Furnace dust from Exxaro Sands KZN
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RUGHUBIR, N. and BESSINGER, D. Furnace dust from Exxaro Sands KZN. The 6th International Heavy Minerals Conference ‘Back to Basics’, The
Southern African Institute of Mining and Metallurgy, 2007.
Furnace dust from Exxaro Sands KZN
N. RUGHUBIR* and D. BESSINGER*
Research and Development, Exxaro
Exxaro Sands KZN produces—among other heavy mineral products—high titania slag, which is
used as feedstock to the pigment industry. The titania slag is produced from the smelting of
ilmenite in two 36 MW DC furnaces. In this smelting process carbon monoxide gas is produced.
This gas is cooled, scrubbed and washed to remove entrained dust. This paper provides details on
the gas cleaning plant system and the chemistry, mineralogy and size distribution of the
particulate material contained in the off-gas stream. Characterization of the dust shows that is
consists mainly of ilmenite, but with relatively high levels of SiO2 and MnO. Smelting of the dust
in a 500 kW DC furnace was carried out to investigate the potential for recovering valuable
elements from the dust.
Introduction furnace gas off-take. The connection between gas off-
Exxaro Sands KZN produces—among other heavy mineral take and off-gas duct allows for axial movement
products—high titania slag, which is used as feedstock to without risking gas leakage and external loads on the
the pigment industry. Titania slag is the primary product furnace roof. The same design is used at the connection
from the ilmenite smelting process utilizing anthracite as to the evaporation cooler inlet flange.
reductant and electricity as the source of power. Exxaro • Evaporation cooler ‘W20’—the evaporation cooler
Sands KZN is equipped with two 36 MW DC arc furnaces. vessel provides a reaction volume for the water to
The secondary product from the ilmenite smelting process evaporate after injection into the CO gas transferred in
is low manganese pig iron (LMPI), intended for the off-gas duct from the furnace. The shell of the
consumption by the ductile iron market. The resulting vessel is cooled indirectly by two cooling water
carbon monoxide gas from the ilmenite smelting process circuits. Due to the water, which is sprayed
passes through gas cleaning plants where the gas is cooled, (temperature controlled) into the hot off-gas, the
scrubbed, and washed to remove entrained dust. The clean temperature of the furnace off-gas in the outlet of the
gas can be distributed throughout the site via a gas evaporation cooler is reduced to approximately 500°C
distribution plant or can be flared. The dust is slurried and via injection water lances. The gas leaves the
then settled in a gravity separator (clarifier/thickener), evaporation cooler either through the bottom outlet to
returning clean water back to the plant water system. The the gas cleaning plant or via the top outlet to the
dust is returned to the mine for rehabilitation back into the emergency stack and the safety valve. Only one mode
dunes. A history and general description of the plant has of operation is possible since both rotary hood water
previously been given (Kotzé et al.). seals, B12 (shut-off valve in the flow direction of the
This dust is currently considered a waste product with emergency stack) and B13 (shut-off valve in the
zero value. There is a strong probability that in future the direction of the gas cleaning plant) are mutually
cost of managing this waste stream will increase interlocked.
substantially. There, however, exists potential for unlocking • Safety valve ‘B11’—the safety valve limits the pressure
some value from this dust. This paper investigates this by in the furnace off-gas system by preventing air
first looking at the chemical characteristics of the dust penetration into the gas system using a defined water
particles and then explores a potential smelting process for level in the water seal of an immersion bell. If the
the recovery of the valuable constituents. system pressure increases above a defined value (i.e.
due to material bridge collapse or eruptions in the
furnace) the momentary gas surplus flows via the safety
Furnace off-gas description valve into the atmosphere. The immersion bell of the
A schematic diagram of the furnace off-gas layout is given safety valve is set in such a way that gas flows
in Figure 1. The gas cleaning plant essentially consists of immediately once the set value has been exceeded. On
the following major components the other hand, in case of reduced pressure in the
• Furnace off-gas duct—the furnace off-gas duct furnace, the water level drops only a little due to the
transfers the hot and dusty CO gas from the furnace large volume of water available in the tank. The
off-take (at approximately 1700°C) to the Theisen off- resulting seal level between the bell and the interior
gas cleaning plant, which is designed as a disintegrator duct prevents the penetration of air into the gas system
based wet scrubber plant. The furnace off-gas system • Raw gas’ rotary hood water seal ‘B12’—the ‘raw gas’
consists of 5 segments, which are cooled indirectly by rotary hood water seal B12 regulates the raw gas path
several cooling water circuits. The bottom section of through the emergency stack ‘A12’. The rotary hood
the off-gas duct is spray cooled and is connected to the water seal operates as a quick-action valve with
FURNACE DUST FROM EXXARO SANDS KZN 43Raw gas operation Clean gas operation
Water seals A12
Raw gas stack A17
Clean gas stack
Safety valve
K12
B11
Pressure control
Rotary hood
valve (B12)
W20-Evaporation To gas distribution plant
cooler
K16
Furnace Pressure control
Rotary hood
# 1/# 2 valve (B13) F16
Cooling Water separator
water
B17/B18 gas directional
control valve
V15
Disintegrator
Theisen sump water seal
Figure 1. Furnace off-gas system at Exxaro Sands KZN
integrated turn caps. In the closed position, the hood is cooling and wash tower can be adapted to relevant
immersed on all sides in water, thus blocking off the operational requirements.
flow of gas. The basic setting for the ‘raw gas’ rotary • Theisen disintegrator gas washer ‘V15’—the
hood water seal B12 is the normally open position, disintegrator gas washer ‘V15’ functions to separate the
which means the valve is closed during normal plant solid particles carried in the gas and compensates for
operation but in any case of emergency the valve will the pressure losses in the gas system. The gas from the
be opened and the raw gas will leave the plant through gas cooler enters the volute casing in an axial direction
the emergency stack. In closed position, compressed air through the two inlet branches as in the case of a
is applied to the retracted cylinder for the ‘raw gas’ double-flow radial fan.
rotary hood water seal. • Water separator ‘F16’—in the gas outlet of the
• Ignition device ‘A 12/17’ for emergency/clean gas disintegrator V15 there is a mixture of cleaned gas and
stack—the raw/clean gas at the emergency/clean gas the solid-loaded water which are separated from each
stack head is ignited by means of a separate ignition other in the water, separator. The gas-water mixture
device using Sasol gas as fuel. The flame emerges at enters the separator through the tangentially arranged
the chimney stack outlet and ignites the furnace gas inlet branch. Initial separation of the water droplets
• Plant rotary hood water seal ‘B13’—the plant rotary from the gas is achieved here by means of centrifugal
hood water seal ‘B13’ regulates the gas path through forces. A mushroom-shaped insert in the separator
the gas cleaning plant. B13 is similar in design and further accelerates the gas-water mixture by way of its
operation to B12 described earlier. The basic setting for cross-sectional construction, similar to a Venturi tube,
the B13 is the normally closed position this means the so that thorough separation of gas and liquid is
valve is open during normal plant operation, but in a achieved as the result of the inertia forces and
case of emergency the valve will close and the raw gas centrifugal forces. The flow rate of the gas water
will leave the plant through the emergency stack (B12 mixture is reduced once again in the centre section of
is then opened). the centrifugal water separator where it is routed into
• Cooling and washing tower ‘W14’—gas cooling and the upper section of the centrifugal separator. The
coarse dust separation take place in the cooling and water droplets that were separated run downward along
washing tower ‘W14’. The hot furnace gas is routed via the wall and through the drain pipe into the circulation
the furnace off-gas duct and the evaporation cooler into water basin. The gas, saturated with water vapour
the first stage of the cooling and washing tower where corresponding to the prevailing temperature, is now
it is sprayed with water. This cools the gas and at the available as clean gas with the required residual dust
same time separates coarse dust. The coarse dust is content.
carried with the water into the sump basin. Agitators • Off-gas slurry system—the hot water from the furnace
are provided to avoid sludge deposits in the basin. The disintegrator discharges into a slurry sump (Theisen
second stage of the cooling and washing tower serves sumps). As the cooling water from the disintegrator
the purpose of best possible precleaning and residual contains a high concentration of solids, a mixer is used
cooling to the required clean gas temperature. This is to ensure the solids do not settle out in the sump.
achieved by further spraying the gas with water by a Pumps are used to transfer the hot water (65°C) to the
special mushroom-shaped device in the cooling and water plant discharging into the splitter box at the
wash tower. This device is adjustable so that the clarifier/thickener. From here the hot water can be
44 HEAVY MINERALS 2007diverted to the clarifier/thicker. The clarifier/thickener The samples were analysed by ICP. The forms of iron
is called a Multised (derived from ‘multiple seeding’). and titanium were determined by standard wet chemical
The Multised reduces the expected maximum techniques. Mineralogical (X-ray diffraction, optical and
concentration of suspended solids in the inflow down to scanning electron microscopy) and size (Malvern
less than 80 mg/l. The Multised performs the dual mastersizer-ultrasonic sample dispersion) analyses were
function of clarification and thickening, all in one unit. also carried out.
The Multised is fitted with lamella plates in the The chemical analyses of the two samples were similar,
clarification section, thereby ensuring that with Fe (21 to 25% Total Fe) and Ti (49 to 52% TiO2)
exceptionally high rise rates can be maintained, while being the major components. The samples were enriched in
still producing water with a low turbidity. This results Si (6 to 7% SiO2), Mn (3.3 to 3.7% MnO), K (0.6 to 0.7%
in the required construction area being substantially K2O), Al (~1.15 % Al2O3) and perhaps also Mg (1.1 to
reduced, in comparison to conventional clarification/ 1.3% MgO). This compares to values of 0.3 % SiO2, 1.1%
thickening equipment. MnO, 0.03 % K2O, 0.3% Al2O3 and 0.5% MgO typically
A picket-fence scraper underneath the clarification found in the ilmenite. X-ray diffraction analyses of these
section allows sludge thickening in excess of 50% solids in samples indicate that ilmenite is the main phase present,
the underflow. It must be noted that, although the Multised with some rutile, metallic iron and the M3O5 solid solution
has a sludge thickening section, it is not a sludge blanket phases also present. Some carbonaceous particles were also
clarifier. Thickened sludge is discharged from the identified (0.2% C present as per bulk chemical analyses
concentrate cone by a sludge waste pump to the for both samples). The X-ray diffraction pattern of sample
sludge/slurry storage tank. Dust from the Fumex extraction DB533 is given in Figure 2.
plant (mainly metal treatment dust) is also transferred to the The following phases were identified:
storage tank from where slurry tankers remove it and Armalcolite - (Mg, Fe)Ti3FeO10 (M305 phase)
transport it to the mine for disposal on the residue dam. The Haematite - Fe2O3
amount of metal treatment dust is relatively small when Ilmenite - FeTiO3
compared to the amount of furnace dust generated. The Metallic iron - Feº
clarified overflow from the Multised will flow under Magnetite - Fe3O4
gravity to the off–gas hot well. The water is then circulated Quartz - SiO2
by the circulation pumps to packed cooling towers and Rutile - TiO2
discharges into the off gas cold well. From here the cooled The particles were identified as comprising the following
water is transferred back to the furnace disintegrator via phases:
cold water recirculation pumps. FeTi-oxides with the following variations:
• FeTi-oxides resembling typical ilmenite compositions
Characterization of furnace dust (dominant)
The thickened sludge, also known as the thickener • Rutile and Fe-rich rutile (subordinate)
underflow, was sampled and characterized. Two samples • FeTi-oxides with elevated Ti contents, resembling
were taken at the underflow position: pseudobrookite and armalcolite-type compositions
• Sample DB533—dust collected from the thickener (occasionally).
underflow, in the form of slurry directly from the The particles are mainly dense, but may also be porous,
underflow valve with the pores filled with silicates (mainly SiO 2 , but
• Sample DB534—dust collected from the thickener sometimes also containing some K and Al). In addition, the
underflow, in the form of wet solid that had porous particles are sometimes multi-phases containing
accumulated on the floor below the underflow valve. various FeTi-oxide grains.
Furnace dust slurry
100 Ilmenite
90
80
70
60 Rutile
Lin (Counts)
Metallic iron
50
40
Armalcolite
30
20
10
0
0 10 20 30 40 50 60 70
2-Tieta-scale
Haematite Fe2O3
Ilmenite FeTiO3 Metallic iron
TiO2 Feo
Rutile Magnetite Fe3O4
Armalcolite (Mg, Fe) Ti3FeO10
Figure 2. Diffractogram of sample DB533
FURNACE DUST FROM EXXARO SANDS KZN 45Ilmenite
Ilmenite
Spherical FeTi-oxides (DB534) Original ilmenite grains (DB533)
Dense
Ilmenite Porous
FeTi-ox Porous
Porous
Various FeTi-oxides (DB534)
Dense and porous FeTi-oxides (DB534)
Fe-metal Granular haematite
Fe-sulphide
Fe-oxide
Ilmenite
Fe-sulphide
Occurrence of Fe-phases (bright) (DB534 and DB533
Figure 3. Dust samples under microscope
Metallic iron (sometimes containing Ti) occurring as: For the first sample (Sample DB533) a clear bimodal
• Inclusions in the FeTi-oxides distribution was observed. The first peak is situated at a
• Interstitial in the porous FeTi-oxide particles particle diameter of between 0.1 and 1 μm, while the
• Free-milling spherical particles (rare) maximum of the second peak is situated at approximately
Haematite is also present as: 300 μm. For the second sample (sample DB534) there is
• Network of recrystallized laths or grains, cementing one major peak, observed below 1 μm. For this sample
other grains approximately 37% of the material is smaller than 1 μm. It
• As a rim around metallic Fe. is again believed that the peaks situated below 1 μm are
Others: mainly due to fume condensates, whereas the larger
• Carbonaceous (reductant) particles particles are from the carry-over of particulate material into
• Silicates (again, mainly SiO 2 , but sometimes also the off-gas system. No work was carried out to confirm the
containing K and Al) presence of fume condensates. It is, however, well known
• Fe-sulphide and Fe-sulphate as a rim surrounding that elements such as silicon (fume as SiO) and manganese
metallic iron. The Fe-sulphate is probably an oxidation (fume as metal vapour) do present as fumes at high
product from the Fe-sulphide, perhaps forming during temperatures. If the -1 μm fraction is mainly due to fume
the quenching and thickening of the slurry. condensate products (enriched in elements such as Si and
Mn), the chemical quality of this dust might be improved
Of interest is the difference in the size distribution of the by separating this fine material from the larger sized
two samples, with the sample taken from the thickener material.
underflow (sample DB533) being relatively coarse (d50 =
189 μm) while the sample taken off the floor below the
valve (sample DB534) being fine (d 50 = 6.1 μm). It is Smelting of dust in a 500 kW furnace
believed that the first sample might be from coarse material A bulk furnace dust slurry sample was obtained for
that had settled at the bottom of the tank and was obtained smelting tests. This sample was dried via electrically heated
when the underflow valve was opened to take the sample. It drying pans. During this drying process the fine dust
might therefore not necessarily be a representative sample agglomerated into a hard cake. This required some light
of the smelter dust. Despite this the chemistry and milling to produce the required feed material used during
mineralogy of the two samples are similar. Particle size the smelting tests. Approximately 3 t of dry furnace dust
analyses of the two samples are shown in Figure 4. was obtained during this operation.
46 HEAVY MINERALS 2007% %
10 100 10 100
DB 5.33 DB 5.34
90 90
80 80
70 70
60 60
50 50
40 40
30 30
20 20
10 10
0 0 0 0
0.01 0.1 1.0 10.0 100.0 100.0 0.01 0.1 1.0 10.0 100.0 100.0
Particle diameter (μm) Particle diameter (μm)
Figure 4. Particle size distribution for the dust samples. Both differential and cumulative analyses are shown
Table I
Chemical analyses of slag obtained from smelting tests (analyses in mass %)
Description SiO2 Al2O3 FeTotal TiO2 MnO K2O Cr2O3 V2O5
Some typical specifications for chloride grade slagA5 A5
TiO2 91% A2 TiO2 90%
FeO 4%
FeO 3%
TiO2 91%
A1
A4 FeO 4%
Mn(Ti)Si-bearing
spherulites
Figure 5. SEM micrographs of a high SiO2 slag with FeTi oxides set in a matrix of a glassy phase containing spherulites
is that some marketable products may be produced, which VISSER, C. Characteristics of some high titania slags.
can generate revenue as apposed to returning dust back to Heavy Minerals 1997, Johannesburg, South African
the mine site. Saleable iron will also be produced from the Institute of Mining and Metallurgy, 1997,
smelting process, which will be an additional benefit. The pp. 151–156.
treatment of the furnace dust will minimize the total site
waste and has the potential of producing some saleable KOTZÉ, H., BESSINGER, D., and BEUKES, J. Ilmenite
products. Smelting at Ticor SA. Southern African
Pyrometallurgy 2006, R.T. Jones (ed.), South African
References Institute of Mining and Metallurgy, Johannesburg,
BESSINGER, D., DU PLOOY, H., PISTORIUS, P.C., and 5–8 March 2006, pp. 203–214.
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