Process and apparatus for treatment of excavated landfill material in a plasma fired cupola

Excavatged landfill material is treated in a plasma fired cupola in a process wherein hazardous material such as PCB's are volatilized and consumed in an afterburner above the cupola and hazardous materials containing heavy metals are fixed in vitreous material made molten within the cupola and resulting in a non-leachable solid product.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention relates to treating materials so that hazardous and toxic 
materials contained in them are either destroyed or made safe. 
Landfills have in the past been collection sites for a wide variety of 
discarded objects and materials. Some contain substantial levels of toxic 
and hazardous chemicals whose clean-up is being required by government 
regulation. Of concern are, for example, materials containing heavy metals 
such as lead, nickel and chromium or toxic halogen containing chemicals 
such as polychlorinated biphenyls (PCB's). 
For convenience, excavated landfill material will be referred to as ELM. 
Incineration processes have been used or proposed to treat the ELM by 
burning it along with other combustible material, typically municipal 
garbage. Such incineration processes are feasible but may involve large 
flow rates of potentially harmful materials and the incinerator ash that 
results as an end product may not be environmentally benign; potential for 
leaching of toxic heavy metals may still exist. 
One type of system for treating ELM is a pyrolyzer such as that disclosed 
in copending application Ser. No. 027,775, filed Mar. 18, 1987 by Levin 
and assigned to the present assignee. Such a pyrolyzer produces vitreous, 
i.e., glass like, material by electrical heating in a chamber operated in 
a substantially closed or pyrolytic manner. Such a system incurs high 
energy costs which it is desirable to minimize It is however successful in 
producing vitrified material in which the heavy metals can be trapped in 
the glass matrix of the slag. 
A plasma fired cupola is a known apparatus previously disclosed for such 
purposes as metal recovery as in Fey et al. U.S. Pat. No. 4,530,101, July 
16, 1985; Dighe et al., "Plasma Fired Cupola and Innovation In Iron 
Foundry Melting", AFS transaction paper, 1986; Dighe et al. U.S. Pat. No. 
4,761,793, Aug. 2, 1988; U.S. Pat. No. 4,780,132, Oct. 25, 1988; and U.S. 
Pat. No. 4,769,065, Sept. 6, 1988; and copending applications Ser. No. 
047,808, filed May 8, 1987, now U.S. Pat. No. 4,828,607 issued May 9, 
1989, and Ser. No. 212,851, filed June 29, 1988, now U.S. Pat. No. 
4,853,033 issued Aug. 1, 1989 by Dighe et al. and Ser. No. 226,712, filed 
Aug. 1, 1988, now U.S. Pat. No. 4,889,556 issued Dec. 26, 1989 by Dighe, 
all assigned to the present assignee. The foregoing descriptions are 
incorporated herein by reference for general information on the structure 
and operation of plasma fired cupolas. 
Cupolas, not plasma-fired, are presently known for metal and mineral 
melting that utilize a shaft with coke and blown air, sometimes enriched 
with oxygen, through tuyeres near the bottom. These units require such 
amounts of air that fine particles of charged material and even, in some 
instances, vitrified material, may be blown upward. Additionally, they 
normally achieve maximum temperatures of only about 3000.degree. F. 
Among the purposes of the present invention are to provide an effective and 
economical treatment for excavated landfill material (ELM) containing 
toxic and hazardous materials. In the process of the present invention, a 
plasma fired cupola is used for treatment of ELM. The cupola is a vertical 
shaft with a charge door proximate the top thereof. A plasma torch is 
provided and located in a tuyere proximate the bottom of the cupola and 
the plasma torch has a feed nozzle. The plasma torch is electrically 
energized and produces a plasma from air. Air is fed through the nozzle 
and is heated to a high temperature by the plasma torch and supplied into 
the cupola. 
In starting operation of the system, the cupola is partly filled with a 
carbonaceous fuel, such as coke or a mixture of coke and coal, which is 
ignited. When adequate operating temperature is reached, the charge 
material is feed through the charge door. The ELM is normally supplied 
along with reactive agents such as additional coke and a fluxing agent, 
such as limestone. 
Conditions are maintained for melting the ELM in the cupola to form a 
vitreous slag. In addition, metal supplied with the ELM, such as iron and 
copper, becomes molten and will separate from the slag gravimetrically. 
The coke reduces oxides of such metals to yield the metal itself. The 
cupola is tapped to take out the vitreous slag and the molten metal. 
The off gases from the cupola are allowed to rise to an afterburner located 
above the cupola for destruction of any toxic and hazardous materials 
contained in the off gases. The vitreous slag from the cupola is allowed 
to cool and produce non-hazardous solid material from which heavy metals 
such as chromium, lead and nickel, occurring as oxides, are substantially 
non-leachable. 
In accordance with the present invention, preferably about 6% of a charge 
is fuel. The fuel is sufficient to supply all of the combustion gases for 
the afterburner in which PCB's or the like are burned. 
A layering technique is employed for the material supplied into the cupola 
through the charge door. That is, a distinct layer of fuel is provided 
between layers of limestone, ELM or mixtures thereof. The layering helps 
reduce back pressure in the system. The gas flow rate is reduced to a 
lower level than that employed for foundry melting, such as preferably 
about 0.6 cubic feet per minute per square inch of cupola cross-section as 
compared to about 0.9 cubic feet per minute. Further, the amount of 
limestone supplied is adjustable for optimizing the basicity of the slag 
to achieve a desired flow rate. 
These and other aspects of the invention will become better understood from 
the description hereinafter

PREFERRED EMBODIMENTS 
FIG. 1 illustrates a plasma fired cupola 10 for use in the practice of the 
present invention which bears many common aspects to the apparatus 
employed for the iron foundry melting and recycling of steel belted tires 
described in the above referred to sources. The cupola itself is a 
vertical shaft 12 with a refractory lining 13. In the vertically upward 
part of the cupola 12 is a charge door 14 for charging excavated landfill 
material (ELM) as well as reactive agents such as a carbonaceous fuel 
which may be coke or a mixture of coke and coal, normally the latter, and 
a fluxing agent such as limestone. Air also enters through the charge door 
14. 
Proximate the bottom of the cupola 12 are disposed plasma torches 16 each 
within a tuyere 17 extending into the cupola and having a shroud nozzle 
18. Each torch 16 is supplied with a gas such as air through line 16a that 
is introduced in an arc between energized, spaced, electrodes to be 
ionized and form a plasma. Blast air tangentially enters just in front of 
the torch 16 through line 18a and nozzle 18. The blast air is added to the 
plasma and is heated and then enters into the cupola 12 in its heated 
state. The air entering through line 18a may be at ambient temperature or 
preheated up to about 1200.degree. F. 
Above the cupola 12 is an afterburner chamber 20 into which the off gases 
from the cupola 12 rise. The afterburner is provided with a stack igniter 
22 but it is not necessary to supply fuel in addition to the off gases 
themselves. 
The ELM can be fed directly into the charge door 14 such as by using a skip 
bucket carried by a skip hoist or by a feeder belt. The ELM does not have 
to be preprocessed to any appreciable degree in terms of sizing or drying 
and is generally used on an as received basis. The cupola shaft diameter 
is sized to accept the biggest size of ELM which might, for example, be an 
engine block or refrigerator. In other respects, it is of course suitable 
if desired to subject the ELM to a pretreatment through a shredder or the 
like to reduce the size of individual pieces; preferably the cupola is 
designed so that this is not necessary. An economical mix of coke and coal 
is also fed into the skip bucket which is then elevated and dumped into 
the charge door by means of a skip hoist. 
In general, coke is the preferred fuel and reducing agent and will be 
referred to herein. As opposed to coal, it provides more firm support for 
the other solid materials added in a charge and ensures adequate gas flow. 
For economy, varying amounts of coal may be mixed with the coke. In 
general, the fuel should be at least about 25% coke. 
In start-up, the cupola 12 filled with coke up to several inches above 
tuyere 17. The coke bed 30 is ignited by the plasma heated air which is 
fed at the bottom of the cupola through the tuyere 17. When the coke bed 
is burning and the cupola refractory is sufficiently heated, the charge 
material consisting of ELM, coke (which may include some coal), and the 
fluxing agent is fed through the charge door. This system start-up 
normally takes only about 2 to 3 hours. The coke, besides providing the 
energy of combustion, also provides a porous matrix inside the cupola 
shaft so that the ELM does not form a mat and cause plugging. Also both 
coal and coke provide carbon monoxide to the off gas which upon ignition 
in the afterburner completely destroys the PCB's. No supplemental fuel is 
required to provide heat in the afterburner 20. However, if a fuel such as 
natural gas is readily available and economical it may be burned to supply 
heat and allow a corresponding reduction in the amount of coke in the 
cupola. 
The ELM, as it travels down the cupola shaft 12, is first heated by the hot 
gases rising from the melt zone at the bottom of the cupola. This 
countercurrent heat exchange is one of the primary reasons for the energy 
efficiency of the plasma fired cupola. The PCB's are evaporated from the 
ELM and exit the cupola along with the other off gases, typically CO, 
CO.sub.2, N.sub.2. 
The top gases rise to the afterburner unit 20 where they are mixed with the 
combustion air entering through the charge door 14 and ignited by the 
stack igniter 22 to produce sufficient temperature for destruction of 
PCB's. The standard requirement is that such materials be subjected to a 
temperature of at least about 2200.degree. F. for a time of at least about 
2 seconds and these conditions can be adequately, readily met in the 
present process. 
Upon traveling down the cupola shaft 12, the ELM enters the melt zone (in 
coke bed 30 proximate to and above tuyere 17) where the temperatures are 
in the range of from about 3000.degree. F to about 4500.degree. F. All 
constituents of the ELM melt at this temperature and form a vitreous slag 
plus the metallic portion of the ELM that also melts. 
The temperature and chemistry of the melt zone are controlled (by air and 
coke supplied) to achieve desired performance. In general, most oxides, 
e.g., iron and copper, are reduced to provide the metal itself in a molten 
state. The heavy metal oxides of metals such as chromium and nickel may 
not be reduced but instead are dissolved in the slag. Some metals, such as 
zinc and lead, are likely to have their oxides reduced and the metals 
vaporized. Such latter metals are reoxidized in the afterburner 20 and 
will be collected from the afterburner discharge 
The plasma torch power is preferably adjusted so that silica (SiO.sub.2), 
contained within the ELM is reduced to produce silicon. The silicon 
dissolves in the molten metal and forms, with molten iron, a useful and 
valuable ferrous alloy that can be sold on the foundry market. The 
vitrified ELM and the metal is continuously tapped through a spout 24 at 
the bottom of the cupola using a skimmer 24a and dam 24b arrangement. The 
molten stream from the spout 24 is about 2500.degree.-2800.degree. F. The 
slag is separated from the metal by this arrangement to produce blocks of 
slag and metal ingots. 
FIG. 2 shows an enlarged view of a spout 24. Molten slag 40 and molten 
metal 42 collect at the bottom of the cupola shaft 12. On the outside of 
the cupola the lighter slag 40' is confined by a skimmer 24a and is tapped 
off. 
The heavier metal flows under the skimmer 24a and over the dam 24b into 
whatever mold collects it. The instances where fly ash or the like is 
injected into the slag it would be introduced into the slag 40'. 
The gases exiting the afterburner 20 may be exhausted to the atmosphere, 
preferably only after going through a scrubber or other air pollution 
control equipment with collection of flyash. 
According to an optional form of the invention, the hot combustion gases 
exiting the afterburner 20 are sent to a recuperator to preheat the blast 
air and also the combustion air of the afterburner. According to another 
optional form of the invention, the hot gases from the afterburner are 
sent to a boiler to generate steam which may be used for process 
requirements or to generate power. 
Still a further variation is to supply fine waste material, such as fly ash 
from the afterburner 20 or elsewhere, such as utility boilers and 
incinerators, through the nozzle 18 installed in the tuyere region at the 
base of the cupola. Bin 18b and line 18c is shown for this purpose. This 
material is simultaneously vitrified along with the ELM for convenient 
disposal. In addition or alternatively, flyash containing, for example, 
oxides of lead or zinc, may be fed directly into the molten slag at the 
bottom of the cupola 12 proximate the spout 24 or in the spout itself 
before the material solidifies. This helps to ensure such oxides are 
dissolved in the slag for safe disposal. 
While using the apparatus for the process for treatment of ELM, it is also 
possible to add other waste material, both combustible and non-combustible 
through the charge door. The plasma fired cupola is a flexible apparatus 
suitable for use with a wide range of feed compositions. 
The fuel supplied with a charge of ELM provides carbon to produce 
sufficient amounts of carbon monoxide to serve as the fuel in the 
afterburner 20 for assured destruction of PCB's and the like. For this 
purpose it is preferred that the carbon fuel (coke) in the charge make up 
about 6% or more, by weight, of the process material supplied into the 
cupola. 
It has been found desirable for the ELM, the coke or other carbonaceous 
fuel, and the limestone or other fluxing agent to be layered rather than 
mixed in the cupola in order to reduce back pressure. For example, after 
initially starting operation with coke 30 supplied in the cupola, a first 
layer of ELM 32 can be fed into the cupola, followed by a coke layer 33, 
then a limestone layer 34 followed by a coke layer 35. Then the sequence 
may be repeated up to near the charge door 14. A coke layer separates 
layers of the ELM and limestone materials, which may, if desired, be mixed 
together in a single layer. 
For improved operation of the plasma fired cupola for treating ELM the coke 
(or carbon fuel) bed height is well regulated. The initial coke charge 30 
fills the cupola up to a level above the tuyere 17 (essentially a hollow 
tube) through which the blast air heated by the torch 16 enters the 
cupola. A level of about five inches to about 10 inches above the top of 
the tuyere 17 is normally preferred to give a desirable ratio of CO to 
CO.sub.2. 
Greater depths, in the range up to about three feet above the tuyere 17, 
for charge 30 are suitable if the intent is to produce grater quantities 
of CO for use as fuel in the afterburner 20. The process that occurs is 
that the blast air enters the cupola and reacts with the coke in an 
exothermic reaction to form largely CO.sub.2. The CP.sub.2, if exposed to 
reducing effects of more coke in a relatively greater depth of coke, will 
react to form CO in an endothermic reaction: CO.sub.2 .fwdarw.2CO, called 
the Boudard reaction. Therefore, the available choices involve operating 
in a range from maximizing the heat produced in the coke bed by using a 
coke charge sufficient for high CO.sub.2 off gas production to maximizing 
the CO available in the afterburner by use of increased coke. 
The initial coke bed height is maintained, with normal variations up and 
down, around that level of the charge material. 
It is suitable and preferred to employ a range of from about 6% to about 
25% of carbon bearing fuel and about 10% to about 45% of limestone 
relative to the total charge of supplied material in the cupola. The coke 
ratio blast rate and torch power is adjusted based on melt rate 
requirement such that the CO/CO.sub.2 molar ratio in the top gas is in the 
range of 0.2 to 3.0. 
The amount of limestone is preferably adjusted for optimum basicity to 
yield good flow characteristics in the slag at the moderate temperatures 
which are easily attained. Basicity is defined as the ratio: 
##EQU1## 
At a value below about 0.3 the slag is highly viscous and flows very 
slowly. Above about 0.7, the slag very friable so it serves less well a an 
encapsulator of hazardous materials that may be contained in it. 
Therefore, it is preferred to have a basicity within the range from about 
0.3 to about 0.7. 
The gas flow rate (through 18) is reduced to an even lower level than the 
case of foundry melting for awarding slag elutriation in the melt zone. 
The flow rate in the cupola is about 0.5 cubic feet per minute (per sq. 
in. of cupola cross-section) or less and preferably about 0.2 to about 
1.0. 
In FIG. 3, the system of FIG. 1 is shown with the addition of a recycle 
loop 50 that includes a draft fan 52 for recycling some of the off gases 
from the cupola 12 back to the torch nozzle 18. The recycle loop 50 also 
includes a trap 54 for particles (fly ash) that can be reinjected through 
bin 18b or can be injected into the slag, in addition to other forms of 
disposition. 
The recycle loop 50 is an option that is more beneficial if the ELM 
contains relatively larger quantities of oxides of valuable metals desired 
to be recovered. Oxides in the charge tend to be reduced by the carbon 
fuel but if excess air is present the reaction will tend to reverse and 
reform the metal oxides. To create a more strongly reducing atmosphere one 
may draw off some of CO and N and have it re-enter the nozzle to lower the 
relative amount of oxygen. Typically the gases exiting the cupola are at 
about 800.degree. F. and re-enter at about 200.degree. F. The recycled off 
gases help promote the reduction reaction while permitting use of less 
coke than may be necessary if it alone were counted on to take care of the 
oxygen in the blast air. 
FIG. 4 shows a further variation referred to as a plasma-fired cupola with 
a "below charge take-off". As compared with FIG. 1, the top of the cupola 
12 is changed so the afterburner 20 receives off gases from a point below 
the charge level. A fan and air pollution control equipment, not shown, 
would be provided after the afterburner. 
In FIG. 4, a charge door 14' is provided at the top that minimizes air 
entry. The door 14' is supported by a hoist cable 60 that in the closed 
position holds the door 14' against the door frame 62. A charge 64 of 
material is applied on the upper surface of the door. When the door is 
lowered, the material enters the cupola and becomes part of charge 64' 
which extends up past the gas take-off. The door 14' can be promptly 
closed so a highly reducing atmosphere is maintained. 
The table below gives examples of representative ELM compositions and other 
conditions for their treatment in the plasma fired cupola substantially in 
accordance with FIG. 1, for example. Example I has been actually performed 
and verified to process the material into a stream of lag and a stream of 
metal, essentially a ferrous alloy containing about 4% silicon. The off 
gases produced afterburner temperatures in excess of 2200 .degree. F. 
CO/CO.sub.2 
______________________________________ 
EXAMPLES 
No. Item I II III 
______________________________________ 
1. ELM Charge Mix (weight %) 
Glass % 31.3 20 20 
Steel % 26.3 24 20 
Bricks % 6.3 7 7 
Concrete % 6.3 7 7 
Rocks % 6.3 7 7 
Ash % 6.3 7 7 
Clay % 6.3 7 7 
Wood % 6.1 5 4 
Industrial Elec. 2.9 10 15 
Hardware % 
Copper % 1.0 1 1 
Tires % 0.9 5 5 
2. Carbonaceous Fuel (% of ELM 
Charge) 
Coke % 8.0 4 3 
Coal % -- 4 3 
3. Limestone (% of ELM Charge) 
37.0 30 20 
4a. Blast Rate Scf/min/t/hr 
275 300 300 
4b. Blast Rate Scf/min/in.sup.2 cupola 
0.33 0.37 0.37 
5. Torch Power Kw/ton/hr 
620 600 550 
6. Co/CO2 Ratio 1.0 1.0 0.7 
______________________________________ 
Various other changes may be made consistent wit the general teachings 
hereof.