Ferrovanadium smelting (vanadium slag - vanadium pentoxide - vanadium trioxide - metal vanadium - ferrovanadium - vanadium aluminum alloy - vanadium carbonitride - vanadium battery)

(1) Reduction period

The reduction operation first needs to melt the steel scraps and ferrosilicon, add the refining slag returned from the refining period, and then add a small amount of V2O5. The slag formed after smelting is called lean slag, and its V2O5 content is less than 0.35%. Pour out the lean slag, transfer to the second step of smelting in the reduction period, add aluminum particles, and control the Si and V contents in the alloy. Take fev40 as an example, it is required to ensure

Carry the ingredients in table 5.5.3 to the refining period.

Table 5.5.3 alloy composition

(2) Refining period

The purpose is to desilicate and increase the content of vanadium. V2O5 and lime are added to the slag together with excess silicon to increase the vanadium content in the alloy and meet the requirements of fev40. The composition of the product is shown in table 5.5.4.

Table 5.5.4 product composition

Vanadium rich slag produced in refining period is returned to reduction period for refining. Typical composition of vanadium rich slag is shown in table 5.5.5.

Table 5.5.5 composition of vanadium rich slag

See table 5.5.6 for the distribution of furnace charge in each smelting phase.

Table 5.5.6 burden distribution of each smelting phase /%

(3) Operation process

First, after the last furnace is discharged, the furnace top is tilted back, the slag and the residual slag on the furnace slope are removed, and the mixed magnesia with sufficient viscosity (brine: magnesia brick powder: Magnesia =1:3:5) is used to quickly repair the lining damage at high temperature, and the taphole is blocked. After the furnace is repaired, a certain amount of refining slag shall be padded on the furnace bottom. After the steel scraps are added, drop or replace the electrodes according to the burning condition of the electrodes, check each system, and power on after it is normal. At this time, use high voltage and low current, and immediately pour into the refining slag existing in the liquid state of the previous furnace. After the refining slag is returned, add phase I mixture. Increase the current to the maximum value according to the arc stability. After the completion of phase I mixture, try to push the furnace charge to the center area of three-phase electrode. When the furnace burden melts to a certain extent, ferrosilicon can be added in batches for reduction, and the basicity of the slag can be adjusted at the same time. After the ferrosilicon is fully reduced, aluminum block is added for reduction when the alkalinity is appropriate. The reduction reaction is intense, and the power is cut off when the flame is large. When V2O5 in slag is ≤ 0.35%, lean slag can be poured out. Low voltage and low current shall be used during slag pouring. At the later stage of slag pouring, it shall be slow and checked with a pull rod to prevent molten iron from pouring out. After the lean slag is poured, use an iron bar to dip the slag sample and send it to the laboratory for analysis of vanadium pentoxide content.

Secondly, after the phase I lean slag is poured out, the high voltage is used to feed. With the addition of phase II mixture, the current gradually increases to the maximum value. After the furnace charge is basically melted, add ferrosilicon for reduction. At the same time, adjust the basicity of the slag, continue to add ferrosilicon for reduction, and then add aluminum dilution slag. The slag discharge is the same as that of phase I.

During the final refining, it is the same as the charging for power supply in phase II. Adjust the amount of materials in the refining period according to the alloy composition. First melt the furnace materials with high voltage and high current, and then adjust the basicity of the furnace slag. When the basicity of slag is appropriate, small voltage and large current shall be used for temperature rise in time according to the length of arc. When the slag and alloy have appropriate temperature and fluidity, stir, take alloy samples and send them to the laboratory for analysis of V, Si, C, P and s components, and discharge them normally. During tapping, the refining slag shall be poured out from the slag outlet with small voltage and current, and the tap hole shall be opened, then the power shall be cut off for tapping, and then the casting shall be completed.

4.4 technical and economic indicators

Generally, the recovery rate of vanadium can reach 97% ~ 98%; Lean slag contains vanadium, V2O5 ≤ 0.35%; Smelting time, 80min/t. The consumption per 1t of fev40 is shown in table 5.5.7.

Table 5.5.7 typical unit consumption of smelting 1tfev40 (kg/t)

4.5 main equipment

Ferrovanadium is produced by silicothermic reduction method and smelted in ferroalloy electric furnace. The typical capacity is 840 ~ 2500kVA, the typical voltage is 150~250v, and the current is 4000~4500a. Furnace cover, bottom and wall shall be built with magnesia bricks. Graphite electrode is used for operation, with electrode diameter of 200~250mm. The representative equipment of a plant in Panzhihua is as follows.

① Transformer parameters. Specification: hsk7-3000/10, capacity: 2500kVA, primary voltage: 10000v, secondary voltage: 121, 92/210160v, rated current: 6870a.

② Electric furnace parameters. Specification: 3T electric arc furnace, electrode diameter: Φ 250mm, furnace shell: inner diameter Φ two thousand and nine hundred × 1835mm, polar circle: Φ 760mm, electrode stroke: 1300mm.

③ Electrodes. Graphite electrode, gb-3072-82, Φ 250mm。

5 smelting ferrovanadium by thermit method

5.1 basic principles

Because vanadium has many valence states, the principle of producing ferrovanadium by thermit method can usually be described by the following reaction. 

3V₂O_5(s)+10Al＝6V+5Al₂O₃   (Al)=-368.36kJ/mol    

(Al)=—681180+112.773T (J/mol)

3VO₂+4Al＝3V+2 Al₂O₃     (Al)=-299.50kJ/mol  

(Al)=-307825+40.1175T  (J/mol)

V₂O₃+2Al＝2V+Al₂O₃       (Al)=-221.02kJ/mol  

(Al)=-236100+37.835T  (J/mol)

3VO+2Al＝3V+Al₂O₃       (Al)=-195.90kJ/mol  

(Al)＝-200500+36.54T (J/mol)

It can be seen that the above reactions are all negative values, which are easy to carry out in thermodynamics. In terms of reaction exothermic value, aluminothermic reaction can completely meet the heat required for spontaneous reaction, which is called aluminothermic method. In fact, the reaction is explosive (under adiabatic condition, the reaction temperature can reach about 3000 ℃), so the reaction speed must be artificially controlled.

The reduction reaction with vanadium trioxide consumes 40% less aluminum. However, when smelting high vanadium ferrovanadium by thermit method, the heat of reaction is obviously insufficient, so it is impossible to maintain the automatic reaction, so it is necessary to supplement some heat. At present, the method of supplementing heat by energizing is called electro thermit method. Of course, side effects can also be used. Ferrovanadium alloy with high vanadium content and less impurities can be produced by thermit smelting.

5.2 ferrovanadium smelting process and equipment by thermite method

(1) Raw materials

① Vanadium pentoxide: v2o598 grade conforming to gb3283-87 standard. Particle size: 55 × fifty-five × 5mm。

② Aluminum bean: al>99.2%, fe<0.13%, c<0.005%, si<0.1%, p<0.05%, s<0.0016%, particle size: 10~15mm.

③ Lime: Cao ≥ 85%, mgo<5%, SiO2 ≤ 3.5%, s ≤ 0.15%, P ≤ 0.03%, ignition loss ≤ 7%.

④ Iron filings: C ＜ 0.40%, particle size ＜ 15mm.

⑤ Returned slag: that is, slag (corundum slag) produced by thermite method, with particle size of 5~10mm.

The ideal process condition for ferrovanadium burden smelting by thermit method is that the reaction heat per unit charge is 3140~3350kj/kg. The aluminum content shall be 100%~102% of the theoretical amount required for V2O5 reaction. Generally speaking, increasing the amount of aluminum in the aluminothermic reaction can make the reaction complete and sufficient and achieve a high vanadium recovery rate. However, when the amount of aluminum exceeds a certain limit, the excess aluminum will enter the alloy and fail to meet the quality requirements; On the other hand, due to the high aluminum content in the alloy, its specific gravity is reduced, which affects the settling speed of the alloy in the slag, increases the alloy inclusion in the slag, and reduces the vanadium recovery rate; At the same time, due to the increase of aluminum consumption, the production cost increases and is uneconomical.

The calorific value of aluminothermic reaction exceeds the required value, so inert materials such as returned slag, lime and crushed alloy are added to the furnace charge to reduce the calorific value of the furnace charge and ensure the smooth reaction. Inert materials shall be added at 20%~40% of V2O5 as appropriate.

Output of ferrovanadium = (input metal V × Vanadium yield%) / vanadium content in alloy%

Steel scrap addition = ferrovanadium output × (1 - vanadium content of the alloy% - impurity mass of the alloy%)

Since aluminothermic reaction becomes spontaneous reaction after aluminothermic reaction, the reaction time is short and difficult to control, so the quality of proportioning process directly affects the quality of ferrovanadium products. Therefore, proportioning must be accurate (calculation and weighing) and evenly mixed to avoid furnace burden segregation.

All raw materials for ferrovanadium production shall be thoroughly dried to avoid splashing during smelting.

(3) Main smelting equipment

① Mixer: select according to the situation.

② Reaction furnace: the cylindrical furnace shell made of cast iron or steel is reinforced with steel clamping ring outside and lined with magnesia bricks. In order to improve the service life of magnesia bricks, the inner wall of the furnace is knotted with ground corundum slag and brine mixture. Magnesia can be laid at the bottom of the furnace, and then baked and dried. The whole furnace can be placed on a movable flat car. The size of the furnace depends on its output. Generally, the inner diameter is 0.5~1.7m and the height is 0.6~1.0m.

③ Reaction chamber: smelting space with exhaust hood system. It is a place for smelting by thermite method.

(4) Smelting operation

Ferrovanadium smelting is carried out in a cylindrical furnace. The preparation process of smelting furnace is divided into three processes: furnace laying, knotting and furnace drying. The lining of ferrovanadium smelting furnace is divided into long-term layer and temporary layer. The long layer is built with magnesia bricks and high alumina bricks in three sections, and the temporary layer is tied with return slag. The resistance to rapid cooling and heating is poor. Bricks are easy to be damaged during furnace dismantling. Good furnace lining knotting quality is the key to prevent furnace leakage. The knotting strength is moderate to avoid difficult furnace dismantling. At the same time, the knotting layer at the bottom of the furnace body is thicker than the upper half. In addition, other impurities with low melting point shall not be mixed in the tying material; The joint between furnace body and bottom must be plugged tightly.

When smelting ferrovanadium, the smelting furnace shall be hoisted onto the flat car first. When the lower part is used for ignition, a small amount of furnace charge shall be loaded into the bottom of the furnace barrel, the base charge shall be distributed, some mixed V2O5 powder and aluminum powder shall be placed on the surface, and some ignition agents such as BaO2, potassium chlorate or magnesium chips shall be placed on the surface. Then the flat car is sent to the smelting room. After ignition with ignition agent, gradually add all furnace burden from the upper part according to the reaction conditions. The feeding speed should be appropriate. If the feeding speed is too fast, the reaction speed of furnace burden is fast, the furnace temperature rises, and the splashing is serious, which increases the loss of vanadium and aluminum; If the feeding speed is too slow, the reaction is slow, and the smelting temperature is low, the slag will bond too early, the slag iron separation is incomplete, the alloy agglomeration is not good, and the vanadium recovery rate will decrease. Experience shows that it is appropriate to control the feeding speed at 160~200kg/ (m2.min).

When the upper ignition is adopted, the furnace charge shall be added into the furnace first and then ignited. This method generally adopts the lower ignition method because of the intense reaction, heat concentration and serious splashing of furnace charge.

After the smelting furnace is dismantled, the alloy ingot shall be cooled by water quenching, then the alloy surface shall be finished, then the iron shall be smashed, broken, screened, packaged, and finally warehoused.

The slag is hoisted to the crushing system. After treatment, part of the slag is returned as batching slag, part is used to tie the furnace lining, and the remaining slag is sold to the refractory plant.

(5) Technical and economic indicators

① Output: depending on the volume of the furnace, it is between 500~1000kg, but not more than 2000kg.

② Product quality: generally, products containing 75%~82% vanadium can be obtained. Other components (%) are 1.0~1.5si; 1.0~2.0Al； 0.15~0.2C； ≤0.05S； ≤0.025P。

③ Vanadium recovery rate: generally 85%~90%, up to 95%.

(6) Methods to improve vanadium recovery

Due to the intense reaction of aluminothermic method, some metal beads will be mixed in the slag, and the slag contains high vanadium. In order to improve vanadium yield, the following two methods are generally used.

① Heating sedimentation method

After the completion of aluminothermic reaction, a heating settling agent composed of ferric oxide and aluminum particles is added to the slag surface immediately, which has two purposes:

First, the exothermic reaction of the precipitant keeps the slag in the molten state, which is conducive to the separation of slag and ferrovanadium, and the alloy continues to decline; Second, when the iron aluminum alloy produced by the precipitator reaction drops through the slag layer, the vanadium oxide in the slag that has not been reduced and the alloy particles suspended in the slag are adsorbed to improve the yield of vanadium. Generally, this method can increase the yield by more than 2%.

The settling agent can be added manually or mechanically (such as spray gun). It should be pointed out that the increased iron content in this part should be taken into account in the calculation of ingredients to avoid the reduction of vanadium grade due to excessive iron in the alloy.

② Electrothermal method

After the thermite reaction is completed, the flat car shall be immediately sent to the electric heater, and the slag shall be electrically heated to maintain the molten state of the slag, so that the alloy will continue to decline, so as to improve the vanadium yield.

Smelting ferrovanadium by 6-carbon reduction method

In laboratory or small-scale production, carbothermal method can be selected. The chemical reaction equation for smelting ferrovanadium is:

V₂O₅+C=2VO₂+CO↑   (C)＝49070-213.42T(J/mol)  

 2VO₂+C=V₂O₃+CO↑   (C)=95300- 158.68T (J/mol)   

V₂O₃+C=2VO+CO↑   (C)=239100-163.22T(J/mol)   

 VO+C=V+C↑        (C)=310300- 166.21T (J/mol)   

                         V₂O₅+7C=2VC+5CO↑     (C)= 79824- 145.64T (J/mol)   

The above reduction reactions are endothermic reactions, which can only be carried out with electricity to supplement heat. At the same time of forming carbide reaction, the free energy will be greatly reduced, so the reaction will be sharply enhanced, resulting in the formation of carbon alloy containing a certain proportion. In fact, the alloy produced under this condition contains 4%~6% carbon. Therefore, low-carbon ferrovanadium cannot be produced by carbon reduction in industry. However, in the laboratory, low carbon ferrovanadium can be produced by high temperature and high vacuum. Some foreign factories use similar methods to produce ferrovanadium containing 38%~40%v, 2%~3%c and 5%~12%s. This alloy cannot be used for most vanadium containing alloy steels, so the carbothermal method is rarely used.

7 direct smelting of ferrovanadium with vanadium slag

At home and abroad, there are many methods of directly smelting ferrovanadium with vanadium slag, most of which are in the research state, and few of which are actually produced. The direct smelting of ferrovanadium from vanadium slag is carried out in two steps. Firstly, the iron (iron oxide) in vanadium slag is reduced by selective reduction in an electric arc furnace with carbon, ferrosilicon or silicon calcium alloy, so that most of the iron is separated from the vanadium slag, while the vanadium remains in the vanadium slag. In this way, the pre reduced vanadium slag with high v/fe ratio is obtained.

In the second stage, the pre reduced vanadium slag after iron removal is reduced with carbon, silicon or aluminum in an electric arc furnace to obtain ferrovanadium alloy.

Panzhihua Iron and Steel Co., Ltd. and CITIC Jinzhou Ferroalloy Company in China have also tested the direct smelting of ferrovanadium with electric furnaces.

8 smelting ferrovanadium with vanadium trioxide

In the production of high vanadium ferrovanadium, vanadium trioxide can be smelted by thermit method, which can save the consumption of aluminum reductant and reduce the production cost. Different from the ordinary method of smelting FEV with V205 outside the furnace, the smelting equipment is smelted in the electric arc furnace because the heat of the reaction between V203 and aluminum is insufficient and cannot be carried out automatically. There are three purposes for using electric arc furnace, one is to supplement the insufficient heat during smelting with V2O3, the other is to improve the recovery rate of vanadium, and the third is to make the temperature in the furnace meet the requirements that the slag can be discharged and the molten iron can be cast into the ingot mold. The GFE electric furnace in Germany has a volume of 5M3, a power of 1.2MVA, a 4.5t three-phase electric arc furnace, a graphite electrode diameter of 300mm, and the lining is all knotted with this slag (corundum slag) without refractory bricks. It only needs to be repaired with slag each time.

① Weigh and mix V2O3, aluminum powder (particles), steel chips and lime into the storage tank, and place the mixture tank on the charging device on the top of the electric furnace with the material forklift;

② Melt some steel chips for about 5 ~ 10 minutes;

③ Then add the mixture into the furnace with electromagnetic vibration valve for smelting for about 50 minutes (the voltage is 130v);

④ After 5 minutes of pouring and slagging, the melt is cast into the arc ingot mold lined with the slag in the molten state (temperature: 2100 ℃);

⑤ After the metal is cooled in the ingot mold for one day (500 ℃), the alloy is put into the pool for quenching after demoulding, and then the high vanadium iron is obtained through finishing and crushing. In addition to being used for making up the furnace, the surplus slag can be sold away.

The smelting time of ferrovanadium is about 1 hour, the furnace charge is prepared at one time, and no other furnace charge is added during the smelting process. The power consumption of each furnace is about 1900kwh, 2T alloy and 2.4t slag containing 2 ~ 3% v can be obtained, and the vanadium recovery rate can reach 97%.

9 new ferrovanadium smelting technology

(1) Chengde Branch of Hebei Iron and steel company invented a clean production process of ferrovanadium alloy. Through integrated innovation, the two ferrovanadium production processes of electro silicothermic and electro aluminothermic methods are implemented in the same site, realizing the production capacity of full brand ferrovanadium alloy, and making full use of the complementarity of raw materials and materials in the process of smelting ferrovanadium alloy. The ferrovanadium powder produced by the crushing of ferrovanadium products is directly used in the production of ferrovanadium nitride without returning to the process for secondary smelting. The slag, lining and process waste of the electro aluminothermic process are recycled for the reuse of the electro silicothermic furnace, The high efficiency and clean production of ferrovanadium smelting have been realized; The smelting lean slag, dust removal ash and industrial wastewater produced by the two processes are recycled to reduce the comprehensive energy consumption of the process flow, improve the resource utilization rate of smelting ferrovanadium alloy, and make the whole process free of waste water and slag discharge. In line with the concept of circular economy, the two ferrovanadium production processes of electro silicothermic method and electro aluminothermic method are implemented in the same site for integrated overall design and construction to reduce the project cost.

(2) Zhejiang xinwanfei Technology Co., Ltd. has invented a preparation method of nitrided ferrovanadium alloy to prevent bonding after alloy firing, which includes the following steps: Step 1: mix vanadium containing compounds, powdered carbonaceous reductant, iron powder and binder, and press them into semi-finished balls with a diameter of 30 ~ 60mm; Step 2: during feeding, the semi-finished ball is mixed with granular carbonaceous reductant, and then dried, carbonized and nitrided, cooled and discharged. In this way, feeding and discharging are conducted every 6-8 hours. The semi-finished balls are separated by granular carbon reductant, which, on the one hand, increases heat transfer, makes the temperature rise faster, and reduces energy consumption; on the other hand, the burned balls are separated from each other, so that continuous feeding and discharging can be achieved, which greatly reduces the production cost and improves the production efficiency; The nitrided ferrovanadium alloy prepared by this method has stable quality and high density.

(3) Panzhihua Yinjiang Jinyong industry and trade company has invented a method of smelting ferrovanadium from Panzhihua Xichang vanadium raw materials. The specific method and steps are as follows: weigh the above raw materials and auxiliary materials according to the formula, put the weighed raw materials and auxiliary materials into the mixing equipment to fully mix them until they are uniform, and then put them into the reaction device preheated to 200-300 ℃ for fire reaction. After the reaction, add 30-50kg retarder, cool them for 48-72h, and finally separate the slag and alloy; Ferrovanadium alloy products are inspected, finished, packaged and warehoused. Compared with the prior art, the process step is simplified, which not only improves the safety, but also reduces the production cost; The high vanadium iron product of the invention has high vanadium grade, low sulfur and phosphorus level and low aluminum and silicon content, which effectively reduces the harm of impurities that may be brought into the steel grade and has a positive significance in promoting the deoxidation and alloying of steel production.

(4) Panzhihua Iron and Steel Group Co., Ltd. has invented a production process for smelting ferrovanadium alloy by electro thermic method. The key improvement is to provide a low-cost, easy to control and non-corrosive slag mixing step for furnace lining. The production process is: A. batching; B. Reduction smelting; C. Slag mixing; D. Casting and crushing according to conventional methods; The improvement is as follows: in step C, iron oxide is added to adjust the slag, and the amount of iron oxide is 15 ~ 20% of the weight of aluminum; After the iron oxide is melted, when the furnace temperature reaches 1600 ~ 1900 ℃ and the aluminum content in the ferrovanadium alloy liquid is less than 0.5%, the ferrovanadium alloy liquid and slag are discharged together. The addition of iron oxide is conducive to the removal of residual aluminum, reducing the aluminum content in the alloy to 0.2% ~ 0.5%. At the same time, the slag state can be adjusted to reduce the melting point of the slag, so that the smelting can proceed smoothly, the metal in the slag can settle into the alloy solution, and the vanadium yield can reach 97% ~ 99%, so as to reduce the smelting cost of ferrovanadium, improve the product quality, and increase the enterprise benefit and resource utilization.

(5) Central South University invented a method for producing ferrovanadium alloy from stone coal acid leaching solution. The process includes: adding ammonium (ammonia) to stone coal acid leaching solution to remove aluminum; Neutralize and precipitate iron and vanadium compounds after aluminum removal or oxidation; Iron vanadium compound purification; The process flow of the invention is short, the reagent consumption is small, the production cost is low, the product quality is good, the environment is friendly, and it is easy to realize industrial production.

(6) Hunan Metallurgical Materials Research Institute invented nitrided ferrovanadium alloy and its preparation method. The composition of ferrovanadium nitride alloy is: vanadium 47-81%, nitrogen 8-16%, iron 4-44%, carbon ≤ 1%, silicon ≤ 1.50%, aluminum ≤ 0.01%, manganese ≤ 0.50%, sulfur ≤ 0.05%, phosphorus ≤ 0.05%; The method is to mix the powdered vanadium compound, carbonaceous reductant and iron powder, add the binder, press the block, dry it and put it into the metallurgical furnace with protective atmosphere for carbothermal reduction, nitriding and sintering reaction. The temperature of carbothermal reduction section is 900 ℃ -1200 ℃, the temperature of nitriding section is 1200 ℃ -1350 ℃, the temperature of sintering section is 1350 ℃ -1490 ℃, and the total reaction time is 2-18 hours. This method simplifies the traditional process, greatly shortens the reaction cycle, reduces the equipment investment and production cost, and the obtained product has stable quality, high vanadium yield, high density and low melting point in steel.

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Reference: production process and equipment of vanadium and titanium products, Beijing: Chemical Industry Press, author: zoujianxin et al., January 2014

Physical chemistry of vanadium and titanium, Beijing: Chemical Industry Press, by zoujianxin, 2016

(Sichuan Key Laboratory for comprehensive utilization of vanadium and titanium resources [Panzhihua university], cnzoujx@sina.com ）

Sichuan vanadium titanium material engineering technology center