508 CHAPTER 6. INDUSTRIAL APPLICATIONS
Table 6.16: Physical properties of coal constituents.
Property Coal Mineral matter Pyrite
Mohs hardness
[MHo]
2.2 - 2.5 2.0 - 4.5 6.0 - 6.5
Electrical conduc-
tivity [S/m]
10
6
-10
11
10
4
-10
6
1-10
2
Dielectric constant 2.0 - 2.5 4.5 - 7.8 5.2 - 8.6
Magnetic suscepti-
bility [m
3
/kg]·10
8
-0.5 20 - 200 0.34 - 1.5
Density [kg/m
3
] 1150 - 1500 2400 - 3900 4800 - 5000
Various techniques have been investigated for the extraction of pyrite, rang-
ing from froth flotation to electrostatic separation to microbiological desulphur-
ization, with varying degree of success. The physical techniques are based on of-
ten large dierences between the values of physical properties of coal and pyrite,
as is shown in Table 6.16. Magnetic separation has been recognized as a poten-
tially cost-eective and e!cient method of desulphurization and considerable
research and development work has been conducted in this area, particularly
with the advent of HGMS, OGMS and rare-earth-based permanent magnetic
separators.
Although it has been extensively demonstrated that both wet and dry mag-
netic separation methods are able remove up to 90% of pyritic sulphur, and a
significant amount of ash (up to 60%), at combustible recovery greater than
85%, no magnetic desulphurization process has proven to be su!ciently cost-
eective and e!cient, on an industrial scale, in removing pyrite and ash. Fur-
ther research into magnetic methods of desulphurization was considerably scaled
down in the nineteen nineties. This scale-down coincided with identification of
cleaning options for environmental control strategies by the US Department of
Energy, magnetic methods not being among them [B36].
WetHGMSofcoal
Wet high-gradient magnetic separation used to be a popular technique for labo-
ratory and pilot plant investigations into coal desulphurization. Numerous re-
search papers and reports were published in the seventies and eighties of the last
century. They are often associated with the names of Trindade [T8], Luborsky
[L14], Wechsler [W24] and Male [M35, M36]. The results show that the e!ciency
of sulphur and ash removal is strongly dependent on the origin of the coal and
on the recovery of clean coal. It can be seen in Fig. 6.39 that with increasing
reduction of ash and sulphur concentrations, the losses of clean coal into the
magnetic fraction become unacceptably high. More recent reports [K29, D18]
on the subject confirmed the potential of HGMS to recover high percentage of
sulphur and ash, but at the cost of high losses of combustible matter.
Even though most studies suggest that desulphurization by wet HGMS is