Grinding Mill Liners- An Overview

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Summary

The history of mill liners has evolved over time and this paper tracks the changes and development of liner technology. It provides an overview of the primary types of liners, the design and the materials of construction. The design of a mill liner is driven by application, materials of construction, casting technology and handling issues. The choice of solid steel, grid liners, rubber or composites and magnetic liners is based on many factors.

 The methods used for monitoring wear and predicting eventual change out time are many and varied. The contribution of discrete element modeling (DEM) analysis in predicting wear profile is discussed.

 Mill trajectory and DEM software programs are used with great effect to predict the effect of lifter height, face angle, high low lifters or mill speed. In the field, trial and operational experience, is used or improvements/design concepts at one operation are applied to another. Broken lifters, accentuated wear and other problems can be better understood using hardess testing, chemical analysis and photo micrograph analysis of pearlitic or martensitic structure along with fracture face observations form part of typical root cause analysis. However given correct and sound liner and lifter, feed and discharge design media selection, and materials of construction, the majority of premature liner or bolting failure is normally attributed to incorrect operational methods such as excessive high ball to feed ratio, incorrect mill speed relating to lifter ball throw, incorrect media selection, “tramp” material entering the mill charge from upstream or recirculating process and understanding the specific mill operation in question.

 It is hoped that this paper will benefit mill operators who are faced with challenges of improving wear life and at the same time optimizing mill throughput and performance.

 Introduction

 Liner design has a significant effect on mill performance and mill liner life. The process of getting it right is an evolutionary process over the life of the project. Usually this is a joint continuous improvement process by the Plant Superintendent and the Mill Liner Supplier. Excess liner wear results in high liner material costs, increased downtime and impacts on plant throughput. Improved liner design can result in improved mill grinding performance and the best economic balance between liner life and cost.

 The history of liner design development fell under the maintenance department where longer life was the main consideration to reduce cost. Over the last twenty years new cost engineering approaches and mill modeling has highlighted the mill performance issues with the development of liner profiles and improved materials.

 The other change that has occurred is the advent of large diameter SAG mills. It became apparent to the operators that the liner/lifter profile and mill speed had a significant impact on mill performance. The gains from good liner design and selection also became more important because of the change from multiple mill lines to single train large mills.

 This paper looks at the issues in liner design and selection in operations and some of the findings in the field.

 MILL LINER TYPES AVAILABLE

 For smaller mills the liners are handled manually but for larger mills hydraulic liner handlers are mandatory.

Solid liners

These are single units with integral lifters and fewer pieces but the downside is a higher scrap weight. These have few pieces and are easier to install. Once the lifter section is worn the mill performance drops away. 

 

Grid liners

 These were developed in South Africa and are suited to higher critical speed mills and are very economic for highly abrasive ores. They are light weight and the balls infill the liners and provide an effective wear material. They are manufactured in manganese steel which often spreads (metal flow) and makes removal difficult. Safety inside the mill from dropping balls is also an issue if personnel enter the mill. 

High low double wave liners

 These are commonly used and convenient but the correct wave angle needs to be specified for a particular mill. This is a difficult profile to get right first up. Some modifications normally required as the plant progresses from initial commissioning stages to fully operational and changing ore characteristics through the mine life.

High low lifters

 These are economical and convenient as the original high lifter becomes a low and by alternating is later changed out to be a high lifter. The mill wears to give a good lift profile. These are used in the primary SAG mill at Casposo, Argentina to good effect. This is applicable to mills where ore packing is not an issue.

Wedged liners

 These are liner blocks wedged in by bolted lifter bars which allows simple castings of the liner blocks. These have fallen out of favor.

MILL LINER MATERIALS AVAILABLE

The selection of material is ore specific and mill specific depending on the pulp environment environment, ball size and mill speed. The following material compositions (%) are used.

Austentitic manganese steel

 This is used for grid liners and work hardens with use and can withstand extreme impact without breakage. Its biggest disadvantage is the difficulty of removal after many months of use because it spreads.

 Low carbon chrome moly steel

These are 350 BN hardness and are generally used for AG and SAG mills. It has excellent wear characteristics with good impact resistance.

High carbon chrome moly steel

 These are 350 BN hardness and used in SAG mills with higher carbon and chrome contents.

Nihard liners

 These are 550 BN hardness and were used in rod mills and ball mills where impact is low for this brittle material. It is extremely wear resistant but has been superceded by the chrome moly irons and chrome moly white iron.

High chrome iron liners

 These are 650 BN hardness and have superior abrasion resistance and are used in rod and ball mills. They are lower cost than chrome moly white irons.

 Chrome moly white iron liners

These are 700 BN hardness and considered to be the best for abrasion resistance in large ball mills. This is due to the large volume of primary complex carbides and full martensitic matrix.

 Rubber liners

 Rubber is commonly used in ball mills because of its long wear life, lower cost and easy change out. It can be used in primary mills as rubber steel composites (Polymet). Rubber steel composites are gaining in popularity because of much better economical outcomes. The bolts and fittings are much lighter than for steel liners. Rubber alone is not suitable for primary mills. (Powell, 2012)

 Tensile strength should be at least 20 Mpa and hardness should be between 55 and 70 durometer on the A scale. The rubber should have an elongation factor of 500 to 600%.

Magnetic liners

 The lining system consists of permanent magnets embedded in rubber mouldings. These are more expensive than rubber liners but can give years of trouble free operation with minimal wear. They are more applicable where wear is a primary consideration.

CURRENT LINER DESIGN

Liner design is an evolutionary process where small improvements are made over time resulting in positive outcomes. Heavy reliance is made on the mill Vendor and the liner supplier. Guarantee for performance and liner life is very hard to extract because of the inherent risks and difficulty in quantifying unknowns. There is also no guarantee that the operators will follow the liner supplier’s advice to prevent damage. Most suppliers work on a trial and error approach with iterations of improvement based on findings. Whilst this appears to be slow and a ramp up approach with a loss of potential production the industry has not developed a better way. (Connelly, 2011)

 Good Design Examples

 Influence of lifter height

 It has been established in the field that a higher lifter height will increase throughput but result in excessive wear over a lower height. Monitoring the wear on the liner is also relative to the height of the lifter bar in use. Monitoring liner height and undertaking mill profiles with a piano rod profiler or a measuring instrument. Monitoring the profile during life allows calculating charge trajectories.

Optimising design is about maximizing impact grinding in AG/SAG mills, ensuring cascade action for regrind mills and avoiding impact on the mill shell, minimizing ball breakage, maximizing mill throughput and balancing wear life with throughput and liner cost.

 Finite stress analysis

 Modern computer aided finite stress analysis (FEA) can highlight the impact of normal stress on a liner and shear stress which can be many times more severe. This results in changes to the liner which overcomes high stress points.

 Testing wear rates

 The Bond Abrasion test is a good predictor of liner wear. Tests using liner panels have been notoriously misleading because they don’t represent a full mill condition. Plant trials using parallel trials for multiple lines can yield meaningful results. Ore changes from variability in the pit can influence the service life of liners under test.Historical records can be used but the ore must be similar and operating conditions should be similar.

Mill drilling

The bolt hold drilling is determined by the mill manufacturer. Mills drilled for rubber cannot accommodate steel liners without changes to the hole configuration. A far wider spacing of holes is required on larger mills.

Bad design examples

Noisy mills

 A distinctly noisy or mill that rattles suggests grinding media is impacting on the liners and causing unnecessary wear and possibly damage. Incorrect speed, poor feed blending and incorrect liner profile can contribute to this. Other negatives are ball breakage and liner cracking. Reduced mill efficiency is a direct result. The liner bolts may also loosen because of the incessant pounding they receive with resultant leaking around the bolt holes.

 Loose or broken bolts

 Mill liner bolts are consumables. Usually a forged head into a cast liner so some bedding in is inevitable. Broken bolts are a sure sign of quality issues but it can also be caused by a multitude of other problems. Loose bolts are bad and can lead to shearing of the bolt.

 Bolts becoming loose after a liner change are not uncommon. And a scheduled rattle (torque multiplier)  gun tightening 24 hours after a liner change is recommended. With large SAG mills the discharge end may require two intervals of tightening. The bolts should not loosen after that unless over worn in which case the liner speed needs to be carefully checked.

 Not recognising mill differences

 Regrind mills require a motion to maximize abrasion not high energy impact. Densities and viscosity are also more of an issue to be taken into account. Minimising circulating load and minimizing over grinding is more of an issue.

For primary mills high impact is critical in maximizing efficiency and dropping the load at the toe of the charge is highly desirable. Impact breakage is far more important than abrasion grinding.

Excessive wear

This could be a result of incorrect profile and too much slippage.High wear could be due to the incorrect liner material or the very abrasive nature of the ore.

Throughput and liner life

It is not uncommon for liner wear to impact on mill throughput and efficiency. Mill throughput drops markedly when new liners are installed .At Bougainville the single stage ball mills gave best performance at or near the end of the liner life. Bedding in or poor performance after a liner change out is not uncommon. Design changes should be made to overcome this.

If the mill throughput drops off over the life of the liners then the liners should be changed out sooner.

MILL LINER WEAR

The liner in the mill serves to protect the mill shell from wear but also transfers energy to the charge for impact breakage.

Liner wear measurement was the responsibility of the maintenance personnel but now is the responsibility of the Metallurgist because of the demand to increase mill throughput and efficiency. It is possible to represent the wear graphically using a piano rod measuring device or a mill mapper (electronic gauge).

For rubber, nails can be punched into the rubber to determine the profile. Ultrasonic thickness gauges can also be used.

It is best to produce data from multiple locations in the mill and preferably at the same location so as to get accurate and consistent readings. This is easier said than done.

With mill liner inspections these should be on a regular basis rather than as the opportunity arises. Some of the things to look for are broken balls, loose bolts, broken lifters or cracking, damage around the bolt hole, the shape of the balls, excess grate wear and pegging of the grate with balls or rock as well as any tramp metal present. Excessive metal flow from peening causing the gaps between liners to lessen and increase the internal stress in the liners (especially for chrome/moly steel material). 

LINER PROFILES & TRAJECTORIES

 

MillTraj

MillTraj by Liner Design Services can produce visual plots of mill trajectory (Figure 5). This is a very useful tool for SAG mills and prediction accuracy depends on the ore type, viscosity and ore properties. One can evaluate the space height ratio and wider lifter bar spacing.

Utilising charge trajectory predictions is not straight forward and lifter bar face angles can if not taken into account lead to erroneous predictions if not taken into account. Liner bar heights are easy to predict but high low lifter profiles under new and worn profiles become more difficult to predict accurately.

Mill speed affects the trajectory and the liner profile should be selected to suit the mill operating speed. The final liner selection can be based around the desired operating window. For large SAG mills the new lifter speed may be 73% critical but with worn lifters the mill may be running at 80% critical some 6 months later.

DEM modeling

 The Discrete Element Method is a numerical tool for modeling (CSIRO).When running the effects of wear, lifter height on charge motion can be observed directly

 COMMISSIONING

 This is a particularly nonstandard operating period and care should be taken not to run empty or noisy mills during feed ramp up. There are cases where a set of liners have been damaged under such conditions. 

Usually for a SAG mill at start up a much lower ball charge should be used and built up over time. The mill should be operated at low speeds and the feed blend should be neither too soft nor too hard. A new mill and soft oxide ore is an extremely high risk scenario for liner damage.  The outcome can be catastrophic. High volumetric fillings are recommended and not relying on instruments such as load cells but stopping to visually check the mill load is essential.

Inching the mill to avoid a frozen charge should be built into the mill start up programme.

CONCLUSIONS

 Mill throughput and efficiency gains through iterative liner lifter configurations are common over time based mainly on trial and error. Substantial gains can be achieved in the design, selection of liners based on mill trajectory modeling, supplier selection of lifter bar heights, angles, and spacing.

The height of the lifter bar is critical and should be determined by the ball size, desired wear life balanced against mill throughput. Face angles can be determined by simulation to give impact at the toe of the charge. Field trials are the acid test and comparing predictions with actual data in use. Sensible and controlled changes in consultation with the operator and liner supplier over time results in an optimized outcome. This results in the liner profile and material to be optimized with respect to mill performance, liner cost and liner life.

• Maximise ball height drop for AG/SAG mills to maximize impact grinding.

• Avoid impact on the mill shell ensuring balls and rock land on the toe of the charge.

• Maximise liner life by providing protection to the mill liner with lifter bars.

• Prevent ball breakage by promoting impact directly onto the toe of the charge.

• Check the liners and the mill contents regularly and update the profiles to predict change out.

 REFERENCES

 METS Pty Ltd Internal Image Library 2012

 Growth Steel Ltd internal Image library 2013

 Powell, M.S, Hilden, M.M., Weerasekara N, Toor P, Franke J, Bird M, 2012. ‘A More Holistic View Of Mill Liner Management’. Paper presented at the 11th AusIMM Mill Operators Conference, Hobart,TA.

 D.Connelly, 2011. ‘Trends With Selection and Sizing of Large grinding Mills-What’s Available in the Marketplace’.Paper Presented at the  IIR Mill Optimisation Conference, Perth,WA.