Primera tarea... primera

Queridos colegas,
les muestro el texto a continuación y les encargo que lo vayan leyendo. Mucho ojo, para el primer examen parcial lo van a necesitar, así que mejor vayan traduciendo si no son muy duchos con el Inglés. Ah, también les va a servir en el laboratorio, porque las primeras prácticas tienen mucho que ver con esto.

Saludos


Electrowinning and Refining
The electrowinning of metals, or the production of metals from their ores once put in solution or liquefied, is the oldest industrial electrolytic process. Sodium metal was first prepared in 1807 by the English chemist Humphrey Davy, who obtained it using electrolysis of molten sodium hydroxide. Several industrially important metals (the active metals which react with water) are produced commercially today by electrolysis of molten salts.
Metal deposition rates are key parameters in developing an effective electrowinning unit. Units are most effective when installed in rinse tanks immediately following drag-out tanks. Electrowinning is not a viable recovery technology for all metals. While the process works well for metals with high electro-potential, such as gold, silver, copper, cadmium and zinc, it does not work as well on others, such as chromium. Nickel can be recaptured also. However, the process is very pH sensitive and must be rigorously maintained for any deposition to occur. For metals which electrowinning works, the following criteria are important in the operating performance of electrowinning units:
· Cathode surface area: Because metal deposition rates are related to available surface area, maintaining properly working cathodes is important. Two cathode types exist, flat-plate and reticulated cathodes, each with its own advantages. Flat-plate cathodes can be cleaned and reused, and plated metals recovered. Reticulated cathodes have a much higher deposition rate, compared to flat-plate cathodes. However, they are not reusable and must be sent off for recycling.
· Current density: The deposition rate of metals onto cathodes increases with higher current densities. However, "excess" current is wasted on converting water to hydrogen and oxygen gas, instead of plating out desired metal contaminants.
Copper Refining
Copper is widely used to make electrical wiring and in other applications that utilize its high electrical conductivity. Crude copper, which is usually obtained by pyrometallurgical methods, is not suitable to serve in electrical applications because impurities greatly reduce the metal's conductivity. A purity > 99.7% is therefore required these applications. Purification of copper is achieved by electrorefining. Large slabs of crude copper serve as the anodes in the cell, and thin sheets of pure copper serve as the cathodes.
The Amarillo refinery houses 2,400 tanks in a single building
The electrolyte consists of an acidic solution of CuSO4. Application of a suitable voltage to the electrodes causes oxidation of copper metal at the anode and reduction of Cu2+ to form copper metal at the cathode. This strategy can be used because copper is both oxidized and reduced more readily than water.

Electrolysis cell for refining of copper. As the anodes dissolve away, the cathodes on which the pure metal is deposited grow in size.
The impurities in the copper anode include lead, zinc, nickel, arsenic, selenium, tellurium, and several precious metals including gold and silver. Metallic impurities that are more active than copper are readily oxidized at the anode but do not plate out at the cathode because their reduction potentials are more negative than that for Cu2+. However, less active metals are not oxidized at the anode. Instead, they collect below the anode as a sludge that is collected and processed to recover the valuable metals. The anode sludges from copper-refining cells provide one fourth of U.S. silver production and about one eighth of U.S. gold production


Aluminum Production by Electrowinning
Aluminum compounds, primarily the oxide in forms of various purity and hydration, are fairly widely distributed in nature. The feldspars, the most common rock-forming silicates, make up nearly 54% of the earth's crust; in these, aluminum has replaced up to half the silicon atoms in SiO2. The major ore of aluminum is bauxite, a hydrated aluminum (III) oxide (Al2O3.xH2O).
In the industrial Bayer process, bauxite is concentrated to produce aluminum hydroxide. When this concentrate is calcined at temperatures in excess of 1000oC, anhydrous aluminum oxide, Al2O3, is formed. Anhydrous aluminum oxide melts at over 2000°C. This is too high to permit its use as a molten medium for electrolytic formation of free aluminum. The electrolytic process commercially used to produce aluminum is known as the Hall process, named after its inventor, Charles M. Hall. The purified Al2O3 is dissolved in molten cryolite, Na3AlF6, which has a melting point of 1012oC and is an effective conductor of electric current. In the following schematic diagram of the electrolysis cell graphite rods are employed as anodes and are consumed in the electrolysis process. The cell electrolytic reaction is:
2Al2O3 + 3C --> 4Al(l) + 3CO2(g)

Typical Hall process electrolysis cell used to reduce aluminum. Because molten aluminum is more dense than the molten mixture of Na3AlF6 and Al2O3, the metal collects at the bottom of the cell. (reference)
The cells are ed to use 8,000 A and upwards, and a given cell requires about 5 V although only 2.1 V are theoretically required to decompose aluminum oxide. The excess 2.9 V, plus the heat of combustion of carbon, is used as heat to keep the cell warm. The production of one ton of aluminum requires about 65-70 GJ (18-20 MWh) and about half a ton of carbon. The process is generally nonpolluting but is a heavy consumer of electricity with approximately 36% of the electricity used in the Faradaic process, the rest being lost as heat.

Magnesium Production by Electrowinning
Electrowinning of magnesium is also a major industrial process, and is the most commonly used industrial process for magnesium production although other methods such as chemical reduction of magnesium compounds at high temperatures with carbon, calcium carbide, or ferrosilicon have been used. The raw material for magnesium production is well brines or more commonly seawater, which is about 0.13% magnesium by mass.
The Mg2+ ion is precipitated from seawater by addition of CaO (lime), produced by calcining (strong heating) of CaCO3 (limestone or oyster shells). The insoluble Mg(OH)2 is removed by filtration. Acidification of a slurry of this solid and an aqueous MgCl2 solution with HCl converts the Mg(OH)2 to soluble MgCl2, which is recovered as solid MgCl2 by evaporation. This is dried to the hydrate MgCl2.1.5H2O, which is used as the feedstock to an electrolytic cell.
The cell electrolyte is a molten mixture containing about 25% MgCl2 - 15% CaCl2 - 60% NaCl, and the cell is operated between 700oC and 750oC. The cell electrolytic reaction is:
MgCl2 --> Mg(l) + Cl2(g)
The magnesium produced is about 99.9% pure as it comes from the cell, since neither sodium nor calcium is reducible more easily than is magnesium. The chlorine produced in the cell is converted to HCl for use in the acidification process by reaction with natural gas and steam or in a hydrogen/chlorine burner.

Electrowinning
Electrowinning is employed in PWB shops to remove metallic ions from concentrated rinse water, spent process solutions, and ion exchange regenerant. An electrowinning unit consists of a rectifier and a reaction chamber that houses anodes and cathodes. In the simplest design, a set of cathodes and anodes are set in the reaction chamber containing the electrolyte. When the unit is energized, metal ions are reduced onto the cathode. The rate at which metal can be recovered (i.e., plated onto the cathode) from solutions depends on several factors, including the concentration of metal in the electrolyte, the size of the unit in terms of current and cathode area, and the species of metal being recovered.
Electrowinning is different from other recovery technologies (e.g., evaporation, ion exchange) in that an elemental metal is recovered rather than a metal bearing solution. The recovered metal is usually not pure enough to be used as anode material in plating processes. More often, it is sold as scrap metal.
Electrowinning is particularly applicable for removing metal from solutions containing a moderate to high concentration of metal ions (>3,000 mg/l). Below 1,000 to 2,000 mg/l of metal, the conventional electrowinning process becomes very inefficient. Therefore, it is not thought of as a "compliance" technology, i.e. a technology that will meet wastewater discharge standards. Rather its benefit is in recovering valuable metals that would otherwise be converted to metal hydroxide sludge by the wastewater treatment system.
High surface area (HSA) electrowinning, developed during the 1970s and commercialized in the 1980s with the reticulate cathode design, extends the applicability of this technology to low concentration solutions and in some cases HAS electrowinning may serve as a compliance technology for specific wastestreams. HAS units employing reticulate cathodes are designed as flow though tanks where the electrolyte passes through a series of cathodes. Each reticulate cathode is made up of thread-like material that is woven into a sheet and given a metalized surface. During use, the ions plate onto the surface of the cathode (up to 5 to 10 lbs./ft2). The cathodes are subsequently removed and sold as scrap.
Commercial units employ a variety of strategies designed at increasing plating efficiency at the relatively low metal concentrations found in typical electrolytes available for electrowinning. This is usually accomplished by design innovations that focus on causing motion of the electrolyte across the surface of the cathode or increasing the surface area of the cathode (e.g., HAS). Solution movement reduces the effect of concentration polarization, a condition where the thin film of electrolyte surrounding the cathode is depleted of metal ions. A high cathode surface area permits efficient operation at low metal concentrations.
Electrowinning is applied to a wide variety of chemical solutions found in the PWB industry. Metals that are most commonly recovered by electrowinning are copper, gold, and silver. For practical purposes, the degree to which a metal can be recovered by electrowinning depends on its position in the electromotive series. In general, metals that have more positive standard electrode potentials plate more easily than the ones with less positive potentials. As an illustration, the more noble metals, such as silver and gold, can be removed from solution to less than 1 mg/l using flat plate cathodes whereas with copper and tin, a concentration in the range of 0.5 to 1 g/L or more is required for a homogeneous metal deposit.
Applications
Ion Exchange Cation Spent Regenerant. Ion exchange is employed in a variety of configurations in PWB shops, ranging from closed-loop treatment of single rinse systems to a major component (particularly in small shops) of the waste treatment system handling combined rinse streams from a majority of the wet processes. The metal-rich spent cation regenerant is an ideal and logical candidate for electrowinning. For most PWB ion exchange installations, sulfuric acid is the cation regenerant of choice which is a particularly favorable electrowinning electrolyte.
The ion exchange-electrowinning combination is most efficient with copper-bearing rinses that may be combined from several sources or accomplished at the location of a single process. Although ion exchange may also be employed on tin-lead and gold rinse systems, cation resins primarily employed to remove lead or gold are not usually regenerated on-site due to the difficulty of the cycle or the expense of the required regenerant.
Drag-out Tanks. Since the efficiency of electrowinning falls off as the concentration of metal falls, electrowinning of rinse water is, in general, less efficient that that of ion-exchange regenerant where ion exchange serves to concentrate metal. Nevertheless, electrowinning is quite effective at greatly reducing the introduction of metal into flowing rinses ultimately treated by the shops main waste treatment system when employed on drag-out rinses.
Drag-out tanks are rinse tanks, initially filled with water. Parts are first rinsed in this tank, then proceed to the flowing rinse system. In the most efficient configuration, a drag-out rinse is placed after a heated process tank, and the contents of the drag-out tank are returned (recovered) to the process tank as evaporative loss make-up. The level of the drag-out tank is made-up with fresh water, thereby maintaining the overall concentration of the drag-out tank below that of the process tank. This simple arrangement is quite effective at reducing the mass of metal entering the flowing rinses, but the efficiency of the drag-out tank is a function of the temperature (evaporation rate) of the process fluid. Cool process fluids create little evaporative headroom and little drag-out fluid can be returned.
A major source of copper-bearing drag-out in the PWB shop is the copper electroplating tanks. Today, the overwhelming electrolyte of choice for PWB copper electroplating is copper sulfate. This bath is generally maintained at 80°F or below making it less than an ideal candidate for conventional drag-out recovery rinsing. Some shops have opted for a closed loop ion-exchange/electrowinning system to handle the flowing rinses of the copper electroplating process. A second effective alternative is to employ a drag-out rinse tank connected to an electrowinner, through which the drag-out tank solution is continuously circulated. In this configuration, the metal concentration of the drag-out tank is maintained at a low level (determined by the introduction rate due to drag-in, the metal removal rate of the electrowinner, particularly at low concentration levels which can be greatly enhanced by the use of high-surface-area cathodes), thereby reducing the drag-out of metal to the flowing rinse. A properly sized electrowinning unit can maintain the drag-out tank metal concentration well below 100 mg/L, compared to the 14-25 g/L copper concentration contained in the process fluid. The effect being, with the drag-out and electrowinning configuration, the introduction of copper into flowing rinses will be reduced by two orders of magnitude when compared to a standard flowing or counterflowing rinse system.
In the copper sulfate example, the drag-out tank gradually accumulates sulfuric acid which is not an unfavorable environment for PWBs and therefore, only rarely will the tank need to be dumped. When applied to other plating or preparation processes, the build-up of constituents (the electrowinning is only removing metal) from the process fluid may require more frequent dumping of the drag-out tanks, lowering the overall efficiency of the system.
The drag-out electrowinning configuration can also be employed after tin-lead, nickel and gold plating, with good to excellent results. Recovery of gold from drag-out tanks is common for obvious economic reasons although many shops also opt for ion exchange of gold rinse water which is another effective method of gold recovery. Nickel recovery from dragout tanks following nickel electrolytic plating is less common. Nickel plating baths are generally heated to above 120°F, making conventional drag-out recovery effective. Also many shops only plate nickel on connector edges (only a small portion of the PWB is immersed and the drag-out is much lower compared to copper electroplating) and a significant percentage of PWBs may receive no nickel plating at all, making an investment in nickel drag-out recovery less attractive. On the other hand, the value of nickel is 3 to 4 times that of copper making nickel recovery more economically attractive to shops that do full panel nickel plating or otherwise generate above-average nickel drag-out.
Tin-lead plating, like copper sulfate, is generally performed at low temperature making the drag-out, electrowinning configuration attractive. The move in the industry away from tin-lead to tin-only plating and the competition from ion exchange of tin-lead flowing rinses has limited the use of electrowinning. Although the drag-out with electrowinning configuration will perform nearly as effectively with tin lead as with copper, a few factors combine to make the overall system somewhat more expensive and less attractive strategically. The tin-lead plating operation is basically the only source of lead in the rinsewater of a PWB shop (other sources may contribute minute amounts). Cation exchange essentially can remove nearly all lead from the rinses from this operation thereby preventing any lead from reaching the conventional waste treatment system. Elimination of lead from rinsewater available from ion exchange may be viewed favorably to the reduction of lead achieved by electrowinning. Also, the common electrolyte for tin-lead plating is flouboric acid which necessitates the use of expensive, precious-metals-coated anodes.
Tin is not usually recovered (as it is usually not regulated) except incidentally along with lead in ion exchange or electrowinning systems.
Spent Process Fluids. It is possible to electrowin several spent process fluids to recover metal and/or to reduce the burden on the general waste treatment system. While there are several spent process fluids found that are easily electrowinned, it is not an extremely common practice due mainly to a combination of competing technologies or treatment methods, and the cost of handling irregularly timed or one-time dumps of potential electrowinning candidates.
A large process fluid waste stream that can be electrowinned is spent micro-etchant. The currently favored micro-etchant chemistries are sulfuric-persulfate or sulfuric-peroxide. These baths contain 20-40 or more g/L of copper when spent. Both are strong oxidizing solutions and the spent bath is usually reduced with sodium meta-bisulfite or other reducing agent before electrowinning. With very high copper concentrations and the favorable sulfuric acid-based electrolyte, electrowinning proceeds at very high efficiencies, near coppers theoretical maximum plating rate of 1.19 g/amp-hour. A unit capable of delivering 500 amps can easily plate a pound of copper out of such solutions in a single hour, the proceeds from which can easily cover the cost of energy and depending on the level of automation and the amount of labor involved in the handling and preparation of the electrolyte, may also cover the operating costs.
A study at one circuit board shop involving copper recovery from microetchant demonstrated that an 80 to 90% electrowinning efficiency could be achieved while reducing metal content to 1 mg/l in the waste stream (ref. 7). Two types of cathodes were used in an air-sparged electrolytic recovery cell during a two-step operation. Flat reusable stainless-steel sheet cathodes were first used to reduce the copper concentrations of the solution from 20 grams per liter down to 500 milligrams per liter. The cathodes are removed and the copper is peeled off (up to 2 lbs. of copper per cathode). The second step employs disposable high-surface-area cathodes that collect up to 3 lbs. of copper per cathode and reduce the copper concentration of the solution below 1 mg/l. After two years, the original stainless-steel cathodes were still in use and nearly 1,000 lbs. of copper has been collected. The process reportedly reduced hazardous waste generation by more than 35 tons. Another benefit is a reported 50% reduction in the cost of operating the wastewater treatment system.
Electrowinning of micro-etchant competes with common and simple treatment methods. In the case of sulfuric-peroxide, chilling is very effective. Both sulfuric-peroxide and sulfuric-persulfate spent baths can be shipped off-site for copper recovery. This option is often attractive to small shops that produce only a hundred gallons of spent micro-etchant/year.
Another large process waste stream is spent sulfuric acid dips. These, too, are readily electrowinned. The concentration of copper in these spent baths, however, is low, usually 1 g/L or less. While electrowinning can reduce the copper concentration much lower, efficiency at this concentration is reduced and plating times per unit of copper recovered much higher.
Spent electroless copper is also an electrowinning candidate, but more effective, easier methods are available. Spent electroless copper is produced steadily by bail-out (to make room for frequent additions) and the bath itself is relatively short-lived. Most shops prefer to treat the bailout by passing it through activated foam canisters that cause copper to plate-out on the media surface. The effluent from these cartridges is nearly metal-free.
Spent gold baths are commonly electrowinned although credits for gold content can also be obtain by shipping the spent bath for off-site recovery.
Other spent electroplating fluids (tin, tin-lead, copper, nickel) are not produced in large quantities. Copper sulfate baths may last for several years and the timing of the dump is based solely on analysis. Similar bath lives are common for nickel and tin-lead making an electrowinning recovery strategy for these baths uncommon.
Restrictions. Solutions containing hydrochloric acid, or the chlorine ion in general, are usually not processed using electrowinning since electrolysis of these fluids can result in the evolution of chlorine gas. Fluoboric acid electrolytes, such as tin-lead fluoborate, generally require platinized anodes, affecting the cost-effectiveness of electrowinning such solutions. Solutions containing chelated metals, reducing agents, and stabilizers are more difficult for direct application of electrowinning.
Nickel recovery using electrowinning is possible, but it requires close control of pH and is therefore performed less frequently than for metals such as copper and gold.

ELECTROWINNING AS A PART OF A TOTAL TREATMENT STRATEGY.
Almost all electronic component manufacturers use a combination of treatment methods to maximize effluent quality. These methods can include, precipitation, ion exchange, micro/ultrafiltration, electrowinning, chemical reduction, etc. These methods when use optimally can reduce toatal metal output to less than 0.1 ppm. When not used correctly, operational costs accelerate rapidly and metal levels in the effluent rise to higher levels.Electrowinning support for your batch treatment streams can dramatically reduce both cost and metal loadings on your primary treatment methodology. Electrowinning can rapidly recover hundreds of pounds of heavy metals, reduce chrome, oxidize cyanide, and destroy organics, all at minimal operational costs. Reducing the load on the primary treatment system allows the implimentation of a water reduction strategy without worries of becomming non-compliant.The cost to electrowin is very low. A 3 phase 300 amp rectifier at 6 volts draws less than 10 amps. This will produce 24 pounds of copper per day. The cost at $ 0.11 per kwh is $6.36 per day. What does it cost you to make, filter, handle, and haul 24 pounds of Copper based sludge away? Our product is either a copper sheet (very high volume cells) or copper pellets.Most copper in in the electronics industry is from batch dumps of microetches and dragouts. In the printed circuit board industry, much comes from electroless copper dumps. Note that all of these sources are concentrated sources, very suitable for electrowinning. If you took all of your concentrated sources and electrowinned them to recover the copper, how much would this reduce the load on you treatment system? A side benefit of electrowinning is that it oxidizes organics such as chelation compounds and formaldehyde. Add a touch of chlorine and cyanide is destroyed. There are many advantages to having an electrowinning system.That big dumpster out back will go away for good as will big sacks of sludge. The labor and chemical cost reduction is very large when you switch to electrowinning. The cost in time and chemicals it takes to batch treat a load is very high because very few batch treatment operations are automated and the operator must determined how much of what to add with only a pH controller to guide him. It is much easier for an operator to just dump in a bunch of metal reducing agent (DTC, Sodium Borohydride, etc.) to drop the metal out than to adjust to the correct pH, measure, the add an exact amount of precipitant. The result is much higher treatment costs.An electrowinning system needs little operator attention. Once started, it just runs until there is no more metal to plate out. Usually the visual appearance of the solution will tell you how close to finished you are. Once finished, the solution is pH adjusted if necessary and bled into an onstream treatment system directly.Electrowinning, while simple in appearance, can also be simple in operation given some precautions are taken. The first thing to remember with all electrowinning systems is inherent in the system. The systems must conduct electricity so they have metal components. These metal components are subject to corrosion and due to the electrochemical nature of these systems, the corrosion can be very rapid. Proper maintance is very important. These systems must be maintained by using passive coatings to protect metal components. Wear or breaks in the coatings will result in eventual downtime and higher maintance.

Reglas de la asignatura

1720. Corrosión y Protección.

Asignatura Teórico-Práctica.- 9 créditos (6 y 3, respectivamente).

Martes y Jueves 9:00 a 10:30
Salón Seminario I, Edificio D.

Asistencia mínima: 80 % (5 faltas máximo, 2 retardos equivalen a una falta, 10 minutos tolerancia)

Calificación Total:

Dos exámenes parciales + seminario + trabajo escrito (no más de 5 páginas) = Calificación de teoría

Calificación de Laboratorio (según profesor).

Teoría 67%
Laboratorio 33%

Total 100%

Tareas: 100% de tareas entregadas indispensable.

No se guardan calificaciones.
No se aceptan oyentes
No se acepta que se “encimen” horarios... se sugiere dar de baja alguna de las dos materias.

La calificación en examen final se promediará con el laboratorio.

Trabajo escrito:
No más de 5 páginas.
Bibliografía completa (no menos de tres libros consultados).


Temario:

Repaso electroquímico.
Celdas electroquímicas
Ecuación de Nerst
Diagramas de Pourbaix.

Corrosión
Técnicas electroquímicas para el seguimiento de la corrosión
i. Pérdida de peso.
ii. Extrapolación de Tafel.
iii. Resistencia a la polarización.
iv. Fundamentos de Impedancia.

EXAMEN PARCIAL

Formas de corrosión.
i. Corrosión por picaduras
ii. Corrosión en resquicios
iii. Corrosión filiforme
iv. Corrosión microbiológica
v. Corrosión asistida por esfuerzos (SCC y fatiga)
vi. Corrosión atmosférica

Protección anticorrosiva.
Protección catódica
i. Definición
ii. Criterios
iii. Protección por ánodos de sacrificio
iv. Protección por corriente impresa
Inhibidores
i. Clasificación
ii. Mecanismos
iii. Orgánicos
iv. Inorgánicos
v. Fase vapor
Recubrimientos.
i. Generalidades.
ii. Metálicos
iii. Inorgánicos
iv. Orgánicos

EXAMEN PARCIAL

reglamento general de exámenes

Queridos amigos, les anexo un enlace al Reglamento General de exámenes ya que puede ser de su interés, por tratarse de nuestro marco legislativo. La calificación de la asignatura tendrá que seguir estos lineamientos. Léanlo y con toda confianza acérquense a preguntar.

Saludos y siéntanse en libertad de comentar sus inquietudes al respecto.

http://xenix.dgsca.unam.mx/oag/consulta/

¿Qué opinan de esta imagen? ¿Han visto alguna cosa similar en su ciudad? La corrosión está por todas partes, ya que se trata de un proceso espontáneo. Y en esta foto está mostrada una farola del Puerto de Santa Cruz, pero podríamos mostrar las picaduras en la ventanería de aluminio cerca del mar o los problemas de corrosión en un automóvil cualquiera.