FIELD OF THE INVENTION
The present invention relates to a fabrication process for high strength wear
resistance grinding rolls with enhanced performance and higher operational life.
BACKGROUND OF THE INVENTION
In coal-fired thermal power plant, a pulveriser is the first major component,
whose performance substantially dictates the total power plant efficiency.
Desired size reduction of the coal, uniform flow rate and higher classifier
efficiency are three important measures which decide the pulveriser
performance. Grinding rolls in the bowl mill play an important role in the coal
pulveriser which is used to crush the coal lumps into the fines and which is
transferred to the boiler section. The major concern of the grinding rolls during
the course of coal pulverization is its "harsh abrasive action with the coal" due to
higher ash and silica content present in the coal. Due to this, surface of the
grinding rolls gets worn out faster and it reaches to the situation where it needs
to be replaced. This involves the plant shutdown and ultimately affects the plant
efficiency.
US patent 8147980B2 (2012) teaches a metal matrix ceramic composite (MMCC)
wear part and a process of manufacturing thereof. In this MMCC, the wear
portion formed by a ceramic cake impregnated by metal (SG Iron), wherein the
ceramic cake comprises of Al2O3, ZrO2, fine ceramic powder (Al2O3) any one of
the carbide materials such as boron carbide, silicon carbide and tungsten carbide
and the sodium silicate binder. These powders and binders are mixed in a
flexible holder and the mixture is hardened to form a ceramic cake. In order to

provide the adequate strength, the cake is heated to a temperature between 80-
220°C. The ceramic cake is reinforced with the SG iron and finally shaped into
the grinding roll of bowl mill. It was observed that the life of the grinding roll
improved substantially (quantitative value of the operational life has not been
mentioned in this patent.)
Gronvall et al. disclosed a biomaterial elongated insert member for grinding rolls
in patent application EP 2512680 Al (2012). The insert member comprised of a
core of a first material having a first hardness, and the body formed of a second
material having a second hardness which enclosed the core. The first hardness
was greater than the second hardness. The advantage is that the reinforced
insert members are less likely to dislodge from the grinding roll due to the
elasticity created between the insert member and the grinding roll. Mirchandani
et al. (US 0011965 Al) (2011), developed a wear resistance metal matrix
composite which comprises inorganic particles dispersed in a matrix material.
Melting temperature of the inorganic particle is greater than the melting
temperature of the matrix.
Poncin et al. (US 7513295 B2 (2009)) disclosed cast parts with enhanced wear,
abrasion and impact resistance including a method of their production. The
method comprised of placing two or more powdered raw materials in a mold and
then after adding a molten casting metal to the casting mold. The molten casting
metal generates an in-situ chemical between the powders and finally provides a
particulate porous conglomerate. The powder raw material selected from the
group consisted of ferro-alloys, oxides, nickel, nickel alloy, iron alloy, titanium
alloy etc.

Francois et al. (US 6399176 Bl (2002) described composite wear component
produced by centrifugal casting in which ceramic pad (in the form of honeycomb
structure) is reinforced in the molten metal. The wear component produced by
centrifugal casting consists of a metal matrix whose wear surface comprises
inserts (made of ceramic material, 20-80 wt.% Al2O3 and 80-20 wt.% ZrO,
Particle size range: 0.7-5.5 mm) which is having a high abrasion resistance
properties. The idea of using aluminium oxide and zirconium oxide having
relatively different properties was to adjust the hardness, toughness and thermal
expansion coefficient of the composite ceramic. Zirconium dioxide has the
advantage of having an expansion coefficient which is close to that of metal. In
addition, it contributes to higher toughness, which reduces the risk of breakage.
In addition, ZrO2 particle present in the AI2O3 increases the resistance of the
AI2O3 to cracking and hence increases the toughness greater than that of each
component considered individually (namely ZrO2 or AI2O3). Various geometries
were proposed in their invention in order to solve the infiltration of the liquid
metal within the ceramic phase. For the efficient infiltration of the liquid metal
and to reduce the risk of propagation of the crack, honeycomb type of pad was
fabricated. Finally, Ceramic inserts (hardness: 1600 Hv, expansion coefficient :~8
* 10"6/K) reinforced with the pig iron matrix (hardness: 750 Hv) and thus
composite wear component was fabricated.
EP 0234026 A2 (1987), EP 0192972 Al (1986), US 4604781 (1986), US
4610401 (1986), US 4389767 (1983) also discuss about the development of
wear resistance surface layer.

OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a fabrication process
for high strength wear resistance grinding rolls with enhanced performance and
higher operational life.
Another object of the invention is to propose a fabrication process for high
strength wear resistance grinding rolls with enhanced performance and higher
operational life, in which ceramic materials having higher hardness, high
strength, higher wear resistance including density, wettability and coefficient of
thermal expansion comparable to molten SG iron is selected.
A still another object of the invention is to propose a fabrication process for high
strength wear resistance grinding rolls with enhanced performance and higher
operational life, in which an effective ceramic binder is selected.
A further object of the invention is to propose a fabrication process for high
strength wear resistance grinding rolls with enhanced performance and higher
operational life, in which an octagonal structured carbon steel die for shaping the
ceramic mixture is fabricated and used.
A still further object of the invention is to propose a fabrication process for high
strength wear resistance grinding rolls with enhanced performance and higher
operational life, in which the octagonal structured ceramic pads are reinforced
with molten SG iron.

SUMMARY OF THE INVENTION
The objective of the present invention is to fabricate the high strength wear
resistance grinding roll with enhanced performance and operational life which
ultimately leads to higher plant efficiency. According to the invention, the object
is achieved by reinforcing the ceramic material into a metal (ceramic materials
has higher hardness and wear resistance than that of metal). In the present
invention, first ceramic materials is selected to have higher hardness and wear
resistance. The selected ceramic materials have comparable density, wettability,
coefficient of thermal expansion with the molten metal (SG iron). The binder
chosen is sodium silicate. All the ceramic powders are mixed with the binder and
cast into an octagonal shaped die. The casting is kept overnight for drying. In
order to improve the green strength, the ceramic pads are sintered at around
~1400°C. These fabricated ceramic pads are reinforced (by arranging the pad on
the outer periphery of casting die roll) with the molten metal during the
centrifugal casting of the grinding roll.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure la is the octagonal shaped free standing ceramic pad while Figure lb and
c shows the male and female slots in the pad, respectively to tight fit the pads
into the centrifugal casting die.
Figure 2 shows a ceramic pad showing its dimensions.
Figure 3 is a grinding roll in which ceramic pads are reinforced.
Figure 4 is octagonal structured free standing ceramic pad

Figure 5 is the schematic illustrating the process steps to fabricate the ceramic
pad reinforced grinding roll.
Figure 6 is the grinding roll reinforced with the octagonal structured ceramic pad.
DETAILED DESCRIPTION OF THE INVENTION
Grinding roll in the bowl mill has significant impact in the coal pulveriser, which
ultimately affects the power plant efficiency. However, in the present situation,
coal contains the higher amount of ash and silica content which is highly abrasive
material and can wear out the roll frequently. Hence, there is a need to develop
high strength wear resistance grinding rolls. This present invention discloses to
develop the high strength wear resistance grinding roll with enhanced
performance and higher operational life. To achieve this objective, ceramic
materials having higher hardness and wear resistance are considered. Three
different type of powders i.e. Aluminum Oxide (Al2O3) (55-60 wt.%), Zirconium
oxide (ZrO2) (35-40 wt.%), and Titanium Diboride (TiB2) (5-10 wt.%) are chosen
for fabricating the ceramic pads. Sodium silicate (Na2SiO3) is used as a binder. All
the above mentioned ceramic powders are mixed with the binder and cast in an
octagonal structured die. In order to provide the strength, this green compact is
sintered at around ~ 1400°C for 2-3 hours. The fabricated octagonal structured
ceramic pads are arranged in the outer periphery of a centrifugal casting die and
the molten SG iron is poured in the die. The complete procedure for developing
the roll is shown in Figure 5. Thus, the grinding rolls reinforced with octagonal
structured ceramic pads is fabricated (as shown if Figure 6).

WE CLAIM :
1. A fabrication process for high strength wear resistance grinding rolls with
enhanced performance and higher operational life, comprising the steps of :-
selecting at least three different types of ceramic materials having higher
hardness and greater wear resistance compared to SG iron;
crushing the selected materials in powder form, wherein the ceramic materials
are selected as aluminium oxide (55-60 wt%) zirconium oxide (35-40 wt%)
and titanium diboride (5-10 wt%);
selecting a binder such as sodium silicate;
mixing the ceramic powders with the binder;
fabricating an octagonal structured carbon steel die;
forming ceramic pads by casting the ceramix powder and binder mixture;
sintering the ceramic pads at about 1400°C for about 2-3 hrs; and
producing the grinding roll by reinforcing the ceramic pads in outer periphery of
the die including pouring molten SG iron in the die.
2. The process as claimed in claim 1, wherein the selected ceramic materials are
having density, wettability and co-efficient of thermal expansion compatible to
molten SG iron.

ABSTRACT

The invention relates to a fabrication process for high strength wear resistance
grinding rolls with enhanced performance and higher operational life,
comprising the steps of : selecting at least three different types of ceramic
materials having higher hardness and greater wear resistance compared to SG
iron; crushing the selected materials in powder form, wherein the ceramic
materials are selected as aluminium oxide (55-60 wt%) zirconium oxide (35-40
wt%) and titanium diboride (5-10 wt%); selecting a binder such as sodium
silicate; mixing the ceramic powders with the binder; fabricating an octagonal
structured carbon steel die; forming ceramic pads by casting the ceramic
powder and binder mixture; sintering the ceramic pads at about 1400°C for
about 2-3 hrs; and producing the grinding roll by reinforcing the ceramic pads
in outer periphery of the die including pouring molten SG iron in the die.