Predicting the size distribution in the
product and the power requirements
of a pilot scale Vertimill
INTRODUCTION
• Vale, the second largest mining company in
the world, has instigated a detailed study for
the utilization of Vertimills as an alternative to
conventional tube ball mills in pellet feed
grinding applications.
INTRODUCTION
• Three samples were prepared with mixtures
of different feeds. Normally, the grinding
circuits in pelletizing plant receive the product
from different mines with different properties
in size distributions, hardness, density etc.
Then, the preparation of distinct mixtures for
testing is important.
COMMINUTION MODEL
dm (t )
i

1
i  S m (t )   b S m (t )
i i
ij j j
dt
j 1
COMMINUTION MODEL
P

Si  S  
H
E
i
SE
2
ln E   1 ln x   2 ln x 
S1

 xi 1 
 xi 1 
  1   .

Bi , j   
 x 
 x 
 j 
 j 

SAMPLES
100
% Passing
80
60
40
Mix2
Mix 3
20
Mix 4
0
0.01
0.10
1.00
Size (mm)
10.00
VERTIMILL PILOT TESTS
• The Vertimill was operated with screw speed
of 87 rpm in direct circuit configuration closed
with a Derrick Vibrating Screen. Samples of
new feed, mill discharge and screen oversize
and undersize were collected during the tests
for solids density and size distribution
analysis.
BALL SIZE DISTRIBUTIONS
Sample
Mix 2
Mix 3 and 4
Ball size (mm)
Weight (%)
Weight (%)
25
30.7
-
19
34.6
40.7
15
29.7
-
12
5
42.5
9
-
16.8
Total
100
100
LAB TESTS
• Each batch grinding test is carried out in a
diferent time interval in dry basis. An
additional grinding test is carried out with
water at the desired solids concentration.
LAB TESTS
• The tests are designed to reach the desired
product size distribution specified as a P80
value. In order to achieve this, grinding times
are calculated assuming first order breakage
and the previous test result at a shorter
grinding time.
LAB TESTS
Batch Tube Mill Test
Mill Diameter (mm)
254
Mill Length (mm)
254
Ball Charge Level - J (%)
40
Voids - U (%)
100
Critical Speed (%)
70
Charge Lifters
Yes, 8
PARAMETERS
• The grinding parameters were determined
using a specialized optimization software
developed
by
Mineral
Technologies
International, Inc. – MTII called BatchMillTM.
The software interpolates the size distribution
of each milling time in order to determine the
model parameters
PARAMETERS
Selection Function
Sample
S1E S1E*
t/kWh t/kWh
1
2
Breakage Function



Error
NSR
Mix 2
3.784 5.108 0.115 -0.435 1.508 2.348 0.414 0.277
Mix 3
9.623
Mix 4
3.857 5.207 0.009 -0.401 2.307 2.661 0.837 0.459
12.99
0.626 -0.312 2.500 5.549 0.635 0.230
1
The parameter S1E was multiplied by 1.35 to correct for the higher
efficiency of the Vertimill with respect to the standard ball mill.
SIMULATIONS
• Data from mass balance of each test was used
to perform simulations using the HerbstFuesternau ball mill model implemented in
the Modsim plant-wide Simulator. The feed
and product size distributions of the mill were
considered to check the accuracy of the
model.
MIX 2 Simulation
Cumulative % smaller than
100
80
60
Feed
40
Product
Herbst-Fuerstenau
20
0
0.010
0.100
Particle size, mm
1.000
MIX 3 Simulation
Cumulative % smaller than
100
80
60
Feed
40
Product
Herbst-Fuerstenau
20
0
0.010
0.100
Particle size, mm
1.000
MIX 4 Simulation
Cumulative % smaller than
100
80
60
Feed
40
Product
Herbst-Fuerstenau
20
0
0.010
0.100
Particle size, mm
1.000
CONCLUSIONS
• As a preliminary conclusion it is very likely that
the grinding mechanisms that prevail in a
Vertimill are the very same grinding
mechanisms that occur in the conventional
ball mill.
• The difference between these two types of
mills is basically their energy efficiency.
CONCLUSIONS
• It is recommendable to validate this
methodology with coarser ore samples
because it is possible that other factors may
influence the product size distribution and the
net power drawn in the Vertimill, as for
example the settling velocity in the
classification and the grinding zones of the
Vertimill.
QUESTIONS?