2.1. Fused oxide-fluoride electrolysis
Like other elements of RE, neodymium (Nd) is transformed into metal by the oxide-fluoride electrolysis into a fused salt electrolyte [5, 16, 17]. Usually, Nd oxides are dissolved in an anhydrous electrolyte of Nd fluorides and Li fluorides (LiF) to improve electrical conductivity and increase the fluidity. The electrolyte is maintained in the range of 800 oC to 1100 oC and is initially melted by resistance heating. Both the electrolyte and the deposited metal can be kept liquid by the joule effect [18, 19].
On an industrial scale, the electrolytic cell (or furnace) operates in atmospheric conditions with current densities of ≈ 6.5 A/cm2 at the anode and ≈ 1 A/cm2 at the cathode; features graphite or molybdenum crucible, graphite anodes, molybdenum (Mo) or tungsten (W) cathode and voltage conductive plates (Fig. 1). The cell is externally lined with layers of insulating refractory materials and austenitic stainless steel. A potline is composed of a set of cells electrically connected in series, normally installed in buildings named potroom.
When Nd is prepared, carbon can be used in the anode and cathode. In the case of Nd alloys (e.g. NdFe), graphite electrodes are used at the anode and Fe (including steel and low carbon steel) at the cathode. In other words, if the desired product is metallic Nd, only the anode is the consumable electrode. On the other side, when the NdFe alloy is prepared both electrodes are consumed in the process.
[Fig. 1- Simplified diagram of 10 kA RE electrolytic cell – side view (adapted from Ref. ) ]
Particularly in relation to the consumption of the C anode, oxygen is released on the electrode surface and converted to carbon oxides (reactions 1 and 2), namely CO and CO2. These two gases also react electrochemically with the cathode. Usually, the reaction occurs in a three-phase interface (solid, liquid and gas), formed by the cathode, the electrolyte and the gas .
C + 2O2−→CO2 + 4e− (1)
C + O2−→CO + 2e− (2)
Several studies on the fused salts electrolysis using halogenated RE compounds (oxide-chlorides and oxide-fluorides) have been reported in the literature in last decades [16, 17, 21, and 22]. Comparatively, the Nd electrodeposition from oxide-fluorides is considered more attractive for mass metal production in which the possible decomposition reactions of the oxides-fluorides can be simplified as follows:
Nd2O3 + 3 C → 2 Nd + 3 CO (3)
Nd2O3 + 3/2 C → 2 Nd + 3/2 CO2 (4)
NdF3 + 3/2C → Nd + 3/2 CF2 (5)
NdF3 + 3C → Nd + 3 CF (6)
According to Stefanidaki et. al. , in fused LiF – Nd2O3 – NdF3 systems, the neodymium metal is probably produced by the electro-reduction of Nd fluorides in the cathode, while in the anode occurs oxidation of Nd oxyfluorides generating oxygen (Fig. 2) conforming to the following reactions (7) and (8):
2[NdF6]3− + 6e− = 2Nd(s) + 12F− (7)
3[NdOF5]4− + 6e− = 3/2O2 (g) + 3Nd3++15F− (8)
[Fig. 2– Schematic diagram of partial electrochemical reactions of the Nd electrolysis under smooth operation]
Despite numerous physicochemical and electrochemical properties of the oxide-fluoride electrolysis process have been extensively studied, still remains the challenge for address environmental and economic burdens which significantly reduce the so-called anodic effects (AE) that interfere in the operation of the cell and consequently, to a rise in emissions of fluorine-containing compounds .
AE are manifested by the increase in voltage (V) of the electrolytic cell and if a power system with limited voltage supply is used, also by the decrease in amperage (A). The rise in voltage (overvoltage) can lead to an overheating of the electrolytic cell, which in turn, promotes greater metallic Nd solubility and to excessive interactions of the metal with the electrolyte elements. These occurrences prevent a smooth electrolytic operation and promote the formation of non-metallic deposits (or similar to the slag) in the cathodes and other parts of the cell , in addition to increasing energy consumption and decreasing the current efficiency (CE).
2.2. Anode effects and PFC emissions
One of the main process parameters related to AE is the oxides concentration in the electrolyte, in which the overvoltage can result from the oxides deprivation in a given region of the cell. In the electrolysis of Nd oxide-fluorides, high levels of oxides in the electrolytic cell produce the precipitation of oxyfluorides elements that cannot be reduced , while low levels result in a decrease in electrical conductivity. In this way, the transport of electrical charges is no longer supported by the standard (theoretical) reaction of Nd2O3 electrolysis.
Moreover, in the study conducted by Dorren et al. , during the electrolysis of molten salts, a resistive C-F film formed on the anode surface can be decomposed to form the PFC gases (reactions 5 and 6). Measured data reported in the literature on RE electrolysis indicate that in the occurrence of AE, the gases CF4 and C2F6 are released [25, 26]. The emissions of CF4 and C2F6 can be generated from the following reactions:
NdF3 + 3/4C → Nd + 3/4 CF4 (9)
NdF3 + 3/4C → Nd + 1/2 C2F6 (10)
PFC can also be formed electrochemically by forming COF2 and COF (reactions 11 and 12) which are unstable and react spontaneously with C to form CF4 and CO (reactions 13 and 14) [4, 25, 26], according to the reactions presented below:
Nd2O3 + 2 Nd2F3 + 3 C → 4 Nd + 3 COF2 (11)
Nd2O3 + NdF3 + 3 C → 3Nd + 3 COF (12)
2COF2 + C → CF4 + 2 CO (13)
4COF + C→ CF4 + 4 CO (14)
In addition, depending on the replacement rate of LiF-NdF3-Nd2O3 and the Nd2O3 solubility in the electrolyte, other fluorine emissions occur. Under conditions of positive oxidizing atmosphere, the F reacts with the water contained in the cell forming HF (reaction 15) [18, 27].The theoretical decomposition of Nd fluoride and H2O producing HF  is presented (reaction 16) below:
F2 + H2O → 2HF + O2 (15)
H2O + 2/3 NdF3 + C → 2/3Nd + CO + 2HF (16)
Recently, analyzes conducted on a laboratory and industrial scale have indicated that the generation of PFC can also take place at low voltage [4, 25, 26]. This particular process, known as low voltage AE (LV-AE) or partial AE (P-AE) or smooth AE (S-AE), can happen over a long period of time, with little or no indication of interference in the metal deposition and a marked increase in the voltage of the electrolytic cell.
The LV-AE is analogous to that seen in recent years in the Hall Heroult process, in which the LV-AE of aluminum primary production occurs through the same fundamental mechanisms as conventional AE. However, one of the main differences regarding AE is related to the probability of how and when LV-AE is propagated within the cell [28, 29].
Another category of AE, discussed in the literature (mostly in the aluminum industry) are the so-called no propagating AE (NP-AE), these refer to the slow movement of PFC levels in the background which in terms of magnitude are much smaller when compared to the PFC peaks resulting from AE and LV-AE .
Nonetheless, in the investigation carried out by Vogel et.al.  the presence of NP-AE during the continuous monitoring of the outlet gases can be noted in the electrolysis of fused Nd salts (Fig. 3).
[Fig. 3 – Continuous off-gas measurement showing AE, LV-AE e NP-AE and related CO, CO2 and CF4 emissions (adapted from Ref. .]
In summary, the emission of PFC is inherent to the process and their rate can be highly variable, depending on the concentration of oxides, current density of the anode and the occurrence of AE. All of these parameters are interrelated and will dictate whether eventually the LV-AE will exceed the limit of the voltage required to generate AE . Usually, PFC concentrations rise during conventional AE and rapidly, depending on the production control system and technology, decrease to atmospheric levels when this effect ceases [31, 32].
2.3 Laboratory and industrial accounting of PFC emissions and GWP
In response to the requirements for a sustainable metals management concerning their technical bases for process improvements, i.e. set goals and develop strategies for reducing GHG contributions from RE metals production to the global warming and climate change, Vogel and Friedrich [17, 32], Zhang et. al. , Cai et. al. , Kjos et. al.  carried out analyzes of PFC emissions on a laboratory and industrial scale.
The figures below shows the emission factors of CF4 and C2F6 in kg per RE kg metal and alloy produced by fused salt electrolysis reported in the published literature and respective CO2eq emissions over time horizon of 20 years (Figs. 4 and 5) and time horizon of 100 years (Figs. 6 and 7). In the figures, the number of cell technology (CT) is related to the production of: Nd (1, 2, and 3); Nd-Pr (4, 5, and 6); Dy-Fe (7, 8) and La (9, 10).
The emission factor of both PFC are estimated based on: the operation of electrolytic cell under laboratory conditions (CT: 1); measurements at production site (CT: 2, 3, 6, 7, 9); time integrated and laboratory analysis (CT: 4, 5, 8, 10). The reference studies for CT: 1 ; 2, 3, 7  and 4, 5, 6, 8, 9, 10 .
The colormap represents the intensity (quantity) of the respective PFC emissions converted into CO2eq (kg/RE kg) according to the metric values updated in IPCC Fifth Assessment Report (AR5), as result of new scientific knowledge and also due to the changes in lifetimes and radiative efficiency (RE) caused by changing atmospheric background conditions . The values of global warming potential (GWP) relative to CO2eq used for CF4 and C2F6 are given in the Table 1.
Lifetimes and global warming potentials of CF4 andC2F6
Industrial Designation or Common Name
Global Warming Potential (GWP)
20 years horizon (2)
PFC − 14
PFC − 116
(1) IPCC Fourth Assessment Report – AR4 
(2) IPCC Fifth Assessment Report – AR5 
[Fig. 4 – CF4 emissions factors and respective CO2eq (kg/RE kg) from RE fused salt electrolysis – GWP time horizon of 20 year.].
[Fig. 5 – C2F6 emissions factors and respective CO2eq (kg/RE kg) from RE fused salt electrolysis – GWP time horizon of 20 year.].
[Fig. 6 – CF4 emissions factors and respective CO2eq (kg/RE kg) from RE fused salt electrolysis – GWP time horizon of 100 year.].
[Fig. 7 – C2F6 emissions factors and respective CO2eq (kg/RE kg) from RE fused salt electrolysis – GWP time horizon of 100 year.].
[Table 1. Lifetime and global warming potentials of CF4 and C2F6]
Take into account the technical operation indicators, the differences between the emissions factors and the same RE metal, e.g. kg CF4 per kg Nd (from 9.50E-03 kg/kg to 7.39E-02 kg/kg), can be initially explained by the technological conditions of production of each RE metal and the methods applied for measurement the air streams from the fused oxide-fluoride system.
In the context of climate change, a critical issue regarding to the PFC emissions in the electrolysis of Nd oxide-fluorides, including other RE metals and their alloys, is the absence of standard methods or protocols for measuring the emission factors of these gases, similar to the emission inventories applied in the primary aluminum industry, which allows an reliable evaluation of CO2 equivalent emissions from the RE metal and alloys production.
Generally, industries and government have used GHG inventories to support benchmarking and process improvement initiatives in which the development of strategies to mitigate PFC emissions in industrial units is perceived as the basis for a well-formulated processes improvement plan. Under the umbrella of United Nations Framework Convention on Climate Change, the measurements of PFC enables creation of more accurate inventory through use of facility specific emissions factors rather than default uses .
Inventories of PFC emissions are also important for several market mechanisms of Kyoto Protocol, including emissions trading, Clean Development Mechanism (CDM) and Joint Implementation (JI). Precise and verifiable emissions reductions are required to participate in the sale of credits generated under these programs. That is, accepted and validated measurements procedures are important to verify reductions and facilitate trading .