Unused refractory products are easy to recycle with only simple treatment processes required. It is also possible to reuse or recycle materials with superficial contamination by implementing suitable treatment protocols. However, materials which are heavily contaminated are much more difficult to exploit.
A typical treatment protocol can be envisaged as follows:
- programmed dismantling and immediate on-site sorting
- transport of differentiated lots to the recycling station
- re-sorting on a conveyor belt
- initial crushing and magnetic separation
- milling, sieving and magnetic separation
- separation of different grades of materials by chemical nature
- treatment of fines, leaching and water treatment
- removal of fraction not suitable for exploitation to landfill
A large scale recycling facility would allow mixing of lots of similar chemical nature to offer a more consistent secondary material in terms of chemistry and particle size. It would also allow the definition of a technical data sheet for the secondary materials which can be met over time, a condition essential to gain the confidence of new users.
Clearly the general treatment protocol described above must be adapted for different refractory products and their degree of contamination.
- in glassmaking, a large proportion of the AZS type electrocast refractories have been successfully reprocessed for almost 20 years. Valoref, a subsidiary company of the Saint-Gobain group, is specialised in the salvaging and treatment of used refractory products from this sector. They now claim that for a complete dismantling and refurbishment, 60% of the old refractory is recycled, 24% goes to Class 3 landfill, 14% to Class 2 landfill and only 2% to Class 1 landfill. This company currently has a processing capacity of 40,000 tonnes per year.
- in the iron and steel sector, refractories containing magnesia-carbon have been the subject of particular attention. Incorporation of recycled product in new magnesia-carbon material leads to parasitic reactions between water, released by the resin binder during polymerisation, and Al4C3 and AlN phases, formed from the Al starting powder (antioxidant), which gives rise to gas emissions (CH4 and NH3). Vesuvius has developed a treatment of the superheated vapour which allows transformation of the aluminium carbides and nitrides into stable alumina.
The same problem is encountered when producing magnesia based concretes containing recycled grains.
Used refractory products with appreciable hexavalent chromium content has led to the development of many treatment processes (~ 30 publications in the form of patents and articles). The principles most often advanced are:
- carboreduction leading either to Cr2O3, or directly to chromium metal
- reduction in liquid medium by reducing agents (sugars, sulphites…)
- thermal treatment at a temperature higher than the stability of CrO3, with or without quenching during cooling
- absorption by organic substrates or micro-organisms
Although processes for treating hexavalent chromium exist, it is rare for a company involved in recycling refractory products to be equipped with them. There are a number of reasons for this:
- the quantities involved have become smaller because, for certain applications, the problem has been circumvented by adopting other, often lower performance, materials (e.g. containing spinel)
- it is not clear which is the most profitable treatment route
- the investment required for the installations and the cost of the treatment including employing people is high compared to the cost of putting the waste in landfill
Thus, agreement has yet to be reached regarding the optimal exploitation of used refractory products containing chromium.