| Phosphate rock resources play an important role in grain production and phosphorus chemical industry.The sedimentary phosphorite with middle-low grade have become the main target of utilization.Its impurities,such as SiO2 and MgO,are relatively high.The valuable mineral and gangue minerals coexist closely and distribute finely,commonly known as collophane.It needs beneficiation to meet the requirements of subsequent phosphate chemical industry.Flotation is the main mineral processing method.It is a complex physicochemical process for mineral separation,involving solid,liquid and gas phases and exploiting the differences in wettability of mineral interfaces and using bubbles as a carrier.The flotation efficiency is related to particle size,and it is difficult to recover fine particles with the size below 20 micron by flotation due to their smaller mass and higher surface area.The interfacial regulation could enhance the flotation of fine minerals,including the wettability of mineral/water interface induced by flotation reagents,the interaction between mineral particles and the interaction between mineral particles and bubbles.Therefore,study on the interfacial regulation of fine apatite flotation is of great significance for enhancing the flotation and promoting the efficient utilization and sustainable development of fine-grained phosphate rock resources with middle-low grade.In this research,the objects are fine apatite,dolomite and quartz single minerals with mean size of 14 micron from sedimentary phosphorite,as well as siliceous-calcareous and calcareous-dolomitic phosphate ores with middle-low grade.The properties of phosphate ores and the surface properties and flotation behavior of fine minerals were studied firstly.Using surface analysis and measurement methods such as atomic force microscopy（AFM）,X-ray photoelectron spectroscopy（XPS）,contact angle measurement by captive-bubble method with molecular dynamics simulation and in-situ characterization of aggregate sizes by focused beam reflectance measurement（FBRM）,we studied the effects of sodium oleate as a collector and sulfuric acid/phosphoric acid as a depressant on the wettability of apatite interface,effects of dispersants and kerosene emulsion by sodium oleate on the dispersion and hydrophobic aggregation between fine apatite particles,and effects of conditioning scheme of reagents on the adhesion of carbonates to bubbles under acidic condition.The research results could lay a theoretical foundation for enhancing the fine apatite flotation.The research results are as follows:（1）There were the cryptocrystalline banded and massive structures for the low-grade siliceous-calcareous and calcareous-dolomitic phosphate ore.Their valuable minerals were collophane with fine-grain and closely associated with gangue minerals.There were higher impurities of SiO2,Al2O3,MgO and Fe2O3 for siliceous-calcareous phosphate ore,and silicon and aluminium mainly occurred in the quartz,sericite and clay minerals,magnesium mainly occurred in the dolomite and iron mainly occurred in the pyrite.The gangue minerals of calcareous-dolomitic phosphate ore were mainly dolomite,and the content of other impurities were relatively low.（2）There were the higher purity and crystallinity,rougher surface and larger specific surface area for the apatite single mineral from a weathered phosphorite than those from a primary phosphorite,but they had no significant difference in surface electrical properties and floatability.Due to the smaller mass and larger specific surface area,there was the worse floatability,slower flotation rate and larger collector dosage for fine apatite than that with a size range of 45-75 micron.There was the better floatability for fine apatite with sodium oleate as a collector under neutral and alkaline conditions,while quartz could not be floated,and therefore they could be separated by direct flotation.The entrainment rate of fine quartz was higher than that of quartz with a size range of 45-75 micron.The floatability of fine apatite decreased under acidic condition,while there was still better floatability for the dolomite,and therefore they could be separated by reverse flotation.（3）The chemisorbed oleate and calcium oleate precipitate physically adsorbed coexisted on the apatite surface under alkaline condition.There was mainly the adsorption of oleate monolayer on apatite surface detected at a low equilibrium concentration of sodium oleate,and the non-polar hydrocarbon chain formed a hydrophobic membrane,which made the apatite interface hydrophobic.When the equilibrium concentration of sodium oleate reached its critical micelle concentration(1×10-3 mol/L),there was mainly the adsorption of oleate bilayer on apatite surface detected,and the polar heads strongly interacted with the calcium atoms of the apatite surface and water molecules,making the apatite interface hydrophilic.The dissolution rate of fine apatite in acidic media was rapid and mainly controlled by chemical reaction.Its dissolved ionic concentration depended on initial H+concentration and pulp pH after dissolution,which was independent of particle size.The dissolved H2PO4-ions from apatite and the ionized H2PO4-by phosphoric acid,as a potential determining ions of apatite,produced hydration by hydrogen bond and made the apatite interface hydrophilic.（4）The dispersion stability of fine apatite was weaker than that of quartz under natural condition.The dispersants could significantly enhance the dispersion stability of fine apatite to avoid the heterocoagulation in the following descending order:sodium hexametaphosphate>water glass=sodium silicate>sodium carbonate.The dispersion stability was attributed to electrostatic repulsion and hydration repulsion,and the hydration repulsion had a greater influence on the stability than electrostatic repulsion.Sodium hexametaphosphate could not be used as a dispersant in the direct flotation of phosphate ores since it made the apatite hydrophilic.（5）Hydrophobic agglomeration of fine apatite mainly depended on the particle hydrophobicity.Under neutral and alkaline conditions,the appropriate concentration of sodium oleate made the apatite particles hydrophobic,and hydrophobic attraction between particles was detected,and hydrophobic attraction was greater than electrostatic repulsion,thus forming hydrophobic aggregates which had a loosely branched chain structure.Under acidic condition or in presence of excessive sodium oleate concentration,the apatite particles were hydrophilic and the hydration repulsion force was detected between the particles,resulting in the weakening or even dispersion of the aggregates.The hydrophobic agglomeration of fine apatite could be formed quickly at a low stirring speed,which was attributed to the dominant role of hydrophobic attraction.It was dynamically monitored that the aggregates would be destroyed as the stirring speed increased,and when the stirring speed was reduced again,the aggregates could be recovered.This indicated that the process of hydrophobic agglomeration was reversible.The finer the particle size of apatite was,the more powerful the ability of hydrophobic agglomeration was.In addition,the micron sized kerosene droplets obtained by mechanical agitation could enhance the hydrophobic agglomeration of fine apatite more significantly than the nanoscale kerosene droplets obtained by ultrasonic wave.This was attributed to its effect of bridging between hydrophobic apatite particles induced by sodium oleate,which further increased the size and strength of aggregates.The size of aggregates increased with the increase of kerosene dosage and the aggregates had a compact spherical structure.Emulsified kerosene with sodium oleate not only improved the flotation speed of fine apatite,but also showed good selectivity in flotation of the artificial mixtures between fine apatite and quartz.（6）In the presence of collectors,carbonates would steadily adhere to CO2 bubbles generated by themselves under acidic condition and be floated.Therefore,the removal rate of dolomite and calcite in the reverse flotation of calcareous phosphate ore with middle-low grade could be improved by adding sulfuric acid after collector GJBW or adding the collector GJBW and sulfuric acid simultaneously.When the phosphoric acid was used as a depressor,at a low dosage,the similar flotation index could be obtained as sulfuric acid,because its ionized H2PO4-had the ability to depress the apatite and buffer the pH of slurry,which was suitable for the phosphate ore containing calcite.In addition,there was only the small amount of CO2 bubbles produced due to the low dosage of phosphoric acid.Therefore,the order of adding phosphoric acid and collector had no obvious influence on the flotation index. |
