Tantalum recycling from waste of electrical and electronic equipment

The possibility of tantalum recycling from waste of electrical and electronic equipment was investigated. Study was carried out using basic physical and chemical methods, ie. mechanical separation via crushing, leaching of silver layer in diluted HNO3, grinding and oxidation of anodes and thermic reduction with metallic reducing agent. A recovery rate of anodes was determined at 96%, and recycling efficiency of tantalum to pure form was determined more than 50%. Also was made mass balance.


Introduction
The necessity of miniaturization of electrical and electronic equipment led to the search for the capacitors on the performance of providing relatively high electrical capacity and stability under working conditions.Metallic tantalum is used for the manufacture of capacitors, because from this metal can be produced a dielectric layer with a high electric capacity.
Typical tantalum capacitor (Figure 1), otherwise Ta and Ta2O5, also consists of other factors, such as: MnO2 (solid electrolyte), Ag (high chemical resistance layer), graphite (layer between MnO2 and Ag), Sn and Ni (components of solder and wires), epoxy resin (protective layer against mechanical damage).Not only the form of tantalum (pure metal and oxide), but also other abovementioned components, determines the process of recycling of tantalum capacitors.There are many methods of tantalum recycling from waste electrical and electronic equipment (WEEE).Acknowledged practice is mechanical separation of tantalum by breaking and milling [1,2].Otherwise, it is possible to burn the epoxy resin in 500÷1000 0 C for 1÷5 hours oxidation is also obtained Ta2O5 [2][3][4], which can be reduced most commonly with Na, Mg or Ca [4,5].Tantalum capacitors can be leached e.g. in aqua regia, concentrated HCl, NaOH or KOH [1].The leaching capacitors by using iron chlorides (FeClx) tantalum chloride is obtained, which is then reduced with Mg [2].Other methods, such: exfoliation of printed circuit boards containing capacitors [6], tantalum recovery by Cu-Fe-C alloy doped Fe2O3 [3], the electrochemical reduction of dissolved tantalum in molten salt by dysprosium ions [5], are reported.
Typically, the tantalum recycling from WEEE (mainly form printed circuit boards) is a multistep process, and the final product may be impure with other elements; therefore, it has still not been development a simple and effective recycling process.
The aim of this study was to develop a process of tantalum recycling from WEEE.Special attention was paid to the use of diluted solutions, low temperature and safe reagents.

Experimental
Development process of tantalum recycling from WEEE consists the following steps: 1. coarse breaking the capacitors and grinding the tantalum anodes, 2. leaching anodes in diluted HNO3, 3. grinding the anodes, 4. oxidation, 5. pressing the oxidised anodes with reducing agents, 6. thermic reduction.The material used for this study was tantalum capacitors to though-hole mounting (Figure 1), weighting approx.210 g.

Coarse breaking and grinding
Capacitors were broken (Figure 1), anodes were mechanically separated from wires and resin and grinded.Due to the generation of harmful combustion products of epoxy resin [2][3][4] was not used from burning process.

Leaching
Leaching was primarily aimed at get rid of Ag from the anodes: Efficient leaching of Ag requires concentration of HNO3 above 3M: the best in the range 8÷10M.In addition, the epoxy resin pieces float on the solution surface -they were mechanically removed.
Leaching was lasted 12 h.

Grinding the anodes
Deprived of Ag and most of resin, anodes were grinding.
The purpose of grinding was to allow uniform for future oxidation.

Oxidation
After oxidation was obtained anodes to Ta2O5 "concentrate" [1][2][3]: In addition, the residual epoxy resin reacts with oxygen to form Si and additionally are formed combustion products (CO2 and H2O) [1], and graphite layer -to CO2.Oxidation was carried in furnace at 900 0 C for 1h.

Pressing the oxidised anodes with reducing agents
Reduction conditions must be such that the material containing Ta2O5 (oxidised anodes) had a minimal physical contact with bottom and wall of crucible and other impurity sources [4].For this purpose, the oxidised anodes (per 12 g) together with respectively reducing agents were pressed to moulded pieces.It has been applied 20% excess of reducing agents relative to the mass resulting from the stoichiometry.

Thermic reduction
Reducing agents were Al, Mg and Si (all reduce of Ta2O5 in process conditions, have high affinity to oxygen and are safe to use).Reductions are as follows: The reduction with Al and Mg were carried at 650 0 C, and with Si at 1000 0 C for 9 h duration under Ar atmosphere.

Mass balance
Table 1 shows the mass balance of materials starting from breaking the anodes to thermic reduction.
Tantalum anodes losses were resulted of the layered construction and structure of capacitors.Pressing the oxidised anodes with reducing agents is associated with material loss of 8%.where: mgrinded anodes -mass of grinded anodes, and manodes -mass of obtained (recovered) anodes (all masses in g).

Analysis of the chemical composition
The capacitors consists mainly of Ta and Mn, less Ag and Sn (Figure 2A).The confirmation of leaching Ag, Sn and also Mn in HNO3 is showed in Figure 2.These impurities are passed to solution through chemical and mechanical ways.In oxidised anodes are: Ta2O5, manganese tantalum oxide and Mn(Ta2O6) (Figure 3), which are oxidation products.Oxidation is resulted in increased concentration of Ta (Figure 2 and Table 2).After thermic reduction, only in case of moulded piece with Mg was obtained pure Ta (Figure 4).The reduction was incomplete, because beside Ta is also formed a complex compound (Mn0.6Mg1.4)Mg2(Ta2O9).Mg was also in MnO form (Figure 4).The main impurities are Mn and Mg (Table 3).

Recycling efficiency of tantalum
Because analysis are without pattern, they do not exclude oxygen in oxidised anodes and reduced moulded pieces.The calculations resulted that in 12 g of oxidised anodes is 1.788 g oxygen and moulded piece with Mg has 1.229 g oxygen.Considering these values, real percentage content Ta in oxidised anodes is equal 81.5% (%Taoxidisedanodes) and in moulded piece with Mg -77.1% (%Tamp), of which 38.6% is occur in bounded form, and 61.4% in pure Ta form (%Tapure,mp).Mass of Ta, which is contained in oxidised anodes in then:

Discussion and conclusions
The presented method of tantalum recycling from WEEE consists of segregation of tantalum anodes and obtaining pure Ta.Mechanical treatment by breaking the epoxy resin is an alternative to burning.This operation is effective and easy to implement.
This study has shown the ability and efficiency of leaching of Ag layer counting tantalum anodes by using 8M HNO3.
Recovery rate of tantalum anodes is at 96%.Such a high rate achieved while maintaining the simplicity of this process.
Impurity, which has failed to get rid in a satisfactory degree, is Mn.It should be investigate the possibility of get rid of MnO2 layer, for example by several leaching in various and fresh solutions.At each stage of process, remained a part of Mn in material.It proves, than MnO2 has high resistance to diluted by HNO3.
Only Mg partially reduced Ta2O5 in moulded piece to pure Ta.In this case, recycling efficiency of Ta is estimated at 57%.This value is relatively low, but instead a part of Mg to reduce Ta2O5, is formed mixture of Mn and Mg oxides.Therefore, it should be investigate the thermodynamic reasons of this phenomenon.
Oxidised anodes or reduced moulded pieces can be considered as a high content concentrate of Ta (81.5 and 77.1% respectively).However, if the goal is to receive a pure Ta, reduced moulded pieces requires further processing, as it contains reduction by-products, namely oxides of reducing agents.Reduced moulded piece may be dissolved in e.g.HCl, CH3COOH or diluted aqua regia, which in turn would give pure Ta.Further studies are required.

Figure 4 .
Figure 4. X-ray diffraction analysis of the moulded piece of oxidised anodes with Mg after thermic reduction

Table 1 .
Mass balance in steps: coarse breaking and grinding