Introduction and general considerations
Introduction and general considerations
The recycling of scrap containing tungsten is by no means a modern procedure. On the contrary, it has been established for many decades. Even before anyone though about recycling of glass, paper, and aluminum—as is common today in most industrialized countries—a considerable amount of tungsten was being reclaimed from different types of scrap. The reason can be easily explained: most tungsten ores contain less than 1 wt% WO3 and very seldom higher concentrations. It is evident that the least valuable scrap, like low-grade grinding sludge, contains about 15 times more tungsten than an average ore. Most scrap materials are even richer in tungsten than ore concentrates.
It is the high intrinsic value in anything containing tungsten, even in smaller concentrations, which makes tungsten scrap an interesting and worthy material for reuse. Due to low abundance of tungsten, which requires the mining and ore dressing of a huge ore tonnage to gain a small amount of tungsten, it was always a “valuable” metal.
Tungsten Recycling Methods
A variety of tungsten recycling methods were developed. They can be divided into four groups: hydrometallurgy, melting metallurgy, direct recycling, and semidirect recycling.
In principle, most tungsten containing scrap qualities can be processed by this method, which converts tungsten to “virgin” APT. However, it should be kept in mind that it is an expensive, high-energy-consuming and chemical-waste-producing procedure. In regard to the economy, preferably more impure scrap qualities, which are not easily converted by other methods, should pass this way. The scrap must first be oxidized by air, chemical, or electrical energy, in order to transfer tungsten to the hexavalent state, which is soluble in alkaline solutions. From here on the entire procedure, as for ore concentrates, in required.
Typical scrap material for hydrometallurgical treatment are grinding sludges, floor sweepings, contaminated powder qualities, tungsten-copper, and solid pieces, which can be dissolved by electrolysis.
In regard to tonnage of tungsten fed into this conversion type, it is the method mostly used (even for scrap) for converting in a more economic and ecologically preferable manner. One of the main reasons, besides those already mentioned in the introduction, might be that APT is the general intermediate for most tungsten products, with the exception of melting metallurgy. Furthermore, no additional contamination does occur, but in contrast purity will be increased.
2. Melting Metallurgy
Supperalloys, satellites, menstruum WC, and cast eutectic carbide require very pure scrap while, for steel melting metallurgy products, impure scrap can also be used. The advantage of the latter is that all oxidic substances(such as gringding media, etc.) end up in the slag, and traces of foreign elements, which are soluble in the metallic melt, are distributed homogeneously.
Any tungsten going in this direction cannot be recycled again by other methods, with the exception of re-melting. It finally ends up in loss by recycling & dilution.
3. Direct Recycling
As direct recycling is understood, the as-supplied material is transformed to powder of the same composition by either chemical or physical treatment, or a combination of both. Prerequisites for direct recycling are:
The composition of the scrap must be the same as for the final product.
The scrap must be of high purity, not only in regard to foreign substances but also in quality.
The process must offer the possibility of converting the scrap to powder of a metallurgical acceptable form.
No contamination by foreign material during processing.
Direct recycling is combined with a minimum of energy consumption, chemical waste, and lowest production costs.
4. Semidirect Recycling
Semidirect reclamation can be applied to binary or ternary-phase alloys. One component is dissolved chemical, leaving the other phase(s) intact. By the dissolution, the integrity of the structure is lowered and attrition can take place.
In conclusion, it can be stated that the importance of tungsten scrap recycling will continue to grow considerably in the future. This will mainly be driven by economic, ecological, and natural-resource preserving considerations. The share of direct recycled scrap will increase as soon as the organization of scrap collection and separation at the places of generation has improved. Scrap should always be kept as pure as possible. This must be understood not only in regard to contamination by foreign materials, but also in regard to the grade purity. As an example, different grades of cemented carbide throw away inserts should be kept separated. The old philosophy—scrap is chunk—is not any longer valid. On the contrary, scrap is a very valuable material and has to be treated accordingly. Pure grade scrap will bring a much better price or yield and is easier to convert.
A further prerequisite for an increase in direct recycling is the necessity for the consumer to learn to deal with reclaimed materials, which require modified production conditions.
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