Monday, 13 January 2020

Are modern flotation cells too big?

When I first saw the flotation plant at the Nchanga copper mine in 1969 I was impressed with the sheer scale of the operation, and the multitude of Denver Sub-A flotation machines arranged in parallel banks, each bank containing 20 1-m³ cells. A year later, what were considered to be huge 8.5-m³ Wemco Fagergen cells were introduced into the oxide flotation circuit.
When I made a return to Nchanga in 2012 all these cells had been replaced by much bigger machines, but even they were not of the size used on many large copper concentrators today. 
Nchanga's derelict former sulphide flotation plant
Nchanga's current sulphide roughers
The Applications Symposia of the last few MEI Flotation conferences have highlighted the continued increase in size of machines. The race to increase cell size has become almost a competition between manufacturers, the leading players being Metso and Outotec, who manufacture cells of over 600-m³ capacity, and FLSmidth, with the largest cell in the Western world, the 660-m³ SuperCell.
The FLSmidth SuperCell
At Flotation '19 Outotec described the commissioning of the first two Outotec e630 TankCells®, with 630-m³ of effective flotation volume, at Buenavista del Cobre Cu-Mo concentrator plant in Northern Mexico as the first cells in two existing rougher lines. Commissioning was finished in March 2018 and since start-up the plant has reported increased copper recoveries while maintaining the final grade.
Leading the race at the moment, however, is Chinese company BGRIMM Technology Group, who described the installation of the 680-m³ KYF-680 Machine, which is currently operating at the DeXing Copper Mine in China to reprocess the tailing.
But is the race ending and have machines now reached a limiting size? This was the question asked by Stephen Neethling, of Imperial College, UK, at Flotation '19, who showed that as cells get larger they become more carrying capacity constrained, while a paper from Eriez Flotation Division, USA, suggested that the approach of exploiting economies of scale and building increasingly larger unit operations is flawed, as there is a significant reduction in energy efficiency as the conventional tank designs become larger.

In last month's E&MJ, Carly Leonida conducted an excellent interview on flotation for the 21st century with Thierry Monredon, Metso's global manager for flotation. He felt that 600-m³ cells would definitely be the maximum size used in the foreseeable future, suggesting that it is often cheaper to have two banks of 300-m³ cells instead of one bank of 600-m³ cells.
Currently, it is operations with low grades and high throughput, for example, copper mines, that have opted for 600-m³ cells. “Grades have been decreasing, and to maximize efficiency, these operations need to have high throughput,” said Monredon. “In these cases, large diameter flotation cells are compulsory, but again, 600-m³ is really the limit. That’s clear for me, for our company, and I believe that’s what we’ve seen from our competition as well.”
So, what are your views on this?  I would be particularly interested in hearing from those of you who have had experience of working with such large machines.


  1. Very interesting and relevant point raised --involves many aspects and looking forward to comments from experts in theory and practice of flotation.

  2. Interesting points of view. Fortunately there are great technology companies with customers as partners to keep on pushing the boundaries of what is possible in all minerals processing technologies.
    Regardless the size, I am excited to see where we will end up at the end of this decade, then we will probably be surprised as to what will be achieved 2 decades from now.
    Exciting world of minerals processing is the best place to be!
    Dirk Slabbert, Director at SPS, Johannesburg, South Africa

  3. I certainly like the question being posed. The same thing with DMS cyclones they become larger for economies of scale and then the benefit of going larger starts dropping significantly.
    Ian du Plessis, Johannesburg, South Africa

    1. Yes, and what of grinding mills? I believe (may be wrong) that the largest tumbling mill is Metso's 12.8 m diameter SAG mill. Will these grow in size, or maybe become obsolete by the end of the decade?

  4. Size definitely saves space and more economical. But I feel factors like retention time (time it takes for a bubble laden particle) to travel from a given point in the cell to be out into froth launder is the eventual benchmark .

  5. Perhaps another question would be is there even a place for mechanical flotation cells in the future?. As per your article there has been little innovation over several decades in mechanical cells apart from making them larger and "tweeking" the bubble generation systems

    1. Good question Steve, but what might be the alternative to mechanical cells? Columns maybe?

    2. Hi Barry, how about pneumatic flotation? High collision frequency, very short residence time, small cell volume, and therefore high capacity!

      Duong Hoang

      Maelqwyn Mineral Services
      Helmholtz Institute Freiberg for Resource Technology

  6. I am still not recommending 600 m³ cells yet for a few reasons:

    i) They may take up smaller space, but the heavier weight of the cells requires larger foundations, so there is no saving on capex. Cranes to remove mechanisms for maintenance also need upgrades.

    ii) A large benefit that is marketed is that they consume less energy. I see this as a possible flaw rather than a benefit. If we look at the kW/m³, we see that all of that bubble-particle interaction is happening in a small volume around the impellor. This goes against the trend where all other modern flotation reactors are trying to intensify the shear in order to maximize the collisions.

    iii) The Buenavista paper shows that the performance was comparable, but at the start of roughing, where it *may* just have been scalping the fast floating particles, leaving the mids to the existing rougher train. In this way, we dont see how the big cell would have done with collecting slower floating particles. A line of large cells *may* not have had the same performance.

    iv) Finally, there has to be a compelling reason to go larger. Since 100% of your recovery are coming from these rougher cells, you need to make sure that they are good. If you have enough space (projects >100 kt/d may start to run into layout issues), and your throughput rate allows for say 5 to 10 cells in a comfortable bank, then why go larger?

    In large low grade copper roughers, the carrying capacity is not an issue, since the mass pull is relatively low. We do see it more of an issue in the cleaners. In roughers if your carrying capacity cannot keep up, then I assume that you are pulling like crazy, so recovery cant be an issue. The bit you miss will float in the next cell. Usually 80% of your mass collected is in the first couple of roughers, so they are area overloaded whether you like it or not.

    I'd be surprised to hear if there are still a lot of new columns going in given the recent market successes in pneumatic cells. Roughers and scavengers will likely remain mechanical, but cleaners seem to be leaning toward compact pneumatic cells, froth washing included!

    All of these opinions will change when evidence is presented.
    I had similar views on the 300 m³ cells until the Chuqui comparison was published!

  7. Adam's detailed note is very informative and gives a deep understanding of what goes on in a flotation circuits having roughers/cleaners and scavengers.
    Anything very big in an operating plant has problems when some simple maintenance tasks have to be done; more so when the quantity of feed availability at rated rate and characteristics vary as it happens so often in India.

  8. Pushing the adage "more and/or bigger is better" is always a two edged sword. I've seen many a mill being operated at higher than designed and produce below spec results. The push is more due to lower grades than anything else.

    My experience is the big tank cells are needed for throughput. I remember the Morenci flotation mill processing 80,000 tons per day with Denvers and they took up alot of real estate. The added benefit is tank cells act a bit like columns, creating better grades in the froth. The down side is fine particles that would float in a column cell have problems hanging on to a bubble all the way up when bounced around by an agitator. The bigger the cell, the more problems floating fine particles. As fine particle flotation is becoming a serious concern with the lower grade ores, I would think there is a definite limiting factor to the size of the cell.

    Bret Cousins, Consultant, Calgary, Canada


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