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A Kinetics Study on the Hydrometallurgical Recovery of Vanadium from LD Converter Slag in Alkaline Media

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Abstract The pig iron from steel manufacturing plants contains a high concentration of vanadium-bearing materials smelted with iron ore. During the oxidation process of the molten pig iron with oxygen lances, vanadium transfers to the slag. In this
  A KINETICS STUDY ON THE HYDROMETALLURGICAL RECOVERYOF VANADIUM FROM LD CONVERTER SLAG IN ALKALINE MEDIA Amirhossein Shahnazi 1 , Fereshteh Rashchi 1 , Ehsan Vahidi 1,21 School of Metallurgy and Materials Engineering, University of Tehran, Tehran, 11155, Iran 2 Department of Civil and Environmental Engineering, University of South Florida, Tampa, FL33620, USAKeywords: LD Converter Slag, Vanadium, Alkaline Leaching, Kinetics, Shrinking Core Model Abstract The pig iron from steel manufacturing plants contains a high concentration of vanadium-bearingmaterials smelted with iron ore. During the oxidation process of the molten pig iron with oxygenlances, vanadium transfers to the slag. In this research, recovery of vanadium from LD(Linz– Donawitz)converter slag of steelmaking plant was investigated. The leaching residue wascharacterized by XRD and XRF techniques. The maximum vanadium recovery was achieved atoptimum leaching conditions of 70 °C, S/L: 1/15, sodium hydroxide concentration: 3 M and leaching time: 150 minutes. The dissolution rate increased with rising sodium hydroxideconcentration and temperature and decreasing particle size. The experimental data are treated graphically to explain the kinetics of the vanadium recovery process using shrinking core model(SCM). As a result, the controlling regimes in the SCM were analyzed separately using liquid film diffusion control, solid product diffusion control, and reaction control mechanisms. Introduction Vanadium is commercially important as a constituent of several alloys and catalysts. Nowadays,vanadium is mostly recovered as a by-product from secondary sources or from industrial wastestreams such as titaniferrous magnetite, fly ash, spent catalysts, and petroleum coke.Steelmaking operations are specifically concerned by this problem due to the production of ahuge quantity of by-products [1]. These by-products, including LD converter slag, sludge, flyash, spent catalyst, ash, and alloyed scrap, contain a notable amount of heavy metals which maycause environmental problems. Solvent extraction is one of the techniques being increasinglyused for the recovery of vanadium [2]. Apart from vanadium ore, secondary resources such astitanmagnetite, ilmenite and slag from the ferrous industry are a major source of supply. The pigiron from steel manufacturing contains a majority of vanadium-bearing materials smelted withiron ore. In a heat resistant shaking ladle the molten pig iron is oxidized with oxygen lances,causing the vanadium to be transferred to the slag. Normally, the vanadium in LD converter steel slag (coming from Linz–Donawitz steelmaking process) is ca. 5% as vanadium pentoxide(V 2 O 5 ). This slag is the world's principal raw material for vanadium production[3, 4].Moskalyk and Alfantazi [4] reported that a roast–leach process is used in South Africa for the production of vanadium oxides from magnetite ores while some of the slag was converted toferrovanadium. A portion of the slag was exported to Europe for the production of vanadiumoxide. The researchers mentioned that alkaline roasting was the main process at that time 425  employed to extract vanadium pentoxide from vanadium slags. They also reported thatmetallurgical slags from China and Russia contain 12–20% V 2 O 5 while South African slagcontains about 25% vanadium pentoxide. Moskalyk and Alfantazi [4] reported another process used in Russia in their review paper published in 2003. The process involved a roast/leachoperation using either kilns or multi-hearth furnaces to recover the V 2 O 5 after the addition of sodium carbonate, chloride, or sulfate during roasting. Lime was used in the roasting stagewithin Russia, and the sodium vanadates were leached out with water. Ammonium vanadateswere precipitated from the solution upon adding ammonia and controlling the pH via sulfuricacid. The lime present in the slag causes difficulties by forming an insoluble calcium vanadate,instead of the more soluble sodium vanadate. The formation of calcium vanadate is prevented byadding pyrite (FeS 2 ) and sodium phosphate to the ore/slag mixture. Calcium is thereby tied upeither as a sulfate or a phosphate during salt roasting and calcium vanadate formation issuppressed. Another alternative is to allow the formation of calcium vanadate during saltroasting and solubilize this compound by leaching this roasted material by sulfuric acid or carbonate solutions. Aarabi et al. [5] studied on extraction of vanadium from LD converter slagsusing acidic leaching. They achieved maximum vanadium recovery of ca. 95% at the maximumleaching condition of 70 ºC, S/L: 1/15, acid concentration: 3 M, agitation rate: 600 rpm, particle size: finer than 850 μm and leaching time: 150 minutes. They showed that in dissolution of vanadium in sulfuric acid there are two stages in the kinetics of leaching that control the processes. They found that the kinetics of leaching at low temperature is controlled by chemicalreactions for both short (first 15 minutes of leaching time) and long (last 120 minutes of leachingtime) periods based on the shrinking core model (SCM).In the present study the effects of different leaching parameters: particle size, alkaline concentration, reaction temperature, and solid - liquid ratio (S/L) on the extraction of vanadium from LD converter slag have beeninvestigated using salt roasting and sodium hydroxideleaching. The kinetics of the leaching process was studied using SCM. Experimental MaterialsThe LD converter slag was collected from disposal sites of waste materials of Esfahan SteelCompany, Iran. The crushed and ground sample was divided into three size fractions: <0.850mm, 0.850–1.40 mm and 2.36–1.40 mm. Sodium Hydroxide (purity 99%) was obtained fromMerck. Sodium carbonate (purity 99%) used as the salt in the roasting experiments was fromMerck as well.AnalysisMineralogical and chemical analyses of the sample were done using X-ray diffraction analyzer (XRD, model Philips, Netherlands) and X-ray fluorescence spectroscopy (XRF, modelS4explorer, Germany). The XRD pattern and the major minerals present in the sample areshown in Fig. 1.   426   Fig. 1. XRD pattern of the LD converter slag (before roasting) The XRD result revealed that the major phases in the slag are Portlandite (Ca(OH) 2 ), calciumiron oxide (CaFe 2 O 5 ), calcium silicate (Ca 3 SiO 5 ) and calcium vanadium oxide (Ca 2 V 2 O 5 ). Table1 presents the chemical analysis of the slag, obtained from XRF.Table 1.   Major elements in the sample before roastingCompound CaO Fe 2 O 3 SiO 2 Vwt.% 53.45 17.82 14.35 0.45Experimental Procedure  Alkaline roasting: Alkaline roasting was done to change vanadium compound to a soluble form.50 g sodium carbonate and 200 g sample for 15 min in ball mill were mixed. The roasting stagewas conducted by placing the sample mixed with sodium carbonate 20% in a muffle furnace for a certain temperature and time. The roasting condition applied was 1000 °C and 2 h time [6].  Alkaline leaching:  The roasted sample was leached in sodium hydroxide at a certaintemperature, S/L ratio, and time. The leaching experiments were performed at atmospheric pressure in a Pyrex reactor with 3 necks, one for the condenser, one for thermometer, and the lastserved either for the inlet of the sample or for withdrawal of samples at regular time intervals.The reaction mixture was agitated with a magnetic stirrer and indirectly heated on a hot platethrough a water bath. For each leaching test, the sample and solution of predetermined concentration were charged into the Pyrex reactor and the mixture was well mixed at 400 rpm.After a specific leaching time, a Buckner funnel equipped with a filter was used for the filtrationstep. The slurry was filtered and the filtrate was analyzed by titration method to determine theamount of vanadium in the leaching solution [7]. The temperature range applied was 25, 40, 50,   427  and 70 °C and at specific time intervals a 10 mL sample was taken by pipette. The sample wasfiltered, and the solution was analyzed for V by titration. To determine the effect of particle size,6.66 g of the sample was leached in a 100 mL of 3 M sodium hydroxide solution at 70 °C for three different size fractions (<0.850 mm (20 mesh), 0.850–1.40 mm (14-20 mesh) and 1.40-2.36mm (8-14 mesh)). Results and Discussion Effect of Sodium Hydroxide Concentration Fig. 2 illustrates the recovery curves of vanadium as a function of NaOH concentration fromLDconverter slag for the particle size of below 0.850 mm, at 70 °C, S/L of 1:15 g/mL, 90 minutes, particle size 0.850 mm, 400 rpm. As seen, the vanadium recovery increased with increasing the NaOH concentration up to 3 M and leveled out at higher hydroxide concentrations.   01020304050607080901000 1 2 3 4 5 6    R   (   %   ) [NaOH ](mol/lit)  Fig. 2. Effect of NaOH concentration on vanadium extraction (at 70 °C, S/L of 1:15g/mL, 90 min, particle size 0.850 mm)Effect of Particle SizeFig. 3 shows leaching recovery as a function of particle size from LD converter slag for 70 °C,S/L of 1:15 g/mL withthree size fractions in a time range of 0 to 150 minutes.As seen, particlesize has a significant effect on the dissolution of vanadium and maximum extraction wasachieved for the finest size of below 0.850 mm. 428  01020304050607080901000 20 40 60 80 100 120 140 160    R   (   %   ) Time (min) P.S<850 μ m 850μ m<P.S<1.40mm1.40mm<P.S<2.36mm  Fig. 3. Effect of particle size on vanadium recovery (at 70 °C, 3 M NaOH, S/L of 1:15g/mL) Effect of Solid to Liquid RatioEffect of S/L ratioonvanadium recoveryat 70 ºC, 3M sodium hydroxide concentration, 90 minutes and 0.850 mm particle sizeis presented in Fig. 4.This figure shows thatincreasing the S/L ratio to 1:15 leads to 94% vanadium recovery and above this ratio the vanadium recovery decreases. Generally, increasing the S/L ratio would increase the extraction of vanadium. This, however, is limited by the concentration (more precisely, activity) of proton ions available in the solution . This effect could be attributed to theincrease in the percent solid, which enhances the interaction between the ions, thus reducing the proton ions concentration [8, 9].    01020304050607080901000 0.05 0.1 0.15 0.2 0.25      R      (      %      ) S/L (g/mL)  Fig. 4.   Effect of S/L ratio on vanadium extraction (at 70°C, 3 M NaOH, 90 min, particlesize 0.850 mm) 429
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