Flocc Dosing on Demand by Werner Loeser rev01

Dosing on Demand

© Water & Tailings Pty Ltd – chemical dosing on demand for optimised thickener and plant operation

optimized chemical dosing for tailings thickeners

fig. 1: feed well with high turbulence

Beside facilitating a sound tailings

dam management, thickeners are

employed to recover a significant

amount of water which is crucial

for the ore processing operations

upstream of the thickener and for

dust suppression.

Without the thickener overflow / supernatant, certain mining operations wouldn’t

be possible; thickeners are often bottlenecks, and downtimes can result in

significant production loses.

Despite their importance, thickener operation is often far from

optimal, and negative impacts on the ore recovery in ore processing

facilities upstream of the thickener like scrubbers, spirals, upstream-

classifiers, cyclones and band-filters are widely neglected.

THICKENERS are crucial facilities on mine sites…


fig. 2: overflow launder heavily loaded with sediments

…however, due to cost restriction the buffer

volume is kept as small as possible and so called

‘high-rate’ thickeners are common on mine sites.

…‘high-rate’ refers to the characteristic settling velocity, which needs

to be higher for thickeners with a smaller volume / shorter retention

time to achieve the same overflow quality.

High-rate thickeners allow smaller footprints, are smaller sized and

therefore cost significant less. However, the cost savings come with

some downsides:

THICKENERS are buffers providing required retention time for settling…

• operation is less stable due to reduced volume / cross-section;

• operation without chemicals / flocculants not possible, thus higher operational costs;

• they can be extremely sensitive regarding non-optimal flocculant dosing regimen.

Dosing on Demandoptimized chemical dosing for tailings thickeners


The thickener design has to address

• flow rates / inflow

• load / tons of solids per m3

• type and composition of solids / fines

• chemical dosing: flocculants / coagulants

• environmental parameters (temperature etc.)

THICKENERS and design parameter…


It should be noted, that the terms ‘coagulation / flocculation’ have different meanings for various industries / realms, however, in general it describes

‘a process of contact and adhesion whereby the particles of a dispersion form larger-size clusters for the purpose of settling’.

The removal of fines in a (high-rate) thickener is based on physico-chemical kinetics involving:

• coagulation / flocculation processes for flocc forming, and

• separation of formed floccs by gravity / settling



fig. 3: basic thickener operation

THICKENERS …basic setup…(…that rarely works)


THICKENERS & instrumentation…


Trying to operate a thickener with only

• a bed level sensor

• infrared light adsorption, or

• submersible sonar, or

• electromechanical displacement

and

• a bed pressure / bed mass sensor

• pressure gauge / ceramic capacitive bridge

might work if the inflow composition and flow rate don’t vary over time.

fig. 4: bridge and feed launder

However, changes in ore body, raw material, plant operations prior to the thickener

and temperature that impacts the kinetics, will likely require a more sophisticated

instrumentation setup.




fig. 5: underflow density over time




additional instrumentation to control bed level, underflow & overflow clarity:

• flow meters – feed, underflow, overflow

• magnetic inductive inline flow meter

• density meter - underflow

• nucleonic gamma ray attenuation

• turbidity meter - overflow

• infrared beam scattered light

• settling velocity meter – feed well

• type Alcotech / BASF

fig. 6: Alcotech




• dosing rates are determined via lab tests which can only be snapshots…

• for ‘performance surety’, the recommended dosage will cover often the worst case scenario 24/7

While all these instrumentations will assist in controlling the thickener, they won’t

prevent flocculants in the overflow.

The instrumentation listed above will facilitate some level of dosage control, e.g. by

linking the dosage to inflow rates, and / or Alcotech readings, however, they can’t

reliably control the flocculant residual in the overflow.

Till now, such operation regimens and the associated higher chemical costs due to

overdosing are widely accepted.


This PPP outlines why this usual adopted flocculant dosing regimen

will likely have far more disadvantages than just the ones usually

linked to poor thickener performance and / or overdosing, e.g.:

Importance of optimized flocculant dosing…


• higher maintenance

• higher risk of downtime and associated production losses

• poor beaching in tailings dams if underflow is too thin

• higher pump&pipe wear if underflow is too thick

• higher chemical costs due to overdosing

• poor water recovery

…however, something commonly overseen and neglected:

…Correct flocculant dosing is the single most important factor in

the operation of a high rate thickener!

Dosing on Demand



The tailings / inflow into a

thickener can contain minerals,

clay, inert solids, oxidized metals

and organics - these matters have

to be removed to allow the

overflow to be reused as process

water in processes upstream of

the thickener.

Floc forming kinetics…

The ability to remain in suspension depends on the particle size and SG; colloids have

a large surface area / mass ratio which is the reason why

• electric surface charges, and

• Brownian movements & van der Waals forces

need to be considered to allow the forming of larger and denser flocs.


Matters that cause turbidity…


fig. 7: types of colloids

Dosing on Demand



Hydrophilic colloids can remain in a stable suspension and may require chemical

reaction with a coagulant to destabilize the suspension and allow floc forming.

Floc forming kinetics…

The floc forming kinetics for hydrophobic colloids are more physical than

chemical. As both types of colloids are usually present in tailings, the kinetics

become very complicated and complex.

The added flocculating / coagulating agents (e.g. Fe3+ or Al3+, and / or

polyelectrolytes) will induce aggregation of the colloids by manipulating (prior

chemical reaction might occur) the surface charge and the zeta potential to allow

the Brownian movement and the van der Waals forces to destabilize the

suspension and to grow aggregates / flocs.

Therefore the surface charge & zeta potential become the most crucial

parameter to determine the flocculation / coagulation potential in the

thickener and in the overflow!

Dosing on Demand



The water recovered as overflow is required as process water for the operation of

equipment upstream of the thickener:

Performance threats through overflow with high flocculant residuals…

• scrubbers

• spirals

• upstream classifier

• vibration screens

• cyclones

• band filters

The purpose of this equipment is removal of fines and gunk from the raw material to

achieve the highest possible ore recovery!

The physical principles for this equipment are the same that are valid for a

thickener, however, for the best separation of tailings from the ore it is

absolutely crucial that the non-ore particles are remaining in suspension!

If the colloids are already forming agglomerates with a higher SG, all the

beneficiation equipment will struggle to recover the valuable ore.

All equipment upstream of the thickener receiving process water with active flocculants will

show compromised performance with negative impacts on overall ore recovery.

Dosing on Demand




fig. 8: ore processing plant - schematic

Dosing on Demand




fig. 9: upstream classifier

fig. 10: spirals

Dosing on Demand




fig.10: scrubbers

fig. 11: scrubbers with band filters in the front

Dosing on Demand



Conclusion…

While all the commonly employed instrumentation assists thickener

performance, it won’t facilitate control of the flocculant residuals in the overflow.

However, for the best ore recovery possible, the control of flocculation

potential in the overflow is a must!

This requires new technologies to measure the surface charge and zeta

potential in tailings.

Currently the University of Wollongong, in cooperation with Water & Tailings Pty

Ltd, are developing and testing suitable charge analyser instruments that will

allow optimised flocculant dosing depending on actual demand (DoD), thus

minimising the risk of higher flocculant concentrations in the overflow.

It can be assumed that the return on the required investment for retrofitting such

a new technology will be significantly less than 1 year due to higher ore

recovery, savings in chemicals and optimized thickener operation.

Dosing on Demand

End of Presentation – Thank you for your interest!

Werner M. Loeser – Specialist Water Engineer

+61 (0) 421 585 365

+61 (0) 280 056 910

[email protected]

39 Kalang Rd., Kiama Heights N.S.W. 2533



mailto:[email protected]

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Flocc Dosing on Demand by Werner Loeser rev01