METHOD FOR CLEANSING AND DECONTAMINATING SOIL AND THE EQUIPMENT USED FOR THIS PURPOSE

Abstract: Method for the cleansing and decontamination of soil by (a) the extraction of soil contaminated principally by hydro­ carbons or other organic material, heavy metals and/or cyanides, by mixing the contaminated soil with water of specific pH as extractant, (b) separating the cleansed or at least partially cleansed soil from the water, (c) separating the contaminants from the water and (d) recirculating the water as extractant, whereby the soil, possibly pretreated in sieves, is washed intensively with an aqueous extractant in a washer provided with a stirrer, at a solids content of 60-80% by weight and a pH that is controlled during and af­ ter the washing to values dependent on the nature of the contaminants.

Description: TITLE OF INVENTION Method for cleansing and decontaminating soil and the equipment used for this purpose. The invention refers to a method for the cleansing and decontamination of soil by (a) the extraction of soil contaminated principally by hydrocarbons or other organic material (lighter than water), heavy metals and/or cyanides by mixing the contaminated soil with water of a specific pH as extractant, (b) separating the cleansed or at least partially cleansed soil from the water, (c) separating the contaminants from the water and (d) recirculating the water as extractant. This method is known from the "Handboek Bodemsaneringstechnieken" of 1 July 1983, pp. 17 and 18, where the possibility of cleansing soil by means of extraction with water is given. In this Handbook it is stated that although experiments have been conducted on quite a wide scale, only one extraction technique is applied in practice, namely, the washing of soil by means of high-pressure steam. In Netherlands Patent Application 8202593 particular attention is paid to the decontamination of soil contaminated with cyanides : the soil is mixed with an aqueous caustic solution to a dilution of 36 - 50% solids, at a pH of about 11, and alkali hypochlorite is added while stirring vigorously, so that the cyanide is oxidized to cyanate and after this more alkali hypochlorite is added at a pH of 6 - 7 and a solids content of 21 - 33%, so that the cyanate is dissociated to gaseous carbon dioxide and nitrogen. Netherlands Patent Application 8100879 describes a method for the removal of undesirable constituents from soil by screening off the contaminated soil from the adjacent uncontaminated soil and then extracting the contaminants in situ from the contaminated soil and draining off the extract so formed and purifying it according to well-known methods. With regard to the extraction method it is stated that the extractant can be introduced on or into the contaminated ground in any suitable manner, for example by spraying and/or by injecting. From experiments, however, it has been found that with this method of extraction of soil, the contaminants are not taken up or are too slowly taken up by the extractant. The extraction method is now one of the improvements to which the method according to the present invention refers. From Netherlands Patent Application 8200155 a method is known for the cleansing of soil containing contaminated sand by washing the soil on a sieve. In this way more intimate contact between the extractant and the soil containing the sand is obtained because the extractant can then be passed more often through the soil containing the contaminated sand. This method is restricted to soil containing sand because with soil containing clay the sieving operation is more difficult to implement. From the American Patent Specifications 4.336.136 and 4.392.944 a method is known for the removal of hydrocarbons from soil by washing the soil, whereby a maximum solids content of 50%, preferably 20% by weight, is used. In German Patent Application 2.911.399 and in Belgian Patent Specification 894.733 attention is paid primarily to the removal of heavy metals from soil, in which complex-forming substances and caustics are introduced in the soil. Investigations have shown that it is possible to develop a general process for the cleansing and decontamination of soil containing the contaminants listed in the introduction, the method being characterized by the fact that the soil, if desired after a pre-treatment with sieves, is intensively washed with water in washing equipment, provided with a stirrer, which mixture having a solids content of 60 - 80% by weight, at pH values during and after washing dependent on the nature of the contaminants. The washing equipment consists preferentially of a scrubber with two or more compartments, each provided with a stirrer. This "attrition scrubber" is well-known in the mining industry (see Mineral, coal, chemical processing equipment, Bulletin No. ASM-l). A characteristic feature of the mixing process in the scrubber is the intensive mutual contact of the soil particles owing to the abrasive rubbing between the particles whereby contaminants on the soil particles are transferred to the water extractant better and faster. Contaminants in the form of particles are also pulverized by the mechanical forces in the scrubber acting on the solid particles, whereby they dissolve more rapidly in the aqueous phase. Because coarse material such as rubble and gravel can have adverse effects on the operation of the scrubber, coarse material is first remove from the soil before it is washed with an alkaline solution and introduced in the scrubber. The solids content in the scrubber is adjusted to 60 - 80% by weight, preferably to about 70% by weight, so that contaminants are shaken loose from the soil particles and transferred to the aqueous phase. This desorption process takes place particularly favourably at a pH of ll or higher. If the soil is contaminated only by hydrocarbons, the pH may be set at about the value 9, while in the presence of cyanides and heavy metals it is preferable to use a pH of about 11 - 12. A higher temperature in the scrubber improves the transfer of contaminants to the water. The rate of cleansing of kerosene-contaminated soil in the scrubber and its dependence on the pH is shown in Table A. The data refer to soil containing 4100 mg of kerosene per kg of dry soil, whereby the quantities of kerosene remaining in the soil after the indicated number of minutes of intensive stirring in the scrubber, for a fixed solids content of 70% by weight, are given in the table, expressed in mg/kg. TABLE A Mixing time (minutes) pH 3 10 30 7 1800 650 260 9 390 270 255 11 290 250 210 The optimum conditions in the scrubber can be ascertined as functions of the contaminants present and the further processing of the materials. In the presence in the soil of cyanides and heavy metals it has been found that a pH value of 11 - 12 in the scrubber is preferable in order to transfer the maximum amount of contaminants from the soil to the aqueous phase. At lower pH undesirable substances could be formed from the cyanides. The heavy metals can be removed together with the cyanides because the heavy metals dissolve as complexes or as compounds. The rates of removal from the soil of the metals zinc, chromium, nickel and copper, in the scrubber, at a pH of 11.5 are given in Table B, expressed in mg per kg of dry material. TABLE B Time Zn Cr Ni Cu (minutes) 0 1500 320 43 140 3 920 130 24 20 10 320 136 12 8.4 30 220 82 8.5 7.1 Experiments have been made which show how rapidly cyanide contaminants are transferred to the aqueous extractant in the scrubber. These experiments refer to soil contaminated by electroplating waste, containing 930 mg of cyanide compounds per kg of dry soil. After extraction at a pH of 11.5 and with a solids content of 70%, it was found that 10 minutes of washing in the scrubber reduced the cyanide content of the soil to 42 mg per kg of soil. After intensive washing in the scrubber, the mixture of partially cleansed soil and liquid (water), into which the greater part of the contaminants have been transferred, is fed to a pump tank in which it is diluted with recirculation water to make a suspension suitable for further processing, having a solids content of preferably 30%. After this the cyanide, present in ionic form in the water, is oxidized to cyanate by the addition of an oxidant, whereby the pH is maintained at 11 - 12. Hypochlorite or hydrogen peroxide is used as oxidant. In the neutralization tank which follows, the pH is reduced to 7 or less, whereby the cyanate is dissociated to nitrogen and carbon dioxide. The results obtained after oxidation of the contaminated soil by NaOCl or by H2 02, which before scrubbing contained 930 mg of CN per kg of soil and after washing for 10 minutes in the scrubber still contained 42 mg of CN per kg of soil, are summarized in Table C. The table shows that after an hour of oxidation with NaOCl the soil contained 0.08 mg of CN per kg of soil and after an hour of oxidation with H202 less than 0.05 mg of CN per kg of soil. TABLE C. Oxidant Time (minutes) 0 15 30 60 NaOCl 42 6.9 3.1 0.08 H202 42 0.78 0.42 < 0.05 It is also possible to carry out the oxidation earlier, in the scrubber ; in this case the oxidation of the cyanide is done at a higher solids content than when the soil-water mixture is first further diluted in a reaction vessel. If there are organic compound contaminants present together with the cyanide, the use of hypochlorite can lead to the formation of organo-chlorine compounds hydrogen peroxide is then to be preferred as oxidant. In practice a molar excess of oxidant, with respect to the cyanide, will be used, whereby the preferred excess will be 5 : 1 with respect to the stoichiometric ratio of oxidant: cyanide. The results obtained under these conditions show that for soil containing 930 mg of CN per kg, when oxidation takes place in the scrubber for 10 minutes with an excess of oxidant of 5 : 1 with respect to the stoichiometric quantity and at a pH of 11.5, the CN contamination remaining in the ground was less than 0.05 mg per kg of soil, whereby it should be noted that the detection limit for CN was 0.05 mg/kg. The cyanate ion is then further oxidized in the pump tank to CO2 and N2. In the next stage the soil is separated out from the suspension according to well-known methods and finally the aqueous phase is purified according to well-known techniques, this last falling outside the scope of the present patent application. The equipment for the implementation of the method, which comprises, amongst other things, a scrubber, pump tank, separator, buffer vessel, mixing vessel, second separator and floatation tank, should preferably be constructed in such a way as to be transportable in order that it can be set up to work at any desired location. For this purpose the equipment is arranged and assembled in such a way that it can be built into transportable containers. In itself it is known from Netherlands Patent Application 8200155 that such equipment or plant should be transportable but it is not stated there that the equipment should be built into containers for this purpose.

Claims: CLAIMS 1. Method for the cleansing and decontamination of soil byr (a) the extraction of soil contaminated principally by hydrocarbons or other organic material, heavy metals and/or cyanides, by mixing the contaminated soil with water of a certain pH as extractant, (b) separating the cleansed or at least partially cleansed soil from the water, (c) separating the contaminants from the water and (d) recirculating the water as extractant, characterized by an intensive washing of the soil, possibly pre-treated in sieves, by the aqueous extractant in a washer provided with stirrer, whereby the solids content is 60 - 80% by weight and the pH during and after washing is controlled according to the nature of the contaminants. 2. Method as in Claim 1 characterized by washing in the washer at a pH of 9 or more when the soil to be cleansed contains hydrocarbons and at a pH of 11 or more when the soil contains cyanides and heavy metals. 3. Method as in Claims 1 and 2, characterized by the use of a scrubber as the washer with two or more compartments, each provided with a mechanical stirrer. 4. Method as in Claim 3, characterized by the use of an "attrition scrubber" as the scrubber. 5. Method as in Claims 1 - 4, characterized by the oxidation of cyanides in the washer. 6. Equipment for the implementation of a method according to Claims 1 - 5, characterized by the fact that the various component parts such as the scrubber, pump tank, separator, buffer vessel, mixing vessel, second separator and floatation tank are assembled in transportable containers.

Inventors: JONKER C.
Applicants: WEGENBOUWMAATSCHAPPIJ J. HEIJMANS B.V.
Application year: 1985
Main classification: B09B5
IPC codes: B09C1, C02F9