Muğla yöresi kromitlerden alkali füzyon yöntemiyle kromat üretimi

Abstract

Geleneksel kromat üretim prosesinde kromit konsantresi, Na2C03 ve CaO ile birlikte bir döner fırında, 1100-1 150°C de oksitleyici kavurma işlemine tabi tutulmaktadır. Ancak kromit konsantresi, bu metotla işlenebilmesine imkan vermeyecek miktarda silisyum içerebilir. Bu bileşen, fırında olumsuz şartlar oluşturarak, örneğin N^CC^ ile reaksiyona girip fırın duvarlarında yapışkan ürünler oluşturarak, prosesin işlemesine engel olabilmektedirler. Ayrıca kromit konsantresinde bulunacak yüksek miktardaki alüminyum içerikleri de Na2C03 in fazla harcanmasına yol açabilmektedir. Son yıllarda geliştirilen ve alkali tüzyon prosesi adı verilen yöntemde ise, kromit konsantresi, gereken miktardan daha fazla miktarda NaOH kullanımı ve ortama hava gönderilmesi şekliyle, 550-650°C sıcaklıklarında işleme tabi tutulmakta ve yüksek yüzdelerde kromata dönüşüm sağlanmaktadır. Bu prosesin bir diğer özelliği, prosesin kromit konsantresinin muhtevasından bağımsız olmasıdır. Diğer bir ifade ile, alüminyum ve silisyum içeriklerinden etkilenmemesidir. Bu çalışmada, Muğla-Fethiye yöresinden alınan kromit konsantresinden alkali füzyon yöntemiyle kromat ekstraksiyonu gerçekleştirilmiştir. Alkali füzyon kademesinde 550, 600 ve 650°C sıcaklıklarında, 4/1, 5/1, 6/1 ve 7/1 olarak alınan NaOH/Cr203 karışım oranlarında sırasıyla 20, 40, 60 ve 90 dakika sürelerde çalışılmıştır. Bu çalışmalar sonunda çalışma sıcaklığı olarak 650°C, NaOH/C^C^ karışım oranı olarak 6/1 ve füzyon süresi olarak 60 dk, optimumlar olarak tesbit edilmiştir. Alkali tüzyon çalışmasının daha sonraki kademeleri olan ve füzyon ürünündeki serbest NaOH in geri kazanılmasını sağlayan metanol liçi çalışması, üründeki kromatı çözeltiye almak amacıyla gerçekleştirilen su liçi çalışması ve nihai ana çözeltinin temizlenmesi amacıyla yapılan alüminyumun giderilmesi işlemleri de ilave çalışmalar olarak ortaya konmuştur. Metanol liçi kademesinde optimumlar 50°C çalışma sıcaklığı, K/S oranı 1/4 ve liç süresi 60 dk olarak belirlenirken su liçi kademesinde bu optimumlar, 25°C çalışma sıcaklığı, K/S oranı 1/5 ve liç süresi 20 dk olarak tesbit edilmiştir. Çözelti temizleme kademesinde ise 80°C de farklı sürelerde yapılan ön ısıtma sonrasındaki 30 ve 60 dk lık kaynatma işlemlerinde, çözeltide mevcut alüminyumun yaklaşık %70-80 'i giderilebilmiş ancak bir optimum seçme olanağı olmamıştır. Çalışmada ayrıca alkali füzyon reaksiyonunun kinetik incelemesi de yapılmış ve reaksiyonun partikül yüzeyinde gerçekleşen reaksiyon kontrollü olduğu tesbit edilmiştir. Deneysel datalardan yola çıkılarak yapılan hesaplamalar sonucu, alkali füzyon reaksiyonunun aktivasyon enerjisinin 22,6 Kcal/mol olduğu bulunmuştur.

Chromite is the only ore mineral of metallic chromium and chromium compounds and chemicals. Because of this fact, chromite and chrome ore are used synonymously in trade literature. In commercials markets, chrome ore is the term commonly used. Chromite, because of properties imparted by its chromium content, is used in refractories and in special purpose molding sands for metal casting. There are many other minerals containing some amounts of chromium, but none are commercial sources of the element. Chromium and chromite have many diverse uses that, directly and indirectly, critical affect vast segments of our modern system. Most important are probably the metallurgical applications wherein chromium is a component of heat-, abrasion-, corrosion- and oxidation-resistant and high-strength alloys of many types. Chromium chemicals are used in leather tanning, in pigment, dyes and mordants, printing, chemical process industries, photography, metal plating, pure chromium metal production and for many other purposes. Chromite is a necessary constituent in basic refractories indispensable for the production of steel, copper, cement and glass. Chromite is a variety, or more properly stated, a composition range, of the spinel group of minerals. This range can be expressed as (Mg, Fe2+)0 (Cr, Al, Fe3+)203. The composition also can be generalized as R2+O.R3+203. Chromite was first discovered in 1 848 in the Harmancık, Bursa area of north western Turkey. Since then Turkey has had an important part in the world chromite market. For some years Turkey's production led the world market and she always been among the first six countries. Chromite deposits in Turkey are scattered throughout the ultrabasic rocks. There are 710 known chromite deposits and groups of occurences. Their distribution does not show any recognisable pattern, but from geographical point of view they can be grouped in 6 major areas. The order of relative importance of these areas can be given as follows; 1. Guleman, Elazığ area, eastern Turkey 2. Fethiye-Köycegiz-Denizli area, south-western Turkey 3. Bursa-Eskisehir area, north-western Turkey 4. Kayseri-Adana-Mersin area, southern Turkey 5. Kopdag area, eastern Turkey xiv 6. Iskenderun-Islahiye-Maraş area, southern Turkey. Apart from these areas, some scattered chromite deposits are also known elsewhere. From the available data chromite reserves are estimated to be about 31 m. tonnes at 30-48% Cr203 grade. A low grade, large reserve chromite field in Kızılyürek-Yataardıç area, Karsantı, Adana has been under investigation. The data shows the reserve of the explored part is estimated to be about 72 m. tonnes at the 6% Cr2Û3 grade. With the completion of exploration this figure may reach 200 m. tonnes. In the world, the main suppliers in 1980 were the Republic of South Africa (43%), Russia and former States in Soviet Union (21%), the Philippines (12%), Albania ( 1 1%), Turkey (8%) and others (5%) The strategic nature of chromite is obvious. In Table 1, world chromium consumption and market shares are given. Table 1 World chromium consumption and market shares cEstimaled Commercial process presently used to chemically treat chromite concentrates include an oxidizing roast of the chromite with sodium carbonate and lime in a rotary kiln at a temperature of 1000 to 1150°C. The amount of reagents and a diluent is controlled so that the reaction mixture remains as a solid phase in the kiln. If concentrates that have been produced from chromite deposits contain too much silicon, it is difficult to process by this method. The silicon forms molten, sticky reaction products, which cause balls and rings of material to form in the kiln, hindering its operation. The aluminum content in resources is also high, resulting in an excess consumption of reagents. A simplified flowsheet for the alkali fusion method to recover sodium chromate from chromite concentrates is shown in Figure 1. Briefly the procedure consists of reacting the chromite at 550 to 650°C with an excess of molten NaOH under oxidizing conditions. The general equation for the chemical reaction involved is shown below. FeO.Cr203 + 4NaOH + 7/402 -> 2Na2Cr04 + 1/2Fe203 + 2H20 (1) All of the experiments were performed in a small stainless steel open-top reactor measuring 9 cm in diameter and 1 0 cm in deep. The reaction mixture was stirred with a mixer and air was sparged into the mixture through a stainless steel tube. xv The reactor was placed in an electric furnace and contents were emptied by removing the reactor from furnace and pouring the contents into a tray. The analytical procedure used to determine metal extractions is described below. The metal extractions are defined as the amount of the various metals in the chromite converted to a water-soluble form by the fusion reaction. The chromium extraction values represent the amount of chromium in the chromite converted to soluble sodium chromate. *t*öteft. Chromite 1^ Makeup NaOH t concentrote { Air 1 Oxidation and fusion 550°-650° C Solidified fusion product Leoch slurr Solid-liquid separation [ Solid-liquid separation ı Recycle NaOH Water vapor { Purification »- Crystallization Residue to disposal Al and Si compounds Na2Cr04 producl Figure 1 Flowsheet for chemical processing of chromite by alkali fusion method XVI The variables studied in this work were reaction time, temperature and sodium hydroxide to Cp^O^ ratio. The NaOH to O2O3 ratio was based on the Cr203 of concantrate. The mixing rate and airflow ( 1,65 1/min) were kept the same for all experiments. The concantrate was mines 200 mesh. The chromite concantrate from Muğla-Fethiye was studied under a variety of conditions. Reaction times from 20 to 90 min were used at temperatures of 550°, 600° and 650°C, and the NaOH to Cr203 ratio was varied from 4: 1 to 7: 1. At ratios lower than 4:1, the reaction mixture became too viscous to stir. As indicated in Figure 2, the optimum conditions were obtained 96,5 pet at a 6:1 NaOH to O2O3 ratio and fusion time of 60 min at 650°C. NaOH/Cr203=6/1 100 90 80 70 c B 60 o CO "£ 50 LU Ö 40 30 20 10 0* 10 20 30 40 50 60 70 Fusion Time (min) ?" 650 °C I 600 °C +. 550 °C 80 90 100 Figure 2 Cr extractions vs fusion time obtained with a 6: 1 NaOH to Cr203 ratio The solidified fusion product from alkali fusion reaction was crushed and ground to mines 3 mm. The product then was leached with methanol to remove the majority of the excess NaOH while only removing a trace of the Na2Cr04. This seperation can be accomplished because of the large difference in solubility of the compounds in methanol. The methanol solution then can be evaporated to recover the NaOH, which can be recycled to fusion reactor. XVII The solids from methanol leach were leached with water to recover the Na2CrÜ4. The variables studied in methanol and water leach were leaching time, temperature and the ratio of the solids to liquid. The optimum experimental results obtained were 85,6% at a 1:5 solid to methanol ratio at 50°C in methanol leach (60 min) and 98.8% at a 1 :5 solid to water ratio at 25°C (20 min). The major impurities that were solibilized in the aqueous solution by the fusion reaction were silicon and aluminum. Magnesium was a major impurity in the chromite concentrates but did not become soluble to any extent. The aluminum extraction tended to follow the same trend as chromium extraction. This would be expected because aluminum substitutes for chromium in the chromite lattice. As the chromium was reacted, the aluminum also would be exposed and react with NaOH. In solution purification step, a soluble silica compound (waterglass, Na2SiÛ3) was added to the solution so that the ratio of silicon to aluminum was adjusted to form the compound NaAlSi04, which then precipitated from solution. The solution was initially heated to 80°C for 30 min to 120 min after waterglass was added, then the solution was boiled for 30 min to l'h. The variables studied in this step were the time at initial heating step and time at boiling temperature. Aluminum extraction ranged from 71,8 to 79, 1 pet of the total amount in the aqueous solution. The oxidation reaction kinetics of chromite with molten sodium hydroxide was also studied in the temperature range of 550-650°C. The grain size of chromite was kept constant throghout the reaction period and the kinetic curves were analysed in consideration of the core-model. The rate of reaction was well expressed by the following equation, l-(l-X)1/3=k.t (2) Where k is rate of reaction, t is time and X is fractional conversion. The kinetik energy for the alkali fusion reaction was calculated as 22,6 Kcal/mole. As a result, alkali fusion process involves reacting the chromite with fused NaOH under oxidizing conditions to form sodium chromate. The sodium chromate is then recovered by methanol and water leach steps. Chromium extractions as high as 96,5 pet were obtained from Muğla-Fethiye chromite concentrate. The Na2CrC<4 product produced from this method is a basic industrial chemical and can be used to produce the other common chromium compounds in commercial use.