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Paste Supplement
intensifies the mixing process. Due to the
design of a BHS Twin-Shaft batch mixer, it’s
possible to achieve 95% homogeneity within
only 30 seconds of mixing time. This can be
achieved with a relatively low mixing speed of
the shafts, between 20 to 30 rpm. This low
mixing speed reduces energy consumption,
reduces wear and avoids stress on the particles
to be mixed.
In all mix designs with cementitious
materials, as in paste and concrete applications,
cement is the most important and most costly
raw material, typically 10% of the mix design
and 90% of the cost. Only a twin-shaft mixer can
guarantee the most efficient use of this high
value raw material. Various tests have shown
that a twin-shaft batch mixer can produce
concrete of a given strength with less cement then
other mixing methods still being utilised today.
Twin-shaft mixer applications:
batch or continuous
There are many different mixing applications
that require different types of twin-shaft mixers.
While the application may be batch or
continuous, the three-dimensional mixing
action, the mixing cycle, or retention-time never
changes. The difference between batch and
continuous is based upon how the mixer
discharges the final product.
For batch processing, the product discharges
along the centre of the mixer between both
mixing shafts using a rotary gate that is the
length of the mixing trough. A major portion of
the mixed materials are discharged through the
opening created by gravity. The remainder of the
material is forced out by the broad mixing
blades with minimal residual product. The risk
of mix segregation is therefore extremely low
compared to other types of mixers. This
discharge process is controllable as the rotary
discharge gate system is equipped with
adjustable opening positions that increase or
decrease the outflow during discharge.
For continuous processing, the discharge can
either be through the bottom or alternatively as
an overflow design. The bottom discharge uses
a smaller controllable rotary door which would
be more common for applications using larger
coarse aggregate and for dry non-pumpable
mixes. The smaller rotary door is located on the
opposite end of where the materials would
enter the mixer. The door would control the
discharge rate equal to the feed rate by
monitoring the weight contents via load-cells at
the same time as containing the optimum
mixing capacity inside the mixing chamber.
The overflow discharge is used for more selfconsolidating mixes that contain fine particles.
Once the optimum mixing level is achieved, the
continuous process is an automatic overflow
through an opening on the
opposite end of where the
materials enter the mixer.
Mixing testing
For batch and continuous, bottom
or overflow applications, the
mixer capacity must be
determined through specific lab
testing of the same ingredients
(mix design) to determine the mix
cycle or retention time. Having
the data of how long it takes to
blend specific materials
homogenously prior t o
discharging and the required
production rate, will determine
the specific mixer type and
capacity. This lab testing must be performed
using a similar mixer that would guarantee the
same mixing results at a much smaller capacity.
The scale-up is then direct from the lab mixer
including the guaranteed points of capacity, mix
time, quality, amount of cement, water usage,
etc.
The twin-shaft mixer is also an important
solution for applications with unique
operational requirements. For example, ice for
cooling and/or steam for heating is possible.
Concrete formulas with coarse aggregate sizes
up to 180 mm can also be processed. Scientific
studies have confirmed that many types of
cementitious blends can be processed within
the 30 second mix time.
Non-conditioned tailings and new
development of special mixing tools
It is well known that for many mining
applications using non-conditioned tailings; the
mixing process is even more demanding. The
mixer is required to break down the
agglomerates and distribute them evenly to
achieve a high level homogenous state. This
extra task required from the mixer, would
increase the mixing cycle and/or retention time,
which could lead to the need for larger capacity
mixers and or multiple mixers in a circuit to
achieve the required throughput.
To meet the needs of the marketplace and
avoid multiple mixers, BHS began a test
program with several clients and after
performing many lab mixing tests using various
non-conditioned tailings developed special
mixing tools, called “Duplex Tools.”
These “Duplex Tools” increase the number of
mixing arms and blades by 100%. As shown in
the attached figure, there are duplicate arms
located 180° from the standard configuration in
all positions. This array of mixing tools
essentially increases the mixing action two-fold.
Therefore a typical mix cycle that would take
To being a mixing evaluation and project, the
engineer must look at these steps
120 seconds is reduced to 60 seconds by
utilisation of Duplex Tools. The Duplex Tools
design also requires improvements in the
mixing shafts, seals, bearings and drive systems
as well as to take into account the additional
weight and the lateral forces generated during
the mixing cycle.
Summary
To being a mixing evaluation and project, the
engineer must look at the following steps
shown in the diagram. The first question is the
process and mix design. These parameters
include batch or continuous as well as the mix
design (cement, fly ash/slag, coarse and fine
aggregate, water, air entrainment and other
admix components). The other important
information is the mixing and batch cycle time,
throughput/hour, annual capacity, aggregate
sizes (largest to smallest), design density,
design strength and downstream discharge
components. With this data, the mixer
technology can be selected and optimised.
The principal advantages of the twin-shaft
mixing technology becomes even more evident
with larger mixing batch sizes and larger
machine capacity. This places different demands
on the mixing machine. The twin-shaft batch
mixer must guarantee the same mixing results
within the same mixing time, regardless of the
size of the mixer. Therefore, the mixer
manufacturer must ensure the precise scale-up
design of the mixing tools and drive system. In
summary, for dry or wet production of
cementitious powders blended with other fine
and/or coarse aggregate particles that require
uniform homogeneous consistency at rapid
mixing rates with reduced operational costs, the
twin-shaft mixer is the most optimum and
effective solution available. IM
JUNE 2016 Supplement | International Mining P5