Fundamental Study On The Removal Of Organics From Sodium Aluminate Solution Via Froth Flotation

The negative effect of organics on the global alumina production process has been for decades.Under the conditions of high temperature（323～553 K）,high pressure（1.0～4.5 MPa）,high concentration of alkali(Na₂O>2.10 mol·L^-1),high ionic strength(I>1.8mol·L^-1)and long-term circulation of liquor,high molecular weight organics such as humic acid and lignin in bauxite degrade continuously and generate hundreds of different types of organics in the solution.In the past two decades,the harm of organics to industrial alumina production,such as loss of caustic soda and aluminum hydroxide,formation of rich foam,inhibition of seed decomposition and other problems,has become more and more obvious.Due to the high temperature,strong alkalinity,high ionic strength,and high organic concentration in the Bayer liquor,the existing organic removal methods such as adsorption method,solution roasting method and lime causticizing method are difficult to overcome the problems of high cost,low efficiency,and complex process.In this thesis,the interfacial structure,micellarization behavior,and the interaction law between organic molecules of organics were studied by surface tension method,fluorescence probe method,and thermodynamic calculation combined with Langmuir-Blodgett（LB）film balance technique,in response to the problems of unclear occurance and interfacial behavior of organics in the sodium aluminate solution.On this basis,a froth flotation method was proposed to realize the efficient enrichment and economic removal of various organics in the sodium aluminate solution.At the same time,the flotation removal law of different organics under different conditions was discussed.In addition,based on the fact of high total organic carbon concentration and rich foams in industrial sodium aluminate solutions,the technical feasibility of the froth flotation method for the efficient and economic removal of organics from the industrial sodium aluminate solution was verified without the use of any additives,and the basic flow was proposed.The main conclusions are as follows:（1）The interfacial behavior of single organic matter was clarified in the synthetic sodium aluminate solution.The surface tension-concentration（γ-lgC）curves of organics in the synthetic sodium aluminate solution were determined and it was clear that organics such as sodium oxalate and sodium benzoate had good surface activity in the sodium aluminate solution.The effects of factors such as temperature and alkali concentration on their surface activity were revealed.Under the same conditions,the order of the critical micelle concentrations of the organics in the synthetic sodium aluminate solution(C_Na2O=2.42 mol·L^-1,α_k=1.45)was:sodium tartrate>sodium oxalate>sodium citrate>sodium benzoate>sodium laurate>sodium stearate.The formation of micelles was verified by pyrene fluorescence probe method and the results showed that the critical micelle concentrations of sodium laurate and sodium stearate were respectively 1×10^-3.4 mol·L^-1 and 1×10^-3.6mol·L^-1,which were consistent with the results obtained by surface tension method.The thermodynamic calculations of the micelle formation of organics in the sodium aluminate solution showed that the process tended to proceed spontaneously in a disordered state.At 298 K,the order of the maximum surface adsorption of organics was:sodium oxalate>sodium benzoate>sodium laurate>sodium citrate>sodium stearate.The minimum average molecular cross-sectional area and the thickness of the adsorption layer of organics increased with the increase of their carbon chain.The increase of temperature led to the increase of the minimum average molecular cross-sectional area and the thickness of the adsorption layer of organics,while the maximum surface adsorption decreased.（2）The interfacial behavior of the mixture of organics was elucidated in the synthetic sodium aluminate solution.Binary organic mixtures such as sodium oxalate/sodium benzoate and sodium citrate/sodium laurate were studied to determine their critical micellar concentrations at different temperatures in the synthetic sodium aluminate solution.By comparing the results with those of single system,it was demonstrated that sodium oxalate inhibits the formation of micelles in sodium aluminate solution for the mixtures,while long-chain organics such as sodium stearate promote this process.The micelle formation ability of the organic mixtures in the sodium aluminate solution diminishes with increasing temperature.Sodium oxalate and sodium citrate have different effects on the surface adsorption and average molecular cross-sectional area of other organics in sodium aluminate solutions.The intermolecular interaction parameters and critical micelle concentrations of the mixed system of organics in sodium aluminate solution were calculated based on the Gibbs adsorption equation and Clint model with a non-ideal mixing model of surfactants,and the results showed that the calculated results were in good agreement with the experimentally values.In addition,the interfacial behavior of the ternary organic mixture system is more complicated and requires further study.（3）Multilayer adsorption was found in the synthetic sodium aluminate solution.The aggregation behavior and interfacial structure of low molecular weight organics such as sodium benzoate and medium molecular weight organics such as sodium laurate and their mixtures at the interface of air/sodium aluminate solution were analyzed under different spreading concentrations and other conditions.Theπ-A isotherm of the Langmuir monolayer and the microscopic morphology of LB film showed that the collapse pressure of the subphase of sodium aluminate solution increased with the increase of the organic spreading concentration.Theπ-A isotherm shifted to the left and the minimum average molecular area of the organics decreased.The micellar sizes of sodium stearate and sodium laurate at the air/solution interface increased with the increase of the spreading concentration.In addition,the introduction of sodium palmitate and sodium oleate in the mixtures changed the height and morphology of the LB film of sodium stearate.（4）Technical prototype of froth flotation for organic removal was presented in sodium aluminate solutions.Compared with the synthetic sodium aluminate solution,the industrial sodium aluminate solution has stronger foaming ability,smaller size（distribution）of bubbles and better stability.Sodium dodecyl sulfate（SDS）was selected as the foaming agent and collector,and the foam enrichment separation process of organics such as sodium benzoate in the synthetic sodium aluminate solution was studied.The results showed that the removal efficiency and enrichment ratio of organics increased with the increase of organic concentration and temperature.Meanwhile,the tight multilayer adsorption structure formed by the organics on the surface of sodium aluminate solution promoted the efficient enrichment of organics at the air/solution interface by froth flotation.The removal efficiency of sodium stearate by froth flotation reached higher than 29%at 333 K,with an enrichment ratio ranging from 2.42 to 3.02.In addition,two different industrial sodium aluminate solutions were chosen to verify the feasibility of froth flotation in practical production.Without use of any additives,the enrichment ratio of organics in Zunyi（ZY）Bayer liquor exceeded 2.71,while it reached 3.46 in Nanshan（NS）Bayer liquor.After solid-liquid separation and drying of the concentrated foam,the total organic carbon content in the solid sample exceeded 20%.Finally,the basic flow of organic removal from sodium aluminate solutions by froth flotation was proposed.