Post on 07-Apr-2018
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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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Vane Feedwell Performance
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Vane Feedwell Performance
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Vane Feedwell Performance
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
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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