Recovery of metallic values from disposed nickel–titanium rotary endodontic files

Endodontics is a branch of dental medicine that deal with dental problems that advanced to tooth pulp. The first aim of endodontic treatment is prevent tooth losses. Whatever tooth disease, endodontic treatment can be result in tooth saving. Before the widespread use of nickel-titanium alloys in endodontics, stainless steel rotary files are used for endodontic root canal treatment. Stainless steel rotary file is not elasticity as Ni – Ti rotary file in curved tooth root canal. However, 20 percent of rotary files is made of stainless steel. Nickel – titanium based alloys are used for endodontic root channel treatment as rotary files. Due to their flexibility, strength and bio-compatibility nickel- titanium based superalloys are widely used for dental canal treatment. Moreover, these alloys are high corrosion resistance materials. and show super elasticity and shape memory behavior. In a tooth canal treatment, one endodontic Ni – Ti rotary file may be used several times. It is suggested that Ni – Ti rotary files should be used just once by endodontic authorities. Although, after adequate sterilization, Ni – Ti rotary files use several times, abrasion loss of these files obstacle to further use. Nickel – titanium based endodontic rotary files are disposed together with medical wastes. Ni – Ti rotary file disposes are not recovered metallugically. In Turkey, number of endodontic root canal treatments is done around 2 million per year, among them % 80 used Ni – Ti rotary files. Elimination of oxides is necessary for titanium melting and elemental accumulation changes nickel –titanium based alloys' properties. Nickel – titanium based alloys are produced with very pure raw materials and also production methods, like Vacuum Arc Remelting (VAR) or Vacuum Induction Melting (VIM) is expensive. Due to their reduced size the volume / surface ratio of Ni – Ti rotary files are suitable for oxidation. The oxide layer, disposed Ni – Ti rotary files melting is result in material losses. In industry, nickel containing scrap is used as a nickel source in steel production. The use of nickel containing scrap for steel production is 14 % while the use of metallic nickel is 65 %. The majority of the primary nickel output is directed to the production of ferronickel for stainless steel production. As in the recycling of nickel, mixed titanium alloy scrap types are converted to ferrotitanium for producing titanium alloyed steel as a titanium source. The amount of titanium alloys scrap which converted to ferrotitanium is 50 percent approximately. High quality titanium alloys are used in very small scale and recycling procedure of high quality titanium alloys are also very small scale. Recycled from titanium containing scrap as a very pure oxide can be appropriate for titanium production. Pure titanium oxides (Anatase, Rutil) are used as a pigment or photocatalytic materials that more valuable than ferrotitanium.xxiv In this work, a recycling process is suggested for disposed endodontic Ni – Ti rotary files. Ni – Ti alloy part was separated from holder and rubber. Specified metallurgical recycling process of disposed Ni – Ti rotary files is not known, hydrometallurgical methods are prefered as an effective recycling of Ni – Ti rotary. Due to the high corrosion resistance of nickel – titanium alloys, conventional leaching methods are not effective. Corrosion resistance behavior is caused by compact oxidation layer of nickel – titanium based alloys. Oxidation layer of Ni – Ti rotary files was characterized by SEM – EDX characterization method. Acid baking process and autoclave leaching techniques , that are non – conventional but effective leaching techniques are used for recycling process design. In preliminary studies, sulphuric acid, hydrochloric acid and nitric acid had been tried at different concentration and temperature. Dissolving of Ni – Ti rotary files within sulphuric acid is more effective than the other two acids. High temperature and high sulphuric acid concentration increased the dissolution effect, the highest dissolution rate is observed with 14 % wt hot sulphuric acid solution. Leaching capabilities of sulphuric acid was enhanced by acid baking process. In this process, Ni – Ti rotary file sample was heated with concentrated sulfuric acid for two hours. After that few water drops added to hot sample – acid mixture. Dissolution of Ni – Ti rotary file became very quickly. Sample / acid weight ratio in acid baking experiments has been selected as 1/3, 1/5, 1/7 and 1/9. Increasing acid the amount is improving the metal dissolution but, the dissolution is not efficient when acid weight ratio is above 7. In addition, high acid ratio inhibited nickel dissolution from Ni – Ti rotary file. Ni – Ti rotary files totaly dissolved under pressure via autoclave leaching process. Autoclave has been manufactured by 316 L type quality stainless steel containing teflon cup which prevents acid attack to stainless steel. The effect of temperature, concentration an leaching time has been evaluated. Autoclave leaching solution was prepared as 2,5 M and 5 M sulphuric acid solution. Dissolving time parameters have determined as 30, 60, 90, 120 minutes. Temperature parameters have also selected as 150, 175, 200, 225, 250 oC degrees. 150 oC, 120 minutes, 1/5 sample / acid ratio parameters and 250 oC, 30 minutes, 1/5 sample / acid ratio parameters are border conditions for 100 % dissolving. Hence, furnace energy consumption during heating has measured with power analyzer. At these temperature parameters, power values are so closed. Heating time is decisive for energy consumption at these conditions that are work for total dissolution. After leaching parameter optimization, all of Ni – Ti rotary files was dissolved at 250 oC, 30 minutes, 1/5 sample / acid ratio conditions and solution named "black liqueur"containing nickel and titanium ion was obtained. Titanium and nickel ions were precipitated by pH adjustment with NaOH. At pH 1, titanium ions was precipitated as Ti(OH)4, while Nickel ions were precipitated at pH 10 as Ni(OH)2. In order to prevent the formation of rutile phase, titanium hydroxide was dehydrated at 300 oC. The anatase was characterized with XRD and Raman spectroscopy. Nickel hydroxide was dehydrated at 600 oC to convert as nickel oxide that was characterized with XRD. Ni – Ti rotary files' holder is gold and nickel coated brass that gold content of holder is 0.6 percent. Gold is biocompatible metal. Nickel coating has been made for inhibition of gold – copper diffusion. The reason of choosing brass for holder manufacturing is high torsion yield. In consequence of containing high valuable metals, holders were recycled. Metal out of gold were dissolved in 63 % hot nitric acid solutions. Dissolving time parameters were selected as 6, 8, 10, 12, 18 hours. The undissolved metal part was characterized with XRF. 12 hour treatment time was optimum enrichment condition for which gold containing of undissolved part is 76 percent. The solution obtained from holder recycling, contains copper, zinc and nickel ions. These ions were precipitated by pH adjustment with NaOH. Copper ion was precipitated as Cu(OH)2 at pH 8 value and zinc ion was precipitated as Zn(OH)2 at pH 10 value. Zinc was precipitated with together nickel ions at pH 10. Like nickel hydroxide precipitated from "black liqueur", Cu(OH)2 and Zn(OH)2 were dehydrated at 600 oC. At the end of the dehydration process, CuO and ZnO were obtained. These oxide was characterized with XRD. NiO peaks have not detected with XRD analysis of dehydrated ZnO particles. Leaching with acid baking process was not work efficiently. Experimental studies were showed that disposed Ni – Ti rotary files can be recycled by autoclave leaching and pH adjustment with caustic soda and all of the products are considered as a raw material.