Crushing, Screening & Conveying

Crushing, Screening & Conveying 2017-04-04T06:57:13+00:00
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Crushing Work Index (CWi) results variation (6 replies)

2 years ago
Subhash-Kumar-Roy 2 years ago

Is there any standard regarding the CWi test result variation? We experience a 15% standard deviation of the 20 pieces impact test results.

Helena Russell
2 years ago
Helena Russell 2 years ago

For some comparative measurements -

Starkey, J. and Meadows, D., Comparison of ore hardness measurements for grinding mill design for the Tenke project.
CWi on 6 samples varied from 5 to 11.1 with average of 8.0

2 years ago
Obergruppenfuhrer 2 years ago

I have read a paper on this test, and can confirm that it is exceptionally "noisy". I've observed that seemingly inconsequential procedures (such as rotating a specimen by 90 degrees) can have double (or halve) the work index result. This test is used in a lot of modelling, and just has to live with the fact that rocks in this size range give noisy results. You would see similar "noise" in point load index or unconfined compressive strength measurements (also tests on coarse rocks).

For designing a primary crusher it doesn't cost much to put in a bigger motor (and the rest of the drive system), so you could design to 75th percentile or higher with a minor cost increase (your operators won't break the crusher as often)! Primary crushers are usually volume limited and the motor power not very important (but KGHM has a harder ore than normal).

Secondary/tertiary crushers in closed circuit are a bit more site-dependent; but 75th percentile is a reasonable starting value.

2 years ago
Subhash-Kumar-Roy 2 years ago

Not many mines use this CWi parameter yet, in Indonesia. Will this parameter good enough to be used in predicting the Crusher throughput in production? Something like 48 hours looks ahead.

Jean Rasczak
2 years ago
Jean Rasczak 2 years ago

I've seen papers for taconite mining, exceptionally hard and tough rock that show significant variations in crushability and grind ability just from the drilling and blasting practices. It turns out that a higher powder factor in the mine can have significant beneficial effects all the way through the concentrator. Some of these papers are at the following site.

Victor Bergman
2 years ago
Victor Bergman 2 years ago

When one tries to predict plant throughput using model parameters one needs to be ready to develop a model first, then one needs to be willing to understand the ore variability in the ore body. At the end of the day’s one manage to get to know how different ore types in the ore body produce different performance.

There are numerous crusher models that can be adapted, but neither a model neither a parameter obtained from a test will give you the answer you are looking for.

Predicting throughput is a complex process but is doable. It requires that you manage to get support from your management team because you will need to use resources to prepare your model.

If your management team believes that they need a model then they can make a decision to ask you to start the work to develop this model. Your management team needs to believe that predicting tonnage is beneficial for their business.

Many mines in the world have these models and they use it to correlate it to energy consumption and to profit, so the answer to the question of having or not having a model has been answered. Having a model to be used to predict the production pays off. A well calibrated model can help to guide your management decisions.

The calibration of a model is an ongoing process for a period of time. If you don't collect all the data you need to calibrate the model then your model will miss guide you.

So the first question is: does your management believe that your company needs a crusher model and will they provide the resources to develop it? After this has been answered I would suggest that you hire people who have dedicated their life to develop these models.

Jean Rasczak
2 years ago
Jean Rasczak 2 years ago

There are two users of crushers that tend to have opposing goals. What's good for one is a disaster for the other. When someone talks crushing efficiency you have to know who their market is and how efficiency is defined.

The first group is aggregate and industrial mineral producers. These people want to produce as few fines as possible because fines are their waste material. To them crushing efficiency means maximizing the yield of coarse aggregate and minimizing the production of fines. The shape of the crushed rock can also be important.

The second is us. We ultimately want to crush and grind everything to a very fine size anyway. In the appropriate size ranges, crushing is cheaper than grinding, so making as many fines as possible while crushing is a plus for us. To us, crushing efficiency is generally measured by how many ultra-fines are generated while producing product fine enough for the next process down the line. The ultimate crushing plant would crush everything to -150 microns or a similar size.

Beside the ore being pre shattered by blasting, there are other parameters that affect the throughput of a crushing plant. These include, but are not limited to:

Variations in the ore body. Different ore zones crush differently.
Variations in the state of wear parts. Primary gyratory especially tend to develop a "skirt" at the bottom of the mantle profile as they wear out that tends to slow down the passage of ore through the crusher. This draws power but does little crushing.

Closed side setting of the crushers: One plant I worked at made all crushed material pass through a 12 mm screen before it was sent to the Concentrator Plus 12 mm product was re-circulated. If closed side setting got too large on the crushers, the circulating load from the screens built up and made the crushers look like they were very ineffective. Throughput rate on the crushers was great, but not enough was getting out of the circuit through the screens. The solution was not to open the crusher settings up to allow a higher feed rate; it was to close the setting to produce more -12mm that could get out of the circuit. The crusher throughput went down, but the circulating load problem went away and the overall plant throughput went up.

Conditions of any screens in the circuit: This was especially noticeable about 35 years ago when the industry was transitioning from steel to elastomer screen decks. The elastomer decks had much less open area and at first screened much less efficiently. This also caused a very high circulating load and limited crusher throughput. There were also problems with the profile of the sides of the holes. Screens with hour glass hole side profiles held their screening size much better over the life of the deck than ones that tapered out towards the undersize. Steel decks never lasted long enough for this to be a problem.

Moisture in the ore: Lots of moisture in the ore makes screening efficiency get worse, again slowing down throughput for the crushing plant.

Crusher feed chutes. If a cone crusher is not fed evenly around the opening, crusher throughput will be reduced.
Control systems. If the crushing chamber level is not optimized, crusher performance will suffer. Running "safe" with low chamber levels will make coarser product and reduce crushing rates.

Working with Nordberg standard and short head crushers, we found that when adjusted for ore density the optimized throughput rates for taconite, a very hard, tough ore, were essentially the same as the handbook values for limestone published by the manufacturer. As long as there was sufficient motor horsepower and sufficient hold down force between the adjusting ring and frame it didn't really matter how hard or soft the ore was. The crushing rates stayed about the same.

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