The project’s objectives are essentially environmental and technical-economic, and in particular:
- Environmental benefits
- complete elimination of the use of lubricating oils and coolants or their emulsions in machining of the titanium grade 5;
- increase the useful life of cutting tools (WC-Co coated with a triple layer ceramic), up to 40% more than the current values in the case of turning, or 260% in the case of milling. This results in less waste production and a lower level of contamination of the chips worked;
- full recycling of the titanium scraps, no longer contaminated with organic coolant without any pre-treatment for sintering;
- recycling process in which the only energy input occurs in the sintering step by FATS techniques; with an energy saving estimated in 40-60% compared to remelting in an induction furnace under vacuum. It is estimated that sintered parts obtained through recycling with sintering possess have an embodied energy lower by 45% than similar pieces obtained by fusion; similar estimations could be applied for the CO2 footprint;
- no need to clean the scraps (no use of soaps, detergents and pickling acids), neither the machined components: liquid nitrogen simply evaporates and returns to the air, without any pollution;
- recycling by sintering of scraps obtained from cryogenic processing in nitrogen liquid allows, for the first time, to form a closed cycle which does not produce scraps, because the eventual chips are recyclable within the same production cycle;
- creation of lighter components due to sintering with minimum densification, for high temperature applications in the automotive field and racing in general, with the consequent possibility to increase operating temperatures (increasing of the efficiency), reduce consumption (lightweight components) and increase useful life of the component (high resistance to thermal oxidation, lower centrifugal forces in action).
- realization of innovative high-performance components, in addition of the company’s standard production: in particular the higher oxygen content in the sintered products will lead to higher breaking loads, partially at the expense of toughness, while the production of porous structures will increase drastically the specific resistance of the components made;
The environmental problem
The use of recycled metals at industrial level is becoming increasingly important both to meet the demands of the public about the environmental conservation and protection and as an attempt to reduce costs, possibly shortening the supply chain.
Metal recycling has an important role, providing environmental benefits in terms of energy saving, reducing waste volumes and the reduction in emissions deriving from the energy savings.
In the case of titanium, recycling is even more important as the metallurgical extraction process that leads to the titanium sponge involves high labor, energy and capital intensity. Furthermore, additional steps of crushing and repeated melting of the sponge are required to remove inclusions and achieve the required level of uniformity. The multiple stages of primary metallurgical processes mean that titanium has a high embodied energy although, quantitatively, is the fourth most abundant metal, constituting about 0.62% of the earth’s crust.
It is obvious to note that the primary metallurgy of titanium involves another environmental problem, namely the generation of chloride waste (Zheng and Okabe, 2008). In particular, during the TiCl4 reduction, intermediate compound, is produced a considerable amount of magnesium chloride. To partially solve this environmental problem, this material is placed immediately in a recycling cell. This cell firstly separates the magnesium metal and then collect the chlorine gas. Both of these components are reused for the industrial production of titanium. Despite these productive problems, nowadays, the world market for titanium, in all its forms, is increasing.
Titanium and its alloys are used in various fields, but the reference markets are aerospace and aeronautics that, alone, absorb over the 80% of global production, using mainly Ti Grade 5 (Ti6Al4V), very fine material. If we consider the industrial sectors, the use of titanium in 2011 reached its historical record, reaching over 30% growth over the previous year. Among the industrial applications, chemical, power and desalination plants are representing the largest market share, followed by medical and automotive sectors.
So, recycling of titanium is of fundamental importance, both environmentally and economically. But while the recycling of titanium scraps and bigger pieces can be somehow managed by acid pickling and recasting in appropriate conditions (Veronesi, 2013), this usually does not apply to chips produced during machining operations for material removal. The problem resides in the specific metallurgy of titanium, which tends to show excessive interstitial absorption of oxygen, nitrogen and carbon in the chips, with irreversible changes of the properties of the alloy (increasing of the resistance but decreasing of toughness). Furthermore, due to the low thermal conductivity of titanium, conventional machining requires the use of large amounts of metalworking fluids that further contaminate the chips. For these reasons, the recycling of titanium chips today is not environmentally and economically advantageous: would require the use of mixtures of hydrofluoric and nitric acid with losses of up to 10% and the production of special waste (sludge).
To achieve the ambitious goals listed above, will be necessary the development of the following actions:
- implementation of the coolant system with liquid nitrogen;
- modification of the machine tool to operate with coolant liquid nitrogen;
- changes to the system for conveying and collecting the chips;
- turning, milling and drilling tests with total recovery of chips, their chemical and particle size analysis and acquisition of optimal processing parameters, compared to standard cooling lubrication systems;
- preparation of preliminary pressing system chip (briquetting) functional to the next step of sintering;
- adaptation of the SPS system for the rapid sintering to the theoretical density of the pressed chips: obtaining a master curve of sintering;
- Study and optimization of sintering conditions with the varying of the temperature and of the pressing pressure;
- characterization of the obtained sintered (Ti recycled) and assessment of their machinability and characterization of chips obtained;
- realization of finishing operations on sintered (drilling, deburring, grinding) and recovery of scraps and their reintroduction into the pressing stage of the chips from bar virgin;
- energy and mass balance of the new recycling process of the Ti chips; assessment of the eventual cost and environmental effectiveness, considering the benefits of lower consumption of tools, increased productivity, lower energy consumption, lower number of non-compliance;
- Simplified LCA of components in Ti from recycled chips compared to similar components made from virgin Ti.
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