GB2623823A - Process and product - Google Patents

Abstract

A process is disclosed for reducing the content of leachable contaminants in fly ash which comprises the steps of contacting fly ash with water having a pH of from 6 to 12 to form a washed ash slurry; and filtering the washed ash slurry to obtain a processed fly ash comprising a reduced content of leachable contaminants and a wastewater filtrate. The wastewater filtrate may be treated with an inorganic acid such as nitric acid, a transition metal such as ferrous sulphate and then basified with, for example, lime. This produces a precipitate containing metal washed from the fly ash. Also disclosed is a treated fly ash aggregate wherein the fly ash comprises at most 0.5 mg/l leachable chromium, at most 1 mg/l leachable lead and at most 0.45 mg/l leachable zinc, wherein the content of the leachable chromium, lead and zinc is determined in accordance with the method set out in BSI standard BS 12457. Further disclosed is an integrated plant for carrying out the fly ash washing and wastewater treatment stages.

Description

PROCESS AND PRODUCT

The present invention relates to a process for reducing the content of leachable contaminants in fly ash and a product obtained by said process.

The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

Fly ash is a coal combustion product which is entrained in flue gases typically produced by power plants. For example, fly ash is generated in biomass plants by burning wood, crops, animal manure and/or similar biomass materials. It can also be produced from combustion of solid waste in an energy from waste (EfW) facilities.

Fly ash largely consists of generically spherical particles having a typical particle size of from 10 to 100 pm. Fly ash predominantly comprises oxides of aluminium, calcium, iron and silicon.

The contaminants in fly ash pose a risk to human/animal health and the environment.

According to waste acceptance criteria test results for fly ash, this material typically does not qualify as inert waste. This is at least partly due to a significant content of certain leachable contaminants, as demonstrated on Figure 1.

For many years, fly ash has been disposed in landfill, for example, by mixing it with canal dredging to cap landfill. Disposing fly ash in landfill is not only very expensive for producers but also damaging to the environment. Therefore, this way of utilising ash presents itself as the least preferred option of disposal in the waste hierarchy.

The recycling of fly ash has become an increasing concern in recent years due to rising landfill costs and current interest in sustainable development. Furthermore, the circular economy agenda has been leading to an increase in EM and biomass combustion and production of higher volumes of fly ash. Moreover, current thermal plant operators face significant costs when disposing air pollution control residues (APCr) and these are set to rise with increasing legislative pressure to reduce disposal of fly ash into landfill. All of these aspects necessitate development of further solutions for fly ash reconditioning/recycling.

One previously proposed process for recycling fly ash is a carbonation process, which involves treating fly ash with pressurised carbon dioxide to precipitate heavy metals as insoluble salts. Although this is a recovery option, such a process requires considerable energy expenditure and additional infrastructure to generate and deliver high-pressure carbon dioxide and subsequently process the treated fly ash.

Another previously proposed recovery option involves desalination of fly ash via heating and/or adding chemicals, such as inorganic acids and/or salts thereof. Such processes tend to be complex and expensive, and typically require many unit operations to recondition fly ash to a satisfactory extent.

One solution to the problem of fly ash utilisation is to use fly ash in certain applications, such as in concrete products, for example as a replacement for concrete aggregates. Aggregates are essential in concrete products for a number of reasons, including their dimensional stability and abrasion resistance but also cost.

There is a constant requirement for aggregates within the construction industry forcing more environmentally negative quarries to be opened. The process of producing raw aggregate from quarrying of rock and sand not only destroys the landscape leaving destructive pits but also exposes species of all kinds to permanent negative impacts of mining. Thus, the use of fly ash in concrete products could offer a viable alternative to quarrying aggregates.

However, it has been found that when unprocessed fly ash is used as an aggregate in concrete, substantial leaching of hazardous and/or undesirable components (certain heavy metal ions and chlorides) occurs over time, which is not only of environmental concern but also surprisingly affects properties of concrete, e.g. leads to its swelling and/or makes it more brittle and hence prone to breakage.

It would be desirable to develop a simple, economical and environmentally friendly way of recycling/reconditioning fly ash, resulting in processed waste that is particularly suitable for use as an aggregate for concrete products. More specifically, it would be desirable to recycle/recondition fly ash in a way that consistently produces non-hazardous processed fly ash for use in concrete products without the need for using complex infrastructure, chemicals and considerable amounts of energy. Ideally, the processed fly ash could: (i) be used in the same way as non-waste; (ii) be stored and used with no worse environmental effects than the concrete aggregate material that it is intended to replace; (iii) have a sufficiently minimised content of leachable contaminants, both itself and when used as an aggregate in concrete products; and/or (iv) impart improved properties (e.g. enhanced durability) to concrete products made using the processed fly ash.

SUMMARY OF THE INVENTION

The present invention is based on a surprising finding that it is possible to recycle/recondition fly ash for use in concrete products by washing fly ash with water. Specifically, the present inventors have found that by washing fly ash with water, it is possible to stop undesirable components from leaching to a significant extent from the treated fly ash. Thus, the process of the invention enables the production of processed fly ash product with reduced content of leachable contaminants, which is important from environmental perspective and renders the product particularly suitable for use in concrete product applications. In particular, the process of the invention enables reduction in the content of leachable chromium, lead and zinc, as well as leachable copper and chlorides. These five leachable contaminants (particularly chromium, lead and zinc) have been found to be most relevant both from environmental standpoint and in terms of affecting the properties of concrete products prepared using the reconditioned fly ash composition.

The process of the invention further allows to avoid the use of chemicals and complex infrastructure and reduces the number of unit operations required for effective fly ash recycling/reconditioning. As such, it presents a simple, economic and consistent way of recycling/reconditioning fly ash.

In addition, the invention generally provides a sustainable and environmentally friendly option for recycling/recovering fly ash, because: It is a recovery process with little to no emissions to the environment from the process, and therefore is a 'green' solution to the problem of fly ash utilisation.

* It allows for reduction in amount of combustion wastes that must otherwise be disposed in landfill.

* It enables reduction in air pollution caused by transporting the waste to suitable landfills and pre-treatment facilities.

* It provides processed fly ash product that can conveniently be used in the same way as non-waste, particularly when used in concrete products.

According to the present invention, there is provided a process for reducing the content of leachable contaminants in fly ash comprising the steps of: (i) contacting fly ash with water to form a washed ash slurry, wherein the water has a pH of from 6 to 12; and (ii) filtering the ash slurry to obtain a processed fly ash comprising a reduced content of leachable contaminants and a wastewater filtrate.

In a preferred aspect, the invention provides a process for reducing the content of leachable contaminants in fly ash originating from a biomass power plant and/or from an energy from waste (EfW) power plant.

The present invention also provides a plant for conducting the process for reducing the content of leachable contaminants in fly ash, wherein the plant comprises: (a) a mixing tank equipped with an agitator, adapted to contact fly ash with water; (b) a water supply fluidly connected to the mixing tank; (c) a vessel adapted to store fly ash prior to the contacting in the mixing tank and a means connected to the mixing tank and adapted to feed fly ash from the vessel into the mixing tank; (d) a filtering apparatus disposed downstream of the mixing tank and configured to filter the ash slurry; and (e) a means adapted to transfer the ash slurry from the mixing tank to the filtering apparatus.

In another aspect, the invention provides a process for treating the wastewater filtrate originating from the process for treating fly ash according to the invention, the process comprising: (a) acidifying the wastewater filtrate to form a first solution; (b) adding a transition metal salt into the first solution to form a second solution; (c) basifying the second solution to form a suspension comprising treated wastewater filtrate and a precipitate.

The present invention further provides a plant for conducting the process for treating the wastewater filtrate originating from the process for treating fly ash according to the invention, wherein the plant comprises: (a) a wastewater filtrate treatment mixing tank equipped with an agitator, adapted to contact wastewater filtrate with an acid, a transition metal salt and a base; (b) an acid dosing tank, a transition metal salt dosing tank and a base dosing tank, said dosing tanks being fluidly connected to the wastewater filtrate treatment mixing tank; (c) a means adapted to feed the wastewater filtrate into the wastewater treatment mixing tank; and (d) a decant tank adapted to store the treated wastewater filtrate.

The invention also provides an integrated process for reducing the content of leachable contaminants in fly ash and treating the wastewater filtrate originating from said process. Also provided is a plant for carrying out the integrated process.

Also provided is the processed fly ash (reconditioned fly ash) obtained according to the process for treating fly ash of the invention.

The present invention also provides an aggregate for use in concrete, said aggregate consisting essentially of the reconditioned fly ash having a reduced content of certain leachable contaminants.

In another aspect, there is provided a method for manufacturing an aggregate for use in concrete products, the method comprising: (a) reducing the content of leachable contaminants in fly ash by contacting fly ash with water to form a washed ash slurry, wherein the water has a pH of from 6 to 12, and filtering the ash slurry to obtain a processed fly ash having a reduced content of leachable contaminants and a wastewater filtrate; and (b) forming the aggregate from the processed fly ash.

Also provided is a method for manufacturing concrete using the aggregate of the invention.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification, including definitions, will be taken. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the present invention, suitable methods and materials are described below. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

When an amount, concentration, or other value or parameter is given as either a range, preferred range or a list of upper preferable values and/or lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range.

It will be appreciated that the use of the terms "first" and "second", and the like, in this patent specification is merely intended to help distinguish between similar features, and is not intended to indicate the relative importance of one feature over another feature, unless otherwise specified.

Unless stated otherwise, pH and ionic strength values referred to in the present specification are the values as calculated at 25 °C.

DESCRIPTION OF FIGURES

Figure 1 shows an exemplary profile of contaminants of fly ash generated in a biomass plant.

Figure 2 shows an exemplary setup for conducting the process for reducing the content of leachable contaminants in fly ash according to the claimed invention Figure 3 shows an exemplary setup for conducting the process for treating wastewater filtrate originating from the process for treating fly ash according to the present invention.

Figure 4 shows concrete blocks made using processed fly ash obtained in the process of the present invention.

DETAILED DESCRIPTION Contacting

The present invention provides a process for reducing the content of leachable components (such as heavy metal ions and chlorides) in fly ash.

Step (i) of the process for treating fly ash in accordance with the present invention involves contacting fly ash with water to form a washed ash slurry.

During the contacting in step (i), fly ash is washed with water. That is, the contacting in step (i) involves or consists essentially of washing fly ash with water to form a washed ash slurry. This has been found to reduce the content of leachable contaminants (particularly lead, zinc, and chromium but also copper and chlorides). Without wishing to be bound by theory, it is believed that washing with water 'unlocks' these components from the solid constituents of fly ash, for example by turning the compounds into their hydrated forms.

Step (i) of the process starts with feeding a portion of water into a mixing tank (typically, at least 30% of the total amount of water used in step (i), preferably at least 50%).

Subsequently, fly ash and any remaining water are added together, and the resulting mixture is agitated to wash the fly ash. The present inventors have surprisingly found that feeding a portion of water first in accordance with the amounts described above ensures substantially homogenous distribution of leachable contaminants in the processed ash.

Typically, the weight ratio of water to fly ash in step (i) is from 1:1 to 5.5:1. Conveniently, the weight ratio of water to fly ash is from 2:1 to 5:1. Generally, excessive amounts of water are not used in the process of the invention. It is known that solubility of poorly soluble salts of heavy metals increases with an increasing amount of solute (e.g. water). Based on this, one might expect that better washing results might be achieved with relatively high amounts of water. However, it has been surprisingly found that gradually increasing the amount of water does not result in improved reduction of the amounts of certain leachable contaminants in the processed fly ash.

Preferably, the weight ratio of water to fly ash in step (i) is from 2.5:1 to 4.5:1, even more preferably from 2.5:1 to 4:1. The present inventors have found that such weight ratios (particularly the ratio of from 2.5:1 to 4:1) provide an advantageous balance between maximising the reduction of leachable contaminants in the processed fly ash and minimising the issues with the mechanics of the plant wherein the process is conducted.

Furthermore, such weight ratios ensure that the amount of wastewater filtrate generated during the process is minimal and hence easy to handle/process. The processing of the wastewater filtrate is discussed in the later parts of the specification.

The washing of the fly ash in step (i) is typically carried out at a temperature of at most °C (hereinafter the "contact temperature"). Preferably, the contact temperature is from 2 to 30 °C, even more preferably from 5 to 25 °C. The term "contact temperature" as used herein includes the temperature of fly ash and water immediately prior to the contacting. These relatively low contact temperatures allow for considerable reduction in energy usage (e.g. no heating is required during washing) and hence provide a significant economic advantage over processes that require one or more heating steps (e.g. melting at high temperatures) to treat fly ash. It will be appreciated by the skilled person that the temperature during the contacting in step (i) might rise to some extent due to the presence of certain components in the ash, wherein exposing these components to water generates some thermal energy.

Typically, the fly ash is washed with water for from 10 to 75 minutes, such as from 20 to 50 minutes (hereinafter "contact time"). Preferably, the contact time is from 25 to 40 minutes. With respect to these ranges, the skilled person will appreciate that for some fly ashes (e.g. those comprising relatively coarse particles), it may be preferred to use contact times closer to the upper limits of these ranges, e.g. to ensure thorough contact between larger ash particles and water and/or control an elevation in temperature (as described above), if observed.

It has been surprisingly found that, in the context of the present invention, shortening the contact (washing) time significantly improves the reduction of the amount of leachable contaminants in the processed fly ash. This is counterintuitive, as one would expect that shorter contact times do not adequately reduce the content of leachable contaminants in the ash, due to at least insufficient exposure of the components of fly ash to water.

The water used for the contacting in step (i) typically has a pH of from 6 to 12. Preferably, the process comprises at least one step of contacting fly ash with water having a pH of from 6 to 8. The process of the invention preferably uses mains water. Since the pH of the water is from 6 to 8, the water contains substantially no added acids, bases or indeed acidic/basic salts. The present inventors have found that fly ash can be treated using water, without the need for the use of chemicals (including hazardous chemicals). Therefore, the present invention provides for a simple and economic process for treating fly ash to yield a non-hazardous material suitable for use in concrete products.

If the pH of water used in the contacting is from 6 to 8, it is preferred that the ionic strength of the water is at most 0.05. Conveniently, the water has a pH of from 6.2 to 7.8 and an ionic strength of at most 0.045. Preferably, the water has a pH of from 6.3 to 7.7 and an ionic strength of at most 0.040.

Surprisingly, it has been found that the wastewater filtrate (see step (ii) below) generated in the process for reducing the content of leachable contaminants according to the invention can be successfully reused at least once for contacting the fly ash to reduce the amount of leachable contaminants. Thus, the process for reducing the content of leachable components can be repeated using at least some of the wastewater filtrate and another batch of fly ash. This further improves efficiency and economy of the overall process.

Accordingly, in a preferred aspect, at least a portion of the wastewater filtrate is recycled into step (i) of the process (i.e. a subsequent step of contacting fly ash with water to form a washed ash slurry). That is, in one embodiment, the contacting in step (i) is repeated using at least a portion of the wastewater filtrate and another batch of fly ash. Advantageously, up to 80% by weight of the wastewater filtrate is recycled into step (i), such as from 50 to 80% by weight, for example from 60 to 80% by weight. Typically, the wastewater filtrate suitable for recycling has a concentration of chlorides of up to 50,000 mg/I. Any remaining wastewater filtrate can be sent to the wastewater filtrate treatment process described later in the specification.

Therefore, in a preferred aspect, the process of the invention comprises: )(a) contacting a first portion of fly ash with a first portion of water to form a first washed ash slurry, wherein the first portion of water has a pH of from 6 to 8; (ii)(a) filtering to separate the first washed ash slurry into a first processed fly ash and a first wastewater filtrate; (i)(b) contacting a second portion of fly ash with a second portion of water to form a second washed ash slurry, wherein the second portion of water has a pH of from 6 to 12 and comprises at least a portion of the first wastewater filtrate; and (ii)(b) filtering to separate the second washed ash slurry into a second processed fly ash and a second wastewater filtrate.

As stated above, it is preferred that up to 80% by weight of the wastewater filtrate generated in step (ii)(a) is recycled (fed) into step (i)(b), such as from 50 to 80% by weight, for example from 60 to 80% by weight.

At least some of the second wastewater filtrate can be advantageously further reused in a subsequent contacting step and/or can be sent directly to the wastewater treatment process described later in the specification. Typically, the wastewater filtrate is no longer recycled once it has reached a concentration of chlorides of 50,000 mg/I.

In one embodiment, the process comprises at least one further sequence of contacting and filtering steps, wherein contacting steps in each subsequent sequence use at least a portion of the wastewater filtrate originating from a filtering step of an immediately preceding sequence. The wastewater filtrate originating from a filtering step of an immediately preceding sequence is preferably used in an amount of up to 80% by weight, such as from 50 to 80% by weight, such as from 60 to 80% by weight.

For example, the second wastewater filtrate can be reused in contacting a third portion of fly ash with a third portion of water to form a third washed ash slurry, wherein the third portion of water has a pH of from 6 to 12 and comprises at least a portion of the second wastewater filtrate. Up to 80% by weight of the second wastewater filtrate is recycled, such as from 50 to 80% by weight, for example from 60 to 80% by weight. Then, filtering is carried out to separate the third washed ash slurry into a third processed fly ash and a third wastewater filtrate, etc. The wastewater filtrate is preferably sent to the wastewater treatment process once it has reached a point of saturation. In one embodiment, the wastewater filtrate is sent to the wastewater filtrate treatment process once the filtrate has reached a concentration of chlorides of 50,000 mg/I.

The washing of fly ash is typically carried out using an agitator. It is preferred that the agitator is a high shear mixer. Preferably, the mixing speed of the high shear mixer is from 30 to 75 rpm, more preferably from 35 to 70 revolutions per minute (rpm). In a preferred embodiment, step (i) is carried out in a substantially round tank equipped with a high shear mixer and a plurality of baffles. For example, 4 sets of 125 mm baffles can be used.

Although other setups (e.g. tanks equipped with conventional or low shear mixers and/or tanks with no baffles) have been shown to remove leachable contaminants from fly ash to some extent, the present inventors have found that using such setups surprisingly lead to hot spots of leachable contaminants in the washed ash slurry and the processed ash, i.e. regions with relatively elevated concentration of leachable contaminants. The use of a substantially round tank, a high shear mixer and a plurality of baffles has been shown to substantially eliminate hot spots of leachable contaminants in the washed ash slurry and the processed fly ash.

Filtering Step (ii) of the process for treating fly ash in accordance with the present invention involves filtering the ash slurry in a filtering apparatus to obtain a processed ash (typically in the form of a cake) and a wastewater filtrate. This produces a processed ash comprising a reduced content of leachable contaminants (particularly leachable metal contaminants and leachable chlorides).

Following the contacting (washing), the washed ash slurry is transferred into a filtering apparatus, wherein step (ii) of the process is carried out. The time between the completion of step (i) and the initiation of step (ii) is the so-called "loading time", during which the washed ash slurry is transferred into the filtering apparatus. Preferably, the washed ash slurry is transferred into the filtering apparatus using a fast-acting pump, such as a centrifugal pump. The loading is typically carried out at a pressure of from 5 to 10 bar, preferably from 7 to 9 bar. It has been found that the time between the completion of step (i) and the initiation of step (ii) is time sensitive. Specifically, the present inventors have found that leaving the washed ash slurry in the mixing tank for some time or gradually feeding the slurry into the filtering apparatus causes settlement of solids, which over time results in damage to the equipment for transferring the slurry into the filtering apparatus and to the filtering apparatus itself. Furthermore, doing so results in uneven loading of the slurry into the filtering apparatus, resulting in an unfavourable moisture content of the processed ash, which results in poor quality of the ash cake and in turn necessitates additional unit operations (washing, drying and/or filtering) to treat fly ash. Furthermore, an elevated moisture content poses a risk of damaging the filtering apparatus and reduces product quality. For this reason, it is preferred that the time between the completion of step (i) and the initiation of step (ii) of the process is less than 45 minutes, such as less than 40 minutes, preferably less than 35 minutes. It has been found that the loading time of less than 35 minutes consistently minimises the above drawbacks. A typical preferred loading time is from 15 to 35 minutes, such as from 20 to 35 minutes.

Preferably, step (ii) is carried out in a filtering apparatus comprising a membrane filter press system. Such systems have been found to be most effective in removing as much moisture as possible from the washed ash slurry.

Typically, the filtering in step (ii) is carried out for from 2 to 35 minutes, such as from 5 to minutes. Preferably, the filtering time is from 5 to 25 minutes. If a membrane filter press is used, the filter time refers to the membrane squeeze time.

The filtering in step (ii) is typically carried out under the pressure of from 3 to 12 bar, such as from 4 to 10 bar. If a membrane filter press is used, the filter pressure refers to the pressure of the membrane filter press. It is most preferred that the filtering in step (ii) is carried out under the pressure of from 5 to 9 bar. The present inventors have found that if the filtering pressure is below 5 bar, certain ash cakes require an additional filtering stage before they can be applied in concrete products, due to the presence of relatively high amounts of leachable contaminants. Conversely, if the pressure is above 9 bar, certain ash cakes become too compact, such that additional infrastructure and unit operations are required to process the ash before it can be converted into an aggregate for use in concrete products.

The filtering is carried out so as to achieve a dry matter content of at least 60% in the processed ash. Preferably, the dry matter content in the processed ash is from 65 to 90%, more preferably from 65 to 90%.

Optionally, additional water can be fed into the filtering apparatus for contacting the washed ash slurry prior to the initiation of step (ii). This may be preferred in certain embodiments, for example when ashes comprise relatively coarse particles. Doing so improves lifetime of the filtering apparatus by minimising blockages caused by coarser ash particles. In one embodiment, additional water is fed into the filtering apparatus using one or more flushing lines. If additional water is fed into the filtering apparatus, it is typically fed in an amount of from 40 to 80% water by weight of fly ash, such as from 50 to 70%.

Preferably, the additional water is fed in an amount of from 60 to 70% by weight of fly ash.

Once the filtering cycle has been completed, the processed ash (typically in the form of a cake) can be transferred (e.g. dropped) into a container for storage or processing into an aggregate for use in concrete products. The processed ash is preferably dried. Drying can be carried out using methods known in the art, preferably by air-drying.

Preferably, the process for reducing the content of leachable contaminants in fly ash is combined with the process for treating the wastewater filtrate originating from the treating of fly ash (described later in the specification) to form an integrated process. This allows for maximisation of economic and environmental advantages by purifying the wastewater filtrate contaminated with hazardous components (e.g. heavy metals and organics). In particular, doing so removes the need for storing and/or transporting the wastewater filtrate into a wastewater treatment facility.

Processed Ash The process of the invention produces a processed/reconditioned fly ash composition comprising a reduced content of leachable contaminants. The process is particularly effective in reducing the content of leachable chromium, lead and zinc. It is also effective in reducing the content of leachable copper and chlorides. These five leachable contaminants (particularly chromium, lead and zinc) have been found to be most relevant both from environmental standpoint and in terms of affecting the properties of concrete products prepared using the reconditioned fly ash composition.

The process is also typically effective in reducing the content of other leachable contaminants, such as arsenic, mercury, molybdenum, barium, nickel, selenium and cadmium.

Accordingly, the invention provides a processed fly ash (typically in the form of a cake) obtained according to the process for reducing the content of leachable contaminants in fly ash according to the invention.

The content of leachable components in the processed fly ash obtained by the process of the invention, the aggregate of the claimed invention and the concrete prepared using the aggregate of the invention can be measured in accordance with the method set out in BSI standard BS 12457.

The composition of fly ash can be determined using ICP-OES (Inductively Coupled Plasma -Optical Emission Spectrometry) and/or IC (Ion Chromatography).

Typically, the reconditioned fly ash composition comprises at most 0.5 mg/I leachable chromium, at most 1 mg/I leachable lead and at most 0.45 mg/I leachable zinc. For example, the invention provides a reconditioned fly ash composition comprising at most 0.5 mg/I leachable chromium, at most 1 mg/I leachable lead, at most 0.45 mg/I leachable zinc, at most 0.035 mg/I leachable copper and at most 750 mg/I leachable chlorides. For example, the invention provides a reconditioned fly ash composition comprising at most 0.5 mg/I leachable chromium, at most 1 mg/I leachable lead, at most 0.45 mg/I leachable zinc, at most 0.035 mg/I leachable copper, at most 0.00015 mg/I leachable mercury, at most 0.25 mg/I leachable molybdenum, at most 2.7 mg/I leachable barium, at most 750 mg/I leachable chloride, at most 0.00015 mg/I leachable cadmium, at most 0.0015 mg/I leachable nickel, at most 0.030 mg/I leachable selenium and at most 0.0085 mg/I leachable arsenic.

In one embodiment, the composition comprises at most 0.4 mg/I leachable chromium, at most 0.85 mg/I leachable lead and at most 0.35 mg/I leachable zinc, such as at most 0.4 mg/I leachable chromium, at most 0.85 mg/I leachable lead, at most 0.35 mg/I leachable zinc, at most 0.025 mg/I leachable copper and at most 650 mg/I leachable chloride. For example, the composition comprises at most 0.4 mg/I leachable chromium, at most 0.85 mg/I leachable lead, at most 0.35 mg/I leachable zinc, at most 0.025 mg/I leachable copper at most 0.00010 mg/I leachable mercury, at most 0.15 mg/I leachable molybdenum, at most 2.5 mg/I leachable barium, at most 650 mg/I leachable chloride, at most 0.00012 mg/I leachable cadmium, at most 0.0010 mg/I leachable nickel, at most 0.025 mg/I leachable selenium and at most 0.0075 mg/I leachable arsenic.

In a preferred embodiment, the composition comprises at most 0.36 mg/I leachable chromium, at most 0.77 mg/I leachable lead and at most 0.31 mg/I leachable zinc. More preferably, the composition comprises at most 0.36 mg/I leachable chromium, at most 0.77 mg/I leachable lead, at most 0.31 mg/I leachable zinc, at most 0.019 mg/I leachable copper and at most 550 mg/I leachable chlorides. Preferably, the composition comprises at most 0.36 mg/I leachable chromium, at most 0.77 mg/I leachable lead, at most 0.31 mg/I leachable zinc, at most 0.019 mg/I leachable copper, at most 0.00008 mg/I leachable mercury, at most 0.13 mg/I leachable molybdenum, at most 2.2 mg/I leachable barium, at most 550 mg/I leachable chloride, at most 0.00009 mg/I leachable cadmium, at most 0.0005 mg/I leachable nickel, at most 0.022 mg/I leachable selenium and at most 0.0069 mg/I leachable arsenic. It has been found that the reconditioned fly ash comprising these leachable contaminants (particularly chromium, lead and zinc) in the above amounts can be used effectively in the same way as non-waste and can impart improved properties (e.g. enhanced durability) to concrete products made using the reconditioned fly ash.

Preferably, the leachable contaminants described above (i.e. chromium, lead, zinc etc.) are substantially homogenously distributed in the processed fly ash. Fly ash produced by energy plants typically contains 'hot spots' of contaminants due to the process it is produced by and being a dry material and in consequence can often be described as a heterogenous waste. These hotspots can be detrimental to properties of concrete produced using fly ash. The reconditioned fly ash obtained in accordance with the invention preferably contains substantially no hot spots of leachable contaminants. Preferably, this can be verified by measuring, in a single batch of the processed fly ash, the concentration of leachable contaminants in multiple (at least five) samples taken randomly from different parts of the processed fly ash and comparing the concentrations of the leachable contaminants between the samples. The concentration of a given leachable contaminant in the processed fly ash will vary by 20% or less between all samples, preferably by 15% or less, most preferably by 10% or less.

Aaareqate After fly ash has been treated in accordance with the process described above, the processed ash (cake) having a reduced content of leachable metal and other contaminants is typically dried/cured and then broken up to particles of various sizes to form an aggregate having a reduced content of leachable contaminants, for use in concrete products. The formed aggregate is of the lightweight type. Due to significantly reduced content of leachable contaminants, the processed fly ash as present in such an aggregate is treated as effectively non-waste.

Thus, the invention also provides a method for manufacturing an aggregate for use in concrete products, the method comprising: (a) reducing the content of leachable contaminants in fly ash by contacting fly ash with water to form a washed ash slurry, wherein the water has a pH of from 6 to 12, and filtering the ash slurry to obtain a processed ash and a wastewater filtrate; and (b) forming the aggregate from the processed ash.

The amounts of leachable contaminants in the aggregate of the invention are in line with those described above in respect of the reconditioned fly ash composition comprising a reduced content of leachable contaminants.

For example, the invention provides an aggregate for use in concrete products, said aggregate comprising at most 0.5 mg/I leachable chromium, at most 1 mg/I leachable lead and at most 0.45 mg/I leachable zinc, such as at most 0.5 mg/I leachable chromium, at most 1 mg/I leachable lead, at most 0.45 mg/I leachable zinc, at most 0.035 mg/I leachable copper and at most 750 mg/I leachable chlorides, further wherein said aggregate consists essentially of reconditioned fly ash.

In one embodiment, the aggregate comprises at most 0.4 mg/I leachable chromium, at most 0.85 mg/I leachable lead and at most 0.35 mg/I leachable zinc, such as at most 0.4 mg/I leachable chromium, at most 0.85 mg/I leachable lead, at most 0.35 mg/I leachable zinc, at most 0.025 mg/I leachable copper and at most 650 mg/I leachable chloride.

Preferably, the invention provides an aggregate for use in concrete products, said aggregate comprising at most 0.36 mg/I of leachable chromium, at most 0.77 mg/I of leachable lead and at most 0.31 mg/I of leachable zinc, further wherein said aggregate consists essentially of reconditioned fly ash. More preferably, the aggregate comprises at most 0.36 mg/I of leachable chromium, at most 0.77 mg/I of leachable lead, at most 0.31 mg/I of leachable zinc, at most 0.019 mg/I leachable copper and at most 550 mg/I leachable chlorides, further wherein said aggregate consists essentially of reconditioned fly ash. Preferably, the aggregate comprises at most 0.36 mg/I of leachable chromium, at most 0.77 mg/I of leachable lead at most 0.31 mg/I of leachable zinc, at most 0.019 mg/I of leachable copper, at most 0.13 mg/I of leachable molybdenum, at most 0.00008 mg/I of leachable mercury, at most 2.2 mg/I of leachable barium, at most 550 mg/I of leachable chloride, at most 0.0069 mg/I of leachable arsenic, at most 0.00009 mg/I of leachable cadmium, at most 0.0005 mg/I of leachable nickel and at most 0.022 mg/I of leachable selenium, further wherein the aggregate consist essentially of reconditioned fly ash. Preferably, the leachable contaminants described above (i.e. chromium, lead, zinc etc.) are substantially homogenously distributed in the aggregate. The aggregate comprising these leachable contaminants (particularly chromium, lead and zinc) in the above amounts can be treated as non-waste and can impart improved properties (e.g. enhanced durability) to concrete products made using the aggregate.

It is to be understood that all aspects and embodiments defined in respect of the process for reducing the content of leachable contaminants in fly ash as described above apply to the method for manufacturing an aggregate for use in concrete products of the present invention.

Typically, the aggregate particles have a particle size of less than 2 mm, such as less than 1 mm. The particle size range can be adjusted by breaking up the dried and cured ash to a greater or lesser extent, if so desired. The formed aggregate is suitable for use in concrete as a replacement for sand.

The aggregate can be combined with cement and water to form a concrete product. Optionally, conventional aggregates (such as sand or natural aggregates) could be used in conjunction with the aggregate of the invention.

An exemplary formulation of a concrete product (dry matter) using the aggregate of the invention comprises 4 parts of natural aggregate, 2 parts of the aggregate of the invention and 1 part cement (parts per volume).

The invention also provides a concrete comprising the aggregate of the present invention.

Also provided is a method of manufacturing concrete comprising blending the aggregate of the invention with cement, water and optional components such as other aggregates. The aggregate of the invention can be present in the concrete in an amount of from 15 to 50% by volume, such as from 20 to 40% by volume, preferably from 25 to 35% by volume. Similarly, the aggregate of the invention can be mixed in these amounts with other components in accordance with the above method. As stated above, the aggregate of the invention is most preferably used as a sand replacement in concrete formulations.

The aggregate of the invention has been found to accelerate drying of concrete prepared using the aggregate. This is because the aggregate accelerates the rate of hydration of the components of the cement. Thus, also provided is the use of the aggregate of the invention for improving the rate of drying of concrete.

The aggregate of the invention has also been found to improve thermal resistance of concrete prepared using the aggregate. Thus, also provided is the use of the aggregate of the invention for improving thermal resistance of concrete.

The content of leachable components in the aggregate of the invention and in the concrete product made using the aggregate can be measured in accordance with the method set out in BSI standard BS 12457.

Plant for the process for treating fly ash The invention also provides a plant for conducting the process for reducing the content of leachable contaminants in fly ash as described above. Typically, the plant comprises: (a) a mixing tank equipped with an agitator, adapted to contact fly ash with water; (b) a water supply fluidly connected to the mixing tank; (c) a vessel adapted to store fly ash prior to the contacting in the mixing tank and a means connected to the mixing tank and adapted to feed fly ash from the vessel into the mixing tank; (d) a filtering apparatus disposed downstream of the mixing tank and configured to filter the ash slurry; and (e) a means adapted to transfer the ash slurry from the mixing tank to the filtering apparatus.

The mixing tank is preferably a substantially round tank equipped with a high shear mixer and, preferably, a plurality of baffles. For example, 4 sets of 125 mm baffles can be used.

The use of a substantially round tank, a high shear mixer and a plurality of baffles has been shown to substantially eliminate hot spots of leachable contaminants in the washed ash slurry and the processed ash cake.

The water supply may comprise a water storage tank from which the water is fed into the mixing tank. Alternatively, or additionally, water may be fed directly from a mains supply.

Preferably, the filtering apparatus is a membrane filter press system. Typically, the membrane filter press system is configured to filter the washed ash slurry under a pressure of from 4 to 10 bar. Preferably, the membrane filter press system is configured to filter the slurry under a pressure of from 5 to 9 bar.

The means for transferring the ash slurry from the mixing tank to the filtering apparatus is preferably a fast-acting pump, such as a centrifugal pump capable of operating under a pressure of from 5 to 10 bar, preferably from 7 to 9 bar. The means is conveniently configured to load the slurry into the filtering apparatus in less than 40 minutes, such as in less than 35 minutes. Preferably the means is configured to load the slurry into the filtering apparatus in from 20 to 35 minutes. For example, the means can comprise one or more 75 kW centrifugal pumps capable of achieving a flow of 1000 dm3 per minute.

Advantageously, the vessel for storing fly ash comprises a silo or a dome. Preferably the vessel comprises a silo equipped with a weight measurement system adapted to weigh out a predetermined mass of fly ash. This allows accurate measurement and delivery of a desired quantity of fly ash. Preferably, the system comprises one or more load cells. In one embodiment, the means for feeding fly ash comprises one or more of a screw conveyor, air slide, bucket conveyor or pipeline for transporting fly ash under pressure. Preferably, the means comprises a screw conveyor. Thus, in a preferred embodiment, the vessel comprises a silo equipped with a weight measurement system adapted to weigh out a desired quantity of ash and the means comprises a screw conveyor.

Conveniently, the plant further comprises a container disposed downstream of the membrane filter press system adapted to handle/store processed fly ash.

Advantageously, the filtering apparatus comprises an outlet adapted to discharge the wastewater filtrate.

Typically, the plant also comprises a wastewater filtrate holding tank fluidly connected to the membrane filter press. For example, the wastewater filtrate holding tank may be connected to the filtering apparatus by an outlet for discharging wastewater filtrate.

As explained above, the wastewater filtrate can advantageously be recycled into step (i) of the process for treating fly ash. Accordingly, the plant preferably also comprises a means adapted to recycle the wastewater filtrate into step (i) of the process.

In a preferred embodiment of the invention, the plant for the process for reducing the content of leachable contaminants in fly ash is combined with the plant for the wastewater filtrate treatment process (described below) to form an integrated plant. In such an integrated plant, it is preferred that the wastewater filtrate holding tank (if present) is fluidly connected to the wastewater treatment tank (see below). The wastewater filtrate can also be fed directly from the filtering apparatus into the wastewater filtrate treatment process, if so desired (see below).

As explained above, in certain embodiments it may be preferred to feed additional water into the filtering apparatus to maximise lifetime of the apparatus. Therefore, the plant optionally further comprises a means adapted to feed additional water into the membrane filter press. Preferably, the means comprises one or more flushing lines.

Wastewater treatment process The wastewater filtrate originating from the step of filtering the washed ash slurry is typically contaminated with certain soluble parts of the ash that have been washed out, primarily various forms of salt and heavy metal contaminants (e.g. chromium, copper, lead and zinc). As such, it is desirable to treat the wastewater filtrate to remove the contaminants present therein. For the avoidance of doubt, it is noted that the wastewater filtrate that has been recycled one or more times to the contacting step of the process for reducing the content of leachable contaminants in fly ash can also be treated in this way. Preferably, the wastewater filtrate is treated in accordance with the process described below once it has reached a concentration of chlorides of 50,000 mg/I.

The wastewater filtrate treatment process of the invention involves: (a) acidifying the wastewater filtrate to form a first solution; (b) adding a transition metal salt into the first solution to form a second solution; (c) basifying the second solution to form a suspension comprising treated wastewater filtrate and a precipitate.

Typically, step (a) involves acidifying the wastewater filtrate to a pH of from 2 to 4, preferably from 2 to 3. Conveniently, step (a) is carried out using a solution comprising an inorganic acid. Preferably, HNO3 is used as an inorganic acid. Although any strong inorganic acid could adjust the pH the wastewater filtrate to the desired levels, HNO3 has been found to be the most efficient inorganic acid from the perspective of removing undesired components and not introducing further undesirable components into the wastewater filtrate. For instance, although HCI and H2504 could adjust the pH in step (a) to the desired levels, these acids typically increase the concentrations of chlorides and sulphates in the wastewater filtrate to excessive levels, which is not desirable if the treated wastewater is to be deemed safe for discharge into sewers.

It has been found that adding the transition metal salt (step (b)) ensures improved precipitation of heavy metal contaminants in step (c) and facilitates their settlement and removal. The amount of the transition metal salt is not particularly limited, but the salt is typically added in an amount of up to 3% by weight of the wastewater filtrate. Conveniently, the transition metal salt is an iron (ii) salt, preferably Fe504, which has been found to be the most effective salt for the removal of heavy metal contaminants. Preferably, the amount of solid FeSO4 added in step (b) is such that the weight ratio of the wastewater filtrate to Fe504 is from 1000:1 to 100:1.

Typically, step (c) involves adjusting the pH such that the formed suspension has a pH of from 6 to 10, preferably from 6 to 9. It has been found that the compounds present in the wastewater filtrate originating from the process for treating fly ash according to the invention are least soluble at these pH values, resulting in improved treatment of the wastewater filtrate.

Conveniently, the basifying in step (c) is carried out using an alkali metal hydroxide or oxide and/or alkaline earth metal hydroxide or oxide. Advantageously, the pH is adjusted in step (c) using lime. Preferably, hydrated lime is used for this purpose, since it has been found to readily form the precipitate comprising heavy metal contaminants.

Thus, in a preferred embodiment, the present invention provides a process for treating the wastewater filtrate, the process comprising the steps of: (a) acidifying the wastewater filtrate using HNO3 to form a first solution; (b) adding an iron (II) salt (preferably Fe504) into the first solution to form a second solution; (c) basifying the second solution (preferably using lime) to form a suspension comprising treated wastewater filtrate and a precipitate, wherein the suspension has a pH of from 6 to 9; The process is typically carried out at a temperature of up to 35 °C, such as up to 30 °C, preferably from 5 to 25 °C. The total contact time between the wastewater and the reagents (steps (a) to (c) combined) is generally up to 15 minutes, more typically up to 10 minutes.

It is to be understood that the inorganic acid, iron (ii) salt and/or base as noted above could be used as solids (where appropriate) or as solutions, e.g. aqueous solutions.

Following step (c), the resulting suspension is preferably transferred into a decant tank where the suspension is allowed to settle. In this way, substantially all of the treated wastewater filtrate can be decanted to separate it from the resulting sediment (precipitate). In this way, the treated wastewater filtrate is obtained. The resulting (second) suspension containing the precipitate and residual amounts of the treated wastewater filtrate can then be filtered to separate further amounts of the treated wastewater filtrate. The filtering can be carried out using any conventional means, for example using a filter press.

The process for treating wastewater filtrate originating from the process for treating of fly ash is very effective in removing lead, zinc, chromium, copper and mercury from the wastewater filtrate, which is important from environmental and commercial standpoints.

Conveniently, the process of the invention reduces the concentration of chromium in the wastewater by at least 80%, the concentration of copper by at least 70%, the concentration of lead by at least 90%, the concentration of zinc by at least 85% and the concentration of mercury by at least 97%. Advantageously, the process of the invention reduces the concentration of chromium in the wastewater by at least 85%, the concentration of copper by at least 80%, the concentration of lead by at least 95%, the concentration of zinc by at least 90% and the concentration of mercury by at least 99%.

As a result of the wastewater filtrate treatment process of the invention, the undesired components (e.g. heavy metals) are precipitated as insoluble inorganic salts/hydroxides. These contaminants are removed as a cake via filtration as described above. This process also renders the treated wastewater filtrate substantially safe for discharge to sewers.

The present inventors have surprisingly found that the treated wastewater filtrate can be recycled into the step of contacting fly ash to reduce the amount of leachable contaminants. Thus, the process for reducing the content of leachable components can be repeated using the treated wastewater filtrate and another batch of fly ash. This further improves efficiency and economy of the overall process.

Thus, in a preferred aspect, at least a portion of the treated wastewater filtrate is recycled into step (i) of the process (i.e. a step of contacting fly ash with water to form a washed ash slurry). That is, the contacting in step (i) is repeated using at least a portion of the treated wastewater filtrate and another batch of fly ash. Advantageously, at least 50% of the treated wastewater filtrate, such as at least 7 5 % is recycled into step (i). Preferably, substantially all treated wastewater filtrate is recycled.

Therefore, in one aspect, the process of the invention comprises: (i)(a) contacting a first portion of fly ash with a first portion of water to form a first washed ash slurry, wherein the first portion of water has a pH of from 6 to 8; ( )(a) filtering to separate the first washed ash slurry into a first processed fly ash and a wastewater filtrate; (i D) treating the wastewater filtrate in accordance with the process described above; ( )(b) contacting a second portion of fly ash with a second portion of water to form a second washed ash slurry, wherein the second portion of water has a pH of from 6 to 12 and comprises at least a portion of the treated wastewater filtrate; and (10(b) filtering to separate the second washed ash slurry into a second processed fly ash and a second wastewater filtrate.

It is to be understood that this aspect of the invention can be combined with the aspect of recycling into step (i) the wastewater filtrate that has not been treated in accordance with the process as described above. For example, following step (i), the wastewater filtrate can be recycled into step (i), then purified following the filtering and subsequently reused as treated wastewater filtrate in step (i). In that case, the references to "second portion of fly ash", "second portion of water", "second washed ash slurry", "second processed fly ash", "second wastewater filtrate" are to be understood as those to "third portion of water", "third portion of fly ash", etc. In a preferred embodiment, the process for treating the wastewater filtrate originating from the process for treating of fly ash is combined with the process for reducing the content of leachable contaminants in fly ash (as described above) to form an integrated process. The wastewater filtrate can be directly fed into the wastewater filtrate treatment, or temporarily stored (e.g. in a wastewater filtrate holding tank) before the treatment is carried out, for example with a view to combining several batches of wastewater filtrate prior to the treatment.

Therefore, the invention provides an integrated process for reducing the content of leachable contaminants in fly ash and treating the wastewater filtrate originating from the treatment of fly ash, the process comprising: (a) reducing the content of leachable contaminants in fly ash as described above; (b) acidifying the wastewater filtrate to form a first solution; (c) adding a transition metal salt into the first solution to form a second solution; (d) basifying the second solution to form a suspension comprising treated wastewater and a precipitate.

As noted above, to obtain the treated wastewater filtrate, the precipitate in the suspension is allowed to settle such that substantially all of the treated wastewater filtrate can be decanted to separate it from the sediment (precipitate). This is preferably carried out in a separate decant tank. The resulting suspension can then be filtered to separate further amounts of the treated wastewater.

It is to be understood that the "wastewater filtrate" in this aspect of the invention could have been used one or more times for the purpose of contacting fly ash to reduce the content of leachable contaminants.

Plant for the wastewater treatment process The present invention further provides a plant for conducting the process for treating the wastewater filtrate, wherein the plant comprises: (a) a wastewater filtrate treatment mixing tank equipped with an agitator, adapted to contact wastewater filtrate with an acid, a transition metal salt and a base; (b) an acid dosing tank, a transition metal salt dosing tank and a base dosing tank, said dosing tanks being fluidly connected to the wastewater filtrate treatment mixing tank; (c) a means adapted to feed the wastewater filtrate into the wastewater treatment mixing tank; and (d) a decant tank adapted to store the suspension comprising treated wastewater and a precipitate.

The plant may further comprise a filtering apparatus adapted to filter the second suspension comprising residual amounts of wastewater filtrate and the precipitate, to separate the treated wastewater filtrate from the precipitate and a means adapted to transfer the second suspension from the decant tank into the filtering apparatus. The means conveniently comprises a pump, such as a centrifugal pump.

In a preferred embodiment of the invention, the plant for the wastewater filtrate treatment process is combined with the plant for the process for reducing the content of leachable contaminants in fly ash (described above) to form an integrated plant. In such an integrated plant, it is preferred that the wastewater filtrate holding tank (if present) is fluidly connected to the wastewater treatment tank (see below). Alternatively, the wastewater filtrate can be directly fed into the wastewater filtrate treatment process (see below). Either way, the integrated plant comprises a means (c) for feeding the wastewater filtrate into the wastewater treatment mixing tank. The means typically comprises one or more dosing lines.

Therefore, the present invention also provides a plant for carrying out the integrated process for reducing the content of leachable contaminants in fly ash and treating the wastewater filtrate originating from said process, the plant comprising (a) a mixing tank equipped with an agitator, adapted to contact fly ash with water; (b) a water supply fluidly connected to the mixing tank; (c) a vessel adapted to store fly ash prior to the contacting in the mixing tank and a means connected to the mixing tank and adapted to feed fly ash from the vessel into the mixing tank; (d) a filtering apparatus disposed downstream of the mixing tank and configured to filter the washed ash slurry; and (e) a means adapted to transfer the ash slurry from the mixing tank to the filtering apparatus; (f) a wastewater filtrate treatment mixing tank equipped with an agitator, adapted to contact wastewater filtrate with an acid, a transition metal salt and a base; (g) an acid dosing tank, a transition metal salt dosing tank and a base dosing tank, said dosing tanks being fluidly connected to the wastewater filtrate treatment mixing tank; (h) a means adapted to feed the wastewater filtrate into the wastewater treatment mixing tank; and (i) a decant tank adapted to store the suspension comprising treated wastewater and a precipitate.

The plant may further comprise a filtering apparatus adapted to filter the second suspension comprising residual amounts of the treated wastewater filtrate and the precipitate, to separate the treated wastewater filtrate from the precipitate and a means adapted to transfer the second suspension from the decant tank into the filtering apparatus.

Preferably, the plant comprises a means adapted to recycle the treated wastewater filtrate into the mixing tank wherein fly ash is contacted with water.

The filtering apparatuses above may be two separate filtering apparatuses or the same filtering apparatus.

The invention will now be described with reference to the following non-limiting examples.

EXAMPLES

Raw ash was tested via ICP-OES (Inductively Coupled Plasma -Optical Emission Spectrometry) and IC (Ion Chromatography). Figure 1 shows an exemplary profile of contaminants of fly ash generated in a biomass plant.

Example 1 (Fly ash treatment) 6000 kg water (corresponding to about 33% of total water used in the contacting step) having a pH of 7.6 was added to a rounded mixing tank equipped with a high shear mixer and 4 sets of 125mm baffles. Fly ash (6000 kg) originating from a biomass plant was then introduced via a screw conveyor into the mixing tank together with extra water (pH of 7.6) so as to achieve the weight ratio of water to fly ash of 3:1. The resulting suspension was agitated for 30 minutes (mixing speed of 65 rpm) to produce a washed ash slurry.

Immediately following the washing, the slurry was pumped via a centrifugal pump into a membrane filter press. The loading time was 25 minutes. The slurry was filtered under a pressure of 8 bar. The filtering time was 9 minutes. The cake was then air dried for 24 hours and analysed for the content of leachable contaminants. The results are shown in the Table 1 below.

Table 1

Leachable Fly Ash Washed Ash components 0119/0 (mg/I) Chromium 0.30000 0.00680 Lead 26.00000 0.06100 Zinc 9.70000 0.04900 Copper 0.05700 0.01000 Chloride 23400.00000 310.00000 Arsenic 0.00670 0.00510 Barium 12.00000 0.13000 Cadmium 0.00015 0.00003 Mercury 0.00017 0.00001 Molybdenum 0.07500 0.00870 Nickel 0.00500 0.00050 Selenium 0.00440 0.00160 The content of leachable contaminants in samples obtained by treating other fly ash samples (originating from biomass and EfW plants) in accordance with the process of the invention are also shown in Table 2.

Leachable Table 2

components Treated ash sample (mg/I) 1A 1B 2A 2B 2C 3 4A 4B 5 6 7 8 9 Chromium 0.079 0.13 0.055 0.037 0.026 0.022 0.12 0.082 0.36 0.19 0.0024 0.22 0.20 Lead 0.2 0.3 0.063 0.14 0.091 0.13 0.25 0.34 0.77 0.63 0.043 0.14 0.21 Zinc 0.066 0.096 0.31 0.3 0.11 0.12 0.069 0.088 0.19 0.055 0.027 0.15 0.31 Copper 0.0071 0.0043 0.0026 0.0028 0.0031 0.0058 0.0077 0.013 0.019 0.0048 0.0061 0.013 0.013 Chloride 300 490 2 26 16 8 44 130 270 200 340 6.3 12 Arsenic 0.0029 0.0032 0.0014 0.0013 0.0013 0.0022 0.00016 0.00016 0.0030 0.00016 n/m 0.0016 0.0019 Barium 0.23 0.25 0.52 0.76 0.78 0.63 0.68 0.78 0.57 0.43 1.90 0.55 0.38 Cadmium 0.00006 0.00004 0.00003 0.00003 0.00003 0.00003 0.00004 0.00006 0.00006 0.00004 0.00003 0.00003 0.00003 Mercury 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 000005. 0.00004 0.00002 0.00001 0.00001 or less or less or less or less or less or less or less or less or less or less Molybdenum 0.026 0.03 0.008 0.0039 0.0027 0.0026 0.021 0.034 0.07 0.07 0.096 0.018 0.019 Nickel 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 Selenium 0.0031 0.0033 0.00066 0.00026 0.00035 0.00041 0.003 0.0043 0.0081 0.0062 0.00096 0.0031 0.0043 As can be seen, the process consistently reduces the contents of leachable contaminants in fly ash to low levels. Similar results have been obtained following recycling of the wastewater filtrate into the contacting step. The treated fly ash cake is non-hazardous and can be processed into an environmentally friendly aggregate for use in concrete products.

The process has also been demonstrated to reduce the contents of leachable contaminants to low levels even in respect of ashes comprising significantly higher than typical contents of leachable contaminants in (e.g. about 10 mg/I, chromium, about 20 mg/I lead, about 15 mg/I zinc).

Example 2 (Making concrete) The reconditioned fly ash cake obtained according to the process of the invention was used as an aggregate in the preparation of concrete. Dry matter was formed by mixing 4 parts of natural aggregate, 2 parts of the aggregate of the invention and 1 part cement (parts per volume). The concrete blocks produced using the aggregate of the invention are shown on Figure 4.

The prepared blocks had smooth finishes and no visible swelling. The treated fly ash can be used in a concrete mix that gives C35 strength (i.e. designed to withstand 35 Newton/28 day strength), which concrete finds applications in building external walks for commercial structures or areas that bear constant heavy loads from industrial machinery. The reconditioned fly ash as present in such a concrete product is treated effectively as a non-waste product.

Example 3 (leachable contaminants in concrete) Concrete prepared using the aggregates of the invention has tested for the content of certain leachable contaminants. The results are shown in Table 3 below.

Leachable Table 3 Concrete 3 components Sample Ong/0 2 Chromium 0.00026 0.00092 0.00031 Lead 0.00400 0.00130 0.00009 Zinc 0.00560 0.00370 0.00360 Copper 0.00350 0.00320 0.00120 Chloride 11.00000 0.86000 1.60000 Arsenic 0.00067 0.00049 0.00062 Barium 0.13000 0.03800 0.03000 Cadmium 0.00003 0.00003 0.00003 Mercury 0.00001 0.00001 0.00001 Molybdenum 0.00110 0.00110 0.00110 Nickel 0.00050 0.00050 0.00050 Selenium 0.00025 0.00025 0.00025 Alternative concrete additives available on the market, such as cement kiln dust (CKD), bypass dust (BPD), blast furnace slags and mortars have been shown to contain significantly higher contents of leachable contaminants, e.g. up to about 2.7 mg/I leachable lead. As such, the aggregate of the invention can be advantageously used in the manufacture of an environmentally friendly concrete product.

Example 4 (Wastewater filtrate treatment) Wastewater filtrate originating from the process for reducing the content of leachable contaminants was treated using 20% (v/v) HNO3, ferrous sulphate and lime. The formed suspension was allowed to settle, and the treated wastewater filtrate was decanted and tested for the content of selected contaminants. The results are shown in Table 4 below.

Table 4

(wastewater filtrate treatment process) Parameter Units Raw Treated % Sample Sample reduction

A A

pH n/a 12.3 7.6 Electrical pS/cm 48300 45500 Conductivity Suspended mg/I 460 20 95.70% Solids Chromium mg/I 0.01 0.00055 94.50% Copper mg/I 0.39 0.087 77.70% Lead mg/I 130 0.17 99.90% Mercury mg/I 0.00007 <0.00001 >99% Zinc mg/I 18 0.06 99.70% It can be seen that the process is effective in treating the wastewater filtrate originating from the process for reducing the content of leachable contaminants in fly ash.

Furthermore, integrating the wastewater filtrate treatment process into the process for treating fly ash according to the invention enables significant economic and environmental advantages.

Whilst the accompanying claims set out particular combination of features described herein, the scope of the present invention is not limited to the particular combinations hereafter claimed but instead extends to encompass any combination of features herein disclosed.

Claims (29)

1. CLAIMS1. A process for reducing the content of leachable contaminants in fly ash comprising the steps of: (i) contacting fly ash with water having a pH of from 6 to 12 to form a washed ash slurry; and (ii) filtering the washed ash slurry to obtain a processed fly ash comprising a reduced content of leachable contaminants and a wastewater filtrate.
2. 2. A process according to Claim 1, wherein at least a portion of the wastewater filtrate is recycled into step (i), preferably up to 80% by weight, such as from 50 to 80% by weight or from 60 to 80% by weight.
3. 3. A process according to Claim 1 or 2, wherein the process comprises at least one step of contacting fly ash with water having a pH of from 6 to 8.
4. 4. A process according to any of Claims 1 to 3, wherein the process comprises: (0(a) contacting a first portion of fly ash with a first portion of water to form a first washed ash slurry, wherein the first portion of water has a pH of from 6 to 8; (ii)(a) filtering to separate the first washed ash slurry into a first processed fly ash and a first wastewater filtrate; (i)(b) contacting a second portion of fly ash with a second portion of water to form a second washed ash slurry, wherein the second portion of water has a pH of from 6 to 12 and comprises at least a portion of the first wastewater filtrate; and (ii)(b) filtering to separate the second washed ash slurry into a second processed fly ash and a second wastewater filtrate.
5. 5. A process according to Claim 4, wherein the water in step (0(a) has an ionic strength of at most 0.05.
6. 6. A process according to Claim 4 or 5, wherein up to 80% by weight of the first wastewater filtrate is used in step (i)(b), such as from 50 to 80% by weight or from 60 to 80°/0 by weight.
7. 7. A process according to any of the preceding claims, wherein the contacting is carried out using a weight ratio of water to fly ash of from 1:1 to 5.5:1, such as from 2.5:1 to 4.5:1, preferably from 2.5:1 to 4:1.
8. 8. A process according to any of the preceding claims, wherein the contact time is from to 75 minutes, such as from 20 to 50 minutes, preferably from 25 to 40 minutes.
9. 9. A process according to any of the preceding claims, wherein the contact temperature is at most 35 °C, such as from 2 to 30 °C, preferably from 5 to 25 °C.
10. 10. A process according to any of the preceding claims, wherein the contacting is carried out using a substantially rounded mixing tank equipped with a high shear mixer and a plurality of baffles.
11. 11. A process according to any of the preceding claims, wherein prior to the contacting at least 30% by weight of water used in the contacting is fed into a mixing tank wherein the contacting is carried out, preferably at least 50% by weight.
12. 12. A process according to any of the preceding claims, wherein the time between the completion of the contacting and the initiation of the filtering is less than 45 minutes, such as less than 40 minutes, preferably from 15 to 35 minutes.
13. 13. A process according to any of the preceding claims, wherein the filtering is carried out at a pressure of from 3 to 12 bar, such as from 4 to 10 bar, preferably from 5 to 9 bar.
14. 14. A process according to any of the preceding claims further comprising a step of drying the processed ash.
15. 15. The processed ash obtainable by the process according to any of Claims 1 to 14, preferably wherein the ash comprises at most 0.5 mg/I leachable chromium, at most 1 mg/I leachable lead and at most 0.45 mg/I leachable zinc, wherein the content of the leachable chromium, lead and zinc is determined in accordance with the method set out in BSI standard BS 12457.
16. 16. The processed ash according to Claim 15, wherein the leachable chromium, lead and zinc are substantially homogenously distributed in the processed ash.
17. 17. A plant for conducting the process according to any of Claims 1 to 14, the plant comprising: (a) a mixing tank equipped with an agitator, adapted to contact fly ash with water; (b) a water supply fluidly connected to the mixing tank; (c) a vessel adapted to store fly ash prior to the contacting in the mixing tank and a means connected to the mixing tank and adapted to feed fly ash from the vessel into the mixing tank; (d) a filtering apparatus disposed downstream of the mixing tank and configured to filter the ash slurry; and (e) a means adapted to transfer the ash slurry from the mixing tank to the filtering apparatus.
18. 18. The plant according to Claim 17, wherein the vessel is equipped with a weight measurement system adapted to weigh out a predetermined mass of fly ash.
19. 19. The plant according to Claim 17 or 18, wherein the mixing tank is a substantially round tank equipped with a high shear mixer and a plurality of baffles.
20. 20. The plant according to any of Claims 17 to 19 further comprising a means adapted to recycle the wastewater filtrate into the mixing tank.
21. 21. An integrated process for reducing the content of leachable contaminants in fly ash and treating the wastewater filtrate originating from reducing the content of leachable contaminants, the process comprising: (a) reducing the content of leachable contaminants in fly ash according to any of Claims 1 to 14 to obtain a processed fly ash and a wastewater filtrate; (b) acidifying the wastewater filtrate to form a first solution; (c) adding a transition metal salt into the first solution to form a second solution; (d) basifying the second solution to form a suspension comprising treated wastewater filtrate and a precipitate.
22. 22. A process according to Claim 21, wherein step (a) comprises acidifying the wastewater filtrate using an inorganic acid to form the first solution having a pH of from 2 to 4, step (b) comprises adding an iron (II) salt to form the second solution and step (c) comprises basifying the second solution to form the suspension haying a pH of from 6 to 10.
23. 23. A process according to Claim 22, wherein the inorganic acid in step (a) is HNO3, the iron (II) salt in step (b) is Fe504 and the basifying in step (c) is carried out using lime.
24. 24. A process according to any of Claims 21 to 23, further comprising a step of recycling at least a portion of the treated wastewater filtrate into the step of contacting fly ash with water.
25. 25. A plant for performing the process according to any of Claims 21 to 24 comprising a plant according to any of Claims 17 to 20; and: (f) a wastewater filtrate treatment mixing tank equipped with an agitator, adapted to contact wastewater filtrate with an acid, a transition metal salt and a base; (g) an acid dosing tank, a transition metal salt dosing tank and a base dosing tank, said dosing tanks being fluidly connected to the wastewater filtrate treatment mixing tank (h) a means adapted to feed the wastewater filtrate into the wastewater treatment mixing tank; and (i) a decant tank adapted to store the treated wastewater filtrate.
26. 26. A plant according to Claim 25 further comprising a means adapted to recycle the treated wastewater filtrate into the mixing tank adapted to contact fly ash with water.
27. 27. An aggregate for use in concrete, said aggregate comprising at most 0.5 mg/I leachable chromium, at most 1 mg/I leachable lead and at most 0.45 mg/I leachable zinc, wherein the content of leachable chromium, lead and zinc is determined in accordance with the method set out in BSI standard BS 12457, further wherein said aggregate consists essentially of reconditioned fly ash.
28. 28. The aggregate according to Claim 27, wherein the leachable chromium, lead and zinc are substantially homogenously distributed in the reconditioned fly ash.
29. 29. Concrete comprising the aggregate according to Claim 27 or 28.