Selective oxidation of vanadium prior to iron and phosphorus

A collaboration between SSAB Oxelösund, SSAB Tunnplåt, LKAB, MEFOS, Jernkontoret and LTU/MIMER financed by MISTRA started with the aim to survey the possibilities to recover vanadium from converter-slag. Pellets produced by LKAB have a V-content between 0.11 and 0.13%. SSAB, Swedish Steel AB, runs their blast furnaces with 100% LKAB pellets. As a result vanadium ends up in the converter slag (LD-slag). Recirculation of LD-slag as a slag former in the blast furnace practised at SSAB increases the vanadium load in the LD-converter. As a consequence the LD-slag produced by SSAB is very vanadium rich, 3- 5 weight % V2O5. The annual production of LD-slag at SSAB Oxelösund and SSAB Tunnplåt Luleå is approximately 350,000 tonnes. After the accident in Lillpite in northern Sweden where 23 heifers out of 98 cattle died of acute vanadium toxicity, LD-slag has been forbidden since 1992 as fertilizer in Swedish agriculture- and forestry industry. There is no other application of LD-slag partially due to the risk of vanadium leaching. As a result 170,000 tonnes each year is put on landfill. This corresponds to 4500 tonnes vanadium metal annual, with a currently potential value of US$ 130 millions. This can be compared with the aggregated world V-production of 51,000 tons in 2004. The possible benefits with V-recovery from LD-slag are dual; a sellable and valuable vanadium product and a sellable and usable slag. The steel industry is by far the biggest consumer of vanadium products, mainly ferrovanadium. Vanadium has many favourable properties in steels and as a result widely used in different types of steel; where HSLA-steels has been of great commercial importance. The best available technology for V-recovery from iron and steelmaking processes is based on pre-oxidation of hot metal from iron-making prior to the decarburization step. However pre-oxidation of hot metal at SSAB is not further considered due to the risk of disturbance of the main steel production line. The concept studied in this master thesis is based on reduction of LD-slag, producing a metal phase mainly consisting of V, P and Fe. If the V-content in LD-slag can be reduced below 0.3%, it can be used externally as a road building material. The next step is selective oxidation of vanadium prior to iron and phosphorus producing a V- rich slag phase, with O2 or CO2 as oxidant. To be able to sell the V-slag for direct FeV-production the ratios, V/Fe>1 and V/P > 500, must be reached. This report describes the results and the experiments carried out in purpose to examine the possibility of selective oxidation of vanadium. According to thermodynamics V-oxidation occurs before P- and Fe-oxidation. Initial small scale tests on selective oxidation were made with and without slag formers, an Al2O3- and a SiO2-based slag system were considered. Without slag formers the slag froze and target ratios could not be reached, however the main phase had high V- and low P- and Fe-contents. Phosphorus was mainly distributed in iron droplets in the slag. This confirms the hypothesis, that iron droplets were the main reason why target ratios could not be achieved in earlier investigations at MEFOS. With slag formers liquid slags were obtained for both cases, the investigation could serve as the basis for the larger scale tests carried out in a 150 kg induction furnace. The target ratios, V/Fe>1 and V/P>500, could then be reached in the 150 kg induction furnace tests with CO2 as oxidant, hence selective oxidation of vanadium prior to iron and phosphorus is possible. Obtained ratios with the Al2O3-based slag system were; V/Fe = 1.4 and V/P = 1094, and in the SiO2-based slag system; V/Fe = 1.5 and V/P = 556.

Author: Lindvall, Mikael

Source: Lulea University of Technology

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