1. Introduction - Firenze University Press

(raw) materials pre-heating and AS recovery. The specifications for these two utilities are listed in
Table 2.
Table 1. Main assumptions taken in the Aspen Plus ® model
Block Assumption
S-SDECOM
Operating conditions: T= 440ºC, P=1bar
AS/S/W=3:2:1
Extraction %: Mg = 80%, Fe = 60%, Ca = 100%
DISSOLUT T=80ºC; Total conversion of XSO4 species to X 2+
TANK 1
PPI
PPII
CRYST
CARBONAT
T=40ºC; Complete dissolution of NH3 into water producing a
30~40% solution of NH4OH
Total precipitation of Fe and Ca as Fe(OH)2 and Ca(OH)2
pH =9-9.5; T=30ºC
Precipitation of Fe and Ca as Fe(OH)2 and Ca(OH)2
pH =9.5-11.5; T=30ºC
Water is evaporated so that the recovered product has 75% -w/w of
AS and 25 %-w/w of water.
Carbonation with Flue Gas from the limekiln
pCO2~=20 bar, P=80 bar
90% of Mg(OH)2 carbonation according to reaction R4
MSCOMPRE 6 polytropic compressors with 80% efficiency and intercoolers
TURBINE Isentropic; Decompression to ~5.6 bar
Table 2. Data for the limekiln’s flue gas defined as a utility in the Aspen Plus ® model
Utility ID
Composition
(mol %)
107
TIN
(°C)
TOUT
(°C)
FG500
H2O 5.9
CO2 21.7
500 460
FG440
O2 6.9
440 350
N2 65.5
2.2. Results – Mass and exergy
The aim of this simulation is to evaluate the performance of a pilot scale mineral carbonation plant
running on waste heat from the limekiln. In order to process 600 kg/hr of flue gas (7 m 3 at 500 ⁰C
and 80 bar) i.e, ~190 kg of CO2, 500 kg/hr of serpentinite and 750 kg/hr (later recovered) of AS are
required. The amounts of residue produced are quite significant, 298 kg/hr. However, this material
is very rich in SiO2 (>70%) making its future processing for Si recovery an auspicious possibility.
Ideally, the ~36 kg/hr of iron and calcium hydroxide products are redirected to the steelmaking
industry. Although this appears to be a modest quantity of by-product, note that, at a larger scale,
the mineralisation of, e.g., all the CO2 emissions of a single steel company operating in Finland
(Ruukki), is enough to replace up to 18% of iron ore raw material with FeOOH [12].
Out of the 500 kg/hr of processed serpentinite it is possible to produce 275.3 kg/hr of Mg(OH)2.
Assuming a 90% carbonation efficiency, 275 kg/hr of MgCO3/Mg(OH)2, with a content of 90%
MgCO3, are produced. Figure 2 presents a schematic diagram of the process including mass balance
results.