AU2005201669A1 - Beneficiation of mineral fines - Google Patents

Description

COMPLETE

SPECIFICATION

FOR A STANDARD

PATENT

in the name of AGR SCIENCE TECHNOLOGY PTY LTD ABN 200 876 216 27 entitled "BENEFICIATION OF MINERAL

FINES"

Filed by: AGR Science Technology Pty Ltd 772 Fairfield Road Moorooka QLD 4105

AUSTRALIA

P.O. Box 210 New Farm QLD 4005

AUSTRALIA

TITLE

"BENEFICIATION OF MINERAL FINES" FIELD OF THE INVENTION This invention relates to a method for treating fine-grained or plastic or expansive sands, clays or silts. Particularly, this invention relates to treating said fine-grained mineral matter in order to produce materials of use in civil- or earth-works, which may then be disposed of or applied at reduced cost, or at added value.

BACKGROUND

Marine harbours, canal estates, waterways, wharfs, shipping channels, berths, etc.

require regular dredging and maintenance. These dredged silts amass to a significant annual volume, from many perspectives, and especially process perspectives; environmentally, chemically, industrially, or civilly.

For instance, dredge materials and silts resulting from the Capital works and Maintenance works of a city's shipping and cargo port can be of the order of one million m 3 per year.

While dredging of waterways constitutes the bulk of the silt disposal problem in developed coastal areas, mining operations which produce slimes, or fine-grained spoils also can represent a significant source of silt problems.

While the sandy components of such dredgings or spoils can find myriad uses in landscaping, concrete, drainage and other construction applications, the use or disposal of the sulphide-bearing, high organic, fine-grained or plastic or expansive sands, clays and silts can prove to be quite intractable on such a large scale.

Sea dumping of dredge material particularly is increasingly being restricted by Environmental Protection Authorities on the grounds that such practices cause excess silting and turbidity of marine environments, which lead to the ultimate destruction of fish and marine animal habitats. This issue particularly applies to ports or waterways which do not have proximate access to deep, open waters with fast-flowing currents to carry sediments out to sea.

In the case of land dumping of mine spoils, the costs of landfill are becoming increasingly prohibitive and there is a consequent need for the beneficiation of such spoils in order to enhance their value sufficiently to ensure their use, rather than disposal.

Thus in the absence of outright disposal of such dredgings or spoils, treatment options of a significant scale are required to deal with them, and render them potentially useful. Accordingly, options to exploit the potential of these treated silts must also be of like scale.

Therefore, a treatment regime for dredged silts or clays should seek to tailor the silt properties to their ultimate application, e.g. CBR (where CBR refers to "California Bearing Ratio") and swelling or consolidation index for reuse options such as land-fill or reclaimed land sub-grade soil for road, marine or airport works, which are of appropriate scale to the silt or clay disposal problem.

Agents for such beneficiation often include waste lime or alkali sludges, ash, pozzolanics, metal salts, and agents for soil hardening, drainage, consolidation, settlement, or compaction.

Improved soil properties may be gauged in terms of parameters such as CBR, MDD, drainage, plasticity, bearing capacity, consolidation, shrinkage, or trafficability. A brief description of the major engineering parameters follows.

By compacting a soil, air and water voids are removed and the soil mass becomes incompressible and thus stronger and less prone to shrinkage or subsidence. Minerals will compact well if their gradings (distribution of particle sizes) are suited to maximum space filling large particles will have voids between them, which eversmaller particles can pack into. Ideally graded silicates with zero air or water voids will result in a mass with a theoretical density of 2.66 g/cm 3 Clay minerals such as kaolinite or smectite would pack to a maximum theoretical density of 2.40 g/cm 3 When compaction is undertaken with too little water, the mineral particles are not able to easily move about and pack properly. With too much moisture, the water itself occupies space as an incompressible volume and also reduces the effective mineral density. Therefore, there is an optimum moisture content (OMC) at which a particular mix or grading of minerals will achieve its greatest density (maximum dry density

MDD).

Clays are a special case, whereby they absorb water and swell into the available space. The compaction curves for clays therefore tend to be shallow, with indistinct MDD peaks.

Compaction is the first step to strength formation in soils or silts. The stabilisers which neutralise the clays and allow soil particles to be brought into close contact with each other can then go on to promote soil mineral reactions which create interparticle bonding. The nature and extent of these interparticle bonds are what generate soil strength.

Soil strength is measured by the load a soil block can bear before it fails either by shattering (brittle failure) or by irreversibly deforming, bulging or slumping (plastic failure). For example, rigid blocks concrete) will not deform or compress and are prone to brittle failure, while plastic blocks clay) will bulge and strain or compress (strain) to a great extent before failing. The load (stress) at which failure occurs depends on the strength of the bonding between the mineral particles in either case.

Elasticity is the "recoverable" strain rate of a soil subjected to extremely low stresses, defined as the initial slope of a stress-strain graph. A high elastic modulus indicates a brittle soil, while a low elastic modulus suggests the soil is plastic.

CBR is one of the key design parameters for fill materials. This value represents the load bearing capacity of a soil compact as a ratio of a standard crushed gravel -3compact. While there is no direct relationship between strength and CBR and other engineering parameters such as UCS, it has been shown to be a very effective criterion by which to design the thickness of sub-base, base and surfacing.

A number of options are presented in Prior Art for dewatering or stabilising such dredge material or mine spoils, demonstrate that these materials can be processed for further use, as in the following examples: Thomas (US Patent 6343559): reuse of dredged material, physically de-watered, for levy building.

Lassiter (US Patent 4036752): waste clay slurries are mixed with sand and overburden to give in-place landfill.

Ribas (US Patent 4235562): similar to Lassiter, claiming that the added sand and pebble, and possibly gypsum, will accelerate de-watering.

Bracegirdle (US Patent 6299380): a process for combining sludge and dredge material with co-existing organic matter and unspecified pozzolanic material to stabilise and bind the sludge.

Others have sought to stabilise and beneficiate dredgings and mine spoils using polymers and additional aggregate material for reinforcement of the treated soils: Eden and Griffin (US Patent 4134862): wet soil is rendered dry and compactable for backfill by mixing with lime and a starch-acrylonitrile copolymer.

Koo (US Patent 5389702): soil is hardened by addition of polymers and setting agents.

Jacquart and Gremont (FR Patent 2842128): dredgings stored in a deposit, treated with hydraulic binder and mixing with extra materials, e.g. waste foundry sands, slags, polypropylene fibres, cements, for use in road construction.

Chu and Shakoor (1997; Proceedings 22 d International Symposium on Engineering Geology and the Environment, Athens, 2, 1687): silt and clay are mixed with shredded tyres to give light-weight fill material with improved drainage characteristics.

Novish et al (US Patent 6042305): dredge material is solidified by cementitious materials, Ca, Si, or Al containing materials, and reinforced by the addition of fibres, such as glass fibres, for fill.

-4- The use of metal salts to achieve stabilisation of poor soils was demonstrated by Bauer et al (CS Patent 195848), in which dilute aqueous electrolytes, e.g. Fe, Co, Ni, Mg, Zn salts and possibly alcohols were sprayed on soils to increase their bearing capacity for use in construction of roads.

The ability of lime in particular to stabilise marine silts and sediments for their reuse in land reclamation works has been studied extensively, as in the examples of Panda and Rao (1998; Marine Georesources and Geotechnology, 16, 335), Rajasekaran and Rao (1996; Ocean Engineering, 23, 325), or Rajasekaran and Rao (1997; Ocean Engineering, 24, 867). Rao and Rajasekaran (1994; Ocean Engineering, 21, 29) have also studied lime mixing methods such as lime injection for improving the behaviour of soft marine clays.

Likewise, processes for cementation of silts, dredgings, and mine spoils, or their treatment with hydraulic materials to render them useful for civil- or earth-works are well documented: Kamon et al. (1989; Zairyo, 38, 1092): silty dredgings are improved by cement, calcium aluminate hydrate and gypsum, suitable for subgrades and embankments.

Okumura et al (2000; Coastal Geotechnical Engineering in Practice, Proceedings of the International Symposium, Yokohama, 1, 697): dredged soil is mixed with cement and bubbles (to reduce density), to form stable platforms for ports and airports.

Yoshida (JP Patent 50087965): dredging sludges mixed with hydraulic aluminous cement have high compressive strength and can be used in civil works.

Sasaki et al (JP Patent 11140443): high moisture soils are dehydrated and solidified by cement, gypsum, lime, polyaluminium chloride, aluminium sulphate and/or fly ash.

Krizek et al. (1977; Proc. Conf Geotech. Pract. Disposal Solid Waste Matter, 517): lime mixed with dredged slurry opened and stabilised the soil structure, improving its permeability and swell/shrink characteristics.

Kapland and Robinson (US Patent 4539121): mud sludges and dredgings are converted into load supporting masses by the addition of blast furnace slag.

a Komori et al. (US Patent 5658097): soil CBR is improved by the addition of lime, Al, Fe, and Si oxides (including sewage sludge ash) for pavements.

0 Ii et al. (US Patent 5456553): have a related patent to Komori et al., whereby soil is strengthened by mixing with Al, Fe and Ti oxides from metal (steel and aluminium) works.

Smith and Webster (US Patent reissue Re29783): alternatively to Ii et al. and Komori et al. produced reclaimed landfill by hardening waste sludges containing Al, lime and sulphate, possibly also incorporating fly ash or aggregate materials.

Chin et al. (1997; Geotechnical Special Publication, 65, 177): lime admixture and soil cements for stabilising shallow marine deposits to support excavations for construction of tunnels.

Vaghar et al. (1997; Geotechnical Special Publication, 65, 105): lime admixture reduces drying time and assists fill compaction of harbour bottom sediments and marine clays.

In terms of sourcing stabilising agents, Hart et al. (1993; Waste Management, 13, have demonstrated that waste lime lime sludges from lime calcination processes) rather than fresh lime is suitable for soil admixture, to produce structural fill material.

Show et al. (2003; Journal of Materials in Civil Engineering, 15, 335) indicated a beneficial reuse for incinerator fly ash in soft soil stabilisation.

OBJECT OF THE INVENTION While a great volume of prior art addresses the issue of improving drainage or bearing capacity of dredged materials and wet soils, the present inventors have realised that treatment options for a given dredge material or silt may be tailored to meet desired soil parameters or specifications in that fine-grained, or plastic, or expansive sands, clays or silts are beneficiated through treatment by a source of polyvalent cations to mitigate their moisture reactivity, then by a second agent to build up the desired engineering or soil properties, as determined by the ultimate application criteria for the soil.

The present inventors have also realised that there are two principal mechanisms by which soft, unstable, plastic or expansive soils are stabilised, listed as follows: 1) through the neutralisation of repulsive surface charges on the fine soil particles and their agglomeration or coagulation, and the reduction of their moisture reactivity, by the application of multivalent cations.

2) through improving the soil microstructure and chemistry to enhance compaction or drainage as in the case of application of cementitious, hydraulic, or pozzolanic materials (polymers, lime, cement or ash including fly ash, or boiler ash, and the like), or inclusion of harder, reinforcing, better draining, or inert material.

The inventors have also realised that to mitigate or stabilise the reactive components of a fine silt, sand or clay (for example, but not limited to, dredge material) requires dosing or treating it with a very low percentage of a soluble polyvalent cation.

Sources ofpolyvalent cations may include, but are not limited to, cement, lime, waste or contaminated lime, gypsum, spent pickling acid, alumina cake, slag, fly ash, boiler ash, concrete batch plant returns, aluminium or magnesium processing swarf, sludge, dross or salt cake, remelt furnace scrubbers, smelter pot liner, or mill scale. The inventors have further realised that additional bulk properties or engineering properties of the soil so amended can then be further developed to a desired specification by additional or further treatments with cementitious matter, or with further alteration to the microstructure and chemistry using oversize material to the existing soil grading e.g. but not limited to fly ash, boiler ash, bagasse ash, building rubble, shredder floc, shredded tyres, paper pulp, alumina or salt cake, remelt furnace scrubber material, mill scale, crushed or waste glass, abrasives, silica fume, or mineral sands processors or ceramics manufacturers byproducts. Additional cementitious matter may be in the form of, but not limited to, cement, concrete batch plant returns, reclaimed concrete or cement waste, lime (hydrated, quick, agricultural, or otherwise), lime sludge, waste lime, gypsum (waste or otherwise), silica fume, fly ash, bottom ash, boiler ash, bagasse ash, brickworks byproducts, aluminium smelter pot liner, or pozzolanic matter.

-7- The present inventors have also realised that the process by which said soil is beneficiated creates a useful means by which the silt and the beneficiation agents sourced from waste streams are removed from the environment.

The present invention relates to the process of identifying the necessary target soil parameters for beneficial reuse of the silt or clay, identifying the optimum treatment options and treatment materials, potentially from (but not limited to) other existing waste streams, and applying them to produce a beneficiated silt suitable for use as fill material or sub-base in civil- or earth-works.

SUMMARY OF THE INVENTION This invention provides a means of beneficiation of fine mineral silt, sand or clay wherein the chemistry and microstructure of said mineral matter is altered by the addition and admixture of a low dose of a source of soluble polyvalent cations (with or without cementitious properties, e.g but not limited to cement, lime, waste or contaminated lime, gypsum, spent pickling acid, alumina cake, slag, fly ash, boiler ash, concrete batch plant returns, aluminium or magnesium processing swarf, sludge, dross or salt cake, remelt furnace scrubbers, smelter pot liner, or mill scale), to stabilise the reactive components, then further treated with material to adjust the bulk soil chemistry, microstructure or grading using material with or without cementitious properties (e.g but not limited to any combination of the following: cement, concrete batch plant returns, reclaimed concrete or cement waste, lime (hydrated, quick, agricultural, or otherwise), waste lime, gypsum (waste or otherwise) or pozzolanic matter, alumina cake, slag, fly ash, boiler ash, bagasse ash, concrete batch plant returns, aluminium or magnesium processing swarf, sludge, dross or salt cake, building rubble, shredder floc, shredded tyres, paper pulp, remelt furnace scrubber material, aluminium smelter pot liner, or mill scale, crushed or waste glass, abrasives, silica fume, or mineral sands processors or ceramics manufacturers byproducts) to produce the desired soil properties of for example, but not limited to, strength, flexure, rigidity, compaction, Attaberg limits, swell, moisture retention, permeability, drainage, or CBR.

-8- Preferably, the dose of matter for stabilisation of the reactive components is in the range of 0.001 to 15 of the dry soil weight, with additional material in the range of0.001 to 50 of the dry soil weight being dosed for achievement of the specified soil properties.

More preferably, the former dose is in the range of 0.05 to 2 and the latter dose ranges from 0.05 to 15 Even more preferably, the former dose is in the range of 0.05 to 0.25 and the latter dose ranges from 0.05 to 5 In another aspect, the invention provides a method of beneficiating fine mineral silt, sand or clay wherein the chemistry and microstructure of said mineral matter is altered by the addition and admixture of a source of polyvalent cations (for example, but not limited to, cement, lime, waste or contaminated lime, gypsum, spent pickling acid, alumina cake, slag, fly ash, boiler ash, concrete batch plant returns, aluminium or magnesium processing swarf, sludge, dross or salt cake, remelt furnace scrubbers, aluminium smelter pot liner, or mill scale) to produce inorganic floccules giving the desired soil properties of for example, but not limited to, strength, flexure, rigidity, compaction, Attaberg limits, swell, moisture retention, permeability, drainage, or

CBR.

In another aspect, the invention provides a method of beneficiating fine mineral silt, sand or clay wherein the chemistry and microstructure of said mineral matter is altered by a combination of any or all of the abovementioned processes to produce the desired soil properties of for example, but not limited to, strength, flexure, rigidity, compaction, Attaberg limits, swell, moisture retention, permeability, drainage, or

CBR.

It is another aspect of this invention wherein undesired soil properties are mitigated by any combination of any of the abovementioned properties.

In another aspect, this invention provides a soil with pre-determined chemistry or microstructure having any or all of the specified properties of, for example but not -9limited to, strength, flexure, rigidity, compaction, Attaberg limits, swell, moisture retention, permeability, drainage, or CBR, produced by any combination of the abovementioned processes.

Throughout this Specification, the ranges stipulated are inclusive rather than exclusive.

The invention described herein may be understood more clearly by the non-limiting examples listed under the following "Preferred Embodiments".

BRIEF DESCRIPTION OF THE TABLES AND FIGURES Table 1: Chemical properties of spent galvanisers' pickling acid, for use as coagulant in stabilising dredged silt materials.

Table 2: Development of UCS in silt stabilised with 1% coagulant 1% hydrated lime.

Table 3: Measured CBRs of maintenance dredge silt stabilised with 1% coagulant 1% hydrated lime.

Table 4: Mineralogical composition of spent aluminium smelter pot liner lime.

Table 5: Development of UCS in silt stabilised with 2.9% coagulant, 0.6% alum, 0.875% hydrated lime, and 4% spent pot liner lime.

Table 6: Measured CBRs of maintenance dredge silt stabilised with 2.9% coagulant, 0.6% alum, 0.875% hydrated lime, and 4% spent pot liner lime.

Figure 1: Measured compaction curves for silt stabilised using 1% hydrated lime and 1% coagulant sourced from spent galvaniser's pickling acid. The points represent the observed density data, with curves serving as guides for the eye only. The diagonal lines represent the ZAV traces for mineral compacts of specified densities.

Curve 1: 2.40 g/cm 3 for clay minerals (kaolinite and smectite), Curve 3: 2.66 g/cm 3 for silica (quartz), and Curve 2: 2.60 g/cm 3 for the mineralogical mixture found in dredged material.

Figure 2: Measured compaction curves for silt stabilised using 2.9% spent pickling acid, 0.6% alum solution, 0.875% hydrated lime, then followed by 4% spent pot liner lime. The points represent the observed density data, with curves serving as guides for the eye only. The diagonal lines represent the ZAV traces for mineral compacts of specified densities. Curve 1: 2.40 g/cm 3 for clay minerals (kaolinite and smectite), Curve 3: 2.66 g/cm 3 for silica (quartz), and Curve 2: 2.60 g/cm 3 for the mineralogical mixture found in dredged material.

Figure 3: Plan of mixing and curing facility for dredged silt stabilisation.

Figure 4: Cross section F F through plan of mixing and curing facility for dredged silt stabilisation, showing earthen bund construction.

PREFERRED EMBODIMENTS Example 1) A marine silt dredged from cargo ship berths has the following properties: Mineralogy: Smectite Kaolinite Quartz Feldspar Pyrite Calcite 1.2 40.4 23.6 33.1 1.3 0.4 Engineering parameters: Density

MDD

OMC

Consolidation Swell Material 1.18mm

PL

PI

CBR

1.33 t/m 3 1.54 t/m 3 23.2 30.8 3.7% 100% 24% 62 4 From the mineralogy of this particular dredged material, it can be seen that the lowgrade engineering properties are largely a function of the fine grade of the material -11 and the quantity of poorly reactive clays kaolinite), rather than the reactivity of quantities of highly reactive clays smectite).

Since the CBR is adequate for sub-grade landfill in a pavement construction project, the principal criteria of reduced moisture loss rate and reduced swell were specified for the beneficial reuse of this material. Stabilisation and desiccation of the silt were to be achieved with waste lime treatment.

A contaminated or waste lime sourced from a petroleum refinery with 5.2 calcium fluoride, 9 bituminous matter and an equivalent alkalinity of 62 calcium hydroxide was used for achieving the necessary soil properties.

Addition and admixture of this waste lime at a level of 0.35 of the dry silt mass was deemed suitable for neutralising the swelling smectitic component, while an additional 0.10 of this lime was necessary to sufficiently neutralise the clay surface charges and reduce their moisture sorption to achieve the desired rate of moisture loss.

The total waste lime complement was added and mixed in a single operation and the soil cured for 14 days. The swell of the resultant stabilised soil was reduced to 2.4 while its moisture retention capacity (at pF 1.39; where pF represents the matric potential, or soil suction pressure to be overcome when removing moisture) reduced from 21.2 to 3.7%.

Example 2) A landfill project for an airport will require up to 50,000 m 3 of a flexible medium strength sub-grade with CBR in the range 7 9, and minimum swell and moisture sorption capacity. This quantity of fill may be sourced from a nearby port dredging operation.

The abovementioned silt can be treated with 0.45 of the abovementioned waste lime, as a first step to reducing its moisture sorption capacity and reactivity, along with an additional 2 fresh hydrated lime for the extra cementation needed to achieve the required strength. The lime additions may be concurrent, by spraying slurried stabilisers onto a conveyor belt of the dredge material directly from the barge, or by soil injection of the stabiliser slurry into a spread stockpile of the silt.

Silt so treated will show a CBR value of 9, a swell of 0.2 and a moisture retention capacity (at pF 1.39) of 3.9 after 14 days curing.

Example 3) The abovementioned dredged silt is to also form an upper, intermediate layer to the sub-grade of Example with an additional specification of minimum soil dispersibility.

In addition to the 0.45 waste lime for primary control of the reactivity and moisture retention of the soil, a further I fresh hydrated lime and 2 byproduct alumina hydrate filter cake from an aluminium extrusion plant was prescribed in order to generate a cementitious calcium aluminate phase, to create some rigidity within the soil matrix.

The waste lime, fresh hydrated lime and alumina hydrate filter cake could be tined into a bed of the silt and aged for 14 days. After curing, the resultant stabilised soil had a CBR value of 7, a swell of 0.3 a moisture retention capacity (at pF 1.39) of 2.2 and a dispersibility index of 1 (on a scale of 1 to 5, where 1 represents a ball of soil which retains its integrity in moisture, while 5 disintegrates into a clay slurry).

For comparison, the treated sub-grade soil of Example 2) had a dispersibility index of between 2 and 3.

The strengthening of the soil mass through cementation (either inter-particle bridging or aggregation) need not generate or facilitate the types of compaction necessary to give good particle interlock or friction. The addition of sufficient inert fines (as in the present Example) makes for high CBR values.

Example 4) A cation exchange mechanism is also partially responsible for the reduction of swell and improved compaction and aggregation, while cementation and interparticle bonding is responsible for development of strength and structure in stabilised silts. Both of these processes are at work when silt is treated with coagulant and lime.

-13- Dredged silt material is admixed (for example through a ribbon mixer) with 1% of its weight in coagulant consisting of spent galvanisers pickling acid of composition shown in Table 1, then admixed with a further 1% of its weight in hydrated lime.

The maximum dry density (MDD or compaction) curves obtained from mixtures in which lime (be it hydrated lime, or a waste lime) is the key stabiliser appear sharpened compared to unstabilised soil MDD curves, and have lost their clay-like behaviour. The mineral particles therein are hardened and made "inert" or unreactive to water. Figure 1 illustrates the sharpening of the MDD curve of dredged silt material stabilised with 1% spent galvanisers pickling acid coagulant and 1% hydrated lime.

Unconfined compressive strength develops over 28 days while the stabilised silt mass undergoes embrittlement, as evident by the increase in UCS and corresponding increase in elastic modulus and decrease in strain at failure, as shown in Table 2. The CBR value and swell of the stabilised silt are also given in Table 3.

Example 5) As with Example the balance of cation exchange and interparticle bonding mechanisms can be adjusted to optimise the CBR and other mechanical or engineering properties of a silt.

Silt is mixed with 2.9% coagulant as described by Table 1 and 0.6% alum (28% aluminium sulphate solution), followed by 0.875% hydrated lime, and 4% spent pot liner lime as described by Table 4.

As in the previous Example the MDD curve for this stabilised mixture shown in Figure 2 appears sharpened and shows better compaction compared to untreated silt.

The strength and elastic modulus of this stabilised silt grow with ageing, as shown in Table 5. As a result of this particular combination of coagulation and cementation of the silt components from the additives, a CBR value of 16 and a swell of 0.6% are attained (Table making the material a useful sub-grade for airport runways (source: US War Department, Office of the Chief of Engineers (1947), "Flexible Pavements, -14- Engineering Manual Part XII, Airfield Pavement Design", Chapt. 2, Washington DC,

USA).

Example 6) 250 t of maintenance dredge silt is stockpiled in "Canvacon" impervious lined and covered earthen dams laid out as illustrated in Figure 3. The approximately by 14.5 m 2 dams A, B, C, D, and E are constructed of earthen walls 1.5 m high with a work platform accessible by an earthen ramp and an access area A checked surface drain for removal of surface and stormwater runs alongside the bunds. Each dam is lined with "Canvacon" HDPE impervious liner (6) and affixed to the bund walls by the weight of overlying ballast, as illustrated in Figure 4.

The silt is brought from the dredge by tip-trucks or semi-tippers and deposited in 50 t batches in each of the dams A, B, C, D, or E, in turn. In the first instance, the silt is allowed to settle and dewater within the dam, while the runoff is pumped away. The first of the stabilising agents, 1.5 kl of spent galvanisers' pickling acid, is then poured or sprayed over each pile and mixed and turned through using an excavator. Within a week of this action, 0.4 t of hydrated lime followed by 2 t of spent aluminium smelter pot line lime are likewise mixed and turned through each pile. The silt-additive mixture is then regularly turned, approximately weekly, with an excavator, while water liberated by the stabilisation process is daily pumped out of the dams. After at least 28 days from the lime addition step, the resultant fill grade material may be removed from the dams and used in civil works, while the dams are cleared to receive another barge-load of silt for a fresh processing cycle.

Table 1: Ferric Chloride Ferrous Chloride Zinc Chloride Aliphatic Hydrocarbons Cadmium Lead 12-18% 2 8% <0.1% 0.1% <5 mg/kg 300 mg/kg Table 2: Day Ucs (kPa) Failure Strain Elastic Modulus (MPa) 7 280 14 285 3.3 18.8 28 323 2.8 23.4 Table 3: OMC CBR Swell 29.5 4.5 2.6 Table 4: Portlandite 10 Fluorite 12 Graphite 18 Hydrogarnet 45 Table Day UCS Failure Strain Elastic Modulus (kPa) (MPa) 7 233 14 245 4.2 15.6 28 268 4.3 15.0 Table 6: OMC CBR Swell 30.0 16.0 0.6 -16-

Claims (4)

1. mixing together a low grade soil with sources ofpolyvalent cations, sufficient to cause a stabilising reaction within the expansive and plastic components of former entity,

2. additionally mixing said mixture with agents for modifying soil microstructure,

3. ageing said mixture for a period sufficient to drive the reactions between the reacting entities to equilibrium, and

4. ageing said mixture for a period sufficient for the admixture to develop the desired engineering or physical properties. 12) A method according to Claim 11, whereby the sources of polyvalent cations may be applied as solids. 13) A method according to Claim 11, whereby the sources of polyvalent cations may be applied as liquids. 14) A method according to Claim 11, whereby the sources of polyvalent cations may be combinations of multiple sources of polyvalent cations. A method according to Claim 11, whereby the sources of polyvalent cations are sourced from a waste or by-product stream. 16) A method according to Claim 11, whereby the sources of polyvalent cations possess additional coagulant properties. -18- 1 0 17) A method according to Claim 11, whereby the sources of polyvalent cations possess additional cementitious or pozzolanic properties. 18) A method according to Claim 11, whereby the agents for modifying soil microstructure may be inert filler materials. 19) A method according to Claim 11, whereby the agents for modifying soil microstructure may possess additional coagulant properties. 20) A method according to Claim 11, whereby the agents for modifying soil microstructure may possess additional cementitious or pozzolanic properties. 21) A method according to Claim 11, whereby the agents for modifying soil microstructure may possess additional combinations of coagulant or cementitious or pozzolanic properties. 22) A method according to Claim 11, whereby the agents for modifying soil microstructure may be combinations of multiple sources. 23) A method according to Claim 11, whereby the agents for modifying soil microstructure are sourced from a waste or by-product stream. 24) A method according to Claim 11, whereby the combination of sources of polyvalent cations and agents for modifying soil microstructure may possess additional combinations of coagulant or cementitious or pozzolanic properties. A method according to Claim 11, whereby the low grade soil may be a fine- grained, or plastic, or expansive sand, clay or silt. 26) A method according to Claim 11, whereby the low grade soil may be marine mud or dredged silt material. 27) A method according to Claim 11, whereby Step 1 and Step 2 occur simultaneously. -19- I 0I 28) A method according to Claim 11, whereby Step 1 and Step 2 occur sequentially. DATED: this twenty-first day of April 2005. AGR SCIENCE TECHNOLOGY PTY LTD