Carbon Dioxide Sequestration By Accelerated Carbonation of Waelz Slag

CARBON DIOXIDE
SEQUESTRATION BY ACCELERATED CARBONATION OF WAELZ SLAG

G. CAPPAI*°, G. DE GIUDICI**, D. MEDAS**, A.
MUNTONI*°, A. NIEDDU*, G. ORRU?*, M. PIREDDA*

* DICAAR, Department of Civil and Environmental
Engineering and Architecture, University of Cagliari, Piazza d?Armi 1, 09123,
Cagliari, Italy

° Institute of Environmental Geology and Geoengineering,
National Research Council (IGAG-CNR), Piazza D?Armi 1, 09123 Cagliari, Italy

**Department of Chemical and Geological Sciences, via Trentino
51, 09127, Cagliari, Italy

Key words: CO₂, Waelz Slag, Liquid to Solid Ratio,
CO₂ Pressure

Accelerated carbonation of
alkaline industrial residues has been recognized in recent years as an
advantageous method for sequestering CO₂ produced by point-source
emissions. The process, consisting in exploiting the content of reactive
alkaline oxides to fix CO₂ in the form of solid carbonates, is
indeed a permanent, safe and sustainable method of carbon storage. Some waste
materials, if compared to natural resources, may possess higher reactivity for
carbonation and be generally available near to CO₂ point sources,
reducing the need for raw materials consumption and the cost of the process.
Moreover, natural or accelerated carbonation processes applied to industrial
residues could lead to an improvement of both waste physical characteristics
and leaching behavior for a number of inorganic contaminants. Waelz slag is a
by-product of a metallurgical process aimed at recovering zinc from waste
products such as electric arc furnace dusts. Since the Waelz process can be
performed by using lime as a flux agent, a molten residue characterized by a
high alkalinity and metals content is obtained. Leaching may exceed the limit
values for disposal in non hazardous waste landfills, therefore the current common
management strategy consists of inertization by mixing with hydraulic ligands
which, in turn, significantly increases the mass and volume to be disposed and
negatively affects the possibility of reuse. In this frame, the application of
the accelerated carbonation process on Waelz slag appears interesting, both for
the reactive alkaline species found in the material and for the potential
stabilizing effect on metal mobility induced by carbonation.

In this paper, the possibility to apply accelerated
carbonation on Waelz slag is presented and discussed, with the aim of assessing
the influence of operating conditions on CO₂ sequestration capacity
and reaction kinetics as well as on the slag environmental and
mineralogical properties. In particular, the effect of parameters such
as liquid-to-solid (L/S) ratio (0, 0.2, 0.4, and 1 l/kg), CO₂
pressure (1, 3, 5, 10 and 20 bar) and process duration (up to 240 hr), was
evaluated. Several accelerated carbonation tests were performed at 25°C in a
pressurized reactor on Waelz slag according to a semy dry route. Table 1 resumes the different operating conditions adopted.

After each experiment, the CO₂
sequestration capacity was assessed by applying total carbon content analysis
on both fresh and carbonated residues. The degree of sequestration was also
evaluated by comparing the amount of CO₂ sequestered in each test
with the theoretical maximum amount, the latter evaluated by assuming the full
stoichiometric conversion to carbonates of the available CaO and MgO content. The
effect of the treatment on the leaching behaviour was evaluated through the
compliance leaching test (EN 12457-2).

The effect of the adopted operating variables on the
carbonation yields achieved was studied by multiple regression analysis, consisting
in fitting the set of data in a polynomial function. The equation obtained
describes the behavior of the response in the experimental region as a function
of the independent variables (Pressure, L/S ratio).

Table 1. Outline of the accelerated
carbonation experiments

The operating parameters, such as L/S ratio and total pressure
tested in the present study affected differently the extent of CO₂
sequestration and the reaction kinetics. Generally, lower
CO₂ uptake and Ca conversion were attained at dry conditions and
lower CO₂ pressures.

The maximum absolute value of 6% was achieved at a L/S=0.4
and at an operating pressure of 20 bar. The corresponding degree of
sequestration in terms of Ca conversion yield was of 28.5%. The minimum value
of 2% was achieved adopting a L/S = 0 and at the operating pressure of 1 bar,
with a Ca conversion yields limited to 9.5 %.

Figure 1 shows the effect on CO₂
uptake and reaction kinetics of CO₂ pressure for L/S = 0.4 L/kg and
T= 25°C. It can be observed that the higher pressure values improved the
reaction kinetics, probably due to both the increased CO₂
dissolution and the higher Ca²⁺ mobilization from the solid matrix.
As far as the effect of L/S is concerned, in general CO₂ sequestration
increased as higher L/S values were adopted, though no further positive effects
were observed by increasing the L/S from 0.4 to 1 L/kg.

The mineralogical changes occurred
during carbonation test were observed by means of XRD and FTIR
analysis and revealed that the main carbonation products were a double salt of
Ca and Mn (kuthnaorite) and amorphous carbonate.

The carbonation treatment allowed also
for an improvement of the environmental behavior of the Waelz slag; the mobility
of Pb and Zn was reduced and the corrosive nature of the waste was mitigated due
to the partial neutralization of pH induced by carbonation.

Figure 1. CO₂ uptakes attained
during the accelerated carbonation tests as a function of CO₂ Pressure
(L/S=0.4 L/kg; T= 25°C)