Claims:We Claim:
1. A process for the production of industrial grade magnesium aluminate spinel (MgAl2O4) with little expansion / shrinkage comprising admixing Magnesia (MgO>96%) with pure Alumina (Al2O3 > 99%)and firing at elevated temperature.

2. A process as claimed in claim 1, wherein the proportion of Magnesia is 90 to 10 % and Alumina is 10 to 90 % by wt.

3. A process as claimed in claim 1,wherein the proportion of Magnesia is 75 – 80% and Alumina is 20 – 25 % by wt.

4. A process as claimed in claims 1, wherein the proportion of Magnesia is 78% and Alumina is 22 % by wt.

5. A process as claimed in claims 1 and 4, wherein the Magnesia used is of industrial grade.

6. A process as claimed in claims 1 to 5, wherein the admixture of Magnesia and Alumina is made using PVA solution and pressed in carver plate at 20 T pressure and the same dried.

7. A process as claimed in claims 1 to 6, wherein the admixture of Magnesia and Alumina is fired at about 1200 to 1550o C for about 2 hours and cooled to room temperature.

8. A process as claimed in claims 1 to 6, wherein the admixture of Magnesia and Alumina is fired at about 1400oCfor about 2 hours and cooled to room temperature.

9. A process as claimed in claims 1 to 6, wherein the product Magnesium Aluminate spinel is having low expansion properties (2.8% linear) after firing at about 1200o C, 1400o C, and 1500o C for about 2 hours respectively with cooling the samples to room temperature each time after firing.

10. A process as claimed in claims 1 to 6, wherein the product Magnesium Aluminate spinel is having very low shrinkage (1 % linear) of the developed samples when fired at 1500o C for about 2 hours and cooled to room temperature.

11. Spinel bricks and castables made of magnesium aluminate spinel by the process as claimed in claims 1 to 9.

Dated: this 19th day of March, 2018

, Description:

A PROCESS FOR THE PRODUCTION OF INDUSTRIAL GRADE MAGNESIUM ALUMINATE SPINEL (MgAl2O4) AND SPINEL BRICKS AND CASTABLES MADE THEREFROM

FIELD OF THE INVENTION:

This invention relates to process development of industrial grade spinel formation and study of its expansion / shrinkage properties. This invention particularly relates to a process development of industrial grade magnesium aluminate spinel (MgAl2O4) from industrial grade magnesia (>96% MgO) and alumina (Al2O3> 99 %).The invention also relates to spinel bricks and castables made there from.

BACKGROUND OF THE INVENTION AND PRIOR ART:
Magnesia is widely used as refractory for its high temperature properties for lining of steel making furnaces in various refractory forms like magnesia-carbon (MgO-C) bricks, magnesia-alumina spinel (MgAl2O4), magnesite, magnesite -chrome and for basic castables and ramming mass etc.However MgO has high co-efficient of expansion (13.5x10-6/K at 1000oC), low spall resistant and hydration resistant in presence of water. It is also corroded specially in presence of FeO, Fe2O3 forming Magnesio-Wusite/ Magnesio-ferrite solid solution; hence often Carbon is added for its non-wettable characteristics by slag.
MgAl2O4 has long been considered an important ceramic material. MgAl2O4 possesses useful physical, chemical and thermal properties, both at normal and elevated temperatures. This spinel contains a lot of interstitial vacant Octahedral and Tetrahedral sites and is considered to prevent slag corrosion by trapping large amount of bivalent and Trivalent ions (Mn++, Fe++, Fe+++).It melts congruently at 2135o C which shows high resistance to attack by most of the acids and alkalis and has low electrical losses. It is used as a refractory in lining of steel-making furnaces, transition and burning zones of cement rotary kilns, checker work of the glass furnace regenerators, sidewalls and bottom of the steel ladles, glass furnaces and melting tanks. Hence the MgO-Al2O3 spinel is always a matter of interest of Refractory researchers. In the existing art there are reports of some scholarly research articles in the field which are given below as references:
I) Effect of raw materials on formation and densification of magnesium aluminate spinel – R. Sarkar and S. Sahoo, Ceramics International, Vol 40, Issue 10, Part B, Dec, 2014, P16719 – 16725.
II) Densification and characterisation of magnesium aluminate spinel from commercial grade reactants: Effect of milling and additives – S. K. Mohan, Dissertation for M. Tech. submitted to National Institute of Technology, Rourkela, Odisha, Department of Ceramic Engineering, July, 2016.
III) A review of magnesium aluminate (MgAl2O4) spinel synthesis, processing and applications – I. Ganesh, J. International Materials Reviews, Vol 58, 2013 ; Issue 2.
IV) Synthesis and densification of magnesium aluminate spinel: effect of MgO reactivity – H S Tripathi et al, Ceramics International, Vol 29, 2003, pp 915 – 918.
In none of the said prior arts the present invention disclosed herein has been published or reported.

OBJECT OF THE INVENTION:
The main object of the invention is to develop a process for the production of industrial grade magnesium aluminate Spinel (MgAl2O4) by using industrial grade magnesia with little expansion / shrinkage.

Another object of the invention is to develop a process for the production of magnesium aluminate spinel economically.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:

Fig. 1 shows graphical comparison for Expansion / Shrinkage of MgO + Al2O3 compositions measured after firing and cooled at different temperatures

Fig. 2 shows volume change and formation of spinel after repetitive firing at 1200o C, 1400o C and 1550o C for 2 hours

Fig. 3 shows volume change and formation of spinel after repetitive firing at 1550o C for 2 hours

Fig. 4 shows XRD of different compositions fired at 1550o C for 2 hours

Fig. 5 shows XRD of different compositions after firing at 1200o C, 1400o C, and 1550o C for 2 hours

Fig. 6 shows MgO – Al2O3 sample fired at 1550o C for 2 hours

Fig. 7 shows MgO – Al2O3 sample fired at 1200o C, 1400o C and 1550o C for 2 hours

DETAILED DESCRIPTION OF THE INVENTION:

According to the invention there is provided a process for the production of industrial grade magnesium aluminate spinel (MgAl2O4) with little expansion and shrinkage which comprises admixing Magnesia ( MgO >96 %) with pure Alumina ( Al2O3>99 %) and firing at elevated temperature. The proportion of Magnesia used in the process ranges from 75 – 80% and Alumina from 20 – 25% by wt. The preferred proportion of Magnesia is 78% and Alumina 22% by wt. In one embodiment of the invention the Magnesia used is of industrial grade.

The invention includes spinel bricks and castables made from the magnesium aluminate spinel produced according to the invention.

The Process of production of magnesium aluminate spinel with different embodiments and experimental data and inferences are given below :
Industrial grade magnesia (>96% MgO) and high purity alumina (>99% Al2O3) was taken with varying compositions as mentioned in the Table-1. The samples were thoroughly mixed using PVA solutions and pressed in carver press at 20T pressure to form small cylindrical samples with 20 mm diameter and 5mm.thickness. Samples were dried at 200oC for 48 hours, and the dimensions were measured accurately.
Sample ID MgO% Al2O3%
P1 90 10
P2 50 50
P3 22 78
P4 10 90

Table: 1 : Experimental composition of MgO and Al2O3%

Samples were fired at 1200o C, 1400o C, and 1500o C for 2 hours respectively in furnace. Few samples were fired repetitively at all temperatures mentioned above. The cooled samples were measured to find the expansion or shrinkage occurred in the samples. These samples were studied in XRD tests to find the percent of Spinel at different stages and a thorough investigation have been made to correlate the percent of expansion/shrinkage with formation of spinel in the matrix for present system with respect to firing the above mentioned temperatures.

MgO-Al2O3samples (20mm F, 5 mm thickness) after firing was measured accurately to determine the volume of the samples and percent expansion and shrinkage were calculated based on initial sample volume. In all cases average of four sample data was taken. It was observed that after firing at 1200o C for 2 hrs all the four compositions (P1, P2, P3 and P4) in Table-2 and Figure-1 of the accompanying drawings, there is expansion characteristics of sample and after firing at 1400o C for 2 hrs all other samples except P3 have shrinkage property. All samples after firing at 1550o C showed shrinkage where P2 sample had the maximum expansion at 1200o C and minimum shrinkage at 1550o C.
Sample No. 1200 C/2hrs 1400 C/2hrs 1550 C/2hrs
P1 0.97 -12.14 -4.35
P2 11.47 -3.8 -21.24
P3 7.35 4.35 -3.12
P4 1.66 -3.03 -12.15

Table-2: Expansion / Shrinkage of MgO+Al2O3 Compositions
measured after firing and cooled at different temperatures

In another experiment the sample for all above compositions were fired incrementally i.e. first fired at 1200o C for 2 hrs and cooled and again fired at 1400o C for 2 hrs and again cooled and finally fired at 1550o C for two hours and final expansion was measured after cooling at room temperature. These samples and the samples fired at 1550o C for 2 hrs were tested in XRD for identification of different phases and quantifications of spinel formed during the experimental procedure. The results of samples fired are given in Fig-2 and Fig-3 of the accompanying drawings respectively.
From the X ray diffraction (Fig.-4 of the accompanying drawings) of the four compositions fired at 1550o C/2 hours it was observed that full formation of spinel in composition P3 was observed whereas in rest compositions either free alumina or magnesia was observed. In the repeated fired compositions (Fig.-5 of the accompanying drawings) also the spinelisation in composition P3 (22% MgO, 78% Al2O3) is fully completed. The repeated fired composition P3 has expansion with full spinelisation whereas in only fired at 1550o C/ 2 hrs there is little shrinkage and comparatively less spinelisation.

The microstructure of the samples (Fig.-6 & Fig.-7 of the accompanying drawings) also confirms full spinelisation in P3 compositions.

USEFULNESS OF THE INVENTION:

At present steel vessels are prepared by using MgO – C bricks which have carbon up to 12%. For making clean steel, carbon free vessels are required. This industrial grade spinel can be used in making spinel bricks which will be used in making steel vessels for making ultra low carbon steel whose vessel life will be better.

INDUSTRIAL APPLICABILITY:
Industrial grade spinel can be used in making high quality refractory bricks and castables that can be used in steel, glass and cement industry for lining of different vessels.
Although the invention described herein mentions a few embodiments, it could be possible for a person skilled in the art to develop other similar embodiments as well beyond those disclosed herein with little modifications. However, all such modifications are deemed to be within the scope of the invention claimed herein.