Study On Stabilization Treatment Of Arsenic-containing Waste Residues

Arsenic-containing waste residues belong to the hazardous wastes containing arsenic specified in the list of"Hazardous Waste List".These residues have leaching toxicity,and their random stacking without the required disposal treatments will cause serious arsenic pollution in water and soil.As arsenic is often associated with non-ferrous metals,a large amount of arsenic-containing water/acid will be produced during the mining and smelting of non-ferrous metal ore.The lime-iron salt method and the sulfide precipitation method can be used in the treatment of arsenic-containing water/acid to produce calcium arsenate residue and arsenic sulfide residue,respectively.These two types of precipitated slags are the main sources of arsenic-containing waste residues and have strong environmental hazards.This thesis takes is aiming at the treatment of the calcium arsenate residue and arsenic sulfide residue provided by an environmental protection company.After the basic characteristics of the two types of residues were acquired,stabilization methods via H₂O₂-Ca O oxidation stabilization and wet milling of zero-valent iron were developed,followed by clafication of the stabilization mechanism.The main findings are as follows:（1）The composition of the calcium arsenate residue was complex.Most of the present arsenic and calcium were combined in the form of amorphous compounds.The leaching concentration of arsenic was measured as high as 650 mg/L,beingmuch higher than“less than 1.2 mg/L”as required by the"Hazardous Waste Landfill Pollution Control Standard"（GB 18598-2019）.The stabilizing effect of Ca O on calcium arsenate residue is better than that of iron salt and aluminum salt,but the addition of 20%（wt%）of Ca O can only reduce the leaching concentration of arsenic to 2.5 mg/L.Therefore,we developed a two-step method to stabilize the calcium arsenate residue:the first step is oxidation conversion,and the second one is reagent stabilization.According to the new method,the treatment by using2%H₂O₂ and 6%Ca O sucessively decreased the arsenic leaching to 0.97 mg/L.The results of XPS and XRD show that the oxidation of H₂O₂ reduced the proportion of As（Ⅲ）in calcium arsenate residue from 33.8%to 22.7%.Ca O can increase p H,increase active calcium ions,and form more insoluble Ca₃（As O₄）₂·2H₂O crystal compound,after treatment,the arsenic occurrence form is transformed from unstable acid-soluble state to more stable reducible state and residue state,which increases the stability of arsenic in calcium arsenic slag.（2）The arsenic content in the arsenic sulfide residue as as high as 32%,and As and S were mainly combined in the form of amorphous precipitation,being accompanied by some elemental S and As₂O₃ formed by oxidation.The arsenic sulfide residue was strongly acidic,and the leaching concentration of arsenic was as high as 18484.9 mg/L,which is very difficult to handle.A method for treating arsenic sulfide residue with reduced iron powder as a stabilizer and wet ball milling was established.The amount of reduced iron powder added was n（Fe）:n（As）=2:1,and the ball to powder ratio was 20:1.Under the conditions of ball milling time of 2 h and ball mill speed of 300 r/min,the leaching concentration of arsenic can be reduced from 18484.9 mg/L to 56.3 mg/L,and the stabilization efficiency of arsenic sulfide residue can reach 99.7%.The results of SEM,XRD and XPS show that under the action of ball milling,reduced iron powder can produce corrosion products in an acidic water environment.The corrosion products make the arsenic oxide and amorphous arsenic sulfide in the arsenic sulfide slag disappear and transform into a combined form with the corrosion products of the reduced iron powder.After treatment,the occurrence form of arsenic changes from the acid-soluble,reducible,and oxidizable states to a stable residue state,and the proportion of the residue state increases from 13.0%to 83.7%.The stabilization treatment of arsenic in arsenic sulfide slag is realized.