Melting furnace and process for the inertization of hazardous substances by vitrification

A tank furnace for the inertization of non-flammable batch, which contains hazardous substances, metals and less than 10% by weight of carbon, by vitrification with glass forming aggregates by producing a glassy melt. The furnace has a batch charging device and a tank, on the rim of which a furnace crown rests, electrodes for heating the melt, a supply of oxidizing gases to the melt and, as an outlet for the melt, an overflow channel which can be heated. Nozzles are installed in the bottom of the tank to introduce oxidizing gases in order to reduce the eluate values, and the electrodes are immersed in the melt from above. It is preferable to install the nozzles at sloping sidewalls of the tank which slope upwardly and outwardly.

BACKGROUND OF THE INVENTION 
The invention concerns a melting furnace for the inertization of 
non-flammable hazardous charging material containing metals and less than 
10% by weight of carbon, by vitrification with glass forming aggregates by 
producing a glassy melt, with a charging device and a tank on the rim of 
which rests a furnace crown, with electrodes to heat the melt, with the 
means to supply oxidizing gases to the melt, and with a heatable overflow 
channel for draining the melt. 
Mayer-Schwinning, et al ("Vitrification process for the inertization of 
residual products during noxious gas removal in thermal waste disposal 
units", VGB Kraftwerkstechnik 70 (1990), issue 4, pages 332 to 336) and 
U.S. Pat. No. 5,035,735; 5,032,161 and 4,944,785 teach the application of 
tank melting furnaces for vitrification purposes. However, these furnaces 
do not possess a means for introducing oxidizing gases into the glass 
melt. 
The vitrification of environmentally hazardous waste materials has proved 
to be an extremely safe conservation method for such waste materials, but 
certain conditions must be maintained during the process, so that the long 
term success of the vitrification is not compromised. A possible weakness 
of vitrification is the release of soluble components by washing out with 
water, a process also described as elution. The degree of elution depends 
on, amongst other factors, the concentration of the soluble materials and 
their intrinsic solubility, but also on the specific surface of the glass 
mass (broken or ground glass) and on the atomic structure of the glass 
(crystalline, amorphous). 
The above-mentioned paper by Mayer-Schwinning, et al describes several 
elution analysis and demonstrates that, on the whole, the concentrations 
of contaminants in the eluates conform with the legal standards and 
regulations. 
Contaminants of the type mentioned above often contain more or less carbon, 
whereby the carbon content is usually between 3 and 4 weight percent, but 
on occasions can be as much as 10 weight percent. This carbon is formed 
when power stations and refuse incineration units operate with under 
stoichiometric combustion, in order to keep the NO.sub.x content as low as 
possible. This leads to the formation of carbon in the form of very fine 
particles. In addition, carbon dust, produced by grinding active carbon 
contaminated with other environmental poisons, including heavy metals and 
heavy metal compounds, is also occasionally blown into such units. The 
heavy metals producing the highest level of contamination are copper, 
zinc, iron, nickel and lead. 
However, the presence of carbon in the melt leads to reduction processes 
and is counter productive, as it leads to the separation of the metals, 
part of which at least remain in colloidal solution in the melt. Such 
metallic constituents in the melt can be washed out, i.e., eluted, when 
the glass is deposited at a land fill site, and are therefore a risk to 
the environment. This behavior is examined in the elution analysis 
mentioned above, and such tests have shown that the elution values for 
heavy metals increase following vitrification carried out under reducing 
conditions. 
U.S. Pat. No. 3,592,151 describes a refuse incinerator in the form of a 
shaft, in which glass is placed in the bottom, heated by immersed 
electrodes and, if necessary, by fossil fuel burners, and maintained in a 
molten state. Refuse, with a random composition, is burned above the 
molten glass, which acts as a replacement for a conventional grate. In 
this process non-volatile and partially dangerous combustion products, 
with the exception of metals, are absorbed by the glass and combine with 
this to form a slag. Nozzles are used to introduce at least a part of the 
combustion air through the slag from below, and so this air is preheated. 
However as large quantities of combustion air are required, equivalent 
large amounts of heat are removed from the slag. In this process the 
amount of slag is increased by broken glass from the refuse, so that the 
additional glass and molten metal must be drained periodically. A 
reproducible process with controllable low elution values is not 
mentioned. 
The present invention does not concern such a refuse incinerator, but 
concerns a melting furnace for the disposal of charging material which 
contains at the most 10 weight percent of carbon, and therefore, in 
itself, does not constitute a combustible material. 
Patent application WO 93/02974 describes a melting furnace of the type 
described initially, in which oxygen is introduced into the melt, and in 
which contaminated waste containing organic and/or carbon compounds can be 
oxidized and vitrified. A charging device for the charging material is 
installed centrally, i.e., in the furnace axis. Plate electrodes are 
installed on opposite sides of the furnace below the glass surface. At a 
distance above the furnace bottom there is at least one horizontal, 
perforated inconel pipe, which is used to produce a curtain of gas bubbles 
between the electrodes in order to increase the specific electrical 
resistance of the melt. The gas bubbles also produce currents in the melt 
and distribute the charged material, while heating this simultaneously 
from below, in order to achieve fast melting at relatively low 
temperatures. However, the perforated pipes cannot be cooled sufficiently, 
even by the oxygenated gas which flows through, as this would cause 
blockage of the holes. Therefore, the pipes are subjected to the 
aggressive effect of the glass and the oxygen at high temperatures, which 
results in a drastic reduction in the lifetime of the pipes, especially 
when the amount of oxygen is high. Furthermore, as a result of the 
relatively high location of the perforated pipes, the lower part of the 
melt, in which metallic components collect, is not sufficiently included 
in the circulation and oxidation of the melt, and contains elutable 
components. 
SUMMARY OF THE INVENTION 
It is the object of the invention to provide a melting furnace of the type 
described initially, which can also be operated at high melting 
temperatures with gases with a high oxygen content and which reduces the 
formation of eluates from non-oxidized components of the charging 
material. 
This object is achieved according to the invention with the melting furnace 
described initially, in that nozzles for the introduction of the oxidizing 
gases are installed in the bottom of the furnace, and electrodes are 
immersed in the melt from above. 
The oxidizing operation, which starts at the bottom, reduces the amount of 
metal entering a colloidal solution in the melt. Furthermore, the 
ascending current of gas bubbles improves the homogenization of the 
complete melt, so that high temperatures can be achieved in an overflow 
channel for the melt, even when the bath is deep. The ascending gas 
bubbles produce circulatory movements within the complete melt. 
Comparative eluate analyses which have been carried out have shown that 
with furnaces according to the invention, fewer heavy metals and heavy 
metal compounds can be washed out of the end product. 
The installation of nozzles in the furnace bottom means that the melt can 
be kept at a significantly higher temperature level than in 
state-of-the-art furnaces, even when oxygen is introduced. 
Further advantageous embodiments of the invention are obtained when: 
the nozzles are installed in that part of the inside surface of the furnace 
bottom which rises from the inside to the outside, as this results in 
favorable currents in the melt, 
the nozzles are distributed around a closable drain installed in the axis 
of the furnace as this produces a toroidal current, and 
if the charging unit installed above the furnace crown is designed so that 
the charging material is added to the melt above the nozzles at a minimum 
of two locations off center, as this results in a faster distribution of 
the raw material on the surface of the melt.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1 the tank 1 has an internal surface 2, which comprises 
two truncated cones 3 and 4 which abut one another, and which have a 
common vertical axis A--A. A discharge opening 5 is provided, which is 
concentric with the above mentioned axis A--A, and through which any metal 
which collects can be removed. The tank 1 has a rim 6, running round the 
top, upon which a furnace crown 7 rests. A number of electrodes 8, of 
which only one is depicted in the figure, are distributed equidistantly 
around the circumference of the furnace in the area of the joint between 
the rim 6 and the furnace crown 7, whereby said electrodes are immersed 
from above into the melt 9, which extends upwards as far as a level 10 
determined in the furnace design. 
A charging unit 11, which is only shown schematically in the figure, and 
which is used to feed the charging material in the tank 1, is situated 
above the furnace crown 7, whereby the charging material comprises the 
hazardous substances for disposal. If necessary, glass forming aggregates 
can be added in special cases. No combustion process takes place in the 
charging unit 11. 
Two nozzles 12 are installed in the bottom of the tank 1, which are 
connected to a supply of oxidizing gas, not shown, and from which a stream 
of ascending gas bubbles 13 emanate. The design of the charging unit 11 
and the distribution of the nozzles 12 are chosen such that the reactive 
gas bubbles 13 reach the surface of the melt underneath the charging 
material 14, which floats on the melt. In this way, which is not shown in 
detail in the figure, the piles of charging material 14 are pushed apart, 
so that in reality the complete surface of the melt 9 is covered with 
charging material. 
The nozzles 12 are also referred to as bubblers. Although only two such 
bubblers are shown in the figure, it is of course possible to install 
numerous such bubblers, distributed around the tank 1. The bubblers 
produce bubbles, which ascend and cause colder melt from the bottom of the 
tank 1 to be transported to the top and into the sphere of influence of 
the electrodes, before returning to the bottom of the furnace. This 
results not only in a homogenization of the temperature distribution, but 
also in a more intensive heat transfer from the melt 9 to the charging 
material 14. 
A discharge device 15 is attached to the side of the tank 1, such that the 
main axis of the discharge device 15 is aligned with a radius of the tank 
1. The discharge device 15 includes a horizontal channel 16, the lowest 
point of the opening 17 of which lies below a horizontal level which is 
defined by the bottom ends of the electrodes 8. A vertical channel 18 
leads from the overflow channel 16 to a horizontal channel section 19, at 
the end of which a weir 20 is provided, the upper surface of which defines 
the fluid level of the melt 9. The active end of a further heating 
electrode 21 is provided in the vertical channel 18. A heating chamber 22, 
which is mostly open at the bottom, is provided above the vertical channel 
18, the horizontal channel section 19 and the weir 20, and in which 
heating chamber several radiation heating elements 23 in the form of 
horizontal rods are installed. Further radiation heating elements 25, with 
the same form of horizontal rods already described, are also installed on 
the outside of the shaft 24 which leads from the weir 20. 
The ascending gas bubbles 13 ensure that a sufficiently high temperature is 
always maintained in the melt 9 in front of the opening 17, so that a 
continuous passage of material is assured in accordance with the amount of 
charging material added. It can be stated that typical quantities for each 
nozzle 12 lie in the region of approximately 0.5 to 0.6 normal cubic 
meters of air per hour. Such extremely low volumes of air are not 
sufficient to support the combustion of larger amounts of refuse; while 
the introduction of pure oxygen is not permissible for refuse 
incineration, as the refuse would then burn explosively. 
As is apparent from the foregoing specification, the invention is 
susceptible of being embodied with various alterations and modifications 
which may differ particularly from those that have been described in the 
preceding specification and description. It should be understood that we 
wish to embody within the scope of the patent warranted hereon all such 
modifications as reasonably and properly come within the scope of our 
contribution to the art.