Effect of slag basicity and reduction time on carbothermal reduction behaviour of P-bearing steelmaking slag

ABSTRACT

To realize the efficient reduction and recycling of phosphorus resources in P-bearing steelmaking slag, it is necessary to study slag composition and phase changes during carbothermal reduction under different conditions (slag basicity and reduction time). The thermodynamic and kinetic behaviours of carbothermal reduction dephosphorization of P-bearing steelmaking slag are systematically discussed as well. The results show that the reduction rate of FeO and P₂O₅ show an upward trend with the increase of reduction time at 1823 K. However, the reduction rate of FeO does not change much within 50 minutes. In the range of slag basicity from 0.5 to 2.5, the formation of Ca₃(PO₄)₂ increases with the increase of slag basicity. However, the increase of slag basicity is not good for the occurrence of carbothermic reduction gasification dephosphorization reaction. Taking into account the higher formation of Ca₃(PO₄)₂ and the better dephosphorization effect of carbothermic reduction gasification, the appropriate slag basicity for carbothermic reduction gasification dephosphorization is 1.0–1.5. Carbothermal reduction gasification dephosphorization reaction of P-bearing steelmaking slag is a second-order reaction, and the mass transfer of P₂O₅ in the slag is the rate-limiting step of the whole process. The kinetic equations that describe the variation of P₂O₅ content in slag with time are yielded through regression. At 1823 K, by controlling the appropriate slag basicity (1.0–1.5) and reduction time (at least 20 minutes), the gasification dephosphorization rate of the carbothermal reduction can reach more than 80%, which can realize efficient recovery of phosphorus resources in P-bearing steelmaking slag.

KEYWORDS:

• P-bearing steelmaking slag
• carbothermic reduction
• slag basicity
• reduction time
• phase changes
• thermodynamics
• kinetics

Disclosure statement

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

List of variables
Variables	=	Definitions
$a_{\mathrm{CaO}}$	=	activity of CaO
$a_{{\mathrm{P}}_2{\mathrm{O}}_5}$	=	activity of P2O5
$a_{\mathrm{Si}{\mathrm{O}}_2}$	=	activity of SiO2
$a_{\mathrm{C}{\mathrm{a}}_3{(\mathrm{P}{\mathrm{O}}_4)}_2}$	=	activity of Ca3(PO4)2
$a_{\mathrm{CaSi}{\mathrm{O}}_3}$	=	activity of CaSiO3
$a_{\mathrm{C}{\mathrm{a}}_2\mathrm{Si}{\mathrm{O}}_4}$	=	activity of Ca2SiO4
$a_{\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_2{\mathrm{O}}_7}$	=	activity of Ca3Si2O7
$K_1$, $K_2$, $K_3$, $K_4$	=	equilibrium constants
$P_{{\mathrm{P}}_2}$, $P_{\mathrm{CO}}$	=	equilibrium partial pressures of P2 and CO
$k_t$	=	reaction rate constant (cm3 (mol·min)−1)
$C_{{\mathrm{P}}_2{\mathrm{O}}_5}$	=	P2O5 concentration in slag (mol cm−3)
n	=	reaction order
$W_{{\mathrm{P}}_2{\mathrm{O}}_5}$	=	mass fraction of P2O5 in the slag (%)
$\rho$, ${\rho }_L$	=	steel slag density, here is 3.2 g cm−3
$M_{{\mathrm{P}}_2{\mathrm{O}}_5}$	=	P2O5 relative molecular mass (g mol−1)
$C_{{\mathrm{P}}_2{\mathrm{O}}_5}^0$	=	P2O5 concentration in slag at the first sampling point (mol cm−3)
$C_{{\mathrm{P}}_2{\mathrm{O}}_5}^t$	=	P2O5 concentration in slag at any sampling point (mol cm−3)
t	=	reduction time
$\beta$, ${\beta }_L$	=	the mass transfer coefficient of P2O5 in slag (cm s−1)
D	=	the diffusion coefficient of P2O5 in slag (cm2 s−1)
$t_e$	=	the time that the generated P2 bubbles stay at the reaction interface (s)
d	=	bubble diameter
$C_{{\mathrm{P}}_2{\mathrm{O}}_5}^{\ast }$	=	the concentration of P2O5 in slag when the interface reaction reaches equilibrium (mol cm−3)
$k_{\beta }$	=	the apparent rate constant (mol (cm3·s)−1)
$A$	=	interface reaction area (cm2)
$M_L$	=	slag quality (g)
$C_{P_g}$	=	the concentration of gas phase P2 (mol cm−3)
$C_{P_{\mathrm{g}}}^{\ast }$	=	the concentration of P2 when the interface reaction reaches equilibrium (mol cm−3)
${\beta }_g$	=	the mass transfer coefficient of phosphorus in gas phase (cm s−1)
${\rho }_g$	=	gas phase density (g cm−3)
$M_g$	=	gas phase quantity
$L_P$	=	the phosphorus equilibrium distribution ratio

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [grant numbers 51704080, 51874102, 51704085].