DE19724786A1 - Electrochemical process and device for the decontamination of soils and silt contaminated with heavy metals - Google Patents

Abstract

The invention relates to an electro-chemical method and to a device for decontaminating soils and silts contaminated by heavy metals, specially mercury-containing substances. The contaminated substances are extracted with a leaching agent containing hypochlorite ions/hypochlorous acid which dissolves the heavy metal, e.g. mercury. The leaching agent charged with the heavy metal is then transferred into a cathode chamber in which the heavy metal is electrodeposited on the cathode. The depleted leaching agent is then fed into an anode chamber where the chlorite ions are subjected to anodic oxidation to form hypochlorite ions. The leaching agent thus obtained is subsequently re-used for extraction. Easily-adjustable and extremely low-cost recirculation is thus possible.

Description

The invention relates to an electrochemical method for decontamination of heavy metal contaminants, esp special mercury-contaminated substances, in particular Soils and slacks, and a device for electro chemical decontamination of the substances mentioned.

There are a large number of locations worldwide that with heavy metals and especially with mercury are contaminated. These locations include in the Federal Republic of Germany primarily former Indu beach locations. To eliminate this so-called  Contaminated sites are considerable financial and also tech African expenses required.

Although the invention relates to decontamination relates to substances contaminated with heavy metals, the Invention below based on the decontamination of with Mercury contaminated substances explained in more detail, because this represents a preferred embodiment is not meant to be a limitation but rather the presentation will be facilitated.

The substances to be treated do not only include floors and so-called sows, but also other substances such as for example rubber seals and rubble.

So far, for example, contaminated soils cleaned that the contaminated fine fraction of sandy soils was separated by a floor wash, so that clean Coarse grain remained. The one actually to be cleaned The contaminated fine grain is then the substance.

The cleaning of such substances and, for example, also of the above-mentioned rubber seals or rubble into which for example, mercury is diffused nowadays often with the help of a thermal treatment performed. So there are already large-scale plants based on this process. Usually principle applied to the pollutant in the gas phase convince, however, has the disadvantage that it Dilution is achieved and a complex exhaust air agreement is required. This leads to a high one Energy consumption as well as large investment costs. Also Problems of acceptance among the population and the authorities go along with that. It also has a full decon  tamination not reached. Residual concentrations of 10 up to 20 mg / kg are quite common.

The problems that arise when the pollutant is converted to the gas phase can be avoided by extracting the pollutant into a liquid phase. For example, mercury is present in contaminated substances, especially in the "reactor" bottom, in the form of diverse species, which are characterized by the oxidation level (Hg ⁰ , Hg⁺ or Hg ₂ ² ⁺ and Hg ² ⁺) and the type of Differentiate solid binding. Condensed mercury is present in capillaries as Hg °. In addition, it can be present as a precipitated salt or as a cation chemically sorbed on surfaces or complex-bound by an organic matrix. Occluded Hg ^x ⁺ is present in amorphous and crystalline structures. During the extraction, these physical and chemical solid bonds have to be loosened and converted into energetically more favorable bonds in the liquid phase.

From US Pat. No. 3,476,552 it is known that mercury with the help of hypochlorite leaching from sulfidic Ores can be extracted technically. For the assignments The solution of the dissolved mercury becomes activated carbon Applied. This represents an expensive fine clean technology and is great for a partition Quantities extremely uneconomical.

It has also been described as being made out of mercury contaminated substances with the help of halogen-containing lau extract and then cut down. This With regard to this, reference is made, for example, to US Pat. No. 3,732,094. The leached mercury is thereby Reduction pleases, taking a large amount of revival preparation sludge.  

In US Pat. No. 3,639,221 the possibility is also provided wrote the yield by electrolytic in situ Improve hypochlorite production. The one in the DE-OS 42 12 322 described method is this idea taken over and chlorine generated in situ by the fact that Electrodes in a suspension of contaminated substance and saline. The separation of the dissolved mercury in this suspension electrolysis on the cathode, however, poses a mechanical problem because the abrasive suspension flows around it becomes. This leads to abrasion of the deposited Mercury, which is then resuspended, oxidizes and is solved again.

International application WO 94/0 09 167 is also concerned deal with the extraction of mercury compounds contaminated substances and solutions. It will be if a halogenated leaching agent is used.

In particular, the leached with iodine / iodide Mercury separated electrolytically, one conventional electrolytic cell is used.

Generally speaking, leaches with chlorine differ, hypochlorous acid and hypochlorite only through the Process control, for example the pH, since the three Substances by disproportionation or dissociation to be in balance with each other. The mechanism of the Leaching is an oxidation of elemental or single mercury to divalent mercury with subsequent complexation by chloride ions. At a sufficiently high concentration of chloride the stable, readily soluble tetrachloromercurate com plex so that reabsorption of ionic mercury is prevented.  

An alternative to the leachings described above is the decontamination process described in DE-PS 33 12 986 for soils contaminated with heavy metals, especially for mercury. A mixture of HNO ₃ and NaCL is used and the oxidizing effect of nitrate at low pH Exploited values. Here too, chloride must be present to complex the oxidized mercury to prevent reabsorption. Furthermore, this known method also produces a large amount of precipitation sludge to be processed.

All decontamination procedures described so far by extracting mercury by leaching the main disadvantages that on the one hand for a Circulation are not suitable and that others mercury from the leaching agent only unfree is just deposited.

For example, the desired circulation To be able to realize, the leach must return leads and the amount of waste water are minimized, so that Small amount of leaching agent used irreversibly remains. None of the procedures described above such a cycle process control is provided. In addition these methods are also not suitable for this. This in DE-PS 33 12 986 method also has the disadvantage that the low pH <2 relatively high proportion of the soil phases is dissolved. This not only leads to a pronounced wastewater free problem, but also has a high, irreversible sible acid consumption, so that a cycle leadership is hardly possible.

The object of the present invention is a verbes to provide a procedure that is free of  Disadvantages of the prior art methods is and designed as a cycle process can be. It is also an object of the invention to provide appropriate device.

This task is solved by a procedure according to the Teaching the claims.

The simple series connection of anodic leachate tel generation, which then in the circulation in a anodic leaching regeneration passes from Lau tion and made possible by cathodic mercury separation light an easily controllable and extremely inexpensive circulatory process. By the inventive method The leaching can be done with hypochlorous acid operated in a cycle. Regarding the relationship of chlorine, hypochlorous acid and hypochlorite applies otherwise what was said at the beginning, according to which these three Substances are in balance with each other.

In the method according to the invention, there is therefore a sequenced flow through the anode chamber in the stage a), the leach tank in stage b) and Cathode chamber in stage c) instead.

The mercury is in step c) as a metal recovered. A complex preparation of precipitation mud is not required.

The heavy metal or Mercury depleted lye then becomes returned to the anode chamber of stage a). The loaded extract phase is thus removed after the metal  divorce not rejected but in the sense of a waste water minimization.

Another advantage is that little and also extremely cost-effective lye starting material (Table salt) is added because it does not ver is needed. Only a small part is with the cleaned substance or, for example, with the rinse water removed from the process and must be renewed.

The separation of the mercury dissolved in the leaching agent bers is energy-saving because it is on a high level Concentration level takes place. In the fiction This is because the circulating method Leaching agents have a "base concentration" of 10 to 200 mg / l of mercury depending on the substance treated and have the desired maximum residual content. As for the electrolytic metal deposition with the effort decreasing concentration increases significantly, this allows "Base concentration" is an electrolytic deposition of mercury with little effort. A fine clean is only for the liquid to be discharged required. In contrast, the previously known methods in which the leaching agent is not the entire leaching agent is returned Undergo fine cleaning. According to a preferred The embodiment is the leaching agent after the step b) and thus after exiting the leach tank filtered so that no undissolved particles or the like Liches get into the cathode chamber.

Form according to a further known embodiment the anode chamber of stage a) and the cathode chamber of Stage c) together an electrolyser or an elec trolysis device. The two chambers are through  a diaphragm separated. The sequential Flow through the two chambers or the cathode chamber and the anode chamber of this electrolyzer it, therefore, high convection and associated one to maintain a high mass exchange at the electrodes. At Corresponding flow control can also Gaspro production on the electrodes. A Upflow at the cathode promotes the discharge of entste hydrogen, while an anode effluent Chlorine absorption improved. You can also appropriate circulation pumps are used.

The invention also relates to a device for electrochemical decontamination not heavy metal niger, especially mercury contaminated substances, especially soils and silt, according to the teaching of Expectations.

The invention will now be described with reference to the accompanying Figures explained in more detail. Show

Fig. 1 shows the schematic sequence of the procedure according to the invention,

Fig. 2 is a curve relating to the leaching of a contaminated soil (220 mg / kg mercury) and

3 is a graph with respect to the Abreicherungskinetiken of soil leaching at 2 cathode potentials (reference electrode: Ag / AgCl / 3M KCl)..

The substance to be contaminated is filled in a Laugungsbe container 1 . This can be a stirred tank in which the substance to be leached is suspended. However, it can also be a column or a tank in which the leaching agent flows through the substance.

The leaching agent has previously been produced in an anode chamber 2 . For this purpose, a solution containing sodium chloride is fed into the anode chamber 2 at the beginning of the process, in which the chloride ion is anodically oxidized to hypochlorite. The thus obtained hypochlorous acid-containing leaching agent is then discharged from the outlet 14 and transferred to the leaching tank 1 via line 12 and inlet 18 for leaching.

The leaching depends on the mass transfer. Is z. B. elemental mercury condensed in deep capillaries of porous substances or precipitated salt in mineral phases, then the leaching of a low-flow mass is diffusion-controlled and slow. If, on the other hand, it is an average contaminated soil which is leached in a stirred kettle, the leaching takes only a few minutes. This is shown graphically in FIG. 2.

The mercury-laden leaching agent is discharged from the outlet 15 , then filtered (not shown) and transferred via a line 3 and an inlet 19 into the cathode chamber 4 of an electrolysis cell or an electrolyzer 5 . This electrolyzer is divided by a diaphragm 6 .

The residence time in the cathode chamber 4 is loaded / enriched depending on the desired concentration ratio, on the ratio of the cathode surface / cell chamber volume and on the convection.

The residence time can be minimized, for example, by using a metal-coated foam electrode as the cathode 7 . Electrodes consisting of a staggered battery of vertical steel, lead or graphite rods are also suitable. Convection can be significantly increased by pumping around according to arrow 8 . In addition, the cathodic current density can be kept low by a large electrode surface, which increases the current yield with respect to mercury. Nevertheless, hydrogen is produced, the discharge of which is facilitated by an upstream duct.

The depleted of heavy metal leaching agent is recycled through to the outlet 16 from the cathode chamber 4 through a line 9 and the inlet 17 into the anode chamber 2 and thus recycled. As anode 10 , a dimensionally stable, mixed oxide-coated titanium electrode is used for anodic leaching agent production, which is washed around in the outflow in order to obtain maximum absorption of the chlorine. In the anode chamber 2 too, the leaching agent is preferably pumped around according to the arrow 11 . The leaching agent, which is again loaded with hypochlorite ions, is then passed via line 12 back to leaching tank 1 . The electrolytically regenerated leaching agent is thus circulated.

The depletion kinetics obtained with the aid of the method according to the invention at two cathode potentials are shown graphically in FIG. 3.

The depletion results for different mate rialien can be found in the following Table 1.  

The method according to the invention is expedient at a pH of the leaching agent of 4 to 8, performed in particular 5 to 7. The leaching agent can also be buffered at the desired pH, for example with acetic acid / acetate buffer.

Depletion results for different materials

Depletion results for different materials

Claims (8)

1. Electrochemical process for the decontamination of heavy metal contaminated, in particular mercury contaminated, in particular soils and silt, in which

• a) chloride ions are anodically oxidized to hypochlorite ions in a leaching agent in an anode chamber,
• b) the contaminated substance is treated with the leaching agent from step a) containing hypochlorite ions / hypochlorous acid,
• c) the leaching agent loaded in heavy metal in stage b) is transferred into a cathode chamber in which the heavy metal is at least partially separated and separated electrolytically on a cathode and
• d) the leaching agent obtained in step c), depleted in heavy metal, is returned to the anode chamber of step a) and treated there again as in step a).

2. The method according to claim 1, characterized in that that the leaching agent is filtered after step b), before it is transferred to the cathode chamber. 3. The method according to claim 1 or 2, characterized records that the cathode chamber of step c) and Anode chamber of stage a) together an electrolyzer form and are separated by a diaphragm. 4. The method according to any one of the preceding claims, characterized in that a metal as cathode coated foam electrode or a staggered arranged battery of vertical steel, lead or Graphite rods is used. 5. The method according to any one of the preceding claims, characterized in that a mixed oxide as anode layered dimensionally stable titanium electrode becomes. 6. The method according to any one of the preceding claims, characterized in that the leaching agent in step a) at the anode in the outflow and in stage c) at the Cathode is moved in the upstream.   7. Device for the electrochemical decontamination of heavy metal contaminated, in particular mercury contaminated substances, in particular soils and silt, from an anode ( 10 ) having an anode chamber ( 2 ), a leaching tank ( 1 ) and a cathode ( 7 ) having cathode chamber ( 4 ), thereby characterized in that the anode chamber ( 2 ) of the leach tank ( 1 ) and the cathode chamber ( 4 ) are connected in series and in a circuit. 8. The device according to claim 7, characterized in that the anode chamber ( 2 ) and the cathode chamber ( 4 ) form an electrolyzer ( 5 ) and are separated from one another by a diaphragm ( 6 ).