Basic Research On Catalytic Oxidation Of High-sulfur And High-arsenic Refractory Gold Concentrate And Tailings In Three-phase Circulated Fluidized Bed

With the depletion of high-grade ore resources, it is a trend to develop and utilizerefractory gold ore and tailings, especially high-sulfur and high-arsenic goldconcentrate (HGC) and tailings. To achieve a satisfactory recovery, an oxidativepretreatment stage is required before applying any conventional treatment. Refractoryore pretreatment processes mainly include roasting oxidation, pressure oxidation andbacterial oxidation. Roasting oxidation would release SO₂ and As₂O₃ and pollute theenvironment; Pressure oxidation processes require high-pressure and high temperatureequipments. Bio-oxidation process requires long time for pretreatment and is sensitiveto arsenic. Nitric acid oxidation method can significantly reduce the reactiontemperature and pressure. Nitric acid oxidation method can be divided into N1TROXprocess, ARSENO technology and COAL method. Although pressure is lower inARSENO technology and COAL method than in high pressure method, there are still2 to 7 atmospheres of pressure. In N1TROX process, nitrogen oxides recovery systemis complex. Considering the above problems and the characteristics of three-phasecirculated fluidized bed, the process of catalytic oxidation of high-sulfur andhigh-arsenic refractory gold concentrate and tailings in three-phase circulatedfluidized bed was suggested. Therefore the development of a cost-effective, clean,atmospheric pressure, low temperature, NO_X easy reuse and recycling rates arerelatively high pre-treatment methods have become the current needs.In this paper, first of all, Through XRD, MLA (Mineral Analysis dissociation) andpoint scan, line scan and surface scan, it was revealed that the gold concentrate andtailings in this paper is of refractory kind and the gold in HGC and tailings isencapsulated as fine grained particles in the crystal structure of the mineral matrix.Then, nitric acid oxidation ability was analysed from three aspects: the structure of nitrate and nitric acid molecules, the electrode potential of nitric acid, and catalyticaleffect of NO₂. Thermodynamic analysis was conducted from three aspects: chemicalreaction heat, enthalpy and Gibbs free energy. It was found that HGC and tailingswere oxidized by nitric acid is feasible in theory.Under the guidance of the above theories, this paper researched four problems:Direct oxidation of HGC by HNO₃ in the intermittent reactor; Hydrodynamics inthree-phase circulated fluidized; Direct oxidation of HGC by nitric acid in three-phasefluidized bed; NOx circulated catalytic oxidation of HGC and tailings in three-phasecirculating fluidized bed. The conclusions can be drawn as follows:(1) The HGC was used as raw materials. The conversion of iron was made as theindicator. Through orthogonal experiment and single factor experiment, the influencesof main factors on the rate of conversion of iron were conducted. Furthermore, thekinetics of HGC oxidation by dilute nitric acid in batch reactor was investigated.â‘ The effects of particle size (50-335μm), reaction temperature (25-85℃), initialacid concentration (10-30%, wt.) and stirring speed (400-800 rpm) on the ironextraction rate (C_r) were determined. It is obvious that C_r increases with the rise ofinitial nitric acid concentration, reaction time and stirring speed, but decreases withthe increase of particle size.â‘¡the kinetics of HGC oxidation by dilute nitric acidunder in batch reactor was investigated. The reaction process conforms to shrinkingcore model.â‘¢The activation energies were determined to be 10.70 KJ/mol in the10% HNO₃ and 12.25 KJ/mol in the 25% HNO₃. Oxidation kinetics indicates that theprocess of HGC oxidation by dilute nitric acid in batch reactor is diffusion controlled.It is necessary to accelerate fluid turbulence and decreases liquid film.(2) Air, water and HGC were used as gas phase, liquid phase and solid phase,respectively. Through theoretical analysis and experimental study, hydrodynamics inthree-phase circulated fluidized were studied. The effects of gas distribution plate andoperation conditions on the flow Characteristics were conducted.â‘ average gasholdup increased with gas velocity exponentially; liquid circulation velocity increasedwith gas velocity, and gas-liquid mass transfer mainly occurred in the upper tube; average gas holdup increased with amount of solid particles.â‘¡Under the sameoperating conditions, gas holdup is higher in the circulating fluidized bed withmicropore gas distribution board than in the circulating fluidized bed with sparse gashole distribution board.â‘¢At the initial stage, with the increase in gas flow rate,pressure drop between two points dropped. The pressure drop of upper part is thelargest, the lower part is the second, the central is minimum. With a further increase ingas flow rate, pressure drop slightly decrease.â‘£Under the experimental condition,the correlation of average gas holdup in circulating fluidized bed with sparse poredistribution board and micro-pore distribution board areε_g=0.08U_g^0.86andε_g=0.12U_g^0.99, respectively.(3) The HGC was used as raw materials. The conversion of iron in three-phasecirculated fluidized bed was made as the indicator. Through single factor experiment,the influences of main factors on the rate of conversion of iron were conducted.Furthermore, the kinetics of HGC oxidation by nitric acid in three-phase circulatedfluidized bed was investigated.â‘ In three-phase circulated fluidized bed, theconversion of iron increases with the rise of initial nitric acid concentration, reactiontemperature and air velocity, but decreases with the increase of particle size.â‘¡Thereaction process of HGC oxidation by nitric acid in three-phase circulated fluidizedbed conforms to shrinking core model.â‘¢The activation energies were determined tobe 43.2KJ/mol. Oxidation kinetics indicates that the reaction process of HGCoxidation by nitric acid in three-phase circulated fluidized bed ischemically-controlled. Reaction model is 1-(1-C_r)^?=5987.325e^-5187.14/Tt andreaction rate is(?).(4) The HGC and tailings were used as raw materials. The conversion of iron inNOx circulated catalytic oxidation process was made as the indicator. The experimentof NO_X circulated catalytic oxidation of HGC and tailings in three-phase circulatedfluidized bed was conducted. Through XRD, point scan, line scan and surface scan, the effect of NOx circulated catalytic oxidation were studied.â‘ The conversion ofiron in NOx circulated catalytic oxidation process can be as high as 97.59%. Afterdepth pretreatment, the maximum volume Fe of the single-point is only 0.63%; S andAs contents of a single point is 0. Regardless of line or surface scan, As content is 0.Gold minerals can be completely oxidized in NOx cycle three-phase circulatingfluidized bed.â‘¡The cyanidation leaching rate of HGC and tailings after depthpre-treatment is 96.38% and 100%, respectively(The amount of sodium cyanide is3000g/t; liquid-solid ratio is 3:1; particle size -25μm; leaching time is 24 h; pH is9-10), proving that NOx circulated catalytic oxidation process indeed improve the rateof gold leaching.â‘¢It is feasible to pretreat HGC and tailings using three-phasecirculating fluidized bed NO_X catalytic oxidation method. The apparatus is simple andis easy to operate. There is no waste and the environment was protected.