1. Bauxite Residue Valorisation and Best Practices Conference , 08. 05
Combined Carbothermic Reduction of Bauxite Residue and Basic Oxygen Furnace Slag for Enhanced Recovery of Fe and Slag Conditioning
Buhle Xakalashe, Bernd Friedrich

2. Carbothermic Reduction of Bauxite Residue
Fe recovered to metal phase
(low iron production compared
to primary resources)
High Al2O3 containing slag
High operating temperatures
Fluxed smelting
Lower operating temperatures
‘conventional fluxes‘
Lime, dolomite, wollastonite,
quartz, silica sand
Non conventional fluxing
Enhanced Fe recovery, slag conditioning
Aluminium of Greece Bauxite Residue
Components
(wt.%)
Fe2O3
Al2O3
CaO
SiO2
TiO2
Nd (ppm) Sc
(ppm)
Na2O
Bauxite residue
43.5 24
10.2
5.5
5.6
110
130 2
3C + Fe2O3 = 2Fe + 3CO (1) BR

3. Basic Oxygen Furnace Slag
Fluxing capability
Contains Fe
BOF-slag
By-product in converter steel making
High production volumes
Main uses: aggregates for road
construction and fertilizers
Extraction of Fe and use of gangue components would be a high value application
German BOF-slag
Components
(wt.%)
Fe2O3
Al2O3
CaO
SiO2
TiO2
Na2O
BOF-slag
23.1
1.4
45.1
14.4
1.1
0.1
BOF
Slag BR

4. Iron Production - Combined smelting of BR and BOF-slag
Increases of 16 to 25% theoretically achievable for BOF-slag additions of 23 wt.% to 33 wt.% (FactSage 6.4)

5. Experimental Procedure
100kW DC electric arc furnace
(operated 3-6 kW)
Batch mass: 1.5 kg BR
Lignite coke: 0.1 x BR (reductant)
CaO: 0.3 x BR (Baseline fluxing)
BOF-slag: 0.3 x BR (combined fluxing)
Target temperature: 1500oC oC
Holding time: 1 hour
Controlled cooling (slow cooling)
Slag cooling: 20oC/min

6. Product Slag – Theoretical vs Experimental
Thermochemical modelling in agreement with experimental results
(exception to Na in high CaO containing slag)
Low residual Fe achieved in the slag (high recoveries to metal)
Components (wt.%)
Fe2O3
Al2O3
CaO
SiO2
TiO2
Na2O
Bauxite residue
43.5 24
10.2
5.5
5.6
2.0
BOF slag
23.1
1.4
45.1
14.4
1.1
0.1
30% CaO fluxing (exp.)
1.7
33.8
49.0
6.9
0.2
30% CaO fluxing
(Theoretical)
1.6
31.2
50.4
7.1
7.2
2.1
30% BOF slag fluxing
(exp.)
3.7
34.5
31.6
15.7
7.8
2.9
30% BOF slag fluxing (Theoretical)
3.6
35.1
31.7
15.2
7.4
2.4

7. Elemental Recoveries
Experimental
Product Fe Al Ca Ti Si Na
30%
Lime
Slag
2.9
105.6
95.0
92.4
94.1
7.5
Metal
87.9 -
0.8
Total
90.9
93.2
BOF-slag
5.0
96.8
91.3
90.1
109.6
97.9
91.5
0.2
1.8
96.5
90.3
111.4
A closing mass balance observed, based on comparable product masses to theoretically predicted masses

8. XRD analysis of BR and produced slags
Both slags contain comparable phases which can be optimised for recovery of valuable elements (Al, Ti and Sc)
The mayenite phase for the CaO fluxed slag can be attributed to the suffient availability CaO

9. Summary and Outlook
Combined smelting of bauxite residue and BOF-slag was carried out successfully
Combined recovery of iron from bauxite residue and BOF-slag was achieved
BOF-slag is a promising fluxing agent in bauxite residue smelting
Outlook
Optimisation of BOF-slag additions for iron recovery (quality and quantity of the produced metal)
Optimisation of the combined smelting process for downstream recovery of valuable elements (slag chemistry and mineralogy)