Stabilization Of Arsenic-bearing Sludge By Carbide Slag

In China,sulfuric acid industry is developing rapidly,increasing the yield of sulfuric acid.Because of the treatment of arsenic-bearing wastewater from the sulfuric acid plants,a large amount of arsenic-bearing sludge was produced,which could pose severe threats to surrounding ecological environment and human health due to arsenic（As）release from such sludge caused by weathering process and rainwater leaching.Therefore,the arsenic-bearing sludge needs to be disposed of promptly and reasonably.Stabilization technologies could be practicable for the treatment of As-bearing sludge via reducing its environmental risks,which have strong applicability and high economic efficiency.Carbide slag,a solid waste from acetylene production industry,could be used as a calcium-containing agent for the stabilization treatment of As-bearing sludge,since it’s mainly composed of Ca（OH）₂.The application of carbide slag for As-bearing sludge stabilization totally agrees with the purpose of“treating waste by waste”,promoting green development of circular economy.In this study,carbide slag was used as the main stabilizing agent,while bleaching powder acted as an auxiliary oxidant when needed.Nine different As-bearing sludge samples from four sulfuric acid production plants in Yunnan Province,China were studied.Specific methods of stabilization treatment and related mechanisms were investigated.The main contents of the dissertation are summarized as follows:The basic physical and chemical properties,elemental composition and content,leaching toxicity,and phase composition of the nine kinds of As-bearing sludge and carbide slag were analyzed.It was found that the composition of these As-bearing sludge is complex,and the leaching concentration of As is extremely high,greatly exceeding the limit value of As in the“Identification Standards for Hazardous Wastes”.Therefore,they are urgently needed to be disposed of reasonably.Based on the basic composition and characteristics of As-bearing sludge,optimization studies on the technological conditions of stabilization,oxidation and maintenance were carried out to develop a cost-effective method for the stable disposal of As-bearing sludge.The study found that the four kinds of sludge which mainly contain As（V）in nine kinds of sludge can be directly stabilized with 30%carbide slag,while the five kinds of sludge with As（III）higher need to add 6%-15%bleaching powder for oxidation treatment.The leaching concentration of arsenic in nine kinds of As-bearing sludge after stabilization treatment was lower than 2.5 mg L^-1,and the stabilization rate of arsenic reached more than 98.5%.The treated sludge can maintain long-term stability under the sealed storage conditions of normal temperature simulating landfill conditions.Based on the market prices of carbide slag and bleaching powder,the cost of the stabilization treatment of such sludge was estimated to be no more than 210 yuan/t,which is about 1/3 of the existing technology.The morphological distributions of As as arsenate and arsenite at different p H conditions were simulated by MINTEQ.Under the strong alkaline and oxidative conditions,the synergistic effect of calcium carbide slag and bleaching powder on the stabilization of arsenic-bearing sludge was clearly defined.Based on the valence analysis of As,the contents of As（III）in the original sludge leachates were variant,thus controlling the content of As（III）was the key to stabilization treatment.The mineral compositions of treated and untreated sludge were analyzed and it was found that the formation of calcium arsenic minerals was the main mechanism of stabilization treatment.Through SEM-EDS analysis,the positive effect of the gelation of carbide slag on stabilization treatment was proved.