Removal of Metal-Cyanide Complexes from Aqueous Solutions with Raw and Acid Activated Sepiolite

1. Removal of Metal-Cyanide Complexes from Aqueous Solutions with Raw and Acid Activated Sepiolite Vildan ONEN and Esra YEL Selcuk University, Faculty of Engineering Architecture, Mining Engineering Department Konya/TURKEY

2. CN has been the most preferred solvent in the extraction of gold and silver ores over a century due to its strong complexing capability, availability, relatively low cost and its well-known chemistry. Wastewaters generated in these operations often contain cyanide species at various levels. Three cyanide forms are common in gold mining process waste solutions : Metal-Cyanide Complexes

3. Sepiolite • It is a natural fibrous clay mineral. Structually, it consist of blocks and channels running paralel to the fibre axis. • It has a large specific surface area and high chemical and mechanical stability. • It can be used both as raw mineral and as in activated form. Acid activation is the process of increasing porosity, and the surface area, as a result of : Replacement of (Na+, K+) between the layers and Al3+ inside of the octahedral layer with H+ ions (Ca2+, Mg2+) Fe3+ • These characteristics of sepiolite are related to its powerfull adsorbent properties and its ability to adsorb organic or inorganic ions. • This study was conducted to propose a method for the removal of metal cyanide complexes from aqueous solutions by using this cheap, easily achievable mineral; sepiolite.

4. Materials • In this study, sepiolite sample of Eskisehir-Sivrihisar (Turkiye) region were utilized. • The specific surface area of raw sepiolite was 293 m2/g. • The sample was sieved and particle sizes of <0.106, 0.106 -0.300 and 0.300-0.600 mm were used in the study. • Experiments were performed with synthetically prepared Metal cyanide stock solutions : [Zn(CN)4]2- (1g/100 ml), [Cu(CN)3]2- (1,4g/100ml) and [Fe(CN)6]4- (5g/100ml) and diluted in required rates in the trials.

5. Method Activation • Besides raw mineral forms, acid-activated sepiolite samples were also employed in the trials. • Suspensions of 10% by weight were prepared with 0.5, 0.75, 1.0 and 1.25 N HNO3 of 65%. • mixed 6 hours at 70˚C • rinsed with distilled water until their pH reached approximately 5.5 • filtered and dried in 60˚C. • The specific surface area of sepiolite increased from 293 to 310 m2/g for 0.75 N and then decreased to 210 m2/g at 1.0 and 1.25 N.

6. Experimental • The experiments were performed in batch system obtained by adding 50 mg mineral into 100 ml metal cyanide solutions in varying concentrations. • Flasks were placed in the shaker after their initial pH value was controlled at room temperature (20-22°C) and 220 rpm speed. • As a result of the preliminary experiments, equilibrium time was set as 4 hours. • On the samples both metal and cyanide analyses were performed. • The same application was repeated seperately for raw and acid activated sepiolite samples of <0.106, 0.106-0.300 and 0.300-0.600 mm particle sizes. • Isotherm experiments were performed in the same manner with different initial complex concentrations

7. Results And Discussion • Normalized data presented in the Figure represent the separate removal of the metal and cyanide ions, instead of the removal as a whole complex. • Despite the negligible metal reductions, 50, 62 and 80 % CN removals were achieved for [Zn(CN)4]2-, [Cu(CN)3]2- and [Fe(CN)6]4- complexes, respectively. • These CN removals in the order from weak complex to strong one are surprising since the strong complex was expected as almost non-degradable. • These are the results of only one condition and one particle size... Change of remaining (a) Metal and (b) CN- concentration with detention time for 0.106-0.300 mm particle size raw sepiolite

8. Results And Discussion • The results of all trials, raw and acid activated sepiolite samples of • <0.106, • 0.106-0.300 and • 0.300-0.600 mm particle sizes. are presented comparatively for both metal and cyanide ions in the following Figures for • [Zn(CN)4]2-, • [Cu(CN)3]2- and • [Fe(CN)6]4-

9. for ZnCN for CuCN for FeCN Me+ Removals The highest metal removal achieved in [Zn(CN)4]2- and metal removal performances decreased from weak complex to strong complex. CN removals Whereas, the highest CN removal achieved in [Fe(CN)6]4- and CN removal performances increased from weak complex to strong complex.

10. Results And Discussion • Both metal and CN removal efficiencies of three different particle sizes are close to eachother, indicating the negligible effect of particle size on removal. • Although acid activation was expected to improve adsorption, the resultant metal and CN removal performances of raw and acid-activated sepiolites are not distinct for three complexes.

11. Results And Discussion It is known that in order to get adsorption isotherms, several trials of different initial compositions should be done. The representative particle sizes and activation conditions were selected for isotherm experiments for each complex. The selected conditions are presented in the Table.

12. Results And Discussion • One component-two dimensional adsorption was firstly examined and their appropriateness to Freundlich, Langmuir and Tempkin isotherms which are basic adsorption equations were checked. • According to determination coefficients, the adsorption of cations and anions were characterized in different isotherms. • Paralel to the graphical findings, metal and CN ions fit different isotherms supporting the adsorption of each one separately with a different mechanism.

13. Results And Discussion Adsorption performances of raw and acid activated sepiolites were characterized by recoveries and adsorption capacities of metal and CN ions separately. Although Zn recovery percentages are the highest of all complexes, capacity for Zn is quiet small (1-2 meq/g) while for the other two metals are around 20 meq/g. CN adsorption findings are more parallel to recovery findings. Raw sepiolite showed similar or better performance than acid-activated sepiolite. Therefore, acid activation is not suggested as a performance developing mineral modification for [Fe(CN)6]3- adsorption as a result of this study.

14. Conclusion • Sepiolite can remove metal-cyanide complexes from solutions by adsorbing the metal and CN ions separately. • For [Zn(CN)4]2-, [Cu(CN)3]2- and [Fe(CN)6]4-, CN adsorption capacities were 23, 15 and 38 meq/g whereas metal adsorption capacities were 1, 22, and 18 meq/g, respectively. • Particle size has no significant effect on system performance. Up to 0.600 mm particle sizes were studied here and medium size, i.e., 0.106-0.300 mm was suggested considering the required effort for crushing and grinding the mineral

15. Conclusion • Acid activation did not improve metal-CN adsorption of sepiolite. Unlike the common tendency in cation adsorption applications, in which acid activation suggested to increase adsorption capacities. • It was also indicated that anion adsorption capacity was not directly proportional to the surface area for CN removal. Instead, surface structure is more effective. • The adsorption of cations and anions were characterized in different isotherms, supporting the adsorption of each one separately with a different mechanism. The metal and the CN in the structure of the complex behaved independently in the system

16. Thank you for your interest... Vildan ONEN and Esra YEL vonen@selcuk.edu.tr