Study On The Effect Of Impurity Ions On Arsenic Precipitation From Scorodite By Hydrothermal Method

Most of arsenic in nature is associated with non-ferrous mineral resources.A large number of arsenic containing wastewater is produced in the smelting process of non-ferrous metals,which is characterized by high acidity,high arsenic concentration and rich in various heavy metals(Cu²⁺,Zn²⁺),alkali metals?Na⁺,K⁺?,alkaline earth metals(Mg²⁺,Ca²⁺),etc.Because of its advantages of high stability,high theoretical arsenic content and easy solid-liquid separation in crystalline state,scorodite is recognized as an ideal arsenic curing product in recent years.In the process of arsenic precipitation by hydrothermal scorodite method,the impurity ions in the arsenic containing waste water have a significant effect on the arsenic precipitation process and the phase and composition of arsenic precipitate,thus affecting the stability of arsenic precipitate.In this paper,the mechanism of heavy metals(Cu²⁺,Zn²⁺),alkali metals?Na⁺,K⁺?,alkaline earth metals(Mg²⁺)and other impurity ions in arsenic bearing wastewater on the process of arsenic precipitation by hydrothermal scorodite method,as well as the effect of them on the precipitation rate of arsenic,the phase and composition of arsenic recipitate,the micro morphology,the particle size,and the metastable iron phase transition behavior of the process are studied.The following results are obtained:?1?Compared with the improved atmospheric method and hydrothermal method,the hydrothermal method is better than the improved atmospheric method in the micro morphology and stability of arsenic precipitate.When the hydrothermal method is used to precipitate arsenic,the arsenic content in arsenic residue is as high as 29.61%,the arsenic precipitation rate is as high as 96.4%,and the arsenic concentration in the leaching toxicity test is as low as 0.12mg/L,reaching the concentration of general solid waste leaching toxicity as It is lower than the standard of 0.5mg/L,so hydrothermal method is more suitable for the harmless treatment of arsenic containing wastewater.?2?Due to the catalysis of Cu²⁺/Cu⁺ion pair,the oxidation rate of ferrous is accelerated,which promotes the hydrothermal precipitation reaction of metastable basic iron sulfate.When the initial concentration of Cu²⁺is 20 g/L,the precipitation rates of As and Fe are 96.9%and 74.9%respectively.As a result of the same ion effect,the increase of Zn²⁺concentration leads to the increase of SO₄^2-concentration in the system,which promotes the oxidation of Fe SO₄ion pair,makes Fe³⁺more easily reach the supersaturation state,leads to the formation of metastable phase of jarosite,and forms arsenic precipitate mainly composed of scorodite with trace jarosite and basic ferric sulfate.When the initial mass concentration of Zn²⁺is 20 g/L,the mass fractions of As,Fe and S in arsenic slag are 23.5%,28.5%and 3.7%respectively,in which Zn is mainly coated on the surface of scorodite in the form of polymer.?3?Alkali metal ions have a significant effect on the arsenic deposition process and the phase composition of arsenic precipitate.When Na⁺and K⁺exist in the arsenic precipitation system,metastable iron phases such as Na-jarosite and K-jarosite are more likely to form.The increase of Na⁺concentration is beneficial to the formation of Na-jarosite,while the formation of scorodite is gradually inhibited.When the concentration of Na⁺reaches 10 g/L,the main phases of arsenic precipitate are ferric arsenate sub-hydrate?Fe As O₄?0.75H₂O?and Na-jarosite,and the contents of Na and S in arsenic precipitate are 1.7%and 4.6%respectively.The presence and concentration of K⁺in the system lead to the increasing content of K-jarosite and the rapid increase of iron deposition rate,and the formation of K-jarosite leads to the obvious change of the micro morphology of arsenic precipitate.At the same time,the leaching toxicity test results show that the existence of Na⁺,K⁺leads to the decrease of precipitate stability.?4?The existence of Mg²⁺in the system will lead to the increase of S content in the precipitate,and the iron deposition rate will also increase from 61.6%to 75.5%.Through the kinetic equation fitting,it is found that the ferrous oxidation is a two stage reaction,and with the increase of the initial Mg²⁺concentration,the rate of ferrous oxidation is accelerated.The molecular structure infrared spectrum analysis of the arsenic deposition precipitate shows that the ferrous arsenate partially forms the basic ferric arsenate sulfate?BFAS?phase at high temperature.?5?According to the thermodynamic analysis of the arsenic precipitation process,the standard Gibbs free energy of the three phases of Na-jarosite,K-jarosite and iron arsenate is relatively close at high temperature,and there is a phenomenon of competitive precipitation,and the reaction process is entropy increasing enthalpy increasing,which can be carried out spontaneously at high temperature;the stable structure total energy of the three phases of Na-jarosite,ferric arsenate sub-hydrate and iron arsenate is fitted.It is found that the ferric arsenate sub-hydrate has poor stability and is easy to decompose or transform into scorodite.The experimental results show that the formation of metastable phase of Na-jarosite can be inhibited by properly reducing the initial p H,reaction time and reaction temperature,which is beneficial to obtain high purity arsenolite precipitate,and the transformation of ferric arsenate sub-hydrate to scorodite can be realized by prolonging the reaction time.