3436
The Oxidation of Ferrous Ions as a Limitation to the Purification
of Leaching Solutions by Iron Precipitation
F.K. Crundwell*, N.B. du Preez, J. Christie
CM Solutions Metlab (Pty) Ltd, Johannesburg, South Africa
ABSTRACT: The hydrometallurgical production of metals of high purity in their salt form suitable for their
incorporation in the cathode material of lithium-ion batteries is transforming the process routes for cobalt,
nickel, and manganese. The removal of iron from solution by precipitation in the pH region 3 to 6 is a key
purification step in the hydrometallurgical production of many of these battery metals. This reaction can be
conducted at both atmospheric conditions and pressure conditions. In both cases, a residual concentration of
iron remains that is higher in concentration than expected from application of the thermodynamic equilibria
for jarosite or ferric hydroxide. Usually, this purification step consists of two reactions, that is, the oxidation
of ferrous ions and the precipitation of the ferric ions. In this paper, we argue that the residual iron that is not
removed is due to the kinetic mechanism of the oxidation of ferrous ions. Results from pilot plant operation are
used to support the model proposed.
Keywords: iron removal, ferrous oxidation, precipitation, battery metals
INTRODUCTION
Precipitation is an important step in both the purification
of metals and the production of final battery metal prod-
ucts. Precipitation of either impurities or the battery metal
is used extensively in the production process. For example,
cobalt hydroxide and nickel hydroxide are well-known
intermediates while magnesium, calcium and iron precipi-
tation steps are crucial in the hydrometallurgical purifica-
tion of these metals.
While precipitation is well-known and has been prac-
ticed for decades, the high-purity requirements for battery
metal salts raise additional challenges. Particularly interest-
ing is the removal of iron by the precipitation of ferric ion
from solution. The precipitation reaction of iron either as
hydroxide, Fe(OH)3, or as oxyhydroxide, FeOOH, or as
hydroxide sulphate, HFe3(SO4)2(OH)6, should produce
solutions that are very low in iron at the appropriate pH
value. Shown in Figure 1 is a plot of the concentration of
iron in solution expected from the thermodynamic rela-
tionship for the precipitation reaction in g/L of iron as a
function of the pH. These results show that at a pH value
of about 5, the iron concentration should be well below
0.1 mg/L. However, plant results collected over a multi-
year period and shown in Figure 2 indicate that there
appears to be a minimum value for concentration of iron in
the product stream.
In practice, the precipitation of ferric ions is preceded
by oxygen mass transfer and the oxidation of ferrous ions
to ferric ions. The reaction scheme is as follows. There are
three reactions: (i) oxygen dissolution from the gas phase to
the aqueous phase, commonly referred to as gas-liquid mass
transfer, Equation [1], (ii) the oxidation of ferrous ions to
ferric ions, Equation [2], and (iii) the precipitation of ferric
The Oxidation of Ferrous Ions as a Limitation to the Purification
of Leaching Solutions by Iron Precipitation
F.K. Crundwell*, N.B. du Preez, J. Christie
CM Solutions Metlab (Pty) Ltd, Johannesburg, South Africa
ABSTRACT: The hydrometallurgical production of metals of high purity in their salt form suitable for their
incorporation in the cathode material of lithium-ion batteries is transforming the process routes for cobalt,
nickel, and manganese. The removal of iron from solution by precipitation in the pH region 3 to 6 is a key
purification step in the hydrometallurgical production of many of these battery metals. This reaction can be
conducted at both atmospheric conditions and pressure conditions. In both cases, a residual concentration of
iron remains that is higher in concentration than expected from application of the thermodynamic equilibria
for jarosite or ferric hydroxide. Usually, this purification step consists of two reactions, that is, the oxidation
of ferrous ions and the precipitation of the ferric ions. In this paper, we argue that the residual iron that is not
removed is due to the kinetic mechanism of the oxidation of ferrous ions. Results from pilot plant operation are
used to support the model proposed.
Keywords: iron removal, ferrous oxidation, precipitation, battery metals
INTRODUCTION
Precipitation is an important step in both the purification
of metals and the production of final battery metal prod-
ucts. Precipitation of either impurities or the battery metal
is used extensively in the production process. For example,
cobalt hydroxide and nickel hydroxide are well-known
intermediates while magnesium, calcium and iron precipi-
tation steps are crucial in the hydrometallurgical purifica-
tion of these metals.
While precipitation is well-known and has been prac-
ticed for decades, the high-purity requirements for battery
metal salts raise additional challenges. Particularly interest-
ing is the removal of iron by the precipitation of ferric ion
from solution. The precipitation reaction of iron either as
hydroxide, Fe(OH)3, or as oxyhydroxide, FeOOH, or as
hydroxide sulphate, HFe3(SO4)2(OH)6, should produce
solutions that are very low in iron at the appropriate pH
value. Shown in Figure 1 is a plot of the concentration of
iron in solution expected from the thermodynamic rela-
tionship for the precipitation reaction in g/L of iron as a
function of the pH. These results show that at a pH value
of about 5, the iron concentration should be well below
0.1 mg/L. However, plant results collected over a multi-
year period and shown in Figure 2 indicate that there
appears to be a minimum value for concentration of iron in
the product stream.
In practice, the precipitation of ferric ions is preceded
by oxygen mass transfer and the oxidation of ferrous ions
to ferric ions. The reaction scheme is as follows. There are
three reactions: (i) oxygen dissolution from the gas phase to
the aqueous phase, commonly referred to as gas-liquid mass
transfer, Equation [1], (ii) the oxidation of ferrous ions to
ferric ions, Equation [2], and (iii) the precipitation of ferric