Recovery rhodium from dilute waste solution by electrodyalisis method

Rhodium is one of the precious metals which is rarest and valuable. It is a noble metal and it has high resistants to corossion. It is found nickel and copper ores with the other platinum group metals.It is commonly use as catalytic converters in automobiles. It is alloyed with the other precious metals because of high corrosion resistants and high temperature resistants and used production of white gold. White gold is often plated with a thin rhodium layer to improve its apperiance of brightness and avoid to nickel allergy. The other uses of rhodium are electrocoating of thermocouple, electrodes, optic instruments, mamography...etc. Rhodium has areas of usage, but it is found rarely in the earth and it takes so much time, money to get out from ores. The industrial extraction of rhodium is complex because the ores are mixed with other metals such as palladium, silver, platinum and gold. It is found in platinum ores and extracted as a white inert metal that is difficult to fuse. South africa is the main exporter of the rhodium, followed by Russia.Almost 10 mine is found for refining rhodium. Refining rhodium from the ore is complex and diffucult process.So it is important the recovery rhodium from the secondary sources. Some methods are using to recovery rhodium from residues as sedimentation, liquid - liquid solvent extraction, adsorpsion, electrolysis...etc. But generally rhodium recovery as solid form. It is complex to change the rhodium solid to liquid form and it takes lots of time (almost nine months), money and full attetion to process. Rhodium is very stable, inert and complex metal that not intract with chemicals easily. It is important to recovery rhodium from solution by liquid form, exactly. Rhodium recovery by zinc sedimentation from the residue solutions but lots of parametre like pH, temperature.. etc is affect the system effiency and a little mistake could be locked the system Solvent extraction method is used for the rhodium recovery from residue solutions. by different extraction and stripping solutions are investigate to improve the effiency. The conventional solvent extraction is , not possible for rhodium, solvent extraction was coupled with the use of SnCI2 to :alter the aqueous chemistry of rhodium and render it extractable. In this study, electrodialysis, is used to recovery rhodium from dilute solution, which is the one of the electrochemical membrane separation method. Membrane technology is increasingly become important today and used in a wide range of applications. A membrane is a permselective polymer. In this study, ion exchange membranes are used. Ion exchange membranes are made of a polymeric material attached to charge ion groups. Two type of membranes are used; anion exchange membrane and cation exchange membrane. Anion exchange membranes contain fixed cation groups, and cation exchange membranes contain fixed anion groups on polymeric matrix to decided to permselectiviy of the membranes. Anion exchanges membranes are allowed to transfer anion and cation exchange membranes are allowed to transfer cation from the membrane to the other side. Electrical potential difference is occoured a driving force to move the ions from one side to another side of the membrane, in the electrodyalisis process. Electrodialysis method is used in this study by three different cell design. Firstly, three compartament electrodyalisis cell, which include both anion exchange membrane and cation excahnge membrane together, is tested. System is worked very slowly. Efficiency of the rhodium transfer is not enough to recovery rhodium from diluate solution because the electrodyalisis cell is not have optimum conditions yet. The membranes have to be invetigated by seperatly. And second step, two compartement electrodyalisis cell is designed so anion exchange membrane and cation exchange membrane can be investigated separately. All type of electrodyalisis, stainless steel is used as cathod and lead is used as anod which areas are the same as 5,5x6 cm. Membranes' areas are 4x4 cm. All membranes get wet before used like give at their description. Firstly, two compartament electrodyalisis process is used for testing cation exchange membrane. CMI-7000S cation exchange membrane is used firstly. To finding, the optimum working conditions, we test the system at different current densities. İnitial working solution is including 200 ppm Rh+3, and varies current densities as 75 A/m2 , 150 A/m2 , 200 A/m2 and 300 A/m2 are applied. While 300 A/m2 current density is applying, the electrodyalisis proses get warmer to approach 50-60 ⁰C. That temperature is high for membranes' life time, so 300 A/m2 current density is not suitable for the process, however the effiency of the transfer is increas while the applied current density is increasing,too. At 200 A/m2 current density is give the best result for the transfer rodium to CEM. So 200A/m2 current density is chosen for the optimum working condition. Different initial working solutions are prepared which include 50 ppm, 100 ppm, 200 ppm and 400 ppm Rh3 for testing initial solution affect on the proses. The process is worked at fixed 200 A/m2 current density. While the concentration is increased, the transfer amount of rhodium from cation exchange membrane is increased, too. But 400 ppm initial solution not give the same increased result as well. Because rhodium atoms reduction on the membrane area by the hydrogen atoms and foul the membrane's pores. So, the efficiency of the process is reduce. At high initial concentrations, membranes efficiency reduced because of the fouling on the membranes pores. At 200 ppm Rh+3 initial solutions, the electrodyalisis process give the best efficiency. Optimum initial concentration is found 200 ppm. Consumption of the proses enerji is increase while initial concentration is decrease, because of decreasing of the solutions' conductivity. At low concentrations, amount of ions is decrease so solution of conductivity is decrease, too. Different anolyte and catolyte solution concentration was investigated about rhodium transfer effiency. Firstly, catolyte was chancing per 4 hour as 0 ppm Rh+3solution,and then catolyte solution was chancing per 4 hours as 100 ppm Rh+3 solution, and results were comparisioned. Two different cation exchange membrane were used for that working. For MC-3470 CEM, increasing of the catolyte concentration is not decrease the effiency of the rhodium transfer from the membrane. But for the CMI-7000S CEM, increasing of the catolyte concentration is decrease the effiency of the rhodium transfer from the membrane. Different membrane having different selectivity of ions and create different resistance to process. So if the process is used for the industrial area, the process must be done by multiple steps. Anion exchange membranes was investigate at two compartement electrodyalisis. Two different anion exchange membranes are used, Fumatech and American. Catolyte was 200 ppm Rh+3 solution, anolyte was 13 g/lt H2SO4 solution and almost 4 hour later all the anions transfered from the anion exchange membrane. The prosess is very fast. Anion atoms move faster than cation atoms in the solution, so the prosess finished quickly. For optimising the electrodyalisis system, anion exchange membrane areas could be reduced. The last step is four comportament electrodyalisis system The anion exchange membrane areas were reduced to 2x2 cm because of the anion mobility is faster than the cations.The cation exchange membranes areas the same as 4x4 cm. At the last four compartement electrodyalisis system, the object is, rhodium transfer from the diluate concentration to high concentration. I. department is include 50 ppm rhodium, II department include 100 ppm rhodium, III department include 100 ppm rhodium and IV department include 150 ppm rhodium solution.All the department include 15 g/lt H2SO4. Between I-II departments cation exchange membrane is found, between II-III departments anion exchange membrane is found and between III-IV departments cation exchange membrane is found. The electrodyalisis system worked at fixed 50 A/m2 and 100 A/m2 for 4 hours and investigate rhodium transfer between I-II departments and between III-IV departments. For continious electrodyalisis system, rhodium transfer is occured but transfered amount of rhodium is lower than the batch system. Concentration driving force is not exist at the continious series and therefore pressure driving force must be applied to system for increasing the transfer of rhodium effiency. Continious electrodyalisis system was worked at two different current density but the transfered rhodium amount wasn't changed. It is showed the membrane area is effective than the cathod area. For increasing the rhodium transfer amount, membrane areas must be increase as much as possible. For the different membranes, different system potansiyel is occoured so it is important the choose the correct membrane for the system.Different membranes has different resistance and different ion selectivity at various conditions. And the system temperature is important.At high temperature is reduce the membranes' life. Conclusion, at industrial applications, membrane area is effective from than the current density so membrane area is increase as much as possible. For the continious electrodyalisis system, rhodium transfered from low concentration to high concentration so concentration driven force is removed and the system effiency is decrease , pressure driven forced must be applied to increas the system efficiency.