PASTE BHS-SONTHOFEN_proof 25/05/2016 11:12 Page 1
Paste Supplement
Twin-shaft mixing
Barry Perlmutter, President & Managing Director of BHSSonthofen Inc, looks at twin-shaft mixing technology for
paste backfill
BHS “Duplex
Tools” for nonconditioned
tailings
veryone has a general idea what mixing
means, but describing the mixing process
is not so simple. The objective of any
mixing process is to distribute and blend
materials with different characteristics, and
different quantities as evenly as possible,
resulting in a homogenous mixture.
Introducing proportional movement with the
optimal intensity into the materials achieves
effective homogeneous mixing results. A simple
one directional movement with inadequate
acceleration of the materials would obviously
not achieve the same mixing results. This would
be more akin to stirring rather than mixing.
The amount of relative movement that is
introduced is a critical factor. For successful
mixing there must be a complete distribution of
all materials and, at the same time, achieving
the highest probability that all recipe particles
located in a specific position at the beginning
can be found at any random location in the
mixing chamber at the end of the process. This
is the only way to ensure that a consistent and
repeatable result is achieved.
E
Three-phases of a mixing cycle
The mixing cycle occurs in three phases;
charging the mixer, the mixing process, and
discharging. The sequence in which the
individual materials to be mixed are filled into
P4 International Mining | JUNE 2016 Supplement
cons istent paste viscosity or measured spread.
Many paste and concrete formulas also involve
small quantities of chemical additives which
also must be evenly distributed. The time
required per mixing cycle as well as the costs of
energy, wear and maintenance will determine
the economics of the concrete production.
Twin-shaft mixing technology and
three-dimensional mixing
The twin-shaft batch mixer has become the
preferred technology for paste production in
many countries. The twin-shaft mixer was
invented for concrete production at the end of
the 19th century and registered for patent by
BHS-Sonthofen GmbH. The initial reasoning was
to replace manual mixing with a shovel in which
a line of aggregate, cement and water
positioned on the ground, was repeatedly
shovelled from right to left. This concept was
later abandoned and over a number of
development stages, was replaced by today’s
highly dynamic movement process used for
numerous applications worldwide.
Twin-shaft mixers are equipped with mixing
blades on both mixing shafts that are
geometrically arranged so they follow the
pattern of an interrupted spiral. This motions
the materials to be mixed in a screw-like pattern
both along the mixing shafts and on each shaft
in opposite directions. On the ends of both
shafts the mixing blades are positioned in a
counter direction so they can transport the mix
onto the opposing shaft. This way, the materials
are constantly rotating around the mixing
trough. At the same time, the material rotating
process also takes place in an inward turning
spiral. This results in an intensive threedimensional movement of material.
the mixer can have a significant impact on the
efficiency of the mixing cycle. The mixing cycle
overlaps the charging process and the discharge
cycle. To achieve optimum results, effective
discharging methods should be taken into
consideration to avoid risks of segregation.
Costs for energy and wear should be as low as
possible and finally, it is very important that the
mixing cycle takes place as quickly as possible.
In paste backfill production, selecting the
Intense three-dimensional mixing
best mixing technology is crucial for both the
The two mixing circuits overlap in the middle to
quality of the final product to be blended and
further increase the intensity of the relative
the efficiency of the production process. The
motion. This creates a high turbulent zone in the
optimum mixing system is one that rapidly
middle of the mixing trough and significantly
disperses all
ingredients evenly
throughout the
mixing trough and
completely
surrounds all the
coarse or fine
aggregate particles
with the slurry
cement particles.
Specific for paste
production, the
mixers job is to
break down the
unconditioned
BHS twin-shaft mixer produces a 95% homogenous mixture in a 30 second
tailings to achieve
mixing time