1. Introduction - Firenze University Press

2. Process development
2.1. Background
During earlier studies [6, 10, 11], several batch experiments have been performed to verify the
effects of different process changes such as temperature and CO2 pressure on the process chemistry.
It was found that the reaction steps, which both generate some heat (see Section 4), could be
performed at room temperature (20–30 °C). An increase in temperature to 70 °C had hardly any
effect on the extraction stage kinetics. In carbonation the shape of the precipitated particles changes
with the temperature. This restricts the applicable carbonation temperatures below 30–40 °C. The
effect of carbon dioxide pressure was limited to changes in process kinetics, the precipitation rate
being slower at lower partial pressures of CO2. Thus, 20–30 °C was chosen as a suitable process
operation temperature interval. At these temperatures the kinetic reaction rates are acceptable [10]
and solubility and volatility of gaseous components such as NH3 and CO2 are beneficial for the
process.
Steel converter slag was reported to contain calcium as free lime (CaO), larnite (Ca2SiO4) and
various calcium-iron compounds that seem not to react with ammonium salt solvents. The
dissolution reactions of lime and larnite are presented as (R1) and (R2). Carbonate precipitation
chemistry can be summarised as (R3) and (R4). X in the reaction equations represents Cl - , NO3 - or
CH3COO - (acetate), depending on the chosen salt.
( s)
2NH
Xaq
H Ol
CaX
aq 2NH
OHaq
(R1)
CaO 4
2
2
4
aq H Ol
CaX
aq CaO
SiO ( s)
2NH
OHaq
2CaO SiO2
( s)
2NH4
X
2
2
2
(R2)
4
aq CO ( g)
NH CO aq H Ol
2NH4 OH 2
4 3
(R3)
2
CO aq CaX aq CaCO ( s)
2NH
X aq NH 4 2 3
2
3
4
2
(R4)
In the batch experiments it was also observed that if solvents with molarities higher than 1.0 mol/L
or solid-to-liquid ratios higher than 100 g/L were used, also some iron and manganese was extracted
from the steel slag, decreasing the purity and whiteness of the process solution and the produced
carbonates [10-12]. Thus, these specifications were used in the current work. Mainly ammonium
chloride solutions were used, both in modelling work and in experiments, since the available data
were most complete for this solvent, and also because it is cheaper than the other two ammonium
salts. Some observations of experimental work with ammonium nitrate will be presented in later
sections.
In larger scale, the process should preferably be operated on continuous basis to achieve
presumably lower operational costs and better adaptability to changes in feedstock quality.
Problems arising especially from the continuous operation as identified already by [9, 13] are the
losses of solvent components (NH3 and water vapour, ammonium and calcium salt precipitates) and
dissolution of excess carbon dioxide as bicarbonate and carbonate ions in the carbonation step. The
solvent losses cause an unnecessary increase in process costs, both as a need of a solvent make-up
but also as a need for purification of precipitates and purged gases. On the other hand, if excess
dissolved carbon species are recycled from carbonation to the extraction unit, solid calcium
carbonate is precipitated on the slag particles, lowering the overall production rate of pure carbonate
product (PCC). These problems exist to some extent also in a batch type process.
2.2. Process modelling
Thermodynamic modelling and simulation software Aspen Plus 7.2 were utilised to study the
possibilities to decrease losses of both carbonate product and solvent components. The software
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