Method for recovering metals from waste

A method for recovering metals from metals-containing wastes, and vitrifying the remainder of the wastes for disposal. Metals-containing wastes such as circuit boards, cathode ray tubes, vacuum tubes, transistors and so forth, are broken up and placed in a suitable container. The container is heated by microwaves to a first temperature in the range of approximately 300.degree.-800.degree. C. to combust organic materials in the waste, then heated further to a second temperature in the range of approximately 1,000.degree.-1,550.degree. C. at which temperature glass formers present in the waste will cause it to melt and vitrify. Low-melting-point metals such as tin and aluminum can be recovered after organics combustion is substantially complete. Metals with higher melting points, such as gold, silver and copper, can be recovered from the solidified product or separated from the waste at their respective melting points. Network former-containing materials can be added at the start of the process to assist vitrification.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This application is a continuation-in-part of application Ser. No. 
07/866,780, filed Apr. 1, 1992. 
The present invention relates to a method for recovering metals from 
wastes. In particular, the present invention relates to a method for 
recovering re-usable metals from waste electronic components, and 
vitrifying the waste residue using microwave energy. The United States 
Government has rights in this invention pursuant to Contract No. 
DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse 
Savannah River Company. 
2. Discussion of Background 
The proper disposal of all kinds of wastes is an important issue. In 
particular, the disposal of hazardous wastes: biological, chemical and 
radioactive, is of concern to generators, regulatory officials and the 
public. These waste materials can present a hazard if they re-enter the 
environment. Furthermore, there is the related issue of reduction of waste 
volume and minimization of disposal space. Although progress has been made 
in reducing the volume of wastes generated and in recycling some 
components of the wastes, there remains a large volume of material that 
must be safely disposed of. 
Many solid wastes, including waste electronic components such as used 
circuit boards, vacuum tubes, transistors, relays, wiring, television 
screens and computer monitors, remote controls, personal computers and 
calculators, contain metals, organic compounds and potentially leachable 
constituents. With the ever-increasing use of electronic devices in our 
society, there is a growing interest in the safe disposal of these types 
of wastes. Furthermore, even though the amount of metal in a single 
discarded electronic component is small, many millions--perhaps billions 
--of such components are discarded each year. This number is increasing 
along with the growing number of electronic devices in business, industry, 
military and household use. In the U.S. alone, such wastes may contain 
many thousands of pounds of potentially useful, valuable metals, including 
gold, silver and copper. There are no known, routinely-used, 
cost-effective methods for recovering the various metals found in a 
variety of waste electronic components. 
A number of techniques have been used in stabilizing and encapsulating 
hazardous wastes and the literature abounds with descriptions of these. A 
particularly effective technique, called vitrification, is the 
encapsulation of wastes in glass. Glass is very stable against chemical 
attack. Vitrification has been studied for decades in connection with 
radioactive wastes. Typically in vitrification, the waste is slurried with 
glass frit into a glass melter where the glass is heated until it is 
molten. The waste is incorporated into the glass matrix in such a way that 
the final, cooled product will resist leaching of the waste for very long 
periods of time. 
In other applications of vitrification, electrodes are placed directly in 
contaminated earth, which typically has a significant silicate component, 
and a voltage applied. The resistance of the ground results in sufficient 
joule heating to vitrify the waste in-situ. 
Heat can be applied to wastes using a variety of electrical and thermal 
heating processes. The use of electric melting concepts is well known for 
incorporating waste into glass and the use of microwave energy is a 
favored technology for treating halogenated hydrocarbons. Varma (U.S. Pat. 
No. 4,935,114) brings toxic wastes into contact with a bed of 
non-metallic, absorbing particles, such as activated carbon, then heats 
the waste to 500.degree.-600.degree. C. to destroy the wastes chemically. 
In all electrical and thermal waste treatment processes, there is a need 
for a method for recovering re-usable materials, including precious metals 
from waste electronic components. Preferably, the method should be simple, 
flexible and effective, reduce the overall volume of waste substantially, 
and produce a stable, durable product. 
SUMMARY OF THE INVENTION 
According to its major aspects and broadly stated, the present invention is 
a method for recovering metals from wastes, and encapsulating and 
immobilizing the residue of the wastes for disposal. The method can be 
used to recover precious metals from waste electronic components and 
wastes that may contain potentially hazardous constituents. Metals 
recovery, encapsulation and immobilization are accomplished in a simple 
process that can be accomplished remotely and using equipment that can be 
easily transported. The method of the present invention results in 
significant volume reduction and in the formation of a highly durable 
waste glass product. The method comprises the steps of applying microwave 
energy to the waste to raise its temperature sufficiently to combust 
organic material present in the waste, then continuing to apply microwave 
energy to the waste to further increase its temperature up to the range of 
approximately 1,100.degree. C. to 1,550.degree. C., or more. When the 
waste is held at this second, higher temperature, it will melt and 
vitrify, assuming it has sufficient glass formers such as silicates in it. 
If it does not, glass formers can be added. Once vitrified, the waste is 
allowed to cool and may subsequently be disposed of. Metals contained in 
the waste may be recovered at several stages of the process: 
low-melting-point metals during the organics-combustion stage, and 
higher-melting-point metals during melting of the waste or after the waste 
has cooled. 
An important feature of the present invention is the separation and 
recovery of re-usable metals, including precious metals, from the waste 
residue. Surprisingly, it has been found that metals contained in the 
wastes separate out as the wastes are heated and melted, and remain 
substantially separated from the remainder of the wastes when the melted 
wastes cool. These metals may be recovered at several stages of the 
process: low-melting-point metals such as tin and aluminum are recoverable 
during or after organics combustion, and higher-melting-point metals such 
as gold, silver, and copper, after the vitrified waste cools. If desired, 
metals may be separated from the wastes according to melting point during 
heating and melting of the wastes. 
Another important feature of the present invention for wastes containing 
glass-forming components is that it requires no additive other than heat. 
For wastes such as fiberglass, syringes and circuit boards, there are 
sufficient glass formers present and the effect of the process is to 
immobilize the wastes safely, without additives, to reduce the volume by 
50% or more, and to recover any metals present in the wastes. For medical 
wastes such as syringes and intravenous items, organics are combusted 
before remaining waste constituents are vitrified. 
Another feature of the invention is the ability to adjust the composition 
of the final waste glass product. Whether glass formers are added to 
compensate for a shortage of silicates or other glass formers in the 
wastes, or whether the wastes have sufficient glass formers to begin with, 
the product has good leaching resistance and can be disposed of in an 
ordinary landfill. Alternatively, the product can be used as a filler in 
materials such as concrete, asphalt, brick and tile. If the wastes contain 
certain contaminants such as lead and arsenic, chemical compounds can be 
added to the melt that chemically bind these contaminants, as is 
well-known in the chemical arts. 
Still another feature of the invention is the use of microwaves to heat the 
wastes. Microwaves allow careful control of the temperature of the wastes, 
yet enable a rapid temperature increase, and can heat the wastes to higher 
temperatures than many conventional melters. Susceptors can also be used 
to aid the heating operation. Susceptors are materials that absorb 
microwaves quickly and then radiate heat energy to heat adjacent materials 
that are somewhat slower to respond to the microwaves. The use of this 
hybrid heating concept--microwave energy plus heat energy--allows 
metals-containing wastes to be treated in a microwave process. 
Aside from efficiency, an advantage of accurate temperature control is the 
ability to adjust the rate of organics combustion to the process off-gas 
system. A too-rapid combustion of organics can overwhelm the off-gas 
system and allow discharge of particulates to the atmosphere. Another 
advantage of the use of microwaves is that they enable much smaller 
melters than the conventional joule-heated variety: a microwave melter is 
small enough to be transported to the source of the waste or moved from 
place to place at a disposal site. Microwave melters can also be made for 
remote handling of wastes when the wastes are highly radioactive or 
especially hazardous. 
Other features and advantages of the present invention will be apparent to 
those skilled in the art from a careful reading of the Detailed 
Description of a Preferred Embodiment presented below.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The present invention is a method for recovering metals from wastes and 
immobilizing the waste residue in a stable, durable matrix. The method is 
appropriate and economical for wastes containing potentially hazardous, 
leachable constituents because of the high level of stability required and 
the generally high costs of safe disposal of these wastes. The present 
method is especially well-suited for wastes containing silicates, such as 
electronic components and syringes, but silicates or other materials can 
be easily added to any waste to produce the composition required for 
vitrification. 
Crucial to the method is the use of microwaves for heating the wastes, 
because of the higher temperatures that can be reached, the greater 
control available with microwaves and the faster rate of temperature 
increase. Use of hybrid heating concepts assists this operation. 
Referring now to FIG. 1, there is shown a flow chart of a method for 
treating wastes according to a preferred embodiment of the present 
invention. The method is carried out generally as follows: 
1. Break up the waste materials into pieces to assist melting, and place 
into a container that can withstand high temperatures. 
Breaking up the wastes into small pieces, preferably pieces on the order of 
0.5 cm or smaller in size, assists placement in a container for melting, 
and also facilitates melting and mixing with glass formers (if needed to 
ensure complete vitrification). The wastes may be cut, crushed, or ground 
or pulverized to produce a powder if desired. The wastes are then placed 
into a crucible of a ceramic or other material that can withstand high 
temperatures, that is, temperatures of approximately 2,000.degree. C. 
Suitable containers include fused silica or alumina crucibles. 
Wastes that can be treated by the method include syringes and other 
metals-containing medical wastes, and electronic components such as 
circuit boards, vacuum tubes, transistors, integrated circuits, relays, 
switches and wiring, television screens and computer monitors, light 
bulbs, etc. These types of electronic components are found in a wide 
variety of products, including computers, television sets and other 
audiovisual equipment, power supplies, remote controls, etc. 
2. Using microwave energy, heat the wastes to a first temperature T.sub.1 
to combust organic materials in the wastes. 
The wastes are maintained at the first temperature until any organic 
materials present in the wastes are substantially combusted, that is, 
reduced to ash. The first temperature is preferably in the range of 
approximately 300.degree. C. to 800.degree. C., however, the optimum 
temperature for combustion may vary depending on the composition of the 
wastes. Temperatures outside this range may also be useful for some types 
of wastes. To reduce emissions of combustion byproducts to the 
environment, the off-gas generated during organics combustion may be 
vented through an off-gas system that removes particulates. 
Organic compounds typically generate particulates and off-gas as they 
decompose. Some organic compounds decompose rapidly when heated to a 
specific temperature range, generating very large amounts of off-gas, but 
decompose relatively slowly at temperatures outside this range. In 
general, the more rapid the decomposition, the more rapid the generation 
of off-gas and particulates. Depending on the composition of the wastes, 
off-gas constituents may include carbon monoxide (CO), carbon dioxide 
(CO.sub.2), nitrogen oxides (NO, NO.sub.2), halogen compounds and volatile 
organics. If present in sufficient concentrations, some of these gases can 
ignite and burn, potentially damaging process equipment and presenting a 
safety hazard to personnel. There is a need to control the release of 
these gases so they will not ignite uncontrollably and can be handled by 
an off-gas system. 
Heating the wastes to a first temperature, preferably a temperature at 
which any organics in the wastes decompose sufficiently slowly to be 
handled by the off-gas system, and maintaining that temperature until the 
organics are substantially combusted, ensures that off-gases are released 
in a controlled manner to prevent spontaneous ignition. 
The rate of temperature rise is preferably at least approximately 
100.degree. C. per minute, however, different rates may also be useful as 
long as the rate is controlled. Rapid heating is desirable to ensure an 
efficient process, however, if the rate is too fast the off-gas system may 
be unable to cope with the amount of airborne particulates and the volume 
of off-gas produced by the decomposing organics. The optimum rate and 
temperature T.sub.1 will therefore depend on the types and quantities of 
organics known to be in the wastes, and the capabilities of the off-gas 
system. 
If desired, the off-gas may be monitored to determine when organics in the 
wastes are substantially combusted. For example, the off-gas CO.sub.2 or 
NO.sub.x concentration may be measured, and the temperature of the waste 
maintained at the first temperature until the concentration drops below a 
predetermined value. Alternatively, the temperature may be maintained at 
the first temperature for a predetermined period of time, depending on the 
composition of the wastes, the particular apparatus used for treating the 
wastes, and the amount of waste material to be treated. The optimum time 
is best determined by a modest amount of observation and experimentation 
for each particular application. 
At temperature T.sub.1, no significant melting of silicates (or other glass 
formers) will take place, although other waste constituents may melt or 
soften. However, low-melting-point metals may separate out from the wastes 
and settle to the bottom of the container. By way of example, tin (Sn) has 
a melting point T.sub.m =232.degree. C., zinc (Zn), a melting point of 
419.degree. C., and aluminum (Al), a melting point of 660.degree. C. If 
desired, these metals may be recovered from the waste before proceeding to 
Step 3. 
3. If desired, break up the combusted wastes by crushing, milling, 
pulverizing or some other suitable process. 
4. Using microwave energy, heat the wastes to a second temperature T.sub.2 
to melt the wastes. Step 4 may be carried out in the same container and 
microwave heating apparatus as Step 2, or in a different container and 
different apparatus, depending on the amount and composition of the wastes 
to be treated, and the capacity of the apparatus. 
The wastes may be placed inside a susceptor to facilitate melting, and an 
industrial-type microwave containing the proper protection against arcing 
and damage is strongly preferred. It is well known that metals strongly 
reflect microwaves instead of absorbing them, therefore, waste materials 
that contain significant amounts of metals and alloys cannot be safely and 
efficiently melted by microwaves alone. The susceptor preferably contains 
a substance with good dielectric coupling characteristics, that is, a 
substance whose orbital electrons are directly coupled with the incident 
microwaves. The rise in temperature that results from direct coupling is 
known as "internal heating"; other forms of heating (radiant, convection, 
conduction) do not involve direct coupling and are known as "external 
heating." When the susceptor is heated by microwaves, it readily absorbs 
microwave energy and, as its temperature rises, radiates heat energy to 
melt any metals contained in the wastes. The non-metal waste constituents 
are melted by "hybrid heating," a combination of microwave energy and heat 
energy. 
At temperatures in the range of approximately 1,100.degree. 
C.-1,550.degree. C., glass formers in the wastes will melt, as will metals 
having melting points in this temperature range. The wastes are maintained 
at temperature T.sub.2 until the glass formers and at least a portion of 
these metals melt, preferably for at least approximately five minutes. 
5. If needed, add glass formers to the wastes. 
If there are insufficient silicates or other glass formers present in the 
wastes, the wastes may not be completely vitrified. In that case, 
silicates or other materials that contain network formers, including but 
not limited to borosilicate glass, quartz, fiberglass, alumina, boron, 
germanium oxide (GeO.sub.2), and phosphorus pentoxide (P.sub.2 O.sub.5) 
can be added to the wastes and the microwave energy reapplied. The term 
"network former" as used herein, refers to components such as silicon (Si) 
that make up the backbone structure of a glassy material. Depending on the 
composition of the wastes and the metals to be recovered, other materials 
such as glass modifiers and intermediates may be added. 
Once the mixture of waste, glass formers and other additives (if present) 
is heated to a temperature in the range of approximately 1,100.degree. C. 
-1,550.degree. C., the glass formers will melt and encapsulate the wastes. 
Alternatively, the wastes may be analyzed before treatment to determine 
whether sufficient glass formers are present for vitrification. Then, the 
types and amounts of glass formers and other additives needed to provide 
the desired final product composition are added to the wastes, either 
before heating to the first temperature (Step 2) or before heating to the 
second temperature (Step 4). 
For example, the composition of circuit boards depends on the end se, the 
manufacturer, and the date of manufacture. The substrate of a circuit 
board can be composed of any of a variety of polymeric or ceramic 
materials, or a combination of materials. Some types of boards are 
reinforced by glassy fiber mats that have a high silica content. If the 
wastes being processed include circuit boards, the type of board 
determines whether or not network formers must be added prior to 
vitrification. Typically, fiber-mat-reinforced boards can be vitrified 
without additives, while unreinforced boards may require supplemental 
glass formers. 
6. Recover metals from the wastes. 
As the wastes are heated to temperature T.sub.2 and maintained at 
approximately that temperature (Step 4), any metals with melting points 
lower than T.sub.2 will melt. Surprisingly, the melted metals do not 
remain mixed with the remainder of the wastes, but tend to separate from 
the wastes and settle to the bottom of the container. Furthermore, these 
metals remain separated as the melted wastes cool. As a result, the cooled 
waste is not an amorphous mixture of metals, ash, etc., but contains 
well-defined metallic nodules or spheres in a glassy substrate. After 
cooling, the metal spheres may be separated from the waste residue by any 
convenient means. 
Many waste materials--and mixtures of different types of materials 
--contain a plurality of metals or alloys with different melting points 
T.sub.a &lt;T.sub.b &lt;. . . &lt;T.sub.n. To recover a metal or alloy having a 
lowest melting temperature T.sub.a, the wastes are heated to a third, 
intermediate temperature T.sub.3, where T.sub.a .ltoreq.T.sub.3 &lt;T.sub.b, 
and held at T.sub.3 until that metal melts and separates from the 
remainder of the wastes. After the first metal is recovered, the wastes 
are heated to a next intermediate temperature T.sub.b .ltoreq.T.sub.3 
&lt;T.sub.c to recover the metal with the next lowest melting point. This 
process may be repeated for all recoverable metals and alloys that have 
melting points less than approximately 1550.degree. C. Depending on the 
types of wastes being treated and the particular selection of process 
equipment, metals with higher melting points may also be recovered, for 
example, palladium (T.sub.m =1,552.degree. C.) and platinum (T.sub.m 
=1,769.degree. C.). 
By way of example, a mixture of waste electronic components may contain 
silver (T.sub.m =961.degree. C.), gold (T.sub.m =1,063.degree. C.), and 
copper (T.sub.m =1,083.degree. C). To separate and recover these metals, 
the wastes are heated to a temperature T.sub.3, where 961.degree. 
C..ltoreq.T.sub.3 &lt;1,063.degree. C., and held at that temperature until 
the silver melts and can be removed from the remainder of the waste. The 
wastes are then heated to a temperature 1,063.degree. C..ltoreq.T.sub.3 
&lt;1,083.degree. C. to melt and recover the gold, then to a temperature of 
at least 1,083.degree. C. to melt and recover the copper. Then, the 
temperature is raised to the second temperature to melt and vitrify the 
waste residue. 
Alternatively, the wastes are heated to a sufficiently high temperature for 
substantially all potentially recoverable metals to separate out as a 
single metallic phase. Each species of metal may then by separated and 
recovered by any suitable chemical or physical technique. For example, 
wastes that contain silver, gold and copper may be heated to a temperature 
of at least 1,083.degree.0 C., and held at that temperature until the 
metals separate out from the remainder of the waste. 
7. Recover the waste glass product. 
The product is in the form of a chemically stable, homogeneous glassy 
matrix that encapsulates the remaining waste constituents, having a volume 
less than approximately 50% of the original waste volume. The composition 
of the product depends on the types of wastes being treated, process 
temperatures, and whether or not additional glass formers were added. The 
product is a stable, durable material that meets Environmental Protection 
Agency (EPA) leaching standards, thus, may safely be disposed of in an 
ordinary landfill. Alternatively, the product may be crushed or pulverized 
and used as a filler in construction materials such as brick, concrete, 
asphalt, tile, fencing and roofing materials, as a road underlay, and so 
forth. 
As noted above, maintaining the wastes at the first temperature T.sub.1 
(Step 2) ensures that organics are destroyed in a controlled manner. 
Combustion of organics before vitrification has the additional advantage 
of ensuring a more reliable waste glass composition. If the wastes are 
heated directly to the second temperature T.sub.2, the organics may 
decompose so rapidly and uncontrollably that the off-gas cannot be safely 
handled by the process off-gas system. Even if all the organics in the 
wastes are decomposed to ash and off-gas, some of the off-gas may be 
trapped in the melted waste in the form of bubbles. Depending on the types 
and amounts of organics originally present in the wastes, this could 
result in a porous product rather than a solid, homogeneous and uniform 
waste glass. If, on the other hand, organics are substantially combusted 
before melting the waste at the second temperature, the final product is 
more homogeneous, substantially nonporous, and satisfies EPA leaching 
standards. 
Referring now to FIG. 2, there is shown an apparatus for treating waste 
materials according to a preferred embodiment of the present invention. An 
apparatus 10 includes a microwave oven 12, an exhaust outlet 14, a 
condenser 16 (a liquid nitrogen cold finger or some other suitable type of 
condenser) to trap volatile organics and reduce particle emissions to the 
environment during operation of apparatus 10, and a vacuum pump 18. 
Microwave oven 12 is any suitable type of microwave oven having a 
sufficient power output to treat the desired quantities of wastes. Oven 12 
may be lined with a refractory material, such as refractory fiber board. 
Apparatus 10 may also include filters, scrubbers, and so forth for 
treating off-gas and particulates released by materials heated in oven 12, 
and devices for monitoring off-gas constituents (not shown). The method of 
the present invention may be implemented in apparatus 10, or indeed in any 
apparatus that has sufficient heating capability to treat the desired 
types and quantities of wastes. 
A quantity of wastes 20 is placed inside a suitable container 22, 
preferably a fused silica or alumina crucible. Container 22 is positioned 
inside a susceptor 24. Refractory bricks 26 may be placed around crucible 
24 to protect the microwave cavity of oven 12 from heat released during 
combustion of organics (Step 2). A thermocouple unit 28, for example, an 
Inconel.TM.-shielded K-type thermocouple, may be used to monitor the 
temperature of wastes 20 by means of a probe 30. Unit 28 may include a 
pyrometer or other suitable devices connected to the control system of 
oven 12 to control the temperature. Apparatus 10 may be placed inside a 
fume hood (not shown). 
In operation, wastes to be treated in apparatus 10 are sectioned, crushed 
and placed in container 22. Container 22 is placed inside susceptor 24 in 
oven 12, and wastes 20 are heated to a temperature T.sub.1 sufficient to 
combust organics. Wastes 20 are maintained at a temperature of 
approximately T.sub.1 until any organics in the wastes are substantially 
combusted. Off-gas constituents may be monitored continuously if desired. 
After the organics are reduced to ash, emissions are no longer observed 
and wastes 20 are heated to a second temperature T.sub.2 for 
vitrification. 
The method according to the present invention is illustrated in the 
following non-limiting examples: 
EXAMPLE 1 
A 100-gram circuit board sample was cut into pieces, pulverized, and placed 
into a fused silica crucible. The crucible was placed in an 800 W 
microwave oven lined with refractory fiber board. In addition, alumina 
refractory bricks were placed around the crucible to further protect the 
microwave cavity from the heat released during the ashing process. The 
sample was heated to a first temperature T.sub.1, and maintained at that 
temperature for 84.5 minutes to reduce the pulverized board to ash. The 
sample was then transferred to a 6,400 W microwave and heated in 
approximately 12 minutes to a second temperature T.sub.2, and maintained 
at that temperature for approximately 20 minutes. 
Several samples were treated, at temperatures T.sub.1 ranging from 
300.degree. C. to 800.degree. C. and T.sub.2 =1,400.+-.50.degree. C. The 
total weight loss due to volatilization of organics was approximately 56 
g; the total volume reduction was greater than 80%. 
EXAMPLE 2 
Non-reinforced circuit board samples were broken up and heated to a first 
temperature T.sub.1 as described in Example 1. After organics combustion 
was substantially complete, the remaining waste material was milled for 60 
minutes to break up the combusted mass. Borosilicate glass frit in an 
amount of 10 wt. % of the waste material was added, and the resulting 
mixture was milled for an additional 30 minutes. The temperature was 
raised to 1,400.+-.50.degree. C. and held for 25 minutes, at which time 
the mixture had melted but not completely vitrified. Additional frit (up 
to 50 wt. % of the original waste sample) was added incrementally, while 
the temperature was maintained at 1,400.+-.50.degree. C. The final product 
was a black, glassy mass. Upon removal from the crucible, it was observed 
that a large, magnetic metallic sphere had settled to the bottom of the 
crucible, indicating that the metals in the sample had separated from the 
remainder of the waste. Typical weight losses due to organics combustion 
were approximately 45%; total volume loss ranged between 52.5% and 81.5%. 
EXAMPLE 3 
Several commercially-available transistors were analyzed by scanning 
electron microscopy and energy dispersive spectroscopy (SEM/EDS) to 
determine the metal content (primarily tungsten, with small quantities of 
gold, copper and silica). The transistors were crushed and mixed with 
borosilicate glass frit in a ratio of 1:2 by weight. The mixture was 
placed in an alumina crucible, positioned inside a zirconia/beta silicon 
carbide susceptor, placed inside a 6,400 W microwave oven and processed as 
described above. Total processing time was approximately 15 minutes with a 
maximum temperature of 1,478.degree. C. 
The end product was a blue-green vitreous mass. Destruction of the 
ceramic/wire portion of the transistor was achieved, however, the 
processing temperature was not sufficiently high to melt the tungsten base 
plates, which remained substantially intact. 
EXAMPLE 4 
A sample consisting of connector pieces from a computer hard drive and 
crushed transistors was prepared. Organics were combusted in an 800 W 
microwave oven, then the remaining wastes were transferred to an alumina 
crucible and placed inside a zirconia/beta silicon carbide susceptor. 
This assembly was placed inside a 3,200 W microwave. An optical pyrometer 
was used to control the temperature of the wastes. After organics 
combustion, the waste was heated to a temperature range of 
1,100.degree.-1,150.degree. C. for approximately 15 minutes to melt any 
gold present (T.sub.m =1,063.degree. C). The temperature was then raised 
to approximately 1,400.+-.50.degree. C., held at that temperature until 
the waste residue vitrified and cooled. The resulting product was a black, 
glassy mass, with the tungsten base plates of the transistors 
substantially intact. 
Metals were recovered from the wastes at several stages of the process. 
Low-melting point metals (Sn, Al) melted and separated out from the wastes 
during organics combustion (ashing). The metallic spheres formed during 
ashing were removed using either magnetic or mechanical separation 
techniques. Several small metallic spheres were recovered from the melt, 
and the remaining separated metals were removed after the melted wastes 
had cooled. SEM/EDS analyses of the spheres confirmed that a wide variety 
of metals could be separated from the wastes, including Au, Ag, Sn, Pb, 
Fe, Al, Cu, Ti, Ni, Mn, Zn and Si. Because of the wide range of melting 
points of the metals and alloys used in manufacturing electronic 
components, metals may be recovered at several points in the process: 
after organics combustion (Step 2), during heating to the second 
temperature (Step 4), and after vitrification (Step 5). This feature 
allows considerable flexibility for treating different types of wastes. By 
way of example, low-melting-point metals may be removed after organics 
combustion (Step 2) and the remaining metals after cooling of the 
vitrified wastes (Step 5). Alternatively, at least some of the metals may 
be separated by melting point in Steps 2 and/or 4, reducing the need for 
reprocessing of the metal spheres/nodules obtained after completion of 
these steps to separate out the different elements. 
The wastes may be analyzed to determine whether additional glass formers 
are needed for vitrification. Some wastes, such as fiber-reinforced 
circuit board materials, electron tubes, and so forth, already contain 
sufficient glass formers for vitrification. Other types of wastes may need 
additional glass formers, either supplied in pure form or as a different 
type of waste material (for example, light bulbs and other waste glass, 
syringes, etc.). Depending on the composition of the wastes, addition of 
modifiers and intermediates may also be useful. 
Use of the method allows recovery of metals from heterogeneous wastes 
(i.e., wastes that contain a variety of constituents), while destroying 
potentially hazardous organics and reducing the overall waste volume by 
more than 50%. The waste residue is contained in a stable, nonporous 
glassy matrix that meets EPA leaching standards, thus, the final product 
may safely be disposed of in an ordinary landfill, or used as a filler in 
construction materials. The method may be carried in any type of apparatus 
that is capable of heating the wastes to the desired process temperatures 
T.sub.1 and T.sub.2, including apparatus such as that shown schematically 
in FIG. 2. Alternatively, organics combustion and vitrification may be 
carried out in different microwave heaters, or the wastes may be processed 
continuously rather than batch-wise. 
The above-described method may include additional process steps without 
departing from the spirit of the invention. For example, there are many 
well-known chemical compounds that attach themselves chemically to certain 
hazardous materials such as lead and arsenic. If immobilization in a 
stable, non-hazardous form (rather than recovery) of such materials is 
desired, these chemicals can be added to the wastes before vitrification 
so that they have an opportunity to immobilize the hazardous material even 
more effectively. 
It will be apparent to those skilled in the art that many changes and 
substitutions can be made to the preferred embodiment herein described 
without departing from the spirit and scope of the present invention as 
defined by the appended claims.