Part 1: Experimental Design and Efficiencies
This study attempts to achieve optimal recovery of diamond and cobalt from polycrystalline diamond (PCD) blanks. In nine experimental runs of 5 days’ duration, cobalt-containing PCD was leached in aqua regia at atmospheric pressure between 60 °C and 80 °C. Using two reactors in parallel, the temperature, ultrasound irradiation time, solid-to-liquid ratio, and PCD size were varied to find out which parameters are beneficial and could possibly accelerate the process. PCD weights and cobalt content in solution were monitored as well. It was found that aqua regia accumulated more dissolved cobalt at 60 °C than at 80 °C, probably due to volatile reagents being less available over time. With added ultrasound and at low S/L ratios, i.e., close to 15 g/L, the leaching time for D14 to reach a 90% leach mark was reduced to three days, a significant shortening. PCD type D18 with a thickness of 3.5 mm were not leached to completion after five days. Leaching temperature had more impact on the results than ultrasound. These findings were reinforced by the mass balance in which a small discrepancy was found. The PCD lost a fraction of weight that could not be explained by the weight of dissolved cobalt. From EDS (Energy Dispersive Spectroscopy) data and the nature of PCD, this fraction probably consisted of, oxygen from oxides in the PCD, iron or single diamond grains that were broken off by the impact of the ultrasound.
The term polycrystalline diamond (PCD) describes a variety of amorphous compounds mostly or wholly consisting of microscopically small diamond grains. A single crystal of natural diamond is anisotropic in terms of its mechanical and thermodynamic properties, including tensile strength and thermal conductivity, for instance. Most PCD will have a random arrangement of individual grains, resulting in a quasi-isotropic compound. However, there are forms of PCD that are made in a different way and that have different properties. Binderless PCD (Sumidia), CVD crystals [1
], and monocrystalline dies in general will not be discussed herein. The conditions needed for diamond powder to form a framework are extreme. Only in the region of 50 kbar and at a temperature of 2000 °C will the desired reaction happen on reasonable time scales [2
]. Cobalt is used as a solvent catalyst in the production of PCD; without it, the reaction would require even more pressure and a higher temperature.
The leaching solution in this case has the colloquial name “aqua regia” because it was found to dissolve noble metals such as gold or platinum; early records of its use date back centuries [5
]. Aqua regia ensures an oxidation environment. More specifically, the aqua regia was mixed from 3 parts Merck KGaA fuming hydrochloric acid 37%, Emsure ACS/ISO quality and one part PanReac ApplicChem nitric acid 65% ISO analysis quality.
The solution is a mixture of hydrochloric acid, HCl, and nitric acid, HNO3
. Both are strong acids, and at a ratio of 3:1, reactions (1) to (3) occur [6
The formation and transport of molecular chlorine gas and NOCl has been found to occur within minutes to hours [6
]. Baghalha et al. [7
] concluded that the 3:1 mixing ratio maximizes the production of chlorine per unit mass of reactants, and is to be favored when chlorine is the desired oxidizing agent. However, if the desired reaction requires only low pH or different oxidizing agents, this ratio or aqua regia itself may not be suitable. This study intends to extract cobalt from PCD as a chloride, CoCl2(aq)
, and therefore, uses aqua regia, or NOCl to be precise. The desired reaction in this case is the oxidation and dissolution of cobalt into aqua regia, which is achieved in the following redox-reactions:
The Pourbaix diagrams by Huang et al. [8
] show that the equilibrium for this reaction should be on the right side of the balance, since the divalent cobalt cation is not only stable at pH << 1, but also at pH > 1. In many cases, the most cost- and energy-efficient way to extract metal from gangue or scraps is to oxidize and dissolve it in a leaching solution. There are many examples; the well-established Caron process is one of them [9
]. It is used to treat lateritic nickel ores by reduction roasting and subsequent leaching for the purpose of obtaining a nickel-bearing solution while separating nickel from iron [10
The polycrystalline diamond in aqua regia can be seen as a solid compound particle where its metallic components react with the solution. The reaction front moves inwards and leaves a layer of inert diamond grains behind. That is why the model of the shrinking unreacted core (SCM) is applied [12
]. According to the model, the leaching rate may depend on the reaction or diffusion of educts and products from the reaction site; in reality, it is often a mixture of both effects. If a linear relationship is found in the plots of Equations (6) and (7), over time, the model is confirmed and the apparent rate constant can be extracted from the slope. Equation (6) is the Ginstling-Brounshtein (D4) model. The D4 model is another type of diffusion three-dimensional model, in contrast to widely used Jander model, as reported by Khawan and Flanagan [14
]. If a solid particle has a spherical or cubical shape, a contracting sphere/cube model can be applied, as shown in Equation (7).
X is a dimensionless variable that represents the relative change in the amount of substance or concentration, which is why in leaching processes, the yield is taken for X, for example. In this study, k will be indexed with “D” or “R”, depending on whether the diffusion- or chemical reaction-controlled model was applied.
From this apparent rate constant, the activation energy for the reaction can be obtained by plotting the natural logarithm of k over the reciprocal temperature in an Arrhenius plot, or by using the following equation [15
The slope of the Arrhenius plot delivers the activation energy. Furthermore, the k-rate coefficient can be used to calculate the Gibbs energy accompanying this reaction using Equation (9)
The main aim of this work was to study the kinetics of cobalt removal from polycrystalline diamond blanks using an ultrasound-assisted leaching process; no reports of such a process exist in the literature. Two mathematical models will be tested in order to determine the activation energy and rate coefficient. An additional thermochemical analysis was included to provide a better explanation of the behavior of cobalt in a water solution at different pH-Eh values using an Eh-pH diagram.
F.K. and S.S. conceptualized and managed the research. S.S. co-wrote the paper. S.G. contributed the SEM and EDS analysis of the PCD surface. B.F. supervised personnel, coordinated resources, and co-wrote the paper. F.K. performed the experiments and wrote the paper. All authors have read and agreed to the published version of the manuscript.
This research was funded by Projektträger (PtJ) Jülich, Grant Number 005-1902-0147.
We would like to thank to Redies Deutschland GmbH & Co. KG for providing PCD samples as well as additional equipment.
Conflicts of Interest
The authors declare no conflict of interest.
- Dekempeneer, E.; Jacobs, R.; Smeets, J.; Meneve, J.; Eersels, L.; Blanpain, B.; Roos, J.; Oostra, D.J. Rf plasma-assisted chemical vapour deposition of diamond-like carbon: Physical and mechanical properties. Thin Solid Films 1992, 217, 56–61. [Google Scholar] [CrossRef]
- General Electric Research Laboratory. Man-made diamonds. Nature 1955, 176, 50–56. [Google Scholar]
- Bovenkerk, H.P.; Bundy, F.P.; Hall, H.T.; Strong, H.M.; Wentorf, R.H., Jr. Preparation of Diamond. Nature 1959, 184, 1094–1098. [Google Scholar] [CrossRef]
- Strong, H.M.; Chrenko, R.M. Diamond growth rates and physical properties of laboratory-made diamond. J. Phys. Chem. 1971, 75, 838–1843. [Google Scholar] [CrossRef]
- Lavoisier, A.L. Elements of Chemistry, in a New Systematic Order. Containing All the Modern Discoveries. In Dover Histories and Classics of Science; Dover Publications: New York, NY, USA, 1965. [Google Scholar]
- Massucci, M.; Clegg, S.L.; Brimblecombe, P. Equilibrium Partial Pressures, Thermodynamic Properties of Aqueous and Solid Phases, and Cl2 Production from Aqueous HCl and HNO3 and Their Mixtures. J. Phys. Chem. A 1999, 21, 4209–4226. [Google Scholar] [CrossRef]
- Baghalha, M.; Khosravian, G.H.; Mortaheb, H.R. Kinetics of platinum extraction from spent reforming catalysts in aqua-regia solutions. Hydrometallurgy 2009, 95, 247–253. [Google Scholar] [CrossRef]
- Huang, J.H.; Kargl-Simard, C.; Oliazadeh, M.; Alfantazi, A.M. pH-Controlled precipitation of cobalt and molybdenum from industrial waste effluents of a cobalt electrodeposition process. Hydrometallurgy 2004, 75, 77–90. [Google Scholar] [CrossRef]
- Moskalyk, R.R.; Alfantazi, A.M. Nickel laterite processing and electrowinning practice. Miner. Eng. 2002, 15, 593–605. [Google Scholar]
- Olanipekun, E.O. Kinetics of leaching laterite. Int. J. Miner. Process. 2000, 60, 9–14. [Google Scholar] [CrossRef]
- De Graaf, J.E. The treatment of lateritic nickel ores—A further study of the caron process and other possible improvements. Part, I. Effect of reduction conditions. Hydrometallurgy 1979, 5, 47–65. [Google Scholar] [CrossRef]
- Chang, J.; Zhang, E.-D.; Zhang, L.-B.; Peng, J.-H.; Zhou, J.-W.; Srinivasakannan, C.; Yang, C.-J. A comparison of ultrasound-augmented and conventional leaching of silver from sintering dust using acidic thiourea. Ultrason. Sonochem. 2017, 34, 222–231. [Google Scholar] [CrossRef] [PubMed]
- Wen, C.Y. Noncatalytic heterogeneous solid-fluid reaction models. Ind. Eng. Chem. 1968, 9, 34–54. [Google Scholar] [CrossRef]
- Khawam, A.; Flanagan, D.R. Solid state kinetic models- Basics and Mathematical Fundamentals. J. Phys. Chem. B 2006, 110, 17315–17328. [Google Scholar] [CrossRef] [PubMed]
- Wedler, G. Lehrbuch der Physikalischen Chemie; Wiley-VCH: Weinheim, Germany, 2010. [Google Scholar]
- Han, K.N.; Meng, X. The Leaching Behavior of Nickel and Cobalt from Metals and Ores—A review. Extr. Metall. Copp. Nickel Cobalt 1993, 1, 709–733. [Google Scholar]
- Kiessling, F.; Gürmen, S.; Stopic, S.; Friedrich, B. Advances in synthesis of metallic powders using ultrasound assisted leaching process from polycrystalline diamond blanks—Process design, (First part). Metals 2020, in press. [Google Scholar]
- Grénnman, H.; Salmi, T.Y.; Murzin, D. Solid-liquid reaction kinetics—Experimental aspects and model development. Rev. Chem. Eng. 2011, 27, 53–77. [Google Scholar] [CrossRef]
- Han, K.N.; Lawson, F. Leaching behavior of cobalt in acid solutions. J. Less Common. Metals 1974, 38, 19–29. [Google Scholar] [CrossRef]
eH-pH diagram for Co-Cl-H2O at 25 °C and 80 °C.
SEM image of ground and polished PCD surface, 5 µm class (dark grey areas are the bridged diamond grains with cavities where traces of cobalt also show up as lighter shades).
Measuring magnetic properties of PCD a: magnet support, b: ∅ 5 mm by 8 mm Nd-Alloy magnet, c: PCD blank, d: probe, e: handheld teslameter.
Data plot of teslametric samples, (left): as measured, (right): normalized to initial value.
Teslametric measurements—comparison of process parameters. “Full Ultrasound” refers to the full-time use of ultrasound.
Teslametric measurements—comparison with respect to bath temperature
SCM plots for D14 and D18, red: 80 °C, blue: 60 °C.
SCM plot from liquid samples week 1.
Arrhenius plots for D14 and D18.
Plots of ln(k) over PCD blank size and ultrasound time fraction (D14).
ΔG0 trends for D14 and D18.
Energy dispersive X-ray spectroscopy image of ground and polished PCD surface, also 5 µm class.
|Content (Weight %)||C||O||Fe ||Co|
Dimensions of PCD samples with grain size of 5 µm.
|Blank Type||Symbol||Diameter [mm]||Height [mm]||Weight [g]||Volume [mm3]||Surface Area [mm2]||Surface/Volume [mm−1]|
|Mant® MSD-14-005||D14||4.05 ± 0.09||2.00 ± 0.04||0.099||25.71 ± 1.40||51.14 ± 1.97||1.99 (+0.20|−0.18)|
|Mant® MSD-18-005||D18||5.22 ± 0.02||3.50 ± 0.02||0.299||74.90 ± 0.77||100.22 ± 0.68||1.338 (±0.023)|
Parameters for the leaching of cobalt from polycrystalline diamond blank.
|ID||TBath [K]||PCD Type||Leaching Time [h/d]|
in the Presence of Ultrasound
|W2R1||333||D14 + D18||0||30|
|W2R2||353||D14 + D18||0||30|
|W3R1||333||D14 + D18||0||45|
|W3R2||353||D14 + D18||0||45|
|W4R1||333||D14 + D18||8 h/d||15|
|W4R2||353||D14 + D18||8 h/d||15|
|W5R1||333||D14 + D18||8 h/d||30|
|W5R2||353||D14 + D18||8 h/d||30|
|W6R1||333||D15 + D18||8 h/d||45|
|W6R2||353||D15 + D18||8h/d||45|
|W7R1||333||D14 + D18||24 h/d||15|
|W7R2||353||D14 + D18||24 h/d||15|
|W8R1||333||D14 + D18||24 h/d||30|
|W8R2||353||D14 + D18||24 h/d||30|
|W9R1||333||D14 + D18||24 h/d||45|
|W9R2||353||D14 + D18||24 h/d||45|
Activation energy EA (J/mol) derived from the apparent rate constants.
|Week No.||PCD Type||δln(k) = ln(k353K) − ln(k333K)||δ/T = (1/353K) − (1/333K)||RIdeal [J*K−1*mol−1]||EA = −RIdeal*(δln(k)/(δ/T))|
© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).