Metallurgical apparatus – Means for melting or vaporizing metal or treating liquefied... – By separating metal in a molten mass from undesired...
Reexamination Certificate
1999-08-19
2001-10-09
Kastler, Scott (Department: 1742)
Metallurgical apparatus
Means for melting or vaporizing metal or treating liquefied...
By separating metal in a molten mass from undesired...
C266S227000, C266S200000
Reexamination Certificate
active
06299827
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for vitrifying heavy-metal-containing residues having a chlorine content above 10% by mass from flue gas cleaning, in particular filter dust and residues from flue gas scrubbing. In the process, the residues are melted in a melting furnace and are then discharged, with fluxes being able to be added to condition the melt. The invention also relates to a melting furnace for carrying out the process. A process of this generic type is disclosed by DE 43 40 754 A1, for example, and a Deglor furnace is described, for example, in EP 0 633 411 A1.
2. Discussion of Background
It is known to treat heavy-metal-containing residues, such as filter ash from the incineration of refuse, by melt processes. At temperatures of approximately from 1300 to 1400° C., these residues are generally melted without fluxes in an electrical melting furnace, discharged from the furnace and then cooled, so that a vitreous residue is formed which can be landfilled without problem.
In the Deglor process, heating above the melt as well ensures that, on account of sufficiently high temperatures in the gas space, a considerable proportion of the heavy metals is evaporated. The exhaust gas taken off from the furnace by a fan is cooled and passed through a bag filter, in which the heavy metals which have condensed out are collected as condensate. This condensate can be treated in a metal works to recover the heavy metals. The molten residues are discharged from the furnace via a gas-tight siphon and then cooled, in which case a vitreous product is formed. The heavy metal evaporation contributes to the fact that the quality of the glass product permits reusability. This makes up a considerable advantage of the Deglor process in comparison with other known melting processes in which, because of the lack of heating in the upper furnace, there is no evaporation of the heavy metals.
The Deglor process has proved most useful in the vitrification of, residues having a comparatively low content of metal chlorides, metal sulfates and metal sulfites, as are typically present in filter dust.
However, residues also arise in which the filter ash is admixed with the flue gas cleaning product which has a high chlorine content. Thus, to remove sulfur dioxide and hydrochloric acid, the exhaust gases are subjected to a lime scrubbing, calcium compounds, in particular CaSO
3
, CaSO
4
and CaCl
2
, arising as byproducts which are to be melted and vitrified together with the filter ash. Typically, this product comprises from 10 to 20% by mass of chlorine, predominantly in the form of CaCl
2
. This compound has a boiling point (>1600° C.) far above the customary operating temperatures. This means that these chlorides cannot be transferred to the vapor phase effectively enough. At the same time, only a small portion of chlorine can be incorporated into the glass.
In order to be able to melt this residue with a high salt content likewise in an environmentally compatible manner, the applicant has suggested a process in which an alkaline flux in the form of an oxide, hydroxide or carbonate is added to the residues, after mixing the stoichiometric ratio between the sum of the alkalines and the chlorine being greater than 0.75, preferably greater than 1 (DE 196 03 365.9). The admixture of suitable alkali-metal-containing additives and the assurance of a sufficiently high exhaust gas volume significantly increases the evaporation rate of the chlorides.
Since the chlorides predominantly evaporate as NaCl, disadvantageously, a considerably greater amount of condensate is produced in which the heavy metals are then present in a form so dilute that treatment is no longer worthwhile.
A further disadvantage is that on account of the additives additionally required, the additional rate of exhaust gas and the greater quantity of material to be evaporated off, the energy requirement of the Deglor unit increases.
The two last-mentioned disadvantages are also present if, instead of lime, alkali-metal-containing reagents, such as NaHCO
3
, for example, are used for the flue gas cleaning, as a result of which the addition of alkali-metal-containing fluxes to the melting furnace is no longer necessary to increase the evaporation rate.
SUMMARY OF THE INVENTION
The invention attempts to avoid all these disadvantages. Accordingly, one object of the invention is to provide a process and an apparatus for vitrifying heavy-metal-containing residues having a high chlorine content from flue gas cleaning of the abovementioned type, in which the amount of the required alkali-metal-containing additives is reduced, and the throughput of residues can be increased. Furthermore, the electrical power required and the exhaust gas rate are to be reduced, so that, overall, lower operating costs arise. In addition, a more economical recovery of heavy metals from the condensate is to be made possible.
According to the invention this is achieved, in a process, wherein heavy-metal-containing residues are melted in a melting furnace which includes a main furnace and a discharge furnace by separating, in the main furnace, the components of the residues which cannot be incorporated into the melt into two fractions, the first fraction comprising the readily volatile components and the second fraction comprising the salt-like less volatile components, and the first fraction is evaporated off, conducted out of the furnace together with the exhaust gas and collected as condensate, and the second fraction is separated from the surface of the melt as a liquid salt and is discharged.
The invention is based on the unexpected finding that, in the operation of the melt plant as described in DE 196 03 365.9, on the surface of the vitreous melt a stable mobile salt layer can develop, which consists virtually exclusively of CaCl
2
, NaCl and KCl and comprises, in particular, a heavy metal concentration which is lower, compared with the condensate. According to the invention, all of the salt no longer is evaporated off together with the other volatile constituents of the residue.
According to the invention this is achieved in a melting furnace wherein by a lateral outlet for the salt floating on the glass melt is provided in the outer wall of the main furnace. The lower edge of this lateral orifice is arranged at a height of approximately from 1 to 10 cm above the melt level determined by the outlet block.
The advantages of the invention are, inter alia, that the condensate is so highly enriched with heavy metals that treatment for heavy metal recovery is worthwhile. In comparison with the prior art, in addition, the amount of alkali-metal-containing fluxes can be reduced, and the furnace temperature can be decreased, so that, firstly, the service life of the furnace parts is increased, and, secondly, the electrical power required can be reduced. There is a significant increase in the throughput of residue, so that overall the operating costs of the plant can be decreased.
It is particularly expedient if a mass ratio between the first fraction, that is the condensate, and the second fraction, that is the liquid salt, is set which is in the range from 0.1 to 10, depending on the composition of the residue. The size of the mass ratio between the first and second fractions is set by the choice of the process parameters, preferably temperature, exhaust gas rate and amount of alkali-metal-containing additives. It is important that the evaporation of the salt, that is of the second fraction, is restricted. This occurs, for example, by the amount of exhaust gas and/or the temperature above the melt of the amount of alkali-metal-containing additives are reduced.
Finally, advantageously, a thickness greater than 1 cm, preferably from 3 to 10 cm, is set for the second fraction situated on the surface of the melt in the main furnace. This ensures a clean separation of the salt layer from the glass melt situated beneath. In this case, layer thicknesses in the upper range are advantageous if the residues are fed into t
ABB Alstom Power (Switzerland) Ltd
Burns Doane Swecker & Mathis L.L.P.
Kastler Scott
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