Method of Recovering Valuable Metals from Waste

Abstract

A method of recovering valuable metals from a waste including: heating the waste at a temperature and for a period of time such that a glass fiber does not melt but degrades to the extent that it becomes pulverizable, wherein conditions of the temperature and period of time are selected from a group consisting of a range of more than or equal to 750° C. and less than 800° C. for 30-40 minutes, a range of more than or equal to 800° C. and less than 900° C. for 10-40 minutes, a range of more than or equal to 900° C. and less than 950° C. for 10-30 minutes, a range of more than or equal to 950° C. and less than 1000° C. for 10-20 minutes, and a range of 1000° C. for about 10 minutes; removing the degraded glass fiber; and recovering valuable metals contained in the waste.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of application Ser. No. 12/186,333 filed on Aug. 5, 2008, which claims the benefit of priority to Japanese Patent Application No. 2008-144732, filed on Jun. 2, 2008, of which full contents are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of recovering valuable metals from wastes such as IC circuits board and printed wiring board.

2. Description of the Related Art

Printed wiring boards commonly used in personal computers and cell phones contain significant amounts of metals used therein.

Specifically, printed wiring boards have an insulation board such as glass epoxy substrate and semiconductor elements, capacitors, resistors, and wirings in combination formed thereon, and generally, the organic component content is said to be 32%; the glass component content, 38%; and the metal component content, about 30%.

Most of the organic matter is an epoxy resin, and approximately 66% of the glass components is SiO₂.

In particular, the metal materials are said to contain copper in the greatest amount and additionally valuable metals such as tin, iron, lead, nickel and gold in an amount of 0.1% in the printed wiring board. Because these wastes contain a great amount of precious valuable metals, various methods of recovering the metals were developed and commercialized.

Patent Document 3, Ueno et al. (U.S. Pat. No. 6,336,601 B1), discloses a waste-printed-circuit board treatment method including the steps of: heating up for dry-distilling the waste printed circuit boards having copper foil retaining solder in at least a part of the surface to the extent that the waste becomes brittle, at the temperature less than or equal to 500° C.; pulverizing the dry-distilled material of the waste by e.g. metallic-ball collision to the brittle waste; and separating the pulverized material of the waste into board resin component and metal component. The upper limit of 500° C. in the heating-up step is set for saving the heating energy, and suppressing the evaporation and diffusion of solder as well as the generation of hazardous gas.

Patent Document 4, JP 1990-88725, discloses a method of recovering copper from waste printed circuit boards by heating the boards at a temperature more than or equal to 800° C. up to a melting temperature thereof, while introducing air (oxygen) for carbonizing a resin, and pulverizing the heated boards. More specifically, the heating temperature is set at more than or equal to 800° C. and less than or equal to 900° C. (or a melting point of copper i.e. 1085° C.), and the holding period of time is set at 45 to 90 minutes. The minimum temperature, 800° C., is provided in order to maintain a predetermined grade of copper, and the maximum temperature, 900 (or 1085)° C., is provided in order to save heating energy. The heated boards are pulverized by a pulverizer, e.g. a ball mill magnetic separator before screening through a screen classifier.

• Patent Document 1: Japanese Patent Application Laid-Open No. 2003-301225.
• Patent Document 2: Japanese Patent Application Laid-Open No. 2001-259603.
• Patent Document 3: U.S. Pat. No. 6,336,601 B1.
• Patent Document 4: Japanese Patent Application Laid-Open No. 1990-88725.

SUMMARY OF THE INVENTION

The conventional recovery methods described above, i.e., methods of recovering metals by dissolving the printed wiring board in acid or heating it and then oxidizing or pulverizing it, have the following problems:

<1> The glass fiber therein melts in the heating step into a melt-solidified solid state containing the metals inside.

<2> Incineration at high temperature leads to increase of the loads both on the apparatus and the environment.

<3> Fusion or electrolysis at high temperature demands great amounts of fuel and power.

<4> Although it is possible to dissolve the glass fiber in acid more efficiently by pulverization thereof, the fibrous material is tough and thus difficult to pulverize.

An object of the present invention, which was made to solve the problems above, is to provide a method of recovering valuable metals from waste that is an integrated industrial waste containing a glass fiber, an epoxy resin, and valuable metals such as copper, iron, gold, aluminum, the method comprising the steps of: heating the integrated industrial waste at a temperature and for a period of time in such a fashion that the glass fiber does not melt but degrades to the extent that the glass fiber becomes pulverizable, wherein conditions of the temperature and the period of time applied to the integrated industrial waste are selected from a group consisting of a range of more than or equal to 750° C. and less than 800° C. for 30 to 40 minutes, a range of more than or equal to 800° C. and less than 900° C. for 10 to 40 minutes, a range of more than or equal to 900° C. and less than 950° C. for 10 to 30 minutes, a range of more than or equal to 950° C. and less than 1000° C. for 10 to 20 minutes, and a range of 1000° C. for about 10 minutes; removing the degraded glass fiber; and thereafter, recovering the valuable metals contained in the integrated industrial waste.

As described above, the method of recovering metals from waste according to the present invention has the following advantages:

<1> The method allows recovery of valuable metals in simple steps without need for an additional step of pulverizing the melt-solidified glass fiber.

<2> The method does not demand heating or fusion at high temperature and is thus, lower in concern about environmental pollution, and can be used in countries where stricter laws and regulations are imposed.

<3> The method demands smaller amounts of fuel and power and is thus economical.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 (A) is a photograph showing printed wiring boards heated respectively under temperature/period-of-time conditions in the range of 300° C. for 10 minutes and 750° C. for 20 minutes.

FIG. 1 (B) is an English translation in table form of the Japanese characters shown in FIG. 1 (A).

FIG. 2 (A) is a photograph showing printed wiring boards heated respectively under temperature/period-of-time conditions in the range of 750° C. for 30 minutes and 1000° C. for 40 minutes.

FIG. 2 (B) is an English translation in table form of the Japanese characters shown in FIG. 2 (A).

FIGS. 3 to 34 are photographs of a heated printed wiring board.

FIG. 35 depicts plots for natural logarithmic values of the inverse of period of time [ln(min⁻¹)] as a function of values of the inverse of absolute temperature [K⁻¹] obtained on the basis of Tables 1, 2, where the straight dotted line represents just an eye-guide for four plots.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Hereinafter, favorable embodiments of the present invention will be described in detail with reference to drawings.

Examples

<1> Wastes to be Processed

The industrial wastes processed by the recovery method according to the present invention are industrial wastes such as printed wiring boards.

These industrial wastes are characteristic in that they are integrated materials of a glass fiber, an epoxy resin and valuable metals such as copper, iron and gold.

<2> Heating Step

These industrial wastes are heated at a particular temperature for a particular period of time.

The temperature and the period of time are such that the glass fiber does not melt but decomposes.

The relationship between the temperature and the period of time was determined in many tests.

The results showed specifically that the industrial waste is heated favorably under a condition in the range of 500° C. for 20 minutes or more and 1000° C. for 10 minutes or less.

More favorable results are found to be obtained, when the industrial waste is processed under a condition in the range of approximately 750° C. for 30 minutes or more to 1000° C. for 10 minutes or less.

Here, the “favorable results” mean that the glass fiber and the valuable metals are separated and only the valuable metals are recovered easily.

<3> Reason for Selecting the Temperature and the Period of Time

<3-1> Determination of Upper Limit

The temperature and the period of time above are selected, because the glass fiber melts when heated under a temperature/period-of-time condition severer than the temperature/period-of-time condition above and solidifies itself when cooled.

The melted glass fiber solidifies itself into a solid state, while enclosing the metals therein, prohibiting recovery of the metals from the state once formed.

For that reason, the metals have been recovered after the metals and the glass fiber are both melted at high temperature, by using the difference in specific density.

However, by the method according to the present invention, wherein the waste is heated up to 1000° C. for 10 minutes or less, the glass fiber does not melt, eliminating the possibility of the glass fiber melt-enclosing the metals.

<3-2> Determination of Lower Limit

As for the lower limit heating condition, the glass fiber sheet, when heated at 500° C. for 20 minutes or less, retains its original shape and still contains the metals therein without separation, although the surface thereof turns brown in color.

The glass fiber sheet in that state is resistant to pulverization under pressure and cannot be separated from the metals.

<3-3> Determination of Optimal Temperature and Period of Time

However, the glass fiber sheet and the metals are separated actually, when heated under a condition in the range of 500° C. for 20 minute or more to 750° C. for 30 minutes or less.

It is thus possible to make the metal components sediment and collect the metal sediment, while separating it from the glass fiber sheet, for example, by gravimetric sorting by using a liquid.

The lower limit heating condition is more preferably approximately 750° C. for 30 minutes or more.

In the temperature/period-of-time condition above, the glass fiber sheet apparently retains its plate shape as a sheet, but is pulverized easily under external force, for example by the force when the sheet is held with fingers.

In addition, the metals do not melt in the temperature range.

Therefore, the glass fiber is pulverized easily by slight vibration or pressurization by roller, favorably allowing recovery of the metal components as they are.

<4> Test Results

Printed wiring boards were heated in various temperature conditions of upward from 300° C. for 10 minutes at intervals of 100° C. and 10 minutes.

The results obtained in respective temperature/period-of-time conditions are shown in photographs of FIGS. 3 to 46, and the results are summarized in the following Table 1.

TABLE 1
The marks in respective columns are as follows:
x The fiber sheet and the metals retain their shapes and are not separated
  from each other. In particular at the low temperature side, the fiber sheet
  is only discolored. The fiber sheet retains its original shape, even when
  pressurized by hand.
Δ The fiber sheet and the metals are separated from each other. Thus, the
  valuable metals can be collected. However, the fiber sheet retains its
  shape and is resistant to pulverization.
∘ Apparently, the fiber sheet retains its shape. However, it is pulverized
  easily by slight pressurization or by the force when it is held with
  fingers. Because the glass fiber can be pulverized easily into powdery,
  it is possible to separate the valuable metals easily from the powder.
□ The glass is solidified into an aggregate state, while holding the metals.
  Aggregates as hard as rock are obtained when the waste is heated at
  higher temperature for an elongated period. The metals, which are
  entrapped in the melted glass, are difficult to separate.

Further, each temperature in Table 1 is converted into a value of the inverse of absolute temperature (1/T) [K⁻¹], and each period of time in Table 1 is converted into a natural logarithmic value of the inverse of period of time (ln(1/t)) [ln(min⁻¹)], as shown in Table 2 below. Among a plurality of relations between the converted “1/T” and “ln(1/t),” only a relation at each critical point of the maximum temperature and the maximum period of time, above which melt-solidification occurs instead of degradation for the glass fiber, is highlighted in Table 2 below.

TABLE 2

Still further, the above maximum temperature and maximum period of time at each of four critical points highlighted in Table 2 are plotted in FIG. 35.

In general, at a time when the maximum period of time of a preferred range for degradation (without melt-solidification) of the glass fiber has elapsed at a constant temperature, the glass fiber is in a state that a substantially total amount thereof degrades, but further melt-solidification thereof is suppressed to an acceptably small level. The glass fiber at the above four critical points shown in Table 2 and FIG. 35 is in such a state.

In a similar fashion, at a time when the minimum period of time of a preferred range for degradation (without melt-solidification) of the glass fiber has elapsed at a constant temperature, the glass fiber is in a state that an acceptably large amount thereof degrades, but a substantially negligible amount thereof is melt-solidified. Although only one clear critical point of the minimum temperature and the minimum period of time (750° C. for 30 minutes) is explicitly shown in Table 1, the glass fiber at the critical points would probably be in such a state.

In other words, the minimum/maximum temperature and period of time of a preferred range for degradation without melt-solidification of the glass fiber are determined by the above-mentioned competitive relation between degradation and melt-solidification of the glass fiber. Such a competitive relation means that the glass fiber is converted into the degraded glass fiber while the degraded glass fiber is further converted into the melt-solidified glass. In the preferred range, the former conversion (degradation) is enhanced but the latter conversion (melt-solidification) is suppressed. More specifically, the minimum/maximum temperature and period of time are determined on the basis of a difference of velocity obtained by subtracting a melt-solidification velocity from a degradation velocity, both of which are proportional to exp(−Ems/(kT)) and exp(−Ed/(kT)), respectively, where: “Ems” and “Ed (<Ems)” denote the activation energies of melt-solidification and degradation, respectively; “k” denotes Boltzmann constant; and “T” denotes the temperature.

For example, the above-mentioned difference of velocity, which is proportional to the inverse of period of time “1/t” for degradation and melt-solidification, is represented by “v_d0×exp(−Ed/(kT))−v_ms0×(−Ems/(kT))” (Ed<Ems), which is a subtraction between double exponential functions. The symbols “v_d0” and “v_ms0” denote the constant velocities of degradation and melt-solidification, respectively. In principle, the relation between “−ln(1/t)” and “1/T” could not comply with the Arrhenius equation (constituted by a single exponential function) if Ems is much higher than Ed, because the difference of velocity “v_d0×exp(−Ed/(kT))−v_ms0×(−Ems/(kT))” maintains a double exponential function. On the other hand, the relation between “−ln(1/t)” and “1/T” could comply with the Arrhenius equation if Ems is higher than but close to Ed, because the difference of velocity “v_d0×exp(−Ed/(kT))−v_ms0×(−Ems/(kT))” could be approximated to “(v_d0−v_ms0)×(−Ed/(kT))” constituted by a single exponential function.

In this example according to the present invention, as shown in Table 2, four pieces of relation of (1/T [K⁻¹], −ln(1/t) [ln(min⁻¹)]) at the maximum temperature and the maximum period of time are: (0.786×10⁻³, −2.30); (0.818×10⁻³, −3.00); (0.853×10⁻³, −3.41); and (0.912×10⁻³, −3.69). And, as shown in FIG. 35 (see the straight dotted line), it could be safely said that such a relation exhibits substantially a linearity, which would result in the presumption that the activation energy of melt-solidification of the glass fiber (Ems) is higher than but close to the activation energy of degradation of the glass fiber (Ed), and that the degradation is higher in constant velocity than the melt-solidification (v_d0>v_ms0). In view of the above, the minimum/maximum temperature and period of time of the preferred range for degradation without melt-solidification of the glass fiber are determined by the applicant. Although such analysis could not be carried out for the minimum temperature and the minimum period of time on the basis of the data in Table 1, substantially a similar result would probably be obtained by the applicant.

As a consequence, the temperature and the period of time for degradation without melt-solidification of the glass fiber in this example are not result effective variable. For example, the quality of the recovered valuable metals does not depend upon one critical point of temperature and period of time but depends upon a range defined by two critical points: the minimum temperature and period of time; and the maximum temperature and period of time, in which the degradation is enhanced but the melt-solidification is suppressed. Accordingly, it could be safely said that the temperature and the period of time do not affect the quality of the recovered valuable metals in a direct or straightforward fashion, and that the maximum temperature and period of time are not determined merely for the purpose of saving the energy (cost) but determined principally for the purpose of pursuing the quality of the recovered valuable metals. The preferred range shown in Table 1 is determined by a competitive relation between degradation and melt-solidification, and therefore, is determined by their respective activation energies, Ed and Ems, and by their respective constant velocities, v_d0 and v_ms0. Needless to say, it would not have been obvious for a person having the ordinary skill in the art to try to focus only upon the temperature/period of time for degradation without melt-solidification, and such a person could not have reached the preferred range of the temperature/period of time merely by experimental trial-and-error optimization.

(Comparison with prior art disclosed in Patent Document 3): In Patent Document 3 (Ueno et al., U.S. Pat. No. 6,336,601 B1), it is described that dry-distilled material of the waste is pulverized by metallic-ball collision to the brittle waste, and the upper limit of temperature for heating up the waste is set at 500° C. for the purpose of saving the heating energy, and suppressing the evaporation and diffusion of solder as well as the generation of hazardous gas. On the other hand, in this example according to the present invention, the waste is heated at a temperature and for a period of time in such a fashion that the glass fiber does not melt but degrades to the extent that the glass fiber becomes pulverizable, which does not need any means for such metallic-ball collision. Furthermore, the preferred temperature for such pulverization of the glass fiber in this example is more than or equal to 750° C. (see Table 1), which does not have any overlap with the temperature range whose upper limit is 500° C. in Patent Document 3.

(Comparison with prior art disclosed in Patent Document 4): In Patent Document 4 (JP 1990-88725), it is described that the temperature is set at more than or equal to 800° C. and less than or equal to 900° C. (or 1085° C.), and the holding period of time is set at 45 to 90 minutes. On the other hand, in this example according to the present invention, the waste is heated for much shorter period of time than such a “45 to 90 minutes” (see Table 1). As mentioned above, the period of time for degradation of the glass fiber in this example is not result effective variable. The maximum period of time is not determined merely for saving the energy (cost), but determined principally for pursuing the quality of the recovered valuable metals. By the applicant's discovery and analysis on the basis of Tables 1, 2 and FIG. 35, the temperature as well as the period of time of the preferred range for degradation without melt-solidification of the glass fiber are determined.

There was no change in the samples if they were heated for an extended period of time, even though the results obtained are not described herein.

<5> Heating in Oxygen-Free State

It is possible to heat wastes in oxygen-free state, for example by using an electric furnace.

In this way, it is possible to recover valuable metals as they are without formation of oxide films, because the surface of the valuable metals such as copper are not oxidized.

If there is a concern about the damage of the heating unit in electric furnace, the waste may be heated in a low-oxygen state generated by burning carbon additionally as the heat source.

<6> Step of Removing Glass Fiber

When heated in a temperature/period-of-time condition in the range above, in particular when heated in the range of 500° C. for 20 minutes or more and 750° C. for 30 minutes or less, the glass fiber sheet mostly retains its original shape but is separated from the valuable metals.

It is thus possible to recover the valuable metals, by separating the glass fiber sheet from the valuable metals in the later separation step by using an optimal method such as gravimetric separation.

Further, when heated in the range of 750° C. for 30 minutes or more, the glass fiber degrades to such a degree that it is pulverized easily into powder by application of slight external force, such as that when the glass fiber is held with fingers, or by application of vibration or pressure.

On the other hand, the glass fiber does not melt, because the heating temperature/period-of-time condition is 1000° C. for 10 minutes or less, and thus, the metal components remains as they are without entrapment in the melted glass fiber.

It is thus possible to separate valuable metals from the glass fiber, removing the glass fiber and recovering the valuable metals, by a known simple method such as sieve classification or gravimetric classification.

Claims

1. A method of recovering valuable metals from waste that is an integrated industrial waste containing a glass fiber, an epoxy resin, and valuable metals such as copper, iron, gold, aluminum, the method comprising the steps of:

heating the integrated industrial waste at a temperature and for a period of time in such a fashion that the glass fiber does not melt but degrades to the extent that the glass fiber becomes pulverizable, wherein

conditions of the temperature and the period of time applied to the integrated industrial waste are selected from a group consisting of a range of more than or equal to 750° C. and less than 800° C. for 30 to 40 minutes, a range of more than or equal to 800° C. and less than 900° C. for 10 to 40 minutes, a range of more than or equal to 900° C. and less than 950° C. for 10 to 30 minutes, a range of more than or equal to 950° C. and less than 1000° C. for 10 to 20 minutes, and a range of 1000° C. for about 10 minutes;

removing the degraded glass fiber; and thereafter,

recovering the valuable metals contained in the integrated industrial waste.

2. The method of recovering valuable metals from waste according to claim 1, wherein, in the step of heating the integrated industrial waste, the heating is carried out in an oxygen-free state.