Ion Exchange Resins Tailored for Effective Recovery and Separation of Rhenium, Molybdenum and Tungsten

Separation of rhenium from molybdenum in primary solutions 
Use of SBA resins for collective sorption of rhenium and molybdenum followed by selective desorption of molybdenum by alkali and rhenium by concentrated nitric acid has been suggested earlier _[5]. We would like to introduce a resin which is able to selectively separate rhenium from molybdenum in the solution and which resin is easily eluted by ammonia solution. 

For our consideration, it is important that under low to strong acidic conditions molybdenum exists as very large anions or in cationic form while rhenium is presented by much smaller ReO₄- anion. By use of sieving effect, the latter can be effectively separated from molybdenum by gel type resin Puromet MTA1721.^{[6, 7]}

Due to smaller size of anion, rhenium easier diffuses inside of the resin bead. Large molybdenum anions sorbed mostly by surface of the resin. Puromet MTA1721 resin and artificial solutions were used in the kinetics study.^[6] 

Investigation of kinetics was undertaken in static mode. The study was run in seven beakers per experimental point. To simulate virtually constant concentration of metals in the outer solution such large volume of the later to the resin as 2,000 to 1 was used. Composition of the artificial solution was the following, mg/l: 

Mo – 3200; 
Re – 12.5; 
P – 70.0; 
Cl – 16.0; 
SO₄^2- – 100 g/l; 
pH = 1.2. 
Phosphate anions were to simulate arsenates. 

As the kinetic curve on the figure below shows, molybdenum reaches maximal loading in about 4 hours of contact while rhenium continues loading up to about 25 hours. 

Then sorption study in column was performed on the similar solution, mg/l: 
Mo – 3426; 
Re – 18.9; 
P – 70.0; 
Cl – 16.0; 
SO₄^2- – 100 g/l; 
pH = 1.5. 
Flow rate of the feed was 10 bed volumes per hour (BV/hour).

Kinetics Study for Sorption of Rhenium and Molybdenum by MTA1721 Resin

Assuming that molybdenum is mostly adsorbed on the surface of the resin the separation number can be further improved via two-step desorption. At first step, the most of molybdenum is to be flushed out by fast and short rinsing by diluted alkali. Then, at the second step, rhenium is to be desorbed by the slower flow of more concentrated alkali. 

Sorption of rhenium from different acids in absence of molybdenum 
Nowadays significant amount of rhenium comes from copper smelters. Their off-gas treatment systems generate rinsing and produced sulfuric acid streams containing rhenium. These streams do not contain a meaningful concentration of molybdenum. To date, there are three copper smelter plants in the world, which have installed Puromet MTA1701 resin for rhenium recovery from sulfuric acid. 

The figure below demonstrates the simplest example of rhenium sorption from sulfuric acid by macroporous WBA resin Puromet MTA1701. Affinity of the Puromet MTA1701 resin for rhenium remains high at different concentrations of sulfuric acid. 

For other experiment, the solution of the following composition was used: Re – 0.85 g/l; Na₂SO₄ – 103 g/l; pH 3.1. Full capacity for rhenium on Puromet MTA1701 resin, achieved in column experiment, was 162.2 g/l. Desorption from the loaded resin was done with 12.5% ammonia solution. Full rhenium desorption could be done with less than 10 BV of the eluent. Rhenium concentration at the pick of elution profile reached 35 g/l. Rate of desorption was above 94%. 

When such rhenium sources are processed as spent catalysts or superalloy scraps, there is no such problematic impurity as molybdenum. Common metals in the process solutions are nickel, aluminum, iron etc. They do not interfere with sorption of rhenium, which is in anionic form. However such solutions have different anionic background. Often nitric or hydrochloric acids are used, either as individual leaching agents or as their mixture. So, it is important to know how rhenium is recovered from these acids. 

Sorption of rhenium from solution with chloride matrix remains very effective. Isotherms in the figure below show similar capacities as they are obtained at sorption from sulfuric acid. 
 

Application of SBA resins for rhenium recovery is known for long time.^{[2-5, 8]} Purolite A600 product is a Type I, SBA resin on gel matrix with chloride exchange capacity around 1.5 eq/l. This resin was compared with A170* for rhenium sorption from nitric and hydrochloric acids of different strength at initial rhenium concentration 100 mg/l.

Rhenium sorption on Puromet MTA1701 resin is less sensitive to chloride presence than common SBA resin. 

Nitrates depress sorption of rhenium much stronger than chlorides. As it is clear from the figure below, both resins demonstrate very similar capabilities for rhenium sorption though Puromet MTA1701 still has some advantage. But one should remember that desorption of rhenium from SBA resin is much harder ^{[2-5, 8]}.

Recovery of Molybdenum 

Sorption of molybdenum from concentrated acids 
Some manufactures are to deal with nitric-sulfuric molybdenum solutions generated by etching processes. Acid concentration in obtained solutions is high. So, recovery of molybdenum from such solutions may have practical interest. 

Three resins were compared for this task. The figure above represents isotherms of molybdenum sorption from mixture of 1M HNO₃ and 2M H₂SO₄. 

First of all, it is clear that anion exchange resin Purogold MTA1930 has much less sorption capacity under such acidity than two other products. It can be explained by two reasons. Firstly, high concentrations of bisulfate and sulfate anions, and especially nitrate anions, strongly compete with anionic forms of molybdenum for ion exchange sites. Secondly, under such high acidity molybdenum mostly exists in cationic forms, which are not sorbed by anion exchange resin. 

Isotherm of sorption for Puromet MTS9500 resin has very convex upwards profile at lower concentrations of the metal. At the same time this resin becomes not sensible to increasing of molybdenum concentration above 2.5-3 g/l; the metal loading on the resin remains the same. 

At some molybdenum concentration the capacity of Puromet MTS9570 resin becomes similar to Puromet MTS9500 and exceeds the later with further concentration increasing. 

Thus Puromet MTS9500 resin can be recommended for molybdenum recovery from its lean solutions in concentrated acids while Puromet MTS9570 resin is more suitable at higher concentrations of the metal. Another advantage of Puromet MTS9500 resin can be better tolerance to presence of ferric iron under so high acidity. However, the later assumption is to be verified by experiment yet. 

It is interesting to notice that Puromet MTS9500 resin has as phosphonic acid groups and weak base anion exchange function as well. Possibly both acid and base functions of the resin make input in its high molybdenum selectivity. 

At sorption from 100 g/l sulfuric acid solution, the Puromet MTS9570 resin capacity for molybdenum reaches 80 g/l at equilibrium concentration of the metal 2 g/l. 

Desorption can be effectively done by about 5 BV of 15-25% ammonia providing a rate of desorption above 90%. The second figure below demonstrates elution profiles for these two resins. They were loaded with molybdenum in columns from solution with molybdenum concentration 20 g/l. Full molybdenum capacity was 84 g/l for Puromet MTS9570 resin and 64 g/l for Puromet MTS9500. 

Presumably, molybdenum sorption by resins with phosphonate functionality has cation exchange mechanism. However, the formation of heteropoly acid function via condensation of molybdate anion with phosphonate groups should not be disregarded as well. 

Indirectly, the later statement can be supported by the fact of very effective desorption of molybdenum from this resin by aqueous ammonia. In this case, the heteropolyacid structure is to be hydrolyzed under alkaline conditions and produced molybdate anions are repulsed from the cation exchange, i.e., negatively charged matrix. 

Regardless, the sorption mechanism of the Puromet MTS9570 and Puromet MTS9500 resins can be used for molybdenum recovery from concentrated nitric and sulfuric acids. 

Elution of molybdenum from these resins can be easily done by ammonium solution (figure below). 

Processing of molybdenite concentrates 
Block scheme in the figure below represents a generalized flowsheet for recovery of molybdenum from concentrates via autoclave leaching. 

Conditions of leaching can be varied by pressure, nitric acid concentration, temperature etc. If leaching is curried out under ambient pressure then a higher concentration of nitric acid is required. 

The autoclave leaching of molybdenite concentrates results in increasing of total acidity due to the oxidation of sulfide minerals. 

Purogold MTA1930 resin can be used in the process. Before the first sorption stage acidity of the leachate is corrected to pH 0.8-0.9. Sorption at so low pH is required to avoid co-precipitation of molybdenum with iron. At this stage, most of the molybdenum is extracted from the leachate. The sorption contact time for the resin is around 2.5-3 hours. 

Molybdenum loading of the resin depends on the concentration of molybdenum in the feed and is affected by nitrate anion. The typical resin capacity for molybdenum is about 90 g/l at the first stage. 

After further correction of pH to 2–2.3, the second sorption can be done. At this stage, resin capacity increases to approximately 120 g/l as per molybdenum. 

Full desorption can be effectively done by about 5 BV of 15–25% ammonia. Desorbates can have molybdenum concentration up to 75 g/l and, after additional concentrating can be used for crystallization of ammonium paramolybdate (APM). 

Due to proper bead size and good mechanical stability, the Purogold MTA1930 resin can be used for sorption from filtrated “clean” solutions or directly from pulps. Elimination of solids separation operations in the resin in pulp (RIP) process significantly reduces costs.

Conversion of Sodium Paratungstate (SPT) to Ammonium Paratungstate (APT) 
In hydrometallurgy, the tungsten mineral concentrates can be processed via alkaline (soda ash) or acidic routes. At either route, the conversion of SPT to APT may be required for further production of pure tungsten compounds, items and alloys. 

One of the methods to do this operation is the application of ion exchange resins. 

Processing of carbonate PLS 
Carbonate routes for the processing of tungsten mineral concentrates produce solutions with a high tenor of sodium carbonate. 

So, an investigation of tungstate sorption from concentrated sodium carbonate solutions was undertaken. Strong base anion exchange resins must be used for such kinds of solutions. 

Increasing carbonate concentration in the feed solution affects the loading capacity of the resin for tungsten. 

The figure below illustrates the ability of Purolite A500Plus resin to uptake tungsten from carbonate solution. It can be noticed that there is no significant concentration of the metal on the resin. The effect of carbonate concentration on tungsten resin capacity is demonstrated by the second figure below.

Obtained in desorbate ammonium salt of tungstic acid can be concentrated and separated via crystallization. 

Processing of acidic PLS 
Direct ion exchange sorption of tungsten from carbonate solutions is not so effective because of relatively low capacity of the resin under alkaline conditions. 

Another route implies acidification of the pregnant solution. It leads to polymerization of tungstate anions. Sorption of polymerized anions significantly increases operating capacity of the resin. 

Sorption from acidic solutions also implies application of weak base anion exchange resins. Thus elution can be easily done by aqueous ammonia yielding rich APT. The later is more suitable for further concentrating and crystallization of APT. Results of Purolite A110 resin testing for tungsten recovery from sufuric acid solution is presented in the figure below.

Sorption test was run in a column packed with 200 ml of Puromet MTS9570 resin. Flow rate of the feed was 70 ml/hour. As it was assumed, the Puromet MTS9570 resin did not uptake rhenium from solution but loaded molybdenum and ferric iron. Copper sorption was negligible.

In this process a separation factor of molybdenum over rhenium fixed in the resin was the following:

Of course, this approach to the separation of molybdenum from rhenium can not be directly applied to those real solutions, which contain ferric iron because Puromet MTS9570 resin binds the iron too strongly. However, if to undertake measures for reduction of iron to divalent state before sorption, then this resin can perform well. Alternatively, the sorbed ferric iron must be eluted by 6N hydrochloric acid either by another suitable reagent to recover the resin capacity.

Removal of tungsten impurity from molybdenum
Tungsten contamination affects the ductility of molybdenum. Molybdenum to be used for the production of wire, foil and other rolled materials must have less than 0.01% of tungsten. Production of molybdenum monocrystals requires less than 10-4-10-5% of tungsten impurity.

The problem is that both metals are able to form cross-contaminated polyanions. However, under definite acidity, tungsten tends to form isopolyanions, while molybdenum yet remains mainly in a monomeric form. This minor difference in the behavior of molybdenum and tungsten in aqueous solutions makes the use of ion exchange possible for the removal of tungsten from molybdenum.

After screening work done among many candidates the Puromet MTS9840 resin has been chosen as the most effective material for molybdenum purification.

Investigation of sorption isotherms (the figure below) demonstrated that Puromet MTS9840 resin has high potency for selective sorption of tungsten from the concentrated molybdate solution.

For sorption in column the following composition paramolybdate and paratungstate in water was used: Mo – 140 g/l; W – 0.9 g/l. The feed flow rate was 1 BV/hour. Main results of the refinery process are shown in the table below. 

Under the given conditions, the breakthrough occurs after the 26th BV. It provides convenient operation and considerable tungsten loading on the resin.

A desorption test was done by ammonia solution for a resin sample loaded with tungsten in the column experiment. Desorption smoothly run with a small volume of desorbent (see figure below).