584
M. Maccagni and E. Guerrini
Through this reaction, we reach an equilibrium calcium-sulphate in solution that
has to be controlled to avoid, or at least minimize, the possible formation of CaSO 4
scales, mainly into the heat exchangers, which need a careful work to be removed.
We demonstrated in our demo plant the possibility to produce very pure lead (Pb
> 99.99%). To reach this high purity, we need to remove all the impurities nobler than
lead to avoid their co-plating with lead. This can be achieved through cementation
using, in the cementation reactor, the electrolytic produced Pb flakes as the reactant
or any kind of Pb scrap. The relevant chemical reaction is shown here below.
Me
n +
+ n
2 Pb = Me + n
2 Pb
2+
(Me = Ag, Cu, Cd, . . .)
(7)
After the cementation, the solution now is pure enough to be fed to the electrolytic
unit but needs a careful polishing filtration to remove even any particle of cement that,
in the electrolytic unit, can be incorporated into the deposit decreasing its quality.
The electrowinning flow cell unit is the core of the process; it is unique and very
different from the typical cells used in the traditional electrowinning.
The plating mechanism is the opposite of the traditional electrowinning cells:
instead of trying to plate a very smooth lead cathode plate, in the FAST cell, we would
like to produce flakes that do not strongly adhere to the cathode. These plated flakes
are easily stripped off the cathode and transported out of the cell by the electrolyte
flow that has to be higher than 1.5 m/s.
The cell design is very peculiar. The most important feature is the gap between
the two electrodes that is very narrow for the following reasons:
• having to reach a high linear velocity of the electrolyte, a narrow gap helps to
achieve the required velocity with a reasonable pump flow rate;
• having to reduce the ohmic drop in solution to reasonable value and being it
function of the gap between the electrodes, a narrow gap reduces this ohmic drop
keeping low the cell voltage and, consequently, the energy cost.
Of course, this gap has to be “big” enough to avoid the formation of short circuits
that can occur when the plated Pb contacts the anode before being stripped to avoid
the re-dissolution of lead decreasing the faradic efficiency.
The FAST cell was proven capable of operating at very high current densities (up
to 50,000 A/m
2 ). The best compromise for an industrial application seems to be in
the range of 8000–12,000 A/m
2 . Due to the lead solubility, the temperature of the
electrolyte in the cell, as well as one of the solutions in any other unit of the process,
has to be kept quite high.
Another important peculiarity of the cell is the use of bipolar electrodes: on one
face, they work as a cathode, and on the other one, they work as an anode. Some
different material candidates such as graphite and some metallic electrodes were
tested. These different electrode materials proved to have a strong impact on the cell
design, on the adhesion to the cathode, and on the formation of anodic PbO 2 . For
this last mentioned problem, the appropriate working conditions through the control
of the free ammonia concentration of the electrolyte play a very important role.
M. Maccagni and E. Guerrini
Through this reaction, we reach an equilibrium calcium-sulphate in solution that
has to be controlled to avoid, or at least minimize, the possible formation of CaSO 4
scales, mainly into the heat exchangers, which need a careful work to be removed.
We demonstrated in our demo plant the possibility to produce very pure lead (Pb
> 99.99%). To reach this high purity, we need to remove all the impurities nobler than
lead to avoid their co-plating with lead. This can be achieved through cementation
using, in the cementation reactor, the electrolytic produced Pb flakes as the reactant
or any kind of Pb scrap. The relevant chemical reaction is shown here below.
Me
n +
+ n
2 Pb = Me + n
2 Pb
2+
(Me = Ag, Cu, Cd, . . .)
(7)
After the cementation, the solution now is pure enough to be fed to the electrolytic
unit but needs a careful polishing filtration to remove even any particle of cement that,
in the electrolytic unit, can be incorporated into the deposit decreasing its quality.
The electrowinning flow cell unit is the core of the process; it is unique and very
different from the typical cells used in the traditional electrowinning.
The plating mechanism is the opposite of the traditional electrowinning cells:
instead of trying to plate a very smooth lead cathode plate, in the FAST cell, we would
like to produce flakes that do not strongly adhere to the cathode. These plated flakes
are easily stripped off the cathode and transported out of the cell by the electrolyte
flow that has to be higher than 1.5 m/s.
The cell design is very peculiar. The most important feature is the gap between
the two electrodes that is very narrow for the following reasons:
• having to reach a high linear velocity of the electrolyte, a narrow gap helps to
achieve the required velocity with a reasonable pump flow rate;
• having to reduce the ohmic drop in solution to reasonable value and being it
function of the gap between the electrodes, a narrow gap reduces this ohmic drop
keeping low the cell voltage and, consequently, the energy cost.
Of course, this gap has to be “big” enough to avoid the formation of short circuits
that can occur when the plated Pb contacts the anode before being stripped to avoid
the re-dissolution of lead decreasing the faradic efficiency.
The FAST cell was proven capable of operating at very high current densities (up
to 50,000 A/m
2 ). The best compromise for an industrial application seems to be in
the range of 8000–12,000 A/m
2 . Due to the lead solubility, the temperature of the
electrolyte in the cell, as well as one of the solutions in any other unit of the process,
has to be kept quite high.
Another important peculiarity of the cell is the use of bipolar electrodes: on one
face, they work as a cathode, and on the other one, they work as an anode. Some
different material candidates such as graphite and some metallic electrodes were
tested. These different electrode materials proved to have a strong impact on the cell
design, on the adhesion to the cathode, and on the formation of anodic PbO 2 . For
this last mentioned problem, the appropriate working conditions through the control
of the free ammonia concentration of the electrolyte play a very important role.