According to the production practice of a factory, when electrolytic silver is used, about 40% to 50% of platinum and palladium in the anode plate enter the electrolyte and accumulate. The activated carbon is used to selectively adsorb platinum and palladium dispersed in the electrolyte, and then desorbed and recovered by nitric acid. The recommended process is shown in Figure 1.
Figure 1 Activated carbon adsorption, desorption of platinum and palladium
The activated carbon used in the plant is a medicinal activated carbon. After sieving, take 40 to 60 mesh for use. The preparation of activated carbon and carbon column is to heat the industrial dilute nitric acid of 1:1 to 90-100 ° C, and slowly add activated carbon to the hot dilute nitric acid after the solid-liquid ratio is 1:10 until the brown gas is no longer released. 6~12h), the nitric acid was discarded by decantation, and the column was washed 3 times with an equal amount of distilled water. After the column is packed, it is further distilled with distilled water until the pH is 4 to 5 for use. The carbon column is a glass tube with a diameter of 70 mm, a total of 8 tubes, 7 columns in series, and 1 column for use. Each column is packed with 1 kg of activated carbon, and the high-position tank is 5 to 6 m above the carbon column.
The electrolyte is heated and concentrated at 80-90 ° C for 4 h, and then added with 10% NaOH to adjust the pH to 1.5-1.8 (free nitric acid 1-2 g ∕L), and then continuously passed through the series at a flow rate of 100-150 mL/min. Root carbon column. After the first column is adsorbed, the platinum and palladium are saturated and taken out, and the second column is changed to one direction, and the alternate cassia is connected in series to the tail end as the seventh column. So on and so forth.
A carbon column saturated with platinum and palladium was desorbed with 1:1 industrial nitric acid. The desorbed solution was passed through a carbon column at a flow rate of 75 to 100 mL/min, and platinum and palladium were recovered by taking 2.5 L of platinum-palladium rich liquid each time. Primary and secondary depletion of the spent liquor and freshly replenished 1:1 industrial nitric acid are returned in countercurrent for the next desorption. The desorption process passes through a total amount of 25 to 30 L per room.
The test results show that the total recovery of adsorption and desorption of platinum and palladium is Pt1027% and Pd96.5%. The adsorbed solution contains less than 1 mg/L of platinum and palladium. It can be considered to return to the electrolysis process after removing copper and lead .
The adsorption capacity of activated carbon is calculated according to the content of platinum and palladium in the desorbed solution of the second column. The total adsorption capacity of palladium is at most 72.5 mg ∕g, and platinum is greater than 6.9 mg ∕g.
The desorbed carbon column is washed with distilled water until it is neutral, and then it can be recycled. After 4 regenerations, the adsorption capacity did not decrease.
The platinum-palladium rich liquid obtained by desorption is obtained in a ratio of platinum to palladium of 1:6-7. The silver was first precipitated with silver chloride by treatment with 12 mol of hydrazine L hydrochloric acid, and the silver chloride was washed with 3 mol/L of hydrochloric acid after filtration. In addition to the silver rich liquid and the washing liquid, solid ammonium chloride is added to make the palladium precipitated as crude ammonium chloropalladate, and the palladium salt is purified by dichlorodihydropyridinium twice and then calcined or reduced with 10% hydrazine hydrate. A sponge palladium containing more than 99.9% palladium was obtained.
The mother liquor after palladium is precipitated by heating, and the nitrate is precipitated with ammonium chloride, and then directly hydrolyzed and purified, and reduced with 10% hydrazine hydrate to obtain sponge platinum containing more than 99% of platinum.
The above industrial tests also prove that:
(1) The activated carbon of 40 to 60 days is larger than the adsorption capacity of 20 to 40. The palladium is increased by 71% and the platinum is increased by 32%.
(2) The electrolyte concentrated by heating at 80 to 90 ° C for 4 hours, the adsorbed palladium is increased by about 1 time, and the platinum is increased by nearly 4 times compared with the electrolyte which is not heated.
(3) Activated carbon has poor adsorption to lead, but can adsorb ruthenium . When the electrolyte contains ruthenium 0.26 g ∕L, the concentration of ruthenium in the desorption liquid can reach 10.03 g ∕L.
(4) When the pH value was adjusted with sodium hydroxide, the Na + in the human electrolyte accumulated, but after a period of use, no effect was found on the quality of the electrolytic silver.
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