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Introduction
Motivation
Experimental
Results & Discussion
Gibbsite auto-precipitation experiments
Bauxite residue flocculation experiments
Gibbsite crystallisation experiments
Conclusions
.
• Alumina is refined from bauxite ore via the Bayer process, as show in the schematic left.
• The mud separation and washing stage is an important unit operation in the process, shown as enlarged schematic right.
Figure 8. Gibbsite auto-precipitation with no additive and 100 ppm HX800.
0
5
10
15
20
25
30
35
40
45
0 5 10 15 20
Me
an
No
. W
t. C
hd
. L
en
gth
(u
m)
Time (h)
Blank
10 mmol/L Na-gluconate
3 mmol/L Na-gluconate
1 mmol/L Na-gluconate
0.1 mmol/L Na-gluconate
0
100
200
300
400
500
600
1 10 100 1000Chord Length (um)
CL
D (N
o. / C
han
nel)
t = 0
t = 1 min
t = 10 mins
t = 20 mins
t = 40 mins
Figure 2. Time variance of bauxite ore quality at Jamaican mine1.
Jacketed Heater
Lab Jack
Figure 4. Baskerville pilot autoclave used to digest bauxite ores.
Eurotherm Control
Figure 7. Experimental setup for flocculation and gibbsite crystallisation studies.
Data processing Unit
FBRM Probe
Automated stirrer
Circulating water bath
Crystalliser and flocculation setup
Online PC
Figure 15. FBRM data showing the effect of gluconate additive on gibbsite crystallisation.
Loss of fines
Fines forming
• Alumina producers often experience premature alumina crystallisation (gibbsite auto-precipitation) in the mud separation and washing stage: Al(OH)4- <==> Al(OH)3 + OH-
• Recent changes to bauxite ore quality at a Jamaican Bayer refinery saw major losses of alumina and caustic from the process, attributable to certain mineral phases in the bauxite.
Figure 1. At left, process flow diagram of Bayer Process illustrating its cyclic nature, and right, flows at each stage (n) in counter-current decantation (O = overflow; U = underflow).
Figure 3. Mineral transformations of bauxite ores due to weathering
Figure 5. Baskerville Eurotherm process control unit.
Figure 6. Parr 4560 benchtop autoclave used for digestion.
Table 1. Bayer process terminologies and concentrations, and their molar equivalents.
• Bauxite ores were digested at 150 oC for 30 minutes (A/C ≈ 0.65) to give bauxite residue seeds used in the study.
• The effect of additives on gibbsite auto-precipitation was monitored at 75 oC.
• Gibbsite crystallisation and bauxite residue flocculation were monitored by FBRM.
Figure 9. Gibbsite auto-precipitation with no additive, 2 mmol L-1 CaCO3 and MgCO3.
Figure 11. Gibbsite auto-precipitation with additive and a range of gluconate doses.
Figure 10. Gibbsite auto-precipitation with no additive, 20 mmol L-1 CaCO3 and LiOH.H2O.
Figure 12. (a) Far view; (b) close view. Gibbsite auto-precipitation with goethite seed present (fine
needle-like crystals).
Figure 14. FBRM and turbidity data as a function of gluconate dose for the flocculation of bauxite residue
with 702 g/t HX300 flocculant.
• Gluconate behaviour is complex.
• Flocculation negatively affected at low doses, but positive affected at high doses.
• The effect may involve surface adsorption or complexation of the residue/flocculant.
• Gibbsite crystallisation deteriorates markedly as the gluconate dose increases.
• Gluconate suppression mechanism is likely due to adsorption onto gibbsite seed surfaces.
Figure 16. Change in CLD when monitoring gluconate effect on gibbsite crystallisation.
Acknowledgements This work was funded by West Indies Alumina Company (Windalco). Technical and logistical assistance were received from the UC RUSAL group and Aughinish Alumina Limited. 1Mr. Desmond Lawson is credited with the production of Figure 2.
The effect of additives on gibbsite auto-precipitation and bauxite residue flocculation when processing goethitic bauxites
Keddon Andre Powell1, Benjamin Kieran Hodnett1* and Luke Jonathon Kirwan2 1. Materials and Surface Science Institute, University of Limerick, Limerick, Ireland
2. Researcher, Aughinish Alumina Limited, Askeaton, Limerick, Ireland Corresponding author: [email protected]
Figure 13. FBRM and turbidity data as a function of gluconate dose for the flocculation of bauxite residue
with 202 g/t 9779 flocculant.
• Goethite is an active seed in the premature crystallisation (auto-precipitation) of gibbsite in the mud circuit.
• Sodium gluconate is an effective additive for mitigating against premature crystallisation of gibbsite.
• Sodium gluconate negatively affects bauxite residue flocculation and gibbsite crystallisation, this warrants further investigation, due to the potential impact it may have on these unit operations.
Bauxite Storage
Desilication Caustic Soda
Mined Bauxite
Digestion
Slurry
Heat + Pressure
Liquor Flashing Liquor Heating
Spent Liquor Mud Separation
Liquor
Evaporation
Pregnant Liquor
Filtration
Cooling Heating
Wash
Water
Precipitation
Classification
Washing
Calcination
Steam
Steam
Alumina
Heat (1000 0C)
Gibbsite Seed
Mud
Disposal
Mud
Washing
Spent Liquor
Water
O n
On+1
Un-1 Un
Stage n - 1 Stage n Stage n + 1
Red Bauxite
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Yellow-Red
Bauxite
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Yellow Bauxite
Boehmite Kaolin Anatase (Gibbsite) (Hematite) (Goethite) (Apatite)
Boehmite Kaolin Anatase Gibbsite Hematite Al-Goethite
[Organics
(CH2O)]
Crandallite
Wavellite
Variscite
( eFeFe 32 )
• Order of effect: HX800 ≈ MgCO3 < LiOH.H2O < CaCO3 < Gluconate.
• Gluconate is effective in the concentration range 2 - 10 mmol L-1.
( Al 3+ )
( )
𝑃𝑂4
3−
Little change in coarse end
Bayer Process Terminology
Bayer Process Concentration (gL-1)
Molar Equivalent Terminology
Molar Equivalent Concentration (mol L-1)
A as Al2O3 156 NaAl(OH)4 3.06
C as Na2CO3
240
NaOH 1.47
NaAl(OH)4 3.06
TTS as Na2CO3 300 Na2CO3 5.66
Carbonate as Na2CO3 60 Na2CO3 0.57