ASM Metals Handbook - Desk Edition (ASM_ 1998) WW part 11 ppt

Leaching Processes Leaching is a separation process that uses aqueous solutions. A suitable aqueous environment is selected which can decompose the mineral containing the valuable metal. The objectives of leaching are: • Production of a compound for further processing by pyrometallurgical techniques • Production of a metal from impure metal or metal compounds that have been prepared by a pyrometallurgical process • Direct production of a metal from an ore or concentrate Selection of a particular objective depends on economic factors and the involved thermodynamic and kinetic conditions of the system. The theoretical possibility is limited by thermodynamic constraints, whereas kinetic constraints relate to the overall time required and affect the reactor size and design. The thermodynamics of leaching are concerned with the ability to decompose a particular compound so that it will selectively dissolve and become stable in the aqueous solution used for leaching. Proper calculations can predict the maximum amount of mineral that can be leached until the system reaches equilibrium. The rate of dissociation and dissolution is kinetically controlled. Figures 9 and 10 are schematic diagrams of a mineral/water interface on a microscopic scale. The concentration of the active chemical in the bulk of the solution is greater than the concentration near the surface of the mineral. This occurs because the active chemical is removed from solution by reaction at the mineral surface. Figure 9 shows that the rate at which the reaction or decomposition of the mineral will occur is determined by: (1) the diffusion of reactant, R, from the bulk of the solution to the surface of the mineral; (2) the reaction of the reagent with the surface to form a soluble species; and finally, (3) the diffusion of the product metal species, M, away from the surface. In Fig. 10, diffusion of R through the porous product layer and diffusion of M through the product layer are also possible (Ref 15). Fig. 9 Schematic diagram of a mineral surface showing complete dissolution in water Fig. 10 Schematic diagram of a mineral surface showing decomposition in water and generation of a porous layer of residue on the surface The leaching of a compound in an aqueous environment can take several forms. Simple dissolution reactions in water, acid, and alkali can be represented by Eq 6, 7, and 8 respectively: CuSO 4(s) + nH 2 O CuSO 4 ·nH 2 O (aq) (Eq 6) ZnO (s) + + H 2 O (Eq 7) Al 2 O 3(s) + + H 2 O (Eq 8) Anionic base exchange, oxidation-reduction, and water-soluble complex formation reactions can be shown by the reactions shown in Eq 9, 10, and 11, respectively: CaWO 4(s) + CaCO 3(s) + (Eq 9) CuS (s) + + (Eq 10) CuO (s) + + 2NH 3 + H 2 O (Eq 11) Oxygen pressure and pH are the most important parameters for determining the chemical properties of aqueous solutions. Predominance area diagrams can be plotted for a metal/H 2 O system in the form of a pH/log p diagram showing regimes of stability for the metal, ions, and insoluble metallic compounds, such as hydroxides (Ref 16). The leaching method used depends on the physical condition of the ore and the inherent mineralogy. Simple leaching processes include in situ leaching, heap leaching, and agitation leaching. In situ leaching refers to mineral dissolution with the ore "in place" underground. In situ leaching is usually done in worked-out stops of high-grade mines, support pillars left behind after mining, or low-grade deposits. This type of treatment requires the surrounding rock to be tight and impermeable to solution flow in order to contain the leaching solution. This process is very time consuming but has low treatment costs, low equipment requirements, low capital costs, and the ability to treat low-grade ores. The heap-leaching process is similar to in situ leaching since it does not require extensive leaching equipment, such as tanks, slurry pumps, and thickeners; hence, heap leaching requires low capital and maintenance costs. Therefore, the principal feed materials for heap leaching are low-grade ores, ores not amenable to flotation, and discarded waste rock from previous processing with metal values below milling grade. The leachant is sprayed or pumped over a heap of ore. As it percolates through the heap, it dissolves the desirable compounds from the ore. An agglomeration step prior to leaching of the fine ore has shown significant improvement in precious metals recovery (Ref 17). Agitation leaching is usually used with well-disseminated, fine-grained, high-grade ores. This requires extensive crushing and grinding before leaching in order to expose the solution to the minerals where agitation improves the process kinetics. Because of the extensive amount of equipment required for agitation leaching, recoveries of over 90% and short residence times are requirements of the process. Agitation is accomplished by either bubble action using compressed air or by mechanical agitation using impellers. Pachuka tanks are commonly used in air-agitated leaching processes (Ref 18). When high-intensity agitation is required mechanical agitation with impellers is usually used. Marine propellers produce an axial flow, paddles cause a tangential flow, and turbine impellers produce a combined radial and axial flow pattern (Ref 19). Pressure leaching is similar to agitation leaching except that the process is done at elevated pressures and temperatures. Agitation leaching under normal pressure is limited to 100 °C (212 °F). As pressure on the solution is raised, the boiling point of water can be elevated. Thus, higher leaching temperatures can be employed by increasing the pressure. Therefore, the main objective of pressure leaching is to enhance the kinetics of metal dissolution by permitting higher operating temperatures and by increasing the solubility of gaseous species that may take part in the leaching reaction. The oxidation of sulfides in aqueous solutions exemplifies the need for increased gas solubility. At atmospheric pressure metal sulfides are insoluble even in strong acidic solutions. Increasing the temperature and pressure of the system increases the solubility of oxygen in solution. The increase in oxygen solubility and temperature causes rapid oxidation of some metal sulfides to sulfates making them soluble in acid solutions. This ability to achieve oxidation in the leaching step by increasing the pressure can eliminate the need for pretreatment steps, such as oxidation roasting of sulfide ores. Pressure leaching is generally done in a stainless steel or titanium autoclave for high strength and corrosion resistance at higher temperature and pressure. Linings of glass, lead, or refractory brick may be used under severe corrosion conditions. Most autoclaves are equipped with agitators for mixing. Both vertical and horizontal autoclaves are used in hydrometallurgy. The Sherritt-Gordon process (Fig. 11) uses pressure leaching during the production of nickel and cobalt metals (Ref 20). Fig. 11 Sherritt-Gordon process flow diagram for nickel and cobalt production Solution Purification Leaching with strong solutions usually produces an aqueous stream containing the desired metal values as well as some impurities due to the complexity of the mineral ore. Processes for the purification of the leaching solution prior to metal recovery include precipitation, solvent extraction, and ion exchange. Precipitation involves the removal of ionic species from solution as compounds. Precipitation is accomplished by making adjustments to the solution which cause formation of compounds that are no longer soluble in the solution. The concept of the solubility product is used to predict, and to perform calculations concerning, precipitation from solution. The solubility product for a given compound is defined as the product of the concentrations of cation and anion of the compound of interest, each raised to the power of its proportion in the compound. For example, the solubility product for a hypothetical compound MX 2(s) would be [M 2+ ] [X - ] 2 = K s (Eq 12a) based on the reaction M 2+ + 2X - = MX 2(s) (Eq 12b) If the product of the concentrations (left-hand portion of Eq 12a) exceeds the value of the solubility constant (K s ) in a solution, precipitation of the compound occurs. When used as a purification technique in hydrometallurgy, precipitation can be initiated by one of a few different methods. For example, precipitation can be done by addition of chemicals. If the appropriate cation or anion is added to a solution, it will force the precipitation of a specific compound or compounds by exceeding the solubility products. Because metallic sulfides in general have very small solubility products, addition of sulfide anions (as hydrogen sulfide) causes precipitation of insoluble metal sulfides. Precipitation can also be accomplished by the evaporation of water from the solution. As the water evaporates, the concentrations of all ionic species increase until one or more solubility products are exceeded and precipitation occurs. Changes in the pH of solutions can also be used to precipitate compounds (Ref 16). As the pH of a solution is increased and the solution becomes more basic, the hydroxide ion (OH - ) concentration increases and solid hydroxides precipitate. Iron, copper, cobalt, and nickel are precipitated selectively as hydroxides in solutions by raising the pH with milk of lime to 2.5, 5.8, 8.3, and 9.4, respectively. Precipitation can be used in a process to remove the impurities as well as to concentrate metal values in the form of a compound. Desired metal values can also be removed from an impure solution and concentrated in a solid compound. The recovery of sulfides of nickel, copper, lead, and zinc from leaching solutions as precipitates requires further purification, but provides a low-cost treatment method with very low concentrations of metal values in the leach solution. Solvent extraction is a chemical process used to purify and concentrate a given species from aqueous solution. This is accomplished by recycling an organic solution, which selectively exchanges the metal species of interest between an impure aqueous feed solution and a pure fresh aqueous solution. The process relies on the immiscibilities of organic and aqueous solutions as well as the stabilities of the metal species in them. Purification is then achieved by extracting a metal species from the impure aqueous solution to the organic solution and then stripping the metal species from the organic solution back to fresh aqueous solution. A typical solvent-extraction flow sheet is shown in Fig. 12. The organic must be selective to the species being purified, and the reaction must be reversible so that the metal species can be transferred from impure to fresh aqueous solutions via the organic phase. Solvent-extraction chemistry can roughly be separated into two types of reactions: solvation and exchange reactions. Fig. 12 Typical flow diagram for solvent extraction Solvation involves transfer of neutral molecular species between aqueous and organic. In the transfer from the aqueous to the organic phase, the neutral species simply dissolves in the organic solution. The organic phases used for solvation can be alcohols, ethers, esters, ketones or phosphorus-containing compounds such as trialkyl phosphates and trialkyl phosphine oxides. Exchange reactions involve the formation of specific bonds between metal species and active compounds in the organic phase. The metal species forms an organic salt with the active organic compounds, which then dissolves in the organic phase. Exchange reactions can be cationic or anionic depending on the system involved (Ref 21). The organic solutions typically used for exchange reactions are made up of a carrier, an extractant, and a modifier. The carrier or diluent is the inert organic that makes up approximately 90% of the solution and acts as a vehicle for carrying the active extractant. Common diluents are kerosine, and naphthene. The extractant or active agent is the compound that contains the functional group capable of chemically reacting with the particular metal species in the aqueous phase. The modifier, which is usually an alcohol, is added because it increases extracting power, increases selectivity, improves phase separation, and prevents formation of solid organic compounds. Solvent extraction is typically performed in a combination of mixer-settler units to allow the countercurrent flow of organic and aqueous solutions from stage to stage. Ion exchange is accomplished by interchange of metal ions between aqueous solutions and a solid, insoluble resin. The chemistry of the ion-exchange process is similar to the solvent extraction systems and is sometimes used as a substitute for solvent extraction to avoid problems of emulsion formation and solvent loss due to entrainment. The ion-exchange process involves adsorption followed by elution. Adsorption is the removal of metal ionic species from an aqueous solution when that solution is passed through a bed of ion-exchange resins. Elution is the recovery of the metal ionic species in fresh solution by passing a suitable fresh solution through the previously loaded resins. Ion-exchange resins are classified as cationic resins and anionic resins. Cationic resins exchange cationic species and are made of strong acid or weak acid groups and exchange H + ions. Anionic resins are strong bases, such as quaternary ammonium group bases, and weak bases, such as secondary or tertiary amine group bases. In most cases, chloride ions exchange with anions in solution (Ref 22). The important properties of ion-exchange resins are capacity, selectivity, and mechanical properties. The capacity of a particular resin is the amount of a specific inorganic group that the resin will hold per unit weight or volume. The affinity of a resin for different ions in solution varies. This selectivity of one ion over the other is described as a distribution coefficient, K, where K = (% equivalent of ions in resin)/(% equivalent of ions in solution). For a particular resin, the selectivity coefficient varies with the species of interest making it possible to purify a particular metal ion from a complex solution. Because ion-exchange resins are used over and over as the transfer media for purification, they require good mechanical properties. Resins must be durable and resistant to breakage, must have low chemical degradation, and must be insoluble in aqueous solutions. The ion-exchange equipment involves fairly high capital costs and a large plant area due to the large amounts of in-process material. However, properly run ion-exchange facilities result in up to 99% efficiency during normal operation. Metal Deposition Once an ore concentrate has been leached and purified, the metal of interest must be recovered from solution. Three common techniques of metal reduction from aqueous solution are cementation, hydrogen reduction, and carbon adsorption. Cementation, or metallic replacement, is a classical process for recovering metals from aqueous solution. Cementation is essentially the precipitation or discharge of a noble or less-reactive metal in favor of a more-reactive metal. The basic reaction between the two metals is electrochemical in nature and can be represented by the reaction in the following equation for the reduction of cadmium by zinc metal: + Zn (s) Cd (s) + (Eq 13) The above electrochemical reaction can be separated into two half-reactions: Zn (s) + 2e - (Eq 13a) + 2e - Cd (s) (Eq 13b) When the active zinc metal is added to a solution containing relatively noble cadmium ions, reduction takes place in microcells producing cadmium metal. Since the two half-cell reactions require transfer of electrons it is essential that the solid is a conductor of electricity. The tendency of one metal to displace or reduce another metal from solution is based on the electromotive series of metals (Ref 23). In general, when two metals are considered, the one that is more electropositive will tend to reduce a less-electropositive metal from solution. Also, the greater the difference in potential between the two metals is, the greater is the driving force for the reaction. However, even a small difference in potential results in an extensive degree of reduction. Reduction of copper ions from solution by metallic iron using cementation is industrially practiced: + Fe (s) Cu (s) + (Eq 14) Copper sulfate solution is fed through open launders containing steel scrap where the displacement reaction occurs and produces a very pure copper that can be recovered in the bottom of the launders. Cementation cones are commonly used now for the deposition of metals (Ref 24). The rate at which cementation reactions occur depends on initial concentrations, temperature, agitation, polarization characteristics of different metals, and addition agents (Ref 25). Gaseous reduction of metals from aqueous solution can be done with reducing gases, such as hydrogen, carbon monoxide, and sulfur dioxide. Hydrogen is the most widely used because it is relatively inexpensive. The reaction products from carbon monoxide and sulfur dioxide have to be further treated after the reduction step. The use of reducing gases also involves a replacement reaction: + H 2(g) Cu (s) + (Eq 15) The tendency of the above reaction to produce copper can be increased by increasing the pressure of hydrogen gas since the reaction moves to the right. A similar reaction for nickel at room temperature and atmospheric pressure does not occur. However, if the reaction is carried out in an autoclave that permits high hydrogen pressures and temperatures, nickel can be reduced from the solution. Although the mechanism by which nickel is reduced from ammoniacal solutions is more complex, nickel is reduced commercially at high temperature and pressure in the presence of ammonia (Ref 20). Carbon adsorption, or reduction of metals from aqueous solution, is used almost exclusively to recover the noble precious metals, such as gold and silver. This process is based on the principle that these metals can be reduced out of solution by solid carbon at low temperatures and deposited in metallic form on the carbon. In a typical carbon-adsorption process, the metal leaching solution is fed to carbon columns and the metal is almost completely removed from the solution by adsorption on the solid carbon. After the carbon is loaded, it is removed from the circuit and the metal is stripped away. The carbon can also be added to the leaching liquor and agitated without the requirement of carbon columns, known as the "carbon in pulp" process. Desorption or stripping is done by passing a hot, caustic stripping solution over the column. Stripping is followed by electrowinning from solution to produce a very pure gold or silver product. A typical flow sheet, including carbon adsorption, is shown in Fig. 13. The loaded carbon can also be burned, which leaves a gold- or silver-rich ash. Fig. 13 Heap-leaching charcoal-adsorption process for gold ores low in silver References cited in this section 15. M.E. Wadsworth and J.D. Miller, Hydrometallurgical Processes, Rate Processes of Extractive Metallurgy, H.Y. Sohn and M.E. Wadsworth, Ed., Plenum Press, 1979, p 133-241 16. T. Rosenquist, Principles of Extractive Metallurgy, 2nd ed., McGraw-Hill, 1983, p 438-441 17. "Agglomeration- Heap Leaching Operations in the Precious Metals Industry: U.S. Bureau of Mines Information Circular 8945," 1983 18. A.G.W. Lamont, Air Agitation and Pachuca Tanks, Can. J. Chem. Eng., Aug 1958, p 153 19. J.Y. Oldshue, Fluid Mixing Technology, Chemical Engineering Division, McGraw-Hill, 1983 20. J.R. Boldt, Jr. and P. Queneau, The Winning of Nickel, International Nickel, New York, 1967 21. P.J. Bailes, C. Hanson, and M. Hughes, Liquid-Liquid Extraction-Metals, Chem. Eng. (UK), Aug 30, 1976 22. K. Dorffner, Ion Exchangers, Properties, and Applications, Ann Arbor Science Publishing, 1973 23. D. Jones, Principles and Prevention of Corrosion, 2nd ed., Prentice Hall, 1996, p 43 24. R.H. Spedden, E.E. Malouf, and J.D. Prater, Cone-Type Precipitates for Improved Copper Recovery, J. Met., Vol 18, 1966, p 1137-1141 25. J.D. Miller, Cementation, Rate Processes of Extractive Metallurgy, H.Y. Sohn and M.E. Wadsworth, Ed., Plenum Press, 1979 Electrometallurgical Processes Electrometallurgy, or electrolytic processing, deals with the production of metals from ions by application of electrical energy. In contrast with electrochemical processing, where chemical reaction produces electricity as in batteries or corrosion, electrolytic processing uses electricity to perform chemical functions as in metal extraction. Therefore, corrosion is also known as "extractive metallurgy in reverse." As has been discussed thus far, extraction usually implies reduction of a compound, where as corrosion implies oxidation of a metal into a compound. The term electrowinning is used to describe the recovery of metals from a solution or electrolyte on a negative electrode, or cathode, while the positive electrode, or anode, is inert to the ongoing reaction. Electrorefining entails a purification process in which the anode is made of solid impure metal that actively dissolves in the electrolyte; deposition of pure metal occurs on the cathode. These solutions could be generated during leaching and/or purification, as discussed in hydrometallurgy. Fused salt electrolysis is the production of metals which cannot be electrolyzed from aqueous solution due to their relative positions in the electromotive series and, therefore, are reduced from molten salts. In other words, production of hydrogen from its oxide (water) is, energy-wise, more favorable than the production of metal from its compound. Electrowinning and electrorefining concepts are equally applicable to molten salt electrolysis. If the metal compound has high melting point and is difficult to melt, it may be dissolved in an inert carrier electrolyte (molten salt) which is more stable than the metal compound of interest. The Hall-Héroult process for aluminum production essentially follows this scheme where alumina is dissolved in molten cryolite (sodium-aluminum fluoride). Basic Concepts The electromotive series is a listing of the standard half-cell potentials with respect to a reference electrode. The reactions described in establishing an electromotive series are referred to as electrochemical reactions. Electrochemical reactions involve oxidation (loss of electron) and reduction (gain of electron) and can be arranged in an electrochemical (galvanic) cell as shown in Fig. 14. In general, the valence state of the metal is increased in oxidation and decreased in reduction. The oxidation state of a pure metal atom is zero which gets positively ionized by losing electrons. The electrons are picked up by another ionic species, which gets reduced in the solution. If this other ionic species is another metal ion that is reduced on the cathode, the process is known as cementation. It should be memorized that the ions that move towards the anode are anions (negatively charged) and those that are attracted by the cathode are cations (positively charged). The ionic species in the solution (acidic) could be simply protons that can be reduced on the cathode as hydrogen gas. Fig. 14 Electrochemical (galvanic) cell The potentials on the electromotive series are the minimum theoretical volts required for depositing the metals from a molar solution saturated with its own ions at 25 °C (77 °F), and are measured against a standard reference electrode. For the example of copper metal deposition using zinc metal (Eq 13), an electrochemical cell can be set up, as in Fig. 14, by using a copper electrode as M in a solution of copper sulfate and a zinc electrode as M 2 in a solution of zinc sulfate. If the external circuit is short circuited, electrons will flow from the zinc electrode (anode) to the copper electrode (cathode) as zinc dissolves, which causes deposition of copper metal. By placing a voltmeter between the cathode and the anode, the potential difference between the oxidation and reduction half-reactions can be measured. Because measurement of potential requires measurement between the electrodes, an absolute potential of any half-reaction cannot be measured. The theoretical dissociation potential required for electrolysis of a compound is given by -cG/nF, where G is the Gibbs free energy change of the formation of the compound and is a function of temperature and activities of the ions, n is the number of electrons transferred in the oxidation-reduction reaction, and F is the Faraday Constant (96,486 C). The theoretical current required for metal deposition from the solution is given by Faraday's law, which states that 1 gram- equivalent weight of any metal can be deposited by passing 95,500 ampere-sec of charge (1 faraday). The product of the theoretical potential and current is the theoretical power required. The current and energy efficiencies of an electrolytic cell may be measured as the percent ratios of these theoretical values and the actual amounts of potential applied and current consumed. Efficient electrolytic cells are characterized by high current and energy efficiencies. The theoretical dissociation potential must be exceeded to allow deposition of the metal of interest, but should be kept as low as possible for a higher energy efficiency. Current, in general, is directly linked with the deposition rate, although higher current also implies higher ohmic losses and I 2 R heating. Electrowinning The electrowinning arrangement usually recycles the acid between the leaching operation and the electrowinning operation for economic efficiency. A simplified diagram of the leaching/electrowinning process is presented in Fig. 15. For example, in the leaching step, the sulfuric acid is consumed as the metal dissolves, according to the reaction in the equation: MO (s) + H 2 SO 4(aq) H 2 O + MSO 4(aq) (Eq 16) Fig. 15 Simplified block diagram showing the cyclical nature of the leaching/electrowinning process The leaching solution, H 2 O + MSO 4(aq) , is transferred to the electrowinning step for deposition of the metal. The leaching solution is fed into cells which consist of inert anodes (usually made of lead) and cathodes (which may be made of stainless steel, aluminum, or starter sheets of metal M), arranged as parallel alternating plates of cathodes and anodes. In the electrowinning step, the power supply forces electrons to the cathode, and metal sulfate is reduced to produce the pure metal: MSO 4(aq) + 2e - M (s) + (Eq 17) At the anode, oxygen is produced by decomposition of water and the regeneration of acid is accomplished by the reaction in Eq 19, and recycled back to the leaching step (Eq 16): H 2 O + O (g) + 2e - (Eq 18) + 2H + H 2 SO 4 (Eq 19) In this case, the potential required between cathode and anode in electrowinning is of the order of 1.25 to 1.75 V. This potential is the combination of the decomposition potential defined earlier (the reversible potential between the cathode and the anode), the ohmic drops or IR losses due to the resistance of the electrolyte, connections and conductors, and polarization of the cell (Ref 23). The decomposition potential is measured only when the cell reactions take place under conditions of infinitely low current or net zero current. In electrolysis, where current flows at a finite rate, additional voltage drops occur as a result of phenomena that take place near the electrode solution boundaries. These extra voltages are called overvoltages and are caused by reactants not being supplied to electrodes as fast as products are removed (concentration overvoltage) or by [...]... of secondary metals (which often are a series of precious metals) in a concentrated state The secondary metals are recovered from the concentrated form in a separate process Metals made by smelting of sulfides, such as copper, lead, and nickel, usually are refined by electrolysis The impurities that contaminate these metals are most often associated with sulfides themselves The precious metals gold,... Grjotheim, C Krohn, M Malinovsky, K Matiasovsky, and J Thonstad, Aluminium Electrolysis: Fundamentals of the Hall-Heroult Process, Aluminium-Verlag GmbH, Dusseldorf, 1982 27 A.W Schlechten and C.A Natalie, Extractive Metallurgy, Metals Handbook: Desk Edition, ASM International, 1985 General Introduction to Casting Thomas S Piwonka, The University of Alabama Introduction METAL CASTING is the manufacturing... horizontally parted molds (a) Parallel to parting line (b) Between 0 and 90° to parting line (c) 90° to parting line Arrows indicate the direction of metal flow Pouring metal down a sprue causes the metal to accelerate due to gravity The increased velocity caused by this acceleration leads to turbulent flow and the formation of inclusions One method of avoiding turbulent flow is to use some form of counter-gravity... place where shrinkage is expected is at the centerline of the casting, which is the last part of the casting to solidify In these alloys, tapering the casting cures the problem Fig 13 Solidification (freezing) mode for pure metals and alloys (a) Freezing mode in pure metals, in which the freezing range (liquidus-to-solidus interval) approaches zero Crystallization begins at the mold wall and advances... solidification rate is, the greater the departure from equilibrium will be Departures from equilibrium exaggerate segregation They also depress the liquidus and solidus temperatures Indeed, it is common for the last metal to solidify as a eutectic because of departures from equilibrium during solidification It is important to remember in analyzing castings that there is this departure from equilibrium caused... extract metals cheaply, efficiently, and in an environment-friendly manner References cited in this section 5 T Rosenquist, Principles of Extractive Metallurgy, 2nd ed., McGraw-Hill, 1983 23 D Jones, Principles and Prevention of Corrosion, 2nd ed., Prentice Hall, 1996, p 43 26 K Grjotheim, C Krohn, M Malinovsky, K Matiasovsky, and J Thonstad, Aluminium Electrolysis: Fundamentals of the Hall-Heroult... Fig 11 Counter-gravity pouring systems allow the rate of fill to be controlled precisely; it can be speeded up or slowed down as necessary to compensate for changes in the cross-sectional area of the casting In filling the mold from the bottom, the rate of rise in the casting cavity should be no greater than 0.5 m/s (1.65 ft/s) for aluminum alloys and no greater than 0.3 m/s (1 ft/s) for copper-base... counter-gravity gating systems Counter-gravity pouring systems operate at the same speed as conventional pouring lines; a number of techniques, such as rotating the mold around a horizontal axis at the bottom of the mold after pouring, are available to prevent molten metal from draining out of the casting cavity until solidification is complete Fig 11 Schematic of the operations of the counter-gravity... approximately 940 °C (1725 °F), and the Hall-Héroult cell operates at temperatures of approximately 960 to 1000 °C (1760 to 1830 °F) with a power rating of 10 to 12 kWh/kg aluminum Fig 16 Hall-Héroult aluminum production cell with self-baking anode Source: Ref 5 Electrochemically, aluminum is reduced at the cathode from an ionic state to a metallic state by: Al3+ + 3e- Al(l) (Eq 20) This simplified reaction... have already solidified, the first rule to follow in establishing progressive solidification is not to attempt to feed a thick section through a thin section Each thick (slow-to-solidify) section must be isolated from thin (fast-to-solidify) sections and provided with its own riser The riser for these thick sections may require a gate; however, in many cases it is not feasible or economical to place . Fundamentals of the Hall-Heroult Process, Aluminium-Verlag GmbH, Dusseldorf, 1982 27. A.W. Schlechten and C.A. Natalie, Extractive Metallurgy, Metals Handbook: Desk Edition, ASM International,. 1979, p 13 3-2 41 16. T. Rosenquist, Principles of Extractive Metallurgy, 2nd ed., McGraw-Hill, 1983, p 43 8-4 41 17. "Agglomeration- Heap Leaching Operations in the Precious Metals Industry:. in hydrometallurgy. The Sherritt-Gordon process (Fig. 11) uses pressure leaching during the production of nickel and cobalt metals (Ref 20). Fig. 11 Sherritt-Gordon process flow diagram for