Note: Descriptions are shown in the official language in which they were submitted.
CA 02640857 2008-10-09
METHODS AND COMPOSITIONS FOR COATING SULFUR BLOCKS
FIELD OF THE INVENTION
[0001] The invention relates to methods and compositions for coating blocks of
elemental sulfur for storage.
BACKGROUND OF THE INVENTION
[0002] Raw sulfur is obtained in large quantities from several sources
including sour
natural gas processing, heavy oil upgrading and light oil during refining.
[0003] The vast majority of the hydrogen sulphide removed from the sour gas
and oil is
converted into elemental sulfur. Elemental sulfur is at various times, sold
and shipped in
molten liquid form, processed into one of several solid particles for handling
and where
logistics and/or lack of demand dictate, poured and solidified into
aboveground storage
blocks. Molten (liquid) sulfur is red in color and when solid is a bright
yellow color.
[0004] Elemental sulfur may be sold to customers around the world and is used
primarily in the manufacture of sulfuric acid for a variety of uses including
the
manufacture of phosphate fertilizers. A smaller proportion of the sulfur
market/supply is
also used in making products including pharmaceuticals, plastics and rubber.
[0005] Importantly, when the sulfur production rate is higher than the demand
for sulfur,
sulfur is most often stored in blocks near or at a plant site where the sulfur
originated
with the result that permanent or very long-term storage of sulfur in some
locations is
required. At such storage facility sites, sulfur blocks may be as much as 20 m
in height,
200m wide and have lengths greater than 500m. Smaller size blocks may also be
made.
Industry forecasts generally indicate an increasing surplus of sulfur in
traditional markets
such as Canada and Kazakhstan.
[0006] The long-term storage of sulfur blocks as described above poses a
number of
problems as discussed below.
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[0007] One problem is fugitive dust emissions from the degradation of sulfur
blocks by
exposure to the weather, including degradation by sunlight, wind, rain and
atmospheric
freeze/thaw cycles.
[0008] In addition, degradation may occur by various sulfur consuming bacteria
such as
thiobacilli oxidans that is omnipresent in the environment and will naturally
populate
sulfur surfaces. Metabolites from these organisms include sulfuric acid.
[0009] Thus, as a result of these degradation mechanisms, the sulfur itself
and the
various by-products make their way into the surrounding environment, including
the
surrounding soil and ground water leading to various levels and types of
contamination
of the groundwater and the surrounding land. Furthermore, any precipitation
that comes
into contact with the sulfur and that is trapped by the blocks or in a
containment facility
requires testing and treatment prior to release of the contaminated water back
to the
environment.
[0010] Further still, in addition to the environmental issues associated with
sulfur block
degradation, another problem associated with sulfur blocks is that they are
often
perceived as unsightly due to the bright yellow color of the elemental sulfur.
Large piles
of sulfur are often visible from a great distance and as a result, there is a
negative public
perception to the presence of sulfur in or around a community.
[0011] Finally, there is a fire-risk associated with large and dormant piles
of sulfur. While
relatively difficult to ignite, burning sulfur is a highly undesirable and
polluting reaction
that is potentially fatal to people and may be highly contaminating to the
environment
depending on the severity of a fire.
[0012] As a result, there has been a need for a technology to minimize the
above
problems relating to the short, medium and long-term storage of sulfur. More
specifically,
there has been a need for effective coating compositions that can minimize the
risk of
degradation, that have long-term stability when exposed to the weather
elements, that
can enhance the visual appearance of the sulfur blocks and that can minimize
the fire-
risk associated with the long-term storage of sulfur blocks. In addition,
there has been a
need for a coating system that is relatively inexpensive and that can be
readily removed
from the sulfur blocks for ultimate recovery of the sulfur.
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[0013] A review of the prior art indicates that no such coating compositions
have been
created or used. For example, while the following references teach various
coating
compositions, none are directed to inexpensive and effective compositions
described by
the present invention. More specifically, US Patent 4,238,536 discloses a
method of
coating coal with a gel; US Patent 6,344,236 discloses an acrylic copolymer
coating
composition; US Patent 5,330,788 discloses a temporary acrylic polymer or
copolymer
coating composition; US Patent 4,087,572 discloses a method of preventing
environmental erosion with an organic polymer latex and a silicone; and US
Patent
2,854,347 discloses a method of erosion control of mineral products with
rubber
compounds.
SUMMARY OF THE INVENTION
[0014] In accordance with the invention, there is provided a method of sealing
elemental
sulfur against atmospheric exposure comprising the steps of: a) preparing a
coating
composition comprising an effective blend of silicone and aqueous sodium
silicate and;
b) applying the coating composition to the elemental sulfur to create a
continuous barrier
between the elemental sulfur and the atmosphere.
[0015] In various embodiments the silicone:sodium silicate composition is
25:75 (w/w) to
75:25 (w/w), preferably 50:50 (w/w) to 30:70 (w/w) and more preferably 50:50
(w/w).
[0016] The method may further include adding a coloring agent to the coating
composition.
[0017] The invention also provides a composition for sealing elemental sulfur
comprising
an effective blend of silicone and aqueous sodium silicate.
DETAILED DESCRIPTION
[0018] In accordance with the invention, effective sealing and coating
compositions for
coating sulfur blocks are described together with methods of coating sulfur
blocks to
provide an effective and removable weather-resistant barrier to the sulfur
blocks, which
at the same time allows the sulfur to 'breathe'.
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[0019] As described below, preferred compositions for preparing sulfur block
coatings in
accordance with the invention include those that:
a. can be readily applied onto the exterior surfaces of sulfur blocks to
provide an even coating;
b. are heat and cold resistant to and beyond normal atmospheric
temperatures;
c. have good adherence to the sulfur;
d. are resistant to the penetration of water and sulfur
bacteria;
e. allow for gases but not water to pass through the coating composition;
f. can be readily removed from the sulfur blocks;
g. are fire-resistant;
h. are relatively inexpensive; and
i. can be colored.
[0020] Suitable compositions in accordance with the invention include
effective mixtures
of silicone and sodium silicates. In addition, the mixtures may include
appropriate
coloring agents. Neutral browns/greens/blues are preferred colors but any
color is
possible and may be selected in accordance with the invention.
[0021] The practical range of blends of silicone:silicates are 25:75 (w/w) to
75:25 (w/w),
preferably 50:50 (w/w) to 30:70 (w/w) and more preferably 50:50 (w/w). A
preferred
blend of 50:50 wt% silicone:silicates provides effective liquefying/heat-
spraying
properties enabling the use of the composition with conventional spraying
equipment. In
addition, this composition after application onto a sulfur block can be
readily removed by
scraping. Silicone:silicates blends in the range of 50:50 wt% and 30:70 wt%
were
particularly fire-resistant. Coloring agents may be added to the composition
to impart
desired colors. Alternatively, paints may be applied to the exterior of a
cured coating.
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Sodium Silicate
[0022] Sodium silicate is a highly effective sealant. Commercial solutions of
sodium
silicate typically include approximately 40 wt% sodium silicate in water. The
solutions
have the appearance of clear water and can be applied to surfaces as paint.
Silicone
[0023] Silicone, or more specifically polymerized siloxanes or polysiloxanes,
are mixed
inorganic-organic polymers with the general chemical formula [R2Si0], where R
=
organic groups such as methyl, ethyl, and phenyl. These materials consist of
an
inorganic silicon-oxygen backbone (...-Si-O-Si-O-Si-0-...) with organic side
groups
attached to the silicon atoms. In some cases organic side groups can be used
to link two
or more of these -Si-0- backbones together. By varying the -Si-0- chain
lengths, side
groups, and crosslinking, silicones can be synthesized with a wide variety of
properties
and compositions and can vary in consistency from liquid to gel to rubber to
hard plastic.
Curing of silicones is highly variable and is based on the specific chemistry
of the
silicones. One class of silicones is moisture curing silicones that cure upon
exposure to
atmospheric moisture. In accordance with the invention, silicate/silicone
solutions having
sufficient viscosity to enable effective working properties and that result in
a rubbery
cured product while maintaining acceptable non-flammability properties are
within the
scope of the invention as understood by those skilled in the art.
[0024] Compositions in accordance with the invention are prepared by mixing
appropriate volumes of sodium silicate (in water) with the silicone to create
the desired
wt% mixtures in the cured product. The water in the sodium silicate solution
will promote
composition curing (and will be substantially vaporized during curing) over a
time period
sufficient to enable acceptable composition working times. Generally,
compositions in
accordance with the invention will be a balance between the desired properties
of final
cured consistency (flexible rubber to hard rubber), fire-resistance and
working time.
Increasing silicate concentration in the final composition will generally
improve fire
resistance properties and working time but will result in a product that is
more rigid and
less viscous. In contrast, increasing silicone concentration will result in a
more flexible
coating (thereby providing a thicker coating) but that will have decreased
fire resistance.
Increased silicone concentration is also more expensive. Conventional
residential
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silicone caulking materials from a local hardware store were utilized in
preparing test
compositions.
Methods of Application
[0025] In order to effectively apply the silicone:silicates coating
compositions, it is
preferred that the silicone:silicate compositions are applied at a temperature
of at least
C. While with appropriate equipment the compositions may be applied at lower
temperatures, practically temperatures above 5 C are preferred. A texture gun
or an air
spray gun with a wide nose can be used for spraying the silicone and sodium
silicate
solution in addition to brushing equipment.
Surface Preparation
[0026] The coating compositions should be applied to clean surfaces. If
necessary, the
outer surfaces of the sulfur blocks should be cleaned of debris such as dust
and dirt and
should be dry.
Application
[0027] The silicone:silicate solutions may be applied by spraying, rolling,
brushing or the
like. Multiple coats of the solution may be applied and will usually require
approximately
2 hours drying time between applications depending on the atmospheric
conditions.
[0028] Silicone:silicate compositions may be applied by appropriate spraying
equipment
that may heat the composition to promote curing of the composition. Brushing
or rolling
of a silicone:silicates composition may also be used.
[0029] Ideally, thicknesses in the range of 1-5 mm and preferably 3mm will
provide both
satisfactory coating and performance properties at a reasonable price.
Examples
[0030) A mixture of silicone: silicates
was
prepared to a final concentration of 50:50 (silicone:silicates) wt%. The
liquid mixture
was applied to a sulfur block that had been cleared of surface debris by
brushing. The
semi-solid mixture (having a rubbery texture) was applied by brush to both the
vertical
and horizontal surfaces and fully solidified on the sulfur block within
approximately 12
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hours. No significant dripping or creep of the mixture was observed on the
vertical
surfaces. Within 24 hours, the coating had the appearance of a hard stone-like
surface.
The coating was flexible and could be broken by hand with difficulty. Small
sulfur blocks
(a few kilograms each) coated with the coating composition as described above
are
hereinafter referred to as control blocks.
[0031] Other blocks were prepared with 25:75 wt% (silicone: silicates). The
composition
prior to coating showed acceptable working viscosity and resulted in a harder
but thinner
coated surface after curing. A further set of blocks were prepared with 75:25
wt%
(silicone: silicates). The composition showed acceptable working viscosity and
resulted
in a more rubbery surface after curing.
Burn Test
[0032] Control blocks were subjected to direct heating of the coating with an
open flame
placed against the coating. This test showed insignificant combustion when the
open
flame was directly against the coating. The coating would quickly self-
extinguish when
the open flame was removed. The surface temperature of the coating during this
test
was 1380 C.
[0033] The coating could subsequently be scraped from the sulfur blocks with a
scraping tool and brushed from the control blocks for disposal.
Freeze/Thaw Cycle Test
[0034] In this test, control blocks were a) immersed in water for 24 hours; b)
towel-dried
and placed in a -20 C freezer for 16 hours and c) stored at 20 C for 24
hours. The
above cycle was repeated 10 times. Mass change data was collected as well as
photographic data.
[0035] These tests showed that after 10 cycles, the coating on the control
blocks did not
crack. In some samples, the underlying sulfur block would crumble but the
coating
remained intact.
Sunlight/Heat Test
[0036] Control blocks were placed in an enclosure and subjected to full
spectrum visible
light and an ultraviolet source. The enclosure was temperature and humidity
controlled
cycling between 90 C and 100% humidity to 0 C and 20% humidity over a 7 day
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period. After recovery, the control blocks were immersed in water to measure
any mass
change.
[0037] These tests showed that the control blocks had satisfactory
sunlight/ultraviolet
resistance as no appreciable change in the coating was observed at the
conclusion of
this test.
Abrasion Test
[0038] Control blocks were subjected to a stream of blown sand onto one side
of the
blocks simulating sand-carrying wind up to 50km/hour.
[0039] These tests showed that the control blocks had satisfactory abrasion
resistance
as no appreciable degradation in the coating was observed during the test.
Specifically,
sand particles were observed bouncing off the coating without eroding the
coating
surface.
[0040] While the present invention has been described and illustrated with
respect to
preferred embodiments and preferred uses thereof, it is not to be so limited
since
modifications and changes can be made therein which are within the full,
intended scope
of the invention as understood by those skilled in the art.
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