Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Reexamination Certificate
2000-01-03
2002-04-30
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
C266S201000, C266S236000, C266S216000, C266S218000, C075S416000
Reexamination Certificate
active
06379416
ABSTRACT:
TECHNICAL FIELD
Present invention relates to a method for melting an incineration residue containing salts such as an incineration residue of municipal solid waste or the like, and an apparatus therefor.
BACKGROUND ART
In disposal of incineration residues produced by incineration of municipal solid waste, industrial waste and the like, there is an obligation to make some heavy metals insoluble. There is also demand for decreasing the volume of the incineration residue with tightness of final disposal spots. Therefore, as processing for making heavy metals in the incineration residue insoluble, and decreasing the volume of the incineration residue, melting is carried out.
Various melting furnaces are used for melting incineration residues, which include a melting furnace in which a reducing atmosphere is maintained, for example, an electric resistance melting furnace for melting, as disclosed in Japanese Unexamined Patent Publication No. 7-225013. In melting using such a melting furnace, an incineration residue is put in the melting furnace in which a reducing atmosphere is maintained, and temporarily stayed in the melting furnace to be separated into molten slag mainly comprising oxides, and molten salts comprising salts such as chlorides and the like, so that the molten slag and the molten salts are separately discharged.
However, when an incineration residue containing salts is melted in the melting furnace in which a reducing atmosphere is maintained, the obtained slag contains a large amount of chlorine in spite of the separate discharges of the molten slag and the salts. The slag produced under such processing conditions contains several percentages of chlorine depending upon the composition thereof. Since the slag is in a state in which chloride is readily eluted, a large amount of chlorine is eluted in an elution test of the slag. It is thus predicted that the use of the slag as aggregate for civil engineering and construction causes various problems of metal corrosion, etc. Therefore, the slag is not used as the aggregate.
When the slag obtained by melting the incineration residue is used as aggregate for civil engineering and construction, there is now no standard for the chlorine content. However, the content of chlorine ions is 0.02% or less under JIS standards (R5210) for portland cement, and a value of less than 0.01% (100 mg/kg) is required in industry. This industrial requirement value corresponds to 10 mg/l in terms of the concentration of chlorine eluted in the elution test according to the method of deciding soil environment standards (Notification No. 46 of Environment Agency). Therefore, on the basis of the industrial requirement value of portland cement quality, the upper limit of the allowable concentration of eluted chlorine of slag is thought to be 10 mg/l.
According to the test results obtained by the inventors, as shown in
FIG. 6
, as the chlorine content increases, the amount of eluted chlorine of slag increases. Therefore, in order to keep the concentration of eluted chloride at the above value (10 kg/l) or less, the chlorine content of slag must be decreased to 1% or less. However, particularly, when an incineration residue containing salts is processed by the above-described method in which it is melted in a reducing atmosphere, and then temporarily stayed in the furnace, a large amount of chlorine is mixed in slag, causing difficulties in decreasing the chlorine content to 1% or less.
On the other hand, the molten slag in the melting furnace is present on a high-temperature molten slag layer mainly comprising oxides having melting points of 1300 to 1500° C., and the molten salt layer is consequently heated to a high temperature. When the molten slat layer is heated to a high temperature, low-boiling-point substances contained in the molten salt layer, such as alkali metal salts such as sodium chloride, potassium chloride, and the like, and chlorides of heavy metals such as zinc, lead, cadmium, and the like, are vaporized because of their relatively low boiling points.
At this time, the melt in the furnace is covered with the unmelted incineration residue which functions as a thermal insulating layer for preventing upward heat transfer from the high-temperature melt, and thus the temperature of the vapor phase in the furnace is very lower than the temperature of the melt. Therefore, the vaporized low-boiling-point substances are condensed and solidified by cooling in the vapor phase, and discharged as dust together with exhaust gases.
The above-mentioned vaporization of the low-boiling-point substances causes the occurrence of a large amount of dust mainly comprising chlorides of alkali metals, causing various troubles in the exhaust gas processing system. Namely, of the low-boiling-point substances, alkali metal salts have adhesion, and thus adhere to an exhaust gas duct to cause clogging, and promote clogging of a dust collector, causing the problem of deteriorating the processing capacity. This causes a trouble in the continuous operation of the melting furnace.
Furthermore, when the molten salts stayed in the furnace are discharged, the molten salts on the molten slag layer cannot be completely discharged in order to prevent contamination with the molten slag. Therefore, the molten salt layer is present to some extent in the furnace. As a result, various problems occur due to the presence of the molten salt layer.
First, the production of the molten salt layer in the furnace causes erosion of the furnace wall refractory of a portion in contact with the molten salts to increase the repair cost of the furnace. Of the above-described conventional techniques, the method using an electric resistance melting furnace causes a short circuit phenomenon that a current flowing between electrodes is concentrated in the molten salt layer with a low electric resistance because the electric resistance value of the molten salts is lower than the value of the molten slag. As a result, the temperature of the molten slag layer cannot be maintained at a predetermined value, causing difficulties in discharging the molten slag.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide a method for melting an incineration residue containing salts, which is capable of suppressing elution of chlorine, and obtaining slag for aggregate containing no metal.
It is another object of the present invention to provide a method of melting an incineration residue and an apparatus for carrying out the method, which are capable of suppressing the production of alkali metal dust even in melting an incineration residue containing salts, thereby causing no trouble in an operation of an exhaust gas processing apparatus.
It is a further object of the present invention to provide a method of melting an incineration residue and an apparatus for carrying out the method, which produce substantially no molten salt layer in a melting furnace even in melting an incineration residue containing salts.
In order to achieve the objects, first, the present invention provides a method for melting an incineration residue containing salts, comprising the steps:
adding a component adjustor to the incineration residue containing salts to adjust a component ratio determined by the following equation in the range of 0.7 to 2.0;
Component ratio (molar ratio)=(Ca+Mg)/(Si+Al) charging the incineration residue having the adjusted component ratio to a melting furnace maintained in a reducing atmosphere, and melting the incineration residue to form a melt;
staying the melt in the melting furnace to separate the melt into a molten slag layer, a molten salt layer, and a molten metal layer;
fractionating the molten slag and discharging it; and
rapidly cooling the discharged molten slag.
Secondarily, the present invention provides a method for melting an incineration residue containing salts, comprising the steps of:
charging an incineration residue containing salts to a melting furnace containing a melt;
melting the incineration residue; and
maintaining the temperature of the
Matsui Satoshi
Nakahara Keisuke
Shinagawa Takuya
Banks Tima McGuthry
Frishauf, Holtz Goodman, Langer & Chick, P.C.
King Roy
NKK Corporation
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