Electrolytic refining
Vacuum distillation
Melting of indium in a vacuum of 0.13 Pa at 600-950 C allows to remove low-boiling impurities of metals such as mercury, cadmium, zinc, arsenic. The melting was carried out in an induction furnace in a high purity graphite crucible. Impurity content after vacuum purification is less than 10-4 %, loss of indium - 5-10 %.
Zone melting and pulling the ingot from the melt
Indium is melted in a boat (zone melting) or crucible (stretching method) made from pure quartz or high purity graphite in vacuum or in an argon or hydrogen. This method is suitable only for purification from nickel, copper and silver.
Thallium refining
The thallium purification process includes alkaline refining of the metal with the addition of an oxidizing agent, electrolysis with a soluble anode, and zone melting or drawing from the melt.
The alkaline refining is especially effective in the presence of oxidizing agents (KN03 or NaN03). However, in such a case up to 770° of Tl is dissolved by the molten alkali, the metal must be leached from the alkali melt and precipitated from the leach liquor as Tl(OH)3. The refining is carried out at 350°. After the alkaline refining the thallium is taken for electrolysis with a soluble anode. Zone melting is the last operation.
Thallium of a purity of 99.9995% is obtained from the metal containing 99.98%Tl by a combination of electrolytic refining and zone melting.
26. Explain production of lithium metal.
The main commercial method for the production of lithium involves the electrolysis of lithium chloride in a molten medium. Methods based on thermal reduction with metals have also been used recently. These methods are also used for the production of lithium alloys
The electrolysis is carried out in a LiCl + KCl mixture (1 : 1 by weight), whose composition is close to that of the eutectic mixture (the eutectic mixture contains 60 mole% LiCl and melts at 352°.
Diagram of a bath for the electrolytic production of Li.
The bath consists of a steel jacket (wall thickness 4 to 5 mm) lined with talcum stone or magnesite; the anodic space is separated from the cathodic space by an alundum (Al2O3) partition that does not reach the bottom of the electrolyzer. The lithium metal formed in the electrolysis flows to the surface of the molten electrolyte and collects under the cover of the cathodic space. The lithium is scooped from the cathodic space and cast into ingots. The electrolysis is carried out at 400 to 430°. The metal produced contains at least 98% lithium, the main impurities being sodium, magnesium (up to 1%), iron (up to 0.04%), potassium (0.01 to 0.05%).
27. Create a modern technological scheme of processing of mineral raw materials containing lithium.
The commercial products of the processing of lithium concentrates are lithium salts (LiCl, Li2C03, Li2S04) and lithium hydroxide.
The first stage in the processing involves the decomposition of the concentrates, which may be accomplished by several methods. The most important methods are: decomposition with sulfuric acid, with lime, and the sulfate process. The chlorination-roasting method is also used occasionally.