Tumbling Mill (Power, Morrell Empirical)

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Revision as of 03:42, 21 December 2022 by md>Scott.Munro (→‎SysCAD)
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Description

This article describes the Morrell Empirical (Morrell E) method for estimating the power draw of a tumbling mill.[1]

The Morrell E model is a set of empirical equations based on the performance of the theoretical Morrell Continuum model. The model was originally intended to be simpler, and therefore easier to use in practice, than the theoretical Morrell C method.

Model theory

Figure 1. Schematic of a tumbling mill showing key dimensions.

Morrell developed a simpler, empirical model of power input to the motor of a tumbling mill based on the theoretical Continuum approach.[1]

Morrell's Empirical relationship for tumbling mill power draw is:

where:

and

  • is a calibration constant, for overflow mills and for grate mills
  • is mill diameter inside liners (m)
  • is the effective length of the mill (m)
  • is the density of the total charge (t/m3)
  • is fraction critical speed (frac)
  • is length of the cone end (m)
  • is length of the cylindrical section (belly) of the mill inside liners (m)
  • , and are empirical parameters
  • is the volumetric fraction of the mill occupied by balls and coarse rock (v/v)
  • is the fraction of critical speed at which power draw is maximum (frac)
  • is the density of ore (t/m3)
  • is the density of balls (t/m3)
  • is volumetric fraction of interstitial void space in the charge, typically 0.4 (v/v)
  • is the volumetric fraction of interstitial grinding media voidage occupied by slurry (v/v)
  • is the volume fraction of solids in the mill discharge (v/v)

The length of the cone end, (m), is:

where is the cone angle, measured as the angular displacement of the cone surface from the vertical direction.

Excel

The Morrell Empirical mill power model may be invoked from the Excel formula bar with the following function call:

=mdMillPower_MorrellE(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:


where:

  • is the diameter of the discharge trunnion (m)
  • is the mass fraction of solids in the discharge pulp (% w/w)
  • is the density of liquids (t/m3)
  • is the power the mill would draw if configured with a grate discharge (kW)
  • is the power the mill would draw if configured with an overflow discharge (kW)


Figure 2. Example showing the selection of the Parameters (blue frame), and Results (light blue frame) arrays in Excel.

SysCAD

The Morrell Empirical power 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
MorrellE
HelpLink ButtonModelHelp.png Opens a link to this page using the system default web browser. Note: Internet access is required.
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).
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).
Jb Input/Display Volumetric fraction of the mill occupied by balls (including voids).
Voidage Input/Display Volumetric fraction of interstitial void space in the charge. Usually 0.4.
VoidFillFraction Input/Display Volumetric fraction of interstitial grinding media voidage occupied by slurry.
ConeAngle Input/Display Angular displacement of the cone surface from the vertical direction.
DischargePulpDensity Display Mass fraction of solids in discharge slurry.
SolidsSG Display Specific Gravity or density of solids.
LiquidsSG Display Specific Gravity or density of liquids.
BallSG Input/Display Specific Gravity or density of balls.
NoLoadPower Display Power input to the motor when the mill is empty (no balls, rocks or slurry).
NetPower.Grate Display Charge motion power, including losses, for a grate discharge mill.
NetPower.Overflow Display Charge motion power, including losses, for an overflow discharge mill.
GrossPower.Grate Display Gross power input to the motor, grate discharge mill.
GrossPower.Overflow Display Gross power input to the motor, overflow discharge mill.

See also

References

  1. 1.0 1.1 Morrell, S., 1996. Power draw of wet tumbling mills and its relationship to charge dynamics. Pt. 2: an empirical approach to modelling of mill power draw. Transactions of the Institution of Mining and Metallurgy. Section C. Mineral Processing and Extractive Metallurgy, 105.