Sunday, 12 May 2013

Energy recovery potential in comminution processes

My posting of 23rd April discussed Tim Napier-Munn’s forthcoming keynote lecture, which will be presented at next year’s Comminution ’14. Tim asked the question “Is progress in energy efficient comminution doomed?”

Comminution processes are cited as having an efficiency of less than 1%. Even if the efficiency of these processes could be increased to 5%, as is suggested to be possible, the conclusion would remain that comminution processes are very inefficient, with some 95% of the input energy lost to the environment as heat. This is an interesting observation as one could suggest that, although comminution systems are very inefficient in producing new surface energy, they should be very efficient in producing heat. On the other hand, high efficiency in generating heat might be off-set by a limit on the energy that can be recovered.

This is the theme of a very interesting and thought-provoking paper by Peter Radziszewski of McGill University, Canada, which has recently been published in Minerals Engineering. Four issues are addressed in the paper: heat generated in comminution, potential energy recovery, different means to increase energy recovery in comminution processes and avenues to possible implementation.

Peter shows that at the discharge of the comminution circuit, the working fluid mineral slurry has captured a certain amount of energy that can be defined by its heat capacity and temperature. The greatest potential for energy recovery is therefore found at the discharge of the grinding circuit, which of course coincides with the input to the flotation circuit. It is at this location that one would find the highest slurry temperature. Also, at this location one could either divert the energy carrying slurry into a separate heat conversion system or find a heat conversion system that has the potential to be integrated or retrofitted directly with the flotation circuit. In the literature a number of workers indicate that increased temperatures might be advantageous to flotation. It is suggested that more focused research should be initiated to explore in more detail the effect of increased slurry temperatures on flotation performance.

However, the paper recognises the challenges of insulating and sealing a comminution circuit, the implementation of a capture and energy conversion system, and effects that the application of an energy recovery system might have on other dimensions to mineral processing such as flotation, wear and water recovery.

Insulating and sealing an existing comminution circuit is in itself a non-trivial challenge. The minimal requirement of an insulating system for a rotating mill such as a SAG or ball mill would be that it does not increase the downtime for liner change-outs. Further, it would be unrealistic to completely seal an existing comminution circuit. However, measures can be made to reduce such loss by covering mill trunnions and sumps. In this analysis, only SAG and ball mills were considered. However, higher intensity stirred mills, and in general higher intensity grinding, may provide higher slurry discharge temperatures and therefore higher energy recovery potentials. This observation may justify the development of new, yet higher intensity, grinding systems that may provide both increased throughput and higher energy recovery potentials, resulting in higher overall comminution efficiencies.

Therefore, it might be time to propose that the desired goal of comminution processes is not only to grind a given ore to a target granulometry, but also to capture and recover the heat generated in comminution, but despite the promise of increased comminution circuit efficiency and potentially substantial annual energy savings, there remain a number of challenges before such energy harvesting technologies can be brought successfully to the mining industry.

Food for thought though? Let’s have your opinions.

7 comments:

  1. Hi Barry, it would be interesting to see if some colleague has data about the heat loss comparing steel/cast iron versus rubber/composite liners in a ball mill.
    Adrian Villanueva, BASF Mining Solutions, Germany

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    1. Excellent question. Unfortunately, I have no data. However, based on the thermal conductivity of steel (50 to 80 W/m C) and of (natural) rubber (about 0.13 W/m C), one could expect that the heat loss through conduction in a rubber/composite lined mill to be quite a bit smaller than in a steel/cast iron liner mill. One would need data to validate this difference.
      Peter Radziszewski

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  2. Hello Barry,
    Its an interesting study for the forthcoming challenges in beneficiation. In fact major COP of beneficiation circuit lies in comminution part.
    I wonder if; 1) there is some equipment/process to recover heat/energy from the mill discharge slurry. 2) Will it be economical and, 3) how to utilize recovered heat/energy?

    Ashish, Hindustan Zinc Ltd., India

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    1. Thank you for the question.
      There is some indication in the paper on these three points which can be summarised as follows:
      1) Potentially yes. Stirling engines, organic Rankine cycles are potential alternatives that convert heat into motion which then requires a generator to convert into electricity. On the other hand, thermoelectric generators provide the possibility of converting heat directly into electricity.
      2) The focus of the paper was to evaluate the energy recovery potential in comminution processes. The results show that there is good energy recovery potential which can potentially lead to recovering some good value. It did not look at the costs related to developing such a system and consequently the paper did not look at the pay back on such a development.
      3) Once the energy is converted to electricity, it can be recycled back into the plant electrical grid.
      Peter Radziszewski

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  3. I do not think that there is a need to insulate any part of the grinding circuit. Without doing the calculations I believe that the heat capacity of the mineral pulp is so high, and in combination with today's low mill retention times, that the heat loss per ton of ore processed would be very low. What we experience as a lowering of the temperature after grinding is often the result of dilution with colder water.

    But I agree that we can use the generated warmth provided by the mills. An example; at LKAB, Sweden; the secondary and tertiary grinding mills and corresponding magnetic separators are in a separate water recovery circuit where the hot water is sent back as dilution water to the LIMSes. This results in a warm magnetite pellet feed that reduces the fuel consumption, and give higher capacity in the pelletizing plants.
    Bertil Pålsson, Lulea University of Technology, Sweden

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    1. I like the cited example from LKAB. This is a perfect example of how the thermal energy potential in processing plants can be used to reduce the costs of processing. Thank you.

      With respect to “heat loss per ton of ore processed would be very low”, please consider the observation made in response to Mr Villanueva question above. I believe the case can be made that this assumption might hold true for rubber lined mills, but in the case of steel lined mills the assumption might not.

      Independent of whether this assumption holds true or not, I would agree with you that “What we experience as a lowering of the temperature after grinding is often the result of dilution with colder water.”

      However, if one decides to pursue the recovery of comminution energy in the slurry, one can take the view illustrated in the LKAB case and explore ways and means to heat up the “colder water” at the cyclones through heat transfer...
      Peter Radziszewski

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  4. Thank you Barry for this discussion topic!

    Slurry is an energy carrier.

    Once one understands that slurry is indeed an energy carrier and that potentially some 99% of the energy input into comminution can be found in that energy carrier then one can start to focus on how one can minimise losses in the energy carrier, maximise the energy carrying capacity of the energy carrier, and most importantly how one can convert the energy found in the energy carrier into a useable form.

    With the still evolving development of thermoelectric generator technologies, the possibility to convert heat, even low yield heat, directly into electricity now becomes an alternative for the mining industry to consider as a means to increasing comminution efficiency.

    The first step in this quest to capture and convert the energy in a slurry is to determine how much energy there is.

    Please note that anyone completing a circuit survey already obtains data on flow (water, solids) around a given circuit. Adding one more measurement column to a circuit survey data sheet for temperature and then using a thermometer (there are some very good infared thermometers available on the market) to capture temperature at all circuit sampling points will provide the data of evaluating how much energy is in the slurry coming into and leaving the mill, how much energy is coming into and leaving the hydro-cyclone and how much energy is coming into and leaving the circuit.

    Once you have the temperatures of the different streams in the circuit along with the different mass flows, you can use equation (4) in the paper to determine the heat loss for a control volume around a mill, a cyclone or a circuit. And then, you can evaluate any options to reduce heat losses, maximise heat capacity and energy conversion alternatives and the associated costs.

    Peter Radziszewski

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