| Ludwigite ore owes its extremely high comprehensive utilization value and significant strategic position due to abundantly associating valuable elements,i.e.,boron,iron and magnesium.The reserve of ludwigite resources is greater than 0.28 billion tons in the eastern regions of Liaoning province,where the reserve of B2O3 is 21.84 million tons,accounting for 57.88%of the national total reserves.However,with the features of poor grade,numerous mineral components,fine mineral crystal size and complicated phase structure,the complex ore brings about critical problems in mining,separating and smelting processes.For a long time,under the established idea of traditional developing and utilizing routes,a portion of boron and most of magnesium in the ore is abandoned,resulting to resources waste and environment pollution.The problem is particularly acute in low grade refractory ludwigite ore.In recent years,a great number of new processes have been proposed and studied for the comprehensive utilization of ludwigite.Nevertheless,none of them have been successfully put into industrial application.Based on the summary of published work,a new process of metallizing reduction-magnetic separation,applying magnetic substance to steelmaking,applying non-magnetic substance to prepare magnesium sulfate and boron acid,and applying boron sludge as pellets additives was put forward in this study.For the key steps of the new process,researches were carried out systematically,including metallizing reduction-magnetic separation on ludwigite,sulfuric acid leaching-crystallization on non-magnetic substance,applying boron sludge as pellets additives.Meanwhile,the carbothermic reduction thermodynamics,non-isothermal decomposition kinetics,solution thermodynamics and valuable elements migration patterns of the new process were studied.The thermodynamics analysis showed that,iron oxides can be reduced to metallic iron with solid carbon while oxides of boron and magnesium cannot be reduced in the raw material.The equilibrium phase composition analysis indicated that,in order to reduce iron oxides completely,with the carbon ratio 1.0,the reduction temperature should be higher than 1050?.The exploration experiment used boron-bearing iron concentrate under laboratory conditions showed that,with the magnetic field intensity 50 mT,reduction temperature higher than 1250? and reduction time longer than 20 min,the recovery ratio of iron in magnetic substance can reach above 85%,while the recovery ratio of boron in non-magnetic substance can reach above 70%.The decomposition of serpentine and ascharite was the main cause of mass loss in the non-isothermal decomposition process in inert atmosphere.At the end of decomposition,(Mg,Fe)2SiO4 and(Mg,Fe)2FeBO5 were the two main phases in the sample.The average E value of structural water decomposition was 277.97 kJ/mol based on FWO method(277.17 kJ/mol based on KAS method).The mechanism model function of structural water decomposition was confirmed by Satava method and Popescu method.The form of the most probable model function was G(a)=(1-a)-1-1(integral form)and f(?)=(1-?)2(differential form),and its mechanism was chemical reaction,and the pre-exponential factor 1gA was 15.69 s-1.Under the conditions of this study,the optimum technological parameters for metallizing reduction and magnetic separation on ludwigite ore were obtained as follows:reduction temperature 1250?,reduction time 60 min,carbon ratio 1.4,ore size 0.500?2.000 mm,and coal size 0.500?1.500 mm.The recovery ratio of iron in magnetic substance was 88.02%,while the recovery ratios of boron and magnesium in non-magnetic substance were 88.86%and 94.60%,respectively.The magnetic substance could be used as high-quality raw material for steelmaking,and the non-magnetic substance could be used for further recoverying boron and magnesium.During the carbothermic reduction process,the phase transition of iron-bearing minerals were obtained as follows:Fe3O4/(Mg,Fe)2FeBO5/MgFe2O4?FeO?Mg2-xFexSiO4?Fe;that of boron-bearing minerals were as follows:MgBO2(OH)/(Mg,F e)2FeBO5?Mg2B2O5.The research results of the acid leaching-crystallization for non-magnetic substance indicated that,at ordinary pressure,leached at 90? for 15 min with the ratio of liquid/solid 7,92.04%boron and 87.43%magnesium in non-magnetic substance was leached into the liquor separated from silicon gathering in leaching residue.The leaching residue could be used as high-quality raw material for praparing industrial silicon products.Conducted in water-ethanol mixture system with the volume ratio of ethanol/original liquid 1.5 for 30 min at room temperature,97.23%magnesium can precipitate out with the initial concentration of 0.8 mol/L,and the product was MgSO4·7H2O.After distillation and collection of ethanol in the mixture system,the liquor was concentrated to recycle boron acid at room temperature with the first recovery rate 83.52%.During the acid leaching-crystallization process,the boron-bearing phase transition was obtained as follows:Mg2B2O5(non-magnetic substance)?H3BO3(leaching liquor)?H3BO3(water-ethanol mixture system)?H3BO3(primary product);that of magnesium-bearing phases were as follows:Mg2B2O5/Mg2SiO4(non-magnetic substance)?Mg2+(leaching liquor)?MgSO4·7H2O(primary product);that of silicon-bearing phase was as follows:Mg2SiO4(non-magnetic substance)?SiO2(leaching residue,amorphous state).The amorphous state SiO2 had an adverse effect on filtration operation.The study on boron sludge applied to oxidized pellets production as additive was carried out under laboratory conditions.The results showed that,within certain limits,boron sludge can improve the comprehensive strength of oxidized pellets and maintain the reduction swelling index.The bentonite content decreased to 0.3%when adding 1.0%boron sludge additive and the pellets met the requirements of blast furnace.When the combination additive content was 0.8%,bentonite content can be further decreased to 0.2%,and the reduction swelling index was better than base pellet,while the comprehensive strength was not much different from each other.The new process proposed in this study achieved the goal of separation and extraction for multi elements simultaneously,and the corresponding products could be used as quality materials in steel industry and chemical industry.Meanwhile,the pollution problems caused by boron sludge were furthest reduced.It enriched the techniques and theories of the comprehensive utilization for low grade ludwigite ore,and provided important theoretical foundations for the design development and industrial production of its green and efficient utilization routes.The exploration will advance the ludwigite utilization progress,furthermore,it can give some reference and inspiration to the exploitation on other special resources. |
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