|Title of Invention||
"A PORCESS FOR THE PREPARATION IMPROVED HIGH ALUMINA SELF FLOHING CASTABLE"
|Abstract||A process for the preparation of improved high alumina self flowing castable The present invention relates to the development of a manufacturing process for production of dense cement free high alumina castables for industrial application with improved properties that of low cement, ultra low cement and conventional castables mentioned earlier. It also ensures elimination of vibration during casting.|
|Full Text||This invention relates to a process for the preparation of improved high alumina self flowing castable. Refractory castables comprise of hydraulic setting high temperature cement and graded refractory aggregates. Various types of castables are prepared either by casting or by ramming. These castables give better thermal efficiency and more trouble free service compared to conventional brick lining. However, the conventional type of castables have some serious draw backs viz there is deterioration of strength at intermediate temperature ( 600 to 800°C ) and lowering of refractoriness due to higher amount of cement content in the mix resulting in higher content of CaO in the composition. Low cement castables are recent generation monolithics. These castables have much higher cold crushing strength and higher mechanical properties at elevated temperature because of less cement content and correspondingly less CaO content in the final mix. But certain constraints are encountered in application i.e. (1) Vibration is essential (2) Special mixture are required for efficient mixing (3) Some ingredients which are not readily available are to be used for preparing the composition.
The main object is to provide a process for the manufacture of improved cement free high alumina self flowing castable which obviates the above noted drawbacks.
Another object of the present invention relates to the development of a manufacturing process for production of dense cement free high alumina castables for industrial application with improved properties that of low cement, ultra low cement
and conventional castables mentioned earlier. It also ensures elimination of vibration during casting.
Accordingly the present invention provides a process for the preparation of improved high alumina self flowing castable which comprises mixing fused alumina of particle size -80 +110 to -200 mesh, 15 to 35 wt% coarse & fine sillimanite sand, 15-20 wt% Al2O3 of particle size 250 to 325 mesh, adding 10 to 15 wt% reactive silica solution and 0.05 to 0.10 wt% additive such as herein described, maintaining the pH in the range of 5 to 8, casting the resultant mixture, drying and firing the dried castable at a temperature in the range of 500°C to 1300°C, to get the desired alumina castable, the said process characterized in using additive which provides cold crushing strength and higher mechanical properties to the obtained castable at elevated temperature.
The general relationship between pH and stability of silicic acid or colloidal silica is discussed in brief for better understanding of the complexity of bond mechanism.
Above about pH 6 or 7 in the absence of soluble salts silica bears a strong negative charge and gel formation is rather slow. Gelling is most rapid at pH 5 -6. From 2 to 5, the charge on silica or silicic acid is low so that presumably the molecules can readily collide, However, in this region the rate of condensation between polysilicic acid molecules depends on the concentration of hydroxyl ions; the OH-1 ion therefore acts a catalyst for the condensation reaction. Below about pH 2, the rate of polymerization increases with greater acidity. The rate in this region below this pH is proportional to the concentration of H+.
The reason for the minimum rate of polymerisation of silicic acid around pH 1 to 3 has been attributed due to catalytic action of traces of Flouride ion [ Since pure form of silicic acid also contain traces of flouride ion]. It has been observed that it is almost impossible to prepare solutions of silicic acid free from traces of Flouride ion . As little as 0.1 millilitre of Flouride ion per litre has a marked effect.
From pH 3 to 7 colloidal silica usually gel rapidly within the range pH 5 to 7. The reason is attributed by the fact that in faintly acidic range silica particles pass through isoelectric point where the particles are not charged and therefore come together rapidly. Thus below above pH 5, rate of reaction between colloidal silica particles is proportional to
(1) The no. of collisions &
(2) The concentration of OH ions which catalyse the reaction when collision occurs. Above pH 4.5 , the polysilica acid molecules or particles begin to absorb OH ions and become negatively charged resulting fewer collisions resulting decrease in the rate of gel formation. The maximum rate of gelling is usually within the from pH 5 to 8 but exact point depends on various facts especially concentration of particle size.
When colloidal silica are titreated upto pH 10 to 11.5, there is a strong point of inflection. This is due to dissolution or depolymerisation of polymerised silica.
The effect of additives in very minute quantities have
pronounced effect on change of pH and subsequently gelation
period. The additive are HF, (NH4 )2 HPO4 , (NH4 )2 SO4 , Sod.
hexameta Phosphate, MgCl2 , MgSO4 , MgO, & CaO. Depending upon the
requirement for setting time, 0.1% (NH4 )2 SO4 is added for which
a considerable time of gelation is obtained when a concrete
/pumpable retains its flow characteristics for a considerable
time. But when 0.1% MgO is added, setting is quicker.
Process in Brief :
The different ingredients namely fused alumina, sillimanite beach sand, Fine alumina, Reactive Silica solution bond and additives are mixed intensively in a mixture. The major ingra-dients namely Fused alumina, Sillimanite beach sand and fine alumina are mixed first and followed by additives and bonds. The whole mass is mixed for 2 to 3 minutes which leads to form the product mass which has flowing property suitable for casting in different shapes as per the requirements. The mass is cast in wooden or steel mould and kept for 24 hours after which
the product is de-moulded and dried on air for 24 hours followed
by drying at 110°C for 24 hours. The material develop mechanical strength increase corrosion resistant and improved high mechanical strength. This products do not have any weakness in
strength in the intermediate temperature and product is suitable
in the entire temperature range of 110° to 1600°C. The materi-
als are fired on different temperature like 500, 800, 900 1600°C
and have been characterised. The invention is described in the
following 4 experiments :
The process of the present invention is illustrated by the
following examples which should not be construed to limit the
scope of the present invention.
Example - 1
Fused Alumina of the following particle size
-8 + 10 mesh - 40%
-10 + 30 mesh - 10%
- 200 mesh - 35%
& fine Al O 15%
were throughly mixed with to which 15% reactive silica solution &
0.1 % MgO were added.
The mixture is then cast into cubical moulds. After 24 hours the
test pieces are removed from mould and allowed for drying for 24
hrs in air and then in drier at 110°C for 24 hrs. The dried test
pieces were fired at 500°C, 800°C, 1300°C and 1600°C.
The cubes were then characterised as follows :
The RUL test were conducted on test pieces fired at 1300°C.
The porosity, CCS, PLCR & RUL were determined following standard
This constitutes the aggregate having Al2O3 content 93% +.
Example - 2
600 gm of sillimanite sand (320 gm coarse & 280 gm fine)
with fused Al 0 aggregates in the following ratio were mixed 2 3
-8+10 mesh 280 gm
fine Al 0 120 gm
Reactive silica bond to the extent of 18% and add .1%
(NH ) SO & 1% (NH ) PO were added and. Mixed uniformly and
4 2 4 4 3 4
cast into cubical moulds. After 24 hrs, the test pieces were
o removed from mould allowed to dry in air and in oven (at 110 C)
o for 24 hrs each. The test pieces were then fired to 500 C,
o o o 800 C, 1300 C & 1600 C for property evaluation. This constituents 70% Al 0 content. 2 3
Example - 3
50% sillimanite sand coarse/fine (4 : 1) and 40% fused Al O
(8 +10) & 10% fine Al 0 were taken and intimately blended in
which 16% reactive silica solution and 0.1% sodiun
hexametaphosphate were added. Requiste quantity of water was
added thereafter. The test pieces were air dried and then at
110°C and fired to 500°C, 800°C 1300°C & 1600°C and and their
properties were evaluated. This constitutes 80% alumina
Example - 4
Fused Alumina as the following particle size
-8 + 10 mesh 40%
-10 + 30 mesh 10%
- 200 mesh 35%
& fine Al2O3 15%
These fraction were throughly mixed to which 15% reactive silica solution with 0.1% HF for the attainment of longer storage life.
The properties of Reactive silica sol and dense self flowing castable are given in Table I & Table II.
Table I : Characteristics of Reactive silica sol (colloidal SiO )
SiO content 40%
Density (gm/cc) 1.25
Viscosity 12.55 +
(Measured on 4 B cup )
Particle Size 14 nano metre
Table II : Properties of the dense self flowing castable
Al2O3 - 90% Al2O3 - 80% Al2O3 - 70%
CCS 400-650 400-650 400-650
App.Porosity (%) 8-10 8-10 10-12
PLCR at 1600°C for 5 hrs
RUL Ta 1600-1680°C + Ta 1600-1650° + Ta 1550-1600° +
Ta >1680°C Te >1650°C Te> 1650°
Advantages : (i) Higher hot strength (Hot modulus of rupture & Creep in compression ) and higher thermal shock resistance.
(ii) Pumpable and self flowing nature of the material after
proper mixing (iii) No vibration & no special mixer are required as in the case
of low cement castable & ultra low cement castable.
(iv) No deterioration of strength in the intermediate
temperature range (600 - 800°C).
(v) Use of lighter aggregate may be incorporated in the mix.
1. A process for the preparation of improved high alumina self flowing castable which comprises mixing fused alumina of particle size -80 +110 to -200 mesh, 15 to 35 wt% coarse & fine sillimanite sand, 15 -20 wt% Al2O3 of particle size 250 to 325 mesh, adding 10 to 15 wt% reactive silica solution and 0.05 to 0.10 wt% additive such as herein described, maintaining the pH in the range of 5 to 8, casting the resultant mixture, drying and firing the dried castable at a temperature in the range of 500°C to 1300°C, to get the desired alumina castable, the said process characterized in using additive which provides cold crushing strength and higher mechanical properties to the obtained castable at elevated temperature.
2. A process as claimed in claim 1 wherein the additive used is selected from HF, (NH4)2SO4, (NH4)2HPO4, Sodium hexameta phosphate, MgO, MgCI2, MgSO4, CaO.
3. A process for the preparation of improved high alumina self flowing castable substantially as herein described with reference to the examples.
|Indian Patent Application Number||909/DEL/1997|
|PG Journal Number||41/2004|
|Date of Filing||09-Apr-1997|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRAIL RESEARCH|
|Applicant Address||RAFI MARG NEW DELHI-110001,INDIA|
|PCT International Classification Number||C04B 7/32|
|PCT International Application Number||N/A|
|PCT International Filing date|