In the magnetic part, the cadmium content was consistently above 1% except for pieces of 10 to 2 mm in which the cadmium content fell to 0.6%.
In the non-magnetic powders, pieces of 20 to 5 mm had a content below 1% but only represented 4.5% by weight of the batteries.
In other words, more or less all of the fractions had to be treated as they all contained cadmium.
- In 1995, INTER-RECYCLING proposed in the USA the construction of a fixed centre and a mobile treatment unit based on a similar approach. This experiment was discontinued because of the impossibility of proving that there was no cadmium in the fibres produced by separator crushing and the flexible plastics.
The designer never carried out tests on an industrial scale to confirm his theories: only the powder, 50% of the weight of the battery, was to be treated. The other fractions were free from cadmium and so could be sold as they were.
The hydrometallurgical methods have been tested many times over, and one of them is still used to day by SAFT AB in their plant in Oskarshamn (Sweden) in addition to their thermal process and for the treatment of their sludge.
The process is based on the following:
Cadmium electrolysis on a rotary aluminium cathode. The cadmium is then converted into cadmium oxide in their plant and used for the production of new batteries.
A process of this type is not economically viable in an independent recycling plant as it can only treat a low percentage of the waste from the production of nickel-cadmium batteries. And the process cannot in any event be used to treat used batteries.
Another significant experiment was developed by TNO in 1994. The flow sheet below gives details of the process (Figure 2).
In 1995, LETO RECYCLING, which took part in this development, set up an industrial pilot plant.
Unfortunately, it quickly became apparent that the cost of treating used sealed batteries was no lower than the cost of a thermal process. On the other hand, recycling the cadmium into a carbonate and nickel into chloride was less profitable than recycling the products produced by the thermal process, mainly owing to the low degree of purity of these products. TNO attempted to improve recycling by purifying the products by electrolysis. TNO was then producing pure cadmium cathodes and nickel cathodes with cobalt. The cost of retreating the end-products by electrolysis made the new experiment economically uncompetitive.
2.3 - Thermal Process
These were the most commonly used processes in the past and with a few exceptions they are still in use today.
They fall into two categories:
• Open processes
• Closed processes
Figure 3 shows the thermal technology for the treatment of Ni Cd batteries.
Open processes are based on the principle of direct ferronickel production and vaporisation of the cadmium, which is allowed to burn on the surface of the furnace. The cadmium is converted into cadmium oxide with a relatively high impurities content. This process calls for heavy capital expenditure, both for capturing the particles above the furnace and treating the captured air. It will be remembered that these furnaces work at a temperature of 1400°C and have to be charged with a liquid
heel. This necessarily means slow steady charging so as to avoid explosions caused by moisture in the batteries, among other things, when the furnaces are being recharged.
No plant recycling nickel-cadmium batteries currently uses this type of process, not just because of the risks of explosion, but also because of the difficulty of capturing the particles. The standards in respect of the cadmium content of the air in treatment plants reach exposure limit values that it is veiy hard to comply with.
In Europe, the threshold values are from 25 to 30 (xg/Nm3. In the USA, they range from 2 to 10 (ig/Nm3.
The products obtained by these processes are ferronickel in the form of an alloy, which
could be recycled more effectively than that of the nickel-iron residues obtained by the same process, if these alloys could be titrated.
Rectifying one or another content is practically impossible with an induction furnace, the type of furnace most often used. The inconsistency of the analyses meant that recycling could not be optimised.
On the other hand, the cadmium was recovered in the form of very impure cadmium oxide, with high nickel, zinc and carbon contents. This made it necessary to add one or two distilling furnaces to the treatment furnace to convert the cadmium oxide into cadmium metal with an average purity of 99.95% and which still had to be refined to obtain a purity of 99.99%, or else sold unrefined at an unfavourable price.
A variant of this process was developed in Australia by AUSMELT in 1998: their process used a closed induction furnace but with an air feed to burn the cadmium in the furnace and recover it in the form of cadmium oxide. The process produced nickel matte which had to be sent to the nickel refiners and highly contaminated cadmium oxides that had to be sent to the producers of primary cadmium. This required a higher investment than closed furnaces and the products extracted were somewhat less effectively recycled than the nickel-iron and cadmium metal residues.
2.3.2. - Closed Processes
The closed furnace process is thus the process used to treat over 90% of the batteries and waste products from nickel-cadmium batteries.
The principle consists of putting the products to be treated into a vessel placed in a chamber which is closed and sealed with the exception of an outlet for the following:
the gases produced by the decomposition of organic materials, vaporisation of the water and decomposition of the hydroxides or oxides;
the gaseous cadmium which has to be cooled to change into the liquid and then the solid phase in either a confined space or in water.
Heating is usually done by heating elements. The temperature is built up gradually and by stages so as to obtain, if necessary, the successive gasification of the different constituents, i.e. water, organic materials and cadmium.
The purity of the cadmium obtained is between 99.9 and 99.95%. It usually needs to be refined to obtain a purity of 99.99%, the only readily marketable quality for, among other things, the production of nickel-cadmium batteries.
The residue is an association of nickel and iron, not an alloy. Depending on the types of furnace and how they are used, these residues have a cadmium impurity content of 0.1 to 0.01%.
The furnaces used can operate:
at atmospheric pressure, in which case the distillation chamber is pressurised and the condensation chamber under negative pressure; these furnaces are both the cheapest and the simplest;
at low pressure to help expel the cadmium from the gas distillation chamber;
at very low pressure to speed up distillation (ALD furnace made in Germany). These are the most expensive furnaces, but they are also the fastest.
Energy consumption per kilo of batteries treated is the same in all three cases. The energy savings achieved with a high vacuum furnace, because it operates at low temperature, is in fact offset by the high amount of energy required in order to maintain the high vacuum.
The constantly increasing use of sealed cells that cannot be disassembled and which have an organic materials content of 5 to 10%, has meant the introduction of two pretreatments.
Removal of the plastic casing:
Numerous power packs have a hard plastic shell accounting for 12 to 15% of their weight. The shell can be broken and separated from the individual nickel-cadmium elements by means of, for example, a magnetic separator.
Pyrolysis or oxidation of the elements (or possibly of the power pack as is).
Pyrolysis is done in a sealed furnace, with no air coming in. The organic materials are broken down into simple organic chains (CHn) at a temperature below the cadmium distillation or cadmium oxide sublimation temperature. The gases obtained can be burned at high temperature (1000°C) in a post-combustion chamber producing CO2 and water vapour. It is important to wash the gases so as to prevent dioxins and inhibit the contaminants that could appear.
The main recycling companies using these processes are mentioned below.
• SAFT AB in Sweden, INMETCO in the USA, KOBAR in Korea, NIPPON RECYCLE CENTER in Japan use furnaces that process one tonne in 16 hours.
• ACCUREC in Germany uses high vacuum furnaces that process 500 kg in 12 hours.
• S.N.A.M. in France uses furnaces that work at atmospheric pressure and which process one tonne in 24 hours.
These processes use very little water and do not call for high capital investment for the treatment of aqueous effluent. However, they do call for increasingly heavy investment in air treatment in order to comply with the increasingly stringent air discharge limits; these apply not only to cadmium but also to nickel, CO, CO2, NOx, dioxins and furans.
TODAY'S BATTERY RECYCLING COMPANIES 1/ EUROPE
The companies involved in the processing of NiCd batteries at the industrial scale are: SAFT AB (Sweden), ACCUREC GmbH (Germany) and SNAM (France).
The recycling capacity expressed in metric tons of NiCd batteries (industrial and portable) is presented below.
A brief description of each company and of the technology used for recycling NiCd batteries is given below. The data presented are related to the treatment of batteries of european origin. In addition, the recycling companies are processing materials coming from all around the world. The transboundary movement of used batteries for recycling purpose is submitted to the Basle Convention administrative rules.
Ni-Cd BATTERY RECYCLING CAPACITY OF PROCESSING PLANTS LOCA TED IN EUROPE
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