On-line Measurement Of Sodium Aluminate Solution By Conductance-refraction Method

Bayer is a widely used production method in modern alumina production,and sodium aluminate solution runs through the whole process of Bayer alumina production,of which the concentration and properties of its components play a crucial role in alumina production.On-line detection of sodium aluminate solution is particularly important with the improvement of automation of metallurgy business.Based on the traditional conductivity model,a refractive index model was introduced in this paper to establish a conductor-refractive model(CRF)by studying the relationships between the refractive index and conductivity of sodium aluminate solution and components of solution,so as to realize automatic online detection of sodium aluminate solution.The main research results of the paper are as follows:The relationship between the conductivity and refractive index of sodium aluminate solution and solution composition and temperature was studied.When the Na2O concentration(NK)was between 200～240 g·L-1 and the molecular ratio(αK)was between 2.6～3.0,the conductivity of sodium aluminate solution increases with the rise of temperature and has a quadratic function with the temperature.For every 10 g·L-1 increase in NK,the conductivity decreases by 0.35 s·m-1.The conductivity is a quadratic function of the aK,and the conductivity increases by 0.55 s·m-1 for every 0.1 increase in the αK.The refractive index decreases with the increase of temperature and αK of aluminate solution,and increases with the increase of NK.For every 1℃ increase in temperature,the refractive index increases by about 1.6×10-4·The αK increased by 0.1,and the refractive index decreases by about 1.05×10-3.For every 10 g·L-1 increase in NK,the refractive index increases by about 1.95×10-3The three-factor quadratic orthogonal experiment was adopted to establish the conductance-refractory model(CRF)of aluminate solution with the NK ranging from 200 to 240 g·L-1 and the αK ranging from 2.6 to 3.0.The result in significance test of regression equation is good.The conductivity model is as follows:σ=389.87-3.867×10-2Nk+1.667×10-1T+50.486αK2-272.95αK,F=29.05,The refractive index model is as follows:n=1.74569.4688×10-4Nk-0.1388αK-2.4251×10-3T+4.1623×10-4NkαK+7.8695×10-4TαK,F=18.3469.Steepest descent method,Newton method and genetic algorithm are used to solve the model.Newton method has a small number of iterations but a low accuracy,and the genetic algorithm has no solution due to the limitation of convergence speed.Therefore,the model is solved by the Steepest descent method with a high accuracy.After using MATLAB software programming solution and error analysis,the relative error of NK and αK of aluminate solution is not more than ±5%,the average error of NK is 2.76%,the average error of αK is 3.22%,while the precision of Al2O3 is 0.27 g·L-1 and the precision of NK is 0.51 g·L-1.By studying Raman spectra of sodium aluminate solution under different temperatures and concentrations,the influence of aluminate ion on the conductivity and refraction of solution was studied.When the concentration of aluminate ion in solution is fixed,the conductivity of solution will be reduced by the conversion of Al(OH)4-to Al2O(OH)62-or by the conversion of aluminum ions with a higher degree of polymerization.However,the refractive index is almost unaffected by the ion structure of aluminate ion in the range of NK of 200～240 g·L-1 and αK of 2.6～3.0.The influence of impurity ion on the conductivity and refractive index of the sodium aluminate solution was studied,which provides a theoretical basis for noise reduction of the model.The conductivity of sodium aluminate solution decreases with the increase of the concentration of impurity ion,among which,the influence of impurity on the conductivity is Sodium acetate<Sodium sulfate<Sodium carbonate<Sodium silicate<Sodium oxalate.The influence of sodium acetate less than 10 g·L-1 on the conductivity of the solution can be ignored.The influence of sodium sulfate and sodium carbonate concentrations within 10 g·L-1 on the conductivity was no more than 0.5 s·m-1;For every 1 g·L-1 increase in sodium silicate concentration,the conductivity decreases by about 0.4 s·m-1.For each 1 g·L-1 increase in sodium oxalate,the conductivity decreases by about 0.7 s·m-1.The refractive index of the solution increased with the increase of impurity ion concentration.The refractive index increases with the increase of impurity concentration.When sodium sulfate,sodium carbonate and sodium silicate increased from 2 g·L-1 to 10 g·L-1,the refractive index of the solution increased by about 1×10-3.When sodium acetate and sodium sulfate rose from 2 g·L-1 to 10 g·L-1,the refractive index of the solution increased by about 1.5 ×10-3.