(271g) Thermodynamics and Kinetics for KOH Leaching of Potassium Alunite from Copper tailings

Thermodynamics and kinetics
for KOH leaching of potassium alunite from copper tailings

Mengjie Luo, Youfa Jiang, Ping Li,
Xingfu Song*, Jianguo Yu*

National Engineering Research Center for Integrated Utilization of
Salt Lake Resources, East China University of Science and Technology, Meilong
Road 130, Shanghai, China, Postcode 200237.

mengjie_luo@ecust.edu.cn, xfsong@ecust.edu.cn,
jgyu@ecust.edu.cn

Abstract:

Five to fourteen billion tons of mine tailings per year worldwide
will be discharged from the contemporary mining industry. Tailings as waste is
not only the loss of valuable resources, but also brings serious environmental
impacts and ecological damages. In Fu Jian province of China, about 18000
ton/day copper tailings are discharged from the production line of the
extraction of Cu from copper mine. Through mineral flotation, we can get the
refined copper tailings with 72.67% alunite, 19.30% kaolinite, and 6.86%
quartz. Alunite is considered a potential resource for both the production of
alumina and potash fertilizer. In this work, a novel gradient leaching process
is proposed to recover potassium and aluminum resoureces from copper tailings.
In the proposed gradient leaching process, alunite from tailings is selectively
extracted, while impurities of kaolinite and quartz, are still remained in the
leaching residues. Here, we shall investigate reaction thermodynamics, reaction
kinetics and dissolution mechanism of alunite from copper tailings in KOH
solution.

It is deduced that there exist three reactions during tailings
leaching with potassium hydroxide, as shown in Eq(1)-(3). Using the HSC
chemistry software, ¦¤_rG_¦È and ¦¤_rH_¦È
for three reactions with KOH in molten state and aqueous state are calculated
at 303 K-493 K, as shown in Figure 1. It is found that under the same reaction
conditions, the thermodynamic tendency of reactions with KOH is potassium
alunite > kaolinite > quartz.

KAl₃(OH)₆(SO₄)₂
+ 6KOH = 2K₂SO₄ + 3KAlO₂ + 6H₂O                   
       (1)

Al₂O₃¡¤2SiO₂¡¤2H₂O
+ 6KOH = 2K₂SiO₃ + 2KAlO₂ + 5H₂O                         (2)

SiO₂
+ 2KOH = K₂SiO₃ + H₂O                                      
        (3)

Thermodynamically, the reaction of potassium alunite in KOH
solution might become preferential and dominant in the leaching process of
tailings. Therefore, the gradient leaching of potassium alunite from tailings
is proposed, where potassium alunite is dissolved firstly, and then following
kaolinite and quartz dissolution. Leaching experiments are carried out to test
the thermodynamical prediction, as shown in Figure 2. Using 50% KOH to leach
tailings at 323 K, potassium alunite is dissolved into solution with above 90%
recovery, while kaolinite and quartz as impurities are still kept in residue.
It is recommended to control the silicon ions concentration as small as
possible in the solution in order to maintain the high potassium leaching ratio
and to reduce the expensive KOH consumption. Therefore, the leaching
temperature should be maintained below 373 K.

Figure 1. ¦¤_rG_¦È
for reactions (1)-(3) with KOH in molten state (A) and aqueous state (B)
calculated by HSC chemistry software

Figure 2. Experimental results
of K, Al, Si leaching ratio at different leaching temperature

Thermodynamic
analysis and experimental results prove that it is feasible to extract
potassium alunite from copper tailings into the alkaline solution at 323 K-363
K and to keep kaolinite and quartz in the leaching residues avoiding the
traditional desilication step, being a green and closed-loop gradient leaching
process. The dissolution mechanism of potassium alunite from tailings is that
individual aluminum atoms at the surface react with hydroxide ions to form
activated complexes and sulfate tetrahedral at the surface depart intact, as
shown in Figure 3. According to the proposed dissolution mechanism, the
shrinking core model with the surface chemical reaction control is used to
predict the leaching kinetics of alunite from tailings at 323 K-363 K with an
acceptable accuracy, where the apparent activation energy is 44.48 kJ•mol^-1 and the order of reaction with respect to KOH is
1.76, as shown in Eq(4).

Figure 3. The proposed dissolution mechanism

Reaction model: 1-(1-X)^1/3 = 2122.16 C_KOH^1.76e^{-44480/(8.314T)}t                     
(4)

Keywords: copper tailings, gradient leaching, thermodynamics, kinetics,
dissolution mechanism