the effect of process variations on slurry rheology, washer
TRANSCRIPT
Fiona Sofra, Rheological Consulting Services, Australia
Adrian Knight, Rheological Consulting Services, Australia
The effect of process variations on slurry rheology, washer performance and control strategy
•Lateritic ore processing
•Following leaching, pregnant liquor is recovered via counter current washing/decantation (CCD).
•Multiple washing trains operating in parallel
•Flocculants induce rapid settling
•Following washing, tailings are pumped for further thickening and final disposal
The process
•Last washer in the train was experiencing exceedingly high rake torques with over torque occurring often
•Underflow and pipeline blockages, washer train shutdowns had occurred as a result of production of excessively thick/high yield stress underflows
•Excessive turbidity in last washer overflow - operators reacted by increasing floc dosage rates
•However, the underflow SG/solids concentration was relatively stable and within operational limits
•Some trains operating with greater stability than others – not known why at the outset of the study
The Problem
•Underflow yield stress directly related to flocculation (type, dose, conditions), causing raking and pumping problems
So…
•Assumption that suboptimal flocculation was causing operational issues
Therefore…
•Investigation of sediment rheology flocculated under various conditions will determine optimum flocculation conditions and address operational issues
Preconceptions/assumptions
•Shear yield stress was primary measurement in all instances
•Samples collected over 11 days
•Unflocculated material testing
•Last washer feed ‘bucket’ flocculation
•Varied flocculant type and dosage rate
•Concentration of flocculated sediment and generation of shear yield stress profiles for partially sheared flocculated material
Testwork
DV
H
Speed TorqueTorsionHead
Sample
Vane
DV
H
Speed TorqueTorsionHead
Sample
Vane
•Unsheared shear yield stress of flocculated sediment using custom built cylinder
•Flocculated sediment shear history effects
•PSD measurement of all collected sample
(after site testwork)
Testwork - Unsheared
•Trialled 10, 20, 30 and 50gpt of each flocculant type (1, 2 and 3)
•Inconclusive results despite consistent shear history.
•The “best performing” flocculants and doses also had most favourable PSDs (more coarse) in terms of achieving a lower yield stress for a given solids concentration.
Effect of Flocculant Type and Dose
Flocculant Type
0
50
100
150
200
250
300
350
400
450
500
0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65
Yie
ld S
tre
ss (Pa
)
Solids Mass Fraction (x)
Day 3 LWF 50gpt Floc 1
Day 8 LWF 50gpt Floc 2
Day 10 LWF 50gpt Floc 3
• typical results at all doses tested
• inconclusive due to inconsistent PSD – PSD effects dominated
Mean: 34mm d50: 8mm d90: 92mm
Mean: 41mm d50: 8mm d90: 130mm Mean: 50mm
d50: 10mm d90: 161mm
Thickener feed PSD Variation
0
50
100
150
200
250
0 2 4 6 8 10 12
Par
ticl
e S
ize
(u
m)
Day
d50d80d90
• high variability, especially in coarse fraction
Flocculant Dose
– typical results for all flocculants within dose ranges tested
Effect of PSD Variations on Yield Stress
Mean: 63mm d50: 16mm d90: 198mm
Mean: 46mm d50: 8mm d90: 147mm
Mean: 45mm d50: 9mm d90: 138mm
Preliminary finding…
High feed PSD variability effects dominate flocculation effects
Therefore…
Optimising flocculant type or dose will have little effect without prior consideration of process variability and control
Two washers - different control strategies, with different results
A) Bed height B) Bed pressure
Control Strategy A: Bed Height Control
Bed Height
Increase UF Pump Speed
Decrease Floc Dosage
Decrease UF Pump Speed
Increase Floc Dosage
Control Parameter Control Responses Other Influences
Downstream Controls
Operator Inputs
Downstream Controls
Overflow turbidity
High Reading
Low Reading
Bed Height response to underflow pump
IN PRACTICE •Bed height responds rapidly to changes in UF pump speed •Makes UF pump speed good control variable for this strategy
Bed Height response to floc dose
IN PRACTICE •Bed height responds relatively quickly to changes in floc dosage •Minimises fluctuations in floc dosage rate to mostly well within ± 50% of set points
Bed Height Control – effect on rake pressure
IN PRACTICE •Provides relative operational stability for long periods of time •Bed height and rake pressure stay mostly within ± 25% of set points
Control Strategy B: Rake Pressure Control
Rake Pressure
Increase UF Pump Speed
Decrease Floc Dosage
Decrease UF Pump Speed
Increase Floc Dosage
Control Parameter Control Responses Other Influences
Downstream Controls
Operator Inputs
Downstream Controls
Overflow turbidity
High Reading
Low Reading
IN PRACTICE •As rake pressure increases, UF pump cannot operate efficiently •Rake pressure and UF pump are out of sync and do not stabilise
Rake pressure response to underflow pump
IN PRACTICE •Floc dose is out of sync with rake pressure due to temporal differences •Floc dose also used to control overflow clarity and is unreliable as control parameter
Rake pressure response to floc dose
IN PRACTICE •Significant amounts of fluctuation in both rake pressure and bed height •Bed height and rake pressure fluctuate to in excess of ± 50% of set points
Rake Pressure Control – effect on bed height
Control Philosophy Comparisons
Washer Control Strategy A
Parameter Maximum Deviation (%) Average Deviation (%)
Bed Height 23.46 5.75
Rake Pressure 31.96 11.59
Washer Control Strategy B
Parameter Maximum Deviation (%) Average Deviation (%)
Bed Height 83.43 20.24
Rake Pressure 66.08 17.00
•Rheological assessment showed that operational variations, mostly feed rate and PSD effects dominate flocculation effects in terms of rheological issues
•Optimising flocculant type or dose will have little effect without prior consideration of feed variability and process control
•Analysis of control strategy shows some strategies can exacerbate effects of process variation whereas other control strategies can smooth these effects
•Operator intervention must be considered and operators trained to recognise ‘knock-on’ effects of manual changes made.
Conclusions
Gather historical feed and operating data during testwork planning phase
Be prepared to question long-held operational assumptions
In hindsight…