METHOD FOR RECOVERING METAL POWDER FROM PLATINUM PASTE AND METHOD FOR REGENERATING PLATINUM PASTE

Abstract

The present invention relates to a technique for recovering and recycling a platinum paste. The present invention provides a method for recovering a metal powder from a platinum paste formed by mixing a solid component composed of a metal powder including at least a platinum powder or a platinum alloy powder and an organic component including at least an organic solvent, the method including removing the organic component by heating the platinum paste at a recovery temperature set in a temperature range of 300 C. or higher and 500 C. or lower. The recovered metal powder can be recycled into a platinum paste equivalent to a new product by mixing the metal powder with a solvent etc.

Claims

1-8. (canceled)

9. A method for recovering a metal powder from a platinum paste formed by mixing a solid component composed of a metal powder including at least a platinum powder or a platinum alloy powder and an organic component including at least an organic solvent, comprising the steps of heating the platinum paste at a recovery temperature set in a temperature range of 300 C. or higher and 500 C. or lower, thereby to remove the organic component and recover the metal powder.

10. The method for recovering a metal powder from a platinum paste according to claim 9, comprising the steps of treating a platinum paste corresponding to at least one of the following conditions: (a) a platinum paste viscosity has been changed by 20% or more compared with the viscosity at the time of production of the platinum paste; (b) one or both of a platinum paste viscosity ratio of 0.4/s to 4/s (0.4/4) or a platinum paste viscosity ratio of 4/s to 20/s (4/20) as measured by Brookfield viscometer are changed by 10% or more with respect to the platinum paste viscosity ratio at the time of production; and (c) a solid component content has been changed by 2% or more with respect to the solid component content at the production time.

11. The method for recovering a metal powder from a platinum paste according to claim 10, wherein the organic solvent as the organic component includes ethylene glycol, propylene glycol, ethylene glycol monophenyl ether, benzyl alcohol, kerosene, -butyrolactone, N-methylpyrrolidone, butyl carbitol, butyl carbitol acetate, terpin oil, -terpineol, texanol, menthanol and/or menthanol acetate.

12. The method for recovering a metal powder from a platinum paste according to claim 11, wherein the organic component includes a resin.

13. The method for recovering a metal powder from a platinum paste according to claim 12, wherein the resin includes at least one of a cellulose derivative, a butyral resin, a polyvinyl acetate resin, an amino-based resin and an alkyd resin.

14. The method for recovering a metal powder from a platinum paste according to claim 13, wherein the solid component includes at least one of a ceramic powder and insoluble particles.

15. The method for recovering a metal powder from a platinum paste according to claim 14, wherein the platinum paste is preliminarily heated at 150 C. to 200 C. before being heated at a recovery temperature.

16. A method for recycling a platinum paste, comprising the steps of producing a platinum paste by mixing an organic component, which includes at least an organic solvent, with a metal powder recovered by the method defined in any of claim 9.

17. The method for recovering a metal powder from a platinum paste according to claim 13, wherein the organic component includes a surfactant.

18. The method for recovering a metal powder from a platinum paste according to claim 14, wherein the solid component comprises at least one of the group of alumina, zirconia, carbon powder or diamond.

19. The method for recovering a metal powder from a platinum paste according to claim 15, wherein the preliminary heating is conducted for a period of between 5 minutes and 60 minutes.

20. The method for recovering a metal powder from a platinum paste according to claim 9, wherein the platinum paste has a platinum concentration chosen from the group of (i) at least 99% platinum concentration by mass, or (ii) at least 40% platinum concentration by mass.

21. The method for recovering a metal powder from a platinum paste according to claim 20, wherein the platinum paste comprises a platinum alloy comprising at least one additional metal chosen from the group of palladium, gold, silver, rhodium and iridium.

22. The method for recovering a metal powder from a platinum paste according to claim 9, wherein the platinum paste has an average particle size of between 0.05 m and 5 m.

23. The method for recovering a metal powder from a platinum paste according to claim 9, wherein the step of heating the platinum paste proceeds at a temperature elevation rate of between 5 C./minute and 20 C./minute.

24. The method for recovering a metal powder from a platinum paste according to claim 23, wherein the step of heating the platinum paste includes a period during which the recovery temperature is held constant for between 5 minutes and 60 minutes.

25. A method for recovering a metal powder from a platinum paste formed by mixing a solid component composed of a metal powder including at least a platinum powder or a platinum alloy powder and an organic component including at least an organic solvent, comprising the steps of heating the platinum paste to a recovery temperature between 300 C. and 500 C., removing the organic component, and recovering the metal powder.

26. A method for recycling a platinum paste, comprising the steps of producing a platinum paste by mixing an organic component with a metal powder, wherein the organic component includes at least an organic solvent, heating the platinum paste to a recovery temperature between 300 C. and 500 C., removing the organic component, and recovering the metal powder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 is a view illustrating the results of TG-DTA analysis of a platinum paste in a first embodiment.

[0033] FIG. 2 is a view illustrating a change rate in surface area of a platinum powder in the first embodiment.

[0034] FIG. 3 is a view illustrating a change rate in specific surface area of a solid component recovered repeatedly from a platinum paste in a second embodiment.

[0035] FIG. 4 is a view illustrating a change rate in resistance of an electrode produced from a platinum paste recycled repeatedly in the second embodiment.

[0036] FIG. 5 is a view illustrating the results of measuring the electrode properties of an electrode produced from a platinum paste recycled in a third embodiment.

DESCRIPTION OF EMBODIMENTS

First Embodiment

[0037] Hereinafter, embodiments of the present invention will be described. In this embodiment, a preliminary test for estimating preferred conditions for recovering a metal powder through a heating test was conducted for a platinum paste which is not degraded (within 1 hour after production). The platinum paste used here was obtained by mixing a platinum powder (particle size: 0.7 m) with texanol as a solvent, and further mixing ethylcellulose as a resin.

[0038] 20 mg of the platinum paste was taken, and subjected to TG-DTA analysis. The platinum paste was heated at a constant rate with a temperature elevation rate of 10 C./min during analysis, and a weight loss was measured. FIG. 1 illustrates the results of TG-DTA analysis. The platinum paste started losing the mass at 136.8 C., and stopped losing the weight at 250 C. or lower. This suggests that an organic component including a solvent and a resin is eliminated at 250 C. or lower. Ethyl cellulose as a resin component was confirmed to be eliminated by the catalytic action of the platinum powder. Because the analyzed amount in TG-DTA analysis is very small, there is the possibility that the measured temperature at which the organic component was eliminated was lower than actual temperature, but it is thought when the heating temperature is 300 C. or higher, the organic component can be completely removed.

[0039] Next, the platinum paste was heated and held at each temperature of 300 to 600 C. for 30 minutes to remove the organic component to recover a platinum powder, the specific surface area of the platinum powder was measured, and a change rate with respect to the specific surface area before production of the paste was evaluated. The result of the evaluation is shown in FIG. 2.

[0040] FIG. 2 illustrates the result showing a change rate in specific surface area of the recovered platinum powder, and for this platinum powder, the specific surface area decreases in heating at 500 C. A decrease in specific surface area may be caused by sintering of the platinum powder, and therefore it can be determined that the heating temperature is required to be 500 C. or lower for preventing a decrease in specific surface area.

[0041] Thus, from this embodiment, it was shown that the platinum powder may be recovered from the platinum paste. In heating of the platinum paste, the organic component was combusted and removed by the catalytic action of the platinum powder. It was also shown that the platinum powder obtained by removing the organic component in this way can be expected to resist a change in physical properties due to sintering etc. when the upper limit of the heating temperature is appropriately set, so that the platinum powder may be reused.

Second Embodiment

[0042] In this embodiment, a test was conducted in which a platinum powder was recovered from a predetermined platinum paste, and the platinum powder was recycled into a new platinum paste. The recovery of the platinum powder and recycling of the paste were repeated multiple times. In this embodiment, the following two platinum pastes were used. The platinum paste was used within 1 hour after production.

[0043] (I) Platinum paste I . . . mixture of a platinum powder (particle size: 0.7 m) with -terpineol as a solvent and ethylcellulose as a resin. Solid component content: 90.9%.

[0044] (II) Platinum paste II . . . mixture of a platinum powder (particle size: 0.7 m) and an alumina powder with -terpineol as a solvent and ethylcellulose as a resin. Solid component content: 89.1%.

[0045] In this embodiment, the platinum pastes I and II were subjected to a two-stage heating treatment in which the platinum paste was heated at 150 C., and then heated at 350 C. as a method for recovering a platinum powder (solid component). Specifically, first the platinum paste was heated and held at 150 C. for 60 minutes, then heated to 350 C., and held for 30 minutes. The solid component (platinum powder and alumina powder) was recovered from the two-stage heating treatment, and the same solvent was mixed with the solid component to recycle the platinum paste. The recovery of the solid component and recycling of the solid component into the paste were considered as one set, and performed three times, and the specific surface area of the solid component recovered at each time was measured (the sum of the specific areas of the platinum powder and the alumina powder for the paste II). For the platinum paste recycled at each time, electrodes were prepared, and the resistivity was measured. The resistivity was measured in the following manner: three linear electrodes of 0.5 mm20 mm were printed, the thicknesses and the resistance values of the electrodes were measured, the average was determined, and the resistance value per thickness (m//10 m) was calculated.

[0046] The change rate in each of specific surface area of the powder and resistivity as measured at each time was calculated based on the measured value for the initial platinum paste before recycling. The results are shown in FIG. 3 (powder specific surface area) and FIG. 4 (resistivity).

[0047] It is apparent from FIG. 3 that under the heating condition (two-stage heating at 150 C. and 350 C.) in this embodiment, the properties of the platinum powder are not changed even when the platinum powder (solid component) is repeatedly recovered multiple times. FIG. 4 shows that the platinum paste recycled in this way is almost unchanged from the initial state.

Third Embodiment

[0048] In this embodiment, recovery of a platinum powder and recycling of a paste were performed for a platinum paste degraded due to long period storage. The platinum paste treated here is a platinum paste for formation of an electrode in an oxygen sensor. The platinum paste is a dispersed paste (solid component content: 83.71%) obtained by mixing powders: a platinum powder, a zirconia powder, an alumina powder and a diamond powder with -terpineol as a solvent, and further adding ethylcellulose as a resin for imparting a viscosity. The platinum paste was stored for 4 months after production, and the viscosity of the platinum paste increased from 260 Pa.Math.s (at the time of production) to 508 Pa.Math.s (after elapse of 4 months).

[0049] 5 g of the degraded paste was heated at 450 C. for 30 minutes to eliminate the organic component, the solid component was then recovered, and a fresh solvent was mixed with the solid component to recycle a platinum paste.

[0050] For the degraded platinum paste, the weight of the solid component was 4.18 g after the platinum paste was heated at 450 C. for 30 minutes. Accordingly, 99% of the organic component was removed by the heat treatment. A solvent (-terpineol) and a resin (ethylcellulose) were mixed with the solid component to produce a platinum paste having the same concentration as that of the original platinum paste (before degradation). The viscosity of the recycled platinum paste was measured, and the result showed that the viscosity was 268 Pa.Math.s, and almost the same as the viscosity of the platinum paste before degradation (viscosity: 260 Pa.Math.s).

[0051] Next, the recycled platinum paste was applied and fired to form a 10 m-thick electrode on a YSZ green sheet, and electrode properties were evaluated. For evaluation of the electrode properties, impedance measurement was performed from 100 kHz to 0.1 Hz with a bias-free amplitude of 20 mV at 700 C. in the air. The measurement was also performed for the platinum paste before degradation. The results of the measurement are shown in FIG. 5. The recycled paste has almost the same electrode properties as those of the platinum paste before degradation. Accordingly, it was shown that the platinum powder and solid component recovered in this embodiment were also effective for recycling of a degraded platinum paste.

[0052] For the degraded platinum paste, a platinum powder was recovered by two-stage heating (two-stage heating at 150 C. and 450 C.) as a heating treatment as in the second embodiment, and similar impedance measurement was performed. The recycled paste here showed the same electrode properties as those of the platinum paste before degradation.

Fourth Embodiment

[0053] In this embodiment, recovery of a platinum powder and recycling of a paste were performed for a plurality of platinum pastes having different types of organic solvents and resins as organic components. The same platinum powder (particle size: 0.7 m) as in the first embodiment, and a platinum-25 mass % palladium alloy powder were provided, and various kinds of organic solvents and resins were kneaded with the powders to produce platinum pastes (solid component content: 90%). The viscosity was measured, and the platinum pastes were stored for 1 month to be degraded. After elapse of 1 month, the viscosity was measured to confirm that the viscosity was changed by 10 to 15%.

[0054] 5 g of each of the degraded platinum pastes was taken, and heated for 30 minutes at each of temperatures of 200 C., 250 C., 300 C., 400 C., 500 C. and 550 C. The weight of the recovered solid component was measured to determine the organic component removal ratio. The surface area of the recovered platinum powder was measured. Further, an organic component identical to that at the time of production was mixed with the recovered platinum powder to recycle a platinum paste, and the viscosity of the platinum paste was measured. Possibility of platinum powder recovery was determined from the organic component removal ratio, the change rate in specific surface area of the platinum powder and the viscosity of the recycled platinum paste, each of which was measured as described above.

[0055] The results of determining possibility of platinum powder recovery in this embodiment are shown in Table 1. For determination criteria, samples for which the organic component removal ratio was less than 50% was rated , samples for which the organic component removal ratio was 50% or more and less than 95% was rated , and all of these samples were determined as being recovery impossible. Samples for which the organic component removal ratio was 95% or more, and the difference between the viscosity of the recycled platinum paste and the viscosity of the platinum paste at the time of production was within 10% were determined as being platinum powder recovery possible and rated . Samples for which the organic component removal ratio was 95% or more, but the difference between the viscosity of the recycled platinum paste and the viscosity of the platinum paste at the time of production exceeded 10% were rated based on the assumption that substantially the platinum powder was not recovered. Further, for the specific surface area of the recovered platinum paste, samples for which the specific surface area decreased by more than 10% were rated NG because recovery of the platinum powder could be impossible based on the determination that sintering occurred. Table 1 shows the results of measuring the removal ratio in heating of a resin alone as a reference to examples.

TABLE-US-00001 TABLE 1 Paste configuration Solid Heating temperature No. component Organic solvent Resin 200 C. 250 C. 300 C. 400 C. 500 C. 550 C. 1 Pt powder Texanol Ethylcellulose NG 2 -terpineol NG 3 Butyl carbitol NG 4 Texanol Alkyd resin NG 5 Polyvinyl butyral X NG 6 PtPd powder Texanol Ethylcellulose NG 7 Ethylcellulose alone X X X X 8 Polyvinyl butyral alone X X X X: organic component removal ratio is less than 50%. : organic component removal ratio is 50% or more and less than 95%. : organic component removal ratio is 95% + change in viscosity of recycled paste is more than 10%. : organic component removal ratio is 95% + change in viscosity of recycled paste is 10% or less. NG: recovery is impossible due to sintering of powder.

[0056] From Table 1, even when the organic solvent and the resin as the organic component were variously changed, a platinum powder was recovered by heating at 300 to 500 C., and recycled into a platinum paste (Nos. 1 to 6). Further, a metal powder was successfully recovered from a metal paste having a platinum-palladium alloy as the metal powder (No. 6).

[0057] From the results of studies in this embodiment, it can be confirmed that possibility of recovery of the platinum powder from the platinum paste is also influenced by the catalytic action of the platinum powder itself. From the results of No. 7 and No. 8, the resin which is not volatile unlike the solvent cannot be sufficiently removed alone even by heating at a relatively high temperature, and remains. The resin is almost totally fired and eliminated by heating at 300 C. or higher when it exists in the platinum paste, i.e. it coexists with the platinum powder. This may be because the catalytic action of platinum (platinum alloy) accelerates oxidation and decomposition of the resin.

INDUSTRIAL APPLICABILITY

[0058] As described above, according to the present invention, a metal powder equivalent to that in the initial stage of production can be recovered in simple and easy steps from a platinum paste which would have been discarded and sent to a recovery step previously. Accordingly, a platinum paste can be simply and easily recycled. In the present invention, the loss of platinum can be expected to decrease, leading to a reduction in cost of the platinum paste. The present invention relates to a technique for recycling platinum, a precious metal receiving attention in terms of price fluctuation, and contributes to effective utilization of platinum.