Research On The Release And Migration Patterns Of Cd、Zn、As In Mixed Slag From Pyrite Mining And Smelting

Pyrite is the most widely distributed sulfide mineral, and has been used as the mainraw material to extract sulfur and make sulfuric acid. In the second half of the lastcentury indigenous method began to rise, which was adopted to refine sulfur, or extractsulfur and sulfuric acid for the sake of benefits in many regions. However, theindigenous method was rather backward and primitive with a low utilization rate ofpyrite resources, meanwhile, most slags were abandoned in the valley and mineralresources have mostly dissipated into the space, which resulted in heavy metalspollution. In order to seek suitable ways to solve the problems of slag threatsfundamentally, this dissertation discussed the patterns of heavy metal release andmigration by studying on slag samples and surrounding soil left over from pyritesmelting using indigenous methods.The whole slag yard takes an area of about1.35×106m2with an average elevationof1130m, and the stacking amount reaches about2.7×106m3. The slag density measuresaround1.14t/m3, and it is estimated that the stacking amount has exceeded above3.43×106t within the whole yard.The size of slag particles has big spans with coarse surface and irregular shapes. Itshows the sign of obvious corrosion and less heavy metals on the slag surface. The basicelements are observed in slags by XRF experiment. The major phase is SiO2in slags,including a large quantity of sediments, especially jarosite sediments （MFe3(SO4)2(OH)6）, which are attached to the slag surface, and they exsit as the form of salts suchas metallic oxide, sulphate, silicate, arsenic acid and carbonate. The surface waterdisplays obvious acidification with the pH average value of2.47. Through examingheavy metals in the stream water, the heavy metal amount has exceeded level Ⅲstandard of groundwater. During the biological experiment, the mixed microbes havebeen found in water samples, such as thiobacillus ferrooxidans and thiobacillusthiooxidans, which has accelerated the oxidation and release of heavy metals.The whole slag yard including surrounding soil is in a weak acidulous environment.The slags contain more heavy metals, especially As (Arsenic is a kind of metalloidelement, this paper collectively referred to as heavy metal for the convenience ofdescription.) and Zinc, which poses a threat to the environment. However, the leachingvalue of heavy metals is quite lower. Heavy metals are observed in the surrounding soilwith high content. Compared to standard values of soil quality, As has exceeded119 times more than the standards, also Ni, Zn, Cu, Pd and Cd are beyond allowableconcentrations. It indicates heavy metals in slag have migrated extensively with a widescope of effects on the environment after a long period of rain leaching.Through speciation analysis, it can be concluded that leachable state of Cd, Zn andAs measures27.98%,2.47%, and0.08%, respectively. It shows that, although the slaghas no leaching toxicity in a short term, it still contains higher leachable content andwill release gradually with the effect of stacking time, rain and microorganism, whichcauses higher environmental toxicity. Arsenic(As) exists as the form of residual fractionand reaches99.92%, and it shows that As forms relatively stable crystal structure duringthe weathering process and As is enriched as a result of continuous release and loss ofother heavy metals. The heavy metal experiment shows that, the heavy metal iscontinuously releasing and even accelerates the release under the conditions of air andmicrobes. The release model of heavy metals is obtained based upon the dynamicmodel.The migration experiment is designed to explore the permeation and diffusion ofleaching liquor of heavy metals in surrounding soil. The results indicate that themigration attribute varies as the concentration changes in solutions. When thepermeation coefficient of leaching liquor is lower than that of water in farmland soil, thehigher the leaching concentration, the lower the permeability coefficient, in other words,the permeability coefficient is inversely proportional to the concentration in solutions.Under the same permeation rate, the higher the concentration is, the lower thepermeability coefficient is. Both permeability and diffusion coefficient of unsaturatedsoil are less than that of the saturated soil, it means permeation and diffusion coefficientare proportional to the moisture content.Finally, the present research sets the exposure factors consistent with our nationalconditions by means of exposure and migration, and designs a risk assessment softwareaccording to toxicological data. Through RBCA risk assessment system calculation,people who have activities within the limits of sampling spots suffer from higher healthrisks. Therefore, repaire measures should be taken to reduce the risk up to an acceptablehealth leve.