Endodontide kullanılan nikel–titanyum esaslı kullanılmış döner diş eğelerinden metalik değerlerin geri kazanımı

Abstract

Nikel – titanium esaslı alaşımlar, endodontide diş kanal tedavisinde kanal eğesiolarak yaygın bir şekilde kullanılmaktadır. Diş kanal tedavisinde Ni – Tialaşımlarının tercih edilmesinin başlıca sebepleri; yüksek mukavemet göstermeleri,biyo uyumlu olmaları, yüksek korozyon dayanımına sahip olmaları, süper elastiklikgösterebilmeleri ve şekil hafızalı özellik kazandırılarak tekrarlı kullanılabilen birürün haline getirilebilmeleridir. Ülkemizde ve dünyada, endodontide Ni – Ti alaşımıdöner diş eğeleri ile tedavi sistemleşmiş bir hal almıştır. Tek bir dişin tedavisi bilefarklı tip ve boylarda Ni – Ti alaşımı eğeler yardımı ile gerçekleştirilmektedir. Dişkanal tedavisinde Ni – Ti alaşımının her bir eğe için bir veya iki kullanımlık olmasıönerilmektedir. Ancak sterilizasyon yapılsa bile eğelerde meydana gelen aşınma,eğenin aşındırma kabiliyetini kaybetmesine sebep olmakta ve tekrarlı kullanımısınırlandırmaktadır. Ni – Ti alaşımı döner diş eğeleri sınırlı kullanımlarından sonratıbbi atıklarla birlikte toplanmakta ve bilinen bir metalurjik kazanım sürecine tabitutulmamaktadır.Ülkemizde yılda iki milyona yakın kanal tedavisi yapılmakta olup, bu kanaltedavilerinin % 80'inde Ni – Ti alaşımı döner diş eğelerinin kullanıldığı TürkiyeCumhuriyeti Sağlık Bakanlığı'nca rapor edilmiştir. Bu eğeler, ağırlıkça % 55 – 60nikel ile % 40 – 45 titanyum içermektedir. Tutucu kısımları, çok defa pirinçalaşımdan imal edilmekte olup ağız florasında korozyona maruz kalmaması için altınile kaplanmıştır. Ni – Ti alaşımı uç kısımlar nikel ve titanyum kaynağı olarakdeğerlendirilebilecek niteliktedir. Altın ve nikel kaplamalı pirinç alaşımı tutucukısmın geri kazanımı da dahil edildiğinde, bu işlemin geri kazanım maliyetinidüşürmesi beklenebilir.Bu tez kapsamında döner diş eğesi olan, nikel ve titanyum esaslı uç kısmının, derişikasitte çözme, sülfürik asitte pişirme ve karıştırmasız otoklav ortamında derişiksülfürik asitte çözme gibi yüksek korozif ortamlarda, farklı liç teknikleri ilehidrometalurjik olarak geri kazanımı sağlanmaya çalışılmıştır. Liç işlemlerinde asitkonsantrasyonu, sıcaklık ve süre parametreleri incelenmiştir. Bu yöntemleriçerisinden tam çözünme veriminin sağlandığı, otoklav ortamında derişik sülfürikasitte liç işlemi için optimizasyon yapılmıştır. Geri kazanım sonunda kostik soda(NaOH) ile pH ayarlaması yapılarak Ti(OH)4 ve Ni(OH)2 elde edilmiş, dehidratasyonile birlikte NiO ve TiO2 (anatas) fazlarına ulaşılmıştır.Altın ve nikel kaplamalı pirinç alaşımından imal edilen tutucu kısım ise nitrik asitortamında ısıtılarak liç edilmiş, altından gayrı metallerin tamamına yakını çözeltiortamına alınarak altın kaplama, alaşımdan ayrılmıştır. Altından gayrı metaller,kostik soda ile pH ayarlaması yapılarak çöktürülmüş ve çöken metal hidroksitlerinkatı / sıvı ayrımı yapıldıktan sonra dehidratasyon ile su molekülleri yapıdanxxiiuzaklaştırılarak, CuO, ZnO elde edilmiştir. Altın ve nikel kaplamalı pirinç alaşımıtutucu kısmın Ni – Ti alaşımı uç ile beraber geri kazanımı, bu konuda tasarlanacakgeri kazanım sürecini daha da cazip hale getirilmiştir.

Endodontics is a branch of dental medicine that deal with dental problems thatadvanced to tooth pulp. The first aim of endodontic treatment is prevent tooth losses.Whatever tooth disease, endodontic treatment can be result in tooth saving. Beforethe widespread use of nickel-titanium alloys in endodontics, stainless steel rotaryfiles are used for endodontic root canal treatment. Stainless steel rotary file is notelasticity as Ni – Ti rotary file in curved tooth root canal. However, 20 percent ofrotary files is made of stainless steel. Nickel – titanium based alloys are used forendodontic root channel treatment as rotary files. Due to their flexibility, strengthand bio-compatibility nickel- titanium based superalloys are widely used for dentalcanal treatment. Moreover, these alloys are high corrosion resistance materials. andshow super elasticity and shape memory behavior.In a tooth canal treatment, one endodontic Ni – Ti rotary file may be used severaltimes. It is suggested that Ni – Ti rotary files should be used just once by endodonticauthorities. Although, after adequate sterilization, Ni – Ti rotary files use severaltimes, abrasion loss of these files obstacle to further use. Nickel – titanium basedendodontic rotary files are disposed together with medical wastes. Ni – Ti rotary filedisposes are not recovered metallugically. In Turkey, number of endodontic rootcanal treatments is done around 2 million per year, among them % 80 used Ni – Tirotary files.Elimination of oxides is necessary for titanium melting and elemental accumulationchanges nickel –titanium based alloys' properties. Nickel – titanium based alloys areproduced with very pure raw materials and also production methods, like VacuumArc Remelting (VAR) or Vacuum Induction Melting (VIM) is expensive. Due totheir reduced size the volume / surface ratio of Ni – Ti rotary files are suitable foroxidation. The oxide layer, disposed Ni – Ti rotary files melting is result in materiallosses.In industry, nickel containing scrap is used as a nickel source in steel production. Theuse of nickel containing scrap for steel production is 14 % while the use of metallicnickel is 65 %. The majority of the primary nickel output is directed to theproduction of ferronickel for stainless steel production.As in the recycling of nickel, mixed titanium alloy scrap types are converted toferrotitanium for producing titanium alloyed steel as a titanium source. The amountof titanium alloys scrap which converted to ferrotitanium is 50 percentapproximately. High quality titanium alloys are used in very small scale andrecycling procedure of high quality titanium alloys are also very small scale.Recycled from titanium containing scrap as a very pure oxide can be appropriate fortitanium production. Pure titanium oxides (Anatase, Rutil) are used as a pigment orphotocatalytic materials that more valuable than ferrotitanium.xxivIn this work, a recycling process is suggested for disposed endodontic Ni – Ti rotaryfiles. Ni – Ti alloy part was separated from holder and rubber. Specifiedmetallurgical 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, conventionalleaching methods are not effective. Corrosion resistance behavior is caused bycompact oxidation layer of nickel – titanium based alloys. Oxidation layer of Ni – Tirotary files was characterized by SEM – EDX characterization method. Acid bakingprocess and autoclave leaching techniques , that are non – conventional but effectiveleaching techniques are used for recycling process design. In preliminary studies,sulphuric acid, hydrochloric acid and nitric acid had been tried at differentconcentration and temperature. Dissolving of Ni – Ti rotary files within sulphuricacid is more effective than the other two acids. High temperature and high sulphuricacid concentration increased the dissolution effect, the highest dissolution rate isobserved with 14 % wt hot sulphuric acid solution.Leaching capabilities of sulphuric acid was enhanced by acid baking process. In thisprocess, Ni – Ti rotary file sample was heated with concentrated sulfuric acid for twohours. After that few water drops added to hot sample – acid mixture. Dissolution ofNi – Ti rotary file became very quickly. Sample / acid weight ratio in acid bakingexperiments has been selected as 1/3, 1/5, 1/7 and 1/9. Increasing acid the amount isimproving the metal dissolution but, the dissolution is not efficient when acid weightratio is above 7. In addition, high acid ratio inhibited nickel dissolution from Ni – Tirotary file.Ni – Ti rotary files totaly dissolved under pressure via autoclave leaching process.Autoclave has been manufactured by 316 L type quality stainless steel containingteflon cup which prevents acid attack to stainless steel. The effect of temperature,concentration an leaching time has been evaluated. Autoclave leaching solution wasprepared as 2,5 M and 5 M sulphuric acid solution. Dissolving time parameters havedetermined as 30, 60, 90, 120 minutes. Temperature parameters have also selected as150, 175, 200, 225, 250 oC degrees. 150 oC, 120 minutes, 1/5 sample / acid ratioparameters and 250 oC, 30 minutes, 1/5 sample / acid ratio parameters are borderconditions for 100 % dissolving. Hence, furnace energy consumption during heatinghas measured with power analyzer. At these temperature parameters, power valuesare so closed. Heating time is decisive for energy consumption at these conditionsthat 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 `blackliqueur`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. Nickelhydroxide was dehydrated at 600 oC to convert as nickel oxide that was characterizedwith XRD. Ni – Ti rotary files' holder is gold and nickel coated brass that gold content of holderis 0.6 percent. Gold is biocompatible metal. Nickel coating has been made forinhibition of gold – copper diffusion. The reason of choosing brass for holdermanufacturing is high torsion yield. In consequence of containing high valuablemetals, holders were recycled. Metal out of gold were dissolved in 63 % hot nitricacid 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 wasoptimum enrichment condition for which gold containing of undissolved part is 76percent.The solution obtained from holder recycling, contains copper, zinc and nickel ions.These ions were precipitated by pH adjustment with NaOH. Copper ion wasprecipitated as Cu(OH)2 at pH 8 value and zinc ion was precipitated as Zn(OH)2 atpH 10 value. Zinc was precipitated with together nickel ions at pH 10. Like nickelhydroxide precipitated from `black liqueur`, Cu(OH)2 and Zn(OH)2 were dehydratedat 600 oC. At the end of the dehydration process, CuO and ZnO were obtained. Theseoxide was characterized with XRD. NiO peaks have not detected with XRD analysisof dehydrated ZnO particles.Leaching with acid baking process was not work efficiently. Experimental studieswere showed that disposed Ni – Ti rotary files can be recycled by autoclave leachingand pH adjustment with caustic soda and all of the products are considered as a rawmaterial.