Alüminyum elektroliz hücrelerinde taban kaplama malzemesi adayı tib2-bn kompozitlerinin bileşime bağlı olarak aşınma dayanımlarının ölçülmesi
Measuring wear resistance of tib2-bn composites which are as a candidate for the base coating material in aluminium electrolysis cells
Yazar: EDA ERGÜN
Danışman: PROF. DR. İSMAİL   DUMAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Üretim Metalurjisi ve Teknolojileri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2012
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Alüminyum, fiziksel ve mekanik özellikleri sayesinde ön plana çıkarak özellikle otomotiv, makine ve gıda endüstrisinde geniş bir kullanım alanına sahiptir. Ergimiş tuz elektrolizi ile üretilen alüminyumun kazanılmasında çok fazla enerji harcanmaktadır. Bu enerji sarfiyatı günde ortalama 2-4 kez gerçekleşen anot efektinden ve hücre içindeki sıvı alüminyum yastığının varlığından kaynaklanmaktadır.Enerji verimini arttırmak için sıvı alüminyum yastığı, hücre içinde birikmeden sürekli drenaj yoluyla hücreden dışarı alınmalıdır. Sıvı metal yastığı olmadığı için hücre içinde Eddy?Fuko akımından dolayı oluşan dönme hareketi engellenmiş olacaktır. Oluşan alüminyum zerrecikleri kritik yarıçapı aşarak Stokes kanununa göre en kısa yoldan (düşey yörünge) katot yüzeyindeki alüminyum gölcüğüne ulaşacaktır. Ayrıca anot-katot mesafesinin 5-7 cm'den 2,5-4 cm'ye inmesi mümkün olacaktır. Böylece elektrolit kalınlığı ve dolayısıyla direnci azalacaktır.Ancak bu durumda karbon katot ark üflemesine, kriyolite ve sürekli akış halindeki alüminyum filminin aşındırma etkisine dayanamayacaktır. Bu yüzden katodun üzeri bu etkilere dayanabilecek bir örtü ile kaplanmalıdır. Karbonunkinden daha yüksek elektriksel iletkenliği, alüminyum tarafından daha iyi ıslatılabilirlik özelliği ve yüksek aşınma dayanımı nedeniyle favori malzeme olarak TiB2 öne çıkmaktadır. TiB2'nin umut verici özelliklerinin yanı sıra umut kırıcı özellikleri de var. Bunlardan en önemlileri sinterleme sıcaklığının 2400oC'den yüksek olması ve mekanik işlenebilirliğinin (talaşlı imalat) mümkün olmamasıdır.Seramiklerin mekanik işlemeye elverişli olmadığı bilinir. Ancak mükemmel işlenebilirliği olan ileri bir seramik söz konusu olduğunda ilk akla gelen malzeme BN'dir. TiB2'ye eklendiği zaman ortak sinterleme sıcaklığının birkaç yüz derece aşağıya çekilebildiği bilinmektedir. Yüksek sıcaklık dayanımı hayli yüksektir, fakat elektrik iletkenliği yok denecek kadar düşüktür. Islatılabilirlik özelliği ise son derece kötüdür, sıvı metaller ve curuflar tarafından ıslatılmaz.TiB2-BN kompozitini katot üzerine kaplamak için TiB2-BN'den üretilmiş altıgen fayanslar vakumlu sıcak preste basılarak, aynı malzemelerden talaşlı imalat yoluyla (pafta ile diş çekilerek) üretilen vidalar yardımıyla bağlanmalıdır. Aynı vida dişleri katot karbonundaki deliklere de kılavuz ile çekilmelidir. Fayansın geometrik şekli olarak altıgen seçilmesinin nedeni, bir alanı boşluksuz doldurabilmek için üçgen, dörtgen ya da altıgen formlarından birinin kullanılması gerektiğidir ve minimum malzemeyle maksimum alanı kaplayabilmek için altıgen tercih edilmiştir. Böylece katot üzerinde sıvı alüminyumun akabileceği daha çok derz olacaktır. Tabana vida ile sabitlenmiş her fayans üç komşu fayansı kendisi ile birlikte zemine kilitleyecektir.Bu çalışmada, alüminyum elektrolizinde katot yüzeyini kaplamayı düşündüğümüz farklı bileşimdeki TiB2-BN kompozitlerinin elektriksel iletkenlikleri, mekanik işlenebilirlikleri, aşınma dayanımları incelenmiştir ve bu kompozitler katot karbonu ile kıyaslanmıştır.

Abstract:
Aluminium is the third most abundant element, and the most abundant metal, in the Earth's crust. It makes up about 8% by weight of the Earth's solid surface. Aluminium metal is chemically reactive that native specimens are rare and limited to extreme reducing environments. It is found combined in over 270 different minerals. Almost metallic aluminium is produced from the ore bauxite. Aluminium is remarkable for the metal's low density and for its ability to resist corrosion. Aluminium is a good thermal and electrical conductor. Corrosion resistance can be excellent due to a thin surface layer of aluminium oxide. Due to the physical and mechanic properties of aluminium, it becomes more popular in automotive, machine and food industries with its wide usage area.There are two main processing stages for aluminium production. This stages are alumina refining and aluminium smelting. Alumina refining is Bayer process, aluminium smelting is Hall-Heroult process.The Hall?Héroult electrolytic process was adopted over hundred years ago, and is the major industrial process for the production of aluminium. It involves dissolving alumina in molten cryolite, and electrolysing the molten salt bath to obtain pure aluminium metal. In the Hall?Héroult process alumina (Al2O3), is dissolved in an industrial carbon-lined vat of molten cryolite (Na3AlF6). AlF3 is added into the process to reduce the melting point of the cryolite-alumina mixture. So much energy is consumed in aluminium production with molten salt electrolysis. This energy consumption is derived from electromagneto hydrodynamic forces and the anode effect which realizes 2-4 times per day.During electrolysis, angle of the anode surface wetting by the electrolyte increases with decreasing the amount of alumina. Thus, gas bubbles on the surface increase. Gas bubbles cut the contact of between the anode and the electrolyte. Also, they block the passage of current. The resistance rises and cell voltage reaches very high levels.When alumina is given into the cell, the alumina is not been dissolved into electrolyte and sink to the bottom the liquid metal pad. Sludge occurs and this is a common phenomenon. This leads to continued to lack of oxygen into the cell.The aluminium ped has to be removed from the cell with constantly drainage to before it accumulated there to increase the energy efficiency. The rotary motion which occurs due to Eddy-Fuko current in the cell will be prevented as the liqiuid metal ped is not existing. The occurred liquid metal drops will exceed the critical radius and reach with most shortly way over the aluminium film on the cathode surface according to the Stokes law. Also, decreasing of the distance between anode-cathode from 5-7 cm to 2,5-4 cm will be possible. So, the thickness of the electrolyte and the resistance will decrease.However, in this situation, the cathode carbon can not stand to the arc blow, criyolite and the abrasion effect of the aluminium film which flows constantly. Thus, the surface of the cathode has to be covered with a coat which can resist to these effects. Because of its electrical conductivity which is higher than cathode carbon, good wettability by aluminium and high abrasion resistance, TiB2 is highlighted as the favorite material.Titanium diboride is known as a ceramic material with relatively high strength and durability as characterized by relatively high values of its melting point, strength to density ratio, and wear resistance. TiB2 also has many other properties, such as high hardness, a high Young?s modulus, and a high-temperature strength. It is a candidate material for cutting tools or wear-resistance applications. Titanium diboride is a metallic conductor because of the high corrosion resistance against metallic aluminum and molten cryolite. The attractive combination of hardness, thermal and electrical properties of TiB2 based materials made them suitable for various engineering applications, such as cathode material for Hall-Heroult cell, aluminium evaporation boat, Electro Discharge Machining electrode, armour material, conductive coating and wear components. As well as its advantages, it has also drawbacks. The most important drawback of TiB2 is that, the sintering temperature is higher than 2400 oC and it is not machinable. Because of strong covalent bonding, high melting point and relatively low self-diffusion coefficient, the densification of titanium diboride is needed to be carried out at relatively high sintering temperature. The sinterability of TiB2 is limite, because of a low diffusion coefficient. The low diffusion coefficient is caused by the covalent character of the material. Therefore, TiB2 has been densified by hot pressing. A major problem with using TiB2 materials that have been formed by either hot pressing or pressureless sintering at high temperatures is exaggerated grain growth, which creates strong internal stresses because of anisotropy effects and severe microcracking on cooling. Internal stresses as a consequence of grain growth at higher sintering temperature triggers microcracking, resulting in degradation of mechanical properties.It is kown that the ceramics are not convenient to machining but boron nitride is regarded as the most machinable ceramic. Boron nitride chemical formula is BN. It is a synthetic compounts consisting of boron and nitrogen element. Boron nitride is an intriguing ceramic because of its unique combination of properties such as low density, high melting point, high electrical resistivity, and good chemical stability at high temperatures. Therefore, it is generally used as a refractory material for high-purity alloy melting and crystal growth as well as radiator substrate. Being neighbors to carbon in the periodic table boron and nitrogen elements form a compount, boron nitride, which is isoelectronic with carbon and this similarity leads BN to display similar crystal forms isostructural to the polymorphous of carbon. Therefore, properties of BN are highly dependent on the crystalline modification. Boron nitride has three main polymorphous which are most extensively studied. These are; hegzagonal, wurtzite and cubic form.h-BN: It?s layer is like graphite, and theoretical density is 2.27 g/cm3.w-BN: Closed packed hexagonal form is fabricated under high pressures at relatively low temperatures and theoretical density is 3.48 g/cm3.c-BN: It is fabricated under high pressures and high temperatures and theoretical density is 3.48 g/cm3.At this study, h-BN is used because of the low density. This density is the lowest value of the ceramic materials. h-BN is manufactured by reaction of nitrogen with the mixture of boron oxide and carbon at 1450-16500C. The unique properties of hexagonal boron nitride can be given as follows; high temperatures stability, chemically inert against wetting by glasses, slags, molten oxides, cryolite and most molten metals including aluminium, resists against the attact of mineral acids, an electrically nonconducting material at lower temperatures and the electrical resistivity decreases at higher temperatures, a good thermal conductivty and easy machinability. When BN adds to TiB2, the common sintering temperature can be decreased to a few hundred degrees. Its high temperature resistance is very high but electrical conductivity is very low. Also, its wettability is very bad and it can not be damped by liquid metals and slags.This project was divided into three major tasks;- Manufacturing of the ceramic materials,- Designing the ceramic materials in industrial cells.- Pilot scale testing of composition.To coat TiB2-BN composite on the cathode, TiB2-BN hexagonal tiles were mould with could-press and bound with the help of screws which are produced with the same material and by machining method (screw cutting with bil stock). On the holes on cathode carbon, the same screw thread was applied. The reason of being chosen hexagonal as tile geometrical shape is that, it supplies a structure without hollow such as triangle and tetragon. So the hexagon is prefered since the maximum area can be covered with minimum material and on the cathode surface, there will be more lap joints through which liquid aluminium can flow easily. The tiles fastened on the cathode floor with screws lock their three neighbour tiles to the bottom with themselves.The aim of the project was to develop the ceramic based materials, technology, and necessary engineering packages to retrofit existing aluminium reduction cells in order to reduce enegy consumption required for making primary aluminium. This study extends the life of aluminum electrolysis cells provide energy savings, the carbon cathode stand to the arc blow, cryolite and the abrasion effect of the aluminium film which flows constantly. Also, the electrical conductivity, machinability and wear resitance of TiB2-BN composite will be better than cathode carbon.At this study, the electrical conductivity, machinability and wear resitance of TiB2-BN composite which can be coated on cathode surface in aluminium electrolysis, are analized and compared with cathode carbon.- Machining of TiB2-BN composite varies according to the rate of BN. As a result of the experiments seems to have increased machinability when the ratio of BN is increased.- Electrical conductivity were decreased by increasing the ratio of BN.- Amount of the wear on the composite is reduced by decreasing the ratio of BN.