5
Filtrate Solids Higher Than Expected
The filters were equipped with monofilamt bags which were
used during lab testing. During the first days of commis-
sioning, the amount of filtrate solids was far higher than
what was expected from the lab test. Apart from that, the
filters were reaching 170 t/h at a speed of less than 1 rpm
at a moisture in the range of 21–22 %w/w. It was decided
to change the filter bags to needle felt. This took about two
weeks to get a full set of needle felt bags for one filter to site.
Immediately after the installation of these bags, the filtrate
became clear with 1 g/l solids and the issue was solved.
After Two Years Operation
Segment Corrosion
After two years of operation the segments showed severe
corrosion. During decision making it was decided that with
a pH 10 the use of mild steel for segments and filtrate
pipes should be possible although the standard MoC is
ss304 for these parts. But this was decided to stay within
the project budget. In the third year of operation the repair
of the segments started as a first step to deal with this issue
because of maintenance budget restraints. This repair strat-
egy was continued for almost three years. Then the decision
was made to change to ss304 segments.
Filtrate Pipes Corrosion
After the issue with segment corrosion had become aware,
it was only a question of time, when this issue will reach
the filtrate pipes which were made of mild steel as well. But
as this issue was expected to come, the planning could be
made at an early stage for a change to a centre barrel with
stainless steel filtrate pipes. One barrel was changed to a
new barrel and the two original barrels were repaired and
equipped with stainless steel filtrate pipes. Now the plant
has a rotating spare barrel which is a smart way of minimiz-
ing the risk of loosing production.
After Five Years Operation
Change in Particle Size Distribution
In the period of 5 … 7 years after commissioning the filter
was building thicker and thicker cakes and the automatic
filter control was reducing the speed to almost minimum
speed. As a consequence, the operators reduced the floc-
culant dosage to the filter step by step and finally did not
add any flocculant. A filter feed sample was taken and the
particle size distribution was measured. This revealed that
the medium particle size had increased from 25 µm to 40
µm what explained the situation very well. The nice side
effect was the reduction in operation cost by more than
Euro 200,000 per year with the total stop of flocculant dos-
age. And, in addition, the moisture improved i.e., reduced
by more than 1 %w/w. However, this was not helpful for
the process, because the cement mixer requires a minimum
moisture and 20 …21 %w/w is too low for propper mix-
ing. Therefore, makeup water addition was increased.
CONCLUSION
Mining operations are designed to run for many years or
decades. Therefore, it is important to do a proper selection
and sizing of the equipment during the engineering phase
in order to ensure a smooth commissioning of the plant.
It is important that the filter sizing has some safety margin
incorporated in order to ensure that the equipment fullfill
Figure 9. View of installed Bovac Disc Filter with 176 m2
filtration area
Figure 10. View of new filter barrel with stainless steel
filtrate pipes
Filtrate Solids Higher Than Expected
The filters were equipped with monofilamt bags which were
used during lab testing. During the first days of commis-
sioning, the amount of filtrate solids was far higher than
what was expected from the lab test. Apart from that, the
filters were reaching 170 t/h at a speed of less than 1 rpm
at a moisture in the range of 21–22 %w/w. It was decided
to change the filter bags to needle felt. This took about two
weeks to get a full set of needle felt bags for one filter to site.
Immediately after the installation of these bags, the filtrate
became clear with 1 g/l solids and the issue was solved.
After Two Years Operation
Segment Corrosion
After two years of operation the segments showed severe
corrosion. During decision making it was decided that with
a pH 10 the use of mild steel for segments and filtrate
pipes should be possible although the standard MoC is
ss304 for these parts. But this was decided to stay within
the project budget. In the third year of operation the repair
of the segments started as a first step to deal with this issue
because of maintenance budget restraints. This repair strat-
egy was continued for almost three years. Then the decision
was made to change to ss304 segments.
Filtrate Pipes Corrosion
After the issue with segment corrosion had become aware,
it was only a question of time, when this issue will reach
the filtrate pipes which were made of mild steel as well. But
as this issue was expected to come, the planning could be
made at an early stage for a change to a centre barrel with
stainless steel filtrate pipes. One barrel was changed to a
new barrel and the two original barrels were repaired and
equipped with stainless steel filtrate pipes. Now the plant
has a rotating spare barrel which is a smart way of minimiz-
ing the risk of loosing production.
After Five Years Operation
Change in Particle Size Distribution
In the period of 5 … 7 years after commissioning the filter
was building thicker and thicker cakes and the automatic
filter control was reducing the speed to almost minimum
speed. As a consequence, the operators reduced the floc-
culant dosage to the filter step by step and finally did not
add any flocculant. A filter feed sample was taken and the
particle size distribution was measured. This revealed that
the medium particle size had increased from 25 µm to 40
µm what explained the situation very well. The nice side
effect was the reduction in operation cost by more than
Euro 200,000 per year with the total stop of flocculant dos-
age. And, in addition, the moisture improved i.e., reduced
by more than 1 %w/w. However, this was not helpful for
the process, because the cement mixer requires a minimum
moisture and 20 …21 %w/w is too low for propper mix-
ing. Therefore, makeup water addition was increased.
CONCLUSION
Mining operations are designed to run for many years or
decades. Therefore, it is important to do a proper selection
and sizing of the equipment during the engineering phase
in order to ensure a smooth commissioning of the plant.
It is important that the filter sizing has some safety margin
incorporated in order to ensure that the equipment fullfill
Figure 9. View of installed Bovac Disc Filter with 176 m2
filtration area
Figure 10. View of new filter barrel with stainless steel
filtrate pipes