 2.50 4.00 2.50 2.50 1.00 1.00 2.50 2.50 2.50 2.50 4.00 4.00 1.00 4.00
to be taken of temperature monitoring and the verification of preferably constant leaching conditions. The current examination discloses the results of leaching cobalt from a cutting insert in formic acid (CH2O2) and hydrogen peroxide (H2O2). 2. Materials and methods A design of experiments under assistance of the software Modde 11 served for a target-oriented execution including the mutual interaction of examined factors. The selected CCF (Central Composite Face centered) design forms a part of the response surface methodology. Altogether 17 tests facilitated the analysis of the three variable parameters temperature and molar concentration of H2O2 as well as CH2O2. Additionally, three incorporated repeated center experiments assisted for the determination of the reproducibility and other statistical quality factors. In continuation, Table 1 presents the used parameters of the design. Further theoretical background to this topic can be obtained by the Modde 11 User Guide [19] or Myers et al. [20].
Fig. 1. Schematic representation of the double-walled reaction vessel with position of the specimen inside.
Table 2 Identification of the experimental setup equipment.
2.1. Experimental setup
Instrument
Specification
Reactor
Double-walled, cylindrical, flange DN100, 1000 ml LAUDA Proline Edition X P 8 C IKA RH basic
Thermostat Magnetic stirrer (only stirring function) Pt100 sensor (class B)
The experiments applied a borosilicate double-walled reaction vessel with a nominal volume of 1000 ml with a cap attached by a steel clamp. According to Fig. 1, the cap possessed five ground glass joints corresponding to DIN 12242, four 29/23 and one 14/23. The smaller comprised the reflux condenser with the outlet for nitrogen as purging gas. Its inlet, a Pt100 temperature sensor, a sampling port and the centrally positioned glass bar with the sample occupied the other ones. In addition, a magnetic stirrer at about 200 min− 1 ensured a homogenous intermixing of the solution. A thermostat connected to the vessel together with the Pt100 sensor established an automatic temperature regulation to keep the desired level inside the reactor. The software Wintherm Plus 3.4 recorded the actual temperature. For proving preferably constant reaction conditions during the experiment, the periodically removed liquid samples were measured for determining the prevailing pH and EORP after cooling to room temperature. Table 2 lists the exact characterizations of the used experimental items. A cutting insert provided by courtesy of CERATIZIT Austria GmbH served as test substrate. In reference to Table 3, the hard metal with medium grain size contained a low amount of cobalt and composite carbides. Although ISO 1832 determines the geometry, the exact surface of the embedded and polished specimen in contact with the reactants was ascertained to be 160 mm2. All experiments employed a total liquid volume of 500 ml at the beginning. Hence, the specific
pH meter ORP meter
Jumo 902250/32-415-1001-1-3-300-042500/000 WTW inoLab pH 7310, electrode: WTW SenTix81 Hanna instruments HI2211 pH/ORP Meter, electrode: HI3131B
Table 3 Quality of applied cutting insert. Specification
Value
Co WC Composite carbides Grain size Delivery state Geometry
6.0 wt% 93.4 wt% 0.6 wt% Medium Sintered, nonmachined SNUN120412 – ISO 1832
proportion amounted to 0.32 mm2/ml whereas the common s/l ratio cannot be applied because only the defined surface was contacted with the solution. Initially, the liquid was heated to the desired temperature. The trial started by immersion of the specimen from the cap into the liquid.
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During the assessment, the extraction of solution samples out of the vessel after 5, 10, 20 and 30 min ensured the monitoring of cobalt concentration. An ICP-MS in accord with ÖNORM EN ISO 17294-2 performed the measurement of the elemental concentration in the leachant. A multi-element standard solution with β = 10 mg/l for cobalt provided the calibration within 0.50–100 μg/l. For analysis, a triple dilution occurred by applying the ratios 1:100, 1:1000 and 1:10,000.
Anyhow, some discrepancy emerged for a few experiments during the regression analysis. For instance, Fig. 3 displays the received data points (spots) of the experiment No. 4, a center trial. Apparently, the dotted line for the diffusion-controlled model (n = 0.5) improperly characterizes the dissolution mechanism. Hence, the necessary generalization of Eq. (1) resulted in the empirical law of Eq. (2) with n, the power coefficient as second factor.
2.2. Specimen preparation
mg mg ⎤⋅t n [minn] = y ⎡ ⎤ k⎡ ⎣ l⋅minn ⎦ ⎣ l ⎦
Generally, for diffusion-limited reactions the transport of the species to the reaction surface or away from it represents the rate determining step. Often this mechanism predominates in heterogeneous reactions like the dissolution of solids. Here, the complete conversion rate decreases over time demonstrated by n = 0.5. In contrast, for a chemically restricted process the kinetics of the reaction itself constitute the limiting factor. The overall reaction rate stays constant displayed by the value n = 1. Accordingly, the power coefficient facilitates conclusions about the reaction type. Nevertheless, a secure verification of the mechanism implies an invariant surface area and concentration [21]. For the evaluation, a simple logarithm performed the mandatory linearization of Eq. (2) for linear regression analysis with the function “LINEST” of MS Excel 2010 to obtain the values for k as well as n. Finally, a statistical analysis by the software Modde 11 served for the compilation of quadratic equations about the dependences of k and n on the three process parameters T, cH2O2 and cCH2O2.
As specified by Habashi [21], heterogeneous reactions (between solids and fluids) display only an evaluable reaction rate if the surface area and the concentration stay constant. Alternatively, a decreasing concentration or surface area provokes a falling reaction rate. Hence, for a reliable detection of reaction- or diffusion-controlled mechanism the surface area and the concentration have to be kept almost constant. Thus, main obstacles in reaction rate investigations consist of geometrical implications. For example, assumptions and mathematics compensate the changing boundary surface area in shrinking core models to derive the respective model equation. In order to avoid this issue and to find a more simple form of equation for the time dependency, more attention was paid to the sample preparation. A large liquid volume (500 ml) in relation to the surface (160 mm2) helped to diminish effects of dissolved ions in the solution. Very detailed information about modeling reaction rates can be found in Habashi [21] and Levenspiel [22]. For the purpose of bypassing the three-dimensional task, the specimen was hot-embedded in Struers ClaroFast, a thermoplast. Grinding and polishing steps, the last being a 1 μm diamond finish, ensured equal conditions of all substrate surfaces. The Fig. 2 represents a sketch of a final test specimen as applied during the experiments. Hence, the accordingly prepared sample evidently featured only one possible reaction plane and consequently the surface area stayed almost constant. Here, the indicated drill hole served for fixation of the sample on the beforehand stated glass bar in the reactor (Fig. 1). Prior tests verified the chemical stability of the resin in the liquid phase. Additionally, no preferential leaching of the contact zone between substrate and synthetic material appeared which could influence the results.
3. Hydrometallurgical framework The pourbaix diagrams for cobalt and tungsten carbide disclose a simplified hydrometallurgical setting of this investigation. As represented by Fig. 4, the acidic region shows the predominance of a soluble Co2 + ion and insoluble tungsten oxides. Though, the setting for tungsten species comprises more complexness. Typically, in acidic media tungsten tends to form various complexed isopolytungstate ions in dependence of pH and time. At the moment the knowledge of occurring species remains limited, especially concerning their thermodynamic data. Additionally, literature indicates the chance of forming peroxotungstates in the presence of H2O2 [23]. From the thermodynamic point of view above a potential of E0 ≥ − 0.28 V the dissolution of cobalt proceeds, like specified by Eq. (3) [24]. Anyhow as verified by screening tests [25] the used acids alone could not accomplish a leaching of the cobalt out of the specimen. Only acids combined with an oxidation agent (H2O2) achieved a lixiviation.
2.3. Model equation and examination scheme A mathematical correction compensated the removed cobalt and volume by sampling. The parabolic law (Eq. (1)) based on Habashi [21] served as model, since literature supposed a diffusion-controlled reaction mechanism. The letter k represents the rate coefficient, t the passed time and y the concentration of Co in the liquid phase.
mg mg ⎤⋅t 0.5 [min0.5] = y ⎡ ⎤ k⎡ ⎣ l⋅min0.5 ⎦ ⎣ l ⎦
(2)
Co2 + (a) + 2e− ↔ Co (s) E0 = −0.28 V
(3)
Consequently, the proposed cathodic reaction follows Eq. (4) [24]. The addition of H2O2 causes higher oxidation potentials in the acidic
(1)
Fig. 2. Model of embedded cutting insert for reaction rate law determination with indication of lixiviation pathway.
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Fig. 3. Comparison of different model equations for experiment No. 4.
solution (Table 5). Hence the overall reaction follows Eq. (5).
H2 O2 (a) +
2H+ (a)
+
2e−
↔ 2H2 O (l)
E0
= 1.78 V
H2 O2 (a) + 2H+ (a) + Co(s) ↔ 2H2 O (l) + Co2 + (a) E0 = 2.06 V
Table 4 Performed tests with received data from the regression analysis and coefficient of determination.
(4)
No.
(5)
Like stated in Fig. 4, in this area the stable tungsten species displays tungsten oxide hydrate. Correspondingly, corrosion investigations propose the formation of tungsten oxide according to Eq. (6) in aerated 1 mol/l H2SO4 [26]. In the region of 2.5 < pH < 3.5 the formed oxide dissolves as soluble isopolytungstate in coexistence with a hydrated WO3 as given by Eq. (7) [27].
WC (s) + 5H2 O (l) ↔ WO3 (s) + CO2 (g) + 10H+ (a) + 10e−
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17
(6)
15WO3 (s) + 21H2 O (l) ↔ H3 W6O21−3 (a) + 9(WO3 ·2H2 O) (s) + 3H+ (a) (7) Apart from the unwanted tungsten carbide oxidation, the system features another possible side reaction, the decomposition of H2O2. In accordance with literature, the disintegration rises with temperature and an increasing concentration of catalysts such as ions of nickel, gold or platinum [28]. Likewise, WC and cobalt further destabilize H2O2 [29]. For instance, Eq. (8) describes the decomposition reaction (calculated by HSC 8.1.4) for 298.15 K and 1 bar in an aqueous solution.
2H2 O2 (a) = O2 (g) + 2H2 O (l) ΔG = −206 kJ
mg
k⎡ ⎤ ⎣ l ⋅ minn ⎦ 5.85 3.94 2.67 5.83 2.93 3.75 4.42 1.98 4.69 4.75 5.92 2.63 5.02 3.83 3.39 7.82 5.28
n [−]
R2
0.35 0.32 0.47 0.31 0.55 0.45 0.43 0.67 0.45 0.33 0.25 0.50 0.43 0.39 0.38 0.40 0.33
0.9847 0.9801 0.9908 0.9929 0.9519 0.9911 0.9997 0.9967 0.9923 0.9923 0.9669 0.9774 0.9981 0.9966 0.9918 0.9968 0.9984
coefficient of determination (R2) for the respective tests, most of the variance is accounted for in conformity with the variables. Only one experiment (No. 5) deviates more than the others. This reveals a good consistency with the derived empirical model (Eq. (2)) and a low experimental error. The Table 5 lists the mean value (x ) and sample variance (s) of the pH, EORP, room and reactor temperature of each experiment to display the mandatory steady conditions. As stated in Table 2, the temperature
(8)
4. Results The Table 4 presents the calculated values for the factors k and n of Eq. (2) in relation to the varied parameters. As demonstrated by the
Fig. 4. Pourbaix diagrams for W-C-H2O and Co-H2O calculated by FactSage 7.1 including the FactPS 2011 database.
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influence of examined parameters on the coefficients k and n of the generalized empirical rate law (Eq. (2)). Later in this chapter, the response contour plots constitute the achievement in a more demonstrative form. During the statistical evaluation, experiment No. 16 had to be excluded by reasons of the normal probability plot (outlier). Regardless, the summary of fit displays proper characteristics of the executed regression. A value of Q2 > 0.5 indicates a good predictive ability, Q2 > 0.7 is considered to be large. Hence, only a minor predictive error occurs during new trials. Furthermore, the model validity shows proper characteristics, since values higher than 0.25 proof no lack of fit. Especially, a model validity of nearly one implies that the error provoked by the model equation is very small compared to that of the experimental procedure. The reproducibility describes the response variation in terms of the center points caused by the experimental procedure [19]. The prediction plots in Fig. 5 reveal the relationship between the variable parameters and the factors of the model equation. In detail, each plot indicates the impact of one variable on one model coefficient, while the two others exhibit intermediate conditions regarding the experimental limits. Some of the repeated central trials are located outside the 95% confidence interval for the model equation of n and k due to scatter. Concerning the influence of varied parameters, n and k display an opposite behavior. With reference to the temperature, the power factor reaches a minimum at about 335 K, while k attains a peak. Furthermore, the applied acid concentration constitutes a substantial effect. Comparatively low emphasis is indicated by the amount of used oxidizing agent. For example, there exists some probability of a constant reaction rate coefficient inside the confidence interval, whereas H2O2 can incorporate slightly negative consequences in matters of the power coefficient. The augmentation of the oxidant demonstrates no positive implication, as apparent in Fig. 6. In order to advantageously affect the power coefficient, either a low or high temperature with small amounts
Table 5 Statistical values of the experiments to demonstrate the constancy of the conditions. No.
xpH [−]
spH [−]
x ORP [mV]
sORP [mV]
xRT [K]
xBathT [K]
sBathT [K]
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17
1.06 1.04 1.13 1.12 1.51 1.11 1.09 1.59 1.21 1.12 0.84 1.23 1.50 1.37 0.74 1.62 1.03
0.01 0.03 0.08 0.03 0.04 0.01 0.01 0.00 0.01 0.03 0.02 0.05 0.02 0.01 0.05 0.02 0.03
545 616 588 574 524 560 548 517 549 559 592 554 543 528 577 502 548
13 26a 29a 12 3 9 4 9 9 7 42a 33a 15 9 2 3 8
292.15 291.15 292.15 292.15 292.15 293.15 292.15 293.15 293.15 293.15 292.15 292.15 294.15 292.15 292.15 293.15 291.15
353.11 303.16 353.12 328.15 353.16 303.15 328.16 303.15 328.16 328.16 328.15 353.11 328.16 303.15 303.15 353.16 328.16
0.06 0.01 0.14 0.03 0.06 0.01 0.03 0.01 0.03 0.03 0.04 0.04 0.02 0.01 0.01 0.05 0.04
a
First registered value deviant.
Table 6 Summary of fit for the model received by Modde 11.
k n
R2
Q2
Model validity
Reproducibility
0.92 0.93
0.68 0.81
0.94 1.00
0.79 0.54
sensor belonged to the tolerance class B. Thus, a measuring tolerance of ± 0.45–0.70 K existed in the range of 303.15–353.15 K, which in comparison to sBathT caused the main uncertainty. The Table 6 presents statistical factors about the regression quality of the quadratic model equation compiled by Modde 11 to describe the
Fig. 5. Prediction plots of the n- and k factors within a 95% confidence interval (dotted line).
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Fig. 6. Response contour plots for n (top) and k (bottom) as functions of the investigated parameters.
and the formation of an oxide layer on the grains emerges to a certain extent as one potential reason for such deviations. Hochstrasser et al. [13,14] mentioned a possible contact corrosion between cobalt and WC. Thus, the anodic dissolution of the cobalt causes a local increase of the pH with oxidation and dissolution of the WC. Accordingly, the dissolution of the built oxide forms the rate determining step. Hence, always a combined leaching of cobalt and WC in acid and neutral media appears. In the presented experiments no visible oxide formation took place and corresponding to Barbatti et al. [12] the leaching of hard metals should therefore be diffusion-controlled. With reference to Fig. 6, power coefficient values near 0.5 can be actually obtained for a large range of compositions. Reasons for a power coefficient below 0.5 theoretically include the formation of an oxide layer as supplementary diffusion obstacle. However, the decomposition of H2O2 seems more plausible. Otherwise, higher values suggest a detachment of WC grains or a dissolving skeleton reducing the diffusion resistance.
of acid should be chosen. The peak for the rate factor is reached at a temperature of around 335 K as well as concentrations of 6 mol/l CH2O2 and 1 mol/l H2O2.
5. Discussion One issue of the leachant is the application of H2O2 and therefore the unwanted decomposition reaction according to Eq. (8). Nevertheless, the lixiviation without oxidant appears impossible within a reasonable amount of time as examined in screening tests [25]. A critical aspect concerning the oxidant refers to the desirable steady conditions of the system. The examined media showed no perfectly constant status, since at least four values of the EORP differed notably (Table 5). However, the media with the lowest oxidant concentration exhibits the fastest leaching kinetics. With regard to the acid consumption, no abnormalities arose due to the small specimen surface (160 mm2) and low penetration depth in relation to the applied volume (500 ml). As already stated, some investigations in the literature explain potential corrosion mechanisms and therefore an explanation for a deviation from the classic parabolic diffusion law. For instance, Barbatti et al. [12] disclose that the binder-free path restricts the binder metal leaching in acid media. Therefore, the remaining WC forms a carbide skeleton with the advancing reaction, which serves as diffusion barrier. Nevertheless, at approximately ESCE = 800 mV, the oxidation of WC
6. Conclusion and outlook In conformity with literature and the present experimental results, the leaching of a hard metal cutting insert in CH2O2 and H2O2 displays a diffusion restricted mechanism for some analyzed concentration regions. Moreover, the current investigation discloses how to positively influence the leaching properties of cemented carbides. Likewise, this 244
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carbides (US 4234333), 1980. [16] A.M. Human, H.E. Exner, The relationship between electrochemical behaviour and in-service corrosion of WC based cemented carbides, Int. J. Refract. Met. Hard Mater. 15 (1–3) (1997) 65–71. [17] S. Sutthiruangwong, G. Mori, Corrosion properties of Co-based cemented carbides in acidic solutions, Int. J. Refract. Met. Hard Mater. 21 (3–4) (2003) 135–145. [18] J.M. Tarragó, G. Fargas, E. Jimenez-Piqué, A. Felip, L. Isern, D. Coureaux, et al., Corrosion damage in WC–Co cemented carbides: residual strength assessment and 3D FIB-FESEM tomography characterisation, Powder Metall. 57 (5) (2014) 324–330. [19] MKS Umetrics AB, User Guide to Modde 11, (2015) Malmö. [20] R.H. Myers, D.C. Montgomery, C.M. Anderson-Cook, Response Surface Methodology: Process and Product Optimization Using Designed Experiments, 3rd ed., John Wiley & Sons, Hoboken, 2011. [21] F. Habashi, Extractive Metallurgy: General Principles, Gordon and Breach, New York, 1969. [22] O. Levenspiel, Chemical Reaction Engineering, 3rd ed., Wiley, New York, 1999. [23] E. Lassner, W. Schubert, Tungsten: Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds, Kluwer Academic/Plenum Publishers, New York, 1999. [24] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics: A Ready-reference Book of Chemical and Physical Data, 90th ed., CRC Press, Boca Raton, 2009. [25] G. Kücher, S. Luidold, C. Czettl and C. Storf (in press), First evaluation of semidirect recycling methods for the reclamation of cemented carbides based on a literature survey, Revista Matéria. [26] J.D. Voorhies, Electrochemical and chemical corrosion of tungsten carbide (WC), J. Electrochem. Soc. 119 (2) (1972) 219–222. [27] K.M. Andersson, L. Bergström, Oxidation and dissolution of tungsten carbide powder in water, Int. J. Refract. Met. Hard Mater. 18 (2–3) (2000) 121–129. [28] A.F. Holleman, E. Wiberg, N. Wiberg, Lehrbuch der anorganischen Chemie, 102nd ed., de Gruyter, Berlin, 2007. [29] Europäische Forschungsgesellschaft Dünne Schichten e. V, Workshop “Entschichtung, Recycling und Wiederaufbereitung”, (2015).
report reveals the relationship among temperature, oxidant as well as acid concentration, the reaction rate and the power coefficient. However, some divergence between the classical law of diffusion and the actual investigation appeared. On the one hand, the decomposition and consumption of the oxidant may have caused slightly drifting conditions inside the reactor, while on the other the carbide skeleton as diffusion barrier could have grown inconstantly. Certainly, further investigations have to place emphasis on the H2O2 stabilization and the WC skeleton formation. Supplementarily, no oxide formation could be observed under the experimental conditions. As the present experiments intend to promote the semi-direct recycling technology, the lixiviation time will have to be extended to obtain completely leached substrates. After pulverization of the remaining carbide skeletons, a sintering test will reveal the product quality of the hard metals based on recycled powder. Naturally, the recovery of cobalt from the solution by hydrometallurgical techniques plays an important role for implementation of the technique and has therefore to be investigated. Acknowledgment The financial support by the Austrian Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development (CDL-TM) is gratefully acknowledged. References
Gregor Kücher began his education in the Werkschulheim Felbertal, a grammar school with apprenticeship in mechatronics. In continuation, he studied from 2009–2015 metallurgy at the Montanuniversitaet Leoben including a semester abroad at the Universidad Tecnológica Nacional, Córdoba Argentina, in 2013. His master thesis covered decoating processes for cutting inserts as a pretreatment step for the zinc process. Since 2015 he forms part of the Christian Doppler Laboratory for Extractive Metallurgy of Technological Metals at the same university as research associate. His current tasks comprise the development of a hydrometallurgical recycling technology for contaminated worn hard metal substrates.
[1] G. Gille, A. Meier, Recycling von Refraktärmetallen, in: K.J. Thomé-Kozmiensky (Ed.), Recycling und Rohstoffe, TK-Verlag, Neuruppin, 2012, pp. 537–560. [2] K.B. Shedd, Tungsten Recycling in the United States in 2000, http://pubs.usgs.gov/ circ/circ1196-R/. [3] Karhumaa T, Kurkela M, Review of the hardmetal recycling market and the role of the zinc process as a recycling option, Proceedings of the 18th International Plansee Seminar 2013, HM13/1–11. [4] R. Schiesser, Wertstoffrecycling wolframhältiger Sekundärrohstoffe (Dissertation), TU Wien, 2003. [5] H. Meyer, Verfahren zur Rückgewinnung von Ta(Nb)C-WC-TiC-Mischungen (DE 2726817 A1), (1979). [6] Shwayder WM, Method of disintegrating sintered hard carbide masses (US 3438730), 1969. [7] N. Kopylov and B. Polyakov, Method of disintegrating sintered hard metal carbide bodies and recovering hard metal carbide powder (WO 99/07905 A1), 1999. [8] T. Kojima, H. Itoh, R. Sasai, T. Shimizu, Recycling process of WC-Co cermets by hydrothermal treatment, J. Mater. Sci. 40 (19) (2005) 5167–5172. [9] S. Gürmen, S. Stopic, B. Friedrich, Recovery of submicron cobalt-powder by acidic leaching of cemented carbide scrap, in: N. N. (Ed.), Perspectives of the Non-ferrous Metals Industry in an Era of Consolidation, GDMB, Clausthal-Zellerfeld, 2005, pp. 1725–1740. [10] C. Edtmaier, R. Schiesser, C. Meissl, W.D. Schubert, A. Bock, A. Schoen, et al., Selective removal of the cobalt binder in WC/Co based hardmetal scraps by acetic acid leaching, Hydrometallurgy 76 (2005) 63–71. [11] D. zur Megede, E. Heitz, Das Korrosionsverhalten von HartmetallVerbundwerkstoffen in chloridhaltigen wäßrigen Lösungen, Mater. Corros. 37 (5) (1986) 207–214. [12] C.F. Barbatti, F. Sket, J. Garcia, A. Pyzalla, Influence of binder metal and surface treatment on the corrosion resistance of (W,Ti)C-based hardmetals, Surf. Coat. Technol. 201 (6) (2006) 3314–3327. [13] S. Hochstrasser-Kurz, C. Latkoczy, S. Virtanen, P.J. Uggowitzer, P. Schmutz, Analytical characterization of the corrosion mechanisms of WC-Co by electrochemical methods and inductively coupled plasma mass spectroscopy, 208th ECS Meeting, 2005, pp. 251–262. [14] S. Hochstrasser-Kurz, D. Reiss, T. Suter, C. Latkoczy, D. Günther, S. Virtanen, et al., ICP-MS, SKPFM, XPS, and microcapillary investigation of the local corrosion mechanisms of WC–Co hardmetal, J. Electrochem. Soc. 155 (8) (2008) C415–C426. [15] Ghandehari MH, Process for recovering metal carbide powder from cemented
Stefan Luidold started his professional education at the School of Mechanical Engineering in Steyr and then studied industrial environment protection, branch of study process engineering, at the Montanuniversitaet Leoben, from 1994 to 2001. Since April 2001 he is employed at the same university at the chair of Nonferrous Metallurgy, where he was writing his doctoral thesis about an alternative process for the production of niobium powder for the company CBMM, Brazil. In 2013 he got the venia docendi in the field of Nonferrous Metallurgy. From 2008 to 2014 he was the head of the two Research Studio Austria “Aufarbeitung von sondermetallhaltigen Reststoffen” and “Alternative Raw Materials of Technological Metals”. Today he is the head of the Christian Doppler Laboratory for Extractive Metallurgy of Technological Metals. Christoph Czettl at first came in contact with the field of metallurgy at the Technical and Vocational College for mechanical engineering and metallurgy in Leoben. In the years 2001–2007 he finished his master's degree in metallurgy at the Montanuniversitaet Leoben. Thereafter he was project manager of “Design of CVD Coatings” at the Materials Center Leoben. In 2011 he joined CERATIZIT Austria GmbH, since then he works as group leader of the cemented carbide and coating development. Finally, he received his doctoral degree in 2014. Christian Storf visited the technological stream at the Bundesrealgymnasium Reutte and then studied industrial engineering with focus on mechanical engineering at the University of Applied Sciences Kempten, Bavaria, from 2009 to 2013. After writing his diploma thesis on the influences of grain growth inhibitors in steel milling he got an established member of the R & D Carbide and Coating Development at Ceratizit Austria GmbH in 2014. Since then he mainly deals with the development of cemented carbide substrates including conception, implementation and coordination of experiment series, material property measurements and application tests especially recycling technologies and its influence on cemented carbide properties.
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International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Lixiviation kinetics of cobalt from cemented carbides a,⁎
a
b
Gregor Kücher , Stefan Luidold , Christoph Czettl , Christian Storf a b
MARK
b
CD Laboratory for Extractive Metallurgy of Technological Metals, Nonferrous Metallurgy, Montanuniversitaet Leoben, Franz-Josef-Str. 18, A-8700 Leoben, Austria CERATIZIT Austria GmbH, Metallwerk-Plansee-Str. 71, A-6600 Reutte, Austria
A R T I C L E I N F O
A B S T R A C T
Keywords: Semi-direct recycling Reclamation Cemented carbides Leaching Hard metals Chemical kinetics investigation Rate law Lixiviation
This report aims to disclose interactions between semi-direct recycling concepts, corrosion analysis of hard metals and possible leaching mechanisms. Accordingly, an examination reveals chemical kinetic dependencies via leaching a cutting insert in formic acid inclusive an oxidant. Currently, contradictions and issues in literature slow down the progress of semi-direct technologies in comparison to direct and indirect recycling concepts. Additionally, corrosion investigations show helpful explanations for leaching mechanisms of cobalt from cemented carbides, which so far are not connected to reclamation reviews. In order to obtain reliable data from experiments, the statistical software Modde 11 assists to apply a design of experiments and to perform a multiple regression analysis. Nevertheless, the experimental setup decides the result of the rate law investigation. The sample geometry and leaching conditions in the reactor affect the outcome in the form of a kinetic model equation. Finally, an empirical kinetic law with its dependencies inside the experimental boundaries presents potential ways to favorably influence cobalt leaching out of cemented carbides.
1. Introduction Indirect recycling dominates today's recycling technologies for cemented carbide scrap. About 85% of the total recycled hard metal amount in Germany [1] and at least half in the USA [2] are treated accordingly. The zinc process, a direct method, handles the residual quantity [2,3]. Consequently, the semi-direct techniques only play a minor role caused by technological obstacles, but offer advantages in comparison to the prevalent processes. The semi-direct recycling of hard metals rests upon the idea of leaching the binder metal (mostly cobalt) as well as coatings without affecting the hard phases (commonly WC, tungsten carbide). Subsequently, the remaining brittle WC can be mechanically crushed and reused without further processing. Thus, the produced tungsten carbide powders facilitate a direct application for the production of new cemented carbide commodities. In addition, common hydrometallurgical methods retrieve the lixiviated binder out of the solution. Hence, semi-direct methods potentially omit process steps like solution purification, SX, crystallization, reduction and carburation in contrast to primary or indirectly recycled powder [1,4,5]. Naturally, there exist reasons why no considerable industrial process is currently in operation. Numerous authors [4–7] reveal that one main issue - the partial oxidation of WC - contaminates the recyclate and hinders the leaching reaction through limited diffusion of the reactants. Also chemically resistant coating materials represent a
⁎
diffusion barrier and a potential impurity [7]. Generally, the dissolution of the binder metal slows down, depending on the grain size as well as binder content if bulky material is leached without mechanical support [4,6]. Schiesser [4] for instance discloses leaching times of approximately 30–160 days for a cube of 1 cm3 depending on material grade and conditions in acetic acid. Therefore, some studies examine hard metals with high cobalt content, for example Kojima et al. [8], or pulverized scraps like Gürmen et al. [9] to achieve a more reasonable extraction rate. Especially the inefficient mechanical disintegration of untreated hard metals seems not feasible as a result of high wear rates. A lack of knowledge remains in literature concerning the very bases of a potential technique. Beside the stated issues in literature, only Edtmaier et al. [10] reported the reaction rate of semi-direct leaching of a bulky substrate once. Others like Kojima et al. [8] inform roughly about the time dependency. The report of Gürmen et al. [9] deals with pulverized fractions and further available information results from patents. Normally, these did not comprise details about reaction rate relations. Additionally, a variety of available publications concerns the corrosion properties of hard metals as well as possible dissolution mechanisms [11–14], beneficial regions for the selective cobalt leaching [15,16] and the formation of a carbide skeleton [15,17,18]. Generally, scientific information about semi-direct methods is hardly available and therefore a necessity for an accurate experimental design and specimen preparation persists for a proper assessment. Side effects caused by the geometry should be excluded, while more care has
Corresponding author. E-mail addresses: [email protected] (G. Kücher), [email protected] (S. Luidold).
http://dx.doi.org/10.1016/j.ijrmhm.2017.10.004 Received 6 July 2017; Received in revised form 12 October 2017; Accepted 14 October 2017 Available online 16 October 2017 0263-4368/ © 2017 Elsevier Ltd. All rights reserved.
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Table 1 Individual experiments with their parameters of the compiled CCF design. No.
T [K]
cCH2O2 [mol/l]
cH2O2 [mol/l]
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17
353.15 303.15 353.15 328.15 353.15 303.15 328.15 303.15 328.15 328.15 328.15 353.15 328.15 303.15 303.15 353.15 328.15
6.00 6.00 6.00 3.75 1.50 3.75 3.75 1.50 3.75 3.75 6.00 3.75 1.50 1.50 6.00 1.50 3.75
1.00 1.00 4.00 2.50 4.00 2.50 2.50 1.00 1.00 2.50 2.50 2.50 2.50 4.00 4.00 1.00 4.00
to be taken of temperature monitoring and the verification of preferably constant leaching conditions. The current examination discloses the results of leaching cobalt from a cutting insert in formic acid (CH2O2) and hydrogen peroxide (H2O2). 2. Materials and methods A design of experiments under assistance of the software Modde 11 served for a target-oriented execution including the mutual interaction of examined factors. The selected CCF (Central Composite Face centered) design forms a part of the response surface methodology. Altogether 17 tests facilitated the analysis of the three variable parameters temperature and molar concentration of H2O2 as well as CH2O2. Additionally, three incorporated repeated center experiments assisted for the determination of the reproducibility and other statistical quality factors. In continuation, Table 1 presents the used parameters of the design. Further theoretical background to this topic can be obtained by the Modde 11 User Guide [19] or Myers et al. [20].
Fig. 1. Schematic representation of the double-walled reaction vessel with position of the specimen inside.
Table 2 Identification of the experimental setup equipment.
2.1. Experimental setup
Instrument
Specification
Reactor
Double-walled, cylindrical, flange DN100, 1000 ml LAUDA Proline Edition X P 8 C IKA RH basic
Thermostat Magnetic stirrer (only stirring function) Pt100 sensor (class B)
The experiments applied a borosilicate double-walled reaction vessel with a nominal volume of 1000 ml with a cap attached by a steel clamp. According to Fig. 1, the cap possessed five ground glass joints corresponding to DIN 12242, four 29/23 and one 14/23. The smaller comprised the reflux condenser with the outlet for nitrogen as purging gas. Its inlet, a Pt100 temperature sensor, a sampling port and the centrally positioned glass bar with the sample occupied the other ones. In addition, a magnetic stirrer at about 200 min− 1 ensured a homogenous intermixing of the solution. A thermostat connected to the vessel together with the Pt100 sensor established an automatic temperature regulation to keep the desired level inside the reactor. The software Wintherm Plus 3.4 recorded the actual temperature. For proving preferably constant reaction conditions during the experiment, the periodically removed liquid samples were measured for determining the prevailing pH and EORP after cooling to room temperature. Table 2 lists the exact characterizations of the used experimental items. A cutting insert provided by courtesy of CERATIZIT Austria GmbH served as test substrate. In reference to Table 3, the hard metal with medium grain size contained a low amount of cobalt and composite carbides. Although ISO 1832 determines the geometry, the exact surface of the embedded and polished specimen in contact with the reactants was ascertained to be 160 mm2. All experiments employed a total liquid volume of 500 ml at the beginning. Hence, the specific
pH meter ORP meter
Jumo 902250/32-415-1001-1-3-300-042500/000 WTW inoLab pH 7310, electrode: WTW SenTix81 Hanna instruments HI2211 pH/ORP Meter, electrode: HI3131B
Table 3 Quality of applied cutting insert. Specification
Value
Co WC Composite carbides Grain size Delivery state Geometry
6.0 wt% 93.4 wt% 0.6 wt% Medium Sintered, nonmachined SNUN120412 – ISO 1832
proportion amounted to 0.32 mm2/ml whereas the common s/l ratio cannot be applied because only the defined surface was contacted with the solution. Initially, the liquid was heated to the desired temperature. The trial started by immersion of the specimen from the cap into the liquid.
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During the assessment, the extraction of solution samples out of the vessel after 5, 10, 20 and 30 min ensured the monitoring of cobalt concentration. An ICP-MS in accord with ÖNORM EN ISO 17294-2 performed the measurement of the elemental concentration in the leachant. A multi-element standard solution with β = 10 mg/l for cobalt provided the calibration within 0.50–100 μg/l. For analysis, a triple dilution occurred by applying the ratios 1:100, 1:1000 and 1:10,000.
Anyhow, some discrepancy emerged for a few experiments during the regression analysis. For instance, Fig. 3 displays the received data points (spots) of the experiment No. 4, a center trial. Apparently, the dotted line for the diffusion-controlled model (n = 0.5) improperly characterizes the dissolution mechanism. Hence, the necessary generalization of Eq. (1) resulted in the empirical law of Eq. (2) with n, the power coefficient as second factor.
2.2. Specimen preparation
mg mg ⎤⋅t n [minn] = y ⎡ ⎤ k⎡ ⎣ l⋅minn ⎦ ⎣ l ⎦
Generally, for diffusion-limited reactions the transport of the species to the reaction surface or away from it represents the rate determining step. Often this mechanism predominates in heterogeneous reactions like the dissolution of solids. Here, the complete conversion rate decreases over time demonstrated by n = 0.5. In contrast, for a chemically restricted process the kinetics of the reaction itself constitute the limiting factor. The overall reaction rate stays constant displayed by the value n = 1. Accordingly, the power coefficient facilitates conclusions about the reaction type. Nevertheless, a secure verification of the mechanism implies an invariant surface area and concentration [21]. For the evaluation, a simple logarithm performed the mandatory linearization of Eq. (2) for linear regression analysis with the function “LINEST” of MS Excel 2010 to obtain the values for k as well as n. Finally, a statistical analysis by the software Modde 11 served for the compilation of quadratic equations about the dependences of k and n on the three process parameters T, cH2O2 and cCH2O2.
As specified by Habashi [21], heterogeneous reactions (between solids and fluids) display only an evaluable reaction rate if the surface area and the concentration stay constant. Alternatively, a decreasing concentration or surface area provokes a falling reaction rate. Hence, for a reliable detection of reaction- or diffusion-controlled mechanism the surface area and the concentration have to be kept almost constant. Thus, main obstacles in reaction rate investigations consist of geometrical implications. For example, assumptions and mathematics compensate the changing boundary surface area in shrinking core models to derive the respective model equation. In order to avoid this issue and to find a more simple form of equation for the time dependency, more attention was paid to the sample preparation. A large liquid volume (500 ml) in relation to the surface (160 mm2) helped to diminish effects of dissolved ions in the solution. Very detailed information about modeling reaction rates can be found in Habashi [21] and Levenspiel [22]. For the purpose of bypassing the three-dimensional task, the specimen was hot-embedded in Struers ClaroFast, a thermoplast. Grinding and polishing steps, the last being a 1 μm diamond finish, ensured equal conditions of all substrate surfaces. The Fig. 2 represents a sketch of a final test specimen as applied during the experiments. Hence, the accordingly prepared sample evidently featured only one possible reaction plane and consequently the surface area stayed almost constant. Here, the indicated drill hole served for fixation of the sample on the beforehand stated glass bar in the reactor (Fig. 1). Prior tests verified the chemical stability of the resin in the liquid phase. Additionally, no preferential leaching of the contact zone between substrate and synthetic material appeared which could influence the results.
3. Hydrometallurgical framework The pourbaix diagrams for cobalt and tungsten carbide disclose a simplified hydrometallurgical setting of this investigation. As represented by Fig. 4, the acidic region shows the predominance of a soluble Co2 + ion and insoluble tungsten oxides. Though, the setting for tungsten species comprises more complexness. Typically, in acidic media tungsten tends to form various complexed isopolytungstate ions in dependence of pH and time. At the moment the knowledge of occurring species remains limited, especially concerning their thermodynamic data. Additionally, literature indicates the chance of forming peroxotungstates in the presence of H2O2 [23]. From the thermodynamic point of view above a potential of E0 ≥ − 0.28 V the dissolution of cobalt proceeds, like specified by Eq. (3) [24]. Anyhow as verified by screening tests [25] the used acids alone could not accomplish a leaching of the cobalt out of the specimen. Only acids combined with an oxidation agent (H2O2) achieved a lixiviation.
2.3. Model equation and examination scheme A mathematical correction compensated the removed cobalt and volume by sampling. The parabolic law (Eq. (1)) based on Habashi [21] served as model, since literature supposed a diffusion-controlled reaction mechanism. The letter k represents the rate coefficient, t the passed time and y the concentration of Co in the liquid phase.
mg mg ⎤⋅t 0.5 [min0.5] = y ⎡ ⎤ k⎡ ⎣ l⋅min0.5 ⎦ ⎣ l ⎦
(2)
Co2 + (a) + 2e− ↔ Co (s) E0 = −0.28 V
(3)
Consequently, the proposed cathodic reaction follows Eq. (4) [24]. The addition of H2O2 causes higher oxidation potentials in the acidic
(1)
Fig. 2. Model of embedded cutting insert for reaction rate law determination with indication of lixiviation pathway.
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Fig. 3. Comparison of different model equations for experiment No. 4.
solution (Table 5). Hence the overall reaction follows Eq. (5).
H2 O2 (a) +
2H+ (a)
+
2e−
↔ 2H2 O (l)
E0
= 1.78 V
H2 O2 (a) + 2H+ (a) + Co(s) ↔ 2H2 O (l) + Co2 + (a) E0 = 2.06 V
Table 4 Performed tests with received data from the regression analysis and coefficient of determination.
(4)
No.
(5)
Like stated in Fig. 4, in this area the stable tungsten species displays tungsten oxide hydrate. Correspondingly, corrosion investigations propose the formation of tungsten oxide according to Eq. (6) in aerated 1 mol/l H2SO4 [26]. In the region of 2.5 < pH < 3.5 the formed oxide dissolves as soluble isopolytungstate in coexistence with a hydrated WO3 as given by Eq. (7) [27].
WC (s) + 5H2 O (l) ↔ WO3 (s) + CO2 (g) + 10H+ (a) + 10e−
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17
(6)
15WO3 (s) + 21H2 O (l) ↔ H3 W6O21−3 (a) + 9(WO3 ·2H2 O) (s) + 3H+ (a) (7) Apart from the unwanted tungsten carbide oxidation, the system features another possible side reaction, the decomposition of H2O2. In accordance with literature, the disintegration rises with temperature and an increasing concentration of catalysts such as ions of nickel, gold or platinum [28]. Likewise, WC and cobalt further destabilize H2O2 [29]. For instance, Eq. (8) describes the decomposition reaction (calculated by HSC 8.1.4) for 298.15 K and 1 bar in an aqueous solution.
2H2 O2 (a) = O2 (g) + 2H2 O (l) ΔG = −206 kJ
mg
k⎡ ⎤ ⎣ l ⋅ minn ⎦ 5.85 3.94 2.67 5.83 2.93 3.75 4.42 1.98 4.69 4.75 5.92 2.63 5.02 3.83 3.39 7.82 5.28
n [−]
R2
0.35 0.32 0.47 0.31 0.55 0.45 0.43 0.67 0.45 0.33 0.25 0.50 0.43 0.39 0.38 0.40 0.33
0.9847 0.9801 0.9908 0.9929 0.9519 0.9911 0.9997 0.9967 0.9923 0.9923 0.9669 0.9774 0.9981 0.9966 0.9918 0.9968 0.9984
coefficient of determination (R2) for the respective tests, most of the variance is accounted for in conformity with the variables. Only one experiment (No. 5) deviates more than the others. This reveals a good consistency with the derived empirical model (Eq. (2)) and a low experimental error. The Table 5 lists the mean value (x ) and sample variance (s) of the pH, EORP, room and reactor temperature of each experiment to display the mandatory steady conditions. As stated in Table 2, the temperature
(8)
4. Results The Table 4 presents the calculated values for the factors k and n of Eq. (2) in relation to the varied parameters. As demonstrated by the
Fig. 4. Pourbaix diagrams for W-C-H2O and Co-H2O calculated by FactSage 7.1 including the FactPS 2011 database.
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influence of examined parameters on the coefficients k and n of the generalized empirical rate law (Eq. (2)). Later in this chapter, the response contour plots constitute the achievement in a more demonstrative form. During the statistical evaluation, experiment No. 16 had to be excluded by reasons of the normal probability plot (outlier). Regardless, the summary of fit displays proper characteristics of the executed regression. A value of Q2 > 0.5 indicates a good predictive ability, Q2 > 0.7 is considered to be large. Hence, only a minor predictive error occurs during new trials. Furthermore, the model validity shows proper characteristics, since values higher than 0.25 proof no lack of fit. Especially, a model validity of nearly one implies that the error provoked by the model equation is very small compared to that of the experimental procedure. The reproducibility describes the response variation in terms of the center points caused by the experimental procedure [19]. The prediction plots in Fig. 5 reveal the relationship between the variable parameters and the factors of the model equation. In detail, each plot indicates the impact of one variable on one model coefficient, while the two others exhibit intermediate conditions regarding the experimental limits. Some of the repeated central trials are located outside the 95% confidence interval for the model equation of n and k due to scatter. Concerning the influence of varied parameters, n and k display an opposite behavior. With reference to the temperature, the power factor reaches a minimum at about 335 K, while k attains a peak. Furthermore, the applied acid concentration constitutes a substantial effect. Comparatively low emphasis is indicated by the amount of used oxidizing agent. For example, there exists some probability of a constant reaction rate coefficient inside the confidence interval, whereas H2O2 can incorporate slightly negative consequences in matters of the power coefficient. The augmentation of the oxidant demonstrates no positive implication, as apparent in Fig. 6. In order to advantageously affect the power coefficient, either a low or high temperature with small amounts
Table 5 Statistical values of the experiments to demonstrate the constancy of the conditions. No.
xpH [−]
spH [−]
x ORP [mV]
sORP [mV]
xRT [K]
xBathT [K]
sBathT [K]
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17
1.06 1.04 1.13 1.12 1.51 1.11 1.09 1.59 1.21 1.12 0.84 1.23 1.50 1.37 0.74 1.62 1.03
0.01 0.03 0.08 0.03 0.04 0.01 0.01 0.00 0.01 0.03 0.02 0.05 0.02 0.01 0.05 0.02 0.03
545 616 588 574 524 560 548 517 549 559 592 554 543 528 577 502 548
13 26a 29a 12 3 9 4 9 9 7 42a 33a 15 9 2 3 8
292.15 291.15 292.15 292.15 292.15 293.15 292.15 293.15 293.15 293.15 292.15 292.15 294.15 292.15 292.15 293.15 291.15
353.11 303.16 353.12 328.15 353.16 303.15 328.16 303.15 328.16 328.16 328.15 353.11 328.16 303.15 303.15 353.16 328.16
0.06 0.01 0.14 0.03 0.06 0.01 0.03 0.01 0.03 0.03 0.04 0.04 0.02 0.01 0.01 0.05 0.04
a
First registered value deviant.
Table 6 Summary of fit for the model received by Modde 11.
k n
R2
Q2
Model validity
Reproducibility
0.92 0.93
0.68 0.81
0.94 1.00
0.79 0.54
sensor belonged to the tolerance class B. Thus, a measuring tolerance of ± 0.45–0.70 K existed in the range of 303.15–353.15 K, which in comparison to sBathT caused the main uncertainty. The Table 6 presents statistical factors about the regression quality of the quadratic model equation compiled by Modde 11 to describe the
Fig. 5. Prediction plots of the n- and k factors within a 95% confidence interval (dotted line).
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Fig. 6. Response contour plots for n (top) and k (bottom) as functions of the investigated parameters.
and the formation of an oxide layer on the grains emerges to a certain extent as one potential reason for such deviations. Hochstrasser et al. [13,14] mentioned a possible contact corrosion between cobalt and WC. Thus, the anodic dissolution of the cobalt causes a local increase of the pH with oxidation and dissolution of the WC. Accordingly, the dissolution of the built oxide forms the rate determining step. Hence, always a combined leaching of cobalt and WC in acid and neutral media appears. In the presented experiments no visible oxide formation took place and corresponding to Barbatti et al. [12] the leaching of hard metals should therefore be diffusion-controlled. With reference to Fig. 6, power coefficient values near 0.5 can be actually obtained for a large range of compositions. Reasons for a power coefficient below 0.5 theoretically include the formation of an oxide layer as supplementary diffusion obstacle. However, the decomposition of H2O2 seems more plausible. Otherwise, higher values suggest a detachment of WC grains or a dissolving skeleton reducing the diffusion resistance.
of acid should be chosen. The peak for the rate factor is reached at a temperature of around 335 K as well as concentrations of 6 mol/l CH2O2 and 1 mol/l H2O2.
5. Discussion One issue of the leachant is the application of H2O2 and therefore the unwanted decomposition reaction according to Eq. (8). Nevertheless, the lixiviation without oxidant appears impossible within a reasonable amount of time as examined in screening tests [25]. A critical aspect concerning the oxidant refers to the desirable steady conditions of the system. The examined media showed no perfectly constant status, since at least four values of the EORP differed notably (Table 5). However, the media with the lowest oxidant concentration exhibits the fastest leaching kinetics. With regard to the acid consumption, no abnormalities arose due to the small specimen surface (160 mm2) and low penetration depth in relation to the applied volume (500 ml). As already stated, some investigations in the literature explain potential corrosion mechanisms and therefore an explanation for a deviation from the classic parabolic diffusion law. For instance, Barbatti et al. [12] disclose that the binder-free path restricts the binder metal leaching in acid media. Therefore, the remaining WC forms a carbide skeleton with the advancing reaction, which serves as diffusion barrier. Nevertheless, at approximately ESCE = 800 mV, the oxidation of WC
6. Conclusion and outlook In conformity with literature and the present experimental results, the leaching of a hard metal cutting insert in CH2O2 and H2O2 displays a diffusion restricted mechanism for some analyzed concentration regions. Moreover, the current investigation discloses how to positively influence the leaching properties of cemented carbides. Likewise, this 244
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report reveals the relationship among temperature, oxidant as well as acid concentration, the reaction rate and the power coefficient. However, some divergence between the classical law of diffusion and the actual investigation appeared. On the one hand, the decomposition and consumption of the oxidant may have caused slightly drifting conditions inside the reactor, while on the other the carbide skeleton as diffusion barrier could have grown inconstantly. Certainly, further investigations have to place emphasis on the H2O2 stabilization and the WC skeleton formation. Supplementarily, no oxide formation could be observed under the experimental conditions. As the present experiments intend to promote the semi-direct recycling technology, the lixiviation time will have to be extended to obtain completely leached substrates. After pulverization of the remaining carbide skeletons, a sintering test will reveal the product quality of the hard metals based on recycled powder. Naturally, the recovery of cobalt from the solution by hydrometallurgical techniques plays an important role for implementation of the technique and has therefore to be investigated. Acknowledgment The financial support by the Austrian Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development (CDL-TM) is gratefully acknowledged. References
Gregor Kücher began his education in the Werkschulheim Felbertal, a grammar school with apprenticeship in mechatronics. In continuation, he studied from 2009–2015 metallurgy at the Montanuniversitaet Leoben including a semester abroad at the Universidad Tecnológica Nacional, Córdoba Argentina, in 2013. His master thesis covered decoating processes for cutting inserts as a pretreatment step for the zinc process. Since 2015 he forms part of the Christian Doppler Laboratory for Extractive Metallurgy of Technological Metals at the same university as research associate. His current tasks comprise the development of a hydrometallurgical recycling technology for contaminated worn hard metal substrates.
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Stefan Luidold started his professional education at the School of Mechanical Engineering in Steyr and then studied industrial environment protection, branch of study process engineering, at the Montanuniversitaet Leoben, from 1994 to 2001. Since April 2001 he is employed at the same university at the chair of Nonferrous Metallurgy, where he was writing his doctoral thesis about an alternative process for the production of niobium powder for the company CBMM, Brazil. In 2013 he got the venia docendi in the field of Nonferrous Metallurgy. From 2008 to 2014 he was the head of the two Research Studio Austria “Aufarbeitung von sondermetallhaltigen Reststoffen” and “Alternative Raw Materials of Technological Metals”. Today he is the head of the Christian Doppler Laboratory for Extractive Metallurgy of Technological Metals. Christoph Czettl at first came in contact with the field of metallurgy at the Technical and Vocational College for mechanical engineering and metallurgy in Leoben. In the years 2001–2007 he finished his master's degree in metallurgy at the Montanuniversitaet Leoben. Thereafter he was project manager of “Design of CVD Coatings” at the Materials Center Leoben. In 2011 he joined CERATIZIT Austria GmbH, since then he works as group leader of the cemented carbide and coating development. Finally, he received his doctoral degree in 2014. Christian Storf visited the technological stream at the Bundesrealgymnasium Reutte and then studied industrial engineering with focus on mechanical engineering at the University of Applied Sciences Kempten, Bavaria, from 2009 to 2013. After writing his diploma thesis on the influences of grain growth inhibitors in steel milling he got an established member of the R & D Carbide and Coating Development at Ceratizit Austria GmbH in 2014. Since then he mainly deals with the development of cemented carbide substrates including conception, implementation and coordination of experiment series, material property measurements and application tests especially recycling technologies and its influence on cemented carbide properties.
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