ENRICHMENT ON BANGKA TIN SLAG ’ S TANTALUM AND NIOBIUM OXIDE CONTENTS THROUGH NON-FLUORIDE PROCESS

The electronic, automotive, and aerospace new technology’s high dependence on tantalum makes this metal one of the technology-critical elements [1]. Researches on the endurance of tantalum supply chain investigated the endurance improvement mechanism such as optimization of other sources (e. g. tin slag, scraps, etc.), recycling, material substitution, and hoarding [2]. Efforts to widen the knowledge of tantalum around environment were done especially that 9. Nitriding and carbonitriding: мonograph / Chatterjee-Fisher R., Eisel F. V., Hofmann R. et. al.; A. V. Supov (Ed.). Moscow: Metallurgy, 1990. 280 p. 10. Interdependence between stress and texture in arc evaporated Ti–Al–N thin films / Falub C. V., Karimi A., Ante M., Kalss W. // Surface and Coatings Technology. 2007. Vol. 201, Issue 12. P. 5891–5898. doi: https://doi.org/10.1016/j.surfcoat.2006.10.046 11. Stout K. Y., Dong W. P., Mainsah E. A Proposol for Standardisation of Asserment of Three–Dimensional Mikro-Topography-Part 1: Snrface Digitisation and Parametric Characterisation. Birmingham: The University of Birmingham, 1993. 21 р. 12. Chen X., Chen G. On the thermally induced cracking of a segmented coating deposited on the outer surface of a hollow cylinder // Surface and Coatings Technology. 2009. Vol. 203, Issue 9. P. 1114–1120. doi: https://doi.org/10.1016/j.surfcoat.2008.10.002

The aforementioned explanation of the tantalum scarcity and researchers' effort to keep the tantalum supply chain, to venture materials that, one never thinks before, contain this metal such as sea and freshwater, and to find the best technique to maximally pull this valuable element out of the unwanted matters such as hydrometallurgy draw researchers' attention to enrich the contents of tantalum.

Literature review and problem statement
Some information above becomes a challenge for the researchers to carry out the next research in order to raise the tantalum supply.In general, as is common in the case with tantalum, tantalum extracted from tin slag is followed by niobium [9].Tin slag as a secondary source of tantalum and niobium was reported in prior researches [9,10].Tin slag primarily spreads across Nigeria [10], Zaire [20], Brasil [12], Thailand [13] and Indonesia [9].Studies of tin slag leaching with acid and alkaline solution to upgrade the concentration of tantalum and niobium have been done in previous researches [14,15].This paragraph summarizes prior studies which exploited fluoride acid to enrich TNO contents from tin slag.The dissolution of HCl, HF, and NaOH in tin slag which involved either alkali-acid order reagents or acid-alkali order reagents increased tantalum recovery rate, from 60 % to 93 %, and niobium recovery rate, from 29 % to 78 % [20].HF or (HF+H 2 SO 4 ) dissolution in tin slag's (Ta+Nb) 2 O 5 whose contents were over 25 % produced around 85 % recovery rate [21].Tin slag's 3.4 % Nb 2 O 5 and 3.05 % Ta 2 O 5 underwent several processes of leaching, namely caustic solution leaching, alkali pugging, alkali fusion, acid leaching, and HF & H 2 SO 4 leaching.The alkali processes and acid leaching optimally produced Ta 2 O 5 and Nb 2 O 5 , 10.4 % and 10.6 % [14].Among 4 %, 8 %, 16 %, and 32 % concentrations, dissolving 8 % HF in tin slag produced an optimal yield ratio of tantalum and niobium, 2.01 and 2.09 [15].
Chlorination was also used to enrich tin slag's TNO concentration.The summary of related investigations is provided in this paragraph.Tin slag's 7.5 % tantalum and 5.2 % niobium underwent HCl leaching (called low-grade composition (LGC)).Meanwhile, the other was HCl-leached followed by NaOH leaching (called high-grade composition (HGC)).Next, both LGC and HGC were chlorinated and added with Cl 2 +N 2 or Cl 2 +CO+N 2 in 200-1,000 o C. On the one hand, LGC in 1,000°C within 24 hours extracted more than 95 % of tantalum and niobium.On the other hand, at the same temperature and time, HGC chlorination produced 65 % tantalum and 84 % niobium [22].Recovery refractory metal applied to tin slag which engaged HCl leaching and chlorination roasting optimally produced 82.1 % Nb 2 O 5 and 84.7 % Ta 2 O 5 [19].HCl and NaOH dissolution in tin slag containing 0.33 % Ta 2 O 5 and 0.64 % Nb 2 O 5 optimally upgraded tantalum and niobium concentration to 0.52 % and 1.18 % [15].
In this study, the tin slag which was used is from Bangka Islands.The tin slag will be further referred to as Bangka tin slag (BTS).Prior investigations that used BTS informed: BTS contained 2.7 % (Ta,Nb) 2 O 5 [11], the study upgraded BTS' TNO contents [9,19], the thermodynamic analysis was done in the study of upgrading BTS' REE [20].

The aim and objectives of the study
This research aims at enhancing the concentration of Bangka tin slag's tantalum and niobium through leaching processes using non-fluoride chemical substances.
To achieve this goal, the following objectives were set: 1. Characterizing BTS through XRF.

2. Procedure for conducting the experiments and determining the indicators samples' properties
Below is the layout of research stages which tantalum and niobium underwent during investigation.The details are systematically given under Fig. 1.
The first characterization of BTS was SEM and XRF.In the first process, BTS was roasted at 900 o C, quenched, and dewatered.Next, the results of these processes were characterized using XRF and SEM.After being dewatered, BTS was sieved with size distribution classifications of +100, -100+150, -150+200, -200+250, and -250 mesh.The sizes which were used in the next leaching were -200+250 mesh.
The first leaching exploited NaOH 8M.Then, the residues were divided into 2.The first one was HClO 4 -leached with 0.1, 0.4, and 0.8 M concentrations while the other underwent 0.8 M HClO 4 and 0, 0.8, 1.6, and 2.4 M H 2 SO 4 leaching.All the leaching processes were carried out at 25 o C within 2 hours.All residues were characterized using XRF.Meanwhile, the characterization of filtrates from iron and calcium elements involved AAS, and those from niobium and tantalum elements were characterized through ICP-OES.Fig. 1 illustrates all the research schemes.

Results of enrichment on Bangka tin slag's tantalum and niobium oxide contents through the non-fluoride process
The sub-chapters of the research results are as follows: (1) XRF and SEM characterization, (2) BTS 900 o C-roasting, quenching, dewatering, and sieving, (3) NaOH leaching, and (4) HClO 4 leaching and HClO 4 followed by H 2 SO 4 leaching.

1. XRF and SEM Characterization
The first BTS characterization was conducted using XRF as shown in Table 1 and SEM as illustrated in Fig. 2, a.All the elements of BTS were split into 3 parts: the valuable oxides are tantalum-niobium; major other oxides (MOO); elements and minor other oxides (EMO).Tantalum and niobium are the two metals whose contents would be upgraded.Major other oxides (MOO) are the high-concentration of other oxides including SiO 2 , CaO, TiO 2 , Al 2 O 3 , Fe 2 O 3 , Sn, and Zr.Elements and minor other oxides (EMO) are the chemical elements not categorized as tantalum-niobium and MOO.Below are XRF characterization results.Table 1 shows the three highest MOO elements, namely SiO 2 , TiO 2 , Fe 2 O 3 , and EMO (83.15 %).

Table 1
The results of Bangka tin slag's XRF characterization

3. NaOH Leaching
The roasted, quenched, dewatered, and -200+250 meshsieved sample was 8M NaOH-leached at 25 o C within 2 hours.The residues and filtrates characterization results are presented in Table 2.The results reveal an increase in niobium and MOO, and a decrease in tantalum and EMO.NaOHleached filtrates have a small amount of niobium, 0.205 ppm, and tantalum, <0.1 ppm.In other words, the concentration of both metals is very low.

4. HClO 4 Leaching and HClO 4 Followed by H 2 SO 4 Leaching
The next step is the observation of NaOH leaching results.In this stage, the residues were split into two parts.The first part was 0.1 M, 0.4 M, and 0.8 M HClO 4 -leached while the second one was leached two times.The first leaching engaged 0.8 M HClO 4 while the other leaching exploited 0, 8, 16, and 24 M H 2 SO 4 .Then, all the leaching results were XRF-characterized.Fig. 4 shows further information.
On the one hand, Fig. 4, a provides the results of HClO 4 leaching which show an increase in tantalum and niobium contents higher than those not processed (as in pure BTS) and those undergoing RQDS and NaOH leaching.The maximum rise in the amount of niobium and tantalum contents through HClO 4 leaching is 1.28 % and 0.79 % respectively.On the other hand, Fig. 4, b provides the results of HClO 4 and H 2 SO 4 leaching with their concentration variations.The results show a rise in niobium and tantalum contents higher than those undergoing only HClO 4 leaching.The highest rise in the amount reaches 1.57 % for niobium and 0.94 % for tantalum.

4. 1. MOO Leached with HClO 4 and HClO 4 Followed by H 2 SO 4
The contents of MOO leached with only HClO 4 and both 0.8M HClO 4 and H 2 SO 4 are provided in Fig. 5.In Fig. 5, a, HClO 4 leaching with its concentration variations shows a dominant contents decrease and the start of this decrease is 0.1M HClO 4 .On the other hand, TiO 2 and Zr are the two oxides whose contents rise from 0.1M to 0.4M.HClO 4 leaches and 0.8M HClO 4 and H 2 SO 4 leaching with its concentration variations show that all concentration variations produce a small amount of niobium and tantalum.Table 3 will give further information.
Table 3 The results of BTS leaching with 8M NaOH, HClO 4 with its concentration variations, 0.8M HClO The results of all leaching processes together with their chemicals' concentration variations in the above table imply that the non-fluoride solutions produce a small rise in the niobium and tantalum contents.

Discussion of results of enrichment on Bangka tin slag's tantalum and niobium oxide contents
The dominant oxide compounds produced ceramic structures in tin slag.The fragility of these structures was caused by the expansion of cracks in the materials before tin slag deformed.Roasting and water quenching on tin slag functioned to increase the number of pores.The rise in pores enlarged other oxides wetting areas.The enlargement of these zones helped accelerate the leaching of other oxides which would be dissolved.
The following is a description of the roasting and water quenching process: a -tin slag has a three-dimensional porous structure where a particle stone house, free other oxide particles, and both fully and partially-locked other oxides particles lie on its surface and in its inside.In other oxide particles, either fully or partially locked, there is a direct contact area next becoming the wetting zone when given a solvent; b -roasting made the particle stone house more porous and also expanded the wetting areas; c -the mismatch of heat expansion in the multiphase materials during water quenching process suddenly produced spontaneous micro-cracks; d -the thermal cracks eventually led to fractures, size reduction, and the increase in the porous and surface areas.Fig. 6 illustrates the occurrence of porous surface change, thermal cracking, fractures, and size reduction.By extending the porous areas, (1) the particle wetting surface widened, (2) the process of thermal cracking, fractures, and size reduction succeeded, and (3) so did the wetting process of fully and partially-locked other oxides surface areas.The results of SEM-EDS characterization at 100 times magnification of BTS and BTS RQD are in Fig. 3.
The roasting and water quenching effects on tin slag did not result in the oxide elements compounding (Fig. 7).In that figure, there is no intersection of the Gibbs free energy equation (ΔG) with the temperature variables.Meanwhile, the results of XRF characterization of several-sizes samples show that SiO 2 has the dominant contents (Fig. 2).In Table 4, the value of ΔG 25⁰ <0 implies that the NaOH solution can dissolve Nb 2 O 5 , Ta 2 O 5 , and MOO at 25 o C. On the other hand, the decrease in EMO contents from 83.15 % to 58.16 % (Fig. 12) shows that the percentage of EMO solubility in NaOH solution is much greater than that of MOO solubility.Meanwhile, the greater EMO solubility can raise the MOO contents from 16.15 % to 40.87 % (Table 1, 2).Fig. 5, a shows the results of HClO 4 leaching.In this figure, the contents of SiO 2 , Fe 2 O 3 , Sn, CaO, and Al 2 O 3 decrease as the concentration of the leaching solution increases.The thermodynamic analysis of HClO 4 dissolution is in Fig. 8.In Fig. 8, the Pourbaix diagrams show silica, alumina, and tin oxides forming elemental ions at pH=1 (HClO 4 =0.1M),pH=0.397(HClO 4 =0.4M), and pH=0.097(HClO 4 =0.8M).After the filtrate contents were HClO 4 -leached, the contents of iron and calcium oxide reduced as shown in Table 3.
Compared to the MOO contents in BTS, those which were HClO 4 -leached rise (Fig. 5, a, Table 1, 2).This increase occurs particularly in SiO 2 from 6.56 % to 11.73 % (Table 1).The rise in MOO contents occurs when EMO solubility is much greater than that of MOO.The 0.8 M HClO 4 leaching followed by H 2 SO 4 and its concentration variations resulted in an optimum TNO content, 2.51 %.The calculation result of EMO contents after being AAS-characterized and both HClO 4 and H 2 SO 4leached shows a reduction (Fig. 12, b) and the iron and calcium solubility in filtrates (Table 3).
The cerium and yttrium ions on the Pourbaix diagrams (Fig. 11) show the solubility of cerium and yttrium oxides in H 2 SO 4 solution between pH-1 and 0.
The 0.8 M HClO 4 leaching followed by H 2 SO 4 and its concentration variations reduced the EMO contents (Fig. 12, b).In addition, the contents of leached EMO also have a smaller percentage than those in BTS.
The upgrading of tantalum and niobium in BTS through double leachings and leaching duration variation, especially with HCl (20 minutes in the initial leaching and 50 minutes in the subsequent leaching), shows an increase in TNO (Ta 2 O 5 +Nb 2 O 5 ) contents from 1.95 % (0.8 % Ta 2 O 5 +0.15 % Nb 2 O 5 ) to 2.67 % (1.56 % Ta 2 O 5 +1.11 % Nb 2 O 5 ) [15].The leaching duration variation in the (second) leaching can be considered for further studies of enhancing Ta 2 O 5 and Nb 2 O 5 contents.Even though the researchers here are successful to have slightly increased the contents of both tantalum and niobium, their study still has some drawbacks.

4 . 1 .
Bangka tin slag's tantalum and niobium oxide contents through the non-fluoride process Materials and apparatus used in the experiment The tin slag was taken from a tin smelter in Bangka Belitung Islands in Indonesia.The investigation exploited sodium hydroxide (technical solution), perchloric acid (p.a), and sulfuric acid (p.a).These chemical elements are comprised of (a) 8M NaOH, (b) 0.8, 1.6 and 2.4 M H 2 SO 4 , and (c) 0.1, 0.4 and 0.8 M HClO 4 .

Fig. 2 .Fig. 3 .Fig. 1 .
Fig. 2. The contents of oxides and elements in various grain sizes.Note: in the X-axis, the unit of size is mesh

Fig. 6 .
Fig. 6.Оccurrence of porous surface change: a -Bangka tin slag where there is valuable oxide with a small wetting area; b -Porous surface change; c -thermal cracking and fracture condition; d -particle size reduction

Fig. 9 Fig. 8 .
Fig. 5, a shows the results of HClO 4 leaching.In this figure, the contents of SiO 2 , Fe 2 O 3 , Sn, CaO, and Al 2 O 3 decrease as the concentration of the leaching solution increases.The thermodynamic analysis of HClO 4 dissolution is in Fig. 8.In Fig. 8, the Pourbaix diagrams show silica, alumina, and tin oxides forming elemental ions at pH=1 (HClO 4 =0.1M),pH=0.397(HClO 4 =0.4M), and pH=0.097(HClO 4 =0.8M).After the filtrate contents were HClO 4 -leached, the contents of iron and calcium oxide reduced as shown in Table3.Compared to the MOO contents in BTS, those which were HClO 4 -leached rise (Fig.5, a, Table1, 2).This increase occurs particularly in SiO 2 from 6.56 % to 11.73 % (Table1).The rise in MOO contents occurs when EMO solubility is much greater than that of MOO.Fig.9shows the thermodynamic analysis of HClO 4 leaching results in EMO.The Pourbaix diagrams illustrate

Table 4
∆G of Nb 2 O 5 , Ta 2 O 5 , and MOO solution in NaOH

Table 5
∆G of MOO solution in H 2SO 4