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Sizing of Ball Mill (5 replies and 2 comments)
It is important to know if the grindability testing program tested several ore types to determine the ore hardness variability. Also, how were selected the samples to perform the testing program?, were they selected considering their spatial distribution in the deposit?
It is not very clear if the contractor reviewed the results from the grindability testing program or performed a new testing program to determine the Bond Ball Mill Work Index. If two or more testing programs were performed, the closing size to determine the Bond Ball Mill Work Index should be the same ( 150 microns). Try to talk to the lab to get more details on the testing program if you don't have enough information on the lab procedures.
Your Bond Work Index (Wi) is 15.36 KWH/t. Your contractor got 11.30 KWH/t. The difference is 4.06 KWH/t which represents 36 % : this is big! Do not wait to discuss this with your contractor when the ball mill will be installed and commissioned. It can be very late and you could lose a lot of money.
I suggest you this:
- Verify and make sure you are using the same units. In this range, you can have a difference of not far than 2 KWH/t between short ton and metric ton. The letter T for ton should always have a qualifying adjective( M for metric and S for short). Write MT for metric ton and ST for short ton for peace of mind.
- Call a meeting with your contractor in order to find an agreement;
- Propose to choose 3 renowned laboratories and send samples to them in order to determine the Bond Work Index. This is a very good investment in this case.
- Pay a big attention to sampling.
-The mean of these 3 results of Work Index will be the reference for both of you if they are similar or in an acceptable bracket.
Depending on ore types in the deposit, work index as determined in Lab may vary. Accordingly proper sampling with testing of different ore types for grindability tests are important based on which plant design is made.
The contractor/technology provider must do their own tests as done to confirm the Lab tests. If there is a variation as in present case, repeat tests should be done both in Lab and by Contractor on representative samples to generate the basic design data. As such Contractor need to give the performance guarantee of the system with flexibility to process all ore types with necessary penalty clauses in the binding Contract .
Definition of ore types and selection of adequate number of samples from within each ore type volume is critical. One interpretation of your comments is a single sample run by you and by the consulting/engineering company. A single sample is not enough - should run enough samples to understand the distribution of Work Index for each ore type.
it is done by computer now
here are some useful tools for doing it by hand https://www.911metallurgist.com/blog/eq
Based on test work I have designed a flow sheet for the Beneficiation of Hemetitic Iron ore. I have selected Ball mill for the grinding of rejects coming from Jig for further recovery of iron values from WHIMS. Optimum mesh of grind is 0.15 mm . From Laboratory testwork Bond work index determine is 15.36 kWH/t for Jig rejects. Ball Mill is in close circuit. Recirculating load is 250% for ball mill.
The contractor/technology supplier who are involved in the erection / installation of plant find after testing of ore at their end, that Bond work index of the Jig rejects is 11.30 kWH/t. They insist that they will design the Ball Mill at 11.30 kWH/t.
Problem is that for the Bond work index suggested by contractor, size of mill will be short, Motor and gear box for the mill also get short. There is a Play of money in ball mill sizing.
What are the problems plant people faces if we will go with Ball Mill selected on the basis of work index 15.36kWH/t. If there is a case of a overgrind, then how it can be controlled through closed circuit hydrocyclone. How overgrinding of particle can be minimized inside the ball mill.