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84CAPACITOR GRADE POWDERS FROM SCRAP OF TANTALUM CAPACITORS PRODUCTS
V. M. Orlov", T. Yu. Prokhorova*", and E. N. Kiselev"
aTananaev Institute of Chemistry - Subdivision of the Federal Research Centre "Kola Science Centre of the Russian Academy of Sciences", Apatity, 184209 Russia *e-mail: tantal@chemy.kolasc.net.ru
DOI: 10.24411/9999-0014A-2019-10114
Regeneration of tantalum from secondary sources of raw materials provides up to 10% of world demand for tantalum (in the USA - up to 25%) [1]. Capacitor scrap makes up about 20% of the amount of recycled tantalum returning to the market [2]. This scrap is an anode, rejected at different stages of the manufacture of capacitors. trimming tantalum wire used as a lead, a snap-out of compact metal and its waste during its manufacture.
Some methods of using scrap provide for a full processing cycle ending with the production of tantalum compounds and their subsequent reduction to metal [3-6]. Direct use of sintered anodes scrap by hydrogenation, grinding, dehydrogenation followed by agglomeration and deoxidation didn't ensure the high quality of the powder [7]. The possibility of processing the anodes of electrolytic and oxide-semiconductor capacitors into high-quality capacitor powder without the use of refining electron-beam remelting is shown in [8, 9]. Schematic diagrams for processing various types of scrap from the production of tantalum capacitors to produce conditioned capacitor grade powder with a specific charge of 5500-8400 CV/g were proposed in [10].
It was of interest to consider the possibility of using high-purity tantalum scrap to produce capacitor powders with a specific charge of 100000 CV/g.
Waste capacitor powder, rejected sintered anodes, scrap of wire used as lead were used as a raw material. The metal was oxidized in a muffle furnace at 800°C for 4 h. The resulting pentoxide was analyzed for the content of metallic impurities by atomic emission spectroscopy with photographic recording of the spectrum. The reduction was carried out with magnesium vapor at a temperature of 830°C and a residual pressure of argon in the reactor for 5-6 h. The apparatus and method of reduction are given in [11].
Data on the content of impurities in Ta2O5 obtained from various types of scrap is shown in the Table 1. It can be seen that the samples differ mainly in the content of niobium. Depending on the type of waste, the specific surface area of Ta2O5 ranged from 0.3 to 1.9 m2g-1, which affected the characteristics of the obtained tantalum powder. The dependence of the specific surface of tantalum powder on the specific surface of pentoxide is shown in Fig. 1a. Since the powders obtained by the reduction of the tantalum oxide compounds with magnesium vapors have the mesoporous structure [11], the value of their surface area is determined by the surface area of the pores (Fig. 1b). At the same time at least 50% of the surface falls on pores with a diameter of less than 10 nm.
Table 1. Impurity content in Ta2Ö5.
No. Impurity content, %
Scrap type Fe Ni Cr Nb Si
1 Powder 0.0017 < 0.0003 < 0.0003 0.045 < 0.001
2 Cuttings 0.0018 0.0002 < 0.0003 0.300 0.002
3 Wire 0.0012 0.0010 < 0.0003 < 0.004 < 0.002
ÏSHS2019
Moscow, Russia
(a) (b)
Fig. 1. (a) Dependence of the specific surface of the Ta powder on the specific surface of Ta2O5 and (b) cumulative distribution curves of the pore area in the Ta powders with different surfaces.
The SEM images of tantalum powders obtained from precursors with different characteristics presented in Fig. 2 give an idea of the morphology of the particles. It is seen that they have the same appearance and similar sizes. If the precursor was pentoxide which was obtained by burning a compact metal (wire, cuttings) Ta powders had a smaller surface area. This affected the characteristics of the anodes made from them (Table 2). The specific surface area of the used powder (S), temperature (J), and the duration of sintering of the anodes (t), their diameter shrinkage (Ad/d), specific charge (Q) and leakage current (I) are shown in Table 2; anodizing voltage is 16 V.
(a) (b)
Fig. 2. The SEM images of tantalum powder particles with specific surface area. m2/g: (a) 14, (b) 4.5. Ta205 obtained by burning of: (a) powder, (b) compact metal.
Table 2. Characteristics of the anodes.
No. Scrap type S, m2/g T, °C / x, min Ad/d, g/cm3 tgS, % Q, cv/g I, A/C
Powder 1250 / 15 8.5 47 148700 0.001
1 14
1300 / 20 19 22 94200 0.001
2 Cuttings 5.2 1300 / 20 8.9 46 138300 0.001
3 Wire 4.6 1300 / 20 8.0 47 125600 0.0008
The use of powders with a lower specific surface area makes it possible to raise the sintering temperature of the anodes without a significant decrease in the specific charge. Despite the higher nickel content in the pentoxide, the lowest leakage current was observed in the anodes made from the powder obtained from the recycling of waste wire. Thus, some types of tantalum
scrap can be used as a raw material without the use of deep chemical processing or refining
electron beam remelting and for the production of magnesium thermal capacitor powders with
a specific charge of more than 100000 CV/g.
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