Grinding & Classification Circuits

Grinding & Classification Circuits 2017-03-23T09:46:37+00:00
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Are Electricity Costs Stoping Mining (17 replies)

Maya Rothman
1 year ago
Maya Rothman 1 year ago

Electricity is one of the industry’s main cost drivers: higher energy costs will force closure of mine shafts.

The key is to improve energy productivity, regardless of the source, starting with best practice operations. CEEC is supporting the global mining industry to address this issue by collating the best- in-class papers on efficient mineral processing on CEEC's open web site, and more recently, launching the CEEC energy curve, the tool for analysis of current operating efficiency. We encourage you to use the energy curve program as the starting point for efficiency gains.

Helena Russell
1 year ago
Helena Russell 1 year ago

It's unfortunate but energy costs are going to rise and unless the mined commodity price also increases, the cut off grade will have to increase resulting in a shorter mine life. In the case of Gold, the head grades have been decreasing worldwide and the only fix is an increase in the price of gold. Mining companies, in general, are going to have to get more creative in how the buy and consume energy.

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

I feel we have to innovate to come out with most energy efficient fracture systems with greater reduction ratios;I am also not clear which branch of Engineering is most appropriate to take it up but characteristics of ore to fine particle generation has to be integrated.

Helena Russell
1 year ago
Helena Russell 1 year ago

You are absolutely correct. It's all done by compression at present. There is significant mineral processing research in comminution.

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

Unfortunately, most of the research is "technical paper oriented" and we need to focus on energy.When I was a student in early '60s I was told(may be from Gaudin's classical text book) that only 1% of the energy spent is effectively used in comminution.In 2015, with all this good work, I do not know wheather it is still 1% or more. Some hard questions, as started now, have to asked.

Helena Russell
1 year ago
Helena Russell 1 year ago

Agreed. Energy consumed in comminution is a significant cost.

JohnnyD
1 year ago
JohnnyD 1 year ago

Energy management systems and related protocols are in place such as ISO 50001. A recent operating example is New Gold's New Afton Cu-Au Mine in British Columbia, a collaborative effort with the provincial energy utility BC Hydro.
Perhaps improved characterization of ore body variability at the pre-production stage and the intent to leave more waste tonnage in-situ or underground before it is ever conveyed out of the pit or from underground is a realistic target. Gekko Systems underground pre-concentration displayed in the Python was engineered on this premise.

Alan Carter
1 year ago
Alan Carter 1 year ago

When I was a young metallurgist we did a heat balance on a 1450 kW rod mill and an 1850 kW ball mill grinding taconite. Taking the temperatures of the ins and outs, the mill skin was about the same temperature as the building, assuming a modest mechanical loss of 1%, and knowing the specific heat of the slurry components, we could not detect any energy that wasn't converted into heat in the mill product. This was for a rod mill and ball mill that by all standards were running efficiently. This in a plant where electricity was 1/3 of the total production cost and grinding mills consumed over half of the electricity. Power consumption and cost per ton produced were very important to us.

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

Thanks for coming out with such detail and for reiterating the importance of reducing power consumption. Instead of more simulation studies/mathematics loaded comminution equations, we should look at the weight of mills for a given purpose/better breakage and classification systems to reduce circulating loads etc.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

I am not clear on the results you are describing. But when an empty mill is rotated, the power draw is a measure of energy loss in the drive train. The mill shell itself should be reasonably balanced. When a grinding ball charge is added, the mill becomes unbalanced, with energy required to lift the balls to cascading height. A lot of friction is also developed in the dry charge, increasing the charge and mill temperature. For dry grinding of cement, this energy raises the temperature of the media, cement and mill shell considerably. With wet grinding, the temperature is controlled by heat loss in the slurry flow. There is no doubt that size reduction is achieved in the process. The question is how much of the applied power simply lifts the media and produces heat, and how much causes breakage. Bond and other empirical results for mill design do not specifically measure breakage energy, but do use breakage functions in the determination of power requirements. Other tests such as the JKTech pendulum test give a direct energy measurement. Were any such tests included on the feed material to give an idea of the power used for breakage? 

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

I agree with what is said; However, I personally feel that more focused research to reduce power consumption in comminution is required. So much money spent on R&D, so many Seminars on comminution and large number of technical papers are in fact, I feel, are shifting the focus into different directions but very little which says "do this and one will reduce so much energy per ton broken".
Please take my comments in the spirit with which I made.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

I completely agree with your view - theory has to be proved by practice. The problem is usually not a theoretical issue at all. A holistic view from mine to market is to my mind the only way to optimize energy efficiency. However, research definitely has its place in the overall scheme of things. Research for its own sake can turn up some surprises.

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

Thank you! My main point is also that R&D is the backbone of any discipline; but very little is done on "how to reduce power consumption". For me, a time has come for mineral industry to encourage a group of equipment manufacturers to identify where it is possible so that it becomes the starting point on R&D to go interdisciplinary(material science/mechanical/electrical etc) to come out with energy efficient fracture mechanics.

Alan Carter
1 year ago
Alan Carter 1 year ago

Minnesota's climate is near arctic in the winter.

The rod mill feed was near freezing (32 to 33F) and the incoming water was about 40 F. The discharge, mill shell, and surrounding building were about 64F, thus no heat loss or gain to the surroundings. We used the motor design data to estimate the difference from electricity input to shaft horsepower output (96%).

As stated, according to the Bond index of the ore, the mill size, and the feed and product F80 and P80, they were grinding efficiently. They were using the energy the Bond equations predicted. Similar results for the ball mill with incoming feed at 40F and product near 68F. The Bond equations apparently include all the inefficiencies of rod and ball mill grinding in the power prediction.

By the way, the power required to rotate an empty mill is not the "tare power" of a loaded mill. Look up the friction coefficient for lubricated steel on brass, which is what trunnion bearings are. It is surprisingly large. A loaded mill weighs a lot more and uses a lot more energy to overcome trunnion friction than an empty one. Same with the gears on a mill drive. Trunnion and drive friction can account for many hundred horsepower on a large mill, between 5 and 10% of total mill power. It's enough to account for most of the diameter correction in Bond's equation. Most of this heat is conducted into the shell and ultimately the mill discharge slurry.

When we needed to melt the ice on a tailings thickener during a winter shutdown we ran the ball mills with water only. They still converted nearly 100% of the drive energy into water heat and drew essentially the same power as they did before the shutdown grinding ore.

Ball wear was insignificant compared to grinding ore, but our ore is considered very abrasive. Grinding ore each ball mill would need about 4000 pounds of new balls a day to maintain power draw. Empty, we didn't have to charge it for over a week. When we restarted, no make up charge was needed.

I don't know what the answer is, but I think we can all agree that rod, ball, and AG/SAG mills are very inefficient at making little ones out of big ones.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Many thanks for the detailed information - a very interesting operation!

I would like to see more information from other operations where the actual power used for breakage is compared with the power needed to run the machinery. The only power reduction potential I have seen up to present is increased use of blasting technology and the use of HPGR units. Perhaps low volume units such as jaw and gyratory crushers and rolls crushers are more power efficient than large grinding mills. Stirred mills, vertical or horizontal, may also give increased power application efficiency. I have not looked at any detailed information on this, but no doubt a lot of work has been done. Of course, maintenance and wear parts costs also have to be added into the energy equation.

Many people have thought about rock breakage over many years, it is hard to see where a breakthrough (!) is going to come from. One major factor often mentioned is avoiding breaking rocks that don't need to be broken, whether by improved mining selectivity, or rejection stages after each breakage stage, either on the main stream or on recycle streams. The use of screens on grinding mill discharge instead of cyclones can have major benefits in reducing mill circulating loads while avoiding overgrinding. 

Maya Rothman
1 year ago
Maya Rothman 1 year ago

Very insightful information from very knowledgeable sources. I would only add that, it appears that what is required is an integrated resource plan (IRP). The utilities have been doing IRPs for years in energy supply. The IRP for mining will be more complex and will be different for every ore body.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Yes - many people in the research area of the industry have done a lot of work on pointing out the cost and inefficiency of energy consumption in the mining industry, but it comes down to an individual mine doing a detailed study on all aspects of its operations, from geomorphology of the ore body to the market for the final product, using energy application as the focus. This must include the energy cost of maintenance and replacement equipment.

Which operating company will divert resources into this - one of the majors or a junior start-up? It is not clear where the expenditure incentive lies, when other issues are taking priority, such as ramping up production in a low demand, highly competitive market, or simply getting a new mine into production. There may already be examples of energy surveys carried out by research and development groups, but I have not seen any. Also, it seems apparent that most mines will find issues with energy efficiency, particularly in rock breakage. Is there a commitment, in times of industry downturn, to spend in order to save? Equally, when times are good, why worry about energy efficiency? This is painting a rather gloomy picture, but why else does little of a practical nature seem to be happening in this area?

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

A very accurate assessment. The driving force will be the cost of energy.

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