Surface-deposited products

Organochromium Catalysts. Several commercially important catalysts utilize organ ochromium compounds. Some of them are prepared by supporting bis(triphenylsilyl)chromate on siUca or siUca-alumina in a hydrocarbon slurry followed by a treatment with alkyl aluminum compounds (41). Other catalysts are based on bis(cyclopentadienyl)chromium deposited on siUca (42). The reactions between the hydroxyl groups in siUca and the chromium compounds leave various chromium species chemically linked to the siUca surface. The productivity of supported organochromium catalysts is also high, around 8—10 kg PE/g catalyst (800—1000 kg PE/g Cr). 

Internal surfaces of all tubes were severely attacked (Fig. 4.29). A brown deposit layer consisting of magnetite, iron oxide hydroxide, and silica covered all surfaces. Deposition was thicker and more tenacious along the bottom of tubes. These deposits had a distinct greenish-blue cast caused by copper corrosion products beneath the deposit. Underlying corrosion products were ruby-red cuprous oxide crystals (Fig. 4.29). Areas not covered with deposits suffered only superficial attack, but below deposits wastage was severe. 

HONO the mean flux was an emission of 1 ng Nm s but this includes periods both of emission and deposition. On several occasions, no concentration gradients were detected. The direction of the flux was dependent on NOj concentration, with emission observed only when NOj concentration was less than 10 ppb. The process of HONO exchange appears to be regulated by the net result of small deposition flux to the surface and a surface chemistry production of HONO from NOj. Fluxes of PAN deposition were measured using a chamber technique " and were small (less than 0.5ng Nm s ). 

Beryllium, like its neighbours Li and B, is relatively unabundant in the earth s crust it occurs to the extent of about 2 ppm and is thus similar to Sn (2.1 ppm), Eu (2.1 ppm) and As (1.8 ppm). However, its occurrence as surface deposits of beryl in pegmatite rocks (which are the last portions of granite domes to crystallize) makes it readily accessible. Crystals as large as 1 m on edge and weighing up to 60 tonnes have been reported. World reserves in commercial deposits are about 4 million tonnes of contained Be and mined production in 1985-86 was USA... 

Where particulate matter (in the form of corrosion products of iron oxide) is present in returning condensate, it often contains copper, nickel, and zinc oxides as well. This debris can initiate foaming (through steam bubble nucleation mechanisms) leading to carryover. It certainly contributes to boiler surface deposits, and the Cu usually also leads to copper-induced corrosion of steel. 

As corrosion products develop, so the rate of 02 diffusion reduces and the rate of general etch corrosion slows down. But in practice, the presence of surface deposits tends to promote various forms of localized corrosion such as tuberculation, pitting corrosion, and stress corrosion, and consequently the rate of corrosion continues unabated. 

Surface deposition is the most important parameter in reduction of the free and aerosol attached radon decay products in room air. If V is the volume of a room and S is the surface area available for deposition (walls, furniture etc), the rate of removal (plateout rate) q is vg S/V, always assuming well mixed room air. vg is the deposition velocity. 

Volcanic and Other Surface Deposits. Sulfur is recovered from volcanic and other surface deposits by a number of different processes, including distillation, flotation, autoclaving, filtration, solvent extraction, or a combination of several of these processes. The Japanese sulfur deposits are reached by tunnel, and mining is done by the room-and-pillar, chamber-and-pillar with filling, and cut-and-fill systems. Sulfur was historically extracted from the ore by a distillation process performed in rows of cast-iron pots, each containing about 180 kg of ore. Each row of pots is connected to a condensation chamber outside the furnace. A short length of pipe connects each pot with a condenser. Brick flues connect combustion gases under the pots. Sulfur vapor flows from the pots to the condensation chamber where the liquid sulfur is collected. The Japanese ore contains 25—35 wt % sulfur. This method has been superseded by other sources of sulfur production. 

Even though paleoaltimetric data from internal structural elements of orogenic belts and plateaus represent much needed complementary information to those derived from surface deposits or weathering products we caution about the uncritical use of stable isotopic data from deeper Earth environments in paleoaltimetric studies. It is highly desirable to obtain reliable thermometric, structural, and isotopic tracer data before attempting any paleoaltimetric reconstruction in such environments, as uncertainties exist about the fluid pathways and mechanisms responsible for fluid transport into the ductile crust. Maybe more importantly, it is imperative to document that the timing of meteoric water-rock interaction can be dated precisely, especially within thermally and kinematically rapidly evolving tectonic environments such as extensional detachment systems. 

Magnesium methoxide has been found to be a very effective neutralizer. However, on a damp day or with a damp paper, the solution tends to precipitate prematurely and leave surface deposits on the treated paper. Methyl magnesium carbonate is also effective but much less sensitive to water. Both these products produce adequate alkaline reserves in paper. Since methanol is used as the solvent, the deacidification should be conducted in a well-ventilated hood. 

In the analysis of CVD reactions, it is important to recognize the rates of the various processes. The slowest rate will be controlling, and which one is the slowest or fastest can depend on gas as well as surface conditions. For example, surface reactions may be fast at high surface temperatures. In this case, the CVD process will tend to be limited by the rate at which reactants can get to the surface or products leave it. For this situation, the fluid dynamic boundary layer phenomena will govern the deposition rate. On the other hand, at low pressures diffusion is very rapid and the rate at which surface reactions proceed will tend to govern the deposition rate. Alternatively, low surface temperatures will have low reaction rates, and this will govern no matter how much material diffuses to the surface. 

In an environment with a constant redox condition (e.g., permanently aerated and/or constant pH), a condition not uncommon in industrial and environmental situations, corr could shift in the positive direction for a number of reasons. Incongruent dissolution of an alloy could lead to surface ennoblement. Alternatively, as corrosion progresses, the formation of a corrosion product deposit could polarize (i.e., increase the overpotential, i), for) the anodic reaction as illustrated in the Evans diagram of Fig. 4. Polarization in this manner may be due to the introduction of anodic concentration polarization in the deposit as the rate of transport of dissolved metal species away from the corroding surface becomes steadily inhibited by the thickening of the surface deposit i.e., the anodic half-reaction becomes transport controlled. 

The pesticide residues remaining in mans food and in animal feed may exist as a surface deposit, penetrate the cuticle to a limited extent, or be transported to various sections of the plant or animal by systemic action. The pesticide, once deposited, is usually metabolized or broken down to its end products by various means. Systemic pesticides often exhibit complex breakdown patterns which can differ somewhat, depending on whether a plant or mammalian metabolic mechanism is involved ... 

These processes occur by precipitation through evaporative concentration of a solute in the aqueous medium until its dissolution capacity is exceeded. Then, a solid is formed and deposited either as a sediment or on a nearby surface. These products are called evaporites. A typical example is the deposition and formation of calcium carbonate stalactites and stalagmites. Evaporation is a major process in arid areas and it influences the chemistry of surface waters. That is why in saline lakes, inland seas, or even in estuaries, evaporites of NaCl or NaCl/KCl and deposits of CaS04 and CaC03 are formed. Here, CaS04 generally precipitates first, and then NaCl. 

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