The present invention relates to an improved method for the production of Fly Ash Based Abrasion Resistance Ceramics.
The present invention particularly relates to an improved method for the production of fly ash based abrasion resistant ceramics.
i) The following steps of mixing in a ball mill raw materials in the
proportion range of i
Fly ash : 5 to 60% by weight
Calcined alumina : 40 to 94% by weight
Additives : 0to2% by weight
Water : 60 to 80% of total dry material
ii) dewatering of wet mix, sieving of the powder by a known
process
iii) compacting said mixture into desired shapes by uniaxial pressing
iv) drying shaped article over night at 110°C in an electrical oven
v) sintering the dried articles in a furnace in the temperature range
of 1300°C - 1500°C with soaking for a one hour uniformly followed by cooling and unloading from the furnace.
It is possible to develop a fly ash based abrasion resistance ceramics that can be used as liner in the coal carrying ducts in the power station. The central focus of this study is to use the byproduct or the wastes of a process as a protective material (as a part of composition) for the safe conveyance of the original material, ie. coal.
The most common abrasion resistance alumina ceramics available in commercial scale comprises of ~ 85% of calcined alumina with other minor additives like clay, talc, MnQ2 etc. The sintering temperature employed for these compositions are close to 1600°C. With the rising cost of energy it has been the endeavour of the applicant to develop a new material which reduces cost of product and at the same time improve the properties of the material. There are few efforts in the direction by many researchers around the world, for example, Masonry et al. in their patent vide No.JP2180747 dated 13th July 1990 described a process for producing wear resistant alumina ceramics employing additives like MgO and Si02 to calcined Al2O3 which can be sintered at a temperature range of 1300°C - 1500°C. Similarly Koji et al. in their patent vide No.JP9221354 dated 26th August 1997 described a process for producing wear resistance alumina ceramics by using SiO2, MgO and CaO as additives in calcined alumina which can be sintered at a low temperature. Similar studies have also been reported in patents vide No.CN1081175B, CN1165124, JP3080873B2, JP1834793C and JP5044428B etc.
There are two important observations in most of these studies:
a) The additives used are SiO2, CaO, MgO etc. in A12O3.
b) The amount of additives are in the range of 5 to 15 wt. %.
Keeping the prior art in view, it is prudent to use a material that can provide all the above additives rather than adding them separately. This will also probably minimize the additive content to less than 5% by weight. The material in such a case can be "Fly ash" which is a byproduct of thermal power station.
Fly ash is an environmentally hazardous material and hence it needs to be utilized to produce value-added products. It is inexpensive and available in plenty. It contains all the above additives in a amorphous phase. Therefore, fly ash can be added to alumina to produce abrasion resistant material. This idea was utilized in Indian Patent Specification No. 187947 titled "An improved process for the production of wear resistance ceramics using fly ash". This Indian Patent suffers various drawbacks and therefore needs modification in terms of the following:
1) For good abrasion resistance in Al2O3, Fe2O3 is kept in minimum, which is considered an undesired impurity. The fly ash contains lot of free iron, which deteriorates the abrasion resistance property.
2) The additives used in the above Patent Specification are many and the amounts as high as 20%. Problems associated with number of additives are of non-uniform mixing especially for milling time of 3-4 hrs. Also the large amount of additives influence the abrasion resistance properties drastically which is difficult to control in the production stage.
3) The firing temperature in compositions cited in the patent are in the order of 1500°C and of longer duration. There is a need to develop composition, which are sintered at lower temperature and lower duration without many additives.
4) The patent uses only one FA composition. However, since the composition of FA vary from source to source, it is beneficial to develop composition using two different source of FA for the purpose of comparison.
Keeping the disadvantages experienced in the conventional methods in view, it was felt to develop an improved process for fly ash based abrasion resistance ceramics.
The improved method described in this application takes into account all the above points, which can be summarized as:
i) benefaction of fly ash was carried out
ii) the amount of additives was 2% maximum
iii) the firing temperature of one composition was as low as ~
1300°C iv) detailed mechanical properties viz. Modulus of Rupture (MOR),
Modulus of Elasticity (MOE), Hardeness, Fracture Toughness
(FT) were carried out for almost all compositions v) two different sources of fly ash were used with wide range of
composition vi) one step drying and sieving was introduced in the modified
process vii) fly ash has been added in a wide range of 5% to 60%.
Thus the main objective of the present invention is to provide an improved process for the production of fly ash based abrasion resistance ceramics.
The other objective of the present invention is to reduce the additive content from ^ 15% to < 2%.
Yet another objective is to sinter a fly ash alumina composition at a very low temperature < 1300°C.
Still another objective of the present invention is to characterize the material for its other mechanical properties besides the abrasion resistance.
In the method of present invention, calcined alumina is replaced by an inexpensive waste material fly ash, in the range of 5 to 60% with very low additive contents to produce an abrasion resistant ceramic material. Mullite (3A120 2SiO2) is the major phase of alumina - fly ash mixture. The presence of small amount of MgO facilitates reaction sintering and densification of the material. The needles like mullite crystal acts as a reinforcing phase in alumina matrix resulting in better abrasion resistance properties. The additives used in this type of process in excess amount (> 5%) in general produce liquid phase during high temperature sintering resulting loss in mechanical properties and wear resistant properties.
The present invention provides an improved method for the production of fly ash based abrasion resistant ceramics, which comprises the following steps -
i) passing fly ash through ferro-filter to remove iron impurities;
ii) wet mixing intimately by ball milling the raw materials in the range of:
Fly ash : 5 - 60%
Cancined A12O3 : 40 - 94%
Additives (MgO and/or : 0 - 2%
°Mn02)
Water : 60 — 80% of total power weight
iii) dewatering and drying of the wet mix using combination of plasters drying and electrical heating,
iv) uniform mixing of 2-3 wt % of an organic binder such as herein described in water to the dried mass,
v) compacting said mixture into desired shapes by a known method at room temperature drying for overnight,
vi) sintering the dried article in a furnace at a temperature varying between 1280°C and 1585°C with uniform soaking for a period of 1 hour,
vii) cooling and unloading from the furnace the sintered article.
According to this invention, two different sources of fly ash were used for this work. The fly ash may be selected in the following composition range:

(Table Removed)
The average particle size of fly ash may vary from 3 to 5 microns. Alpha content is calcined Al2O3 may be 97% minimum with an average particle size in the range of 1 to 1.5 microns. The additives may be selected are MgO & MnO2.
Intimate wet mixing in ball mill for 3 to 4 hrs is required. Dewatering may be carried out by a combination of plaster drying and electrical heating at 110°C. According to yet another embodiment of the invention, organic binder_may be selected from Polyvinyl Alcohol (PVA), Polyethylene Glycol (PEG)_ and Dextrin. The binder may be in the range of 2_to3%_ of the dry mix. The forming pressure during compaction may be in the range of 300 to 400 Kg/cm2. Room temperature drying for overnight is sufficient prior to firing. Sintering of the dried components is effected in an electric / gas operated furnace.
The wear resistant ceramic material produced by the process of present invention has the following range of properties

(Table Removed)
The following examples are given way of illustration of the invention and should not be construed to limit the scope of present invention.
Example 1
240g of beneficial fly ash (A1203 = 25-30%, Si02 = 55-65%, CaO = 3-4%), 160g of calcined alumina are uniformly mixed with 250 mk of tap water in pot mill for a duration of four hours. No additives were added to the composition. Dewatering of wet mix is done by combination of drying in plaster of Paris mould followed by drying in an electrical oven at 110°C for overnight 10 ml of diluted polyvinyl alcohol was added to dried powder and mixed thoroughly in a mortar and pestle. Articles of 80 mm * 20 mm * 8 mm rectangular bars are made by compaction at -350 Kg/cm2 pressure.
These bars are dried at room temperature overnight. Finally, sintering was carried out at 1585°C for a period of 1 hour and the properties obtained are furnished in Table 1.
Example 2
160g of beneficial fly ash (A1203 = 25-30%, Si02 = 55-65%, CaO = 3-4%), 160g of calcined alumina are uniformly mixed with 250 ml of tap water in pot mill for a duration of four hours. No additives were added to the composition. Dewatering of wet mix is done by combination of drying in plaster of Paris mould followed by drying in an electrical oven at 110°C for overnight. 10 ml of diluted polyvinyl alcohol was added to dried powder and mixed thoroughly in a mortar and pestle. Articles of 80 mm * 20 mm * 8 mm rectangular bars are made by compaction at -350 Kg/cm2 pressure.
These bars are dried at room temperature overnight. Finally, sintering was carried out at 1585°C for a period of 1 hour and the properties obtained are furnished in Table 1.
Example 3
72g of beneficial fly ash (A1203 = 10-15%, Si02 = 65-70%, Fe203 = 2-4%), 320g of calcined alumina 4g of Mn02 and 4g of light MgO are uniformly mixed with 250 ml of tap water in pot mill for a duration of four hours. No additives were added to the composition. Dewatering of wet mix is done by combination of drying in plaster of Paris mould followed by drying in an electrical oven at 110°C for overnight 10 ml of diluted polyvinyl alcohol was added to dried powder and mixed thoroughly in a mortar and pestle articles of 80 mm * 20 mm * 8 mm rectangular bars are made by compaction at -350 Kg/cm2 pressure. These bars are dried at room temperature overnight. Finally, sintering was carried out at 1280°C for a period of 1 hour and the properties obtained are furnished in Table 1.
Example 4
40g of beneficial fly ash (A1203 = 10-15%, Si02 = 65-70%, Fe203 = 2-4%), 356g of calcined alumina and 4g of light MhO are uniformly mixed with 250 ml of tap water in pot mill for a duration of four hours to the composition. Dewatering of wet mix is done by combination of drying in plaster of Paris mould followed by drying in an electrical oven at 110°C for overnight. 10 ml of diluted polyvinyl alcohol was added to dried powder and mixed thoroughly in a mortar and pestle. Articles of 80 mm * 20 mm * 8 mm rectangular bars are made by compaction at - 350 Kg/cm2 pressure.
These bars are dried at room temperature overnight. Finally, sintering was carried out at 1500 C for a period of 1 hour and the properties obtained are furnished in Table 1.
Example 5
20g of beneficial fly ash (A1203 = 10-15%, Si02 = 65-70%, Fe203 = 2-4%), 376g of calcined alumina and 4g of light; MgO are uniformly mixed with 250 ml of tap water in pot mill for a duration of four hours. No additives were added to the composition. Dewatering of wet mix is done by combination of drying in plaster of Paris mould followed by drying in an electrical oven at 110°C for overnight. 10 ml of diluted polyvinyl alcohol was added to dried powder and mixed thoroughly in a mortar and pestle. Articles of 80 mm * 20 mm * 8 mm rectangular bars are made by compaction at ~ 350 Kg/cm2 pressure.
These bars are dried at room temperature overnight. Finally, sintering was carried out at 1500°C for a period of 1 hour and the properties obtained are furnished in Table 1.
Tablet : Properties of Alumina- Fly ash system used in this work
(Table Removed)

Note : a : Thermal Expansion Coefficient (RT - 1000°C); MOR : Modulus of Rupture (Three point bend test); MOE : Modulus of Elasticity (Resonant Frequency Technique); HV05 : Vickers hardness at 5N load; COV : Coefficient of variation in hardness data; FT : Indentation Fracture Toughness at 5 ON load : AVL : Adjusted volume loss from sand abrasion test.
The typical AVL value of commercial wear resistance alumina lies in the range of 4 - 6 mm3.
The Table 1 implies that:
1. A combination of mullite and Al2O3 phases is beneficial for better Abrasion resistance properties.
2. More additives in the composition lead to poor abrasion resistance Properties observed in Example 3.
Main advantages of the present invention are :
1. The present invention replaces upto 60% AI2O3, by inexpensive and environmentarry hazardous fly ash to develop an abrasion resistant ceramic material.
2. The amount of additives has been reduced to just 2% maximum, thus overcoming the problems due to in homogeneous mixing of many additives.
3. The soaking period at high temperature can be minimized to just 1 hour by proper selection of additive viz. MgO in the present system.
4. The sintering temperature of as low as 1280°C can be employed for densification of a typical A1203 fly ash system with suitable additives.
5. Many properties of the fired composition are measured and can be correlated to high abrasion resistance of the material.

WE CLAIM:
1 . An improved method for the production of fly ash based
abrasion resistant ceramics which comprises the following steps:
i) passing fly ash through ferro-filter to remove iron impurities, ii) wet mixing intimately by ball milling the raw materials in the range of:

(Table Removed)
iii) dewartering and drying of the wet mix using
combination of plasters drying and electrical
heating,
iv) uniform mixing of 2-3 wt54 of an /organic binder in
water to the dried mass,
v) compacting said mixture into desired shapes by a
known method of room temperature and drying for
overnight,
vi) sintering the dried article in a furnace at a
o o
temperature varying between 1280 C and 1585 C with
uniform soaking for a period of 1 hour, vii) cooling and unloading from the furnace the the sintered article.An improved process as claimed in Claim 1 wherein said fly ash has a composition in range of A1203 : 10-15%, Si02 : 65-70% and Fe203 : 2-4% with loss of ignition in the range of 4-6%, the average particle size being in the range of 3-5 micron as determined by sedigraph.
2. An improved process as claimed in Claim 1 wherein said fly ash has a composition in the range of A1203 : 25-30%, Si02 : 55-65% and Fe203 : 3-4% with loss of ignition in the range of 1-2%.
3. An improved process as claimed in Claim 1 or 2 wherein alpha A1203 content in calcined A1203 is 97% minimum with an average particle size in the range of 1-1.5 micron.

5. An improved process as claimed in Claim 1 wherein sieving the binder mixed powder is carried out for ease in pressing.
6. An improved process as claimed in Claim 1 wherein the sintering is carried out in electrical / gas / coal operative furnace or kiln.
7. An improved process as claimed in Claim 1 wherein A1203 in the fly ash composition is sintered at a low temperature of 1280°C with one or more additives.

8. An improved method for the production of fly ash based abrasion resistant ceramics substantially as herein described.