GB2073780A - Purification of Molybdenum Compounds by Electrodialysis - Google Patents

Abstract

The anolyte side (12) of a cation exchange membrane (11) is contacted with an aqueous solution of the compound; in particular technical grade molybdenum oxide, and an electrical potential is applied between anode and cathode (14,15) across the membrane (11) so as to effect transport of cationic impurities in the compound through the membrane into the catholyte (13); the purified compound being subsequenty recovered from the solution. Advantages over conventional purification techniques are efficient impurity removal with lower molybdenum losses. <IMAGE>

Description

SPECIFICATION Purification of Molybdenum Compounds by Electrodialysis The invention relates to the purification of molybdenum compounds.

Technical MoO3 is produced by roasting molybdenite in a multi-hearth furnace and contains appreciable amounts of impurities which have to be removed before processing of the compound to produce Mo metal. Conventionally, technical MoO3 is purified to remove impurities such as alkali metals, alkaline earth metals, copper, iron, etc., by a lengthy multiple stage slurry/decant process, the MoO3 being stirred with water and/or dilute acid, e.g hydrochloric acid, before each decanting step. The wash liquors, apart from containing soluble impurities, also contain appreciable amounts of suspended and soluble molybdenum compounds, sometimes as much as 10~15% of the Mo throughout. This Mo must then be recovered in the form of ferric molybdate by the addition of ferric chloride in the presence of an oxidising agent. These residues are normally recycled.

British Patent Specification No. 1 397,458 describes a method of purifying a technical molybdenum oxide product obtained by roasting molybdenite, in which the lengthy wash processes described above are replaced by a single stage wash step at elevated temperature using nitric acid and ammonium nitrate. This process has the advantage that efficient impurity removal can be achieved with correspondingly less Mo losses.

The invention provides a method for purifying molybdenum compounds, e.g., molybdic oxide and particular technical grade MoO3, comprising contacting the anolyte side of a cation exchange membrane with an aqueous solution or slurry of the respective compound, and applying an electrical potential across the membrane such as to effect transport of cationic impurities in the compound from said solution through the membrane into the catholyte and subsequently recovering the purified compound from the solution or slurry.

A method according to the invention is applied to the purification of molybdenum compounds, in particular technical molybdenum oxide (produced by roasting molybdenite) whereby cationic impurities such as alkali and alkali earth metals, copper and iron are transported through the membrane leaving a purified aqueous slurry on the anolyte side of the membrane without substantialy transport of anions through the membrane. This method achieves the advantage over conventional wash purification processes which can be obtained by the process proposed in British Patent Specification No.

1,397,458 namely efficient impurity removal with lower molybdenum losses.

Advantages of the invention include the possibility of reducing losses during the purification processes, the reduction of effluent generation, and the possibility of operation as an automated continuous process.

Examples of processes according to the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a cell for use in demonstrating the invention; Figure 2 is a flow sheet for electrodialysis of technical grade Molybdic Oxide; Figure 3 is a diagrammatic illustration of a flowing cell system, Figure 4 is an exploded view of the cell of Figure 2, and Figures 5(a) and (b) are diagrammatic representations of a dynamic system and static single cell, respectively, in accordance with the invention.

The following commercially available cation membranes have been found suitable for use in a process according to the invention. These include the following: 1. BDH Na+ exchange membrane made by Ashai Glass Company Limited of Japan.

2. lonac MC-3470 cationic exchange membrane manufactured by Sybron Corporation.

3. lonics 61-AZL389 cationic-transfer membrane manufactured by lonics Incorporated.

4. Dupon Nafion 427 and 425 membranes made by Dupont.

The anolyte is the process stream slurry or solution containing a molybdenum compound. Cations are removed from the anolyte through the membrane into the anolyte which is a recirculating solution maintained acidic to prevent precipitation of salts which would foul the equipment. It is important that anolyte conditions are such as to provide that valuable molybdenum cornpounds are generally in anionic form and not therefore lost through the membranes.

Although the electrode reactions (see Figure 5), namely the evolution of oxygen and hydrogen, are of secondary importance, this may not be the case in an intergrated plant. The metals are produced by hydrogen reduction at points downstream in the process and therefore at least one electrode byproduct may be utilised. This affects the economic viability of the electrodialysis process.

Having established that appropriate chemically resistant and physically robust selective membranes are commercially available, the next most important factor to be considered is the optimisation of energy requirement in a process generally considered as energy intensive. Power consumption is always greater than the theoretical value, i.e. that stipulated by Faradays Law, because of polarisation, resistance effects and the current efficiency being less than one hundred per cent.

The current efficiency, a measure of the power consumed in the desired electrochemical processes, is low because of such factors as current leakage through manifolds, back diffusion of counter ions and undesirable side reactions. The factors affecting the cost of power in electrodialysis are discussed in great detail in the literature. Of utmost importance in optimising the current efficiency is the critical current density. The current efficiency increases with current density until a value is reached where the transfer rate of ions is so reduced through the membrane boundary layer that the electrical resistance, and therefore the voltage and power, rise dramatically. The current density must be maintained below this critical value for maximum economy.

Electrodialysis of Technical Grade Molybdic Oxide Technical grade molybdic oxide (TGMO) is produced by roasting molybdenite concentrates, normally in a multihearth roaster. Typically TGMO contains the following ranges of impurity elements: 0.2 - 3 percent Fe 0.04 - 1 percent Cu 0.03 - 1 percent Ca 0.01 - 0.3 percent Mg 0.001 - 0.04 percent Na The majority of these elements are soluble under neutral to acidic aqueous conditions and may be removed by washing. The conventional purification process involves washing, ammonia digestion, filtration, chemical purification and crystallisation, the product being ammonium molybdate which is the starting point for a range of pure molybdenum products.Unfortunately, a proportion of the molybdenum in TGMO is present in soluble forms and/or as fine colloidal compounds. This results in a higher than desirable molybdenum content of wash and filtrate streams which must be treated to recover the molybdenum. This treatment produces undesirable low grade byproducts which undergo costly recycling. TGMO purification processes which improve molybdenum recovery to refined oxide are therefore of considerable importance.

During the leaching of TGMO with a HNO3/HCI solution soluble molybdenum is present in various anionic, isopoly acid forms and is likely to be in cationic form at pH values less than 1.8 although such species are probably highly polymerised. As the majority of impurities exist in cation form in weakly acid conditions it is possible to obtain high yield purification by electrodialysis of these impurities through a cation transfer membrane.

Table 1 gives the results of a typical conventional multistage wash for purifying technical grade MoO3 (TGMO) obtained by roasting molybdenite.

For each wash the TGMO was stirred for 1 hour at 70 1 800C, with the required volume of demineralised water, then allowed to settle before decanting off the mother liquor before proceeding to the next wash. After the final wash the TGMO was filtered and dried for assay.

Example 1 An electrodialysis cell as illustrated in Figure 1, was used, the cell being constructed from Perspex and having the overall dimensions of 1 2"x6"x6". The cell (10) was internally divided at the centre by a watertight frame containing a cation exchange membrane (11), into an anode compartment (12) and a cathode compartment (13). The membrane (11) was a propriety brand as supplied.

The anode (14) was constructed from a piece of coiled Pt wire, while the cathode (15) was a 4"x4" sheet of stainless steel.

250 g TGMO was placed in the anolpte compartment together with 2.5 litres deminralised water and 24 ml conc. HN03, the pH of the liquor being 1.1. The wash liquor in the catholyte compartment consisted for 2.5 litres dilute HCI (1% v/v).

The contents of both compartments was stirred and the voltage applied. The unit was run for 48 hours, after which time the refined oxide was filtered and dried for assay. The process flow sheet is given in Figure 2.

Example 2 The conditions for this Example were as for Example 1, except that 25 ml conc. HCI was used in place of 25 ml conc. HNO3 to wash the TGMO.

Example 3 The conditions were the same as those for Example 1, except for examining the effect of more concentrated acid. In this example 100 ml conc. HNO, was used instead of 25 ml.

Example 4 The conditions were the same as those for Example 3, except for incorporating a continuous tap water wash in place of dil. HCI in the catholyte compartment.

Example 5 In this Example the TGMO (250 g) was first slurried with 50 ml cold conc. HNO3. This mixture, after standing for several hours with occasional rabbling, was placed in the cell together with 2.51 demin and electrodialysed with a continuous tap water wash.

Example 6 The conditions were the same as those for Example 5, except that the TGMO was slurred with a mixture of 25 ml conc. No3/25 ml conc. HCI before adding 2.5 litres demineralised water and proceeding as in Example 5.

As can be seen from these results, (taking Example 6 as being the best purification achieved) a far more effective purification is produced by electrodialysis, than by the conventional wash process. In fact the Cu and Na figures are almost as good as specification for final refined product.

The treated MoO3 will be further purified by ammonia digestion and selective crystallization of ammonium molybdate.

Table I

Fe Cu Ca ~ Mg Na K starting material 1.28 0.066 0.15 0.087 0.033 0.15 Conventional route 0.41 0.002 0.13 0.023 0.013 0.053 Example 1 0.075 0.002 0.12 0.02 0.013 0.048 Example 2 1.19 0.002 0.13 0.02 0.018 0.087 Example 3 0.16 0.001 0.135 0.023 0.01 0.053 Example 4 0.84 0.006 0.11 o.Q22 0.02 0.058 Example 5 0.25 0.001 0.035 0.021 0.01 0.047 Example 6 0.062 0.001 0.016 0.021 0.01 0.026 Refined MoO3 (from Example 6) 0.005 0.002 0.004 0.0005 0.002 0.003 Refined MoO3 Specification 0.002 0.001 0.003 0.012 0.006 0.014 A significant advantage to be gained by this process is that no soluble Mo will be generated for recovery and recycle as ferric molybdate. The trace of Fe present in the oxide will remain insoluble during the ammonia digestion stage.

A sample of purified MoO3 obtained from Example 6 above, was subject to refinement by an ammonia digestion/evaporation process.

18 g purified MoO3, from Example 6 was digested with stirring for 1 hour in a mixture of 14 ml 0.88 NH4OH and 100 ml demineralised water. The mixture was filtered from insolubles (0.2 g) and the filtrate (ph=8.9) concentrated by evaporation to approximately 20 ml. After cooling, the ammonium molybdate was filtered and dried (16.8 g) before calcining for 2 hours at 4000C to produce 14.5 g refined MoO3.

Refined MoO3 produced by an electrodialysis treatment of TGMO according to the invention followed by ammonia digestion and then evaporation and calcination is of acceptable quality for Mo metal production.

As can be seen, the Fe and Cu figures are slightly above specification and presumably limited treatment may still be necessary. The small amount of residues this will produce, should not create a recycle problem.

It is advantageous to operate the process in a flowing cell as illustrated in Figures 2 and 3. In this equipment large membrane areas are available enabling large current flow. The respective solutions are passed through the anode and cathode spaces on opposite sides of the membranes such that the reaction may be completed in a short residence time in the cell bank. Suitable equipment is available from Asahi Glass Co. Limited, Sybron Corporation and V.C. Filters Limited.

The cell bank (2) comprises a series of anolyte chambers defined by frame members (21), a series of catholyte chambers defined by frame members (22), separated by ion-exchange membranes (23) and anodes (24) and cathodes (25) in each the respectivs chambers. Manifolds (26,27) and 28,29) are provided respectively for circulating anolyte and catholyte continuously through the chambers (21,22) from the respective storage tanks (30,31). A rectifier unit (33) provides current to the anodes and cathodes. The particular multiple cell unit shown in Figure 3 is manufactured by V. C. Filters Limited.

The following example demonstrates the process using a continuous multiple cell unit.

Example 7 3 kg TGMO is leached for six hours in an aqueous slurry containing 50 to 60 weight percent solids, 150 ml conc. HCI and 150 ml conc. HNO, at ambient temperature. The slurry is diluted with demineralised water and (or recycled anolyte solutions to approximately 12 litres. The diluted slurry is continuously dewatered using cyclone or a settling arrangement and the overflow containing a low percent solids is pumped as anolyte to a V.C. Filters laboratory two cell electrodialysis unit with a dipole electrode of stainless steel and platinised titanium between the two cells. The anolyte is continuously recirculated to reslurry and continue the leaching of impurities from the dewatered solids.

Catholyte is a weak solution (pH2) of hydrochloric acid whose conductivity is maintained by continual dosage of further acid. Electrodialysis causes a 2 percent catholyte volume increase which is bled off continuously along with a small quantity of fine precipitate.

Electrodialysis is carried out for 21 48 hours at a current density of 10 to 20 mA/cm2 on the unit which has a total membrane area of 422 square centimetres.

The process is stopped. Anolyte is decanted and given a single wash/decant to remove residual acid. All solutions are used to make up the next batch of anolyte as anolyte volume loss per cycle is 5 to 25 percent.

Ammonia digestion of the electrodialysed TGMO, followed by limited solution pruification to remove some copper and iron, gives ammonium molybdate and thence refined molybdic oxide products consistently better than specification.

The use of coarse TGMO and allowing the majority of solids to settle out prior to pumping diluted slurry to electrodialysis has reduced the likelihood of equipment abrasion, notably the membranes which have withstood three months of almost continuous use without sign of failure. Temperature rises a maximum of 1 00C during a cycle. Platinised titanium anodes and lead and stainless steel cathodes are being carefully examined for signs of deteriorating. Catholyte acidity must be maintained although pH remains relatively steady even after use over several cycles. Anolyte filtrate recycle may be used.

Claims (11)

Claims

1. A method for purifying molybdenum compounds which comprises contacting the anolyte side o a cation exchange membrane with an aqueous solution or slurry of the respective compound, and applying an electrical potential across the membrane such as to effect transport of cationic impurities in the compound through the membrane into the catholyte and subsequently recovering the purified compound from the solution of slurry.

2. A method as claimed in Claim 1 wherein the compound to be purified is technical grade molybdic oxide.

3. A method as claimed in Claim 2 wherein the molybdic oxide is leached in an acid slurry or solution prior to the electrodialysis thereof when said cationic impurities are removed.

4. A method as claimed in Claim 3 wherein solids, or at least a proportion thereof, are removed from said slurry or solution before it is passed into a cell for electrodialysis.

5. A method as claimed in any preceding claim wherein the current density during electrodialysis ol the molybdenum compound is in the range 10 to 20 mA/cm2.

6. A method as claimed in any preceding Claim wherein the pH and conductivity of the catholyte wash liquor is controlled by adding hydrochloric acid.

7. A method as claimed in Claim 6 wherein a controlled flow of hydrochloric acid is continuously added to the catholyte wash liquor.

8. A method as claimed in any preceding Claim, wherein said aqueous solution or slurry is continuously circulated through an electrodialysis cell or cells for a specific period during which said cationic impurities are removed therefrom.

9. A method of purifying molybdenum compounds, substantially as hereinbefore described with reference to Figure 1 or Figures 2 and 3 of the accompanying drawings.

10. A method of purifying molybdenum compounds substantially as hereinbefore described in any one of the examples.

11. A molybdenum compound when purified by a method as claimed in any preceding claim.