GB2413323A - Improvements to processes for the preparation of iridium acetate - Google Patents

Abstract

In a method for forming iridium acetate, a process for the removal of barium sulfate from a liquid reaction mixture comprising a suspension of barium sulfate particles in an aqueous solution of iridium acetate is modified. A solution of a high molecular weight anionic polymer flocculant emulsion is added to the reaction mixture, the barium sulfate is permitted to form flocs and the mixture is filtered. In particular, the flocculant is a polyacrylamide. A method for forming iridium acetate comprising a step of sintering iridium metal with an excess of barium peroxide is modified by sintering at a temperature in excess of 700 {C and below 900 {C. In particular, the sintering is between about 750 {C and 815 {C and is carried out for between one and five hours.

Description

241 3323

PRECIOUS METAL SALTS

The present invention concerns precious metal salts, and more especially it concerns a process for the preparation of iridium acetate.

A process for the preparation of iridium acetate starting from iridium metal sponge has been described by a Russian paper (Russian Journal of Inorganic Chemistry, 37, 915 (1992). This process was carried out at laboratory bench scale, and scaling up has not proved facile. A process was also described by Watson in Proceedings of the Precious Metal Catalyses Seminar held October 21-23, 1996, in Houston, Texas, USA. The process described by Watson was a multi-step synthesis starting from iridium metal in the form of iridium sponge, involving the steps of: a) Sintering Ir sponge with barium peroxide; b) Leaching sintered product using acetic acid/water; c) Filtration to remove unreacted metal; d) Precipitate Ba with sulphuric acid; e) Filtration to remove barium sulphate and f.) Concentrate filtrate solution by evaporation.

We have found certain technical difficulties and low yields in the commercial implementation of the above synthesis, principally, but not exclusively, in step e), where we found that barium sulphate tends to form in extremely fine particle sizes that tend to blind filters, and possibly may pass through certain filters, thus contaminating the product filtrate. We have tried to adjust process conditions to give larger particle sizes, but the low solubility of barium sulphate in the process liquor means that it is difficult to grow crystals. The use of filter aids was also considered but carried the probability of increased losses of expensive iridium. Other conventional process steps such as increasing the temperature of filtration were found to be largely ineffective. In the process liquor, the concentration of barium is unusually high, and dilution simply adds process cost in a downstream concentration step.

Flocculating agents are primarily used in the treatment of waste streams, for example wastes from mining, from paper-making or in the treatment of sewage, where the liquor is essentially water having neutral pH of about 7. In general, the use of flocculants in mainstream chemical processes is not favoured, because it adds a complex component which may be difficult to remove or cause undesired contamination of product.

Additionally, the process liquor in this step is quite acidic, generally of the order of pH 3.

However, since the problem was still unsolved, a number of different commercial flocculating agents were tried. Most of these gave surprisingly little or no result, but one class has been found to be unexpectedly successful. The present invention concerns, in one aspect, the use of this class of flocculating agents.

Accordingly, the present invention concerns a process for the removal of barium sulphate particles from a liquid reaction mixture comprising the barium sulphate particles suspended in a solution of iridium acetate, which process comprises adding to the reaction mixture an effective amount of a solution of a high molecular weight anionic polymer flocculent emulsion, permitting the barium sulphate to form floes and filtering the mixture.

The desired flocculent is a very low charge, very high molecular weight flocculent emulsion, especially comprising a polyacrylamide, more especially also comprising light hydrocarbon distillates, typically BetzDearborn AE111 5. They are generally supplied as emulsions, and are desirably prepared for use by adding the appropriate amount of emulsion to water, suitably of the order of 1 OOg emulsion to 50 litres water. Desirably the emulsion is added slowly to a vigorously stirred quantity of water, for example over a period of 20 minutes. The flocculent solution is most preferably allowed to mature for a period before use, for example 30 to 60 minutes, which it is believed allows the flocculent molecules to unwrap and be more effective.

The matured flocculent solution is then added slowly, for example over a period of 20 minutes, to a stirred reaction mixture. The reaction mixture is desirably at a temperature above ambient, for example at 40 to SO C.

Flocculation begins immediately the matured flocculent solution is added, and should be substantially complete by the time addition is completed. If desired, a further period may be allowed before filtering. If stirring of the reaction mixture is stopped, green streaks of iridium acetate solution can be observed within 5-20 seconds.

The flocculated reaction mixture can be filtered using conventional equipment such as filter presses.

In another aspect, the present invention provides a further improvement in the iridium acetate process. It will be appreciated that a key process step is the solubilizing of the iridium metal sponge by sintering with barium peroxide, as this governs the overall process yield. We have discovered that the temperature of sintering can have a remarkable effect on the yield of this step, and, additionally, the mixing of the two components can have a surprising and significant effect on yield. Initial yields, following the teaching of the Russian paper and sintering at 600 C, were from 60 to 80%. Initial trials were carried out to determine the yield at lower temperatures, however sintering at lower temperatures (400 C) for longer times gave significantly poorer yields than sintering at 600 C. It has now been found that yields can readily be increased above 95%, and under optimum conditions in excess of 98%.

Accordingly, the present invention provide an improvement in the iridium solubilizing step in the iridium acetate process, wherein a mixture of iridium metal sponge with an excess of barium peroxide is heated to sinter the mixture, characterised in that the temperature of sintering is in excess of 700 C and below 900 C.

Preferably, the temperature of sintering is from 725 to 825 C, especially from about 750 to about 815 C.

Good control of temperature and other parameters is recommended in order to form an open barium iridate. It is speculated that excessive heating, for example above 900 to 1,000 C, or for excessive times, may cause a transition in the form of barium iridate which is less amenable to further processing.

Desirably, the barium peroxide is used in a weight ratio of approximately from 3:1 to 4:1 with respect to the iridium sponge, most preferably approximately 4:1.

Sintering may conveniently be carried out by heating the reaction mixture in a vessel such as an alumina sagger within an oven for more than I hour. Preferred sintering times are approximately 5 hours; any period in excess of this tends to yield a hard and tough sinter product that is difficult to dislodge from the sagger without breaking the sagger.

Alternative process variants to firing in saggers to form the barium iridate may be worth exploring, with the aim of reducing the excess of barium peroxide and associated downstream processing costs.

As mentioned above, mixing can also contribute a surprising amount to yields. It will readily be understood that good mixing of the reactant components is desirable.

However, we have discovered that excessive mixing leads to increased density of the mixture, which adversely affects the efficiency of the sintering process, and adversely affects the yield of this step. Accordingly, the invention further provides the improvement wherein mixing of iridium metal sponge with barium peroxide is carried out for a time sufficient to provide a visually homogeneous mixture, but not for a time such that there is any significant increase in mixture density.

The particle size of the iridium sponge is not especially critical, and in any event the common route of formation tends to give similar particle sizes. The raw materials should be such as to permit the formation of an intimate mixture.

The process of forming iridium acetate may be further improved, if desired, by using a process variation in which a different precipitation step removes barium from the liquor, and potentially permits recycling of barium. We have found that the use of oxalic acid instead of sulphuric acid causes the formation of an easy-to-filter precipitate. Although filtration is improved compared to barium sulphate, the barium oxalate is more soluble in the process liquor. Accordingly, the filtrate may still need to be treated with sulphuric acid in order to remove the remaining barium.

In the above process variation, the barium oxalate recovered may be calcined, for example at 550 C, to form barium carbonate. The barium carbonate may be mixed with iridium sponge and sintered, for example at I l DO C, without a separate step of calcining to form barium peroxide. Yields of dissolvable iridium of the order of 96% were observed. Such a barium recycle could be of particular interest in the event of supply uncertainties or other increased costs.

A modification to the above barium precipitation process variation comprises the use of phosphoric acid, which precipitates barium phosphate.

The following experimental results demonstrate certain aspects of the present invention: Tests on Flocculants ml water and the necessary amount of 20% sulphuric acid were placed in a round bottom flask and heated to 90 C. 2% iridium acetate solution, prepared according to the Russian process was then added over 90 minutes. The final mass of slurry was 220 g.

Filtration took 2.25 hours giving a rate of 0.12 hrs/g barium sulphate. The filtrate contained 1.27% Ir. Particle size distribution of the barium sulphate peaks at 1.1 1lm.

Solutions of commercial flocculating agents were made up according to the following: Magnafloc 919 0.25 g + 1.5 ml industrial methylated spirits ("IMS"), diluted with water to 100 ml; 20 g of this solution diluted to g with water Zetag 84 5 g in 45 g water Magnafloc 351 0.25 g + 1.5 ml IMS diluted with water to 100 ml; 20 g of this solution diluted to l OOg with water Magnafloc 368 0.5 g + 3 ml IMS, diluted with water to 100 ml; 10 g of solution is diluted to 50 g with water Magnafloc 592 0.5 g + 3 ml IMS, diluted with water to 100 ml; 10 g of solution diluted to 50 g with water Magnafloc 1011 0.5 g+3mlIMS,diluted with water to lOOml; lOgof solution diluted to 50 g with water BetzDearborn AE 1115 0.2 g diluted to 20 g with water ml samples of iridium acetate/barium sulphate slurry were taken from a pilot plant, and treated with the prepared flocculent solutions. The results are tabulated below.

Flocculant Amount added Result Magnafloc 919 1 ml No effect Zetag 84 1.5 ml Slight signs of settling Magnafloc 351 1-2 ml Little effect Magnafloc 592 2 ml Little effect. On standing, some signs of settling Magnafloc 1 0 11 2 ml Little effect. On standing a little solid settled BetzDearborn AE I 115 4 ml Solid BaSO 4 flocculated and sank to bottom Magnafloc 368 2 ml No effect. No effect on standing It can readily be seen that, surprisingly, only one of the flocculants had any significant effect. Further tests showed that the flocculated material filtered quickly and well.

Desirably, the flocculent solution is allowed to stand for 45 minutes or more to mature.

Further tests were carried out to assess the effect of sintering temperature on yield of dissolved iridium. Samples of barium peroxide were ground together with iridium sponge in a 4:1 ratio by weight, by hand in a mortar and pestle until a homogeneous light grey mixture was obtained. Sintering was carried out under the conditions listed in the table below. After firing, the materials were reground in a mortar and pestle and then dissolved in a 1:1 glacial acetic acid/water mixture and filtered on a weighed GF/C paper.

A note of the filtration time was made. The residue on the paper was assumed to be undissolved iridium, and the solubilization yield was calculated. The test parameters and results are shown in the table below: Temp ( C) Time (hours) Time of filtration % Ir dissolution (mine) 650 Abandoned - too ND 700 4 120 98.24 750 60 98.84 800 60 97.34 750 1 2 91.11 It can readily be seen that higher sintering temperatures are preferred, and that even at a suitable temperature, sintering should be carried out for more than 1 hour. r 9

Claims (7)

1. In a method for forming iridium acetate, a process for the removal of barium sulphate from a liquid reaction mixture comprising barium sulphate particles suspended in an aqueous solution of iridium acetate, which process comprises adding to the reaction mixture an effective amount of a solution of a high molecular weight anionic polymer flocculent emulsion, permitting the barium sulphate to form floes and filtering the mixture.

2. A process according to claim 1, wherein the flocculent is a polyacrylamide.

3. A process according to claim I or 2, wherein the flocculent comprises light hydrocarbon distillates.

4. A process according to any of the preceding claims, wherein the flocculent emulsion is used in the form of a matured solution.

5. In a method for forming iridium acetate by a step of sintering iridium metal sponge with an excess of barium peroxide, the improvement comprising sintering at a temperature in excess of 700 C and below 900 C.

6. A method according to claim 5, wherein the temperature of sintering is from about 750 C to about 815 C

7. A method according to claim 5 or 6, wherein sintering is carried out for a time of more than one hour up to approximately 5 hours.