8. Stabilization and Solidification

8/12/2019 8. Stabilization and Solidification

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HAZARDOUS WASTE

MANAGEMENT TECHNOLOGY

Stabilization and Solidification (S/S)

Lecturer: Dr Tan Kok Weng

Email: [email protected]


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Introduction

Stabilizationand solidification (S/S) have been

widely used in management of hazardous

wastes.

These technologies are being applied to:

i. Treatment of industrial wastes

ii. Treatment of wastes prior to secure landfill disposal

iii. Treatment of contaminated land where largequantities of soil containing hazardous substances

are encountered.


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Stabilization process where additives are mixed withwaste - Contaminants are fully or partially bound by theaddition of supporting media, binders or other modifiers

to minimize the rate of contaminant migration from

the waste to reduce the toxicity of the waste.

Solidificationprocess employing additives Liquid or semi liquid to solid form

Objective of S/S would encompass both inreduction in waste toxicity and mobility as well asimprovement in t

he engineering properties of thest

abilized material.


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Definitions

Stabilization is a process employing additives (reagents) toreduce the hazardous nature of a waste by converting thewaste and its hazardous constituents into a form that

minimizes the rate of contaminant migration intoenvironment, or reduces to level of toxicity.

The purpose of addition of reagents:

Improve the handling and physical characteristic of the waste

Decrease the surface area across which transfer or loss ofcontaminants can occur

Limit the solubility of any pollutants contained in the waste

Reduce the toxicity of the contaminants


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Fundamental physical and chemical mechanisms thatcontrol the effectiveness of the stabilization reagent

R&D is important - New reagents are developed or

existing reagent are modified and adapted to newand different wastes.


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Successful stabilization employs one or moreof the following mechanisms.

1. Macro-encapsulation

2. Micro-encapsulation

3. Absorption

4. Precipitation


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1. Macro encapsulation (waste block)

Hazardous waste constituents are physically entrapped in alarger structural matrix

That is, hazardous waste constituents are held in discontinuous

pores within the stabilizing materials The entrapped materials are free to migrate

The stabilized mass may breakdown over time due to imposedenvironment stresses including

Repeated cycles of wetting and drying

Freezing

Thawing

Introduction of percolating fluids

Physical loading stresses


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2. Micro-encapsulation (waste particle)

Hazardous waste constituents are entrapped withinthe crystalline structure of the solidified matrix at amicroscopic level

As a result, even if the stabilized materials degradeinto relatively small particle sizes, most of thestabilized hazardous wastes remains entrapped.

Microencapsulation is more effective because it

entrapped the hazardous waste within the crystallinestructure of the solidified matrix at microscopic level.

As a results, even if the stabilized materials degradeinto relatively small particle size, most of the stabilizedhazardous wastes remains entrapped.


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3. Absorption

Contaminants are taken into the sorbent in very much the same way asponge takes on water

Adsorption requires the addition of solid material (sorbent) to soak upor absorb the free liquids in the waste

This process is primarily employed to remove free liquid to improvethe waste-handling characteristics

Most common absorbent:

Soil

Fly ash

Cement kiln dust

Lime kiln dust

Sawdust

Clay minerals including bentonite, kaolinite, vermiculite and zoelite


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Organic waste adsorbed to anorganophilic clay


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4. Precipitation

Precipitation process will precipitate contaminants fromwaste, resulting in a more stable form of the constituentswithin the waste

Precipitates such as Hydroxides

Sulfides

Silicates

Carbonates phosphates

Are then contained within the stabilized mass as part ofthe material structure


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Binder used to denote a reagent that contributes tothe strength gain associated with stabilization

The factors that affect the reagent:

Concentration of the contaminants

Quantity of the reagent

Synergistic effects of multiple contaminants and reagents

1. Portland Cement (Cementitious S/S )

2. Pozzolan (Cementitious S/S )

3. Thermosetting Organic polymer (Polymer S/S)

4. Thermoplastic (Polymer S/S)


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1. Cement

Portland cement, made by firing a mixture of limestoneand clay (or silicate) in a kiln at high temperatures

The kiln produce a clinker, which is ground to a powder

that is a mixture of Ca, Si, Al and iron oxides Waste materials are mixed with cement followed by the

addition of waster for hydration (if necessary when waterdoes not presence)

The hydration of the cement forms a crystalline structure,consisting of calcium aluminosilicate

This result in a rock-like, monolithic and hardness mass

Concrete particulate composite consisting of hydratedcement and aggregate


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Cement-based stabilization is best suited forinorganic wastes, especially those containingheavy metals

Cause the high pH of the cement is able to retainthe heavy metal in the form of insoluble hydroxideor carbonate salts within the hardened structure.

Lead, copper, zinc, tin and cadmium are likely

bound in the matrix by chemical fixation, forminginsoluble compounds

Mercury is predominantly held by physicalmicroencapsulation


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2. Pozzolans

The reaction of aluminosilicious material, lime andwater results in the formation of Pozzolanic

concrete

Fly ash, ground blast furnace slag and cement kilndust

The theory of stabilization is similar to cement.

High pH environment is well suited to treat heavymetal.


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3. Thermosetting Organic Polymers

Hazardous wastes may be stabilized through anorganic polymer process that involves mixing of a

monomer , such as urea-formaldehyde, that actsas a catalyst to form a polymeric material

A sponge like mass is thereby formed, trappingsolid particles of hazardous waste within matrix

Micro-capsulation

However, it will leave some un-trapped,particularly liquid wastes


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4. Thermoplastic materials

Hazardous waste may be stabilized by blending moltenthermoplastic materials with wastes at high temperatures

When cooled down, the molten will solidified, the wastewill thermo-plastically coated and typically containerized(put in drums) for ultimate disposal

The used of thermoplastic has received attention for mixedwaste, that is, waste that is both hazardous and radioactive

Type of molten thermoplastic material:

Asphalt, paraffin, bitumen, polyethylene, polypropylene and sulfur


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TechnologiesTraditional in-situ vitrification

The traditional in-situ vitrification process employs an array ofelectrodes placed vertically into waste or contaminated soil,and an electric current is passed through the soil between theelectrodes.

The heat generated from the resistance of the soil to thepassage of the current is referred to as Joule heating.

As the heated soil melts progressively downwards, theelectrodes are allowed to sink through the melted soil,enabling melting depths of 7 m or more.

An off-gas hood covers the entire melt and some distancearound the outside edge to control release of gases andairborne particles generated within or near the melt.


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The off-gases are drawn into the hood by the negative pressurecreated by a fan, then treated in a process train before beingdischarged to the atmosphere.

When the melting has progressed to the desired depth, the powerto the electrodes is shut off and the melt is allowed to cool.

The electrodes are left in place in the melt and are sawn off at the

ground surface. New electrodes are installed at each new meltlocation.

The final melt is smaller in volume than the original waste andassociated soil due to: Removal of volatile contaminants;

Reduced void space; Higher density of glass relative to waste materials.

Each melting produces a single block shaped monolith of glass.Most vitrification projects require multiple, overlapping melts tocover the area and the volume of the contaminated site.


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Planar in-situ vitrification

Like traditional in-situ vitrification, planar in-situvitrification employs the same Joule heating principle butdiffers in the application of electric current and in thestarter path configuration.

In planar in-situ vitrification, the current travels betweenpairs of electrodes, causing two parallel planar melts toform.

As the melts grow downwards and spread laterally, theyeventually meet in the centre of the electrode array andfuse together into one melt. The final planar melt has thesame size and shape as a traditional in-situ vitrificatn melt.

Technologies


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In-situ Plasma Vitrification (Circeo and Martin, 1997)


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S/S technology The advantages:

Low cost because the reagents are widely

available and inexpensive

Can be used on a large variety of contaminants

Can be applied to different types of soils

Equipment simple compare to other treatment

process

High effectiveness

Sharma (2004)


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S/S technology The disadvantages:

Contaminants are not destroyed or removed

Large volume of the treated waste

Volatile organic compounds and some

particulates may come out during treatmentprocess

Delivering reagents deep into the wastes andmixing them evenly is difficult

In situ S/S site may not be redeveloped

Long-term efficiency of S/S is still uncertain

Sharma (2004)


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References:

LaGrega, M.D., Buckingham, P.L., Evans, J.C.2001, Hazardous Waste Management, 2nd edi,McGraw-Hill.

Additional references

Sharma, H. D., and Lewis, S. P. 1994. WasteContainment Systems, Waste Stabilization, andLandfills: Design and Evaluation. Wiley, NewYork.

Lizzie Grobbel and Zhijie Wang . A Review ofstabilization/Solidification (S/S) Technology forWaste Soil Remediation. [Online source]
http://www.dfat.gov.au/dept/annual_reports/06_07/financials/index.htm


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Key terms:

Term Definition

Stabilization process where additives are mixed with waste to

minimize the rate of contaminant migration from the

waste and to reduce the toxicity of the waste.

Solidification process employing additives by which the physical nature ofthe waste is altered during the process

8. Stabilization and Solidification

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