Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
2001-10-26
2003-02-18
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C588S256000
Reexamination Certificate
active
06521040
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for processing chromium oxide-containing substances, for example, stainless steel slag as discharged in refining stainless steel, chromium slag as discharged in producing chromium compounds such as sodium bichromate, slag of molten wastes, sewage sludge, slag of molten sewage sludge and the like, thereby to prevent the release of Cr
6+
from those substances.
BACKGROUND OF THE INVENTION
Stainless steel slag as discharged in refining stainless steel, and chromium slag as discharged in producing chromium compounds such as sodium bichromate, contain chromium oxides at a level of a few %. A part of those chromium oxides is often oxidized to Cr
6+
, depending on the working conditions, and Cr
6+
is released from them.
Where stainless steel slag and chromium slag are used as materials in roadbeds, materials in temporary works, fillers in civil engineering reclamation works and the like, it is indispensable that Cr
6+
is not released from the slag.
Recently, recycling of slag as formed by melting ashes from garbage incinerators, sludge and the like in roadbeds, tiles and the like is now under consideration in the industry. However, the slag formed from some types of ashes from garbage incinerators, sludge and the like will often release Cr
6+
to make it difficult to recycle the slag.
Accordingly, for safely recycling such slag in roadbeds, fillers in civil engineering reclamation works and the like, there have been proposed some new techniques of preventing the release of Cr
6+
from the slag.
For example, for preventing the release of Cr
6+
from stainless steel slag, there has been disclosed a method of adding aluminum ash and magnesia-based industrial wastes to stainless steel slag (see Japanese Patent Application Laid-Open (JP-A) No. Hei-6-171993)
When industrially carrying out the disclosed method, the intended aluminum ash and magnesia-based industrial wastes are previously introduced into a slag-receiving kiln, into which molten stainless steel slag is poured, stirred and mixed by the energy of the falling slag melt.
In this method, however, the slag melt is highly viscous, and as to this is different from molten steel, and it is difficult to produce a homogeneously mixed condition by the stirring and mixing means assisted only by the energy of the falling slag melt, resulting in the failure to completely stabilize all chromium oxides in the slag to thereby prevent the release of Cr
6+
from the slag.
Another method was proposed, of adding a divalent S compound such as FeS or the like to molten slag resulting from decarburizing and refining stainless steel. The resulting slag is then reduced to recover Cr in the slag. The slag is then stirred with an inert gas stream introduced thereinto, thereby making the slag have an S content of not less than 0.20% by weight so as not to release Cr
6+
therefrom (see JP-A Hei-8-104553).
In this method, an inert gas stream is introduced into the slag melt to homogeneously mix the slag melt with the additive, thereby stabilizing the chromium oxides existing in the slag. This method is effective in preventing the release of Cr
6+
from the slag, but requires the troublesome operation of introducing such an inert gas stream into the reduced slag melt while using the additive for lowering the viscosity of the slag melt. Therefore, the method is not economical.
In addition, the method is further troublesome in that, if the additive is added to the slag melt in the refining furnace in order to ensure the homogeneous mixing of the slag melt with the additive, the additive will contaminate the steel melt.
On the other hand, for preventing the release of Cr
6+
from chromium slag as discharged in producing chromium compounds, a method has been employed of reductionally roasting the slag to thereby reduce Cr
6+
to Cr
3+
for rendering the slag harmless. Economically, however, this method is troublesome in that it requires a roasting step.
Sewage sludge is incinerated to reduce its volume prior to being used for reclamation. To prevent the release of Cr
6+
from the incinerated ashes, a method is used for controlling the air ratio during incineration to be less than 1 (see Journal of the Drainage Works Association of Japan, Vol. 38, No. 378, pp. 29-32, 1994).
However, as pointed out in this publication, it is extremely difficult to ensure the optimum operation for a variety of sludges derived from various sites and having different properties, and the same applies also to the incineration of industrial wastes.
To reduce Cr
6+
in chromium oxide-containing substances at room temperature, there is generally employed a method using ferrous sulfate as the reducing agent. In this method, however, it is difficult to efficiently lower the Cr
6+
content of the processed substances to an environmentally acceptable level.
Where chromium oxide-containing substances are reduced or incinerated according to the methods noted above, and the thus-processed substances are recycled in roadbeds, fillers in civil engineering reclamation works, temporary works and the like, they are checked as to whether or not chromium oxides were fully reduced, prior to being shipped. If the processed substances contain non-reduced chromium oxides, they cannot be recycled in roadbeds, fillers in civil engineering reclamation works, temporary works and the like.
OBJECTS OF THE INVENTION
An object of the present invention is to solve the problems in the prior art noted above, and to provide an industrial, simple and economical method for processing chromium oxide-containing substances, such as stainless steel slag, chromium slag, slag of molten wastes, sewage sludge, slag of molten sewage sludge, soil contaminated with chromium oxides and the like, thereby to completely prevent the release of Cr
6+
from those chromium oxide-containing substances.
Another object of the invention is to provide a method for processing chromium oxide-containing substances such as those mentioned above within a short period of time without significantly increasing the volume of the substances processed to attain the intended results.
Still another object of the invention is to provide a method of recycling chromium oxide-containing substances, and to provide materials for roadbeds, fillers in civil engineering reclamation works, materials for temporary works, materials for civil engineering and construction works, etc.
SUMMARY OF THE INVENTION
Having carefully studied the problems in the prior art noted above, the present inventors have found that, in the prior art in which the defined reduction of chromium is attained in a high-temperature condition for melting, roasting or incineration, the reduction condition is difficult to control and the defined condition is not economical.
On the basis of this finding, we have tried methods of reducing Cr
6+
at room temperature or at temperatures near room temperature. Specifically, we have made various studies and experiments for reacting various chromium oxide-containing substances such as those mentioned above with different aqueous solutions containing a variety of reducing agents, thereby to reduce Cr
6+
in those substances.
As a result, we have found that, when water that contains sulfur having a valence less than 6, especially water that contains sulfur ions and sulfur as released from the non-aged material of blast furnace slag, is used, then chromium existing in chromium oxide-containing substances can be reduced with high reactivity in an industrial, simple and economical manner. On the basis of this finding, we have achieved the objects of the present invention.
In addition, based on this finding, we have further studied and have made the following findings [1] to [3]:
[1] Using any of sulfur-containing substances listed below as <1> to <4>, the release of Cr
6+
from chromium oxide-containing substances is extremely effectively prevented.
<1&g
Kishimoto Yasuo
Kumagai Masato
Matsunaga Hisahiro
Sakuraya Toshikazu
Tobo Hiroyuki
Kawasaki Steel Corporation
Marcantoni Paul
Young & Thompson
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