Molten alloy method

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

Wetzel (75) atomized both molten wax and a molten alloy in a Venturi nozzle and established empirical equations for expressing the magnitudes of the effects of operating conditions on the particle-size distribution of the spray. An advantage of molten alloy is that a permanent record of the spray is obtained and large numbers of particles may be sized by physical methods. 

Figure 15 Two selected methods for metallic glass preparation (a) in the melt-spinning process the molten alloys are projected against a spinning wheel (b) in the twin-roUer quenching technique the melt is cooled between two rotating wheels. In both processes, the aim is to achieve ultrafast cooling...

Continuous ribbons can be obtained at speeds of the order of 1 Km/minute. Molten alloys can be injected into the space between spring loaded and cooled rollers spinning in opposite directions. Removal of heat in such a set-up is even better than when a single roller is employed because of the effective use of both the surfaces of the ribbon. Methods have also been developed where drops (instead of jets) of the liquid are... 

Methods based on quenching from the melt, such as piston and anvil, double piston, torsion catapult and roller casting (Luborsky, 1980) permit larger quantities of amorphous materials to be obtained. The last of these, also known as melt spinning, is probably the method most used to obtain amorphous ribbons of thickness 10-60 pm, width up to 17 cm and virtually infinite length. In this method, the molten alloy is propelled through a small hole or a slot onto a massive, cold metallic disc rotating... 

The technique used, especially the rate of quenching, influences the catalytic activity of the alloy. A commonly used method is melt spinning or strip casting, in which a rotating wheel or disk is brought into contact with the molten alloy. The catalyst comes out in the form of ribbons no more than a few nanometers in thickness. Another technique that is likely to be increasingly practiced is ion sputtering, which has already been used to prepare Fe-Co-Si, Ni-Al-Cr, and Ni-Co (Otsuka Chemical Company, 1982). 

Sur] Solubility measurements in molten alloys, volumetric method 1550-1700°C, < 30 mass% Cr, <0.1 MPaN2... 

Fig. 1. Schematic representation of the melt spinning method (a) external melt spinning, (b) internal melt spinning. The alloy is contained in a quartz vessel A and is induction heated by means of the coils B. The molten alloys is ejected by means of an argon pressure (D).

Fig. 10. Production of amorphous wires by quenching a jet of a molten alloy ejected from a nozzle into water that rotates with the turning wheel. On the left side a sample of a 100 p.m diameter wire obtained with this method. (From Baltzer and Kiinzi, 1987.)...

One of the pyrometallurgical methods used is the Imperial Smelting Technique. A roasted zinc concentrate is charged together with coke into a blast furnace. At 1000°C zinc is reduced and its vapor passes from the top of the furnace into a condenser. Here the zinc vapor is cooled by molten lead and the two metals form a molten alloy, which is allowed to cool to 44f)°C. At this temperature the metal system has separated into a lead phase and a zinc phase. The lead is circulated for continued cooling purposes. Crude zinc produced by this process contains about 2% Pb, 0.3% Cd and 0.05% Fe. It is refined by distillation in two columns [33.5]. In the first one, zinc is purified from lead and iron. The separation is based on the fact that the boiling point of zinc is 907°C, while lead boils at 1749°C and iron at 2861°C. In the second column, zinc is separated from cadmium (boiHng point 767°C). Zinc metal with 99.9% purity is obtained. 

Type metal was an alloy with low melting point that was utilized to make the type for printing. The composition may be 81% lead, 15.5% antimony and 3.5% tin. The molten alloy expands on solidification and thus fills the moulds very well. This gives the type sharp edges. The technique has entirely been replaced by new printing methods. 

The method of liquid dynamic compaction (LDC) (Chin et al. 1986, Tanigawa et al. 1986) is based on the process of gas atomization (Anand et al. 1980). In gas atomization a stream of molten alloy is broken into a spray of fine particles by a jet of high-velocity gas and the rapidly solidified particles are collected. In LDC, a cooled substrate is placed beneath the atomization core at a distance such that most of the sprayed droplets are partially solidified. The rapidly solidified alloy builds up on the substrate at controllable rates, which can easily exceed 1 cm/min. Rapid solidification is made possible by the supercooling of the high-velocity atomized particles and the good thermal contact with a water-cooled copper substrate. 

In atomization, a stream of molten metal is stmck with air or water jets. The particles formed are collected, sieved, and aimealed. This is the most common commercial method in use for all powders. Reduction of iron oxides or other compounds in soHd or gaseous media gives sponge iron or hydrogen-reduced mill scale. Decomposition of Hquid or gaseous metal carbonyls (qv) (iron or nickel) yields a fine powder (see Nickel and nickel alloys). Electrolytic deposition from molten salts or solutions either gives powder direcdy, or an adherent mass that has to be mechanically comminuted. 

Another familiar commercial method is the immersion or hot-dipping process. The article to be coated is immersed in a molten metal bath. Usually httie else is done to change the properties of the coating, which adheres to the surface upon removal of the article from the bath. For a successful coating, an alloying action must take place between the components to some extent. Zinc and tin coatings are appHed to sheet steel by hot-dipping. 

Heating and Cooling. Heat must be appHed to form the molten zones, and this heat much be removed from the adjacent sohd material (4,70). In principle, any heat source can be used, including direct flames. However, the most common method is to place electrical resistance heaters around the container. In air, nichrome wine is useflil to ca 1000°C, Kanthal to ca 1300°C, and platinum-rhodium alloys to ca 1700°C. In an inert atmosphere or vacuum, molybdenum, tungsten, and graphite can be used to well over 2000°C. 

Magnesium. This molten salt electrolysis process is the current principal method of magnesium production. The graphite anodes can be either round or rectangular in nature (see Magnesiumand magnesium alloys). 

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