CC BY
45
9. Nitriding and carbonitriding: Monograph / 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 p.
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
В даному до^дженш вивчено вплив без-фторидних розчитв, включаючи 8 М ^ОН, 0,8, 1,6 i 2,4 М H2SO4 та 0,1, 0,4 I 0,8 М НС1О4, на збшьшення вм^ту оксиду танталу i тобю в результатi вилуговуван-ня. Перед вилуговуванням оловяний шлак Бангка (ОШБ) характеризувався за допо-могою РФА. Потiм шлак обжарювався при 900 °С, гартувався i зневоднювався. Далi ОШБ пройшов процес просювання з розмiр-ною класифтащею +100, -100 +150, -150 +200, -200 +250 i -250 меш. Шсля цього шлак -200 +250 меш витравлювали за допо-могою 8 М ^ОН. Потiм вилужений продукт дшили на два, один з яких витравлювали з використанням 0,1, 0,4 i 0,8 М НС1О4, а решту - за допомогою 0,8 М НС1О4, а потiм 0, 0,8, 1,6 та 2,4 М H2SO4 при 25 °С протягом двох годин. Для характеристики вых залиштв використовувався РФА, а для фiльтратiв - ААС, а також 1СП-ОЕС. Дане дослiдження обумовлене дефi-цитом танталу i його статусом як одного з найважливших технологiчних елементiв. На додаток, бшьш^ть попередтх дослi-джень дозволили збшьшити вм^т оксиду танталу i тобю завдяки використанню фторидног кислоти, в той час як в даному дослiджент розглянутi тшьки безфто-ридш розчини. Результаты показують, що перхлоратна кислота з подальшим вилуговуванням Ырчаною кислотою злегка тд-вищуе вм^т танталу i тобю. Однак цей метод е найбшьш ефективним серед ^ОН, НС1О4 i НС1О4 з подальшим вилуговуванням за допомогою H2SO4. Даний висновок е формою наукових зусиль з тдтримки наяв-ностi танталу за рахунок використання непотрiбних вiдходiв олов'яног плавки
Ключовi слова: вилуговування, оксид тобю танталу (ОНТ), оловяний шлак
Бангка, НаОН,, НС1О4 -□ □-
UDC 802
|DOI: 10.15587/1729-4061.2018.129914|
ENRICHMENT ON BANGKA TIN SLAG'S TANTALUM AND NIOBIUM OXIDE CONTENTS THROUGH NON-FLUORIDE PROCESS
Sulaksana Permana
Doctor of Engineering in Metallurgy and Materials* Е-mail: sulaksana@yahoo.com Shiva Frida Vincia Bachelor of Engineering in Metallurgy and Materials* Е-mail: shivafv07@gmail.com Anggita Amilia Bachelor of Engineering in Metallurgy and Materials* Е-mail: anggitamilia@gmail.com Ahmad Maksum Master of Engineering in Metallurgy and Materials Department of Mechanical Engineering Politeknik Negeri Jakarta Kukusan, Beji, Depok, Indonesia, 16425 Е-mail: ahmad.maksum@mesin.pnj.ac.id Kurnia Setiawan Widana Master of Engineering in Chemical Engineering Center for Nuclear Minerals Technology National Nuclear Energy Agency of Indonesia Pasar Jum'at, Jakarta, Indonesia, 12440 Е-mail: kurnias@batan.go.id Johny Wahyuadi Soedarsono Doctor of Engineering, Professor* Е-mail: jwsono@metal.ui.ac.id *Centre of Mineral Processing and Corrosion Research Department of Metallurgy and Materials Universitas Indonesia Depok, Jawa Barat, Indonesia, 16424
1. Introduction
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
which merges with sea and freshwater [1]. Investigations on material and future sources revealed that the source of tantalum from tin slag was 13 % among all supplies [3] and a forecast in 2013 implied a deficit in 2015 and 2016 [4]. One of the measures to obtain the secondary sources of valuable elements is the metallurgical slag and several explorations presented the acquisition of valuable elements from metallurgical slag [5, 6]. Hydrometallurgy was used by prior researches to upgrade the concentration of elements [7, 8].
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.
2. 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+H2SO4) dissolution in tin slag's (Ta+Nb)2O5 whose contents were over 25 % produced around 85 % recovery rate [21]. Tin slag's 3.4 % Nb2O5 and 3.05 % Ta2Os underwent several processes of leaching, namely caustic solution leaching, alkali pugging, alkali fusion, acid leaching, and HF & H2SO4 leaching. The alkali processes and acid leaching optimally produced Ta2Os and Nb2O5, 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 Cl2+N2 or Cl2+CO+N2 in 200-1,000 oC. 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 % Nb2O5 and 84.7 % Ta2O5 [19]. HCl and NaOH dissolution in tin slag containing 0.33 % Ta2O5 and 0.64 % Nb2Os 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)2O5 [11], the study upgraded BTS' TNO contents [9, 19], the thermodynamic analysis was done in the study of upgrading BTS' REE [20].
3. 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. 900 oC-roasting, quenching, and dewatering BTS.
3. Leaching -200+250 mesh BTS by 8M NaOH and dividing the residues into two groups.
4. Leaching group one with 0.1, 0.4, and 0.8 M HClO4 and leaching group two with 0.8 M HClO4 followed by 0, 0.8, 1.6, and 2.4 M H2SO4 at 25 oC within 2 hours.
4. Materials and methods for enrichment on Bangka tin slag's tantalum and niobium oxide contents through the non-fluoride process
4. 1. Materials and apparatus used in the experiment
The tin slag was taken from a tin smelter in Bangka Be-litung 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 H2SO4, and (c) 0.1, 0.4 and 0.8 M HClO4.
The researchers used a ball mill (Toptek Topvert G1), a sieve, a Barnstead Thermolyne furnace, a ceramic container, a magnetic stirrer (Thermo Scientific CIMAREC), Scanning Electron Microscopy (SEM), X-Ray Fluorescence (Bruker handheld XRF analyzer), Atomic Absorption Spectroscopy (AAS - PerkinElmer Analyst 400), and Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES PerkinElmer Optima 8300).
4. 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 oC, 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 HClO4-leached with 0.1, 0.4, and 0.8 M concentrations while the other underwent 0.8 M HClO4 and 0, 0.8, 1.6, and 2.4 M H2SO4 leaching. All the leaching processes were carried out at 25 oC 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.
Bangka Tin Slag
A • 900 oC Roasting , A •
> Water quenching > Sieving >
and Dewatering
<-.F •
0.1 M
<~IF •
<-lF •
•R<- 0.4 M
0M
<-lF •
•R<- 0.8M <■
<-lF •
•R< 1.6 m
<-lF •
2.4 M ^
<-lF •
0.8 M ^
HClO4 Leaching 2h-25 oC
<HF •
•R
H2SO4 Leaching 2h-25 oC
£
HClO4 Leaching 0.8M NaOH-2h-25 oC
F = Filtrate ; R = Residue
Characterization^ ) XRF ( ) AAS ( ) ICP-OES ( ) SEM Fig. 1. Research scheme which tantalum and niobium underwent during investigation
5. 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 oC-roast-ing, quenching, dewatering, and sieving, (3) NaOH leaching, and (4) HClO4 leaching and HClO4 followed by H2SO4 leaching.
5. 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 SiO2, CaO, TiO2, Al2O3, Fe2O3, 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 SiO2, TiO2, Fe2O3, and EMO (83.15 %).
Table 1
The results of Bangka tin slag's XRF characterization
Nb (%) Ta (%) SiO2 (%) TiO2 (%) Fe2O3 (%) Sn (%) Zr (%) CaO (%) Al2O3 (%) EMO (%)
0.47 0.23 6.56 2.38 2.36 1.78 1.33 1.22 0.52 83.15
ilikik
Nb
■ Ta
■ SiO2
■ TiO2
■ Fe2O3
■ Sn
■ Zr
■ CaO
■ A12O3
BTS
+100
-100+150 -150+200 -200+250 -250
Fig. 2. The contents of oxides and elements in various grain sizes. Note: in the X-axis, the unit of size is mesh
Roasting, quenching, dewatering, and sieving on BTS left some physical distinctions. The differences in appearance between pure BTS and roasted, water-quenched, and dewatered BTS are shown below.
,., ves/r
-r
Jfgi
5. 2. BTS 900 oC Roasting, Quenching, Dewatering, and Sieving
The results of BTS 900 oC roasting, quenching, dewa-tering, and sieving (RQDS) are revealed in Fig. 2. +100 and -100+150 mesh BTS samples represent the three dominant contents of oxides, namely SiO2, TiO2, and Fe2O3. On the other hand, in sizes -150+200, -200+250, and -250 mesh, SiO2, Sn, and Fe2O3 become the three dominant contents of oxides.
a b
Fig. 3. The SEM-EDS characterization at 100 times magnification: a — Bangka tin slag; b — Bangka tin slag after being roasted, water quenched, and dewatered
The SEM-EDS characterization which had been given a magnification of 100 times of BTS and BTS which had been 900 oC-roasted, quenched, and dewatered (called as BTS-RQD) are in Fig. 3. BTS-RQD has a bigger grain size and a more open touch surface area.
F
8
6
4
2
0
8
6
4
2
0
5. 3. NaOH Leaching
The roasted, quenched, dewatered, and -200+250 mesh-sieved sample was 8M NaOH-leached at 25 oC 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. NaOH-leached 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.
Table 2
After-NaOH leaching XRF characterization of Bangka tin slag's residues and filtrates
Nb Ta SiO2 TiO2 Fe2O3 Sn Zr CaO Al2O3 EMO
Residue (in %)
0.72 0.16 11.73 3.23 3.99 6.49 4.07 4.36 2.61 62.64
Filtrate (in ppm)
0.205 <0.1 not. available not. available not. available not. available not. available not. available not. available not. available
5. 4. HClO4 Leaching and HClO4 Followed by H2SO4 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 HClO4-leached while the second one was leached two times. The first leaching engaged 0.8 M HClO4 while the other leaching exploited 0, 8, 16, and 24 M H2SO4. Then, all the leaching results were XRF-characterized. Fig. 4 shows further information.
Both 0.8M HClO4 leaching and 0.8M HClO4 & 0 M H2SO4 leaching produced near results (see the red arrows). The 0.8M HClO4 leaching reveals 1.28 % tantalum and 0.79 % niobium whereas 0.8M HClO4 and 0 M H2SO4 leaching represents tantalum and niobium, with 1.27 % and 0.71 %, respectively.
On the one hand, Fig. 4, a provides the results of HClO4 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 HClO4 leaching is 1.28 % and 0.79 % respectively. On the other hand, Fig. 4, b provides the results of HClO4 and H2SO4 leaching with their concentration variations. The results show a rise in niobium and tantalum contents higher than those undergoing only HClO4 leaching. The highest rise in the amount reaches 1.57 % for niobium and 0.94 % for tantalum.
5. 4. 1. MOO Leached with HClO4 and HClO4 Followed by H2SO4
The contents of MOO leached with only HClO4 and both 0.8M HClO4 and H2SO4 are provided in Fig. 5. In Fig. 5, a, HClO4 leaching with its concentration variations shows a dominant contents decrease and the start of this decrease is 0.1M HClO4. On the other hand, TiO2 and Zr are the two oxides whose contents rise from 0.1M to 0.4M.
Sulfuric acid concentration b
Fig. 4. The residue results of tantalum and niobium's leaching: a — BTS & BTS roasted, water quenched, dewatered, —200+250 mesh-sieved, and 8M NaOH as well as HClO4-leached with its concentration variations; b — BTS & BTS roasted, quenched, dewatered, —200+250 mesh-sieved, and 8M NaOH, 0.8M HClO4,
and H2SO4-leached with its concentration variations
What 0.8M HClO4 and H2SO4 leaching with its concentration variations produced can be seen in Fig. 5, b. SiO2 shows a rise in all concentration variations while a fall is observed in the contents of TiO2, Fe2O3, Sn, Zr, CaO, and Al2O3 which underwent from 0.8M to 1.6M H2SO4 leaching. All MOO elements went up from 1.6M to 2.4M H2SO4.
-Si02 -Ti02 -Fe203 -Sn -Zr -CaO A1203
BTS R-Q-D-S 8 M NaOH
0.1 M 0.4 M 0.8 M
Perchloric acid concentration b
Fig. 5. The results of MOO's residues leaching: a — BTS & BTS roasted, water quenched, dewatered, —200+250 mesh-sieved, and 8M NaOH as well as HClO4-leached with its concentration variations; b — BTS & BTS roasted, quenched, dewatered, -200+250 mesh-sieved, and 8M NaOH, 0.8M HClO4, and H2SO4-leached
HClO4 leaches and 0.8M HClO4 and H2SO4 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, HClO4 with its concentration variations, 0.8M HClO4, and H2SO4 with its concentration variations
Element 8M NaOH HClO4 concentration variation 0.8M HClO4+H2SO4 concentration variation
0.1M 0.4M 0.8M 0 M 0.8M 1.6M 2.4M
(PPm) (PPm) (PPm)
Nb* 0.205 0.208 0.207 0.206 0.206 0.364 0.450 0.222
Ta* n.a n.a n.a <0.1 <0.1 0.516 0.537 0.011
Fe** 2 82.33 626.9 1,281 1,278 1,821 2,356 2,942
Ca** n.a 3,466 3,874 4,672 4,675 5,123 2,011 3,824
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.
5. 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.
Wetting area
Note: Characterized by * - ICP-OES;** - AAS
water guenching
Fig. 6. Occurrence 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
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 (AG) with the temperature variables. Meanwhile, the results of XRF characterization of several-sizes samples show that SiO2 has the dominant contents (Fig. 2).
a
d
c
Temperature (degree Celcius)
Fig. 7. The Ellingham diagram of MOO up to 900 °C
In Table 4, the value of AG25°<0 implies that the NaOH solution can dissolve Nb205, Ta205, and MOO at 25 °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).
Table 4
AG of Nb205, Ta205, and MOO solution in NaOH
No. Reaction Equation AG25. (kcal/mol)
1 Nb205+2Na0H=2NaNb03+H20 -30.400
2 Ta205+2Na0H=2NaTa03+H20 -45.235
3 Si02+2Na0H=Na2Si03+H20 -21.228
4 6Ti02+2Na0H=Na2Ti6013+H20 -23.582
5 Fe203+2Na0H=2NaFe02+H20 -4.262
6 Sn02+2Na0H=Na2Sn03+H20 Not found in database
7 Zr02+2Na0H=Na2Zr03+H20 -5.954
8 Al203+2Na0H=2NaA102+H20 -8.165
Fig. 5, a shows the results of HCIO4 leaching. In this figure, the contents of Si02, Fe203, Sn, CaO, and Al203 decrease as the concentration of the leaching solution increases. The thermodynamic analysis of HCIO4 dissolution is in Fig. 8. In Fig. 8, the Pourbaix diagrams show silica, alumina, and tin oxides forming elemental ions at pH=l (HC104=0.1M), pH=0.397 (HC104=0.4M), and pH=0.097 (HC104=0.8M). After the filtrate contents were HClO4-leached, the contents of iron and calcium oxide reduced as shown in Table 3.
Compared to the MOO contents in BTS, those which were HClO4-leached rise (Fig. 5, a, Table 1, 2). This increase occurs particularly in SiO2 from 6.56 % to 11.73 % (Table 1). The rise in MOO contents occurs when EMO solubility is much greater than that of MOO.
Fig. 9 shows the thermodynamic analysis of HClO4 leaching results in EMO. The Pourbaix diagrams illustrate
the examples of cerium and yttrium oxides and the formation of cerium and yttrium ions at pH=1 (HClO4=0.1M), pH=0.397 (HClO4=0.4M), and pH=0.097 (HClO4=0.8M). Both ions also exhibit cerium and yttrium oxides dissolved in HC04.
Eh (Volts) 2.0
Si-Cl-H20-System at 25.00 C
1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
Si02(I)
Si03(0H)(-3a)
Si2H6
H20 Limits
2
*A*pH=i pH=0.09(7)|
pH=0.39(7)
Eh (Volts) 2.0
10
12
14 pH
Al-Cl-H20-System at 25.00 C
pH=0.09(7)
pH=0.39(7)
Eh (Volts) 2.0
Sn-Cl-H20-System at 25.00 C
Sn02
pH=0.09(7)
pH=0.39(7)
Fig. 8. The Poubaix diagrams of: a — silica; b — aluminum; c — tin oxides
The 0.8 M HClO4 leaching followed by H2SO4 and its concentration variations resulted in an optimum TNO content, 2.51 %. The calculation result of EMO contents after being AAS-characterized and both HClO4 and H2SO4-leached shows a reduction (Fig. 12, b) and the iron and calcium solubility in filtrates (Table 3).
a
b
c
The cerium and yttrium ions on the Pourbaix diagrams (Fig. 11) show the solubility of cerium and yttrium oxides in H2SO4 solution between pH-1 and 0.
The 0.8 M HClO4 leaching followed by H2SO4 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 (Ta2O5+Nb2O5) contents from 1.95 % (0.8 % Ta2O5+0.15 % Nb2O5) to 2.67 % (1.56 % Ta2O5+1.11 % Nb2O5) [15]. The leaching duration variation in the (second) leaching can be considered for further studies of enhancing Ta2O5 and Nb2C>5 contents.
Table 5
AG of MOO solution in H2SO4
No. Reaction Equation AG25o (kcal/mol)
1 SiO2+H2SO4
2 TiO2+H2SO4=TiOSO4 +H2O Not found in database
3 Fe2O3+3H2SO4=Fe2(SO4)3+3H2O -39.437
4 SnO2+2H2SO4=Sn(SO4)2+2H2O 2.726
5 ZrO2+2H2SO4=Zr(SO4)2+2H2O -14.254
6 Al2O3+3H2SO4=Al2(SO4)3+3H2O -37.994
Even though the researchers here are successful to have slightly increased the contents of both tantalum and niobium, their study still has some drawbacks.
b
Fig. 9. The Poubaix diagrams of: a — cerium; b — yttrium oxides
Si02(I)
Eh (Volts) Si-S-H20-System at 25.00 C 2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
Si2H6
Si03(0H)(-3a)
H20 Limits
10 12
14 pH
Eh (Volts) 2.0 1.5 1.0 0.5
-0.5 -1.0 -1.5 -2.0
Ti-S-H20-System at 25.00 C -Ti0(H202)(+2a)-
Ti02
.Ti5O17jni0C>19
TiH2
10 12
14 pH
Eh (Volts) Fe-S-H20-System at 25.00 C 2.0 r
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
pi(+3a) Fe(+2a)\ FeO*OH
Fe0.877S
Fe H20 Limits
10 12
14 pH
Sn(S04)02
Eh (Volts) Sn-S-H20-System at 25.00 C 2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
H20 Limits
6 d
10 12
Eh (Volts) 2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
Al-S-H20-System at 25.00 C
A13(OH)4(+5a)
AIO(OH)
A1H3 -__
H20 Limits
14 pH
10 12
14 pH
Fig. 10. The Pourbaix diagrams of: a — silica; b — titanium; c — iron; d — tin; e —aluminum oxides
b
a
c
e
a b
Fig. 11. The Pourbaix diagrams of: a — cerium; b — yttrium oxides
Perchloric acid concentration
a
Sulfuric acid concentration b
Fig. 12. The residues of leached TNO, MOO, and EMO: a — The HClO4 and its concentration variations leaching decreases the MOO contents, b — The 0.8M HClO4 leaching followed by H2SO4 and its concentration variations decreases the EMO contents
First, this study exploits high acid chemical solutions. Therefore, the filtrates as a leaching product are toxic. Secondly, this investigation involves tin slag only from Bangka Island, Indonesia. In fact, samples from various countries will produce varied and comparable results so that tantalum and niobium from the concerned regions' tin slag can be exploited as well as possible for human welfare.
For the development of further studies, other researchers are advised to use organic chemical solutions such as phyt-ic acid so that the resulted filtrates are not toxic. On the other hand, increasing the tantalum and niobium contents through this method has a serious consequence as follows. The dissolution of elements and minor other oxides (EMO) raises the contents of Ta, Nb, and EMO itself. Increasing the contents of EMO will raise the contents of uranium (U) and thorium (Th) in EMO, leading to a rise in the effect of radiation on BTS.
6. Conclusions
1. XRF characterization reveals the three highest MOO elements that include SiO2, TiO2, Fe2O3, and EMO (83.15 %)
while that of SEM shows that BTS-RQD has a bigger grain size and a more open touch surface area.
2. The 900 oC-roasting, quenching, dewatering, and sieving of BTS results in the three dominant contents of oxides, SiO2, TiO2, and Fe2O3.
3. 8M NaOH leaching shows a rise in niobium and MOO, and a reduction in tantalum and EMO.
4. HClO4 leaching produces an increase in the contents of tantalum and niobium higher than those of pure BTS and those undergoing RQDS and NaOH leaching. On the other hand, HClO4 followed by H2SO4 leaching produces an increase in niobium and tantalum contents higher than those leached with only HClO4.
All the above investigations imply that the rise in both tantalum and niobium is relatively low.
Acknowledgements
This work was financially supported by the Directorate of Research and Community Engagement, Universitas Indonesia under the 2018 TADOK program with contract number 1351/UN2.R3.1/HKP.05.00/2018.
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