Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2000-01-21
2001-06-12
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S637000, C423S275000, C502S340000
Reexamination Certificate
active
06245315
ABSTRACT:
FIELD OF INVENTION
This invention relates to a process for the production of high density hydration resistant sintered lime. This invention particularly relates to a process for the production of high density hydration resistant sintered lime from relatively pure limestone or calcite containing less than 2% impurities. The sintered lime are prepared by the process of this invention useful as an intermediate raw material for the production of lime refractories which has a potential and large scale application as lining materials in kilns and furnaces in Steel and Cement Industries.
BACKGROUND OF THE INVENTION
The steel technology is rapidly changing with the introduction of bigger capacity Basic Oxygen Furnace, stringent conditions in ladle metallurgy with higher operating temperature. This needs superior quality basic refractories for high service performance. Magnesite and dolomite either tar or pitch bonded with graphite/carbon addition are traditionally used in these furnaces for achieving high slag corrosion resistance and superior refractory properties. 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 dissolved in liquid steel reacts with refractory constituents like SiO
2
/Cr
2
O
3
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 utilized 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. Still the lime will be a potential refractory if highly dense lime is produced, with an excellent hydration resistance and stability.
The basic raw material for production of sintered lime is natural limestone. High purity limestone is easily available in India and other parts of the world. Since the refractoriness of lime is very high, sintering of lime requires a high temperature which is not usually available in the industries. Therefore, the achievement of good quality sintered lime from purer natural lime stone lump needs a temperature above 2000° C. The low flux lime stone (<2% impurity) generates favourable liquid phase above this temperature. Whereas, high flux limestone (>3% impurity) can be densified at a relatively lower temperature, but the product does not find use owing to its lower densification and low performance.
Reference may be made to the work of L. L. Wong and R. C. Bradt (1. Am. Ceram. Soc. Bull., 69 (7)1183-89 (1990) wherein it has been shown that impure limestone resulted poor densification at high firing temperature, 1600° C.
L. Xintian et. al (2. Brit. Ceram. Trans. 93 (4)150-153 (1994) tried to develop CaO sinter with improved hydration resistance by the addition of A
1
2
0
3
. However, hydration resistance properties were inferior to the present work which is being reported herein.
Vezikova et. al (3. Refractories 33 (1-2)85-89 (1992) reported the development of lime sinters with the addition of TiO
2
. The material was fired at 1750° C. which is much higher than the current work which is being sought to be protected.
Addink et. al (4. U.S. Pat. No. 4,795,725 (1989) developed a CaO refractory composition based on lime. To avoid hydration of lime Addink et. al used a thermosetting novolak type phenol resin in the batch so that it coates the CaO grains and prevent its hydration. The present work does not use any resin to protect the CaO grains.
Cassens (5. U.S. Pat. No. 4,463,100(1984)) developed a refractory material based on CaO by converting CaO into a different compound
2
CaO. SiO
2
.
Neville et. al (6. U.S. Pat. No. 4,843,044 (1989)) developed a lime based refractory material which contained lime sinter along with monosodium phosphate and alkali metal polyphosphate. No such binder has been used in the work reported by us.
For the improvement in the densification and hydration resistance, it is necessary to introduce mineralizer or additive in a reactive lime. This will help sintering at relatively lower temperature which improve the quality of the product to a great extent.
OBJECT OF THE INVENTION
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 than 2% impurities) variety of limestone which will generate high quality sintered lime to provide maximum hydration resistance to atmospheric moisture.
Yet another objective of the present invention is to maximize the grain size and their homogenity in the matrix.
Still another objective is to provide a compact microstructure which will contain less number of pores, with uniform distribution in the grain matrix.
Another objective of the invention is to prepare sintered lime with relatively large grain size uniforming distributed in the matrix.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly the present invention provides a process for the production of high density hydration resistant lime sinter, which comprises washing limestone having impurity of less then 2% to remove external impurities, 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 3 hours, hydrating the calcining mass, drying the hydrated mass by known methods, deagglomerating by known methods the dried hydrated mass, mixing to the said hydrated dried mass 1 to 4 weight percent additives such as transition metal oxides capable of forming low melting compound, rare earth metal oxides capable of forming solid solution, or mixture thereof, pelletizing the resultant mixture at pressure of at least 1000 Kg/cm
2
, sintering the pellets so obtained at a temperature in the range of 1550 to 1650° C. for a period in the range of 2 to 4 hours and allowing the resultant sinter to cool naturally.
In an embodiment of the present invention the transition metal oxide additive used may be such as ferric oxide, titanium dioxide, cooper oxide, vanadium pentoxide or mixture thereof.
In another embodiment of the present invention the rare earth metal oxides additives used may be such as 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 highly basic and ferruginous slag magnesia is known to be comparatively better than calcia. If the grain size of calcia is sufficiently large, the slag & hydration resistant will be significantly improved. The sintered lime prepared by the process of the present invention possess relatively large grain size uniformly distributed in the matrix. This not only improves 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 limestone 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 oxid
Banawalikar Deepak Gangadhar
Banerjee Gautam
Biswas Jnan Ranjan
Das Samir Kumar
Ghosh Arup
Council of Scientific & Industrial Research
Griffin Steven P.
Ladas and Parry
Nguyen Cam N.
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