CONTAMINATED SOILS

The present invention relates to the treatment of contaminated soil and more particularly but not exclusively to the extraction of toxic material from soil.

Soils contaminated with toxic and other contaminants pose a significant health risk. Whilst currently used techniques have been effective they are in general inefficient and relatively costly.

It is an object of the present invention to provide a method and apparatus for treating contaminated soil and in a relatively efficient manner.

According to one aspect of the present invention there is provided apparatus suitable for use in the treatment of contaminated soil, the apparatus comprising a pre-sizing device for separating the soil into at least one group of relatively fine solids and at least another group of relatively coarse solids, contaminated soil treatment means for receiving the group of relatively fine solids for treatment with a solvent; separating means for at least partially separating the contaminated solvent mixture from the treated soil; and solvent treatment means for treating, or at least partially treating at least part of the contaminated solvent mixture after separation from the soil.

Contaminants usually concentrate in or on the smaller particles. Whilst this can vary in some cases the contaminants will be concentrated on around 20% of the total soil.

It will be appreciated therefore that by separating out the soil that does not contain the contaminants can significantly reduce the amount of soil required to be handled by the separation parts of the apparatus.

The pre-sizing device can take any suitable form and may for example comprise a wet or dry screening device or may for example comprise an elutriation process. In

one preferred form a wet screening device is used.

The solvent treatment means may include a treatment chamber and means for generating conditions within the chamber sufficient to dissociate components in the contaminated solvent mixture delivered thereto into constituent ions and thereafter cause oxidation or other chemical reaction of at least some of the constituent ions.

In one form, the treatment chamber may comprise a pair of electrodes therein operable to produce an electric arc. There may further be provided means for delivering the contaminated solvent mixture into the chamber and into proximity with the arc generated by the electrodes. The delivery means may include a conduit disposed within one of the electrodes, the conduit having an outlet end arranged adjacent the electrode gap*

The apparatus may further include residual solvent separation means which in one form may comprise a dryer for heating the treated soil for removing for residual solvent contained therein. Advantageously, the residual solvent is arranged for return to the separation means or other part of the process. In one form the coarse material separated in the pre-sizing device may be delivered to and mixed with the soil in the residual solvent separation means or downstream thereof. This means that the coarse particles can be recombined with the treated finer particles.

The apparatus may further include solvent recovery means which may include means for receiving solvent which includes means for separating the contaminated solvent mixture into a first fraction and second contaminated fraction, the contaminated fraction being arranged for delivery to the solvent treatment means. This separating means may comprise distilling means or the like.

Advantageously, the first fraction is returned to the contaminated soil treatment means. In the particular form of the invention where the sizing device comprises a wet screening device, the solvent recovery means may separate the water from the solvent and contaminants so that it can be suitably treated and optionally recycled for example in the sizing device. Part or all of the water may be used.

In one form the contaminated soil treatment means may comprise a vessel adapted to receive the contaminated soil and solvent therein so as to form a slurry. The vessel may include mixing means for mixing the contaminated soil and solvent together.

The separation means may comprise a settling tank which at least in part separates the contaminated solvent mixture from the treated soil. The separating means may further include a filter for receiving the contaminated solvent mixture from the settling tank. In another arrangement the separation means may comprise a centrifuge or the like.

A heat exchanger may be arranged to utilise the heat generated by the solvent treatment to at least assist in operating the dryer.

According to another aspect of the present invention there is provided a method of treating contaminated soil comprising the steps of pre-sizing the soil so as to separate it into at least one group of relatively fine solids and another group of relatively coarse solids contacting the contaminated soil in the first group with the solvent so as to form a slurry, transferring the slurry to separating means for at least partially separating the contaminated solvent mixture from the treated soil and transferring at least part of the contaminated solvent mixture to a solvent treatment means. Preferably, conditions are produced in the treatment means to dissociate components in the solvent mixture into constituent ions for subsequent oxidation of at least some of the constituent ions.

A preferred embodiment of the invention will hereinafter be described with reference to the drawing, in which:

Figure 1 is a block diagram of apparatus according to one form of the present invention.

The apparatus shown in Figure 1 comprises a pre-sizing device 10 which receives the contaminated soil via line 12 which may include a conveyor belt or the like and separates it into a group of smaller sized particles and a group of coarser particles. The group containing the smaller particles is passed to soil treatment means 20 via line 14.

The soil treatment means may be in the form of an extraction vessel having a motorised

stirrer with a soil delivery line 14 and solvent delivery line 46 for feeding materials to the vessel for agitation and mixing together. Known toxics include but are not limited to polychlorinated biphenols (PCB), dichloro benzene (DCB), polychlorinated dibenzodioxin (PCDD), polychlorinated dibenzo furan (PCDF), diclorin, aldrin, parathion and malathion. In fact, the process is applicable to any contaminant, toxic or otherwise, which is able to be dissolved. For example, heavy metals including copper, chromium, and arsenic in combination with insecticides such as pentachlorphenol or DDT may be treated. The introduced solvent may include make-up or fresh solvent delivered along line 25 and may also include recycled solvent which is generated downstream in a process as will be described later. The soil treatment means 20 produces a slurry which is transferred along line 24 to separating means 30 via line 24. The separating means 30 may comprise a settler tank which produces a clarified solvent which is drawn off and if desired pass to a filter, centrifuge or the like. Thereafter the treated soil can pass along line 32 to residual solvent treatment means 60 which may be in the form of a rotary dryer. The dryer is operable to vapourise residual solvent in the contaminated soil and the cleansed soil is discharged through line 62. The coarse soil particles may be returned to the treated soil via line 16 and mixed before discharge. The vapourised solvent may be condensed and returned to the settling tank or elsewhere. Dependent upon the number and concentration of contaminants more than one solvent extraction stage may be necessary and more than one solvent may be used.

The solvent containing the extracted toxics is passed along line 34 and fed into a solvent recovery means 40 which may comprise a distillation column which includes a condensor for producing a purifier recycled solvent which passes along line 46 and is returned to the soil treatment means. This device also produces a bottom fraction which passes out along line 42. This fraction contains concentrated toxics. In addition, where water is present this may be separated and passed out through line 44 for further treatment or possible reuse of some or all in the sizing device 10. When the solvent has a higher boiling point than the contaminant then the solvent is the bottom fraction in the distillation column and the contaminant is condensed from the overhead vapours. When water is also present a further separation stage may be necessary or it may be able to be removed as a sidestream, top or bottom in the distillation operation.

Some solvent may be recuperated from the bottom fraction by passing it through for example a re-boiler, the output of which feeds to the distillation column. The concentrated remainder of the bottom fraction which may include waste water, solvent and toxins is delivered via line 42 to solvent treatment means 50.

The solvent treatment means may includes a chamber and means for generating an electric arc within the chamber so as to disassociate components in the contaminated solvent mixture into constituent ions. The chamber contains an oxidising agent for oxidising the constituent ions. It will be appreciated that other treatment apparatus could be used.

An example of a suitable device is described in Australian Patent Specification No. 80053/87. In general, the apparatus includes a pair of electrodes between which there is produced an electric arc reaching temperatures of 12000°F to 16000°F. The mixture is brought into proximity of the arc. One way of accomplishing this is to provide that one of the electrode includes a central conduit or passageway. Material is fed through the central conduit and is emitted in the region of electrode gap. The oxidation products of the apparatus may be fed into a second pair of electrodes for disintegration if required.

As well as or alternatively to the contaminants may be recovered for re-use, stored or destroyed by some other means such as for example, in a molten metal bath, incineration or the like. The reduction in the volume of material can make this economic.

Heat may be recovered from the combustion occurring in the apparatus to heat air which can be passed through a heat exchanger for use in connection with the rotary dryer.

Combustion gases from the treatment means 50 are passed via line 32 through gas treatment station 70 which may comprise a flue-gas scrubber and which may include a water or other scrubber liquor treatment apparatus. The products of the scrubber are waste water and scrubbed gas. The scrubbed gas is passed to a stack.

There are significant benefits obtained from the apparatus and method according

to the preferred form of the invention. A single solvent can be used in the extraction/toxics concentration route. Toxics are concentrated in a relatively low volume of material. Further low temperature processing does not chemically modify the soils, thus minimised problems for disposal of cleaned residues. In addition, high destruction temperatures and high combustion efficiencies are obtained using the apparatus.