Title of Invention

A PROCESS FOR THE PRODUCTION OF MAGNESITE-CHROME AGGREGATES USEFUL AS REFRACTORY MATERIAL

Abstract

Magnesitechrome refratories are combination of magnesia and chrome ore with Cr2O3 content varying in a wide range. These refractories possess good high temp properties and moderately volume stability at elevated temperature. The major application areas of these refractories are in the steel, cement and copper industry. Mag-chrome refractories, till recently was being produced from magnesia and massive chrome ore. However, as this variety of ores is depleted, the alternate friable chrome ore has become the other option. The present invention relates to a process for the production of magnesite-chrome aggregates from friable chrome ore with Cr2O3 content in the range of 5 to 30%. The process consists of mixing of sintered magnesia and friable chrome ore, milling, incorporation of additives and sintering in the range of 1550 deg.C to 1750 deg. C. The selected additives either form liquid phase or changes the phase assemblage and reduces dihedral angle. The advantage of this process is development of low porosity magnesite-chrome aggregates from a relatively cheaper raw material by single firing. The direct bonding and exsolution of chrome spinel in the magnesia grains are observed in the microstructure of aggregates.

Full Text

The present invention relates to a process for the production of magnesite-chrome aggregates useful as refractory material. These sintered aggregates will be used as a synthetic raw material for the manufacture of shaped and unshaped magnesite-chrome refractories having large-scale application in steel, cement and copper industries. Magnesite-chrome refractories are combination of magnesia and chrome ore with Cr2O3 content varying in a wide range. These refractories possess good high temp properties and moderately volume stability at elevated temperature. The major application areas of these refractories are in the steel, cement and copper industry. Magnesite-chrome refractories are useful owing to its high refractoriness, high temperature stability, moderately low thermal expansion and comparatively neutral nature. The Cr2O3 content changes depending upon the application areas. The major application of these refractories are in LD converters, ladles, VOD/AOD, EAF, copper furnace, cement rotary kilns. These refractories are also used in secondary steel refining vessels because of their resistance to wide varities of slags and then stability at high temperature. Till recently, mag-chrome refractories were produced from a mixture of magnesia and massive variety of chrome ore. However, massive chrome ore is being depleted throughout the world and the alternate friable chrome ore is available. But the deficiency of this type of chrome ore is that it cannot be used directly for brick manufacturing. Initially, the friable chrome ore is mixed and reacted with magnesia at high temperature for the development of Magnesite-chrome aggregates (prereacted grains). These grains are used as intermediate raw materials for the manufacturing of mag-chrome bricks. Reference may be made to American Ceramic Society Bulletin, Vol 40, No. 8, pp 498-502 (1961); wherein friable chrome ore was mixed with magnesia, ball milled, pelleted and fired above 1760°C to obtain aggregates. In this process the high temperature of firing made mag-chrome aggregates a costly material and could not be used for industrial production. Thus, the process was not suitable for the large-scale economic production. The pure mineral chromite has the formuia FeCr2O4 and is found rarely in nature. Naturally occurring chromite is a solid solution of the simple spinels with the composition (Fe2+, Mg2+, Cr3+, Al3+, Fe3+)2 04. It has a refractoriness of 2135°C. SiO2 is the most detrimental impurity in chrome ore. High SiO2 forms low melting silicate while low SiO2 increases bursting expansion. The optimum SiO2 content is around 3 wt%.. The CaO content of the ore should be 1% to minimize flux formation and the desirable level of Cr2O3 is above 40 wt%. Magnesia-chrome aggregate is prone to bursting expansion particularly when developed from friable chrome ore. This is a phenomenon, which is related to unequal diffusion of Cr2O3 (from MgO Cr2O3) when it comes in contact with iron oxide (FeO. Fe2O3) of slag. Reference may be made to the symposium of British Ceramic Society (Editors A.T. Green et al) pp 488 - 512 (1953); where it is described that this characteristic is related to the mixed crystal formation from normal and inverse spinels when both ferrous and ferric ions are present. Chrome ore, which originally has high percentage of Cr2O3 have expanded the most. The chrome ore which has low FeO contents and which is replaced to some extent by Al2O3 expands least. Magnesite-chrome was originally developed to improve the spalling resistance of Magnesite brick which otherwise is an excellent refractory in the presence of basic and ferruginous slag. Direct bonded mag-chrome refractories have the characteristics of high hot strength, dimensional stability and improved thermal shock resistance. Reference in Interceram, Vol-16, No. 1 pp 32-35 (1967) describes the mechanism of bond formation between grains of chrome spinel and its surrounding magnesite grains. The direct bond formation generally takes place between 1500° to 1700°C by reducing the gap formed between chrome spinel and periclase. Reference may be made to American Ceram Soc. Bull, Vol 69, No. 7, pp 1177 to 83 (1990) ; where the effect of attrition milling on the sintering of MgO-Cr2O3 was described. In this reference the raw material used was electrofused MgO-Cr2O3 powder In this process high densification was achieved in the temperature range of 1600° to 1700°C. However the raw materials was electrofused which is very costly. Reference may also be made to US Patent 40, 08. 092 (1977) where the mag-chrome products were developed in incorporating CaO or SrO additive and sintering was carried out preferably above 1800°C. The specific gravity of the products corresponds to a total porosity of 9.5 % to 1 i .90%. The drawback of the process is the sintering temperature of 1800°C, which is a costly proposition, and total porosity is quite high which deteriorates the refractory properties. The main drawbacks of the hitherto known processes are: 1) The sintering temperature is above 1760°C when the raw materials are processed by milling such as ball mill and some additives are used. 2) In case of attrition milling the sintering temperature is in excess of 1600°C. However, the raw materials used are electrofused MgO-Cr2O3 powder, which is extremely costly. The main object of the present invention is to provide a process for the production of magnesite-chrome aggregates useful as refractory material which obviates the above noted drawbacks. Another object of the present invention is to produce mag-chrome aggregates from different reactive magnesia. Yet another object of the present invention is to utilize friable chrome ore for the development of mag-chrome aggregates which is otherwise considered to be as a waste material. Again another object of the present invention is to increase the efficiency of the sintering process by the addition of suitable additive. Still another object of the present invention is to generate dense aggregate at a comparatively lower sintering temperature than that developed till now. Mag-chrome refractories, till recently was being produced from magnesia and massive chrome ore. However, as this variety of ores is depleted, the alternate friable chrome ore has become the other option. The present invention relates to a process for the production of magnesite-chrome aggregates from friable chrome ore with Cr2O3 content in the range of 5 to 30%. The process consists of mixing of sintered magnesia and friable chrome ore, milling, incorporation of additives and sintering in the range of 1550°C to 1750°C. The selected additives either form liquid phase or changes the phase assemblage and reduces dihedral angle. The advantage of this process is development of low porosity magnesite-chrome aggregates from a relatively cheaper raw material by single firing. The direct bonding and exsolution of chrome spinel in the magnesia grains are observed in the microstructure of aggregates. Accordingly the present invention provides a process for the production of magnesite chrome aggregates useful as refractory material which comprises mixing magnesia and friable chrome ore in a ratio to obtain a composition in the range of 5 to 30% Cr2O3, adding to the mixture additive such as 1 to 4 wt% TiO2, 1 to 5 wt% ZrO2, milling the mixture for a period in the range of 3 to 24 hours by conventional processes, drying the resultant mixture at a temperature in the range of 95 °C to 110°C for a period in the range of 24 to 40 hours to obtain a mixed dry powder, adding and mixing an organic binder to the dried mixed powder and uniaxially pressing the resultant powder at a pressure in the range of 800 to 1200kg/cm2 to obtain pellets, drying the pressed pellets at a temperature in the range of 100 to 110°C for a period in the range of 24 to 36 hours, sintering the dried pellets at a temperature in the range of 1550 to 1750°C with a holding period in range of 2 to 4 hours at the peak temperature to obtain magnesite-chrome aggregates. In an embodiment of the present invention the raw materials used may be such as caustic magnesia , sintered magnesia and friable type chrome ore. In another embodiment of the present invention the magnesia used may be of purity greater than 96%. In yet another embodiment of the present invention the Cr2O3 content in the friable chrome ore may be in the range of 40-60%. In still another embodiment of the present invention the raw materials may be separately crushed and ground to pass through 60 mesh BS sieve before mixing and milling. In a further embodiment of the present invention the milling may be done by conventional processes such as attrition milling, pot milling, vibro milling . In yet another embodiment of the present invention the milling may be effected with or without an organic solvent. In still another embodiment of the present invention the binder used before pressing may be such as polyvinyl alcohol, dextrin, glycol in the range of 4-7 wt%. The process steps of the present invention for the production of magnesite chrome aggregates involves: 1) mixing magnesia such as sintered, caustic of greater than 96 % purity and friable chrome ore having 40 - 60 % Cr2O3 content, in a ratio to obtain a composition in the range of 5-30% Cr2O3, 2) adding 1-4 wt% TiO2 or 1-5 wt% ZrO2 to the mixture, 3) milling the mixture for a period in the range of 3-24 hours with or without the presence of an organic solvent, 4) drying the resultant mixture at a temperature in the range of 95°C to 110°C for a period in the range of 24-40 hours to obtain a mixed dry powder, 5) adding and mixing 4 - 7 wt % of an organic binder to the dried mixed powder and uniaxially pressing the resultant powder to pelletise at a pressure in the range of 800-1200kg/cm2, 6) drying the pressed pellets at a temperature in the range of 100-110°C for a period in the range of 24-36 hours, 7) sintering the dried pellets at a temperature in the range of 1550 to 1750°C with a holding period in range of 2-4 hours at the peak temperature to obtain the desired magnesite-chrome aggregates. In the present invention emphasis is given to develop dense mag-chrome aggregates with a phase assemblage of minimum remnant chrome ore and exsolution of secondary spinels over MgO grains. The selected additive TiO2or ZrO2 enhances the densification process by lowering the dihedral angle in Mag-chrome. TiO2 helps liquid phase sintering in MgO by the formation of liquid phase at 1707°C. The sintered magnesite-chrome aggregates were characterized by determining properties like (1) Bulk density and apparent porosity (2) High temperature flexural strength (3) X-ray and (4) Microstructure. Bulk density was measured by xylene displacement method under vacuum using Archimedes principle. High temperature strength was determined by three point bend test at 1300°C. Phase identification was carried out by x-ray diffraction and microstructure of the polished samples was observed by optical microscope. The improved high temperature properties of mag-chrome bricks can be obtained when the Magnesia and chrome spinel grains are direct bonded. This is achievable when the chrome ore is massive in nature. However, similar properties are also obtained when prereacxed grains are used by utilizing friable chrome ore and magnesia, In the latter case it is desirable to develop homogeneous microstructure with chrome spinels in the magnesia grains formed by exsolution. The prior art details reveal that the sintering temperature for the production of mag-chrome aggregate is in excess of 1760°C. Otherwise when the sintering temperature is as low as 1600°C the raw materials is electrofused MgO-Cr2O3 which is extremely costly. The novelty of the present invention is the development of low porosity ( The magnesia when used in the sintered form suppress the spinelisation reaction with Cr2O3 at the spinel formation temperature. This reduces the rate of expansion due to spinel formation and promotes sintering. The selected additive TiO2 on the one side forms liquid phase with MgO and enhances liquid phase sintering. On the other hand both TiO2 and ZrO2 change the phase assemblage in the spinel grains and reduces the dihedral angle. In addition to this milling particularly by attrition grinds the powder to submicron level and increases the energy content of the system. As a result densification is substantially enhanced. Thus the inventive steps of the present process are as follows : 1) Adding TiO2 or ZrO2 as additive. 2) Utilisation of sintered magnesia raw materials. 3) Attrition milling of the mixture. The following examples are given by way of illustration of the present invention and therefore should not be construed to limit the scope of the present invention. Example - 1: Sintered magnesia and friable chrome ore were separately crushed and ground to pass through 60 mesh BS sieve. Magnesia powder was mixed with chrome ore in such a proportion to obtain 18% Cr2O3 of the batch. TiO2 of 3 wt% was added to the batch. The resultant mixture was attrition milled for 5 hours in presence of acetone as dispersing medium. The material was then dried at 100°C for 24 hours and subsequently mixed with 6 wt% PVA solution (5% concentration) as a green binder. The powder was then pressed uniaxially at 1000Kg/cm2. The briquettes were dried for 24 hours and sintered at 1550°C with 2 hours soaking. The heating rate was maintained at 5°C/min. The sintered aggregates had an apparent porosity of 2% and bulk density of 3.63 gm/cc. XRD study showed major peak of magnesia and medium peak of chrome spinel. The microstructure, which was developed, had exsolved spinels on magnesia grains. Example - 2: Sintered magnesia and friable chrome ore were separately crushed and ground to pass through 60 mesh BS sieve. Magnesia powder was mixed with chrome ore in such a proportion to obtain 5% Cr2O3 of the batch. ZrO2 of 3 wt% was added to the batch. The resultant mixture was attrition milled for 4 hours in presence of acetone as dispersing medium. The material was then dried at 100 C for 24 hours and subsequently mixed with 6 wt% PVA solution (5% concentration) as a green binder. The powder was then pressed uniaxially at 800Kgcm2. The briquettes were dried for 24 hours and sintered at 1550°C with 2 hours soaking. The heating rate was maintained at 5°C/min. The sintered aggregates had an apparent porosity of 1% and bulk density of 3.53 gm/cc. XRD study showed major peak of magnesia and minor peak of chrome spinel. The microstructure, showed larger grains of magnesia and exsolved spinel are not visible. Example-3: Friable chrome ore was sieved through 60 mesh BS sieve and vibroground in a tubular grinder containing Al2O3 rods for 2 hours. As received caustic magnesia [lightly calcined (say 1100°C)] was mixed with chrome ore in such a proportion so as to obtain 18% O2O3 of the batch. TiO2 of 4 wt% was also added to the batch and it was mixed in a fluidised bed mixer for 5 minutes. The batch was then moved with 5 wt% PVA (5% PVA solution) and pressed uniaxially at 1000 Kg/cm2. The briquettes were dried for 24 hours and sintered at 1750°C with 2 hours soaking. The heating rate was maintained at 5°C/min. The sintered aggregates had an apparent porosity of 5% and bulk density of 3.35 gm/cc. XRD study showed major peak of magnesia and medium peak of chrome spinel. The microstructure, which was developed, had exsolved spinels on magnesia grains. Example-4: Friable chrome ore and sintered magnesia were separately crushed, passed through 60 mesh BS sieve and vibroground in a tubular grinder containing Al2O3 rods as grinding media for 2 hours. The raw materials were proportioned to obtain 18% Cr2O3 content of the batch. TiO2 of 3 wt% was added to the batch and mixed in a fludized bed mixer for 5 minutes. PVA to the tune of 5 wt% (5% PVA solution) was added as binder, mixed and uniaxially pressed at 1000 Kg/cm2. The pellets were dried for 24 hours and sintered at 1750°C with 2 hours soaking. The sintered aggregates had an apparent porosity of 6% and bulk density of 3.37 gm/cc. XRD analysis showed the peak of magnesia and chrome spinel. Example-5: Friable chrome ore was passed through 60 mesh BS sieve and vibroground in a tubular grinder containing Al2O3 rods as grinding media for 2 hours As received caustic Magnesia and ground chrome ore was in such a proportion so as to obtain Cr2O3 content of 5 wt%. ZrO2 of 3 wt% was added to the batch and mixed in fluidized bed mixer for 5 minutes. PVA to the tune of 4 wt% {5% PVA solution) was added as binder, mixed and pressed uniaxially at a pressure of 800 Kg/cm2. The briquettes were dried and sintered at 1700°C with 2 hours soaking. The mag-chrome aggregates showed an apparent porosity of 2% and bulk density of 3.28 gm/cc, XRD pattern showed major magnesia peak with minor spinel peak. Magnesia grains were seen in the microstructure and exsolved spinels are not visible. Example-6: Friable chrome ore and sintered Magnesia were separately crushed, passed through 60 mesh BS sieve and vibroground for 2 hours. Desired proportions of raw materials were mixed according to their chemical analysis to obtain 5% Cr2O3 content in the mix. TiO2 of 1 wt% was added to the batch and then mixed homogeneously in a fludised bed mixer for 5 minutes. PVA to the tune of 6 wt %(5% PVA solution) was mixed with the batch as green binders and pressed uniaxially at 1200 Kg/cm2. The pellets after drying for 24 hours at 100°C, was subjected to sintering at 1700°C for 3 hours holding period. The sintered material showed a bulk density of 3.32 gm/cc and apparent porosity of 2%. The XRD analysis showed major peak of Magnesia and the minor peak of chrome spinel. Grain to grain binding of magnesia was observed in the photomicrographs. Flexural strength at 1300°C was 700 Kg/cm2. The main advantages of the present invention are: 1. Development of low porosity mag-chrome aggregates at a firing temperature of 1550°-1750°C which is low considering the type of grinding system used. 2. The raw materials used are relatively cheaper compared to that used in other low temperature processes. Friable chrome ore used is considered to be waste material. 3. A wide range of mag-chrome aggregates can be obtained by this process with a homogeneous microstructure, which has exsolution of chrome spinel in magnesia grains.

Documents:

87-DEL-2002-Abstract(17-1-2008).pdf

87-DEL-2002-Claims(17-1-2008).pdf

87-DEL-2002-Correscpondence-Others(17-1-2008).pdf

87-DEL-2002-Description (Complete)(17-1-2008).pdf

abstract.pdf

claims.pdf

correspondence-others.pdf

correspondence-po.pdf

description complete.pdf

form-1.pdf

form-18.pdf

form-2.pdf

form-3.pdf