Thursday, 20 January 2011

Hydrometallurgy- is it ‘bucket chemistry’?

I know this will incur the wrath of some hydrometallurgists, but it is a question that has been with me for 42 years!

In 1969 I was offered my first job, as a metallurgical engineer with Nchanga. The interview in London was a very relaxed affair, where the young recruiter basically told me what the job of a mineral processor entailed and how wonderful it was to work in Zambia. I expressed reservations that I knew nothing at all about copper mining or mineral processing, but he shrugged this off with “don’t worry- it’s only bucket chemistry”, which really didn’t clarify matters.

Nchanga Tailings Leach Plant, 1972
It was 4 years later that this statement came back to haunt me. My first two years on the concentrator had introduced me to mineral processing, which was exciting and dynamic, following the mined ore through crushing and grinding to its separation into concentrate and tailings. After this, the practical experience commissioning the tailings leach plant had been invaluable, but my final few months spent on the high grade leach plant felt like being put out to pasture. I found the process dull and static compared to mineral processing, the chemistry was fairly simple, and metallurgical testwork was tedious, mainly involving stewing concentrates with acid of varying strengths for varying times.

I have always felt that comminution is the most important process in ore to metal, and so should be the most intensively researched. It is not however- in 2008 forty comminution papers were published in peer-reviewed journals, froth flotation, the most important mineral separation technique, accounted for 96 papers, and 180 papers on hydrometallurgy were published. Of all the minerals engineering disciplines, hydrometallurgy is the only one with its own dedicated peer-reviewed journal.

A major reason for this intensive hydrometallurgical research effort is that in the 1980s there was a revolution, with the development of a new branch of hydrometallurgy, biohydrometallurgy.

However, another reason contributing to the profusion of hydrometallurgical research is that it can be relatively simple and cheap to perform. Comminution research requires expensive equipment and specialist researchers, while hydrometallurgical research at its very basic level, such as simple leaching, can be carried out in facilities of school chemistry standard with researchers having only a rudimentary knowledge of chemical principles. Although there are many excellent institutes devoted to high quality hydrometallurgical research, there are many more low calibre institutes around the world producing equally low calibre scientific papers which flood the editorial offices of reputable journals. Minerals Engineering, for example, rejects around 60% of all received papers, a very high proportion being hydrometallurgy related. The majority of these involve very simple experiments, involving stewing minerals in acids of varying strengths and temperatures, and reporting the results; identical work, in fact, to the routine testwork during my last months at Nchanga.

My transfer to the leach plant was the catalyst which influenced my decision to leave Nchanga and Zambia in August 1973, and I finally knew for sure what the recruitment officer was referring to when he said “it’s only bucket chemistry.”


  1. Interesting blog I agree with you that it can be but on the other hand to get really efficient processes at low cost " bucket biology" or "bucket Chemical Engineering" might also be applicable.
    Nevill Rice, Independent Education Professional, UK

  2. A very apt description for a discipline very much underpinned by chemistry and the need to study these processes on the kg and L ("bucket") scale due to the inhomogeneous nature of ores. Begs the questions though to why there isn't more chemists in the industry????
    Paul Breuer, CSIRO, Australia

  3. Thanks Nevill and Paul.
    I suppose the main purpose of the blog was to highlight the amount of low calibre work submitted to peer-reviewed journals, putting unnecessary pressure on editors and reviewers. You are two of my best Minerals Engineering hydrometallurgy reviewers, but the papers that I send to you for refereeing are those which have a good chance of final acceptance. A very high proportion of hydromet papers submitted are of the ‘stew it and see’ type that I refer to in the blog. These I reject without recourse to referees. Interestingly they nearly all end with the same profound conclusions:
    1. Leaching rate increases with increase in temperature.
    2. Leaching rate increases with decrease in particle size.
    3. Leaching rate increases with increase in stirring speed.
    What more can I say?

  4. Nevill Rice • Perhaps one is getting out of the bucket stage when autoclaves and high pressure are needed but the chemistry under these conditions can be very interesting e.g. the classic bayer and Sherritt Gordon Ni/Co processes. The best practical bucket chemistry (literally) that I have seen was at a site - I won't honour it with the name plant - belonging to Coogee Chemicals near Perth making reagents like copper sulphate and sodium silicate.Their main tools seemed to be hoses and shovels apart from the fairly large buckets/small tanks.
    I am pleased you are filtering the submissions Barry - I would be most surprised if they came to any other conclusions than the ones you listed. I can't recall if it was for MEng or Hydromet but one submission I reviewed a few years ago didn't even bother with graphs but just included a few colour photos of beakers with varying shades of blue solutions. I have yet to see it published anywhere.
    Nevill Rice, Independent Education Professional, UK

  5. I was hoping for this topic to arise. This is because of the misconception surrounding hydrometallurgy whereby it is a simple, clean technology, easy to grasp in general and by chemists/chemical engineers in general. Let's throw in the pure academics and stock promoters and we have the complete picture. Well, bad news across the board: hydromet has nothing to do with bucket chemistry, unless you have a robust batch to continuous modelling tool, yet all models are just models, unless they are backed by Real Data, at each at every step. Hydromet plants are entirely flow-behavior dependent. Everything must flow and the phases must separate, the discharge streams split, recycled upstream/directed downstream, and you need to close all balances, including water . What you put in must come out, and if possible, come up with a product. A real life thickener/CCD/filter system does not even remotely resemble a benchtop conceptual leach-wash set-up....nor does a continuous pressure autoclave have much in common with the little Parr, as effective as it is for a few base line tests....Bottom line: chemistry is predictable, physics is not...I learn this all over again at each plant commissioning....Need to know more?....please write to cheers, Alex.
    Alex Mezei, SGS Mineral Services, Canada

  6. Why are so many lousy hydrometallurgy papers submitted? Simple. As you said, hydrometallurgy experiments are easy to perform but crushing, milling etc, are not. But there still should be more hydrometallurgy papers because it is so important. Why?
    1. The further you go away from the ore, the greater the variability in process selection. There are not that many ways to crush/mill say, a Cu ore. But how many ways are there to get to Cu metal e.g. flotation (and all of its complexity) through to concentrate; countless numbers of Cu leaching options, SX variations, EW, etc. And all of this for a complex range of Cu minerals in a complex range of gangue minerals which all greatly complicates both the chemistry and hydrometallurgy.
    2. Having invested the capital in getting the milled ore to the “recovery” and perhaps adding all that acid into your plant, the value of a marginal 1% extraction/recovery can easily be 10X more valuable than an extra 1% throughput. Hence, the hydrometallurgical process has huge leverage. It all comes down to NPV.
    But there shouldn’t be a competition between mining/mineral processing disciplines. We’re not training enough professionals for of any of them and nor are we doing enough R&D in any of them. And the biggest mining/mineral processing companies are often the worst offenders. Do we promote the industry well enough to combat the BANANAs (build absolutely nothing anywhere near anything) and to promote an exciting, valuable, responsible, intellectually complex industry to bright young teens entering their final years of high schools? After all, do we really need any more forensic scientists and lawyers?
    Stephen Grocott, Rio Tinto, Australia

  7. Seeing as modeling has come up in discussion even if its not the main thrust of the article, and I have been involved modeling copper and gold heap leach systems for a very long time I thought I'd add a couple of comments.

    The biggest problems that tend to arise in modeling leach systems is either that people try and publish model results with no practical data to back it up (something that can occasionally be understood due to commercial companies being somewhat reluctant to allow data to be published) or that it can be surprisingly easy to generate something that looks right for a single set of data. Getting a model that is, say, properly scalable from lab to full size heap is possible but its not at all easy

    The biggest problems from the commercial side is that people dont always realise that you cant model that which you dont fully understand, and that a model can still be useful even if it does not give the right answers.
    Chris Bennett, Swansea University, UK

  8. Dear Barry. You have raised a nice question which looks to be difficult to answer. Although I am not a hydrometallurgist, however, my opionion is that hydrometallurgy includes "bucket chemistry" but it doesn't stop there. When it comes to the selection of best suited processes it is "bucket chemistry/batch testings", but when it comes to apply the results for designing or optimization of a real plant (involving continuous processes and reactors design) it becomes "minerals or hydrometallurgical engineering". Bottom line, I think the answer is yes and no.
    Akbar Farzanegan, University of Tehran, Iran

  9. Can of worms opened now - hydrometallurgy rests on chemistry, but goes far beyond plain chemistry, as Alex points out. Bucket is still OK as a term, but think big bucket - like 1 000 m3 leach tanks in some bioleach operations. Try running one of those as a batch operation...

    Modelling is how I make my living, particularly hydrometallurgy modelling. The biggest risk in modelling is believing the model. You absolutely have to back up any model with good hard data - anything else is a great disservice to modelling and to hydrometallurgy. Modelling makes great sense, but use it to guide experimental work, then plug the real data back into the model and keep at it till the model replicates the data beore you use the model output for anything else.

    As experienced people become more scarce (we greybeards aren't going to be around forever, and industry has allowed a huge experience gap to form) and computers more powerful, I reckon we're going to see modelling becoming much more of a central tool, so let's all use it properly and not go discrediting something that hydrometallurgy is going to need ever more as we all ride off into the sunset...
    Mike Dry, Arithmetek Inc., Canada

  10. richard afoakwa akowuahBarry,As mike said even if you consider Agitation in leaching (i.e. both CIL and CIP), in practice it is carried out on slurries in continous leaching configurations involving several tanks.since selection of agitation system will depend on several factors at each batch stage which includes: viscosity,solid settling rate,pulp density,Air and oxy injection in all it could be yes or no depending on the exact process
    Richard Afoakwa Akowuah, Mining & Metals Professional, Ghana

  11. Mike, I agree with the comment that believing the results from your model is crucial. I have been involved with a feasibility project for the last year and included modelling and designing of a process based on the Sherritt Gordon Ni/Co leach process. What we found is that, for a process of this complex nature (and any process really..), modelling of various scenarios are extremely important. Understanding variability in things like feed mineralogy, operational changes etc. are all important for design purposes and if you don't use software that support quick convergence to handle numerous scenarios, you will not get enough information to get the full picture to properly size the equipment. The variance in reaction kinetics for different scenarios unfortunately is not that easy to predict in these models, and are all based on experimental or production data that could be at a different states of equilibrium. So the information you obtain from modelling are predictive, but definitely have limitations and the impact of that needs to be understood. That is where the years of experience comes in on which we younger ones rely on!
    Lisa Roux, Hatch Africa, South Africa

  12. Lisa thanks for the interesting comments. I would like to add some ideas here. The first and most obvious is that a model is only as good as its inputs, if you have to make many assumptions due to a lack of testwork data the model will have some shortcomings. Many people think they can replace testwork by doing models. I would suggest that this is not the case and that good testwork leads to good models.

    The chemistry principles must also hold in the models and I know modelers go to great lengths to make sure this is indeed the case. Most difficult is the interaction and interaction coefficients that are part of complex metallurgical systems. I think we need to obtain more inputs from operating plants, the models of operating plant can be a good starting point. My hypothesis is that we can obtain the interaction coefficients from the operating plant and the conditions at which they would apply. Does this make sense?

    The I must agree with Mike that as skills become more scarce we will rely more on models. These models will become great teach tools, training tools, control tools and even design tools if we can do the correct testwork to find the critical model inputs.
    Herman Schwarzer, Lycopodium Minerals, Australia

  13. Definitely room for the use of models in teaching. At their best they contain the knowledge of a lot of smart people.

    Model development is often driven by the fact that a process is either no longer fully understood - i.e. the people who did understand it have moved on and are no longer available - and building the model can be a way of recovering that understanding. The other situation is that there are a lot of people who are experts in their little part of a process but dont fully appreciate the interactions between all of the different parts. Models formalise the knowledge available (i.e. convert it into maths) and will in effect hold that knowledge for as long as the model is used and maintained.
    Chris Bennett, Swansea University, UK

  14. I think Chris has expressed an important idea extremely well. What I've seen quite a lot over the years is that, particularly for a novel process, people focus on the new bit and don't really examine how it interacts with the other unit operations. One example was in bioleach - the lab work looked great but when the whole circuit was piloted the water balance didn't. Overflowing tanks all round. In that case the circuit was modelled, but using a spreadsheet and too many over-simplified assumptions. When the guys went back and modelled it using Aspen Plus, the water imbalance showed up clearly. Could have saved an expensive pilot plant campaign from being a bit of an ambarrasment...

    Mike Dry, Arithmetek, Inc., Canada

  15. Looks like we're morphing this discussion into modelling. That OK, Barry?
    Mike Dry

  16. Mo problem at all Mike. Good to see healthy debate in the group, which is what I hoped for when I asked the initial provocative question.

    Anyway, I am used to going off on tangents, as I am sure many of my past CSM students will testify.

    Keep talking!!

  17. With a couple of exceptions most of hydromet is just as Barry described it. Gold cyanidation and copper oxide leaching are generally extremely boring. The interesting cases are what us consultants usually spend all our time on and in my experience very few of them get off the ground. In the last 20 years it was either bucket chemistry or it did not go ahead (many of the Ni laterites should not have gone ahead but did, so point still proved). There are the odd successful exceptions such as Sepon, Tenke and Skorpion, but look at all the massive investments in copper autoclave leaching, laterite leaching, platinum leaching, thiosulfate gold leaching, zinc oxides leaching, tank bioleaching, etc, etc. that have just never gotten out of the laboratory. Pyro won over almost all of them.

    In conclusion, Hydromet can be very interesting, but not usually in industrial applications, just in development that leads nowhere. (If my clients read this: this does not apply to your project!!!)

  18. I am a little bit late, but feel necessary to comment. "Looking at hydrometallurgy from the narrow hole of leaching" could be a good topic for this discussion. I am sorry that no real hydrometallurgist has commented here, so I have to remind you a couple of important issues;(I do not consider myself a real hydrometallurgist, but have been in the business enough to share some comments);

    1. Hydrometallurgy starts with leaching that could be the easiest or hardest bit, but continues with separation and purification and recovery. Despite almost a century of developments, these areas are still evolving.

    2. Adam- Pyro processes: which new smelter has been builtrecently? Perhaps, all heap leach, dump leach, bio-leach, and stirred tank leach processes are Pyro? is that a new classification?

    3. Recent trends show that the future of non-ferrous industry has been tied to hydrometallurgy, because of economical and environmental issues.

    4. Can you list some of the most important contributions of "comminutors" over the past 50 years?(other than SAG mills and HPGR)

    5. Hydrometallurgy is a new science, and therefore more evolving in terms of research. Arguably, there is more pure science in hydro than crushing/grinding.

    6. I think it would be better if we try to improve the existing knowledge with some constructive discussions instead of discrediting a particular section, just because we do not like/understand it.

    Then a hydrometallurgist may call comminution "hammering" or flotation "mud chemistry". Regardless of what we call them, all of these are important.

    Journal of mineral engineering goes its way, and hydrometallurgy goes its own way as well. Whether which one is more successful, or attracts more readership in the field of hydrometallurgy will be judged by the scientific community. But personally, if my paper is rejected from hydrometallurgy, I would never submit it to a general mineral processing journal like Minerals Engineering.

    Let's widen our knowledge...

  19. It's a pity you didn't leave your name, anonymous, as it is difficult to argue with a faceless one.

    If you had read my initial post, you would see that I was not griping about hydrometallurgists in general. I said "Although there are many excellent institutes devoted to high quality hydrometallurgical research, there are many more low calibre institutes around the world producing equally low calibre scientific papers which flood the editorial offices of reputable journals."

    My tirade was against the legions of institutes around the world who carry out simple 'stew it and see' tests, because they are simple and cheap to perform, and require little scientific knowledge or methodology. Subsequent papers flood journal offices, not only Minerals Engineering, but also Hydrometallurgy, as I am aware from discussions I have had with their editors.

    There is much splendid work in hydrometallurgy, as there is in comminution, but such work is performed by responsible, high calibre workers.

  20. Hi Barry

    The fields are fundamentally different. New developments in minerals processing are centered on a new machine - HPGR, Knelson, and the list goes on.

    New machinery is not that evident in hydrometallurgy. It is boring, just tanks, and then there are those odd things called autoclaves.

    And you are right, the Hydrometallurgy journal is flooded with papers from "stew it" type research. And all that bio stuff – is it really hydrometallurgy? Should it not be a bio journal somewhere else?

    I have been a hydromet for almost thirty years now (since 1984). There is an alarming tendency for those outside of hydromet (min proc, pyro) to associate hydromet with leaching. This is so very short sighted. What makes hydromet interesting is not that I can bioleach this or that, but how does this fit into a whole process, one that recovers the metals and produces high value products, rejects the impurities efficiently and recycles the reagents in an optimal manner.

    Hydromet is very different in another respect: generally in min proc and pyro there is a part-by part replacement: HPGR vs same other crusher, electric furnace vs flash furnace, etc. In hydromet, what is it? This tank for that tank? No, it is more abstract, this chemistry for that, but not only in this tank, but in the whole process. Much more difficult for a young engineer to say I commissioned this plant, because once we are operating, there is usually no major change for 30 years! No part-by-part swop out, but whole-by-whole!

    And then there is my observation that the min proc and pyro guys that I have come across do not get reagent recycle. Sure water is recycled in float plants, and there are reverts in the pyro guys world, but wholesale recycle of the reagents is not present. How that effects the operation and control of hydromet plants, is a mystery to most who are not hydromets. Balancing the water, acid, spent etc, in a complex refinery like nickel-copper refineries is not as trivial as it seems.

    While I am on a rave, there is another major difference. Hydromet plants are usually pull-plants, not push-plants. In min proc and pyro, you push material through, getting production up by forcing material through the crusher, mill etc. Hydromet plants are usually pull plants - you cannot get production through unless the conditions are right, and then once they are right, only then can you let the plant pull more feed through. Okay, a subtle concept, but there it is, one that the journals will never publish if I wrote on it! Can you imagine the reviewer’s face, sitting at his/her university desk? Reject!

    All the best, Barry, and thanks for all your good work,

    Frank Crundwell
    CM Solutions, Johannesburg


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