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Pregrinding systems














3


Roller Press (suite 3)















3.7

Problem of the roller pressure:

















The
force generated on the rollers to complete the work of comminution comes from
3 factors:




* The number of hydraulic cylinders








* The diameter of the cylinder








* Operating pressure









The formula calculating
the force is as follows:






















With:








F is the force generated in kN








d is the roll diameter in m








n is the number of cylinder








p is the pressure in bar









The specific force is given by the formula:






























With:








S is the specific force in kN/m2








D is the diameter of the rollers in m








l is the width of the
rollers (or the bed of material) in m








Example:








* Rollers diameter: 1,6 m








* Rollers Width: 0,9 m








* Diameter of pistons
(hydraulic cylinder): 0,53 m







* Number of cylinders: 4








* Operating Pressure: 110 bar








Calculate the force
generated and the specific strength:








































Generally, the hydraulic
pressure is between 50 and 150 bar.








The
specific force generated is between 6500 and 8000 kN/m2 depending on the
manufacturer.





Obviously,
the permitted specific force must consider the problem of wear roller which
is still current.





The
constant shock of the rollers is absorbed by a nitrogen accumulator system
installed behind the moving roller,




allowing smooth operation of the press.















3.8

Operating parameters:















3.8.1

Feed granulometry









The maximum size of the
feed recommended varies from one manufacturer to another.






But it is in any way
dependent on the diameter of the rollers.








A commonly accepted value is 50 mm.









Another means for
determining the maximum size is given by the formula:


















3.8.2

Rollers speed









The circumferential speed
of the rollers is a key parameter of the operation of the press.






Its metric unit is m/s (meters per second).









This
speed determines the throughput of the press and consequently the power
consumption of the press.





But the speed of the
rollers must also adapt to the particle size of the fresh feed.







Indeed,
if the feed includes a large proportion of fine particles (90μ), the press
tends to vibrate if the roller speed




is high (1,71,9 m/s).









Result, a regular operation is not possible.









This
can happen in the case of the circuit with recirculation of cakes, because
there are usually a large proportion of




fines in it.













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The
diagram here below shows the influence of the recirculation of cakes on the
speed of the rollers:


































































We
see that the greater the circulation factor and the more it is necessary to
reduce the speed, so the output.





The
circumferential speed of the rollers is between 1 et 1,6
m/s depending on the conditions of use.











3.8.2

Gap between rollers









The gap between the two
rollers equals the thickness of the material bed.







This is a critical
parameter since it is directly proportional to the total flow rate of the
press.






The distance between the
rollers is primarily dependent on the press dimensions.






Is estimated to be
between 0,013.D and 0,02.D (D being the diameter of the rollers in mm)






If this method is used, we have:
































































3.8.3

Operating Pressure









It
is known that the grinding efficiency depends on the specific power
transmitted to the material bed and the same




specific energy depends
on the absorbed power.








The
absorbed power is also correlated to the transmitted force itself correlated
to the operating pressure.





We
must therefore find the best setting for this pressure which actually depends
on three parameters:




* Initial settings of the
nitrogen accumulators







* Pressure setting of the hydraulic system








* Initial gap between rollers









A
compromise must be found between these 3 parameters to achieve the proper
operation pressure.



3.8.4

Problem of moisture









Some materials can be
very wet such as limestone, slag, coal, shale, pozzolan or clay.






Obviously,
a drying plant must be installed in order to dry these materials prior to
their introduction in the press.





However,
a certain percentage of residual moisture (up to 2%) can be beneficial in
order to stabilize the feed.





This is particularly the
case for grinding slag material.














3.9

Wear problems of the rollers:

















As
was mentioned in the introduction, the beginnings of the roller presses have
been quite difficult following




the big problems of
premature wear of material at the surface of the rollers.







In
addition to the costs associated for the repair or replacement of the rollers
and the decrease of production,




it was not recommended to
optimize the ball mill.








Great efforts have
therefore been made to find lasting solutions.








Although
manufacturers continue their research, we can say today that the roller press
is a reliable machine.





Without going into
complicated metallurgical considerations, the proposed solutions are:





* Forged rollers reloaded








* Rollers with bainitic bandages








* Rollers with composite bandages








* Rollers with hard alloy pins (abrasive)








* Rollers with hardened
surfaces (fullhardfacing CHF)







* Rollers with Hexadur profile








(informations collected
in the suppliers technical brochures and available online)






The roller surfaces thus
can have the following profiles:

















































According
to the same manufacturers, the lifetime of some of these profiles are tens of
thousands of hours.









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