As Outokumpu Technology’s Andrew Okely (System Sales Manager – Minerals Processing in Australia) and Peter Bourke (Global Technology Manager – Flotation Process) point out, in recent years, many headlines in flotation have been increasingly devoted to advances on the size front. Technology suppliers strive to design greater capacity, higher performance units and ensure that existing technologies do a better job. Outokumpu Technology’s TankCell® is a case in point.  The successful TankCell family, whose cells range from 5 m³ to the previous highest of 200 m³, has recently added a 300 m³ model – now the largest flotation cell available in the world [but not yet in operation]. So, with a 300m³ cell in the market, the obvious questions to ask are do large flotation cells offer anything new to the marketplace and is biggest always best?

Well, like most things in life, the answer is yes – and no.  Undoubtedly, there are both economic and processing benefits to the innovative use of proven technologies such as larger cells.  However, size does actually matter and, in some instances, smaller units actually do the job better.

Consider a plant requiring 1, 800 m³ of rougher/scavenger volume.  Here are four possible scenarios for the provision of this volume:

• 18 x 100m3 cells, in two rows of nine
• 12 x 150 m³ cells, in two rows of six
• nine 200 m³  cells in a single line
• six 300 m³  cells in a single line

The table above indicates the relative capital cost (ex works), footprint, installed power and air requirement for such a scenario.  Ten years ago, 100 m³ cells were the most powerful and efficient ‘new kids on the block’.  Yet nowadays, had we used the 100 m³ cells versus the latest 300 m³ units, it would have cost 50% more. The advantages in capital cost are therefore significant, but the savings do not just exist in initial acquisition of the larger cells. Footprint is also an obvious area for cost saving  – 18 x 100 m³ units use 117% more space than 6 x 300 m³ units. Power and air requirements for the 100 m³ units are also higher – at 28% and 50% respectively. And the economic advantages of larger cells obviously continue in areas such as maintenance and servicing of the units.   

When smaller can be better….

Demand for larger cells has in part been driven by the desire to exploit larger, lower grade orebodies. Economies of scale like those demonstrated above have made lower cut-off grades for orebodies feasible, despite a decrease in the real value of most metals [until recently]. In many cases, the use of larger-sized technologies has meant a compromise on process design. Plant surges, lack of circuit flexibility and lack of redundancy have been difficult issues for large throughput circuits. 

Froth stability is critical to good flotation performance and is almost impossible to achieve without a minimum level of mineralisation in the froth. Higher grade ores need more froth surface area to remove sufficient froth and therefore maximise recovery. There exists a limit to the mass of mineral a froth can carry out of the cell, exceed this and your recovery will suffer.

Thus recovery from higher grade ores can be restricted if cells are too large and lack the required froth surface area.

However, the converse of this is also true. The use of larger flotation cells for low grade orebodies has provided a distinct process advantage, namely greater froth stability. As the froth surface area to cell volume ratio is lower in larger cells, they proportionately have less froth surface area to stabilize and are particularly efficient at floating mineral from very low grade orebodies. Thus, on low grade ores, reducing the overall froth surface area and the mineral mass required to stabilise that froth provides for significant improvements in flotation cell performance. The new 300 m³ cells are particularly suited for roughing and scavenging duties on low-grade ores due to this low surface area to cell volume ratio. Some suppliers have further enhanced the froth stability in larger cells through innovative froth-creating technology and customised launder designs.

Large cells must, however, also maintain their efficiency at recovery of both coarse and fine particles.  To this end Outokumpu Technology, for example, has not just one flotation mechanism but offers a range of proven mechanisms and launder styles to best suit specific duties.

Where’s the limit? Like all things in mineral processing, the answer depends upon the ore. As a rule, caution should be exercised and froth surface area considered rigorously when selecting cells for ore bodies where the headgrade is 10% of the concentrate grade or greater. The largest available cells may not be the most suited in this case.

Interestingly, the largest mechanical cleaner cells (a duty where high percentages of the feed mass are generally recovered to the concentrate) are presently 100m3 in volume. This limitation is the result of the froth area restriction that large cells have, inherently. Overcoming the relationship between froth surface area and cell volume, i.e making the froth removal step an order of magnitude more efficient, is one of the challenges for the next generation of flotation cell designs.