Leaching of Turkish Oxidized Pb–Zn Flotation Tailings …
449
mineralogical alterations, and dissolved ions also affect the floatability of particles.
But it seems that the best way to recover the fine valuable metals from Turkish waste
material is hydrometallurgical leaching.
Larba et al. (2) used CA as an alternative lixiviant in the presence of NaCl for
zinc oxide dissolution [2]. They obtained 70% Zn dissolution at 25 °C, 0.5 M CA
concentration, 350 rpm impeller speed, 60 min leaching time, and 50 mL/g L/S ratio.
At 50 °C, 90.4% Zn dissolution was obtained at 60 min leaching period with 0.05 M
CA concentration. Larba et al. (2) also found that mixtures of 0.05 M CA with 0.05 M
chloride anion increased Zn dissolution to 98% at 40 °C in 10 min [2]. Larba et al.
(2) found that NaCl or chloride ions both increased the Zn dissolution by 8.6% and
significantly reduced leaching time from 60 min to 10 min at 40 °C leaching temperature [2]. Since Larba et al. (2) used pure chemical substance (5%ZnO/αAl 2 O 3 ) in
their study, they dissolved only Zn with very dilute CA in 60 min [2]. The presence
of chloride anions along with H
+ from CA catalysed the dissolution of Zn and was
involved in the rate controlling step. Larba et al. (2) calculated the activation energies
for ZnO-citric acid leaching as 45.7 kJ/mol and ZnO-citric acid-chlorides leaching
as 96.9 kJ/mol [2].
Boukerche et al. (3) found that 2.0 M nitrates (NaNO 3 ) with 0.5 M citric acid
dissolved Zn totally after 30 min while 92% were dissolved in the presence of 2.0 M
chlorides (NaCl) after 270 min [3]. SEM analyses have shown the surface morphology corroded/pitted with nitrate anions. Among three anions tested in this study, only
sulfates (Na 2 SO 4 ) gave very low Zn dissolution. Furrer and Stumm (4) explained the
dissolution process by low molecular weight organic acids as follows: The adsorption of chelating anion (citrate) will transfer a considerable electron density into
the coordination sphere of the Zn metal cation (citrate contains several donor atoms
that are able of forming mononuclear bi- and polydentate surface chelates) [4]. As a
result, Zn metal-oxygen bonds are polarized and weakened consequently lowering
the energy barrier for the dissolution of Zn metal atoms. The explanation given above
indicates clearly the importance of the anionic species of organic acid (ligand) in the
dissolution process. Thus, two kinds of positive species (protons and the positively
charged surface of Zn) and two negative species [citrates and anions (Cl
− , NO 3
−
and SO 4
2− )] may react together and simultaneously in several ways: (1) H
+ attack
(protons with Zn surface species), (2) anions reactions (Cl
− , NO 3
− , SO 4
2− with
Zn surface species), (3) citrate complexation with Zn surface species, and (4) H
+
combination with citrates to form molecular citric acid. This shows the complexity
of the system studied.
Leaching by organic acids is an attractive alternative that is emerging in green
technology for the removal of metals due to the biodegradability of the organic acids
and the toxicity of inorganic leaching agents. Organic acids are weak and usually
have –COOH. Green organic acid leaching results with CA with and without NaCl,
OA, AA, TA, MA, and FA are presented. Reagent type, reagent dosage, leaching
temperature, leaching time, and S/L ratio are optimized in the leaching tests. Table 1
shows basic inorganic and tested organic acid properties and chemical bond structures. For inorganic acids acidity changes, HCl > H 2 SO 4 > HNO 3 and organic acids
ascorbic < formic < malic < citric < tartaric < oxalic acids.
449
mineralogical alterations, and dissolved ions also affect the floatability of particles.
But it seems that the best way to recover the fine valuable metals from Turkish waste
material is hydrometallurgical leaching.
Larba et al. (2) used CA as an alternative lixiviant in the presence of NaCl for
zinc oxide dissolution [2]. They obtained 70% Zn dissolution at 25 °C, 0.5 M CA
concentration, 350 rpm impeller speed, 60 min leaching time, and 50 mL/g L/S ratio.
At 50 °C, 90.4% Zn dissolution was obtained at 60 min leaching period with 0.05 M
CA concentration. Larba et al. (2) also found that mixtures of 0.05 M CA with 0.05 M
chloride anion increased Zn dissolution to 98% at 40 °C in 10 min [2]. Larba et al.
(2) found that NaCl or chloride ions both increased the Zn dissolution by 8.6% and
significantly reduced leaching time from 60 min to 10 min at 40 °C leaching temperature [2]. Since Larba et al. (2) used pure chemical substance (5%ZnO/αAl 2 O 3 ) in
their study, they dissolved only Zn with very dilute CA in 60 min [2]. The presence
of chloride anions along with H
+ from CA catalysed the dissolution of Zn and was
involved in the rate controlling step. Larba et al. (2) calculated the activation energies
for ZnO-citric acid leaching as 45.7 kJ/mol and ZnO-citric acid-chlorides leaching
as 96.9 kJ/mol [2].
Boukerche et al. (3) found that 2.0 M nitrates (NaNO 3 ) with 0.5 M citric acid
dissolved Zn totally after 30 min while 92% were dissolved in the presence of 2.0 M
chlorides (NaCl) after 270 min [3]. SEM analyses have shown the surface morphology corroded/pitted with nitrate anions. Among three anions tested in this study, only
sulfates (Na 2 SO 4 ) gave very low Zn dissolution. Furrer and Stumm (4) explained the
dissolution process by low molecular weight organic acids as follows: The adsorption of chelating anion (citrate) will transfer a considerable electron density into
the coordination sphere of the Zn metal cation (citrate contains several donor atoms
that are able of forming mononuclear bi- and polydentate surface chelates) [4]. As a
result, Zn metal-oxygen bonds are polarized and weakened consequently lowering
the energy barrier for the dissolution of Zn metal atoms. The explanation given above
indicates clearly the importance of the anionic species of organic acid (ligand) in the
dissolution process. Thus, two kinds of positive species (protons and the positively
charged surface of Zn) and two negative species [citrates and anions (Cl
− , NO 3
−
and SO 4
2− )] may react together and simultaneously in several ways: (1) H
+ attack
(protons with Zn surface species), (2) anions reactions (Cl
− , NO 3
− , SO 4
2− with
Zn surface species), (3) citrate complexation with Zn surface species, and (4) H
+
combination with citrates to form molecular citric acid. This shows the complexity
of the system studied.
Leaching by organic acids is an attractive alternative that is emerging in green
technology for the removal of metals due to the biodegradability of the organic acids
and the toxicity of inorganic leaching agents. Organic acids are weak and usually
have –COOH. Green organic acid leaching results with CA with and without NaCl,
OA, AA, TA, MA, and FA are presented. Reagent type, reagent dosage, leaching
temperature, leaching time, and S/L ratio are optimized in the leaching tests. Table 1
shows basic inorganic and tested organic acid properties and chemical bond structures. For inorganic acids acidity changes, HCl > H 2 SO 4 > HNO 3 and organic acids
ascorbic < formic < malic < citric < tartaric < oxalic acids.