Hazardous or toxic waste destruction or containment – Containment – Solidification – vitrification – or cementation
Reexamination Certificate
1999-02-26
2001-10-30
Suchfield, George (Department: 3672)
Hazardous or toxic waste destruction or containment
Containment
Solidification, vitrification, or cementation
C210S195100, C210S202000, C405S129250, C405S129270, C405S129300, C588S256000, C588S259000
Reexamination Certificate
active
06309338
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method for the treatment, in particular stabilization of materials containing environmentally noxious constituents, especially from waste incineration plants for protection against the release of environmentally foreign or environmentally noxious compounds during transport, use or disposal of the stabilized material. The method combines a washing-out of salts with the formation of coatings on the surfaces of the particles whereby the release of salts, metals etc. from the particles is reduced, and the treated material can therefore more easily be reused or landfilled, and, if desired, a subsequent thermal treatment of the coated product formed by the coating, e.g. in the incineration plant, in order to obtain in the plant a sintering together with the slag.
The washing-out is carried out by means of a ferrous solution whereafter the material is oxidized so that the ferrous iron used with the material forms coatings of ferric oxides on the material and thereby immobilizes heavy metals in particular. The salt containing washing-water is also oxidized, whereby the metal ion content, if any, is reduced substantially by the sedimentation of ferric oxides. The method results in a stabilized solid product and a fairly pure salt solution. The method can be used in connection with all granular materials, which contain environmentally noxious compounds, such as slag, fly ash, flue gas cleaning products, sludge ash, industrial dust, contaminated soil or combinations thereof, in particular however in connection with residuals from waste incineration plants.
BACKGROUND ART
By the incineration of waste, a reduction of the amount of waste is obtained, and it is made possible to use the released energy for power and/or heating purposes, but at the same time there is obtained a concentration of environmentally foreign or environmentally noxious constituents, such as heavy metals, in the ash and the other solid particle fractions, and a flue gas which also carries hazardous compounds, both mineral and organic, such as halogen compounds, is also obtained.
The flue gas from the incineration can be cleaned with respect to solid as well as gaseous contaminating constituents by means of various known methods whereby further particle fractions of flue gas cleaning products (FCP) can be produced.
When ash fractions and other by-products from the incineration of waste are used or landfilled, a dissolution and leaching of the environmentally foreign or environmentally noxious compounds can take place resulting in a contamination of the environment.
This effect can be removed or reduced to an acceptable level by extracting the environmentally noxious compounds for reuse or disposal of reduced volumes, or the compounds can be immobilized by coating the particles and/or binding the compounds chemically by means of additives or thermal treatment.
It is known to return boiler ash, filter ash, flue gas cleaning products and grate screenings and to sinter or melt these materials together with the slag, e.g. by introducing them in the rotary kiln inlet in a grate/rotary kiln plant.
Likewise, much experience has been obtained by melting together various of these fractions in order to obtain stabilized materials.
Processes are known for extracting, with e.g. acid containing water, a series of metals from the ash products, which are thereafter made into briquettes for being returned to the incineration plant so that the material can form part of the slag.
It is also known to add cement, silicates, iron oxide or phosphate, whereby the materials are stabilized markedly with respect to the washing-out of heavy metals.
Thus, it is possible to achieve that certain residual products or materials can be used as secondary building materials or be deposited.
For more detailed descriptions of the said methods reference is made to the literature, e.g. the references cited in the following.
However, the ash from waste incineration plants is typically subject to such variations that it has been difficult to provide a safe and economical method for securing the reuse of the ash products.
In the method known from the Danish Patent Application No. 6379/89 there is, in principle, also used ferrous compounds in connection with stabilization. In this method aluminum silicates, lime, ferrous iron and water are mixed to a liquid mass and subsequently dried to a concrete-like solid waste product, but this method does not comprise a washingout step, and ferrous iron is only one of several stabilizing chemicals which lead to the formation of a concrete-like product.
Likewise, it is known from Japanese Patent Application No. J 62183896 that coal ash can be treated with iron containing compounds in order to improve the utility of the ash, but also this method does not comprise a combination with washing-out or subsequent oxidation and, if desired, thermal treatment.
From e.g. EP Patent No. 0 536 268 B1 it is known to introduce ashes and other flue gas cleaning products in the combustion zone of a waste incineration plant for thereby obtaining a melting together with the slag and thereby an immobilization of heavy metals etc. in the slag, but one may hereby obtain an undesired release of noxious constituents to the flue gas, e.g. Pb, Hg, As and Zn.
It is the main object of the invention to provide such a safe and economical method for the stabilization of the residual products from waste incineration that they can be used without risk as filling materials or be landfilled without contamination of the environment.
It is also an object of the invention to provide a plant for carrying out the said method.
DISCLOSURE OF THE INVENTION
According to the invention there is provided a method as defined for the treatment of materials containing environmentally noxious constituents by combining washing-out and stabilization, and by this combination there is obtained a substantial stabilization of the metal compounds in the material by the formation of iron oxide coatings on the particles as well as a substantial washing-out of salts, avoiding that the salt-containing washing-water after the process contains substantial concentrations of especially metals. By means of an optional subsequent thermal treatment in an incineration plant there can be obtained a further stabilization of the residual product by sintering/melting together with the other slag in the plant.
This is obtained by means of a method of the type defined above, the said method being characterized by the features set forth in the claims. Preferred and/or special embodiments of the method are defined in the dependent method claims.
The invention also comprises a plant for carrying out the method of the present invention, the said plant being characterized by a silo containing materials containing environmentally noxious constituents; a reactor to which the said materials are fed; a tank and a return water tank and silos containing ferrous salt solution and, if desired, other chemicals, washing-water and the chemicals being fed to the reactor, the reactor contents being stirred; a sedimentation tank to which the washing-water is fed; a centrifuge or filter press in which the precipitate formed is dewatered; and means for aerating the water in the tank for decreasing the pH and oxidation of the water. Preferred and/or special embodiments of the plant are defined in the dependent plant claims.
The material to be stabilized is mixed in a reactor in a solution containing 0.2 to 5% of ferrous iron in an L/S ratio (liquid/solid) between 1 and 10, preferably 2-5, a pH regulation possibly being required for obtaining a pH in the mixture in the interval from 5 to 12, in particular from 8 to 11.7 additionally, stabilization may be carried out with other chemicals such as silicon (Si), aluminum (Al) and phosphate (Po
4
−3
). The L/S ratio is adapted to inter alia the properties of the residual material in question, including the amount of easily soluble salts which must and can be washed out, and the porosity, particle size distribution and types of
Babcock & Wilcox Volund ApS
Larson & Taylor PLC
Suchfield George
LandOfFree
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