Paste Desulfurisation

Desulfurisation depends on the fact that lead carbonate has a much lower solubility than lead sulfate. Hence lead sulfate can be converted to the carbonate by treatment with a solution of sodium or ammonium carbonate. With the use of ammonium carbonate the reaction is shown in Equation 10.3. The paste is agitated with a strong carbonate solution for one or two hours until the S content is reduced from around five per cent to less than 0.5 per cent. 

The ammonium carbonate solution can be regenerated by first reacting the ammonium sulfate solution with lime, as in Equation 10.4, to form gypsum, which is separated by filtration to leave an ammonium hydroxide solution. This is then treated with CO2 by absorption in a packed tower to reform ammonium carbonate solution as in Equation 10.5. CO2 can be generated from kiln gases from the burning of limestone to produce the lime required for the first stage. 

With the use of sodium carbonate, the solution pH is higher and there is a tendency to produce a basic carbonate (hydrocerrusite), according to Equation 10.6  

With treatment at high pH or temperatures above 55°C there is a tendency for sodium to enter the solid phase as NaPb2(C03)2(0H), and if sodium carbonate is present in excess, the double salt Na2C03.2PbC03 can form. These effects will incorporate sodium into the desnUnrised paste, as can the presence of NaCl in the solution, which is usually the case for industrial grade soda ash. Hence one to two per cent Na is usually always present in paste desulfurised by this method. Conversion of lead to carbonate is also lower at around 85 to 90 per cent and the sodium must be discarded in smelter slag (Queneau et al, 1998 Stout, 1993). 

An alternative to carbonates for desulfurisation is sodium hydroxide producing hydrated lead oxide according to Equation 10.7  

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The sodium sulfate solution resulting from past desulfurisation can be filtered, evaporated and crystallised to produce sodium sulfate for sale. It is used in water treatment and for detergent manufacture as the quality from this source is sufficiently free of metals for this purpose. In the case where sodium carbonate is used the residual solution is usually neutralised with waste acid to remove any residual carbonate before evaporation and crystallisation. 

The TBRC is represented by the Kaldo process, developed and operated by Boliden in Sweden and detailed in Chapter 7. This process is suitable for both primary and secondary smelting and can accept whole batteries if required. The Kaldo system at the Ronnskar smelter has adequate facilities for the capture of sulfur and paste desulfurisation is unnecessary. 

Another option with the RSR process is the use of SO2 during paste desulfurisation leaching with sodium carbonate however, this will generate additional sulfate at around 50 per cent more than generated from the removal of the lead sulfate content alone. 

As an example, indicative costs are provided for the treatment of scrap lead-acid automotive batteries to produce refined lead using battery breaking and paste desulfurisation, followed by melting and reduction in a short rotary furnace, as described in Chapters 10 and 11. A typical small scale unit handling 35 000 tonnes per year of batteries is considered with a lead production of around 19 000 tonnes per year. Relevant production data is as follows ... 

This results in a cleaner separation with no sodium present in the desulfurised paste (Reynolds, Hudson and Olper, 1990). 

The primary smelting stage feed varies from whole drained batteries to disintegrated whole batteries, mixed paste and metaUics after separation of plastics, pastes only after separation of metallics and plastics, and pastes after desulfurisation. Some processes are unsnited to different types of feeds. 

In this case batteries are fully separated, metallic components are simply melted and drosses from that operation together with desulfurised battery pastes are subjected to hydrometaUurgical extraction of lead followed by electrowinning from the leach solution. As detailed in Chapter 9, solutions used for lead electrolytes are fluosilicates as used in the Betts refining process, fluoborates and chlorides. 

After desulfurisation, battery paste material contains PbO, Pb(OH)2, PbC03, Pb02 and some fine metallic Pb. A common problem with hydrometaUurgical extraction is that both metallic lead and Pb02 are not soluble in the leaching solutions. A common approach is to make use of the reaction shown in Equation 11.16, by the addition of lead powder as required under acidic conditions. 

Desulfurised paste is leached with fluosilicic acid as spent electrolyte to dissolve lead, as in Equation 11.18 ... 

The dried paste is leached in spent electrolyte to dissolve lead in accordance with Equation 11.18. Lead concentration is raised to around 150 g/L or up to 200 g/L but must maintain a free fluosilicic acid concentration of at least 50 g/L to prevent hydrolysis and precipitation of lead compounds. Lead recovery to the leach solution is about 95 per cent and the leach residne containing 30 per cent lead and two to three per cent sulfur can be recycled to the desulfurisation step, or part discarded after conversion to a slag in an electric furnace. 

The Ginatta process also uses a fluorborate electrolyte and is based on the leaching of battery pastes which have been desulfurised with ammonium carbonate. The electrowinning process is licensed to MA Industries, who promote it in conjunction with their battery breaking technology (Ginatta, 1984). 

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