Rare Earth Electrolysis: key issues for measurement of greenhouse gases from oxide-uoride systems

Over the past decade, the reduction of greenhouse gases (GHG) has been recognized as one of the key factors for sustainable primary metal production, in which the rare earth (RE) industry can be affected both in terms of price and use by GHG reduction policies and non-tariff technical barriers. From environmental and economic standpoint, the peruorocarbons (PFC) emissions generated in RE electrolysis during events known as anode effects (AE) are strong infrared-absorbing GHG and play an important role for RE metals process improvements. However, there is no standard methodology to account these GHG emissions from RE metal production industry and the assessment of the contribution of PFC emissions from different technologies to the global warming is urgently needed. This paper focuses on the analysis of PFC measurements from RE metal production in terms of GHG inventory and sustainable production. The state of art of RE fused oxide-uoride electrolysis, particularly of neodymium electrolysis, provides the technical fundamentals for the evaluation of PFC emissions factors reported in scientic articles. Based on International Panel on Climate Change (IPCC) standard methods and US Environmental Protection Agency (EPA) and International Aluminium Institute (IAI) protocol applied to analogous industrial process, the analysis of key issues for estimate CF 4 and C 2 F 6 emission factors from electrolytic RE production indicates the additional renements are necessary to optimize the accuracy of total PFC emission amount from each currently RE technology. Additionally, the selection of emission estimation technique (EET) or mix EET should be considered on case-by-case basis as to their purposes and suitability for a particular process and facility. Finally, this paper highlights the technological implications related to the PFC emissions measurements and trends towards to set goals and develop strategies for GHG mitigation. related GHG emissions and GWP from RE metal production reported in scientic database shows that EET applied in the studies demands further investigations to establish a robust GHG inventory. In order to perceive a base for continuous process enhancements and hold the climate change mitigation and is essential the development of a standard methodology to account the PFC of different RE metal technologies. In this way, contrary to common sense, public and private sector initiatives on a national and global scale associated with GHG mitigation policies can lead and have to the development of new techniques, processes and materials, combining the reduction of GHG emissions and socio-environmental risks with the resources economy in the RE supply chain.


Introduction
Rare earth elements (REE) are recognized as essential transition resources for high-tech industries and have attracted worldwide attention from countless productive sectors. The unique qualities of REE, such as chemical stability, excellent thermal conductivity, high ductility and corrosion resistance, have made these metals an important material in many of the modern renewable energy, transportation, communication, medical and alloy industries, structural steel, among others [1,2].
On the other hand, the advance of urbanization and the construction of new structures, the electronics use and the change to cutting-edge and low-carbon technologies impose, in addition to a continuous demand, a series of technological challenges when this issue is associated with a sustainable primary metals production [1,3], especially in relation to the REE.
Nowadays, fused salt electrolysis has been used on a large scale for the industrial production of RE metals (lanthanum, cerium, praseodymium, neodymium, dysprosium) and their alloys (Nd-Pr, Pr-Nd-Dy, Dy, Nd-F, among others) in which the oxides-uorides electrolysis became the main manufacturing method and China the world producer, consumer and major exporter [4]. Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js However, even in the face of several technical and economic advances achieved in the production of RE metals, over more than 80 years of exploration and practice in the Chinese industry, still persist problems related to high energy consumption and air pollution. Given the different types of RE and their respective alloys, on average energy consumption is approximately 10 kWh/kg and the uorine-containing compounds gas emission factor is 0.03 kg/kg [5].
Fluorine-containing compounds, speci cally per uorocarbons (PFC) -CF 4 , C 2 F 6 and C 3 F 8 are emitted during the RE metals production and from processes that range from the semiconductors industry to the primary aluminum production. The PFC is considered one of the major greenhouse gas (GHG) due their high global warming potentials (GWP), even at a few parts per trillion, and long atmospheric lifetimes [6].
Although concentrations of PFC in the atmosphere were near zero a few decades ago, including other halogenated gases that not natural abundances have been found and natural abundances have been assumed to be insigni cant [7], these gases have increased rapidly as they have been incorporated into a variety industrial process and commercial and household uses. When PFC is counted in terms of CO 2 eq, the GWP values are thousands to ten thousands higher [6,7].
As a result of the high potentials for long-term impacts in relation to climate change, international efforts have been made to reduce emissions not only from PFC, but also from hydro uorocarbons (HFC) and sulfur hexa uoride (SF 6 ) [8]. Initiatives in the aluminium and semiconductor industries have conducted several researches and technical studies in order to mitigating PFC emissions and concomitantly, improving the e ciency of control systems and gases treatment in the production process [9].
Additionally, Mancheri et.al [10] outlines that new policies with stricter environmental standards, illegal mine closure and comparatively high cost of importing concentrates around the world, reduce the advantages of China with low production costs and low prices of REE for domestic industries at upstream of the RE supply chain. That is to say, the rational use of resources can certainly affect the capital expenditure (CAPEX) and operational expenditure (OPEX) of these companies in the long term and promote the use of more e cient technologies with less environmental impact [11].
Besides, over the last decade, the number of disputes over actions, or inaction, related to climate mitigation and adaptation efforts have grown in importance. As of July 2020 climate change cases have been led in 38 countries, with at least 1550 litigations. These cases are forcing greater climate disclosures, ending greenwashing on the subject of climate change and energy transition. Moreover, the laws codifying national and international responses to climate change have recognized new rights and created new duties, compelling governments and corporate actors to purse more ambitious climate change mitigation and adaptation actions [12].
Therefore, the development of new equipment and techniques, associated with energy conservation and environmental protection represents an inevitable path for the improvement of RE metals industry [5]. As cleaner production technologies and recycling processes are achieved, the importance of international standardization of the products in the RE chain is highlighted. Indeed, in 2015, the International Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js Organization for Standardization (ISO) established a Technical Committee (ISO/TC 298) of RE, composed of 34 countries and, so far, 5 working groups [13].
In this way, Chinese companies have been strongly incited to deploy signi cant work to reduce the energy consumption and emissions from the RE fused salt electrolysis, in addition to accomplish the environmental, occupational health and safety compliances. From this perspective, this paper analyses the PFC emissions measurements and their role in the production processes improvements of RE metals and sustainable production, with emphasis on the neodymium (Nd) electrolysis oxide-uoride system and GHG inventory.
The rst section presents the state of art of the metallic Nd production, considering the main technical aspects of RE electrolysis, as well as the analysis of the emissions reported from previous publications. Then, is discussed the International Panel on Climate Change (IPCC) methods [14] and US Environmental Protection Agency (EPA) and International Aluminium Institute (IAI) protocols [15] for quantifying PFC gases applied in the primary aluminum industry, known as analogous process for RE electrolysis. Finally, the last section emphasized, vis-à-vis standard methodologies, the key issues for measurement of emission factors from the RE metal and their alloys production and indicates the trends for evaluating and reducing GHG emissions.
2. State-of-art 2.1. Fused oxide-uoride electrolysis Like other elements of RE, neodymium (Nd) is transformed into metal by the oxide-uoride electrolysis into a fused salt electrolyte [5,16,17]. Usually, Nd oxides are dissolved in an anhydrous electrolyte of Nd uorides and Li uorides (LiF) to improve electrical conductivity and increase the uidity. The electrolyte is maintained in the range of 800 o C to 1100 o C 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/cm 2 at the anode and ≈ 1 A/cm 2 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 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 CO 2 . 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 [20].
Several studies on the fused salts electrolysis using halogenated RE compounds (oxide-chlorides and oxide-uorides) have been reported in the literature in last decades [16, 17, 21, and 22]. Comparatively, the Nd electrodeposition from oxide-uorides is considered more attractive for mass metal production in which the possible decomposition reactions of the oxides-uorides can be simpli ed as follows: According to Stefanidaki et. al. [16], in fused LiF -Nd 2 O 3 -NdF3 systems, the neodymium metal is probably produced by the electro-reduction of Nd uorides in the cathode, while in the anode occurs oxidation of Nd oxy uorides generating oxygen (Fig. 2) conforming to the following reactions (7)  Despite numerous physicochemical and electrochemical properties of the oxide-uoride electrolysis process have been extensively studied, still remains the challenge for address environmental and economic burdens which signi cantly reduce the so-called anodic effects (AE) that interfere in the operation of the cell and consequently, to a rise in emissions of uorine-containing compounds [23].
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 Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js cathodes and other parts of the cell [20], in addition to increasing energy consumption and decreasing the current e ciency (CE).

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-uorides, high levels of oxides in the electrolytic cell produce the precipitation of oxy uorides elements that cannot be reduced [18], 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 Nd 2 O 3 electrolysis.
Moreover, in the study conducted by Dorren et al. [24], during the electrolysis of molten salts, a resistive C-F lm 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 CF 4 and C 2 F 6 are released [25,26]. The emissions of CF 4 and C 2 F 6 can be generated from the following reactions: PFC can also be formed electrochemically by forming COF 2 and COF (reactions 11 and 12) which are unstable and react spontaneously with C to form CF 4  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 [28].
Nonetheless, in the investigation carried out by Vogel et.al. [25] 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, CO 2 and CF 4 emissions (adapted from Ref. [25].] 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 [30]. 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].

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. 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 [17]; 2, 3, 7 [31] and 4, 5, 6, 8, 9, 10 [4].
The colormap represents the intensity (quantity) of the respective PFC emissions converted into CO 2eq (kg/RE kg) according to the metric values updated in IPCC Fifth Assessment Report (AR5), as result of new scienti c knowledge and also due to the changes in lifetimes and radiative e ciency (RE) caused by changing atmospheric background conditions [33]. The values of global warming potential (GWP) relative to CO 2eq used for CF 4 and C 2 F 6 are given in the Table 1. explained by the technological conditions of production of each RE metal and the methods applied for measurement the air streams from the fused oxide-uoride system.
In the context of climate change, a critical issue regarding to the PFC emissions in the electrolysis of Nd oxide-uorides, 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 CO 2 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 speci c emissions factors rather than default uses [15].
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 veri able 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 [15].

Ipcc Standard Methodology For Quantifying Pfc Emissions
In the primary aluminum industry, three methods established by the IPCC 2006 Guidelines for National Greenhouse Inventories are used to quantify PFC emissions [14]. The IPCC Tier 1 method comprises the estimation of PFC emissions from the smelter by the multiplication of the annual metal production with a given coe cient based on the technological class used, namely: Center Worked Prebake (CWPB), Side Worked Prebaked (SVPB), Vertical Stud Soderberg (VSS) and Horizontal Stud Soderberg (HSS). This method is applied in industries where there is no system for monitoring and recording AEs and is less accurate than the IPCC Tier 2 and 3 methods.
The IPCC Tier 2 and 3 methods, involve the statistical analysis of conventional AE (frequency -AEF and duration -AED) and emissions coe cients (CF 4 slope coe cient and mass ratio of C 2 F 6 emissions in relation to CF 4 ). In general, calculations are carried out on the monthly average data and then summed to obtain an annual issue. The emission factors of CF 4 and C 2 F 6 can be calculated using speci c coe cients of the technology employed (Tier 2) and/or direct measurements performed at the smelter (Tier 3) [14].
The IPCC Tier 3 is applied to de ne a long-term relationship between the measured emissions and the operating parameters and to verify the correlation of the AE and the production levels. This method is recognized for presenting highest precision for building an inventory of PFC emissions and provides two Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js models for quantifying the gases: slope and overvoltage. The slope is a parameter in which, when multiplied by the minutes of AE per cell/day, it results in an industry-speci c CF 4 emission factor (Eq. 1): EF CF4 = S CF4 * AEM * MP

Measurement of CF 4 and C 2 F 6 emissions based on EPA and IAI Protocol
In the EPA and IAI protocol, two alternatives are given for the measurement and calculation of emissions of CF 4 and C 2 F 6 per kg of metal, there are: 1) measurements of PFC carried out based on sampling using stainless steel bags or cylinders and 2) measurements conducted directly on the production site. In both, the slope and overvoltage coe cients and the mass ratio of C 2 F 6 /CF 4 can be evaluated. The recommended calculation procedures for the analysis of PFC emissions include information on how to measure: The gas ows from the cells exhaust ducts and the potroom area, weighting the speed, temperature, pressure, as well as the sectional area of the duct; The total emissions of CF 4 and C 2 F 6 per minute of AE and sampling time (ST); The mass ratio or weight ratio of C 2 F 6 /CF 4 ; The total metal production as a function of ST; Calculation of PFC gases from secondary emissions in relation to ST; The secondary emissions and the adjustment of the quantity of CF 4 and C 2 F 6 issued; The total emission factor of each PFC; The slope CF 4 emission coe cient and the CF 4 emission coe cient in overvoltage.
While primary (exhaust ducts) and secondary (fugitive) PFC emissions are analyzed based on sampling bags or onto sorbent columns and the exhaust gas collection e ciency (GC), the emission factors of the  As long as the gas collection e ciency (GC) is greater than 90%, the total of PFC gases can be divided by It should be noted that since the fugitive emissions are measured, the Eq. (4) must be adjusted in order to calculate PFC emission factor without the variable GC, avoiding underestimation and outsized uncertainty. The same procedure given by the Eq. (4), (5) or (6) and (7) can be applied for the quanti cation of the C 2 F 6 emission factor, but replacing the value of the variable M of the Eq. (4) with the corresponding number in kg per mol of C 2 F 6 (0.138).
The slope and overvoltage emission coe cients presented before in the Eqs. (1) and (2)  Regardless of whether fugitive PFC emissions are estimated based on GC or measured by the equipment, the uncertainty values associated with the results achieved must be computed to indicate the uncertainty coe cient of the Tier 3 method application. One statistical approach estimates uncertainties by using error propagation equation [15,34]. The approximation is used to combine emission factor, activity data and other estimation parameter ranges by categories and GHG (Eq. 10): Where: U total : the percentage uncertainty in the product of the quantities that are added (%).  Under typical circumstances, the approximate of all random variables is accurate as long as the coe cient variation is less than 0.3. Where uncertain quantities are be combined by addition or subtraction, a simple equation can be derived for the uncertainty of the sum, expressed in percentage terms (Eq. 11): ⋃ total = U 1. X 1 2 + U 2. X 2 2 + ⋯ + U n . X n 2 | X 1 + X 2 + …X n |

11
Where: U total : the percentage uncertainty in the sum of the quantities that are added (%). This term uncertainty is based upon 95 percent con dence interval.
U i and x i : the percentage uncertainties and uncertain quantities associated with them, respectively (%).
From the perspective of applying the PFC measurement protocol (Tier 3) proposed by the EPA & IAI, it is emphasized that the current technologies in the primary aluminum industry present gas collection e ciency between 80 to ≥ 97%. That is, the PFC emissions that are not collected directly from the cell's exhaust duct and escape into the environment often vary from 3% to a maximum of 20%, depending on the technology [15].

Key issues for evaluating of PFC emissions from RE metal production
Face the standard methods applied in the primary aluminium and the published PFC emission results from the RE metals electrolysis process, the analysis of PFC emission factors requires exceptional attention to some methodological aspects underlined here, namely: 1) the AE data correlation with the emissions measured in the exhaust ducts and fugitive emissions, 2) analysis of emissions in the background, 3) evaluation of the ow homogeneity in the ducts system for collecting the gases from the cell and releasing to the atmosphere, as well as sampling time.
The proper correlation of the AE data with the measured PFC emissions presupposes the statistical analysis of the conventional AE monitored on the potline by the production control systems. This allows determining the conditions considered normal or stable operation of the potline and the process parameters (e.g. percentage of AE manual termination, cell control and feeding strategy) that can affect the Tier 3 coe cients during the measurements.

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Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js Additionally, there are transient circumstances, e.g. startup of a new cell and restart of the potline due to the replacement of cells or their components (except exchange of anodes), power outages and testing of raw material that do not characterize the process routine of the RE production. Thus, care must be taken to ensure that AE measurements are representative and to proper account the PFC emissions and any reduction in collection e ciency.
On the other hand, background sampling aims to identify all interference that may occur in the analyte signal coming from locations close to the potroom or industrial site. The levels of background emissions of CF 4 and C 2 F 6 must be insigni cant when comparing the average concentrations of the primary and secondary emissions. Interference in the analyte signal can be observed in the study by Zhang et.al [31], in which the C 2 F 6 emissions data resulted in negative values, after deducting the emissions in the background (see Figs. 5 and 7).
Regarding to the ow homogeneity, the evaluation of this variable aims to avoid measurement errors that can be caused due to the sampling points and concentration of the gases. Experiences in the aluminum industry have indicated that PFC concentrations diverge across the sectional area of the duct, as the gas ows collected from the cell by the exhaust system can remain segregated for a certain distance even after being merging [15].
Commonly, homogeneity is tested by injecting a small constant ow of a tracer gas into the exhaust duct of one of the reduction cells, which is subsequently monitored at ve points, equally spaced along the duct sectional area. For an adequate de nition of the sampling point, the concentration of this gas cannot vary by more than 10% at any candidate sampling point. This is particular important for the de nition of the emission coe cient in overvoltage and the weight ratio of C 2 F 6 /CF 4 . In either case, the measurement requires a minimum of 72 hours of gas collection and the inclusion of normal productive operation cycles, e.g. electrode changes, among others [15].
An indication of signi cant variation in homogeneity in the gas ow can be observed in the experiments conducted by Kjos and collaborators [26], in which the gases produced by electrolytic cells used for the production of various RE alloys resulted in values of CF 4 extremely high and above the capacity of the measuring equipment, even operating without signal from conventional AE. Thus, the homogeneity and the sampling point aims to determine a value of the emission coe cient reliable and conforming to the cycles of the production process.
The tracer gas can also be used to analyze the ow volumetric rate (velocity in Nm 3 /min) from the duct upstream of the sampling point [35]. This method is particularly advantageous as it allows the evaluation of short-term variations in the duct ow and when the measuring instruments can quantify the tracer gas and the PFC simultaneously. The tracer compound is selected based on knowledge of the process and must be stable, non-reactive gas that is not otherwise found in the native air stream. In this way, additional caution related to the use of SF 6 as tracer gas must be considered since it is used in electric power systems, in medium and high voltage switchgear for insulation and breaking. Emissions of SF 6 from equipment operation can occur if subject, e.g. to high ambient temperature and heat produced by the current passing through the circuit breaker and corrosion due to the external environment, such as salt spray from the ocean and pollution [36].
Therefore, in the absence of the emission coe cients standard values for each RE metal and their alloy, the selection of emission estimation technique (EET), e.g. sampling or direct measurement, mass balance, mathematical models or other engineering calculations, or even the mix of EET should be carefully considered on case-by-case basis as to their purposes and suitability for a particular technology (types of cell) and facility site.

Technological implications and trends
Pang et. al. [5] claimed that RE oxide-uoride electrolysis commercial technologies have shown marked improvements over the past decade. Technical-economic indicators presented to an increase of ≈ 10% in CE and a reduction of ≈ 11% in V, when comparing the 25 kA and 30 kA cells in relation to the 4 kA up to 6 kA.
Speci cally in relation to Nd production, the increase in amperage has conducted to a relative reduction in electricity consumption (from 11-13 kWh/kg up to 9.5-11 kWh/kg). Although, the voltage still remains high (8-10V) compared to the aluminum industry, the changes in the cell design, with focus on energy conservation and reduction of PFC emissions, lead to the upturn in CE (from 65% up to 80%) and better productivity [5].
In  Furthermore, cells equal to or less than 6 kA, in which the gas collection system of the RE production line is unique and located a few meters high from the electrolytic cells, may have PFC emissions factors much lower than those in that the exhaust systems are coupled to the cells individually. This is because the ventilation of gases, from all other cells in the potroom, tends to dilute the emissions resulting from the AE of a cell and even change the gas collection e ciency. Recently, in the production systems of Nd-Pr, Dy-Fe and La, operating in atmospheric conditions, the gas collection e ciency was found to be approximately 57% [4].
Another relevant aspect is related to the LV-AE and NP-AE emissions. Even though the RE fused salt RE electrolysis is very similar to primary aluminium metal process, sharing some features, such as the use of carbon anodes and electricity for heating the electrolytic cell, a major difference is the quantity of current applied for electrolysis that is determined according to Faraday Law and by the current actually being used to electrodeposit the RE metal [18]. The increase in amperage leads to a greater probability of different concentrations of the oxide within the cell and these conditions can limit the AE propagation and consequently make the LV-AE and NP-AE more frequent in cells with high amperage.
Hence, it is important to note that PFC emissions from AE and LV-AE represents not only an operational process expense [16], but also a capital cost for the RE metal production industry and an environmental cost for the governments and society. These AE represents disturbances in the process of electrolysis that has a long-term impact on the operation of the cell e ciency [28] and environment, reducing the lifetime of electrolytic cell components and the quality of product, increasing the energy and materials consumption and the generation of waste stream.
In order to overcome potential challenges related to the anode effect and PFC monitoring, some precise estimation methods uses linear models quantifying the total amount of PFC based on a single cell parameter (e.g. AE, polarized AE duration, etc.). Similarly, innovative and non-linear models using an approach that identify and account the PFC emissions related to AE, L-AE and N-AE have been developed and applied to estimate the total amount of PFC emissions in the smelter industry [30]. affords an updated and sound scienti c basis for supporting the preparation and continuous improvement of national GHG inventories. It also provides background and guidance for estimating PFC from generalized AE (also known as high voltage anode effect) and LV-AE in the primary aluminium production using a range of methods.
Thus, the development of speci c EET for RE industry and laboratory analyses of uorine-containing compounds considering the production cycles is essential to determine the potential indicators and variables for each technology were most likely associated with different AE and respective PFC. This Loading [MathJax]/jax/output/CommonHTML/fonts/TeX/fontdata.js approach lets, primarily, to nd the correlation between the operating conditions and the emissions generated. Secondly, to evaluate the necessary adjustments for the processes improvements, as well as the potential errors and the general reliability related to the EET applied.
That is to say, the management of preconditions that generate PFC by e.g. control of the materials feeding and dissolution in the electrolyte have been recognized as the best probably way to prevent GHG emissions for the existing RE production technologies. On the other side, the development and use of new technologies, such as Liquide State Cathode Electrolysis (LSCE) and Solid Oxide Membrane Electrolysis (SOM), represents alternatives for more e cient production and mitigation of GWP in the RE metals industry [40].
Pang et.al. [5] pointed out that since 2007 Chinese researchers have been conducting studies with LSCE. Experimental results using this technology for the Nd production showed EC from 87.1 up to 90.6% and a signi cant reduction in energy consumption and uorine-containing compounds emissions in the gas [5].
The SOM process, developed in the USA, uses a membrane that conducts oxygen ions as part of the anode and does not produce PFC. Recently, the application of this process for the production of Nd and Dy-Fe for the magnet industry was reported by Guan et.al [40].
Finally, although research efforts by the academy, private sector and governments together have accomplished substantial progress, an estimation of GHG and respective contributions to the global warming from RE metals industries still require performing a systematic and robust EET in other to de ne PFC emissions factors speci c for each currently RE metals technology, counting also their products and shares in the RE supply chain.

Conclusions
Studies on PFC emissions from RE metal industry production have emerged as an important feature of ongoing efforts to promote process improvements, cleaner production and GHG mitigation. This owes in large part to the growing number of environmental concerns regard to the climate change and sustainability of primary metal production. It is possible to observe that in recent years, the industry of RE earth metals and their alloys production has achieved signi cant improvements in the reduction of energy consumption and emissions of GHG. However, the analysis of estimate amount of PFC, related GHG emissions and GWP from RE metal production reported in scienti c database shows that EET applied in the studies demands further investigations to establish a robust GHG inventory. In order to perceive a base for continuous process enhancements and hold the climate change mitigation and adaptation goals is essential the development of a standard methodology to account the PFC of different RE metal technologies. In this way, contrary to common sense, public and private sector initiatives on a national and global scale associated with GHG mitigation policies can lead and have leading to the development of new techniques, processes and materials, combining the reduction of GHG emissions and socio-environmental risks with the resources economy in the RE supply chain.