Metal removal

The lowest concentration of metal ions in solution is achieved when the pH of the solution reaches the minimum solubility for that particular metal ion. The practical maximum working pH of magnesium hydroxides is about 8.5. Attempting to raise the pH above this level will result in overdosing, and any unreacted magnesium hydroxide contributes to the sludge. 

There are two mechanism at work when Mg(OH)2 is being used to neutralize metal-bearing acidic waste. First, the supply of hydroxyl ion by dissolution for metal precipitation as the hydroxide see reactions (10.2) and (10.3)  

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Duolite C-433 4.5 1.19 Acrylic-DVB type very high capacity. Used for metals removal and neutralization of alkaline solutions. 

In addition to then use in bonded and coated products, both natural and manufactured abrasive grains are used loose in such operations as polishing, buffing, lapping, pressure blasting, and barrel finishing. AH of these operations are characterized by very low metal removal rates and are used to improve the surface quaUty of the workpiece. 

Many factors other than current influence the rate of machining. These involve electrolyte type, rate of electrolyte flow, and other process conditions. For example, nickel machines at 100% current efficiency, defined as the percentage ratio of the experimental to theoretical rates of metal removal, at low current densities, eg, 25 A/cm. If the current density is increased to 250 A/cm the efficiency is reduced typically to 85—90%, by the onset of other reactions at the anode. Oxygen gas evolution becomes increasingly preferred as the current density is increased. 

Surface Finish. As well as influencing the rate of metal removal, electrolytes also affect the quality of surface finish obtained in ECM. Depending on the metal being machined, some electrolytes leave an etched finish. This finish results from the nonspecular reflection of light from crystal faces electrochemicaHy dissolved at different rates. Sodium chloride electrolyte tends to produce a kind of etched, matte finish when used for steels and nickel aHoys. A typical surface roughness average, Ra is about 1 ]lni. 

An attraction of the ECAM technique is the very fast rates of metal removal attainable by the combined effects of sparking and ECM. Eor example, in comparison to hole drilling rates for EDM and ECM, respectively 0.1 and 5.0 mm/min, rates of 15-40 mm/min may be achieved by ECAM. The ECAM technique can be appHed in all the ways discussed for ECM, thus surfaces can be smoothed and ddUed. Turning is also possible, as is wire machining (17). 

Eor drilling, the discharge action occurs at the leading edge of the tool, whereas ECM takes place on the side walls between the tool and the workpiece. The combined spark erosion and ECM action yields fast rates of metal removal. Because ECM is stiU possible, any metallurgical damage to the components caused by the sparking action can be removed by a short period (eg, 15 s) of ECM after the main ECAM action. Currents of 250 A at 30 V are typically used in the process. 

The process operates at 1200°C and < 400 Pa (3 tort) and has a cycle time of 20—24 hours. The reactor is opened at the flange and the metal removed. Energy usage in the furnace is 7-7.3 kWh/kg magnesium. A similar process is used by Brasmag (Minas Givras, Bra2il) (56). 

Hard-burned magnesias may be used in a variety of appHcations such as ceramics (qv), animal feed supplements, acid neutralization, wastewater treatment, leather (qv) tanning, magnesium phosphate cements, magnesium compound manufacturing, fertilizer, or as a raw material for fused magnesia. A patented process has introduced this material as a cation adsorbent for metals removal in wastewater treatment (132). 

In metallurgical practice, sodium uses include preparation of powdered metals removal of antimony, tin, and sulfur from lead modification of the stmcture of siHcon—aluminum alloys appHcation of diffusion alloy coatings to substrate metals (162,163) cleaning and desulfurizing alloy steels via NaH (164) nodularization of graphite in cast iron deoxidation of molten metals heat treatment and the coating of steel using aluminum or zinc. 

High Speed Steels. Toward the latter part of the nineteenth century, a new he at-treatment technique for tool steels was developed in the United States (3,17) that enabled increased metal removal rates and cutting speeds. This material was termed high speed steel (HSS) because it nearly doubled the then maximum cutting speeds of carbon—low alloy steels. Cemented carbides and ceramics have since surpassed the cutting speed capabiUties of HSS by 5—15 times. 

Electrokinetics. Electrokinetics is a tested technology that has been used for over half a century to dewater and stabilize soils, and has recently been investigated for in situ use at hazardous waste sites (23). Primarily used for metals removal, the technology utilizes an electrical field to generate a flow and concentration gradient in porous and semiporous soils. 

Heavy Metals Removal. Heavy metals should be removed prior to biological treatment or use of other technologies which generate sludges to avoid comingling metal sludges with other, nonhazardous sludges. 

Table 12. Effluent Levels Achievable in Heavy Metal Removals...

Chemical Oxidation. Chemical oxidation can be appHed ia iadustrial wastewater pretreatment for reduction of toxicity, to oxidize metal complexes to enhance heavy metals removal from wastewaters, or as a posttreatment for toxicity reduction or priority pollutant removal. 

The total emissions of hazardous air pollutants from a CGCC plant having wet cleanup are expected to be at least an order of magnitude lower than those achievable from a modem coal-fired steam plant (41). Metals removal in hot-gas cleanup systems is still under development. 

Effluents from both dye works and dyehouses are treated both before leaving the plant, eg, neutrali2ation of acidic and alkaline Hquors and heavy metal removal, and in municipal sewage works. Various treatments are used (34). 

M. M. Cook and co-workers, "Sodium Borohydride Reductions—Novel Approaches to Decolorization and Metals Removal iu Dye Manufacturiug and Textile Effluent Applications," 203rd National Meeting of the American Chemical Society, San Erancisco, Apr. 5—10,1992. 

Eor the cover-coat direct-on process, a ferric sulfate [10028-22-5] Ee2(S0 2> etch is included in the metal pretreatment for rapid metal removal. It is designed to remove ca 20 g/m (2 g/ft ) of iron from the sheet metal surface. Hydrogen peroxide [7722-84-1/, H2O2, is added intermittently to a 1% ferric sulfate solution to reoxidize ferrous sulfate [7720-78-7] EeSO, to ferric sulfate. 

Suction Limitations of a Pump Whenever the pressure in a liquid drops below the vapor pressure corresponding to its temperature, the liquid will vaporize. When this happens within an operating pump, the vapor bubbles will be carried along to a point of higher pressure, where they suddenly collapse. This phenomenon is known as cavitation. Cavitation in a pump should be avoided, as it is accompanied by metal removal, vibration, reduced flow, loss in efficiency, and noise. When the absolute suction pressure is low, cavitation may occur in the pump inlet and damage result in the pump suction and on the impeller vanes near the inlet edges. To avoid this phenomenon, it is necessary to maintain a required net positive suction head (NPSH)r, which is the equivalent total head of liquid at the pump centerline less the vapor pressure p. Each pump manufacturer publishes curves relating (NPSH)r to capacity and speed for each pump. 

Chemical precipitation can remove 95 percent of the suspended solids, up to 50 percent of the soluble organics and the bulk of the heavy metals in a wastewater. Removal of soluble organics is a function of the coagulant chemical, with iron salts yielding best results and lime the poorest. Metal removal is primarily a function of pH and the ionic state of the metal. Guidance is available from solubihty product data. 

So far, we have been talking in our case study about the advantage of an oxide layer in reducing the rate of metal removal by oxidation. Oxide films do, however, have some disadvantages. 

Types of damage can be classified as uniform or localized metal removal, corrosion cracking or detrimental effects to the environment from the corrosion products. Local attack can take the form of shallow pits, pitting, selective dissolution of small microstructure regions of the material or cracking. Detrimental effects are certainly not the case with buried pipelines, but have to be considered for environments in vessels and containers. It is usual, where different results of reactions lead... 

The passage of electrons from the metal to the electrolyte is not directly related to metal removal, but has an indirect connection due to the electron neutrality law ... 

If the amount of metal removal by erosion is significant the surface will probably be continually active. Metal loss will be the additive effect of erosion and active corrosion. Sometimes the erosion rate is higher than that of active corrosion. The material selection judgment can then disregard coirosion and proceed on the basis of erosion resistance provided the corrosion rates of aetive surfaces of the alloys considered are not much different. As an example of magnitudes, a good high-chromium iron may lose metal from erosion only a tenth as fast as do the usual stainless steels. 

A useful property of liquids is their ability to dissolve gases, other liquids and solids. The solutions produced may be end-products, e.g. carbonated drinks, paints, disinfectants or the process itself may serve a useful function, e.g. pickling of metals, removal of pollutant gas from air by absorption (Chapter 17), leaching of a constituent from bulk solid. Clearly a solution s properties can differ significantly from the individual constituents. Solvents are covalent compounds in which molecules are much closer together than in a gas and the intermolecular forces are therefore relatively strong. When the molecules of a covalent solute are physically and chemically similar to those of a liquid solvent the intermolecular forces of each are the same and the solute and solvent will usually mix readily with each other. The quantity of solute in solvent is often expressed as a concentration, e.g. in grams/litre. 

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