Dry Stacking of Cycloned Tailings W. Lugão - VOGBR, Brazil M. Almeida - VOGBR, Brazil A. Guimarães - VOGBR, Brazil F. Magalhães - VOGBR, Brazil S. Mohallem - ARCELORMITTAL, Brazil

Introduction Tailings at ArcelorMittal Mineração’s Serra Azul Mine are currently disposed of in a tailings dam, which is at the end of its useful life. Another tailings disposal facility is required and there is no place for another conventional dam. A drained stacking scheme was proposed, with the following advantages over conventional dams: - It allows tailings to be stored in stacks, which poses less risk as it does not involve the storage of water; - It will be located in an area that was occupied by a fines stockpile (sinter feed), which is why no additional land will have to be purchased; - It allows mining operations to proceed without interruption.

IMAGE: 09-14-2007 – GOOGLE EARTH SERRA AZUL MINE TAILINGS DAM SINTER FEED

IMAGE: 07-30-2011 – GOOGLE EARTH SERRA AZUL MINE TAILINGS DAM DRY STACKING TAILINGS

CONTAINMENT DIKE SEDIMENT Design Concept DRY STACKING DRYING PONDS- OVERFLOW TAILINGS CONTAINMENT DIKE SEDIMENT CYCLONE

WASTE/TAILINGS CO-DISPOSAL Design Concept A WASTE/TAILINGS CO-DISPOSAL WASTE PILE DRY STACKING

ROCK-FILL STARTER EMBANKMENT Design Concept Internal drainage DRAIN - SECTION JIG TAILINGS ROCK-FILL STARTER EMBANKMENT

Example: Plant level 1,030 m – tailings/waste co-disposal Design Concept Example: Plant level 1,030 m – tailings/waste co-disposal A WASTE PILE UNDERFLOW TAILINGS OVERFLOW / UNDERFLOW TAILINGS JIG TAILINGS ROCK-FILL STARTER EMBANKMENT SECTION A TAILINGS CELLS 100m 25m 8m

Schematic profile (section A) Design Concept Schematic profile (section A) OVERFLOW/UNDERFLOW UNDERFLOW DIKES WASTE PILE JIG DIKES ROCK-FILL - STARTER EMBANKMENT

Design Concept Conception: Underflow dikes – vertical drainage!!! Overflow – drainage in top and base!!!

Pilot Tests – Total Tailings

Pilot Tests - Cyclone UNDERFLOW TAILINGS CYCLONE

Tailings characterisation Percent finer than Grain size - millimeters Jig Tailings Total Tailings AM01 Underflow AM02 Underflow Overflow

Table - Permeability test summary Table - Test results Sample SG Full gradation Compaction Void ratio Clay (%) Silt (%) Sand (%) Gravel (%) woptimum (%) rmax (g/cm³) emax emin Total tailings 3.717 4.4 60.9 34.7 0.0 - AM-1 Underflow 3.741 1.8 30.6 67.4 0.2 1.01 0.71 AM-2 Underflow 3.838 1.4 33.6 64.7 1.09 0.77 Overflow 3.405 7.2 77.2 15.6 13.0 2.208 Jig 3.544 5.0 65.5 29.3 Table - Permeability test summary Sample Re-moulding conditions Permeability (m/s) Relative compaction (%) Degree of compaction (%) Moisture (%) rdry (g/cm³) AM-1 Underflow 60 - 11.6 2.039 1.10e-06 AM-2 Underflow 70 11.7 2.038 1.40e-06 Overflow 85 12.8 1.878 4.20e-08

Table - Triaxial test results Table 6 Parameters for materials used in stress-strain and stability analyses Table - Triaxial test results Sample Effective strength parameters Total strength parameters c’ (kPa) f’ (°) c (kPa) f (°) AM-1 Underflow 3.89  29.7 26.5 5.00  31.2 17.20  18.7 Overflow 4.05 30.1 5.20 11.6  Table - Parameters for materials used in stress-strain and stability analyses Material Density  (kN/ m³) c’ (kPa) f’ (°) Deformation modulus (MPa) n (Poisson’s ratio) Foundation 17.0 15 28 20,000 0.20 Rock-fill 23.0 42 80 0.25 Underflow 22.8 30 20 0.30 Overflow 20.0 2 26 4.0 0.40 Jigue 36 27.5 Waste 22.0 34 40 0.33

Stress-strain analysis MATERIALS FINITE ELEMENT MODEL (SIGMA) FINAL STACK CONDITION

Stability Analyses The safety factor for the critical failure surface is 1.97 and is therefore satisfactory. Stability analysis results for waste rock/tailings co-disposal – Global – Non-Circular failure.

Operation OVERFLOW TAILINGS – DISPOSAL SEQUENCE

Building the tailings cells

ROCK-FILL - STARTER EMBANKMENT Situation - December 2012 DRYING PONDS ROCK-FILL - STARTER EMBANKMENT UNDERFLOW LEVEL 1000.5 m CONTAINMENT DIKE OVERFLOW LEVEL 1000 m

Final Remarks Stability analyses have arrived at a satisfactory safety factor, assuming the long-term (drained) final stack condition; the maximum predicted subsidence in dry stacks will be acceptable even in the most critical long-term condition; It is suggested that the cyclone system operating techniques should be reviewed, as the system is now working with overflow rates in excess of design rates. As a result, underflow requirements are being filled with jig tailings currently available at the mine for such purpose. It is also recommended that alternative methods should be developed for carrying overflow tailings from dewatering ponds to the storage basins, including alternative deposition methods, with a view to improving the process which is currently undertaken using trucks. An investigation campaign is programmed, composed of SPT, CPTU, Vane, permeability and laboratory tests; ArcelorMittal will continue monitoring the performance of the system.

Contact: wlugao@vogbr.com.br Thank you! Contact: wlugao@vogbr.com.br