Silver oxide batteries
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Silver oxide batteries are often found as button cells in equipment as watches, cameras or hearing aids. Since each battery only contains a small amount of silver a lot of batteries are needed before starting to process them.
The type of battery can be decoded by the type number, batteries with SRxx or TRxx where xx is a number is most often silver oxide button cells. See the battery nomenclature reference below. There are other batteries from example Varta or Duracell that doesn't follow the standard codes. Look at the document on the first post by Juan Manuel Arcos Frank on GRF.
The battery is made up of a stainless steel casing with silver oxide and zinc electrodes with an alkaline (sodium hydroxide or potassium hydroxide) electrolyte. When the battery is discharged the electrodes are converted to silver and zinc oxide.
Silver button batteries contains about 30-35% silver by weight.
Extracting the silver from the batteries are quite easy, crush the battery to access the electrodes, then wash it to get rid of the electrolyte and dissolve the silver in nitric acid. The silver nitrate can be washed off the undissolved steel and then filtered. From the leach the silver could be cemented with copper or dropped as silver chloride.
The washing removes the alkaline electrolyte which keeps down the amount of acid that is needed to dissolve the silver and zinc.
Originally silver oxide batteries contained 0.2% of mercury but since 2004 there have been mercury free silver oxide batteries produced.
Mercury can be removed in two different ways.
- Distillation. By distilling off the mercury under vacuum the remaining material is free from mercury and can be refined by traditional methods.
- Wet chemistry. First the content of the batteries is dissolved in nitric acid and filtered off. The silver is precipitated as silver chloride and filtered off. The higher solubility of mercury lets it stay in solution and can be cemented out with zinc dust. The waste water will still contain some mercury and should be treated accordingly and not discharged.
Alternative to refining
If you don't feel it is worth the hassle with refining the batteries can be sold. For example,
- Mercury Refining LLC, buyer of silver oxide batteries.
- GRF : All about silver button batteries by Juan Manuel Arcos Frank
- GRF : Large military silver oxide battery
- Wikipedia : Silver oxide battery
- Wikipedia : Battery nomenclature
- Silver recovery from spent silver oxide button cells
- CSIR : Reclamation of Mercury from Used Silver Oxide Watch Batteries (pdf)