Title of Invention

"A PROCESS FOR THE PRODUCTION OF HIGH DENSITY HYDRATION RESISTANT LIME SINTER"

Abstract

A process for the production of high density hydration resistant lime sinter A process for the production of high density hydration resistant lime sinter by washing lime stone having imurity of less than 2% to remove external impurities by known methods if any, crushing the said washed limestone to a size 25 mm or below, calcining the limestone at a temperature in the range of 1000 to 1150°C for a period in the range of 2 to 5 hours, hydrating the calcined mass, drying the hydrated mass by known methods deagglomerating the hydrated mass by known methods, mixing to the said hydrated dried mass 1 to 4 weight percent additives selected from transition metal oxides capable of forming low melting compound, rare earth metal oxides capable of forming solid solution, or mixture thereof, pelletising the resultant mixture at a pressure of atleast 1000 kg/cm2, sintering the pellets so obtained at a temperature in the range of 1450°C to 1800°C for a period in the range of 2 hours to 15 hours allowing the resultant sinter to cool naturally to obtain high density hydration resistant v lime sinter.

Full Text

This invention relates to a process for the production of high density hydration resistant lime sinters. This invention particularly relates to a process for the production of high density hydration resistant lime sinters from relatively pure limestone or calcite containing less then 2% impurities. The sinter is useful as a base raw material for the production of lime refractories which has a potential and large scale application in Steel and Cement Industries. The fast changing steel technology with the introduction of bigger capacity Basic Oxygen Furnace, stringent conditions in ladle metallurgy with higher operating temperature needs superior quality basic refractories. Magnesite and dolomite enriched with graphite/carbon are traditionally used in these furnaces because of high slag corrosion resistance and superior refractory property. Lime/calcia inspite of being strong contender of magnesite and dolomite is not used as a refractory material for its high tendency towards atmospheric hydration. When the refractoriness and thermodynamic stability in the presence of carbon is considered, lime would be even better refractory material than dolomite or magnesite in the steel making conditions. Non metallic inclusions are formed when an element dissloved in liquid steel reacts with refractory constituents like Si02/Cr203 etc of lining. The other advantage of lime refractories is its resistance to these dissolved elements, and thus formation of non metallic inclusions is reduced and this ultimately helps to improve the cleanliness of steel. The advantages of lime is utilised in some other countries by comixing it with dolomite/magnesite in brick making or cosintering at an early stage. Pure lime refractory is not used due to its perishing tendency. However, it was felt that if highly dense lime was produced, it could develop an excellent refractory owing to its stability, slag resistance and providing cleanliness of steel. The basic raw material for producing lime sinter is lime stone. High purity lime stone is available easily not only in India but throughout the world. The higher refractoriness of lime creates difficulty to produce dense lime sinter in the industrially available furnace temperature. Therefore, achievement of good quality lime sinter from purer lime stone lump needs a temperature above 2000°C. The low flux lime stone (3% impurity) can be densified at a relatively lower temperature, but the product is not suitable owing to its lower densification. Reference may be made to the work of L.L. Wong and R.C. Bradt published in American Ceramic Society Bulletin, Vol 69, No. 7, 1990 wherein it has been shown that impure limestone resulted poor densification at high firing temperature, 1600°C. It is therefore, necessary either to introduce mineraliser/additive or convert the lime stone into a reactive mass or both, bring down the sintering temperature for achieving improvement in the quality of the product. The main objective of the present invention is to provide a process for the production of high density hydration resistant lime sinter. Another object is to use purer (less then 2% impurities) variety of lime stone which will provide maximum hydration resistant to atmospheric moisture. Yet another objective of the present invention is to maximize the grain size and their homogeneity in the matrix. Still another objective is to provide sinter having microstructure which comprise of an average uniform grain size and even distribution of pores, which should be minimum in order to restrict the moisture in the grain matrix. Accordingly, the present invention provides a process for the production of high density hydration resistant lime sinter which comprises washing lime stone having imurity of less than 2% to remove external impurities by known methods if any, crushing the said washed limestone to a size 25 mm or below, calcining the limestone at a temperature in the range of 1000 to 1150°C for a period in the range of 2 to 5 hours, hydrating the calcined mass, drying the hydrated mass by known methods deagglomerating the hydrated mass by known methods, mixing to the said hydrated dried mass 1 to 4 weight percent additives selected from transition metal oxides capable of forming low melting compound, rare earth metal oxides capable of forming solid solution, or mixture thereof, pelletising the resultant mixture at a pressure of atleast 1000 kg/cm2, sintering the pellets so obtained at a temperature in the range of 1450°C to 1800°C for a period in the range of 2 hours to 15 hours allowing the resultant sinter to cool naturally to obtain high density hydration resistant lime sinter. In an embodiment of the present invention the transition metal oxide additive used may be selected from ferric oxide, titanium dioxide, copper oxide, vanadium pentoxide or mixture thereof. In another embodiment of the present invention the rare earth metal oxides additives used may be selected from cerium oxide, lanthanum oxide or mixture thereof. The hydration is minimized not only by achieving higher densification. Uniform grain growth is also needed to the extent as far as possible. Densification along with grain growth will make the lime grain more hydration resistant and slag resistant refractory. In the presence of high basic and ferruginous slag magnesia is known to be comparatively better than calcia. If the grain size of calcia is sufficiently large, the slag resistance will be significantly improved. Therefore, another objective of the invention is to develop lime sinter with relatively large grain size uniformly distributed in the matrix. This will not only improve the resistance towards hydration but also improve the flexural strength at elevated temperatures. Lime has a theoretical density of 3.32 gm/cc. Achievement of this density in case of a purer variety of lime stone containing less than 2 percent impurity is extremely difficult at the commercially available calcination temperature. Thus to achieve high densification at relatively lower temperature one has to start with reactive lime and thus the choice of precursor becomes very much important. In fact it has been found that hydroxide of lime produce much finer CaO grain than that received from carbonate of calcium. The natural lime stone which is the carbonate of calcium can be converted into hydroxide form by pre-calcination and followed by hydration of carbonate derived oxide. The pre-calcination temperature in the range of 1000° to 1150°C is very much related to the characteristics of lime stone such as grain size, impurity content etc. The selection of precalcination temperature is primarily done from the knowledge of Differential Thermal Analysis peak. When decomposition of lime hydroxide occurs an enormous volume expansion takes place and results fine particles of CaO. The reactivity can be ascertained by measuring the specific surface area in relation to pre-calcination temperature. To achieve high densification the fine particles of hydroxides needs pelletisation under high pressure either by pelletising machine or under uniaxial pressure by hydraulic press machine at pressure varying from 1000 to 1500 kg/cm2. The pellets for briquetting machine was pillow shaped (20mm x 10mm) and for hydraulic press the dimension is 25mm x 25mm high. The pellets were then finally calcined at a temperature of 1600° to 1700°C. In the double calcination process although densification is attained at relatively lower temperature (below 1700°C), it is difficult to achieve substantial grain growth. Additives play an important role here to control the grain growth and are required to be incorporated in the reactive powder before pelletisation. There are two different classes of additives which facilitate grain growth by different mechanisms. A group of additives favour grain growth by formation of low melting compound while another class of additives are effective by formation of defects as a result of solid solution. In the solid state process the cation (Ca2 + ) of lime is substituted by the cation of additive. According to vacancy difference, cation vacancy or defective structure will be created in the crystal lattice of lime. The lime sintering is thus enhanced at a much lower temperature due to excess stored energy in the vacant site of lime lattice. The disadvantage of liquid forming additive is, if added above certain amount the refractory property will be deteriorated. Moreover, liquid state sintering forms subrounded grains, which impairs the hot strength. In double stage calcination process an additional low temperature calcination is required and the process requires little more energy for generation of reactive lime. The lime sinters were characterised by determining three properties like (I) Bulk Density and Apparent Porosity (2) Hydration resistance and (3) Microstructure. Bulk density and apparent porosity were measured by xylene penetration method under vacuum and the porosity was measured in xylene using Archimedes principle. The hydration resistance property was determined by measuring the fine dust generated below 35 mesh Tyler in 95% relative humidity at 50°C for a testing period of 3 hours. The lime sinters of grain size -5 +10 BS mesh was subjected to this atmosphere in a humidity cabinet and the weight gain as well as fine dust as % weight loss was measured after the experiment. The grain size of lime and their size distribution were determined by evaluating microstructure under optical microscope. The experiment was carried out on the polished section of the sample which was subjected to precise thermal etching. The grain size and their distribution were measured by a sophisticated Image Analyser. The invention will now be described with the help of the following examples for carrying out the process in actual practice. However, these examples should not be construed as to limit the scope of invention. EXAMPLE - 1 The limestone was washed and then crushed to 25 ram size, calcined at 1000°C for two hours and cooled the calcined mass in the furnace. The sample was then hydrated by allowing the reaction of calcined mass with water and then dried at 110°C for 6 hours. The calcium hydroxide powders was briquetted at a pressure of 1000 kg/cm2 dried and finally sintered at 1650°C for 2 hours holding period. The result showed BD-3.12 gm/cc, apparent porosity of 0.8% and hydration loss of 1%. The average grain size of lime was 47 micron. EXAMPLE - 2 The limestone was washed and then crushed to 25 mm size, calcined at 1100°C for 2 hours. The calcined mass was cooled by air-quenching from 1100°C to room temperature followed by hydration with water. The calcined hydrated limestone was briquetted at a pressure of 1200 Kg/cm2, dried and finally sintered at 1650°C for 2 hours. The bulk density achieved was 3.18 gm/cc with apparent porosity of 0.3% and hydration loss was 0.7%. EXAMPLE - 3 The limestone was washed and then crushed to 25 mm size, calcined at 1150°C for 2 hours. The calcined mass was motor generated for 1150°C. The calcined hydrated limestone was briquetted at a pressure of 1500 Kg/cm2, dried and finally sintered at 1650°C for 2 hours. The result obtained was bulk density - 3.10 gm/cc with 2.4% apparent porosity and hydration loss of 1.7%. EXAMPLE - 4 Limestone was washed and then crushed to 25 mm size, calcined at 1100°C for 2 hours and cooled the calcined mass in the furnace. The calcined mass was hydrated with water and then dried at 110°C for 6 hours. The calcined hydrated limestone was mixed with 2 weight percent titanium dioxide. Briquetting of the mixture was done under a pressure of 1200 Kg/cm2, dried and finally sintered at 1600°C for 3 hours. The bulk density achieved was 3.20 gm/cc with apparent porosity of 0.3% and hydration loss was 2.8%. EXAMPLE-5 Limestone was washed and then crushed to 25 mm size, calcined at 1100°C for 2 hours and cooled the calcined mass in the furnace. The calcined mass was hydrated with water and then dried at 110°C for 6 hours. The calcined hydrated limestone was mixed with 2 weight percent Ferric Oxide. Briquetting of the mixture was done under a pressure of 1200 Kg/cm2, dried and finally sintered at 1600°C for 3 hours. The bulk density achieved was 3.19 gm/cc with apparent porosity of 0.5% and hydration loss was 1.9%. The grain size was 149 micron. EXAMPLE - 6 lime stone was washed and then crushed to 25 mm size, calcined at 1100°C for 2 hours and cooled the calcined mass in the furnace. The calcined mass was hydrated with water and then dried at 110°C for 6 hours. The calcined hydrated limestone was mixed with 2 weight percent Lanthanum Oxide. Briquetting of the mixture was done under a pressure of 1200 Kg/cm2, dried and finally sintered at 1600°C for 3 hours. The bulk density achieved fwas 3.20 gm/cc with apparent porosity of 0.3% and hydration loss was 2.5%. The grain size was 108 micron. EXAMPLE - 7 Limestone was washed and then crushed to 25 mm size, calcined at 1100°C for 2 hours and cooled the calcined mass in the furnace. The calcined mass was hydrated with water and then dried at 110°C for 6 hours. The calcined hydrated limestone was mixed with 2 weight percent Cerium Oxide. Briquetting of the *-mixture was done under a pressure of 1200 Kg/cm , dried and finally sintered at 1600°C for 3 hours. The bulk density achieved fwas 3.23 gm/cc with apparent porosity of 0.1% and hydration loss was 3.5%. The grain size was 95 micron EXAMPLE - 8 Limestone was washed and then crushed to 25 mm size, calcined at 1100°C for 2 hours and cooled the calcined mass in the furnace. The calcined mass was hydrated with water and then dried at 110°C for 6 hours. The calcined hydrated limestone was mixed with 2 weight percent Vanadium Pentoxide. Briquetting of the mixture was done under a pressure of 1200 Kg/cm2, dried and finally sintered at 1600°C for 3 hours. The bulk density achieved was 3.05 gm/cc with apparent porosity of 0.4% and hydration loss was 3.8%. The grain size was 100 micron. EXAMPLE - 9 Limestone was washed and then crushed to 25 mm size, calcined at 1100°C for 2 hours and cooled the calcined mass in the furnace. The calcined mass was hydrated with water and then dried at 110°C for 6 hours. The calcined hydrated limestone was mixed with 2 weight percent Copper Oxide. Briquetting of the *~mixture was done under a pressure of 1200 Kg/cm , dried and finally sintered at 1600°C for 3 hours. The bulk density achieved fwas 3.08 gm/cc with apparent porosity of 0.3% and hydration loss was 2.1%. The grain size was 95 micron. EXAMPLE - 10 Limestone was washed and then crushed to 25 mm size, calcined at 1100°C for 2 hours and cooled the calcined mass in the furnace. The calcined mass was hydrated with water and then dried at 110°C for 6 hours. The calcined hydrated limestone was mixed with 2 weight percent Ferric Oxide. Briquetting of the «2 mixture was done under a pressure of 1200 Kg/cm , dried and finally sintered at 1450°C for 15 hours. The bulk density achieved was 3.14 gm/cc with apparent porosity of 0.2% and hydration loss was 2.5% The grain size was 155 micron. EXAMPLE - 11 Limestone was washed and then crushed to 25 mm size, calcined at 1100°C for 2 hours and cooled the calcined mass in the furnace. The calcined mass was hydrated with water and then dried at 110°C for 6 hours. The calcined hydrated limestone was mixed with 2 weight percent Lanthanum Oxide. Briquetting of the mixture was done under a pressure of 1200 Kg/cm2, dried and finally sintered at 1600°C for 3 hours. The bulk density achieved fwas 3.10 gm/cc with apparent porosity of 0.2% and hydration loss was 2.1%. The grain size was 129 micron. EXAMPLE - 12 Limestone was washed and then crushed to 25 mm size, calcined at 1100°C for 2 hours. The calcined mass was cooled by air quenching from 1100°C to room temperature followed by hydration with water. The calcined hydrated limestone was briquetted under pressure 300, 500, 750, 1000, 1500, 2000 & 2500 Kg/cm2. Brique^es were fired a£ 1650°C for 2 hours. The fired density measured indicate that highest bulk density ( 3.18 gm/cc) was achieved at 1500 Kg/cm2. No significant improvement was achieved beyond fabrication pressure of 1500 Kg/cm2. The main advantages are : (1) Relatively high densif ication of lime can be achieved by double calcination technique. (2) High densif ication and high resistance to hydration can be achieved by final calcination at the temperature range 1450°C to 1650°C with varying soaking time. (3) Additives help the improvement in grain size and hydration resistance without deteriorating the high temperature load bearing capacity. We claim : 1. A process for the production of high density hydration resistant lime sinter which comprises washing lime stone having of less than 2% to remove external impurities by known methods if any, crushing the said washed limestone to a size 25 mm or below, calcining the limestone at a temperature in the range of 1000 to 1150°C for a period in the range of 2 to 5 hours, hydrating the calcined mass, drying the hydrated mass by known methods deagglomerating the hydrated mass by known methods, mixing to the said hydrated dried mass 1 to 4 weight percent additives selected from transition metal oxides capable of forming low melting compound, rare earth metal oxides capable of forming solid solution, or mixture thereof, pelletising the resultant mixture at a pressure of atleast 1000 kg/cm2, sintering the pellets so obtained at a temperature in the range of 1450°C to 1800°C for a period in the range of 2 hours to 15 hours allowing the resultant sinter to cool naturally to obtain high density hydration resistant lime sinter. 2. A process as claimed in claim 1 wherein the transition metal oxide additive used is selected from ferric oxide, titanium dioxide, copper oxide, vanadium pentoxide or mixture thereof. 3. A process as claimed in claims 1-2 wherein the rare earth metal oxides used is selected from cerium oxide, lanthanum oxide or mixture thereof. 4. A process for the production of high density hydration resistant lime sinter substantially as herein described with reference to the examples.

Documents:

444-del-1999-abstract.pdf

444-del-1999-claims.pdf

444-del-1999-correspondence-others.pdf

444-del-1999-correspondence-po.pdf

444-del-1999-description (complete).pdf

444-del-1999-form-1.pdf

444-del-1999-form-19.pdf

444-del-1999-form-2.pdf

444-del-1999-form-3.pdf

444-del-1999-petition-138.pdf