448
M. Kaya et al.
Introduction
Strengthened environmental legislation has prompted Pb–Zn producers to consider
at least three approaches to the problem: (1) Disposal of the oxidized Pb–Zn flotation
tails in environmentally stabilized forms. This is accomplished through mixing with
cement-forming or silicate-binding additives. (2) Recycling of tailings to recover Zn
and Pb units and to produce higher grade concentrates (with minimum impurities
such as Fe and As) in smaller quantities for further treatment. (3) Recovery of valuable
metals from the tails for economic recovery and detoxification of the residues [1].
Zn is one of the most important non-ferrous metals after aluminum and copper.
There is an increasing demand for Zn in the world. Primary (ores) and secondary (tailings, waste) production of Zn can be achieved by hydro- and/or pyro-metallurgical
processes. Hydrometallurgical methods mainly contain acid, alkaline, or ammonia
leaching. An acid dissociation constant, K a , (also known as acidity constant or acidionization constant) is a quantitative measure of the strength of an acid in solution.
It is the equilibrium constant for a chemical reaction known as dissociation in the
context of acid–base reactions:
K a = [A
−
]
∗
[H
+
]/[HA]
(1)
For many practical purposes, it is more convenient to discuss the logarithmic
constant, pK a :
pK a = − log 10 [K a ]
(2)
The more positive the value of pK a , the smaller the extent of dissociation at
any given pH—that is, the weaker the acid. A weak acid has a pK a value in the
approximate range −2 to 12 in water. Acids with a pK a value of less than about −2
are said to be strong acids. Strength of an acid is defined by its pK a value; the smaller
the number, the stronger the acid. The pK a value of HCl is approximately −7, the pK a
value of H 2 SO 4 is approximately −2, and the pK a value of HNO 3 is approximately
1.3. Acidity constants of inorganic acids tested in this study are in the range of 10
−3
and 10
−5 . So HCl is the strongest of all inorganic and organic acids tested. HNO 3 is
the weakest inorganic acid. From an environmental point of view, HNO 3 and organic
acids should be tested to dissolve flotation tailings. Inorganic sulphuric acid (SA)
leaching is the most commonly used process in the Zn–Pb industry today.
In general, leaching efficiency increases with increasing fineness of particles.
Flotation tails are ground, and if there is already fine material, then there is no
need for further grinding other then deagglomeration. However, fine particles may
produce aggregation and cause reduced dispersion problems during leaching. Gravity
pre-concentration and/or further flotation concentration of these fine tails are very
difficult and not feasible. The increasing demand for Zn and Pb has required intensive
studies on new hydrometallurgical processes for extraction of these metals from
low grade primary oxide ores or secondary tails. Surface composition, oxidation,
M. Kaya et al.
Introduction
Strengthened environmental legislation has prompted Pb–Zn producers to consider
at least three approaches to the problem: (1) Disposal of the oxidized Pb–Zn flotation
tails in environmentally stabilized forms. This is accomplished through mixing with
cement-forming or silicate-binding additives. (2) Recycling of tailings to recover Zn
and Pb units and to produce higher grade concentrates (with minimum impurities
such as Fe and As) in smaller quantities for further treatment. (3) Recovery of valuable
metals from the tails for economic recovery and detoxification of the residues [1].
Zn is one of the most important non-ferrous metals after aluminum and copper.
There is an increasing demand for Zn in the world. Primary (ores) and secondary (tailings, waste) production of Zn can be achieved by hydro- and/or pyro-metallurgical
processes. Hydrometallurgical methods mainly contain acid, alkaline, or ammonia
leaching. An acid dissociation constant, K a , (also known as acidity constant or acidionization constant) is a quantitative measure of the strength of an acid in solution.
It is the equilibrium constant for a chemical reaction known as dissociation in the
context of acid–base reactions:
K a = [A
−
]
∗
[H
+
]/[HA]
(1)
For many practical purposes, it is more convenient to discuss the logarithmic
constant, pK a :
pK a = − log 10 [K a ]
(2)
The more positive the value of pK a , the smaller the extent of dissociation at
any given pH—that is, the weaker the acid. A weak acid has a pK a value in the
approximate range −2 to 12 in water. Acids with a pK a value of less than about −2
are said to be strong acids. Strength of an acid is defined by its pK a value; the smaller
the number, the stronger the acid. The pK a value of HCl is approximately −7, the pK a
value of H 2 SO 4 is approximately −2, and the pK a value of HNO 3 is approximately
1.3. Acidity constants of inorganic acids tested in this study are in the range of 10
−3
and 10
−5 . So HCl is the strongest of all inorganic and organic acids tested. HNO 3 is
the weakest inorganic acid. From an environmental point of view, HNO 3 and organic
acids should be tested to dissolve flotation tailings. Inorganic sulphuric acid (SA)
leaching is the most commonly used process in the Zn–Pb industry today.
In general, leaching efficiency increases with increasing fineness of particles.
Flotation tails are ground, and if there is already fine material, then there is no
need for further grinding other then deagglomeration. However, fine particles may
produce aggregation and cause reduced dispersion problems during leaching. Gravity
pre-concentration and/or further flotation concentration of these fine tails are very
difficult and not feasible. The increasing demand for Zn and Pb has required intensive
studies on new hydrometallurgical processes for extraction of these metals from
low grade primary oxide ores or secondary tails. Surface composition, oxidation,