20. Explain the differences extraction of selenium and tellurium from waste of industry
The selenium content of the anode slime varies with the raw material and can reach> 20%.
Roasting with sodium carbonate
The anode slime is first treated with H2SO4 solution to remove the Cu then mixed with Na2CO3 and H2O to give a stiff paste. The paste is extruded or pelletized, and dried. It is then roasted at 530-650 °C in a stream of oxygen air. The roasted pellets are ground and leached with H2O to dissolve Na2SeO4. Na2SeO4 is crystallized from solution and reduced with active carbon to Na2Se, which is redissolved in H2O and oxidized with air to elemental Se.
Tellurium can be recovered as a by-product in the treatment of Pb, Cu, Bi, precious metals, Ni ores, and from sulfuric acid plants.The main source is Cu anode slimes, Cu anode slimes contain 0.5-10% Te.Tellurium is mostly present in the form of intermetallic compounds of silver, copper, and sometimes gold [Ag2Te,Cu2Te, and (Ag, Au)Te2].
Roasting
Pretreated decopperized slimes are roasted with soda ash,
Whereby Te is converted to insoluble Na2TeO3 and Na2TeO4:
After water leaching, Se is recovered as soluble Na2SeO4. The residue containing tellurium (Na2TeO4), Pb, and precious metals is either sent to a dore furnace, or treated with sulfuric acid:
The residue containing Pb and precious metals is separated for further treatment. H2TeO4 is reduced to tellurium by treatment HCl and SO2, or reduced by Na2SO3 to TeO2, or precipitated as Cu2Te by adding Cu. Cu2Te and TeO2 can be leached with dilute NaOH (with Aeration in the case of Cu2Te) to form Na2TeO3 solution which is further prepared for electrolysis.
21. Compare methods of decomposition of tantalum and niobium containing rare metal raw materials.
Ta and Nb ores
Ta and Nb ores are poor. Ta and Nb minerals are extracted simultaneously with other valuable minerals.The basic method of Ta nad Nb ores enrichment – gravitational, which gives concentrate consisting tantalite or columbite. Further enrichment of minerals separation is carried out by electromagnetic and flotation.
Decomposition of tantalite-columbite with hydrofluoric acid
The finely-divided tantalite (columbite) is decomposed with hot concentrated hydrofluoric acid. The main decomposition reactions are (where, R - Ta and Nb):
The digestion of concentrates with hydrofluoric acid is carried out in steel tanks covered with lead and lined with graphite blocks. The stirrers may be prepared from monel metal, an alloy of nickel containing 27 to 29% copper
Decomposition of Tantalite and columbite concentrate with sodium hydroxide
Explain production of high purity gallium.
Production of very pure gallium
Extremely pure gallium is needed for certain purposes (semiconductor technology, etc.).
Gallium of a high degree of purity (99.9999%) is usually produced by a combination of several purification methods: a) electrolytic refining; b) vacuum distillation of the impurities; c) zone melting
Zone melting of gallium is carried out either in a high vacuum or in hermetically sealed tubes filled with gallium. The tubes may be made of poly(vinyl chloride). The liquid gallium crystallizes with some difficulty because of its tendency to supercool. Hence, forced cooling is employed. The zone melting is conveniently carried out in an instrument with a rotating drum having a poly (vinyl chloride) tube (filled with gallium) coiled over it. A part of the drum is immersed in water.
Gallium of a degree of purity of 99.9999% was produced by using a tube 5 mm in diameter and 100 mm long, the zone moving at a rate of 2 to 4 cm/hour. 99.999% pure gallium was used as the starting material. The yield of the pure metal was about 50%.
Gallium is produced by electrolysis of an alkaline solution of sodium gallate, containing 5 to 100g Ga/l.
Gallium concentrates (oxides or hydroxides) of different degrees of purity may be used for the preparation of the electrolyte. During the electrolysis most of the impurities remain in the solution. The electrolysis is carried out at 50 to 60° in baths made of glass, porcelain or organic materials, using stainless steel anodes and cathodes.
The electrolysis is carried out at a bath voltage of 4.5 V and a current density of 0.3 to 0.5 A/cm2. The metal produced by the electrolysis is washed first with distilled water and then with hot hydrochloric acid. This treatment results in the removal of zinc, copper, lead, iron, and some other metallic contaminants. Successive treatments with hydrochloric and nitric acids are occasionally used. The washed metal usually contains 99.9 to 99.957% gallium