Mill

This page is dedicated to information related to a Chidliak mill to process the kimberlite

Milling in diamond mining is very important, has multiple trade-offs throughout and is actually very simple as compared to most mining mills.

The mill is one part of processing the kimberlite, so it is best to look at processing the kimberlite from the beginning through to selling of the rough diamond parcels.

What is known now? The preliminary economic assessment from July 2016 has deemed a mill size to process 2,000 tpd of kimberlite material. Processing is simple...but thinking about this single number is too simplistic and it can cause headaches by just focusing on one single number.

Stage 1 - Drilling and blasting.

Over at the proposed ruby mine in greenland (across the water from Chidliak), drilling and blasting is an absolute art. These rubies and other precious stones are large in size and can easily break in a production setting. They have very carefully set aside drilling/blasting patterns to remove overburden material without damaging the remaining ore material. Probably small...but many more drill holes are needed. In kimberlite/diamond mining, the diamonds are smaller than rubies and are a bit tougher. The energy from a drill and blast will go with the path of least resistance. In kimberlite mining, that energy should works it way through the kimberlite rock and not specifically targeting the diamonds themselves. LDD RC drilling is another story as the tri-bit blades are causing friction all over the place and do not care if they smash kimberlite rock or a diamond. Bench blasts with standard drill holes are probably perfectly fine. No additional work or cost is needed. Similar to underground mining. The difference with underground mining is if you use a raise bore machine to create your slot raises...that raise bore would not care between kimberlite material and diamonds. Using drop raises might actually be better for diamond mining and actually cost less (traditionally) than raise boring. 2,000 tpd is a reasonable target for material to feed the mill.

Stage 2 - Crushing of material.

This is a crucial step in diamond mining. In other mining commodities, you only want to crush the rock as cheaply as possible while still going through the rest of the mill. In diamond mining, you want to crush the rock to a minimum size equal or slightly above the largest diamond size you expect to get...and if the largest diamond size only comes in once every year or two...a reduction in crushed size may be called for. This is one of the trade-offs. Some current technologies are looking at putting a 'red flag' x-ray box in front of the crusher...just in case a larger than life diamond is contained in the kimberlite material. This technology is a tough sell as it may reduce the throughput significantly and therefore should only really be considered if there is a significant chance of that happening and where it isn't practical to increase the minimum crushed size. Crushing and settings are not straight forward when it comes to diamond mining. Crushing of lots of diamonds is a disaster...crushing of a small amount of diamonds might be economically pleasing. 2,000 tpd is a reasonable target for crushing of material. What goes in the crusher should come out. Some smaller bits will be removed.

Stage 3 - DMS Unit. Dense Media Separation is taking advantage of the differences between the density of the diamond and other heavy minerals and the rest of the kimberlite material.  2,000 tonnes of crushed material is fed into the system and the lighter materials gets separated out and the remaining material gets sent onto to the next stage. The result is a heavy mineral concentrate. This stage is the heart and soul of processing of the kimberlite material.

STOP - This is the where the simplicity of a single tonnage rate is too much. The input into the next stage is equal to the heavy mineral concentrate and that heavy mineral concentrate can vary significantly between pipes and even within a pipe. Clear example with Chidliak already. CH6 produces a much smaller concentrate than CH7 produces. What does the next stages of processing have to be designed to handle? 20 tonnes of concentrate or 200 tonnes of concentrate per day??? Those are two very different numbers and two potentially legitimate numbers. Can there be a buffer or a concentrate stockpile set up? The design stage of the mill is quite crucial in doing the right thing for the life of the mine. Chidliak should be designing the ability to stockpile concentrate that can be processed later on with kimberlite material that produces less concentrate...or it needs to beef up the units beyond the DMS unit to handle the peak loads expected. Beefing up the units might lead to higher capital costs...these might not be able to be avoided.

Stage 4 - X-Ray or twin X-Ray sorter. This is where the concentrate is sent through and the fluorescence of the diamonds stick out like a sore thumb and a blast of air hits that 3D target where the diamond exists and the diamond is flung to the side to be cleaned, valued, sold later on. Some of the diamonds may be contained within some of the harder minerals and those will eventually be set a side as COR (coarse ore rejects). That material is usually stock piled for re-processing (at a slower rate) later in the mine life.

Stage 5 - Grease table - This is the remaining diamonds that made it past the x-ray sorter. Diamonds naturally repel water. The concept of the grease table is that the diamonds will adhere to the grease and prefer not to get washed away with water that is sprayed all over them. The other material get sprayed and pushed out and down out of the grease area of the table. The remaining grease is than sorted through for the diamonds.

Stage 6 - Boiling or caustic fusion - The resulting rough diamond's need to be finally cleaned. This can be done using caustic fusion (where all other material is basically destroyed) or an industrial boil of the diamonds is completed to leave the diamonds nice and clean, ready to be valued and sold.

The outcome is the design of the DMS unit can be set to one throughput rate, but the design of the x-ray sorter needs to take into account variances in the kimberlite population.

Potential Flaws?

CH7 - Domain 5 diamonds did not naturally repel grease, so the grease table itself was useless for processing that material. In the end, caustic fusion was used to recover as many domain 5 stones as possible during the bulk sample process. This domain 5 material is a small amount compared to the overall kimberlite material. Adjustments could be made when processing domain 5 or maybe not.

Faraday 2 - Another project in Canada (Kennady diamonds) recently completed some bulk sample processing and one of their kimberlite pipes ran into an issue with x-ray sorting. It turned out that the background minerals were prone to be luminescent to x-rays in addition to the diamonds themselves. They didn't even try and use the x-ray sorters...and instead chose to use caustic fusion for the concentrate material. In a production setting, this would be a costly endeavour...so figuring out the best way tot use the x-rays might be a technical hurdle that Kennady Diamonds need to figure out.

Diamond processing is pretty simple...but the thought into the throughput requirement before and after each stage needs to be done in order to complete a reasonable and effective design.

Other details:

Sieve size isn't mentioned above...but the reality is any diamonds or material below a certain sieve size should be turfed as quickly as  possible in the process. Anywhere between 1.18 mm and 1.50 mm and below will be turfed in a production setting. Drill and blast material can be set onto some screening and some of the material below 1.5 can be sent away. After the crushing, some more screening and removal of material under 1.5 can be completed.  Most, if not all, material below 1.5 mm should not be gone and the rest of the processes can occur on the remaining material.

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