CaCo₃-Ca?OH?₂-Fe??? Process For Arsenic Removal From Dirty Acid Wastewater And Solidification Of Arsenic In Lime-Arsenic Slag

Sulfuric acid is widely used in agriculture,metallurgy industry,national defense,and textile industry,which also plays an important role in producing raw chemical materials.It was estimated that sulfuric acid-containing wastewater in China amounted to 100 billion tons in 2015,and 30%of them were from smelting off-gas.During the production process,wastewater,known as dirty acid wastewater?DAW?,is the main by-product.DAW typically contains 0.5?1.0 M sulfuric acid,high-concentrated arsenic and some extremely toxic heavy metals.In general,the concentration of arsenic in DAW ranges from 0.5 to 3 g L^-1.In some cases,the concentration of arsenic in DAW could be as high as 10 g L^-1.A direct discharge without effective removal of arsenic from DAW could cause serious damages to the ecological environment.To treatment the DAW discharging from sulfuric acid plant,this study developed an efficient and economical method to remove arsenic from DAW through a continuous three-stage co-precipitation processing,which includes?1?formation of gypsum by addition of calcium carbonate,?2?neutralization of the filtrate with lime,and?3?adsorption of residual arsenic by ferric sulfate.In the 1^st stage,the gypsum formed from using Ca CO₃ showed higher crystal purity than those using traditional neutralizers such as Ca O and Ca?OH?₂,and the precipitates are recyclable due to the fact that carbon dioxide?CO₂?emitted in neutralization could prevent gypsum from shielding the surface of the neutralizer.In the 2^nd stage,Ca?OH?₂ was used to get a p H of 11.9 for arsenic removal.CO₃^2-shows little effect on arsenic removal rate and stability of precipitates.In the 3^rd stage,Fe₂?SO₄?₃ was utilized to further remove arsenic to make the concentration below than 0.2 mg/L by adsorption at p H 9.0.Using this novel 3-stage processing,the gross weight of As-containing hazardous waste was sharply reduced to as high as 50%.The property research of high-arsenic content lime-arsenic slag was studied in terms of its corrosivity,chemical composition,sedimentation property,surface morphology,arsenic forms distribution,arsenic leaching toxicity.The results showed that the lime-arsenic slag had a good sedimentation property,and arsenic was present mainly in exchangeable and residual forms.Because of high-arsenic content?29.3%?and arsenic leaching concentration was higher than 5mg/L,the lime-arsenic slag must need for stabilization/solidification treatment.In addition,the lower of the leaching liquid p H,the higher of the arsenic leaching concentration.The arsenic leaching concentration would also increase,due to SO₄^2-and CH₃COO^-reaction with calcium ions.The arsenic leaching concentration of lime-arsenic slag would be in vacuum drying and air dry.And the arsenic leaching concentration of lime-arsenic slag by vacuum drying was lower than it by air dry.XRD analysis shows that a lot of calcium carbonate was formed in air dry.On the contrary,there were some limes in vacuum drying.The TG analysis also has proved it.This process reduced the p H of lime arsenic slag leaching liquid,so the arsenic leaching concentration would increase.From the EDS analysis,CO₂ from the air would react with lime,calcium arsenate and calcium arsenite in air dry,so it increased the arsenic leaching concentration of lime-arsenic slag.SEM analysis shows,the surface morphology of lime-arsenic slag by vacuum drying was honeycomb membrane,but it changed into cellular honeycomb in air dry.Through the above study shows,the lime-arsenic slag was hazardous waste and it needs to be stabilized treatment.Then,a kind of lime-arsenic slag,containing large amounts of arsenic,was stabilized/immobilized by lime,cement and medicament.The influence of arsenic leaching concentration with curing agent was investigated under sealed and smashed condition.The results showed that the leaching concentration of arsenic were 2.99 mg/L and 146.5 mg/L in the sealed and smashed condition,when the addition of lime was 30%.According to the results of XRD and TG,CO₂ from the air would react with lime,calcium arsenate and calcium arsenite under the smashed condition,which leaded to release arsenic.And the surface morphology of it has changed from compact to disperse.However,compression molding could increase the compactness of sample and reduce it poriness.Such processes could make arsenic leaching concentration low to3.43 mg/L under the smashed condition for 28 d,which below the national standard?5 mg/L?.