外文翻译---利用冶金渣副产物去除酸性矿山废水中的污染物

Removal of pollutants from acid mine wastewaterusing metallurgical by-product slagsD. Feng a,∗, J.S.J. van Deventer a, C. Aldrich ba Department of Chemical and Biomolecular Engineering, The University of Melbourne, Melbourne, Vic., 3010, Australiab Department of Chemical Engineering, University of Stellenbosch, Private Bag X1, Matieland, 7602, Stellenbosch, South AfricaReceived in revised form 8 January 2004; accepted 12 January 2004AbstractTheremoval of pollutants from acid mine drainage using metallurgicalby-product slags was studied in laboratory scale. Metallurgicalby-product furnace slags were used as sorbents for metal ions and dispersed air column flotation was employed for the solid/liquid separationof the loaded slags. Batch sorption/pH/kinetic studies were conducted using simulated Cu and Pb bearing wastewater. The calcium glasstype of slags had high surface area and porosity. Promising result was succeeded from the combined process of slag sorption/flotation on thetreatment of an acid mine drainage from a South African gold mine.© 2004 Elsevier B.V. All rights reserved.Keywords: Furnace slag; Sorption; Flotation; Wastewater treatment; Acid mine drainage1. IntroductionVarious methods exist for the removal of toxic metalions from aqueous solution, viz. ion exchange, reverse osmosis,precipitation and adsorption, among others. Adsorptionis by far the most versatile and widely used process.Activated carbon has been the standard adsorbent for thereclamation of municipal and industrial wastewaters. Owingto the high-cost of activated carbon, production of itslow-cost alternatives has been the focus of research in thisarea for years. These sorbents for the heavy metals sorptionranged from natural materials to industrial and agriculturalby-products, such as fly ash, carbonaceous material, metaloxides, zeolites, moss, hydroxides, lignin, clays, biomass,peanut hulls, pyrite fines, goethite and coral sand.Furnace slags as metallurgical by-products are beingused as fillers or in the production of slag cement. It hasbeen reported that granulated furnace slag can be convertedinto an effective adsorbent and used for the removal ofdyes [1,2] and metal ions [3,4]. Alkaline-based slags asnon-conventional sorbents for various heavy metal ionscombine ion-exchange and sorption properties with anacid-neutralising ability. Acid mine water is an unavoidableby-product of the mining and mineral industry, especiallyas far as the oxidation of sulphide minerals is concerned.Acid mine waters typically contain high concentrations ofdissolved heavy metals and sulphate and can have a highturbidity and pH values as low as 2. These conditions mayprohibit discharge of untreated acid mine waters into publicstreams, as they have a detrimental effect on aquatic plantand fish life. Similarly, ground water pollution caused by thedrainage of acid mine water is an equally serious problem.Traditionally, acid mine water is neutralised by treatmentwith lime, resulting in concomitant precipitation of iron,aluminium and other metal hydroxides. However, since theminimum solubilities for the different metals usuallyfoundin the polluted water occur at different pH values and the hydroxide precipitates are amphoteric in nature, maximumremoval efficiency of mixed metals cannot be achieved at asingle precipitation pH level. Conventional sorbents are notacceptable in such a mal-condition as acidic high-turbiditymine drainage. Slags can be used as low-cost adsorbentsand neutralising agents and viable alternatives to the combinationof much more expensive activated carbon or ionexchange resins and lime.Slags exist often in a powdered form and are mainly appliedas dispersions. Downstream of the reaction tank, asuitable solid/liquid separation is generally necessary. Flota-tion offers various advantages for the scope of separation,compared with other processes such as filtration, sedimentationor centrifugation [5] and constitutes a known method ineffluent and water treatment [6,7]. Combination of adsorptionand subsequent flotation has proven to be an effectivemethod for the removal of heavy metals from wastewaterstreams [8–10].The present study involves an examination of the sorptioncapacities of two different slags for Cu and Pb removal fromwastewater streams. Batch sorption/pH/kinetic studies wereconducted in laboratory scale using simulated Cu and Pbbearing wastewater. Flotation of slags following ion sorptionoffers an effective way for solid/liquid separation. Alsoreported is the successful use of the novel technique in thetreatment of an acid drainage from a gold mine.2. Experimental workTwo furnace slags, viz. iron making slag and steel makingslag, were obtained from Saldanha Steel South Africain the form of powder with a mean particle size of 24.5and 24.1 m, respectively. The size distribution was: 100%and100m; 90% and 45 m; 22% and 10 m; and 1.2%and 1m for the iron slag, compared to 100% and 100m;90% and 45m; 23% and 10m; and 1.5% and 1m forthe steel slag. The chemical composition of the slags expressedas oxides in mass percentage is shown in Table 1.The XRD spectra obtained by a DRON X-ray diffractometerindicated that the calcium glass was the major phase inthe two slags. The slags were washed with distilled waterto remove the adhering impurities for three times and driedat 200 ◦C. The dried slags were stored in a desiccator forExperiments.HCl and NaOH (analytical grade from Merck) were usedto adjust the solution pH. The Cu and Pb stock solutionswere prepared by dissolving their corresponding chloride ornitrate salts (CuCl2 ·5H2O and Pb(NO3)2, analytical gradefrom Merck) in distilled water. The ion concentrations instock solutions were about 5000 mg/l. A cationic flocculant (polyamine type) was obtained from Monica, South Africa.Sodium dodecyl sulphate (analytical grade from Sigma) was used as a collector in flotation.Batch sorption experiments were carried out at an ambienttemperature of about 18 ◦C on a roller (60 min−1) using1 l screwed cap plastic bottles. The sorpt ion isotherm studieswere conducted by varying the initial ion concentrations.After a contact time of 24 h, the reaction mixture was filteredthrough a 0.45 m membrane filter (Millipore) and the filtratewas analysed for ion contents. The loading of the slagswas determined by the difference of the ion contents beforeand after adsorption equilibrium. All the kinetic experimentswere carried out at a constant temperature of 18 ◦Cin a 1 l round bottomed transparent plastic reaction vesselimmersed in a water bath. The solution in the vessel wasagitated with a glass impeller at a fixed speed of 600 min−1.All the experiments were duplicated with only the averagevalues being reported.The metal ion contents in solutions were determined byVarian Inductively Coupled Plasma (ICP). The solution pHvalues were detected by CRISON Micro pH 2000. Electrokineticmeasurements for the determination of point ofzero charge (PZC) were carried out on a Laser Zeta meter(Rank Brothers, Cambridge). The surface area of the samplewas measured by BET method on Micropores (ModelASAP 2010, Micromeritics Instrument Corporation, Norcross,GA). The common inorganic anions were determinedby a DIONEX AI450 ion chromatograph with a conductingdetector. For turbidity (NTU units) measurements, a nephelometerfrom HF Instruments was used.The test flotation column was made of glass having adiameter and height of 35 and 300 mm, respectively. Thesparged air was distributed through a sinter glass with poresize 4 (10–15m). The experiments were conducted at anambient temperature of 18 ◦C. The froth was overflowedautomatically. A certain amount of surfactant was added tothe slag slurry and was allowed to condition for 2 min priorto flotation in a beakerstirred by a magnetic stirrer. In thetreatment of the acid mine drainage, the cationic flocculantwas added to the slag slurry and conditioned for 3 min priorto the addition of the collector. The flotation efficiency wasdetermined by the removal of the slags through the changeof the solution turbidity.3. Results and discussion3.1. Sorption equilibriaTwo important physico–chemical aspects for the evaluationof the sorption process as a unit operation are the equilibriaof sorption and the kinetics. Sorption equilibrium isestablished when the concentration of metal in a bulk solutionis in dynamic balance with that of the interface. Fig. 1shows typical sorption isotherms of Cu2+ and Pb2+ on thetwo slags, respectively. The slurry pH was maintained at 5.5 and the slag doses were 2 g/l.Fig. 1.Sorption isotherms of Cu2+ and Pb2+ on the slags. IS representsiron slag and SS steel slag. The maximum standard deviation was within3%.As can be seen from Fig. 1, the iron slag had a muchhigher sorption capacity for heavy metals than the steel slagand Cu2+ had a higher loading on the slags than Pb2+ interms of molar numbers.The sorption isotherms of uptake metals by the slags couldbe expressed as Langmuir isotherms. The metal sorptionconstants for the Cu2+ and Pb2+ on the slags were calculatedfrom Langmuir plots and are given in Table 2. TheLangmuir parameters, Qmax (mg/g) and K (l/mg), are relatedto saturation capacity and the sorption binding constant, respectively.The sorption data for the slags were fitted toLangmuir isotherm equations. The sorption data in respectof the both metals provided an excellent fit to the Langmuirisotherm, giving high correlation coefficients for the slags.As can be seen from Table 2, the saturation capacity ofCu2+ was 88.50 mg/g for the iron slag and 16.21 mg/g for thesteel slag. The saturation capacity of Pb2+ was 95.24 mg/gfor the iron slag and 32.26 mg/g for the steel slag. The sorptionbindingconstants of the iron slag were much higherthan those of the steel slag, in well accordance with thefact that the iron slag had a much higher sorption capacitythan the steel slag. An adsorption capacity of around 40 mgPb2+/g on a granulated blast-furnace slag was reported forthe size fraction of <0.25mm [11]. This slag had a similarcomposition to the iron slag in this study, consisting of34% SiO2, 44% CaO, 6.4% Al2O3, 2.45% MgO and 0.5%Fe2O3. However, the iron slag had a much higher adsorptioncapacity than the reported slag in the literature, likelydue to a much smaller particle size and a higher surface areafor the iron slag.Fig. 2.Dependence of the slag loading with Cu2+ and Pb2+ on equilibriumsolutionpH. IS represents iron slag and SS steel slag.3.2. Effect of pH on sorptionThe original solution concentrations were about 200 mg/l,the slag doses were 2 g/l and the contact time was 24 h. Fig. 2illustrates the pH dependence of the slag loading withCu2+and Pb2+, respectively. Clearly, the metal uptake was quitelow at low pH levels. However, with an increase in solutionpH, a significant enhancement in sorption was recorded forboth slags, with the optimum pH values for the metal sorptionby the iron and steel slags being 3.5–8.5 and 5.2–8.5,respectively. This behaviour can be explained by consideringthe surface charges of the slags which depend, to a largeextent, upon the values of PZC of silica (about 2.3) andalumina (about 8.2) [12]. The composite PZC of the adsorbentslags, as determined by electrophoretic measurements,was found to be 3.2 for the steel slag and 4.8 for the ironslag. For the iron slag, the particle surface had a positivecharge density in the range of pH < 4.8. Under these conditionsthe uptake of metals would be quite low due to electrostaticrepulsion. With increasing pH, i.e. beyond PZC ofthe iron slag (>4.8) the negative charge density on the surfaceof the iron slag increased, thereby resulting in a suddenenhancement in the metal adsorption onto the iron slag.Likewise, the steel slag had a quite low metal loading ataround pH < 3.2 and a higher metal loading at pH > 3.2.In addition, the precipitation of heavy metal ions may contributeto the higher metal loadings on the slags at highersolution pH.3.3. Sorption kineticsThe slag doses were 2 g/l and the slurry pH was maintainedat 5.5. The initial concentrations for Cu2+ and Pb2+solutions were around 200 mg/l, respectively. The sorptionkinetic result is shown in Fig. 3. The sorption equilibriumfor the steel slag could reach within 1 h, while thesorption equilibrium for the iron slag could only reachtill 24 h.Fig. 3.Sorption kinetics of Cu2+ and Pb2+ on the slags. IS representsiron slag and SS steel slag.3.4. Sorption mechanismIn view of the nature of the slags, an exchange interactionof the slag glass with thesolution can be expressed as follows[11]:=SiOCa + 2HOH → =SiOH2 + Ca2+ + 2OH− (1)It can be expected that in acid environment the above reactionwould shift to the left due to the high concentration ofhydrogen ions. The basic slags had neutralising effect followingthe above route.Different amounts of slags were added into 1 l distilledwater and stirred at 600 min−1 for 24 h. Table 3 shows t hechanges of the solution pH and Ca2+ concentrations. Clearly,the calcium ion exchanged with the hydrogen ion and wasreleased from the slags into the solutions, while the solutionpH increased. This confirms that the reaction in Eq. (1)occurred in the contact of the slags with solutions. After 24 hcontact, the calcium concentration was much lower for thesteel slag than for the iron slag at the same doses. It can beinferred that the iron slag had a much higher ion exchangeability than the steel slag, which was in agreement with thesorption equilibria. In the presence of divalent heavy metalions (M2+) in solutions, the above equation can be describedas:=SiOCa +M2+ → =SiOM + Ca2+ (2)The slags had high surface area and porosity as indicatedin Table 1, which provided the basis for metal sorption aswell. As indicated by the microstructures of the slags inFig. 4. Typical topographic image for the original iron slag by SEM. Barlength is 1 m and the magnification 10000×.Fig. 5. Typical topographic image for the original steel slag by SEM. Barlength is 1 m and the magnification 10000×.Figs. 4–6, both slags revealed porous structures and the ironslag had smaller pores than the steel slag. After sorption,some loose flocs appeared at the slag surface as indicated inFig. 6. These flocs could be alumino silicates of copper orcopper hydroxide complexes depositing on the porous slagsurface.Fig. 6. Typical topographic image for the copper loaded iron slag bySEM. Bar length is 1 m and the magnification 10000×.3.5. Slag flotationSome loose and fairly small flocs appeared in the slurryafter 24 h slag adsorption. The solid/liquid separation bysimple sedimentation was not complete in short times (upto 10 h) and required a further stage of filtration. Bothion-exchange and metal precipitation contributed to theremoval of metal ions from wastewater by the slags. Thehydroxide species of the metal ions increased the turbidityof solutions after the adsorption. In addition, the presenceof some fine slag particles could also result in an increase insolution turbidity and this can, however, be avoided by usingcoarse slag particles. In order to remove the suspendedsolids, dispersed air column flotation was employed for thesubsequent separation of loaded slags from solutions. Theinitial Cu and Pb concentrations were 200 mg/l and the slagdoses were 2 g/l. The solution pH was maintained at 5.5.After 24 h contact, the slurry was subjected to flotation atan air flowrate of 200 ml/min. The average bubble size inthe flotation column was around 0.62 mm, based on frothimage analysis. The flotation result is shown in Table 4.As can be seen from Table 4, at a dose of 15 mg/lSDS thesolution turbidity after flotation was already under 1.00 NTU(tap water turbidity) for the slag slurry systems. Both Cu2+and Pb2+ concentrations further decreased after flotationand this effect became more prominent with increasing theSDS doses. This was attributed to the complexation of heavymetal ions with the collector SDS, as confirmed in the blanktests without slags. Adsorptive slag flotation, using smallamounts of slags as carriers, was found to be very effectivefor the treatment of solutions containing heavy metals suchas copper and lead.Settling should also be an effective process leaving a clearsupernatant with the aid of a typical flocculant. Settling issimpler and relatively cheaper. However, flotation is fasterand more effective in recovering fine particles in comparisonwith settling. In the current study, fine slag powderswere used as the sorbents. Therefore, flotation was chosenas the subsequent solid/liquid separation process. In casethat coarse slag particles are used for wastewater treatment,settling process should be favourable. Sorption of pollutantsin packed columns with coarse slag particles can be efficient,despite a lower adsorption area. This will avoid theuse of the subsequent flotation or settling for solid/liquidseparation.3.6. Treatment of an acid mine drainage by flotation ofadsorptive slagsThe acid mine water used in the experiments was sampledfrom the drainage of a South African gold mine. The acidmine water had a very low pH of 2.03 and a very highsulphate concentration, as indicated in Table 5.The acid mine water is neutralised with lime in site, resultingin concomitant precipitation of iron, aluminium andother metal hydroxides. Treatment with lime requires a shortreaction period, owing to its high solubility (0.15%). However,since the minimum solubilities for the different metalsusually found in the polluted water occur at different pHvalues and the hydroxide precipitates are amphoteric in nature,maximum removal efficiency of mixed metals cannotbe achieved at a single precipitation pH level around 7. Theslags combine ion-exchange and sorption properties with anacid-neutralising ability and may offer an alternative for theremoval of metal ions from the acid drainage.Batch sorption experiments were conducted with 1 l of theacid mine drainage and the contact time was 24 h. The effectof slag doses on the removal of ions from the acid minedrainage is shown in Table 5. The slurry pH increased toneutral pH as the slag dose increased up to 30 g/l. Most of theheavy metals such as Cu, Pb, Cr were removed at an iron slagdose of 30 g/l and the residual concentrations could meet thedischarge standards (<0.05 mg/l). The iron slag had a betterperformance in heavy metal ion removal than the steel slag.The precious metal ions such as Au, Ag, Pt, Pd, Rh and Ruwere only removed to a small extent. In general, the preciousmetal ions exist in the forms of anionic complexes withchloride in the acidic environment, such as AuCl4−, PdCl62−and PtCl42−. The anionic complexes could not physicallyadsorb onto the negatively charged slag surface at pH >ZPC and could not exchange with Ca2+ in the slags. Theremoval of some of the precious metal ions may be attributedto the co-flocculation with the Fe(OH)3 flocs formed due tothe increased pH.The other anionic ions except for PO43−were also difficult to remove owing to the same reason forthe anionic precious metal complexes. PO43− was removedas low solubility calcium phosphate precipitates. Ca and Mgcould form some difficult-to-dissolve precipitates with F−,PO43− and SO42− depositing on the slag surface.After 24 h contact with 30 g/l slags, a substantial amount of loose andfairly small flocs appeared in the slag slurry.a Ion concentration in mg/l.Direct flotation by SDS was not very effective for thesolid/liquid separation. A cationic flocculant polyamineat an optimised concentration of 100 mg/l was added intothe slurry and conditioned for 3 min prior to the additionof SDS. The fine flocs and slags formed large andwell-structured aggregates with the addition of polyamine,which were readily removed by the subsequent flotationprocess. The flotation result is shown in Table 6. Obviously,the residual concentrations of all the base metal and preciousmetal ions in the solution were low enough for dischargeinto the public streams, solution pH was neutral and thea Ion concentration in mg/l.solution turbidity was under 1.0 NTU (tap water) after theslag neutralisation and sorption, flocculation and flotationprocess. The cationic flocculant not only flocculatedthe softfine particles to form large aggregates, but also served as acarrier for the removal of anions such as anionic preciousmetal complexes due to the electrostatic attraction. Afterflocculation, the aggregates were more resistant to shearingand suitable for the subsequent flotation. The iron slag had abetter performance than the steel slag for the mine drainagetreatment in terms of the final residual ion concentrations.Finally, it is believed that the concept of adsorptive slagflotation should be stressed by using the non-conventionalsorbents of metallurgical by-product slags and high capacityflotation techniques. The case study only focused on thetreatment of acid mine drainage and it can be expected thatthis process may lead to an alternative future technology inthe field of inorganic and organic contaminants removal4. ConclusionsThe iron and steel making slags were appropriate sorbentsfor heavy metal removal from aqueous solutions. Theslags combined ion-exchange and sorption properties withan acid-neutralising ability. The iron slag had a much highersorption capacity for metals than the steel slag owing to itshigher surface area, higher porosity and higher ion-exchangeeffectivelyseparate the slags, yielding very low solution turbidities.Advantages when compared with settling were observedin terms of process kinetics and quality of the supernatants.The adsorbing slag flotation is particularly attractive for theremoval of heavy metal ions from acid streams and has obtainedpromising result on the treatment of an acid minedrainage from a South African gold mine.ability. Flotation following the slag sorption could effectively References[1] V.K. Gupta, S.K. Srivastava, D. Mohan, Ind. Eng. Chem. Res. 36(1997) 2207.[2] K.R. Ramakrishna, T. Viraraghavan, Waste Manage. 17 (8) (1997)483.[3] S.K. Srivastava, V.K. Gupta, D. Mohan, J. Environ. Eng. Div. Am.Soc. Civ. Eng. 123 (1997) 461.[4] V.K. Gupta, A. Rastogi, M.K. Dwivedi, D. Mohan, Sep. Sci. Technol.32 (1997) 2883.[5] K.A. Matis, A.I. Zouboulis, I.C. Hancock, Sep. Sci. Technol. 29(1994) 1055.[6] Th.F. Zabel, in: P. Mavros, K.A. Matis (Eds.), Innovations in Flotation Technology, 1992, Kluwer Academic, Dordrecht, The Netherlands,p. 431.[7] A.I. Zouboulis, K.A. Matis, K.A. Stalidis, G.A. in: P. Mavros, K.A.Matis (Eds.), Innovations in Flotation Technology, 1992, KluwerAcademic, Dordrecht, The Netherlands, p. 475.[8] C. Aldrich, D. Feng, Minerals Eng. 13 (10–11) (2000) 1129.[9] J. Rubio, F. Tessele, Minerals Eng. 10 (7) (1997) 671.[10] A.I. Zouboulis, K.A. Kydros, K.A. Matis, Water Res. 29 (1995) 1755.[11] K.K. Panday, G. Prasad, V.N. Singh, Water Res. 19 (1985) 869.[12] S.V. Dimitrova, D.R. Mehandgiev, Water Res. 32 (11) (1998) 3289.利用冶金渣副产物去除酸性矿山废水中的污染物摘要在实验室规模，研究利用冶金副产物渣料去除酸性矿物排泄的污染物。