process may contain an abnormally high quantity of cadmium impurities, which makes the zinc-based end-product more difficult to sell and contributes to reduce its value.
As regards the nickel, it has to be isolated by precipitation or the preparation of a selective solution so as to separate it from the manganese.
With the majority of these processes, a sorting operation is necessary. The basic purpose of these processes is in fact recycling of the chief components, zinc and manganese. The forms in which they are recycled and the market for which they are intended often preclude too high a content of one or another impurity, whether it be mercury, lead, cadmium and sometimes even nickel. The purpose of this mandatory sorting is to extract the type(s) of battery and/or accumulator responsible for these impurities. Very brief economic examination shows that, whatever the reason for sorting, its cost only increases fractionally if it is pursued beyond its initial objective.
Moreover, the cost of treating nickel-cadmium batteries is quite clear - the supplier contributes to the cost, i.e. that part of the cost not covered by recycling the nickel and the cadmium - and significantly below the cost for saline and alkaline batteries.
Nickel-cadmium batteries: 0.2 to 0.3 euros / kg Saline and alkaline batteries: approx. 1 euro / kg
With NiMH batteries, the advantage is even greater, since recycling the nickel far outweighs the cost of treatment and gives these batteries a positive value: 0 to 0.50 euros per kg depending on the price of nickel.
This difference, added to saving an additional stage in the process, largely covers the extra cost incurred by sorting.
In conclusion, even if the hydrometallurgical processes can in theory accept the presence of NiCd, NiMH and Li-Ion batteries, for both technical and economic reasons a sort is usually done in order to send the NiCd and NiMH batteries to treatment centres using specific processes.
The most compelling example is the BATENUS process developed by PIRA in 1992 or thereabouts. In the laboratory, this process made it possible to separate all the metals, including lead, nickel, cadmium, lithium, etc.
At pilot level, it quickly became apparent that setting up a plant was becoming very complicated. In the end, the plant that had started to be built in Germany was never completed. Today, certain battery treatment centres, in Spain among other countries, are using a simplified version of this process, including a preliminary sorting operation designed to remove undesirable batteries and accumulators.
With the pyrometallurgical, or thermal processes, the presence of nickel-cadmium and NiMH batteries does not, again, pose a problem in principle. Irrespective of the type of furnace used, the nickel contained in the batteries is combined with the ferro-manganese that the process makes possible. However, the problem is the cadmium, which will be vaporised and - if the surface of the furnace is in contact with the air -will burn to produce cadmium oxide which will be captured in the air treatment system. It will combine with the zinc oxide which already contains numerous impurities, including lead.
Battery recycling plants using this type of process but without any preliminary sorting operation have 3 problems:
The presence of highly reactive, if not explosive, batteries, including lithium batteries, can result in serious incidents.
The presence of cadmium in the zinc-based end-product means that the product is not economically viable.
A very poor "green" image: the air coming out of the furnace contains metals regarded as environmentally hostile and is often suspected of carrying dioxins. The particles coming from the filters are then handled successively by one, two or three specialised treatment centres. This sort of structure makes it very difficult, among other things, to trace the cadmium.
As with the hydrometallurgical processes, and in the light of increasingly stricter environmental controls, plants of this type are seeking to treat only products that have been sorted and from which have been removed batteries and/or accumulators containing materials incompatible with their remits or with the risks they are prepared to accept.
In other words, the mixtures treated in plants which could accept any battery and accumulator mixes are in fact sorted. For both economic and environmental reasons, the NiCd, NiMH and Li-Ion batteries are usually separated and are no longer treated in these plants.
2/ SPECIFIC PROCESSES FOR THE TREATMENT OF NICKEL CADMIUM BATTERIES
The methods specific to the treatment of nickel cadmium, and thus of NiMH, fall into three categories:
2.1 - Mechanical Process
In Europe, two companies at least have tried to develop mechanical processes.
- In 1989, M.G. Horn of IRELAND ALLOYS LTD presented to the 6th International Cadmium Conference in Paris the experimental study of a process based on crushing industrial battery plates followed by screening into different size grading values and magnetic separation. The purpose was to concentrate the cadmium so as to have to treat only a minimal fraction of the weight of the complete battery.
The results, based on 3 tests, were as follows:
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