Pitfalls in Vezin sampling for finely crushed materials
C Kruger, E van Tonder
Anglo American Platinum has adopted a strategy of best practice principles
(BPP) for sampling, mass measurement, analysis, and metal accounting for
more than a decade now. When new plants are designed, the most suitable
sampling equipment and sampling protocols are implemented and material
flows and plant layouts are designed around sampling strategies. A few
years ago, a crushing plant for a high-grade matte stream was commissioned.
The design called for the high-grade material to be crushed to 95%
passing 2 mm, and the design allowed for a single-cutter primary Vezin
sampler taking increments from a gravity flowing stream at regular
intervals while filling a tanker for pneumatic offload at the base metal
refinery. This was a single point of sampling and analysis that would
measure the wanted output from the smelting complex and the input to the
refineries. In addition, the design incorporated the functionality to collect
increments in an ‘interleaved’ fashion from the Vezin sampler for internal
precision checks.
The metal accounting strategy for Anglo Platinum involves the ongoing
measurement of all inputs, outputs, and stock levels for all operations for
platinum group metal (PGM) and base metal (BM) content. Included are
yearly stocktakes, usually done on 1 February, across the smelters and
refineries (at least the base metal refinery). In the accounting period
immediately following the introduction of the new crushing and sampling
facility, a trend was developing whereby the absolute difference between
the physical stock and theoretical stock (PGMs predominantly) increased
for the two respective sites on an ongoing basis. The smelter showed an
increase in physical stock compared to the theoretical stock (sender) and
the base metal refinery showed an increase in theoretical stock against
physical stock. Because of this anomaly, an additional stocktake was held
in September for the period in question, and it was confirmed through the
physical stock measurement results that the PGM accounting problem was
indeed pointing towards the evaluation of the high-grade matte stream.
This paper showcases parts of the process that took place to explain
the poor accountabilities for the two sites. Previous results from the
sampler ratification process (of the interleaved sampling design) were reanalysed,
a physical inspection was carried out on the sampling system
again, and the sampler performance was monitored (quality control
parameters) for the period.
From these investigations, it was found that airflows inside the Vezin
introduced the sampling bias. A change in sampling protocol was
introduced, as well as certain physical changes to the sampling equipment,
to eliminate the airflow and thus the bias conditions significantly. The
accounting period that followed the phased upgrading process showed that
the accounting between the smelter and base metal refinery had been
restored to within statistical limits (difference between physical and
theoretical stock below 5% relative to theoretical stock).
Keywords: sampling, metal accounting, Vezin sampler, sampler bias.