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Nickel cadmium

About

  • Anode: cadmium  
  • Cathode: nickel oxide hydroxide  
  • Electrolyte: potassium hydroxide  
  • Type: Rechargeable  
  • Nominal cell voltage: 1.2 volts  
  • Energy density: 50–150 W·h/L  
  • Monthly self-discharge rate: 10% 

Pros:

  • Rugged - NiCd cells can function from -20° to +60°C 
  • Rapid delivery - NiCd cells can deliver all their energy in as little as 1 hour 

Cons:

  • Outdated - NiCd has mostly been replaced by other battery chemistries 
  • Toxic materials - cadmium is a toxic metal that is potentially fatal to humans 
  • Memory effect - NiCd cells lose their capacity unless regularly fully discharged 
Nikkel cadmium

General information about nickel cadmium 

Wet cell nickel cadmium batteries were invented in 1899 by the Swedish engineer Waldemar Jungner, but have since taken many different shapes and capacities. They come in a variety of sizes, from standard commercial AA and AAA to large, ventilated back-up power formats.   

While they were once the preferred battery of choice in all sorts of appliances, from power tools to cameras, NiCd cells have largely been phased out due to the toxicity of cadmium and the superior capacity and price point of nickel metal hydride cells. The main selling point of nickel cadmium technology today is its ability to rapidly deliver its entire charge over as little as an hour or so in a variety of circumstances (functioning at temperatures between -20 to +60°C). They remain one of the most rugged battery types, and for this reason are sometimes still used in the airline industry. Another consideration with NiCd and NiMH batteries is their ‘memory effect’. If the cells are only partially discharged before being recharged, over time they effectively lose capacity as they lose the ability to discharge completely.  

They are sometimes found in specific medical devices and standby power sources for emergency lighting and such. Cadmium telluride is another cadmium-based cell chemistry and is the most common type of thin-film solar cell in production. 

A confusing memory effect

True memory effect refers to cyclical memory - a notion that if the battery is repeatedly only partially discharged before recharging occurs, it will ‘remember’ how much energy was required and become incapable of supplying more than that. This phenomenon came from an aerospace experiment and only applies to old sintered-plate NiCd cells that are repeatedly discharged to the exact same level before recharging. It is extremely rare but nonetheless blamed far too often for poor battery performance. 

Modern NiCd batteries do not suffer from cyclical memory effect, but rather a phenomenon known as crystalline formation, which leads to a similar gradual voltage depletion. Crystalline formation occurs when NiCd batteries are overcharged and not maintained properly by periodically fully discharging and recharging them. The nickel and cadmium surfaces found in the electrodes slowly get covered in crystal formations, decreasing the active surface area of the metals and thus the battery’s performance. In advanced stages these crystal formations can penetrate the separator, causing rapid self-discharge and possible electrical shorting. 

Cadmium from around the world

Cadmium is mined and distilled from the earth, but is virtually never found alone in isolated deposits. Instead, it is heavily associated with zinc ores, such as sphalerite. Thus, most of the world’s primary cadmium is produced as a byproduct of zinc recovery from such ores. The world’s largest producers of cadmium today are China (34%), South Korea (18%), and Japan (9%), though many other countries contribute to the global cadmium supply. 

Most common applications

Next to older power tools and camera’s, NiCd batteries are sometimes found in specific medical devices and standby power sources for emergency lighting and such. Cadmium telluride is another cadmium-based cell chemistry and is the most common type of thin-film solar cell in production. 

Cameras

Why recycle nickel cadmium batteries? 

The primary concern with nickel cadmium batteries is the toxicity of cadmium, which accounts for 6 to 18% of the battery. Exposure to this heavy metal is known to cause cancer and target various parts of the body. This is why the sale of nickel cadmium batteries has been banned within the EU since 2009, except in the case of medical use, alarm systems and emergency lighting. 

Process highlights

Drainage

A first pretreatment step for wet-cell cadmium batteries is to carefully drain the liquid electrolyte from the solid components by opening the casings. This step must be performed with great caution, as the electrolyte contains potassium hydroxide (KOH), a highly corrosive base. Direct contact with KOH can damage the nerves in the skin, preventing you from feeling the severity of chemical burns. Once safely drained, the liquid electrolyte can be processed separately, while the solid components are dried and combined with dry-cell cadmium batteries for further processing. 

Comminution and physical separation

Comminution in a hammer mill and physical separation are standard steps for NiCd recycling, followed by pyrometallurgical processes. 

Pyrometallurgical process

Pyrolysis is carried out to decompose and vapourise cadmium content; the black mass stream (after physical separation is completed) is heated to around 900°C under vacuum pressure in an inert atmosphere. 

This inert, low atmospheric pressure environment facilitates the decomposition of cadmium as a pure metal at relatively low temperatures and stops oxidation from occurring. The result is 99.9% pure cadmium, collected in vapour form and condensed in a separate chamber. Once the cadmium and other vapourised components such as zinc are extracted, temperatures rise further to turn the mix molten. This allows impurities to rise to the surface, which are then poured / scraped off the top to form slag. 

The remaining fraction in the furnace consists of an alloy mix of iron, nickel and cobalt which is poured into ingots, cooled, and used for the production of stainless steel among other things.

Pyrometallurgy in more detail

Decomposition of cadmium hydroxide and nickel hydroxide into cadmium and nickel takes place at 300° and 230° respectively. Without any reducing agent added to the mix, pressure needs to be regulated to 10-4 bar to allow decomposition of cadmium oxide at 850 - 900° resulting in cadmium vapour. With a reducing agent, cadmium boils at 767°. Either way, the goal is to turn cadmium-oxide into cadmium and then vapourise it, collect it (along with water and oils that evaporate along the way) and condense it back into a liquid / solid form. The resulting cadmium is 99.9% pure and can be used to make new NiCd batteries. The temperature then increases further until the contents within are entirely molten, so that impurities can be scraped off the surface and pure metal alloys can be rendered into ingots for commercial industrial use.

Some facilities recycle nickel metal hydride in the same batch as nickel cadmium batteries, as the two batteries share many similarities, including their basic cathode and electrolyte materials. However, if the rare earth metals found in NiMH batteries are to be recovered, extra recycling steps are required.

End products

  • Polymer plastics 
  • Papers 
  • Steel 
  • Nickel, iron, and cobalt alloy 
  • Cadmium 
  • Slag 
Cadmium