Limestone Crushing in India
Limestone Crushing in India

Limestone Crushing in India

Thyssenkrupp Industries India Pvt Ltd, Pimpri, Pune (henceforth tkII) offers a variety of solutions to different types of limestones, which are geographically located in different areas of India.
India is a rich country in minerals with a lot of deposits of coal, iron ore, limestone, etc. Out of which limestone is one of the most important minerals in many parts of India right from Jammu & Kashmir in north to Tamil Nadu in South. Different limestones in various areas are not the same owing to their geological formation and hence every mine's limestone is different than that of others. Eg. Limestone in North Karnataka/Telangana belt is hard, dry, with very less amount of silica whereas that of Chhattisgarh belt is medium hard, Marley type with clay. Different types of limestones ideally requires different type of crushing solutions.
Thyssenkrupp Industries India Pvt Ltd, Pimpri, Pune (henceforth tkII) offers variety of solutions to different types of limestones, which are geographically located in different areas of India. E.g. Limestone in Rajasthan (Udaipur and surrounding area) is hard, difficult to crush but easy to grind (Bond index ranging from 9-13 kWh/ST) however limestone in Karnataka region is very hard, but relatively easy to crush and difficult to grind (as bond index is between 13-17 kWh/ST) limestone. Limestone in eastern Madhya Pradesh is full of clay, very hard to crack, but easy to grind (Bond index ranging from 9-13 kWh/ST) and that appearing in Odisha region is very soft, with clay and Marley type.
For different limestones, tkII can offer different crusher. Following is the range of crushers, that tkII can offer which can cover all types of limestone, currently available in India. Speciality of tkII is, it can offer all types of solutions for very high production rate of up to 2,500 tonne per hour or customised solutions can be offered for higher capacity.
Let's understand different types of crushers that tkII can offer
1.    Single or Double shaft type impact hammer crushers:
Impact hammer crushers are used when generally compressive strength of limestone is <150 MPa, Moh's hardness value is < 4 and max. Silica content is approximately 8 per cent. These crushers can take up to 10 per cent moisture for single shaft and maximum 20 per cent moisture for double shaft. Construction of an impact hammer crusher comprises of a rotor made from cast manganese steel discs, fixed on forged shaft, usually machined in square or pentagonal shape for better distribution of generated stresses.
A rotor discs comprises of 5 or 6 hammer axles mounted peripherally on which swinging hammers weighing from 90 to 160 kg are mounted. Hammers are also of cast manganese steel. This crusher crushes limestone, initially with impact when rows of hammers hit the bolder, then with impact and shear between anvil block and hammers and finally attrition takes place between grate bars and hammers. Due to grate bars fixed below rotor, material won't come out till it achieves desired size. Generally double shaft hammer crusher can cater to most of the limestones that are occurring in India (except Tamil Nadu). This crusher can handle up to 20 per cent of clay provided it is fed with stones in between. The combination of clay and stones ensures self-cleaning of grate bars.
2.    Impact crusher or Impactor:
tkII is capable of making single rotor impactor, which is used for hard limestones with compressive strength of up to 175 MPa. However such crusher is not recommended for sticky or Marley type limestones which causes clogging of chambers. It comprises of fabricated housing with Hardox liners and cast manganese steel grinding path. The rotor is generally a cast steel body with six rows of fixed blow bars, which can be reversed to get more life. As it does not have any grate assembly at the bottom, such crusher has application mainly for products size between 75 to 100 mm which it can generally produce from 1.5 m bolder.
3.    Double roll crusher/Sizer:
If the plant and mines are located separately and client needs only primary crushing in mines, then double roll crusher or sizers is a good option. These crushers are also used when the limestone is highly sticky with no stones e.g. the one appearing in Southern parts of India. Here the toothed rolls rotating in opposite direction compress the material between two rolls and crush it. Double roll crusher works on the principle of inertia and sizer works on the principle of torque. Speed of rotation in sizer is slow at 60-80 rpm whereas that in double roll crusher is high at 200-230 rpm. However limitation of these sizers/ roll crushers is crushing ratio which can be max. 1:5.
4.    Jaw crushers:
For very hard limestone & for primary crushing, tkII can offer jaw crushers wherein it can take very hard limestones up to 400 MPa compressive strength. But crushing ratio is limited to max. 1:5. in jaw crusher, there will be one reciprocating jaw which compresses the material between another fixed jaw & crushing takes place. There will not be much size control of the product hence such crusher is generally used where there is a secondary crushing planned in the circuit.  Product capacity for such crushers is also limited to 500 Tons per hour.
Selection of crushers based on Applications of limeston
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Maximum use of limestone is for cement production, but apart from that, limestone is also used as flux in steel making, as reaction agent in flue gas desulphurisation (FGD) systems in power plants, for emission reduction etc. Most (>90 per cent) of commercially mined limestone is used for cement production. The type of raw meal grinding systems in cement plant can have impact on selection of crushing system for limestone. 
Based on onward grinding system, it is desirable to choose a crusher.
  • Ball mills: As the ball mills requires lesser product size of < 25 or 30 mm, a hammer type crusher is highly recommended for such applications. The advantage that hammer type crusher provides for limestone is high crushing ratio which ensures -25 mm can be achieved in single pass/stage. For most parts of Indian limestone, tkII double rotor hammer crusher is suitable & can easily crush to -25 mm.
  • High pressure grinding roll: As high pressure grinding rolls are sensitive to feed size, it is desirable to feed them with product from hammer crusher. Also the size of < 40 - 60 mm may not be possible in single stage in any other crusher so hammer crusher has an edge if you wish to use HPGR for raw meal grinding.
  • Vertical roller mills: As VRM can accommodate larger product size of < 75 - 90 mm and the same can be done in single stage in crusher like impactor, a single stage impactor can be a good solution for crushing. tkII can offer single stage impactor crusher for such requirements.
tkII is capable of offering all above systems. So for most plants which are in between 5000 TPD to 10000 TPD, the crushing plant capacity turns out to be 1200 TPH to 2000 TPH, tkII can offer either impactor or hammer crusher. Following is the comparison of impactor hammer crusher. Ramp height for Hammer crusher is less compared to Double rotor impactor. As every mine's limestone is customised, it is advised to refer the requirement to us so that customised solutions can be offered.
Impact Hammer Crusher    Impactor (Fixed blow bar type)
Crushes by Impact, shear & Attrition. Suitable for medium hard materials.
Hammer crusher will be self-cleaned when hard stones will be fed with the sticky material.    Crushes by Impact and shear.
Suitable for medium hard and hard materials.
Not suitable for sticky materials
Impact is due to heavy swing hammers
(135 to 160 kg)    Impact is due to Fixed blow bars fixed by side wedges.
Reduction ratio is high up to 1:100 in single stage    Reduction ratio is limited up to 1:20 per rotor stage (for higher reduction two rotor compound impactor is used)
Can accept higher moisture up to 20% and higher clay and silica    Moisture is up to 10 % and silica up to 6%, limitation due to wear
Crusher product size is governed by the gap in the grate bars. Within limits product size is adjusted by adjusting impact plates and grinding path
Consistent product size throughout the use, no adjustments needed to compensate for wear. This crusher is far more suitable for HPGR which is sensitive to input product size.    Frequent adjustment of impact walls/ grinding path necessary to compensate the rotor blade wear & grinding/ impact plates. In impactor, with wear of grinding path & blow bars, product size changes.
Due to swinging hammers, part of shock loads absorbed in the hammer deflection and remaining load transferred to the rotor bearings.Due to rigid construction of rotor, 100% shock goes to rotor bearings.
Hidden cracks are left in the crushed products - that is favorable for onward grinding. This crusher produces finer product granulometry which is favorable to increase throughput of HPGR / Ball Mill.    Cubical products without any hidden cracks and coarse product granulometry limits the capacity of the VRM / Ball mill.
In case of double shaft hammer crusher, each rotor shares theoretically equal loads - hence installed power is lower (2 x 800 kW for say 1200 TPH at -40 mm) as well as consumed power per ton (0.68 kWh/T) lower than compound Impactors. In case of compound rotor Impactors each rotor handles 100% material - hence installed power (2 x 1200kW) as well as consumed power (1 to 1.1 kWh/T) are higher.
ABOUT THE AUTHOR:
Sanjeev Nargund, Senior GM - Crusher & Screen Design at Thyssenkrupp industries India Pvt Ltd.
He can be contacted on:  Sanjeev.nargund@thyssenkrupp.com
Milind Kulkarni, Senior Manager - Marketing at Thyssenkrupp industries India Pvt Ltd.
He can be contacted on:  milinds.kulkarni@thyssenkrupp.com
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