Eriez's StackCell technology uses two-stage flotation.

Froth flotation is the most important industrial process for upgrading various mineral systems. Most operating concentrators have a flotation line. Most of these operations use mechanical tank units to meet most of the flotation requirements.

Slot batteries have some advantages. First, they can float incompletely dissociated ore, thereby achieving initial separation with high recovery rates. However, this does not necessarily reach a practical level of abundance. Secondly, they are relatively robust and easy to handle, which facilitates handling of ore variability and operator training. Third, the operation of the stirred tank unit has been proven to be scalable, in other words, the feed to the unit can be increased or decreased by making the unit larger in a well-known manner. Finally, the magnification rules are simple and time-tested. This means that flotation performance can be tested in batch mode on very small laboratory equipment, and the results can be reliably extrapolated to large processes.

Despite these advantages, there is a significant trade-off between lower efficiency and higher cost.

It is generally believed that the flotation separation process depends on at least two consecutive steps. In the first step, ore particles and bubbles effectively collide to form aggregates.

When these bubble particle aggregates gently float to the top of the device and add the foam phase, the second step occurs. The first step is to induce turbulence by adding a large amount of mechanical energy, thereby enhancing collision and adhesion, especially for fine particles. In contrast, the second step is improved by low-energy static fluid conditions, especially for coarse particles.

Therefore, in a traditional tank, the input of kinetic energy is selected as a compromise between the recovery of fine particles and coarse particles. In practice, tailings are usually enriched in both size classes, as shown in Figure 1, which illustrates the size of the metals contained in the final tailings streams of world-class copper concentrators.

It can be observed here that about 85% of the unrecovered ore is in the fine and coarse range of conventional flotation.

Due to this compromise, the ore particles may combine with bubbles, then be accelerated by the fluid (called "falling back" in flotation terminology) several times and then be successfully removed during the froth stage. This internal circulation in the unit will cause the overall efficiency to decrease, which can only be compensated by increasing the residence time in the cell. Compared with the flotation cell, increasing the volume of the flotation cell by making the flotation cell too large has a direct effect. Influence. More effective process engineering applications can be used for design and operation.

The solution to this problem has been determined before.

The basic idea is to divide the flotation cell into two independent isolation cells. In the first tank, the fluid environment is optimized for high-bubble particle contact (called "particle collection" in flotation terminology). In the second tank, the fluid environment is optimized for static density separation, thereby minimizing the number of particles captured in the internal "return" loop. This type of flotation equipment is sometimes referred to as a "two-stage flotation cell".

Eriez developed and patented a two-stage device called StackCell in 2008. Today, the company has more than 30 comprehensive installations in the field of coal flotation. The cross-sectional view of Stack-Cell is shown in Figure 2. The feature of this design is that there is a tank inside the tank. Air and feed slurry enter the inner tank where they are mixed with the high shear rotor and stator mechanism. The inner tank is bounded on all sides, and the aerated pulp can only exit through the narrow gap between the top of the tank wall and the rotating lid. This special design means that when bubble particle aggregates form and flow through the inner tank, they cannot return. In the outer tank, low energy is ideal for promoting the density separation (flotation) of bubble particle aggregates, and since there is no mechanically induced forced convection, "fallback" is minimized.

A few years ago, a 1.2-meter-diameter device was tested for gold sulfide flotation in South America. In comparison, StackCell runs side-by-side with traditional mechanical cell electrolyzers, and can achieve a considerable degree of metallurgical separation (measured by grade and recovery rate) within about one-sixth of the residence time of traditional electrolyzers. In another recent side-by-side comparison, at the site of a copper porphyry beneficiation plant, a row of three StackCells was operated together with a set of five 8-meter-diameter conventional rough removal cell units. This article summarizes this comparison. The complete experimental methods and results were presented at the Canadian National Mineral Processors Conference in Ottawa earlier this year.

In this comparison, continuous sampling can be performed at the feed and outlet (tails and concentrate) of the coarse tank and the clear tank. Receiving the same feed is a sequence of three StackCells with a diameter of 0.6 m, as shown in Figure 3. Upsampling on each of these cells allows researchers to approach the plant coarsening and scavenger library and the mass balance around each StackCell under the same feeding conditions.

Finally, the pulp from the factory was taken to a commercial laboratory and used for batch Denver testing. This is a standard test used to evaluate the performance of industrial tank batteries.

Generally, industrial tank batteries require two to three times the retention time to achieve the same degree of separation as the Denver batch test.

The typical recovery results of this test are shown in Figure 4. All three flotation units produce concentrates of comparable grade. In this figure, the dynamic response of StackCell sequence (diamond), Denver batch test (square), and commercial cell (triangle) are shown.

As expected, the multiplier for converting the Denver curve to a commercial tank battery is about 2. This is a well-known "rule of thumb" for the amplification of the time-tested mechanical tank battery discussed in the introduction of this article. The big news is that the StackCell result is on the left side of the Denver curve. In other words, the flotation speed in StackCells is almost three times faster than Denver's batch test, or about six times faster than the traditional cell.

What can explain this dramatic and improved result?

Recall that in traditional trough batteries, high energy is required to combine bubbles and ore particles, but the same conditions can cause separation and fall. The only way to deal with this trade-off is to allow multiple cycles of formation and regression by radically increasing the required retention time. Even so, a lot of fine and coarse particles will be lost, as shown in Figure 1. It is expected that the internal "fallback" cycle will also occur in the Denver laboratory unit, although due to the significant reduction in length, the impact may be smaller compared to the scale involved in industrial-scale tank batteries. These results strongly demonstrate the potential of the two-stage device, in contrast to two variants of the traditional stirred tank design with an inherent built-in "back-off" loop. Obviously, the improvement and commercialization of the two-stage device can change the rules of the game in the mining industry. Based on these results, it has shown the potential to significantly reduce device size, installation costs, and operating energy costs.

To this end, Eriez is currently working closely with a number of mining companies, including several of the world's largest mining companies, to test the limitations of this technology and expand it to a larger number.

ERIC BAIN WASMUND is the Global Managing Director and Global Testing Manager of Eriez Flotation Division, LANCE CHRISTODOULOU; MIKE MANKOSA is Eriez's Executive Vice President.

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