Removal Of Organics From Sodium Aluminate Solution By Foam Flotation

More than 95%of alumina is currently produced by the Bayer process worldwide.Owing to high temperature,high pressure,and circulation of the spent liquor,a large amount of the concentrated organics are dissolved into the sodium aluminate solution.These organics have a significant negative impact on alumina production by Bayer process.Therefore,an efficient and economical method to remove organics from the sodium aluminate solutions is urgently required.Rich foam is generally found in the sodium aluminate solution（Bayer liquor）because of high surface activity for sodium-bearing organics,the removal of organics by foam flotation was then proposed from the sodium aluminate solution.The interfacial behavior of organics in the sodium aluminate solution was firstly studied by surface tension.The effect of organics on the viscosity of the sodium aluminate solution was then investigated.Foam flotation was finally explored for removal of organics from the sodium aluminate solution.The conclusions were listed as follows:（1）Remarkable surface activity was observed for sodium oxalate,sodium benzoate,sodium tartrate,sodium citrate,sodium stearate and sodium oleate in the sodium aluminate solution.Sodium stearate with long chain and sodium citrate with multiple hydroxyl groups notably reduced the surface tension of the sodium aluminate solution compared to sodium oxalate and sodium benzoate.The surface activity followed the sequence:sodium citrate>sodium tartrate>sodium stearate>sodium oleate>sodium benzoate>sodium oxalate.Meanwhile,the order of CMC was:sodium oxalate>sodium tartrate>sodium citrate>sodium benzoate>sodium oleate>sodium stearate.Furthermore,the long hydrophobic chain raised cross-sectional headgroup area(A_min)and saturated adsorption layer thickness（δ）but reduced Gibbs surface excess(Γ_max).Elevating temperature raised CMC,A_min andδ,but decreasedΓ_max.（2）Sodium oxalate,sodium benzoate,sodium tartrate and sodium citrate significantly affected the viscosity of the sodium aluminate solution.The increase in concentration of the above four organic substances within the range of 0～0.7 g·L^-1 initially reduced the viscosity of the sodium aluminate solution,while increasing concentration of organics over 0.7g·L^-1 then raised the viscosity of the sodium aluminate solution.the corresponding extreme point was near to the CMC of organic substances.Meanwhile,the viscous flow activation energy and pre-exponential factor A both decreased firstly and then increased with the increase of organic concentration,which were well consistent with the variation of viscosity.Moreover,the viscous flow activation energy and viscosity of Bayer liquor increased significantly with increase in TOC more than 1.0 g·L^-1compared with that for the synthetic sodium aluminate solution,.（3）Uniform bubble size with great thickness and high stability occurred in the sodium aluminate solution containing sodium citrate compared with solution containing sodium oxalate.The order of foam stability of the sodium aluminate solution containing various organics followed the order:sodium citrate>sodium tartrate>sodium benzoate>sodium oxalate.Increasing temperature decreased the foam stability.Meanwhile,increase of foam volume raised the organics removal rate and reduced the organics concentration ratio in the foam.The sequence of removal rate of organics was:sodium citrate>sodium tartrate>sodium benzoate>sodium oxalate.Adding cationic surfactant A slightly increased the organic removal rate.The sodium citrate removal rate reached 27.52%in 100 m L foam at 333K,and the concentration ratio in the foams was more than 4.0 in 20 m L from 810 m L at 333K.