Vanadium slags

Imports and Exports. The United States has long been a significant importer of vanadium slags, but imports of pentoxide were negligible until they rose quickly to 850 metric tons ia 1974, and 2000 t ia 1975 (mostly from the Repubhc of South Africa). Peatoxide imports thea decliaed to 1400 t ia 1980 with Finland being the maia and South Africa the minor suppHers. In recent years, U.S. imports of ammonium and potassium vanadates and of other vanadium compounds have been 100—200 t/yr, mainly from the U.K., Germany, and the RepubHc of South Africa. 

High temperature corrosion may occur when superheater temperatures are above 500 to 550 °C and the sodium or vanadium slag is molten. 

The recovery of vanadium from these slags is of commercial interest because of the depletion of easily accessible ores and the comparatively low concentrations (ranging from less than 100 ppm to 500 ppm) of vanadium in natural deposits (147,148). In the LILCO appHcations the total ash contained up to 36% 20 (147). Vanadium is of value in the manufacture of high strength steels and specialized titanium alloys used in the aerospace industry (148,149). Magnesium vanadates allow the recovery of vanadium as a significant by-product of fuel use by electric utiUties (see Recycling, nonferrous LffiTALS). 

Ferrovanadium. The steel industry accounts for the majority of the world s consumption of vanadium as an additive to steel. It is added in the steelmaking process as a ferrovanadium alloy [12604-58-9] which is produced commercially by the reduction of vanadium ore, slag, or technical-grade oxide with carbon, ferrosiHcon, or aluminum. The product grades, which may contain 35—80 wt % vanadium, are classified according to their vanadium content. The consumer use and grade desired dictate the choice of reductant. 

Silicon Reduction. The preparation of ferrovanadium by the reduction of vanadium concentrates with ferrosiUcon has been used but not extensively. It involves a two-stage process in which technical-grade vanadium pentoxide, ferrosiUcon, lime, and fluorspar are heated in an electric furnace to reduce the oxide an iron alloy containing ca 30 wt % vanadium but undesirable amounts of siUcon is produced. The siUcon content of the alloy is then decreased by the addition of more V2O5 and lime to effect the extraction of most of the siUcon into the slag phase. An alternative process involves the... 

Vanadium powder can be prepared by substituting V2O2 for the V20 as the vanadium source. The heat generated during the reduction of the trioxide is considerably less than for the pentoxide, so that only soHd products are obtained. The powder is recovered from the product by leaching the slag with dilute acid. 

Most foreiga vanadium is obtained as a coproduct of iron and titanium. South Africa, Norway, and Finland are suppHers. Chile produces slag from an iron operation. AustraUa s first vanadium operation started produciag fused pentoxide flake from a vanadium mine ia 1980. Russia and the People s Repubhc of china produce slag and pentoxide from iron—titanium ores. 

The production of metals which form very stable oxides by tire aluminothermic process, such as manganese, clrromium and vanadium is carried out with reactants at room temperature which react to provide enough heat to raise the temperature of the products to high temperatures at which the whole system is liquid. The metal phase which is produced can therefore separate from the liquid slag which is formed. The production of clrromium serves as a useful... 

The usual extraction procedure is to roast the crushed ore, or vanadium residue, with NaCl or Na2C03 at 850°C. This produces sodium vanadate, NaV03, which is leached out with water. Acidification with sulfuric acid to pH 2-3 precipitates red cake , a polyvanadate which, on fusing at 700°C, gives a black, technical grade vanadium pentoxide. Reduction is then necessary to obtain the metal, but, since about 80% of vanadium produced is used as an additive to steel, it is usual to effect the reduction in an electric furnace in the presence of iron or iron ore to produce ferrovanadium, which can then be used without further refinement. Carbon was formerly used as the reductant, but it is difficult to avoid the formation of an intractable carbide, and so it has been superseded by aluminium or, more commonly, ferrosilicon (p. 330) in which case lime is also added to remove the silica as a slag of calcium silicate. If pure vanadium metal is required it can... 

The combustion process creates deposits that stick to all fireside surfaces. These deposits contain carbonaceous materials and often voluminous quantities of ash, slag, sulfur, and vanadium residues, especially when lower grade fuels are used. 

The presence of asphaltenes, originating in the fuel, acts as a trap for vanadium, nickel, and sodium (which promote slagging and sulfur corrosion)-, these asphalthenes often contain sulfur compounds, which simply add to the contaminant load. Additionally, asphaltenes act as precursors to spherical stack solids (cenospheres), which are exhausted with the flue gases as stack emissions. 

The vanadium-containing ash contributes significantly to the formation of sticky, adherent slags, to the oxygen-induced corrosion of hot-end fireside metals surfaces, and to the acid-induced corrosion of cold-end surfaces. 

The degree of slagging, is, in turn, closely related to the concentration of vanadium, nickel, and sodium compounds present in the fuel, and the types of low melting-point oxides and complex sticky deposits formed under specific boiler temperatures and prevailing conditions. These deposits are difficult to remove online with soot blowers, but... 

Furnace area and superheater slagging may occur at low furnace or superheater temperatures (below 450-500 °C) due to high vanadium content in fuel oil. These high levels of vanadium in the fuel reduce the eutectic temperature of the noncombustibles, creating a molten deposit that holds unbumed carbon and contributes to a thickening of the slag. The trapped carbon is unavailable for combustion, and this process consequently reduces the overall fuel combustion efficiency. 

Reducing the amount of vanadium and sodium deposits formed through the use of slag modifiers can also effect good control of acid corrosion by S03. Vanadium and sodium deposits form low-melting-point slags and are a major contributor to the formation of S03. 

This formulation is designed to control combustion and slagging problems associated with vanadium, sodium, and sulfur in heavy residual fuel oils. 

The preparation of ferrovanadium by this route is carried out batchwise in refractory-lined open reactors, with vanadium pentoxide, aluminum powder, iron scrap and lime or fluorspar constituting the charge. The reactions once initiated, proceed briskly to completion. The reaction heat is sufficient to melt the ferrovanadium and the alumina-lime/fluor-spar slag, which readily separate due to density difference. The aluminothermic ferroalloy product contains practically no carbon. 

Slags and sludges Copper converter slag vanadium sludge (from aluminum industry) ferroalloys slag... 

Vanadium/sodium enhanced corrosion of metal occurs when molten slag containing vanadium compounds forms on metal system parts. The steps in the corrosion sequence are listed below ... 

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