In deposit formation

Studies of 2.65 Ga gold systems in the Yilgarn Craton, Western Australia, have emphasised the interaction of intrinsically reduced (CH4 H2 N2 HCI) and oxidized (CO2 SO2) anhydrous fluids as well as hydrous fluids in deposit formation. Carbonate veins have between -4 and -6 %o, typical of mantle values and primary sulfate have of 8 to 12 %o. 

In general each potential corrosion problem that may result in deposit formation on heat transfer surfaces, has to be considered in its own right. The brief description of some of the additives available to combat corrosion shows that there are numerous ways in which the problems may be handled. The final choice will depend on effectiveness required, the cost of the additive and the associated costs. 

In Chapter 8 crystallisation and scale formation are discussed and the effect of pH was demonstrated as being a factor in deposit formation. Furthermore the equilibrium of ions in solution is likely to be affected by the presence of electric currents and the current density. As a consequence, a characteristic feature of cathodically protected metallic surfaces in contact with solutions containing mineral ions, notably sea water, is the formation of deposits or cathodic protection scale. The use of controlled scale formation is mentioned as a method of corrosion control in Chapter 14. Cox [1940] proposed the deliberate formation of calcium deposits on steel by the imposition of large cathodic currents to act as anticorrosive self healing layer. 

Nowadays, ultrafiltration (UF) or microfiltration (MF) membrane processes are widely used because of their ability to remove particles, colloidal species and microorganisms from different liquids feeds. However a limitation inherent in the process is membrane fouling due to the deposition of suspended matter during filtration. Therefore the understanding of formation and transport properties of particle deposits responsible for membrane fouling is a necessary step to optimize membrane processes. Thus it is necessary to obtain local information in order to analyze and model the basic mechanisms involved in deposit formation and then to further predict the process operation. Besides, it is also useful to control the deposit formation and to plan preventive or curative actions with a controlled efficiency. Nonetheless, local parameters such as cake thickness and porosity are hardly reachable with conventional techniques. 

In most cases, CVD reactions are activated thermally, but in some cases, notably in exothermic chemical transport reactions, the substrate temperature is held below that of the feed material to obtain deposition. Other means of activation are available (7), eg, deposition at lower substrate temperatures is obtained by electric-discharge plasma activation. In some cases, unique materials are produced by plasma-assisted CVD (PACVD), such as amorphous siHcon from silane where 10—35 mol % hydrogen remains bonded in the soHd deposit. Except for the problem of large amounts of energy consumption in its formation, this material is of interest for thin-film solar cells. Passivating films of Si02 or Si02 Si N deposited by PACVD are of interest in the semiconductor industry (see Semiconductors). 

The last technique commonly employed to deposit metals for compound semiconductors is electroplating (150). This technique is usually used where very thick metal layers are desired for very low resistance interconnects or for thick wire bond pads. Another common use of this technique is in the formation of air-bridged interconnects (150), which are popular for high speed electronic and optoelectronic circuits. 

Supercritial boilers use all-volatile treatments, generally consisting of ammonia and hydrazine. Because of the extreme potential for deposit formation and steam contamination, no soHds can be tolerated in supercritical once-through boiler water, including treatment soHds. 

Thin films of photochromic glass containing silver haUde have been produced by simultaneous vacuum deposition of siUcon monoxide, lead siUcate, aluminum chloride, copper (I) chloride, and silver haUdes (9). Again, heat treatment (120°C for several hours) after vacuum deposition results in the formation of photochromic silver haUde crystaUites. Photochemical darkening and thermal fade rates are much slower than those of the standard dispersed systems. 

Feed Slurry Temperature Temperature can be both an aid and a limitation. As temperature of the feed slurry is increased, the viscosity of the hquid phase is decreased, causing an increase in filtration rate and a decrease in cake moisture content. The limit to the benefits of increased temperature occurs when the vapor pressure of the hquid phase starts to materially reduce the allowable vacuum. If the hquid phase is permitted to flash within the filter internals, various undesired resiilts may ensue disruption in cake formation adjacent to the medium, scale deposit on the filter internals, a sharp rise in pressure drop within the filter drainage passages due to increased vapor flow, or decreased vacuum pump capacity. In most cases, the vacuum system should be designed so that the liquid phase does not boil. 

Iron and manganese Fe (ferrous) Fe" (ferric) Mn+ Discolors water, and results in the formation of deposits in water lines, boilers and other heat exchangers. Can interfere with dying, tanning, paper manufacture and various process works. 

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