Life Cycle Assessment of Tantalum and Niobium Recycling from Hard Metal Scrap

: Secondary hard metal contains valuable tantalum and niobium, which could be recovered after chemical recycling of the scrap; however, the environmental impacts of their recycling have not been earlier quantified. This study provides gate-to-gate life cycle inventory data on tantalum and niobium recovery from the Ta − Nb-rich residue after the leaching of cobalt in the chemical recycling of hard metal and first assessment of the environmental impacts of tantalum and niobium coproduction. The environmental impacts were quantified using life cycle assessment (LCA) based on data acquired by process simulation. Two processes were evaluated: one based on conventional HF leaching used in the primary production of tantalum and niobium and one prospective HF-free process using NaOH. The results show that environmental impacts of Ta − Nb recycling can outperform primary production environmentally if the Ta and Nb content in the raw material is high enough. At the process level, a benefit is gained even with a lower content, but at the product level, higher contents are required for tantalum recovery to be worthwhile. In HF-based recycling, increasing the Ta and Nb contents each from 2.5 to 5 wt % decreases the value of global warming potential (GWP) of Ta recycling from 1.24 times the GWP of primary tantalum production to 0.72 times the GWP of primary tantalum production. The environmental impacts of the recycling processes mostly originate from the background processes. The most burdening process hot spots of recycling included the leaching and effluent treatment stages for the HF-based process in which HF and lime were the largest contributors. For the HF-free process, the largest contributions were due to NaOH used in the caustic conversion as well as oxalic acid in the solvent extraction.


■ INTRODUCTION
Hard metal, also known as cemented carbide, is a material that consists mainly of a tungsten carbide (WC) phase and a cobalt binder and is widely used in the manufacturing industry.The global recycling rate of hard metal has been estimated to be between 40 and 55% depending on the application.Cutting tool scraps, which are easier to handle by the current recycling technologies, also have the highest recycling rates.Global producers consider even recycling rates above 80% to be possible, with higher rates being promoted by the accumulation of scrap at large users, which makes scrap more readily available to metal producers and recyclers. 1hemical hard metal recycling processes can treat any quality of scrap material and typically recover the tungsten and cobalt contained in the scrap material.Hard metals may also contain carbides, nitrides, or carbonitrides of other metals used, e.g., coatings or grain growth inhibitors. 2 Of these, there has been interest in recovering the tantalum and possibly niobium contained in the solid residue following the recovery of cobalt by leaching.Cubic carbides of Ta and Nb are used in considerable amounts in, e.g., cutting tools that have a high recycling rate. 3th tantalum and niobium are included in the EU list of critical raw materials.Tantalum is an important material for the tooling industry and cannot be replaced in its applications without loss of performance.10% of EU tantalum consumption of 395 tonnes was used for carbides between 2012 and 2016.Production of tantalum and niobium is geographically highly concentrated.There is no production of tantalum or niobium in the EU, but various recyclers process secondary tantalum materials. 4rom certain secondary materials such as hard metals and Ta-containing alloys, tantalum can be well recycled.The postconsumer recycling rate of tantalum in general, however, is very low as the recyclability of tantalum from their main use in capacitors remains virtually nonexistent. 5Considering the criticality of both metals, it is increasingly necessary to recover them from the secondary raw materials for which functional recycling is possible.For hard metal, recycling using the zinc process results in a product with the same composition as in the original scrap, and if contained in the raw material, tantalum and niobium are thereby also recovered.In chemical recycling of hard metal, they remain in the leach residue following tungsten and cobalt recoveries by leaching from which they could be recovered, but it appears to not typically be done in the industry.
Environmental impacts of the recovery process used are essential information when determining the sustainability of recycling from any specific raw material.The information on environmental impacts of tantalum and niobium production is currently limited, although some life cycle inventory (LCI) data and impact assessment results have been previously reported.For niobium production, all available life cycle information is for production from pyrochlore.Alves and dos Reis Coutinho 6 presented LCI data based on process mass balance for ferroniobium and Nb 2 O 5 production but did not calculate the impact assessment results.Dolganova et al. 7 reported life cycle assessment (LCA) of ferroniobium production based on primary information.Da Silva Lima et al. 8 performed LCA of ferroniobium and Nb 2 O 5 production from industrial data.For tantalum, some LCA data are available as a part of the life cycle of tantalum capacitors, 9 including its recovery. 10,11The environmental impacts of tantalum and niobium recovery from hard metal applications have not been evaluated, and assessment of the impacts of the coproduction of both metals remains unavailable in public literature.
In this study, the goal was to provide detailed unit processlevel LCI data on possible tantalum and niobium recycling routes using hard metal chemical recycling residue as the input material.As there are no primary plant data publicly available on the studied processes, the recovery of tantalum and niobium from hard metal recycling residue was generated by process simulation using the SIM module of HSC Chemistry 10. 12 Mass and energy balances from the simulations were used in compiling the LCI.This LCI data can also be used in further LCA studies to include Ta and Nb coproduction impacts.LCIA was conducted in GaBi LCA software 13 to identify environmental hot spots of the process, the knowledge of which can be used to guide process development in a more sustainable direction.The impacts of tantalum and niobium recovery from scrap raw material with different compositions were studied to help identify under which conditions the recovery process may be environmentally beneficial.

■ MATERIALS AND METHODS
The simulation parameters and process chemistry were determined based on review of the literature.Process models were built using HSC Sim 10 based on the literature values and typical metallurgical operation.The process simulation was used to compile an LCI, and last, an impact assessment was conducted using GaBi software.
Process Description.The feed into the recycling processes was the leach residue that remains after cobalt has been leached out of the residue left after leaching of oxidized hard metal scrap.The feed composition was based on the residue composition in hard metal chemical recycling modeled by Aromaa et al. 14 The hard metal scrap composition was 11% by weight Co, 0.5% Cr 3 C 2 , 0.2% VC, 2.5% TaC, TiC, and NbC, with the remainder being WC.After two leaching steps to recover first tungsten and then cobalt, the remaining residue contains 20.2 and 24.2 wt % of the tantalum and niobium compounds, respectively.Two scenarios were modeled for the recovery of Ta and Nb from the leach residue.Detailed process descriptions are available in the Supporting Information (Section S1), and schematic diagrams of the flowsheets are presented in Figure 1.
Scenario 1 (SCE1): the leaching residue is roasted followed by leaching with HF and H 2 SO 4 .Methyl isobutyl ketone (MIBK) is used to coextract both metals from the solution, after which they are selectively stripped using dilute HF and H 2 SO 4 for Nb and distilled water for Ta.After solvent extraction (SX), both are separately Figure 1.Technical boundary used in the study for the tantalum and niobium recovery processes from cobalt leaching residue from hard metal scrap chemical recycling.The modeled system is shown in blue, and the expanded system is shown in orange.precipitated as hydroxides using ammonia.The solids are then calcined to produce Ta 2 O 5 and Nb 2 O 5 .Abatement and waste management includes scrubbing of gaseous emissions from leaching, as well as sulfate removal and neutralization of the produced wastewater.Solid wastes are modeled as hazardous waste.
Scenario 2 (SCE2): the leaching residue is roasted followed by caustic conversion using NaOH which is, in turn, followed by water leaching.A quaternary ammonium salt dissolved in an organic diluent is used to coextract both metals from the solution, after which they are selectively stripped using diluted nitric and oxalic acids for Nb and diluted nitric acid for Ta.After SX, both are separately precipitated as hydroxides using ammonia.The solids are then calcined to produce Ta 2 O 5 and Nb 2 O 5 .Waste management includes the neutralization of the produced wastewater.Solid wastes are modeled as hazardous waste.
The leach residue is first subjected to a roasting pretreatment, after which tantalum and niobium are solubilized.In SCE1, this takes place using HF and H 2 SO 4 as the leaching agents, and in SCE2, caustic conversion using NaOH followed by water leaching is employed.The solution after leaching and before SX contains 9.2 g/L of Ta and 6.7 g/L of Nb in SCE1 and 1.1 g/L of Ta and 0.82 g/L of Nb in SCE2.The pregnant leach solution (PLS) in primary production from ores with HF-based leaching typically contains 10−50 g/L Ta and 15−65 g/L Nb. 15 The PLS concentrations in SCE1, which are also based on leaching with HF, are lower but in a similar magnitude to primary processing.SX is used in both scenarios to separate tantalum and niobium from each other.The two scenarios use different extractants and stripping chemicals, but in both scenarios, tantalum and niobium are first coextracted, which is followed by the stepwise stripping of first niobium and then tantalum.The separated tantalum and niobium are the precipitated as hydroxides using ammonia, and the precipitates are calcined into Ta 2 O 5 and Nb 2 O 5 .SCE1 recovers 73.8% of tantalum and 78.5% of niobium contained in the feed material.The recoveries in SCE2 are 58.5 and 71.0% for tantalum and niobium, respectively.
In the choice of the flowsheet options and units, preference was given to industrially-proven process options and units.This was done in an attempt to minimize the uncertainty that is related to the simulation parameters.The process in SCE1 is based on primary industrial production of tantalum and niobium, and the flowsheet closely follows this process after the pretreatment of the leach residue.The modeled flowsheet for SCE1 thereby uses industrially commonly used unit processes, but in the absence of representative industrialscale data, most of the design parameters used in the simulation were based on laboratory-scale batch experiments.It is possible that an optimized industrial process could, e.g., provide better yields and reagent recycle.The process in SCE2 is not representative of industrial production but is mainly based on pilot-scale experimental data 16−18 for the leaching and SX stages, while the following units are similar to those in SCE1.Detailed simulation parameters and their sources are presented in the Supporting Information in Tables S1 and  S2 for SCE1 and SCE2, respectively.Goal and Scope.The goal of the LCA was to study the gate-togate environmental impacts of tantalum and niobium recovery from the leach residue that is left after cobalt is leached during the chemical recycling of hard metal scrap and to compile detailed process-level LCI data.The analysis was performed with GaBi LCA software using the ecoinvent 3.8 database.The functional unit was defined as treatment of 1 tonne (mt) of the tantalum and niobium containing leach residue left after cobalt recovery from hard metal scrap.
The scope was limited to tantalum and niobium recovery from the leach residue; other elements contained in the residue were assumed to not be recovered and, thereby, disposed as waste.The geographical boundary of the study was set as Europe, and whenever possible, average European values were used to model the background processes.The system boundary is presented in Figure 1.
The avoided burden approach was used to address end-of-life allocation as it adequately captures the infinite recyclability of metals as a key property. 19The products from recycling were considered to substitute for the primary production of equivalent products.Allocation between the two coproducts of the systems was done based on the mass and economic value of the products.A substitution ratio of 1:1 was used, as metals are infinitely recyclable and hydrometallurgical treatment can recover products of sufficient quality.Data for the primary production were estimated based on data from the ecoinvent 3.8 database for mining impacts and the process simulation for SCE1 with average columbo-tantalite concentrate composition for refining impacts.
A sensitivity analysis was conducted to study the effect of uncertainty in the simulation parameters on the environmental impact assessment results.The effects of variation in different recycling process parameters [solid−liquid ratio, HF concentration, H 2 SO 4 concentration, NaOH concentration, aqueous to organic (A/ O) ratio in Nb stripping, A/O ratio in Ta stripping, Ta extraction in Nb stripping, and precipitation pH] were studied by varying these parameter values in the process simulation by ±20% and recalculating the corresponding LCIA results.These parameters were chosen for varying reasons.The chemicals used for the solubilization of tantalum and niobium (HF, H 2 SO 4 , and NaOH) can, for some processes, have a substantial impact on the LCIA results.For many of the parameters related to the SX stage, the representative literature was not available, and the parameter values are therefore only estimates.Changing the original parameter values by 20% was considered substantial enough to show the possible sensitivity of the LCIA results to the observed process parameters.
Inventory Analysis.In the absence of primary data, process simulation can be used to collect LCI data and study the impacts of metallurgical processes.−23 In this work, simulation parameters such as process conditions, reagent concentrations, and yields were mainly gathered from the literature on laboratory-scale processes or estimated based on knowledge of similar hydrometallurgical operation.This was considered in the uncertainty and sensitivity analyses.The simulation parameters and their sources are presented in detail in the Supporting Information in Tables S1 and S2 for SCE1 and SCE2, respectively.The flowsheets used in this work are shown in the Supporting Information in Figures S1 and S2 for SCE1 and SCE2, respectively.The LCI was collected based on the mass and energy balances calculated from the process simulation.Electricity consumption was also calculated based on the simulation.The used electricity calculation method has been previously detailed in Elomaa et al. 20 and Rinne et al. 23 Reactors, thickeners, filters, and kilns were included in the calculation.Energy used for heating was estimated for high-temperature operations.The final LCI data for the studied system are available in Table 1.
Largest differences in the consumption of chemicals, water, and energy in the two scenarios are found in the precipitation and calcination steps.The leaching and SX steps are, by nature, different in the chemicals they consume, but the consumption is mainly on a similar level of magnitude.In leaching, the electricity consumption is higher in SCE2 as is the water consumption, which leads to a larger amount of wastewater and raffinate produced and a more dilute solution entering SX.The stream volumes following SX, however, are smaller in SCE2, which is likely the cause of the smaller consumption of chemicals and utilities in the following unit processes of precipitation and calcination.Therefore, the final amount of wastewater entering the treatment process and the treated effluent exiting the system are similar in both scenarios.

■ RESULTS
Results were obtained for the environmental impacts based on process simulation.Simulation was used to determine the chemical consumption, produced wastes, and mass balances in the system.These were quantified as environmental impacts in the LCA.
Impact Analysis.The LCIA results are shown in Figure 2 for each of the process steps (roasting, leaching, SX, precipitation, calcination, and abatement and effluent treatment) in SCE1 and SCE2. Figure 2 also includes the main contributing chemicals and utilities.
In SCE1, leaching was the largest contributing unit process to most of the impact categories (FC, EP, POCP, and AP).However, regarding GWP, abatement and effluent treatment were unit processes with the largest impact.Both leaching and abatement and effluent treatment contributed almost equally to the oxidation of ODP.In SCE2, leaching was also the largest contributor to several of the environmental impact categories (GWP, FC, and EP), but SX was shown to have the largest effect on POCP, ODP, and AP.The largest contributing unit processes accounted for 31.6−68.6% of the total impacts in SCE1, whereas in SCE2, the contribution of the largest contributing unit process was even higher, between 44.1 and 81.4% of the total impacts.
The analysis shows that the modeled recycling processes had no meaningful impacts originating from direct emissions to the environment, but all impacts were dominantly due to upstream processes such as production of the chemicals used in the process.In SCE1, HF was clearly the largest contributor to FC, EP, POCP, ODP, and AP.Only exception was made by GWP, where lime used in the effluent treatment stage was the largest contributor.In SCE2, NaOH was found to be the largest contributor to GWP, FC, and EP, whereas oxalic acid used in the SX stage was the largest contributor to POCP, ODP, and AP.The largest contributing chemical accounted for 26.4− 48.0% of the total impacts in SCE1 depending on the impact category.In SCE2, the largest contributing chemicals contributed between 40.8 and 79.5% of the total impacts.The results here highlight the importance of critical evaluation and minimization of the chemicals used in metallurgical processing when possible.A detailed contribution analysis of the chemicals and utilities is shown in Figure 2.
Compared to each other, SCE1 and SCE2 differed in how they contributed to different impact categories.SCE1 had larger impacts on GWP, FC, and AP by 26.5, 42.2, and 32.9%, respectively.In EP, POCP, and ODP, the SCE2 impacts were larger by 71.8, 366.9, and 879.6%, respectively.This indicates that although traditional HF-based recycling of Ta and Nb may have a slightly higher GWP, the novel HF-free process has vastly higher POCP and ODP values.Again, the results here highlight the critical evaluation of the environmental impacts�in this studied case, a process that may at first sound greener exposes dramatically high EP, POCP, and ODP values.
The impacts were allocated between the two coproducts to facilitate easier comparison to primary production of the same products.It was found that in HF-based recycling (SCE1), the recycling producing Ta 2 O 5 outperformed primary production with lower impacts in EP, POCP, and ODP, whereas GWP, FC, and AP were lower in primary production.In HF-free recycling (SCE2), the impacts in recycling were larger in all categories when compared to those of primary production.The quantitative results are shown for both scenarios in Figure 3 where the impacts have been scaled so that 100% represents the value of the primary production impacts.The chosen allocation key resulted in more of the impacts being allocated to the more valuable Ta 2 O 5 , and therefore in most impact categories, the impacts of Nb 2 O 5 were found to be lower than primary production.Here, the ODP in SCE2 forms the only exception.
The effect of changing the tantalum and niobium contents in the hard metal scrap entering the chemical recycling process was also examined as increased tantalum and niobium content in the hard metal scrap will also result in higher tantalum and niobium contents in the leach residue after cobalt recovery.The impacts of the total unallocated process in SCE1 were below those of primary production for all compositions.Closest values to primary production were found for GWP, FC, and AP, which were 87.5, 87.9, and 87.1% of primary production impacts, respectively.For the highest studied carbide content of 15 wt %, GWP, FC, and AP remain the impact categories closest to primary production values at 43.3, 55.2, and 53.9%, respectively.It was found that increasing the content of tantalum and niobium from 2.5 wt % each to 3.75 wt % decreases the impacts of Ta 2 O 5 production by recycling in SCE1 to an equal level (or below) with primary production.The impacts of Nb 2 O 5 production by recycling were then clearly below the primary production impacts in all cases.Further increase in the tantalum and niobium contents (up to 5 or 7.5 wt %) decreases gradually the environmental impacts of recycling.For instance, increasing the tantalum and niobium contents in the raw material from 2.5 to 5 and 7.5 wt % would decrease the GWP of the recycling from 1.24 times the GWP of primary tantalum production to 0.72 and 0.61 times the GWP of primary tantalum production, respectively.The results are shown in Figure 4.
Sensitivity and Uncertainty.The response in the LCIA results to a 20% disturbance in the uncertain simulation parameter values showed that the model was sensitive to some of the investigated parameters; both scenarios were very sensitive to the solid−liquid ratio, A/O ratio in Nb stripping, and the concentration of chemicals used to solubilize Ta and Nb, HF and H 2 SO 4 in SCE1 and NaOH in SCE2.Opposite to this, the responses in both cases to changes in the values of tantalum extraction in the niobium stripping, precipitation stage pH, and A/O ratio in tantalum stripping were very modest.The results of the analysis are presented in the Supporting Information in Figure S3.
The model in SCE1 was most sensitive to the concentration of the leaching chemicals and the solid−liquid ratio in all indicators.An 20% increase in the HF concentration increased the studied impacts between 8.4 and 12.0%, whereas a decrease resulted in a 7.6−10.6%decrease.Similarly, increase and decrease in the H 2 SO 4 concentration had a substantial effect, with an 8.2−22.0%increase and a 5.1−15.1% decrease, respectively.An increase in the solid−liquid ratio decreased the impacts between 0.6 and 12.6%, and a decrease in the ratio increased the impacts between 13.7 and 23.3%.The model in SCE2 was most sensitive to the NaOH concentration in GWP, FC, and EP.In POCP, ODP, and AP, it was most sensitive to the A/O ratio in Nb stripping.A 20% increase in the NaOH concentration increased the studied impacts between 21.2 and 21.6%, and a decrease decreased the impacts between 15.9 and 16.2%.The effect of change in the A/O ratio corresponded to either a 16.8−22.7%increase or a 21.9−29.5% decrease.Effect of changing tantalum and niobium content in the hard metal scrap on the evaluated environmental impacts of (a) tantalum pentoxide, (b) niobium pentoxide, and (c) total unallocated process.Production by recycling from 1 mt of the studied raw materials (cobalt leach residue from hard metal scrap chemical recycling) is compared here to primary production of the same products.Impacts have been scaled so that primary production impacts equal 100%.

■ DISCUSSION
In HF-based recycling (SCE1), leaching and effluent treatment were found to be the main contributing process steps to the gate-to-gate environmental impacts of Ta 2 O 5 and Nb 2 O 5 production.For HF-free recycling (SCE2), caustic conversion and SX were found to be the main contributing process steps.In both modeled processes, the production of the chemicals used in the largest contributing process steps were found to be the main cause for the high impacts.In SCE1, these included HF in particular, and in SCE2, they were NaOH and oxalic acid.Da Silva Lima et al. 8 also found that processing chemicals were the major contributor to the environmental impacts of Nb 2 O 5 production in a process of Nb 2 O 5 production as a coproduct of ferroniobium production from pyrochlore.In their process, the chemicals used for Nb 2 O 5 processing include KOH, NaOH, and H 2 SO 4 , and their consumption was shown to contribute highly to almost all impact categories.The results are not directly comparable to the current study as the raw materials, coproducts, and processes are different.In general, however, the impact assessment results are in the same range of magnitude apart from POCP.The impacts of Nb 2 O 5 produced from pyrochlore assessed by Da Silva Lima et al. 8 are lower compared to impacts from the primary process based on HF-leaching that modeled in the current study using columbo-tantalite raw material, which is reasonable considering the high contributions of the HF-leaching process step and the effluent treatment it necessitates.
The HF-free process modeled as SCE2 shows some promise to possibly be a less environmentally harmful process for the separation and refining of tantalum and niobium.In GWP, FC, and AP, the HF-free process performed better than the HFbased process.However, in EP, POCP, and ODP, it performed substantially worse, and the impact of POCP and ODP was multiple times larger when compared to the HF-based process.These impacts in the HF-free process were found to be mostly due to the use of oxalic acid in the SX stage.−26 However, currently, organic chemicals including oxalic acid are mainly produced from nonrenewable fossil resources. 23This highlights the importance of the chemical selection in the process and numerical observation of the environmental impacts of the use of those chemicals, which might under other circumstances be considered green but should be determined case by case.Here, the environmental impacts could potentially be substantially decreased with the use of another less environmentally intensive stripping chemical.Alternatively, development steps in oxalic acid production could benefit the downstream processing impacts.Oxalic acid is currently produced from, among others, fossil feedstocks, but there are studies in the scientific literature that have examined the possibility of using other feedstock options, such as CO 2 , which could potentially increase the sustainability of oxalic acid production. 27ased on the studied indicators, the recovery of tantalum and niobium from hard metal scrap by chemical recycling becomes increasingly environmentally beneficial when the Ta and Nb content in the scrap increases.For 2.5 wt % Ta and Nb each, 5 wt % total in the hard metal scrap, EP, POCP, and ODP values of the total unallocated process are substantially below those of primary production.The values of GWP, FC, and AP, however, are close to those of primary production.GWP, FC, and AP remain the indicators with values closest to primary production for all investigated Ta and Nb contents, but their values decrease steadily in comparison to those of primary production as the carbide content increases in the scrap raw material.From a total process point of view, the production of tantalum and niobium oxides by recycling from hard metal scrap is environmentally less intensive than their coproduction from ores.
To determine possible environmental benefits on the product level for each of the coproducts separately, the results must be allocated between the products.Tantalum is the more interesting and valuable of the two metals and will likely drive any possible recovery, and the impacts of the process are thereby allocated more heavily to tantalum.As a result of this, the allocated impacts of the coproduction of niobium are smaller than primary production from coltan ore for all of the studied raw material compositions.From a circular economy perspective, the aim should naturally be to functionally recycle all materials, but when comparing solely the environmental impacts allocated to the tantalum product in the studied case, recycling does not always perform better than primary production.The recovery process modeled in SCE1 was environmentally beneficial for tantalum recovery for all compositions only when looking at the EP, POCP, and ODP indicators.For GWP, FC, and AP, 3.75 wt % Ta and Nb each, 7.5 wt % total carbides in the hard metal scrap was required for the recycling to environmentally equal primary production.For material with even higher Ta and Nb content, recycling provides environmental benefit in comparison to primary production.To ensure environmental recycling of tantalum and niobium from hard metal scrap where they only exist in low concentrations, recycling by the zinc process could be considered.It does not enable the recovery of each metal as pure products as chemical recycling does but rather produces a powder with the same composition as the scrap that was fed into the process and which can be used in the production of new hard metals.
The modeling choices made in the study, along with used tools, data sources, and assumptions, affect how the results can and should be interpreted.Of the modeling choices, the methods used for coproduct allocation and end-of-life allocation can markedly affect how impacts are calculated for the system and its products, and it is necessary to understand their impact.Economic allocation, which considers the mass of the two produced products and also their economic value, was used for coproduct allocation as it was assumed that recycling operators would be more interested in the tantalum.This allocated more of the impacts to the tantalum product, while allocation purely based on mass would allocate the impacts more evenly.
The avoided burden approach was used for the end-of-life allocation to partition the impacts of the primary production of material and its recovery.It is also known as the end-of-life recycling (EOLR) approach and equates recycling with a decrease in primary production.Calculated in this manner, if the impacts of recycling are smaller than what can be credited as avoided primary production at the time of material recovery, then there is an environmental benefit to recycling.Another common method is the recycled content (RC) approach, which assumes a different driver for recycling compared to EOLR, and impacts are therefore calculated at different points in the life cycle.The effect of the chosen method can be better seen on a full product life cycle level, whereas in the current study with a limited scope of interest and a product with a short use phase, the difference in results is likely to be small.Process simulation is a flexible tool that can be used to create specific and precise LCI data that consider the particularities of the process under study.It does, however, also add some sources of uncertainty such as unavailability of representative data for simulation and upscaling of laboratory scale data to industrial scale.Another source of uncertainty is the limitations of the databases used for both process simulation and the LCA.HSC Chemistry, which was used for the process simulation, contains an extensive thermochemical database but does not include all compounds involved in the process, making the prediction of solution properties difficult.This may lead to errors in the model in the solution density and heat balance.
For the studied raw materials, information on their processing and chemical behavior is limited in literature.The process simulations built for this study would benefit from additional experimental or industrial data.Particularly information on the solid−liquid ratios and separation, abatement technologies, and energy consumption in addition to parameters related to the large contributors of leaching and SX would increase the accuracy of the model.
The process model is built for industrial scale but based on data that is mainly on laboratory or pilot scale, which increases the uncertainty associated with the results somewhat.Process parameters based on laboratory scale batch experiments may not be translatable as such to industrial operations where the process could be expected to be operated continuously for reasons of efficiency.Some case studies suggest that process simulation as a LCI generation tool is likelier to overestimate impacts in some impact categories, while underestimations are much smaller. 28An optimized industrial process may, for example, more efficiently manage recycling of chemicals and achieve better yields compared to a laboratory scale batch experiment.Continuously operated pilot scale experiments offer somewhat better translatability to industrial operation when it comes to, e.g., retention times which are reflected directly in the electricity consumption.The effect on this uncertainty is less impactful for the comparison done within the study as all recycling and primary processes were simulated in the same manner with the same assumptions, and thirdparty data were used only for the mining included in the primary production.It should, however, be carefully considered in comparison with other LCAs.
The uncertainty in the models has been considered in the conclusions and recommendations of this LCA.This study provides gate-to-gate LCI data and impact assessment results for recovery of tantalum and niobium from hard metal scrap recycling residue in the EU.The LCI data have been provided on a unit process level to enable omission of one or more unit if necessary for further use.The data can be used to estimate the environmental impacts of tantalum and niobium recovery from hard metal in the absence of primary plant data with reasonable reliability.Alterations to the data may, however, be necessary if the composition of the hard metal scrap material of interest were to be substantially different from the one used in the current study.It was shown that, for example, the share of tantalum and niobium carbides in the scrap material markedly affects the environmental impacts of their recovery.In comparison to primary coproduction of tantalum and niobium, the recycling process was less environmentally intense for all studied compositions, but for the tantalum product specifically, a relatively high content in the raw material was necessary to achieve environmental benefit.Overall, the results are encouraging of tantalum and niobium recovery from hard metal.
Detailed process descriptions and chemistries, flowsheets, simulation design parameters, and environmental impacts of tantalum and niobium recovery from hard metal by chemical recycling, studied by simulation-based LCA (PDF) ■

Figure 2 .
Figure 2. Evaluated environmental impacts of tantalum and niobium recycling from 1 mt of the investigated secondary raw material (cobalt leach residue from hard metal scrap chemical recycling).

Figure 3 .
Figure 3. Evaluated environmental impacts of tantalum pentoxide and niobium pentoxide production by recycling of 1 mt of the investigated secondary raw material (cobalt leach residue from hard metal scrap chemical recycling) compared to primary production in (a) SCE1 and (b) SCE2.Impacts have been scaled so that primary production impacts are equal 100%.

Figure 4 .
Figure 4. Effect of changing tantalum and niobium content in the hard metal scrap on the evaluated environmental impacts of (a) tantalum pentoxide, (b) niobium pentoxide, and (c) total unallocated process.Production by recycling from 1 mt of the studied raw materials (cobalt leach residue from hard metal scrap chemical recycling) is compared here to primary production of the same products.Impacts have been scaled so that primary production impacts equal 100%.

Table 1 .
Gate-to-Gate LCI for Each of the Process Steps per 1 mt Leaching Residue from Cobalt Leaching in Hard Metal Scrap Chemical Recycling

AUTHOR INFORMATION Corresponding Author Mari
Lundström − School of Chemical Engineering, Department of Chemical and Metallurgical Engineering, Aalto University, FI-00076 Aalto, Finland; Email: mari.lundstrom@aalto.fi