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Thread: Hardgrove Index

  1. Hardgrove Index

    Dear Experts,

    I am working in Bulk handling equipment company one of our client specfication given hardgrove index for designing the bucket wheel cutting force.
    I am not able to understand how hardgrove index no convert in cutting resistance of material. The material is coal having density 0.83 T/

    please give me reply if you have.



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    Hardgrove Grindability Tester

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    Hardgrove Grindability Tester
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  2. #2
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    Hardgrove Index

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  3. Unhappy Cutting resistance is quite a grind.

    HGI refers to the grinding or pulverising of coal and has little or no relevance to cutting resistance.
    Neither has the density of 0.83tm-3 got anything to do with cutting resistance.
    I can do a site test for you or the client.

  4. Quote Originally Posted by louispanjang View Post
    HGI refers to the grinding or pulverising of coal and has little or no relevance to cutting resistance.
    Neither has the density of 0.83tm-3 got anything to do with cutting resistance.
    I can do a site test for you or the client.
    Dear sir,

    Thank you for your reply. However can you discribe the testing method for find out the cutting resistance at site?

    Thanking you.


  5. Hello,

    Force / torque required for bucket wheel is corresponding to the sum of following resistances:

    1) Cutting resistance 2) Lifting resistance 3) Friction against chute / walls in bucket wheel 4) Filling resistance of the material in bucket.

    In general the black bituminous coal, is classified as hard coal and soft coal. The coal mining engineers would be the right people to say about the conventionally understood boundary line of HGI to speak about hard coal and soft coal. For time being we can say hard coal as having HGI less than 52 HGI and soft coal for HGI in the range say 52 to say 65 HGI. The implication on sr. nos. 1), 2), 3) and 4) would be as below:

    Sr. no. 2): It is work / resistance against gravity and so HGI has no implication.

    Sr. no. 3): The literature (and DIN) simply mentions one value of coefficient of friction between coal and steel plate. This means it has no relevance to coal HGI (or it has insignificant influence).

    Sr. no. 4): The filling resistance depends upon friction coefficient between steel plate and coal, and more particularly to the lump size in relation to bucket, and moisture. Thus this also implies HGI has no relevance to filling resistance.

    Sr. no. 1): The difference in HGI of black coal is likely to have marginal influence. This value is represented by cutting resistance per unit length (mm or cm or inch). This arises when bucket mouth pushes into bulk material to detach bucket side material from rest of the material in stockpile. The bucket edges primarily do not act as cutter / crusher, so very small proportion of material will suffer breakage. The breakage will have relevance to HGI in context of resistance.
    During this detachment process most of the surrounding material is forced to reshuffle either toward stockpile or toward (in) bucket, for allowing bucket wall to pass. This reshuffling of material will mainly contribute to resistance under sr. no. 1. This reshuffling will be least for spherical shape and maximum when material granules / lumps are of odd shapes and sharp edges which tend to interlock among themselves (like bricks in a wall). This resistance will be also more for large size lumps, because material quantity being reshuffled will be larger.

    As for single material as a black coal, I think material lumps will not be different in shape, due to change in HGI. Their shape will be primarily same when being crushed by one type of crusher (say ring granulator, which is common prior to stockpiling). Thus it appears HGI difference between soft black coal and hard black coal will not have meaningful change as far as total power for bucket wheel is concerned. However, to honour your customer and to do business you may keep additional 5% contingency specifically for HGI less than usual, compared to the power being selected for black coal (which defecto already implies soft black coal or hard black coal).

    The typical formulae for calculations of sr. nos. 1) to 4) are publicly known but their exact inputs are proprietary information of manufacturer of such machines.

    It is clarified that some time, word soft coal and hard coal is also used for lignite and black coal respectively. But here this reply considers it as 2 variants in single material black coal.

    As for the hundreds of coal handling plants I came across for Indian requirement, as I remember; the specifications usually mention black bituminous Indian coal having hardness in the range of 50 to 60 HGI, and crusher power on the basis of 50 HGI (some time buyers also extend this range from 45 to 65 HGI, possibly for precautionary measure to ensure suitability of equipment for wider variation in coal characteristics).

    The word scooping resistance of centrifugal bucket elevator is synonymous to sum of sr. nos. 1) and 4) mentioned here.

    Regarding power, it will be product of resistance and speed, as per usual formula. Thus for particular bucket wheel operating at less speed will have less power and less wear. But same bucket wheel operating at more speed with more discharge / minute will have more power and more wear.

    Ishwar G. Mulani
    Author of Book: Engineering Science And Application Design For Belt Conveyors (new print November, 2012)
    Author of Book: Belt Feeder Design And Hopper Bin Silo
    Advisor / Consultant for Bulk Material Handling System & Issues.
    Pune, India.
    Tel.: 0091 (0)20 25871916

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