Carbon deposition solution-precipitation processes

The core and shell type of particulates are similar to one of the deposit morphologies formed on an Fe-Ni alloy from CO at temperatures above 500°C where the core consisted of a metal particle in the size range 0.09 to 0.2 pm, with a shell thickness typically of 0.04 jjm(23). The structure of the particles, i.e. a carbon layer on metal, is comparable to the laminar film on the metal, suggesting that the carbon in the shell has been precipitated. Free metal particles have not been observed on the iron foils that could serve as active centres for growth directly from the gas phase. Therefore, it must be concluded that a solution-precipitation process plays a part in determining the final morphology of the core / shell particles, but further details of the mechanism of growth cannot be established at present. 

Sepiolite and palygorskite have a rather special composition and seem to be related to specific mineral parageneses. They appear to be stably associated with montmorillonite, corrensite, serpentine, chert, sulfates, carbonates and various salts. They are found in deposits typified by processes of chemical precipitation or solution-solid equilibria (Millot, 1964) and are therefore rarely associated in sediments with large quantities of detrital minerals. Their chemical environment of formation is in all evidence impoverished in alumina and divalent iron. Their frequent association with evaporites, carbonates and cherts indicate that they came from solutions with high chlorinity. 

The conditions of deposition of iron from ionic solutions are depicted more visually in Fig. 43, on which curves of the beginning of deposition of hydroxide and silicate-carbonate sediments are entered. Amorphous Fe(OH)3 hydroxide can be deposited in a wide pH range from solutions with various ratios of ferrous and ferric iron activities. However, already at pH > 3, > flpgs in the solution. Consequently, if free oxygen is introduced into the solution, precipitation of Fe(OH)3 hydroxide, in which this oxygen is completely fixed, should theoretically occur. The actual course of this process is determined by the rate of the reaction ... 

To these sets of primary and secondary reactions related to solvents, one has to add the eontributions of salt anion reduction, which usually forms metal halides and M AXy species (A is the main high oxidation-state element in the salt anion and X is a halide, such as chloride or fluoride). Most of the produets of aetive metal surface reactions are ionic compounds that are insoluble in the mother solution, and therefore, precipitate as surface films. It should be added to this picture that possible polymeric species can be formed, espeeially in alkyl carbonate solvents, whose reduction forms polymerizable species sueh as ethylene or propylene. Hence, the surface films formed on active metal electrodes are very complicated. They have a multilayer structure perpendicular to the metal surface, and a lateral, mosaic-type composition and morphology (i.e. containing mixtures and islands of different compounds and grains). Such a structure may induce very non-uniform current distribution upon metal deposition or dissolution processes, which leads to dendrite formation, a breakdown of the surface films, etc. These situations are demonstrated in Fig. 13.6 active metal dissolution leads to the break-and-repair of the surface films, thus forming mosaic-type structures. 

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. 

In the 1980s, zinc precipitation was replaced by a method involving the passing of the solution over activated carbon to adsorb the precious metals, which are then stripped from the charcoal by a hot caustic solution. Electrowinning removes the precious metals from this solution, depositing them on the cathode. One benefit of this process is that the filtration or deareation steps of the zinc precipitation are not required. Thus the environmental hazard of the zinc salts is eliminated. 

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