NL8703081A - Treating polluted soil to remove metal cpds. - by percolation with recirculated aq. reagent soln., esp. EDTA, then washing with water - Google Patents

Abstract

Method for removing metals from metal cpds., present in any form in porous surface soil material, comprises (i) filling the material into a container having a permeable base, surface area at least 10 sq. m and 0.5-5m tall, the material being mixed, no later than during the filling step, to produce a homogeneous permeable mass; (2) spraying the material with an aq. soln. of a reagent (A) which reacts with the metal cpd., so that is percolates through, using 40l/hr of soln. per 0.25 sq.m. of material surface; (3) recycling the eluate, via a reflux container, with removal of the contaminating cpd. and with replacement of any (A) lost in the process; (4) repeating the cycle until the metal ion content of the material has been reduced to an acceptable level, then washing out residue (A) with water at pH 3-11. The material may be lightly compacted (pref. by vibration) and opt. mixed with sand or other additives. (A) is esp. a 2-40 wt. % soln. of EDTA with pH about 6.

Description

* N907 / CK / th B.V. Project Office for Industrial Waste P.B.I.

Sittard

METHOD FOR TREATING SOIL

The invention relates to a method for removing metals from one or more metal compounds, which are in any form in loose rock material, from this material, by placing it in an open reservoir with one or more - with said metal compound (s). spray the reactant, drain the newly formed metal compound (s) from the rock material and remove the metal from the removed solution.

Here, "rock material" is understood to mean surface soil which contains undesirable metal compounds, such as compounds of Cd, Cu, Hg, Pb and 10 Zn and other harmful substances.

The method described in the preamble is known, inter alia, from U.S. 4,473,532, but is intended for treating ores, a material which is almost always permeable. Apart from in-situ leaching methods, for example according to EP 0 059 020, other methods are known in which contaminated soil is treated as a slurry with an aqueous reagent, for example according to JP 52072368 and JP52107964. The "Soil Remediation Techniques Manual, 1.1, July 1983" also refers to the extraction of contaminated surface soil without, however, providing further essential details.

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It is also known to use suitable reagents for this purpose. 3,634,070 mentions EDTA in this connection, however, according to the examples for extremely small amounts of soil.

There is a general view that further development of 5 soil remediation techniques is necessary, see inter alia "NRC-Handelsblad" 12 September 1987, page 7 or "Energy and Waste Management" no. 4, April 1987.

Lag, according to U.S. 3, 634, 070 the use of laboratory scale EDTA for the hand, for the application of larger scale EDTA and under well-defined conditions, both in terms of treating the soil with an aqueous EDTA solution, recovering the fairly expensive EDTA used requires a method which is the subject of the present invention.

EDTA forms complexes with existing metal compounds. The full name is: sodium salt of Ethylene Diamine Tetra-Acetic Acid and is extensively described inter alia as "dissolvine E 39" of the "sequestering agents" of AKZO-Chemie. Normally, 1 mole of this complexing agent binds 1 mole of metal.

Incidentally, the invention is not limited to the use of EDTA, but depending on the contamination, an aqueous solution of an acid, a lye, another complexing agent or even water can also be used. The use of a device as described in U.S. Pat.

4, 473, 532 is central - as stated in the preamble - but the specifications indicated as general are insufficient for the treatment of soil.

The method is carried out continuously by: a. Filling one or more reservoirs with the contaminated soil, each on a permeable bottom surface with a surface of at least 10 m2 and a height of 0.5 to 5 m, and these masses during the filling of reservoirs or to be treated in advance such that a homogeneous permeable mass is obtained.

-2- Γ; b, then sprinkle every quarter m2 of the surface of this mass (s) with a pre-prepared aqueous solution of a reagent such that at least 40 l of solution per hour percolates through the mass (s).

5 c. Recirculate and regenerate the eluate through a return tank and continuously separate part of the substances to be removed, adding so much of reagents to the sprinkling solution that the losses resulting from the regeneration are compensated.

10 d. Continue the recirculation until the metal ions have been removed to a value permissible in the soil in question, after which the reagent still present is washed away with water with a pH value ranging from 3 to 11.

It is essential that the soil is soaked homogeneously and channel formation is prevented. This can be done by mildly compacting the soil with a mechanical vibrator, but it may also be necessary to add sand or another additive to increase permeability, optionally followed by the mild compaction to avoid said channel formation.

The recycled liquid is partly regenerated with the addition of chemicals and water required for this. As the reagent, an EDTA solution with a pH value of about 6, and a content of between 2 and 40 m%, is preferably used.

The liquid is preferably supplied with drippers, for which reason the reasons have already been stated. A number of pressure-compensating drippers known per se are preferably used, known inter alia under the trade name Revaho. With these drippers, which are also used for plant watering, it is quite possible to apply a known defined amount of liquid per surface to, or in the heap of, soil. The use of drippers makes the treatment simple and in fact an accelerated natural leaching process. This makes it possible to deliver the materials after treatment with guarantees on leachability.

According to the method of the invention, preferably only a part of the eluate is regenerated continuously. This occurs because: a. The eluate is neutralized with Ca (OH) 2 and / or NaOH to a pH value of approximately 12; -3- f i A - * 5 b. the neutralized solution is passed to a first settling tank and the sludge settled therein is separated, washed and dewatered and discarded as waste; c. acidify almost all of the remaining liquid to a pH value of approximately 6 and return it to the return tank for 10 reuse.

A further step that minimizes the use of EDTA is that:

a. part of the filtrate and washing liquid from the dewatered sludge is acidified to a pH value of approximately 2, whereby EDTA

15 precipitates; b. the precipitate formed in its liquid settles in a second settling tank and is then led to the final acidification stage, dissolving this EDTA sludge at a pH value of approximately 6 and with the filtrate and washing water of the filter and the clear liquid of the first settling tank is returned to the return tank also for reuse.

The part of the eluate that recirculates directly, that is to say without regeneration, is related to the amount to be regenerated in practice between limits of 1: 5 and 100: 1. This depends on the concentration, or on the residual concentration of the metal ions in the soil under treatment and the residence time of the leachate in the soil mass.

Where pH values are stated, these are the most appropriate values; deviations are permissible, provided that other values do not adversely affect the principle and the efficiency of the method.

It will be clear that the solid product recovered must find an appropriate deposit. Metallurgical reprocessing is highly eligible.

Another method of regenerating the separated part of the eluate consists in that it is subjected to an electrolysis process, in which on the one hand metals are continuously separated and on the other hand an EDTA solution which is recycled to the solution to be prepared. For this purpose, conventional commercially available products are:. "".

Available electrolysis cells, applicable.

Also, when using this method, the use of EDTA can be minimized by acidifying part of the solution, separating the solid EDTA formed and returning it to the preparation tank 5. In principle, this procedure does not deviate from the previously described method.

The concentration of the aqueous EDTA solution is not critical; the use of a solution for sprinkling the heap of soil with a concentration between 1 and 40 m%, preferably between 2 and 20 m%, can be used; a concentration of 5 m% was excellent.

The invention has the following advantages: a. Possibility of delivering the soil or industrial residues with guarantees on leaching behavior; 15 b. the process is simple and easy to control.

The invention is explained with a drawing. This shows: figure 1: a flow sheet with chemical regeneration of the used solution; Figure 2: a flow sheet with electrolytic regeneration of the used solution.

Figure 1 forms a mass of soil -1 with dimensions and density as described above. The mass 1 is formed by the layer-wise application in flat surfaces of soil pre-sorted according to degree of contamination in a reservoir.

25 When talking about a "mass of soil" it can also mean several masses that are treated simultaneously. This can for instance be the case if soil is treated with different kinds of contamination, such as one mass with soil contaminated with various heavy metals and one mass with, for example, (zinc) slag. It may also be possible for one mass with a high degree of contamination to be treated simultaneously with one with a low degree of contamination. The treatment of the last mass may of course be shorter. The same mass of soil can be treated successively with multiple solutions with extraction reagents if two or more kinds of impurities are present.

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Furthermore, it may be necessary to increase the permeability of the soil. The soil is then mixed with sand beforehand, which can be up to 50 m%. Particle size is also important in this regard. This can lead to separating (sieving) the material into fractions and / or wholly or partially reducing the material to make the substances to be removed more accessible.

The mass 1 rests on a permeable bottom 2, which can be of any suitable configuration, provided it is not attackable by the solution. The liquid penetrating through the bottom 2 is collected on an impermeable bottom 3, which has a slope to a collection point A.

The liquid collected at A is pumped with a pump 4 into a return-eluate tank 5 and from there passed to a large number of nozzles or drippers 6 above or into the mass 1; one dripper per surface of approximately 1.15 x 0.50 x 0.50 m 6.

The permeable bottom 2 preferably consists of (coarse) gravel, sand and optionally drainage pipes, which can, among other things, bring the eluate to a collection point A.

In this case, a prepared EDTA solution is assumed for the removal of metals. This is located in a tank 7 and can supply the drippers in whole or as far as necessary. The solution in this tank is partly prepared in a mixing tank 8 with a concentration of EDTA of approximately 5 m%, at a pH value of 6, but will partly also come from the regeneration part.

This part can be described as follows: part of the eluate is drained at B and passed to a neutralization tank 9, where Ca (OH) 2 and / or NaOH solution is added until the pH value is about 12.

The resulting suspension is led from the neutralization tank 30 to a settler 10, where the metal hydroxide precipitate formed can settle. The settled metal hydroxide precipitate is pumped into a filter press installation 11. In the filter press, the precipitate, containing 2 - 10Z dry matter, is dewatered, discontinuously or otherwise, into a filter cake with 40 to 50Z dry matter content.

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The remaining percentage of the filter cake consists of EDTA solution. This EDTA solution is largely recovered by washing the filter cake with washing water with a pH value of approximately 12 during a filter cycle. The washed filter cake is collected in containers per filter-5 cycle and is the waste product from the process, which contains the heavy metals. which have been removed from the ground bed. The clear supernatant from the settling tank, the filtrate and the washing water from the filter press are discharged to a collection point C.

10 From the collection point C, the liquid is pumped to a split point D, where the total flow is divided into 2 partial flows, each of which undergoes a separate treatment. Most of the total flow is added from point D to a mixing tank 12. The other part, being the surplus of EDTA-15 solution, is added to an acidification tank 13. The surplus of EDTA solution is caused by dilution of the eluate by adding lime milk, NaOH solution and washing water.

In the acidification tank, the pH value is adjusted to approximately 2 by adding 30 m% HCl solution in batches in quantities of approximately 1 to 4 m3. At this pH value, the EDTA precipitates from the solution as organic acid in the form yan crystals.

The suspension obtained is discharged from the acidification tank to a settler 14. In the settler 14, the EDTA crystals are separated as EDTA sludge. The EDTA sludge is added to the mixing tank 12, so that as much EDTA as possible is reused.

In the mixing tank 12, the EDTA solution is made suitable for reuse again. This suitability can be described as follows: a. The major part of the total flow from point D is mixed with the EDTA sludge coming from the settler 14; b. the loss of EDTA during the regeneration process is here supplemented, if necessary, by adding 40 m% EDTA-35 solution; -7- r * 1. - '·· <\ S J,' i * 'c. after correcting the pH value by adding HCl solution or or a NaOH solution, the solution is ready for reuse; d. from the mixing tank 12 this solution is then pumped to tank 7 and can be used as leachate.

The clear supernatant from the settler 14 is partly reused as dilution water in mixing tank 8 and partly as rinsing water to a rinsing water tank 15. In the final phase (rinsing phase), part of this liquid is discharged to a sewage system.

At the end of the cleaning process, which lasts until about 1 to 4 months after the start of percolation, the EDTA residues are rinsed from the mass of soil 1 by percolation with rinsing water. The exiting rinse water can undergo the entire regeneration procedure as described for regeneration of the eluate. On the other hand, the leaving rinse water can also be added directly to the acidification tank 1.3 to recover the EDTA as a solid.

Figure 2 shows the same process as described in figure 1 up to the regeneration part, which starts at B.

The regeneration part in figure 2 can be described as follows:.

part of the eluate is drawn off at B and passed to an electrolysis unit 8. In the electrolysis unit 8, the heavy metals, such as metal or metal oxide, are separated from the eluate by passage of electric current in one or more electrolysis cells. These metals are separated in batches from the electrolysis unit by cleaning the electrode material and removed as residual material.

The cleaned EDTA solution is fed from the electrolysis unit 8 to a mixing tank 9. In the mixing tank 9, this EDTA solution is made suitable for reuse by adding, if necessary, 40 m% EDTA solution and correcting the pH by adding HCl or NaOH solution.

After making suitable for reuse, the EDTA solution 35 is pumped from mixing tank 9 to tank 7 and can be reused there as leachate.

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The mixing tank 9 also serves to prepare the EDTA solution at the start of the cleaning process. At the end of the cleaning process, approximately 1 to 4 months after the start of percolation, the EDTA residues from the mass of soil 1 are rinsed by percolation 5 with rinsing water from rinsing water tank 13, as already described.

The exiting rinsing water is drained from point B and undergoes the same treatment as the eluate in the electrolysis unit 8. The purified rinsing water from electrolysis unit 8 is directly discharged to an acid tank 10. In the acid tank 10 10 this rinsing water, which contains EDTA , brought to a pH of about 2 by adding HCl solution. The EDTA precipitates as organic acid at this pH value in the form of crystals.

The suspension formed is led from the acidification tank 10 to a settling device 11. The EDTA crystals are discharged as EDTA-15 sludge from the settling device to a storage tank 12.

The stored EDTA sludge can then be used as a raw material for the preparation of EDTA solutions.

The invention is not limited to EDTA, but acid and caustic solutions can also be used depending on the impurities.

The principle of dripping a soil bed can also be applied for: a. Controlled biochemical or bacteriological treatment and then extraction or flushing with other solutions; 25 b. leaching of granular industrial residues in which unwanted, leachable components are removed or useful components are removed.

Example:

An amount of loose soil with the following impurities: Cd, Pb and Zn was treated according to the method of the invention. To this end, 10 m3 of soil was raised and vibrated to a height of 1 m on a gravel layer of 10 cm thick.

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Above the bed, 40 drippers of the type as described were placed. The amount of liquid supplied, in this case an aqueous EDTA solution of 5 m% with a pH value of 6, was dosed such that all added liquid percolated through the layer of soil. The amount of eluate was then 40 l / h.

After a period of 3 months of continuous treatment, the content of the above metals was found to have decreased by 92% of the original value, which was determined from soil samples taken.

After this was still. Rinsed with acidified water for 15 days while recovering the remaining EDTA. The relatively dry soil, which had been drained for 5 days, could be returned as "clean soil".

15 The use of EDTA exceeded the calculated amount by only 4.6%.

Finally, variations are possible on the described method. For example, the eluate can be cleaned or post-cleaned immediately after collection or at another time by means of electrolysis.

If the soil to be cleaned contains components that each require a specific extraction medium, they can be added simultaneously (such as DTPA, HEDTA, gluconates, sulphite, pyrosulphite, and / or oxalate / oxalic acid). Certain conditions can also be created in the ground bed, such as a reducing - oxidizing acidic or basic environment. If necessary, wetting agents can be added back to the extraction agent for better contact between extractant and contaminants.

Although spraying according to the given specification is preferred, the invention is not strictly limited thereto and the supply of extraction liquid from below is not excluded in advance, or the extraction liquid can be alternately supplied from above or below during the cleaning of the soil bed and the eluate is discharged from below or above the bed for regeneration or recirculation, respectively.

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Claims (20)

1, A method of removing metals from one or more metal compounds, which are in any form in loose rock material, from this material, by spraying it in an open reservoir with one or more reagent reacting with said metal compound (s), which drain newly formed metal compound (s) from the rock material and remove the metal from the removed solution, characterized in that: 10 a. fill one or more containers (1) with the contaminated soil, each on the permeable bottom surface ( 2) with a surface of at least 10 m2 and a height of 0.5 to 5 m and to treat these masses during filling of the container (s) or beforehand such that a homogeneous permeable mass is created; 15 b. Then sprinkle every quarter m2 of the surface of this mass (es) with a pre-prepared aqueous solution of a reagent, such that at least 40 l of solution per hour percolates through the mass (es). c. Recirculate and recirculate the eluate through a return tank and continuously separate a portion of the materials to be removed, adding so much of reagents to the sprinkling solution that the losses due to regeneration are compensated. d. Continue the recirculation until the metal ions have been removed to a value permissible in the soil in question, after which the reagent still present is washed away with water with a pH value ranging from 3 to 11. Method according to claim 1, characterized in that the soil mass is compacted mildly, for example with a mechanical vibrator. A method according to claim 1, characterized in that sand or another additive is added to the soil. Method according to claims 1 and 3, characterized in that the soil to which sand or another raw material has been added is mildly compacted. -11-> Process according to claims 1-4, characterized in that the aqueous solution of the reactant is an EDTA solution with a pH value of approximately 6 and a content of between 2 and 40 m%. A method according to claims 1-5, characterized in that the recirculated liquid is partly regenerated with the addition of chemicals and water required for this. Method according to claims 1-4, characterized in that the sprinkling takes place with drippers (6). preferably pressure compensating drippers. 8. Method according to claims 1-7. characterized in that: a. the eluate is neutralized with CaCOH2 and / or NaOH to a pH value of about 12; b. the neutralized solution is passed to a first settling tank (13) and the sludge settled therein. is separated, washed and dewatered and removed as waste; c. acidify almost all of the remaining liquid to a pH value of approximately 6 and return it to the return tank (5) for reuse. A method according to claims 1-8, characterized in that: a. A part of the filtrate and washing liquid of the dewatered sludge is acidified to a pH value of approximately 2, whereby EDTA precipitates; b. the precipitate formed in its liquid settles in a second settling tank (14) and is then led to the last acidification tank (12), dissolving it as EDTA sludge at a pH value of approximately 6 and with the filtrate and washing water of the filter and the clear liquid from the first settling tank (10) is returned to the return tank (5), also for reuse. .. A method according to claims 1-9, characterized in that the amount of liquid which recirculates without regeneration is related to the amount to be regenerated between the limits of 1: 5 and 100: 1. \ f '-12- $ V! £ 11. Method according to one or more of claims 1-9, characterized in that more than one mass of soil of deviating composition and / or contamination are treated simultaneously, if desired each with its specific treatment time and reactant. Method according to claims 1-11, characterized in that the soil is pre-mixed with sand or another additive up to 50 m%. 13. Method according to claims 1-12, characterized in that the soil to be treated is separated according to particle size and / or wholly or partly reduced before treatment. Method according to claims 1-10, characterized in that the permeable layer (2) consists of sand or gravel, optionally provided with a drainage system. Method according to claim 1, 3, 4, 7, 8, 9, 10, 11, characterized in that the formed, optionally pretreated masses are dripped with an acid, lye or liquid for controlled bacteriological treatment, or leaching in granular industrial residues. A method according to claims 1-12, characterized in that the supply of reactant solutions via the drippers (6) can be effected by arranging the drippers (6) in the mass (s) of material. 17. Process according to claims 1-9, characterized in that: a. The eluate is post-cleaned after neutralization by means of electrolysis; b. the eluate is cleaned immediately after collection from the ground bed by electrolysis. 18. A method according to claims 1 to 12, characterized in that: a. A combination of extractants can be used for each contaminated component in the soil; b. certain conditions can be created in the ground bed by the order of supply of the various extraction means, such as reducing, oxidizing, acidic or basic medium; -13- t t c. other chemicals, for example DTPA, HEDTA, gluconates, sulfite, pyrosulfite, oxalate / oxalic acid, can also be used for applying these conditions. d. For better contact between the extractant and the contaminants present, wetting agents can be added to the extractant. A method according to claims 1-16, characterized in that: a. The extraction liquid can also be supplied from below by means of a system; 10 b. the extraction liquid is alternately supplied from above or below during the cleaning of the ground bed and the eluate is discharged from below or above the ground bed for regeneration or recirculation, respectively. 20. Method as described and explained with an example. - Λ f -14-