FIELD OF THE INVENTION
The present invention relates to a method of producing alumina grinding media
balls for coal pulverization in thermal plant. These balls are produced by cold iso-
static press and sintered at 1570-1650°C preferably in the range of 1580-1620°C
using both microwave furnace and in conventional gas fired kiln.
BACKGROUND OF THE INVENTION
Coal pulverization in thermal power plant is normally done in tube/bowl mill. The
process and the associated equipment is very costly because of high power
consumption and high consumption of grinding media. Due to the poor quality of
Indian coal, the grinding balls need rapid replacement as they wear out fast.
Use of wear resistance medium (e.g. manganese steel or high chrome steel)
makes the process more costly because of large power consumption and high
abrasion of the medium.
In a tube mill, primarily crushed coal having average size range of 25mm is
introduced through a screw conveyor into a rotating tube mill lined with
replaceable liners and filled with toughened, impact and wear resistance steel
balls. The balls have the function of crushing and grinding the coal by their
kinematics motion within the mill.
SUMMARY OF THE INVENTION
It is known that the impact energy consists of medium volume, medium density
and impact velocity. These parameters control lift, stagnation and breakage.
Impact energy plays a very important role to initiate crack nucleation. The
breakage at a constant impact increases with impact velocity. The critical level of
impact energy is dependent on the velocity and not the mass which controls
breakages leading to reducing the power consumption. Accordingly, the present
inventors recognized that the ball mass can be reduced by reducing the density
of the ball, so that in pulverizing mill the impact velocity is high. It is possible to
reduce the density of grinding media without greatly sacrificing breakage
efficiency, but gaining significantly in reducing power consumption:
According to the invention, use of high wear resistance alumina grinding media
substantially reduces the power consumption in the mill. The balls are not only
wear resistant but are significantly lighter than the prior art steel balls. The
ceramic grinding media reduces power consumption as most of the energy
regained to run the pulverizer is consumed in lifting the medium to a height
which ensures the maximum potential energy of the medium prior to free fall.
According to the prior art, ball to the mill diameter ratio of 0.01 to 0.02 is
maintained. At this ratio, it is established that the impact energy is very small
but impact velocity is very high. Effective pulverization of coal is to generate
stress wave velocity which dominated the process. Since it is not an energy
controlled process, a large velocity obtained from the balls should be the
configuration of maximize grinding efficiency.
The development of ceramic grinding media has been of interest worldwide for a
long time. US 3486706 teaches dense water impervious ceramic dispersing
media for small ball mills formed from hard ceramic particles such as alumina
bonded together with vitrified feldspar or the like formed on palletizing disc and
fired in a rotary furnace. US Patent 4,430,279 (1984) teaches a process
comprising two step fabrication process involving hot isostatic pressing as a final
step to produce yttria stabilized zirconia grinding media in the preparation of
magnetic disc coating. US Patent 5,502,012 (1996) describes a method of
producing yttria stabilized zirconia based fused ceramic beads, and application to
grinding and to dispersion in wet medium. Further, US Patent 7,487,928 (2009)
disclosed a method of producing ceramic grinding balls made of fritted ceramic
comprising a compound of alumina silicate and in particular mullite, zirconia and
alumina. The method is based on granulation process of obtaining ceramic balls.
The processes described are mostly not suitable for producing dense ceramic
balls as the pressure applied in such processes are very small. The technology of
isostatic pressing is thus prominent in preparing ceramic grinding media from
powders.
Cold isostatic pressing (CIP) is a known material processing technique in which
high pressure is applied to ceramic powder in a sealed elastomer container
shaped for the application. The powder is converted from a loose aggregate into
a partially dense compact that has sufficient green strength to permit careful
handling and transfer to the following process operation. There are two
variations of the CIP process depending on the way the elastomer mold is
utilized. In the "dry-bag" process, the elastomer mould is fixed to the pressure
vessel and the equipment is designed for high production rate of small parts
more typically in ceramics than in metals. In the "wet-bag" process, the mould is
filled outside the vessel, transferred in, pressure compacted, and removed. In
this process, multiple molds can be processed simultaneously according to the
size of the pressure vessel.
One of the first patent using this technology is the fabrication of spark plug
insulators (US Patent 362926 in 1920). The technology has further been
established as a fabrication tool for producing variety of ceramic components as
described in many US Patents such as 3,59,1903 (1971), 3,698,843 (1972),
4,330,251 (1982), 5,221,542 (1993), 5,244,623 (1993) and 8,062,200 (2009).
Further, the cold isostatic pressing has been used for making grinding media in
laboratory scale using wet bag method and that in continuous scale using dry
bag method. Though several processes are available for making balls using
isostatic pressing, none of them are suitable for coal grinding application. US
2005 0200035 (2005), describes a method for making spheres from a multi-
carbide material, comprising the steps of: providing fine particles of said multi-
carbide material; admixing said fine particles of said multi-carbide to form a
mixture; binding said fine particles of said multi-carbide material using an in inert
binding agent; subdividing said bound mixture into a plurality of aggregates each
having mass approximately equal to that of a desired sphere to be formed;
applying heat to said plurality of aggregates sufficient to cause said fine particles
of said multi-carbide material to fuse into said spheres; and cooling said fused
spheres such that said fused spheres retain their spherical shape below their
melting point. The heat treatment of such balls were attempted with a thermal
plasma torch, a vertical thermal tube, melt atomization and arc melting methods.
Further, it is known that hot isostatic pressing is used to form balls of high
performance ceramics like in bearings with high value of the product. Materials
like zirconia, silicon nitride etc. have been manufactured by hot isostatic pressing
followed by precision machining to obtain the final products (US Patent 7029623,
20090267257, 6696376). However, these production processes are very
expensive and cannot be used for conventional grinding media manufacturing.
Ceramic grinding media are typically used in ceramic industry, paint industry,
pharmaceuticals industry etc. for producing fine powders and slurries. These
materials are hard, abrasion resistant and much lighter in weight than steel balls,
and inert to many chemicals. The grinding media are available in different sizes
and are generally manufactured using cold isostatic pressing depending on the
requirements. The fabrication of such equipment are capital intensive and has to
be imported. Further, running continuously such equipment is not cost effective
since the spare parts are also to be imported. Therefore, an urgent need arises
to indigenously design and fabricate a cold isostatic press capable of producing
uniform pressure for fabricating grinding media at room temperature using
ceramic in particular alumina powders for developing high performance product
which can be used for coal grinding applications.
The ceramic grinding media are generally fired at a very high temperature. The
cycle time is high and not cost competitive. In order to address this issue, the
alumina grinding media developed in this invention was fired in pure microwave
kiln with volumetric heating technology resulting in better fired product which
withstands high impacts. Further, these balls were also fired in batch kilns at
high temperature.
The steel balls used in prior art are prone to rapid wear with abrasive coal. The
concept of introducing ceramic grinding media in coal grinding was new. Attempt
has been made by American Cyanamid Company (US Patent No. 751,458) to
overcome the problem of high wear and also to prevent contamination by steel
balls which is detrimental to the ground product. The reference suggested a
grinding medium for tumbling mills comprising a core of lead and outer wearing
surface comprising a hard ceramics with an average specific gravity being 7.
However this type of balls pose manufacturing difficulties of breakage problems.
Moreover having specific gravity more than 7 does not reduce power
consumption appreciably from the existing practice of steel balls. In an Indian
Patent 237499 (2009) jointly applied by BHEL and IISc, Bangalore, it has been
pointed out from the theoretical calculations that ceramic balls can be used for
coal grinding applications. However, it was observed that the balls available in
the market are not suitable (table 1) for such stringent applications from both
technological and economical point of view. As a result of which an indigenous
method of developing a system capable of the application of equal hydrostatic
pressure technology on the product needs to be developed.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention to propose a process to produce low
mass alumina grinding media ball to achieve uniform compaction during coal
pulverization.
Another object of the present invention is to propose a process to produce low
mass alumina grinding media ball to achieve uniform compaction during coal
pulverization, which allows the balls to have high impact and wear resistant
properties by appropriate densification of these balls at 1570-1650°C preferably
in the range of 1580-1620°C both by conventional and microwave sintering
route.
A further object of the invention is to propose a process to produce low mass
alumina grinding media ball to achieve uniform compaction during coal
pulverization, which uses the equal hydrostatic pressure technology.
According to the invention, at least 60-120 balls /h preferably 80-100 balls /h of
sizes in the range of 20-80 mm in diameter preferably in 30-50 mm diameter
range of alumina balls can be produced. The two important parameters like wear
resistance and impact resistance of such products were studied. Further, the
wear resistance of the product was confirmed through coal grinding experiments
conducted in the laboratory for coal pulverization. It was noted that more than
8,000 impacts can be achieved by the balls which is very essential in coal
pulverization process.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 Schematic of alumina ball manufacturing process according to the
invention.
Figure 2 Schematic of a mould assembly.
Figure 3 Schematic of microwave fired ball used for micro-hardness
measurement.
Table 1. Mechanical and wear properties of fired alumina balls obtained from
different sources.
DETAIL DESCRIPTION OF THE INVENTION
The present invention is based on manufacturing of alumina grinding media bails
by application of equal hydrostatic pressure technology for coal pulverization by
tube mill in thermal power station using ready to press alumina powder
consisting of alumina exceeding 95% and a mixture of other compounds like
CaO, SiO2 and MgO of total 4-5 % on the fired basis. The ready to mix powders
are having granules in the size range of 25-40 µm with loss on ignition in the
range of 3.4-3.8%.
Standardised powder was fed manually to the moulds to fabricate alumina
grinding balls in the diameter range of 30-50 mm using equal hydrostatic
pressure in the range of 300-450 Kgs/ cm2 resulting the green densities in the
range of 2.34-2.38. Figure 1 is a schematic of the process from raw materials to
final product development. The primary and secondary moulds formed part of
the system (Figure 2). Such dual mould type helps in forming the required shape
of the product using inner mould (1) and making the mould damage resistant
with the outer covering (2). Four cavity moulds are in place in the system in
order to produce an output of at least 60-120 balls /h preferably 80-100 balls /h.
A soft metal alloy fitted with the rubber forms the top cover of the mould (3)
which seals the bottom rubber mould by a simple locking device (4). Releasing of
balls is performed by the use of compressed air. The semi-automatic system is
capable of performing both feeding and releasing operation from the top side
which is advantageous during operation. The indigenization of this machine with
improved features has drastically reduced the cost of such machines as
compared to similar systems available elsewhere in the world.
The alumina balls thus produced were sintered in a temperature range of 1580-
1620°C both in conventional gas fired batch kiln and in a pure microwave kiln. A
30 kW Microwave sintering system was used for firing 50-55 Kgs batch alumina
balls at a temperature range of 1590-1610°C. The application of microwave
energy allows volumetric heating of alumina balls resulting in lower cycle time by
15-20 % and with a power savings in the range of 15-20 %. This technique of
heat treatment of alumina balls is unique and provides uniform heat treatment
of the object resulting in better densification and properties of the product. In
the conventional method, the dried balls were sintered in a batch type gas fired
kiln.
It was observed that the conventional method of firing resulted bulk density in
the range of 3.76 - 3.80 g/cc, while the microwave sintered balls resulted in the
same in the range of 3.78 - 3.80 g/cc. The most important observation in this
invention is the observation of high impact resistance of exceeding 10,000
impacts achieved in microwave fired balls than their conventionally processed
counterparts. This may be achieved due to volumetric nature of heating of the
products in a microwave field. However, in the absence of large microwave kiln,
all the Grinding media balls were sintered in conventional gas fired batch kiln so
as to achieve properties such as micro hardness (HV5) of approx. 13 GPa,
indentation fracture toughness > 4.2 MPam1/2. The impact resistance is in the
range of 8,000-10,000 units which is 4 to 5 times higher than previously
manufactured balls by uniaxial pressing of 90 % alumina mix powder having bulk
density of 3.30 - 3.35 g/cc, Vicker's hardness of ~ 10 GPa and volume loss of 8-
10 mm3. The high impact resistance value of such balls can be explained based
on the homogenous densification during pressing and sintering besides of course
high alumina content in the composition. High impact resistance of grinding
media is a critical parameter for coal grinding in tube mills in the thermal power
stations.
The abrasion volume loss was measured using ASTM G65 standard for alumina
composition made by this invention and compared with that of high chrome steel
balls used for coal grinding. It was noted that the average value is 2.28
mm3 for product developed in this invention against that of average 3.15 mm3
for high chrome steel. However, the most significant observation is the average
mass loss i.e. 8.7 g for alumina against average 24.57 g of steel. This result
indicated three time better wear resistance of alumina balls developed in this
invention than that of steel balls presently used. Further, the grinding of coal
was carried out in a tube mill using separately steel balls and alumina balls. It
was noted that the weight loss from steel balls was 125 g per one MT coal
grinding against 45 g from alumina balls for same grinding amount. This result is
in close agreement with laboratory data of mass loss and volume loss reported
using ASTM standard. This results indicate that alumina balls made in the
present invention is much superior than that of present high chrome steel balls
used for coal grinding.
Table 1 compares the mechanical wear properties of different types of alumina
grinding media available in the market and also high chrome steel balls. It was
noted that the product developed in this study shows least volume loss among
all the competitive products. This is another confirmation of suitability of such
materials for coal grinding applications where the crushing effect of coal is
velocity controlled by stress wave propagation than mass controlled especially for
the grinding of bigger coal chunks.
The present invention can be better explained with suitable examples :
Example 1 : Alumina with nominal composition 90 % was isopressed using wet
bag isostatic pressing method and fired in a conventional kiln of 1580 oC/2h to
produce balls of size range 30-50 mm. The fired components resulted bulk
density in the range of 3.30 3.35 g/cc, water absorption <0.1 % Vickers
hardness (HV5) of 10 and sand abrasion volume loss of 8-10 mm3 on bar sample.
The balls could withstand upto 2000 -2400 impacts.
Example 2: Alumina with nominal composition of 92% was fabricated using the
present invention using the present method and fired in a conventional kiln of
1590 °C/2h to produce balls of size range of 30-50 mm. The fired components
resulted bulk density in the range of 3.44-3.46 g/cc, water absorption < 0.1 %
Vicker's hardness (HV5) of 11-11.5 GPa and abrasion volume loss in the range of
6-7 mm3 on bar sample.
Example 3 : Alumina with nominal composition of minimum 95 % was fabricated
using the present invention and fired in a conventional kiln at 1600 C/2h to
produce balls of size range of 30-50 mm. The fired components resulted bulk
density in the range of 3.76-3. 80 g/cc, water absorption <0.1 % Vicker's
hardness (HV5) of ~ 13 GPa and abrasion volume loss of bars in the range of
2.25-2.35 mm3 on bar sample. The impact resistance of such balls was in the
range of 8000-8500 impacts. These balls are suitable for coal grinding
applications.
Example 4 : Alumina with nominal composition of minimum 95 % was fabricated
using the present invention and fired in a 30 kW pure microwave kiln at 1600
C/2h to produce balls of size range of 30-50 mm. The fired components resulted
bulk density in the range of 3.78-3.80 g/cc, water absorption <0.1 %, Vicker's
hardness (HV5) of 13 GPa and abrasion volume loss of bars in the range of
2.2-2.3 mm3 on bar sample. The impact resistance of such balls was in the
range of 10000-10500 impacts. These balls are best suitable for coal grinding
applications. In order to understand the high impact resistance of these balls
compared to other processing methods, a 50 mm microwave fired ball (5) was
sliced in the middle and a flat specimen was formed (6) as depicted in figure 3.
The both side flat sample (6) was polished with emery paper from 200 grit, 320
grit, 600 grit and 1000 grit followed by lapping with diamond paste with 8, 3 and
1 µm. The polished sample was thermally etched in a 6 kW microwave furnace at
1400 °C for 30 min. The micro hardness was measured at five different points on
the polished and etched sample as depicted in figure 3 covering entire diameter
of the ball. A very low load of 200g was used for micro hardness in order to
avoid any interference from high load related defects in the sample during
measurement. The results given in figure 3 indicated that the hardness is
uniform at ~ 13 GPa throughout the entire diameter of the ball. This result is a
confirmation of uniform densification achieved by volumetric heating using
microwave energy. Further, this uniform compactness has resulted in very high
impact strength of the microwave fired balls.
While preferred embodiments have been shown and described, it should be
understood that process adopted by change of process and selection of alumina
mix powder can be made therein without departing from the invention in its
broader aspects. Various features of the invention are defined in the following
claims.
WE CLAIM:
1. A process for producing alumina grinding media balls for coal pulverization
in thermal power station in an equal hydrostatic pressure method, the
process comprising the steps of:
- providing ready to press alumina powder consisting of alumina exceeding
95 % by weight;
- providing additive in total wt percent of 4-5 on fired basis selected from a
group of compound consisting of CaO, SiO2 and MgO;
- feeding the additive-mixed dried alumina powder in a dual covering
mould;
- fabricating the alumina balls in four cavity mould at 60-120 balls /h
preferably 80-100 balls /h of sizes in the range of 20-80 mm in diameter
preferably 30-50 mm diameter by applying equal hydrostatic pressure;
and
- sintering the balls both in conventional gas fired kiln and in pure
microwave kiln to achieve high impact resistance in a temperature range
of 1570-1650 °C preferably in the range of 1580-1620 °C.
2. The process as claimed in claim 1, wherein the additive mixed spray dried
powders are in the granulated form with average granule size 25-40 µm
with loss on ignition in the range of 3.4-3.8 %.
3. The process as claimed in claim 1 or 2, wherein the size of balls is in the
range of 20-80 mm preferably in the range of 30-50 mm, and wherein the
isostatic pressure is in the range of 300-450 Kgs/cm2 resulting green
density in the range of 2.34-2.38 g/cc.
4. The process as claimed in claim 1, wherein the dual covering mould allows
to obtain desired shape of the ball without damaging the mould protected
by an outer lining.
5. The process as claimed in claim 1, wherein both feeding and releasing of
the balls are performed on the same side of the mould and compressed
air is employed for releasing the product.
6. The process as claimed in claim 1, wherein the produced balls having fired
density exceeding 3.76 g/cc, water absorption <0.1 %, Vicker's
hardness ~ 13 GPa.
7. The process as claimed in claim 1, wherein a uniform micro hardness of
the balls through its entire diameter is achieved resulting in high impact
strength.
8. The process as claimed in claim 1, wherein the impact resistance achieved
for the balls is more than 10000 units by volumetric microwave heating
and exceeding 8,000 impacts in conventional heating.

ABSTRACT

The invention relates to a process for producing high impact resistant ceramic
balls which are suitable for coal pulverization in thermal power stations. The
process is based on the application of equal hydrostatic pressure technology
which is capable of developing maximum fluid pressure of 1000 Kgs/cm2 and
producing balls with a minimum output of 60-120 balls /h preferably 80-100 balls
/h of sizes in the range of 20 -80 mm in diameter preferably in 30-50 mm
diameter range. The mold design plays an important role to produce defect free
components. The balls having nominal composition of alumina content of
exceeding 95 % and with other additives were fired at 1570-1650 °C preferably
in the range of 1580-1620 °C both in pure microwave kiln and in gas fired batch
kiln. The balls produced are having green density >2.30 g/cc, Fired bulk density
>3.76 g/cc with <0.1 %, water absorption. The balls are having micro-hardness
~ 13 GPa, fracture toughness of > 4.2 MPa m1/2 volume loss of < 3.0 mm3. Most
significant result of the balls is the impact resistance withstanding capability of
ball exceeding 8,000 impacts. The balls produced in this development will be
used for grinding coal in thermal power station promising huge savings in
electrical energy due to low density and high abrasion resistance of these
products. These balls also can be used in other industries for low abrasion and
high grinding efficiency.