Ball Mill (Overfilling)

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Description

This article describes several methods for estimating the maximum volumetric flow capacity of an overflow ball mill, including the Shi (2016) and Arbiter (1990) approaches.[1][2]

Model theory

Shi (2016)

Figure 1. Overflow discharge tumbling mill profile showing Shi's assumed charge and slurry pool areas.

Shi estimated the residence time of an overflow ball mill by considering the volume of slurry present in the grinding media interstices and slurry pool.

This was accomplished by adopting Morrell's simplified charge geometry and calculating the volume of slurry resident in each of the areas A-C in Figure 1:[3]

A. In the grinding media interstices above the slurry pool level
B. In the grinding media interstices below the slurry pool level
C. In the slurry pool

The total volume of slurry in the mill is therefore the sum of slurry in areas A-C.

Estimating the slurry volume in areas A-C requires definition of the simplified charge geometry and slurry pool level for the mill in question.

Charge position

The simplified charge geometry is defined by three parameters:

  • the angular position of the toe of the charge, (rad)
  • the angular position of the shoulder charge, (rad)
  • the radius of the inner charge surface, (m)

Morrell's (1996) equations are used to define the charge position, i.e.[3]

The position of the toe of the charge is defined as:

where:

  • is the volumetric fraction of the mill occupied by balls and coarse rock (including void space and interstitial slurry) (v/v)
  • (frac) is the theoretical fraction critical speed

The fraction of critical speed at which centrifuging actually occurs, (frac), is:

The position of the shoulder of the charge is:

The inner charge surface radius is:

where the fraction of total charge in the active region, (frac), is:

The time taken to travel between the toe and shoulder of the charge during one revolution, (s), is:

where the mean rotational rate, (rev/s), is:

The time taken to travel between the shoulder and toe of the charge in free flight during one revolution, (s), is:

where the mean radial position (m), is:

Slurry hold-up below pool level

The volume of slurry held up below the pool level in areas B and C, (m3), is:

where:

  • is effective mill length (m)
  • is cross-sectional area of the slurry pool (m2)
  • is cross-sectional area occupied by the grinding media, excluding balls, below the slurry pool level(m2)

This implementation of the Shi model computes as an effective mill length from:

where is the volume of the mill (m3) and is the radius of the mill (m).

The volume of the mill is calculated as the sum of a cylinder and two right circular frustums:[4]

wher:

  • is the length of the cylindrical (belly) section of the mill (m)
  • is the radius of the discharge trunnion (m)
  • is the cone angle, measured as the angular displacement of the cone surface from the vertical direction (rad)

is computed from:

where is the radius of the discharge trunnion (m), and is the height of the slurry pool level over the trunnion lip (m).

The value of may be estimated as , where is the slurry height above the trunnion level inside the mill (m):[5]

and:

  • is an empirical coefficient related to the viscidity of the slurry (Morrell suggests , Shi applies )
  • is the volumetric discharge rate of pulp from the mill (m3/s)
  • is acceleration due to gravity (m/s2)

is computed as:

where the dynamic charge porosity, (v/v), is:

Slurry hold up above pool level

The volume of slurry held up in the grinding media interstices above the slurry pool level in area A, (m3), is:

where:

Residence time and overfilling

The total volume of slurry hold up in the mill is (m3).

The residence time of slurry in the mill, (s), is then:

The axial velocity of slurry through the mill, (m/s), is:

Shi suggests the following residence time limits for overflow discharge ball mills (s):

where is the diameter of the mill (m), i.e. .

The maximum volumetric flow rate of the mill, (m3/s), at the residence time limit may be back-calculated using the above relations. However, as the height of slurry above the trunnion lip (), and hence residence time (), is a function of flow rate, no analytical solution is available and a numerical method is required for computation.

Arbiter (1990)

Arbiter describes two approaches for identifying the overfilling of an overflow discharge ball mill:

  1. . the critical flow number method
  2. . the axial flow velocity method

Arbiter's limits are rearranged to yield the following relations for the maximum volumetric discharge rate of slurry from a mill:

where:

  • is the critical flow number volumetric slurry discharge rate limit (m3/h)
  • is the axial velocity volumetric slurry discharge rate limit (m/s)
  • is mill diameter (m)
  • is mill rotational speed (rev/min)
  • is the volumetric charge fraction in the mill (v/v)

Overfilling may be a risk if the actual volumetric flow rate from the mill approaches or exceeds either of or .

Additional notes

Shi estimated the residence times for a database of 121 overflow ball mills. These residence times are presented as cumulative frequency distributions in Figure 2, allowing the performance a given mill to be ranked against the database.

Figure 2. Cumulative frequency distribution of the volume-based residence time of 121 mills in Shi's database (after Shi, 2016).[1]

Arbiter's relations were developed from a limited database of five mills, the largest of which was 5.3 m diameter x 6.4 m length.

Excel

Shi

The Shi overflow discharge ball mill overfilling model may be invoked from the Excel formula bar with the following function call:

=mdMillOverfilling_Shi(Parameters as Range)

Invoking the function with no arguments will print Help text associated with the model, including a link to this page.

The Parameters array and model results are defined below in matrix notation, along with an example image showing the selection of the same arrays in the Excel interface:

Figure 3. Example showing the selection of the Parameters (blue frame), and Results (light blue frame) arrays in Excel.

Arbiter

The Arbiter overflow discharge ball mill overfilling model may be invoked from the Excel formula bar with the following function call:

=mdMillOverfilling_Arbiter(Parameters as Range)(Parameters as Range)

Invoking the function with no arguments will print Help text associated with the model, including a link to this page.

The Parameters array and model results are defined below in matrix notation, along with an example image showing the selection of the same arrays in the Excel interface:

Figure 4. Example showing the selection of the Parameters (blue frame), and Results (light blue frame) arrays in Excel.

SysCAD

The Ball Mill Overfilling Indicator model is an optional calculation for tumbling mill units. If selected, the input and display parameters below are shown.

Tag (Long/Short) Input / Display Description/Calculated Variables/Options
Shi
MillDiameter Input/Display Diameter of the mill (inside liners).
BellyLength Input/Display Length of the cylindrical section (belly) of the mill (inside liners).
TrunnionDiameter Input/Display Diameter of the trunnion (inside liners).
ConeAngle Input/Display Angular displacement of the cone surface from the vertical direction.
FracCS Input/Display Fraction critical speed of the mill.
Jt Input/Display Volumetric fraction of the mill occupied by balls and coarse rock (including voids).
ViscidityCoeff Input Coefficient of viscidity.
V1 Display Volume of slurry below slurry pool level.
V2 Display Volume of slurry above slurry pool level.
V1 Display Total volume of slurry in charge and pool.
AxialVelocity Display Axial velocity of slurry flow through the charge and pool.
tRes Display Residence time of slurry in mill.
tLimit Display Shi's suggested lower limit of residence time for mill. Based on mill diameter.
Feed.SLQv Display Volumetric flow rate of slurry (solids + liquids) in mill feed.
Feed.SLQv.Limit Display Estimated volumetric flow rate of slurry (solids + liquids) in mill feed at tLimit.
Overfilled True/False Indicates if tRes is less than tLimit, i.e. mill is overfilled.
Arbiter
MillDiameter Input/Display Diameter of the mill (inside liners).
FracCS Input/Display Fraction critical speed of the mill.
Jt Input/Display Volumetric fraction of the mill occupied by balls and coarse rock (including voids).
Feed.SLQv Display Volumetric flow rate of slurry (solids + liquids) in mill feed.
Feed.SLQv.NqLimit Display Estimated volumetric flow rate of slurry (solids + liquids) at Arbiters critical flow number limit.
Feed.SLQv.VmaxLimit Display Estimated volumetric flow rate of slurry (solids + liquids) at Arbiters axial velocity limit.

See also

References

  1. 1.0 1.1 Shi, F., 2016. An overfilling indicator for wet overflow ball mills. Minerals Engineering, 95, pp.146-154.
  2. Arbiter, N., 1991. Dimensionality in ball mill dynamics. Mining, Metallurgy & Exploration, 8(2), pp.77-81.
  3. 3.0 3.1 Morrell, S., 1996. Power draw of wet tumbling mills and its relationship to charge dynamics. Pt. 1: a continuum approach to mathematical modelling of mill power draw. Transactions of the Institution of Mining and Metallurgy. Section C. Mineral Processing and Extractive Metallurgy, 105.
  4. Gupta, A. and Yan, D.S., 2016. Mineral processing design and operations: an introduction. Elsevier.
  5. Morrell, S., 2016. Modelling the influence on power draw of the slurry phase in Autogenous (AG), Semi-autogenous (SAG) and ball mills. Minerals Engineering, 89, pp.148-156.