W.L. Oliveira Filho, UFOP‐D.R. Silva, SAMARCO

F.E. Almeida, SAMARCO

Thickening of iron ore tailings slimes using sub-aerial deposition: field experimental observations

• Introduction

• Background

• Field experimental studies

• Analyses of experimental data

• Conclusion

Contents

Introduction

Desiccation & Sub-aerial Studies Timeline

BackgroundAlternative disposal techniques for slimes

• Types• sub-aerial deposition• Thickened• Paste• filtered

• Common aspects• Intermitent: cycles of waiting and disposal periods• Physical processes: sedimentation, consolidation and desiccation

BackgroundDesiccation

• Importance • The most effective phenomena for rehabilitation work, optimizing storage, and

reducing risks regarding containment structure failure

• Triggering mechanisms• Surface drying• Lowering the GWT

• Driving force and Phases• Suction• 1D shrinkage• 3D shrinkage (cracking)

• Comprehensive works• Abu-Hejleh & Znidarcic (1995) and Yao et al. (2002)• Konrad & Ayad (1997) • Fujiyasu (1997)

Background

Constitutiverelations

Compressibility

Permeability

Cracking function

a - function

Main input relationships for analyses with CONDES (Yao et al. 2002)

Field experimental studies

Goal & strategies

• To gain some understanding of the main mechanisms that play a role in the sub-aerial method applied to Samarco’s slimes

• To investigate consolidation separately from desiccation using a field experiment

• To focus on slimes desiccation because was lesser known and more challenging

Field experimental studiesSite & operations• Location: inside the Germano tailings impoundment (Samarco Mineração S.A.) in

Mariana, MG.

• Impoundment figures: 3 m high ring dyke, confining an area of 4,850 m2.

• Foundation: 2 m of coarse siliceous tailings, grading from fine sand to medium silt, underlain by a deep layer of iron tailings slimes.

• Ground water table: at the contact of those layers, 2 m below the surface.

• Drainage system: stop logs installed at the lower part (bottom at 2% slope)

• Access: a pier to the centre of the testing area for instrumentation maintenance and sampling operations

• Filling: slimes pumped from an adjacent slimes pond at the Germano impoundment

• Water cover: 5 to 10 cm deep during the filling process and consolidation period to prevent early desiccation.

• Surface water removal: for the desiccation part of the test.

Field experimental studiesInstrumentation, testing & instalation

• Devices: • Geotechnical: Thermistors, settlement devices (staff gauges), tensiometers,

piezometers, time domain reflectometry probes (TDRs)• Climate: Weather station and a class A pan test

• Placement: • Thermistors and TDR probes launched at certain pond elevations during the deposit

filling (movable position)• All other instruments at fixed positions

• Testing• Periodic sampling using a stationary sampler• Gravimetric water content determination• Specific gravity and bulk density.

Field experimental studies

Overview of the testing site

Analyses of experimental data

Analyses Data/Parameters

Consolidation

Compressibility and Permeability parameters

A (kPa-1) 2.5438

B -0.1920C (m/dia) 9.45 x 10-4

D 4.2370Z (kPa) 0.0495

Boundary conditions Bottom hp= variable

Top Surcharge nullSpecific gravity - G 3.89Filling rate (m/day) 0.0603Filling period (day) 34

Desiccation

Cracking function parameters

A 0.3859B 0.0508C 1.3665D 0.626 x 1018

Boundary conditions

Bottom hp= -0.55 mTop Evap. rate =

0.002 m/day

Input data for analyses with CONDES

Analyses of experimental data

Progress of the deposit height (at day 84 starts desiccation)

Analyses of experimental data

Progress of volumetric water content (day 84 starts desiccation)

Analyses of experimental data

Progress of gravimetric water content (desiccation starts at day 84)

Analyses of experimental data

Progress of bulk densities (desiccation starts at day 84)

Analyses of experimental data

Progress in solids content (desiccation starts at day 84)

Analyses of experimental data

Progress of porepressure at base (desiccation starts at day 84)

Analyses of experimental data

Meteorological data during field experiments (desiccation starts at day 84)

Analyses of experimental data

Progress of evaporation with data from the weather station and Class A device

Analyses of experimental data

Progress of cracking (a) day 89, (b) day 92, (c) day 94

Conclusions• Characterization of the desiccation behaviour of a fine tailings from the iron

ore milling operations was successfully reached.

• A test section was built and monitored, using an extensive sort of instruments and tests.

• Material behaviour and boundary conditions were assessed such as settlement, water content, bulk densities, solids content, porewater pressures, evaporation rates, etc.

• Cracking morphology has also been described.

• Sub-aerial deposition conditions was examined as a part a large study of alternative methods for slimes.

• A brief comparison of field data with a numerical modelling of the problem was presented and the results have shown consistent agreement.

• Overall, the research seems to suggest that reasonable efficiency with slimes thickening can be achieved by the sub-aerial disposition method.

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