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.docx

removal of pollutants from acid mine wastewater using metallurgical by-product slags d. 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 africa received in revised form 8 january 2004; accepted 12 january 2004abstractthe removal of pollutants from acid mine drainage using metallurgical by-product slags was studied in laboratory scale. metallurgical by-product furnace slags were used as sorbents for metal ions and dispersed air column flotation was employed for the solid/liquid separation of the loaded slags. batch sorption/ph/kinetic studies were conducted using simulated cu and pb bearing wastewater. the calcium glass type of slags had high surface area and porosity. promising result was succeeded from the combined process of slag sorption/flotation on the treatment 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 metal ions from aqueous solution, viz. ion exchange, reverse osmosis, precipitation and adsorption, among others. adsorption is by far the most versatile and widely used process.activated carbon has been the standard adsorbent for the reclamation of municipal and industrial wastewaters. owing to the high-cost of activated carbon, production of its low-cost alternatives has been the focus of research in this area for years. these sorbents for the heavy metals sorption ranged from natural materials to industrial and agricultural by-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 being used as fillers or in the production of slag cement. it has been reported that granulated furnace slag can be converted into an effective adsorbent and used for the removal of dyes 1,2 and metal ions 3,4. alkaline-based slags as non-conventional sorbents for various heavy metal ions combine ion-exchange and sorption properties with anacid-neutralising ability. acid mine water is an unavoidable by-product of the mining and mineral industry, especially as far as the oxidation of sulphide minerals is concerned. acid mine waters typically contain high concentrations of dissolved heavy metals and sulphate and can have a high turbidity and ph values as low as 2. these conditions may prohibit discharge of untreated acid mine waters into public streams, as they have a detrimental effect on aquatic plant and fish life. similarly, ground water pollution caused by the drainage of acid mine water is an equally serious problem. traditionally, acid mine water is neutralised by treatment with lime, resulting in concomitant precipitation of iron, aluminium and other metal hydroxides. however, since the minimum solubilities for the different metals usually found in 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 a single precipitation ph level. conventional sorbents are not acceptable in such a mal-condition as acidic high-turbidity mine drainage. slags can be used as low-cost adsorbents and neutralising agents and viable alternatives to the combination of much more expensive activated carbon or ion exchange resins and lime. slags exist often in a powdered form and are mainly applied as dispersions. downstream of the reaction tank, a suitable solid/liquid separation is generally necessary. flota-tion offers various advantages for the scope of separation, compared with other processes such as filtration, sedimentation or centrifugation 5 and constitutes a known method in effluent and water treatment 6,7. combination of adsorption and subsequent flotation has proven to be an effective method for the removal of heavy metals from wastewater streams 810. the present study involves an examination of the sorption capacities of two different slags for cu and pb removal from wastewater streams. batch sorption/ph/kinetic studies were conducted in laboratory scale using simulated cu and pb bearing wastewater. flotation of slags following ion sorption offers an effective way for solid/liquid separation. also reported is the successful use of the novel technique in the treatment of an acid drainage from a gold mine.2. experimental worktwo furnace slags, viz. iron making slag and steel making slag, were obtained from saldanha steel south africa in the form of powder with a mean particle size of 24.5 and 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 for the steel slag. the chemical composition of the slags expressed as oxides in mass percentage is shown in table 1.the xrd spectra obtained by a dron x-ray diffractometer indicated that the calcium glass was the major phase in the two slags. the slags were washed with distilled water to remove the adhering impurities for three times and dried at 200 c. the dried slags were stored in a desiccator forexperiments. hcl and naoh (analytical grade from merck) were used to adjust the solution ph. the cu and pb stock solutions were prepared by dissolving their corresponding chloride or nitrate salts (cucl2 5h2o and pb(no3)2, analytical grade from merck) in distilled water. the ion concentrations in stock 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 ambient temperature of about 18 c on a roller (60 min1) using 1 l screwed cap plastic bottles. the sorption isotherm studies were conducted by varying the initial ion concentrations. after a contact time of 24 h, the reaction mixture was filtered through a 0.45 m membrane filter (millipore) and the filtrate was analysed for ion contents. the loading of the slags was determined by the difference of the ion contents before and after adsorption equilibrium. all the kinetic experiments were carried out at a constant temperature of 18 c in a 1 l round bottomed transparent plastic reaction vessel immersed in a water bath. the solution in the vessel was agitated with a glass impeller at a fixed speed of 600 min1. all the experiments were duplicated with only the average values being reported.the metal ion contents in solutions were determined by varian inductively coupled plasma (icp). the solution ph values were detected by crison micro ph 2000. electrokinetic measurements for the determination of point of zero charge (pzc) were carried out on a laser zeta meter (rank brothers, cambridge). the surface area of the sample was measured by bet method on micropores (model asap 2010, micromeritics instrument corporation, norcross,ga). the common inorganic anions were determined by a dionex ai450 ion chromatograph with a conducting detector. for turbidity (ntu units) measurements, a nephelometer from hf instruments was used. the test flotation column was made of glass having a diameter and height of 35 and 300 mm, respectively. the sparged air was distributed through a sinter glass with pore size 4 (1015m). the experiments were conducted at an ambient temperature of 18 c. the froth was overflowed automatically. a certain amount of surfactant was added to the slag slurry and was allowed to condition for 2 min prior to flotation in a beaker stirred by a magnetic stirrer. in the treatment of the acid mine drainage, the cationic flocculant was added to the slag slurry and conditioned for 3 min prior to the addition of the collector. the flotation efficiency was determined by the removal of the slags through the change of the solution turbidity.3. results and discussion3.1. sorption equilibriatwo important physicochemical aspects for the evaluation of the sorption process as a unit operation are the equilibria of sorption and the kinetics. sorption equilibrium is established when the concentration of metal in a bulk solution is in dynamic balance with that of the interface. fig. 1 shows typical sorption isotherms of cu2+ and pb2+ on the two 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 much higher sorption capacity for heavy metals than the steel slag and cu2+ had a higher loading on the slags than pb2+ in terms of molar numbers. the sorption isotherms of uptake metals by the slags could be expressed as langmuir isotherms. the metal sorption constants for the cu2+ and pb2+ on the slags were calculated from langmuir plots and are given in table 2. the langmuir parameters, qmax (mg/g) and k (l/mg), are related to saturation capacity and the sorption binding constant, respectively. the sorption data for the slags were fitted to langmuir isotherm equations. the sorption data in respect of the both metals provided an excellent fit to the langmuir isotherm, giving high correlation coefficients for the slags. as can be seen from table 2, the saturation capacity of cu2+ was 88.50 mg/g for the iron slag and 16.21 mg/g for the steel slag. the saturation capacity of pb2+ was 95.24 mg/g for the iron slag and 32.26 mg/g for the steel slag. the sorption binding constants of the iron slag were much higher than those of the steel slag, in well accordance with the fact that the iron slag had a much higher sorption capacity than the steel slag. an adsorption capacity of around 40 mg pb2+/g on a granulated blast-furnace slag was reported for the size fraction of 0.25mm 11. this slag had a similar composition to the iron slag in this study, consisting of 34% sio2, 44% cao, 6.4% al2o3, 2.45% mgo and 0.5% fe2o3. however, the iron slag had a much higher adsorption capacity than the reported slag in the literature, likely due to a much smaller particle size and a higher surface area for the iron slag. fig. 2. dependence of the slag loading with cu2+ and pb2+ on equilibriumsolution ph. 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. 2 illustrates the ph dependence of the slag loading with cu2+ and pb2+, respectively. clearly, the metal uptake was quite low at low ph levels. however, with an increase in solution ph, a significant enhancement in sorption was recorded for both slags, with the optimum ph values for the metal sorption by the iron and steel slags being 3.58.5 and 5.28.5, respectively. this behaviour can be explained by considering the surface charges of the slags which depend, to a large extent, upon the values of pzc of silica (about 2.3) and alumina (about 8.2) 12. the composite pzc of the adsorbent slags, as determined by electrophoretic measurements, was found to be 3.2 for the steel slag and 4.8 for the iron slag. for the iron slag, the particle surface had a positive charge density in the range of ph 4.8) the negative charge density on the surface of the iron slag increased, thereby resulting in a sudden enhancement in the metal adsorption onto the iron slag. likewise, the steel slag had a quite low metal loading at around ph 3.2. in addition, the precipitation of heavy metal ions may contribute to the higher metal loadings on the slags at higher solution ph.3.3. sorption kineticsthe slag doses were 2 g/l and the slurry ph was maintained at 5.5. the initial concentrations for cu2+ and pb2+ solutions were around 200 mg/l, respectively. the sorption kinetic result is shown in fig. 3. the sorption equilibrium for the steel slag could reach within 1 h, while the sorption equilibrium for the iron slag could only reach till 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 interaction of the slag glass with the solution can be expressed as follows11:=sioca + 2hoh =sioh2 + ca2+ + 2oh (1)it can be expected that in acid environment the above reaction would shift to the left due to the high concentration of hydrogen ions. the basic slags had neutralising effect following the above route. different amounts of slags were added into 1 l distilled water and stirred at 600 min1 for 24 h. table 3 shows the changes of the solution ph and ca2+ concentrations. clearly, the calcium ion exchanged with the hydrogen ion and was released from the slags into the solutions, while the solution ph increased. this confirms that the reaction in eq. (1) occurred in the contact of the slags with solutions. after 24 h contact, the calcium concentration was much lower for the steel slag than for the iron slag at the same doses. it can be inferred that the iron slag had a much higher ion exchange ability than the steel slag, which was in agreement with the sorption equilibria. in the presence of divalent heavy metal ions (m2+) in solutions, the above equation can be described as:=sioca +m2+ =siom + ca2+ (2)the slags had high surface area and porosity as indicated in table 1, which provided the basis for metal sorption as well. 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. 46, both slags revealed porous structures and the iron slag 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 or copper hydroxide complexes depositing on the porous slag surface.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 slurry after 24 h slag adsorption. the solid/liquid separation by simple sedimentation was not complete in short times (up to 10 h) and required a further stage of filtration. both ion-exchange and metal precipitation contributed to the removal of metal ions from wastewater by the slags. the hydroxide species of the metal ions increased the turbidity of solutions after the adsorption. in addition, the presence of some fine slag particles could also result in an increase in solution turbidity and this can, however, be avoided by using coarse slag particles. in order to remove the suspendedsolids, dispersed air column flotation was employed for the subsequent separation of loaded slags from solutions. the initial cu and pb concentrations were 200 mg/l and the slag doses were 2 g/l. the solution ph was maintained at 5.5. after 24 h contact, the slurry was subjected to flotation at an air flowrate of 200 ml/min. the average bubble size in the flotation column was around 0.62 mm, based on froth image analysis. the flotation result is shown in table 4. as can be seen from table 4, at a dose of 15 mg/l sds the solution 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 flotation and this effect became more prominent with increasing the sds doses. this was attributed to the complexation of heavy metal ions with the collector sds, as confirmed in the blank tests without slags. adsorptive slag flotation, using small amounts of slags as carriers, was found to be very effective for the treatment of solutions containing heavy metals such as copper and lead. settling should also be an effective process leaving a clear supernatant with the aid of a typical flocculant. settling is simpler and relatively cheaper. however, flotation is faster and more effective in recovering fine particles in comparison with settling. in the current study, fine slag powders were used as the sorbents. therefore, flotation was chosen as the subsequent solid/liquid separation process. in case that coarse slag particles are used for wastewater treatment, settling process should be favourable. sorption of pollutants in packed columns with coarse slag particles can be efficient, despite a lower adsorption area. this will avoid the use of the subsequent flotation or settling for solid/liquid separation.3.6. treatment of an acid mine drainage by flotation ofadsorptive slagsthe acid mine water used in the experiments was sampled from the drainage of a south african gold mine. the acid mine water had a very low ph of 2.03 and a very high sulphate concentration, as indicated in table 5. the acid mine water is neutralised with