Process Route

Lead is a critical material since it has been widely used in engineering, technology application and product design due to the property of corrosion-resist [1]. More than 95% mined lead comes from ores, that is, Galena (PbS) Cerussite (lead carbonate) and Anglesite (lead sulfate). Generally, Cerussite and Anglesite are called secondary minerals because they are derived from galena by natural actions, for example, weathering. Although Galena contains approximately 86.6% of lead, ores of commercial importance just ranges from 2% to 20% in that Galena is often found to be mixed with other minerals. So, there are only 4% recoverable lead in ores. As a result, lead refinery demand that comes from recycling, mostly from reclaimed batteries, accounts for more that half of the total.[2]

main steps:

1. Lead battery breaking and separation
2. Metal lead reduction
3. Lead refining

                                                Figure 1  Lead battery processing diagram

1. The first step -- Lead battery breaking and separation [3]

In this step, battery paste, metallic lead, polypropylene and polyethylene will be recovered on the breaking and separation plant. The exhausted batteries are taken from a basin onto the feeder through that is controlled by PLC. Thus, the batteries are crushed in a Hammer crusher and the electrolyte will come out. In this way, the first separation of acid and the other component of battery is realized. After that, the breakers of lead battery are reduced into small pieces by Hammer mill. These small substances, passing through Sieve, can be removed from residual electrolyte and battery paste. Finally, the plastic and lead fragments will be left in the sieves grid and sent to hydrodynamic system. After sink＆float, the plastics are separated from metal and metallic lead will remain at the bottom.

2. The second step -- Metal lead reduction [3]

Battery paste will be converted in raw metallic lead after desulphuration and Gas phase reduction in this step. Lead paste, once react with Ammonia carbonate and Soda, can remove the contained Sulphur. After calcination, rough PbO is obtained. Then the yellow lead gained from the desulfurization reaction can be converted into raw metallic lead when reacting with carbon monoxide (CO) or hydrogen(H2) between 1000 degree Celsius and 1300 degree Celsius.

3. The third step -- Lead refining [3]

This process is also known as ‘Softening of Lead’ because impurities that presented in raw lead will be removed. Thus, the lead will be rendered soft. In this step, the important impurities that are presented in the battery recycling process are moved in this order: copper, tin, arsenic, antinomy and bismuth. The whole process is composed of a series of reaction with several chemical agents under different temperature.  

Mining and concentrating from ores

Main steps:

1. Flotation separation
2. Extraction
3. Refining
4. The first procedure -- Flotation separation [4]

Diluted by water, the finely crushed ore is stirred violently in a tank filled with some suitable chemical such as turpentine. Finally, an oily forth that contains most of metallic substances will floats to the top while the gangue will sink to the bottom.

2. The second procedure – Extraction [4]

Carbon monoxide is produced under 1200 degree Celsius at first, then it will react with the metallic oxides, producing carbon dioxide and molten metal. Nonmetallic substance will form a slag and be removed. In that step, few metallic and nonmetallic impurities (copper and sulfite) might still attach on the product. Then, based on the different melt point between lead and impurities, purify the product again.

3. The third procedure – Refining [4]

First melt lead and then cool it below the freezing point of copper. When copper crystallizes, it will be removed along with any remaining nickel, cobalt and zinc by degreasing. Then the mixture containing lead is converted into another softener, in which a small amount of zinc is added, and as the solubility of gold and silver in zinc is greater than that of lead, when cooled to temperatures below 370 degree Celsius (higher than the melting point of lead), the purer lead is finally obtained.

References:

[1]: Lead - Element information, properties and uses | Periodic Table. Royal Society of Chemistry - Advancing excellence in the chemical sciences. Available at: http://www.rsc.org/periodic-table/element/82/lead [Accessed on May 09, 2019]

[2]: (August 23, 2013) Lead processing. Encyclopædia Britannica. Availiable at: https://www.britannica.com/technology/lead-processing. [Accessed on May 09, 2019]

[3]: Pyrometallurgical lead reduction in rotative furnaces.  STC Italy: Pyrometallurgical lead reduction in rotative furnaces. Available at: http://lead.stcitaly.com/lead-battery-recycling-process/pyrometallurgical-lead-reduction-in-rotative-furnaces.html [Accessed on May 09, 2019]

[4]: (August 23, 2013) Lead processing. Encyclopædia Britannica. Availiable at: https://www.britannica.com/technology/lead-processing#ref82077 [Accessed on May 09, 2019]