After 18 years operation of 65tph ball mill, feed end cone has developed a crack. It was welded as per procedure taking all precautionary measures, but this crack has further propagated after 24 hrs operation and now a series of welding it is being operated with a close watch. This mill is 2 compartment mill with simple mechanical dynamic separator and is discharge end drive. Will you please help me to sort out the reason for cracks.

The reason for the cracks is that the mill end castings and flanges are among the most stressed parts of a cement works. The data that I have suggests that the mean time between failures for such parts is about 19 years. Your mill has operated for 18 years therefore is has exhibited typical behaviour. Now that you have cracks you need to have a rigorous inspection and repair regime. What means are you using for inspection? Magnetic particles? Dye penetration or ultra-sonic? Presumably the mill supplier has told you the material the cone is manufactured from and the correct materials for the welding repair. The depth of penetration of the cracks is also a critical factor. If the cracks are right through the casting then any repair will only extend the life of the part by a short time. I would suggest ordering and preparing for a replacement.

Is there any international benchmark regarding maintenance cost for a cement plant? What is a reasonable figure per ton of cement or ton of clinker? I know it depends on the age of plant, technology, the country, etc.

As you say there is no true reply to your question, however if you can reduce your total maintenance costs below US$5 per tonne then you will be among the best factories in the world!