Atık banyo çözeltilerinden iyon değişimi ile altın kazanımı
Recovery of gold from residual bath solutions by ion-exchange
Yazar: EGEMEN BULUŞ
Danışman: PROF. DR. MAHMUT ERCAN AÇMA
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2016
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Altın, atom ağırlığı 197, kimyasal sembolü Au olan, yumuşak, biçimlendirilebilir, parlak sarı renkte ve çok değerli bir metaldir. Altın, güzel görünümlü rengi sebebiyle sıklıkla dekoratif amaçlar doğrultusunda bir kaplama metali olarak kullanılmıştır. Bunun yanında altın geçmişten beri kıymetli bir metal olarak benimsenmiş ve mücehverat, takı yapımı için kullanılagelmiştir
Altın, günümüzde, geçmişteki kullanım amacından ziyade, sınai amaçlar doğrultusunda kullanılır duruma gelmiştir. Bilhassa uzay teknolojisinde, elektronik sanayinde altın, kararma ve donuklaşma direnci, kolayca lehimlenebilmesi, yüksek elektriksel iletkenliği ve kızılötesi (sıcaklık) yansıtıcı özellikleri ile giderek kendisine daha fazla kullanım alanı bulmaktadır.
Altın geri kazanımı işleminde pek çok yöntem bulunmaktadır, bunlardan bazıları yıkama, iyon değişimi ve adsorplamadır. Altın adsorplama işleminde aktifleştirilmiş karbon, ticari sorbentler, mantarsı biyokütleler gibi pek çok malzeme kullanılabilmektedir. 
Literatür çalışmaları ve altın kazanımı konusunda faaliyet gösteren şirketlerin uygulamalarına bakıldığında, altın kazanımı konusunda yıllardır kullanılagelmekte olan ve verimini kanıtlamış ticari iyon değişim sorbentlerinin kullanımı, oldukça sık görülen ve mantıklı bir yöntemdir. Bu ticari amaçlı hazırlanmış sorbentler, oldukça yüksek kazanım yüzdelerine sahip, kolay ulaşılabilir ve yeniden kullanılabilirdirler. Bu ticari amaçlı sorbentler, değerli metal kazanımında kullanım ile birlikte aynı zamanda yer altı sularından fenol, Bisphenol-A gibi zararlı maddelerin uzaklaştırılması uygulamalarında da sıklıkla karşımıza çıkmaktadırlar. Bu çalışmada, çapraz bağlı polistiren matriksli, zayıf bazik bir iyon değişim sorbenti olan, Lewatit MP 62 WS isimli Lanxess Kimya sorbenti ile yapılan altın adsorplama işlemleri sonucunda, yüksek verimli bir geri kazanıma ulaşmak amacıyla optimum koşullar belirlenmiştir. Bu ideal koşullar, sorbent miktarı, çözelti miktarı, süre, sıcaklık ve stok çözelti konsantrasyonu gibi fiziksel etkiler şeklinde açıklanmıştır. Adsorpsiyon işlemlerinin tamamlanması sonrasında, atomik adsorpsiyon spektrometresi aracılığıyla çözeltilerin içerisinde ne kadarlık bir altın konsantrasyonu kaldığı belirlenmiştir. Daha sonra, bu adsorpsiyon işlemleri sonucu elde edilen veriler, adsorpsiyon karakteristiklerini tanımlamakta sıklıkla kullanılan Freundlich ve Langmuir izoterm modellerine uyarlanmış ve bu modellerden hangisiyle daha yüksek oranda bir uyumun görüldüğü gözlemlenmiştir. Lewatit MP 62 WS ile siyanürlü atık banyo çözeltisinden altın kazanımında, adsorpsiyonun karakteristik olarak Langmuir izoterm modeliyle daha yüksek bir uyum gösterdiği görülmüştür, bu da adsorpsiyonun, adsorbent yüzeyinde, sabit bir sayıda, hepsi aynı enerjiye sahip ulaşılabilir alanlarda gerçekleştiğini, tersinir olduğunu ve tek katmanlı olduğunu ifade etmektedir. Adsorpsiyon işlemlerinden sonra desorpsiyonun kral suyunda gerçekleşip gerçekleşmediğinin teyidi için 84 saat sure, 0,5 gr sorbent, 45 °C sıcaklık ve 411,6 ppm altın konsantrasyonuna sahip çözelti ile gerçekleştirilen adsorpsiyon deneyleri sonucu elde edilen sorbent, 500 ml kral suyunda 2 saat süre ile muamele edilmiş ve kral suyuna geçen altın konsantrasyonu AAS analizi ile incelenmiştir.

Abstract:
Gold, whose atomic weight is 197 and chemical symbol is Au, is a soft, shiny, formable and very precious metal. From past to present, it's been widely used with decorative purposes as a coating metal due to it's beautiful colour. Besides that, gold has always been adopted as a precious metal and hence it's been used in jewellery.
Nowadays, gold is more frequently being used with industrial purposes rather than it's former decorative or jewellery purposes. With it's superior features such as tarnish resistance, easy solderability, high electrical conductivity and infrared reflectivity, day by day, it's taking place in industrial applications increasingly. 
There are several methods of gold recovery, some of those are leaching, ion-exchange and adsorption. Several materials can be used for adsorption of gold such as activated carbon, commercial sorbents and fungal biomasses.
When the literature researches and applications of the companies which are operating in gold recovery are considered, using of commercial ion-exchange sorbents which are widely being used for years and proved their efficiency in such applications is a logical method. These sorbents which prepared with commercial purposes have quite high recovery percentages, also they are easy accesible, cheap and reusable. Apart from recovery applications of precious metals, these commercial sorbents can be frequently seen also in applications such as removal of phenol, Bisphenol-A from ground waters.
In these study, gold was recovered by a commercial sorbent, Lewatit MP 62 WS in order to reach high efficient recovery. Best recovery conditions were explained in adsorption time, amounts of sorbent, temperature and concentration of solution. The remaining gold concentrations were measured by atomic adsorption spectrometer (AAS). After determining the optimum parameters, adsorption data applied on Freundlich and Langmuir adsorption models which are widely used for determine the characteristics of adsorption. 
In experiments, the cyanide solution which has 411,6 ppm gold concentration and 3 other diluted cyanide solutions which are diluted by 75/100 (310 ppm), 50/100 (207 ppm) and 25/100 (103,5 ppm) were used. In the first series of experiments, in order to observe the influence of time on gold adsorption, the concentration of gold after 0,5-1-1,5-2-3-4-5-24-36-46-60-72 and 84 hours of adsorptions investigated by AAS analysis, 50 ml flasks, 0,5 g sorbent and 25 ml solutions were used, temperature was constant and 25 Celcius. In the second step, experiments were carried out in order to determine the affect of sorbent amount on the adsorption of gold. 50 ml flasks, 25 ml solutions and 25 Celcius constant temperature were used, for each weight of 0,1-0,2-0,25-0,3-0,4-0,5 grams, the mixtures of sorbent and solution were waited by 2-6-12-24 hours and after this hours, the concentrations of residual gold after adsorptions were investigated by AAS analysis. In the third series of experiments, different temperatures were used in order to determine the affect of temperature on gold adsorption. 5 different temperatures were used, these are 25-30-35-40-45 Celcius. In these step of experiments, 25 ml solutions, 0,25 g sorbent and 50 ml volumetric flask were used, 100 rpm stirring tempo were determined as constant and in this step temperature controlled magnetic stirrer was used, after the experiments, concentrations of the residual golds in solutions were determined by AAS analysis. Lastly, in the fourth series of experiments, influence of the different initial gold concentrations on the adsorption of gold were investigated. 50 ml flasks, 25 ml solutions and 25 Celcius constant temperature were used. The solutions which has 103,5-207-310,5 and 411,6 ppm gold concentrations were mixed with 0,25 g and 0,50 g amount of sorbents, after the experiments, concentrations of residual gold in solutions were determined by AAS analysis. After all this experiment results were graphed, for all investigated parameters, optimum results were determined for the adsorption of gold from cyanide solution.
In results, the best recovery conditions of gold from cyanide solution by Lewatit MP 62 WS was determined as 0,5 g for amount of sorbent, 84 hours for adsorption time, 45 Celcius for temperature and 103,5 ppm for initial gold concentration. When the adsorption times were investigated, it can be stated that by the end of 84 hours, the gold concentration in solution is almost zeroied and the gold is almost totally adsorbed. When the temperature was investigated, 45 Celcius was the best option which ensures the best recovery conditions and  there was a strict change with changing temperature for Lewatit MP 62 WS that indicates adsorption with Lewatit is temperature dependent. When the amount of sorbent investigated, 0,5 g amount of sorbent found as the best option. When the concentration investigated, it's found that with the solution whose initial gold concentration was 103,5 ppm, best results have obtained, in adsorption phenomenon, efficiency mostly starts to decrease after a certain level of concentration due to decreasing number of the available sites for adsorption, the reason for such result is considered to be due to that fact.
In adsorption models, the results obtained during the adsorption are applied on Freundlich and Langmuir adsorption models. It's seen that adsorption of gold from cyanide solution by Lewatit MP 62 WS fitted to Langmuir adsorption model which indicates that adsorption occured in limited region of surface and it is one layer adsorption.  After determination of these steps, in order to prove that it's possible to take gold in aqua regia for the further desorption steps, the adsorbent which collected the maximum amount of gold under parameters of 84 hours, 0,5 g sorbent, 45 °C temperature and 411,6 ppm gold concentration was treated with aqua regia and after the concentrations were determined by AAS. Only in concentration parameter, 411,6 ppm gold concentration value were chosen instead of the optimum value due to the less amount of stock solution left. 500 ml aqua regia were prepared in order to carry out the desorpiton step. The sorbent which collected the maximum amount of gold under the parameters of 84 hours, 0,5 g sorbent, 45 °C and 411,6 ppm gold concentration was treated with 500 ml aqua regia for 2 hours. After this desorption step, aqua regia were diluted with water in order to observe the gold concentration in it in AAS analysis. Stock solution was containing 0,15 ppm Ag, 411,6 ppm Au and 196,64 ppm Cu before adsorption, and after adsorption with below mentioned parameters, it's seen that Ag concentration was decreased to 0,14 ppm, Au concentration was decreased to 2,681 ppm and Cu concentration was decreased to 54,54 ppm. When the concentrations in aqua regia were investigated, it's seen that the concentration of Ag in aqua regia is 0,74 ppm, the concentration of Au in aqua regia is 177,7 ppm and the concentration of Cu in aqua regia is 186,84 ppm. It's determined that %43,34 of the gold can be taken into aqua regia. Under the parameters of 84 hours, 0,5 g sorbent amount, 411,6 ppm gold concentration and 45 °C temperature, 0,052 g gold can be taken from 25 ml solution and 0,022 g of these gold can be taken into aqua regia.