GEOPHYSICAL TECHNIQUES APPLIED TO BLASTING DESIGN

A. L. Vieira1, J. C. Koppe2 1- VALE

2- Federal University of Rio Grande do Sul

Summary

• Introduction

– Problem

– Objectives

• Mine site

• Methodology

• Results

• Conclusions

Introduction

• Iron ore production in Brazil is the most important mining operation.

• Brazil is one of the world’s largest iron ore producer.

• Two regions: Minas Gerais and Pará.

• Ore: BIF with high level of weathering.

• Drill & Blasting.

Problem

• The intensive weathering acting on banded iron formation produced deep soil profiles and extensive rock alteration.

• In many cases hard material was preserved from the rock alteration process resulting in a mixture of boulders, soil and soft rocky material.

• Large blocks of hard rock – soft material Problem for blasting and excavation

8 m

Objectives

– The presence of large blocks of rock amid soil and weathered rocks represents a challenge for blast design.

• Develop a methodology based on geophysical survey to obtain a geological model that can assist on specific blast design in case of occurrence of hard and friable materials in a mineral deposit.

Mine site

• Aboboras mine: 40 km from Belo Horizonte, MG.

• Ore production: 21 Mt ROM / year.

• Quadrilatero Ferrífero (QF).

• The lithologies of the QF are characterized by a metamorphic complex represented by supracrustal sequences of volcano sedimentary rocks, clastic and chemical sedimentary rocks, ultramafic, mafic and felsic bodies especially intruded into the Archean lithologies.

• Itabirites = iron ore (43 to 64%). Hard and friable.

• Bench: 10 m.

• Drill hole Ø: 200 mm.

• 4 x 7 m; 4 x 8 m.

• ANFO, emulsion.

Methodology

• To study the problem involving the occurrence of boulders amid soil and weathered rocks were selected 4 geophysical methods.

• The main objectives were to differentiate the friable from the compact (hard) itabirites and identify the location of the boulders in the soil.

Methodology

• Considering the properties of the rock types and soil four methods were selected to be used in this research:

– seismic,

– electro resistivity,

– induced polarization and

– ground penetrating radar.

Methodology

• Selection of the study area working bench.

• After selecting the study area the geophysical survey was carried out considering 14 sections covering an

area of 15,000 m2(150 x 100 m).

Geophysical sections (14)

• The data obtained were compared and integrated to generate the geological/geophysical model.

• After the geophysical survey a drilling campaign was performed to validate the model.

This model was the base for the blast design

Results

• Electro resistivity

• Method based on the eletric resistivity of materials.

• Mineralogical composition, porosity, water, salts.

SuperSting R8/IP

• The analysis considered the differences in the profiles: based on the magnitude in high apparent resistivity zones (ZAR - whose values are above 50 000 ohm.m), and areas of low apparent resistivity (ZBR - whose values are below 2600 ohm.m).

• The electric profiles allowed observing at depths ranging from 3 to 30 m. The data were fairly consistent in the differentiation of high and low resistivity anomalies.

• Induced polarisation

• Geoelectric Method based on measurements of

voltage variations as function of time and frequency.

• The IP analyses considered differences in the profiles

on the basis of magnitude in high chargeability

apparent zones (ZAC) whose values are above 60 ms

and low chargeability apparent zones (ZBC) whose

values are below 10 ms.

3D modeling – software

3D

Blast Design

• The blast pattern (burden × spacing) at the Aboboras mine is 3 × 7 m or 4 x 8 m, powder factor around 200 g/t.

• The new blast design: based on the geological model developed following the steps described before.

• Essentially the change was done in the sense of increasing the number of blastholes in areas where boulders or hard itabirite were located and decreasing the number of blastholes in areas of friable itabirite.

• The blasting test was performed in the area where the geophysical survey was carried out.

Conclusions

• The methods of GPR combined with IP, resistivity and seismic showed to be effective in modeling the contrast among hard, friable materials and soil.

• Resistivity and IP were the most effective geophysical methods.

• Sismic refracion showed two types of materials.

• GPR showed some descontinuities related to regional faults.

• The blast design based on geophysical/geological models resulted in significant improvement in the fragmentation of the iron ore at the Aboboras mine.

• With the new specialised blasting design the formation of boulders was eliminated.

• Thanks VALE.

• THANK YOU!