Kıymetli metallerin liçinde kullanılan tiyoürenin bozunma reaksiyonunun incelenmesi
Başlık çevirisi mevcut değil.
- Tez No: 100552
- Danışmanlar: Y.DOÇ.DR. A. EKREM YÜCE
- Tez Türü: Yüksek Lisans
- Konular: Maden Mühendisliği ve Madencilik, Mining Engineering and Mining
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1999
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 51
Özet
ÖZET Bu tez kapsamında; kıymetli metallerin tiyoüre ile liçinde yaygın olarak kullanılan demir sülfat ve sodyum sülfıtin farklı pH ve sıcaklık koşullarında tiyoürenin bozunması üzerindeki etkileri araştırılmış; kullanılan reaktiflerin konsantrasyon ve miktarları ile tiyoürenin bozunma oram arasındaki ilişkinin matematiksel bir fonksiyon ile tammlanabilme olanakları araştınlmıştır.Deneysel çalışmalar dört temel grupta yapılmıştır. 1-Liç Reaktiflerinin Orijinal pH Ve Eh Değerleri Ölçümleri Tiyoürenin, demir sülfat ve sodyum sülfıt ile birlikte kullanıldığı liç ortamında her bir reaktifin bireysel olarak pH ve Eh değerlerinin araştırıldığı seri deneyler yapılmıştır. 1,0M tiyoüre ile farklı başlangıç pH değerlerinde, toplam 2 saatlik karıştırma boyunca; çözelti pH-Eh değerlerinde önemli bir değişiklik olmamaktadır. pH= 1,0-2,0 arasında pH değerleri duraylılıklanm korurken; doğal pH'da, pH değerleri duraylılıklarmı kaybetmektedir. Farklı başlangıç demir sülfat konsantrasyonlarında, 2 saatlik karıştırma boyunca; çözelti pH-Eh değerlerinde önemli bir değişiklik olmamaktadır. Çözelti başlangıç demir sülfat konsantrasyonu arttıkça pH değerleri düşmektedir. Farklı başlangıç sodyum sülfıt konsantrasyonlarında, 2 saatlik karıştırma boyunca; çözelti pH-Eh değerleri geniş bir aralıkta değişim göstermekte; karıştırma süresi arttıkça pH değerleri düşmektedir. Çözelti başlangıç sodyum sülfıt konsantrasyonu arttıkça pH değerleri de artmakta; pH ve Eh değerleri daha duraylı hale gelmektedir. 2-Tiyoüre Ve Demir Sülfat İkili Sisteminde Tiyoürenin Bozunması Bu grupta farklı başlangıç konsantrasyonlarda tiyoüre içeren çözeltilere, değişen konsantrasyonlarda demir sülfat ilave edilerek; taze çözelti, 2 ve 4 saatlik karıştırma deneyleri sonucunda tiyoüre bozunma oranlan ile pH ve Eh değişimleri incelenmiştir. Düşük konsantrasyonlarda (0,1M) tiyoüre içeren çözeltilerde, demir sülfat konsantrasyonuna bağlı olarak tiyoüre hızla bozunmakta; yüksek tiyoüre konsantrasyonlarında bozunma oram daha az olmaktadır. Yüksek konsantrasyonlarda (1,0M) tiyoüre içeren çözeltilerde, tiyoüre/demir sülfat molarite orammn“2”den küçük olduğu durumla büyük olduğu durumlarda; tiyoüre iki farklı bozunma karakteristiği göstermektedir. vuYüksek tiyoüre konsantrasyonlarında (1,0M), tiyoürenin %50 oranında formamidin disülfit formuna bozunduğu tiyoüre/demir sülfat molarite oranının 2 saatlik karıştırma süresi sonunda“4”; 4 saatlik karıştırma sonunda“3”olduğu görülmektedir. Gerek düşük gerekse yüksek tiyoüre konsantrasyonlarında, çözelti tiyoüre/demir sülfat molarite oram“1”iken TU bozunumu %100 değerlerine ulaşmaktadır. 3-Tiyoüre, Demir Sülfat Ve Sodyum Sfilfit Üçlü Sisteminde Tiyoürenin Bozunması Başlangıç pH değeri 1,0 ve 2,0 olan tiyoüre/demir sülfat molarite oram“2”olacak şekilde hazırlanmış çözeltilere farklı oranlarda sodyum sülfit ilave edilerek 2 saatlik sabit karıştırma süresinde deneyler yapılmış; tiyoüre bozunma oranlan ile pH ve Eh değişimleri incelenmiştir. Çözelti içinde sodyum sülfit konsantrasyonu arttıkça tiyoüre bozunumu azalmakta; pH değerleri duraylılıklanm korumaktadır. Tiyoürenin %50 oramnda formamidin disülfit formuna bozunduğu durumda tiyoüre/demir sülfat/sodyum sülfit molarite oram 1,0M tiyoüre için 1/2/3,0* 10“2; 0,5M tiyoüre için ise 1/2/4,4* 10”2 olarak bulunmuştur.. 4-Çözelti Sıcaklığının Tiyoüre Bozunumuna Etkisi Doğal pH' da ve farklı sıcaklıklarda (20°C, 30°C, 40°C ve 60°C), tiyoüre/demir sülfat mol oranı“2”olacak şekilde hazırlanmış tiyoüreli çözeltilerde, toplam 2 saatlik karıştırma süresince tiyoüre bozundurma deneyleri yapılmış; tiyoüre bozunma oranlan ile pH ve Eh değişimleri incelenmiştir. Çözelti sıcaklığının 40°C a kadar olduğu durumlarda tiyoüre bozunması dengeli bir artış göstermekte; 60°C hk çözelti sıcaklığında 2 saatlik kanştırma süresince tiyoüre bozunurluğu düşmektedir. Bu durum tiyoürenin önce hızla formamidin disülfit formuna bozunduğu ve ardından da geri dönüşümsüz tiyoüre formuna dönüştüğü şeklinde yorumlanmaktadır. Tiyoüre bozunurluğundaki bu düşüş çözeltideki başlangıç tiyoüre konsantrasyonu artışına bağlı olarak şiddetlenmektedir. vnı
Özet (Çeviri)
SUMMARY In this study, the effect of iron sulfate and sodium sulphide, which are commonly used in thiourea leaching, upon the decomposition of thiourea has been studied at different pH and temperature conditions and in an environment free of ore; it has been investigated to find a way of defining the relationship between the concentration and quantities of the reactants used and decomposition rate of thiourea in terms of aînathematical function. It is being searched for finding solutions against the remarkable extent of environmental pollution which is thought to be caused by the cyanide process used in the enrichment of the valuable metals like gold and silver. Majority of these solutions are focused on the neutralization of cyanide by products of cyanide process, using natural and chemical methods and the rest is utilizing alternative reactants which have less or none pollutive effect upon environment. Potential advantages of the reactants which are considered to be the alternatives to cyanide are: 4 1. They have faster solubility kinetics, 2. They are applicable to refractory type ores, 3. They are more selective for gold and silver as compared to cyanide. Amongst them; Thiourea, Thiocyanate, Thiosulfate and Halogens have been studied and resulting reports were issued. There has also been made various studies on dissolving of gold in Cyanamide, Cyanaform, Malononitrilin type reactants, but no industrial applications were established up today because of the economical point of view. Thiourea (Thiocarbamite) With the chemical formula CS(NH2)2; this is a compound with colorless, rhombic prismatic structure and contains sulfur. It is quite similar with urea in terms of chemical behaviours. It takes place in the group of mercaptans of which chemical reaction points don't change. It's solubility in water is fairly low (14,2 g in 100 g of water at 25°C) and it melts between the temperatures 178 -182°C. Although thiourea has a carcinogenic effect upon skin exposure, it is claimed to be of less pollutive and toxic effect on environment. It has been in long term use in photography and photocopying industries, production of insecticides, paint and pharmaceutical industries. IXReaction Chemistry and Kinetics Main advantages of thiourea over cyanide leaching are summarized below:. Thiourea acts in compliance with the metals like copper and iron dissolved in small amounts in the solution.. It's toxicity is very low and can be used as a fertilizer.. The leach kinetics of thiourea is five times greater than that of cyanide. In an acidic thiourea solution, in the presence of an oxidizing agent, thiourea forms metal complexes with metal ions. The mechanism of the general reaction by which thiourea forms metal complexes with metal ions is given as follows : Me + X(TU)=> Me(TU)x+ + e“ In the above equation, ”Me“ represents the metal and ”TU“ represents thiourea. This equation can be written for gold as follows: Au° + 2CS(NH2)2 => Au [CS(NH2)2 ]2+ + e”(1) In the literature, it is given that the ideal gold and silver solubility in thiourea leach is obtained when 50% of thiourea is oxidized into the formamidine disulphide form. In thiourea leaching, where the oxidizing agents like air and oxygen are insufficient, in addition to the naturally existing formamidine disulphide in the system, oxidants like iron sulfate, hydrogen peroxide, ozone and permanganate are also added to the system. In the presence of these oxidants, thiourea undergoes a stepwise oxidation. The first reversible oxidation product is formamidine disulphide, which is a selective substance for gold and silver. At this stage, the reaction is as follows: 2CS(NH2)2 o [NH2(NH)CSSC(NH)NH2]+2 + 2H+ + 2e (2) Formamidine disulphide is a very reactive oxidizer itself and it provides sufficient gold solubility in a short time. Formamidine disulphide oxidizes in time and depending on it's potential value, it is converted to irreversible thiourea, elemental sulphur and cyanamide [CN(NH2)2] forms, thus enhancing the thiourea consumption. The decomposition of thiourea to the final products is dependent to the composition of solution as well as the potential of oxidants used in high quantities. In order to decrease the excessive oxidation of thiourea to irreversible products and reactant consumption, pH and Eh values of solution should be well controlled. The oxidation of formamidine disulphide is reversible and it is possible to keep the reaction under control by controlling the potential. The oxidation reaction of formamidine disulphide to irreversible products takes place as follows: NH2(NH)CSSC(NH)NH2 => CS(NH2)2 + S° + CN(NH2) (3) It is accepted that, formamidine disulphide is capable of oxidizing gold by itself and it is more effective than the other oxidants. This can be seen in the following equation which is a combination of the equations number (1) and number (2) in which two electrons are transferred at + 0.04 V potential.2Au° + 2CS(NH2)2 + NH2(NH)CSSC(NH)NH2 + 2H+ => 2Au[CS(NH2)2]2+ (4) Above equation can be written for silver as follows: 2Ag° + 4CS(NH2)2 + NH2(NH)CSSC(NH)NH2 + 2lT => 2Ag[CS(NH2)2]3+ (5) As mentioned above, in addition to the naturally existing formamidine disulphide in the system, oxidants like iron sulfate, hydrogen peroxide, ozone and permanganate are also used in the thiourea leaching. According to the previous studies carried out about these oxidants, Fe+3/Fe+2 ion couple is stated to be the most convenient oxidant with an oxidation potential of +0.77 volt. The oxidation reaction of thiourea with Fe+3 is as follows: Fe+3 + CS(NH2)2 => Fe+2 + 1/2(NH2(NH)CSSC(NH)NH2) (6) It has been given that in the dissolution of gold and silver, in the presence of thiourea and Fe+3 ions in the media and at different pH values, 1 mol of thiourea reacts with 1 mol of Fe+3 ions in stochiometric terms. 2Au° + 4CS(NH2)2 + 2Fe+3 => 2Au[CS(NH2)2 ]2+ + 2Fe+2 (7) 2Ag° + 6CS(NH2)2 + 2Fe+3 => 2Ag[CS(NH2)2 ]3+ + 2Fe+2 (8) Thiourea leach is limited by two factors depending upon the pH value of the solution:. when pH>3, precipitation of ferric ions as Fe(OH)3,. when pH>4.3, Cast oxidation of thiourea. Although thiourea is used in the leaching of refractory type ores containing gold and silver, it may be necessary to use stronger oxidants in order to break sulphide matrix, and usage of thiourea at high concentrations may become necessary. The decomposition rate of thiourea increases with the potential increases of the system. In order to prevent the thiourea from excessively oxidizing to other products; when sulfur dioxide is introduced to the solution via using sodium sulfite with the purpose of reducing formamidin disulphide which forms after the oxidation of thiourea, ideal leaching condition has been observed to occur with a 1/1 mol ratio. Also, in a similar study, it was shown that at high thiourea concentrations providing low redox potential values was necessary and sulphur dioxide contributes to controlling the potential. The equations of reactions representing the oxidation of Fe+3 by sulphur dioxide and inhibition of thiourea decomposition are given as follows: 2Fe+3 + S02 + 2H20 => S04“2 + 2Fe+2 + 4lT (9) [NH2(NH)CSSC(NH)NH2] + S02 + 2H20 => 2[CS(NH2)2] + S04”2 + 4H+ (10) Experimental studies were carried out in four basic groups: XI1. Measurements for Original pH and Eh Values of Leach Reactants Series of experiments have been conducted for determining pH and Eh values of each reactant individually in a leach environment in which iron sulfate and sodium sulphide were used together with thiourea. With 1,0 M thiourea and different initial pH values, throughout an agitating period of 2 hours no significant change was observed in pH and Eh values of the solution. In the interval of pH =1,0 and pH = 2,0; pH values kept their stability and at natural pH, pH values lost their stability. At different initial iron sulfate concentrations, throughout an agitating period of 2 hours no significant change was observed in pH and Eh values of the solution. As initial iron sulfate concentrations increase, pH values decrease. At different initial sodium sulphide concentrations, throughout an agitating period of 2 hours, pH and Eh values of the solution change in wide range and as the agitation period increases pH values decrease. As initial sodium sulphide concentrations increase, pH values also increase and pH and Eh values become more stable. 2. Decomposition of Thiourea In The Thiourea and Iron Sulfate Double System In this group, into the solutions of different initial concentrations of thiourea, iron sulfate of changing concentrations was added and after 2 -4 hours of agitating periods, fresh solution was examined in terms of thiourea decomposition rates and pH and Eh changes. In the solutions containing low concentrations of thiourea (0,1M), thiourea decomposes fastly, depending upon iron sulfate concentrations; at high concentrations of thiourea decomposition rate decreases. At high concentrations of thiourea containing solutions (1,0 M), thiourea shows two different decomposition characteristics; when molarity ratio of thiourea/iron sulfate is smaller than“2”and the ratio is grater than“2”. At high concentrations of thiourea (1,0 M), when a 50% of thiourea decomposes into formamidine disulphide form, at the end of 2 hour decomposition period thiourea/iron sulfate molarity ratio is observed to be“4”and at the end of 4 hour decomposition period this ratio is“3”. At both low and high thiourea concentrations, thiourea decomposition reaches to 100% when thiourea/iron sulfate molarity ratio of the solution is“1”. 3. Decomposition of Thiourea In The Thiourea, Iron Sulfate and Sodium Sulphide Triple System Into the solutions, prepared to have initial pH values 1,0 and 2,0 and thiourea/iron sulfate molarity ratio is“2”, different amounts of sodium sulphide was added and solutions were examined at the end of agitating period of 2 hours: thiourea decomposition rates and pH and Eh changes were examined. As the sodium sulphide concentration of the solution increases, thiourea decomposition rate decreases, pH values keep their consistency. When thiourea decomposes into formamidine disulphide form at a rate of %50, thiourea / iron sulfate / sodium sulphide molarity ratio for 1,0M thiourea becomes 1/2/3,0* 10-2 and for 0,5M thiourea becomes 1/2/4,4* 10“2. xu4. Effect of Solution Temperature on Thiourea Decomposition At natural pH and different temperatures (20°C, 30°C, 40°C and 60°C), in the solutions with thiourea/iron sulfate molarity ratio of ”2", thiourea decomposition tests were done throughout an agitating period of 2 hours and thiourea decomposition rates and pH and Eh changes were examined. At the solution temperatures up to 40°C, thiourea decomposition rate increases gradually; at 60°C of solution temperature, thiourea decomposition rate decreases during the agitation period of 2 hours. This situation is interpreted as the thiourea first fastly decomposes to formamidine disulphide form and then it decomposes to irreversible thiourea form. This decrease in the decomposition rate of thiourea decrease sharply depending on the initial thiourea concentration. xui
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