Calculating Power Draw when sizing Ball MillsI’m interested to know when sizing a new ball mill, as to why recirculating load has no effect on the power calculation, although it will have effect on volumetric capacity naturally.For instance a mill of 300tph with a 300% recirculating load would have 1200tph going through effectively, but if we reduce the recirculating load to 200% at 900tph it will not change the power calculation.However if we increase the fresh feed by 100 tph the power calculation goes up as does the size of mill required.Not being a commination expert by any means, I wonder if we engineer too much risk into capital equipment these days, or am I just working with the wrong calculations.

See previous comments

ARABINDA BANDYOPADHYAY ( Bandyo)I am not a designer but had been in operation for more than 20 years. In my experience the mill size calculated from Bond's index is not sacrosanct as per as capacity is concerned. Significant increase in new feed capacity can be effected by control of classification efficiency. This increase can be as high as 30% in smaller mills. The consideration should be mill & the classification system together and not mill alone in isolation.250% circulating load is not to be treated as something "holy" but it should be managed for the best or to say loosely the optimum output. And output is not grind size alone but also the % solids of the mill circuit output. Many times optimum % solids required in the beneficiation circuit dictates the requirement from the grinding circuit. Selection from Bond's law is the starting point for mill sizing but needs to be corrected with many factors as the law holds for 8' by 8' mill only. Of course no one, including myself, have addressed the original question posed by Mr. Keith and what we are writing are the actual observations in operation. I guess the answer would require a population balance of each fraction and how that changes with increase or decrease of circulating load or increase of fresh feed. Difficult to put the details here but may be someone could do it.

June 11, 2014

Tadimety RaoVery good dimension ; as said, do not try to analyse a ball mill performance in isolation; the hydro cyclone which looks so small in size in comparison to the mill and which does not make much noise nor heat has a major say in understanding griding circuits. Dr.T.C.Rao

June 12, 2014

Venu GopalHi, I am not an expert but am into basic sizing of mills. I agree to the answer given by Peter above hold good because mill power is calculated for power drawn by the cylinder (mill) considering media and slurry loading. Any excess feed in terms of fresh or recirculating for that particular mill power drawn will not affect. however, lesser feed will definitely draw lesser power.

June 12, 2014

Kenneth ArmstrongIf you run a ball mill in open circuit the chance of large particles surviving the trip is low but they do break into medium sized particles. At the discharge end the outflow is a sample of the slurry there. There will be way too many oversize particles for most intended applications. The solution is to classify the discharge by size and return the oversize to the feed. Bonds work assumed a circulating load of 250% so that a reference set of conditions could be met for testing. By removing particles that were already smaller than the desired size this both prevents overgrinding (grinding too small) and frees up volume for the larger particles that are returned.

Bond says that a work index is the measure of the energy to reduce from a feed to a product size under these conditions. This is energy per tonne of raw feed, the higher the recirculation the lower the residence time, but the power draw of a mill is a fixed number. It is, simplistically, the energy required to lift/tumble a ball charge that occupies 40% of the volume.

Efficiency is another story. Bond's equation has several factors related to the physics of the mill and the desired reduction. These are used with the Work Index to estimate the capacity of a given mill. Another factor that, IIRC, Bond did not investigate is the percent circulating load. Studies have demonstrated what is intuitively obvious, that the higher the CL the more efficient the circuit. This is because particles of acceptable size are being removed faster, preventing the wastage of energy to make them even smaller and freeing up energy to use on the oversize. A 400% circulating load will allow more tonnes per hour of on-specification product that does 250%, and even higher re-circulation is better. The trade-off becomes the cost of the pumps and the classifying equipment.

Cyclones are not efficient classifying devices but they are cheap and low maintenance compared to the spiral and rake classifiers that preceeded them, and require less water. An alternative to them is wet screening which can be done down to around 100 microns if the

solids volume exceeds 50%. The solids returning to the ball mill contain lower amounts of fines, zero being ideal. This can add another 10 percent or more capacity while reducing the amount of undersize that might not concentrate. There are a couple of hundred circuits on the planet that use screens. Derrick Screens are specialists at this.

KC Armstrong Sr, Mineral Processor, Allnorth Consultants.

June 14, 2014

B P RaviThe limited tests in lab work index on some iron ores indicated that varying test sieve and circulating load [CL]indicated that the work index value increased with increase in finesse of MOG [ increase in sieve number] and decrease in circulating load. The increase in work index value may be due to finer product [ P 80] and decrease in net gms produced per mill revolution [GBP]. The decrease in GBP is a result of requirement of more number of revolutions required for producing more finished material as weight of finished material is inversely proportional to [1+CL]. How ever in practice the use of 2 stage classification and use of Derrick stack sizer is believed to increase mill throughput and reduce the grinding cost maintaining the MOG. Probably, the increase in mill throughput is due to prevention of over grinding finished material. How ever detailed test data sharing by lab test people, mill designers and classifiers designers is solicited to under stand the phenomenon.

June 14, 2014

Enzo ArtoneI am not sure I agree with everything Ken is saying. I believe that as circulating load in a ball mill increases beyond a critical point the mill power draw begins to fall so there is an adverse effect on grinding efficiency (apart from the obvious physical problems it causes like high scats rates, balls discharged from mill, sumps overflowing, cyclones clogging up etc.). But getting back to the basic question, if circulating load is too high, then it really is saying the mill (or calculated power) is too smal, therefore beyond a critical CL an allowance should be made in sizing a mill, question is what is the critical CL??

June 16, 2014

Kenneth ArmstrongEnzo:

The energy draw in a ball mill is friction plus the work to lift the load of balls and slurry, mostly balls, up the wall so that it can free fall and create grinding. It has absolutely nothing to do with rate at which the slurry is being replasced by new or recirculating feed. How in the world can a ball mill motor know the rate at which the slurry is passing through body of the mill? Your list of negative effects is hard to comprehend. Higher scat rates are good for the operation. good balls can be sorted easily and recycled, and no good engineer would try to operate an mill at a CL that is not compatible with the available classification equipment, both being chosen at the same time.

The really good paper that I have describes test results from 100 to 500% CL on several minerals including coal. Above that the incremental gains were very diminished. 500% would require about twice the pumping and classification capacity vs. the reference 250% and the laws of CAPEX and OPEX apply. It may not make economic sent to use anything higher than 250%. It should, though, when overgrinding causes significant recovery reduction.

Sizing a mill is straightforeward. If the rock needs 10 kwh/t which is based on a CL of 250% and the plant requires 150 tph, then the mill must deliver 1,500 kw at steady state. That will not be the size of the attached motor which must be capable of starting the mill, up to twice as much nameplate power. Power draw is basically related to volume after all the efficiency factors are put in.

The reference CL is 250% and as far as I know there is no physical upper limit until the feed chute plugs. The premise of this thread, that power draw and CL are related, is wrong to begin with. And for what it's worth, I only do these calculations for a small operation. For large operations I bring in a world-scale consultant in grinding and accept the recommendations returned by someone who does not rely on linkedin.

KC Armstrong

June 16, 2014

ARABINDA BANDYOPADHYAY ( Bandyo)I agree with Mr. Enzo that higher circulating load is not necessarily healthy. In closed circuit operation of any mill, the mill discharge has a higher % of intermediate particles than the finished size range. The hydrocyclones separate the desired size and circulates back the coarse. Due inefficiency ( cyclone efficiency is never more than 60-70% at 200 mesh

separation) of the hydrocyclones a good amount of liberated fines also circulate back to the mill. In a normal operation the cyclone underflow contains anywhere between 25-35% of the liberated fines. And that is considered a good operation. Higher circulation would essentially mean, for a particular mill, more of such fines circulating in the system. Even if the feed chute does not get choked, which is an extreme situation, there will be more slurry in the system than the pumps or cyclones can handle and thus the new feed will have to be reduced to bring back the flows to normalcy. This situation is very often faced when the spigots/apex of the cyclones wear out and are not replaced in time. Double stage classification increases classification efficiency and reduces recirculation of fines and thus enables increased new feed from the same mill than designed. Same is the case with Derrick screens which are very efficient. Ease of installation or cost is another matter. A simple mass balance calculation considering different circulating loads can explain this. I would also like to emphasise that a particular mill-cyclone combination cannot be used to achieve a wide range of grind requirements. It can be done only within a limit that too at the cost of new tonnage unless care is taken to maintain the classification efficiency. Agreed 250% circulation is only a base but that does not mean that that the mil could be run with 400% circulating load without affecting new feed tonnage or out put quality both terms of size & pulp density. Similarly mills cannot be operated at too low a circulating load either, it will grind too coarse and the mass balance will go awry. Circulating load has to be brought back to around 200-300% by increasing new feed to stabilise the circuit. To me it is a mystery how Mr. Bond had hit upon 250% circulating load which seems to be optimum for most of the normal grinding requirements.

June 17, 2014

Tadimety RaoDr.Bando brought the subject into a well focussed point; thank you, Dr.Bando. While reading these comments on this very important operation,I am sure all of you would join me In saluting Bond, a Great man , with no tools of mathematics and well documented data ,could suggest some process on which all of us are dependent even today For me, we Mineral Engineers ,have to keep on saluting such "Great Pioneers and Thinkers" Dr.T.C.Rao

June 17, 2014

Itumeleng Kohitlhetsethis is a great exposure to us the still crowling metallurgists keep up the good work colleques.

Why do ball mills have such high recirculating loads?The ball mill grinds. The cyclones separate. But the pump neither grinds nor separates. So what is the purpose of having such a big pump to maintain high circulating loads in our grinding circuit?

Metcom’s recent grinding bulletin addresses this question from the perspective of Classification System Efficiency (CSE). Simply put, CSE is an emerging efficiency metric that is equal to the percentage of 'coarse' material in the ball mill, and thus also equal to the percentage of useful ball mill power. 

Click the link below read this bulletin and learn more about Classification System Efficiency (CSE).

Expertise in Mineral Grinding Systems...metcomtech.com•WHY DO WE HAVE SUCH A HIGH RECIRCULATING LOAD ON OUR BALL MILL? The ball mill grinds. The cyclones separate. But the pump neither grinds nor separates. So what purpose is served by having such a big pump, which is expensive to...See previous comments

Osvaldo BascurYou need the right blend of fines and coaser particles for proper reology in the mill. See Dynamill in app store.

February 23, 2013

Osvaldo BascurYou need the right blend of fines and coaser particles for proper reology in the mill. See Dynamill in app store.

February 23, 2013

Justin Spaldingis Dynamill available for android?

February 23, 2013

Joe van NiekerkThe most significant factor causing high recirculating loads in grinding circuits is classifier efficiency. Hydraulic classifiers such as hydrocyclones conventionally find their way into grinding circuits but they are very inefficient. For example: in a mill circuit grinding to 80% passing 75 micron the cyclone underflow will typically contain 12% to 14% minus 75 micron material i.e. mill feed rate x circulating load % x 12% to14% returning to the mill. The cyclone overflow will also exhibit a wide particle size distribution with top size of circa 300 micron.

Enter the Derrick StackSizer screen, a physical classifier that gives a clean cut size and a mill product stream exhibiting a narrow particle size distribution (very beneficial for downstream processes). Derrick Corporation claims that replacing a hydrocyclone with a StackSizer screen in a mill circuit results in up to 40% reduction in mill circuit recirculating load. This equates to a massive reduction in mill power consumption per ton of ore milled.

However, I can imagine that retrofitting a StackSizer screen in a mill circuit will pose certain challenges e.g. the mill will all of a sudden be too large for the application, the mill discharge pump will be too large for the application, etc.

I would like to hear from anybody who has executed such project.

February 24, 2013

Romke KuyvenhovenHigh circulating loads are because of two reasons, (1) to avoid overgrinding, that is, if you would grind finer and thus recirculate less, then your product would be too fine (using a typical ball mill) and your process would become even less energy-efficient and (2) as Joe indicates, cyclone inefficiency, because cyclone separate on the basis of specific density and not particle size, which is even worse considering that that ore particles typically are heavier

and smaller and gangue particles lighter and larger. My understanding is that circulating load can reduce from 250% to 100% when using screens instead of cyclones, but I wonder if that will work with large mining operations of say over 100.000 tpd as is typical in Chile.

February 24, 2013

Osvaldo BascurI understand that Dynamill is only available for IOS. You could suggest them to see if it could be made available for android. The effect of the Sump Pump of the cyclone performance are critically demostrated. Most of the Comminution Design Applications are static, The proper time constants and inertia of the belt, pump, sump, SaG, Ball Mill, Hydrocyclone banks and hydrostatic pulp pressure are never considered in Static Design Applications.

It is a lot of fun to play with Dynamill to fine tune the operation to get the maximum throught while getting the right particle size distribution in the cyclone overflow while having your type of ore, sump size, mill sizes, limited amount of water and number of cyclones with vortex finder and spigot dimensions. You can also try to find the proper control strategy depending on your type of mill disturbances ( size distribution, ore hardness, specific energy consumption.)

February 24, 2013

Ninoslav PavlovicJoe van Niekerk, I agree with you. However, it is necessary to analyze the process from the beginning before taking any actions. Also ore characteristics are very important for further consideration.

Application of StackSizer screen is good solution in processing of hard, abrasive and dens material which has a big commintuion resistance. In this case, mechanical classification can replace successfully hydraulic classification.

However, I think that efficient grinding media and suitable ball mill will additionally improve size reduction and decrease contamination of coarse particles. For this purpose, intensive lab and pilot investigation has to be carried out.

Regards

February 24, 2013

Kenneth ArmstrongThe higher the circulating load the more efficient the grinding action. The trade-off is the size of the pump(s) and clasifying device(s). Considering the wasted energy and losses due to overgrinding, we should really be planning for 5-600% circulating loads. It's also a fact that screening is more efficient if it can be used. Considering that grinding takes so much of the energy in a plant, tyhere differencers cannot be overlooked as they have been. Think green.

KC

February 24, 2013

Winifred Amoahcirculating load is the amount of material returning to a comminution equipment (in this case ball mill) in a closed circuit system. for every material entering into a system, the products to be obtained are dependent on the nature of the material as of the time of entry into the system and the conditions offered the material a it is in the system. from my point of view, i believe a very coarse material will yield a higher circulating load. since milling is the first stage of comminution and as a matter of fact a part of disintegration, the size of feed to the mill will be greatly influenced by the prior disintegration processes i.e. blasting and crushing. if the efficiency of blasting and crushing is low, it is most likely to have very coarse material as mill feed and therefore depending on the mill design parameters, a lot of coarse material relative to the fines will be produced at the classification stage and thus a lot of circulating load. on the other hand, if the nature of the mill feed is as expected and grinding efficiency is faulty, then u should be looking at the mill's design, operational parameters and factors. u should be looking at factors such as the mill type relative to the ore; the speed of the mill which is inversely dependent on the mill diameter; the energy required to grind which depends on the work index of the material, the feed size and the expected product size; the residence time of the feed which will depend on the throughput; the ball sizes and quantity; the mill liners and lifters and the forces at play in the mill. a deviation in the parameters used to run the mill will cause a deviation in the expected products and thus may increase the amount of material that will circulate in the system in a closed circuit.

February 28, 2013

Jordi MuñizMy experience is based in dry ball mill milling and in vertical stirred wet milling for ultrafine slurries of non metallic minerals.

As has been said in this discussion, high recirculating loads results in overgrinding and gives wider particle size distributions to the finished products. This can be an advantage or an inconvenient depending on the desired fineshed product performance at the final application, therefore, this is a particular tool for designing your product characteristics.

On the other hand, dry ball mill milling has to be completed with ultrafine air dynamic classifiers, playing similar role than hydrocyclons. For such air classifiers, the yield for classification highly depends on the fineness concentration at feed, beign desireable at least a 70%. Therefore, high recirculation also has a good effect on this classifying performance.

Further classifying after a first classifying stage can be done, then, the products could be splited in two for having both an even finer product and another fine but very vertical product. The purpose for such a different classifying and grinding configurations is obtaining real special top performing products that should be improving the final application to pay back such an energy investment in all those stages.

Please considere that dry ball mill milling is a very unefficient energy process (more that 95% of energy is lost by means friction heat, therefore you would not consider the high overload as a way to grind coarser feeds, this being a common mistake. In the milling process design, ball mill load and feed particle size should play the most important role for a proper energeticaly susteinable performance.