Lead Paste Desulphuration

June 24, 2016

Lead Paste Desulphurization


The content of Sulphuric acid in the paste is considered a matter of pollution and becomes more dangerous when the paste is put in the furnace to produce pure lead.

During this phase the furnace releases gases that are mainly constituted by SOx. Some countries rules require that the peak point never exceed the 200 mg/Nm3. Some others out of Europe, require that the daily average never exceed this data. So, a desulphurization system is required.

This plant, using some reagents, special reactors and a crystallization units, neutralize the content of H2SO4 avoiding all environmental problems. In the other hand, the result from the neutralization plant is sodium sulphate Na2SO4 that is in detergent degree purity, that can be used in detergents or in glass production.

From the exhaust battery crushing and components separation process two fractions are obtained containing:

  • grids and poles
  • Pb paste (PbO, PbO2 and PbSO4), where PbSO4 is 50% (S = 6%).

The desulphurization of the Pb-paste is made in water with adding sodium carbonate and traces of caustic soda in the B01 reactions tanks, obtaining the transformation from PbSO4 to PbCO3 according to the following reaction:

PbSO4 + Na2CO3 > PbCO3 + NaSO4

During this treatment the sulphuric acid (H2SO4) present or eventually added is neutralized with caustic soda, according to the following reaction:

H2SO4 + 2NaOH > Na2SO4 + H2O

The machines composing the plant are:

  • Paste desulphurized section and sulphuric acid neutralization section (B01).
  • Desulphurised paste de-watering (filter press B02) section with washing and drying to eliminate as well as the sodium sulphate.
  • Crystallization anhydrous sodium sulphate (Na2SO4) section (B09).
  • Anhydrous sodium sulphate drying section (B10).
    The process is shown on the FLOW SHHET picture.
  • Environmental pollution for the gaseous emission of SO2/SO3 in theatmosphere.

With the present desulphurisation process, the PbSO4 turn to PbCO3 and the melting in the same furnace of the PbCO3 involves advantages in the process as follows:

  • Minimum or no necessity of reagents added, it is expected about 5% of reagents on the charge instead of previus 23%.
    More exploitation of the furnace capacity for the inprovments of lead (95%) in the charge, instead of about 78% with sulphur paste.
    Therefore it is foreseen a productivity increase of the rotating furnace of about + 20%.
  • Less time cycle (about 20%), and less fuel energy consumption (16%) due to the less chemical reactions and lower furnace temperature in the furnace during melting (Carbonate specifi c heat – 356 cal/Kg, Sulphate – 535 cal/Kg).
  • Of course less production of slags (27% less) due to the lack of sulphur and iron compounds that produce slags.

Desulphurized paste
Metallic yield (as Pb oxides and carbonate): 87% about
Residual sulphur content (as PbSO4): 0,13% maximum, on d.w.
Residual sulphur content (as Na2SO4): 0,2% depends on the effi ciency condition of the filter
Moisture: 10% maximum, on d.w.
Filter press Pb Paste screenning