WO1997030757A1

WO1997030757A1 - Treatment process for contaminated waste

Info

Publication number: WO1997030757A1
Application number: PCT/CA1997/000114
Authority: WO
Inventors: Rikkert J. Vos
Original assignee: B.C. Research, Inc.
Priority date: 1996-02-23
Filing date: 1997-02-19
Publication date: 1997-08-28
Also published as: AU1715297A

Abstract

A process for stabilizing leachable heavy metals contained in a waste material. The waste material is contacted with carbon dioxide. The invention is of particular application in the stabilizing of lead, particularly in waste material produced by the combustion of solid waste.

Description

TREATMENT PROCESS FOR CONTAMI ATED WASTE

FIELD OF THE INVENTION

This invention relates to a process for stabilizing leachable heavy metals in waste material. The invention finds particular application in the stabilization of lead in incinerator ash so that the lead cannot be leached into the environment .

DESCRIPTION OF RELATED ART

The incineration of solid wastes, particularly municipal garbage, is finding increasing application worldwide. It has the advantage of reducing the cost of landfill disposal and, of course, saving landfill space. However, incineration has its own problems. It results in air emissions and the production of ash. The ash is a problem in that it contains heavy metals, particularly lead, which can be leached out from any disposal site. There are a number of tests to determine leachability, for example, the United State Environmental Protection Agency has published the Federal Registry, Volume 40, Part 261, Appendix II, Method 1311 entitled Toxicity

Characteristic Leaching Procedure (TCLP) 1993, p.p. 66 to 82. As an example of the Canadian regulations the Province of British Columbia has published Regulation 63/88, O.C. 268/88, Waste Management Act, Special Waste Regulations, February, 1988 (S.W.E.P.) .

Thus, although the use of incineration has undoubtedly been an advantage, there are still problems to be solved and the present invention addresses one particular problem, namely the leaching of heavy metals, especially lead, from the ash produced in the incineration of solid wastes. The invention is such that the treated ash does not contain lead that is leachable. This means that the ash can be disposed as a non- hazardous waste or used in, for example, the production of cement and concrete. SUMMARY OF THE INVENTION

Accordingly, the present invention provides a process for stabilizing leachable heavy metals contained in a waste material that comprises contacting the waste material with carbon dioxide. The reaction may be carried out in the presence of water.

In a preferred embodiment, the heavy metal is lead. In a further preferred embodiment, the waste material is ash, particularly ash produced from a municipal incinerator.

The carbon dioxide may be added in gaseous or solid form, the latter as dry ice. The carbon dioxide might also be the carbon dioxide in the air, that is air may be used as a source of carbon dioxide.

The process may be carried out at atmospheric pressure but it is desirably carried out in a closed vessel. This permits the use of pressure greater than atmospheric pressure which reduces the reaction time. Pressures up to about 90 pounds per square inch gauge (psig) have been used. Higher pressure may be used to speed the reaction rate.

The process may be controlled by monitoring the pH during the addition of the carbon dioxide. Typically, the process results in a decrease in pH from approximately 12 to 11.5 or lower.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is illustrated in Figure 1, which is a flow diagram showing the stabilizing of fly ash with carbon dioxide. DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 1 shows a closed reactor 10 having an inlet 12 for ash and a mixer 14. The reactor 10 may be a batch or continuous reactor. There is an inlet 16 to receive carbon dioxide from a carbon dioxide storage tank 18. Water is added through an inlet 20. Typically water is added in an amount of about 40 to 65% moisture content. The stabilized ash slurry from the reactor 10 leaves through an outlet 22 and passes to a dewatering unit 24. Water from the dewatering unit 24 is fed back to the reactor 10 via inlet 20. Make-up water is added as desired, through a conduit 26. Sludge from the dewatering unit 24 leaves the unit 24 through outlet 28. This sludge typically contains about 30% moisture and is sent for disposal or use. For example, it may be used as an inert filler in the production of concrete or cement.

In an alternative, illustrated to the right of Figure 1, effluent from the reactor 10 may be fed to a dryer 30. The slurry from the dryer is disposed of in the same way as the effluent from the dewatering unit 24.

The following example illustrates the invention:

Fly ash obtained from the Greater Vancouver Regional District (GVRD) in Vancouver, British Columbia, failed to pass the British Columbia SWEP test or the USTCLP test.

It was treated with water and carbon dioxide in apparatus as schematically illustrated in Figure 1. Carbon dioxide was added at 90 psig in a pressure vessel for tests 1, 2 and 3 and at atmospheric pressure for test 4. The period of reaction was about 0.5 hrs. The quantity of carbon dioxide added ranged from about 8.00% to 14.03% based on the volume of the wet ash. In general the amount of carbon dioxide can be determined by routine experiment as the amount needed to stabilize the lead.

The following results were achieved:

It is clear from the above results that the process of the present invention has taken an ash unacceptable under the known tests and produced a solid product that easily passes the known tests. Furthermore, the pronounced alkalinity of the untreated ash has changed in the treated ash to about neutral.

Although the present invention has been described in some detail by way of example for purposes of clarity and understanding, it will be apparent that certain changes and modifications may be practised within the scope of the appended claims.

Claims

I CLAIM :

I. A process for stabilizing leachable heavy metals contained in a waste material that comprises contacting the waste material with carbon dioxide.

2. A process as claimed in claim 1 in which the heavy metal is lead.

3. A process as claimed in claim 1 in which the waste material is produced by the combustion of solid waste .

4. A process as claimed in claim 1 in which the carbon dioxide is added as a gas, liquid or solid.

5. A process as claimed in claim 1 in which the carbon dioxide is added diluted with air or supplied from the air.

6. A process as claimed in claim 1 in which water is added.

7. A process as claimed in claim 6 that includes monitoring the pH during the addition of carbon dioxide.

8. A process as claimed in claim 6 including dewatering the treated waste.

9. A process as claimed in claim 8 in which dewatering is carried out by evaporation or filtration.

10. A process as claimed in claim 9 in which the effluent from the dewatering is recycled to be added to the untreated waste material.

II. A process as claimed in claim 1 carried out in an enclosed vessel .

12. A process as claimed in claim 1 in which water is absent from the process.

13. A process as claimed in claim 12 in which pressure and/or heat is applied to increase the stabilization rate.