When flash flotation and gravity recovery are both used, how should they interface?

In the first section, the use of gravity to recover coarse gold and flash flotation to recover fine gold and sulphides is discussed. The most frequent joint application of the two recovery methods is the use of gravity recovery to recover GRG from the flash flotation concentrate. The incentive for such treatment usually lies in the increased NSR obtained with very high-grade gold concentrates. The overall evaluation of the benefit of gravity in this case should include the potential lower payment of gold left in the flotation concentrate, because of its lower grade.

Gold Flash Flotation Ball Mill

A second, often overlooked, incentive lies in the minimization of gold losses from the flash flotation concentrate, when further upgrading is required. The two objectives (maximizing NSR and flotation recovery) are not always easily pursued. Figure 2 shows two flowsheets, each aimed at maximizing one of the two objectives. The first flowsheet redirects the gold room tailing to the grinding circuit for additional GRG liberation or gravity recovery. Note that “coarse” gold losses are more likely to occur, because flotation cleaners, with their typically deep froths, are not designed to recover coarse gold. If the flash concentrate requires cleaning, it makes more sense to direct the flash flotation concentrate to a dedicated cleaner stage, rather than the overall cleaner treating the conventional rougher concentrate. This is shown in the second flowsheet of Figure 2. Such circuits are small thus inexpensive and often treat very high value products whose grades are much higher than the rougher flotation concentrates produced by conventional flotation. Thus combining these two very dissimilar products makes little sense. Similarly, because the gold grade of the gold room tailing is either equal to or greater than that of the final flotation concentrate, it is added to it, to minimize gold losses. At Cadia, the tailing of the Falcon SB52001 that treats the flash flotation concentrate is directed to a dedicated cleaner circuit that achieves a recovery of 98%. The gold room tailing is added to the final copper concentrate, after scavenging with a small Falcon SB unit.

The use of a regrind circuit treating a high-throughput, relatively coarse rougher concentrate stream is becoming common for low-grade gold-copper ores. When such a circuit is used, gravity recovery from the circulating load of the regrind circuit is very attractive, but it should be pursued with high-Gs units, typically 150 Gs or more, owing to the very fine size distribution of the GRG.

The potential user must understand the two hurdles to overcome when processing flash flotation concentrates with batch centrifuge concentrators (BCCs). Firstly, flash flotation concentrates have a high sulphide content, which is the “gangue” from which GRG must be recovered. These separations are inherently more difficult than recovery from a silicate gangue. Secondly, and maybe more importantly, whereas recovery from the primary circuit is a multi-pass application, flash flotation concentrate are usually processed only once by BCCs. This can lower gravity recovery very significantly, especially for GRG finer than 37 μm.

At the Cadia mill, gravity recoveries of 15 to 18% are achieved with a Falcon SB4000 and gold room, compared to a GRG content of 68%. The SB4000 recovers approximately 50% of the gold in the flash flotation concentrate (which contains 30 to 40% of the gold in the ore). The low overall gravity recovery is much more a reflection of the low flash flotation recovery and the single-pass nature of the application than any failings of the Falcon SB. The low flash flotation recovery is largely due to the coarse grinding size, a Pgo of 200 to 220 pm, which fails to keep the finer GRG in the circulating load. This will be analyzed later.

At the Mourro do Ourro plant, high gravity recoveries (over 50%) were achieved by treating the flash concentrate of an oxide ore with three BCCs in series (to minimize the drawback of the lack of circulating load). The BCC concentrate was then processed on a shaking table, although large pans were later used, providing some 10% more recovery. Rare are the applications that can justify such a recovery effort. When the oxide ore was mined out, the gravity circuit was shut down because the sulphide ore that is now being treated has a lower content of finer GRG present is a higher s.g. gangue (i.e. the flash flotation concentrate).

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