ATMOSPHERIC DIRECT LEACHING OF SPHALERITE CONCENTRATE IN SULFURIC ACID USING OZONE AS OXIDIZING AGENT

• Main Author: Sukamto, Kostiawan
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/63401
• Institution: Institut Teknologi Bandung

A conventional method for producing zinc from sphalerite concentrate is usually through Roasting-Leaching-Electrowinning (RLE) route, which contributes to around 85% of overall zinc production in the world. The major challenges in applying RLE process are the emission of SO2 gas and the formation of zinc ferrite during a roasting process that leads to the requirement to carry out the leaching in two stages, namely using dilute acid followed by concentrated acid. An alternative for avoiding the emission of SO2 gas and the formation of zinc ferrite in the RLE process is by performing direct leaching, in which sphalerite concentrate is directly leached without passing thorough a roasting process. During its development stage, direct leaching was started by the application of pressure leaching technology and later on by atmospheric leaching. Direct leaching of sphalerite concentrate at atmospheric pressure is regarded as a leaching method with lower energy consumption, easier process control, requires cheaper of maintenance and construction materials than the RLE and pressure leaching processes. One of key factors for successful application of direct atmospheric leaching process for sulfide minerals is the selection of effective and inexpensive oxidant that can result in high metals extraction percentages with faster leaching reaction kinetics. Ozone is an inexpensive oxidizing agent that can be directly generated from air. Ozone, which has a standard reduction potential of 2.07V, is a strong oxidizing agent, which is stronger than oxygen and hydrogen which has standard reduction potential of i 1.23 V and 1.77 V, respectively. In this research work, the effectiveness of using ozone as an oxidant in the atmospheric direct leaching of sphalerite concentrate from Bogor, West Java in sulfuric acid solution was studied. The investigation began with preparation of sphalerite concentrate, which involved drying, sampling with a conning-quartering method, and sieving to obtain the particle size distribution of concentrate according to the testworks that will be done. After the preparation process, the chemical and mineralogical compositions of the sphalerite concentrate sample of were analyzed by X-ray Fluoroscence (XRF), Atomic Absorption Spectrophotometer (AAS), X-ray Diffraction (XRD) and thin polished section of mineragraphy analysis. After accomplishing a characterization of the sample of concentrates, an experiment to determine the method of ozone injection method that provides the best ozone solubility the contact between ozone and slurry was carried out. The injection method studied was an injection system by using gas diffuser and Mazzei injector, in which the ozone used in the experiment, was derived from an ozone generator produced by a corona discharge method. A series of leaching experiments was conducted in a 1000 mL solution reactor. The independent variables of the experimental works were sulfuric acid concentration, particle sizes distribution of sphalerite concentrate, temperature, solid-liquid ratio, stirring speed, and the injection rate of oxygen being used as raw material for the ozone generation. In each condition of the leaching test, measurements of dissolved Zn, Fe, Cu, and Pb concentrations as a function of time were performed by using AAS. Based on the data of dissolved Zn, Fe, Cu, and Pb concentrations the extraction percentages of zinc selectivity of leaching to Fe, Cu and Pb were then determined. In addition, ozone solubility data for each condition of leaching experiment was collected by conducting measurements using an ozone detector. The kinetics of sphalerite concentrate leaching process was analyzed by adopting shrinking core and shrinking particle models. The investigation results showed that ozone injection method that gives with the best ozone solubility was the ozone injection system with Mazzei injector followed by agitation of the slurry in the reactor using a stirrer. The highest zinc extraction of 99,96% was obtained form the leaching of sphalerite concentrate in 2M sulfuric acid solution with solid-liquid ratio (g/mL) of 50/1000, acid at room temperature and oxygen injection rate of 1 lpm, using particle size distribution of -200#, and stirring speed 420 rpm. The extraction percentage of zinc was consistently enhanced by the increase of sulfuric acid concentration from 0.5M to 2M, in which a further increase of sulfuric acid concentrations from 2M to 2.5 and 3M oppositely reduced extraction percentage of zinc, due to lowering ozone solubility in concentrated acid solution. Finer particle size of sphalerite concentrate gave better ozone solubility in the leach slurry and resulted in a higher zinc extraction percentage. Variations of stirring speed at the range of 120-420 rpm did not significantly affect the extraction percentage of zinc. Extraction percentages of zinc tend to decrease by lowering ozone solubility in solution at higher solid/liquid ratio, while the increase of temperature reduced the extraction percentage and leaching rate due to diminishing of ozone solubility at higher temperature. Leaching of sphalerite concentrate in sulfuric acid using ozone as oxidant was not adequately selective to Fe with average selectivity value of 0.39, while selectivity of leaching to Cu and Pb were quite good with average selectivity values of 0.74 and 0.88, respectively. Kinetics analysis revealed that the rate of leaching of sphalerite concentrate in sulfuric acid solution using ozone as an oxidant is controlled by diffusion through the porous layer of solid product with activation energy of 4.727 kcal/mole.