Maximise your recoveries in a flash

Flash flotation is the instantaneous flotation of liberated, high grade `particles from the recirculating load in the grinding circuit. The concept has been around for almost eighty years, although it has only been recently that large scale flash flotation circuits have become a reality. The process is widely known however not widely understood.

 

 

 

As valuable minerals are soft in comparison to the host rock, with a significantly higher solids density, they tend to concentrate in there-circulating load of the grinding circuit.

 

This can lead to the over-grinding of the valuable minerals until they report to the cyclone overflow, which in turn can lead to lower recoveries in the flotation circuit. In comparison, the flotation conditions are significantly better in the cyclone underflow stream than in the cyclone overflow (flotation feed) due to the presence of fast-floating liberated particles.

 

The cyclone underflow particles also tend to have cleaner, less oxidised surfaces and are not too fine for flotation (i.e. less slimes).Therefore, installing a specially designed flotation cell in the grinding circuit can significantly improve plant performance. There are various flash flotation designs in the marketplace, but as this process is quite complex to get right, it is particularly important to chose a supplier with experience and a proven track record. The typical design of Outotec’s flash flotation, SkimAir is shown in Figure 1.

 

 In the SkimAir flash flotation units, the feed to the unit comes from a distribution box which splits the cyclone underflow. Typically, one-half to two-thirds of the cyclone under-flow stream is diverted into the SkimAir, with the remainder of the stream returning to the mill feed. Dilution water is also added to the feed to reduce the solids density to approximately 65%.

Although optimal flotation performance is at a lower solids density, this would require more dilution water, and as the majority of feed reports to the tailings, the additional water returns to the mill, reducing the mill throughput. Therefore, it is a balancing act between mill throughput and flash flotation performance.

 

The suitably sized particles in the feed are contacted with air in the mechanism, the fast-floating liberated particles are floated to the surface and then recovered to the concentrate. The mass pull to concentrate is typically only 1-2% and can be of high enough grade to be directly mixed with the conventional circuit final concentrate.

 

The particles in the feed that are too coarse to suspend and float are segregated to the tailings via a conical-shaped tank floor. These particles simply by-pass the SkimAir flash flotation unit and return to the mill for further grinding. Therefore, the size of the particles in the feed to the unit can be as large as 10mm.In other words, the unit acts as a flotation cell and a classifier.

 

The design of a suitable flash flotation cell requires extensive experience. There are many pitfalls when treating such high solids densities and a wide particle size distribution. These include poor solids suspension and sanding and inadequate mixing of water and solids in the feedpipe.

 

Additionally, the froth booster cone needs to be correctly sized to ensure the correct concentrate flow and grade is achieved. A poorly operating flash flotation circuit can have a significant detrimental impact on mill throughput and performance.

 

It is now clear that the flash flotation can pre-vent the over-grinding of valuable minerals and the generation of slimes. Another important advantage of using flash flotation is that the SkimAir provides a buffer for the conventional flotation circuit when the feed grade is highly variable. When feed grades are high, the SkimAir can recover the additional metal to produce a more stable feed to the conventional flotation circuit. This advantage is often overlooked.

 

As the fast-floating liberated mineral is removed in the grinding circuit, the footprint of a greenfield’s flotation circuit can also be reduced. The SkimAir concentrate particle size is also coarser than conventional flotation so when added to the final concentrate it improves concentrate dewatering, lowers filter costs and reduces final concentrate moisture.

 

There are two main flash flotation circuit options depending on the ore characteristics. The first method is conventional flash flotation, where the SkimAir unit is used as a stand-alone cell to produce a high-grade concentrate (see Figure 2).

 

The second main method, Flash Roughing, unique to Outotec, comprises two stages, with the SkimAir as a rougher, operating at longer residence times and higher mass pulls so as to produce a lower-grade concentrate at high recoveries. The SkimAir concentrate is then treated through a Flash Cleaner TankCell to produce a high-grade concentrate. If the SkimAir concentrate is high in free gold, a gravity separation device is installed between the Flash Rougher and Flash Cleaner to recover free gold directly to the gold room.

 

Flash flotation technology has successfully been applied to the processing of many minerals –with the SkimAir, for example, there have been significant successes in applications such as gold, platinum group metals, copper, nickel, lead and silver.

 

So, how do you know if your process will benefit from flash flotation? Firstly, you need to perform laboratory flotation tests. Taking Outotec’s specialised test procedure as an example, if the laboratory float shows high recoveries for the valuable mineral (40% to 80%) with low overall mass yields(less than 5%) then flash flotation will most likely be suitable.

 

Secondly, a good understanding of the ore mineralogy in the cyclone underflow is also a necessity. Lastly, if required, pilot-scale units can be used, some capable of treating between250 kg/hr and 3 tph of solids. The results can then be modelled using computer simulation software, from which full-scale performance can be predicted.

 

 

 

There have been over 200 SkimAir units installed in operations world-wide. Some recent case studies are described in Table 1and shown in Figures 3, 4 and 5.

 

Flash flotation is not new. The flash flotation process is highly complex, so an experienced supplier is recommended. It is an exact science to optimally design flash flotation cells to handle such coarse particles with high solids densities, yet still keep them running with high availabilities. However for a minimal investment, the performance improvements can be staggering.