Alkaline and zinc-carbon batteries
About
- Anode: zinc
- Cathode: manganese dioxide
- Electrolyte: potassium hydroxide / ammonium chloride
- Type: primary / rechargeable
- Nominal cell voltage: 1.5 volts
- Energy density: 250 - 434 W·h/L
- Monthly self-discharge rate: 0.3%
Pros:
- Value for money - alkaline cells offer a great shelf life & energy density for the price
- Low toxicity - manganese, steel and zinc are all relatively harmless materials
Cons:
- Performance - alkaline and zinc-carbon are inferior to many new battery chemistries
- Internal resistance - alkaline cells have a high internal resistance
- Leakage - when left in devices for too long alkaline cells often leak corrosive materials
General information on zinc carbon
Alkaline and zinc-carbon batteries are similar enough that they are recycled together. The zinc-carbon battery was the first commercially available dry-cell battery in production. Experiments in the late 19th century by Georges Leclanché and Carl Gassner led to the refinement and wide-spread use of zinc-carbon batteries in the first ever flashlight, out of which the battery manufacturer Eveready was born.
Zinc-carbon batteries have a zinc casing which acts as the anode, and a carbon rod surrounded by manganese dioxide acting as the cathode, hence the name. It is a relatively inefficient battery design with low capacity, poor durability, and a significant risk of leaking, but it remains in production today due to the unequalled low production cost.
General information on alkaline
The modern alkaline battery was invented in the 1950s by a Canadian engineer working for Eveready. It was an update on zinc-carbon batteries, replacing the ammonium-chloride or zinc chloride electrolyte with alkaline potassium hydroxide, and replacing a solid zinc casing with an internal zinc-powder for the anode. These changes meant an improved capacity and durability, and a higher energy density.
Since their commercial debut, alkaline batteries have been exceedingly popular. Today they account for around 80% of batteries manufactured in the United States, while within the EU, 47% of annual battery sales are alkaline. In 1992 rechargeable alkaline batteries came on to the market, but they are far less efficient than other rechargeable battery types and can only be recharged a few times at best.
Most common applications
Alcaline and zinc carbon batteries are mostly used for common small, household devices. For example for remotes, toys, clocks,…
Why recycle zinc-carbon batteries?
While modern alkaline and zinc-carbon batteries are relatively low in toxic and volatile substances, they are the most numerous batteries in use today, so recycling them has a very large impact. It also helps to offset the resources mined from the earth to make new alkaline and zinc batteries, of course.
Older alkaline and zinc-carbon batteries may also contain small amounts of mercury which is highly toxic, and zinc-carbon batteries that have no outer plastic cover are prone to leakage and possible short-circuits.
Process highlights
General
Alkaline and Zinc battery streams follow a relatively typical recycling process. They require very little specialised treatment as they use simple elements and present no major health or environmental hazards, though there are still several different methods used around the world, and more are being researched every day.
Comminution
In most cases, the batteries are first shredded in a grab shredder or cutting mill to break them down into particles between 1 and 3 millimetres in diameter. However, some recyclers skip the shredding process and put whole batteries into blast furnaces to recycle zinc and manganese primarily.
Depending on the comminution method used, the next separation steps may vary, if not in method than at least in their efficiency. Better comminution makes for purer output streams.
Physical separation
Magnetic separation is used to retrieve the steel casings of the alkaline batteries.
Dense Medium Separation (DMS) retrieves plastics and paper from the heavier black mass materials. Then DMS is repeated to separate out light plastics from heavy plastics.
Sieving is also a common option with this battery type, though it requires intensive comminution to break particles down based on material, at which point the black mass can dbe retrieved as the finest particle size in the mix - 0.5 to 0.75 millimetres.
Pyrometallurgical processing
Once plastics, papers, and steel casings have been separated out into their own streams, they are ready to be sent to steel works and factories for refinement, production or fuel purposes.
Most recycling facilities then take a pyrometallurgical route to deal with the remaining stream - the black mass. In the case of alkaline and zinc batteries, the black mass consists principally of zinc, manganese-dioxide, and potassium, though there are often trace amounts of other metals such as iron, steel, and mercury, as well as carbon. These are mostly in ore / sulphide compound forms, so chemical bonds must be broken down to separate the metals from organic matter, which then usually takes the form of slag. For the process below, a Waelz-rotary kiln is often used.
- Separation of mercury from chlorine ions occurs at 450°C with the help of a vacuum.
- Zinc is reduced from zinc-oxide at temperatures above 920°C and collected as a vapour to be further distilled.
- Manganese remains in a solid-state throughout the pyrolysis process, though it is reduced to manganese-oxide due to the carbon present in the mix.
- The remaining materials at the end of these pyrometallurgical processes are manganese-oxide and iron.
- The iron will be separated out for processing, while the remaining potassium and manganese-oxide content will end up as slag.
Alternative hydrometallurgical methods
Alternatively, hydrometallurgical routes are being explored too, mostly on a lab scale at present. This process looks something like the following:
After mechanical processing to recover a metal stream (through magnetic and density difference separation), the leftover black powder contains carbon, manganese and zinc.
This powder is leeched to produce a solution rich in manganese and zinc.
This solution is then purified, resulting in manganese and zinc salts.
End products
Most recycling methods for alkaline and zinc batteries retrieve the following:
- Polymer plastics
- Papers
- Steel
- Zinc
- Manganese-enriched slag
Future trends
As resources get scarcer and humanity prioritises responsible and sustainable practices, it is quite possible that zinc batteries will be phased out completely. Perhaps followed soon after by alkaline, as new, more efficient and readily available battery chemistries are explored.
Nonetheless, due to the sheer number of alkaline and zinc batteries in use today, new recycling methods are always being explored to improve the process.
