Pereparation Of Superfine Bismuth Oxide By Silver-zinc Slag And The Relative Basic Theory Study

Our country is in rich of bismuth resources of which the reserves and yields are in the first rank in the world.However,our country falls behind in the bismuth industry,and particularly the market competitiveness of deep-processing bismuth products is very weak.This work aims to transform this resource advantage to industrial advantage.The valuable metals of the raw material of silver-zinc slag containing substantial bismuth,zinc,gold and silver were extracted firstly,then the technology of preparing superfine bismuth oxide powder with high purity using chemical precipitation method was studied,as well as the technology of that with hydro-thermal method was explored.The key target of this paper is to control the particle size during the whole process.The solution containing HCl,NaClO₃ and NaCl was used to leach silver-zinc slag and the influences of liquid-to-solid ratio and the component dosage of leachate on the leaching rate of bismuth were investigated.The appropriate conditions of leaching silver-zinc slag are that the liquid-to-solid ratio is 10:1,the NaClO₃ quality is 15g,the NaCl quality is 60g,the HCl volume is 120mL,the extraction temperature is 80℃and the extraction time is 5h.The leaching rate of bismuth is 100% approximately,the slag yield is 12%-16%,the content of silver and gold of leaching residue reach 70%and 1%under these conditions.The silver was further extracted with hydrazine hydrate by reducing the residue that was leached by ammonia solution,and the gold was further extracted with NaClO₃ and H₂SO₄ solution from that leaching solution.The recovery rate of silver and gold gets up to 95%at right conditions.The Eh-pH diagrams of Me-Cl^--H₂O system were drawn based on thermodynamic equations of that system.For Bi-Cl^--H₂O system,if the solution potential(Eh) is in 0-1.4V,BiOCl will be dissolved when the acidity of solution is high(pH＜3) and be stable when pH =3-11.For Ag-Cl^--H₂O and As-Cl^--H₂O system,as long as Eh is controlled at a certain range,arsenic and silver exist in form of ion when pH value varies from -2 to 12,while if Cl^- concentration is too high,Ag will precipitate in form of AgCl.For Pb-Cl^--H₂O system,Pb exists in the form of PbCl₂ at the region of pH＜6 and Eh＜1.4V,it's difficult for PbO2^- to dissolve because of the rigid demand of alkali concentration.For Fe-Cl^--H₂O system,Fe exists in the form of Fe²⁺ as long as Eh＜0.6V and there is no oxidizer,hydrolysis precipitation happens when pH＞6,and Fe can be removed in the form of Fe³⁺ when Eh is 0.6-1.3V and pH＜1.4.For Sb-Cl^--H₂O system,Sb exists in the form of SbCl₃ when Eh＜0.8V and HCl concentration is high,but solid Sb(OH)₃ and Sb₂O₄ form with the increase of pH value.The process route of the preparing high-pure bismuth oxide using chemical precipitation method with the leachate from silver-zinc slag was put forward.It is "purification-hydrolysis- transformation".The purification process is that the Pb is firstly removed to 3-10mg/L by silver-zinc slag under the conditions of pH value of 0.3-0.4,slag dosage of 100g/L and reaction time of 15min,and Ag is then removed to 1mg/L by a little NaI.The appropriate conditions of preparing BiOCl with purified solution using hydrolysis method are that liquid-to-solid ratio is 8-10:1,neutralizer is 40%Na₂CO₃ solution or dilute ammonia,pH value is 0.8 and the hydrolysis time is 20-30 min.The impurities of Fe,Zn,Cu and other impurities are removed by washing the BiOCl solid using 1 mol/L H₂SO₄ and deionized water.The appropriate conditions of preparing bismuth trioxide with BiOCl are that the liquid-to-solid ratio is 5:1,the alkali concentration is 4mol/L,the reaction temperature is 95℃and the reaction time is 1h.The impurities of SO₄^2- and Cl^- ions are removed by washing Bi₂O₃ solid using 1mol/L NaOH solution and deionized water.The product ofα-Bi₂O₃ with diamond shape and particle size of about 20μm is obtained at these conditions above.The content of Cu,As,Sb,Fe,Si of the product is a few parts in 100,000 and that of Ag,Pb,Zn,Fe is only a few ppm,which fulfils the requirement of high-purity bismuth trioxide.Superfine bismuth trioxide powder was prepared using chemical precipitation method by adding ammonia into Bi(NO₃)₃ solution that was prepared by HNO₃ and the coarse high-pure bismuth oxide obtained above.The appropriate conditions are that the Bi(NO₃)₃ concentration is 0.3mol/L,the feeding speed is 2.5mL/min,the ammonia concentration is 1:10(volume ratio),the end pH value is 9 and the stirring rate is 600r/min.The preparation experiment in scale-up was carried out with 500g bismuth oxide at these conditions.The precursor powder of Bi(OH) ₃ is gained by drying the filter cake for 2h at 105℃.The spherical Bi₂O₃ is gained by roasting the precursor powder for 2h at 600℃.Its grain size is 28.6nm according to the calculation.SEM results show that the particles are spherical with the average size of about 0.2μm.The product has a bright orange color.The particles agglomerate seriously in the product.The particle size of bismuth oxide powder was analyzed and controlled from the aspects of theory and experiment.In the early nucleation period,the explosive nucleation occurs by feeding at varying rate.The nucleation process is controlled in a relatively short time,and a smaller particle is achieved by separating the process of nucleation and growth.The surfactant of PVA-124,sodium oleate and sodium dodecyl benzene sulfonate can inhibit agglomeration of particles,the appropriate dosage is 4×10^-3mol/L,0.8×10^-3mol/L and 6g/L respectively.The particle with narrower size and high dispersion ability can be obtained and its average particle size is 0.05μm,0.07μm and 0.123μm,respectively.The effect mechanism of polyvinyl alcohol on the agglomeration of particle is the steric clash function,and the effect mechanism of sodium oleate and sodium dodecyl sulfate is the dual-layer(static) stability function.The possible reason of agglomeration phenomena during the drying process was analyzed.It was found that the residues of water of wet powder can be removed and the reunion phenomena can be restrained by washing the powder with ethanol or azeotropic distillating with n-butanol.The particle with high dispersion ability is obtained under these operations and its average particle size is 0.118μm and 0.116μm respectively.The theory and technology of preparing superfine bismuth trioxide using hydro-thermal method were explored.The influences of reaction temperature,reaction time,filling degree,pH value,as well as such factors as the type of reaction medium on the crystal size and microstructure were investigated.The higher the temperature,the greater the average particle size,the wider the particle size is.The longer the reaction time,the coarser the grain,the larger average particle size is.The particle size decreases with the increase of pH value.The impact of reaction medium on grain size is not obvious.The appropriate conditions of preparing Bi₂O₃ with hydro-thermal method are that the reaction temperature is 120℃,the reaction time is 2h,filling degrees is 90%,the pH value is≥11.The average particle size obtained under these conditions is about 80nm.Meanwhile it was also found that the product with different microstructure and crystal can be obtained with the alteration of pH value and reaction medium.