Monday, 29 September 2014

Does the choice of regrind mill and mill media affect downstream performance?

There has been much discussion recently on the relative energy efficiencies of various comminution machines, and this is as it should be, as comminution is such a high consumer of energy.
But does the type of particle breakage mechanism have an effect on subsequent processes, notably flotation? This is the subject of an interesting paper by workers from Australia's JKMRC, recently published in Minerals Engineering.
Stirred mills have been widely used for regrinding, and are acknowledged to be more energy efficient than tumbling mills, but these two types of mills present different particle breakage mechanisms during grinding. In the study, the effect of regrinding by both mills on surface properties and subsequent mineral flotation was studied, using chalcocite as the mineral example. A rod mill and a stirred mill with the same stainless steel media were used to regrind rougher flotation concentrates. Different chalcocite flotation recovery was achieved in the cleaner stage after regrinding in tumbling and stirred mills. The factors contributing to the different recovery included particle size, the amount of created fresh surfaces, surface oxidation and the redistribution of collector carried from rougher flotation.
It was determined that the predominant factor was the different distribution of collector resulting from different particle breakage mechanisms in the stirred and tumbling mills. In the tumbling mill, the impact particle breakage mechanism predominates, causing the collector to remain on the surface of newly produced particles. In the stirred mill, the attrition breakage removes collector from the surface, and decreases particle floatability. Furthermore, the type of grinding media in the stirred mill also influences the subsequent flotation, again due to the change of particle breakage mechanisms. The chalcocite flotation recovery was improved by the addition of more collector in the cleaner stage. More collector addition was required after regrinding in the stirred mill than in the rod mill to restore the flotation recovery to the same level.
The authors conclude that the selection of regrinding mills and grinding media should not only depend on the required energy efficiency, but also on the properties of the surfaces produced for subsequent flotation.
But what do you operators think? How significant do you think this effect on downstream performance might be compared with the economics of energy consumption? Has anyone else researched or observed the effects mentioned in the paper?
To me it is another good example of how comminution and flotation are inextricably linked.

13 comments:

  1. Thank you for the information. One should note that there are several breakage mechanisms in any tumbling mills. The predominant mechanisms range from impact to attrition. One is dominant in SAG other one is more dominant in ball milling. As all we know SAG uses mostly impacts, however ball uses both impact and good amount of abrasion that is supposedly take all the added collector from the surface. Single categorization for tumbling mills as impact is quite insufficient. In addition, from this article it is understood that ball milling is quite inefficient due to high abrasion. Any other choice?
    E.Tugcan Tuzcu, Dama Engineering Co., Turkey

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  2. Please see our research about regrinding mill in Sungun copper concentration plant. Its topic is “A new approach for evaluating the performance of industrial regrinding mills based on grindability and floatability” and published in minerals engineering journal, Volume 49, August 2013, Pages 116–120
    Hamed Dehgani, University of Amirkabir, Iran

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  3. I would like to split you question into two parts: first let us answer whether the choice of regrind mill and mill media affect the characteristics of the product from the total comminution circuit; depending on the answer to this, we should proceed to answer your full question.
    Rao,T.C., Inda

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  4. This is a very appropriate discussion, we were only discussing today how we can combine our fine grinding expertise with our flotation expertise, and ways in which we can optimise both processes as a package.
    Nick Wilshaw, Grinding Solutions Ltd, UK

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  5. Thanks Barry for posting this discussion here. I think this kind of discussion is a very good way to connect the academic fundamental research with the real plant operation. The feedback and comments from the industry experts will be an important guideline to our future research.
    These days, regrinding is becoming increasingly common due to the need to process more and more low grade and complicated ores. Regrinding not only further reduces particle size and increases the liberation but also produces new surfaces and changes the pulp chemistry, both of which are critical for subsequent flotation. Therefore, we believe it is important to study the influence of various regrinding conditions (e.g. grinding media, pulp chemistry, particle breakage) on subsequent flotation through the modification of particle surfaces.
    Apart from the effect of particle breakage mechanisms on subsequent cleaner flotation, we have also conducted some studies focusing on regrinding chemistry, some results have also been published on Minerals Engineering:
    1. The separation of chalcopyrite and chalcocite from pyrite in cleaner flotation after regrinding, Volume 58, April 2014, Pages 64–72 http://www.sciencedirect.com/science/article/pii/S0892687514000119

    2. Importance of oxidation during regrinding of rougher flotation concentrates with a high content of sulphides Volumes 66–68, November 2014, Pages 165–172

    http://www.sciencedirect.com/science/article/pii/S0892687514001241


    I believe there are still a lot of research work we need to do since regrinding is becoming more common and we will identify more problems. I hope we can get more interesting comments and discussions here, which will be highly valuable for our future research.

    Regards
    Xumeng Chen from JKMRC, Australia

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  6. As a grinding media designer we have had customers specify media chemistries as they have less consumption of their flotation agents in the downstream process. We did look at this closely and have found that some floatation agents do indeed have higher consumption with higher Fe ball content. In checking this we have found that other floatation agent makers do not have that problem. In conclusion the higher Cr balls were not needed, just changing the chemical supplier allowed the use of more economic media.
    Just one piece of your puzzle.
    Mark Addison, General Manager - Sino Grinding (Americas) Inc., USA

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    Replies
    1. Hi Barry, your question is certainly an interesting one.
      I can say that my experience from conducting flotation testwork programmes over the years has been that at least in some ores the type of regrind media used can have an effect on subsequent flotation. For example, gains in recovery and/or grade are seen when using a high-Cr or an inert ceramic-type media over that seen with mild steel.
      However, as Mark has mentioned above it would also be necessary to trial alternative reagents to determine whether the effect is consistently seen or an artifact of using a particular manufacturers product / type of reagent.
      Obviously any decision on the type of media to use in the plant will need to weigh up the additional costs of specialist media against any additional revenues from extra recovery or savings in reagent costs.
      Patrick Hegarty, Cornwall, UK

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  7. My previous post dealt purely with the chemistry of the media. The second issue is that of breakage mechanism. Alumina and other ceramic-type media are significantly less dense than steel. My experience is that using a similar charge size distribution in a similar tumbling mill will typically require a longer grind time to achieve the same target p80. The breakage mechanisms and charateristics must therefore be different and this in turn will have an effect on surface chemistry and reagent adsorption.
    So it appears that we are seeing both a physical effect in breakage characteristics and an effect from the chemistry of the media.
    My experience of media effects comes from conducting regrind tests during project testwork where we were only looking at results to determine whether grade and/or recovery could be improved in subsequent cleaner stages. Further investigative work would be required to fully understand the mechanisms involved and their relative contributions to downstream performance.
    Patrick Hegarty, Cornwall, UK

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  8. The efficiency of the classifiers is also important when we look at the regrind circuit and not just the mills. Creating slimes is not going to help flotation. The achieved classification efficiency at full scale can be quite poor (in my limited experience). Simple preventative maintenance practices can make dramatic differences.
    Thomas Rivett, Outotec, Australia

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  9. As usual, another insightful observation Barry, that will cause many of us to toss and turn at nights thinking about regrinding and subsequent down stream processing.
    I think this question is much broader than the just the choice of regrind mill and the media that it uses.

    If we think of a typical regrinding mill embedded in a flotation circuit the following usually occurs:
    * All or some portion of the rougher/scavenger concentrate reports to a sump where it is pumped to a cyclone;
    * The cyclone (or other classification device) separated the solids in the cyclone feed such that the fines report to the overflow and the coarse particles flow through to the regrind mill;
    * The coarse particles enter the regrind mill and undergo some form of size reduction as well as experience some surface chemical modifications (through interactions of the form: mineral-mineral; mineral-media; mineral-solution; mineral-gas; mineral-reagent; media-solution; media-gas; media-reagent; solution-gas; solution-reagent; gas-reagent; reagent-reagent. All will have an impact on the pH, Eh, dissolved oxygen content, temperature and oxygen demand of the pulp.);
    * The regrind mill discharge flows into a sump where it is pumped to a cyclone for re-classification (usually with the fresh feed to the regrinding circuit), where the finer fractions pass through to the cyclone overflow and the coarse particles head back to the mill via the cyclone underflow for further size reduction

    What the down stream process actually receives (in most instances) is a mixture of particles from different sources. That is:
    * A portion of the rougher/scavenger concentrate that is already below the cut size of the cyclone and report to the cyclone overflow without passing through the regrinding mill; and
    * A portion that is made up of coarse particles that have spent a variety of times in the regrinding mill to attain the correct particle size to escape the regrind mill/cyclone circuit.

    The surface chemistry of these two populations can be vastly different. For example, particles that by-pass the regrinding mill often have surface chemistry that is generated during primary grinding, conditioning and rougher/scavenger flotation. In some instances this surface chemistry may be less than ideal, and be contaminated with oxidation products, calcium salts and grinding media corrosion products that may make it difficult for these particles to be recovered during the first cleaner stage. Their flotation rate may be lower than optimal. Whereas, the freshly ground "coarse" particles may actually have cleaner, "pristine" surfaces that display excellent hydrophobicity and have superior flotation kinetics. When the two particle populations are brought together the interaction of the two different particle types may produce grades and recoveries that are inferior to those expected.
    /cont.....

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    1. ....cont/
      The choice of classification and regrinding mill technology (the combination of mill and media type) will give markedly different flotation characteristics. Most of the time these characteristics only become apparent once the operation starts up and commissions its regrind mill! This is because the equipment selection has been based on somebodies "best guess" of what size the regrinding circuit should be grinding to. In some instances this size is not based on good liberation data, but a historic perspective of what should work!

      What is apparent is that if the mill selection is completed based on achieving a particle particle size distribution, then tradition dictates that a tumbling mill will enable the user to grind down to around 20 to 25 microns comfortably, albeit inefficiently in terms of power. A tower mill (of some description) will enable the user to grind to P80 values of 15 microns, and the stirred mill technologies like the IsaMill and SMD will achieve P80 values down to 5 microns (at least), and can be used in some applications to replace ball tower mills.

      It is also apparent that current laboratory tests to determine the flotation response of a rougher/scavenger concentrate to regrinding are inadequate. In most instances, for simplicity the entire rougher/scavenger concentrate is reground in the laboratory to the desired particle size distribution and floated. This invariably showed a superior flotation performance compared to that achieved in the plant. The current laboratory procedures do not adequately take into account:
      * The actual plant circuit configuration;
      * The plant pulp chemistry; and
      * In may instances the correct liberation size.

      Regrinding is an important part of any concentrator, as it is an integral part of achieving a saleable concentrate grade, yet its design, simulation in the laboratory and application is poorly executed.

      Chris Greet, Magotteaux, Australia

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  10. Thanks a lot for the discussion
    Recently, we examined the floatability of regrind feed and discharge in the varies size fractions in Sarcheshmeh Copper Complex (Cu mineral was Chalcopyrite).
    We observed that, particle size decreased significantly due to regrinding while floatability of the feed was more than discharge. Moreover, floatability of each fraction (e.g. -400, +400-325 , ...) in mill feed and discharge was studied. Results indicated that, the floatability of all fractions in the mill feed was more than the mill discharge. it means that electrochemical interactions between the grinding media and Chalcopyrite let to decrease in floatability of material.
    Mohammadreza Garmsiri, National Iranian Copper Industries Co., Shahid Bahonar University of Kerman, Iran

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  11. What form do the electrochemical reactions take? I presume perhaps wrongly, you are talking about regrinds in mills using steel media not ceramic ?

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