1. Introduction - Firenze University Press

p
p
1 1.
5
p
H 1 1.
5
2O
p
mixture
H
, using
2O
H 2O
p
1
1
p
p
With input values H2O = 1002.7*10 -6 Pa*s, slag=2604 kg/m 3 and solvent =1008 kg/m 3 , (4) yields
mixture= 1102*10 -6 Pa*s. Together with =1074 kg/m 3 , and with experimental values n=5 1/s and
d=0.09 m, Rev =39471. In other words, the mixing situation is clearly turbulent. Power number in a
vessel without baffles for a turbine stirrer is then estimated to 1.25, resulting finally to energy
dissipation of 0.076 W/kgsolution. In a fully baffled reactor, resulting to better mixing, the power
number would be approximately 6, yielding to 0.369 W/kgsolution [16].
If it is further assumed that the ratio of mixer diameter and vessel volume is maintained constant in
scale-up, this means that plainly mixing of slag and ammonium salt solvent in extraction requires
20.8 kW/25 ton slag, or 0.8 kW/ton slag. Related to PCC production, this equals to 1.9 kW/ton
PCC. If the experimental residence times (Table 3) would be used for the larger scale, this would
mean 0.8 kW/ton slag * (25/60) h = 0.33 kWh/ton slag. However, these numbers do not include the
electrical losses of the system (90% efficiency would yield to 23.1 kW/ 25 ton slag), nor they are
optimised considering the mixer dimensions and reactor size relation. When compared to heat
duties of the different reactors (Table 8), it can be seen that also the mixing energy has a small, yet
significant role for the overall energy balance of the process.
For the carbonation reactor, the energy dissipation in mixing would be lower since the high gas
flow through the reactor lowers the solution density and viscosity. Also, depending on the gas inlet
arrangement, the mixing caused by the gas flow itself could be sufficient to obtain the required
dispersion of solution components.
5. Studies on additional process units
5.1. Separation equipment
As mentioned in Section 2.2, various options exist for continuous separation of solids from the
aqueous streams. For steel slag separation, a gravitational settling tube (Fig. 7) was tested
experimentally [18]. The utilised plastic tube with a circular cross-section had a volume of 8.64 L
(length 110 cm, diameter 10 cm), and it was used with a feed flow of 40 L/h. The underflow was
maintained at ~7 L/h, a value high enough to guarantee a steady stream for the concentrated particle
suspension, resulting thus to an overflow of 33 L/h.
The feasibility of using the settling tube was tested with spherical Ballotini glass beads (p = 2634
kg/m 3 , d p = 163 µm) in water. Tests were performed with the settling tube positioned horizontally
(0°) and at an angle of 10°. The beads settled within 0-22 cm from the feed orifice on the horizontal
tube and approximately within the same range in the tube with 10° angle. The distances where the
settling occurred were observed visually during the experiments. Because of the large size and high
density of the glass beads the separation efficiency was ~100%.
To obtain an estimate with separation efficiencies with the actual steel slag, tests were performed
with the steel converter slag (p = 2604 kg/m 3 , d p = 96 µm) that was used also as an input material
for the modelling work described in Section 2. Angles of 30° and 45° were used with water as the
liquid phase, but 45° was tested also with 1 mol/L NH4Cl. Sedimentation was observed between 0-
83 cm (0-80 cm for 45°), although the determination was quite challenging due to a very turbid
suspension.
After the process had reached a steady-state, a sample was taken from the outflow to determine the
concentration of particles. The samples were filtrated, dried in an oven at 105 °C and weighed.
Calculated from (5), the separation efficiencies were 99.9% for both 30° and 45° experiments.
228
H 2O
(4)