It is very difficult to provide an appropriate response to your question. The aspects of the arrangement of the comminution phases are complex and depend on the numerous factors as follows: ore hardness and size distribution and aimed size reduction ratios. Apparently, secondary crushing before the SAG diminishes the grinding power consumption but, the high participation of the fine particles will deteriorate the grinding efficiency and the power consumption will be higher. In addition, the fine particles will be recirculated and the result will be the power consumption increase. I met, during my operational and consulting experience the both comminution arrangements, but the selection of optimal scenarios was based on the study of the experimental and/or operational data. The study of the screening circuit is another required action. I met a situation that required the separation of the fine size fraction before the grinding, in order to increase the SAG efficiency. Sorry, I cannot provide you a general law of the arrangement of the comminution phases. The knowledge of the aspects mentioned above is required.

I suppose you have a dry comminution concept. Please do not forget the HPGRs. I combined the crushers, SAGs and HPGRs, but unfortunately, I do not know your required grinding size. In addition, please think to the crushers maintenance cost that can be higher than SAG maintenance cost. It is a very interesting project and its results can be surprising under incidence of efficiency and cost criteria. I met some similar situations. I and can mention Canada, Kazakhstan and Mauritania. The required production capacity is another important aspect.

Circuit simulation is an excellent way to effectively assess the performance of alternatives. I'd be happy to help you with this.

I think you're very correctly identified the difficulty of the question. I agree with you that there is no single answer. Some experts believe that the secondary crushing effectively used for hard ore, crushing critical size effectively for soft ore.

This question nevertheless remains very important to today's time. Now the maximum capacity of SAG mills reached to 5000 t/h or 35-37 million t/y. But unfortunately (or fortunately) this maximum achieved by pre-crushing.

The secondary crusher has additional provisions: increased nu ball mills, increasing the size of balls in SAG mill, increasing ball load in SAG mill, etc.

Simulation allows a wider view any process. Unfortunately I have a program for SAG mills and grinding circuits. Computational testing showed that:

Please define your problem. What is the feed size, product size, ore hardness, capacity? Why should you be focused on SAG circuit, is it given?

Ok for example calculate capacity SAG mill (8m x 3m): Wi = 12; P80 = 100000; T80 = 2000; Ball charge = 15%; speed = 0.8 of critical; ball size = 100

There is no definite rule for crushing circuits for any ore and depends on characteristics of material in the deposit. The first option should be AGM...which can be done with primary crushing followed by AGM as practiced in Kudremukh. For SAG, may be two stages crushing followed by SAG better option. However, the right crushing circuit can be decided only after initial test work including competency test of ore of particular size as grinding media.

If the Crushers and SAG are not able to ensure the aimed hourly production, I have the doubts concerning the FAG grinding scenario. Please do not forget Wi = 12 kWh/t. The ore is not very hard, but it is not soft. I think, the audit of the comminution plant, including the screening, is the right way to the comminution plant optimization. Our opinions are a general character; we do not know the flowsheet and the responses of the grinding and screening equipment to the size change. We do not know the recirculation degree of the screens. Under these conditions, I think, we are not able to provide the appropriate solution. As previously mentioned, the Novoi ore characteristics are very important. The simulation and the model of the process are, in fact, the final step of the project development. My opinion is based on the experience acquired from direct operation of the different comminution plants (soft and hard ores, dry and wet process), including the Crushers, FAG, SAG, HPRG, RMs and fine grinding including BMs, Vertimills and ISA Mills. I worked in favor to Kudremukh, I think in 2000 or 2001, if my memory is OK, but the ore are maybe very different of Novoi ore. In the mining industry is not recommended to accept the use of the general laws. Even in the case of the same ore, because of its quality variation (daily, weekly…), the process efficiency and the product quality can significantly vary. The stability (STD) of the process efficiency and product quality is a required target associated to the production capacity. Really, we can discuss and provide our opinions but without to effectively provide the right way necessary to the problem solving.

In addition to my previous comment: My opinion, is not possible to indicate an appropriate/right way, without to know the operational people opinions. The operator’s experience and well process knowledge is the most valuable information. Please note, the Novoi Plant operation did not start yesterday. Thanks.

The question is not "what is better?” it is "what does the ore want to do?". The case of a SAG mill followed by scats (oversize in mill discharge) crushing is usually used to reduce the oversize material by crushing because a) there is not enough competent coarse material in the SAG mill to do this at the required rate or b) there is a hard component in the mill feed which is resistant to breakage by the operating conditions in the mill. Pre-crushing part of the SAG feed on the other hand is a means of increasing mill throughput by reducing the coarse lump breakage energy required in the mill by reducing the quantity of coarse material which has to be broken by impact with balls or coarse ore lumps. You need to review the grinding characterisation data for the Novoi ore(s) and operating practices to determine which approach will achieve your objective. Simulations will be helpful if properly interpreted.

You probably figured out that I am a supporter of pre-crushing.

For supporters of crushing critical class I have a question: Why to the SAG mill is make critical class?

I want to know the opinion of experts in these flowsheet. Which you choose? Because the next question would be: Two grinding flowsheet: SAG mill or HPRG. What is better?

To select the "best" circuit in your case I would need to review your operational grinding data and/or grinding characterisation testwork results you have for the ore in question. There is no general "best" grinding circuit, only the best circuit for a particular set of ore breakage characteristics.

We can go on making general statements about the alternative circuits but it would be much more productive if you define the circuit, e.g. single stage SAG, SAG/ball, what performance you expect of the circuit and of course what are the ore's grinding characteristics.

I totally agree with you. But I want to compare these two flowsheet. Determine their '+' and '-'. For example, why the critical class load in the SAG mill?

A pebble crusher is the way to go for a greenfields project. SAG mill grates generally limit pebble extraction rates to around 25%, so you can't expect a throughput increase of more than 5-10% depending on the ore. To pebble crush, you need to port the grate. This removes any fine media, so achieving a fine grind size in a single stage becomes difficult.

Full secondary crushing is not recommended as these circuits are unstable and grind-outs (i.e liner damage) can occur frequently without experienced operators. You typically need a pebble crusher anyway because you fill the mill up with critical size material and constipate the mill! Not a good option.

Partial secondary crush circuits allow a stable load to develop and protect the liners. This is a good option for brownfields expansion targeting ~50% throughput increase. Partial secondary crushing also typically increases power efficiency.

If the ore is really hard, you might just have to bite the bullet and go three stage crush ball mill. Remember, pebble crushing and secondary crushing will coarsen the transfer size and may overload the secondary mill.

If you have identified that the ore is suitable for SAG milling go ahead and select a suitable SAG/Ball Mill combination (without pebble crushing if characterisation indicates it is not required). This will give you a cost effective, easy to operate circuit. You will need to recirculate trommel/screen oversize from the SAG mill so in the layout make provision for a pebble crusher installation in the conveying arrangement. If the ore changes and pebble crushing is required it will be relatively easy to install the pebble crusher. If the ore remains unchanged you can get another 5-10% more throughput by including a pebble crusher.

If after installation of the basic SAG/Ball Mill circuit you want a significant increase in throughput (>10%) you can consider pre-crushing part of the SAG mill feed. Since you already have a pebble conveying system in place, with or without a pebble crusher installed, it will be possible to install a pre-crushing circuit without hindering the operation of the grinding circuit.

If you feel there is a need to install pre-crushing for a green field grinding circuit, it suggests you are SAG mill limited (because of ore hardness), in which case an HPGR/Ball Mill circuit should be considered.

Yes, 5-10 years ago SAG mills was designed with pebble crusher, however over the last 5 years in some plant pebble crusher was replaced by secondary crusher. Unstable work of secondary crushing doesn't take a place in practice. Use of the equipment makes 90... 92%.

There is a basic principle of crushing: don't load into a crusher a ready size. For a SAG mill similarly, but only on the contrary: don't load the pieces which it can't crush.

When you load large pieces in a mill, you spend is useless the electric power: large pieces rotate in a mill, but crushing doesn't happen. Large pieces occupy mill volume, and it was possible to give to ball. Kinetic energy of a ball is 2.5 times more, than at large pieces of ore.

Therefore the tendency of reduction of the feed size and increase ball charge (15...20%) is observed the last 5 years.

Change the opinion on full secondary crushing. Through 2... 3 years pebble crushing won't be.

Your opinion is entirely right. Your ore is not very hard, but it is hard. That is right, is not recommended to increase the feed size, hopping in an increase of kinetic energy of ore particle. In the case of the hard ore (I met this situation in the case of the hard magnetite) the size reducing is preponderantly carried out by abrasion and not by ore fracture. Under these conditions you are obliged to increase the ball size and, finally to spend the power. I do not know the required size of the final product, after the grinding but, I am sure, you will obtain a particular size distribution and oversize flow rate will be higher. Another negative aspect generated by the feed size increase will be the over charge of the SAG, the increase of the unit power consumption etc. Based on the experimental and operational results, you can find an optimal point of SAG operation, including the feed size, power consumption, ball consumption and production capacity, excepting the size of the ground product. Taking account of this aspect, logically, you will need a second reducing phase. Sure, you think to the crushing but you can find other more economic variants, as HPGRs. Here is the key of my right response. I do not know your operational parameters (flow rate, SAG characteristics, required size distribution of the ground ore etc.). Under these conditions, I am not able to provide my opinion. You know, the crusher power consumption is lower than SAG power consumption, but maintenance cost of the crushers are higher. HPGRs seem to be the best. They accept max. 50 mm feed size, the maintenance costs are lower than the crushers and the power consumption is not very high in comparison with the crusher power requirement. Really, this equipment seems to be OK, after the SAG, but without knowledge of your existing comminution circuit and required operational parameters, I am not able to provide you the best connection of the secondary grinding circuit. You have a big advantage in comparison with you. You know very well your flowsheet and the daily operational experience is available and known. Our opinions are the suppositions only. We try to the find the right way without to know it. In conclusion, I agree with your opinion, to add a secondary grinding/crushing size after the SAG, without be able to specify you the equipment type and its connection with the SAG circuit. You will be obliged to review the screens. Thank you and my colleagues for your and their time.

Pre-crushing feed to the SAG mill may also increase the SAG Mill POWER consumption. Others have played with pre-crushing down to 50 mm but what is the point? You may as well replace the SAG mill with a large Ball Mill. HPGR is now developed to the point that it is actually being considered as a replacement for SAG technology. Something to think about. HPGR has many advantages over the SAG, starting with capital cost and power costs as well as maintenance. The way I see it anyhow.

Yes High pressure is now considered as replacement to a SAG mill. In 2008 I reported (Comminution 08) to need the feed size reducing to 40-50mm. To me asked a question: if the feed size is 40mm, why a SAG mill is better from the High pressure? I answered "may be".

Now past 5 years and it is possible to tell shortcomings HPRG:

- high sensitivity to humidity of ore

- for hard and very hard ores HPRG application is problematic