SOLUBLE COMPOUND OF VANADIUM

Abstract: A gas mixture containing hydrogen sulphide is contacted with an aqueous alkaline solution to form bisulphide and thereby remove the hydrogen sulphide from the solution. The bisulphide reacted with a soluble vanadium (V) compound to form sulphur. The vanadium is subsequently reoxidised. The soluble vanadium compound is Na<u3>uVO<u4>u . 1/4NaOH.12H<u2>uO. It is prepared by adding sodium hydroxide to an aqueous solution containing vanadium (V) at elevated temperature, and then cooling the mixture. The amount of sodium hydroxide added is sufficient to increase the mole ratio of Na to V in the solution (preferably from at least 2:1) to at least 3:1 (and preferably to at least 6:1).

Description: Soluble Compound of Vanadium This invention relates to the preparation of a soluble compound of vanadium and to its use in gas purification. In the well known Stretford process for removal of hydrogen sulphide from a gas mixture containing hydrogen sulphide, the gas mixture is washed with an al aline aqueous solution (typically of sodium carbonate) so as to convert the hydrogen sulphide tσ (sodium) bisulphide. The bisulphide is then reacted with a soluble vanadium (V) compound which is dissolved in the aqueous solution. The bisulphide is consequently oxidised to sulphur, the vanadium being reduced to oxidation state IV. The vanadium is then reoxidised to oxidation state V by reaction with an oxidising agent which is also contained in the solution. The oxidising agent is of a kind which can be reoxidised by air or oxygen. The oxidising agent is typically anthraquinone 2, 7 disulphom ' c acid. Thus, continuous regeneration of vanadium in oxidation state V is made possible. Moreover, the hydrogen sulphide is converted to sul hur which may be readily collected as a vendible product. In practice, the aqueous solution of vanadium (V) compound, alkali, and oxidising agent is prepared in a first vessel- and then passed into a second vessel where it is contacted with the gas mixture. The resulting sulphur-containing mixture is returned to the first vessel where it is blown with air to reoxidise the vanadium, to regenerate the oxidising agent and also to help the sulphur to rise to the surface of the liquid in that vessel. The sulphur may be collected from the surface. Substantially all the hydrogen sulphide is removed from the gas mixture, which is simply passed through the second vessel. Typically, a sequestering (or stabilising) agent such as sodium tartrate is included in the solution. Alternative oxidising agents to anthraquiπone which also function as sequestering agents are known. Vanadium (in oxidation state (V) has been added in various forms to the alkaline liquor, for example: 1. As sodium etavanadate. 2. By dissolving commercial vanadium pentoxide in the alkaline liquor. 3. In the form of a soluble polyvanadate of sodium and ammonium. 4. As ammonium metavanadate. Whichever of these forms of vanadium is used, there is a loss of vanadium from the liquor by progressive precipitation of an insolubl form of vanadium, which may contain vanadium in oxidation state IV. In addition, some forms such as vanadium pentoxide are found to be difficult to dissolve and if they can be dissolved, this can be done only slowly. In the art, this is considered to be an important disadvantage. A further disadvantage associated with vanadium pentoxid is that it generally contains some vanadium in oxidation state IV (ie. it contains some V ? ^ W ^ 1C ^ 1S inactive in the Stretford and analogous processes. In addition, ammonium is considered to be undesirable. by many users of the Stretford process who consequently do not use ammonium metavanadate or ammonium polyvanadate. There is thus a need for a new soluble compound of vanadium (V for use in the Stretford process. We have discovered that a soluble compound of vanadium of formula: Na 3 V0 4 .l/4 NaOH.12H 2 0 is surprisingly soluble in aqueous solution. Moreover, this compound is relatively stable in aqueous solution: the propensity of vanadium to come out of solution is relatively small. We have also found that the compound dissolves relatively quickly in water. The existence of this compound is reported by Chretien and Lelong in Compt. Rend. Ser. C 262 (6), pp 478-479 (1966). It is also referred to in Gmelins Handbuch der Anorganischen Che ie, System-Num er 48, Teil B - Lieferung 2 (1967), pp 382-383. An X-ray diffraction stud of the compound has been reported by E. Till mans and W H Baur, Acta Crystallographica, B27, pp 2124-2132 (1971), who found it hexagonal, space-group P 3 c 1, a=b=12.038 Angstroms, c=12.833 Angstroms. The X- ray diffraction method is probably the best way of characterising the OMPI vanadium compound. One characteristic of this vanadium compound that has been reported is a marked tendency for it to pick-up carbon dioxide from the atmosphere. It is believed that carbonate is formed, though, in general, an appreciable proportion of hydroxide remains. We have also found that the compound tends readily to give up at least some molecules of water of crystallisation on being heated. We have found that the compound is still very soluble even after pick-up of carbon dioxide and/or at least partial loss of its water of crystallisation. No method of preparing this compound has, we believe, been reported. According to the present invention, there is provided a method of removing hydrogen sulphide from a gas mixture containing hydrogen sulphide, including the steps of contacting the gas mixture with an aqueous alkaline solution to form bisulphide, dissolving a soluble compound of vanadium (V) in the solution, reacting the bisulphide with the soluble compound of vanadium (V) with the bisulphide to form sulphur, the vanadium being reduced to oxidation state IV, and reoxidising the vanadium to oxidation state V, wherein the soluble vanadium compound is: Na 3 Y0 4 .l/4NaOH.12H 2 0 or a product formed by driving off at least some of the molecules of water of crystallisation therefrom, and/or by the compound picking-up carbon dioxide. The invention also provides a method of preparing a vanadium compound of formula: Na 3 V0 4 .l/4NaOH.12H 2 0 wherein at elevated temperature an aqueous solution, containing 10 to 100 g/1 of vanadium (in odixation state V) has added to it ' sufficient sodium hydroxide to increase the mole ration of Na to V in the solution to at least 3, preferably to at least 6, and most preferably to 6.5 to 6.7 and then the solution is cooled so as to form crystals of the compound of vanadium. The solution mmay be of sodium metavanadate, or sodium orthovanadate or indeed any other compound of sodium, vanadium (V) and oxygen. The solution is preferably sodium vanadate liquor. It may be prepared by methods well known in the art. For example, sodium vanadate liquor may be prepared in accordance with the OMPI common practice in the industry, by roasting or fusing low grade vanadium bearing materials with soda ash, caustic soda or sodium salts in odixising atmosphere thereby converting the vanadium to sodium vanadate, and then extracting the so-formed sodium vanadate by leaching with water. The vanadate liquor preferably contains from 35 to 60 g/1 of vanadium. We have found that with sodium vanadate liquor below a vanadium concentration of 35 g/1, the yield of the oroduct decreases with decreasing vanadium concentration. Above a vanadium concentration of 60 g/1, we believe the magma of crystals which forms on cooling the liquor will be so dense that it will be difficult to handle. Typically the sodium vanadate liquor contains from 2 to 4.5 moles of sodium for every mole of vanadium. The elevated temperature is preferably at least 50°C, but is preferably below 100°C. The sodium hydroxide is preferably added in solid form so as to avoid dilution of the vanadate liquor. Care should be taken to avoi excessive temperature rise during the addition of the alkali. It is desirable to dissolve the alkali as quickly as possible. Thus, the vanadate liquor is preferably stirred vigorously throughout the additio of the alkali. The amount of sodium hydroxide which needs to be added depends on how much sodimm there is in the starting solution. Typically the mole ratio of Na to V in the starting solution is in the order of 2:1 up to about 4.5:1 though it may frequently be in the order of 4:1 if the starting solution is sodium vanadate liquor. In these circumstances sufficient sodium hydroxide may be added to increase the mole ratio to at least 5:1, preferably to at least 5.5:1 and most preferably to 6:1 to 7:1. As a result of dissolving the hydroxide in the vanadate liquor a clear solution is generally formed. This solution is preferably passed through a fine "polishing" filter to remove any particles of impurity and thus ensure that there is a clear solution. The solution is then preferably allowed to cool, preferably to ambient temperature. Desirably, the solution is stirred during cooling. As the solution cools so a thick magma of fine crystals forms. The crystals may be recovered by conventional means, such as filtration and ceπtrifuging. As much of the mother liquid as possible is desirably removed during the recovery of the crystals in view of their relatively high solubility. It is also desirable not to wash the crystals with water or other aqueous medium. This is to as to avoid URE; O PI - dissolving them. The crystals may, if desired, be dried at a temperature in the order of 45°C. If desired, before drying, the crystals may be purified by recrystallisation from water. Little change in composition is found in the case of the sodium salt. Generally, there may be a reduction of one percent in the total sodium present. The vanadium salt according to the invention is found empirically to have a composition (excluding water of crystallisation) as follows: % by weight Na 34.50 V 23.52 0 balance, as determined by recrystallising to constant weight. Typically, the vanadium salt forms trigonal crystals which appear white to the eye. We believe that Na 3 V0 4 .l/4Ha0H.12H 2 0 is more soluble than vanadium oxides and sodium salts of vanadium previously used in the Stretford process. In addition, we believe that at least the novel sodium salt of vanadium has a greater stability than sodium ammonium polyvanadate. These advantages are illustrated below: 1. Solubility (in a g V/lOOg solution) Table 1 Ammonium Fused Sodium Na 3 V0 4 NaOH.12H 2 0 metavanadate V 2°5 ammonium . . polyvanadate- — Temp °C 20 20 20 80 30 70 30 70 Water 0.25 1.29 Insoluble 0.45 4.2 3.8 12.3 Fresh Stretford Liquor 0.18 2.1 5.2 12.8 Spent Stretford Liquor - 1.5 2.0 9.0 • OMPI Fro these results, it can be seen that under all the conditions examined, the new vanadium compound according to the invention is considerably more soluble than any of the alternatives. 2. Stability It has been reported that vanadium catalysts used in the Stretfo process diminish in activity with time due, it is claimed, to precipitation of part of the vanadium, and some users believe thi is related to the nature of the vanadium ionic species in soluti Stability is usually tested by passing hydrogen sulphide gas, heavily diluted with nitrogen as an inert carrier gas, through freshly prepared Stretford liquor for several hours, and then sealing the vessel to exclude air. A stabiliser, typically tartaric acid, is present in the liquor (although citric acid ma be used instead). After some lapse of time, say 1 to 4 weeks, the liquor is filtered and any insoluble vanadium is estimated by analysis. The following results have been obtained using this procedure. The results relate to a 2 litre solution containing 1.7 g/1. Table 2 Insoluble Vanadium (g) from Sodium ammonium Na V0 4 .l/4NaOH.12H 2 0 1 week 0.303 0.572 2 weeks 0.508 0.564 3 weeks 0.615 0.447 4 weeks 0.756 0.393 While the mechanism is not clear it seems that vanadium at first precipitated from the new vanadium compound according to the invention redissolved, whereas the commonly used polyvanadates show progressive precipitation. We have also found there to be a marked improvement in stability when no stabiliser is present in the liquor. Example 1 In this example, un-neutralised sodium vanadate liquor was used as a starting material. Four litres of un-neutralised vanadate liquor containing, in total, 196g (3.84 moles) of vanadium, and 374g (16.25 moles) of sodium (mole ratio of Na:V 4.2:1) were heated to 60°C, the liquor being stirred during the heating. The stirring was continued and 307.2g of sodium hydroxide pellets were added gradually to the mixture. This increases the mole ratio of Na:V. The stirring was continued and the mixture allowed to cool to room temperature, filtered and sucked dry through a vacuum filter to give 1.733kg of crude sodium salt of vanadium according to the invention. The resulting crystals were then raised in temperature to 105°C so as to drive off all moisture. It was found that as a result of this process 515 by weight of the dry solid was lost, ie. the solid contains approximately 51% by weight of uncombined water. It was found that the resultant solid contained 24.55* by weight of vanadium and 35.6% by weight of sodium. The filtrate was also subjected to analysis to find out how much vanadium it contained. It was found that the filtrate contained 10.4g/l of vanadium, the total volume of the filtrate being 2.721. It is to be appreciated that the preceding analysis of the crude sodium salt of vanadium was performed on only a small portion of that compound. The remainder was dissolved in 1.5 litres of demineralised water at 80°C, filtered hot and cooled. The resulting crystals were sucked dry by means of a vacuum filter and it was found that 1.017kg of crystals were obtained. An analysis of these crystals was conducted. A sample was taken and heated to 105°C to drive off all the uncombined water. It was found that the crystals contained 54 * by weight of water of crystallisation. An analysis of the anhydrous crystals showed that they contained 24.9% by weight of vanadium and 35.1% by weight of sodium. This corresponds within the confines of the standard experimental error to a formula of a 3 V0 4 . l/4NaOH.12H 2 0. The filtrate was found to contain 33g/l of vanadium, and there was in total 1.51 litres of such filtrate. The above results show that the yield of vanadium in the pure sodium compound according to the present invention was 59.8% by weight. In order to verify that the compound prepared in a manner described above is that reported in the literature under the form Na 3 V0 4 NaOH.12H 2 0, a crystal of the substance was coated with UHU cement and mounted on a fibre. It was then subjected to an X-ray diffraction study. The crystal was stable for long enough for it dimensions to be deduced from oscillation and Weissenberg photogr It was found that the substance crystallises in the hexagonal sys cell dimensions a=b=12.05 Angstroms and c=12.86 Angstroms. These conform well to those obtained by Tillimans and Baur. Example 2 The compound of vanadium prepared by the method according invention was employed in the Stretford process on a pilot plant s The aqueous solution used to treat the gas mixture containing hyd sulphide contained sodium carbonate (as the alkali) and anthraqu n 2, 7 disulphom'c acid (as the oxidising agent) at conventional concentrations, t In experiment A, sodium metavanadate was used as the vanad compound. In addition, a sequestering agent (or stabilising agent is sometimes called) sodium tartrate is present in conventional quantities. (The sodium tartrade being formed in situ by addition solution of tartaric acid). Experiment B involved the substitution of Na 3 V0 4 .l/4NaOH.12 for the sodium metavanadate. Experiment C involved the omission of the sequestering age the solution, which was otherwise identical to that used in Experi In each Experiment, the starting concentration of vanadium 1.7g per litre. In each Experiment, the solution was blown with a as to regenerate the oxidising agent. Periodic additions of the v compound were made to the solution so as to restore its vanadium c to 1.7 g/1. Similarly, in Experiments A and B, periodic additions tartaric acid were made to replenish the tartrate. In each Experiment the rate of loss of vanadium from solut measured. In experiments A and B, the rate of loss of the tartrat al so measured. The results as shown in Table 3 below. It can be seen tha significantly less vanadium is lost in Experiments B and C than in Bt Experiment A. Moreover, the results obtained in Experiments B and C are roughly comparable with one another, indicating that the method of removing hydrogen sulphide makes it possible to perform the Stretford process efficiently without using a sequestering or stabilising agent for the vanadium. Table 3 Experiment Rate of entry of Hrs of Operation Rate of Rate of H 2 S (in 1/hr) loss of lass of tartrate V %/day %/day 4.4 145 6.3 15.7 3.8 167.5 7.6 8.6 3.8 263 7.9

Claims: CLAIMS 1. A method of removing hydrogen sulphide from a gas mixture containi hydrogen sulphide, including the steps of contacting the gas mixtu with an aqueous alkaline solution to form bisulphide, dissolving a soluble compound of vanadium (V) in the solution, reacting the bisulphide with the soluble compound of vanadium (V) with the bisulphide to form sulphur, the vanadium being reduced to oxidatio state IV and reoxidising the vanadium to oxidation state V, wherei the soluble vanadium compound is: Na 3 V0 4 .l/4NaOH.12H 2 0 2. A method of preparing a vanadium compound of formula: Na 3 V0 4 .l/4NaOH.12H 2 0 wherein at elevated temperature an aqueous solution containing in 10 to 100 g/1 of vanadium in oxidation state V has added to it sufficient sodium hydroxide to increase the mole ratio of Na to V in the solution to at least 3 and then the liquor is cooled so as to form crystals of the compound of vanadium. 3. A method as claimed in claim 2, in which the mole ratio is increas to at least 5:1. 4. A method as claimed in claim 2 or claim 3, in which the starting solution is sodium vanadate liquor, formed by roasting of fusing l grade vanadium bearing materials with soda ash, sodium hydroxide or a sodium salt in an oxidising atmosphere, and then leaching the resulting solids with water. 5. A method as claimed in any one of claims 2 to 4, in which the mole ratio of Na to V is increased to at least 6:1. 6. A method as claimed in any one of claims 2 to 5, in which the mole ratio of Na to V is increased from a value in the range 2 to 4.5. a value of at least 5.5. 7. A method as claimed in any one of claims 2 to 6, in which the solu contains 35 to 60 g/1 of vanadium. / ^BU REΛ ( °MPI ~ " ΪPO~ 8. A method as claimed in any one of claims 2 to 7, in which the elevated temperature is above 50°C and below 100°C. 9. A method as claimed in any one of claims 2 to 8, in which the sodium hydroxide is added in solid form. \ 10. A method as claimed in any one of claims 2 to 9, in which the solution is cooled to ambient temperature. AMENDED CLAIMS (received by the International Bureau on 8 June 1979 (08.06.79)) 1. A method of removing hydrogen sulphide from a gas mixture containing hydrogen sulphide, including the steps of contacting the gas mixture with an aqueous alkaline solution to form bisulphide, dissolving a soluble compound of vanadium (V) in the solution, reacting the bisulphide with the soluble compound of vanadium (V) with the bisulphide to form sulphur, the vanadium being reduced to oxidation state IV and reoxidising the vanadium to oxidation state V, wherein the sol ble vanadium compound is: Na 3 V0 4 .l/4NaOH.12H 2 0 or a product formed by driving off at least some of the molecules of water of crystallisation from the compound, and/or by the compoun picking up carbon dioxide. 2. A method of prepar ng a vanadium compound of formula: * Na 3 V0 4 .l/4Na0H.12H 2 0 wherein at elevated temperature an aqueous solution containing in 10 to 100 g/1 of vanadium in oxidation state V has added to it sufficient sodium hydroxide to increase the mole ratio of Na to V in the solution to at least- 3 and then the liquor is cooled so as to form crystals of the compound of vanadium. 3. A method as claimed in claim 2, in which the mole ratio is increased to at least 5:1. 4. A method as claimed in claim 2 or claim 3, in which the starting solution is sodium vanadate liquor, formed by roasting of fusing low grade vanadium bearing materials with soda ash, sodium hydroxide or a sodium salt in an oxidising atmosphere, and then leaching the resulting solids with water. 5. A method as claimed in any one of claims 2 to 4, in which the mole ratio of Na to V is increased to at least 6:1. 6. A method as claimed in any one of claims 2 to 5, in the role ratio of Na to V is increased from a value in the range 2 to 4.5. to a value of at least 5.5. ^ -r- n"c OMP 7. A method as claimed in any one of claims 2 to 6, in which the solution contains 35 to 60 g/1 of vanadium. 8. A method as claimed in any one of claims 2 to 7, in which the elevated temperature is above 50°C and below 100°C. 9. A method as claimed in any one of claims 2 to 8, in which the sodium hydroxide is added in solid form. 10. A method as claimed in any one of claims 2 to 9, in which the solution is cooled to ambient temperature. STATEMENT UNDER ARTICLE 19 The Applicants wish to amend claim 1 of the above application by adding to it after the chemical formula the words "...or a product formed by driving off at least some of the molecules of water of crystallisation from the compound, and/or by the compound picking carbon dioxide." Accordingly, three copies of an amended set of claims are attached hereto.

Inventors: COUCH C
Applicants: COUCH C, MUREX LTD
Application year: 1979
Main classification: C01B17
IPC codes: B01D53, C01B17, C01G31