Filtered out - minerals processing
Global trends influencing the minerals industry include processing of lower grade and more complex ores, productivity, environmental responsibility, and energy and water conservation. All these add to costs at a time when companies are looking to maximise efficiency. Ian Townsend reports
A first step in minerals processing is concentration or beneficiation, where valuable minerals are separated from waste or tailings. Most separations are conducted from slurries after the ore has been fine ground in wet mills to achieve liberation. Froth flotation, where the surface tension allows preferential separation of sulphide base metal and platinum group minerals, results in a concentrate that must be dewatered ahead of transport, smelting or refining. The global trend has been towards lower grade and more complex ores that need finer grinding to achieve liberation. Particle size was typically 80% <75μm, but is now likely to be 80% <20 to 45μm, and can be as fine as 80% <8μm – the finer the particles are, the more difficult it is to dewater.
This separation of solids from liquids has two principal processes –
1. Cake filtration – to form a solid filter cake with or without in-situ cake washing for solute removal.
2. Polishing or clarification filtration – to remove traces of solid from a bulk liquid stream. Slurry properties including flow rate, solids’ concentration, particle size, density and shape, liquid viscosity, slurry temperature and corrosive properties determine the filtration method.
Filtration has long been the Cinderella of minerals metallurgical processing. Hidden at the tail end of the plant, it has received less attention than comminution and flotation. But it is also an important process in hydrometallurgy and refining. Purification often involves leaching of concentrate or calcined concentrate, followed by precipitation of impurities and recovery of pure metal. Filter cakes of leach residues and precipitates must usually be washed to maximise recovery of valuable solute. Large vacuum belt filters with counter current cake washing are commonly used for leach residues and precipitates such as jarosite. Pressure filters are used for finer leach residues or precipitates that are more difficult to dewater.
Considerations for filter selection in hydrometallurgical processes include slurry temperature and composition that determine materials of construction, and propensity for cloth blinding by precipitation of salts in the filter media. The moving cloth on horizontal belt vacuum filters and tower presses helps preserve cloth permeability by more effective washing.
There are many different filter designs, sometimes unique to niche applications. The driving force for filtration is the differential pressure across the filter cake and filter media. The principal methods for both cake and polishing filtration are vacuum filtration for easy dewatering, where liquid is sucked through the filter media at differential pressure 0.6-0.8 bar, and pressure filtration for more difficult dewatering, achieved by forcing liquid through the filter media at above atmospheric pressure. Differential pressure can be as low as 1-3 bar, more typically 8-16 bar, and as high as 150 bar for exceptional applications such as clay filtration.
Vacuum filters are unlikely to achieve the required 8-12%w/w cake moisture content with fine concentrates, and pressure filters have been the preferred choice for most recent mine developments. The lower cake moisture achieved by pressure filters reduces transport costs, and decreases or eliminates energy consumption in drying before smelting or refining.
Finer concentrate increases the likelihood of filter media blinding. This is significant as replacement of filter cloth is the highest running cost item. Efficient cloth washing is important to prevent blinding, extend cloth life and reduce maintenance time for cloth changing. Complex ores are inherently variable, and concentrate characteristics can change over short periods. Modern pressure filters adjust automatically to changes in feed to maintain consistent throughput and cake moisture.
The waste product or tailings from flotation plants have traditionally been deposited in dams. While this continues for most new mines, there is a growing trend to paste disposal or dry stacking. It is driven by the need to increase water recovery, ensure stable disposal in seismic areas, and minimise the deposition area in mountainous terrain. Dry stacking may be a condition for permitting in areas of environmental sensitivity.
Tailings moisture for dry stacking is typically 14-18%w/w, which can usually be achieved at lower differential pressure than concentrate filtration. The filtration rate of tailings is lower than that of concentrates because of the lower solids density, hydrophilic nature of the particles and the presence of fine or platy gangue minerals such as clays and talc. The high mass flow and lower filtration rate of tailings requires large filtration areas, meaning installations with multiple filters of the biggest sizes available. Recent proposals have included installations of 20 or more 560m2 filters. A major challenge is managing filter cloths on such large installations.
The filtration of iron ore fines for the production of furnace-ready pellets is a particular demanding application as there is a high throughput of dense, abrasive solids. Pelletising cake moisture is 8-9%w/w, and must be consistent for optimum binder addition and maximum pellet strength. Strong pellets minimise breakage and recycle during sintering, thereby increasing throughput and reducing energy consumption. Vacuum disc filters were used for iron ore fines, but have been superseded by pressure filters and ceramic disc filters. The latter are similar in appearance to traditional vacuum discs, but use microporous ceramic sectors in place of filter cloths. Ceramic sectors have long life and create suction by capillary action without the need for high kilowatt vacuum pumps. Ceramic disc filters are used widely in ferrochrome plants and in base metal concentrators with coarser grinds.
The filtrate from vacuum belt or pressure filters always contains some suspended solids. It is important to remove these trace solids before operations such as solvent extraction or electro-refining. Polishing filter designs are sometimes specific to certain applications. For example, dual media filters are used to remove both solids and entrained organics between solvent extraction and electrowinning. Simple, lowpressure cloth media polishing filters are the industry standard in electrorefining for removal of suspended anode slimes from circulating electrolyte. Removal of suspended solids reduces the potential for nodule formation, which in turn improves current efficiency and reduces impurity entrainment in cathodes.
More uses of filtration in mining and metallurgy include wet scrubbing of roaster and smelter off gas, and remediation of acid mine drainage. Environmental protection invariably involves some form of filtration and will become ever more important.
The main trends in filtration development for mining and metallurgical applications are summarised in this table (opens in pop-up window).
Ian Townsend, email: firstname.lastname@example.org