NL8603266A - METHOD FOR TREATING CONTAMINATED SOIL - Google Patents

Description

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N739 / AT / th B.V. Project Office for Industrial Waste P.B.I.

Sittard

METHOD FOR TREATING CONTAMINATED SOIL

The invention relates to a method for removing metals of metal compounds from soil contaminated with these compounds, by treating this soil with an aqueous solution of complexing agents, removing the used solution from the soil and, if desired, for reuse. to regenerate.

The principle of removing metals from metal compounds from soil contaminated with these compounds, for instance by using an EDTA solution, is in principle obvious, in view of the metal-binding properties of EDTA. It concerns a complex formation with metal ions. The full name of EDTA is: sodium salt of Ethylene Diamine Tetra-Acetic Acid and is discussed in detail as "dissolvine E 39" of "sequestering agents from AKZO-Chemie".

15 Normally, 1 mole of this complexing agent binds 1 mole of metal.

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The present invention involves the use of EDTA in a continuous process for cleaning soil soil in a soil bed containing unwanted metals, including Cd, Cu, Hg, Pb, Zn and other harmful substances.

Essential to the invention is that the EDTA solution used for the "purification" is regenerated and reused so that no more of this substance is used in the continuous process than is necessary to bind the relevant metal ions and to delete. The total consumption can therefore already be determined in advance by means of analyzes.

It is also important in which way the soil itself undergoes the purification process: the soil must be homogeneously soaked, which, in order to prevent channel formation for liquid, the heap of soil that is treated must usually be compacted gently, for example with a mechanical vibrator or stamper. This will usually not even be necessary for sand. The heap of soil is moistened by means of a dripper system, which is placed in or on the heap of soil. As a result, the discharge of the gaseous reaction products, if gas development takes place, to the ambient air is reduced and annoying atomization, such as occurs when using spray nozzles, is avoided.

For the economy of the process, the scale on which it is performed is important, in other words, the absolute amounts of soil and amounts of EDTA solution, seen in the time frame of treatment.

According to the invention, the method is carried out continuously by: a. Forming one or more piles of the contaminated soil, each on a permeable bottom surface, with a surface of at least 30 m2 and a height of 0.5 to 2 m, and this pile (heaps) if necessary, compacting gently, such that a homogeneous permeable mass is created; b. then sprinkle every quarter m2 of the surface of this heap with a pre-prepared aqueous solution of a reactant such that at least 40 l of solution per hour percolates through the heap;

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-2- *: c. recirculating the eluate through a return tank, adding so much reactant to the solution to be sprinkled that losses of reactant are compensated for; 5 d. continue the recirculation until the metal ions have been removed to a permissible value in the soil concerned, after which EDTA still present is washed away with water with a pH value which can vary from .3 to 11. This value depends on the extent to which one wishes to remove residues; e. partly, the recycled liquid is regenerated with the addition of chemicals and water required for this.

As the reactant, an EDTA solution with a pH value of about 6 is preferably used; 15 f. the liquid is preferably supplied with drippers for which the reason has 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 hope 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 25 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; 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 approx. 6 and return it to the return tank for reuse.

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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

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 concentration. residual concentration of the metal ions in the soil under treatment and the residence time of the leachate in the soil heap.

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. Conventional commercially available electrolysis cells can be used for this purpose.

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 mixing tank. In principle, this procedure does not deviate from the method already described above.

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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; b. the process is simple and easy to control.

The invention is elucidated with a drawing. This shows with: 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 pile of soil 1 with dimensions and density as described above. The heap 1 is formed by the layer-wise application in flat surfaces of soil pre-sorted according to degree of contamination.

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

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 the separation (sieving) of -5- t> f * Y fi f * **

Ci Ij / "S V: * w · ** V« £ * ** -v 1 »* y reduce the material to fractions and / or partially or completely reduce the material to make the substances to be removed more accessible.

The heap (possibly heap) rests on a permeable bottom 2, 5 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 Δ is pumped with a pump 4 into a return eluate tank 5 and from there it is led to a 10 large number of nozzles or drippers 6 above or in the heap 1; one dripper per surface of approximately 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 6.20, 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 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 - 10% dry matter, is dewatered, discontinuously or otherwise, to a filter cake with 40 to 50% dry matter content.

The remaining percentage of the filter cake consists of EDTA solution. This EDTA solution is largely recovered by washing the filter cake during a filter cycle with washing water with a pH value of approximately 12. The washed filter cake is collected in containers per filter cycle, and is the waste product from the process -6- 8603266 4 , which contains the heavy metals removed from the soil 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.

5 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 EDIA-10 solution, is added to an acidification tank 13. The surplus of EDTA solution is caused by dilution of the eluate by adding milk of lime, 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 about 1-4 m 3. At this pH value, the EDTA precipitates from the solution as organic acid in the form. of 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 solution; c. after correcting the pH value by adding HCl solution or or a NaOH solution, the solution is ready for reuse; Γ ίΐ t)

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-7- »r * 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 5 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 takes up to about 1 to 4 months after percolation, the EDTA residues are rinsed out of the 10 heap of soil 1 by percolation with rinsing water. The leaving rinse water can undergo the entire regeneration procedure as described for regeneration of the eluate. Alternatively, the exiting rinse water can also be added directly to the acidification tank 13 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 are fed through electric current in one or more electrolysis cells. separated from the eluate as metal or metal oxide. These metals are separated in batches by cleaning the electrode material from the electrolysis unit and removed as residual material.

The cleaned EDTA solution is supplied 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 adjusting the pH by Adding HCl or NaOH solution.

After making suitable for reuse, the EDTA solution 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 heap 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 slurry formed is led from the acidification tank 10 to a sinker 11. The EDTA crystals are removed as EDTA-15 sludge from the settler 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, rinsing with acidified water was continued for another 15 days, with recovery of the EDTA still present. 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%.

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Claims (14)

1. U 0 ά v 0 A 1. A method for removing metals from metal compounds from soil contaminated with these compounds, by treating this soil with an aqueous solution of reactants, separating the used solution from the soil and, if desired, regenerating it for reuse, characterized in that the method is carried out continuously by: a. one or more. to form piles of the contaminated soil, each on a permeable bottom surface (2) with an area of at least 10 m2 height of 0.5 to 2 m and, if necessary, mildly compact this pile (piles), such that a homogeneous permeable mass arises; b. then sprinkle each quarter m2 of the surface of this heap (heapes) with a pre-prepared aqueous solution of a reactant such that at least 40 1 solution per hour percolates through the heap (heapes); c. recirculating the eluate through a return tank (5), adding so much reactant to the solution to be sprinkled that the losses due to regeneration are compensated for; d. continue the recirculation until the metal ions have been removed to a value permissible in the soil in question, after which EDTA still present is washed away with water with a 25 µm value which can vary from 3 to 11. 2. Process according to claim 1, characterized in that the aqueous solution of the reactant is an EDTA solution with a pH value of approximately 6 and a content between 2 and 40 m%. Method according to claims 1-2, characterized in that the recirculated liquid is partly regenerated with the addition of chemicals and water required for this. Method according to claim 1, characterized in that the sprinkling takes place with drippers (6). ξ ·. Λ Λ - <- A a —11 ~ 5. Process according to claims 1-4 *, characterized in that: a. The eluate is neutralized with Ca (OH) 2 and / or NaOH to a pH value of approximately 12; b. the neutralized solution is passed to a first settling tank (13) and the sludge settled therein is separated, washed and dewatered and discarded; 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. 6. Process according to claims 1-5, characterized in that: 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), 20 also for reuse. A method according to claims 1-6, characterized in that the amount of liquid which recirculates without regeneration is proportional to the amount to be regenerated between the limits of 1: 5 and 100: 1. Method according to one or more of claims 1 to 6, characterized in that more than one heap of soil of deviating composition and / or contamination are treated simultaneously, if desired, each with its specific treatment time and reactant. 9. Method according to claims 1-8, characterized in that the soil 30 is previously mixed with sand up to 50 m%. A method according to claims 1-9, characterized in that the soil to be treated is separated according to particle size before the treatment and / or is reduced in whole or in part. f * Λ n £ Λ v v v j i- y o _12_ Method according to claims 1-10, characterized in that the permeable layer (2) consists of sand or gravel, optionally provided with a drainage system. 12. A method according to claim 1, 3, 4, 7, 8, 9, 10, 11, characterized in that the formed, optionally pretreated, heaps 5 are dripped with an acid, lye or liquid for controlled bacteriological treatment, or leaching of granular industrial residues. Method according to claims 1-12, characterized in that the supply of reactant solutions via the drippers 10 (6) can be effected by arranging the drippers (6) in the heap (heaps) of material. 14. Method as described and explained with an example. h "v *> 'f -13-