Dissolving precious metal concentrates using HCl + Cl₂ (known as the “aqueous solution chlorination method”) is a common technique in precious metal metallurgy. After grinding spent catalysts, oxidants such as NaClO, NaClO, and H₂O₂ are added to a hydrochloric acid medium, or Cl₂ gas is introduced. Under heating and stirring, platinum group metals can be dissolved. The advantages are its simplicity, its ability to be implemented on a large scale in acid-resistant enamel-lined reactors, and the ability to further enrich the platinum group metal concentrate from the leachate using methods such as displacement and precipitation, followed by separation and refining into finished products. In the 1970s, research on this technology for treating waste automotive catalysts began abroad (41-46). Early studies showed low leaching rates. Leaching with HCl + Cl₂ at approximately 90℃ for 4 hours yielded only about 90% leaching rate for Pt and Pd, and 80% for Rh.
The low leaching rate was due to the long-term effects of high-temperature automotive exhaust gases, causing some metal particles adhering to the catalyst support surface to penetrate into the inner layer of the support, or to be partially sintered and aggregated, partially encapsulated by the support through glazing, or transformed into chemically inert oxides. To improve the leaching rate, the following measures were implemented:
If platinum group metals penetrate into the inner layer of the support or are encapsulated through glazing, the waste catalyst should be finely ground or calcined to transform it into an alumina state, or a small amount of dilute acid or alkali solution should be used to dissolve part of the support, exposing the dissolved surface of the platinum group metals;
If platinum group metals exhibit sulfidation or phosphating, oxidative roasting or reduction roasting in nitrogen containing 3% H₂ at 800℃ should be used to destroy the sulfides and phosphides;
To reduce HCl consumption, a portion of… 4. Increase the Cl concentration in the leachate. After AlCl3·6H2O crystallizes out, the HCl is regenerated by hydrolysis and reused.
Pre-treat with a strong reducing agent (such as alkaline sodium borohydride solution) to convert some sparingly soluble oxides into readily soluble metals, then leach the platinum group metals with hydrochloric acid and an oxidizing agent.
Improve the leaching rate by applying a low-frequency alternating electric field during dissolution, or by pressurizing to 105 Pa and introducing nitric oxide catalysis.
