RU2087564C1 - Method of cleaning platinum black - Google Patents

Abstract

FIELD: hydrometallurgy. SUBSTANCE: method consists in reprecipitation of chloroplatinate into chloroplatinite with oxidant and halogen-containing compound mixture to form intermediate diaquo complex in acidic acid. EFFECT: improved cleaning degree.

Description

 The invention relates to the metallurgy of non-ferrous metals, in particular to hydrometallurgical methods for the purification of noble metals.

 Precipitation methods using insignificant solubility of some alkali metals and ammonium hexahalogenoplatinates have long been used to separate them in many technological schemes and analysis. As a rule, accompanying base metals always precipitate, and from platinum iridium, palladium, and to a lesser extent rhodium. (Behavior and forms of coprecipitation of platinum metals in hydroxide precipitates of nitration of chloride solutions. Thesis. XV Chernyaevskogo meeting on the chemistry, analysis and technology of platinum metals. M. 1993, S. 309, 308, 310, 274, 275).

 To reduce the amount of crystallizable and contaminating impurities, as a rule, they go along the path of optimizing the precipitation process of chloroplatinates: variations in the concentration of ammonium or potassium chloride, changes in ionic background, alcoholization of solutions, the use of ultrasonic deposition, while at the same time significantly reducing the yield of the target salt.

 Despite the different methods and techniques for the deposition of hexachloro- or hexabromoplatinates due to the inevitable co-crystallization of impurities, precipitation cannot be dispensed with as the most technologically simplest process.

 However, even repeated recrystallizations do not always lead to the desired result. Experience shows that impurities of non-ferrous and ferrous metals with a content of 1.5 to 2% in the raw material of raw platinum reduce their content in a single redistribution no more than 50 70 times. At the same time, it is recommended to carry out 2-, 3-stage deposition, paying attention to the fact that the greatest pollution occurs in the last phases of precipitation.

 This option requires high costs for the processing of large volumes of formed filtrates and wash water. Even a carefully conducted process for producing single crystals of chloroplatinates (which is absolutely unacceptable in technology) does not make it possible to get rid of isomorphic impurities.

 Analogs are known where platinum is purified from other noble metals accompanying it in many products, namely palladium and iridium. It should be specially noted that this also uses a change in the properties of palladium and iridium compounds with a change in their oxidation state, but all this refers to noble impurities that pollute platinum. In our case, the actions of the reagents are aimed at changing the shape of the presence in the solution, respectively, of platinum, and not metal ions of impurity composition.

In technological practice, in the processing of concentrates containing the sum of platinum metals for the selective precipitation of platinum in the form of ammonium chloroplatinate, iridium (IV) in the form of (NH ₄ ) ₂ IrCl _{6 is} converted to iridium (III), and palladium (IV) from (NH ₄ ) ₂ PdCl ₆ to palladium (II) (NH ₄ ) ₂ PdCl ₄ .

 As indicated above, this is done for the sole purpose of preventing their coprecipitation with platinum. The recovery processes themselves are carried out by various methods, which is called refining of solutions in refining.

 There are a number of methods to reduce the oxidation state of iridium and palladium. Various reducing agents or their combinations are used: with oxalic acid and its salts, sugar, alcohol (Chernyaev I.I. Shirokova V.N. On the reduction of ammonium chloroiridate with sugars. Izv. Institute for the Study of Platinum, 1938, issue 15, p. 63 -99). After separation of platinum in the form of a precipitate of its chloroplatinate, the reduced complexes of iridium and palladium are again oxidized and converted to their original form.

 Thus, as mentioned above, this analogue according to the principles laid down in it is quite far from the proposed method.

 The closest technical solution aimed at purifying platinum from impurities of non-ferrous and ferrous metal ions is K.K. Klaus (I. Plaksin. Metallurgy of precious metals. M. 1958, p. 351), according to which complex compounds of other platinum metals are converted into derivatives of lower valency before platinum is precipitated as chloroplatinate.

 The essence of the proposed method is to conduct the reaction of recrystallization (reprecipitation) of insoluble chloroplatinate, using a different chemical form of platinum in solution, specially obtained for this. The chemical justification for such a procedure is the facts of a sharp change in the solubility of its salts (in particular, halides) from the oxidation state of the central atom, known in the chemistry of coordination compounds of platinum.

 Thus, the overall purification scheme appears to be the sequential conduct of the following steps.

 1. The translation of sparingly soluble chloroplatinate into highly soluble potassium chloroplatinate by reduction.

 2. The return of chloroplatinate to its original chemical form through the formation of an intermediate complex using the oxidation reaction.

 3. Isolation of platinum metal from the precipitated salt.

During the reaction for the reduction of dirty K ₂ PtCl ₆ , a solution of potassium chloroplatinite is formed, into which impurities also pass. The return of chloroplatinate to chloroplatinate is carried out with a mixture of an oxidizing agent, for example, hydrogen peroxide with a halogen-containing compound, in our case KCl, is used.

 When treated with an oxidizing agent in an acidic medium, a null-charged diaquacomplex (structural formula 1) is formed, which has low solubility and a different crystal lattice compared to chloroplatinate, which does not contribute to the isomorphic inclusion of impurities.

 Further substitution of coordinated bound water molecules with chloride ions serves only to bring platinum to its original chemical state. A significant purification of platinum black from impurities occurs only when the oxidation of the platinum ion is carried out in an acidic environment at pH <1.

 The required acidity of the solution for carrying out both the reduction reaction and subsequent oxidation is set by hydrochloric acid, which is introduced in excess into the process of neutralizing hydrazine hydrate.

 In the case of the oxidation of chloroplatinite by hydrogen peroxide in a neutral medium followed by precipitation of chloroplatinate with potassium chloride, a complex anion of the form is formed (structural formula 2), (Chernyaev II, Selected Works. Complex compounds of platinum. M. Nauka, 1973, p. 226), no purification of platinum black occurs.

 A new set of essential features has arisen: the precipitation of chloroplatinate into chloroplatinate by its reduction, followed by the return of chloroplatinate to chloroplatinate with a mixture of an oxidizing agent and a halogen-containing compound, with the formation of an intermediate diaquacomplex in an acidic medium. This new combination of essential features led to a new technical result of a more complete purification of platinum black from impurities.

The conditions for the specific implementation of our proposed method will be given as examples, however, the main advantages of the operations we cite are as follows:
the transfer of platinum metal into a water-soluble state is carried out only once. Already this allows to significantly reduce the amount of harmful gas emissions to be trapped chemically, and, as a result, reduce environmental stress and save on absorption reagents;
the large difference in the solubility of platinum (II) and (IV) salts allows us to significantly simplify the hardware scheme (reduce the number of devices, their working volume), reduce the heating and evaporation of large masses of solution with significant energy savings;
the process uses a unified reducing reagent (hydrazine hydrate), which allows both the conversion of platinum (IV) to platinum (II) and the reduction of purified chloroplatinate to a metal powder.

 Example.

15 kg of wet (approximately 30%) K ₂ PtCl ₆ are introduced into a 160-liter enameled reactor equipped with a mechanical stirrer with a controlled stirring speed of 60 120 rpm, 40 l of distilled water are added, 40 l of distilled water are added, stirring is turned on and heated to a temperature of 70 80 ^o C.

 Upon reaching a predetermined temperature, a solution of previously prepared hydrazine chloride is introduced in 200 ml portions into the initial pulp. A hydrazine chloride solution is prepared by neutralizing a hydrazine hydrate solution (30% concentration) with 37% hydrochloric acid until a pH <1 is reached.

 After the reduction reaction, the end of which is easily determined by the color change of the solution from yellow to red and the end of the evolution of nitrogen gas, the solution is filtered.

 Hydrogen peroxide (30% concentration) and a 25% KCl solution are added in small portions of 200 ml into the resulting solution with stirring in an amount of up to 10 L.

 At the end of the process, the reaction mixture is cooled, filtered on a suction filter, filled with a filter cloth of a fluorlon brand (pore diameter of about 3 μm), and washed.

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Claims (1)

 A method for purifying platinum black, including the conversion of chloroplatinate to chloroplatinite by reduction with subsequent isolation of refined platinum black, characterized in that prior to the isolation of refined platinum black, chloroplatinite is oxidized to chloroplatinate by treating a chloroplatinite solution with a mixture of an oxidizing agent and a halogen-containing compound in an intermediate compound.