CS256980B1 - A method for recovering platinum from PVC membranes of bipolar electrolysers - Google Patents

Abstract

The method for the regeneration of platinum from PVC membranes of bipolar electrolyzers for the production of ammonium peroxodisulfate or peroxodisulfuric acid consists in eluting platinum for three hours using boiling aqua regia acting on the membranes coiled into a spiral and placed in a glass vessel equipped with a reflux condenser. The maximum elution is achieved at an elution module of 2.2 1/kg. To reduce the content of sulfuric acid and lead in the extract, the membranes are pre-equilibrated with nitric acid at a concentration of 10% by weight. To reduce platinum losses due to mechanical loss of the extract, water equilibration is included in the ratio of 2.2 1/kg of membranes. Precipitation in the form of chloroplatinate is carried out from the extract and water broth with solid ammonium chloride in the amount of 50 g/1 in the presence of ethyl alcohol in such an amount as to produce an ethyl alcohol solution with a concentration of 30% by volume and subsequent rinsing of the precipitate on the filter with ethyl alcohol with a concentration of 50% by volume. The obtained ammonium chloroplatinate contains at least 40% platinum with a platinum recovery rate of at least 93%.

Description

The invention relates to a method for regenerating platinum from PVC membranes of bipolar electrolyzers for the production of peroxodisulfates or peroxodisulfuric acid.

During the electrode process, the platinum anode, depending on the current density, gradually dissolves and migrates to the cathode, and the finely dispersed platinum particles are firmly captured by a PVC membrane tightly fitting the carbon cathode in an amount of 0.33 to 0.51 g/piece. Platinum regeneration from these membranes has not yet been carried out. The procedure for platinum regeneration from bipolar electrolyzers described in patent document NDR 115 097 (1974) ensures only the dissolution of platinum on the surface of the corobone (essentially carbon) cathodes, since at low temperatures and low flow rates of the leaching solution, perfect elution of platinum from the entire volume of the membrane is not possible.

Under the platinum regeneration conditions specified in the above-cited patent, according to our experience, a maximum of 50% of platinum can be regenerated from PVC membranes. Another disadvantage of this process is the need for extensive circulation equipment for the circulation of a strongly corrosive special lye (a mixture of hydrochloric acid, sulfuric acid, chlorine, hydrogen peroxide) and the need for additional electrolyzers for obtaining platinum from dilute solutions.

The methods of platinum extraction using ammonium chloroplatinate described in the literature are based on the precipitation of concentrated solutions of chloroplatinic acid (50 to 200 g platinum per liter) with a saturated solution of ammonium chloride. Since only dilute solutions (1 to 2 g platinum per liter) are formed during the elution of platinum from PVC membranes with aqua regia, their concentration by evaporation is necessary. This also increases the concentration of undissolved lead and sulfuric acid salts, which are adsorbed in significant quantities by the membrane. When applying the described extraction methods based on concentrated solutions, we obtain contaminated ammonium chloroplatinate at low yields.

The above-mentioned shortcomings are solved by a method of regeneration of platinum from PVC membranes according to the invention that is easy to use in terms of equipment and highly effective. This method consists in treating the spiral-wound membranes for a maximum of three hours with boiling aqua regia, preferably with an elution module of 2.2 l/kg, in a glass vessel equipped with a reflux condenser. Elution with aqua regia is preferably preceded by boiling the membranes with diluted nitric acid with a concentration of at least 10% by weight. Precipitation of ammonium chloroplatinate from acidic solutions with a maximum sulfuric acid content of 300 g/1 and a platinum content of 5 to 10 g/1 with solid ammonium chloride in an amount of 50 to 60 g/1 in the presence of ethyl alcohol (minimum content of 30% by volume in the reaction mixture).

In order to maximize the degree of regeneration, in addition to a temperature above 100 °C, the elution modulus (the ratio between the volume of the leaching solution and the weight of the regenerated membranes) is crucial, as can be seen from the values given in Table 1 and the graph in Figure 1, where the value of the elution modulus in 1/kg is plotted on the x-axis and the value of the degree of platinum regeneration in percent is plotted on the y-axis.

Table 1

Elution module

(1/kg) Grade regeneration 0.80 0.97 1.16 1.18 2.00 2.20 2.70 (%) 71.0 81.3 87.0 90.0 94.2 95.5 96.6 With larger of the given data than 2.2 1/kg. it is obvious, that the maxima elution is reaches practically during elution module To avoid changing the elution modulus evaporator light bulbs, needs to be done regenerate·

The apparatus for eluting platinum from PVC membranes is shown in Fig. 2. The apparatus consists of a glass vessel £ immersed one-third in an oil bath £. The vessel is equipped with a reflux condenser £.

Since the membranes contain, in addition to platinum, a large amount of sulfuric acid and lead, which make it difficult to extract platinum with ammonium chloride, it is necessary to boil the membranes before elution with nitric acid solution at a concentration of at least 10% by weight.

The residue of the caustic soda with a significant platinum content is removed by boiling with water in an amount of at least 2.2 l/kg of membrane. The caustic soda elution is combined with the water broth and evaporated to a platinum content of 5 to 10 g/l. The sulfuric acid content must not exceed 300 g/l. Then precipitation is carried out with solid ammonium chloride in the presence of ethyl alcohol with a concentration of at least 30% by volume to eliminate ammonium chloroplatinate. The salt obtained is rinsed with ethyl alcohol with a concentration of at least 50% by volume and dried at 100 °C in a hot-air dryer.

Example

Four pieces of sorted PVC membranes weighing 2,000 g with a platinum content of 6 g are rolled up gradually into a spiral and placed in a glass container 1. equipped with a stopper with a reflux condenser 2. 4.4 liters of nitric acid with a concentration of 10% by weight are poured into the container. The container is placed in an oil bath 2 reaching one third of its height. The bath is heated to 105 °C.

This arrangement achieves intensive circulation of the elution bath based on the thermosiphon principle. Boils (

for one hour. This removes a significant portion of the sulfuric acid and lead from the membranes. The bath is evacuated using a water pump and the vessel is filled with 4.4 liters of aqua regia prepared from 35% hydrochloric acid by weight and 65% nitric acid by weight in a ratio of 3:1 and extracted for three hours at boiling. The extract containing 4.45 g of platinum is evacuated into a collecting tank and the vessel is filled with 4.4 liters of warm water (preferably steam condensate) and boiled for one hour. The rinsing water containing 1.20 g of platinum is evacuated into a common collecting tank.

The extract combined with the rinsing water is evaporated to one liter and, after cooling, filtered through a suitable acid-resistant filter material (e.g. a sheet of non-woven fabric based on polypropylene fibers). This removes the ballast dispersion of carbon, silicic acid, sulfate and lead chloride and thus prevents contamination of the final product. The sulfuric acid content after evaporation of the solution must not exceed 300 g/1. 50 g of ammonium chloride is added to the thus prepared chloroplatinic acid solution and, after proper mixing, 500 ml of ethyl alcohol with a concentration of 96% by volume is added. The resulting suspension of ammonium chloroplatinate is allowed to sediment for 24 hours at temperatures of minus 5 to 0 °C. After decantation of the clear portion, the precipitate is filtered through an acid-resistant filter. The precipitate is rinsed with ethyl alcohol with a concentration of 50% by volume cooled to 0 °C. The extracted salt is dried in a hot air oven at 100 °C.

From the above number of membranes, 14.15 g of ammonium chloroplatinate with a platinum content of 40% by weight is obtained. The residue on the membrane is 0.2 g of platinum, the mother liquor contains 0.1 g of platinum per liter. The overall degree of platinum recovery is 93 to 95%.

Claims (2)

Method for the regeneration of platinum from PVC membranes of bipolar electrolysers for the production of peroxodisulphates or peroxodisulphuric acid, characterized in that the membranes coiled in a spiral are treated for at least three hours by boiling royal bow, preferably with an elution modulus of 2.2 l / kg in glass a flask equipped with a reflux condenser, whereupon the resulting acid solution having a maximum sulfuric acid content of 300 g / l and a platinum content of 5 to 10 g / l is precipitated with 50 to 60 g / l of solid ammonium chloroplatinate in the presence of ethyl alcohol added in such an amount; to form a solution having a concentration of at least 30% by volume. 2. A method according to claim 1, characterized in that the membranes are avoided with nitric acid, the concentration being at least 10% by volume being eluted by the royal bowel.