reducing the cost base through metallurgical examination of grinding balls

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“Reducing the Cost Base through Metallurgical Examination of Grinding Balls” Johann Petrick Metallurgist, MTS Metallurgical Testing Services Presented at : MATERIALS & MAINTENANCE ADVANCEMENTS FOR MINING 2016 Kalgoorlie, October 14th, 2016

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“Reducing the Cost Base through Metallurgical Examination of

Grinding Balls”

Johann Petrick Metallurgist, MTS Metallurgical Testing Services

Presented at : MATERIALS & MAINTENANCE ADVANCEMENTS FOR MINING 2016 Kalgoorlie, October 14th, 2016

1. Overview 2. Types of Grinding Media 3. Manufacture process 4. Wear 5. Testing 6. Failures 7. Quality Control 8. Summary

Contents

∗ Comminution represents 40-50% of whole operational cost. ∗ Grinding media represents 40–45% of the total cost of

comminution. ∗ Grinding circuits in use worldwide:

∗ 53% Ball mills ∗ 38% SAG or AG

∗ Grinding media quality influence ∗ Ball consumption ∗ Power consumption ∗ Product quality ∗ Reagent use ∗ Efficiency of downstream processing and extraction

Overview

Grinding Media Materials

Metals

Abrasion Resistant Steels

Mn-Steels 1-1.5%C, 12.14%Mn

Low Alloy Steels Pearlitic

Martensitic

Stainless Steels

Austenitic

Alloyed Cast Irons

Ni-Cr grades (Ni-Hard)

2.6-3.3%C, 2%Cr, 4%Ni

3%C, 9%Cr, 5%Ni

High Cr Cast Irons

1.5-3%C, 12-30%Cr

Non-Metals

Types of Grinding Balls

100mm - 150mm balls

100mm - 150mm balls

75mm - 100mm grinding rods

25mm - 100mm balls

12mm - 38mm balls

<2mm balls

No balls

No balls

Grinding Ball Size

Skew rolling

Manufacturing Processes: Forged Grinding Balls

Upset forge

Copyright © 2010 Pearson Education South Asia Pte Ltd

Raw Material (steel bars)

Heating Induction or

furnace

Quenching Tempering Shipping Inspection and Testing

Individual Sand or Die moulds

Manufacturing Processes: Cast Grinding Balls

Automated production line

Raw Material Smelting

Cleaning Heat treatment Shipping Inspection

and Testing

∗ Global capacity for grinding media >5 million tons per year ∗ China is the biggest producer of grinding balls (2.3Mt) ∗ MolyCop (1.2Mt), Donhad, Scaw, Magotteaux, ME Elecmetal

Origins of Manufacture

0

500,000

1,000,000

1,500,000

2,000,000

2,500,000

China SouthAmerica

NorthAmerica

Europe Russia & CIS Middle East,SE Asia

Australia Africa

Capa

city

(ton

s)

Global capacity of the grinding media industry

Wear

∗ Abrasive wear ∗ Impact wear ∗ Corrosive wear ∗ Mills need to be refilled

continuously with new grinding balls as old ones wear out.

∗ Low Wear Rate ∗ No Premature Failures ∗ Low Cost ∗ Attributes

∗ Size, Shape, Density ∗ Free from flaws ∗ Hardness (Surface and Internal) ∗ Microstructure ∗ Chemistry

The Optimum Grinding Ball

Visual Assessment

∗ Casting defects ∗ Shrinkage and gas porosity

∗ Forging defects ∗ Laps, pits, grooves

∗ Cracks ∗ Shape

∗ Out of roundness (sphericity)

∗ Size ∗ Density

Sample Preparation

∗ Sectioning ∗ Minimise damage ∗ Water jet ∗ Abrasive disc

∗ Mounting ∗ Polishing ∗ Etching

Hardness

∗ Surface & 5mm below ∗ Hardness traverse

∗ Average Volumetric Hardness (AVH)

∗ Indicates Wear Performance

500

520

540

560

580

600

620

640

660

0 10 20 30 40 50 60 70Ha

rdne

ss (H

V30)

Distance from Surface (mm)

AVH 2mm spacing

AVH Traditional

Results 2mm spacing

Results Traditional

626

623

Microstructure

Low Alloy Steel

∗ Martensite ∗ Tempered ∗ Untempered

∗ Retained Austenite ∗ Bainite ∗ Pearlite

Core

Surface

Half Radius

Tempered Martensite

Martensite & Bainite

Retained Austenite

Martensite & Bainite

Pearlite

Retained Austenite

Microstructure

Defects

∗ Surface decarburisation ∗ Segregation ∗ Hot laps or folds ∗ Cracks

Hot Laps

Hot Laps

Hot Laps

Surface decarburisation

Segregation

Segregation Cracks

Microstructure

High Cr Cast Irons

∗ Dendrites ∗ Segregation ∗ Primary, secondary and

eutectic carbides ∗ Austenite or martensite

matrix

Chemical Analysis

∗ OES / Leco ∗ Nitrogen ∗ Oxygen ∗ Hydrogen ∗ Fisheye fractures ∗ <2ppm

∗ Calculated hardenability or % Carbides

∗ Splitting, half balls, shelling, capping, surface spalling and misshapen balls

∗ Change of shape reduces the grinding efficiency. ∗ The grinding effectiveness is greatly reduced when a ball

splits as the grinding energy depends on the weight. ∗ Fisheyes

∗ Hydrogen embrittlement ∗ Segregation ∗ High hardness

Premature Failures

Fisheye Fracture

Localised intergranular fracture

∗ Develop and Maintain Quality Control (QC) Program ∗ Inspection of each shipment to verify correct supply ∗ Statistical sampling for further testing. ∗ Basic testing (Surface hardness) on larger sample size ∗ Comprehensive test program on small number of

samples ∗ Examination of scats ∗ Failure analysis

Quality Control

∗ Correct selection of grinding media for application ∗ Understanding of the Metallurgical properties

influencing performance ∗ Develop and Maintain Quality Control (QC) Program ∗ Ensure constant supply of good quality grinding balls ∗ Cost benefits of a structured testing regime. ∗ Reduction in wear and failures ∗ Enables better focus on other milling variables by user

Summary

∗ Moema, J.S., Papo, M.J., Slabbert, G.A., and ZIMBA, J. (2009). Grinding media quality assurance for the comminution of gold ores. World Gold Conference 2009, The Southern African Institute of Mining and Metallurgy.

∗ Aldrich, C. (2013). Consumption of steel grinding media in mills – A review. Minerals Engineering 49. pp77–91.

∗ Massola, C., Chaves, A.P. and Albertin, E. (2016). A discussion on the measurement of grinding media wear. Journal of Materials Research and Technology. 5(3), pp 282-288.

∗ Dennis, M.V., Purdue, J.D. (1989). Development of alloy steel grinding balls for semi-autogeneous grinding. SAG 1989, University of British Columbia, Dept. of Mining and Mineral Process Engineering, pp 461-476.

∗ Nass, D.E. (1974). Steel grinding media used in the United States and Canada. Materials for the Mining Industry, Barr, E.Q. (Ed.), Climax Molybdenum Co., Greenwich, CT, pp 173-188.

Bibliography