reducing the cost base through metallurgical examination of grinding balls
TRANSCRIPT
“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
∗ 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