PROFESSIONAL TECHNICAL AND SCIENTIFIC PAPERS

 

Field-portable X-ray fluorescence analyzer for chemical characterization of carbonate-bearing base metal tailings: case study from Namib Pb-Zn Mine, Namibia

 

 

S. Lohmeier^I; D. Gallhofer^II; B.G. Lottermoser^III

^IInstitute of Disposal Research, Department of Mineral Resources, and Institute of Mining Engineering, Department of Surface Mining and International Mining, Clausthal University of Technology, ClausthalZellerfeld, Germany
^IIInstitute for Earth Sciences, University of Graz, Graz, Austria
^IIIInstitute of Mineral Resources Engineering, RWTH Aachen University, Aachen, Germany

Correspondence

 

 

---

ABSTRACT

Reprocessing of historic gold tailings is a common activity in South Africa, while base metal tailings offer similar reprocessing potential. The historic base metal tailings of the Namib Pb-Zn mine (Erongo Region, Namibia) still contain valuable resources of Pb and Zn in carbonate-rich matter. Micro-analysis of primary ore minerals identifies some galena as argentiferous and sphalerite as the principal host of Cd. This study demonstrates that knowledge of primary ore mineralogy helps to reveal the hosts of valuable commodities (Ag, Cd) and that field-portable X-ray fluorescence tools allow precise and accurate determinations of major and minor elements like Zn and Cd in such carbonate-rich material. Although there are limitations to directly determine the contents of certain element (i.e., As, Sb, In), linear correlations allow prediction of the likely abundance of these elements. Providing that the inter-mineral and inter-element relations are understood and there is consistency in sampling and analytical methodology, portable X-ray fluorescence analysis is an effective method to evaluate the chemical characteristics of base metal tailings for a range of major and trace elements.

Keywords: base metal tailings, trace elements, portable X-ray fluorescence analyzer

---

 

 

Introduction

In times of increasing demand for resources (European Commission, 2010, 2017), political and economic uncertainties, and worldwide changing supply chains, old tailings dumps are coming back into focus as a potential resource of formerly not extracted commodities. In Namibia alone, there are more than 250 abandoned mine sites (Salom and Kivinen, 2020), of which many still have potential for containing valuable commodities. Many of these Namibian tailings dumps are from colonial times and can be related to what is nowadays called small-scale mining; however, there are also younger mining residues that result from modern or large-scale mining operations and comprise large tonnages. In conjunction with changes in mining style and technologies, the focus of mining changed from time to time during production. One example is the Namib Pb-Zn mine in Namibia's Erongo region (Figure 1), where mining focussed first only on galena, while valuable sphalerite was lost to tailings; during later production activities, a sphalerite concentrate was also produced and silver was mined as by-product. Beside Pb and Zn, which are elements of traditional industrial interest, base metal tailings bear certain potential for other commodities, which were formerly not of interest, but are nowadays desired raw materials (Mudd et al., 2017; Werner et al., 2017). In the case of Namib Pb-Zn tailings (Figure 2A, B), the old tailings dump has potential for In, Cd, and Ag, as shown by Lohmeier et al. (2024). However, the potential of old tailings dumps is frequently not known and an evaluation is often avoided due to presumably high costs related to cost-intensive and time-consuming laboratory analyses.

The aim of this study is to show that data from primary samples allow assignment of the host(s) of trace metals to specific minerals and that portable X-ray fluorescence (pXRF) can be used to screen carbonate-bearing base metal tailings, such as the Namib Pb-Zn tailings, for certain elements (e.g., Cd, Ag). Moreover, an indirect estimate of the quantities of some minor and trace elements (e.g., In, Sb) in this material is also possible, provided that the mineralogical and geochemical compositions of the tailings are understood and the hosts of these elements are known. However, this study also points out the limits of pXRF and why conventional data obtained by XRF, inductively coupled plasma mass spectrometry (ICP-MS) or atomic emission spectrometry (AES), and electron microprobe (EMP) are still needed to obtain reliable results.

 

Background

Mining site

The Namib Pb-Zn Mine, formerly known as Deblin Mine or Namib Lead Mine, is located within the Rössing Mountains in the Dorob National Park in Namibia's Erongo Region (22°31'17.53"S; 14°45'41.16''E; Figure 1). The closest town is Swakopmund, about 25 km to the southwest. Access to the mining site is via the paved B2 connected to a small gravel road. The Namib Pb-Zn deposit was discovered during exploration activities in the 1930s; however, mining of base metal ores and production of a Pb concentrate only started in 1968. This was later supplemented by the production of a sphalerite concentrate and additional Ag (Snowden, 2014). After mining stopped in 1992, some exploration activities were carried out in 1992 and 1993 by Iscor Namibia and tentative reprocessing of tailings for Zn was tested by African Exploration in the mid-1990s (CCA, 2013; Snowden, 2014). The mine site was then abandoned for several years, before Kalahari Mineral Limited started drilling for primary ore and carried out a resource estimation of the potential of the tailings in 2007/2008. The mine site was then sold to North River Resources (CCA, 2013; Hahn et al., 2004; Tenova Mining and Minerals, 2014), which outlined a remaining indicated primary ore reserve of 710 000 t at a grade of 2.4% Pb, 7.0% Zn, and 50 g/t Ag related to four orebodies and an inferred resource of 409 000 t at a grade of 2.2% Pb, 6.0% Zn, and 38 g/t Ag (NLZM, 2023). There are additional resources in close-by gossans (NLZM, 2023). Limited mining and processing activities restarted in 2019; however, the mine site has been under care and maintenance since early 2020.

Two tailings dumps result from the former mining activities. The northern larger tailings dump comprises about 2.75M m³ of tailings material, while the smaller southern dump contains about 1.25M m3 already tentatively reprocessed tailings material (Figure 2; Hahn et al., 2004). The remaining bulk tailings resource is estimated at 2.75M m3 at a grade of 2.54% Zn, 0.21% Pb, and 7.0 g/t Ag (northern dump) plus 1.25M m3 at a grade of 2.14% Zn, 0.15% Pb, and 7.9 g/t Ag (southern dump) by Hahn et al. (2004), while NLZM (2023) reported a measured tailings resource of 260 000 t at a grade of 0.3% Pb, 2.2% Zn, and 7.5 g/t Ag and an indicated tailings resource of 350 000 t at a grade of 0.3% Pb, 2.1% Zn, and 7.7 g/t Ag.

There are no data for the inferred tailings resource by NLZM.

 

Materials and methods

Sampling

Eighteen surface samples, each weighing about 5 kg, were taken in 2019 from the northern tailings dump, which contains tailings material only from former processing activities; the southern tailings dump comprises already tentatively reprocessed material (Figures 1, 2). Samples were collected along vertical profiles and directly from exposed tailings faces (Figure 1B, 2A, B) and included different grey-yellow-brown-red coloured samples to account for different production cycles and thus possible geochemical heterogeneities in the tailings mass. Larger solidified chips are present, which disintegrate easily to smaller pieces/grains of silt to sand size (particle data are provided in Lohmeier et al., 2024). In addition, two ore samples, representative of the principal ore mineralization according to the mine geologists, were taken from new stockpiles (Figure 2C, D).

Sample processing and laboratory-based analysis

Tailings samples were air-dried and subsequently homogenized. A representative aliquot was milled to analytical fineness using a WC swing mill in the Department of Mineral Processing at RWTH Aachen University. Milled powders were sent to Australian Laboratory Services (ALS, Loughrea, Ireland) for conventional X-ray fluorescence spectroscopy (XRF) of major elements (Al, Ca, Fe, K, Mg, Mn, Na, P, Si, Ti), for ICP-MS after HNO3-HF-HGO4 and HCl digestions for certain trace elements (Dy, Er, Eu, Gd, Ho, Nd, Pr, Sm, Tm), and for infrared spectroscopy of C and S. Loss on ignition (LOI) was determined by sintering at 1000°C. In addition, samples were analysed at SGS Bulgaria (Bor Laboratory, Serbia) by ICP-MS after HNO3-HF-HGO4 and HCl digestion, for Ag, Al, As, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, In, K, La, Li, Lu, Mg, Mn, Mo, Nb, Na, Ni, P, Pb, Rb, Sb, Sc, Se, Sn, Sr, Ta, Tb, Te, Th, Ti, Tl, U, V, W, Y, Yb, Zn, and Zr. Samples having Ag > 10 μg/g, Pb > 10 000 μg/g, and/or Zn > 10 000 μg/g were reanalysed by AES using the same digestion approach. All sample packages included the analyses of duplicates and external and laboratory internal reference materials for quality control. Analytical data are documented in Lohmeier et al. (2024).

EMP analysis on primary ore was performed at the Institute of Disposal Research (IDR) at Clausthal University of Technology (TUC) using a Cameca SX FIVE instrument to determine trace element contents of sphalerite, galena, pyrite, marcasite, and cassiterite. Reconnaissance microanalyses by energy-dispersive X-ray spectrometry (EDX) showed that no elements other than those analysed were detectable in the respective minerals. The instrument was then operated in wavelength-dispersive mode at 15 kV and 100 nA for sphalerite and at 15 kV and 20 nA for all other mineral phases. X-ray lines, spectrometer crystals, and reference materials for cassiterite, galena, marcasite, pyrite, and sphalerite are provided in Table I. All analyses were checked for line and peak overlaps and the background sides were accordingly adjusted. Detailed results of sphalerite (153 analyses), galena (80 analyses), pyrite (58 analyses), marcasite (17 analyses), and cassiterite (89 analyses) can be requested from the authors.

Portable X-ray fluorescence spectroscopy

For pXRF analysis, Chemplex sample cups were filled with milled tailings powders and then backfilled with stuffing fibre. Prolene^TM thin films of 4.0 μηι thinness were used to guarantee simple but comparable analytical settings. As X-ray penetration of portable devices varies for each element, usage of these very thin films guarantees sufficient X-ray penetration depth both for light and heavy elements (Demirsar Arli et al., 2020; Potts et al., 1997) so that bias is minor to negligible and is the same for all samples and reference materials. Analyses were carried out at IDR (TUC) using a Niton XL3t 900 pXRF instrument equipped with a 50 kV Ag target X-ray tube, connected to a radiation protection chamber. All analyses were done in 'environmental mode - minerals with Cu/Zn, with a total measurement time of 100 s and were repeated five times. Results are presented in Table II. Reproducibility and homogeneity were tested by preparing two sample cups of each pulp; results were almost identical. The device was calibrated using the following certified reference materials (CRM): (1) OREAS 24b, 24c, 36, 37, 112, 131b, 132b, 133b, 134b, 160, 623, and 932 of the OREAS pXRF Zn-Pb-Ag sulfide kit, (2) RTS-3a and MP-1b of CANMET Mining and Mineral Sciences Laboratories, Canada, and (3) SRM 2780 of the National Institute of Standards and Technology (USA).

Calibration factors, slope, and intercepts were obtained using the provided CorrectCalc software program of the Niton device. A set of six CRM (OREAS 36, 131b, 132b, 133b, 134b, 623) was analysed three times at the start and end of each measuring day and once after every sample (including duplicates) to monitor drift of the instrument and assure quality control over the measurement period of five days (Figure 3).

Mineralogical and geochemical characterization of the old Namib Pb-Zn tailings

Namib Pb-Zn tailings are composed of relict galena and sphalerite, set into carbonate gangue, as the main mineral phases. In addition, minor pyrite, pyrrhotite, magnetite, quartz, graphite, apatite, biotite, phlogopite, and muscovite are present and relate to the primary ore mineralogy, as well as rare arsenopyrite, cassiterite, marcasite, rare-earth element fluorcarbonates (parisite), scheelite, and zircon. In contrast, gypsum, lepidocrocite, anglesite, and some goethite are most likely due to post-processing weathering under arid conditions (see Lohmeier et al., 2024 for more details). This mineralogical composition is also reflected by the element abundance of Fe (15.46-30.98 mass%), Ca (8.79-8.87 mass%), S (6.51-14.65 mass%), Zn (0.89-9.57 mass%), Si (2.54-6.87 mass%), Pb (0.16-5.69 mass%), Na (0.01-3.41 mass%), Mn (0.82-2.81 mass%), Al (0.56-1.57 mass%), Mg (0.25-1.31 mass%), and K (0.32-0.94 mass%). In addition, Cu (108-1479 μg/g), Sr (132-482 μg/g), Ti (420-839 μ^), As (86-587 μ^), P (218-349 μg/g), and Cd (32-399 μg/g) values are largely in the 100 μg/g range, whereas Ba (37-125 μg/g), Rb (35.7-93.2 μg/g), Ag (6.26-83 μg/g), Bi (3.02-45.70 μg/g), Sb (4.5-43.7 μg/g), Sn (13.1-40.2 μg/g), In (4.08-40.00 μg/g), Cr (9-22 μg/g), Ni (9.2-12.8 μg/g), V (3.7-17.7 μg/g), and Zr (3.90-8.10 μg/g) values are mostly in the 10 μg/g range or lower. Se concentrations are mostly < 2 μg/g (data are documented in Lohmeier et al., 2024). Consequently, almost all elements are present over quite large concentration ranges, although the concentration scales of individual elements are different. Pb and Zn still constitute quite large contents in these tailings and are thus clearly the elements of highest economic interest; however, Ag, Cd, In, and Sb are also present in certain quantities.

 

Results

Hosts of trace elements in primary ore

Backscattered electron (BSE) images revealed two different sphalerite generations by texture, which are not distinguishable in hand specimens. One generation comprises porous crystals; the other has a compact appearance (Figure 4A, B). The quite uncommon, very dark visual colour of sphalerite is attributed to very high Fe contents of ~ 7-10.5 mass% (Figure 4C), colloquially designated as marmatite. EMP analysis shows that compact sphalerite has on average 9.5 mass% Fe (8.6-10.4 mass% Fe) and 0.20 mass% Cd, while porous sphalerite has on average 8.4 mass% Fe (7.3-9.4 mass% Fe) and 0.19 mass% Cd (Figure 4C, D). Rarely, sphalerite has trace concentrations of Pb (< 0.3 mass% Pb; 8 analyses). Sb (< 0.05 mass%) and In (< 0.05 mass%) contents in sphalerite are below the lower analytical detection limit (LOD) of the EMP device.

There is only one galena generation present (Figure 4E, F), which has, in general, quite low trace element content, with Fe as the most abundant trace element (< 0.3-1.6 mass% Fe; av. 0.4 mass% Fe; Figure 4G). About one-third of all galena crystals have trace Ag contents (max. 0.10 mass%; Figure 4H), but most Ag contents are below the LOD of the EMP device (< 0.06 mass% Ag). There is no obvious relation between the Ag and Fe abundance, nor are there any other apparent inter-element relations. Cd (< 0.08 mass%), In (< 0.06 mass%), Sb (< 0.07 mass%), and Zn (< 0.46 mass%) concentrations in galena are below the LOD of the EMP device.

Similar to sphalerite, one pyrite generation has a porous and the other a compact appearance (Figure 5A, B); however, both show similar trace element abundance so no differentiation is made here. The same applies to pyrite and marcasite, although the marcasite database is distinctly smaller than the pyrite database (Figure 5C). The most abundant trace element in pyrite and marcasite is Zn, varying between < 0.4 and 1.9 mass% (Figure 5D). Pb concentrations in the sulfides range between < 0.2 and 0.4 mass%. Only marcasite shows single Ag (~ 0.06 mass%; 2 analyses) and Cd (~ 0.05 mass%; 1 analysis) values above the LOD of the EMP device (< 0.05 mass% Ag; < 0.04 mass% Cd), whereas In (LOD < 0.05 mass%) and Sb (LOD < 0.06 mass%) concentrations in pyrite and marcasite are all below the LOD of the EMP device.

Cassiterite crystals (Figure 5E, F) have a very restricted trace element spectrum, with Zn (< 0.4-2.3 mass%; av. 1.0 mass%) and W (< 0.1-1.5 mass%; av. 0.5 mass%) being the most abundant substituents for Sn (74.6-78.4 mass%; av. 76.7 mass%). In addition, Fe is detected by EMP with concentrations varying between < 0.19 and 0.7 mass% (av. 0.3 mass%). In concentrations vary between < 0.11 and 0.24 mass% (av. 0.18 mass%). The trace element contents of Ag (< 0.05 mass%), Cd (< 0.05 mass%), S (< 0.04 mass%), Sb (< 0.05 mass%), and Pb (< 0.21 mass%) are below the LOD of the EMP device. The general substitution of Sn by the cations Fe, In, W, and Zn is expressed in the Sn vs. Z(Fe + In + W + Zn) plot in Figure 5G. In concentrations should be taken with caution as there is some bias by InLa-SnL« interference. However, some crystals have trace element concentrations above 1.5x the highest obtained interference value and thus above the lower interference limit (Figure 5H).

Precision and accuracy of portable X-ray fluorescence data

The quality of the linear relationship/regression between the certified value of a CRM (the 'true' value) and the value obtained via pXRF is expressed by the coefficient of determination (R²), which is at best 1.00. The R² value indicates whether a specific element can be principally analysed by pXRF with acceptable quality or not. However, similar to conventional XRF (e.g., Rousseau, 2006), it has to be assured that the matrix of the CRM is ideally the same or at least similar to that of the sample material to be analysed (De Winter et al., 2017; Hou et al., 2004; Lu et al., 2022). Very good R² values were obtained for Ag, As, Ba, Bi, Ca, Cd, Cu, K, Mn, Pb, Rb, Sb, Se, Sn, Sr, Ti, and Zn (R² = 0.99-1.00), and for Fe, Si, and Zr (R2 = 0.95-0.98). Values are good for V (R² = 0.93); however, R2 is low for Cr (R² = 0.85) and Mg (R² = 0.86), and, in particular, for Al (R² = 0.53) and Ni R = 0.44) (Figure 6; Table III). Thus, pXRF should be principally capable of analyzing all elements having R² > 0.95 with a good to acceptable quality. To eliminate, or at least weaken, the matrix effect, the calculated slope and intercept values of CRM-pXRF pairs were used for external calibration of the portable Niton XL3t 900 tool.

Precision (the measure of analytical reproducibility or repeatability) and accuracy (the measure of correctness, meaning the proximity of analytical results to the true value) are common parameters used to evaluate the quality of (geo)chemical analyses. Using the criteria of Jenner (1996) and Piercy and Devine (2014), the precision of pXRF values can be assessed via the percent relative standard deviation (RSD), while the accuracy of pXRF values is expressed via the relative difference (%RD). RSD and %RD are calculated as follows:

Using Equation [2] can lead to negative values. If negative values resulted, these values are shown in Table III for %RDICP-MS/AES and %RDxrf to represent the entire value range. However, averages (av |%RDicp-ms|, av |%RD_xrf|) are based on absolute values only.

The most abundant elements (Al, Ca, Fe, K, Mn, Pb, Si, Zn) and several minor and trace elements (Ag, Ba, Bi, Cd, Ni, Rb, Sn, Sr, Zr) show, on average, excellent (RSD 0%-3%) to good (RSD 7%-10%) precision, with the exceptions of As, Cu, and Ti (av. RSD > 10%) (Table III). The major elements Al, Ca, Fe, K, Mn, Si, and Zn and the minor/trace elements Sn and Sr have excellent to good precision throughout, whereas precision is influenced by one outlier each for Ba, Bi, Zr, and by two outliers for Pb. The Ag and Cd values are influenced by more than two outliers. Cr, Mg, Sb, Se, and V cannot be analysed in Namib Pb-Zn tailings by pXRF due to the element contents being too low to be detected in this analytical setting.

The average accuracy of pXRF data, compared with ICP-MS/ AES and XRF data, is rather variable. When compared with XRF data, Ca, Fe, K, Mn, Ti, and Si are within an acceptable accuracy range (%RDxrf < 20), while Al values (|%RDxrf| > 80) are distinctly out of range. However, except for Fe and Mn, there are outliers within all element datasets. Considering only averages, only Cd, Cu, Fe, Rb, Sr, and Zn show acceptable accuracy compared with ICP-MS/AES data, of which the Cu, Fe, and Rb datasets contain outliers. Accuracy is distinctly out of range for Al, Ba, Bi, Ni, Sn, and Zr, with |%RD_icp-ms/_aes| values > loo, and is similarly poor for Ag, As, K, Pb, and Ti, with |%RD_icp-ms/_aes| values between 23 and 67 (Table III). However, the poor average accuracy of Ag (|%RD_icp-ms/_aes| = 47) can be partly explained by Ag values close to the LOD of the pXRF device, because elevated Ag values (> 50 μg/g) can be analysed with generally good to excellent (|%RDI_cp-ms/ _aes| < lO) accuracy. A similar relationship is observed for Pb, for which higher concentrations (> 4OOO μg/g) can be determined with good to excellent accuracy, while accuracy is poor for lower Pb concentrations. Cd also shows this general relation; however, the accuracy is excellent for higher Cd (> 4O μg/g) and good for lower Cd contents.

 

Discussion

Geochemical relations

EMP analyses reveal the presence of Cd (up to O.24 mass%) in sphalerite. In addition, few marcasite crystals have up to O.O5 mass% Cd. However, galena, pyrite, and cassiterite have no detectable Cd concentrations, identifying sphalerite as the principal Cd carrier. About one-third of all galena crystals have Ag up to O.lO mass%. In addition, accessory marcasite can have low Ag concentrations (< O.O6 mass%). Marcasite abundance in the bulk ore is very low (< O.Ol vol.%, according to microscopy), so galena is the principal identified Ag carrier, not excluding the presence of other Ag-bearing minerals. In concentrations in sphalerite are < O.O5 mass%, although substitution of Zn by In and Cd in sphalerite is common (e.g., Cook et al., 2OO9, 2Oll; Johan, 1988; Xu et al., 2O2l) and sphalerite is the most common host of In in base metal deposits (Cook et al., 2Oll; Schwarz-Schampera and Herzig, 2OO2). Rare cassiterite has only low In concentrations, indicating that most In is in sphalerite but at concentrations below O.O5 mass%. The presence of rare cassiterite explains also the low Sn concentrations of primary ore and very likely of the tailings. Sb was not detectable by EMP in any tested mineral phase; however, Sb concentrations in tailings and primary ore are low (< 44 μg/g; data are in Lohmeier et al., 2O24). Minor Pb can substitute for Zn in sphalerite and for Fe in pyrite, and minor Zn can substitute for Fe in pyrite and for Sn in cassiterite; however, the principal carriers of Zn and Pb are abundant sphalerite and galena. Moreover, no secondary Zn-bearing mineral is observed and

the tailings material has not been subject to any pyrometallurgical modifications so that the presence of Zn-bearing oxides is very unlikely. Anglesite is a typical weathering product of natural galena deposits and is also known from anthropogenically affected environments (e.g., Keim and Markl, 2O15; Lara et al., 2Oll). It is a very soluble secondary Pb mineral (Keim and Markl, 2O15), the presence of which indicates that weathering of the tailings material is not advanced and only slightly affected the uppermost part of the tailings dump, otherwise cerussite or pyromorphite-group minerals would be present (cf. Keim and Markl, 2O15; Lara et al., 2Oll). Therefore, destruction of argentiferous galena and redistribution of silver from galena is very unlikely and not indicated in bulk tailings, so galena is the main identified Ag carrier in the tailings material. W is mostly within accessory scheelite, explaining concentrations of ~ 7 μg/g in the primary ore and up to ~ 66 μg/g in tailings (data in Lohmeier et al., 2O24). Cassiterite can have trace concentrations of W, but these concentrations are negligible. Average As concentrations in tailings of ~ 18O μg/g can be explained, at least in part, by the presence of rare arsenopyrite (data in Lohmeier et al., 2O24). We only can speculate about the host(s) of Bi and Cs; however, bulk concentrations are relatively low so most of these elements can simply occur as substituents in other minerals; e.g., Bi can occur in traces in pyrite and other sulfides with values below LOD of the EMP device or can form Bi minerals in comparable settings (e.g., Callaghan, 2OOl; Fitros et al., 2O17; Wang et al., 2O2O).

Reconnaissance investigation of relict metals and other commodities of interest in fine-grained tailings materials-so-called slurries or impoundment cell material-is mostly based only on geochemical analysis of these materials, frequently by only checking for those elements of interest and leaving detailed mineralogical investigations aside. This is because slurries are usually too finegrained to allow microscopic investigations (e.g., Lohmeier et al., 2O2la) so XRD is the method of choice to reveal the mineralogical composition. However, XRD fails to detect minor and trace mineral commodities ('5% rule' and/or the minerals have no 'conspicuous' XRD pattern) when many different mineral phases are present (e.g., Khan et al., 2O2O). Moreover, neither bulk geochemical data nor XRD data reveal the inter-element relations of the mineral phases present. In many cases, inter-element relations and the host(s) of selected trace elements can be assumed based on experience with similar material, but bulk geochemical data are sometimes misleading, resulting in erroneous assumptions.

Prediction of geochemical composition from portable X-ray fluorescence data

Laboratory-based XRF analyses combined with ICP-MS/AES analyses are well-established techniques for routine analysis of major and trace elements in geological materials. In addition, both methods have been successfully applied to determination of the composition of tailings (e.g., Hahn et al., 2OO4; John Morrell et al., 1996; Othmani et al., 2O15; Souissi et al., 2O13; Struthers et al., 1997). In the last 3O years, pXRF tools were developed and constantly improved so that they are now frequently used for screening and selecting in mining and environmental-related tasks (Lemière, 2O18). However, the excitation energy (most portable tools work with 4O or 5O kV X-ray tubes (Lemière, 2018)) is too low for screening for many (critical) elements commonly present at low element concentrations (compare Gallhofer and Lottermoser, 2O18). Nevertheless, comparison of laboratory-based XRF and ICP-MS/ AES data can be used to identify specific element relations that are due to intrinsic element and/or mineral relations. In case of Namib Pb-Zn tailings, the trace elements In and Cd substitute for Zn in sphalerite, and Ag and probably Sb substitute for Pb in galena. Moreover, these are generally common minor or trace elements in Pb-Zn ores (Cook et al., 2009; George et al., 2015).

The chemical compositions of Namib Pb-Zn tailings, obtained by pXRF, are shown in Table II. For comparison of pXRF and ICP-MS/AES and XRF data, linear regression functions were calculated for pXRF-ICP-MS/AES and pXRF-XRF datasets. R² values are reported in Table III. Very good correlations (R2 > 0.95) were only obtained for Ca, Fe, and Mn, relative to XRF values, and for Ag, Cd, Cu, Fe, Pb, Sr, and Zn, relative to ICP-MS values (Ag, Cd, Pb, and Zn are shown in Figure 7A-D). K shows good correlations (R2 > 0.90) for both pXRF-ICP-MS and pXRF-XRF data pairs. Rb values (R2icp-ms = 0.89) are also of acceptable quality. However, correlations for all other elements, independent of whether pXRF-ICP-MS or pXRF-XRF datasets are considered, are poor, with R2 values of 0.14 to 0.64 (Table III). In this case, poor correlations were also found for metals/metalloids like As and Ni, for which very good R2pXRF-icp-MS data pairs have previously been obtained for slags (Lohmeier et al., 2021b).

Portable XRF provides neither precise nor accurate data for any of the critical elements-As (86-587 μg/g),ess Bi (3-46 μg/g), and Sn (13-40 μg/g)-defined as those elements ential for (modern) economy but (very) vulnerable to disruptions in the mining chain (USGS, 2018), in carbonate-bearing Namib Pb-Zn base metal tailings; As was only ranked amongst the critical elements for a limited time. Moreover, pXRF cannot determine Cs, In, Sb, and W using the chosen analytical setting. However, calculation of correlation coefficients for element pairs from ICP-MS and pXRF datasets reveals clear positive correlations of Zn (analysed by pXRF) with Ag (R² = 0.93), Cd (R² = 0.98), In (R² = 0.91), Pb (R2 = 0.91), and Sb (R² = 0.96; analysed by ICP-MS/AES; Figure 7E-I). In addition, there is a good correlation between Zn and Pb data (R2 = 0.93) obtained by pXRF (Figure 8K, L). Good, clearly positive correlations are revealed for Pb (analysed by pXRF) with Ag (R2 = 1.00), Cd (R2 = 0.92), In (R² = 0.90), Sb (R² = 0.94), and Zn (r2 = 0.94; analysed by ICP-MS; Figure 8F-J). Moreover, good positive correlations are present for Cd (analysed by pXRF) and Ag (R2 = 0.93), In (R² = 0.93), Pb (R² = 0.92), Sb (R² = 0.94), and Zn (R2 = 0.99; analysed by ICP-MS; Figure 8A-E). Neither Cs nor W show good correlation with elements measurable by pXRF. The same applies for the rest of the bulk dataset, indicating that the inter-element relations in the group Ag-Cd-In-Pb-Sb-Zn are due to intrinsic element and mineral relations, explained by the substitution of Cd and In for Zn in sphalerite, and probably by the coupled substitution of (Ag+Sb) for Pb in galena.

Therefore, Ag, In, and Sb can be indirectly estimated via pXRF analyses of Cd and Zn, for which both elements can be determined with good precision (Cd: av. RSD = 7.42; Zn: av. RSD = 1.19) and good accuracy (Cd: av. |%RD_ICP-Ms| = 6.89; Zn: av. |RD_ICP-Ms| = 5.84). However, determination via Zn values is favoured because all Zn values by pXRF are of excellent precision, while Cd values are mostly of good precision only. In addition, estimation of In and Sb via pXRF data of Pb is feasible because Pb can be analysed in this setting with good precision (av. RSD = 5.56), but only moderate accuracy (av. |%RSD_ICP-MS| = 39). Hence, linear regressions [3] to [5] allow semi-quantitative estimation of the In content in Namib Pb-Zn tailings:

Linear regressions [6] to [8] allow semi-quantitative estimation of the Sb content in Namib Pb-Zn tailings:

It has to be mentioned that linear regressions [3] to [8] are only applicable for carbonate-bearing Namib Pb-Zn base metal tailings. However, when inter-element relations are known for a material of interest, linear regression functions can be developed for the respective material. These linear functions allow then an indirect semi-quantitative estimation of the element contents. This does not imply that conventional laboratory-based analyses are not necessary, because portable tools can simply not analyse all elements, as for example Na, and data are always calculated to a sum of 100% when using only the internal calibration of the portable tool. However, when financial resources are limited, acceptable data can be obtained when only a small conventional laboratory-based dataset is available, provided that inter-element relations are known for the tailings and/or primary ore material and that portable tools are calibrated with an appropriate CRM. Moreover, this indirect approach can only be used when there are clear inter-element relations, elements are not present in various mineral phases, and the material is quite homogeneous, as in case of slags (compare Lohmeier et al., 2021b). As shown for Namib Pb-Zn, Cd and In are related to sphalerite (In concentrations in accessory cassiterite and Cd concentrations in pyrite are unimportant) and Ag is related to galena (Ag concentrations in pyrite are unimportant). In contrast, the host of Bi is not known. It is very likely that the trace Bi forms no separate mineral phases, but occurs as substitutions for other elements in various minerals, so there are no explicit correlations between Bi and elements measurable by pXRF. In case of Sn, cassiterite is the major host of Sn; however, Sn can also be incorporated in other minerals and Sn concentrations are low. As primarily occurs as rare arsenopyrite (FeAsS) and is thus related to two elements that are present in diverse mineral phases, so there is no explicit correlation with another element for As.

Linear regressions [9] to [11] allow calculation of Ag in carbonate-bearing Namib Pb-Zn base metal tailings:

These regressions are provided because Ag can be directly analysed via pXRF with good precision, but only poor accuracy, so an additional evaluation/quality check of Ag values can be done via other elements.

 

Namib Pb-Zn tailings resource and reprocessing

Namib Pb-Zn tailings are still a noteworthy resource of Pb and Zn and contain Ag, Cd, In, and Sb, which are of interest for (modern) industry (Lohmeier et al., 2024). Hahn et al. (2004) state that the old tailings still contain 2.54% Zn, 0.21% Pb, and 7.0 μg/g Ag, which matches quite well with recent company data provided by NLZM (2023), with average element contents of 2.2% Zn, 0.3% Pb, and 7.5 μg/g Ag. Considering a tailings volume of 2.75M m³, as outlined by Hahn et al. (2004), and an average density of 2.42 g/cm3, this would translate into about 6.6 Mt (for density estimation, see Lohmeier et al., 2024). This resource is less than the remaining measured and indicated resource of the Namibian Rosh Pinah Mine of 19.94 Mt at a grade of 7.34% Zn, 1.83% Pb, and 27.71 μg/g Ag (Trevali, 2023) and is distinctly lower than the Zn resource of the Namibian Skorpion Mine of 24.6 Mt at a grade of 10.6% Zn (Borg et al., 2003); however, the Namib Pb-Zn tailings are easily accessible and the material is already milled. In the mid-1990s, reprocessing of these tailings was considered, resulting in the construction of the younger tailings dump (Hahn et al., 2004). However, processing technology was less advanced in the 1990s than today and extraction of valuable quantities of Zn failed only due to problems with pyrrhotite suppression during flotation (Snowden, 2014). To date, the processing of Zn-Pb(-Cu) ores is still the most challenging of all ore types; in particular, when Fe-rich sphalerite is present, which has a similar density as pyrrhotite (Bulatovic, 2007). However, a new approach via sequential flotation, instead of the usually applied bulk Pb-Zn(-Cu) flotation, was successfully realized in some mining projects (for detail, see Bulatovic, 2007), so reprocessing of Namib Pb-Zn tailings seems possible. Moreover, valuable commodities of Ag, Cd, In and Sb will be directly extracted with Pb and Zn because these are contained in the Pb- and Zn-bearing minerals, and will upgrade a future Pb-Zn concentrate. Preparation of a saleable concentrate and further processing of such a concentrate will be challenging (see Lohmeier et al., 2024). However, the Southern African processing and metallurgical industry has experience in base metal extraction over many decades (e.g., Dworzanowski, 2019) and has already managed to treat complex tailings (e.g., Guest et al., 1988; Svoboda et al., 1988) and other kinds of Pb- and Zn-containing secondary raw materials (Reuter et al., 1997).

 

Conclusion

Namib Pb-Zn tailings contain elevated base metal concentrations, with Ag, Cd, In, and Sb as additional elements of commercial interest. Whenever possible, assessment of fine-grained tailings should not be performed solely on tailings, but should be combined with (detailed) mineralogical and geochemical investigations of primary ore and host rock(s) to be sure that the origin of trace elements-provided they are of interest-is understood. Portable XRF can be used as a complementary semi-quantitative tool for screening carbonate-bearing base metal tailings for target metals (Pb, Zn). Some elements of interest cannot be directly analysed via a portable tool, because element concentrations (In, Sb) are simply too low. Provided that 1) the mineralogical composition of the tailings material is known, 2) the host mineral phases of the elements of interest are known, 3) there are clear inter-mineral and inter-element relations, and 4) appropriate certified reference materials are available, then selected elements (In, Sb) can be determined via proxies (Zn, Cd, Pb) using simple linear regression functions. This does not mean that conventional geochemical analyses are not necessary, but reasonable semi-quantitative results can be obtained via portable tools, thereby reducing analytical costs and saving time. Base metal tailings like those of the Namib Pb-Zn mine are an underestimated source of elements/metals of interest for industry, particularly in times of declining resources and worldwide political and economic uncertainties.

 

Acknowledgements

This work was supported by the German Federal Ministry of Education and Research (BMBF) and is part of the sub-Saharan based LoCoSu project; grant number 01DG16011. Thanks to S. Garoeb and M. Punzel from Namib Pb-Zn for free access to the sampling site in 2018 and for an exciting above-ground mine visit in 2019. U. Hemmerling is thanked for preparation of polished (thin) sections (Clausthal University of Technology (TUC), Department of Mineral Resources (IMMR)). Thanks to D. Nordhausen for technical assistance and a warming coffee during many hours at the microprobe and to F. Türck for his patience and support during many long days of pXRF analysis (both: IMMR, TUC). We are grateful to L. Weitkämpfer, D. Gürsel, and P. Ihl (RWTH Aachen University, Department of Processing) for providing free access to powder preparation equipment and their never-ending patience.

Authors contribution

Conceptualization: BGL, SL; sampling: DG, SL; methodology: SL; validation: SL; formal analysis: SL; data curation: SL; writing -original draft preparation: SL; writing - review and editing: BGL, DG

 

References

Borg, G., Kärner, K., Buxton, M., Armstrong, R., van der Merwe, S.W. 2003. Geology of the Skorpion supergene zinc deposit, southern Namibia. Economic Geology, vol. 96, pp. 749-771. https://doi.org/10.2113/gsecongeo.98.4.749        [ Links ]

Bulatovic, S.M. 2007. Flotation of sulfide ores. Handbook of Flotation Reagents: Chemistry, Theory and Practice. Bulatovic, S.M. (ed.). Elsevier, Amsterdam. pp. 367-400. https://doi.org/10.1016/C2009-0-17332-4        [ Links ]

Callaghan, T. 2001. Geology and host-rock alteration of the Henty and Mount Julia gold deposits, Western Tasmania. Economic Geology, vol. 96, pp. 1073-1088. https://doi.org/10.2113/gsecongeo.96.5.1073        [ Links ]

Colin Christian and Associates CC (CCA). 2013. Proposed recommissioning of Lead and Zinc Mine on EPL 2902. Volume 1: Environmental impact assessment and environmental management plan including closure and rehabilitation, December.         [ Links ]

Cook, N.J., Ciobanu, C.L., Pring, A., Skinner, W., Shimizu, M., Danyushevsky, L., Saini-Eidukat, B., Melcher, F. 2009. Trace and minor elements in sphalerite: A LA-ICP-MS study. Geochimicaet Cosmochimica Acta, vol. 73, pp. 4761-4791. https://doi.org/10.1016/j.gca.2009.05.045        [ Links ]

Cook, N.J., Ciobanu, C.L., Williams, T. 2011. The mineralogy and mineral chemistry of indium in sulphide deposits and implications for mineral processing. Hydrometallurgy, vol. 108, pp. 226-228. https://doi.org/10.1016/j.hydromet.2011.04.003        [ Links ]

Demirsar Arli, B., Simsek Franci, G., Kaya, S., Arli, H., Colomban, P. 2020. Portable X-ray fluorescence (p-XRF) uncertainty estimation for glazed ceramic analysis: Case of Iznik tiles. Heritage, vol. 3, pp. 1302-1329.https://doi.org/10.3390/heritage3040072        [ Links ]

De Winter, N.J., Sinnesael, M., Makarona, Ch., Vansteenberge, S., Claeys, P. 2017. Trace element analyses of carbonates using portable and micro-X-ray fluorescence: Performance and optimization of measurement parameters and strategies. Journalof Analytical Atomic Spectrometry, vol. 32, pp. 1211-1223. https://doi.org/10.1039/C6JA00361C        [ Links ]

Dworzanowski, M. 2019. Base metals production in Southern Africa provides the common thread between all the country branches in the SAIMM. Journal of the South African Institute of Mining and Metallurgy, vol. 119, pp. 50-51.         [ Links ]

European Commission. 2010. Critical raw materials for the EU. Report of the ad-hoc working group on defining critical raw materials. European Commission, Brussel.         [ Links ]

European Commission. 2017. Study on the review of the list of critical raw materials - criticality assessments. European Commission, Brussel.         [ Links ]

Fitros, M., Tombros, S.F., Williams-Jones, A.E., Tsikouras, B., Koutsopoulou, E., Hatzipanagiotou, K. 2017. Physicochemical controls on bismuth mineralization: An example from Moutoulas, Serifos Island, Cyclades, Greece. American Mineralogist, vol. 102, pp. 1622-1631. https://doi.org/10.2138/am-2017-6125        [ Links ]

Gallhofer, D., Lottermoser, B.G. 2018. The influence of spectral interferences on critical element determination with portable X-ray fluorescence (pXRF). Minerals, vol. 8, no. 320. https://doi.org/10.3390/min8080320        [ Links ]

George, L., Cook, N.J., Ciobanu, C.L., Wade, B.P. 2015. Trace and minor elements in galena: a reconnaissance LA-ICP-MS study. American Mineralogist, vol. 100, pp. 548-569. https://doi.org/10.2138/am-2015-4862        [ Links ]

Guest, R.N., Svoboda, J., Venter, W.J.C. 1988. The use of gravity and magnetic separation to recover copper and lead from Tsumeb flotation tailings. Journal of the South African Institute of Mining and Metallurgy, vol. 88, pp. 21-26.         [ Links ]

Hahn, L., Solesbury, F., Mwiya, S. 2004. Report: Assessment of potential environmental impacts and rehabilitation of abandoned mine sites in Namibia. Communications of the Geological Survey of Namibia, vol. 13, pp. 85-91.         [ Links ]

Hou, X., He, Y., Jones, B.T. 2004. Recent advances in portable X-ray fluorescence spectrometry. Applied Spectroscopy Reviews, vol. 39, pp. 1-25. https://doi.org/10.1081/ASR-120028867        [ Links ]

Jenner, G.A. 1996. Trace element geochemistry of igneous rocks: geochemical nomenclature and analytical geochemistry. Geological Association of Canada, Short Course Notes, vol. 12, pp. 51-77.         [ Links ]

Johan, Z. 1988. Indium and germanium in the structure of sphalerite: An example of coupled substitution with copper. Mineralogy and Petrology, vol. 39, pp. 211-229.         [ Links ]

John Morrell, W.J., Stewart, R.B., Gregg, P.E.H., Boan, N.S., Horne, D. 1996. An assessment of sulphide oxidation in abandoned base-metal tailings, Te Aroha, New Zealand. Environmental Pollution, vol. 94, pp. 217-225. https://doi.org/10.1016/S0269-7491(96)00059-0        [ Links ]

Khan, H., Yerramilli, A.S., D'Oliveira, A., Alford, T.L., Boffito, D.C., Patience, G.S. 2020. Experimental methods in chemical engineering: X-ray diffraction spectroscopy - XRD. Canadian Journal of Chemical Engineering, vol. 98, pp. 1255-1266. https://doi.org/10.1002/cjce.23747        [ Links ]

Keim, M.F., Markl, G. 2015. Weathering of galena: Mineralogical processes, hydrogeochemical fluid path modeling, and estimation of the growth rate of pyromorphite. American Mineralogist, vol. 100, pp. 1584-1594. https://doi.org/10.2138/am-2015-5183        [ Links ]

Lara, R.H., Briones, R., Monroy, M.G., Mullet, M., Humbert, B., Dossot, M., Naja, G.M., Cruz, R. 2011. Galena weathering under simulated calcareous soil conditions. Science of the Total Environment, vol. 409, pp. 3971-3979. https://doi.org/10.1016/j.scitotenv.2011.06.055        [ Links ]

Lemière, B. 2018. A review of pXRF (field portable X-ray fluorescence) applications for applied geochemistry. Journal of Geochemical Exploration, vol. 188, pp. 350-363. https://doi.org/10.1016/j.gexplo.2018.02.006        [ Links ]

Lohmeier, S., Lottermoser, B.G., Schirmer, T., Fuchsloch, W 2021a. Reprocessing potential of pegmatite tailings for rare metal extraction and brick fabrication, Uis, Namibia. South African Journal of Geology, vol. 124, pp. 639-662. https://doi.org/10.25131/sajg.124.0015        [ Links ]

Lohmeier, S., Lottermoser, B.G., Schirmer, T., Gallhofer, D. 2021b. Copper slag as a potential source of critical elements - a case study from Tsumeb, Namibia. Journal of the Southern African Institute of Mining and Metallurgy, vol. 121, pp. 129-142.http://dx.doi.org/10.17159/2411-9717/1383/2021        [ Links ]

Lohmeier, S., Gallhofer, D., Lottermoser, B.G. 2024. Geochemical and mineralogical characterization and resource potential of the Namib Pb-Zn tailings (Erongo Region, Namibia). Journal of the Southern African Institute of Mining and Metallurgy, vol. 124, no. 8, pp. 447-460.         [ Links ]

Lu, J., Guo, J., Wei, Q., Tang, X., Lan, T., Hou, Y., Zhao, X. 2022. A matrix effect correction method for portable X-ray fluorescence data. Applied Sciences, vol. 12, no. 568. https://doi.org/10.3390/app12020568        [ Links ]

Mudd, G.M., Jowitt, S.M., Werner, T.T. 2017. The world's lead-zinc mineral resources: Scarcity, data, issues and opportunities. Ore Geology Reviews, vol. 80, pp. 1160-1190. https://doi.org/10.1016/j.oregeorev.2016.08.010        [ Links ]

Namib Lead and Zinc Mining (NLZM). 2023. Operational data. http://www.namibleadzinc.com/operations.html [Assessed 7 March 2023].         [ Links ]

Othmani, M.A., Souissi, F., Bouzahzah, H., Bussière, B., Ferreira da Silva, E., Benzaazoua, M. 2015. The flotation tailings of the former Pb-Zn mine of Touiref (NW Tunisia): mineralogy, mine drainage prediction, base-metal speciation assessment and geochemical modelling. Environmental Science and Pollution Research, vol. 22, pp. 2877-2890. https://doi.org/10.1007/s11356-014-3569-1        [ Links ]

Piercy, S.J., Devine, M.C. 2014. Analysis of powdered reference materials and known samples with a benchtop, field portable X-ray fluorescence (pXRF) spectrometer: evaluation of performance and potential application for exploration lithogeochemistry. Geochemistry: Exploration, Environment, Analysis, vol. 14, pp. 139-148. https://doi.org/10.1144/geochem2013-199        [ Links ]

Potts, P.J., Williams-Thorpe, O., Webb, P.C. 1997. The bulk analysis of silicate rocks by portable X-ray fluorescence: Effect of sample mineralogy in relation to the size of the excited volume. Geostandard Newsletter, vol. 6, pp. 29-41. https://doi.org/10.1111/j.1751-908X.1997.tb00529.x        [ Links ]

Reuter, M., Sudhölter, S., Krüger, J. 1997. Some criteria for the selection of environmentally acceptable processes for the processing of lead- and zinc-containing flue dusts. Journal of the South African Institute of Mining and Metallurgy, vol. 97, pp. 27-37.         [ Links ]

Rousseau, R.M. 2006. Corrections for matrix effects in X-ray fluorescence analysis - a tutorial. Spectrochimica Acta Part B, vol. 61, pp. 759-777. https://doi.org/10.1016/j.sab.2006.06.014        [ Links ]

Salom, A.T., Kivinen, S. 2020. Closed and abandoned mines in Namibia: a critical review of environmental impacts and constraints to rehabilitation. South African Geographical Journal, vol. 102, pp. 1-17. https://doi.org/10.1080/03736245.2019.1698450        [ Links ]

Schwarz-Schampera, U., Herzig, P.M. 2002. Indium - Geology, Mineralogy, and Economics. Springer. https://doi.org/10.1007/978-3-662-05076-7        [ Links ]

Snowden. 2014. Namib lead zinc project. Project no. J2178. Mine development plan.         [ Links ]

Souissi, R., Souissi, F., Chakroun, H.K., Bouchard, J.L. 2013. Mineralogical and geochemical characterization of mine tailings and Pb, Zn, and Cd mobility in a carbonate setting (Northern Tunisia). Mine Water and the Environment, vol. 32, pp. 16-27. https://doi.org/10.1007/s10230-012-0208-2        [ Links ]

Struthers, S., Brumley, J., Taylor, G. 1997. An integrated system for recycling base metal mine tailings. Proceedings of the 14th Annual Meeting of the American Society of Mining and Reclamation, Austin, Texas, 10-15 May 1997. American Society of Mining and Reclamation, pp. 579-592.         [ Links ]

Svoboda, J., Guest, R.N., Venter, W.J.C. 1988. The recovery of copper and lead minerals from Tsumeb flotation tailings by magnetic separation. Journal of the South African Institute of Mining and Metallurgy, vol. 88, pp. 9-19. https://hdl.handle.net/10520/AJA0038223X1865        [ Links ]

Tenova Mining and Minerals. 2014. North River Namib lead/ zinc project. Feasibility Study, M8168-1300-001, Section: 2 Introduction and project background.         [ Links ]

Trevali. 2023. Operational data. https://trevali.com/operations/operations/rosh-pinah [Assessed 9 March 2023].         [ Links ]

United States Geological Survey (USGS). 2018. Draft critical mineral list - summary of methodology and background information. U.S. Geological Survey technical input document in response to secretarial order no. 3359.         [ Links ]

Wang, L., Wang, Y., Huang, S., Wei, R., Sun, Z., Hu, Q., Hao, J. 2020. The gold occurrence in pyrite and Te-Bi mineralogy of the Fancha gold deposit, Xiaoqinling gold field, southern margin of the North China craton: Implications for ore genesis. Geological Journal, vol. 55, pp. 5791-5811. https://doi.org/10.1002/gj.3637        [ Links ]

Werner, T.T., Mudd, G.M., Jowitt, S.M. 2017. The world's by-product and critical metal resources. Part III: A global assessment of indium. Ore Geology Reviews, vol. 86, pp. 939-956. https://doi.org/10.1016/j.oregeorev.2017.01.015        [ Links ]

Xu, J., Ciobanu, C.L., Cook, N.J., Slattery, A., Li, X., Kontonikas- Charos, A. 2021. Phase relationships in the system ZnS-CuInS2: Insights from a nanoscale study of indium-bearing sphalerite. American Mineralogist, vol. 106, pp. 192-205. https://doi.org/10.2138/am-2020-7488 ♦         [ Links ]

 

 

Correspondence:
S. Lohmeier
Email: stephanie.lohmeier@tu-clausthal.de

Received: 11 Mar. 2023
Revised: 31 May 2023
Accepted: 11 Jun. 2024
Published: August 2024

 

 

ORCID:
S. Lohmeier http://orcid.org/0000-0003-2556-2096
D. Gallhofer http://orcid.org/0000-0003-2139-7847
B.G. Lottermoser http://orcid.org/0000-0002-8385-3898