Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing single metal
Reexamination Certificate
2001-02-28
2002-11-26
Phasge, Aruns S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing single metal
C423S043000, C423S050000, C423S055000, C423S064000, C423S065000
Reexamination Certificate
active
06485629
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to processes and materials for use therein for separating heavy metals from solutions or substrates, in particular from contaminated soil, sludges, sediments, and industrial residues. The invention also relates to processes and materials particularly suitable for metal recovery.
BACKGROUND OF THE INVENTION
If an aqueous solution containing a chelating or complexing 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. 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.
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, 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 treat the top soil or ground water, 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.
In co-pending European patent application 97870004.5. a process is described which enables the specific extraction of heavy metal from a water-immiscible substrate by contacting the substrate with [S,S]-EDDS, thereby forming an extract solution containing a complex of the heavy metal and [S,S] EDDS, whereafter the extract solution can be separated from the substrate and then in a separation step, the heavy metal can be separated from the complex and from the extract solution, in particularly by electro winning or precipitation.
It is preferred that the whole substrate/extract solution is treated under the same conditions and in the same separation process and under the same process conditions. However, in many instances, the water-immiscible substrate comprises various heavy metals and thus various complexes with EDDS are formed. The inventors have observed that the separation of mixtures of metal ions from EDDS complexes can be cumbersome.
They have now found an improved process to facilitate the removal of mixtures of heavy metal ions from EDDS complexes from a solution, whereby first the heavy metal ions in the EDDS complexes are displaced by one single heavy metal ion. The displaced heavy metal ions can then be recovered from the solution. In particular, they have found specific process conditions to be used to displace the heavy metal ions with one single heavy metal ion and specific conditions to separate or recover the displaced heavy metal ions from the solution.
Subsequently, the remaining EDDS complex comprising one single displacement heavy metal ion can be recovered by any of the separation methods described in co-pending European patent application 97870004.5, such as in particularly by electrolysis, electrowinning or precipitation.
Thus, the conditions of the process to separate or recover heavy metal ions can be optimised for one single complex-type and this thus allows more effective and costefficient heavy metal removal or separation from substrates or solutions.
SUMMARY OF THE INVENTION
The invention provides a process for separating a source of a first heavy metal ion or mixture of heavy metal ions, (Me
1
), from a solution comprising a complex of said Me
1
and EDDS, (Me
1
-EDDS), by displacement of said Me
1
with a source of a second heavy metal ion (Me
2
) by addition to the solution of a salt of said Me
2
.
In particular, the invention provides a process for separating a first source of a heavy metal ion or a mixture of heavy metal ions (Me
1
) from a solution comprising a complex of said Me
1
and EDDS (Me
1
-EDDS), comprising the steps of:
obtaining a solution having a pH 1 to 6, comprising the Me
1
-EDDS;
b) addition to the solution of step a) of a second source of a heavy metal ion (Me
2
) which is soluble in the solution having a pH 1 to 6, preferably in a molar stoichiometric excess to the EDDS which forms part of Me
1
-EDDS, to produce a solution comprising a complex of Me
2
and EDDS (Me
2
-EDDS);
c) addition of an alkaline material to the solution of step b) to increase the pH to at least 8.5, preferably at least 9, thereby obtaining a precipitate of Me
1
, preferably in the form of a salt of Me
1
;
d) optionally removing the precipitate of step c) from the solution.
The EDDS is preferably [S,S] EDDS.
REFERENCES:
patent: 6264720 (2001-07-01), Schowanek et al.
patent: 0 853 986 (1998-07-01), None
T-C. Chen, et al., “Selection and Test of Effective Chelators for Removal of Heavy Metals from Contaminated Soils,” Can. J. Civ. Eng., vol. 22, (1995) pp 1185-1197.
P. M. Jardine, et al., “Fate and Transport of Ethylenediaminetetraacetate Chelated Contaminants in Subsurface Environments,” Geoderma, Vol. 67, (1995) pp. 125-140.
S. B. Martin, Jr., et al., “Recycling EDTA After Heavy Metals Extraction,” ChemTech, (Apr. 1996) pp 23-25.
Feijtel Tom Cornelis Jan
Schowanek Diederik Rudolf
Phasge Aruns S.
The Procter & Gamble & Company
Waugh Kevin L.
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