Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR UPGRADING ASBESTOS TAILINGS
TECHNICAL FIELD
The technical field generally relates to upgrading asbestos tailings, and more
specifically relates to a process for manufacturing refractory materials from
asbestos tailings.
BACKGROUND
Asbestos has been widely used by manufacturers and builders because of its
desirable physical properties such as sound insulation, tensile strength,
resistance to fire, electrical and chemical damage, and affordability.
Asbestos
deposits normally occur in certain types of silicate rock which contain about
5 to
10% by volume of asbestos fibres. Consequently, separation of the fibres from
asbestos ore leaves large quantities of by-products (also referred to herein
as
"asbestos tailings") which accumulate near extraction or processing sites. As
asbestos deposits were widely harvested throughout the 20th century, large
quantities of asbestos tailings are now available and can serve as raw
materials
in novel commercial applications.
Canadian patent No. 1,216,403 describes a process for manufacturing a granular
product from asbestos tailings, suitable for use as a refractory material and
obtained from calcined asbestos tailings. The granular material obtained was
found to have numerous advantages over silica sand, including having superior
refractory properties as well as being devoid of noxious free silicate dusts.
Canadian patent No. 2,130,330 describes a process for manufacturing a
refractory material from asbestos tailings. The process includes mixing ground
particles of the asbestos tailings with a magnesium oxide-based slurry to
obtain a
mixture, and calcining the mixture in a rotary kiln in order to obtain a
refractory
material.
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However, such refractory materials typically contain a relatively high amount
of
dust particles, which can cause a number of problems to the user.
Also, if introduced into the rotary kiln, the dust particles are dehydrated
and
calcined along with the rest of the material, which consumes additional
energy.
Furthermore, humid material or a water-based slurry is typically introduced
into
the rotary kiln, which also requires additional energy to be dried.
Consequently, a
portion of the rotary kiln is used for drying, and less surface area is
available for
dehydration and calcining of the asbestos tailings.
In light of the above, upgrading asbestos tailings still has a number of
challenges.
SUMMARY
In one general aspect, a process for upgrading a serpentine-containing
material
is provided. The process includes the steps of:
a) feeding the serpentine-containing material into a first reactor;
b) pre-treating the serpentine-containing material in the first reactor to
obtain a pre-treated material, the pre-treating comprising:
drying the serpentine-containing material; and
removing dust particles from the serpentine-containing material;
c) feeding the pre-treated material into a second reactor; and
d) dehydrating and calcining the pre-treated material in the second
reactor to obtain a calcined material.
In some embodiments, drying of the serpentine-containing material is performed
at a temperature between 300cC and 500cC.
In some embodiments, the dust particles removed from the serpentine-containing
material have a particle size of 150 microns or less.
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In some embodiments, the first reactor is a fluid bed reactor.
In some embodiments, the fluid bed reactor is one of a back mix fluid bed
dryer
and a plug flow fluid bed dryer.
In some embodiments, the fluid bed reactor is a vibrating fluid bed reactor.
In some embodiments, the fluid bed reactor comprises a perforated plate
comprising perforations having a size between 2.5 mm and 4 mm.
In some embodiments, a residence time of the serpentine-containing material in
the fluid bed reactor is between 5 minutes and 8 minutes.
In some embodiments, the dehydrating of the pre-treated material is performed
at
a temperature between 700cC and 900t.
In some embodiments, the calcining of the dehydrated material is performed at
a
temperature between 900cC and 1500t.
In some embodiments, the process further comprises removing residues having
a particle size larger than 2 cm from the serpentine-containing material.
In some embodiments, the residues having a particle size larger than 2 cm are
removed from the serpentine-containing material prior to step a).
In some embodiments, the second reactor is a rotary kiln.
In some embodiments, the rotary kiln comprises a dehydrating zone where the
pre-treated material is dehydrated and a calcining zone where the dehydrated
material is calcined.
In some embodiments, the rotary kiln further comprises a cooling zone for
cooling
the calcined material.
In some embodiments, a residence time of the pre-treated material within the
dehydrating zone is between 3 minutes and 5 minutes.
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In some embodiments, a residence time of the dehydrated material within the
calcining zone is between 10 minutes and 25 minutes.
In some embodiments, step d) comprises removing dust particles from the
second reactor.
In some embodiments, the dust particles removed from the second reactor have
a particle size of 150 microns or less
In some embodiments, the serpentine-containing material includes asbestos
tailings.
In some embodiments, the pre-treated material is stored at a storage
temperature prior to step c).
In some embodiments, the storage temperature is between 300cC and 500cC.
In some embodiments, the process further comprises the step of: e) processing
the calcined material to obtain a refractory material.
In some embodiments, step e) comprises crushing the calcined material.
In some embodiments, step e) comprises sieving the calcined material.
In another general aspect, there is provided a refractory material, obtained
by the
process described herein.
In some embodiments, the refractory material comprises about 5 wt% or less of
material having a particle size of less than 150 microns.
In some embodiments, the refractory material comprises about 2 wt% or less of
material having a particle size of less than 150 microns.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a process flow diagram of a prior process for manufacturing
refractory
materials from asbestos tailings;
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Figure 2 is a process flow diagram of a process for manufacturing refractory
materials from asbestos tailings, according to an embodiment of the invention;
Figure 3 is a diagram representing a pre-treatment in a fluidized bed,
according
to an embodiment of the invention;
5 Figure
4 is a scheme showing a system for manufacturing refractory materials
from asbestos tailings, according to an embodiment of the invention;
Figure 5 is a graph showing granulometric compositions (US Mesh) of (i) a
refractory material obtained using the process of Figure 2; and (ii) a
comparative
refractory material obtained using the process of Figure 1; and
Figure 6 includes Figures 6A and 6B; Figure 6A shows zones of a rotary kiln
used for dehydration and calcination of a material obtained from the process
of
Figure 1; and Figure 6B shows zones of a rotary kiln used for dehydration and
calcination of a material obtained from the process of Figure 2.
DETAILED DESCRIPTION
Various techniques that are described herein enable upgrading of serpentine-
containing materials such as asbestos tailings. It is understood that by
"upgrading" of a material, it is meant that the material is transformed by a
series
of process steps in order to obtain a product which may be sold, or be used
for a
certain purpose. For example, in some scenarios, upgrading of asbestos
tailings
allows obtaining a calcined material, which can be further processed to obtain
a
refractory material. It is therefore understood that some of the techniques
described herein enable the manufacture of refractory material from asbestos
tailings.
Asbestos tailings typically contain a substantial portion of hydrated
magnesium
silicates referred to as serpentine. Other components which occur with
serpentine include brucite Mg(OH)2 and hematite-magnetite Fe203-Fe304.
Serpentine can be dehydrated and calcined in order to produce sintered angular
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shaped granules which can be useful as sandblasting or heat accumulating
material, or to produce granular products useful as foundry mold sands.
Typically, dehydration and calcining of serpentine involves heating the
serpentine
to a temperature of above 1200t, whereby the following chemical reactions can
take place:
1) dehydration reaction typically occurring between 600 and 900cC to
form an anhydrous magnesium silicate:
Mg3 (Si205)(OH)4 ¨> 3 Mg0.2SiO2 + 2 H20
2) conversion of the anhydrous magnesium silicate into forsterite
Mg2SiO4 and free silica Si02, which typically occurs above 900cC:
2 (3Mg0 = 2Si02) ¨> 3 Mg2SiO4 + Si02
3) reaction of forsterite with free silica, typically occuring above
1000t, thereby forming enstatite MgSiO 3:
Mg2SiO4 + Si02 -> 2 MgSiO3
The mixture of forsterite and enstatite thereby obtained typically has a high
melting point above 1700t, which can make the material suitable for the
aforementioned applications.
It is understood that the term "asbestos tailings" as used herein refers to by-
products of the separation process of the asbestos fibres from asbestos ore.
The
asbestos tailings can be recovered from or near former asbestos production
sites
or facilities. The processes, systems and compositions described herein are
exemplified using "asbestos tailings" as starting material, but it is
understood that
other serpentine-containing materials can be used. The serpentine-containing
materials as referred to herein include materials wherein the primary
component
is serpentine and which include, for example, between 30 wt% and 100 wt%
serpentine, between 50 wt% and 95 wt% serpentine, between 70 wt% and 95
wt% serpentine, or between 80 wt% and 95 wt% serpentine; with the balance to
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100 wt% being at least one of a metal (such as iron, magnesium and/or nickel),
a
metal oxide (such as iron oxides (FeO, Fe203 and/or Fe304), magnesium oxides
and/or nickel oxides) and a metal derivative (such as a magnesium hydroxide or
an iron hydroxide). One non-limiting example of a serpentine-containing
material
includes between 90 wt% and 99 wt% serpentine, and between 1 wt% and 10
wt% iron oxides. Another non-limiting example of a serpentine-containing
material includes between 90 wt% and 95 wt% serpentine, and between 5 wt%
and 10 wt% iron oxides.
Referring to Figure 1, a comparative prior process 100 for the manufacture of
a
refractory material from asbestos tailings 102 is provided. The process 100
includes directly introducing the asbestos tailings 102 into a rotary kiln
104. The
asbestos tailings 102 is dried 106, dehydrated 110 and then calcined 114 in
the
rotary kiln 104 in order to obtain a calcined material 116 which can be
recovered
from the rotary kiln 104. The calcined material 116 is then typically crushed
in a
crushing step 118 and the crushed material 120 thereby obtained can be sieved
in a sieving step 122 in order to obtain a refractory material 124. It is
understood
that by "directly introducing the asbestos tailings", it is meant that the
asbestos
tailings are introduced into the rotary kiln 104 without any pre-treatment or
pre-
drying step. Depending on the time of the year and the environmental
conditions,
the asbestos tailings 102 can therefore include a certain amount of moisture
as
well as a certain amount of unwanted material (also referred to as
impurities).
It is understood that the term "moisture" can refer to water which can, for
example, originate from rain or snow, and can be deposited onto the asbestos
tailings 102. Such "moisture" is to be differentiated from water molecules
which
are embedded into certain molecular or ionic components of the asbestos
tailings
such as serpentine (such as in the form of hydrated magnesium silicates).
Therefore, it is understood that a drying step can typically suffice for
removing
moisture, for example by vaporizing the moisture, whereas a dehydration step
(or
dehydration reaction) is typically needed (as step 1 above), to remove water
molecules or hydroxide moieties embedded into some molecular or ionic
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components of the asbestos tailings 102. It is therefore understood that a
drying
step is typically performed at a lower temperature than a dehydration step.
For
example, a drying step for drying asbestos tailings can be performed between
100cC and 500cC, or between 300cC and 500cC, wherea s a dehydrating step
can be performed between 600cC and 900cC, or betwee n 700cC and 800t. It is
understood that the "unwanted material" or the "impurities" which can be
present
in the asbestos tailings 102, can include residues which have a particle size
larger than several cm, such as 2 cm, and/or materials which were initially
not a
by-product of the asbestos extraction process, such as wood-based or plastic-
based residues. Such residues can for example be by-products of other
unrelated processes, or waste material which was stored in conjunction with
the
asbestos tailings, or deposited at the same storage site than the asbestos
tailings.
Now referring to Figure 2, a process 200 for manufacturing refractory
materials
from asbestos tailings 102, according to an embodiment of the invention, is
provided. The asbestos tailings 102 can be recovered from or near former
asbestos production sites or facilities. In some embodiments, the asbestos
tailings 102 can initially be screened in a screening step 202, for example
using a
vibrating screen, in order to remove unwanted material and/or impurities (such
as
residues having a particle size larger than 2 cm), thereby obtaining screened
asbestos tailings. The process 200 includes feeding (or introducing) the feed
material 205 into a first reactor. The asbestos tailings 102 and/or the
screened
asbestos tailings are subjected to a pre-treatment 206 in the first reactor,
and the
pre-treated material obtained is then fed into a second reactor such as a
rotary
kiln 204 for further processing. It is understood that the asbestos tailings
102
and/or the screened asbestos tailings will be generally referred to herein as
feed
material 205. It is therefore understood that the feed material 205 can
include
screened asbestos tailings, asbestos tailings 102 (i.e., asbestos tailings
which
have not been screened) or a combination thereof. In some embodiments, the
second reactor includes a rotary kiln, a shaft furnace, multiple hearth
furnaces or
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a calcining oven. In some embodiments, the second reactor can further include
a
dust collector or a dust-collecting assembly.
In some embodiments, the pre-treatment 206 includes drying the feed material
205, and/or removing dust particles from the feed material 205 in order to
obtain
pre-treated asbestos tailings 208. In the embodiment shown in Figure 2, the
pre-
treatment 206 includes drying the feed material 205 and removing dust
particles
from the feed material 205. In some embodiments, the pre-treatment 206
including drying of the feed material 205 and removing of the dust particles
from
the feed material 205, can be performed in a single processing step. For
example, the first reactor can include a dryer such as a spray dryer, a drum
dryer, a pulse combustion dryer, a freeze dryer or a fluid bed dryer. In some
embodiments, the first reactor can further include a dust collector or a dust-
collecting assembly.
Still referring to Figure 2, in some embodiments, the pretreatment 206 which
includes drying of the feed material 205 and removing of the dust particles
from
the feed material 205, can be performed in a fluidized bed. In other words, in
some embodiments, the first reactor is a fluid bed reactor (also referred to
as a
fluid bed dryer). Hot gas or hot air 207A can be fed into the fluid bed
reactor, and
dust-containing gases 207B can be recovered therefrom. In some scenarios, the
drying step can be performed at a temperature of between 300cC and 500cC. In
some scenarios, the dust particles removed from the feed material 205 have a
particle size of 150 microns or lower.
Still referring to Figure 2, the pre-treated asbestos tailings 208 is fed into
a
second reactor such as rotary kiln 204. The pre-treated asbestos tailings 208
can
then be subjected to a dehydrating step 210 in the second reactor, in order to
obtain dehydrated asbestos tailings 212, and the dehydrated asbestos tailings
212 can be subjected to a calcining step 214 in the second reactor, in order
to
obtain a calcined material 216. In some embodiments, the step of dehydrating
210 the pre-treated asbestos tailings 208 is performed at a temperature
between
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600cC and 900cC, or between 700cC and 900cC. In som e embodiments, the step
of calcining the dehydrated asbestos tailings 214 is performed at a
temperature
between 900cC and 1500cC or between 1200cC and 1500 cC. In some
embodiments, the steps of dehydrating and calcining can be performed in the
5 rotary kiln 204.
It is understood that the term "rotary kiln" as used herein refers to a
pyroprocessing device used to raise the temperature of materials to a high
temperature (such as a calcination temperature) in a continuous process. The
rotary kiln is typically a cylindrical vessel, inclined slightly to the
horizontal and
10 rotatable about its longitudinal axis. When a rotary kiln is used, the
feed material
205 is typically fed into the upper end of the cylindrical vessel and rotation
of the
vessel allows the material to gradually move down towards the lower end of the
vessel. The material may undergo a certain amount of stirring and mixing. It
is
also understood that the rotary kiln can be used in a co-current configuration
(wherein hot gases pass along the kiln in the same direction as the movement
of
the material) or in a counter-current configuration (wherein hot gases pass
along
the kiln in the opposite direction as the movement of the material). The hot
gases
may be generated by an external furnace, or may be generated by a flame inside
the rotary kiln. Different types of fuel can be used, such as natural gas,
oil, coke,
coal, pulverized petroleum coke, pulverized coke, or a combination thereof. In
some embodiments, the rotary kiln is rotating at between 0.75 and 1.25 rpm. In
some embodiments, the asbestos tailings are introduced into the rotary kiln at
a
rate of up to 10 tons per hour, for example between 8 and 10 tons per hour. In
some embodiments, the temperature within the rotary kiln is a temperature
gradient varying from between about 300cC at the fe ed end to about 2500cC
proximate to the flame and/or at the discharge end.
Still referring to Figure 2, in some embodiments, the calcined material 216
can be
further processed in order to obtain a refractory material 224. For example,
the
processing of the calcined material 216 can include crushing the calcined
material 216 in a crushing step 218 to obtain a crushed material 220, and
sieving
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the crushed material 220 in a sieving step 222 in order to obtain the
refractory
material 224. For example, the crushing step 218 can be performed in a
crusher,
such as a jaw crusher, a gyratory crusher, a cone crusher, an impact crusher,
a
mineral sizer or a combination thereof. In some scenarios, the crushing step
218
can allow breaking of the calcined material into smaller pieces, for example
to
obtain a desired average particle size for the pre-treated asbestos tailings
208,
and/or to remove clumps of calcined material which may have formed during one
of the dehydrating and the calcining steps 210, 214.
In some embodiments, the crushed material 220, or the calcined material 216
can be sieved, for example to remove the bigger and/or smaller particles of
the
material, and/or obtain a desired particle size distribution. For example, the
sieving step 222 can include multiple sieving sub-steps to remove selected
particle sizes with each sub-step.
In some embodiments, at least one of the dehydrating step and the calcining
step
includes removing dust particles. For example, dust can be removed from the
rotary kiln during operation. A dust collector can allow collecting dust from
one
end of the rotary kiln in order to recover dust-containing gases 226, which
can be
further processed. In some implementations, the dust particles removed from
the
second reactor have a particle size of 150 microns or less (i.e., 100 Mesh
US).
Now referring to Figure 3, the pre-treatment 206 can be performed using a
fluid
bed dryer. In some embodiments, the feed material 205 is introduced into a
fluid
bed dryer 230. For example, the fluid bed dryer 230 can be a back mix fluid
bed
dryer, or a plug flow fluid bed dryer. Hot gas 232 required for drying can be
generated in a hot gas generator 234, for example by using fuel such as oil,
natural gas, coal or any other suitable fuel available. In some scenarios, the
fuel
can be burned in a combustion chamber 236 using a burner 238, and the hot gas
232 can be conveyed to and introduced into the fluid bed dryer 230 using a fan
240. The hot gas 232 can be introduced in the fluid bed dryer 230 at the
bottom
of the fluid bed dryer 230, through a perforated plate 242. In some
embodiments,
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the perforated plate 242 includes perforations having a size between 1 mm and
5
mm, or between 2.5 mm and 4 mm, or between 2.7 mm and 4 mm. In some
embodiments, the temperature of the hot gas can be between 100`C and 500cC,
or between 300cC and 500cC. In some embodiments, th e hot gas can be
introduced in the fluid bed dryer at a flow rate between 10000 CFM and 20000
CFM, between 13000 CFM and 17000 CFM, or at about 15000 CFM. Typically,
the feed material 205 is conveyed from a feed end 230A of the fluid bed dryer
230 towards a discharge end 230B of the fluid bed dryer 230. In the fluid bed
dryer 230, the feed material 205 comes in contact with the hot gas 232, which
can dry the feed material 205. In some embodiments, the fluid bed dryer 230 is
a
vibrating fluid bed dryer, in which vibrations can cause the feed material 205
to
be displaced from the feed end 230A to the discharge end 230B. In some
implementations, the residence time of the feed material 205 in the fluid bed
reactor is between 3 and 20 minutes, between 5 and 15 minutes, between 5 and
10 minutes, or between Sand 8 minutes. In some scenarios, the combined action
of vibrations and fluidization can ensure uniform drying and obtaining desired
product moisture. In some embodiments, the fluid bed dryer 230 is configured
such that an outlet gas 244 exiting from the fluid bed dryer 230 contains a
certain
amount of dust which was present in the feed material 205 (in such case, the
outlet gas 244 can be referred to as a dust-containing gas). The dust-
containing
gas 244 can be passed through a dust collector 246. For example, the dust
collector 246 can be a bag filter dust collector. The dust collector 246 can
allow
obtaining a dust-free gas 248.
In some embodiments, the pre-treatment includes measuring the humidity of the
feed material 205, and selecting at least one of operational parameters of the
fluid bed dryer (i.e., at least one of the temperature of the hot gas, the
residence
time of the material inside the fluid bed dryer, the flow rate of the hot gas,
and the
size of the perforations of the perforated plates) according to the humidity
of the
feed material. For example, in some scenarios wherein the humidity of the feed
material is between 5% and 25%, the temperature of the hot gas can be set
between 350cC and 450cC or of about 400cC, the perf orations can be selected
to
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have a diameter between 2.7 mm and 4 mm, the flow rate of the hot gas can be
between 13000 CFM and 17000 CFM, and the residence time can be between 5
minutes and 8 minutes.
Now referring to Figure 4, a system for manufacturing refractory materials
from
asbestos tailings 102 is provided, according to an embodiment of the
invention.
In some embodiments, the asbestos tailings 102 can be recovered from or near
former asbestos production sites or facilities and can be conveyed, for
example
using trucks 250, to a container 252. The asbestos tailings 102 can then be
conveyed to a screening device 254, for example using a conveyor belt 255. In
some embodiments, the screening device 254 can include a vibrating screener,
such as a TyrockTm screener. The screened asbestos tailings and/or the
asbestos tailings 102 (generally referred to as feed material 205, as
explained
above), are fed into a fluid bed dryer 230. The feed material 205 is dried in
the
fluid bed dryer 230 and dust is removed from feed material 205 using a dust
collector 246. The dried and dust-lean asbestos tailings (also referred to as
pre-
treated asbestos tailings 208) are then conveyed to be dehydrated and calcined
in rotary kiln 204.
Still referring to Figure 4, it is understood that the pre-treated asbestos
tailings
208 can either be directly fed into the rotary kiln 204, or be first conveyed
to a
feeding container 256 in order to be fed into the rotary kiln 204 at a later
or
desired time. The pre-treated asbestos tailings 208 can be conveyed to the
feeding container 256 using, for example, a conveyor 257. In some
embodiments, the temperature of the feeding container 256 is controlled such
that the temperature of the pre-treated asbestos tailings 208 can remain
substantially equal to the temperature inside the fluid bed dryer 230. In
other
words, the temperature of the pre-treated asbestos tailings 208 can be
controlled
and/or maintained before feeding the pre-treated asbestos tailings 208 into
the
rotary kiln 204.
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Still referring to Figure 4, in some embodiments, the second reactor such as
the
rotary kiln 204 can be provided with a dust collecting assembly 258. It is
understood that the dust collecting assembly 258 can be provided at the feed
end
of the rotary kiln or at the discharge end of the rotary kiln. In the
embodiment
shown in Figure 4, the rotary kiln 204 is in a counter-current configuration,
with
the hot gases flowing from the discharge end of the rotary kiln 204 to the
feed
end of the rotary kiln 204. The dust collecting assembly 258 is therefore
connected to the feed end of the rotary kiln 204. In some embodiments, dust-
containing gas 259 is removed from the rotary kiln 204, and routed through the
dust collecting assembly 258. In some embodiments, the dust-containing gas 259
is introduced into a cyclone 260 for separating larger solid residues present
in the
dust-containing gas 259. The larger solid residues 262 can be recycled back to
the feeding container 256, while cycloned dust-containing gas 264 exists the
cyclone 260 and can then be fed into a dust collector 266, or a series of dust
collectors 266. The dust collectors 266 can separate the dust from the dust
containing gas and a dust-lean or dust-free gas 268 can be recovered or
released.
Now referring to Figure 6A, when the asbestos tailings 102 are directly fed
into
the rotary kiln 204 without being subjected to a pretreatment 206 as shown for
example in Figure 1, the configuration and temperature profile of the rotary
kiln
104 includes a zone which is suitable for the drying of the asbestos tailings.
As
can be seen in Figure 6A, a portion of the rotary kiln 104 is therefore
required in
order to dry the asbestos tailings 102, which diminishes the available surface
area for the dehydrating and calcining steps in the rotary kiln 104.
Now referring to Figure 6B, the pre-treated asbestos tailings 208 are fed into
the
rotary kiln 204. As explained, the pre-treated asbestos tailings 208 are fed
into
the rotary kiln 204 at a high temperature. Therefore, the dehydrating step can
start taking place immediately after the feeding into the rotary kiln 204, and
both
the dehydrating and the calcining steps can be performed using larger surface
areas of the rotary kiln 204. This configuration can allow for the rotary kiln
204 to
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operate at higher rates, and therefore allow obtaining a higher amount of
calcined
material 216 in the same amount of time. In some embodiments, the rotary kiln
204 includes a dehydrating zone where the pre-treated material is dehydrated,
and a calcining zone where the dehydrated material is calcined. A temperature
5
gradient is typically provided inside the rotary kiln 204, as a flame provides
heat
to the rotary kiln 204 from one side of the rotaty kiln. In some
implementations,
the calcining zone extends from the source of the flame, proximate to the
discharge end of the rotary kiln 204, to a point where the temperature is of
about
900t. In some implementations, the dehydrating zon e extends from the feed
10 end of
the rotary kiln 204, to the point where the temperature is of about 900t.
In some implementations, the rotary kiln 204 includes a cooling zone for
cooling
the calcined material. In some scenarios, the residence time of the pre-
treated
asbestos tailings in the dehydration zone is between 2 minutes and 10 minutes,
or between 3 minutes and 5 minutes. In some scenarios, the residence time of
15 the
dehydrated material in the calcining zone is between 10 minutes and 40
minutes, between 10 minutes and 30 minutes, between 10 minutes and 25
minutes or between 15 minutes and 20 minutes.
In some scenarios, using a pre-treatment 206 which includes both drying and
removing dust particles prior to the dehydrating and calcining steps can allow
obtaining a refractory material which includes less material having a particle
size
of 150 pm or lower, with the same crushing steps and sieving steps being
performed on the calcined materials. Obtaining a refractory material including
less material having a particle size of 150 pm or lower can be advantageous
for
various reasons, as explained above. In some scenarios, the refractory
material
obtained according to the invention (i.e., using a process including a pre-
treatment 206 as described herein), includes substantially less material
having a
particle size of 150 pm or lower than a process which does not include a pre-
treatment. In some scenarios, the refractory material obtained according to
the
invention includes 5 wt % or less of material having a particle size of 150 pm
or
lower. In some scenarios, the refractory material obtained according to the
invention includes 2 wt % or less of material having a particle size of 150 pm
or
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lower. In some scenarios, the refractory material obtained according to the
invention includes 1.5 wt % or less of material having a particle size of 150
pm or
lower.
EXAMPLES
Example 1
Referring to Figure 5, experiments were conducted to compare the granulometry
of refractory materials obtained without using a pretreatment (Refractory
material
A in Figure 5) and with a pretreatment (Refractory material B in Figure 5). In
Figure 5, each one of the experimental data points shows the proportion (in
wt%)
of particles of material A or B which is retained by a mesh screen (positive
mesh
values) or which passes through a mesh screen (-140 mesh value).
To obtain Material A, humid asbestos tailings were screened using a TyrockTm
screener in order to remove materials having a size larger than 2 cm, and then
directly introduced into a rotary kiln to be dried, dehydrated and calcined.
The
calcined material obtained was crushed and sieved to obtain refractory
material
A. A dust collector was operated to remove dust particles from the rotary
kiln.
To obtain Material B, humid asbestos tailings were screened using a TyrockTm
screener in order to remove materials having a size larger than 2 cm. The
screened material was introduced into a fluid bed dryer provided with a
perforated plate having perforations between 2.7 mm and 4 mm, with hot air
introduced at a temperature of about 400cC and at a flow rate of about 15000
CFM. Dust particles smaller than about 150 microns were collected from the
fluid
bed dryer using a dust collector. The pre-treated material recovered from the
fluid
bed dryer was introduced at about 400cC into a rota ry kiln to be dehydrated
and
calcined. The temperature of the pre-treated material was maintained at about
400cC between exiting the fluid bed dryer and being introduced into the rotary
kiln. The calcined material obtained was crushed and sieved to obtain
refractory
material B, using the same crushing and sieving conditions as for obtaining
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17
material A. A second dust collector was also operated to remove dust particles
from the rotary kiln, using the same dust-collecting conditions as for
material A.
It has been shown that Material B contains about 1.5 wt% of material having a
particle size smaller than 150 pm, whereas Material A contains about 10 wt% of
material having a size of 150 pm or less. It has therefore been shown that pre-
treating the asbestos tailings prior to introducing the material into the
rotary kiln
can allow for a reduction of the dust particles in the calcined material. It
is
believed that only operating a dust collector to remove dust particles
directly from
the rotary kiln does not allow removing as many dust particles, because the
dust
particles can agglomerate with the bulk of the material during the dehydration
and/or calcination processes.
