Development And Application Of Precipitation-adsorption Technology Using Lanthanum Compounds For Efficient Fluoride Removal From Fluoride-containing Effluent

As mineral grades continue to decrease and mineral compositions become increasingly complex,the concentration of fluorine in the metal smelting system gradually increases,resulting in a large amount of fluorine-containing wastewater,resulting in a series of technical difficulties and environmental risks.The development of high-efficiency defluoridation technology for smelting fluorine-containing wastewater is the key to realize the green upgrading and sustainable development of the metallurgical industry.In view of the disadvantages of high residual fluorine concentration and large amount of slag in the traditional lime precipitation defluorination process,this paper proposes a two-step process of lanthanum salt precipitation-adsorption to achieve deep defluorination of fluorine-containing wastewater.Based on the analysis of the thermodynamics of the La-F-H₂O series solution,the paper clarifies the chemical behavior and precipitation path of La-F,reveals the influence mechanism of sulfate on the defluoridation process,and optimizes the process parameters of lanthanum salt defluoridation in sulfate-containing wastewater.For the liquid after precipitation defluorination,Mg-Fe-La adsorbent is used for deep defluorination.The effect of the composition,structure and preparation parameters of the adsorbent on the effect of fluorine removal has been ascertained,the mechanism of Mg-Fe-La adsorption and defluorination has been revealed,and the process parameters of adsorption and defluorination have been optimized.The main conclusions are as follows:（1）When the La/F ratio is≥1:3.05,most of the fluorine exists in the form of La F_x^3-xcomplexes,and the fluorine removal effect cannot be achieved.When 1:3.20≤La/F molar ratio≤1:3.10,it is favorable for the formation of La F₃.The La F_x^3-x existing at the same time tends to be adsorbed on the surface of La F₃ particles,forming a colloidal solution containing a large amount of La F₃·La F_x^3-x suspended matter.When the optimized La/F ratio was 1:3.10,the fluorine concentration was 6.42 mg/L,and the fluorine removal rate was 97.86%.Considering the existence of positively charged La F_x^3-x and La F₃·La F_x^3-x,a method of coagulating lanthanum salt and silicate to remove fluorine was proposed.When the La/F ratio is 1:3.00 and the amount of Na₂Si O₃ added is 0.50 g/L,the fluorine concentration is 2.24 mg/L,and the fluorine removal rate is 99.25%.When the dose of Na₂Si O₃ was increased to 1.00 g/L,the residual fluorine concentration could be further reduced to 0.80 mg/L.（2）In the solution containing SO₄^2-,lanthanum salt precipitation was carried out to remove fluoride.It was found that the precipitates generated in the simulated wastewater containing SO₄^2-were mainly irregular and massive La F₃ crystals.However,amorphous particles,which were deduced to be La[SO₄]F,were also present in the precipitate.Compared with La F₃,the F/La stoichiometric coefficient in La[SO₄]F is smaller and the utilization efficiency of La atoms is lower.Therefore,as the SO₄^2-concentration in the simulated wastewater increases,the residual fluorine concentration increases.The addition sequence of adding lanthanum salt first and then SO₄^2-can inhibit the formation of amorphous La[SO₄]F,which is beneficial to reduce the residual fluorine concentration.When the La/F molar ratio was 1:3.0,adding La（NO₃）₃?6H₂O and SO₄^2-（0.02 mol/L）sequentially could reduce the fluoride ion concentration in the simulated wastewater to 6.9 mg/L.A lower residual fluorine concentration（<2 mg/L）can be obtained by further increasing the La/F molar ratio,but it may cause the risk of La³⁺residue.（3）Co-precipitation method to prepare Mg-Fe-La adsorbent for adsorption and defluorination of low-fluorine wastewater after precipitation defluorination treatment.The optimal preparation conditions are Mg:Fe:La element ratio of 8:2:1 and precipitation The end point p H was 8.0.Using Mg-Fe-La to defluoride low-fluorine wastewater,the solution with an initial fluorine concentration of 10 mg/L can be reduced to 1.02 mg/L after defluorination,and the solution with an initial fluorine concentration of 5 mg/L can be reduced to to 0.29 mg/L,the adsorption and defluorination process conforms to the second-order kinetics,and its fitting coefficient R²=0.9941,the adsorption process is dominated by chemical adsorption,and the adsorption behavior conforms to the Langmuir isotherm adsorption model,indicating that the fluoride ion in Mg-Fe-La The adsorption on the composite material is monolayer adsorption,and the theoretical maximum adsorption capacity is 137.54 mg/g.The mechanism of adsorption and removal of fluorine is the effect of adsorption on the surface of the adsorbent and the complexation of fluorine ions and the metal and fluorine in the Mg-Fe-La composite adsorbent.The coexistence of impurity ions Cl^-,SO₄^2-,and CO₃^2-all have an inhibitory effect on the adsorption process,the influence of the three ions on the adsorption and removal of fluorine is from large to small:CO₃^2->SO₄^2->Cl^-.