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Thread: coal grinding

  1. #1
    Join Date
    Aug 02

    Unhappy Coal Grinding

    I want to have your experiences and comments for coal grinding in ball mill.We are using 3 types of coal in different percentages and different grindability hardgrove y´ndex.For example:
    Type A:30% HGI=49
    Type B:35% HGI=55
    Type C:35% HGI=36

    With this my´xture we are aiming to have certay´n amount calori and sulphur value.We are not taking account of grindability factor.
    In ball mill what can be the reaction of this situation?

    Also another problem whivh I want to share with you is:
    The percentage of total sulphur is changing nearly 30% between inlet coal mixture and outlet dust coal product.
    The outlet amount is high.
    We are taking hot gas from preheater system after passing thruogh a cyclone.

    I am waiting your comments.
    Thanks a lot for your interest.
    Sincerely Yours

  2. #2
    Join Date
    Jul 02


    We have available an UNUSED Hosokawa Grinder, Dryer.

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  3. Thumbs down

    I am afraid that this query is well outside my field of solids handling experience. It would appear that this milling process has a significant, adverse on the sulphur content of the coal, causing losses with the take-off gas, but I have no idea how to deal with the problem. Can any reader with more relevant experience comment on the situation?

    Lyn Bates

  4. #4
    Lawrence K. Nordell

    Lawrence K. Nordell

    President and CEO

    Conveyor Dynamics Inc.

    Conveyor Dynamics Inc.

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    Coal Grinding - Comments

    Dear Levo

    I interpret that you pose two questions:
    1. Identify grinding behavior in a ball mill with three different coal feedstocks
    2. Why is the sulphur content increasing in ball mill product coal dust?

    ANSWER 1:
    I offer a way to investigate the answer as the feedstock changes. Hopefully, you have heard of PBM (Population Balance Model) tool. This is a reactor birth and death model. Its two principle components are functions B & S: B for Breakage Function and S for Selection Function.

    "B" defines the individual breakage properties for each type of coal (your A,B;C), by size fraction, in the range of interest. Its breakage parameters are produced in a laboratory breakage experiment (usually drop hammer test). Using particle mass and drop hammer energy, the test produces a surface energy constant ( particle progene that produce broken surface area liberated for energy applied). This is similar to Kick or Rittenger.

    "S" defines the rate of coal breakage which are the result of the stirring action of all materials in the mill to wit:
    1. mill size - configuration in 3-dimensions
    2. mill speed
    3. ball content by size, type (steel?), and volume occupied
    4. coal feed content by feed size, and quantity by type A,B;C
    5. slurry volume (if applicable), density, and strength (rheology)
    6. retention time
    7. mill lifter-liner configuration
    8 exit product control of flow (open, grated,...)
    9. production by type and size distribution
    10. recycle specs from cyclones

    By observing (sampling or other measure technique, (usually photo image of particle sizes )) the feed size, and the product size, including the loss of dust, a Section Function(S) can be determined (approximated) that then describes the output from any input configuration. The technique is used extensively in copper, gold, iron ores, platinum, aluminum, and many others. the trick here is the application of multiple ore properties. I believe Conveyor Dynamics, Inc., Dr. Ming Song, has produced the only multi-ore PBM. This technology has been sold to Metso's Mineral Process Engineering Division. General treatment of this approach can be found in the literature -some references: Prof Leonard Austin, Prof John A. Herbst (Minocad), or JK Tech in Australia (JK Simet).

    Another way to define "S" is to use the Discrete Element Model(DEM). The 3-D mill coal be modeled with a representative population of balls and coal by type. The energy spectra, for each strength, size distribution of coal, and their breakage properties (in steady-state), are produced. This is the equivalent of the Selection Function in the PBM. THe DEM produces a limited range of particles which can then be extended to map the field.

    ANSWER 2:
    I am not a metalurgist. So, this is a stab at the observation of more sulphur. When the coal is fractured, it breaks along weaker grain boundaries. These weaker boundaries may contain larger amounts of sulphur. This could be tested in the above "B" drop hammer tests by micrographing the surfaces for their content in the finer size range. Others, of this forum may have a more accurate answer.

    Again, Metso is now the owner of our mineral process engineering mathematical modeling techniques PBM, DEM, Ore Fracture, mill measurement analyses. They have comprehensive particle photo imaging capabilities. I suggest you ask them for assistance of a commercial nature. Reference: Prof. John A. Herbst phone 808-324-6100 in Hawaii or locate him on the web.
    Get a copy of SAG 2001 in Vancouver on ore comminution.

    Our website might give you some visual aids.
    my email is:

    Good Luck in Your Endeavor,

    Lawrence Nordell
    Convyor Dynamics, Inc.
    1111 West Holly St.
    Bellingham, Washington USA
    ph 360-671-2200
    fx 360-671-8450

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