Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
2000-05-30
2002-09-03
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C588S253000, C588S253000
Reexamination Certificate
active
06444183
ABSTRACT:
This application is an application under 35 U.S.C. Section 371 of International Application No. PCT/FR98/01993, filed on Sep. 17, 1998.
The present invention relates to an agent for removing heavy metals contained in a medium, in particular an aqueous effluent.
Legislation in the field of heavy metal discharges to the natural environment is constantly changing. Industrial effluents, such as, for example. those originating from factories in the chemical, metallurgical, electronics and mechanical engineering industries, and aqueous liquors from the washing (or purification) of flue gases from plants for the incineration of household refuse or industrial waste, in particular of industrial effluents of the spent sulphuric acid type, are media likely to contain heavy metals. Likewise, certain soils are contaminated by the presence of such metals.
Thus, in the field of aqueous liquors for washing flue gases from plants for the incineration of household refuse, the most well known process for removing heavy metals consists of a basic precipitation carried out with lime; the settling/separation stage which follows is generally improved by the incorporation of a flocculating agent.
However, this process exhibits a number of disadvantages.
On the one hand, this type of effluent to be heated often has a very high content of inorganic salts, which can hinder the complete precipitation of the heavy metals.
On the other hand, a very large volume of sludge is generated by the precipitation with lime; this sludge, after filtration and compaction in the cake form, must currently be placed in suitable landfill sites.
Furthermore, the sludge composition obtained can be difficult to stabilize (or immobilize) by current techniques. Now, future regulations relating to the storage of final special waste make it necessary to stabilize (or immobilize) the cake before it can be allowed onto the storage site, in order to very substantially reduce the leaching of this type of waste.
The aim of the present invention is in particular to provide a means which makes possible an efficient removal (or scrubbing) of heavy metals, in particular cadmium, nickel and lead, as well as, in some cases, mercury, and which does not exhibit the abovementioned disadvantages.
To this end, the present invention provides a novel agent for removing (or scrubbing) heavy metals present in a medium, the said agent being a mixed product comprising at least one alkali metal silicate, at least one alkali metal carbonate, at least one compound of the phosphate type and, optionally, at. least one carrier, in particular a clay.
It also relates to the use of the said agent for removing heavy metals, in particular cadmium, nickel and lead, as well as, in certain cases, mercury, contained in a medium, in particular an aqueous effluent.
It also relates to an agent for stabilizing (or immobilizing) the heavy metals comprising the said agent for removing heavy metals.
The Applicant has found, surprisingly, that the use of an agent for removing heavy metals as defined above made possible very efficient removal of heavy metals from the medium containing them, in particular cadmium, nickel and lead and, when it comprises a sulphur compound, mercury, and, advantageously, an improvement in the sludge obtained/supernatant liquid settling and separation, a degree of improvement in the aptitude for stabilization of this sludge, that is to say a degree of improvement in the ability of the latter to withstand leaching, and a decrease in the residual calcium content in the precipitate with respect to what is obtained with the process with lime described above.
Thus, one of the subject-matters of the invention is an agent for removing (or taking up) heavy metals contained in a medium, characterized On that the said agent comprises:
at least one compound consisting of an alkali metal silicate, hereinafter known as compound (A),
at least one compound consisting of an alkali metal carbonate, hereinafter known as compound
at least one compound of the phosphate type, hereinafter known as compound (C), and
optionally at least one carrier.
Heavy metals is understood to mean in particular metals with a valency greater than or equal to 2, preferably equal to 2, and in particular those chosen from antimony, arsenic, bismuth, cadmium, chromium, cobalt, copper, tin, manganese, mercury, molybdenum, nickel, gold, lead, thallium, tungsten, zinc, iron or metals from the actinide family.
The heavy metals particularly targeted by the present invention are chromium, copper, iron, zinc and, to a greater degree, cadmium, nickel and lead.
Certain embodiments of the agent according to the invention are advantageously used when the medium to be treated contains, as heavy metals, at least mercury.
The heavy metals to be removed are usually in the form of ions, in particular in the form of their respective cations (for example, Cr
3+
, Cu
2+
, Ni
2+
, Fe
2+
, Fe
3+
, Cd
2+
, Hg
2+
, Pb
2+
or Zn
2+
).
It should be noted that the agent according to the invention can also prove to be effective in removing metals such as aluminium and so-called radioactive metals, such as uranium or thorium.
The medium to be treated is preferably liquid.
This medium can thus be composed of an aqueous effluent, in particular an aqueous industrial effluent (that is to say, an aqueous effluent originating from an industrial process).
This medium can be, for example, an aqueous effluent formed by the aqueous liquors from the washing (or purification) of flue gases from the incineration of waste, in particular of household refuse, industrial waste or hospital waste, by aqueous liquors from washing solid material, such as earth, containing heavy metals or by aqueous surface treatment effluents; it can be an aqueous effluent originating from a factory in the chemical, metallurgical, electronics or mechanical engineering industry.
The compound (A) is an alkali metal silicate, in particular sodium or potassium silicate.
The compound (A) is advantageously a sodium silicate. The said sodium silicate then generally exhibits an SiO
2
/Na
2
O molar ratio of between 0.5 and 3.8, for example equal to approximately 2.
The compound (B) is an alkali metal carbonate, in particular a sodium or potassium carbonate. The compound (B) is advantageously a sodium carbonate.
The ratio by weight of the compound (B) to the compound (A) can vary within relatively broad ranges of values. Nevertheless, this compound (B)/compound (A) ratio by weight is generally between 0.5 and 3.5, preferably between 1.1 and 2.5, in particular between 1.5 and 2.0.
When it contains at least one carrier, the mixed product for removing the heavy metals from a medium according to the invention, which can be used to purify the said medium, can be regarded as a composite product formed:
of at least one active principle (or precipitation agent) composed of;
at least one compound (A) consisting of an alkali metal silicate,
at least one compound (B) consisting of an alkali metal carbonate,
at least one compound (C) of the phosphate type; and
of at least one carrier (or substrate).
The carrier optionally contained in the agent according to the invention is preferably a clay.
The clay then contained in the agent according to the invention can be of natural or synthetic origin.
The clay thus employed in the agent according to the invention advantageously exhibits a high content by weight of Al
2
O
3
; this content is, for examples between 20 and 40%.
A clay of lamellar or phyllosilicate structure can be employed according to the invention.
It is thus possible to use a clay chosen from the group comprising kaolinites and serpentines.
The clay can also be chosen from the group comprising montmorillonites, bentonites (in particular alkaline bentonites), talc and mica.
The clay chosen preferably belongs to one of these two groups.
A clay of chlorite-type structure can optionally be employed.
The agent according to the invention generally contains, as clay, a montmorillonite or a bentonite.
The content of carrier, in
Kiefer Jean-Claude
Mottot Yves
Johnson Edward M.
Rhodia Chimie
Silverman Stanley S.
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