The present invention relates to a method of dusting-preventive treatment
of powder such as cement that tends to scatter dust upon handling or during transportation
As a technique for prevention of dusting of powder such as cement
that tends to scatter dust upon handling or during transportation or storage,
a method of adding a fibril-forming polytetrafluoroethylene (hereinafter polytetrafluoroethylene
is simply referred to as PTFE) to the powder having dusting property, stirring
them to exert compression/shear force so as to form PTFE fibril from fibril-forming
PTFE thereby suppressing dusting has been developed.
The dusting-preventive treating technique described above is disclosed,
for example, in Japanese Published Examined Patent Application No. 24872/1993 (JP-H05-24872
B4). When a fibril-forming PTFE is added to a cement powder and fibril-forming
treatment of exerting compression/shear force is applied by using the technique
disclosed in this patent publication, since the cement powder is in a state of
being covered and agglomerated in cobweb-like nets of PTFE fibrils, a dusting-preventive
cement with no scattering of cement powder in atmospheric air can be obtained.
Further, Japanese Published Examined Patent Application No. 32877/1977
(JP-S52-32877-B4) discloses a technique of mixing a fibril-forming PTFE with a
powdery material and exerting compression/shear force to the mixture at a temperature
in the range of 20 to 200°C, thereby forming PTFE fibril and suppressing dusting
of the powdery material.
As described above, occurrence of dusts can be prevented by applying
the dusting-preventive treatment to the cement powder or the like based on the
technique as disclosed in Japanese Published Examined Patent Application No. 24872/1993
and Japanese Published Examined Patent Application No. 32877/1977.
The methods of preventing dusting by the nets of the PTFE fibrils
described above involve a drawback in that strong air pressure, suction force
or shearing force exerts on the dusting-preventive powdery product when it is transported
under pressure or transported under suction by a pneumatic transportation apparatus
used generally for powder transportation or when transported at a high speed by
a screw conveyor or a rotary feeder at high speed rotation as a powder transportation
method, the PTFE fibrils is partially broken or a portion of powdery particles
is detached from the nets of the PTFE fibrils to remarkably lower the dusting-preventive
performance. Accordingly, in the transportation step of the dusting-preventive
powder products in a production site has a problem that it is inevitable to use
a transportation machine not exerting strong external force to the product such
as a belt conveyor and transportation of the product undergoes restriction to some
extent in view of the apparatus and in view of the quantity. In addition, even
when the dust-forming powder deteriorated with the dusting-preventive performance
is stirred at a temperature of 20 to 200°C again to exert compression/shear force,
the dusting-preventive performance can not be recovered. Accordingly, for the
recovery of the dusting-preventive performance, it is necessary for again adding
an appropriate amount of fibril-forming PTFE and stirring them at a temperature
in the range of 20 to 200°C to exert compression/shear force.
A subject intended to be solved in accordance with this invention
is to provide a dusting-preventive treating method capable of advantageously recovering
or improving the dusting-preventive performance of the dust-forming powder whose
dusting-preventive performance has been lowered during transportation, by a relatively
simple step and with economical advantages.
Disclosure of the Invention
A method of dusting-preventive treatment of powder having dusting
property according to the present invention comprises: a first dusting-preventive
treatment step in which fibril-forming polytetrafluoroethylene is added to a powder
having dusting property, and a compression/shear force is exerted to the mixture
at a temperature in the range of 20 to 200°C to thereby regulate powder so as to
have a flow value of 150 mm to 200 mm; and a second dusting-preventive treatment
step in which a compression/shear force is exerted again at a temperature in the
range of 50 to 150°C, without newly adding fibril-forming polytetrafluoroethylene,
to the resultant powder after the powder treated in the first step has deteriorated
in the dusting-preventive performance during transportation with transporting means
due to such an external force that the polytetrafluoroethylene fibrils partly cleave
or powder particles partly fall off the polytetrafluoroethylene fibril nets.
Brief Description of the Drawings
Best Mode for Carrying Out the Invention
- Fig. 1 is a graph showing a relation between compression/shear force (amount
of work) in the dusting-preventive treatment and a dusting-preventive performance;
Fig. 2 is a graph showing a relation between the flow value and a dusting-preventive
performance of a powder; and Fig. 3 is an explanatory view for the steps showing
a dusting-preventive treating method of a preferred embodiment.
The flow value is an index that usually indicates the softness of
mortar and the measuring method is provided in JIS R5201. In this invention, the
flow value measured by the same method for the cement put to a dusting-preventive
treatment is adopted as an index that indicates the degree of fibrillation of PTFE.
In the measuring method, a flow cone having 100 mm lower inner diameter, 70 mm
upper inner diameter and 60 mm height is set at the center of a flow table of 300
mm diameter and a specimen is filled inside of the flow cone. After removing the
flow cone, the specimen is dropped together with the flow table from the height
of 100 mm. This procedure is repeated by ten times (at a rate of once per one sec)
and the diameter of the specimen after spreading is measured and defined as a
flow value. As shown in Fig. 2, when the dusting amount of the powder is small
and the dusting-preventive performance is high, the fluidity of the powder is low
and the flow value decreases.
In the dusting-preventive treatment for the dust-forming powder by
a fibril-forming PTFE, when the fibril-forming PTFE is added to the dust-forming
powder, and compression/shear force is exerted at a temperature of 20 to 200°C,
dispersion of the fibril-forming PTFE to the dust-forming powder and initial formation
of fibrils from the fibril-forming PTFE particles occur at the initial stage. Then,
fibrils develop rapidly and the dusting of the powder subsides rapidly into a plateau
state. Then, when compression/shear force are further exerted, the dusting-preventive
performance is lowered, finally, to reach a stage where re-dusting is observed.
Fig. 1 shows the process schematically.
The particles of the fibril-forming PTFE are dispersed into the dust-forming
powder at an appropriate temperature and fibrillated into fibrils under the compression/shear
force as described above (region A in Fig. 1). When compression/shear force further
exerts, fibrils grow and trap the dust-forming powder particles to improve the
dusting-preventive performance (region B in Fig. 1). Growing of fibrils corresponds
to increase of the aspect ratio of the fibril (fibril length/cross sectional area)
by elongation of the fibril by the compression/shear force. When the fibril is
elongated continuously as it is, its diameter is reduced excessively and the fibril
is broken to loss the performance of trapping the dust-forming powder. With a micro
point of view, not all fibril-forming PTFE particles cause fibrillation simultaneously
but some particles start fibrillation just after the start of compression/shear
force while other particles are fibrillated at a delay. Then, when compression/shear
force continue further, breakage of fibrils formed previously and growing of fibrils
occur concurrently. The equilibrium state between them is a plateau state (region
C in Fig. 1). Then, when compression/shear force further exert continuously, breakage
of fibrils becomes predominant and, finally, it reaches a stage where re-dusting
is observed (region D in Fig. 1).
In the dusting-preventive treatment for the dust-forming powder by
the fibril-forming PTFE, it is considered that the state of the region C shown
in Fig. 1 provides a high dusting-preventive effect and economical advantage. However,
in a case where the dusting-preventive performance is lowered under the effect
of external force, for example, during transportation, the dusting-preventive performance
can not be recovered even by stirring again at a temperature in the range of 20
to 200°C and provision compression/shear force.
According to this invention, even when an external force exerts, for
example, during transportation of dusting-preventive powder products to cause
partial breakage of PTFE fibrils or to cause partial detachment of the powder particles
from the nets of PTFE fibrils, the dust-forming powder can be made dusting-preventive
by way of a second dusting-preventive treatment step of fibril-forming or re-fibril-forming
of not yet fibrillated PTFE or partially fibrillated PTFE, which is mixed and dispersed
in the dust-forming powder, with no further additional charge of PTFE. Further,
upon production site transportation of powder products such as shipping or bulk
loading on trucks, transportation and dusting-preventive treatment of applying
compression/shear force can be exerted simultaneously, for example, by the use
of a heating screw conveyor to provide an advantage of not requiring additional
step for the dusting-preventive treatment.
PTFE used herein is a material that can be fibrillated, manufactured
by emulsion polymerization as described in US Patent No. 2559752, for which either
an emulsion type or powder type PTFE can be used.
Control for the flow value in the first dusting-preventive treatment
step means control for the extent of fibrillation of PTFE, which is conducted
generally by adjusting the compression/shear force to PTFE although details are
different depending on the addition amount of PTFE, the kind of the dust-forming
powder and the treatment temperature. Specifically, it can be attained by controlling
the speed of stirring blades and the stirring time, for example, of a mixer.
In a case when the dusting-preventive treatment for the dust-forming
powder by fibrillation of PTFE is conducted only for once, it is optimal to set
the state near the plateau state (region C in Fig. 1) in view of the dusting-preventive
effect and utilization efficiency of the fibril-forming PTFE and the flow value
in this case is less than 150 mm, although different somewhat depending on the
kind of the powder.
It is necessary that PTFE fibrils capable of recovering dusting-preventive
performance in the second dusting-preventive treatment step after transported
by a transportation apparatus such as a transportation machine should remain within
a relatively small aspect ratio, which corresponds to the region B in Fig. 1. The
flow value in this case is within a range from 150 mm to 200 mm. On the other hand,
the treated product in the region A of Fig. 1 at the first dusting-preventive treatment
step, that is, a treated product processed in a short stirring time where PTFE
fibrils are not yet grown can not recover the dusting-preventive performance in
the second dusting-preventive treatment step. Further, the product in the region
C of Fig. 1, that is, a treated product for which the dusting-preventive effect
by fibrillation of PTFE is in the plateau state or the treated product in the region
D of Fig. 1, that is, a treated product in which PTFE fibrils are broken being
processed for a long stirring time can neither recover the dusting-preventive performance
by the second dusting-preventive treatment step.
The temperature condition for exerting compression/shear force in
the second dusting-preventive treatment step is 50 to 150°C. According to the
result of the test to be described later, no recovery or improvement in the dusting-preventive
performance can be observed at a temperature lower than 50°C, whereas deterioration
of the quality of the dust-forming powder such as false set of cement is observed
at a temperature higher than 150°C.
The addition amount of PTFE in the first dusting-preventive treatment
step is preferably from 0.01 to 0.5 parts by weight based on 100 parts by weight
of the dust-forming powder. When the content of PTFE is less than 0.01 parts by
weight, no sufficient dusting-preventive performance can be obtained. On the other
hand, when it is more than 0.5 parts by weight, it results in a phenomenon that
the dust-forming powder is set into a marshmallow-like state, in the first dusting-preventive
treatment step. Further, identical or different kind of dust-forming powder may
be added in the second dusting-preventive treatment step. In this case, it is preferred
that PTFE is added somewhat in a great amount in the first dusting-preventive
The method according to this invention is applicable to any of dust-forming
powders and the most preferred are those powders to be produced in a great amount
and transported by a great amount, for example, on ships, wagons or automobiles
and they can include, for example, cement powder, hydrated lime powder, quicklime
powder, calcium carbonate powder, slag powder, fly ash, talc and gypsum.
A preferred embodiment of this invention is to be described with
reference to the drawings.
Fig. 3 is an explanatory step views of applying the dusting-preventive
method according to this invention to portland cement produced in a cement plant.
In the production step for portland cement, clinker after baking is
rapidly cooled with air, incorporated with additives such as gypsum and then pulverized
in a finishing mill to control the grain size. Cement in a hopper 10 just after
the processing has residual heat at about 100°C. When fibril-forming PTFE is added
from a PTFE hopper 11 to the cement 10 at high temperature and compression/shear
force are given by mixing and stirring using a mixer 13, fibrillation of PTFE proceeds
and a great number of the cement particles are trapped in the nets of the fibrils
of the partially fibrillated PTFE to obtain dusting-preventive cement.
The optimal amount of the fibril-forming PTFE added in this process
depends on various conditions such as the dusting-preventive treating temperature,
the structure of a mixer giving compression/shear force and the addition method.
In this embodiment, 0.03 parts by weight of the fibril-forming PTFE is added based
on 100 parts by weight of the cement.
In this embodiment, an emulsion type product under the commercial
name of "Teflon (registered trademark) K-20" is used for the fibril-forming PTFE,
but a powder type product under the commercial name of "Teflon (registered trademark)
K-10" may also be used.
Further, cement having a residual heat at 100°C is used in this embodiment,
but for the cement product with lowered temperature, it may be adopted a method
of adding quicklime in a hopper 14 and water in a hopper 12 to the mixer 13 and
elevating the temperature by the heat of hydrating reaction, or a method of elevating
the temperature by a heating device.
In this embodiment, compression/shear force are exerted by using
the mixer 13 in the first dusting-preventive treatment step. Since the adaptability
of the mixer is different depending on the kind of the cement powder, the form
of PTFE, the method of temperature elevation and the amount of water added, it
is preferred to select a mixer having a. good fibril-forming efficiency, among
pan type mixer, two axes pug mill type mixer, roller type mixer, for example, based
on the result of a preliminary test. In a case where the roller type mixer is used,
since the cement powder can efficiently undergo a composite effect of compression/shear
and rolling between the roller and the bottom surface of the mixer, this is suitable
to the dusting-preventive treatment of cement which is intended to reduce the
addition amount of water as less as possible.
The dusting-preventive cement produced in the mixer 13 by the step
described above is contained by way of a chain conveyor and bucket elevator 15
into a product silo 16 and then transported by way of a pressure transportation
pipe 17 by pneumatic transportation and loaded on a cement tanker 18 or a cement
transportation vehicle 19, and transported under bulk loading on the ship or the
vehicle to a destination and then transported again through the pressure transportation
pipe 20 by pneumatic transportation to a product silo 21 at a relay station.
Subsequently, the dusting-preventive cement is sent from the product
silo 21 by way of a chain conveyor and a belt conveyor 22, exerted with compression/shear
force under heating by a heating screw conveyor 23 as the second dusting-preventive
treatment step to recover the dusting-preventive performance, and then transported
in a state not packed as it is or packed each by a predetermined amount in a bag,
and then transported by way of a cement transportation vehicle 24 to a destination.
By pneumatic transportation and high speed transportation for several times in
the course from the production of dusting-preventive cement to shipping of the
products, PTFE fibrils are partially broken or a portion of powder particles is
detached from the nets of the PTFE fibrils to bring about a phenomenon of remarkably
lowering the dusting-preventive performance. However, since the PTFE contained
in the dusting-preventive cement is re-fibrillated by heating and compression/shear
force in the second dusting-preventive treatment step by the heating screw conveyor
23 before shipping, to trap the detached cement particles again by the nets of
the fibrils, the dusting-preventive performance of the dusting-preventive cement
is recovered or improved.
In this embodiment, description has been made to a case where dusting-preventive
cement treated by the first dusting-preventive treatment step in a cement plant
as a place of generating dust-forming powder is transported by way of a relay station,
the products are sometimes transported directly under bulk loading from the place
of production to a place of consumption of a large quantity, for example, a dam
construction site. In such a case, the second dusting-preventive treatment may
be conducted at the dam construction site as a place of use.
As described above, in this embodiment, the fibril-forming PTFE is
added to a cement having residual heat at about 100°C and compression/shear force
are exerted in the first dusting-preventive treatment step in the mixer 13 to treat
the dusting-preventive cement in which cement particles are trapped by the nets
of the fibrils of the fibril-forming PTFE such that the flow value is within a
range from 150 mm to 200 mm. Subsequently, while the dusting-preventive performance
of the cement may sometimes be lowered during transportation by the external force
given to the treated products during short range transportation, this embodiment
can provide an effect of recovering or improving the dusting-preventive performance
of the cement by heating and compression/shear force in the second dusting-preventive
treatment step in the heating screw conveyor 23 which re-fibrillate the PTFE.
Further, this invention can provide another effect of recovering and
improving the dusting-preventive performance of the dusting-preventive cement
during transportation process with no additional charge of PTFE and with no additional
provision of a dusting-preventive treatment facility. This means that the performance
of the dusting-preventive cement can be recovered simply even in a construction
site, for example, by merely attaching a heating device to the screw conveyor.
In this embodiment, description has been made to an example of applying
the dusting-preventive treatment method according to this invention to the dust-forming
cement but the invention is not restricted only thereto but is applicable generally
also to other dust-forming powder.
[Comparative Example 1]
While stirring cement having a residual heat at about 100°C in the
mixer 13 for applying the dusting-preventive treatment, a fibril-forming PTFE
emulsion was added for treatment at a ratio of 0.03 parts by weight of PTFE solids
based on 100 parts by weight of cement to form a dusting-preventive cement having
a flow value of 150 mm, which was transported by the chain conveyor and the bucket
elevator 15 and contained in the product silo 16 and then transported by pneumatic
transportation by way of the pressure transportation pipe 17 to the cement transportation
vehicle 19. Then, the products were re-transported again by pneumatic transportation
from the cement transportation vehicle 19 by way of the pressure transportation
pipe 20 to another product silo 21. Powder was sampled at five positions in this
process and each dusting amount was measured by a digital powdery dust meter. The
result of the measurement is shown in table 1.
Place for measurement
Dusting amount (cpm)
Flow value (mm)
Cement in hopper 10
Cement discharged from mixer 13
Cement discharged from product silo 16
Cement discharged from cement transportation vehicle 19
Cement discharged from product silo 21
As can be seen from Table 1, the cement as dust-forming powder applied
with the dusting-preventive treatment and decreased with the dust forming amount
increases the dust forming amount by being put under transportation process such
as a chain conveyor and pneumatic transportation to remarkably deteriorate the
dusting-preventive performance of the dusting-preventive cement. This shows that
the fibrils were partially broken or a portion of cement particles was detached
from the nets of the PTFE fibrils.
While stirring cement kept at a temperature of 80°C by a roller type
mixer having a heating and temperature keeping device, a fibril-forming PTFE emulsion
was added for processing at a ratio of 0.03 parts by weight of PTFE solids based
on 100 parts by weight of the cement, and the dusting-preventive performance was
compared while varying the treating time. The result is shown in Table 2. For the
evaluation of the dusting-preventive performance, the dusting amount was indicated
as "AA" in a case of less than 50 cpm, as "BB" in a case of 50 to 150 cpm and as
"CC" in a case of 150 cpm over.
Treating time (min)
Flow value (mm)
Dusting amount (cpm)
The treating time of about 0 to 2 min corresponds to the region A,
about 3 to 5 min corresponds to the region B and about 7 to 13 min corresponds
to the region C and more treating time corresponds to the region D in Fig. 1.
When the temperature of the specimen for each of the treating times
was lowered to a room temperature and external force was given to the PTFE fibrils
by stirring for 10 min in a pan type forced pug mixer, fibrils were broken to form
dust-forming powder. Subsequently, the specimen returned to the dust-forming powder
was stirred while being kept at a temperature of 80°C for 10 min to give compression/shear
force by a roller type mixer having a heating and temperature keeping device and
the dusting amount of each specimen was measured. The result is shown in Table
First dusting-preventive treatment step
Specimen undergoing external force
Second dusting-preventive treatment step
Treatment time (min)
Dusting amount (CPM)
Dusting amount (CPM)
Dusting amount (CPM)
As can be seen from Table 3, it was possible to recover the dusting-preventive
performance by the second dusting-preventive treatment step only for the specimen
treated in the first dusting-preventive treatment step such that the flow value
was within a range from 150 mm to 200 mm, that is, only for the specimen put under
the treatment for the time of 3 min and 5 min corresponding to the region B shown
in Fig. 1.
When a dusting-preventive cement having a flow value of 160 mm produced
in Example 1 with the treatment time of 5 min in the first dusting-preventive treatment
step was stirred for 10 min in a pan type forced pug mixer with the temperature
being lowered to a room temperature to give external force to the PTFE fibrils,
the fibrils were broken to form a dust-forming powder. Subsequently, the specimen
returned to the dust-forming powder was stirred for 10 min in a roller type mixer
having a heating and temperature keeping device as a second dusting-preventive
treatment step to exert compression/shear force again, and the dusting amount for
each specimen was measured. In this case, the heating temperature was set to 10°C,
20°C, 50°C, 75°C, 100°C, 150°C, 200°C and 250°C, the dusting-preventive performance
and false set of eight types of dusting-preventive cement treated at each temperature
were investigated, and the result of the overall evaluation is shown in Table 4.
The false set was expressed by the indentation of a Vicat probe as "AA" for less
than 6 to 10 mm, as "BB" for 10 to 15 mm and as "CC" for 15 mm over, while the
overall evaluation was expressed as "AA" for very much excellent, "BB" for excellent
and "CC" for existent level.
Re-heating temperature (°C)
As can be seen from Table 4, in a case where the heating temperature
was lower than 50°C, no recovery was observed for the dusting-preventive performance
which was lowered after the dusting-preventive treatment. While false set of cement
occurred in a case where the heating temperature is higher than 150°C. Thus, it
has been confirmed that 50 to 150°C is an optimal range for the treatment temperature
in the second dusting-preventive treatment step to the cement applied with the
first dusting-preventive treatment.
After the dusting-preventive treatment by the fibril-forming PTFE,
the dusting-preventive performance of the dust-forming powder the dusting-preventive
performance of which is lowered during transportation can be recovered or improved
by a relatively simple step and with an economical advantage.