Limestone

Limestone is calcareous sedimentary rock composed of mineral calcite (CaCO3) which upon calcination yields lime (CaO) for commercial use. Cement and Iron & Steel are the major consumers of limestone. It is also consumed in chemical industries like calcium carbide, bleaching powder, soda-ash, precipitated calcium carbonate, etc. Besides, it is used as fluxing material in ferro-alloys, pelletization plants, foundries and in the production of sponge from iron, as refining materials in the production of sugar; as an additive in glass industry; as a coating material in fertilizers industry; and in the cooking of range and grass in the paper industry.

The details of specifications on limestone required by various industries are discussed below:

1. Cement Industry : -
Limestone containing about 45% CaO and above is usually preferred for the manufacture of cement. Magnesia content in the stone should ideally be less than 3%, although as high as 5% MgO is used by the industry. The proportion of Al2O3 + Fe2O3: SiO2 should be 1:2:5. Magnesia sulphur and phosphorus are regarding as most undesirable impurities. The presence of phosphorus and P2O5 more than 1% slows down the setting time of Portland cement. The cement industry in India in general prescribed the following specification of limestone.

CaO 42% (min.) Al2O3 2 to 4% (min.), Fe2O3 1 – 2% SiO2 12 to 16% and MgO 4% (max.)
2. Iron & Steel Industry : -
Limestone is used in this industry both in the blast furnace to lower the temperature of melting and second to from calcium silicate by combining with silica of the iron ore, which comes out as slag.

i. Blast furnace: - Specifications of limestone in various steel plants in India for use in Blast furnace are given below:
Plant Size CaO MgO SiO2 Al2O3 T. I.
BSP +50mm
10% (max.)
-25mm
5% (max.) 		42%
1% 		8% - - 6.58%
1%
BSL +25mm
5% (max.)
-85mm
5% (max.) 		45% 6% max 5% max - -
RSP +50mm
10% (max.) 		44% - - 2% max 9%
1%
BSP -25mm
10% (max.)
+75mm
10% (max.) 		45% - 7.5% max - 12% max
IISCO 75mm
10% (max.)
-25mm
10% (max.) 		46% ---------10%------------ 11% max
TISCO -75mm
+25mm
o/s 10% up to 100m
u/s up to 10% 		6% - - - 12% max
(ii) Steel Melting Shop: - For this purpose good fluxing limestone low in acid constituents such as silica, alumina, sulphur and phosphorus is required. The limestone should be dense, massive, preferably fine grained, compact and non-fretting on burning. The specifications of limestone required by steel plants in India for SMS purpose are given below:
Plant Size CaO MgO SiO2 Al2O3 T. I.
BSP (a) O. H. -50mm
5% (max.)
(b) L. D. 40-80mm
5% (max.)
 49%
53% 		4% max.
2% max. 		1.8% max. 1.7% max 5% max.
BSL L. D. -30mm
5% (max.)
+60mm
5% (max.) 		50% 3.5% max. - - -
DSP 75-125mm
+125mm
10% (max.)
-75mm
10% (max.) 		48% 3.5%max - - 5%max
RSP 80mm
10% (max.)
-40mm
10% (max.) 		51% - - - 5%
1.5%
TISCO +125mm
5% (max.)
-75mm
10% (max.) 		50% - 4.0% max. - -
TISCO (a) O. H. -75+35mm
(b) L. D. -75+35mm
-35mm
5% (max.) 		53% 2% max.
2% max. 		- - 4% max.
1% max.
From the above it is seen that limestone is L. D. furnace must be of high purity with low silica content as far as possible. Some plants are insisting on as low as 0.5% SiO2.

The BIS (IS: 10345 – 1982) has prescribed the under mentioned guidelines for flux limestone for use in steel plants:
Characteristics Grade-I (for use in lime manufacturing) Grade-II (for use in steel making) Grade-III (for use in Iron making)
CaO 52% min. 50% min. 42% min.
MgO 2% max. 3% max. 9% max.
Acid insoluble 2% max. 4% max.
out which SiO2 should not exceed 1.8% 		10% max.
out which Al2O3 should not exceed 2.5%
Preferred size * * 25-60mm, 25mm not to exceed 5% and 60mm not to exceed 10% * 25-60mm, 25mm not to exceed 5% & 60mm not to exceed 10%
The size range other than specified above shall be subject to agreement between the supplier and the purchaser.

COMPARATIVE TABLE OF CHEMICAL, PHYSICAL & SOME ENGINEERING PROPERTIES OF SMS GRADE LIMESTONES FROM JAISALMER AND GOTAN, RAJASTHAN
S. No. Item / Property Trials at Bhilai Steel Plant

Trial, JaisalmerBase, Gotan Limestone
Limestone
M/s RSMDC Gotan
1. Period considered 19 to 30 May, 1988 15-31 Dec. 1987
2. Total limestone calcined in trial period 3500 tonnes --
3. Average Limestone Analysis % %
(a) Chemical: CaO MgO SiO2 LOI (b) Screen -35mm -40mm +80mm 54.88 0.73 0.90 43.07 21.66 26.06 0.04 53.10 2.04 1.64 42.58 24.00 -- --
4. Average Lime Analysis CaO LOI Reactivity Undersize (-15mm) 87.7 6.88 325cc 10.9 80.8 7.40 367cc 33.5
5. Operational factors in Kiln i) Yield of LD grade lime from limestone charged into the kiln


49.89% (overall yield 60.8%)
38.87%
6. Operational factors in converters
i) Specific lime consumption


80 Kg / T (corrected)


106 Kg / T

LABORATORY EVALUATION
7. i) Disassociation started
ii) Finished
iii) Range 		720° C
977° C
257° C		 725° C945° C220° C
8. DTA Peak Temperature 930° C 900° C
9. Weight Loss (total) 42.0% 43.02%
10. Decrepitation Index (-15mm%) 1.42 16.0
11. Crushing Strength (Kg / cm2) 320 358
3. Chemical Industry : -
Generally quick lime is used in the manufacture of calcium carbide. The calcium carbide manufactures generally prefer lime containing 95% CaO (min.), not more than 3% SiO2, not exceed 2%. Though chemical composition is the guiding factor, the physical characteristics of the limestone are also very important.

For the manufacture of bleaching powder also, fat lime-containing CaO 95% and above is required. Total Fe2O3 + Al2O3 + MnO2 should be less than 2%; MgO should be below 2% and SiO2 less than 1.5%. Bleaching powder is prepared by absorption of chlorine by dry hydrated lime. The hydrated lime should not contain more than 2% of excess water. Iron and oxides tend to discolour the bleached material. Magnesia renders the bleaching powder hydroscopic. Silica and clay impede solution and setting of bleaching powder.

Limestone suitable for bleaching powder can be advantage utilized by alkali industry for the manufacture of soda ash and caustic soda.
4. Sugar Industry : -
In sugar industry lime is used for the clarification of juice form can be beets. Milk of lime 1% in volume of cane juice is added to pre-heated juice. Limestone to be used in sugar industry must be high in active lime (CaO S 80% min.) must be low in iron, alumina and silica, etc. Magnesia should be less than 1%. Excess of silica in un-desirable because it separates as a gelatinous precipitate, which covers the sugar crystals and retards their growth and filtration. Magnesia is objectionable because magnesia carbonate is soluble in sugar juice. Presence of Iron tends to color the finished product.

The BIS (IS: 3204 – 1978) stipulates the following specifications of limestone for use in the preparation of bleaching powder, caustic soda and calcium carbide and sugar.
Characteristics Requirement in percent by mass for
Bleaching Caustic Calcium Sugar
Powder Soda Carbide
1. L O I 46.0 46.0 46.0 44.0
2. SiO2 (max.) 0.75 -- 1.0 2.0
3. Fe2O3 (max.) 0.15 -- 0.25 --
4. CaO (min.) 54.0 53.0 54.0 50.0
5. MgO (max.) 2.0 1.0 0.8 1.0
6. Mn2O3 (min.) 0.06 -- -- --
7. CO2 (min.) 42.0 42.0 42.0 41.0
8. S (max.) -- -- 0.1 --
9. P (max.) -- -- 0.1 --
10. Al2O3 + Fe2O3 (max.) -- -- 0.50 1.50
11. SiO2 + Al2O3 + Fe2O3 (max.) -- 3.0 -- --
5. Glass Industry : -
Glass industry required high limestone (CaCO3: 94.5%) combine CaCO3 and MgCO3 should be 97.5%. Iron and other coloring matter like carbon are regarded as objectionable. Fe2O3 should be up to 0.20% (max.). For colorless glass, limestone should not be more than 0.04%; and for bottle glass Fe2O3 up to 0.05% is used. The BIS specifications (IS: 977 – 1973) for limestone for use in glass industry are as follows:
1. Silica as SiO2    2.5%
2. Total iron (Fe2O3) (a) Calcite or marble (b) Limestone (c) Dolomite Limestone or Dolomite   0.05% 0.10% 0.15%
3. Lime (as CaO) 53.00%
4. Total lime and magnesia (as CaO + MgO) 54.50%
6. Fertilizers Industry : -
The latest use of limestone introduced in the country is in the manufacture of calcium ammonium fertilizers, limestone is used only as carrier. For this purpose limestone should contain MgCO3 + CaCO3 85% (min.) SiO2 5% (max.) and acid insolubles 14% (max.)

7. The Chemical requirements of limestone for use in foundries as per BIS specifications (IS: 4140 – 1978) are given below:
S. No. Characteristics Requirement in percent by mass for Grade-I Grade-II Grade-III
1. Calcium Oxide (CaO) min. 52 50 45
2. Silica (SiO2) max. 1.5 3 5
3. Mixes oxides R2O3 (Fe2O3 + Al2O3) max. 1 1 1
4. Magnesia oxide (MgO) max. 2 3 5
5. Insoluble matter, max. 0.5 1 2
6. Sulphur and phosphorus Trace Trace Trace
Future Trends : - it is noticed specifications are prescribed by the steel industry should be 52% (min.) and SiO2 1% (max.). Besides, the physical characteristics are important. It should be non-decapitating and should have low content of alkalis.

In the mineral inventory as on 01.01.1985, the reserves of limestone have been classified in to cement, BF, SMS and chemical grade taking in to consideration (a) reported contents CaO, MgO, and SiO2; (b) Judgments of the exploration agencies without any separate chemical analysis data and (c) actual reported use.

Where chemical analysis data has been taken as a criterion, the parameters are as under:
Cement CaO MgO 42 to 45% 5% (max.)
BF CaO CaO + MgO SiO2 42% (min.) 48% (min.) 9% (max.)
SMS CaO MgO SiO2 48% (min.) 3% (max.) 4% (max.)
Chemical CaO MgO SiO2 50% (min.) 2% (max.) 2% (max.)
For BF and SMS grade limestone physical form i.e., hard lumpy is important. Currently for LD converters very low silica limestone not susceptible to decapitation is required. Thi