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Increasing the Energy Efficiency of Processing

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Increasing the Energy Efficiency of Processing Increasing the Energy Efficiency of Processing Joe Pease General Manager Xstrata Technology August 2007 1 Can we double the energy efficiency of grinding ? • Diminishing returns on research ? • Radical new technologies? • 10 years or longer ? Mineral comminution consumes about 3% of the world’s energy. So improving the efficiency of grinding has long been an important objective for operators and researchers. Much progress has been made. Entire research institutions like the JKMRC have dedicated decades of study with almost the sole objective of improving comminution efficiency. Does all this work mean that the big gains have been made, and that we are on diminishing returns from our research ? Has conventional technology reached its limit ? Is equipment just getting bigger, but not more efficient ? Do we need radical new technologies like microwave grinding to make a step change in grinding efficiency ? A new collaborative research program, AMSRI has a $20 M budget and the ambitious target to double grinding efficiency in 10 years. Is this a realistic target, given the enormous of amount of work already done? No, we don’t think 10 years is a reasonable target. We think it can be done today ! 2 The tools are already available • Laboratory grinding and flotation characterisation • Quantitative mineralogy • Basic thermodynamics In fact, processing efficiency can be significantly increased by using tools already developed by our previous research. We don’t need radical new technologies, we need to combine the tools we already have in the right way. These tools are : •Laboratory grinding and laboratory flotation tests •Quantitative mineralogy • basic thermodynamics of smelting and refining To grind the right mineral in the right place to the right size. In many cases this approach can double the energy efficiency of processing. Note that I refer to processing efficiency, not grinding efficiency. Grinding is not an end in itself. It is just one step in the processing chain. It cannot be considered in isolation of the downstream process. This is why I have included smelting thermodynamics in the list of tools. It is no point reducing grinding energy if this increases smelting energy. 3 We already have the tools to double the efficiency of processing : • grind the right streams, • in the right place, • to the right size In fact, processing efficiency can be significantly increased by using tools already developed by our previous research. We don’t need radical new technologies like microwave grinding, we need to combine the tools we already have in the right way. These tools are : •Laboratory grinding and laboratory flotation tests •Quantitative mineralogy • basic thermodynamics of smelting and refining To grind the right mineral in the right place to the right size. In many cases this approach can double the energy efficiency of processing. Note that I refer to processing efficiency, not grinding efficiency. Grinding is not an end in itself. It is just one step in the processing chain. It cannot be considered in isolation of the downstream process. This is why I have included smelting thermodynamics in the list of tools. Sometimes we should increase grinding energy, if we can reduce smelting energy by more. 4 Processing Efficiency … Energy used by grinding ` Energy Impacted by grinding So of course, the real question is about the energy efficiency of the whole process, not just the grinding step. It is no point reducing grinding energy if it increases downstream energy by more. This is the same logic we all understand about mine-to-mill. One of the advances in the last decade has been to recognise the importance of the mine-to-mill interaction. Most of us will be familiar with the classic example of the mining engineer who reduced his blasting cost by 20 c/tonne, only to reduce SAG mill throughput by 10% and increase milling cost by $1/tonne. Sometimes the right answer is to spend a bit more and blast finer in the mine, and save much more in the mill. Well of course, the same applies between the mill and the smelter. Yet how many of us check the thermodynamics of smelting, and see what we can do in the mill to reduce smelting energy ? Maybe that’s not a fair question, because we don’t have simple tools to help us do it. Have you ever seen a grinding model that includes some simple smelting thermodynamics to help you minimise processing energy ? Which research group is working on it ? We have research proposals to develop more and more sophisticated grinding simulations, to seek the last little bit of grinding efficiecny. But where do they consider smelting energy ? I think this is a huge oversight, and a huge opportunity – we are just like the mining engineer who tried to reduce his blasting cost with no consideration of what it would do to grinding efficiency. 5 Regrind minor stream Circulating loads A common circuit Inefficient grinding Steel harms chemistry Don’t regrind concentrate fine enough Grind whole tonnage too fine This is a flowsheet of a common circuit. How can we use existing tools to reduce the energy need. Often the primary grind can be coarsened. We don’t need complete liberation for roughing, we just need to liberate enough gangue away from the valuable mineral to get high rougher recovery. We can then achieve the liberation we want for cleaning by regrinding a smaller tonnage of rougher concentrate (or a part of it). This is the biggest single impact on grinding energy – why grind big tonnages of silica finer than you need to? Grinding too fine up front also causes us to put in more roughing flotation capacity than we really need. Coarsening primary grind also has other advantages for tailings storage and reactivity. 6 Where is the lowest energy place to remove this impurity ? • Mine • Preconcentration • Roughing • Cleaning • Smelting/refining Pentlandite in gangue Grind it out. Or add Silica and limestone, and melt it out at 13500C Improving energy efficiency means asking a simple question : where is the cheapest, lowest energy place to remove this impurity ? • in the mine (by dilution control) – requiring no energy • in a preconcentration stage (removing some gangue while it is still coarse) • in roughing – ie by grinding the whole mill feed • in cleaning – ie by grinding a much smaller tonnage of rougher concentrate • in the smelter – by melting it out ? Of course, cleaner feed tonnage is much lower than rougher feed tonnage. So ideally, so it is obviously better to grind the lower tonnage of rougher concentrate rather than rougher feed. So long as you grind rougher feed just enough to get recovery into the rougher concentrate. The photo of the composite shown is taken from a paper on Nickel metallurgy. The gray is pentlandite, the white is MgO. Excellent work was done in the flotation circuit to allow recovery of this low grade composite. Even so, this particle causes a problem for the smelter – MgO causes highly viscous slag. The smelter has to add Silica and lime fluxes, then heat the whole lot up to 13500C to remove the MgO. This consumes a lot of energy – over 10 times the energy it would take to grind the MgO out. The high temperatures also make smelting difficult and reduce refractory campaign life. So to improve energy efficiency, the concentrator should regrind this particle – so long as the mill can still recover the pentlandite. 7 How can we use existing tools better ? • Simplify use of quantitative mineralogy • Make better use of regrinding • Look at the mill-smelter interface I think there three areas where we can improve our use of the existing tools to improve energy efficiency, which I will discuss in the next few slides. 8 Quantitative Mineralogy … …. See the forest for the trees What do we want to achieve in each part of the circuit, eg • Primary grind and rough for recovery – Look for gangue liberation, not mineral liberation • Regrind and clean for grade – Look for mineral liberation and entrainment Distinguish symptoms from causes • Eg fines in tails may mean … – … you are grinding too fine – OR you are grinding too coarse ! Because quantitative mineralogy is so powerful, sometimes we overcomplicate it. It can generate so much data that we get lost in detail, and can’t see the forest for the trees. We need to keep it simple, and apply the 80/20 rule. Avoid the temptation to find out everything about every mineral in every liberation class in every stream. Ask a few simple questions about what you are trying to achieve in each stage of the process. And look for root causes rather than symptoms. The mineralogy can only be interpreted with a good understanding of how the circuit is operated. Two simple questions are : 1. in roughing, how coarse can I grind and still be able to get good recovery to rougher concentrate. You don’t need to worry about grade yet, that is, you don’t need to worry about the mineral liberation, but rather the gangue liberation – is enough gangue liberated from the mineral to allow it to float into rougher concentrate. 2. In cleaning, the question changes to how fine you need to grind to make a high grade concentrate. Now mineral liberation is important. So is entrainment of liberated gangue. If there are coarse composites in concentrate that could be easily liberated, these are “low hanging fruit”. Even if you are already at concentrate grade, by removing the easy impurities you make room to “pull” the rest of the circuit harder. This point is an example of distinguishing root causes from symptoms. You will always find fine liberated values in cleaner tailings and rougher tailings.
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