Return to the Main Menu ~
Return to the Rubber Menu
Rubber Mill Liners
Mills are essentially rotating drums..... creating a crushing (cataracting) and grinding (cascading/attrition) effect through which the material inside is reduced in size and ground to a fine powder, the purpose being to efficiently access and extract / liberate the precious minerals / metals inside the ore.
This is obviously a highly abrasive process and causes great wear on the inside body of the mill. Historically was the inside of the mill lined out with steel plates, and replaced when worn through before causing damage to the mill body istelf.
Around the 1960's was there a move towards lining mills with a rubber material, which gained prominence over a period of time as many user's resistance to rubber liners reduced.
The obvious advantages of a rubber liner include...... ease of replacement (rubber being substantially lighter than steel), no metal contamination of product, and last but not least lower noise levels.
Like all good things, does rubber as a liner medium for mills also have certain disadvantages which include.......
+ a limit on the operating speed of approx. 80% of critical speed,
+ where the operating temperature exceeds 75'C (unless a synthetic polymer based rubber is used),
+ where there is organic substances present (unless Nitrile Rubber is used),
or in certain primary milling applications where large incoming rock (ore) must be handled.
Much can be said about the relative cost, but this is often affected by the material cost of rubber vs. steel (which have varied over the years), the quality of steel used (obviously will high grade steels be more expensive but last longer), the thickness of the liner (the thicker the more expensive but the longer it will last), configuration of lifter bars (which can have a subtle impact on the cost but also the performance of the mill).
MORE ON RUBBER MILL LINERS
Over the years has the technology of rubber liners developed to the point where certain aspects have become the standard. Although various companies have come up with more developments, are the following features pretty standard for rubber mill liners......
The best all-round rubber formula is based on either Natural Rubber, or a blend of Natural Rubber (NR) and Styrene Butadiene Rubber (SBR), with an approx. loading of 65 phr of HAF (N339) Carbon Black, and only the other bare essential ingredients.
The physical properties of such a compound should be as follows...
Tensile Strength: 23 MPa, Elongation at Break: 550%, Hardness 65 shore A / IRHD
Any attempt to cheapen the compound by addition of other ingredients only reduce it's wearing properties and actually increase the operating cost of the liner as it needs to be replaced more often.
ADVENT OF LIFTER BARS
One of the unique aspects of a mill is that it should not have a smooth liner, and it was early on found that better performance and life was achieved by inserting higher bars along the length of the mill, subsequently called Lifter Bars.
These Lifter Bars not only stop the load from sliding as the mill rotates, but also lifts the load and contributes towards the cascading effect required for crushing in addition to grinding. In the ultimate can the configuarion of the Lifter Bars be used to fine-tune the trajectory / fall profile.
The requirement of Lifter Bars ideally suited rubber, for starters because they are by nature large objects and much easier to handle during installation and replacment than steel bars. But just as importantly, did they facilitate a very simple fastening system whereby the Lifter Bars contained slots that are used for bolting into position (using what became known as T-bolts), and at the same time also holding the Liner Plates in position.
MORE ON MILLING
TYPES OF MILLS
In order to serve customers properly, must there be a good understanding of what constitutes the different types of mills, for which the following classification is suggested.
Method of discharge
-Batch / bottom discharge
-Scoop discharge (more commonly known as a grate discharge mill, but actually a misnomer)
Use of grinding media
-Conventional, using grinding media extensively
-SAG (semi autogenous), containing the feaures of a AG mill, but also using a smaller amount of grinding media
-Autogenous (AG), designed to use no grinding media and the charge conatining large particles that act as the grinding media
Type of grinding media
-Rods, normally steel
-Balls, normally steel
-Pepples, from ceramics
Position in the circuit
Although to a large extent falling outside the scope of the mill liner supplier, is it necessary to have a good understanding of the extent to which the product must be ground.... which some factors fall within the scope of the mill design and other outside the area of responsibility of the mill-liner supplier.
If the ore is not ground small enough, will we not be able to access all the precious minerals, and will product be lost. Likewise, if the ore is ground too small, will precious material also be lost as the extremely fine grains will not be "caught" by the concentration process and go out with the waste.
FACTORS THAT AFFECT THE TRAJECTORY
PERCENTAGE OF CRITICAL SPEED
For those who want to be in the know, is it important to be able to calculate the critical speed of a mill. Not only does it indicate whether rubber liners will be suitable or not, but can it give you important information about the fall-parabola inside the mill.
The formulas for calculationg % of Critical Speed are ....
CS = 42.3 / sqr. root of D, where ....
CS is Critical Speed in RPM,
D is inside diameter of mill in meter
%CS = CS / AS * 100, where .....
%CS is Percentage of Critical Speed, being Actual Speed expressed as a percentage of Critical Speed
AS is Actual Speed in RPM
Unfortunately can the % of Critical Speed seldom be changed. So, other variables must be used.....
LIFTER BAR CONFIGURATION
This has firstly to do with the number of lifter bars, but can often not be changed since the mill already contains holes in the shell where the lifter bars are bolted in.
The variables that can be used to control the extent of grind, lies firstly in the width & height, and secondly in the profile of the Lifter Bar.
-The most important variable is the width of the Lifter Bar in relation to the width of the Liner Plates, as well as the extent to which it stands proud of the Liner Plate surface. This is often referred to as the H over S ratio, and the greater this value the greater the cataracting effect.
-Having selected the desired ratio, can the effect of the Lifter Bars further be adjusted by "shamfering".
+a slight shamfer of approx. 10 to 30 degrees across part or the whole of the leading edge, gives a more gentle cataracting effect.
+a shamfer of approx. 45 degrees across part of the leading edge, reduces the cataracting significantly and gives a more constant trajectory profile over the lifetime of the Lifter Bar
+a slight shamfer of approx. 10 to 30 degrees across part of or the whole top edge of the Lifter bar is a more radical approach that also reduces the cataracting effect, but possibly gives longer life
CARE OF MILL LINER
In order to prevent your mill liner from being damaged (mainly applicable to rubber liners, but also relevent to steel liners), should certain aspects not be neglected
-Not only does crushing and grinding happen when the load and grinding media fall onto itself, but this very aspect is what protects the liner from severe wear...... the load acts as a kind of cushion, So....
+Never run a mill unless it has been established by calculation taking into account the various factors that affect the fall-parabola, that the load falls onto itself and is not being thrown against the wall of the mill
+Never run a mill at much less than it's minimum operating level.... this will also cause the load to fall on the liner instead of the load
-Never feed a mill with larger / heavier ore than what the liner was designed to cope with.
-Never run a mill dry, unless it was designed to do so
-Monitor and record the thickness and condition of the liner regularly and take steps to replace it before damage is done to the mill body self
MILL LINER DESIGN
The design of a mill liner is a highly scientific art, as it involves a series of very specific steps and requirements... Not only must it be designed to give the right grinding peformance, but also to last as long as possible giving highest output over the period.
Furthermore does the design not just require intimate knowledge of the various formulations involved, but also piecing together all the parts so that they will fit, and can be replaced by having to strip out the minimum number of parts. With all the computing power available these days, should no mill liner manufacture be attempted without proper scale drawings of all the components.
-The following phases can be identified.....
+Conseptualisation, of what will fit and satistfy the requirements of the application
+Initial design, also needed for costing purposes
+Detail design calculations
+Detail design drawings
-At all stages should all parties involved in the job be consulted, best in the form of regular meetings where aspects of the design is presented and opportunity given to raise queries and potential problems
+After installation is it advisable that a debriefing meeting be held to establish what problems were encountered and amending the design in order to prevent a recurrance when the job is done again