Rising to the top - froth flotation
Michael Forrest talks to Dr Jussi Vaarno about advances in froth flotation, a key component in recovering minerals from ore.
Although there is a long-term trend towards obtaining copper, lead and zinc through hydrometallurgy, the majority of these metals are recovered from sulphide minerals, primarily chalcopyrite, galena and sphalerite.
Traditionally sulphides have been roasted – burnt with carbon to disassociate the sulphur – which is an environmentally challenging process. The result of roasting was a matte that could be refined by pyrometallurgy to produce the refined metal. Other methods include leaching with a variety of chemicals to provide a pregnant liquor that can be subjected to electro-winning. In a number of deposits, the sulphide minerals have already been oxidised by atmospheric weathering, allowing the minerals to be leached directly using a weak acid.
Before these processes can take place, the sulphide minerals have to be recovered from the ore. This is achieved through crushing and grinding to a predetermined size to liberate the minerals, and then via a process to form a concentrate. Smelters require concentrates assaying around 25% copper and a similar amount for lead and zinc. Some deposits contain more than one sulphide mineral and may also be associated with gold and silver – separation of the various phases is consequently paramount.
A successful method of achieving this separation is through the using of flotation, a technique that relies on the hydrophobic nature of some minerals. Froth flotation is an adaptable technique based on the ability of air bubbles to selectively adhere to the surface of minerals while they are held in a water-based slurry. Hydrocarbons and sulphide minerals are naturally hydrophobic and remain dry, while other minerals are wetted and remain in the liquid phase. Sulphides, buoyed by the attached air bubbles, float to the surface and are collected. It is possible, with the addition of a number of chemicals, to modify the surface characteristics of these minerals to enhance the separation of gangue (waste) and from each other. Froth flotation is particularly effective in the concentration of fine-grained deposits where gravity separation fails to produce high recovery rates.
It is not only the controlled addition of chemicals that is important. Froth flotation is dependent on a number of inter-related components where a change in the setting of one impacts on the others. For example, a change in the particle size will demand revision of the feed rate, air flow, pulp density or even the physical components of the system. This can make it difficult to predict the performance, although new systems have gone some way to predict outcomes.
Finnish company Outotec started to apply froth flotation at its Orijarvi mine in the south of the country. Here the massive volcanogenic sulphide deposit contained copper, lead, zinc and silverbearing minerals. The early flotation cells were of small capacity and incurred high operating and maintenance costs, factors that stimulated research to improve their performance. By the late 1930s, more selective flotation was achieved through individual cells each within its own circuit and customised to a specific ore. By the late 1960s, the cell size had grown to 3m3 capacity.
Another long-term trend has been declining ore grades as the most profitable deposits are worked out. Today, copper grades of between 0.5% and 1% are considered viable, while combined lead/zinc grades have also fallen. The difference between 0.5% and 1% grades requires double the amount of mining for the same amount of recovered copper. This extra tonnage has to be processed in a similar manner, resulting in greater throughput at the crushing and grinding stage, and also in the flotation stage if sulphides are to be recovered. ‘The necessity to develop larger units has been with us since we published the first 16m3 cells in the 1970s,’ explains Dr Jussi Vaarno, Director of Flotation and Hydromet Equipment at Outotec. To achieve the bubble-particle interaction, the mineral slurry must be kept in motion without excessive turbulence. With coarse particles, strong yet gentle mixing is preferred to minimise the detachment of the bubbles from the particles. For fine and medium-sized particles the bubble-particle impact frequency must be maximised, which requires the generation of strong shear forces within the slurry to ensure contact. Another criteria is to design the rotor (usually downward tapering) to allow smooth start-up under full load. ‘One of most important developments has been the aeration of the slurry,’ says Vaarno. Blower systems are necessary to maintain bubble and froth formation independent from mixing intensity. Maintaining optimum froth conditions is imperative and requires constant supervision. Outotec uses advanced image analysis to monitor the froth motion and bubble generation, where output is coupled to air feed, froth depth and reagent addition controls to provide the best result. In operating conditions however, uncontrolled plant surges and fluctuations in flotation feed can occur. These need to be monitored upstream, and a compensating signal transmitted to maintain the cell-level control before the surges interfere with cell performance. These systems are automatic and are built in to the flowsheet.
‘All these processes have been developed within advanced flotation plants,’ states Vaarno. Overriding these is the scaling-up of the cells to meet the high ore throughputs in modern mines. Circular tank design has proved the answer, as it offers ideal mixing and maximises the number of collisions between mineral particles and air bubbles, as well as reducing short-circuiting. Large cells mean fewer units, with commensurate savings on construction costs, piping, cables, instrumentation and auxiliary equipment. However, with tanks holding large tonnages, reliability and process control inputs are critical. The move towards large cells has accelerated over recent years: in 2007 the company produced the world’s largest flotation cell at 300m3, which was swiftly superseded in 2012 by its 500m3 model. ‘These units are particularly effective in large, lowgrade copper-gold mines found in Chile and Australia,’ says Vaamo. Of particular note are advances in rotor design providing separator areas for pumping and air dispersion, and the use of electron impedance tomography to provide surface level control and the option to optimise froth properties.
Dr Jussi Vaarno, email@example.com