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HOMOGENISATION EVALUATION EXERCISE:
BACKGROUND
A kiln feed has to be produced based on a 2 component raw blend.
One orboth of the components may be produced from a front section
in a limestonebench. If necessary correction materials such as
shale or limestone sweetenermay be applied.
A representative front section in the limestone bench has been
selected asbasis for the evaluation (taken from an actual case).
This is shown on sheet 1:
The front section is 170 m long, with a burden of 3 m and a
maximum benchheight of 10 m (the scale ratio on sheet 1 is:
vertical : front direction = 5:1).The tonnage of the block
delineated by the length, the bench height and the
burden is: 12750 ton (density 2.5 T/m3). If required this
tonnage has to beentered into a pre-homogenisation stockpile of
12750 ton, distributed in 200layers.
The limestone block, as defined above, has been investigated by
18 drillholes, separated by 10 m in the front direction and sampled
each meter in thevertical direction. Each sample has been analysed
for CaO. The overallaverage of all samples, computed as a simple
average, is 42.65% CaO, whichtherefore is the overall average of
the 12750 ton. Further, it is assumed thatthe overall average of
the total limestone component is 42.65% CaO as well.
A completely homogeneous shale is available, which for each
hourly tonnage,HTMi, has an average grade, Mi, of 10%CaO. The error
on this average,
computed as the standard-deviation Mi is: 0.01%CaO.
A completely homogeneous limestone sweetener is also available.
Thecorresponding figures forHTMi of this component are: Mi, =
54%CaO and
Mi = 0.01%CaO.
The raw blend set point for CaO = 42%.
If homogenisation and/or proportioning shall be avoided it is
required that theallowable grade variation of all quantities HTMi
are within the variation interval:Mi +/- 0.05%CaO (1 SD). This is
obviously the case for the shale and thelimestone sweetener but not
for the material from the limestone bench.
OBJECTIVE
In the following (sheet 2 through 6) five different scenarios
are described.They define five different ways for extracting,
pre-homogenising and blendingof two raw blend components, in order
to end up with a variation interval of 42+/- 0.05%CaO in the kiln
feed. For each scenario the corresponding values of
Mi and Mi have been computed for the quantity HTMi from each
blendcomponent.
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If the variation interval, Mi +/- M , for the limestone
component proves to begreater than 42.65 +/- 0.05%CaO a
proportioning simulation test has beencarried out (sheet 7 through
10). This test proves if it will be possible toeliminate the
variation through proportioning of the quantities HTMi. The
criteria
for success is that 0 Wi 1 for all simulated blends, where Wi is
theblending fractions (raw mix fractions on sheet 7 through 10) of
the twocomponents in question.
The objective of the exercise is then to find out:
What are the practical consequences of each scenario for
theextraction of the materials along the limestone front. (Hint: It
might beuseful to redraw the stockpile block of sheet 1on a profile
with thesame scale in the vertical- and front direction, and to
enter theboundary line between the specified limestone components
on theprofile).
Which of the scenarios is the most suitable with respect to:o
Controlling the raw blend/kiln feed quality safely.
o Extracting the blend components from the front in a
practically
feasible way.o Obtaining the most representative samples for
forecasting the
quality of the blend components during the on-line quality
controlof the stockpile (Hint: use the information on the
covariancefunctions shown for each component, in particular the
max.covar. value, remembering that the greater this is the greater
the
error on the estimated grades of the components will be). Which
homogenising equipment should be selected to comply with the
found solution.
