Grinding Theory Sent

Vertical Mills used for • Pre-grinding of clinker Finish grinding of • Coal • Raw materials • Cement • Slag

Roller Mill: Definition: A roller mill is characterised in that a bed of loosely packed material is compacted and thereby ground between two rolling surfaces pressed against each other, and that at least one of the rolling surfaces is a roller. The gap between the rolling surfaces is not fixed but varies with change in the material properties.

Evaluation and calculations are based on • Theoretical considerations • Basic data obtained from test work mainly with roller presses

Operational data: • Table speed (rpm) • Grinding press. (bar) or (kN/m2) • Product fineness (sieve residue, Blaine, etc.) • Capacity (tph) • Mill Motor (kW)/Friction factor (-) • Grinding bed thickness (mm) • Vibration level (mm/s)

Process inside the roller mill Evaluation of: • Max pressure pmax (MPa) • Max grinding bed thickness (mm) • Mass flow through roller gap (tph) • Spec. Power per roller pass (kWh/t) • Circulation factor for roller (-) • Friction factor µ (-) • Retention time in mill (sec) • Flow of material on the table • Slip between roller and table • Wear

Roller Press, basic considerations L: length of compaction zone R: roller radius W: roller width T: roller force α: nip angle [radian] δ: gripping angle [radian] α small, therefore tg α≈α ⎛1 1⎞ δ = α1 + α2 = L ⋅ ⎜⎜ + ⎟⎟ ⎝ R1 R2 ⎠

Illustration of the gripping angle δ and the nip angle α.

Specific grinding pressure kT R: D: W: T:

T kT = W ⋅ D1

roller radius roller diameter roller width roller force

2 (kN/m )

Roller press

Vertical mill

Ring roller mill

Formulas and general characteristics of vertical mills.

Vertical Roller Mill DO µ T kT DR W DM n v i N P

Table diameter Friction coefficient Roller pressure per roller Spec. roller pressure Roller diameter Roller width Mean diameter of track Table speed Velocity at DM Number of rollers Power Production

Vertical and horizontal components of roller load

[m] [radian] [kN] [kN/m2] [m] [m] [m] [rpm] [m/s] [-] [kW] [t/h]

Vertical Roller Mill

DO µ T kT DR W DM n v i N P

Table diameter Friction coefficient Roller pressure per roller Spec. roller pressure Roller diameter Roller width Mean diameter of track Table speed Velocity at DM Number of rollers Power Production

[m] [radian] [kN] [kN/m2] [m] [m] [m] [rpm] [m/s] [-] [kW] [t/h]

N = i ⋅ μ ⋅ T ⋅ v = i ⋅ μ ⋅ k T ⋅ D R ⋅ W ⋅ D M ⋅ π ⋅ n 60 For the standard Atox mills i=3, DR=0.6*D0, W=0.2*D0, DM=0.8*D0 and n=56*D00.5, which inserted gives:

N = 0.844 ⋅ μ ⋅ k T ⋅ D O2.5

Friction coefficient µ Cement raw material 0.09 +/- 0.02 Coal 0.10 +/- 0.02 Cement 0.06 +/- 0.01 Calculations based on identical rolling point

Material movement on the table Radial and tangential velocity influenced by •Friction coefficient of material (mat. type, moisture content, granulometry, aeration level) •Surface condition of grinding elements •Shape of grinding table (flat, spherical, dam ring height) •Table speed

Material movement on the flat table

Material tracks, absolute movements

Material movement on the flat table

Material tracks relative to table

Material thickness and flow on the flat grinding table.

Material layer in percent of table diameter

Net layer fed to the rollers (tangential speed for material lower than table speed)

Material layer on curved grinding surface.

Grinding bed thickness limited by vibrations

Optimisation of mill operation. •Increase of friction coefficient of material: Inject water on table • Higher power uptake • Lower vibration level •Increase thickness of material: Increase height of dam ring • Higher power uptake • Lower vibration level • Decreased grinding efficiency •Increase thickness of material: Reduce table speed (to 80-100%) • Lower vibration level (for finer materials) • Increased friction factor but lower power uptake

Slip between roller and table

Determination of roller-table slip

Slip between roller and table

Polysius

Slip between roller and table

MPS

RM

Slip between roller and table

Loesche

ATOX

Wear profiles in an Atox Mill hard, dry material Wear profile on roller segment Centre Æ Max. wear

ÅRolling point Max. wear

Wear profile on table segment Highest table speed

Lowest table speed

Slag grinding on OK mill with typical wave pattern from squeezed out material

Clinker grinding. Smooth wear surface