Vanadium Pentoxide from Vanadium Slag: Roasting to V2O5 Flake

From Ore to Oxide: Why the Slag Route Dominates

Vanadium pentoxide is the pivot point of the entire vanadium chain. Nearly all ferrovanadium, most vanadium catalysts, and the electrolyte for vanadium flow batteries are made from V2O5, and the great majority of that oxide is not produced from a clean vanadium mineral at all. It is recovered as a by-product of steelmaking: vanadium-bearing magnetite is smelted to vanadium pig iron, the pig iron is blown to vanadium slag, and the slag becomes the true feed for the oxide plant.

This matters to anyone buying or specifying V2O5, because the slag route fixes the impurity profile of the product. A slag-derived oxide carries iron, silicon, titanium, chromium, manganese and sulphur in proportions set by the ore body and by the steelmaking practice that produced the slag, not by the oxide plant. Two suppliers offering the same 98% V2O5 assay can therefore deliver very different material in terms of silicon, chromium and alkali content, and those differences show up downstream in alloy recovery, catalyst poisoning or electrolyte stability.

Smelting Vanadium Ore to Vanadium-Bearing Pig Iron

The typical ore is a vanadium-bearing titanomagnetite with roughly 0.3-1.4% V, iron around 55-60% and TiO2 in the region of 10-14%. It is reduced with coke in a blast furnace or in an electric smelting furnace to produce a pig iron that carries the vanadium in solution rather than in the slag. Keeping vanadium in the metal is the whole point of this first step: vanadium is readily oxidised, so the smelting conditions must stay reducing and the slag must be kept from becoming an oxidising sink for vanadium.

The resulting vanadium pig iron normally contains about 0.4-1.2% V, with silicon below 0.5% and titanium around 0.3%. The low silicon is deliberate. Silicon is a stronger deoxidiser than vanadium in the blowing step, so a high-silicon pig iron burns silicon first, raises the heat, and produces a slag that is high in silica and low in vanadium. Low silicon and controlled temperature are what make a rich vanadium slag possible.

Ore quality variation is felt immediately here. Iron grade, V-to-Fe ratio, TiO2 level and the alkali and sulphur content all change the economics of the furnace, and a drop in the V-to-Fe ratio raises the volume of pig iron that must be blown to obtain a given tonnage of slag. For a buyer of V2O5, the practical consequence is that oxide output is tied to steel and pig iron production rates, not to oxide demand, which is why vanadium prices move on steelmaking news as much as on vanadium news.

Converter Blowing and the Chemistry of Vanadium Slag

The pig iron is blown with oxygen in a top-blown converter, or in a ladle in smaller operations, under conditions designed to oxidise vanadium preferentially and to freeze the oxide into a slag that can be skimmed. Temperature control is the critical variable. The blow is run relatively cool, and the oxygen potential is raised in stages, so that vanadium oxidises before carbon does; if the heat runs too hot and too carbon-hungry, vanadium reports to the steel and the slag becomes lean.

The slag produced in this step is the ore of the oxide plant. Its composition controls everything downstream: the roasting reagent demand, the leach yield, the impurity load, and the final grade of the flake.

Component	Typical range in converter vanadium slag	Process consequence
V2O5 equivalent	10-18%	Sets the tonnage of slag per tonne of oxide
Total iron as FeO	25-45%	Metallic iron must be removed before roasting
SiO2	13-20%	Forms sodium silicate in roasting; consumes alkali
TiO2	About 8-12%	Can form insoluble titanates; lowers leach yield
Cr2O3	1-5%	Co-leaches as chromate; needs separation or a low-chrome grade
MnO	2-6%	Partially leaches; affects solution purification
CaO and MgO	2-8% combined	Raises alkali demand and slag melting point

Two structural features of the slag determine leachability. First, most of the vanadium sits in a spinel phase, typically a vanadium-bearing magnetite or a vanadium-chromium spinel, which is chemically stubborn: the vanadium in the spinel is in the trivalent state, insoluble in water and only slowly attacked by acid. Second, the slag is dense and hard, so it must be finely ground before any reaction can reach the locked vanadium. Both facts point to the same conclusion: the vanadium must be both oxidised and structurally broken down before it can be dissolved.

Crushing, Milling and Iron Removal

After skimming and cooling, the slag is crushed and ball-milled to a particle size below about 0.1 mm. Grinding fineness is a direct lever on vanadium yield: coarse particles leave unreacted spinel cores that pass through roasting untouched and report to the leach residue as lost vanadium, while over-grinding raises milling energy and makes the subsequent leach and filtration slower.

Metallic iron is a separate problem. Converter slag carries entrained semi-steel and metallic iron shot, which is not only dead weight but actively harmful downstream: in the roasting step it consumes oxidant and can form insoluble iron-vanadium compounds, and in the leach step it consumes acid or reduces vanadium back to the tetravalent state. The usual practice is to remove it magnetically, either with a drum or belt magnetic separator after coarse crushing or with an air classifier plus magnetic separator on the ground product. The recovered iron is returned to the steel plant as a coolant or scrap, so the separation pays twice.

Ground slag is normally blended and stockpiled to homogenise composition before roasting. Because the roasting reagent addition is calculated from the vanadium and silica assays, an unblended stock forces the plant to run conservatively high on alkali, which increases reagent cost, raises the sodium content of the product and increases the salt load in the effluent.

Salt Roasting: Converting V2O3 to Soluble Vanadate

Roasting is the conversion step. The objective is to oxidise the trivalent vanadium in the spinel to pentavalent vanadium and simultaneously to combine it with sodium oxide to form a water-soluble sodium vanadate, typically sodium metavanadate or sodium orthovanadate depending on the sodium-to-vanadium ratio and the temperature. Neither oxidation alone nor alkali addition alone will do the job: vanadium pentoxide itself is only slightly soluble, so the sodium combination is what makes the water leach viable.

The reagents are sodium carbonate, sodium sulphate, or a mixture, sometimes with sodium chloride in older practice. Sodium carbonate is the more effective alkali but the more expensive and the more aggressive toward refractories; sodium sulphate, often available as a by-product, is cheaper and is reduced to sulphide and then re-oxidised in the bed, which generates sulphur dioxide in the off-gas and requires gas cleaning. The choice is an economic and environmental trade-off, not a technical one.

Roasting is carried out in a rotary kiln or on a sintering grate at temperatures in the region of 800-900 degrees Celsius, held below the point at which the charge softens and starts to stick to the kiln. Retention time and bed agitation must be sufficient for oxygen to reach the spinel particles. Under-roasted charge retains insoluble trivalent vanadium and shows up as a low leach yield plus a high vanadium content in the residue; over-roasted charge forms refractory calcium and iron vanadates and can also volatilise vanadium, both of which lose metal and contaminate the off-gas dust.

Three controls keep the kiln honest: the alkali-to-vanadium ratio, expressed as a sodium oxide to V2O5 molar ratio; the roast temperature and the free-oxygen level in the kiln; and the leach yield measured on each batch of calcine. In practice a plant will run the ratio slightly above the stoichiometric requirement, because unreacted alkali is recovered in the leach liquor and recycled, whereas unreacted vanadium is lost to residue.

Leaching, Purification and Precipitation

The hot calcine is leached with water, usually in counter-current stirred tanks, at a solids loading and temperature chosen to keep sodium vanadate in solution while rejecting the bulk of the iron, calcium and silica into the residue. The pregnant liquor is then clarified; the residue reports to tailings, and its vanadium content is the plant's single most closely watched loss figure.

Purification follows in stages. The first is desilication, usually by adding a small amount of aluminium salt or by controlled pH adjustment, which precipitates a sodium aluminosilicate and removes the silica that would otherwise gel and foul the filters. The second is removal of phosphorus and arsenic where the ore carries them, typically by precipitation with magnesium or calcium salts at a controlled pH. Chromium, if present in the slag, co-leaches as chromate and must be reduced and precipitated if a low-chromium oxide is required.

The purified liquor is then heated and treated with an ammonium salt, usually ammonium sulphate or ammonium chloride, to precipitate ammonium metavanadate. The precipitation is pH and temperature sensitive: acidification shifts the equilibrium toward the metavanadate and then toward polyvanadates, and the crystalline form and crystal size determine how well the product filters and how much liquor it retains. Ammonium metavanadate is filtered, washed and then either dried and sold as a chemical or calcined.

Calcination, or fusion, converts ammonium metavanadate to vanadium pentoxide, drives off ammonia and water, and melts the product so that it can be cast into flake. Fusion temperature must be high enough to give a fluid melt and low enough to avoid excessive volatilisation of vanadium and contamination from the fusion vessel. The melt is poured onto a cooled casting wheel or into a water-cooled mould and breaks into the characteristic dark red-orange flake that the ferrovanadium and catalyst industries specify.

Product Specification and Grade Structure

Vanadium pentoxide for metallurgical use is bought against a grade such as the 98% grade within the vanadium pentoxide standard GB/T 3283, and against a size description: fused flake, broken plate, or powder of a stated mesh. Assay alone is not a sufficient specification, because a 98% product and a 99% product differ mainly in the impurity suite rather than in vanadium itself, and it is the impurities that decide performance downstream.

Characteristic	Metallurgical flake, typical	High-purity grade, typical	Why it matters
V2O5	98% min	99% min or higher	Charge calculation in FeV smelting; catalyst activity
Silicon as SiO2	0.20-0.50%	0.10% max	Raises slag volume and aluminium consumption in FeV
Iron as Fe2O3	0.20-0.50%	0.05-0.10% max	Dilutes the alloy; colours catalysts and electrolyte
Sodium and potassium as Na2O plus K2O	0.30-1.00%	0.10% max	Residual roasting alkali; affects catalyst and battery life
Sulphur	0.05% max	0.02% max	Carried into steel; off-gas in calcination
Phosphorus	0.03-0.05% max	0.01% max	Harmful in rail and forging steels
Chromium	0.05-0.20%	0.02% max	Chromate carry-over; electrolyte and catalyst poison
Loss on ignition	0.30% max	0.20% max	Indicates residual moisture, ammonium salts or under-calcination
Size	Flake 3-5 mm thick, under 20 mm face	Powder, stated mesh	Reaction rate in aluminothermic smelting; dissolution rate

For the ferrovanadium customer, the practical specification is narrower than the table suggests. An aluminothermic charge wants flake, not fines, and it wants a stable V2O5 assay lot to lot, because the aluminium addition is computed from that assay. Fines are carried off in the fume and destabilise the ignition front, and a lot that assays 97% instead of 98% silently adds about 8-9 kg of unnecessary aluminium per tonne of oxide to the correct stoichiometry.

Impurity Control, Safety and Environmental Compliance

Vanadium pentoxide is a hazardous material and must be handled as one. Dust and fume are respiratory irritants and sensitisers, and occupational exposure must be controlled by enclosure, local exhaust ventilation and appropriate respiratory protection, with exposure monitoring against the national occupational exposure limit for vanadium pentoxide dust and fume in GBZ 2.1. Skin and eye contact with the powder or with the alkaline roast liquor causes irritation and burns; the alkaline liquors are corrosive and must be handled in lined or stainless systems with proper personal protective equipment.

The roasting stage is the environmental pressure point of the whole process. Sulphur dioxide from sulphate-based roasting, dust carrying vanadium and alkali, and ammonia vapour from the precipitation and calcination stages all require capture and treatment. Effluent carries residual vanadium, ammonia nitrogen and sulphate, and the discharge must meet the vanadium industry pollutant emission requirements of GB 26452, which sets limits on vanadium, ammonia nitrogen, sulphate and pH as well as on particulate emissions. Modern plants close the loop by recycling leach liquor and recovering ammonium sulphate as a fertiliser by-product, which reduces both reagent cost and effluent load.

Solid residue is the other legacy issue. Leach tailings are bulky, contain iron, titanium and residual vanadium, and must be managed in a lined facility with leachate collection. Where the economics allow, a second-stage recovery pass on the tailings is worth considering: the residual vanadium in tailings is often 1-2% V2O5, which is low but not negligible on the scale of a large plant.

Fire and reactivity hazards are modest but real: vanadium pentoxide is an oxidiser and should be stored away from combustible dusts and reducing agents, kept dry, and segregated from acids and ammonium salts. Warehouses should be ventilated, floors sealed and washed rather than swept dry, and spill cleanup should be wet rather than dry to avoid generating airborne dust.

Purchasing and Acceptance Practice

A workable purchase specification for V2O5 fixes five things: the grade against GB/T 3283, the full impurity suite rather than assay alone, the physical form and size, the sampling and analysis method, and the traceability of the lot to the production batch. Sampling should follow the ferroalloy and bulk material sampling principles of GB/T 4010 and ISO 3713, and the analysis should be by a method agreed in advance, since vanadium assays by titration and by X-ray fluorescence can differ by tenths of a percent and a tenth of a percent on a tonne of oxide is real money.

Buyers should also ask for the roasting route behind the product, because it predicts the residual alkali and sulphur profile, and for the chromium figure, because a slag-derived oxide from a high-chrome ore body can carry several tenths of a percent of chromium that will show up in the steel or in a catalyst. A certificate of analysis without a chromium and a sodium figure is incomplete for metallurgical use.

Three common purchasing mistakes are worth naming. Comparing offers on V2O5 assay alone, when silicon and sodium drive the downstream cost. Accepting powder where the process needs flake, then discovering the fume losses at ignition. And ignoring lot-to-lot assay stability, which matters more than the average assay, because the smelting charge is calculated from each individual lot.

FAQ

Q: Why is vanadium pentoxide made from slag rather than from a vanadium ore concentrate?
Because most vanadium is a by-product of steelmaking: vanadium-bearing magnetite is reduced to pig iron, the pig iron is blown to a rich vanadium slag, and that slag is a cheaper and more concentrated feed than a primary vanadium mineral.

Q: What is the purpose of salt roasting in V2O5 production?
Roasting with sodium carbonate or sodium sulphate oxidises the trivalent vanadium locked in the slag spinel to pentavalent vanadium and combines it with sodium oxide to form a water-soluble sodium vanadate, which is what makes the water leach economically viable.

Q: Why must metallic iron be removed from vanadium slag before roasting?
Entrained iron consumes oxidant, can form insoluble iron-vanadium compounds that depress leach yield, and reduces vanadium in the leach circuit. It is removed magnetically after crushing and milling below about 0.1 mm.

Q: What purity of V2O5 is needed for ferrovanadium production?
The metallurgical 98% grade of GB/T 3283 is the normal feed, supplied as fused flake 3-5 mm thick with limited fines. Aluminothermic charges are calculated from the lot assay, so lot-to-lot consistency matters as much as the minimum figure.

Q: Which impurities limit V2O5 in catalyst and battery applications?
Silicon, iron, sodium plus potassium, sulphur, phosphorus and chromium. High-purity grades restrict these to a fraction of the metallurgical limits, because alkalis and chromium shorten catalyst life and degrade flow battery electrolytes.

Q: How should vanadium pentoxide be stored and handled?
Dry, ventilated, segregated from acids, ammonium salts and combustible dusts, with spill cleanup done wet to avoid airborne dust. Dust and fume must be controlled by enclosure and local exhaust, with exposure monitored against the GBZ 2.1 occupational limit for vanadium pentoxide.