A new horizon for lead paste desulfurization

Desulfurization is a key step in the recycling of lead batteries. In the first step (Figure 1) of recycling the casing and the electrolyte is separated off before the pure lead and lead paste are processed.

Traditional desulfurization of the paste typically uses sodium hydroxide or sodium carbonate. The reactions are:

PbSO4 + Na2CO3 ⇒ PbCO3 + Na2SO4

PbSO4 + 2NaOH ⇒ Pb(OH)2 + Na2SO4

H2SO4 +Na2CO3 ⇒ Na2SO4 + H2O+ CO2

H2SO4 + NaOH ⇒ Na2SO4 + H2O

The chemistry of the traditional sodium carbonate-based desulfurization is a straightforward reaction.

Among the main advantages, it is possible to highlight the easy availability of the reagent, the well known and studied mechanisms of reaction the process is based on, the possibility to use the obtained by-product (sodium sulfate) as an intermediate in the chemical industry and the stability of the reagent and its product.

In summary this hydrometallurgical approach resulted in savings of a variety of production expenditure: principally the eduction of slags generated (-65%); iron consumption reduced by 90%; less fuel required (-15%); less oxygen required (-15%) when using oxy-fuel burners; higher productivity; faster cycle; less lead lost with slags; and, revenue from sodium sulfate covers some of the additional costs (chemicals and energy).

The environmental impact was also lessened.

The desulfurization of lead paste has become so essential in the ULAB recycling activities: this chemical reaction via sodium carbonate or sodium hydroxide leads to the reduction of the environmental impact (less SO² emissions and slags production) and to an increase in productivity of the subsequent lead paste smelting operations.

It has however limitations related to the relatively poor market for sodium sulfate (a by-product of the process), also important here is the high level of equipment corrosion which is part of the process and to the high costs of the chemicals used.

Urea as the answer

The U4Lead process by STC uses an amino compound — urea, as chemical for the desulfurization of paste and electrolyte neutralization process.

The simplified reaction can be summarized as:

CO(NH2)2 + PbSO4+H2O

⇒ PbCO3 + (NH4)2SO4

CO(NH2)2 + H2SO4 + H2O

⇒ (NH4)2SO4+CO2

The U4Lead patent was published in November 2019 but the installation of this technology on an industrial scale was started in 2020 as a subproject of a larger project for a new complete battery recycling plant entirely supplied by STC to our Nigeria plant.

The U4Lead process by STC, protected by national and international patents, is based on the use of urea as chemical for the desulfurization of paste and electrolyte neutralization. The U4Lead solution offers the following advantages (Figure 2):

• PbSO4 conversion >97% = low residual sulfur (< 0.3%)

• No side reactions ⇒ no sodium/ lead double salts formation

• Further reduction of iron addition ⇒ less slag production, natural gas/oxygen consumption, etc.

• Ammonium sulfate valuable as a fertilizer

• No Na2S