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ball mill discharge grind size (13 replies and 14 comments)
Not enough information.
Ball mill dimensions? Cyclone diameter? Circuit feed rate? Circulating load ratio? Circuit configuration (fresh feed to cyclones or to ball mill)?
Dear collegue the size change you want to achieve is quite large, so changing normal Hydrocyclone dimensions or operating conditions will not be enough.... so may need to change the size of your cyclone (increase), but this will reduce you flexibility just one cyclone in operation.... Increasing feed density will increase circulation load and decrease circuit efficiency (you have to be aware of increasing density in the mill may overload the mill)....... One option is to incline the hydrocyclones 45 degree or more..... syphoning the cyclone will be other option....
Thank you Castro, inclination of the cyclones is one proposal that is on the cards because the concentrator has just been commissioned and the cyclones were installed wrongly with no inclination to them
Hi collegues, a month back i asked for help to increase grind size but i did not have enough data. now i have the data. i am currently grinding to an average of 110 microns but i would like to increase to 150. mill throughput is now at 70 t/hr. the ball mill is in closed circuit with the cyclones. mill diameter inside liners is 4m, effective grinding length is 5.3m, critical speed is 76.5%, ball charge 33%. cyclone diameter is 380mm, 80mm spigot, 140mm vortex finder. circulating load is about 180% with fresh feed into the ball mill. i hope this is enough for you to help me
I thing all you can do is
1)try to grind coarser by reducing the residence time in your mill if your grind is fine.(put more in let water)
2)if your grind is already coarse,means with 90 micron your circulating load is high than increase you cyclone feed density and push a bit you pressure to 100kpa.
Note that i will like also to have all the parameters that philip is asking for a better guidance.
Additional options include reducing ball charge or increasing size of balls.
If the mill discharge is already too fine (content of desired product is low) changing classification will not achieve end. Do you know particle size distribution of mill discharge?
thanks Robert, i will embark on a project to determine what ball size to top up the mill. i will aim at topping up with one size.
the ball mill discharge P80 is about 0.455mm with top size material being about 4.75mm. also wanted to find out if circulating load has any effect on my grind size.
What is fraction of material meeting desired product size in the existing mill discharge?
Hi Guys
Could You be more specific like:
- What is the total ball charge.
- What is the slurry density at ballmill outlet.
- When was the last classification done.
- What is the wear rate of balls in the mill.
- What is the power consumed by the mill now.
- Any idea on the initial classification of balls charged into the mill.
Changing the cyclone geometry may not help as the mill is grinding to its design, so better way is to alter the grinding parameters.
Regards
Krishna
Hi Krishna,
total ball charge is 29%
slurry density at ball mill discharge is 78 - 80% solids
that classification was done about 3 weeks ago
wear rate of the balls is approx 0.8kg/t
the mill is currently consuming 1450 KW
initial balls charged on size up was 65/80 mm sizes. but currently we are now only topping up using 78 mm
70 t/hr on 1400 kW is nuts. That is 20 kW/t. You are pulverising it. If you want 150 um at 70 tpd, you must cut your ball load & get rid of the load via your cyclones yes.
Quick look at your data tells me your mill could run at 140 TPD ++
I'd need to run a power model to see the maximum power your 4 x 5.3 m ball mill can draw.
As o.castro.soto said, your cyclones are too small to cut where you want them to. 150um is the job of a 20" cyclone (500mm). At 70 TPH, you'd only need 1 of those 500 mm cyclones.
I would not waste my time trying to incline those 15" cyclones. Just buy a 1 x 20".
For now, what you can do is install the largest possible vortex you can on your 15" cyclones and start reducing the apex diameter. Try a +/- 65 mm
What is your cyclone overflow % solids? See if you can increase the cyclone feed density by cutting cyclone feed water. Yes, this will reduce your feed pressure. That is what you need now. Have you calculated your D 50?
You can increase this the cyclone overflow % solids to 50-55 to help lower your cyclone efficiency and cut coarser.
What operating problems are you having at your current grind size?
Do you not have ore supply to increase plant tonnage?Is this a leaching or flotation concentrator?
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Thanks David. to answer your questions:
cyclone overflow is currently 42%
D50 is about 300um
its a flotation concentrator
we are currently grinding to 106um so w have changed the ore type from oxides to sulphides and i want to try to achieve the design grind size for sulphides. one problem we have is that we are sending over ground concentrates to the filter so we are having problems with settling in the thickener as well as the filter press. out final tails assay show that our gold is being lost as fines. its a copper / gold concentrator
we dont have problems with ore supply. i was considering increasing throughput as well but did not know if that was a good thing to do.
the operation has no money so i know proposing buying a bigger cyclone will be a problem so i will try and angle the cyclones by about 20 degrees and see if i can grind coarse.
you talked about over charging the mill, - i will try to reduce the ball charge but the last inspection it was 29% but the design say 33%
i will consider increasing tonnage through the mill with the same power consumption
Well, cutting the ball load will do nothing for you now. It will increase the circulating load which you are not able to unload yet.
You mean D50 = 30um not 300, correct?
Cut your cyclone feed water and bring your cyclone overflow up.
You will be OK to +50%
If you do have the ore, feed it.
Review https://www.911metallurgist.com/blog/troubleshooting-hydrocyclones
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Thanks David, i will try and implement these changes. i will definitely be back on this forum to give feedback
when you said D50 do you mean 50% of mill discharge particles passing a certain screen size? is it the same way i get my D80 on the ball mill discharge sizings.
in my earlier conversation i said we are grinding to 106 um, that is actual P80 to the flot circuit from the cyclone overflow not the ball mill product size.
https://www.911metallurgist.com/wp-content/uploads/2016/02/sizing-select-cyclones.pdf
particle size of which 50% reports to the overflow and 50% to the underflow, or the so-called D50 point.
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Just to shown you that if you really are at 38um, your will need 50um maybe... and 10" cyclones are not designed for that. Larger +20" are best suited for this coarse cut.
in the mean time = increase your cyclone feed/overflow density.
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Thanks David, i am now with a clear picture. i have also implemented what you and other colleagues suggested, increasing my vortex find and reducing the spigot size. i have managed to get to 125 um from 106 um. You were also right that our current operating power index of 20 Kw t / hr was too high for our throughput of 70 t/ hr. what i found out from the design criteria was that, when they determined the ball mill work index they used a closing screen of 106 um but our operational P80 Target is 150 um. This gave us a Wi of 21 so i want to ask if its correct to re calculate the operating Wi using a 212 um screen as the closing screen since i want to get 150 um as grind size so that i can get a correct Wi and lower my Power on the ball mill
can you just increase your TPH tonnage? what is your cyclone overflow density now?
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I have collected some average Ball Mill Bond Work Indices (BWI) and the specific gravity of the two minerals, quartz and basalt, from the literature. (See attachment)
Your BWI value for the quartz is 21 (very hard materials category) where the mean from the literature is 15 (hard materials category). Your BWI value for basalt is 21 where the average from the publications is 19. I suggest you continue to test regularly your ores in order to understand why these differences and use an external laboratory as reference from time to time. And share findings if possible.
Great participation BAUD thank you!
Yea, Bond Work Index 21 stands in between Mt Milligan & Copper Mountain.
https://www.911metallurgist.com/blog/table-of-bond-work-index-by-minerals
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Thank you David. This site is unique in the world in what it offers in the profession. When googling, I was agreeably surprised by the quality and quantity of reading materials it contains and I have to take time to dig out. Congratulations David for this masterpiece!
I am working in a plant with single stage grinding using a ball mill we operate the mill power at around 1400KW& the feed size is 20mm. we use 78mm/64mm steel balls. the ball mill is in closed circuit with classification cyclones. there are 3 cyclones and we operate 2 with a pressure of about 90KPa. the Cyclone Overflow is around 95 microns. I want to increase the size to about 150 microns. how do I do it? I currently have 90mm spigots and 140mm vortex finders.
The mill motor of the ball mill is 2900KW.