New flotation-cell designs and process-control tools keep bubbling to the surface

By Russell A. Carter, Managing Editor

In the never-ending battle to gain tighter control over flotation circuit performance, plant operators already have a wide spectrum of tactics and tools to choose from, ranging from eyeball inspection of froth characteristics based on prior experience, up to complete optical- or statistics-based control systems. Recent technological developments from two companies—one well-known to the industry, the other not quite as familiar—add even more options to the operator’s arsenal.

Floating the Fines
According to Wolfgang Krieglstein, sales and product manager, Siemens AG Mining, a mounting challenge to minerals processing is the issue of steadily declining head grades for valuable minerals in ore.

He explained beneficiation performance must provide a balance that minimizes energy consumption and maximizes recovery. However, decreased head grades in ore often mean finer disseminations of valuable minerals, and as a result, increasingly finer grinding and re-grinding may be necessary. This presents a challenge to flotation circuit design to provide acceptable recovery and product grade while coping with larger volumes of fine and ultra-fine particles passing through the circuits.

With its HybridFlot flotation cells, Siemens offers technology it says is more capable than conventional cell types for recovering these fine and ultra-fine particles. Both recovery and enrichment results achieved in industrial-scale applications and in pilot works using Siemens HybridFlot cells operating as a pre-rougher stage show fine fractions can be extracted and then enriched to final product grade in only one additional step, eliminating any need to recirculate concentrate. And, indications are encouraging that feed flow to conventional circuits, minus the fines removed at an upstream point of the process, can significantly improve the overall performance of the complete downstream flotation process.

Results obtained so far indicate the selectivity and enrichment capability of the flotation technology as applied to cleaner cells also is encouraging, although this application has not yet undergone pilot evaluation. These results indicate the Siemens HybridFlot technology can offer an opportunity to eliminate multiple cleaner stages and high recirculation loads.

In certain applications it also has been shown the very short retention time of the feed material in the Siemens cells can significantly depress undesired, slow-floating minerals, which often float into the froth product due to the much longer retention time of the feed pulp in conventional cells. And, test work done so far indicates that, on a partial process level at least, HybridFlot technology offers significant potential savings in energy, process gas and water, compared with conventional technology of equivalent capacity.

At Minera Los Pelambres, Chile, for example, two 16-m3 Siemens HybridFlot cells have been installed as pre-roughers in the plant’s copper-moly selective process. There are several benefits from this application. First, extracting the fine fractions and directly producing a final concentrate eliminates the need to recirculate the material. Second, the downstream process is fed by the fines-depleted tails from the Siemens cells, enhancing conventional processing.

A similar setup involving Siemens cells will be installed at Codelco’s Andina Division, also in the copper-moly selective process, in the second half of 2011, where a 16-m3 Siemens cell will be combined with a 2-m3 cell (see sidebar) to produce final concentrate quality. Successful piloting as an additional rougher line in the copper-moly selective process also has been conducted at Codelco’s El Teniente Division.

Crafting a Better Image
Meanwhile, Finnish company Numcore Oy claims it can offer new imaging technology that increases the efficiency of flotation processes, as it is now possible to employ measurement devices based on impedance tomography to create real-time 3-D images from inside the pipelines and tanks used in mineral processing.

The technology opens up entirely new possibilities to control flotation processes, said Jukka Hakola, Numcore’s vice president of sales and marketing. “Problems emerge if the froth cannot carry the load of mineral particles or the process otherwise becomes disturbed. The froth bed then disappears, and restarting the process wastes valuable time,” Hakola said. “If the solid matter content and bubble size in the froth bed changes, and solid matter starts to gather under the froth, this can cause the froth to become rigid or to collapse. When this situation can be predicted, the problem can be solved by changing the operating parameters.”

With Numcore’s measurement devices, the size and quantity of air bubbles and the solid matter content of the froth bed can be monitored by means of electric conductivity distribution. Real-time performance is a key functionality in this technology; the system can continuously provide the operator with current data on what is happening in the flotation cells.

“Controlling a mineral concentration process has largely been based on experience-derived know-how. Now that operators can ‘look’ inside the process, it is possible for them to maintain an optimal mix all the time,” said Hakola, who cautioned that measuring, by itself, does not create added value—rather, the results should be used to control the operation. Consequently, Numcore has, in co-operation with a few key customers, developed measurement technology to better serve the routine requirements of day-to-day plant operation.

The company’s partners include Outotec, and in the initial phase, the Geological Survey of Finland’s Outokumpu Mineral Processing Laboratory.

“At the moment, our sensors are in use at two concentration plants, where we have been able to see in practice which parameters need to be controlled and how we can really numerically prove the benefits that can be achieved for the customer,” said Hakola.

Numcore’s technology is currently undergoing test work at Inmet Pyhäsalmi Mine Oy’s copper and zinc mine at Pyhäsalmi, among others. According to Seppo Lähteenmäki, processing mill manager, the system has provided accurate information on the condition of the froth bed, and the technology has functioned reliably.

“The device has provided clear benefits for those operators who have received training for it and actively monitored the data provided by the system. The device appears to be so useful, we are seriously considering buying it after the test period,” he said.

Depending on the diameter of the pipeline or tank, Numcore’s measurement devices are configured as either a flow-through sensor or as a probe-type sensor for installation inside large pipelines or tanks.

By supplying a weak alternating current to the electrodes of the sensor, it is possible to measure conductivity differences between the phases; for example, for liquids inside pipelines and tanks, as different substances show different conductivity values. The actual image is created by means of inverse calculation, however, and this is where Numcore’s core expertise lies.

“Our CoreApus flow-through sensor lends itself for analyzing material flows inside pipelines of less than one meter in diameter. Because processing tanks are generally larger than this, we have developed the probe-type CoreHydra sensor which can analyze a larger area of the tank.” said Hakola.

According to the company, both sensor types can accurately measure interfaces between liquids and solids, but CoreApus is also applicable for imaging rapidly flowing liquids.

HybridFlot: Capturing the Fine and the Coarse
The Simine Hybrid Flot technology combines a pneumatic spray-in approach with a column method, and does not employ an impeller or agitator. The pneumatic component creates very small gas bubbles and injects high kinetic energy, as feed pulp and gas are mixed inside the chambers of the ejector system before being sprayed into the cell. This results in increased recovery of fine particles. The column system creates bigger bubbles combined with lower kinetic energy insertion to capture coarser particles. The combination of short retention time of pulp in the machine plus additional launders reduces the risk of losing particles already captured, and results in a higher concentration of valuable minerals in the generated froth product.

A new addition to the 2-m3 and 16-m3 HybridFlot tank lineup is a 0.03-m3 laboratory cell. Using this small cell, operators can perform quick evaluations of flotation processes involving different ores and flotation chemical recipes. The laboratory cell is typically integrated into a continuous, closed-cycle flotation circuit; however, it can also be used in open-circuit continuous or batch mode, if required.