SINTERING:
The first
stage of the process is to mix new concentrate and secondary
input materials with recycled materials in a proportioning
plant. Some concentrates contain up to 30% sulphur, but
the sinter mix is approx. 6% total sulphur content (crushed
tp 6 mm) and this compromises up to 80%
of the sinter machine feed.
The purpose of the Sintering process is to:
Desulpherise
(0.4 - 0.8 %S in output sinter)
Agglomerate
(fuses the mass into lumps)
Render
porous (lumps of porous sinter to permit a gas solid
reaction in the blast furnace shaft)
Yield refractory lumps of sinter. (Must not soften at
temperatures lower than 1140 degrees C.)
The
sinter mix is made up a blend of materials inorder to:
optimise
the economic margin (£'s):
provide sinter of the correct chemical composition
ensure good quality lump sinter for the ISF shaft and
slagging operations.
A 25mm.
layer of sinter mix is fed onto the sinter machine (3m width)
and ignited using natural gas burners. Once the ignition
layer is established the main layer feed hopper increases
the sinter machine bed depth to approx. 370 mm. depth. Updraught
sintering then proceeds using air volumes up to 120,000
Nm3/hr., with a total sinter mix feed to the machine of
up tp 200 tonnes per hour. Sinter output will have a typical
assay:
Zn 40%, Pb 20%, CaO 4.5%, SiO2 4.2%, FeO 11 - 14%, Cd 0.3%.
During the sintering process sulphur is burnt off as sulphur
dioxide gas. The sinter gases are scrubbed with water, dried,
passed through electrostatic precipitators (demisters),
then through heat exchangers into the converter.
The converter
is packed with Vanadium Pentoxide catalyst which promotes
the conversion of SO2 into SO3. The sulphur trioxide gas
is absorbed in Sulphuric acid and diluted back to 96% or
98% product acid. White acid is produced by bleaching with
hydrogen peroxide. Product acid is transported to chemical
companies by road or ship.
