This invention relates to the treatment of contaminants and will more particularly, though not exclusively, to the treatment of contaminants removed from contaminated soil. In International Patent Application PCT/AU93/00646 there is described a method and apparatus for removing toxic wastes or other contaminants from soil using a thermal desorption technique. Figures 1 to 3 of the accompanying drawings relate to the particular method and apparatus described. FIG 1 shows a schematic view of a first embodiment of the apparatus;

FIG 2 is a schematic view of a retort of a second embodiment; and FIG 3 is a sectional view taken on line 3-3 on FIG 2.

Referring to FIG 1, a retort 10 has walls 11 of e.g. stainless- or alloy steel capable of withstanding temperatures of > 1000°. The retort is closed by a conical or dome-like roof 12. On one side, a discharge chamber 13 has a weir 14 over which the treated soil tumbles into a discharge pipe 15 to a cooler 16 and then to a storage unit 17.

A substantially vertical spiral 18, rotated by a motor (not shown), has a tubular body 19 connected at its upper end to a soil feed pipe 20 connected to a supply 21 of contaminated soil, which may be in powder/fine granule form from a crusher (not shown). The lower end of the tubular body 19 terminates just above the floor 22 of the retort 10. A helical flyte 23 about the tubular body is separated into a lower spiral 24 and upper spiral 25.

The retort 10 is surrounded by a heating chamber 26, lined with refractory material, and has a gas exhaust stack 27 which incorporates a gas scrubber 28 (or other pollution control equipment).

A gas passage 29 connects the upper portion of the retort 10 to an "off gas" pipe

30 connected to the flame zone of a combustion chamber 31. LPG (or other fuel) 32 and air 33 are supplied to the combustion chamber 31 via suitable inlets 34, 35 and a passage

36 transfers the hot exhaust gases 37 from the combustion chamber 31 to the heating chamber 26.

The apparatus may be mounted on a vehicle or suitable transport frame to enable it to be transported so that the contaminated soil is treated on-site.

The operation of the apparatus will now be described.

Contaminated soil e.g. contaminated with pesticides or PCB's is crushed in the crusher and fed down the interior of the tubular body 19 of the spiral 18.

The retort 10 is heated to a temperature of approximately 500-1000°C by the hot exhaust gases 37 in the heating chamber 26.

The soil rises up the retort 10 and is circulated by the lower spiral 24, the soil coming into contact with the hot retort wall 11. At the level of the break between the spirals 24, 25, it partially settles (i.e. is less agitated) before being raised and circulated by the upper spiral 25. The contaminants are heated to the gaseous phase and are given off from the soil and due to the residence time of the soil in the retort, the contaminants may be at least partially destroyed, burnt or pyrolysed.

The treated soil falls or tumbles over the weir 14 into the discharge pipe 15 to be cooled by the cooler 16 and then conveyed to a storage unit 17.

The hot "off gas" is directed by the dome-like roof 12 of the retort 10 into the gas passage 29 and thereby by "off gas" pipe 30 to be burnt in the combustion chamber 31. The gas may have a residence time of e.g. 1-10 seconds at a temperature of e.g. 700- 1400° which will ensure all the contaminates (e.g. PCB's) are fully incinerated.

The hot exhaust gases from the combustion chamber 31 are directed to the heating chamber 26 to heat the retort, before being exhausted via the gas exhaust stack 27 and the gas scrubber 28.

It will be noted that the gases given off by the contaminants in the soil undergo a two-stage treatment process to ensure full removal and destruction of same.

The construction of the spiral 18, with its upper and lower spirals 24, 25, ensures efficient agitation of the soil in the retort and a long "residence" time of the soil particles in contact with the retort wall 11.

In a modified embodiment, the "off-gas" may be directed from the gas passage 29 to a secondary burner 40 which is used to preheat and/or dry the incoming soil feed before or as it passes along the soil feed pipe 20.

In an alternative embodiment, the "off-gas" is drawn from the gas passage 29 to a condenser 41, where it is cooled and placed in a contaminant storage unit 42 (e.g. for later destruction).

The air 33 for the chamber 31 may be pre-heated by a transfer of heat from the

cooler 16 for the treated soil.

The desorption of the contaminants from the soil may be assisted/accelerated by the pumping, of stripping gas 43 into the retort 10; and by the application of a pressure or partial vacuum in the retort 10 by a pump 44. Referring now to FIGS 2 and 3, the retort 110 is enclosed in the combustion chamber 131 for direct heat transfer to the retort 110.

The retort 110 is in the form of an annulus (see FIG 3) with an inner retort wall 111 connected to an outer retort wall 111 A by radially extending heat transfer fins 11 IB (which assist in transferring the heat to the soil). The soil from the supply 121 is transferred to the retort 110 by a screw conveyor

120 (with motor 120 A) and passes down the annular chamber 150 defined by the inner and outer retort walls 111, 111A, the mixing of the solid being assisted by the heat transfer fins 11 IB.

The shaft 119 of the vertical spiral 118 is driven by a motor 119A and the upper and lower helical flytes 125, 124 cause the soil to be agitated within the inner chamber 151 of the retort 110, defined by the inner retort wall 111.

A portion of the treated soil passes over the weir 114 and enters the discharge pipe 115 (to be cooled and stored as hereinbefore described). The balance is recycled to the annular chamber 150 and mixed with the incoming soil from the screw conveyor 120 for further treatment. The hot "off-gas" is drawn from the retort 110 via gas passage 129 and can be supplied to the combustion chamber 131; to the secondary burner to preheat and dry the incoming soil; or to the condenser to be cooled and stored, as hereinbefore described.

The rapid circulation of the soil in the retort 10, 110 ensures efficient heat transfer from the retort walls 11, 111, 111 A to the soil and better mixing of the soil to ensure all contaminants are desorbed from the soil.

For high boiling point (B.P.) contaminants the application of a partial vacuum to the retort 10, 110 by pump 44 will lower the B.P.

In certain installations, all or part of the heating for the retort may be provided by electrical heating, obviating the need for a combustion chamber.

The residence time of the soil in the retort required to desorb the contaminants will depend, inter alia, on:

1. the temperature of the retort;

2. soil particle size;

3. physical characteristics of the soil;

4. pressure in the retort; and 5. local treatment regulations.

It is possible to separate the contaminants (e.g. where different storage/handling is required for different toxicities) by treating the soil at e.g. 300°C to remove one or more contaminants, and the re-treatment at e.g. 600°C to remove the balance. Alternatively, two or more retorts may be provided in series to treat the soil in cascades, with the retort operating at increasing temperatures doing the series.

The present invention is particularly related to the treatment of the waste components extracted as a result of the process described above. In such a process one or more of the waste components may be volatilised depending on the operating conditions and the form and concentration of the waste components. As described above, the contaminated components can be transferred to a condenser of the direct or indirect type. In many treatments the contaminants being transferred to the condenser will contain moisture (which may be in the form of vapour) and often solid particles arising from the condensation of volatilised components or from carry over. The subsequent treatment of the condensate can cause problems depending essentially upon the contaminants being handled.

For example, if plasma destruction techniques are to be used to treat the condensate, problems can be encountered due to the presence of the moisture in the condensate because it greatly increases the volume of material to be treated rendering such destruction processes uneconomic. Furthermore, the presence of solid particles can interfere with or in some instances prevent the plasma destruction operation.

Contaminants containing DDT can provide additional problems. DDT can be found in soils which have contained cattle dips. In addition contaminants found in soils in this application may further include arsenic and other pesticides. DDT is one example of a contaminant which can turn to a solid when condensed and in addition may coat the condenser. The portion of the arsenic which is volatilised during the treatment is also condensed to form a solid. It will be appreciated that the formation of the solids and the

coating of the condenser can cause serious problems in the treatment of these contaminants.

In applications where the contaminated soils contain PCBs, generally the condensed volatiles form two liquid phases (aqueous and hydrocarbon). In another example application where the soils have been in timber treatment sites, these soils are likely to be contaminated with one or more of at least copper chrome arsenate, pentachlorphenol and creosote. Organochlorines, hydrocarbons and at least arsenic of heavy metals will be volatilised in the retort and condensing problems similar to that described with reference to DDT containing soils may be encountered. It is an object of the present invention to provide an improved method and/or apparatus for treatment of contaminants and/or for preparation of contaminants for subsequent destruction and which is suitable for use with thermal desorption apparatus of the type described in the aforementioned International application.

According to one aspect of the present invention there is provided a method of treating a gaseous mixture which may contain some solids, moisture and/or carryover containing contaminants the method including the steps of condensing the gas mixture and thereafter concentrating the concentrate for subsequent treatment, collection and/or destruction.

According to another aspect of the present invention there is provided apparatus for treating a gaseous mixture which may contain some solids, moisture and/or carryover containing contaminants including condenser means and concentrater means downstream of the condenser means for receiving the condensate and concentrating or separating it.

In one form the concentrating step comprises transferring the condensate to a settling tank where separation and as such concentration is effected by gravity. In another arrangement concentration may be effected through solvent extraction, washing, distillation, adsorption, chromotography or any other suibable technique.

Under certain circumstances, it is desirable that the concentrate is filtered prior to the concentration step.

A filter is incorporated into the system between the retort and condenser to prevent the carryover of solids to the condenser. The temperature of the gases to the filter is usually kept above the temperature of the solids in the retort to prevent condensation in the ducts or the filter. The filter may be of a type which can be cleaned by a reverse

pulse of air. The soil removed by the filter may be returned to the cleaned soil from the retort. The filter medium may be ceramic, metallic, a fabric, or other suitable material.

The filter may also be operated below the temperature of the solids in the retort to provide selective condensation of fume. In this mode the temperature is usually kept above the dew point of the gases.

It may be further desirable to feed the condenser wash to the condenser to remove any solids which occur in the condenser during the condensing process. The condenser wash may include a solvent.

Where a solvent wash and/or solvent extraction technique is used, solvent may be returned to a solvent storage tank whereupon at a selected contaminant concentration it can be removed for further treatment, destruction and/or collection.

The solvent from the solvent storage tank can be recycled for use in the solvent extraction and/or wash stages described above.

One of the major reasons for condensing the volatiles from the retort rather than incinerating them in the after burner would be because of restrictions on incineration or simply economic reasons. It will be appreciated that ultimate disposal methods other plasma destruction could be used such as base catalysed dechlorination.

A preferred embodiment of the invention will hereinafter be described with reference to FIG 4 of the accompanying drawings which is a flow sheet for treatment of a single liquid phase condensate. The dotted line shows the main flow lines of contaminant.

As can be seen in the drawing, gaseous material from the retort which is described in FIGS 1 to 3 enters the condenser 100 whereupon the contaminants are condensed and the condensates optionally transferred to the condensate or storage drum 102 as shown. Prior to entering the condenser 100 the gaseous mixture passes through a filter 120 to prevent carryover of solids to the condenser. If desired the condenser 100 can receive a condenser wash in the form of a solvent from solvent tank 108 to ensure that all contaminants are removed from the condenser. Incondensibles such as other contaminants that cannot be condensed and solids not removed from the gas stream in the condenser, can be discharged from the condenser via line 200 for treatment or destruction such as by incinerations, scrubbing, carbon absorption or the like.

The condensate collected in the condenser drum 102 can be transferred to a filter

104, or other separating means such as a centrifuge, gravity settler by way of example, where further particulate matter can be collected. There is also provided means via line

202 for washing the filter solids collected by means of a solvent or further decontamination of those solids. The condensate is transferred from the filter 104 to a condensate wash or extraction device 112 where it is also treated with a solvent.

The solvent wash containing the contaminants is returned to a solvent tank 108 where it is stored. The solvent tank contaimng the condenser wash is adapted to be recycled for a selected period of time using the pump 110. Once the contaminants in the solvent have reached a selected concentration they can be removed via line 204 for destruction such as by plasma destruction techniques.

An example of a plasma destruction device is described in the specification of Australian

Patent Application No. 80053/87.

Solids removed from the filter and the liquids removed via line 206 from the condensate wash can be further treated or disposed of in any suitable manner.

Three examples of contaminant treatment for particular applications are set out below. In each case, soils with the stated contaminants are firstly treated in an apparatus as described with reference to FIGS 1 to 3.

Case 1: Former Cattle Dips These contain various contaminants including DDT and its derivatives (DDE,

DDD) and arsenic as well as other pesticides. The DDT turns to a solid when condensed.

This solid DDT may coat on the condenser or report to the condensate. DDT is slightly soluble in water which usually forms the bulk of the condensate. The portion of the arsenic which is volatilised is also condensed to form a solid. A possible treatment to produce a DDT concentrate suitable for plasma destruction may include:

(a) Solvent wash of condenser to remove deposited DDT.

(b) Filtration of the condensate to remove solids.

(c) Solvent extraction or solvent wash of the solids removed from the condensate to remove DDT. This may utilise the condenser wash solvent.

(d) Solvent extraction of the condensate. This may also use the condenser wash solvent.

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(e) Further cleanup of the condensate using conventional means.

(f) Possible further concentration by e.g. distillation of the DDT laden solvent prior to plasma destruction.

(g) Solvent extraction or acid washing of the filtered solids to remove arsenic (if required).

(h) Treatment or disposal of this extract.

(i) Return of remaining decontaminated solids to blend with product from the retort.

It may be possible to fix the filtered solids and dispose to landfill thus avoiding steps (c), (g), (h) and (i). Case 2: PCB Contaminated Soil

Retorting trials with PCB contaminated solid gave two liquids phases (aqueous and hydrocarbon) when the volatiles driven off from the retort were condensed.

These would be gravity separated for treatment as necessary.

This treatment may require solvent extraction or distillation to concentrate the PCBs for plasma or other means of destruction.

Solids may also be present in either liquid phase if carried over from the retort. Case 3: Timber Treatment Site Soils

These soils are likely to be contaminated with one or more of at least c.c.a (copper chrome arsenate), pentachlorophenol, and creosote. Organochlorines, hydrocarbons and at least arsenic of heavy metals will be volatilised in the retort. Treatment along the lines of Case 1 could be utilised although the condenser wash may not be necessary.

Finally, it is to be understood that the inventive concept in any of its aspects can be incorporated in many different constructions so that the generality of the preceding description is not to be superseded by the particularity of the attached drawings. Various alterations, modifications and/or additions may be incorporated into the various constructions and arrangements of parts without departing from the spirit or ambit of the invention.