Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Reexamination Certificate
1997-04-16
2001-06-05
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
C075S010150, C075S010290, C075S010300, C075S500000, C075S658000, C075S694000, C075S640000
Reexamination Certificate
active
06241797
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for reducing oxidic slags or combustion residues above a metal bath.
2. Description of the Related Art
Oxidic slags more or less completely oxidized occur, for instance, in the incineration of waste or gasification of waste. Attempts have been made to subject such slags occurring during waste incineration or waste gasification to metallurgical aftertreatments immediately thereupon in order to obtain slags that may be used as starting substances or additive components in the cement industry on account of their puzzolanic properties. However, in addition to involving a relatively complex process, this also involves capital expenditures that are relatively high in the waste incineration field such that plants of that kind must be conceived especially with a view to the respective aim set. Yet, waste incineration slags as well as dusts occur in a number of communal incineration plants, wherein the high capital expenditures required for metallurgical aftertreatment would considerably increase costs for disposal, in particular, in small-scale waste incineration plants.
SUMMARY OF THE INVENTION
The invention aims at providing a process for reducing oxidic slags or combustion residues above a metal bath, in which oxidic slags of different origin and loaded with different heavy metal oxides or toxic substances are processed centrally thereby providing the opportunity of homogenizing the charging substances by simple mixing techniques, such as, for instance, mixed bed techniques, and controlling the final concentration of heavy metals within the slag more precisely by applying simpler metallurgy. To solve this object, the invention substantially resides in a process in which the metal bath comprising an iron alloy comprising metals nobler than iron, such as, e.g., an FeNi or FeCu or Sn alloy, is provided, whose redox potential is adjusted such that reduction of FeO to Fe is avoided. In principle, the reduction of iron oxide to iron can be avoided by means of a pure iron bath or a steel bath, yet relatively high temperatures are required in that case. At temperatures of above 1700° C. undesired problems occur with the refractory lining. By using for the bath an iron alloy comprising metals nobler than iron and whose redox potential is adjusted such that FeO is not reduced to Fe, a slag is immediately formed at substantially lower process temperatures, from which slag heavy metals have been separated and in which chromium, if present, has been safely converted from the hexavalent into the trivalent stages. The remaining heavy metal content of the slag is in the range of that of naturally occurring rocks, the reduced slag melt exhibiting excellent puzzolanic and good hydraulic properties. The overall heavy metal content of slags reduced in the manner according to the invention is substantially lower than that of fly ash mixed cement used by comparison, wherein, due to the relatively high content of bivalent iron, which may amount up to 20% within the slag, a reduction potential offering a significant protection against corrosion in steel-concrete structures remains when using such slags, mortar or concrete. At the same time, it is ensured that toxic hexavalent chromium is safely reduced to non-toxic trivalent chromium. The redox potential adjusted for the desired incomplete reduction of the slag simply may be defined via the free enthalpy as follows:
RT ln p
o2
is to be between 250 and 420 kJ/mol O
2,
wherein on grounds of the temperature dependence of the free enthalpy values of about 320 kJ/mol O
2
at 1400° C. and about 290 kJ/mol O
2
at 1500° C. are preferably adjusted. Adjustment advantageously is effected in a manner that an oxygen partial pressure of approximately 10
−10
atm is obtained, wherein the ratio of CO/CO
2
may be between 1:1 and 1:10.
In order to ensure, at such relatively low process temperatures, that the reduction takes place quantitatively in the desired manner, advantageously flush gas may be introduced through the metal bath via bottom tuyeres, thereby enhancing reduction kinetics.
In order to ensure that iron oxide will not be reduced to iron, the carbon content of the metal bath must not become too high. The use of graphite electrodes for heating the metal bath is not beneficial for that reason. Advantageously, the invention provides for heating the metal bath indirectly, in particular, inductively, thereby safeguarding a sufficiently low carbon content within the bath.
With a view to providing a precise process control and obtaining a uniform purity of the slag, Al, FeSi or C is advantageously added to the metal bath in order to adjust its redox potential. If an iron-nickel alloy is present, the invention provides for proceeding in a manner that the C content is maintained at below 5% by weight, preferably below 4% by weight, in order to prevent the formation of nickel carbides.
The process according to the invention in the first place is suitable for disposing of waste incineration slags of different origin centrally. In order to ensure that a completely oxidized slag is actually present, the oxidic slags before being discontinuously charged onto the metal bath are melted in an oxidizingly operated reactor and are transferred into a fore-hearth, from which Cu is tapped in the form of a melt at temperatures of above 1500° C. A meltdown oxidizing reactor of that kind may be charged with a mixed bed slag via its charging opening so as to ensure a uniform quality of the charging slag by simple mixing techniques. It is also feasible to charge wet slag into a meltdown oxidizing reactor of that kind, the slag in that case being dried first, preheated afterwards and, in addition, liquefied. The gas passing through the slag must have a sufficiently high oxygen partial pressure such that all of the oxidizable components contained in the slag, such as unburnt matter, residual carbon, scrap metals, such as, for instance, iron, aluminium or the like, are burnt and oxidized. The same holds for sulfur compounds and chloride compounds. The process may be controlled in a manner that the furnace offgases have temperatures approximately ranging between 200° C. and 300° C. such that possibly formed dioxins and furans primarily will remain in the slag, a dynamic equilibrium between synthesis and oxidative degradation thus building up within the slag column. In doing so, chloride and sulfur compounds are decomposed quantitatively, free chlorine and SO
2
being drawn off along with the furnace offgases due to the high oxygen partial pressure adjusted. At the same time, it is ensured that with the simultaneous introduction of filter dusts the latter are directly introduced into the silicate slag melt under strictly oxidizing conditions, thereby forming a heavy metal sink within the silicate slag melt at a temperature of below 1350° C. Thus, heavy metal is retained quantitatively. By the fact that a fore-hearth is connected to the oxidizingly operated reactor, the temperature in that fore-hearth may be raised and brought to above 1500° C., for instance. At that temperature, practically all of the copper compounds and, in particular, also the intermediarily formed copper silicate dissociate. The metallic liquid copper formed collects in the sump of the fore-hearth of the shaft furnace and from there can be tapped periodically and hence be recovered in a relatively pure form. A copper content of more than 85% may be realized without difficulty. All the other heavy metals, such as primarily zinc and lead are slagged almost completely under the above-mentioned conditions.
In order to maintain the desired temperatures of above 1500° C. for the purpose of dissociating copper silicate, the fore-hearth advantageously may be heated by means of auxiliary burners while effecting overstoichiometric combustion within the gas space. Alternatively, hot offgases from a melter cyclone may be introduced, which will flow through the shaft furnace under energetic utilization of the waste heat.
In order to co
"Holderbank" Financiere Glarus AG
Andrews Melvyn
Pillsbury Madison & Sutro LLP
LandOfFree
Process for reducing oxidic slags does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for reducing oxidic slags, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for reducing oxidic slags will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2512357