Extraction of Vanadium from an Industrial Waste T.K. Mukherjee and C.K. Gupta Metallurgy Division, Bhabha Atomic Research Centre, Bombay 400 085, India CONTENTS Page ABSTRACT 190 1. INTRODUCTION 190 2. GENERATION OF VANADIUM 190 3. SOLUTION PURIFICATION 192 4. PRECIPITATION OF VANADATE SALTS 193 A. Sodium Hexavanadate 193 B. Ammonium Metavanadate 193 C. Ammonium Hexavanadate 194 D. Calcium Vanadate 194 E. Ferrous Vanadate 195 5. CONVERSION OF VANADIUM COMPOUNDS TO METALLIC PRODUCTS 195 A. Ferrovanadium 196 Β. Vanadium-Aluminium Alloys 198 6. REFINING OF VANADIUM-ALUMINIUM ALLOYS" TO DUCTILE GRADE VANADIUM 201 A. Electron Beam Melting 201 Β. Fused Salt Electron Refining 201 7. CONCLUSIONS 203 REFERENCES 205 189 Vol. 11, Nos. 1-4, 1993 Extraction of Vanadium from an Industrial Waste ABSTRACT Among the various industrial wastes mentioned above, Bayer sludges from aluminium extraction plants Vanadium-bearing Bayer sludge, an industrial waste are quite rich in vanadium and, therefore, stand as a generated during recovery of alumina from bauxite, is highly attractive vanadium resource in countries like the widely recognised as an important secondary resource of USSR, France, Turkey and India, all of which have vanadium. Processes for the extraction of vanadium in bauxite ores associated with vanadium pentoxide. In the the form of compounds, alloys and metal from such a present paper, an effort is made to describe the mode of source essentially forms the subject matter of this paper. generation of this type of vanadium resource, purifica- The theory and practice of precipitation of various tion of the leach liquor formed by water leaching of the vanadate salts like sodium hexavanadate, ammonium sludge, precipitation of various vanadate salts from the metavanadate, ammonium hexavanadate, calcium vana- purified solution, conversion of these salts to vanadium- date and ferrous vanadate by chemically processing the bearing metallic products and finally refining these re- vanadium-bearing alkaline solutions generated from duced vanadium metallic products to ductile grade metal.. water leaching of vanadium sludges are described first. Conversion of these chemically produced vanadate compounds into ferrovanadium and vanadium-aluminium alloys by using an aluminothermic reduction technique 2. GENERATION OF VANADIUM SLUDGE is next presented. Finally, postreduction refining tech- Bauxite ore is generally associated with varying niques like electron beam melt refining with or without quantities of T1O2, Fe203, S1O2, CaO, P2O5, etc. a prior pyrovacuum treatment in the solid state and mol- Bauxite ores found in the USSR, France, Turkey and ten salt electrorefining process are described for obtaining India are also known to carry 0.01 to 0.056% vanadium ductile grade vanadium metal from various grades of as an oxide. Bauxite ore is universally treated by the aluminothermically prepared vanadium intermediates. Bayer process (Fig. 1) which yields pure AI2O3 and also always yields an unreacted residue called red mud and sometimes a second waste in the form of vanadium 1. INTRODUCTION sludge if the oxide is associated with V2O5. During the The various resources of vanadium can be broadly caustic soda digestion of bauxite, about 25 to 30% of classified /l/ as primary, by- or co-products and indust- vanadium and phosphorous present in the ore gets rial wastes. Primary resources are processed solely for codissolved along with aluminium minerals. The in- vanadium and include ores containing vanadium minerals soluble residue (red mud) which contains the undissolved like patronite, bravoite, sulvanite, elavidite and roscoe- part of bauxite and unrecoverable alkali is generated at a lite as well as materials like clay. The by- or co-product rate of 0.3 to 0.4 Τ for every ton of bauxite processed. group of vanadium resources are not worked exclusively A typical analysis of a red mud from an Indian for vanadium but also for other elements like uranium, aluminium plant is Fe2Ü3 35.5, AI2O3 23, T1O2 17.2, iron, titanium, phosphorous, etc. and include carnotite, S1O2 5, Na20 4.8 and V2O5 0.34 in weight percent. titaniferous magnetite, vanadiferous iron ores and phos- The leach liquor after separation from the red mud is phate rocks. Finally, a host of industrial wastes like treated for the precipitation of aluminium hydroxide. flyash from oil combustion, spent catalysts generated The vanadium value present in the leach liquor does not during desulphurisation of crude oils, waste sulphate precipitate initially along with Al(OH)3 but as its liquor from the Ti02-based pigment manufacturing concentration gradually builds up due to recycling of the plants, slime tailings generated during beneficiation of caustic liquor, a stage is reached when vanadium acts as a the phosphate ore and vanadium-bearing sludge rejected poison in the growth of the hydrated alumina seeds and during the treatment of bauxite ore by the Bayer process adversely influences the process efficiency. Moreover, if happen to contain vanadium in significant concentrations the vanadium in the circuit is carried over to the final to warrant recovery. aluminium metals, it will bring down the electrical 190 Ύ.Κ. Mukherjee and C.K. Gupta High Temperature Materials and Processes BAUXITE Fig. 1: Bayer process for the recovery of AI2O3 and vanadium sludge from vanadium bearing bauxite ore. 191 Vol. II, Nos. 1-4, 1993 Extraction of Vanadium from an Industrial Waste conductivity significantly. It is, therefore, mandatory to to upgrading and purification of lean solutions. Crystal- separate the excess vanadium from the circuit by cooling lization and selective precipitation are the two classic down the liquor to below 25 'C at which point a methods reported for the separation of phosphate from complex sodium vanadate salt of formular 2Na3V04, the vanadium-bearing leach liquors. NaF, 19H2O along with an isomorphous compound of In the crystallization process patented /3/ way back phosphorous precipitates out. in 1948, the highly alkaline solution is partially neu- This complex vanadium salt is sandy in texture and tralised prior to separation to disodium phosphate contains 5 to 20 wt% V2O5 besides free alkali, phos- crystals. It was claimed that separation efficiency depen- phate, silica, alumina and fluoride. The sodium complex ded on the ratios of vanadium to phosphorous in the salt generated by the M/s. Indian Aluminium Co. liquor; the higher the ratio of V2O5 to P2O5, the better (INDALCO) located in the eastern part of India, for the separation. According to another publication /4/ a example, contains 15% V2O5 and 3% P2O5. The V2O5 to P2O5 ratio of 1.86 was considered optimum vanadium sludge from the Hindustan Aluminium Co. for removal of most of the phosphate. In an investiga- (HINDALCO) located in the northern part of India is tion conducted by the National Metallurgical Laboratory produced at a rate of 1000 T/year /2/ and is relatively (NML) of Jamshedpur, India /5/, a leach liquor analysed richer in its vanadium content which has been analysed as 8 g/1 V and 22.7 g/1 PO4 was subjected to the crystal- as 17.5 - 18% V 0 and 1 to 1.2% P 0 . The Bharat 2 5 2 5 lization process. It was found to be possible to crystal- Aluminium Company (BALCO), yet another Indian lize out 16.5, 54.3, and 82.8% of the phosphate by producer of aluminium, on the other hand, rejects adopting a degree of evaporation on the order of 25, 50 vanadium sludge containing only 5 to 9% V2O5. and 60%, respectively. These three batches of crystals, Irrespective of the grade of the sludge, water however, contained 9.2, 39.4 and 46.2% of vanadium leaching yields a vanadium-bearing alkaline solution that present in the starting solution. It is, therefore, apparent carries sodium phosphate as an undesirable impurity. that crystallization is not an effective means for selective separation of phosphate. Also any effort to partially Depending on the V2O5 and P2O5 contents of the neutralise the leach liquor before crystallization did not sludge and the pulp density employed during the water succeed in improving separation efficiency. leaching, the leach liquor may analyse as 8 to 20 g/1 V2O5 or higher with a V2O5 to P2O5 ratio of 0.6 to The selective precipitation process involves adding 15. Vanadium value can be recovered from such solu- CaCl2 to the leach liquor to precipitate phosphorous tions in a variety of ways after adequate purification. according to the following reaction: 2Na3P04 + 3CaCl2 -> Ca3(P04)2 + 6NaCl (1) 3. SOLUTION PURIFICATION In the same work, selective precipitation of phos- Since sodium vanadate present in the sludge phate from a leach liquor analysed as 8 g/1 V and 22.7 exhibits high solubility in water, repeated leaching of g/1 PO4 was attempted using a stoichiometric amount of fresh batches of sludge in the same liquor can easily CaCl2 as per Eq. 1. It was found that about 75% of the generate a solution containing 20 g/1 V2O5 or higher as phosphate could be precipitated at a pH of 6. The loss mentioned earlier. During such a leaching process, con- of vanadium due to coprecipitation was 6.85%. Phos- centrations of phosphate, silicate and fluoride also build phate precipitation was found to be more complete as the up considerably. Separation of these impurities is pH was gradually raised beyond 6 and a maximum necessary to prevent not only inhibition of precipitation phosphate precipitation of 88.9% could be achieved at a of vanadate salts but also contamination. Purification of pH of 9 but the corresponding vanadium loss increased the leach liquor is, therefore, an essential step in the significantly to a value of 31.35%. According to work recovery of vanadium products. It should, however, be done by the U.S. Bureau of Mines /6/, phosphorous and mentioned here that classic methods of purification can fluoride from a vanadium-bearing carbonate solution serve the purpose adequately and it is not mandatory to could be removed by precipitation with CaCl2 and MgO.