Model of the system of calcium sulphate, calcium sulphide and water

Abstract

Calcium sulphide and calcium sulphate are permanent components in the solid and liquid
waste of oil shale industry in North East of Estonia due to the limestone containing basement,
Solid calcium sulphide in contact with water according to the solubility product value will add
to the liquid phase sulphide anions, which reacting with water are forming HS- and H2S.
Hydroxide ions formed at the same processes will change the pH value of water.
In present paper the model is presented for describing the equilibrium state of the given
system. For calculation of concentrations of seven different particles present in the liquid 2+ 2 + phase (Ca , so/-, s -, HS-, H2S, OH-and H3O ) seven equations are composed.
Concentrations of so/- and s2
-ions in liquid phase at the equilibrium state are negligible and
were not taken into account when solving composed equations. Despite of low values of
concentrations of both ions in the liquid phase they are influencing the equilibrium state of the
system,
First and second step dissociation constants of H2S (accordingly presented as Ka1 and Ka2)
from literature were used for mathematical solution of equations. It was found that pK.1 value
for the first step of ionization ofH2S in water at 25°
C is 7.01 having low deviation rate of the
values published by different authors. The value of pKa2 for second step of ionization of H2S,
is scattering in wide range, from 13.l-;, 17.1 (at t = 20°
C) and from 13.78-;, 15.19 (at t =
25°
C), Calculated concentrations of the components in equilibrium state of the system are
significantly depending on the pKa2 value, Using developed model and average values of pK.1
= 7.01 and pKa2 = 14.63 at t = 25°
C, the calculated pH value was estimated 12,5, The
proposed model is useful for modeling systems containing CaS and CaSO4, which are quite
common in polluted areas with limestone basement,