Study On The Solidification Of Arsenic-containing Waste Residues By Chemical Bonding Materials And Their Mechanism Of Action

The arsenic-containing waste,produced in the process of mining,selecting and smelting of non-ferrous metal minerals,is liable to be re-dissolved into the environment by natural forces,such as precipitation and wind during storage process,resulting in serious secondary environmental pollution.Solidification/stabilization technology is one of the main ways of dealing with toxic and hazardous solid wastes internationally.Traditional cement-based materials are widely used,due to their simple process and low cost,however,they have some shortcomings such as poor durability and low curing capacity.Therefore,two kinds of chemically bonded ceramics,blast furnace slag?BFS?-fly ash?FA?based alkali activated materials?AAMs?and copper slag based oxalate acid-base cement,were used to solidify arsenic-containing solid wastes including calcium arsenates waste and ferrous arsenates waste according to their physiochemical characteristics.Chemically bonded ceramics have the properties of fast setting,high compressive strength and warm reaction and can effectively immobilize arsenate-containing solid wastes through physical encapsulation,physical adsorption and chemical bond.This research can not only resolve the problems about the industrial solid wastes,but also make the idead that‘waste control by waste'and‘pollution control by waste'come true.BFS-FA based AAMs are used to solidify/immobilize the calcium arsenate waste,and the influences of the factors including BFS/FA mass ratio,modulus?WGN?and dosages?WGC?of activators,curing regim and addition amount of arsenate waste on the solidified forms were researched.The results showed that when BFS/FA was 2.0,WGN was 0.84,WGC was 4.0%and the addition amount of arsenate waste was up to 40%,the compressive strength of the solidified form reached 27.19MPa and the arsenic leaching concentration was 1.36 mg/L,far lower than the national standard limited value?5.0mg/L?.The structure and phase composition of Arsenic calcium slag solidified body were characterized by SEM/EDS,FTIR and XRD.In addition,Metakaolin was used instead of slag and fly ash,and sodium arsenate was used instead of Arsenic-calcium slag.And then,the reaction conditions of the curing system were simulated by hydrothermal method,and thealuminosilicate solid with arsenic was synthesized and analyzed.Through characterization and simulation,the solidification mechanism of Arsenic-calcium slag solidified by alkali-activated chemically bonded materials is proposed for the first time in this paper.It is the result of physical encapsulation,adsorption,transformation of highly soluble arsenic-containing mineral into more stable mineral phases and the partial substitutionof SiO₄^4-by AsO₄^3-into the long chain of Silicon alumina oxygenOxalate acid-base cement with fast setting behavior and high compressive strength was prepared through the acid-base reaction between iron-rich copper slag and oxalic aicd.The effects of factors such as addition amount of oxalic acid and retardant,particle distribution of copper slag and curing regime on the physical property and setting behavior of oxalate cement.It was shown that when the addition amount of oxalic acid was 14%,borax was 8%and the residue on 80 mesh siveve was no higher than 5%,the initial setting time was up to 45min and the compressive strength reached 39.6MPa.Moreover,the oxalate acid-base cement paste specimen showed good water resistance.The formation mechanism of oxalate cement was investigated by means of XRD and FTIR.Ferrous oxalate with chain-like structure was formed during the hydration reaction,and acted as binder phase to connect the unreacted falyte.H⁺from oxalic acid promotes the dissolution of ferrous of copper slag,and the dissolved Fe²⁺and C₂O₄^2-form a stable five-member chelating ring via coordination bonds.Eventually,FeC₂O₄·2H₂O chemical bonded macromolecules with chain-like structure is formed.Moreover,there are incompletely reacted iron olivine?Fe₂SiO₄?and iron-silica oxide(Fe_2.95Si_0.05O₄)in the curing body of oxalate acid-base cement,which can play an aggregate role in the curing body.Solidification of arsenic and iron slag was conducted using copper slag-based oxalate acid-base cement.The results shows that the maximum solidification capacity can reach 50%,and the initial solidification time is 34 minutes.For 3 days of natural curing,the compressive strength of arsenic slag solidified body is 15.56MPa,and the arsenic leaching concentration is 4.12mg/L,which meets the national standard requirements for safe storage of hazardous wastes.It is believed that amorphous arsenic is the main reason for high arsenic leaching from arsenic-iron slag.Based on the characterization and analysis of the structure,the solidification and stabilization of arsenic-iron slag by copper slag-based oxalate acid-base cement is the result of transformation of amorphous arsenic into arsenopyrite,the formation of pharmacosiderite by the reaction of free arsenic and iron in the system,and physical encapsulation of arsenic-containing waste and adsorption of free arsenic by Silica gel phase produced during the solidification.Under the three fixed sealing,copper slag-based oxalate acid-base cement has achieved good solidification effect on arsenic and iron slag.