GB2185558A - Method of decontaminating soils - Google Patents

Abstract

Soils which have been affected with high-boiling or not easily decomposable organic pollutants are decontaminated by direct flame treatment in a rotating drum. The temperature is in the range 600-1,200 DEG C. Catalytic inserts of copper, vanadium or molybdenum steel in the drum stir the soil. Combustion or decomposition of the pollutants may be promoted by alkali, alkaline earth oxides, hydroxides or carbonates or by magnesium, aluminium iron or zinc powders. Adjusting the stoichiometric ratio of oxygen to fuel promotes either complete combustion or splitting of the pollutants into low molecular fragments for recovery from the flue-gas.

Description

SPECIFICATION Method of decontaminating soils The invention relates to a thermal method of decontaminating soils which have been affected with high-boiling, or not easily decomposable, organic pollutants.

Polluted soils of this type are an environmental burden and hinder utilization or or re-utilization of the ground affected. There is also the riskthatfurther distribution of the pollutants in the soil will affect the ground water.

The pollutants musttherefore be removed from the soils contaminated by them, and the method used should be as economical and effective as possible.

The most economical method known at present is heat treatment. It is known from the German Specif ication 3216771 Al in a first stage to heat polluted soils to a temperature sufficientto allow the contaminating substances, and/ortheir undesired conversion products to pass into a gaseous form and, in a second stage, to refine the gaseous products. The optimum temperature for the first reaction stage is regarded as being in the region of between 200 and 400"C.

According to the German Specification 3427532 C1 contaminated masses of soil are "heated red-hot" at between 600 and 800.060C and the flue gases released are subsequently burnt.

These methods are not sufficient, however, for complete decontamination of soils if the soils have been contaminated with particularly high-boiling or not easily decomposable, organic pollutants, such as high-temperature-resistant softeners, polychlorinated aromatics or mazut.

Because of absorption or complex-forming processes with the soils (the degree to which these pro cesses take place depending upon the nature of the soil) the high-boiling or not easily decomposable pollutants, unlesstheyare present in small quantities only, are not completely removed by the said methods or are degraded to form gaseous products.

These are partly retained in the soil masses orare converted into other more stable and less volatile substances such as dioxins and remain in this form in the treated soils.

The invention therefore sets out to provide a method by means of which it is possible to dec ontaminate economically and effectively soils which have been affected with large quantities of high boiling or not easily decomposable substances.

This object is attained by a method of dec ontaminating soils which have been affected with high-boiling or not easily decomposable organic pol lutants, by heat treatment, characterized in that the contaminated soil material is treated by directflame treatment in a rotarydrum.

It has been found that when contaminated soils are su bjected to a d irect fla me treatment the high- boiling or not easily decomposable pollutants are re moved without trace. What is crucial is the factthat the soil can dwell in the reaction zone oftheflame, whereby a changing surface is produced so that there are no large agglomerates enclosing the pol lutants. This has two effects. In addition to a proper heating of the soil material the pollutants come into direct contact with a highly reactive medium and are thereby forced more vigorously to react. Even flameresistant, thermostable softeners react in the reaction zone ofthe flame.It is possible, by selecting the reaction conditions and by selecting the heating and flame temperature and the quantity of the oxygen supplied, to influence the reaction of the pollutants.

Thus an excess of oxygen leadstothorough com- bustion,while a quantity of oxygenwhich isequiva- lent to the quantity of the fuel introduced, leads essentiallytoasplitting ofthehigh-boiling ornoteasily decomposable pollutants. Binding energies with the soil material are substantially overcome in the flame zone, so that no pollutants, decomposition or conversion products are retained.

On account of the slow rotation ofthe drum during the treatment a constantly renewed surfaceofthe soil material is produced in the flame treatment zone.

This importantfeature is substantially increased when the interior of the rotating drum has a static insert by which the soil material is more vigorously circulated. In this way the effectiveness is increased and the dwell periods of the soil material in the rotating drum can be reduced.

It has been found that the decomposition or combustion of the high-boiling or not easily decompos- able pollutants is considerably accelerated if the static insert of a rotating drum reactor consists of a steel or is coated with a steel which contains at least one ofthe three metals copper, vanadium or molybdenum. These steels can also be alloyed with additional metals, without their catalytic activity being affected. A similar but less effective, catalytic effect would also be achieved if the rotating drum consisted of a copper-, vanadium- or molybdenumcontaining steel or were coated on the inside with a steel of thins type. This would be less advantageous, however, from the economic point of view.

A further acceleration of the decomposition or combustion of the high-boiling or not easily decomposable pollutants is achieved if slight quantities of alkali or alkaline earth oxides, hydroxides orcar bonates are mixed with the contaminated soil before the flame treatment according to the invention. To avoid affecting the subsequent biological reactivation of the decontaminated soil the addition of alkali substances should not be excessive. However, depending upon the original basicity of the soil material, a quantity of between 100 and 5000 ppm of these alkali substances may therefore be mixed with the contaminated soil.

Decontamination can be further facilitated, in part icularwhen halogen-containing organic compounds are present as pollutants, if slight quantities of reactive metal powders such as magnesium, aluminium, iron orzinc powders are mixed with the soil before the flame treatment according to the invention. The quantities ofthese additions should preferably not exceed 5000 ppm, in case they affect subsequent biological reactivation.

The heat treatment of substances in a rotating drum is knownperse. Suitably equipped rotating drums are available in various branches of industry and can,where necessary after appropriate refitting, be used for the method according to the invention.

In orderto carry out a preferred form of the method, the rotating drum is continuously charged atthe end opposite the flame with a contaminated soil mass. The soil mass introduced is conveyed to the flame either by a screw conveyor or, when the rotating drum is lying obliquely, bytherotational movement. In the process the soil is preheated by heating from outside or where appropriate by the flue gases as well, and is dried and crumbled. Atthe other end ofthe drum there is a burner, through which a solid, liquid or gaseous fuel, together with oxygen or an oxygen-containing gas, preferably air, are blown into the rotating drum in such a way that as large as possible a flame is produced. This flame is pointed directlytowardsthe arriving soil mass.In the flame zone there are also the catalytically active static inserts of copper-, vanadium- or molybdenumcontaining steel. The rotational movement and the conveying speed are set in such a way that each particle of soil comes into contact with the flame before the soil mass is removedfrom the rotary drum. The soil mass treated according to the above method is effectively free of pollutants and after cooling and appropriate regeneration can once again be used as soil material.

Depending upon the pollutant removed and the treatment conditions the flue gases may comprise only carbon dioxide and water. In such a case they may then be led off directly through the flue. If they contain gaseous pollutants such as HCI or NOxfor example they mustfirst be cleaned. If they contain low-molecularfragments ofthe pollutants these can be separated and re-used as raw materials. It is usually necessarythereforeto cool the flue gases rapidly. An advantage of the rapid cooling of the flue gases is thatthis prevents the re-formation of pollutants (such as for example dioxins) in the discharge gas.

The treatmenttem perature depends upon the nature ofthe pollutant and the contaminated soil. For economic reasons, the lowest usabletemperature range, is preferred, and this lies at approximately 600"C.Th is is possible with predominantly arenac- eous soils and with those pollutants which, although not easily volatilized, can be decomposed relatively easily. Substantially argillaceous soils, which have been contaminated with the same pollutants, require treatment temperatures approximately between 50 and 100 C higher. Pollutants, which can be converted by heat into more resistant compounds the heat, re quire treatmenttemperatures of up to 1 200"C.

Such high temperatures are generally not achieved by burning fuel with air, but are usually achieved by spraying in precisely-determined quantities of a mixture consisting of a high-quality fuel and a small surplus of oxygen.

Irrespective ofwhich organic pollutants are removed from the contaminated soil, the stoichiometric ratio of oxygen to fuel should usually lie within the range offrom 1:1 to 4:1. The course ofthe pollutant decomposition can be affected by varying the quantities of oxygen within this range. As a substantial oxygen surplus typically leads to complete combustion of the pollutants, a quantity of oxygen equi valent to the fuel supplied, or only a slight oxygen surplus (e.g. an oxygen/fuel ratio offrom 1:1 to 1.3:1) results predominantly in a splitting of the pollutants into low-molecularfragments, which are separated from the flue gases for re-use where appropriate as raw materials.

The flue gases contain the low-molecularfragments in an activated state. In order to prevent further cracking or combustion reactions or there- formation of large molecules, in the preferred method according to the invention the flue gases are cooled as rapidly as possible. To this end they are drawn off immediately behind the flame zone and are directly cooled. This cooling is preferably carried out by quenching with a high-boiling oil, such as anthracene oil for example, in which the low-molecular fragments are soluble.

The said cracking processes are preferably moreover also catalysed by static inserts present in the rotating drum and consisting of or comprising surfaces consisting of copper-, vanadium- or molybdenum-containing steel, and by the additions of alikali substances or metallic powder. In this way therespective reaction temperature can be lowered by approximately 50to 100 and the through-putspeed of the soil masses can be increased.

Claims (11)

1. A method of decontaminating soils which have been affected with high-boiling or not easily decomposable organic pollutants, by heat treatment, characterized in thatthe contaminated soil material is treated by direct flame treatment in a rotary drum.

2. A method according to Claim 1, characterized in thatthe soil material is treated at a temperature in the range of between 600 and 1200 C.

3. A method according to Claim 1 or 2, characterized in that the soil material is moved relative to the rotary drum by means of a static inserttherein.

4. A method according to Claim 3, characterized in thatthe static insert consists of a copper-, vanadium- or molybdenum-containing steel.

5. A method according to Claim 3, characterized in that the static fitting has a surface of a copper-, vanadium- or molybdenum-containing steel.

6. A method according to any of Claims 1 to 5, characterized in that for directflame treatment a fuel mixed with oxygen or an oxygen-containing gas is blown into the rotating drum.

7. A method according to any of Claims 1 to 6, characterized in that the stoichiometric ratio of oxygen to fuel lies in the range of between 1:1 and 4:1.

8. A method according to any of Claims 1 to 7, characterized in setting a stoichiometric ratio of oxygen to fuel in the range of between 1:1 and 1.3:1 wherebythe pollutants are not burnt or are only part iallyburnt, but are split into low molecular fragments which are separated from the flue gases and are used again as raw materials.

9. A method according to any of Claims 1 to8, characterized in that the flue gases are drawn off im mediately behind the flame zone and are quenched.

10. A method according to any of Claims 1 to 9, characterized in that between 100 and 5000 ppm of alkali or alkaline earth oxides, hydroxides or carbonates are mixed with the contaminated soil to be treated.

11. A method according to any of Claims 1 to 10, characterized in that up to 5000 ppm of a magnesium, aluminium, iron or zinc powder are mixed with the contaminated soil to be treated.