| The shortage of zinc supply in china is becoming a serious problem with the increasingly rising demand for zinc metal. In recent years, one third of zinc supply per year has to be imported from abroad. Although the zinc oxide ore resources are relatively abundant and available in many mines in China, their low quality for processing, such as low grade, high oxidation degree, high content of slime and complex mineral composition, prevent them from being economically and effectively beneficiated and smelted, leading the mined ores always to be only stacked and even discarde as wastes. Thus, it is quite urgent to develop a new technology to treat and use this kind of zinc reserves, supporting a sustainable development of china's zinc industry.In this thesis, a newly developed combined flowsheet,"ammonia leaching-extraction-electrolytic deposition-leaching residue flotation (LEEF Process)", for treating extremely low-grade zinc oxide ores with high content of calcium was studied. The main characteristics of the ores studied mainly include low zinc grade (<8%zinc), high content of calcium and (CaO:>22%), high content of slime, complex ore structure and fine dissemination of the minerals.In this study, the basic theory aspects of the key steps of the flowsheet are investigated as emphases, and feasibilities of individual unit operations and the whole flowsheet of the LEEF Process has been confermed by primary condition tests. The main conclusions can be summarized as follows.(1) Basic theory of the ammonia leaching system of the zinc oxide oresThe solubility of zinc oxide ores is dependent on total nitrogen (ammonia-ammonium) concentration, dosage of ammonium hydrogen carbonate and pH values of the solution. The pH range is varied from6.5to11according to stable precipitation of coordination ions formed by zinc and ammonia. Higher leaching rates can be reached at pH values of10to11of the leaching solution.The results of comparison experiments show that the leaching efficiency for an ammonia-ammonium complex system is superior to that for an ammonia water-only or ammonium-only system. So, an ammonia-ammonium system was adopted as the leaching system for the leaching tests. The possible existence forms of zinc of various solution systems were determined by thermodynamic calculation. The results indicate that zinc in Zn(II)-NH3-NH4HCO3-H2O system is mainly in the form of [Zn(NH3)4]2+.Kinetics study of the ammonia leaching process is affected by stirring intensity, the total ammonia concentration, stirring time and reaction temperature. The leaching process follows a shrinking unreacted core model, which can be described by the kinetic equation, i.e.1-(1-α)1/3=k·t. The leaching process is controlled by the external diffusion. The apparent activation energy of the leaching reaction is6.489KJ/mol.(2) Basic theory of M5640extraction systemThe extraction of zinc is the key procedure in the process. The basic theory of zinc ammonia solution extraction was studied. The results obtained show that the chemical formula of extraction complex can be expressed as ZnR2. A linear relation is found between the maximum capacity of zinc presented in organic phase and M5640concentration. The saturated capacity of zinc in NH3-NH4HCO3system containing1V/O M5640is determined as0.16g/L. The process of zinc extracted by M5640is exothermic and the isobaric enthalpy is equal to-5.66J/mol. Thus the extraction process should be better operated at normal temperature since increasing temperatures are unfavourable to zinc extraction. The equilibria isotherm of zinc extraction shows that the extraction process should adopt cross flow. Kinetics analysis of M5640extraction system has carried out, applying the method of invariable interface. The extraction process in an invariable interface pool has been preliminarily categorized into chemical reaction control.(3) Activating effect of ammonia leaching on leaching residue flotationFor the first time, the effect of ammonia leaching on the residue flotaion was investigated. It has been discovered that the floatability of zinc and lead sulphide minerals remained in the residues have been apparently improved after being leached. The phenomena is referred to as "dissolution activation".In order to prove the feasibilityes of the individual steps of the process, including the ammonia leaching, the extraction-electrowinning, the leaching residue flotation, and the feasibility of the whole flowsheet, primary condition tests are also carried out, and the main conclusions can be drawn as follows: (1) The feasibility of ammonia leachingA complex agent of NH3-NH4HCO3was selected as leacing agent. The leaching performance is determined through preliminary tests and optimization tests, and the feasibility of ammonia leaching is confirmed.The leaching efficiency of zinc is dependent on the dosages of ammonia and ammonium for the Zn(Ⅱ)-NH3-NH4HCO3-H2O system. Increasing grinding fineness of the roe, extended leaching time, increasing reaction temperature and stirring speed can enhance the leaching efficiency of zinc.In order to supply reference data for technology tests in future, response surface design method was adopted to study the parameters of the zinc oxide ore ammonia leaching. The leaching efficiency of zinc is about83.07%under the optimized conditions, i.e. C(NH3) of4.95mol/L, liquid-solid ratio of6:1, reaction temperature of323.05K, grinding fineness of74μm accounting for95%, C(NH4HCO3) of2.5mol/L, leaching time of2h and stirring speed of400rpm.(2) The feasibility of extraction-electrowinningIt has been proved that zinc can be extracted from ammonia leaching solution by aldehyde oxime extracting agent (M5640). The feasibility of the extraction technology has been confirmed and has been patented in China.The extractant, pH value of solution and total ammonia concentration are the dominant factors for the extraction operation. The extraction and strpping tests were carried out. The experimental results show that for the ammonia leaching solution with zinc concentration of9.72g/L and pH of9.71, the optimal extraction parameters are extractant concentration of35%in organic phase, phase ratio (O/A) of2:1, extraction time of3min. The zinc concentration through a single stage extraction is65.18%. The optimal parameters for stripping are:sulfuric acid concentration of40g/L, phase ratio (O/A) of1:1. The efficiency of strippng during three minutes can reach100%.The zinc enrichment process by cross-flow extrctions was also studied. The results indicate that under the conditions of extractant concentration of35%, phase ratio of2:1, zinc concentration in feeding solution of9.57g/L, pH of9.56, extraction time of3min, deionized water used as scrubbing, pH of1.3～1.5, phase ratio of1:1, scrubbing time of3min, stripping phase ratio of1:1and sulfuric acid concentration of150g/L, stripping time of3min, the zinc concentration presented in stripping solution can reach55.84g/L, and thus meeting the demand for zinc electrowinning after going through seven stages of cross-flow extraction-scrubbing-stripping.With regard to the developed electrowinning, the effect of organic matters on electrowinning was studied. The results show that the organic matters should be reduced to a certain degree by using15g/L activated carbon before the electrowinning process.(3) The feasibility of leaching residue flotationThe feasibility of leaching residue flotation was also demonstrated by primary experiments.The results of zinc-lead bulk flotation tests indicate that the leaching residue exhibits a much better floatability. The recovery of the ammonia leached lead and zinc minerals are49.34%and57.03%, respectively. The recovery of zinc reported in the bulk concentrate from the rough concentrate can reach as haigh as76.65%. A new concept of "dissolution activation" was put forward and a patent has been applied for.(4) The feasibility of the whole flowsheetUsing the LEEF Process, both the zinc oxide minerals and zinc sulfide minerals can be fully recovered due to the advantages of ammonia leaching and flotation. The feasibility of the whole flowsheet has been experimentally demonstrated,and a patent of the LEEF Process has also been applied for.
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