Hydrometallurgical conversion of lead paste into lead oxide

Currently used lead oxide recovery processes from exhausted batteries have several disadvantages:

  • High energy consumption due to the high temperatures in the furnaces;
  • Environmental problems due to dust and gas emission from the furnaces;
  • Limited lead recovery;
  • In order to be economically competitive, plants have to treat approximately 20.000 - 30.000 tons of batteries per year.

The lead oxide recovery process developed by STC carries out the separation of the various lead battery compounds through mechanical, chemical and physical technologies, an innovative approach in comparison to current recovery processes. An important aspect of the STC process for the recovery of lead oxide is the separation of antimonial lead, present in the batteries’ terminals, from the lead-calcium, present in the electrodes. Today many battery producers sill use antimonial lead in grid preparation. This application will probably disappear in the next future.

The STC process is extremely convenient as far as energy consumption and the environmental impact are concerned because it works at maximum 600°C and no atmospheric emission is necessary.

The single recovered materials can be recycled, directly or after simple operations at low temperatures, as raw materials in the battery production. In a near future, the absence of high temperature furnaces and the consequent absence of expensive protection systems for the environment will make investments economically profitable also for small plants (15.000 t/year). The STC lead oxide recovery process consists of the following steps:

  • emptying the acid solution from the opened batteries;
    The single battery is drilled in many points in order to make the electrolyte solution drain gently from the case. The Acid solution is collected in a tank, filtered and then concentrated in a heat pump evaporator. It is possible to use the concentrated solution in the production of new batteries.
  • Cutting of battery covers with terminals from containers
    A robotized arm takes the batteries from a bin and puts them on a conveyor belt. This belt transports the batteries to a computer vision system where the dimensions and the battery type is automatically recognized through a computer system. If the battery is not recognized it will be rejected and an operator will insert the information about the unknown battery in a data base. Subsequently a laser cutting system is used to cut the top of the battery in an exact point. The battery top is crushed and the polypropylene is separated from the antimonial lead of the terminals. This separation is carried out by an hydro gravimetric system.
  • Milling of batteries’ solid content
    The containers of the batteries are crushed in a hammer mill. Sieves and hydro gravimetric separation are used to separate plastic from paste and metal grids.
  • Melting of different metals coming from terminals and grids
    Since the obtained metal from the previous separation process do not have impurities, refining of lead is not necessary for the further lead recycling. A simple fusion of the lead in an electric induction furnace is sufficient.
  • Superdesulphurization of the battery paste
    The heart of the STC process is the conversion of the paste from paste sulphate to pure lead oxide. This process takes place in a two stadium reaction. Ball mill reactors are used in order to remove nearly completely the lead sulphate. The by-product of the desulphurization reaction can be sodium sulphate, reusable in the detergent industry, or ammonium sulfate, to be used as fertilizer. The desulphurized paste is then calcinated to obtain lead oxide. This calcination is carried out at 600°C in inert atmosphere. 

This new process has several advantages as opposed to traditional processes:

  • Possibility of an immediate recycling of nearly all battery components;
  • Reduction of energy consumption because of the exclusion of thermal operations in furnaces at high temperatures (> 1000°C);
  • Elimination of gaseous emissions and dust;
  • Reduction of slag;
  • Higher yield of lead;
  • Valorization of by-products.

If high purity of oxide is necessary, STC suggests a specific process to obtain ultrapure lead oxide from lead battery recycling.

The obtained lead oxide can be used as it is in mixture with oxides obtained from battery producers through mills or the Barton process, or can be reduced to metal lead through a thermal reduction and hydrogen.

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