Magnetite And Heavy Media Separation
Introduction
Heavy media separation (HMS, also referred to as dense media separation or sink-and-float separation) is a process of effective filtration that achieves separation via different densities. HMS dates back centuries, with the first properly designed system observed in publications around 1935. Typically, a slurry is formed using a predetermined medium at a set density. The material that needs separating is added to the slurry, and the light density material floats on top of the slurry whereas the denser materials sink through the slurry to the bottom. The choice of slurry media is an important consideration as it is this which determines how easy it is to separate what has been filtered from the slurry itself. Whilst skimming the light density material off the top may seem facile enough, it will not ensure complete removal of the slurry. Similarly, pumping or pouring away the slurry will not remove it in its entirety from the denser materials(1). As such, the slurry material used must be easy to remove. It must also be simple to make up to a desired density. A common organic example may be acetylene tetrabromide, diluted in a suitable solvent, but this is toxic. Ferrosilicon is another popular choice, but a much more widely used material for slurry production is magnetite. A common deployment would be finely ground magnetite suspended in water or a potassium- and sodium chloride saturated brine solution. Many other examples exist, depending on the nature of the material due to be separated.
Magnetite In Heavy Media Separation For Coal
The separation of coal from other materials is a leading use case for magnetite assisted heavy media separation. The process by which a material such as coal is ‘cleaned up’ by way of having gangue or other valueless components removed, thereby leaving the desired mineral and a waste stream, is called beneficiation. Coal beneficiation can be achieved through several methods including jig washing, froth flotation and heavy media separation. Data shows that HMS is the leading method of coal beneficiation in the United States, Australia and South Africa, commanding 52, 60 and 85% of the market respectively(2). The method for beneficiation used is determined by the nature of the coal itself, and overall beneficiation is becoming a more important consideration for processors as thinner and marginal coal seams are increasingly mined.
As a general rule(3), coal beneficiation using magnetite in HMS is employed as follows:
- Where a separation density of 1.2 to 2.2 g cm-3 is required, magnetite is used alone
- Where a separation density of 2.2 to 2.9 g cm-3 is required, magnetite is used alongside another dense material such as ferrosilicon
- Where a separation density in excess of 2.9 g cm-3 is required, magnetite is not used
A detailed overview of selection processes for HMS is available(4). In simple terms, coal beneficiation using magnetite in HMS works by mixing water with magnetite at a specific density to form a slurry. By introducing pulverised coal to this system, the ‘good’ clean coal will float to the top and impurities will sink to the bottom. The clean coal can then be skimmed off. Coal has densities in the region of 1.3 to 1.4 g cm-3, whereas shale–type materials have densities exceeding 2.2 g cm-3. Being able to easily modulate the slurry density of magnetite in water is an attractive property and has led to its widespread use in HMS(5). In Poland for example, between 700 and 800 g of magnetite are used to beneficiate one tonne of coal(6) and it remains a popular choice despite the need for importing all of the magnetite required, and initial results for magnetite replacement in HMS are far from mature. Such quantities of magnetite are typical. Magnetite HMS is regarded as an efficient, easily controlled and accurate process(7).
Contemporary plants utilise a cyclone to accelerate the separation process(8), with the centrifugal forces forcing the isolation of coal and waste. Rheology and stability of the dense media need to be taken into account - too high a viscosity and the clean coal and the gangue waste may not be able to effectively move through the media leading to poor separation - and as such must be controlled(9). Viscosity modifiers may be used to modulate this, although it may come at the cost of stability. As size decreases, the impact of media density is more keenly felt at higher densities(10) and so feed characteristics (flow rate etc.) must be managed carefully; computational models are often used to optimise HMS flows.
Advanced Developments
Researchers have shown over the years a development of fluidized bed reactors that make use of the density properties of magnetite to achieve separation of coal from the ash waste. In a dry and gas-fed fluidized bed system where the HMS medium is magnetite and coal (authors note that fine coal is readily available and more will be generated by the operation of the fluidized bed), separation of coal from wastes proceeded well across a range of magnetite particle sizes and it was found to be broadly tolerant of unimproved coal in a variety of conditions(11). Other researchers have shown that both dry and wet fluidized bed reactors can be effective with magnetite particle sizes in the 0.15 to 0.3 mm range(12), which is easily within the reach of any processing operation, maintaining uniform separation over varying pressures. The advantage of such technologies is that they are continuous in their operation. Inverted gravity fed fluidized beds have been demonstrated as effective - negating the need for high pressure or cyclonic systems - with 82% recovered high quality coal at an impressive throughput of 6.8 T m-2 h-1(13).
Naturally, coal beneficiation is not a completely efficient process and some coal may remain in the wastes. Additionally, coal waste in general from other processes can contain coal that is viable for further use, and applying a magnetite HMS process to this can lead to reduced overall wastes(14).
HMS Using Magnetite In Non-Coal Scenarios
Spodumene is a leading source of lithium - becoming an ever more important metal in the modern global economy - and exists as hard rock deposits(15). The only other alternative for lithium production is from brines, which is energy intensive and often low yielding. Researchers have used magnetite HMS as a method of isolating the spodumene from the other rock material. In their testing, 840 μm graded mined material produced a concentrated grade of 6.1% Li2O which represents approximately a 50% lithium recovery, with 50% of the original rock mass routed to tailings (waste) and an overall loss of lithium of just 8%(16). Researchers praised the simple operation and low energy requirements of the magnetite HMS process. Another material which may be separated by magnetite HMS is andalusite, an ore of aluminium(17). Research has also shown that magnetite HMS can be used with crude aggregate materials, producing viable components for the manufacture of concrete(18).
Summary
- Heavy media separation is a technique that relies on different densities across a slurry medium (such as magnetite in water) to separate materials
- HMS using magnetite is often used in the beneficiation of coal
- Contemporary plants use cyclonic systems to aid in the separation
- Specialist and high performance systems may use fluidized bed reactors for separation, still relying on the density gradient provided by magnetite
- HMS using magnetite has other uses, including in lithium ore beneficiation
References:
1 Handbook of Flotation Reagents: Chemistry, Theory and Practice, S.M. Bulatovic, Elsevier, Amsterdam, 2015
2 Coal Combustion Products, T. Robl, A. Oberlink and R. Jones (eds.), Woodhead, Cambridge, 2017
3 The Coal Handbook, D. Osborne (ed.), Woodhead, Cambridge, 2023
4 J. Bosman, J. S. Afr. Inst. Min. Metall., 2014, 114, 529
5 J. Nyssen et al., Z. Geomorph., 2012, 56, 23
6 D. Gadja et al., IOP Conf. Ser.: Mater. Sci. Eng., 2018, 427, 012036
7 M. W. Mikal and D. G. Osborne, Coal Preparation, 1990, 8, 111
8 E. J Mayer and I. K. Craig, Miner. Eng., 2010, 23, 791
9 R. Sripriya et al., Coal Preparation, 2007, 27, 78
10 D. Behera, SSRN (Preprint), DOI: 10.2139/ssrn.4700776
11 J. Zhou et al., Fuel, 2019, 243, 509
12 J. He et al., Particulate Sci. Tech., 2016, 34, 173
13 K. van Netten et al., Miner. Eng., 2018, 126, 101
14 S. T. L. Harrison et al., Minerals, 2002, 12, 1519
15 B, Tadesse et al., Miner. Eng., 2019, 139, 170
16 C. E. Gibson et al., Minerals, 2021, 11, 649
17 Y. Zhang et al., Miner. Proc. Extractive Metall., 2013, 113, 60
18 I. D. MacKenzie, Proc. Am. Concrete Inst., 1954, 55, 133
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