Page 101 - Materials Chemistry, Second Edition

82 Life Cycle Assessment of Wastewater Treatment

onto a conveyor belt. The production speed can be considered equal to r = 1.0 t/h.
Downstream of the conveyor belt is placed a mechanical selection system realized
by means of three sieves that intercept the batteries according to their cross-sectional
diameter. The nominal power of the geometric sorting line is equal to Pn = 11.0 kW.
Following this, chemical selection is carried out using a scanning X-ray machine to
identify stacks of a certain type: first alkaline, then zinc carbon. Once selected, the
batteries are cut along their longitudinal section. Batteries with the same chemical
characteristics are fed into a tumbling machine together with an equal volume of
water; the mechanical action and washing allow the contents, called black paste
(BP), to be extracted from the metal. The black paste is fed into the hydrometallurgi-
cal recycling process.

5.3.2 energy conSumption caLcuLation
An analysis was carried out to evaluate the energy consumption of the chemical
treatment plant for alkaline batteries, assuming a steady-state operation. The total
energy consumption for each batch of treated batteries was given by

214 845. kWh + 69 13. kWh + 58 56. kWh + 97 6. kWh = 440 14. kWh (5.1)

for a total of 1584.5 MJ.
In this sum, the first term is the total consumption due to the various equipment
used in the different elementary phases; the second term is the consumption of the
scrubber; the third term is the consumption of the electrolytic cell; and the fourth
term is the consumption of the evaporator/concentrator. The materials covered after
each batch are listed below:

• Final BP, which after drying, may represent a secondary raw material to be
used for the manufacture of new batteries
• High-purity zinc oxalate, which is a raw material valuable for the produc-
tion of pure metallic zinc
• Manganese (IV) oxide, which is a valuable product for the manufacture of
new batteries
• In summary, 66 kg of final BP is obtained after a treatment of 5 h and
53 min, with a total energy consumption of 330.15 kWh, which corresponds
to 1188.5 MJ.

5.4 LCA STUDY

5.4.1 functionaL unit, goaL, and aim of the Study
The present LCA takes into account the process of selection of exhausted alkaline bat-
teries and the subsequent hydrometallurgical process used to recycle the resulting mass.
The approach used is “cradle to gate,” as other steps of the battery life cycle (production,