1. Introduction
The demand for tantalum capacitors is steadily increasing due to the popularity of small electronic products. At the same time, small electronic products are eliminated rapidly, which represents a large amount of waste [
1,
2,
3]. Tantalum is a transition element with an atomic number of 73, an atomic weight of 180.95, and a melting point of about 3000 °C (2980 ± 20 °C), which is slightly lower than tungsten and rhenium. The resources of tantalum, which usually co-exist with niobium, are mainly from the coltan or columbite [
4,
5], making refinement difficult due to their similar chemical characteristics. Tantalum was discovered in 1802, one year after niobium, and the average amount of tantalum in the Earth’s crust is 2 ppm [
6]. Therefore, it is feasible to recover tantalum from tantalum capacitors that do not contain niobium [
7,
8,
9]. Tantalum has high capacitance/unit volume, high thermal stability, and high oxidation resistance which can be used in electronic and biotechnology industries [
10,
11,
12]. In the year 2016, about 34% of tantalum was used in manufacturing capacitors, followed by superalloy, chemical, sputtering target, mill product, and carbide industries [
11,
13].
Epoxy-coated solid electrolyte tantalum capacitors (EcSETCs) consist of electrodes, an epoxy resin, and wires. Electrodes include cathodes, anodes, and dielectrics. Anodes and dielectrics are made of tantalum and a small amount of its oxide powder, while cathodes are made of manganese oxide, graphite, and silver paste. The epoxy resin, which has silicon added to enhance its thermal durability, comprises halogenated compounds.
Tantalum-rich electrodes are sealed with epoxy resin. Hence, the removal of epoxy resin is conducted before any recovery process. Many methods can remove the epoxy resin, such as combustion [
14], pyrolysis [
15,
16,
17,
18], solubilization [
19,
20], and supercritical water treatment [
21]. After removing the epoxy resin, the recovery of tantalum mainly chooses one of two methods: the hydrometallurgy process and the chlorination process. In the hydrometallurgy process, leaching agents such sulfuric acid (H
2SO
4) [
22,
23], hydrofluoric acid (HF) [
24], or a mixture of both [
14,
15], all applied at normal temperatures, have been investigated. Previous studies have indicated that pressure leaching using HF has better leaching efficiency [
7]. After leaching, several extractants such as MIBK [
25], CHO [
26], and Alamine 336 [
17,
27] have been studied. The drawbacks of traditional extraction are high volatility and low thermal stability. To overcome these shortcomings, ionic liquids are studied in hydrometallurgy [
24,
28,
29]. In the chlorination process, chlorine gas or hydrogen applied to tantalum and niobium raw ore [
6] or ferrous chloride applied to tantalum capacitors [
15,
30,
31,
32] have both been studied.
In most of the studies mentioned above, the recovery of tantalum is higher in the hydrometallurgy process but the purify of tantalum is higher in the chlorination process. In this study, a feasible recycling process that combines the advantage of hydrometallurgy and chloride metallurgy was designed. After first removing the epoxy resin from EcSETCs, selective leaching was conducted to remove the manganese and increase the recovery of tantalum. Previous studies on leaching manganese from the different wastes were researched [
33,
34,
35,
36,
37,
38] to investigate the best leaching efficiency and increase the recovery rate of manganese. Low concentrate ordinary inorganic acid, which is applied to dissolve the manganese through selective leaching, was chosen instead of hydrofluoric acid, which is more dangerous and a pollutant. The parameters were investigated, such as acid concentrate, liquid–solid ratio, and reaction time to increase the leaching efficiency of manganese. Then, manganese hydroxide was obtained through chemical precipitation. Furthermore, chlorination with ferrous chloride was used in the recovery of tantalum. The manganese hydroxide and the tantalum chloride were separated individually as the final productions in this study.
4. Conclusions
As the demand for tantalum continuously increases, natural resources are mined and decrease gradually. Finding alternatives to mitigate this situation has become critical. The EcSETCs used in this study contain over 20 wt.% of tantalum and 8% of manganese, which have a great potential to provide a tremendous second resource. In this study, the epoxy resins must be removed from the EcSETCs by pyrolysis at 600 °C for 10 min, grinding, and washing. Next, the separation of iron and nickel through a magnetic separation is conducted to affect the efficiency of selective leaching. In summary, 3M of hydrochloric acid and the liquid–solid ratio of 40 mL/g at 25 °C for 32 min are chosen as the optimal parameters for selective leaching. With the optimal conditions, the leaching efficiency of Mn can achieve 99%. As Ta does not dissolve in hydrochloric acid, the leaching efficiency of Ta is almost 0%. It means that selective leaching is successful and feasible. After the filtering process, the leaching solution is divided into clear Mn ion-rich solution and Ta residues. By adjusting the pH value to 12, Mn(OH)2 will precipitate as one of the final productions. Then, Ta residues are washed and dried. Additionally, 48 wt.% of Ta residues and 52 wt.% of FeCl2 are mixed evenly and heated at 450 °C for 3 h by tube furnace. The gas of TaCl5 will generate and condense into the solid as the final production. In this study, the purity of Mn(OH)2 and TaCl5 is 98.5% and 99%, respectively; the recovery of Mn and Ta is 98% and 94%, respectively.