Leaching Thermodynamics of Oil Shale Waste Key Components

About 60% of
the Estonia`s fuel balance is covered by local low-grade fuel, oil shale (OS),
which is utilized mainly in heat and power production, but also in shale oil
production. Safe disposing of mineral waste (in 2013 Eesti Energia (the main
power producer in Estonia) deposited 8.0 Mt oil shale ash (OSA)) and control of
atmospheric emissions (13.4 MT CO₂ according to Eesti Energia) are
among the most serious problems caused by the extensive use of OS [1]. Today,
about 90% of the OSA is wet deposited in the ash fields near the power plants [2]. The OSA is classified as a
?hazardous waste? due to its high free lime content which dominates leaching
(pH 12 - 13). On the other hand, this makes OSA (or leaching wastewater) an attractive
candidate for CO₂ mineralization.

The
aim of the study was to elaborate the leaching equilibriums of the main water-soluble
Ca-compounds of OSAs,
as a basis for mathematical models which simulate the processes from leaching
to aqueous carbonation.

Pulverised
firing cyclone ash (PFA), an ash mixture from circulated fluidized bed
combustion (CFBCA) and solid heat carrier ash (SHCA) were selected as Ca?rich raw
materials. The weight percentages of the OSA components in focus of current
study are given in Table 1.

Table 1. Main components in OS ashes, wt%.

The PFA and
CFBCA contain considerable amount of free lime (especially PFA) and anhydrite,
which in water systems form portlandite and gypsum, respectively. The SHCA is characterized
by high sulfide content (mainly as CaS), which disseminates toxic compounds
into water systems and causes atmospheric pollution. The aqueous suspensions (OSA/H₂O
= 1/2 - 1/250) were shaken for 3 - 3.5h and filtrated immediately. The
binary and ternary model systems containing respective amounts of pure Ca(OH)₂
(as Ca²⁺ source), CaSO₄*2H₂O (as SO₄^2-
source), and CaS (as S^2- source), were composed according to the
compositions of selected OSAs and studied for comparison.

The equilibrium
concentrations of the main ions (Ca²⁺, SO₄^2-
accordingly ~33.0 ? 39.0, ~12.0 ? 16.0 mM) as well as pH and electrical
conductivity were similar in all saturated OSA based model systems. Concentrations
of sulfide differed furthest ~0.01 - 0.02 (in PFA
and CFBCA) and 0.2 - 0.3 (in SHCA). The lowest pH values occur in SHCA
systems whereas the highest values are found in PFA systems ? this is due to
the initial free lime (CaO) content which affects pH the most. From the
conductivity results, at saturated concentrations of CFBCA and SHCA the values
are higher (9.5 - 10.0 mS/cm) as compared to PFA systems (9.0 - 9.6 mS/cm).
This reflects the fact that higher CaS concentration has a stronger influence on
the conductivity (the ions remain in aqueous phase). The Ca²⁺-
and SO₄^2-- ions at saturated concentrations present
similar values in all model systems based on pure components, being slightly
higher in ternary model systems due to the dissolution and oxidation of CaS. The concentrations of Ca²⁺ - and
SO₄^2- - ions (accordingly ~30 and ~10 mM)
in the SHCA ? H₂O system are lower compared to CFBCA and PFA real water systems (accordingly ~40
and ~16 mM).
The leaching ability of CaS clearly manifests itself in SHCA based systems
characterized by the highest initial CaS content. While the S^2--ion concentration
in the leachates of CFBCA and PFA remains negligible (0.015 mM), it is up to 20
times higher in case of SHCA systems. Even though the initial S^2- - ion
source concentration difference in the compared ashes is not twenty-fold, the
leaching of CaS in SHCA systems is favoured as compared to other OSA based
systems. Comparing the results of model
systems and OSA based systems, the results coincide. The differences are caused
by other active components residing in the ash systems.

Equations (1) - (5) represent the dissociation equilibriums for model
systems:

The thermodynamic equilibrium
constants (K_eq1 ? K_eq5) of dissolution
reactions (1) - (5) for three types of OSAs (PFA, CFBCA and SHCA) were
evaluated on the basis of comparative leaching experiments with model systems
and real ash based systems. These constants could be used in future studies,
firstly, to estimate the dissolution kinetics of the main ash components and
secondly, to design a pilot-scale OSA carbonation reactor.

[1]. https://www.energia.ee/-/doc/10187/pdf/concern/annual_report_2013_eng.pdf

[2]. Kuusik, R.; Uibu, M.; KirsimƤe, K.; MƵtlep, R.;
Meriste, T. (2012). Open-air deposition of Estonian oli shale ash: formation,
state of art, problems and prospects for the abatement of environmental impact.
Oil Shale, 29(4), 376 - 403.