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Building mills from the ground up

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article image Outotec mill foundation

Grinding mill foundations need to be solid as a rock, Jeff Belke writes.
 
It's hard to work unless you have a solid foundation on which to build.
 
This is true in all jobs.
 
In mining, if you don't have a solid foundation - a concrete one, for your grinding mill, you can easily put your mill out of action and cost yourself millions in unnecessary downtimes, all for the lack of a little preparation.
 
In the beginning it is the job of the mill supplier to specify the right mill related loads, which is then used by the engineer to design the mill's foundation.
 
Mill loading diagrams are inherently complicated but most suppliers will ensure the diagrams are comprehensive enough to include all necessary information but are still as unambiguous and straightforward as possible.
 
For example, some mill suppliers do not state the dynamic loads generated by an operating mill, i.e. drive train and charge related load variations.
 
This may make it simpler to comprehend but will cause problems when it comes to designing the foundation.
 
Consideration of static loads is not enough; capacity to accept the dynamic loads while achieving acceptable vibration levels must also be designed into any mill foundation.
 
When designing the foundations, suppliers typically provide a diagrammatic foundation layout, which can be used to give a much wider picture.
 
During this early phase engineers should consider the mill supplier data, which is often easy to misinterpret, especially in dial pinion mills where one pinion experiences a downward load and then undergoes uplift.

Local conditions also play a role, where small inaccuracies in strata data can compromise mill performance.
 
In general, local concrete design standards should be applied, particularly in relation to seismic requirements.
 
Outside influences such as adjacent mills and vibrations from nearby equipment will also affect your foundations.
 
Mills of the same size and speed are particularly prone to generating vibration issues for each other. Installing adjacent mills on separate slabs is not always enough to avert problems.
 
The construction phase must be planned so that installation and maintenance access is appropriately placed.
 
Now to building.
 
The mill foundation is often the largest concrete structure on a mine, and also the one most likely to being contractors unstuck, so there are some key elements to consider.
 
During the pre-pour, ensuring correctness of cast-ins should be a given as poorly arranged cast-ins can cause lasting problems such as base plate instability.
 
Just as in important is the proper location of these cast-ins within the reinforcing to ensure they do not move during pouring.
 
The reinforcing also has to be properly installed, as once it is covered with concrete it is impossible to check without destroying the base.
 
Make sure the reinforcing is not too close to the surface, as it may expose it or allow fluids in - causing concrete cancer.
 
While larger concreting jobs are typically carried out in stages, delays between pours can make it difficult to obtain a good bond between the concrete layers.
 
This can result in a dry joint which leads to serious cracking and independent movement.
 
Once movement is established then the vibrations from the grinding mill quickly increases.
 
If the design and the pour are all carried out with no hitches then the completed foundation needs to be properly grouted.
 
First ensure the concrete is scabbled in preparation and fit hold down bolts with sleeves to allow tightening stretch.
 
Only the underside of the base plates should be grouted, and it is important to ensure there are no air pockets underneath.
 
Chamfer the grout edges away from the base plat to encourage water dissipation.
 
If there any process solutions on site which may degrade the concrete over time, a highly likely occurrence on a mine, then the concrete must be sealed.
 
Producing a good foundation takes attention to detail in the planning, design and construction phases. Careful management of those phases will ensure your mill is built on a solid foundation, with no problems.

Jeff Belke is the chief application engineer - grinding mills for Outotec .

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