23. Explain processing of thallium containing rare metal raw materials.

Thallium is found in several ores. One of these is pyrites, which is used to produce sulfuric acid. Some thallium is obtained from pyrites, but it is mainly obtained as a by-product of copper, zinc and lead refining. Thallium is also present in manganese nodules found on the ocean floor.

Tl production

Although the amounts of thallium in ores, including those of lead and zinc, are small compared with the amounts in salts, rock, etc., only the former play a significant role in the winning of the metal. They' can be economically worked only if the thallium becomes concentrated in the side products in the course of processing the ore to obtain the main metal, and if other metals are present in this side product to an extent that justifies their extraction. Only a small part of the thallium that occurs naturally. Is therefore available for extraction thallium is present only as a trace element in nonferrous metal concentrates. Only in a few smelting works does the thallium become sufficiently concentrated in particular process steps for extraction to be practical.

Thallium produced as a by-product of the extraction of nonferrous metals is of only minor importance, both quantitatively and economically. If the raw materials are in solution, the thallium is precipitated by cementation, and is then processed like a solid raw material.

24. Explain differences of extractions methods of indium from waste of industry.

Indium is one of the least abundant minerals on Earth. It has been found uncombined in nature, but typically it is found associated with zinc minerals and iron, lead and copper ores. It is commercially produced as a by-product of zinc refining.

In production from Zinc Circuits

During smelting processes for the recovery of base metals, indium concentrates in by-products such as residues, flue dusts, slags, and metallic intermediates. The processes for the recovery and production of indium are often complex and sophisticated; they are characterized by a low direct extraction efficiency. Therefore, the recycling of the indium contained in intermediate products is important.

In the first step, most of the zinc is removed by leaching with dilute sulfuric acid. The residue is leached with dilute hydrochloric acid to dissolve the indium. Tin is removed from the indium solution by neutralization to pH

Further neutralization then precipitates indium. The indium residue is leached with sodium hydroxide to give crude indium hydroxide as an intermediate product. Dissolution of this indium hydroxide in dilute hydrochloric acid gives an indium solution which is purified by cementation of copper and arsenic with iron, followed by cementation of tin and lead with indium. Indium is recovered from the purified solution by cementation with aluminum.

25. Critically evaluate production of high purity indium and thallium.

Chemical methods

• Melting under a layer of alkali.

The resulting blister indium melt under a layer of sodium hydroxide at a temperature of 320-350 C. This is achieved by removal of most of the zinc, aluminum, lead, tin, transitioning into the molten alkali.

• Melting under a glycerol layer, containing potassium iodide and iodine.

This method is used for the purification of indium from thallium and cadmium. Impurities of these elements having in comparison with the indium high affinity for iodine, pass into the solution of glycerol.