Yerinde uranyum ekstraksiyonu yöntemi üzerine deneysel ve teorik modelleme

Abstract

Petrol, kömür ve tabii gaz gibi enerji kaynaklarının yakın bir gelecekte tükeneceği bilindiğinden 2000 yıllarında dünya enerji tüketiminin %25'nin nükleer santrallerden sağla nacağı tahmin edilmektedir. Nükleer enerji, kaynağını teşkil eden uranyum madeni ülkemiz rezervleri de dahil olmak üzere dünyanın çeşitli bölgelerinde genellikle düşük tenörlü(%l den az) rezervler şeklinde bulunmaktadır. Özellikle klasik maden cilik yöntemleriyle ekonomik ve teknik açıdan değerlendirile- miyen uranyum rezervleri için yerinde uranyum ekstraksiyonu yöntemi yeni bir çözüm olmaktadır. Bu çalışma iki amaca yönelik olarak gerçekleştirilmiş tir. Deneysel olarak yürütülen birinci amaç ile, yerinde uranyum ekstraksiyonu yöntemiyle, uranyum madeninden uranyum ekstraksiyonunda oksitleyici olarak kullanılan hidrojen pe- roksitin bozunma hızına ve uranyum çözünürlüğüne pH ve kelat etkileri incelenmiştir. Yerinde değerlendirme yöntemi için bir sakınca teşkil eden hidrojen peroksit bo sunmasının yavaş latılması ve uranyum çözünürlüğünün mekanizması araştırılmış tır. İkinci amaç ile, yerinde uranyum ekstraksiyonu.yöntemi nin başarıyla uygulanmasında önemli bir yer tutan besleme ve. üretim kuyularının maden bölgesi içinde yerleşimi ve bu kuyu lar arasında çözeltinin akış yollarının matematik mode İlenmesi hedef tutularak, yeni bir nümerik çözüm yöntemi olan sınır ele manları tekniğinin kullanılabilirliği araştırılmıştır. Deneysel olarak gerçekleştirilen birinci grup çalışma da, yerinde uranyum ekstraksiyonu şartlarında bir litrelik Autoclave Engineers, Erie (ABD) yapımı karıştırıcı. 1 1 otoklav kullanılarak ekstraksiyon denemeleri yürütülmüştür. Ekstraksi- yon işleminde çözücü olarak asidik ve karbonat sistemleri de- nenmi-ş ve ortama oksitleyici olarak hidrojen peroksit ilave edilmiştir. Yapılan denemeler sonucunda karnotit ve uraninit yapısındaki uranyum cevheri için çözücü olarak karbonat sistem lerinin kullanımının asidik sistemlere göre daha üstünlüğe sa hip olduğu anlaşılmıştır. Bunun yanısıra hidrojen peroksitin uranyum ekstraksiyonu verimini daima olumlu yönde etkileyen uygun bir oksitleyici olduğu gösterilmiştir. -v- Yerinde uranyum ekstraksiyonu yönteminde oksitleyici olarak hidrojen peroksitin kullanılmasının pekçok yararlarının bulunmasının yanısıra kısa bir sürede su ve oksijene bozunma- sı, kullanımı açısından bir sakınca teşkil etmektedir. Çok kü- ÇÜk miktarlarda bile olsa ağır ve geçiş metallerinin bir kısmı hidrojen peroksitin bozunmasmda katalizör olarak önemli rol oynamakta, ayrıca ortamda bulunan demir tuzları bozunma hızı nı artırmaktadır. Bu sebeple ikinci aşamada gerçekleştirilen deneysel çalışmada, yüksek pH'a sahip karbonat çözeltisinde hidrojen peroksit bozunmas ma demir etkisinin azaltılması araştırılmıştır. Dört ayrı test şartı hazırlanarak, çalkala yıcı üzerinde gerçekleştirilen denemelerde hidrojen peroksit bozunmasına demir etkisinin Hampene OH-3, DEG, EDG ve EDTA kelatlarınm kullanılmasıyla azaltılabileceği belirlenerek hidrojen peroksitin stabilitesi sağlanmıştır. Karbonat çözeltisinde hidrojen peroksitin bozunma hı zının bazı kelatlarla yavaşlatılarak olumlu neticeler alınma sı üzerine otoklavda iki farklı pH'a sahip karbonat çözelti- siyle bir seri uranyum ekstraksiyonu denemeleri gerçekleşti rilmiştir. Denemelerde, ekstraksiyon süresince, hidrojen pe roksitin bozunma hızına ve uranyum çözünürlüğüne pH ve Hampene OH-3, DEG, EDG, hidroksiasetik asit kelatlarınm etkileri in celenmiş, kelat konsantrasyonundaki artışın hidrojen peroksit stabilitesini artırdığı gözlenmiştir. Yine denemeler sonucun da uranyum ekstraksiyonu işleminin iki farklı bölgede yürüdü ğü, birinci hızlı ekstraksiyon bölgesinde uranyum çözünürlü ğünün a = A + B İn G şeklinde zamanın logaritmasına bağlı bir modele uyduğu, ikinci bölgede ise çözünürlüğün birinci bölgeye göre daha yavaş bir hızla yürüdüğü ve hidrojen perok sit yerine oksijen tarafından kontrol edildiği ortaya çıkarıl mıştır. Yerinde değerlendirme proseslerinin matematik modellen- mesinde kullanılan Önemli eşitliklerden biride gözenekli ortam da akışı ifade eden iki boyutlu kararlı hal akış denklemidir. Yerinde uranyum ekstraksiyonu yöntemiyle ilgili teorik mo.d el leme çalışmasında, bu akış denklemi ve bununla ilgili sınır şartları kapsayan kısmî diferansiyel denklemler için sınır elemanları yöntemi kullanılarak, önce temel, bağıntılar türetil miş, bilgisayar programı geliştirilmiş ve nümerik çözüme ula şılmıştır. Sonuçta, gözenekli ortamın homojen bir yapıya sahip olması durumunda, besleme ve üretim kuyularının tespiti ve bu -VI- kuyular arasında akışkan akımı yollarının modellenmesinde sı nır elemanları tekniğinin kullanılabilecek emin ve sağlıklı bir yöntem olduğu anlaşılmıştır. Maden rezervinin bulunduğu bölgenin farklı geçirgenliklere sahip olması, heterojen yapı daki gözenekli ortam, durumunda aynı nümerik çözüm yönteminin prosesin mod ellenme s ine kısmen cevap vermekte olduğu, özellik le alt bölgelerin birleştiği köşe noktalarında yetersiz kaldı ğı gözlenmiştir.

The. decrease in production and the increase, in the price and demand for finite energy resources, such as oil and natural, gas, has forced mam/ countries to meet their energy needs by using alternate energy supplies. Thus, the studies on these sources of energy, such as fossil fuel, nuclear, geo thermal, sol ar, wind, hydrogen, and wave energy has initialized. Since it is known that the finite energy resources, such as oil, coal and natural gas, will be depleted in near future, it is predicted that: twenty-five percent of the energy used all over the world will be supplied from nuclear power stations, in 2000,' s. Uranium ore which forms the source of nuclear energy, is mostly found as low grade reserves (less than 1%) in different; regions of the world. In situ extraction of uranium using the "bore hole raining" technique, has proven to be one of the most economical process for recovering this metal, especially from lower grade deposits that cannot, be mined and processed by conventional methods. This newly developed technique consists of production wells, injection wells, and monitor wells. A solution of water and selected chemicals (the lixiviant) is pumped into the ore body through the injection wells. This lixiviant flows through the ore, dissolving the uranium mineral, and is recovered at the production wells. It is then pumped to a recovery plant, where the mineral is separated and the lixiviant is recyled to the uranium deposit. This study mainly consists of two parts, which are experimental and theoretical modeling studies. In the con tent of the laboratory work the effect of chelates on the extraction rate of uranium and also on the decomposition rate of hydrogen peroxide which is used as an oxidant during the uranium leaching by in situ techniques, were studied. Hydrogen peroxide may be more effective than oxygen for in-situ extraction. However, field operations often encounter difficulties with the decay of hydrogen peroxide in the well bore, causing decreased injectivity -VIII- due to gas locking of the well bore vicinity. It is the purpose of these experiments to evaluate various chelating agents for their iron sequestering ability to promote the life of hydrogen peroxide and to investigate the mechanism of uranium solubility under in situ conditions. During the implementation of the theoretical part, work performed on the mathematical modeling of the placing of the injection and production wells, and also determination of the pattern of stream-lines. For this purpose, the applicability of "Boundary Element Methods'/which is a new numerical solution technique was investigated. The uranium ore used for the extraction experiments was obtained from the Pumpkin Buttes area in Powder River Basin of the State of Wyoming. In this area the uranium deposits are in the form of sandstone which is very suitable for in-situ leaching process. Predominant minerals are the uranyl vanadates, yellow. orange carnotite, uraninite and yellow greenish tyuyamunite. Bulk mineralogy are quartz and feldspar with minor amount of dolamite and calcite. The uranium in the composite is 1110 ppm» During the implemantation of the first part of expe rimental work, leaching experiments were conducted using one liter autoclave made by Autoclave Engineers, Erie under in situ uranium leaching conditions. All autoclave tests were, made under 6.8 atm. nitrogen pressure. Cooling coils were inserted in the units. Temperature was maintained at 12-13 degrees centigrade which is appropriate to the in-situ formation water at Collins Draw. This was accomplished by using two fluid streams. One was circulated through a large ice bath and the other through a refrigerator. The two streams with ligh and low temperatures, were blended with a value to produce the desired temperature in the autoclave. All samples were agitated continuously at 200 rpm except for brief intervals to sample the solution. All tests were run with initial solution to solids in the ratio of 10:1. The experiments were, repeated with acid and carbonate systems in order to determine the most suitable lixiviant for this leaching process. Also hydrogen peroxide ?IX- was added as an oxidant to the leaching media. The systems used as lixiviant are as follows: a. NaoC0o/NaHC0o-Ho0o system 2 3 5 1/. *. b. (NH4)2C03/NH4HCO -HO system c. HoS0,-Ho0o system 2 4 2 2 d. HNO" system The experiments performed showed that the carbonate systems are superior to the acid systems on the besis of recovery and insensitivity to pH changes, and hydrogen peroxide as an oxidant allways increases the yield of uranium during leaching operations. The ef f ectiteveness of hydrogen peroxide as an oxidant for the extraction of uranium by in-situ techniques was demonstrated both in the laboratory and this is substan tiated in field operations elsewhere. Hydrogen peroxide may be more effective than oxygen. However, during the application of in situ uranium leaching technique, field operations often encounter difficulties with the decay of hydrogen peroxide in the well bore and in deposit. The problems of premature peroxide decay are most pressing when high pH solutions are used because in in-situ leaching high pH conditions tend to accelerate the rate of hydrogen peroxide decay. It has been shown that a large number of heavy and transition elements act as catalysts for hydrogen peroxide decomposition. On the other hand, the presence of iron salts increase the decay rate of H"0". It is the purpose of second stage of the laboratory work is to investigate the effects of chealeting agents for their iron sequestering ability to lengthen the effective life of hydrogen peroxide in carbonate solutions» For this reason, four different test conditions were prepared for evaluating chelating agents. All tests were conducted on Junior Orbital Lab-Line shaker in this part. The chelating agents Hampene OH-3, DEG and EDG of the Hampshire Organic Chemicals Division of the W.R.Grace and Co, and EDTA were all found to decrease the decomposition rate of hydrogen peroxide in high pH carbonate solutions -x- at the presence of iron. Once was possible to promote the effective life of hydrogen peroxide by using various chelating agents, a series of uranium leaching experiments were run in the autoclave using carbonate solutions at two different pH levels. Tests conditions were in situ conditions. Extraction period was 48 hours for each leaching. Before starting the leaching, hydrogen peroxide at concentration of 1 gram/liter and various iron chelating agents at different concentrations were added to the system. During leaching, 2 mlsamples were taken from the leaching solutions at different times and U3O0 and hydrogen peroxide concentrations were measured. The effects of the solution pH and the presence of the chelating agents on the decomposition of hydrogen peroxide and on the solubility rate of uranium were investigated by these experiments. The results indicated that the chelating agents Hampene OH-3 and Hampshire DEG lengthen the life of hydrogen peroxide, but hydroxyacetic acid has no positive effect on the decay rate of hydrogen peroxide. Increasing the concentration of the sequestering agent increases the stability of hydrogen peroxide. Consequently, the problem of decomposition of hydrogen peroxide encountered when used as an oxidant during in situ recovery of uranium, was eliminated. Results of the leaching experiments also showed that uranium leaching operation occurs in two distinct regions. The first region is the rapid initial extraction region observed during the. period at which the hydrogen peroxide is relatively stable. Obviously in this region, solubility of uranium mechanism occurs at the presence of hydrogen peroxide. The uranium solubility at this region can be expressed by a relationship which depends on the logarithm of time, as follows: a = A + B In 6 The second region starts from the poiint where all of the hydrogen peroxide in the autoclave has', been decomposed to -XI- oxygen and water. In this region, the leaching rate is controlled by oxygen rather than hydrogen peroxide and solubility rate is lower than the first region. One of the most important equation used in mathema tical modeling of in situ, processes is the two dimensional Steady state flow equation which represents the flow in porous media. This partial differential equation and releated boundary conditions are: î."q = I^i Mx^y^ (x,y) t A "q."? = M ( x, y ) ( x, y ) e L "q » K^P /M(x,y) dL = ÎİQİ where A is closed and connected region, L is the boundary of A, q is flow vector equal to reservoir thickness times Darcy velocity, Q. is injection or production rate,. is Dirac Delta function, n is outer unit normal to a boundary, K is hydraulic conductivity, P is pressure and M(x,y) is normal component of flow on the reservoir boundary. In the content of theoretical modeling study, the fundamental relationships for the abovementioned flow equation and its boundary conditions were derived using "Boundary Element Methods". Then, a computer program solving this systems of equations is developed, and finally a numerical solution was reached. It was concluded that, if the porous media is homogeneous, then location of injection and production wells, and also the modelling of flow pattern between these wells can be accomplished successfully by using Boundary Element Methods. In the case of having a re gion with different subregions (heterogeneous media), the same numerical solution technique may be partially applied to the modelling of the process.lt was observed that the method was unsatisfactory especially at the corners between subregions.