Optimization of selective Copper, Nickel and Zinc sulphide precipitation of industrial wastewater

Treatment of wastewater which arised from mining, electroplating, metal finishing and refining processes, is crucial with the aim of conservation and reuse of natural resources. For this purpose, metal seperation and recovery from industrial effluents with using the method of chemical precipitation is becoming common day by day. It is known that industrial treatment and recovery methods such as electrochemical applications are required to make technological investments which cause to large amount of costs and also have dissadvantages related to adapt to the challenges of different processes. Hydroxide precipitation, which has basic applications fairly well known, stands out in the industry. Although the process has an ease of application and ability to create an economical controlled system, it creates high volumed sludge wastes and related long term waste management problems. Metal sulphide precipitation is an alternative way, providing the possibility of precipitation of metal ions in a wide pH range and capable of rapid precipitation. Also metal sulphide precipitates which are obtained from precipitation process can be reused with feeding to the melting process. The process is realized with cheap chemicals such as Na2S and shows high solubility differences in low pH values. As a consequence of these advantages, metal sulphide precipitation is becoming increasingly important. Providing selective recovery of heavy metals from asidic wastewater solutions in low pH values as following raising pH value and precipitation of metal hydroxides ensure decreasing the values below to wastewater discharge limits. Thus the process becomes environmentally friendly and an economical method. The scope of this thesis the purpose of experiments is recovery of copper, nickel and zinc selective sulphide precipitation with using Na2S as sulphur resource. For the investigation of experimental datas and identification of reaction potentials and pH values, Eh - pH diagrams of Cu-S-H2O, Ni-S-H2O, Zn-S-H2O, Cu-Ni-S-H2O, Ni-Zn-S-H2O, which based on room temperature, were drawn with FactSage programme. In this way, the optimum pH range for the precipitation reactions, which are performed in the synthetic solution and contain single metal ion or mixed metal ions, determined. Then experimental studies were carried out in three groups. First two group studies have been examined by working in synhetic solution to optimize process conditions. As the first step, pH and Na2S values optimized in single metal ion contained synthetic solutions. Also reaction time was determined with experimental studies. For the following step, all three types of metal ions were tested in synthetic solutions. Based on data obtained from first two stages, last experiments were carried out as selective sulfide precipitation from refinery waste solutions and precipitation efficiency was determined. Main steps of experiments were explained with a diagram in thesis. For preperation of synthetic solutions CuSO4.5H2O, NiSO4.6H2O, ZnSO4.7H2O Merck chemicals and Na2S were distilled in determined volume of water. During experimental studies all pH adjustments made with w %10 NaOH and w %10 H2SO4 solutions. But last group of experiments were held in strong asidic refinery waste solutions. That made the pH adjustment step a bit more complicated. Because of this during industrial applications the waste water solution buffer action should be taken into account. After pH adjustment, Na2S solution were added to main solution and reaction started. Then the solution filtered and precipitates watered, dryed and analyzed. Also residual solution was analyzed and residual metal ion concentration were calculated. Experimental studies with the aim of reaction time investigation prooved that precipitation rates are quite high beginning from first minutes. Total metal sulphide precipitation reactions complete around 45 minutes to 60 minutes. These results show that metal sulphide precipitation reactions occur quickly when they compared with the other precipitation reactions. The most two important factors which affect precipitation of metal ions by using Na2S, are determined as controlling the pH and free sulphide concentration in the solution. During experimental studies, the pH of solution measured by pH electrode before the reaction started. However pH electrodes, which could be integrated to the system, are suggested to use in industrial applications due to the controlling difficulties at high volume rates. In this way subsequent pH variations during the reactions can be prevented, sulphide ion species can be followed and potential hazardous gas (H2S) formation can be controlled. Also the system required to have a homogeneous mixing usage. For the sulphide precipitation with Na2S from synthetic solutions, firstly copper ions precipitated as CuS between pH 2-2,5; then nickel ions as NiS and zinc ions as ZnS were precipitated in pH 5,3-5,5. Over these pH values with the aim of controlling the results, pH tried through 5,75 as maximum point. All pH variated experiments showed that around optimized pH range precipittaion efficiency increases through the maximum value of range. However it was observed that while pH value changes up to maximum value in the range, selectivity decreases. Consequently, for Cu2+ ions in pH=2,3, Ni2+ ions and Zn2+ ions in pH=5,3 high yield was obtained. Another parameter is the effect of Na2S ions which are added through the solution. When the experiments are made for examine this effect, precipitation efficiency increases with the rising stoichiometric amount. When the stochiometry metal ions in the solution (mol) : Na2S ions (mol) in the solution increased to 1:1,75 the maximum precipitation yield obtained. But access free sulphide ions shared by all metal ions in mixed and wastewater solutions. On the other hand, during precipitation from wastewater solutions selectivity rate decreases with rising Na2S stoichometric amount. The mixed metals synthetic solution experiments and wastewater solution experiments show the same result according to rising Na2S stoichoimetric amount. Even though Na2S stoichoimetric amount is less more than one to one, all metal ions react with free sulphide in the solution. In case optimized pH values are not provided in the system strangely free sulphide ions are shared by all metal ions thus coprecipitation occurs. From this point of view, in industrial applications it could be required to measure free sulphide ion concentration in the system by using a selective ion electrode or a vessel design which allows homogeneous sulphide ion distribution. As the last part of the experimental studies, refining wastewater solutions are treated. This refining wastewater solution contains 10 g/L Cu2+, 0,3 g/L Ni2+ ve 3 g/L Zn2+ ions. For the precipitation of copper ions from wastewater solution in pH=2,3 as CuS with approximately 100% efficiency and the residual copper ion concentration in the solution decreased to 40 mg/L. In pH=5,3 residual nickel ion concentration in the solution decreased to 4 mg/L and 44% precipitation efficiency achieved. When the precipitation reaction completed in pH=5,3 the residual zinc ion concentration in the solution decreased to 6 mg/L and precipitation efficiency rate as 52% obtained. These results proof that in optimized pH and Na2S stoichoimetric amount values, residual metal ion cencentrations in the solutions decreases with rising pH however available selectivity rate is not achieved and metal ions coprecipitate. Add more, precipitates which held in pH 2,3 characterized with XRD and showed CuS and Cu2S peaks. When pH in solution increased to 5,3 precipitates showed NiS33.3Zn32.3 peaks. According to these thesis experimental results, optimized metal sulphide precipitation process with main reasons as basic application, high reaction rates, economical sulphide resources is suggested to use in refining waste water solutions as an alternative way to the hydroxide precipitation process.