Note: Descriptions are shown in the official language in which they were submitted.
CA 02315841 2000-07-27
A SYNTHETIC AGGREGATE AND A PROCESS FOR THE PRODUCTION THEREOF
FIELD OF INVENTION
The present invention relates to a synthetic aggregate and a process for
producing the synthetic aggregate. More particularly, the process of the
invention is
directed at converting a waste material into the synthetic aggregate utilizing
a molten
sulphur binder. The invention also relates to a process for immobilizing
environmental
contaminants and converting them into a physical state wherein they are non-
hazardous
to the environment.
BACKGROUND OF INVENTION
Numerous waste products or materials are generated by a variety of
industries. These waste products include contaminated sand and soil,
metallurgical
slag, ash, saw dust, wood shavings and various other mineral and organic by-
products
generated by industries such as the oil and gas, petrochemical, mining, pulp
and paper,
timber and construction industries. Costly solutions are often required to
address the
treatment and disposal problems presented by such waste materials in order to
ensure
that such treatment or disposal occurs in a safe and effective manner.
Several options currently exist for dealing with these types of waste
materials. First, the waste materials may be disposed of in sanitary
landfills. However,
this solution tends to be costly and may be environmentally undesirable due to
the
potential for the leaching of the wastes into the surrounding soil and
groundwater.
Second, these wastes may undergo washing or some form of chemical treatment
aimed
at de-contamination of the waste. This solution also tends to be costly and
may result in
the production of some amount of contaminated by-products, such as waste
water,
which similarly requires treatment prior to disposal. As well, the
decontaminated
waste, such as decontaminated soil waste, may have no use and therefore must
typically
be stockpiled in some fashion, taking up land space.
Finally, the waste material may be encapsulated using a Portland cement
binder in order to produce a concrete-like mass. However, the use of Portland
cement
as a binder tends to be costly due to the materials used and the necessary
processing
requirements for Portland cement. In addition, the use of Portland cement may
present
difficulties when actually using the encapsulated materials for construction
due to the
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relatively short time which is typically available for the delivery of the
concrete to a
potential construction site prior to the commencement of the setting of the
concrete. In
addition, environmental concerns continue to be raised with respect to the use
of
contaminated waste materials in Portland cement concrete when being used for
construction purposes. Thus, such concrete is usually restricted by
environmental
regulations to industrial construction sites.
The treatment of sulphur-rich petroleum oil and natural gas products, as
well as some ores, produces a voluminous amount of sulphur as a by-product.
While
there are available uses for sulphur, such as the manufacture of fertilizers,
there has
tended to be a substantial oversupply of sulphur resulting from the ever
increasing
desulphurization of primary products such as petroleum oil, natural gas and
ores. As a
result of environmental concerns due to potential contamination by sulphur
stock-piled
on site, safe, effective and cost efficient solutions are similarly required
for the treatment
or disposal of such sulphur.
One use of this sulphur has been the development of sulphur cements and
concretes. Sulphur concretes are materials in which aggregate is bound
together by
elemental sulphur. The concretes are typically cast in the molten state and
bonding
occurs upon cooling of the molten mass as the sulphur crystallizes. It has
been found
that on solidifying from the molten state that elemental sulphur tends to be a
relatively
good binder for conventional aggregates such as sand, gravel and stone. The
physical
properties of the elemental sulphur have an effect upon the properties of the
resulting
sulphur concrete. Generally speaking, these properties tend to be superior to
the
properties found in Portland cement concrete. For instance, sulphur concrete
typically
displays rapid strength development, high strength (tensile, compressive,
flexural and
fatigue), low permeability, low thermal conductivity, low electrical
conductivity and
high corrosion and wear resistance. Further, these properties may be modified
and
improved by the inclusion of various modifiers or plasticizers in the sulphur
cement or
concrete to produce what is referred to as modified sulphur cements and
concretes.
For instance, United States of America Patent No. 4,134,775 issued January
16, 1979 to Schwoegler describes the production of shaped articles from a dry
blend
composition containing "sulphur and a particulate solid additive inorganic
material."
The dry blend is heated to fuse the sulphur and then formed into the desired
shape by
extrusion or pelletization. The purpose of the Patent is produce a dry blend
that does
not settle or stratify during shipping and storage, thereby obviating the need
for re-
mixing of the blend when required to be used by the consumer. The dry blend
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10
maintains its uniformity until use by the consumer. United States of America
Patent No.
5,004,799 issued April 2, 1991 to Kohls et. al. also describes the formation
of modified
sulphur concrete into discrete self-sustaining pellets which can be shipped
and stored
for indefinite periods and re-melted to yield usable concrete.
The purpose of these Patents is to render the sulphur or modified sulphur
concrete more amenable to transport and storage. The Patents do not deal with
the
treatment or disposal of waste materials nor do they deal with the production
of a
synthetic aggregate from the compositions. United States Patent No. 5,569,153
issued
October 29, 1996 to Mallow et. al. does describe a method of immobilizing
toxic waste
material by forming concrete shapes. However, the process and the resulting
concrete
shape require the use of hydraulic or Portland cement.
Specifically, a synthetic aggregate is first created from a pozzolana, a
calcium hydroxide containing material, hydrothermal cement reactant, hydraulic
cement and the toxic waste material. The waste material must be admixed with a
liquid
and used as a waste slurry in the mixture or sufficient water must be added to
provide a
moldable consistency of the mixture. The mixture is permitted to cure and is
then
formed into the aggregate by any typical grinding technique. The synthetic
aggregate is
then mixed with a modified sulphur cement, a pozzolana and sand to form a
concrete
product. As a result, the synthetic aggregate and process of this Patent tend
to be costly
and involve other disadvantageous features as discussed above due at least in
part to
the use of hydraulic cement therein.
United States of America Patent No. 4,428,700 issued January 31, 1984 to
Lennemann describes the use of a modified sulphur cement or concrete as a
barrier or
backfill material for the containment of waste materials. In particular, a
container of
waste material, which is placed within an excavation, is surrounded by
modified
sulphur cement or concrete which has been placed in the molten state into the
excavation around the container and then allowed to cool or harden. The Patent
does
not discuss or deal with the formation of a composition comprising the sulphur
cement
and the waste material, nor does it deal with the formation of a synthetic
aggregate
therefrom.
Therefore, there remains a need in the industry for a process for treating or
otherwise dealing with waste materials in a relatively cost effective and
environmentally
safe manner as compared to conventional processes. In addition, the process
preferably
permits the utilization of sulphur which is presently produced in an abundant
supply
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by various industrial processes. More particularly, there remains a need in
the industry
for a process for converting a waste material into a synthetic aggregate and a
need for a
synthetic aggregate so produced which may be used in the construction and
other
industries. Further, the process for converting the waste material into the
synthetic
aggregate preferably utilizes sulphur as a binder. Finally, there is a need
for a process
which utilizes sulphur to encapsulate the waste material and thereby produce a
synthetic aggregate. Preferably, the synthetic aggregate will provide a
replacement for
conventional aggregate in construction, the production of which conventional
aggregate
tends to be costly, energy consuming and often environmentally undesirable.
SUMMARY OF INVENTION
The present invention relates to a process for treating or otherwise dealing
with waste materials in a relatively cost effective and environmentally safe
manner as
compared to conventional processes. The process also preferably permits the
utilization
of sulphur which is presently produced as a by-product by various industrial
processes.
The present invention further relates to a process for converting a waste
material into a
synthetic aggregate and to the synthetic aggregate so produced. The process
for
converting the waste material into the synthetic aggregate preferably utilizes
sulphur as
a binder. Finally, the present invention relates to a process which utilizes a
sulphur
binder to encapsulate a waste material and thereby produce a synthetic
aggregate.
In a first aspect of the invention, the invention relates to a process for
converting a waste material into a synthetic aggregate. The process comprises
the steps
of:
(a) creating a molten sulphur binder;
(b) mixing an amount of the waste material into the molten sulphur binder
such that the waste material is substantially coated by the molten sulphur
binder to produce an encapsulated waste material;
(c) separating the encapsulated waste material into discrete particles having
a
size suitable for use as an aggregate to produce the synthetic aggregate;
and
(d) cooling the molten sulphur binder to harden the encapsulated waste
material.
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The process comprises preheating the waste material prior to the mixing
step in order to minimize any cooling of the molten sulfur binder as a result
of the
mixing step. Preferably, the waste material is preheated to a temperature of
about the
temperature of the molten sulfur binder. More particularly, the waste material
is
preferably preheated to a temperature of between about 120 and 200 degrees
Celsius.
Further, the cooling step may cool the molten sulphur binder to any
temperature sufficient to harden the encapsulated waste material. However,
preferably
the cooling step comprises cooling the molten sulphur binder to a temperature
of less
than about 90 degrees Celsius. The cooling step may be performed at any time
throughout the process as necessary to produce the hardened encapsulated waste
material.
Finally, in the process, the separating step may comprise any conventional
method or process for separating the encapsulated waste material into the
desired
discrete particles. However, preferably, the separating step comprises
pelletizing the
encapsulated waste material to produce the synthetic aggregate. Any
conventional
pelletizing process may be used. However, the pelletizing process preferably
utilizes
extrusion or rolling techniques.
In a second aspect of the invention, the invention relates to a synthetic
aggregate. The synthetic aggregate comprises: a waste material; and an amount
of a
sulphur binder for substantially coating the waste material such that the
waste material
is encapsulated thereby.
The sulphur is stabilized by a portion of the waste material or by a mineral
additive. The stabilizer comprises a portion of the waste material. Any
mineral additive
capable of having a stabilizing effect may be used. In a further alternative,
the stabilizer
may be comprised of slag, preferably a finely ground metallurgical slag. In a
still
further alternative, the stabilizer may be comprised of fly ash. No chemical
stabilizer
such as polymerized sulphur, or plasticizer such as cyclopentodiene is used.
T'he
synthetic aggregate is formed in the absence of a chemical stabilizer,
polymerizer, or
modifier.
The waste material may be comprised of any industrial waste product or
by-product. Preferably, the waste material is comprised of any contaminated,
toxic or
hazardous waste product or by-product from the oil, gas, petrochemical,
mining, pulp,
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paper, timber and construction industries. For instance, the waste material
may be
comprised of fly ash, contaminated sand from heavy oil production,
contaminated soil,
slag such as metallurgical slag, waste mill dusts such as precipitated steel
mill dust, coal,
petroleum coke, saw dust, wood shavings, and sludges containing pesticides.
DETAILED DESCRIPTION
The within invention is directed at a synthetic aggregate and a process for
producing the synthetic aggregate. The process converts a waste material into
the
synthetic aggregate by encapsulating the waste material with a sulphur binder.
Thus,
the synthetic aggregate is comprised of the waste material and an amount of
the sulphur
binder.
The waste material is comprised of any material or product which
conventionally requires treatment prior to its disposal or storage due to
largely
environmental concerns. In particular, the waste material will typically
present
concerns with respect to environmental contamination such as by the leaching
of any
toxic or hazardous substances to the environment. However, any material or
product
produced as waste or a by-product of any industrial process may be used. The
waste
material may be comprised of a single type of waste material or may be
comprised of
two or more types of waste material mixed together and used in combination to
produce the synthetic aggregate. In addition, the waste material is used in a
solid state,
that is, as a substantially dry, finely divided material, in contrast to being
utilized in a
liquid state, as a waste slurry.
The finely divided waste materials preferably have the following particle
size distribution: a minimum of 5 %, by weight, of the particles must be less
than 80
microns in size. Not more than 10 %, by weight, of the particles should be
greater than 1
mm in size. Not more than 5%, by weight, of the particles should be greater
than 3 mm
in size. Not more than 2%, by weight, of the particles should be greater than
5 mm in
size. The finely divided particles are believed to facilitate the formation of
the stable
and durable crystal structure of the sulphur matrix.
The finely divided waste materials should have the aforementioned
particle size distribution in order to provide sufficient stabilization to the
synthetic
aggregate. In the event the waste materials have larger particle sizes that
are outside the
range of the aforementioned particle size distribution, particle size
reduction needs to be
implemented to obtain the designated particle size distribution.
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The waste material containing the environmental contaminants is
preferably comprised of one or more of the following: contaminated sand,
contaminated soil, non-metallurgical or metallurgical slag or coke, ash, fly
ash, saw
dust, wood shavings, produced sand and any mineral or organic by-products
generated
by industries such as the oil and gas, petrochemical, mining, pulp and paper,
timber and
construction industries. Produced sand is contaminated sand resulting from
heavy oil
production. More particularly, produced sand results from pumping operations
where
heavy or viscous oil is present in geological formations containing sand.
Typically,
produced sand may contain heavy metals, chlorides and light aromatic
hydrocarbons
including benzene and other carcinogens.
This invention targets a wide range of environmental contaminants
contained in a variety of waste materials. These waste materials are more
severely
contaminated with environmental contaminants than materials previously
utilized to
make sulphur concrete. The envirorunental contaminants become immobilized
within
the synthetic aggregate and resistant to leaching, such that the synthetic
aggregate is
substantially environmentally nontoxic and in compliance with U.S.
Environmental
Protection Agency standards. Heavy metals and small amounts of chlorides are
generally the only contaminants present in flyash and slag, which may be
utilized in
sulphur mortars and concretes. In addition to heavy metals and large amounts
of
chlorides, such environmental contaminants as carbon, silicones, hydrocarbons
and
pesticides are effectively treated and converted into envirorunentally
nontoxic synthetic
aggregate.
Larger particle size waste materials, on the order of at least about 5 mm do
not present the same degree of environmental concern as the more finely
divided waste
materials on the order of less than about 1 mm. Thus, the larger particle size
waste
materials expose a lower surface area to leaching conditions, such as water
run-off, than
more finely divided waster materials which expose large surface areas to
leaching
conditions and consequently present a more severe threat of environmental
contamination than larger particle size waste materials.
Conventional sulphur concrete, mortar and cement generally utilizes
finely divided fillers such as silica flour, limestone, mica, diatomaceous
earth,
vermiculite and perlite. These materials usually do not contain substantial
quantities, if
any, of envirorunental contaminants. Chemical stabilizers, polymerizers and
modifiers
are also often included.
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The waste materials in the present invention are contaminated, and
generally have little or no value, In most cases, the waste materials are
actually an
environmental liability and costly disposal problem for the producer.
The amount of waste material used in this invention can vary from about
65 weight % to about 95 weight %, preferably about 70 weight % to about 90
weight %,
of the synthetic aggregate. The amount of contaminant in the waste material
can vary
from about 0.2 weight % to about 95 weight %, and preferably about 2 weight %
to
about 90 weight % . The amount of environmental contaminants in the aggregate
can
vary from about 0.5 weight % to about 70 weight %, and preferably about 1
weight % to
about 65 weight % of the synthetic aggregate.
The waste material is encapsulated within a sulphur binder. The sulphur
binder is comprised of elemental sulphur. Typically, below about 90 degrees
Celsius,
crystallization of the sulphur occurs and there is a crystallographic
conversion from the
monoclinic to the more thermodynamically stable orthorhombic form. This
conversion
is typically complete in about 20 hours. The elemental sulphur may be obtained
from
any source. However, typically, the sulphur is a by-product of the de-
sulphurization of
petroleum oil, natural gas or ores.
The finely divided waste material with the designated particle size
distribution acts to stabilize the structure of the sulphur binder in order to
produce or
maintain the desired properties in the synthetic aggregate. Specifically, the
finely
divided waste material stabilizer acts to minimize the formation of, or the
reversion of
the structure of the sulphur binder over time into, an unstable structure
affecting the
durability and other properties of the synthetic aggregate. More particularly,
the finely
divided waste material stabilizer preferably inhibits the formation of, or
reversion into,
crystals and minimizes the size of any crystals which are formed. It is
believed that
crystal growth may be restricted as the particles of the finely divided waste
material
stabilizer serve as nucleation sites inducing the formation of many small
crystals instead
of fewer larger ones. This is accomplished without the addition of a chemical
stabilizer,
such as polymeric or polymerized sulphur, or any modifier.
It has been found that the presence of larger or macro crystals in the
sulphur binder tends to reduce the durability of the synthetic aggregate upon
exposure
to freezing-thawing cycles. Sulphur has a relatively high thermal expansion
coefficient
and a low thermal conductivity. Thus, when a sulphur binder containing
adjacent
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macro crystals of sulphur undergoes temperature changes, movement is believed
to
occur between the macro crystals which rnay break or severely weaken the
structure of
the sulphur binder.
Thus, where the waste material is finely ground with the particle size
distribution, already discussed, the finely ground waste material acts as the
stabilizer.
No chemical stabilizer is added.
It has further been found that the stabilizer may also be comprised of a
finely ground mineral additive that preferably has the following particle size
distribution: a minimum of 5%, by weight, of the particles must be less than
80 microns
in size. Not more than 10 %, by weight, of the particles should be greater
than 1 mm in
size. Not more than 5%, by weight, of the particles should be greater than 3
mm in size.
Not more than 2%, by weight, of the particles should be greater than 5 mm in
size. T'he
mineral additive may comprise finely ground metallurgical slag. Other specific
finely
ground materials or substances which have also been found to be useful as
stabilizers
include phosphoric gypsum, metallurgical slag, coke, fly ash and produced
sand.
In addition, where necessary to enhance the strength or other properties of
the synthetic aggregate, the synthetic aggregate may be further comprised of a
filler.
'The filler may be any conventional material used in the production of cement
or
concrete which acts as a strength enhancing agent or reinforcing agent.
Typically, such
fillers are inert materials which provide a fibrous reinforcement to the
cement. The filler
may also modify the viscosity of the mix and thus reduce any tendency for
separation
and may reduce the required amount of binder by filling the voids between the
particles
of the waste material. For instance, the filler may be comprised of carbon
fibers, steel
fibers, glass fibers, wood fibers or other organic fibers. However, in the
preferred
embodiment, the filler is comprised of wollastonite. The filler content in the
synthetic
aggregate should generally be no more than about 20 weight %, and preferably
no more
than about 10 weight % of the synthetic aggregate.
The process for converting the waste material into the synthetic aggregate
may be performed on either a batch or continuous basis. The process comprises
the
steps of creating a molten sulphur binder, mixing the waste material into the
molten
sulphur binder to produce an encapsulated waste material, separating the
encapsulated
waste material into discrete particles to produce the synthetic aggregate and
cooling the
molten sulphur binder to harden the encapsulated waste material and thereby
produce
a hardened synthetic aggregate.
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T'he creating step includes heating the sulphur binder to a temperature
greater than the melting point of sulphur to produce the molten sulphur
binder. In
particular, the sulphur binder is preferably heated to a temperature of equal
to or
greater than about 120 degrees Celsius such that the sulphur binder melts. In
the
preferred embodiment, the sulfur binder is heated to a temperature of between
about
120 and 200 degrees Celsius. As described above, the sulphur binder is
comprised of
elemental sulphur.
The sulphur binder may be heated in or by any conventional heating
device, vessel or apparatus or by any conventional heating method or process
suitable
for heating sulphur. In the preferred embodiment, the heating step is
performed in a
heating vessel wrapped with insulation to inhibit any heat loss therefrom. The
temperature of the sulphur binder in the vessel may be monitored and
controlled by any
conventional temperature control or monitor. In the preferred embodiment, the
heating
element of the vessel is connected to a thermostat. In addition, one or more
temperature
sensors are installed and preferably connected to a digital temperature read-
out device.
An amount of the waste material is then mixed into the molten sulphur
binder. The mixing step is performed until the waste material is substantially
coated by
the molten sulphur binder such that the waste material is encapsulated by the
molten
sulphur binder. Thus, the mixing step produces an encapsulated waste material.
In
addition, the waste material is preferably substantially uniformly mixed into
or
throughout the molten sulphur binder such that a substantially uniform
encapsulated
waste material is produced. The coating or encapsulation of the waste material
substantially binds and immobilizes the contaminants within the waste
material. Thus,
the encapsulation reduces or minimizes any potential for the leaching or
escape of the
waste material or the contaminants from the synthetic aggregate to the
environment.
'The mixing step may be performed by any conventional mixing device,
vessel or apparatus or by any conventional mixing method or process capable of
mixing
the waste material into the molten sulphur binder in order to produce the
encapsulated
waste material.
In addition, the waste material may be pre-heated prior to the mixing of
the waste material into the molten sulfur binder in order to minimize any
cooling of the
sulfur binder as a result of the mixing step. Thus the preheating of the waste
material is
beneficial from a practical standpoint in order to avoid any necessary
reheating of the
molten sulfur binder, as well as to assist in the reduction or elimination of
any moisture
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in the mixture of the waste material and the molten sulfur binder, which
moisture may
lead to undesirable foaming of the mixture. However, this preheating step is
considered
to be preferred only.
Where the preheating step is performed, the waste material is preferably
pre-heated to a temperature of about the temperature of the molten sulfur
binder, which
is preferably between about 120 and 200 degrees Celsius. In the event the
addition of
the waste material reduces the temperature of the molten sulfur binder, the
resulting
mixture of the waste material and the sulfur binder may need to be re-heated
prior to
the step of separating the encapsulated material into discrete particles
depending upon
the particular process or equipment utilized to perform the separating step.
As well, as indicated above, an effective amount of a filler may also be
added to or mixed with the molten sulphur binder. The filler may be combined
with the
sulphur binder at any step or time within the process prior to the step of
separating the
encapsulated waste material into discrete particles in order to ensure that
the filler is
substantially uniformly mixed throughout the molten sulphur binder prior to
producing
the synthetic aggregate. For instance, the filler may be combined with the
sulphur
binder prior to heating the sulphur binder. Thus, the filler may simply be
combined
with the sulphur, prior to the step of creating the molten sulphur binder. The
filler may
be added and mixed into either the unmolten sulfur or the molten sulphur
binder by
any conventional mixing device, vessel or apparatus or by any conventional
mixing
method or process capable of mixing the filler into the unmolten sulfur or the
molten
sulphur binder, as required.
In addition, the filler may be preheated prior to combining the filler with
the molten sulfur binder in order to minimize any cooling of the molten sulfur
binder as
a result of this combining step. Thus, as with the preheating of the waste
material, the
preheating of the filler is also beneficial from a practical standpoint in
order to avoid any
necessary reheating of the molten sulfur binder, to assist in the reduction or
elimination
of any moisture in the mixture of the waste material, the filler and the
molten sulfur
binder and to minimize or reduce any undesirable foaming of the mixture.
However,
this preheating step is also considered to be preferred only.
Where the step of preheating the filler is performed, the filler is preferably
preheated to a temperature of about the temperature of the molten sulfur
binder. More
preferably, the filler is preheated to a temperature of between about 120 and
200 degrees
Celsius. Alternately, the filler may be preheated to a temperature higher than
the
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desired temperature of the molten sulfur binder, but lower than the igniting
temperature of the sulfur, and added to the unmolten sulfur in a uniform
fashion to
melt the sulfur.
The amount of the waste material to be mixed into the molten sulphur
binder, and the relative proportions of the sulphur binder and the waste
material, will
vary depending upon various factors including the type of waste material used,
the
desired properties of the resulting synthetic aggregate, the desired
workability of the
mix, the particular process being used to perform the separating step and the
temperature regime of the process.
A. Produced Sand Based Aggregates
Mix
Number Com osition
A.1 Produced Sand (dried) - 65-80% (average-70%)
Sulfur - 20-35
A.2 Produced Sand - 70%
Sulfur - 30
A.3 Produced Sand - 65%
Fly Ash - 5
Sulfur - 30
A.4 Produced Sand - 70%
Fly Ash - 5
Sulfur - 25
A.5 Produced Sand - 70%
Phosphogypsum - 5%
Sulfur - 25
A.6 Produced Sand - 66%
Ground Slag - 5
Sulfur - 29
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B. Slag / Fly Ash Based Agg-gates
Mix
Number Com osition
B.1 Barren Slag - 35%
Ground Slag- 42%
Sulfur - 23
B.2 Barren Slag - 51
Fly Ash - 29
Sulfur - 20
B.3 Ground Slag - 70%
Fly Ash - 5
Sulfur - 25
Notes: -All of the aggregates produced from these mixes were produced
through extrusion pelletizing
-Further mixes used in testing shaking or rolling pelletizers were either:
(a) Produced Sand - 70%
Sulfur - 30 %; or
(b) Produced Sand - 75%
Sulfur - 25
-The mix temperature range for shaking or rolling pelletizers is
between about 130-170 degrees Celsius.
Following the production of the encapsulated waste material, the
encapsulated waste material is separated into discrete particles having a size
or sizes
suitable for use as an aggregate to produce the synthetic aggregate. The size
and shape
of each discrete particle formed by the separating step need not be the same.
In other
words, the size and shape of the discrete particles need not be uniform as
long as the
size and shape of each discrete particle is suitable for use as an aggregate.
Further, as
long as the size or sizes of the discrete particles are suitable for use as an
aggregate, the
actual particle size may vary depending upon the desired use or uses of the
synthetic
aggregate.
Any conventional device, apparatus, method or process capable of
separating the encapsulated waste material into discrete particles having the
desired
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size may be used. However, in the preferred embodiment, the separating step is
comprised of pelletizing the encapsulated waste material to produce the
synthetic
aggregate. Most conventional pelletizing apparatuses and any conventional
pelletizing
processes may be used. However, preferably, such apparatuses or processes use
extrusion or rolling techniques.
Finally, the process is also comprised of cooling the molten sulphur binder
to harden the encapsulated waste material. As a result, a hardened synthetic
aggregate,
ready for use, is formed. The molten sulphur binder may be cooled to any
temperature
permitting the hardening of the encapsulated waste material, and in particular
the
hardening of the sulphur binder coating the waste material. However,
preferably the
molten sulphur binder is cooled to a temperature of less than about 90 degrees
Celsius.
The cooling step may be performed actively, by any conventional cooling
device,
apparatus, method or process, or passively by simply permitting the heat to
dissipate
from the sulphur binder.
The cooling step may be performed at any time following the production
of the encapsulated waste material by the mixing step. In the preferred
embodiment in
which the encapsulated waste material is pelletized, the cooling step may
partially occur
prior to the pelletization so that the encapsulated waste material has a
consistency
compatible with the pelletization process. In addition, the synthetic
aggregate will tend
to have a consistency suitable for pelletization due to the relatively high
amounts of
finely divided waste material that are used to form the synthetic aggregate.
Furthermore, the pelletization process may cool the molten sulphur binder
during the
pelletization to form the discrete particles. Finally, the sulphur binder may
be
adequately cooled during the pelletization to produce a hardened synthetic
aggregate
ready for transport or storage. However, alternately, the sulphur binder may
require
further cooling following the pelletization process.
In the preferred embodiment, one pelletizer includes a hopper inlet, a
casing chamber with a rotating screw conveyor and a stationary perforated dye
at the
end of the casing. Following the preparation of the encapsulated waste
material, the
encapsulated waste material is transferred to the pelletizer. Once in the
pelletizer, the
encapsulated waste material is conveyed through the pelletizer by turning the
handle of
the rotating screw conveyor towards the perforated dye. As the encapsulated
waste
material is squeezed through the perforated dye, the sulphur binder, and thus
the
encapsulated waste material, are cooled by spraying water on the outside of
the dye. As
a result, the encapsulated waste material is formed into discrete particles,
being hard
-14
CA 02315841 2000-07-27
cylindrical pellets, which break off at the dye face. The pellets or synthetic
aggregate are
produced ready for handling and do not require any further cooling.
Alternately, the pelletizer may include a preferably sloped flat or
corrugated sheet attached to a vibrator. The encapsulated waste material is
discharged
on the sheet and separated into discrete particles which then roll along the
sloped sheet
forming the pellets. In a further alternative, the pelletizer may include a
rotating drum
preferably sloped along its axis. The encapsulated waste material is
discharged into the
drum and separated into discrete particles which then roll into pellets.
The resulting synthetic aggregate tends to be relatively strong and durable
as compared to other natural and synthetic aggregates. In addition, the
synthetic
aggregate prevents, or at least minimizes, the leaching of any contaminants
from the
waste material. As well, the synthetic aggregate has been found to have lower
electric
and thermal conductivities than most natural aggregates, and in particular
natural
gravel. These qualities are thought to occur as sulphur tends to be both a
dielectric and
an insulator. Finally, the synthetic aggregate may be crushed, remelted and
reformed as
desired or required for any particular application without any substantial
loss of
strength or other properties of the synthetic aggregate.
Accordingly, one of the specific applications of the synthetic aggregate is
its use in the construction industry. For instance, its lower electrical
conductivity may
be beneficial in the use of the synthetic aggregate as a base cover for
electrical
substations. As well, the lower thermal conductivity may be beneficial in the
use of the
synthetic aggregate for backfilling trenches with water pipes to protect them
from
freezing or for road sub-base or foundation construction to prevent frost
heave action on
pavements or foundations.
The synthetic aggregate is quite effective in immobilizing environmental
contaminants and is uniquely suited for a variety of construction
applications, including
an aggregate for Portland cement concrete, asphalt paving materials,
compactible gravel
fill for road foundations or road sub-bases. The particle size of the
synthetic aggregate
can vary from about 50 microns up to about 100 mm, preferably about 150
microns to
about 25 mm depending on the application. Fine aggregate such as sand,
generally
varies from above about 150 microns to about 5 mm. Coarse aggregate such as
gravel
used in asphalt generally varies from about 300 microns to about 25 mm.
-15
CA 02315841 2000-07-27
The synthetic aggregate of the present invention is better in quality than
state-of-the-art aggregates in terms of hardness, strength and overall
durability.
The following examples and tables provide test results and data which
serve more fully to illustrate the invention.
-16
CA 02315841 2000-07-27
Table 1: Contaminant Content of Flyash
Fly Ash* Fly Ash*
Contaminant (ppm) (%, by weight)
barium, Ba 10000 1
beryllium, (as 2 0.0002
Be0)
boron, B 500 0.05
chromium, Cr 50 0.005
copper,Cu 20 0.002
gallium, Ga 50 0.005
lead, Pb 200 0.02
manganese, Mn 100 0.01
molybdenum, 20 0.002
Mo
nickel, Ni 20 0.002
strontium, Sr 3000 0.3
yttrium (as 20 0.002
Y203)
zirconium (as 200 0.02
Zr02)
Total Contaminants (%, by weight of flyash) 1.42
* Semi-quantitative spectrographic analysis of a typical,
Class F coal fly ash, produced in Alberta.
Table 2: Contaminant Content of Slay
Slag** Slag**
Contaminant (ppm) (%, by weight)
barium, Ba 3000 0.3
chromium, Cr 590 0.059
copper,Cu 6000 0.6
lead, Pb 900 0.09
manganese, Mn 8000 0.8
nickel, Ni 50 0.005
strontium, Sr 1000 0.1
zinc, Zn 26000 2.6
arsenic, As 200 0.02
cadmium, Cd 6 0.0006
germanium, Ge 70 0.007
tin, Sn 300 0.03
antimony, Sb 200 0.02
cobalt, Co 200 0.02
thallium, TI 10 0.001
chloride, CI 20 0.002
Total Contaminants 4.65
(%, by weight of slag)
** Analysis of a heavily contaminated metallurgical slag
-17
CA 02315841 2000-07-27
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CA 02315841 2000-07-27
Table 6: Contaminant Content Comparison of Synthetic
Aggregate and Sulphur "Mortar" Containing Flyash
Contaminant Contenty weight of
(b material)
Sulphur "Mortar"Synthetic
with Flyash Aggregate
Contaminant (%, by weight)~'~(%, by weight)
chromium, <0.0030 0.0211
Cr
nickel, Ni <0.0012 0.0032~3~
lead, (Pb) 0.012 0.405
copper, (Cu)0.0012 0.0178
(1 ) Contaminant
content
in mortar
is based
c on the
chemical
analysis
in Table
1
Contaminant
content
in mortar
assumes
a theoretical,
maximum
flyash
content of
60% , by
weight of
mortar.
(2) Contaminant
content
in synthetic
aggregate
produced
from waste
mill dust
(C), see
Table 5
(3) Contaminant
content
in synthetic
aggregate
produced
from waste
sludge (A),
see Table
5.
Table 7: Contaminant Content Comparison of Synthetic
Aggregate and Sulphur Concrete Containing Slaa
Contaminant Contenty weight rial
(b
Sulphur ConcreteSynthetic
with Slag Aggregate
Contaminant (%, by weight)t'~(%, by weight)
cadmium, 0.00048 0.0031
Cd
lead, Pb 0.072 0.405~2~
arsenic, 0.016 0.0563 ~3~
As
chloride, 0.0016 0.0882
(CI)
(1 ) Contaminant
content
in concrete
is based
on the baseline
slag chemical
analysis
presented
in Table
2. Contaminant
content
in concrete
assumes
a maximum
slag
content of
80%, by
weight of
concrete.
(2) Contaminant
content
in synthetic
aggregate
produced
from waste
mill dust
(C), see
Table 5.
(3) Contaminant
content
in synthetic
aggregate
produced
from waste
sludge (A),
see Table
5.
(4) Contaminant
content
in synthetic
aggregate
produced
from waste
sand, see
Table 3.
-21
CA 02315841 2000-07-27
Table 8: Leachability Assessment of Synthetic Aggregate
Containing Heavily Contaminated Steelmill Dust
Waste MillWaste Leachate from SyntheticItalian Residentiall
Parameter Dust (C) Mill Agg. Agricultural Criteria
(%, by Dust Containing Waste Mill(ppm) or (mglL)Z
weight) (C) Dust (C)
(ppm) (mglL)'
Total PesticidesNA NA NA 0.001
antimon Sb NA NA < 0.2 20.0
arsenic As 0.00177 17.7 < 0.2 20.0
barium Ba 0.00771 77.1 0.45 700
be Ilium Be 0.0001 < 1.0 < 0.01 4.00
boron B 0.00076 7.6 0.48 NA
cadmium Cd 0.0122 122 0.79 3.00
chromium Cr 0.0845 845 0.02 100
chromium, hex. 0.000003 0.03 < 0.1 5.00
(Cr+s)
cobalt Co 0.0008 8.00 < 0.01 40.0
co er Cu 0.0708 708 0.02 100
c snide water 0.00005 < 0.5 NA Com lex 50 / Free
soluble 10
o lead Pb 1.62 16,200 30.9 100
man anese Mn NA NA 3.46 NA
mercur , total 0.000121 1.21 < 0.1 1.00
H
mol bdenum Mo 0.0018 18.0 < 0.01 10.0
nickel Ni 0.0095 95.0 0.04 150
selenium Se 0.00018 1.80 < 0.2 3.00
silver A 0.0002 2.00 0.01 20.0
tin Sn 0.0055 55.0 < 0.05 50.0
thallium TI 0.0001 < 1.0 0.06 1.00
uranium U NA NA < 0.2 NA
vanadium V 0.0023 23.0 < 0.01 200
zinc Zn 8.08 80,800 33.5 300
( zirconium (Zr) NA NA < 0.1 NA
Notes:
NA - Not Applicable
1: Synthetic Aggregate mix design of 25%, waste mill dust (C), 50% waste sand,
25% Sulphur (see Table 5).
Leachate extracted as per TCLP Method (EPA SW-846, Method #: 1311 ).
2: Regulatory criteria is applied to the dispersible material in question
(waste dust or synthetic aggregate leachate)
-22
CA 02315841 2000-07-27
Table 9: Alberta Transportation and Utilities (AT & U)
Aaareaate Gradation Specifications
AT&U Spec. AT&U Spec.
for Asphalt for Pit-Run
Concrete Gravel
Sieve Size Pavement Fill Desi on 6, Class
Desi nation nati 80
1, Class
16 .
mm) Min. % PassinMax. % PassinMin. % PassinMax. % Passin
80 100 100 100 100
50 100 100 55 100
40 100 100
25 100 100 38 100
20 100 100
16 100 100 32 85
12.5 80 92
70 84
5 50 65 20 65
1.250 26 45
0.630 18 38
0.315 12 30 6 30
0.160 8 20
0.080 4 10 2 10
Alberta Transportation and Utilities is the government body responsible for
public
works construction specifications in Alberta. Table 9 provides Alberta
aggregate
gradation specifications for two construction applications.
Firstly, a gradation specification for an aggregate used in asphalt paving
materials is provided. The second specification is for so-called "Pit-Run"
aggregate. Pit-run gravel is a high-bearing capacity, compactible gravel fill
used
typically as a foundation or road sub-base.
T'he present invention is capable of producing synthetic aggregate meeting
these gradations. Futhermore, the synthetic aggregate gradation is not limited
to
these gradations. Additional variability in the aggregate gradation is
possible,
depending on the application.
-23
CA 02315841 2000-07-27
Leachate Tests
The leachate tests were performed to determine the leaching of
various contaminants from untreated produced sand as compared to the synthetic
aggregate. The produced sand was obtained from the Norcen Energy site in Elk
Point, Alberta, Canada. The synthetic aggregate was produced utilizing the
produced sand as the waste material. In addition, the synthetic aggregate was
prepared from the following:
sulphur - 30 % by weight
waste material (produced sand) - 70 % by weight
The leachate tests were performed in accordance with Method No. 1311,
"Toxicity
Characteristic Leaching Procedure" (TCLP), United States Environmental
Protection Agency (USEPA) Module SW-846 (Update I, July 1992.). The leachate
data appears in Table 8 and illustrates the effectiveness of the synthetic
aggregate
matrix to immobilize heavy metal contaminants, thereby minimizing the ability
of
the waste material to contaminate the environment. In essence, the data
demonstrates that the synthetic aggregate produced by the present invention is
substantially environmentally non-toxic.
-24
CA 02315841 2000-07-27
Strength Tests
The strength tests were performed by forming mass test cubes, which were
then used to determine compressive strength upon destroying the cubes in a
compression tester. Strength results related to some of the compositions from
which
synthetic aggregate was produced, as outlined above, are as follows:
Table 10
Strength Results
Mix Average Strength
Number MPa
A.1 14.0
A.4 20.2
A.6 19.3
Notes: -Results for Mix A.1 were averaged over the strength of 9 cubes with a
minimum of 7.4 MPa and a maximum of 19.0 MPa.
-Results for Mix A.4 were averaged over the strength of 6 cubes with a
minimum of 17.6 MPa and a maximum of 24.4 MPa.
-Results for Mix A.6 were averaged over the strength of 6 cubes with a
minimum of 16.4 MPa and a maximum of 21.8 MPa.
-All values were 24 hour strengths.
-25
CA 02315841 2000-07-27
DurabilitX Tests
The durability tests were performed by subjecting the test specimens
(actual synthetic aggregate) to numerous freeze-thaw cycles. The particulars
of the
materials and mix parameters for these tests are as follows:
(a) Testing Procedure
1. 190 grams of synthetic aggregate, 5-15 mm in size, is placed in a 1.5
inches x 3.0 inches x 2.0 inches plastic container.
2. The synthetic aggregate undergoes one freeze-thaw cycle every 24
hours. The cycle is induced by submerging the aggregate in
tapwater for 9.0 hours, draining the water from the sample and
placing the sample in a freezer for the next 15. 0 hours. Five cycles
are achieved each week.
3. For the purposes of this invention, each week the samples are
qualitatively examined and compared to a control group of
aggregates that have not undergone freeze-thaw cycling. In
addition, another control group aggregate has been tested which is
a synthetic aggregate produced from expanded slate, a material
which has been on the market for several decades.
4. The aggregate samples are subjected to the following hand tests:
Test (a) - scratching to evaluate surface integrity;
Test (b) - rolling over corners and edges attempting to break off
pieces of aggregate; and
Test (c) - attempting to shear aggregate pieces manually to evaluate
strength loss.
5. It should be noted that the durability tests performed were
qualitative only and were not standard durability tests. It is
anticipated that once a specific process and synthetic aggregate are
selected for commercial application that standard durability tests
will be conducted on the synthetic aggregate.
-26
CA 02315841 2000-07-27
(b) Testing Results
Eleven different synthetic aggregate samples were subjected to freeze-thaw
testing. The majority were produced by extrusion pelletizing and
crushing, but two samples were produced by crushing sheets of material
and one sample was produced by plate-vibration pelletizing.
A rating system with a scale of 1 to 5 was used to evaluate degradation
under freeze-thaw testing. The aggregate received a rating for each of the
three hand tests (a) - (c) previously described. A "5" corresponded to the
undiminished properties of the control sample which had undergone no
cycling. A "1" corresponded to a badly deteriorated sample which
crumbles under moderate finger pressure.
The number of cycles listed is the point at which deterioration was first
noticed. However, the test results set out below reflect the current status of
ongoing durability tests with respect to some of the mixes. Therefore,
where testing is ongoing and no degradation or deterioration has been
noted to date, the number of cycles to deterioration is not specifically
provided. Rather, the test results merely indicate the number of cycles
conducted to date.
Strength results related to some of the compositions from which synthetic
aggregate was
produced, as outlined previously, are as follows:
Table 11
Results of Freeze-Thaw Cog Durabili Tests
Mix No. of Test Test Test Observations
No. C cles (a) (b) (c)
Ratin Ratin Ratin
A.1* 35 4 3 5 aggregate appears to be
becoming slightly crumbly
at
the corners but is still
maintaining overall integrity
and stren th
-27
CA 02315841 2000-07-27
A.2 35 4 4 4 aggregate becoming "softer"
and more crumbl
A.4 45 5 4 4 aggregate noticeably harder
(more difficult to scratch)
than
mixes A.1 and A.2; aggregate
more prone to shear into
halves
rather than crumble
A.5 25 3 3 3 aggregate deteriorated
quickly,
was crumbly and weak to
begin with; aggregate with
phosphogypsum is weak under
freeze-thaw
A.6 45 5 4 4 very similar to mix A.4;
aggregate was noticeably
harder than others; not
crumbly but tended to shear
more easily than control
sam le
B.1 - 5 5 5 no degradation noticed
after 55
cycles; very strong and
hard
a a ate
B.2 40 4 3 4 initially hard and strong;
although aggregate is still
hard
and has maintained its
integrity, it has become
more
crumbly around the edges
and
corners; some strength
loss is
evident as aggregate is
more
Basil sheared
B.3 - 5 5 5 no deterioration after
45 cycles;
ver hard and stron a a
ate
A.7* 20 2 2 2 aggregate became crumbly,
soft
and weak very quickly;
very
oor freeze-thaw durabili
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CA 02315841 2000-07-27
A.8* 30 3 2 2 aggregate was originally
very
hard; aggregate tended
to
fracture into small pieces
after
35 c cles
A.9* - 5 5 5 no deterioration noticed
after
27 cycles; rounded aggregate
(not an ular like other
sam les)
Notes: A.1* - mix of 70% Produced Sand and 30% Sulfur
A.7* - mix of 70% Produced Sand and 30% Sulfur (crushed
unconsolidated quenched sheets)
A.8* - mix of 70% Produced Sand and 30% Sulfur (crushed vibrated
quenched sheets)
A.9* - mix of 70% Produced Sand and 30% Sulfur (aggregate formed from
a vibrating trough)
(c) Test Summary - Ranking of aggregates according to freeze-thaw durability
(in declining order of durability):
Produced Sand Based Aggregates:
1. Mixes A.9, A.6 and A.4
2. Mixes A.1 and A.2
3. Mix A.8
4. Mixes A.7 and A.5
Slag / Flyash Based Aggregates:
1. Mixes B.1 and B.3
2. Mix B.2
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