AU2016203225B2 - Separation of fine particles from a powder - Google Patents

Abstract

ABSRACT Disclosed herein is a method and an apparatus for separating fine particles of fly ash from a fly ash powder. The method comprises delivering the fly ash powder to a chamber whereby a pile of settled fly ash forms at a lower portion of the chamber and an atmosphere comprising fine particles of fly ash forms above the settled fly ash; and drawing a product stream from the atmosphere above the settled fly ash, wherein the product stream comprises the fine particles of fly ash entrained therein. 1/3 A IN C / N S LO A S C Ca N N N (N N N 2' o '~~- t 4tN 'N~-. e S S ~ N N I ~2&Vt+dt N~ 1N N NN 'N 'NQ NS\N. 0 0 N N $ 'N 1' l.a -C IN.x N S $ 10 4--'' -x 2 ~N, N CC N S Ca N N N N / 'IN 'N~~ N ' 2 N 5 N" N N S N NN 4' NO '("~ S N N N 0 ~ N\C( NN '~ N N 1 ' '*--~--'~N N N 17 l.a N N N I It CC N N N\ N N 11 N N N /1 $ N $ N NJ Figure.

Description

ABSRACT

Disclosed herein is a method and an apparatus for separating fine particles of fly ash from a fly ash powder. The method comprises delivering the fly ash powder to a chamber whereby a pile of settled fly ash forms at a lower portion of the chamber and an atmosphere comprising fine particles of fly ash forms above the settled fly ash; and drawing a product stream from the atmosphere above the settled fly ash, wherein the product stream comprises the fine particles of fly ash entrained therein.

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Figure.

SEPARATION OF FINE PARTICLES FROM A POWDER

Technical Field

[0001] The present invention relates to methods and apparatus for separating fine particles from coarse particles in a powder. In particular, the present invention relates to methods and apparatus for separating fine particles of fly ash from a fly ash powder.

Background Art

[0002] Fly ash is generated by coal fired power stations when coal is burned, for example to generate electricity. Disposal of fly ash is an enduring challenge in the industry, with power stations currently moving away from the traditional "wet disposal" methods, where fly ash is dumped as a wet slurry into ponds, because of environmental concerns and its requirement for large areas of storage. So-called "dry disposal" methods, where the fly ash is conveyed to remote storage silos and subsequently compacted as land fill, are therefore becoming more common. However, whilst dry disposal methods are an improvement in many respects over wet disposal methods (e.g. they reduce leachate to ground waters), they still involve the disposal of a relatively high amount of material, some of which might be beneficially used elsewhere.

[0003] In this regard, fly ash is used in significant quantities in industries such as the cement and concrete industry, where the use of fly ash is environmentally desirable for at least two reasons. Firstly, this beneficial utilisation of fly ash reduces the volume of waste dumped in landfill. Secondly, a reduced volume of cement is used in concrete made with fly ash. Reduced cement consumption conserves limestone reserves and helps reduce greenhouse gas emissions.

[0004] The finer fly ash particles are of most use in the cement and concrete industry as they enhance the durability of hardened concrete. Presently, the fly ash powder is drawn from the remote storage silos, and separators or classifiers are used to separate the fine particles of fly ash from the coarse particles. As would be appreciated, the infrastructure requirements and costs associated with operating and maintaining such separators and classifiers can be significant.

Summary of Invention

[0005] In a first aspect, the present invention provides a method for separating fine particles

of fly ash (having a particle size of less than about 45 microns) from a fly ash powder. The

method comprises delivering the fly ash powder to an upper portion of a chamber whereby

coarse particles of fly ash fall through the chamber relatively quickly, forming a pile of

settled fly ash at a lower portion of the chamber, and fine particles of fly ash fall relatively

slowly, remaining suspended in an atmosphere above the settled fly ash. The method further

comprises creating a vacuum in a second chamber that is in fluid communication with the

atmosphere above the settled fly ash, the vacuum causing an airflow that is effective to draw

a product stream from the atmosphere above the settled fly ash, wherein the product stream

comprises the fine particles of fly ash entrained therein.

[0006] In the present invention, the fly ash powder is delivered to the chamber (e.g. a storage

silo such as those presently used for dry disposal methods) using any conventional technique

(e.g. via compressed air in a pipeline (pneumatic conveying), bucket elevators or other

mechanical devices, etc.) in a manner whereby the chamber contains a mixture of fly ash

powder that has rapidly settled at the bottom of the chamber and an atmosphere above the

settled powder which includes air and suspended fly ash powder. The principle of this

invention is that the coarser particles of fly ash tend to settle first in the chamber, whilst finer

particles of fly ash tend to remain airborne, and it is these finer particles that can be drawn off

in the product stream.

[0007] The present invention advantageously does not require the use of the relatively

complicated equipment and processes traditionally used to separate fine particles of fly ash

from coarse particles. Further, the finer particles of fly ash are separated from the bulk fly

ash powder at around the same time as the fly ash is delivered to the chamber and in an

essentially one step process. Additional advantages of the present invention, and specific

embodiments thereof, will be described below.

[0008] Adjusting the rate at which the product stream is drawn from the atmosphere inside

the chamber can vary the fineness of the particles being collected. A relatively high airflow

will result in drawing off fine and slightly coarser sized particles, whilst a relatively low

airflow will draw off only the very fine particles. As such, in some embodiments, a rate at

which the product stream is drawn from the atmosphere above the settled fly ash is adjustable

in response to the particle size of the fine particles of fly ash entrained in the product stream.

[0009] In some embodiments, a rate at which the product stream is drawn from the

atmosphere above the settled fly ash is adjustable in response to an amount (e.g. a measured

amount) of the settled fly ash at the lower portion of the chamber. It has been found that the

rate of settlement of fly ash in the chamber will vary with the level of the settled fly ash in the

chamber, with (for a constant rate of drawing) marginally finer material being drawn when

the chamber is empty and marginally coarser material when the chamber is near full. This

variance can be accounted for such that material having a relatively narrow particle size range

is obtained. For example, a higher rate of drawing may be used when the chamber is near

empty and lower rate of drawing when chamber is near full. Level indicators or continuous

level measurement on the chamber may, for example, be used to provide an indication of the

amount of the settled fly ash.

[0010] In some embodiments, a location in the chamber from where the product stream is

drawn is adjustable in response to a particle size of the fine particles of fly ash entrained in

the product stream. As would be appreciated, lowering an intake via which the product

stream is drawn from the atmosphere will move it towards the settled fly ash and result in

more coarse particles being entrained in the product stream.

[0011] In some embodiments, a plurality of product streams are drawn from the atmosphere

above the settled fly ash. The use of multiple product streams is another way in which the

rate at which the product stream is drawn from the atmosphere above the settled fly ash may

be increased. Depending on factors such as the size of the chamber and relative locations of

the intakes for the product streams, this may result in a greater rate of production of the fine

fly ash particles, or result in the production of a product containing a relatively higher

proportion of medium sized fly ash particles (which may be useful in some applications).

[0012] In some embodiments, the fly ash powder is turbulently delivered to the chamber.

Turbulently delivering the fly ash powder into the chamber will ensure that the powder is

well mixed with air, thus enabling a relatively larger proportion of the fine particles of fly ash

in the fly ash powder to become entrained in the atmosphere for drawing off. Such a delivery

may help to improve the yield, purity, consistency of particle size, etc. of the fine particles of

fly ash.

[0013] The fly ash powder is delivered to an upper portion of the chamber, whereupon it falls

through the chamber and mixes with air as it does so.

[0014] In some embodiments, for example, the fly ash powder is pneumatically conveyed to

the chamber. In such embodiments, the fly ash powder arrives in the chamber already well

mixed with an airstream, and it may not be necessary for it to be delivered to an upper portion

of the chamber.

[0015] In some embodiments, the method further comprises measuring an amount of the

settled fly ash at the lower portion of the chamber. In some embodiments, the method may

further comprise removing a portion of the settled fly ash from the chamber (e.g. when the

measured amount of the settled fly ash exceeds a predetermined amount). In this manner, the

chamber never becomes overfull and the method can be operated continuously.

[0016] In some embodiments, the method further comprises allowing the fine particles of fly

ash entrained in the product stream to settle in the second chamber, where they can be

relatively easily collected at a convenient time. The settled particles of fly ash can then be

collected in a traditional manner, for example, by opening a gate at the bottom of the second

chamber and directing the flow of fine fly ash to a transport vehicle waiting underneath.

[0017] In a second aspect, the present invention provides an apparatus for separating fine

particles of fly ash (having a particle size of less than about 45 microns) from a fly ash

powder. The apparatus comprises a first chamber comprising an inlet for receiving a flow of

fly ash powder into the first chamber; a lower portion configured to receive settled fly ash;

and an outlet from which a product stream comprising entrained fine particles of fly ash can

be drawn from an atmosphere above the settled fly ash. The apparatus also comprises a

second chamber for receiving the product stream, the second chamber comprising an exhaust

fan for creating a vacuum in the second chamber that causes an airflow that draws the product

stream into the outlet.

[0018] In some embodiments, the apparatus further comprises a controller for controlling a

rate at which the product stream is drawn into the outlet (e.g. in response to a particle size of

the fine particles of fly ash entrained in the product stream).

[0019] In some embodiments, the apparatus further comprises a settled fly ash measurer for

measuring an amount of the settled fly ash in the first chamber, wherein a rate at which the

product stream is drawn into the outlet is adjustable (e.g. by the controller) in response to the

measured amount of the settled fly ash.

[0020] In some embodiments, the outlet comprises a conduit between the first chamber and

the second chamber. In some embodiments, the outlet is located at an uppermost portion of the first chamber. In some embodiments, the apparatus comprises a plurality of outlets from which product streams comprising entrained fine particles of fly ash are drawn from the atmosphere above the settled fly ash in the first chamber.

[0021] In some embodiments, the inlet is configured to receive a turbulent flow of fly ash

powder. In some embodiments, the inlet is located at an uppermost portion of the first

chamber.

[0022] In some embodiments of the present invention, the apparatus of the second aspect

may be used to perform the first aspect and vice versa.

[0023] As would be appreciated, although described herein primarily with respect to

separating particles of different sizes from fly ash powders, the present invention has broader

application. Indeed, the present invention may be used to separate particles having

practically any size ranges from a powder (e.g. granulated blast furnace slag, hydrated lime,

powdered food products, etc.). As such, in a third aspect, the present invention provides a

method for separating fine particles of a particulate material from a powder of the material.

The method comprises delivering the powder to a chamber whereby a pile of settled coarser

particles forms at a lower portion of the chamber and an atmosphere comprising the fine

particles forms above the settled particles, and drawing a product stream from the atmosphere

above the settled particles, wherein the product stream comprises the fine particles entrained

therein.

[0024] Similarly, in a fourth aspect, the present invention provides an apparatus for

separating fine particles of a particulate material from a powder of the material. The

apparatus comprises a first chamber comprising an inlet for receiving a flow of the powder

into the first chamber; a lower portion configured to receive settled coarser particles; and an

outlet from which a product stream comprising entrained fine particles of the particulate

material can be drawn from an atmosphere above the settled particles. The apparatus also

comprises a second chamber for receiving the product stream.

[0025] Specific features of the third and fourth aspects of the present invention are as

described herein with reference to the first and second aspects of the present invention,

mutatis mutandis.

Brief Description of Drawings

[0026] Embodiments of the present invention will be described in further detail below with

reference to the following figures, in which:

[0027] Figure 1 depicts a perspective view of an apparatus for separating fine particles of a

particulate material from a powder in accordance with an embodiment of the present

invention;

[0028] Figure 2 depicts a side view of the apparatus for separating fine particles of a

particulate material from a powder of Figure 1; and

[0029] Figure 3 depicts a partial side view of the second chamber of an apparatus for

separating fine particles of a particulate material from a powder in accordance with an

alternative embodiment of the present invention.

Description of Embodiments

[0030] As noted above, the present invention provides a method and an apparatus for

separating fine particles of a particulate material from a powder (and, in particular, fine

particles of fly ash from a fly ash powder). As described below, fine particles of fly ash are

separated from more coarse particles in fly ash powders using settlement principles and a

controlled airflow.

[0031] The method of the present invention comprises delivering the fly ash powder to an

upper portion of a chamber, whereby coarse particles of fly ash fall through the chamber

relatively quickly, forming a pile of settled fly ash at a lower portion of the chamber, and fine

particles of fly ash fall relatively slowly, remaining suspended in an atmosphere above the

settled fly ash. A vacuum created in a second chamber which is in fluid communication with

the atmosphere above the settled fly ash causes an airflow that draws a product stream

comprising the fine particles of fly ash entrained therein from the atmosphere above the

settled fly ash.

[0032] The apparatus of the present invention comprises a first chamber comprising an inlet

via which a fly ash powder can be delivered into the first chamber; a lower portion onto

which fly ash can settle; and an outlet via which a product stream comprising entrained fine

particles of fly ash can be drawn from an atmosphere above settled fly ash. The apparatus

also comprises a second chamber for receiving the product stream, the second chamber comprising an exhaust fan for creating a vacuum in the second chamber that causes an airflow that draws the product stream into the outlet.

[0033] Fly ash powders contain generally spherically-shaped particles having a wide

distribution of particle sizes, typically from about 0.5 microns to about 300 microns in

diameter. A fly ash powder will include particles of fly ash having a wide range of sizes, and

which can be broadly classified as fine sized particles and coarse sized particles. As used

herein the term "fine particles of fly ash" (and similar terms) is intended to encompass fly ash

particles having a particle size of less than about 45 microns (e.g. less than about 40 microns,

less than about 35 microns, less than about 30 microns, less than about 25 microns or less

than about 20 microns). Coarse fly ash particles are fly ash particles having a particle size of

above about 45 microns. Fine fly ash conforms to AS 3582 Supplementary Cementitious

Materials for use with Portland Cement. Part 1: Fly Ash as a fine grade fly ash.

[0034] Whilst the size of fly ash particles can be measured using scanning electron

microscopes, laser diffraction techniques, etc., such techniques are not routinely used in

practice due to their relative expense. Instead, particle size is usually determined (and

specified) with reference to material that passes through sieves having a given pore size. For

example, the relevant Australian Standard for fine grade fly ash specifies that 75% (at least)

of the product must pass through a 45 micron sieve.

[0035] The fly ash produced by coal-fired power plants is usually transported off site and

stored in large chambers in the form of closed silos or bins (known in the art as storage silos)

of typical size 20 - 2,000 cubic metres before its further (wet or dry) processing. The fly ash

may be transferred from the power plant to the chamber using any conventional technique,

for example, in a continuous process such as via pneumatic conveying (in which the fly ash

powder is delivered to the storage silo using compressed air in a pipeline) or in a batch

process such as via bucket elevators or other similar mechanical devices.

[0036] In the present invention, once the fly ash powder reaches the (first) chamber, it is

delivered to the chamber in a manner whereby a pile of settled fly ash forms at a lower

portion of the chamber and an atmosphere comprising fine particles of fly ash forms above

the settled fly ash. The fly ash powder may, for example, either enter the chamber as a

mixture of powder and compressed air (e.g. if pneumatically conveyed to the chamber, as

described below) or mix with air as it falls into the chamber (e.g. if delivered using a bucket

elevator, as described below). Hence, post-delivery, the chamber contains a mixture of fly ash particles that have rapidly settled at the bottom of the chamber, and an atmosphere above the settled particles which includes air and suspended fly ash particles. The principle of this invention is that the coarser particles of fly ash tend to settle first in the chamber whilst finer particles of fly ash tend to remain airborne for a while, and it is these particles that are able to be drawn off using air currents.

[0037] The powdered fly ash may be delivered to any portion of the (first) chamber, provided

that a pile of settled fly ash forms at a lower portion of the chamber and an atmosphere

comprising fine particles of fly ash forms above the settled fly ash. Typically, the inlet of the

first chamber is located at an upper (e.g. uppermost) portion of the chamber such that optimal

mixing of air and fly ash powder can occur during delivery of the fly ash powder into the

chamber. In some embodiments, for example, the fly ash powder may be delivered through

an aperture in the roof of the chamber. In some embodiments, the fly ash powder may be

delivered through a conduit that extends into the chamber and has an outlet in an upper

portion of the chamber. In some embodiments, the fly ash powder may be delivered through

an aperture in a side wall of an upper portion of the chamber.

[0038] Any delivery technique which achieves the result described above may be used in the

present invention. Typically, the fly ash powder is turbulently delivered to the (first)

chamber, which ensures that, if not already, the fly ash powder mixes with air immediately

upon entry into the chamber. The turbulent delivery of more fly ash powder into the chamber

also ensures that there is a constant mixing action, causing finer particles of fly ash that may

have settled to become re-suspended.

[0039] Turbulent delivery of the fly ash powder into the chamber may, for example, be

achieved by delivering the fly ash powder to an upper portion of the chamber, where it then

falls through the chamber and mixes with air. The coarser particles of fly ash relatively

quickly fall onto the pile of settled fly ash at the bottom of the container, whilst the finer

particles of fly ash fall more slowly and remain entrained in the atmosphere above the settled

fly ash for a while.

[0040] Turbulent delivery of the fly ash powder into the chamber may, for example, be

achieved by delivering the fly ash powder into the chamber at a relatively high velocity

and/or pre-mixed with air, as would be the case, for example, when the fly ash powder is

pneumatically conveyed to the chamber. In such embodiments, it may not be necessary to

deliver the fly ash powder to an upper portion of the chamber, although the amount of settled fly ash which the chamber may be able to hold before needing to be emptied may be reduced if the fly ash powder was delivered too far from the ceiling of the chamber.

[0041] The fly ash powder may be delivered to the chamber as a continuous stream (e.g. in

embodiments of the apparatus of the present invention in which a pneumatic conveyor

delivers fly ash powder into the chamber via the inlet) or in a batch wise method (e.g. in

embodiments of the apparatus of the present invention in which a bucket elevator delivers fly

ash powder into the chamber via the inlet). The inlet of the first chamber is correspondingly

configured to receive the flow of fly ash powder resulting from any of such delivery methods.

It is within the ability of a person skilled in the art to determine which of these delivery

methods would be best suited to a particular application, as well as whether a turbulent

delivery might be advantageous, based on factors such as the physical characteristics of the

fly ash powder, the rate of production of the fly ash powder, the distance between the power

station and storage silos, equipment already available on-site, etc.

[0042] In the method of the present invention, a product stream comprising the fine particles

of fly ash entrained therein is drawn from the atmosphere above the settled fly ash. Any

suitable method or apparatus may be used to draw the product stream from the atmosphere

above the settled fly ash.

[0043] Typically, the product stream is drawn from the atmosphere above the settled fly ash

by an airflow, such as that which can be generated by an exhaust fan (e.g. a dust fan, such as

those commonly used to extract air from industrial spaces and sold, for example, by Aerotech

or Dustcotech). In such embodiments, the apparatus of the present invention further

comprises an airflow generator (e.g. an exhaust fan) for generating an airflow that draws the

product stream comprising entrained fine particles of fly ash from the atmosphere into the

outlet. Alternatively, a vacuum generator may be used in some embodiments to generate a

suitable airflow.

[0044] In the apparatus of the present invention, the product stream is drawn from the first

chamber into the second chamber, with the first and second chambers being joined in any

suitable manner. In some embodiments, for example, the outlet of the apparatus of the

present invention comprises a conduit between the first chamber and the second chamber.

Such a conduit may have any form, provided that the product stream can be drawn

therethrough. In some embodiments, the conduit leads directly from the first chamber to the

second chamber. In alternative embodiments (such as those described below, where the outlet is "trouser leg" shaped), benefits may be obtained by providing a more torturous path for the product stream between the first and second chambers. Potential benefits include, for example, an even more controllable separation of particles sizes of the fine fly ash, with relatively larger fine particles of fly ash not being able to traverse the torturous path, or a "fail safe" configuration where, in the event of a malfunction, the extractor fans, second chamber, etc. do not become swamped by a large volume of fly ash.

[0045] Any suitable technique may be used to modify the composition of the product stream in order to obtain a desired product (e.g. fine particles of fly ash having a narrow particle size distribution or particles all below a desired size). For example, adjusting the volume or rate of the airflow at which the product stream is drawn from the first chamber can vary the fineness of the particles being collected. High airflow will result in drawing off fine and some mid-sized particles (i.e. particles at the larger end of the fine particle spectrum, as described above), whilst a low airflow will draw off only the very finest of fine particles. Accordingly, in some embodiments, a rate at which the product stream is drawn from the atmosphere above the settled fly ash is adjustable (e.g. in response to a particle size of the fine particles of fly ash entrained in the product stream). In some embodiments of the apparatus of the present invention, the apparatus may, for example, have a controller for controlling a rate at which the product stream is drawn into the outlet (e.g. in response to a particle size of the fine particles of fly ash entrained in the product stream).

[0046] The airflow at which the product stream is drawn from the first chamber (and transferred to the second chamber in the apparatus) can be adjusted using any suitable methods or apparatus. For example, speed controllers on the motors of the airflow generators (e.g. in the form of dust collector fans) may be used, or multiple airflow generators may be provided, with the number in service controlled according to the airflow required. In embodiments where two (or more) airflow generators are used, their operation may be controlled in order to increase or decrease the rate of airflow (and hence product stream) drawn from the chamber, or to maintain the airflow (or at least some airflow) in the event of one of the airflow generators becoming temporarily overloaded or covered in fly ash particles (thereby reducing its efficiency).

[0047] In some embodiments, a plurality of product streams may be drawn from the atmosphere above the settled fly ash. The apparatus of the present invention may, for example, comprise a plurality of outlets from which product streams including entrained fine particles of fly ash can be drawn from the atmosphere above the settled fly ash in the first chamber. In such embodiments, the product stream can be drawn from different areas of the atmosphere, which may be useful as the first chamber fills up, or if a greater rate of extraction of the product stream is required. The rate at which the product stream is drawn out of each outlet may be the same, or may be independently adjustable to best suit the circumstances.

[0048] Another factor which may influence the particle size (and particle size distribution) of the fine fly ash is the distance above the settled fly ash from where the product stream is collected. As will be appreciated, the fineness of the particles of fly ash in the atmosphere above the settled fly ash will generally increase with distance above the settled fly ash (although other factors, such as how turbulently and from where the fly ash powder is delivered to the chamber will also affect this). The product stream may be drawn from anywhere in the atmosphere that results in it containing entrained particles of fine fly ash, and can be determined by a person skilled in the art for any given method or apparatus. In some embodiments therefore, a location in the chamber from where the product stream is drawn may be adjustable in response to an estimated or measured particle size of the fine particles of fly ash entrained in the product stream. Such adjustments may, for example, be provided by multiple outlets arranged at different heights in the first chamber, or by an outlet that can move upwardly and downwardly in response to a measured amount of settled (coarse) fly ash or a measured particle size in the product stream. In some embodiments, for example, the outlet may be located at an uppermost portion of the first chamber (e.g. the outlet may be defined by a hole in the ceiling of the chamber). Embodiments in which the outlet is located in a lower position are possible but lowering the outlet too far may affect the fineness of the fly ash particles that can be drawn from the atmosphere (as the finer particles will settle relatively slowly and remain above the outlet for longer), and may necessitate that settled fly ash be removed from the chamber more frequently.

[0049] The rate of settlement of fly ash particles in the first chamber will also vary with the level of the settled fly ash in the chamber, with marginally finer material becoming entrained in the product stream when the chamber is empty and marginally coarser material when the chamber is near full. This can be accounted for such that material of a substantially consistent fineness can be obtained by providing a settled fly ash measurer (e.g. level indicators or continuous level measurement, as described below) for measuring an amount of the settled fly ash in the first chamber, where the rate at which the product stream is drawn into the outlet is adjustable (e.g. by the controller described below) based on the measured amount of the settled fly ash in the first chamber (i.e. higher airflow when the storage silo is near empty and lower airflow when storage silo is near full).

[0050] In some embodiments therefore, it may be beneficial to measure an amount of the

settled fly ash at the lower portion of the (first) chamber. Such measurements will also

enable the continuous monitoring of the amount of settled fly ash, to ensure that the first

chamber does not become overfull, as well as to ensure that the method and apparatus draw

off appropriately sized fine particles of fly ash and operate in the most efficient manner.

[0051] Measurements of the amount of settled fly ash in the (first or second) chambers may

be made either using level indicators on the sides of the chamber, or via other means, such as

by a weight of the chamber or by estimation based on factors such as the volume of fly ash

powder delivered into the first chamber over a period of time.

[0052] In some embodiments, the settled (coarse) fly ash can be removed from the (first)

chamber when a measured amount of the settled fly ash exceeds a predetermined amount.

The predetermined amount may, for example, be reached when the amount of settled fly ash

in the chamber hinders operation of the method and apparatus and needs to be removed (e.g.

by being transferred to a truck for disposal) or production needs to be slowed down. Whilst it

will depend on a number of factors specific to the particular application, an upper amount of

settled (coarse) fly ash of above about 50%, 60%, 70% or 80% of the capacity of the (first)

chamber would trigger the removal at least a portion of the settled fly ash from the chamber.

Again, whilst it will depend on a number of factors specific to the particular application, once

the amount of settled (coarse) fly ash the first chamber drops to below about 10%, 20% or

% of the capacity of the chamber, then removal of the settled fly ash could be stopped.

[0053] The particle size of the fine particles of fly ash entrained in the product stream may be

measured (continuously or at periodic intervals), using the techniques discussed above, for

example, or estimated based on operational data obtained previously. As will be appreciated,

the ability to fine-tune the particle range distribution of particles in the product stream may

result in a more highly desirable product (e.g. a product having a guaranteed particle size

distribution or maximum particle size).

[0054] Notwithstanding the relative expense of doing so, it may, in some circumstances be

appropriate to measure the particle size of the fine particles of fly ash actually entrained in the

product stream in real time order to provide a product having a consistent fineness. In such

circumstances, the apparatus of the present invention may have a particle size measurer for measuring (or otherwise deriving) a particle size of the fine particles of fly ash entrained in the product stream. In some embodiments, the rate at which the product stream is drawn into the outlet is adjustable by a controller based on the measured particle size of the fine particles of fly ash entrained in the product stream.

[0055] The particle size of the fine particles of fly ash entrained in the product stream may be

measured using any suitable technique, including using a scanning electron microscope or by

laser diffraction. The results of these measurements can be fed back to a controller, which

may (if necessary) adjust the rate at which the product stream is drawn from the atmosphere

(e.g. by speeding up or slowing down extractor fan(s)) or adjust the location from which the

product stream is drawn.

[0056] In other circumstances, empirical techniques, optionally combined with data from

earlier separations, may produce suitable operating conditions and products.

[0057] The product stream drawn from the atmosphere above the settled fly ash comprises

entrained particles of fine fly ash which, as noted above, can be beneficially used in the

concrete industry (for example). The fine particles of fly ash may be collected from the

product stream using any suitable technique.

[0058] In some embodiments, for example, the method of the present invention further

comprises allowing the fine particles of fly ash entrained in the product stream to settle in

another chamber. In such embodiments, the product stream may, for example, be drawn into

another chamber (e.g. the second chamber in the apparatus of the present invention) by an

airflow, with the air component of the airflow being vented to the atmosphere via a dust filter

through which the entrained particles cannot pass. Once separated from the air by the dust

filter, the fine particles of fly ash would typically settle at the bottom of the (second)

chamber, where they are able to be subsequently collected. The second chamber in the

apparatus of the present invention may therefore be configured to enable settled fine particles

of fly ash to be removed. Suitable chambers may, for example, be provided in the form of

storage silos having structures similar to those commonly used in the art - i.e. generally

cylindrical, with a lower portion that tapers towards a gate which can be opened to release the

settled fine fly ash particles for collection.

[0059] Similarly to that discussed above in relation to the first chamber, an amount of settled

fine fly ash particles in the (second) chamber can be measured, with a prompt being provided

when the level of settled fine fly ash reaches a predetermined amount (e.g. where there is enough fine fly ash to fill a truck, or if production needs to be slowed down because the second chamber is becoming too full).

[0060] In alternative embodiments, however, the fine particles of fly ash may be collected

without necessarily requiring a second silo (or the like). For example, when the product

stream is drawn out of the atmosphere in the first container, the fine fly ash particles can be

transferred directly to a collection point (e.g. a truck waiting underneath the silo), for

example by creating a vacuum inside the truck using an extraction fan. In such embodiments,

the truck itself may provide the second chamber of the apparatus of the present invention.

[0061] Specific embodiments of an apparatus and method for separating fine particles of fly

ash from a fly ash powder in accordance with the present invention will now be described

with reference to the Figures.

[0062] Referring firstly to Figures 1 and 2, shown is a fly ash processing plant 10 in

accordance with an embodiment of the present invention. Fly ash processing plant 10 has

two storage silos 12A and 12B, of the type currently used to store powders of particle size 1

100 micron. Silos 12A and 12B have a typical size of 20 - 2,000 cubic metres, and are

capable of receiving up to about 250 tph of powder via large diameter pipes (not shown).

Silos 12A and 12B are elevated above the ground and have gates (not shown) at their

lowermost portions via which settled (coarse) fly ash can be removed by flowing into a truck

14 situated below the solos 12A, 12B.

[0063] Fly ash processing plant 10 also has a smaller silo in the form of fine particles silo 16.

Fine particles silo 16 is joined to silos 12A and 12B via conduits in the form of trouser leg

pipes 18A and 18B respectively. In the embodiment shown, trouser leg pipes 18A and 18B

extend between uppermost surfaces of the silos 12A and 12B (respectively) and silo 16. In

alternative embodiments, however, the trouser leg pipes may extend some distance into one

or more of the silos.

[0064] On an uppermost portion of fine particles silo 16 sits a dust filer 20, which includes a

conventional multi-speed fan and dust filtering material (not shown). Fine particles silo 16 is

elevated above the ground and has a gate 22 (see Figure 2) at its lowermost portion via which

settled (fine) fly ash can be removed by flowing through conduit 24 into the waiting truck 14.

[0065] In operation, fly ash powder (not shown) is delivered to the storage silos 12A and 12B

using compressed air in a pipeline (pneumatic conveying) or bucket elevators (or other

mechanical devices), neither of which are shown in the Figures for clarity. In the embodiment shown, the fly ash powder is delivered to silos 12A and 12B via openings 26A and 26B (respectively) at their upper portions.

[0066] Depending on its form of delivery, the powder either enters the storage silos 12A and

12B as a mixture of powder and compressed air or mixes with air as it falls into the silos.

Hence, storage silos 12A and 12B contain a mixture of air and suspended powder (the inside

of the silos is not shown in the Figures). The principle of this invention is that the coarser

particles of fly ash settle first in the silos 12A and 12B following the normal rules of Stokes

Law. Finer particles of fly ash remain airborne for a time in the atmosphere above the settled

particles, and these particles are drawn off to the fine particles silo 16 using air currents, as

will be described below.

[0067] The fine particles silo 16 is connected to the storage silos 12A and 12B by 'trouser leg'

pipes 18A and 18B. Airflow to carry the fine particles through each 'trouser leg' pipe 18A,

18B is created by using fan-forced dust collector 20, which extracts air from fine particles

silo 16 such that a small vacuum is created inside the silo. The creation of this vacuum inside

the fine particles silo 16 causes a product stream containing air in which the fine particles of

fly ash are entrained to be drawn from the uppermost portions of silos 12A and 12B, through

trouser leg pipe 18A and 18B, respectively, and into fine particles silo 16. Once the product

stream enters the fine particles silo 16, the fine particles can start to settle to the bottom of

silo 16, where they can subsequently be collected via gate 22. Any fine fly ash particles

which get drawn into the fan-forced dust collector 20 will not be vented to the outside

atmosphere, but will be retained on the dust filter (not shown) of the fan-forced dust collector

20. If necessary, periodic agitation of the dust filter would cause these fine particles of fly

ash to fall back into a lower portion of the fine particles silo 16 and subsequently settle.

Typically, the dust filter will be a fabric dust filter, although other forms of dust filers such as

reverse pulse fabric filters could be used.

[0068] Trouser leg pipes 18A and 18B have their distinctive shape so that the dust filter

cannot be smothered with fine fly ash in the event that the fan-forced dust collector 20 is

switched off. In such circumstances, the fly ash in the section of the trouser leg pipes 18A,

18B projecting from its respective storage silo 12A, 12B would fall back into the silo 12A,

12B.

[0069] As discussed above, adjusting the airflow to the fine particle silo 16 can vary the

fineness of the particles being collected. High airflow will result in drawing off fine, and some mid-sized particles, whereas a low airflow will draw off only the very fine particles. In this specific embodiment, airflow can be adjusted by one of two methods. Firstly, by the use of speed controllers (not shown) on the motors of the dust collector fan 20. Secondly, by the use of multiple dust collectors 20, 20 (see Figure 3) and controlling the number in service according the airflow required.

[0070] As also discussed above, the rate of settlement in the storage silos 12A and 12B will

vary with the level of the settled (coarser) fly ash particles in the storage silo; marginally finer

material will be delivered to the fine particles silo 16 when the storage silo 12A, 12B is

empty and marginally coarser material when the storage silo 12A, 12B is near full. This can

be countered and material of a consistent fineness achieved by fitting the storage silos 12A,

12B with level indicators, shown generally as 28 (in Figure 2). The airflow to the fine

particle silo 16 may be adjusted depending on the level of powder in the storage silos 12A,

12B i.e. a higher airflow when the storage silo is near empty and lower airflow when storage

silo is near full. The level indicators 28 would usually be positioned so that it is possible to

maintain the level of settled fly ash in the silos 12A, 12B between about 30 - 70% of

capacity.

[0071] Using the embodiment depicted in the figures, it is envisaged that the storage silos

12A, 12B could receive up to about 250 tph (tonnes per hour) fly ash powder, with about 15

tph fine grade fly ash being removable from the fine particles silo 16.

[0072] It will be appreciated that the present invention provides a number of new and useful

results. For example, specific embodiments of the present invention may provide one or

more of the following advantages:

• fine particles of fly ash are separable from a fly ash powder without requiring the

separators or classifiers presently used;

• existing fly ash storage facilities can be utilised, with only relatively small

additional infrastructure modifications being required;

• minimal additional handling of the fly ash is required; • the fineness of the fine particles of fly ash can be adjusted simply by controlling

the rate at which the product stream is drawn from the atmosphere;

Sa relatively narrow particle size distribution of fly ash can be obtained, making the

product more desirable to consumers;

• with few moving parts, repair and maintenance costs are likely to be significantly

reduced compared to conventional methods;

• with few moving parts, it is possible for the process to be operated unmanned,

hence saving on labour costs; and

* power consumption is likely to be only approximately one fifth compared to

conventional methods.

[0073] It will be understood to persons skilled in the art of the invention that many

modifications may be made without departing from the spirit and scope of the invention. All

such modifications are intended to fall within the scope of the following claims.

[0074] In the claims which follow and in the preceding description of the invention, 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 invention.

Claims (20)

CLAIMS:

1. A method for separating fine particles of fly ash from a fly ash powder, the method

comprising:

delivering the fly ash powder to an upper portion of a chamber whereby coarse

particles of fly ash fall through the chamber relatively quickly, forming a pile of

settled fly ash at a lower portion of the chamber, and fine particles of fly ash fall

relatively slowly, remaining suspended in an atmosphere above the settled fly ash;

and

creating a vacuum in a second chamber that is in fluid communication with the

atmosphere above the settled fly ash, the vacuum causing an airflow that is effective

to draw a product stream from the atmosphere above the settled fly ash, wherein the

product stream comprises the fine particles of fly ash entrained therein,

wherein the fine particles of fly ash have a particle size of less than about 45 microns.

2. The method of claim 1, wherein a rate at which the product stream is drawn from the

atmosphere above the settled fly ash is adjustable in response to the particle size of the

fine particles of fly ash entrained in the product stream.

3. The method of claim 1 or claim 2, wherein a rate at which the product stream is drawn

from the atmosphere above the settled fly ash is adjustable in response to an amount of

the settled fly ash at the lower portion of the chamber.

4. The method of any one of claims 1 to 3, wherein a location in the chamber from where

the product stream is drawn is adjustable in response to the particle size of the fine

particles of fly ash entrained in the product stream.

5. The method of any one of claims 1 to 4, wherein the product stream is drawn from the

atmosphere above the settled fly ash by a vacuum generated by an exhaust fan in the

second chamber.

6. The method of any one of claims 1 to 5, wherein a plurality of product streams are drawn

from the atmosphere above the settled fly ash.

7. The method of any one of claims 1 to 6, wherein the fly ash powder is turbulently

delivered to the chamber.

8. The method of any one of claims 1 to 7, wherein the fly ash powder is pneumatically

conveyed to the chamber.

9. The method of any one of claims 1 to 8, wherein the fly ash powder is delivered to an

upper portion of the chamber by a bucket elevator.

10. The method of any one of claims 1 to 9, further comprising removing a portion of the

settled fly ash from the chamber.

11. The method of any one of claims 1 to 10, further comprising allowing the fine particles of

fly ash entrained in the product stream to settle in the second chamber for collection.

12. An apparatus for separating fine particles of fly ash from a fly ash powder, the apparatus

comprising:

a first chamber comprising:

an inlet for receiving a flow of fly ash powder into the first chamber;

a lower portion configured to receive settled fly ash; and

an outlet from which a product stream comprising entrained fine particles of

fly ash can be drawn from an atmosphere above the settled fly ash; and

a second chamber for receiving the product stream, the second chamber comprising

an exhaust fan for creating a vacuum in the second chamber that causes an airflow

that draws the product stream into the outlet,

wherein the fine particles of fly ash have a particle size of less than about 45 microns.

13. The apparatus of claim 12, further comprising a controller for controlling a rate at which

the product stream is drawn into the outlet.

14. The apparatus of claim 12 or claim 13, further comprising a settled fly ash measurer for

measuring an amount of the settled fly ash in the first chamber, wherein a rate at which

the product stream is drawn into the outlet is adjustable in response to the measured

amount of the settled fly ash.

15. The apparatus of any one of claims 12 to 14, wherein the outlet comprises a conduit

between the first chamber and the second chamber.

16. The apparatus of any one of claims 12 to 15, wherein the outlet is located at an uppermost

portion of the first chamber.

17. The apparatus of any one of claims 12 to 16, comprising a plurality of outlets from which

product streams comprising entrained fine particles of fly ash can be drawn from the

atmosphere above settled fly ash in the first chamber.

18. The apparatus of any one of claims 12 to 18, wherein the inlet is located at an uppermost

portion of the first chamber.

19. The apparatus of any one of claims 12 to 18, wherein the inlet is configured to receive fly

ash powder from a pneumatic conveyor.

20. The apparatus of any one of claims 12 to 18, wherein the inlet is configured to receive fly

ash powder from a bucket elevator.