What Controls the Leaching of Magnesite with Concentrated Solutions of Hydrochloric or Nitric Acid?

ABSTRACT

The kinetics of the leaching of natural magnesite with hydrochloric or nitric acid has been studied. Relatively high values of the apparent activation energy (from 46.5 to 70.0 kJ mol⁻¹), but especially the low values of the apparent reaction order for H⁺ cations, $\mathit{n}$, have shown that the whole process is controlled by the surface chemical reaction. The value of $\mathit{n}$ decreases continuously with increasing activity of H⁺ cations in the range of pH 5 to −1.4. At $\mathrm{pH}<2.6$ there is a transition from the kinetic regime characterized by $\mathit{n}\cong$ 0.5 to another one with $\mathit{n}$ close to zero.

KEYWORDS:

• Chemical dissolution of magnesite
• hydrochloric and nitric acid
• shrinking particle model
• reaction order
• chemical reaction-controlled process

Acknowledgments

This work was supported by the Slovak Grant Agency for Science (Grant ID: 1/0503/23).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notation

$\mathit{a}$	=	stoichiometric coefficient for H+ cations in Equation (1)(1) $\mathit{MgC}{O}_3\left(\mathit{s}\right)+2H^{+}\left(\mathit{aq}.\right)\to \mathit{M}{g}^{2+}\left(\mathit{aq}.\right)+H_2\mathit{O}\left(\mathit{l}\right)+\mathit{C}{O}_2\left(\mathit{aq}.\right).$(1) ;
$a_{Ab}^0$	=	initial activity of H+ cations in bulk lixiviant;$\mathit{\hspace{0.17em}}{a}_{Ab}^0$= ${\gamma }_A^0{c}_{Ab}^0/\left(1\mathrm{mol}{\mathrm{m}}^{-3}\right)$; –
A	=	H+ cations
B	=	MgCO3
$c_{Ab}^0$	=	initial concentration of H+ cations in bulk lixiviant; mol m−3
CV	=	covariance of the particle size distribution
$E_A$	=	apparent activation energy of reaction (1); J mol−1
$\mathit{k}$	=	rate constant obeying Arrhenius law, $\mathit{k}=k_0\exp \left(-E_A/R_G\mathit{T}\right)$; –
$k_0$	=	constant (pre-exponential factor) in Arrhenius equation; mol m−2 s−1
$\mathit{k}$	=	process parameter defined by Eq. (4); m2 mol−1
${\mathit{M}}_B$	=	molar mass of MgCO3; kg mol−1
${\mathit{B}}_i$	=	apparent order of chemical reaction (1) with respect to H+ cations; –
$r_A^0$	=	initial reaction rate defined by Equation (5)(5) $r_A^0=\mathit{k}{\left(a_{Ab}^0\right)}^{\mathit{n}},$(5) ; mol m−2 s−1
$\mathit{R}$	=	initial radius of spherical solid (ore) particles; m
$\bar{R}$	=	mean of particle radius distribution; m
$R_G$	=	gas constant = 8.314 J mol−1 K−1
SF NSPM =	=	“simplified” non-porous shrinking particle model
$\mathit{t}$	=	reaction (leaching) time; s
$t_r$	=	time required for complete conversion of solid (ore) particle, defined by Eq. (3); s
T	=	temperature; K
$w_B^0$	=	initial weight fraction of ${\mathit{B}}_i$ in magnesite ore; –
$x_B$	=	fraction of reagent B reacted; in present study, fraction of magnesium (carbonate) dissolved; –
Greek letters	=
${\gamma }_A^0$	=	activity coefficient of H+ cations in leach solution at $\mathit{p}$; –
${\rho }_S$	=	density of solid (ore); kg m−3
Superscripts	=
$\mathit{exp}.$	=	value(s) determined from experiment(s)
$\mathit{sim}.$	=	value(s) obtained by numerical simulation(s)

Note: The symbols used only locally and explained in the text are not listed.

Additional information

Funding

Research funded by Agentúra Ministerstva Školstva, Vedy, Výskumu a Športu SR [VEGA 1/0503/23].