Grinding & Classification Circuits

Grinding & Classification Circuits 2017-04-04T06:57:16+00:00
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Grinding for Liberation and Flotation (8 replies and 2 comments)

Alan Carter
2 years ago
Alan Carter 2 years ago

For me it is interesting to see a lot of discussion around the topic of reducing costs for grinding, but very little comment or effort apparently being put into understanding the requirements of the grinding in the first stage. The question of why are we grinding to some target grind, how efficient should that grinding be, do we care if we generate excessive 10 um particles, is not being asked. For a flotation process have we really understood the whole liberation versus recovery, and optimization between primary and regrind. A very good laboratory once told me, it is about liberating gangue so that it can be rejected in the rougher stage, while not loosing too much of the good stuff. This has to be one of the most significant areas where operational cost savings can be made. It start to ask the question are we generating too much fines and using the wrong equipment in the first place.

Helena Russell
2 years ago
Helena Russell 2 years ago

The mindset of throwing away liberated tailings as soon as possible is the key to processing taconite economically in Minnesota. And it's a very good point. Why grind tailings to -53 microns, the liberation size of the magnetite, if a third of the incoming ore can be thrown away at 4000 microns or larger?

Unfortunately it is much easier to implement with magnetic separators and magnetite than with a process that is more size sensitive like flotaton. It also requires significant rock zones in the ore that are barren at a coarse size, like banded cherty taconite.

Still it's a very effective way of saving grinding energy and should be the focus of more research. Just think of the savings if a copper plant could throw away a third of its ore after crushing and grinding to 4000 microns without throwing away a lot of copper?

I don't know how the coarse separation would be made because I'm not that familiar with copper ore, maybe heavy media, some sort of sorting, coarse material flotation, something like that?

Tony Verdeschi
2 years ago
Tony Verdeschi 2 years ago

Interesting comment theme reducing milling costs at the primary stage, this is when finer grinding is the largest energy consumption and therefore higher treatment costs.
One way to redice this is that the mine give us a fairly thin material as possible so as to reduce the size with explosives is less expensive. In step crushing costs are also reduceria and milling step tendrianos granulumetria optimal and thus reduce milling costs of both energy and steel consumption.

In South African Platinum Mills the technology is called MFMF or MF2: Mill-Float-Mill-Float.

Although High Pressure Grinding Rolls are also doing a lot to reduce Comminution energy.

Two major "offences" in grinding - 1. under grind and 2. over grind.

  1. You have not maximized the targeted recovery.
  2. You have wasted energy.

Grind priorities can be assisted with best practice ball use, blended sizes of compatible grades can give better results. By adjusting the percentages of ball sizes in the blend fine tuning can be tweaked. This works both ways for coarser or finer grind targeting.

Bill Fraser
2 years ago
Bill Fraser 2 years ago

All unit operation from blasting , material degradation in handling and transfer points, size reduction processes should be critically analysed for a particular deposit to minimize energy cost including grinding cost. Its advisable to to recover metal values as most coarse size as viable so that subsequent grinding is minimized. Pre-concentration will greatly reduce the energy requirement. Whatever over-grinding is done, aim should be to recover maximum metal values through suitable technologies.

Many times we follow blindly thinking we don't know much. But most of the time the process we use is obsolete. Similarly to day grinding is out dated. We have new innovative technologies to follow. We are doing wrong thing using ball mills and sag mills when we have new proved technology. Why waste energy to grind. .pre concentration is one such new technology. HPGR, verti mill. Coarse flotation, flash flotation, heap leaching 1mm particles. Similarly many other energy saving equipment may be used.

Maya Rothman
2 years ago
Maya Rothman 2 years ago

I am an advocate of looking at sensor assisted machine sorting, which is an established technology in the industrial minerals industry, but is ignored by the base metal industry. Great reductions in grinding costs can be achieved for ores that lend themselves to sorting (comments about banding), especially if sorting is focused on cutoff grade ores. Think about it. Ore grade assays are derived from blast hole drillings, some 8 m apart. Blasting mixes up the ore, scooping does it again. I suggest to assay a number of (400 ton) truck loads to get the truth about selective mining. Why worry about grinding when there is low hanging fruit in the operation? Just sort the 4 - 6" fraction, costs will be modest and will be recovered by allowing the Mill grade to go up. That means more ore has to be mined, plus the tonnage to be backhauled, the Mine Super's bonus will go up, output will go up and management may even remember that a mineral dresser made the suggestion!

David Kano
2 years ago
David Kano 2 years ago

Gents this is indeed a conversation that can take on many guises. I believe that the efficiency should start in the pit with optimal blasting as this is generally acknowledged as a cheaper form of energy than mechanical and electrical power required downstream in the comminution circuit. Next is the opportunity to reduce the amount of "waste" going into the separation process by early separation by optical sorting methods, the net result is a reduced comminution circuit focusing on a concentrated feed to the downstream processing plant. As it was suggested everyone gets hung up on the efficiency of comminution. How about a look at what the downstream process requires in terms of optimal conditions for separation and minimization of product losses?

John Koenig
2 years ago

I don't agree that blasting is more efficient; I am happy to be proven wrong. But time and time again I have asked advocates of blasting for the FULL energy utilisation - blasting vs. crushing; and every time the advocate goes quiet.

Tony Verdeschi
2 years ago

​I can only talk about one industry, taconite mining, where this has definitely been proven. Harder blasting (higher powder factor) has positive effects all the way from dipper tooth wear to grinding to -53 microns in the Concentrator.
I can recommend some papers done by an acquantance of mine, who has made a career of persuing this, first at USS Minntac and now on his own.

Bob Mathias
2 years ago
Bob Mathias 2 years ago

We should all agree that "selective" feed of higher grade ore into the first grinding stage is a great plan. A no brainer I suspect.
I can only imagine how this can be done as, pre-crusher, I have little focus and after final grind is completed, I again have little experience. So all of your comments are well respected.
However, during the grinding process, we do have some expertise and there have been several great leaps in this area with new technologies and products that do NEW things to improve efficiencies.
Recently, SGI have begun to promote our free service for media evaluation based on data sheets that we need for modeling - never hesitate to take advantage of this service even as a check to what you have happening at this time.

2 years ago
JohnnyD 2 years ago

My approach is to perform a recovery versus primary grind (P80) curve as well as mineralogy to determine the liberation size. This will establish at what P80 the recovery of the valuable mineral(s) will start to decrease. Then perform mineralogy on the rougher conc to establish what regrind P80 is required. An Isamill is excellent for regrinding because it produces a tight size distribution (less fines).

I think that concentrators should perform grinding surveys to establish optimum ball size and %solids in the ball mill and the optimum %solids in the cyclone. Efficiency in the ball mill, cyclones and grinding circuit can be established. There are ways to calculate these. Not an expensive exercise but it can save concentrators a significant amount of money in power.

My approach is to perform a recovery versus primary grind (P80) in the lab. This will establish at what P80 the recovery start to decrease. It will provide an idea of the max P80 that can be used; thus, the energy cost would be reduced. As for regrinding, mineralogy on the rougher concentrate would have to be done to determine the liberation size (P80). Isamills are excellent for regrinding because they tend to produce less fines and a tighter size distribution.

I recommend that concentrators determine the ball mill efficiency, cyclones efficiency and grinding circuit efficiency. There are methods to increase the efficiency of the grinding circuit. Also, when the cyclones and ball mill operate efficiently, less fines are produced. Performing this exercise is not expensive and can have a significant impact on energy costs.

Victor Bergman
2 years ago
Victor Bergman 2 years ago

We have put a great deal of time into secondary grinding by gaining much better control of the forming of a small ball population. Having the media remain round (Spherical) during deployment life is now achievable it now comes down to a ball size and compatible grade selection in the control of this smaller ball population.

We have customers looking exactly at this for fine tuning the grind.

Here overgrind in fine particle discharge will be the main alert as the enemy.

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