Lead-acid batteries
About
- Anode: lead
- Cathode: lead dioxide
- Electrolyte: sulphuric acid / water gel solution
- Type: Rechargeable
- Nominal cell voltage: 2.1 volts
- Energy density: 60-75 W·h/L
- Monthly self-discharge rate: 3 - 20%
Pros:
- Cheap production - Lead acid batteries are the cheapest production batteries around
- Efficient recycling - lead-acid is easily and efficiently recycled
- Rugged & reliable - these batteries work in a range of circumstances
Cons:
- Self-discharge - Lead-acid batteries slowly lose their charge over time
- Low energy density - this is part of the reason they are so large
- Toxic contents - both lead and sulphuric acid are dangerous substances
General information on lead-acid
Lead-acid batteries were one of the very first battery technologies, invented in 1859 by Gaston Planté and made commercially available in 1881. It is also the world’s first rechargeable battery, and somewhat miraculously, it is still in great demand even today.
These cells offer poor energy density and high self-discharge rates, but are still in production due to their ability to supply high surge currents, such as those needed to start a car engine, and most importantly, their low low cost of manufacturing, maintenance, and recycling. In 2014 global lead-acid battery sales totalled 44.7 billion USD, making it the best-selling battery in the world, followed by lithium-ion, NiMH, and NiCd respectively.
Recycling of lead-acid batteries also began quite some time ago. In 1912 the ‘starter battery’ was introduced to the market by Cadillac, offering an alternative to cranking car engines to get them started. Due to the high value of lead-acid and the relative ease of disassembling and extracting it from car batteries, recycling initiatives appeared soon after as a for-profit business.
Most common appliances
Today lead-acid batteries serve a surprising variety of uses, due largely to their low production cost and simple, rugged reliability. Aside from car ignition systems, lead-acid cells are used to power golf carts and powered wheelchairs, boats, solar power systems, back up batteries for emergency situations, temporary traffic lights, and more.
Why recycle lead-acid batteries?
Lead-acid battery recycling is already well established due to one simple reason - it’s profitable! But money aside, lead-acid cells are primarily made up of lead, a toxic metal that can build up within our systems, and sulphuric acid, a highly corrosive, harmful substance in itself. If not disposed of correctly, these batteries can poison water supplies, seep into the land, and harm us and our environment significantly.
The last incentive to recycle lead-acid is a positive one - because it’s easy and extremely efficient. Generally speaking, battery recycling is energy intensive and costs more than mining new materials from the earth. But the lead-acid battery is an excellent example of closed loop recycling. 99% of all lead used in batteries is recycled in Belgium! Some sources estimate that around 98% of all lead-acid batteries are recycled globally.
Process highlights
A growing concern in lead-acid battery recycling is the purity of the input stream from battery sorting facilities. Lithium-ion car batteries now exist and are often visually identical to lead-acid starter batteries. There have even been examples of lithium-ion batteries being disguised as lead-acid batteries - with a brick or heavy weight inside the package to replicate the weight of a lead-acid battery - for financial gain. Such batteries present challenges for recyclers where it is not always easy to identify the contents.
As a result there are increasing reports of lithium-ion car batteries getting mixed into lead-acid streams, which is extremely dangerous due to the volatility of lithium-ion. These cells can cause fires and lead to explosions if they are heated up, punctured, or physically damaged, which is exactly what the lead-acid recycling process entails. This is illustrated in this video which shows a lithium-ion battery entering a lead-acid battery recycling process - https://www.youtube.com/watch?v=Mg7iJEBm2qA
Thus, inspection of sorted battery streams before the recycling process is a necessary first step to remove lithium-ion cells along with any rubbish or other inappropriate materials.
The specifics of many battery recycling processes are confidential or patented as they involve private research and investments in development. However, lead-acid battery recycling is an open book due to its traditional simplicity and how long the process has been around.
Comminution
The first step is a hammer mill to break the batteries apart, releasing the liquid acid electrolyte which drains out of the cells and is collected for further processing.
Density separation
The broken up battery matter consists almost entirely of lead and plastics. This stream goes through density separation in a water vat, where plastics float to the surface and heavy metals sink, separating them very efficiently.
Pyrometallurgic process
The lead metal is smelted in a furnace along with some added elements to free the lead metal from any impurities. Once the mixture is completely molten, these impurities float to the surface to form dross (defn: foreign matter, dregs, or mineral waste, in particular scum formed on the surface of molten metal) which is scraped away. The purified lead is then poured into various sized ingots, cooled, and sent off to metal works, often to be directly reused in lead-acid battery production.
Plastics are also heated to a semi-liquid state and shaped into small uniform polypropylene pellets which can be used either for fuel in appropriate facilities or in the production of new plastics.
Neutralisation and distillation
Lastly, the sulphuric acid solution extracted at the beginning of the process is also treated, typically in one of two ways. Either the acid is neutralised (by the addition of a substance similar to commercial baking-soda), processed, and released as water (with carbon dioxide being produced as a byproduct), or it undergoes distillation to separate sodium sulphate crystals out of the acid, which can be used in laundry detergents among other things.
If you want to review the science of the reaction between acids and bases, you might like to take a look at the following link:https://sciencing.com/type-sulphuric-acid-reacts-alkaline-8414641.html
Lead batteries have a very high recycle fraction and are currently the closest thing to a ‘closed loop’ battery system, where the parts of old batteries (lead, plastics, acid) are recycled to make new batteries.
End products
- Lead ingots
- Plastic polypropylene pellets
- Sodium sulphate / water
Future trends
Lithium-ion batteries are increasingly found in lead-acid battery streams, creating very dangerous situations. Some sources quote a ten-fold increase in reported incidents of this. This happens, at least in part, due to the fact that in many parts of the world people have to pay to have their used lithium-ion cells recycled, whereas old lead-acid batteries are a profitable commodity. This creates an incentive to pass off lithium-ion batteries for lead-acid ones. This is being tackled by sorting facilities bit by bit to ensure a safer recycling process down the line. Moves by regulators to improve the mandatory labelling of batteries will aid in this mission.
While lead-acid batteries are possibly the most recycled commodity on the planet, lead has also traditionally been one of the worst historical pollutants. Since the 1980s lead contamination in the air has dropped by 99%, but lead smelting is an old, environmentally unfriendly process that can and is being improved upon. Companies like Aqua Metals are developing alternatives to pyrometallurgy for the treatment of lead, such as electroplating and hydrometallurgical processes.
