AU2004100541A4 - Method For Treating and Soluble Heavy Metal Concentrates at Source - Google Patents

Description

Description Specification Description FIELD OF THE INVENTION Treatment of soluble heavy metal concentrates usually involves diversion of the wastes through third party treatment processes, which adds to logistical and treatment costs. The present invention allows for on site treatment directly in the receptacle in which the treated material will be transported off site. The invention relates to a method for converting a soluble metal concentrate or slurry, which is environmentally unacceptable, to a stabilized spadable material that is environmentally acceptable for fertilizer application or tip disposal. The method involves mixing with said waste material, which is in the form of a soluble heavy metal concentrate or unstable sludge, with a particulate alkali reagent that contains an effective amount of calcium oxide to stoichiometrically form insoluble metal hydroxides. The resultant exothermic reaction results in the thermosetting of a stabilized spadable solid, thereby resulting in an environmentally acceptable material.

BACKGROUND OF THE INVENTION Metal plating industries are faced with significant environmental problems associated with soluble heavy metal waste liquors. Significant work has been done in recent year in developing methods for safely handling these materials and for neutralizing their troublesome characteristics so they can be safely disposed of. Stabilization processes either reduce the solubility, chemical reactivity, or physical entrapment of the waste. Stabilization processes typically include adjusting pH, converting metals to hydroxides, or establish oxidation-reduction conditions in the waste to prevent solubilisation and/or leachability to ground water.

The most commonly used inorganic stabilization processes include mixing the waste product with inorganic materials such as quicklime, hydrated lime, fly ash, cement kiln dust, lime kiln dust, Portland cement, and pozzolanic materials.

Although a significant amount of work has already been done to treat contaminated waste materials, there is still a considerable need in the art for improved methods for treating and neutralizing such materials. Furthermore, stricter governmental regulations are also making on-site treatment of hazardous waste more desirable.

DETAILED DESCRIPTION OF THE INVENTION Typically metal plating waste liquors will not meet environmental laws and/or regulations. After treatment in accordance with the present invention, it is converted to a relatively innocuous spadable solid which will meet such regulations, and will thus become environmentally acceptable, meeting leachate criteria for tip disposal, or for solids comprising of trace elements, such as iron and zinc, the material is valuable as a fertilizer additive. Furthermore, the method of the present invention can be performed on-site, thus, not requiring the transportation of contaminated substances that may be subject to even more restrictive regulations. In addition, cost savings due to reduced logistics via external treatment plants has economic and environmental benefits.

The waste material to be treated is placed into a receptacle, ideally in which the stabilized material will be transported off site. An ideal receptacle is a lift-on 9m3 waste bin or skip which can carry up to 12 tonnes, which standard lift-on trucks can service.

An effective amount of a reagent material is then mixed with the waste material or sludge. The effective amount will typically be that minimum amount needed to cause the sludge to become non-flowable. A typical liquid plating waste or slurry will be a water based concentrate which has a density in the range of about 1.2 1.5 kg/litre. A flowable material in the above density range will typically be made non-flowable by adding enough reagent material to increase its density to at least about 1.5 1.8 kg/litre.

The reagent material suitable for this reaction will contain about 70 wt. of calcium oxide.

Typically the reagent material is selected from the group consisting of quicklime, hydrated lime, fly ash, kiln ash, flue dust, and flue ash. The reagent must generate sufficient hydroxide to stoichiometrically react with the metal cations to form insoluble metal hydroxides.

After mixing the reagent material with the soluble metal concentrate the temperature of the mixture will exothermically increase to 70'C 90'C. Care must be exercised to add the latter half of the reagent in a slow controlled manner to avoid the reaction mixture boiling over. However, the higher the heat, the better the curing stage, during which the slurry turns into a spadable solid.

In a preferred embodiment of the present invention, the concentrate is a flowable dense solution or slurry and the effective amount of alkali reagent is at least that amount which will cause it to become non-flowable after reaction.

In another preferred embodiment of the present invention, the calcium oxide containing material is selected from the group consisting of quicklime, hydrated lime, fly ash, kiln ash, flue dust, and flue ash; providing sufficient hydroxide to stoichiometrically react with the metal cations to form insoluble metal hydroxides.

In yet other preferred embodiments of the present invention, the soluble heavy metal concentrate is selected from spent galvanizing pickling acids; from spent heavy metal plating concentrates, including cadmium, nickel and reduced chromium (VI) salt solutions, or from similar heavy cationic metal concentrates, including flowable precipitated sludge, isolated in a receptacle such as a skip, tank or bin.

The following example is for illustrative purposes only and should not be taken as limiting the present invention in any way.

EXAMPLE 1 A 6 m3 volume of spent galvanizing acid liquor was added to 9 m3 skip and treated in accordance with the present invention. The waste acid, containing 5 %wt. zinc and 7 %wt. iron as their chloride salts, with a residual of 4 %wt. hydrochloric acid, and a density of -1.3 kg/L, was pumped into a 9 -3m3 skip. A portable mixer was suspended across the bin walls with the 450mm impeller some 400mm below the surface.

Quicklime was added from a 1 tonne bulkibag suspended over the bin by means of a forklift. A sock arrangement at the bottom of the bulkibag was dangled close to the surface within a metre of the stirrer and the first tonne added to the mixture within 15 minutes. The reaction mixture was then stirred for another hour, during which the temperature exothermically increased to 90°C. After 6 hours and cooling to 50°C, a second tonne of quicklime was added with mixing. The impeller was withdrawn from the slurry after adding the reagent and before the thermoset occurred.

The skip was allowed to cool over 24 hours for complete curing. The Toxicity Characteristics Leaching Procedure (TCLP) testing proved that the leachate characteristics of the stabilized solids met landfill criteria.

Claims (4)

1. A method for treating environmentally unacceptable soluble heavy metal concentrate that cause it to be environmentally acceptable, which comprises: isolating said soluble heavy metal concentrate, usually acidic, having a density of about 1.2 to kg/litre, containing about 10 30 wt.% of soluble metals as salts, in a receptacle; mixing an effective amount of alkali material with said concentrate to increase its density to about 1.5 1.8 kg/litre, wherein the reagent material contains at least about 70 wt.% of calcium oxide or alkali equivalent; mixing the alkaline treated blend whilst the exothermic reaction generates temperatures of 70 0 C 90 0 C, and then allowing the mixture to sets into a spadable solid on cooling; repeating steps through if the resulting spadable solid of step is sloppy or not yet environmentally acceptable; and transporting the in situ stabilized material in the recptacle, such as a skip, tank or bin, directly to the point of disposal or application.

2. The method of claim 1 wherein the soluble heavy metal concentrate is selected from spent galvanizing pickling acids; from spent heavy metal plating concentrates, including cadmium, nickel and reduced chromium (VI) salt solutions, or from similar heavy cationic metal concentrates, including flowable precipitated sludge, isolated in a receptacle such as a skip, tank or bin.

3. The method of claim 1 wherein the reagent material is selected from the group consisting of quicklime, hydrated lime, fly ash, kiln ash, flue dust, and flue ash; providing sufficient hydroxide to stoichiometrically react with the metal cations to form insoluble metal hydroxides.

4. The method of claim 1 wherein the contaminated waste material is a flowable material having a density in the range of about 1.2 1.5 kg/litre. The method of claim 1 wherein the treated material results in a density about 1.5 1.8 kg/litre of in situ spadable solid material, which can be transported directly to tip or fertilizer application.