An overview for the utilization of wastes from stainless steel industries

Resources,Conservation and Recycling 55 (2011) 745–754Contents lists available at ScienceDirectResources,Conservation andRecyclingj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /r e s c o n r ecReviewAn overview for the utilization of wastes from stainless steel industriesZhang Huaiwei ∗,Hong XinSchool of Materials Science and Engineering,Shanghai University,Shanghai 200072,Chinaa r t i c l e i n f o Article history:Received 12April 2010Received in revised form 14March 2011Accepted 16March 2011Keywords:Stainless steel wastesComposition and characteristics Leaching behaviors StabilizationSmelting reductiona b s t r a c tSignificant quantities of wastes are generated as the waste materials or byproducts every day from stain-less steel processes.According to the origins and characteristics,the stainless steel wastes can be mainly classified into two categories,slags and dusts.They usually contained considerable quantities of valuable metals and materials.This paper summarized and analyzed the generation,composition,characteristics and the leaching behaviors of the most of wastes obtained from the stainless steel processes.On this basis,a review of several methods for treating the various stainless steel wastes was made.It is very essential not only for recycling the valuable metals and mineral resources but also for protecting the environment.Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.Contents 1.Introduction............................................................................................................................................7452.Compositions and characteristics of stainless steel wastes ...........................................................................................7462.1.Chemical composition and phase analysis.....................................................................................................7462.1.1.Stainless steel slag....................................................................................................................7462.1.2.Stainless steel dusts...................................................................................................................7472.2.Leaching behavior of stainless steel wastes....................................................................................................7493.The treatment of stainless steel wastes................................................................................................................7503.1.The recovery of metals from stainless steel wastes............................................................................................7503.1.1.Hydrometallurgical techniques ......................................................................................................7503.1.2.Pyrometallurgical techniques ........................................................................................................7503.1.3.Physical separation ...................................................................................................................7523.2.Stabilization/solidification .....................................................................................................................7524.Conclusion..............................................................................................................................................753References..............................................................................................................................................7531.IntroductionNowadays,the production of stainless steel is one of the fastest-growing sectors of the manufacturing industry all over the world.In the stainless steel making process,lots of wastes are generated from the factories.Approximately one tonne of stainless steel wastes was originated when producing three tonnes of stainless steel.It is not only a crisis of quantity but also a crisis of arising from toxic ingredients,such as the chromium,lead,nickel,cadmium that could pose both occupational and environmental health threats (Huiting∗Corresponding author.E-mail address:zhw5984@ (Z.Huaiwei).and Forssberg,2003,2004;Ma and Garbers-Craig,2006;Durinck et al.,2008a,b ).In the stainless steel production,the stainless steel wastes are mainly generated in two forms,the stainless slags and the stainless dusts.The stainless slags mainly include the gangue contents and some of metal oxides (Lopez et al.,1997;Ding and Merchant,1999;Huiting and Forssberg,2003,2004;Perrine et al.,2006;Tossavainen et al.,2007;Das and Prakash,2007;Maria et al.,2008).The stainless slag could be used as the cement adhesives and roadbed mate-rials after the treatment of stabilization/solidification process or other methods (Bi and Lin,1999;Motz et al.,2001).The dusts are the most important by-product of stainless steelmaking operations.Approximately 1–2%(mass fraction)of scrap charged in the smelt-ing furnaces enters into the exhaust gases and then is converted into the dusts (Zunkel,1996).The stainless dusts also contain a lot of0921-3449/$–see front matter.Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.doi:10.1016/j.resconrec.2011.03.005746Z.Huaiwei,H.Xin/Resources,Conservation and Recycling55 (2011) 745–754gangues,but the contents were very low contrasted with the stain-less slags.Otherwise,there were lots of iron oxides in the stainless dusts,and the mass percent was easily reached about40%,which were even higher than some of iron ore.The contents of chromium and nickel were much more than the stainless slags and the mass fraction and existence forms were similar to those of laterite nickel ores,so the recycling and utility value of them was much more than the stainless slags(Li and Tsai,1993;Laforest and Duchesne, 2006;Tang et al.,2008;Ma and Garbers-Craig,2009).They could be used as the raw materials of smelting reduction ironmaking and steelmaking processes.The stainless steel wastes consist of lots of hazardous element, such as Cr,Pb,Ni and so on.These materials are harmful to the environment,especially to the human health.How to handle the stainless wastes and recover the valuable metals from them are very important at the present period not only for the resource and economy but also for the environment.The objective of this paper was to describe the physical and chemical characteristics of the various types of stainless steel wastes for enhancing the under-standing of the stainless steel industry wastes and summarize data that could be used to evaluate the potential environmental impacts associated with its application.The methods of treating the wastes and recovering the valuable metals from them were also mentioned in the article.positions and characteristics of stainless steel wastes 2.1.Chemical composition and phase analysis2.1.1.Stainless steel slagThe different slags from various iron and steel making processes were shown in Fig.1(Durinck et al.,2008b).The stainless steel slags mainly includes the gangue content,such as CaO,SiO2,Al2O3, MgO,and some metal oxides(Lopez et al.,1997;Ding and Merchant, 1999;Bi and Lin,1999;Huiting and Forssberg,2003,2004;Perrine et al.,2006;Tossavainen et al.,2007;Das and Prakash,2007;Maria et al.,2008).The typical chemical compositions of stainless steel slags from some different industries are shown in Table1.From this table,the composition of the slags from various steps were approximately similar,some inconspicuous distinctions between them were because of the difference of the materials and refining processes in the various factories.The stainless steel slags con-tained much less iron oxides than the ordinary steel slags(Bi and Lin,1999;Motz et al.,2001).Perhaps the most important charac-teristic of stainless slag is that they contained lots ofnon-ferrousFig.1.The processes of stainless steel slags.metal elements,especially the higher amount of Cr(1.5–4.5%)and Ni(0.05–0.45%).The phase analyses of the stainless steel slags were reported in many articles(Li and Tsai,1993;Lopez et al.,1997;Huiting and Forssberg,2004;Tossavainen et al.,2007;Tang et al.,2008;Zhang et al.,2008;Ma and Garbers-Craig,2009).Huiting and Forssberg have already made a systematic research about the physicochem-ical and mineralogical properties of the stainless steel slag which were provided by Bergslagens Stålservice AB in Sweden.The XRD of the AOD and EAF stainless steel slags were showed in Figs.2and3 respectively(Huiting and Forssberg,2004).These pictures indi-cated that the main phase compositions of the stainless steel slags were mainly present in the forms of complex oxides,silicates and the valuable metal elements,such as Cr and Ni,which were existed in the slags as the form of Cr–Fe–Ni spinel both AOD and EAF. Chemical phase analysis of some metals had been made,which are shown in Table2(Huiting and Forssberg,2004).It was interest-ing to note that some pure metals,such as almost all the nickel metals and part of the iron and chromium metals,were existed in the slags,so it is possible to recover these metals directly through the methods of gravity or magnetic separations.Particle sizes wereTable1Compositions of various stainless slags.Component(mass%)12345CaO45.5–47.654.146.950.8843.27SiO226.3–31.226.533.533.2929.71Al2O3 1.65–9.69 4.91 2.3 3.2210.07MgO 3.65–7.30 6.3 6.22 4.788.59MnO 1.4–2.1 1.02 2.6 2.94 1.86Na2O0.05–0.08–0.17–0.15K2O0.050.140.11–0.04Fe 1.75–2.40 2.03 1.56 1.36(Fe2O3) 1.43(Fe2O3) Cr 2.48–4.28 1.71 3.22 2.84(Cr2O3) 2.39(Cr2O3) Cu0.05–0.12––––Ti0.13–0.68––0.50(TiO2) 2.06(TiO2) Zn0.02–0.030.010.011––Pb0.04–0.050.005–––Ni0.22–0.450.20.075––Slag style and data source Italy,AOD slagLopez et al.(1997)Sweden,AOD slagHuiting and Forssberg(2003)Sweden,EAF slagHuiting and Forssberg(2003)Spain,Reduction slagMaria et al.(2008)Spain,Ladle transferslagMaria et al.(2008)Z.Huaiwei,H.Xin /Resources,Conservation and Recycling 55 (2011) 745–754747Fig.2.X-ray diffraction of EAFsample.Fig.3.X-ray diffraction of AOD sample.also analyzed (Huiting and Forssberg,2004).The results investi-gated that the size distributions were quite wide,about from 0to 300␮m,and chromium and nickel were almost distributed in all the size fractions.The same results were detected in some otherTable 2Chemical phase analysis of some metals.Element EAF slags AOD slags FeTotal 1.56 2.03Metal0.520.67Metal/total Fe (%)33.3324.51CrTotal 3.22 1.71Metal1.220.46Metal/total Cr (%)37.8926.90NiTotal 0.0750.20Metal0.070.19Metal/total Ni (%)93.3395.00researches (Nolasco-Sobrinho et al.,2003;Zhang et al.,2008;Ma and Garbers-Craig,2009).The mineralogical compositions of the first two samples in Table 1which from Italy are shown in Table 3(Lopez et al.,1997).The slags mainly composed of dicalcium and tricalcium silicates,calcium–aluminium silicates,free lime,periclase and chrome-nickel-magnesium oxide.This was also the most typical composition of the stainless slags.2.1.2.Stainless steel dustsThere were some reports about stainless steel dusts (Li and Tsai,1993;Nolasco-Sobrinho et al.,2003;Laforest and Duchesne,2006;Tang et al.,2008;Ma and Garbers-Craig,2009).Stainless steel dusts were the mixtures of metals and slags which mainly entered into the collecting equipments.In additional,some of dusts came from the flue gas pipelines because of the violent agitation of molten748Z.Huaiwei,H.Xin/Resources,Conservation and Recycling55 (2011) 745–754Table3Mineralogical composition of the slags.Slag ElementCa Si Al Mg Fe Cr,Ni and Mn1MeliliteMerwiniteC2SC2S·SiO2Silicate Melilite Silicate Periclase Chromites Chromites2MeliliteMerwiniteTi–CaOxideSilicate Melilite Silicate Periclase Chromites Chromites3MeliliteMerwiniteC2SC2S·SiO2CalciteFluoriteSilicate Melilite SilicatePericlaseMgO·Cr2O3Chromites␣-Fe ChromitesC2S,dicalcium silicate.steel in the furnace during the stainless steel making processes.Table4shows a typical composition of the stainless steel dust fromthe metallurgy plants of South African and Canada(Laforest andDuchesne,2006;Ma and Garbers-Craig,2009).The stainless dustsalso contained a lot of gangues,but the contents were much lowercontrasted with the slags.Otherwise,there were lots of iron ele-ments in the stainless dusts,and the mass fraction could reach morethan40%,which even was higher than some iron ores.The contentsof chromium and nickel in the stainless dusts were much morethan slags.The compositions of the dusts were similar to the lat-erite nickel ore.They were of great recovering values,for example,utilized as the raw materials of iron or steel smelting processes.The phase structures and the formation mechanism of the stain-less steel dusts were studied by some researchers(Dreisinger et al.,1990;Delhaes et al.,1993;Li and Tsai,1993;Nolasco-Sobrinhoet al.,2003;B.Peng et al.,2004;J.Peng et al.,2004;Peng et al.,2007;Tang et al.,2008).The structures of the dusts,the similar asthe stainless steel slags,mainly consist of complex oxide and sili-cate,but the component contents of them were significant variance(Tang et al.,2008).The complex metal oxides played a dominantrole in the stainless steel dust.Some reviewers have studied themicrograms of the stainless dusts which obtained from Canada.Thesecondary electron(SE)images were illustrated in Fig.4(Laforestand Duchesne,2006).They thought that micro-shape of the dustswas spherical,elongated grain,fine grains and irregular particlesTable4Compositions of different stainless dusts.Component123CaO 6.5912.99.02SiO2 5.76 4.81 5.14Al2O30.740.40.64MgO 4.25 5.44 3.63MnO 5.88 5.08–Na2O 1.010.60.60K2O0.480.970.72P2O50.040.040.30Fe2O327.731.651.3Cr10.99.9916.3(Cr2O3)Ti0.0960.0480.08(TiO2)Zn 5.2 4.110.96(ZnO)Pb 1.40.360.29(PbO)Ni 4.1 2.2 6.70(NiO)Materials and data source CanadaLaforest andDuchesne(2006)South AfricanMa andGarbers-Craig(2009)ChinaTang et al.(2008)through the pictures.The inner parts of spherical and elongatedgrains were composed of Fe–Cr–Mn–O and C–O–Cr respectively,probably a variation of magnetite and the chromium carbonate.Thespherical grain was covered with a crust of different composition ofsilica glass in the form of Ca–Si–Al–O(Fig.4a and b).They also inves-tigated that spheres with internal holes probably were resultedfrom the degassing during the dusts formation processes(Fig.4c).The main existing forms of nickel and chromium in the stainlesssteel dust were the Cr–Fe–Ni spinel(Huiting and Forssberg,2004;Park,2007;Li et al.,2009).Some other researchers indicated thatnickel was mainly present in the forms of pure nickel metal or nickeldroplets in stainless steelflue dustsfines(Geldenhuis and Home,2002).Tang and Peng considered that the formation mechanism ofthe stainless steel dusts was classified into two stages,precursorappearance and accumulation of small particulates(B.Peng et al.,2004;J.Peng et al.,2004;Peng et al.,2007;Tang et al.,2008).Theyobtained the photo with a high speed camera at1000frames persecond.It was showed in Fig.5.They thought that the dusts were thedroplets due to the breakdown of bubbles in the surface of smelt-ing bath during the stainless steel process.The same results wereobtained in another article(Guézennec et al.,2005).Some other for-mation mechanisms were reported.Stephen considered that thevolatile wastes were congregated in order to keep lower surfaceenergy(Stephen et al.,1983).Other researchers investigated thismechanism particularly,and both of them indicated that the pro-cess wasfinished through the electrostatic forces(Li and Tsai,1993;Linak and Wendt,1993).Another mechanism was that the volatilemetallic compounds were congregated through the collision andbond among gas phase materials(MeAloon,1990;Kola,1990).Somebody else thought that the formation mechanism of dustsduring the stainless steel making process were four possible peri-ods of evaporation of metal,ejection of slag,ejection of metal andentrainment of solids(Dreisinger et al.,1990;Delhaes et al.,1993;Nolasco-Sobrinho et al.,2003).All of these mechanisms contain twoprogresses.One was the diffuse stage and another was the adsorp-tion stage with some principal particles,of course,they wouldlead to some of chemical or physical changes in these particlessurface.All analyses(Li and Tsai,1993;Wu and Themelis,1993;Nolasco-Sobrinho et al.,2003)showed that the stainless steel dusts wereveryfine.Around70–90%(in weight)of the dusts consisted of theparticles which were smaller than5␮m in diameter,and even mostof them were actually smaller than1␮m.There were still somelarge solid agglomerates(about10–30%in weight)in the dusts,thediameter could be more than150␮m.Particle size distribution ofthe dusts greatly depended on its moisture content.As a result,theZ.Huaiwei,H.Xin/Resources,Conservation and Recycling55 (2011) 745–754749Fig.4.SEM pictures in secondary electrons(SE)mode of a random dispersed samples of stainless steel dust.dusts would probably contain relatively large particles if they have been stored for a long period.2.2.Leaching behavior of stainless steel wastesThe release of the heavy metal elements or some other harm-ful materials from wastes will cause environmental problems such as water pollution and soil pollution,so the leaching tox-icity of stainless wastes was investigated by some researchers. Chromium element was the most harmful one in the stainless wastes(ICDA,2007).Toxicity Characteristic Leaching Procedure (TCLP)was adopted.The leaching standards and methods were reported in some articles(US EPA,1997,2003).The leaching results of two different stainless steel slags from Italy and China are shown in Table5(Lopez et al.,1997)and Table6 (Zhang et al.,2008;ICDA,2007).From the two tables,it could be concluded that almost all the heavy metals by the leaching test were lower than the detection limits.Therefore,the pollution risks of the heavy metals in the stainless steel slag were very low.So,in the most areas,the stainless slags could only treat as thecommonFig.5.Picture taken from a sequence at1000per second.(a)Before bubble emission,(b)bubble emerging at bath surface,(c)disruption of the bubble,(d and e)formation of liquid jet,and(f)emission of jet drop.750Z.Huaiwei,H.Xin /Resources,Conservation and Recycling 55 (2011) 745–754Table 5TCLP results for the slags from Italy (mg/L).Element Cr (total)Pb Cd Zn Ni Cu Hg Content 1.0<0.5<0.1No <0.5<0.5No Standard<5<5<1<10<10<10<0.2Table 6TCLP results for the slag s from China (mg/L).Element Cr (total)Cu Hg Ni Pb Zn Cd EAF slag 0.098±0.011No <0.001No No No No AOD slag 0.170±0.009No <0.001No No No No Standard<5<10<0.2<10<5<10<1wastes,not the hazardous.In general,the stainless slag could be landed or abandoned directly,but they might bring some dangers because of containing the heavy metal elements.Prior to the uti-lization or landfill,the stainless slags were often treated by some other methods.The leaching behaviors of the stainless steel dusts were also investigated (Proctor et al.,2000;Ma and Garbers-Craig,2006;Laforest and Duchesne,2006;Tang et al.,2008).The leaching results were very similar among these papers.The typical TCLP results of the stainless steel making process dusts obtained a Canadian indus-try are presented in Table 7(Laforest and Duchesne,2006).The results showed that the total Cr (9.7mg/L)and Zn (93.9mg/L)con-centrations in the leaching solutions were over the value of Toxicity Characteristic Regulatory Level,and Pb and Ni concentrations were under the regulatory level.Thought the nickel concentrations of the TCLP test were lower than the standard value,it was important to note the high concentration (2.3mg/L)had been reached.In any case,the stainless dusts are the hazardous wastes.Some researchers investigated that the leaching levels were believed to be limited substantially when chromium or other haz-ardous metals were contained in a spinel phase-AB 2O 4with A being a bivalent cation and B being a trivalent cation (Kühn and Behmenburg,2000;Kühn et al.,2000;Tossavainen and Forssberg,2000;Kühn and Mudersbach,2004).Based on this research,Kühn and Mudersbach performed a large number of extraction tests (Fig.6)on industrial stainless steel electric arc furnace wastes and educed an empirical formula factor to relate the overall wastes composition to the chromium leaching:Empirical formula factor =0.2MgO +1.0Al 2O 3+n FeO x −0.5Cr 2O3Fig.6.Chromium leaching as a function of factor.‘n ’is a number between 1and 4,depending on the oxidation state of the wastes.When the factor is below 5wt%,a high chromium leaching level is observed.When it is above 5,chromium leaching level is low.3.The treatment of stainless steel wastes 3.1.The recovery of metals from stainless steel wastes3.1.1.Hydrometallurgical techniquesThe hydrometallurgical process mainly consists of four steps:roasting,leaching,purification,electrowinning.It is a usual method for extracting the non-ferrous metals,such as zinc,lead,copper and so on.It was reported that hydrometallurgical techniques have been used for recovery of Zn from stainless steel wastes through the alkaline leaching in some commercial processes (Lindblom et al.,2002;Menad et al.,2003;Kelebek et al.,2004;Dutra et al.,2006).Dutra et al.reported that the most significant zinc recovery rate was 74%after conventional leaching for 4h with a 6M NaOH solution.The leaching process could be only used to the stainless steel wastes which contained some specific metals,such as zinc,aluminium,lead and so on.Although the hydrometallurgical techniques have lots of advantages,as low energy consumption and simple oper-ation,the pyrometallurgical techniques were the main methods for recovering the valuable metals from the stainless steel wastes because of the special characters and structures of the wastes.3.1.2.Pyrometallurgical techniques3.1.2.1.Smelting reduction techniques.Smelting reduction process was a recent trend in the development of iron or steel making processes (Basu,2002).Keeping this in mind,smelting reduction process was also developed for the treatment of solid wastes which originated from the metallurgical industry,especially steel plant dusts.The process was based on the reduction of selected metal oxides (such as iron oxides,chromium oxides nickel oxides and so on)at a high temperature by means of a carbonaceous or the other reducing agent.By this process,the valuable metals could be recov-ered through forming the alloy with the melting iron which could be easily separated from the innocuous slags.Laboratory scale reduction smelting experiments have been carried out using mixtures of stainless steel wastes from some researchers (Takamitsu et al.,2004;Takano et al.,2005).The results indicate that about 90%of total iron,95%of chromium,almost 100%of nickel in the wastes could be recovered as mixing molten metals at the proper temperature,additives and reductants.The typical smelting reduction flow chart in the factory for treating the stain-less steel wastes was showed in Fig.7(Denton et al.,2005).Z.Huaiwei,H.Xin/Resources,Conservation and Recycling55 (2011) 745–754751Table7TCLP results for the stainless steel dust from Canada(mg/L).ElementCr(total)Pb Cd Zn Ni Cu Hg Content9.70.4No93.9 2.3No NoStandard<5<5<1<10<10<10<0.2Fig.7.Typical processflow for stainless steel plant waste processing.The industrial processes of smelting reduction to treat the stain-less wastes mainly include INMETCO process of America(Hanewald and Dombrowski,1985;Money and Hanewald,2000)and the STAR process of Kawasaki Steel Group in Japan(Hascgawa,1998; Ham et al.,2000;Uetani and Bessho,2001).The amelioration has been adopted comparing with the classical processes.During the IMETCO process,as was shown in Fig.8(Lockwood,2005),annu-lar furnace and electric arc furnace were respectively applied in the pre-reduction and melting stage.The technique was classed into three stages,raw material processing,pre-reductionprocess-Fig.8.Theflow chart of IMETCO process.752Z.Huaiwei,H.Xin/Resources,Conservation and Recycling55 (2011) 745–754Fig.9.Processflow of the shaft furnace for dust recycling.ing and melting and casting processing.The reduction rate of iron and nickel could reach80%and95%in the stage of pre-reduction, while the reduction of chromium was processed in the electric arc furnace.Japan’s Kawasaki Steel Corporation has developed a smelting reduction process for recovering chromium and nickel dusts with the shaft furnace,as was shown in Fig.9(Ham et al.,2000).The advantage of the process was that thefine dusts could be directly used as raw material without the pellet making process and could be recovered almost all the metals without secondary pollution. Double-nozzle was used in the operation process.Raw materials were injected into the furnace from the upper spray nozzle and the lower spray nozzle was mainly used for the heat compensation. As the coke combustion,the temperature was very high between the two nozzles,the metal oxides could be reduced easily into the hearth,while the volatile components,such as zinc,etc.,could be discharged from the roof of furnace.Metal alloys could be used as raw materials to return to the steelmaking process.The total recovery rate of metals could be up to98%.3.1.2.2.Thermal plasma technology.In the last30years,the plasma technology used to treat the stainless wastes was researched by some institutions and labs with the development of science and technology.The benefits of plasma technology are small-scale, short payback period of investment and the special advantage of recovering the valuable metals such as Cr and Ni.The principium of plasma technology is that the fuel gas dissociated into atoms through high temperature(3000◦C)produced by the plasma torch. The atoms arefiring in thefirebox and the temperature of central blaze could reach20000◦C.The metal oxides in the stainless steel wastes were deoxidized rapidly by the reducing agents under high temperature circumstance and separated in the condenser for the different boiling points of various metals.A schematic diagram of a DC arc plasma torch was shown in Fig.10(Dutra et al.,2006).The plasma furnace styles were various such as converter,induc-tion furnace,shaft furnace and so on.The methods were very effective to recover not only the nickel and chrome but also the other metals such as zinc and lead(Hascgawa,1998;Ham et al., 2000;Lockwood,2005).The applications of the plasma process for treating the steelmaking wastes have been summarized in some papers(Ye et al.,2003).The stainless steel wastes,solvents and cokes mixed according to a certain percentage were plunged into the plasma furnace with a certain degree of feeding system.On average,the chromium recovery was over90%,nickel recovery close to100%and the zinc and lead were up to70%.ThecontentsFig.10.Schematic diagram of a DC arc plasma torch.of hazardous elements in the stainless steel wastes could reach the standards of landfill after the proper treatment.The comparatively mature processes of plasma technology for the recovery of valuable metals from the stainless steel wastes mainly include SCANDUST process of SKF group in Sweden(Smith et al.,1989),Tetronics process of TRD Company in Britain(Cowx and Roddis,1986;Chapman and Cowx,1991),Environplas process in South Africa(Schoukens et al.,1993;Goff and Denton,2004) and Ilserv process produced by Acciai Speciali Temi Company and Harsco Company in Italy(Brascugli et al.,1997).3.1.3.Physical separationPhysical separation processes are usually used to remove smaller,more contaminated particles.These processes include cen-trifugation,sedimentation,gravity separation,magnetic separation and so on.It was reported that magnetic separation or gravity sep-aration methods have been used for recovery of Ni and Cr from stainless steel wastes in some commercial processes,but the most were still staying on the laboratory studies(Lopez et al.,1997;Das et al.,1997;Geldenhuis and Home,2002).Gravity separation is an effective method for separating miner-als of significantly different densities.Magnetic separation is widely used for the recovery of ferromagnetic metals from non-ferrous metals and other non-magnetic wastes.Geldenhuis had made an investigation to recover nickel from the stainless dustfines.The difference in magnetic properties and density between oxide and metallic particles could be utilized to separate these phases.More than50%of the nickel could be recovered with magnetic and gravity separation(Geldenhuis and Home,2002).Some articles reported that it was possible to further recover most of such Cr and Ni from the stainless wastes using gravity separation or magnetic separa-tion methods.It was necessary to further grind the waste particles to release the alloy and metal oxide grains because higher Cr–Ni concentrate was required(Das et al.,1997;Huiting and Forssberg, 2004).From these studies,the gravity separation and magnetic sep-aration are used to recover the pure nickel and chromium metals or alloys from the wastes,on the other hand,some metals existed in the silicate phases or metal oxides were impossible to recover by these methods.It was evident that more efforts need to be made about physical separation processes for recovering valuable metals from the stainless wastes.3.2.Stabilization/solidificationStainless steel wastes contain lots of heavy metals as Cr,Ni and Pb,it was always perceived as the hazardous materials.Sta-bilization/solidification(S/S)processes were found to be one of the effective technologies whereby heavy metal elements-containing。