Muğla boksitlerinden alumina üretiminde verimlilik koşullarının incelenmesi /
Yazar: AHMET ALP
Danışman: DOÇ.DR. ALİ OSMAN AYDIN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü
Derece: Doktora, 1996
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Alümina = Alumina ; Boksit = Bouxite ; Muğla = Muğla ; Üretim metalurjisi = Production metallurgy

Özet:
ÖZET Ülkemizde birincil öncelikli rezerv olarak adlandırılan iki boksit yatağından birisi, özellikle 22,7 milyon ton rezerve sahip, yüksek A1203 tenörü ve düşük silis yüzdesi ile Muğla-Milas bölgesi boksitleridir. Halen alumina üretiminde değerlendirilemeyip parça cevher olarak satılan bölge boksitlerinin alumina üretiminde değerlendirilebilmesi olanaklarının araştırılması bu çalışmanın ana amacım oluşturmaktadır, özellikle, halen ülkemizin tek alüminyum üretim tesisi olan Seydişehir böhmitik tip boksit cevherlerinin Bayer prosesi ile değerlendirildiği göz önüne alınarak, Milas bölgesi diasporitlerinin de alkali ortamda çözündürülebilmesi imkanları araştırılmıştır. Bu amaçla, alkali katkılı örnekler ön kalsinasyon sonrası sodyum hidroksit çözeltisi ile üç edilmişlerdir. Muğla-Milas yöresi Savran, Göbekdağı, Yağhdağ ve Kayaderesi yataklarından temin edilen ve 200 mesh elek alt olacak şekilde öğütülen cevher örnekleri ile bunların kanştinlmasıyla hazırlanan bir harmanın öncelikle kimyasal analizi, yoğunluğu ve tane boyut analizi tesbit edilmiştir. Boksit cevher örneklerinin ve harmanın statik metodla kalsinasyonu gerçekleştirilerek belirli sıcaklıklarda uğradığı ağırlık kayıpları tesbit edilmiştir. Aynı numunelerin ve belirli mol fraksiyonlarında alkali katkısı yapılmış boksit harmanının termal özellikleri (TG, DTG ve DTA) dinamik yöntemle de incelenmiştir. 0,5 mol fraksiyonunda CaO ve Na2C03 ilavesi yapılmış harman belirli sıcaklıklarda kızdırılarak, katı fazda meydana gelen reaksiyon ürünleri ve muhtemel bileşikleri tesbit etmek için X-ışını çalışmaları gerçeHeştirilmiştir. Öğütülmüş boksit cevherinden A1203 ve Si02 ekstraksiyon verimine belirli mol kesrinde alkali (CaO ve/veya Na2C03) ilavesinin, kalsinasyon sıcaklık ve süresinin, Uç süresinin, çözücünün alkali molar konsantrasyonunun, kan/sıvı oranının ve harmanın tane boyutunun etkileri araştırılmıştır. Elde edilen sonuçlardan, bu tip diasporitik boksit cevherlerinin düşük sıcaklık ve alkali konsantrasyonunda çözündürülebilmesi için gereken optimum koşullar belirlenmiştir, ön kalsinasyon öncesi 0,5 mol fraksiyonunda CaO ve Na2C03 nın boksite ilavesi ile optimum verimlere ulaşılmıştır. Uygun kalsinasyon sıcaklığı endüstriyel çalışma güçlükleri de göz önüne alınarak 900 °C, kalsinasyon süresi 2 saat, liç süresi ise 3 saat olarak belirlenmiştir. Karışımların 1,5 M Na20 dan daha fazla alkali içeren çözeltilerde liç edilmesinin alumina ekstraksiyonuna fazla bir katkı sağlamadığı, katı/sıvı oranının ise 1/30 olması ile optimum sonuçların elde edildiği gözlenmiştir. Ayrıca, harmanın tane boyutunun ekstraksiyon verimine etkisinin fazla olmadığı da ortaya konmuştur.

Abstract:
THE INVESTIGATION OF THE EFFICIENCY CONDITIONS IN THE ALUMINA PRODUCTION FROM MU?LA BAUXITES Aluminium, the most common metal, forms approximately 8 % of the Earth's crust. The increasing areas of the use of aluminium has given rise to the need for alumina. Nowadays aluminium can only be produced from bauxites carrying certain properties. Bauxite was the term introduced by Berthier for sediments in alumina from the vicinity of Les Baux in the Alpilles, France. The term bauxite ore is applied to bauxites which are economically mineable at present or in the foreseeable future, containing not less than 45-50 % A1203 and not more than 20 % Fe203 and 3-5 % combined silica. Bauxite deposits originate from weathering or soil formation with enrichments of aluminium. Its composition is similar to mat of aluminium hydrate willi two water molecules. Thus we can consider it as a chemical mixture of a trihydrate such as gibbsite and a monohydrate such as diasporite and bpehmite. Bauxite has different colours ranging from cream to brown. The aluminium silicates form 40 % of aluminium minerals in the nature. It's known that there are 250 kinds of aluminium containing mineral in the Earth's crust. Aluminium oxide found naturally in crstalline is called corundum. The type containing some impurities is called as emery-stone. Additionally, aluminium oxide is usually found in nature in combinations with silica and other minerals. The powder aluminium oxide which is used in industry, is called alumina or alumin. White alumina has hexagonal or cubic crystalline systems. Hexagonally crystallined aluminium oxide is denned as a-alumina and cubicly crystallized as y-alumina. y-alumina will change to the a-crystalline system at high temperatures. Usually boehmitic bauxites (AI2O3.H2O) and gibbsitic bauxites (A1203.3H20) but rarely diasporic bauxites (A1203.H20) are used for the production of alumina. Although, today the most important alumina ore is bauxite, technological developments have shown that some other minerals such as clay, kaolin, nepheline, alunite, andalusite, dişten, silimanite and leucite may also be used for obtaining alumina. The bauxites may be classified in five different groups depending on the mineral composition of the bauxite. - Pure gibbsitic bauxites - Gibbsitic bauxites containing quartz - Mixed (gibbsitic + boehmitic) bauxites - Boehmitic bauxites - Diasporic bauxites ixThe ratio of the percentages of A1203 and Si02 in the bauxite is known silica module and shown as : AhCh % Silica Module (Mgi) = 3İU2 /o The silica module should not be lower from seven in order to produce alumina from bauxites by alkaline methods. The bauxites may be classified depending on silica module and Fe203 % as follows : - High alumina containing bauxite, Msi > 20 - Alumina containing bauxite, Ms; = 10-20 - Silica containing bauxite, Ms = 4-10 - High silica containing bauxite, M& < 4 - High iron containing bauxite, Fe203 % > 25 - Iron containing bauxite, Fe203 % = 10-25 - Little iron containing bauxite, Fe203 % < 10 It's also possible to classify bauxites depending on the base module. The base module can be expressed according to the following equation : Base Module (B) = A1203 % - 2 Si02 % The base module should at least be fourty to produce alumina from bauxites by alkaline methods. Meanwhile, more than 90 % of the extracted bauxite ores are used in the production of alumina and aluminium. In Turkey, exploration activities started in 1939 and were accelerated following after 1960. Some large reserves suitable to industry are found in the region of Mortaş and Doğankuzu in Seydişehir. Today it is estimated that Turkey has a potential of 430 million tonnes of bauxite, which consist of 51 million tonnes boehmitic minerals, 47 million tonnes diasporic minerals and 332 million tonnes ferrous minerals. The ratio of Turkey's bauxite reserves to world's bauxite reserves is approximately 1.23 %. The Muğla-Milas bauxite ores, which are evaluated for extracting alumina in this study, have 22.7 million tonnes reserve. There are various methods used in the production of alumina. Briefly, they are acidic, electrothermic, sintering and basic methods. The use of the Bayer method, which is a basic method, is very common in the world, and also used in Seydişehir aluminium plants in Turkey. Generally, acidic processes are applied to the aluminium ores which aren't evaluated economically by Bayer process. In thismethod, the ore containing aluminium is calcinated and then leached by acidic (H2S04, HNO3, HCl,etc.) solutions. In the electrothermic processes, ore and CaC03 or BaC03 mixtures are melted in an electrical furnace. Calcium aluminate or barium aluminate, occured in melted liquid phase is leached by alkaline solution and dissolved. In the sintering processes, ore is mixtured with lime or soda or sodium sulphate and coal and sintered between 1 100 and 1300 °C. This mass is dissolved by water or sodium hydroxide solution. The basic methods widely used in the world are the most important alumina production processes. More than 90 % of the world alumina production is achieved by Bayer process. Sometimes bauxite with lime or limestone is mixed and leached by sodium hydroxide or sodium carbonate. Alumina is dissolved as sodium aluminate in the alkaline solution. Briefly, this process is occured in five steps : - Raw material preparation - Dissolving and redmud filtration - Decomposition and filtration of hydrate - Calcination - Evaporation At present, Turkey has a large deposits of diasporic bauxite ores which are not used in alumina production. It is obvious that in future these ores will gain importances in the production of alumina. The main objective of this study is to evaluate the diasporic bauxites in the region of Muğla-Milas from the viewpoint of its usage in the production of alumina. Diasporic bauxite samples used in the studies were obtained from the region of Muğla-Milas. Four samples were carefully cleaned and ground to -200 mesh. Homogenous mixture was prepared using equal amounts of these four ore samples and analysed chemically. The chemical composition was 57.91 % A1203, 21.33 % FeA, 3.84 % Si02, 2.20 % Ti02, 2.10 % CaO, 1.02 % FeS2, 1.65 % C02, 0.52 % other. After calcination applications, weight loss of the mineral due to heating was determined to be 11.08 % of the bauxite mixture. Specific gravity of the bauxite mixture was determined as 3.435 g/cm3. Calcination was carried out statically and dinamically. In the experimental studies made by the static method, samples taken at a certain weight were heated at constant temperatures (100, 200, 300, 400, 500, 600, 700, 800, 900, 1000 and 1 100 °C) for two hours. Thus, weight changes of samples due to dehydratation and decomposition at certain temperatures were recorded. In the dynamic studies, Netzcs equipment, capable of drawing the curves of differential thermal analysis (DTA), thermogravimetric analysis (TG) and derivatografic thermogravimetric analysis (DTG), simultaneously was used. Dynamic studies were carried out under atmospheric conditions. The increase of temperature in furnace and the sensitivity were chosen as 10 °C/min., 0.2 mV respectively. xiThe four samples and their mixture were calcinated both statically and dinamically. However, bauxite mixed with lime and soda by changing the molecule fraction of CaO and Na20, were calcinated only dinamically. Calcination studies showed that the quickest dehydration, i.e. the weakening of crystalline network, occured at 550 °C. The first stage of alumina production was done by taking this fact into account. CaO and Na2C03 in certain molecule fractions were added to a bauxite sample of 3.00 gms. This mixture was calcinated at 550 °C after being ground in an agate mortar. Then more than 20 % of the equivalent amount of alkaline oxide (Na20 and/or CaO) was added to form alkaline aluminate, silicate, ferrate and titanate. This amount is 0.0289 molecules of alkaline oxide for 3.00 gms of molecule fraction of CaO and Na20 in our experiments. The weights of CaO and Na2C03 used in our experiments are given in the table : Table. The weight of alkaline oxides added to bauxite ores. The mixtures were calcinated and cooled. The calcinated mixtures were leached for two hours in a 100 ml NaOH solution, containing 150 gms/1 Na20, at boiling temperature with a standart mixing rate. Experiments were carried out in a stainless steel container with a back cooling system, by using a heating device with a magnetic mixer. After these process, the solution was rested and filtered using a G-3 type crucible. Thus, the effect of lime-soda fraction on leach was investigated and the amounts of A1203 and Si02 were determined. After completing the first part of the experiments, we found that the leach efficiency was the best for the alkaline fraction of Xcao:0.5 and XNa2Co3:0.5 by also taking into account the amount of Si02 passing into the solution. In the second part, the effect of temperature on the extraction efficiency of A1203 was investigated by considering this fraction. The mixtures prepared for this purpose were leached in the same manner after being calcinated at temperatures of 500, 600, 700, 800, 900, 1000, 1100 and 1200 for two hours. Although the highest extraction efficiency of A1203 was found more than 80 % at 1200 °C, 900 °C was prefered as an optimum calcination temperature because of passing high amount of Si02 into the solution and the general disadvantages of high temperature processes as well. xuAt the latter studies, bauxite samples mixed with CaO and Na2C03 at 0.5 molecule fraction were calcinated for 1, 2, 3 and 4 hours at 900 °C. The calcinated samples were leached for two hours after cooling. Concantration of 100 ml NaOH solution was 150 gms/1 Na20. The efficiency of the alumina extraction was found as 76.74% for four hours calcination time. To determine the optimum leaching time, bauxite samples, mixed with CaO and Na2C03 at 0.5 molecule fraction and calcinated for two hours were leached for 1, 2, 3, 4 and 5 hours with 100 ml NaOH solution containing 150 gms/1 Na20. The optimum leaching time was determined to be three hours because of the high alumina extraction and the low silica extraction. The calcinated samples as mentioned above were leached for three hours in a 100 ml NaOH solution. The concentrations of the solvent were choosen as 0.0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 molecule Na20. The preferred optimum concentration of the solvent was 1.5 molecule Na20. Because the efficiency of alumina extraction was almost constant in the solutions having more than 93 gms/1 Na20 concentration. The prepared bauxite samples were leached for three hours in 30, 60, 90, 120, 150, 180 and 210 gms of NaOH solutions with a specific gravity of 1.11539 containing 1.5 molecule Na20. In addition to these studies, the effects of the grain size of the bauxite ore on the efficiency of the alumina and extraction of silica were examined too. xni