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Selective Precipitation Platinum - Ammonium Chloride Precipitation Method

This method is a classical method in platinum production. Palladium and iridium can be separated by boiling or adding weak reducing agents to keep the low valence state from being precipitated by ammonium chloride. During the operation, the solution is boiled, then solid ammonium chloride is directly added and stirred continuously, which forms a yellowish ammonium chloroplatinate [(NH4)2PtCl6] precipitation. The operation stops until the addition of ammonium chloride does not produce precipitation. After cooling and filtering, the precipitated ammonium chloroplatinate was washed with 10% NH4Cl solution.

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Platinum precipitation solution mainly contains platinum, palladium (a small amount of rhodium, iridium) and base metals. When platinum content is absolutely dominant, ammonium chloride precipitation method is generally used. Ammonium chloride reacts with platinum group metals to form (NH4)2MeCl6 salts, in which (NH4)2PtCl6 has the lowest solubility and precipitates in light yellow or lemon yellow crystals. The reaction is as follows:

H2PtCl6+2NH4Cl=(NH4)2PtCl6↓+2HCl

The solubility of the same type of iridium and osmium salt is 6~7 times higher than that of platinum salt, and the solubility of palladium and ruthenium of the same type is 300~600 times higher than that of platinum salt, so platinum can be separated from other metals by the difference of solubility in the washing process.

Since this method is a classic method, it is generally suitable for solutions with an absolute dominant platinum content and a high platinum ion concentration (preferably 50~100 g/L). If platinum is high, the separation effect is not good, platinum precipitation is not complete and the solution is polluted.

Before adding ammonium chloride, the feed solution should be oxidized properly to ensure the existence of platinum in the form of platinum (IV). Control pH=1 and heat to 80 oC, add solid ammonium chloride under slow stirring, stop stirring until excess of a little ammonium chloride no longer causes the solution to form yellow precipitation, and rapidly cool the feed liquid to room temperature, and then filter after static clarification. The filter cake is washed 2 to 3 times at room temperature with 10% ammonium chloride solution, and the liquid-solid ratio is between 1 to 2 during washing, so that the palladium content in the precipitation is less than 1%. At this time, the precipitation rate of platinum can reach 98%~99%. The lotion is combined with the filtrate to extract palladium, and the precipitation is used to produce platinum.

When platinum is precipitated with ammonium chloride, generally 1%~3% of platinum remains in the mother liquor. When the concentration of platinum is 50 g/L, the direct yield is 97%~99%, while when the concentration of platinum is 10~20 g/L, the direct yield is 91%~92%. When the content of base metal is high, it will seriously pollute the precipitation of platinum.

The production practice shows that coprecipitation is easy to occur if the valence state of the metal is not properly controlled. The impurities brought into the platinum ammonium salt by co-precipitation and adsorption are mainly removed by washing with ammonium chloride aqueous solution. The more times of washing, the more thoroughly the impurities are removed. However, a large amount of washing liquid will dissolve the platinum ammonium salt, resulting in dispersion loss, and the platinum in the washing liquid is mixed into the palladium, which must be removed during the ammonia complexing process of palladium, so that platinum and palladium can be dispersed again, while the platinum in the palladium ammonium salt precipitation Impurities cannot be completely removed by washing.

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