Remelting

Before this treatment, the cassiterite content of the ore is increased by removing impurities such as clay, by washing and by roasting which drives off oxides of arsenic and sulphur. The crude tin obtained is often contaminated with iron and other metals. It is, therefore, remelted on an inclined hearth the easily fusible tin melts away, leaving behind the less fusible impurities. The molten tin is finally stirred to bring it into intimate contact with air. Any remaining metal impurities are thereby oxidised to form a scum tin dross ) on the surface and this can be skimmed off Very pure tin can be obtained by zone refining. 

Crude lead contains traces of a number of metals. The desilvering of lead is considered later under silver (Chapter 14). Other metallic impurities are removed by remelting under controlled conditions when arsenic and antimony form a scum of lead(II) arsenate and antimonate on the surface while copper forms an infusible alloy which also takes up any sulphur, and also appears on the surface. The removal of bismuth, a valuable by-product, from lead is accomplished by making the crude lead the anode in an electrolytic bath consisting of a solution of lead in fluorosilicic acid. Gelatin is added so that a smooth coherent deposit of lead is obtained on the pure lead cathode when the current is passed. The impurities here (i.e. all other metals) form a sludge in the electrolytic bath and are not deposited on the cathode. 

Add 40 ml. of ethyl alcohol to 21 -5 g. of 70 per cent, ethylenediamine solution (0 -25 mol) dissolve 36 -5 g. of adipic acid (0 -25 mol) in 50 ml. of a 6 1 mixture of ethyl alcohol and water. Mix the two solutions, stir and cool. Filter off the resulting salt and recrystalliae it from 60 ml. of a 6 1 ethyl alcohol - water mixture, and dry the salt in the air. Heat the salt in an atmosphere of oxygen-free nitrogen or of carbon dioxide in an oil bath until it melts (ca. 160°) the product will sohdify after a short time. Reduce the pressure to 15 mm. of mercury or less and raise the temperature of the oil bath until the product remelts (about 290°) and continue the heating for 4r-5 hours. Upon coohng, a nylon type polymer is obtained. 

Terbium has been isolated only in recent years with the development of ion-exchange techniques for separating the rare-earth elements. As with other rare earths, it can be produced by reducing the anhydrous chloride or fluoride with calcium metal in a tantalum crucible. Calcium and tantalum impurities can be removed by vacuum remelting. Other methods of isolation are possible. 

The Mam2en process uses ethylene gas as a direct refrigerant in a two-stage process (77,78). The first stage slurry is centrifuged, partially remelted, and fed to the second centrifuge. 

The crystal stmcture of glycerides may be unambiguously determined by x-ray diffraction of powdered samples. However, the dynamic crystallization may also be readily studied by differential scanning calorimetry (dsc). Crystallization, remelting, and recrystallization to a more stable form may be observed when Hquid fat is solidified at a carefully controlled rate ia the iastmment. Enthalpy values and melting poiats for the various crystal forms are shown ia Table 3 (52). 

Because of the vast quantities of scrap steel available from automobiles and appHances, recycling of steel cans has been growing at a relatively modest rate. As of 1992 up to 30% of steel cans are returned and remelted (see Recycling, ferrous metals). 

The effects of variations in composition, cleanliness, stmcture, and mechanical properties of electroslag remelted (ESR) 35NiCrMoV12.5 steel have been reported. This steel which ties between Grades 2 and 3 of ASTM specification A723 is widely used in Europe (145). 

An alternative process is electroslag remelting (ESR). More oxide inclusions are found in ESR steel than in VAR steel, but thek size and distribution are such that they normally have no noticeable adverse effect on properties (141). 

Vacuum arc remelting often is used to develop optimum solidification stmctures. Electroslag remelting, which utilizes a molten pool of slag in which the electrode is immersed, yields a cleaner and purer material. 

Lead—Calcium—Aluminum Alloys. Lead—calcium alloys can be protected against loss of calcium by addition of aluminum. Aluminum provides a protective oxide skin on molten lead—calcium alloys. Even when scrap is remelted, calcium content is maintained by the presence of 0.02 wt % aluminum. Alloys without aluminum rapidly lose calcium, whereas those that contain 0.03 wt % aluminum exhibit negligible calcium losses, as shown in Figure 8 (10). Even with less than optimum aluminum levels, the rate of oxidation is lower than that of aluminum-free alloys. 

The retorts must be opened, the reaction products removed, and the retorts filled with raw materials and resealed. The typical cycle is 8—10 hours. Capacity is controlled by the number of retorts used and the number of furnaces available. The metal crowns are removed, remelted, and cast iato iagots, or alloyed and then cast. 

MetaHothermic magnesium is recovered in the soHd state. Magnesium produced in this manner is then remelted and refined for subsequent casting. 

Standard Specification for Magnesium Ingot and Stick for Remelting," specification B 92/B AnnualF>oo/e ofASTM Standards, Vol. 2.02,... 

Health and Safety Factors. Malononitrile is usually available as a soHdifted melt in plastic-Hned dmms. Remelting has to be done carefully because spontaneous decomposition can occur at elevated temperatures, particularly above 100°C, in the presence of impurities such as alkaHes, ammonium, and 2inc salts. Melting should be carried out by means of a water bath and only shordy before use. Occupational exposure to malononitrile mainly occurs by inhalation of vapors and absorption through the skin. Malononitrile has a recommended workplace exposure limit of 8 mg/m, an LD q (oral, rats) of 13.9 mg/kg, and is classified as slight irritant (skin irritation, rabbits). Transport classification RID/ADR 61, IMDG-Code 6.1, lATA/ICAO 6.1. 

The obvious destination for nickel waste is in the manufacture of stainless steel, which consumes 65% of new refined nickel production. Stainless steel is produced in a series of roasting and smelting operations. These can be hospitable to the various forms of nickel chemical waste. In 1993, 3 x 10 t of nickel from nickel-containing wastes were processed into 30 x 10 t of stainless steel remelt alloy (205,206) (see Recycling, nonferrous metals). This quantity is expected to increase dramatically as development of the technology of waste recycle coUection improves. 

For off-site transportation, the phosphoms is loaded into railcars for transfer to the sites where it is used directly as a raw material or burned and hydrated to phosphoric acid. During shipping, the phosphoms is allowed to soHdify in the cars. The railcars are commonly double walled with a jacket that can be heated with steam or hot water so that the phosphoms can be remelted on-site for transloading to local storage tanks. For overseas shipping, tanktainers with reinforced superstmcture for safe handling are used. Formerly, full tanker ships were in use. 

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