RECYCLING OF LEAD FROM USED BATTERIES.
Dr. Yashpal Singh (With inputs from Ms Deeksha Shukla)
Published On 17/05/2018
First Update On 16/09/2020
Also see Archives-Recycling of Lead-Acid Batteries (2011). www.wealthywaste.com for earlier version
Used batteries are classified as a hazardous waste under the Basel Convention and need to be handled accordingly in order to prevent damage to human health or to the environment. Exposure to lead can cause adverse effects on many parts of the body. The organs that could be most affected are the brain and nervous system, kidneys, blood, and the reproductive system of both sexes. Lead in certain forms is also considered a possible carcinogen. Emissions of lead dust and fumes can also pollute the soil and poison the ground water with negative effects on the plant, animal and human ecosystems.
The old car batteries are an efficient resource and feed a major recycling industry across the globe. Recycling keeps the toxic materials out of the landfill. With the production of recycled lead requiring just 35-40% of the energy needed to produce lead from ore, recycling the battery is also easier and much less energy intensive than producing new lead from ore
Returning used lead batteries for recycle has been long practiced. With more than 95% lead and high prices of lead, it presents a profitable and environmentally sustainable solution. Lead production by recycling consumes considerably less energy (less than half) than lead production through ore. According to a report by US Environmental Agency, the rate of lead battery recovery is almost 99 % which is the highest recycling rates among other known recycled products as newspaper (63 %), aluminum cans (55 %), tires (40.5%), glass container (32.5 %) and polyethylene terephthalate (PET) bottles (32.2 %)
Modern car batteries consist of a polypropylene casing, plates (consisting of grid and paste), connectors/poles and bridges and polypropylene separators as insulators between the plates. The paste consists of Pb, PbSO4 and PbO2. Grid metal, poles and bridges may constitute 44% and the paste about 56% of the lead bearing components.
Lead battery recycling needs high investments in air and water pollution control and it may not be profitable for small smelters to operate under strict controls. This promotes non compliant smelting and an unhealthy competition between non compliant and compliant smelters. Many industrialized countries have a well organized system for collection of Battery wastes. Countries such as Sweden, Germany and Italy operate adjustable levy systems which are related to lead price. A levy is imposed on new batteries and is used to fund the collection and recycling process to make it more economical. In the United States, many states have retailers to collect the used battery when consumers purchase new ones, some also have a cash deposit on new batteries which is refunded after the return of the used battery. In Europe, most retailers are under obligation to take back spent batteries. In Germany, this has catalyzed a return rate of almost 95% for starter batteries and almost 100% for industrial batteries.
In India, the Batteries (Management and Handling) Rules 2001 as amended in 2010 provides specific procedures for collection, recycling and reuse of used batteries. It specifies that manufacturers, importers, assemblers, re conditioners, dealers (including bulk) and recyclers will ensure that 90% of the batteries sold by them are collected back by them and for which they have to keep records as prescribed. It has also been provided that the dealer would give an appropriate discount for every used battery returned by the consumer. A deposit refund scheme is also in place. Consumers and bulk consumers have to ensure that used batteries are not disposed of in any manner other than by depositing with the dealer/manufacturer/registered recycler/importer/reconditioner. A system of registration of recyclers through the MoEF and CC has been prescribed. The Batteries Management and Handling rules requires retailers to sell used batteries to registered smelters who use recycling technologies that do not have any harmful impact on environment.
The process of recycling includes the following steps
- Draining the batteries and collected the acid in barrels where flocculation may be employed to settle impurities. The super-natant clean acid is decanted and packed for sale. The remaining sludge is neutralized with lime and filtered. The filter cake may be charged in the melting and reduction furnace.
- Next the grid packs are removed form the battery and fed to a perforated grinding drain which rotates in a water basin. The grinding process separates the grids from the separators and the paste from the grids. The perforations act as a sieve. The water is kept neutralized to prevent corrosion. The fines are separated and carried away with water. Alternatively whole batteries may be crushed in a hammer mill and then fed to the grinding/washing drum for separation.
- The slurry from the grinding drum is pumped into sedimentation tanks where the solids settle and the clarified liquid is sent back to the water basin of the grinding machine. The sludge is recovered (sun drying or filter press) and dried Sludge is the main feed for the melting and reduction operation which will produce almost pure lead.
- Coarse materials from the grinding mill (basically grids and separators) leave the grinding drum at its lower end. Separators and grids are segregated by hand sorting in a slow moving transmission belt.
- Empty battery cases, covers with attached poles, bridges and remaining grid parts are charged to a wet hammer mill where metal parts and remaining paste are separated from plastics. This is also passed through a perforated drum where solids and slurry are separated. The Slurry obtained from here is added to the slurry obtained from the grinding drum. The solid metal parts supplement the feed of the low temperature melt.
- The plastic residues can be either pelletised and recycled for battery covers, disposed in Hazardous waste sites or can be used as fuel in Cement factories.
- The filter cake from step 3 is charged to a short rotating drum furnace where the charge is melted with slag forming constituents lie Sodium Carbonate and reduction additives like Fe dust and Coal. The reaction time is about 2 to 3 hours. The metal lead settles at the bottom. When enough lead has accumulated, it is tapped into a mobile ladle and transported in liquid state to the refining kettle.
- The slag which still has some lead is mixed with the feed material for a second or third cycle for optimum recovery of lead. The quantities of slag produced may ultimately be too large to continue the operation. Low Pb content (<9% Pb) slag can be tapped and sent for disposal.
- The coarse fraction of the crushed battery is fed to a crucible furnace, melting kettle or rotary drum furnace with the addition of sodium carbonate. Slag is skimmed off.The melt is cast into ingots or transferred in liquid stage to the refining kettle.
- Crude lead originating from battery scrap is normally alloyed with copper and Antimony (with traces of Cs, Sn, As and Zn). In order to remove these impurities, the lead melt is Sulphurised and stirred. Addition of sulfur leads to a Pb/Cu2S and to some extent Sn, As and Zn dross which can be skimmed. This step needs to be carried out at least two times. The remaining Sb, Sn and As in the molten lead can be oxidized by blowing air or oxygen rich enriched air into the melt with stirring with or without sodium Nitrate. The oxidation process is considered to be complete when only Pbo2 is formed.
Air Pollution Control
Most of the elements and compounds involved in lead smelling are hazardous. It is important that all fumes, gases and dusts that are generated during the different production steps should be collected and treated in an air pollution control system consisting of a hot dust chamber or cyclone for the removal of coarse dust particles. The off gases from the cyclone are conveyed to a wet gas cleaning system which consists of a venturi scrubber for the collection of fine particles. The off gas from the venturi washer enters a wet scrubber with lime within the scrubber liquid. The So2 in the off gas reacts with lime to from gypsum which is insoluble in water and precipitates and is removed. The slurry is pumped again to the wet scrubber. The sludge produced in the sedimentation tanks for the scrubber liquids is recovered through filter presses. The extracted water is re-circulated back to the scrubbers whilst the filter cake is sent back is the milling and remelting furnace.