Yerel Nikel Cevherlerinden Nikel Pik Demir (npd) Üretimi

Abstract

Nikel, manyetik özelliğe sahip periyodik tablonun 8B grubuna ait bir element olup ergime sıcaklığı 1453 °C ve yoğunluğu 8,908 g/cm3’tür. Nikel yerkabuğunda en fazla bulunan 24. elementtir ve 80 ppm konsantrasyona sahiptir. Korozyona, paslanmaya ve ısıya olan dayanıklılığı nedeniyle nikelin en büyük kullanım alanı paslanmaz çelik üretimi ve yüksek mukavemete sahip alaşım üretimidir. Tüm dünyada üretilen nikelin yaklaşık % 60’tan fazlası paslanmaz çelik üretiminde kullanılmaktadır. Son dönemlerde FeNi yerine özellikle Nikel Pik Demir in (NPD) daha çok üretilip kullanılmaya başlanması nedeniyle, bu çalışmada Van yöresi lateritik nikel cevherlerine NPD üretmek amacıyla indüksiyon fırını kullanılarak doğrudan redükleyici ergitme uygulanmıştır. Ekonomik olarak işlenebilen nikel cevherleri lateritik ve sülfürlü cevherler olarak ikiye ayrılmaktadır. Lateritik nikel cevher rezervleri tüm nikel cevherlerinin ortalama %70’ini oluşturmakta ve gün geçtikçe kullanımı artmaktadır. Lateritik cevherler tropikal ve sub-tropikal bölgelerde limonit [(Fe,Ni)O(OH).nH2O] ve garnierit [(Ni,Mg)3Si2O5(OH)] vb. içeren ultramafik kayaçaların yağmurlar ile yıkanması ile oluşan ikinci bir yoğunlaştırılmış tabakadır. Sülfürlü nikel cevherleri ise magmatik hareketler sonucu oluşmuştur ve çoğunlukla doğada bakır cevherleri ile beraber bulunmaktadırlar. Lateritik cevherlerden pirometalurjik yöntemler, hidrometalurjik yöntemler ve kombine yöntemler (piro-hidrometalurjik) yöntemler ile nikel üretimi mümkündür. Bu çalışmada Van yöresi lateritik cevherlerinden nikel pik demir (NPD) üretmek amacıyla öğütme yapılmış, cevher ortalama 303 µm tane boyutuna getirilmiştir. Öğütülen cevher ve kullanılacak redüktan etüvde kurutulduktan sonra her bir deney için cevherden 100 g tartılıp farklı stiokiometrilerde redüktan, curuflaştırıcı karıştılıp 1550-1650ºC’de redüktif ergitme işlemi uygulanmıştır. Elde edilen alaşımlar, kullanılan lateritik cevher, redüktan (metalurjik kok), curuflaştırıcı ve curuflar, XRD (X-Işınları difraktometresi), XRF (X-Işınları floresans spektrometresi), AAS (Atomik absorpsiyon spektrometresi), ve EPMA (Elektron Prop Mikro Analizör) analiz teknikleri ile karakterize edilmiştir. İlk deney setinde 100 g lateritik nikel cevheri ve değişen stokiometrilerde redüktan (metalurjik kok) ilavesi ile metal kazanım verimleri incelenmiştir. En yüksek verim P6 kodlu %30 redüktan/cevher oranına sahip numunede elde edilmiştir. Ni verimi % 77,76, Co verimi % 75,23 ve Cr verimi % 48,88 olarak gerçekleşmiştir. Alaşımdaki Ni, Co ve Cr konsantrasyonları sırasıyla % 3,00, % 0,15 ve % 4,05’tir. İkinci deney setinde en yüksek verimi elde edilen P6 numunesi farklı prosess sürelerinde redüktif ergitmeye tabi tutularak sürenin metal kazanım verimine etkisi incelenmiştir. Artan reaksiyon süresiyle metal kazanımının arttığı görülmüştür. 35 dakika süre sonucunda % 91,99 Ni, % 93,87 Co, % 69,40 Cr, metal kazanım verimini elde edilmiştir. Alaşımdaki Ni, Co ve Cr konsantrasyonları sırasıyla % 3,13, % 0,17ve % 5,07’dir. Üçüncü deney setinde %10 sabit redüktan/cevher oranına sahip P2 nununesi sırasıyla % 2, % 4, % 6, % 8 ve % 10 oranlarında CaO ilavesi yapılarak redüktif ergitmeye tabi tutulmuştur. En yüksek Ni verimi % 6 CaO ilavesi yapılan numunede elde edilmiştir. Ni verimi % 78,18, Co verimi % 66,58 ve Cr verimi % 16,29 olarak gerçekleşmiştir. Alaşımdaki Ni, Co ve Cr konsantrasyonları sırasıyla % 4,47, % 0,20 ve % 1,98’dir. Dördüncü deney setinde % 30 sabit redüktan/cevher oranına sahip P6 nununesi sırasıyla % 2, % 4, % 6, % 8 ve % oranlarında CaO ilavesi yapılarak redüktif ergitmeye tabi tutulmuştur. En yüksek Ni verimi % 10 CaO ilavesi yapılan numunede elde edilmiştir. Ni verimi % 93,46, Co verimi % 86,45 ve Cr verimi % 27,82 olarak gerçekleşmiştir. Alaşımdaki Ni, Co ve Cr konsantrasyonları sırasıyla % 3,88, % 0,19 ve % 2,48’dir. CaO ilavesi genel olarak çalışma sıcaklığını düşürmüş Ni, Co verimlerine olumlu etki etmiştir. Cr verimini düşük redüktan/cevher oranlarında artırmış, yüksek redüktan/cevher oranlarında düşürmüştür. CaO Ni ve Co verimine etkisi yüksek redüktan/cevher oranlarında daha etkin olmuştur.

Nickel is the one of the most important metal that has many application area application areas in the industry and there are a lot of kinds of nickel products such as refined metal, powder, sponge etc. 62 % of metallic nickel is used in stainless steel, 13 % is consumed as superalloy and non ferrous alloy because of its superior corrosion and high temperature properties. Nickel ores can be classified in two group as sulphide ores and lateritic ores. Although 70 % of land based nickel ores are laterites, 60 % of primary production is from sulphide ores. The importance of lateritic ores is increasing due to the increase in nickel prices and decrease in reserves of sulphide ores. Nickel laterite typically occur in tropical or sub-tropical regions where weathering of ultramafic rocks containing iron and magnesium for along time . These deposits usually exhibit different layers due to weathering conditions. The first layer is silica rich layer and after limonite layer dominated by goethite a (FeOOH ) and hematite (Fe2O3). Then a saprolite ((Ni,Mg)SiO3.nH2O) layer rich in magnesium and basal elements. Lastly there are altered and unaltred bed rocks. Between the saprolite layer limonite layer there is usually magnesium rich transititon layer (10-20% Mg), with iron called serpentine (Mg3Si2O5(OH)4).For an ideal laterite deposit, the limonitic layer is not very suited to upgrading, while some upgrading the magnesium-rich saprolitic layer is also limited for the nickel concentration. This is the main difference between lateritic and sulfidic ores that can be beneficiated from 10 % to % 28 There are some common pyrometallurgical, hydrometallurgical and combined (pyro-hydrometallurgical) methods which are used for the extraction of nickel from lateritic ores. HPAL (High temperature pressure acid leaching) is generally used to recover metallic nickel and cobalt from laterite nickel ores. It is more suitable for the plants processing ores with low magnesium oxide and aluminum oxide content. Lateritic ores are exposed to hot acidic leaching around ~250 °C to dissolve nickel and cobalt under high pressure. Solvent extraction is commonly used with HPAL processes. Resin-in-pulp methods have also been exposed to selectively separate nickel and cobalt directly from the leach solution, however this method is not being used commercially at present. The main disadvantage for HPAL is the high cost of titanium autoclaves and maintaining cost. Process is complex and difficult to control due to the high pressure and heating of the process. AL (Atmospheric leaching) is being replaced with HPAL due to low costs and more suitable for the smaller scale plants. AL includes direct leaching of laterite ores in the organic or inorganic acids and obtaining Ni, Co hydroxides in the solution. Solution can be enriched by using SX and metallic nickel and cobalt are recovered by EW or precipitating. In the heap leaching process, milled ore is fed by dilute acid from the top, and nickel and cobalt are digested. Collected solution is treated for metal recovery. In the agitation leach, crushed and ground ore is leached in a heated tank. Temperature has a beneficial effect on the metal recovery with decrease of the process duration, Caron Process was first developed by Caron in the 1920s however this process was firstly used after World War II in Cuba. This process Candbe apllied to high iron limonitic ores and tolerates more Mg than other acid leaching processes. In this process, ore is blended and dried, then reduced in a roaster by using hydrocarbon fuel and air around 700 °C. The product is generally iron-nickel alloy. Hot and reduced ore is cooled in a roaster under reducing atmosphere and quench in ammoniacal ammonium carbonate solution in the tanks. Ni and Co are precipitated as carbonate form from solution. The recovery is lower compared to pyrometallurgical and hydrometallurgical processes. The first step of this process also consumes high energy. Ferronickel smelting of laterite ores is generally performed by using fossil fuels (coal, oil, natural gas, etc.) as reductant in a rotary kiln. Nickel and cobalt are firstly reduced because iron has greater affinity for oxygen. The product is charged to converter for refining after discarding slag containing unreduced iron oxide, magnesium and silica. The end-product is ferronickel alloy which contains 25% nickel. This unrefined ferronickel is refined using soda ash, calcium containing compounds to remove sulphur content. Air is blown through molten and desulphurized ferronickel to oxidize carbon, phosphorus and other impurity elements. This process is energy intensive but new furnace technologies reduce the energy costs. Nickel Pig Iron (NPI) production is a new trend which was first developed about 50 years ago but not used commercially until some Chinese pig iron producers changed their production methods into nickel pig iron without new investments. NPI production first began in blast furnaces using low grade laterite ores imported from Indonesia, Philippines and New Guinea. The process is almost same as pig iron production. The difference is that the ore contains more nickel. The blast furnace products contain 2-10 % of nickel. The trend is to use electric arc furnace to reduce operations costs. In this study, experiments were performed in an induction furnace with graphite crucibles. Lateritic ores were employed as raw material to produce nickel pig iron from East Anatolian Region. The effect of reductant ratio, process time and flux addition was examined. East Anatolian Region raw lateritic nickel ore samples were crushed and ground by using a jaw crusher, cone crusher, roller crusher and a vibratory cup mill. Average particle size of 303 μm was calculated by using screen analysis for the ground ore. Ore was homogenized at the end of the mill treatment process. Homogenized ore was characterized by using different analyzing techniques including XRD, XRF and AAS. Also amount of fixed carbon, volatile materials and ash in metallurgical coke were analyzed. Quartz, magnetite, hematite and magnesium chromium oxide phases and slightly nickel iron oxide. In the first experimental set, effect of reductant amount was carried out. Lateritic ores and coke were dried in dryer at 105 °C for 2 hours. 100 g ore and metallurgical coke (from 5g to 35g) were mixed and charged into an induction furnace which is commercially designed for F9 and F10 graphite based crucibles. These mixtures were held for 25 minutes in the furnace at temperature range of 1600-1650 °C. It was observed that charged mixtures began to melt around 10th minute (1350-1400 °C) and reached the maximum temperature about 15th minute. Metallic and slag phases were obtained after smelting. Slags were discarded and grinded. Magnetic-metallic and non-magnetic parts of the slags were separated by using magnetic separation process. Magnetic parts were added to the metallic phase to re-melt. The homogenized metal buttons were characterized using XRF, AAS, EPMA techniques. P6 the sample with % 30 reductant/charged ore ratio has the highest Ni recovery efficiency as 77,76 % and also has 75.23 % Co, and 48.88 % Cr recovery. Metal concentrarions in the alloy are 3.00 % for Ni, 0.15 % for Co and 4.05 % for Cr. In the second experimental set, effect of process duration was carried out. P6 mixture was smelted in different process durations from 15 min. to 35 minutes. The same experimental and characterization procedures as in the first experimental set were employed for the second experimental set with different process times. Ni and Co concentrations in the alloy slightly change with the increase of process duration but their recovery efficiencies change rapidly with increasing in the time. The highest recoveries were achieved for the experiments conducted with the addition of 30% of metallurgical coke with 91,99 % Ni, 93.87 % Co, and 69.40 % Cr at the processes times of 35 minutes. Metal concentrations in the alloy are 3.13 % for Ni, 0.17 % for Co and 5.07 % for Cr. In the third experimental set, different amount of flux (CaO) were added as 2 %, 4 %, 6 %, 8 %, 10 % flux/charged ore ratio to sample with 10 % constant reductant/charged ore ratio. CaO addition has beneficial effect on metal recovery decreasing slag temperature. The highest recovery in nickel was performed with % 6 flux addition/charged ore by 78.18 % Ni with 66.58 % Co and 16.29 % Cr recovery with 4.47 % Ni with 0.20 % Co and 1.98 Cr concentration at 10 % constant reductant/charged ore ratio. In the fourth experimental set, different amount of flux (CaO) were added as 2 %, 4 %, 6 %, 8 %, 10 % flux/charged oreratio to sample with %30 constant reductant/charged ore ratio. The highest recovery in nickel was performed with % 30 flux addition/charged ore by 93. 46 % Ni with 86.45 % Co and 27.82 Cr concentration with 3.88 % Ni, 0.19 % Co and 2.48 % Cr concentration at 30 % constant reductant/charged ore. CaO addition has benefical effects on Ni an Co recoveries and decreases melting temprature of slag. Cr recovreies also slightly increases at low reductant/charged ore ratio however decraseses rapidly at high reductan/charged ore ratio. CaO has more beneficial effects at high reductan/charged ore ratio.