the secret to thickener performance

8/6/2019 The Secret to Thickener Performance

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The secret to thickener

performance

Richard Triglavcanin

Brandt Henriksson


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Operating Principles of Thickener Feedwells Most thickeners today operate using flocculation The principles for flocculating slurries have advanced in

thickeners since the development of High Rate Thickeners in the

mid 1960s Feed systems generally have the following objectives:

To utilise the kinetic energy of the feed stream toefficiently mix dilution liquor (if required) and flocculant

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into the solids suspension To dissipate this kinetic energy prior to promoting

aggregate growth To distribute the flocculated feed evenly into the body of

the thickener without shearing the formed aggregates.

Most current thickener feed systems fail to achieve these criteria!


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Thickener Feed Systems Why is it important?Failure of the feed system to create efficient mixing ofdilution water and flocculant is a major source of

operational problems:

Increased flocculant consumption

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Reduced underflow density Reduced overflow clarity Reduced unit area throughput Rake problems bogging, donuts, Lower availability Increased operator involvement (controllability)


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Current Feedwell Design

There are currently two main options for thickener feedwells: Closed and Open bottom designs

Closed bottom feedwells featurea bottom restriction and deflector

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plate This minimises feed slurry short-

circuiting and uncontrolled feeddilution

Feed is evenly distributed at theexit and hence into the thickener.


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Current Feedwell Design Open bottom designs are generally based on a simple open cylinder. Feed dilution is uncontrolled from below the feedwell.

Open bottom feedwellst icall demonstrate feed

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short circuiting and no controlover feed dispersion into thethickener, especially asdiameter increases


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Feedwell Size Does Matter In both closed and open bottom feedwells, aspect ratio becomes a

critical issue As feedwell diameter to height ratio increases, high density feed

tends to short circuit resulting in significant inefficiencies.

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Low Aspect Ratio Design 1:1 to 1.5:1 Without baffles, shear rates are high in the lower half of the feedwell High shear rates at the feedwell exit cause aggregate breakage on

exit This results in lower underflow density, lower settling rates and

increased flocculant use to compensate.

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Low Aspect Ratio Design 1:1 to 1.5:1 With correct baffling, high shear rates are located in the top half of

the feedwell for good mixing and where dilution and flocculant

dispersion takes place The bottom half of the feedwell shows low shear ideal foraggregate growth

Aggregate size exiting to the thickener is maximised .

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Medium Aspect Ratio Design 2:1 to 4:1 Base case modelling for a gold tailings, 6.5m diam x 2m deep closed

bottom feedwell with Autodil

Feed short-circuits the feedwell. Dilution flow is poor. Poor flocculation and uneven exit distribution dominates.

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Medium Aspect Ratio Design Shelf &Directional Autodilution

Modification to the dilution ports so the dilution flow complimentsthe feed flow improves mixing energy

Addition of a shelf holds-up incoming dense feed and reducedshort-circuiting Solids distribution is even with strong dilution water flow Improved solids distribution exiting the feedwell.

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Current Design in Feed Systems

The last quantum change in feedwell design was more than 15years ago with the introduction of self-dilution systems Autodil and

E-Duc. The fact that this was some 15 years ago demonstrates thecomplexity and difficulty of true feedwell understanding andinnovation

As thickeners et lar er in diameter to accommodate ever-

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increasing throughputs their feedwells also increase in diameter,however feedwell depth does not increase in equal proportion andshort-circuiting of solids leads to poor performance

Feedwell design for 6m diameter and greaterrequires particular design consideration.


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Introducing the Vane Feedwell

Kolwezi 8m

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Introducing the Vane Feedwell

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Bloom Lake


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Vane Feedwell Solids Distribution

Solids, flocculant anddilution water are wellmixed in the upperzone of the feedwell

Dilution flows arestrong and

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comp emen eslurry flow

Strong aggregategrowth in lower zoneof feedwell

Even distribution orlarge aggregates tothickener tank.


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AMIRA / CSIRO Conclusions

good feedwell performance by all modeled criteria good flocculation , . well mixed . residence time was

maintained .. final discharge symmetry was good , although notentirely uniform.

QUOTE;

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,appears to be excellent

Although most of the feed energy was dissipated in the feedwellthe shear rates were moderate in most regions .

Importantly the shear rates in the exit region were moderate,avoiding potentially disruptive aggregate breakage . By keepingmost of the energy dissipation and shear high in the feedwell, aroundthe shelf and vanes, mixing was provided early in the flocculationprocess to aid flocculant dispersion and effective dilution.


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Vane Feedwell Performance

A recent retrofit feedwell has clearly demonstrated what I would describe as a step change in feedwell performance.

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reg ane, anager o aStrategic Sales, AshlandWater Technologies

Mineral sands slimes Australia7m diameter feedwell


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Results:- reduction in flocculant usage 10 - 20%- increase in throughput 10%

- less variability in underflow density- less variability with changing slimes- Dramatic improvement in operability

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Avge flocc dose g/t

0

100

200

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500

600

700

Dec-07 Jan-08 Feb-08 Mar-08 Apr-08 May-08 Jun-08 Jul-08

Retrofit


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Controlling Feed Entry Velocity

Another important aspect in large diameter feedwells isfeed velocity

Maintaining the energy to achieve slurry dispersion andmixing at half the normal design flow is a challenge

This roblem is solved b introduction of a variable area

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feed pipe.


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Controlling Feed Entry Velocity Solids distribution at half design flow rate. Controlled feed

entry velocity on right.

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Conclusions Understanding feedwell performance in the design phase is vital The Vane Feedwell represents the first major innovation for 15 years

It is testament to the difficulty involved in understanding what isrequired for good feedwell hence thickener performance Mixing, diluting, flocculating, shearing and energy dissipation whilst

growing aggregates and then releasing them uniformly without

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aggregate rea age nto t e o y o a t c ener, a w t n an v a afeedwell!

The conclusion drawn is that feedwells must have two zones, an upperand lower zone

It is not true that all thickeners perform in a similar manner; the truth isthere is an operating cost and consequence based on the performanceof the feedwell

The feedwell is the secret of thickener performance


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The secret to thickener

performance

the secret to thickener performance

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