Specialized metallurgical processes – compositions for use therei – Compositions
Reexamination Certificate
1999-07-21
2001-07-24
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
C075S722000, C075S743000, C205S560000, C423SDIG001
Reexamination Certificate
active
06264720
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to processes and materials for separating heavy metals from substrates such as contaminated soil, sludges, sediments, and industrial residues. In particular it relates to such processes wherein the heavy metals can be recovered for industrial use or can be disposed of separately from the substrate. Additionally, the invention relates to such processes wherein conduct of the process can reduce environmental and other pollution problems due to heavy metal loading without an associated increase or shift in environmental problems due to some other contaminant. The invention also relates to processes and materials particularly suitable for metal recovery.
BACKGROUND TO THE INVENTION
If an aqueous solution containing a chelating agent is exposed to an environment (e.g., a waste material) containing a compound of one or more metals which can be chelated by that chelating agent, some or all of the metals will form a chelate with that chelating agent. A wide variety of chelating agents are known, for instance as reviewed by Chen et al at pages 1185 to 1197 of Can.J.Civ.Eng.Vol 22, 1995. The equilibrium complexation constants for the various chelating agents with the various metals indicates the relative affinity and stability of any particular chelate and, when there is competition between metals, which metals will be chelated in preference to others.
It is known that some chelating agents will preferentially chelate divalent metals such as calcium, magnesium and ferrous iron and are sometimes referred to as hardness complexing agents, while other chelating agents have a particular tendency to form a chelate with heavy metals such as copper, lead, cadmium, zinc, nickel or mercury.
As shown in the article by Chen, there is an extremely large number of chelating agents out of which a selection could be made, but in practice interest has concentrated on relatively few. For example citric acid and NTA are commonly used as hardness complexing agents and EDTA is usually the material of choice for chelating heavy metals. Amongst a wide variety of other chelating agents that are discussed in the literature are various phosphates, phosphonates and various imino acids.
Contamination of soils, sediments and municipal or industrial wastes by heavy metal pollutants is a major environmental problem. For instance there are areas of ground which are contaminated by industrial waste containing heavy metal such that there is a risk of the heavy metal getting into ground water or crops, and there are large volumes of river and sea sediments which are contaminated with toxic heavy metals.
Some major engineering works have been conducted and are being proposed which are designed to extract the top soil or ground water, acid mine drainage, or the sediments or other municipal or industrial wastes in such a way as to reduce the heavy metal contamination, but they all suffer from an inherent problem. This problem arises from the fact that chelants (or other chemical reagents) that may be used for attempting to remove the heavy metal contamination from the soil tend to be relatively ineffective unless they form a very strong chelate with the relevant metal. If they do form a strong chelate, then the result of the process is merely to transfer the environmental problem from a contaminated substrate (which is often solid and reasonably concentrated) to a vast volume of a dilute solution of a stable chelate of the heavy metal.
For example, the article by Chen et al reports the screening of 190 chelating agents (including all those mentioned above) and examines in particular the performance of ADA (acetamido imino diacetic acid), SCMC (amino carboxyalkyl thio proponoic acid) and PDA (pyridine dicarboxylic acid) but does not make any clear recommendations.
Other authors have concentrated on the use of EDTA and have noted its power for extracting heavy metals but have also noted the difficulty of recovering the heavy metal and recycling the EDTA (for instance Jardine et al in Geoderma 67 (1995) 125 to 140 and Martin et al Chem Tech April 1996 pages 23 to 25).
Other authors have concentrated on other ways of treating polluted sediments or ground waters from polluted soil and some of these processes have involved a biological treatment. However the same general problem remains, namely that either removal of the heavy metal is inefficient or the pollution problem is transferred from the soil or sediment in favour of a vast volume of dilute contaminated solution.
Further, the use of EDTA and other preferred chelating agents has the risk of the chelating agent subsequently causing further contamination of the environment, since they persist in the environment.
OBJECT OF THE INVENTION
An object of the invention is to provide processes and materials for enabling the efficient extraction of heavy metal from a substrate without associated further contamination due to the introduction of an environmentally persistent chelating agent. A further object of the invention is to provide processes and materials for separating heavy metals from a substrate under conditions so as to provide a heavy metal concentrate which has a sufficiently high concentration that either it can be used economically as a source of heavy metal or has relatively low volume such that storage or dumping in a controlled environment is economically and environmentally convenient.
SUMMARY OF THE INVENTION
The invention provides a process for separating heavy metal from a water-immiscible substrate, wherein the process comprises extracting the metal from the substrate by contacting the substrate with an aqueous treatment solution containing active [S,S]-EDDS and thereby forming an extract solution of heavy metal chelate, separating the extract solution from the substrate, and then separating the heavy metal from the extract solution.
EDDS is ethylene diamine disuccinic acid or ethylene di-imine butane dioic acid. It can exist in various optical isomeric forms. The form which must be used in the invention is the [S,S] form.
By “active” [S,S]-EDDS we mean [S,S]-EDDS in the form of the ion, the free acid or the alkali metal salt or any other chemical form such that it is capable of acting as a chelating agent for heavy metals.
The invention also includes materials suitable for use in such a process. Included amongst these materials is a composition comprising active [S,S]-EDDS and one or more materials selected from (1) microorganisms or enzymes for promoting release of heavy metal from the substrate, (2) biodegradable surfactant for promoting release of heavy metal from the substrate and/or emulsifying hydrophobic material from the substrate into the aqueous treatment solution, (3) biodegradable hardness chelating agent for preferentially chelating calcium and/or magnesium and/or ferrous metal from the substrate, (4) flotation, coagulation or flocculation agent, (5) acid, base or buffer for altering or controlling the pH of the substrate to a working optimum range, (6) oxidizing or reducing agents to facilitate release of the metals from certain fractions of the substrate, and (7) solvents.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
EDDS
The [S,S]-EDDS is a powerful chelating agent for heavy metals and is readily biodegradable in all environmental compartments by indigenous microorganisms. For instance it is rapidly and substantially completely degraded in a suitable environment within 28 days, and usually within 14 days or less in a batch test. The [S,R] and [R,R] isomers are not readily biodegradable according to EU and OECD regulations.
Our current investigations into [S,S]-EDDS suggest that it is unique amongst all known chelating agents for its combination of strong chelating activity with respect of heavy metals and its biodegradability. The heavy metals are usually one or more of Cu, Cd, Hg, Zn, Pb, Ni and Cr.
EDDS is included in the list of chelating agents in the article by Chen. The use of EDDS compounds as allegedly bio
Feijtel Tom Cornelis Jan
Schowanek Diederik Rudolf
Andrews Melvyn
Brown Catherine U.
Dunham Kelly McDow
Reed T. David
The Procter & Gamble Co.
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