Mechanical Technology

Mechanical Technology — November 2015

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Materials handling and minerals processing

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lthough there has always been

the need to obtain representa-

tive samples in mineral par-

ticulate systems, there has not

been a proven technology that can take

a representative sample horizontally from

pressurised tailings’ pipelines.

This is according to Rolf Steinhaus,

sampling specialist at Multotec Process

Equipment, who says that incorrect

sampling solutions are often employed on

the tailings section of minerals processing

plants and particularly so in gold, plati-

num and base metal industries.

“Metallurgical accounting is very dif-

ficult when the data or results produced

are from these inaccurate arbitrary sample

material collections. Meaningful metallur-

gical accounting is essential because plant

operators need to have accurate informa-

tion as to what comes into and leaves a

plant in order to establish how efficient the

actual beneficiation process is and what

yields or recovery can be expected from

the metal or precious mineral. Reliable

reporting and corporate governance calls

for representative sampling, using cor-

rectly designed sampling equipment and

implemented protocols that will produce

accurate data,” Steinhaus points out.

Multotec has developed a mechani-

cal design for the True Pipe

sampler

and, together with Dr Paul Roberts, has

taken out a patent on the product. The

prototype has been extensively labora-

tory tested and the results compared

favourably with a correctly designed

vezin sampler used as a control. Data

generated has been very encouraging

and Multotec is now in a position to

consider industrialising the unit together

with a partner.

This sampling system comprises

pipework and valves that will isolate and

correctly delineate a sample increment

under controlled conditions. Operation

of the valves is synchronised, resulting in

symmetry between two separate biasing

mechanisms occurring in the vicinity of

the two relevant valves, which will cancel

each other out. What is misplaced into

the sample at one end is displaced from

the sample at the other.

“The principle of symmetry has been

statistically proven for this

particular design. The True

Pipe sampler will ul-

timately have ap-

plications in high

pressure slurry lines with pressures of

up 2 000 kPa and slurry line velocities

of up to 6.0 m/s, however testing for

less challenging applications needs to

be done,” says Steinhaus.

During the development stage and

as a separate investigation, the slurry

stream was carefully split into two equal

streams, each carrying 50% of the flow.

“Statistically the results re confirmed

that, due to segregation effects, partial

stream sampling is not advisable and

that splitting the main stream into partial

streams is once again (like any sample

stream diversion) never an acceptable

shortcut when sampling representatively.

Prior to the development of this innovative

sampling system for tailings pipelines,

all measurements taken would be con-

sidered as non-representative and would

therefore have been biased,” he adds.

Plant tailings streams are generally

pumped out in pipelines to slurry and

tailings dams, a process that, typically,

does not lend itself to conventional cross-

cut sampling methods. Current sampling

devices include injector or poppet sam-

plers, which are pneumatically driven

arms that collect small increments of

fluid from the centre of the stream via an

opening in the shaft. Sampling practices

of sampling part of the stream, part of the

time is not acceptable or at all reliable

for heterogeneous particulate systems.

“One needs to be able to take a full

crosscut or slice of the stream, due to

the fact that segregation of material is

always present in particulate systems.

This methodology provides an accurate

and precise composite sample if cuts are

taken frequently,” says Steinhaus.

Another unsatisfactory sampling

system being used is the T-piece bleed

off valve, which bleeds off a portion of

the stream. “This is an

ad hoc

material

extraction via a valve often situated ar-

bitrarily on horizontal pipe columns and

results will differ depending on the loca-

tion and orientation of the device around

Specialist equipment supplier to the minerals processing industry,

Multotec, has designed a True Pipe

Sampler that can provide correct

sampling across pressurised tailings streams.

Pipe sampler for tailings’ streams

Multotec has

developed a

mechanical design

for the True Pipe

sampler.

the diameter of pipe. Larger heavier

particles will be collected preferentially

if the device is situated at the bottom of

the pipe. Again, this is not representa-

tive of the whole but regrettably, in the

absence of anything better, users often

believe that the imperfect sample result

will suffice,” Steinhaus says. Numerous

sampling errors occur, with the result

that potentially poor control decisions are

made based on an inaccurate specimen

collected using an inadequately designed

and limiting design device.

The pressure pipe sampler is another

example of a typically used slurry col-

lection device. Part of the stream is

sampled continuously via a probe within

the dilated housing of the sampler body

pipeline. Steinhaus points out that it is

incorrectly assumed that the turbulence

which occurs in the dilated section of

the pipe with or without turbulence bars,

will overcome sample delimitation and

extraction errors due to some homogeni-

sation. This is definitely not the case as

it is still not a full cross-stream sample.

Endemic to these three slurry collec-

tion devices is their inability to provide

a representative and unbiased sample.

“However, because they are all low-

cost devices, they are often considered

an attractive option. In simple terms,

the widespread use of these devices

underpins the lack of understanding of

sampling theory. The principles of cross-

stream sampling, on the other hand,

have been proven,” adds Steinhaus.

“One needs to remember that repre-

sentative sampling of a stream, particu-

larly in a pressurised environment, is a

complex process. There are up to six or

more types of sampling errors that can

occur when trying to extract a sample in-

crement from a whole stream or a portion

of the particulate stream. The True Pipe

sampler addresses the majority of these

challenges and provides a sample with

integrity under controlled conditions,”

Steinhaus concludes.