TECHNICAL FIELD

This disclosure relates to a process for remediation of impounded coal ash, such as may be held in a storage pond.

BACKGROUND ART

Coal ash is a by-product from the burning of coal in thermal power stations. Worldwide, large quantities of coal ash have been building up over many decades. For example, the Ash Development Association of Australia estimates that there is in total 650 million tonnes of coal ash stored throughout Australia. In New South Wales alone, it is estimated that there are some 216 million tonnes of coal ash currently held in storage at power stations which are currently in operation.

Further, it is understood that, together, coal ash produced in the USA, India and China represents 80% of world production. By way of comparison, Australia represents less than 1% of global production.

Due to the presence of a number of contaminants, most coal ash is unable to be used, unless special exemptions are granted by relevant authorities. Thus, to date, coal ash has been stored, such as in storage ponds which may be located adjacent to a thermal power station. However, such storage can result in water and ground contamination, and may in some cases result in migration of contaminants.

It is to be understood that, if any prior art is referred to herein, such reference does not constitute an admission that the prior art forms a part of the common general knowledge in the art, in Australia or any other country.

SUMMARY OF THE DISCLOSURE

Disclosed herein in a first aspect is a process for remediation of impounded coal ash. The process may be used to remediate coal ash that is impounded in a pond, such as may be located in the vicinity of a coal-fired power station. The process may also be employed to treat coal ash prior to the coal ash being sent to a pond.

The process can comprise removing coal ash as a slurry from a storage pond that comprises the coal ash. In the case where the coal ash is not held in a storage pond, a slurry may be produced from the coal ash to enable it to be treated in the process as disclosed herein. Thus, the step of removing coal ash as a slurry from a storage pond is to be interpreted broadly. Removal of coal ash as a slurry may be achieved, for example, by dredging the pond.

The process can also comprise subjecting the slurry to a separation treatment in which fines are separated from coarser material present in the slurry. As set forth below, this step may produce a product (e.g. a bottom ash/granular fill material which may be employed in e.g. road construction).

The process can further comprise passing the separated fines to a flotation stage in which unbumt carbon and organic matter (when present) are separated from the fines. For example, unbumt carbon can be present in the coal ash as a result of incomplete combustion in coal burners of the thermal power station. Organic matter may be present as a result of dredging the coal ash from e.g. a storage pond. As set forth below, the unburnt carbon and organic matter may be recycled as a kind of biomass for rebuming in suitable burners at the thermal power station.

The process can additionally comprise passing an underflow of the flotation stage that comprises the fines to a metals extraction stage in which metal contaminants that are present in the fines underflow are removed, to produce a fines stream that is substantially free of such metals. In some cases, such metals may be recovered as a valuable by-product of the present process.

The process can still further comprise dewatering the substantially metals-free fines stream to reduce the water content to an acceptable level to produce a first fine ash product. For example, this first fine ash product may represent an ‘unclassified fly ash’. Because of the reduction of contaminants therein (e.g. unburnt carbon, organic matter, metals, etc.) such a product may be a ‘compliant’ material. In this regard, the first fine ash product may be used freely as a bulk fill material, such as for road construction.

The process as disclosed herein is able to be deployed to remediate impounded/stored coal ash which may otherwise be unusable. Through such remediation, the process can produce one or more useful products, and may be able to convert substantially all of the coal ash into usable products. The steps as outlined above enable recovery and treatment of the coal ash to create a range of valuable products. Further, such products may be ‘tailored’ to comply with customer specifications, as well as with Australian (or other relevant) Standards and relevant Environmental Protection Authority regulations. Further, to ensure reliable and consistent quality of product, the process as disclosed herein may adopt modem process control, automation and monitoring techniques.

As set forth above, in some embodiments, during the slurry separation treatment in which fines are separated from coarser material, the coarser material may be collected as a granular fill material product (e.g. a ‘bottom ash’ which may be sold to market).

In some embodiments, the slurry separation treatment may comprise a coarse filtration stage. For example, the coarse filtration stage may employ a screen filter.

In some embodiments, the process may comprise additional stages to produce additional (e.g. further) products. In this regard, the process may comprise further treating the dewatered first fine ash product (‘unclassified fly ash’). In this further treatment, the dewatered first fine ash product may be subjected to thermal treatment (e.g. a drying stage) in which moisture content may be further reduced. For example, the thermal treatment of the dewatered first fine ash product may comprise an indirect heating apparatus. The indirect heating apparatus may be a rotary dryer. In this further treatment, the resultant dried first fine ash product may then be passed to a particle separation stage. In the particle separation stage, a second fine ash product may be produced. The second fine ash product can represent a ‘classified fine ash’. It may be sold to market as e.g. a supplementary cementitious material.

In some embodiments, during the dried first ash particle separation stage, a coarser particulate material may be separated from the second fine ash product.

In some embodiments, the separation of fine particles from coarse particles during particle separation of the dried first fine ash may be achieved by using centrifugal force. For example, the centrifugal force may be generated by means of cyclonic apparatus.

In some embodiments, the process may comprise further treating the coarser particulate material collected during the dried first ash particle separation stage. In this further treatment, the coarser particulate material may be subjected to an aggregation process in which an aggregated ash product is produced. The aggregation process may, for example, comprise geopolymerisation or sintering or a combination thereof. The aggregated ash product produced by the aggregation process may be regarded as a ‘structural -grade light weight aggregate’ or ‘SLWA’ product which may be sold to market.

In some embodiments, the metals extraction stage may comprise a separation stage to which the fines underflow is passed. In the separation stage, the fines present in the underflow may be separated into a slurry and a liquor. The separation stage may comprise a leaching process which can be operated to produce the liquor and the slurry, with the latter comprising leached solids. For example, the leaching process can comprise a series of leaching tanks/vessels which can be operated in co- or counter-current flow. Typically, a leaching reagent such as an acid is employed in the leaching process. The leaching process can be operated to leach metals (and other contaminants) into the resultant liquor. The leaching process may comprise a hydrometallurgical leaching process. The separated slurry from the separation stage (i.e. comprising the leached solids) may be passed to the dewatering stage as the fines stream. Conversely, the separated liquor may be passed to a metals recovery plant of the metals extraction stage.

In some embodiments, the metals recovery plant may comprise one or more treatment processes for treating the liquor from the separation stage. For example, the treatment processes may include: magnetic separation, chemical precipitation, electrowinning, etc.

In some embodiments, operating conditions of one or more treatment processes of the metals recovery plant may be adjusted to enhance efficacy. For example, chemical reagents may be added to one or more of the treatment processes. Further, the separation stage and/or the one or more treatment processes may employ elevated temperatures and/or pressures, or any combination thereof.

In some embodiments, the slurry separated from the liquor in the metals extraction stage may be washed prior to passing the separated slurry to the dewatering stage as the fines stream. In this further treatment, a wash liquor may be used to wash the slurry. The wash liquor may be recycled to the separation (e.g. leaching) stage.

In some embodiments, dewatering the metals-free fines stream may comprise subjecting the metals-free fines stream to dewatering apparatus. For example, the dewatering apparatus may comprise a screw press. Dewatering may be performed to substantially remove excess water (e.g. to produce the first fine ash product - ‘unclassified fly ash’).

Also disclosed herein in a second aspect is a process for remediation of impounded coal ash. The process can be used to remediate coal ash that is impounded in a pond, such as may be located in the vicinity of a coal-fired power station. The process may also be employed to treat coal ash prior to the coal ash being sent to a pond.

The process of the second aspect can comprise removing coal ash as a slurry from a storage pond that comprises the coal ash. Removal of coal ash as a slurry may be achieved by, for example, dredging the pond. As above, the step of removing coal ash as a slurry from a storage pond may comprise producing a slurry from otherwise stockpiled coal ash to enable it to be treated in the process as disclosed herein.

The process of the second aspect can also comprise subjecting the slurry to a separation treatment in which fines are separated from coarser material present in the slurry.

The process of the second aspect can further comprise collecting the separated coarser material as a granular fill material product (e.g. a ‘bottom ash’ which may be sold to market as a granular fill material such as for use in road construction).

The process of the second aspect can be deployed to remediate impounded/stored coal ash, which may otherwise be unusable, to produce a useful granular fill material product. The fines separated during the separation treatment can be further remediated by the process of the first aspect as set forth above.

In some embodiments of the process of the second aspect, the slurry separation treatment may comprise a coarse filtration stage. For example, the coarse filtration stage may employ a screen filter, in which the granular fill material may be retained by the filter. The granular fill material product may be produced by removing the granular fill material retained by the filter.

In some embodiments of the process of the second aspect, the granular fill material may be crushed to produce a granular fill material product with a more consistent particle size. Also disclosed herein in a third aspect is a process for remediation of coal ash particles. The process of the third aspect can be used to remediate coal ash prior to the coal ash being sent to a pond.

The process of the third aspect can comprise removing unclassified fly ash from a filtration stage in a coal-fired power station.

The process of the third aspect can also comprise passing the unclassified fly ash to an aggregate plant in which the unclassified fly ash is subjected to an aggregation process to produce an aggregated ash product.

In some embodiments of the process of the third aspect, the aggregation process may comprise geopolymerisation or sintering or a combination thereof. The resultant remediated coal ash can, as required, be further remediated by the process of the first and/or second aspects, as set forth above.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with reference to the accompanying drawings in which:

Figure l is a concept flow diagram, set out in simple block diagram form, of an embodiment of a process for remediating coal ash.

Figure 2A shows an example of a floating barge with hydraulic dredging apparatus for use in the process, and Figure 2B shows an example of a hydraulic pump with mechanical fingers for deployment from the hydraulic dredging apparatus of Fig. 2A.

Figure 3A shows an example of a static screen filter for use in the process, and Figure 3B shows a schematic figure illustrating the internals that govern how such a static screen filter would operate in the process. Figure 4A shows an example of a cylindrical flotation cell for use in the process, and Figure 4B shows a schematic figure illustrating the internals that govern how such a flotation cell would operate in the process.

Figure 5A shows an example of a leach tank for use in the process, and Figure 5B shows a schematic figure illustrating the internals of the leach tank.

Figure 6A shows an example of an electrodeposition tank for use in the process, and Figure 6B is a schematic that illustrates the principle behind copper electrolysis by way of example.

Figure 7A shows an example of a screw press for dewatering for use in the process, Figure 7B shows a schematic figure illustrating the internals that govern how such a screw press would operate in the process, and Figure 7C shows an example of dewatered coal ash.

Figure 8A shows a diagram of the mechanical construction of an indirectly- heated rotary dryer for use in the process, and Figure 8B shows how the vanes inside such a rotary dryer can be constructed.

Figure 9A shows an example of cyclonic particle separators for use in the process, and Figure 9B shows a schematic figure illustrating the internals that govern how such cyclonic particle separators operate.

Figure 10A and Figure 10B show schematic figures from opposite ends illustrating the internals that govern the operation of a ribbon blender for use in the process to mix binder with dry particles, Figure 10C shows an example of a roll compaction press to agglomerate particles for use in the process, Figure 10D is a schematic illustrating the concept of roll pressing for use in the process.

Figure 11 shows an example of a steam -heated j acketed curing reactor for use in the process to make a structural light weight aggregate. Figure 12 shows an example of a rotary kiln for use in the process to make structural light weight aggregate when the sintering method is used.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the drawings and defined in the claims, are not intended to be limiting. Other embodiments may be utilised and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings can be arranged, substituted, combined, separated and designed in a wide variety of different configurations, all of which are contemplated in this disclosure.

The follow description discloses an embodiment of a process for remediating coal ash. From the remediated coal ash, up to four ash products can be produced in varying proportions. The process can be controlled to predominantly produce one or more of these four ash products. The remediation process is also designed and operated such that the ash products comply with customer specifications, Australian Standards and EPA regulations. The process is further designed such that it can be certified by environmental authorities (for example the NSW EPA), allowing the sale of ash products directly, without the need for special exemptions, etc.

Figure l is a concept flow diagram, set out in simple block diagram form, of a process 10 for remediating coal ash to form up to four ash products. The total process 10 is divided into what may be considered nine ‘stages’, with each stage being described in further detail below.

In general, to produce the feed for the process 10, coal ash slurry is removed from a pond 12 containing coal ash such as by a hydraulic dredge 14 in a dredging stage 11, which is described in more detail below with reference to Figs. 2 A and 2B. In instances where the coal ash is not held in a storage pond, but is e.g. held in a stockpile, a slurry can be produced from the coal ash stockpile to then enable it to be treated in the process 10.

The slurry that is dredged from the pond 12 by the dredge 14 (or that is produced from stockpiled coal ash) is pumped by a slurry pump 16 to a separation stage 20. The slurry pump 16 can pump directly from the dredge 14, or the dredge can pump the slurry to e.g. a holding barge from which the dredged coal ash slurry is then subsequently passed (pumped) to the separation stage 20.

In the separation stage 20 fines are separated from coarser material present in the slurry. The separation stage 20 employs a coarse filter 22, which is described in more detail below with reference to Figs. 3 A and 3B. The coarser material is collected on and then passes from the filter 22 to form a granular fill material product 24. The properties of the coarse granular fill material product 24 are described in greater detail below with reference to Example 1.

Fines present in the slurry pass through the coarse filter 22 to form a finer slurry, which is pumped by slurry pump 31 to a flotation stage 30. In flotation stage 30, flotation is performed in a flotation cell 32 to separate carbon, organic matter (when present) and other contaminants from the fines slurry. Organic matter (e.g. plant material, pond life, etc.) can be present as a result of dredging the coal ash from a storage pond. As explained below, the separated carbon, organic matter and other contaminants can be recycled to the power station as a kind of biomass for combustion in the boiler burners. The flotation stage 30 is described in more detail below with reference to Figs. 4A and 4B.

The underflow from flotation stage 30 is pumped by a slurry pump 34 to a metals recovery area 40 which comprises a metals extraction stage 50 and a metals recovery plant 60. In the metals extraction stage 50, metals present in the underflow are typically subjected to a leaching process (e.g. acid leaching), whereby metal contaminants present in the fines underflow are leached out of the solids present in the underflow. The leaching process is operated to produce a metals-leached solids product and a metals-rich liquor. The leaching process is described in more detail below with reference to Figs. 5A and 5B.

The substantially metals-free fines stream from the metals extraction stage 50 is pumped by slurry pump 54 to a dewatering stage 70. Conversely, the metals-rich liquor from the metals extraction stage 50 is pumped by a liquids pump 52 to the metals recovery plant 60. The metals recovery plant 60 comprises a number of different processing stages aimed at recovering metals contained in the liquor and also aimed at converting these metals into useful products 62 which can then be sold. These processing stages are described in more detail below with reference to Figs. 6 A and 6B.

In dewatering stage 70, the metals-free fines slurry is dewatered to remove a significant amount of moisture by using a screw press 72. The screw press 72 ‘squeezes’ water from the fines present in the slurry to produce a first fine ash product 74. Recovered water can be recycled back to the hydraulic dredge 14. The dewatering stage 70 is described in more detail below with reference to Figs. 7A, 7B and 7C. The properties of the first fine ash product 74 are also described in greater detail below with reference to Example 2.

In one version of the process 10, the process can be terminated after the dewatering stage 70, whereby the final product of the process is the fine ash product 74. As explained in more detail in Example 2, the fine ash product 74 is suitable for use as bulk fill for road construction and mine fill. However, the process 10 can be operated with additional stages for the production of an ash that is suitable for use as a supplementary cementitious material, as well as a structural light weight aggregate. These additional stages will now be described.

A proportion (or all) of the first fine ash product 74 is passed from the dewatering stage 70, using a conveyor 76, to a thermal treatment process 80 in which the moisture content is further (substantially) reduced. The thermal treatment process involves indirectly heating the first fine ash product using a rotary dryer 82. This stage is described in more detail below with reference to Figs. 8A and 8B.

The dried first fine ash product is then transported by means of a roots blower 84 to a particle separation stage 90. In stage 90, coarser particles are removed from the dried first fine ash product such as by centrifugal force, typically, by use of a cyclone 92. The fine (overflow) particles recovered from particle separation stage 90 form a second fine ash product 96 that is suitable for use as a supplementary cementitious material. This is described in more detail below with references to Figs. 9A and 9B. The properties of the second fine ash product 96 are also described in greater detail below with reference to Example 3.

In another variation of the process 10, the process is terminated after the particle separation stage 90, wherein the final products are the second fine ash product 96 (overflow) and a remaining coarser particulate material 98 (underflow). However, typically the process 10 comprises an additional stage 100 for the production of an aggregated ash product that is suitable for use as a structural light weight aggregate. This additional stage will now be described.

The coarser particulate material (underflow) from particle separation stage 90 is transported by a roots blower 94 to an aggregation treatment stage 100. In aggregation stage 100, the coarser particulate material is aggregated, typically by one or by a combination of geopolymerisation and sintering to form an aggregated ash product 104. This is described in more detail below with references to Figs. 10A through 10D, Fig. 11 and Fig. 12. The properties of the aggregated ash product 104 are also described in greater detail below with reference to Example 4.

Specific details of the process 10 for accomplishing each of the operational stages 11 and 20 to 100, as outlined above, will now be described.

Coal Ash Slurry Removal Stage 11 (Fig, 2) Coal ash waste, such as that generated by coal combustion in a coal-fired power station, is often stored in ponds in proximity to the power station. As the coal ash waste is typically denser than the pond liquid, the majority of ash will settle to the bottom of the pond over time. Once accumulated, it can be covered by organic matter growth, pond silt, etc.

In the coal ash removal stage 11, settled ash within the pond 12 is disturbed by mechanical means to generate a pumpable suspension. Typically, hydraulic dredging 14 is used. As shown in Figure 2A, the hydraulic dredging stage 14 typically employs a floating hydraulic dredge barge 15. The dredge barge is deployed into the pond 12 that contains the coal ash waste. The barge 15 comprises a boom and arm arrangement 17, the arm being articulated to extend towards the bottom of the pond 12. As illustrated by Figure 2B, a bottom of the arm 18 is typically fitted with apparatus for dislodging and disturbing the settled coal ash. In the example shown, the apparatus comprises a hydraulic pump 19 having mechanical fingers 18. The mechanical fingers 18 are remotely activated and are used to disturb the settled coal ash. The disturbed coal ash is sucked in as a slurry by the pump 19.

To enhance its pumping as a slurry, it may be required to mix additional water into and within the pump 19. For example, should the slurry pump 19 require a source of cleaner water than the disturbed pond water (e.g. for proper pump operation and to prevent pump fouling and blockage, and to also minimise the consumption of pond and/or fresh water), water extracted during the dewatering stage 70 can be recycled back to the slurry pump 19. In each case, the dredged slurry is pumped directly or indirectly by slurry pump 16 to the solids separation stage 20. For example, the dredged slurry can be pumped to a holding barge, with the filled holding barge then being moved into proximity of the separation stage 20, with the dredged slurry being pumped from the filled holding barge to the solids separation stage 20.

Solids Separation Stage 20 (Fig, 3) In solids separation stage 20, fines are separated from coarser material to maximise the quality of downstream products, but also to protect/optimise downstream process equipment. Further, the separated coarser material can form a first product of the process.

In this regard, pond coal ash can comprise fly ash from coal combustion and bottom ash from the boilers, along with other waste material from the plant that has been deposited in the pond and that has settled into the deposited ash. Typically, during coal ash slurry removal, these materials are disturbed and removed by the dredge 15. In some cases, the pond coal ash may additionally contain asbestos waste, in either cement or fibrous form. The coarser material removed in the separation stage 20 can therefore comprise bottom ash, other waste material, and asbestos in cement (i.e. captive) form.

In the separation stage 20, the slurry is pumped by pump 16 to a static screen filter 22 in which it undergoes coarse filtration. As illustrated in Figs. 3 A and 3B, the slurry passes via an inlet 21 into a vertical, agitated accumulation tank 23. From tank 23, the slurry overflows into, and is evenly distributed across, a top weir 22A of the screen filter 22. The slurry then flows down over a screen surface 25 of the static screen filter 22 which is inclined and contains perforations. The size of the perforations on the screen surface 25 is preselected to stop coarser material from passing therethrough. Coarser solids present in the slurry that cannot pass through the perforations on the screen surface 25 are pushed down the face of the screen surface by hydraulic action. The coarser solids are collected at the bottom of the incline at a solids outlet 26. Finer particles, i.e. particles that are smaller than the perforations on the filter surface 25, and liquids, pass through the perforations and collect as a fines slurry in the receptor 27. The fines slurry is pumped from the receptor 27 via an outlet 28 to flotation stage 30 by slurry pump 31 (Fig. 1).

The coarser material collected from the solids outlet 26 may be collected to form the granular fill material product 24, a first product of the process. An optional crushing step can be performed on the granular fill material product 24 (such a step may be omitted if the product 24 comprises asbestos). The properties of the coarse granular fill material product 24 are described in greater detail below with reference to Example 1.

Flotation Stage 30 (Fig, 4)

The fines slurry is pumped by slurry pump 31 from the separation stage 20 to the flotation stage 30. Pond coal ash usually contains unburnt carbon. High or variable unburnt carbon content can be a significant problem in road construction, a primary end-use of ash products. Pond coal ash can also contain organic matter; for example, pond grass growing at and beneath the pond surface. Further, pond coal ash may comprise asbestos in fibrous and concrete form. The static screen filter 22 can remove asbestos in concrete form, however, asbestos in fibrous form can still be present in the fines slurry pumped to the flotation stage 30. The flotation stage 30 is designed and operated to separate these further contaminants from the fines slurry.

In the flotation stage 30, flotation is carried out using one or more flotation cells 32 that are designed and operated using a froth flotation separation procedure. Fig. 4A shows a typical cylindrical froth flotation cell 32. Figure 4B is a schematic of the internals of the froth flotation cell 32.

The fines slurry pumped by slurry pump 31 enter towards the top of the flotation cell 32 via a feed port 33. The fines in the slurry entering the flotation cell are kept in suspension in the cell 32 through the use of an agitator 36. The agitator 36 is also designed to act as a sparger through which pressurised air is caused to enter the cell 32 as air bubbles 35. A frothing agent (e.g. surfactant) is typically also added to the water within the cell 32 to promote formation of froth 37. The amount and type of frothing agent is adjusted to ensure adequate recovery in the froth 37 of unburnt carbon and other lightweight foreign materials present in the fines slurry (organic matter, etc.). The frothing agent causes hydrophobic particles, such as unbumt carbon and other lightweight foreign materials present in the ash slurry to attach to the air bubbles 35. The bubbles 35, along with the attached hydrophobic particles, float to the surface, forming the froth 37. In this way, the unbumt carbon and other lightweight foreign materials (organic matter, asbestos fibre, etc.) are separated from the ash slurry. The froth 37 comprising such contaminants is collected in a launder 38, from which the froth is eventually passed to the boilers of a nearby power station for combustion in the boiler burners as a biomass. During combustion, the unburnt carbon and organics are incinerated, with fibrous asbestos being vaporised back to its basic elements. The vaporised asbestos is caught in the power station bag filters, with its hazardous fibrous form having been destroyed.

The composition of a treated ash slurry produced by the flotation stage 30 is constantly monitored to ensure the fraction of unwanted materials reporting to the treated ash slurry is minimised. The frothing agents can be adjusted accordingly, as can the residence time in the flotation stage, to thereby increase the purity of the treated ash slurry.

A treated ash slurry, from which the unbumt carbon and other lightweight foreign materials have now been separated, discharges from the bottom of the flotation cell 32 as an underflow 39. This treated ash slurry underflow 39 is pumped by slurry pump 34 (Fig. 1) to the metals recovery area 40.

Metals Recovery Area 40 (Figs. 5 and 6)

In the metals recovery area 40, heavy (and other) metals are removed. In this regard, pond coal ash can contain significantly high levels of heavy metals and other contaminants which are dangerous to the environment and toxic to human health. The type and concentration of these contaminants varies depending on the geological history of the coal that the ash was produced from, as well as when and where the ash was deposited within a coal ash pond. When levels of contaminants approach the limits set out by governing authorities (such as the NSW Coal Ash Order of 2014), contaminants need to be extracted and/or neutralised through further remedial processes.

In this regard, the metals recovery area 40 comprises a metals extraction stage 50 and a metals recovery plant 60.

Metals Extraction Stage 50 (Fig, 5)

The treated ash slurry from the flotation stage 30 is pumped via pump 34 to the metals extraction stage 50. In the metals extraction stage 50, a hydrometallurgical leaching process is performed to leach heavy (and other) metals from the treated ash slurry using leaching reagents. Reagents such as a strong acid or a strong base are employed to dissolve into aqueous solution metals and other contaminants in the treated ash slurry. Typically, for acid leaching, a strong acid such as hydrochloric or sulphuric acid is employed. Similarly, the efficacy of the leach is optimised by operating the leach tanks at specific (e.g. elevated) temperatures and/or pressures.

For example, where copper needs to be leached, then sulphuric acid is added to leach the copper as soluble copper sulphate. In some cases, oxidising agents are used, e.g. when iron and arsenic need to be extracted from the treated ash slurry. Such oxidising agents promote the formation of iron as ferric and, subsequently, the precipitation of iron and arsenic as scorodite.

Fig. 5 A illustrates a typical leach tank 52 and Fig. 5B is a schematic of the internals of the leach tank 52 for use in the leaching process. Typically, the leaching process comprises a series of such tanks 52, whereby the liquor overflow from a first tank is fed into a feed inlet of a next tank, and so on. The leaching process is operated continuously whereby slurry continually flows (or is pumped) from one tank to the next. The number and size of the tanks is selected to produce sufficient residence time for the leaching reactions to achieve the required extent of metal leaching. The leaching process can be operated in co-current flow mode or it can be reconfigured in a counter-current flow mode. The leach tank 52 comprises a treated ash slurry inlet 53 and a liquor overflow 54. The leach tank 52 further comprises a rotary mixer 56 which is operated to ensure homogeneity within the tank, by continuously mixing the contents of the tank.

The mixer 56 is powered by a variable speed motor 57 whereby the speed of mixer rotation can be adjusted as required.

The leaching process results in a substantially metals-free fine ash slurry and a metal-containing liquor. The metals-free fine ash slurry is separated from the metal-containing liquor, typically by means of a filtration device provided after the leaching process. During filtration, the metals-free fine ash slurry is washed with a wash liquor to achieve two main purposes: 1) neutralisation of any remaining added leach reagents within the fines slurry; and 2) decrease the metals present in the metals-free fine ash slurry interstitial liquor.

The now-washed, substantially metals-free fine ash slurry is pumped by slurry pump 54 (Fig. 1) to the dewatering stage 70. Conversely, the metals-containing liquor (including wash liquor) is pumped by a liquids pump 52 to the metals recovery plant 60.

Metals Recovery Plant 60 (Fig, 6)

The main purpose of the metals recovery plant 60 is to recover metals from the metals-containing liquor in useful forms. For example, the aim is to recover a pure metal form or at least a metal concentrate that can be on-sold to other refining operations.

In the metals recovery plant 60, a combination of physical and chemical treatments is used to separate the metals present in the metals-containing liquor produced by the leaching process. The exact type and configuration of processes required depend on the composition of the metals-containing liquor. Typical metal contaminants that require removal from the liquor comprise copper, iron, arsenic, lead, mercury, etc. as well as sulphates, etc. For example, when solid contaminants that exhibit magnetic properties are present in the liquor, the metals recovery plant 60 comprises a magnetic separation stage. The magnetic separation stage enables separation of magnetisable contaminants from the liquor.

When the solid contaminants require chemical treatment, precipitation of different metals at various pH levels, along with suitable precipitation agents, can be employed.

A metal that is typically present in coal ash in high concentrations is copper. Where copper is leached during metals extraction stage 50 using sulphuric acid, it is typically present as copper (II) sulphate in the metal s-containing liquor. The copper in this form can then be recovered electrolytically by electrodeposition.

Fig. 6A shows a suitable electrodeposition tank 63 and Fig. 6B schematically illustrates the electrodeposition tank chemistry. The electrodeposition tank 63 comprises a negatively charged cathode 66 and a positively charged anode 68. The anode 68 and cathode 66 are submerged in an electrolyte 65 held in the tank 63. The electrolyte 65 comprises the metal s-containing liquor plus any required additives. A power supply 64 provides an electrical current to the cell. Under the influence of the electrical current, copper (II) ions in solution migrate towards and deposit at the cathode 66, forming a layer of solid copper on the surface of the cathode 66. At the cathode 66, the copper (II) ions are reduced according to the equation:

^Cu (i) + 2e~ Cu _(s)

The reduction order of metal ions such as copper and iron differs so, by controlling current density, copper can be preferentially separated from other contaminants present, such as iron. The cathode can be periodically removed/replaced for recovery of the deposited copper. To balance the reduction of copper (II) occurring at the cathode 66, water at the anode 68 is oxidised, forming oxygen (as O2 gas) and hydrogen ions.

Another common contaminant in coal ash is arsenic. Where arsenic is present, it is first separated using a froth flotation cell, as previously described. Arsenic is collected from the top launder with the froth. The collected arsenic is then reacted with iron, as ferric, to form a hydrated iron arsenate material scodorite. The scorodite is filtered and recovered as a solid. Scorodite is quite stable in acidic soils and can be disposed of in this form. The ferric required can comprise iron recovered elsewhere in the metals recovery plant 60 or from the addition of a reagent, such as ferric sulphate.

Following the recovery of metals from the metal s-containing liquor in the metals recovery plant 60, the concentration of metals remaining in the liquor will be minimal. Consequently, the liquor is able to be returned to the pond 12 (Fig. 1).

Dewatering Stage 70 (Fig, 7)

As set forth above, the metals-free fine ash slurry from metals extraction stage 50 is pumped by slurry pump 54 to the dewatering stage 70. In dewatering stage 70 excess moisture is removed from the metals-free fine ash.

Fig. 7A illustrates a typical screw press 72 in which dewatering is achieved. Depending on the capacity required, multiple screw presses operating in parallel are typically used.

Fig. 7B provides a schematic of the internals of the screw press. The main chamber 71 of the screw press 72 comprises a screw 73 located within an inclined cylindrical body 75. The metals-free fine ash slurry is pumped into the screw press 72 by the slurry pump 54. In the main chamber 71, water is squeezed out of the fine ash slurry by the action of the screw 73 on the slurry. The water flows back down the chamber, passes through a filter and is then collected at a bottom outlet 77 of the chamber 71 as a filtrate. This filtrate can be returned for use in the hydraulic dredging system 14 or to the coal ash pond 12 (Fig. 1).

Dewatered fine ash is collected from an upper end chute 78 of the chamber 71. Fig. 7C shows an example of a dewatered fine ash product 74 (i.e. first fine ash product 74 - Fig. 1). The fine ash product 74 is typically sold as a second product of the process for use as a general bulk fill in road construction or for filling mine voids. The properties of the first fine ash product 74 are described in greater detail below with reference to Example 2.

Depending on process economics and/or customer demand, a portion (or all) of the first fine ash product 74 can be sent forward to further treatment stages (80 - 100) of the process 10. The division of the fine ash product into a product for sale and a portion sent forward to further treatment stages can be performed manually or automatically (for example by use of a flop gate). Typically, the fine ash is divided at stage 70 with no discrimination based on chemical or physical characteristics, such as particle size.

An advantage of the process 10 is that the proportion of the fine ash product 74 that is sent forward to further treatment stages is readily adjustable based on current market conditions. For example, when the demand for the fine ash product 74 is high compared to the subsequent products of treatment stages 80 - 100, a greater percentage (e.g. all) of the fine ash product 74 is sold directly. When the converse is true, a greater percentage of the fine ash product is further processed into the downstream products of treatment stages 80 - 100.

Thermal Treatment Stage 80 (Fig, 8)

The selected proportion of fine ash product from the dewatering stage 70 is transported by means of a solids conveyor 76 to the thermal treatment stage 80. In thermal treatment stage 80, the moisture content of the fine ash product is further reduced, resulting in a dried fine ash product. Typically, an indirectly-heated rotary dryer 82 is used in the thermal treatment stage 80. Fig. 8A depicts the indirectly-heated rotary dryer 82. The indirectly-heated rotary dryer 82 comprises a rotating drum 89 enclosed in a furnace 81. The furnace 81 of the rotary dryer is externally heated via burners 87. The fine ash product 74 from the dewatering stage 70 enters the rotating drum 89 of the rotary dryer 82 through a material inlet 83.

Piping 86 for gas and air feeds the burners 87, with the combustion of gas in the burners 87 providing thermal energy/heat to the rotating drum 89 (i.e. the thermal energy produced by gas combustion heats the rotating drum 89). The gas combustion also releases exhaust gases to the atmosphere, including carbon dioxide, via exhaust 85. Ideally, solar energy plants are installed in the vicinity of the coal ash ponds, allowing solar energy to be used as the thermal energy source for dryer 82. Using solar energy reduces the carbon footprint of the process 10, compared to using gas combustion.

Figure 8B shows vanes 89A present inside the rotating drum 89. The vanes are attached to the internal walls of the rotating drum 89. The vanes 89A facilitate continuous turning over and breaking down of the fine ash into individual particles, as the fine ash is thermally treated. The internal walls of the rotating drum 89 and the vanes 89A are thus heated by the furnace 81. Contact between the first fine ash product and the heated internal walls and vanes dries the first fine ash product as it progresses through the drum 89.

Vapour is produced in the rotating drum 89 as the first fine ash product is dried. Dried first fine ash product and vapours exit the rotating drum at an exit 88 (discharge breech). At the exit 88, a vapour stream is collected to be separated from a dried first fine ash product. The vapour stream comprises water, entrained dried first fine ash product, and any other vaporised materials. The vapour stream is cooled to condense water present. The condensed water can be returned to the dredge pump 19 and/or to the coal ash pond 12 (Fig. 1).

The concentration of the entrained dried first fine ash product in the vapour stream is closely monitored. If this is too high, the vapour is filtered (for example using a bag filter) to recover the entrained dried first fine ash product. The recovered entrained dried first fine ash product is added into the dried first fine ash product that exits the rotating drum 89 at exit 88. The dried first fine ash product separated from the vapour stream at the exit 88 is transported to a particle separation stage by the roots blower 84 (Fig. 1).

Particle Separation Stage 90 (Fig, 9)

The dried first fine ash product from the thermal treatment stage 82 is transported by the roots blower 84 to the particle separation stage 90 (Fig. 1). Typically, particle separation stage 90 employs cyclonic separation.

Fig. 9A shows an example of a pair of cyclonic particle separators 92 for use in the particle separation stage 90. The cyclonic separators 92 are operated to separate coarser particles from the dried first fine ash product to produce the second fine ash product 96 (i.e. as an overflow from the cyclonic separators 92). This represents a third product of the process 10.

Fig. 9B is a schematic showing the internals of the cyclonic particle separator 92. The dried first fine ash product from the thermal treatment stage 80 is fed tangentially into the feed chamber 91 of the cyclonic particle separator 92 by a fan blower. The tangential orientation of the chamber 91 feeding into the cyclonic particle separator 92 causes a spiral vortex of the feed stream to be formed.

As is known, lighter (finer) fine ash materials have less inertia and are more easily influenced by the vortex. The lighter fine ash material thereby flows upwards. The lighter fine ash material is collected via an overflow outlet 95 as the second fine ash product 96, which is suitable for use as a supplementary cementitious material. The properties of the second fine ash product 96 are described in greater detail below with reference to Example 3.

Larger (coarser) fine ash materials have more inertia and are less easily influenced by the vortex. The larger fine ash material thereby flows downwards. The larger fine ash material is collected via an underflow outlet 97 as a coarser particulate material product 98. Whilst the coarser particulate material product 98 may be used as produced, typically it is further processed.

Separation of particles by the cyclonic particle separator 92 also generates a hot air stream. The hot air stream is either recycled back to the thermal treatment stage 80 or is passed to the inlet 91 of the fan blower of the cyclonic separator 92.

Aggregation Stage 100 (Figs. 10 and 11)

The coarser particulate material 98 from the particle separation stage 90 is transported by means of a roots blower 94 (Fig. 1) to the aggregation stage 100 (Fig. 1). In the aggregation stage 100, an aggregated ash product 104 is formed from the coarser particulate material 98 (i.e. as a fourth product of the process 10).

Two known aggregation processes can be employed in the aggregation stage 100, namely, geopolymerisation and sintering. To facilitate geopolymerisation, a concentrated aqueous solution of alkali activator is mixed with the coarser particulate material 98 prior to agglomeration.

Figs. 10A and 10B show the schematics of a ribbon blender 110 for such mixing of alkali activator. The ribbon blender 110 comprises a trough 112 in which the alkali activator is mixed with the coarser particulate material 98. Blending is achieved through the movement of helical ribbons 113 through the mixture of particulate material 98 and alkali activator located within the trough 112. When the sintering process is used, a bonding agent is added to the trough 112 to facilitate the agglomeration process.

Figs. 10C and 10D show the schematics of a roll compaction press 120. The roll compaction press 120 is used to achieve agglomeration. The blend of particulate material 98 and alkali activator (and bonding agent when present) is fed into the press 120 via a mechanically driven feed hopper 122. In the roll compaction press, agglomeration occurs as blended material 124 is squeezed under pressure between the two opposing press rollers 126. The resultant pressurised product can be collected and allowed to (continue to) geopolymerise.

In the aggregation stage 100, a hybrid model can be employed which incorporates aspects of both geopolymerisation and sintering. The coarser particulate material 98 is first subjected to mixing in the ribbon blender (i.e. with bonding agent added). The blended material 124 then undergoes particle agglomeration in the roll compactor 120 to form an uncured sheet. The uncured sheet is then broken into pieces by a disintegrator before being heat-cured to form the agglomerated product.

Fig. 11 shows a steam -heated jacketed curing reactor 130 used to cure the disintegrated sheet to thereby form the agglomerated product. The disintegrated material is typically fed via a heated screw conveyor into a top of the curing reactor 130 as a continuous stream, and steam (e.g. super-heated) is then continuously fed into and through the jacket of the reactor. The heat-cured material is removed (typically through a rotary valve at the bottom of the reactor 130, the rate of which will control the curing time in the reactor).

Instead of being passed to the curing reactor 130, the disintegrated sheet can be passed to a sintering fumace/kiln 140, such as shown in Fig. 12. The broken pieces of un cured sheet are sintered by heating in the sintering furnace/kiln 140 in a continuous operation. The sintering furnace/kiln 140 comprises a cylindrical drum 142 mounted on support rollers 141 which allow the drum to rotate. The disintegrated sheet is fed into a preheating end 144 of the sintering fumace/kiln 140. Combustion of gas in a burner located at the preheating end 144 generates energy to heat the disintegrated sheet. Exhaust from the burner discharges into the atmosphere. The disintegrated sheet moves through the rotating drum 142, through to a sintering part 146 of the sintering furnace/kiln 140. The disintegrated sheet is sintered as it moves through the sintering part 146. The sintered sheet is cooled and discharged from the sintering fumace/kiln discharge 148. The heat-cured and/or sintered material is then sieved to separate particles of differing sizes, with the different grades being sold as aggregated ash products 104 (i.e. a fourth product of the process 10).

The aggregated ash products 104 typically provide structural -grade lightweight aggregates. Both fine and coarse aggregates can be produced and sold to the precast concrete products industry. The properties of the aggregated ash products 104 are described in greater detail below with reference to Example 4.

As well as aggregating the coarser particulate material produced in the fines separation stage, aggregation can be performed on unclassified fly ash particles that are obtained directly from one or more coal-fired power stations. The unclassified fly ash is produced in a filtration stage in the coal-fired power station in which classified fly ash is collected for sale. Typically, the unclassified fly ash is sent to a coal ash pond (but can be passed to the present process 10). Instead, the unclassified fly ash can bypass the pond/process 10 and be passed directly to the aggregation stage. The hybrid aggregation process as described above can be used to produce an aggregated ash product from the unclassified fly ash. This can represent a further product of the process.

Examples

Non-limiting Examples of the process and various stages thereof will now be provided.

Example 1

In this Example, a process for producing a coarse granular fill material (such as 24 in Fig. 1) will be described, along with a characterisation of the coarse granular fill material.

The coarse granular fill material 24 was produced from the coal ash pond slurry stream 16 by passing that stream through a static screen filter (such as 22 shown in Figs. 1 and 3). The coarse granular fill material 24 was collected at the solids outlet of the static screen filter.

The coarse granular fill material 24 was observed to have consistent particle size and was therefore deemed suitable as a basic granular fill for road construction.

Example 2

In this Example, a process for producing a bulk fill material for road construction and mine fill (such as 74 in Fig. 1) will be described, along with a characterisation of the bulk fill material.

The bulk fill material 74 was produced through a series of steps that started with the coal ash pond slurry 16 stream being passed through a static screen filter (such as 22 shown in Figs. 1 and 3). Fine ash slurry was collected at the screen receptor 27 (Fig. 3) and was pumped to a flotation cell (such as 32 shown in Figs. 1 and 4). In the flotation cell, unburnt carbon and organics (when present) were separated and a treated fine ash slurry was pumped as an underflow to metals recovery. In a metals extraction process (e.g. stage 50), metals and other contaminants from the fine ash slurry were extracted from the fine ash by hydrometallurgical leaching to produce a substantially metals-free fine ash slurry and a metal s-containing liquor. The metals-free fine ash slurry was separated from the liquor by filtering and was pumped to a dewatering screw press (such as 72 shown in Figs. 1 and 7). The dewatered metals-free fine ash produced a bulk material 74.

The bulk fill material 74 was analysed and generally had: consistent particle size; consistently low unbumt carbon; controlled moisture content; a low concentration of the following contaminants (as per Table 1 of the NSW Coal Ash Order 2014): mercury, cadmium, lead, arsenic, boron, chromium, copper, molybdenum, nickel, selenium, zinc; as well as a suitable pH.

Example 3 In this Example, a process for producing a supplementary cementitious material (SCM) (such as 96 in Fig. 1) will be described, along with a characterisation of the SCM.

The SCM 96 was produced by further treating a portion (or all) of the fine ash product 74 from dewatering. The fine ash product 74 was conveyed 76 to an indirectly-heated rotary dryer (such as 82 in Figs. 1 and 8). In the indirectly- heated rotary dryer, the fine ash product was thermally treated to reduce its moisture content. A dried fine ash product and a vapour stream were produced and collected at the dryer outlet 88 (Fig. 8), with the dried fine ash product being separated (i.e. filtered) from the vapour. The separated dried fine ash product was then transported to a cyclonic particle separator (such as 92 in Figs. 1 and 9). In the cyclonic particle separator, coarser particulate material 98 (Fig. 9) was separated from the SCM 96 using centrifugal force.

The SCM 96 was analysed and generally had the following properties: fineness of particle size; low loss-on-ignition; low moisture content; low sulphate content.

Example 4

In this Example, a process for producing aggregated ash products (such as 104 in Fig. 1) will be described, along with a characterisation of the aggregated ash products.

The aggregated ash products were produced by collecting the coarser particulate material 98 (Fig. 1) from the cyclonic particle separator of Example 3. The coarser particulate material 98 was mixed with a bonding agent in a ribbon blender (blender 110 in Figs. 10A and 10B). The mixed material was then agglomerated in a roll compactor (such as 120 in Figs. 10C and 10D) to form an uncured sheet. The uncured sheet was broken into pieces by a disintegrator. This product was not allowed to geopolymerise (such as by being heat-cured in a steam -heated j acketed curing reactor - e.g. 130 in Fig. 11). Instead, the broken pieces of uncured sheet were sintered by being heated in a sintering furnace/kiln (such as 140 in Fig. 12). A number of aggregated ash products 104 were finally produced by sieving the heat-cured material to separate the particles into differing grades.

The aggregated ash products 104 were analysed and generally met AS 2758.1 2014 with regard to the following properties: particle density; water absorption; bulk density; grading; particle shape; flakiness; durability; wet and dry strengths; abrasion resistance; weathering resistance; alkali reactivity; shrinkage; impurities; size/dimensional variations.

Further Variations

Variations and modifications may be made to the process as previously described without departing from the spirit or ambit of the disclosure.

For example, it is to be understood that the characteristics of coal ash may differ to the extent that variations to the above process may be appropriate. Other unit operations can be included in the overall process in line with good engineering practices, in particular, for the extraction and recovery of metals, the conservation of water, and the minimisation of waste streams.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the process.