Ready salted - seawater in mineral processing
Michael Forrest talks to Hal Aral, from the Commonwealth Scientific and Industrial Research Organisation, Australia, about trials employing saline or seawater in mineral processing.
Water has a vital role in mining and mineral processing. As mineable grades decline, the tonnage of ore through the mine and mill increases, and so does the amount of water used in processing. In temperate and equatorial latitudes there is no shortage of water, although the disposal of ‘used’ water presents an environmental challenge. In other parts of the world, water can be at a premium. Mines operating in deserts in Chile, Western Australia and southwest USA can face severe constraints on the availability of fresh water, or find that available ground water is saline or hypersaline. Mines based on islands with insufficient rain-retaining topography may have a similar water supply problem.
Hal Aral and his colleagues in the Minerals Down Under Flagship Programme at the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia, have been researching the use of water in mining and processing. ‘Mining companies have been working hard to conserve and recycle fresh water in their operations, and, in some cases, have reached the end of achievable savings and are looking at alternatives. For those properties within a reasonable distance of the coast, seawater may be a viable option, while inland, the use of a brackish, saline or even hypersaline ground water might provide an alternative resource,’ he says.
‘Our research at CSIRO is very much work in progress, yet we have found that seawater and saline waters can be used effectively in a number of mineral beneficiation processes, both through lab-based experimentation, and also in reviewing a number of plants around the world that are already using saline waters in processing.’
The ‘saltiness’ of sea and saline groundwater is due to dissolved natural chemicals. For example, the composition of the seawater feed to a desalination plant in Perth, Australia, is shown here:
However, desalination is expensive in energy terms at around US$0.5m3 and in the disposal of salts derived from the process – although some have commercial values as industrial feedstocks.
Mines use a great deal of water in grinding the ore, in gravity and magnetic separation, in transporting ore and tailings as a slurry, in flotation and in hydrometallurgical processes. ‘The objective was to examine each of the common processes in beneficiation and the effect on recovery using fresh water (specific gravity [SG] – SG~1.00), seawater (SG~1.03) and hypersaline (SG~1.23). These were carried out under exactly the same laboratory conditions’, remarks Aral.
Milling is the first stage and liberates the individual minerals from the ore so they can be separated. Using a lab-scale quartz mineral sample of less than eight millimeters, the amount of fines (less than one millimetre) generated in a mill containing distilled water, sample and alumina grinding balls over a fixed time increased by 32.7%. In a repeated test using seawater, the amount of fines increased to 36.1%. Hypersaline water did not perform well, giving a value of less than that of distilled water. Before further processing or disposal, the milled product may require washing with fresh water.
Forces of nature
Gravity separation is a common and benign process used on a wide range of ores from gold to heavy mineral sands. In general, higher SG waters require lower flow rates to keep particles suspended, therefore less pumping power is needed to separate two materials. The CSIRO experiment has used a hindered counter current funnel-like separate or at a flow rate of 0.5-1.25l per minute on a 50:50 sample of coarse quartz and ilmenite with a density difference of 2.07SG. In both sea and fresh water experiments, the quartz and ilmenite have been well separated, considering only one cycle of treatment has been applied. However, mineral separation in hypersaline water is rather poor, and there has also been a 14% loss as the sample sticks to the separator’s walls when drained, possibly because of the higher viscosity and the fluid’s surface tension properties.
Sea and hypersaline water, therefore, require lower flow rates to keep particles suspended, meaning less pumping power is required to separate the two minerals from each other. It appears that the hypersaline waters need to be diluted with fresh water to a certain level for better performance, and that the level of acceptable performance needs to be determined by laboratory-scale tests.
Wet magnetic separation is used in a variety of mines. In the experiment, equal parts of ilmenite (magnetic) and zircon in distilled water, seawater and hypersaline water have been tested. Similar results have been obtained using the first two, but greater losses and less efficient separation occurs using hypersaline, attributed to greater density and viscosity. Some non-magnetic mineral has been found in the magnetic fraction, which may reflect this viscosity and surface tension entrainment. Surfactant use may improve this result and further research is needed.
Flotation is an important method in recovering sulphides such as chalcopyrite. The process relies on chemically-induced attachment of gas bubbles to the mineral’s surface to ‘float’ it in an agitated slurry. This test involved a feed from an Australian copper mine while the hypersaline water was from Western Australia. The Eh and pH were closely controlled and a high purity sodium isobutyl xanthate has been used as a flotation collector. Copper recoveries for distilled water and seawater in a rougher flotation are equal at around 78%. However, a significant deterioration in copper recoveries occurs when hypersaline water is used to around 64% of the copper content.
Tests have also been carried out on agitated and autoclave leaching of copper and nickel ores. Chlorine ions in saline water enhance copper recoveries in the former, although the chlorine might create problems in its downstream processing, particularly in electrowinning when hazardous chlorine gas is released.
Autoclaved nickel ores tests by CSIRO and the A J Parker Centre, Waterford, Australia, indicate that recoveries increase by three per cent when saline waters are used. Their use also reduces the concentration of impurities, such as aluminium, in the process liquors. This, in turn, decreases reagent consumption downstream, offering significant cost savings. In gold ore autoclaving, chlorine levels above 100ppm lower cyanide recovery and result in sulphur forming.
‘The CSIRO programme has confirmed that saline waters are acceptable in a number of applications and that further research will help to resolve some of the outstanding issues. One of these is corrosion of the plant. Well designed components from suitable metals will help, but there is no way to completely eliminate corrosion. However, there are ways of dealing with corrosion and minimising its harmful effect – such as the use of corrosion inhibitors, rubber-lined tanks, PVC pipes and special paints,’ notes Aral.
In Chile, Indonesia and Australia, a number of mines are already using saline and seawater in processing. For example, at the Kalgoorlie Consolidated Mines ‘Superpit’, hypersaline paleochannel waters, containing 30-200g/l dissolved elements, are used in the plant and to transport tailing slurries. The effect is increased cyanide consumption as Mg(OH)2 buffers the aqueous solution and forces plants to operate at values around pH 9.0.
At Minera Michilla Copper in Chile, seawater is used in the direct leaching of ores. The resulting leach solutions require washing with demineralised water before sending to the electrolytic plant to avoid generating dangerous chlorine gas. This water is obtained from the seawater, piped 15km to the plant and distilled in three desalination plants with a daily capacity of 2,300m3. It also provides drinking water and is used for make-up in the SX-EW plant.
‘There are a number of other examples. As the industry works in remote locations and treats lower grade ores, the demand for treatment water will rise. The practical experience and on-going research will assist in recovering metals in these often arid and isolated locations,’ says Aral.